查看更多>>摘要：A tilted fiber Bragg grating (TFBG) hydrogen sensor based on Pd/Au composite nanofilms was proposed. Pd and Au films with thickness of 25 nm and 35 nm were deposited on the surface of TFBG by magnetron sputtering method. Pd is a face centered cubic structure, which is capable of adsorbing hydrogen atoms. Accordingly, the effective refractive index of Pd/Au composite film changes with the variations of hydrogen concentrations, which will change the intensities of cladding modes resonant wavelength in TFBG transmission spectrum. The experimental results illustrated that when the hydrogen concentration changes by 1.02%, the intensity of cladding mode resonant wavelength (1558.4 nm for instance) decreases by 1.628 dB, and the hydrogen concentration sensitivity of 1.597 dB/% was obtained, which is almost 16 times higher than that of the current similar hydrogen sensor. The average response and recovery time were 37 s and 49 s, respectively. In addition, the proposed sensor can be made inherently temperature-insensitive by referencing all wavelengths to the wavelength of the core mode resonance of the grating, which is isolated from the fiber surroundings.
查看更多>>摘要：Metasurfaces exhibit promising capacities in the manipulation of electromagnetic (EM) waves, as to diverse dimensions like amplitude, phase, and polarization. Despite many methods have been used to improve regulatory capabilities of EM waves, the efficiency and working bandwidth has yet to be developed. In this paper, an ultra-wideband and high-efficiency reflective polarization converter metasurface (PCM) is proposed. Both theoretical analysis and simulation results show that the polarization conversion ratio (PCR) is above 89% in the ultra-wideband from 2.28 THz to 6.75 THz, and the relative bandwidth reaches 99%. Moreover, the multi-bit digital coding metasurface elements are designed by using Pancharatnam-Berry (PB) phase principle. The coding metasurface arrays are designed by using different coding sequences, which can be used to realize the free adjustment of scattering angle of the reflected EM waves. In addition, the radar cross section (RCS) reduction performances of the designed metasurface array antennas based on different coding elements are studied. The results present that it can significantly reduce the RCS by -20 dB over a wide frequency range from 2.1 THz to 6.9 THz. The proposed metasurface has great application potential in manipulation of terahertz wave and contributes to RCS reduction.
查看更多>>摘要：We demonstrate the temperature distribution and 2.79 mu m laser performance in a high-quality Er:YSGG single crystal fiber (SCF) with size of Phi 0.85x8 mm(3) for the first time. Compared with 2x2x8 mm(3) Er:YSGG bulk crystal, the simulation results show that the center maximum temperature of Er:YSGG SCF is much lower, and the temperature rising and total thermal deformation are evidently reduced, indicating that the SCF possesses more efficient thermal dissipation. Under 970 nm laser diode pumping, a maximum output power of 636 mW with slope efficiency of 20.2% is realized. More importantly, a high beam quality factors M-x(2)/M-y(2) of 1.28/1.29 is obtained, which is close to the fundamental TEM00 mode, suggesting that the Er:YSGG SCF can act as a promising candidate for mid-infrared laser seed source.
查看更多>>摘要：This paper proposes a theoretical scheme to synthesize white laser source based on the cascaded sum frequency generation (CSFG) model with stimulated Raman adiabatic passage (STIRAP). In the CSFG model, the signal laser, intermediate laser and output laser correspond to red, green and blue (RGB) lasers respectively. The results show that in general the red laser is completely converted to the blue laser, and there is no apparent intermediate green laser. Moreover, under the counterintuitive order, the pump parameters have a wider adjustment range of system conversion efficiency than the coupling delay parameters. We synthesize the white laser source through the following steps. First, we change the pump intensity to roughly adjust the conversion efficiency of the system, and then vary the coupling delay parameters to fine-tune RGB lasers output power ratio. Compared with the scheme based on frequency conversion with Stark-chirped rapid adiabatic passage to synthesize white laser source, the RGB lasers output of the new scheme can achieve better stability. The results of the investigation can provide a theoretical foundation for the research of novel optical devices.
