Optical Receiver Phase Control Circuit for Stable DQPSK Interference
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Solution Overview
Problem
High-speed optical data transmission systems face challenges with inter symbol interference (ISI) due to polarization mode dispersion (PMD) and chromatic dispersion (CD), which affect the accuracy of phase control in DQPSK and other advanced modulation formats, leading to reduced tolerance to noise and frequency drifts.
Innovation Solution
A phase control circuit and method that utilize a dither signal, a mixer, an adder, and an integrator to minimize errors in DQPSK-modulated signals, combined with a low-pass filter and power detector to remove clock signal components, ensuring accurate phase control by fine-tuning delay interferometers in optical receivers.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If multi-level signaling (DQPSK) is used to increase spectral efficiency, then the symbol rate is reduced and ISI is decreased, but the system becomes more sensitive to phase control errors and frequency drifts
Solution Approach 1:
The patent applies dithering (adding a small sinusoidal phase modulation) to the local oscillator signal to intentionally induce controlled phase variations. This vibration-like perturbation enables the phase detector to generate an error signal that drives the phase control loop, allowing the system to maintain optimal phase synchronization despite the increased sensitivity of multi-level signaling to phase errors
Solution Approach 2:
The patent implements a closed-loop feedback mechanism where the phase detector continuously monitors the phase difference between the received DQPSK signal and the local oscillator, generates an error signal, and feeds it back to control the local oscillator phase via a phase shifter. This feedback loop dynamically corrects phase drifts and maintains phase synchronization, thereby improving reliability while preserving the spectral efficiency benefits of DQPSK
2Measurement precision
If the delay interferometer is used to detect DQPSK signals, then differential phase detection is enabled, but the system requires precise delay matching which is sensitive to frequency drifts
Solution Approach 1:
The patent makes the delay interferometer dynamically adjustable by introducing a controllable phase shifter in the local oscillator path. This allows the system to adapt the phase relationship between the signal paths in real-time, compensating for frequency drifts and maintaining optimal interference conditions despite variations in operating frequency
Solution Approach 2:
The patent changes the phase parameter of the local oscillator signal dynamically through phase control. By adjusting the phase of the local oscillator based on error signals from the phase detector, the system maintains precise delay matching and optimal interference conditions even when frequency drifts occur, thereby improving adaptability while preserving detection precision
3Reliability
If chromatic dispersion and PMD are present in the transmission fiber, then signal quality degrades and ISI increases, but equalization requires complex processing that may not be sufficient at high speeds
Solution Approach 1:
The patent performs preliminary phase synchronization and compensation before the main detection process by using the phase control loop to pre-align the local oscillator phase with the incoming signal. This preliminary action reduces the burden on subsequent equalization processing and improves overall signal quality without requiring excessively complex equalization algorithms
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The solution effectively maintains good interference conditions and reduces bit error rates in optical receivers for DQPSK and advanced modulation formats, even under conditions of chromatic dispersion and differential group delay, while minimizing costs and power consumption.
Implementation Method 1
two parallel delay interferometers (DI) are inserted in the optical path at the receiver side to convert the differential phase modulation in to intensity modulation
Implementation Method 2
The in-phase path is provided to the in-phase Mach-Zehnder modulator 42. The quadrature path is provided to the quadrature Mach-Zehnder modulator 43
Data Source
AI summary
This invention relates to a phase control circuit for an optical receiver (1). The phase control circuit (9, 19) comprises a non-linear element (22) and a power detector (24). The non-linear element (22) has a rectifying characteristic, inputs the received electrical signal (7, 17) and provides a rectified signal at its output. The power detector (24) provides an error signal which is used to obtain a phase control signal (5) which is output by the phase control circuit. The invention further relates to a corresponding method for phase control of an optical receiver (1).