We describe a compact, end-pumped all-solid-state, and Q-switched intracavity optical parametric oscillator (OPO). With this design, the relaxation oscillation type of multi-pulses produced in the intracavity OPO is eliminated. The intracavity OPO is based on periodically poled RbTiOAsO4 (PPRTA) as the nonlinear material and is pumped by a compact diode-pumped Yb:YAG Q-switched laser at a wavelength of 1.030 μm. The pulse width (FWHM) is about 11 ns at the full pump power for both the signal and idler pulses. Output energies of 384 μJ at idler wavelength and 615 μJ at signal wavelength are obtained. We demonstrate a Q-switched IPOPO oscillator that operates the signal and idler wavelengths without relaxation oscillation type multi-pulses and with fast wavelength tuning ability for both signal and idler wavelength.
© 2007 Optical Society of America
Tunable mid-infrared lasers of moderate energy are needed for many applications, such as molecular spectroscopy, photoacoustic spectroscopy and remote sensing of the atmosphere [1, 2]. Some applications, like chemical lidar , require all-solid-state mid-infrared laser sources. An optical parametric oscillator (OPO) pumped by NIR (near infrared) neodymium based lasers is well suited for such applications. In the past decade, there has been considerable interest in developing mid-infrared sources, especially OPO. A number of pulsed and cw OPOs have been built [3–5]. However, most of these systems are external cavity OPOs where the pump laser and OPOs are separated parts. In order to obtain high efficiency and high energy output at MIR (mid-infrared) wavelengths, a high energy pump laser is usually needed.
A way to reduce the requirements on the pump laser is to use the intracavity approach. A primary advantage of the intracavity approach over an external pump is the fluence inside the cavity (greater by a factor of (1+R)/(1-R), where R is the reflectance of the output coupler mirror). The second advantage is to increase the effective nonlinear interaction length due to multiple passes of the pump pulse through the crystal . The intracavity OPO was first studied in the 1960s . However, intracavity pumped OPOs based on all-solid-state lasers with new materials, such as PPLN, PPRTA and non-critically phase-matched KTP et. al. , were studied after the 1990s. Recently, several intracavity OPO prototypes have been reported [8–15]. Most of the IPOPO (intracavity pumped OPO) studies concentrate on cw-IPOPO or low pulse energy with high repetition rate IPOPO. Only a few papers address high pulse energy IPOPO [13–15]. Because the laser fluence inside intracavity OPO is so intense, relaxation oscillation type multi-pulses are produced [14,15]. Paiss et. al.  reported the relaxation oscillation type multi-pulses IPOPO of the non-Q-switched pump laser. Chuang and Burnham  investigated the Q-switched intracavity PPLN-OPO that also produced relaxation oscillation type multi-pulses. Dabu, et. al.  also studied an intracavity OPO that produced a single pulsed output, but only with signal wavelength output, 1.57-μm. Because of material that Dabu, et. al.  used inside cavity, idler wavelength inside cavity is absorbed in the KTP crystal. PPLN-OPO has some unsolved issues such as thermal effects, photorefraction, and restricted aperture sizes. PPRTA on the other hand has good crystal quality, minimal thermal effects, good optical transparency and a broad tuning range extending to about 5.8 μm. The main difficulty faced in the IPOPO is the problem of the relaxation oscillation type multi-pulses produced in IPOPO output when IPOPO produce both signal and idler wavelength. Here we report our experimental investigations on an IPOPO that eliminates the relaxation oscillation type multi-pulses on both signal and idler wavelength. The nonlinear material, PPRTA, is used in this IPOPO, and an IPOPO is pumped within a Q-switched Yb:YAG laser cavity.
