Danail Chuchumishev

Passive mode-locking of a diode-pumped Nd:YVO(4) laser by intracavity SHG in PPKTP.

Experimental results on passive mode-locking of a Nd:YVO(4) laser using intracavity frequency doubling in periodically poled KTP (PPKTP) crystal are reported. Both, negative cascaded chi((2)) lensing and frequency doubling nonlinear mirror (FDNLM) are exploited for the laser mode-locking. The FDNLM based on intensity dependent reflection in the laser cavity ensures self-starting and self-sustaining mode-locking while the cascaded chi((2)) lens process contributes to substantial pulse shortening. This hybrid technique enables generation of stable trains of pulses at high-average output power with several picoseconds pulse width. The pulse repetition rate of the laser is 117 MHz with average output power ranging from 0.5 to 3.1 W and pulse duration from 2.9 to 5.2 ps.

Subnanosecond, mid-IR, 0.5 kHz periodically poled stoichiometric LiTaO3 optical parametric oscillator with over 1 W average power.

We report a subnanosecond mid-IR tunable optical parametric oscillator based on periodically poled stoichiometric lithium tantalate (PPSLT), pumped by an amplified single frequency microchip laser at 1064 nm at a repetition rate of 0.5 kHz. Using a 20 mm long PPSLT crystal polled with three different domain periods (30.2, 30.3, and 30.4 μm) and changing the temperature of the crystal from 20°C to 265°C, we achieved wavelength tuning between 2990 and 3500 nm. The high nonlinearity of the used medium and the large aperture (3.2 mm) ensure maximum idler output energy of ~2 mJ in the whole tuning range, corresponding to 18% idler conversion efficiency and more than 1 W of average power. 270 ps idler pulse durations were obtained as a result of the 818 ps pulse duration of the pump.

High-energy picosecond OPO based on PPKTP

Output energy of 1 mJ is obtained for the 380 ps long idler pulses at 2800 nm from a short cavity singly resonant 500 Hz optical parametric oscillator employing PPKTP and a near-diffraction-limited, single frequency, sub-nanosecond pump source at 1064 nm.

52-mJ, kHz-Nd:YAG laser with diffraction limited output

We present Nd:YAG, diode pumped amplifier system emitting up to 52-mJ pulse energy with 1.6-ns pulse duration and near diffraction limited beam, operating at 0.75-kHz repetition rate.

Efficient tissue ablation using a laser tunable in the water absorption band at 3 microns with little collateral damage

Lasers can significantly advance medical diagnostics and treatment. At high power, they are typically used as cutting tools during surgery. For lasers that are used as knifes, radiation wavelengths in the far ultraviolet and in the near infrared spectral regions are favored because tissue has high contents of collagen and water. Collagen has an absorption peak around 190 nm, while water is in the near infrared around 3,000 nm. Changing the wavelength across the absorption peak will result in significant differences in laser tissue interactions. Tunable lasers in the infrared that could optimize the laser tissue interaction for ablation and/or coagulation are not available until now besides the Free Electron Laser (FEL). Here we demonstrate efficient tissue ablation using a table-top mid-IR laser tunable between 3,000 to 3,500 nm. A detailed study of the ablation has been conducted in different tissues. Little collateral thermal damage has been found at a distance above 10-20 microns from the ablated surface. Furthermore, little mechanical damage could be seen in conventional histology and by examination of birefringent activity of the samples using a pair of cross polarizing filters.

Near diffraction limited pulses with 52-mJ, 1.2 ns at 0.5 kHz, generated by Nd-based MOPA

In this work, we report the amplification of laser pulses from a near diffraction limited, passively Q-switched Nd:YAG master oscillator (0.58 mJ, 1.2 ns at 0.5 kHz) up to 52-mJ in a Nd:YVO4 preamplifier and two diode pumped boost YAG-amplifiers, whilst preserving pulse duration, beam quality and linear polarization.

Picosecond energy scalable kHz OPO/OPA tunable in 3–3.5 μm mid-IR spectral range

We demonstrate 740 ps, 4.1-mJ tunable mid-IR PPSLT based OPO/OPA pumped by 38-mJ, 800 ps, Nd-laser system operated at 0.5–1kHz repetition rate.

Mid-IR Laser Tissue Ablation with Little Collateral Damage Using a Laser Tunable in the Water Absorption Peak

A comprehensive experimental study of mid-IR laser tissue ablation within the water absorption peak is presented. A novel all-solid-state table-top sub-ns mid-IR laser designed for efficient tissue ablation have been used for observation of wavelength-dependent effects on the ablation of hard and soft tissue.

