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  • Potassium Titanyl Phosphate (KTiOPO4, KTP)

    Potassium Titanyl Phosphate (KTiOPO4, KTP) is the most commonly used in both commercial and military lasers, including laboratory and medical systems, range-finders, lidars, industry systems, and optical communication.

    KTP is a positive biaxial crystal, with the principal axes X, Y, and Z (nz>ny>nx) parallel to the crystallographic axes a, b, and c, respectively.

    To overcome the gray track or photorefractive breakdown
    problem for KTP commonly used in practices, AOTK has
    developed one improved technique to grow Super-KTP, which has up to 1.3-1.5 times higher gray track resistance comparing with common flux grown KTP, for high power density laser systems applications. For more information, please feel free to contact AOTK.

    AOTK's KTP advanced properties
     ? Large nonlinear optical coefficients
     ? Broad temperature and spectral bandwidth
     ? Wide angular bandwidth and small walk-off angle
     ? High electro-optic coefficient and low dielectric constant 
     ? Large figure of merit for an optical waveguide modulator
     ? Non-hygroscopic, chemically and mechanically stable

    Typical Applications of KTP
     ? Frequency doubling (SHG) of Nd-doped lasers for green/red output
     ? Frequency mixing (SFM) of Nd laser and diode laser for blue output
     ? Parametric sources (OPG, OPA and OPO) for 600 nm-4500 nm tunable output
     ? E-O modulators, optical switches, directional couplers
     ? Optical waveguides for integrated NLO and E-O devices

    Basic Properties

    1. Structual and Physical Properties
    Crystal Structure
    Orthorhombic, point group mm2
    Lattice Parameters
    a = 6.404? b = 10.616? c = 12.814? Z = 8
    Density
    3.01 g/cm3
    Mohs Hardness
    �5
    Melting Point
    ~1172oC
    Transition Temperature
    936oC
    Specific Heat 0.1643 cal/g�C
    Thermal Conductivity
    0.13 W/cm/�K
    Electrical Conductivity
    3.5x10-8s/cm (c-axis, 22�C, 1KHz)
    Hygroscopic Susceptibility
    No
    Dielectric Constant
    eeff  = 13.0, e11 = 11.6, e22 = 11.0, e33 = 15.4
    Color
    Colorless

    2. Linear Optical Properties
    Transparency Region
    350 - 4500 nm
    Refractive Indexes
    at 1064 nm
    at 532 nm
    nx = 1.7377,  ny = 1.7453,  nz = 1.8297
    nx = 1.7780,  ny = 1.7886,  nz = 1.8887
    Sellmeier Equations (? in �m) 
    nx2 = 3.0065+0.03901/(?2-0.04251)-0.01327?2
    ny2 = 3.0333+0.04154/(?2-0.04547)-0.01408?2
    nz2 = 3.3134+0.05694/(?2-0.05658)-0.01682?2
    Therm-Optic Coefficients
    dnx/dT = 1.1 x 10-5/�C
    dny/dT = 1.3 x 10-5/�C
    dnz/dT = 1.6 x 10-5/�C

    3. Nonlinear Optical Properties
    Phase Matching SHG Wavelength
    497 - 1800nm
    Nonlinear Coefficients
    d31 = 6.5 pm/v
    d32 = 5.0 pm/v
    d33 = 13.7pm/v
    d24 = 7.6 pm/v
    d15 = 6.1 pm/v
    Effective Nonlinearity Expressions
    deff(II) ?/FONT> (d24 - d15)sin2fsin2q - (d15sin2f + d24cos2f)sinq
    For type II SHG of a Nd:YAG Laser at 1064nm
    PM angle: ? = 90�, F = 23.5�
    Effective SHG coefficient:  deff ?/FONT> 8.3xd36(KDP)
    Angular acceptance: 20 mrad-cm
    Temperature acceptance: 25�C-cm
    Spectral acceptance: 5.6 ?/FONT> -cm
    Walk-off angle: 4.5 mrad (0.26�)
    Electro-Optic Coefficients
      r13
      r23
      r33
      r51
      r42
    Low frequency (pm/v)9.5
    9.5
    15.7
    36.3
    7.3
    9.3
    High frequency (pm/v)
    8.8
    13.8
    35.0
    6.9
    8.8
    Optical Damage Threshold
    > 450MW/cm2,  (@ 1064nm, 10ns, 10Hz)

