GB2323466A

GB2323466A - Improvements in or relating to lasers

Info

Publication number: GB2323466A
Application number: GB8412489A
Authority: GB
Inventors: Gary Cook
Original assignee: UK Secretary of State for Defence
Current assignee: UK Secretary of State for Defence
Priority date: 1983-05-25
Filing date: 1984-05-16
Publication date: 1998-09-23
Anticipated expiration: 2004-05-16
Also published as: GB8412489D0; GB8314368D0; GB2323466B; FR2920257A1; DE3419562C1; IT8448257A0; CA1341338C

Abstract

A composite lasing medium, 11, comprising a plurality of separated lasing mediums 33, 32, the separated lasing mediums being so arranged that the excitation traverses the mediums in series, the separated lasing mediums being selected so that each medium traversed by the excitation transmits the excitation for all the mediums further from the source of excitation. The invention provides a composite lasing medium which has, in the case of a pulsed electromagnetic excitation from xenon flash discharge tubes 4, a wide optical absorption and emission band, increased conversion efficiency due to both the excitation of the two separated lasing mediums and the fluorescent emission of one medium exciting the other medium and the protection of the inner medium 32 (Sulforhodamine-B in methanol) from the deleterious effects of the excitation absorbed by the outer medium 33 (Coumarin 102 in methanol).

Description

IMPROVEMENTS IN OR RELATING TO LASERS The present invention relates to lasers, particularly those which utilise pulsed optical excitation.

In a lasing medium the electromagnetic (e-m) absorption spectrum is the combination of the absorption spectra of an active component and a host material. The main pumping bands of a lasing medium correspond to maxima in the e-m absorption spectrum of the lasing medium and these in turn correspond to maxima in the e-m absorption spectrum of the active component of the lasing medium.

Other maxima in the e-m absorption spectrum of the lasing medium may correspond to maxima in the em absorption spectrum of either the active component or the host material or both. Pumping the lasing medium at wavelengths corresponding to the main pumping bands results in the most direct excitation of the energy levels associated with the lasing action, giving the highest conversion efficiency between the energy expended in pumping the lasing medium and the energy delivered in the laser output.

Known lasers which utilise pulsed optical excitation to achieve lasing action have a low conversion efficiency due to the range of wavelength encompassed by the main pumping bands of the lasing medium being considerably smaller than the range of wavelength generated by the pulsed optical excitation. The range of wavelength not utilised by the main pumping bands may generate lasing action by means of indirect excitations, but in the main the energy delivered at these wavelengths is dissipated as heat in the lasing medium.

Further, some of these wavelengths may cause damage to the lasing medium by photodissociative action. In all cases, energy supplied by the optical excitation to the lasing medium not converted to laser output radiation may lead to a further degradation of the conversion efficiency of the laser from its already low value.

The object of the present invention is therefore to provide protection for the lasing medium from the damaging effects of pulsed optical pumping, to obtain lasing action from a wider range of pumping wavelengths and an increased conversion efficiency from a lasing medium than has been previously available and as a result to obtain a wider bandwidth output from the lasing medium.

According to the present invention there is provided an optically excited pulsed laser having a composite lasing medium, the composite lasing medium comprising a plurality of separated lasing mediums in series, the separated mediums being so arranged that each medium traversed by the excitation transmits those regions of the spectrum required for the excitation of the main pumping bands of all the mediums further from the source of the optical excitation.

The order in which the mediums are placed for traversal by the excitation ensures the removal by mediums closer to the source of excitation wavelengths able to cause damage to mediums further from the source of excitation. Absorption of the wavelengths able to cause damage may be accomplished by either the active component or the host material or both in the lasing medium in which absorption occurs.

The separate mediums may comprise gas, liquid, gel or solid lasing mediums as required.

An advantage to be gained from the presence of separate lasing mediums is that the random fluorescence output of one medium may assist in the pumping of another medium.

A further advantage arising from the use of a plurality of separate mediums is that the mediums may be chosen to provide a lasing output from the composite lasing medium encompassing a wide wavelength range, which if in the optical region may be a 'white light' output.

A further advantage arises from the 'white light' output where a narrow bandwidth laser output is required to be tunable over a wide range of wavelengths. Single lasing medium dye lasers have been used to provide tunable lasers with the required bandwidth, but only with the complication of changing the dye to that appropriate for the wavelength range required. With the present invention a single laser will be tunable over all the wavelengths comprising the 'white light' output.

Embodiments of the invention will now be described with reference to the accompanying drawings, in which Figure 1A shows schematically'in cross section an embodiment of the invention, utilising two solid lasing mediums in the laser cavity.

Figures 1B and 1C show the absorption and lasing output spectra of the lasing mediums shown in Figure lA.

Figured 2A shows in cross section a schematic representation of a working embodiment of the invention utilising two liquid lasing mediums.

