Near infrared and visible tunability from a diode pumped Nd3+ activated strontium barium niobate laser crystal

Tunable laser action from a diode pumped Nd³⁺ activated Sr_xBa_1-x(NbO₃)₂ crystal has been obtained. A wide continuous tuning range of 43 nm in the near infrared region (1050–1093 nm) has been demonstrated due to the broad ⁴F_3/2→⁴I_11/2 emission band present in this system. Simultaneously, 20 nm of tunability in the green region (525–545 nm), as well as 7 nm in the blue one (455–462 nm) have been achieved by tunable self-frequency conversion processes using the quasiphase matching technique, without any angle or thermal tuning of the laser crystal. The results evidence that the system is a reliable candidate for compact tunable coherent devices based on diode pumped Nd³⁺ active ions, which are of interest in many technological and scientific applications. PACS 77.84.Dy; 42.79.Nv; 42.60.Fc; 42.55. Xi     

500 nm continuous wave tunable single-frequency mid-IR light source for C-H spectroscopy

A computer controlled tunable mid-IR light source, based on single resonant difference frequency generation (DFG), is experimentally investigated. The DFG process is pumped by an external cavity tapered diode laser, tunable over a spectral range of 30 nm. Grating feedback to the single mode channel of the tapered diode narrows the spectrum and allows for tuning of the emitted spectrum in the range from 780 to 810 nm. The DFG process takes place intra-cavity in a high finesse diode pumped 1064 nm solid state Nd:YVO₄ laser cavity, using periodically poled LiNbO₃ as the nonlinear material. Based on this new approach, a tunable single-frequency output power exceeding 3 mW was obtained in the mid-IR tuning range from 2.9 to 3.4 ??m.     

Electro-optic tuning of a periodically poled LiNbO3 optical parametric oscillator and mixing its output waves to generate mid-IR tunable from 9.4 to 10.5 μm

We demonstrate electro-optic spectral tuning in a pulsed periodically poled LiNbO₃ (PPLN) optical parametric oscillator (OPO) in ns regime. A 3-cm-long LiNbO₃ crystal is segmented in three equal sections; the outer sections are periodically poled. The center segment is of single domain whose refractive index is changed by electro-optic effect. Applying voltage from 0 to −5000 V, the OPO signal and idler waves are tuned from 1.932 to 1.912 μm and 2.368 to 2.40 μm, respectively. The signal and idler waves obtained are difference-frequency-mixed in a 10 mm long AgGaS₂ crystal to produce mid-IR tunable from ∼10.5 to ∼9.4 μm, which matches the tuning range of a CO₂ laser.     

Upconversion white-light emission in Ho³⁺/Yb³⁺/Tm³⁺ tridoped LiNbO₃ single crystal

Ho³⁺/Yb³⁺/Tm³⁺ tridoped LiNbO₃ single crystal exhibiting intense upconversion white light under 980?nm excitation has been successfully fabricated by the Czochralski method. The tridoped LiNbO₃ single crystal offers power dependent color tuning properties by simply changing excitation power. Efficient three-photon blue upconversion emission and two-photon green and red upconversion emissions have been observed. In addition, the red emission of Ho³⁺ originates dominantly from the nonradiative decay of green emission. The LiNbO₃ with upconversion white light will be a potential laser candidate material.     

Discretely tunable erbium-doped fiber ring laser selecting ITU-T grids of 273 channels × 50-GHz spacing in <Emphasis Type="Italic">C</Emphasis>- and <Emphasis Type="Italic">L</Emphasis>-band regions

We report the fabrication of a discretely tunable erbium-doped fiber ring laser with a novel scheme that can select ITU-T grids of 50-GHz spacing in both C and L bands. Wavelength selections of 273 channels are demonstrated with a signal to source spontaneous emission ratio of 60 dB/nm and excellent power flatness over 111-nm bandwidth. This was realized by incorporating both a solid Fabry–Pérot interferometer and a fiber Fabry–Pérot tunable filter into the ring laser. To our knowledge, our experimental result has the widest tuning range ever reported for a discretely tunable fiber laser with 50-GHz channel spacing that matches the ITU-T grids and is the first equipment that can operate in both C and L bands. PACS 42.55.Wd; 42.60.Da; 42.60.-v     

