Etalon Free Spectral Range Measurement Using RF Modulation
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Solution Overview
Problem
Existing methods for measuring the free spectral range (FSR) of etalons are limited in precision, especially for etalons with FSR smaller than 10 GHz, due to the resolution constraints of optical spectrum analyzers and tunable lasers, allowing only up to 4 parts per million error.
Innovation Solution
A modified Pound-Drever-Hall technique is employed, involving RF modulation of laser light, reflection through a circulator, and conversion to an electrical signal, with the RF modulation frequency scanned to detect peak-to-peak minimum voltage, achieving precision beyond one part in 10^4 without requiring a high resolution optical spectrum analyzer or tunable laser.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If traditional transmission spectrum mapping methods are used, then the measurement process is simple, but the measurement precision is limited to 4 parts per million due to instrument resolution constraints
Solution Approach 1:
The patent replaces the mechanical/optical scanning system (tunable laser or optical spectrum analyzer) with an electrical RF signal generation and detection system. By substituting the optical measurement path with an electrical domain implementation using RF modulators and mixers, the system achieves higher precision measurement without being constrained by optical instrument resolution limits.
Solution Approach 2:
The patent transforms the measurement approach by changing from direct optical frequency domain measurement to time domain RF signal processing. By converting the optical FSR measurement problem into an equivalent RF frequency measurement problem through modulation and mixing, the system achieves better than one part per 10^4 precision using electrical signal processing instead of optical spectrum analysis.
2Adaptability or versatility
If optical spectrum analyzer or tunable laser is used, then the measurement can be performed, but the precision deteriorates for etalons with FSR smaller than 10 GHz due to resolution limits
Solution Approach 1:
The patent replaces the optical spectrum analyzer or tunable laser system with an RF-based measurement system. By substituting the optical domain instruments with electrical RF signal generation and mixing equipment, the system overcomes the resolution constraints that limit optical methods for small FSR values, achieving high precision measurement for etalons with FSR below 10 GHz.
Solution Approach 2:
The patent moves the measurement from the optical frequency dimension to the electrical RF dimension. By mapping the optical FSR measurement problem into the electrical domain through RF modulation and mixing, the system accesses a different measurement dimension where higher precision is achievable regardless of the optical FSR size, enabling accurate measurement of small FSR etalons.
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 method provides precise measurement of the FSR with better than one part in 10^4 precision, effectively addressing the limitations of existing techniques for etalons with small FSR, particularly those below 10 GHz, and is useful for dense wavelength division multiplexed systems.
Implementation Method 1
generating a RF source signal and RF modulating the laser light with the RF source signal to produce an RF modulated laser signal
Implementation Method 2
At the photo detector, the reflected RF signal is converted to an electrical signal
Implementation Method 3
the amplified electrical signal is mixed with a RF delayed source signal
Data Source
AI summary
Methods, systems, apparatus and devices for using a modified PDH technique to measure the FSR of an etalon with one part per 104 precision. An embodiment of the method for measuring the free spectral range of an etalon can include generating a laser light from a laser source, generating a RF source signal, RF modulating the laser light with the RF source signal to produce an RF modulated laser signal, coupling the RF modulated laser signal through a circulator to the etalon, coupling a reflected RF signal from the etalon through the circulator to photo detector, converting the reflected RF signal to an electrical signal at the photo detector, amplifying the electrical signal, mixing the amplified electrical signal with a RF delayed source signal, linearly scanning a frequency of the RF source signal, and monitoring a peak-to-peak mixer voltage Vmixer during the linear scanning of the RF source signal frequency to detect a peak-to-peak minimum voltage when the RF modulation frequency is tuned approximately to a free spectral range of the etalon, the result having a precision greater than one part per 104 without the use of a high resolution optical spectrum analyzer or a tunable laser. This method is especially useful for etalons with small FSR (less than 10 GHz) because this method does not require a high resolution OSA or tuneable laser. As the ITU grid for DWDM becomes denser, this method will have a larger impact on the FSR measurement of etalons.