Patent Publication Number: US-7593646-B2

Title: Upgraded optical communication system with increased transmission capacity and method

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This Application is a divisional application of U.S. application Ser. No. 10/394,921, filed Mar. 22, 2003 and entitled “Upgraded Optical Communication System with Increased Transmission Capacity and Method.” 

   TECHNICAL FIELD 
   The present invention relates generally to optical communication networks and, more particularly, to an upgraded optical communication system with increased transmission capacity and method. 
   BACKGROUND 
   Telecommunications systems, cable television systems, and data communication networks use optical networks to rapidly convey large amounts of information between remote points. In an optical network, information is conveyed in the form of optical signals through optical fibers. Optical fibers comprise thin strands of glass capable of transmitting the signals over long distances with very little loss. The optical signals have at least one characteristic modulated to encode audio, video, textual, real-time, non-real-time and/or other suitable data. 
   In an optical network, transmitter and receiver optical components as well as the transmission fiber and amplifiers are selected, configured and positioned for predefined transmission rates. In addition, node electronics are configured to operate at the predefined rate. Upgrading of the network to increase transmission rates, and thus capacity, typically includes replacing, reconfiguring and/or repositioning optical components in the node and fiber as well as replacing the node electronics. 
   SUMMARY 
   An upgraded optical communication system with increased transmission capacity and method are provided. In one embodiment, conventional transmitters and receivers of a wavelength division multiplexed (WDM) network may be replaced with return-to-zero M-ary phase shift keying (RZ-mPSK) transmitters and receivers to increase transmission capacity without upgrading other optical components. 
   More specifically, in accordance with a particular embodiment of the present invention, a method for upgrading a WDM optical communication system includes replacing an installed transmitter operable to transmit a data stream at a defined bit rate with an RZ-mPSK transmitter operable to transmit the data stream at the defined bit rate and at least one other data stream at the defined bit rate together in an RZ-mPSK signal having a combined bit rate at least double the defined bit rate and a symbol rate equal to the defined bit rate. An installed receiver operable to receive the data stream at the defined bit rate is replaced with an RZ-mPSK receiver operable to recover the data stream at the defined bit rate and the at least one other data stream at the defined bit rate from the RZ-mPSK signal. 
   Technical advantages of one or more embodiments of the system and method may include doubling or otherwise increasing the transmission capacity of an optical channel by replacing non return-to-zero (NRZ) based or other suitable installed transmitter and receiver, or transponders, with RZ-mPSK-based transmitter and receiver, or transponders. Other advantages of one or more embodiments may include maintaining the same speed electronics and optical components designed and installed for the lower transmission capacity transponders. For example, existing multiplexer, demultiplexer, optical filters, and optical amplifiers as well as polarization mode dispersion and chromatic dispersion control components designed and installed for NRZ based transponders may be used by the higher rate RZ-mPSK-based transponders. In addition, transmission distances and fiber may be maintained. Thus, lower equipment and operational cost per bit rate may be provided as well as faster time to implement an upgrade. 
   It will be understood that none, some, or all embodiments may include the above enumerated technical advantages. It will be further understood that the method and system may include other technical advantages that will be apparent from the following figures, descriptions, and claims. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  illustrates one embodiment of an optical communication system; 
       FIG. 2  illustrates one embodiment of an upgraded optical transmitter for the optical communication system of  FIG. 1 ; 
       FIG. 3  illustrates one embodiment of an upgraded optical receiver for the optical communication system of  FIG. 1 ; 
       FIG. 4  illustrates one embodiment of a method for upgrading the optical communication system of  FIG. 1 ; 
       FIG. 5  illustrates one embodiment of a method for transmitting a signal in the upgraded optical communication system of  FIG. 1 ; and 
       FIG. 6  illustrates one embodiment of a method for receiving a signal in the upgraded optical communication system of  FIG. 1 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  illustrates an optical communication system  10  in accordance with one embodiment of the present invention. In this embodiment, the optical communication system  10  is a wavelength division multiplexed (WDM) system such as a dense WDM (DWDM) system in which a number of optical channels are carried over a common path at disparate wavelengths. It will be understood that the optical communication system  10  may comprise other suitable single channel, multichannel or bi-directional transmission systems. 
