US 20050270616 A1
A system and method for programmable phase compensation of optical signals is disclosed. The systems and methods include the use of a polarization-independent spatial light modulator (PI-SLM) so that the state of polarization (SOP) of the incoming optical signal need not be known. The system includes a first dispersive module that spatially separates the optical signal into its frequency components. The frequency components are spread over the active area of the PI-SLM. The active area of the PI-SLM includes an array of independently programmable addressable regions capable of altering the phase of the light incident thereon. An exemplary application of the invention is chromatic dispersion compensation. By knowing the amount of chromatic dispersion in the optical signal, or alternatively, by knowing the amount of chromatic dispersion to be introduced into the optical signal downstream, the appropriate phase adjustments can be made to each frequency component of the signal. The phase-adjusted frequency components are then recombined via a second dispersive module to form a compensated optical signal.
1. An optical processing system for altering the phase of the frequency components of channel optical signals traveling in different WDM channels so as to reduce chromatic dispersion effects in the channel optical signals, comprising:
a wavelength division demultiplexer for demultiplexing the channel optical signals;
a plurality of optical fibers connected to the wavelength division demultiplexer each for carrying one of the demultiplexed channel optical signals;
polarization-independent chromatic dispersion compensation means arranged in each optical fiber for independently performing chromatic dispersion compensation of each channel optical signal;
2. A system according to
3. A system according to
4. A system according to
5. A system for programmably adjusting the phase of the frequency components of an optical signal of arbitrary polarization to compensate for chromatic dispersion in the optical signal, comprising:
means for dispersing the frequency components of the optical signal;
programmable phase-adjusting means for programmably adjusting the phase of one or more of the frequency components; and
means for combining the phase-adjusted frequency components to form a compensated optical signal.
6. A system according to
7. A system according to
This application is a divisional under 37 CFR 1.53(b) of U.S. application Ser. No. 10/178,949 filed Jun. 24, 2002, which claims the benefit under 35 U.S.C. 119(e) of U.S. Provisional Application Ser. No. 60/303,763 filed Jul. 6, 2001, which applications are incorporated herein by reference.
The present invention relates to optical communications and the processing of optical signals, and in particular relates to systems and methods for adjusting the phase of optical signals having an arbitrary polarization.
The transmission of information over optical fibers is becoming pervasive. This is motivated, at least in part, because optical fiber offers much larger bandwidths than electrical cable. Moreover, optical fiber can connect nodes over large distances and transmit optical information between such nodes at the speed of light.
There are, however, a number of physical effects that limit the ability to transmit large amounts of information over an optical fiber. One such effect is called “chromatic dispersion,” which refers to the spreading of a pulse of light (i.e., an “optical signal” or “lightwave signal”) due to the variation in the propagation velocity of the different optical frequencies (or equivalently, wavelengths) making up the pulse.
Chromatic dispersion has two root causes. The first is due to the fact that silica of the optical fiber, like any optical material, has an index of refraction that is frequency-dependent. This is referred to as “material dispersion.” The second cause is due to the nature of the propagation of light down the fiber and is referred to as “waveguide dispersion.” The power distribution of the light between the core and the cladding of the fiber is a function of frequency. This means the “effective index” or “propagation constant” of the waveguide mode is a function of frequency as well, which causes the optical signal to disperse as it travels down the fiber.
In optical fiber communication systems, chromatic dispersion causes individual bits to broaden, since each bit is composed of a range of optical frequencies that separate due to their different propagation velocities. Such broadening eventually leads to intersymbol interference due to overlap of adjacent bits, which results in unacceptable data transmission errors. Chromatic dispersion compensation is usually needed to obtain the required performance in lightwave transmission systems operating at per channel data rates of 10 Gb/s or above. For example, the dispersion of a standard single mode fiber (SMF) at the key lightwave communications wavelength of 1550 nm is roughly 17 ps/nm-km. For a 10 Gb/s transmission system, the optical bandwidth per channel is typically a minimum of 0.1 nm, and is often greater. Transmission through a 30 km span of SMF would lead to a chromatic dispersive broadening of the signal of 51 ps, which is 50% of the bit period (100 ps).
