Terahertz-wave Parametric Generator Research Articles

High-repetition-rate injection-seeded terahertz-wave parametric generation pumped by a 1.5 μm sub-nanosecond laser

Injection-seeded terahertz (THz)-wave parametric generation (is-TPG) using a nonlinear optical crystal of MgO-doped lithium niobate (MgO:LiNbO3) has made it possible to obtain THz waves with high peak powers and narrow linewidths. To achieve stable THz-wave generation from the MgO:LiNbO3 crystal under strong excitation, the photorefractive effect in the MgO:LiNbO3 crystal would be crucial, which could be eliminated by using a longer pump wavelength than the conventional 1-μm spectral band. Here, we propose to use a 1.5-μm near-infrared (NIR), high-repetition-rate, sub-nanosecond laser source based on a unique Er,Yb:glass planar waveguide amplifier (PWA) as the pump source for the is-TPG using the MgO:LiNbO3 crystal. At a repetition rate of 25 kHz, the frequency tunability from 1.3 to 2.5 THz was obtained with a maximum THz-wave average output power of 0.14 mW at 1.77 THz.

Efficient Terahertz Parametric Source Using Flat Pump Beam

A flat pump beam was used to improve efficiency and miniaturize an injection-seeded terahertz-wave parametric generator (is-TPG). The improved power density of the pump beam and interaction length of the terahertz-wave reduced the required pump beam energy by > 85% compared with the conventional system. The reduced generation threshold made it possible to construct is-TPG spectroscopic systems with pump beams of ≤ 3 mJ.

Multi-Wavelength Terahertz Parametric Generator Using a Seed Laser Based on Four-Wave Mixing

In this study, we developed a multi-wavelength terahertz-wave parametric generator that operates with only one injection seeding laser. Tunable lasers used as an injection seeder must be single-frequency oscillators, and conventional multi-wavelength terahertz-wave parametric generator requires basically the same number of lasers as the number of wavelengths. In order to solve this problem, we developed a new external cavity semiconductor laser that incorporates a DMD in its wavelength-selective mechanism. In this process, stable multi-wavelength oscillation from a single laser was made possible by efficiently causing four-wave mixing. This seed laser can be applied to practical real-time terahertz spectroscopy by arbitrarily switching the desired wavelength to be generated and the interval between multiple wavelengths.

Cascaded effect in a high-peak-power terahertz-wave parametric generator.

We demonstrate megawatt-level terahertz (THz)-wave generation via a Stokes-seed-injected THz-wave parametric generator and study the cascaded effect. The optical-to-THz conversion efficiency was 1.72 × 10-3, and the peak power was conservatively estimated to be 1.09 MW using the pulse width of the pump. More than 80% of the THz-wave energy came from primary parametric generation, with the rest coming from high-order parametric amplification. Clear cascaded Stokes spots of second to fourth order were observed, and the factors affecting the high-order parametric process are discussed. The cascaded parametric effect is beneficial for achieving a higher optical-to-THz conversion efficiency, thereby improving the performance of high-peak-power THz-wave parametric sources.

Injection-seeded terahertz-wave parametric generator with timing stabilized excitation for nondestructive testing applications.

An injection-seeded terahertz (THz)-wave parametric generator (is-TPG) with a footprintthe size of an A3 paper is presented. We improved the measurement performance of the is-TPG source for nondestructive inspection applications. A high pulse repetition rate up to 70 kHz and a low pulse timing jitter of a few tens of picoseconds, which is approximately one ten-thousandth of the conventional is-TPG, were achieved. THz waves exhibited excellent performance with a maximum average output power of 20 µW, a monochromatic spectrum linewidth of ∼20GHz, and a frequency tuning range of 1.7-3.0THz. This was achieved by designing the entire system configuration from the pump laser source to THz-wave generation. A new double-pass all-solid-state optical amplifier was developed with a high gain and low noise using an externally pulse-modulated laser diode (LD) as the master oscillator source. An achromatic optical injection system was developed for the is-TPG with a 40% reduction in the conventional path length. They were housed in a single enclosure in two layers. LDs and optical fiber amplifiers could be rack-mounted, and the outputs were delivered to the housing via optical fibers. The developed THz-wave source performed nondestructive imaging of a human hair sample fixed with Scotch tape on a test pattern in an envelope by irradiating 2.1THz waves. A clearly recognizable THz-wave image of an enclosed hair with a spatial resolution close to the THz wavelength was obtained.

Numerical Modeling of an Injection-Seeded Terahertz-Wave Parametric Generator

We have constructed a numerical model for injection-seeded terahertz-wave parametric generators in which the pump depletion, the duration of the pump pulse, as well as the spatial modes of the pump and seed beams are taken into consideration. Compared with other models which only provide the parametric gain under the assumption of low pump depletion, our model can make the predictions on the energy as well as spatial and temporal profiles of the generated terahertz-wave. Besides, the appropriate value of nonlinear coefficient of MgO:LiNbO3 in terahertz regime is difficult to find in the published works. To solve this problem, Miller’s rule is applied for the wavelength scaling of the nonlinear coefficient during calculation. We present a detailed description of the model and show that its predictions agree well with the reported experimental results. The presented model allows the performance to be estimated when designing an injection-seeded terahertz-wave parametric generator.

