Wavelength reconfigurable laser transmitter tuned via the resonance passbands of a tunable microresonator

The present invention relates to a laser transmitter capable of being configured to transmit one of a plurality of wavelengths. Specifically, the laser transmitter may be reconfigured using the resonance passbands of a tunable microresonator coupled with a fixed grating.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is related to U.S. patent application Ser. No. 10/116,800 entitled “Waveguide-Bonded Optoelectronic Device” filed Apr. 5, 2002, which application is hereby incorporated by reference in its entirety.

BACKGROUND INFORMATION

1. Field of the Invention

The present invention relates to a laser transmitter capable of being configured to transmit one of a plurality of wavelengths. Specifically, the laser transmitter may be reconfigured using the resonance passbands of a microresonator.

2. Description of Related Art

More and more applications are using photonic links to transmit signals. For example, in antenna remoting applications, a RF signal can be modulated onto an optical carrier and subsequently sent over an optical link to a remote site for possibly additional processing and for remote RF wave transmission from an antenna or an antenna farm. Wavelength Division Multiplexing (WDM) technologies have much to offer in this application. First, many signals can be multiplexed into one strand of optical fiber via WDM, thus alleviating the need to deploy a large number of fibers for the remoting of an antenna farm. Second, the WDM channel multiplexers hold a tremendous advantage in insertion loss over conventional optical combiners as the number of multiplexed channels increases. For example, an 8:1 WDM multiplexer will demonstrate an optical insertion loss of only 5 dB. In comparison, a conventional optical power-combiner has an optical insertion loss of about 10 dB. For 16:1 and 32:1 multiplexing, the combined losses of WDM multiplexers stay constant at approximately 7 dB.

Transmission of signals via photonic links may also be used in a number of other applications. For example, many systems contain a mixture of analog and digital data that originates from a variety of sensors. Typically, the outputs of these sensors are linked together with affiliated processors and displays via a versatile and reconfigurable network. As discussed above, WDM optical networks offer bandwidth enhancements with a concurrent reduction in the physical number of cables and connectors. These extra cables and connector interfaces represent potential on-board service-points. Minimization of these on-board service-points increases reliability. Reconfigurable WDM networks and components provide a platform with the capabilities to perform changing missions, address changing environments, and reduce the variety of components stocked in the maintenance inventory. Another desire, for some systems, is to reduce the overall power consumption.

The International Telecommunications Union (ITU) has specified a grid of wavelengths (near λ˜1550 nm) as the standard carrier frequencies for Wavelength Division Multiplexing (WDM) networks. Depending upon the desired number of transmission channels, these wavelengths (a.k.a. ITU-channels) are designed to be 50 GHz (=0.4 nm) or 100 GHz (=0.8 nm) apart. By standardizing the carrier frequencies, the ITU hopes to ensure that a WDM network's key passive components, such as wavelength multiplexers/demultiplexers, routers/switches, etc., are spectrally compatible with one another, even though they might be developed by different manufactures. Hence, the emission wavelengths of WDM transmitters are designed to align precisely with the ITU-grid.

Previously, distributed feedback (DFB) or distributed Bragg reflector (DBR) laser structures were commonly employed to provide WDM transmitters with emission wavelengths that align precisely with the ITU-grid. DFBs or DBRs provide single mode laser diodes that emit a wavelength near an ITU-channel. Thermal tuning (at a rate of 0.1 nm/° C.) is then used to move the diode's lasing wavelength into precise alignment with the desired ITU-channel. Finally, an external passive element, typically a Fabry-Perot etalon, is used in a feed back loop as a “wave-locker” to maintain the precise alignment of the wavelength over the long term.

There are several disadvantages to the prior art approach. First, the tuning speed via thermal heating/cooling is slow, typically in the millisecond range. Second, the number of distinct DFB/DBR laser “models” needed multiplies rapidly as the number of wavelength channels increases. In addition, each DFB/DBR laser “model” requires a slightly different fabrication procedure. Finally, the wavelength control feedback loop with the “wave-locker” is physically cumbersome, increasing the size of the transmitter package substantially.

