Yixin He

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.

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

Fast Terahertz Imaging based on Compressive Sensing

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

Widely-tunable terahertz parametric oscillator based on MgO-doped near-stoichiometric LiNbO3 crystal

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

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

we demonstrate an active terahertz imaging method based on compressive sensing. With all modulation matrices engraved on one mask, the fast imaging has achieved. Recognizable result of letter “H” has obtained with PSNR of 10.21.

High-sensitivity attenuated total internal reflection continuous-wave terahertz imaging

A widely tunable, high-energy terahertz wave parametric oscillator based on 1 mol. % MgO-doped near-stoichiometric LiNbO3 crystal has been proposed with 1064 nm nanosecond pulsed laser pumping. The tunable range of 1.16 to 4.64 THz was obtained. Under the pump energy of 150 mJ/pulse, the maximum THz wave output energy of 12.56 μJ was achieved at 1.88 THz, corresponding to the THz wave conversion efficiency of 7.61×10-5 and the photon conversion efficiency of 1.14%, respectively. Moreover, the THz half maximum (FWHM) beam diameters of MgO:SLN TPO measured at 4 cm from the output surface were 7.42 mm and 6.06 mm in the vertical and horizontal directions, respectively.