AU5953290A - Segmented, fibre coupled diode laser arrays - Google Patents

Description

Segmented, Fibre Coupled Diode Laser Arrays

Field of the Invention

This invention relates to a phase-locked, fibre coupled diode- fibre array laser consisting of groups of coherently phase-locked arrays emitting diffraction limited laser beams which are fibre 5 coupled to produce a larger diameter phase-locked laser beam, itself being diffraction limited. The invention has applications in the industrial, medical, defence and mass entertainment fields.

Summary of the Prior Art

Phase-locked, diode laser arrays are well known in the art.

10 However, they have a major defect in that they are not coherently phase-locked, larger arrays tending to phase-lock in pockets, a characteristic which leads to multi-lobed outputs. Another defect of prior art diode laser arrays arises from the fact that over 60% of the electrical input power into the array appears as heat within the 5 array itself. As these prior art diode arrays are scaled to higher power levels by increasing the number of diodes within a single array, the effects of the thermal problems become more severe leading to defective phase-locking of the array as a whole. To ensure the coherent phase-locking of a prior art diode laser array it

20 is necessary to position each of the laser diodes within about ten microns (10-3 cms) of each other within the array so that they can be optically coupieά to each other, a process referred to as evanescent coupling of the diodes within the array. However, the . size of the coherently phase-locked array is still limited in

^'SUBSTITUTE SHEET practice.

The present invention overcomes the defects of prior art systems by limiting the cross-sectional area of the laser diode array on a given substrate so that each such diode array emits a diffraction limited laser beam which is fibre coupled to produce phase-locked diffraction limited laser beams which in turn combine together to form a phase-locked, diffraction limited laser output beam of the single, fibre end face aperture of said system. To achieve such operation of groups of diode arrays, they have to be ι o separated for optimum cooling, but collectively coupled optically for optimum phase-locking, two contradictory requirements which are balanced in this invention. To effectively cool the arrays, a given array has to be of a size that provides good phase-locking resulting in a single iobed output beam and experience indicates

15 that the optimum size for such arrays lies in the 100 - 1 ,000 diode range depending on the type of diode lasers involved. For example, a 1 ,000 diode array would have a circular cross-sectional area of about 300 microns (3 x 10-2 cms) in diameter. By fibre coupling such arrays, the whole array can then be phase-locked with the 2o phase-locking being achieved via the fibre array output face of the invention.

Background of the Invention

Semiconductor diode lasers provide a most effective method of converting electrical energy into laser beam energy with 25 conversion efficiencies in excess of 30% being achievable in practice. Diode lasers are of two basic types, namely, those with

SUBSTITUTE SHEET their active regions emitting the laser beam parallel to the substrate on which the said laser diode has been deposited and those which emit their laser beam outputs perpendicular to the said substrate. In the former type of laser diode, the output beam is of 5 a small rectangular cross-section whilst in the latter it is of a circular cross-section which makes it easier to match into optical components, for example, optical lenses and optical fibres.

This invention allows for the coherent phase-locking of diode arrays by connecting each diode to an optical fibre or an optical ι o fibre tape to accommodate the rectangular cross-section output beam more effectively. The opposite ends of the said fibres are then placed in a bundle of such fibre ends to form a phase-locked output aperture - partially transmitting mirror combination which can be scaled to the required output power level simply by adding is more fibre ends to said bundle. In this way, an array of individual laser diodes can be phase-locked together via the interactions occurring at the fibre bundle output mirror end face. In this way, a series of single laser diodes can be phase-locked into a coherent array which is scaleable to high power levels. The heat generating 20 laser diodes can be well cooled in the invention because they are not packed too close together. Also the passive optical fibres introduce relatively little loss in the invention when effectively coupled to the diode lasers, in fact losses as low as a few percent can be achieved. 25 ^• There are three phase-locking techniques that can be utilised to phase-lock the output beam of the invention. Firstly, the fibre core separation in the output aperture can be comparable to the

SUBSTITUTE SHEET core diameter so that evanescent coupling, that is direct core to core optical coupling occurs. Secondly, diffractive effects can be used to direct a portion of the output of one core into its neighbours within the array via a partially reflecting output mirror.

5 Thirdly, a micro lens array can be used to couple the output of each fibre forming the fibre bundle end face so that a parallel beam can be reflected off a plane, partially transmitting output mirror back into the fibre array with phase-locking occurring between the lens array and the output mirror.

