WO2014128499A1

WO2014128499A1 - Multi-pass slab amplifier with zig-zag paths

Info

Publication number: WO2014128499A1
Application number: PCT/GB2014/050540
Authority: WO
Inventors: Young Kwon
Original assignee: Powerlase Photonics Limited
Priority date: 2013-02-21
Filing date: 2014-02-21
Publication date: 2014-08-28
Also published as: GB201303129D0; GB2513098A

Abstract

There is provided a method of using a slab laser gain medium. The method comprises transmitting a light beam through the slab laser gain medium (101)on a first trajectory (105b) using a first mirror(109), wherein the first trajectory comprises a zigzag trajectory. The method further comprises redirecting the light beam through the slab laser gain medium (101) on a second trajectory (105d) using the first mirror (109). The slab laser gain medium may be a so-called "Innoslab" and with the help of two mirrors (107,109) multiple passes (105b-d) through the slab laser gain medium (101) are realized. Using zig-zag paths through the slab instead of straight paths the amplification for every pass is increased and the pump energy is more efficiently used.

Description

MULTI-PASS SLAB AMPLIFIER WITH ZIG-ZAG PATHS

The present disclosure relates to a method of using a slab-shaped, or slab, optical medium. In examples, the optical medium is a laser gain medium. In particular, the present disclosure relates to a method of transmitting a laser beam through a slab laser gain medium and a method of amplifying radiation. More particularly, the present disclosure relates to a method of increasing the pump extraction from a slab laser gain medium.

Background

For power scaling of high power diode pumped lasers, efficient extraction of pump energy from gain medium at high pump density with uniformly excited large volume is needed. Disk lasers use a very thin gain medium, and heat removal from a large face, but are known to be difficult to increase the pumped volume. Rod lasers use large cylindrical gain medium directly cooled by water but suffer from a large thermal lensing effects.

Slab lasers use large rectangular gain medium cooled by water/gas from the side face.

Advantageously, slab lasers may be cooled at the large faces in order to reduce the thermal effect on the beam quality. The rectangular shape provides a large cooling surface and under uniform pumping conditions, stress induced birefringence effects can be reduced. Nevertheless, thermal and stress induced effects, such as thermal lensing, are still a problem and can severely degrade the optical quality of the laser beam and limit the optical power.

In slab lasers, the optical beam may pass straight through the length of the gain crystal in a single pass, parallel to the optical axis.

Slab lasers may also be used in a so-called "zigzag bounce" or "zigzag" configuration. In the zigzag geometry, the optical beam traverses through the crystal at an angle to the optical axis using total internal reflection at surfaces of the crystal. The resulting "zigzag path" is not parallel to the optical axis of the crystal. In these designs, the zigzag path is usually introduced into the slab by shaping the gain medium such that the laser beam enters and exits the crystal at a fixed angle defined by the slab geometry and the number of zigzag bounces. By using the zigzag configuration, the impact of thermal effects are cancelled out and efficient use of pumped volume is enhanced through the overlapped beam along zigzag path.

US 6,654,163 discloses a so-called "Innoslab" laser using a slab gain medium and a folded single pass configuration. The Innoslab amplifier consists of a longitudinally, partially pumped slab crystal. The design improves efficiency by using a folded single pass configuration, where beam expansion on every passage through the slab balances the increase of power and intensity. This is said to improve the efficiency by homogeneous saturation of the gain medium and constantly keeps the intensity away from the damage threshold of the optical components. Notably, it can be understood that US 6,654,163 discloses a straight pass geometry through the slab. A problem with this geometry is that it requires a complex setup for pumping. Furthermore, the laser beam suffers from thermal gradient effects in the cooling direction, resulting in a practical limit to how much the power may be scaled without sacrificing the beam quality. The present disclosure provides a means for addressing these deficiencies.

Summary

Aspects of an invention are defined in the appended independent claims.

The inventor has recognised that systems using a straight pass through a slab gain medium have low extraction efficiency. The inventor has further recognised that systems using multiple straight line passes through the gain medium require a complex pumping arrangement and, nevertheless, suffer from problematic thermal gradients in the cooling direction. This limits the extent to which the optical power output of such systems may be scaled-up.

The present disclosure relates to a method of using a slab laser gain medium and an optical arrangement for a slab laser gain medium. In particular, the present disclosure relates to using one mirror to redirect the light two or more times on two or more different trajectories through the gain medium.

