Saturable Bragg Reflector Research Articles

Z-scan measurements of the nonlinear refractive index of a pumped semiconductor disk laser gain medium

We measure the nonlinear refractive index of an antiresonant 1050 nm semiconductor disk laser (SDL) gain medium using a reflection-type z-scan system, with a 1064 nm, 10-ps-pulse laser as a probe, and a fiber-coupled 808 nm diode pump laser for carrier injection. Empirically, the nonlinear refractive index is found to depend approximately linearly on applied pump intensity, having a value of −1.5(0.2) × 10−12 cm2/W at zero excitation but increasing to take on positive values at typical SDL operating conditions. The focal lengths of corresponding Kerr lenses calculated using typical SDL pulse intensities and spot sizes are sufficiently short to be comparable to SDL cavity mirrors, implying that Kerr lens modelocking may be responsible for the behavior described in recent reports of self-mode-locked SDLs.

Gain-Matched Output Couplers for Efficient Kerr-Lens Mode-Locking of Low-Cost and High-Peak Power Cr:LiSAF Lasers

We present a detailed investigation of Kerr-lens mode-locking (KLM) in diode-pumped Cr:LiSAF lasers employing gain-matched output couplers (GMOCs). Single-mode diodes (660 nm, 130 mW) and tapered diodes (685 nm, 1W) were investigated as diode pump sources. We have shown that, by reducing the gain-filtering effect, GMOC enhances t he effective self amplitude modulation depth and provides a significant improvement in both efficiency and robustness of KLM operation. From the single-mode diode pumped system, 13 fs pulses with an average power of 25 mW and a peak power 25 kW have been generated at an incident pump power of only 120 mW (21% optical-to-optical conversion efficiency). The tapered diode pumped Cr:LiSAF oscillator provided pulses as short as 14.5 fs with 106 mW average power and 60 kW peak power at an incident pump power of 940 mW (11% optical-to-optical conversion efficiency). A femtosecond tuning range extending from 807 nm to 919 nm was also realized with sub-50-fs long pulses. To our knowledge, demonstrated optical-to-optical conversion efficiencies, peak powers and fs tuning ranges are record results for Cr:LiSAF laser oscillators. The obtainable pulsewidth and fs tuning range are currently limite d by the dispersion/reflectivity bandwidth of the available optics.

Intracavity KTP optical parametric oscillator driven by a KLM Nd:GGG laser with a single AO modulator

Abstract An intracavity KTiOPO4 (KTP) optical parametric oscillator (OPO) pumped by a Kerr lens mode-locking (KLM) Nd:GGG laser near 1062 nm with a single AO modulator was realized for the first time. The mode-locking pulses of the signal wave were obtained with a short duration of subnanosecond and a repetition rate of several kilohertz (kHz). Under a diode pump power of 8.25 W, a maximum output power of 104 mW at signal wavelength near 1569 nm was obtained at a repetition rate of 2 kHz. The highest pulse energy and peak power were estimated to be 80 μJ and 102 kW at a repetition rate of 1 kHz, respectively. The shortest pulse duration was measured to be 749 ps. By considering the Gaussian spatial distribution of the photon density and the Kerr-lens effect in the gain medium, a set of the coupled rate equations for QML intracavity optical parametric oscillator are given and the numerical simulations are basically fitted with the experimental results.

Effects of resonator input power on Kerr lens mode-locked lasers

Using the ABCD matrix method, the common stability region between the sagittal and tangential planes of a four-mirror Kerr lens mode-locked (KLM) laser cavity is obtained for different ranges of input power. In addition, the effect of the input power on the Kerr lens sensitivity is investigated. Optimal input power and position for highest Kerr lens sensitivity in the stability region are presented and self-starting regime has been achieved. Results show that the resonator input power has a great influence on designing the KLM lasers which can be used in fabricating an optimal femtosecond laser.

Diode-pumped Kerr-lens mode-locked Yb:LYSO laser with 61fs pulse duration.

A stable diode pumped Kerr-lens mode-locked (KLM) Yb:LuYSiO5 (Yb:LYSO) laser of generating 61 fs pulses at a central wavelength of 1055.4 nm is experimentally demonstrated. This is, to the best of our knowledge, the first demonstration of femtosecond KLM operation in Yb:LYSO laser, and it is believed that 61 fs is the shortest pulse duration ever produced from an Yb-doped orthosilicate laser. The average output power of the mode-locked laser is 40 mW and the repetition rate is 113 MHz.

