Application Cases

Experiment Title: Research on Cavity Parameter Optimization in Dual-Wavelength External Cavity Resonance and Frequency Summation

Testing Equipment: Voltage Amplifier, Photodetector, PZT, etc.

Experiment Process:

Figure 1: Experimental Setup for Dual-Wavelength External Cavity Resonance and Frequency Summation

• M: Cavity Mirror

• PPLN: Periodically Poled Lithium Niobate Crystal

• LEN: Mode-Matching Lens

• PBS: Polarizing Beam Splitter

• PD: Photodetector

• PID: Proportional-Integral-Derivative Controller

• HVA: Voltage Amplifier

• PZT: Piezoelectric Ceramic

• R: Reflecting Plane Mirror

The experimental setup for generating 589nm sodium yellow light through the external cavity resonance and frequency summation of 938nm and 1583nm lasers using periodically poled lithium niobate (PPLN) crystals is shown in Figure 1. To optimize beam quality, single-mode fibers are used to shape the output laser beam, achieving a coupling efficiency of over 50%. The external cavity is a butterfly-shaped ring cavity formed by two plane mirrors (M1 and M2) and two concave mirrors (M3 and M4) with a radius of curvature of 50mm. This cavity design not only meets the requirements of dual-wavelength resonance and frequency summation but also allows the two seed beams to enter from two different cavity mirrors, reducing experimental complexity. The nonlinear crystal used in the experiment is a PPLN crystal with dimensions of 10mm x 3mm x 1mm, which is placed on a temperature-controlled furnace. Since the PPLN crystal uses type-I phase matching for frequency summation, the 938nm laser requires polarization along the e1 axis, and the 1583nm laser also requires polarization along the e1 axis, which can be achieved by adjusting the respective wave plates. The crystal and the butterfly-shaped ring cavity are shown in Figure 2.

Figure 2: Crystals and Butterfly-Shaped Ring Cavity Used in the Experiment: (a) Crystal; (b) Butterfly-Shaped Ring Cavity

The two fundamental beams are cascaded-locked with the butterfly-shaped ring cavity using frequency locking technology. The detector converts the measured optical signal into an electrical signal and inputs it into subtractor 1. The subtractor subtracts the two signals to produce an error signal (H-C1). Due to the presence of the nonlinear crystal in the external cavity, if the laser is resonant with the cavity, the reflected light from mirror M2 will be linearly polarized, and the two divided beams will have equal intensities, resulting in a zero difference. If the laser is not resonant with the cavity, the reflected light from M2 will be elliptically polarized, and the two divided beams will have unequal intensities, resulting in a non-zero difference. This error signal, which is generated by the change in the polarization state of the reflected signal with the cavity mode detuning frequency, has an odd function characteristic and can be used for frequency locking of the laser and the cavity. Finally, this error signal is fed back to the voltage amplifier-controlled PZT through PID1 after setting the proportional gain and integral bandwidth, and the cavity mode frequency is locked to the output frequency of the 938nm laser by the expansion and contraction of the PZT. Similarly, the locking process for the 1583nm laser output frequency to the cavity mode frequency involves the reflected beam from mirror M4 going through the same process (H-C2) to produce an error signal, which is then fed back to the modulation port of the 1583nm laser through PID2 after setting the proportional gain and integral bandwidth. By controlling the frequency of the 1583nm laser, its frequency is locked to the cavity mode frequency. In this way, the cavity mode frequency is locked to the output frequency of the 938nm laser, and the output frequency of the 1583nm laser is locked to the cavity mode frequency, achieving phase correlation locking among the three and stabilizing their frequencies relative to each other. Additionally, the transmitted signal from mirror M3 is detected by another photodetector (PD5) and input into an oscilloscope for data collection. After achieving dual-wavelength resonance in the butterfly-shaped ring cavity, the 938nm and 1583nm fundamental beams interact with the PPLN crystal inside the cavity to generate 589nm fundamental light, and some experimental results are measured and analyzed.

Experimental Results:

Figure 3: Variation of 589nm Summation Light Power with PPLN Crystal Temperature

Since the phase matching in the experiment is temperature-dependent, changing the temperature can optimize the phase matching parameters in the summation process. Figure 3 shows the variation of the output power of the 589nm summation light at different crystal temperatures. It can be seen that temperature significantly affects the summation conversion efficiency of the crystal. When the temperature is 112.5°C, the crystal's summation conversion efficiency is the highest, and the maximum summation light power obtained is 204.3mW. The experimental results are lower than the theoretical calculation results. This difference may mainly be due to incomplete mode matching in the experiment.

Figure 4: Variation of 589nm Summation Light Output Power with the Input Power of 1583nm Fundamental Light. Solid Line: Theoretical Simulation Results; Points: Experimental Results

Figure 5: Variation of the Reflected Power of the 938nm Fundamental Light from the External Cavity with the Input Power of the 1583nm Fundamental Light. Solid Line: Theoretical Simulation Results; Points: Experimental Results

Figure 6: Variation of the Reflected Power of the 1583nm Fundamental Light from the External Cavity with the Input Power of the 1583nm Fundamental Light.: (a) When there is no 938nm Resonant Laser in the Cavity; (b) When there is a 938nm Resonant Laser in the Cavity. Solid Line: Theoretical Simulation Results; Points: Experimental Results

To compare with theoretical results, at a crystal temperature of 112.5°C, the 938nm and 1583nm lasers are resonantly summed. The power of the 938nm fundamental light is 300mW, and the power of the 1583nm fundamental light varies from 0mW to 500mW. However, regardless of the power variation, the dual-wavelength external cavity resonance system remains in an undercoupled impedance state. Figure 4 shows the variation of the 589nm summation light output power with the input power of the 1583nm fundamental light, with the solid line representing the theoretical simulation results and the points representing the experimental results. It can be seen that as the input power of the 1583nm fundamental light increases, both the theoretical simulation and experimental results show an increase in the 589nm summation light power. When the power of the 1583nm fundamental light is 500mW, the highest summation light power of 589nm is obtained at 204.3mW. At this time, the summation conversion efficiency for the 938nm fundamental light entering the cavity is 61.1%. The experimental results are in good agreement with the theoretical simulation results. Figure 5 shows the variation of the reflected power of the 938nm fundamental light from the external cavity with the input power of the 1583nm fundamental light. Similarly, the solid line represents the theoretical simulation results and the points represent the experimental results. It can be seen that as the input power of the 1583nm fundamental light increases, the reflected 938nm power decreases, while the input 938nm power remains constant, indicating that as the input power of the 1583nm light increases, the summation conversion efficiency gradually increases, and the consumed 938nm fundamental light power gradually increases. The experimental results are also in good agreement with the theoretical simulation results. Figure 6 shows the variation of the reflected power of the 1583nm fundamental light from the external cavity with the input power of the 1583nm fundamental light when there is no 938nm resonant laser in the cavity (a) and when there is a 938nm resonant laser in the cavity (b). The solid line represents the theoretical simulation results and the points represent the experimental results. It can be seen that as the input power of the 1583nm fundamental light increases, the reflected 1583nm power also increases accordingly. However, when there is a 938nm resonant laser in the cavity, due to the consumption of a portion of the 1583nm laser by the summation, the reflected 1583nm laser power is lower compared to the case when there is no 938nm resonant laser in the cavity. The experimental results are also in very good agreement with the theoretical simulation results.

Voltage Amplifier Recommendation: ATA-2161

Figure: Specifications of the ATA-2161 High Voltage Amplifier

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