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A compact efficient high-repetition-rate doubly-resonant dual-wavelength KTP optical parametric oscillator (OPO), with output power up to 3.65 W and tuning ranges of 2.088-2.133 μm/2.171-2.122 μm for signal/idler waves, was deployed for terahertz (THz) generation in a GaSe crystal. Based on difference frequency generation (DFG), the THz wave was continuously tunable from 730.9 μm (0.41 THz) to 80.8 μm (3.71 THz), believed to be the first report of a compact high-repetition-rate widely-tunable THz source. The maximum THz average power reached 1.2 μW at 1.54 THz and the corresponding DFG efficiency was 7.8 × 10−7, entirely suitable for portable applications. The utility of the THz source was also demonstrated through spectroscopy and imaging experiments.
© 2016 Optical Society of America
1. Introduction
The terahertz (THz) spectral range (30–3000 μm) which falls between the microwave region and the infrared has captured much attention due to its importance in spectroscopy, imaging, communication, radar, etc. However, among all the existing electronic and photonic based THz sources, the generation of widely tunable coherent THz waves in the frequency range of 0.3-3 THz for high-resolution spectroscopy and hyper-spectral imaging is still challenging. The most efficient and widely applicable method is parametric down-conversion of two laser wavelengths into the THz frequency range by difference frequency generation (DFG), which also possesses the merits of compactness in size, high peak power, room-temperature operation, etc. A multitude of nonlinear optical crystals are available such as PPLN, GaSe, ZGP, DAST, GaP, GaAs [1–8], in which GaSe is an ideal candidate in general considering its wide transparency region, high second-order nonlinear coefficient, suitable birefringence, and low THz absorption, among other properties [9].
On the other hand, the laser wavelengths applied in DFG are usually around 1.06 μm or 1.55 μm, the most common and direct method for dual-wavelength generation. Considering the quantum efficiency from the infrared (IR) laser to the THz regime, pumping at longer wavelength should be more beneficial. As a result, powerful CO2 lasers around 10 μm were investigated for generating THz and the average power achieved was up to 260 μW [7,10]. However, the CO2-laser wavelength is not continuously tunable thus the generated THz-wave is discrete in frequency. As generation of high-brightness mid-IR with solid-state/fiber lasers is difficult, DFG using 2-μm lasers either directly emitted by thulium/holmium doped lasers or generated by nearly degenerate optical parametric oscillators (OPOs) pumped by 1.06 μm neodymium lasers is the best alternative approach. Considering the technical development, efficiency and linewidth, type II OPOs are preferable. Continuous-wave, synchronously pumped and Q-switched 2-μm OPOs based on PPLN and KTP have been deployed for THz generation [8,11,12]. Recently, we reported a compact THz source pumped by a diode-end-pumped acousto-optical (AO) Q-switched nearly degenerate 2-μm KTP OPO giving average output power of 0.6 μW at 1.244 THz with a diffusion-bonded (DB) GaAs nonlinear crystal [13]. Nevertheless, it is difficult to re-establish the phase-matching (PM) condition in DB-GaAs if the interacting wavelengths are changed, thus the output THz frequency was fixed.
In this letter, a compact and highly-efficient 2-μm laser at a repetition rate of 25 kHz is demonstrated based on a diode-end-pumped acousto-optical (AO) Q-switched Nd:YVO4 laser and two identical KTP crystals arranged in a walk-off compensated configuration. Benefitting from the optimized optical coupling system and nonlinear crystal, the average and peak output powers of the KTP OPO over the whole tuning range were significantly enhanced and the beam quality was greatly improved. An 8-mm GaSe crystal was used for DFG, producing THz wave covering the frequency range from 0.41 THz to 3.71 THz. Detecting with a 4.2-K Si bolometer, the maximum output voltage reached 352 mV, corresponding to an average THz power of 1.2 μW, nearly double that of our former best result. To the best of our knowledge, this is the first reported widely tunable high-repetition-rate compact THz parametric source.
