Optical Hybrid Interferometry for Instantaneous Laser Frequency Sensing
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Solution Overview
Problem
Existing methods for measuring instantaneous frequencies of lasers, particularly in applications like absolute distance, speed, and vibration measurements, LIDAR, and optical coherence tomography, face challenges due to ambiguity in distinguishing between positive and negative chirps and reduced accuracy with low-frequency beats, often requiring long fiber delay lines.
Innovation Solution
A system utilizing an optical measurement unit with an interferometer structure to split a light signal into two angle diversity signals with a fixed phase shift, processed by a control unit to generate a complex signal for determining instantaneous frequency, reducing reliance on Hilbert transforms and enabling on-chip integration.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If Hilbert transforms are used to estimate laser frequencies, then frequency measurement capability is achieved, but ambiguity in distinguishing between positive and negative chirps occurs and accuracy is reduced with low-frequency beats
Solution Approach 1:
The light signal is split into multiple delayed versions using a delay line, creating multiple interferometer paths. Each path provides a separate measurement channel that, when combined, resolves the ambiguity of chirp direction and improves low-frequency beat accuracy without requiring Hilbert transforms
Solution Approach 2:
An optical hybrid coupler is introduced as an intermediary element to combine the delayed light signals in a controlled manner. This mediator enables direct complex-domain signal processing that eliminates the need for Hilbert transforms while providing unambiguous chirp distinction and improved frequency measurement accuracy
2Measurement precision
If long delay lines are used to improve frequency measurement accuracy, then measurement precision is improved, but device size and complexity increase
Solution Approach 1:
The system replaces traditional mechanical or fiber-based long delay lines with an integrated photonic delay line implemented on a chip. This substitution maintains the required delay for accurate frequency measurement while dramatically reducing device size and integration complexity, enabling on-chip implementation
Solution Approach 2:
The patent transitions from one-dimensional fiber delay lines to two-dimensional integrated photonic circuits. By utilizing planar waveguide structures and compact interferometer designs on a chip, the system achieves the necessary delay characteristics in a compact footprint, reducing overall device complexity and size
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
Enables accurate, efficient, and real-time measurement of instantaneous frequencies with reduced DSP complexity, supporting fast and large-range laser frequency drifts, and adaptable to varying conditions, suitable for compact and integrated systems.
Implementation Method 1
output, via an optical hybrid coupler, at least two angle diversity signals based on a difference between a first and second signal formed by splitting the at least portion of the light signal, wherein the second signal is delayed relative to the first signal
Data Source
AI summary
According to an aspect of the present inventive concept there is provided a system for measuring frequency of a light signal from a chirped laser source, said system comprising: an optical measurement unit configured to receive at least a portion of the light signal, and to output, via an optical hybrid coupler, at least two angle diversity signals based on a difference between a first and second signal formed by splitting the at least portion of the light signal, wherein the second signal is delayed relative to the first signal, and wherein a pair of signals of the at least two angle diversity signals have a fixed phase shift relative to each other; and a control unit configured to: receive the at least two angle diversity signals, generate a complex signal, based on the at least two angle diversity signals, and determine an instantaneous phase of the complex signal for determining an instantaneous frequency of the light signal.