Application Cases

Experimental Name: Research on High-Power Near-Infrared Optical Frequency Comb System

Test Equipment: Power amplifier, signal generator, optical frequency shifter, detector, RF analyzer, bandpass filter amplifier, etc.

Figure 1: High-Power Near-Infrared Optical Frequency Comb System

Experimental Process:
Figure 1 shows the block diagram of the high-power near-infrared optical frequency comb system. The high-power pulse output from the amplifier passes through a beam splitter, with approximately 1W of power used for detecting the carrier-envelope offset frequency and repetition rate, while the remainder is directly fed into the acousto-optic frequency shifter. In the frequency comb system, the key parameters requiring precise control are the repetition rate (fᵣ) and the carrier-envelope offset frequency (f₀). With the assistance of AOFS-based feedforward CEP control technology, we achieved real-time control of the carrier-envelope offset frequency f₀.

Figure 2: Repetition Rate Locking of the Pulse

When phase-locked loop technology is applied to the optical frequency comb system, the fiber laser itself acts as a voltage-controlled oscillator, with the repetition rate fᵣ of its output pulses being the parameter to be locked, as shown in Figure 2. The detector outputs a series of electrical pulse signals upon detecting the optical pulses. This signal appears on the RF analyzer as a series of harmonic peaks centered around the pulse repetition rate. After passing through a bandpass filter amplifier, this signal yields a single-frequency sinusoidal signal fₓ. This sinusoidal signal fₓ is then mixed with a reference signal fᵣb from the signal generator, and a low-pass filter extracts the low-frequency error signal f_error = fₓ - fᵣb. After amplification by an integrating amplifier (PID), a power amplifier drives the piezoelectric ceramic within the laser cavity. By adjusting the laser cavity length, the repetition rate locking of the laser is achieved.

Experimental Results:

Figure 3: (a)(b)(c) Drift of the laser repetition rate under free-running conditions and (d) the repetition rate after locking

With the help of a frequency counter, the pulse repetition rates under free-running and locked conditions were measured (as shown in Figure 3). Under free-running conditions, the laser's repetition rate, similar to the carrier-envelope offset frequency, exhibits both fast and slow drifts, and its variation lacks a clear pattern. This is primarily because the cavity length of the unlocked laser is susceptible to various factors such as mechanical vibrations, temperature fluctuations, airflow, etc. Among these, temperature changes mainly affect the slow drift process of the laser repetition rate fᵣ. As shown in Figures 3(a), (b), and (c), these data were obtained from the same laser at different time intervals. It is evident that the slow variation of the laser repetition rate is quite sensitive to environmental changes. Through the phase-locking circuit, we achieved precise locking of the laser repetition rate, with the result shown in Figure 3(d). The measured locking precision for fᵣ was 0.8 mHz, with a center frequency of 64.3756 MHz. Consequently, we obtained an optical frequency comb pulse output with both the repetition rate and the carrier-envelope offset frequency precisely locked.

Power Amplifier Recommendation: ATA-3090C Power Amplifier

Figure: ATA-3090C Power Amplifier Specifications and Parameters