GB2167203A - Optical processing apparatus - Google Patents

Abstract

An optical processing apparatus incorporates a body of optically non- linear material. An electric field is applied across the body (1) and first and second coherent light beams (B1, B2) are directed onto the crystal (1) so as to encode a grating structure within the body by which a read out beam (B3) may be diffracted to produce an output beam (B4) whose form is modulated by the form of the first and second beams. The apparatus includes means (7) for controlling the electric field, so as to vary the strength of the grating structure. The body (1) is preferably a crystal of potassium tantalate niobate held above its Curie temperature. <IMAGE>

Description

SPECIFICATION Optical processing apparatus This invention relates to optical processing apparatus. In particular the invention relates to optical processing apparatus of the kind incorporating a quantity of an optically non-linear material, and means for directing first and second coherent light beams onto the quantity so as to form a grating structure within the quantity by which a third light beam may be diffracted to produce an output beam whose form is dependent on the form of the first, second and third beams.

Such an apparatus is hereafter referred to as as optical processing apparatus of the kind specified.

It is an object of the present invention to provide an improved optical processing apparatus of the kind specified.

According to the present invention in an optical processing apparatus of the kind specified said quantity consists of a material such that the strength of said grating structure is dependent on an electric field applied across said quantity, and the apparatus further includes means for controlling said electric field.

In one particular apparatus in accordance with the invention, said means for controlling is responsive to the output of a detector means responsive to said output beam, thereby to control said electric field in such a manner as to regulate the intensity of said output beam.

In another particular apparatus in accordance with the invention said apparatus includes a detector means responsive to light of a prepredetermined frequency and positioned so as to intercept said output beam, and said means for controlling comprises means for applying an electric field at said pre-determined frequency across said quantity.

Two optical processing apparatus, in accordance with the invention, will now be described by way of example only with reference to the accompanying drawings in which: Figure 1 is a schematic diagram of the first apparatus; and Figure 2 is a schematic diagram of the second apparatus.

Referring firstly to Fig. 1, the first apparatus to be described includes a single crystal of potassium tantalate-niobate (KTN) 1 the material having the general formula KTaxNG1-xo3 the crystal being held above its Curie temperature. Edge electrodes 3, 5 are provided on the crystal 1, the electrode 3 being connected to earth, and the electrode 5 being connected to a variable DC voltage supply 7. A light detector 9, whose function is described hereafter, is also provided, an output of the detector being connected to the voltage supply 7.

In use of the apparatus a laser (not shown), is arranged, via a system of beam splitters and other optical components (also not shown) to direct two coherent light beams, indicated as B1 and B2 in Fig. 1, into the crystal 1. The apparatus is arranged such that beam B1 focuses the Fourier transform of an image of an object being monitored (not shown) in the crystal 1 whilst the beam B2 focuses the Fourier transform of a reference pattern (also not shown) in the crystal 1. The supply 7 then provides a voltage across the electrodes 3, 5 such that an electric field is present across the crystal 1 the beams B1, B2 being subsequently removed.

As discussed in Optics Communications, Volume 35, number 1, pages 45-48, KTN when held above its Curie temperature is in a non-ferroelectric phase in which quadratic electro-optic effects are exhibited. Thus only when the electric field is present across the crystal, will the beams B1, B2 encode an effective grating representative of the product of the two Fourier transforms within the crystal 1.

The strength of the grating structure, i.e. the efficency with which the grating will diffract an incident light beam, is regulated by the strength of the electric field, the grating structure persisting in the crystal 1 after the removal of the beams B1, B2.

To 'read out' the grating, a third light beam, 83 is directed onto the crystal 1 such that it is diffracted by the encoded grating to produce an output beam 84 propagating in a direction determined by Bragg's law, the beam B4 containing a representation of the product of the two Fourier transforms with the intensity of the beam B4 being dependent on the strength of the grating. The detector 9 is arranged to produce an output signal representative of the intensity of the beam B4. The supply is arranged to utilise the output signal of the detector to vary the voltage applied across the electrodes 3, 5 in such a manner that the intensity of the beam B4 as detected by the detector 9 tends to assume a predetermined magnitude. Thus automatic regulation of the intensity of the beam B4 is achieved.

It will be appreciated that as the grating will only persist in the crystal when the electric field is present across the crystal, the supply 7 may also be used to switch the grating "on" or "off" thus providing a means of controlling the time periods at which the grating is encoded.

