Patent Publication Number: US-8983244-B2

Title: Optical interferometer

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present invention claims priority from U.S. provisional patent application No. 61/438,017, filed Jan. 31, 2011, which is incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates to optical interferometers, and in particular to interferometers used in differential phase shift keying (DPSK) optical demodulators. 
     BACKGROUND OF THE INVENTION 
     In optical communications, optical signals are encoded with digital information at a transmitter site, propagated towards a receiver site, and decoded at the receiver site. The optical signals are encoded by modulating amplitude and/or phase of the signals. Phase modulation generally allows for a better bandwidth utilization. However, phase modulated signals are more difficult to decode, because the decoding requires a transformation of phase modulation into amplitude modulation that can be detected. To transform the phase modulation into the amplitude modulation, a reference optical signal is mixed with the transmitted phase-modulated signal in an optical interferometer. The amplitude-modulated interference signal is then detected by a photodetector. 
     One can use the optical signal itself as the reference optical signal. In a so-called differential phase shift keying (DPSK) interferometer, a transmitted optical signal is split into two portions of equal amplitude, one portion is delayed relative to the other portion by a time delay corresponding to duration of one bit of information, and the two portions are combined to provide an optical interference signal. Referring to  FIG. 1A , a planar lightwave circuit (PLC) DPSK interferometer  100  of prior art includes a 1×2 waveguide splitter  102 , upper and lower waveguides  104  and  105 , respectively, and a 2×2 optical coupler  106 . A phase-modulated optical signal  108  is coupled to an input port  101  of the DPSK interferometer  100 . The optical signal  108  is split by the 1×2 waveguide splitter  102  into two portions  109  and  110  of equal amplitude, which propagate in the upper and lower waveguides  104  and  105 , respectively. Since the upper waveguide  104  is longer than the lower waveguide  105 , the portion  109  will be delayed relative to the portion  110 . The lengths difference of the upper and lower waveguides  104  and  105  is selected so as to delay the portion  109  by one bit duration of the phase-modulated signal  108 . The two portions  109  and  110  interfere with each other in the 2×2 optical coupler  106 . A differential photodetector pair  114  is coupled to output waveguides  111  and  112  of the 2×2 optical coupler  106  to detect an interference signal. 
     Turning now to  FIG. 1B , the optical signal portions  109  and  110  are illustrated by means of phase and amplitude time diagrams  121  and  122 ;  123  and  124 , respectively. The lower phase diagram  123  illustrates the optical signal portion  109 , which is the portion  110  delayed by one bit. An amplitude time diagram  125  illustrates time dependence of the interference signal&#39;s amplitude. When phases of the optical signal portions  109  and  110  are equal, the amplitude is equal to 1.0 due to constructive interference, and when the phases are opposite (that is, one is π, and the other one is 0), the amplitude is equal to 0 due to destructive interference. The amplitude-modulated signal  125  can be detected by the differential photodetector pair  114 . 
     By using four values of phase, 0, π/2, π, and 3π/2, one can further improve bandwidth utilization of optical phase modulation. This variety of DPSK modulation is termed differential quadrature phase shift keying (DQPSK) modulation. Referring to  FIG. 2 , a prior-art DQPSK interferometer  200  is shown. The DQPSK interferometer  200  includes the 1×2 splitter  102  coupled to two DPSK interferometers  201  and  202  having branch waveguides  204 ,  205 ,  206 , and  207  providing phase delays of 0, π/2, π, and 3π/2, respectively, the long branch waveguides  204  and  206  providing additional one-bit delays relative to the short branch waveguides  205  and  207 . Phase shifters, not shown, are used to fine tune phase delays in the branch waveguides  204 ,  205 ,  206 , and  207 . Two differential photodetector pairs  114  are used to detect the optical interference signals. 
