Alexander M. Heidt

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.

Time-Domain Ptychography

One of the most robust techniques solving the so-called phase problem in X-ray diffraction imaging is ptychography. It produces the correct real-space image if the illumination beam is known [1], but works even if it is unknown [2]. In 2015 we were the first to extend ptychography to the time domain and further to the reconstruction of temporal objects. In comparison to existing algorithms, ptychography minimizes the data to be recorded and processed, and thereby significantly reduces the computational time for reconstruction.

Limits of coherent supercontinuum generation in normal dispersion fibers

We study the largely unexplored transition between coherent and noise-seeded incoherent continuum generation in all-normal dispersion (ANDi) fibers and show that highly coherent supercontinua with spectral bandwidths of one octave can be generated with long pump pulses of up to 1.5 ps duration, corresponding to soliton orders of up to N=600. In terms of N, this corresponds to an approximately 50 times increase of the coherent regime compared to anomalous dispersion pumping. In the transition region between coherent and incoherent spectral broadening, we observe the manifestation of nonlinear phenomena that we term incoherent cloud formation and incoherent optical wave breaking, which lead to a gradual or instantaneous coherence collapse of supercontimuum (SC) spectral components, respectively. The role played by stimulated Raman scattering and parametric four-wave mixing during SC generation in ANDi fibers is shown to be more extensive than previously recognized: their nonlinear coupling contributes to the suppression of incoherent dynamics at short pump pulse durations, while it is responsible for non-phase-matched parametric amplification of noise observed in the long pulse regime. We further discuss the dependence of SC coherence on fiber design, and present basic experimental verifications for our findings using single-shot detection of SC spectra generated by picosecond pulses. This work outlines both the further potential as well as the limitations of broadband coherent light source development for applications such as metrology, nonlinear imaging, and ultrafast photonics, among others.

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.

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.

Dispersion measurement of ultra-high numerical aperture fibers covering thulium, holmium, and erbium emission wavelengths

We present broadband group velocity dispersion (GVD) measurements of commercially available ultra-high numerical aperture fibers (UHNA1, UHNA3, UHNA4, UHNA7, and PM2000D from Coherent-Nufern). Although these fibers are attractive for dispersion management in ultra-fast fiber laser systems in the 2 μm wavelength region, experimental dispersion data in the literature is scarce and inconsistent. Here we demonstrate the measurements using the spectral interferometry technique covering the typically used erbium, thulium, and holmium emission bands. The results are characterized in terms of the standard-deviation uncertainty and compared with previous literature reports. Fitting parameters are provided for each fiber allowing for the straightforward replication of the measured dispersion profiles. This work is intended to facilitate the design of ultra-fast fiber laser sources and the investigations of nonlinear optical phenomena.

Self-injection linear polarization locking of a fiber laser

We present a novel approach for linear polarized operation of a fiber laser not requiring any intra-cavity components, but relying on an external self-seeding effect. A small fraction of back-reflected polarized light created in an external cavity consisting of fused coupler, polarizer and fiber Bragg grating is sufficient to seed and lock the laser to linear polarized operation. This approach enables the construction of polarized monolithic fiber lasers, minimizes intra-cavity losses and drastically reduces parasitic amplified spontaneous emission. We experimentally demonstrate strong polarization locking in an Yb-doped fiber laser with extinction ratio of 500:1 at 1154 nm.

Self-optimizing additive pulse mode-locked fiber laser: wavelength tuning and selective operation in continuous-wave or mode-locked regime

Additive pulsed-mode mode-locked fiber lasers are known for their wide range of operating states, which can be achieved by tuning the artificial saturable absorber with built-in polarization controllers. We have equipped our laser with three motorized polarization controllers and online monitoring. We recorded three dimensional high-resolution maps of the oscillation states by stepping trough all possible polarization controller settings. In addition, we demonstrate the single and multi-objective optimization by a genetic algorithm of the fiber laser towards a desired operating state. To the best of our knowledge this is the first demonstration of multi-objective optimization of a fiber laser. Doing so, we can selectively operate the cavity in pulsed and continuous-wave mode and tune the wavelength of the laser emission by more than 55 nm. The high wavelength tunability is possible because the polarization controllers and the inline-polarizer act as a tunable birefringent filter.

Low-noise supercontinuum sources based on all-normal dispersion fibers: exploring their prospects and limitations

Supercontinuum (SC) sources based on optical fibers exhibiting flat and normal group velocity dispersion (GVD) in the entire wavelength region of interest, so-called all-normal dispersion (ANDi) fibers, have become a well-accepted alternative to the more conventional approach of pumping fibers in the anomalous GVD region. The conservation of a single ultrashort pulse and the suppression of noise-amplifying nonlinear effects during SC generation make ANDi fibers particularly attractive for ultrafast and noise-sensitive applications. Here we discuss current possibilities and future prospects, but also the limitations of ANDi fiber SC sources, focusing particularly on coherence and noise performance for various pump pulse regimes.