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High-gain mid-infrared optical-parametric generation was demonstrated by simple single-pass configuration using PPMgLN devices pumped by giant-pulse microchip laser. Effective mid-infrared wavelength conversion with 1 mJ output energy from 2.4 mJ pumping using conventional PPMgLN could be realized. Broadband optical-parametric generation from 1.7 to 2.6 µm could be also measured using chirped PPMgLN.
© 2016 Optical Society of America
1. Introduction
Nonlinear wavelength conversion pumped by an intense pump source is useful method to realize coherent mid-infrared (MIR) light sources for various applications, such as optical communication, laser processing, and material spectroscopy [1–3]. Combination of high-gain nonlinear device and high-power microchip laser (MCL) is suitable for this purpose. Various types of MCLs at continuous-wave and Q-switched operation have been reported [4–6]. Quasi-phase matching (QPM) [7–9] can realize efficient and arbitrary wavelength conversion compared to conventional birefringent phase matching. MIR light generation by QPM optical parametric oscillation/generation (OPO/OPG) using periodically poled LiNbO3 (PPLN) device have been realized by pumping of the MCL or other high-power pump source [10–14].
Last several years, we have reported giant-pulse microchip laser (GP-MCL) with > MW peak power at sub-nanosecond pulse duration [15,16], which is an attractive light source for various laser processing and nonlinear wavelength conversion. The GP-MCL has been already used in practical applications, such as laser ignition of gasoline engine [16], efficient green/ultra-violet wavelength conversion [17], and THz-light generation/detection [18,19]. Also recently, we have developed high-energy MIR-OPO using a large-aperture periodically poled Mg-doped LiNbO3 (PPMgLN) pumped by conventional flush-lamp pumped Q-sw Nd:YAG laser [20,21].
In this paper, we report on MIR-OPG using highly efficient PPMgLN devices pumped by the GP-MCL. This combination can give various advantages as improvements of damage resistance by shorter pulse duration, realization of single-pass operation without cavity mirrors, and shrinking of total-system size, compared to the conventional flush-lamp pumping. The MIR-OPG characteristics using a conventional PPMgLN with fixed QPM period and a chirped PPMgLN with chirped QPM period are evaluated for comparison.
2. GP-MCL and PPMgLN
A palm-top sized GP-MCL with its head size of 40mm x 43mm x 30mm is shown in Fig. 1. A 1.1 at.% Nd:YAG crystal and a passive Q-switching Cr4+:YAG crystal (initial transmission of 30%) were set in the temperature controlled head at 25°C, and pumped at quasi continuous-wave operated laser diode (120W, 808 nm). Total cavity length was 12 mm. Linearly polarized output with 0.7 ns duration in 1.064 µm wavelength could be obtained. Output energy of the GP-MCL varies on its operation frequency, and typical pulse energy of 3 mJ at 30Hz operation was used in this experiments. The output beam has nearly Gaussian shape with beam divergence ~1 mrad and M2 ~3.
For MIR-OPG experiments using a PPMgLN device pumped by a 1.064 µm source, QPM period Λ ~30 µm is required at room-temperature operation. Various PPMgLN devices with fixed QPM period and chirped QPM period were prepared, which were fabricated by the temperature elevated field poling technique. Further information on preparation of the PPMgLN devices is shown in our previous reports [20, 21].
3. OPG using conventional PPMgLN with fixed QPM period
Experimental set up for single-pass MIR-OPG using a conventional PPMgLN with fixed QPM period is shown in Fig. 2. The PPMgLN with broad anti-reflection (AR) coating for pump and OPG waves was placed in a temperature controlled oven at 25°C. Output energy and spectrum of the OPG-output signal and idler waves were measured after 2-stage filtering optics to eliminate other waves. The 1st one is a dielectric mirror mainly for pump-wave elimination, with transmittance of <0.5%, 46%, and 92% for 1.064 µm, 1.55 µm, and 3.39 µm, respectively. The 2nd one is a commercial infrared-range optical filter (Spectrogon Inc.) for each wave measurement. The signal-pass filter (BBP-1430-1770-D, band-pass type) has a transmittance of 79% for 1.55 µm. The idler-pass filter (LP-2000, longwave-pass type) has a transmission of 93% for 3.39 µm. Both infrared filters can eliminate undesired waves between UV to MIR range. The original output energy for both signal and idler waves (Es0 and Ei0, respectively) just after the PPMgLN were calculated from the measured energy (Es and Ei) by including transmittance of the 2-stage filtering optics. The pumping beam was weakly focused by a focusing lens with f = 400 mm, and the beam size at the input face of the PPMgLN was tuned by changing the length L between the PPMgLN and the focusing lens. The maximum available energy before PPMgLN was 2.4 mJ.