A subnanosecond pulsed laser-source for mid-infrared LIDAR

A new subnanosecond pulsed laser-source for mid-infrared (MIR) LIDARbased on difference-frequency generation (DFG) in periodically poled LiNbO₃ (PPLN) is developed. An output pulse energy of up to 36 pJ is achieved by amplification of the pump and signal laser. A pulse duration of Δτ=538 ps has been measured using silver thiogallate as a non-linear device for an upconversion cross correlation experiment. The mid-infrared pulse duration corresponds to a spatial resolution of Δs≈8 cm in the LIDAR system. PACS 07.07.Df; 42.60.Da; 42.65.Ky     

74% Slope efficiency from a diode-pumped Yb3+:LiNbO3:MgO laser crystal

An optimization of the laser action performance from a diode-pumped Yb³⁺-doped LiNbO₃:MgO crystal has been carried out. In this sense, efficient laser action at 1.06 m when pumping with a fiber-coupled laser diode at 980 nm has been demonstrated, achieving laser slope efficiencies as high as 74%. The influence of output mirror transmittance on both pumping threshold and laser slope efficiency has been investigated, and the parameters of relevance in laser dynamics (emission cross section and optical losses) have been determined. Under the experimental conditions leading to maximum slope efficiency, the pump power at threshold was 300mW, and the pump-to-laser conversion efficiency was 40%. PACS 42.55.Xi; 42.55.Rz; 42.60.Lh     

Atomic and molecular spectroscopy with a continuous-wave, doubly resonant, monolithic optical parametric oscillator

Doppler-broadened atomic and molecular spectra were observed with a one octave tunable, continuous-wave, doubly resonant, monolithic optical parametric oscillator (OPO) using 5% MgO-doped LiNbO₃ as a non-linear crystal with a birefringent phase-matching configuration. By tuning the frequency of a pump laser, longitudinal mode selection over 20 successive modes, corresponding to a 60 GHz span, was possible, owing to the simple structure of the monolithic OPO. Continuous frequency tuning was achieved using an external waveguide-type electrooptic phase modulator (EOM). By changing the modulation frequency of the EOM, frequency tuning of the optical sidebands over 12 GHz was possible, which is larger than the one free spectral range of the monolithic cavity of 3 GHz. We could observe the Cs-D₁ (894 nm), Cs-D₂ (852 nm), Rb-D₁ (795 nm), acetylene R9 (1520 nm) and P9 (1530 nm) transitions with the single monolithic OPO.     

Tunability range of 245 nm in a diode-pumped Tm:BaY2F8 laser at 1.9 μm: a theoretical and experimental investigation

Extremely wide wavelength tuning ranges of up to ∼300 nm around 1.9 μm are theoretically predicted in a Tm-doped BaY₂F₈ crystal, on the basis of near-infrared measurements of emission and absorption cross sections. A tunability interval of 245 nm, from 1849 nm to 2059 nm, has been demonstrated by room-temperature laser experiments using a 8% Tm-doped crystal. PACS 42.55.Xi; 42.55.Rz; 42.60.Pk; 42.70.Hj; 42.60.Lh     

Tunable multilayer narrowband filter containing an ultrathin metallic film and a lithium niobate defect

By using lithium niobate (LiNbO₃) as a defect layer, a tunable multilayer narrowband reflection-and-transmission filter containing an ultrathin metallic film is proposed. Due to the voltage dependence of the refractive index for LiNbO₃, tunable optical filtering properties have been theoretically investigated based on the calculated wavelength-dependent reflectance and transmittance. The dependences of peak wavelength on the applied voltage and the angle of incidence are numerically illustrated. The results reveal that in addition to working as a tunable filter, it can also be expected to act as a refractometric optical sensor.     

