Application Cases

Experiment Name: Experimental Study on Power Stabilization and Laser Noise Suppression Using Mach-Zehnder Interferometer

Test Equipment: Voltage amplifier, Signal generator, Oscilloscope, Photodetector, Beam splitter, Piezoelectric ceramic, etc.

Experimental Process:

Figure 1: Experimental setup for improving 1550nm laser performance using MZI

The MZI used in the experiment consists of two beam splitters (BS1, BS2) and plane mirrors (M1, M2), with the plane mirrors coated with high-reflection films. The BS1, BS2, M1, and M2 are arranged at the four corners of a diamond configuration. Piezoelectric ceramic PZT3 is attached to the back of M1, allowing control of the MZI phase through a voltage amplifier (HV). To ensure the output beam direction remains unchanged after passing through the MZI, two 22.5-degree plane mirrors are placed at the input and output of the MZI. The output 1550nm light field from the MZI is split into two beams using an optical splitter composed of a half-wave plate (HWP) and a polarizing beam splitter (PBS). The beam with higher power can be used for subsequent measurements and preparation of 1550nm non-classical light fields. An optical splitter is placed in the subsequent optical path to extract a small portion of light as an error signal for locking the MZI phase. The effect of different beam splitter ratios on the intensity noise of the 1550nm laser is then investigated.

A 65Hz triangular wave generated by a signal generator (SG) is used to scan PZT3 in the MZI. The photodetector detects changes in the output photocurrent of the MZI, and the detected signal is input to an oscilloscope. By recording the minimum and maximum values of the MZI interference signal on the oscilloscope and adjusting the parameters of the proportional-integral-derivative (PID) controller, the MZI can be locked at different transmittance levels. The photocurrent signal from the detector PD is amplified by the PID and voltage amplifier (HV) and applied to PZT3 in the MZI to lock the relative phase of the two arms. Changes in the DC signal voltage from the PD also affect the MZI transmittance (TMZ). By setting different PID parameters to lock the MZI at various transmittance levels (Tlock), the effect on the intensity noise of the 1550nm laser is studied.

Experimental Results:

The experiment first investigated the effect of the MZI on the power stability of the 1550nm laser. The power fluctuation of the 1550nm laser passing through an unlocked MZI over 5 hours was measured using a power meter. Then, the power fluctuation when the MZI was locked at the optimal operating point was measured.

Figure 2: 5-hour power fluctuation of the 1550nm laser: (a) after passing through an unlocked MZI, (b) when the MZI control loop is closed.

As shown in Figure 2, (a) and (b) represent the output power of the laser over 5 hours before and after locking the MZI, respectively. Calculations show that the power fluctuation of the 1550nm laser decreased from ±0.20% to ±0.09%. Figure 2 demonstrates that the power stability of the 1550nm laser improved after locking the MZI.

Next, the effect of different beam splitter ratios near R=90% on suppressing the intensity noise of the 1550nm laser was studied. Three beam splitters with ratios of R=90.9%, 92.5%, and 95% were used. The intensity noise curves of the output 1550nm laser as a function of analysis frequency were measured and compared for the three cases after locking the MZI.

Figure 3: Relationship between the intensity noise of the 1550nm laser passing through the MZI and the analysis frequency for different beam splitter ratios.

Curve (i) in Figure 3 represents the intensity noise spectrum measured with a spectrum analyzer (SA) after the 1550nm laser passes through an unlocked MZI in the 10kHz-50kHz range. Curves (ii), (iii), and (iv) represent the intensity noise spectra when R is 95%, 92.5%, and 90.9%, respectively, after the MZI is locked. From Figure 3, it can be concluded that in the 10kHz-35kHz range, all three MZIs reduced the intensity noise of the 1550nm laser to varying degrees. However, beyond 35kHz, none of the MZIs reduced the intensity noise. Among the three, the MZI with R=90.9% performed the best, reducing the noise by approximately 8dB at 15kHz. At this point, the MZI was locked at Tlock=85%.

Based on the above results, the MZI with R=90.9% was found to be the most effective for noise suppression. Therefore, the intensity noise of the 1550nm laser was further investigated when the R=90.9% MZI was locked at Tlock=45%, 65%, and 85%. The intensity noise power spectra were measured using an SA at different Tlock values.

Figure 4: Relationship between the intensity noise of the 1550nm laser and the analysis frequency at different MZI transmittance levels.

Curve (i) in Figure 4 represents the intensity noise spectrum measured after the 1550nm laser passes through an unlocked MZI in the 10kHz-50kHz range. Curves (ii), (iii), and (iv) represent the intensity noise spectra when the R=90.9% MZI is locked at Tlock=45%, 65%, and 85%, respectively. From the figure, it can be observed that in the 10kHz-35kHz range, all three transmittance levels suppressed the intensity noise of the 1550nm laser to varying degrees, with Tlock=85% showing the best suppression effect. At 15kHz, Tlock=85% resulted in approximately 6dB lower noise compared to Tlock=45%.

Voltage Amplifier Recommendation: ATA-2168

Figure: ATA-2168 High-Voltage Amplifier Specifications

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