WO1987002519A1

WO1987002519A1 - Laser apparatus

Info

Publication number: WO1987002519A1
Application number: PCT/AU1986/000296
Authority: WO
Inventors: Benny Allan Greene
Original assignee: Benny Allan Greene
Priority date: 1985-10-08
Filing date: 1986-10-08
Publication date: 1987-04-23

Abstract

Laser apparatus includes structure (11, 12) which defines a chamber (9) of annular or part-annular cross-section. A partially reflecting mirror (15) allows recovery of an emitted laser beam from the chamber (9). The structure (11, 12) includes opposed substantially curved reflective surfaces (11, 12a) providing radially spaced boundaries for the chamber (9) for reflecting light to or from a generally radial light path (8) within the chamber. When the apparatus is operated with excited laser material in the chamber, the aforesaid light includes light derived by stimulated emission from the laser material.

Description

TITLE: "LASER APPARATUS"

FIELD OF THE INVENTION

This invention relates to laser apparatus which may be configured for operation as a laser amplifier or as a laser oscillator or resonator.

BACKGROUND ART

Prior gas laser devices have generally utilised mode volumes of elongate .cylindrical or quasi-cylindrical configuration. Such volumes have been considered necessary for satisfactory stability and coherence, and- for adequate suppression of parasitic modes. However, it is difficult with these prior devices to efficiently pump and stimulate emission from the whole mode volume. The mode volume ideally equals the pumped volume but this condition is only approached in present devices by longitudinal discharge excitation of the gas, which is only possible at low pressure and has poor cooling characteristics. Transverse excitation, which can operate at higher pressures, always yields a quasi-rectangular pumped volume section, which is not efficiently used by stable resonator modes, and only marginally better used by unstable modes, as the "edges" or the "centre" of the pumped volume are neglected. SUMMARY OF THE INVENTION

It is an objective of the invention to provide laser apparatus in which the pumped, ie excited, volume is more efficiently utilised for stimulated emission.

The invention accordingly provides laser apparatus comprising structure which defines a chamber of annular or part-annular cross-section, means to recover an emitted laser beam from the chamber, wherein said structure includes opposed substantially curved reflective means providing radially spaced boundaries for said, chamber for reflecting light to or from along a generally radial light path within said -chamber, and wherein,^" when said apparatus is operated with excited-laser material in said chamber, said light includes light derived by stimulated emission from said material.

In one embodiment of the invention, said chamber is relatively narrow transversely to said cross-section so as to be configured as an annular disc. The admission means advantageously then includes a central port disposed co-axially with said cha ber, while the reflective means preferably includes a conical reflective surface for deflecting an axially directed input beam as a radially divergent beam in said chamber. The reflective means may further include an outer reflective .surface of truncated internal conical configuration for directing said light path generally annularly axially with respect to said chamber.

The aforesaid laser apparatus of disc configuration may be employed as a gas laser which can be pumped by a discharge directed axially (ie low operation pressure) or transversely (ie high pressure operation) . The axial discharge is preferably tailored to avoid excessive current densities in the gas nearest the inner electrode.

In a further embodiment of the invention such chamber is an annular or part-annular segment of a sphere and said reflective means includes opposed spherical reflective surfaces, one extending about the other.

The spherical reflective surfaces may be concentric. Preferably, the apparatus includes a light admission^"" port disposed to direct input light radially,- whereby said reflective surfaces multiply reflect said input light, and/or light generated thereby, generally radially back and forth as it arcuately traverses said chamber.

For the purpose of suppressing parasitic laser modes, the aforesaid apparatus may be hybridised with a secondary laser device to provide a laser input light beam for said apparatus for seeding said chamber with oscillations in the radial mode. Alternatively or additionally, the apparatus may further comprise plural radially extending baffles disposed in said chamber.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be further described, by way of example only, by reference to the accompanying drawings, in which:

Figures 1 and 2 are highly schematic plan and side elevational views of a first embodiment of laser apparatus according to the invention, comprising a gas laser in which the mode volume is of disc con iguration;

Figures 3 and 4 are highly schematic like views of a second embodiment of laser apparatus according to the invention, comprising a solid state laser amplifier of disc configuration;

Figure 5 schematically depicts a laser oscillator of disc configuration in accordance with the invention;

Figure 6 depicts an alternative method of extracting the output laser beam from the laser oscillator shown in Figure 5;

Figure 7 is a highly schematic representation of a laser stack amplifier according to the invention, employing multiple mode volumes of disc configuration; and Figure 8 is a highly schematic cross-sectional view of a further embodiment of laser apparatus according to the invention, comprising a gas laser of spherical configuration.

