Eye-safe Laser Research Articles

Brewster-oriented passive Q-switch intracavity optical parametric oscillator

The behaviour of passively Q-switched solid-state lasers containing intracavity optical parametric oscillators (OPOs) is described and a model to optimize this system's performance is developed. Meanwhile, an Nd:YAG laser-pumped KTiOPO4 (KTP) intracavity OPO for eye safe laser output is studied experimentally. A Brewster-oriented bis-(4-dimethylaminodithiobenzil)-nickel (BDN) dye film is employed in laser cavity, which plays the double role of a passive Q-switch and a Brewster plate, to Q-switch the light to increase its intensity and polarize the light so that the polarization plane of fundamental irradiation is perpendicular to the Z-axis of KTP serving as the OPO nonlinear crystal, thereby providing type II phase matching conditions to generate 1.572μm eye-safe signal laser. Eye-safe signal radiations of 6.6mJ per pulse at 1.572μm wavelength and pulse width as narrow as 6.0ns (FWHM) were obtained. Peak power was up to 1.1MW.

Low-cost laser rangefinder with crystal-controlled accuracy

We present a novel laser ranging system that offers improved accuracy and low-cost implementations for short to medium range target distance and velocity measurement. This is accomplished through the emission of an eye-safe amplitude modulated laser beam, whose echo return is processed with a phase-computing discrete Fourier transform (DFT). The data for the DFT are collected directly from the received signal through equivalent time sampling, and the automatic gain control (AGC) function is moved to that part of the system where its varying signal propagation time will not impact measurement accuracy. All emission and data collection signals are derived directly from a single crystal oscillator, which establishes the system's accuracy. The system is also capable of resolving the distance/velocity ambiguity problem inherent in conventional hetero/homodyne laser ranging systems, and, lastly, digital data can be transmitted and received concurrently with the distance measurement process.

Simulation of nonlinear optical absorption in ZnSe:Co 2+ crystals

The paper presents theoretical approach to simulation of nonlinear optical absorption in zinc selenide crystals doped with cobalt (II) ions (ZnSe:Co 2+), which was reported by us earlier (Opt. Laser Technology, V. 35, (2003), 169). We used ZnSe:Co 2+ crystals as saturable absorbers for generation of giant-pulse eye-safe laser radiation. It was found that minimal optical losses (maximal final transmission) occurred for ZnSe samples containing 1.6×10 19 cm −3 of Co 2+ ions. Band structure and photoinduced molecular dynamics simulations were performed to explain the parabolic dependence of optical losses versus Co 2+ concentration. The minimum was shown to be the result of photoinduced anharmonic electron–phonon interaction.

Broad band antireflection coating on silicon from 1.5 to 6 μm spectral band

Silicon is one of the easily available, low cost and stable material transmitting in 1.5–6μm spectral band. Silicon’s optical application includes thermal imaging, motion sensors and forward looking infrared (FLIR) technology. In this work a broad band antireflection coating on silicon substrate for 1.5–6μm spectral band is reported. In the design, a gradient index approach is used which is a very useful tool to achieve broad band antireflection coating. To realize this gradient index profile, the refractive index is assumed to increase from 1.45 (thorium fluoride chosen as low index material) to 3.42 of silicon in five discrete optimized steps. The design was conceived with thorium fluoride as first layer next to air and each other four gradient layers were replaced by two equivalent layer combination of silicon and thorium fluoride. This nine layer coating deposited by e-beam evaporation system has a total thickness of only 879nm. Experimental results shows average transmission 96% in 1.5–6μm region on a 5mm thick silicon substrate. This coating also has 96.3% transmission in the 1.54μm eye safe laser and 96.5% average transmission in 3–5μm wave bands. The coating was environmentally stable, tested as per MIL standard.

Compact efficient all-solid-state eye-safe laser with self-frequency Raman conversion in a Nd:YVO4 crystal.

An efficient compact eye-safe laser at 1525 nm is presented by use of self-frequency Raman conversion in a diode-pumped actively Q-switched Nd:YVO4 1342-nm laser. At an incident pump power of 13.5 W, the self-stimulated Raman laser produces 1.2 W of 1525-nm average output power at a repetition rate of 20 kHz. The corresponding peak power at 1525 nm is generally greater than 10 kW for repetition rates from 5 to 20 kHz.

Efficient sub-nanosecond intracavity optical parametric oscillator pumped with a passively Q-switched Nd:GdVO4 laser

An efficient diode-pumped passively Q-switched Nd:GdVO4/Cr4+:YAG laser was employed to generate a high-repetition-rate, high-peak-power eye-safe laser beam with an intracavity optical parametric oscillator (OPO) based on a KTP crystal. The conversion efficiency for the average power is 8.3% from pump diode input to OPO signal output and the slope efficiency is up to 10%. At an incident pump power of 14.5 W, the compact intracavity OPO cavity, operating at 46 kHz, produces average powers at 1571 nm up to 1.2 W with a pulse width as short as 700 ps.

High-power efficient diode-pumped passively Q-switched Nd:YVO 4/KTP/Cr 4+:YAG eye-safe laser

A high-power efficient eye-safe laser is experimentally realized by using an intracavity optical parametric oscillators of a diode-pumped passively Q-switched Nd:YVO 4/Cr 4+:YAG laser. Considering the thermal lensing effects, the cavity length was designed to allow mode matching with the pump beam and to provide the proper spot size in the saturable absorber. With an incident pump power of 14.5 W, the compact intracavity OPO cavity, operating at 58.5 kHz, produces average powers at 1573 nm up to 1.56 W and peak powers higher than 5 kW.

Modeling the absorption spectra of Er3+ and Yb3+ in a phosphate glass

Absorption spectra of Er3+ and Yb3+ ions, codopants in a phosphate glass, are reported at 8 K and at wavelengths between 350 and 1600 nm. Detailed structure appearing in the spectra, associated with individual multiplet states, LJ2S+1, of Er3+(4f11) and Yb3+(4f13) is interpreted using a ligand-field coordination sphere model to characterize the microscopic environment surrounding the rare earth ions in multiple sites. Inhomogeneous broadening of the spectra is likely due to different configurations of PO4 tetrahedra clustered about a caged rare earth ion in the amorphous host. Similarity between the Er3+ spectrum in the glass and in the spectrum of single-crystal LiErP4O12, where Er3+ occupies sites of C2 symmetry, suggests that an averaged site symmetry of C2 is a reasonable approximation for Er3+ and Yb3+ ions in the phosphate glass. Calculated splitting of multiplet states by the ligand-field cluster model are compared with energy levels derived from the observed absorption peaks and well-defined shoulders. Inhomogeneous broadening of the spectra limit the precision in establishing the energy of the multiplet splittings, but the analysis is useful for modeling studies of the Er:Yb:phosphate glass as an eye-safe laser (1.53 μm). The splitting of the Yb3+(4f13)2FJ states is determined using parameters obtained from the Er3+ set by means of the three-parameter theory. No adjustments were made to the Yb3+ parameters that predict multiplet splittings in reasonable agreement with experimental data.

Judd–Ofelt analysis of the Er3+(4f11) absorption intensities in phosphate glass: Er3+, Yb3+

A spectroscopic Judd–Ofelt investigation has been performed on Er3+ in the doubly doped phosphate glass:Er3+, Yb3+ in order to assess its potential as a laser system. The standard Judd–Ofelt model was applied to the room-temperature absorption intensities of Er3+(4f11) transitions to determine the intensity parameters: Ω2=6.28×10−20 cm2, Ω4=1.03×10−20 cm2, and Ω6=1.39×10−20 cm2 in the phosphate glass host. The intensity parameters are used to determine the radiative decay rates (emission probabilities of transitions) and branching ratios of the Er3+ transitions from the excited-state J manifolds to the lower-lying J′ manifolds. The radiative lifetimes of these excited states are determined from the radiative decay rates. The predicted decay rates and radiative lifetimes are compared to those of Er3+ transitions in other glass hosts. The quantum efficiency of the eye-safe laser transition I13/24→4I15/2 (1.54 μm) of Er3+ is approximately 80% in the phosphate glass host.

ZnSe:Co 2+—nonlinear optical absorber for giant-pulse eye-safe lasers

ZnSe:Co 2+ crystals were grown by the Bridgeman technique. Optical absorption measurements showed non-uniform distribution of Co 2+ ions along the as-grown crystals. Using Pfann formula the distribution coefficient of Co 2+ ions between crystal and melt was estimated to be 0.5. Transmission dependence on laser power at λ=1535 nm was investigated for different Co 2+ ions concentration. Maximal final transmission was found for ZnSe samples containing 1.6×10 19 cm −3 of Co 2+ ions. Obtained ZnSe:Co 2+ samples were used as saturable absorbers to generate giant-pulse eye-safe laser radiation. KIGRE QE-7S rod placed in resonator cavity was used as active element.

20-Hz operation of an eye-safe cascade Raman laser with a Ba(NO3)2 crystal

Operation of a 1.598-μm eye-safe third-Stokes Raman laser with a Ba(NO3)2 crystal pumped by a 1.064-μm Nd:YAG laser is described. We observed a substantial decrease in the output energy during the first 50 s of the continuous operation at 20 Hz. The energy drop is ∼76% of the initial third-Stokes output. We confirmed negative thermal lensing and thermally induced birefringence in the crystal. With a concave cavity mirror at a matched curvature to the thermal lensing, we obtained an output energy of 11 mJ at 20 Hz. TEM00 output was also obtained with a smaller pump-beam diameter with a highest conversion efficiency of 15.5% for a pumping power of only 45 MW/cm2 (0.9 J/cm2).

