Abstract:
A device and technique for aligning an optical carrier signal (e.g., a soliton pulse train) with a data signal in a transmitter. According to the invention, the device is configured to analyze the radio frequency (RF) spectrum of the transmitter&#39;s output. In one implementation, the device evaluates the amount of energy in a certain frequency band located near a selected null of the RF spectrum. In another implementation, the device examines the shape of the RF spectrum within that frequency band. In either case, based on the analysis, the device adjusts the phase of the clock signal driving an electro-optic (E/O) modulator in the transmitter. Such adjustment reduces misalignment between the optical carrier signal and data resulting, e.g., from thermal effects in the E/O modulator. The device may be used, e.g., in long-haul optical transmission systems operating at 10 GBit/s.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to communication equipment.  
           [0003]    2. Description of the Related Art  
           [0004]    Transmission of optical signals through fiber-optic networks is widely used in modern communication systems. In particular, long-haul, high data-rate wavelength division multiplexed (WDM) optical transmission is an important component of optical networking. One known way to accomplish long-haul transmission is by using soliton optical pulses. Due to special non-linear optical characteristics, a soliton pulse is less susceptible to chromatic and polarization mode dispersion than, e.g., a rectangular pulse. As such, soliton pulses can provide relatively low bit error rates and therefore high reliability for optical transmission.  
           [0005]    [0005]FIG. 1 shows a typical prior art system  100  for transmitting data using soliton pulses. System  100  is configured to convert an electronic data stream  102  into an optical signal  104 . System  100  comprises a laser  106  that generates a continuous wave (CW) beam of light. This beam is fed into an optical fiber and delivered to a first electro-optic (E/O) modulator  108 . Modulator  108 , also called a pulse carver, is configured to generate an optical pulse train of soliton pulses based on control signals from a modulator driver  114  receiving an electrical input signal  112 . Signal  112  may be a sine wave at a reference clock frequency. The output of modulator  108  is a soliton pulse train  118 . Depending on the type of E/O modulator, the frequency of pulse train  118  may be equal the frequency of signal  112  or harmonically related to it. Pulse train  118 , also called an optical carrier signal, is fed into a second E/O modulator  110  configured to modulate said pulse train based on control signals from a second modulator driver  116  receiving data stream  102 . The output of modulator  110  is optical signal  104 . In different types of transmitters not using soliton pulses, an optical carrier signal analogous to carrier signal  118  may be a different periodically modulated optical signal.  
           [0006]    One problem with system  100  is that it requires synchronizing optical carrier signal  118  and electronic data stream  102 . Such synchronization is difficult to maintain due to often occurring and, in general, poorly controllable phase drifts in E/O modulators. As a result of phase drift, carrier signal  118  and data stream  102  may become misaligned causing inaccuracies in signal  104 .  
           [0007]    FIGS.  2 A-B illustrate the effect of misalignment of signals  102  and  118  on signal  104 . As shown in FIG. 2A, when signal  102  is properly aligned with signal  118 , modulator  110  transmits or blocks a carrier-signal pulse depending on the logical input to driver  116 . However, as shown in FIG. 2B, when signals  102  and  118  are misaligned, the shape of a transmitted pulse is distorted and/or a pulse is not properly blocked. Distorted pulses do not have the correct soliton waveform required for propagation through a long-haul optical fiber. In addition, misalignment may result in the transmission of portions of carrier-signal pulses that ideally should not be transmitted. Both of these effects may result in increased bit error rates at a receiver.  
         SUMMARY OF THE INVENTION  
         [0008]    In a preferred embodiment, the present invention is a device and technique for aligning an optical carrier signal (e.g., a soliton pulse train) with data in an optical transmitter. The device is configured to analyze the radio frequency (RF) spectrum of the transmitter&#39;s output. In one implementation, the device evaluates the amount of energy in a certain frequency band located near a selected null of the RF spectrum. In another implementation, the device examines the shape of the RF spectrum within that frequency band. In either case, based on the analysis, the device adjusts the phase of the clock signal driving an electro-optic (E/O) modulator in the transmitter. Such adjustment reduces misalignment between the optical carrier signal and the data resulting, e.g., from thermal effects in the E/O modulator. The device may be used, e.g., in long-haul optical transmission systems operating at 10 GBit/s.  
           [0009]    According to one embodiment, the present invention is an apparatus for reducing misalignment between a carrier signal and a data signal, the apparatus comprising: (a) an analyzer configured (i) to analyze an input signal corresponding to the carrier and data signals, and (ii) to generate a control signal based on the analysis; and (b) a phase shifter configured to introduce a phase shift between the data signal and a clock signal using the control signal, wherein the carrier signal is based on the clock signal.  
