WO1998028827A1

WO1998028827A1 - Sagnac raman amplifiers and cascade lasers

Info

Publication number: WO1998028827A1
Application number: PCT/US1997/023856
Authority: WO
Inventors: Mohammed N. Islam
Original assignee: Photonics Innovations Research, Inc.
Priority date: 1996-12-23
Filing date: 1997-12-22
Publication date: 1998-07-02
Also published as: US5778014A; AU5903098A

Abstract

A distributed gain medium, such as an optical fiber (24), is configured as a Sagnac interferometer or loop mirror (30), and this mirror (30) is used as at least one of the two reflectors (22) in the amplifier or laser (26). The distributed gain medium produces optical siganl gain through nonlinear polarization, that may cascade through several orders. The Sagnac interferometer or loop mirror (30) defines two optical paths that will support both common mode and difference mode optical signals. Pump fluctuations resulting from higher cascade orders are at least partially rejected through the difference mode signal path, thereby reducing the overall effects of pump fluctuation. The result is a broadband optical resonator, suitable for use at a variety of different wavelengths, including 1.3um and 1.55um wavelengths. Amplifiers based on this technology are 'four-level', providing a pass through signal even when the pump laser is not functioning.

Description

SAGNAC RAMAN AMPLIFIERS AND CASCADE LASERS

Background and Summary of the Invention

The capacity of telecommunication systems has been increasing

by an order of magnitude every three to four years since the mid 1 970s, when

optical fibers were first introduced into these systems. Optical fibers provide a

four-order-of-magnitude bandwidth enhancement over twisted-pair copper

wire, and over the past 20 years engineers have been mining this bandwidth-

rich medium. By about 1 986 the theoretical loss limits on optical fibers had

been reached, and many wondered whether the capacity increase would begin

to saturate by the early 1 990s. However, quite to the contrary, with the

introduction of erbium-doped fiber amplifiers (EDFAs) around 1 990 to replace

electronic repeaters, the capacity has grown by almost two orders of

magnitude since. Thus every prediction of telecommunication system trends

prior to 1 990 are incorrect because they did not take into account the

paradigm change associated with EDFAs. Also, from a practical standpoint,

experience shows that no matter how much bandwidth is made available,

people will find innovative ways of using it.

Although EDFAs have had a significant impact in the past five

years, they are not without problems. First EDFAs work at an optical

wavelength near 1 .55 micrometers (μm), yet most of the terrestrial fibers

installed in the United States during the 1 970s and up through the mid 1 980s are designed for operation at 1 .3 μm. Thus thousands of miles of 1 .3 μm

terrestrial fiber have already been laid and this presents major difficulties in

upgrading to the higher bandwidth EDFA technology. Some have sought to

combine EDFAs with dispersion compensators, in an effort to correct the

wavelength mismatch. However this approach does not permit further

upgrading based on wavelength-division-multiplexing, and it therefore is not

seen as the best solution. Others are experimenting with new glass

formulations that might provide the advantages of EDFAs at the shorter 1 .3

μm wavelength. However, currently no glass formulation has proven to be

commercially viable.

Aside from the wavelength mismatch, EDFAs are also inherently

prone to signal loss when the pump laser fails. EDFAs are systems of the type

known as "three level" systems that will not allow the optical signal to pass

through unless the pump laser is operative. Reliance on three level systems

can have catastrophic consequences for the reliability of fiber networks. To

overcome this, much expense is required to provide redundancy. A more

reliable system would be a "four-level" system that simply provides no gain

when the pump laser is off, but otherwise allows the optical signal to pass

through the system.

The present invention is such a four-level system, employing

stimulated Raman scattering amplifiers based on fused silica fibers. Unlike

current EDFA technology, the present invention works well at 1 .3 μm. Indeed,

the present invention employs a broadband technology that will work well over

a wide range of different optical wavelengths. Stimulated Raman scattering amplifiers work on an entirely

different principle than EDFAs. Stimulated Raman scattering amplifiers are

based on nonlinear polarization of the dielectric silica host, whereas EDFAs are

based on the doping of glass fibers with rare earth ions. Signal amplification in

Raman amplifiers is due to stimulated scattering accompanied by the excitation

of molecules into a vibrational state. In contrast, signal amplification in EDFAs

is due to stimulated emission accompanied by relaxation of the excited ions to

the ground state. Thus Raman amplifiers and erbium-doped amplifiers work on

entirely different physical principles.

The nonlinear polarization in Raman amplifiers is third order in

electric field strength, resulting in a nonlinear index of refraction and gain that

are both proportional to the instantaneous pump intensity. In contrast, the

medium polarization is linear in the EDFA.

From a functional standpoint, stimulated Raman scattering

amplifiers can be pumped at any wavelength (i.e. , there is no pump absorption

band), while the signal gain characteristics are determined by the optical

phonon spectra. By comparison, the pump and signal band characteristics

(i.e., the spectrum and center wavelength) of EDFAs are fixed by the rare

earth atomic resonances. This means that stimulated Raman scattering

amplifiers are capable of cascading to higher orders or longer wavelengths,

whereas EDFAs present no opportunity to cascade in wavelength.

Cascading is the mechanism by which optical energy at the pump

wavelength is transferred, through a series of nonlinear polarizations, to an

optical signal at the longer signal wavelength. Each nonlinear polarization of the dielectric produces a molecular vibrational state corresponding to a

wavelength that is offset from the wavelength of the light that produced the

stimulation. The nonlinear polarization effect is distributed throughout the

dielectric, resulting in a cascading series of wavelength shifts as energy at one

wavelength excites a vibrational mode that produces light at a longer

wavelength. This process can cascade through numerous orders.

The ability to cascade makes stimulated Raman scattering

amplifiers very desirable, for it allows operation over a wide range of different

wavelengths. There is, however, a significant problem with Raman amplifiers

that has. not heretofore been overcome. Virtually every light source or pump

produces some intensity fluctuation. When Raman amplifiers are allowed to

cascade through several orders, the pump source intensity fluctuations are

combinatorially multiplied, and very rapidly result in enormous intensity

fluctuations that have heretofore made systems virtually unusable.

Compounding the problem the gain produced by this nonlinear response in the

third order electric field strength is proportional to instantaneous pump

intensity. Thus there is no opportunity to "average out" intensity fluctuations

over time. Moreover, the gain produced by Raman scattering is, itself, an

exponential effect. All of these properties have lead most to conclude that

stimulated Raman scattering amplifiers and cascade lasers are not suitable in

general purpose telecommunication applications.

The present invention attacks this major problem by recognizing

that higher order intensity fluctuations are a distributed effect (everywhere present in the distributed gain medium that produces the optical signal gain) that can be significantly reduced by a reflector structure that rejects intensity fluctuations originating in this distributed effect. The present invention

employs a reflector structure that defines two optical paths within the distributed gain medium, configured to support both common mode and difference mode optical signals. By choosing a configuration that propagates higher order intensity fluctuations in the difference mode, much of the unwanted amplification of pump fluctuations is rejected.

Although numerous configurations are possible, one embodiment

employs a Sagnac interferometer as one of the two optical resonator reflectors. The Sagnac interferometer employs an optical coupler with both ends of a fiber loop (a distributed gain medium) connected to its light splitting ports. The coupler thus establishes two optical paths, a clockwise path and a counterclockwise path. Signals are compared at this optical coupler, with common mode signals being substantially reflected and difference mode signals being at least partially rejected through a rejection port associated with the optical coupler. Although intensity fluctuations originating at the pump (at the pump wavelength) are amplified, any intensity fluctuations resulting from higher order stimulation of the distributed gain medium are at least partially

rejected as difference mode signals.