Development of a helicopter-mounted eye-safe laser radar system for distance measurement between power transmission lines and nearby trees

Development of a helicopter-mounted eye-safe laser radar system for distance measurement between power transmission lines and nearby trees

Highly efficient minilaser with transverse pulsed semiconductor pumping for eye-safe laser range-finding

The lasing characteristics of a Cr, Yb, Er:glass minilaser transversely pumped by a pulsed laser diode array are investigated. A single 30-ns pulse of energy 3 mJ, and a beam divergence of 4 mrad was generated for an electric pump pulse energy of 0.5 J in the Q-switching mode.

Optical design and evaluation of a three-dimensional imaging and ranging system based on time-correlated single-photon counting.

The design and operation of a noncontact surface profilometry system based on the time-correlated single-photon-counting technique are described. This system has a robust optomechanical design and uses an eye-safe laser that makes it particularly suitable for operation in an uncontrolled industrial environment. The sensitivity of the photon-counting technique permits its use on a variety of target materials, and its mode of operation does not require the continual presence of an operator. The system described has been optimized for a 1-25-m standoff, has a distance repeatability of <30 microm, and has a transverse spatial resolution of approximately 60 microm at a 2-m standoff and approximately 400 microm at a 13-m standoff.

Development of a Helicopter-Mounted Eye-Safe Laser Radar System for Distance Measurement between Power Transmission Lines and Nearby Trees

An on-board eye-safe laser radar system capable of measuring the distance between high-voltage power transmission lines and nearby trees has been developed. An eye-safe laser operating atl.572,μm wavelength was used for the safety of the human comea. The system can be mounted on the helicopter for general-patrol surveys as it is compact, lightweight, and consumes low power. Clearance on the opposite side of the flight position can also be measured. This system can be applied to all voltage level transmission lines from 66 kV to 500 kV. Clearance accuracy below ±93 cm was obtained in our practical power transmission lines. Software running on personal computers was also developed to simulate the expected minimum clearance at field offices.

Eye-safe laser radiation fromstimulated Raman scattering frequency self-conversion inKGd(WO4)2:Nd3+

Under short-pulsed longitudinal pumping at 600 nm we obtained intracavitystimulated Raman scattering frequency self-conversion of the1352.5 nm laser line in KGd(WO4)2:Nd3+. Thewavelength which was produced is eye-safe at 1539.5 nm and theconversion efficiency was 1.26% with uncoated faces of the 3 cmlong crystal.

Growth of Tm, Ho-codoped YLiF 4 and LuLiF 4 single crystals for eye-safe lasers

YLiF 4 and LuLiF 4 single crystals codoped with thulium and holmium were successfully grown by the Czochralski Technique. The segregation coefficients of the dopants in these crystals were measured and the influence of the dopants on the lattice parameters was investigated. The distribution of birefringence along the a-axis was in the order of 10 −7 in each crystal. Laser performance of both YLiF 4 and LuLiF 4 codoped with thulium and holmium was compared.

Spectroscopic properties at 1.351 μm of Nd3+-doped KY(WO4)2 and KGd(WO4)2 single crystals for Raman conversion

Abstract Spectroscopic properties around 1.35 μm are investigated for Nd 3+ -doped KY(WO 4 ) 2 and KGd(WO 4 ) 2 bi-axial single crystals. Unlike previously published works, all measurements have been done along optical axes. Our aim is to evaluate the feasibility of an eye-safe laser emission into the range 1.51–1.57 μm. A complete Judd–Ofelt analysis and excited state absorption (ESA) have been undertaken so that we can give laser gains at 1.35 μm. Then we compare several schemes of Raman laser actions.

The energy matrix using determinantal product states applied to Ho:YAG

A program written in C is applied for calculations of energy levels and wavefunctions of the lanthanides or actinides. Standard non-relativistic phenomenological operators are used. Simple determinantal product states are used to form the basis set. The complete energy matrix is diagonalized with all operators interacting simultaneously thus allowing mixing of all quantum numbers. The crystal field splitting can also be studied via the standard crystal field hamiltonian as well as the magnetic field influence on the energy levels. The program is here applied to the technologically interesting trivalent ion holmium which is used to form extremely strong magnets as well as the medical eye-safe laser Ho:YAG. Both experimentally fitted and calculated self-consistent crystal field parameters are used to study the energy structure. Crystal ionic polarizabilities with electron correlation included are partly calculated. Zeeman split levels are also studied.

Infrared and visible emission of Pr 3+, Eu 3+, Yb 3+/Er 3+ in Ca 4Gd(BO 3) 3O (GdCOB)

In the present study, several luminescent ions are incorporated in the Ca 4Gd(BO 3) 3O (GdCOB) matrix. Structural perturbation is evidenced by the 5 D 0→ 7 F 0 emission of Eu 3+ ions. The non-radiative multiphonon deexcitation is evaluated by measuring the lifetimes of different ions. Europium and praseodymium are used as potential red phosphors and erbium for 1.55 μm eye-safe laser oscillation.