Frequency-shifted Solitons Research Articles

High-energy 2.8-μm Ultrashort Pulses Generation in an Er:ZBLAN Fiber Amplifier

Compact, high-energy ultrashort pulsed fiber lasers in the 2.8 μm wavelength have attracted extensive interest in many scientific and industrial applications. Here, we demonstrated the generation of μJ-level ultrashort pulses from a quasi-all-fiberized Er:ZBLAN amplifier employing a 2.8-μm frequency-shifted soliton as a seed laser. With backward pumping, the pulse energy was enhanced to 0.282 μJ when the average output power was amplified to 1.023 W. Mechanisms for the generation and amplification of ultrashort pulses were theoretically revealed by numerical simulation, and further cascaded indium fluoride (InF3) fiber to obtain a 4.26-μm redshift soliton numerically. This compact fiber amplification system consisting of frequency-shifted-based seed pulses and fluoride fiber amplifiers will be of practical interest in applications.

All-normal dispersion supercontinuum vs frequency-shifted solitons pumped at 1560 nm as seed sources for thulium-doped fiber amplifiers.

We present a direct comparison between two types of femtosecond 2 µm sources used for seeding of an ultrafast thulium-doped fiber amplifier based on all-normal dispersion supercontinuum and soliton self-frequency shift. Both nonlinear effects were generated in microstructured silica fibers, pumped with low-power femtosecond pulses at 1.56 µm originating from an erbium-doped fiber laser. We performed a full characterization of both nonlinear processes, including their shot-to-shot stability, phase coherence, and relative intensity noise. The results revealed that the solitons show comparable performance to supercontinuum in terms of relative intensity noise and shot-to-shot stability, despite the anomalous dispersion regime. Both sources can be successfully used as seeds for Tm-doped fiber amplifiers as an alternative to Tm-doped oscillators. The results show that the sign of chromatic dispersion of the fiber is not crucial for obtaining a stable, high-quality, and low-noise spectral conversion process when pumped with sub-50 fs laser pulses.

Soliton detuning of 68.5 THz in the near-infrared in a highly nonlinear suspended core tellurite fiber

We demonstrate an isolated phenomenon of soliton self-frequency shift over a bandwidth of 68.5 THz in the near-infrared spectral range. We designed, fabricated, and characterized a suspended core microstructured fiber made of highly nonlinear tellurite glass. The fiber is designed for a flat anomalous dispersion to leverage on Raman scattering-assisted frequency shift of a fundamental-order soliton. We demonstrate a prominent soliton with an input-power-based tunability in the spectral range from the 1560 nm central pump wavelength up to around 2400 nm central wavelength of the frequency-shifted soliton, when the fiber is pumped with 90 fs pulses from a commercial, fiber-based, mode-locked laser.

Jitter-free, dual-wavelength, ultrashort-pulse, energetic fiber sources using soliton self-mode conversion.

We demonstrate an energetic dual-wavelength ultrashort pulsed source by exploiting the inherent features of the newly discovered process of soliton self-mode conversion (SSMC) in a multimode fiber. The generated pulses are at wavelengths of 1205 nm and 1273 nm, respectively, and the pulse energies are approximately 30 nJ. The natural group-velocity-locking feature of SSMC ensures minimal relative timing jitter, hence highlighting the utility of exploiting the new degrees of freedom afforded by field of multimode nonlinear fiber optics. The relative timing jitter is evaluated by measuring the power fluctuations of generated sum-frequency signals. When compared to a conventional fiber based dual-wavelength source based on traditional frequency-shifted solitons, the relative timing jitter is found to be reduced by greater than 11 dB. Since this process is wavelength-agnostic within the transparency window of optical fibers, our source provides an attractive means of achieving integrated multi-color ultrashort pulse sources for a variety of applications.