2 Experimental detail and results
The schematic of the experimental setup is shown in Fig. 1. The IPOPO is configured as a singly resonant oscillator. The IPOPO cavity is 10 cm long with two flat mirrors, M1 and M2. Mirror M1 has high reflectance at 1.03 μm (R>99%), 1.4–1.65 μm (R>95%), and 2.8–3.8 μm (R>95%). IPOPO mirror M2 has high transmittance (T>96%) at 1.03 μm, high transmittance between 2.8–3.8 μm (T>85%) and high reflectance (R>95%) between 1.4–1.65 μm. Fold mirror M3 has high reflectance at 1.030 μm (R>99%) and high transmittance between 1.4–1.65 μm (T>90%) and 2.8–3.8 μm (T>85%). The pump source for the IPOPO was a Yb:YAG laser pumped at 940 nm by InGaAs laser diode stack. The Yb:YAG (doped 15% Yb) laser crystal is 3 mm thick and 3 mm in diameter with a dichroic coating on the pumped face (HR > 99% @ 1.03 μm, HT > 80% @ 0.940 μm) and an anti-reflection coating on the other side (R <0.2% @ 1.03 μm). The Yb:YAG laser cavity is 30 cm in length and consists of mirror M1 and the pump face of the Yb:YAG crystal. The Yb:YAG crystal is placed in a specially designed holder to obtain good thermal contact between the crystal and holder. The pump wavelength is tuned to the peak absorption of the Yb:YAG crystal by a thermoelectric cooler (TEC) which controls the temperature of the pump laser diode. The laser diode pump pulse duration of ~ 1 ms is used. Q-switched operation is obtained by using a fused silica acousto-optic (A-O) modulator. This plano-plano A-O modulator is AR coated at the wavelength of 1.03 μm and has an insertion loss of less than 0.5%. Wavelength tuning as well as the linewidth narrowing is accomplished by two crystal quartz intracavity BRT (birefringent tuner) plates, 2 mm and 0.5 mm thick. The Z-cut PPRTA crystal is 1.5 mm thick, 5 mm wide and 20mm long. The grating period of the PPRTA is 39.6 μm. In order to reduce the insertion loss of the PPRTA, the end faces of the PPRTA are antireflection coated for the pump wavelength of 1.030 μm, a signal wavelength between 1.4–1.65 μm and an idler wavelength between 2.8 μm to 3.8 μm. The AR coatings on the mirrors and crystals are virtually constant over the wavelength range of 1.03 ± 0.02 μm with the AR coating centered at 1.03 μm wavelength. PPRTA crystal is mounted inside an aluminum holder.
With no IPOPO components inside resonator, the simple end pumped Yb:YAG laser produces an output energy of up to 8.4 mJ with 10 Hz repetition rate and pulse width of 27 ns (FWHM) with 245 mJ pump input and a 50% output coupler2. The tuning range of the Yb:YAG laser is 1.024–1.050 μm that corresponds to 1.445–1.469 μm for signal wavelength and 3.51–3.69 μm for idler wavelength for the PPRTA with the 39.6 μm poling period. The Yb:YAG laser output with no IPOPO components falls to about 2 mJ at both ends of its tuning range. After intracavity components, PPRTA crystal and intracavity mirrors, are installed, the temporal profile of the pump laser pulse stretches to more than 200 ns, as shown in Fig. 2. The temporal profile of the IPOPO MIR pulse is also recorded at same time. It is seen that the FWHM of MIR pulse extends to more than 100 ns and relaxation oscillation type multi-pulses are also present. Similar pulse shapes were also observed by Chuang and Burnham  and Paiss et. al. . There are multi-spikes produced in NIR output, signal and idler output. From Fig. 2, we observe that the IPOPO pulse intensity is built up rapidly, overshooting the steady state level. We believe that the relaxation oscillation type multi-pulses are caused by the back conversion between the build up of 1.03-μm pump laser and the successive parametric process. When the pump energy is increased further, the temporal profile of 1.03-μm laser remains almost the same as that for low pump energy.
In order to generate a single pulse output from the IPOPO without the stretched-multi-pulses, several output mirrors (M1) with different reflectivities at 1.03 μm were tested. Because similar results were obtained when we reduced the reflectance of M1 mirror, we only report the result when the output coupler (mirror M1) has 72% reflectance at 1.03 μm, high reflectance between 1.4–1.65 μm (>95%) and 2.8–3.8 μm (>95%). Figure 3 shows the laser pulse profiles at four different output energies. At low pump energy with output energy of 65 μJ at the idler wavelength, a single output pulse is obtained, as shown in Fig. 3(a). When the pump energy is increased, multiple pulses began to appear again, as shown in Fig. 3(b). At output energy of 84 μJ, two pulses are observed and the back conversion between the pump laser and the IPOPO output sets in. As pump energy is increased further and idler output energy reaches more than 100 μJ, as shown in Fig. 3(c) and Fig. 3(d), multi-pulses are observed in the IPOPO output. With further increase in the pump energy, the temporal profile of IPOPO output becomes relaxation oscillation type multi-pulses, same as Fig. 2. Because the advantage of the intracavity configuration is to increase the efficiency of the system by using high fluence inside a laser cavity, we did not further reduce the reflectance of the output coupler, mirror M1. Instead of reducing the reflectance of the output coupler M1 at 1.03 μm, we have developed an alternative approach.
