WO1989005048A1

WO1989005048A1 - Laser

Info

Publication number: WO1989005048A1
Application number: PCT/GB1988/000999
Authority: WO
Inventors: Colin Whitehurst; Terence Alan King
Original assignee: The Victoria University Of Manchester
Priority date: 1987-11-14
Filing date: 1988-11-14
Publication date: 1989-06-01
Also published as: GB8726724D0; AU2728088A

Abstract

A laser comprising a laser medium (17) located in a cavity positioned between two reflectors one of which (20) is highly reflective and the other of which (2) is partially reflective. The laser medium is excited to generate an output beam of radiation which is transmitted through the partially reflective reflector (2) which comprises a body (4) which is transparent to the output beam and which tapers down from a first relatively large surface (2) that faces the other reflector to a second relatively small surface. The first surface is planar and supports a partially reflective coating, and the body is dimensioned such that a beam of radiation directed towards the first surface from the other reflector is totally internally reflected and as a result is directed through the second surface. The output beam emerges from the second surface.

Description

- 1 - LASER

The' present invention relates to a laser. Lasers comprise a laser medium located in a cavity between two reflectors one of which is highly reflective and the other of which is partially reflective. The laser medium is excited by for example an electrical discharge source or a flashlamp to generate an output beam of radiation which is transmitted through the partially* reflective reflector.

Relatively large diameter laser beams, for example of more than 2mm outside diameter, are conventionally launched into small diameter fibre optic cables by using focussing lenses and providing for precision relative movements between the lens system and the fibre end so that the laser beam is accurately focussed on the fibre end. This requires precision components which must be moved in a highly accurate and controlled manner if an acceptable launch efficiency is to be obtained. Thus two problems are presented, firstly the generation of an output beam of small diameter, and secondly the launching of the beam into a fibre of small diameter.

In conventional systems used for launching high energy pulsed lasers into small diameter cables a further problem arises in that it has been found that a small misalignment between the fibre end and the laser output beam can result in the destruction of the input face of the fibre, leading to complete loss of laser transmission. This means that a trial and error approach for focussing the output of a high energy pulsed laser onto a fibre end cannot be used and further adds to the difficulty of using such systems. Thus there is a general need for an optical device capable of condensing laser beams into optical fibres. The need arises with laser beams having a variety of characteristics: for example beams from pulsed or continuously operating lasers, beams from lasers operating at different wavelengths, and beams of high or low quality. Low quality beams are characterised by large divergence and large intensity profile variations. The beams may be generated by various types of laser, such as those called excimer, argon ion, pulsed dye and metal vapour lasers.

A variety of devices have been proposed for coupling the output beam of an optical beam source such as a laser into an optical fibre. Examples are described in the following patent specifications: GB-A-2180367, GB 1541787, GB 1499359, GB 1486637, EP-A2-0194842, EP-A2-0192164, EP-A2-0170561, EP-A1-0046546 and US 3779628. The devices described in these documents include tapered optical coupling devices which receive the output beam of the source in a relatively large input face and guide the radiation by reflection to a relatively small output face which is integral with or is in abutment with an optical fibre. All the described devices are proposed for use as additional components to the light source, such as a laser, and are not themselves components of the light source itself.

It is an object of the present invention to provide an optical device which obviates or mitigates the problems outlined above.

According to the present invention there is provided a laser comprising a laser medium located in a cavity positioned between two reflectors one of which is highly reflective and the other of which is partially reflective, and means for exciting the laser medium to generate an output beam of radiation which is transmitted throug.h the partially reflective ■reflector, characterised in that the partially reflective reflector comprises a body which is transparent to the output beam and which tapers down from a first relatively large surface that faces the said one reflector to a second relatively small surface, the said first surface is planar and supports a partially reflective coating, and the body is dimensioned such that a beam of radiation directed towards the said first surface from the said one reflector is totally internally reflected and as a result is directed through the second surface, whereby the said output beam emerges from the second surface.

The body may comprise an integral optical fibre the free end of which defines the second surface. The body may be a right circular cone, the said first surface being perpendicular to the cone axis.

The surface ~of the body other than the first and second surfaces may be coated with a material of different refractive index to that of the body to produce a stepped index optical boundary.

Embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which:

Fig. 1 is a schematic side view of a device for use in an embodiment of the present invention;

Fig. 2 illustrates a device of the type shown in Fig. 1 provided with a coating to give the device a stepped index structure;

Fig. 3 is a schematic sectional illustration of a device of the type described in Figs. 1 and 2 mounted in a connector assembly; Fig. 4 is a schematic illustration of a first embodiment of the present invention incorporating a device of the type illustrated in Fig. 3, and

Fig. 5 is a schematic illustration of a second embodiment of the present invention incorporating a device of the type illustrated in Fig. 3.

Referring to Fig. 1, the illustrated device comprises a right circular cone 1 having a ground and polished optically flat first surface or entry face 2 which is perpendicular to the cone axis indicated by broken line 3. The tapering surfaces 4 of the cone, which are optically transparent subtend straight lines on any plane through the axis 3. An integral fibre 5 extends from the tip of the cone, the end of the fibre 5 also being cleaved and polished and defining a second surface.

If an optical beam 6 is directed into the entry face 2 in a direction substantially parallel to the cone axis 3 that beam is totally internally reflected and as a result is directed down towards the fibre 5. The cone angle a, that is the angle subtended at the cone tip by a diameter of the entry face 2, is selected such that for the particular frequency of the beam 6 substantially all of the light is reflected back towards the axis 3 each time the beam impinges on one of the walls 4. An angle a of for example less than or equal to 6° is generally sufficiently narrow to achieve total internal reflection. The entry face 2 may be for example 8mm in diameter, the cone length may be for example 100mm, and the fibre 5 may be one or two metres long and have a diameter of 1mm.

Light rays travelling down the cone from the entry face 1 increase their angle of reflection with the curved surface 4 by the cone angle with each - 5 -

reflection. Therefore, if the light is permitted to reflect too many times the angle of reflection rapidly increases until it reaches a point where the beam may be reflected back towards the entry face 2. To overcome this problem it is necessary to keep the initial reflection angle as low as possible and the number of reflections down to a minimum, e.g. below four, irrespective of the distance between the cone axis and the ray axis. This is done to avoid the light ray exceeding the critical angle of the optical fibre into which the beam is to be launched by selecting a small cone angle and a relatively long cone. This is relatively easy to manufacture as such a structure can be drawn from a preform.

Referring now to Fig. 2, the cone and fibre may be given a stepped-index structure by coating the cone having a refractive index nl with a layer having a refractive index n2. This is entirely analogous to the structure of conventional stepped-index fibre optic cables.

Whether an integral fibre 5 is provided, or the output surface of the device is the cone tip, the output surface may require coupling to a main transmission fibre optic cable. A simple conventional butt joint can be used however between the output surface and the main fibre optic cable. The losses inherent in butt-jointing two multi-mode fibres can be of the order of 30%, but when a high initial launch efficiency has been obtained using the above-described device there is an adequate margin to make losses even of this magnitude acceptable elsewhere in the transmission system.

The light emerging from the cone tip will yield a much higher energy/power density than that launched - 6 -

into the entry face. This property makes the device ideal for launching, high laser energies into optical fibres. Light input directly at the entry face of the cone is funnelled down to the cone tip. This eliminates the need for complicated focussing optics.

The input power density is kept low and well below any surface damage threshold values, so problems with surface imperfections and microinclusions are minimised. The cone tip can be integrated into the fibre, or directly terminated thus precluding the need for vacuum or liquid couplers. Thus the device overcomes the three problems of high energy fibre launching: core overspill, surface pitting hotspots and movement of focal spot with laser launch angle. The laser beam will always emerge from the same point even if the beam launch angle varies slightly. The output is always circular and because the cone does not actually focus the laser beam but simply funnels the light through multiple reflections then any localised hotspots are lost in the diffuse concentration of light. Physical protection of the device can be achieved by sealing heat shrinkable plastic (fluorocarbon-based for low refractive index) onto the. curved cone surface, thereby maintaining a stepped index profile, strengthening the device, and affording protection from scratches, etc.

The device may be fabricated from any convenient optically transmissive material for the particular wavelength or operation and glass or silica are obvious choices.

