Abstract:
A wavelength monitoring device for monitoring a beam of light is disclosed having a beam splitter, with opposing first and second spaced apart faces, for receiving optical radiation from the beam of light to be monitored. In operation the first face reflects a first portion of the optical radiation to a first photodiode. The second face includes a grating for reflecting a second portion of the optical radiation to the first photodiode. The grating also reflects a third portion of optical radiation to a second photodiode. The light received by the second photodiode corresponds proportionally to optical power of the incident beam of light. The first photodiode is for detecting a wavelength characteristic of the composite beam and is located so as to receive the first portion and the second portion of optical radiation after the first portion and the second portion of optical radiation have optically interfered to form a composite beam. The first face and the grating are oriented and spaced from one another so that the first and second portions of the optical radiation optically interfere with one another along a path toward the first photodiode.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims priority of U.S. Provisional Patent Application No. 60/513,505 filed Oct. 22, 2003, entitled “Laser Wavelength Locker System”, which is incorporated herein by reference for all purposes. 

   FIELD OF THE INVENTION 
   The present invention relates to wavelength monitoring and control devices. More specifically, it relates to a wavelength locker that is used to precisely maintain the operating wavelength or frequency of a laser. 
   BACKGROUND OF THE INVENTION 
   With the growth of optical communication systems, the need for laser sources operating at well defined spectral frequencies or wavelengths has arisen. Wavelength Division Multiplexing (WDM) systems employ several laser systems, each of them modulated at a unique working wavelength. The modulated optical signals are subsequently multiplexed, and spectrally separated channels are combined and delivered through one or more optical fibers. At a receiver end the channels are separated by way of their wavelengths being demultiplexed and routed to individual detectors. 
   Although WDM systems significantly increase the capacity of a single optical fiber, it comes at a price; control of the wavelength accuracy of each individual laser source must be maintained. Any significant wavelength drift of any channel will cause signal degradation of that channel, or perhaps other adjacent channels at the receiver end. The wavelengths of semiconductor laser sources used in optical WDM systems should be controlled to within a fraction of the channel spacing defined by the ITU grid. 
   A wavelength monitor (WM) is commonly used in conjunction with laser systems to monitor changes in the wavelength or frequency of the emitted radiation. The WM can be used as an independent device, or can be combined with a laser system forming a wavelength locker (WL) to stabilize and maintain the operational wavelength of one or more lasers by detecting the relative change in the operating wavelength, then generating a feedback signal proportional to the deviation of the working wavelength from its nominal value. The feedback signal is further used to adjust the operating wavelength until the feedback signal is reduced to an acceptable level. 
   Different wavelength monitoring and locking technologies have been used in the past. One type of wavelength locker is based on thin-film interference filters as disclosed in U.S. Pat. Nos. 4,309,671; 6,122,301; 6,144,025; 6,411,634. A common deficiency of a filter-based approach is that a plurality of filters are required, wherein each filter can be used for locking a relatively small number of neighboring ITU channels. To cover a broad spectral range, such as C and L telecommunication bands, a large inventory of different filters is required; this increases the cost, inventory required, and manufacturing complexity. The problem becomes even more difficult with reduction in channel spacing due to increased filter fabrication cost and complexity. 
   Another commonly used type of WM employs Fabry-Perot etalons and is based on multi-beam interference, as disclosed in the following U.S. Pat. Nos. 5,825,792; 6,005,995 and US Patent Application US 2003/0063871, all incorporated herein by reference. The thickness of an etalon and the refractive index of the material define the free spectral range (FSR) that corresponds to the spacing of wavelength locked channels. The etalon surface reflectivities should be controlled to achieve a required finesse that defines desired amount of wavelength discrimination. 
   There are several problems associated with etalon-based wavelength monitors. An etalon-based WM in a front-facet configuration usually requires a beam splitter or tap to redirect part of the output beam onto the WL. This leads to increase in cost, complexity and packaging spatial requirements of the laser system. 
