Abstract:
An optical demodulator and accompanying method(s) that demodulates a DQPSK signal employing a single optical delay interferometer comprising a free-space Michelson interferometer having two optical paths, connected to a 1×2 coupler. Positioned within an arm of the Michelson interferometer is a phase shifter that produces a phase difference of π/2 between the two paths. The resulting demodulator is compact, reliable, and may be constructed to be substantially immune from undesirable thermal sensitivities.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates generally to the field of optical communications and in particular to methods and apparatus for demodulating and receiving optical signals having Differential-Quadrature-Phase-Shift-Keying (DQPSK) format(s).  
       BACKGROUND OF THE INVENTION  
       [0002]     Optical DQPSK is a promising modulation format that is attracting considerable commercial attention as a result of its high receiver sensitivity, high spectral efficiency (SE), high filtering and dispersion tolerance(s). Of particular interest, DQPSK may be used in combination with amplitude modulation to achieve even higher spectral efficiencies.  
         [0003]     In optical DQPSK transmission, data is conveyed by an optical phase difference between adjacent bits. In order to detect the data contained within a DQPSK transmission, an optical demodulator is used to convert the phase-coded signal into intensity-coded signals. Typically, such optical demodulators are constructed from a pair of optical delay interferometers (ODIs).  
         [0004]     Unfortunately, contemporary optical demodulators so constructed are quite complex, requiring precise control of the absolute phase difference between the two arms of each of the two ODIs, and precise length matching among the multiple optical paths prior to any data recovery circuits. In addition, conventional ODIs are fiber-based or planar-waveguide-based, which are temperature sensitive and therefore require precise temperature control and stabilization, particularly when employed in high performance optical transmission systems.  
       SUMMARY OF THE INVENTION  
       [0005]     I have developed an optical demodulator that, together with accompanying method(s), demodulates a DQPSK signal without exhibiting the infirmities that plague the prior art. More particularly, this inventive optical demodulator and method employs a single optical delay interferometer comprising a free-space Michelson interferometer having two optical paths, connected to a 1×2 coupler. Positioned within an arm of the Michelson interferometer is a phase shifter that produces a phase difference of π/2 between the two paths.  
         [0006]     This innovative demodulator construction—from a single free-space Michelson interferometer—results in a demodulator that is compact, reliable, and may be constructed to be substantially immune from undesirable thermal sensitivities. 
     
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0007]     A more complete understanding of the present invention may be realized by reference to the accompanying drawing in which:  
         [0008]      FIG. 1  is a schematic of a generalized, PRIOR ART DQPSK receiver having two optical delay interferometers for demodulation;  
         [0009]      FIG. 2  is a schematic of a DQPSK demodulator according to the present invention; and  
         [0010]      FIG. 3  is a flowchart depicting the inventive method according to the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0011]     With initial reference to  FIG. 1 , there is shown a generalized, PRIOR ART optical DQPSK demodulator  100 . With such a PRIOR ART optical demodulator, an optical DQPSK signal  110  having 2 bit/symbol say, is amplified through the effect of an optical amplifier  120 , the output of which is subsequently filtered by an optical filter  130  and then split by 1×2 optical coupler/splitter  140 .  
         [0012]     Since a DQPSK signal comprises two tributaries, the 1×2 split of the optical coupler  140  is necessary to provide signal(s) to the two optical delay interferometers (ODIs)  150 ,  152  each including a delay loop  155 ,  156  and a phase shifter  157 ,  158 , respectively. (Note that the phase shift in this exemplary discussion is shown as +π/4 and −π/4). As noted before, these two phase shifts have to be precisely controlled and maintained. More specifically, for 40-Gb/s DQPSK systems, the free spectral range (FSR) of the ODI is approximately 20 GHz. The tolerance to the frequency mismatch resulting from non-perfect phase shifts is less or about ±0.5 GHz. Additionally, the fiber-based or planar-waveguide based ODIs usually exhibit a temperature sensitivity of about 1 GHz/° C., so the temperature of the ODIs has to be controlled and maintained within less or about 0.5 ° C., which is quite demanding.  