     The PLC-based DPSK and DQPSK interferometers  100  and  200  share a common drawback of temperature dependence of the phase shifts, as well as polarization sensitivity. Complicated temperature control is usually required to achieve a reliable and stable operation of PLC devices. In U.S. Pat. No. 7,259,901 by Parsons et al., PLC interferometers of different branch length, one providing a delay slightly bigger than a bit delay, and the other providing a delay slightly smaller than the bit delay, are used to tune a PLC interferometer by uniformly varying temperature of the entire PLC chip. In US Patent Application Publication US2007/0177151 by Isomura et al., a separate heating elements and a highly conductive, thermally matched spacer are used to control temperature of the PLC chip with precision required for stable and reliable phase demodulation. 
     A free-space Michelson interferometer, which is not as sensitive to temperature as the PLC interferometers  100  and  200 , can be used for DPSK demodulation of optical signals. Referring now to  FIG. 3 , a Michelson interferometer  300  includes a half mirror  302  having 50% reflectance and first and second mirrors  304  and  306  spaced from the half-mirror  302  by distances L and L+cΔt/2, where Δt is bit duration and c is speed of light. A free space optical beam  301 , carrying the phase-modulated signal, impinges on the half-mirror  302 . Output interference signals  308  and  310  are detected by separate photodetectors, not shown. Michelson interferometer DPSK/DQPSK demodulators are known. By way of example, Michelson interferometer DPSK/QPSK demodulators have been disclosed in U.S. Pat. No. 7,411,725 by Suzuki et al. and in U.S. Pat. Nos. 7,489,874 and 7,526,210 by Liu. Detrimentally, Michelson interferometers of the prior art tend to be bulky and have a slow phase delay adjustment time as compared to their PLC counterparts. Furthermore, the prior-art PLC interferometers  100  and  200 , and the prior-art Michelson interferometer  300  art can only operate at a single fixed bit rate of a phase-modulated optical signal. A different interferometer is required to operate at a different bit rate. 
     It is a goal of the present invention to provide a DPSK/DQPSK interferometer that would combine a compact size, a good thermal stability, and quick phase delay adjustment time with an option to adjust or switch the bit delay for operation at different bit rates. 
     SUMMARY OF THE INVENTION 
     An optical interferometer of the invention for demodulating a differential phase shift keying (DPSK) optical signal includes a planar lightwave circuit (PLC) chip having a splitter, a coupler, and a phase adjuster integrally formed therein, and at least one free space delay line optically coupled to the PLC. In operation, the splitter splits the optical signal into equal portions, the phase adjuster adjusts the relative phase of the optical signal portions, and the free space delay line provides one-bit delay between the portions of the optical signal. The delayed signals are mixed in the PLC coupler. The free space delay line can be made variable to adjust the bit delay for operation at different bit rates, and/or for optimization of the interferometer performance during calibration and/or operation in the field. 
     In accordance with the invention there is provided an optical interferometer comprising: 
     a planar lightwave circuit including 
     an input port, first and second intermediate ports, and first and second output ports; 
     an optical waveguide splitter comprising an input waveguide coupled to the input port and first and second output waveguides, for splitting an input optical signal coupled to the input port and propagating in the input waveguide into first and second optical signals propagating in the first and second output waveguides of the splitter, respectively, wherein the second output waveguide of the splitter is coupled to the first intermediate port;
 