2. Experimental setup
The experimental setup is shown in Fig. 1, which is similar to that in [13]. The laser diode, Nd:YVO4 laser crystal, cavity mirrors, dichromatic filter, optical chopper and the detector were identical to the former scheme. However, in order to achieve widely tunable THz radiation, the dual-wavelength OPO requires better power stability over a widely tunable wavelength range. The coupling lens was replaced by a 1:2 imaging system to achieve better mode-matching between the pump, fundamental and OPO resonant waves. Furthermore, the KTP crystals (CRYSTECH Inc.) were cut at θ = 51.18° and φ = 0°, which was intended to generate signal and idler waves at around 2.116 μm and 2.141 μm, respectively, based on theoretical correction for the KTP dispersion, considering that the highest net gain for DFG in GaSe is located at 1.5-1.8 THz. After filtering the residual fundamental laser, an 8-mm GaSe crystal was used for direct DFG. The length of the dual-wavelength OPO was only 150 mm and the entire THz source was less than 220 mm, compact enough for portable applications.
3. Experimental results and discussion
3.1 Dual-wavelength KTP OPO around 2 μm
The output characteristics of the dual-wavelength KTP OPO pumped by the 1.06-μm Nd:YVO4 laser are shown in Fig. 2, in which the performance of the Q-switched Nd:YVO4 laser is shown for comparison. It should be noted that here the KTP facets were normal to the fundamental laser beam without angle tuning, that is, the PM condition was at θ = 51.18° and φ = 0°. When Q-switched at 25 kHz, the pump threshold of the OPO was about 2.32 W and the maximum output power of 3.65 W was achieved at 18.09-W pumping, corresponding to an optical-optical conversion efficiency of 20.18% and slope efficiency of 23.15%. Notably, when the Q-switched Nd:YVO4 laser power was 7.86 W, 46% of the fundamental power was extracted and converted to OPO output. The 25 kHz repetition rate was experimentally optimized considering the fundamental average and peak power which were related to thermal lensing and properties of the laser crystal itself. The highest optical-optical efficiency of 20.5% was obtained when the pump power was 10.01 W and the 2-μm output power was 2.054 W.
With an extended InGaAs PIN detector (EOT ET-5000) and a 500-MHz digital oscilloscope, the temporal pulse shape of the KTP OPO output was measured, demonstrating a decreasing trend from 7.1 ns near threshold to 3.13 ns at maximum output power, narrowed by 32% than the former result in [13]. This was attributed to the improved mode distribution in the laser cavity for better Q-switching, and apparently, the enhanced peak power (> 20 kW) for each wavelength was good for increasing the frequency conversion efficiency. The timing jitter was negligible because both wavelengths came from the same doubly resonant OPO (DRO) cavity. The beam quality of the KTP OPO was measured with a knife edge and the M2 was improved to 1.99 and 2.03 in the horizontal and vertical planes, respectively, benefiting from the walk-off compensating crystal configuration and better mode-matching with a 1:2 coupling lens.
The output spectra were monitored with a Yokogawa AQ6375 optical spectrum analyzer. The signal and idler wavelengths were 2.123 μm and 2.134 μm respectively, yielding a frequency interval of 0.73 THz at maximum OPO output power. Although there was some deviation from the theoretically expected behavior, it was possible to reach the best working point through slightly angle tuning. By symmetrically changing the internal PM angle of the two KTP crystals from 50.98° to 51.69°, the corresponding dual-wavelength laser could cover the range from 2.088 μm to 2.171 μm, that is, in frequency intervals from 0 to 5.5 THz. Degeneracy occurred at a PM angle of 51.07°. Figure 3 and Fig. 4 show the output spectra and tuning characteristics of the 2-μm KTP OPO in the x-z plane of the crystal, respectively. The linewidth of the 2-μm laser was around 0.7 nm and there was no significant variation at different wavelengths. The maximum output power gradually decreased during tuning because of increased Fresnel reflection loss. However, the power was greater than 2.65 W throughout the wavelength range, greatly improved as compared with [13].
