Abstract
Generation of data streams for two dimensions comprises compensation for a nonideal response of a signal path in an optical communications signal. The data streams are converted to analog electrical signals which drive two dimensions of an electrical-to-optical converter. Output of the electrical-to-optical converter is coupled through an optical link to an optical-to-electrical converter.
Claims
1. A method of optical communications, the method comprising: generating a data stream for each of two dimensions; converting the data streams to analog electrical signals which drive two dimensions of an electrical-to-optical converter; and coupling output of the electrical-to-optical converter through an optical link to an optical-to-electrical converter, wherein generation of the data streams comprises compensation for a first nonideal response of a signal path.
2. The method of claim 1, wherein the signal path is part of the electrical-to-optical converter.
3. The method of claim 1, wherein the signal path is part of the optical-to-electrical converter.
4. The method of claim 1, wherein the signal path includes an amplifier.
5. The method of claim 1, wherein the signal path is a digital-to-analog converter.
6. The method of claim 2, wherein the signal path includes a Mach-Zehnder modulator.
7. The method of claim 1, wherein the first nonideal response is linear.
8. The method of claim 1, wherein the first nonideal response is differential propagation delay.
9. The method of claim 1, wherein the first nonideal response is crosstalk.
10. The method of claim 1, wherein the first nonideal response is a non-orthogonal response.
11. The method of claim 1, wherein the first nonideal response is nonlinear.
12. The method of claim 1, wherein generation of the data streams comprises compensation for a second nonideal response of another signal path.
13. The method of claim 1, wherein the data streams comprise N-bit words.
14. An optical communications system comprising: an electrical-to-optical converter, output of which is coupled through an optical link to an optical-to-electrical converter; a compensation processor to calculate compensation for a first nonideal response of a signal path and to generate, while taking into account the calculated compensation, a data stream for each of two dimensions; and digital-to-analog converters to convert the data streams to analog electrical signals which drive two dimensions of the electrical-to-optical converter.
15. The optical communications system of claim 14, wherein the signal path is part of the electrical-to-optical converter.
16. The optical communications system of claim 14, wherein the signal path is part of the optical-to-electrical converter.
17. The optical communications system of claim 14, wherein the signal path includes an amplifier.
18. The optical communications system of claim 14, wherein the signal path is the digital-to-analog converters.
19. The optical communications system of claim 15, wherein the signal path includes a Mach-Zehnder modulator.
20. The optical communications system of claim 14, wherein the first nonideal response is linear.
21. The optical communications system of claim 14, wherein the first nonideal response is differential propagation delay.
22. The optical communications system of claim 14, wherein the first nonideal response is crosstalk.
23. The optical communications system of claim 14, wherein the first nonideal response is a non-orthogonal response.
24. The optical communications system of claim 14, wherein the first nonideal response is nonlinear.
25. The optical communications system of claim 14, wherein the compensation processor is to calculate compensation for a second nonideal response of another signal path.
26. The optical communications system of claim 14, wherein the data streams comprise N-bit words.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
(1) Further features and advantages of the present invention will become apparent from the following detailed description, taken in combination with the appended drawings, in which:
(2) FIGS. 1A and 1B are block diagrams schematically illustrating transmission of an input data signal through a conventional optical communications system;
(3) FIG. 2 is a block diagram schematically illustrating transmission of an input signal through an optical communications system in accordance with an embodiment of the present invention;
(4) FIG. 3 is a block diagram schematically illustrating principal components and operations of a compensation processor in accordance with a first embodiment of the present invention.
(5) FIG. 4 is a block diagram schematically illustrating principal components and operations of a compensation processor in accordance with a second embodiment of the present invention;
(6) FIG. 5 is a block diagram schematically illustrating principal components and operations of a compensation processor in accordance with a third embodiment of the present invention;
(7) FIGS. 6A and 6B are block diagrams schematically illustrating principal components and operations of a compensation processor in accordance with a fourth embodiment of the present invention; and
(8) FIGS. 7A and 7B are block diagrams schematically illustrating principal components and operations of a compensation processor in accordance with a fifth embodiment of the present invention.
(9) It will be noted that throughout the appended drawings, like features are identified by like reference numerals.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
(10) The present invention provides a method and system for compensation of chromatic dispersion in an optical communications system. FIG. 2 is a block diagram schematically illustrating principal operations in accordance with the method of the present invention.