an optical waveguide coupler comprising first and second input waveguides and first and second output waveguides, wherein the first input waveguide of the coupler is coupled to the first output waveguide of the splitter, the second input waveguide of the coupler is coupled to the second intermediate port, and the first and second output waveguides of the coupler are coupled to the first and second output ports, respectively; and
 
a phase shifting element for generating a relative optical phase shift between optical signals propagating in the first and second input ports of the coupler; and
 
a first free space optical delay line coupled between the first and the second intermediate ports of the planar lightwave circuit, for delaying the second optical signal relative to the first optical signal by a delay time corresponding to a bit duration of the input optical signal.
 
     In one embodiment, the free space optical delay line is variable to accommodate different bit rates of the optical signal. Also in one embodiment, the planar lightwave circuit comprises third and fourth intermediate ports coupled to the first output waveguide of the splitter and the first input waveguide of the coupler, respectively. An interferometer of the latter embodiment further includes a second free space optical delay line coupled between the third and the fourth intermediate ports of the planar lightwave circuit, for delaying the first optical signal relative to the second optical signal. 
     In accordance with another aspect of the invention there is further provided an optical interferometer comprising: 
     a planar lightwave circuit including 
     an input port, first, second, third, and fourth intermediate ports, and first, second, third, and fourth output ports; 
     an optical waveguide splitter comprising an input waveguide coupled to the input port and first, second, third, and fourth output waveguides, for splitting an input optical signal coupled to the input port and propagating in the input waveguide into first, second, third, and fourth optical signals propagating in the first, second, third, and fourth output waveguides, respectively, of the splitter, wherein the fourth and the third output waveguides of the splitter are coupled to the first and the second intermediate ports, respectively;
 
an optical waveguide coupler comprising first, second, third, and fourth input waveguides and first, second, third, and fourth output waveguides,
 
wherein the first and the second output waveguides of the splitter are coupled to the third and the first input waveguides of the coupler, respectively;
 
wherein the fourth and the second input waveguides of the coupler are coupled to the third and the fourth intermediate ports, respectively; and
 
wherein the first, the second, the third, and the fourth output waveguides of the coupler are coupled to the first, the second, the third, and the fourth output ports, respectively; and
 
at least three phase shifting elements for generating relative optical phase shifts between the first and the second; the second and the third; and the third and the fourth optical signals propagating in the respective input ports of the coupler; and
 
a first free space optical delay line coupled between the first and the fourth intermediate ports; and between the second and the third intermediate ports of the planar lightwave circuit, for delaying the third and the fourth optical signals relative to the first and the second optical signals by a delay time corresponding to a bit duration of the input optical signal.
 
