All Solid State Mid-IR Laser

Zhang Xiao-Juan, Yang Hai-Bo, Xu Hong, Cai Jun Science and Technology on Electro-Optical Information Security Control Laboratory

He Bei, China neietyj@163.com

Abstract—A Tm laser pumped Mid-IR all solid-state laser is reported. A thulium laser is used to directly drive a ZnGeP2 optical parametric oscillator (OPO) with 1.99μm output to generate the 3～5μm wavelength. Diode pumped Tm:YAP laser operates on 1.99μm, repetition frequency of 10KHz, the optical to optical conversion efficiency is 28%. The all solid-state Mid-IR laser runs at room temperature and achieves over 1W at 3～5μm with an optical to optical conversion efficiency of 5.4% starting from the pump diode.

Keywords-Tm:YAP laser; Mid-IR solid state laser; OPO; ZnGeP2 crystal

I. INTRODUCTION Coherent radiation sources operating in middle infrared are

the subjects of ongoing research due to their usefulness in such a wide variety of applications. High peak power, high repetition rate pulsed Mid-IR lasers are in particular desired for uses [1]. It high desirable to have a Mid-IR laser that is lightweight, compact, and requires minimum electrical input power.

Solid-state lasers have some merits such as high efficient, compact and versatile, avoiding the shortcomings such as incompact, toxic, corrosive and cryogenic cooling of traditional Mid-IR laser. So Mid-IR solid-state lasers become the research focus in all countries.

Now, the most efficient Mid-IR sources have been made using 2μm solid-state lasers driving nonlinear optical oscillator (OPO). Zinc Germanium Phosphide (ZnGeP2) is the better choice for a pump laser to generate the Mid-IR laser [1].

Diode pumped lasers based on Tm3+-doped or Ho3+-doped solid-state media operating in the 2μm spectral band which is an effective way. The Ho laser performance depends on temperature sensitive upconversion and Tm to Ho energy transfer processes. At room temperature the energy transfer is not complete and there is reduction in the effective energy storage lifetime of Ho. To improve the gain and allow sufficient extraction of high pulse power pulses the laser crystal must be cooled to the temperature of liquid-state nitrogen. Tm does has a lower stimulated emission cross-section[2] (about 6×10-20cm2, only 40% of Ho) which leads to lower gain [3].

Now techniques for fabricating the ZnGeP2 crystal make the crystal less loss when it is pumped at the shorter wavelengths. The Tm laser wavelength depends on the crystal host the Tm ions are doped into. The energy transition occurs

between the 3H4 and the 3H6 levels, the laser wavelength will be in the range from 1.91μm to 2.03μm. The Ho laser wavelengths are host dependent and range from 2.05μm to 2.15μm. Obviously, the loss is lower when the Tm laser pumps ZnGeP2 OPO than the Ho laser pumps ZnGeP2 OPO. Thus, it was recognized that with an improved lower loss crystal with improved gain one could utilize the lower gain Tm laser output and use it to pump a ZnGeP2 OPO. The Tm laser can pump ZnGeP2 OPO with lower pump power, reduced input power means that it does not need to use more complicate Ho-Tm system that has to be cooled. The lower gain of Tm leads the higher threshold pump power, which can be settled by optimizing the pump parameters and increasing the pump power [4].

The conversion efficiency of Tm:YAP, which is the better performance than Tm:YAG, is higher, doubled the efficiency of Tm:YAG [5]. In addition, the laser host YAP is a biaxial crystal and it has a rhombic structure, which generate the laser is the line-polarized. But the laser of YAG is not line-polarized, and YAG has a cubic structure, which leads to thermal birefringence and consequent depolarization losses when thermally loaded. The anisotropy structure of YAP induces thermal birefringence, which can increase the conversion efficiency. The b-axis polarization YAP crystal, the strongest absorption feature has a width of 4 nm, and is centered around 795 nm. This is significantly broader than the strongest absorption feature (at 785nm) for Tm:YAG [6], So the Tm:YAP laser is more tolerant of variations in the temperature of a laser diode pump. Therefore, Tm:YAP is appropriate crystal for 2μm laser.

In this paper we describe a solid-state, high efficiency Mid-IR [ZnGeP2(ZGP)] optical parametric oscillator (OPO) pumped by a pulsed Tm:YAP laser, generating the 3～5μm wavelength through nonlinear frequency transformation technique.

