Passive Q-switching of short-length Tm-doped silica fiber lasers by polycrystalline Cr:ZnSe microchips

We demonstrate passive Q-switching of short-length double-clad Tm³⁺-doped silica fiber lasers near 2 μm pumped by a laser diode array (LDA) at 790 nm. Polycrystalline Cr²⁺:ZnSe microchips with thickness from 0.3 to 1 mm are adopted as the Q-switching elements. Pulse duration of 120 ns, pulse energy over 14 μJ and repetition rate of 53 kHz are obtained from a 5-cm long fiber laser. As high as 530 kHz repetition rate is achieved from a 50-cm long fiber laser at ∼10-W pump power. The performance of the Q-switched fiber lasers as a function of fiber length is also analyzed.     

Near-infrared emissions and quantum efficiencies in Tm-doped heavy metal gallate glasses for S- and U-band amplifiers and 1.8 μm infrared laser

Intense 1.8 μm and efficient 1.48 μm infrared emissions have been recorded in Tm³⁺-doped alkali-barium-bismuth-gallate (LKBBG) glasses with low phonon energies under the excitation of 792 nm diode laser. The maximum emission cross-sections for 1.8 and 1.48 μm emission bands are derived to be 6.26×10⁻²¹ and 3.34×10⁻²¹ cm², respectively, and the peak values are much higher than those in Tm³⁺-doped ZBLAN glass. In low-concentration doping, the full-widths at half-maximum (FWHMs) of the two emission bands are 223 and 122 nm, and the quantum efficiencies of the ³F₄ and ³H₄ levels are proved to be ∼100% and 86%, respectively. When the doping concentration increases to 1 wt%, the quantum efficiency of the ³H₄ level is reduced to 60% due to the cross-relaxation processes in high-concentration doping. Efficient 1.8 μm infrared emission in Er³⁺/Tm³⁺-codoped LKBBG glass has also been achieved under the excitation of 970 nm diode laser, and the probability and the efficiency of non-radiative energy transfer from Er³⁺ to Tm³⁺ are as high as 354 s⁻¹ and 58.4%, respectively. Efficient and broad 1.8 and 1.48 μm infrared emission bands indicate that Tm³⁺-doped LKBBG glasses are suitable materials in developing S- and U-band amplifiers and 1.8 μm infrared laser.     

High-power 3.8 μm tunable optical parametric oscillator based on PPMgO:CLN

The experimental results of a high-power 3.8 μm tunable laser are presented on a quasi-phase-matched single-resonated optical parametric oscillator in PPMgO:CLN pumped by a 1064 nm laser of an elliptical beam. Theoretical analyses of the PPMgO:CLN wavelength tuning are presented. The pump source was an acousto-optical Q-switched cw-diode-side-pumped Nd:YAG laser. The beam polarization matched the e-ee interaction in PPMgO:CLN. When the crystal was operated at 90 °C and the pump power was 150 W with a repetition rate of 10 kHz, average output power of 22.6 W at 3.86 μm and 63 W at 1.47 μm was obtained. The slope efficiency of the 3.86 μm laser with respect to the pump laser was 17.8%. The M² factors of the 3.86 μm laser were 1.74 and 4.86 in the parallel and perpendicular directions, respectively. The mid-IR wavelength tunability of 3.7-3.9 μm can be achieved by adjusting the temperature of a 29.2 μm period PPMgO:CLN crystal from 200 °C to 30 °C, which basically is accorded with the theoretic calculation.     

Injection-seeded Tm:YAG laser at room temperature

The solid-state, tunable, narrowband, high pulse energy and high reliability lasers are attractive source for LIDAR system. In this paper, we demonstrated a diode pumped injection-seeded 2 μm Tm:YAG laser. By inserting two F-P etalons into the laser cavity, linear-polarized single-frequency seed-laser was achieved at a wavelength of 2013 nm, with a maximum output power of 60 mW. Long-term and short-term frequency stability for the seed-laser were 1.27 × 10^− 7 and 97 Hz/μs, respectively. High power Q-switched laser was operated using a bowtie cavity, the bidirectional output of which was favorable for the injection-seeded. After injecting the seed-laser to the power-laser, single-frequency, nearly transform-limited pulsed 2 μm laser was obtained. As much as 2.0 mJ output energy was achieved at an operating repetition rate of 15 Hz, with a pulse width of 356.2 ns.     