BEST MODES FOR CARRYING OUT THE INVENTION

In the gas disc laser 10 illustrated in Figures 1 and 2, the cylindrical mirror 11 -is a highly reflecting mirror which forms the radially outer boundary of a chamber 9 of annular cross-section and comprises one end of an optical resonator. The other end is formed by a partially reflecting mirror 15 which is circular and provides a port for emission of an output laser beam.. A conical optical element 12 couples the two mirrors. The (lower) concial surface 12a of element 12 provides the radially inner boundary of chamber 9 and is at 45 to the horizontal, as shown in Figure 2. Surface 12a is highly reflective at the laser wavelength. A single ray path 8 is shown to indicate the light path. The output exits in a downward direction from the output mirror 15. The laser gas fills the chamber 9 between mirrors 11 and 15. The gas containment, ie the external gas tight housing of the laser, is not shown. The spacing between mirrors 12 and 15 may be much shorter than shown. The gas is excited by electrical discharge between cylindrical inner electrodes 13, and cylindrical outer electrodes 16 (radial excitation) . Plasma discharge containment is accomplished by two insulating discs 14.

Alternatively, transverse excitation is required for some applications (eg TEA CO_ or multiatmosphere lasers). In this case, the discharge takes place between the discs 14, which are now conductors. Electrodes 13 and 16 are not then required.

Only the simplest case is shown. In practice, adjustments may be made to minimise plasma current density gradients in the radial direction. Specially shaped quasi-conic mirrors may replace the cone 12 to allow specific output energy profiles transverse to the direction of propogation.

Plural radial baffles or vanes 18 damp unwanted oscillations in the gas by spoiling.

In Figures 3 and 4 a solid state disc laser 10' is shown. The laser material 20 is an annular disc occupying an annular-section chamber 9' bordered by mirrors 12', 25, with the ratio of radii of the removed centre piece to the extremity of the disc varying according to application. The device will accept T M or any other mode with radial symmetry for amplification. The input beam 23 is reflected by the 45 cone 12* into a 360 fanning-out beam, which is amplified in material 20.

The output 24 is shown as an annulus, simulating TEM _nι mode in output. The beam is coupled to the output by means of 45 cone-section mirrors 25, 26, 27. If mirror 27 is a cone, rather than a cone section, the beam emulates TEM_nf) mode.

The laser medium is pumped, coherently or incoherently, from the top 36 of the disc laser material.20, ^• from the bottom 37, or from both sides. Similarly, cooling can be applied to either or both faces of the disc.

The outer face 29 of the disc, which runs arounds its circumference, may be-anti-reflection coated, or cut at Brewster's angle rather than the 90 cut shown in Figure 4.

The suppression of non-radial modes of oscillation/amplification may. be highly preferable, especially for large discs. Baffles or vanes 18' assist in this task as before. These vanes may be cut into the laser material, and used to introduce cooling also. The impact of these on beam quality will be very small. By tilting the cone section mirror 27, an elliptical beam may be output. This may be necessary for some high aspect ratio amplifiers.

The cone 12' need not fill the "hole" in the annulus material. That is, the thickness of material used is not totally dictated by the size of the hole. This will depend on the configuration used. .For example, the configuration shown in Figures 3 and 4 can be double-passed by replacing the cone-section mirror 25 with a cylindrical mirror. In this case, energy density considerations may require a larger "inner radius" for the annulus, to prevent damage to the laser material due to high- energy in the returning beam.

In Figure 5, laser oscillator^" configurations are shown. The laser material may be solid, liquid, or gas. The laser material is present in two annular discs, an upper disc 9a and a lower disc 9b. The two discs are optically coupled to form a resonator by highly reflecting cone or cone-section mirrors 12a, 12b. The ends of the optical resonator are formed by a highly reflective coating 40 on the cylindrical outer face of the upper disc, and a partially reflecting coating 42 on the cylindrical outer face of the lower disc. The output is directed away by mirror 44. The inner cylindrical face of both discs may be anti-reflection (AR) coated. Alternatively, the optical resonator can be formed by cylindrical mirrors 40', 42' not connected to the discs. In this case, the outer faces may be AR coated or cut at Brewster's angle, with the mirrors 40', 42' then being replaced by cone-section mirrors.

Pumping of the discs may be from the top 45, bottom 47, in-between 48, or any combination of these configurations. The pump may be incoherent or coherent. A typical pump may consist of Xenon flashlamps.

Cooling of the discs may be by liquid coolant flowing across any or all of the top and bottom faces of the individual discs.

The top surface of the upper disc, and the bottom surface of the lower disc, ^" may be coated to reflect the pump wavelengths back into the laser material in the cases where upper (45) and lower (47) pumping are not used.

