Abstract:
An etched grating based chip provides a portion of each of a plurality of input optical signals from a plurality of laser diodes as optical feedback to the plurality of laser diodes, and couples the remaining light from the laser diodes onto an optical fiber, all the while maintaining a small form-factor, and meeting strict conditions regarding laser beat frequency. The present invention is applicable for both a single laser diode at a single wavelength and for an array of diodes at multiple wavelengths, which are multiplexed together in accordance with the present invention. The economics of laser diodes is much improved by decoupling the wavelength locking segment from the gain segment of the diode. Furthermore, the additional wavelength stability of such a locked diode will improve the performance and the economics of the network.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present invention claims priority from U.S. Provisional Patent Application Nos. 61/073,152 and 61/073,045, both filed, Jun. 17 2008, which are incorporated herein by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates to a wavelength locker for a laser diode, and in particular to an integrated wavelength locker and wavelength division multiplexer for one or more lasers. 
       BACKGROUND OF THE INVENTION 
       [0003]    The next generation of Ethernet is proposed to increase operating speeds to 100 Gbit/s. In July 2007, the IEEE 802.3 Higher Speed Study Group presented a project authorization request (PAR), which included 100 GBit/s data rates to support multi-mode and single-mode links, which will enable distances of up to 40 km to be travelled. Such high transmission speeds have become possible due to wavelength division multiplexing (WDM). WDM is a technology which multiplexes multiple optical carrier signals on a single optical fiber by using different wavelengths from many individual or arrayed laser diodes. Common examples of 100 Gbit/s technology include multiplexing: 1) 40 channels at 2.5 GBit/s; 2) 10 channels at 10 GBit/s; and 3) 4 channels at 25 GBit/s. The advantage of using multiple channels is in the fact that each individual channel can run at a fraction of the cumulative transmission speed thus allowing for easier electronic signal processing. 
         [0004]    The major disadvantage of using WDM systems is the requirement of having a multitude of laser sources, i.e. one per channel, with each laser source operating at a distinct wavelength under varying environmental conditions. In order to stabilize the laser wavelength at each individual channel, many different solutions have been proposed. Unfortunately, these solutions suffer from increased costs and requirements to manage large inventories of lasers. Moreover, such multitude of lasers requires a multiplexer that combines all the wavelengths into a single fiber. Accordingly the cost still remains the major obstacle for mass-deployment of the 100 Gigabit Ethernet. 
         [0005]    In WDM passive optical networks (PONs), multiple wavelengths are used to increase the upstream and downstream bandwidth available to end users. As opposed to other PON architectures, WDM-PON can provide more bandwidth over longer distances by devoting more optical bandwidth to each user, and by increasing the link loss budget for each wavelength by eliminating splitter losses. The success of WDM-PON deployments will rely heavily on the availability of lasers locked to a specified grid. Traditional Fabry-Perot cavity lasers do not possess the wavelength stability necessary for WDM-PONs. The use of wavelength-locked distributed feedback lasers (DFBs) is prohibitively expensive for applications in PON networks. Consequently, a cheaper integratable solution is necessary for WDM-PON deployments. The proposed invention overcomes many shortcomings associated with traditional Fabry-Perot lasers and provides wavelength locking stability better than DFB laser diodes at a fraction of the cost. 
         [0006]    Diode Pumped Solid State (DPSS) lasers are quickly replacing existing lasers in many bio-photonic, medical, and industrial marking applications. DPSS lasers feature diode lasers, e.g. at 808 nm, for pumping host ions, e.g. Nd, in a solid state medium. The solid state medium then lazes at a wavelength longer than the pump laser, e.g. at 1064 nm. 
         [0007]      FIG. 1  illustrates a pump absorption line of a DPSS laser, which at 808 nm is quite narrow. Accordingly, slight detuning from this wavelength will, at the very least, change the absorption depth, and quite likely change the net absorption of pump radiation, drastically altering the DPSS laser performance in terms of power and beam quality. Thus locking the wavelength of the pump diode is critical to the operation of the DPSS laser itself. Unfortunately, as the ambient temperature changes or as the diode current is altered in order to vary the pump power, the diode lasing wavelength shifts due to the change in diode temperature, at a rate of ˜0.3 nm/° C. 
         [0008]    A method of locking the DPSS pump diodes is by the use of a volume holographic grating to feed back a portion of the emission from the diode, selectively at 808 nm. This solution has several drawbacks: first, the pump diode must be collimated in one dimension to make the technique effective, which requires additional alignment of an additional component; second, the technique is not rugged due to the bulk-optic nature of the solution; and third, the reliability of volume holograms have not been proven, and there is suspicion of susceptibility to moisture ingress over time. 
         [0009]    CWDM networks are designed and specified as a low cost alternative to DWDM, with the main cost driver being the laser sources. The laser sources are usually fiber-coupled DFB diode lasers. The diode lasers themselves are costly because of the integrated DFB sections, which add to the processing steps, reduce the die-per-wafer count by increasing size, and reduce yield due to combined manufacturing tolerances of the diode laser gain section and the DFB. 
         [0010]    The packaging of the diode lasers is also costly mainly due to the fiber coupling. The standard practice for fiber coupling of the diodes is to use 1 or preferably 2 lenses which are aligned and spaced to image light from the diode exit facet to the tip of the fiber, while accounting for the elliptical mode of the laser. It is possible to reduce the parts cost of these lenses and the alignment by incorporating spot-size converters to the exit of the laser. As with the DFB integrated to the end of the laser, a spot-size converter will introduce additional manufacturing complexity and yield issues to the laser itself. 
