# A Miniature Pockels Cell with Novel Electrode Geometry

^{*}

## Abstract

**:**

_{12}GeO

_{20}). This neither transversal nor truly longitudinal geometry, results in electrical field distribution along the sensing beam path that provides high modulation depth. Demonstrated performance level is in agreement with theoretical studies. Delta-sigma polarization detection method allows high linearity of the detector transfer function and measurement independent on laser intensity variations. Channel gain equalization process necessary for accurate delta-sigma normalization is provided by a walk off prism.

## 1. Introduction

_{12}GeO

_{20}(BGO). BGO is transparent over a wide wavelength range and is a low-cost material readily available without optical defects. BGO crystals have cubic symmetry, space group 23 and are without birefringence but they possess optical activity. BGO shows Faraday rotation with a Verdet constant of approximately 70 rad/Tm. Absence of birefringence is crucial for successful FOCS construction [6]. Since BGO exhibits diamagnetic properties Faraday rotation temperature compensation of FOCS can be realized [7].

_{41}[10

^{−12}m/V ] = 3.67, λ = 633 nm [8] and that is sufficient for FOVS construction. Additionally, normalized temperature dependence of the BGO optical activity is negative [9] and can be used to compensate for the temperature dependence of the Pockels effect in the crystal [10].

## 2. Pockels Sensing Setup

_{3}) with fast and slow axes parallel to the major and minor axes of the refraction ellipse is placed behind the Pockels cell. It spatially divides the laser beam into two components with mutually orthogonal polarizations. Irradiations of both beams depend on the light source intensity in the same way. Using this fact we can calculate the retardance Φ that is not dependent on the intensity of the light source by difference-over-sum method. This concept is illustrated in Figure 1. Beam irradiations are measured using two segments of a quadrant photodiode (4QPD), followed by a dual transimpedance (DUAL TIA) stages. Stages are identical, based on dual operational amplifier, having the same transimpedance gains.

_{3}crystal instead of a polarizing prism and making the separate optical paths parallel. This fact also enables the use of the quadrant photodiode thus matching optoelectronic conversion gains as much as possible.

## 3. Pockels' Cell Construction and Modeling

_{12}GeO

_{20}– BGO crystal as the electro-optic crystal. It has cubic symmetry, space group 23 and it is without birefringence, but possesses optical activity. The BGO crystal is cut parallel to the (1 1 0) plane and is 2 mm thick. We assume that the z axis is orthogonal to this plane and that the light wave travels along the z axis. In order to optimize geometry of the Pockels cell electrodes let us now consider a Pockels cell with neither transverse nor truly longitudinal configuration as displayed in Figure 2.

_{x}and E

_{z}components. Since the point group of BGO is 23 electrooptic tensor has the form:

_{x'}and n

_{y'}:

_{AB}and measured voltage U: U

_{AB}= MU, 0 < M < 1.

_{0}= 633 nm [8]:

- r
_{41}is 3.67·10^{−12}m/V - n
_{0}= 2.55

_{π}is:

_{0}= 0.63 mm and beam diameter w at the sensing point z = 0.5 m is:

## 4. Signal Processing

## 5. Experimental Results

_{1/2}of the transimpedance stages are:

_{0}is the beam irradiation, ρ is the optical activity and k is a constant that models optical losses and optoelectronic conversion efficiency [11–13].

_{Δ/Σ}obtained from Δ/Σ sensing method, as described above. The magnitude of the AC excitation on the crystal electrodes was varied by means of the autotransformer and measured with the voltmeter connected to a PC using a serial port. The form of the measurement screen can be seen in Figure 5 together with results obtained for an excitation voltage of 71.5 V (amplitude). The green and magenta signals represent output voltages of the transimpedance stages (left axis). The white signal represents the response R (right axis, denoted Theta on the screen). All signals are functions of time denoted by the Time axis in [s]. Harmonics diagram displays frequency content of the response signal (digital filter implements a cut off above 200 Hz). Numeric displays present mean response value MEAN R (0.113253 deg), response (RMS R) calculated from the frequency bin at 50 Hz, and response (R) value calculated from the waveform, also in degrees. Lower part of the screen contains functions for controlling the electronic signal processing circuit necessary to implement gain equalization.

^{−4}[V

^{−1}]. Since optical rotatory power at λ = 632.8 nm is 18°/mm, then optical activity ρ = 36°. Therefore efficiency factor M is 0.537. For the case of conventional longitudinal electrode configuration, c is 2.39258 × 10

^{−4}[V

^{−1}], therefore the M is 0.423.

## 6. Conclusion

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**MDPI and ACS Style**

Petricevic, S.J.; Mihailovic, P.; Radunovic, J.
A Miniature Pockels Cell with Novel Electrode Geometry. *Sensors* **2009**, *9*, 5298-5307.
https://doi.org/10.3390/s90705298

**AMA Style**

Petricevic SJ, Mihailovic P, Radunovic J.
A Miniature Pockels Cell with Novel Electrode Geometry. *Sensors*. 2009; 9(7):5298-5307.
https://doi.org/10.3390/s90705298

**Chicago/Turabian Style**

Petricevic, Slobodan J., Pedja Mihailovic, and Jovan Radunovic.
2009. "A Miniature Pockels Cell with Novel Electrode Geometry" *Sensors* 9, no. 7: 5298-5307.
https://doi.org/10.3390/s90705298