Negative Group Delay Dispersion Research Articles

Compact compressor based on unparallel gratings

The pulse compressor is one of the essential components in a high-power laser system, which is often bulky. Here, we propose a compact compressor based on a Treacy compressor with two unparallel gratings and a mirror. Two gratings provide a negative group delay dispersion, and the mirror has two functions. One is to make the beam enter the compressor twice, and the other is to make the optical path between the grating pair folded to reduce the volume of the compressor. The relation between the group delay dispersion and the incident angle in three-dimensional space is derived. The results show that a small spatial incident angle can produce a large negative dispersion when the perpendicular distance between the gratings is the same. The parameter limits of the designed structure are also discussed, and the volume of compact compressor under the simulated parameters is two-thirds of the conventional compressor when the constraints are satisfied. This work is applicable to the optimal design of grating-based compressors with different parameters.

Tailoring octave-spanning ultrashort laser pulses using multiple prisms.

We demonstrate a novel pulse shaper in which an incident laser beam is angularly dispersed by a first prism, and then it is split into separate beams using multiple prisms. Since this new pulse shaper offers independent control of the amplitude and phase of the separate beams, it can produce pulses having desired temporal shapes. Furthermore, it imposes a significant amount of negative group delay dispersion (GDD) over an octave spectrum near visible, which can compensate for a positive GDD accumulated in the process of spectral broadening. Consequently, single-cycle or few-cycle laser pulses can be produced without the need for chirped mirrors.

Construction of a Mirror-Dispersion-Controlled Kerr Lens Mode-Locked Ti:Sapphire Pulse Laser Oscillator

We have successfully constructed a Kerr lens mode-locked (KLM) Ti:sapphire oscillator to generate ultrashort pulses of ~18 fs. The oscillator consists of only 5 elements including a pair of double-chirped mirrors to balance negative group delay dispersion in the cavity. The bandwidth of the mode-locked laser oscillator spectrum is from 600 nm to 950 nm and pulse energy is 1.5 nJ. Both the output spectrum and the output power were stable against environmental disturbance. For the pulse characterization, a second harmonic generation frequency resolved optical gating method was used.

Compact and adjustable compensator for AOD spatial and temporal dispersion using off-the-shelf components.

Random access multiphoton microscopy using two orthogonal acousto-optic deflectors (AODs) allows sampling only particular regions of interest within a plane, greatly speeding up the sampling rate. AODs introduce spatial and temporal dispersions, which distort the point spread function and decrease the peak intensity of the pulse. Both of these effects can be compensated for with a single dispersive element placed a distance before the AODs. An additional acousto-optic modulator, a custom cut prism, and a standard prism used with additional cylindrical optics have been demonstrated. All of these introduce additional cost or complexity and require an extended path length to achieve the needed negative group delay dispersion (GDD). By introducing a telescope between a transmission grating and the AODs, we correct for spatial and temporal dispersions in a compact design using only off-the-shelf components, and we show that the GDD can be tuned by translation of the telescope without adjustment of any other elements.

Broadband negative group velocity dispersion in all-dielectric metamaterial and its role in supercontinuum generation.

All-dielectric metamaterials conforming to an optical reflectionless potential (ORP) offer broadband, omni-directional suppression of reflection. Though they are predicted to possess broadband negative group velocity dispersion (GVD), ultrashort pulse propagation through such materials has not been studied so far, to the best of our knowledge. In this work, we demonstrate negative GVD and group delay dispersion over broadband covering visible to near-infrared wavelengths. We investigate the role of ORP in supercontinuum generation (SC), which is observed to be polarization independent. The negative GVD in ORPs is interesting for pulse compression, phase compensation, dispersion engineering, and controlled SC generation.

Operating characteristics of a SESAM-assisted mode-locked laser oscillator with the location of the SESAM position

To establish the operating characteristics of a saturable absorber mirror (SESAM)-assisted mode-locked laser oscillator depending on the location of the SESAM, we designed a laser oscillator with the SESAM toward the middle of the cavity. We then compared it with a conventional laser oscillator that has the SESAM at the end of the cavity. The characteristics of the mode-locked pulses generated from each laser oscillator were studied along with the effective loss and wavelength-dependent loss of the SESAM, and the stability criterion against cw generation (positive group-delay dispersion (GDD) regime) and soliton dynamics (negative GDD regime). We demonstrate that no spatial chirp occurs in an output beam generated from the designed laser oscillator unlike the conventional laser oscillator when a prism pair is used for intracavity dispersion compensation. Furthermore, we applied the configuration to a Herriott-multipass cavity laser oscillator, which delivers sub-μJ pulse energy.

