[Instrument usage skills] Working principle and adjustment skills of Pockels cell

Abstract: It is known that when a directional electric field is applied to a nonlinear birefringent crystal (or other optical medium) with non-centrosymmetry, its birefringent optical properties will change with the modulation of the electric field. In memory of the famous German scientist Friedrich Pockel, this electro-optical effect in which the refractive index change is linearly related to the electric field is called the Pockles effect; the nonlinear crystal device to which a modulation voltage is applied is generally called a Pockels cell and is widely used in electro-optical Q-switched lasers and pulse selectors. Nonlinear crystals that are widely used in Pockels cells include KD*P, BBO, LiNbO3, KTP, etc. This article will give a more detailed introduction to the application attributes of this device from a technical perspective based on the two functions of Q-switching and pulse selection.

Figure 1 Electro-optical Q-switched laser

1. Introduction to working principle

(1) Electro-optical Q-switching

A typical electro-optical Q-switched pulse laser (refer to Figure 1), its core components include laser cavity, electro-optical modulation system, polarizer, etc. When the Pockels cell is at zero voltage, due to the existence of the quarter-wave plate in the cavity, the laser cavity is in a state of high loss and low Q value; on the contrary, when an external voltage is applied, the pulse passes through the Pockels cell back and forth, and the polarization is flipped 90 degrees. At this time, the laser cavity is in a state of low loss and high Q value, and laser oscillation can be achieved. In order to achieve the expected experimental results, during this process, the attitude of the Pockels cell needs to be accurately adjusted to ensure that the propagation direction of the laser is strictly parallel to the optical axis of the crystal, so as to achieve the effect of strictly inverting the polarization by 90 degrees. Otherwise, not only will the best effect of Q-switching not be achieved, but the output energy will be reduced and the temporal quality will be degraded, resulting in damage to the optical components.

Figure 2 Pulse selector

(2) Pulse selection

Using the combination of Pockels cell and Glan prism, pulse repetition frequency adjustment or specified pulse selection can also be realized. For the specific principle, please refer to Figure

2. The high repetition frequency pulse sequence passes through the Pockels cell, and the repetition frequency, time delay and pulse width of the Pockels cell voltage signal are controlled to ensure that the time window of the voltage falls on the required signal pulse. During this process, it is necessary to ensure that only the polarization direction of this pulse changes, while the polarization of other pulses does not change. Using polarization selection devices such as Glan prisms can accurately select the required pulses.

2. Safe operation

(1) Personal safety

It is strongly recommended to wear laser protective goggles during dimming. Especially while polarizer adjustment is in progress, the reflected beam may be reflected in the vertical direction. In terms of safety, the beam should be reflected downward as much as possible during this operation. At the same time, using safety shields can also prevent potential eye damage.

(2) High voltage safety

Try to avoid operating the Pockels cell while the power is on;

During normal operation, the top cover needs to be closed to prevent accidental contact with the high-voltage circuit.

3. Pockels box adjustment skills

(1) Propagation of divergent beams in birefringent crystals

Based on the anisotropic characteristics of birefringent crystals, the phase changes experienced by divergent beams when propagating in them are angularly symmetrical. If a polarizer is placed behind it, interference fringes will be formed at multiple angles. During this process, special attention needs to be paid to two special directions, the o-axis and e-axis of the crystal. The polarization of the beam propagating in these two directions does not change, and the final light spot distribution is a typical Isogyre shape. The core technique of Pockel adjustment is to generate a divergent beam, and then use the relative position or symmetry distribution of the transmitted light spot as a reference to measure whether the adjustment is appropriate.

Figure 3 Two typical Isogyre shapes

Distribution without crossed polarizers (left) Distribution behind crossed polarizers (right)

(2) Pockels cell adjustment in Q-switched laser

As shown in Figure 1, when installing or optimizing the Pockels cell in the Q-switched laser:

First, ensure that the light beam passes through the center of the Pockels cell as much as possible to avoid light cut.

Second, use a He-Ne laser or LD light source to get a divergent beam through a piece of lens paper.

A polarization selecting device orthogonal to the polarizer in the cavity is placed behind the Pockels cell. Place a card behind the Pockels cell. If it is not directly observed through crossed polarizers, the left picture in Figure 3 can also be used as a basis for judgment. Adjust the two knobs of the Pockels cell (Pitch and Yaw) so that the transmitted light is directly in the center of the crosshairs. If you observe from behind the crossed polarizers, use the right picture in Figure 3 as the basis for judgment. Similarly, adjust the Pitch and Yaw knobs to make the entire orthogonal pattern as symmetrical as possible, and the cross lines are symmetrically distributed.

After adjusting Pucker's posture, remember to remove or block the external laser light source, and then take out the external lens paper and polarizer.

After the laser is running, the posture of the Pockels cell can be further adjusted to ensure that no laser is generated in the cavity when the Q-switched voltage is zero applied.

The output laser energy and pulse shape can be optimized by adjusting the voltage and time delay of the Pockels cell.

(3) Pockels box adjustment in pulse selector

The Pockels cell attitude adjustment method in the pulse selector is exactly the same as the above method, and the actual working laser can be used without the need for external light sources. Since it already has two orthogonal polarizing elements, you only need to add an additional piece of lens paper.

Before installing the Pockels cell, adjust the two polarizers to achieve complete orthogonality.

Strictly follow the above steps to achieve a perfect appearance of the Pockels box.

Check the adjustment effect: When no voltage is applied to the Pockels cell, observe the laser energy parameters behind the second polarizer. It should be in the weakest state at this time.

4. References

Thibault Dartigalongue and François Hache, "Precise alignment of a longitudinal Pockels cell for time-resolved circular dichroism experiments," J. Opt. Soc. Am. B 20, 1780-1787 (2003)

“POCKELS CELL ALIGNMENT IN A Q-SWITCHED LASER”, GOOCH & HOUSEGO Company Product Manual

Nisperuza, Daniel & Botero, G & Bastidas, Alvaro. (2010). Precise alignment of a longitudinal Pockels cell for Q-switch operation Nd:YAG laser. Revista Cubana de Física. 27. 63-65.

_______________________________________________________________________________________________________________________________________________

Long press to identify the QR code in the picture and join the knowledge sharing!