Fiber Optic Laser Rangefinder Phase Error Reduction
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Solution Overview
Problem
Current laser rangefinders face limitations in achieving accurate length measurements due to variable phase delays, systematic range errors, low zero-point measurements, and cyclic errors, which restrict their ability to measure distances greater than a few meters with high precision, especially in applications like the National Radio Astronomy Observatory's Robert C. Byrd Green Bank Telescope.
Innovation Solution
A fiber optic based laser rangefinder system utilizing a direct digital synthesizer (DDS) to output reference signals, an isolated diode laser for intensity modulation, and a computing device to calculate distance by sampling the difference in frequency of the reference and returned modulated optical signals, combined with a pulsed laser for absolute distance measurement, improves accuracy and reduces noise.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional laser rangefinders are used for large-scale metrology, then distance measurement capability is provided, but measurement precision deteriorates due to variable phase delays, systematic range errors, and cyclic errors
Solution Approach 1:
The patent replaces conventional electronic phase detection with an optical frequency comb reference system. The laser frequency comb provides a stable, traceable frequency reference that eliminates the cyclic errors and phase delays inherent in electronic systems. This substitution of the reference system fundamentally improves measurement precision and reliability for large-scale metrology applications.
Solution Approach 2:
The patent changes the operating parameters by using frequency combs with line spacings in the radio frequency range (e.g., 100 MHz to 10 GHz) rather than conventional lower frequency modulation. This parameter change enables precise phase measurement while extending the unambiguous measurement range, thereby improving both precision and reliability simultaneously.
2Stability of the object's composition
If fiber optic cables are used for signal transmission, then signal stability is improved, but device complexity increases due to integration requirements
Solution Approach 1:
The patent integrates multiple functions into a single portable unit: the laser frequency comb generator, the photodetector system, the signal processing electronics, and the display interface are all combined in one device. This universal design provides signal stability through fiber optic connections while managing complexity through functional integration rather than adding separate systems.
Solution Approach 2:
The patent uses fiber optic cables as intermediaries to connect the laser comb source to the photodetector and reference systems. These fiber connections provide stable, immune-to-interference signal transmission while being easily integrable into portable devices, thus improving signal stability without proportionally increasing complexity.
3Length of stationary object
If high modulation frequencies are used, then measurement range is extended, but signal-to-noise ratio deteriorates
Solution Approach 1:
The patent employs periodic modulation of the laser frequency comb at radio frequencies (e.g., 100 MHz to 10 GHz). This periodic action allows the use of lock-in detection and synchronous sampling techniques that maintain high signal-to-noise ratios even at high modulation frequencies, thereby extending measurement range without sacrificing precision.
Solution Approach 2:
The patent substitutes conventional electronic modulation and detection with optical frequency comb modulation and photodetector-based detection. This substitution enables high-frequency operation with maintained signal integrity because the optical domain provides broader bandwidth and lower noise floors compared to electronic systems.
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 system achieves enhanced accuracy in distance measurement beyond 100 meters by minimizing phase detection nonlinearities and noise, allowing for precise determination of absolute distances with improved signal-to-noise ratio and reduced power consumption.
Implementation Method 1
an isolated laser source that receives the first signal and outputs an optical signal
Implementation Method 2
a pin diode detector coupled to collecting lens
Implementation Method 3
direct the optical signal to free space, a collecting lens positioned adjacent to the collimating lens adapted to receive a modulated optical signal from free space
Data Source
AI summary
Laser rangefinders and methods of using laser rangefinders are disclosed. One embodiment of a laser rangefinder includes a first DDS (direct digital synthesizer) outputting a first reference signal, an isolated laser source that receives the first signal and outputs an optical signal, a collimating lens coupled to the isolated laser source adapted to direct the optical signal to free space, a collecting lens positioned adjacent to the collimating lens adapted to receive a modulated optical signal from free space, a pin diode detector coupled to collecting lens, a second DDS outputting a second reference signal, and a computing device adapted to receive the first reference signal, the second reference signal, and the received modulated optical signal and calculate a distance.