Optical differential-phase-shift-keyed demodulator apparatus and method

A system and method for demodulating an optical differential-phase-shift-keyed (DPSK) input signal using a Fabray-Perot etalon filter. In one embodiment the system receives a transmitted wavefront from the etalon filter and uses a detector to generate an electrical waveform from the transmitted wavefront. A comparator is used to receive an output from the detector and to generate a signal in accordance with each phase shift (i.e., bit transition) in the optical DPSK input signal. A latching flip-flop receives an output from the comparator and generates a digital signal representative of the bit pattern of the DPSK input signal. The system and method does not require the precisely matched dual optical paths of a Mach-Zehnder interferometer, and therefore is substantially less susceptible to thermal effects that could influence the operation of a conventional Mach-Zehnder interferometer in DPSK demodulation operations.

FIELD

The present disclosure relates to demodulators used with optical differential-phase-shift-keyed (DPSK) communication, and more particularly to an interferometer of less complex construction that is able to demodulate an optical DPSK signal without the need for optical path length control schemes that need to be matched exactly to the data rate, and that might compromise performance of the interferometer.

BACKGROUND

Optical Differential-Phase-Shift-Keyed (DPSK) demodulators have traditionally been formed by using the well known Mach-Zehnder interferometer. This device has essentially two “arms” that form distinct optical paths that are used to detect phase shifts in the optical DPSK signal. The Mach-Zehnder interferometer requires a physical length that is determined by the data rate of the communication link with which it is being used. The two arms of the Mach-Zehnder interferometer must maintain a differential optical path length equal to the distance that light travels in one bit period. As a result, it is highly sensitive to temperature and other environmental factors that cause variations in the physical dimensions of the optical components that it uses.

To address the undesirable influence that thermal factors have on the performance of present day Mach-Zehnder interferometers that are used in DPSK optical demodulator applications, these devices have typically employed a control loop to regulate the path length of one of the optical paths. As will be appreciated, this adds significantly to the complexity and cost of the interferometer.

Other apparatuses and methods for demodulating a DPSK optical signal have involved optical filter discriminator approaches that attempt to detect the phase shift changes in the bit pattern of the optical DPSK signal. However, such approaches typically only use the transmitted wavefront portion of the input optical signal, and not the larger reflected energy provided by the reflected wavefront component of the signal. Such approaches further typically do not attempt to make use of both the transmitted and reflected energy components of the optical DPSK signal. Using both the transmitted and reflected energy 1) increases the received signal, and 2) improves the signal-to-noise ratio, since common-mode noise of the dual detectors is cancelled out. Time alignment of the transmitted and received pulses is less difficult than alignment of the two arms of a Mach-Zehnder demodulator.

SUMMARY

The present system and method is directed to a system for demodulating an optical DPSK signal that is more sensitive and less complex and less costly construction than other forms of optical demodulator systems.

In one embodiment the system makes use of a Fabry-Perot etalon filter for receiving an optical DPSK input signal and generating a wavefront component therefrom each time a phase shift occurs in the optical DPSK input signal. A detector system detects the wavefront component output from the etalon filter to enable bit transitions in a data sequence of the optical DPSK input signal to be detected, and the optical DPSK input signal to be reconstructed as a digital output signal. A comparator is used to compare the electrical signal with a reference threshold signal, and an output of the comparator is used to signal a phase shift (i.e., bit transition) in the optical DPSK input signal. A logic component receives an output of the comparator and generates a digital output signal indicative of the bit pattern of the optical DPSK signal.

In another embodiment the detector system makes use of a first detector for receiving a transmitted wavefront component of the optical DPSK input signal, and a second detector for receiving a reflected wavefront component of the DPSK input signal. A comparator receives the output from each detector and generates a signal indicative of a phase shift in the wavefronts of the signals output from the etalon filter. The signal path lengths relative to the comparator are matched during initial construction of the system. The comparator output is fed to an edge-triggered logic component, which may comprise a latching flip-flop that generates a digital output signal indicative of the bit pattern of the optical DPSK signal. The physically small size of the etalon filter serves to significantly reduce or substantially eliminate the susceptibility of the etalon filter to thermal changes that could significantly affect the performance of a conventional Mach-Zehnder interferometer.

The various embodiments and methods of the present disclosure are substantially unaffected by thermal changes that typically adversely affect performance of other dual arm demodulator systems. The present system and method is expected to find utility in both free-space and optical fiber implementations.

