Choosing the right Pockels cell

Aleksej Januško, Product Manager – Electro-Optics

A practical guide to crystals, applications, and drivers.

Modern lasers no longer just produce light, they need to control it with nanosecond speed. Across manufacturing, medicine, scientific research, telecommunications, and defense, laser systems increasingly rely on ultrafast optical switches to shape pulses, isolate amplifiers, and extract energy at exactly the right moment. At the center of this capability is a deceptively simple component: the Pockels cell.

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Three EKSMA Optics Pockels cells side by side, showing their clear apertures and high-voltage connectors

First described by Friedrich Pockels in 1893, the linear electro-optic effect that bears his name allows an applied voltage to change the polarization dependent refractive index of a crystal almost instantaneously, fast enough to switch a laser beam's polarization state in nanoseconds. This has made the Pockels cell a fast, reliable switching element for Q-switching, cavity dumping, pulse picking, and regenerative amplification in laser systems worldwide.[1]

Yet not all Pockels cells are created equal. Crystal material and wavelength together set the half-wave voltage a cell will need, and for two of the three materials, BBO and RTP, the required aperture pushes that voltage higher still. Cell geometry, crystal length, and whether the cell uses one crystal or several, is the main lever for bringing that voltage back down, at the cost of a larger, more expensive cell. And once the voltage is set, the driver's electronics decide how fast the cell can be switched. None of these choices are made in isolation. Getting a Pockels cell right means choosing crystal material, its geometry, and the driver together, not picking each one on its own.

Inside the Pockels cell

At its core, a Pockels cell is an electro-optic component with no moving parts. It doesn't block or open a path itself. Instead, an electric field changes how a crystal affects the polarization of light passing through it, and a pair of polarizers convert that change into transmission or reflection.

A working cell places an electro-optic crystal between two polarizers set at 90° to each other, as shown in figure 1.

With the voltage off, the crystal induces no birefringence, so light passes through still aligned with the input polarizer's axis. Because the second polarizer is crossed relative to that axis, the laser beam is reflected and does not reach the detector.

When the driver applies the cell's half-wave voltage, a value set by the crystal material, its length, and the beam's wavelength, the crystal induces exactly a half-wave retardation between the two polarization components. That rotates the light's polarization by 90°, aligning it with the second polarizer axis, so the beam passes through. If not aligned, extinction ratio, and laser power output, drop drastically.[2]

Figure 1: basic working principle of a Pockels cell.

Sometimes, though, the half-wave voltage needed for single-pass rotation is simply too high, as it can risk damaging the crystal or exceed what the driver's electronics can deliver. A double-pass configuration offers a way around it: a mirror placed after the crystal sends the beam back through it, so the light picks up retardation twice per round trip. Driving the cell at only the quarter-wave voltage is then enough, because the two quarter-wave contributions from the double pass add up to a full half-wave retardation, achieving the same 90° rotation for half the drive voltage (figure 2).[2]

A Pockels cell doesn't just switch between two states. Because the induced retardation scales continuously, and in principle without limit, with the applied voltage, the polarization rotation can be set to any intermediate value between 0° and 90°. Most laser applications, though, use the cell purely as a fast, two-state switch: closed or open.

Figure 2: quarter-wave operation scheme of a Pockels cell.

Choosing the crystal material

Not every Pockels cell crystal exhibits the same properties, and the choice of material has a direct impact on switching speed, voltage requirements and long-term reliability. Three materials dominate real-world use: DKDP (also called KD*P), BBO, and RTP (closely related to KTP).

Property DKDP BBO RTP / KTP
Field configurationLongitudinalTransverseTransverse
Half-wave voltage< 6.8 kV a< 4.6 kV b< 1.8 kV b
Duty cycle< 5%< 50%< 5%
Repetition ratekHzMHzMHz
Max HV pulse durationmssµs
Piezoelectric resonancesHighLowModerate
HygroscopicYesYesNo
Wavelength range400–1100 nm200–2000 nm400–2700 nm
Clear aperture< 350 mm< 12 mm< 15 mm

Table 1: comparison of DKDP (KD*P), BBO and RTP/KTP crystal based Pockels cells.

a At 1064 nm, independent of aperture. b At 1064 nm and 3.5 mm clear aperture diameter.

