mid-IR Laser System Research Articles

Refining the Performance of mid-IR CPA Laser Systems Based on Fe-Doped Chalcogenides for Nonlinear Photonics

The chirped pulse amplification (CPA) systems based on transition-metal-ion-doped chalcogenide crystals are promising powerful ultrafast laser sources providing access to sub-TW laser pulses in the mid-IR region, which are highly relevant for essential scientific and technological tasks, including high-field physics and attosecond science. The only way to obtain high-peak power few-cycle pulses is through efficient laser amplification, maintaining the gain bandwidth ultrabroad. In this paper, we report on the approaches for mid-IR broadband laser pulse energy scaling and the broadening of the gain bandwidth of iron-doped chalcogenide crystals. The multi-pass chirped pulse amplification in the Fe:ZnSe crystal with 100 mJ level nanosecond optical pumping provided more than 10 mJ of output energy at 4.6 μm. The broadband amplification in the Fe:ZnS crystal in the vicinity of 3.7 μm supports a gain band of more than 300 nm (FWHM). Spectral synthesis combining Fe:ZnSe and Fe:CdSe gain media allows the increase in the gain band (~500 nm (FWHM)) compared to using a single active element, thus opening the route to direct few-cycle laser pulse generation in the prospective mid-IR spectral range. The features of the nonlinear response of carbon nanotubes in the mid-IR range are investigated, including photoinduced absorption under 4.6 μm excitation. The study intends to expand the capabilities and improve the output characteristics of high-power mid-IR laser systems.

Diode-side-pumped watt-level high-energy Q-switched mid-IR Er:YLF laser.

We report on a powerful mid-IR diode-side-pumped tunable Er:LiYF4 (Er:YLF) laser electro-optically Q-switched with the help of a KTiOPO4 crystal. At a 20 Hz repetition rate, the laser pulses with output energy of 82 mJ and 13 ns duration at the wavelength of 2.67 µm are obtained. At higher repetition rates (up to 50 Hz), one can extract up to 20 mJ from the laser cavity. The developed mid-IR laser source demonstrates high peak (up to 6.3 MW) and average (up to 1.7 W) power. Realized wavelength tuning provides access for megawatt-peak power-level nanosecond laser pulses over the 2667-2851 nm wavelength region, which are highly demanded for mid-IR laser systems development and light-matter interaction study in the view of extreme-state creation in liquids and solids, paving the way to novel microprocessing techniques.

Single-Nanosecond-Pulse Lasing in Heavily Doped Fe:ZnSe

We investigated room-temperature pulsed lasing in heavily doped Fe:ZnSe single crystals. The active elements were pumped by a Q-switched Cr <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> :Yb <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> :Ho <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> :YSGG laser operating at 2.87 μm. Our results show that the generation of short laser pulses has a deep high-frequency modulation associated with relaxation dynamics in Fe:ZnSe. The lasing regime obtained in this study provides a straightforward way to generate mid-IR single nanosecond pulses at moderate pump energies. Moreover, we found a relation between the lasing pulse duration and the concentration of Fe <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2+</sup> doping ions, and we experimentally demonstrated pulse shortening in heavily doped active crystals. Single-pulse lasing with an FWHM pulse duration of ~2.8 ns was achieved in ZnSe crystals doped with 2.3 · 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">19</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> Fe <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2+</sup> ions. The demonstrated single-pulse lasing regime is applicable for seeding high-power mid-IR laser systems.

Amplification properties of polycrystalline Fe:ZnSe crystals for high power femtosecond mid-IR laser systems

Abstract In this Letter, a multi-pass amplifier based on polycrystalline Fe:ZnSe element grown by the chemical vapor deposition (CVD) method and doped through diffusion technique is realized with output energy of more than 3 mJ at 4.5 μm. The crystal used in the experiment has external doping layers with peak Fe ions concentration of (1-2)х1019 сm−3 located on the surface and Fe ions diffusion depth of hundreds of microns. The crystal demonstrates small-signal gain per pass of 5.7 (corresponding to effective gain of 7.1 cm−1 taking into account 8 mm total length of the crystal) for short pump pulse (40-ns) under the optimal pump fluence of around 0.8 J/cm2 and upper-level lifetime of 670 ± 10 ns under moderate cooling to 7-8 °C, which makes such a crystal very attractive for mid-IR femtosecond multi-pass amplifiers and generators. Moreover, properties of the polycrystalline Fe:ZnSe elements grown by CVD and doped through diffusion and CVD methods with higher Fe ions concentration are also studied at different temperatures and compared with a single crystal element.