Unlike EDFAs, the system of the invention provide true four-level amplification, in which the optical signal is not blocked when the pump is off. A telecommunication system based on the present invention would therefore inherently have greater reliability and tolerance to fault. For a more complete understanding of the invention, its objects

and advantages, reference may be had to the following specification and to

the accompanying drawings.

Brief Description of the Drawings

Figure 1 is a graph depicting Raman gain as a function of

frequency shift for fused silica at a pump wavelength of 1 μm;

Figure 2 is a diagrammatic illustration of one configuration for the

optical resonator in accordance with the present invention;

Figure 3 is a diagram of another embodiment of the optical

resonator of the invention, useful in further understanding the counter-

propagating optical paths;

Figure 4 is yet another embodiment of the optical resonator of the

invention employing dichroic couplers for wavelength discrimination;

Figure 5 is another embodiment of the optical resonator

employing one or more grating reflectors in addition to the Sagnac

interferometer or loop reflector;

Figure 6 illustrates yet another embodiment, employing an uneven

coupler;

Figure 7A illustrates another embodiment that includes a Fabry-

Perot filter;

Figure 7B depicts in detail the Fabry-Perot filter employed in the

embodiment of Figure 7A; Figure 8 illustrates a generalized Sagnac Raman cascade laser,

providing bidirectional output with a single coupler;

Figure 9 illustrates another Sagnac Raman cascade laser

employing a dichroic coupler;

Figure 10 illustrates a Sagnac Raman cascade laser employing a

dichroic mirror at the cavity end;

Figures 1 1 A-1 1 D illustrate possible configurations for future

systems employing the optical resonator system of the invention;

Figures 1 2A-1 2B illustrate further configurations employing

modulation instability (Ml) amplification;

Figures 1 3A-1 3C are graphs useful in understanding the operation

of the Ml amplification embodiments;

Figures 14A-14B illustrate synchronously pumped systems

employing the invention;

Figures 1 5A and 1 5B illustrate the invention implemented as a

Mach-Zehnder interferometer;

Figure 1 5C illustrates the invention employing a Michelson

interferometer;

Figure 1 5D illustrates the invention employing an interferometer

using two arms of a polarization-maintaining (PM) fiber;

Figure 1 5E illustrates the invention employing two Sagnac loop

mirrors connected with a linear region;

Figure 1 5F illustrates the invention implemented using a figure-

eight laser configuration; Figure 1 6A-1 6B are illustrations that accompany the equations

used to generate values for Table II presented below.

Description of the Preferred Embodiments

The present invention provides a structure for reducing the

inherent noise pumping in cascading optical resonators such as Raman

amplifiers and cascade lasers. More specifically, the preferred embodiments

combine Sagnac interferometer technology with Raman amplifier technology to

achieve performance improvements that neither technology, by itself, has

heretofore been able to deliver. To provide a better understanding of the

amplification mechanism at work in the present invention, some knowledge of

the Raman effect will be helpful. Described below, the stimulated Raman

scattering effect is a result of third order nonlinearities that occur when a

dielectric material (such as an optical fiber) is exposed to intense light. The

third order nonlinear effect is proportional to the instantaneous light intensity.

This distinguishes Raman gain and other third order nonlinearities from other

distributed gain media, such as erbium-doped media.

Stimulated Raman Scattering

Stimulated Raman scattering is an important nonlinear process

that can turn optical fibers into amplifiers and tunable lasers. Raman gain

results from the interaction of intense light with optical phonons in the glass,

and the Raman effect leads to a transfer of power from one optical beam (the

pump) to another optical beam (the signal) . An interesting property of Raman gain, the signal is downshifted in frequency (upshifted in wavelength) by an

amount determined by the vibrational modes of the glass. The Raman gain

coefficient g_r for silica fibers is shown in Figure 1 . Notably, the gain g_r

extends over a large frequency range (up to 40 terahertz [THz]), with a broad

peak centered at 13.2 THz (corresponding to a wavelength of 440 cm^'1 ). This

broad behavior is due to the amorphous nature of the silica glass and means

that the Raman effect can be used as broadband amplifiers. The Raman gain

depends on the composition of the fiber core and can vary with different

dopant concentrations.

The optical resonator of the invention employs a distributed gain

medium comprising a material that produces optical signal gain due to third

order nonlinearities in the material, in which the gain is proportional to the

intensity of the light passing through the medium. By way of background, the

response of any dielectric to light becomes nonlinear for intense

electromagnetic fields, and optical fibers are no exception. This nonlinear

response is related to anharmonic motion of bound electrons under the

influence of an applied field. The induced polarization P from the electric

dipoles is not linear in the electric field E. Rather, it satisfies the more general

relationship described in equation (1 )

P = €₀ [χ ^{( 1 ) ,}E *^■ χ ^{< 2 )} : EE + χ ^{( 3 ) •} EEE ... ] C )

where e₀ is the vacuum permitivity and χ^{'^] (j = 1 ,2, ...) is the jth order

susceptibility. To account for the light polarization effects, χ^& is a tensor of rank j + 1 . The linear susceptibility A"^{1 1} represents the dominant contribution

to P. Its effects are included through the refractive index n and the

attenuation coefficient a. The second order susceptibility χⁱ²⁾ is responsible for

such nonlinear effects as second harmonic generation and sum-frequency

generation. However, this second order susceptibility is nonzero only for

media that lack an inversion symmetry at the molecular level. Since silicon

dioxide is a symmetric molecule, χⁱ²⁾ vanishes for silica glasses. As a result,

optical fibers do not normally exhibit second order nonlinear effects.

Nevertheless, dopants introduced inside the fiber core can contribute to

second harmonic generation under certain conditions.

The third order susceptibility Λ'⁽³⁾. which is responsible for phenomena such as third harmonic generation, four-wave mixing and nonlinear

refraction, are present in optical fibers. It is this third order nonlinearity that is

operative in the present invention. These third order nonlinear effects are

identifiable as being variable in proportion to the intensity of the light. In

contrast, nonlinear effects produced by erbium doping are due to atomic

resonance within the material and the effect does not vary in proportion to the

instantaneous light intensity but rather with the integral (average) of light

intensity with respect to time (energy), over the upper state (stimulated

emission) lifetime of the erbium atoms.

To further explore the mechanism at work in stimulated Raman

scattering amplifiers and cascade lasers, a comparison with erbium-doped fiber

amplifiers (EDFAs) is provided in the following section. Raman Amplifiers and Erbium-Doped Fiber Amplifiers Compared

To provide a further understanding of Raman amplification,

Table I compares stimulated Raman scattering amplifiers (SRS) with erbium-

doped fiber amplifiers (EDFA) .

TABLE I

EDFAs SRS based on principle of doping glass based on nonlinear polarization of the fibers with rare earth ions dielectric silica host glass is a passive host and the nonlinear response is third order in medium polarization responsible for electric field strength; consequently the field interaction is generated by the nonlinear index of refraction and the doping ions; the medium gain are proportional to the pump polarization is linear intensity signal amplification is due to signal amplification is due to stimulated emission accompanied by stimulated scattering accompanied relaxation of the excited ions to the by the excitation of molecules into a ground state vibrational state pump and signal band characteristics pump can be at any wavelength (i.e. (i.e., spectrum and center there is no pump absorption band), wavelength) are fixed by the rare while the signal gain characteristics earth atomic resonances, no are determined by the optical phonon opportunity to cascade spectra, implies ability to cascade gain spectrum characteristics can be gain spectrum characteristics can be modified by glass codopants: glass modified by glass codopants: glass host affects the Stark level positions, host determines the phonon transition homogeneous and spectrum distribution and Stokes inhomogeneous line widths, shift (shift between pump and signal) nonradiative decay characteristics pumping can be either co- or pumping can be either co- or counter-propagating with respect to counter-propagating with respect to the signal the signal three level system so absorbs signal four level-like system to signal when pump absent transmits without amplification when pump absent fiber lengths can be relatively short typical require relatively long fiber (several meters) lengths (a kilometer or more) intrinsically polarization insensitive although intrinsically polarization sensitive, polarization scrambling in long lengths of fiber creates an effective polarization insensitivity As the above Table I suggests, Raman amplification has a number

of attractive features. First, Raman gain exists in every fiber; hence Raman

gain is a good candidate for upgrading existing fiber optic links. Second,

unlike EDFAs, there is no excessive loss in the absence of pump power, an

important consideration for system reliability. Third, the gain spectrum is very

broad (bandwidth of greater than 5 THz around the peak at 1 3.2 THz), so that

it can be used to amplify multiple wavelengths (as in wavelength division

multiplexing) or short optical pulses. Also, Raman amplification can be used

for distributed amplification, which may be especially valuable for ultra-high-

bit-rate systems. Finally, by varying the pump wavelength or by using

cascaded orders of Raman gain, the gain can be provided over the entire

telecommunications window between 1 .3 μm and 1 .6 μm, for example. In

contrast, EDFAs only operate near the 1 .55 μm window.