Interplay between Raman self-frequency shift and cross-phase modulation in the vector-soliton of a birefringent fiber

Competition between cross-phase modulation (XPM) interaction and Raman-induced red-frequency shift is observed in the orthogonally polarized components (in the form of optical pulses) of a vector-soliton propagating in the fundamental mode of a birefringent fiber exhibiting negative third-order dispersion and group velocity birefringence. Both pulses experience opposite frequency shifts due to XPM, which reinforces the Raman self-frequency shift of one pulse while it hinders the self-frequency shift of the other. A large redshift in one of the pulses of the vector-soliton suggests transfer of a greater part of its energy to the dispersive radiation, leading to an asymmetry in the longitudinal evolution of the amplitudes as well as the frequencies of both the pulses of the vector-soliton. To support our analysis, an analytical approach is developed in order to understand the interplay between the Raman effect and XPM. The findings of the analytical approach are in good qualitative agreement with the numerical results, which is consistent with the results of Phys. Rev. E70, 016615 (2004)PLEEE81063-651X10.1103/PhysRevE.70.016615. The derived analytical relations accommodate additional nonlinear interactions originating from the XPM, as well as mutual Raman interaction, which makes our approach much superior to that used in Phys. Rev. E70, 016615 (2004)PLEEE81063-651X10.1103/PhysRevE.70.016615. It is also possible to understand the interaction between two frequency-shifted solitons in a true single-mode fiber or other waveguide system exhibiting negative third-order dispersion by a slight modification of our theoretical approach.

All-fiber mid-infrared source tunable from 6 to 9 μm based on difference frequency generation in OP-GaP crystal

We report the first fully fiberized difference frequency generation (DFG) source, delivering a broadly tunable idler in the 6 to 9 μm spectral range, using an orientation-patterned gallium phosphide (OP-GaP) crystals with different quasi-phase matching periods (QPM). The mid-infrared radiation (MIR) is obtained via mixing of the output of a graphene-based Er-doped fiber laser at 1.55 μm with coherent frequency-shifted solitons at 1.9 μm generated in a highly nonlinear fiber using the same seed. The presented setup is the first truly all-fiber, all-polarization maintaining, alignment-free DFG source reported so far. Its application to laser spectroscopy was demonstrated by the absorption spectrum measurement of ν4 band of methane in 7.5 - 8.3 µm spectral range. The system simplicity and compactness paves the way for applications in field-deployable optical frequency comb spectroscopy systems for gas sensing.

All-in-fiber amplification and compression of coherent frequency-shifted solitons tunable in the 1800–2000 nm range

We report an all-fiber, all-polarization maintaining (PM) source of widely tunable (1800–2000 nm) ultrashort pulses based on the amplification of coherent self-frequency-shifted solitons generated in a highly nonlinear fiber pumped with an Er-doped fiber laser. The system delivers sub-100 fs pulses with energies up to 8.6 nJ and is built entirely from PM optical fibers, without any free-space optics. The all-fiber alignment-free design significantly increases the suitability of such a source for field deployments.

Intermodal Čerenkov radiation in a higher-order-mode fiber

We demonstrate an intermodal Čerenkov radiation effect in a higher-order-mode (HOM) fiber with a mode crossing (i.e., two guided modes having the same propagation constant at the same wavelength). A frequency-shifted soliton in the vicinity of the mode-crossing wavelength emits a phase-matched dispersive wave in a different propagation mode. We develop a theoretical explanation for this nonlinear optical effect and demonstrate that the mode crossing in HOM fibers can be utilized to achieve simultaneous wavelength and mode conversion; the strength of this intermodal nonlinear interaction can be tuned by controlled fiber bending.

Subpicosecond fiber-based soliton-tuned mid-infrared source in the 97–149 μm wavelength region

We present a fiber-based mid-IR source of ultrafast laser pulses tunable in the 9.7-14.9 microm spectral region based on difference-frequency mixing between an Er:fiber laser and a first-order frequency-shifted soliton pulse. We have measured subpicosecond pulses at a repetition rate of 37 MHz, with a maximum power of 1.5 microW.