Modeling of plasma-controlled surface evaporation and condensation of Al target under pulsed laser irradiation in the nanosecond regime

Phase transition on the surface of an aluminium target and vapour plasma induced by laser irradiation in the nanosecond regime at the wavelengths of 1.06 and 0.248 μm with an intensity of 10⁸-10⁹ W/cm² in vacuum are analysed. Particular attention is paid to the wavelength dependence of the observed phenomena and the non-one-dimensional effect caused by the Gaussian laser intensity distribution. A transient two-dimensional model is used which includes conductive heat transfer in the condensed phase, radiative gas dynamics and laser radiation transfer in the plasma as well as surface evaporation and back condensation at the phase interface. It is shown that distinctions in phase transition dynamics for the 1.06 and 0.248 μm radiation result from essentially different characteristics of the laser-induced plasmas. For the 1.06 μm radiation, evaporation stops after the formation of hot optically thick plasma, can occasionally resume at a later stage of the pulse, proceeds non-uniformly in the spot area, and the major contribution to the mass removal occurs in the outer part of the irradiated region. Plasma induced by the 0.248 μm laser is much more transparent therefore evaporation does not stop but continues in the subsonic regime with the Mach number of about 0.1.     

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.     

A novel dispersion-shifted single mode optical fiber design with ultra-high-bit-rate and very low loss for long-haul communications

In this paper, a novel dispersion-shifted multi-clad optical fiber with very small bending loss and ultra-high bit-rate applicable for large capacity information transmission systems is presented. To decrease dispersion and higher-order dispersion effects at λ = 1.55 μm, a weighted pulse broadening factor and genetic algorithm (GA) optimization technique is used. Compared to the works reported previously, this method can precisely set the zero-dispersion wavelength. This kind of dispersion-shifted fibers has dispersion, dispersion slope, mode field diameter (MFD), effective area and quality factor within −1.40 × 10⁻⁴ to −8.44 × 10⁻² ps/km nm, 3.06 × 10⁻² to −4.07 × 10⁻² ps/km nm², 5.56−5.85 μm, 119.25−176.42 μm² and 3.49-5.27 at λ = 1.55 μm, respectively. Besides, by applying dispersion at λ = 1.55 μm as the cost function, dispersion of about 1.31 × 10⁻⁸ ps/km nm is obtained. Thus, this novel optical fiber can be used in long-haul high information-transmission capacity communication systems.     

A widely tunable (5-12.5 μm) continuous-wave mid-infrared laser spectrometer based on difference frequency generation in AgGaS2

A widely tunable (5-12.5 μm) continuous-wave (cw) mid-infrared (mid-IR) laser spectrometer based on difference frequency generation (DFG) by mixing an external-cavity diode laser (ECDL) with a Ti:Sapphire laser in an AgGaS₂ crystal is described. The wide tunability was achieved by tuning laser wavelength associated with crystal angle tuning under type II phase matching condition. A maximum output power of about 66 nW was obtained at 8.06 μm. High resolution spectrum of methane (CH₄) over more than 10 cm⁻¹ near 7.7 μm has been recorded to evaluate the performance of the developed DFG-based mid-IR laser spectrometer.     

The Na LGS generated by laser on the near space aircraft platform

We present a new configuration of the Na laser guide star (LGS) system with the laser source placed on the near space aircraft platform. Model of the Na LGS with the new configuration is established and the working principle is analyzed in detail. Under the rural environment with 23 km visibility and the H-V 5/7 atmospheric turbulent model, performances of the Na LGS with the new configuration and the conventional configuration are calculated, respectively. Results show that performance of the Na LGS can be significantly improved by using the new configuration, the uplink transmittance can be improved from 64.7% to 99.8%, diameter of the Na LGS can be decreased from 3.03 m to 0.36 m, and number of photons received can be improved from 536 photons cm⁻² s⁻¹ to 828 photons cm⁻² s⁻¹.     

Surface temperature evolution in pulsed laser action of millisecond range

An originally developed multi-wavelength pyrometer (12 wavelengths in the range 1.001-1.573 μm, 50 μs acquisition time for each photodiode, 800 μm spatial resolution, 900-3500 °C brightness temperature range) is used to measure brightness temperature under the pulsed action of Nd:YAG laser (HAAS-HL62P) on stainless steel (INOX 304L) substrates. Specially developed “notch” filters (10⁻⁶ transparency at 1.06 μm wavelength) are applied to avoid the influence of laser radiation on temperature measurements. The true temperature is restored on the basis of method of multi-colour pyrometry. The accuracy of brightness temperature measurements is examined by comparing the temperature evolution for pulses with different durations but with the same value of energy density flux.The influence of the following parameters is studied keeping the remaining ones constant: pulse duration (6-20 ms, rectangular pulse shape), energy per pulse (10-33 J, rectangular pulse shape), pulse shape (three types of triangulars and one rectangular). Finally the evolution of surface temperature for pulses with more complex shapes but with the same pulse duration and energy per pulse is compared.     

