Abstract:A 473-nm-laser-tuned whispering gallery mode (WGM) silica microbubble resonator integrated with iron oxide particles is demonstrated in this paper. Owing to the photo-induced thermal effect, the WGM resonance wavelength could be tuned by adjusting laser power density of the illuminating light absorbed by the iron oxide particles. A wavelength tuning sensitivity of 0.03 nm/(mW.mm-2) and a tuning range of 0.18 nm are experimentally achieved. Moreover, the influence of ambient temperature on the WGM spectral characteristics is experimentally studied, and a silica-microbubble-based reference scheme is demonstrated to compensate for the temperature-caused resonance wavelength variation. The proposed laser-tuned microresonator has great potential in optical modulation and high-precision optical filtering applications.

Abstract:Microwave-to-optical phase synchronization techniques have attracted growing research interests in recent years. Here, we demonstrate tight, real-time phase synchronization of an optical frequency comb to a rubidium atomic clock. A detailed mathematical model of the phase locking system is developed to optimize its built-in parameters. Based on the model, we fabricate a phase locking circuit with high integration. Once synchronized, the fractional frequency instability of the repetition rate agrees to 6.35×10-12 at 1 s and the standard deviation is 1.5 mHz, which indicates the phase synchronization system can implement high-precision stabilization. This integrated stable laser comb should enable a wide range of applications beyond the laboratory.

Abstract:In order to improve radio over fiber (RoF) system performance and reduce system complexity, a novel RoF system scheme with continuously tunable optical carrier-to-sideband ratio (OCSR) was proposed. Using polarization multiplexing technology, the OCSR can be tuned from –36 dB to 26 dB by adjusting a polarization controller (PC). The effects of polarization angle of PC, modulation index and extinction ratio on OCSR were studied by theoretical analysis and experimental simulation, in which the results of theoretical calculation and experimental simulation are in good agreement. It is found that the receiver sensitivity can be improved 7.8 dB by tuning the OCSR from 10.41 dB to 0 dB. The power penalty is 0.07 dB for the 0 dB OCSR after 40 km transmission, which means the impact from the fiber link can be ignored.

Abstract:In this paper, a frequency-domain mathematical model is deduced for polarization-division multiplexing (PDM) coherent optical orthogonal frequency division multiplexing (CO-OFDM) system with transceiver in-phase and quadrature (IQ) imbalances. A novel training symbol structure is designed in which the mirror-subcarriers of pilot subcarriers were modulated with zero signal so that the channel distortion with transceiver IQ imbalances can be estimated. It proves that the channel distortion and transmitter IQ imbalances cannot be separated using the training symbols; therefore, the channel equalization method was used to recover the signals. Simulation results show that at 2 dB transmitter amplitude imbalances, 15°transmitter phase imbalances, 100 Gbit/s transmission rate, 1 040 km standard signal-mode fiber link, and 1×10-3bit error rate, 23.5 dB optical signal-to-noise ratio (OSNR) is necessary for the proposed method. However, the compared schemes cannot achieve effective transmission.

Abstract:The supercontinuum (SC) generation (SCG) based on the active nonlinear fiber has attracted much attention due to its advantages in the energy amplification and spectral broadening of the output pulses. The active step-index single-mode fiber is proposed for the SCG. A flat normal-dispersion profile ranging from 1 240 nm to 2 550 nm can be obtained by tuning the germanium (Ge) concentrationand core size, where the values of the dispersion slope are between −0.01 ps/nm2/km and 0.05 ps/nm2/km. Furthermore, the output pulse from the 5 m Er-doped normal-dispersion fiber (NDF) with g0=9 m-1 has a peak power of 2.8 kW and a 20 dB spectral bandwidth of 507 nm ranging from 1 262 nm to 1 769 nm by using the 1 550 nm and 1 ps Gaussian pulse with a peak power of 10 kW as the pumping source.

Abstract:In order to solve the problem that the traditional frequency domain least mean square (FD-LMS) algorithm will lose efficacy with the increase of differential mode group delay (DMGD) when the algorithm is used for demultiplexing of the 6×6 mode division multiplexing (MDM) system, an improved FD-LMS demultiplexing algorithm is proposed. By improving the error signal calculation method, the convergence performance of the output signal of the equalization filter is improved, and the steady-state error of the algorithm is reduced. Besides, the equalization performance of the traditional FD-LMS algorithm is compared with the improved FD-LMS algorithm. Simulation results show that the improved FD-LMS algorithm has great advantage over the traditional FD-LMS algorithm in demultiplexing performance on the premise that the computation complexity does not significantly increase. The optical signal to noise ratio (OSNR) penalty of the improved FD-LMS algorithm is 2.6 dB lower than that of traditional FD-LMS algorithm at a transmission distance of 80 km with DMGD is 50 ps/km.