Despite the advantages, Raman amplifiers also have a number of

difficulties that need to be considered before applying the technology in systems. First, a major drawback is that typically high pump powers are

required (typically peak powers on the order of 1 watt) . The need for high

pump power can be satisfied by using a combination of high power

semiconductor laser diodes or cladding-pumped fiber lasers, together with

fibers that have increased Raman gain cross-section and smaller core affective

area. For example, recently Raman amplifiers with gain coefficients of 0.1

dB/mW have been demonstrated, and further improvements could increase this

efficiency to 0.1 5 dB/mW, or higher. In this regard, see E.M. Dianov, A. A.

Abramov, M.M. Bubnov, A.V. Shipulin, A.M. Prokhorov, S.L. Semjonov, A.G. Schebunjaev, G.G. Deviatykh, A.N. Guryanov and V.F. Khopin, Opt. Fiber

Tech. 1 , 236 ( 1 995).

In comparison, EDFAs pumped at 1480 nm have a gain

coefficient of about 6 dB/mW and EDFAs pumped at 980 nm have a gain

coefficient of around 10 dB/mW in optimized configurations.

Aside from the high power requirements, Raman amplifiers have

been known to cause interchannel interference in multiple wavelength

systems. In particular, as different wavelengths pass through each other, the

shorter wavelength signal tends to transfer energy to the longer wavelength

signal. As a consequence, the gain levels end up being different for different

wavelength signals. However, since the pulse trains walk through each other,

due to group velocity dispersion, the interchannel interference effect tends to

wash out.

Third, in comparison with other fiber amplifiers like EDFAs,

Raman amplifiers require longer fiber lengths (typically on the order of a

kilometer or more) . Whereas this may not be a difficulty in long haul

networks, it prevents their usage in latency-sensitive applications.

In addition to the above difficulties, particular attention must also

be given to the handling of spurious signals and noise. A major source of

noise in Raman amplifiers arises from the coupling of intensity fluctuations in

the pump laser to the signal. This problem is absent in EDFAs, because of the

very long upper state lifetime that is characteristic of EDFAs. However, the

coupling of intensity fluctuations in the Raman amplifier can be significantly

reduced by arranging the pump and signal so that they are counterpropagating through the amplifier. When this is done the pump fluctuations are averaged

and the crosstalk between pump and signal is significantly reduced . A second

source of noise in the Raman amplifier is double-Rayleigh scattering within the

amplifier itself. This can be partially compensated by placing an interstage

isolator within the amplifier for the signal to reduce the multiple path

interference.

Examples of Several Preferred Configurations

Referring to Figure 2, a first embodiment of the optical resonator

has been illustrated at 20 the optical resonator employs at least two reflectors

and a port for coupling to a source of light. Specifically, reflector 22 may be

any reflective structure such as a mirror. Reflector 24 is a loop reflector such

as a Sagnac interferometer. A further explanation of the loop reflector 24 will

be presented below. In the illustrated embodiment the light source 26 is a

pumped fiber laser coupled through WDM port 28 to the optical resonator.

The illustrated embodiment is fabricated using optical fiber (e.g. fiber optic

cable).

The Sagnac interferometer that serves as reflector 24 is

fabricated from a length of optical fiber that may be suitably coiled to

accommodate the physical packaging requirements. The Sagnac

interferometer comprises a fiber loop 30, typically a kilometer or more in

length. The fiber loop is established using a coupler such as 50:50

coupler 32. The 50:50 coupler defines two signal paths, such that half of the

light from light source 26 travels around loop 30 in a clockwise direction and half of the light from light source 26 travels around loop 30 in a

counterclockwise direction.

To illustrate the concept behind the 50:50 coupler 32, refer to

Figure 3. Figure 3 illustrates an alternate embodiment of the invention in

which the coupler 32a is shown using discrete bulk components. Coupler 32a

uses a half-silvered mirror 34 positioned midway between a trio of grin

lenses 36, 38 and 40. Light emitted from pump 26 and injected through

WDM coupler 28 emits through lens 36. Depending on the opacity of the

mirror 34, a portion of the light is reflected into lens 38 and a portion is

passed through mirror 34 into lens 40. This splitting causes a portion of the

light to travel clockwise through fiber loop 30 and a portion of the light to

travel counterclockwise through loop 30. A comparable result is achieved by

the 50:50 coupler 32 shown in the embodiment of Figure 2.

The optical resonator of the invention can function as a laser, and

also as an optical amplifier. When configured as a laser the optical resonator

requires no signal input (other than the light supplied by pump light

source 26) . The reflectors 22 and 24 establish a resonant cavity, producing

the laser effect. When configured as an optical amplifier, an optical signal is

injected into the optical resonator and this signal is then amplified by the

optical energy introduced by the light source 26. In the embodiment of

Figure 2, an optional signal input WDM port 42 is provided to allow the optical

resonator to be used as an optical amplifier. If the optical resonator is being

configured as a simple laser, the signal input WDM port can be omitted . A

similar signal output WDM port 44 supplies the output of the laser or optical amplifier. If desired, a rejection port 46 can also be provided, as an output of

the 50:50 coupler 32.

Noise Rejection

The Sagnac interferometer defines two optical paths (one

clockwise and the other counterclockwise) . These two optical paths support

both common mode and difference mode optical signals. To illustrate, assume that a continuous wave burst of light is injected in WDM 28 via light source

26. The CW burst enters the Sagnac reflector 24; half of the energy

propagates in a clockwise direction and half of the energy propagates in a

counterclockwise direction. After propagating through the Sagnac reflector,

the continuous wave burst is then reflected back in the direction of WDM 28,

where the burst then reflects from reflector 22 and is again transmitted to the

Sagnac reflector, where the cycle repeats. The CW burst thus resonates

between the two reflectors 22 and 24, growing in energy at the resonant

frequency. This is the common mode signal path. The system is designed to

reflect the common mode signal between reflectors 22 and 24, whereby the

laser effect or optical amplification occurs.

Now consider a noise burst signal that originates at some random

location along fiber loop 30. For purposes of the illustration, assume that the

noise burst is injected at a location designated by N in Figure 2. Some of the

energy of the noise burst (that which propagates in the clockwise direction)

passes out through rejection port 46 where it is not returned to the system.

The remainder (propagating in the counterclockwise direction) is reflected within the system and therefore retained. Because the signal paths of the noise burst are unbalanced (difference mode), a portion of the noise burst energy (approximately half of the energy) is lost, thus lowering the noise level within the system. The noise burst originating in the fiber loop travels in a difference mode, in which one optical path is retained within the system and the other optical path is discharged through rejection port 46. This is how the invention is able to reduce higher order amplification of pump source fluctuation. The higher orders originate (through the Raman effect) within the

fiber loop and are thus treated as difference mode signals. The following section explains this further.

