Lithium Triborate Crystal Research Articles

Low repetition rate, hybrid fiber/solid-state, 1064 nm picosecond master oscillator power amplifier laser system

We report on an investigation of a passively mode-locked picosecond ytterbium-doped fiber laser with a repetition rate close to 1 MHz. The mode-locking operation was induced by a butt-coupled semiconductor saturable absorber mirror (SESAM) and stabilized in the normal dispersion regime by means of spectral filtering of a narrow bandwidth (0.1 nm) fiber Bragg grating (FBG). A pulse duration of about 50 ps with maximum output power of 20 mW at 1064 nm were achieved after amplification in the double-pass fiber preamplifier. This compact and reliable all-fiber laser operating at 1064 nm is intended to seed bulk amplifiers in a hybrid fiber/solid-state configuration. Employing a two-stage Nd:YVO4 amplifier, we obtained 10 W average output power, which preserved both the quasi-diffraction limited beam quality and spectral purity of the seeder. Second harmonic generation (SHG) with 50% conversion efficiency was demonstrated in type-II potassium titanyl phosphate (KTP) and type-I angle phase matched lithium triborate (LBO) crystal. Furthermore, a space independent numerical model was developed in order to study the dependence of intracavity pulse energy on the main design parameters of the master oscillator (cavity length, FBG bandwidth, SESAM modulation depth) within the stability range (single pulse per round trip in CW mode-locking operation) of the fiber laser master oscillator.

Intracavity pumped sum-frequency mixing for an emission at 491.5 nm

We present an Nd:LuVO4 laser emitting at 1066 nm intracavity pumped by a 912 nm diode-pumped Nd:GdVO4 laser. A 880 nm diode laser is used to pump the Nd:GdVO4 crystal emitting at 912 nm, and the Nd:LuVO4 laser emitting at 1066 nm intracavity pumped at 912 nm. Intracavity sum-frequency mixing at 912 and 1066 nm was realized in a lithium triborate crystal to reach the blue range. We obtained a continuous-wave output power of 2.35 W at 491.5 nm with a pump laser diode emitting 26.2 W at 880 nm. The power stability is better than 3.5% and the laser beam quality M2 factors are 1.31 and 1.23 in the horizontal and vertical dimensions respectively.

Palm-top size megawatt peak power ultraviolet microlaser

The development of a very compact, highly efficient, megawatt peak power, subnanosecond pulse width, 266 nm ultraviolet (UV) microlaser is reported. It contains a specially designed passively Q-switched Nd∶YAG/Cr 4+ ∶YAG microchip laser whose high output peak power of 13 MW enables an efficient wavelength conversion without using any optics before the nonlinear crystals. The subnanosecond pulse width region, which delivers high peak power even at moderate pulse energy, is very useful for an efficient wavelength conversion. We achieve 73% second harmonic generation efficiency using a LiB 3 O 5 (lithium triborate) crystal and 45% fourth harmonic generation efficiency using a β−BaB 2 O 4 (β -barium borate) crystal. As a result, we obtain 650 μJ, 4.3 MW peak power, 150 ps, and 100 Hz pulse output at 266 nm. We use an original design for the nonlinear crystal holders to reduce the size of the microlaser. This palm-top size 266 nm UV microlaser will be useful for many applications, such as materials microprocessing, pulsed laser deposition, UV laser induced breakdown spectroscopy, and photoionization.

A high power, continuous-wave, single-frequency fiber amplifier at 1091 nm and frequency doubling to 545.5 nm

We present a high power single-frequency ytterbium fiber amplifier system with an output power of 30 W at 1091 nm. The amplifier system consists of two stages, a preamplifier stage in which amplified spontaneous emission is efficiently suppressed (>40 dB) and a high power amplifier with an efficiency of 52%. Two different approaches to frequency doubling are compared. We achieve 8.6 W at 545.5 nm by single-pass frequency doubling in a MgO-doped periodically poled stoichiometric LiTaO3 crystal and up to 19.3 W at 545.5 nm by frequency doubling with a lithium-triborate crystal in an external enhancement cavity.