         [0013]     Continuing with our discussion of the PRIOR ART apparatus shown in  FIG. 1 , optical signals output from the ODIs  150 ,  152  are received by balanced detectors  160 , 162 , the output of which is provided to clock and data recovery circuitry  170 ,  172 . As can be readily appreciated at this point, each of the “branches” of the PRIOR ART demodulator  100  permits the extraction of the two tributaries comprising the DQPSK signal by the clock data recovery circuitry  170 ,  172 , respectively. As also noted before however, the four optical paths starting from the optical coupler  140  and ending at the four detectors situated in the two balanced detectors  160  and  162  have to have essentially the same length. In addition, the electrical path length between the balanced detector  160  and the clock and data recovery circuitry  170  has to be essentially equal to that between the other balanced detector  162  and its respective clock and data recovery circuitry  172 . More specifically, for 40-Gb/s DQPSK systems, the bit period is 50 ps. Consequently, the tolerance to delay mismatch resulting from unequal path lengths is only about 10% of the bit period or 5 ps, which translates into only about 1 mm in length in optical fiber! 
         [0014]     As noted before and as can be readily appreciated, such a PRIOR ART implementation is quite susceptible to variations in temperature, and any temperature variations that may exist between the two ODIs  150  and  152 . As a result, in order to provide such temperature control and stabilization, additional performance monitoring and feedback control components are required which unfortunately, adds to the complexity and cost of such PRIOR ART implementations.  
         [0015]     Turning now to  FIG. 2 , there is shown a schematic of a DQPSK demodulator  200  constructed according to the inventive teachings of the instant application. As can be readily observed from that  FIG. 2 , this inventive demodulator  200  uses a single ODI—based on a free-space Michelson interferometer comprising a beam splitter  220  and two reflectors (mirrors)  230  and  240 —the reflectors being positioned substantially perpendicular to the plane formed by the signal optical paths. This arrangement results in two distinct optical paths, each having a characteristic path length of L and L+ΔL, respectively. The path length difference ΔL is such that the resulting delay is about a bit period of the signal. For 40-Gb/s DQPSK, ΔL is about 15 mm in free-space. If we use a FSR of 25 GHz so that the ODI can be used for multiple wavelength channels that are on the ITU 50-GHz channel grid, ΔL is about 12 mm in free-space.  
         [0016]     The first optical path having a characteristic path length of L includes those paths between optical splitter  220  and reflector  240 . The second optical path having a characteristic path length of L+ΔL includes those paths between the optical splitter  220  and reflector  230 . In addition, and as shown in this  FIG. 2 , one of the optical paths (in this example, the second optical path) may include a π/2 phase shifter  280 , and/or a thermal/athermal waveplate  270 , which may advantageously be coupled or otherwise combined with the phase shifter  280 .  
         [0017]     A single DQPSK signal having 2 bits/symbol is split into two optical signals ( 215 ,  217 ) through the effect of a 1×2 optical coupler  210  (e.g., a 3 dB coupler). The optical coupler  210  splits the single DQPSK signal light into two separate signals,  215 ,  217 , each exhibiting substantially equal power(s). These two split signals  215 ,  217  are directed into the interferometer where portions traverse the two optical paths.  
         [0018]     More specifically, the split optical signal  215  strikes the beam splitter  220  (Point A) where it is further split. A first portion of that further split signal  215  is directed to reflector  240  (Point E) where it is reflected back to beam splitter  220  (Point C). This path, defined by the round trip between the beam splitter  220  and reflector  240 , exhibits a path length of L.  
         [0019]     It should be noted that reflectors (mirrors)  240 , and  230 , preferably have a reflectivity of essentially 100%.  
         [0020]     A second portion of that further split signal  215  is directed to another reflector  230  (Point G) from which it is reflected back along an optical path to beam splitter  220  (Point C). This second optical path, defined by the round trip between the beam splitter  220  and reflector  230 , exhibits a path length of L+ΔL. Upon striking Point C, the two split signals interfere with each other both constructively- and destructively. Without losing generality, the constructive interference component emits from Point C and is directed to a first detector  250 , while the destructive interference component emits from Point C and is directed to a second detector  260 . The difference between the signals received by the detectors  250  and  260 , which can be obtained through a differential amplifier situated inside a differential amplification unit  290 , is then used to recover the first-tributary of the original DQPSK signal.  
         [0021]     Similarly, the split optical signal  217  strikes the beam splitter  220  (Point B) where it is further split. A first portion of that further split signal  217  is directed to reflector  240  (Point F) where it is reflected back to beam splitter  220  (Point D). This path exhibits a path length of L.  
