« PreviousContinue »
METHOD AND SYSTEM FOR EQUALIZING
PMD USING INCREMENTAL DELAY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to fiber optic transmission and communication of information, and more specifically to a solution to equalizing Polarization Mode Dispersion.
2. Related Art
POLARIZATION-MODE DISPERSION (PMD)
Fiber optics technology is revolutionizing the telecommunications field. The main driving force is the promise of extremely high communications bandwidth. At high bandwidths, a single beam of modulated laser light can carry vast amounts of information—equal to hundreds of thousands of phone calls or hundreds of video channels.
However, pulse broadening limits the effective bandwidth and propagation distance of an optical communication signal. Because of the inherent dispersive nature of an optical fiber medium, all portions of a light pulse do not travel the same distance through an optical fiber causing pulse broadening.
FIG. 1 illustrates how pulse broadening arises from varying light propagation delays which eventually distorts light output. Digital input pulses 100 are input to an optical fiber medium 110. The amplitude-modulated pulses are generated by a modulated laser source, such as a direct-modulated laser or an externally-modulated laser.
Different portions of a light pulse encounter varying propagation delays arising from, inter alia, the varying lengths of reflected paths within optical fiber 110. For clarity, three paths are illustrated which correspond to a relatively straight, short path 100a, a reflected, intermediate length path lOOfc, and a relatively long, reflected path 100c. Due to the varying propagation delays, see, e.g., the At delay in arrival time between 100a and lOOfc, the combined optical output is distorted. Thus, a photoreceptor detecting the output pulses 100fl-100c will generate a distorted output 120.
As shown in FIG. 2, such pulse broadening can lead to intersymbol confusion. "Pulse broadening" is called "dispersion" or "spreading" because of the non-uniform way in which parts of the incident signal 200 propagate through a dispersive fiber medium. In a mild form of dispersion, the transitions between ON and OFF states observed at a receiver are not as abrupt and distinct as the transitions that were originated by a transmitting laser. More severe blurring in the time domain limits the useful bandwidth of the path.
In FIG. 2, dispersion effects have broadened two closely spaced pulses to the extent that they are almost indistinguishable, as indicated by a question mark in the output signal 220. This will cause an information bit to be received erroneously, with perhaps disastrous results on network communication and customer dissatisfaction.
Several refinements have been made to reduce dispersion and increase the useful bandwidth. First, single-mode fiber was developed having a slender core such that there is essentially only a single light path through the fiber. Secondly, the distributed feedback (DFB) laser was developed with an extremely narrow distribution of output wavelengths. This technique minimizes chromatic dispersion caused by the fact that different wavelengths traverse the
length of the fiber over longer periods of time. Finally, a dispersion-shifted fiber material was produced to minimize the increased time-v.-wavelength dependency at a specific wavelength of fifteen hundred and fifty nm. common in
5 telecommunication applications.
Cumulatively, recent improvements in fiber materials and transmitter devices have reduced pulse dispersion and increased working bandwidth. "Lightwave" technology has advanced at such a pace that the bandwidth capabilities have
1° more than doubled every two years. As a result, working bandwidths, expressed in terms of digital bit-per-second rates, have escalated from 500 Mbps to 10 Gbps.
PROBLEM SOLVED BY THE INVENTION
15 These progressively more exotic refinements have brought the technology to a new bandwidth barrier: Polarization-Mode Dispersion (PMD). Previously, PMD was insignificant in magnitude relative to other dispersive effects, but now it is a limiting factor. It is well known that
light can be polarized and that, for a given beam of light, this polarization can be expressed in terms of two orthogonal axes that are normal to the axis of propagation. As a beam of light propagates through a fiber, the light energy present along one such polarization may leak into the other polarization.
This leakage would normally be of little consequence (lightwave receivers will detect both polarizations), except that real world fibers carry different polarizations at slightly
30 different time delays due to reflection. This effect can be on the order of 10-20 picoseconds (ps) in a 100 km fiber and becomes important when the modulating pulses are 50-100 ps. in width. To complicate matters, the polarization dispersion within a given fiber changes as a function of time and
35 temperature. Therefore, an effective PMD compensation mechanism must monitor and adapt to the changes so as to keep PMD to a minimum.
