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TUNABLE CHIRP, LIGHTWAVE MODULATOR
FOR DISPERSION COMPENSATION
TECHNICAL FIELD 5
This invention relates to lightwave communication systems and, more particularly, to modulation devices for use in such systems.
BACKGROUND OF THE INVENTION io
Long haul fiber transmission systems have historically been limited by such factors as fiber loss, fiber dispersion, fiber nonlinearities, and amplifier noise. With the advent of practical optical amplifiers, these systems are effectively no longer subject to loss limitations. Rather, the significant system limitation has become dispersion. Present fiber optic transmission systems use transmitters employing direct current modulation of laser diodes. When modulated, these lasers produce pulses exhibiting large, uncontrolled wavelength shifts called "chirp." In the presence of a dispersive medium such as an optical fiber, the chirped pulses can be severely distorted when they finally reach a remote receiver.
System limitations imposed by dispersion may be 25 countered by the use of dispersion-shifted fiber, "zerochirp" transmitters, dispersion equalization, and soliton propagation. The use of dispersion-shifted fiber attempts to minimize dispersive loss within the transmission band whereas the use of zero-chirp transmitters attempts to maintain the transmission wavelength constant, preferably at the attenuation loss minimum for the fiber. Dispersion equalization, both at the receiver and at the transmitter, has been utilized to substantially compensate for the effects of dispersion on the transmitted pulses. Soliton propagation is dependent upon the presence of small amounts of dispersion at'the transmission wavelength.
In the prior art, zero-chirp transmitters have been developed to ameliorate the problem caused by chirped pulses transmitted at high bit-rates over dispersive optical fiber. Generally, the term chirped pulses refers to pulses those whose wavelength swings dynamically about a central wavelength. The amount of chirp varies randomly as the transmitter operates thereby causing 45 the chirped pulses to incur penalties in such systems. These penalties involve additional loss which limits the maximum transmission length or maximum transmission bandwidth due to intersymbol interference in the transmitted data pulses. In conventional optical fiber, the 50 wavelength at which the fiber loss minimum occurs does not necessarily coincide with the wavelength at which zero dispersion occurs. Thus, when the lightwave transmitter produces optical pulses at a particular wavelength corresponding to the loss minimum of the 55 fiber, wavelength chirping of the pulses causes pulse spreading because different wavelength components of the pulses experience different amounts of dispersive delay. Dispersion is higher for wavelength components of the pulses not at the zero dispersion wavelength. By removing chirp from the transmitted pulses, it has been thought possible to produce pulses with the minimum spectral width substantially at a single desired wavelength using a zero-chirp transmitter and thereby assure that only a small amount of pulse spreading is experienced. In general, such zero-chirp transmitters include an external modulator coupled to a laser. One external modulator which has been proposed is a lithium nio
bate, Mach-Zehnder interferometer using push-pull drive signals on the separate arms of the interferometer. See, for example, Koyama et al., /. Lightwave Technol, Vol. 6, No. 1, pp. 87 et seq. (1988) and Namiki et al., Proc. of Seventh International Conf, on Integrated Optics and Optical Fiber Communication, paper 19D4-2 (1989).
While the zero-chirp transmitter offers a potentially attractive solution for high bit-rate transmission at a wavelength "in the" optical fiber exhibiting non-zero dispersion, better performance has been predicted by using pulse compression techniques in the transmitter to achieve a dynamically varying chirp in a broad negative region. See Koyama et al., J. Lightwave Technol, Vol. 6, No. 1, FIG. 8, p. 91 (1988). Subsequent to this prediction, however, the art continues to express the need for lowering the transmitter chirp to zero as the method for combatting dispersion when communicating away from the zero dispersion wavelength of the optical transmission fiber. See, Okiyama et al., /. Lightwave Technol, Vol, 6, No. 11, p. 1686, 1691 (1988).
SUMMARY OF THE INVENTION
External modulation of lightwave signals is controlled in order to minimize the transmission power penalty caused by chromatic dispersion in an optical fiber transmission system by adjusting a modulation chirp parameter to any substantially fixed value in a predetermined, controllable manner. External modulation is accomplished, for example, in a dual waveguide device wherein substantially identical input optical beams are supplied to the waveguides and wherein each waveguide is subject to its own individual, mutually exclusive control. Modulation signals are applied to each waveguide via the separate control. Moreover, control signals are applied to each waveguide for adjusting the modulation chirp parameter to a desired non-zero, substantially fixed value. Typically, the desired value of the chirp parameter is one which provides the lowest fiber dispersion penalty for the system. Modulated lightwave signals emerging from the waveguides are combined to form a single output signal suitable for transmission over an optical fiber.