   Referring to  FIG. 1 , the WDM system  10  includes a WDM transmitter  12  at a source end point or node and a WDM receiver  14  at a destination end point or node coupled together by an optical link  16 . The WDM transmitter  12  and WDM receiver  14  each comprise a card shelf or other module including transmitters, receivers, a switch fabric and control. The WDM transmitter  12  transmits data in a plurality of optical signals, or channels, over the optical link  16  to the remotely located WDM receiver  14 . The WDM transmitter  12 , WDM receiver  14  and optical link  16  may form part of a long-haul, metro ring, metro core or other suitable network or combination of networks. 
   The WDM transmitter  12  includes a plurality of optical transmitters  20  and a WDM multiplexer  22 . The optical transmitters  20  may form part of a transponder or other node element. Each optical transmitter  20  generates an optical information signal  24  on one of a set of distinct wavelengths λ 1 , λ 2  . . . λ n  at a certain channel spacing. For example, in a particular embodiment, channel spacing may be 100 Gigahertz (GHz). The channel spacing may be selected to avoid or minimize crosstalk between adjacent channels. The optical information signals  24  comprise optical signals with at least one characteristic modulated to encode audio, video, textual, real-time, non-real-time or other suitable data. The optical information signals  24  are multiplexed into a single WDM signal  26  by the WDM multiplexer  22  for transmission on the optical link  16 . The optical information signals  24  may be otherwise suitably combined into the WDM signal  26 . 
   The WDM receiver  14  receives, separates and decodes the optical information signals  24  to recover the included data. In one embodiment, the WDM receiver  14  includes a WDM demultiplexer  30  and a plurality of optical receivers  32 . Each optical receiver  32  may be coupled to the demultiplexer  30  through a polarization mode dispersion compensator (PMDC)  34  and a tunable chromatic dispersion compensator (TDC)  36 . The optical receiver  32  may form part of a transponder or other node element. 
   The WDM demultiplexer  30  demultiplexes the optical information signals  24  from the single WDM signal  26  and sends each optical information signal  24  to a corresponding PMDC  34 , TDC  36  and optical receiver  32 . Each PMDC  34  compensates for polarization mode dispersion. Each TDC  36  compensates for chromatic dispersion. Each optical receiver  32  optically or electrically recovers the encoded data from the corresponding signal  24 . As used herein, the term each means every one of at least a subset of the identified items. 
   The optical link  16  comprises optical fiber  40  or other suitable medium in which optical signals may be transmitted with low loss. In one embodiment, the optical fiber  40  may comprise SMF fiber. Interposed along the optical link  16  are one or more optical amplifiers  42 . The optical amplifiers  42  increase the strength, or boost, one or more of the optical information signals  24 , and thus the WDM signal  26 , without the need for optical-to-electrical conversion. 
   In one embodiment, the optical amplifiers  42  comprise discrete amplifiers  44  and distributed amplifiers  46 . The discrete amplifiers  44  comprise rare earth doped fiber amplifiers, such as Erbium doped fiber amplifiers (EDFAs), and other suitable amplifiers operable to amplify the WDM signal  26  at a point in the optical link  16 . The optical amplifiers  2  may include dispersion compensation fiber (DCF)  50  to provide in-line dispersion compensation control. 
   The distributed amplifiers  46  amplify the WDM signal  26  along an extended length of the optical link  16 . In one embodiment, the distributed amplifier  46  comprises a distributed Raman amplifier (DRA). The DRA  46  may include one or more backward, or counter-pumping source lasers  52  coupled to the optical link  16 . The amplification signal from the counter-pumping laser  52  is launched in a direction of travel opposite that of the WDM signal  26  and thus is counter-propagated with respect to the WDM signal  26 . The DRA  46  may include one or more forward, or co-propagating pumping source lasers coupled to the optical link  16 . The amplification signal from the forward-pumping laser is launched in the same direction of travel as the WDM signal  26  and thus is co-propagated with respect to the WDM signal  26 . 
   The Raman pump source  52  comprises semiconductor or other suitable lasers capable of generating a pump light, or amplification signal, capable of amplifying the WDM signal  26  including one, more or all of the included optical information signals  24 . The pump source  52  may be depolarized, polarization scrambled or polarization multiplexed to minimize polarization sensitivity of Raman gain. 
