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Dual-frequency MHz-repetition 1.5-μm optical parametric generation is demonstrated from a tandem system pumped by a picosecond Nd:YVO4 bounce laser. An average power of 1.0 W is obtained over the range 1571–1630 nm, corresponding to an optical-optical conversion efficiency and slope efficiency of 16% and 23%, respectively. Terahertz-wave with a frequency of 3.2 THz is also generated from the system in combination with an organic nonlinear DAST crystal.
©2011 Optical Society of America
1. Introduction
High-average-power short-pulse terahertz (THz) sources have been used in many fields including molecular spectroscopy [1], biomedicine [2], communications technology [3], and nondestructive testing [4]. Difference-frequency generation [5–10] using the organic nonlinear crystal 4’-dimethylamino-N-methyl-4-stilbazolium tosylate [11,12] (DAST-DFG) pumped by a dual-frequency laser source in the 1.5-μm wavelength region has led to efficient terahertz generation in the nanosecond regime. However, there are few reports of picosecond THz sources based on DAST-DFG.
Group velocity dispersion (GVD) in optical elements such as nonlinear crystals and mirrors affects dual-frequency generation in synchronous optical parametric oscillators (OPO) [13,14]. Furthermore, those systems require a complicated optical system including a dual resonant cavity.
In this present paper, a 1.5-μm dual-frequency optical parametric generator (OPG) is proposed using only a few optical elements based on periodically poled stoichiometric lithium tantalate (PPSLT) pumped by a Nd-doped vanadate picosecond bounce laser [15–18]. With this system, dual-frequency output with a total average power of 1.0 W and peak power of 0.27 MW at wavelengths of 1581 nm and 1598 nm was obtained with a slope efficiency of 23%. We also address 3.2 THz generation by combining the system with the DAST crystal.
2. Experimental setup
2.1 High-Power Picosecond Bounce Laser System
The PPSLT crystal exhibits higher damage threshold for 1 µm pump sources in comparison with a conventional PPLN crystal. To avoid optical damage of the crystal and increase the reliability of the system, we used the PPSLT crystals.
Figure 1 shows the setup for a compact dual-frequency OPG system (PPSLT-OPG) consisting of two PPSLT crystals pumped by a Nd-doped vanadate picosecond bounce laser. The bounce laser is composed of a passively mode-locked picosecond master laser and a side-diode-pumped Nd-doped YVO4 amplifier, and its wavelength was 1064nm. The master laser had an average power of ~1 W and a pulse width of 7.4 ps at a pulse repetition frequency (PRF) of 100 MHz. A pulse selector, formed from a rubidium titanyl phosphate (RTP) Pockels cell synchronized with the pulses from the master laser, was used to fix the pulse repetition frequency of the master laser output at 1.0 MHz. The master laser power injected into the amplifier was ~10 mW.
Fig. 1 Setup of the picosecond dual-frequency PPSLT-OPG system pumped by a Nd-doped vanadate bounce laser.
The amplifier was a side-diode-pumped a-cut 1.0% Nd:YVO4 slab crystal with dimensions of 20 mm × 5 mm × 2 mm. The pump face (20mm × 5mm) and two laser faces (5mm × 2mm) of the crystal were anti-reflection coated for 808nm and 1064nm, respectively. The crystal was mounted on a copper block. A temperature of the block was maintained to be ~20 ◦C by a water re-circulating chiller. The output from a continuous-wave 808-nm diode array stack was focused as a line onto the face of the amplifier by a cylindrical lens (f = 25 mm). The amplified master laser beam was retroreflected to the amplifier by two mirrors and a spherical lens (f = 100 mm). Figure 2(a) plots the average output power of the bounce laser as a function of the pump power. A maximum output power of 13.8 W was measured at a pump power of 88 W, corresponding to a peak power of 2.2 MW. Figure 2(b) shows an intensity autocorrelation trace of the output. It has a width of 6.4 ps assuming Gaussian pulses.
Fig. 2 (a) Output power of the vanadate bounce laser as a function of the pump power at a PRF of 1 MHz. (b) The intensity autocorrelation of the bounce laser pulses.
2.2 Dual-Frequency Picosecond PPSLT- OPG System
In order to control the temperatures of the PPSLT crystals individually, the crystals were mounted onto individual ovens. With this system, 1.5 μm dual-frequency output was obtained. The output from the bounce laser system was recollimated by a cylindrical lens VCL (f = 100 mm) and focused to a 0.2-mm-diameter spot into the PPSLT crystal (crystal1) with a domain-inverted period of 30.9 μm and dimensions of 4 mm × 3 mm × 35 mm by a spherical lens (f = 250 mm). It was vertically polarized to satisfy type-0 phase matching. The pump and signal outputs from the PPSLT crystal were relayed to a second PPSLT crystal (crystal2) with a domain-inverted period of 30.9 μm and a crystal length of 15 mm. The lengths of two crystals then were experimentally determined, so that the dual-frequecny output had equal powers at two peak wavelengths. Both surfaces (Y-Z plane) of the crystals had anti-reflection coatings for 1064 nm. After passing through the PPSLT crystals, the pump and signal beams were retroreflected by a spherical lens (f = 100 mm) and the flat mirror M1 designed for high reflectivity at 1064 and 1500 nm. The amplified signal was then output using a beamsplitter (BS).