Longitudinal field

DKDP (KD*P)

DKDP (KD*P) is the most widely used crystal for Q-switching. It uses a longitudinal electro-optic configuration, meaning the operating voltage is essentially independent of the crystal's aperture (figure 3). This lets DKDP Pockels cells be scaled to very large clear apertures. At 1064 nm wavelength the half-wave voltage of a single-crystal DKDP Pockels cell such as Eksma Optics' PC12SR-1/1-1064 is specified at < 6.8 kV, independent of its aperture.[3]

The trade-off is that DKDP is highly hygroscopic, requiring hermetically sealed housing. DKDP also shows more pronounced piezoelectric ringing after each high-voltage pulse than the other two materials: applying a voltage excites acoustic resonances, causing the crystal to repeatedly expand and contract as the vibration decays over time, which reduces the extinction ratio and consequently limits it to lower pulse repetition rates (typically well under a few kilohertz). Figure 4 compares this ringing behavior in DKDP against BBO. Still, DKDP cells remain a solid, low-cost choice for single-shot or low-repetition-rate Q-switching applications.[4]

Vλ/2 at 1064 nm
< 6.8 kV
Design
Single crystal

DKDP Pockels Cells

Transverse field

BBO

BBO is a transverse-field crystal: the electrodes are arranged along the sides of the crystal rather than at its optical faces, and the operating voltage scales with the ratio of electrode spacing to crystal length. It exhibits the lowest piezoelectric ringing of the three materials. That makes it well suited to repetition rates up into the megahertz regime. Also, it handles high duty cycles and long high-voltage pulses well.[5]

Its main trade-off relative to DKDP is a lower aperture, and relative to RTP, a higher drive voltage, which is why it is often used in quarter-wave operation modes. For example, the half-wave voltage of Eksma Optics' most popular BBO Pockels cell, the PCB4D-C-1064, at 1064 nm is < 4.6 kV (at a clear aperture diameter of 3.5 mm). Here, the voltage is brought down by a double crystal design, which is very popular with BBO Pockels cells, but unfortunately at the cost of an additional crystal.[8]

Vλ/2 at 1064 nm
< 4.6 kV (⌀ 3.5 mm)
Design
Double crystal

BBO Pockels Cells

Transverse field

RTP / KTP

RTP/KTP are close relatives, and both are set apart by one standout property: very low half-wave voltage. For example, Eksma Optics' PCR4-1064 RTP Pockels cell has a half-wave voltage of only < 1.8 kV at 1064 nm wavelength and a clear aperture diameter of 3.5 mm. A lower drive voltage means smaller, faster, and less expensive driver electronics.[6]

RTP/KTP also support high repetition rates up to the megahertz regime with short HV pulses, are non-hygroscopic so no sealed housing is required and reach furthest into the infrared. Their piezoelectric ringing sits between the other two, depending on the exact repetition rate and pulse duration used. The main trade-offs are a small aperture and low duty cycle.[4][5][6]

Example
PCR4-1064
Vλ/2 at 1064 nm
< 1.8 kV (⌀ 3.5 mm)
Design
Double crystal (standard)

RTP / KTP Pockels Cells

Figure 3: longitudinal (a) and transverse (b) electrode configurations.

Optical response to a single high-voltage pulse

BBO 3 × 3 × 25 mm

120 80 40 0 Uₔ / mV 120 80 40 0 U₄ / mV

DKDP ⌀ 12 × 24 mm

120 80 40 0 Uₔ / mV 120 80 40 0 U₄ / mV

DKDP ⌀ 9 × 20 mm

120 80 40 0 Uₔ / mV 120 80 40 0 U₄ / mV
Figure 4: Optical response of BBO and DKDP crystal based Pockels cells affected by a single high voltage pulse.[4][5]
BBO - low ringing DKDP - high ringing HV pulse applied

Main applications: Q-switching, pulse picking, and regenerative amplification

The same fast polarization switch described in the previous chapters does three distinctly different jobs inside real laser systems, depending on where it sits and how it's timed. The three shown below are the most common applications of a Pockels cell in laser systems.

Q-switching is the most common. The cell sits inside the laser cavity alongside a polarizer, controlling whether the beam can build up there. With the cell off, the beam generated in the cavity picks up enough polarization rotation each round trip to be deflected out at the polarizer, keeping the cavity too lossy to lase while the gain medium builds up a large population inversion. Switching the cell cancels that rotation, so the beam now survives the round trip aligned with the polarizer and stays in the cavity, building up rapidly as it bounces between the mirror and the output coupler, until the stored energy dumps out through the partially reflective output coupler as a short, high-peak-power pulse rather than a continuous, low-power beam (figure 5). This is the standard choice behind nanosecond-pulsed solid-state lasers used in everything from laser marking and range finding to LIDAR.