     In one embodiment, the free space optical delay line is variable to accommodate different bit rates. Also in one embodiment, the interferometer further comprises fifth, sixth, seventh, and eights intermediate ports coupled to the third and the first input waveguides of the coupler and the second and the first output waveguides of the splitter, respectively. An interferometer of the latter embodiment further includes a second free space optical delay line coupled between the fifth and the eighth; and the sixth and the seventh intermediate ports of the planar lightwave circuit, for delaying the first and the second optical signals relative to the third and the fourth optical signals. 
     In accordance with another aspect of the invention, there is further provided a method of differential phase shift keying demodulation of a first phase-modulated optical signal having a first bit duration, the method comprising: 
     (a) providing a planar waveguide circuit optically coupled to a free space delay line, the planar waveguide circuit having a waveguide splitter, a phase shifter, and a waveguide coupler integrally formed therein; 
     (b) receiving the first optical signal in the planar waveguide circuit; 
     (c) splitting the first optical signal into two parts of substantially equal magnitude using the waveguide splitter; 
     (d) adjusting relative optical phase of the two parts of the first optical signal using the phase shifter; 
     (e) delaying one of the two parts relative to the other of the two parts of the first optical signal by a time substantially equal to the first bit duration, using the free-space delay line; and 
     (f) upon completion of steps (b) through (e), coherently mixing the two parts of the first optical signal in the waveguide coupler. 
     In one embodiment, the method further includes steps of 
     (g) receiving a second phase-modulated optical signal in the planar waveguide circuit the second signal having a second bit duration different from the first bit duration; 
     (h) splitting the second optical signal into two parts of substantially equal magnitude using the waveguide splitter; 
     (i) adjusting relative optical phase of the two parts of the second optical signal using the phase shifter; 
     (j) adjusting the free-space delay line to provide a delay substantially equal to the second bit duration; 
     (k) upon completion of step (j), delaying one of the two parts relative to the other of the two parts of the second optical signal by a time substantially equal to the second bit duration, using the free-space delay line; and 
     (l) upon completion of steps (g) through (k), coherently mixing the two parts of the optical signal in the waveguide coupler. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments will now be described in conjunction with the drawings in which: 
         FIG. 1A  is a plan view of a prior-art planar lightwave circuit (PLC) differential phase shift keying (DPSK) interferometer; 
         FIG. 1B  shows a plurality of time traces of amplitude and phase of optical signals in the DPSK interferometer of  FIG. 1 ; 
         FIG. 2  is a plan view of a prior-art PLC differential quadrature phase shift keying (DQPSK) interferometer; 
         FIG. 3  is a plan view of a prior-art free-space Michelson interferometer suitable for DPSK demodulation; 
         FIGS. 4A to 4D  are schematic views of embodiments of DPSK/DQPSK interferometers of the invention including a waveguide loopback; a waveguide V-joint; two free-space delay lines; and a star coupler, respectively; 
         FIGS. 5A and 5B  are schematic views of embodiments of a symmetric DPSK/DQPSK interferometer of the invention; 
         FIG. 6  is a schematic view of an embodiment of a DQPSK interferometer of the invention; 
         FIG. 7  is a block diagram of a method of demodulation of a DPSK/DQPSK modulated optical signal according to the invention; 
         FIGS. 8A and 8B  are plan and side views, respectively, of the DQPSK interferometer of  FIG. 6 ; and 
         FIG. 9  is a three-dimensional rendering of a prototype of a DQPSK interferometer of  FIGS. 8A and 8B . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     While the present teachings are described in conjunction with various embodiments and examples, it is not intended that the present teachings be limited to such embodiments. On the contrary, the present teachings encompass various alternatives, modifications and equivalents, as will be appreciated by those of skill in the art. 