   To achieve an etalon response function with a desired contrast, operation at a nearly normal incidence angle is required. Because the set point is positioned in the middle of the etalon amplitude modulation response curve, a significant amount of light is reflected and can potentially be coupled back to the laser source. If not rejected, that light will cause performance degradation. To reject the fed-back light, an optical isolator is positioned between the laser diode (LD) and the WL, increasing the product cost, package complexity and spatial requirements. 
   A third group of wavelength lockers employs wavelength-selective devices based on two-beam interference, such as a Mach-Zehnder interferometer (see for example U.S. Pat. No. 6,549,548). This type of WL has a sinusoidal spectral response and, for a given ITU channel spacing, exceeds the capture range and the contrast of the etalon-based WL counterpart. At the same time, the WL disclosed in U.S. Pat. No. 6,549,548 is based on a complex birefringent waveplate filter system that uses several components and requires precise fabrication and assembly. It also requires a beam-splitter to redirect part of the output beam onto the WL. This type of WL is expensive, complex and adds significant cost to the laser system as a whole. 
   It would be, therefore, desirable to provide a simple WL device that overcomes the disadvantages of the existing wavelength lockers while providing inexpensive fabrication and reduced packaging complexity. 
   SUMMARY OF THE INVENTION 
   In view of the foregoing, it is an object of the invention to provide a wavelength locker of reduced complexity that can be inexpensively fabricated in high volumes. 
   It is another object of the invention to provide WL device with a small amount of optical feedback so that the need for an optical isolator between the WL and laser diode (LD) is eliminated, thus reducing the package size and complexity. 
   While the solution presented below is concerned with a front-facet WL arrangement, it can be easily adopted to a back-facet WL configuration by those skilled in the art. 
   The present invention provides a WL that integrates the functions of a beam splitter and a wavelength discriminating element in a single optical component. The beam splitter function is required to redirect the fractions of the output laser beam towards the wavelength and power monitoring photodiodes (PDs). The desired wavelength selectivity is achieved by introducing an optical path difference between at least two interfering beams that reach the wavelength monitoring PD. The WL according to the present invention consists of a single optical element that employs diffraction grating and a submount with wavelength and power monitoring PDs. A fraction of the output laser beam is split by the optical element and is directed to a PD to monitor the output laser power. Another fraction of the output laser beam is split by the optical element and directed to a wavelength monitoring PD. The wavelength-monitoring portion of the beam includes at least two individual beams with introduced optical path difference required to achieve wavelength selectivity. 
   In accordance with the invention there is provided, a wavelength monitoring device for monitoring a beam of light, comprising:
         a) a beam splitter, having opposing first and second spaced apart faces, for receiving optical radiation from the beam of light to be monitored,
           one of the first and second faces for directing a first portion of the optical radiation to a first location,   the other of the first and second faces including an optical structure thereon or thereabout for directing:   a second portion of the optical radiation to the first location, a third portion of optical radiation to the second location,   
           b) a first photodiode disposed to receive optical radiation present at the first location after the first portion and the second portion of optical radiation have optically interfered to form a composite beam, the first photodiode for detecting a wavelength characteristic of the composite beam; and,   c) a second photodiode disposed to receive the optical radiation present at the second location, wherein the third portion of optical radiation at the second location corresponds proportionally to optical power of the incident beam of light;
           wherein the first face and optical structure are oriented and spaced from one another so that the first and second portions of the optical radiation optically interfere with one another along a path toward the first photodiode. Direction of the optical beams to the above-identified locations is accomplished through reflection, refraction or diffraction, as known to those skilled in the art.   
               