Cascaded nonlinearities in high-power femtosecond optical parametric oscillator

A femtosecond optical parametric oscillator (OPO) pumped by the second harmonic of a Yb:KGW solid-state oscillator was investigated. A comparison between lithium triborate (LBO) and barium borate (BBO) crystals was carried out, with particular attention to the impact of phase mismatch on pulse characteristics. The phase mismatch induces cascaded nonlinearity, which acts as an effective nonlinear refractive index. Negative group delay dispersion of the resonator and the effective nonlinear refractive index allows soliton-like pulse formation with varying pulse durations and a bandwidth-limited time-bandwidth product. Deviation from the soliton equation was observed when intracavity power was varied. The measured pulse durations were lower than the classical soliton duration, indicating that the pulses were not classical solitons. A Z-scan experiment was used to determine nonlinear refractive indices under conditions identical to those in the oscillator cavity. Unexpectedly, the signs of nonlinear refraction as measured by the Z scan and indirectly observed in the OPO proved to be the opposite. This sign reversal could be interpreted in terms of the competing cascaded nonlinearities corresponding to self-phase modulation and cross-phase modulation between the signal and the pump pulses in the nonlinear OPO crystal.

Highly-Dispersive Mirrors With Advanced Group Delay Dispersion

We report on a novel starting design structure of highly-dispersive mirror (HDM) which combines quarter-wavelength structure and multi-cavities, introducing more wavelength-dependent storage times of incident radiation and group delay (GD). Based on such structure, a HDM with new advanced group delay dispersion (GDD) of −20 000 fs2 in the wavelength range of 1035–1045 nm is designed, fabricated and characterized. Our proposed initial multilayer structure is instructive in designing HDM aiming at introducing large GDD. Our HDM further pushes the maximum negative GDD allowed by the design’s total physical thickness to its limitation. It offers an ideal trade-off between GDD and total mirror thickness. The HDM has been applied in a fiber-chirped system, providing a considerable compensation of GDD reaching −200 000 fs2 with only 10 reflections between two mirrors, and laser pulse compression from 2.8 ps to 213 fs is realized.

Compensating chirp of attosecond X-ray pulses by a neutral hydrogen gas

It is demonstrated by numerical simulations that the attosecond chirp of high order harmonic pulses in the 530 to 1000 eV range can be partially compensated by the negative group delay dispersion of unionized molecular hydrogen gas. The transmission of X-rays through gas with the required pressure-length product for optimum chirp compensation is higher than 10% in the photon energy range that covers the K-edge of oxygen and L-edges of iron, cobalt, and nickel.

General method of passive optical pulse peak intensity stabilization through controlled self-phase modulation and over-compression

A method of peak intensity stabilization of optical pulses is presented which is based on the combination of self-phase modulation (SPM) and an optical compressor. Spectral broadening via SPM can occur for intense laser pulses propagating through a nonlinear medium. For Gaussian pulses, this nonlinear broadening results in a nonlinear spectral phase, which positively chirps the pulse. Because this is a nonlinear process, the amount of spectral broadening depends on the intensity of the laser pulse with increased broadening for higher intensities. Here we propose a technique of peak intensity stabilization by accumulating a modest amount of nonlinear spectral phase on a pulse via SPM and subsequently introducing a negative net group delay dispersion (GDD) with an optical compressor. For the appropriate value of compressor GDD, the energy-dependent chirp is balanced such that the peak intensity of the pulse is stabilized. Simulations of this method for realistic pulse parameters predict an increase in peak intensity stability by well over an order of magnitude.

Grating configurations to compress free-electron laser pulses.

The optical layout of soft X-ray grating compressors designed to provide both positive and negative group-delay dispersion (GDD) is discussed. They are tailored for chirped-pulse-amplification experiments with seeded free-electron laser sources. Designs with plane or concave gratings are discussed, depending on the sign of the GDD to be introduced.

Generation of high-power few-cycle lasers via Brillouin-based plasma amplification

Strong coupling stimulated Brillouin backscattering (sc-SBS) in plasma is potentially an efficient method of amplifying laser pulses to reach exawatt powers. Here, we report on a new regime of brillouin-based plasma amplification, producing an amplified pulse with a duration of 5 fs and unfocused intensity of 6 × 1017 W/cm2. The results are obtained from 2D particle-in-cell simulations, using two circularly polarized pump and seed pulse with Gaussian transverse profile, both at an intensity of 2.74 × 1016 W/cm2, counter-propagating in a 0.3nc plasma. The significant compression of amplified seed is achieved as a result of sc-SBS amplification as well as additional compression by the interplay between self-phase modulation and negative group delay dispersion. We show that the amplified seed retains high beam qualities since the filamentation can be prevented due to the fast compression. This scheme may pave the way for few-cycle laser pulses to reach exawatt or even zetawatt regime.

Optimization of broadband semiconductor chirped mirrors with genetic algorithm

Genetic algorithm was applied for optimization of dispersion properties in semiconductor Bragg reflectors for applications in femtosecond lasers. Broadband, large negative group-delay dispersion was achieved in the optimized design: The group-delay dispersion (GDD) as large as $$-3500\,\hbox {fs}^{2}$$ was theoretically obtained over a 10-nm bandwidth. The designed structure was manufactured and tested, providing GDD $$-3320\,\hbox {fs}^{2}$$ over a 7-nm bandwidth. The mirror performance was verified in semiconductor structures grown with molecular beam epitaxy. The mirror was tested in a passively mode-locked Yb:KYW laser.