Such a broadening is unacceptably large and would lead to a large error rate. The problem becomes much more acute with higher data rates, such as 40 Gb/s per channel systems currently under development. The problem will even become more acute for the anticipated higher data rate systems presently being contemplated. Further details about the nature of chromatic dispersion in optical fibers and the consequences for optical networks can be found in the book by Ramaswami and Sivarajan, entitled Optical Networks, a Practical Perspective, Morgan Kaufmann Publishers, in chapter 2.3.
Efforts have been made in the past to develop systems and methods for compensating for the effects of chromatic dispersion. For example, dispersion-compensating fibers (DCF) have been developed that have the opposite sign of dispersion compared to conventional single mode fibers have been developed and are widely deployed as compensators. However, the DCF technique lacks the ability to easily fine tune the spectral variation of the dispersion and involves a relatively large insertion loss for long fiber links. Chirped fiber Bragg gratings can also compensate fixed amounts of dispersion, but only for one WDM channel at a time. Both techniques lack the ability to reprogram or programmably fine tune the amount of dispersion and its spectral profile, which is likely to be needed to develop higher rate lightwave communication systems.
A number of workers have used programmable pulse shapers to programmably compensate chromatic dispersion in high-power femtosecond pulse amplifiers and in nonlinear optical pulse compression systems. A variety of spatial light modulator (SLM) types have been used, including liquid crystals, acousto-optic modulators, and deformable mirrors.
By way of examples, the use of a deformable-mirror SLM to correct chromatic dispersion is described in the paper by E. Zeek et al., Pulse compression by use of deformable mirrors, Opt. Lett, 24, 493-495 (1999). The use of an arrayed waveguide grating (AWG) rather than a bulk diffraction grating as the spectral disperser is described in the paper by H. Takenouchi et al., entitled 2×40-channel dispersion-slope compensator for 40-Gbit/s WDM transmission systems covering entire C- and L-bands, presented at the Optical Fiber Communications Conference (OFC), sponsored by the Optical Society of America, Anaheim, Calif., March 2001; however, in this paper a fixed phase mask is used in place of an SLM, with the result that the dispersion is not programmable. Further, the article by C. Chang et al. entitled Dispersion-free fiber transmission for femtosecond pulses by use of a dispersion-compensating fiber and a programmable pulse shaper, Opt. Lett. 23, 283-285 (1998) describes chromatic dispersion compensation using a liquid crystal SLM.
These and the other efforts described in the cited references all have the shortcoming that the operation of the dispersion compensation system depends on the SOP and/or that the system is not sufficiently programmable to handle the dispersion slope and higher-order dispersion terms or to reprogram the dispersion profile to accommodate changes in the length of optical fiber links in a switched optical networking environment. The dependence of a chromatic dispersion compensation system on the SOP of the input lightwave is major shortcoming because the SOP of light having traveled through an optical fiber system is scrambled and can vary with time, resulting in polarization-dependent loss (PDL). Further, the inability to robustly perform phase encoding of the signal reduces the ability to accurately compensate for the chromatic dispersion characteristics of a given optical fiber system.
Accordingly, what is needed is a system and method that can programmably compensate, with a high degree of accuracy, an optical signal for chromatic dispersion effects of an optical fiber, while also being insensitive to the SOP of the light signal being processed.
The present invention relates to optical communications and the processing of optical signals, and in particular relates to systems and methods for adjusting the phase of optical signals having an arbitrary polarization. The present invention finds particularly utility in correcting, reducing or otherwise adjusting chromatic dispersion in optical signals.
The present invention provides the capability to programmably control pulse broadening due to chromatic dispersion in chromatically dispersive media, and in particular in optical fiber communications systems and networks. This capability allows optical fiber lightwave communication systems to run at higher speeds over longer distances by compensating chromatic dispersion, which is regarded as a key impairment for high-performance lightwave communication systems. The present invention can be applied both to very high-speed time-division multiplexed (TDM) and to wavelength division multiplexed (WDM) optical communications. In the case of WDM systems, several WDM channels can be independently compensated and can be programmed to achieve nearly arbitrary dispersion profiles in order to match the system requirements. The chromatic dispersion compensator can handle input optical signals with arbitrary and unspecified state of polarization, and may be configured to provide substantially zero PDL.