Disc surfaces 49 are desirable in some applications for reflection and containment of pumnp energy (rather than the coatings 49a, 49b) and to define the coolant flow paths.

If the mirror 44 is not used, the device will produce radiation in a plane. This is essential for some applications. One method of centrally extracting the output beam 50 of Figure 5 is shown in Figure 6. Cone 12a' is highly reflective. The laser light strikes the cone-section mirror 12b' at Brewster's angle. Circular plate 52 is a waveplate to ensure the correct proportion of light is transmitted through to the output. The output beam is "radially" polarised.

Figure 7 shows a disc stack amplifier incorporating the features already discussed. The optical elements 12a¹ ' ' and 12b¹' ' are partially reflecting, whilst the cone mirror 12c is nearly 100% reflecting. Such a device can produce very high mean or peak powers, in an extremely compact device.

The discs are not always a single disc, but may be made up of radial segments (ie pie-slices).

Some advantages of the disc laser include the following:

a) extremely good pump coupling to laser material and significantly improved utilisation of pumped volume by substantially eliminating laser material pumped but not used, leading to higher efficiency;

2) excellent surface area/volume ratio for laser material; 3) good access for cooling by simple techniques (water) ;

4) extremely compact for given excited or pumped volume of material;

5) mechanically simple and rugged;

6) versatile output beam characteristics, eg ability to convert longitudinal modes in the resonator to transverse modes in the output; and

7) disc uses some crystal boules optimally, as ' "slices" may be simply ^"made from the^" end of the cylindrical boule. Boules with unusable centres are even more well suited.

The resonator (optical) constituted by the cylindrical mirror and the output mirror can be configured as a coherent or quasi-coherent resonator. That is, if assembly tolerances are such that the 90 conic section is concentric with the cylindrical mirror, to within a small fraction of the operating wavelength, - then the output will be - coherent transversely. The stability or instability of the resonator is controlled in the normal manner by the selection of the mirror surface of the output mirror.

If the 90 conic section mirror is eccentric with respect to the cylindrical mirror then the resonator length will vary transversely across the output beam. The coherence of the output beam is reduced accordingly. It is not strictly incoherent since the coherence of different parts of the beam will depend on their proximity in terms of transverse displacement within the beam. If the useful bandwidth of the laser line being used is less than the resonator free spectral range (C/21) then any such eccentricity will produce non-viable resonator lengths at some points on the circumference of. the cylindrical mirror. The output beam will then form a 'star' pattern in transverse section.

It will thus generally be understood that the degree of coherence of transverse samples of the beam can be controlled by varying the eccentricity of the components.

Figure 8 is a sectioned elevation through the centre of a spherical laser. The device is radially (circularly) symmetrical in plan. Spheres 60 and 62 are concentric spheres. Sphere 60 is polished and/or coated so that its outer surface i's almost 100% reflecting at the laser wavelength. Sphere 62 is likewise polished and/or surface coated on its inner surface.

The intervening volume 9'¹' of annular cross-section between spheres 60 and 62 is filled with a laser gas. If spheres 60 and 62 were metallic, or conducting, they would serve as electrodes for a radial discharge through the gas. The entire volume 9 ' ' ' could be so excited.

Within the volume 9' ¹ ', there are a very large number of potential resonators, such that self oscillation and other parasitic modes .may quickly deplete the population of excited centres.

This can be overcome by:

a) "seeding" the spherical laser with radiation which has the desired spectral qualities and which follows the chosen optical resonator path (hybridisation); or

b) insertion of baffles, or vanes, to restrict the number of available resonant modes by spoiling Q; or

c) a combination of a) and b).

In Figure 8, the spherical laser is being "seeded" with a CW laser 65 through a circular window 67. Its optical resonator will consist of a plane back mirror 68 and the convex surface of sphere 60. This is a novel version of a classic unstable resonator, with the output of laser 65 being a thin annulus (TEM-.. ode) captured by the inner surface of sphere 62. The propogation path of this annulus is shown in simplified form in Figure 8. The conic baffles 8'¹' help reduce unwanted modes of oscillation and amplificaton.

The output is taken at 70, the opposite pole of the sphere from window 67, the input.

The spherical laser can be any kind of gas or liquid laser. Several examples are:

a) Low Pressure CE CO_.

In this case the spherical laser is essentially an amplifier for the seed laser 65. Extremely long gain lengths are possible with very little wastage of excited volume.

b) Longitudinally Excited Atmospheric (LEA) CO_ .

