MXPA96006712A

MXPA96006712A - Synchronized polarization and phase modulation using a periodic wave form with complex harmonics for improved performance of opt transmission systems

Info

Publication number: MXPA96006712A
Application number: MXPA/A/1996/006712A
Authority: MX
Inventors: S Bergano Neal
Original assignee: Tyco Submarine Systems Ltd
Priority date: 1995-12-29
Filing date: 1996-12-19
Publication date: 1997-06-01

Abstract

The present invention relates to an apparatus method that provides improved performance by modulating the optical phase and polarization of a signaptic, with a periodic waveform having a harmonic content that is more complex than that associated with a simple sinuosidal waveform. A phase modulator receives an optical signal in which data has been modulated at a predetermined frequency. The phase modulator modulates the phasic phase in a continuous form with the periodic waveform with complex harmonics, where the fundamental phase modulation frequency is equal to the same predetermined frequency in which the data is modulated on the optical signal. . In another illustrative embodiment of the invention, a polarization modulator further processes the signaling by modulating the polarization state of the signal in a continuous form with the periodic waveform with complex harmonics, wherein the fundamental polarization modulation frequency is equal at the same predetermined frequency in which the data are modulated on the optical signal. In addition to continuously modulating, the polarization modulation is performed in such a way that the average value of the polarization state over each modulation cycle is substantially equal to

Description

SYNCHRONOUS POLARIZATION AND PHASE MODULATION USING A PERIODIC WAVE FORM WITH COMPLEX HARMONICS FOR IMPROVED TSTON OPTTPA TRAM SYSTEMS PERFORMANCE Technical Field The invention relates to the optical transmission of information and more particularly to synchronous polarization and phase modulation, using a periodic waveform with complex harmonics for improved performance of optical transmission systems. Antecedent is of the Invention Transmission trajectories of very long optical fibers, such as those used in transcontinental or submarine terrestrial light wave transmission systems, which employ optical amplifying repeaters, are subject to decreased performance due to a set of quality deteriorations. of transmission that accumulate over the length of the optical fiber comprising the transmission path. Typically, these impairments vary with time and cause a random fluctuation in the signal-to-interference ratio of the received signal. These deteriorations can arise from cumulative interference effects, caused for example by burned holes by polarization ("PHB") in the fiber adulterated with erbium used in optical amplifiers and waveform distortions caused by chromatic dispersion and optical non-linearities REF: 23362 through the transmission path. The coded state-of-polarization of the optical signal released to depolarize the optical carrier can improve the signal-to-interference received by reducing PHB. In my patent application of the U.S.A. Co-pending Serial No. 08 / 312,848, a setup is described wherein the bias coded frequency is chosen as the frequency of the synchronizer that defines the bit rate of the transmitter. This technique can provide more efficient use of optical bandwidth in complemented systems with wavelength division ("WDM"). This encoding with synchronous bit polarization is also an advantageous compensation, particularly between the two low speed and high speed coding regimes. In addition to the synchronous polarization encoding, overlapping phase modulation (PMM) can dramatically increase the eye opening of the received data pattern. The increase in eye aperture results from PM conversion into synchronous bit-width ("AM") modulation through chromatic dispersion and nonlinear effects in the fiber. SUMMARY OF THE INVENTION In accordance with the present invention, a method and apparatus providing improved performance is provided by modulating the optical phase and polarization of an optical signal with a periodic waveform having harmonic content that is more complex than that associated with a simple sinusoidal waveform. A phase modulator receives an optical signal in which data has been modulated at a predetermined frequency. The phase modulator modulates the phase of the optical signal in a continuous form with the periodic waveform with complex harmonics, where the fundamental phase modulation frequency is equal to the same predetermined frequency in which data is modulated on the signal optics. In another illustrative embodiment of the invention, a polarization modulator further processes the optical signal by modulating the state-of-polarization of the signal in a continuous form, with the periodic waveform with complex harmonics, wherein the modulation frequency of fundamental polarization is equal to the same predetermined frequency in which data is modulated in the optical signal. In addition to being continuously modulated, the polarization modulation is performed in such a way that the average value of the polarization state on each modulation site is substantially equal to zero. Brief Description of the Draw Figure 1 shows a simplified block diagram of a phase modulated transmitter mode according to the present invention. Figures 2 to 5 show exemplary waveforms used to direct the phase modulator in Figure 1.

