Abstract:
A tunable semiconductor laser device is presented. The device comprises a laser structure formed by at least two waveguides and an active region located within at least a segment of one of the waveguides; and comprises a tunable spectral filter optically coupled to the laser structure. The tunable spectral filter includes at least two filtering elements, at least one of them being a microring cavity.

Description:
FIELD OF THE INVENTION 
   This invention is generally in the field of optical devices and relates to a tunable laser using a microring resonator. 
   BACKGROUND OF THE INVENTION 
   Widely tunable semiconductor lasers are important elements for next generation optical communications systems and possibly for other applications such as testing, biomedical, inspection, etc. 
   Current methods embedded in widely tunable semiconductor lasers are generally divided into: (1) externally tuned lasers (external cavity laser—ECL) and lasers utilizing a monolithical solution based on a distributed Bragg reflector laser (DBR)—see Table 1. In the ECL, the laser cavity is comprised of a semiconductor chip and external spectrally sensitive elements (mainly gratings) that serve as an out of chip wavelength selective mirror. Tuning of the laser is performed by rotating or modifying the external gratings (by applying an external field such as heat, stress, etc.). This type of tunable laser is problematic due to the packaging and environmental reliability of the hybrid device, and is used mainly as a laboratory device and not in optical communications systems. In lasers utilizing a monolithical solution (based on a DBR), all the laser parts are realized on a single chip. Here, two generic solutions exist: 
   (a) The two laser mirrors are made of sampled (SGDBR) or super structure gratings (SSGDBR) each to generate a spectral sequence of high transmission peaks (spectral comb). This is schematically illustrated in FIG.  1 . The two combs of the two mirrors can be aligned by current injection such that a spectral peak of one mirror overlaps the spectral line of the other (Vernier tuning) [G. Sarlet, G. Monthier, R. Baets, “Wavelength and mode stabilization of widely tunable SG-DBR and SSG-DBRlasers”, IEEE Photonics Technology Letters, Vol. 11 no. 11 1999 pp 1351]. 
   (b) Grating-Assisted Codirectional Coupler with Sampled Reflector (GCSR). One laser mirror is comprised of sampled or super structure gratings and the laser active region is coupled to this mirror via a narrow bandpass filter (realized as a long period grating assisted coupler). This is schematically illustrated in FIG.  2 . The tuning is performed by current injection to the sampled, superstructure gratings and the bandpass filter is tuned (also by current injection) to overlap one of the spectral reflectivity peaks of the mirror. 
   SUMMARY OF THE INVENTION 
   The present invention relates to a monolithically integrated tunable laser, in which at least one microring resonator is used (replacing the superstructure gratings in the conventional devices of the kind specified) to generate a spectral comb of frequencies. By this, the main shortcomings of both conventional monolithical methods can be overcome. 
   The term “microring resonator” refers to (a) any configuration where light follows a closed loop path: circular, elliptical or any other annular structure; and (b) any microcavity circular, elliptical square or any shape disk structure. 
   Some of the benefits of using microring resonators are: 
   1. The spectral transmission of gratings consists of a single transmission peak. To generate the required sequence of spectral peaks, complex gratings have to be employed (variable period, segments of different periods etc.) This is accomplished by a difficult fabrication process having adverse effects on yields and prices. Microring resonators generate by nature a periodic sequence of spectral peaks. 
   2. The fabrication of gratings necessitates very fine lithography (sub 0.25 micrometer lines and spacings), which can be performed only by expansive, special tools (e.g. direct e-beam writing) while microring resonators can be defined by conventional lithography of 1 μm lines and slightly below 1 micron spacing. 
   3. Sampled or super structure gratings are usually long (˜1 mm) while the ring dimension (diameter) can be much smaller (10 to 100 μm). This reduces significantly the overall laser size. 
   4. The microring resonator can be employed as a mirror, intracavity filter etc. resulting in a higher level of design flexibility and configuration variety. 
   The present invention provides for a tunable semiconductor laser in which a part of a tuning element is a microring cavity coupled to a laser structure, and serves as a tunable spectral sequence filter. This is different from prior art microring based devices, since there a laser itself is implemented as a microring, or an external ring is used for improving the spectral quality of a laser, but not for tuning (S. Park, Seong-Soo Kim, L. Wang, and Seng-Tiong Ho “Single-Mode Lasing Operation Using a Microring Resonator as a Wavelength Selector”, IEEE J. of QUANTUM ELECTRONICS, VOL. 38, NO. 3, 2002, pp. 207; B. Liu, A. Shakouri, and J. E. Bowers “Passive microring-resonator coupled lasers”, Applied Physics Letters, Vol. 79, Num. 22, 2001, pp. 3561. 