Specialty Yb fiber amplifier for microchip Nd laser: Towards ∼1-mJ/1-ns output at kHz-range repetition rate

We demonstrate and optimize, for a mJ/ns release at the wavelength 1.064 μm, the operation of a compact laser system designed in the form of a hybrid, active-passive, Q-switched Nd³⁺:YAG/Cr⁴⁺:YAG microchip laser seeding an Yb-doped specialty multi-port fiber amplifier. As the result of the amplifier optimization, ∼1 mJ, ∼1 ns, almost single-mode pulses at a 1-10-kHz repetition rate are achieved, given by a gain factor of ∼19 dB for an 11-μJ input from the microchip laser. Meanwhile, a lower pulse energy, ∼120 μJ, but a much higher gain (∼25 dB) are eligible for the less powerful (0.35 μJ) input pulses.     

Carbon monoxide laser emitting nanosecond pulses with 10 MHz repetition rate

Actively mode-locked electron-beam-sustained-discharge CO laser producing a train of ∼5-15 ns (FWHM) spikes following with repetition rate 10 MHz for both single-line and multiline mode of operation in the mid-IR range of ∼5 μm was experimentally studied. Total laser pulse duration was ∼0.5 ms for both mode-locked and free-running laser. Specific output energy in multiline CO laser mode of operation was up to 20 Jl⁻¹ Amagat⁻¹ and the laser efficiency up to 3.5%. The active mode-locking was achieved for single-line CO laser mode of operation in spectral range 5.2-5.3 μm. This sort of radiation can be used for pumping an optical parametric amplifier for optical stochastic cooling in relativistic heavy ion collider, for laser ablation, and for studying vibrational and rotational relaxation of CO and NO molecules.     

Generation of tunable infrared radiation in rubidium titanyl phosphate crystal for the optical measurement of number density, absorption cross-section of methane gas

Tunable near-infrared radiation has been generated in a rubidium titany1 phosphate (RTP) crystal by employing non-collinear difference-frequency mixing (DFM) technique. The input radiation sources are Nd:YAG laser radiation and its second harmonic pumped dye laser radiation. For the generation of 2.0 radiation, the maximum value of the conversion efficiency (quantum) obtained in the process is 49% from the dye (0.6945 μm) to the infrared (2.0 μm) radiation in the 7.9-mm-long crystal. The generated tunable mid-infrared radiation has been used to measure the number density, absorption cross-section and minimum detectable concentration of methane gas in its 2ν₃ band in a multi-pass cell at 30.075 Torr pressure. The number density and column density of the methane molecules are found to be 1.068×10¹⁸ cm⁻³ and 3.02×10²¹ cm⁻², respectively, whereas the minimum still-detectable concentration at 1.658 μm wavelength is estimated to be 4.523×10¹⁷/cm³.     

Designing of endlessly single mode polarization maintaining highly birefringent nonlinear micro-structure fiber at telecommunication window by FV-FEM

An endlessly single mode highly polarization maintaining nonlinear microstructure fiber at telecommunication window is reported via full-vector finite element method. By taking three ring hexagonal PCF with suitable fiber parameter such as air hole diameter in cladding region d = 0.8 μm, pitch 2.3 μm and introducing four symmetrical large air holes near core region d′ = 2 μm, single mode (V_eff ≤ π), small effective mode area 2.7 μm², nonlinear co-efficient 44.39 W⁻¹ km⁻¹, high phase birefringence of the order of 10⁻³ and group birefringence of the order of 10⁻⁴ with beat length 0.3 μm at wavelength 1.55 μm are achieved.     

Pulsed infrared radiation transmission through chalcogenide glass fibers

Two different kinds of chalcogenide glass IR fibers were evaluated relative to transmission of pulsed IR radiation produced by several laser sources in the wavelength range from 1 to 10 μm. Fibers composed either from As-Se-Te or from As₂S₃ glass, of 250, 500, 750 and 1000 μm and 250, 750 and 1000 μm core diameters were studied, respectively. Attenuation measurements were obtained as a function of the laser energy input and as a function of curvature, wherever this was possible. The output beam quality was also studied using a beam profiler. The lasers used were a Q-switched Nd:YAG laser, emitting at 1.06 μm, a free-running or Q-switched Er:YAG laser emitting at 2.94 μm and a tunable pulsed CO₂ laser emitting in the range of 9.3-10.6 μm. The fibers exhibited better behavior when tested with the Er:YAG laser and they were found fragile in pulsed radiation from the Nd:YAG and the CO₂ laser. The output beam profiles generally showed a central multi-spiking energy distribution.     