Figure 6 shows a simplified block diagram of a polarized modulated phase and transmitter mode according to the present invention. Figure 7 shows a simplified block diagram of an alternate embodiment of the polarized and phase modulated transmitter, wherein the polarization and phase modulation is achieved in a single step according to the present invention. Figure 8 is a simplified block diagram of a transmission system architecture, incorporating the principles of the invention. Detailed Description L. Figure 1 shows a simplified block diagram of an exemplary assembly that facilitates the practice of the invention. As illustrated, the invention includes a laser 100 to produce a continuous wave optical signal ("CW") 101.

The optical signal 101 is transmitted to a data modulator 102, which modulates the signal to impart information in a conventional manner, to produce a modulated optical information signal 103. The data modulator 102 receives the signal data 116 to be imparted to the optical signal 101 from a data source 104. The data modulator 102 modulates the optical signal 101 at a frequency determined by a clock 106 by a synchronization signal on the line 117. The optical information signal 103 is transmitted from the modulator data 102 to a phase modulator 108. The phase modulator 108 encodes the phase of the optical carrier (i.e. the optical signal 101) in which the data signal has been modulated. In accordance with the principles of the present invention, the phase modulator 108 is directed by a periodic waveform 112. The periodic waveform 112 is generated using a waveform generator 110 and a variable delay element. The waveform generator 110 generates a periodic control signal 111. The periodic control signal 111 has the same fundamental frequency as the clock 106, and in addition is locked in phase to the clock 106 by a synchronization signal on the line 118. The relative delay between the signal data 116 conveyed in the optical information signal 103 and the periodic waveform 112, are determined by the variable delay element 107. As illustrated in FIG. 1, the variable delay element 107 is it couples to receive the periodic control signal 113 from the waveform generator 110. The delay is adjusted to facilitate the optimization of the transmission performance of a system, using the phase modulator described above. For example, the delay can be adjusted to maximize the signal-to-interference ratio or Q factor received at a remote receiver. It is emphasized, however, that the variable delay element 107 is considered optional and can be eliminated in some applications of the invention. The manner in which the periodic waveform 112 directs the phase modulator 108 can be described by examining the electric field of the optical signal in which the phase modulator acts: where ? is the optical carrier frequency, f (tj is the phase angle of the optical signal 105 and A (t) is considered to be the actual field amplitude and includes the intensity modulation.The optical phase 0 (t) is considered to be is addressed with a periodic function / (x), as described by the equation below: Here a is the optical phase modulation index, or is the phase modulation frequency (corresponding to the bit rate), f defines the relative delay between PM and the data bits and? denotes an arbitrary displacement. The time function f (x) is produced in the phase 110 waveform generator. By introducing the phase f as an additional selectively adjustable parameter, various amplitude errors adversely affect performance when a modulation format is used without Return to zero ("NRZ") can be reduced. These amplitude errors can be caused by a variety of factors including amplifier interference, chromatic dispersion and fiber nonlinearities. As mentioned above, the AM generated from the conversion of phase and polarization modulation that is caused by an interaction between the signal and the chromatic arrangement and non-linear fiber reflection index, can be beneficial if the phases of AM are adjusts appropriately with respect to the data. A graphical method for assessing the impact of impairments to a signal other than interference is known to those with ordinary skill in the art such as an eye diagram. The AM that is generated can "open" the eye of the receiver data and compensate for eye closure caused by error amplitude types. By properly adjusting phase f, eye opening can be improved. In operation, the phase f is adjusted by the variable delay element 107 until the signal-to-interference ratio ("SNR") of the received optical signal is optimized. Figures 2 to 5 show periodic waveform examples f (t) used to phase-modulate the optical information signal 103 (Figure 1). The waveform illustrated in Figure 2 is a simple sinusoidal signal as described in the aforementioned Co-pending Application. The waveforms illustrated in Figures 3 to 5 are examples of waveforms that include more elaborate harmonic content, which is designed to improve the performance of optical transmission systems in accordance with the principles of the present invention. The waveform illustrated in Figure 3 is a square wave limited in