   The WDM transmitter  12 , WDM receiver  14  and link  16  are selected, designed and configured to transmit data at a specified or otherwise defined bit rate. For example, in operation each optical transmitter  20  may transmit a 10 Gigabits per second (Gbit/s), 20 Gbit/s, 40 Gbit/s, 80 Gbit/s or 160 Gbit/s channel. These channel rates may represent or approximate the actual bits per second transmitted. For example, a 40 Gbit/s channel may carry 40 Gbits/s payload data and 3 Gbits/s of additional overhead bits. Thus, a 40 Gbit/s channel transmits approximately 40 Gbit/s data, and in the above example, transmits a total of 43 Gbit/s. 
   The electronics of the optical transmitter  20  as well as the optical components of the WDM transmitter  12 , optical components of the link  16 , optical components of the WDM receiver  14  and the electronics of the optical receiver  32  are all selected, configured, positioned or otherwise designed for transmission at the defined bit rate. Thus, at the defined bit rate, dispersion, nonlinear effects, optical signal-to-noise ratio (OSNR), bit error rate (BER) and/or Q factor are within acceptable tolerances. For example, the DRA  46  is designed to obtain high chromatic dispersion compensation and high OSNR at the designed bit rate. In a particular example, the WDM transmitter  12  may have an installed base of 40 Gbit/s non return-to-zero (NRZ) transmitters  20  each transmitting a 40 Gbit/s channel transporting 43 Gbit/s of data. In this embodiment, the NRZ transmitters  20  each operate at a corresponding 40 Gigahertz (GHz) clock rate and the optical components of the WDM transmitter  12  are all configured for 40 Gbit/s channels. Similarly, the WDM receiver  14  may have an installed base of NRZ receivers  32  each receiving and recovering a 40 Gbit/s channel and optical components configured for 40 Gbit/s channel. The link  16  may have installed and configured optical components such as amplifiers  42  and dispersion compensation controls configured for the 40 Gbit/s channels. 
   As described in more detail below, the transmission of one, some or all of the channels may be upgraded to double or otherwise increase the designed for, or defined, bit rate of each channel while maintaining the electronics speed of the WDM transmitter  12  and the WDM receiver  14  as well as the installed post-transmitter optical components of the WDM transmitter  12 , the installed pre-receiver optical components of the WDM receiver  14  and the installed optical components of the link  16 . Accordingly, an upgrade may be implemented in a short period of time with lower equipment and operational cost on a per bit basis. 
   The optical transmitters  20  may each be upgraded to a return-to-zero M-ary phase shift keying (RZ-mPSK) transmitter each capable of encoding a plurality of data streams each at the system designed bit rate to generate a RZ-mPSK signal having a combined bit rate that is a multiple of the system designed bit rate. The data streams may comprise discrete data streams or different portions of a common data stream. Similarly, the optical receivers  32  may each be upgraded to RZ-mPSK receivers each capable of decoding and recovering the plurality of data streams at the system designed bit rate. 
   In the RZ-mPSK format, information is encoded in the phase of the optical signal such that the phase takes one of the M possible values. Because each value of the phase corresponds to a L bits, the symbol rate (“S”) is the bit rate (“B”) divided by L, where M is equal to 2 L . Thus the transmission bit rate requires the use of only B divided by L rate electronics. 
   In a particular embodiment, an installed NRZ transmitter  20  is upgraded to a return-to-zero differential quadrature phase shift keying (RZ-DQPSK) transmitter and a corresponding NRZ receiver  32  is upgraded to a RZ-DQPSK receiver. One, some or all of the transmitters and receivers may be upgraded. Thus, the WDM system may be upgraded on a pay as you go basis. In this embodiment, the phase of the optical signal takes one of four possible values: 0, π/2, π and 3π/2. Since each value of the phase corresponds to a pair of bits, the symbol rate is exactly half the combined bit rate, which is the initial, designed for bit rate. The bit rate thus requires the use of only B/2 electronics. RZ-DQPSK has a compact spectrum and high tolerance to non-linear effects. The resulting upgraded DWM system may have a Q factor that is only 2 dB or less, and even 1 dB, compared to an NRZ based system. Details of the RZ-DQPSK transmitter and RZ-DQPSK receiver are described in connection with  FIGS. 2 and 3 , respectively. 