   Thus, according to a broad aspect of the present invention, there is provided a tunable semiconductor laser device comprising a laser structure formed by at least two waveguides and an active region located within at least a segment of one of the waveguides; and a tunable spectral filter optically coupled to the laser structure, said tunable spectral filter including at least two filtering elements, at least one of the filtering elements being a microring cavity. 
   The coupled microring may filter a frequency comb, and an additional tunable bandpass filter can be used to select a specific frequency of the comb. In this case, the coupled microring can be large to generate a frequency comb with the required spacing. Here, the microring can be fixed and only the bandpass filter has to be tuned. The bandpass filter may be implemented as another small microring, or a grating assisted coupler that transfers light of a specific frequency band from one output of the coupler to the other (e.g., “Grating-Assisted Codirectional Coupler Filter Using Electrooptic and Passive Polymer Waveguides”, Seh-Won, Ahn and Sang-Yung Shin, IEEE Journal on Selected Topics in Quantum Electronics, Vol. 7, No. 5, September/October 2001, pp. 819-825). 
   The diameter of the coupled microring can be relatively small such that the spacing between the comb frequencies is larger than required, and a tuning mechanism of the ring is utilized such that the frequency comb can be tuned to intermediate frequencies. Similarly, the bandpass filter may be implemented as another smaller ring, or a grating assisted coupler. 
   The coupled microring can filter a frequency comb and an additional filter can be used to filter another frequency comb with a different spectral spacing. The tuning is preferably performed using the Vernier effect. The additional filter may be another microring, a sampled grating, or superstructure gratings. Alternatively, the additional filter may be another microcavity, e.g. Fabry Perot microcavity. The tuning mechanism can be thermal, current injection, electro-optic etc. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In order to understand the invention and to see how it may be carried out in practice, a preferred embodiment will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which: 
       FIG. 1  is a schematic illustration of a prior art SGDBR- or SSGDBR-based laser utilizing Vernier Tuning Principle; 
       FIG. 2  is a schematic illustration of a prior art CGSR-based laser; 
       FIG. 3  schematically illustrates a laser device according to one embodiment of the invention; 
       FIGS. 4A and 4B  illustrate the tuning principles utilized in the device of  FIGS. 3A-3B ; 
       FIGS. 5A  to  5 E exemplify several specific designs of the laser device of  FIGS. 3A-3B ; 
     FIGS.  6  and  7 A- 7 B are schematic illustrations of laser devices according to another embodiment of the invention; 
       FIGS. 8A  to  8 D are schematic illustrations of yet another embodiment of the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 1 and 2  show prior art laser devices lasers of the kind utilizing the monolithical solution (based on a DBR). Laser structures according to the invention utilize microring resonators instead of at least superstructure or sampled gratings. The lasers can be fabricated by an epitaxial growth on InP, or other materials. In the non-limiting examples presented below, the InP based lasers that dominate the optical communications lasers are emphasized. The active laser layer and waveguide is referred to as 1.55 layers (the material composition made to emit light at 1.55 micrometer). Other waveguide layers are denoted as a 1.3 layer and a 1.14 layer, both transparent to the laser wavelength. They are deposited by a process of epitaxial regrowth over the processed wafer. 
   Referring to  FIGS. 3A and 3B , a laser structure according to one embodiment of the present invention is schematically illustrated. This configuration is generally similar to the prior art SGDBR or SSGDBR structures, but differs therefrom in that one of the two gratings ( FIG. 3A ) or both of them ( FIG. 3B ) is replaced by microring resonator(s). Imposing a small difference between the free spectral range of the ring and the second tuning element (another microring or Bragg grating) results in Vernier turning. This difference can be generated by a material selection or by size difference. 
     FIGS. 4A and 4B  illustrate more specific designs of the above configuration. The example of  FIG. 4A  shows that the diameter of the microring resonator can be selected such that the spacing of the spectral peaks (the “free spectral range” or FSR) can be the actual spacing required from the laser (e.g. 25 GHz). A large ring is required to accomplish this feature. Here, the tuning is performed only by a very slight tuning of the rings relatively to each other to apply the Vernier effect, and due to this small tuning, only a very small amount of power (current) is required. Thus, by using a large ring with small FSR, the small tuning is required only for the Vernier effect. The example of  FIG. 4B  shows that the ring can be made smaller, such that the free spectral range is higher (e.g. 200 GHz). In this case, to address each spectral line (e.g. the same 25 GHz grid), one has to tune the spectralsequence to the selected spectral line (8 such lines within the 200 GHz) and then select a single line by the Vernier effect. In other words, by using a smaller ring with higher FSR, the tuning is required both for shifting the comb and for the Vernier effect. This flexibility does not exist for the sampled or superstructure lasers, because implementation of e.g. 25 GHz spaced spectral lines over the band of interest will require the implementation of about 150 different grating periods, which is not feasible in manufacturing, since the most commonly implemented sampled gratings support 5000 Hz spacing. However, rings that support a dense comb of frequencies are large rings that are very easy to fabricate. This configuration is best implemented if rings are realized as a passive waveguide structure to allow for current tuning.  