Femtosecond laser pulses in a Kerr lens mode-locked thin-disk ring oscillator with an intra-cavity peak power beyond 100 MW

We report 860 W average power, ∼440 fs laser pulse generation inside a 15.2 MHz thin-disk ring oscillator under development for intra-cavity high-order harmonic generation (HHG). This has been achieved by placing a Kerr lens mode-locking setup in a low-pressure chamber. The intra-cavity pulse energy is ∼57 µJ with an estimated peak power of 130 MW, which is the highest, to the best of our knowledge, in ring type oscillators. By focusing these laser pulses, we can expect a peak intensity high enough for demonstration of intra-cavity HHG at the multi MHz repetition rate to facilitate experiments in photoelectron spectroscopy.

Femtosecond petawatt laser

AbstractThe high‐power femtosecond laser has now become an excellent scientific tool for the study of not only relativistic laser–matter interactions but also scientific applications. The high‐power femtosecond laser depends on the Kerr‐lens modelocking (KLM) and chirped‐pulse amplification (CPA) technique. An all‐Ti:sapphire‐based 30‐fs PW CPA laser, which is called the PULSER (Petawatt Ultrashort Laser System for Extreme Science Research) has been recently constructed and is being used for accelerating the charged particles (electrons and protons) and generating ultrashort high‐energy photon (X‐ray and γ‐ray) sources. In this review, the world‐wide PW‐level femtosecond laser systems are first summarized, the output performances of the PULSER‐I & II are described, and the future upgrade plan of the PULSER to the multi‐PW level is also discussed. Then, several experimental results on particle (electron and proton) acceleration and X‐ray generation in the intensity range of mid‐1018 W/cm2 to mid‐1020 W/cm2 are described. Experimental demonstrations for the newly proposed phenomena and the understanding of physical mechanisms in relativistic and ultrarelativistic regimes are highly expected as increasing the laser peak intensity up to over 1022 W/cm2 ∼1023 W/cm2.

A Review of Cavity Design for Kerr Lens Mode-Locked Solid-State Lasers

We provide a critical review of the fundamental concepts of Kerr lens mode-locking (KLM), along with a detailed description of the experimental considerations involved in the realization of a mode-locked oscillator. In addition, we review recent developments that overcome inherent limitations and disadvantages in standard KLM lasers. Our review is aimed mainly at readers who wish to realize/maintain such an oscillator or for those who wish to better understand this major experimental tool.

High-brightness fiber laser-pumped 68 fs–23 W Kerr-lens mode-locked Yb:CaF_2 oscillator

By using a high-brightness fiber pump laser, we demonstrate a pure Kerr-lens mode-locked (ML) Yb:CaF(2) oscillator. The laser delivers 68 fs pulses with 2.3 W average power at 73 MHz repetition rate and an optical-to-optical efficiency of 33% is achieved. To the best of our knowledge, this is the first demonstration of Kerr-lens mode-locking in Yb:CaF(2). Incidentally, we report here the highest average power ever achieved for a sub-100-fs active Kerr-lens ML Yb-bulk oscillator.

Lateral Oxidation of AlAs for Circular and Inverted Mesa Saturable Bragg Reflectors

A model of the thermal oxidation kinetics that is based on the continuity equation is adapted to the fabrication of circular saturable Bragg reflector (SBR) mesa structures, as well as inverted mesa array structures. A comparison between theoretical calculations and experimental measurements shows good agreement, making the model a useful tool for the controllability of the structure-dependent oxidation process. The model is applied to the wet oxidation of buried AlAs layers for the fabrication of ultra-broadband, high index contrast, III-V/ AlxOy SBRs. The rate of oxidation is found to be dependent not only on the oxidation temperature, but also on oxidation time, mesa geometry, and AlAs layer thickness. The two distinct large-area SBR structures, the circular mesa and the inverted mesa, are fabricated.