Rejection of Higher Order Fluctuations As previously noted, the Raman amplifier is capable of cascading through multiple orders. With each cascade order there is a corresponding

shift in optical wavelength. The wavelength shift corresponds to a predetermined Stokes wavelength. Thus to achieve a 1310 nm signal wavelength four cascaded orders of Stokes shift would be employed, namely: 1 1 17 nm to 1 175 nm to 1240 nm to 1310 nm. Similarly, a fifth Stokes shift, based on the previous cascaded orders, would produce an output wavelength at 1480 nm.

Cascading is a desirable property; it allows the system designer to

shift the pump wavelength to any number of different desired signal wavelengths. Thus commercially available, high-powered pumps can be wavelength shifted to match the wavelength of the signal being amplified. However, cascading comes at a price. Pump fluctuations are amplified

combinatorially, as Table II demonstrates. Table II shows how a 10% intensity

fluctuation at the pump cascades exponentially with each cascaded order.

Table II compares two cases. Case 1 assumes a 10% fluctuation introduced

in the first step, using a simple Fabry-Perot (linear) cavity so that there is no

rejection of the fluctuation burst. Case 2 assumes a 10% fluctuation . introduced in the second step, using a Sagnac Raman laser cavity with a 50%

rejection of the fluctuation burst. Thus Case 2 shows the improvement

achieved using the principles of the invention.

TABLE II

In the specific example illustrated in Table II we are considering

only one noise burst, entered in the first step. The fluctuation is reduced to

1 /3 by using the invention. Compare the 61 % fluctuation in Case 1 with the

22% in Case 2. The values in Table II are based on the following model.

Assume that the systems compared in both cases start with a

pump and then cascade three orders (e.g., 1 1 1 7 nm pump, cascade to 1 175

nm, 1 240 nm and then 1 310 nm) . We can specify the gain at each

successive order to be 1 /2 of the previous order. A gain in the first step of

10 dB = 1 0x has been assumed. In this model the gain in the earlier stages is

higher than in the later stages, because the earlier stages are robbed of power

by the later stages during the cascading process. In general, the gain required

at each stage for lasing is going to be such that the gain balances the loss.

Thus, pumping higher orders corresponds to a loss and earlier stages must

therefore have more gain. For simplicity, pump depletion and the resulting

gain saturation have been neglected. Case 1 illustrates how a 10% noise

fluctuation grows to a 61 % fluctuation after three stages. Case 2 shows how

that same noise fluctuation is amplified only 22% due to the 50% rejection in

the Sagnac mirror for the higher stages. In Table II, note that the initial 10%

fluctuation is reduced to 5 % upon first reflection from the Sagnac mirror. This

corresponds to 50% of the difference mode energy being rejected through the

rejection port.

The equations used to generate the values shown in Table II will

now be described with reference to Figure 1 6A and 1 6B. In Figure 1 6A two optical signal paths are shown being fed into and out from a 50:50 coupler.

The input signals E. and E₂ produce output signals E₃ and E₄, respectively

according to the following equations:

In the above equations j = ,f^~ , corresponding to the phase of —

Propagation through a fiber of length L is given by the following equation:

E_ta^jφ, in which φ corresponds to the following phase shift calculation:

Φ 2 — n-L . λ

Figure 1 6b shows the signal propagation within a Sagnac loop

mirror that comprises a 50:50 coupler. The input electric field E_in is split at the

coupler, propagating in clockwise and counterclockwise directions,

corresponding to electric fields E₃ and E₄. These fields are related to the input

field E_ιn according to the following equations:

1

The effect of the Sagnac loop mirror is to produce a reflected field E_ref that

corresponds to the common mode of propagation, and to produce a rejected

field E_out that corresponds to the difference mode of propagation. The

common mode and difference mode signals are thus described by the following equations:

1 . ^■"ref ^~~ [ clockwise ^counterclockwise)

- common mode reflection

1 ^aout ~ ~ ^£ 'clockwise -^counterclockwise) - difference mode rejection

As the above Table shows, even a modest pump fluctuation (in

this example a 10% fluctuation) is multiplied again and again through each cascaded order. This is why Raman amplifiers have not been considered

generally useful in the past. However, the invention overcomes this problem

by adopting a structure that places the distributed gain medium in a difference

mode signal path, such that higher order pump fluctuations are at least

partially rejected .

In each of the embodiments illustrated above, and also in the

embodiments described below, light source 26 can be any suitable source of

optical energy. Because the Raman effect relies upon intense optical energy,

high power semiconductor or cladding-pumped fiber lasers are presently

preferred. A suitable high power source is available from Spectra Diode

Lasers, Inc., San Jose, California. The wavelength of the optical energy from light source 26 will, of course, be chosen to match the desired application. By

way of example, in an embodiment designed for 1 .3 μm telecommunication

applications, the light source 26 provides light at a wavelength of 1 1 1 7 nm.

This light is introduced through the wavelength division multiplexing (WDM)

coupler 28. The optical signal to be amplified, injected through WDM

coupler 42, may be at a wavelength of 1 300 nm to 1310 nm. The injected

signal propagates in the clockwise direction around loop 30 and is then

removed using WDM coupler 44. Due to the frequency downshift (wavelength

upshift) of the Raman effect, the wavelength of the light source 26 is

upshifted to match that of the signal. Although a 1 .3 μm amplifier example is

presented here, the configuration illustrated in Figure 2 and the embodiments

described elsewhere in this specification can be configured to work at other

wavelengths as well. Thus the light source 26 can be any suitable wavelength

to match the application (not necessarily at 1 1 1 7 nm) and the two WDM

couplers 42 and 44 can be designed for any desired signal wavelengths (not

necessarily between 1 300 nm and 1 310 nm) .

The resonant cavity of the embodiment illustrated in Figure 2 (and

also illustrated in Figure 3) lies between reflector 22 and reflector 24. In the

illustrated embodiment the optical fiber disposed between these two reflectors

serves as the light transmissive medium. The Sagnac reflector 24 is fabricated

using a distributed gain medium comprising a material that produces optical

signal gain through third order nonlinearities in the material, characterized by a

gain that is proportional to the intensity of the light passing through the

medium. Although reflector 22 is shown as a discrete mirror in the embodiments illustrated so far, it will be appreciated that reflector 22 could be any form of reflector, including a simple metallic coating evaporated onto the fiber end. Thus the invention can be implemented as an all fiber configuration.

Some of the embodiments yet to be described use other forms of reflectors for reflector 22.

One advantage of using the Sagnac reflector 24 is its inherent broadband properties. Unlike some other systems that are restricted by the laws of physics to operate at a single resonant frequency dictated by doping, the present invention operates over a broad range of frequencies, the operating frequency being dependent principally upon the frequency of the input signal.

Of course, if desired, frequency-selective gratings or frequency-selective filters can be employed within the laser cavity if precise wavelength control is desired.

One significant advantage of the invention results from the union of the Sagnac loop mirror with the Raman amplifier technology.

Conventionally, a large source of amplitude jitter in Raman lasers arises from the pump fluctuations that become greatly amplified in the highly nonlinear cascaded Raman process. Advantageously, the Sagnac loop mirror results in a quieter amplifier (and also a quieter laser) due to its difference mode noise rejection properties. The Sagnac loop tends to dampen noise at frequencies larger than the inverse round-trip time of the loop cavity. For example, for a 2 kilometer (km) long fiber loop, noise at frequencies larger than 100 kilohertz

(kHz) will be partially rejected via the rejection port 46. Also, as previously pointed out, spurious signals and noise injected at some arbitrary point along

the loop are also attenuated.