         [0022]     A second portion of that further split signal  217  is directed to another mirror  230  (Point H) from which it is reflected back along an optical path to beam splitter  220  (Point D). This second optical path exhibits a path length of L+ΔL. Upon striking Point D, the two split signals interfere with each other both constructively and destructively. Without losing generality, the constructive interference component emits from Point D and is directed to a third detector  255 , and the destructive interference component emits from Point D and is directed to a fourth detector  265 . The difference between the signals received by the detectors  255  and  265 , which can be obtained through another differential amplifier inside the differential amplification unit  290 , is then used to recover the second-tributary of the original DQPSK signal.  
         [0023]     Shown further in that  FIG. 2 , is a π/2 phase shifter  280  interposed in the optical path traversed by optical signal  217 , and having a path length of L+ΔL. This π/2 phase shifter  280  introduces an optical phase delay of π/2 between path A-G-C and path B-H-D. Those skilled in the art will quickly recognize that such a phase shifter may be implemented through the application of a suitable thin-film coating, applied to a suitable transparent substrate  270  or the mirror  230 . The phase shifter can also be interposed in the optical path traversed by optical signal  215 , and having a path length of L. Note that not shown in  FIG. 2  are precise phase controls that ensure that a +π/4 (or −π/4) phase shift between the path A-E-C and A-G-C, and a −π/4 (or +π/4) phase shift between the path B-F-D and B-H-D at the signal center frequency.  
         [0024]     From this  FIG. 2 , it should be readily apparent to those skilled in the art that the inventive DQPSK demodulator allows the beam splitter, reflectors/mirror(s), and an entire optical package so constructed to be shared by two tributaries. In addition, the use of the π/2 phase shifter ensures that the two tributaries are also aligned correctly with respect to each other, essentially independent of changes in laser frequency and ambient temperature. Accordingly, this inventive design permits the construction of a compact, yet highly reliable demodulator.  
         [0025]     In those instances where source laser frequency is locked with sufficient precision, this inventive demodulator may be made athermal and passive, thereby permitting the DQPSK tributaries to be received without any monitoring and feedback control. The athermal operation of the ODI can be achieved by fixing the free-space path length using an athermal material, so no temperature stabilization is required.  
         [0026]     Alternatively, if an adjustable demodulator is desired—for tracking the laser frequency drift, say—a temperature sensitive waveplate  270  may be interposed along an optical path. Shown in the  FIG. 2  is a thermal/athermal waveplate  270 , positioned in the optical path taken by optical signal  217 , and having a path length of L+ΔL. For design and or construction convenience, the waveplate  270  may be combined with phase shifter  280 .  
         [0027]     Of further advantage—because the size of beam splitter  220  may be much larger than the beam size of the optical signals  215 ,  217 , the four detectors may be optically coupled directly to the beam splitter  220  with, for example, fiber-coupled lenses  253 ,  257 ,  263 ,  267 . The fiber connections can be made having matched length(s) so that not any additional fiber or other coupling mechanism(s) are needed. As a result, demodulators constructed according to the inventive teachings of the present application exhibit low loss and permit a more compact design while, at the same time enhancing the manufacturability and reliability.  
         [0028]     Finally with reference to  FIG. 2 ., the detector outputs are appropriately subtracted to obtain the differences between the detected constructive interference signals and the detected destructive interference signals. This is performed by the differential amplification unit  290 . The results, after clock and data recovery, recover the two tributaries of the original DQPSK signal.  
         [0029]     Turning now to  FIG. 3 , there is shown a flowchart which depicts an overview of the inventive method. As indicated by Block  310  of that  FIG. 3 , a DQPSK signal is split into two signals exhibiting substantially equal power. These two signals are then introduced into a Michelson interferometer where they strike a beam splitter and are further split into two, sub-signals each (Block  320 ).  
         [0030]     The sub-signals that are split from the same equal power signal(s) traverse two different paths within the Michelson interferometer, wherein each of the paths have a different length (Block  330 ).  
         [0031]     The light emission due to the constructive interference of the two sub-signals is directed to a first detector, while the light emission due to the destructive interference of the two sub-signals is sent to a second detector. (Block  340 ). One tributary of the original DQPSK signal is determined from the difference between these two detected signals (Block  350 ).  
         [0032]     At this point, while we have discussed and described our invention using some specific examples, those skilled in the art will recognize that our teachings are not so limited. Accordingly, our invention should be only limited by the scope of the claims attached hereto.