Micro laser Doppler vibrometer DC~2.5MHz | SIMTRUM Photonics

Application

• Ultrasonic material vibration testing
• High-temperature, low-temperature and thin-walled structure vibration tests
• Vibration mode testing of MEMS devices
• Vibration testing for mechanical fault diagnosis
• Vibration testing of electroacoustic equipment such as speakers
• Industrial online quality inspection
• Non-destructive testing of materials
• Detection of pipeline vibration status and natural frequency
• Modal testing and ODS deformation analysis
• Health monitoring of bridge and dam structures
• Environmental vibration assessment and monitoring

The core working principle of the laser Doppler vibrometer is based on the Doppler effect and optical interference. The following is a detailed description of the working principle of each step:

1.  
2. Laser Emission and Optical Path Design

• After the laser is expanded and collimated, it is split into two beams by a beam splitter: one is the measurement beam, and the other is the reference beam.

• The measurement beam is precisely focused onto the surface of the object to be measured by a focusing lens, while the reference beam is directly guided to the interferometer or detector to maintain a stable optical path length.

2. Generation and Characteristics of Doppler Frequency Shift

• When the object vibrates along the direction of the measurement beam, it will have a relative motion with the reflected measurement beam, causing a frequency shift of the measurement beam, that is, the Doppler frequency shift (Δf).

• The frequency shift is directly related to the vibration velocity, and the quantitative relationship is: Δf = 2v・cosθ/λ (v is the vibration velocity of the object, θ is the angle between the measurement beam and the vibration direction of the object, and λ is the wavelength of the laser).

• When the object approaches the light source, the frequency of the reflected light increases (positive frequency shift); when it moves away, the frequency decreases (negative frequency shift), and the direction of the frequency shift corresponds to the vibration direction.

3. Optical Interference and Signal Generation

• The reflected measurement beam (with frequency shift) and the reference beam (with unchanged frequency) are recombined, satisfying the interference conditions (similar frequencies, consistent vibration directions, and stable phase difference).

• The superposition of the two beams forms a beat frequency signal (time variation of interference fringes), and the beat frequency is exactly equal to the Doppler frequency shift Δf, with the light intensity varying periodically with the beat frequency.

• The interference process amplifies the detectability of the frequency shift signal, allowing the weak vibration frequency shift to be captured by the subsequent detector.

4. Signal Detection and Processing

• The photodetector converts the intensity changes of the interference light into an electrical signal.

• The electrical signal undergoes preprocessing: filtering, amplification, and analog-to-digital conversion.

• The beat frequency signal is analyzed through digital signal processing algorithms (such as Fourier transform, lock-in amplification) to extract the value and direction of the Doppler frequency shift Δf.

5. Inverse Calculation of Vibration Parameters

• Based on the quantitative relationship between frequency shift and velocity (v = Δf・λ/(2cosθ)), the instantaneous vibration velocity of the object is calculated.

• By integrating the vibration velocity, the vibration displacement is obtained: x(t) = ∫v(t)dt + x₀ (initial phase calibration is required); through frequency analysis, the complete parameters of the vibration, such as frequency, amplitude, and phase, can also be obtained.