In the embodiments described above a 50:50 coupler 32 (and

32a) was selected, as it provides the most general purpose example of the

principles of the invention. Depending on component availability or special

requirements for a particular application, different embodiments of the optical

resonator may be used. Referring to Figure 4, a dichroic coupler 32b has been

used to provide frequency selectivity. In the embodiment of Figure 4 the

dichroic coupler provides nominally 50:50 coupling over the cascade Raman

order wavelengths, but a ratio that is closer to 100:0 for the signal

wavelength. Thus, for a 1 .3 μm system the 50:50 coupling would be

provided for wavelengths less than 1 300 nm and the 100:0 coupling would be

provided for wavelengths greater than 1 300 nm. The advantage of this

configuration is that it is easier to make a balanced Sagnac interferometer, and

the fiber in the Sagnac interferometer may be packaged more simply. One

possible disadvantage of this configuration is that the dichroic coupler may be

more difficult or expensive to implement.

The embodiment of Figure 4 also illustrates that the signal can

also be input directly into the laser cavity at a location between the two

reflectors 22 and 24. Specifically, the signal input WDM port 42b is

positioned in the cavity adjacent WDM coupler 28. Also illustrated in Figure 4

are the use of polarization controllers 46 and 48. Polarization controllers may

also be used in a similar fashion in the embodiments illustrated in Figures 2

and 3. Figure 5 illustrates another embodiment in which the reflector 22

has been replaced by a series of grating reflectors 50 and 52. The grating

filters may be selected to provide 100 percent reflection at selected

wavelengths, such as at 1 1 75 nm and 1 240 nm. The advantage of the

configuration of Figure 5 is that a narrow pump line width can be achieved.

The disadvantage is that the configuration is more complicated and more

expensive to fabricate.

Figure 6 illustrates yet another embodiment in which the Sagnac

reflector 24 is constructed using a coupler 32c having an unequal coupling

ration, for instance 60:40. By unbalancing the Sagnac reflector the system

will tend to further reject noise bursts that randomly occur in the loop. This

will serve to dampen out any mode locking or Q-switching tendencies.

However, the unequal coupling leads to a leakage at various wavelengths, so that higher pump powers may be required to account for the reduced

efficiency.

Figure 7A depicts a hybrid configuration employing a Fabry-Perot

wavelength filter 54 to narrowly select the Raman pump orders. A detailed

depiction of the Fabry-Perot filter is shown in Figure 7B. The fiber is split into

two segments 56 and 58 and separated to define an air gap 60. The cleaved

ends of the fiber segments are coated as at 62 with a nominally high

selectivity coating (R > 90%) at the wavelengths of interest. The cleaved

faces are aligned parallel to each other and piezoelectric transducers 64 may

be used to adjust the air gap width. Ideally, the air gap width L can be

adjusted so the free-spectral range of the Fabry-Perot interferometer (Δf = c/2nL) will match the reflection at the various Raman orders (spaced by

Δf = 1 3.2THz). Thus a single Fabry-Perot interferometer can be used to

replace the multiple gratings 50 and 52 of the Figure 5 embodiment, because

the transmission function is a periodic function of frequency. For example, for

an air gap index n = 1 , the spacing should be 1 1 .36 μm for Δf = 13.2THz.

Alternatively, the spacing may be some integer multiple of this fundamental

width. The fiber Fabry-Perot interferometer can also be replaced with a bulk

interference filter, which can be rotated to adjust the peak transmission

frequencies.

Sagnac Raman Cascade Lasers

The embodiments illustrated so far have been primarily illustrating

how the optical resonator of the invention can be used as an optical amplifier.

Thus in the preceding examples, a signal input port is provided into which the

signal to be amplified is injected . The invention is not limited to amplifiers, however. As will be illustrated below, the invention can also be used to

develop cascade oscillators or cascade lasers. These may be used in a number

of different applications, including upgrading existing fiber links. In particular,

the following will describe various configurations for constructing Sagnac

Raman cascade lasers. Figure 8 illustrates how a bidirectional output can be

achieved with a single coupler. Figure 8 is configured as a Sagnac

interferometer-based cascade Raman laser that provides bidirectional outputs

labeledΛ_0Ut. An intracavity coupler 66 provides these outputs. Note that unlike

the previously described Raman amplifiers, the configuration of Figure 8 is an oscillator and does not require a signal input. The output at the left (at

port 68) should be stronger than the output at the right (port 70) so the right

output may be used for monitoring purposes.

A dichroic coupler 32d is used in the embodiment of Figure 9 to

implement the Sagnac reflector. The output of this oscillator Λ_out exits from

the external cavity port of the Sagnac loop mirror 24. The dichroic coupler

can be selected to provide 50:50 coupling over the cascade order and 100:0

coupling at the λ_out wavelength.

A further embodiment of Sagnac interferometer-based cascade

Raman laser is shown in Figure 10. In this embodiment a dichroic mirror 72 is

used at the cavity end. Note that the dichroic mirror is reflective for cascade

order wavelengths and is partially or completely transmitting for the Λ_out

wavelength.

The Sagnac interferometer-based cascade Raman lasers described

in the preceding examples (Figures 8, 9 and 10) may be used in numerous

applications, including upgrading existing fiber links, remote pumping of

EDFAs, or other applications requiring different wavelengths of light. In this

regard, the embodiments illustrated in Figures 8-10 are merely exemplary, and

there may be other possible configurations employing the principles of the

invention. By way of further illustration, Figures 1 1 A-1 1 D show how to

apply the technology of the present invention to different situations.

Figure 1 1 A illustrates how an existing fiber link can be upgraded

using the Sagnac interferometer-based cascade Raman laser of the invention.

In Figures 1 1 A-1 1 D the optical resonator of the invention is designated by the

abbreviation SRCL and given reference numeral 80 in the drawings. In

Figure 1 1 A the optical resonator 80 is attached to the terminal end of an

existing fiber link 74. In this application the SRCL 80 should operate at

1 240 nm. A counter-propagating configuration is shown. This configuration

is preferred as it minimizes pump fluctuation coupling .

Figure 1 1 B illustrates how the optical resonator 80 may be used

for remote pumping of an EDFA amplifier. The EDFA amplifier is illustrated at

76. In this case the wavelength output of optical resonator 80 provides light at 1480 nm. The SRCL 80 may be adjusted for 1480 nm operation by using

five cascade orders from and 1 1 1 7 nm pump, or six cascade orders from a

1060 nm pump.

Figure 1 1 C depicts how to upgrade a 1 530-1 550 nm

transmission line with higher gain or distributed amplification using SRCLs

operating between 1450 nm to 1480 nm. In the illustrated system EDFAs 76

are also used . The wavelength out of the SRCL optical resonator 80 may be

on the order 1450 nm to 1480 nm.

Figure 1 1 D illustrates a distributed amplification system that

employs a distributed erbium-doped fiber amplifier (DEDFA) . This erbium-

doped fiber amplifier is described by M.N. Islam and L. Rahman, IEEE Journal, Lightwave Technology 1 2, 1 952 ( 1994) . This configuration may be

appropriate for very high bit rate systems employing soliton shepherding, as

described below.

Distributed Amplification for High Bit Rate Systems

Future fiber optic networks will be operating in the 1 530-

1 550 nm wavelength range with bit rates approaching 100 Gb/s or more.

With higher bit _*rates, higher powers are required . Therefore as the bit rate is

increased, the amplification level and pump power must increase. In addition,

for high speed, time division multiplexed (TDM) systems, short pulse

propagation may require pulse control or soliton control mechanisms that are

distributed throughout the fiber. A combination of EDFAs and distributed

Raman amplification, such as those illustrated in Figures 1 1 C or 1 1 D may

provide the appropriate transmission line for 40 Gb/s and beyond . In

particular, the discrete (Figure 1 1 C) or distributed (Figure 1 1 D) EDFAs can

provide the basic gain at some signal power level with high efficiency. Then

Raman amplification can provide the necessary boost in power needed for the

high bit rate systems. Moreover, for WDM applications it may be possible to

use a combination of EDFA and Raman amplification to broaden or flatten the

gain spectrum.