band, or a square wave with finite transition regions. This waveform is used to place a variant phase in time at the edges of data bits. As it is known by those with skill in the specialty, a phase variant in time is equal to a change in frequency through the relationship: at where? f represents frequency deviation. The waveforms illustrated in Figures 4 and 5 are provided as examples of a practical approach to a ramp function. The waveform illustrated in Figure 4 is a sinusoidal, with sinusoidal frequency modulation and is described by the following expression: f (t) = een (t + 0.6"sin (t)) (4) The waveform illustrated in Figure 5 is generated by a series of sinusoidal signals that are described by the following expressions: f (t) = sin (t) + 0.4 * sin (2t) + 0.2 * sin (4t) (5 Figure 6 shows an alternate embodiment of the invention, wherein a polarization modulation function has been added to the output of the apparatus illustrated in Figure 1. The laser 300 produces an optical signal CW 301. The optical signal 301 is transmitted to a data modulator 302 that modulates the signal to impart information in a conventional manner, producing a modulated optical information signal 303. The data modulator 302 receives the data to be imparted to the optical signal 301 from a data source 304 on the line 335 and modulates the optical signal 301 at a frequency determined by a clock 306 by a clock signal 337. The optical information signal 303 is transmitted from the modulator data 302 to a phase modulator 308, which modulates the phase of the optical information signal 303. The phase modulator 308 operates as described above when referring to FIG. 1. The phase modulated signal 330 emerging from the Phase modulator 308 is directed to a bias modulator 311 that modulates the state-of-polarization ("SOP") of the optical information signal 303. The bias modulator 311 op was to change the SOP of the optical information signal, so that it has no preferred SOP averaged over the modulation period. According to this, the output signal 314 has a degree of polarization that is substantially zero and is said to be polarization coded. In an example of the operation of the bias modulator 311, the SOP of the optical information signal 303 plots a large full circle in the Poincaré sphere. Alternatively, the SOP of the optical signal can reciprocate on the Poincaré sphere. In any case, the average value of the SOP over each modulation cycle is substantially equal to zero. An example of a polarization modulator 108 that can be employed in the present invention is described in U.S. Pat. No. 5,32 ^, 511 particularly in Figure 3 of that reference. According to the present invention, the bias modulator 311 is directed by a periodic waveform 316 produced by a bias waveform generator 312. As with the embodiment illustrated in Figure 1, the periodic waveform 316 may be take any of the exemplary shapes illustrated in Figures 2 to 5, in accordance with the principles of the invention. The periodic waveform has the same fundamental frequency as the clock 306, and in addition it is clocked in phase to the clock 306. The relative delay between the signal data modulated in the signal 303 and the periodic waveform 315 generated in 312, is it adjusts by a variable delay element 313. The manner in which the periodic waveforms 317 and 316 direct the phase modulator 308 and the bias modulator 311 respectively, can be described by examining the electric field of the optical signal in which it acts the phase modulator. In X-Y coordinates, these components can be expressed as follows: l («iW **) r (l,> E <) = A? t) e (6) (• '«•« .. in E> (') * AS ') é (7) where ? is the optical carrier frequency, fx (t) and fv (t) are the phase angles of optical signal 314 and Ax (t) and Ay. (t) are considered real field amplitudes and include intensity modulation. In principle, each possible SOP of an optical signal having these electric field components can be obtained by varying the ratio A? / Ay, while maintaining the value of (Ax2 + A) constant and varying the relative phase difference fx -fy between 0 and 2p. However, the polarization modulator 308 serves to modulate the SOP of the optical signal by varying only the phases f and fy, which is sufficient to provide an SOP whose average value over a modulation cycle is zero. This phase modulation can be described as: F,?"? , + a, (nr + H'1 + *. («f + * F2) (8) F "< '> -?, * 0 / ¡(Or + ?,) **, /, «* +) (9) As indicated by equations 8 and 9, the phase modulator 308 imparts the same phase modulation to both X and Y components of the optical signal 303, since it has the same phase modulation index a. According to this, the phase modulator 303, optic marrow of the signal 303 without modulating the polarization of the optical signal. The reason why the phase modulator 308 does not modulate the polarization is because the polarization modulation of the optical signal is proportional to the difference between the phases 0. and fv, and that difference is not affected by the phase modulator 308 and which modulates both 0"and f and by equal amounts. There are two categories of phenomena that can convert