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1.
Opt Express ; 27(20): 27797-27806, 2019 Sep 30.
Artigo em Inglês | MEDLINE | ID: mdl-31684541

RESUMO

An actively Q-switched dual-wavelength intracavity Raman laser based on a coaxially pumped dual-crystal (Nd:YAG and b-cut Nd:YAP) fundamental configuration was theoretically and experimentally investigated. Stable dual-wavelength Stokes output at 1176 nm and 1195 nm was subsequently obtained by the Raman conversion from an a-cut YVO4 crystal. A total average power of 1.8 W was produced at 10 kHz pulse repetition frequency under an incident diode pump power of 15.8 W, in which the power levels at the two Stokes wavelengths were nearly equivalent. The power proportion and the interval between the dual-wavelength Stokes pulses could be manipulated actively by changing the pump focal position or pump wavelength.

2.
Opt Express ; 27(16): 22808-22818, 2019 Aug 05.
Artigo em Inglês | MEDLINE | ID: mdl-31510566

RESUMO

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.

3.
Opt Express ; 27(16): 23585-23592, 2019 Aug 05.
Artigo em Inglês | MEDLINE | ID: mdl-31510633

RESUMO

A multi-direction bending sensor based on the 18-core photonic crystal fiber (PCF) is demonstrated in this paper. The design of the sensor is discussed theoretically and experimentally. The PCF serves as the laser gain medium and the sensing medium simultaneously in the fiber laser sensor. The operating wavelength of the proposed PCF laser sensor is about 1032.32 nm. A CMOS image capture system is used to acquire the distribution of supermodes in the PCF laser. Based on the normalized intensity distribution of supermodes, six directions can be measured. The sensor also shows the ability to measure bending radius within 0.11 m. Then, the thermal effects of the bending sensor have been analyzed and the sensing system shows a low temperature crosstalk.

4.
Appl Opt ; 58(14): 3913-3920, 2019 May 10.
Artigo em Inglês | MEDLINE | ID: mdl-31158209

RESUMO

The spectrum acquired on the optical instrument usually contains the pure spectrum and undesirable components such as baseline and random noise. However, the intensity of the baseline, which seriously submerges the spectrum, is the primary limitation of spectral applications. Thus, baseline correction has become one of the most significant challenges for spectral applications. In this paper, we propose a doubly reweighted penalized least squares method to estimate the baseline. This method utilizes the first-order derivative of the original spectrum and established spectrum as a constraint of similarity. Meanwhile, the doubly reweighted strategy achieves a better effort. Considering the drawbacks of the weighting rules for the adaptive iteratively reweighted penalized least squares method, we adapt a boosted weighting rule based on the softsign function, which performs well when the spectrum contains high noise. The simulated results confirm that the proposed method yields better outcomes. The proposed method can be applied to Raman and near-infrared spectra as well, and the result shows that it can estimate various kinds of baselines effectively.

5.
Chem Commun (Camb) ; 55(55): 7950-7953, 2019 Jul 04.
Artigo em Inglês | MEDLINE | ID: mdl-31219114

RESUMO

A new promising nonlinear optical crystal C25H25NO4S2 (HDB-T, P21 space group) exhibits significant macroscopic second-order optical nonlinearity about 1.5 times larger than that of the benchmark OH1 crystal benefiting from optimized orientated polar HDB cations. Widely tunable monochromatic terahertz-wave from 0.1 to 20 THz in the HDB-T crystal was realized for the first time.

6.
Opt Express ; 27(6): 9241-9249, 2019 Mar 18.
Artigo em Inglês | MEDLINE | ID: mdl-31052731

RESUMO

A high-energy and tunable mid-infrared source based on BaGa4Se7 crystal was demonstrated by single-pass difference-frequency generation (DFG). Orthogonally polarized wave at 1064 nm (λ1) and tunable idler wave (λ2) generated by KTP-OPO, which could be tuned in the wavelength range of 1360-1600 nm, were used as the DFG dual-wavelength pump. The pump parameters including total pump energy and energy ratio were studied. Maximum pulse energy of 5.72 mJ at 3.58 µm was obtained at the dual-wavelength pump energy of 58.4 mJ/pulse. The wavelength tuning range was 3.36-4.27 µm with a flat tunability. Moreover, a saturation phenomenon of DFG output was observed and experimentally inferred to be related to the input energy of λ2 in the BaGa4Se7 crystal.