For 100 Gb/s TDM systems using picosecond pulses, several

problems need to be addressed in fiber transmission lines longer than several

kilometers beyond the usual considerations of loss, dispersion and nonlinear

index of refraction. First, "gentler" or more gradual changes in gain and amplitude are necessary to avoid generation of unwanted dispersive waves.

For example, discrete EDFAs spaced by more than 25 km may be

inappropriate. Second, the length bit rate product is limited by spontaneous

emission noise that always accompanies coherent amplification. J.P. Gordon

and H.A. Haus, Opt. Lett. 1 1 , 665 ( 1986) have studied this effect and they

show that the most deleterious problem is a random shift in the soliton's

carrier frequency with a corresponding change in its velocity. Third, soliton

self-frequency shift (SSFS) causes a continuous downshift in the mean

frequency of pulses propagating in optical fibers. J.P. Gordon, Opt. Lett. 1 1 ,

662 ( 1 986) has shown that for solitons the shift is a strong function of pulse

width r(Δ ∞ t^"4) . SSFS must be avoided for two reasons: 1 ) the frequency

shift changes the timing of pulses in the system; and 2) SSFS couples

amplitude fluctuations to frequency and timing fluctuations.

These deleterious effects for ultra-high bit rate systems can be

controlled by employing both distributed amplification as well as some sort of

distributed filtering for pulse or soliton control. In fact, K.J. Blow, N.J. Doran

and D. Wood, "Suppression of the Soliton Self-Frequency Shift by Bandwidth-

Limited Amplification, " Journal of Optical Society of America B., Volume 5,

pp. 1 301 -1 304 (1 988) have shown theoretically that the bandwidth limited

gain provided by Raman amplification can serve as a distributed frequency

filter to counteract SSFS. Likewise, this filtering will also reduce the effect of Gordon-Haus jitter. Therefore, the configurations illustrated in Figures 1 1 A

and 1 1 D, even perhaps Figure 1 1 C, can be particularly advantageous for 40

Gb/s and beyond TDM systems. Here we use not only the gain provided by Raman amplification but also the distributed bandwidth limiting that Raman

amplifiers can provide.

Modulational Instability Amplifiers Pumped bv Saonac Raman Lasers

Although Raman amplification such as in the configuration of

Figure 1 1 A may be used to upgrade existing fiber links, another more efficient

method may be to use modulational instability (Ml) amplification. For example,

the copropagating setup of Figure 1 2A may be employed for Ml in an existing

fiber link, where in particular the SRCL frequency is adjusted to the peak of the

Ml gain as described below. This frequency separation between pump and

signal is generally much smaller than the frequency separation used in Raman

amplification and the frequency difference also depends on the pump intensity.

Just like in Raman amplification, the main advantage of Ml gain is that it is

inherent to the glass fiber so it is present in every fiber. Ml is 4-photon

parametric amplification in which the nonlinear index of refraction is used to

phase match the pump and sidebands. For Ml gain the pump wavelength

must lie in the anomalous group velocity regime (Figure 1 3A) and proper phase

matching requires that pump and signal be copropagating. Since in the

copropagating configuration pump fluctuations can affect the bit error rate of

the system, it is particularly important that a quiet pump such as in Figure 8-

10 be employed.

Ml amplifiers can be more efficient than Raman amplifiers because

the gain coefficient for Ml is about a factor of five larger than for the Raman

process, as described by R.H. Stolen, "Nonlinear Properties of Optical Fibers, " Chapter 5 in Optical Fiber Telecommunications, New York, Academic Press (1979). Both Ml and Raman amplification arise from the third order

susceptibility χ^{3) in optical fibers. The real part of χ^{3), the so-called nonlinear

index of refraction n₂, is responsible for Ml, while the imaginary part of X⁽³⁾

associated with molecular vibrations corresponds to the Raman gain effect. In

fused silica fibers about 4/5ths of the n₂ is an electronic, instantaneous

nonlinearity caused by ultraviolet resonances, while about 1 /5th of n₂ arises

from Raman-active vibrations (e.g., optical phonons) (see further description in

M.N. Islam, Ultrafast Fiber Switching Devices and Systems, Cambridge,

Cambridge University Press [1 992]) . The imaginary part of this latter

contribution corresponds to the Raman gain spectrum of Figure 1 .

Parametric amplification is usually inefficient in long fibers due to

the requirement for phase-matching. However, Ml can act as self-phase-

matched because the nonlinear index of refraction is used to phase-match the

pump and sidebands. This is particularly true when operating near the zero

dispersion wavelength λ₀ in fibers. Ml involves two pump (P) photons that

create Stokes (S) and anti-Stokes (A) photons. Consequently, the pump in Ml

can lie on the short or long wavelength side of the signal (in Raman the pump

must always be at a shorter wavelength than the signal) . To illustrate the Ml

gain, consider the gain coefficient as derived in R.H. Stolen and

J.E. Bjorkholm, IEEE J. Quantum Elect. , QE-1 8, 1062 ( 1 982) :

9 = ( γP) 2 _ Δ K + γP (2) The first term under the square root corresponds to the third order nonlinearity

that couples the pump photons to the sidebands. The second term

corresponds to the phase mismatch between the waves and it consists of two parts: one due to the wave-vector mismatch at the different wavelengths and

the other due to the increase in nonlinear index induced by the pump. The

nonlinearity parameter is defined as

Also, assuming that we are operating near the zero dispersion wavelength γ_Q,

the propagation constant can be expanded as

Δk =

2nC " ^{♦ ■}f <λ, - λd Ω² (4)

where

Q = f_p - f_s = f_a - f_p (5)

If we are in the anomalous group velocity dispersion regime, then D > 0,

3D/3Λ > 0, (Λ_p - λ₀) > 0, so that Δk < 0. This is the regime of Ml and we see

that the nonlinearity helps to reduce the phase mismatch (i.e., the two parts in

the second term in equation [2] are of opposite sign) . There is gain for Ml and

the gain is tunable with the pump power. As an example, the power gain

coefficient 2g is plotted in Figure 1 3B for operation in the anomalous group velocity regime. The peak gain (g_pβak = yP) occurs at Δk_peak = -2 P. The

range over which the gain exists is given by 0 > Δk > -4 P. Thus, we see

that the peak gain is proportional to the pump power and the Δk range is

determined by the pump power. Consequently, from equation (3) we see that

the bandwidth can be increased by increasing the pump power, increasing the

nonlinear coefficient n₂ or decreasing the effective area A_βff. Alternately,

for a given required frequency range over which gain is required, the pump

requirements can be reduced by increasing the effective nonlinearity (n₂/A_βff).

An example of the pump power dependence of the gain and the double-sided

frequency spectrum of the gain is shown in Figure 1 3C which is taken from

G.P. Agrawal, Nonlinear Fiber Optics, 2nd Ed . , New York, Academic Press

(1995).

The Ml amplifier could be particularly attractive for distributed

amplification of ultra-high bit rate soliton transmission systems. As discussed

in the last section, distributed amplification and filtering may be required for

systems of 40 Gb/s or higher speeds. Mollenauer and coworkers have shown

that for soliton-based systems the concept of "sliding-guiding filters" is

advantageous. L.F. Mollenauer, J.P. Gordon and S.G. Evangelides, Opt. Lett.