FIGS. 5A-5E  show some examples of suitable configurations of the current embodiment of the invention. In the figures, 1.55 layer is an active layer, 1.3 and 1.14 Layers are passive layers. The phase element is a conventional element in DBR like lasers and its functionality is to match the overall phase of the laser cavity. 
   In the example of  FIG. 5A , passive rings (e.g. 1.14 layer) are horizontally coupled to laser waveguide (1.55 layer) and a passive or auxiliary waveguide (e.g., the same 1.14 layer of the rings) is coupled to the passive rings. In the example of  FIG. 5B , passive rings (e.g. 1.14 layer) are horizontally coupled to passive waveguides (e.g., 1.14 layer), which are the continuation of the laser active waveguide (1.55 layer). According to the example of  FIG. 5C , passive rings (1.3. layers) are directly vertically coupled to the laser waveguide (1.55 layer).  FIG. 5D  exemplifies passive rings (1.3. layer) coupled vertically to the passive continuation (1.14 layer) of the laser waveguide (1.55 layer). The vertical coupling can be wavelength independent or dependent. In  FIG. 5E , passive rings (1.3 layer) are coupled horizontally or vertically to passive waveguides (1.3 layer), and the latter are vertically coupled to either the laser waveguide (1.55 layer) or the passive waveguides (1.14 layer) that are continuations of the laser waveguide. A coupler with or without (w/wo) spectral bandpass can be used, namely, a coupler either with an additional filtering element or without spectral characteristics. 
   Reference is now made to  FIG. 6  showing a laser structure according to another embodiment of the invention. This configuration is generally similar to the prior art GCSR structure, but differs therefrom in that the sampled or superstructure grating is replaced by a ring cavity. Similar to the examples of the previously described embodiment, in the embodiment of  FIG. 6 , the diameter of the microring resonator can be selected such that the spacing of tile spectral peaks (the “free spectral range”) can be the actual spacing required from the laser (e.g. 25 GHz). Here, in distinction to the previous examples, the tuning can be performed by a slight tuning of the bandpass filter only. Due to the slight tuning, only a very small amount of power (current) is required. The ring can be made smaller, such that the free spectral range is higher (e.g. 200 GHz). In this case, to address each spectral line (e.g., on the same 25 GHz grid), one has to tune the ring in order to shift the spectral sequence to the selected spectral line (8 such lines within the 200 GHz) and then shift the bandpass filter accordingly. This flexibility does not exist for the sampled or superstructure, because the implementation of 25 GHz spaced lines over the entire frequency band of interest (e.g. the band of optical communication) requires about 150 different grating periods which is not feasible in manufacturing. The ring that supports this feature, is a large ring that is very easy to fabricate. Since the ring filter cannot be used directly as a mirror, one has to provide a pass for the light to be coupled back to the laser cavity. It should be noted, although not specifically shown, that the device of  FIG. 6  can be terminated with gratings (on the top WG) similar to that of FIG.  3 A. 
   The principles of the embodiment of  FIG. 6  have several possible implementations. According to one of them, the device may be generally similar to the CGSR configuration but with a ring replacing the sampled gratings, and using the facet as a reflector. Other possible implementations are shown in  FIGS. 7A and 7B . In the example of  FIG. 7A , in addition to the most general configuration of  FIG. 6 , the bandpass filter is replaced by a small ring (with large FSR). This allows also direct back coupling to the laser without the use of the facet reflector. In the example of  FIG. 7B , an arc segment is used for directly returning the filtered light back into the laser waveguide, thus eliminating the need for facet or gratings reflection. 
   Turning now to  FIGS. 8A  to  8 D, there are illustrated four designs, respectively, of a laser device according to yet another embodiment of the invention utilizing a tunable filter within the laser cavity. Here, the Vernier effect is implemented by a dual ring based tunable filter (at least two rings are required but more rings can be used as well) located on the right side of the top waveguide. This tunable filter module replaces the functionality of the two separated ring mirrors of the configuration of FIG.  3 . In the examples of  FIGS. 8A and 8B , facet reflection is utilized, while the devices of  FIGS. 8C and 8D  need no facet reflection due to the use of an arc segment. 
   Those skilled in the art will readily appreciate that various modification and changes can be applied to the embodiments of the invention as hereinbefore exemplified without departing from its scope defined in and by the appended claims. 
   