DETAILED DESCRIPTION

Referring toFIG. 1, an optical differential-phase-shift-keyed (DPSK) demodulator system10is illustrated in accordance with one embodiment of the present disclosure. The system10includes, in this example, a Fabry-Perot etalon filter12, an optical detector14, a comparator16, and a latching, edge triggered flip-flop18. Components14,16and18may be viewed as collectively comprising a detector system20. The Fabry-Perot etalon filter12will be referred to as the “etalon filter12” for discussion purposes.

The etalon filter12receives an optical DPSK input signal22and generates a transmitted wavefront24and a reflected wavefront26therefrom. The transmitted wavefront24is directed to an input of the detector14, such as a high-speed InGaAs detector. The detector14produces an electrical output signal at an output28, indicated by waveform30. As long as no phase shifts (i.e., bit transitions) occur in the optical DPSK input signal22data sequence, the amplitude30aof the transmitted wavefront24is constant. However, when a phase shift of 180 degrees (corresponding to a bit transition) occurs in the optical DPSK input signal22data sequence, a decreased-intensity pulse occurs on the transmitted wavefront24, while a corresponding increased-intensity pulse occurs on the reflected wavefront26. The width of pulse depends on the finesse (i.e., resolution) of the etalon filter12.

With brief reference toFIG. 2, curves32and34illustrate two different waveforms produced by a carrier wave length at approximately 1550 nanometers for two different values of etalon filter12finesse. The filter finesse of a value of 150 produces the curve32, which represents an output signal from the etalon filter12. The signal represented by curve32forms a significantly narrower pulse with less energy than the output signal represented by curve34, which is produced with a finesse value of 300. However, the narrower pulse may have a better signal-to-noise ratio. The pulse width should be optimized for the best signal-to-noise ratio for the desired data rate, within the practical constraints of producing high finesse filters.

With further reference toFIG. 1, the electrical waveform30at the output28of the detector14is fed into the inverting input36of the comparator16. A reference threshold level signal38is applied to the non-inverting input40of the comparator16. The output42of the comparator16may be a logic “0” level signal when the magnitude of the signal on the non-inverted input36is less than the value of the threshold level signal38on the inverting input40. For example, this might produce a logic “0” output signal on the output42of the comparator16. However, when the value of the signal30on the inverting input36increases beyond the threshold level signal38on the non-inverting input40, the output42of the comparator16may go to a logic level “1” value. The output waveform44present on the output42of the comparator16forms a digital saw-toothed waveform, as shown inFIG. 1.

When the output waveform44on output42changes to a logic 1 level, the latching flip-flop18receives the output waveform44on its input46and generates a digital output waveform50on its output48. The latching flip-flop18is only triggered on the trailing edge of each pulse of the output waveform44(i.e., only when there is a phase shift in the optical DPSK input signal22data sequence. As a result, the digital output signal50from the latching flip-flop18represents a reconstruction of the bit sequence of the optical DPSK input signal22.

The optical principle upon which use of the system10is predicated is based, in part, on an unconventional application of the transient shift in the transmission and/or reflection characteristics of the etalon filter12that occurs when the phase of the optical DPSK input signal22is shifted over a relatively short period of time. In general, the conventional application of a convention Fabry-Perot etalon filter is based on its steady state operation as depicted inFIG. 3. If δ is an integral multiple of 2π radians at the operational wavelength range of interest, the transmitted amplitude At(t), to within the internal absorption limitations of the etalon filter12, equals the incident amplitude Ai(t), while the reflected amplitude Ar(t) equals zero. This situation is dependent upon constructive interference of the reflecting wavefront component24internal to the etalon filter12. Typically it is assumed that the phase of an electromagnetic wave varies continuously in space and time only due to propagation. However, if the phase φ(t) is shifted at the source, this additional phase term will affect the wavefront, or signal, transmitted (At(t)) through (i.e., transmitted wavefront component24) and reflected (Ar(t)) from the etalon filter12(i.e., reflected wavefront component26), as shown in Equations (1) and (2) below. The delayed phase terms in the summations occur as a result of the multiple reflections internal to the etalon filter12.

where:θ—angle of incidence of incoming wavefront;Ai(t); θ′—angle of refraction of wavefront inside of etalon filter;n′—refractive index of etalon filter material;n—refractive index of surrounding material;r—reflection coefficient from outside of the etalon filter material;r′=−r—reflection coefficient from inside of the etalon filter material;t—transmission coefficient from outside into the etalon filter material;t′=t—transmission coefficient from inside to outside of the etalon filter material;h—thickness of the etalon filter;λo—free space wavelength of wave;c—free space speed of light; andv=c/λo—frequency of the optical input wave signal (i.e., signal22).