Influence of air pressure on soliton formation in hollow-core photonic bandgap fibers

Soliton formation during dispersive compression of chirped few-picosecond pulses at the microjoule level in a hollow-core photonic bandgap (HC-PBG) fiber is studied by numerical simulations. Long-pass filtering of the emerging frequency-shifted solitons is investigated with the objective of obtaining pedestal-free output pulses. Particular emphasis is placed on the influence of the air pressure in the HC-PBG fiber. It is found that a reduction in air pressure enables an increase in the fraction of power going into the most redshifted soliton and also improves the quality of the filtered pulse at high powers. This allows a scaling of the output pulse energy toward the microjoule level.

Powerful wavelength-tunable ultrashort solitons in a solid-core photonic-crystal fiber

A solid-core photonic-crystal fiber (PCF) with an effective mode area of 20 microm2 is used to demonstrate the generation of sub-100-kW, 30-70 fs wavelength-tunable solitons within a wavelength range of 1300-1800 nm at a repetition rate of 18 MHz. An energy of 2.9 nJ per pulse is achieved for a 35 fs soliton PCF output centered at 1770 nm. Our numerical analysis supports experimental findings and suggests that frequency-shifted solitons in solid-core PCFs can be scaled up to a submegawatt level of peak powers.

Spectral interference of frequency-shifted solitons in a photonic-crystal fiber

Frequency-shifted solitons in a highly nonlinear photonic-crystal fiber (PCF) are shown to give rise to high-visibility interference fringes in PCF output spectra, indicating flat spectral phase profiles of individual solitons in the PCF output. This experimental finding, supported by numerical simulations, suggests a promising method of fiber-format pulse shaping and an attractive technology for few-cycle pulse synthesis through a coherent addition of frequency-shifted solitons generated in a highly nonlinear fiber.

Two-photon absorption-induced effects in femtosecond coherent anti-Stokes Raman-scattering microspectroscopy of silicon photonic components

We study the effects related to two-photon absorption (TPA) in the microspectroscopy of the silicon photonic components based on coherent anti-Stokes Raman scattering (CARS) of femtosecond pulses. With 300-fs pulses of 1.24-μm Cr:forsterite laser radiation delivering pump and probe fields and a frequency-shifted soliton output of a large-mode area photonic-crystal fiber employed as a Stokes field, pronounced TPA effects have been observed in the CARS microspectroscopy of silicon components for pump-pulse intensities exceeding 10 GW/cm2.

Coherent anti-Stokes Raman scattering microspectroscopy of silicon components with a photonic-crystal fiber frequency shifter

Coherent anti-Stokes Raman scattering (CARS) microspectroscopy of silicon components is demonstrated with pump and probe fields delivered by a mode-locked Cr:forsterite laser and the frequency-shifted soliton output of a photonic-crystal fiber as a Stokes field. CARS microspectroscopy is shown to allow a visualization of microscale features and defects on the surface of silicon wafers, offering much promise for online diagnostics of electronic and photonic silicon chip components.

Soliton self-frequency shift with diffraction-suppressed wavelength variance and timing jitter

Diffraction-controlled wavelength dependence of the effective mode area Seff(λ) in optical fibers can serve as a mechanism limiting the soliton self-frequency shift induced by the Raman effect in materials with retarded nonlinearity. By numerically solving the generalized nonlinear Schrodinger equation modified to include the Seff(λ) dependence, we show that, as the central wavelength of the soliton increases, the waveguide mode tends to become less compact, slowing down the soliton self-frequency shift. As a result, for optical fibers with a steep Seff(λ) profile, wavelength uncertainties and the timing jitter of the frequency-shifted soliton induced by input power fluctuations can be substantially reduced compared with fibers with a weak Seff(λ) dependence.

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.

Ultrashort-pulse-formation mechanism by slow-loss saturation and self-phase modulation in solid-state lasers

It is shown that, under the balance between slow-loss saturation and self-phase modulation, the solid-state laser is capable of generating a chirped frequency-shifted quasi soliton. The gain saturation, negative group-velocity dispersion, or additional phase modulation stabilizes the quasi soliton.