(11) As shown in FIG. 2, the optical communications system is represented by electrical-to-optical (E/O) and optical-to-electrical (O/E) converters 2 and 6 separated by an optical link 4 represented by a system transfer function T(). In accordance with the present invention, signal distortions introduced by the system transfer function T() are compensated by deriving a compensation function C() that optimizes system performance. A compensation processor 10 uses the compensation function C() to filter the input signal x(t) in the electrical domain, thereby producing a predistorted input signal x.sup.1(t) 12 given by x.sup.1(t)=x(t) cony C() where cony is the convolution function.
(12) The predistorted input signal x.sup.1(t) 12 is then converted to a corresponding predistorted optical signal E.sup.1.sub.IN() by the E/O converter 2 and transmitted through the optical link 4 to the receiver. Within the receiver, the incoming optical signal E.sup.1.sub.OUT() is converted by the O/E converter 6 into a corresponding output signal y.sup.1(t). As may be seen in FIG. 2, the received optical signal E.sup.1.sub.OUT() is the predistorted optical signal E.sup.1.sub.IN() multiplied by the transfer function T() of the optical communications system. When the distortions introduced by the compensation function C() exactly counterbalance those introduced by the system transfer function T(), the output signal y.sup.1(t) generated by the O/E converter 6 will be a substantially undistorted version of the original input data signal x(t). Thus the present invention provides a technique of compensating optical distortions impressed upon optical signals traversing the optical link 4, by predistorting the input data signal x(t) in the electrical domain at the transmitting end of the optical link 4. Because dispersion compensation is accomplished in the electrical domain, a substantially arbitrary compensation function C() can be implemented, thereby facilitating effective compensation of even severe optical dispersion.
(13) Various methods may be used to derive the compensation function C(). In the example of FIG. 2, the optical signal E.sup.1.sub.OUT() at or near the O/E converter 6 is monitored (at 14), in order to detect signal quality parameters indicative of dispersion in the optical communications system. In preferred embodiments, the signal quality parameters comprise a direct measurement of dispersion as a function of wavelength across the wavelength band of interest. However, other signal quality parameters such as, for example, the bit error rate or eye closure may be used as proxies for the dispersion. Any of the signal quality parameters may be detected based on an optical signal obtained by tapping the optical link 4 (as shown by solid lines in FIG. 2), or alternatively by analysis of the output signal y(t) generated by the E/O converter 6 (as shown by the dashed line in FIG. 2). In either case, a compensation function C() which optimizes the detected parameters can then be derived (at 16) deterministically and/or adaptively, using known techniques.
(14) It should be noted that the functional step of deriving the compensation function C() can be implemented by any suitable combination of hardware and software, which may be co-located with the receiver, the transmitter, or any other location. In embodiments in which the detected parameters comprise direct measurement of dispersion, the compensation function C() can be derived to minimize (and preferably eliminate) the total dispersion. Where bit error rate and/or eye closure are used as proxies, then the compensation function C() would be derived to optimize these values.
(15) FIG. 3 is a block diagram schematically illustrating principal elements and operations of the compensation processor 10, in accordance with a first embodiment of the present invention. As shown in FIG. 3, the input signal x(t) is filtered by a digital filter 18 which accepts the compensation function C() as an input. Various known digital filter types may be used to implement the digital filter 18, such as, for example, Finite Impulse Response (FIR) filters, Infinite Impulse Response (IIR) filters, and Fast Fourier Transform (FFT filters). Alternatively, the digital filter 18 may be implemented using a Random Access Memory Look-up Table (RAM LUT), as will be described in greater detail below with reference to FIGS. 4-7. In either case, the digital filter 18 generates successive sample values 20 of the predistorted signal x.sup.1(t) 12, at a sample rate which is about double the bit-rate of the input signal. A high-speed digital to analog converter (DAC) 22 can then be used to convert these sample values 20 into an analog predistorted signal x.sup.1(t) 12. If desired, the analog predistorted signal x.sup.1(t) 12 can be conditioned, for example by means of a filter 24 and a low noise amplifier (LNA) 26, in a conventional manner, to remove out-of-band noise and to scale the amplitude of the analog signal to the dynamic range of the E/O converter 2. In the illustrated embodiment, the E/O converter 2 comprises a tuned optical source 28 such as a narrow band laser coupled to a conventional optical modulator 30. In this case, the analog predistorted signal x.sup.1(t) 12 can be used as an input to control the optical modulator 30 in a manner known in the art.
(16) As mentioned above, the digital filter 18 may be implemented in a variety of ways. FIG. 4 schematically illustrates an embodiment in which the digital filter 18 is implemented using a RAM LUT 32. This technique exploits the fact that the input signal x(t) is a substantially undistorted binary signal, in which successive bits can be unambiguously discriminated. This greatly simplifies the problem of digitally filtering the input signal x(t), because a direct relationship can be defined between the input signal x(t) and each sample value 20 of the predistorted signal x.sup.1(t) 12.