To nullify the effects of PMD, researchers have suggested application of an adaptive compensation device in an optical
40 path at the receive end just before the receiving transducer. These compensators typically employ a detector for analyzing the relative partitioning and delay of the incoming signal along two orthogonal polarizations. The compensators correct a data signal by purposefully adding delay selectively to
45 one polarization or another. A controller interprets the findings of the delay analyzer and manipulates adjustable delay elements so as to compensate for the polarization-dependent delay differences caused by the imperfect fiber transmission path. However, these techniques are not practical in tele
50 communication applications, such as, long-haul optical fiber communication.
The variable delay elements are usually optical fibers that are either heated or squeezed to alter their propagation characteristics. While these elements are adaptable to labo
55 ratory electronic control techniques, they are inadequate in terms of reproducibility and predictability of response. They are also impractical for use in a commercial traffic-bearing fiber network wherein recovery time following an equipment or power failure should be minimized. See, e.g., Ozeki,
60 et al., "Polarization-mode-dispersion equalization experiment using a variable equalizing optical circuit controlled by a pulse-waveform-comparison algorithm," OFC '94 Technical Digest, paper TuN4, pp. 62-64; Ono, et al., "Polarization Control Method for Suppressing Polarization Mode
65 Dispersion Influence in Optical Transmission Systems", Journal of Lightwave Technology, Vol. 12, No. 5, May 1994, pp. 89-91; Takahasi, et al., "Automatic Compensation Tech3
nique for Timewise Fluctuating Polarization Mode Dispersion in In-line Amplifier Systems", Electronics Letters, Vol. 30, No. 4, February 1994, pp. 348^19; and WO 93/09454, Rockwell, Marshall A.; Liquid Crystal Optical Waveguide Display System (each of which is incorporated in its entirety 5 herein by reference).
What is needed is a method and system for testing the ability of various receivers to withstand the effects of PMD and equalizing those effects. Further, a PMD compensation method and system is required which is reliable, responsive, 1° and effective in commercial telecommunication networks.
SUMMARY OF THE INVENTION
The present invention provides a system and method for testing the ability of various receivers to withstand the 15 effects of PMD and compensating for polarization mode dispersion (PMD) in an optical data signal using optical switch elements to provide incremental delays between different polarization modes of the optical data signal.
In one embodiment, a PMD simulator receives a signal, breaks the signal into its various polarization modes, and provides an appropriate variable time delay element in the path before the receiver. A controller will vary the frequency and the amount of dispersion that each of the polarization modes of light signals will have as they come out of the fiber. A number of receivers can be tested utilizing the same test pattern to compare performance, thereby providing a uniform basis of comparison and equalizing their performance. The simulator can also be utilized for quality control, e.g. by gathering time variant dispersion data from a computer on a network and utilized this information in the simulation system controller to test all of the other circuits against what was actually measured.
To provide the time delays, for example, a polarization 3J mode separator separates the optical data signal into first and second orthogonally polarized optical signals. A first variable switching delay element provides a first incremental propagation delay for the first polarized optical signal. A second variable switching delay element provides a second 4Q incremental propagation delay for the second polarized optical signal. A controller controls optical switches in the first and second variable switching delay elements to set first and second incremental propagation delays.
In particular, the first and second polarized optical signals 45 are incrementally delayed relative to one another so as to compensate for polarization mode dispersion. A beam combiner then combines the first and second polarized optical signals to form an optical output data signal which can be detected accurately and reliably by a receiver without the 50 effects of polarization mode dispersion. In this way, optical data signals can be transmitted over greater distances along a long-haul fiber optic dispersive medium at even greater bit-rates and bandwidth.
In one aspect of the present invention, a beam splitter 55 diverts a portion of an input optical data signal to a delay detector. The delay detector detects a relative delay between orthogonal polarization modes of the optical data signal due to polarization mode dispersion. The controller then uses the detected relative delay to control the first and second vari- 60 able switching delay elements so to counteract the detected relative delay.
