Method and apparatus for monitoring performance of optical transmission systems

An optical transmission system, comprising:

• an optical transmitter comprising an optical source for generating an optical signal, a signal modulation arrangement for modulating the optical signal with a high speed data stream, and a dither modulation arrangement for modulating the optical signal with a low speed, small amplitude dither signal having a precisely controlled dither modulation depth fixed at a known value, the known modulation depth being encoded in the dither signal;
• an optical receiver;
• and
• a monitoring arrangement comprising an optical tap for tapping a portion of the optical signal, an opto-electronic conversion device for converting the tapped portion of the optical signal to an electrical signal, a dither signal detection arrangement for detecting the dither signal in the electrical signal and for determining the known modulation depth from the detected dither signal, and a calculation arrangement responsive to the electrical signal for estimating a total optical signal power of the optical signal and for estimating a dither amplitude of the optical signal.

A method for monitoring the performance of a wavelength division multiplexed optical transmission system, the method comprising:

• at each optical transmitter of the transmission system, modulating an optical signal with a low frequency dither signal to provide a modulated optical signal having a known modulation depth, the low frequency dither signal encoding the wavelength of the optical transmitter;
• tapping a portion of the optical signal;
• measuring a total power of the tapped portion of the optical signal, and measuring dither amplitudes of each distinct dither signal detectable in the tapped portion of the optical signal;
• and
• estimating both signal and noise components of the tapped portion of the optical signal by comparing the measured dither amplitudes to the measured total power.

A method for monitoring the performance of an optical amplifier, comprising:

• modulating an optical signal with a low frequency dither signal to provide a modulated optical signal having a known modulation depth;
• tapping portions of the optical signal at both an optical input and an optical output of an optical amplifier;
• measuring total powers and dither amplitudes for both tapped portions of the optical signal;
• and
• estimating signal and noise components for both tapped portions of the optical signal based on the measured dither amplitudes, the measured total powers and the known modulation depth.

Apparatus for monitoring performance of an optical transmission system, comprising:

• an optical tap for tapping a portion of an optical signal at an optical output of an optical amplifier;
• an opto-electronic conversion device for converting the tapped portion of the optical signal to an electrical signal;
• and
• a sampler for sampling the electrical signal to derive analog samples;
• an analog to digital converter for digitally encoding the analog samples;
• and
• a microcontroller operable to correlate the digitally encoded samples with a known pseudorandom sequence to estimate the dither amplitude.

An optical transmitter, comprising:

• an optical source for generating an optical signal;
• a signal modulation arrangement for modulating the optical signal with a high speed data stream;
• and
• a dither modulation arrangement for modulating the optical signal with a low speed, small amplitude dither having a precisely controller dither modulation depth fixed at a known value, the dither modulation arrangement comprising:
• an opto-electronic conversion device for sensing the optical signal and for converting the sensed optical signal to an electrical signal;
• a sampler for sampling the electrical signal to derive analog samples
• an analog to digital converter for digitally encoding the analog samples;
• a microcontroller operable to estimate the dither amplitude based on the digitally encode samples;
• and
• a dither amplitude control arrangement responsive to the estimated total signal power and the estimated dither amplitude for fixing the dither amplitude at the known value, the dither amplitude control arrangement comprising:
• a digital to analog converter responsive to a digital code supplied by the microcontroller to define an analog signal level;
• a chopper responsive to a signal supplied by the microcontroller and to the analog signal level to define pulses having a pulse amplitude substantially equal to the analog signal level;
• and
• a controllable current source responsive to the pulses to modulate a bias current of the optical source.

A wavelength division multiplexed optical transmission system, comprising:

• a plurality of optical transmitters, each optical transmitter comprising an optical source for generating an optical signal at a distinct wavelength, a signal modulation arrangement for modulating the optical signal with a high speed data stream, and a dither modulation arrangement for modulating the optical signal with a distinct low speed, small amplitude dither signal having a precisely controlled dither modulation depth fixed at a known value;
• a plurality of optical receivers, each optical receiver for receiving an optical signal from a corresponding optical transmitter;
• and
• a monitoring arrangement comprising an optical tap for tapping a portion of the optical signals, an opto-electronic conversion device for converting the tapped portion of the optical signals to an electrical signal, and a calculation arrangement responsive to the electrical signal for estimating a total optical signal power of the optical signals and for estimating a dither amplitude of each optical signal.