Investigation of optical homogeneity and photorefractive properties of lithium-niobate single crystals by the Raman-spectroscopy and laser-conoscopy methods

Photorefractive properties and structural and optical homogeneity of (1) LiNbO₃:Cu crystals ([Cu] = 0.015 mas %) grown from a congruent melt, (2) nominally pure stoichiometric crystals grown from a melt with 58.6 mol % of Li₂O (LiNbO_3stoich), and (3) nominally pure congruent crystals (LiNbO_3congr) have been studied using the Raman-spectroscopy method with excitation in the UV, visible, and near-IR ranges; the laser-conoscopy method; and the electron paramagnetic resonance-spectroscopy method. In optically uniaxial LiNbO₃ crystals, a weak optical biaxiality has been revealed, which is attributed to an insignificant deformation of the optical indicatrix. This deformation can be caused both by the initial structural inhomogeneity of crystals and by the photorefractive effect. It has been shown that, under the action of light, charge exchange of copper cations Cu²⁺ → Cu⁺ takes place in the crystal LiNbO₃:Cu ([Cu] = 0.015 mas %). The LiNbO₃:Cu crystal exhibits photorefractive properties not only because of the occurrence of intrinsic defects with electrons localized at them, as is the case with the LiNbO_3stoich and LiNbO_3congr crystals, but also due to the charge exchange of copper cations under the action of the laser radiation.     

Fast and wide-range continuously tunable Šolc-type filter based on periodically poled LiNbO3

We present a fast and wide-range continuously tunable Šolc-type filter based on periodically poled LiNbO₃ (PPLN) in this paper. The filter is formed in PPLN by applying a biased dc electric field along the y-axis, and the tuning of a transmitted central wavelength is realized by applying another dc electric field along the z-axis. The numerical results demonstrate that the tuning range covers as much as 16 nm, and the dependence of the transmitted central wavelength shift on the control electric field, shows a nearly linear relation with a tuning rate of 0.95 kV/mm per nm. PACS 42.65.-k; 78.20.Jq; 42.79.Ci     

Squeezing by second-harmonic generation in a monolithic resonator

We have measured the intensity fluctuations of the second-harmonic mode generated in a MgO:LiNbO₃ external monolithic cavity pumped by a Nd:YAG laser. The cavity has mirror coatings for both the fundamental and the second-harmonic mode. We scan the cavity using the electro-optic effect of the crystal and observe that the second-harmonic beam of 2 mW exhibits a quantum noise reduction of 40(±5)%. In addition, we report on the active frequency stabilization of the monolithic device used in our squeezing experiments. Several fast tuning parameters such as the electro-optic effect, the photo-elastic effect, and the laser frequency have been investigated. With these tuning parameters the monolithic resonator can be locked on double-resonance at the phase-matching temperature, which is a prerequisit for observing squeezing in a cw-regime.     

High-power single-wavelength SOA-based fiber-ring laser with an optical modulator

A semiconductor optical amplifier (SOA) fiber-ring laser (SOAFRL) utilizing a fiber-Bragg grating (FBG) and lithium niobate (LiNbO₃) modulator is demonstrated. The laser operates at a wavelength of 1547.64 nm, which is equal to the Bragg wavelength in the saturation region. By removing the LiNbO₃ modulator in the ring, the laser shows a single-wavelength output, which has a lower peak power. The experimental results show that when reaching the saturation level, the system with the LiNbO₃ modulator shows a higher saturation current and peak power compared to that of the system without the modulator. The effect of varying the modulation frequency on the laser output power is investigated. By incorporating the LiNbO₃ modulator in the laser cavity, the side-mode suppression ratio (SMSR) of the laser is significantly improved and a higher peak power can be obtained at a higher current.     

