Enhanced soliton self-frequency shift of ultrashort light pulses

Photonic-crystal fibers are used to study scenarios of soliton self-frequency shift for laser pulses with initial pulse lengths much less than the Raman-mode period of the fiber material. A typical frequency shift of subnanojoule Ti: sapphire-laser pulses with an initial duration of about 30 fs transmitted through a fiber with a core diameter of about 1.6 μm and a length of about 7 cm exceeds 100 THz. The rate of soliton self-frequency shift is radically increased by reducing the initial pulse width.     

Adaptive control of femtosecond soliton self-frequency shift in fibers

The Raman shift of a subpicosecond soliton in 100 m of fiber is controlled adaptively by pulse shaping before launching into the fiber. We use a deformable-mirror-based shaper to control the spectral phase of the input pulse.     

Photonic analog-to-digital converter using soliton self-frequency shift and interleaving spectral filters

We propose a novel ultrafast photonic analog-to-digital converter that uses the soliton self-frequency shift in an optical fiber as an optical power-to-frequency conversion mechanism and a set of interleaving spectral filters as the optical comparators. Our method does all the signal processing in the optical domain and requires binary receivers in only the electronic domain. In contrast to the usual exponential scaling, the simultaneous binary search architecture that we propose results in a flash analog-to-digital converter with remarkable linear scaling between the number of comparators and the number of bits resolved.     

Soliton self-frequency shift decelerated by self-steepening

Self-steepening of ultrashort light pulses is shown to reduce the soliton self-frequency shift (SSFS) induced by the Raman effect in an optical fiber. We derive an analytical expression for the SSFS that conserves the number of photons and allows the SSFS to be calculated for arbitrary frequency profiles of fiber dispersion and Raman gain without a numerical solution of the pulse evolution equation. The accuracy of this analytical approach to SSFS calculation is tested by numerical simulations based on the generalized nonlinear Schr?dinger equation.     

Broadband self-pulsating fiber laser based on soliton self-frequency shift and regenerative self-phase modulation

We demonstrate experimentally and numerically the operation of a self-pulsating fiber laser based on the cascaded effects of soliton self-frequency shift and self-phase modulation spectral broadening. The combination of those two effects triggers and sustains the propagation of picosecond pulses in the cavity. At one of the outputs, the laser emits a supercontinuum with spectral width in excess of 150 nm at the repetition rate of 95 kHz.     

Widely tunable femtosecond soliton pulse generation at a 10-GHz repetition rate by use of the soliton self-frequency shift in photonic crystal fiber

We demonstrate a soliton self-frequency shift of approximately 120 nm in a fiber with 1.56-microm pulses generated at a 10-GHz repetition rate by an actively mode-locked laser. A highly nonlinear photonic crystal fiber with a length of only 12.6 m and a nonlinear coefficient of 62 W(-1) km(-1) is used to achieve such broadband operation. The wavelengths of the resulting sub-300-fs solitons can be tuned effectively by adjusting the input power. The maximum output power of the solitons exceeds 200 mW.     

Demonstration of soliton self-frequency shift below 1300 nm in higher-order mode, solid silica-based fiber

We demonstrate soliton self-frequency shift of more than 12% of the optical frequency in a higher-order mode solid, silica-based fiber below 1300nm. This new class of fiber shows great promise for supporting Raman-shifted solitons below 1300nm in intermediate energy regimes of 1 to 10nJ that cannot be reached by index-guided photonic crystal fibers or air-core photonic bandgap fibers. By changing the input pulse energy of 200fs pulses from 1.36 to 1.63nJ we observe Raman-shifted solitons between 1064 and 1200nm with up to 57% power conversion efficiency and compressed output pulse widths less than 50fs. Furthermore, due to the dispersion characteristics of the HOM fiber, we observe redshifted Cerenkov radiation in the normal dispersion regime for appropriately energetic input pulses.     