3.2 THz generation in GaSe
The dual-wavelength KTP OPO output was incident into the GaSe crystal directly without focusing to avoid thermally induced damage. The GaSe sample was 8 mm in length with a clear aperture width of 16 mm. There were no AR coatings on the surfaces with single-pass optical loss at 2 μm of around 31.5% and an absorption coefficient of around 0.005 cm−1. An optical chopper (Stanford Research Systems, SR540) was used to modulate the high-repetition-rate signal to 25 Hz before detection. Furthermore, a 1-mm-thick germanium (Ge) wafer with HR coatings at 2.0-2.3 μm and a 1-mm-thick black polyethylene (PE) plate were placed before the entrance window of the bolometer, in order to block the residual 2-μm laser beam. Considering that the effective nonlinear coefficient for type o-e→e DFG in GaSe is deff = d22∙cos2θ∙cos(3φ), φ = 0° was applied for maximum conversion efficiency and simply rotating the crystal in x-z plane enabled phase matching. When the KTP OPO was tuned in the range of 2.132-2.157 μm (e-wave) and 2.126-2.101 μm (o-wave), the obtained THz wavelength varied in the range from 730.9 μm (0.41 THz) to 80.8 μm (3.71 THz). Figure 5 shows the THz tuning characteristics in wavelength and frequency. Clearly, theoretical predictions and experimental results accorded with each other very well.
The bolometer used for detection was calibrated to be 2.89 × 105 V/W and connected to a digital oscilloscope to read the signal intensity. Figure 6 shows the maximum THz output voltage versus frequency. A typical pulse shape is also shown as the inset, from which we can see the chopping frequency at 25 Hz and the saturated peak at high repetition rate owing to slow thermal response of the detector. With the dual-wavelength OPO at 2.117 μm and 2.141 μm, the maximum output voltage was 352 mV at 1.54 THz corresponding to an average output power of 1.2 μW and a DFG conversion efficiency of 7.8 × 10−7 from 2 μm to 195 μm. With an expected pulse width of less than 3 ns, the maximum peak power exceeded 150 mW. The decline of the output power on the low-frequency side is attributed to decreased quantum efficiency and on the high-frequency side results from increased THz absorption in the GaSe crystal. As expected for GaSe [12,14], there were significant absorption peaks at around 0.6 THz, 1.1 THz, 1.9 THz and 2.5 THz, evidenced by the dips in the spectrum in Fig. 6. The polarization of the THz wave was found to be the same as that of the longer pump wave, measured with a 30-μm wire grid (MicroTech Instruments, Inc), which confirmed type o-e→e PM in DFG. The power instability (rms) of the THz wave at 1.54 THz was around 5% from 10-minute statistics of peak-to-peak intensity fluctuation of the modulated THz signal shown on the oscilloscope, and it dropped to below 2% if averaged by 16 times, good enough for various applications.
Fig. 6 Maximum THz output voltage versus frequency for an 8-mm-long GaSe crystal. The inset shows a typical THz signal from the bolometer.
3.3 Discussion
In order to achieve a wider tunable range and higher THz power, a longer and better-quality GaSe crystal always helps to enhance conversion efficiency, and a boxcar averager is useful for obtaining the weak signals at both ends of the tuning curve. Quantitatively, with the parameters deff = 54 pm/V, ns≈ni≈2.5 and nT≈3.3 at 1.54 THz, the THz intensity generated from DFG process is calculated by the coupled-wave equations [15], the results of which are shown in Fig. 7. We conclude that not only the interaction length, but the absorption coefficients and pump intensity also affect the nonlinear optical conversion efficiency. As measured with a THz time domain system (TDS), the crystal used in the experiment had an absorption coefficient of around 4 cm−1 at 1.54 THz and the pump intensity for each wavelength was around 3 MW/cm2, which was far from the ideal condition. If the absorption coefficient is simply reduced to 1 cm−1, the output power can be increased by 2.76 times, and the maximum power can be increased by almost an order at optimum interaction lengths. Enhancing the pump intensity through focusing should also work, but thermal accumulation induced damage in GaSe may occur under high-repetition-rate operation.
Fig. 7 THz intensity generated for different pump intensity (each pump wavelength), crystal length and absorption coefficient. (a) and (b) are calculated for GaSe absorption coefficients of 4 cm−1 and 1 cm−1, respectively.
