Laser Doppler Vibrometer Faraday Rotator Polarization Stability
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Solution Overview
Problem
Laser Doppler vibrometers using long optical fibers face challenges with polarization rotation, leading to unstable measurement light polarization states and potential interference issues, which can prevent vibration measurement.
Innovation Solution
Incorporating a faraday rotator in the sensor head to rotate the polarization of the measurement light 45°, allowing for bidirectional polarization orthogonality, thereby ensuring consistent interference between local and measurement light without being affected by polarization rotation in the optical fibers.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Length of stationary object
If long optical fibers are used to separate the sensor head and measurement section, then the device can be installed in distant locations and the sensor head can be miniaturized, but polarization rotation occurs in the optical fibers causing unstable measurement light polarization states
Solution Approach 1:
The patent applies preliminary anti-action by introducing a Faraday rotator that pre-rotates the polarization of the local light by 45 degrees before it enters the optical fiber. This preliminary polarization adjustment counteracts the unpredictable polarization rotation that occurs during light transmission through long optical fibers, ensuring that the measurement light and local light maintain appropriate polarization relationships for interference detection regardless of the fiber's polarization mode dispersion characteristics.
2Reliability
If polarization diversity configuration is used to handle polarization rotation, then measurement stability is improved, but device complexity and cost increase due to requiring two sets of optical interferometer and optical detector
Solution Approach 1:
The patent extracts the polarization adjustment function from the complex polarization diversity configuration and concentrates it into a single Faraday rotator element. Instead of using two complete optical interferometer and detector sets to handle different polarization states, the invention extracts only the essential polarization rotation capability and implements it through the Faraday rotator, thereby maintaining measurement stability while significantly reducing device complexity and cost.
3Measurement precision
If polarization rotation in optical fibers is compensated, then interference measurement can be maintained, but the phase difference control becomes difficult due to environmental temperature changes
Solution Approach 1:
The patent introduces the Faraday rotator as an intermediary device between the light source and the optical fiber. This intermediary actively adjusts the polarization state of the local light by 45 degrees, creating a polarization relationship that is robust against the phase differences and rotation effects introduced by environmental temperature changes in the optical fiber. This intermediary approach maintains interference measurement capability without requiring complex real-time phase difference control.
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
This configuration enables a low-cost, highly stable laser Doppler vibrometer that can measure vibrations at maximum signal-to-noise ratio, even with long optical fibers, by maintaining consistent polarization states for interference.
Implementation Method 1
The sensor head includes a faraday rotator that rotates a polarization 45°
Implementation Method 2
a second optical coupler configured to generate interference light by causing the measurement light having propagated through the first optical path and the local light to interfere with each other
Implementation Method 3
a laser light source that generates laser light
Implementation Method 4
an optical detector that receives the interference light
Data Source
AI summary
A laser Doppler vibrometer includes a first optical coupler configured to split laser light from a laser light source into two beams of laser light, and one of which is as irradiation light and an other is local light, a sensor head configured to receive the irradiation light come from the first optical coupler through a first optical path, irradiate a measurement target with the irradiation light through a second optical path, receive, through the second optical path, measurement light obtained when the irradiation light is reflected by the measurement target, and cause the measurement light to propagate through the first optical path, a second optical coupler configured to generate interference light by causing the measurement light having propagated through the first optical path and the local light to interfere with each other and an electrical processing system configured to calculate a vibration state of the measurement target from the interference light. The sensor head includes a faraday rotator that rotates a polarization 45°.