RTP single crystal (Rubidium Titanyle Phosphate – RbTiOPO₄) is an isomorph of KTP crystal which is used in nonlinear and Electro Optical applications. Its transmission range is 350nm to 4500nm. RTP single crystals are recently grown by Top Seeded Solution Growth (TSSG) from self-flux and now commercially available in our company.

RTP is a very promising material for Electro Optical Q-switch applications at high repetition rates. The Q-switch is built using two RTP elements in a temperature compensating design. The unique properties of RTP, including high electrical resistivity and a high damage threshold(about 1.8 times of KTP), result in a Q-switch with outstanding properties. These Q-switches have been reported at 100 kHz, with no sign of piezoelectric ringing and hotorefractive damage at high peak power. Moreover, its operating voltages are low compared to most E-O materials.

• An excellent crystal for Electro Optical applications at high repetition rate
• No Piezoelectric Ringing
• Low Insertion Loss
• Thermal Compensating Design
• High damage threshold
• High Extinction and Contrast Ratio
• Non-hygroscopic

RTP’s applications

• E-O application
• Optical Parametric Amplifiers (OPA) and Oscillators (OPO) application;
• SHG Nd: lasers at 1064nm

E-O RTP’s Specifications:

We recommend to use a thermally compensated pair of RTP crystals for this application. Using RTP crystals as an E-O Q-switching device, please follow the below:

      E-O RTP(Cr/Au Coating Electrodes )                As Grown RTP Crystal

       RTP pockels cell-I                            RTP pockels cell-V  

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Structural and Physical Properties

Crystal Structure	Orthorhombic
Lattice Parameters	a = 12.96 A°, b = 10.56 A°, c = 6.49A°
Density	3.6 g/cm3
Melting Point	1000 °C
Ferroeletric transition temp.	810°C
Mohs Hardness	5
Thermal Expansion Coe.,/°C	a1=1.01x10-5, a2=1.37x10-5, a3=-4.17x10-6
Hygroscopic Susceptibility	No
Dielectric Constant	Eeff = 13.0
Color	Colorless
Ionic conductivity	10-8 W/cm(room temperature, 10kHz)

Optical properties

Transmission range		350-4500 nm
Absorption coefficient		< 0.05 % / cm @ 1064 nm < 4 % /cm @ 532 nm
Nonlinear Coefficients		d15 = 2.0 pm/V; d24 = 3.6 pm/V; d31 = 2.0 pm/V
		d32 = 3.6 pm/V; d33 = 8.3 pm/V
		deff for type II SHG@1064nm= 2.39 pm/V
Electro-optic coefficients (pm/V)		r13=10.6, r23=12.5; r33=35(x-cut); r33=38.5(y-cut)
Damage Threshold		10 ns 600 MW/cm2 @ 1064 nm
Contrast Ratio		20 dB @ 633 nm
Static Half Wave Voltage @1064nm		1445 V for a pair of 4x4x10mm3
		1,600 V for a pair of 4x4x20mm3
		1,700 V for a pair of 6x6x7mm3
		2,400 V for a pair of 6x6x20 mm3
Sellmeier coefficients		n2(λi) = A + B/(λi2-C) - Dλi2
	A		B	C	D
nx	2.15559		0.93307	0.20994	0.01452
ny	2.38494		0.73603	0.23891	0.01583
nz	2.27723		1.11030	0.23454	0.01995

PMangle　

PM angle	Handbook of nonlinear optical crystal	http://www.coretech.com.cn/RTP.htm	Optical materials/Bright Crystal Technology Inc.	deff (pm/V)
I	(51.82o, 0o)	(53.14o, 0o)	(52.7o,0o)	0
	(39.67o, 90o)	(41o, 90o)	(41o, 90o )	0
II	(69.54o, 90o)	(77.09o, 90o)	(76o, 90o)	1.93
	(90o, 44.95o)	(90o, 59.75o)	(90o, 57o )	2.58

RTP Sellmeier Equations:

Thermal Expansion Coe.,/°C:

Damage Threshold (GW/cm²):

Wavelenth	1064nm (10ns)	532nm (10ns)	532nm (35ps)
RTP	0.86	0.59	15

Nonlinear Optical RTP vs KTP:

Crystal	PM Wavelenth(nm)	1064nm PM	deff (pm/V)	Walk-off angle (mrad)	Angular acceptance (mrad)	Spectral acceptance (cm-1)	Temperature acceptance (oC)
RTP	>760.5 (type I) >1034 (type II)	(90o, 57.0o)	2.58	6.5	4.5	3.7	3.1
KTP	>742.5 (type I) >999 (type II)	(90o, 23.6o)	2.45	4.1	7.1	3.9	12.7

Electro-optic (E-O) crystal 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 (G) will be induced by Dn to G = 2pDnL, where L is crystal length, for KD*P, again as an example, G = pLn3or63E/l. 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(G/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 RTP,BBO, KTP, LiNbO3 crystals are usually employed in this scheme.

The half-wave voltage (Vp) is defined as the voltage at G = p, for example, Vp=l/(2no3r63)for KD*P and Vp=ld/(2no3r22L) for LiNbO₃, where l is light wavelength and d is the distance between the electrodes.