         [0011]    The cost of the laser sources is minimized by allowing them to operate without temperature stabilization, which requires optical components in the system to have flat transmission bandwidths of 10 to 13 nm. In spatially dispersive devices, e.g. AWGs, etched gratings, bulk gratings and prisms, a flat transmission bandwidth comes at the cost of reduced transmission, which comes into play when signals are multiplexed onto a single fiber. If an array of laser diodes, e.g. anywhere from 2 to 18, are to be multiplexed together, thin-film filter-based multiplexers are difficult to employ because of the close physical proximity of the laser diodes. Accordingly, a planar waveguide based solution is called for; however, such solutions are usually spatially dispersive, resulting in unwanted insertion loss. 
         [0012]    An object of the present invention is to overcome the shortcomings of the prior art by providing a reliable, compact method of locking laser diode wavelengths with little excess loss utilizing a planar reflective grating that enables simultaneous locking and multiplexing of laser sources. The proposed solution reduces the cost of individual laser sources, eliminates the multiplexer, and greatly reduces the inventory of lasers. The combined effect is disruptive cost reductions that become the enabler of the wide deployment of 100 GBit/s Ethernet. 
         [0013]    The shortcomings of the prior art can be overcome if the mode conversion for fiber coupling, the multiplexing, and the wavelength locking are performed by the same planar waveguide optical chip. 
       SUMMARY OF THE INVENTION 
       [0014]    Accordingly, the present invention relates to a planar lightwave circuit (PLC) chip for simultaneously wavelength locking a plurality of lasers emitting input signals at different wavelengths and multiplexing the plurality of input signals together, comprising: 
         [0015]    a plurality of input ports at the edge of the PLC chip for launching the plurality of input signals from the plurality of lasers; 
         [0016]    a plurality of input waveguides, one end of each input waveguide optically coupled to a respective input port, for guiding the plurality of input signals away from the plurality of input ports; 
         [0017]    a slab waveguide region optically coupled to the other end of each of the input waveguides, the slab waveguide region including a plurality of slab input ports for receiving the plurality of input signals, and a slab output port; 
         [0018]    a diffraction grating formed in an edge of the slab waveguide region for reflecting a first portion of each input signal back to the respective slab input port for transmission to the laser, thereby locking the laser at the desired wavelength, and for multiplexing second portions of the input signals together into a single output beam; and 
         [0019]    an output waveguide extending from the slab output port for guiding the output signal away from the slab waveguide; and 
         [0020]    an output port on an edge of the PLC chip for outputting the output signal. 
         [0021]    Another aspect of the present invention relates to a planar lightwave circuit (PLC) chip for wavelength locking a laser emitting an input signal at a desired wavelength, comprising: 
         [0022]    an input port at the edge of the PLC chip for launching the input signal from the laser; 
         [0023]    an input waveguide optically coupled at a first end to the input port for guiding the input signal away from the input port; 
         [0024]    a slab waveguide region optically coupled to a second end of the input waveguide, the slab waveguide region including a slab input port for receiving the input signal, and a slab output port; 
         [0025]    a diffraction grating formed in an edge of the slab waveguide region for reflecting a first portion of the input signal back to the slab input port for transmission to the laser, thereby locking the laser at the desired wavelength, and for directing a second portion of the input signal to the slab output port forming an output beam; and 
         [0026]    an output waveguide extending from the slab output port for guiding the output signal away from the slab waveguide; and 
         [0027]    an output port on an edge of the PLC chip for outputting the output signal. 
         [0028]    Another feature of the present invention provides a planar lightwave circuit (PLC) chip for simultaneously wavelength locking a plurality of lasers emitting input signals at different wavelengths and multiplexing the plurality of input signals together, comprising: 
         [0029]    a plurality of input ports at the edge of the PLC chip for launching the plurality of input signals from the plurality of lasers; 
         [0030]    a slab waveguide region optically coupled to the input ports; 
         [0031]    a diffraction grating formed in an edge of the slab waveguide region for reflecting a first portion of each input signal back to the respective input port for transmission to the laser, thereby locking the laser at the desired wavelength, and for multiplexing second portions of the input signals together into a single output beam; and 
         [0032]    an output port on an edge of the PLC chip for outputting the output beam. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0033]    The invention will be described in greater detail with reference to the accompanying drawings which represent preferred embodiments thereof, wherein: 
           [0034]      FIG. 1  illustrates a pump absorption line of a DPSS laser; 
           [0035]      FIG. 2  illustrates a wavelength locker/multiplexer planar lightwave circuit chip in accordance with the present invention; 
           [0036]      FIG. 3  illustrates a portion of a diffraction grating from the chip of  FIG. 2 ; 
           [0037]      FIG. 4  illustrates another embodiment of a wavelength locker/multiplexer planar lightwave circuit chip in accordance with the present invention; 
           [0038]      FIG. 5  illustrates another embodiment of a wavelength locker/multiplexer planar lightwave circuit chip in accordance with the present invention; and 
           [0039]      FIG. 6  illustrates a portion of an alternative diffraction grating from the chip of  FIGS. 2 ,  4  or  5 . 
       
