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        description:

        Lithium Triborate (LiB3O5 , LBO)

        Introduction

        Lithium Triborate (LiB3O5 or LBO) is an excellent nonlinear optical crystal discovered and  developed by FIRSM, CAS (Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences). Fujian CASTECH Crystals, Inc. (CASTECH)has the exclusive rights to produce, manufacture and market the patented LBO crystal and its NLO devices. The patent number is 4,826,283 in USA, 2023845 in Japan and 88 1 02084.2 in China.

        CASTECH's LBO is featured by 

           broad transparency range from 160nm to 2600nm (see Figure 1);
           high optical homogeneity (dn»10-6/cm) and being free of inclusion;
           relatively large effective SHG coefficient (about three times that of KDP);
           high damage threshold;
           wide acceptance angle and small walk-off;
           type I and type II non-critical phase matching (NCPM) in a wide wavelength range;
           spectral NCPM near 1300nm.
         

        CASTECH offers LBO of 

           strict quality control;
           large crystal size up to 30x30x30mm3 and maximum length of 60mm;
           AR-coating, mounts and re-polishing services;
           a large quantity of  crystals in stock;
           fast delivery(10 days for polished only, 15 days for AR-coated).
         

                                                  Table 1. Chemical and Structural properties of LBO

        Crystal Structure

        Orthorhombic, Space group Pna21, Point group mm2

        Lattice Parameter

        a=8.4473Å, b=7.3788Å , c=5.1395Å , Z=2

        Melting Point   

        About 834℃

        Mohs Hardness   

        6

        Density         

        2.47 g/cm3

        Thermal Conductivity  

        3.5W/m/K

        Thermal Expansion Coefficient

        ax=10.8x10-5/K, ay= -8.8x10-5/K, az=3.4x10-5/K

         

                                                Table 2. Optical and Nonlinear Optical Properties of LBO

        Transparency Range

        160-2600nm

        SHG Phase Matchable Range

        551 ~ 2600nm  (Type I)      790-2150nm (Type II)

        Therm-optic Coefficient

        (°C, λ in μm)

        dnx/dT=-9.3X10-6
        dny/dT=-13.6X10-6
        dnz/dT=(-6.3-2.1λ)X10-6

        Absorption  Coefficient

        <0.1%/cm at 1064nm    <0.3%/cm at 532nm

        Angle Acceptance

        6.54mrad-cm    (φ, Type I,1064 SHG)
        15.27mrad-cm    (q, Type II,1064 SHG)

        Temperature Acceptance

        4.7°C-cm   (Type I, 1064 SHG)
        7.5°C-cm   (Type II,1064 SHG)

        Spectral Acceptance

        1.0nm-cm (Type I, 1064 SHG)
        1.3nm-cm (Type II,1064 SHG)

        Walk-off Angle

        0.60° (Type I  1064 SHG)
        0.12° (Type II  1064 SHG)

        NLO Coefficients

        deff(I)=d32cosΦ     (Type I  in XY plane)
        deff(I)=d31cos2θ+d32sin2θ   (Type I  in XZ plane)
        deff(II)=d31cosθ  (Type II in YZ plane)
        deff(II)=d31cos2θ+d32sin2θ   (Type II in XZ plane)

        Non-vanished NLO susceptibilities

        d31=1.05 ± 0.09 pm/V
        d32= -0.98 ± 0.09 pm/V
        d33=0.05 ± 0.006 pm/V

        Sellmeier  Equations(λ in μm)     

        nx2=2.454140+0.011249/(λ2-0.011350)-0.014591λ2-6.60x10-5λ4
        ny2=2.539070+0.012711/(λ2-0.012523)-0.018540λ2+2.0x10-4λ
        4
        nz2=2.586179+0.013099/(λ2-0.011893)-0.017968λ2-2.26x10-4λ4

         
         

        SHG conversion efficiencies of more than 70% for pulse and 30% for cw Nd:YAG lasers, and THG conversion efficiency over 60% for pulse Nd:YAG laser have been observed by using CASTECH’s LBO crystal.

        LBO's applications

        * More than 480mW output at 395nm is generated by frequency doubling a 2W mode-locked Ti:Sapphire laser (<2ps, 82MHz). The wavelength range of 700-900nm is covered by a 5x3x8mm3 LBO crystal.
        * Over 80W green output is obtained by SHG of a Q-switched Nd:YAG laser in a type II 18mm long LBO crystal.
        * The frequency doubling of a diode pumped Nd:YLF laser (>500µJ @ 1047nm, <7ns, 0-10KHz) reaches over 40% conversion efficiency in a 9mm long LBO crystal.
        * The VUV output at 187.7 nm is obtained by sum-frequency generation.
        * 2mJ/pulse diffraction-limited beam at 355nm is obtained by intra-cavity frequency tripling a Q-switched Nd:YAG laser.

        LBO's Non-Critical Phase-Matching

        Table 3. Properties of type I NCPM SHG at 1064nm

        NCPM Temperature
        Acceptance Angle
        Walk-off Angle
        Temperature Bandwidth
        Effective SHG Coefficient

        148°C
        52 mrad-cm
        1/2
        0
        4°C-cm
        2.69 d36(KDP)


         

        AS shown in table 3, Non-Critical Phase-Matching (NCPM) of LBO is featured by no walk-off, very wide acceptance angle and maximum effective coefficient. It promotes LBO to work in its optimal condition. SHG conversion efficiencies of more than 70% for pulse and 30% for cw Nd:YAG lasers have been obtained, with good output stability and beam quality.

