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Recent development of megawatt peak power, giant pulse microchip lasers has opened new opportunities for efficient wavelength conversion, provided the output of the microchip laser is linearly polarized. We obtain > 2 MW peak power, 260 ps, 100 Hz pulses at 266 nm by fourth harmonic conversion of a linearly polarized Nd:YAG microchip laser that is passively Q-switched with [110] cut Cr4+:YAG. The SHG and FHG conversion efficiencies are 85% and 51%, respectively.
©2011 Optical Society of America
1. Introduction
Ultraviolet (UV) laser sources are needed for a variety of applications, such as, ultrafast UV spectroscopy, photolithography, micromachining, etc. High peak power, pulsed UV lasers are normally identified with large, difficult to own and maintain laser systems that limit their applications. There is a need for compact, easy to maintain and use UV laser sources.
Passively Q-switched microchip lasers offer a great advantage of sub-nanosecond pulse- width operation, resulting in a high peak power output in a very compact size [1–4]. The high peak power of these microchip lasers can also be used for efficient wavelength conversion [5–7]. Recently, we have developed megawatt peak power microchip lasers for laser ignition for use in automobiles [8–10]. However, Nd:YAG microchip lasers used in these systems use either [100] cut Cr4+:YAG single crystal [8], or Cr4+:YAG ceramics [9,10], as the saturable absorber for passive Q-switching. This generates a laser output with unstable polarization that is not suitable for efficient wavelength conversion.
In this paper, we present a very compact microchip laser that uses [110] cut Cr4+:YAG for passive Q-switching to generate stable, linearly polarized output pulses. Although this concept had been reported earlier [11], it had been verified only for low pulse energies (< 1 mJ). We apply this concept to generate high peak power, linearly polarized pulses at 1064 nm, from which we obtain second harmonic generation (SHG) using Type I LiB3O5 (Lithium Triborate, LBO), and fourth harmonic generation (FHG) using Type Iβ-BaB2O4 (β-barium borate, BBO). The SHG conversion efficiency was 85%, whereas the green-to-UV conversion efficiency in the FHG stage was 51%.
As a consequence, we obtain a stable pulse train having > 2 MW peak power with 260 ps pulse width at 266 nm wavelength and 100 Hz repetition rate. We believe that this is the first demonstration of megawatt level peak power at UV wavelength using microchip lasers.
2. Laser structure
The laser structure is shown in Fig. 1 . A 4 mm-thick 1.1 at. % [111] cut Nd:YAG crystal (Scientific Materials Corp.) was pumped in the quasi-continuous-wave (QCW) regime by a fiber-coupled, 120 W, 808 nm laser diode (600 μm core diameter, 0.22 NA, JOLD-120-QPXF-2P of Jenoptik) at 100 Hz. A 30% initial transmission, [110] cut Cr4+:YAG crystal (Scientific Materials Corp.) was used for passive Q-switching. A flat coupler with a transmission of 50% was used at the output. The total cavity length was 11 mm. A TE cooler (30 W, 30x30 mm peltier made by KELK Ltd., temperature controlled by ILX Lightwave LDT-5948) was used to maintain the Nd:YAG and Cr4+:YAG crystal temperature at 25°C. The laser was air-cooled.
Fig. 1 Laser structure. The output is linearly polarized along the <001> crystallographic axis of Cr4+:YAG.
3. Fundamental wavelength characteristics
Using the laser structure described above, we could obtain stable, linearly polarized laser oscillation at 1064 nm. An output pulse train of 3 mJ, 365 ps pulses at 100 Hz was obtained for 100 W, 300 μs width, 100 Hz, QCW pumping. This resulted in an output peak power of 8.2 MW. The output beam was linearly polarized with a polarization ratio better than 100:1. The direction of polarization was stable and no fluctuation was observed during six months’ of usage.
The output beam diameter was 1 mm approximately and the M2 factor was measured to be 3.4. We aimed for maximum output energy, rather than an ideal Gaussian beam.
4. Second harmonic generation
The SHG experiments and results have been reported in detail in an earlier paper [12]. Here, we briefly describe them for completeness. We chose LBO crystal for SHG due to its high enough damage threshold and a relatively large angular acceptance bandwidth that permits effective SHG with multi-mode laser radiation. We performed SHG in the critical phase matching (CPM) regime, since any crystal temperature control mechanism would diminish the size-advantage offered by compact microchip lasers.