    
    
     DETAILED DESCRIPTION 
       [0040]    By handling the wavelength locking function on a locking/multiplexing chip separate from the laser diode chip, the laser diode costs are significantly reduced by process simplification, increasing the number of laser dies per wafer, and yield improvements. Wavelength locking via selective feedback from within the locking/multiplexing chip requires a reflective geometry, i.e. an etched reflective grating. Etched gratings also enable a plurality of input laser signals to be multiplexed onto a single output fiber. Moreover, mode conversion, such as disclosed in U.S. Provisional Patent Application No. 61/073,045 filed Jun. 17, 2008, which is incorporated herein by reference, can be accomplished on a planar optical waveguide in the locking/multiplexing chip by a process modification to standard waveguide manufacturing processes. 
         [0041]    With reference to  FIGS. 2 and 3 , a wavelength locker/multiplexing (MUX) planar lightwave circuit (PLC) chip  1 , according to the present invention, includes a plurality of input waveguides  2   1  to 2 8  having chip edge input ports  3  for optically coupling to respective laser diodes provided on laser diode arrays  4   a  and  4   b.  The laser diodes are capable of emitting light over a range of wavelengths corresponding to the gain profile of the material, but in the absence of external feedback will lase at an arbitrary wavelength near the peak gain. Each of the input signals travels from the laser diode arrays  4   a  and  4   b,  along the respective input waveguide  2   1  to  2   8  until being launched into a slab waveguide region  6  via slab input ports 7. The input wavelength signals travel across the slab waveguide region  6  and are incident on an etched concave reflective diffraction grating  10 , such as the ones disclosed in U.S. Pat. No. 7,151,635, issued to Enablence Inc, which is incorporated herein by reference, which reflects, directs, focuses and multiplexes all of the input wavelength signals of a first selected order to a slab output port  11 . 
         [0042]    The grating  10  is comprised of a plurality of triangular teeth, each tooth consisting of a wide facet F and a short sidewall S. The slab input and output ports  7  and  11  are disposed along a Rowland circle  5  defining the focal plane of the diffraction grating  10 . An output waveguide  12  optically couples the slab output port  11  with a chip edge output port  13 , which is typically coupled to an output waveguide, such as an optical fiber. The slab waveguide region  6 , the input waveguides  21  to  28 , and the output waveguide  12  are formed of optical core material surrounded by cladding material mounted on a substrate, as is well known in the art. Any one or more of the input waveguides  2   1  to  2   8 , and the output waveguide  12  can be omitted by positioning one or more of the input ports  3  and the output port  13  at the edge of the slab waveguide  6 , i.e. at the same position as the slab input and output ports  7  and  11 . Typically, since the slab input ports  7  are spaced about 20 um apart, mounting lasers, which are 300 um wide, is impractical; however, certain embodiments may benefit from slab launching and/or slab retrieval. 
         [0043]    For an etched grating  10  with pitch Λ, refractive index n, vacuum wavelength λ, and operating in diffraction order m 1 , the input and output angles from the grating  10 , measured relative to the normal to the grating  10  at its center, are given by: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       m 
                       1 
                     
