Abstract:
A WDM receiver that includes a single, delay interferometer coupled to an N-channel wavelength-demultiplexing filter provides for the simultaneous conversion of a multiplex of N differential-phase-shift-keyed modulated optical signals of different wavelengths into N intensity-modulated optical signal channels. The Nintensity-modulated optical signals may be individually detected by a bank of N photodiode circuits to recover the modulated data stream or used for optical amplification, regeneration, processing, control, or modulation.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention relates generally to the field of optical telecommunications, and in particular, to optically demodulating differential-phase-shift-keyed (DPSK) signals in a wavelength-division-multiplexed (WDM) communication environment. 
   2. Description of the Related Art 
   In state-of-the-art, wavelength-division-multiplexed optical communication systems, multiple, individually modulated wavelengths of light are multiplexed into a single fiber and routed to a remote receiving location. At the receiving location, it is common to separate the WDM channels using an optical wavelength-demultiplexing filter in order to route each channel to its own, dedicated demodulator. 
   In a system that employs differential-phase-shift-keyed modulation, a dedicated DPSK demodulator is used within the receivers of each WDM channel. A DPSK demodulator may be implemented using an optical delay interferometer (DI) (e.g., a Mach-Zehnder interferometer) followed by an optical-to-electrical (O-E) converter. The function of the DI is to convert a phase-coded optical signal into an intensity-modulated signal before the signal is presented to the O-E converter. The function of the O-E is to convert the resulting optical intensity modulation to an electrical current or voltage that may be further processed in the electrical domain. 
   The DI operates by splitting the DPSK input signal into two components and delaying one component with respect to the other by a time delay T d , where the time delay is usually set to be an integer multiple of the bit duration T. In most cases, but not exclusively, the delay T d  is chosen to equal the bit duration T of the modulated bit stream. The two relatively delayed components of the phase-coded input signal are then combined at an output coupler of the DI. The relative optical phase difference between the two signals within the DI is set to produce, in the absence of phase changes, constructive interference at one of the DI output ports and destructive interference at the other. When single-ended detection is employed by the demodulator, only one output is brought out of the DI to an O-E converter that is typically implemented using a photodiode and associated circuitry. When balanced detection is employed by the demodulator, both outputs are brought out of the DI to two such O-E converters. 
   In a single-ended implementation, a single output port of the DI is coupled to a single photodiode detector and circuitry to translate the optical intensity out of the DI into a recovered bit stream. 
   In a balanced implementation, each of two ports out of the DI is coupled to its own photodiode, and the difference between the electrical outputs of the photodiode circuits are used to recover the bit stream. 
   For a DPSK-modulated WDM system of N channels, it is typical to employ an N-channel optical wavelength-demultiplexing filter, followed by N dedicated DPSK demodulators, one for each receiver. 
   SUMMARY OF THE INVENTION 
   Problems in the prior art are addressed in accordance with principles of the present invention by employing a single delay interferometer (e.g., a Mach-Zehnder interferometer) to simultaneously convert a multiplex of N DPSK-modulated optical signals of different wavelengths into two versions of a multiplex of N intensity-modulated optical signals. Each version is next separated into N individual channels by an N-channel wavelength-demultiplexing filter, and each channel is then individually detected by one of a bank of N photodiode circuits. 
   In one embodiment, the present invention is a receiver for an optical input signal. The receiver includes a delay interferometer (DI) adapted to receive the optical input signal, and
         a first wavelength-demultiplexing filter (WDF), wherein a first output of the DI is coupled to an input of the first WDF.       

