WO1989010018A1

WO1989010018A1 - Laser with variable-direction output beam

Info

Publication number: WO1989010018A1
Application number: PCT/AU1989/000147
Authority: WO
Inventors: Barry Luther-Davies
Original assignee: The Australian National University
Priority date: 1988-04-05
Filing date: 1989-04-05
Publication date: 1989-10-19

Abstract

A laser beam generator for use in cutting and machining operations includes an optical resonator. The resonator "cavity" is the core of a flexible optical fibre (10) having an input end (11) and an output end (12). The ends of the cavity are defined by a phase-conjugate mirror (14) at or adjacent to the input end of the fibre and by a partially reflecting mirror (13) at or adjacent to the output end of the fibre. Light is input to the resonator from a light source (20). All light reflected back into the optical fibre by the partially reflecting mirror (13) is reflected with phase conjugation by the phase-conjugate mirror (14), and this reflected light, amplified when reflected or amplified by a laser amplifier (16), when it arrives back at the partially reflecting mirror, has the same characteristics as the light reflected initially by that mirror. The light transmitted through the partially reflecting mirror as an output beam is focused by a lens (19) or similar device. Because the optical fibre (10) is flexible, the output beam can be moved to illuminate a required point or zone.

Description

TITLE: "LASER WITH VARIABLE-DIRECTION OUTPUT BEAM"

TECHNICAL FIELD

This invention concerns lasers. More particularly, it concerns a laser which includes an optical resonator which has as its "cavity" a flexible optical fibre, at one end of which is located a phase-conjugate mirror and at the other end of which is located a partially transmitting, conventional mirror. Because the optical fibre is flexible, the output beam of the laser can be directed to any desired location. Such a laser is useful in, for example, the cutting or machining of a workpiece.

BACKGROUND TO THE INVENTION

Phase conjugate mirrors are relatively new. A comprehensive review of the features, properties and known applications of phase conjugate mirrors is given in the article by David M Pepper, entitled "Applications of Optical Phase Conjugation", which appeared in the Scientific American, volume 254, No 1, January 1986, pages 56 to 65. That article discusses the mechanism of phase conjugation and indicates (page 59 of the reference) how a phase conjugate mirror may be used to cancel the distortion introduced by optical fibres. The author. Pepper, acknowledges that this phenomenon was first demonstrated by D J Gunning and D C Lind - see Gunning and Lind's paper entitled "Demonstration of Image Transmission Through Fibers by Optical Phase Conjugation", published in Optics Letters, volume 7, pages 558 to 560, 1982. Pepper also discusses the use of a phase conjugate mirror and an ordinary mirror to form an optical resonator.

It is known that high power lasers can be used to machine or cut an object. Examples of this use of laser technology are found in surgical procedures and in the machining and cutting of moving workpieces. In such applications, the output beam from a fixed or immobile laser beam generator has to be directed on to precise locations on the object that is to be cut or machined. The most common arrangements for performing this function consist of complex arrays of optical components such as prisms, mirrors and the like, in conjunction with mechanical swivel joints and/or mechanical slides. Such arrangements must necessarily be constructed with close mechanical tolerances to ensure that the laser beam striking the object does not become misaligned as the mechanical system is moved.

To avoid the disadvantages of such mechanical beam direction systems, it has been proposed to use flexible optical fibres to ^' direct a laser beam to a location from which it can be focused to produce a high intensity spot on the object.

In an optical fibre system, in order to obtain a high brightness of the output beam, the transmission through the fibre must be single mode transmission. This means that the fibres must have very small core diameters, of the order of a few times the wavelength of the light beam that is being transmitted. However, many cutting or machining applications require laser beams having a power of about 1 kW continuous wave (CW) operation, or 5 MW pulses of nanosecond duration. For the transmission of such high average or peak powers through an optical fibre without damage to the materials from which the optical waveguide is constructed, the core diameter of the fibre must be of the order of 1 mm. But if a laser beam is fed into a 1 mm diameter optical fibre to initially match or excite a single lowest order mode of the fibre, this mode pattern is not maintained as the beam passes through the fibre, and the power becomes distributed very quickly between a large number of high order modes of the fibre. Consequently, the output beam from the optical fibre is highly divergent and has a much lower brightness than the beam which is input into the fibre from the laser beam generator. In many instances, focusing such an output beam will not produce a spot of a sufficiently high intensity to perform the machining or cutting operation. Thus the use of flexible optical fibres to direct high intensity laser beams for machining or cutting operations has been impractical hitherto.

DISCLOSURE OF THE PRESENT INVENTION

It is an object of the present invention to provide a laser beam generating system which utilises a flexible optical fibre to direct the generated beam in a desired direction, and which can be used to provide relatively high power, high brightness beams for machining or cutting applications (though the invention is not limited to such applications) .

