Abstract:
A fiber-optic laser source capable of emitting multiple useful wavelengths at the same time, including multiple ITU grid wavelengths, is disclosed. The laser preferably comprises a diode-pumped rare-earth doped fiber amplifier as gain medium, a periodic filter to define a first set of possible lasing wavelengths corresponding to ITU grid lasing wavelengths, and a resonant filter for defining a subset of the first set of wavelengths. The arrangement is enclosed in a ring resonator. The ring resonator may be made, in part or in total, with polarization-maintaining optical fiber. The resonator may also comprise in-line optical isolators to ensure unidirectional operation and to eliminate undesired reflections. A gain-flattening filter may also be included within the resonator depending on the desired output characteristics of the laser. The laser may be designed to operate in a set of single cavity longitudinal modes. The disclosed laser may also comprise output wavelength and/or power definition and stabilization control.

Description:
TECHNICAL FIELD OF THE INVENTION 
     This invention relates to lasers and, more specifically, to multi-wavelength lasers such as may be suitable for dense wavelength division multiplexed (DWDM) applications. It also pertains to methods by which such lasers operate. 
     BACKGROUND OF THE INVENTION 
     Wavelength division multiplexing, or WDM, is a known method for increasing the capacity of a fiber optic communication system. WDM increases capacity by permitting a single optical fiber to carry multiple optical carrier signals having differing wavelengths. After transmission, at the receiving end, the multiple wavelengths are demultiplexed to separate the signals. 
     The concepts behind WDM have been extended to use a set of closely-spaced wavelengths in the 1550 nm window. The International Telecommunication Union (ITU) has proposed the use of a grouping or grid of wavelengths in this window. As presently configured, the channels are anchored to a reference at 193.10 THz and equally spaced in frequency, the closely spaced grids having channels 100 GHz or 50 GHz apart. In the wavelength range between 1528.77 nm and 1560.61 nm, the ITU 100 GHz grid comprises 41 channels. This method of WDM is known as dense wavelength division multiplexing, or DWDM. 
     Communication systems to implement this scheme thus must have access to emissions at each of the grid wavelengths. This could be accomplished by having a multitude of different laser sources, each having an emission wavelength corresponding to a respective one of the grid wavelengths. It would be far more convenient, cost-effective and efficient, however, to have the capability of producing different wavelengths without having to increase correspondingly the number of different laser sources. A single laser source capable of having multiple emission wavelengths at ITU grid wavelengths is therefore desirable. This type of laser source would also prove advantageous in sparing and hot sparing configurations for DWDM, and would allow for reconfigurable DWDM optical communication networks and network elements. Such an apparatus should be capable of producing signals across the widest possible wavelength range, and provide consistent optical output power across its entire wavelength operating range. 
     SUMMARY OF THE INVENTION 
     These and other ends are met in the present invention through provision of a laser apparatus comprising a gain module which is pumped by applied pump radiation, the pump radiation exciting the gain module and thereby achieving lasing action. The laser apparatus also has a control module, with the control module including a periodic filter arranged to receive optical energy from the gain module and having relatively higher transmissivity in certain predefined frequency bandwidths to define a first set of frequency passbands, and a resonant filter arranged to receive optical energy from the periodic filter and defining a second set of frequency passbands, the second set of frequency passbands being a subset of the first set of frequency passbands. 
     The laser apparatus is preferably configured as a ring laser resonator. 
     The control module may further comprise an optical circulator with the periodic filter and the resonant filter being coupled using the optical circulator. The first set of frequency passbands may correspond to International Telecommunication Union frequency grid recommendations. 
     The periodic filter may be a transmission filter or a reflection filter. If it is a transmission filter, it may be any one of a number of specific types of transmission filters including a fiber Fabry-Pérot micro-etalon transmission filter or a fiber-coupled Fabry-Pérot micro-etalon transmission filter. If it is a reflection filter, it may be any one of a number of specific types of reflection filters including a sampled fiber Bragg grating or a set of sampled fiber Bragg gratings. The periodic filter may also be tunable. 