Accordingly, a first aspect of the invention is a system for programmably adjusting the phase of the frequency components of an optical signal of arbitrary polarization. The system includes a first dispersive module arranged to receive and disperse the optical signal into its frequency components. A polarization-independent spatial light modulator (PI-SLM) having an active area comprising a plurality of independently programmable addressable regions is arranged to receive the frequency components on the active area. The PI-SLM may be, for example, a liquid-crystal SLM adapted for polarization-independent operation, or a programmably deformable mirror. A controller is coupled to the PI-SLM. During operation of the system, the controller causes the PI-SLM to independently adjust the phase of one or more of the frequency components.
In an example embodiment of the invention, the phase-adjustment is performed to alter chromatic dispersion in the optical signal.
A second aspect of the invention is a method of programmably adjusting the phase of the frequency components of an optical signal of arbitrary polarization to adjust the amount of chromatic dispersion in the signal. The method includes spatially dispersing frequency components of the optical signal onto a polarization-independent spatial light modulator (PI-SLM) over an active area having a plurality of independently programmable addressable regions. The method further includes independently adjusting one or more of the addressable regions to alter the phase of the corresponding frequency components incident thereon. The phase-altered signals are then recombined to produce a compensated optical signal.
A third aspect of the invention includes the above described method, and involves adjusting the polarization of the optical signal frequency components so as to reduce any polarization-dependent loss (PDL) due to dispersing the optical signal into its frequency components and/or recombining the phase-altered frequency components to form the compensated optical signal.
A fourth aspect of the invention involves using the chromatic dispersion compensation system of the present invention to compensate for chromatic dispersion in channel optical signals in a wavelength-division multiplexed (WDM) signal. This is accomplished by dividing up the active area of the PI-SLM into sets of addressable regions corresponding to the frequency components of the different channel optical signals, and then compensating the frequency components of each channel signal. The compensated channel signals can then be detected, transferred to another optical system, or recombined with a multiplexer to form a compensated WDM signal.
Like reference symbols in the various drawings indicate like elements.
The present invention relates to optical communications and the processing of optical signals, and in particular relates to systems and methods for adjusting the phase of optical signals having an arbitrary polarization. In the following detailed description of the embodiments of the invention, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration specific embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized and that changes may be made without departing from the scope of the present invention. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is defined only by the appended claims.
With reference to
System 100 can be used, for example, to compensate, reduce or otherwise alter the chromatic dispersion in an optical signal 110. As chromatic dispersion is a variation in the propagation velocity of the different frequency (or, equivalently, wavelength) components making up the optical signal, chromatic dispersion can be adjusted by imparting an appropriate phase to one or more of the frequency components based on a desired phase vs. frequency relationship. The discussion of system 100 and the various implementations of system 100 emphasizes polarization-independent chromatic dispersion compensation because the present invention is eminently suited to such a function. However, it will be apparent to one skilled in the art that system 100 can perform other polarization-independent phase vs. frequency adjustment functions, such as for example wavefront reconstruction, wavefront alteration, and pulse shaping.
Chromatic dispersion may be present in optical signal 110 and caused, for example, by the signal having passed through a first optical system 120 having chromatic dispersion. Optical system 120 may include, for example, a distance of optical fiber 122 having chromatic dispersion. Optical system 120 may also include other optical components, e.g., laser sources, amplifiers, switches, gratings, routers, lenses, couplers etc., collectively shown as an element 124, that are capable of introducing additional amounts of chromatic dispersion. Optical system 120 thus produces chromatic dispersion in optical signal 110 from one or more sources that, absent compensation, limits the bandwidth and/or fidelity of optical processing system 106 as a whole. In particular, chromatic dispersion causes pulse-broadening that, absent compensation, sets an upper limit for the bit rate period because of intersymbol interference.
A preferred consequence of compensating chromatic dispersion in optical processing system 106 is that it can optimize the usable bandwidth of optical signal 110. For example, performing chromatic dispersion compensation of optical signal 110 to form a compensated (i.e., phase-adjusted) signal 126 may be necessary to successfully transmit information through a second optical system 130, which may itself include sources of chromatic dispersion, such as an optical fiber 132 as well as other sources 124 of chromatic dispersion.
