WO2007129069A2

WO2007129069A2 - Side-pumped laser device

Info

Publication number: WO2007129069A2
Application number: PCT/GB2007/001660
Authority: WO
Inventors: Michael John Damzen
Original assignee: Imperial Innovations Limited
Priority date: 2006-05-08
Filing date: 2007-05-08
Publication date: 2007-11-15
Also published as: GB0609061D0; WO2007129069A3

Abstract

A monolithic solid-state laser device is disclosed. The laser device is side pumped with a diode laser (32) and reflects the laser beam (29) from one face (21) of the solid state gain medium (22) to ensure gain uniformity. The solid state gain medium (22), output coupler (30) and back reflector (26) are integrally formed so that the resulting monolithic laser design provides a compact, rugged and self-contained laser device.

Description

SIDE-PUMPED LASER DEVICE

BACKGROUND OFTHE INVENTION

The invention relates to a side-pumped laser device.

Solid state lasers are an important class of laser source that exhibit high power emission over a broad range of wavelengths. A solid state laser comprises a gain medium that provides the laser amplification, a resonator for providing optical feedback and a pump source. The gain medium comprises a host material, such as a crystal or glass, that is doped with an active ion. The active ion absorbs light at a pump wavelength and emits light at a laser wavelength. The laser resonator is typically formed with dielectric or metallic mirrors. The pump source provides the light at the pump wavelength, which must then be launched into the gain medium where it is absorbed by the active ions. Diode lasers provide highly efficient pump sources for solid state lasers. This is because diode lasers exhibit a number of desirable characteristics when used as a pump source, including: high-power, high directionality and high efficiency. Furthermore, the composition of a diode laser can be altered so that the spectral range of the emitted light is substantially within the narrow range of wavelengths absorbed by the active ions.

Two main pump geometries have been developed in order to launch the light emitted by the diode laser into the gain medium. Namely, end-pumping and side-pumping. In end-pumping, the light emitted by the diode laser follows a path that is substantially co-linear with the path that the laser follows through the laser resonator. In side- pumping, the light emitted from the diode laser follows a path that is not co-linear with the path that the laser follows through the laser resonator. In fact, side-pumping is frequently performed by launching the pump light at an angle perpendicular to the path that the laser follows through the laser resonator. The light emitted by a diode laser exhibits a poor beam quality, so it is difficult to focus the pump light into the small area required by end-pumping. Furthermore, high-power end-pumping risks overloading the thermal properties of the gain medium, resulting in the destruction of the gain medium or other undesirable thermal effects. By contrast, side-pumping is not limited by the need to focus to a small area. Thus, the pump light may be launched along an area spanning the entire length of the gain medium, hi comparison to end- pumping, side-pumping allows significantly higher pump powers to be launched into the gain medium, which leads to the possibility of a significantly higher output laser power.

However, side pumping is not without problems. After being launched into the gain medium, the light emitted from the diode laser is progressively absorbed across the gain medium. For this reason, the intensity is at a maximum at the side of the gain medium nearest the diode laser, and at a minimum at the side furthest from the diode laser. The local population inversion, and thus the local gain, is dependent on the local intensity of the pump. Thus, a side-pumped geometry causes the local gain to vary across the cross-section of the laser. There is also a nonuniformity in the heating and temperature across the cross-section of gain medium that induces non uniform refractive changes. The non-uniform gain and thermally induced refractive index changes have a detrimental effect on the stability of the laser resonator and the beam quality of the emitted laser radiation.

One solution to overcome the effects of non uniformity is to employ a zig-zag laser arrangement in which the beam reflects alternatively from the pump face and the face that is opposite the pump face. This arrangement can lead to averaging of the non uniformity experienced by the zig-zag beam path. This arrangement is usually employed when the gain region extends over most of the cross section of the gain medium and when the gain medium has a long length to allow multiple alternate reflection from the parallel faces of the gain medium.

An alternative solution is required when the gain media exhibits a strong absorption of the pump light, which may absorb the light emitted by the diode laser over very short distances (e.g. of only a few hundred microns) whilst the gain medium cross section may be many times longer than this (e.g. several millimetres). An alternative geometry in this case is the bounce geometry where the beam to be amplified undergoes a single reflection at grazing angle of incidence from the pump face in any 5 single transit through the gain medium.

