FIELD OF THE INVENTION
The present invention relates generally to lasers, and specifically to stabilization of lasers operating in multiple modes.
BACKGROUND OF THE INVENTION
FIG. 1 is a schematic diagram showing operation of a lasing system 18, as is known in the art. System 18 comprises two mirrors 20 and 22 separated by a distance L. In order for system 18 to lase i.e., to resonate, at a wavelength λ, a medium 24 between mirrors 20 and 22 must provide gain, and an effective optical path length Leff between the mirrors must be an integral number of half-wavelengths. Quantitatively,
wherein m is a positive integer, n is a refractive index of medium 24, f is the frequency corresponding to the wavelength λ, and c is the speed of light.
From equation (1c), a separation Δf of lasing frequencies is given by
Each such lasing frequency corresponds to a longitudinal cavity mode. Since f=c/λ, Δf≈−c·Δλ/λ2 so that equation (2) can be rewritten to give a separation Δλ of lasing wavelengths:
FIG. 2 is a graph of intensity I vs. wavelength k illustrating cavity modes for system 18, as is known in the art. A curve 30 represents an overall gain of medium 24 in system 18. Peaks 32A and 32B, with separation AX, show the cavity modes present in system 18, each node corresponding to a different value of m. As is evident from FIG. 2, there are many possible cavity modes for system 18.
Optical communications within fiber optic links require that the laser carrier have as small a frequency spread as possible, particularly when multiple wavelengths are to be multiplexed on a single fiber. Thus, for efficient communication only one cavity mode should be used, and optimally the frequency spread within the mode should be minimized. Typically, methods for stabilizing the frequency of the laser include utilizing distributed feedback (DFB) lasers and/or distributed Bragg reflector (DBR) lasers. DFB lasers have a frequency-selection grating built into the laser chip, the grating being physically congruent with the gain medium. The grating in a DBR laser is external to the gain medium. The gratings in DFB and DBR lasers are part of the semiconductor material, which is unstable. DFB and DBR lasers are therefore typically externally stabilized utilizing an external wavelength reference in order to achieve good stability.
FIG. 3 shows the effect of adding a tuning element such as a fiber grating to system 18, as is known in the art. A curve 34 shows the resonance curve of the fiber grating, which has a bandwidth ΔλG of the same order as Δλ, the separation between the longitudinal cavity modes. If the grating is optically coupled to system 18, then mode 32A is present, and other modes such as mode 32B, are suppressed.
FIG. 4 is a schematic diagram showing a gain medium 38 coupled to a fiber grating 50, as is known in the art. Gain medium 38 is formed from a semiconductor gain element 44 having a laser gain region 42. Light from region 42 exits from a facet 56 of region 42 to a medium 46, and traverses medium 46 so that a lens 48 collects the light into a fiber optic 52. Fiber grating 50 is mounted in fiber optic 52, which grating reflects light corresponding to curve 34 of FIG. 3 back to region 42. The mirrors of the laser cavity comprise a rear mirror which in this example is a back facet 57 of the semiconductor gain element, and an output coupling mirror which in this example is fiber grating 50. The rear and output coupling mirrors could also be reversed. In the reversed configuration the rear mirror would be the fiber grating and the output coupling mirror would be back facet 57 of the semiconductor gain element.
It is desirable to eliminate parasitic reflections due to surfaces and interfaces internal to the cavity. To eliminate parasitic reflection from the facet of the semiconductor closest to the fiber grating, in this case facet 56, that facet is usually anti reflection coated. It is also useful to anti reflection coat a tip 49 of the fiber closest to the semiconductor gain element to again reduce parasitic reflections. Preferably, grating 50 is written directly at the end of the fiber optic facing the laser. Alternatively, a length Lf of a fiber 63 is interposed between lens 48 and fiber optic grating 50. Thus region 42, medium 46, fiber optic 63 and grating 50 form a resonant system 60 corresponding to region 24 of FIG. 1. This architecture is generally known in the art as an external cavity laser or more specifically as a fiber grating laser (FGL).
System 60 has an effective optical path length Leff given by:
L eff= n 1 ·L 1 +n 0 ·L 0 +n f ·L f +n g ·L gef (4)
n1 is a refractive index of region 42;
L1 is a length of region 42;
n0 is a refractive index of medium 46;
L0 is a length of medium 46;
nf is a refractive index of fiber 63;
Lf is the length of fiber 63.
ng is a refractive index of grating 50; and
Lgef is an effective length of grating 50.
