GB2087584A - Beam splitter - Google Patents

Abstract

The beam splitter comprises a calcite prism 12 and located in the path of one or both of the emergent beams, a half-wave plate 16, 18. It is particularly intended for use with a laser in holography, the half-wave plates 16, 18 enabling the required plates of polarisation to be obtained. The ratio of the intensities of the two emergent beams may be controlled by providing a further half-wave plate 10 upstream of the prism 12. A prism (14) may be arranged in the path of one or both of the emergent beams to compensate for dispersion. <IMAGE>

Description

SPECIFICATION A beam splitter The present invention relates to a beam splitter and seeks in particular to provide a beam splitter for use with high power lasers and multiwavelength lasers.

According to the present invention, there is provided a beam splitter which comprises a calcite prism through which the beam to be split is arranged to pass and operatve to split the beam into two separate beams having mutually orthogonal polarization planes and a half wave plate arranged in the path of at least one of the two separated beams to enable the plane of polarization to be rotated.

Preferably, the beam passes through the calcite prism at the angle of minimum deviation so as to minimize dispersion.

Preferably, again the beam splitter is a variable beam splitter and a half wave plate is arranged before the calcite prism to enable relative intensity of the two separated beams to be regulated.

Conveniently, a second prism having the same dispersion factor as the calcite prism is arranged in the path of one or both of the two separated beams in order to compensate for dispersion introduced into the said beam by the calcite prism.

In the preferred embodiment, half wave plates are arranged in the path of both separated beams in order to allow rotation of the plane of polarization of both said beams.

The invention will now be described further, by way of example, with reference to the accompanying drawing, which is a schematic plan represenatation of the layout of a variable beam splitter in accordance with the invention.

In the accompanying drawing, linearly polarized light from a laser enters into a half wave plate 10 by means of which the plane of polarization can, if desired, be rotated. The light then falls upon a birefringent prism 1 2 made of calcite. Calcite exhibits different refractive indices in mutually orthogonal planes of polarization, the crystal having a slow axis and a fast axis. The components of the polarized light along the slow axis and the fast axis are refracted by different amounts and emerge as two separate beams.

The path of the light through the prism is arranged to correspond to the angle of minimum deviation of the beam. As the angel of deviation is at a minimum, small variations in the angle of incidence can be tolerated and furthermore, the effects of dispersion are minimised. To assist further in reducing dispersion, a prism 14 which may for economy be made of glass rather than calcite is arranged in the path of one (or both as shown in dotted lines) of the two separated beams in order to counteract any dispersion resulting from the calcite prism 1 2 making the beam splitter particularly usefur for use with multi-wavelength lasers.

Two half wave plates 1 6 and 1 8 are arranged in the part of the two separated beams allowing the polarization planes of both the outward beams to be rotated if desired. In holography, the interfering reference and object beam must be of the same polarization and though one half wave plate would suffice to enable this requirement to be met, it is often advantageous to provide two, so as to enable both polarization planes to be varied at will.

By rotating the half wave plate 10 the polarization plane of the incoming light with reference to the slow and fast axes of the calcite crystal can be varied thus enabling the ratio of the intensities of the two output beams to be varied at will.

The apparatus has been described with a beam splitter but of course it can be used merely as an attenuator of only one of the two outward beams is put to practical use. The energy in the other beam, if desired, be absorbed by, for example, a block of metal.

In a practical embodiment all the elements of the beam splitter are arranged in a common housing, not shown in the drawing, which may conveniently be formed mostly of aluminium, the interior surfaces being coated or anodised in order to minimise internal reflections.

Known dielectric coating techniques may be employed at the various optical surfaces also for the purpose of reducing internal reflections.

1. A beam splitter which comprises a calcite prism through which the beam to be split is arranged to pass and operative to split the beam into two separate beams having mutually orthogonal polarization planes and a half wave plate arranged in the path of at least one of the two separated beams to enable the plane of polarization to be rotated.

2. A beam splitter as claimed in claim 1, wherein the beam passes through the calcite prism at the angle of minimum deviation.

3. A beam splitter as claimed in claim 1 or claim 2, in which a half wave plate is arranged before the calcite prism to enable the relative intensity of the two separated beams to be regulated.

4. A beam splitter as claimed in any preceding claim, wherein a second prism is arranged in the path of at least one of the two separated beams in order to compensate for dispersion introduced into the beam by the calcite prism.

