Bias control and method for electro-optic modulators

• US 5,343,324 A
• Filed: 11/15/1991
• Issued: 08/30/1994
• Est. Priority Date: 11/15/1991
• Status: Expired due to Term

• Current Assignee: Sorrento Networks I. Inc
• Original Assignee: Amoco Corporation (BP PLC)
• Inventors: Cheng, Xin, Le, Phat
• Primary Examiner(s): Chilcot, Jr., Richard E.
• Assistant Examiner(s): BACARES, RAFAEL D
• Application Number:
  US07/794,719
• Time in Patent Office: 1,019 Days
• Field of Search: 359/181, 359/184, 359/185, 359/186, 359/187, 359/245, 359/264, 359/153, 359/161, 455/115, 455/116, 455/126
• US Class Current: 398/198
• CPC Class Codes: G02F 1/0123 Circuits for the control or stabilisation of the bias voltage, e.g. automatic bias control [ABC] feedback loops
• CPC Class Codes: G02F 1/212 Mach-Zehnder type

Abstract

An electro-optical modulation system comprising an optical modulator for pulse-modulating an incoming light beam in response to a pulsed RF input signal and to a bias signal and for producing a pulse-modulated optical output which is characterized by a plurality of enveloped pulses, optical detector for producing an electrical signal which is representative of the output from the optical modulator, average detecting device for producing pulse envelope signals which are representative of the envelopes of the pulses, and a bias control circuit, using a plurality of pulse envelop signals, for producing the bias signal as a function of the pulse envelopes.

Claims

1. An electro-optical modulation system that comprises:

2. The system of claim 1, wherein said probe signal generator comprises an RF oscillator, a clock and a switch which is opened and closed by said clock, which receives the output of said RF oscillator and which has said RF signal as an output.

3. The system of claim 2, wherein said error tracking means comprises:

a sample and hold device which receives said pulse envelope signals from said signal producing means, which is gated between an on-cycle and an off-cycle by said clock, and which measures the pulse amplitude of a pulse envelope signal when turned on by said clock; and

integrating means, operating during said off-cycle, for integrating said measured pulse amplitude from said sample and hold device and producing said DC error signal.

4. An electro-optical modulation system that comprises:

optical modulator means for pulse-modulating an incoming carrier light beam in response to a pulsed RF input signal and a bias signal to produce a pulse-modulated optical output that is characterized by a plurality of enveloped pulses;

optical detector means for converting a portion of said pulse-modulated optical output from said output modulator means to an electrical signal that is representative of said output from said output modulator means;

signal filtering means for converting said electrical signal from said optical detector means to pulse envelope signals that are representative of said envelopes of said pulses by averaging said electrical signal over the period of said pulse envelopes; and

a bias control circuit for converting said pulse envelopes signals to said bias signal as a function of said pulse envelopes.

5. In an optical modulator that pulse modulates an incoming laser light beam in response to a pulsed RF input signal and a DC bias input signal to produce a pulse-modulated optical output characterized by a plurality of pulse envelopes, and which has a transfer curve that includes a linear portion that is bonded by a maximum value and a minimum value and nonlinear portions adjacent to said maximum value and said minimum value, apparatus that comprises:

averaging means for converting a pulse-modulated optical output to an electrical signal that is representative of the average of said pulse-modulated optical output that comprises;

a photodetector for converting said pulse-modulated optical output to an electrical output; and

low-pass filter means, receiving said electrical output, for producing said electrical signal; and

a controller for producing from said electrical signal a DC bias input signal that is a function of at least one of the amplitude of said average, the direction of said average, and the symetry of said average of said pulse-modulated optical output that comprises a DC bias supply and a summing amplifier for combining at least the output of said DC bias supply and a signal that is a function of said electrical signal.

6. The apparatus of claim 5, wherein said controller further includes an oscillator for producing an RF signal, a clock, and a switch which is opened and closed by said clock and which applies said RF signal from said oscillator to said summing amplifier.

7. An electro-optical modulation system that comprises:

a Mach-Zehnder modulator for modulating an incoming light beam in response to a pulsed modulation signal from an RF source and a bias signal to produce a pulse-modulated optical output signal that is characerized by a plurality of pulse envelopes;

detector means, having a slow time response relative to said pulse-modulated optical output signal and using said pulse-modulated optical output from said optical modulator means, for producing control signal that are a function of said pulse envelopes that comprises;

a photodetector for converting said pulse-modulated optical output into a electrical signal; and

low-pass filter means for producing, from said electrical signal, said control signal that is representative of said pulse envelopes; and

a bias controller, using said control signals, for producing said bias signal as a function of the level of said control signals and the algebraic sign of said control signals that comprises;

sample and hold means for capturing and measuring the amplitude of said pulse envelopes;

a DC bias supply; and

a signal amplifier that receives as inputs signals from said DC bias supply and from said sample and hold means and that has said bias signal as an output.

8. The system of claim 7, wherein said bias controller further comprises:

a clock for gating said sample and hold means between an on-cycle and an off-cycle; and

an RF oscillator having an output which is gated on and off by said clock and which is sent as an input to said signal amplifier.

9. In an optical modulator that, in response to an RF signal input and a DC bias signal input, produces a pulse modulated optical output that is characterized by a series of pulse envelopes, a metho for controlling the modulation of an incoming light beams that comprises the steps of:

converting said pulse-modulated optical output from an optical modulator to an electrical signal that is representative of said pulse-modified optical output;

sending said electrical signal to a low-pass filter that has a slow time response relative to said pulsed RF input carrier;

converting the output of said low pass filter to a DC bias signal as a function of at least one of the amplitude of said series of pulse envelopes and the sign of said series of said pulse envelope by sending said output of said low-pass filter to a sample and hold circuit that is operated by a clock that has an on-cycle for capturing the magnitude of said output from said low-pass filter and that has an off-cycle and integrating said magnitude during said off-cycle of said sample and hold circuit; and

coupling said DC bias signal to said optical modulator.

10. The method of claim 9, wherein the step of converting the output of said low-pass filter further comprises the steps of:

using said clock to supply an RF oscillator output during said on-cycle; and

summing said clocked RF oscillator output and said integrated magnitude of said output from said low-pass filter.

11. In an optical communications system that comprises a laser light source, an optical modulator that receives said laser light and that, in response to an information signal input and a DC bias signal input, produces a pulse-modulated optical output that is characterized by a series of pulse envelopes, and an optical fiber that joins said modulator output to a receiver, a method for controlling said optical modulator that comprises the steps of:

converting said pulse-modulated optical output from said optical modulator to an electrical signal that is representative of said pulse-modualted optical output;

sending said electrical signal to a low-pass filter that has a slow time response relative to the frequency of said information signal;

converting the output of said low-pass filter to a DC bias signal as a function of the amplitude of said series of pulse envelopes and the sigh of said series of said pulse envelopes by sending said output of said low-pass filter to a sample and hold circuit that is operated by a clock that has an on-cycle for capturing the magnitude of said output from said low-pass filter and that has an off-cycle and integrating said magnitude during said off-cycle of said sample and hold circuit; and

coupling said DC bias signal to said optical modulator.

12. The method of claim 11, wherein the step of converting the output of said low-pass filter further comprises the steps of:

using said clock to supply an RF oscillator output during said on-cycle; and

summing said clocked RF oscillator output and said integrated magnitude of said output from said low-pass filter.