Application Cases

Experiment Title: Femtosecond Laser Ranging Experiment Based on Balanced Optical Cross-Correlation

Testing Purpose:This paper describes a femtosecond laser ranging experiment based on balanced optical cross-correlation, including the composition of the experimental system and the ranging process. It also analyzes the experimental measurement results, discusses the errors and potential problems, and finally summarizes this ranging method.

Testing Equipment:High-power high-voltage amplifier, signal generator, femtosecond laser, polarizing beam splitter, isolator, etc.

Experimental Process:

Figure 1: Ranging Experiment System Based on Time-of-Flight with Balanced Cross-Correlation

According to the requirements of the laser for ranging principles, a femtosecond laser based on nonlinear polarization rotation and semiconductor saturable absorber mirror (SESAM) mode locking was built as the ranging light source. This laser uses a sigma-structure resonator, with the SESAM, which acts as the end mirror, attached to the free end of a piezoelectric ceramic, as shown in the right half of Figure 1. The collimator, which acts as a coupler, is fixed on an electric translation stage to achieve a wider range of adjustment of the laser's repetition rate.

A 980-nm diode laser pumps light into a highly doped Er3+ gain fiber through a wavelength division multiplexer. The amplified pump light is focused onto the SESAM through a polarizing beam splitter (PBS), a wave plate set, and a lens. The saturable absorption effect of the SESAM is used to achieve mode locking. The mode-locked pulse sequence is then reflected by the SESAM, passes through an isolator (ISO), and is coupled into the fiber to realize the periodic circulation of the resonator. Part of the femtosecond laser pulse is output by the PBS.

The repetition rate of the femtosecond pulse sequence output from the laser oscillator is 203.4 MHz, with a central wavelength of 1550 nm and an average power of up to 30 mW, which can meet the needs of the ranging experiment without amplification. At the receiving end, the balance detector feeds the electrical signal back to the proportional-integral servo system, which includes a PI controller and a high-power high-voltage amplifier. The PI controller calculates the control signal to be output based on the feedback error signal. After being amplified by the high-voltage amplifier, it is applied to the PZT to lock the measured distance to an integer multiple of the cavity length.

Figure 2: Intensity Balanced Cross-Correlation Signal

When the target to be measured oscillates harmonically with the stepping motor centered on the lock point, the balanced cross-correlation signal generated is shown in Figure 2. It can be seen from the figure that the middle part of the inverted "S" - shaped curve has good linearity, which ensures that the system has the highest sensitivity when locked. The experiment uses the proportional - integral servo system to feedback - control the elongation of the PZT in the cavity according to the voltage value of this linear part, thereby realizing the adjustment of the cavity length and finally locking the measured distance to an integer multiple of the cavity length. Due to the response bandwidth limitation of the PZT, the system loop lock bandwidth is 1 kHz.

Experimental Results:

Figure 3: Dynamic Distance Measurement Results. (a) Continuous measurement results within 4 s; (b) Partial measurement results within 0.5 s

To verify the real - time performance and accuracy of the ranging system, a signal generator was used to output a 10 - Hz sine signal to control the piezoelectric ceramic and the target reflector to oscillate harmonically with an amplitude of 1 μm. The frequency counter still measured the repetition rate of the laser at a rate of 100 Hz. The calculated distance measurement results are shown in Figure 3(a), and the first 50 sets of data are shown in the partial figure in Figure 3(b).

It can be seen from Figure 3(a) that each measurement value is valid during continuous measurement, and there are no error points (points that deviate significantly from the sine curve). The overall measurement results are stable and well reflect the motion of the target in terms of amplitude and frequency. From the partial enlarged figure, it can be seen that the measurement results fit the sine waveform well. There are as many as six valid values in the vibration process with a stroke of 1 μm, and the adjacent two measurement results at the extreme points can be less than 20 nm.

It can also be seen from Figure 3(a) that there are fluctuations in the upper edge (composed of the maximum values of the sine wave) and the lower edge (composed of the minimum values of the sine wave) of the measurement result waveform, and the fluctuation range is about 200 nm. This is consistent with the measurement results when the target is stationary. Mechanical vibrations of the measured mirror, laser mounting components, and other components, as well as air disturbances, can cause small random changes in the measured distance.

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Figure: ATA - 4051C High - Power High - Voltage Amplifier Specifications

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