AU580713B2 - Mode locked pulsed dye laser - Google Patents

Description

MODE LOCKED PULSED DYE LASER

The present invention relates to Dye Lasers intended for the production of a pulsed light output at a particular wave length.

While the invention is applicable generally within the scope thereof as defined below, it will be described with reference to a dye laser which is pumped with the 511 and 578nm laser emissions from a pulsed copper vapour laser and locked to the 628nm laser emission of gold. The copper and gold are located in spatially separated regions within the same discharge tube.

Although described in these terms for the purpose of illustration, the invention is not limited to these particular elements. In particular, the invention includes such situations as (i) mixtures of copper and elements other than gold in the same discharge tube and (ii) the replacement of both the copper and the element producing the locking wavelength with other atoms or ions with desirable properties e.g. mixtures of strontium and calcium.

One particular application of a dye laser according to the invention is in the treatment of tumours. It is now established that the use of lasers to illuminate tumours previously labelled with suitable dyes is an effective method of treating cancer (see, for example; "Photodynamic Therapy", T.J. Dougherty, Innovations in Radiation Oncology, Edited by L.J. Peters, Springer Verlag, New York 1984). Both continuous and pulsed light have been used. Continuous light of suitable wavelength may be obtained either from conventional discharge lamps of high intensity or from a cw dye laser pumped by an argon ion laser. Pulsed light of the desired wavelength and intensity is obtained either by using a pulsed copper vapour laser to pump a wavelength selectable dye laser, or, more simply, by using the direct output from a pulsed gold vapour laser (Cowled, Grace _ Forbes, Photochemistry and Photobiology, 3__ , no.l, pp 115-117, 1984). In all such cases the wavelength must be chosen within the narrow limits established by the need to obtain adequate penetration of the tissue and by the fact that the wavelength must lie within the absorption band of the dye used to label the tumour. It is found in practice that the wavelengths around the 628nm emitted by the gold vapour laser are particularly desirable.

In addition there is increasing evidence that the use of pulsed light is more effective than the use of continuous light. The use of a pulsed laser emitting at 628nm is very desirable on both these grounds. unfortunately, it is found in practice that the gold vapour laser, which directly emits pulsed 628nm radiation, is (i) expensive to purchase and maintain (because of the quantity of gold needed to ensure adequate working lifetime on a single fill of metal) , (ii) inefficient (thus requiring large electrical supplies and large quantities of cooling water) and (iii) unreliable (because the very high operating temperature of greater than 1750 degrees celsius places great thermal stress on the construction materials and the difficult electrical excitation conditions required limit the lifetime of the electrical switching components) .

It is one of the aims of the present invention to ameliorate these difficulties associated with using the direct 628nm emission from a gold vapour laser.

The present invention consists in laser apparatus for producing a pulsed light output at a desired frequency consisting of a single laser plasma tube having spatially separated regions containing different active media such that the output of the tube contains frequencies characteristic of each medium, one or more dye cells or dye jets and means to apply said pulsed light output to the dye cell or cells or dye jet or jets in such a manner that it or they is or are pumped at a frequency characteristic of one medium to produce an amplified output at a frequency characteristic of the other.

In order that the nature of the invention may be better understood preferred forms thereof are hereinafter described by way of example with reference to the accompanying drawings in which:-

Fig. 1 is a cross-sectional view of a discharge tube for use in apparatus according to the invention;

Fig. 2 is a diagram illustrating the use of the apparatus of Fig. 1 in connection with two dye cells;

Fig. 3 is a diagram similar to Fig. 2 illustrating an alternative arrangement; and

Fig. 4 is a diagram illustrating further arrangement in which a dye cell or jet is pumped longitudinally. Description of the Present Invention

A feature of preferred forms of the invention is the use of two laser wavelengths emitted by different elements contained within a single laser plasma tube, with the wavelength emitted with greater output power being used to pump a dye laser which is frequency locked to the wavelength emitted with the lesser power.

The multi-wavelength pump source is envisioned in particular as a multi-element discharge excited metal vapour laser. The generation of a number of wavelengths from a single discharge tube has the advantage, over a multiplicity of individual lasers, of only requiring a single pulsed discharge switch, gas handling equipment and auxiliary control system.

Although the present invention is not limited to these elements, the description is given in terms of a copper pumped dye laser which is locked to the 628nm emission of gold, the gold being contained within a spatially separated section of the copper laser discharge tube.

As compared with a gold vapour laser a copper laser is (i) less costly, (ii) more efficient and (iii) more reliable. It is thus a more practical source of pulsed laser radiation than a gold vapour laser.

In addition, as only a weak output is needed from the gold atoms in the discharge tube, the gold element can be operated at a temperature well below that needed to obtain maximum 628nm output power. This has the very significant advantages of reducing the required gold vapour pressure, thereby increasing the gold fill lifetime, and of reducing the thermal and mechanical stress on the gold section to approximately that of a copper laser section. This further increases the reliability of the invention when emission from the gold atom is used to provide the frequency locking wavelength.

To be useful in the medical application described above, the output of the copper vapour laser must be used to pump a dye laser and the output of the latter selected in some way to a wavelength near 628nm. There are two principal ways of confining the output of a dye laser to a narrow wavelength interval. Either one uses a tuning mechanism consisting of a wavelength selective device such as a rotatable prism or diffraction grating, together with other optical elements, or one injects a weak beam with a narrow wavelength interval into the dye cell and the output then becomes locked to that wavelength. The former method is less efficient than the latter and usually requires frequent and careful alignment of the optical elements. The frequency locking method is adopted in the present invention.