   In accordance with the invention, there is further provided, a wavelength monitor for monitoring an input light beam, the monitor comprising a beam splitter having first and second spaced apart end faces, one of the end faces having an optical structure thereon for splitting a portion of the input light beam into first second and third sub-beams, wherein the third sub-beam has at least 70% of the power of the input light beam, wherein the first and second sub-beams are directed to first and second photodiodes respectively, the other of the end faces having a surface for directing a fourth portion of the input light beam incident thereupon to the first photodiode in such a manner as to direct the portion of the input light beam along at least a portion of a common path with the first sub beam so that the beams interfere with one another along a common path, wherein the interference is a function of a difference in optical path length traversed by the first sub beam and the fourth portion of the input light before the reaching the common path. Direction of the optical beams to the above-identified locations is accomplished through reflection, refraction or diffraction, as known to those skilled in the art. 
   The features of the invention including construction and operational details will now be more particularly described with reference to the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram of a laser system with WL according to the present invention. 
       FIG. 2   a  is a graph of the WL normalized response and it&#39;s rate of change as a function of the wavelength. 
       FIG. 2   b  is a graph of the WL slope comparison of the normalized slope as a function of wavelength. 
       FIG. 3  is a schematic diagram of the WL in accordance with the first embodiment of the present invention wherein a single block functions as a beam splitter and an interferometer. 
       FIG. 4  is a schematic diagram of the WL in accordance with the second embodiment of the present invention. 
       FIG. 5  is a schematic diagram of the WL in accordance with the third embodiment of the present invention. 
       FIG. 6  is a schematic diagram of the WL in accordance with the fourth embodiment of the present invention 
       FIG. 7  is a schematic diagram of the WL in accordance with the fifth embodiment of the present invention 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  presents an optical layout of a wavelength monitor according to the present invention. A component  104  in the form of an interferometric splitter is disposed between an input waveguide  101  and an optional output optical fiber  110 . The input waveguide  101  may represent an input fiber or a semiconductor laser. Photodiodes (PDs)  114  and  116  are disposed to receive light reflected from the optical component  104 . A lens  102  is provided to collimate light received from the input waveguide  101  and a lens  109  is provided for focusing collimated light received into the optional output optical fiber  110 . 
   In operation, the output from the front facet of the input waveguide  101  propagates through the lens  102  to form a collimated beam  103 . The collimated beam  103  is further split into four sub-beams by the interferometric splitter  104 : a collimated beam  108 , a power monitoring beam  115 , and wavelength monitoring beams  112  and  113 . When the wavelength monitor according to the present invention is used as a front-facet wavelength monitor of a semiconductor laser, most of the power of the collimated beam  103  propagates through the interferometric splitter  104  as a collimated output beam  108  and is coupled by the focusing lens  109  into the output fiber  110 . The beam  108  typically contains more than 80% of the optical power of the initial beam  103 . When the WM is employed in a stand-alone configuration, the input collimating beam  103  is redistributed between the power monitoring beam  115  and the wavelength monitoring beams  112  and  113 . The lenses  102  and  109  are typically aspheric in shape, but other lens types such as a ball or a GRIN lenses can also be employed for the same function. One of the wavelength monitoring beams  112  is split from the incident beam  103  by the front surface  105  of the interferometric splitter  104  is directed onto the wavelength-monitoring PD  114 . The second wavelength-monitoring beam  113  is