Photoacoustic absorption spectrometer for highly transparent dielectrics with parts-per-million sensitivity

A sensitive photoacoustic absorption spectrometer for highly transparent solids has been built and tested. As the light source an optical parametrical oscillator pumped by a nanosecond pulse laser with 10 Hz repetition rate is employed, covering the complete wavelength range from 407 to 2600 nm. A second-harmonic-generation unit extends the range of accessible wavelengths down to 212 nm. A lead-zirconate-titanate piezo transducer, directly coupled to the sample, detects the photoacoustically generated sound waves. Absorption spectra of lithium triborate, lithium niobate, and alpha barium borate crystals with absorption coefficients down to 10(-5) cm(-1) are presented.

Mode-locking of a high power, 888 nm pumped Nd:YVO_4 laser using nonlinear polarization rotation via Type I second harmonic generation

Continuous-wave mode-locking of a laser exploiting the nonlinear polarization rotation (NPR) technique via Type I second harmonic generation is demonstrated for the first time. The NPR is generated by a lithium triborate crystal and transformed into nonlinear cavity losses of a 888 nm pumped Nd:YVO4 laser. Self-starting, reliable mode-locking has been achieved at a high average output power of 20.6 W and a pulse duration of 7.3 ps. Furthermore, transform limited pulses down to 2.7 ps have been demonstrated at 9.9 W.

Structural investigation of Li2O–B2O3–MoO3glasses and high-temperature solutions: toward understanding the mechanism of flux-induced growth of lithium triborate crystal

MoO3 is an important flux for lithium triborate (LiB3O5) crystal growth from high-temperature solutions. Although it has been widely used, the mechanism of the MoO3 flux-induced growth of LiB3O5 crystals is still not very clear. In this paper, we present a spectroscopic investigation of the Li2O–B2O3–MoO3 ternary glasses/solutions, which were prepared from high-temperature MoO3-based solutions for LiB3O5 crystal growth. By combining all the experimental data of Raman and MAS NMR, the types of structural species and the interactions between flux and solute are discussed to understand the MoO3 flux-reduced mechanism of LiB3O5 crystallization from the high-temperature solution. Considering the activities of lithium cations, an isomerization reaction is proposed to describe the structural evolution of the BO4 tetrahedron into the BO3 triangle in boroxol due to the MoO3 flux. The transition between the boron oxide species is essential for the LiB3O5 crystal growth. On cooling, the formed boroxol rings are polymerized by the re-formation of BO4 tetrahedrons again, and gather together to form the LiB3O5 crystal phase. Finally, the MoO3 flux-induced LiB3O5 crystallization may be elucidated with the decrease of the concentration of the BO4 tetrahedron in high-temperature solutions.

201 W picosecond green laser using a mode-locked fiber laser driven cryogenic Yb:YAG amplifier system

We have generated 201 W of green (514.5 nm) average power from a frequency-doubled picosecond cryogenic Yb:YAG laser system driven by a 50 MHz, 12.4 ps mode-locked Yb fiber laser producing 430 W of average power at 1029 nm, using direct pulse amplification. The fundamental beam produced was near-diffraction-limited (M(2)<1.3). Second-harmonic-generation is achieved using a 20 mm long noncritically phase-matched Lithium triborate (LiB3O5) crystal; conversion efficiencies as high as 58% have been observed. At 100 W of 514.5 nm output power, the average M(2) value was 1.35. To the best of our knowledge, this is the highest average power picosecond green pulsed laser.

Compensation for the thermal effect in the second-harmonic generation of a Q-switched nanosecond-kilohertz Nd:YVO4 laser

We report the quantitative relationship between the phase-matching condition and the surrounding temperature in a critical phase-matching second-harmonic generation of a Q-switched Nd:YVO4 laser with a high repetition rate. The thermal effect can be effectively compensated for by appropriately changing the phase-matching angle with respect to the surrounding temperature of a nonlinear optical crystal. A stable wavelength conversion from the 1064 nm fundamental to the 532 nm second harmonic in a type-I critical phase-matching lithium triborate crystal is experimentally achieved and the corresponding conversion efficiency up to 70% is obtained.