     Referring to  FIG. 4A , a differential phase-shift keying (DPSK) optical interferometer  400 A of the invention includes integrally formed planar lightwave circuit (PLC)  401 A and a free-space optical delay line  480  optically coupled to the PLC  401 A. The PLC  401 A includes an input port  411 , first and second intermediate ports  421  and  422 , first and second output ports  471  and  472 , an optical waveguide splitter  402 , an optical waveguide coupler  403 , and two phase shifting elements (heaters)  404 . The splitter  402  has an input waveguide  431 , coupled to the input port  411 , and first and second output waveguides  441  and  442 . The second output waveguide  442  of the splitter  402  is coupled to the first intermediate port  421 . The coupler  403  includes first and second input waveguides  451  and  452  and first and second output waveguides  461  and  462 . The first input waveguide  451  of the coupler  403  is coupled to the first output waveguide  441  of the splitter  402 . The second input waveguide  452  of the coupler  403  is coupled to the second intermediate port  422 . The first and second output waveguides  461  and  462  of the coupler  403  are coupled to the first and second output ports  471  and  472 , respectively. The free-space optical delay line  480  is coupled between the first and the second intermediate ports  421  and  422 , respectively, of the planar lightwave circuit  401 A. The phase shifting elements (heaters)  404  are disposed over the first and second input waveguides  451  and  452  of the coupler  403 . Other types of phase shifting elements known to a person skilled in the art can be used in place of the heaters  404 . 
     In the embodiment shown, the PLC  401 A includes an optional half-wave plate  406  disposed within an optional groove  405  in the PLC  401 A. A lens  407  couples optical signals to and from the free space optical delay line  480 , which includes a reflector  481 . The reflector  481  is movable by a translation stage  482  between positions  481 A and  481 B. An optional quarter-wave plate  483  is placed between the reflector  480  and the intermediate ports  421 ,  422 . The waveplates  406  and  483  interchange polarization of light propagating therethrough, which facilitates reduction of polarization sensitivity of the interferometer  400 A. The quarter-wave plate  483  interchanges polarizations because light passes through the quarter-wave plate  483  twice, before and after reflection from the mirror  481 . The quarter-wave plate  483  in a double-pass is equivalent to the half-wave plate  406  in a single-pass. 
     In operation, an input optical signal  490  coupled to the input port  411  propagates in the input waveguide  431  of the splitter  402 , which splits the input optical signal  490  into first and second optical signals  491  and  492  propagating in the first and second output waveguides  441  and  442 , respectively. The second optical signal  492  is delayed by the free space optical delay line  480  relative to the first optical signal  491  by a delay time corresponding to a bit duration of the input optical signal  490 . The delayed second optical signal  492  interferes with the first optical signal  491  in the coupler  403 , and the interference signal is detected by a differential pair of photodetectors  408  coupled to the output ports  471  and  472  of the PLC  401 A. To maintain DPSK functionality, the phase shifters  404  are operated to generate and maintain a relative optical phase shift between the first and second optical signals  491  and  492  of 180 degrees, or π. Only one phase shifting element  404  can be used for this purpose, but two are preferable for thermal management reasons. 
     In the embodiment shown, the free space optical delay line  480  is a variable delay line. At the position  481 A, the reflector  481  is at a distance d 1  from the PLC  401 A. The distance d 1  corresponds to a bit rate of, for example, 100 GBit/s having one bit duration of 10 ps. The translation stage  482  can be operated to bring the reflector  481  to the position  481 B at a distance d 2  from the PLC  401 A corresponding to a bit rate of 40 GBit/s having one bit duration of 25 ps. Thus, the adjustability of at least 15 ps, corresponding to free space distance difference d 2 −d 1  of at least 2.25 mm, is required to ensure that the interferometer  400 A can be used for demodulation of DPSK modulated optical signals at both 100 GBit/s and 40 GBit/s bit rates. 
     In applications where the bit rate is constant, the free space optical delay line  480  can be a fixed (albeit preferably, one-time adjustable during calibration) delay line. It is advantageous to have a free space delay line even when it is fixed, because it reduces overall size, as well as thermal and polarization sensitivity of the interferometer  400 A. Advantageously, the magnitude of the delay of the free space optical delay line  480  can be set to slightly deviate from one bit duration, which has been found by the inventors to further improve stability of DPSK demodulation. 