1 7, 1 575 ( 1 992) . Specifically, if the center frequency of spectral filters is slid

slowly enough, only soliton pulses can follow the filter by shifting their center

frequency. This abates any linear dispersive waves and suppresses the noise

accumulation. However, the Mollenauer scheme uses discrete amplifiers and

filters, which is inappropriate when pulse widths of a few picoseconds or less

are employed. On the other hand, the Ml-based amplification can effectively provide such a distributed sliding-guiding filter because of the dependence of gain bandwidth on pump intensity. For instance, Figure 12B illustrates a transmission link where the pump is attenuating due to loss in the fiber and perhaps pump depletion. As the pump intensity decreases, the peak gain frequency also shifts. If the pump is at a lower (higher) frequency than the signal, then the frequency shift is downward (upward). A soliton operating near the peak gain frequency will also shift with the gain spectrum, thus separating the soliton from any noise background.

Broader Implications for Sagnac Raman Devices

Beyond the inventions and applications based on Sagnac Raman amplifiers and cascade lasers outlined in Figures 2-1 1 , there are other

extensions of the technology. For example, the same ideas can operate over the entire transparency wavelength range for optical fibers (between 0.3-

2 μm). Beyond the above-described applications in telecommunications, there are also applications such as CATV analog systems around 1 .3 μm, optical time domain reflectrometer (OTDR) and other such optical instrumentation with enhanced sensitivity and amplification in the system monitor bands

around 1 .51 μm and 1 .6 μm. Also, a synchronously-pumped Sagnac Raman cascade laser could be used to reduce the average pump power requirements.

In Figure 14A an intracavity modulator is introduced that is driven by electronics synchronized to the input using a phase-lock loop. Alternately, in Figure 14B cross-phase modulation in the fiber ring is introduced by using a modulation laser that is synchronized to the input stream. Depending on the bit rate of the signal, the electric option of Figure 14A or the all-optical option

of Figure 14B may be more attractive.

The basic configuration of the Sagnac loop may also be

generalized for the Raman amplifiers or lasers. For example, the Sagnac

interferometer can be replaced with Mach-Zehnder interferometer or Michelson

interferometer. Figure 1 5A and 1 5B show two different embodiments of a

Mach-Zehnder interferometer. Each arm of the interferometer is of the same

length. If desired, these embodiments could be implemented using optical

wave guides, such as wave guides that are fabricated upon a silicon substrate.

5 Figure 1 5C illustrates a Michelson interferometer embodiment. Note that the

two arms of the interferometer are of different lengths, corresponding to the

equations set forth in the figure. Also in the case of a pulsed system, the

_ interferometer could also be time-multiplexed over the same fiber. Alternately,

two arms of the interferometer can be the two polarizations in a polarization

maintaining fiber. This is shown in Figure 1 5D. A polarization-maintaining

(PM) fiber defines the two optical paths within the same fiber, the paths being 5 essentially orthogonal to one another. An adjustable polarizer discriminates

between these two paths and serves as the rejection mechanism for the

difference mode signals. Note that the polarization-maintaining fiber is 0 configured in two sections with the axis crossed at the halfway point. This

will undo any effects of walkoff due to birefringence in the PM fiber. Note

that in this embodiment the polarizer functions as the signal comparator in

⁵ place of the coupler used in other embodiments. Further embodiments include

the embodiments illustrated in Figures 1 5E and 1 5F. In Figure 1 5E, the end mirror on the left side of the resonant cavity may be replaced with another

Sagnac interferometer, in which case the configuration would be two loops

connected with a linear region or a figure eight laser configuration. The

figure eight configuration is shown in Figure 1 5F. In this embodiment the

isolator allows optical signals to pass in only one direction. The isolator thus

serves as the rejection mechanism, comparable in function to the rejection port

of other embodiments described above. Furthermore, all of these

configurations can benefit from improvements with higher pump power and

fibers with higher Raman cross-section and smaller affective area. Also, fibers

with various dopings or appropriate polarization properties could be

advantageous. For example, DEDFAs or Tm-doped fibers may be particularly

attractive in applications requiring distributed pulse control or filtering. M.N.

Islam and L. Rahman, IEEE J. Lightwave Tech. 1 2, 1952 (1994).

While the present invention has been described in a number of

different exemplary embodiments, it will be understood that the principles of

the invention can be extended to still further embodiments and that the

embodiments illustrated here are not intended to limit the scope of the

invention as set forth in the appended claims.

Claims

What Is Claimed Is:

1 . In an optical resonator having at least two reflectors and

port for coupling to a source of light and an output, the improvement

comprising:

at least a first one of said reflectors comprising a distributed gain

medium;

said distributed gain medium producing optical signal gain through

nonlinear polarization that cascades through plural orders of wavelength

including a first order and at least one higher order;

said first reflector further defining two optical paths configured to

support both common mode and difference mode optical signals; and

said first reflector having a higher reflectance for common mode

signals than for difference mode signals such that difference mode signals

corresponding to said higher order are at least partially rejected thereby

reducing amplification of fluctuations.

2. The resonator of Claim 1 wherein said distributed gain

medium comprising a material that produces optical signal gain by virtue of

third order nonlinearities within the material and characterized by being

proportional to the instantaneous intensity of light propagating through said

medium.

3. The resonator of Claim 1 wherein said distributed gain

medium produces an optical wavelength shift with each of said plural orders

such that the optical wavelength at said output is longer than the optical

wavelength of said source.

4. The resonator of Claim 1 wherein material produces said

optical signal gain through optically stimulated vibrational modes or electronic

transitions within the material.

5. The resonator of Claim 1 wherein said port is coupled to a

source of light of sufficient intensity such that said distributed gain medium

produces Raman gain.

6. The resonator of Claim 1 wherein said one of said

reflectors comprises a Sagnac interferometer.

7. The resonator of Claim 1 wherein said one of said

reflectors comprises a Sagnac interferometer fabricated from said distributed

gain medium.

8. The resonator of Claim 1 wherein said distributed gain

medium is an optical fiber.

9. The resonator of Claim 1 wherein said optical resonator is configured as an optical signal amplifier and further includes at least one signal input port.

10. The resonator of Claim 1 wherein said first reflector

includes a coupler that establishes said two optical paths.

1 1 . The resonator of Claim 1 wherein said first reflector

comprises a coupler that establishes said two optical paths and an optical fiber system connected to said coupler to support said signal propagation along

both paths.

12. The resonator of Claim 1 wherein said first reflector comprises a coupler that establishes said two optical paths and an optical fiber

loop connected to said coupler to support said signal propagation along both clockwise and counter-clockwise paths within said optical fiber.

13. The resonator of Claim 1 wherein said first reflector includes a coupler that establishes said two optical paths, said coupler functioning as an optical signal comparator that reflects common mode signals and at least partially rejects difference mode signals.

14. The resonator of Claim 1 wherein said first reflector

includes a coupler having a predefined coupling ratio that establishes said two

optical paths.

1 5. The resonator of Claim 14 wherein said predefined coupling

ratio is 50:50.

1 6. The resonator of Claim 14 wherein said predefined coupling

ratio is an unequal coupling ratio.

1 7. The resonator of Claim 1 wherein said first reflector

includes a dichroic coupler having a first predefined coupling ratio at a first

wavelength and a second predefined coupling ratio at a second wavelength,

said dichroic coupler establishing said two optical paths.

18. The resonator of Claim 1 wherein said optical resonator is

configured as an optical signal amplifier and further includes at least one signal

input port disposed between said two reflectors.

1 9. The resonator of Claim 1 wherein a second one of said

reflectors comprises at least one grating reflector that provides maximum

reflectivity at a predetermined wavelength.

20. The resonator of Claim 1 further comprising at least one

wavelength filter disposed between said two reflectors for selecting predefined

pump orders.

21 . The resonator of Claim 1 wherein said resonator functions as a cascade laser.

22. The resonator of Claim 21 wherein said cascade laser is

coupled to an optical system to provide pumping of an erbium-doped fiber amplifier.

