Field of the Invention

This invention relates to a folded, phase-locked, single mode, laser fibre bundle, scaleable laser oscillator system consisting of a folded bundle of single mode optical fibres whose cores have been

5 doped with a lasing ion such as neodynium said laser fibres forming said bundle having optically polished ends, each , end positioned in the same plane and equidistant from each other, forming the end faces of the invention said end faces of said bundle being positioned relative to each other such that each end face can

10 perform a different function in the operation of the invention whose phase-locked output beam is emitted by only one of th© said end faces of the invention. The invention has applications in the industrial, medical, defence and scientific, _, fields.

Summary of the Prior Art i s Prior art, folded laser fibre bundle systems had all of the fibre ends singly bundled together to act as individual, super-radiant laser fibre amplifiers which were not optically coupled to each other. in co-pending patent applications the inventor teaches the art 0 of looped laser fibre, phase-locked, single aperture, single mirror laser oscillators with both side and end pumping arrangements, Ail of these co-pending inventions have single apertures, implying that the output laser beam has to transverse any optical components necessary to either end pump or frequency tune these single 5 aperture looped laser oscillator systems.

This invention provides for a scaleabie, aperture folded fibre laser bundle laser oscillator whose output aperture area is less than twice the cross-sectional area of the said bundle whilst allowing for the side optical pumping of the invention over twice the number of fibres within the said bundle, leading to a relatively compact and efficient optical excitation arrangement.

Background of the Invention

Scaling the output power of lasers has proved to be by no means a simple task. For example, gas lasers must have a length of ι o plasma within their resonators which has to be well confined in order to attain the necessary excitation conditions. In general, gas laser plasma columns have diameters ranging from sub millimeter to about ten centimeters. It then becomes impossible to scale most of these lasers, because the pump power and the effect of

15 such large powers on the plasma itself, prohibits operation in columns more than one centimeter in diameter, the carbon dioxide laser being the well known exception. In the case of carbon dioxide lasers, transverse excitation of the plasma can provide excitation values adequate to generate laser beams of some 5 cms in

2o diameter at power levels up to 25 kilowatts.

When one requires higher power levels a different approach to the scaling of laser beam output powers has to be followed. This can be achieved in two ways, either new techniques are required to scale existing laser beams to much higher power levels or existing

25 laser beams have to be phase-locked together to form what is effectively a single laser beam. In this invention, the smallest

possible laser beam generator, namely a single mode optical fibre, is used and the scaling of its output power is achieved by bundling a large number of these fibre lasers together in a particular manner. Further scaling of the output power is then achieved by bundling

5 together groups of the output apertures of the basic units of tlw invention allowing the output powers achievable to be rapidly increased depending only on the number of the individual inventions used.

This modular approach to high power lasers Is very cost ι o effective because the mass producible basic units, the invention, is simply bundled together to form a scaleable aperture, and hence, scaleable output power.

Summary of the Invention

It is an object of the invention to provide a phase-locked, 15 scaleable, single mode optical fibre bundle laser in a looped configuration such that a compact, diode array pump can be used to excite twice as many fibres as is possible in a straigth bundle, laser oscillator.

Another object of the invention is to provide for ^' the end o pumping of the fibre bundle laser via one of its ends so that the other end can act as clear output aperture.

It is an object of the invention to loop a bundle of single mode laser fibres so that the output aperture end is well clear of the other end yet allowing the invention to benefit from the advantages 5 of external side pumping from a compact diode array source of pumping light.

Another object of the invention is to provide means for grouping together the output aperture ends of several units of said invention such that the output power is scaled simply by adding the power output of each of the individual ends. It is also an object of the invention to provide means of grouping the output ends of several units of the invention such that the output beams of such a scaled multiplicity of the invention are in phase-locked groups of groups, the mode locking being achieved via leakage of laser light from one group of single mode fibres to ι o another using a micro lens array on the output apertures.

Another object of the invention is to achieve phase-locking by feeding laser light from the non output aperture face of one unit of the invention into the rear end face of other units of the invention which form the grouped array of the said invention. i s It is an object of the invention to use a small, cost effective, mass producible laser module (the invention) to provide means of scaling the laser beam to high power levels by grouping the output apertures of such units together such that they are coherently phase-locked to provide what is effectively a single beam output.

20 Another object of the invention is to provide means of steering the output beam by modifying the phases of the individual input beams.