Role of wavelength in photocarrier absorption and plasma formation threshold under excitation of dielectrics by high-intensity laser field tunable from visible to mid-IR

The development of high power mid-IR laser applications requires a study on laser induced damage threshold (LIDT) in the mid-IR. In this paper we have measured the wavelength dependence of the plasma formation threshold (PFT) that is a LIDT precursor. In order to interpret the observed trends numerically, a model describing the laser induced electron dynamics, based on multiple rate equations, has been developed. We show both theoretically and experimentally that PFT at mid-IR wavelengths is controlled by a transition from weak- to strong-field regime of free carrier absorption. In the case of MgF_2 this transition occurs around 3–4 upmum corresponding to the region of the lowermost PFT. The region of the uppermost PFT is reached around 1 upmum and is governed by an interplay of photoionization and weak-field free carrier absorption which manifests itself in both MgF_2 and SiO_2. The PFT observed in considered materials exhibits a universal dependence on the excitation wavelength in dielectrics. Thus, the presented results pave the route towards efficient and controllable laser-induced material modifications and should be of direct interest to laser researchers and application engineers for prevention of laser-induced damage of optical components in high-intensity mid-IR laser systems.

Resolution Enhancement in Wide-Field IR Imaging and Time-Domain Spectroscopy Using Dielectric Microspheres.

Wide-field imaging through dielectric microspheres has emerged in recent years as a simple and effective approach for generating super-resolution images at visible wavelengths. We present, to our knowledge, the first demonstration that dielectric microspheres can be used in a wide-field infrared (IR) microscope to enhance the far field resolution. We have observed a substantial improvement in resolution and magnification when images are collected through polystyrene microspheres. In addition, we demonstrate that spectroscopic imaging with a pulse-shaper based femtosecond mid-IR laser system is possible through the dielectric microspheres, which is a promising first step toward applying this technique to ultrafast IR imaging methods such as pump-probe and 2DIR microscopy.

Table-top high-energy 7 μm OPCPA and 260 mJ Ho:YLF pump laser.

We present a state-of-the-art compact high-energy mid-infrared (mid-IR) laser system for TW-level eight-cycle pulses at 7μm. This system consists of an Er:Tm:Ho:fiber MOPA which serves as the seeder for a ZGP-based optical parametric chirped pulse amplification (OPCPA) chain, in addition to a Ho:YLF amplifier which is Tm:fiber pumped. Featuring all-optical synchronization, the system delivers 260mJ pump energy at 2052nm and 16ps duration at 100Hz with a stability of 0.8% rms over 20min. We show that chirp inversion in the OPCPA chain leads to excellent energy extraction and aids in compression of the 7μm pulses to eight optical cycles (188fs) in bulk BaF2 with 93.5% efficiency. Using 21.7mJ of the available pump energy, we generate 0.75mJ energy pulses at 7μm due to increased efficiency with a chirp inversion scheme. The pulse quality of the system's output is shown by generating high harmonics in ZnSe which span up to harmonic order 13 with excellent contrast. The combination of the passive carrier-envelope phase stable mid-IR seed pulses and the high-energy 2052nm picosecond pulses makes this compact system a key enabling tool for the next generation of studies on extreme photonics, strong field physics, and table-top coherent X-ray science.

Toward a sub-terawatt mid-IR (4–5 μm) femtosecond hybrid laser system based on parametric seed pulse generation and amplification in Fe2+:ZnSe

For the first time, an experimentally measured seed pulse gain of about 2 cm−1 allows possibilities in the scaling power of such a femtosecond laser system in terawatts. The concept of a subterawatt power level hybrid femtosecond mid-IR (4–5 μm) laser system, based on a weak pulse from an optical parametric mid-IR seeder that is amplified in chalcogenide monocrystalline Fe2+:ZnSe, to gain medium has been proposed and designed. The method and approach for optimizing the choice of nonlinear medium, its length, and the required light intensity for the efficient non-linear self-compression of an ultrashort pulse has also been proposed and considered.