The present disclosure relates to using multiple passes through the slab wherein at least a first pass comprises a zigzag trajectory, or path, of internal reflections. Accordingly, power scaling with volume can be achieved by merely scaling the width of the slab. Advantageously, the benefits of zigzag slab lasers, such as reduced susceptibility to thermal effects, are maintained and more of the pumped volume is utilised for greater efficiency. Higher power outputs may therefore be achieved with smaller gain mediums. In an advantageous embodiment, the mirror forming the multiple passes through the gain medium is a cylindrical mirror. By using a cylindrical mirror having a convex mirrored surface, there is provided a system wherein the optical beam is expanded when reflected and redirected by the cylindrical mirror. Beam expansion with every redirected passage through the gain medium balances the increase of power and intensity to advantageously maintain the optical power density incident on the slab gain at a desired level. For example, the optical power density can be maintained at a level below the damage threshold of the gain medium and/or optical component. The amount of expansion, and thereby compensation for the increase in optical power, can simply be controlled by adjusting the curvature of the convex mirrored surface. In yet further advantageous embodiments, the mirrors are cylindrical mirrors having a convex surface with a phase plate. Adjustment of the optical beam's intensity profile can be used to compensate for non-uniform gain, phase and/or absorption profiles across the width of the slab gain medium. Notably, efficient pump energy extraction is achieved from the gain medium. Scaling of the size of the gain medium is provided by scaling the width of the slab to allow multi-pass through the gain medium. There is also provided a system in which uniform pumping of the gain medium may be achieved by many different pumping geometries. That is, the system in accordance with the present disclosure is more flexible in terms of optical pumping geometries. There is also provided a system in which efficient heat extraction may be achieved through the largest faces of the slab. Notably, this is more difficult to achieve with systems using multiple straight line passes through the large face. In embodiments, this is achievable by pumping through the smallest longitudinal faces of the slab. Brief description of the drawing

Embodiments of the present disclosure will now be described with reference to the accompanying drawings in which: Figure 1 shows a slab gain medium in accordance with the present disclosure;

Figure 2 is a diagram of a pumped slab gain medium in accordance with the present disclosure; Figure 3 shows expansion of the optical beam with every passage through the slab gain medium; and Figure 4 shows cross-sections of the beam profile at different locations across the width of the slab gain medium, and also the gain profile across the width of the slab gain medium.

In the figures, like reference numerals refer to like parts.

Embodiments relate to using a laser gain medium but the skilled person will understand that the laser gain medium may be used for other purposes. For example, the medium may be used as a simple waveguide. In another example, the medium may be used as a simple optical phase adjuster. That is, the laser gain medium may simply be described as an optical medium.

The present disclosure refers to a slab. This term relates to the physical shape of the optical medium namely that the medium contains a relative small first dimension (e.g. thickness) and relatively large second and third dimensions (e.g. width and length). The skilled person will understand the term slab is used to refer to a slab-shaped component.

Detailed description

In overview, there is provided a method of using a slab laser gain medium by providing multiple paths through the gain medium, wherein the respective paths are non-overlapping or only partially overlapping. That is, the multiple paths are not collinear. At least a first trajectory or path through the gain medium comprises a zigzag trajectory or path. A zigzag trajectory is a trajectory in which the light passes from one side of the gain medium to the other by way of a series of internal reflections with first and second surfaces of the gain medium. Such a zigzag trajectory will be familiar to the skilled person in the field of slab laser gain mediums and optical fibres, for example.

Figure 1 shows a slab laser gain medium 101 having a longitudinal axis wherein the two smallest faces of the slab gain medium 101 form an input face 113 and an output face 115, respectively. The slab gain medium is arranged to receive an input light beam 103 as an input and output an output light beam 111. The input face 113 is angled to receive an input laser beam 103, parallel to the longitudinal axis, for example, at an angle equal to or greater than the critical angle for total internal reflection. As the skilled person will understand, alternatively, total internal reflection may be achieved by using a regular right-angled slab and introducing the input laser beam at the critical angle, or greater. The slab gain medium 101 is arranged to transmit the received laser beam along an input trajectory 105a from the input face 113 to the output face 115 via a series of internal reflections. The slab resides between two angled mirrors, 107 and 109, positioned on the longitudinal axis, and adjacent the first face 113 and second face 115, respectively. Mirror 109 is positioned at an angle relative to the longitudinal axis so as to receive light from the output face 115 which has followed the input trajectory 105a and redirect that light back through the slab gain medium on a first trajectory 105b to input face 113. Likewise, mirror 107 is positioned at an angle relative to the longitudinal axis so as to receive light from the input face 113 which has followed the first trajectory 105b and redirect that light back through the slab gain medium on a further trajectory 105c to output face 115. The mirrors 107 and 109 are arranged such that light makes two further passes, along second trajectory 105d and exit trajectory 105e, to form output light beam 111. Notably, the configuration uses only one mirror at each end. The first trajectory 105b and second trajectory 105d are formed by a reflection off mirror 109. After entering the slab gain medium at input face 113, the laser beam follows a zigzag trajectory 105a through the medium by way of a series of internal reflections. The first trajectory 105b does not overlap with the input trajectory 105a. In total, figure 1 shows that the laser beam is redirected back through the gain medium four times by the mirrors, with each new pass following a different, non-overlapping zigzag trajectory. The resulting output light beam 111 emerges out of output face 115 having been amplified through the gain medium.