Dispersion and diffraction mechanisms suppressing soliton self-frequency shift in a medium with retarded optical nonlinearity

The frequency dependence of group-velocity dispersion and diffraction-induced increase in the effective mode area in the long-wavelength range suppress the soliton self-frequency shift in a medium with retarded nonlinearity. This effect reduces unwanted variations in the central wavelength, as well as fluctuations of the delay time of the frequency-shifted soliton with respect to the input pulse.     

Broadband tunable Raman soliton self-frequency shift to mid-infrared band in a highly birefringent microstructure fiber

Raman soliton self-frequency shifted to mid-infrared band(λ 2 μm) has been achieved in an air-silica microstructure fiber(MF). The MF used in our experiment has an elliptical core with diameters of 1.08 and 2.48 μm for fast and slow axis. Numerical simulation shows that each fundamental orthogonal polarization mode has two wide-spaced λZDW and theλZDW pairs located at 701/2110 nm and 755/2498 nm along the fast and slow axis, respectively. Using 810-nm Ti:sapphire femtosecond laser as pump, when the output power varies from 0.3 to 0.5 W, the furthest red-shift Raman solitons in both fast and slow axis shift from near-infrared band to mid-infrared band, reaching as far as 2030 and 2261 nm. Also, midinfrared Raman solitons can always be generated for pump wavelength longer than 790 nm if output pump power reaches0.5 W. Specifically, with pump power at 0.5 W, the mid-infrared soliton in slow axis shifts from 2001 to 2261 nm when the pump changes from 790 nm to 810 nm. This means only a 20 nm change of pump results in 260 nm tunability of a mid-infrared soliton.     

Single-laser light source for CARS microscopy based on soliton self-frequency shift in a microstructured fiber

A femtosecond light source comprising two high-intensity beams at different wavelengths is applied to coherent anti-Stokes Raman scattering microscopy. One wavelength is supplied by a Titanium?:?Sapphire oscillator, while the other is derived from that via soliton-self-frequency shift in a microstructured fiber. Clear CARS images are acquired with the frequency difference adjusted to resonances of polystyrene and lipids.     

Polarisation-sensitive four-wave mixing and soliton self-frequency shift effect in the highly birefringent photonic crystal fibre

By adjusting the polarisation state of the pump at 805 nm parallel to slow (x) and fast (y) axes of the highly birefringent photonic crystal fibre with zero dispersion wavelengths 790 nm and 750 nm, this paper demonstrates the efficient polarisation-sensitive four wave mixing involved in pump, anti-Stokes and Stokes signals and soliton self-frequency shift effects induced by the phase-matching between red-shifted solitons and blue-shifted dispersive waves. If the reduction of coupling efficiency to the circular pump laser mode or other circular fibres due to asymmetry of the core is neglected, more than 98% of the total input power is kept in a single linear polarisation. Controlled dispersion characteristic of the doublet of fundamental guided-modes results in achieving light field strongly confined in principal axes of photonic crystal fibre, and enhancing the corresponding nonlinear-optical process through the remarkable nonlinear birefringence.     

Tunable light source for coherent anti-Stokes Raman scattering microspectroscopy based on the soliton self-frequency shift

We present a photonic crystal fiber (PCF)-based light source for generating tunable excitation pulses (pump and Stokes) that are applicable to coherent anti-Stokes Raman scattering (CARS) microspectroscopy. The laser employed is an unamplified Ti:sapphire femtosecond laser oscillator. The CARS pump pulse is generated by spectral compression of a laser pulse in a PCF. The Stokes pulse is generated by redshifting a laser pulse in a PCF through the soliton self-frequency shift. This setup allows for probing up to 4000 cm(-1) with a spectral resolution of approximately 25 cm(-1). We characterize the stability and robustness of CARS microspectroscopy employing this light source.     