It is an object of the invention to narrow the output frequency by inserting appropriate tuning elements into its non output end.

Brief Description of the Drawings

A better understanding of the invention will be obtained from the following considerations taken in conjunction with the accompanying drawings which are not meant to limit the scope of 5 the invention in any way.

Figure 1 shows the folded bundle laser oscillator with both ends along side each other, with a combination end reflector. In this configuration of the invention, the diameter of the single mode core of the laser fibre is equal to or greater than the separation ι o between said cores so that a direct, evanescent optical coupling exists between said cores of said bundle allowing for the generation of a phase-locked output beam for this configuration of the invention.

In Figure 2, we show the invention with the diameter of the i s single mode fibre core much less than the core separation within the said bundle. In this configuration of the invention, an array of micro lenses, matched to the array of the cores is used to phase- lock the output beam of the invention.

In Figure 3, we show the configuration of the invention when 0 one arm of the folded laser fibre bundle is longer than the other, the output beam emerging from the longest arm whilst the invention is end pumped via the end face of the shorter arm.

In Figure 4, we show the invention being frequency tuned by coupling a diffraction grating to the end face of the said shorter 5 arm.

In Figure 5, we show how this scaleable invention can also be scaled by grouping a number of the inventions together with the end

faces of their longer arms being grouped together to form a single, phase-locked output aperture.

Detailed Description of the Invention

In Figure 1, numeral 1 indicates a folded bundle of single mode s laser fibres. Numeral 2 indicates one of the end faces of bundle 1 composed of the optically polished ends of the individual laser fibres positioned equidistant from each other in a single plane. Numeral 3 indicates the other end face of bundle 1, which is identical to the first face indicated by numeral 2, the structure of ι o said fibre ends being characterised by a core diameter equal to or greater than the separation of said cores from each other. Numeral 4 indicates an optically polished laser mirror substrate onto which two different laser mirrors are attached. Numeral 5 indicates a 100% reflecting mirror at the laser

15 wavelength whilst numeral 6 indicates a partially transmitting mirror at the same wavelength. Numeral 7 indicates the phase- locked laser output beam emerging via mirror 6. In Figure 1 , numeral 8 indicates an array of semiconductor light sources used to side pump the two arms of fibre bundle 1 with the electrical power

20 for array 8 being provided by the power supply indicated by numeral 9.

In Figure 1 , laser fibre bundle 1 is also end pumped via face 2 by the output beam, indicated by numeral 10, of the semiconductor diode array indicated by numeral 11 which is powered by the

25 electrical supply indicated by numeral 12. Numeral 13 indicates the optical component used to turn excitation beam 10 through

substrate 4 and mirror 5 into bundle 1 via its end face 2.

In Figure 2, the core of the single mode fibres from bundle 1 is of much smaller diameter that the separation of said cores so that a micro lens array indicated by numeral 14 is required to match 5 said cores in end faces 2 and 3 to mirrors 5 and 6 respectively to allow for the phase-locked output beam indicated by numeral 15. In Figure 3, end face 16 is composed of single mode fibres whose core diameter is equal to or greater than their separation.

Numeral 17 indicates a 100% reflecting mirror at the laser ι o wavelength. Excitation beam 10 is now directed via mirror 17 into end 16 for end pumping said laser fibres forming bundle 1. Numeral

18 indicates a partially transmitting laser mirror which allows for the emission of the phase-locked laser beam indicated by numeral

19 out of the invention. 5 In Figure 4, Numeral 20 indicates a micro lens array which optically couples the small cores of fibre end array indicated by numeral 21 to the diffraction grating indicated by numeral 22. Numeral 23 indicates a second micro lens array, numeral 24 a partially reflecting mirror at the laser wavelength whilst numeral o 25 indicates the frequency tuneable phase-locked laser output beam.

In Figure 5, numeral 26 indicates the phase-locked output beam formed by phase-locking the phase-locked output beams of a scaleable array of the invention. ₅ The invention has applications in the industrial, medical and defence fields and is particularly effective in generating a phase- locked output beam from the end of a very flexible laser fibre

bundle whose other end is looped to provide an effective optical excitation and frequency tuning portion which can be located at a remote site. It should be noted that the length of the individual fibre loops does not affect the overall phase-locking of the output laser beam of the invention provided the length of the smallest fibre loop is much greater than the laser output wavelength.