Figure 1 shows a prior art bounce geometry laser 2 with a gain medium 4 having a pump face 6, a diode laser 8, lenses 10, a first mirror 12 and a second mirror 14. The diagram also shows the path 16 of the laser through the bounce geometry laser 2, the

10 bounce 18 that occurs at the pump face 6 of the gain medium 4 and the laser emission 19. The bounce geometry is used to provide a more uniform gain [1, 2, 3]. A laser resonator is formed between the first mirror 12 and the second mirror 14. At least one of the mirrors has a reflectivity of less than 100%. The laser emission 19 is emitted through this mirror. The lenses 10 are used to control the spatial modes that oscillate

15 in the laser resonator by altering the spot size (i.e. diameter) of the laser radiation within the cavity to match the spot size of the desired spatial mode. The light emitted by the diode laser 8 is launched into the gain medium 4 through the pump face 6. Thus, the side of the laser that is closest to the pump face 6 will experience a significantly higher gain than the side of the laser furthest from the pump face 6. The

20 bounce 18 in the laser path 16 is caused by a reflection at the pump face 6. The bounce 18 has the effect of reversing the side of the laser that is closest to the pump face and the side of the laser that is furthest from the pump face, hi this way, the bounce geometry leads to the laser experiencing a more uniform gain over its cross- sectional area. The bounce geometry laser has been shown to offer some

25 considerable advantages when the gain medium is highly absorbing at the pump wavelength such that the gain is intense within a shallow (few hundred micrometers) inside the pump face and the angle of reflection from the pump face is at a small grazing incidence angle with respect to the pump face (i.e. it is a large angle near 90 degrees with respect to the normal to pump face). As well as providing averaging of

30. . . the non-uniform gain and refractive index, this geometry gives considerable other -A-

advantages. The use of the grazing incidence angle has the advantage of projecting a small diameter laser beam across a long length of pump face. This favours selection of a high spatial quality low order mode that can be most efficiently amplified by the gain medium at grazing angle of incidence. Another advantage of this single bounce geometry is the high gain achieved by the highly absorbing gain medium. The single bounce grazing incidence geometry in a gain medium that is highly absorbing to the pump wavelength is therefore distinct from the conventional zig-zag laser which uses an angle of incidence that is usually much larger than grazing incidence to achieve the multiple reflections from alternative parallel sides of the gain medium and requires a physically long length of gain medium.

The bounce geometry laser 2 provides improved gain uniformity, in combination with the benefits of high gain, high output power and good efficiency in a relatively short length of gain medium that may be achieved by side pumping a solid state laser with a diode laser that is highly absorbed in the gain medium. However, applications also require the lasers to be rugged, compact and self-contained.

SUMMARY OF THE INVENTION

The invention provides a side-pumped laser device comprising: a solid-state laser gain medium; a pump source arranged to side pump radiation into the gain medium to induce population inversion in a shallow region of the gain medium adjacent a first face of the gain medium; a back reflector integrally formed with a second face of the gain medium; and an output coupler integrally formed with a third face of the gain medium, the first, second and third faces being angled to form a resonator defining a beam path contained substantially within the gain medium that is incident upon and reflected from the first face eg at a grazing incidence angle.

Forming the back reflector and output coupler integrally with the gain medium in this way allows a highly compact laser device to be provided. Furthermore, the laser device is relatively unaffected by vibration because the integral formation of the gain medium, the back reflector and the output coupler fixes the alignment between these components. This also improves the reliability and longevity of the device, since re- tuning of the alignment is not required and, indeed, is not possible. The compact size of the laser device means that any changes in the environment surrounding the laser device (such as changes in temperature or humidity) effect all of the components equally, and thus have a minimal effect on the operation of the laser device. Furthermore, the air gaps that separate the resonator components of a conventional laser device are highly susceptible to changes in the surrounding atmosphere. The integral formation of the gain medium, the back reflector and the output coupler, thus further improves the stability of operation. The integral formation between the gain medium, the back reflector and the output coupler allows a straightforward packaging of the device with the pump source, so that the laser device may be supplied as a compact and self-contained product.