What it demands
Hold-off voltage and contrast.

Figure 5: Q-switching scheme.

Pulse picking operates on a laser that is already producing a continuous train of pulses at tens to hundreds of megahertz, which is far too fast for many amplifiers or applications to use directly. A Pockels cell placed between crossed polarizers acts as a gate, switching open for a few nanoseconds at a time to let exactly one pulse through while blocking the rest, reducing the repetition rate down to whatever an amplifier stage or experiment actually needs (figure 6). Because neighboring pulses can be only nanoseconds apart, this application puts the most pressure on switching speed and contrast ratio of any use case here.[8]

What it demands
Switching speed.

Figure 6: Pulse picking scheme.

Regenerative amplification uses the cell as an entry and exit gate for a laser cavity built around an amplifying medium rather than an oscillator. A single seed pulse is switched into the cavity, trapped there for many round trips while it picks up energy from the gain medium on every pass, and then switched back out once it has reached the desired energy, turning a weak seed pulse into a high-energy one without ever letting the rest of the pulse train interfere (figure 7). This is the central part of many high-energy ultrafast (femtosecond/picosecond) laser systems. Unlike Q-switching, which stores energy in the cavity itself and releases it all at once with a single switching event, regenerative amplification starts with an externally supplied seed pulse and needs two switching events per pulse, one to trap it, one to release it. The final pulse energy is set by how many round trips happen in between.[8]

What it demands
Contrast and switching speed together.

Figure 7: Regenerative amplifier scheme.

Each of these three jobs asks something different of the Pockels cell and its driver. Q-switching cares mostly about hold-off voltage and contrast, pulse picking about switching speed, and regenerative amplification about contrast and switching speed together. Matching a Pockels cell and its driver to those demands is exactly what comes next.

Putting it together: selecting the right cell and driver

Specifying a Pockels cell and its driver starts, in practice, with four pieces of information: the beam diameter, the operating wavelength, the required repetition rate, and the required high voltage pulse duration. Figure 8 walks through two real-world examples, from requirements to the final cell and driver choice.

Example 1 - Q-switching

Requirements

Beam diameter
6 mm
Wavelength
1064 nm
Repetition rate
100 Hz
HV pulse duration
1 µs

Pockels cell

D-compact/12-1064

Wavelength
1064 nm
Clear aperture
⌀ 11 mm
Transmission
> 98%
Half-wave voltage
< 6.8 kV

Driver

HVS-D2-3-8.6

Max voltage
8.6 kV
Max rep rate
3 kHz
Pulse duration
35–2000 ns
Rise / fall
< 10.5 / < 9.5 ns
Power supply
integrated

Example 2 - Pulse picking

Requirements

Beam diameter
2.5 mm
Wavelength
800 nm
Repetition rate
1 MHz from 80 MHz
HV pulse duration
few ns, 12.5 ns gaps

Pockels cell

PCR6-800

Wavelength
800 nm
Clear aperture
⌀ 5.5 mm
Transmission
> 98%
Half-wave voltage
< 2.1 kV

Driver

2DP-1000-2.4-Al

Max voltage
2.4 kV
Max rep rate
1 MHz
Pulse duration
0–900 ns
Rise / fall
< 6.5 ns
Power supply
HVS100-2x120-2.6
Figure 8: practical examples on how to choose the right Pockels cell and driver for two different requirements.
01

Laser beam diameter

The clear aperture of the Pockels cell has to be larger than the beam diameter to avoid clipping the beam's edges and to leave room for alignment. For a Gaussian beam, we recommend multiplying the diameter specified at 1/e² by 1.5 to get the effective total beam diameter and adding up to 2 mm to account for alignment. Since the crystal is the most expensive part of a Pockels cell, the aperture is also the biggest driver of cost. For BBO/RTP crystals, a larger aperture also pushes up operating voltage, limiting achievable switching rate. So it's worth reducing beam size before the Pockels cell where possible.[9]

