Based on the tunable electrical and optical properties of graphene, we propose a tunable dual-band graphene-based metamaterial terahertz (THz) absorber. It successively consists of two layers of graphene-ring arrays and a metal ground plate separated by two dielectric layers. Simulation results indicate that either of the absorber’s two absorption bands can be tuned over a broad frequency range while the other absorption band remains unchanged. The designed tunable structure can readily achieve perfect absorption at nearly any two frequency bands dynamically in the THz regime with wide incidence angle and polarization independence. It can find potential applications in photodetectors, solar cells, coherent thermal emitters, and microbolometers.
A reflection-type refractive-index sensor based on the surface plasmon resonance of graphene in the mid-infrared to terahertz regime is proposed. With a simple configuration, the sensor demonstrates high flexibilities for the gas and liquid samples sensing and achieves markedly improved figures of merit of 53.64 and 36.5 RIU − 1, respectively.
All-optical pulse generation opens up a field for ultrawideband (UWB) applications. However, controllable pulse width and pulse type are still challenging. Here, we present a theoretical model and stimulated results of monocycle and doublet waveforms generation using programmable optical photon echo progress. We synthesized instantaneously monocycle and doublet waveforms by adjustment of pulse width, pulse amplitude, pulse position, and time interval of subpulses. We verified the possible application of the proposed method to design U.S. Federal Communications Commission-compliant UWB waveforms, and therefore, it may provide an avenue for waveform generation.
Spectral hole characteristics of Tm3+: YAG under cryogenic temperature was studied. Higher temperature broadens hole width exponentially while leads to hole depth decay, and spectral hole disappeared above 11.2 K. Measurement confirms the prediction.
The explicit formula solutions of cascaded third-harmonic generation (THG) process were derived based on coupled-wave equations. Conversion efficiency (CE) with the simultaneous temporal walk-off effect is theoretically investigated by use of split-step Fourier transform method, and the obtained results show that the CE of cascaded THG can be adjusted by phase mismatch, i.e., ΔkSHG and ΔkTHG. The walk-off effect can be eliminated to some extent by optimizing pump intensity. Using the multiple-grating periodically poled MgO-doped lithium niobate (MgO: PPLN), pumped by 50 fs optical parametric amplifier pulses, the CE of the cascaded THG was achieved (10.8%), and a detailed analysis was presented.
KEYWORDS: Doppler effect, Statistical analysis, Telecommunications, Signal to noise ratio, Systems modeling, Monte Carlo methods, Demodulation, Phase shift keying, Code division multiplexing, Smoothing
In this paper, a new Doppler shift estimator is proposed for Time Division-Synchronous Code Division Multiple Access (TD-SCDMA) over Rician channels. Different from other existing methods, the proposed estimator is developed based on the training sequence, i.e., "midamble" and the statistical properties of demodulation signal. By the help of a training sequence, the received line-of-sight (LOS) signal is first demodulated. Afterwards, its second-order and fourth-order statistics are calculated. Then, by exploiting these statistical results, the Doppler shift estimator is obtained. The proposed estimator employs a smoothing function with corrective factor. By optimizing the corrective factor, an accuracy solution is acquired within the Doppler shift range of possibilities. Simulation results indicate that the proposed estimator is greatly reliable and effective to estimate Doppler shift up to 400 Hz.
The temporal characteristics of a fiber ring laser are reported. The laser has a simple configuration, which contains
an Erbium-doped fiber amplifier (EDFA), a polarization controller (PC) and a coupler. The EDFA has a high saturation
power of 27dBm to provide the gain in the cavity. The PC is used to control the polarization state of light. A 99/1
coupler is used to extract the laser output. There is no filter in the cavity to confine the spectrum of the laser. The simple
laser configuration can operate in different temporal modes with different pump power. In the self mode locking state,
the repetition rate is equal to the inverse of the round trip time of the cavity. The repetition rate of the laser can be
increased by increasing the pump power of the EDFA. This self mode locking phenomena is due to the homogeneous
gain medium in the cavity, rather than the nonlinear effect in the cavity. The nonlinear effect can suppress the self mode
locking phenomena by inserting a short length highly nonlinear fiber in the laser cavity.
A novel FBG obliquity sensor is proposed. In this sensor one half of the FBG is stick to the fixed end of a
cantilever, and the other half remains at freedom state. The difference of the center reflection wavelength of the
two half of the FBG changes linearly with the sine of the obliquity from 0284 to 0393(linear degree 0.9956) . In
this sensor cross sensitivity is alSo eliminated by the different strain coherent of the two half FBG. If combined
with another sensor of the same structure, the sensing of two dimensional obliquity is achieved. The research
demonstrates that the new sensor system has the advantages of simple and compact structure, easy manipulation,
low cost, light weight, etc.
In this paper a four-channel OADM based on fiber gratings with smart temperature stable sets was implemented. It solved the conflict between the tunable capability and the temperature stability of the center wavelength. Using a cantilever beam method, the tunable range of 1nm is obtained. Within the temperature range -20°C ~+ 60°C, the center wavelength shift is less than 0.004 nm/°C and the thermal stability is 6.75 times as that without thermal stable sets. The wavelength spacing is 0.8 nm, which accords with ITU-T G.692 recommendation. 35 dB of the adjacent isolation in the OADM is obtained, which is the best one reported as we known.
In the paper, a simple and effective FBG sensor able to discriminate temperature from strain at a single point had been worked out, by sticking half of it on the fixed end of the polymer equivalent beam, leaving the other half free. The FBG was then separated into two segments with different temperature and strain coefficients, which composed the measuring matrix. The measuring sensitivities of temperature and strain of our setup are 9.58×10-6(1/°C) and 6.72×10-7(1/με) respectively. This device will have wide applications to discriminate between strain and temperature of sensing networks.
Three kinds of add-drop multiplexers (OADMs) based on fiber gratings and circulators are proposed and demonstrated. There are significant homodyne crosstalk reduction of about 24.4dB, 39.3dB and 23.6dB, 24.6dB on the dropped and added channels, respectively, for the best proposed lI-type structure and III-type structure as compared with the conventional I-type structure.
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