Line patterning with large refractive index changes in the deep inside of glass by nanosecond pulsed YAG laser irradiation

Nanosecond (∼100 ns) pulsed (10 Hz) Nd:YAG laser operating at the wavelength (λ) of 1064 nm with pulse energies of 0.16-1.24 mJ/cm² has irradiated 10Sm₂O₃·40BaO·50B₂O₃ glass. It is demonstrated for the first time that the structural modification resulting the large decease (∼3.5%) in the refractive index is induced by the irradiation of YAG laser with λ=1064 nm. The lines with refractive index changes are written in the deep inside of 100-1000 μm depths by scanning laser. The line width is 1-13 μm, depending on laser pulse energy and focused beam position. It is proposed that the samarium atom heat processing is a novel technique for inducing structural modification (refractive index change) in the deep interior of glass.     

A complete study of the line intensities of four bands of CO2 around 1.6 and 2.0 μm: A comparison between Fourier transform and diode laser measurements

Atmospheric carbon dioxide is a key specie for the Earth climate. Two spectral windows at 1.6 μm and 2.0 μm are of particular interest for the in situ and remote monitoring of carbon dioxide from satellite, balloon or airborne platforms using infrared absorption spectroscopy. A precise knowledge of the line strengths is a prerequisite for an accurate concentration retrieval. In this paper, we have revisited in the laboratory the (30⁰1)_III←(00⁰0) and (30⁰1)_II ← (00⁰0) bands of CO₂ near 1.6 μm and the (20⁰1)_III ← (00⁰0) and (20⁰1)_II ← (00⁰0) bands near 2.0 μm by implementing both a high-resolution Connes-type Fourier-transform spectrometer and a tunable diode laser spectrometer equipped with several telecommunication-type semiconductor laser devices. Approximately 200 (respectively 18) transitions of CO₂ have been carefully investigated in spectra recorded with the FT spectrometer (respectively with the tunable diode laser spectrometer). The intensity measurements achieved with both instruments are thoroughly compared to previous instrumental determinations, ab-initio calculations and available atmospheric molecular database.     

Infrared and far-infrared spectroscopy of CH3OH: TeraHertz laser lines and assignments

We use a ¹³CO₂ laser as optical pumping source to search for new THz laser lines generated from ¹³CH₃OH. Nineteen new THz laser lines (also identified as far-infrared, FIR) ranging from 42.3 μm (7.1 THz) to 717.7 μm (0.42 THz) are reported. They are characterized in wavelength, offset, relative polarization, relative intensity, and optimum working pressure. We have assigned eight laser lines to specific rotational energy levels in the excited state associated with the C-O stretching mode.     

Circular and rectangular via holes formed in SiC via using ArF based UV excimer laser

Circular via holes with diameters of 10, 25, 50 and 70 μm and rectangular via holes with dimensions of 10 μm × 100 μm, 20 μm × 100 μm and 30 μm × 100 μm and drilled depths between 105 and 110 μm were formed in 300 μm thick bulk 4H-SiC substrates by Ar/F₂ based UV laser drilling (λ = 193 nm) with a pulse width of ∼30 ns and a pulse frequency of 100 Hz. The drilling rate was linearly proportional to the fluence of the laser, however, the rate decreased for the larger via holes. The laser drilling produces much higher etch rates (229-870 μm/min) than conventional dry etching (0.2-1.3 μm/min) and the via entry can be tapered to facilitate subsequent metallization.     

3D periodic structures grown on silicon by radiation of a pulsed Nd:YAG laser and their field emission properties

Periodic three-dimensional structures were successfully grown on single crystal Si wafers either bare or Au-covered under their exposure to a pulsed radiation of a Nd:YAG laser in vacuum. The structures protrude above the initial wafer surface for 10 μm while their spatial period is about 70 μm. The coupling of the laser radiation to Si surface is related to the thermal non-linear absorption of the near band gap radiation. The structures exhibit an efficient field emission with an average emission current of 5 mA/cm² and is sensitive to the post-treatment of samples. The drawbacks of the emission current densities are discussed.