    Main Applications

    I. SHG and SFG of Nd:Lasers
    KTP exhibits the superior nonlinear and electroptic properties. A combination of high nonlinear coefficient, wide transparency range, and broad angular as well as thermal acceptances makes KTP very attractive for intracavity and extracavity frequency doubler of Nd:YAG laser and other Nd-doped laser applications. The major NLO properties of KTP for frequency-doubling of Nd:YAG or Nd:YVO4 lasers are listed as following table.

    PM Angle
    ? = 90�, F = 23.5�; where q and f are polar angles referring to Z and X axis
    Effective SHG Coefficient
    deff � 8.3xd36(KDP)
    Angular Acceptance
    20 mrad-cm
    Temperature Acceptance
    25�C-cm
    Spectral Acceptance
    5.6 ?/FONT> -cm
    Walk-off Angle
    4.5 mrad (0.26�)

    KTP is also being applied successfully for intracavity mixing of 808 nm diode and 1064 nm Nd:YAG laser to generate blue light and intracavity SHG of Nd:YAG or Nd:YAP lasers at 1300 nm to produce red light. With the development of diode-pumped Nd:lasers, KTP play more and more important role in the construction of the compact visible solid-state lasers. There are some typical results listed as follows: 

     ? 20W green output was generated from CW Nd: YAG laser with intracavity KTP.
     ? More than 80% conversion efficiency and 700mJ green laser were obtained with a 900mJ   injection
        -seeded Q-switch Nd:YAG laser with extracavity KTP SHG.
     ? 3W TEM00 mode-locked green laser was generated by intracavity SHG in a 5.3W  mode-locked diode
        -pumped Nd:YAG laser. 
     ? More than 600mW TEM00 green lasers are obtained from diode-pumped Nd:YAG and  Nd:YVO4 lasers. 
     ? 2.8mW green light was obtained from 50mW LD pumped intracavity Nd: YVO4 mini-  lasers with a
        8.5mm long cavity. 
     ? KTP also exhibits its powerful applications for SHG and SFG laser with wavelength 1000-3400 nm.

    Fig.1 shows Type II SHG phase-matching angle of KTP in X-Y plane. In X-Y plane the slope ?/SPAN>(Dk)/秖 is small. It corresponds to quasi-angular noncritical phase matching, which ensures the double advantage of a small walk off and a large acceptance angle. Otherwise, in X-Z plane the slope ?/SPAN>(Dk)/秎 is almost zero for wavelengths in the range 1.5-2.5 �m and the corresponds to quasi-wavelength noncritical phase matching, which ensures a large spectral acceptance (see Fig 2). Wavelength noncritical phase matching is highly desirable for frequency conversion of short pulses. Fig.2 shows Type II SHG phase-matching angle of KTP in X-Z plane (1.1- 3.4 mm). KTP is seldom used to be phase-matched for SHG of 1.0- 3.45 mm in practices by cut in Y-Z plane, due to its very low non-linear coefficients.

     

     

     Fig. 1. Type II KTP SHG in X-Y Plane

      Fig. 2. Type II KTP SHG in X-Z Plane


    II. OPG, OPA and OPO
    As a lasing material for OPG, OPA or OPO , KTP can most usefully be pumped by the fundamental and second harmonics of a Nd:lasers, or any other source with intermediate wavelength, such as a Dye laser (near 600 nm) and Ti:Sapphire laser (near 700-1000 nm), in parametric sources for tunable output from visible (600 nm) to mid-IR (4500 nm). KTP's OPO results in stable, continuous outputs of fs pulse of 108 Hz repetition rate and miliwatt average power levels in both signal and idler output. KTP's OPO pumped by a 1064 nm Nd:laser has generated more than 66% conversion efficiency for degenerately converting range 1064-2120 nm. Fig.3 & Fig. 4 show KTP OPO pumped by 532 nm & 1064 nm tuning curve in XZ Plane respectively.

