Research on high-precision clock generation technology for synchronization of high-power solid-state lasers

1 Introduction

For high-power solid-state laser devices, large medical equipment, etc., the synchronous trigger system is a very important and indispensable link. The time interval jitter accuracy of the synchronous trigger pulse will affect the operating efficiency and accuracy of these devices. Since such devices require the synchronization of the synchronous trigger signal and the laser, μs-level jitter accuracy is a basic requirement. Physically, the synchronous trigger signal is required to have a time interval jitter accuracy of ns or ps [1].

There are two key elements that determine the timing accuracy of the output trigger signal of this type of synchronous machine: one is the clock accuracy of the synchronous machine; the other is the time delay accuracy of the synchronous machine[2].

This paper proposes a high-precision clock generation technology for the synchronization system of high-power solid-state laser devices[3][4][5]. The input signal is a high-precision repetitive frequency laser. After photoelectric conversion and amplification, it is phase-locked by a high-precision phase-locked loop to obtain a high-precision clock with a strict phase relationship with the input laser. The time interval jitter between the high-precision clock and the main laser obtained by this technology is less than 20ps in peak-to-peak jitter and less than 2ps in RMS. This accuracy index has not been reported in China. At present, this technology has been successfully used in the development of high-precision synchronization triggering systems for high-power solid-state laser devices, and satisfactory results have been achieved.

2 Technical index requirements

The entire synchronization system outputs two types of synchronization trigger signals with different precisions, namely, ns-level signals and μs-level signals. The specific technical indicators of the two types of synchronization signals with different precisions are shown in Table 1.

Technical indicators of input optical pulses:
1) Laser pulse frequency 51.84MHz.
2) Laser pulse width greater than 500ps.
3) Laser power greater than -10dBm.
4) Time interval jitter (RMS) between laser pulses less than 2ps.
5) Laser wavelength: 1053nm.

The technical indicators of the output electrical pulse after photoelectric conversion, amplification and phase locking are as follows:
1) The signal is an electrical signal;
2) The signal leading edge is less than 200ps;
3) The signal level is LVPECL;
4) The time jitter between the output sequence signals is less than 25ps (Pk-Pk), 2.5ps (RMS);
5) The phase jitter between the output electrical pulse signal and the optical pulse is less than 30ps (Pk-Pk), 4ps (RMS);
6) The output signal frequency is 155.52MHz.

From the above six indicators, it can be seen that not only the leading edge of the output signal must be fast enough, but also the requirements for the signal's time jitter technical indicators are very high, especially the phase jitter requirement between the output signal and the input optical pulse is very high. Such high requirements for technical indicators have not been reported in China.

3 Overall technical solution

As shown in FIG1 , a repetition rate laser beam (the laser waveform leading edge is less than 100 ps, ​​the repetition frequency is 51.84 MHz, and the RMS jitter of the optical signal is less than 2 ps) is input from the test laser to the photoelectric conversion and PLL frequency synthesis module to generate a high-precision electrical pulse signal.

Figure 1 Overall technical solution

As shown in FIG1 , a repetition rate laser beam (the laser waveform leading edge is less than 100 ps, ​​the repetition frequency is 51.84 MHz, and the RMS jitter of the optical signal is less than 2 ps) is input from the test laser to the photoelectric conversion and PLL frequency synthesis module to generate a high-precision electrical pulse signal.

4 Low jitter repetition rate photoelectric conversion technology[page]

Figure 2 Low jitter repetition rate photoelectric conversion

As shown in FIG2 , the low jitter repetition frequency photoelectric conversion first completes the photoelectric conversion, and then passes through the shaping circuit to output the electrical pulse for the next stage circuit to amplify and phase-lock output.

Using a LeCroy 6GHz oscilloscope, the frequency and jitter indicators of the optical pulse output by the laser are measured as follows:
1) Signal frequency: 52MHz;
2) Frequency variation: 7.48kHz
3) Pk-Pk jitter: 16ps
4) Rms jitter: 2.03ps

As shown in Figure 3, the phase jitter between the electrical pulse output by the self-developed photoelectric conversion circuit and the optical pulse signal output by the laser was measured using a 6GHz oscilloscope from LeCroy. The measured technical indicators are as follows:
1) Pk-Pk: 14ps
2) Rms: 2.45ps

Figure 3 Signal jitter between the electrical pulse after photoelectric conversion and the input optical pulse

5 Phase-locking and shaping amplification of low-jitter repetition frequency electrical pulses

The technical solution is shown in Figure 4. The low-jitter repetition frequency electrical pulse signal is amplified and then phase-locked and frequency-multiplied by PLL to finally output a 155.52 MHz electrical pulse signal[6][7].

Figure 4 Principle of phase-locking and shaping amplification of low-jitter repetition frequency electrical pulses

Since the output clock signal has very high requirements, the development of the phase-locked loop circuit is the key. The phase-locked loop circuit must have jitter attenuation capability, and should be designed to lock the average phase, that is, insensitive to instantaneous mutations. The phase-locked loop circuit development uses DDS, digital phase-locked loop, analog phase-locked loop, etc. for verification, and the final conclusion is that an analog phase-locked loop must be used at a fixed frequency point.

As shown in Figure 5, the phase jitter between the output electrical pulse after phase locking and the input optical pulse before phase locking was measured using a LeCroy 6GHz oscilloscope. The following indicators are obtained:
1) Pk-Pk: 17ps
2) Rms: 2.62ps

Figure 5 Jitter accuracy test of the clock signal output after phase locking

6 Conclusion

The technical indicators of the final output signal met the design requirements and obtained relatively satisfactory results. This technology has a high reference value for the design of my country's new generation of high-power solid-state laser drivers and other instruments or equipment that require high-precision synchronization, and provides a reliable clock source for the future Shenguang III host synchronization system. It provides a new technical route for related domestic technologies to solve the problems of time jitter and long-distance transmission, and has broad promotion prospects.