                      
                     
                       λ 
                       n 
                     
                   
                   = 
                   
                     Λ 
                      
                     
                       ( 
                       
                         
                           sin 
                            
                           
                               
                           
                            
                           
                             θ 
                             in 
                           
                         
                         + 
                         
                           sin 
                            
                           
                               
                           
                            
                           
                             θ 
                             out 
                           
                         
                       
                       ) 
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
         [0044]    Thus, the arrays of laser  4   a  and  4   b  operating at predetermined wavelengths λ 1  to λ 8 , and aligned to the slab input port positions  7  (θ out ) would all multiplex to the slab output port position  11  (θ in ), if each of the output angles θ out  and wavelengths λ n  are respectively matched to the desired input angle θ in . Meanwhile, for the diffraction grating  10  to provide wavelength selective back-reflection at a second desired order m 2  to the diode lasers aligned to the input port positions  7  (i.e. θ out ), the grating  10  must operate in the Littrow condition given by 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       m 
                       2 
                     
                      
                     
                       λ 
                       n 
                     
                   
                   = 
                   
                     2 
                      
                     Λ 
                      
                     
                         
                     
                      
                     sin 
                      
                     
                         
                     
                      
                     
                       θ 
                       out 
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
         [0045]    where m 2  is the diffraction order, which is different than m 1 , for this Littrow configuration. 
         [0046]    If the grating  10  is to operate in Littrow mode and enable diversion of the light into the slab output port  11 , i.e. at output angle θ in , then the following condition is found by combining equations 1 and 2: 
         [0000]    
       
         
           
             
               
                 
                   
                     sin 
                      
                     
                         
                     
                      
                     
                       θ 
                       in 
                     
                   
                   = 
                   
                     
                       λ 
                       Λ 
                     
                      
                     
                       ( 
                       
                         
                           m 
                           1 
                         
                         - 
                         
                           
                             m 
                             2 
                           
                           2 
                         
                       
                       ) 
                     
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
         [0047]    If only a single laser is to be locked via Littrow reflection and coupled to the slab output port  11 , then suitable choice of the first and second desired diffraction orders m 1  and m 2 , pitch Λ, and angle θ in  are required according to equation (3). A mode converter would be implemented at an edge of the chip  1 , e.g. with segmented waveguides. Accordingly, the locking/multiplexing chip  1 , which also mode converts a single diode laser would be beneficial in overcoming many of the aforementioned cost issues. However, an even more powerful implementation of the mode convertor/wavelength locker is possible, enabling the locking, multiplexing, and mode converting of an array of diodes at different wavelengths. 
         [0048]    In equation (3), the value of the reflection (common port) angle θ in  is a function of the wavelength λ, which in turn is a function of the incident (laser incident) angle θ out . In general, multiple wavelengths at different incident angles θ out  would be redirected at different reflection angles θ in , and not be multiplexed. However if we set the first order m 1  as half the second order m 2    
         [0000]    
       