According to a further aspect of the invention, the first and second variable switching delay elements consist of a series of optical switches optically interconnected by different 65 incremental lengths of optical fiber. 2x2 optical switches are provided for switching between a reference fiber segment
and a delay fiber segment to provide a relative incremental propagation delay. Different 2x2 optical switch configurations can be used including arrangements utilizing semiconductor optical amplifiers or Mach-Zehnder switches.
Compared to other known technologies, the present invention is more reliable and predictable in its response and is therefore more mass-producible. Furthermore, it has an extremely fast response time that is independent of the degree of delay adjustment needed. This is a particular advantage in a mission-critical high data rate optical communications network. The application and commercialization of this invention is very timely as the optical network technologies are approaching the PMD barrier.
Further features and advantages of the present invention, as well as the structure and operation of various embodiments of the present invention, are described in detail below with reference to the accompanying drawings.
BRIEF DESCRIPTION OF THE FIGURES
The accompanying drawings, which are incorporated herein and form part of the specification, illustrate the present invention and, together with the description, further serve to explain the principles of the invention and to enable a person skilled in the pertinent art to make and use the invention, in which:
FIG. 1 illustrates pulse broadening through a dispersive optical fiber;
FIG. 2 shows the intersymbol interference caused by pulse broadening;
FIG. 3A shows an optical communication link having an automatically controlled PMD compensator in accordance with a preferred embodiment;
FIG. 3B is a detailed block diagram of the PMD compensator shown in FIG. 3A in accordance with a preferred embodiment;
FIG. 4 shows a controlled, variable, incremental delay switching element using optical switches and fixed delay elements in accordance with a preferred embodiment;
FIGS. 5A, 5B, 5C and 5D illustrate examples of optical switches in accordance with a preferred embodiment; and
FIG. 6 is a block diagram of a PMD simulator in accordance with a preferred embodiment.
The present invention will now be described with reference to the accompanying drawings. In the drawings, like reference numbers indicate identical or functionally similar elements. Additionally, the left-most digit(s) of a reference number typically identifies the drawing in which the reference number first appears.
DETAILED DESCRIPTION OF THE
A PMD compensator in accordance with a preferred embodiment is based on delay elements that use optical switching to add well-defined increments of delay to polarized portions of an optical signal. An apparatus in accordance with a preferred embodiment is depicted in FIGS. 3A and 3B of the attached drawings.
FIG. 3A shows a typical end-to-end network path. A transmitter 300 transmits an optical data signal through an optical fiber 310 for detection by a receiver 350. For example, transmitter 300 can be a DFB laser modulator or any other type of modulated light source for producing an optical data signal. Optical fiber 310 is a long-haul, singlemode, dispersion-shifted fiber approximately one-hundred
kilometers in length. More generally, any type of optical fiber can be used. Additional fibers, line amplifiers, and/or repeaters can also be included between the transmitter 300 and receiver 350. Receiver 350 can be any suitable photodetector for detecting the modulated optical data signal. 5
A polarization-mode dispersion (PMD) compensator 320 is provided along optical fiber 310 near the receiver 350. According to the present invention, the PMD compensator 320 equalizes PMD which further increases bandwidth and transmission range. For example, given the presence of 1° PMD compensator 320, transmitter 300 can transmit modulated laser data on the order of one to one-hundred Gb/s (gigabits/sec), or more, over a one-hundred km. single-mode fiber without intersymbol interference caused by PMD. Receiver 350 can detect the output reliably and accurately in :5 a telecommunication environment.
FIG. 3B shows a block diagram of the components of the PMD compensator 320 in accordance with one embodiment of the present invention. Optical paths are indicated generally by a loop along the transmission path. The optical data 20 signal traveling through optical fiber 310 enters beam splitter 322 as optical input 315. A portion of the optical input 315 is diverted to a delay detector 327. The delay detector detects delays between two orthogonal polarization modes of the detected light. The operation and implementations of 25 such delay detectors is well-known and need not be described in further detail.
The majority of the optical input 315 passes through beam splitter 322 to a polarization-dependent beam splitter 324. 3Q The polarization-dependent beam splitter is a type of polarization mode separator which separates the optical input 315 into two optical signals that are orthogonally polarized with respect to one another. The two polarized signals travel along two separate paths leading through respective variable 3J switching delay elements 326 and 328. Each of the variable switching delay elements 326 and 328 provides a respective incremental delay to optical signals passing therethrough to equalize or compensate for PMD. After passing through the delay elements 326 and 328, the two orthogonally polarized 4Q optical signals are recombined by beam combiner 340. An optical output 345 having little or no polarization-mode dispersion is then returned to optical fiber 310 and/or receiver 350.
