Frequency response of a continuously tuning narrow-band optical filter based on stimulated Brillouin scattering.

Tunable narrow-band optical filter (TNOF) based on stimulated Brillouin scattering (SBS) has been applied in various applications such as microwave photonics and a high-resolution optical spectrum analyzer (OSA). While the frequency response of a SBS-based filter has always been an important characteristic in the reported studies, few have addressed the issue of the filter response under a continuously tuning condition. When the tuning speed is too fast, the filter response will change and cause spectral distortion. In this paper, the frequency response of SBS-TNOF under a wavelength-swept pump (i.e., continuously tuning) condition is investigated and modeled. Experimental results are in good agreement with the theoretical analysis and verify that the broadening of the SBS-TNOF response is induced by the pump wavelength difference along different positions of the fiber, which can be explained as convolution with broadband pump as well. Based on the widely used Richardson-Lucy deconvolution algorithm and proposed SBS-TNOF response model, the distorted responses are successfully reconstructed and the sweep speed dependency is almost eliminated. Commonly used on-off keying signal is tested using the proposed reconstruction method to assess its performance in the SBS-OSA. Both the overall profile and the detail of the signal spectra are significantly recovered, and the quantitatively evaluation illustrates that the feasible sweep speed can be improved from ∼45 nm/s to over 95 nm/s.

Ultra-narrow-linewidth measurement utilizing dual-parameter acquisition through a partially coherent light interference.

Laser linewidths of the order of 100 Hz are challenging to measure with existing technology. We propose a simple, efficient method to measure ultra-narrow linewidths using dual-parameter acquisition based on partially coherent light interference. The linewidth is obtained using two parameters that are easily extracted from the power spectrum. This method reduces the influence of 1/f noise by utilizing a kilometer-order-length delay fiber and is independent of the fiber-length error for a general situation. Simulation results show that, for a length error less than 10%, the total linewidth measurement error is less than 0.3%. Experimental results confirm the feasibility and superior performance of this method.

Distributed multi-parameter sensing utilizing Brillouin frequency shifts contributed by multiple acoustic modes in SSMF.

A multi-parameter optical-fiber sensor, which is based on multiple acoustic modes in stimulated Brillouin scattering (SBS) effect, for distributed measurement of temperature and strain utilizing standard single-mode fiber (SSMF) is proposed and experimentally demonstrated. By manipulating doping level and refractive index profile of the fiber, the properties of Brillouin gain spectrum (BGS) related to the guided optical and acoustic modes are analyzed. The simulated results indicate that multiple acoustic modes can be excited in single-mode fiber (SMF) and the BGS is composed of multiple peaks corresponding to multiple acoustic modes. Moreover, the temperature and strain sensitivities of different acoustic modes are unequal and the capability of discriminative measurement between temperature and strain can be proved. Simultaneously, the mode field diameter, the dispersion parameter, and the cutoff wavelength are calculated and the results show that parts of SSMF can be used for multi-parameter measurement. However, the accuracy of measurement is varied with the fiber structure parameters. Consequently, in experimental section, two different SSMFs are put into test and both have multiple-peak BGSs although the BGSs show a great difference to each other. The discrimination of temperature and strain is successfully demonstrated by analyzing the coefficients of the Brillouin frequency shifts introduced by different acoustic modes. In the fiber which has a better measurement result, the sensitivities of the fundamental acoustic mode are 1.19 MHz/°C and 62.28 kHz/µÎµ with an accuracy of 0.98 °C and 19.6 µÎµ in 20 km sensing range.

Investigation of channel model for weakly coupled multicore fiber.

We investigate the evolution of the decorrelation bandwidth of intercore crosstalk (IC-XT) based on the modified mode-coupled equations (MCEs) in homogeneous weakly coupled multicore fibers (WC-MCFs). The modified MCEs are numerically solved by combining the fourth order Runge-Kutta method with the compound Simpson integral method. It can be theoretically and numerically observed that the decorrelation bandwidth of IC-XT decreases with transmission distance by fractional linear function. The evolution rule of IC-XT's decorrelation bandwidth is further confirmed by experiments, which can be used as an evaluation criterion for the channel model. Finally, we propose a new channel model with the coupling matrix of IC-XT generated directly from the phase transfer function (PTF), which is in good agreement with the above evaluation criterion. We believe the proposed channel model can provide a good simulation platform for homogeneous WC-MCF based communication systems.

