GB2179489A - Laser apparatus - Google Patents

Abstract

Laser apparatus includes an active medium 9, the constituents of which are substantially fixed relative to each other. Optical pumping energy 11 is supplied by another laser 10 to the active medium 9 which is arranged to move relative to the location at which the pumping energy 11 is supplied. Thus where absorbing states are formed within the active medium, they can be removed from the pumping region at which the energy 11 is supplied and substituted with ground state molecules. Use of the active medium 9 in which its constituents are substantially fixed relative to each other, enables control over its movement to be improved over that which would be possible were it in a liquid state. <IMAGE>

Description

SPECIFICATION Laser Apparatus This invention relates to laser apparatus and is particularly, but not exclusively, applicable to dye laser apparatus.

A dye laser is a device in which certain types of organic molecules are used as the laser active medium and which typically permit tuning of the output laser radiation. Such lasers are termed "dye lasers" because the organic molecules which are active in the laser action are generally those which have good dye properties, but other molecules may also be suitable. The organic molecules of the dye have many rotational and vibrational modes giving a large number of energy levels. These are illustrated in Fig. 1, which shows energy levels of a typical organic dye molecule. Four electronic states are shown, each of these including a number of closely spaced energy levels which effectively form a continuum.

Two of the states shown SO and S1 are singlet states and the other two TO and T1 are triplet states. Molecules which are excited to higher energy levels of an electronic state rapidly iose energy by thermalisation processes until they reach the lower energy level of that electronic state. These transitions are illustrated by arrows 1.

In the laser system shown, pumping energy is applied to the active medium to achieve a population inversion between the energy levels within the electronic states S1 and SO, laser action then occuring as a result of transition between these two states. Pumping is by absorption of optical energy and causes excitation of molecules from the SO state to the S1 state. Thermalisation processes ensure that molecules in the S1 state return to the lowest energy level of S1 which acts as the upper laser level. The lower level of the laser transition may be any of the energy levels of the electronic state SO, permitting tuning across a wide range of wavelengths.This tuning is achieved by including a dispersive component such as a prism or diffraction grating within the laser cavity to ensure that only light at a particular wavelength can oscillate within the laser cavity.

One problem with this system is that there is a possibility of non-radiative transitions occurring between the S1 electronic state and the TO electronic state. The population of the TO state increases in competition with the laser transition from S1 to SO. A further problem is that the state TO has a longer lifetime than the S1 state. Thus molecules in this state can absorb photons of suitable energy and so be excited to energy state T1. In many dye molecules the energies of photons from the transition S1 to SO are the same as the energies for the transitions from TO to T1. Thus photons obtained by laser transitions from S1 to SO can be absorbed by molecules in the state TO.It is therefore desirable to remove the molecules in state TO from the region of laser radiation. One way in which continuous operation of the laser action is made possible is to flow the dye molecules of the active medium, which is liquid, through the laser cavity to physically remove those molecules in the TO state from the pumping energy and to replace them with molecules in the SO state.

The present invention seeks to provide improved laser apparatus.

According to this invention there is provided laser apparatus comprising: a laser including an active medium having constituents which are substantially fixed relative to each other; and means for moving the active medium relative to a location at which pumping energy is applied to the active medium. The constituents referred to may be molecules, atoms or other particles which take part in the laser mechanism. They are substantially fixed such that they remain in substantially the same location in relation to neighbouring constituents but are sufficiently mobile to permit vibrational and/or rotational modes desired for laser action to exist.Thus where an active medium has populated states which can absorb photons of energy corresponding to the energy of the laser transition it can be moved relative to the region where pumping energy is applied, to enable part of the active medium having constituents in a ground state to be supplied with pumping energy. By employing the invention, problems associated with the liquidity of an active medium, such as control of the flow of the active medium, are avoided. Preferably, the constituents of the active medium are substantially fixed by means of a carrier medium, and it is preferred that the carrier medium is a suitable plastic.

Preferably, movement of the active medium is arranged to be rotational i.e. there is a continuous movement in one direction, this being a particularly advantageous mechanical arrangement. Advantageously, the active medium is arranged upon a substrate to give it rigidity and robustness.

