GB2346689A - CCD sensor located behind a controlled Pockel cell for receiving laser light redifferent from ranges - Google Patents

Abstract

A sensor apparatus includes a sensor, such as a CCD 3 which is located behind a Pockels cell 4 which is switched to give rapid gating of an optical input to the sensor 3. Laser light may be used to illuminate a surface or object and the Pockel cell may be switched so that light received from different ranges may be detected.

Description

Sensor Apparatus This invention relates to sensor apparatus and more particularly but not exclusively to apparatus in which optical radiation incident on a sensor is shuttered to provide range gating.

One known type of optical modulator, used for example in switching the output of a laser, uses the linear electro-optic effect, known as Pockels effect. This effect occurs in certain materials, for example non-linear optical polymers or inorganic single crystals such as lithium niobate. A shutter using this effect is called a Pockels cell and operates as a voltage driven polarisation switch. The Pockels cell includes an electro-optic layer placed between two polarisers which may for example be crossed or arranged parallel. Incident light passes through the first polariser which is set at an angle of 45 with respect to the principle axes of the electro-optic layer. On entering the electro-optic layer, the incident beam sets up two separate optical waves with polarisations along the principal axes of the electro-optic layer.

When no voltage is applied, these waves pass through the layer and combine in phase at the output to give a beam which has the same polarisation as the input. Thus, if the output polariser is arranged parallel with respect to the input polariser, the light passes through the Pockels cell with substantially no attenuation. If alternatively, the output polariser is crossed with respect to the input polariser, then no light is transmitted through the Pockels cell where no volts are applied across it.

To change the transmission characteristics of the Pockels cell, voltage is applied across the faces of the Pockels cell to produce an electric field parallel to the direction of light incident on the input polariser. The electric field produces a field induced birefringence in the material which is proportional to the electric field. The field induced birefringence produces a phase delay between the two optical waves in the electro-optic layer, the phase delay being given by the expression rcn 3rV where Q is the phase delay, n is the static refractive index, r is the electric optic constant, V is the voltage across the Pockels cell and A is the free space optical wavelength.

When the phase delay is 71, the delay between the two optical waves as they pass through the electro-optic layer is equivalent to half a wavelength, the voltage at which this occurs being called the half wave-voltage V where Vs= = n3r At the half wave voltage, at the output face of the electro-optic layer, the phase of one of the output waves is reversed with respect to the other and the two waves combine to give a beam which has an output polarisation which is at right angles to the input polarisation. Hence, if parallel polarisers are used, the light transmission through the Pockels cell is zero and with crossed polarisers, the transmission is at a maximum.

According to the present invention, a sensor apparatus comprises a light sensitive sensor, a Pockels cell positioned in front of the sensor, and control means for controlling the optical transmission characteristics of the Pockels cell to control the light incident on the sensor.

The Pockels cell can be switched on and off in under one ns, thus the invention may provide gating of an optical input to a sensor. In this specification, the terms"light"and "optical"include radiation in the infra-red and ultra-violet parts of the spectrum, in addition to visible light. The invention may thus provide fast acting protection for the sensor in the presence of very bright light which might otherwise damage it or cause saturation.

In a preferred embodiment, the sensor is a solid state imager, which may be a CCD (charge coupled device) for example. Such an imager may be used to give an indication of the total amount of light incident on the sensitive area of the imager but may in addition also provide positional resolution or an image of a viewed scene.

In one preferred embodiment of the invention, the CCD sensor includes an image area, an output register which receives signal charge from the image area, a separate multiplication register into which signal charge from the output register is transferred and means for obtaining signal charge multiplication by transferring the charge through a sufficiently high field in elements of the multiplication register. A CCD sensor suitable for this purpose is described in our co-pending European application, publication serial number EP-0-866501-A. Such a CCD sensor is particularly suitable for low light level use but hence is also vulnerable to damage or saturation when exposed to high intensity incident light. Thus its use in sensor apparatus in accordance with the present invention is particularly advantageous.

In one advantageous embodiment of the invention, sensor apparatus includes two Pockels cells positioned in front of the sensor with the principle axes of one cell being rotated relative to those of the other. A thinner Pockels cell is able to operate at a reduced voltage and thus using two thinner Pockels cells instead of one thicker one to obtain the same effect enables lower voltages to be employed. Also, a thinner Pockels cell has a greater field of view than a thicker one. More than two Pockels cells might be included but it is likely that this would lead to increased complexity without any appreciable further improvement.

According to a feature of the invention, a laser arrangement comprises a laser source arranged to direct laser light towards a region and optical apparatus in accordance with the invention for receiving the laser light after it has been returned by surfaces in the region. The surfaces may be any object or substance which causes incident laser light to be reflected or scattered towards the optical apparatus, for example a manmade object, thick mist or cloud.