10 By combining the properties of diode lasers with that of passive optical fibres a very effective diode-fibre hybrid laser is produced which not only allows for the phase-locking of a large number of laser diodes, but does so in a manner which is scaleable to high power levels simply by adding more fibre ends to the output 5 aperture.

Summary of the Invention

It is an object of the invention to provide a diode-fibre hybrid laser oscillator by optically coupling said optical fibre to the output aperture of said laser diode via a matching lens and placing

2o a partially transmitting mirror near the free end of said fibre to form one end of a laser oscillator resonator, the other end being in the form of a mirror of maximum reflection at the lasing wavelength and placed on the outward face of said laser diode.

Another object of the invention is to bundle together said diode-

25 fibre hybrid laser oscillators so that the free, optically polished fibre ends form an output aperture, each of the said fibres being

SUBSTITUTE SHEET optically polished and positioned so as to form a single output aperture which can be phase-locked using a partially transmitting mirror positioned at such a distance from said aperture that a portion of the laser beam emitted from any given fibre core is reflected into neighbouring cores.

It is an object of the invention to provide a laser output aperture where the fibre core separation is comparable to the diameter of said cores so that phase-locking of said invention can occur via a direct core to core evanescent optical coupling. o Another object of the invention is to provide partially transmitting output mirror on the rear surface of a substrate whose front surface is attached to a micro lens array, allowing for the phase-locking of the invention to take place within the said substrate. 5 It is an object of the invention to provide a computer controlled switching sequence of the individual diode-fibre hybrid oscillators such that laser light emitted from one side of the said output aperture emerges before the laser light emitted by the fibres on the opposite end of the aperture diameter thus causing the o output wavelength to tilt and hence the beam propagation direction to be tilted relative to the optically polished face of said output aperture.

Another object of the invention is to provide a diode-fibre hybrid laser system which can be scaled to any practical power 5 output level by simply adding more diode-fibre hybrid oscillators to a bundle of said hybrid oscillators and phase-locking these output beams to provide a single output beam.

SUBSTITUTE SHEE It is an object of the invention to provide a phase-locked output aperture composed of an array of diode fibre hybrid laser oscillators whose individual output powers are relatively low but whose combined, phase-locked array power is relatively high and focussable to a power density that results in efficiency frequency conversion when said output beam is focussed through a non-linear crystal.

Laser diode arrays are difficult to coherently phase-lock and tend to phase-lock in pockets across the array, in the case of iaser diode beams of rectangular cross-sections, a difficulty exists in matching such laser beams to the cores of single optical fibres. However, the coupling of such laser beams to tapes of opticai fibre cores is a much more efficient process because a line of such fibre cores present a rectangular cross-section. With the advent of low optical loss fibres which can be bundled to form a high optical quality output face, it became possible to enhance the power of laser diode arrays by bundling together diode- fibre hybrid laser oscillators. The case with which iaser diodes can be electronically switched allows for the diode-fibre hybrid laser to significantly enhance the properties of passive optical fibres regarding laser beam generation and laser beam steering. Also by selectively switching the individual diode, it is possible to generate images in the output aperture of said diode-fibre hybrid laser systems. It is clear that by adding passive opticai fibres to diode iasers, it is possible to provide for the coherent phase-locking of group of phase-locked diode arrays which effectively provides for the

SUBSTITUTE SHEET equivalent of large, phase-locked diode arrays with a selectively small optical loss due to diode-fibre coupling.

Brief Description of the Drawings

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

Figure 1 shows a schematic layout of the diode array unit of the invention which is fibre coupled via a lens to focus the output o of the array into a single mode fibre.

Figure 2 shows two elements of the invention with their fibre output ends stacked together to allow for a phase-locked laser output beam.

Figure 3 shows the use of an output etalon to couple the light 5 from one of the fibre ends to another so as to produce a single beam phase-locked output. The beam can be increased in intensity for frequency conversion at higher efficiency using a telescope in the reverse mode and a non-linear optical input.

Figure 4 shows a fibre coupled array of laser diode arrays o computer controlled to ensure that the final output beam is phase- locked and fully steerable. This configuration of the invention is frequency tuneable, using non-linear optical effects within the fibre couplers themselves.

