Variable Free-Space Delay Line Optical Interferometer
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Solution Overview
Problem
Existing optical interferometers used in differential phase shift keying (DPSK) and differential quadrature phase shift keying (DQPSK) optical demodulators face challenges with temperature dependence and polarization sensitivity, requiring complex temperature control and being bulky with slow phase delay adjustment, and are limited to operating at a single fixed bit rate.
Innovation Solution
An optical interferometer design incorporating a planar lightwave circuit (PLC) with a splitter, coupler, and phase adjuster, along with a variable free-space delay line, which splits and delays optical signals to adjust bit delay for different bit rates and optimize performance, reducing thermal and polarization sensitivity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Volume of moving object
If a PLC-based DPSK interferometer is used, then the device achieves a compact size, but it exhibits temperature dependence and polarization sensitivity requiring complicated temperature control
Solution Approach 1:
The interferometer is divided into two distinct parts: a PLC-based section for signal splitting and combining, and a free-space section for the delay line. This segmentation allows each part to be optimized independently - the PLC provides compactness while the free-space section provides thermal stability and polarization insensitivity.
Solution Approach 2:
Free-space optical components (lenses, mirrors) are introduced as intermediaries to couple the PLC waveguides and enable the delay line function. These intermediaries transfer the optical signals between the confined PLC environment and the open free-space environment, allowing the system to benefit from both approaches.
2Reliability
If a prior-art Michelson interferometer is used, then temperature control is simplified, but the device becomes bulky with slow phase delay adjustment
Solution Approach 1:
The interferometer is divided into two distinct parts: a PLC-based section for signal splitting and combining, and a free-space section for the delay line. This segmentation allows each part to be optimized independently - the PLC provides compactness while the free-space section provides thermal stability and polarization insensitivity.
Solution Approach 2:
The mechanical adjustment mechanisms found in traditional Michelson interferometers are replaced with integrated PLC waveguide structures for beam splitting and combining. The phase adjustment is achieved through optical path length control in free-space rather than mechanical component adjustment, enabling faster response.
3Device complexity
If a fixed-bit-rate interferometer is used, then the device structure is simplified, but it cannot operate at different bit rates
Solution Approach 1:
The delay line length is made variable rather than fixed, allowing the interferometer to adapt to different bit rates. This is achieved through mechanisms that can dynamically adjust the optical path length in the free-space delay section, enabling the device to maintain proper operation across multiple operating conditions.
Solution Approach 2:
The interferometer design incorporates adjustable parameters (delay line length, phase shifts) that allow a single device structure to perform multiple functions at different bit rates. The free-space delay line can be configured for different delay values, making the interferometer universal across various operating conditions without requiring complete redesign.
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 design achieves compact size, good thermal stability, quick phase delay adjustment, and the ability to operate at various bit rates, improving the reliability and flexibility of DPSK/DQPSK demodulation.
Implementation Method 1
The optical signal 108 is split by the 1×2 waveguide splitter 102 into two portions 109 and 110 of equal amplitude, which propagate in the upper and lower waveguides 104 and 105
Implementation Method 2
The two portions 109 and 110 interfere with each other in the 2×2 optical coupler 106
Implementation Method 3
A differential photodetector pair 114 is coupled to output waveguides 111 and 112 of the 2×2 optical coupler 106 to detect an interference signal
Data Source
AI summary
An optical interferometer for demodulating a differential phase shift keying optical signal includes a planar lightwave circuit and at least one free space delay line optically coupled to the planar lightwave circuit. The planar lightwave circuit has a waveguide splitter, a waveguide coupler, and a phase adjuster. In operation, the splitter splits the optical signal into equal portions, the phase adjuster adjusts the relative phase of the optical signal portions, and the free space delay line provides one-bit delay between the portions of the optical signal. The delayed signals are mixed in the waveguide coupler. The free space delay line can be made variable for adjustment of the bit delay for operation at different bit rates, and/or for optimization of the interferometer performance.