Figure 2B shows the view on A-A of Figure 2A.

Figures 2C and 2D show the absorption and output spectra of the lacing mediums shown in Figures 2A and 2B.

Figure 3A shows in cross section a schematic representation a laser cavity embodying the invention utilising three liquid lasing mediums.

Figure 3B shows the view on B-B of Figure 3A.

Figures 3C and 3D show the absorption and output spectra of the lasing mediums shown in Figures 3A and 3B.

Features of the same description illustrated in more than one Figure have been accorded the same reference numeral.

In Figure 1A, the laser cavity 10 which is substantially cylindrical in shape with closed ends is shown in cross section. The cavity has a circular cross section with the interior surface 5 coated with diffuse white reflectant to direct the light generated by the xenon flash discharge tubes 4 toward the composite lasing medium 11. The xenon flash discharge tubes are equispaced on a circle, shown by the dashed circle 12, about the centre of the cavity. The composite lasing medium comprises two lasing mediums, the inner medium being a cylinder of Nod Glass 1, the outer being a hollow cylinder of Alexandrite 2, separated by a void 3. The Alexandrite is the same length as the cavity and is sealed into holes in the ends of the cavity (not shown) by neoprene gaskets (not shown) to prevent leakage of the circulated water coolant 13 for the xenon lamps. The circulation and cooling equipment is not shown.

The Nd :Glass cylinder is held in the centre of the Alexandrite by three corrugated plastic clips, of which the central clip 14 is shown.

In operation, the flash discharge from the xenon lamps (4) impinges on the Alexandrite lasing medium where part is absorbed as shown by the Alexandrite absorption spectrum in Figure 1B. The range of wavelengths transmitted by the Alexandrite is that required to excite the main pumping bands of the Nd+++ :Glass medium, also shown in Figure 1B. Thus both the Alexandrite and the Nd+++:Glass mediums may be pumped by the xenon flashlamp output.

The range of wavelengths available for laser output from the Alexandrite and Nd Glass composite medium is as shown in Figure 1C. Note that the output from the Alexandrite coincides with the shorter wavelength main pumping band of the Nod glass medium. It is expected that fluorescent.output from the Alexandrite assists in pumping the Nd+++ :Glass medium.

In Figures 2A and 2B a working embodiment of the invention is shown schematically, Figure 2A being a horizontal section of the laser, Figure 2B being a view on A-A of Figure 2A.

The laser cavity 20, is composed of four sections, 21, 22, 23 and 24 located by end plates 25 and 26. The end plates are provided with holes 27 and 28 to accommodate the xenon flash tubes 4 and the composite lasing medium 11. The laser cavity is of substantially elliptical cross section, close wrapped around the flash tubes and the lasing medium. The inner surface 5 of the cavity is coated with diffuse white reflectant. The end plates provide locations for mounting and sealing the lasing mediums by means of plastics gaskets 29 and for sealing the windows 30 to each end plate by means of clamp rings 31. The composite lasing medium comprises an inner dye cell 32 and an outer dye cell 33 separated and contained by cylindrical glass tubes 34 and 35 respectively. The cell was placed within a plane parallel resonator bounded by mirrors Ml and M2, M2 being partly reflecting to provide output coupling. A Phase-R Corporation 6 psi dye laser pump (not shown) is used to circulate the solvent containing dye through the outer cell, via the end plate connectors 36, and a Chemicon Model 150 PTFE diaphragm pump (not shown) is used to circulate the solvent containing dye through the inner cell via the end plate connector 37. The dyes were passed through the cell in the same direction, with the inlet temperature of the dyes at 20 + .05 C, controlled by a water cooled heat exchanger (not shown). The parallel flow direction of the dyes and the temperature control is necessary to minimise thermally generated turbulence in the dye cell. The dyes flow rates were adjusted to ensure that the dyes were replaced between excitations.

The dimensions of the laser are as follows, using the lettering on Figures 2A and 2B.

D: Dye cell length : 300 mm M: Separation of mirrors : 900 mm F: Flash tube - centre of cells : 35 mm C1C2: Cross section dimensions of cavity : 25 mm x 85 mm Inner cell, internal diameter : 10 mm Tube wall Outer cell, internal diameter : 18 mm thickness: 1.5 mm Operating characteristics Inner cell dye : Sulforhodamine B, 1 x 10 4 molar in methanol Outer cell dye : Coumarin 102, 5 x 10 molar in methanol Flash tube energy : 400J total Output characteristics Inner cell dye with outer cell filled with methanol to eliminate focussing effect on relative output : 250 mJ (mean) Inner cell dye with outer cell : 280 mJ (mean) filled with dye, outer cell spoiled Increased output due to prevent lasing to fluorescene pumping by outer cell dye (see below) Inner cell and outer cell combined lasing output : 380 mJ (mean) Beam divergence : 10 m rad (full angle) Firing cycle : single shot : 1 Hz for 10 seconds maximum in burst mode at best energy output. Repetition rate and duration limited by power supply.