split from the beam  111  by the back surface  106  of the interferometric splitter and is also directed onto the wavelength-monitoring PD  114 . The power-monitoring beam  115  is formed by splitting a portion of the incident beam  111  from the back surface  106  of the interferometric splitter  104 . In alternative configurations for example in embodiments 3 through 5 which follow, the power-monitoring beam  115  is formed by splitting a portion of the incident beam  104  from the front surface  105  of the interferometric splitter  104 . The power-monitoring beam  115  is directed onto the power-monitoring PD  116 . The beam  113  is delayed with respect to the beam  112  by the interferometric splitter  104 . Interference of the beams  112  and  113  provides wavelength-selective response of the wavelength locker as shown in the  FIG. 2   a  for 50 GHz channel spacing. The wavelength locking is achieved by keeping the ratio between the signals from the wavelength-monitoring PD  114  and the power-monitoring PD  116  constant. 
   The interferometric splitter  104  can be a wedge-shape or a plane-parallel plate, as is described in more detail below. 
     FIG. 2   a  presents normalized response as a function of the change in LD wavelength for a multi-beam interference device, such as an etalon in accordance with the prior art, and a two-beam interference device in accordance with the present invention. Both devices have a free spectral range that corresponds to 50 GHz channel spacing.  FIG. 2   b  presents respective normalized slopes or normalized response rate of change as a function of the change in the operating wavelength for devices exhibiting normalized responses shown in  FIG. 2   a.    
     FIG. 3  illustrates an integrated wavelength monitor according to the first embodiment of the present invention in accordance with  FIG. 1 . For picture clarity, only the chief rays indicating the centers of the propagating beams are shown. The wavelength monitor comprises an interferometric splitter  304  in the shape of a wedge shaped block with a wedge angle □ shown. A diffraction grating  307  is applied to the back surface  306  of the interferometric wedge  304 . An incident beam  303  collimated by the lens  302  is split into two beams  311  and  312  by the first front surface  305  of interferometric wedge  304 . The beam  311  contains most of the power of beam  303  and propagates through the first surface  305  of the wedge  304  towards the back surface  306 . The beam  312  is reflected by wedge surface  305  and is directed towards the wavelength monitoring PD  314 . Surface  306  of the interferometric wedge  304  integrates the diffraction grating  307 . The diffraction grating  307  can be fabricated, for example, as a surface relief phase grating, by etching respective groves on surface  306  of the interferometric wedge  304 . The collimated beam  311  is further split into 2 beams by the surface  306 : the beam  318  which is formed by a specular reflection from the surface  306 , and the beam  317  which is the grating diffraction order reflected from the diffraction grating  307 . The grating structure is typically optimized to reflect the first diffraction order, but other diffraction orders can be employed instead, as known to those skilled in the art. Optionally another beam (not shown) propagating through surface  306  can be created and coupled through the focusing lens into the output fiber (the beam  108  shown on  FIG. 1 ), The beam  318  refracts through the surface  305  of the interferometric wedge  304  and emerges as a collimated beam  315  that is directed onto the power monitoring PD  316 . The beam  317  refracts through the surface  305  of the interferometric wedge  304  and emerges as a collimated beam  313  that is directed onto the wavelength monitoring PD  314 . The two beams  312  and  313  are laterally offset from each other at the surface of the wavelength-monitoring PD  314  by a lateral shift Δh, as shown in  FIG. 3 . Interference of the beams  312  and  313  occurs in free-space along the chief rays of the beams  312  and  313  and produces a wavelength-selective WL response at the active area of the wavelength-monitoring PD  314  as shown in  FIG. 2   a  for a two-beam interference case. 
   The optical path difference between the chief rays of the interfering beams  312  and  313  can be calculated as: 
   