Referring now to Fig. 2, the second apparatus to be described includes a single crystal of KTN 1 held above its Curie temperature, with electrodes 3, 5 as in the first apparatus, like components thus being correspondingly labelled. The second apparatus differs from the first apparatus, however, in that the electrode 5 is connected to both a DC voltage supply 11, and an AC voltage supply 13. This second apparatus also includes a phase sensitive light detector 15 tuned to the frequency of the AC voltage supply 13.

In use of the apparatus beams B1, B2 are directed onto the crystal 1 as in the first apparatus, the beam B1 again focussing the Fourier transform of the image of an object being monitored in the crystal and the beam B2 again focussing the Fourier transform of a reference pattern in the crystal. The DC supply 11 is used to impose an electric field across the crystal 1 such that a grating structure is encoded in the crystal which is representative of the product of the two Fourier transforms.

The beams B1 and B2 are removed and the AC supply 13 is used to superimpose an AC electric field on the DC electric field across the crystal 1, such that the strength of the grating oscillates at the frequency of the AC field. A read out beam B3 is then directed onto the crystal to be diffracted by the encoded grating to produce an output beam B4' propagating in a direction determined by Bragg's law. Thus by detecting this beam with the phase sensitive detector 15 it is possible to discriminate between the light component within the light detected by the detector 15 which has been diffracted by the grating and thus contains an oscillating intensity component containing the representation of the product of the two Fourier transforms and other components within the light detected by the detector 15 associated with scatter and crystal inhomogeneities.

It will be appreciated that in each of the apparatus described herebefore by way of example, further optical processing steps may be incorporated. In particular, the diffracted beam B4 or B4' will generally be arranged to undergo an inverse Fourier transform such that it becomes spatially modulated with an image representative of the correlation of the image of the object being monitored and the reference pattern used to modulate the beams B1, B2.

It will be appreciated that whilst both of the apparatus described herebefore by way of example are joint transform correlators in which the two beams used to encode each grating are each spatially modulated, and the read-out beam is a plane wave beam, a frequency plane correlator in accordance with the invention in which only one of the beams used to encode the grating is spatially modulated and the read-out beam is also spatially modulated so as to achieve the required correlation function is also possible.

It will however also be appreciated that whilst each of the apparatus described herebefore by way of example performs a correlation function, the invention is also applicable to an apparatus of the kind specified which performs a different function, for example a phase conjugation function by appropriate choice of the form of the beams incident on the quantity of optically non-linear material.

It will be appreciated that as potassium niobate and potassium tantalate form a continuous series of solid solutions, the Curie temperature of the composition KTaxNG, x03 may be varied dependent on the value of x. It is found that the Curie temperature is close to room temperature for values of x around 0.63. It may be advantageous in some cases to add small amounts of transition metal oxides to increase the absorption of light in the visible region of the material.

It will however be appreciated that whilst KTN in a para-electric phase is a particularly appropriate material for use in an apparatus in accordance with the invention as it has large electro-optic coefficients, any optically non-linear material in which the induced grating may be controlled by an applied electric field may, in principle be substituted.

Claims (10)

1. An optical processing apparatus comprising: a quantity of an optically non-linear material; and means for directing first and second coherent light beams onto the quantity so as to form a grating structure within the quantity by which a third light beam may be diffracted to produce an output beam whose form is modulated by the form of the first and second beams, the apparatus being characterised in that said quantity consists of a metal such that the strength of said grating structure is dependent on an electric field applied across said quantity, and the apparatus further includes means for controlling said electric field.

2. An apparatus according to Claim 1 in which said means for controlling is responsive to the output of a detector means responsive to said output beam, thereby to control said electric field in such a manner as to regulate the intensity of said output beam.

3. An apparatus according to either one of the preceding claims in which said means for controlling is used to determine the time at which said grating structure is encoded.

4. An apparatus according to any one of the preceding claims including a detector means responsive to light of a pre-determined frequency and positioned so as to intercept said output beam, and said means for controlling comprises means for applying an electric field at said pre-determined frequency across said quantity.

5. An apparatus according to any one of the preceding claims in which the apparatus is an image correlation apparatus.

6. An apparatus according to Claim 5 in which the apparatus is a joint transform correlator apparatus.

7. An apparatus according to Claim 5 in which the apparatus is a frequency plane correlator apparatus.

8. An apparatus according to any one of the preceding claims in which said quantity is a single crystal of potassium tantalate-niobate held above its Curie temperature.

9. An apparatus according to Claim 8 in which said crystal has the formula KTa0,63Ng0.37O3.

10. An optical processing apparatus substantially as hereinbefore described with reference to the accompanying drawings.