     The laser apparatus may also include means for ensuring unidirectional laser oscillation, which may be an optical isolator, and an optical gain-flattening filter for obtaining an approximately constant laser output power for specified wavelengths of operation of the laser resonator. The optical gain-flattening filter may have a wavelength-dependent loss curve which compensates the wavelength-dependent gain curve of the gain module and may be made up of a set of long-period fiber gratings. It is also possible that the optical gain-flattening filter may be incorporated into the gain module. The laser apparatus may also include at least one polarization controller. 
     The periodic filter may have passbands spaced apart at a frequency spacing of 200 GHz or a sub-multiple of 200 GHz. The center frequency of at least one passband of the first set of frequency passbands may be maintained referenced to a predetermined frequency so as to obtain a laser output with wavelengths according to the International Telecommunication Union frequency grid recommendations. The resonant filter then has a subset of frequency passbands selected from the set of frequency passbands defined by the periodic filter. The resonant filter passband bandwidths may be sufficiently narrow that within each resonant filter passband laser oscillation occurs at wavelengths within a single passband of the passbands of the periodic filter. In another embodiment, each of one or more of the resonant filter passbands may enclose more than one of the passbands of the periodic filter, allowing multiple-wavelength laser oscillation with wavelengths within adjacent passbands of the periodic filter. 
     The control module may also include an optical isolator, and may further include a wavelength reference control module for locking a spectral response of the periodic filter to the first set of passbands. The wavelength reference control module may include a coupler, optically coupled to the other elements in the resonator, for extracting a small fraction of laser radiation from the laser cavity, a photodetector, and a wavelength reference filter for coupling a narrow-band optical signal from the coupler to the photodetector to provide a stable wavelength reference. The wavelength reference filter may be a reflective wavelength reference filter, and, if it is, it may be made up of a temperature-compensated fiber Bragg grating. 
     The control module may further include a spectral selective transmission module which may be made up of a set of fiber-fused Mach-Zehnder interleavers. 
     At least one of the periodic filter and the resonant filter may be only partially reflective to provide a means for extracting optical output energy without the use of a separate output coupler. Also, at least one of the resonant filter and the periodic filter may include means for accessing individual oscillation wavelengths of the laser. The means for accessing individual oscillation wavelengths of laser may be separated output optical fibers. A separate laser output power control module may be inserted in-line with each of the output optical fibers. The means for accessing individual oscillation wavelengths of laser may also be made up of a spectral selective transmission module and a set of filters. The spectral selective transmission module may be comprised of a set of fiber-fused Mach-Zehnder interleavers and the set of filters may be comprised of a set of fiber Bragg gratings. 
     At least one of the periodic filter and resonat filter may have a wavelength-dependent envelope curve, which compensates a wavelength-dependent gain curve of the gain module. 
     The laser apparatus may also include a laser output power control module, inserted in-line with an output of the laser, for setting and maintaining a predetermined level for optical output power of the lasers. The laser output power control module may be made up of a coupler to extract a fraction of laser optical output power, a calibrated photodetector optically coupled to the coupler and producing an electric reference signal, a loop control unit, arranged to receive the electric reference signal, for generating an control signal based on the electric reference signal, and an in-line variable optical attenuator arranged to receive the control signal, for controlling the laser optical output power based on the control signal. 
     The invention also resides in a method of generating a multiple wavelength laser output at several of a set of discrete frequencies, the method comprising the steps of providing pump energy to a gain medium in a laser cavity to excite laser resonances, filtering the laser resonances using a periodic filter to limit possible lasing frequencies to the frequencies in the set; and filtering the laser resonances using a resonant filter to limit possible lasing frequencies to a subset of the frequencies in the set. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a functional block diagram of a presently preferred embodiment of a multi-wavelength laser according to the present invention. 
     FIG. 2 is a functional block diagram of a first presently preferred embodiment of a laser spectral control module according to the present invention. 
     FIG. 3 is a functional block diagram of a presently preferred embodiment of a wavelength reference control module according to the present invention. 
     FIG. 4 is a functional block diagram of a second presently preferred embodiment of a laser spectral control module according to the present invention. 
     FIG. 5 is a functional block diagram of a third presently preferred embodiment of a laser spectral control module according to the present invention. 
