Introduction
When an electric field (E) is applied to an electro-optic (E-O) crystal, the refractive index of E-O crystal will change linearly to electric field. The phenomenon is called linear electro-optic effect. For KD*P crystal, for example, the change of the refractive index (Dn) is Dn = 0.5n3or63E if both the directions of light propagation and electric field are along the z-axis, where no is refractive index without electric field and r63 is electro-optic coefficient of KD*P

If a linearly polarized light passes through an E-O crystal, the phase retardation () will be induced by n to  = 2nL, where L is crystal length, for KD*P, again as an example,  = Ln3or63E/. It is clear that the phase of light will change together with electric field (E). This is called electro-optic phase modulation. If two crossed polarizers are placed at input and output ends of E-O crystal separately, the output intensity of light will be I = I0sin2(/2), where I0 is input intensity. That means the intensity or amplitude of light can also be modulated by electric field. This is called amplitude modulation.
 

longitudinal E-O modulation	transverse E-O modulation

There are two kinds of E-O modulations. One is longitudinal E-O modulation if the directions of electric field and light propagation are the same. The KDP isomorphic crystals are normally used in this scheme. If the directions of electric field and light propagation are perpendicular, it is called transverse E-O modulation. The LiNbO3, MgO:LiNbO3, ZnO:LiNbO3, BBO and KTP crystals are usually employed in this scheme.

The half-wave voltage (V) is defined as the voltage at  = , for example, V=/(2no3r63) for KD*P and V=d/(2no3r22L) for LiNbO3, where  is light wavelength and d is the distance between the electrodes.
 

Electro-Optic Modulator Materials

BBO

BBO crystal with Z-cut is an excellent electro-optic crystal combining good physical properties, it's suitable for high power online orderings. It launchs a super Q-switch for a cw diode pumped Nd:YAG laser with average power >50w.Please refer to NLO Crystal- BBO for more information about BBO crystal.

KTP  

In addition to unique NLO features, KTP also has promising E-O and dielectric properties that are comparable to LiNbO3. These advantaged properties make KTP extremely useful to various E-O devices. 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.

LiNbO3

Crystal Lithium Niobate (LN) has higher transmission, and high contrast ratio at average powers in the KW range. Applications that utilize the large electro-optic coefficients of lithium niobate are optical modulation and Q-switching of infrared wavelengths. the crystal is nonhygroscopic and has a low half-wave voltage. The crystal can be operated in a Q-switch configuration with zero residual birefringence and with an electric field that is transverse to the direction of light propagation. Because piezoelectric ringing can be severe, piezoelectrically damped designs can be very useful. The damage threshold of the intrinsic material at 1.06 microns with a 10 nsec pulse is approximately 3 J/cm2. With appropriate AR coatings, a surface damage threshold of300-500 MW/cm2 can be achieved for the same conditions.

The light propagates in z-axis and electric field applies to x-axis, the refractive retardation will be  = Lnr22V/d. The electro-optic coefficients of LiNbO3 are: r33 = 32 pm/V, r31 = 10 pm/V, r22 = 6.8 pm/V at low frequency and r33 = 31 pm/V, r31 = 8.6 pm/V, r22 = 3.4 pm/V at high electric frequency.
 

MT-Optics,Inc. can provide as large as 6x6x25mm BBO for high power online_ordering. The standard BBO for Q-Switch as: