Benefits of cascaded nonlinear dynamics in hybrid fibers for low-noise supercontinuum generation.

The recent development of fiber supercontinuum (SC) sources with ultra-low noise levels has been instrumental in advancing the state-of-the-art in a wide range of research topics. However, simultaneously satisfying the application demands of maximizing spectral bandwidth and minimizing noise is a major challenge that so far has been addressed with compromise, found by fine-tuning the characteristics of a single nonlinear fiber transforming the injected laser pulses into a broadband SC. In this work, we investigate a hybrid approach that splits the nonlinear dynamics into two discrete fibers optimized for nonlinear temporal compression and spectral broadening, respectively. This introduces new design degrees of freedom, making it possible to select the best fiber for each stage of the SC generation process. With experiments and simulations we study the benefits of this hybrid approach for three common and commercially available highly nonlinear fiber (HNLF) designs, focusing on flatness, bandwidth and relative intensity noise of the generated SC. In our results, hybrid all-normal dispersion (ANDi) HNLF stand out as they combine the broad spectral bandwidths associated with soliton dynamics with extremely low noise and smooth spectra known from normal dispersion nonlinearities. Hybrid ANDi HNLF are a simple and low-cost route for implementing ultra-low noise SC sources and scaling their repetition rate for various applications such as biophotonic imaging, coherent optical communications, or ultrafast photonics.

Low pump power coherent supercontinuum generation in heavy metal oxide solid-core photonic crystal fibers infiltrated with carbon tetrachloride covering 930-2500†nm.

All-normal dispersion supercontinuum (ANDi SC) generation in a lead-bismuth-gallate glass solid-core photonic crystal fiber (PCF) with cladding air-holes infiltrated with carbon tetrachloride (CCl4) is experimentally investigated and numerically verified. The liquid infiltration results in additional degrees of freedom that are complimentary to conventional dispersion engineering techniques and that allow the design of soft-glass ANDi fibers with an exceptionally flat near-zero dispersion profile. The unique combination of high nonlinearity and low normal dispersion enables the generation of a coherent, low-noise SC covering 0.93-2.5 µm requiring only 12.5 kW of pump peak power delivered by a standard ultrafast erbium-fiber laser with 100 MHz pulse repetition rate (PRR). This is a much lower peak power level than has been previously required for the generation of ANDi SC with bandwidths exceeding one octave in silica- or soft-glass fibers. Our results show that liquid-composite fibers are a promising pathway for scaling the PRR of ANDi SC sources by making the concept accessible to pump lasers with hundreds of megahertz of gigahertz PRR that have limited peak power per pulse but are often required in applications such as high-speed nonlinear imaging, optical communications, or frequency metrology. Furthermore, due to the overlap of the SC with the major gain bands of many rare-earth fiber amplifiers, our source could serve as a coherent seed for low-noise ultrafast lasers operating in the short-wave infrared spectral region.

Temporal fine structure of all-normal dispersion fiber supercontinuum pulses caused by non-ideal pump pulse shapes.

We experimentally investigate the spectro-temporal characteristics of coherent supercontinuum (SC) pulses generated in several implementations of silica and soft-glass all-normal dispersion (ANDi) photonic crystal fibers optimized for pumping with Erbium (Er):fiber femtosecond laser technology. We characterize the resulting SC using time-domain ptychography, which is especially suitable for the measurement of complex, spectrally broadband ultrashort pulses. The measurements of the ANDi SC pulses reveal intricate pulse shapes, considerable temporal fine structure, and oscillations on time scales of < 25 femtoseconds, which differ from the smoothness and simplicity of temporal profiles obtained in numerical simulations and observed in previous experiments. We link the measured complex features to temporal sub-structures of the pump pulse, such as pre- and post-pulses and low-level pedestals, which are common in high pulse energy ultrafast Er:fiber systems. We also observe spectro-temporal structures consistent with incoherent noise amplification in weakly birefringent fiber samples. Our results highlight the importance of the pump source and polarization-maintaining (PM) fibers for high-quality SC generation and have practical relevance for many ultrafast photonics applications employing ANDi fiber-based SC sources.

Improved retrieval of complex supercontinuum pulses from XFROG traces using a ptychographic algorithm.

We demonstrate that time-domain ptychography, a recently introduced iterative ultrafast pulse retrieval algorithm, has properties well suited for the reconstruction of complex light pulses with large time-bandwidth products from a cross-correlation frequency-resolved optical gating (XFROG) measurement. It achieves temporal resolution on the scale of a single optical cycle using long probe pulses and low sampling rates. In comparison to existing algorithms, ptychography minimizes the data to be recorded and processed, and significantly reduces the computational time of the reconstruction. Experimentally, we measure the temporal waveform of an octave-spanning, 3.5 ps long, supercontinuum pulse generated in photonic crystal fiber, resolving features as short as 5.7 fs with sub-fs resolution and 30 dB dynamic range using 100 fs probe pulses and similarly large delay steps.

Low-loss and low-bend-sensitivity mid-infrared guidance in a hollow-core-photonic-bandgap fiber.

Hollow-core-photonic-bandgap fiber, fabricated from high-purity synthetic silica, with a wide operating bandwidth between 3.1 and 3.7 µm, is reported. A minimum attenuation of 0.13 dB/m is achieved through a 19-cell core design with a thin core wall surround. The loss is reduced further to 0.05 dB/m following a purging process to remove hydrogen chloride gas from the fiber-representing more than an order of magnitude loss reduction as compared to previously reported bandgap-guiding fibers operating in the mid-infrared. The fiber also offers a low bend sensitivity of <0.25 dB per 5 cm diameter turn over a 300 nm bandwidth. Simulations are in good agreement with the achieved losses and indicate that a further loss reduction of more than a factor of 2 should be possible by enlarging the core using a 37-cell design.

Nanoscale all-normal dispersion optical fibers for coherent supercontinuum generation at ultraviolet wavelengths.

We report on the possibilities of nanoscale optical fibers with all-normal dispersion behavior for pulse-preserving and coherent supercontinuum generation at deep ultraviolet wavelengths. We discuss the influence of important parameters such as pump wavelength and fiber diameter, for both optical nanofibers and nanoscale suspended-core optical fibers. Simulations reveal that by appropriate combination of fiber geometry and input pulse parameters, intensive spectral components well below 300 nm are generated. In addition, the impact of preceding taper transitions used for input coupling purposes is discussed in detail.

All-fibre heterogeneously-integrated frequency comb generation using silicon core fibre.

Originally developed for metrology, optical frequency combs are becoming increasingly pervasive in a wider range of research topics including optical communications, spectroscopy, and radio or microwave signal processing. However, application demands in these fields can be more challenging as they require compact sources with a high tolerance to temperature variations that are capable of delivering flat comb spectra, high power per tone, narrow linewidth and high optical signal-to-noise ratio. This work reports the generation of a flat, high power frequency comb in the telecom band using a 17 mm fully-integrated silicon core fibre as a parametric mixer. Our all-fibre, cavity-free source combines the material benefits of planar waveguide structures with the advantageous properties of fibre platforms to achieve a 30 nm bandwidth comb source containing 143 tones with <3 kHz linewidth, 12 dB flatness, and >30 dB OSNR over the entire spectral region.

Design of all-normal dispersion microstructured optical fibers for pulse-preserving supercontinuum generation.

Recently, the generation of coherent, octave-spanning, and recompressible supercontinuum (SC) light has been demonstrated in optical fibers with all-normal group velocity dispersion (GVD) behavior by femtosecond pumping. In the normal dispersion regime, soliton dynamics are suppressed and the SC generation process is mainly due to self-phase modulation and optical wave breaking. This makes such white light sources suitable for time-resolved applications. The broadest spectra can be obtained when the pump wavelength equals the wavelength of maximum all-normal GVD. Therefore each available pump wavelength requires a specifically designed optical fiber with suitable GVD to unfold its full power. We investigate the possibilities to shift the all-normal maximum dispersion wavelength in microstructured optical fibers from the near infra red (NIR) to the ultra violet (UV). In general, a submicron guiding fiber core surrounded by a holey region is required to overcome the material dispersion of silica. Photonic crystal fibers (PCFs) with a hexagonal array of holes as well as suspended core fibers are simulated for this purpose over a wide field of parameters. The PCFs are varied concerning their air hole diameter and pitch and the suspended core fibers are varied concerning the number of supporting walls and the wall width. We show that these two fiber types complement each other well in their possible wavelength regions for all-normal GVD. While the PCFs are suitable for obtaining a maximum all-normal GVD in the NIR, suspended core fibers are well applicable in the visible wavelength range.

Pulse-preserving broadband visible supercontinuum generation in all-normal dispersion tapered suspended-core optical fibers.