     FIG. 6 is a functional block diagram of a fourth presently preferred embodiment of a laser spectral control module according to the present invention. 
     FIG. 7 is a functional block diagram of a fifth presently preferred embodiment of a laser spectral control module according to the present invention. 
     FIG. 8 is a functional block diagram of a presently preferred embodiment of a resonant filter or a periodic filter with means to access the individual oscillation wavelengths of the laser through separated output optical fibers, according to the present invention. 
     FIG. 9 is a functional block diagram of a presently preferred embodiment of a laser output power control module according to the present invention. 
     FIG.  10 ( a ) is a graph qualitatively illustrating a spectral response of a periodic filter in a presently preferred embodiment of a system according to the present invention. 
     FIG.  10 ( b ) is a graph of a spectral response of a resonant filter in a presently preferred embodiment of a system according to the present invention. 
     FIG.  10 ( c ) is a graph of a combined spectral response of a periodic filter and a resonant filter, in a presently preferred embodiment of a system according to the present invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG.1 is a functional block diagram of a presently preferred configuration for a laser according to the invention. As can be seen in FIG. 1, it is presently preferred to embody the invention as a multi-wavelength fiber-optic ring laser  10 . The configuration of FIG.1 preferably includes several cavity elements, as described below. While for the sake of illustration a specific ordering of elements is shown for the ring configuration described immediately below, it will be readily apparent to one having ordinary skill in the art that cavity elements may be placed within the cavity in relative positions other than those shown and described. Also, while the presently preferred embodiment is as a ring laser, it will be apparent to one of ordinary skill in the art that a linear arrangement of elements could be used as well. 
     A presently preferred embodiment, as depicted in FIG. 1, includes a laser spectral control module  100 . It is presently preferred that this laser spectral control module  100  includes means to define and stabilize the multi-wavelength output spectrum of laser  10  and means to provide a useful output from the fiber laser  10 , as described. It is also presently preferred to use the laser spectral control module  100  in conjunction with means for defining and stabilizing the output power of laser  10 , also as described below. 
     The laser spectral control module  100  is optically coupled to a gain module  20 . Here and elsewhere, “optically coupled” means arranged so that optical radiation may pass from one to the other or vice versa, and possibly passing through one or more active or passive intermediate optical elements along the way. Also, in the figures it will be understood that the solid lines interconnecting the components are intended to depict optical paths, and that thicker solid lines interconnecting the components are intended to depict one or more optical paths. In the presented preferred embodiments the optical interconnecting path is defined with optical fiber. Also, in the figures it will be understood that the dashed lines interconnecting the components are intended to depict electrical connections. 
     It is presently preferred that the gain module  20  be made up of a length of rare earth doped optical fiber. More preferably, in a preferred embodiment the gain module  20  includes a length of erbium-doped optical fiber. It will, however, be apparent to one of ordinary skill in the art that other rare earth and transition metal materials may be used as dopants or co-dopants of the optical fiber in the gain module  20 , in concentrations which may be varied across the radial and/or longitudinal profile of the fiber. It will also be apparent to one of ordinary skill in the art that gain module  20  may be made up of other optical amplifiers, such as: fiber Raman amplifier, semiconductor optical amplifier, rare-earth doped waveguide amplifier or doped solid-state amplifier. Also, supplementary gain modules can be added to the laser  10  in series or parallel with the gain module  20  in order to extend the tuning range and/or to increase the optical output power of the laser  10 . 
     Pump couplers  30  and  40  are used to couple the output of a pump module  50  into the fiber laser  10 . It is presently preferred that the couplers  30  and  40  be fiber-fused wavelength division multiplexer couplers which couple the optical radiation from the pump module  50  into the laser  10 . However, coupling the pump laser light radiation into the ring laser  10  is not limited to this method and may be achieved by any of several other methods known in the art. The pump module  50  may be any one of many commonly available pump laser sources, including an array of pump laser diodes arranged according to a pump redundancy scheme. In the presented preferred embodiment the pump module  50  is constituted by two laser diodes providing both co-propagating and contra-propagating pump radiation into the gain module  20 . 