FIG. 4illustrates an exemplary optical input waveform22having a transmitted wavefront component, represented by waveform52, and a reflected wavefront component, represented by waveform54, where the phase of the optical input waveform22is reversed (shifted by π radians) every 100 picoseconds (pS). In this example waveform52corresponds to transmitted wavefront24inFIG. 1, and waveform54corresponds to reflected wavefront26inFIG. 1. Waveforms52and54also represent waveforms that are generated from the application of equations (1) and (2) given above. The phase transition occurs over a 10 pS interval, designated by reference numeral56. It appears that the magnitude of reflectivity of the etalon filter12is substantially greater than 1, which is a physical impossibility for a passive device such as an etalon filter. This is explained by the fact that the etalon filter12actually releases its stored energy over a substantially shorter time interval than the 100 pS time interval (in this example) that the phase shifting occurs in. Thus, the signal surge represented by waveform54provides a significant signal-to-noise advantage for bit detection purposes and enables phase shifts in the optical DPSK input signal (i.e., the bit pattern) to be directly detected without bit-to-bit comparison. Thus, the system10does not require the two arms of conventional Mach-Zehnder interferometers to detect the phase shift in the optical DPSK input signal22. Furthermore, the system10is much less susceptible to thermal conditions that affect the detectability of conventional Mach-Zehnder interferometers. InFIG. 5, the exemplary output waveform44produced by comparator16is presented in comparison to an exemplary output waveform58from a typical Mach-Zehnder interferometer.

With brief reference toFIG. 6, a system100in accordance with other embodiment of the present disclosure is illustrated. The system100is similar in some respects to the system10, and common components will be designated by reference numerals increased by 100 over those used in connection with the system10.

The system100differs from the system10by making use of both the transmitted wavefront126and the reflected wavefront124from an etalon filter112. The incoming optical DPSK-modulated beam22is linearly polarized so that it is reflected by a polarizing beam splitter172. It passes through the Faraday rotator170and into the etalon filter112. The transmitted wavefront126is received by one half of a differential detector114awhile the reflected wavefront124passes through the Faraday rotator170and the polarization beam splitter172to the other half of the differential detector114b. The Faraday rotator170is a non-reciprocal device that rotates the polarization of a transmitted beam in the same sense relative to the direction of propagation. The thickness of the Faraday rotator170determines the amount of polarization. In this application the polarization is rotated by 45 degrees on each pass, in order that the reflected beam124from the etalon filter122is transmitted by the polarization beam splitter172to the second detector114b.

The waveform produced at the output128aof the first detector114aillustrates the decreased-intensity pulse that occurs each time the detector114adetects a phase shift in the transmitted wavefront126. This waveform is indicated by waveform130a. The second detector114aproduces an increased-intensity waveform130bat its output115from the reflected wavefront124each time a phase shift occurs in the optical DPSK input signal22. The output128afrom the first detector114aand the output115from the second detector114bare fed into the inputs of a comparator116. When a phase shift occurs in the optical DPSK input signal22, the signal on the non-inverting input140of the comparator116will be different than the magnitude of the signal on the inverting input136, thus causing the comparator116to produce a signal144at its output142. The signal144is used to drive a latching flip-flop118that produces a digital output signal150at its output148representative of the bit pattern of the optical DPSK input signal22. Thus, the system100makes use of both the transmitted wavefront126and the reflected wavefront124, and does not require a separate reference threshold input on the comparator116. It will be appreciated that the propagation path lengths between the etalon filter112and the first detector114a, and between the etalon filter112and the second detector114b, will need to be matched to a fraction of the bit period (typically within about 5%) during initial construction and/or tuning of the system100. Differential detectors with a variable optical delay on one input are commercially available. One such component is the “Balanced Photodiode Lab Buddy with Variable Optical Delay Line”, commercially available from Discovery Semiconductors, Inc. of Ewing, N.J. The system100is expected to provide an even higher signal-to-noise ratio for the output signal150of the system100.

The various embodiments and methods described herein thus enable the bit pattern of an optical DPSK input signal to be detected and reconstructed without the use of a conventional dual arm interferometer such as a Mach-Zehnder interferometer. The various embodiments and methods do not require any control loops to be constructed to account for thermal changes in the environment in which the demodulator is being used. The use of a Fabry-Perot etalon filter, which is significantly smaller in physical size than a conventional Mach-Zehnder interferometer, helps to significantly reduce the susceptibility of the filter to thermal changes that could affect performance. The use of a Fabry-Perot etalon filter also significantly simplifies the construction of a DPSK demodulator system, as well as potentially enabling the system to be used in applications and/or environments where significant thermal changes would render conventional Mach-Zehnder DPSK demodulators unreliable.