A key advantage of this invention is its ability to achieve a relatively high spatial quality from a short length of the gain medium. The monolithic gain medium for practical reasons may necessitate a short length of gain material. It is known to those skilled in the art of lasers that a short laser resonator length tends to lead to a small diameter lowest order (TEM₀₀) mode size. For most applications it is desirable for the laser to operate in this lowest single order of mode or in a small subset of the lowest mode orders. This provides radiation output with good spatial diffraction properties of low divergence spreading and good ability to be focused to a small spot size, The gain region must be very small in size if it is provide gain predominately to select just the lowest order mode of modes which have the smallest size. The grazing incidence geometry provides this apparent small aperture as seen by the cavity beam even for a long length of pump face. For instance, at a grazing incidence angle of a few degrees with respect to the pump face a beam of diameter d can overlap with a pump face length of 10-100 times d. The long pump face length allows the use of high power diode pumping and leading to high laser output capability. TMs means that the single bounce laser of this invention can provide a high spatial quality at high power even for a very compact monolithic cavity as described in this invention.

In one embodiment, the beam path is incident on the first face at an angle greater than the critical angle. The conventional definition of the critical angle θ_c, which we adopt throughout this document, is the angle formed between the beam path and the normal to the pump face (i.e. the first face of the gain medium). Under this definition, total internal reflection will occur if the beam path is incident on the pump face at an angle equal to, or greater than, the critical angle. If the beam path is incident on the pump face at an angle less than the critical angle, the beam will not be fully reflected but also partially refracted through the surface. It will be understood that the critical angle depends on the refractive index of the gain medium and the refractive index of the medium that lies on the other side of the pump face, which may be air, another gaseous medium or vacuum, or may be another material in contact with the pump face. The mathematical relationship for the critical angle is sin θ_c = n₂/n_t where n_\ is the refractive index of the gain medium and n₂ is the refractive index of the material in contact with the pump face at which the bounce occurs. In other embodiments, grazing incidence reflection could be used, hi these embodiments, the angle between the beam path and the normal to the pump face is almost perpendicular, so that the beam follows a path that is almost parallel with the pump face.

hi a further embodiment, at least one of the back reflector and the output coupler is formed by coating the second and third faces respectively with a reflective material, hi this embodiment, the back reflector and the output coupler are formed by coating or growing a dielectric or a metallic film onto the end faces of the gain medium. The reflectivity of the back reflector and the output coupler may be controlled during the coating or growth dependent on the desired feedback in the laser resonator.

hi a further embodiment, at least one of the back reflector and the output coupler is formed by adhering or bonding a prefabricated reflective material onto the second and third faces respectively. The prefabricated reflective material may be a dielectric mirror formed on a glass slide, or a specular mirror formed by coating a glass slide with a metallic material. The prefabricated reflective material may then be bonded to the end faces of the gain medium.

In one embodiment, one or both of the second and third faces are in optical contact with a spacer element that spatially separates the second and third faces from a reflecting surface of the back reflector and the output coupler respectively so that the beam path extends into the spacer element. The spacer element may be introduced for purposes such as countering thermal effects in the vicinity of the back reflector and the output coupler. Alternatively, the spacer element may exhibit a non-linear effect used to provide further functionality from the laser device, such as Q-switching, mode-locking or frequency conversion.

In one embodiment, the first, second and third faces are arranged and configured to form a linear resonator, hi this type of resonator, the beam path followed in the first half of one round-trip around the resonator is identical to the beam path followed in the second half of one round-trip around the resonator (although the direction of travel in the second half is opposite to the direction of travel in the first half). One round-trip is defined as the path that the beam follows inside the resonator, starting from any point in the beam path and following the beam path around the resonator until the beam arrives back at the starting point (assuming that no light is emitted by either the output coupler or the back reflector). Alternatively, in a different embodiment the first, second and third faces are arranged and configured to form a ring resonator. In this type of resonator, the beam path followed in the first half of one round-trip around the resonator is different to the beam path followed in the second half of one round- trip around the resonator.

