Abstract
1. Introduction
In recent years, with the development of adaptive optics, quantum optics and optical communications, high-power all-solid-state lasers are required[
At present, twisted-mode cavities, short cavities and ring cavities are the proven methods to obtain single-frequency lasers[
An efficient way to obtain a green laser is by nonlinear frequency conversion[
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In this paper, a single-frequency long-pulse seed laser (with a pulse length of a hundred microseconds) is obtained by externally modulating a super-narrow spectrum linewidth, single-frequency CW fibre laser. After injecting the seed laser into a series of solid-state amplifiers, a 1064 nm long-pulse single-frequency laser is achieved with a high repetition frequency, high power and high beam quality. In addition, based on frequency doubling this fundamental 1064 nm laser, a green laser with an pulse output of 212 mJ is achieved. There have been no similar green lasers reported so far, but in 2006 Yarrow reported on a 1064 nm single-frequency laser with a long pulse duration and an average power of 11.3 W. In 2012, Wang demonstrated a high-energy, single-frequency long-pulse laser with a hundred microsecond pulse length, a pulse energy of 180 mJ, a repetition rate of 50 Hz and a pulse width of in 2012. Lu reported a high-energy all-solid-state sodium beacon laser with a pulse energy of 380 mJ at a repetition rate of 50 Hz with a pulse width of [
2. High-repetition-rate 1064 nm single-frequency laser
The primary problem in developing an all-solid-state single-frequency laser with a high repetition rate, high energy and high beam quality is to obtain a long-pulse single-frequency seed laser with an easily controlled wavelength. The scheme to obtain the seed laser is shown in Figure
As the energy of pulse seed laser is only 0.7 mJ, multi-stage amplification is necessary to obtain a hundreds of millijoule laser. Due to the severe thermal effects of the high-repetition-rate amplification system, the operating current should not be too high. Considering all the requirements – laser output energy, beam quality, system complexity and other factors – a three-stage master oscillator power amplifier (MOPA) system is proposed.
As shown in Figure
3. Long-pulse single-frequency green laser
It is feasible to obtain a conversion efficiency of more than 50% in frequency doubling with a short-pulse fundamental laser. But the conversion efficiency decreases rapidly, as the peak power is much lower in the long-pulse fundamental laser condition. The situation can be improved by increasing the length of the frequency-doubling crystal, but in turn the angle mismatch factor and temperature mismatch factor are also increased, which will limit any further improvement of efficiency and destroy the system stability. It will be advantageous for obtaining a high conversion efficiency to focus the incident laser, because the power density is enhanced as the beam spot size gets smaller. However, the Rayleigh length will also get shorter, which will decrease the conversion efficiency. Therefore, to obtain a high conversion efficiency, we should take into account the combined effects of the crystal length, the phase mismatch, the incident laser aperture and the focusing properties.
Furthermore, the nonlinear crystal should have low absorption for both the fundamental laser and the frequency-doubled laser, because absorption of the fundamental laser in the crystal may produce large thermal gradients, which will result in a temperature mismatch, or even damage to the nonlinear crystal. Taking all these factors into consideration, a lithium triborate (LBO) crystal working in type I noncritical phase matching at a temperature of is an ideal choice. At this temperature, the absorption coefficients of both the fundamental and the harmonic laser are less than 50 ppm, and the reception parameters (angle, linewidth, temperature, etc.) are large.
In addition, beam quality is an important factor affecting the frequency-doubling efficiency of the long-pulse laser – we can obtain a smaller focused laser spot size and a higher intensity with better beam quality from the same incident conditions and focusing system. Following the model in Ref. [
As shown in Figure
4. Results and analysis
Figure
The third amplifier is the main amplifier, giving an output energy of 700 mJ (an amplification factor of 5). The temporal profiles of the pump and final output laser pulse are shown in Figure
The short fibre delayed self-heterodyne method is used to measure the linewidth of the single-frequency laser. The results are shown in Figure
The experiment on the long-pulse single-frequency green laser is carried out using the injected fundamental laser of pulse power 700 mJ – the beam quality and the beam spot size is 5 mm before the focusing lens (). The temperature of the nonlinear crystal LBO is precisely controlled in an oven with a control accuracy of and the damage threshold of the crystal is higher than in order to meet the high peak power requirement of the fundamental laser. The power obtained for the green laser is 212 mJ, with a conversion efficiency reaching 31%, which agrees well with the results of theoretical calculations. The measured beam quality of the green laser is shown in Figure
The stability of the output power and the green laser wavelength are measured over a period of 15 min. Thanks to the excellent stability of the fundamental laser and the nonlinear process, the root mean square (RMS) power stability is better than , with a peak-to-valley (PV) value of better than . Because of the superior features of the fundamental seed laser, the PV value of wavelength is better than and the linewidth of the green laser is 37 kHz. Furthermore, the wavelength is tunable from 532.15 to 532.50 nm.
5. Conclusions
By injecting the long laser pulse (with a duration of hundreds of microseconds) obtained by externally modulating a single-frequency CW fibre laser into multi-stage amplifiers, a 1064 nm single-frequency laser with a linewidth of 18.7 kHz is achieved. The single-shot energy is 700 mJ, with a pulse width of , a repetition rate of 500 Hz, and a beam quality . In the next stage, we carry out an investigation of a green laser based on frequency doubling that 1064 nm single-frequency laser. The output of the single-frequency green laser is 212 mJ, with a beam quality of , a linewidth of 37 kHz, and a conversion efficiency higher than 30%.
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