High Repetition Rate Pulse Trains Research Articles

Design of an Optimized Terahertz Time-Domain Spectroscopy System Pumped by a 30 W Yb:KGW Source at a 100 kHz Repetition Rate with 245 fs Pulse Duration

Ytterbium laser sources are state-of-the-art systems that are increasingly replacing Ti:Sapphire lasers in most applications requiring high repetition rate pulse trains. However, extending these laser sources to THz Time-Domain Spectroscopy (THz-TDS) poses several challenges not encountered in conventional, lower-power systems. These challenges include pump rejection, thermal lensing in nonlinear media, and pulse durations exceeding 100 fs, which consequently limit the detection bandwidth in TDS applications. In this article, we describe our design of a THz-TDS beamline that seeks to address these issues. We report on the effectiveness of temperature controlling the Gallium Phosphide (GaP) used to generate the THz radiation and its impact on increasing the generation efficiency and aiding pump rejection while avoiding thermal distortions of the residual pump laser beam. We detail our approach to pump rejection, which can be implemented with off-the-shelf products and minimal customization. Finally, we describe our solution based on a commercial optical parametric amplifier to obtain a temporally compressed probe pulse of 55 fs duration. Our study will prove useful to the increasing number of laboratories seeking to move from the high-energy, low-power THz time-domain spectroscopy systems based on Ti:Sapphire lasers, to medium-energy, high-power systems driven by Yb-doped lasers.

Designing a single-mode anomalous dispersion silicon core fiber for temporal multiplet formation.

A highly nonlinear single-mode anomalous dispersion silicon core fiber (SCF) is suitably designed and optimized to generate a high repetition rate pulse train in the temporal domain from a single input pulse at a sufficiently shorter optimum length in comparison to silica-based standard fibers used for the same purpose. The large amount of Kerr-induced nonlinearity of a SCF is effectively utilized here such that input Gaussian pulses or pulse trains transform into a highly repetitive temporal multiplet. The effects of free-carrier generation-induced change in absorption and dispersion are included while studying the nonlinear pulse propagation through the SCF. To declare the generated pulse as a superior-graded triplet, a Q parameter, as a function of relative pulse parameters of the individual pulses of a triplet, is defined for the first time, to the best of our knowledge. Different pulse parameters are thoroughly optimized as well as the effect of external gain is examined from the perspective of requirement of shorter fiber length and development of quality triplets. Finally, the work is further extended for the formation of quadruplet pulses by the same type of SCF. It is to be mentioned here that such a methodical study for the generation of a temporal multiplet using a semiconductor core fiber has not been reported earlier.

Nonlinear pulse reshaping in a typically designed silicon-on-insulator waveguide and its application to generate a high repetition rate pulse train

A silicon-on-insulator (SOI) planar waveguide is designed with a small group velocity dispersion (β 2) ∼2.212 (ps2 m−1) and a quite high nonlinear coefficient (γ) of ∼360.57 (W−1 m−1). The designed waveguide is capable of reshaping a super-Gaussian input optical pulse into a parabolic pulse (PP), without any use of external gain. The same waveguide with a relatively longer length can also generate a triangular pulse (TP) by using a positive chirp at the input. In both cases, PP and TP are created at a much shorter optimum length (L opt) of a few millimeters when compared to previously reported works on normal dispersion optical fibers. The interaction of a pulse pair inside such an SOI waveguide is investigated, for the first time as per our knowledge to generate a high-frequency (∼4.8 THz) pulse train, while lower repetition rate (∼180 GHz) pulses are used at the input. This study as a whole enables potential device applications in the domains of tunable high frequency (THz) pulse generators, optical signal processing, and many more.

Femtosecond laser induced structural changes in fluorozirconate glass

Fluorozirconate glasses, such as ZBLAN (ZrF4-BaF2-LaF3-AlF3-NaF), have a high infrared transparency and large rare-earth solubility, which makes them an attractive platform for highly efficient and compact mid-IR waveguide lasers. We investigate the structural changes within the glass network induced by high repetition rate femtosecond laser pulses and reveal the origin of the observed decrease in refractive index by using Raman microscopy. The high repetition rate pulse train causes local melting followed by rapid quenching of the glass network. This results in breaking of bridging bonds between neighboring zirconium fluoride polyhedra and as the glass resolidifies, a larger fraction of single bridging fluorine bonds relative to double bridging links are formed in comparison to the pristine glass. The distance between adjacent zirconium cations is larger for single bridging than double bridging links and consequently an expansion of the glass network occurs. The rarified glass network can be related to the experimentally observed decrease in refractive index via the Lorentz-Lorenz equation.