Injection pulse-seeded terahertz-wave parametric generator with gain enhancement in wide frequency range.

An injection pulse-seeded terahertz-wave parametric generator (ips-TPG) has been demonstrated with gain enhancement in wide tuning range. Theoretical analysis denotes that the compensation of initial Stokes energy is favorable to the THz gain enhancement in wide frequency range, which is attributed to the improvement on interaction of stimulated polariton scattering (SPS) and difference frequency generation (DFG) processes. In the experiment, the THz frequency tuning range from 1.04 THz to 5.15 THz was achieved based on near-stoichiometric LiNbO3 (SLN) crystal. Compared with the traditional terahertz parametric oscillator (TPO) under the same experimental conditions, a significant enhancement of THz output energy was occurred in high frequency range. As the THz frequency increased from 1.9 THz to 3.6 THz, the enhancement ratios from 1.6 times to 34.7 times were obtained. Besides, the 3dB bandwidth of ips-TPG was measured to be 2.1 THz, which was about 2.6 times that of SLN-TPO. This THz parametric source with a relative flat gain in wide frequency range is suitable to a variety of practical applications.

Perspective: Terahertz wave parametric generator and its applications

The injection-seeded terahertz (THz) parametric generator (is-TPG) is one of the most high-power single-longitudinal mode THz-wave sources. Our system is less influenced by scattering, refraction, and multiple reflections by samples because it is a narrow-linewidth source, and the detection area of the THz parametric detector is large. Thus, it is suitable for nondestructive inspection of practical samples in the real world. In 2003, we reported on the development of a mail inspection system that employed a THz parametric oscillator. However, with a dynamic range of less than four orders of magnitude, this system could only identify reagents through thin envelopes. Recently, we succeeded in developing a high-power, highly sensitive THz-wave spectroscopic imaging system with a dynamic range of 100 dB using the is-TPG and a THz parametric detector. Nondestructive inspection of reagents inside thick envelopes and three-dimensional computed tomography of plastics, which attenuate THz-waves by more than 60 dB, were conducted using this system. More recently, we have focused our efforts on a real-time measurement system using a multiwavelength is-TPG, which gives rise to numerous potential applications, given the significantly shorter measurement times. Thus, this system will facilitate the implementation of THz-wave measurements in real-world applications. In this paper, we report on our recent results and provide a perspective on the is-TPG.

Energy scaling of terahertz-wave parametric sources.

Terahertz-wave parametric oscillators (TPOs) have advantages of room temperature operation, wide tunable range, narrow line-width, good coherence. They have also disadvantage of small pulse energy. In this paper, several factors preventing TPOs from generating high-energy THz pulses and the corresponding solutions are analyzed. A scheme to generate high-energy THz pulses by using the combination of a TPO and a Stokes-pulse-injected terahertz-wave parametric generator (spi-TPG) is proposed and demonstrated. A TPO is used as a source to generate a seed pulse for the surface-emitted spi-TPG. The time delay between the pump and Stokes pulses is adjusted to guarantee they have good temporal overlap. The pump pulses have a large pulse energy and a large beam size. The Stokes beam is enlarged to make its size be larger than the pump beam size to have a large effective interaction volume. The experimental results show that the generated THz pulse energy from the spi-TPG is 1.8 times as large as that obtained from the TPO for the same pumping pulse energy density of 0.90 J/cm(2) and the same pumping beam size of 3.0 mm. When the pumping beam sizes are 5.0 and 7.0 mm, the enhancement times are 3.7 and 7.5, respectively. The spi-TPG here is similar to a difference frequency generator; it can also be used as a Stokes pulse amplifier.

High-power, single-longitudinal-mode terahertz-wave generation pumped by a microchip Nd:YAG laser [Invited

We report on the development of a high-peak-power, single-longitudinal-mode and tunable injection-seeded terahertz-wave parametric generator using MgO:LiNbO3, which operates at room temperature. The high peak power (> 120 W) is enough to allow easy detection by commercial and calibrated pyroelectric detectors, and the spectral resolution (< 10 GHz) is the Fourier transform limit of the sub-nanosecond terahertz-wave pulse. The tunability (1.2-2.8 THz) and the small footprint size (A3 paper, 29.7 × 42 cm) are suitable for a variety of applications.

Transform-limited, achromatic injection-seeded terahertz-wave parametric generator

A review to our effort on developing the transform-limited, frequency-agile terahertz-wave parametric generator (TPG) is presented. A frequency-agile THz-wave generation is realized by introducing the injection-seeding method and the optical design for the stationary dispersion-compensation The purity of the THz-wave frequency was dramatically improved to Δv/v<10−4. Simultaneously, the THz-wave output was several hundred times higher than that of a conventional TPG. In addition, a wide frequency tuning with fast tuning speed were realized. The THz-wave frequency can be set randomly or scanned continuously over a frequency range from 0.6 THz to 2.4 THz with narrow linewidth of sub 100 MHz. Furthermore, a tabletop, high-performance THz-wave gas spectrometer based on this achromatic injection-seeded TPG was developed. To demonstrate the potential of this system, we performed the measurement of the absorption line due to rotational transitions of the water molecules and determined their pressure-broadening coefficient.