Fig. 2 Experimental set up for OPG using a conventional PPMgLN. HWP: Half-wave plate, ISO: Faraday isolator, PBS: Polarization beam splitter, OSA: Optical spectrum analyzer.
The OPG-signal wavelength using a PPMgLN with Λ = 30.6 µm was measured to 1.55 µm, and the resulting OPG-idler wavelength was calculated to 3.39 µm. Figure 3 shows the OPG energy (Es0, Ei0) characteristics on pump energy using a pump 1/e2 beam-area size S of 0.59 mm2 at L = 500 mm. Oscillation threshold energy was 0.3 mJ, and maximum Es0 and Ei0 at 2.4 mJ pumping were 696 µJ and 327 µJ, respectively. Total OPG energy reached > 1 mJ with 40% energy conversion efficiency. Ratio of the maximum Es0 and Ei0 is 2.13 ( = 696 µJ / 327 µJ), which is close to an inverse of wavelength ratio 2.19 ( = 3.39 µm / 1.55 µm) and well agreed with the Manley-Rowe relations in the OPG process.
Figure 4(a) presents the pump beam-area size S and maximum pumping intensity Imax on length L at maximum pumping energy of 2.4 mJ. The beam size S varies from 3.34 to 0.59 mm2, and the intensity changed from 103 to 581 MW/cm2 by moving L from 100 to 500 mm. Figure 4(b) shows maximum Es0 and Ei0 on length L at 2.4 mJ pumping. Maximum pumping intensity was calculated to be around 581 MW/cm2 at L = 500 mm with S = 0.59 mm2, which resulted no damage in the PPMgLN device. The maximum intensity value of the 581 MW/cm2 is 2-times higher intensity compared to the damage threshold intensity (~300 MW/cm2) at the conventional 10 ns pulse laser in our previous works, which is the advantage of the sub-nanosecond GP-MCL pumping. Although small beam size of S = 0.59 mm2 (for the 581 MW/cm2 intensity) was used in this demonstration because of pump energy limitation in the GP-MCL, still we realized an efficient wavelength conversion as high as 40% with 1 mJ output energy as shown in Fig. 3. Therefore, higher energy MIR-OPG > 10 mJ output can be easily expected in future by preparing higher energy GP-MCL source with our large-aperture PPMgLN device, up to 10-mm thickness [21].
Fig. 4 (a) Pump beam-area size S and maximum pumping intensity Imax, (b) Maximum OPG-signal and -idler energy, on Length L at pumping energy of 2.4 mJ.
Because the single-pass MIR-OPG using PPMgLN device has no cavity mirrors such as conventional OPO, the output wavelength can be easily tuned by changing of QPM period Λ. Figure 5 shows dependence of the measured OPG-signal wavelength and the calculated OPG-idler wavelength on Λ of various PPMgLN devices with 26.2 to 32.3 µm period. Wavelength range from 1.36 µm to 4.91 µm could be easily obtained by a simple GP-MCL-pumped single-pass OPG set up. Although the wavelength dependence on the QPM period at the OPG is same as that in our previous OPO experiments [21], expanded wavelength range with signal < 1.4 µm and idler > 4.5 µm could be realized by the single-pass operation without specially coated, MIR-range dielectric mirrors.
4. OPG using chirped PPMgLN with chirped QPM period
Although non-periodically poled QPM device such as chirped PPMgLN with chirped QPM period can be expected to handle a broad wavelength range and to enable arbitrary wavelength conversion compared to the conventional QPM-period-fixed PPMgLN device, conversion efficiency per unit-wavelength or per unit-device-length should be reduced. Therefore, high intensity pumping, such as short-pulse operation and waveguide structure, is needed for effective wavelength conversion in case of the chirped PPMgLN operation. The GP-MCL with sub-nanosecond pulse duration can realize higher intensity pumping without PPMgLN damage, and therefore we can expect to realize an effective operation of single-pass broadband MIR-OPG using a chirped PPMgLN by the GP-MCL pumping.
