High pulse energy fiber/solid-slab hybrid picosecond pulse system for material processing on polycrystalline diamonds Download: 596次
1 Introduction
High power, high repetition rate pulse lasers with near-diffraction-limited beam quality have made significant contributions in many applications such as X-ray generation[1], attosecond pulses generation[2] and material processing[3–7]. In particular, the performance of material precise processing varies largely due to different pulse durations. Generally, with the decreasing of pulse duration, the material processing results become better[3]. It can be explained that with the ultrashort pulse duration (i.e., less than one picosecond), laser pulse transfers almost all of its energy to the electrons, rather than the atoms/lattice, and the pulse is shorter than the time takes for the energy of the electronics to reach equilibrium with the lattice, whereas the pulse machining by nanosecond or longer pulse laser may involve a solid-state phase transformation, melting or evaporation of the target due to thermal activation[4]. Although -switch laser processing systems have achieved great successes for their simple and stable schemes, the processing quality and scope of applications are limited, due to nanosecond pulse durations[4–7].
All polarization-maintaining (PM) fiber mode-locked lasers have been confirmed as robust, compact and alignment-free light sources with the output pulse duration of less than 10 ps[8–13]. Particularly, in most schemes of these kinds of mode-locked lasers, the mode-locking is achieved by passive mode-locked devices such as carbon nanotube (CNT) or semiconductor saturable absorber mirror (SESAM), which have simpler mode-locking mechanism than traditional nonlinear polarization rotation (NLPR) mode-locked lasers. On the other hand, due to the fact that lights are always trapped in the PM fiber without any free space optical devices, these structures are less sensitive to external temperature and stress perturbations. As a result, this kind of mode-locked laser shows much more robustness and less output states, compared to traditional NLPR mode-locked lasers, which is an ideal alternative to -switch laser in material processing applications.
Another problem is how to amplify the seed pulse up to desired output pulse energies. Unlike the regenerative amplifier scheme which is hard to stay stable at high repetition rate[14], the master-oscillator power amplifier (MOPA) configuration has been widely used in seed pulse amplification at higher repetition rate. However, for fiber-based amplifiers, because the small mode areas and long interaction lengths for light propagating in optical fiber will cause pulse distortions and pulse break-up, complex chirped pulse amplification (CPA) configuration must be set up in most cases[15–19], which will increase the complexity of systems. Recently, the partially pumped slab laser systems have been widely studied, due to the ability of circumventing nonlinearity by the short reaction distance between the light and gain matter[20–27]. These slab laser systems have provided a novel approach to realize CPA-free amplification with high power ultrafast pulses. Indeed, these slab crystal amplifiers also have some disadvantages. For example, the slab amplifiers need bulk cooling systems for avoiding beam quality degradation induced by the thermal effects. Besides, the slab amplifiers require unique imaging systems for laser amplification, which will make it more difficulty in system adjustment. Despite these shortcomings, these slab crystal amplifiers are still attractive because they promise a compact and robust scheme for high power pulse amplification without complex CPA technique. However, in all these slab laser systems, the seed pulses are produced from solid-state mode-locked/-switch oscillators. Compared with the turn-key all-fiber mode-locked oscillators, these solid-state oscillators are less compact, less robust and harder to build, which make it bulky, costly and far from real-world applications.
Fig. 1. Setup of the all PM fiber mode-locked laser. ISO: isolator, WDM: wavelength division multiplexer, SESAM: semiconductor saturable absorber mirror, FBG: fiber Bragg grating, PD: photonics detector, SG: signal generator, AOM: acoustic optical modulator, RF signal: radio frequency signal.
Here, we first demonstrate an all PM fiber mode-locked laser seeded, hybrid fiber/solid-slab picosecond pulse laser system. The MOPA design is used in this system. Due to the all PM fiber structure of the pulse seed source, the whole laser system becomes much more compact and stable. By establishing a suitable design, we achieve , 10 ps pulses output at the central wavelength of 1064 nm. The beam quality factors in the unstable and stable directions are 1.35 and 1.31, respectively. In order to get better processing efficiency for materials which have low absorption in infrared radiation such as the diamonds[28], picosecond pulses at the central wavelength of 355 nm are generated through third harmonic generation (THG) by using two (LBO) crystals. Thanks to the high pulse energy and beam quality of these ultraviolet (UV) picosecond pulses, we achieve a high performance of material processing on polycrystalline diamonds in the latter experiments.
2 Experimental setup
Our experimental setup includes three main parts: the all PM fiber mode-locked laser, slab laser amplifier and ultraviolet generator.
