Bandwidth Of Gain Spectrum Research Articles

Bandwidth tunable microwave photonic filter based on digital and analog modulation

Abstract A bandwidth tunable microwave photonic filter based on digital and analog modulation is proposed and experimentally demonstrated. The digital modulation is used to broaden the effective gain spectrum and the analog modulation is to get optical lines. By changing the symbol rate of data pattern, the bandwidth is tunable from 50 MHz to 700 MHz. The interval of optical lines is set according to the bandwidth of gain spectrum which is related to the symbol rate. Several times of bandwidth increase are achieved compared to a single analog modulation and the selectivity of the response is increased by 3.7 dB compared to a single digital modulation.

Brillouin gain bandwidth reduction in Brillouin optical time domain analyzers.

This paper proposes a novel method based on differential π-phase-shift long-pulse-width pair to narrow the Brillouin gain spectrum for improving the frequency accuracy in Brillouin optical time-domain analysis (BOTDA) system. This is the first approach to reduce the bandwidth of Brillouin gain spectrum for distributed sensing to the best of our knowledge. The temporal and spectral Brillouin responses for the proposal are analytically solved, and the key parameters such as pulse width and the length of π-phase-shift pulse section are investigated. Theoretical analysis and experimental results demonstrate that the proposal could achieve 17 MHz Brillouin gain spectrum bandwidth and a fixed spatial resolution of 2.5 m simultaneously, without signal-to-noise ratio penalty. This Brillouin gain spectrum is 3 times narrower than that obtained using standard single-pulse based BOTDA method with same spatial resolution, resulting in3times frequency accuracy improvement. Furthermore, such a significantly narrowed Brillouin gain spectrum provides more tolerance to the small temperature change when hot spots are introduced, giving rise to the sharper rising/falling edge of the Brillouin frequency shift profile along the sensing fiber. This way, more precise temperature/strain measurement can be obtained.

Nearly-octave wavelength tuning of a continuous wave fiber laser

The wavelength tunability of conventional fiber lasers are limited by the bandwidth of gain spectrum and the tunability of feedback mechanism. Here a fiber laser which is continuously tunable from 1 to 1.9 μm is reported. It is a random distributed feedback Raman fiber laser, pumped by a tunable Yb doped fiber laser. The ultra-wide wavelength tunability is enabled by the unique property of random distributed feedback Raman fiber laser that both stimulated Raman scattering gain and Rayleigh scattering feedback are available at any wavelength. The dispersion property of the gain fiber is used to control the spectral purity of the laser output.

Modulation instability in lossy silicon optical waveguide

Taking into account the small linear loss of silicon-on-insulator(SOI) waveguide, conditions and gain spectra of modulation instability(MI) induced by combined effects of self-phase modulation and waveguide dispersions are investigated. The impacts of various parameters to gain spectra of MI are analyzed theoretically. Results show that strong MI takes place even in the existence of low light power. The MI peak gain is 2 to 3 orders of magnitude larger than that achieved in optical fibers with the same light power. The linear loss of waveguide impacts gain spectra of MI, even within ultra-short propagation distance. The peak gain, peak gain frequency and bandwidth of gain spectra decrease to 66.828%, 41.683% and 41.6879% of their maxima at propagation distance z = 5mm, respectively.

Modulation instability in silicon optical waveguide considering linear loss

Taking into account the linear loss of silicon-on-insulator (SOI) waveguide, modulation instability (MI) induced by combined effects of self-phase modulation and waveguide dispersions is investigated. The impacts of various parameters to gain spectra of MI are analyzed theoretically, and direct numerical simulation of nonlinear Schroedinger equation is performed as well. Results show that strong MI takes place even in the existence of low light power. The linear loss of waveguide obviously impacts gain spectra of MI, even within ultra-short propagation distance. The peak gain frequency and bandwidth of gain spectra decrease to 41.683% and 41.6879% of their maximum at propagation distance z = 5 mm, respectively.

Modulation instability of dispersion-shifted fiber couplers

The nonlinear coupled-mode equations are rewritten by even and odd modes. We study modulation instability (MI) of dispersion-shifted fiber couplers when either even or odd mode is launched alone by using zero-dispersion wavelength relatively long (quasi-cw) pulses. The result shows that there are new types of MI in both the normal-dispersion and the anomalous-dispersion regimes. MI is concerned with forth-order dispersion and has no relation with third-order dispersion. Quasi-cw can be changed into pulses array under certain conditions. We can extract super short pulse from this. Furthermore, the bandwidth of gain spectra widens and its strength accretes as the input power increases.

Experimental and Theoretical Analysis of NondegenerateUltrabroadband Chirped Pulse Optical Parametric Amplification

Experimental investigations of nondegenerate ultrabroadband chirpedpulse optical parametric amplification have been carried out. Thegeneral mathematical expressions for evaluatingparametric bandwidth, gain and gain bandwidth for arbitrary three-wavemixing parametric amplifiers are presented. In our experiments, atype-I noncollinear phase-matched optical parametric amplifier based onlithium triborate, which was pumped by a 5-ns second harmonic pulsesfrom a Q-switched Nd:YAG operating at 10 Hz, seeded by a 14-fsTi:sapphire laser at 800 nm, was presented. The 0.85 nJ energy of inputchirped signal pulse with 57-FWHM has been amplified to 3.1 μJ at pumpintensity 3 GW/cm2, the corresponding parametric gainreached 3.6×103, the 53 nm-FWHM gain spectrum bandwidth ofoutput signal has been obtained. The large gain and broad gainbandwidth, which have been confirmed experimentally, provide greatpotentials to amplify efficiently the broad bandwidth femtosecond lightpulses to generate new extremes in power, intensity, and pulse durationusing optical parametric chirped pulse amplifiers pumped by powerfulnanosecond systems.

Modulation instability of femtosecond pulses in dispersion-decreasing fibers

Modulation instability (MI) to generate femtosecond pulse in dispersion-decreasing fibers (DDF) is investigated. Higher-order dispersion effect and higher-order nonlinear effects are taken into account when continuous wave or quasi-continuous wave evolves into sub-picosecond or femtosecond pulses by MI in optical fibers. Our research results show that the gain spectrum of the DDF is much wider than that in conventional fibers. The third-order dispersion effect has no contribution to gain spectrum, self-steepening effect reduces the maximum value and gain bandwidth of gain spectrum, and Raman self-scattering effect widens the extent where MI occurs.