We obtained best results for a 10 mm-long LBO, by focusing the fundamental beam to a spot-diameter of 0.72 mm. We obtained 1.7 mJ pulse energy with 265 ps pulse width. This resulted in a peak power of 6.3 MW at 532 nm for an input peak power of 7.4 MW at 1064 nm. The SHG conversion efficiency was 85%. The M2 factor of the 532 nm beam was measured to be 3.
For the SHG results under various other conditions of crystal length and focusing, and for a discussion on the optimum conditions, the reader is referred to our paper on SHG [12].
5. Fourth harmonic generation
A 5x5x5 mm Type I BBO crystal cut at θ = 47.7°, φ = 0° was used for fourth harmonic generation in the CPM regime, at room temperature. The input 532 nm beam was focused with different lenses of focal length, f, as shown in Fig. 2 and Table 1 . The spot-diameter was varied from 1.08 mm to 0.54 mm, with an accompanying confocal length of 128 mm to 32 mm. We would like to point out that the confocal length was much larger than the BBO crystal length, even for the shortest focal length used.
Table 1. Spot-Diameter and Confocal Length for Focusing Lenses Used in Experiment
The fourth harmonic conversion efficiency, for different focus conditions, is shown in Fig. 3 . When the input beam is focused to a spot-diameter of about 1 mm, the conversion efficiency saturates to a little over 50%. We believe that this is due to pump depletion and some phase-mismatch, which is inevitable in the CPM regime. However, when the input beam spot-diameter is reduced to 0.54 mm, we find that, at the resulting high optical intensity, there is a roll-over of the conversion efficiency. We shall explore the reasons for this in the next section. The roll-over in the conversion efficiency can be avoided by focusing the input beamsoftly (2w2ω = 0.94 mm). This reduces the maximum conversion efficiency, but ultimately provides higher output energy by avoiding the roll-over. Further reducing the optical intensity (2w2ω = 1.08 mm), decreases the overall conversion efficiency, indicating an optimum value for the optical intensity to achieve maximum conversion.
The fourth harmonic output characteristics under optimum focus conditions are shown in Fig. 4 . We could obtain a stable pulse train of 562 μJ at 100 Hz with a pulse width of 260 ps. This results in a peak power of 2.2 MW. The stability of the output power, measured over 8 hrs., was within ± 2%.
6. Discussion
There are several factors that can affect fourth harmonic generation leading to a roll-over in the conversion efficiency, as seen in Fig. 3. Pump depletion in combination with diffraction could be one of them [13]. Pump depletion is important under our conditions of operation, since the conversion efficiency is more than 50%. However, as the confocal length is much larger than the BBO crystal length, even at a spot-diameter of 0.54 mm, the diffraction effects are not significant. Hence, the roll-over in the conversion efficiency cannot be attributed to a combination of pump depletion and diffraction.
Another factor which could lead to the roll-over in conversion efficiency is thermal dephasing, in combination with pump depletion. However, at the energy levels achieved by us, a repetition rate of 100 Hz is low for thermal effects to become important. In order to verify this, we reduced the repetition rate, but there was no change in the roll-over.
Still another factor which could be responsible for the roll-over is two-photon absorption in the BBO crystal. The propagation of light in a medium can be described by the following differential equation:
where I is the light intensity,αis the linear absorption coefficient, βTPA is the two-photon absorption coefficient and z is the direction of propagation.Several authors have studied the two-photon absorption in BBO based on the above model [14–16]. If the UV intensity is low, typically below 10 MW/cm2, then the contribution of two-photon absorption is negligible in BBO [14]. In our case, when the input beam is focused to a spot-diameter of 0.54 mm, the UV optical intensity reaches 1 GW/cm2, and two-photon absorption must be taken into account at these intensities. Therefore, we attribute the roll-over in the conversion efficiency to a combination of pump depletion and two-photon absorption.
In order to obtain high conversion efficiency, we had to adopt appropriate weak focusing of the input 532 nm beam, so that the effects of diffraction and two-photon absorption are minimized at the high intensities achieved by us.
7. Conclusion
We have demonstrated multi-megawatt level peak power generation at 266 nm from a microchip laser. This could be achieved due to the linearly polarized, high peak power of the fundamental beam, and by optimizing the conditions for harmonic generation. In particular, it was important to reduce the effect of two-photon absorption during fourth harmonic conversion using the BBO crystal, by suitable focusing of the input beam. Using this method, we have achieved > 2 MW, 562 μJ, 260 ps, 100 Hz pulses at 266 nm. This is a step forward in giant micro-photonics [17], and should be useful for a variety of applications.
Acknowledgments
We acknowledge the support of SENTAN, JST (Japan Science and Technical Agency) and Grant-in-Aid for Scientific Research B No. 21360038 for this work.
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