Potassium Titanyl Phosphate (KTiOPO₄ or KTP)

III. Main Applications

• SHG and SFG of Nd:Lasers (OPG, OPA and OPO | Optical Waveguides)
  
  On the combination of its superior NLO properties (as Table 1), KTP is the most commercial material applied for intracavity and extracavity frequency doubler of Nd:YAG laser and other Nd-doped laser, to get green outputs for versatile applications. The very large temperature bandwidth is KTP's particular advantage to maintain pulsed energy stability of the converted beam. KTP is also being used successfully for intracavity mixing of 0.81mm diode and 1.06mm Nd:YAG laser to generate blue light and intracavity SHG of Nd:YAG or Nd:YAP lasers at 1.3 mm to produce red light. Moreover, with the speedy development of diode-pumped Nd:lasers, KTP becomes more and more important in the construction of the compact visible solid-state lasers. Some typical results are listed as follows:
  
  

Table 1. NLO Properties of KTP for SHG of Nd:YAG Laser

• More than 80% conversion efficiency and 700mJ green laser were obtained with a 900mJ injection- seeded Q-switch Nd:YAG laser by using extracavity KTP SHG;

• 20W green output was generated from CW Nd: YAG laser with intracavity KTP doubler;

• 3W TEM₀₀ mode-locked green laser was generated by intracavity SHG in a 5.3W mode-locked diode-pumped Nd:YAG laser;

• More than 500mW TEM₀₀ green lasers are obtained from diode-pumped Nd:YAG and Nd:YVO₄ lasers;

• 18mW green light was obtained from 50mW LD pumped intracavity Nd: YVO₄ mini-lasers with a 9mm long cavity.
  
  

Fig.1 illustrates Type II SHG phase-matching angle of KTP in XY plane (0.9mm to 1.08mm). In XY plane the slope m(Dk)/ mq is small. It corresponds to quasi-angular noncritical phase matching, which ensures the double advantage of a large acceptance angle and a small walk off. Fig.2 shows Type II SHG phase-matching angle of KTP in XZ plane (1.1mm to 3.4mm).

Fig. 1  Type II KTP SHG in XY Plane

Fig. 2  Type II KTP SHG in XZ Plane

In XZ plane the slope m(Dk)/mq is almost zero for wavelengths in the range 1.5 - 2.5mm and this corresponds to quasi-wavelength non-critical phase matching, which ensures a large spectral acceptance. Wavelength non-critical phase matching is highly desirable for frequency conversion of short pulses. But KTP cut in YZ plane is seldom used to be phase-matched for SHG of 1mm to 3.45mm in practices due to its very low non-linear coefficients.

• OPG , OPA and OPO (SHG and SFG of Nd:Lasers | Optical Waveguides)
  
  

As an attractive material for OPG, OPA or OPO, KTP can most usefully be pumped by a Nd:lasers and their second harmonics, or any other tunable output from visible (0.6 mm) to mid-IR (4.5 mm), such as Dye laser (about 0.6 mm) and Ti:Sapphire Laser (about 0.7-1 mm). 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. Fig.3 shows KTP OPO pumped by 532 nm tuning curve in XZ Plane. KTP's OPO pumped by a 1.064 mm Nd:laser has generated above 66% conversion efficiency for degenerately converting to 2.12 mm.

Fig.3  KTP OPO Tuning Curves
in XZ Plane

The novel and effective application is the non-critical phase-matched (NCPM) KTP OPO/OPA pumped by the tunable lasers. As shown in Fig.4, the NCPM KTP OPO keeps the KTP crystal fixed in X-axis and tunes pumping wavelength (0.7 mm to 1 mm), the output can cover wavelength range from 1.04 mm to 1.45 mm (signal) and from 2.15 mm to 3.2 mm (idler). 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.

Fig.4  Type II KTP NCPM OPO

• Optical Waveguides (SHG and SFG of Nd:Lasers | OPG, OPA and OPO)

With its low optical absorption and high damage threshold, one new low optical loss waveguide fabricated by using relatively simple ion-exchange process on KTP substrate, has created new applications of integrated optics. Table 2 gives the comparison of KTP with other optical waveguide materials.

Table 2. Comparison Electro-Optic Waveguide Materials

Recently, type II SHG conversion efficiency of 20%/W/cm² was achieved by balanced phase matching, in which the phase mismatch from one section is balanced against a phase mismatch of opposite sign from a second section. Furthermore, segmented KTP waveguide has been applied to type I quasi-phase- matchable SHG of 0.76-0.96m m for tunable Ti:Sapphire laser and directly doubled diode laser for 0.43-0.40 mm output. Conversion efficiency in excess of 100%/W/ cm² have been obtained. The KTP crystal, which size is as large as 35x35x1 mm, with -Z or both faces polished for waveguide applications, are available in GOT. Other sizes are of course available according to customers' requests.