The spherical laser in this case is quasi-hybridised with the laser 65. It is not strictly hybrid, since the CW and pulsed lasers do not share a resonator. The discharge volume, and thus the energy available, is very large compared to conventional TEA CO„ lasers. c) Multiatmosphere CO_.

The spherical symmetry produces an excellent containment vessel for high pressure gas lasers. Again, a "seed" laser is used to initiate lasing action at the wavelength and in the resonant mode desired.

For TEM-, input and output beams, the central, unused (optically), vertical axis, can be used to support the inner sphere, and connect electrical and cooling lines.

Multiple output ports can be used. The laser may be used with whole spherical sections removed, for example, in hemisphere configuration.

If the inner surface is not spherical, or if it is spherical, but not concentric with the outer sphere, 3-dimensional standing wave patterns can be induced.

Only small eccentricities are allowable before special techniques are required to homogenise the gas discharge. However, very small eccentricities can produce significant effects in output beam window placement.

The simple concentric sphere examples shown in

Figure 8 can be "double-passed" by not installing the output window 70, however the energy density near window 70 may be exceptionally high, as this is the "focus" of the amplifier in this mode. It is analagous to the Gaussian beam waist in a highly (bi-concave) stable, conventional, 2-dimensional, laser resonator.

The spherical laser can be excited by electrical discharge, or by coherent or incoherent pumping. In addition, its spherical symmetry makes it particularly suited to excitation by chemical reaction at the sphere centre.

The spherical laser is also suited to liquid and solid laser media. In the case of gas, liquid, or solid gain media, pumping from within sphere 60, or from outside sphere 62, may be used.

The characteristics of a sphere laser can be varied through a wide range by adjusting the sphere radii, the ratio of these radii, the relative size of the input and output apertures, the vanes, and the eccentricity of the spheres.

If the spherical laser, is used as a short pulse amplifier, with a TEM (annular) input, then the output can be made to exhibit unique and useful characteristics if desired.

For example, if a 1-picosecond pulse is input to a spherical amplifier, the output pulse will be a train of pulses. The number of pulses in the train, and- the relative timing of the pulses, will depend upon from which point in space the output is observed. Thus, the device has the capability to transmit a self-scanning LIDAR beam which conveys range and angular information.

If the output window 70 is made large (that is, a significant proportion) with respect to the sphere radii, the output beam, instead of approximating TEM_n, , will exit in a conical shape.

The spherical optical resonator can be used without pumping (gain) to achieve the beam characteristics (above) .

The spherical configuration of the invention extends to a device which, when input with a short laser pulse, will output into a core or pulse of the same duration, but scanned in angle without moving parts, with energy not being released to all angles simultaneously, as- would be the case with a simple lens.

Claims

CLAIMS :

1. Laser apparatus comprising structure which defines a chamber of annular or part-annular cross-section, means to recover an emitted laser beam from the chamber, wherein said structure includes opposed substantially curved reflective means providing radially spaced boundaries for said chamber for reflecting light to or from a generally radial light path within said chamber, and wherein, when said apparatus is operated with excited laser material in said chamber, said light includes light derived by stimulated emission from said material.

2. Laser apparatus according to claim .1, wherein said chamber is relatively narrow transversely to said cross-section so as to be configured as an annular disc.

3. Laser apparatus according to claim 2, wherein said admission means includes a central port disposed co-axially with said chamber, said reflective means includes a conical reflective surface for deflecting an axially directed input beam as a radially divergent beam in said chamber.

4. Laser apparatus according to claim 3, wherein said reflective means further includes an outer reflective surface of truncated internal conical configuration for directing said light path generally annularly axially with respect to said chamber.

5. Laser apparatus according to any preceding claim, further comprising solid or liquid laser material occupying a substantial portion of said chamber.

6. Laser apparatus according to claim 1, wherein such chamber is an annular or part-annular segment of a sphere and said reflective means includes opposed spherical reflective surfaces, one extending about the other.

7. Laser apparatus according to claim 6, wherein said spherical reflective surfaces are concentric.

8. Laser apparatus according to claims 6 or 7, further including a light admission port disposed to direct input light radially, whereby said reflective surfaces multiply reflect said input light and/or ^"light generated thereby, generally radially back and forth as it arcuately traverses said chamber.

9. Laser apparatus according to any preceding claim in combination with a secondary laser device to provide a laser input light beam for said apparatus for seeding said chamber with oscillations in the radial mode.

10. Laser apparatus according to any preceding claim, further comprising means for exciting a gas laser material in said chamber.

11. Laser apparatus according to claims 7 and 9, wherein said reflective means comprises spaced electrodes forming said excitation means.

12. Laser apparatus according to any preceding claim further comprising plural radially extending baffles disposed in said chamber to suppress parasitic laser modes.