With continuing reference to
PI-SLM 140 may be one of a number of spatial light modulators that do not depend on the polarization of the input light signal, and that do not impart a polarization to a light signal. More generally, as used herein, a PI-SLM is any component or aggregation of components that defines an active area 144 having multiple, addressable regions 146 for adjusting the phase, and/or amplitude of light wavefronts incident on the regions. For example, the PI-SLM can have multiple, independently addressable regions such as a discrete array of independently addressable addressable regions. Alternatively, the PI-SLM can have multiple, addressable regions that partially overlap. For example, the PI-SLM can be a deformable mirror having multiple, addressable actuators that deform overlapping regions of the active area. Furthermore, other PI-SLMs can vary the phase, for example, by varying the refractive index of the addressable regions. In the example embodiment shown in
Thus, the PI-SLM can adjust the phase, and/or amplitude of the incident frequency components by, e.g., reflection, transmission, diffraction, or some combination thereof. As described further below, in many embodiments, the PI-SLM involves one or more liquid crystal layers, whose birefringence and/or orientation are controlled to provide a desired series of adjustments for each SLM addressable region. For example, the liquid-crystal PI-SLM may include twisted nematic liquid crystals, non-twisted nematic liquid crystals, and/or ferroelectric liquid crystals. In further embodiments, the PI-SLM can include an inorganic electro-optic modulator, e.g., a lithium niobate crystal coupled to a generator providing a spatially addressable E-field, or an acousto-optic modulator coupled to a transducer providing a spatially addressable acoustic wave.
In one example embodiment, PI-SLM 140 is a multi-layer liquid-crystal modulator, such as described in U.S. Pat. No. 5,719,650 (the '650 Patent), which patent is incorporated herein by reference. As described in the '650 Patent and as illustrated in
In the present invention, liquid-crystal-based PI-SLM 140 of
Specifically, if the liquid crystal alignment directions (axes) of arrays 160 and 162 are described as x and y, then the polarization transfer matrix M(ω) for the two-layered liquid crystal PI-SLM 140 is given by:
Thus, to obtain the output electric field vector EOUT(ω) from the input electric field vector EIN(ω) the following operation is performed:
By setting φx(ω)=φy(ω), M(ω) becomes:
Thus, in the case of the liquid-crystal-based PI-SLM 140 of
With reference now to
For a given SLM element 166, both φx(ω) and φy(ω) can be adjusted by applying the appropriate voltage according to a phase vs. voltage calibration. Such voltage can be provided by controller 150, which is calibrated with the necessary pixel phase vs. voltage data, e.g., as look-up table. As long as elements 166 offer a range of phase variation greater than 2π, φx(ω) can be made equal to φy(ω) and can be programmed to any desired value (modulo 2 π).
With reference again to
Ideally, one would like to avoid PDL in any of the elements in system 100. As the input SOP cannot be specified in system 100, one desires zero PDL, as with any optical processing system where the SOP is not maintained. However, first dispersive module 136 used for spectral dispersion can have an associated PDL. As PI-SLM 140 of the present invention is of the type that does not alter the SOP, one can optionally compensate for dispersive-module-induced PDL by inserting a half-wave (λ/2) polarization-adjusting element 176 (e.g., a half-wave plate, Faraday rotator, etc.) anywhere in system 100 between dispersive modules 130 and 148 (in
With continuing reference to
The phase to be imparted to each frequency component 200 of optical signal 110 can be based on information about the chromatic dispersion properties of a particular optical system (e.g., system 120 or 130) as measured or calculated (e.g., based on a model of chromatic dispersion effects of an optical system). Alternatively, information about chromatic dispersion can be acquired empirically by propagating a known optical signal (e.g., optical signal 110 or a test signal) through an optical system and measuring the chromatic dispersion effect.
In an exemplary embodiment of system 100, controller 150 controls PI-SLM 140 based at least in part on a feed forward detection signal from a detection system 220, which samples a portion of optical signal 110 to characterize its chromatic dispersion. In another exemplary embodiment, controller 150 controls PI-SLM 140 based at least in part on a feedback detection signal from detection system 230 that samples a portion of compensated optical signal 126 to characterize the effective reduction in the chromatic dispersion from system 100.
Furthermore, in another exemplary embodiment, controller 150 controls PI-SLM 140 based at least in part on signals from both detection systems 220 and 230.