     FIG. 2  illustrates details of an RZ-DQPSK optical transmitter  80  in accordance with one embodiment. The RZ-DQPSK optical transmitter  80  may be used to replace, or upgrade, existing NRZ or other installed, designed for transmitter  20 . As described in more detail below, the RZ-DQPSK transmitter  80  is a multi-stage modulator. A first stage  82  encodes a set of data streams into a DQPSK signal  86 . A second stage  84  modulates the first stage DQPSK signal  86  using intensity modulation to generate an RZ-DQPSK signal  88 . The first stage  82  and the second stage  84  may be transposed in order. Moreover, the RZ-DQPSK transmitter  80  may include other or different suitable stages and may be otherwise arranged. For example, RZ-DQPSK transmitter  80  may instead modulate a signal using intensity and then DQPSK modulate the intensity modulated signal. In addition, while the present invention is described in connection with an optical communications system, the RZ-DQPSK or other RZ-mPSK transmitters may also be employed in other suitable systems, such as microwave communication systems, for example, to enhance transmission capacity. 
   The first stage  82  includes a continuous wave laser  90 , an optical power splitter  91 , a plurality of phase modulators  92 , a phase shifter  94 , and an optical power combiner  96  coupled by a plurality of optical links. The optical links form a first, or upper arm, and a second, or lower arm, between power splitter  91  and power combiner  96 . A first phase modulator  92   a  is coupled to the upper arm while the phase shifter  94  and a second phase modulator  92   b  is coupled to the lower arm, or vice versa. 
   In particular, the optical links connects power splitter  91  with the first phase modulator  92   a  and the phase shifter  94 . The optical link connects the first phase modulator  92   a  with the power combiner  96 . The optical links connect the phase shifter  94  with a second phase modulator  92   b  and the second phase modulator  92   b  with the power combiner  96 . Each optical link may be an optical fiber and may be formed with varying types of materials that affect the transport characteristics of light flows along optical link. The first stage  82  and/or other portion of the RZ-DQPSK transmitter  80  may be implemented as a planar light wave circuit, discrete elements connected by optical fiber, free space optics or suitably otherwise. 
   Continuous wave laser  90  is an optical light source emitter, operable to generate a carrier signal at a prescribed or selected frequency with good wavelength control. As used herein, continuous wave means a substantially constant, continuous, steady, or otherwise ongoing signal as opposed to a pulse or burst signal. Continuous wave laser  90  may be a distributed feedback laser, tunable laser, non-tunable laser or other suitable energy source operable to provide light energy. Typically, the wavelengths emitted by continuous wave laser  90  are selected to be within the 1500 nanometer (nm) range, the range at which the minimum signal attenuation occurs for silica-based optical fibers. More particularly, the wavelengths are generally selected to be in the range from 1310 to 1650 nanometers but may be suitably varied. 
   Power splitter  91  is any device operable to split an ingress signal into discrete signals or otherwise passively generate discrete signals based on the ingress signal. The discrete signals may be identical in form and/or process or may suitably differ. The power splitter  91  may be a polarization beam splitter operable to split the ingress signal into discrete signals or otherwise passively generate discrete signals of disparate polarization states based on the ingress signal. In one embodiment, the power splitter  91  may be a three-dB optical coupler. 
   The power splitter  91  splits the carrier signal from the laser  90  into a first portion for modulation by the first phase modulator  92   a  and a second portion for modulation by the second phase modulator  92   b.  The phase shifter  94  is operable to adjust the relative phase between the two portions of the carrier signal to an integer multiple of π/2. The phase shifter  94  may be realized in implicit manner, e.g. slight difference in the optical path difference between the arms, so that it is not distinctively recognizable in the actual implementation. 
   The phase modulators  92  each receive a data signal and modulate the phase of the received portion of the carrier signal based on the data signal. In particular, the first phase modulator  92   a  modulates the first portion of the carrier signal with a first data stream (data  1 ). The second phase modulator  92   b  modulates the second portion of the carrier signal with a second data signal (data  2 ). As previously discussed, the first and second data signals may be from discrete sources or may be portions of a common signal. Thus, when a transmitter  20  is upgraded, the first signal prior to upgrade may be transmitted after the upgrade along with an additional signal. Alternatively, a higher bit rate signal may instead be transmitted with a first portion now being the first data signal and the second portion being the second data signal. Thus, the phases of the signals are differentially encoded. The data signals may be precoded NRZ data signals each at the system designed bit rate. Thus, for example, the speed of the electronics need not be upgraded. 
   The optical combiner  96  is any device operable to receive a plurality of signals and combine or otherwise passively generate a combined signal based on the received signals. The power combiner  96  may be a polarization beam splitter operable to receive a plurality of signals and combine or otherwise passively generate a combined signal based on the received signals and their associated polarization, or a 3-dB optical coupler. 
