Marina Starikova

Optical damage studies of mercury thiogallate nonlinear crystals for use in 1-μm pumped optical parametric oscillators

Abstract. We investigated optical damage (surface and bulk) in one of the most promising wide bandgap nonoxide nonlinear crystals, HgGa2S4, that can be used in ∼1-μm pumped optical parametric oscillators (OPOs) and synchronously pumped OPOs (SPOPOs) for generation of idler pulses above 4 μm without two-photon absorption losses at the pump wavelength. The optical damage has been characterized at the pump wavelength for different repetition rates using uncoated and antireflection-coated (mainly with a single layer for pump and signal wavelengths) samples. HgGa2S4 is the most successful nonlinear crystal (both in terms of output energy and average power) for such OPOs, but optical damage inside the OPO has a lower threshold and represents at present the principal limitation for the achievable output. It is related to peak pulse and not to average intensity, and bulk damage in the form of scattering centers occurs before surface damage. Such bulk damage formation is faster at higher repetition rates. Lower repetition rates increase the lifetime of the crystal but do not solve the problem. The safe pump fluence in extracavity measurements is <1  J/cm2, which corresponds to ∼100  MW/cm2 for the 8-ns pulse duration (both values peak on-axis). In the OPO, however, peak on-axis fluence should not exceed 0.3  J/cm2 limited by the formation of bulk scattering centers in orange-phase HgGa2S4. In the nanosecond OPO regime, the damage resistivity of Cd-doped HgGa2S4 is higher and that of the almost colorless CdGa2S4 is roughly two times higher, but the latter has no sufficient birefringence for phase-matching. In SPOPOs operating in the ∼100  MHz regime, the damage limitations are related both to the peak pulse and the average intensities, but here HgGa2S4 seems the best nonoxide candidate to obtain first steady-state operation with Yb-based mode-locked laser pump sources.

LaserBreeze gas analyzer for noninvasive diagnostics of air exhaled by patients

A laser gas analyzer has been designed for determining the composition of exhaled air by means of photoacoustic spectroscopy. The analyzer, based on a broadband optical parametric oscillator and photoacoustic detector, provides high-precision rapid analysis of the multicomponent composition of human exhaled air for dynamic estimation of the efficiency of treating bronchus and lung diseases.

Screening of patients with bronchopulmonary diseases using methods of infrared laser photoacoustic spectroscopy and principal component analysis

Abstract. A human exhaled air analysis by means of infrared (IR) laser photoacoustic spectroscopy is presented. Eleven healthy nonsmoking volunteers (control group) and seven patients with chronic obstructive pulmonary disease (COPD, target group) were involved in the study. The principal component analysis method was used to select the most informative ranges of the absorption spectra of patients’ exhaled air in terms of the separation of the studied groups. It is shown that the data of the profiles of exhaled air absorption spectrum in the informative ranges allow identifying COPD patients in comparison to the control group.

Mercury thiogallate nanosecond optical parametric oscillator continuously tunable from 4.2 to 10.8 μm

We demonstrate an optical parametric oscillator based on two HgGa2S4 crystals with an extremely wide tuning range from 4.2 to 10.8 μm. The HgGa2S4 optical parametric oscillator was pumped by a Q-switched nanosecond Nd : YLF laser at 1.053 μm. The absorption spectrum of ammonia was presented to demonstrate the feasibility of the developed optical parametric oscillator system for spectroscopic measurements and gas detection.

Continuously wavelength tuned optical parametric oscillator based on fan-out periodically poled lithium niobate

A PC-controlled optical parametric oscillator (OPO) based on fan-out periodically poled lithium niobate (MgO : PPLN) structures was developed. Continuous wavelength tuning (2.40–3.85 μm) was realized via linear displacements of fan-out MgO : PPLN structures using a PC-controlled precision motorized translation stage. The total wavelength scanning time in the range of 2.40–3.85 μm was ≤1 min. The OPO was developed as a source of tunable radiation for use in a laser photo-acoustic gas analyzer. Studies of the methane absorption spectrum showed a good coincidence of the experimental and theoretical data.

Effect of post-growth annealing on the optical properties of LiGaS2 nonlinear crystals

High chalcogen volatility and Li interaction with the container walls result in variation of crystal composition and presence of both extended and point defects in as-grown LiGaS2 nonlinear crystals. Annealing in appropriate conditions is used to correct the composition and improve the optical quality. We annealed LiGaS2 in vacuum, in the presence of Li2S, Ga2S3, and S, and studied changes in transmission, photoluminescence and photo-induced absorption. OH groups, S-H and S-S complexes, sulfur vacancies and cation antisite defects (GaLi) are most important. Photo-induced absorption is reversible: It appears after illumination with UV/blue light and disappears after illumination with IR light or by heating the sample.

Twin HgGa2S4 optical parametric oscillator at 4.3-10.78 µm for biomedical applications

We demonstrate an optical parametric oscillator (OPO) based on two HgGa2S4 (HGS) crystals with exceedingly wide tuning range from 4.2 μm to 10.73 μm. The HGS OPO was pumped by Q-switched Nd:YLF laser at 1.053 μm with a 5-7 ns pulse duration. Absorption spectrum of ammonia was presented to demonstrate the feasibility of the OPO system for spectroscopic measurements and gas detection.

Gas analysis in medicine: New developments

In this article we discuss the method of early diagnosis of bronchopulmonary diseases based on the analysis of absorption spectra of biomarkers in the human exhaled air. For the analysis of absorption spectra of human exhaled air gas analyzer based on laser photo-acoustic spectroscopy (LPAS) was designed. A method for analysis of exhaled air samples from patients with lung cancer in comparison with the target and comparison group by LPAS was developed. This work is promising for screening of lung cancer.

Optical damage limits in chalcogenide nonlinear crystals used in 1064nm pumped nanosecond optical parametric oscillators

We investigated optical damage (surface and bulk) in wide band-gap (absorption edge below 532 nm) sulphide and selenide nonlinear crystals that can be used in 1064-nm pumped optical parametric oscillators (OPOs) for generation of idler pulses above 4 μm without two-photon absorption losses at the pump wavelength. The optical damage has been characterized at the pump wavelength for different repetition rates. Surface damage has been studied for uncoated and antireflection-coated (mainly with a single layer for pump and signal wavelengths) samples. Optical damage inside the OPO has a lower threshold and represents at present the principal limitation for the achievable output. It is related to peak and not to average intensities and in many of the studied crystals bulk damage in the form of scattering centers occurs before surface damage. Such bulk damage formation is faster at higher repetition rate. Lower repetition rates increase the lifetime of the crystal but do not solve the problem. In the most successful nonlinear crystal (both in terms of output energy and average power), orange-phase HgGa2S4, the safe pump intensity in extracavity measurements is below 100 MW/cm2 which corresponds to less than 1 J/cm2 for the 8 ns pulse duration (both values peak on-axis). In the OPO, however, peak on-axis fluence should not exceed 0.3 J/cm2 limited by the formation of bulk scattering centers. The damage resistivity of yellow-phase HgGa2S4 or Cd-doped HgGa2S4 is higher and of the almost colorless CdGa2S4 it is roughly two times higher but the latter has no sufficient birefringence for phase-matching.

High-energy 1064-nm pumped optical parametric oscillator for the 6 µm spectral range based on HgGa2S4

The defect chalcopyrite crystal HgGa2S4 has been employed in a 1064-nm pumped optical parametric oscillator to generate <7 ns long idler pulses near 6.3 µm with energies as high as 3 mJ, tunable in a broad spectral range from 4.5 to 9 µm.