This objective is achieved by using an optical resonator with the cavity of the resonator formed by a flexible optical fibre. A phase-conjugate mirror is positioned at or near one end (the input end) of the fibre, and a partially transmitting mirror is mounted close to the other end (the output end of the fibre) . A beam of light generated within the resonator adjacent to the phase-conjugate mirror by a laser amplifier or by the phase-conjugate mirror itself is transmitted to the output end of the optical fibre, where it is output through the partially transmitting mirror. That proportion of the output light beam from the fibre that is reflected back into the optical fibre is transmitted to the input end where it is reflected and (usually) is amplified by the phase-conjugate mirror. When this reflected and amplified light^' reaches the output end of the optical fibre again, as a consequence of the property of reflection from a phase-conjugate mirror, it has the same mode structure as it had when it was first reflected back into the fibre from the partially transmitting mirror. Thus this form of optical resonator operates as a type of laser and generates a laser beam at the output end of the optical fibre that can be directed in any required direction. For cutting or machining purposes, the laser beam can be focused in the usual manner.

Thus, according to the present invention, there is provided a laser beam generator comprising

(a) an optical resonator having a cavity comprising the core of a flexible optical fibre, said optical fibre having an input end and an output end, the cavity being closed at the input end of said optical fibre by a phase-conjugate mirror and being closed at its output end by a partially reflecting mirror;

(b) an input light source connected to the cavity for input of a light beam into the cavity at or adjacent to said input end, said input light beam being directed towards said partially reflecting mirror? and

(c) focusing means mounted in fixed relationship to said output end, for focusing light emanating from said cavity through said partially reflecting mirror.

Normally the phase-conjugate mirror will act as an amplifier of the light reflected from it. If it is not of the type that amplifies the light reflected with phase conjugation, a laser amplifier will be included in the resonator between the input end of the optical fibre and the phase conjugate mirror. Two embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic side view of a laser beam generator constructed in accordance with the present invention and including a laser amplifier-

Figure 2 is a schematic side view of a modified form of the laser beam generator of Figure 1, in which the phase conjugate mirror is an amplifying mirror, so that a laser amplifier is not required.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENT The laser beam generators illustrated in Figures 1 and 2 each have an optical resonator, the cavity of which is .the core of a flexible optical fibre 10, having an input end 11 and an output end 12. A partially transmitting mirror 13, and a phase conjugate mirror 14 define the ends of the elongate cavity. A light source 20 produces a beam of light which is passed into the input end 11 of the optical fibre 10 so that the input light is directed to the output end 12 of the optical fibre. The light source 20 will normally be a laser.

Each partially transmitting mirror 13 is located at or adjacent to the output end 12 of its associated optical fibre 10, and transmits (as beam 15) part of any output beam from the end 12 of the optical fibre 10. The partially transmitting (or output) mirror 13 requires no special characteristics other than being a partial reflector at the wavelength of the light from the optical fibre. Those skilled in this art will appreciate that the reflectivity of each output mirror 13 may be chosen to optimise the output power from the system.

The energy of the light within the optical fibre that is not transmitted as output beam 15 is reflected back into the optical fibre 10,

In the embodiment illustrated in Figure 1, the light that is reflected by the output mirror 13 back into the optical fibre 10 travels through the fibre until it leaves the input end 11 to pass through a laser amplifier 16 before striking the phase-conjugate mirror 14. The phase-conjugate mirror 14 reflects the laser light with phase conjugation back into the laser amplifier 16 and into the optical fibre 10. The presence of the laser amplifier 16 ensures that the phase conjugated input beam into the optical fibre is a twice amplified beam, so that when this beam again reaches the output end 12 of the optical fibre, it will be an amplified version of the initially reflected light that is (due to the phase-conjugated reflection) spatially and temporally coherent with the light initially reflected by the mirror 13. The arrangement illustrated in Figure 1 thus acts as a type of laser. In the embodiment illustrated in Figure 2, the phase-conjugate mirror 14 is of the type that is externally pumped by an excitation light beam 17 and, using the degenerate four wave mixing process, exhibits gain when reflecting incident light. Thus the phase-conjugate mirror 14 of Figure 2 acts as an amplifier of the light that it reflects with phase conjugation. Since light initially reflected from the output mirror 13 traverses the length of the optical fibre 10 to be incident upon the phase- conjugate mirror 14 of Figure 2, the -reflected, amplified, phase conjugated light arrives back at end 12 of the optical fibre 10 with spatial and temporal coherence with the initially reflected light from mirror 13. Thus the arrangement illustrated in Figure 2 also acts as a type of laser.

