The Laser Diode (LD) end-pumped solid-state lasers with excellent output characteristics are extensively used in many fields. The thermal effects of the laser crystal are one of the key factors preventing the laser from achieving high-quality output. In this paper, the thermal effects of a laser at 1 kHz repetition rate were explored by developing a three-dimensional thermal model of an end-Quasi-Continuous Waves (QCW) LD pumped Nd: YAG crystal. Considering the radial and axial heat conduction of the laser crystal, the transient temperature field within the crystal was numerically calculated using a seven-point Finite Difference Method (FDM). The transient thermal effects of the composite crystal were compared with those of the non-composite crystal. The effects of different parameters on the transient thermal effects of the composite crystal were discussed in detail. This simulation work is believed to guide the design of thermally stable cavities for lasers operating at muti-kHz to attain favorable laser characteristics.
To enhance the correlation in the orthogonal directions, a polarization self-modulation scheme with an intra-cavity quarter wave plate in a coaxial pumping orthogonally polarized laser was proposed. This quasi-isotropic cavity was compared with the traditional scheme in terms of the differences in the oscillation between dual components and the intra-cavity eigenstate distribution was obtained. Both theoretical and experimental results indicated that modes were effectively locked in TE and TM directions and dual-eigenstates output was achieved, which provided a half-free-spectrum-range frequency difference in ±45° directions. Q-switching and dual-wavelength-operation did not affect the polarization self-modulation process.
Depth image reconstruction has been of interest in single photon LiDAR. The difficulty of high-accuracy depth image reconstruction, for example, the low-reflection objects are ignored sometimes, results from the signal intensity in the reconstructing, which is heavily affected by the target characteristics. The confidence interval width is utilized to guide the recovery of depth images for achieving high-accuracy depth imaging under the non-negligible differences in target characteristics. This work proposes a confidence interval (CI)-guided depth imaging method, which evaluates the uncertainty of depth estimation with the 95% confidence interval of normal distribution. Three necessary steps exist in process with this CI-guided depth imaging method. Firstly, the noise responses are eliminated using the local gating method. Then, the depth image is reconstructed by a pixel-wise maximum likelihood estimator, and the CI guidance image is calculated from the 95% confidence interval of the mean normal distribution. Finally, the adaptive thresholding segmentation algorithm based on the CI guidance image is adopted to achieve the reconstruction of depth images. The CI-guided depth imaging method presents a unique perspective between signal and noise. This paves the way to improve the depth image recovery accuracy with the state-of-the-art photon-efficient imaging algorithm.
We report a compact and highly stable 1064-nm electro-optic Q-switched laser operating at the repetition rate of 1 kHz. A composite Nd:YAG crystal was used as the gain media and the cavity length was 105 mm. Under the average pump power of 11 W, the output power achieved 2.404 W with the pulse width of 4.558 ns, corresponding to the maximum peak power of 0.527 MW and the optical-to-optical conversion efficiency of 21.85%. The slope efficiency reached 42.69%. The beam quality in the horizontal (Mx2) and vertical (My2) directions were 1.81 and 1.58, respectively. The pulse timing jitter was less than 1 ns, and the average power fluctuation measured within 30 min was 0.83% (RMS). It is believed that such a compact and highly stable pulsed laser with high repetition rate, high peak power, and good beam quality has great potential in the fields of lidar, etc.
A theoretical model was proposed to simulate the broadband second harmonic generation (SHG) based on random quasiphase matching (RQPM) by Fourier transform mothed. A broadband SHG experiment system was built which could obtain the distribution of the SHG signal over a whole ZnSe sample. Both the simulated and experimental results demonstrated that the main feature of RQPM is the linear dependency of the SHG intensity with sample thickness.
Theoretical models for the backscattering intensities of Lambertian tilted plates, spheres and cones in monostatic radar situation were proposed, and their one-dimensional (1D) range profiles were simulated in the terahertz range. Then the 1D range profiles of picosecond and nanosecond pulse incidence are compared, which reveal that more details of object shapes can be obtained with the ultrashort terahertz pulse. The influences of target size, posture, pulse width and waveform were also investigated, respectively.
The dusty plasma sheath formed during the reentry process of hypersonic vehicle will interrupt the propagation of the communication signals, which is called the blackout problem. One of the most effective solutions to the blackout problem is to detect with terahertz wave, which refers to the electromagnetic wave with a frequency range from 0.1 to 10THz. Recently, the propagation characteristics of terahertz wave in dusty plasma have been studied based on the analyzed of the dielectric properties. Unfortunately, the influence of scattering caused by ablation particles is neglected in these studies, which can be ignored no more, especially for high terahertz wave. In this work, the propagation characteristics of dusty plasmas are analyzed considering both intrinsic absorption and scattering. Firstly, the electron densities and collision frequencies have been calculated based on the simulation of flow field around a return capsule model, and then the dielectric properties are analyzed. Secondly, the propagation characteristics are calculated and the results show that with the increase of detection frequency, the transmittance of THz wave in dusty plasma increases due to the decrease of absorption. But for higher frequency, the stronger scattering leads to the decrease of transmittance. Moreover, the influences of the flight speed of the vehicle, the diameter and density of ablation particle are also discussed. This research provides a basis for the selection of the best frequency band for the detection of return capsule.