Abstract:A dispersion-compensating square-lattice photonic crystal fiber for broadband compensation which covers the S, C and L communication bands, i.e. wavelength ranging from 1 460 nm to 1 625 nm is proposed in this paper. Theoretically, it is shown a negative dispersion coefficient of about −595 ps/(nm.km) to −1 288 ps/(nm.km) over S to L bands and −975 ps/(nm.km) at the operating wavelength 1 550 nm. The relative dispersion slope is perfectly matched to that of conventional single-mode fiber of about 0.003 6 nm-1. Besides the proposed photonic crystal fiber shows the large non-linear coefficient of 61.88 W/km at the operating wavelength of 1 550 nm. Moreover, the variation of structural parameters is also studied and discussed here.

Abstract:In order to simultaneously measure the displacements of multiple targets, the laser self-mixing interference (SMI) measurement method based on variational mode decomposition (VMD) is proposed. The SMI signal containing the motion information of the external targets is detected by a photodiode. The VMD can non-recursively decompose the mixed SMI signal into SMI signals corresponding to different external targets. The displacement signals can be reconstructed by fringe counting method and interpolation method. The experimental results show that the displacement signals of different external targets can be reconstructed with half wavelength accuracy, which verifies the correctness and feasibility of this method. This method can improve the measurement efficiency, which offers an effective method for the multi-channel displacement measurement.

Abstract:A precise and noninvasive method for the size and shape measurement of gold nanorods (GNRs) has been proposed based on depolarized dynamic light scattering (DDLS). A home-made DDLS apparatus has been established. By applying depolarized optical path with precise alignment method, the signal-to-noise ratio (SNR) of this apparatus is highly improved. GNRs with three different diameter and length has been precisely measured by using DDLS method as well as scanning electron microscopy (SEM). The thickness of adsorption layer of cetyltrimethylammonium bromide (CTAB) in solution has been taken into consideration. Results show that size measurement of GNRs by using DDLS method agrees very well with that by using SEM. In addition, it is shown that the extinction spectroscopy strongly limited the application of DDLS method by affecting the effective scattering light intensity. Proper laser wavelength should be chosen before the application of this method.

Abstract:The concentrations of CO2 were measured by the supercontinuum laser at normal temperature and variable temperature accurately in this paper. The absorption spectra of CO2 at different concentrations (1.2%—9.0%) were measured in the wavelength range of 1 425—1 445 nm under the optical path of 26.4 m at 293 K and 1 atm. The experimental results showed that the positions of the primary and secondary absorption peaks (1 432 nm, 1 437 nm) were consistent with the HITRAN database. A linear model of concentration and signal intensity at 1 432 nm was established. The maximum relative error of the concentration measurement was 3.3%. The line intensities of 99.9% CO2 in the 1 425—1 445 nm at different temperatures (298—373 K, interval of 15 K) were measured. The influence of temperature changes on the concentration measurement result was corrected and the relative error of the concentration measurement was reduced to 1.4%. Finally, the source of the uncertainty of the entire spectrum measurement system was analyzed and evaluated. This paper demonstrate that the supercontinuum laser can achieve the long-distance measurement of the CO2 under normal temperature or variable temperature environment accurately, which provide an important reference for the long-distance gas detection on site and simultaneous detection of multi-component gases.

Abstract:Experimental results of thermal lens measurements for Tris(2,2'-bipyridyl)iron(II) tetrafluoroborate solution are reported using the dual beam technique. The temporal evaluation of thermal lens was observed at input pump power of I=47 mW. The thermal lens effect was observed with increasing the pump power intensity as well. The evolution time of thermal lens and the fitting of the experimental transient curve were useful to evaluate the parameters of the Tris(2,2'-bipyridyl)iron(II) tetrafluoroborate solution.

Abstract:Terahertz (THz) pulse imaging can be used for biomedicine, material, and food security. However, image quality is lower attributed to the THz time-domain spectroscopy system. Such as, noises and lower space resolution are presented in some THz images. To improve the THz image quality, we proposed a novel method which combining the simulation evolutionary with the symmetric fourth order partial differential equation. The image edge is first detected by using the simulation evolutionary. Then the symmetric fourth order partial differential equation (PDE) is applied on the non-edge image for noise reduction. Finally, the de-noise image is combined with the edge image obtained from the edge detection step. Experiments on four different THz images prove that the proposed method can preserve the edge information during noise reduction.