Dexian Yan

Green laser induced terahertz tuning range expanding in KTiOPO4 terahertz parametric oscillator

532 nm green laser is utilized to achieve terahertz tuning range expanding in KTiOPO4 terahertz parametric oscillator. With the theoretical analysis of the stimulated polariton scattering, an expanded tunability of the KTiOPO4 terahertz parametric oscillator can be realized. A wide terahertz output tuning range from 5.7 to 6.1 THz, from 7.4 to 7.8 THz, from 11.5 to 11.8 THz, and from 13.3 to 13.5 THz was demonstrated in our experiment, and the result well matched the analysis. The maximum terahertz output energy was 1.61 μJ under the pump energy of 140 mJ, corresponding to the maximum THz wave conversion efficiency of 1.3 × 10−5, and the threshold pump energy is about 30 mJ.

Terahertz Imaging Based on Morphological Reconstruction

Terahertz (THz) imaging technology is a developing and promising candidate for biological diagnosis, security inspection, and semiconductor wafer examination, due to the low photon energy, the high transparency, and the fingerprint properties of the THz radiation. However, a major encountered bottleneck is the degradation of image quality caused by the power fluctuation of THz source, interference phenomenon, complex environment, and so on. In this paper, we present the mathematical morphology for THz imaging to improve the image quality, taking advantage of morphological reconstructions. Based on the original THz image of a paper with some letters taken from our continuous THz imaging system, the visibility of objects has been successfully enhanced with the suppression of complex background and improvement of peak signal-to-noise ratio using morphological reconstruction. Moreover, morphological reconstruction with proper structuring element parameter was then performed to a THz image of fresh rat cerebral tissue. It presents a satisfactory result with clearer edges and suppressions of the interference fringes and noises. It is suggested that THz imaging based on morphological reconstruction opens a pathway towards automatic techniques for denoising, recognitions, and segmentations in THz biomedical imaging.

Energy scaling and extended tunability of terahertz wave parametric oscillator with MgO-doped near-stoichiometric LiNbO_3 crystal

A widely tunable, high-energy terahertz wave parametric oscillator based on 1 mol. % MgO-doped near-stoichiometric LiNbO3 crystal has been demonstrated with 1064 nm nanosecond pulsed laser pumping. The tunable range of 1.16 to 4.64 THz was achieved. The maximum THz wave output energy of 17.49 μJ was obtained at 1.88 THz under the pump energy of 165 mJ/pulse, corresponding to the THz wave conversion efficiency of 1.06 × 10-4 and the photon conversion efficiency of 1.59%, respectively. Moreover, under the same experimental conditions, the THz output energy of TPO with MgO:SLN crystal was about 2.75 times larger than that obtained from the MgO:CLN TPO at 1.60 THz. Based on the theoretical analysis, the THz energy enhancement mechanism in the MgO:SLN TPO was clarified to originate from its larger Raman scattering cross section and smaller absorption coefficient.

Numerical Study of Compact Terahertz Gas Laser Based on Photonic Crystal Fiber Cavity

We report a compact terahertz (THz) gas laser based on the cavity photonic crystal fiber (PCF). This compact structure consists of a large air core PCF constructed by high-density polyethylene tubes. The 2.52 THz wave can be generated in the reaction cell filled with the CH3 OH gas. The reaction cell should shrink the volume of THz sources. The saturation intensity is analyzed based on the rate equations theory. Numerical calculations show that the THz wave can realize the fundamental mode transmission and obtain a lower confinement loss of 5.6 dB/m and the flat bending loss spectrum. The optimum pressure, pump threshold are analyzed and the THz output power can reach the magnitude of 10 mW. We combined the THz pump technology and the excellent transmission characteristics of the PCF together. This compact THz gas laser gives a new method to realize the THz source with high beam quality, low transmission loss, well-distributed bending shape and small size.

Remote Gas Pressure Sensor Based on Fiber Ring Laser Embedded With Fabry–Pérot Interferometer and Sagnac Loop

In this paper, we propose a remote gas pressure sensor based on a fiber ring laser embedded with a Fabry-Pérot (F-P) interferometer and a Sagnac loop. A compact optic-fiber F-P interferometer is inserted in the fiber laser and works as the sensing head for gas pressure detection. A wavelength selective filter based on optic-fiber Sagnac loop is used in the fiber laser. The sensitivity of -9.69 nm/kPa is obtained with a narrow 3-dB bandwidth less than 0.02 nm and high signal-to-noise ratio (SNR) ~45 dB. Moreover, the excellent performance of the gas pressure sensor for remote detection is demonstrated.