         
           
             
               
                 
                   
                     m 
                     1 
                   
                   = 
                   
                     
                       m 
                       2 
                     
                     2 
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
         [0049]    and necessarily 
         [0000]      sin θ in =0   (5) 
         [0050]    i.e. θ in =0° from the normal of the grating  10 , then multiplexing and Littrow operation will occur simultaneously for all laser diodes at their required wavelengths. 
         [0051]    A simple etched grating planar waveguide multiplexor would employ matched Gaussian modes at the input and output of the grating focus. In a standard CWDM system the passband of the Gaussian passband MUX would be very narrow, e.g. approximately 2 nm, and would not be acceptable because of laser diode drift with temperature, requiring 5-fold larger passbands, whereby such an increase in passband would create an insertion loss penalty of approximately 3 dB. However, in the integrated wavelength locker/MUX chip  1 , in accordance with the present invention, the wavelength of the diodes are set by the same grating  10 , which multiplexes the light, enabling sharp (Gaussian) passbands to be employed. Again mode conversion occurs at the exit edge of the chip  1 . 
         [0052]    An added benefit of using the integrated wavelength locker/MUX  1  is, if the chip is a silica or a silicon oxy-nitride based device, the wavelength drift with temperature will be almost an order of magnitude less than the drift for a DFB locked laser diode, which eases system requirements at other stages of the network, allowing a larger variety of components, including planar waveguide based MUX/DEMUXes, to be used, potentially lowering system costs. 
         [0053]    The ideal CWDM MUX should enable modulation up to 2.5 Gbit/sec. There is a likelihood that the wavelength locker/MUX chip  1  could operate in multiple longitudinal modes; if diode modulation occurs with the laser modulated 100%, wherein mode beating within a data pulse should not be an issue. However, in applications in which the laser could be on for long time spans, e.g. analog applications such as video data, then the laser modes should be separated by a frequency interval large compared to the data bandwidth, e.g. at least 10-fold larger, to prevent beat noise from being detected. Assuming that the frequencies involved are less than 2.5 GHz, a 25 GHz free spectral range is required for the locked diodes. The free spectral range is related to the total cavity length L (in vacuum) by the relation 
         [0000]    
       
         
           
             
               
                 
                   
                     F 
                      
                     
                         
                     
                      
                     S 
                      
                     
                         
                     
                      
                     R 
                   
                   = 
                   
                     c 
                     
                       2 
                        
                       L 
                     
                   
                 
               
               
                 
                   ( 
                   6 
                   ) 
                 
               
             
           
         
       
     