5 mJ, sub-nanosecond PPSLT OPA at 0.5 kHz, tunable in the water absorption band at 3 microns

Summary form only given. Coherent mid-IR sources (2.5-4 μm) combining high average power and high pulse energy are essential for a great number of industrial, scientific and commercial applications. Furthermore, they have broad medical implications, due to the high water absorption around 3μm. Due to the limited choice of laser materials emitting directly in this spectral range, covering it can be efficiently achieved through nonlinear frequency conversion devices, pumped by Q-switched Nd-laser systems, operating at kHz repetition rates. A fundamental efficiency limitation in short-pulsed OPOs is the available resonant wave build-up time. In order to keep this time period small compared to the pump pulse duration in short cavity OPOs, one should make use of highly nonlinear optical media. Furthermore, pulsed OPOs reaching mJ level output are prone to optical component damage due to the high amount of energy circulating inside the OPO cavity [1]. Mid-IR OPO output can be scaled significantly beyond this threshold by employing an optical parametric amplification (OPA) stage, determining the output energy and efficiency of the whole system [2]. Naturally, given the potential benefits, the search for nonlinear materials for frequency conversion in mid IR have recently intensified [1, 2]. However for really high power (energy) frequency down-conversion devices in the spectral region between 2.5 and 4 μm, periodically poled stoichiometric lithium tantalate (PPSLT) with its low coercive field (0.8 kV/mm), high refraction damage threshold and transparency up to 5 μm, is amongst the very few suitable candidate.Here we demonstrate a highly efficient OPA system with high pulse energy at kilohertz repetition rate, seeded by a singly resonant OPO tunable between 3 and 3.5 μm. The OPO employs a 20 mm long, 10 mm wide, and 3.2 mm (along z axis) thick PPSLT crystal with three polled zones with different domain inversion periods (30.2, 30.3 and 30.4 μm). The OPA stage employs a similar 37 mm long crystal, which together with the OPO crystal is antireflection coated for the pump, signal and idler waves. The OPO cavity is 27 mm long with plane parallel mirrors, highly reflective for the signal wave. The output idler wave is collimated and then focused in the OPA by two CaF2 lens with 100 mm focal lengths. The pump source is a diode-pumped MOPA system providing 35 mJ pulses with high beam quality (Mx2 x My2=1.3 x 1.1), short pulse duration (1.2 ns), at 0.5 kHz repetition rate.The maximum output idler energy is 5.1 mJ, when seeded with 0.52 mJ at 3.04 μm, while the OPA pump energy is 27 mJ, corresponding to idler conversion efficiency of 18.3 % and total conversion efficiency of 48.4 % (Fig1.a). By changing the temperature of the two PPSLT crystals from 40°C up to 265°C and employing the three domain inversion periods, we were able to achieve continuous tunability from 3 to 3.5 μm. In order to measure the idler pulse duration we frequency doubled the idler wave in 3 mm thick KTP crystal. After deconvolution with the response function of the detection system we obtain 580 ps for the SH of the idler, corresponding to an idler pulse duration of 820 ps, expected shorter than the pump pulse duration (1.2 ns) - Fig.1b. The beam quality of the idler wave was Mx2 = 50 and My2 = 44 (Fig.1c). To the best of our knowledge, this is the first sub-nanosecond coherent source, that incorporates high energy pulses (up to 5.1 mJ) with high repetition rate (0.5 kHz) and tunability in this highly relevant for biomedical applications spectral region.

Sub-ns OPO based on PPKTP with 1 mJ idler energy at 2.8 μm

Recently we demonstrated that periodically-poled oxide materials can be employed in short cavity, singly-resonant optical parametric oscillators (OPOs), with very low thresholds and high conversion efficiency. We demonstrated sub-ns pulse durations down to ~250 ps for the signal and idler pulses of 1-10 kHz OPOs based on periodically-poled KTiOPO4 (PPKTP) and pumped at 1064 nm. However, with maximum idler energies of 110 and 50 μJ near 2.8 μm obtained with the 1-ns and 500-ps pump pulse sources, respectively, the energy scaling capability of thick PPKTP remained unexploited. Here we extend these results to the millijoule energy range for the idler by utilizing the full crystal aperture with a powerful pump source specially designed for this purpose.