7.
Opt Express ; 27(8): 10449-10455, 2019 Apr 15.
Artigo em Inglês | MEDLINE | ID: mdl-31052904

RESUMO

An effective method for wavelength tuning in an optical parametric oscillator (OPO) was proposed using noncollinear phase-matching (PM) in a ring cavity. This method was particularly useful for noncritically phase-matched (NCPM) KTP/KTA OPOs where changing the crystal orientation or working temperature is ineffective. A wide tuning range in the eye-safe band from 1572.9 nm to 1684.2 nm was realized pumped by an Nd:YAG laser at 1.06 µm in an NCPM KTP OPO while the internal noncollinear angle was tuned from 0 to 3.1° or the external angle from 0° to 5.8°, with slight variation of the deflection angle of one cavity mirror. The good beam quality and high spectrum intensity of the narrow-linewidth Nd:YAG laser resulted in 33.3% conversion efficiency for the collinear case and above 11% throughout the tuning range. Such OPOs have many potential applications where tunable eye-safe lasers are required, and the proposed wavelength tuning method can be extended to all kinds of OPOs.

8.
Opt Express ; 27(2): 1529-1537, 2019 Jan 21.
Artigo em Inglês | MEDLINE | ID: mdl-30696217

RESUMO

An all-fiber seawater salinity sensor based on intracavity loss-modulated sensing in a fiber ring laser is proposed and experimentally demonstrated. An optical fiber multimode interferometer, which is based on single-mode-no-core-single-mode fiber structure, is cascaded with a fiber reflector and used as a reflected sensing head to enhance loss-modulated depth. It is inserted in a fiber ring laser and the intracavity loss-modulated salinity sensing is induced for the fiber laser's output intensity. The salinity sensitivity is measured to be 0.1 W/‰ with a high signal-to-noise ratio more than 49 dB and narrow full width at half maximum less than 40 pm. The temperature cross-sensitivity characteristic and stability are also analyzed. Considering the errors from cross-sensitivity, stability and resolution of the photodetector, the detection limit of the sensor system is 0.0023 ‰ (0.0002 S/m), which is comparable to the most advanced commercial electronic salinity sensor.

9.
Opt Lett ; 43(22): 5599-5602, 2018 Nov 15.
Artigo em Inglês | MEDLINE | ID: mdl-30439905

RESUMO

We demonstrate a 266 nm ultraviolet (UV) picosecond laser by fourth-harmonic generation of a Nd:YAG laser with a 5.4 mm thick NaSr3Be3B3O9F4 (NSBBF) crystal. A maximum output power exceeding 1 W at 266 nm was obtained (the highest output power being 1.6 W), corresponding to a conversion efficiency of 10.3%. The stability measurements on the NSBBF crystal with a fluctuation of 3.34% at 200 mW within 1 h indicate that it is a promising UV nonlinear optical material for practical applications. In addition, for the first time, to the best of our knowledge, we measured the effective nonlinear coefficient of NSBBF crystal at 266 nm and compared it with that of ß-BaB2O4 crystal.

10.
Opt Express ; 26(16): 20744-20757, 2018 Aug 06.
Artigo em Inglês | MEDLINE | ID: mdl-30119380

RESUMO

Terahertz attenuated total reflection imaging has been used to develop preliminary applications without any in-depth analysis of the nature of present systems. Based on our proposed vertically scanning imaging system, an analysis of optimum prism design and polarization selection for error reduction is presented theoretically and experimentally, showing good agreement. By taking the secondary reflection inside the prism and the prism deflection into consideration, p-polarized terahertz waves are recommended for prisms with a base angle below 31°, leading to minimum error. This work will contribute to the development of more practical application of terahertz attenuated total reflection scanning imaging in various fields with enhanced performance.

11.
Opt Express ; 26(16): 20768-20776, 2018 Aug 06.
Artigo em Inglês | MEDLINE | ID: mdl-30119382

RESUMO

A flexible method of generating stable dual-wavelength laser pulses with tunable power ratio and pulse interval is proposed, through integrating a coaxially end-pumped laser with compound gain media and an intracavity pumped optical parametric oscillator (IOPO). A theoretical model was built by a set of time-domain coupling wave equations containing both the generation of two fundamental waves from a shared pump source and the conversion to signal waves through the parametric process. Simulations showed that by simply varying the pump focal position or pump wavelength, the gains in two laser crystals could be changed, leading to simultaneous change in average power ratio and time interval between two wavelengths. Experimental verifications were performed with combined laser crystals (Nd:YAG and a-cut Nd:YLF) and a nonlinear crystal (KTA), which enabled dual-wavelength signal output in the 1.5-1.6 µm eye-safe region and demonstrated coincident conclusions with theoretical results. As there was no gain competition between two fundamental waves, stable signal output was obtained. Moreover, various wavelength pairs in any wavelength ranges are possible by using different laser crystals and nonlinear crystals. It is believed that this is a promising method for generating simultaneous dual-wavelength laser pulses for applications in lidar, remote sensing, nonlinear frequency conversion, etc.