     
       
         
           
             
               
                 OPD 
                 = 
                 
                   
                     t 
                     
                       cos 
                       ⁡ 
                       
                         ( 
                         
                           α 
                           refr 
                         
                         ) 
                       
                     
                   
                   + 
                   
                     t 
                     
                       cos 
                       ⁡ 
                       
                         ( 
                         
                           α 
                           difr 
                         
                         ) 
                       
                     
                   
                 
               
             
             
               
                 ( 
                 1 
                 ) 
               
             
           
         
       
     
   
   where t is the wedge thickness at the intersection of the chief ray with the grating structure  307 ; α refr  is the refraction angle at the first surface  305  of the wedge  304 ; α difr  is the diffraction angle at the second surface  306  of the wedge  304 . Accounting for the basic refraction and diffraction equations: 
   
     
       
         
           
             
               
                 
                   sin 
                   ⁡ 
                   
                     ( 
                     α 
                     ) 
                   
                 
                 = 
                 
                   n 
                   · 
                   
                     sin 
                     ⁡ 
                     
                       ( 
                       
                         α 
                         refr 
                       
                       ) 
                     
                   
                 
               
             
             
               
                 ( 
                 2 
                 ) 
               
             
           
           
             
               
                 
                   
                     sin 
                     ⁡ 
                     
                       ( 
                       
                         
                           α 
                           refr 
                         
                         + 
                         β 
                       
                       ) 
                     
                   
                   + 
                   
                     sin 
                     ⁡ 
                     
                       ( 
                       
                         α 
                         difr 
                       
                       ) 
                     
                   
                 
                 = 
                 
                   m 
                   · 
                   
                     λ 
                     d 
                   
                 
               
             
             
               
                 ( 
                 3 
                 ) 
               
             
           
         
       
     
   
   the equation (1) for the optical path difference can be re-written as: 
   
     
       
         
           
             
               
                 OPD 
                 = 
                 
                   
                     t 
                     
                       
                         1 
                         - 
                         
                           
                             ( 
                             
                               
                                 sin 
                                 ⁡ 
                                 
                                   ( 
                                   α 
                                   ) 
                                 
                               
                               n 
                             
                             ) 
                           
                           2 
                         
                       
                     
                   
                   + 
                   
                     t 
                     
                       
                         1 
                         - 
                         
                           
                             ( 
                             
                               
                                 m 
                                 · 
                                 
                                   λ 
                                   d 
                                 
                               
                               - 
                               
                                 sin 
                                 ⁡ 
                                 
                                   ( 
                                   
                                     
                                       A 
                                       ⁢ 
                                       
                                           
                                       
                                       ⁢ 
                                       
                                         sin 
                                         ⁡ 
                                         