II. YAP PULSED LASER

A. The design of YAP laser A schematic of Tm laser is shown in Fig. 1.

GaAs diodes provide a pump coupled through a fiber that is focus by the lens into a Tm:YAP crystal, the fiber core diameter is 400μm, and NA is 0.22. Diode pump outputs 795nm laser which injected into YAP laser crystal. The YAP laser crystal is a cylinder with 5mm diameter and 9 mm length,

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the Tm3+ ions in YAP. The YAP laser crystal was polished plane/parallel with the pumped face coated to be HT at 795 nm and 1.99μm. The laser crystal was mounted on a brass heat sink, with indium foil sandwiched between the sides of the laser crystal and the heat sink walls. The heat sink temperature was controlled by a thermoelectric cooler. Acousto-optic modulator in the resonator was driven at 10KHz.

Figure 1. Schematic of the YAP laser.

The components of laser were built in the special radiator that was machined by the duralumin, which the heat exchange coefficient is high, and twelve slices have made out on the underside of radiato; the radiator was an optical flat simultaneity. The heat that laser produced was scattered by the fans.

The design of laser resonator will affect the output performance and the output of OPO. The laser is designed to be TEM00, and the size of laser beam is close to the size of pump laser beam in the YAP crystal (the size of laser beam is less than the size of pump laser) [7], which make the better matching coefficient of pump laser, make the best of pump energy and make the thermal diffraction loss do not increase when the pump power increase. The TEM00 is advantage to improve the efficiency of OPO.

From theory analyzing [8], the pump threshold is reduced when the reflection of output mirror is increased, at the same time, the optical-to-optical conversion efficiency is decreased. Therefore, the request reflection of output mirror to settle the lower pump threshold and the high optical-to-optical is contrary, give attention to the two aspects, the reflection of output cavity mirror is chosen [9].

In Fig. 2, the output power of the Tm:YAP laser is shown for three cavities at the same conditions. The output power of flat-concave cavity is higher, the power of YAP with -100mm radius of curvature output mirror is the highest, but the laser mode is bad. Fig. 3(a) was obtained with -100mm radius of curvature output mirror, and Fig. 3(b) with -500mm radius of curvature output mirror. Fig. 3(a) and 3(b) was obtained on the same distance and pump laser power

Figure 2. The output power of different cavity model at the same conditions.

(a) R=-100mm (b) R=-500mm

Figure 3. Laser beams on the same conditions.

Because the laser mode is very important to the ZGP OPO，the better laser beam quality is easy to generate the mid-IR laser. Fig. 3 shows the laser beam is better with -500mm radius of curvature output mirror, so the laser with -500mm radius of curvature out put mirror is adapt to pump ZGP OPO.

A flat-concave cavity was chosen, the radius of curvature output mirror is -500mm, the input mirror was coated for high transmission of the diode and high reflection of the laser, the output coupler was coated the reflective at two microns

B. The results of YAP laser For the YAP laser over 10W was achieved, optical-optical

conversion efficiency was about 28%. The wavelength spectrum of the YAP laser is plotted in Fig. 4. The divergence of laser is about 8mrad, as shown in Fig.5. Temporal output was observed with the Infrared Detector and the digital scope which displayed a stable train of 150ns pulses, shown in Fig. 6. The Infrared Detector was made in Poland, the manufacturer is Vigo System S.A., the type of detector is PD-10.6-3, the Vol.responsivity is 0.155V/W(@10.6μm), the detectivity is 1.5×107cmHz1/2/W。

Figure 4. Spectrum of Tm:YAP laser output.

Figure 5. The divergence of Tm:YAP laser.

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Figure 6. Pulse of Tm:YAP laser.

III. MID-IR LASER The schematic of the experimental setup of Tm:TAP laser

pumped Mid-IR solid state laser is shown in Fig. 7.

The 1.99μm laser pulse train was focused by lens into the ZnGeP2 crystal. OPO was configured as a single crystal resonator. The ZnGeP2 crystal were cut for TypeΙ phase matching, the resonator configuration was flat-flat, Input mirror was coated for high transmission at 2μm and high reflection at 3～5μm; Output mirror was 50% reflectivity at 3～5μm.

The wavelength spectrum of OPO laser is plotted in Fig. 8. The power performance of OPO is shown in Fig.9. Over 1W output power was achieved in the signal and idler beam, converting at 23% with a slope efficiency of 21% as shown in Fig.9.

Figure 7. The schematic of the setup of Mid-IR laser.

Figure 8. Spectrum of OPO laser output.

Figure 9. Output power performance of OPO.

IV. CONCLUSION The all solid-state Mid-IR laser was built, and achieved

over 1W output power at 3～5μm with an optical to optical conversion efficiency of 5.4% starting from the pump diode. The power and the optical-to-optical conversion efficiency are all higher. If we will increase the performance of ZGP crystal, the higher power of Mid-IR will be achieved.

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