     Any element having optical power, that is, a capability to focus light, can be used in place of the lens  407 . Furthermore, micro-collimators or microlenses, not shown, can be separately coupled to the first and second intermediate ports  421  and  422  of the PLC  401 A to collimate/focus beams of light out of the second intermediate port  422  and into the first intermediate port  421 . The movable reflector  481  can include a metal or dielectric mirror, a retro-reflecting cube, a Porro prism, etc. The heaters  404  can be disposed for selectively heating at least one of: the first  441  and second  442  output waveguides of the splitter  402  and the first  451  and second  452  input waveguides of the coupler  403 . 
     Referring now to  FIG. 4B , a DPSK optical interferometer  400 B is similar to the DPSK optical interferometer  400 A of  FIG. 4A , one difference being that the first output waveguide  441  of the splitter  402  and the first input waveguide  451  of the coupler  403  form a V-joint  485 . A planar lightwave circuit  401 B comprises a third intermediate port  423  at the tip of the V-joint. A reflector  486  is disposed at the third intermediate port  423 . The reflector  486  optically couples the first output waveguide  441  of the splitter  402  to the first input waveguide  451  of the coupler  403 . An optional quarter-wave plate  484  rotates polarization of the first optical signal  491  as it propagates twice through the quarter-wave plate  484 , for reduction of polarization sensitivity. Advantageously, the V-joint  485  allows one to avoid having a waveguide loop  489  in  FIG. 4A , which makes the PLC  401 B of  FIG. 4B  more compact than the PLC  401 A of  FIG. 4A . The distances d 1  and d 2  between the reflector  481  and the PLC  401 B in  FIG. 4B  are also reduced as compared to the distances d 1  and d 2  in  FIG. 4A . 
     Turning now to  FIG. 4C , a DPSK optical interferometer  400 C is similar to the DPSK optical interferometer  400 A of  FIG. 4A , one difference being that a planar lightwave circuit  401 C comprises the third intermediate port  423  coupled to the first output waveguide  441  of the splitter  402 , and a fourth intermediate port  424  coupled to the first input waveguide  451  of the coupler  403 . The interferometer  400 C further includes a second lens  407  and a second, fixed free space optical delay line  488  coupled between the third and the fourth intermediate ports  423  and  424 , for delaying the first optical signal  491  relative to the second optical signal  492 . Advantageously, having two free space optical delay lines  480  and  488  allows a further reduction of size of the interferometer  400 C and thermal sensitivity of the optical path length, because thermal dependences of optical path lengths of the two free space optical delay lines  480  and  488  tend to compensate one another. 
     Referring to  FIG. 4D , a DQPSK optical interferometer  400 D is similar to the DPSK optical interferometer  400 A of  FIG. 4A , the difference being that a planar lightwave circuit  401 D further includes third and fourth output ports  473  and  474 , respectively, and the coupler  403  is a star coupler further comprising third and fourth output waveguides  463  and  464  coupled to the third and fourth output ports  473  and  474 , respectively. Photodetectors  498  are coupled to the four output ports  471  to  474 . Optical interference of the first and second optical signals  491  and  492  in the star coupler  403  causes the output signal to be predominantly sent to one of the output waveguides  461  to  464  in dependence on phase difference between the first and second optical signals  491  and  492 . The output signal is detected by four photodetectors  498  coupled, one by one, to the four output ports  471  to  474 . The four photodetectors  498  can be grouped into differential pairs. 
     Turning now to  FIG. 5A , a DPSK optical interferometer  500 A is similar to the DPSK optical interferometer  400 C of  FIG. 4C , one difference being that a planar lightwave circuit  501 A has a symmetry axis  502 , and elements of the following pairs: the splitter  402  and the coupler  403 ; the first output waveguide  441  of the splitter  402  and the second output waveguide  452  of the coupler  403 ; the second output waveguide  442  of the splitter  402  and the first output waveguide  451  of the coupler  403 ; the second and the fourth intermediate ports  422  and  424 ; and the first and the third intermediate ports  421  and  423  are disposed symmetrically with respect to the symmetry axis  502 . The symmetrical placement of waveguides and couplers results in a reduced sensitivity of the interferometer  500 A to thermal gradients and mechanical stress patterns, since the thermal gradients and mechanical stress patterns tend to be symmetrical with respect to the axis of symmetry  502  of the PLC  501 A. 
     Since the optical path lengths of the first and the second optical signals  491  and  492  within the PLC  501 A are equal, the one-bit optical path difference is generated solely in the free space delay lines  480  and  488 . Therefore, 
     