Dye lifetime : the Sulforhodamine-B dye has operated for 5000 shots over a period of 4 months and is still at 90X of the best output.

Comparison of Figure 2C and 2D shows the criteria for selecting the dyes used and the order in which they are placed with repeat to the excitation. Note that in accordance with the concept of the invention the outer Coumarin 102 dye absorbs at a shorter wavelength than the inner Sulforhodamine-B dye thus protecting the inner dye from the deleterious effects of exposure to radiation at these wavelengths. There is little overlap of the absorption curves of the dyes so that the e-m excitation wavelengths transmitted by the outer dye retain the appropriate wavelengths to excite the inner dye and the Coumarin 102 emission spectrum overlaps the Sulforhodamine-B absorption spectrum leading to pumping of the Sulforhodamine-B dye by the random fluorescent output of the Coumarin 102. Finally, the emission spectra of the dyes show no overlap leading to a 'multi-coloured' output from the laser.

In Figures 3A and 3B a laser cavity 20 of substantially the same construction as that in Figures 2A and 2B is shown. The cavity has a substantially elliptical cross section with the interior surface 5 coated with diffuse white reflectant to direct the light generated by the xenon flash discharge tubes 4 towards the composite laser medium 11. The composite laser medium comprises three dye lasing mediums having the active components, Coumarin 102, 301, in the outermost layers, Coumarin 153, 302, in the middle layers and rhodamine 6G, 303, in the innermost layer The dye layers are of substantially rectangular cross section, with flat glass separators 304 and the dyes are circulated to pass through the cavity transverse to the lasing output, with all the dye layers flowing in the same direction. Transverse flow is used to minimise the circulation induced turbulence found with the longitudinal flow of the previous embodiment. The dye circulation is provided by a pump 305 and temperature control by a heat exchanger 306 for each dye used (one only shown). The connecting pipes 307 and flow spreaders 308 are arranged to have the same cross sectional area as the dye cell. The composite medium is placed in a plane parallel resonator formed by mirrors 309, 310, mirror 309 being partly reflecting to provide output coupling.

In operation, the flash discharge from the xenon lamps 4 impinges on the composite lasing medium. The absorption of light by the successive glass separators can be neglected in comparison to the absorption of light by the lasing mediums.

The Coumarin 102 layers absorbs light from the xenon lamps most strongly in the region of the optical spectrum shown in Figure 3C, these are the main pumping bands for Coumarin 102.

The transmitted light retains the appropriate wavelengths for pumping the main pumping bands of both the Coumarin 153 layers and the Rhodamine 6G layer. After passing through the Coumarin 153 layers the light retains only the wavelengths for pumping the main pumping bands of the Rhodamine 6G layer. Each layer is pumped at the wavelengths corresponding to the layer's main pumping bands, giving the highest possible conversion efficiency in each layer. The range of wavelengths available for laser output for the three layer composite medium is shown in Figure 3D. It can be seen from Figures 3C and 3D that this particular three layer composite lasing medium has an absorption spectrum spanning a wide range of optical wavelengths, and the lasing output occurs over wide range of optical wavelengths giving rise to a 'white light' output.

The protection afforded to the innermost dye lasing mediums may be used so that circulation of the dye is no longer required.

As a result a Rhodamine 6G gel layer may be substituted leading to a reduction in ancillary equipment.

In all the above embodiments it is to be noted that in comparison with known single laser mediums, the cross section of the inventive composite laser medium is large in relation to its length. To ensure that the fluorescent output of each layer is reflected through that layer to induce the stimulated emission and lasing action, a plane-parallel (Fabry-Perot) type resonator is used, with one semi-reflecting mirror to provide the output coupling. Phase conjugate mirrors may be used in place of the standard mirrors.

The criteria used in selecting the lasing mediums used in the above embodiments may be applied to other composite mediums, for example, a gas-vapour laser giving a wide-band infra-red output may use the mediums and materials for construction of the separators as shown in Table 1.

Persons skilled in the art of laser design will be aware that the embodiments described above may be constructed from well known components with the exception of the composite medium. Selection and ordering of the lasing mediums may also be made with reference to well known tables of electromagnetic absorption and emission spectra.