Electro-Optic Modulator Materials

Material			Phase			Amplitude
	e	n	r(pm/V)	k(10-6/¡ã C)	N7r2/e (pm/V)2	r(pm/V)	k(10-6/¡ã C)	n7r2/e (pm/V)2
KTP	15.42	1.80	35.0	31	6130	27.0	11.7	3650
LiNbO3	27.9	2.2	8.8	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	355	1.2	15	124

RTP Crystal

The new crystal RTP is an isomorph of KTP. However, it has higher damage threshold (about 1.8 times KTP), higher resistivity, and no sign of electro- chromism.These are Biaxial crystals and natural Birefringence needs to be compensated by use of two crystal rods specially oriented so that beam passes along the X-direction. The Q-switch is built using two RTP elements in a temperature compensating design. Input beam is polarized along the diagonal of the input face and Z and Y axis are perpendicular to the two side faces. Y and Z faces are rotated by 90° in the second crystal for thermal compensation. The ‘o' ray in the first crystal becomes the ‘e' ray in the second crystal and vice versa, so that the thermal birefringence is compensated. Matched pairs (equal lengths polished together) are required for effective compensation.

The effective E-O constant r c1 (light propagating along the Y axis) is 23.6 pm/V and E-O constant r c2 (light propagating along the X axis) is 20.3 pm/V. The contrast ratio is better for r c2 constant. At repetition rates of 50KHz, the noise due to piezo-electric ringing is less than 3% while that in BBO it is 10% when operated at 30KHz. However in RTP Pockels cells, the half-wave voltage is about 40% and the hold-off is about 25% of that of BBO pockels cell.

crystal	Size	quarter -wave V @ 1064nm	Contrast Ratio	Capacitance
RTP	5 x 5 x 20mm 3	2265V	20dB	6pF
BBO	5 x 5 x 20mm 3	5850V	40dB	< 2pF

These are transverse Pockels devices and the voltage increases linearly with wavelength for a given aspect ratio. BBO is slightly hygroscopic while RTP is not, so hermetical sealed housing is not required. Water cooled BBO Q switches are tested for average powers up to 150W, and RTP may be usable up to these levels at 1064nm. BBO optical bandwidth is 200nm to 2000nm while that of RTP is 400nm to 2500nm.

BBO Crystals

BBO is the electro-optic material of choice for high average power Pockels cell applications at the wavelength range from 200nm to 2500nm. BBO has significant advantages over other materials in terms of laser power handling abilities, temperature stability, and substantial freedom from piezoelectric ringing. Because it relies on the electro optic effect, switching time--aided by the low capacitance of the Pockels cell is very fast. The wide transparency range of BBO allows it to be used in diverse applications

BBO is the electro-optic material of choice for high average power Pockels cell applications at the wavelength range from 200nm to 2500nm. BBO has a high damage threshold and a low dielectric constant and is useful in high repetetion rate, high average power (up to 150W) diode pumped solid state lasers (DPSS lasers). BBO has significant advantages over other materials in terms of laser power handling abilities, temperature stability, and substantial freedom from piezoelectric ringing. Because it relies on the electro optic effect, switching time — aided by the low capacitance of the Pockels cell — is very fast. The wide transparency range of BBO allows it to be used in diverse applications

Electro-optic Pockels cells are used in applications that require fast switching of the polarization direction of a beam of light. These uses include Q-switching of laser cavities, coupling light into and out from regenerative amplifiers, and, when used in conjunction with a pair of polarizers, light intensity modulation. Pockels cells are characterized by fast response, since the Pockels Effect is largely an electronic effect that produces a linear change in refractive index when an electric field is applied, and are much faster in response than devices based on acoustic changes in a material, for example.

Because of crystal symmetry and the desire for the light beam to experience no birefringence in the absence of an electric field, BBO Pockels cells are transverse-field devices.

It has electro-optic coefficients g11=2.7pm/V and g22, g31<0.1 g11. It can be used for Q-Switching a cw diode pumped Nd:YAG laser with average power>50W.

Dimension	3x3x15mm	3x3x18mm	3x3x20mm	3x3x25mm
Vp	8.7kV	7.25kV	6.53kV	5.22kV

KD*P Crystals

KD*P crystal is widely applied for electro-optic application as Q-switch and Pockels cells. KD*P is routinely used for Q-switching applications from the uv out to about 1.1 µm where absorption limits its use in active cavities, although it can be useful at longer wavelengths when a few percent of absorption can be tolerated. KD*P has high optical uniformity and is useful for large aperature applications.

The device of longitudinal Pockels' cell using crystal of >98% deuterated DKDP is normally used for Q-switch of laser radiation. These Pockels' cells are manufactured from the crystals that have been specially selected for their low optical loss and strain free property. The crystal is in the form of a cylinder and two silver ring electrodes are used to produce the longitudinal electric field.

Crystal	KDP	KD*P	ADP
Electro-Optic Coefficient(pm/V)	g63=10.5	g63=26.4	g41(T)=24.5
Longitudinal Half-Wave Voltage Vp(546nm)	7.65kV	2.98kV	9kV

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. Because the crystal is nonhygroscopic and has a low half-wave voltage, it is often the material of choice for Q-switches in military applications. 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 of 300-500 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 G = pLnr22V/ld. 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.

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.