查看更多>>摘要：In this paper, we report a high-power single-frequency all-fiber amplifier at 1064 nm based on cascaded hybrid active fibers. The cascaded gain fiber design, where a piece of active fiber with lower dopant concentration is spliced before the highly doped gain fiber, is proposed to effectively alleviate the thermal loads of high-power amplifier. By virtue of hybrid pump at 976 nm and 915 nm to increase the threshold of mode instability (MI) effect, an output power of 435 W is achieved from the Yb3+-doped fiber with large core diameter of 30 mu m. The beam quality is well maintained with a M2 of 1.40 even at the maximum laser power.
查看更多>>摘要：A novel optical fiber integrated optoelectrode was proposed and miniatured photoelectrochemical (PEC) sensor was realized. In the structure, transparent conductive oxide indium-tin-oxide (ITO) film and photocatalytic material are coated outside of one end of the optical fiber. This tiny optoelectrode first simultaneously realizes the light-guiding ability and the microelectrode function of electroanalysis. Specifically, the detection of trace nitrite is realized by PEC process of the optical fiber/ITO/BiOI optoelectrode. The integrated optoelectrode shows remarkable electrochemical performance, and the sensor has notable PEC detection ability. At the optimal photosensitizer concentration, the optoelectrode sensor has a linear range of 1 nM-1 mu M and a low detection limit of 5 x 10(-10) M for the nitrite sensing. Due to the advantages of integration, fast detection, high sensitivity, and simple operation, the proposed optoelectrode has great potentials in a variety of fields, such as all-fiber integrated devices and various biochemical microanalyses systems.
查看更多>>摘要：The structure optimization for a binary reconfigurable true time delay lines (RTTDL) based on the Mach- Zehnder interferometer (MZI) switches is proposed. Firstly, the number of the variable optical attenuator (VOA), which is generally used to make up for the imperfectness of the MZI switch and improves the RTTDL 's flatness of broadband amplitude response and the delay accuracy, is minimized. Meanwhile, by adopting a pair of photoelectric detectors (PD) between every two MZI switches for power monitoring, the initialization time can be greatly reduced owing to the realization of the parallel initialization. On the other hand, with the help of the PDs, a continuously adjustable delay scheme is put forward by adjusting the allocation ratio of the last MZI switch. The proof-of-principle calculations are carried out to verify the optimization. The results show that the proposed optimization not only reduces the power consumption, the optical link loss and the integration volume caused by the VOAs, but also solves the problem that the binary RTTDL has only discrete delay states.
查看更多>>摘要：A novel photonic analog-to-digital conversion (ADC) scheme based on time-to-frequency mapping is proposed and experimentally demonstrated. In the approach, the amplitude of the input analog signal is firstly converted into a time-varying signal by pulse position modulation, then the time variation is mapped into the optical spectrum with the help of time-to-frequency mapping offered by the chirped optical pulse. The optical spectral information can be directly encoded by using interleaving filters and a binary receiver array, which outputs the digital records of the input analog signal. Compared with the existing photonic ADC schemes, the proposed approach greatly simplifies the system configuration and uses only one modulator with a dispersive element to achieve 2(N) quantization levels. Meanwhile, it also avoids the limitation of optical nonlinearity and realizes the linear mapping between the analog voltage and optical spectrum. A proof-of-concept experiment of the proposed approach based on time-to-frequency mapping is successfully carried out to verify the feasibility of the scheme. In addition, we also discuss the performance limitations induced by the frequency ambiguity in chirped spectra and the timing jitter in sampling.
查看更多>>摘要：A highly tunable design for obtaining double resonance enhanced surface-enhanced Raman spectroscopy (SERS) substrates with ultrasmall vertical nanogaps is proposed by utilizing the plasmonic Fano resonances resulted from the strong coupling between the broad gap plasmon mode of the Au nanocavity and the narrow surface plasmons polaritons. A sandwiched plasmonic nanocavity of Au nanostrip grating upon a gold film separated by a dielectric spacer of nanopillar with ultrasmall nanogap is designed and simulated to obtain the double resonances at the laser excitation and Raman scattering frequencies for Raman signals detection. We demonstrate that by tuning the geometrical and material parameters of the proposed nanocavity, the double resonance frequencies and the SERS enhancement can be readily controlled. Our proposed plasmonic nanocavity exhibits very high SERS enhancement factor up to 1.1x10(10), which are promising for detect specific signals of sport doping drugs and medical diagnoses.
NSTL
Elsevier