     The arrangement of FIG.1 may also include an optical gain-flattening filter  60  arranged within the laser resonator with the purpose of obtaining an approximately constant laser output power for all the specified wavelengths of operation of the laser  10 . The optical gain-flattening filter  60  has a wavelength-dependent loss curve which compensates the wavelength-dependent gain curve of the gain module  20 , in the sense that the effect of the loss curve of the gain-flattening filter  60  when combined with the effect of the gain curve of the gain module  20  results in an approximately flat overall gain curve for the operation wavelength range of the laser  10 . In a presently preferred embodiment the gain-flattening filter  60  may include a set of long-period fiber gratings with a compensating loss curve, in the sense just described. Also, the gain-flattening filter  60  can be inserted into the gain module  20 . In a preferred embodiment the position of the gain-flattening filter  60  within the gain module  20  is such that it ensures minimum excess insertion loss. 
     In the presently preferred embodiment depicted in FIG. 1, the laser  10  includes optical isolators  70  and  80  to ensure unidirectional laser oscillation and to prevent unwanted reflections induced by elements inside the cavity from adversely affecting the operation of the multi-wavelength fiber-optic ring laser  10 . It will, however, be apparent to one of ordinary skill in the art that, in certain applications, the position and/or number of optical isolators may be changed to obtain enhanced performance of the laser  10 . It is also possible that a design for the gain module  20  may already include one or two optical isolators, which may render some of the shown optical isolators unnecessary. 
     A polarization controller  90 , which may include a polarizer, can be used within the laser  10  in order to optimize the laser operation whenever polarization-dependent elements are used within the laser cavity and/or, for certain applications, to define a specific state-of-polarization for the output radiation of the laser  10 . 
     As shown in FIG. 2, a first presently preferred embodiment of the laser spectral control module  100  includes a periodic filter  110  optically coupled to a resonant filter  120  and optically coupled to an output coupler  130  through a three port optical circulator  140 , as depicted in FIG.  2 . The periodic filter  10  is a filter that has high transmissivity for light with frequency within certain frequency bands, i.e., passbands, at approximately constant frequency spacing. In the preferred embodiment, the periodic filter  110  has passbands spaced apart at a frequency spacing of 200 GHz or a sub-multiple of that spacing, such as 100 GHz, 50 GHz, 25 GHz, 12.5 GHz, or other. The periodic filter  110  limits the possible lasing wavelengths of the multi-wavelength ring laser  10  by introducing lower optical loss at the passbands as compared to other wavelengths, with the effect of precluding laser action at the low transmissivity bands. In the preferred embodiment, the center wavelength of one, or more, of the passbands may be kept at or near certain predetermined frequency values so as to obtain a laser output with a wavelength or wavelengths according to International Telecommunication Union (ITU) frequency grid recommendations, or according to another set of wavelengths. For certain applications the periodic filter  110  may be tunable by electrical or mechanical means. 
     The resonant filter  120  is preferably an optical filter with a subset of arbitrary frequency passbands selected from the set of frequency passbands defined by the periodic filter  110 . In the presently preferred embodiment of the invention it is preferred to use a resonant filter with passband bandwidths, which are small enough, that within each resonant filter passband laser oscillation occurs only for wavelengths within a single of the pre-selected passbands of the periodic filter  110 . It will be readily apparent to one having ordinary skill in the art that the passband bandwidth of the resonant filter  120  may enclose more than one of the passbands of the periodic filter  110 , thus allowing multiple wavelength laser oscillation with wavelengths within adjacent passbands of the periodic filter  110 . 
     The output coupler  130  is preferably a fiber-fused coupler. The output coupler  130  is used to provide a useful output from the ring fiber laser  10 . The coupler  130  is provided in line in the resonator so that a portion of the optical energy entering the coupler  130  is coupled out of the resonator as an output, towards the power control module  210 . Another portion of the optical energy entering the coupler  130 , however, passes through the coupler  130  and stays in the resonator  10 . In the presently preferred embodiment, the coupling ratio of the output coupler  130  is such that it maximizes the optical output power. As an illustrative example, the output coupler  130  may be a 10/90 fiber-fused coupler. 