       
         
           
             
               
                 
                   
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     wherein Δt 1  and Δt 2  are one-bit delays for two different bit rates, c is speed of light in vacuum and n is the refractive index of air. 
     The refractive index of air exhibits temperature dependence of about −1 ppm/° C. Over distances of few millimeters of the light travel in air, the air refractive index temperature dependence can create a noticeable phase error. For example, for d 3 −d 1  of 1.171 mm, the phase error is about 9 degrees per 60° C. temperature change. To compensate for such an error, the mirror  481  of the lower delay line  480  can be mounted to a ring  504 , which is mounted to a post  503  extending from the translation stage  482 . The light propagates through an opening  505  in the post  503  and the ring  504 , reflects from the mirror  481 , and propagates back. The thermal expansion coefficient of the ring  504  is selected so as to compensate for optical path length variation with temperature due to thermal dependence of refractive index of air or another gas in which the free space optical delay line  480  is disposed. Thermal expansion or contraction of the ring  504  causes the reflector  481  to shift away from or towards the intermediate ports  421 ,  422 , thereby compensating for thermal dependence of refractive index of air or another filling gas. This mounting arrangement can also be used to mount the mirror  481  in the interferometers  400 A to  400 D. 
     Turning now to  FIG. 5B , a DQPSK optical interferometer  500 B is similar to the DPSK optical interferometer  500 A of  FIG. 5A , the difference being that a planar lightwave circuit  501 B further includes third and fourth output ports  473  and  474 , respectively, and the coupler  403  is a star coupler further comprising third and fourth output waveguides  463  and  464  coupled to the third and fourth output ports  473  and  474 , respectively. The photodetectors  498  are coupled to the four output ports  471  to  474 . Optical interference of the first and second optical signals  491  and  492  in the star coupler  403  causes the output signal to be predominantly sent to one of the output waveguides  461  to  464  in dependence on phase difference between the first and second optical signals  491  and  492 . The output signal is detected by four photodetectors  498  coupled, one by one, to the four output ports  471  to  474 . The four photodetectors  498  can be grouped into differential pairs. 
     Referring to  FIG. 6 , a differential quadrature phase-shift keying (DQPSK) optical interferometer  600  of the invention includes a PLC  601 , the variable free-space optical delay line  480  coupled to the PLC  601 , and the fixed free-space optical delay line  488  coupled to the PLC  601 . The free-space optical delay lines  480  and  488  are similar to those of interferometers  400 C,  500 A, and  500 B of  FIGS. 4C ,  5 A, and  5 B, respectively. The PLC  601  includes an input port  611 , first to eighth intermediate ports  621  to  628 , respectively, first to fourth output ports  671  to  674 , respectively, an 1×4 optical waveguide splitter  602  including three 1×2 splitters  402  connected with an s-bent waveguide  610  and a straight waveguide  620 , a 4×4 optical waveguide coupler  603  including two 2×2 couplers  403 , and four phase shifting elements (heaters)  404 . The splitter  602  has an input waveguide  631  coupled to the input port  611  and first to fourth output waveguides  641  to  644 , respectively, for splitting an input optical signal  690  coupled to the input port  611  and propagating in the input waveguide  631  into first to fourth optical signals  691  to  694 , propagating in the first to fourth output waveguides  641  to  644 , respectively, of the splitter  602 . The fourth output waveguide  644  of the splitter  602  is coupled to the first intermediate port  621 , and the third output waveguide  643  of the splitter  602  is coupled to the second intermediate port  622 . The coupler  603  has first to fourth input waveguides  651  to  654  and first to fourth output waveguides  661  to  664 , respectively. The first and the second output waveguides  641  and  642  of the splitter  602  are coupled to the third and the first input waveguides  653  and  651  of the coupler  603 , respectively, through the fixed free space delay line  488  coupled between fifth and eight; and sixth and seventh intermediate ports  625  and  628 ;  626  and  627 , respectively. The first to fourth output waveguides  661  to  664  of the coupler  603  terminate in the first to fourth output ports  671  to  674 , respectively. The fourth  654  and the second  652  input waveguides of the coupler  603  are coupled to the third  623  and the fourth  624  intermediate ports, respectively. The variable free space optical delay line  480  is coupled between the first and the fourth intermediate ports  621  and  624 , respectively; and between the second and the third intermediate ports  622  and  623 , respectively, of the planar lightwave circuit  601 . 
     In operation, the third and the fourth optical signals  693  and  694 , respectively, are delayed relative to the first and the second optical signals  691  and  692 , respectively, by a delay time corresponding to a bit duration Δt of the input optical signal  690 . In the embodiment shown, the free space delay lines  480  and  488  are substantially of a same length, the delay being generated mostly by the length difference between S-bent waveguide  610  and the straight waveguide  620 . One advantage of having the free space delay lines  480  and  488  of a same length is that thermal dependence of optical path length due to variation of refractive index of air is compensated. However, when the mirror  481  of the variable free space delay line  480  is moved by the translation stage  482 , the lengths of the free space delay lines  480  and  488  will no longer be equal, so that some form of thermal compensation of optical path length may be required. In another embodiment, the free space delay lines  480  and  488  are always of different length. The thermal compensation can include adjusting phase of the phase shifters  404  and/or using the ring  504 , mounted to a post  503  fixed to the translation stage  482 , as shown in  FIG. 5A  for the case of the interferometer  500 A. 
