Application Cases

Experiment Title: Optimization of a Ring Resonator

Testing Equipment:High Voltage Amplifier, Signal Generator, Oscilloscope, Semiconductor Laser, Optical Isolator, Lock-in Amplifier, Proportional Integrator, Detector, Piezoelectric Ceramic, etc.

Experiment Process:

Figure 1: Experimental setup of the ring cavity (solid lines represent the optical path, and dashed lines represent the electrical circuit)

As shown in Figure 1, the experimental setup is depicted. Initially, a triangle wave at approximately 30Hz generated by the signal generator is amplified by the high voltage amplifier into a high voltage scanning signal, which is applied to the piezoelectric ceramic of the frequency-doubling cavity to scan the cavity length. The transmission peak of the fundamental frequency light is observed on the oscilloscope. The position of the matching lens, the focal length, the angle of the ring cavity mirrors, and the cavity length are adjusted to achieve mode matching for the frequency-doubling cavity.

The one-dimensional translation stage beneath M2 is finely adjusted to gradually increase the cavity length. Simultaneously, each cavity mirror is fine-tuned. When the infrared mode monitored by the oscilloscope becomes a "sine wave," the one-dimensional translation stage beneath the loaded five-dimensional stage is adjusted to advance the PPKTP crystal into the ring cavity. Following the same steps as mentioned above, the matching lens and cavity mirrors are fine-tuned until the monitoring mode of the crystal cavity reaches the optimal state. The experiment yields an optimal mode matching efficiency of 87% for the crystal cavity.

Figure 2: Relationship between the power of the 426nm blue light and temperature

After matching the mode of the crystal cavity, a homemade temperature controller is used to scan the temperature. The power of the second harmonic blue light is monitored on the oscilloscope to search for the optimal matching temperature, as shown in Figure 2.

Once the frequency-doubling cavity is matched, a high-frequency modulation signal provided by the lock-in amplifier is added to the current of the main laser of the semiconductor laser. At this point, the 852nm transmission peak is observed on the oscilloscope. The obvious fast fluctuation of the transmission peak proves that the modulation signal has been successfully applied to the laser.

The infrared spectral line detected by the detector is input into the input port of the lock-in amplifier. Inside the lock-in amplifier, this input signal is multiplied by the internal modulation signal of the lock-in amplifier. After filtering, the frequency-doubling cavity's frequency discrimination signal is obtained and output from the output port of the lock-in amplifier. In the experiment, the initial phase, modulation frequency, modulation amplitude, time constant, sensitivity, and other parameters of the lock-in amplifier are carefully adjusted to obtain a high signal-to-noise ratio frequency discrimination signal.

After adjusting the frequency discrimination signal, the bias and gain of the high voltage amplifier are reduced. The signal generator is turned off, and the error signal obtained from the lock-in amplifier is fed through the PI controller into the high voltage amplifier. Then, the bias and gain of the high voltage amplifier are slowly and manually searched.

Experimental Results:

Figure 3: Relationship between the power of the generated blue light and the pump power (triangles represent directly measured values; circles represent actual values considering the output mirror loss; the solid line represents theoretical values.)

Figure 4: Relationship between conversion efficiency and pump power (triangles represent measured values; the solid line represents theoretical values.)

The measured blue light power and second harmonic conversion efficiency as a function of pump power are shown in Figures 3 and 4. It can be observed from Figures 3 and 4 that the theoretical values match well with the experimental values at lower pump powers. However, when the pump power is relatively high, there is some deviation between the experimental and theoretical values.

In summary, using PPKTP as the frequency-doubling crystal and a four-mirror ring cavity as the frequency-doubling cavity, with an input coupling mirror transmission of 7.6%, a ring cavity length of 516mm, a distance of 58mm between the two concave mirrors, and an optimal matching temperature of 318.3K, 42mW of blue light was measured from behind the output mirror M2. Considering the transmission of M2 at 77.2%, the actual 426nm blue light obtained was 54.4mW, with a second harmonic conversion efficiency of 40%.

High Voltage Amplifier Recommendation: ATA-7050

Figure: Specifications of the ATA-7050 High Voltage Amplifier

This material is compiled and released by Aigtek Antai Electronics. For more case studies and product details, please continue to follow us. Xi'an Aigtek Antai Electronics has become a widely recognized supplier of instruments and equipment with a broad product line and considerable scale in the industry. Sample machines are available for free trial.