02

Wavelength

Longer wavelengths need more voltage to reach a full 90° rotation, and that holds for all three crystal families. Once the voltage is known, two crystal-side options exist to bring it down. A double-crystal design splits the required rotation across two crystals driven by the same voltage, so each one contributes only part of it. This roughly halves the voltage needed and has the side benefit of canceling residual birefringence between the pair. RTP-based cells use this double-crystal design as standard; DKDP is usually single-crystal, though double-crystal versions exist; BBO comes in single, double, or even quadruple crystal like Eksma's PCB4Q-C-1030.[8]

03

Repetition rate

This is where the required switching speed and the voltage just established either fit together or don't. A driver can't push both to their limits at once, since the heat it has to dissipate climbs with each. The real limiting factor is how fast that heat can be extracted from the switching electronics to keep them from overheating. A driver engineered to reach several kilovolts is incapable of cycling as fast as one built for a few hundred volts. Eksma's HVS-D2-2.5-9.8 reaches 9.8 kV but tops out at 2.5 kHz. That's the profile a DKDP Q-switch needs, where the voltage is high but the application was never going to run fast, since Q-switched repetition rates are governed by pump and gain recovery time rather than by the driver. At the opposite end, the 2DPB3-6000-1.3-Al reaches 6 MHz at only 1.3 kV, the natural partner for a low-voltage BBO or RTP/KTP cell picking pulses down from an 80 MHz mode-locked train. If the required repetition rate and the voltage set by wavelength and crystal choice don't both fall within what a driver can deliver, you need to revisit the aperture, crystal choice, and configuration to bring the voltage down.[10]

04

HV pulse duration

Selecting single pulses out of an 80 MHz mode-locked train leaves only about 12.5 ns between neighboring pulses, so the driver's minimum HV pulse duration has to be short enough to isolate one pulse cleanly. A Q-switched cavity demands the reverse: held in its high-loss state for the full pump duration, calling for an HV pulse that stretches far longer. Eksma's driver range spans both extremes, minimum HV pulse durations down to 0 ns in some units, and effectively unlimited maximum durations in others, enabled by the pulse-regeneration technique. However, both extremes at once are not possible in the same design.

Rise/fall time often comes up alongside pulse duration, especially for pulse picking and regenerative amplification, where the edge itself has to fit inside a few nanoseconds of neighboring pulses. But a fast edge comes at a cost: a bigger crystal or a higher voltage makes it harder for a given driver to deliver, so a demanding rise/fall time spec can send you back to the aperture or crystal-configuration decisions covered earlier.

Closing thoughts

Choosing the right Pockels cell comes down to a short list of parameters, but each one matters. Beam diameter sets the clear aperture, and with it, most of the cost. Wavelength and crystal material together set the drive voltage. Required repetition rate decides whether a driver can actually deliver that voltage fast enough and required HV pulse duration decides how long it must hold it. Rise/fall time sits on top of all of this, often deciding whether an otherwise well-matched cell and driver can actually keep pace with the pulses they're meant to control.

None of these decisions happen in isolation. A larger aperture, a different crystal choice, higher voltage each ripple into what's achievable for the others. That's really the core idea running through all of this: a Pockels cell is never chosen in the abstract. It's chosen against a specific beam, a specific timing requirement, and a driver capable of delivering both.

From Friedrich Pockels' 1893 discovery of the effect that carries his name to today's Q-switched lasers, LIDAR systems, and high-energy amplifier chains, the physics haven't changed, what has changed is how precisely it can now be engineered around.

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Our catalog covers Pockels cell models, driver electronics and complete pulse picker units. If you cannot find the right spec in it - send us the beam, rate, wavelength and HV pulse duration, and we will come back with a custom spec for you.

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Pockels cell FAQ

  • What is the Pockels effect?

    First described by Friedrich Pockels in 1893, the linear electro-optic effect that bears his name allows an applied voltage to change the polarization dependent refractive index of a crystal almost instantaneously, fast enough to switch a laser beam's polarization state in nanoseconds.

  • What is a Pockels cell used for?

    The Pockels cell is a fast, reliable switching element for Q-switching, cavity dumping, pulse picking, and regenerative amplification in laser systems worldwide. Q-switching is the most common: it is the standard choice behind nanosecond-pulsed solid-state lasers used in everything from laser marking and range finding to LIDAR.

  • Is a Pockels cell the same as an optical shutter?

    A Pockels cell is an electro-optic component with no moving parts. It doesn't block or open a path itself. Instead, an electric field changes how a crystal affects the polarization of light passing through it, and a pair of polarizers convert that change into transmission or reflection. Most laser applications use the cell purely as a fast, two-state switch: closed or open.