In one embodiment an inter-cavity reflector is congruent with a fourth face of the gain medium. Li this embodiment, an additional intermediate reflection occurs at the fourth face of the gain medium. In an alternative embodiment, the beam path is incident on the fourth face at an angle greater than the critical angle so that the additional intermediate reflection occurs by total internal reflection. In a further embodiment, a second pump source is arranged to side pump radiation into the gain medium to induce population inversion in a region of the gain medium adjacent to the fourth face of the gain medium. Thus, amplification can also occur during transit of the laser in the proximity of the fourth face of the gain medium.

hi one embodiment the solid-state laser gain medium comprises a host material of any one of yttrium aluminium garnet, yttrium vanadate, gadolinium vanadate, yttrium lithium fluoride, sapphire or a glass. In one embodiment the solid-state laser gain medium is doped with an active ion comprising any one, or any combination, of neodymium, praseodymium, erbium, thulium, ytterbium, samarium, dysprosium or titanium.

In another embodiment the solid-state laser gain medium exhibits an absorption coefficient defined at the wavelength of the pump radiation greater than or equal to 5 cm^"1. hi this embodiment, the solid state laser gain medium is defined with reference to the absorption coefficient, which is a term that is well known in the art and is dependent upon the characteristics of the host material and the characteristics and doping concentration of the active ion.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the invention and to show how the same may be carried into effect reference is now made by way of example to the accompanying drawings.

Figure 1 shows a prior art bounce geometry laser with an extended cavity. Figure 2 shows a plan view of the monolithic bounce geometry laser of a first embodiment of the present invention.

Figure 3 shows a graph of output power against diode pump power for a monolithic bounce geometry laser of a first embodiment of the present invention.

Figure 4 illustrates a part of the device of Figure 2 in order to show geometric definitions.

I Figure 5 shows a cross-section through the monolithic bounce geometry laser of Figure 2.

Figure 6A shows in further detail a coated mirror applied to an end face of the monolithic bounce geometry laser of Figure 2.

Figure 6B shows in further detail a coated mirror and a spacer layer applied to an end face of the monolithic bounce geometry laser of Figure 2 as an alternative to Figure 6A.

Figure 7 shows in plan view a monolithic bounce geometry laser of a second embodiment of the present invention.

Figure 8A shows a cross-section through the monolithic bounce geometry laser and a possible cooling system for the monolithic bounce geometry laser.

Figure 8B shows a plan view of the monolithic bounce geometry laser and a possible cooling system for the monolithic bounce geometry laser. DETAILED DESCRIPTION

Figure 2 shows a plan view of a monolithic bounce geometry laser 20 of a first embodiment with a gain medium 22 having a first end face 24 with a first mirror 26, a second end face 28 with a second mirror 30 a pump face 25 and a third end face 27.

The pump light emitted by a diode laser 32 is focussed by a optical system 34 and launched into the gain medium 22 through the pump face 25. The path 29 that the laser follows inside the resonator is dependent on the relative angles between the first end face 24, the second end face 28 and the pump face 25. The laser emission 31 occurs through the second mirror 30. Thus, the second mirror 30 is an output coupler and the first mirror 26 is a back reflector.

The term monolithic is used to describe a laser resonator in which each of the resonator components are integrally formed so that the resonator components are in physical connection. For this reason, during a round trip in the resonator, the beam follows a path that passes from one resonator component to the next without passing through a region of the surrounding atmosphere, with the obvious exception of any emission that occurs through either of the mirrors. The first mirror 26 is bonded directly onto the first end face 24 and the second mirror 30 is bonded directly onto the second end face 28. The reflectivity of the first mirror and the second mirror controls the fraction of optical power inside the resonator that is fed back into the resonator and the fraction that is emitted as laser radiation.

The monolithic design of the monolithic bounce geometry laser 20 does not generally require the lenses that are used in prior art bounce geometry lasers for spatial mode control. In general, the shorter distances traversed by the light in the monolithic bounce geometry laser 20 negate much of the divergence and other undesirable spatial effects that degrade the spatial mode. However, if focussing is deemed to be desirable, the first end face 24 and/or the second end face 28 can be polished to the required shape so that the end faces act as monolithic lenses. The gain medium 22 may be any solid state host material such as yttrium aluminium garnet, yttrium vanadate, gadolinium vanadate, yttrium lithium fluoride, sapphire or a glass doped with an active ion. Many active ions are suitable for doping in certain 5 solid state materials. The rare-earth ions neodymium, praseodymium, erbium, thulium and ytterbium are frequently used, but other known active ions such as titanium, samarium or dysprosium may also be used. Generally, it will be understood that the monolithic design of the invention is generic to any suitable solid state gain medium crystalline or other.