There is therefore provided a method of using a slab laser gain medium, the method comprising: directing a light beam through the slab laser gain medium on a first trajectory 105b using a first mirror, wherein the first trajectory 105b comprises a zigzag trajectory; and directing the light beam through the slab laser gain medium on a second trajectory 105d using the first mirror.

Although embodiments described relate to mirrors 107 and 109, in other embodiments mirror 107 and/or mirror 109 are any optical elements having a reflective property such as a prisms or diffractive optical elements. Optionally, the method further comprises directing the light beam through the slab laser gain medium on a further trajectory 105c using a second mirror 107 It can be understood that light passes from a first side of the slab gain medium to a second side of the gain medium along (or "on") a first trajectory (or "path"). That is, the light beam passes out of the other side of the gain medium. The light beam enters the gain medium on one face and passes out of the gain medium from another face. Having exited the gain medium on the other face, the light beam is directed back into the gain medium. The light beam then follows another trajectory (or "path") through the gain medium and exits the gain medium once again. The first trajectory is a zigzag path comprising a series of internal reflections with alternate opposing surfaces of the gain medium.

It can be understood that whilst figure 1 shows five zigzag passes through the slab gain medium, any number of passes may be used in accordance with the present disclosure. That is, the method or device may comprise a plurality of further trajectories. In other embodiments, only the first trajectory is a zigzag trajectory.

However, there is shown an embodiment in which the light on the second trajectory also travels through the gain medium by a series of total internal reflections. That is, in an embodiment, the second trajectory comprises a zigzag trajectory.

It can therefore be understood that, in embodiments, the zigzag trajectory of the first and/or second trajectory comprises a series of internal reflections between a first surface and a second surface of the slab laser gain medium.

In one embodiment, the slab gain medium is pumped through the longitudinal side faces 216 and 217 as illustrated in Figure 2. Advantageously, by arranging embodiments in this way, the largest faces (the faces having the largest surface area) of the gain medium are used for efficient cooling and therefore thermal effects on the beam quality can be reduced.

Figure 3 shows a slab gain medium in accordance with the present disclosure, wherein the slab is positioned between two cylindrical mirrors, 307 and 309. That is, mirror 107 and 109 of figure 1 are cylindrical mirrors. In other embodiments, only one of the mirrors is cylindrical. Although embodiments described relate to at least one cylindrical mirror, in other embodiments, at least one reflective optical element having a negative lensing effect or beam expanding effect in at least one- direction is used. Figure 3 shows a slab laser gain medium 301 having a longitudinal axis wherein the two smallest faces of the slab gain medium 301 form an input face 313 and an output face 315, respectively. The slab gain medium is arranged to receive an input light beam 303 as an input and output an output light beam 311. Input face 313 is arranged to receive an input laser beam 303 at an angle to the longitudinal axis. Light is guided on an input trajectory 305a through the slab gain medium to output face 315.

Cylindrical mirror 309 is positioned at an angle relative to the longitudinal axis so as to receive light from the output face 315 which has followed the input trajectory 305a. Cylindrical mirror 309 has a convex mirrored surface with a radius of curvature and is positioned at a small tilt relative to the longitudinal axis. Light is received by cylindrical mirror 309 and is redirected for a first time back through the slab gain medium on a first trajectory 305b to input face 313.

Cylindrical mirror 307 is positioned at an angle relative to the longitudinal axis so as to receive light from the input face 313 which has followed the first trajectory 305b. Cylindrical mirror 307 has a convex mirrored surface with a radius of curvature and is positioned at a small tilt relative to the longitudinal axis so as to receive light from the input face 313. Cylindrical mirror 307 is arranged to redirect the light back through the slab gain medium on a further trajectory 305c to output face 315 where it is outputted to form transmitted beam 311.

Figure 3 only shows three trajectories through the slab gain medium. However, it can be understood that in embodiments, the system is arranged to provide further trajectories through the slab gain medium. In particular, in embodiments, there is provided a second trajectory formed by a second reflection from cylindrical mirror 309. That is, the first and second trajectories are formed by reflections off the same mirror, namely cylindrical mirror 309.