   The second stage  84  includes an intensity, or clock, modulator  100 . The intensity modulator  100  is operable to modulate the intensity of, or remodulate, the DQPSK signal  86  based on a clock signal. The clock signal may correspond to the designed bit rate. It is a symbol synchronous sinusoidal clock signal, synchronized with the data signal. Other suitable signals or data may be used to provide the data by which intensity modulator  100  modulates the DQSPK signal  86  to generate, or form, the RZ-DQPSK signal  88 . Where the first and second data streams each comprise precoded 43 Gbit/s NRZ data streams, the clock signal may comprise a 43 GHz clock to generate an 86 Gbit/s RZ-DQPSK signal  88 . The resultant RZ-DQPSK signal  88  has an RZ-like intensity waveform with all marks and fifty percent duty ratio, while the data are encoded in the optical phase of each RZ pulse. 
   The intensity modulator  100  may be a Mach-Zehnder interferometer (MZI) or other suitable optical component operable to induce an additional phase shift in one of the arms of the interferometer through voltage-induced refractive index changes and to then combine the portions to generate specified interference and a resultant output signal. 
   In the MZI embodiment, the refractive index of electro-optic material (such as LiNb03) can be changed by applying an external voltage. The interferometer splits the DQPSK signal  86  into two interferometer paths and then combines the two paths interferometrically to generate the RZ-DQPSK signal  88 . The MZI may include a power splitter to split the received optical signal and a power combiner to combine the first and second potions of the signal. Path signals are combined such that there is a constructive interference between two signals in the absence of external voltage. The additional phase shift, introduced in one of the arms of the MZI through voltage-induced index changes, destroys the constructive interference and reduces the transmitted intensity. In particular, there is a complete or substantially complete destructive interference between path signals when the phase difference between two arms equals to π (180 degrees). 
   In a particular embodiment, continuous wave laser  90  may be mathematically expressed, for example, as A cos(2πf c t), where A is amplitude, f c  is the carrier frequency, and t is time. In QPSK modulation, the phase of the carrier signal is modulated and takes on values from the set [0°, 90°, 180°, 270°] corresponding to the symbol set [00, 01, 10, 11], respectively. The RZ-DQPSK signal  88  is intensity modulated DQPSK signal  86  based on a symbol synchronous clock signal, synchronized with the data signal. 
     FIG. 3  illustrates one embodiment of an RZ-DQPSK optical receiver  120 . The RZ-DQPSK receiver  120  may be used to upgrade, or replace, installed, designed for receivers  32 . The RZ-DQPSK receiver  120  includes an optical splitter  122 , a first interferometer  124  coupled by optical link to a first, or upper arm  126  of the optical splitter and a second interferometer  128  coupled by optical link to a second, or lower arm  130  of the optical splitter. A first balanced receiver  132  is coupled by optical link to the first interferometer  124 . A second balanced receiver  134  is coupled by optical link to the second interferometer  128 . The DQPSK receiver  120  may be implemented using discrete elements coupled by optical links, planar waveguide circuit, free space optics and the suitably otherwise. 
   The optical splitter  122  may be the same type of splitter as optical splitter  91  of the RZ-DQPSK transmitter  80  and may split an ingress signal into a first portion provided on the first, or upper arm  126  and a second portion provided on the second, or lower arm  130 . 
   The interferometers  124  and  128  may each comprise MZIs. In this embodiment, each MZI  124  and  128  includes an upper arm and a lower arm. In each MZI  124  and  128 , the phase modulated signal received from the optical splitter  122  is converted to an intensity modulated signal by optically delaying one portion of the signal with respect to the other and applying additional phase shift between the two arms of the MZI. The amount of optical delays equal approximately to the symbol period, which is equal in the DQPSK embodiment to two divided by the combined bit rate. The amount of applied phase shift is equal to π/4 for the first MZI  124  and −π/4 for the second MZI  128 . In each MZI  124  and  128 , the signal is combined interferometrically. The MZIs are adjusted to achieve complete or substantially complete constructive interference at one output port and complete or substantially complete destructive interference at the other output port such that the data signals can be received by the balanced receivers. 
   The balanced receivers  132  and  134  each include two photodetectors, one for each output of the corresponding MZI  124  or  128 . The signal at the output port of each MZI  124  or  128  is detected by the two separate photodetectors. The current of one photodetector is subtracted from the other to recover the corresponding data stream. The first balanced receiver  132  recovers the first corresponding data stream while the second balanced receiver  134  recovers the second data stream. 