Narrow linewidth 1118/559 nm VECSEL based on strain compensated GaInAs/GaAs quantum-wells for laser cooling of Mg-ions

We report on the development of an optically-pumped vertical external-cavity surface-emitting laser emitting near 1120 nm using strain compensated quantum wells. The development is motivated by the need to achieve narrow linewidth emission at ~280 nm via fourth harmonic generation, which is required to cool Mg+ ions. The gain mirror had a top-emitting geometry, was grown by molecular beam epitaxy and comprised GaInAs/GaAs quantum wells strain compensated by GaAsP layers; the strain compensation was instrumental for achieving a dislocation free epitaxial structure without dark lines. We demonstrate VECSEL operation at a fundamental wavelength close to 1118 nm with a linewidth of less than 300 kHz. Using a lithium triborate crystal we achieved frequency doubling to ~559 nm with an output power of 1.1W.

Multiwavelength green-yellow laser based on a Nd:YAG laser with nonlinear frequency conversion in a LBO crystal

We demonstrate a multiwavelength laser in the green-yellow region by means of a diode-pumped neodymium-doped yttrium aluminum garnet laser. This laser system combines a homemade 1074 nm and 1112 nm dual-wavelength laser with extracavity second harmonic generation (SHG) or sum-frequency generation in a lithium triborate crystal to generate visible output at any one of three wavelengths, 537 nm, 546 nm, and 556 nm, by simple temperature tuning, which has an important application in detecting carbon monoxide. The maximum average output power at the three wavelengths (537 nm, 546 nm, and 556 nm) was obtained to be 10.5 W, 0.5 W, and 8.5 W, respectively. The maximum SHG conversion efficiency from the infrared to the visible spectral region was about 51%.

High peak power green light generation using ytterbium fiber amplifier and lithium triborate crystal

We reported on a master-oscillator fiber power amplifier (MOPA) system and a second-harmonic generation (SHG) system. Actively Q-switched Nd:YVO4 laser was used as a seed source of light pluses. The MOPA system produced as much as 76 kW of peak power at wavelength of 1064 nm, with 35 kHz repetition rate and 20 ns pulse duration. With a lithium triborate (LBO) crystal at type II phase matching, a maximum 16 kW of peak power had been demonstrated. The fiber nonlinear effects such as stimulate Brillouin scattering (SBS) and stimulate Raman scattering (SRS) and the SHG conversion efficiency were investigated.

Optical, mechanical and laser-induced damage threshold properties of 1 064 nm, 532 nm frequency-doubled antireflection coating for LBO

1 064 nm, 532 nm frequency-doubled antireflection (AR) coatings with buffer layer of SiO2 between the coating and the substrate were fabricated by the electron beam evaporation technology on the substrate of lithium triborate (LiB3O5 or LBO) crystals. The residual reflectance of the sample is 0.07% and 0.11% at 1 064 nm and 532 nm, respectively. The adhesion and the laser-induced damage threshold (LIDT) of the sample are greater than 200 mN and 18.6 J/cm2. The strengthening mechanism of adhesion and LIDT of the buffer layer of SiO2 were discussed by considering full plastic indentation and shear theory, and spallation of a plated film induced by thermal shock stress, respectively.

Spectral phase retrieval of 8 fs optical pulses at 600 nmby using a collinear autocorrelator with 300-μm-thick lithium triborate crystals

We report on noniterative spectral phase retrieval of 1.1 nJ, 8 fs pulses at 600 nm by using 300-μm-thick lithium triborate crystals in a standard collinear autocorrelator with ≈2 min data acquisition time. This method is simple, sensitive, and immune to the spectral distortion and UV absorption of the linear and nonlinear optics.