Controller 150 includes the necessary power source and logic for independently applying electric fields (voltages) to each of respective addressable regions 146. Suitable power sources and logic are commercially available, e.g., from Cambridge Research and Instrumentation (CRI), Woburn, Mass. Controller 150 can also store appropriate calibration curves for array 145 so that the voltage necessary to impart a desired phase retardance is known. The algorithms can be implemented in computer programs or dedicated integrated circuits or computer-readable media (e.g., floppy disks or compact disks) using standard programming techniques.
Thus, in an exemplary embodiment of the present invention, controller 150 includes a computer system 258 (or may be linked to a computer system) that may be, for example, any digital or analog processing unit, such as a personal computer, workstation, a portion of a console, set top box, mainframe server, server-computer, laptop or the like capable of embodying the programmable aspect of invention described herein. In an example embodiment, computer 258 includes a processor 260, a memory device 262, and a data storage unit 264, all electrically interconnected. Data storage control unit 264 may be, for example, a hard drive, CD-ROM drive, or a floppy disk drive that contains or is capable of accepting and reading a computer-readable medium 268. In an example embodiment, computer-readable medium 268 is a hard disk, a CD, a floppy disk or the like. Computer-readable medium 268 may contain computer-executable instructions to cause controller 150 to perform the methods described herein. An example computer 258 is a Dell personal computer (PC) or Workstation, available from Dell Computer, Inc., Austin, Tex.
In another example embodiment, computer-readable medium 268 comprises a signal 270 traveling on a communications medium 272. In one example embodiment, signal 270 is an electrical signal and communications medium 272 is a wire, while in another example embodiment, the communications medium is an optical fiber and the signal is an optical signal. Signal 270 may, in one example, be transmitted over the Internet 276 to computer 258 and optionally onward to controller 150.
As described above in connection with system 100 of
Preferably, one or both of detection systems 220 and 230 sense the spectral phase of the particular optical signal 110 and/or 126 on a wavelength-by-wavelength basis. Sensing of the spectral phase (or equivalently the frequency-dependent delay τ(ω)) can be achieved by using spectral interferometry techniques, cross-correlation techniques, and/or self-referencing measurement techniques, such as frequency resolved optical gating. Such techniques are described in, e.g., L. Lepetit et al., J. Opt. Soc. Am. B. 12, 2467-2474 (1995), K. Naganuma et al., Opt. Lett. 15, 393-395 (1990), and R. Trebino et al., Rev. Sci. Instrum. 68, 3277-3295 (1997), respectively.
With continuing reference to
Dispersive modules 130 and 148 may further include one or more imaging or relaying optics (e.g., lenses, mirrors, apertures, etc.) for directing the frequency components spatially separated by the dispersive element in module 130 onto PI-SLM 140 or for directing the adjusted frequency components from PI-SLM 140 to the dispersive element in dispersive module 148. Moreover, in additional exemplary embodiments of the present invention, the dispersive modules can be a single optical element that combines the dispersing and directing functions, such as a diffractive optical element (DOE).
Even where individual addressable regions of PI-SLM 140 provide many degrees of control over incident frequency components, the maximum amount of chromatic dispersion that can be compensated or reduced is limited by the spectral resolution of system 100. Generally, the parameters of dispersive module 136 are selected to fully exploit the entire pixel array 145 of PI-SLM 140. In other words, one tries to minimize the range of frequency components 200 on any one pixel 146 while also insuring that all frequency components of interest are incident on at least one pixel. Accordingly, spectral resolution can be made to scale with the number of independently addressable addressable regions 146 of PI-SLM 140.
For example, PI-SLM 140 may have, e.g., at least 2, 4, or 8 addressable regions, and preferably many more, e.g., 64, 128, etc. In any case, to avoid aliasing, spectral variations in the chromatic dispersion of the signal should be slow compared to the frequency width, denoted δf, of one pixel 146. This is equivalent to the requirement that the total duration of the signal to be compensated should be significantly below ½δf. The situation may be modified somewhat for embodiments in which the chromatically dispersed optical signal includes multiple signals on separate wavelength bands. In this case, dispersive modules 130 and 148 and PI-SLM 140 can be tailored to optimize spectral resolution within each band, whereas regions between separate bands may be ignored. Thus, the PI-SLM can have multiple sets of arrays 145, with each array dedicated to a particular wavelength band.
