GB2049267A

GB2049267A - Laser apparatus

Info

Publication number: GB2049267A
Application number: GB8011006A
Authority: GB
Original assignee: Ferranti PLC
Current assignee: Ferranti International PLC
Priority date: 1979-04-17
Filing date: 1980-04-02
Publication date: 1980-12-17
Also published as: SE8002854L; GB2049267B; AU5749180A; AU527734B2; JPS55156380A; JPS6333313B2; DE3013217A1

Abstract

A laser has an optical cavity defined by two end prisms P1 and P2, a corner cube prism CP and a folding prism FP. These components are arranged so that radiation passing from one end reflector to the other passes once through the corner cube prism and twice through the folding prism. Also included in the optical cavity is a laser active medium AM, whilst means are provided for stimulating this active medium into producing laser radiation. A beam splitting polariser BS is included in the optical cavity to produce a laser output beam LO. The active medium may be an optically pumped Nd YAG rod or a discharge-excited gas contained in a tube with Brewster-angle end windows. <IMAGE>

Description

SPECIFICATION Laser apparatus This invention relates to laser apparatus, and in particular to a robust vibration-resistant laser.

Lasers are used in many applications, and in some situations they are subjected to considerable vibration. It is often necessary to maintain the alignment of the optical components making up a laser cavity to a high degree of accuracy, and this presents problems when vibration is present. In many instances a construction which is sufficiently rigid to withstand vibration may well be too massive and heavy to be usable. This applies particularly in situations where size and weight are important, say in an aircraft.

The length of the optical path provided by a laser cavity is one factor determining the characteristics of the laser output, and it is frequently necessary to provide a cavity of greater length than the space available. This problem is overcome by folding the optical path by means of mirrors or prisms of various types. In most cases these too must be accurately positioned. One exception to this arises when a prism known as a corner cube is used, since the respective directions of the input and output optical axes are not affected by limited movement of the corner cube. However, this advantage is offset by the fairly large size and weight of a corner cube prism, compared for example with a right-angled prism. The problem may become more acute if more than one fold is necessary.

According to the present invention there is provided a laser which includes a pair of right-angled end prisms, a corner cube prism and a folding prism together arranged to provide a double-folded optical cavity in which a beam of radiation passing from one end prism to the other passes once through the corner cube prism and twice through the folding prism, the right-angled prisms being arranged with their apex lines normal to the optical axis of the laser, an active medium located within the optical cavity, means for stimulating the active medium to produce laser radiation, and a beam-splitting polar iser located within the optical cavity and arranged to divert part of the radiation out of the optical cavity to form an output beam.

Also according to the invention the corner cube prism and the folding prism may be arranged so that the paths of the radiation entering or leaving the folding prism are in two parallel planes.

The invention will now be described with refer ence to the accompanying drawings, in which Figure l is a schematic diagram of a laser according to a first embodiment; Figure 2 is a similar view of a second embodi ment; and Figure 3 illustrates a third embodiment.

Referring now to Figure 1, this shows a simple double-fold laser according to the invention. The optical cavity of the laser is defined by a pair of right-angled end prisms P1 and P2, each arranged with its apex line perpendicular to the optical axis of the laser. Positioned within the optical cavity defined by the two prisms is a folding prism FP, in the form of a right-angled prism, and a corner cube prism CP.

The positions and alignments of the two prisms FP and CP are such that radiation reflected from end prisms P1 passes through the folding prisms FP to the corner cube prism CP and back through the folding prism FP to the other end prism P2. The laser active medium is shown as being a rod R of a material such as Nd.YAG. excited by light from a flash tube FT, and may produce pulsed or continuous emission. The active medium may be placed in any one of the four "legs" of the optical path.

Since the end prisms P1 and P2 are totally reflecting, a beam-splitting polariser BS has to be included within the optical cavity in order to produce a laser output beam LO. The polariser BS may be, for example, a Nicol prism.