SUBSTITUTE SHEET Detailed Description of the Invention in Figure 1, numeral 1 indicates the substrate onto which the diode array indicated by numeral 2 is mounted. Numeral 3 indicates the electrical power supply which is used to excite diode array 2. 5 Numeral 4 indicates the single, phase-locked laser output beam of diode array 2 whilst numeral 5 indicates a rod lens which focusses said diode array (2) output (4) in a spot diameter of 10 microns

(10-3 cms) or less to match the single mode fibre core indicated by numeral 6. Numeral 7 indicates the cladding surrounding fibre core ι o 6 which both minimises the optical losses from the said core 6 and provides a mechanically strong medium which can protect the fragile fibre core. Numeral 9 indicates an index-matching medium inserted between low reflecting mirror 10 at the optically polished end-face indicated by numeral 11. Numeral 12 indicates a phase-

15 locked output beam of the invention whilst numeral 13 indicates the optical coupler between diode arrays.

In Figure 3, numeral 14 indicates a telescope used to reduce the output beam diameter of the invention such that the intensity of said output beam increases to a level where its frequency

20 conversion, using the non-linear crystai indicated by numeral 15 is efficient. The frequency converted output of the invention is indicated by numeral 16.

In Figure 4, numeral 17 indicates a phase-locked stack of optically polished single mode fibre ends forming the output

25 aperture of the invention. Numeral 18 indicates a micro iens array on the inner face of an optical substrate indicated by numeral 19 which has a partially transmitting mirror on the substrate face SUBSTITUTE SHEET indicated by numeral 20. The ovens/coolers used to vary the operating temperature and hence the output wavelengths of the diode arrays 2 indicated by numeral 22. Numeral 23 indicates the computer used to control both the switching and phase of the iaser beams emitted by diode arrays 2.

Computer controlled switching of the diode arrays of the invention can control the phases of the emitted laser beams to ensure the precise steering of the final phase-locked output laser beam of the invention. ι o The invention allows for outputs of a large number of coherently phase-locked diode arrays to be combined into a single, scaleable, phase-locked output aperture composed of optically polished, anti-reflection coated ends of single mode optical fibres in one of the preferred configurations of the invention. The phase- i s locked output beam of the invention can be frequency tuned either by the temperature tuning of the diode arrays themselves, utilising the non-linear optical effects that can be realised within the single mode fibre cores or by using non-linear crystals and concentrated output beams. The flexible fibre bundle coupler allows for the 20 mounting on robotic arms used in industrial workstations.

SUBSTITUTE SHEET

Claims (8)

I claim 4,

1. A diode-fibre hybrid iaser consisting of:

(a) An electrically excited diode laser which has a 100% reflecting mirror on one of the faces of its lasing

5 , medium, the other face being anti-reflection coated.

(b) A lens to focus the output of said diode laser medium.

(c) A passive optical fibre which accepts the focussed beam of said coupling lens to one of its core end, the other core end by optically polished and mirrored to partially ι o transmit said laser beam generated in the diode-fibre hybrid iaser oscillator.

2. A phase-locked array of the bundled mirrored output ends of the said opticai fibres of said diode-fibre hybrid lasers of Claim 1 , said phase-locking being achieved via direction optical coupling i s between the cores of said optical fibres forming part of the said diode-fibre hybrid laser.

3. A phase-locked array of the bundled mirrored output units of the said optical fibres of said diode-fibre hybrid lasers of Claim 1 , said phase-locking being achieved via the diffraction and reflection

2o of the laser light emitted by a given fibre core is directed into neighbouring fibre cores via reflection off a partially transmitting output mirror.

4. A phase-locked array of the bundled mirror output ends of the said optical fibres of said diode-fibre hybrid lasers of Claim 1 ,

25 said phase-locking being achieved via the use of a micro lens array to match the output of individual fibre cores into the partially

BSTITUTE SHEET transmitting output mirror.

5. A system as described in Claim 1 where a time difference exists between the emission of the light across the output aperture linking to a controlled tilting of the output beam wave front and

5 hence the steering of said output beam.

6. A system as claimed in Claim 1 where the individual diode- fibre hybrid laser oscillators are selectively switched so as to produce a high definition laser output beam image.

7. A system as claimed in Claim 2 above the individual diode- ι o fibre hybrid laser oscillators are selectively, excited so as to produce different power levels across said phase-locked aperture.

8. A system as claimed in Claim 2 which can be scaled to any practical power output level simply by adding more diode-fibre hybrid oscillator to said fibre bundle.

SUBSTITUTE SHEET