To reduce the inventory count of discrete diode elements in a WDM network, various types of wavelength tunable transmitters have been developed in recent years. These include external cavity lasers, as shown inFIG. 1a, whose emission wavelengths are controlled mechanically via the physical alignment of an external grating. The physical alignment of the external grating is made with respect to the optical feedback path in the laser cavity. In other designs, as shown inFIG. 1b, the wavelength, λB(mλB=2Λneff, where m is an integer) of the distributed Bragg reflector (period=Λ) in the diode laser is electrically tuned, via current-induced index (neff) changes, to vary the lasing wavelength. However, these tunable DBR lasers require the use of a phase control-section, in addition to the gain and Bragg sections, to accomplish the quasi-continuous wavelength tuning. Furthermore, the implementation of complex multi-variable algorithms is often necessary to guide the bias-currents of all three diode sections, so that the alignment of wavelength, λBto the ITU-grid can be accomplished and maintained. For more information see Sarlet et. al, “Wavelength and Mode Stabilization of Widely Tunable SG-DBR and SSG-DBR Lasers”,IEEE Photon. Technol. Lett., Vol. 11, No. 11, 1999, pp. 1351-1353.

Another prior art solution can be found in J. Berger, et. al, “Widely tunable external cavity diode laser based on a MEMS electrostatic rotary actuator”,Paper TuJ2-1, OFC2001, Anaheim, Calif. This solution utilizes a MEMS-tuned external cavity diode laser as shown inFIGS. 2aand2b. To achieve compactness, the optical feedback path of this external cavity laser is folded, making the assembly and alignment of the laser chip, lens, diffraction grating, and MEMS-controlled mirror extremely critical for optimal performance. In addition, a relatively large voltage, approximately 140 volts, on the MEMS actuators is needed to rotate the mirrors by ±1.4°. As with other external cavity lasers that are tuned mechanically, the response speed for “hopping” from one ITU-channel to another is slow. Specifically, it takes 15 msec for the MEMS actuators to execute a coarse tuning towards one of the ITU-channels. Finally, “wave-lockers”, used in conjunction with software algorithms, are needed to fine-tune these coarse set points to a precise alignment with the ITU-grid.

SUMMARY

In one aspect, the presently disclsoed technology provides a single laser transmitter whose emission wavelengths can be reconfigured to align with specific frequencies. The laser includes a variable microresonator preferably integrated inside the external laser cavity.

In another aspect, the presently disclosed technology provides a method for wavelength tuning a laser comprising the steps of integrating a variable microresonator in an external laser cavity and tuning the microresonator over a set of Bragg reflection wavelengths.

In accordance with yet another aspect, the presently disclosed technology provides a comb of Bragg reflection wavelengths generated from a sampled grating fabricated in a waveguide and preferably a silica waveguide. The fabrication procedure for these gratings enables verification, a priori, that the Bragg-comb is precisely aligned to a given set of frequencies, for example, to the ITU-grid. Because these gratings are formed in a waveguide, the external cavity laser need only consist of a compact guided-wave structure that is chip-scale integrable. Secondly, the resonance passband of the microresonator in the laser cavity is used to select one wavelength, λB, from the comb as the emission wavelength. Since the tuning of the microresonator's passband can be accomplished via electrical injection, response times on the order of nanoseconds can be achieved.

In accordance with still yet another aspect, the emission wavelengths of the transmitter are locked by the Bragg reflection peaks (λB) of a passive sampled grating. Since λBin these gratings has a low temperature sensitivity of approximately 0.1 Å/° C., the transmitter will demonstrate approximately one tenth the temperature sensitivity of a Group III-V semiconductor, distributed feedback (DFB) laser with respect to environmental temperature perturbations.

In accordance with still yet another aspect of the presently disclosed technology, the transmitter is chip-scale integrable. Specifically, the disclosed embodiments do not require intricate optical cavity designs or mechanical/MEMs devices to accomplish tuning. The solution proposed herein includes a geometric form factor of the waveguide-based Bragg-gratings, which allows utilization of an almost in-line external cavity design using guided-wave components. In addition, the electrical tuning of the microresonator's passband is much more rapid (i.e., with nsec rise times) than are prior art mechanical tunings that have msec response speeds.

In accordance with yet another aspect of the presently disclosed technology, compared with multi-section tunable lasers, is that the presently disclosed technology does not require (i) a multi-variable algorithm to accomplish tuning to a specified wavelength, such as an ITU-wavelength, or (ii) an external “wave-locker” to stay at the specified wavelength. As mentioned above, the Bragg reflection peaks of the sampled grating (SG) are determined a priori. The only tuning required is a coarse overlap of the microresonator's passband to the selected Bragg wavelength. The passive SG acts as an integrated “wave-locker” possessing excellent temperature stability.

DETAILED DESCRIPTION

This technology will now be described more fully hereinafter with references to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein.