                                           ( 
                                           
                                             
                                               sin 
                                               ⁡ 
                                               
                                                 ( 
                                                 α 
                                                 ) 
                                               
                                             
                                             n 
                                           
                                           ) 
                                         
                                       
                                     
                                     + 
                                     β 
                                   
                                   ) 
                                 
                               
                             
                             ) 
                           
                           2 
                         
                       
                     
                   
                 
               
             
             
               
                 ( 
                 4 
                 ) 
               
             
           
         
       
     
       
       
         
           where α is the angle of incidence onto the first surface  305  of the wedge  304 ; n is the refractive index of the wedge  304  material; λ is the wavelength of the propagating light; d is the spacing between the grating groves; m is the order of diffraction; β is the angle of the wedge. 
         
       
     
  
   In the simplest configuration the amount of power in the beams  312 ,  313  and  315  is defined by Fresnel reflections at the air-glass interfaces of the uncoated surfaces  305  and  306  of the WL component  304 . The surfaces  305  and  306  may optionally include coatings for equalization of the power levels in the beams  312  and  313  to increase the contrast of the spectrally modulated interferometric response, as well as to balance the response levels of the monitoring photodiodes  314  and  316 . Because the beams  312  and  313  have lateral offset Δh, the highest modulation contrast is achieved when the center of the wavelength monitoring PD is located at the mid-point between the beam centers. In the preferred embodiment, the power-monitoring PD  316  and the wavelength-monitoring PD  314  are mounted on a common substrate  319 , shown schematically in  FIG. 3 . The wavelength change is monitored independently from the changes in the power of the input beam  303 : the output signal from the wavelength-monitoring PD  314  is normalized by dividing it by the output signal of the power-monitoring PD  316 . The wavelength-monitoring PD  314  and the power-monitoring PD  316  are located on a same side from the collimated beam  303 . 
     FIG. 4  presents the integrated wavelength monitor according to the second embodiment of the present invention. According to the second embodiment, the WM component  404  comprises two plane-parallel surfaces  405  and  406  with diffraction grating  407  fabricated on the rear surface  406 . According to the second embodiment, the output from the waveguide  401  propagates through a lens  402  that forms a collimated beam  403 . The collimated beam  403  is further split into several individual beams by the interferometric WL component  404 . At least three sub-beams are formed by interferometric plane-parallel plate  404 : the main beam which contains most of the power of beam  403  propagates through the WL component  404  and is coupled into the output fiber (not shown in the Figure), a power monitoring beam  415  and wavelength monitoring beams  412  and  413 . The collimated beam  403  is split into two beams  411  and  412  by the first surface  405  of the interferometric plate  404 . The beam  411  typically contains most of the power of the beam  403  and propagates through the first surface  405  of the plate  404  towards the second surface  406 . The beam  412  is reflected by the plate surface  405  and is directed towards the wavelength monitoring PD  414 . The surface  406  of the interferometric plate  404  contains a grating  407 . The grating  407  can be fabricated as a surface relief phase grating by, for example, etching groves on the surface  406  of the interferometric plate  404 . The collimated beam  411  is split into at least two beams by the surface  406 : the beam  417  which is formed by a specular reflection from the surface  406 , and the beam  418  which is formed by diffraction in reflection from the grating structure  407 . The grating structure is typically optimized to reflect the first diffraction order, but other diffraction orders can be employed instead, as known to those skilled in the art. Optionally the pass-through beam (beam  108  shown in  FIG. 1 ) is also formed on the surface  406  and is coupled into the output fiber through the focusing lens. The beam  417  emerges from the interferometric plate  404  after refraction on the surface  405  as a collimated beam  413  that is directed onto the wavelength-monitoring PD  414 . The beam  418  emerges from the interferometric plate  404  after refraction on the surface  405  as a collimated beam  415  that is directed onto the power-monitoring PD  416 . 
   The optical path difference between the chief rays of the interfering beams  412  and  413  in accordance with the second embodiment can be calculated as: 
   
     
       
         
           
             
               
                 OPD 
                 = 
                 
                   
                     2 
                     · 
                     t 
                   
                   
                     
                       1 
                       - 
                       
                         
                           ( 
                           
                             
                               sin 
                               ⁡ 
                               
                                 ( 
                                 α 
                                 ) 
                               
                             
                             n 
                           
                           ) 
                         
                         2 
                       
                     
                   
                 
               
             
             
               
                 ( 
                 5 
                 ) 
               
             
           
         
       
     
   
   where t is the plate  404  thickness; α is the angle of incidence onto the first surface of the plate  404 ; n is the refractive index of the plate  404  material at the working wavelength λ of the propagating light. 
   In the simplest configuration the amount of power in the beams  412 ,  413  and  415  is defined by Fresnel reflections at the air-glass interfaces of the uncoated surfaces  405  and  406  of the WL component  404 . The surfaces  405  and  406  may optionally include coatings for equalization of the power levels in beams  412  and  413  to maximize the contrast of the spectrally modulated interferometric response, as well as to balance the response levels of the monitoring photodiodes  414  and  416 . Because the beams  412  and  413  have lateral offset Δh, the highest modulation contrast is achieved when the center of the wavelength monitoring PD is located at the mid-point between the beam centers. In the preferred embodiment, the power-monitoring PD  416  and the wavelength monitoring PD  414  are mounted on a common substrate  419 , shown schematically in  FIG. 4 . The wavelength change is monitored independently from the changes in the power of the input beam  403 : the output signal from the wavelength-monitoring PD  414  is normalized by dividing it by the output signal of the power-monitoring PD  416 . The wavelength-monitoring PD  414  and the power-monitoring PD  416  are located on the same side from the collimated beam  403 . 
     FIG. 5  illustrates the integrated wavelength monitor according to the third embodiment of the present invention. According to the third embodiment, the WM is constructed so that the grating  507  is fabricated on the front surface  505  of the WM interferometric splitter  504  in the shape of a plane-parallel plate. According to the third embodiment, the output from the input waveguide  501  propagates through a lens  502  that forms a collimated beam  503 . The collimated beam  503  is incident onto the first surface  505  of the plate  504  containing diffraction grating  507 , where it is split into at least three individual beams  511  and  512  and  515 . The beam  511  is formed through refraction of the beam  503  through the front surface  505  of the interferometric splitter  504  and propagates from the first surface  505  of the plate  504  towards the second surface  506 . The beam  512  is formed by specular reflection of the beam  503  from the surface  505  and is directed onto the wavelength-monitoring PD  514 . The beam  515  is formed as a diffraction order in reflection of the beam  503  from the grating  507 , and is directed onto the power-monitoring PD  516 . The grating structure is typically optimized to reflect the first diffraction order, but other diffraction orders can be employed instead, as known to those skilled in the art. The beam  511  is reflected from the plate surface  506  and is directed towards the front surface  505  as a beam  517 . The beam  513  is formed by refracting the beam  517  through the front surface  505 , and is directed onto the wavelength-monitoring PD  514 . The beams  512  and  513  interfere at the surface of the wavelength-monitoring PD  514 , providing required wavelength selectivity of the WL. The optical path difference between the chief rays of the interfering beams  512  and  513  can be calculated using equation (5). The wavelength-monitoring PD  514  and the power-monitoring PD  516  are located on the same side from the collimated beam  503 . In an alternative fourth embodiment the beam  511  is split into two beams at the surface  506 . One beam, corresponding to the beam  108  in  FIG. 1  (not shown in  FIG. 5 ), contains majority of the power of the incident beam  511 , propagates through the surface  506  and is coupled into the output fiber through a focusing lens. The second beam  517  is formed by reflecting a portion of the beam  511  from the plate surface  506  and is directed towards the front surface  505 . The beam  513  is formed by refracting the beam  517  through the front surface  505 , and is directed onto the wavelength-monitoring PD  514 . The beams  512  and  513  interfere at the surface of the wavelength-monitoring PD  514 , providing required wavelength selectivity of the WL. The optical path difference between the chief rays of the interfering beams  512  and  513  can be calculated using equation (5). The wavelength-monitoring PD  514  and the power-monitoring PD  516  are located on the same side from the collimated beam  503 . 