           [0010]    According to another embodiment, the present invention is a method of reducing misalignment between a carrier signal and a data signal, comprising the steps of: (i) analyzing a data-modulated signal corresponding to the carrier and data signals; and (ii) introducing a phase shift between the data signal and a clock signal based on the analysis, wherein the carrier signal is based on the clock signal. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    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:  
         [0012]    [0012]FIG. 1 shows a prior art system for transmitting data using an optical train of soliton pulses;  
         [0013]    FIGS.  2 A-B illustrate the effect of alignment between the carrier and data signals on the output signal in the system of FIG. 1;  
         [0014]    FIGS.  3 A-B show representative spectra of a data-modulated signal produced using an optical pulse train and a pseudo-random data signal having a bit rate of about 10 GBit/s;  
         [0015]    [0015]FIG. 4 shows a system for transmitting data according to one embodiment of the present invention;  
         [0016]    [0016]FIG. 5 shows a block diagram of a power analyzer that can be used in the system of FIG. 4 according to one embodiment of the present invention;  
         [0017]    [0017]FIG. 6 illustrates the operation of the power analyzer of FIG. 5;  
         [0018]    [0018]FIG. 7 shows a block diagram of a spectrum analyzer that can be used in the system of FIG. 4 according to another embodiment of the present invention;  
         [0019]    FIGS.  8 A-B illustrate one type of analysis that can be implemented in the spectrum analyzer of FIG. 7 according to one embodiment of the present invention; and  
         [0020]    [0020]FIG. 9 illustrates the results of the analysis illustrated in FIG. 8. 
     
    
     DETAILED DESCRIPTION  
       [0021]    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. Although the invention is particularly suitable for use with communications equipment, those skilled in the art can appreciate that the invention can be equally applied to other types of electrical and/or optical equipment.  
         [0022]    Before embodiments of the present invention are described in detail, spectral properties of modulated optical signals, such as signal  104  of system  100 , are briefly characterized.  
         [0023]    [0023]FIGS. 3A and 3B show two representative spectra of a data-modulated optical signal produced using an optical pulse train of soliton pulses and a pseudo-random non-return-to-zero (NRZ) data signal having a bit rate of about 10 GBit/s. FIG. 3A shows a spectrum of a data-modulated signal (e.g., signal  104  of system  100 ) when the carrier signal (e.g., signal  118 ) and the data signal (e.g., signal  102 ) are properly aligned. The spectrum exhibits a generally flat background with a sharp peak  302  corresponding to the modulation frequency, i.e., about 10 GHz.  
         [0024]    [0024]FIG. 3B shows a typical spectrum of a data-modulated signal when the carrier and data signals are misaligned. As can be seen in FIG. 3B, the spectral background is no longer flat, but rather, exhibits spectral nulls, e.g., nulls  304  and  306  at about 6 and 17 GHz, respectively. The presence of one or more nulls in the spectrum is indicative of misalignment and may be used to detect and correct the same. The position, shape, and number of nulls depends on certain characteristics of the system, such as modulation frequency, pulse shape, data format, etc. For example, an optical pulse train of soliton pulses modulated with pseudo-random NRZ data having a bit rate of X GBit/s will have a spectral null at about 0.6X GHz.  
         [0025]    [0025]FIG. 4 shows a transmission system  400  according to one embodiment of the present invention. System  400  comprises system  100  of FIG. 1 (already described above) and an alignment device  402 . Device  402  is configured to generate feedback to modulator  108  of system  100  based on signal  104  to maintain carrier signal  118  in alignment with the input data signal (i.e., signal  102 ).  
         [0026]    In one embodiment, device  402  comprises a photodetector  404 , an analyzer circuit  406 , and a voltage-controlled phase shifter  408 . Phase shifter  408  may be, for example, PS-1401 available from Communication Techniques, Inc. of Wippany, N.J. A small fraction of the optical output of system  100  is delivered to photodetector  404  (e.g., a photodiode) using an optical tap. Photodetector  404  is configured to convert an optical tap signal  410  into an electrical signal  414  corresponding to optical signal  104 . Analyzer  406  processes signal  414  and, based on the processing, generates a control signal  416  applied to phase shifter  408 . Based on signal  416 , phase shifter  408  adjusts the phase of clock signal  112  to generate a phase-shifted clock signal  412  applied to driver  114  of modulator  108 . Using signal  412  instead of signal  112 , e.g., helps to compensate for phase drifts of modulator  108  and maintain signals  118  and  102  in better alignment with each other.  
         [0027]    [0027]FIG. 5 shows a block diagram of a power analyzer  500  that can be used as analyzer  406  of system  400  according to one embodiment of the present invention. Power analyzer  500  is configured to generate control signal  416  based on the amount of energy in a certain band located near a spectral null. In one implementation, the band may be centered on null  304  (see FIG. 3B) and have a bandwidth of, e.g., 2 GHz. In a different implementation, a different bandwidth and/or a different null or combination of nulls may be used. In general, control signal  416  causes phase shifter  408  to introduce such phase difference between clock signals  112  and  412  as to maintain the amount of energy in the chosen band/combination of bands at a specific (e.g., maximum) level.  