A linearly chirped, broad AR-coated PPMgLN with QPM period Λ ranging from 32.3 µm to 30.3 µm was prepared, which can cover OPO range between 1.5 µm and 3.5 µm in our previous OPO experiments [21]. Set up for single-pass MIR-OPG using a chirped PPMgLN is shown in Fig. 6. Output energy and spectrum of the MIR-OPG were measured after 2-stage filtering optics. The 1st dielectric mirror was for pump-wave elimination, with a transmittance of <0.5% for 1.064 µm, and an average transmission of around 80% between 1.7 µm to 2.6 µm of broadband OPG range. The 2nd infrared filter (Spectrogon, LP-1300, longwave-pass type) has an average transmission of around 90% between 1.7 µm to 2.6 µm, and can suppress undesired detection of pump wave or parasitic light. Total MIR-OPG energy (Esi) measured after the 2-stage filtering optics is shown in following part, because of the difficulty in calculation of transmittance for broadband OPG spectrum.
Figure 7 shows a detected total (signal + idler) OPG energy Esi on input pump energy at the same pump beam size of S = 0.59 mm2 at L = 500 mm as the conventional PPMgLN case. Threshold pumping energy increased to 1.1 mJ, which is an effect of chirped PPMgLN structure. Measured maximum Esi was about 180 µJ at 2.4 mJ pumping, and we can expect higher OPG energy just after the chirped PPMgLN device.
Figure 8 shows spectrum shape dependence on various pumping energy condition at L = 500 mm, measured using an arrayed InGaAs spectrometer (SOMA Optics, S-2810, wavelength range 1.2 ~2.6 µm). Although longer-wavelength side of the measured spectrum was limited up to 2.6 µm by the limitation of the InGaAs spectrometer, spectrum broadened MIR-OPG output could be obtained by simple single-pass OPG set up using a chirped PPMgLN. Small depletion around 1.98 µm is assumed to be an effect of atmospheric gases. Improved measurement such as vacuum condition is required for an exact spectrum-shape evaluation of the broadband OPG. At the pumping energy of 1.41 mJ, which is near oscillation threshold as shown in Fig. 7, broadened OPG spectrum from 1.8 ~2.5 µm could be measured. At the highest pumping energy of 2.39 mJ, shortest spectrum side reached around 1.7 µm. From the simple calculation of pump wavelength 1.064 µm and the shortest OPG spectrum around 1.7 µm, longest OPG output should have a spectrum range of 2.84 µm, which means a broad range generation with >1 µm spectrum bandwidth could be realized by simple single-pass MIR-OPG of GP-MCL pumping and chirped PPMgLN combination. By further improvements of experimental conditions, such as pump-beam size, maximum pump energy, and QPM chirping structure, we can expect to realize a fully broadband OPG from 1.5 µm to 3.5 µm range as expected.
6. Future scaling up of MIR energy
A single-pass MIR-OPG pumped by a current GP-MCL of ~2.4 mJ with 0.59 mm2 beam-size can generate total OPG energy > 1 mJ without damages in PPMgLN. An energy-amplified GP-MCL with > 10 mJ output energy was already demonstrated for THz wave generation [18]. Also, large-aperture PPMgLN with 10 mm thickness is already realized for high energy OPO [21], and the 10-mm-thick PPMgLN device can handle 1 J pumping energy even in 10 ns operation. Therefore, future scaling up of the MIR-OPG with several mJ output energy can be easily expected by combination of the amplified GP-MCL and the large-aperture PPMgLN.
7. Conclusion
We demonstrated MIR-OPG by combination of PPMgLN device and giant-pulse microchip laser (GP-MCL) with > MW peak power. In case of conventional PPMgLN with fixed QPM period, maximum total output energy of 1 mJ with 40% conversion efficiency could be realized at the pumping energy of 2.4 mJ. Also, in case of chirped PPMgLN with chirped QPM period, broadband OPG ranging 1.7 µm to 2.6 µm could be measured (actual range from 1.7 µm to 2.84 µm expected). The GP-MCL-pumped MIR-OPG using PPMgLN can give advantage for many practical applications, compared to conventional MIR-OPO pumped by flush-lamp pumped laser source.
Acknowledgments
This research was partially supported by JSPS Grant-in-Aid for Scientific Research (C) 25390102 and (A) 15H02030, and Photon-Frontier-Consortium Project by MEXT of Japan.
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