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GRIIRA & Laser Damage
Q:Does PPMgO: LN has gray-tracking issue?
What’s GRIRA for CW SHG at 532nm?
What is the typical damage threshold for CW and pulsed applications?

A:
1.

Gray-tracking:Typically MgO:cLN has neglectable gray-tracking issue.
However, in some material such as KTP, a grey line is observed when the crystal isirradiated by visible or UV light. The induced absorption is attributed to polarons, wherecarriers may in that case be captured e.g. by Ti4+ ions or by Fe3+ impurities in the KTP [1].
Gray tracking is usually reversible and disappears within a few hours after irradiation. Its
strength depends on the crystal quality and thus on the fabrication method, as well assome important parameters such as the pulse duration and pulse repetition rate.

2.

GRIIRA:In niobates, an important difference from KTP is that the induced absorption appears tooccur only for pulses with sufficiently high peak intensity. The GRIIRA (green inducedinfrared absorption) results from the impurity inside the material which has unbalanced
ratio of Li+ and NbO3-. The induced absorption appears to be related to the formation ofpolarons (electrons trapped by antisite niobium ions), also called color centers, which can
result e.g. from two-photon absorption.
The effect can be greatly reduced by doping the material with magnesium oxide (MgO)
and/or by using stoichiometric material, which contains fewer of the intrinsic niobium
antisite defects. At the same time, both measures reduce the tendency forphotorefractive beam distortions, as they increase the ionic conductivity.
In MgO doped material, GRIIA still exists when illuminated with very strong intensity ofvisible/UV beam. Some research result shows that the absorption coefficient of IR
wavelength will change rapidly with the visible beam on/off.

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Figure 1. The change of absorption coefficient and refractive index of the IR wavelength with thepresence of the visible light.

3.

Damage:The laser induced damage threshold (LIDT) is not easy to define for it is related on:
I. Spot size
II. pulse duration
III. repetition rate
IV. wavelength
V. surface quality
I.

Spot size:
For same intensity/energy density, the smaller spot size will have higher LIDT, since
the smaller spot size have better heat dissipation. For example, >300mW 532nm canbe generated in PPLN ridge waveguide with ~6um MFD, which corresponds to >50
times of that obtained in PPLN bulk crystal.

II.

Pulse duration:
The LIDT has different mechanism under the different pulse duration as shown intable below. In long pulse region, the absorption leads to increase of temperatureand the crystal usually get cracked due to the non-uniform thermal distribution in
the crystal. On the other hand, the short pulse can has less pulse energy and it tends
to have dielectric breakdown or ionization for its strong peak intensity.

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Typically the LIDT is inverse proportional to the square root of laser pulse width.
For example, the LIDT of a 100ns pulse is 10 times lower than that of the 1ns pulse.
III. Repetition rate:
For the same intensity and same pulse duration, the lower rep. rate will give ahigher LIDT since the material will have more time to dissipate the heat.
IV. Wavelength:
Typically the LIDT is scaled with laser wavelength since the photon energy is inverse
proportional to wavelength. However, the visible wavelength will cause other issues
such as GRIIA or PR effect in the lithium niobate, as a result the damage threshold
drops quickly to short wavelength.
V.

Surface quality:
The poor surface quality will lead to higher scattering and the LIDT will significantly
degrade. Although all of HCP’s chips are cleaned and inspected the surface quality
before shipment, always clean the end-faces before using will help to avoid possible
damage.

Following is a reference table that gives many test results in different pumping condition
from our customer and internal experiments:

HC Photonics reserves all rights for modification of the designs, specifications, and technologies described here, and all theinformation in this document is not guaranteed to be up to date.

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Reference:[1]

B. Boulanger et al., “Study of KTiOPO4 gray-tracking at 1064, 532, and 355 nm”, Appl. Phys. Lett. 65, 2401
(1994)[2]

H. Mabuchi et al., “Blue-light-induced infrared absorption in KNbO3”, J. Opt. Soc. Am. B 11, 2023 (1994)[3]

Y. Furukawa et al., “Green-induced infrared absorption in MgO doped LiNbO 3”, Appl. Phys. Lett. 78, 1970
(2001)[4]J. Hirohashi et al., “Characterization of GRIIRA properties in LiNbO3 and LiTaO3 with different compositions
and doping”, Proc. of SPIE 6875 687516-1 (2008).

[5]

Y. Furukawa et al., “Optical d a m a g e resistance and crystal quality of LiNbOs single crystals with various[Li]/[Nb] ratios”, J. Appl. Phys. 72 (1992).

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