         [0028]    In one embodiment, power analyzer  500  comprises a bandpass filter (BPF)  502 , an envelope detector (ED)  504 , a low-pass filter (LPF)  506 , and a control signal generating circuit  508 . BPF  502  is configured to pass a portion of signal  414  corresponding to the pass band of the BPF. In one embodiment of the present invention, the pass band of BPF  502  is from about 5 GHz to about 7 GHz. In other embodiments, the pass band of BPF  502  may be configured differently depending, e.g., on the particular spectral null to be used.  
         [0029]    ED  504  is configured to detect the radio frequency (RF) power in the pass band of BPF  502 . In one embodiment, detector  504  may be a Schottky diode whose output voltage is proportional to the RF power in the pass band of BPF  502 . The output signal of ED  504  is a relatively slow changing signal corresponding to the relatively slow phase drift (mostly thermal in nature) of modulator  108  of system  400 . This signal is processed by LPF  506  and applied to circuit  508 .  
         [0030]    In one embodiment, circuit  508  may be an analog circuit. In another embodiment, circuit  508  may include digital circuitry. For example, as shown in FIG. 5, circuit  508  comprises an analog-to-digital converter (ADC)  510 , a digital processor  512 , and a digital-to-analog converter (DAC)  514 . ADC  510  can be a relatively low speed ADC configured to measure the amplitude of the output of LPF  506 . Based on the measured amplitude, processor  512  generates a digital control signal that is then converted by DAC  514  to analog control signal  416  applied to phase shifter  408 .  
         [0031]    [0031]FIG. 6 further illustrates the operation of power analyzer  500  in a 10-GHz system. More specifically, FIG. 6 shows the dependence of the average power of signal  414  in the 2-GHz band centered on spectral null  304  on the relative delay between signals  102  and  112 . Delaying clock signal  112  causes signal  118  to go in or out of alignment with signal  102 . For example, at a delay of about 220 or 320 picoseconds (ps), signals  102  and  118  are properly aligned. As seen in FIG. 6, proper alignment corresponds to a relatively high power level (i.e., −80.5 dBm) in the 2-GHz band. Similarly, when clock signal  112  is delayed by about 270 or 370 ps, the power level is relatively low (i.e., −88.5 dBm) indicating that signals  102  and  118  are misaligned. Therefore, to maintain signals  102  and  118  in alignment, power analyzer  500  may configure phase shifter  408  by way of control signal  416  to apply a time delay, e.g., of about 220 ps to clock signal  112 .  
         [0032]    [0032]FIG. 7 shows a block diagram of a spectrum analyzer  700  that can be used as analyzer  406  of system  400  according to another embodiment of the present invention. Spectrum analyzer  700  is configured to generate control signal  416  based on the spectral shape of signal  414  within a selected frequency range near a spectral null. In one implementation, the frequency range may be centered on null  304  (see FIG. 3B) and be within, e.g., ±3 GHz from the position of said null. In a different implementation, a different frequency range and/or a different null or combination of nulls may be used. In one embodiment, control signal  416  causes phase shifter  408  to introduce such phase difference between clock signals  112  and  412  so as to flatten the shape of the spectrum within the selected frequency range. In different embodiments, different shape criteria for the spectrum may be applied.  
         [0033]    In one embodiment, spectrum analyzer  700  comprises a BPF  702 , a mixer  704 , an LPF  706 , a control signal generating circuit  708 , a sawtooth generator  716 , and a voltage-controlled oscillator (VCO)  718 . BPF  702  is configured to pass a portion of signal  414  corresponding to the pass band of the BPF. In one embodiment of the present invention employed in a 10-GHz system, the pass band of BPF  702  is from about 3 GHz to about 9 GHz. In other embodiments, the pass band of BPF  702  may be configured differently depending, e.g., on the particular frequency range and/or the spectral null to be used.  
         [0034]    VCO  718  is configured to sweep across a selected frequency range, e.g., the pass band of BPF  702 , using a sawtooth waveform from generator  716 . Generator  716  also applies that waveform to circuit  708 . Mixer  704  multiplies the outputs of BPF  702  and VCO  718  to place at DC a portion of the power spectrum of signal  414  corresponding to the instant frequency of VCO  718 . That portion is passed onto circuit  708  via LPF  706  which filters out the relatively high-frequency components also present in the multiplied signal.  
         [0035]    In one embodiment, circuit  708  comprises an envelope detector  720 , an ADC  710 , a digital processor  712 , and a DAC  714 . Detector  720  may be a detecting log amplifier configured to generate an output voltage proportional to the logarithm of in-band power of LPF  706 . In one implementation, detector  720  may have a bandwidth and log-linear range of about 0-500 MHz and 90 dB, respectively. In other implementations, a different suitable detector may be used.  