Direct frequency comb spectroscopy in the extreme ultraviolet

The development of the optical frequency comb (a spectrum consisting of a series of evenly spaced lines) has revolutionized metrology and precision spectroscopy owing to its ability to provide a precise and direct link between microwave and optical frequencies. A further advance in frequency comb technology is the generation of frequency combs in the extreme-ultraviolet spectral range by means of high-harmonic generation in a femtosecond enhancement cavity. Until now, combs produced by this method have lacked sufficient power for applications, a drawback that has also hampered efforts to observe phase coherence of the high-repetition-rate pulse train produced by high-harmonic generation, which is an extremely nonlinear process. Here we report the generation of extreme-ultraviolet frequency combs, reaching wavelengths of 40 nanometres, by coupling a high-power near-infrared frequency comb to a robust femtosecond enhancement cavity. These combs are powerful enough for us to observe single-photon spectroscopy signals for both an argon transition at 82 nanometres and a neon transition at 63 nanometres, thus confirming the combs' coherence in the extreme ultraviolet. The absolute frequency of the argon transition has been determined by direct frequency comb spectroscopy. The resolved ten-megahertz linewidth of the transition, which is limited by the temperature of the argon atoms, is unprecedented in this spectral region and places a stringent upper limit on the linewidth of individual comb teeth. Owing to the lack of continuous-wave lasers, extreme-ultraviolet frequency combs are at present the only promising route to extending ultrahigh-precision spectroscopy to the spectral region below 100 nanometres. At such wavelengths there is a wide range of applications, including the spectroscopy of electronic transitions in molecules, experimental tests of bound-state and many-body quantum electrodynamics in singly ionized helium and neutral helium, the development of next-generation 'nuclear' clocks and searches for variation of fundamental constants using the enhanced sensitivity of highly charged ions.

Estimating intensity fluctuations in high repetition rate pulse trains generated using the temporal Talbot effect

We derive the upper limit of the peak intensity fluctuations for high repetition rate pulse trains generated using the fractional temporal Talbot effect for pulse repetition rate multiplication. Our analysis is general, can be applied to any input pulse shape, and can be used as a design tool to determine the input pulsewidth that will minimize the output amplitude fluctuations for a given multiplication rate.

Corrections to "Estimating Intensity Fluctuations in High Repetition Rate Pulse Trains Generated Using the Temporal Talbot Effect"

Corrections to "Estimating Intensity Fluctuations in High Repetition Rate Pulse Trains Generated Using the Temporal Talbot Effect"

Talbot self-imaging and cross-phase modulation for generation of tunable high repetition rate pulse trains

We report generation of an ultra-high speed tunable pulse source with a repetition rate of 10, 30 or 40GHz by a technique implementing a tunable chirped fibre Bragg grating. An optical pulse train of variable repetition rate is derived in a tunable dispersion compensator via the fractional temporal Talbot effect, acting on the output of a 10GHz mode-locked laser. This ultra-high speed modulation is then used for phase modulation of a spectrally tunable probe by cross-phase modulation in highly nonlinear fibre, which is converted into constant-phase intensity modulation by spectral filtering.

External fibre laser based pulse amplitude equalisation scheme for rational harmonic modelocking in a ring-type fibre laser

The authors have obtained, for the first time to their knowledge, pulse amplitude equalisation in a high repetition rate (up to 22.6 GHz) pulse train, which is generated from rational harmonic modelocking of an Er-doped fibre ring laser, by using an equalising fibre laser. The equalising fibre laser is based on a passively modelocked laser with an NALM (nonlinear amplifying loop mirror) and a Faraday rotating mirror.

Programmable shaping of ultrabroad-bandwidth pulses from a Ti:sapphire laser

We have used a commercially available liquid-crystal spatial light modulator within a reflective optics pulse-shaping apparatus to shape ultrashort pulses with temporal resolution approaching 10 fs. Using the spatial light modulator as a phase modulator, we produce a variety of complex ultrafast waveforms, including odd pulses, high repetition rate (>23 THz) pulse trains, and asymmetric pulse trains. We also show that it is possible to compensate for large amounts of high-order phase dispersion (in excess of 60π) by appropriate cubic- and quartic-phase modulations of the pulse. Finally, we examine the limitations of shaping ultrabroad-bandwidth pulses. We find that, for specific classes of waveforms, Fourier-transform pulse-shaping techniques can be used for pulses with 5-fs durations, which exceed the current state of the art in ultrashort pulse generation. However, synthesis of general waveforms with 5-fs resolution will require compensating for nonlinear spatial dispersion of frequency in the masking plane.

Monolithic multiple colliding pulse mode-locked quantum-well lasers, experiment and theory

We report here on a monolithic multisectioned quantum-well laser that operates as multiple colliding pulse mode-locking source. With this type of operation several pulses are present within a laser simultaneously and it produces high-repetition rate pulse trains (up to 375 GHz) and pulse techniques of around 1-3 ps, as deduced from linear correlation technique measurements. By changing the configuration of the sections 1, 2, 3, or 4 pulse operation can be selected; frequency- and time-domain theories have been developed to explain this type of behavior. >