Replacing the optical path length nL of equation (1b) by that given by equation (4) leads to the following equation giving cavity modes for the system of FIG. 3:
m λ/2=(n 1 ·L 1 +n 0 ·L 0 +n f ·L f +n g ·L gef) (5)
In constructing system 60, it is necessary to adjust and maintain the positions of curve 32A and 34 (FIG. 3) to have their peaks at the same wavelength. Changes in temperature and/or changes in injection current into region 42 and/or mechanical changes affect one or more parameters of the optical path length given by equation (4). Such changes can thus cause mode hopping, which refers to the phenomena whereby mode 32A shifts underneath resonance curve 34 of the fiber grating. When that shift is large enough, an adjacent mode will at some point experience a larger gain and start to lase. These mode hops occur underneath the resonance curve of the fiber grating (curve 34 in FIG. 3) resulting in wavelength shifts and intensity noise when the mode hops. For example, referring to FIG. 3, mode 32B could shift within resonance curve 34 of the fiber grating and resonate instead of mode 32A.
An article titled “1.5-1.6 μm dynamic-single-mode (DSM) lasers with distributed Bragg reflector,” by Koyama et al., in the Vol. 19 (1983) issue of IEEE Journal of Quantum Electronics, which is incorporated herein by reference, describes a method for tuning a DBR by injecting current.
Super structure grating (SSGs) are known in the art as structures which comprise a plurality of gratings distributed along a fiber in a manner such as to provide a spectral response with several peaks. An article titled “Long periodic superstructure Bragg gratings in optical fibers,” by Eggleton et al., in the Vol. 30 (1994) issue of Electronics Letters, which is incorporated herein by reference, describes one such SSG. An SSG in a fiber provides a system having a plurality of relatively highly stable fixed wavelengths.
SSG systems have been implemented in semiconductor devices, to take advantage of the fact that when implemented therein the cavities so formed are tunable. An article titled “Theory, design and performance of extended tuning range semiconductor lasers with sampled gratings,” by Jayaraman et al., in the Vol. 29 (1993) issue of IEEE Journal of Quantum Electronics, which is incorporated herein by reference, describes two such systems which are tuned in a vernier-like manner. Unfortunately, due to the inherent characteristics of the semiconductor, SSG devices implemented in semiconductors are relatively unstable, as is also true for single wavelength DBRs implemented therein.
The information carrying capacity of a single lasing line can be increased by increasing the frequency of modulation of the laser. However such increased frequency widens the laser line width through chirp, which in turn reduces the transmission range due to dispersion. In order to circumvent the effects of dispersion, a method known in the art is to use wavelength division multiplexing (WDM) wherein a plurality of laser lines are each modulated at an intermediate bandwidth. The total bandwidth is then the number of laser lines multiplied by the intermediate bandwidth. For example, instead of modulating one line at 10 Gbit/s, four lines can each be modulated at 2.5 Gbit/s to provide the same information carrying capacity.
Standard ITU-T G. 692 of the International Telecommunications Union (ITU), Place des Nations CH-1211, Geneva 20, Switzerland, defines allowable wavelengths for WDM systems, so that systems implemented by different manufacturers will be compatible with each other. Systems known in the art for implementing WDM use a plurality of lasers, each having a different fixed wavelength corresponding to the ITU standard.
SUMMARY OF THE INVENTION
It is an object of some aspects of the present invention to provide improved methods and apparatus for generating a plurality of laser wavelengths.
In preferred embodiments of the present invention, a semiconductor device comprises an active gain region and a distributed Bragg reflector (DBR), which acts as a first, highly-reflecting mirror at one end of a laser cavity that contains the gain region. The DBR is tuned by, most preferably, varying a current injected into the DBR. The active gain region is coupled at the side opposite the DBR to a fiber optic comprising a super structure grating (FO-SSG) having a plurality of relatively highly stable resonant peaks, the peaks most preferably being separated substantially equidistantly. The FO-SSG acts as a second, partially-reflecting mirror at the other end of the laser cavity. An output of the cavity is derived from light transmitted by the FO-SSG into the fiber optic.
To operate the laser, the gain region is activated, and the DBR is tuned so that a resonant peak of the DBR is aligned with one of the resonant peaks of the FO-SSG. Thus, the laser resonates in a single cavity mode defined by the two resonant peaks, and all other modes are substantially suppressed. By scanning the tuning of the DBR over the range of the FO-SSG, all the different resonant peaks of the FO-SSG may be selected at will, producing corresponding single cavity modes. Coupling a DBR with an FO-SSG combines the advantages of tunability associated with the DBR and stability associated with the FO-SSG for all modes of the cavity. Preferred embodiments of the present invention thus enable a single laser to be used as a generator in a WDM system.
In some preferred embodiments of the present invention, the DBR is written as a super structure grating (DBR-SSG) within the semiconductor device. The spacing of peaks of the DBR-SSG is implemented to be slightly different from the spacing of the peaks of the FO-SSG. The cavity produced by the combination of the DBR-SSG with the FO-SSG is then tuned in a vernier-like manner, by setting one of the peaks of the DBR-SSG to align with one of the peaks of the FO-SSG. Because of the vernier-like spacing relationship between the two SSGs, all other peaks, apart from the aligned pair, are misaligned, so that only one mode defined by the aligned pair resonates and all other modes are suppressed. All the resonant peaks of the FO-SSG may be selected by scanning the DBR-SSG over a range that is substantially the same as the spacing of two peaks of the DBR-SSG.
