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Efficient single-pass optical parametric generator for environmental gas sensing based on periodically poled stoichiometric lithium tantalate Nan Ei Yu, Yonghoon Lee, Yeung Lak Lee, Changsoo Jung, Do-Kyeong Ko, and Jongmin Lee Advanced Photonics Research Institute, Gwangju Institute of Science and Technology, Gwangju 500-712 Korea; e-mail:
[email protected]
Kenji Kitamura and Shunji Takekawa Advanced Materials Laboratory, National Institute for Materials Science, Tsukuba, Ibaraki 305-0044, Japan
Jung Hoon Ro Department of Biomedical Engineering, School of Medicine, Pusan National University, Busan 607-735, Korea Abstract: An efficient 1064 nm-pumped OPG that could be operated at room temperature using a PPMgSLT crystal is presented. 1.6 W total output for a 4.8 W input, power conversion of 50% was achieved.
©2006 Optical Society of America OCIS codes: (190.4400) Nonlinear optics, materials; (190.2620) Frequency conversion; (190.4970) Parametric oscillation and amplification
Generally, remote-sensing applications of various absorptions or differential scattering require broadly tunable mid-IR sources and narrow line-width is also an important requirement for high spectral resolution. Optical parametric oscillators (OPO’s) with injection seeded at single-frequency are mainly used in these applications. However, it is a difficult task that maintaining narrow line-width over a wide tuning range. The system requires complicated alignment for frequency matching between the injection seeding and resonated oscillation. An optical parametric generation (OPG) is an alternative method to obtain coherent tunable laser sources without complicated resonator. In OPG the generated frequency are amplified from quantum noise or an injected signal during a single or a multi-pass through the nonlinear crystal. During the past decade, development of a quasi-phase-matching (QPM) technique based on ferroic nonlinear material has increased the possibilities of efficient OPG. Periodically poled lithium niobate (PPLN) crystals have been extensively used for OPG and OPO’s because of its large nonlinear coefficient, reproducible big size of single crystal wafer and stabilized periodical poling method. However PPLN crystals suffer to photorefractive damage at room temperature operation result in efficiency reduction and poor output beam quality. In this work, we used an MgO-doped stoichiometric lithium tantalate (SLT) crystal to achieve QPM interactions without photorefractive damage at room temperature operation [1] and demonstrated an efficient single-pass OPG. A small signal from an extended-cavity laser (ECL: semiconductor laser at telecommunication range) was also amplified at signal waves. This compact OPG system was applied for low pressure NH3 gas detection as an example of environmental gas sensing around mid-IR range. Fig. 1 shows a schematic diagram of the single-pass OPG and OPA. A diode pumped Q-switched Nd:YVO4 laser was operated at 1064 nm with a repetition rate of 20 kHz and a ECL adopted as a seeding source at signal wavelength. For NH3 gas detection the generated main idler beam passed through a NH3 gas cell (5.2 Torr, 50cm) and was detected by an InSb detector, whereas the other beam passed through a powermeter and monochrometer and used as a reference.
Fig. 1. Experimental setup of OPG and OPA for NH3 gas detection.
A 0.7mol% MgO-doped SLT crystal was grown by the double-crucible Czochralski method [2] and periodically poled structure was fabricated by the electrical poling method at room temperature [3]. The
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QPM period of PPSLT device was 31.4 µm with 35-mm long. The generated signal and idler waves are 1614 to 1650 nm and 3043 to 3122 nm, respectively, according to the device temperature. The generated signal bandwidth was about 2.2 nm at FWHM whereas the pump was about 0.1 nm at 1064 nm. To reduce the spectral bandwidth we applied an injection seeding technique at signal waves using a fiber coupled CW ECL. The narrowness of the spectral bandwidth is limited by the finite bandwidth of both the seed and the pump. The bandwidth of the ECL is negligible compared to the bandwidth of the pump laser. The bandwidth of signal beam could be estimated λs=λ2s λp /λ2p, where λs , λi , and λp are the signal, idler and pump wavelength, respectively, and λp is the bandwidth of the pump. The λs =0.31 nm ( λp = 0.13 nm) and it had good agreement with the measured signal bandwidth calculation. Next, we measured the output power as a function of pump power at room temperature. As shown in Fig. 2 we obtained the signal and idler power of 1.1 W and 0.5W at pump power of 4.8 W. The total slope efficiency (S.E.) was 50% at OPG. For the signal amplification the seed beam was set at the center frequency of OPG such as 1614.4 nm. The CW seeding power of 1 mW was contributed for 3.7 ns because the measured pulse-width of OPG was 3.7 ns at signal wave even the pump laser pulse-width was 10 ns. Thus the seeding power of 1 mW was corresponded to 3.7 pJ for the pulse energy. In the amplification we obtained the maximum energy of 65 µJ. The maximum gain was 72.4 dB from this particular single-pass OPA system. In the previous our report we had estimated an effective nonlinear coefficient of the SLT crystal, the value was low compare to the LiNbO3. However SLT crystal is more suitable for watt-level high power generation owing to the high thermal conductivity and high optical damage threshold without photo-refraction [3]. Wavelength (nm) 4.5
1.2
OPA OPG
1.0
Transmittance (arb. )
Signal output power (W)
1.4
λ= 1614 nm
0.8 0.6
S.E. 34%
S.E. 37%
0.4 0.2 0.0 1
2
3
4
λ= 3122 nm
0.4 0.3 0.2
S.E. 16%
0.1 0.0 1
2
3
4
5
3040
3020
3000
(a)
4.0 3.5 3.0
Vacuum NH3
2.5 2.0 3240
3260
3280
3300
3320
3340
3320
3340
-1
(b)
3220
HITRAN2000
3240
3260
3280
3300 -1
Wavenumber ( cm )
Input power (W)
Fig. 2. Output power vs input power at room temperature.
3060
Wavenumber (cm )
Absorption coefficient (arb.)
Idler output power (W)
0.5
3080
3220
5
Input power (W)
3100
Fig. 3. Output spectra through NH3 gas cell and vacuum cell (a), HITRAN 2000 database of NH3 gas (b).
Finally, we applied the compact single-pass OPG system to the NH3 gas detection. We changed the signal wavelength from 3000 nm to 3100 nm by the crystal temperature change. Fig. 3(a) show output spectra as a function of wave number for the NH3 gas cell and for the vacuum cell. The envelope of vacuum cell reflects the OPG tuning curve. The wavelength resolution of the measured absorption spectrum about the NH3 gas is determined by the temperature step of the PPSLT device and the bandwidth of the OPG spectrum. The characteristic absorption bands of NH3 gas were successfully assigned to the measured spectrum with a reference to that in the HITRAN2000 database as shown in Fig. 3(b). In conclusion, a single-pass OPG for near and mid-IR generation in 1.6 to 3 µm range was demonstrated. An output power of 1.6 W and a slope conversion efficiency of 50% were successfully achieved. A small signal was amplified and the maximum gain was 72.4 dB. The compact OPG system was used for NH3 gas detection around 3 µm, and confirmed the feasibility of the system to realize environmental gas sensing. 5. References [1] N. Yu, S. Kurimura, et. al., Apl. Phys. Lett. 85, 5134 (2004). [2] Y. Furukawa, K. Kitamura, E. Suzuki and K Niwa, J. Cryst. Grow. 197, 889 (1999). [3] N. E. Yu, S. Kurimura, et. al., Apl. Phys. Lett. 84, 1662 (2004).