TABLE 1 GAS LASER COMPOSITE LASER MEDIUM CONSTRUCTION Direction of excitation traversing the composite medium

Separator Medium Output wavelength Glass Iodine 1.3 pm Glass Carbon Monoxide 3.8 - 5.3 um Germanium Carbon Monoxide 10.6 m Ge rmanium Hydrogen Cyanide 200 m Perspex Methanol ~1000 pm

Claims

CLAIMS What is claimed is:1. An optically excited pulsed laser having a composite lasing medium, the composite lasing medium comprising a plurality of separated lasing mediums in series, the separated mediums being so arranged that each medium traversed by the excitation transmits those regions of the excitation spectrum required for the excitation of the main pumping bands of all the mediums further from the source of the optical excitation.
2. A composite lasing medium, for use in a laser, the composite lasing medium comprising a plurality of separated lasing mediums, the separated lasing mediums being so arranged that the excitation to produce lasing action traverses the mediums in series, the separated lasing mediums being selected so that each medium traversed by the excitation transmits the excitation of all the mediums further from the source of excitation.
3. A composite lasing medium as claimed in claim 2 in which the excitation is provided by electromagnetic radiation.
4. A composite lasing medium as claimed in claim 2 in which the excitation is electromagnetic radiation provided by a pulsed inert gas discharge lamp or lamps.
5. A composite lasing medium as claimed in claim 2 in which the separated lasing mediums are gaseous, liquidagel or solid lasing mediums or the composite medium is formed of any combination of gaseous, lquid, gel or solid lasing mediums, the gaseous and liquid lasing mediums being contained in a cell.
6. An optically excited pulsed laser according to any preceding claim in which the said means for illuminating the lasing mediums comprises one or more pulsed inert gas discharge lamps -
7. An optically excited pulsed laser substantially as hereinbefore described with reference to Figures ta, 2b, 2c and td of the drawings.

6. A composite lasing medium as claimed in claim 5 in which the gas or liquid separated lasing mediums are circulated through the cell and flow parallel to the axis of the laser output.

7. A composite lasing medium as claimed in claim 5 in which the gas or liquid separated lasing mediums are circulated through the cell and flow transversely to the axis of the laser output.

8. A composite lasing medium as claimed in claim 2 in which thegepuated lasing mediums are arranged concentrically about the axis of the laser output.

9. A composite lasing medium as claimed in claim 2 in which the separated lasing mediums are of elongated cuboid shape, the long axis of each cuboid lying parallel to the axis of the laser output.

10. A composite lasing medium, for use in a laser, the composite lasing medium comprising a plurality of separated lasing mediums, the separated lasing mediums being operably selected such that in use, during the serial traversal of the separated lasing mediums by the excitation to produce lasing action, each medium transmits the excitation of all the mediums further from the source of excitation.

11. A composite lasing medium as claimed in claim 10, the separated lasing mediums being further selected such that in use the random flourescent output of any one of the separated lasing mediums may provide excitation for another or others of the separated lasing mediums.

12. A composite lasing medium is claimed in claim 11, the separated lasing mediums being further selected such that in use components of the excitation able to deleteriously effect the separated lasing mediums further from the source of excitation are absorbed by the separated lasing mediums closer to the source of excitation.

13. A composite lasing medium as claimed in claim 11, the separated lasing mediums being further selected such that in use the random fluorescent emission spectra do not overlap, thus maximising the wavelength range available in the lasing output. 14. A composite lasing medium as in claim 11 or claim 12 or claim 13 in which the excitation to produce lasing action is provided by a pulsed inert gas discharge lamp or lamps.

15. A laser having a composite lasing medium as hereinbefore described and as illustrated with reference to the drawings in Figure 1A, B and C or Figures 2A, B, C and D or Figures 3A, B, C and D.

Amendments to the claims have been filed as follows 1. An optically excited pulsed laser having: an elongate light-emitting unit defining an axial direction and comprising a plurality of different, transversely-separated lasing mediums of elongated cuboid shape, the long axis of each cuboid being parallel to the axial direction; and means for illuminating by excitation radiation the lasing mediums in series and generally transversely with respect to the axial direction; the excitation radiation spectrum being such as to produce from each of the lasing mediums a simultaneous laser output emitted in an axial direction from the ends of the light-emitting unit, and the lasing mediums being so arranged that each medium traversed by the excitation radiation transmits those regions of the excitation spectrum required for the excitation of the main pumping bands of all the mediums further from the illuminating means whilst absorbing components of the excitation radiation which might affect deleteriously such further mediums.

2. An optically excited pulsed laser according to Claim 1 in which one or more of the said lasing mediums is a fluid circulated through a cell parallel to the said axial direction.

3. An optically excited pulsed laser according to Claim 1 in which one or more of the said lasing mediums is a fluid circulated through a cell in a direction transverse to the said axial direction and the general direction of the excitation radiation.

4. An optically excited pulsed laser according to any preceding claim in which the said lasing mediums are further selected so that in use the random fluorescent output of one or more of the mediums will provide excitation for one or more of the other mediums.

5. An optically excited pulsed laser according to any preceding claim in which the said lasing mediums are further selected so that in use their laser output spectra do not overlap, thus maximising the wavelength range available in the laser output.