FIG. 3depicts a schematic diagram of a reconfigurable laser transmitter (RLT)100having an external optical cavity in accordance with the presently disclosed technology. The output of a gain element20is coupled to a tunable microresonator50via an optical path, which is preferably provided by a waveguide101. A second optical path, which is preferably provided by a waveguide102, provides an optical path from the tunable microresonator50to an optical filter, preferably implemented by an optical grating40. The external optical cavity includes the optical paths101and102, the tunable microresonator50and the optical grating40. The external optical cavity length defines the set of wavelengths the laser transmitter100can support. Among these supported wavelengths, the laser transmitter100will transmit the wavelength that has the largest gain, which corresponds to the resonance frequency of the laser transmitter100. All supported wavelengths will have approximately the same gain when transmitted through the gain element20. In order to select and transmit the desired wavelength from the plurality of supported frequencies or wavelengths, such as a desired ITU-channel, the tunable microresonator50is coupled to the fixed optical grating40. One of the benefits from using a fixed optical grating40is that the fixed optical grating40may be fabricated in materials, such as silicon, that are stable over a desired operating temperature range.

A spectrum of light provided by gain element20is passed through the tunable microresonator50into the optical grating40.FIG. 4aillustrates an example of the plurality of supported wavelengths λ1-λ10that the laser transmitter100can support. The tunable microresonator50is preferably tuned such that one of the passbands51of the tunable microresonator50aligns with one of the wavelengths (λ2for the example depicted byFIG. 4a) of the optical grating40. The tunable microresonator50attenuates a portion of the plurality of supported wavelengths. The optical grating40then reflects a portion of the selected wavelength back through the optical cavity. Thus, a wavelength (λ2in this example) is selected and becomes the wavelength transmitted by the transmitter100.FIG. 4billustrates that the tunable microresonator50may later be tuned such that one of the passbands51aligns with a different one of the wavelengths (λ8for the example ofFIG. 4b) of the optical grating40. Thus, the wavelength λ8is selected (in this example) and becomes the wavelength transmitted by the transmitter100.

FIG. 5illustrates one embodiment of the present technology. InFIG. 5, the gain (or active) element20of this laser is preferably an optical amplifier20. The gain element20is preferably mounted on an integration platform30that serves to facilitate the coupling of optical devices. One common integration platform30is a silicon waferboard. The gain element20is coupled to waveguide101located within the integration platform30. One skilled in the art will appreciate that there are a number of gain elements20well known in the art that may be used. For example,FIG. 6depicts a GaInAsP/InP semiconductor optical amplifier (SOA)23integrated with a spot-size conversion (SSC) taper24. The SSC taper24provides for high coupling efficiency (>50%) to waveguide101on the integration platform30, and allows for relaxed alignment tolerances of ±2 μm between the gain element20and the waveguide101.

The optical output from the gain element20is provided to the tunable microresonator50(implemented as a microdisk50ain this embodiment) via waveguide101. One skilled in the art will appreciate that gain elements, specifically light emitting semiconductor devices, produce a range of wavelengths. Of the range of wavelengths provided to the tunable microresonator50, only the wavelengths that match passbands51of the tunable microresonator50are passed to the optical grating40via waveguide102.

One skilled in the art will appreciate that there are many ways well known in the art to couple waveguides101,102and tunable microresonator50. For example, a thermal annealing approach is discussed in J. M. London, “Preparation of Silicon-on-Gallium Arsenide Wafers for Monolithic Optoelectronic Integration,”IEEE Photon. Technol. Lett., Vol. 11, No. 8, 1999, pp. 958-960, herein incorporated by reference, where wafer-bonding occurs between a III-V and a Si-based device, via intermediate layers of deposited dielectric. This thermal annealing approach can be used to accomplish a heterogeneous integration between the tunable microresonator50, which may be fabricated from a semiconductor material such as GaAs or InP, and the waveguides101,102, which may be fabricated from silica material.