12.
J Biophotonics ; 11(8): e201700386, 2018 Aug.
Artigo em Inglês | MEDLINE | ID: mdl-29633578

RESUMO

Timely and accurate bacterial detection is critical for various health and safety applications, which promotes the continuous development of versatile optical sensors for bacterial investigations. Here, we report a new strategy for bacterial colony sensing using terahertz (THz) imaging with minimal assay procedures. The proposed method utilizes the acute sensitivity of THz wave to the changes in the water content and cellular structures. Single bacterial colonies of 4 bacterial species were directly distinguished using THz imaging by utilizing their differences in THz absorption. In addition, the distribution of mixed bacterial samples has been demonstrated by THz imaging, which demonstrated that the target bacterium could be easily recognized. Furthermore, we investigated the differentiation of bacterial viability, which indicated that bacteria under different living states could be distinguished by THz imaging because of their different hydration levels and cellular structures. Our results suggest that THz imaging has the potential to be used for mixed bacterial sample detection and bacterial viability assessment in a label-free and nondestructive manner.

13.
J Biomed Opt ; 23(3): 1-7, 2018 Mar.
Artigo em Inglês | MEDLINE | ID: mdl-29595016

RESUMO

We demonstrated that different degrees of experimental traumatic brain injury (TBI) can be differentiated clearly in fresh slices of rat brain tissues using transmission-type terahertz (THz) imaging system. The high absorption region in THz images corresponded well with the injured area in visible images and magnetic resonance imaging results. The THz image and absorption characteristics of dehydrated paraffin-embedded brain slices and the hematoxylin and eosin (H&E)-stained microscopic images were investigated to account for the intrinsic differences in the THz images for the brain tissues suffered from different degrees of TBI and normal tissue aside from water. The THz absorption coefficients of rat brain tissues showed an increase in the aggravation of brain damage, particularly in the high-frequency range, whereas the cell density decreased as the order of mild, moderate, and severe TBI tissues compared with the normal tissue. Our results indicated that the different degrees of TBI were distinguishable owing to the different water contents and probable hematoma components distribution rather than intrinsic cell intensity. These promising results suggest that THz imaging has great potential as an alternative method for the fast diagnosis of TBI.

14.
Opt Express ; 26(5): 6371-6381, 2018 Mar 05.
Artigo em Inglês | MEDLINE | ID: mdl-29529829

RESUMO

The imaging diagnosis and prognostication of different degrees of traumatic brain injury (TBI) is very important for early care and clinical treatment. Especially, the exact recognition of mild TBI is the bottleneck for current label-free imaging technologies in neurosurgery. Here, we report an automatic evaluation method for TBI recognition with terahertz (THz) continuous-wave (CW) transmission imaging based on machine learning (ML). We propose a new feature extraction method for biological THz images combined with the transmittance distribution features in spatial domain and statistical distribution features in normalized gray histogram. Based on the extracted feature database, ML algorithms are performed for the classification of different degrees of TBI by feature selection and parameter optimization. The highest classification accuracy is up to 87.5%. The area under the curve (AUC) scores of the receiver operating characteristics (ROC) curve are all higher than 0.9, which shows this evaluation method has a good generalization ability. Furthermore, the excellent performance of the proposed system in the recognition of mild TBI is analyzed by different methodological parameters and diagnostic criteria. The system can be extensible to various diseases and will be a powerful tool in automatic biomedical diagnostics.

15.
Opt Express ; 25(25): 31988-31996, 2017 Dec 11.
Artigo em Inglês | MEDLINE | ID: mdl-29245866

RESUMO

A compact and stable terahertz (THz) source is demonstrated based on difference frequency generation (DFG) pumped by an efficient dual-wavelength acousto-optic (AO) Q-switched solid-state Nd:YLF laser with composite gain media (a-cut and c-cut) in the coaxial pumping configuration. Optimal power ratio and pulse synchronization of the orthogonal polarized 1047/1053 nm dual-wavelength laser could be realized by varying the pump focusing depth and/or pump wavelength. The total power of 2.92 W was obtained at 5 kHz pumped by 10-W laser-diode power at 803 nm. Such an efficient dual-wavelength laser demonstrated good stability and inconspicuous timing jitter benefiting from the suppressed gain competition between two resonating wavelengths. An 8-mm-long GaSe crystal was employed to generate THz waves at 1.64 THz by DFG and the maximum THz average output power was about 0.93 µW. This compact coaxial pumping method can be extended to all kinds of neodymium (Nd) doped laser crystals to produce different dual-wavelength lasers for various THz wavelength generation, which have good prospects for portable and costless applications like imaging, non-destructive inspection, etc.