     The first presently preferred embodiment of the laser spectral control module  100  as depicted in FIG. 2 also includes an optical isolator  150  to prevent unwanted reflections induced by elements outside the cavity from adversely affecting the operation of the multi-wavelength fiber-optic ring laser  10 . 
     As depicted in FIG. 2, the laser spectral control module  100  may also include a wavelength reference control module  160  in order to lock the spectral response of the periodic filter  110  to the ITU grid. In the presently preferred embodiment of the invention, the wavelength reference control module  160  includes, as shown in FIG. 3, a wavelength reference filter  161 , a coupler  162  and a photodetector  163 . The coupler  162  is inserted in-line with the ring cavity laser  10 , optically coupled to the other elements in the resonator, with the purpose of extracting a small fraction of the laser radiation from the laser cavity. As an illustrative example, the coupler  162  may be a 01/99 fiber-fused coupler. However, extracting a fraction of the laser light radiation is not limited to this method and may be achieved by any of other methods known in the art. The narrow-band optical signal coupled by the wavelength reference filter  161  to the detector  163  provides a stable wavelength reference suitable for closed-loop operation. In the closed-loop mode of operation long-term wavelength drift, caused by thermal and/or mechanical-induced fluctuations, in the periodic filter spectral response is prevented by the loop control unit  164 . 
     FIG. 3 illustrates a configuration using a reflective wavelength reference filter  161 . As an illustrative example, the wavelength reference filter  161  may be a temperature-compensated fiber Bragg grating. It will, however, be apparent to one of ordinary skill in the art that stabilized wavelength reference filters other than this one may also be used, such as transmission notch filters. 
     Certain applications make it necessary or desirable to access the individual oscillation wavelengths of laser  10  through different optical fibers. In a presently preferred embodiment of the invention, this is achieved by inserting, in the laser spectral control module  100 , a spectral selective transmission module  170  optically coupled to the optical isolator  150 , as depicted in FIG.  2 . As mentioned, the thicker solid line represents one or more optical fibers. As an illustrative example, the spectral selective transmission module  170  may be a set of fiber-fused Mach-Zehnder interleavers. However, means for accessing the individual oscillation wavelengths of laser  10  through separate output optical fibers is not limited to this device and may be achieved by any of other means or methods known in the art. 
     The invention has been described above in terms of the use of a transmission periodic filter. It is also possible to use a reflection periodic filter. An arrangement using a reflective periodic filter is shown in FIG.  4 . As shown there, the laser spectral control module  100  may include a reflective periodic filter  180  optically coupled to the resonant filter  120  and optically coupled to the output coupler  130  through a four port optical circulator  190 . In FIG. 4, elements numbered the same as those in FIG. 2 indicate elements with like functions. 
     The reflection periodic filter  180  is a filter which has high reflectivity for light with frequency within certain frequency bands, i.e., passbands, at approximately constant frequency spacing. In the preferred embodiment the reflection periodic filter  180  has, similarly to periodic filter  110 , passbands spaced apart at a frequency spacing of 200 GHz or a sub-multiple of that spacing, such as 100 GHz, 50 GHz, 25 GHz, 12.5 GHz, or other. Also similarly to the periodic filter  110 , the reflection periodic filter  180  limits the possible lasing wavelengths of the multi-wavelength ring laser  10  by introducing lower optical loss at the passbands as compared to other wavelengths, with the effect of precluding laser action at the low reflectivity bands of the periodic filter  180 . In the preferred embodiment, the center wavelength of one, or more, of the passbands may be kept within certain values of a predetermined frequency so as to obtain a laser output with a wavelength or wavelengths according to International Telecommunications Union (ITU) frequency grid recommendations, or according to another set of wavelengths. For certain applications the reflection periodic filter  180  may be tunable by electrical or mechanical means. 
     In other embodiments of the invention, either the reflection periodic filter  180  or the resonant filter  120  may partially transmit laser radiation thus providing means for obtaining a useful output from the fiber laser  10 , thus rendering optional the use of a separate output power coupler such as output coupler  130 . FIGS. 5,  6 , and  7  illustrate arrangements for the laser spectral control module  100  where the output of laser  10  is obtained through the reflection periodic filter  180  or the resonant filter  120 . In these Figures, elements numbered the same as those in FIGS. 2 and 4 indicate elements with like functions. As mentioned, the thicker solid lines represent one or more optical fibers. 