Continuous-wave single-frequency 532 nm laser source emitting 130 W into the fundamental transversal mode

The nonlinear effect of second-harmonic generation is an efficient way to realize high-power green laser sources. But when scaling up the harmonic power, many setups reported in the literature have been limited by conversion efficiency degradation or the fundamental laser power. Here we report on the generation of 134 W of cw laser light at a wavelength of 532 nm from a fundamental power of 149 W by second-harmonic generation in an external optical resonator comprising a lithium triborate crystal. The external conversion efficiency was 90%. The harmonic light consisted of a single spectral line. At least 97% of it was emitted into the fundamental transversal mode.

Thermophysical parameters of the LBO crystal

The thermophysical parameters (linear thermal expansion coefficients, thermal conductivities, and heat capacity) of the lithium triborate (LBO) crystal are measured and compared with previously published data.

Yellow-green 523 W laser at 556 nm based on frequency doubling of a diode side-pumped Q-switched Nd:YAG laser

We demonstrate a high-power 556nm yellow-green laser generated by intracavity frequency doubling of a diode side-pumped Nd:YAG laser at 1112nm. A symmetrical L-shaped flat-flat cavity was employed to implement efficient operation of the low-gain 1112nm transition and to achieve good power scalability. The coatings of the cavity mirrors were carefully designed to optimize the performance of the laser, and a 92W continuous wave laser output at 1112nm was achieved when the pumping power of the laser diodes reached 960W. By intracavity frequency doubling of the fundamental laser in a lithium triborate crystal, the maximum power of the frequency-doubled output at 556nm was found to be as high as 52.3W with a pulse repetition frequency of 10kHz. This corresponds to an optical-to-optical conversion efficiency of about 5.4%.

56-W Frequency-Doubled Source at 530 nm Pumped by a Single-Mode, Single-Polarization, Picosecond, Yb$^{3+}$-Doped Fiber MOPA

We report a frequency-doubled green source at 530 nm pumped based on an all-fiber, picosecond, single-polarization Yb 3+-doped fiber master oscillator power amplifier (MOPA) delivering 20-ps pulses at user selectable repetition rates of up to 910 MHz and an average output power in excess of 100 W at 1.06 ?m. The output of the MOPA was frequency-doubled using a lithium triborate crystal. Up to 56 W of green light was generated at a corresponding repetition rate of 227 MHz at an overall conversion efficiency of 56%. The diode-to-green optical power conversion efficiency was 37%.

Towards sympathetic cooling of trapped ions with laser-cooled Mg + ions for mass spectrometry and laser spectroscopy

Sympathetic cooling by laser cooled Mg ions has been proposed as a method for fast cooling of highly charged ions to a very low temperature. The paper describes the construction of the solid state laser system at 279.63 nm required for laser cooling of the Mg ions. The laser system is composed of a fiber laser at 1,118.54 nm and two successive second harmonic generation (SHG) ring cavities for frequency quadrupling. In the first SHG cavity, non-critical phase matching of a lithium triborate (LBO) crystal is used for doubling from 1,118.54 to 559.27 nm. The second SHG cavity uses critical phase matching of a β-barium borate (BBO) crystal for doubling from 559.27 to 279.63 nm. With the aid of Boyd–Kleinmann theory, optimum experimental parameters are calculated and used for an efficient SHG. Besides this, the paper intends to be a shortcut for practical applications of the Boyd–Kleinmann theory for SHG.

Employing SiO2 buffer layer to improve adhesion of the frequency-doubled antireflection coating on LBO

Frequency-doubled antireflection coatings simultaneously effective at 1064 nm and 532 nm were deposited on the lithium triborate (LiB3O5 or LBO) crystals using the electron beam evaporation method. Comparing with the sample without buffer layer, it is found that the adhesion of the sample with buffer layer of SiO2 between coating and LBO substrate is improved significantly from 137.4 mN to greater than 200 mN. And the laser-induced damage threshold is increased by 20% from 15.1 J/cm2 to 18.6 J/cm2. The strengthening mechanism of adhesion of the buffer layer of SiO2 is discussed by considering full plastic indentation and shear theory.