The degree of incremental delay, if any, imparted by the 45 variable switching delay elements 326 and 328 is manipulated by control signals received over respective control lines 331 and 332 from a controller 330. Controller 330 receives data output from the delay detector 327 representing the magnitude of PMD over an output line 329. Con- 50 trailer 330 then processes the data and generates control signals for the variable switching delay elements 326 and 328 to counteract the PMD effects of the long transmission path along fiber 310. The control signals may be of any conventional form including electrical light or electromag- 55 netic depending on the nature of the control lines 331 and 332. In particular, the controller 330 sets optical switches within the variable switching elements 326 and 328 to apply an incremental relative propagation delay between the orthogonally polarized signals which compensates for the 60 delay detected by delay detector 327. Because PMD is compensated through optical switching, an extremely fast response time to detected delay, e.g., on the order of nanoseconds, can be achieved that is independent of the degree of delay adjustment. 65
Note that in FIG. 3B, two delay elements are applied so that either polarization can be retarded with respect to the
other. Another variation can use a single delay element to provide a relative propagation delay. Also, one or more rotatable polarizers can be used at the polarization beam splitter 324 to select a polarization component at any degree of rotation as the two optical signals passing through the switching delay elements 326 and 328 do not necessarily have to be orthogonally polarized.
An important feature in accordance with a preferred embodiment is the implementation of the variable switching delay elements 326 and 328. Each of these switching delay elements have a similar structure and operation. Accordingly, only one switching delay element need be described in detail.
FIG. 4 shows a detailed example of one variable switching delay element 426 used in the present invention. A series of optical switches SW0-SW6 are connected in stages by different incremental lengths of fiber. At each stage, an optical switch SW0-SW6 can switch an optical signal over a reference fiber, segment or a delay fiber segment which is longer than the reference fiber segment so as to introduce an incremental propagation delay. Depending upon how each switch SW0-SW6 is set, an optical output 427 can be delayed by different incremental time intervals.
For example, switch SW0 is a 2x2 optical cross-connect switch having four ports A to D. In one switch configuration, switch SW0 can simultaneously connect port A to port C and connect port B to port D. Optical input 425 would exit port C and travel along a reference fiber segment 440 experiencing no relative propagation delay, e.g., 0 picoseconds. Alternatively, SW0 can switch optical input 425 to pass from port A to exit port D for transmission over a delay fiber segment 441. The delay fiber segment 441 is longer than the reference fiber segment 440 by a predetermined amount calculated to introduce a one picosecond propagation delay compared to the transit time for light traveling through the reference fiber segment 440.
As shown in FIG. 4, the switching stages introduce progressively more delay. In particular, the arrangement of FIG. 4 uses delay values that progress geometrically by powers of two from one to thirty-two picoseconds for the individual stages. However, the switching delay element 426 can represent any integral number delay value from zero picoseconds to sixty-three picoseconds by varying the switched state of the optical switches SW0-SW6. Many other value assignments for the propagation delay are certainly possible and have been contemplated. The use of many low picosecond values is preferred for high data rate communications because the stepwise introduction of large delay values can cause momentary signal disruption. Thus, for extremely high bit rates the range of PMD compensation and quantization steps will be relatively small.
Thus, the above propagation delays in the switching delay element 426 are illustrative and can be varied by adding more or less stages and changing the reference fiber lengths. Further, at each switching stage, other optical delay elements can be used in combination with or instead of the optical delay segments to apply a propagation delay. Also, multiport cross-connect switches having more than two ports on a side, e.g., NxN optical cross-connects where N=3, 4, 8, or more, can be used to allow multiple delay fiber segments to introduce a greater variety of incremental delays at each stage.
Each of the optical switches SW0-SW6 is controlled through control lines 435 based on a control input 431 output from controller 330 to set a desired propagation delay. A further optional delay switch controller 433 is included in