  • Is a Pockels cell a modulator?

    A Pockels cell doesn't just switch between two states. Because the induced retardation scales continuously, and in principle without limit, with the applied voltage, the polarization rotation can be set to any intermediate value between 0° and 90°. Most laser applications, though, use the cell purely as a fast, two-state switch: closed or open.

  • Which crystal should I choose - DKDP, BBO or RTP/KTP?

    DKDP (KD*P) is the most widely used crystal for Q-switching and can be scaled to very large clear apertures, but it is highly hygroscopic and its piezoelectric ringing limits it to lower pulse repetition rates. BBO exhibits the lowest piezoelectric ringing of the three materials, is well suited to repetition rates up into the megahertz regime and handles high duty cycles and long high-voltage pulses well; its main trade-off relative to DKDP is a lower aperture, and relative to RTP, a higher drive voltage. RTP/KTP are set apart by very low half-wave voltage, are non-hygroscopic and reach furthest into the infrared; the main trade-offs are a small aperture and low duty cycle.

  • What voltage does a Pockels cell need?

    Crystal material and wavelength together set the half-wave voltage a cell will need. At 1064 nm, the single-crystal DKDP PC12SR-1/1-1064 is specified at < 6.8 kV, independent of its aperture; the BBO PCB4D-C-1064 needs < 4.6 kV and the RTP PCR4-1064 < 1.8 kV, both at a clear aperture diameter of 3.5 mm. A double-crystal design roughly halves the voltage needed, and a double-pass configuration achieves the same 90° rotation for half the drive voltage.

  • How fast can a Pockels cell switch?

    The linear electro-optic effect changes the refractive index of a crystal almost instantaneously, fast enough to switch a laser beam's polarization state in nanoseconds. Once the voltage is set, the driver's electronics decide how fast the cell can be switched.

  • What do you need to specify a Pockels cell?

    Specifying a Pockels cell and its driver starts, in practice, with four pieces of information: the beam diameter, the operating wavelength, the required repetition rate, and the required high voltage pulse duration.

References

  1. F. Pockels, “Über den Einfluss des elektrostatischen Feldes auf das optische Verhalten piezoelektrischer Krystalle,” Abhandlungen der Königlichen Gesellschaft der Wissenschaften zu Göttingen, Mathematisch-Physikalische Klasse 39, 1–204 (1894).
  2. A. Yariv and P. Yeh, Optical Waves in Crystals: Propagation and Control of Laser Radiation, Wiley, New York (1984).
  3. X. Yin, M. Jiang, Z. Sun, Y. Hui, H. Lei and Q. Li, “Effect of thermally induced birefringence on performance of KD*P electro-optics crystal with rectangular shape,” Applied Optics 56(10), 2893–2900 (2017). DOI: 10.1364/AO.56.002893.
  4. G. Sinkevičius and A. Baškys, “Investigation of piezoelectric ringing in the Pockels cells based on the DKDP and BBO crystal,” Proc. IEEE Open Conference of Electrical, Electronic and Information Sciences (eStream) (2017). DOI: 10.1109/eStream.2017.7950307.
  5. J. Vengelis, G. Sinkevičius, J. Banys, L. Masiulis, R. Grigonis, J. Domarkas and V. Sirutkaitis, “Investigation of piezoelectric ringing effects in Pockels cells based on beta barium borate crystals,” Applied Optics 58(33), 9240–9250 (2019). DOI: 10.1364/AO.58.009240.
  6. H. Albrecht, P. Villeval and C. Bonnin, “Study of RTP crystal used as Electro-Optic modulator,” Advanced Solid-State Photonics, OSA Technical Digest, paper MB3 (2006).
  7. F. J. McClung and R. W. Hellwarth, “Giant Optical Pulsations from Ruby,” Journal of Applied Physics 33(3), 828–829 (1962). DOI: 10.1063/1.1777174.
  8. W. Koechner, Solid-State Laser Engineering, 6th ed., Springer, New York (2006).
  9. N. G. Worku and H. Gross, “Propagation of truncated Gaussian beams and their application in modeling sharp-edge diffraction,” Journal of the Optical Society of America A 36(5), 859–868 (2019). DOI: 10.1364/JOSAA.36.000859.
  10. M. R. Woolston, “Fast Electronic Driver for Optical Switches,” M.S. Thesis, Colorado State University (2012).