16.
Sensors (Basel) ; 17(10)2017 Oct 16.
Artigo em Inglês | MEDLINE | ID: mdl-29035290

RESUMO

A relative humidity (RH) sensor based on single-mode-no-core-single-mode fiber (SNCS) structure is proposed and experimentally demonstrated. The agarose gel is coated on the no-core fiber (NCF) as the cladding, and multimode interference (MMI) occurs in the SNCS structure. The transmission spectrum of the sensor is modulated at different ambient relative humidities due to the tunable refractive index property of the agarose gel film. The relative humidity can be measured by the wavelength shift and intensity variation of the dip in the transmission spectra. The humidity response of the sensors, coated with different concentrations and coating numbers of the agarose solution, were experimentally investigated. The wavelength and intensity sensitivity is obtained as -149 pm/%RH and -0.075 dB/%RH in the range of 30% RH to 75% RH, respectively. The rise and fall time is tested to be 4.8 s and 7.1 s, respectively. The proposed sensor has a great potential in real-time RH monitoring.

17.
Sensors (Basel) ; 17(10)2017 Sep 29.
Artigo em Inglês | MEDLINE | ID: mdl-28961174

RESUMO

Multimode interferometers based on the single-mode-no-core-single-mode fiber (SNCS) structure have been widely investigated as functional devices and sensors. However, the theoretical support for the sensing mechanism is still imperfect, especially for the cladding refractive index response. In this paper, a modified model of no-core fiber (NCF) based on far from cut-off approximation is proposed to investigate the spectrum characteristic and sensing mechanism of the SNCS structure. Guided-mode propagation analysis (MPA) is used to analyze the self-image effect and spectrum response to the cladding refractive index and temperature. Verified by experiments, the performance of the SNCS structure can be estimated specifically and easily by the proposed method.

18.
Opt Express ; 25(8): 8926-8936, 2017 Apr 17.
Artigo em Inglês | MEDLINE | ID: mdl-28437966

RESUMO

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.

19.
Guang Pu Xue Yu Guang Pu Fen Xi ; 37(1): 129-34, 2017 01.
Artigo em Chinês | MEDLINE | ID: mdl-30195280

RESUMO

The regional features, metallogenitic regularities and mineral composition of the hydrothermal sulphide ore have been preliminarily studied. According to the different mineralization period, the patterns of valuable minerals disseminated in ore are complicated, which causes the large changes in the properties of the sulphide ore. The different properties of the sulphide ore may increase the difficulty of the mineral processing and reduce the recovery rate of valuable minerals. Therefore a simple method for rapidly classification of sulphide ore is required to optimize mineral processing flowsheet. Laser Raman spectrometry, as an effective method to analyze the structure of the material is used to identify the component and structure of minerals. The research on the Laser Raman spectra of the large number of sulphide ore samples can reveal the reasons for the difference of the Raman spectra. A new method for classifying the complex sulphide ore using Raman spectroscopy is proposed. The experiment results demonstrate that the properties of the sulphide ore in different mineralization period vary greatly and the fluorescent scattering is mainly produced by gangue minerals. The measured Raman spectral after quenching the fluorescence scattering show the peaks of Raman spectra at 201.62, 242.54, 288.38 and 309.77 cm-1 can be used to identify this kind of complex sulphide ore. The raw ore can be divided into three categories based on the difference of the intensity of fluorescence scattering and the ratio of fluorescence and Raman intensity. The accuracy of the classification method is further validated by the industrial tests. The findings demonstrate the close relationship between Raman spectra and the properties of sulphide ore. The proposed method, which can fast classify the sulphide ore, don't need complex chemical pretreatment before spectra collection. Therefore, this method will have important application value for improving the efficiency of mineral processing.

20.
Appl Opt ; 55(35): 10073-10077, 2016 Dec 10.
Artigo em Inglês | MEDLINE | ID: mdl-27958419

RESUMO

Theoretical analysis of terahertz (THz) generation based on difference frequency generation (DFG) by rotating fan-out poled LiNbO3 is presented. The uncertainty of poling vectors during clockwise and counterclockwise rotation of the fan-out poled LiNbO3 at the center of the crystal input face is investigated. The poling period along the propagation direction of the input pump and signal waves during clockwise and counterclockwise rotation is calculated. The dependence of THz vectors on rotation angles is numerically simulated. The relative intensities of a THz wave during clockwise rotation of the fan-out poled LiNbO3 are calculated.

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