With continuing reference again to
It is worth remarking on the relationship between delay and spectral phase. For complete phase control, PI-SLM 140 only needs to vary the phase at each pixel 146 over a 0-2π radian range, which by itself constitutes a small phase delay. The frequency dependent group delay, however, varies as the derivative of phase with respect to frequency. In particular, frequency-dependent delay π(ω) is related to a spectral phase variation ψ(ω) as shown in EQ. 4:
As mentioned above, there are many specific examples of system 100 of
Transmission System with Liquid Crystal PI-SLM and Diffraction Gratings
Referring now to
Optical signal 110 emanates from the end of an output optical fiber 122 as part of optical system 120 and is incident on first grating 300. The collimation and focusing of frequency components 200 can be accomplished by spacing lens 306 from each of grating 300 and PI-SLM 140 by a distance equal to its focal length F1. Thus, the grating and lens map the frequency content (i.e., components 200) of optical signal 110 onto SLM arrays 160 and 162. Moreover, because of the positioning of lens 306, grating 300, and PI-SLM 140, the spatial extent of any individual frequency component on arrays 160 and 162 is minimized. For each pixel, PI-SLM 140 independently adjusts the phase (and optionally the amplitude) of the frequency components 200 incident on the pixel (in
Second dispersive module 148 includes a second grating 320 and a second lens 326 having a focal length F2 for recombining the adjusted spatially-separated frequency components 204 into adjusted optical signal 126, which can then be coupled to an optical fiber 132 as part of second optical system 130. Like first dispersive module 136, lens 326 is preferably spaced from each of PI-SLM 140 and grating 320 by a distance equal to its focal length F2. In an example embodiment, the focal length of lenses 306 and 326 are the same (i.e., F1=F2=F), and thus the gratings, lenses, and SLM define a “4-F” arrangement.
An advantage of the present invention is that gratings 300 and 320 need not be polarization insensitive, since system 110 as a whole does not rely on knowledge of SOP of optical signal 110.
In other embodiments of system 100 of
Depending on the nature of gratings 200, PDL can be significant. Thus, optionally included in system 100 of
On-Axis Reflection System with Diffraction Gratings
With reference now to
On-Axis Reflection System with Magnification
With continuing reference to
Accordingly with reference now to
Magnification is achieved in the present invention by forming an intermediate image I1 at an intermediate image plane P1 of spectral components 200 formed by grating 300 located at a plane P0. Image I1 is then used as an object for forming a magnified image I2 of the frequency components 200 at a second image plane P2 coincident with active area 144 of PI-SLM 140 using a magnifying optical system 460 arranged between planes P1 and P2. Thus, magnifying optical system 460 relays with magnification frequency components 200 onto active area 144. In general, the magnification provided is such that the optical path of system 100 is shortened as compared to the optical path without the introduction of magnification. The necessary magnification will depend, in part, on the size of active area 144 of PI-SLM 140 and the amount of dispersion of the frequency components provided by dispersive module 136.
With reference to
Because the size of system 100 scales with the size of the active area 144 of PI-SLM 140, it can also be made compact by using an SLM with a smaller active area (i.e., aperture) 144 and smaller addressable regions 146 in addition to, or as an alternative to providing magnification. For example, certain liquid crystal SLMs have addressable regions (pixels) of typically about 100 microns, but also as small as 25 microns, which allowing 512 pixels to fit into a 12.8 mm aperture. Such an SLM is available from the Raytheon Company in Lexington, Mass. A similar SLM with 128 pixels would have an aperture of approximately 3 mm. A liquid crystal SLM from Boulder Nonlinear Systems, Boulder, Colo., has 4096 pixels, with a center-to-center pixel spacing of 1.8 microns and an aperture of 7.4 mm.
Thus, a small PI-SLM 140 has an aperture size of about 5 mm across, and in an example embodiment, has an aperture size of 3 mm or less. With a reflective system 100, using a PI-SLM having an active area of (3 mm×3 mm) and a magnification of 30 can result in system 100 having an overall length as small as, for example, 40 cm, for a 1 nm optical bandwidth. This system could be made more compact by folding the optical path using, for example, fold mirrors or fold prisms.
