A tunable multi-wavelength Brillouin–erbium random fiber laser with a half-open cavity is proposed and experimentally demonstrated. In this simple laser device, stabilized 15-order Stokes lines and 3-order anti-Stokes lines can be obtained by adjusting the pumping power. By adjusting the wavelength of Brillouin pump, the random laser wavelength tuning is realized, which can be tuned to 1550.5 nm-1565.5 nm. In addition, the laser has high wavelength and power stability. The wavelength fluctuation range of 1–10 orders Stokes light is less than 0.01 nm, and the corresponding peak power fluctuation is less than 1.8 dB. The results show that the laser has the advantages of simple structure, many spectral line orders, wide tunable wavelength range and high stability, which makes it have broad application prospects in many fields, such as fiber sensing, microwave photonics, optical imaging, optical communication systems, precision metrology and so on.
A triple-wavelength erbium-doped fiber laser based on seven-core fiber (SCF) interferometer is proposed and successfully implemented. In the structure of the laser, a special structure of single-seven-single core is used to realize laser interference, which is composed of a 6 cm SCF and ordinary fiber optic patch cables fused by convex cone. In the laser with simple structure, triple-wavelength output with central wavelengths of 1530 nm, 1544 nm and 1558 nm are obtained by adjusting the polarization controller, and the optical signal-to-noise ratio (OSNR) of the triple-wavelengths is approximately 60 dB. The laser has high stability in terms of both wavelength and power. In the stability test of 150 min, the frequency drift of three wavelengths is approximately 0.4 nm, and the output wavelength peak power jitter is approximately 1.6 dB. The laser has the characteristics of simple structure and high stability, and has broad application prospects in the fields of optical fiber sensing, wavelength division multiplexing (WDM) optical communication system and microwave photonics.
The scheme of optical digital-to-analog conversion based on weighted fiber coupler is proposed. The simulation results show that when the output signal without filter, the time domain waveform is interfered by beat noise, and the function of optical digital-to-analog conversion cannot be realized. Fortunately, the beat noise is concentrated in the high frequency region which is more than 100GHz by setting the wavelength interval of the directly modulated laser array, and it can be removed effectively by the low-pass filter. However, some peak interference will appear at the time of signal alternation. When the filtering processing matching with the bandwidth of the transform signal, the peak interference can be suppressed. Finally, the optical digital-to-analog conversion with signal rate of 10Gbps and quantization accuracy of 5 bits can be realized.
Traditional biological detection methods in practical applications, such as polymerase chain reaction, fluorescence microscopy, flow cytometry and so on, are commonly limited by label needed, high cost, complex operation, low sensitivity. Fortunately, metamaterial-based terahertz (THz) biosensors have shown great potential in label-free, integrated chip and ultra-sensitive sensing due to their advantages in light collection and miniaturization. However, most of THz metamaterial biosensors are based on transmission spectrum measurement, the one-way transmission will lead to the weak interactions between analyte and THz wave, so the sensitivity will be greatly reduced. Here, we proposed a label-free dual-band THz biosensor with ultra-high sensitivity based on metamaterial absorber. The device consists of the asymmetric cross shaped metal metasurface, hollow sensing channel and a back reflector. The simulation results show that the two resonance modes are excited at 0.626 THz and 1.504 THz and the absorptivity is higher than 95%. With the increasing the refractive index of the analyte, the two absorption peaks have obvious red shift. The maximum sensitivities for mode A and mode B are up to 250 GHz/RIU and 630 GHz/RIU, respectively. By simulating the electromagnetic field distribution of the structure, the absorption sensing mechanism is discussed in detail. The proposed THz metamaterial biosensor exhibits promising applications in chemical and biological detection.
We demonstrate a multi-wavelength Q-switched random fiber laser with the erbium-doped fiber as the gain medium and the Rayleigh scattering as the randomly distributed feedback in a 6 km long single-mode fiber. Q-switched pulses and sub-pulses with different repetition frequencies were generated with the pump power as 182 mW by combining the random cavity resonances and the Q-value modulation effect induced by the stimulated Brillouin scattering and nonlinear multimode interference in the graded-index multimode fiber. The pulse repetition rate increased continuously from 79 kHz to 113 kHz, and the corresponding pulse width exponentially decreased with the continuously increasing pump power, and a shortest pulse width of 1.85 μs was obtained. At the same time, a multi-wavelength spectrum with the equally spaced interval and an optical signal-to-noise ratio of ~31 dB was achieved. This laser has the unique properties different from the conventional resonant cavity lasers, which makes it a very promising light source in optical communication, imaging, and sensing applications.
Based on the insulator to metal transition (IMT) characteristics of vanadium dioxide (VO2), a highly active tunable terahertz (THz) metasurface resonator patterned VO2 cross structures is proposed. The simulation results show that the transmission of the proposed structure at low temperature is higher than 0.7 because of the VO2 in its insulation phase. With the increasing temperature, a strong transmission dip appears at 0.76 THz due to the metal phase of VO2, which indicates the onset of a new resonant mode. The maximum tunable range of transmission for THz wave is 0.06-0.86. Simultaneously, we also demonstrate that the resonant responses of the heating and cooling processes are quite different and the heating process is more sensitive. The transition temperature is close to room temperature and the device can achieve good modulation effect on both TE and TM waves. The TE and TM mode resonances are selective by altering the cross arms, which is almost impossible for other THz devices. So, the resonator can greatly promote practical applications of THz functional devices such as filters, sensors, modulators, and switches.
The statistical characteristics of quantum noise stream cipher system is investigated. Simulation results shows that with the variance of the noise σ2 is changed from 0 to 0.3, the statistical parameter of signals decreases to the floor. When the variance of the noise is more than noise masking threshold, the statistical characteristic of signals is completely submerged in noise. Moreover, when the variance of the noise is equal to {0.001, 0.01}, PB =1 is unchanged with the increase of modulation order. Furthermore, when the variance of the noise is equal to {0.1, 0.2}, the correct probability for Bob PB decreases to a fixed value which is 0.99 and 0.79, respectively. Correspondingly, the correct probability for Eve PE decreases to 0 with the modulation order increasing. And the channel advantage for main channel over wiretap channel can be formed by combining proper modulation order with noise.
An efficient terahertz (THz) metamaterial linear polarization converter is proposed. The structure consists of three layers of metal-dielectric-metal, including an anisotropic super-surface based on bimetallic arc structure as the top layer and a dielectric layer and a metal plate as middle layer and bottom layer, separately. The numerical analysis results show that broadband polarization conversion of a linearly polarized wave in the terahertz band is realized using the structure proposed. The polarization conversion ratio of the linearly polarized wave is more than 90% across a wide frequency range from 1.21 THz to 2.82 THz. The ratio of bandwidth to central frequency can reach 80%. The mechanism of high polarization conversion rate and broadband is analyzed. At the same time, the dependence of the polarization conversion rate of the structure on the incident angle is studied. The results show that the structure has good polarization conversion performance in the incident angle range of 0-60°. Compared with previous designs, the polarization converter has not only simple structure, but also wide bandwidth. It has potential application value in polarization modulation of terahertz wave.
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