After entering the near space, a layer of plasma sheath is formed outside the hypersonic vehicles due to the hightemperature and high-pressure environment. The plasma sheath, which characteristic frequency is similar to microwave, will cause serious impediment to communication signal. This phenomenon is known as the blackout problem. With the rapid development of aerospace industry, plasma sheath blackout has become an urgent problem to be solved. Current research shows that increasing the frequency of electromagnetic wave higher than the plasma characteristic frequency can effectively reduce the shielding effect of plasma. The frequency of terahertz (THz) wave is much higher than microwave, it can propagate through plasma sheath, which provides an effective method to solve the problem of plasma sheath. In this paper, a theoretical model of plasma is established, and the transmission properties of THz wave in plasma is simulated using scattering matrix method. Then a kind of plasma jet is produced in laboratory environment according to dielectric barrier discharge. And the experiments of a broadband THz source and THz time-domain spectrum transmission in this kind of plasma and a 2.52 THz wave reflection imaging of target under plasma shelter are carried out respectively. The transmittance increases with frequency under 0.5 THz and becomes stable at 100% over 0.5 THz, and the result of experiments and simulation are in good agreement. Both theory and experiments show that THz wave has good penetration in plasma jet and can detect targets behind plasma, and this study will lay a theoretical foundation for solving the plasma blackout problem of hypersonic vehicle in near space.
A novel nested anti-resonant hollow core fiber (NAHF), based on Topas, with low loss and flattened dispersion is proposed for efficient transmission of terahertz wave. Finite element method with an ideally matched layer (PML) boundary condition is used to investigate its guiding properties. A cladding structure of nested anti-resonant elliptical rings is introduced to reduce mode power leakage. The NAHF shows a low confinement loss (< 0.29 cm-1 ) and a small effective material loss (< 0.019 cm-1 ) in the frequency range of 0.9-1.5 THz. An ultra-flatted near zero dispersion profile of ±0.029 ps/THz/cm is obtained within a broad frequency range of 0.6-1.5 THz. Furthermore, optimizing the structure parameters in NAHF, higher core power fraction over 80 %, higher effective mode area of ~10-6 μm2 and the bending loss of 3.05×10-5 cm-1 at the bending radius of 10 cm are also achieved.
A typical plasmon-induced transmission metamaterial and its corresponding complementary structure is designed and explored. The results show that the transmission spectra of the two structures are complementary, but the reflection spectra of the complementary structure and the transmission spectra of the ordinary structure are in good agreement. Furthermore, this work integrate photosensitive silicon into the complementary metal-based metamaterials, realizing the optical active control of the plasmon-induced reflection and transmission. This research strategy provides a new way for the study of reflective structures. Moreover, the active control of the reflection and transmission play a role in slow light devices and terahertz filter.
We have demonstrated a high-energy and broadly tunable monochromatic terahertz (THz) source via difference frequency generation (DFG) in DAST crystal. The THz frequency is tuned randomly in the range of 0.3-19.6 THz, which is much wider than the THz source based on the inorganic crystal and the photoconductive antenna. The highest energy of 2.53μJ/pulse is obtained at 18.9 THz corresponding to the optical-to-optical conversion efficiency of 1.31×10-4. The THz output spectroscopy is theoretically and experimentally explained by DFG process and Raman spectroscopy. Meanwhile, a phenomenon of blue light from the KTP-OPO with tunable and multiple wavelengths was firstly observed and explained. Based on our THz source, an ultra-wideband THz frequency domain system (THz-FDS) with transmission mode is realized to measure the ultra-wideband THz spectroscopies of typical materials in solid and liquid states, such as Si, SiC, White PE, water, isopropyl myristate, simethicone, atonlein and oleic acid, etc.. Furthermore, we have studied the THz spectral characteristic of biomedical tissue in the ultra-wideband THz frequency range of 0.3-15THz to study the biomedical response in the entire THz frequency range, which contains more abundant spectral information and was rarely focused with the limit of the THz source.
We set up the terahertz continuous reflectometry imaging system and the spatial resolution of our system was roughly 0.6×0.6mm at 2.52 THz. We also demonstrated that the paraffin embedded traumatic brain injury (TBI) in rat model sample can be differentiated clearly. The results show that the THz reflection intensity of the TBI area was lower than that of normal area. These promising results suggest that THz reflection imaging has great potential as an alternative method for the fast diagnosis of TBI.
One kind of switchable, tri-band, terahertz linear polarizing rotator is presented in this paper, which consists of sandwiched metal chiral metamaterial structure composed of twisted electric field-coupled resonators in C4 symmetry and a VO2 film on substrate for active controlling. The polarizing rotation is switchable with the state change of VO2 from an insulator to metal. Simulated results consistently demonstrate that the switchable rotator exhibits extremely low loss, high polarization conversion ratio and optical activity at the three resonance frequencies. The influence of different geometric parameters of the chiral metamaterial structure is investigated to optimize the multiband rotating response of the polarizing rotator. This switchable terahertz metamaterial-based rotator has various potential applications in terahertz wave controlling and the terahertz functional devices.
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