Application Cases

Experiment Name: Distance Measurement Based on Continuous Light Interference Fringe Counting

Research Direction: Investigation of Underwater Acousto-Optic Modulation Effects

Experimental Objective:
To measure the speed of sound in seawater using femtosecond lasers. The distance traveled by the acoustic wave was determined using a method based on continuous light interference fringe counting, while the flight time of the acoustic wave was measured using cross-correlation techniques. Based on these measurement methods, an experimental optical system was constructed in the laboratory to achieve high-precision measurement of sound velocity in seawater.

Testing Equipment:
ATA-122D broadband amplifier, signal generator, specimen, oscilloscope

Experimental Procedure:
A signal generator, in conjunction with a broadband amplifier (Aigtek, ATA-122D), was used to drive an ultrasonic transducer to emit acoustic pulse signals through the acousto-optic interaction region. The modulated signals of the pulsed light were focused by a lens, converted by a photodetector, and displayed on an oscilloscope.

Figure 1: Schematic Diagram of Acoustic Wave Flight Distance Measurement

The femtosecond laser was locked to an external frequency source (rubidium clock). The position of mirror M3 on the precision displacement stage was adjusted to ensure equal optical path differences between the reference and measurement optical paths. Simultaneously, mirror M4, along with precision displacement stage PDP3, was moved to ensure that continuous light emitted by the solid-state laser produced interference signals between the measurement and reference arms.

Figure 2: Schematic Diagram of Acoustic Wave Flight Time Measurement

To ensure the accuracy of the measured ultrasonic propagation distance, an air-floated platform was used to maintain a horizontal state. Fine adjustments of mirrors M1 and M2, fixed to the platform, ensured parallelism between the two measurement optical paths. Meanwhile, the position of mirror M6 on the displacement stage was adjusted to equalize the optical path differences between the reference and measurement paths. Similarly, the position of mirror M7 was adjusted, and beam splitter BS1 was used to divide the frequency comb into two perpendicular beams. These beams entered the dual Michelson interferometer, where the two measurement optical paths were continuously modulated by acoustic waves. A photodetector (PD) was used to capture the interference signals. During the experiment, temperature, salinity, and sound velocity sensors were placed in the water tank to monitor temperature and salinity in real-time. The measurement results were compared with those from the sound velocity sensor to validate the accuracy and feasibility of the method.

Experimental Setup:
The instruments used in this testing system mainly included a function generator, broadband amplifier, ultrasonic transducer, and photodetector. The function generator emitted signals, which were amplified by the broadband amplifier to drive the ultrasonic transducer. The generated ultrasonic waves passed through the acousto-optic interaction region. The modulated signals of the pulsed light were focused by a lens, converted by the photodetector, and finally displayed on an oscilloscope.

Experimental Results:
A sinusoidal signal of a specific frequency generated by the signal generator was amplified by the power amplifier and connected to the ultrasonic transducer. Continuous ultrasound was observed during the experiment. At higher ultrasonic frequencies, the acoustic wavelength becomes shorter, the diffraction angle increases, and the diffracted light of various orders becomes easier to separate. Experimental results showed that diffracted light generated by ultrasonic transducers operating at frequencies below 2 MHz mixed with the zero-order light, and no effective method currently exists to separate the diffracted light from the original incident light. Therefore, a 2 MHz ultrasonic transducer was used in this experiment. As shown in the figure below, the interference signal diagram between the original incident light and the first-order diffracted light collected in this environment is presented.

Figure 3: Interference Signal Diagram

After expanding the interference signal between the first-order diffracted light and the incident light, it was found that the optical wave corresponded one-to-one with the sinusoidal modulation signal of the acoustic wave, exhibiting good regularity. By interfering the first-order diffracted light with the original incident light, the experimental results demonstrated that the signals obtained by this method were affected by environmental factors, leading to waveform jitter, but overall stability was relatively good. As shown in the figure below, the black curve represents the results obtained from the sound velocity sensor, while the red solid circles represent the results obtained by this method. The measurement results from this experimental method closely matched those from the sound velocity sensor.

Figure 4: Sound Velocity Measurement Results at 23.02°C

Since the intensity of the first-order diffracted light is much weaker than that of the original incident light, when this method was integrated into the entire experimental system, the interference signals obtained from the second measurement optical path did not meet the ideal requirements. Therefore, based on the method of interfering the first-order diffracted light with the original incident light, future efforts should focus on improving the intensity of the first-order diffracted light to enhance the precision of seawater sound velocity measurements.

Figure: Specifications of the ATA-1372A Broadband Amplifier