(17) As shown in FIG. 4, successive bits of the input signal x(t) are latched through a serial-to-parallel converter (SPC) 34 (e.g., a shift register) which converts the serial bit stream into a parallel bit stream 36 having a width of N bits. In the illustrated embodiment N=8, it being understood that other values of N may be used as desired. Each N-bit word of the parallel data stream 36 is then used as an index value to access a random access memory (RAM) look-up table 32 having 2.sup.N registers (not shown). With this arrangement, one or more M-bit (where M will generally be at least four) sample values 20 of the predistorted signal x.sup.1(t) 12 corresponding to each N-bit word can be calculated and stored in each register of the RAM LUT. As each N-bit word of the parallel data stream is latched into the RAM LUT, the corresponding M-bit sample value(s) of the predistorted signal x.sup.1(t) 12 is (are) latched out of the RAM LUT 32. It should be noted that, in order to facilitate accurate synthesis of the predistorted signal x.sup.1(t) 12 at the output of the DAC 22, the Nyquist sampling theorem must be respected. In this case, each register of the RAM LUT 32 will store at least two successive sample values 20 for each N-bit word of the parallel data stream. Because the RAM LUT 32 performs a substantially linear filtering function, it is possible to construct the LUT 32 as a set of two or more Random Access Memory blocks (not shown), if desired. In this case, each RAM block stores a respective portion of the desired numerical value 20 of the predistorted signal component. Thus the outputs generated from each RAM block can be digitally summed, in a conventional manner, to produce the desired numerical value 20. This arrangement allows the LUT 32 to be larger than can conveniently be accommodated within a single RAM block.
(18) Various methods may be used to calculate each of the digital values 20 stored in the look-up table 32. In the example of FIG. 4, the compensation function C() (derived at 16) is used to calculate (at 38) respective M-bit sample values for each of the 2.sup.N possible N-bit words. Each M-bit sample value can then be stored in the appropriate RAM LUT register indexed by its respective N-bit word.
(19) Because chromatic dispersion causes a time domain distortion of an input signal, the sample value of the predistorted input signal 12 at a particular instant (t) will necessarily be a function of the analog waveform of the input signal x(t) within a time window that brackets the instant in question. The width of the time window, measured in symbols, will generally be a function of the maximum dispersion (D) for which compensation is to be provided; the bandwidth (B) of the optical signal; and the symbol interval (S) of the optical signal. For example, consider an optical communications system in which the transmitter generates an optical signal having a bandwidth of B nanometers and a symbol interval of S picoseconds/symbol. In this case, the maximum dispersion (D) that can be compensated is given by the equation D=N.Math.(S/B), where N is the width of the time window, measured in symbols. It will be appreciated that the selected value of N will limit the maximum dispersion (D) that can be effectively compensated. In general, the value of N will be selected based on the expected maximum dispersion of the optical communications system.
(20) In the example of FIGS. 3 and 4, the look-up table 32 uses N=8 successive bits of the input signal x(t) to reference successive sample values 20 of the predistorted signal 12. In this case, the time window used for calculating each sample value of the predistorted signal spans a total of N=8 symbols.
(21) Following the discussion above, it will be seen that each sample value 20 stored in the look-up table 32 can readily be calculated (at 38) by applying the derived compensation function C() to each one of the 2 possible N-bit words. For each N-bit word, the calculated sample value can then be stored in the lookup table register that is indexed by the N-bit word in question. This process will result in the look-up table 32 being loaded with pre-calculated sample values 20 of the predistorted signal 12 which will be accessed, in sequence, as successive bits of the input signal x(t) are latched through the serial-to-parallel converter 34.
(22) It will generally be advantageous to arrange the RAM LUT 32 to output more than one numerical value 20 of the predistorted signal 12 for each bit of the input signal x(t). This can readily be accomplished by calculating the required numerical values 20 for each N-bit word, and storing the resulting set of numerical values 20 in the appropriate register. As each bit of the input signal x(t) is latched through the serial-to-parallel converter 34, all of the numerical values calculated for each unique N-bit word will be output, in an appropriate sequence, from the RAM LUT 32.
(23) In the foregoing discussion, it was assumed that both the DAC 22 and the optical modulator 30 are fully linear, so that these components do not introduce any distortions beyond those accounted for by the system transfer function T(). Depending on how the system transfer function T() is defined, this approach may yield satisfactory results. However, it may be desirable to treat the transfer function T() of the optical fiber span 4 separately from those of the DAC 22 and modulator 30, in which case the compensation function C() will not account for distortions introduced by these elements.