10

For example, the monolithic bounce geometry laser 20 may comprise a gain medium of neodymium-doped yttrium vanadate. Typically, the neodymium ion is pumped at a pump wavelength of 808nm and emits at a laser wavelength of 1064nm, although other pump and laser wavelengths may are also be used. Yttrium vanadate, when

15 doped with neodymium, exhibits a strong absorption at the 808nm pump wavelength, which ensures that the majority of the available pump power will be absorbed in a small region near to the pump face of the gain medium. The resonator properties can be tailored to ensure that the laser's path and its spot size lead to a significant overlap with the pumped region, which improves the efficiency of the laser. Suitable mirrors

20 may be coated onto the end faces of the neodymium-doped yttrium vanadate crystal using a multi-layer deposition of different dielectric materials to construct a mirror with high reflectance or partial transmission. Alternatively, neodymium-doped yttrium aluminium garnet could be used as the gain medium. Neodymium-doped yttrium aluminium garnet can also be used with the benefit that it exhibits a superior thermal

25 conductivity and physical strength than neodymium-doped yttrium vanadate, which may be an important feature for high power laser operation. The gain medium may instead comprise erbium-doped yttrium aluminium garnet. The erbium ion generally exhibits a lower efficiency in converting pump power into laser power, but has the benefit of emitting within a number of commercially important wavelength regions.

30. . . For example, erbium emits in the 1.5-1.7 micron region, which is useful for telecommunications and in applications requiring "eye safe" radiation. Another useful wavelength region of erbium is the wavelength region near 3 microns, which is useful for applications that require operation further into the infra-red, including the favourable water absorption properties and biological and chemical sensing applications. The different pump and laser wavelengths associated with an erbium- doped gain medium requires a different thickness and/or composition of the coatings that are used to construct the end face mirrors, hi these examples of gain media, the active ion dopant (e.g. neodymium or erbium, can be selected to provide a suitably strong absorption for the single bounce grazing incidence geometry.

Figure 3 shows a graph of output power against diode pump power for a monolithic bounce geometry laser 20 comprising a gain medium having an yttrium vanadate host material doped with 1.1 at% of neodymium. The end faces of the first end face 24 and the second end face 28 were angled at 7 degrees with respect to a normal to the pump face 25. The diode laser 32 consisted of a single diode bar that emitted at approximately 808nm. The first mirror 26 exhibited a reflectivity of 100% at the laser wavelength and the second mirror 30 exhibited a reflectivity of 11% at the laser wavelength. An output laser power of 21.8 W was obtained when the monolithic bounce geometry laser 20 was pumped at 4OW with the diode laser 32. However, the monolithic bounce geometry laser 20 can produce a maximum output power of greater than 10OW, dependent on the pump power, which is suitable for laser marking and laser micromachining applications. The slope efficiency, which is the gradient of the straight line shown in the graph, was 72%.

Figure 4 shows further detail of the path 29 followed by the laser inside the resonator. The path 29 comprises an incident ray 40, which makes a first angle 46 with a normal 44 to the pump face 25 of the gain medium 22, and a second angle 48 with the pump face 25 of the gain medium 22. A reflected ray 42 and the third end face 27 of the gain medium are also shown. The incident ray 40 will be reflected by total internal reflection if the first angle 46 is greater than the critical angle. The critical angle is a term that is well known in the art and is dependent on the refractive indices of the gain medium 22 and the medium surrounding the gain medium 22. Alternatively, a coating or other reflective surface 5 may be applied to the pump face 25 so that the incident ray 40 may be reflected without having to meet the requirements of total internal reflection. A so-called "grazing angle" incidence may be used. In this case, the first angle 46 is greater than 70 degrees, so that the incident ray 40 and the reflected ray 42 follow a path that is close to parallel with the pump face 25. A grazing angle bounce is frequently used

10 when the gain medium 22 exhibits strong absorption, because the absorption of the pump in these media, and thus the available gain, occurs within a short distance from the pump face 25. hi this case, a grazing bounce angle ensures that the majority of the path 29 of the laser through the gain medium is situated within a short distance from the pump face 25.

