Ayhan Tajalli

Simple route toward efficient frequency conversion for generation of fully coherent supercontinua in the mid-IR and UV range

Fiber supercontinua represent light sources of pivotal importance for a wide range of applications, ranging from optical communications to frequency metrology. Although spectra encompassing more than three octaves can be produced, the applicability of such spectra is strongly hampered due to coherence degradation during spectral broadening. Assuming pulse parameters at the cutting edge of currently available laser technology, we demonstrate the possibility of strongly coherent supercontinuum generation. In a fiber with two zero-dispersion wavelengths a higher-order soliton experiences a temporal breakdown, without any compression or splitting behavior, which leads to nearly complete conversion of input solitonic radiation into resonant nonsolitonic radiation in the dispersive wave regime. As the process is completely deterministic and shows little sensitivity to input noise, the resulting pulses appear to be compressible down to the sub-cycle level and may thus hold a new opportunity for direct generation of attosecond pulses in the visible to near ultraviolet wavelength range.

Advanced phase retrieval for dispersion scan: a comparative study

Dispersion scan is a self-referenced measurement technique for ultrashort pulses. Similar to frequency-resolved optical gating, the dispersion scan technique records the dependence of nonlinearly generated spectra as a function of a parameter. For the two mentioned techniques, these parameters are the delay and the dispersion, respectively. While dispersion scan seems to offer a number of potential advantages over other characterization methods, in particular for measuring few-cycle pulses, retrieval of the spectral phase from the measured traces has so far mostly relied on the Nelder–Mead algorithm, which has a tendency to stagnate in a local minimum and may produce ghost satellites in the retrieval of pulses with complex spectra. We evaluate three different strategies to overcome these retrieval problems, namely, regularization, use of a generalized-projections algorithm, and an evolutionary retrieval algorithm. While all these measures are found to improve the precision and convergence of dispersion scan retrieval, differential evolution is found to provide the best performance, enabling the near-perfect retrieval of the phase of complex supercontinuum pulses, even in the presence of strong detection noise and limited phase-matching bandwidth of the nonlinear process.

Complete characterization of ultrafast pulses of an Yb-doped fiber amplifier via dispersion scans after compression in a grism compressor (Conference Presentation)

We present the application of the dispersion scan (d-scan) technique in order to fully characterize intensity and phase of ultrafast pulses compressed by a grating-prism (grism) compressor. Ultrafast pulses are generated in an Yb-doped fiber oscillator, mode-locked by non-linear polarization evolution (NPE) and operating at a central wavelength of 1030 nm with a repetition rate of 50 MHz. These output pulses are stretched in a 150 m long fiber to roughly 50 ps and then amplified in an Yb-doped fiber to an average power of 150 mW. A grating-prism combination is used to compensate the second and third order dispersion that was applied to the pulse by the fiber material. For the optimal compression of the pulse, information on its spectral and temporal phase is required. Therefore, the dispersion scan is applied. In this technique, second harmonic spectra of the pulse are generated and recorded for different amounts of dispersion compensation applied to the pulse by the grism compressor. Depending on the chirp of the pulse, the maximum of the generated second harmonic spectra shifts, leading to a characteristic trace. The amount of dispersion is changed by varying the separation of the prisms within the compressor, while the relation of second and third order dispersion is kept almost constant. The separation is changed by a stepper motor in order to enable a fully automated recording of the spectra. The spectral and temporal course and phase of the pulse are then retrieved from the d-scan trace by a genetic computer algorithm.

Improved phase retrieval for dispersion scan

Ultrashort pulse generation has come of age, along with several well established methods that allow characterization of femtosecond pulses in amplitude and phase. While all these methods have been demonstrated to work well for pulses with >10 fs pulse duration, the reliable measurement of few-cycle pulses still poses a challenge that requires sophisticated adaptation of these existing techniques. In the sub-2-cycle regime, the recently devised dispersion-scan (d-scan) technique [1] has recently attracted much interest since it relies on single-beam geometry, is easy to implement, robust against adjustment errors, and can be completely immune against phase-matching limitations [2]. Formally, it is similar to frequency-resolved optical gating (FROG), yet with the noted difference of scanning the added group delay dispersion in the beam path rather than the delay between two replicas of the pulse under test. Spectrally resolving the pulses after nonlinear conversion then gives rise to two-dimensional d-scan traces, from which the pulse shape can be reconstructed by a retrieval algorithm. To this end, the Nelder-Mead (NM) algorithm has been nearly exclusively employed. While NM is a very flexible algorithm and does not require gradients, it turns out to be very slow for complex pulse shapes that require a large number of points, i.e., a large dimension for the retrieval algorithm, for which one often observes local stagnation, slow convergence speed, and the build-up of erroneous spectral phase oscillations. To overcome this limiting problem of the otherwise very promising d-scan technique, we investigated a number of possible solutions, including regularization, generalized projections (GP), and differential evolution (DE), and combinations thereof. Here we demonstrate that the last method can overcome all above-mentioned problems of d-scan retrieval and is remarkably resilient against detection noise. Furthermore, we demonstrate the possibility of retrieving both phase and amplitude of the test pulse.

Parametric amplification of few-optical-cycle pulses

The reliable availability of energetic and very short pulses in the femtosecond regime triggered innumerable developments in research and application in physics, chemistry, and life sciences. Now, a new and superior technology reached maturity in research labs during the last decade, the few-cycle optical parametric chirped-pulse amplifier (OPCPA). The concept is advantageous against Ti:sapphire technology in many aspects, and enables few-cycle pulses with multi-kHz or MHz repetition rates. Furthermore, the technique can be up-scaled in average power due to the rapid development of Yb-based pump lasers (see Fig.1 ). This development paves the way to High Harmonic and tabletop XUV generation and new insights in physics and new fields of applications.

Generation of fast-tunable femtosecond visible radiation via intra-cavity sum-frequency in a high-power ultra-broadband NIR NOPO

High repetition rate femtosecond VIS radiation is normally generated in optical parametric oscillators (OPOs) either by generating VIS light via intra-cavity SHG or SFG [1, 2] or in VIS OPOs employing Ti:sa as a pump laser [3, 4]. Recently we demonstrated an efficient intra-cavity SFG in a non-collinear OPO (NOPO) pumped by thin disk oscillator [5]. Here, we present a NOPO system, pumped by the high power fiber laser, that is capable to deliver simultaneously intrinsically synchronized NIR and VIS femtosecond pulses which are widely tunable, scalable in power and do not require any realignment during the tuning process, therefore offering a possibility of a high tuning speed. The system is based on the unique properties of the NIR NOPO [6], which delivers high power femtosecond widely tunable radiation in a range of 640–1000 with a cavity length adjustment as a tuning mechanism. By introducing a second focus for intra-cavity sum-frequency generation (SFG) of the oscillator light with the residual from SHG fundamental pump light, visible radiation tunable in a range of 396–500 nm with output power up to 1.6W was generated.

Few-cycle pulse characterization using XPW d-scan

We demonstrate a new version of dispersion-scan pulse characterization scheme based on cross-polarized wave generation. We compare the performance of the new device with a SHG FROG by characterizing near-IR pulses of sub-10 fs duration.

Controlling Rogue Waves by Group-Velocity Horizons

We present a way to actively control the appearance or destruction of rogue waves in supercontinuum generation. The scheme also allows manipulating the statistics of extreme events, independently of their generation mechanism.