In some preferred embodiments of the present invention, the laser cavity is stabilized by thermally modulating one or more optical elements, and/or parameters thereof, so as to vary an effective length of the cavity. A method of thermal modulation is described in detail in PCT patent application PCT/IL00/00401 which is assigned to the assignee of the present invention and which is incorporated herein by reference. The modulation generates an error signal which is dependent on a relationship of the oscillating mode with resonant frequency of the cavity, and the error signal is used in a negative feedback loop to ensure that the mode and resonant frequency substantially coincide.
There is therefore provided, according to a preferred embodiment of the present invention, a laser, including:
a grating structure, including two or more gratings having a plurality of different wavelength peaks for reflection of optical radiation therefrom; and
a semiconductor device, including an active region, which is operative to amplify the optical radiation, and a reflective region, which is adapted to reflect the optical radiation at a tunable resonant wavelength of the reflective region, the device being optically coupled to the grating structure so as to define a laser cavity having a single cavity mode defined by tuning the resonant wavelength of the reflective region to overlap with one of the wavelength peaks of the grating structure.
Preferably, the grating structure includes a super structure grating (SSG) written in a fiber optic.
Preferably, fiber optic includes a lens which focuses optical radiation from the semiconductor device to the grating structure.
Preferably, the two or more gratings are adapted to partially transmit optical radiation at the different wavelengths, so as to provide output optical radiation.
Preferably, the reflective region includes a Distributed Bragg Reflector (DBR) written onto the semiconductor device, wherein the resonant wavelength of the reflective region is tuned by a current injected into the DBR.
Preferably, the plurality of different wavelength peaks are substantially equidistantly spaced by a first separation, wherein the reflective region includes a Distributed Bragg Reflector with a super structure grating (DBR-SSG) having a plurality of different wavelength peaks substantially equidistantly spaced by a second separation different from the first separation, so that the first separation is related to the second separation in a vernier-like manner and so that the single cavity mode is defined when one of the grating structure wavelength peaks overlaps with one of the DBR-SSG wavelength peaks
Preferably, the laser includes:
an optical length changer which varies an optical length of at least one of a group of optical elements including the grating structure the active region and the reflective region, so as to vary accordingly an optical length of the laser cavity;
a detector which is adapted to monitor a level of the optical radiation responsive to the variation in the optical length of the at least one of the group; and
a stabilizer which responsive to the measured output from the detector supplies a control signal to the optical length changer to control an optical length of at least one of the group, so that the laser cavity resonates stably in the single cavity mode.
Further preferably, the optical length changer includes at least one of a thermally active group comprising a heater and a thermoelectric cooler, wherein the at least one of the thermally active group is adapted to alter a temperature of at least one of the group of optical elements.
There is further provided, according to a preferred embodiment of the present invention, a method for generating a laser output, including:
providing a grating structure having a plurality of different wavelength peaks for reflection of optical radiation therefrom;
optically coupling a semiconductor device to the structure so as to define a laser cavity between the structure and a reflective region of the device, which is adapted to reflect the optical radiation at a tunable resonant wavelength of the reflective region; and
tuning the resonant wavelength of the reflective region to overlap with one of the wavelength peaks of the grating structure so as to generate a laser output in a single cavity mode defined by the overlap.
Preferably, providing the grating structure includes writing a super structure grating (SSG) in a fiber optic.
Preferably, providing the grating structure includes providing two or more gratings which are adapted to partially transmit optical radiation at the different wavelength peaks, so as to provide output optical radiation.
Preferably, the reflective region includes a Distributed Bragg Reflector (DBR) written onto the semiconductor device, wherein tuning the resonant wavelength of the reflective region includes injecting a current into the DBR.
Preferably, the reflective region includes a Distributed Bragg Reflector with a super structure grating (DER-SSG) having a plurality of different DBR-SSG wavelength peaks substantially equidistantly spaced by a first separation, wherein providing the grating structure includes substantially equidistantly spacing the plurality of different wavelength peaks by a second separation different from the first separation and related to the first separation in a vernier-like manner, and wherein tuning the resonant wavelength includes overlapping one of the grating structure wavelength peaks with one of the DBR-SSG wavelength peaks
Preferably the method includes:
varying with an optical length changer an optical length of at least one of a group of optical elements including the grating structure, an active region of the semiconductor device, and the reflective region, so as to vary accordingly an optical length of the laser cavity;
monitoring a level of the optical radiation responsive to the variation in the optical length of the at least one of the group; and
supplying a control signal to the optical length changer responsive to the monitored level so as to control an optical length of at least one of the group, so that the laser cavity resonates stably in the single cavity mode.
Further preferably, varying the optical length includes altering a temperature of at least one of the group of optical elements using at least one of a thermally active group comprising a heater and a thermoelectric cooler.
The present invention will be more fully understood from the following detailed description of the preferred embodiments thereof, taken together with the drawings, in which: