Abstract:
A multichannel laser is based on an interleaved chirped waveguide grating router. An interferometric modulator is incorporated inside a laser cavity by means of a waveguide grating router and enables independent modulation of any of the wavelengths of the multichannel laser. The interferometric modulator operates independently of the wavelength selection elements of the waveguide grating router used to select the wavelengths of the multichannel laser.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     Related subject matter is disclosed in the concurrently filed application entitled “LASER TRANSMITTER BASED ON A COUPLER AND HAVING A SEPARATE OUTPUT PORT” by the same inventors, C. R. Doerr and C, H. Joyner, both applications being assigned to the same Assignee. 
     TECHNICAL FIELD OF THE INVENTION 
     This invention relates to lasers and, more particularly, to a multichannel laser having a single output port, as well as an improved method of controlling the output of optical signals from a multichannel laser. 
     BACKGROUND OF THE INVENTION 
     To make optimal use of the bandwidth available in a fiber based network, it is desirable to use as many separately detectable wavelength channels as possible and to encode data onto each wavelength at as high a speed as possible. One problem with lasers that deliver multiple channels is that the cavity length for many of them is long, making direct modulation above 1 GHz impractical due to the round trip time of a photon in the cavity. A possible solution is to use an output power tap on the main laser cavity and to modulate this output while the main laser cavity runs in continuous wavelength mode. (see C. H. Joyner et. al. “An 8 channel digitally tunable transmitter with electroabsorbtion modulated output by selective-area epitaxy” PTL vol. 7, no.9, September 1995 pp. 1013-1015 or the pending patent application entitled “Improved tunable transmitter with Mach-Zehnder Modulator,” Ser. No. 09/016,176, filed on Jan. 30, 1998 by C. H. Joyner. In both of the above cases the modulator was external to the laser cavity. 
     For improved modulation rates at low drive levels, it is desired is to integrate a phase modulator capability into a laser transmitter. Moreover, in WDM applications it would also be desirable to combine a number of these integrated modulated lasers together into a single multichannel modulated laser. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, a multichannel laser is based on an interleaved chirped waveguide grating router. An interferometric modulator is incorporated inside a laser cavity by means of a waveguide grating router and enables independent modulation or switching of any of the wavelengths of the multichannel laser. The interferometric modulator operates independently of the wavelength selection elements of the waveguide grating router used to select the frequencies of the multichannel laser. 
     More particularly, in accordance with the present invention, a multichannel laser comprises a waveguide grating router including a first star coupler, a second star coupler, and three interleaved sets of m waveguide grating arms (where m is a number greater than one) interconnecting to a first side of the first and second star couplers, for producing n wavelengths in the multichannel laser (where n is a number greater than one). A second side of the first coupler includes a first connection to a reflecting surface, and a second connection to an output port. A second side of said second coupler includes a first group of n connected waveguides, each of the n waveguides connected through an optical amplifier to a reflecting surface, a second group of n connected waveguides, each of the n waveguides connected through an optical amplifier and a controllable phase shifter to a reflecting surface. A laser signal is generated in each of n cavities formed by one of the first group of waveguides, the first and second couplers, the group of m waveguide grating arms, and the first connection of said first coupler. The magnitude of each of the n laser signals appearing at the output port are controlled by the optical amplifier and phase shifter in each one of the second group of n connected waveguides. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings, 
     FIG. 1 shows, in accordance with the present invention, an illustrative block diagram of an illustrative 3 by 3 star coupler laser transmitter for providing a single wavelength output; 
     FIG. 2 shows a plot of the electric field amplitudes over a range of phase shifts at all three ports; 
     FIG. 3 shows, in accordance with the present invention, a more general illustrative 2 by 2 star coupler laser transmitter for providing a single wavelength output; 
     FIG. 4 shows a first embodiment of a multichannel transmitter with a single output port; and 
     FIG. 5 shows a star coupler with an array of waveguide arms which are triple interleaved and chirped; and 
     FIG. 6 shows, in accordance with the present invention, an illustrative diagram of a multichannel transmitter with a single output port where all the channels are independently modulated at high speed. 
    
    
     DETAILED DESCRIPTION 
     In the following description, each item or block of each figure has a reference designation associated therewith, the first number of which refers to the figure in which that item is first described (e.g.,  101  is first described in FIG.  1 ). 
     The basic schematic of a 3 by 3 coupler is shown in FIG.  1 . The invention includes a star coupler  120  in which the input and output arms (or waveguides) to the left  101  and right  102  of the free-space region [FS]  103  are symmetrically spaced. A wavelength λ x  signal exiting any of the waveguides R 1 -R 3  is radiated into the free space region  103  and arrives somewhat equally at the waveguides L 1 -L 3 . If the phase of the wavelength λ x  signal from each of the waveguides R 1 -R 3  are all equal, then all of the wavefronts constructively combine at the entrance to waveguides L 1 -L 3 . If, however, the phase of the wavelength λ x  signal from each of the waveguides R 1 -R 3  are not the same then some destructive combining of the wavefronts occurs at waveguides L 1 -L 3 . As will be discussed in a later paragraph, the amount of destruction depends on the phase difference between the wavefronts radiated from the waveguides R 1 -R 3 . The laser signal generated in the arrangement of FIG. 1 will operate in a single transverse mode if the width and height of the cavity elements are restricted to a size that will not support multimode operation. 
     If the star coupler  120  input (left) and output (right) arms  101  and  102  are strongly coupled, the efficiency of the 3 by 3 coupler of FIG. 1 is increased. This occurs when the mode profile of each waveguide (e.g., R 2 , L 2 ) overlaps strongly with the mode profile of its adjacent neighbors (i.e., R 1 , R 3  and L 1 , L 3 , respectively). In this manner, wavefronts are created in adjacent waveguides as the waveguides enter the free-space region even though the laser signal may only have been injected into a single waveguide at some distance from the free-space region where the mode profile of the adjacent waveguides did not overlap. Thus, for example a wavelength λ x  signal originating on waveguide R 2 , at some distance from the free-space region  103 , becomes coupled to waveguides R 1  and R 3  near the free-space region  103  and the wavefronts of wavelength λ x  would then exit from each of R 1 -R 3  into the free-space region  103 . The star coupler  120  can be implemented as described in the article by C. Dragone entitled “Optimum design of a planar array of tapered waveguides,” published in J. Opt. Soc. Am. A, Vol. 7, No.11, November 1990 and incorporated by reference herein. 
     The waveguides R 1 , R 2 , and R 3  to the right of FS  103  terminate at facet B that has a high reflection HR coating  108 . Each of these arms R 1 -R 3  contains an amplifier section, A 1 -A 3 , for gain and/or control of the optical power amplitude in that waveguide. The waveguides R 1  and R 3  also contain phase adjustment elements, P 1  and P 3 , which allow the optical phase of that arm to be adjusted either by application of current or voltage signal  104  and  105 , respectively, to P 1  and P 3 . To the left of FS unit  103 , waveguides arms L 1  and L 3  terminate, respectively, at output ports p and p-bar at facet D, which has an antireflection AR coating. Waveguide L 2  contains a wavelength selective element  106  to choose a single wavelength among those allowed by the gain spectrum of the amplifier sections A 1 -A 3 . This element, while illustratively represented schematically as a grating, may also be a waveguide grating router, a coupler, filter or any other optical element used to select wavelength. The main CW laser cavity is defined by the HR mirror  107  on facet D, on the left, wavelength selective element  106 , waveguide L 2 , FS  103 , waveguide R 2 , amplifier A 2 , and the HR mirror  108  on the right. The amplifier A 2 , and hence the laser signal from the transmitter, can be turned on and off via lead  110 . 
     The free-space region FS  103  and the associated waveguides to the right and left, R 1 -R 3  and L 1 -L 3 , behave as a 3×3 coupler. The relationships for conservation of optical power as a function of amplitude and phase among the 6 ports for a lossless star coupler are given by the following equations: 
     
       
         L=AR with A=A −1    
       
     
     Where L is a vector denoting the power in waveguides L 1 , L 2  and L 3 . The variable R is a vector denoting the power in waveguides R 1 , R 2 , and R 3 . A is the matrix of coefficients denoting the phase relationship between vectors R and L given by              A   =     1   /       3                [               -   1     /   2     -     j          3     /   2             1             -   1     /   2     +     j          3     /   2                 1       1       1                 -   1     /   2     +     j          3   /   2               1             -   1     /   2     -     j          3     /   2               ]                   =     1   /       3                [                  -   π                   j                   2   /   3             1                -   π                   j                   2   /   3                 1       1       1                    +   π                   j                   2   /   3             1                -   π                   j                   2   /   3               ]                                    
     With reference to FIG. 2, there is shown the variation in electric field magnitudes |Er|, |Ep|, and |Epbar| (in the waveguides L 1 ; L 2 ; and L 3 , respectively, of FIG. 2) with changes in the phase shifts in the arms R 1  and R 3  relative to the phase in arm R 2 . As shown, for example, if the power density in arm R 2  is twice that in arms R 1  and R 3  and the phase of the electric field vector in waveguide R 1  is rotated −60° relative to R 2  while the phase in R 3  is rotated +60° relative to R 2 , then no power emerges from waveguide L 3  at port p-bar,  201 . However, {fraction ( 9 / 16 )} of the power density (Ep 2 ) in arm L 2  will emerge from arm L 1  at port p,  202 . If the sign of the phase shifts is reversed for arms R 1  and R 3  via the phase shifters P 1  and P 3  respectively, then this condition will reverse and power will be delivered to port p-bar,  203 , while none will emerge from port p,  204 . Thus for the described initial power densities, a phase shift swing of +/−60° will drive the device to modulate (or switch) power between ports p and p-bar. This is to be compared to a conventional extra-cavity Mach-Zehnder interferometer modulator, which requires +/−90° of phase shift. 
     It should be noted that the electric field amplitudes that exist over a range of phase shifts for all three ports are shown in FIG.  2 . Thus, when the relative phase in R 3  is zero and the relative phase in R 1  is zero, the electric field from ports p and pbar are both I unit,  205 , or {fraction ( 1 / 16 )} of the power of the laser. In this manner it is possible to get equal and complementary outputs at the output ports p and p-bar of the laser transmitter of FIG.  1 . There are many other solutions to the set of the above- described transcendental equations. We note that in general the voltage or current required, on control leads  104  and  105 , to produce a phase change in phase shifters P 1  and P 3  of FIG. 1, via an index change, is modest so that such a device is particularly suited to high speed operation. 
     We also note that when the complementary output signal, p-bar, is not required then the device can also be constructed from a 2×2 star coupler with less efficiency. This is shown in FIG. 3 where the arms R 3  and L 3 , phase shifter P 3  and amplifier A 3  have been removed. 
     Returning to FIG. 1, It should be noted that it is also possible to construct the same functionality with higher order n by n couplers with even greater efficiency at the expense of having n phase shifting elements P and n amplifiers A to control. It is also possible to direct arm L 1  to facet A and arm L 3  to facet C, if it is desired to have each facet entirely of one reflectivity type. The Facets B and D are reflective or highly reflective HR, while facets A and C are anti-reflective AR. In principle any of the active elements Pn or An can be located anywhere in arms Ln or Rn (e.g., see the dotted elements A 1 -A 3 , P 1  and P 3  in FIG.  1 ). However, when a phase shifter element P 1 , P 3  is located in arms L 1 , L 3 , the phase shift must be twice the value when these phase elements are located in the arms R 1 , R 3 . This is because when P 1 , P 3  are in arm R 1 , R 3 , the signal traverses in one direction through P 1 , P 3  and is then reflected back through P 1 , P 3  in the opposite direction. The same situation occurs for an amplifier An in arms L 1 , L 3 , i.e., it must also be twice the gain of an amplifier An that is placed in arms R 1 , R 3 . Wavelength selector F  106  may also be located in waveguide R 2  instead. 
     In WDM network applications it is desirable to produce a number of modulated lasers signals as cheaply and cost effective as possible. While the FIG. 1 and 2 modulated laser transmitters offer an improvement over prior modulated laser transmitters, they still only operate on one wavelength at a time. What is desired in WDM systems is to generate multiple wavelengths simultaneously on the same device. 
     In accordance with the present invention, we have combined the function of a 3×3 star coupler (of FIG. 1) with a interleaved-chirped wavelength selection element in a compact way to produce a multichannel transmitter with a single output port, where all the channels are independently modulated at high speed. One embodiment of this arrangement is shown in FIG.  4 . As shown, a triple interleaved- chirped waveguide grating router  401  including two star couplers  402  and  403  interconnected by an array of waveguide arms  404 . The interleaved-chirped waveguide grating router  401  has waveguide arms  404  that are divided into groups of three arms  405 . In each group of three waveguides  405 , every third arm is given an additional path length of λ c /3, while the other two waveguide grating arms in each group remain the same length as for a conventional waveguide grating router. 
     Such a triple interleaved router is described in the pending U. S. patent application entitled “Wavelength-Division-Multiplexing Cross-Connect using Angular Dispersive Elements and Phase Shifters” by C. R. Doerr, Ser. No. 08/923,304, filed on Sep. 4, 1997 and incorporated by reference herein. This patent application describes a technique for implementing a WDM cross-connect using two “interleaved-chirped” waveguide grating routers interconnected by controllable phase shifters, the description of which is incorporated by reference herein. 
     With reference to FIG. 5 there is shown, illustratively, the star coupler  402  and the length of each arm in the array of waveguide arms  404 . In group  405 , the length of the first two arms  501  and  502  have the conventional length, while the third arm  503  has an extra length λ c /3, where λ c  is the desired laser wavelength This pattern is repeated in the other groups of arms in  404 . This triple interleave chirp produces three primary Brillouin image zones  504 - 506 , for each of the group of wavelengths λ 1x λ nx , λ 1y -λ ny , and λ 1z -λ nz. , that are radiated from each of the waveguides  404 . Note that a separate image is formed for each different wavelength in each image zone  504 - 506 . Returning to FIG. 4, each of the wavelengths in the groups λ nx , λ ny , and λ nz  travels over a separate waveguide in the groups  410 ,  411 , and  412 , respectively. 
     In FIG. 4, a laser for each wavelength λ n  is formed using a laser cavity that includes HR  406 , amplifier  407 , a waveguide of group  410 , star coupler  402 , the waveguides  404 , star coupler  403 , path r  408 , and HR  409 . In this manner, the path between port r and ports nx comprise the n carrier wave (CW) lasing cavities for the n wavelengths λ n . Powering amplifier A in arm nx, via lead  420 , turns on the associated laser wavelength λ n . Each of the n wavelengths in λ ny , and λ nz  also has its own output port in groups  411  and  412 , respectively, as well as its own phase shifter,  413  and  414 , respectively. Star coupler  415  is a 2n by 2 coupler which couples each of the wavelengths λ ny  in waveguide group  416  and π nz  in waveguide group  417  to either output p or p-bar under control of control signals  418  and  419 . The operation of star coupler  415  and phase shifters  413  and  414  is the same as that previously described for FIG.  1 . 
     Shown in FIG. 6 is a preferred embodiment of our multichannel transmitter with a single output port. As shown, a triple interleaved-chirped waveguide grating router  601  includes two star couplers  602  and  603  interconnected by an array of waveguide arms  604 . The interleaved-chirped waveguide grating router (WGR)  601  has waveguide arms  604  that are divided into groups of three arms  605 . In each group of three waveguides  605 , every third arm is given an additional path length of λ c /3, while the other two waveguide grating arms in each group remain the same length as for a conventional waveguide grating router. The interleave-chirped WGR  601  may be of the type described in the previously referenced Doerr patent application. 
     Each path between arm r and one of the arms nx comprise a CW lasing cavity for one of the n wavelength lasers. With reference to FIG. 5, the length L is the length of the shortest waveguide in the grating arms  405 . Returning to FIG.  6 . the arms r and nx terminate in HR surfaces  609  and  616 . Powering an amplifier A in arm nx, via a lead  608 , turns on the associated wavelength λn. Ports p and p-bar output a laser signal and its complement, respectively, via an AR surface  617 . For optimal performance all arms x, y, and z should be the same length. Due to possible 4-wave mixing in the common waveguide r, arm r should be kept as short as possible. 
     In operation, a portion of each of the n laser wavelengths is coupled from path r, via star coupler  603 , to each of the three waveguides in each group, e.g.,  605 . In the same manner as was discussed with respect to FIG. 5, the n wavelength signals from each of the n groups  605  are coupled to three of the interleaved chirp-created Brillouin zones  610 - 612 . The resulting wavelengths in groups λ 1x -λ nx ; λ 1y -λ ny ; and λ 1z -λ nz , are outputted over the associated waveguides groups 1x-nx, 1y-ny, 1z-nz, respectively. Under control of signal  608 , amplifier A amplifies the wavelength signals λ 1x -λ nx  on waveguides 1x-nx. The associated phase shifter P and amplifier A set the phase and magnitude, respectively, of each wavelength on waveguides 1y-ny and 1z-nz, under control of control signals  614  and  615 , respectively. The HR surface  616  reflects these wavelengths back through the waveguides 1x-nx, 1y-ny, and 1z-nz; star coupler  602 ; waveguides  604 ; to star coupler  603 . At star coupler  603 , a portion of the wavelength signals λ 1x -λ nx ; λ 1y -λ ny ; and λ 1z -λ nz  go to path r to support laser operation and to one or both of the output ports p and p-bar. The phase vs. power output equations for ports p and p-bar are identical to the those of the above single wavelength device as shown in FIG.  2 . 
     Advantageously, the device of FIG. 6, provides many individually modulated wavelength channels yet is very compact in size. Using phase shift as the modulation mechanism not only allows for modulation at high speed with low current (or voltage), but by choosing the proper modulation waveform, the chirp of the outgoing signal may be tailored as well. For example, the chirp may be eliminated by driving the complementary phase shifter in a push-pull manner or tailored to any degree by asymmetrically driving each phase shifter, in an appropriate manner, to obtain opposite signs in the phase value. Another advantage of this device of FIG. 6, is that because it is used in reflective mode, the optical signal passes through each phase shifter twice. Therefore the phase shifters P may be half the length or run at half the voltage (or current), as compared to using phase shifter P in the arms  620  and  621 . For a similar reason, the amplifiers A are used in arms 1y-ny and 1z-nz rather than in the arms  620  and  621 . 
     In much the same manner as that described in FIG. 3, in an alternate embodiment of the invention of FIG. 6, we may eliminate either the y set  622  or the z set  623  of n waveguides, and the associated amplifiers A and phase shifter PS, and the associated output port p or p-bar. Such an embodiment would, however, be less efficient than that of FIG.  6 . 
     Many of the elements of the present invention may be implemented as described in the previously referenced Doerr patent application. Additionally, while the arrangement of FIG. 6, has been described as using WGR  601 , it should be understood that other types of angular dispersive elements may be utilized. For example, the unit  601  can be an angular dispersive element implemented using a virtually imaged phase array (VIPA) as described in the article by M. Shirasaki entitled “Large angular dispersion by a virtually imaged phase array and its application to a wavelength demultiplexer” , OPTICS LETTERS, Vol. 21, No. 5, March 1996. Another type of angular dispersive element, which may be used, is a well-known reflecting grating. 
     Thus, what has been described is merely illustrative of the application of the principles of the present invention. Hence, other arrangements can be implemented by those skilled in the art without departing from the spirit and scope of the present invention.