15

The first angle 46 will be the angle that is used to describe the path 29. However, it is noted that in the field of bounce geometry lasers it is common to define the bounce with reference to the second angle 48. It is clear that the first angle and the second angle are related because the normal 44 to the pump face 25, by definition, forms an

20 angle of 90 degrees with the pump face 25. Thus, the sum of the first angle 46 and the second angle 48 is 90 degrees.

Figure 5 is a cross-section through the monolithic bounce geometry laser 20, which shows the lateral positions of the diode laser 32, the optical system 34 the gain

25 medium 22 and the pump face 25. The laser is side pumped by a diode laser 32, which may be a single diode laser, a one-dimensional array of diode lasers (known as a diode-bar in the art) or a two-dimensional array of diode lasers (known as a diode stack in the art). The optical system 34 may be used to focus or reshape the output radiation of the diode laser 32 to optimise the fraction of pump light that is incident

,3ft on, and absorbed by, the gain medium 22. In its simplest form the optical system 34 may be a cylindrical lens that is used to focus or collimate the fast axis of the output radiation of the diode laser 32. Alternatively, further focussing lenses may be used to provide more control over the focussing, or to focus the pump light in both axes. The optical system 34 may be designed such that the pump light illuminates the entire pump face 25, or only part of the pump face 25. The optical system 34 may also be used to focus or reshape the pump light so that only a narrow region of the pump face 25 is illuminated. This ensures that the absorbed pump light substantially overlaps with a low order spatial mode of the laser path, thereby improving the spatial quality of the laser emission. The pump face 25 may be coated to minimise reflection of the pump light. The pump light emitted by the diode laser 32 may also be launched without a optical system 34. Although this might lead to less pump light being launched into the gain medium 22, the reduction in overall efficiency may be offset by the associated reduction in the complexity of the laser device.

Once launched into the gain medium 22, the pump light is successively absorbed by the active ions doped in the gain medium 22. The absorption is dependent on the absorption characteristics of the active ions and their concentration in the gain medium 22. As described above, the successively absorbed pump light causes the local gain to vary across the gain medium 22, starting from a maximum at the pump face 25.

Figure 6 A shows the first end face 24 of the gain medium 22 and the first mirror 26 in further detail as constructed according to one example. The first mirror 26 is in physical contact with the first end face 24. The first mirror 26 may be a dielectric mirror that is coated directly onto the first end face 24 or a reflecting surface, such as a metal mirror, that is bonded onto the first end face 24 by a process of adhesive bonding, direct (or thermal) bonding, or any other adhering mechanism. It will be appreciated that the second end face 28 and the second mirror 30 can be constructed in the same way. Figure 6B shows the first end face 24 of the gain medium 22 and the first mirror 26 as constructed according to another example alternative to that of Figure 6A. In this example, the first end face 24 and the first mirror 26 are separated by a spacer 50. A monolithic laser has all of the components in physical contact. However, this does not mean that the mirrors must be directly bonded onto the end faces of the gain medium 22. It is also possible to bond a spacer 50 between the gain medium 22 and the first mirror 22. The spacer 50 may simply be a transparent block, hi this case, the spacer 50 may be used to control thermally affected phenomena that occur at the end faces of the gain medium 22, such as thermal lensing or thermal fracture. Alternatively, the spacer 50 may be a non-linear material that is used to provide further functionality to the laser, such as Q-switching, mode-locking or frequency conversion. Intensity related non-linear materials benefit from inclusion inside the laser resonator because the intensity of the laser radiation is at its highest value inside the resonator. It will be appreciated that the second end face 28 and the second mirror 30 can be constructed in the same way by provision of a spacer.

With reference to the examples of Figure 6A and 6B, it will also be understood that different reflective faces of the same laser device can be constructed using a mixture of these construction examples.