(24) In many cases, the performance of the various devices forming the signal path between the DAC 22 and modulator 30, (i.e. the DAC 22, filter 24, LNA 26 and modulator 30) are well characterized, and this knowledge can be used to calculate a mapping which adjusts the sample values 20 generated by the RAM LUT 32 to compensate for any non-ideal response of these devices. However, improved performance can be obtained by sampling the optical waveform output from the E/O converter 2, and comparing the instantaneous amplitude of the optical waveform with the corresponding sample value output by the RAM LUT. Thus, as shown in FIG. 4, the optical waveform output from the E/O converter 2 is tapped (at 40), converted into a corresponding electrical signal (at 42), and sampled by an analog-to-digital (A/D) converter 44. The digital sample 46 generated by the A/D converter 44 is then supplied to a non-linear processor 48, which also receives sample values 20 of the predistorted signal 12 from the RAM LUT 32. By properly accounting for propagation delays, it is readily possible to match each sample value 20 with its corresponding digital sample 46 from the A/D converter 44. Comparison between these two values (20 and 46) facilitates calculation of a mapping which can be applied to the sample values 20 to compensate any non-ideal response of the entire signal path between (and including) the DAC 22 and the E/O converter 2, and thereby yield a desired waveform of the predistorted optical signal E.sup.1.sub.IN() at the output of the E/O converter 2.
(25) As may be seen in FIG. 4, the mapping calculated by the non-linear processor 48 can be used to drive a non-linear compensator 50 implemented as a second RAM look-up table positioned between the digital filter 18 and the DAC 22. Alternatively, the mapping can be used to adjust (at 52) the M-bit sample values 20 calculated (at 38) using the compensation function C(). In the latter case, sample values 20 output from the digital filter 18 will combine compensation for both dispersion of the link 4 and nonlinearities of the DAC 22 and E/O converter 2.
(26) In the embodiments of FIGS. 3 and 4, a single compensation path is provided for generating the predistorted signal x.sup.1(t) 12 based on the input signal x(t). As will be appreciated, such an arrangement is suitable for modulating a single dimension (e.g., either amplitude or phase) of the input signal x(t). Accordingly, the predistorted signal x.sup.1(t) 12 of the embodiments of FIGS. 3 and 4 will comprise an amplitude or phase modulated version of the original input signal x(t). However, the system transfer function T() of the optical communications system will normally be complex, so that an optical signal traversing the communications system will suffer distortions in both phase and amplitude. This issue is addressed by the embodiments of FIGS. 5-7.
(27) FIG. 5 is a block diagram schematically illustrating an embodiment of the present invention in which a 2-dimensional compensation processor 10 is used to compensate effects of a complex system transfer function T(). In general, the 2-dimensional compensation processor 10 provides an independent compensation path for each dimension of the communications system for which compensation is to be provided. In the embodiment of FIG. 5, it is desired to provide compensation for both amplitude and phase distortions. In this case, a pair of compensation paths generate respective output signals I and Q (at 56 and 58) that represent respective orthogonal (In-phase and Quadrature) components of the predistorted signal 12.
(28) The two output signals I and Q are then supplied to a suitable 2-dimensional optical modulator 30 capable of modulating the amplitude and phase of light generated by the optical source 28. Various optical modulators capable of providing this functionality are known in the art, such as, for example, Mach Zehnder modulators.
(29) As will be appreciated, the 2-dimensional compensation processor 10 can be implemented using multiple compensation paths to generate the predistorted signal x.sup.1(t) 12 in any desired format, such as, for example, polar coordinates. The only limitation here is that a suitable optical modulator 30 must be provided for modulating the appropriate optical components in response to the signal components generated by the 2-dimensional compensation processor 10.
(30) FIG. 6A is a block diagram illustrating an alternative embodiment of the 2-dimensional compensation processor 10 of FIG. 5. In this case, the pair of parallel NM RAM LUTs 32 of the digital filter 18 are replaced by a single N2M look-up table, which outputs parallel M-bit sample values 20 for each N-bit sequence of the input data x(t). As in the embodiment of FIG. 5, these M-bit sample values 20 represent respective orthogonal (e.g. In-phase and Quadrature) components of the predistorted signal x.sup.1(t) 12. Similarly, other signal formats, such as polar coordinates, may equally be used, provided only that the optical modulator 30 is capable of modulating the appropriate optical components in response to the signals generated by the multi-dimensional compensation processor 10.