         [0036]    ADC  710  is configured to measure the amplitude of the output of detector  720 . ADC  710  is further configured to measure the output voltage of generator  716 . Based on these measurements, ADC  710  outputs, e.g., a pair of values corresponding to a frequency within the frequency range swept by VCO  718  and a power level of signal  414  at that frequency. Therefore in each frequency sweep, a power spectrum of signal  414  is measured and output to processor  712  which is configured to analyze the shape of that power spectrum using a set of selected criteria. Based on the analysis, processor  712  generates a digital control signal that is then converted by DAC  714  to analog control signal  416  applied to phase shifter  408 .  
         [0037]    [0037]FIGS. 8A and 8B illustrate one type of analysis that can be implemented in processor  712  according to one embodiment of the present invention. FIG. 8A shows a representative set of power spectra received by processor  712  from ADC  710 . Each spectrum, S(ƒ), is approximated with a second order polynomial, e.g., using Equation (1) as follows:  
           S (ƒ)=α 2 ƒ 2 +α 1 ƒ+α 0   (1) 
         [0038]    A representative result of such approximations is shown in FIG. 8B. Processor  712  evaluates the concavity of a recent spectrum, e.g., using the value of α 2  corresponding to that spectrum. Based on that value, the processor derives a phase shift that needs to be applied to clock signal  112  by phase shifter  408  to flatten out the spectrum (i.e., to minimize α 2 ). Processor  712  then generates a digital control signal corresponding to the derived phase shift. That digital control signal is then converted to control signal  416  by DAC  714  and used by phase shifter  408  to generate phase-shifted clock signal  412 .  
         [0039]    [0039]FIG. 9 shows a set of α 2  values derived from the spectra of FIG. 8B as a function of time delay introduced by phase shifter  408  between clock signals  112  and  412 . As explained earlier in the context of FIG. 6, delaying clock signal  112  causes signal  118  to go in or out of alignment with signal  102 . For example in a 10-GHz system, at the delay of about 220 ps, signals  102  and  118  are properly aligned, while at the delay of about 270 ps, those signals are misaligned. FIG. 9 shows that proper alignment corresponds to low concavity of the spectrum (i.e., near zero) whereas misalignment results in relatively high concavity (i.e., about 6×10 −19  dBm/Hz 2 ). Therefore similar to the results of FIG. 6, to maintain signals  102  and  118  in alignment, spectrum analyzer  700  may configure phase shifter  408  by way of control signal  416  to apply a time delay, e.g., of about 220 ps to clock signal  112 .  
         [0040]    Spectrum analyzer  700  has the advantage of being less susceptible to gradual laser power fluctuations (e.g., that of laser  106  of system  100 ) than power analyzer  500 , whereas power analyzer  500  can be implemented using fewer and/or less expensive components than spectrum analyzer  700 . Therefore depending on the particular application, power analyzer  500  or spectrum analyzer  700  may be used. For example, it may be preferable to use power analyzer  500  with optical transmitters having relatively stable optical power levels. Likewise, spectrum analyzer  700  may be preferred in situations where laser power is relatively unstable.  
         [0041]    Analyzer  406  of system  400  may be implemented using any suitable technology, e.g., as an ASIC or as discrete circuit elements. Alignment device  402  may be adapted to align signals having different data rates (e.g., 10, 20, or 40 GBit/s) and to accept clock signals represented by different waveforms. Furthermore, alignment device  402  may be configured for use with pure electronic circuits, in which situation photodetector  404  may be excluded. In different embodiments, photodetector  404  may be based on any suitable light-sensitive device, such as, for example, a photodiode, a phototransistor, a photogate, photo-conductor, a charge-coupled device, a charge-transfer device, or a charge-injection device. Similarly, as used in this specification, the term “light” refers to any suitable electromagnetic radiation in any wavelength that may be used in an optical transmission system, such as system  100 . Modulators employed in system  100  may be, for example, lithium niobate Mach-Zhender type modulators operating at, e.g., 1550 nm. In various embodiments, digital processors  512  and  712  may be specialized processors designed for their respective circuits  508  and  708  or be part of a different circuit or device connected to analyzer  406 . Furthermore, said digital processors may be configured to use look-up tables for generating their respective digital control signals. In some embodiments, a delay may be applied to the data signal (e.g., signal  102 ) instead of the clock signal (e.g., signal  112 ).  
         [0042]    While this invention has been described with reference to illustrative embodiments, this description is not intended to 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.  
         [0043]    Although the steps in the following method claims, if any, are recited in a particular sequence with corresponding labeling, unless the claim recitations otherwise imply a particular sequence for implementing some or all of those steps, those steps are not necessarily intended to be limited to being implemented in that particular sequence.