Application Cases

Experimental Name: Research on Low-Frequency Band Frequency Noise Suppression Based on Fiber Interferometer

Experimental Content: Fiber interferometers, as structural devices capable of precisely discriminating laser phase, are widely used in testing and measuring laser phase noise. Using reverse thinking, the intensity signal carrying laser phase information output from the fiber interferometer is processed through a specifically designed signal processing module. The feedback signal is then applied to the frequency modulation port of the laser, thereby locking the output laser frequency and effectively reducing frequency noise. This section utilizes a self-developed single-frequency fiber laser integrated with a piezoelectric ceramic (PZT), combined with the frequency discrimination mechanism of an unbalanced fiber interferometer, to achieve effective suppression of frequency noise in the low-frequency band of the output laser.

Test Equipment: Voltage amplifier, fiber interferometer, oscilloscope, signal generator, photodetector, PZT, etc.

Figure 1: Experimental Setup Diagram for Low-Frequency Noise Suppression Based on Unbalanced Fiber Interferometer

Experimental Process:

A delay fiber length of 500 m was selected, corresponding to an interferometer arm difference of 1 km. From the gain transfer function of the fiber interferometer, the 3 dB control bandwidth corresponding to a 1 km arm difference is 120 kHz. However, the phase transfer function reveals rapid phase changes occurring in frequency bands above 20 kHz, which is expected to pose some difficulty for closed-loop control. Therefore, the focus is on examining the frequency noise suppression results within 20 kHz.

To minimize the effects of mechanical vibrations and airflow disturbances from the external environment, the entire fiber interferometer is housed within a sealed plexiglass box lined with vibration-damping foam. The two returning beams of the fiber interferometer interfere at the 50:50 Coupler and are then input into a low-noise photodetector (PD) with a bandwidth of 17 MHz. Subsequently, the converted electrical signal passes through a low-pass filter (LPF) with a 3 dB bandwidth of 100 kHz to filter out high-frequency spurious noise, obtaining the frequency variation error signal. After processing by the Proportional-Integral-Derivative (PID) module, the resulting feedback signal has a small amplitude, in the voltage range of 0~5 V. To achieve effective frequency control, a voltage amplifier (HV) is introduced to increase the range of the feedback signal voltage applied to the PZT to 0~50 V. This enhances the feedback control capability and enables long-term stable frequency noise suppression.

During the experiment, the pump current of the LD was set to 250 mA. The output power from the 70% port was 10.5 mW, the power entering the fiber interferometer was about 4 mW, and the power entering the PD was about 1 mW.

Experimental Results:

Before conducting the frequency noise suppression experiment, tests were performed on the frequency stability and PZT tuning performance of this 1550 nm laser. A scanning F-P cavity combined with an oscilloscope-type acquisition card (Model: PXI-5122, bandwidth 100 M, 14-bit resolution) was used to acquire the signal of a single longitudinal mode of the laser in real-time. Specific results are shown in Figure 2(a). During 24 hours of continuous testing, the drift of the output laser's central frequency was 419.63 MHz, equivalent to a wavelength variation of 3.36 pm.

Figure 2: (a) Frequency stability test results; (b) Relationship between laser wavelength modulation change and the voltage applied to the PZT

Figure 2(b) shows the relationship between the laser wavelength modulation change and the voltage applied to the PZT. A modulation signal was applied to the PZT using a signal generator combined with a voltage amplifier. Within the modulation voltage range of 0~48 V, the central wavelength of the laser changed from 1550.0834 nm to 1550.0907 nm, with a variation amplitude of 7.3 pm.

Voltage Amplifier Recommendation: ATA-2022B High-Voltage Amplifier

Figure: ATA-2022B High-Voltage Amplifier Specifications and Parameters