        As shown in Fig.3, type I and type II non-critical phase-matching can be reached along x-axis and z-axis at room temperature, respectively.(CASTECH develops an assembly of oven and temperature controller for NCPM applications. Please refer to Page 20 for more technical data.)

        LBO's applications

        * By using LBO crystal,over 11W of average power at 532nm was obtained by extra-cavity SHG of a 25W Antares mode-locked Nd:YAG laser (76MHz, 80ps).

        * By using LBO crystal,20W green output was generated by frequency doubling a medical, multi-mode Q-switched Nd:YAG laser. Much higher green output is expected with higher input.

        LBO’s OPO and OPA

        LBO is an excellent NLO crystal for OPOs and OPAs with a widely tunable wavelength range and high powers. These OPO and OPA that are pumped by the SHG and THG of Nd:YAG laser and XeCl excimer laser at 308nm have been reported. The unique properties of type I and type II phase matching as well as the NCPM leave a big room in the research and applications of LBO’s OPO and OPA. Fig.4 shows the calculated type I OPO tuning curves of LBO pumped by the SHG, THG and 4HG of Nd:YAG laser in XY plane at the room temperature. And Fig. 5 illustrates type II OPO tuning curves of LBO pumped by the SHG and THG of Nd:YAG laser in XZ plane.

        LBO's applications


        * A quite high overall conversion efficiency and 540-1030nm tunable wavelength range were obtained with OPO pumped at 355nm.
        * Type I OPA pumped at 355nm with the pump-to-signal energy conversion efficiency of 30% has been reported.
        * Type II NCPM OPO pumped by a XeCl excimer laser at 308nm has achieved 16.5% conversion efficiency, and moderate tunable wavelength ranges can be obtained with different pumping sources and temperature tuning.
        * By using the NCPM technique, type I OPA pumped by the SHG of a Nd:YAG laser at 532nm was also observed to cover a wide tunable range from 750nm to 1800nm by temperature tuning from 106.5°C to 148.5°C.
        * By using type II NCPM LBO as an optical parametric generator (OPG) and type I critical phase-matched BBO as an OPA, a narrow linewidth (0.15nm) and high pump-to-signal energy conversion efficiency (32.7%) were obtained when it is pumped by a 4.8mJ, 30ps laser at 354.7nm. Wavelength tuning range from 482.6nm to 415.9nm was covered by increasing the temperature of LBO or rotating BBO.

        LBO’s Spectral NCPM

        Not only the ordinary non-critical phase matching (NCPM) for angular variation but also the noncritical phase matching for spectral variation (SNCPM) can be achieved in the LBO crystal. As shown in Fig.2, the phase matching retracing positions are λ1=1.31μm with θ =86.4°, φ=0° for Type I and λ2=1.30μm with θ=4.8°, φ=0° for Type II. The phase matching at these positions possess very large spectral acceptances Δλ. The calculated Δλ at λ1 and λ2are 57nm-cm-1/2 and 74nm-cm-1/2 respectively, which are much larger than the other NLO crystals. These spectral characteristics are very suitable for doubling broadband coherent radiations near 1.3μm, such as those from some diode lasers, and some OPA/OPO output without linewidth-narrowing components.

        The crystal holder (free) and oven & Temperature Controller (for NCPM,OPO,OPA applications) are available for BBO & LBO.

        AR-coatings of LBO

        CASTECH provides LBO crystal with the following AR-coatings:
        * Dual Band AR-coating (DBAR) of LBO for SHG of 1064nm.
        · low reflectance (R<0.2% at 1064nm and R<0.5% at 532nm );
        · high damage threshold (>500MW/cm2at both wavelengths);
        · long durability.
        * Broad Band AR-coating (BBAR) of LBO for SHG of tunable lasers.
        * Other coatings are available upon request.

        CASTECH Warranty on LBO Specifications


        * Dimension tolerance: (W±0.1mm)x(H±0.1mm)x(L+0.5/-0.1mm) (L≥2.5mm)
                                                  (W±0.1mm)x(H±0.1mm)x(L+0.1/-0.1mm) (L<2.5mm)
        * Clear aperture: central 90% of the diameter
        * No visible scattering paths or centers when inspected by a 50mW green laser
        * Flatness: less than λ/8 @ 633nm
        * Transmitting wavefront distortion: less than λ/8 @ 633nm
        * Chamfer: ≤0.2mmx45°
        * Chip: ≤0.1mm
        * Scratch/Dig code: better than 10/ 5 to MIL-PRF-13830B
        * Parallelism: better than 20 arc seconds
        * Perpendicularity: ≤5 arc minutes
        * Angle tolerance: Δθ≤0.25°, ΔΦ≤0.25°
        * Damage threshold[GW/cm2 ]: >10 for 1064nm, TEM00, 10ns, 10HZ (polished only)
                                                                 >1 for 1064nm, TEM00, 10ns, 10HZ (AR-coated)
                                                                 >0.5 for 532nm, TEM00, 10ns, 10HZ (AR-coated)
        * Quality Warranty Period: one year under proper use.

        NOTES


        1. LBO has a very low susceptibility to moisture. Users are advised to provide dry conditions for both the use  and preservation of LBO.
        2. Polished surfaces of LBO requires precautions to prevent any damage.
        3. CASTECH engineers can select and design the best LBO crystal for you, if the main parameters of your laser are  provided, such as energy per pulse, pulse width and repetition rate for a pulsed laser, power for a cw laser, laser beam diameter, mode condition, divergence, wavelength tuning range, etc.
        4. For thin LBO crystal, CASTECH can provide free holder for you.