   In another embodiment, the present invention is a receiver for an optical input signal. In this embodiment, the optical input signal is a multiplex of N modulated signals corresponding to different wavelengths and the receiver includes (i) a first component adapted to convert the optical input signal into one or more versions of a multiplex of N intensity-modulated optical signals and (ii) a second component adapted to separate a first version of the multiplex of N intensity-modulated optical signals into N different optical channel signals. 
   In yet another embodiment, the present invention is a method for processing an optical input signal where the optical input signal is a multiplex of N modulated signals corresponding to N different wavelengths and the method involves (a) converting the optical input signal into one or more versions of a multiplex of N intensity-modulated optical signals, and (b) separating a first version of the multiplex of N intensity-modulated optical signals into N different optical channel signals. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other aspects, features, and advantages of the present invention will become more fully apparent from the following detailed description, the appended claims, and the accompanying drawings in which: 
       FIG. 1  depicts N-channel WDM receiver  100  according to the prior art. 
       FIG. 2  depicts receiver  200  for an N-channel DPSK WDM optical communication system according to one embodiment of present invention. 
       FIG. 3  depicts single-ended receiver  300  for an N-channel DPSK WDM optical communication system according to an alternative embodiment of present invention. 
   

   DETAILED DESCRIPTION 
   Reference herein to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment can be included in at least one embodiment of the invention. The appearances of the phrase “in one embodiment” in various places in the specification are not necessarily all referring to the same embodiment, nor are separate or alternative embodiments mutually exclusive of other embodiments. 
   Demultiplexer-Demodulator 
     FIG. 1  depicts N-channel differential-phase-shift-keyed WDM receiver  100  according to the prior art. As shown, receiver  100  includes an N-channel optical wavelength-demultiplexing filter  102  feeding N DPSK demodulators  104 . Each demodulator includes dual-output delay interferometer (e.g., a Mach-Zehnder interferometer)  106  followed by first and second photodiode circuits,  108  and  110 , respectively, and differencing circuit  112 . 
   In operation, a multiplex of independently DPSK-modulated wavelengths of light are fed to the input of N-channel optical wavelength-demultiplexing filter  102 . The filter separates the multiplex into N channels and routes each channel to its own, dedicated demodulator  104 . 
   Each demodulator  104  includes dual-output delay interferometer (DI)  106 , which converts the DPSK-modulated channel signal into two intensity-modulated output signals that are logically inverted copies of each other. Each of the two DI outputs is fed to a photodiode circuit (e.g., implemented using InGaAs photodiodes and supporting circuitry), where the DI output is converted from an optical signal into an electrical signal. The two electrical signals are then forwarded to differencing circuit  112 , which outputs the corresponding difference signal from the demodulator. 
   Demodulator  104  of circuit  100  is an example of a balanced implementation. Alternatively, a single-ended implementation of demodulator  104  might be used, where a single-output DI (or one output of a dual-output DI) is coupled to a single photodiode circuit, and the output of the single photodiode circuit is the output of the demodulator. 
   Demodulator-Demultiplexer 
     FIG. 2  depicts receiver  200  for an N-channel DPSK WDM optical communication system according to one embodiment of present invention. 
   As illustrated, receiver  200  includes delay interferometer  202 , first wavelength-demultiplexing filter  208 , second wavelength-demultiplexing filter  210 , a first set of photodiode circuits  212 , a second set of photodiode circuits  214 , and differencing circuits  216 . 
   Note that a DI with a relative path delay of T d  represents a periodic filter with period 
             1     T   d       =     f   d           
when looked at in the frequency domain. Thus, the demodulating properties of a DI apply equally well to all information-carrying DPSK data signals with a frequency separation of P·f d , where P is an integer quantity. Further, interchanging the output ports of the DI is equivalent to shifting the DI output spectrum by
 
             1   2     ⁢       f   d     .           
Each DI frequency shift of ½ f d  logically inverts the demodulated data signal. Thus, all optical signal carrier frequencies spaced at integer multiples of
 
             1   2     ⁢     f   d           
can be simultaneously demodulated by the same DI.
 