Octave spanning wedge dispersive mirrors with low dispersion oscillations.

A novel concept for octave spanning dispersive mirrors with low spectral dispersion oscillations is presented. The key element of the so-called wedge dispersive mirror is a slightly wedged layer which is coated on a specially optimized dispersive multilayer stack by a common sputter coating process. The group delay dispersion (GDD) of a pulse reflected on a wedge dispersive mirror is nearly free of oscillations. Fabricated mirrors with negative GDD demonstrate the compression of a pulse down to 3.8 fs as good as double angled mirrors optimized for the same bandwidth.

Simultaneous second- and third- order spectral phase control of Ti:sapphire laser pulses using achromatic doublet prisms.

The standard technique commonly utilized to introduce large amounts of negative group delay dispersion (GDD) into the beam path of ultrashort laser pulses with low insertion losses is the use of a pair of prisms in a double pass configuration. However, one disadvantage of this approach is the unavoidable introduction of additional high-order spectral phase errors, most notably third-order dispersion (TOD) due to the characteristics of the refractive index of available optical materials. In this paper we provide an overview of the dispersive properties of more than 100 common types of optical glasses, used either as a bulk stretcher or in a prism compressor configuration. In addition, we present a novel method that enables independent control of GDD and TOD in a prism-only setup. The performance of different prism combinations is analyzed numerically, and design guidelines are given.

Pulse-Width Saturation and Kelly-Sideband Shift in a Graphene-Nanosheet Mode-Locked Fiber Laser with Weak Negative Dispersion

In ultrafast laser systems, graphene has become a popular saturable absorber to provide passive mode-locking with tunable wavelength. The authors elucidate the pulse width saturated mode-locking mechanism, focusing on the soliton compression that can be achieved by strengthening self-phase modulation in the regime of weakly negative group-delay dispersion. This mechanistic understanding allows for an optimized soliton pulse width of 500 femtoseconds or less from an erbium-doped fiber laser, for photonics and optoelectronics applications.

Empirical study of the group delay dispersion achievable with multilayer mirrors

With the help of the most advanced algorithms we obtained many dozens of multilayer dispersive mirror designs to empirically find limits for the maximum achievable negative value of the group delay dispersion (GDD). This value depends on the total thickness of coatings and layer material combination. Nb(2)O(5)/SiO(2) and Ta(2)O(50/SiO(2) combinations are studied in detail, for combinations HfO(2)/SiO(2) and TiO(2)/SiO(2) we obtained estimations for two bandwidths. We also show that reasonable values of third-order dispersion have no significant impact on the obtained results. Current state-of-the-art technology allows to produce designs with total physical thicknesses slightly higher than 10 µm and to achieve maximum negative GDD values corresponding to this total design thickness. Designs with total physical thickness of 15 µm and 20 µm are not realized yet due to high sensitivity to deposition errors.

275 W average output power from a femtosecond thin disk oscillator operated in a vacuum environment

We present an ultrafast thin disk laser that generates an average output power of 275 W, which is higher than any other modelocked laser oscillator. It is based on the gain material Yb:YAG and operates at a pulse duration of 583 fs and a repetition rate of 16.3 MHz resulting in a pulse energy of 16.9 μJ and a peak power of 25.6 MW. A SESAM designed for high damage threshold initiated and stabilized soliton modelocking. We reduced the nonlinearity of the atmosphere inside the cavity by several orders of magnitude by operating the oscillator in a vacuum environment. Thus soliton modelocking was achieved at moderate amounts of self-phase modulation and negative group delay dispersion. Our approach opens a new avenue for power scaling femtosecond oscillators to the kW level.

Direct measurement of group delay dispersion in metamagnetics for ultrafast pulse shaping

In this paper, we explore the use of magnetic resonant metamaterials, so called metamagnetics, as dispersive elements for optical pulse shaping. We measure both positive and negative group delay dispersion (GDD) values in a metamagnetic material using the multiphoton interference phase scan (MIIPS) technique and show pulse temporal profiles numerically. The results are compared with finite element models. These GDD properties of metamagnetics, along with previously shown tunability and loss control with gain media, enable their use in ultrashort pulse optical applications.

Attosecond-chirp compensation with material dispersion to produce near transform-limited attosecond pulses

The duration of attosecond pulses, produced through high harmonic generation from gaseous atoms, was found to be much longer than the transform-limited duration. The inherent attosecond chirp existing in the temporal structure of the attosecond pulses was analysed to obtain a scaling factor in terms of laser intensity and wavelength. We explored the attosecond chirp compensation using various materials with negative group delay dispersion to cover different spectral ranges. From numerical simulations based on the time-dependent Schrödinger equation, we showed that 38 as isolated attosecond pulses could be produced by applying intense few-cycle mid-infrared laser pulses after compensating for the attosecond chirp in Xe.