Directly-Modulated Laser Strain Measurement with Optical Bandpass Filter
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Solution Overview
Problem
Conventional optical fiber strain measurement apparatuses using Brillouin OTDR are costly due to the high price of external modulators required for generating optical pulses, and directly-modulated semiconductor lasers cause wavelength chirping, making it difficult to apply them in BOTDR systems without significant processing.
Innovation Solution
An optical fiber strain measurement apparatus utilizing a directly-modulated semiconductor laser as a light source with a transmitter-side optical bandpass filter to transmit the ON level wavelength and block the OFF level wavelength, allowing for a low-cost compact solution while maintaining high reception sensitivity using a variable-wavelength optical bandpass filter.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If an external modulator is used to generate optical pulses, then the extinction characteristic is improved, but the device cost and complexity increase significantly
Solution Approach 1:
The patent extracts the wavelength selection function from the modulation process by introducing a transmitter-side optical bandpass filter. This filter selectively transmits only the ON-level wavelength while blocking the OFF-level wavelength, thereby achieving high extinction ratio without requiring complex external modulators. The filter isolates the harmful wavelength components that would otherwise require expensive modulation hardware to suppress.
Solution Approach 2:
The patent replaces expensive external modulators with a simpler, more cost-effective directly-modulated semiconductor laser combined with an optical bandpass filter. This substitution uses cheaper components (semiconductor laser + filter) to achieve the same functional outcome (high extinction ratio) that previously required costly external modulation equipment, thereby reducing overall system cost and complexity.
2Device complexity
If a directly-modulated semiconductor laser is used, then the device cost and compactness are improved, but wavelength chirping occurs degrading measurement precision
Solution Approach 1:
The patent converts the harmful effect of wavelength chirping into a beneficial outcome by using the transmitter-side optical bandpass filter. The filter's fixed transmission wavelength acts as a reference that automatically compensates for the chirping-induced wavelength variations. By blocking wavelengths outside the passband, the filter effectively removes the chirping artifacts, allowing the use of simple directly-modulated lasers without sacrificing measurement precision.
Solution Approach 2:
The optical bandpass filter serves as an intermediary component between the directly-modulated laser and the optical fiber under test. It mediates the interaction by selectively transmitting only the desired wavelength range while blocking the chirped wavelengths, thereby protecting the measurement system from the adverse effects of wavelength modulation and enabling precise strain measurements.
3Adaptability or versatility
If the wavelength band of Brillouin scattering changes, then the adaptability to different conditions is improved, but the reception sensitivity decreases
Solution Approach 1:
The patent implements dynamic adaptability through a variable-wavelength optical bandpass filter on the receiver side. This filter can be tuned to match the Brillouin scattering wavelength at different measurement positions along the optical fiber, maintaining high reception sensitivity regardless of wavelength variations. The delay controller dynamically adjusts the reference arm delay to compensate for different propagation times, enabling the system to adapt to changing conditions while preserving measurement sensitivity.
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 the use of a low-priced compact directly-modulated semiconductor laser in BOTDR systems, improving the extinction characteristic and maintaining high reception sensitivity despite changes in the wavelength band of Brillouin scattering.
Implementation Method 1
a light source unit 12 that generates an optical pulse
Implementation Method 2
a transmitter-side optical bandpass filter 14 provided in a stage following the directly-modulated light source 12, and configured to transmit wavelength of an ON level of the optical pulse as the probe light, and block wavelength of an OFF level
Implementation Method 3
Backscattering in an optical fiber includes Rayleigh scattering, Brillouin scattering, and Raman scattering
Implementation Method 4
a receiver-side optical bandpass filter (BPF) 32 that extracts a Stokes component of Brillouin backscattered light
Implementation Method 5
a self-delayed heterodyne interferometer that detects a change in a frequency shift amount of the Stokes component as a phase difference
Data Source
AI summary
Object is to enable a directly-modulated semiconductor laser to be applied as a light source of probe light. A transmission unit configured to generate probe light; and a reception unit including a receiver-side optical bandpass filter that extracts a Stokes component of Brillouin backscattered light from backscattered light which is caused by the probe light in a measurement target optical fiber, and a self-delayed heterodyne interferometer that detects a change in a frequency shift amount of the Stokes component as a phase difference are included. The transmission unit includes a directly-modulated light source configured to generate an optical pulse, and a transmitter-side optical bandpass filter provided in a stage following the directly-modulated light source, and configured to transmit wavelength of an ON level of the optical pulse as the probe light, and block wavelength of an OFF level.