Each output mirror 13 should be designed to reflect only low order modes (that is, low divergence modes) of the optical fibre back into the fibre, for a consequence of the use of the phase-conjugate mirror is that the mode structure- of the light reflected into the optical fibre by the mirror 13 determines the actual mode structure, at the end 12, of the light reflected by the phase conjugate mirror. While the initially reflected light travels through the optical fibre towards the phase-conjugate mirror, it may undergo considerable mode conversion (especially if the optical fibre 10 has a diameter of about 1 mm to permit high power transmission through it without damage to the material of the optical waveguide). The light emerging from the output end 11 of the optical fibre, therefore, may comprise a combination of a very large number of waveguide modes, and this is likely to be highly divergent, lacking spatial and temporal coherence, and thus having a low brightness. However, the phase-conjugate mirror 14 generates a reflected beam with the special property that, after it has travelled back down the optical fibre to the output end 12, the mode conversion that occurred while the light travelled in the opposite direction will be exactly cancelled. Thus,, as indicated above, after a double pass through the optical fibre, with a reflection after one pass from the phase-conjugate mirror 14, the beam of light emerging from the output end 12 will have the same mode structure as the light initially reflected into the fibre by the output mirror 13. A high brightness of the light emerging through end 12 of the fibre 10, and thus a high brightness of the beam 15, is thus maintained.

A known focusing device 19 is used to focus the output beam 15. The focused beam can be made to be incident upon a zone of an object, and moved from one zone to another, by manual or mechanical manipulation of the output end of the arrangement depicted in Figure 1 or Figure 2 (the output arrangement being the end 12 of the optical fibre 10, the partially reflecting mirror 13 and its associated lens or other focusing device 19). Those familiar with phase conjugation devices will appreciate that, in realisations of the present invention, the phase-conjugate mirror 14 must have a response time which is short in comparison with the time required to change the mode pattern emerging from the output end 12 of the optical fibre 10. This mode pattern is likely to vary as the output end of the optical fibre is moved. The phase-conjugate mirror must have high enough response speed to correct for such changes. However, because the output end of the optical, fibre normally will be moved relatively slowly, this is not a severe limitation.

It will also be appreciated that the phase-conjugate mirror 14 will be able to correct only for the mode conversion in the optical fibre 10 if the optical fibre characteristics are constant during the time it takes the light to travel a double pass of the fibre. The very high velocity of light ensures that this condition will be fulfilled with optical fibres having a length ranging from a few metres to several tens of metres. This condition may be a restriction if the fibre length is of the order of a kilometre.

The optical fibre 10 need have no special properties. Any flexible optical fibre that is able to propagate the required laser power without significant loss of power may be used to form the resonator included in the present invention. In order to transmit high powers, as already indicated, the core of the optical fibre should have a diameter sufficiently large that the material of the optical fibre is not damaged by the transmitted laser radiation.

Although the aforementioned article by David M Pepper in Scientific American discloses the use of conjugate-phase mirrors to form optical amplifiers, optical resonators and lasers, there is no disclosure of the use of the combination of a flexible optical waveguide as the cavity of an optical resonator, with the ends of the cavity defined by a phase-conjugate mirror and a partially reflecting conventional mirror, to provide an output laser beam that can be directed to any required direction, and that can have sufficient power to cut or machine a workpiece.

Claims

1. A laser beam generator comprising:

(a) an optical resonator^' having a cavity comprising the core of a flexible optical fibre CIO), said optical fibre having an input end (11) and an output end (12), .the cavity being closed at the input end of said optical fibre by a phase-conjugate mirror (14) and being closed at its output end by a partially. eflecting mirror (13);

(b) an input light source (20) connected to the cavity for input of a light beam into the cavity at or adjacent to said input end, said input light beam being directed towards said partially reflecting mirror; and

(c) focusing means . (19) mounted in fixed relationship to said output end, for focusing light emanating from said cavity through said partially reflecting mirror.

2. A laser beam generator as defined in claim 1, in which the phase-conjugate mirror amplifies the light reflected therefrom.

3. A laser beam generator as defined in claim 1, in which the phase-conjugate mirror reflects the light incident thereon without amplification. said generator including a laser amplifier (16) between the input end of the optical fibre and the phase-conjugate mirror.

4. A laser beam generator as defined in claim 1, claim 2 or claim 3, in which the partially reflecting mirror reflects only low order transmission modes of the optical fibre.

5. A laser beam generator as defined in any preceding claim, in which the light source (20) is a laser.

6. A laser beam generator substantially as hereinbefore described with reference to Figure 1 or Figure 2 of the accompanying drawings.