         [0054]    where c is the speed of light. 
         [0055]    An FSR of 25 GHz requires a cavity length L of 6 mm. Typical laser diodes have an actual length of about 0.75 mm, with a group index of about 3.8, so the laser diodes occupy about 2.85 mm of the allowed space, leaving only 3.15 mm for the locker/MUX chip  1  between the diode arrays  4   a  and  4   b  and the grating  10 , i.e. the length of the input waveguides  2   n  and the slab waveguide region  6 . Low index step devices, e.g. with Δn&lt;0.012, are limited to bend radii of 5 mm or more, and die sizes of the same order of magnitude, suggesting that a higher index step device would be more appropriate for the wavelength locker/MUX chip  1 . If a silicon oxy-nitride based waveguide grating is used, the effective index of the device will be about 1.8. The index step would be chosen to be compatible with the laser diode mode size, and bend radii as small as several 10&#39;s of microns would be acceptable. At an effective index of 1.8, the length of the wavelength locker/MUX  1  can be up to 1.75 mm in length. It should be noted that if mode beating is not an issue, then low or high index devices can be made, whereby the present invention is not restricted by index; however, the embodiment described hereinafter will be for the stricter requirement. 
         [0056]    The FSR of the combined laser diode/locker chip device should be small enough to enable several modes within the passband of the grating  10 , ensuring that there is at least one strong mode to lase with. For a CWDM device, the FSR should be 3 to 10 GHz wide; however, the FSR requirement will depend on the specific application, and be controlled by the length of the lasing optical path. The Gaussian passbands of a CWDM device, i.e. with 20 nm channel spacing, will be typically several nm or many 100&#39;s of GHz in size, whereby the FSR upper limit should not be an issue. 
         [0057]    In a high index step device, the dimension of the waveguides  2   n  and 12, should be about 0.5 to 0.7 μm square. If a Rowland circle design is used, the ends of the waveguides  2   n  and  12  at the Rowland circle, i.e. at ports  7  and  11 , should be separated by some minimum processing gap, e.g. 0.7 μm, in order to ensure that the gaps between cores can be filled with cladding. 
         [0058]    In the fabrication of the etched diffraction grating  10 , the corners of the facets will be rounded. In a projection lithography system (10×), corner roundings with radii of 0.5 μm are typical. Facets, and therefore the facet pitch, should be about 10-fold larger than corner radii in order to minimize the corner radii contribution to grating loss, which suggests a 5 um or larger facet size/grating pitch. 
         [0059]    The present invention is also applicable for use in a one laser/one-wavelength operation, if the wavelength locking function can be moved onto a high-yield, low cost, easily manufacturable chip, which also converts the mode to be compatible to standard SMF-28 fibers. This type of external wavelength locking can provide performance equal to or better than a comparable distributed feedback (DFB) laser, while using a much lower-cost gain chip that requires none of the extra processing steps or lower yields typically associated with DFBs. At a single wavelength operation, space constraints arise possibly from the mode-beating restriction, but also from the requirement that the device be packageable in a standard butterfly module. For a single-wavelength product, chip sizes should be restricted to within a few mm on any dimension. The grating is typically designed for illumination by the far-field of a mode at the grating focus. Consider a low-index device (n˜1.45), with a mode 1/e 2  half-width of about 3 μm. The confocal length of the mode, in the glass medium, is given by 
         [0000]    
       
         
           
             
               
                 
                   z 
                   = 
                   
                     
                       n 
                        
                       
                           
                       
                        
                       
                         πω 
                         0 
                         2 
                       
                     
                     λ 
                   
                 
               
               
                 
                   ( 
                   7 
                   ) 
                 
               
             
           
         
       
     