Recently, coherent pulse-preserving and octave-spanning supercontinuum (SC) generation was theoretically predicted and experimentally shown in photonic crystal fibers (PCFs) with all-normal dispersion behavior. Since this behavior is due only to the all-normal dispersion profile and not to the photonic crystal cladding, other all-normal optical waveguides exhibit these properties as well. We extend this concept to suspended-core fibers and optical nanofibers and show experimental demonstrations of this way of SC generation. We show that optical suspended-core fibers and optical nanofibers of appropriate dimensions exhibit all-normal dispersion and address octave-spanning single pulse SC generation in the visible (VIS) and ultra violet (UV) wavelength range. In addition, we discuss the feasibility of fiber taper transitions for suitable input coupling schemes in sub-micron diameter fibers and show the importance of short adiabatic transition profiles for utilizing high-energy pulses to obtain maximum spectral broadening. They are essential for coherent broadband UV SC generation in optical nanofibers.

Coherent octave spanning near-infrared and visible supercontinuum generation in all-normal dispersion photonic crystal fibers.

We present the first detailed demonstrations of octave-spanning SC generation in all-normal dispersion photonic crystal fibers (ANDi PCF) in the visible and near-infrared spectral regions. The resulting spectral profiles are extremely flat without significant fine structure and with excellent stability and coherence properties. The key benefit of SC generation in ANDi PCF is the conservation of a single ultrashort pulse in the time domain with smooth and recompressible phase distribution. For the first time we confirm the exceptional temporal properties of the generated SC pulses experimentally and demonstrate their applicability in ultrafast transient absorption spectroscopy. The experimental results are in excellent agreement with numerical simulations, which are used to illustrate the SC generation dynamics by self-phase modulation and optical wave breaking. To our knowledge, we present the broadest spectra generated in the normal dispersion regime of an optical fiber.

High quality sub-two cycle pulses from compression of supercontinuum generated in all-normal dispersion photonic crystal fiber.

We demonstrate nonlinear pulse compression based on recently introduced highly coherent broadband supercontinuum (SC) generation in all-normal dispersion photonic crystal fiber (ANDi PCF). The special temporal properties of the octave-spanning SC spectra generated with 15 fs, 1.7 nJ pulses from a Ti:Sapphire oscillator in a 1.7 mm fiber piece allow the compression to 5.0 fs high quality pulses by linear chirp compensation with a compact chirped mirror compressor. This is the shortest pulse duration achieved to date from the external recompression of SC pulses generated in PCF. Numerical simulations in excellent agreement with the experimental results are used to discuss the scalability of the concept to the single-cycle regime employing active phase shaping. We show that previously reported limits to few-cycle pulse generation from compression of SC spectra generated in conventional PCF possessing one or more zero dispersion wavelengths do not apply for ANDi PCF.

Generation of high quality, 1.3 cycle pulses by active phase control of an octave spanning supercontinuum.

Nonlinear pulse compression based on the generation of ultra-broadband supercontinuum (SC) in an all-normal dispersion photonic crystal fiber (ANDi PCF) is demonstrated. The highly coherent and smooth octave-spanning SC spectra are generated using 6 fs, 3 nJ pulses from a Ti:Sapphire oscillator for pumping a 13 mm piece of ANDi PCF. Applying active phase control has enabled the generation of 4.5 fs pulses. Additional spectral amplitude shaping has increased the bandwidth of the SC spectra further leading to nearly transform-limited pulses with a duration of 3.64 fs, which corresponds to only 1.3 optical cycles at a central wavelength of 810 nm. This is the shortest pulse duration achieved via compression of SC spectra generated in PCF to date. Due to the high stability and the smooth spectral intensity and phase distribution of the generated SC, an excellent temporal pulse quality exhibiting a pulse contrast of 14 dB with respect to the pre- and post-pulses is achieved.

Low noise all-fiber amplification of a coherent supercontinuum at 2 µm and its limits imposed by polarization noise.

We report a low noise, broadband, ultrafast Thulium/Holmium co-doped all-fiber chirped pulse amplifier, seeded by an Erbium-fiber system spectrally broadened via coherent supercontinuum generation in an all-normal dispersion photonic crystal fiber. The amplifier supports a - 20 dB bandwidth of more than 300 nm and delivers high quality 66 fs pulses with more than 70 kW peak power directly from the output fiber. The total relative intensity noise (RIN) integrated from 10 Hz to 20 MHz is 0.07%, which to our knowledge is the lowest reported RIN for wideband ultrafast amplifiers operating at 2 µm to date. This is achieved by eliminating noise-sensitive anomalous dispersion nonlinear dynamics from the spectral broadening stage. In addition, we identify the origin of the remaining excess RIN as polarization modulational instability (PMI), and propose a route towards complete elimination of this excess noise. Hence, our work paves the way for a next generation of ultra-low noise frequency combs and ultrashort pulse sources in the 2 µm spectral region that rival or even outperform the excellent noise characteristics of Erbium-fiber technology.