BBO Electro-Optic Modulators
1. Electro-Optic Theory
1.1. Electro-Optic Effect and Pockels cell
Electro-optic effect is the index change of certain crystal while an electric field applied to the crystal parallel to its optic axis. The index change is a function of applied voltage linearly, and dependent on the direction and polarization of the incident beam. The electro-optic effect leads to the ability to control light beams in a variety of ways and is the basis of a number of applications, including light-beam modulators, Q-switches, and deflectors.
F. Pockels first described the electro-optic effect in 1890s. So the devices based on the principle of electro-optic effect are known as Pockels cells.
Common NLO crystals used for Pockels cells are BBO, KDP, DKDP, LiNbO3, LiTaO3, KTP, RTP, ADP, etc.
The simplest form of Pockels cell is a crystal with electrodes fixed to two polished faces perpendicular to the optic axis.
1.2. Electro-Optic Modulator (EOM) and LFM & TFM
Containing one or two Pockels cells, Electro-Optic Modulators (EOM) are widely used with lasers to control the phase, amplitude, or polarization state, or power of a laser beam electrically.
When the electric field is in a direction parallel to the light passing through the crystal, the device is called a longitudinal field modulator (LFM). The KDP isomorphic crystals (KDP, DKDP, etc.) are normally used in this scheme
When the electric field applied orthogonally to the direction of the light beam, the device is called a transverse field modulator (TFM). BBO, KTP, and LiNbO3 crystals are usually employed in this scheme.
1.3. Half-wave Voltage (Vπ) and Extinction Ratio
As a useful figure of merit for comparing E-O devices, half-wave voltage (Vπ) is defined as the voltage at which half-wave retardation (a phase change of π) occurs for a given wavelength, Vπ = l/2gijno3 (gij is the electro-optic coefficient of crystal). For a Pockels cell, it is usually hundreds or even thousands of volts. In many applications it is the limiting factor on the types of electronic drivers available.
Another important performance parameter for EOMs is the extinction ratio, defined as the ratio of the transmission when the device is fully open, to the transmission when the device is fully closed. A high value of the extinction ratio is desirable because it determines the maximum contrast that may be obtained in a system that uses the modulator. Commercial EOMs can have extinction ratios in excess of 1000.
2. CASTON’s BBO Electro-Optic Modulators
2.1 BBO EOM Features:
O UV Transmission
O Low Insertion Loss
O Small Piezoelectric Ringing
O Resonance Free Operation
O High Average Power Handling
O High Contrast Ratio (up to 2000:1)
O High Speed Switching (up to 30 KHz)
O High Damage Threshold (> 20 KW/cm2, CW)
As the common and commercial choice of electro-optic crystal used for high average power TFM applications, BBO has significant advantages over other crystals such as LiNbO3 and KTP. The wide transparency range (from 200nm to 3000nm) of BBO allows it to be used in diverse applications.
It has electro-optic coefficients γ11= 2.7pm/V and γ22, γ31<0.1γ11. It is ideal for Q-Switching of CW DPSS lasers with average power>50W at sub-nanosecond speeds.
Vπ of BBO EOM with single crystal of different apertures are 6.8KV (2.5x2.5 mm), 8.0KV (3x3 mm), and 10.8KV (4x4 mm) at 1064nm. BBO EOM with two crystals in series is suitable for TFM with larger aperture and/or lower Vπ.
CASTON provides Z-cut BBO crystals with high performance, AR-coatings and Cr/Au-electrodes for high power applications.
2.2 CASTON Standard Specifications
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Aperture
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2x2 mm ~ 15x15 mm
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Length
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Up to 30 mm
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Dimensional Tolerance
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(W±0.1mm)x(H±0.1mm)x(L+0.2/-0.0 mm)
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Wavefront Distortion
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< /8 @632.8 nm
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Crystal Internal Quality
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No visible bubbles, inclusions and scattering sites or paths when inspected by a 30mW green laser
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Cutting Type
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Z-Cut
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Clear Aperture
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> Central 90%
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Surface Quality
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20/10 Scratch/Dig per MIL-O-13830A
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Flatness
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/8 @632.8 nm
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Parallelism
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≤ 20 arc seconds
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Perpendicularity
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≤ 5 arc minutes
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Chamfer
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≤ 0.2 mm x 45°
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End Surfaces
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AR/AR coatings (R<0.1% per surface)
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Electrodes
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Chrome Gold (Cr/ Au) coatings on both Y surfaces
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Quality Warranty Period
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one year under proper use
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2.3 CASTON Standard Sizes
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Part Number
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Dimension
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Cut Type
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End Surfaces
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Y-Surfaces
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EO83320
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3x3x20 mm
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Z-Cut
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AR/AR@1064nm
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Au Electrodes
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EO84420
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4x4x20 mm
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Z-Cut
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AR/AR@1064nm
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Au Electrodes
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EO86620
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6x6x20 mm
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Z-Cut
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AR/AR@1064nm
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Au Electrodes
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EO88820
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8x8x20 mm
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Z-Cut
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AR/AR@1064nm
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Au Electrodes
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EO81020
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10x10x20 mm
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Z-Cut
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AR/AR@1064nm
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Au Electrodes
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