Figure 7 shows in section a laser device of a second embodiment. In the second embodiment, the gain medium 22 has an altered rhomboidal cross-section The cross- section of the gain medium 22 is altered so that the resonator is formed between the first mirror 26 located at the first end face 24, and the second mirror 30 located at the third end face 27. The first, second and third end faces are angled so that the path 29 of the laser through the resonator performs two bounces, a first at the pump face 22, and a second at the second end face 28. The second bounce redirects the direction of the laser emission 31. Optionally, a second diode laser (not shown) is arranged to pump the second end face 28 so that further amplification occurs in a part of the path 29 adjacent to the second end face 28. A significant amount of heat is generated in the gain medium 22 during operation of the laser. This heating causes temperature gradients that lead to non-uniform 5 variations in the refractive index and the formation of stress in the gain medium. The non-uniform refractive index of the gain medium distorts the spatial mode of the laser during its transit through the gain medium, which has the effect of reducing the beam quality of the output laser. The non-uniform refractive index of the gain medium may also cause the polarisation state of the laser to become distorted, leading to

10 depolarisation. Significant temperature gradients may subject the gain medium to stresses that are sufficient to cause the gain medium to fracture. The amount of heat generated in a laser is roughly related to the pump power of the laser. Thus, the problems associated with heating of the gain medium are particularly apparent in diode-pumped solid state lasers, which typically exhibit high pump powers.

15 Dependent on the operating power of the laser, it may be necessary to control the gain medium's temperature for optimum laser operation.

Figure 8A shows a cross-section through the monolithic bounce geometry laser 20 that details a possible cooling system for the monolithic bounce geometry laser 20. The

20 gain medium 22 is side-pumped through the pump face 25 with a diode laser 32 that is focussed or reshaped by the optical system 34. A transparent cooling block 70 is placed in thermal contact with the pump face 25 and acts to cool the gain medium 22 in a lateral direction. One or more transverse cooling blocks 72 may be placed in thermal contact with an upper face 74 and a lower face 76 of the gain medium 22. A

25 lateral cooling block 78 may be placed in thermal contact with the face 80 opposite the pump face. The transverse cooling blocks 72 cool the gain medium 22 in a transverse direction that is perpendicular to the pump face 25 and also perpendicular to the direction of transit of the laser or amplifier signal through the gain medium 22. The transparent cooling block 70 and the lateral cooling block 78 cool the gain

3.O₁ medium 22 in a lateral direction. Any one, or any combination of, the transparent cooling block 70, the transverse cooling blocks 72 or the lateral cooling block 78 may be used to cool the gain medium 22. Cooling the gain medium 22 in both the lateral and transverse directions has been found to allow more efficient removal of the heat that is generated during laser operation.

The transparent cooling block 70 may be formed from any material that is transparent to the pump wavelengths emitted by the diode laser 32 that can also conduct heat away from the pump face 25. It is preferable for the transparent cooling block 70 to exhibit a good thermal conductance in order to allow heat to be transferred from the gain medium 22 more efficiently. Diamond and sapphire are both suitable materials for the transparent cooling block 70, although other materials may also be used. The transparent cooling block 70 may be adhered or bonded to the pump face 25 using any conventional mechanism known in the art, such as adhesive bonding or diffusion (or thermal) bonding. The transparent cooling block 70 may be air cooled. Alternatively, metal heat sinks (not shown) may be bonded to the transparent cooling block 70. The metal heat sinks may be air cooled, or may be cooled by pumping water, or the like, through channels in the metal heat sinks. If further faces of the gain medium are pumped then further transparent cooling blocks 70 may be added to the monolithic bounce geometry laser 20 in order to cool these further pump faces.

The transverse cooling blocks 72 may be formed from any material that is capable of transferring heat from the upper face 74 and the lower face 76 of the gain medium 22. There are fewer restrictions concerning the material used to form the transverse cooling blocks 72 than for the transparent cooling block 70, since there is no need for the transverse cooling blocks 72 to transmit at the pump wavelength. Thus, a metal such as copper or aluminium may be used to construct the transverse cooling blocks 72. The transverse cooling blocks 72 may be bonded to the upper face 74 and the lower face 76 using any conventional mechanism known in the art, such as adhesive bonding. A thin layer of graphite, indium or another suitable material may be placed between each transverse cooling block 72 and the upper face 74 and/or the lower face 76, in order to attain a superior thermal connection.