Preferably, the active medium is a dye. The invention is particularly suitable where large organic dye molecules take part in the laser action and where movement of the dye molecules from the location at which pumping energy is applied is necessary for continuous laser action.

The pumping energy may be optical energy, and it may be convenient to arrange another laser to supply the pumping energy.

The invention is now further described with reference to Figs 2, 2a and 3 of the accompanying drawings in which: Figure 2 schematically illustrates apparatus in accordance with the invention; Figure 2a schematically shows part of the apparatus of Fig. 2 in greater detail; and Figure 3 illustrates part of another apparatus in accordance with the invention.

With reference to Figs. 2 and 2a laser apparatus includes a laser 2 which uses an organic dye, Rhodamine 6G, as its active medium.

Rhodamine 6G has electronic states similar to those described previously with reference to Fig. 1, and quenching may occur due to depopulation of the S1 state into the TO state. The laser 2 comprises two mirrors 3 and 4 between which laser action occurs. A diffraction grating 5 is arranged between the two mirrors 3 and 4 to select a desired wavelength of the output laser radiation 6, which passes through the partially reflecting mirror 4.

A rotating member 7, which is disk-like and is of quartz, is also located between the two mirrors 3 and 4 and is rotatable about a shaft 8 in the direction shown by the arrow. The rotating member 7, is arranged to support the active medium such that it moves with the member 7. The constituents or molecules of the active medium are substantially fixed relative to each other by a carrier medium which is a plastic to give a dye bearing plastic film 9. The dye bearing plastic film 9 coats one surface of the member 7. An argon ion laser 10 is used to supply the optical pumping energy required for laser operation, its output beam 11 being focused to irradiate an area of about lum in diameter on a part of the dye bearing plastic film 9 near the circumference of the disk.

During operation of the laser apparatus, the member 7, and hence the film 9 is arranged to rotate in the direction indicated by the arrow, such that the part of the dye to which the pumping energy is applied is continually changed. Thus molecules which are in the TO state are moved from the location at which the optical pumping energy is applied before they become a significant number and quenching of the laser action. Thus continuous laser operation is possible. The diameter of the member 7 is selected to give the required speed of movement of the dye through the location at which pumping energy is applied which in turn depends on the lifetimes of the states of the active medium. A typical speed of rotation which may be required at that location could be 100ms ', which would exist at the circumference of a disc having a radius of 5cm and being rotated at 2.104 revolutions per minute.

With reference to Fig. 3 in another laser apparatus the active medium 12 is carried by a member 13 which is in the form of a flexible belt and is arranged to rotate about two rollers 14 and 15 which are spaced apart. The optical pumping energy is arranged to impinge on the active medium 12 at a location 16 as illustrated by the arrow. Of course, other arrangements are possible, such as that in which the belt itself is dye-bearing plastic.

Claims (11)

1. Laser apparatus comprising: a laser including an active medium having constituents which are substantially fixed relative to each other; and means for moving the active medium to a location at which pumping energy is applied to the active medium.

2. Apparatus as claimed in claim 1 and wherein the constituents of the active medium are substantially fixed by means of a carrier medium.

3. Apparatus as claimed in claim 2 and wherein the setting medium is plastic.

4. Apparatus as claimed in any preceding claim and wherein the active medium is arrange to move through the said location.

5. Apparatus as claimed in any preceding claim and wherein movement of the active medium is arranged to be rotational.

6. Apparatus as claimed in any preceding claim and wherein the active medium is arranged upon a substrate.

7. Apparatus as claimed in any preceding claim and wherein the active medium is a dye.

8. Apparatus as claimed in any preceding claim and wherein pumping energy is optical energy.

9. Apparatus as claimed in any preceding claim and including another laser arranged to supply the pumping energy.

10. Laser apparatus substantially as illustrated in and described with reference to Figs.

2 and 2a of the accompanying drawings.

11. Laser apparatus substantially as illustrated in and described with reference to Fig.

3 of the accompanying drawings.