Figure 1 shows a diagram of a discharge tube suitable for simultaneously obtaining substantial output power from copper atoms and a small amount of laser power from gold atoms contained in a spatially separated region within the same discharge vessel.

The essential features of such a discharge tube as used to obtain pulsed metal vapour laser action are well known in the literature and are present in this device. The drawing shown also incorporates the cold-electrode design described in a previous patent (Application No. PH04674) . The idea of using spatially separated regions containing different active media is also known in the scientific literature (Evtushenko, F.S. et al, Zhurnal Prikladnoi Spektroskopiya, __9_, no.6 pp 939-944 1983). The copper and gold must be in spatially separated regions so that conditions for laser output may be separately optimised by adjustment of the depth of the insulation and other thermal properties. If this is not done and both media are contained within the same section, a common temperature would be reached and at this temperature the vapour pressure of the copper would be much larger than that of the gold and the discharge would be completely dominated by the copper and insufficient excitation of the gold would occur. Since the energy required to excite the gold atoms is greater than that of the copper, the section containing the gold is preferably located at the cathode end of the structure so as to make use of the greater number of high energy electrons present in this region of the discharge. Alternatively, as only weak gold laser output is needed, the gold discharge region may be located in the centre of two separate copper sections; this reduces the amount of energy lost at the ends of the section and thus minimises the amount of energy needed to maintain the gold section at the correct operating temperature.

Figure 2 shows a diagram of how the larger amount of copper vapour laser output is used to pump two dye cell amplifiers which are locked to the weaker output obtained from the gold atoms. It is obvious that one can use either a single, or a number of, dye cell amplifiers.

The wavelength selective properties of mirror 1 allow transmission of the copper laser output at 511 and 578nm to mirror 2 whilst directing the gold laser output through the spatial filter 3 and thence into the transversely pumped dye cell amplifiers 4 and 5. Approximately one third of the copper laser output is directed by mirror 6 and focussing lens 7 into the dye cell 4. In this way the weak gold laser radiation at 628nm emitted by the discharge tube passes through two stages of amplification and results in a beam of substantial output power which is frequency locked to the 628nm line. The dyes used in the dye cell amplifiers can be either single dye^s or a mixture of dyes chosen to maximise the efficiency of the amplifier by matching the absorption and emission characteristics of the dye mix to the pump and frequency locking wavelengths generated by the discharge tube. For.example, the maximum absorption of the dye rhodamine 590 is close to the 511nm copper laser wavelength, but it is the dyes rhodamine 640 or kiton red that have maximum emissiog at the 628nm gold laser wavelength. A mixture of these dyes in the one dye cell amplifier enhances the amount of frequency locked 628nm radiation that is generated.

Typically, the amplification of the 628nm emission will be about 10X in the dye cell 4 and 3X in dye cell 5, with the final laser output being about 30% of the original copper vapour laser output.

Another advantage of this technique is shown in figure 3 where a number of dye cell amplifiers of both the 628nm gold laser wavelength and the 578nm copper laser wavelength are used to provide a very flexible three colour system with the simplicity and reliability of a copper vapour laser. The beam steering optics can be adjusted so as to provide significant output powers at 511, 578 or 628nm.

Figure 4 shows an essentially similar arrangement which differs from that described above only in that the dye amplifiers may be longitudinally rather transversely pumped. This allows the use of either alternative dye cell designs or the use of the well known dye jet as the amplifier medium. A particular advantage of the longitudinally pumped amplifier is the resulting high quality of the output beam.

Although these arrangements contain more optical elements than a single discharge laser directly emitting - all of .its power on the 628nm gold transition, the present design is more reliable and of comparable overall efficiency. The greater reliability is due to the greater reliability of the copper laser and the fact that the dye amplifiers contain only passive components of high ^' intrinsic reliability. The efficiency is comparable to the ^" direct gold system because the copper vapour laser is more efficient than the gold laser and because the dye cell amplifiers are also reasonably efficient. The fact that the designs use only amplifiers rather than oscillator/ amplifier configurations also reduces the cost of the optics and relaxes the need for precise alignment of the optical components. Finally, the fact that only weak gold laser output is required means that a low vapour pressure of gold will suffice and this lowers the operating costs by reducing the amount of gold needed for a metal fill and the amount of gold which is lost from the system. As the temperature of the gold element of the' discharge tube needed to produce the low gold vapour pressure is similar to that of the copper vapour element (1600C) the entire discharge tube can be constructed from materials that exhibit high reliability at this temperature.

Claims (8)

CLAIMS : -

1. A laser apparatus for producing a pulsed light output at a desired frequency consisting of a single laser plasma tube having spatially separated regions containing different active media such that the output of the tube contains frequencies characteristic of each media, one or more dye cells or dye jets and means to apply said pulsed light output to the dye cell or cells or dye jet or jets in such a manner that it or they is or are pumped at a frequency characteristic of one medium to produce an amplified output at a frequency characteristic of the other.

2. An apparatus according to claim 1, wherein said different active media are copper and gold.

3. An apparatus according to claim 2, wherein the gold is located in said spatially separated region being closest to the cathode end of said single laser plasma tube.

4. An apparatus according to claim 2, wherein the gold is located in said spatially separated region being in the middle of two separate copper regions.

5. An apparatus according to any one or more of the preceding claims, wherein the wavelength output of said dye cell or cells or dye jet or jets is selected by frequency locking means.

6. An apparatus according to any one or more of the preceding claims, wherein said dye cell or cells contain a mixture of dyes.

7. An apparatus according to any one or more of the preceding claims wherein said dye cell or cells are longitudinally pumped.

8. An apparatus for treating the human body according to any one or more of the preceding claims.