     In the arrangement of FIG. 5, for example, a resonant filter  120  that partially transmits optical radiation permits some of the optical energy to pass through the resonant filter  120  as an output signal with the rest of the optical energy being coupled back into the resonator. Similarly, in the arrangement of FIG. 6, a periodic filter  180  that partially transmits optical radiation permits some of the optical energy to pass as an output signal while reflecting the rest of the optical energy back into the optical circulator  190  and so to be coupled back into the resonator. FIG. 7 shows an arrangement with a reflection periodic filter  180  used in conjunction with a resonant filter  120  that partially transmits optical radiation. Again, the resonant filter  120  that partially transmits optical radiation permits some of the optical energy to pass as an output signal while reflecting the rest of the optical energy back into the optical circulator  190  and so to be coupled back into the resonator by coupler  130 . 
     In another embodiment of the invention, the resonant filter  120  or the reflection periodic filter  180  may also provide means to access one or more individual oscillation wavelengths of laser  10  through separated output optical fibers. In a preferred embodiment of the invention, this is achieved with an arrangement of a spectral selective transmission module  170  and a set of filters  200 , as depicted in FIG.  8 . As an illustrative example, the spectral selective transmission module  170  may be a set of fiber-fused Mach-Zehnder interleavers and the set of filters  200  may be a set of fiber Bragg gratings. However, accessing the individual oscillation wavelength of the laser  10  through the resonant filter  120  or the reflection periodic filter  180  is not limited to the use of these components and may be achieved by any of other various means and methods known in the art. 
     In another embodiment of the invention, the transmissivity/reflectivity curve of the periodic filter  110  or  180 , and/or of the resonant filter  120  or filters  200  may have a wavelength dependency which compensates the wavelength-dependent gain curve of the gain module  20 , in the sense described earlier for the gain-flattening filter  60 . This may be implemented by adjusting the reflectivity or transmissivity passbands of each individual filter to compensate for the wavelength-dependent gain curve of the gain module  20 , thus rendering optional the use of a gain-flattening filter such as the gain-flattening filter  60 . 
     In another embodiment of the invention, a laser output power control module  210  can be inserted in-line with the output of the laser  10 , as depicted in FIG. 1, in order to set and maintain the optical output power of the fiber laser  10  according to a predetermined value. In the presently preferred embodiment of the invention, the laser output power control module  210  uses, as shown in FIG. 9, a coupler  211  to extract a fraction of the fiber laser  10  optical output power, which is optically coupled to a calibrated photodetector  212 . As an illustrative example, the coupler  211  may be a 01/99 fiber-fused coupler. However, extracting a fraction of the output laser light radiation is not limited to this method and may be achieved by any of other methods known in the art. The loop control unit  213  uses the electric reference signal provided by the calibrated photodetector  212  to ensure closed-loop control of an in-line variable optical attenuator (VOA)  214 . In the closed-loop mode of operation laser output power instabilities caused by fluctuations in laser  10  are reduced. The laser output power control module  210  may further render optional the use of a gain-flattening filter, such as gain-flattening filter  60 , in order to provide wavelength independent laser output power as required in some applications. It will be readily apparent to one having ordinary skill in the art that when access to the individual oscillation wavelength of laser  10  through separated output optical fibers is provided, a laser output power control module  210  can be inserted in-line with each of the output optical fibers, in order to set and maintain the optical output power of each particular oscillation wavelength according to a predetermined value. 
     According to the present invention, the overlap of the passbands of the resonant filters  120  or  200  and the periodic filters  110  or  180  defines a set of passbands with laser transmissivity higher than any other frequency band within the frequency operating range of the fiber laser  10 , therefore restricting the output wavelengths of the fiber laser  10  to within such high-transmissivity passbands. FIG.  10 ( a ) shows an example of a calculated spectral transmissivity curve of such a transmission periodic filter  110  or spectral reflectivity curve of such a reflection periodic filter  180 . FIG.  10 ( b ) shows an example of a calculated spectral reflectivity curve of such a resonant filter  120 . FIG.  10 ( c ) shows an example of a calculated combined transmissivity curve of such resonant filters  120  and such periodic filters  110  or  180  illustrating the resulting high-transmissivity passbands. It will be readily apparent to one having ordinary skill in the art that the spectral response of the resonant filter  120  must be such that only a restricted number of high-transmissivity passbands exists and that the spacing between those high-transmissivity passbands may be arbitrarily defined within the frequency operating range of the fiber laser  10 . 