Polarization- and wavelength-independent SBS-based filters for high resolution optical spectrum measurement.

A polarization-independent narrow passband optical filter with a FWHM bandwidth of ~10 MHz based on stimulated Brillouin scattering (SBS) effect is proposed and experimentally demonstrated. By optimizing the parameters of a depolarizer consisting of a power-splitting Mach-Zehnder interferometer (MZI) structure, degree of polarization (DOP) of the pump source can be reduced to less than 5%. Consequently frequency response of the SBS-based filter remains almost unchanged when the state of polarization (SOP) of the input signal varies and the corresponding polarization-dependent gain (PDG) is less than 1 dB. And the experimental results also indicate that the proposed filter can realize the wavelength-independent operation over a range of ~30 nm. Furthermore, high resolution optical spectrum measurement utilizing this novel filter is investigated. For an input signal with a frequency interval of 30 MHz in arbitrary SOP, spectral details can be clearly observed and the power stability of the measured spectra can be improved from 3~5 dB to less than 0.5 dB, which benefitting from the polarization independent operation of the forementioned filter. Moreover, considering the resolution degradation of the measured spectra induced by the frequency shift in the depolarizer, a sweep speed of 5 nm/s for the pump source and a delayed fiber length of 2000 m in the MZI should be used to guarantee the desired high spectral resolution of ~10 MHz.

High-input dynamic range and selectivity stimulated Brillouin scattering-based microwave photonic filter utilizing a dual-stage scheme.

A high-input dynamic range and selectivity stimulated Brillouin scattering (SBS)-based microwave photonic filter (MPF) is proposed and experimentally demonstrated. By utilizing a pump-splitting dual-stage scheme to mitigate the gain saturation, an input dynamic range of >40 dB and a selectivity of >49 dB are achieved simultaneously for the first time, to the best of our knowledge. The bandwidth is reconfigurable from 50 MHz to 200 MHz by tailoring the Brillouin pump precisely. During the bandwidth turning, the passband ripple is maintained <1.3 dB, and the 20-dB shape factor is 1.2 for the 200-MHz case. The superior performance makes the proposed SBS-MPF one of the best candidates for microwave photonics applications.

Electro-optic chaotic system based on the reverse-time chaos theory and a nonlinear hybrid feedback loop.

A novel electro-optic chaos source is proposed on the basis of the reverse-time chaos theory and an analog-digital hybrid feedback loop. The analog output of the system can be determined by the numeric states of shift registers, which makes the system robust and easy to control. The dynamical properties as well as the complexity dependence on the feedback parameters are investigated in detail. The correlation characteristics of the system are also studied. Two improving strategies which were established in digital field and analog field are proposed to conceal the time-delay signature. The proposed scheme has the potential to be used in radar and optical secure communication systems.

All-optical spectral linewidth reduction of lasers for coherent optical communication.

We propose and experimentally demonstrate an effective spectral linewidth suppression scheme for a commercial tunable laser source. By using a long-cavity, narrow-bandwidth stimulated Brillouin scattering filter and multifrequency-selection mechanism simultaneously, a single-longitudinal-mode laser output with linewidth suppression from ~3 MHz to less than 20 kHz is successfully achieved in the whole C-band. Meanwhile, insertion loss of ~5 dB and a side-mode suppression ratio improvement of ~20 dB at a typical output wavelength of 1552.520 nm are obtained. The scheme is experimentally verified to be wavelength independent in the C-band and characterized with good performance of wavelength and power stability.

Downhill simplex algorithm based approach to holey fiber design for tunable fiber parametric wavelength converters.

We present a new approach to the design of the holey fibers that have ultra-high nonlinearity and dispersion properties optimized for tunable fiber parametric wavelength converters based on degenerated four wave mixing. This hybrid approach combines downhill simplex algorithms with four wave mixing modeling. Exploiting the relations between fiber properties and the converter's characteristics, this method is not only much faster than other methods proposed before but also enables an inverse design of the holey fibers according to the pre-set device characteristics, like conversion gain, tuning range, fiber length and pump power. We then investigate the sensitivity of these characteristics to the small variations in the fiber structural parameters and find adjusting the pump power can to some extent mitigate the impact of the fabrication errors.