Numerical investigation on frequency-shifting-induced spectral compression of femtosecond solitons in highly nonlinear fiber
Introduction
Soliton self-frequency shift (SSFS) is a phenomenon where the central frequency of an optical soliton pulse undergoes a Raman-scattering-induced redshift while propagating in an optical fiber [1]. In the SSFS process, the amount of central frequency shift increases with the peak power and propagation length of input pulse. With this power-dependent feature, SSFS of femtosecond pulses enables the generation of wideband wavelength-tunable light sources for applications in coherent anti-Stokes Raman scattering (CARS) microspectroscopy [2] and material microprocessing [3], and offers an attractive optical quantization scheme for analog-to-digital conversion (ADC) [4], [5]. In these applications, narrow spectrum is often desired for high spectral resolution. For instance, a typical spectral width of excitation light source required for practical liquid-phase CARS application is over one order of magnitude smaller than that of 100-fs laser pulse [2]. Hence, spectral compression of wavelength-tunable ultrashort pulse is deemed necessary. Several spectral-compression techniques have been implemented by utilizing negative-chirped pulse propagating in optical fiber with normal dispersion [6], chirp-free pulse propagating in highly nonlinear fiber (HNLF) with anomalous dispersion [5], or soliton pulse propagating in dispersion-increasing fiber (DIF) [7], [8]. These techniques have been employed for spectral-width control of ultrashort pulses after the SSFS process.
With a careful dispersion management of photonic crystal fiber (PCF), Fedotov et al. have recently demonstrated spectral compression of 50-fs, 1270-nm pulses based on SSFS in a PCF with negative dispersion [9]. However, little attention has been paid to the impacts of fiber third-order dispersion (TOD) and initial chirp (C0) on SSFS-based spectral compression of ultrashort pulses. In this paper, we present a numerical investigation on frequency-shifting-induced spectral compression of 60-fs pulses at 1550 nm propagating in conventional HNLF. It has been found that both the fiber TOD and input pulse parameters can significantly influence the spectral-compression performance. The initial spectral width of 42.1 nm can be compressed to 16.9 nm for a fundamental soliton red-shifted by 109 nm. When the TOD effect is strengthened, greater spectral-compression factor can be obtained. The simulation results also show that the impact of initial chirp on output spectral width is negligible. This investigation provides possibilities of controlling the spectral bandwidth and center wavelength of femtosecond laser pulses with HNLF.
Section snippets
Theoretical analysis
We start with the principle of adiabatic soliton spectral compression in DIF. For a ultrashort pulse with peak power P and temporal width T (half-width at 1/e-intensity point), the soliton order N is given by [1]where γ and β2 are the nonlinear coefficient and the group-velocity dispersion (GVD) parameter of the fiber, respectively. For 0.5 < N < 1.5, the pulse adjusts its shape and width toward a fundamental soliton with N = 1 as it propagates along a fiber. Thus, according to
Simulation results and discussion
Fig. 1 shows the spectral characteristics of frequency-shifting solitons versus the propagation length in the HNLF, where initial peak power of 170 W corresponds to N = 1.0. The center wavelength of the soliton was red-shifted continuously along the propagation length, and the output spectral width decreased with the redshift. At z = 400 m, the initial spectral width of 42.1 nm can be compressed to 16.9 nm for a red-shifted soliton centered at 1659 nm, giving a spectral-compression factor of 2.5. This
Conclusions
Spectral compression of frequency-shifting solitons in HNLF with positive β3 has been numerically analyzed. The simulation results show that a spectral-compression factor of 2.5 can be achieved for a fundamental soliton red-shifted by 109 nm in a HNLF. As the soliton order N increased from 0.8 to 1.3, the spectral-compression factor slightly decreased from 2.7 to 2.2. The analysis of TOD effect on spectral compression has shown that increasing β3 can enhance spectral compression while
Acknowledgment
This work was partially supported by the National Natural Science Foundation of China (Grant Nos. 61077017, 60925019 and 60908024).
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