Modulation crystal (electro-optic)

01

Choose the crystal and size for the half-wave voltage

CASTECH's RTP (rubidium titanyl phosphate) datasheet describes an isomorph of KTP used in nonlinear and electro-optic applications, with a damage threshold about 1.8 times that of KTP, high resistivity, high repetition rate, no hygroscopy and no induced piezoelectric effect with electrical signals up to 60 kHz. It lists transmission from 350 to 4,500 nm, electro-optic coefficients r33 = 38.5 pm/V (Y-cut) and r33 = 35, r23 = 12.5, r13 = 10.6 pm/V (X-cut), and static half-wave voltage at 1,064 nm of 1,600 V for 4 × 4 × 20 mm, 2,400 V for 6 × 6 × 20 mm and 3,600 V for 9 × 9 × 20 mm crystals. [1]

Its KDP/DKDP datasheet describes potassium dihydrogen phosphate and its deuterated form as electro-optic crystals with high coefficients, used in Q-switches and Pockels cells. It lists r63 = 10.3 pm/V (KDP) and 25 pm/V (DKDP), longitudinal half-wave voltage at 546 nm of 7.65 kV (KDP) and 2.98 kV (DKDP), and transmission of 200–1,650 nm (KDP) and 200–2,100 nm (DKDP at 98% deuteration). [1]

Editorial calculation: the RTP values work out to 400 V per millimetre of aperture at 20 mm length (1,600 ÷ 4, 2,400 ÷ 6, 3,600 ÷ 9). A larger clear aperture for a bigger beam therefore raises the drive voltage in proportion; check that the driver can switch that voltage at your repetition rate. [1]

EKSMA Optics' Pockels cell guide compares three crystal families at 1,064 nm: a single-crystal DKDP cell (longitudinal field) needs under 6.8 kV whatever its aperture and scales to clear apertures under 350 mm; a double-crystal BBO cell needs under 4.6 kV and an RTP/KTP cell under 1.8 kV, both at 3.5 mm clear aperture. It gives duty cycle below 5% for DKDP and RTP/KTP and below 50% for BBO, repetition rates in the kHz range for DKDP and MHz for BBO and RTP/KTP, and wavelength ranges of 400–1,100 nm (DKDP), 200–2,000 nm (BBO) and 400–2,700 nm (RTP/KTP). It lists DKDP and BBO as hygroscopic and RTP/KTP as not, and says DKDP's stronger piezoelectric ringing after each high-voltage pulse limits it to lower repetition rates. [3]

Editorial calculation: at 3.5 mm aperture and 1,064 nm, EKSMA's RTP cell needs under 1.8 kV against under 4.6 kV for BBO, about 39% of the drive voltage (1.8 ÷ 4.6), which is why the guide pairs low-voltage BBO or RTP cells with MHz pulse-picking drivers. [3]

Give the supplier the wavelength, beam diameter and required clear aperture, pulse energy, pulse width and repetition rate, crystal material and cut, length, half-wave voltage target, extinction ratio, surface quality, flatness, parallelism, AR coating, electrodes, housing (sealed or not), quantity and any matched-pair requirement.

02

Check damage threshold, extinction ratio, finish and handling

The KDP/DKDP datasheet lists damage thresholds above 5 GW/cm² (KDP) and above 3 GW/cm² (DKDP), absorption of 0.07 /cm (KDP) and 0.006 /cm (DKDP), and 30 dB extinction ratio. It advises keeping the crystals dry (below 50% humidity) in a sealed housing because their polished surfaces are easily moistened, and says CASTECH supplies polishing, coating and sealed housings. [1]

The RTP datasheet lists extinction ratio above 20 dB at 633 nm, maximum length of 25 mm for a 5 × 5 mm aperture, surface quality of 20/10 scratch-dig to MIL-PRF-13830B, flatness of λ/6 at 633 nm, parallelism of 20 arc seconds, perpendicularity within 15 arc minutes, angle tolerance within 0.5°, AR coatings and a one-year warranty under proper use. [1]

CASTECH lists AO and EO crystals and electro-optic devices among its crystal products. [2]

03

Compare contrast ratio, aperture and coating figures from a second crystal grower

Gooch & Housego (G&H) states that it uses proprietary crystal growth, fabrication and polishing to make Pockels cells in BBO, CdTe, KDP and 99% grade KD*P. For a high-end Pockels cell using "99% KD*P" it says buyers "should expect" transmission >99% @1064 nm, intrinsic contrast ratio (ICR) >4000 and voltage contrast ratio (VCR) >3500 @1064 nm, and it lists contrast ratios "up to more than 8000:1" and active apertures from 3mm to 99mm. Its 7100 Series lithium niobate Q-switches, for use at 1064 nm, come in dimensions up to 9x9x25 mm with a damage threshold of 300 MW/cm², an extinction ratio of 100:1 at 633 nm and wavefront distortion of λ/8 at 633 nm. [4] [5]

The contrast figures are quoted in different forms. G&H gives ratios (ICR, VCR, 100:1) while CASTECH gives decibels (30 dB, >20 dB) [4] [5] [1]. Convert to one form before comparing, and ask which wavelength each figure was measured at: G&H's lithium niobate figures are at 633 nm although the Q-switch is used at 1064 nm.

The damage thresholds also use different conditions. CASTECH quotes >3–5 GW/cm² for KDP and DKDP [1], G&H 300 MW/cm² for lithium niobate Q-switches [5], and EKSMA compares drive voltages across DKDP, BBO and RTP cells [3]. Ask each supplier for the pulse length, wavelength and repetition rate behind a damage figure.

Editorial calculation: a 30 dB extinction ratio equals 1,000:1 (10^(30/10)); G&H's ICR >4000 is about 36 dB and its 100:1 lithium niobate figure is 20 dB; 300 MW/cm² is 0.3 GW/cm². [1] [4] [5]

Values are from G&H's Pockels cells and lithium niobate Q-switch pages; they are the maker's stated expectations, not test reports.

04

Compare what suppliers offer

Suppliers linked to this page list electro-optic crystals; each card links to the page reviewed.

The supplier reviewed publishes datasheets for RTP, KDP, DKDP, lithium niobate and other crystals, and lists electro-optic devices and drivers. Ask for measured half-wave voltage, extinction ratio and damage test conditions for the exact size ordered. [1] [2]

05

Trace what each specification controls

This is a functional reference built from the sources cited; it is not one maker's specification.

06

Fill the specification fields with values for your laser

A modulation crystal quotation needs values for your laser. The table explains each field, gives published values where a reviewed source states them, and says what to obtain from the supplier.

07

Send the beam, voltage and handling data together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Laser: [λ; energy; pulse width; rep rate]
Beam: [diameter; clear aperture]
Crystal: [material; cut; length]
Voltage: [Vπ target; driver limit]
Optics: [S/D; flatness; parallelism; AR]
Housing: [sealed; electrodes]
Quantity: [pieces; matched pairs]

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