An improved method has been devised for controlling the DC bias applied to an electro-optical crystal that is part of a Mach-Zehnder modulator that generates low-duty-cycle optical pulses for a pulse-position modulation (PPM) optical data-communication system. In such a system, it is desirable to minimize the transmission of light during the intervals between pulses, and for this purpose, it is necessary to maximize the extinction ratio of the modulator (the ratio between the power transmitted during an “on” period and the power transmitted during an “off” period). The present method is related to prior dither error feedback methods, but unlike in those methods, there is no need for an auxiliary modulation subsystem to generate a dithering signal. Instead, as described below, dither is effected through alternation of the polarity of the modulation signal.
The upper part of Figure 1 schematically depicts a Mach-Zehnder modulator. The signal applied to the electro-optical crystal consists of a radio-frequency modulating pulse signal, VRF, superimposed on a DC bias Vbias. Maximum extinction occurs during the “off” (VRF = 0) period if Vbias is set at a value that makes the two optical paths differ by an odd integer multiple of a half wavelength so that the beams traveling along the two paths interfere destructively at the output beam splitter. Assuming that the modulating pulse signal VRF has a rectangular waveform, maximum transmission occurs during the “on” period if the amplitude of VRF is set to a value, Vπ, that shifts the length of the affected optical path by a half wavelength so that now the two beams interfere constructively at the output beam splitter.
The modulating pulse signal is AC-coupled from an amplifier to the electro-optical crystal. Sometimes, two successive pulses occur so close in time that the operating point of the amplifier drifts, one result being that there is not enough time for the signal level to return to ground between pulses. Also, the difference between the optical-path lengths can drift with changes in temperature and other spurious effects. The effects of both types of drift are suppressed in the present method, in which one takes advantage of the fact that when Vbias is set at the value for maximum extinction, equal-magnitude positive and negative pulses applied to the electro-optical crystal produce equal output light pulses.
In a modulation system designed and operated according to this method (see Figure 2), the modulating pulses are converted to alternating polarity, a small portion of optical output power is sampled by a photodetector, the photodetector output is multiplied by a sample of the alternating-polarity modulating signal, and the product is integrated over time to obtain an error signal. When Vbias is not at the optimum, maximum-extinction value, there is either an overshoot or an undershoot in the output light pulse, such that the integral signal amounts to an error signal that is proportional, in both magnitude and sign, to the difference between the actual and optimum values of Vbias. The integral signal is amplified and added to a DC offset voltage, and the sum fed to a bias control input terminal to drive the modulator toward optimum bias. Normally, the DC offset voltage would be set initially at a maximum-extinction point.
This work was done by William Farr and Joseph Kovalik of Caltech for NASA’s Jet Propulsion Laboratory. NPO-41301
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Electro-Optical Modulator Bias Control Using Bipolar Pulses
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