Application of Single Frequency Semiconductor Laser in Precise Measurement of Light Wave Interference

Application of Single Frequency Semiconductor Laser in Precise Measurement of Light Wave Interference
The application of single frequency semiconductor laser in precision measurement fields such as fiber optic hydrophones and ground listening interferometers is discussed, and the key impact of laser performance on the performance of interferometer systems is analyzed in depth.

Core structure and working principle of the system: The fiber optic hydrophone system is mainly composed of a sensing head and an interferometer (taking M-Z interferometer as an example). The basic principle is that the sound signal (sound pressure Δ p) acts on the sensing head, causing changes in the length and refractive index of the sensing fiber wrapped around the hollow cylinder, thereby introducing changes in the optical path. This small optical path change (i.e. phase change) is detected with high sensitivity by an interferometer.

1. Sensor head: It’s core function is to convert sound vibrations into changes in the optical path of the interferometer. The sensitivity coefficient s is related to factors such as fiber length L, and longer sensing fibers are beneficial for improving system sensitivity.
2. Interferometer: It is the “best weapon” for detecting small phase changes. The output light intensity has a cosine relationship with the phase difference. By stabilizing the static phase bias φ ₀ at the orthogonal operating point ((m+1/2) π), the system can achieve the highest detection sensitivity.
3. Key light source parameters that affect system performance: The article focuses on analyzing the limitations of laser performance on achieving high phase resolution (with a target of ≤ 1 μ rad).
4. Laser frequency noise and linewidth: The frequency noise of the laser can cause interference phase noise, thereby reducing the visibility of interference fringes. For an interferometer with an optical path difference of about 1 meter, to achieve a phase resolution of 1 μ rad, the linewidth of the laser needs to be less than about 30 Hz. This is a very high requirement for the frequency stability of the light source.
5. Laser intensity noise: The relative intensity noise (RIN) of the laser will be directly converted into phase error of the interference signal. To achieve a phase resolution of 1 μ rad at a typical detection light power (~100 μ W), the RIN of the laser needs to be reduced to below -120 dB. This is a very high requirement for the stability of light source intensity.

In summary, by analyzing the fiber optic hydrophone system, the strict requirements for the core light source – single frequency semiconductor laser – in terms of extremely narrow linewidth (high frequency stability) and extremely low intensity noise in precision measurement based on interference principle are elaborated, and the laser frequency stabilization challenges faced in large-scale system applications are presented.