   In embodiments of the present invention, the periodic filtering characteristics of DI  202  are exploited in combination with the DI&#39;s ability to perform phase-to-amplitude conversion to realize a massively parallel, N-channel, phase-to-amplitude converter  202 . 
   In contrast to the prior art, which teaches a single demultiplexer followed by multiple phase-to-amplitude converters and multiple amplitude detectors, embodiments of the present invention are directed to a single phase-to-amplitude converter followed by one or two demultiplexers and multiple amplitude detectors. 
   In the balanced embodiment of the present invention shown in  FIG. 2 , an N-channel multiplex of DPSK modulated signals that are separated in frequency by integer multiples of 
             1   2     ⁢     f   d           
are fed to DI  202 . The DI performs simultaneous phase-to-amplitude conversions on the N channels and outputs versions of the processed multiplex on each of its two output ports  204  and  206 . Output port  204  of DI  202  is coupled to first wavelength-demultiplexing filter  208 , and output port  206  of DI  202  is coupled to second wavelength-demultiplexing filter  210 . Each demultiplexing filter separates its input into N-channels. Next, each channel out of first wavelength-demultiplexing filter  208  is fed to its own photodiode circuit  212 , and each channel out of second wavelength-demultiplexing filter  210  is fed to its own photodiode circuit  214 . Corresponding channels output from the first and second filters are then fed to differencing circuits  216  to form the balanced output for each channel.
 
   Note that, for embodiments of the present invention that employ balanced detection, each DI output port is connected to a separate wavelength-demultiplexing filter. In this case, the two demultiplexers are not restricted to having the same transfer characteristics. This can prove useful in dense WDM scenarios, where tight channel filtering may seriously degrade the signal quality, if the WDM demultiplexer&#39;s filter shape is chosen inappropriately. 
   Single-Ended Embodiment 
     FIG. 3  depicts single-ended receiver  300  for an N-channel DPSK WDM optical communication system according to an alternative embodiment of the present invention. Elements of this embodiment operate similarly to corresponding elements of the embodiment of  FIG. 2 . 
   In this single-ended embodiment, an N-channel multiplex of DPSK-modulated signals that are separated in frequency by integer multiples of 
             1   2     ⁢     f   d           
are fed to DI  302 . The DI performs simultaneous phase-to-amplitude conversions on the N-channels of the multiplex and presents the processed results on its output ports. Note that only one of the output ports of the DI is used in this embodiment. As shown, output port  304  of DI  302  feeds a single, wavelength-demultiplexing filter  306 . The filter separates the multiplex into its individual component channels. Each channel is fed to its own photodiode circuit  308 , which converts the optical amplitude signal to an electrical current or voltage.
 
   In certain embodiments of the present invention, receiver  200  or  300  may be combined with an inter-symbol equalization mechanism that adjusts the magnitude and phase of the received WDM multiplex prior to demodulation. Such equalization serves to improve detection performance of the received signal, in particular, by limiting the impact of inter-symbol interference in the WDM transmission. Details on such an equalizer can be found in co-pending U.S. non-provisional application Ser. No. 10/393,483, “Multi-Channel Optical Equalizer For Intersymbol Interference,” filed on Mar. 20, 2003, incorporated herein by reference in its entirety. Although the wavelength-demultiplexing filters illustrated in  FIGS. 2 and 3  feed O-E converters in the form of a single or double photodiode circuit (depending on whether it is a single-ended or balanced implementation), it should be understood that alternative methods or devices for O-E conversion may be used. Alternatively, the intensity-modulated light might not be converted to the electrical domain at all and might instead be kept in the optical domain for use in optical amplification, optical regeneration, optical processing, or optical modulation applications, as would be understood to one skilled in the art. 
   Note that the wavelength-demultiplexing filter and other elements of the present invention may be implemented by various techniques and in various technologies while remaining within the spirit and scope of the invention. These techniques and technologies include, but are not limited to, integrated optics (including silica on silicon substrate or Si:SiO 2 ), fiber optics, free space optics, thin film, InGaAs, InP, micromirror micro-electro-mechanical system (MEMS) arrays, and optical grating subsystems. 
   Although the receiver of this invention was described in the context of DPSK-modulated data channels, embodiments of the present invention directed to other differential phase modulation schemes (e.g., 8-DPSK and differential quadrature phase shift keyed (DQPSK)) would also be within the principle and scope of the present invention, as would be understood to one skilled in the art. 
   While this invention has been described with reference to illustrative embodiments, this description should not be construed in a limiting sense. Various modifications of the described embodiments, as well as other embodiments of the invention, which are apparent to persons skilled in the art to which the invention pertains are deemed to lie within the principle and scope of the invention as expressed in the following claims.