Continuous-wave high power laser operation and tunability of Yb:LaSc3(BO3)4 in thin disk configuration

We report power scaling of the Yb³⁺:LaSc₃(BO₃)₄ (Yb:LSB) laser material in thin disk configuration. Employing a 300-μm thick Yb(25 at. %):LSB crystal, the continuous-wave output power around 1.0 μm wavelength reaches 40 W for 95 W of pump power at 974 nm; the overall optical-to-optical efficiency and the slope efficiency are 0.43 and 0.48, respectively. Preliminary experiments show continuous tuning of the laser output between 991 nm and 1085 nm. PACS 42.55.Xi; 42.60.Fc; 42.55.Rz     

Widely phase-matched tunable difference-frequency generation in periodically poled LiNbO3 crystal

We report on the development of a laser source in the mid-infrared spectral region based on difference-frequency generation (DFG) in a periodically poled LiNbO₃ (PPLN) crystal. Continuously tunable coherent radiation from 2.75 to 4.78 μm was produced by optical parametric interaction between a diode-pumped monolithic continuous-wave (CW) Nd:YAG laser operating at 1.064 μm and a CW Ti:Sapphire laser tunable from 767 to 871 nm. Temperature-dependent quasi-phase-matched DFG wavelength acceptance bandwidth was studied and characterized. An empiric formula is given to estimate the phase-matched wavelength acceptance bandwidth as a function of the crystal temperature at Λ = 22.5 μm. A large frequency scan of 128 cm⁻¹ (about 78 cm⁻¹ above 1 μW) near 4.2 μm was achieved. The whole absorption spectrum of the P and R branches of the ν₃ band of atmospheric carbon dioxide has been recorded with a single phase-matched frequency scan.     

High-power tunable terahertz generation from a surface-emitted THz-wave parametric oscillator based on two MgO:LiNbO3 crystals

A high-powered tunable terahertz wave (THz-wave) has been parametrically generated via a surface-emitted THz-wave parametric oscillator (TPO) pumped by a multi-longitudinal-mode Q-switched Nd:YAG laser. The effective parametric gain length was enlarged by employing two MgO:LiNbO₃ crystals. The tunable THz-wave radiation from 0.8 to 2.8 THz was realized via varying phase-matching angle between the pump wave and the Stokes wave. The maximum THz-wave radiation was 173.9 nJ/pulse at 1.7 THz as the pump energy was 82 mJ, corresponding to an energy conversion efficiency of about 2.12 × 10⁻⁶ and a photon conversion efficiency of about 0.035%. The first-order, the second-order and the third-order Stokes waves were observed during the experiments.     

Power scaling potential of Yb:NGW in thin disk laser configuration

We report on efficient laser operation of Yb:NaGd(WO₄)₂ (Yb:NGW) in thin disk laser configuration. Using doping concentrations of 10.7 at. % and 15.3 at. % and disk thicknesses between 0.1 mm and 0.4 mm the optimum crystal parameters have been determined. A 0.1 mm thick, 10.7 at. % Yb:NGW disk pumped at 975 nm delivered 18.2 W of output power at 43.6 W of incident pump power with a slope efficiency of 51% and a resulting optical-to-optical efficiency of 42%. Using a 2 mm thick birefringent filter, continuous tuning from 997 nm to 1075 nm has been achieved. PACS 42.55.Rz; 42.60.Lh; 42.70.Hj     

Study of crystals in medium-and long-wavelength IR ranges by surface-electromagnetic-wave spectroscopy

First observation of excitation of surface electromagnetic waves (SEW) is reported in the reststrahlen region in for biaxial crystal KTiOPO₄ (KTP) using a tunable CO₂ laser around 10 μm, and for CaF₂, BaF₂, MgO (cubic crystals), and LiNbO₃ (uniaxial crystal) in the far IR using ith a free-electron laser. The parameters of SEW propagation in these crystals have been obtained by the interference method of SEW phase spectroscopy, and the possibility of determining the complex dielectric permittivity of crystals from the SEW propagation parameters demonstrated in the range of SEW existence. Fiz. Tverd. Tela (St. Petersburg) 40, 237–241 (February 1998)     

Down-conversion in the 4–18 μm range with GaSe and AgGaSe2 nonlinear crystals

The signal and idler outputs from a LiNbO₃ optical parametric oscillator have been down-converted in GaSe and AgGaSe₂ crystals. A many hundred watt peak power output continuously tunable in the range 4–18 μm has been achieved.