         [0060]    where n is the refractive index, and ω 0  is the half-width. For a “low-index” material, this confocal length will be about 40 μm. To ensure far-field illumination of the grating  10 , a distance between grating focus, i.e. the Rowland circle  5 , and the grating  10  of 400 μm should be sufficient. 
         [0061]    By manipulating equation (3) by choice of diffraction orders and pitch, it is possible to choose a diffraction order m 2  such that the Littrow reflection channel is viable for only one wavelength within the gain bandwidth of a typical diode, e.g. approximately 100 nm. The wavelength and the output port  13  can be positioned at different chip edges, e.g. to accommodate packaging, by insisting on a separation between input and output guides  2   n  and  12 , e.g. 45° separation. 
         [0062]      FIG. 4  illustrates an integrated MUX/Wavelength locker chip  21  for sixteen channels, i.e. sixteen laser diodes formed by four arrays  24   1  to  24   4  of four laser diodes positioned along a first edge of the chip  21  optically coupled to chip input ports  23 , drawn to scale for the following parameters: 
         [0063]    Average Refractive Index of the chip material, e.g. highly doped silica (at 1545 nm)=1.8; 
         [0064]    Refractive Index dispersion of the chip material=−1.66×10 −5 /nm; 
         [0065]    Diameter of Rowland Circle  25  defining focal plane of the concave reflective grating  30 , which defines an edge of a slab waveguide region  26 =460 μm or 0.46 mm; 
         [0066]    Minimum spacing between input waveguides  22   1  to  22   16  (only two shown) at slab input ports  27  at the Rowland circle  25 =1.5 μm; 
         [0067]    Minimum bend Radius of Output Waveguide  32  having a slab output port  31  along the Rowland circle  25 , and a chip output port  33  positioned at a second edge of the chip  21 , e.g. perpendicular to the first edge,=100 μm; 
         [0068]    The first predetermined order m 1  (the MUX diffraction order)=2; 
         [0069]    The second predetermined order m 2  (the Locker Diffraction order)=2×m 1 =4; 
         [0070]    The pitch of the grating  30 =7 μm 
         [0071]    The path length of the wavelength locking path is 1.3 mm or less, which meets the criteria of being less than 1.75 mm for an FSR of 25 GHz. 
         [0072]    The grating  30 , which ideally is similar to grating  10 , in the locker/MUX chip  21  serves two purposes: first, provides feedback to the lasers in the laser arrays  24   1  to  24   4 ; and second, multiplexes all output signals from laser diodes to the common output waveguide  32 . If the loss of the grating  30  is negligible, and all of the energy from the grating  30  can be accounted for in the feedback and common paths, then the grating  30  is analogous to a coated reflecting diode facet. The output at the multiplexed edge  33  of the locker/MUX chip  21  is equivalent to the output normally obtained from the edge of a DFB laser, i.e. the MUXing operation is folded into the operation of the laser. 
         [0073]      FIG. 5  illustrates a wavelength locker and mode convertor chip  41  drawn to scale for a single input wavelength signal or channel from a single laser diode  44  optically coupled to a chip-edge input port  43  under the following parameters: 
         [0074]    The locking wavelength of the single laser diode  44 : 1531 nm; 
         [0075]    The average refractive index of the chip material: 1.452; 
         [0076]    The diameter of Rowland circle  45 , which defines the focal plane of a concave reflective diffraction grating  50 , and which defines an opposite edges of a slab waveguide region  56 : 450 μm; 
         [0077]    The angle between the input and output ports  47  and  51  of the input and output waveguides  42  and  52 , respectively: 45°; 
         [0078]    The minimum bend radius for the input and output waveguides  42  and  52 : 5 mm; 
         [0079]    The first desired diffraction order m 1  (the MUX diffraction order)=1; 
         [0080]    The second desired diffraction order m 2  (the Locker Diffraction order)=7; 
         [0081]    The pitch of the grating  50 , which is preferably similar to gratings  10  and  30 =8.3 μm. 
         [0082]    In the locker/mode converter chip  41 , the path length within the mode converter laser feedback section of the die, i.e. the input port  43  to the grating  50 , is only about 1.2 mm, and the overall die size is 3.8×1.4 mm. The minimum bend radius for the input and output waveguides  42  and  52  is 5 mm. Despite the high order of the Littrow waveguide (order  7 ), the nearest overlapping order is at approximately 1340 nm, which will not be within the gain bandwidth of the intended laser  44 . Including the laser diode  44 , a net size of 2.5×4 mm should be achievable, which can be packaged in a small form-factor enclosure. 