     
       
             
           
             
             
             
             
             
             
             
             
           
             
             
             
             
             
             
             
             
           
         
             
               TABLE 1 
             
           
           
             
                 
             
             
               Tunable Laser Technology Approaches 
             
             
               Source: the Yankcc Group, 2001 
             
           
        
         
             
                 
                 
               Tuning 
               Output 
               Switching 
               Manufacturing 
               Laser 
                 
             
             
                 
               Technology 
               Range 
               Power 
               Speed 
               Process 
               Cost 
               Applications 
             
             
                 
                 
             
           
        
         
             
                 
               DFB 
               Narrow 
               High 
               Low 
               Established 
               Medium 
               Long Haul, Ultra Long Haul 
             
             
                 
               DBR 
               Modest 
               High 
               Low 
               Established 
               Medium 
               Metro, Long Haul 
             
             
               → 
               SGDBR 
               Wide 
               Medium 
               Medium 
               Evolving 
               Medium 
               Metro, Long Haul 
             
             
               → 
               SSGDBR 
               Wide 
               Low 
               Medium 
               Evolving 
               High 
               Metro 
             
             
                 
               GGSR 
               Wide 
               Medium 
               Medium 
               Evolving 
               Mediun 
               Metro, Long Haul 
             
             
                 
               VCSEL 
               Wide 
               Low 
               Low 
               Evolving 
               Low 
               Metro 
             
             
               → 
               ECL 
               Wide 
               High 
               Low 
               Evolving 
               Medium 
               Long Haul, Ultra Long Haul