Advantageously, the Pockels cell is controlled to switch between transmissive and non-transmissive states to receive laser light from different ranges. By sending a pulse of laser light and then using the Pockels cell to allow light to be transmitted to the sensor at short intervals, the range of a reflective surface may be determined. Range gating in this way also enables a sensor to be suitable for both day and night operation as during daylight operation, the Pockels cell can be used to ensure that only small amounts of radiation are incident on the sensor, reducing the risk of saturation.

Some ways in which the invention may be performed are now described by way of example with reference to the accompanying drawings in which: Figure 1 schematically illustrates a laser arrangement in accordance with the invention; Figure 2 is an explanatory diagram relating to the operation of the arrangement shown in Figure 1; and Figure 3 shows a sensor apparatus in accordance with the invention which uses two Pockels cells.

With reference to Figure 1, a laser range finder includes a laser 1 having an infra-red output and controlled by laser control 2 to produce a pulsed output at 13kHz. The arrangement includes an optical sensor apparatus comprising a low light level CCD sensor 3 in front of which is located a Pockels cell 4 with crossed polarisers. The voltage applied across the Pockels cell is controlled at 5 to switch the Pockels cell between optical transmissive and optically non-transmissive states. A clock 6 is used to synchronise the operation of the laser control circuit 2 with the voltage applied to the Pockels cell 4. When a pulse of laser radiation is transmitted from the laser 1 in the direction shown by the arrow, the voltage across the Pockels cell 4 is set at zero for an initial time period T1 shown schematically in Figure 2. As the polarisers of the Pockels cell 4 are crossed, the transmission is thus set to zero and any laser light scattered from the atmosphere immediately surrounding the arrangement is blocked from being incident on the CCD 3. At a subsequent time T2 the half wave voltage is applied to the Pockels cell 4 to render it transmissive and light from the region under observation, together with scattered laser radiation from any reflector at a range corresponding to that time is then incident on the CCD 3. The Pockels cell 4 is then gated off and image data transferred from the CCD 3 to a processor 7 for analysis. This may take the form of thresholding the signal, for example, to observe any peaks corresponding to a reflection from an object at the range corresponding to the time period T2. The Pockels cell 4 is gated on at intervals T3, T4 and so on so as to permit laser light reflected or scattered from objects at ranges corresponding to these times to be accepted at the CCD 3. Each of these time periods T2, T3 and so on may be of the order of 100ns although the Pockels cell may be switched at speeds of lns or less. Range gating enables signal returns to be distinguished over background illumination.

Figure 3 shows an optical apparatus in which an optical sensor 8 has two Pockels cells 9 and 10 located in front of it. The Pockels cells 9 and 10 may be touching or spaced apart from one another. The polarisers may be arranged to give the same effect as a single Pockels cell or thicker dimension but because they are thinner, switching can take place at lower voltages which is advantageous. Also, the field of view of the dual Pockels cell arrangement, indicated by a broken line 11 is wider than would be obtained using a thicker equivalent single Pockels cell, this being indicated by the dotted line 12.

Claims (10)

1. Claims 1. A sensor apparatus comprising a light sensitive sensor, a Pockels cell positioned in front of the sensor, and control means for controlling the optical transmission characteristics of the Pockel cell to control the light incident on the sensor.
2. 2. Apparatus as claimed in claim 1 wherein the sensor is a solid state imager.
3. 3. Apparatus as claimed in claim 2 wherein the sensor is a CCD.
4. 4. Apparatus as claimed in claim 3 wherein the CCD includes an image area, an output register which receives signal charge from the image area, a separate multiplication register in to which signal charge from the output register is transferred, and means for obtaining signal charge multiplication by transferring the charge through a sufficiently high field in elements of the multiplication register.
5. 5. Apparatus as claimed in any preceding claim wherein the Pockels cell has crossed polarisers.
6. 6. Apparatus as claimed in any preceding claim and including two Pockels cells positioned in front of the sensor with the principle axes of one cell being rotated relative to those of the other.
7. 7. A laser arrangement comprising a laser arranged to direct laser light towards a region and optical apparatus as claimed in any preceding claim for receiving the laser light after it has been returned from surfaces in the region.
8. 8. An arrangement as claimed in claim 7 wherein the Pockels cell is controlled to switch between transmissive and non-transmissive states to receive laser light returned from surfaces at different ranges.
9. 9. Optical apparatus substantially as illustrated in and described with reference to the accompanyingdrawings.
10. 10. A laser arrangement substantially as illustrated in and described with reference to the accompanying drawings.