The relaxation oscillation type multi-pulses are caused by the interaction between the parametric process that generates the signal and idler from the 1.03 μm pump and the back conversion process in the OPO resonator. With the build up of the signal and idler fluences the back conversion increases rapidly reducing further increase in idler and signal. But in turn the back conversion causes the 1.03 μm pump laser intra-cavity energy fluence to increase which successively leads to the parametric process to increase. An effective method to eliminate the relaxation oscillation type multi-pulses is to prevent the back conversion process from competing with the generation of the MIR wavelength and thus reducing the conversion of the pump wavelength. This is achieved by reducing the signal fluence within the OPO cavity.
From the results of Fig. 3, the intra-cavity power density for NIR wavelength can be estimated. The energy outputs at the idler and signal wavelength are 65 μJ and 62 μJ, respectively, for Fig. 3(a) where only a single pulse IPOPO output is obtained. The signal wavelength energy inside OPO cavity is
Where Eoutput is 62 μJ and Rs is the equivalent signal reflectivity ~ 0.9. The signal energy inside the OPO cavity is 1.2 mJ. The internal power density for the signal wavelength is 15 MW/cm2 with pulse width of 16 ns and beam size of 0.8 mm in diameter for. When output energies reach to 84 μJ for idler wavelength and 83 μJ for and signal wavelength, the relaxation oscillation type multi-pulses start to appear, as shown in Fig. 3(b). The signal energy inside cavity now is 1.6 mJ and the internal power density is 20 MW/cm2. From the above analysis, we expect to prevent the onset of relaxation oscillation type multi-pulses by reducing the signal internal power density inside OPO oscillator below this level.
The reflectance of the intracavity mirror M2 at the signal wavelength was reduced in order to reduce internal power density and stop back conversion. The original intracavity OPO mirror M2 had a high transmittance (T>96%) at 1.03 μm, high transmittance between 2.8–3.8 μm (T>85%) and high reflectance (R>95%) between 1.4–1.65 μm. The new mirror M2 is coated with a reflectance of R~10% over the wavelength range of 1.4–1.65 μm, high transmittance (T>96%) at 1.03 μm and high transmittance between 2.8–3.8 μm (T>85%). The high reflectance mirror (R>99% at 1.03 μm, R>95% at 1.4–1.65 μm, and R>95% at 2.8–3.8 μm) used originally was put back as the back mirror M1. With this new cavity configuration, a single pulse IPOPO output is obtained. Figure 4 shows the temporal profile of the idler and NIR outputs. The idler energy of 384 μJ is obtained with a pulse width (FWHM) of 11 ns. We also obtain 615 μJ output energy at the signal wavelength. It is seen that the internal energy at the signal wavelength is now only 0.75 mJ for a measured signal output energy of 615 μJ and Rs ~ 0.1 and hence well below the onset of relaxation oscillations.
Figure 5 shows the IPOPO output energies at signal and idler wavelengths as a function of the pump energy of laser diode at 940 nm. The output efficiency of IPOPO is 0.91%. This IPOPO design also has fast wavelength tuning ability. By rotating the BRT inside the cavity, the wavelength of NIR pump is changed continuously, thus making the signal and idler wavelength scan continuously. The full wavelength range can be tuned within a second.
In conclusion, we have demonstrated a simple intracavity OPO laser based on PPRTA that generates pulsed Q-switched outputs at idler and signal wavelengths without relaxation oscillation type multi-pulses. A Yb:YAG pump laser was constructed with a large cavity mode size to minimize damage to the PPRTA crystal. Two different cavity configurations are discussed in this paper. By reducing the intra-cavity IPOPO mirror reflectance at the signal wavelength, the relaxation oscillation type multi-pulses are eliminated and a single pulse IPOPO output is obtained. The output energies at idler and signal wavelength are 384 μJ and 615 μJ, respectively. Also, the Q-switched IPOPO oscillator provides rapid wavelength tuning ability for both the signal and idler wavelengths by tuning the pump wavelength using an intra-cavity bi-refringent tuner.
The authors thank Dr. Douglas R. Moore and Dr. Wenhui Shi for their advice and discussions.
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