2.1 All PM fiber mode-locked laser
The all PM fiber mode-locked laser contains a passively mode-locked oscillator, a fiber pre-amplifier and a fiber-based pulse picker (shown in Figure
2.2 Slab laser amplifier and ultraviolet generator
Due to the fact that the Nd: crystal has a large emission cross-section and polarized emission attributed to its natural birefringence, we choose it as the gain medium in latter solid-state amplification stages. Besides, the Nd: crystal also shows much more price advantages over the Yb-based crystal and enough gain bandwidth for pulse amplification as well. The structure of solid-state slab laser amplifier is shown in Figure
Before coupling into the slab amplifier, the seed beam is shaped by beam shaper-1 which consists of two cylindrical lenses and a spherical lens. In the horizontal direction, the beam shaper-1 is used to compress the seed beam to be line-like for mode matching between the seed beam and the pump light. In the vertical direction, the spherical lens is used to focus the seed beam (which is Gaussian distribution). Meanwhile the spherical lens increases the divergence angle of the seed beam. As a result, the beam diameter will be wider for every passage through the crystal which ensures that the optical power density is always similar. The beam shaper-2 is used to restore the amplified laser beam to Gaussian distribution in both the horizontal and vertical directions.
The ultraviolet generator shown in Figure
The output spectra of the all PM fiber mode-locked laser are recorded by an optical spectrum analyzer (Yokogawa-AQ6370B) with the resolution of 0.02 nm. Its RF spectrum is measured by a high speed photo-detector (Thorlabs DET10A/M) which is connected to an oscilloscope (Iwatsu, 400 MHz). The pulse duration is measured by an optical autocorrelator (APE, PulseCheck).
Fig. 3. Output characteristics of the all PM fiber pulse seed source. (a) Spectrum profiles, the inset shows the long term stability of the seed pulses (in the 10% output port) in 2 h. (b) Autocorrelation trace and its Gaussian fitting.
3 Experimental results and discussions
When the pump power is 75 mW, the oscillator laser is self-started mode-locking with the output power of 2 mW. As the pump power increases to 130 mW, 5 mW output power is achieved. Then the output power is amplified to 75 mW with 400 mW pump power of the fiber pre-amplifier. Thanks to the all-fiber construction, we realize a low repetition rate of 30.7 MHz with a compact size, which is difficult to a solid-state mode-locking laser. The fiber-based pulse picker reduces the repetition rate to 500 kHz with diffraction efficiency of , according to the output power of 0.6 mW. Figure
Fig. 4. Output characteristics of the all slab amplifier. (a) Measured (dots) and calculated (line) output power of the slab amplifier. (b) Long term stability of the slab amplifier in 2 h. (c) Beam radius and profile. (d) Autocorrelation trace and its Gaussian fitting.
The seed beam is amplified to by two end-pumped double-pass Nd: pre-amplifiers. In our design, the seed beam will pass through the slab crystal 10 times. At the pump power of 125 W emitted by the LD stack, the output power of the slab amplifier is 3.5 W. As the pump power increases up to 260 W, 19.8 W output power is achieved. The corresponding pulse energy is about , at the repetition rate of 500 kHz. Figure
In order to test this laser system, we carry out an experiment of material processing on polycrystalline diamond samples. The material processing by picosecond pulses can be generally considered as strong evaporation, the critical condition for evaporation can be written as[3]
Fig. 5. SEM images of laser-cut grooves on the diamond surface by 355 nm picosecond pulses with (a) at the repetition rate of 500 kHz (the corresponding pulse energy is ) and (b) at the repetition rate of 10 MHz (the corresponding pulse energy is ), respectively.
According to Equation (
4 Conclusions
In conclusion, we demonstrate an all PM fiber mode-locked laser seeded, hybrid fiber/solid-slab picosecond pulse laser system without using CPA technology. The all PM fiber mode-locked laser is operated at the central wavelength of 1064 nm, and outputs 75 mW, 9.09 ps at a low repetition rate of 30.7 MHz. After the decreasing of repetition rate (i.e., down to 500 kHz) by a fiber-based pulse picker and then energy amplification by two end-pumped double-pass Nd: pre-amplifiers, the pulse energy is amplified to by a partially pumped slab Nd: amplifier. The beam quality factors in the unstable and stable directions are 1.35 and 1.31, respectively. The pulse duration is broaden to 10.89 ps due to finite gain bandwidth of the slab Nd: crystal. picosecond UV pulses at 355 nm are achieved through THG from two commercial LBO crystals. We use these UV picosecond pulses to process polycrystalline diamond samples and experimental results show that the high performance benefits from the high pulse energy and beam quality. This compact, robust and cost-effective UV picosecond pulse system is likely to benefit a number of material processing applications.
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Article Outline
Wei Chen, Bowen Liu, Youjian Song, Lu Chai, Qianjin Cui, Qingjing Liu, Chingyue Wang, Minglie Hu. High pulse energy fiber/solid-slab hybrid picosecond pulse system for material processing on polycrystalline diamonds[J]. High Power Laser Science and Engineering, 2018, 6(2): 02000e18.