Humidity Sensor Based on Fabry–Perot Interferometer and Intracavity Sensing of Fiber Laser

A humidity sensor based on Fabry–Perot interferometer (FPI) and intracavity sensing of a fiber ring laser is proposed and experimentally demonstrated. A compact humidity-sensitive fiber-optic FPI is developed and inserted in a fiber ring laser. Because the output power of the fiber laser is modulated by the reflection loss of the FPI at different ambient humidity, intracavity humidity sensing is induced. The experiment shows that the relative output power of the fiber laser has a good linear response to ambient humidity from 25%RH to 95%RH and the humidity sensitivity of 0.202 dB/%RH is obtained with a narrow 3-dB bandwidth and high signal-to-noise ratio. Furthermore, the excellent performance of the sensor for remote humidity detection is demonstrated. The sensor also shows a low temperature cross-sensitivity, fast time response, and good stability. The proposed sensor has a great potential in high-capacity sensor network and remote detections.

Temperature Self-Compensation High-Resolution Refractive Index Sensor Based on Fiber Ring Laser

In this letter, a temperature self-compensation high-resolution refractive index (RI) sensor based on intracavity intensity-modulated sensing in a fiber ring laser is demonstrated. A fiber-optic multimode interferometer based on single-mode-no-core-single-mode fiber structure is cascaded with a fiber Bragg grating and used as a reflected sensing head to enhance intensity-modulated depth. It is inserted in a fiber ring laser as a wavelength selective filter and the intracavity intensity-modulated RI sensing is induced for the output intensity of the fiber laser. Furthermore, because the lasing wavelength of the sensor system is sensitive to temperature but insensitive to external RI, the temperature self-compensation measurement can be realized. The RI sensitivity is measured to be −4.98 mW/RIU from 1.3349 to 1.3665. Correspondingly, the relative intensity sensitivity achieves −196.1 dB/RIU from 1.3349 to 1.3544 and −744.6 dB/RIU from 1.3544 to 1.3665. The resolution of the fiber laser sensor is obtained as high as

High-repetition-rate, widely tunable terahertz generation in GaSe pumped by a dual-wavelength KTP-OPO

2\times 10^{{{-10}}}

High-average-power, high-repetition-rate tunable terahertz difference frequency generation with GaSe crystal pumped by 2 μm dual-wavelength intracavity KTP optical parametric oscillator

RIU with the signal-to-noise ratio more than 55 dB.

High power, widely tunable dual-wavelength 2 μm laser based on intracavity KTP optical parametric oscillator

High-repetition-rate, monochromatic and tunable terahertz (THz) source is demonstrated. We use an orthogonally polarized dual-wavelength intracavity OPO to complete the type-II phase-matched collinear difference-frequency generation in GaSe. A high average-power 2 μm laser with 12 W output power and good beam quality based on an intracavity KTP OPO is experimentally designed. The KTP OPO is intracavity pumped by an acousto-optical Q-switched side-pumped Nd:YAG with the repetition rate of 10 kHz. Two identical KTP crystals were 7 × 8 × 15 mm3 in size, cut at θ = 51.2°, φ = 0°, which were tuned in the x-z plane to achieve type-II phase-matching. The KTP OPO consists of two identical KTP crystals to reduce the walk-off effect and improve the beam overlap area of the output signal and idler waves. The pulse-width of the 2-μm KTP OPO laser is about 11 ns with the linewidth about 0.8 nm. The focused OPO beam is injected into the uncoated GaSe with the length of 8 mm, and the generated THz wave is detected with a 4.2-K Si-bolometer after focusing with a polyethylene lens. The tunable and coherent radiation from 0.2 to 3 THz has been achieved based on the type-II phase-matching DFG when the two pump waves are in the range of 2.1064 - 2.1272 μm and 2.1516 - 2.1304 μm while symmetrically tuning the phase-matching angle of the KTPs. The maximum output THz average power can reach μW-level around 1.48 THz.