Multi-Mode Laser-Pumped Injection-Seeded Terahertz-Wave Parametric Generator

We developed a Nd:YAG multi-mode laser-pumped injection-seeded terahertz-wave parametric generator (is-TPG), and compared its terahertz (THz)-wave output energy to that of a single-mode laser-pumped is-TPG system. The oscillation threshold of the multi-mode is-TPG was approximately 73% lower than that of the single-mode system, and the multi-mode system produced higher THz-wave energy than that of the single-mode system with the same pump energy. The required seed power for the multi-mode system was much lower than that for the single-mode system. The multi-mode laser was compact, inexpensive, and simpler to handle than a single-mode device. This resulted in a room-temperature THz-wave generation system that could produce sufficient power for THz spectroscopy.

Tunability enhancement of a terahertz-wave parametric generator pumped by a microchip Nd:YAG laser

We have enhanced the tunability of a terahertz-wave parametric generator using MgO:LiNbO3 pumped by a subnanosecond, passively Q-switched, microchip Nd:YAG laser. This pump source allows high-intensity pumping without damage to the nonlinear crystal and generates a narrow linewidth and tunable terahertz wave with injection seeding by an external cavity diode laser for the idler wave. The high-intensity pumping causes a gain curve broadening of the terahertz-wave parametric generation, especially in the high-frequency region. We obtained an output terahertz wave with a tuning range of 0.9-3 THz (100-330 microm), a peak power of 100 mW, and a linewidth of less than 10 GHz. This room temperature operated, palm-sized source promises to be a widely tunable terahertz-wave source that is suited to a variety of applications.

マイクロチップレーザー励起によるポータブル波長可変テラヘルツ光源

マイクロチップレーザー励起によるポータブル波長可変テラヘルツ光源

Frequency-agile terahertz-wave spectrometer for high-resolution gas sensing

A tabletop, frequency-agile terahertz-wave spectrometer was developed based on an injection-seeded terahertz-wave parametric generator. High-resolution (&amp;lt;100MHz) spectra of polar gases can be measured in a wide frequency region (0.6–2.4THz) within a short time (several minutes) with this spectrometer. To demonstrate the potential of this system, the authors performed real-time detection of the absorption line due to rotational transitions of the water molecule and determined their pressure-broadening coefficient.

The generation and linewidth control of terahertz waves by parametric processes

AbstractIn this article, we discuss broadband‐tunable terahertz‐wave parametric generation (TPG) via stimulated polariton scattering and techniques for narrowing the output spectrum of sources based on this phenomenon. The TPG effect, which is observed when a LiNbO3 crystal is excited with nanosecond pulses from a Nd:YAG laser, has a gain bandwidth of over 500 GHz. After carrying out experiments aimed at converting the TPG effect into a high‐power source of terahertz waves, we used our TPG light source for Fourier spectroscopy in the terahertz band. In addition, we were able to narrow the spectral bandwidth of the terahertz emission down to the Fourier limit (spectral purity Δν/ν∼10−4) by applying the method of optical injection seeding to the TPG using light from a wavelength‐tunable semiconductor laser. When we used the light from this injection‐seeded TPG source for the spectroscopy of water vapor in the terahertz region, we confirmed a resolution below 100 MHz (0.003 cm−1). Our final version of an injection‐seeded TPG light source exhibited peak powers of over 300 mW (dynamic range &gt; 106), and a frequency tunability of 0.7 to 2.4 THz. We anticipate that this compact nonlinear‐optics‐based terahertz wave light source, which can operate at ordinary temperatures and is structurally simple, can become an effective tool for scientific and technological research in the terahertz region. © 2003 Wiley Periodicals, Inc. Electron Comm Jpn Pt 2, 86(5): 52–65, 2003; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ecjb.10047

Achromatically injection-seeded terahertz-wave parametric generator

An achromatically injection-seeded terahertz-wave parametric generator was constructed with MgO:LiNbO (3) crystals and a tunable seeder in a stationary dispersion-compensated optical arrangement. Without readjusting the mirrors, we obtained smooth tuning of the terahertz wave over the 0.6-2.6 THz range by adjusting the seeder wavelength alone. We have successfully demonstrated the feasibility of this system for terahertz-wave absorption measurements over a wide frequency range by using low-pressure water vapor.

Tabletop terahertz-wave parametric generator using a compact, diode-pumped Nd:YAG laser

We specifically developed a diode-pumped, Q-switched Nd:YAG laser to act as a pump source for a terahertz-wave parametric generator. A single Q-switched laser pulse (pulse width 12 ns, pulse repetition frequency 40 Hz) produced an output energy of 12 mJ. Utilizing this pump source, we were able to construct a compact (650×350 mm) terahertz-wave parametric generator using LiNbO3 crystals. We achieved a broad terahertz-wave spectrum, ranging from 1.2 to 2.4 THz, with a pump energy of 12 mJ.