Tunable femtosecond laser in the visible range with an intracavity frequency-doubled optical parametric oscillator

We demonstrated experimentally a synchronously pumped intracavity frequency-doubled femtosecond optical parametric oscillator (OPO) using a periodically-poled lithium niobate (PPLN) as the nonlinear material in combination with a lithium triborate (LBO) as the doubling crystal. A Kerr-lens-mode-locked (KLM) Ti:sapphire oscillator at the wavelength of 790 nm was used as the pump source, which was capable of generating pulses with a duration as short as 117 fs. A tunable femtosecond laser covering the 624–672 nm range was realized by conveniently adjusting the OPO cavity length. A maximum average output power of 260 mW in the visible range was obtained at the pump power of 2.2 W, with a typical pulse duration of 205 fs assuming a sech2 pulse profile.

Large-area broad band saturable Bragg reflectors using oxidized AlAs in the circular and inverted mesa geometries

A semiconductor Saturable Bragg Reflector (SBR) is a mirror structure comprising alternating layers of high and low refractive index materials with an incorporated saturable absorber. SBRs can be used to initiate and sustain ultra-short pulses in various laser systems. In order to form ultra-short pulses, SBRs with high reflectivity over a broad wavelength range are required. Furthermore, large-area SBRs facilitate easy integration in a laser cavity. One of the key elements for the realization of broad band SBRs is the development of the thermal oxidation process that creates buried low-index AlxOy layers over large areas. The design, fabrication, characterization, and implementation of broad band, high index contrast III-V/AlxOy SBRs in the form of circular mesas, as well as inverted mesa structures, is presented.

Low-cost, broadly tunable (375–433 nm & 746–887 nm) Cr:LiCAF laser pumped by one single-spatial-mode diode

We describe a low-cost, compact, and efficient Cr(3+):LiCaAlF(6) (Cr:LiCAF) laser that is broadly tunable in the near infrared (746-887 nm) and blue/ultraviolet (375-433 nm) regions of the optical spectrum. The pump source is a 660 nm single-mode-diode with 145 mW of output power. A melt-grown Cr:LiCAF crystal with an extremely high figure of merit above 2000 was used as the gain medium. This enabled the construction of a high-Q-cavity with continuous wave (cw) lasing thresholds as low as 3 mW, slope efficiencies as high as 54%, and output powers up to 63 mW. The stored intracavity power levels were above 30 W. By placing a beta-barium-borate crystal at a second intracavity focus, we could obtain tunable cw blue/ultraviolet radiation with output powers up to 3.5 mW around 400 nm. When mode-locked using a saturable Bragg reflector around 800 nm, the Cr:LiCAF laser produced 95 fs pulses with average powers of 33 mW and peak powers of 3.58 kW at a repetition rate of 85.5 MHz. Extracavity second harmonic generation enables generation of femtosecond pulses around 400 nm. These results demonstrate the possibility to grow Cr:LiCAF crystals with passive losses below 0.15% per cm, which enables construction of low-cost and complexity systems that are pumped only with one low-power single-mode-diode.

Power scaling of a directly diode-laser-pumped Ti:sapphire laser

Improvements in the output power of a directly GaN diode-laser-pumped Ti:Al2O3 laser are achieved by using double-sided pumping. In continuous wave operation, an output power of 159 mW is reported. A tuning range of over 125 nm with output powers in excess of 100 mW is achieved. Pulses of 111 fs duration and an average power of 101 mW are demonstrated by mode locking the laser with a saturable Bragg reflector. Pumping with GaN diode lasers at wavelengths around 450 nm induces an additional parasitic crystal loss of about 1% per resonator roundtrip that is not observed at the conventional green pump wavelengths.

High-power Kerr-lens mode-locked Yb:YAG thin-disk oscillator in the positive dispersion regime

We demonstrate a self-starting Kerr-lens mode-locked (KLM) Yb:YAG thin-disk oscillator operating in the regime of positive intracavity group-delay dispersion (GDD). It delivers 1.7 ps pulses at an average power of 17 W and a repetition rate of 40 MHz. Dispersive mirrors compress the pulses to a duration of 190 fs (assuming sech2 shape; Fourier limit: 150 fs) at an average power level of 11 W. To our knowledge, this is the first KLM thin-disk oscillator with positive GDD. Output powers of up to 30 W were achieved with an increased output coupler transmission and intracavity GDD. We demonstrate increase of the pulse energy with increasing positive intracavity GDD, limited by difficulties in initiating mode-locking.