In the embodiment illustrated inFIG. 5the tunable microresonator50is preferably implemented as a microdisk50a. Further details of the microdisk50aare illustrated inFIGS. 7 and 8.FIG. 7is a perspective view of the microdisk50aintegrated with the waveguides101,102.FIG. 8is a cross-section view of the microdisk50aand integration platform30containing the waveguides101,102. The microdisk50ais preferably comprised of an optically absorptive material whose optical index can be changed. For further information regarding the structure of the microdisk50asee U.S. patent application Ser. No. 10/116,800 entitled “Waveguide-Bonded Optoelectronic Device” filed Apr. 5, 2002, which is hereby incorporated herein by reference. Particularly useful electro-optic materials are Gallium Indium Arsenide Phosphide/Indium Phosphide (GaInAsP/InP) and Gallium Arsenide (GaAs). Vertical proximity is used to evanescently couple light from the waveguide101to the microdisk50a. At specific resonance frequencies, the guided wave in the waveguides101,102is strongly coupled to the microdisk50athat is located above them. The specific resonance frequencies are given by fm=m/τd, where m is an integer and τd=(2πnR)/c, which is the round trip time in the circular microdisk50a, R being the radius of the microdisk50a, n being the refractive index and c being the speed of light. Since fmis dependent on n, the resonance frequencies of the microdisk50acan be varied by changing the bias current provided to the microdisk50a. The resonance frequencies of the microdisk are controlled by controlling the length of time it takes light to travel the circumference of the microdisk50a, which is done by applying a voltage across the contacts106depicted inFIGS. 7 and 8. Thus, the resonance passbands of the GaInAsP/InP microdisk50acan be electrically tuned.

The free spectral range (FSR) of these microdisk resonances is given by FSR=λm2/(2πnR), and is thus inversely proportional to the diameter of the microdisk50a. As will be discussed later, with the FSR reduced tenfold, the vernier-tuning approach offers the advantage of increasing the tunable microresonator's dimensions by the same ratio. Thus, a circular microdisk resonator50aof a diameter equal to 274 μm may be used in an embodiment where a FSR=100 GHz and a ΔF=10 GHz are required.

Another embodiment is illustrated inFIGS. 9 and 10, where the tunable microresonator50is preferably a Fabry-Perot etalon50b.FIG. 9is a block diagram of a reconfigurable transmitter200when a Fabry-Perot etalon50bis used as the microresonator50. The Fabry-Perot etalon50bis preferably formed from a cleaved piece of III-V semiconductor diode (e.g. GaInAsP/InP). Indeed, the etalon50bcan be integrated with the integration platform30that serves to facilitate the coupling of optical devices.FIGS. 10aand10bdepict two ways of integrating the etalon50binto the integration platform30. InFIG. 10athe etalon50bis evanescently coupled to the silica waveguides101and102while inFIG. 10bthe etalon50bis butt-coupled to silica waveguides101and102by placing it in a micro-machined (etched) slot31in integration platform30. InFIG. 10awaveguides101and102form the input portion and output portion of a single waveguide101,102.

FIG. 10is a top view of one approach used in coupling a semiconductor-based Fabry-Perot etalon50bto silica waveguides. One skilled in the art will appreciate that there are many methods of coupling a Fabry-Perot etalon to a silica waveguide known in the art such as passive alignment. Passive alignment is discussed by John V. Collins et al. “Passive Alignment of Second Generation Optoelectronic Devices,”IEEE Journal of Sel. Topics in Quantum Electronics, Vol. 3, No. 6, 1997, pp. 1441-1444 and by M. Cohen et al.IEEE Trans. Components, Hybrids and Mfg Technology, Vol. 15, No. 6, December 1992, both documents being herein incorporated by reference. In the example shown inFIG. 10, the silica waveguide facet101-1preferably has an anti-reflective (AR) coating disposed on the facet closest to the Fabry-Perot etalon50b. The Fabry-Perot etalon50bfacets50b-1,50b-2are preferably not AR coated, rather are preferably simply cleaved. The silica waveguide facet102-1preferably also has an AR coating on the facet closet to the Fabry-Perot etalon50b. The waveguide facets101-1,102-1and the Fabry-Perot etalon facets50b-1,50b-2are aligned to provide efficient coupling between the waveguides101,102and the Fabry-Perot etalon50b. The etalon is tuned by attaching an electrode to the etalon and an electrode to the wafer board similar to a microdisk.

One skilled in the art will appreciate that there are multiple approaches to tuning the passbands51of the tunable microresonator50, which, in case of the presently disclosed technology, results in selecting the Bragg wavelength λBin the reflection spectrum of a fixed optical grating40, which could be a sampled grating fabricated in a silica optical waveguide. Tuning a tunable microresonator50enables the transmitter's emission wavelength to be changed from one wavelength to another wavelength, within a set of specified wavelengths, for example, from one ITU-channel to another ITU-channel. Preferably, the tunable microresonator50is tuned electrically in order to provide fast response times. One skilled in the art will appreciate that any manner of tuning may be used in order to provide response times suitable for the given application. Two electrically tuning approaches will be discussed herein and are provided as examples only.