Off-Axis Reflection System
With reference now to
Optical Processing Systems Implementing the System 100
System 100 can also be implemented in optical processing system configurations other than that shown in
These various implementations are now described in greater detail below.
With reference now to
With reference now to
Optical System Compensation Implementation
With reference now to
Accordingly, a sensor 580 is arranged to be in optical communication with optical system 520 and system 100, wherein the sensor is adapted to sense the chromatic dispersion of the optical system. This may carried out, for example, by providing one or more lightwave test signals 586 having particular characteristics (e.g., a set bandwidth, pulse length and/or pulse shape) through optical system 520, and measuring the amount of chromatic dispersion induced using system 100. System 100, via controller 150, processes the measurements and determines the amount of pre-compensation (
WDM Optical Processing System Implementation
Although the preceding paragraphs refer to compensation of pulse broadening caused by chromatic dispersion, it is noted that optical signals 110 and 126 (
Thus, with reference to
Channel optical signals 110 a, 110 b and 110 c pass through a demultiplexer 600, which separates the channel signals so that they can be coupled into corresponding optical fibers 610 a, 610 b and 610 c. Each of fibers 610 a, 610 b and 610 c is coupled to a system 100. Systems 100, as described above, each adjust the phase of the frequency components of the corresponding channel optical signal so that the signal is compensated for chromatic dispersion, as described above. The result is compensated channel signals 126 a, 126 b and 126 c traveling along optical fibers 610 a, 610 b and 610 c
With reference now to
With reference now to
WDM Compensation Using a Single PI-SLM
With reference now to
Multiplexed optical signal 604 is incident on dispersive module 136. The latter is designed to disperse not only the frequency components 200 a-200 c of the individual channel optical signals 110 a, 110 b and 110 c, but also to disperse the different channels signals relative to one another. Thus, the different channel optical signals 110 a-110 c of multiplexed signal 604 are dispersed to different zones 630 (e.g., 630 a-630 c) on PI-SLM 140, with each zone containing a set of addressable regions 146. Thus, the addressable regions 146 in zones 630 a-630 c are dedicated to compensating the optical frequencies in the wavelength band Δλ centered around each wavelength λa-λc, respectively. This allows different channel optical signals centered at different wavelengths to be independently and simultaneously compensated and optimized. This is advantageous because chromatic dispersion properties vary across the wavelength spectrum, so that different optical signals centered around different wavelength bands will generally require different amounts of chromatic dispersion compensation.
It should be noted that this WDM chromatic dispersion correction approach requires a trade-off between the number of addressable regions per channel optical signal and the number of channel optical signals in the WDM signal. In an example embodiment, the number of addressable regions per channel optical signal is preferably between about 8 and 16. Further, the total number of addressable regions is preferably about 128 or greater.
The invention provides the capability to programmably control pulse broadening due to chromatic dispersion in optical fiber communication and networking systems. This capability allows optical fiber lightwave communication systems to run at higher speeds over longer distances by compensating chromatic dispersion, which is regarded as a key impairment for high performance lightwave communication systems.
The present invention can be applied both to very high-speed time-division multiplexed (TDM) and to wavelength division multiplexed (WDM) optical communications. In the case of WDM systems, several WDM channels can be compensated independently and can be programmed to achieve nearly arbitrary dispersion profiles in order to match the system requirements. The chromatic dispersion compensation system of the present invention can handle input optical signals with an arbitrary and unspecified SOP, and may be configured to provide substantially zero PDL. The programmable nature of the present invention allows for chromatic dispersion to be compensated under a variety of conditions and situations, including re-programming the chromatic dispersion compensation system when there is a change in the optical processing system configuration (e.g., the length of the network) that results in a change in the system chromatic dispersion.
A number of embodiments of the invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. For example, some embodiments may incorporate both pre-compensation and post-compensation. In such case, a pre-compensation system may be used to match the wavelength-by-wavelength chromatic dispersion of an optical signal to the wavelength-by-wavelength chromatic dispersion of a downstream optical system. Thereafter, a post-compensation system can further reduce chromatic dispersion in the optical signal caused by the optical system
Thus, while the present invention has been described in connection with preferred embodiments, it will be understood that it is not so limited. On the contrary, it is intended to cover all alternatives, modifications and equivalents as may be included within the spirit and scope of the invention as defined in the appended claims.