The arrangement operates as follows: Regardless of the polarisation of the radiation emerging from the laser rod R, the effect of total internal reflection by either the folding prism FP or the corner cube prism CP is to produce elliptical polarisation with different phase shifts of the two components of the polarised radiation. Each total internal reflection produces additional, though different, phase shifts of the two components. Hence, the radiation reaching the polariser BS from the folding prism FP will be elliptically polarised. The beam splitter is aligned so that its pass plane coincides with the plane of polarisation of one of the components of the elliptically-polarised radiation, and this component passes through the beam splitter BS to the end prism P2.The other component of the elliptically polarised radiation has its plane of polarisation perpendicular to the pass plane of the beam splitter BS and is reflected out of the optical cavity to form the output beam LO.

The laser described above is very robust, since the prims are relatively solid, and is fairly insensitive to limited movement of the prisms. The plane containing the four "legs" of the optical path is called the "retro-reflective plane" of the folding prism FP, and it is a characteristic of this type of prism that limited rotation about an axis RA which is perpendicularto this plane has no effect on the directions of the four legs of the optical path. Similarly, rotation of the folding prism about an axis parallel to one of these legs would have no effect, so long as this movement was small enough for reflection to take place at the rectangular faces of the prism as shown. A plane perpendicular to the retro-reflective plane and to the axes of the four legs of the optical path is known as the "mirror" plane ofthefolding prism.Rotation of the folding prism in this plane will cause a change in direction of the axes of the optical path. However, the corner cube prism CP is totally unaffected by such a rotation, and due to the double passage of the radiation through the folding prism, the direction of the laser output beam LO is unchanged. Hence the combination of the folding prism and the corner cube prism in the manner specified above renders the laser insensitive to movements of these two prisms. Similarly, the end prisms P1 and P2, which operate by total internal reflection, are relatively insensitive to limited misalignment.

Figure 2 shows a second embodiment which incorporates two modifications to the arrangement of Figure 1. These modifications may be used singly or together. The laser rod Rand flash tube FT of Figure 1 have been replaced by a discharge-excited gaseous active medium contained in a tube LT having Brewster-angle end windows. This type of tube is used for obtaining a continuous output. The optical cavity also includes a Q-switching device QS which may be used to switch the laser output LO on and off. Because of the polarisation effects used to obtain the laser output. the fast and slow axes of the O-switching device should preferably be correctly aligned with its fast axis parallel to the apex line of one of the end prisms.

Athird embodiment is shown in Figure 3, in which the laser active medium is indicated schematically at AM. In this embodiment, the corner cube prism CP is used to introduce a fold in a plane perpendicular to that of the fold introduced by the folding prisms FP, thus producing two retro-reflective planes having the same characteristics. One of the advantages of this arrangement is that the path length of the radiation in the material of the prism, and hence the attenuation, is reduced. In addition. although the folding prism may require to be taller, it may also be truncated, resulting in a reduction in the overall length of the laser.

As already stated, any one or more of the modifications described may be applied to the basic arrangement of Figure 1. Other modifications, wellknown in the laser art, may also be applied.

It is possible to change the output of the laser by rotation of the end prisms so that the relationship between the pass plane of the beam splitter BS and the planes of polarisation of the two components of the elliptically polarised radiation is changed.

Claims

1. A laser which includes a pair of right-angled prisms, a corner cube prism and a folding prism together arranged to provide a double-folded optical cavity in which a beam of radiation passing from one end prism to the other passes once through the corner cube prism and twice through the folding prism, the right-angled prisms being arranged with their apex lines normal to the optical axis of the laser, an active medium located within the optical cavity, means for stimulating the active medium to produce laser radiation, and a beam-splitting polariser located in the optical cavity and arranged to divert part of the radiation out of the optical cavity to form an output beam.

2. A laser as claimed in claim 1 in which the paths of all the radiation entering or leaving the folding prism are in a common plane.

3. A laser as claimed in claim 1 in which the paths of the radiation entering or leaving the folding prism are in two parallel planes.

4. A laser as claimed in either of claims 1 or 2 in which the optical cavity includes a O-switching device.

5. A laser as claimed in any one of claims 1 to 4 in which the laser active medium is a solid medium, the means for stimulating the medium being a discharge tube.

6. A laser as claimed in any one of claims 1 to 4 in which the laser active medium is a gaseous medium, the means for stimulating the medium being a gas discharge through the medium.

7. A laser substantially as herein described with reference to the accompanying drawings.