In one approach of electrically tuning microresonators, the free spectral range (FSR) of the tunable microresonator50is designed to cover the entire tuning range of the transmitter100. The design of the fixed optical grating40used in this example will be discussed in detail later in this specification. Preferably, for the fixed optical grating40, the tuning range covers approximately ten wavelengths (with Δf=100 GHz). Therefore, fm, the mthresonance frequency of the tunable microresonator50, is tuned over a frequency range of δfmof approximately 1,000 GHz.

As previously discussed, in one embodiment the tunable microresonator50may be a Fabry-Perot etalon50bas depicted byFIG. 9having a physical length L. The following equations describe the fractional change in fm(fm˜2×1014/sec for λ˜1550 nm) with respect to its optical length, Lopt:

δ⁢⁢fmfm=δ⁢⁢LoptLopt=δ⁢⁢nn
where n is the refractive index for the waveguide in the Fabry-Perot etalon50b. With a the physical length L of approximately 43 μm, a fractional index change (δn/n) of 0.5% will accomplish a δfmof ˜1,000 GHz. Thus, a tuning current of ˜24 mA can move the passband51depicted inFIGS. 4aand4bfrom λ1to λ10for an etalon50bfabricated from a semiconductor material such as GaAs or InP. The current tuning also enables a fine degree of control in moving the tunable microresonator's passband51from one Bragg peak to the next (e.g. λ2to λ3). For example, for a δfmof ˜100 GHz, a fractional index change of 0.05% is required, which corresponds to a current of ˜2.4 mA. One skilled in the art will appreciate that the fractional index change could be provided through a change in voltage supplied to the tunable microresonator rather than a change in current.

As also previously discussed in another embodiment, tunable microresonator50may be a microdisk50aas depicted byFIG. 7, and more particularly, a microdisk50ahaving a radius R. In the present example, in order to support a FSR of ˜1,000 GHz (e.g. 10 ITU channels), R is preferably selected to be ˜13 μm. A tuning current of ˜25.5 mA will move the microdisk's passband from λ1to λ10. The tuning current also enables a fine degree of control in moving the tunable microresonator's passband51, depicted inFIGS. 4aand4b, from one Bragg peak to the next (e.g. λ2to λ3). For example, a small tuning current of ˜2.55 mA will tune the passband over δfmof ˜100 GHz.

In another approach of electrically tuning microresonators50, a vernier-tuning concept is applied to achieve a potentially even larger tuning range. The vernier-tuning concept is illustrated inFIG. 11. When the tunable microresonator50is tuned, successive passbands51of the tunable microresonator50are moved to overlap, one at a time, with successive Bragg reflection peaks λ1. . . λ4from the sampled grating40, as is illustrated inFIG. 11. The vernier mechanism is often used in multi-section GaInAsP/InP distributed Bragg reflector (DBR) lasers to achieve ultra-broad ranges. These DBR lasers apply the vernier mechanism to internal cavity designs. In the present invention, the vernier-tuning concept is applied to an external cavity design. To achieve the spectrum shown inFIGS. 4aand4b, the tunable microresonator's FSR is designed to be ˜110 GHz, so that the FSR is close to the frequency spacing of (Δf) of 100 GHz between the Bragg reflection peaks. The vernier effect provides an increased tuning range obtained via Δn/n by a lever-factor equal to FSR/(ΔF), where ΔF=FSR−Δf. Thus:

In one embodiment, FSR=100 GHz and ΔF=10 GHz, resulting in a vernier multiplier of 10. Thus, the tuning current required to achieve a tuning range of δfmof ˜1,000 GHz is only several mA. Therefore, a much larger tuning range may be achieved via the vernier effect. In addition, the bandwidths of the passbands51of the tunable microresonator50need only be 10-15 GHz (for a finesse of F˜10) to execute discriminatory selection of a specific Bragg reflection peak.