     

     

     Fig.3 KTP OPO Pumped by 532 nm
    Tuning Curves in X-Z Plane

     Fig.4 KTP OPO Pumped by 1064 nm
    Tuning Curves in X-Z Plane


    The new and effective application is the non-critical phase-matched (NCPM) KTP OPO/OPA pumped by the tunable lasers (as shown in Fig.5). The output can cover wavelength range from 1040 nm to 1450 mm (signal) and from 2150 nm to 3200 nm (idler), by fixed the NCPM KTP crystal fixed in X-axis, and tunes pumping wavelength (700 nm to 1000 nm). Due to the favorable NLO properties of NCPM KTP, as high as 45% conversion efficiency was obtained with narrow output bandwidth and good beam quality.

     


    III. Quasi-Phase-Matched Waveguide
    On low optical absorption and high damage threshold, the low optical loss waveguide fabricated by applying relatively simple ion-exchange process on KTP substrate, has created novel applications of integrated optics. Following table shows the comparison of KTP with other optical waveguide materials.

    Recently, Type II SHG conversion efficiency of above 20%/W/cm2 was obtained by balanced phase matching, in which the phase mismatch from one was balanced against a phase mismatch of opposite sign from a second section. Furthermore, segmented KTP waveguides have been applied to type I quasi-phase-matchable SHG of 760-960 nm for tunable Ti:Sapphire laser and directly doubled diode laser for 400-430 nm output. Conversion efficiency in excess of 100%/W/cm2 have been obtained.

    As large as 35x35x1 mm KTP with Z-cut or both surfaces polished for waveguide applications can be provided by AOTK. Other sizes of course available upon request.

    Electro-Optic Waveguide Materials

    Materials
    n3?/eeff(pm/V)
    g (pm/V)
    eeff (e11e33<, /SUB>)1/2
    n
    KTP
    17.3
    35
    13
    1.86
    KNbO3
    9.2
    25
    30
    2.17
    LiNbO3
    8.3
    29
    37
    2.20
    Ba2NaNb5O15
    7.1
    56
    86
    2.22
    SBN(25-75)
    5.1-0.14
    56-1340
    119-3400
    2.22
    GaAS
    4.0
    1.2
    14
    3.60
    BaTiO3
    1.0
    28
    373
    2.36

    IV. E-O Devices
    KTP's unique NLO features and E-O and dielectric properties make it extremely useful to various E-O devices. Table gives the comparison of KTP with those commonly used E-O modulator materials.

    Electro-Optic Modulator Materials
     
    Phase
    Amplitude
    Materials
    e
    n
    r
    pm/v
    K
    10-6/�C
    n7r2/e(pm/v)2
    r
    pm/v
    K
    10-6/�C
    n7r2/e(pm/v)2
    KTP
    15.42
    1.80
    35.0
    31
    6130
    27.0
    11.7
    3650
    LiNbO3
    27.9
    2.20
    28.8
    82
    7410
    20.1
    42
    3500
    KD*P
    48.0
    1.47
    24.0
    9
    178
    24.0
    8
    178
    LiIO3
    5.9
    1.74
    6.4
    24
    335
    1.2
    15
    124

    When these properties are combined with wide optical bandwidth (>15GHz), low loss, high damage threshold, thermal and mechanical stability, KTP can be expected to replace a considerable volume of LiNbO3 crystals as E-O modulators, especially for mode-locking diode laser pumped Nd:YAG and Nd:YLF lasers as well as Ti:Sapphire and Cr:LiSrAlF6 laser.