In one embodiment, a Mach-Zehnder interferometer having separately controllable waveguides has its input coupled to a CW laser. Drive signals to the electrodes over each of the waveguides are generated to create a sufficient depth of modulation, a proper operating point for the modulator, and a desired value for the modulation chirp parameter. By using peak-to-peak voltage differences on each waveguide, it is possible to represent the value of the modulation chirp parameter as the sum divided by the difference of the individual peak-topeak voltage differences. Both III-V semiconductor and Ti:LiNb03 Mach-Zehnder interferometers have been realized.
BRIEF DESCRIPTION OF THE DRAWING
A more complete understanding of the invention may be obtained by reading the following description of specific illustrative embodiments of the invention in conjunction with the appended drawing in which:
FIG. 1 shows a simplified lightwave transmission system including the transmitter realized in accordance with the principles of the present invention;
FIG. 2 shows a simplified schematic diagram for the controller in FIG. 1;
FIG. 3 shows a simplified perspective view of an exemplary external modulator for use in the system shown in FIG. 1;
FIGS. 4-7 show exemplary drive voltage waveforms and intensity waveforms for the controller and external 5 modulator in FIGS. 1-3; and
FIGS. 8 and 9 show simplified perspective and crosssectional diagrams of a semiconductor embodiment for the external modulator shown in FIG. 1.
DETAILED DESCRIPTION 10
Electrooptic external modulators for optical signals are known to operate with a very low amounts of chirp. In fact, Mach-Zehnder interferometer waveguide modulators have recently been designed to operate chirp- 15 free. Chirp-free modulation has been thought desirable to overcome the transmission limitations imposed by fiber dispersion. However, it has been shown by us that the lowest fiber dispersion penalty is not necessarily obtained for a chirp parameter identically equal to zero. 20 For example, we have shown this to be true for systems operated at 1.5 urn. using a fiber having a zero dispersion wavelength of approximately 1.3 fim. As described below, it is shown that choosing a non-zero modulation chirp parameter am dependent on the fiber dispersion 25 coefficient and fiber transmission distance is advantageous because of the pulse compression provided.
A lightwave transmission system is shown in FIG. 1 including a transmitter, a receiver, and a transmission medium connecting the transmitter to the receiver. The 30 transmitter includes laser 10, lensed optical fiber 13, isolator 14, external modulator 16, controller 24, and data source 22. The transmission medium is shown as lengths of optical fiber 19 interconnected with optical amplifiers 20, so that the combinations of fibers and 35 amplifiers are sufficient to span the distance between the transmitter and receiver 21.
In the lightwave transmitter shown in FIG. 1, laser 10 produces optical signals 12 at a predetermined transmission wavelength for the lightwave transmission system. 40 Laser 10 is operated in a continuous wave (CW) mode or pulsed mode by applying the proper signal to terminal 11 of the laser. For long wavelength systems, laser 10 is typically an InGaAsP/InP semiconductor single mode laser operating nominally at 1.5 u,m, for example. 45 Output optical beam 12 from the laser is coupled into a lensed optical fiber 13 usually called a fiber pigtail. Lensed optical fiber 13 facilitates coupling of the optical beam from the laser to the external modulator.
Isolator 14 is positioned between lensed optical fiber 50
13 and the external modulator as an in-line element to reduce reflections toward the laser from the rest of transmission system. Isolator 14 can also be combined with a polarizer (not shown) to assist in further reduction of reflections back toward the laser. While isolator 55
14 is shown in the transmitter arrangement of FIG. 1, it should be noted that the isolator is an optional, rather than a necessary, element in the realization of the transmitter.
External modulator 16 receives optical signals 12 60 from the laser via input fiber 15. The external modulator includes two separate waveguides which are independently controllable via controller 24. At the input of the external modulator, optical signals from the laser are coupled into each of the waveguides labeled as wave- 65 guide A and waveguide B in FIG. 1. At the output of external modulator 16, the modulated optical signals from each waveguide are combined into a single optical
signal for transmission over the optical fiber transmission medium. Modulation performed by modulator 16 on optical signals 12 is either amplitude modulation or intensity modulation.
Controller 24 receives a digital data signal from data source 22 via lead 23. Controller 24 generates modulation control signals output to external modulator 16 via leads 17 and 18. Modulation control signals from controller 24 are supplied via leads 17 and 18 to control characteristics of waveguides A and B, respectively, so that the data from data source 22 is properly modulated onto the optical signals at the transmission wavelength in the waveguides and so that the desired modulation chirp parameter value is generated in the resulting modulated optical signals. For example, the control signals can adjust the relative propagation velocities of each of the waveguides to permit the desired modulation chirp parameter value to be achieved in accordance with the principles of the invention.
The transmission medium includes a plurality of lengths of optical fiber together with optical amplifiers 20. Optical amplifiers 20 are, for example, erbium doped optical fiber amplifiers for amplifying the modulated optical signals as the propagate along the optical fibers 19. Optical fiber 19 is either a standard silica-based fiber having a loss minimum in the vicinity of 1.3 fim. or a dispersion shifted fiber having its loss minimum in the vicinity of 1.5 jim. The transmission medium is of sufficient length to span the distance from the transmitter to lightwave receiver 21.
Lasers, lensed fiber couplers, isolators, polarizers, data sources, externals modulators, optical fibers, optical amplifiers, and lightwave receivers are commercially available and well known to persons skilled in the art.
In an example from experimental practice, a waveguide Mach-Zehnder interferometer (FIG. 3) is used for external modulator 16 and an amplifier arrangement (FIG. 2) is utilized for controller 24. As shown in FIG. 2, exemplary controller 24 comprises gain-adjustable amplifiers 241 and 243 whose inputs are coupled together to receive the digital data on lead 23. Each amplifier can vary the peak-to-peak amplitude swing of the digital data at its output. Amplifier 241 is shown as an inverting amplifier. Such a function is not necessary to the practice of the invention. Moreover, phase control elements (not shown) such as adjustable delay lines are contemplated for use with one or both of the amplifiers, either preceding or following a particular amplifier. Such phase control elements permit the modulation control signal output on lead 17 to have a different phase relative to that of the modulation control signal on lead 18.
Exemplary external modulator 16 shown in FIG. 3 utilizes titanium in-diffused waveguides 162, 163, 164, and 170 in a 1X1 Y-branch Mach-Zehnder interferometer configuration on a lithium niobate substrate 161. Ground planes 165, 166, and 167 are disposed to maintain electrical isolation between drive electrodes 168 and 169 and to insure that the drive electrodes control their respective waveguides independent of each other. As a result, waveguides 164 and 163 are individually addressable via drive electrodes 168 and 169, respectively. The drive electrodes are of the travelling-wave or lumped-element type. Drive electrode 168 connected to lead 17 is disposed over a portion of waveguide 164 (waveguide "A") whereas drive electrode 169 connected to lead 18 is disposed over a portion of wave
guide 163 (waveguide "B"). In the interferometer to adjust the ratio r so that a particular value of the
waveguide structure, an input Y-branch couples the modulation chirp parameter is achieved. As such, the
optical signal in waveguide 162 into both coplanar strip controller adjusts the modulation chirp parameter by
waveguides 163 and 164 while output Y-branch couples choosing unique peak-to-peak applied drive voltages so
the optical signals from coplanar strip waveguides 163 5 that in the small signal limit ...
and 164 into a single waveguide 170. AV^ —AV^). While this expression is derived for the
Each optical waveguide is designed using standard Mach-Zehnder configuration in the small signal regime, techniques for single mode operation at the transmission it is accurate for relatively large values of modulation wavelength (e.g., 1.5 jun). For the external modulator depth. From the description above, it is now undershown in FIG. 3, the waveguides were produced by 10 stood that, by modifying the digital data signal approdiffusion of titanium z-cut LiNb03 so that the two arms priately in the controller to generate drive signals havof the interferometer are separated by a distance of 250 ing the proper relative magnitude and sign of their amfim, which is approximately 15 times the gap spacing plitudes, it is possible to achieve modulation chirp paused between the coplanar waveguide electrodes. The rameters in the range from — oo <am< + «. electrodes are formed using standard metallic plating 15 The capability of obtaining controlled and arbitrarily techniques (e.g., gold plating). Although not shown, a adjustable values of the modulation chirp parameter is silicon dioxide buffer layer is deposited or grown be- demonstrated in FIGS. 4-7. With respect to these figtween the waveguides and the corresponding elec- ures, a shorthand notation has been introduced for the trodes. Thicknesses of the buffer