The residual pump and undesired second harmonics were removed using a high-reflection filter for 1064 nm (R > 99.9% from Precision Optics) and a long-wavelength-pass filter with a cutoff wavelength of 600 nm (FGL610 from Thorlabs). The output power was measured using a thermal detector (PM30 from Coherent), and its spectrum was measured using an optical spectrum analyzer (Q8381 from Advantest).
3. Experiments and Discussion
Figure 3 shows the wavelength tuning curve of the dual-frequency signal output emitted by the two PPSLT crystals as a function of the crystal temperature. The output from crystal1 was in the range of 1571.0–1625.6 nm. The corresponding idler wavelength ranged from 3.06 to 3.28 µm. The blue symbols plot the signal output from crystal2. The solid curve in Fig. 3 is a simulated tuning curve using the Sellmeier equation [19]; in good agreement with the experimental values, although there is a slight discrepancy due to an error associated with determination of the poling period and the uncertainty in the measured crystal temperatures.
Fig. 3 The PPSLT-OPG/OPA tuning curves. The red and blue symbols plot the signals from crystals 1 and 2, respectively.
Figure 4(a) shows a snapshot of the lasing spectrum from crystal1 at a temperature of 350 K. The central wavelength was measured to be 1586 nm with a bandwidth of ~3 nm. An intensity autocorrelation trace is shown in Fig. 4(b). The signal output had a Gaussian pulse width of 3.7 ps.
Fig. 4 (a) The spectrum of a single-frequency output at a temperature of 350 K. (b) Corresponding intensity autocorrelation traces.
A spectrum of the dual-frequency signal output is shown in Fig. 5 . The temperatures of the two crystals were 380 K and 336 K, with peak wavelengths of 1581 nm and 1598 nm, corresponding to a difference frequency of 1.78 THz. Using this PPSLT-OPG system in combination with a DAST difference-frequency generator, terahertz waves in the range of 0.5–7.0 THz can be generated by tuning the temperature of crystal2 between 380 K and 440 K.
Figure 6 plots the measured dual-frequency output power as a function of the 1-μm pump power. The maximum average power was measured to be 1.0 W at a pump power of 6.3 W. The optical-optical efficiency from the 1064-nm pump laser output relative to the dual-frequency signal output was 16%. The slope efficiency and lasing threshold were measured to be 23% and 2 W, respectively. The total peak power of the dual-frequency signal output was ~0.27 MW with a pulse width of 3.7 ps.
Terahertz generation was demonstrated using the dual-frequency output in combination with a DAST crystal. The temperatures of the two PPSLT crystals were tuned to be 310 K and 377 K, respectively, thereby producing the dual-frequency output with peak wavelengths of 1573.75 nm and 1600.85 nm. The PPSLT-OPG output was focused to a 200-μm-diameter spot incident on a DAST crystal having a cross-section of 10 mm x 10 mm and a 0.8-mm thickness. The highly diffracted THz output, at a lasing frequency of 3.22 THz, was collected onto a liquid-helium-cooled Si bolometer using two parabolic mirrors with a focal length of 50 mm [20].
Figure 7 shows the temporal response of the output from the bolometer. The pump power was modulated at 150 Hz using a chopper. When the pump beam was polarized parallel to the a-axis of the DAST crystal, phase matching between the THz and pump outputs was enabled, leading to an observable signal on the bolometer. Figure 7(b) shows the experimental THz output power as a function of the dual-frequency signal power. The THz generation by DFG process was evidenced by that the output power was proportional to a square of the dual-frequency signal power.
Fig. 7 (a) Temporal evolution of the electric signal from the Si bolometer. (b) Experimental THz output power as a function of the dual-frequency output (pump) power.
4. Conclusion
We have successfully demonstrated 1.5-μm picosecond dual-frequency generation from a tandem PPSLT crystal pumped by a Nd:YVO4 bounce laser. This system can potentially create a high-power (watts), high-intensity (MW) picosecond output without utilizing an oscillator. Over one watt of signal with a peak power of 0.27 MW was achieved at frequencies of 1581 nm and 1598 nm. We have also demonstrated 3.22 THz generation. The system could be used to generate tunable picosecond THz output in the frequency range of 0.5–7.0 THz.
Acknowledgments
The authors acknowledge support from the Ministry of Education, Science, and Culture of Japan from Scientific Research Grants-in-Aid (16032202, 18360031, and 22760034) and the Japanese Society for the Promotion of Science.
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