     FIG. 6  illustrates the integrated wavelength locker in accordance with a fifth embodiment of the present invention, where the interferometric splitter  604  of the WL is constructed as a wedge with diffraction grating  607  fabricated on the front surface  605  of it. According to the fifth embodiment, the output from the waveguide  601  propagates through a lens  602  that forms a collimated beam  603 . The collimated beam  603  is incident onto the first surface  605  of the plate  604  containing diffraction grating  607 , where it is split into at least three individual beams  611  and  612  and  615 . The beam  611  contains most of the power of the beam  603 , refracts through the first surface  605  of the plate  604  and propagates towards the second surface  606 . The beam  615  is formed by specular reflection of the beam  603  from the surface  605  and is directed onto the power-monitoring PD  616 . The beam  612  is formed as a diffraction order in reflection of the beam  603  from the grating  607 , and is directed onto the wavelength-monitoring PD  614 . The grating structure is typically optimized to reflect the first diffraction order, but other diffraction orders can be employed instead, as known to those skilled in the art. The beam  611  is reflected from the wedge surface  606  as a beam  617  and is directed towards the first surface  605 . The beam  613  is continuation of the beam  617  after refraction through the surface  605 , is directed onto the wavelength-monitoring PD  614 . The beams  612  and  613  interfere at the surface of the wavelength-monitoring PD  614 , providing required wavelength selectivity to the WM. The optical path difference between the chief rays of the interfering beams  612  and  613  is defined by equation (4). The wavelength-monitoring PD  614  and the power-monitoring PD  616  are located on the opposite sides from the collimated beam  603 . 
   In an alternative sixth embodiment the beam  611  is split into two beams at the back surface  606 . One beam, corresponding to the beam  108  in  FIG. 1  (not shown in  FIG. 6 ), contains majority of the power of the incident beam  611 , propagates through the surface  606  and is coupled into the output fiber through a focusing lens. The second beam  617  is reflected from the wedge surface  606  and is directed towards the first surface  605 . The beam  613  is continuation of the beam  617  after refraction through the surface  605 , is directed onto the wavelength-monitoring PD  614 . The beams  612  and  613  interfere at the surface of the wavelength-monitoring PD  614 , providing required wavelength selectivity to the WM. The optical path difference between the chief rays of the interfering beams  612  and  613  is defined by equation (4). The wavelength-monitoring PD  614  and the power-monitoring PD  616  are located on the opposite sides from the collimated beam  603 . 
     FIG. 7  illustrates the integrated wavelength locker according to a seventh embodiment of the present invention. According to the seventh embodiment, the WL is constructed so that the grating  707  is fabricated on the front surface  705  of the interferometric splitter  704  in the shape of a plane-parallel plate. According to the embodiment, the output from the input waveguide  701  propagates through a lens  702  that forms a collimated beam  703 . The collimated beam  703  is incident onto the first surface  705  of the plate  704  containing diffraction grating  707 , where it is split into at least three individual beams  711  and  712  and  715 . The beam  711  is defined by refraction of the beam  703  through the surface  705  and propagates towards the second surface  706 . The beam  715  is formed by specular reflection of the beam  703  from the front surface  705  and is directed onto the wavelength-monitoring PD  716 . The beam  712  is formed as a diffraction order in reflection of the beam  703  from the grating  707 , and is directed onto the power-monitoring PD  714 . The grating structure is typically optimized to reflect the first diffraction order, but other diffraction orders can be employed instead, as known to those skilled in the art. The beam  711  is reflected from the plate surface  706  as a beam  717  and is directed towards the first surface  705 . The beam  713  is a continuation of the beam  717  after refraction through the surface  705 , is directed onto the wavelength-monitoring PD  714 . The beams  712  and  713  interfere at the active area of the wavelength-monitoring PD  614 , providing required wavelength selectivity to the WM. The optical path difference between the chief rays of the interfering beams  712  and  713  is defined by equation (5). The wavelength-monitoring PD  714  and the power-monitoring PD  716  are located on the opposite sides from the collimated beam  703 . In an alternative eighth embodiment the beam  711  is further split into two beams at the surface  706 . One beam, corresponding to the beam  108  in  FIG. 1  (not shown in  FIG. 7 ), contains majority of the power of the incident beam  711 , propagates through the surface  706  and is coupled into the output fiber through a focusing lens. The second beam  717  is defined by reflection of the beam  711  from the plate back surface  706  and is directed towards the first surface  705 . The beam  713  formed by refraction of the beam  717  through the surface  705 , is directed onto the wavelength-monitoring PD  716 . The beams  712  and  713  interfere at the surface of the wavelength-monitoring PD  716 , providing wavelength selectivity to the WL. The optical path difference between the chief rays of the interfering beams  712  and  713  is defined using equation (5). The wavelength-monitoring PD  716  and the power-monitoring PD  714  are located on the opposite sides from the collimated beam  703 . 
   While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that several other variances of the presented WM design can be constructed without departing from the scope of the invention. For example, although the embodiments shown are directed to having an incident beam split into three beams that are reflected and a fourth optional beam that is transmitted through the block, it is possible to have the transmitted beam reflected as well, to a distinct location where it may be coupled into an optical waveguide. Alternatively, although not shown, the invention may work in transmission, where all or some of the output ports, including monitoring PDs, are disposed on the opposite side with respect to the input beam. 
   Nowithstanding, the diffractive grating would have to transmit the beams it is currently shown to reflect to distinct locations, wherein two beam have an overlapping path. This could be done by having one of the beams bounce once prior to being transmitted to ensure an optical path length difference between the two beams that propagate and mix in freespace prior to being incident upon the detector. In yet another less preferred embodiment, the block could be replaced with two faces or surfaces of two transmissive substrates having a gap there between. These two spaced apart faces would function in a less efficient and less convenient manner than the preferred and described block. 
   The term block used in this specification is to include a wedge or block having multiple sides, which may or may not be parallel.