     Still referring to  FIG. 6 , although four phase shifters  404  are shown, only three can be used for generating relative optical phase shifts between the first  691  and the second  692 ; the second  692  and the third  693 ; and the third  693  and the fourth  694  optical signals propagating in the respective input ports  651  to  654  of the coupler  603 . The coupler  603  can be made in form of a 4×4 star coupler. Also, the splitter  602  can include a single 1×4 splitter in place of three 1×2 splitters  402  coupled with the S-shaped waveguide  610  and the straight waveguide  620 . Waveguide loopback and/or a V-joint can be used in place of the fixed free space delay line  488 , similarly to those shown in  FIGS. 4A and 4B . When the waveguide loopback is used, the PLC  601  has only four intermediate ports  621  to  624 . The free space delay line  480  can be a fixed delay line, although the variable delay line is preferable, because it allows demodulation of signals at different bit rates as explained above. Furthermore, the waveguides/ports of the interferometer  600  can be made symmetrical, similar to the interferometers  500 A and  500 B of  FIGS. 5A and 5B , respectively. 
     Turning now to  FIG. 7 , a method of differential phase shift keying demodulation of a phase-modulated optical signal is shown. The method can be used with the demodulator  400 A of  FIG. 4A  including the planar waveguide circuit  401 A optically coupled to the free-space delay line  480 , the planar waveguide circuit  401 A having the waveguide splitter  402 , the phase shifters  404 , and the waveguide coupler  403  integrally formed therein as described above. In a step  702 , a first optical signal having a first bit duration, represented by the phase-modulated optical signal  490 , is received at the input port  411  of the demodulator  400 A. In a step  704 , the first optical signal  490  is split by the waveguide splitter  402  into the first and second portions (signals)  491  and  492  having substantially equal magnitude. In a step  706 , relative phase of the two portions  491  and  492  of the first signal  490  is adjusted by operating at least one of the phase shifters  404 . In a step  708 , the second portion  492  is delayed relative to the first portion  491  in the free-space delay line  480  by a time substantially equal to the first bit duration. The steps  704  to  708  may be performed in a different order or simultaneously. Then, in a step  710 , the two portions of the first optical signal are coherently mixed in the waveguide coupler  403 . 
     Advantageously, the demodulator  400 A of  FIG. 4A  allows demodulation of optical signals having differing bit durations. Still referring to  FIG. 7 , in a step  712 , a second optical signal having a second bit duration different from the first bit duration is received at the input port  411  of the PLC  601 . In a step  714 , the second optical signal is split by the splitter  402  into two portions of equal magnitude. In a step  716 , relative optical phase of the two portions is adjusted by operating at least one of the phase shifters  404 . In a step  718 , the free-space delay line  480  is adjusted to provide a delay substantially equal to the second bit duration. In a step  720 , one of the two portions of the second optical signal is delayed relative to the other of the two portions of the second optical signal by a time substantially equal to the second bit duration, using the free-space delay line  480 . Steps  714  to  720  may be performed in a different order or simultaneously. Then, in a step  722 , the two portions are coherently mixed in the waveguide coupler  403 . 
     The method of  FIG. 7  is applicable for operation with any of the interferometers  400 B to  400 D,  500 A,  500 B, and  600  of  FIGS. 4B to 4D ,  5 A  5 B, and  6 , respectively. For the case of the DQPSK interferometer  600  of  FIG. 6 , the input optical signal  690  is split not in two but in four portions  691  to  694 , respectively, which are similarly processed. One might envision splitting the input signal  690  into a different number of portions, depending on the couplers used and the modulation scheme involved. 
     Referring now to  FIGS. 8A and 8B , a DQPSK interferometer  800  is shown. The DQPSK interferometer  800  is a mounted version of the DQPSK interferometer  600  of  FIG. 6 . In the DQPSK interferometer  800  of  FIGS. 8A and 8B , the PLC  601  is mounted on a ceramic plate  802  with a thermal epoxy  804 . The ceramic plate  802  includes a heater and a thermal sensor, not shown, for maintaining the ceramic plate  802  at a steady temperature. Gradient index (GRIN) lenses  807 , corresponding to the lenses  607  of  FIG. 6 , are mounted in mounts  805 . The mounts  805  are affixed, for example soldered or epoxied, to a Pyrex™ plate  806  mounted on the PLC  601 . The top mirror  481  in  FIG. 8A  is a fixed mirror, and the bottom mirror  481  is a movable mirror mounted to a miniature piezoelectric motor-driven translation stage  482 . Of course, various types of translation stages, motors, and lenses can be used, as appreciated by those of skill in the art. 
     Turning to  FIG. 9 , a prototype  900  of the mounted DQPSK interferometer  800  has a bracket-like package  902  on which the ceramic plate  802  is mounted. The translation stage  482  is mounted to the package  902  with a pair of screws  905 . Springs  907  and a pusher  908  are used to prevent wobble of the stage  482 , for angular stability of the movable reflector  481 . A fiber array  904  is attached to the PLC  601  using the Pyrex plates  806 . The fiber array  904  couples the input port  611  seen in  FIGS. 6 and 8A  to a source of a modulated optical signal, not shown, and the output ports  671  to  674  to photodetectors, not shown in  FIG. 9 .