The optical grating40is preferably written such that the wavelengths of the optical grating40are aligned to a desired set of frequencies, for example to the ITU-grid. The optical grating40is preferably a sampled Bragg grating fabricated in a material that has a low temperature sensitivity (on the order of 0.1 Å/° C.), such as silica. The sampled grating exhibits multiple Bragg reflection peaks from one grating. In particular, the λ spacing (Δλ=0.8 nm inFIG. 4a) between the peaks is set by the period of a modulation envelope impressed on the grating profile. As shown inFIG. 4a, one can obtain more than ten Bragg reflection peaks with reflectance>32% in a single grating. By tuning the passband51of the tunable microresonator50, any of the Bragg reflection peaks may be chosen to set the transmitter's lasing wavelength, as is shown inFIGS. 4aand4b. By using the tunable microresonator50coupled to the sampled grating, different wavelengths can be provided without the need for bulky “wave-lockers”. One skilled in the art will appreciate that the passband characteristics of the tunable microresonator50need only offer very modest finesse (in the range of approximately 10-50) for the transmitter100to achieve single frequency oscillation in any of the ten Bragg wavelengths λ1. . . λ10. The lasing characteristics (e.g. side mode suppression ratio (SMSR)) of the transmitter100are mostly determined by the gain element20, spectral width (ΔλB) of the sampled grating and Bragg reflectance of the sampled grating.

In one embodiment, the sampled grating is a UV-induced Bragg grating40fabricated in a waveguide102located at the output end of the transmitter100. See, for example, the embodiment ofFIG. 5. The waveguide102preferably comprises a Ge-doped core that results in high photosensitivity. UV-induced gratings may be written using a transmissive phase mask and an excimer laser having a wavelength of approximately 2480 nm. The high photosensitivity of the Ge-doped core allows for reproducibility in writing the UV-induced gratings. For further information regarding the fabrication of UV-induced Bragg gratings, see W. Ng et. al, “High speed single and multi-element fiber-grating coupled laser transmitter for WDM networks”, Paper FJ5, LEOS'98, Orlando, Fla., which paper is incorporated herein by reference.

The following is a brief discussion of how multiple Bragg reflection peaks are derived from the optical grating40. As illustrated inFIGS. 12athrough12f, the formation of an optical grating40is achieved by the impression of a modulation envelope, defined by the sampling function S(z)203, seeFIG. 12b, on the refractive index profile Δn(z)201of a uniform grating, seeFIG. 12a. Basically, an amplitude-mask is added and cascaded with a conventional phase-mask through which the UV-induced grating is written. In the spatial frequency domain, as shown inFIGS. 12dthrough12f, the uniform grating201(with period Λ) transforms to a δ-function at fs=2π/Λ, as shown inFIG. 12dwhereas the sampling function203transforms to a series of periodic impulses that are separated by 2π/zo, as shown inFIG. 12e. When these two spectra are convolved to form the spatial spectrum of the sampled grating, the series of Fourier components are centered at 2π/Λ and separated by 2π/z1, as shown inFIG. 12f. Each of these spatial Fourier components results in a Bragg reflection peak in the reflection spectrum of the sampled grating. Thus, the λ-spacing [Δλ=λ2/(2πzo)] of the reflection peaks is determined by zo, the period of the sampling function203. For example, a zoof ˜1 mm will lock Δλ to a spacing of 0.8 nm. In addition, the number of Bragg reflection wavelengths can be increased by decreasing the duty cycle, z1/zo, of the sampling function.

In addition to being able to select the wavelength of interest, a transmitter is also provided whose emission wavelengths can be aligned to a specified set of wavelengths, i.e., the ITU-grid.FIG. 13adepicts the transmission spectrum of a Bragg fiber-grating measured in situ. One skilled in the art will appreciate that one can characterize, in situ, the Bragg wavelength (λB), half-width (ΔλB) and the grating's reflectivity (R) as the grating is fabricated. This a priori knowledge ensures that the Bragg wavelengths of the sampled grating are well aligned with the set of specified wavelengths, for example the ITU-channels.FIG. 13bshows the lasing spectrum observed when the gain element20is coupled to the Bragg fiber-grating. As shown, a side mode suppression ratio (SMSR) of >40 dB at λBis achieved.

In another embodiment, a fixed optical-resonator filter designed to the set of frequencies desired can replace the fixed optical grating40. Just as the optical grating40is designed to provide the set of desired frequencies, for example the set of desired ITU-channels, a fixed optical-resonator filter may also be designed to provide the same set of desired frequencies. As an example, the fixed optical-resonator filter may be an etalon.

From the forgoing description, it will be apparent that the present invention has a number of advantages, some of which have been described herein, and others of which are inherent in the embodiments of the invention described herein. Also, it will be understood that modifications can be made to the method described herein without departing from the teachings of the subject matter described herein. As such, the invention is not limited to the described embodiments except as required by the appended claims.