layer and electrodes, waveguides and electrodes in which the subscript A width of the electrodes, and size of the interelectrode 20 refers to waveguide 164 or the drive signals applied to gap are selected using standard techniques to realize a electrode 168 and the subscript B refers to waveguide modulator having broad bandwidth and low Vff, where 163 or the drive signals applied to electrode 169. The Vw is the voltage necessary to achieve a w-phase shift. curves shown in each of these figures represent the For a discussion of design techniques for these types of drive waveforms, and Vb, generated by controller devices, see Optical Fiber Telecommunications II, pp. 25 24, the approximate voltage difference between the 421-65 (S. Miller et al. ed. 1988) and S. K. Korotky, drive waveforms, and the optical intensity of the moduTechnical Digest, Workship on Numerical Simulation and lated signal output by external modulator 16. In each of Analysis in Guided-Wave Optics and Optoelectronics, FIGS. 4 through 7, the drive waveforms are at approxipaper SF2 (1989). Impedance of the electrodes 168 and mately 2.5 GHz in various voltage ratios. 169 has been measured as approximately 43ft where the 30 In FIG. 4, pure phase modulation is obtained with active length of the electrode is approximately 4 cm. An r = 1 (am= »). In order to obtain pure phase modulaend-to-end center conductor resistance has been mea- tion with a modulation index of unity, electrodes 168 sured as approximately 7.9ft. and 169 receive drive voltages from controller 24 which
In order to assure low optical return loss below ap- were in phase and which individually exhibited a peak
proximately — 60 dB, waveguide ends in external modu- 35 to-peak voltage swing corresponding to a It phase shift,
lator 16 were polished in plane at an angle of 6° from the Curve 41 shows the drive signals to the two waveguides
normal with respect to the waveguide longitudinal axis. being substantially in phase and having the appropriate
Fibers 15 and 19 had their mating end faces polished at voltage swings so that AVy4 = AVs=Vff. Curve 42
the corresponding Fresnel angle of 8° 52'. shows a negligible oscillation in the voltage difference
In accordance with the principles of this invention, a 40 between the drive voltages as approximately Vb—V^.
modulation chirp parameter for modulator 16—rather As expected, curve 43 shows a substantially constant
than being allowed to be randomly varying or at- output intensity due to pure phase modulation in the
tempted to be made zero as in the prior art—is adjusted external modulator.
to a substantially fixed, non-zero value in a predeter- Pure amplitude modulation is shown in FIG. 5 mined, controllable manner. The desired value of the 45 wherein the ratio r is equal to negative one and the modulation chirp parameter is typically one which pro- modulation chirp parameter is approximately zero. Pure vides the lowest fiber dispersion penalty for the system. amplitude modulation is achieved with the drive waveHerein, modulation chirp parameter am represents the forms being the negative (complement) of each other, as ratio of phase modulation to amplitude modulation as shown by curves 51 and 52. This corresponds to the performed by external modulator 16. The modulation 50 condition where AV^= — Avb=vt/2. The voltage chirp parameter may be expressed as follows: difference between the drive waveforms oscillates in a am=(d4>/dt)/(dl/dt)/21, where t denotes time and <j> periodic manner as shown in curve 53. Curve 54 shows and I are the instantaneous phase and intensity of the the optical intensity oscillating between a high level and optical output from external modulator 16. Generally, a low level corresponding, for example, to an alternatthe modulation chirp parameter is a function of the type 55 ing pattern of binary zeroes and ones, of modulation, the modulation depth, and the operating FIG. 6 depicts the case of combined amplitude and point of the optoelectronic transducer being used for phase modulation where the ratio r is approximately the transmitter. For the Mach-Zehnder external modu- equal to J and the corresponding modulation chirp lator shown in FIG. 3, which is (1) based on the linear parameter is approximately 2. The drive waveforms are electrooptic effect, (2) biased midway on its switching 60 substantially in synchronism but exhibit different peakcurve, and (3) driven by mathematically similar drive to-peak voltages as shown by curves 61 and 62. The waveforms, am is written in terms of modulated optical drive waveforms generated by controller 24 satisfy the phase velocities for the optical signals in each arm following conditions: AV^=3Vff/2 and Avb=vw/2. (waveguide "A" or waveguide "B") of the interferome- The optical intensity shown in curve 64 is dominated by ter. The phase velocities are written as A/J^and A/J^for 65 amplitude modulation as the intensity switches between the two different arms of the interferometer. As a result, a logically high state and a logically low state, modulation chirp parameter is equal to (l+r)/(l—r), FIG. 7 also depicts the case of combined amplitude where I=a/3b/ai3a- It is the function of controller 24 and phase modulation. The resulting curves 71 through