The advantages of 780nm wavelength adaptation for laser Doppler systems
780nm is in the near-infrared band and has many natural advantages in laser Doppler systems. This wavelength of laser has strong tolerance to interferences such as atmospheric scattering and dust. In outdoor laser speed measurement (such as speed measurement on traffic roads and aircraft on airport runways), the optical signal transmission is more stable, reducing the influence of environmental factors on the measurement. In the field of biomedicine, 780nm laser has moderate penetration into human tissues and can be used for microvascular blood flow monitoring and skin blood flow imaging. It can obtain deep blood flow information without damaging the tissues, thereby facilitating disease diagnosis and research.
Actual measurement scheme design and equipment setup
(1) Test Objectives and Parameters
Focus on the parameters such as insertion loss, rise time, switching extinction ratio and frequency shift characteristics of the 780nm fiber AOM to verify its ability to modulate the laser frequency and intensity in the laser Doppler system, as well as the processing effect on the Doppler frequency shift signal.
(2) Experimental equipment
Multiple models of 780nm fiber AOM (such as SGTF80-780-1P, SGTF400-780-1P) were selected, combined with 780nm narrow-linewidth lasers, high-precision optical power meters, radio frequency signal generators, and spectrum analyzers to construct a laser Doppler simulation test platform. Simulate the Doppler frequency shift generated by the movement of the target to test the modulation performance of AOM.
Key performance measurement and analysis
(1) Measured insertion loss
The AOM insertion loss of different models of 780nm optical fibers was measured. The insertion loss of the SGTF80-780-1P model was less than 3dB, and that of the SGTF400-780-1P model was less than 4dB. Low insertion loss ensures that the laser energy is less attenuated during the modulation process. In the laser Doppler velodmetry system, more optical energy is used to generate the echo signal, enhancing the signal strength and signal-to-noise ratio. If the speed is measured at a distance of 1km, the low-loss AOM can increase the power of the reflected light by approximately 20%, making the speed measurement results more accurate.
(2) Measured rise time
Some models have excellent rise time performance. The rise time of SGTF200-780-1P is less than 12ns, and that of SGTF400-780-1P is less than 8ns. The ultra-fast rise time enables the AOM to respond quickly to changes in modulated signals. When measuring ultra-high-speed moving targets (such as aero engine blades with rotational speeds exceeding 10,000 revolutions per minute), it can capture the instantaneous Doppler frequency shift in real time, providing the system with high refresh rate and high-precision speed measurement data, ensuring the accuracy of industrial equipment monitoring and maintenance.
(3) Measured frequency shift and frequency stability
The 780nm optical fiber AOM frequency shift covers 80-400MHz. The frequency stability under different frequency shifts has been measured. In high-temperature and long-term operating environments, the frequency drift is less than ±0.1%. Stable frequency shift provides precise frequency reference for laser Doppler systems. When measuring fluid flow velocity (such as river and industrial pipeline fluids), through frequency shift compensation and frequency shift measurement, the flow velocity can be accurately calculated, providing reliable data for water conservancy monitoring and industrial process control.
(4) Measured extinction ratio of the switch
The extinction ratio of the switch is ≥50dB. In the actual measurement, the optical power contrast in the "on - off" state is distinct. In laser Doppler signal detection, a clear "on-off" boundary can effectively distinguish signal light from noise light, enhancing the system's anti-interference capability. In complex environments (such as urban traffic roads and multi-source interference) for speed measurement, a high extinction ratio AOM can reduce the false detection rate and ensure the safety of traffic management and autonomous driving.
Actual measurement summary and application suggestions
Through actual measurement, 780nm fiber AOM has demonstrated advantages such as low insertion loss, ultra-fast rise time, stable frequency shift and high extinction ratio in the laser Doppler system, and is suitable for multiple application scenarios. In industrial speed measurement and fluid monitoring, models with frequency shift adaptation (such as 80-200 MHZ) and rise time < 25ns should be given priority. In biomedical high-precision blood flow monitoring, the focus is on models with low insertion loss (<3.5dB) and high extinction ratio. Outdoor long-distance speed measurement focuses on frequency stability and anti-environmental interference performance.
The 780nm fiber AOM, with its outstanding modulation performance verified through actual measurement, has become a reliable core component of laser Doppler systems. From industrial production to biomedicine, from scientific research exploration to traffic management, it provides technical support for precise measurement and promotes the in-depth application and innovative development of laser Doppler technology in various fields.