(31) In the embodiment of FIGS. 4-6, the input signal x(t) is supplied directly to the SPC 34, which converts the serial bit stream x(t) into successive N-bit words of the parallel data stream 36. In some cases, however, it may be desirable to provide a converter 64 to map the serial bit stream x(t) into n-bit symbols 66 upstream of the SPC 34, as shown in FIG. 6B. This arrangement would be particularly suitable for Quadrature Phase Shift Keying (QPSK) encoded traffic, for example. In such cases, the SPC 34 will operate to convert each bit of the n-bit symbol stream 66 into successive N-bit words of a corresponding parallel data stream 36. The values stored in each register of the RAM LUT are calculated to implement a complex filtering operation to output appropriate parallel M-bit sample values 20 of the predistorted signal 12.
(32) Due to various factors (such as, for example imperfections, or electrical cross-talk) the orthogonal (i.e. In-phase and Quadrature) control signals 56, 58 may in fact produce a non-orthogonal response in the E/O converter 2. Accordingly, in the embodiment of FIG. 6A the non-linear compensator 50 is provided as a single 2M2M RAM LUT, instead of separate devices for each compensation path. Each of the 2.sup.2M registers of the non-linear compensator 50 stores previously calculated output numerical values for both control signals 56, 58.
(33) These output numerical values may be calculated in accordance with a mapping function derived from the combined lightwave emerging from the E/O converter 2, as described above with reference to FIG. 4. With this arrangement, the non-linear compensator 50 can readily implement a 2-D mapping which adjusts both components of the predistorted signal 12 in a complementary manner, to thereby compensate for any non-orthogonal response of the E/O converter 2. As mentioned above, the non-linear compensator 50 may be implemented as a separate device (from the RAM LUT 32), or may be embedded within the RAM LUT 32 by applying the mapping function to adjust the sample values stored in the RAM LUT 32.
(34) As may be appreciated, in order to successfully compensate dispersion of the link 4, it is important to maintain precise alignment between each of the signal components 56, 58 of the predistorted signal x.sup.1(t) 12 supplied to the optical modulator 30. However, the effects of independent DACs 22, filters 24 and LNAs 26 for each signal component may cause differential propagation delays between the non-linear compensator 50 and the optical modulator 30. Such differential delay can be compensated by means of a digital filter 60 located in at least one of the signal paths. In general, for an n-dimensional compensation processor 10 (i.e. generating n signal components of the predistorted signal 12) differential propagation delays can be compensated by providing a respective digital filter 60 in at least (n1) compensation paths. For convenience, it may be desirable to provide a digital filter 60 in all of the signal paths, as illustrated in FIG. 6A. In any event, each digital filter 60 can be controlled in a known manner to impose an selected delay, which is calculated to compensate for the differential propagation delays experienced by each of the signal components.
(35) If desired, the non-linear compensator 50 can also be used to implement a mapping between different signal formats. For example, in the embodiments of FIGS. 7A and 7B, the digital filter 18 is designed to generate the predistorted signal x.sup.1(t) 12 in the form of In-phase and Quadrature components. In addition to compensating non-ideal response of the signal paths between the DACs 22 and the E/O converter 2, the 2M2M mapping implemented in the non-linear compensator 50 may also be used to convert these I and Q signal components into a corresponding pair of Amplitude A and Phase component samples (i.e. following a polar coordinate system), as shown in FIG. 7A. Subsequent digital-to-analog conversion of the phase sample by the DAC 22a yields a phase control signal 56 which can be used to drive a conventional phase shifter 62 to modulate the phase of the optical signal. The amplitude of the optical signal can then be controlled using a conventional optical modulator (such as an electroabsorptive modulator (EAM) or a variable optical attenuator) in accordance with the amplitude control signal component 58.
(36) As shown in FIG. 7B, the digital filter 60a can be controlled to calculate the time derivative of the phase component, to yield a frequency sample f(n). In embodiments in which the non-linear compensator 50 is incorporated into the RAM LUT 32, the digital filter 18 can generate the predistorted signal x.sup.1(t) 12 in the form of Amplitude and frequency components directly. In either case, subsequent digital-to-analog conversion of the frequency sample f(n) by the DAC 22a yields a frequency control signal f(t) 56 which can be used to drive the optical source 28 directly. This operation exploits the known characteristic of many narrow-band lasers, which exhibit a frequency dependence on the input current. Accordingly, within a certain range, the frequency of the optical signal emitted by the optical source 28 can be controlled by varying the input current. Here again, the amplitude of the optical signal can be controlled using a conventional optical modulator (such as an electroabsorptive modulator (EAM) or a variable optical attenuator) in accordance with the amplitude control signal component 58.
(37) The embodiment(s) of the invention described above is (are) intended to be exemplary only. The scope of the invention is therefore intended to be limited solely by the scope of the appended claims.