5. A beam splitter as claimed in any preceding claim, wherein half wave plates are arranged in the path of both separated beams in order to allow the rotation of the plane of polarization of both said beams.

6. A beam splitter constructed, arranged and adapted to operate substantially as hereinbefore described with reference to and as illustrated in the accompanying drawing.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (6)

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION A beam splitter The present invention relates to a beam splitter and seeks in particular to provide a beam splitter for use with high power lasers and multiwavelength lasers. According to the present invention, there is provided a beam splitter which comprises a calcite prism through which the beam to be split is arranged to pass and operatve to split the beam into two separate beams having mutually orthogonal polarization planes and a half wave plate arranged in the path of at least one of the two separated beams to enable the plane of polarization to be rotated. Preferably, the beam passes through the calcite prism at the angle of minimum deviation so as to minimize dispersion. Preferably, again the beam splitter is a variable beam splitter and a half wave plate is arranged before the calcite prism to enable relative intensity of the two separated beams to be regulated. Conveniently, a second prism having the same dispersion factor as the calcite prism is arranged in the path of one or both of the two separated beams in order to compensate for dispersion introduced into the said beam by the calcite prism. In the preferred embodiment, half wave plates are arranged in the path of both separated beams in order to allow rotation of the plane of polarization of both said beams. The invention will now be described further, by way of example, with reference to the accompanying drawing, which is a schematic plan represenatation of the layout of a variable beam splitter in accordance with the invention. In the accompanying drawing, linearly polarized light from a laser enters into a half wave plate 10 by means of which the plane of polarization can, if desired, be rotated. The light then falls upon a birefringent prism 1 2 made of calcite. Calcite exhibits different refractive indices in mutually orthogonal planes of polarization, the crystal having a slow axis and a fast axis. The components of the polarized light along the slow axis and the fast axis are refracted by different amounts and emerge as two separate beams. The path of the light through the prism is arranged to correspond to the angle of minimum deviation of the beam. As the angel of deviation is at a minimum, small variations in the angle of incidence can be tolerated and furthermore, the effects of dispersion are minimised. To assist further in reducing dispersion, a prism 14 which may for economy be made of glass rather than calcite is arranged in the path of one (or both as shown in dotted lines) of the two separated beams in order to counteract any dispersion resulting from the calcite prism 1 2 making the beam splitter particularly usefur for use with multi-wavelength lasers. Two half wave plates 1 6 and 1 8 are arranged in the part of the two separated beams allowing the polarization planes of both the outward beams to be rotated if desired. In holography, the interfering reference and object beam must be of the same polarization and though one half wave plate would suffice to enable this requirement to be met, it is often advantageous to provide two, so as to enable both polarization planes to be varied at will. By rotating the half wave plate 10 the polarization plane of the incoming light with reference to the slow and fast axes of the calcite crystal can be varied thus enabling the ratio of the intensities of the two output beams to be varied at will. The apparatus has been described with a beam splitter but of course it can be used merely as an attenuator of only one of the two outward beams is put to practical use. The energy in the other beam, if desired, be absorbed by, for example, a block of metal. In a practical embodiment all the elements of the beam splitter are arranged in a common housing, not shown in the drawing, which may conveniently be formed mostly of aluminium, the interior surfaces being coated or anodised in order to minimise internal reflections. Known dielectric coating techniques may be employed at the various optical surfaces also for the purpose of reducing internal reflections. CLAIMS

1. A beam splitter which comprises a calcite prism through which the beam to be split is arranged to pass and operative to split the beam into two separate beams having mutually orthogonal polarization planes and a half wave plate arranged in the path of at least one of the two separated beams to enable the plane of polarization to be rotated.

2. A beam splitter as claimed in claim 1, wherein the beam passes through the calcite prism at the angle of minimum deviation.

3. A beam splitter as claimed in claim 1 or claim 2, in which a half wave plate is arranged before the calcite prism to enable the relative intensity of the two separated beams to be regulated.

4. A beam splitter as claimed in any preceding claim, wherein a second prism is arranged in the path of at least one of the two separated beams in order to compensate for dispersion introduced into the beam by the calcite prism.

5. A beam splitter as claimed in any preceding claim, wherein half wave plates are arranged in the path of both separated beams in order to allow the rotation of the plane of polarization of both said beams.

6. A beam splitter constructed, arranged and adapted to operate substantially as hereinbefore described with reference to and as illustrated in the accompanying drawing.