     It will be readily apparent to one having ordinary skill in the art that the exact output wavelengths of the fiber laser  10  will be determined by the wavelength dependence of both the overall cavity loss and the gain in the gain module  20 , and by mode-pulling effects and laser dynamics of various natures. It will also be readily apparent to one having ordinary skill in the art that within each of the passbands defined by the overlap of the passbands of the resonant filters  120  or  200  and the periodic filters  110  or  180  several cavity longitudinal modes may attain the laser threshold, resulting in laser emission composed of several nearly equally spaced wavelengths. For certain applications, the cavity of the laser  10  may be designed to ensure single longitudinal mode operation within each such passband, increasing side-mode suppression and reducing the emission line-width and noise. In the presented preferred embodiment of the invention, depicted in FIG. 1, the total length of the laser resonator  10  is defined such that its longitudinal spectral mode structure in combination with the spectral characteristics of the intracavity filters restricts the laser emission to only one well-defined cavity longitudinal mode within each one of the laser resonant passbands. It will be readily apparent to one having ordinary skill in the art that unidirectional laser oscillation can further assist single longitudinal mode operation within each one of the laser resonant passbands. 
     In the preferred embodiments of the ring laser  10  cavity described above, the periodic filter  110  is preferably a fiber or fiber-coupled Fabry-Pérot micro-etalon transmission filter and the reflection periodic filter  180  is preferably a sampled or set of sampled fiber Bragg gratings reflection filters. It will be readily apparent to one having ordinary skill in the art, however, that the specific choice of components for implementing the functions of the periodic filters  110  and  180  is not limited to such components but instead may be transmission or reflection filters made by any other means known in the art, such as: discrete set of fiber Bragg gratings, long-period fiber gratings, fiber interferometers, fiber wavelength-dependent couplers, fiber or fiber-coupled Fabry-Pérot etalon filters, integrated-optic devices, quantum-well structures, and semiconductor waveguides. 
     In the preferred embodiments described above, the resonant filter  120  and filters  200  are preferably a set of fiber Bragg gratings or a set of sampled fiber Bragg gratings. It will be readily apparent to one having ordinary skill in the art, however, that the specific choice of a component for implementing the functions of the resonant filters  120  and filters  200  is not limited to such components but instead may be a reflection filter made by any other means known in the art, such as: fiber or fiber-coupled Fabry-Pérot filter, long-period fiber gratings, fiber interferometers, fiber wavelength-dependent couplers, angle dielectric filter stacks, acousto-optic filters, quantum-well structures, semiconductor waveguides, and integrated optic devices. In other embodiments of this invention the ring laser  10  may include at least one optical switch in conjunction with sets of any of the periodic and/or resonant filters referred above allowing for configurable laser operation. 
     In the presently preferred embodiment, components in the laser  10  resonator are preferably optically coupled using lengths of single-mode fiber. For certain applications, the laser  10  resonator may comprise fiber lengths, in part or in total, made of polarization maintaining single-mode fiber. Also, for certain applications, the laser  10  resonator may also or alternatively include lengths of non-single-mode fiber. 
     As mentioned, the cavity elements in the laser  10  resonator may be placed within the cavity accordingly to different positions other than those shown in the diagrams, which are to be regarded as illustrative examples of such arrangements. Also as mentioned, it will be readily apparent to one having ordinary skill in the art that the present invention may also be implemented in a linear laser resonator configuration. 
     The invention has been described herein using specific embodiments for the purposes of illustration only. It will be readily apparent to one of ordinary skill in the art, however, that the principles of the invention can be embodied in other ways. Therefore, the invention should not be regarded as being limited to the specific embodiments disclosed herein, but instead as being fully commensurate in scope with the following claims.