Six-bit all-optical quantization using photonic crystal fiber with soliton self-frequency shift and pre-chirp spectral compression techniques

In this paper,we propose an optical quantization scheme for all-optical analog-to-digital conversion that facilitates photonics integration.A segment of 10-m photonic crystal fiber with a high nonlinear coefficient of 62.8 W 1/km is utilized to realize large scale soliton self-frequency shift relevant to the power of the sampled optical signal.Furthermore,a 100-m dispersion-increasing fiber is used as the spectral compression module for further resolution enhancement.Simulation results show that 317-nm maximum wavelength shift is realized with 1550-nm initial wavelength and 6-bit quantization resolution is obtained with a subsequent spectral compression process.     

Optimizational 6-bit all-optical quantization with soliton self-frequency shift and pre-chirp spectral compression techniques based on photonic crystal fiber

In this paper, we optimize a proposed all-optical quantization scheme based on soliton self-frequency shift(SSFS)and pre-chirp spectral compression techniques. A 10m-long high-nonlinear photonic crystal fiber(PCF) is used as an SSFS medium relevant to the power of the sampled optical pulses. Furthermore, a 10m-long dispersion flattened hybrid cladding hexagonal-octagonal PCF(6/8-PCF) is utilized as a spectral compression medium to further enhance the resolution. Simulation results show that 6-bit quantization resolution is still obtained when a 100m-long dispersion-increasing fiber(DIF)is replaced by a 6/8-PCF in spectral compression module.     

Observation of filamentation-induced continuous self-frequency down shift in air

We observed a new filamentation nonlinear process: continuous self-frequency down shift inside the filament zone during the propagation of a femtosecond laser pulse in air. The frequency shift depends strongly on the length of the self-guided column (filament). PACS 42.65.Dr; 42.65.Jx; 52.35.Mw     

Chirped femtosecond solitonlike laser pulse form with self-frequency shift

Ultrashort laser pulse propagation in a generalized nonconservative system is considered. Slopes appearing in the form of the third-order time derivative for narrow pulse widths, nonlinear dispersion, and self-frequency shift arising from stimulated Raman scattering are taken into account. An exact analytical solitonlike solution is presented for a femtosecond solitary laser pulse. The stability of the latter has been shown numerically by applying perturbations in amplitude and chirp, as well as adding white noise. The results indicate stability in a broad parameter range. In addition, we have also found that the solution acts as an attractor when starting with a quite arbitrary Gaussian pulse as an initial condition.     

Soliton self-frequency shift of 6-fs pulses in photonic-crystal fibers

Raman soliton phenomena in photonic crystal fibers are shown to allow efficient tunable frequency shifting of sub-10-fs laser pulses. Soliton self-frequency shift in a photonic-crystal fiber with a core diameter less than 2 μm is used to transform the spectrum of a 6-fs 2-nJ Ti: sapphire-laser pulse, dominated by a 670-nm peak, into a spectrum featuring a well-resolved intense spectral component centered at 1064 nm, which is ideally suited as a seed for Nd: YAG- and ytterbium-based laser devices.     

Cancellation of Raman self-frequency shift for compression of optical pulses

We study to which extent a fiber soliton can be manipulated by a specially chosen continuous pump wave. A group velocity matched pump scatters at the soliton, which is compressed due to the energy/momentum transfer. As the pump scattering is very sensitive to the velocity matching condition, soliton compression is quickly destroyed by the soliton self-frequency shift (SSFS). This is especially true for ultrashort pulses: SSFS inevitably impairs the degree of compression. We demonstrate numerically that soliton enhancement can be restored to some extent and the compressed soliton can be stabilized, provided that SSFS is canceled by a second pump wave. Still the available compression degree is considerably smaller than that in the Raman-free nonlinear fibers.     

Soliton self-frequency shift in highly nonlinear fiber with extension by external Raman pumping

A completely fiber-integrated, wavelength-tunable subpicosecond pulse source is demonstrated using the soliton self-frequency shift in highly nonlinear dispersion-shifted fiber from a 1.56-microm 10-GHz 400-fs signal. Solitons as short as 100 fs are obtained at tunable wavelengths as high as 1.72 microm. Raman gain from an external pump is used to extend the soliton self-frequency shift to longer wavelengths.