phase modulation and / or polarization to AM modulation, ie those that are polarization dependent and those that are polarization independent. An example of a polarization-dependent phenomenon is mediated by polarization-dependent loss ("PDL") in the transmission medium and as such, may fluctuate with time, causing further signal fading. An example of an independent polarization phenomenon is mediated by chromatic dispersion and / or a non-linear refractive index in the transmission fiber and as such does not fluctuate in time. As explained below, the AM generated by modulating the bit rate polarization does not contribute significantly to signal fading. When the coded polarization signal encounters an element having PDL, AM modulation may occur at the modulation frequency O and its harmonics (ie 2O, 3O,). The amount of AM and the phase relationship of the AM with respect to the polarization modulation phase; it generally depends on the orientation of the lost axis of the PDL element with respect to the polarization modulation axis. The amount of AM that occurs; it will fluctuate over time because the poise-state of the optical signal varies with time. As will be appreciated by those skilled in the art, a typical fiber optic receiver has an electrical bandwidth at approximately 60% of the data rate. In this way, something of the AM occurs at the bit rate can pass through the receiver to the decision circuit and effect the BER. However, the BER is not affected by AM that occurs in harmonics of the bit rate that has a frequency of 2 O or higher, since these harmonics are blocked by the receiver. In an analysis of AM formation caused by the interaction between the reciprocating SOP of the optical signal and the PDL elements, it can be shown that most of the AM modulation occurs in harmonics of the modulation frequency (ie 2 O and above) and not in the fundamental modulation frequency O. In this way, as noted above, the AM generated by bit-rate polarization modulation does not contribute significantly signal fading, considering that a properly designed optical receiver was used. The AM generated by the polarization and / or phase modulation conversion as a result of the chromatic expression and / or the non-linear refractive index of the optical fiber, can be beneficial, if the polarization modulation is performed at the bit rate. . In Figure 7, the functions of the phase modulator 308 and the bias modulator 311 illustrated in Figure 6 are both incorporated into a single unit 408. In this embodiment of the invention, a simple variable delay element 407 is used. to vary both the polarization modulation and the optical phase modulation. The polarization modulation is given by the difference in the angles fx -02 and is adjusted for a low degree of polarization. The excess phase modulation is given by the average of two angles (0? + 0i) / 2. The operation of this embodiment of the invention is similar to that illustrated in Figure 6 using the following expression: * 1-? 2 > f | -f2 (t). (8) Figure 8 is a simplified block diagram of a transmission system architecture, incorporating principles of the invention. A transmitter 500 that includes a laser, data source, data modulator and clock (not shown) is coupled to a modulator 510. In this exemplary transmission system architecture, the modulator 510 includes both polarization and phase modulation, that is, , it is configured in a similar assembly as the elements illustrated in Figure 4 and described above and thus incorporates the advantageous features provided therein. Alternatively, the modulator 510 can only incorporate the characteristics of the phase modulator as illustrated in Figure 1 (ie, not implement the polarization modulation described above). The receiver 520 is coupled to the modulator 510 via a forward transmission path 530. A monitor 550 is coupled to the receiver 520 to measure performance characteristics of the received optical signal at the receiver 520. The monitor 550 may for example be a speed detector. -of conventional bit error. A return telemetry path 540 couples the monitor 550 to the modulator 510. These performance characteristics include SNR or Q factor. The measured performance characteristic can be transmitted via the telemetry path 540 to the modulator 510. As discussed above, the parameter of phase f (ie relative delay) in equation 2 can then be adjusted in modulator 510 in response to the measured performance characteristic, to optimize the performance of the transmission system. It will be understood that the particular techniques described above are only illustrative of the principles of the present invention, and that various modifications may be practiced by those skilled in the art, without departing from the scope and spirit of the present invention, which is limited by those skilled in the art. following claims:? e notes that in relation to this date, the best method known by the applicant to carry out the aforementioned invention, is that which is clear from the present description of the invention. Having described the invention as above, property is claimed as contained in the following:

Claims

CLAIMS 1. Apparatus, characterized in that it comprises: an optical signal source, for generating an optical signal in which data is modulated at a predetermined frequency; a phase modulator coupled to the optical signal source, to modulate the phase of the optical signal, a periodic waveform generator coupled to the phase modulator, to generate a periodic control signal to control the modulation cycle of the modulator of phase; / a clock coupled to the phase modulator having a frequency that determines the periodicity of the control signal, wherein the clock frequency is equal to the predetermined frequency.
2. The apparatus according to claim 1, characterized in that the optical signal source includes a continuous wave optical signal generator and a data source, the clock is coupled to the data source to set the predetermined frequency in the which modulates the data on the optical signal.
3. The apparatus according to claim 1, further comprising a variable delay element that couples the waveform generator to the phase modulator to selectively vary a relative delay between the optical signal and the control signal. 4. - The apparatus according to claim 1, characterized in that the phase modulator is locked in phase to the clock. 5. The apparatus according to claim 1, characterized in that the phase modulator provides optical phase modulation to the optical signal, while substantially no polarization modulation is imparted to the optical signal. 6. The apparatus according to claim 5, characterized in that the clock is coupled to the phase modulator such that the phase modulation provides optical phase modulation at a frequency that is locked in phase and is equal to the predetermined frequency. . 7. The apparatus according to claim 1, characterized in that the control signal is a function of time f (t) described by f (tj = sin (t + 0.6 * sin (tj). according to claim 1, characterized in that the control signal is a function of time f (t) described by f (t) = sin (t + 0.4 * sin (2tj + 0.2 * sin (4t). in accordance with the claim 1, characterized in that the control signal is a periodic signal having a fundamental frequency and at least one other higher harmonic frequency. 10. - A method for modulating an optical signal, characterized in that it comprises the steps of: receiving an optical signal in which data has been modulated at a predetermined frequency; and modulating in phase the received optical signal with a periodic waveform at the predetermined frequency, wherein the periodic waveform includes a substantial fundamental frequency equal to the predetermined frequency and at least one higher harmonic frequency. 11. The method according to claim 10, characterized in that it further comprises the step of modulating a state of polarization of the optical signal at a frequency that is locked in phase and equal to the predetermined frequency when tracking the polarization of the optical signal on at least a portion of a Poincaré sphere, such that an average value of the polarization state over each modulation cycle is substantially equal to zero. 12. The method according to claim 11, characterized in that it further comprises the step of selectively varying the polarization modulation phase imparted to the optical signal. 13. The method according to claim 11, characterized in that it further comprises the step of selectively modulating the optical signal phase while substantially no polarization modulation is imparted to the optical signal. 14. The method according to claim 13, characterized in that the step of selectively modulating the optical signal comprises the step of selectively modulating the optical signal at a frequency equal to the predetermined frequency at which the signals are modulated. data. 15. The method according to claim 10, characterized in that the periodic waveform is a function of time f (t) described by f? T, >; = sin (t + 0.6 * sin (tJ) 16.- The method according to claim 10, characterized in that the periodic waveform is a function of time f (t) described by f (t /) = sin (t + 0.4 * sin (2t /) + 0.2 * sin (4t) 17. An apparatus, characterized in that it comprises: a polarization modulator adapted to receive an optical signal that is modulated with data at a predetermined frequency and to modulate the polarization state of the optical signal by plotting the polarization of the optical signal over at least a portion of a Poincaré sphere, such that an average value of the polarization state over a modulation cycle is substantially equal to zero; of phase coupled to the polarization modulator, to modulate the phase of the optical signal, and a first periodic wave generator coupled to the phase modulator to generate a first periodic control signal, to control the modulation cycle of the phase modulator, in where the first s The periodic control signal includes a fundamental frequency substantially equal to the predetermined frequency and at least one higher harmonic frequency. 18. The apparatus according to claim 17, characterized in that it also includes a second periodic waveform generator coupled to the polarization modulator to generate a second periodic control signal, to control the modulation cycle of the polarization modulator, in wherein the second periodic control signal includes a fundamental frequency substantially equal to the predetermined frequency and at least one higher harmonic frequency. 19. The apparatus according to claim 17, characterized in that it also comprises a first variable delay element arranged between the first waveform generator and the phase modulator, to selectively vary a relative delay between the optical signal and the first control signal. 20. The apparatus in accordance with the claim 17, characterized in that it further comprises a second variable delay element arranged between the first waveform generator and the bias modulator, to selectively vary a relative delay between the optical signal and the second control signal. 21. - The apparatus in accordance with the claim 19, characterized in that the first variable delay element or the second variable delay element is varied to optimize a predetermined characteristic that is measured by a receiver receiving the optical signal. 22. The apparatus according to claim 21, characterized in that the predetermined characteristic is the signal-to-interference ratio of the optical signal received by the receiver. 23.- The apparatus in accordance with the claim 21, characterized in that the predetermined characteristic is the Q factor of the optical signal received by the receiver. 24.- The device in accordance with the claim 17, characterized in that it also includes a clock coupled to the first waveform generator having a clock frequency that determines the frequency of the fundamental frequency. 25. An apparatus, characterized in that it comprises: a polarization modulator adapted to receive an optical signal that is modulated with data at a predetermined frequency and to modulate the polarization state of the optical signal, by tracking the polarization of the signal optical on at least a portion of a Poincaré sphere, such that an average value of the polarization state over a modulation cycle is substantially equal to zero; a periodic waveform generator coupled to the bias modulator for generating a periodic control signal, for controlling the modulation cycle of the bias modulator, wherein the periodic control signal includes a fundamental frequency substantially equal to the predetermined frequency and minus a higher harmonic frequency. 26.- The device in accordance with the claim 25, characterized in that it also includes a clock coupled to the waveform generator having a clock frequency that determines the predetermined frequency and the fundamental frequency.