Small-signal gain measurements for highly doped and co-doped Er3+:YAG at

Abstract Direct small-signal gain measurements have been made for a 50% doped Er3+:YAG rods for the 2.936 μ m wavelength. We also provide gain measurements for 55% Er3+:YAG and co-doped 55% Er3+:YAG rods. Erbium doped solid-state lasers are typically used in a flash-pumped configuration, where the gain dynamics is highly dependent on the pump–pulse length. In this paper we have reported the small-signal gain over the pulse duration, along with average small-signal gain, so as to provide the laser-engineer with data to efficiently design the mid-IR laser system.

A high power hybrid mid-IR laser source

An efficient hybrid mid-IR laser system comprising a thulium fibre laser, Ho:YAG solid state laser and a zinc germanium phosphide optical parametric oscillator is presented. A 790 nm diode pumped 1908 nm thulium fibre laser operating at 30 W pumps an RTP q-switched Ho:YAG laser emitting 17 W at 40 kHz and 2090 nm. The zinc germanium phosphide optical parametric oscillator efficiently converts this into the 3–5 μm region producing 10.1 W with 59% optical conversion efficiency and an M 2 = 1.5.

Hot-pressed ceramic Cr2+:ZnSe gain-switched laser

The technology of hot-pressed Cr(2+):ZnSe ceramic preparation is reported. Comparative gain-switched lasing of hot-pressed ceramic and CVD grown Cr(2+):ZnSe samples with slope efficiencies up to 10% and output energies up to 2 mJ were demonstrated. This is a milestone in the development of future large scale mid-IR laser systems based on ceramic TM(2+):II-VI materials. This evolving technology has excellent potential as media for practical low cost, high power mid-IR laser applications.

Development of a tunable mid-IR difference frequency laser source for highly sensitive airborne trace gas detection.

The development of a compact tunable mid-IR laser system at 3.5 micrometers for quantitative airborne spectroscopic trace gas absorption measurements is reported. The mid-IR laser system is based on difference frequency generation (DFG) in periodically poled LiNbO3 and utilizes optical fiber amplified near-IR diode and fiber lasers as pump sources operating at 1083 nm and 1562 nm, respectively. This paper describes the optical sensor architecture, performance characteristics of individual pump lasers and DFG, as well as its application to wavelength modulation spectroscopy employing an astigmatic Herriott multi-pass gas absorption cell. This compact system permits detection of formaldehyde with a minimal detectable concentration (1 sigma replicate precision) of 74 parts-per-trillion by volume (pptv) for 1 min of averaging time and was achieved using calibrated gas standards, zero air background and rapid dual-beam subtraction. This corresponds to a pathlength-normalized replicate fractional absorption sensitivity of 2.5 x 10-(10 )cm-1.

Analysis of cavitation dynamics during pulsed laser tissue ablation by optical on-line monitoring

Flashlamp pumped mid-IR laser systems emitting in the 2-3-/spl mu/m wavelength range are widely used for various medical applications, especially for tissue ablation. Explosive evaporation is inevitably associated with this process due to the short pulse durations of these laser systems and the high absorption of tissue and water in this spectral regime. Tissue displacement and dissection occur in liquid environment as a consequence of the induced cavitation. Depending on the application these processes might enhance the tissue ablation but can also cause adverse tissue effects. The ablation dynamics were investigated by evaluating the change in reflected probe-light intensity re-emitted from the application fiber tip. The ablated cavity and the signal was correlated to fast-flash photographs of the event. Based on this reflection signal a water/tissue discrimination system is introduced which can widely support medical laser applications. In laser sclerostomy ab externo, for example, this approach can be used as a feedback system to automatically control the ablation process. With such a system, adverse effects to adjacent tissue in the anterior chamber of the eye can be minimized.