4. Spectroscopy and imaging applications
Based on the tunable THz source shown in Fig. 1, we conducted transmission spectrum measurements with white and black polyethylene (PE) samples, both of which were 1 mm in thickness. The results are shown as solid lines in Fig. 8, where we can clearly distinguish the lattice mode of PE around 2.2 THz [16]. We also compared the results measured with a time-domain system (TAS7500, Advantest Corp.), shown as dotted line in Fig. 8. The deviation of absolute transmittance and absorption peak frequency should be caused by information loss during data processing of the time-domain THz electric field with TDS, where extra interference peaks in addition to the primary one should be cut off to avoid oscillations in frequency domain after Fourier transform [17]. As the spacing between neighboring peaks for thin samples was very short, it was hard to retain all the information while cutting the time-domain signal. And the etalon effects of the TDS results, originating from the multiple beam interference between two surfaces of the sample, were obviously not real. In contrast, the transmission spectra directly measured with our tunable THz source was more reliable.
Fig. 8 THz transmission spectra of white and black PE samples measured with tunable THz source and TDS.
We also performed imaging experiments at different frequencies with the tunable THz source. Using a 9-mm-thick white PE plate with five blind holes on it, the THz scanning imaging results at 1.5 THz and 2.2 THz were obtained, shown in Fig. 9. The holes were 4 mm in diameter and 5.3 mm in depth on the substrate. The sample was mounted on a two dimensional translation stage after the chopper and the scanning steps were 400 μm in both directions. Apparently the general transmission at 1.5 THz was larger than that at 2.2 THz because of their different absorption coefficients (α1.5 = 0.12cm−1, α2.2 = 0.38cm−1). However, the signal contrast at the hole and substrate (η = Ih/Is) at 2.2 THz (η2.2 = 1.61) was found higher than that at 1.5 THz (η1.5 = 1.32), thus the image obtained at 2.2 THz was clearer. It can be explained by the theoretical analysis expressed as and at both frequencies (ΔL = 3.7mm is the thickness difference for substrate and holes), ignoring the surface reflections. The simple experiment verified the idea of disparate imaging quality at different frequencies. Benefiting from the high-repetition-rate operation of the THz source, the scanning process could be very fast and continuous scanning without chopping was also feasible with a DC detector. Extending to multi-spectral or hyper-spectral imaging that can be realized by such a continuously tunable THz source, the spectrum for each pixel of a certain image is also expected, enabling the purpose of finding objects, identifying materials, etc.
Fig. 9 THz imaging for a white PE sample at different frequencies: (a) 1.5 THz; (b) 2.2 THz. The scale values of the coordinates are scanning steps of 400 μm.
5. Conclusions
We have achieved compact, widely-tunable, and high-repetition-rate optical THz generation based on DFG in GaSe crystal using a compact, efficient, and nearly-degenerate 2-μm KTP OPO as the pump source. The dual-wavelength laser was tunable covering the wavelength range from 2.088 μm to 2.171 μm, with a maximum output power of 3.65 W and optical-optical conversion efficiency of 20.18% at 25 kHz. Benefiting from the optimization of mode-matching and crystal orientation, higher dual-wavelength average and peak power, better beam quality and tuning stability were achieved. With an 8-mm-long GaSe crystal, the maximum output THz average power was 1.2 μW at 1.54 THz with a DFG efficiency of 7.8 × 10−7, which was more than twice that of our former results using DB-GaAs crystal. The frequency tuning range covered from 0.41 THz to 3.71 THz and could be further expanded by using a better DFG crystal and improved electronic signal processing system to increase signal-to-noise ratio in detection. Simple spectroscopy and imaging measurements were performed with PE samples to verify related applications. Although there is still much room for improvement, such a microwatt-level widely-tunable THz source is a good alternative for spectroscopy, imaging etc., and especially useful for portable applications with current dimensions less than those of a laptop and further reductions to palm size feasible.
Funding
National Basic Research Program of China (2014CB339802, 2015CB755403); National Natural Science Foundation of China (NSFC) (61675146, 61275102, 61271066); Science and Technology Support Program of Tianjin (14ZCZDGX00030).
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