Potassium Titanyl Phosphate (KTiOPO4,KTP)

Crystal Structure

Orthorhombic, space group Pna2₁,point group mm²

Lattice Parameter

a=6.404?, b=10.616?, c=12.814?, Z=8

Melting Point   

About 1172°C

Mohs Hardness   

5

Density         

3.01 g/cm³

Thermal Conductivity  

13W/m/K

Thermal Expansion Coefficient

α_x=11x10^-6/°C, α_y=9x10^-6/°C, α_z=0.6x10^-6/°C

Transparency Range

350～4500nm

SHG Phase Matchable Range

497～1800nm  (Type II)

Therm-optic Coefficients(/°C)

dnx/dT=1.1X10^-5
dny/dT=1.3X10^-5
dnz/dT=1.6X10^-5

Absorption  Coefficients

<0.1%/cm at 1064nm, <1%/cm at 532nm

For Type II SHG of a Nd:YAG laser at 1064nm

Temperature Acceptance: 24°C-cm
Spectral Acceptance: 0.56nm-cm
Angular Acceptance: 14.2mrad-cm (φ);55.3mrad-cm (θ)
Walk-off Angle: 0.55°

NLO Coefficients

d_eff(II)≈(d₂₄-d₁₅)sin2φsin2θ-(d₁₅sin²φ + d₂₄cos²φ)sinθ

Non-vanished NLO susceptibilities

d₃₁=6.5 pm/V    d₂₄=7.6 pm/V
d₃₂=5 pm/V        d₁₅=6.1 pm/V
d₃₃=13.7 pm/V

Sellmeier  Equations
(λ in μm)     

n_x²=3.0065+0.03901/(λ²-0.04251)-0.01327λ ²
n_y²=3.0333+0.04154/(λ ²-0.04547)-0.01408λ²
n_z²=3.0065+0.05694/(λ ²-0.05658)-0.01682λ²

KTP is the most commonly used material for frequency doubling of Nd:YAG and other Nd-doped lasers, particularly when the power density is at a low or medium level. To date, extra- and intra-cavity frequency doubled Nd:lasers using KTP have become a preferred pumping source for visible dye lasers and tunable Ti:Sapphire lasers as well as their amplifiers. They are also useful green sources for many research and industry applications.

KTP is also being used for intracavity mixing of 0.81μm diode and 1.064μm Nd:YAG laser to generate blue light and intracavity SHG of Nd:YAG or Nd:YAP lasers at 1.3μm to produce red light.

Fig.1 Type II KTP SHG in XY Plane

Fig 2 Type II KTP SHG in XZ Plane

As an efficient OPO crystal pumped by a Nd:laser and its second harmonics, KTP plays an important role for parametric sources for tunable outputs from visible (600nm) to mid-IR (4500nm), as shown in Fig. 3 and Fig. 4.

Generally, KTP’s OPOs provide stable and continuous pulse outputs (signal and idler) in fs, with 10⁸ Hz repetition rate and a mW average power level. A KTP’s OPO that are pumped by a 1064nm Nd:YAG laser has generated as high as above 66% efficiency for degenerately converting to 2120nm.

Fig.3 OPO pumped at 532 in X-Z plane

Fig.4 OPO pumped at 532 in X-Y plane

The novel developed application is the non-critical phase-matched(NCPM) KTP OPO/OPA pumped by the X-cut KTP crystal. As shown in Fig.5, for pumping wavelength range from 0.7μm to 1 μm, the output can cover from 1.04μm to 1.45μm(signal) and from 2.15μm to 3.2μm(idler). More than 45% conversion efficiency was obtained with narrow output bandwidth and good beam quality.

Fig.5 Type II NCPM OPO

In addition to unique NLO features, KTP also has promising E-O and dielectric properties that are comparable to LiNbO₃These advantaged properties make KTP extremely useful to various E-O devices. Table 1 is a comparison of KTP with other E-O modulator materials commonly used:

Phase

Amplitude

Material

ε

N

R(pm/V)

k(10^-6/°C)

N⁷r²/ε(pm/V)²

r(pm/V)

k(10^-6/°C)

n⁷r²/ε(pm/V)²

KTP

LiNbO₃

KD^*P

LiIO₃

15.42  

27.9

48.0

5.9

1.80

2.20

1.47

1.74

35.0

 8.8

24.0

6.4

31

82

9

24

6130

7410

178

335

27.0

20.1

24.0

1.2

   11.7

   42

    8

   15

3650

3500

178

124

From Table 1, clearly, KTP is expected to replace LiNbO₃ crystal in the considerable volume application of E-O modulators, when other merits of KTP are combined into account, such as high damage threshold, wide optical bandwidth (>15GHZ), thermal and mechanical stability, and low loss, etc.

Based on the ion-exchange process on KTP substrate, low loss optical waveguides developed for KTP have created novel applications in integrated optics. Table 2 gives a comparison of KTP with other optical waveguide materials. Recently, a type II SHG conversion efficiency of 20%/W/cm2 was achieved by the balanced phase matching, in which the phase mismatch from one section was balanced against a phase mismatch in the opposite sign from the second. Furthermore, segmented KTP waveguides have been applied to the type I quasi-phase-matchable SHG of a tunable Ti:Sapphire laser in the range of 760-960mm, and directly doubled diode lasers for the 400-430nm outputs.

Materials

r (pm/V)

n

e_eff (e₁₁e₃₃)^1/2

n³r/e_eff (pm/V)

KTP

LiNbO₃

KNbO₃

BNN

BN

GaAs

BaTiO₃

35

29

25

56

56-1340

1.2

28

1.86

2.20

2.17

2.22

2.22

3.6

2.36

13

37

30

86

119-3400

14

373

17.3

8.3

9.2

7.1

5.1-0.14

4.0

1.0