Compact and efficient Cr:LiSAF lasers pumped by one single-spatial-mode diode: a minimal cost approach

In this study, we report a minimal-cost and minimal-complexity Cr:LiSAF laser that is pumped only by one inexpensive single-spatial-mode diode. The pump diode, which was originally developed for DVD-writers, provides 130 mW of output power at 660 nm with an efficiency of 30%. This simple pump source enables the construction of a Cr:LiSAF laser that (i) has an estimated total material cost below US$ 5k, (ii) has a footprint of about 20×30  cm, (iii) does not require active cooling, and (iv) can be driven by batteries. All of these make this system ideal for applications that require portability. In continuous-wave (cw) laser experiments, we have demonstrated lasing thresholds as low as 2 mW, slope efficiencies as high as 52%, and output powers up to 58 mW. A record cw tuning range extending from 780 to 1110 nm has also been obtained. In cw mode-locking experiments using a saturable Bragg reflector at 850 nm, the Cr:LiSAF laser produced 100 fs pulses with an average power of 38 mW at a repetition rate of 235 MHz. Using a more compact laser cavity, we have also obtained 130 fs pulses with an average power of 33 mW at a repetition rate of 757 MHz. The corresponding electrical-to-optical conversion efficiencies in cw and cw mode-locked regimes were 12.8% and 8.4%, respectively. These results show that, with the progress in laser-diode and optical mirror technology in the last decade, reasonable output powers can now be obtained from Cr:LiSAF lasers that are pumped only by one single-spatial-mode diode. We believe that this compact, low-cost, and simplistic Cr:LiSAF laser system may be an attractive source for several applications including amplifier seeding.

Saturation of the all-optical Kerr effect in solids

We discuss the influence of the higher-order Kerr effect (HOKE) in wide bandgap solids at extreme intensities below the onset of optically induced damage. Using different theoretical models, we employ multiphoton absorption rates to compute the nonlinear refractive index by a Kramers-Kronig transform. Within this theoretical framework we provide an estimate for the appearance of significant deviations from the standard optical Kerr effect predicting a linear index change with intensity. We discuss the role of the observed saturation behavior in practically relevant situations, including Kerr lens mode-locking and supercontinuum generation in photonic crystal fibers. Furthermore, we present experimental data from a multiwave mixing experiment in BaF2, which can be explained by the appearance of the HOKE.

Optical trapping with “on-demand” two-photon luminescence using Cr:LiSAF laser with optically addressed saturable Bragg reflector

We demonstrate a diode-pumped Cr:LiSAF laser with controllable and reliable fast switching between its continuous-wave and mode-locked states of operation using an optically-addressed semiconductor Bragg reflector, permitting dyed microspheres to be continuously trapped and monitored using a standard microscope imaging and on-demand two-photon-excited luminescence techniques.

Thermal effects in semiconductor saturable-absorber mirrors

A semianalytical evaluation of laser-beam-induced thermal deformation of III–V semiconductor-based saturable Bragg reflectors is presented. The thermally induced bowing and resulting optical aberrations for different incident laser spot sizes and saturable Bragg reflector dimensions are calculated. Also discussed are the effects of the major physical and optical SESAM and laser parameters.

Operation of femtosecond Kerr-lens mode-locked Cr:ZnSe lasers with different dispersion compensation methods

We employed various low-cost dispersion compensation methods to generate femtosecond pulses from a Kerr-lens mode-locked (KLM) Cr:ZnSe laser operating near 2400 nm. Prism pairs made of CaF2 and MgF2 and slabs of BK7 and YAG were tested. Pulses as short as 92 fs were obtained when a CaF2 prism pair was used in the resonator with a 1% output coupler. With a 6% output coupler and CaF2 prism pair, pulse energies as high as 1.8 nJ were obtained. The KLM operating point was further analyzed for different dispersion compensation scenarios by using the soliton area theorem to determine the nonlinear refractive index (n 2) of Cr:ZnSe. Results gave an n 2 value of (1.2±0.2)×10−18 m2/W in agreement with previous reports.