Interferometric Rangefinder Phase Noise Compensation
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Solution Overview
Problem
Existing absolute distance-measuring technologies face challenges with phase noise and non-linearity in laser sources, limiting measuring range and accuracy, especially beyond the coherence length of the laser, and are complex and costly to implement.
Innovation Solution
Characterizing the phase curve of the laser source in a reference interferometer and using synthetic phase curve generation to correct for non-ideal properties, allowing for accurate distance measurement beyond coherence length with reduced complexity and cost.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a laser source is used for absolute distance measurement, then the measurement can be performed, but phase noise limits the measuring range to distances below the coherence length
Solution Approach 1:
A reference interferometer with a known reference length is introduced as an intermediary system. This reference interferometer measures the phase curve of the laser source independently, allowing the harmful phase noise to be characterized and subsequently removed from the measurement signal, thereby extending the measurable range beyond the coherence length
Solution Approach 2:
The phase curve of the laser source is copied by measuring it in the reference interferometer. This copied phase information is then used to correct the measurement signal from the measuring interferometer, eliminating the need for complex linear tuning mechanisms and extending the measuring range
2Reliability
If the arm length of the reference interferometer is made approximately equal to the target distance to suppress phase noise, then phase noise is reduced, but the system complexity and production costs increase
Solution Approach 1:
The system is segmented into two independent interferometers: a reference interferometer with a fixed, manageable arm length for characterizing phase noise, and a measuring interferometer for actual distance measurement. This segmentation allows phase noise suppression without requiring the reference interferometer to have an arm length equal to the target distance, thereby reducing system complexity
Solution Approach 2:
The reference interferometer acts as an intermediary that characterizes phase noise independently. By using this intermediary system with a practical arm length to capture phase curve information, the main measurement system can operate without the complexity of matching arm lengths to target distances
3Measurement precision
If linear tuning of the laser light source is implemented with sufficient accuracy, then measurement accuracy is improved, but the effort and complexity required increase greatly
Solution Approach 1:
Instead of implementing complex linear tuning mechanisms, the invention copies the actual (non-linear) phase curve of the laser source by measuring it in the reference interferometer. This copied phase information is then used to correct the measurement signal, achieving high measurement accuracy without requiring complex linear tuning hardware
Solution Approach 2:
The invention changes the approach from controlling the tuning parameter (attempting to achieve linear tuning) to measuring and compensating for the actual phase parameter. By using the reference interferometer to capture the actual phase curve and applying it as a correction, the system achieves accuracy without complex tuning mechanisms
4Length of moving object
If the laser source coherence length is increased to extend measuring range, then the measuring range increases, but the system cost and complexity increase
Solution Approach 1:
The reference interferometer serves as an intermediary that characterizes the laser's phase properties. By measuring and compensating for phase noise through this intermediary system, the invention enables extended measuring range beyond the coherence length without requiring lasers with inherently long coherence lengths, thereby avoiding the high costs and complexity associated with such specialized laser sources
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 approach enhances measurement accuracy and range while reducing the complexity and cost of the system, enabling efficient absolute distance measurement with improved tolerance to phase noise and non-linearity.
Implementation Method 1
The laser light sources, in particular semi-conductor laser diodes, have a strong phase noise, which is equivalent to uncontrolled variations of the scale length
Implementation Method 2
The laser light sources, in particular semi-conductor laser diodes, have a strong phase noise, which is equivalent to uncontrolled variations of the scale length
Implementation Method 3
The laser light sources, in particular semi-conductor laser diodes, have a strong phase noise, which is equivalent to uncontrolled variations of the scale length
Implementation Method 4
The signal received is superposed with a second signal which is derived from the emitted light signal. The resulting beat frequency of the heterodyne mixed product, the interferogram, is a measure of the distance to the target object
Implementation Method 5
In a heterodyne interferometer arrangement, a tuneable laser light source is used for the absolute distance measurement
Implementation Method 6
For this purpose, for example, a part of the emitted light is passed via a reference interferometer with a defined reference length. From the resulting beat product, it is possible to derive the wavelength change of the emitted light signal as a function of time on the basis of the known reference length
Implementation Method 7
Transmitting and receiving optical systems down-circuit of which a detector or quadrature detector for heterodyne mixing, A/D converter and digital signal processor are connected are used for emission and for reception in the optical range
Data Source
AI summary
The invention relates to a method for interferometric absolute distance measuring by a frequency modulation electromagnetic radiation on at least one measurable target and for subsequently receiving a retransmitted radiation with a heterodyne mixture, wherein the radiation is guided in a parallel direction via a reference interferometric length. In such a way, a first digitized interferogram of the radiation retransmitted by the target and a second digitised interferogram of the radiation guided on the reference length are obtainable at a reception. According to phase progression data of the second interferogram, a virtual interferogram or a phase progression thereof is synthesized and the distance determination is carried out by comparing the progression face data of the first interferogram with the progression face data of the virtual interferogram.