The cone can be optically polished both on the entry face and the output face. The fibre can be pulled down to any diameter which is convenient, for example anything from 2000um to lOum. As mentioned above the cone angle must be sufficiently small to ensure total internal reflection. The taper angle of the cone should be as regular as possible along the axial length of the cone. The device will still be operative however providing any irregularities in the tapering angle of the cone are not so great as to result in localised loss of total internal reflection.

A device as illustrated in Fig. 1 or 2 can be mounted in a connector assembly of the type illustrated in Fig. 3. Referring to Fig. 3, this shows a right circular cone 1 having a polished entry face 2 and tapering surfaces 4 mounted in a brass tube 7. The diameter of the entry face, which may be for example 5mm, is equal to the internal diameter of the tube 7, and the radial space between the tube 7 and the tapering surfaces 4 is filled with a body of epoxy resin 8.

A further brass tube 9 is secured to the tube 7 and has mounted within it a stainless steel tube 10 through which an integral fibre 5 having a diameter of for example 0.3mm extends from the tip of the cone 1. The body of epoxy 8 encapsulates the fibre 5 and secures all the components together to form a unitary rugged structure. The tube 10 supports a flange 11 behind which a nut 12 engages, a resilient ring 13 being positioned between the end of the tube 9 and the nut 12 to maintain the nut 12 in engagement with the flange 11. The nut 12 forms one component of a conventional optical fibre connector such as an SMA connector. This enables the end of the fibre 5 to be correctly aligned with another optical fibre. The entry face 2 and the end of the fibre 5 are polished.

In accordance with the present invention, a laser can be produced using a connector assembly as described in Fig.* 3 to define an output coupling mirror for the laser. Examples of such arrangements are described in Figs. 4 and 5.

Referring to Fig. 4, the illustrated laser comprises a casing 14 supporting a device of the type described with reference to Fig. 3 in a mounting 15 the orientation of which relative to the casing can be adjusted by micrometer-driven screws 16. The entry face 2 of the device is provided with an optical coating which renders it partially reflective, and the device is located such that the entry face 2 forms the output coupling mirror of a laser assembly including a laser medium 17 and a laser excitation source 18 such as a flashlamp mounted in a laser optical pumping chamber 19, and a mirror 20 of high reflectivity.

The nut 12 of the output coupling device engages a housing 21 the other end of which is engaged by a further nut 22 which retains the end of a main fibre 23.

The operation of the laser is entirely conventional, the entry face 2 serving as the output coupling mirror of the system. The conical body which defines the entry surface 2 ensures that radiation generated by the laser is efficiently delivered to the relatively thin fibre 5 which is integral with the cone and a simple connector can then be used to couple that radiation to the main fibre 23. Correct alignment of the entry face 2, which can be easily achieved using the screws 16, is the only fine adjustment required.

Fig. 5 shows an alternative arrangement to that of Fig. 4, the only difference between the two arrangements being that in the arrangement of Fig. 5 a gas laser medium 24 is provided which is excited by - 9 -

an electrical discharge source 25. Thie indicates that the present invention can be applied to a wide range of different laser sources.

Claims

1. A laser comprising a laser medium located in a cavity positioned between two reflectors one of which is highly reflective and the other of which is partially reflective, and means for exciting the laser medium to generate an output beam of radiation which is transmitted through the partially reflective reflector, characterised in that the partially reflective reflector comprises a body which is transparent to the output beam and which tapers down from a first relatively large surface that faces the said one reflector to a second relatively small surface, the said first surface is planar and supports a partially reflective coating, and the body is dimensioned such that a beam of radiation directed towards the said first surface from the said one reflector is totally internally reflected and as a result is directed through the second surface, whereby the said output beam emerges from the second surface.

2. A laser according to claim 1, wherein the said body is a right circular cone, the said first surface being perpendicular to the cone axis.

3. A laser according to claim 1 or 2, wherein the said body comprises an integral optical fibre, the second surface being defined by the free end of the optical fibre.

4. A laser according to any preceding claim, wherein the surface of the body other than the said first and second surfaces is coated with a material of different refractive index to the body to define a stepped index optical boundary.

5. A laser according to any preceding claim, wherein the said body is encapsulated within a tubular body, the said first surface being located at one end of the tubular body and the said second surface being located in a connector assembly secured to the other end of the tubular body.