23. The resonator of Claim 21 wherein said cascade laser is

coupled to an optical system to provide pumping of a distributed erbium-doped

fiber amplifier system.

24. The resonator of Claim 21 wherein said cascade laser is

coupled to an optical fiber link to provide distributed gain.

25. The resonator of Claim 1 further comprising a plurality of

said resonators, wherein each of said resonators is configured to function as a

cascade laser and each is coupled to an optical fiber link to provide distributed

gain.

26. The resonator of Claim 1 wherein said resonator is tuned to

a modulation instability (Ml) peak.

27. The resonator of Claim 1 wherein said distributed gain

medium comprises a material that produces optical signal gain by virtue of

nonlinear index of refraction within the material.

28. The resonator of Claim 1 wherein said first reflector

includes an optical comparator for discriminating between common mode and

difference mode signals.

29. The resonator of Claim 28 wherein said comparator

comprises an optical coupler.

30. The resonator of Claim 28 wherein said distributed gain

medium is a polarization maintaining fiber and said comparator comprises a

polarizer.

31 . The resonator of Claim 1 wherein said first reflector

comprises first and second optical waveguides defining two optical paths, said

waveguides being coupled to an optical splitter and to an optical comparator to

support common mode and difference mode optical signals.

32. The resonator of Claim 31 wherein at least one of said

waveguides comprises optical fiber.

33. The resonator of Claim 31 wherein at least one of said

waveguides is formed on a substrate.

34. The resonator of Claim 1 further comprising means for

synchronously pumping said resonator that includes electrical or optical signal

feedback to establish synchronization.

35. In an optical resonator having at least two reflectors and

port for coupling to a source of light and an output, the improvement

comprising:

at least a first one of said reflectors comprising a distributed gain

medium;

said distributed gain medium producing optical signal gain through

nonlinear polarization;

said first reflector further defining two optical paths configured to

support both common mode and difference mode optical signals; and

said first reflector having a higher reflectance for common mode

signals than for difference mode signals such that said difference mode signals

are at least partially rejected .

36. The resonator of Claim 35 wherein said distributed gain

medium comprising a material that produces optical signal gain by virtue of

third order nonlinearities within the material and characterized by being

proportional to the instantaneous intensity of light propagating through said

medium.

37. The resonator of Claim 35 wherein said distributed gain

medium produces an optical wavelength shift with each of said plural orders

such that the optical wavelength at said output is longer than the optical

wavelength of said source.

38. The resonator of Claim 35 wherein material produces said

transitions within the material.

39. The resonator of Claim 35 wherein said port is coupled to a

source of light of sufficient intensity such that said distributed gain medium

produces Raman gain.

40. The resonator of Claim 35 wherein said one of said

reflectors comprises a Sagnac interferometer.

41 . The resonator of Claim 35 wherein said one of said

reflectors comprises a Sagnac interferometer fabricated from said distributed

gain medium.

42. The resonator of Claim 35 wherein said distributed gain

medium is an optical fiber.

43. The resonator of Claim 35 wherein said optical resonator is

input port.

44. The resonator of Claim 35 wherein said first reflector

includes a coupler that establishes said two optical paths.

45. The resonator of Claim 35 wherein said first reflector

comprises a coupler that establishes said two optical paths and an optical fiber

system connected to said coupler to support said signal propagation along

both paths.

46. The resonator of Claim 35 wherein said first reflector

loop connected to said coupler to support said signal propagation along both

clockwise and counterclockwise paths within said optical fiber.

47. The resonator of Claim 35 wherein said first reflector includes a coupler that establishes said two optical paths, said coupler functioning as an optical signal comparator that reflects common mode signals and at least partially rejects difference mode signals.

48. The resonator of Claim 35 wherein said first reflector

includes a coupler having a predefined coupling ratio that establishes said two optical paths.

49. The resonator of Claim 48 wherein said predefined coupling ratio is 50:50.

50. The resonator of Claim 48 wherein said predefined coupling ratio is an unequal coupling ratio.

51 . The resonator of Claim 35 wherein said first reflector includes a dichroic coupler having a first predefined coupling ratio at a first wavelength and a second predefined coupling ratio at a second wavelength,

said dichroic coupler establishing said two optical paths.

52. The resonator of Claim 35 wherein said optical resonator is configured as an optical signal amplifier and further includes at least signal one input port disposed between said two reflectors.

53. The resonator of Claim 35 wherein a second one of said

reflectors comprises at least one grating reflector that provides maximum

reflectivity at a predetermined wavelength.

54. The resonator of Claim 35 further comprising at least one

wavelength filter disposed between said two reflectors for selecting predefined

pump orders.

55. The resonator of Claim 35 wherein said resonator functions

as a cascade laser.

56. The resonator of Claim 55 wherein said cascade laser is

coupled to an optical system to provide pumping of an erbium-doped fiber

amplifier.

57. The resonator of Claim 55 wherein said cascade laser is

coupled to an optical system to provide pumping of a distributed erbium-doped

fiber amplifier system.

58. The resonator of Claim 55 wherein said cascade laser is

coupled to an optical fiber link to provide distributed gain.

59. The resonator of Claim 35 further comprising a plurality of

said resonators, wherein each of said resonators is configured to function as a

gain.

60. The resonator of Claim 35 wherein said resonator is tuned

to a modulation instability (Ml) peak.

61 . The resonator of Claim 35 wherein said distributed gain

medium comprises a material that produces optical signal gain by virtue of

nonlinear index of refraction within the material.

62. The resonator of Claim 35 wherein said first reflector

includes an optical comparator for discriminating between common mode and

difference mode signals.

63. The resonator of Claim 62 wherein said comparator

comprises an optical coupler.

64. The resonator of Claim 62 wherein said distributed gain

medium is a polarization maintaining fiber and said comparator comprises a polarizer.

65. The resonator of Claim 35 wherein said first reflector comprises first and second optical waveguides defining two optical paths, said waveguides being coupled to an optical splitter and to an optical comparator to support common mode and difference mode optical signals.

66. The resonator of Claim 65 wherein at least one of said waveguides comprises optical fiber.

67. The resonator of Claim 65 wherein at least one of said waveguides is formed on a substrate.

68. The resonator of Claim 35 further comprising means for synchronously pumping said resonator that includes electrical or optical signal feedback to establish synchronization.

69. An optical resonator, comprising:

at least two reflectors defining a laser cavity; wherein at least a first one of said reflectors is an interferometer comprising a distributed gain medium that produces gain based on nonlinear polarization inherent to the medium.

70. The optical resonator of Claim 69 wherein said interferometer includes an optical comparator for discriminating between common mode and difference mode signals and a mechanism for rejecting at least partially said difference mode signals.

71 . The optical resonator of Claim 69 wherein said interferometer includes an optical comparator having plural input ports coupled to said distributed gain medium.

72. The optical resonator of Claim 71 wherein said comparator produces sum and difference optical signals based on signals supplied by said distributed gain medium.

73. The optical resonator of Claim 72 wherein said comparator includes a rejection port for rejecting at least a portion of said difference optical signals.

74. The optical resonator of Claim 69 wherein said

interferometer is comprised of optical fiber.

75. The optical resonator of Claim 69 wherein said interferometer comprises plural optical waveguides.

76. The optical resonator of Claim 69 wherein said interferometer is a Sagnac interferometer.

77. The optical resonator of Claim 69 wherein said interferometer supports both common mode and difference mode optical signals and has higher reflectance for said common mode than for said difference mode.

78. The optical resonator of Claim 69 wherein said distributed gain medium produces optical signal gain through nonlinear polarization that cascades through plural orders of wavelength, including a first order and at least one higher order; and

wherein said inteferometer supports both common mode and difference mode optical signals and includes a rejection port for at least partially rejecting difference mode signals corresponding to said higher order.