     FIG. 4  illustrates one embodiment of a method for upgrading an optical communications system to double or otherwise enhance transmission capacity. In this embodiment, an existing system is designed for NRZ 40 Gbit/s channels. It will be understood that the method may be used in connection with channels of other rates to double or otherwise increase transmission capacity. 
   The method begins at step  150  in which an existing, installed and operating transmitter  20  transmitting a first 40 Gbit/s channel is replaced with an RZ-DQPSK transmitter  80 . Next, at step  152 , the first 40 Gbit/s channel is coupled to a first phase modulator  92   a  of the RZ-DQPSK transmitter  80 . At step  154 , a second 40 Gbit/s channel is coupled to a second phase modulator  92   b  of the RZ-DQPSK transmitter  80 . Thus, the same 40 Gbit/s speed electronics are employed. 
   Proceeding to step  156 , the multiplexer  22  and other post-transmitter optical components of the WDM transmitter  12  are maintained. At step  158 , the transmission fiber and path, including length, are maintained. At step  160 , amplifiers  42  including DCF&#39;s are maintained. At step  162 , the demultiplexer  30  and other pre-receiver optical components of the WDM receiver  14  are maintained. 
   At step  164 , an existing receiver installed to receive the first 40 Gbit/s channel is replaced with an RZ-DQPSK receiver  120 . At step  166 , an output of the first balanced receiver  132  is coupled to a port for the first channel. At step  168 , an output of the second balanced receiver  134  is coupled to a port for the second channel. Thus, only the transmitter  20  and receiver  32  need be upgraded and the remaining optical components may be maintained. It will be understood that one or more of the optical components may be suitably modified. In operation, the first and second channels are combined into and transmitted in an RZ-DQPSK signal  88  from the upgraded WDM transmitter  12  to the upgraded WDM receiver  14 . At the upgraded WDM receiver  14 , the first and second channels are separately recovered. 
     FIG. 5  illustrates one embodiment of a method for transmitting an RZ-DQPSK signal  88 . In this embodiment, intensity modulation independent of the signal is performed at the second stage  84 , with phase modulation being performed at the first stage  82 . 
   The process begins at step  180  wherein a carrier signal is provided. As described above, this step may be performed by a local oscillator or continuous wave laser  90 , or other means suitable to produce a carrier signal. Next, at step  182 , the carrier signal is split into two discrete arms. As described above, this step may be, for example, performed by the power splitter  91 . 
   At step  184 , the first split signal is phase modulated based on a first data signal. This step may be performed by the first phase modulator  92   a.  Next, at step  186 , the phase of the second split signal is shifted by π/2 radians. As described above, this may be performed by the phase shifter  94 . Next, at step  188 , the phase shifted second split signal is phase modulated based on a second data signal. This step may be performed by the second phase modulator  92   b.    
   Next, at step  190 , the modulated first signal and the modulated second signal are combined to form a DQPSK signal  86 . This step may be performed by the power combiner  96 . At step  192 , the DQPSK signal  86  is intensity modulated. This step may be performed by intensity modulator  100 . Next, at step  140 , the resulting RZ-DQPSK signal  88  is transmitted and the process ends. 
     FIG. 6  illustrates one embodiment of a method for receiving an RZ-DQPSK signal  88 . In this embodiment, a set of balanced receivers is utilized which typically provide a 3 dB improvement in OSNR over single port detectors. 
   The method begins at step  200  in which ingress signal is split into a first portion for recovering a first channel and a second portion for recovering a second channel. This may be performed by power splitter. At step  202 , a first portion is forwarded to a first MZI  124 . At step  204 , a second portion is forwarded to a second MZI  128 . 
   Next, at step  206 , in each MZI  124  or  128 , the forwarded portion of the ingress signal is converted into an intensity modulated signal with complete or substantially complete constructive interference at a first output port and complete or substantially complete destructive interference at a second output port. At step  208 , the first channel is detected with the first balance receiver  132  coupled to the first and second output ports of the first MZI  124 . At step  210 , the second channel is detected with the second balanced receiver  134  coupled to the output ports of the second MZI  128 . In this way, an RZ-DQPSK signal  88  may be detected utilizing electronics operating at half the combined bit rate of the RZ-DQPSK signal. 
   Although the present invention has been described with several embodiments, various changes and modifications may be suggested to one skilled in the art. It is intended that the present invention encompass its changes and modifications as fall within the scope of the appended claims.