Figure 8B shows a plan view of the monolithic bounce geometry laser 20 that details a possible cooling system for the monolithic bounce geometry laser 20. The transverse cooling block 72 attached to the upper face 74 of the gain medium 22, the transparent cooling block 70 and the lateral cooling block 78 are shown. A refractive index element 82 is also shown between the pump face 25 and the transparent cooling block 70. If the material used for the transparent cooling block 70 exhibits a refractive index greater than the gain medium 22, then it is not possible to reflect the laser from the pump face 25 using the mechanism of total internal reflection. The material chosen for the refractive index element has a refractive index lower than that of the gain medium, thereby to support total internal reflection of a beam at the pump face 25. Alternatively, a reflective material may be applied to the pump face 25.

Although the above embodiments refer to a gain medium having a cross-section with a maximum of four faces, any cross-section could conceivably be used. Additionally, the number of diode lasers used to pump the cross-section is dependent only on the desired pump power and the physical limitations involved with arranging the diode lasers so that they may pump the gain medium. For example, a gain medium with an octagonal cross-section, having an output from one face, could conceivably exhibit a beam path with seven bounces. This would allow each of the faces, except the output face, to be pumped in order to increase the gain available to the laser.

It will be appreciated that the features of the embodiments discussed above may be adapted by adding features from the other embodiments. For example, the construction of the mirrors, or the mirrors and spacers shown in Figures 5A and 5B respectively may be combined with any of the embodiments shown in Figures 2, 6 and 7. REFERENCES

[1] "Ultrahigh-efficiency TEM₀₀ diode-side-pumped Nd:YVO₄ laser", Applied

Physics B, volume 76, 2003, pages 341-343

[2] "High-power TEM₀₀ grazing-incidence Nd: YVO₄ oscillators in single and multiple bounce configurations", A. Minassian, B. Thompson and MJ. Damzen, Optics Communications, volume 245, 2005, pages 295-300

[3] US-A-5,315,612

Claims

1. A side-pumped laser device comprising: a solid-state laser gain medium; a pump source arranged to side pump radiation into the gain medium to induce population inversion in a region of the gain medium adjacent a first face of the gain medium; a back reflector integrally formed with a second face of the gain medium; an output coupler integrally formed with a third face of the gain medium; and the first, second and third faces being angled to form a resonator defining a beam path contained substantially within the gain medium that is incident upon and reflected from the first face.

2. The device of claim 1, wherein the beam path is incident on the first face at an angle greater than the critical angle.

3. The device of claim 1 or 2, wherein at least one of the back reflector and the output coupler is formed by coating the second and third faces respectively with a reflective material.

4. The device of claim 1 or 2, wherein at least one of the back reflector and the output coupler is formed by adhering or bonding a prefabricated reflective material onto the second and third faces respectively.

5. The device of any preceding claim, wherein one or both of the second and third faces are in optical contact with a spacer element that spatially separates the second and third faces from a reflecting surface of the back reflector and the output coupler respectively so that the beam path extends into the spacer element.

6. The device of any preceding claim, wherein the first, second and third faces arranged and configured to form a linear resonator.

7. The device of any preceding claim, wherein the first, second and third faces are arranged and configured to form a ring resonator.

8. The device of any preceding claim, further comprising an inter-cavity reflector congruent with a fourth face of the gain medium.

9. The device of any preceding claim, wherein the beam path is incident on the fourth face at an angle below the critical angle.

10. The device of claim 8 or 9, further comprising a second pump source arranged to side pump radiation into the gain medium to induce population inversion in a region of the gain medium adjacent to the fourth face of the gain medium.

11. The device of any preceding claim, wherein the solid-state laser gain medium comprises a host material of any one of yttrium aluminium garnet, yttrium vanadate, gadolinium vanadate, yttrium lithium fluoride, sapphire or a glass.

12. The device of any preceding claim, wherein the solid-state laser gain medium is doped with an active ion comprising any one or any combination of neodymium, praseodymium, erbium, thulium, ytterbium, samarium, dysprosium or titanium.

13. The device of any preceding claim, wherein the solid-state laser gain medium exhibits an absorption coefficient defined at the wavelength of the pump radiation greater than or equal to 5cm^"1.

14. The device of any preceding claim in which the beam impinges at a grazing angle of incidence on the first face.

15. The device of claim 14 in which the grazing angle of incidence is less than 20 degrees