         [0083]    For the embodiments illustrated in  FIGS. 2 ,  4  and  5 , the diffraction gratings  10 ,  30  and  50  are required to operate in two separate modes, a Littrow mode and a “MUX” mode. If the output ports  11 ,  31  and  51  for the multiplexed or output laser signal are in close angular proximity to the Littrow or feedback ports  7 ,  27  and  47 , then possibly the blaze envelope from the gratings  10 ,  30  and  50  will be broad enough to encompass both modes of operation. However, in a general design, it is likely that the blaze envelope is insufficiently wide. The different orders of operation may require that the teeth of the gratings  10 ,  30  and  50  have separate blazes to be effective. According to another aspect of the present invention, two blazes are incorporated simultaneously onto each facet F of each tooth in the gratings  10 ,  30  and  50 . 
         [0084]    With reference to  FIG. 6 , a diffraction grating  70 , according to the present invention, for replacing the diffraction gratings  10 ,  30  and  50  in  FIGS. 2 ,  4  and  5 , includes a plurality of teeth  73 , only two of which are shown, with a relatively wide front facet F and a relatively short sidewall S. Each front facet F includes a first Littrow blazed subdivision section  71  defining a first angle to the incident light (and the grating normal) for reflecting a first portion of the incident light back towards the slab input ports  7 ,  27  and  47 , and a second MUX blazed subdivision section  72  defining a second, different, angle to the incident light (and the grating normal) for reflecting a second portion of the incident light to the slab output port  11 ,  31  and  51 . The diffraction grating  70  could form part of or replace the diffraction gratings  10 ,  30  and  50  from the aforementioned embodiments. 
         [0085]    The facets F are drawn with the following parameters in mind: a pitch and facet total size of 6 to 10 μm (ideally 7 μm), a subdivision of 3 to 6 μm (ideally 4 μm) for the MUX blaze  72 , a subdivision of 2 to 5 μm (ideally 3 μm) for the Littrow or laser feedback blaze  71  (ideally a split of 57%/43% MUX/feedback), and a corner rounding of 0.5 μm therebetween. The angle between the normals of the two facet subdivisions  71  and  72  is a relatively small acute angle, e.g. 1° to 5° compared to the overall facet-to-sidewall angle, e.g. 90°. Accordingly, the front faces of facet subdivisions have an acute angle, e.g. 85° to 89°, therebetween. As seen in  FIG. 6 , the corner rounding has little influence between the subdivisions  71  and  72 , only at the edges of the total facet F. The facets F above can be viewed as a beam splitter, in this case approximately 57%/43% MUX/feedback, although anywhere between 10%/90% MUX/feedback and 50%/50% MUX/feedback is preferable, while anywhere between 10%/90% MUX/feedback and 90%/10% MUX/feedback is possible. The coherent effect of an entire grating  70  with facets F is to create a wavelength specific beamsplitter with approximately 57% transmission and 43% back-reflection. The blaze envelopes of the first and second subdivisions  71  and  72  may overlap somewhat, and careful attention must be paid in actual design to coherent interference between the facet subdivisions  71  and  72  at the Rowland circle  5 ,  25  and  45  of the grating  10 ,  30  and  50 , respectively. 
         [0086]    In the design of  FIG. 4 , if a 40% backreflection/60% forward grating  30  ( 70 ) is implemented, and the chip  21  has a high loss of 5.0 dB per cm, then including the effects of corner rounding on the grating  30  ( 70 ), the designed system would be equivalent to a standard comparison device having a diode with a 30% reflective DFB, no internal DFB loss, followed by a 10% absorption filter (90% transmitted). The mode convertor would then be compared “head-to-head” with a lens-based fiber coupler, which will achieve 1 db to 3 dB of efficiency depending on the sophistication of the optics and the care taken in the alignment. A planar waveguide based converter should be able to achieve 1.5 dB transmission. Finally, keeping in mind that the device of  FIG. 4  has multiplexed multiple channels, an additional 2 dB multiplexing loss must be included in the standard comparison device. 
         [0087]    The laser diodes  4 ,  24  and  44  are susceptible to feedback. Typically, in a standard multiplexer, the back coupling of light from the diffraction grating is minimized; however, in accordance with the present invention, grating feedback is necessary and sought-after. However, feedback caused by reflection of light from the edges of the chips  1 ,  21  and  41  must be reduced. Therefore, the front edges of the laser diode arrays,  4 ,  24  and  44  require anti-reflection coating to reduce loss, and the waveguides within the diodes may need to be slightly angled as they approaches the edge of the diode. The input waveguides  2   1  to  2   8 ,  22   1  to  22   16 , and  42  on the locker/MUX chips  1 ,  21  and  41  will need to be angled appropriately to match the waveguide within the diode. 
         [0088]    Finally, sufficient length will be required at the input or output edges of the chip to accommodate mode converters (&lt;0.5 mm should be sufficient), such as two-dimensional or three-dimensional mode converters, well known in the art, or custom segmented or offset-segmented mode converters disclosed in Applicant&#39;s co-pending application 60/filed Jun. 17, 2008, which is incorporated herein by reference. The mode converters are for converting the mode between the respective lasers and the diffraction grating, and also for converting from the waveguide mode to the fiber mode.