In embodiments, cylindrical mirror 309 and/or cylindrical mirror 307 have convex mirrored surfaces which reflect light outwardly and are otherwise referred to as diverging mirrors in that the reflected beams are expanded. In this embodiment, this effect is used to advantageously expand the beam every time (or every other time, in the case of one cylindrical mirror) the beam is

redirected/reflected back through the gain medium. Expanding the beam decreases the optical power density and thereby balances the increase in optical power that results from light amplification of the optical beam as it propagates through the gain medium. That is, the increase in optical power that occurs with every passage through the gain medium is balanced by expanding the beam to maintain the power density. In this way, the system is afforded a means of maintaining the optical power density within a desired level. For example, the optical power density can be maintained at a level below the damage threshold of the gain medium and/or optical component. The amount of expansion can simply be controlled by adjusting the curvature of the convex mirrored surface.

That is, in an embodiment a cylindrical mirror is arranged to direct light through the slab laser gain medium on a first trajectory and direct light back through the slab laser gain medium on a second trajectory using the same mirror, wherein upon reflection the beam is spread outwardly and expanded in at least one direction.

In an embodiment, first mirror 109 is a cylindrical mirror having a convex surface.

It can be understood the arrangement of figure 3 may comprise any number of trajectories wherein the trajectories are non-overlapping zigzag trajectory. That is, the skilled person will understand there is no upper limit to the number of passes. That is, mirrors 307 and 309 can be arranged to redirect light back through the gain medium any number of times by suitable arrangement.

Beam cross-sections 405a, 405b and 405c of each trajectory 305a, 305b and 305c, respectively, on a plane through the gain medium of figure 3 are shown in figure 4. The cross-sections illustrate the effect of expanding the beam on each reflection. Beam cross-sections 405a, 405b and 405c illustrate the profiles of guided beams 305a, 305b and 305c respectively. Mirrors 307 and 309 have expanded the beam along the width of the slab with each redirected trajectory. Consequently, beam cross- section 405c is wider along the width of the slab than 405b, and accordingly 405b is wider along the width of the slab than 405c.

In another embodiment, mirrors 307 and 309 expand the beam along the width (horizontally) and height (vertically) of the slab gain medium. That is, the beam is expanded in two or more directions by each reflection. Figure 4 also illustrates the gain profile 430 along the width of the slab gain medium. The gain profile 430 is non-uniform across the width of the slab and follows a parabolic shape. Therein, with each beam propagating through a different location of the slab, the beam cross-sections, 405a, 405b and 405c experience a different gain profile across their width. In other words, one side of the beam experiences more gain than the other side of the beam. Non-uniform gain and phase distortions in the amplified beam between different beam trajectories and across the width of the beam result in inhomogeneous beam amplification which degrades beam quality. Non-uniform gain profiles can arise from non-uniform pumping and birefringent losses and phase distortions from the residual thermally induced phase distortion.

Therefore, in another embodiment, mirrors 307 and 309 of figure 3 are cylindrical mirrors having a convex surface with a phase plate to compensate for non-uniformities in the gain and phase across the width of the beam cross-section. The phase plate may be a fixed element or adaptively controlled in an open or closed loop to compensate for the phase non-uniformities. A beam of light reflected off a cylindrical mirror having a convex surface will be expanded and will also have its phase profile altered by the phase plate. Advantageously, shaping the optical beam's intensity profile is used to compensate for non-uniform gain and phase profiles across the width of the beam cross-section. Offsetting the intensity profile to compensate for anisotropic amplification and phase distortion therein provides improved beam quality. Without compensation, the optical beam profile may be amplified and distorted in a non-uniform manner and result in poor quality beam. The shape of the beam's intensity profile can simply be controlled by changing the design of the phase plate or adjusting the phase using an adaptive optical elements in an open loop or closed loop with phase feedback from extra beam profile measurement device.

In another embodiment, the non-uniform gain profile may be non-parabolic. The skilled person will understand that the shape of the gain profile will depend on the cause of the non-uniformity.

However, it may be understood that the phase plate is tailored, or adjusted, to at least partially compensate for the non-uniform gain and phase profile.

In another embodiment, the mirrors 107 and 109 are alternatively diffractive optics arranged to perform the functions of beam expansion and shaping. The skilled person will understand that the slab gain medium is a higher refractive index than its surroundings so that total internal reflection may take place at the interface between the slab and its surroundings. In other embodiments, the slab may also contain regions of lower refractive index material providing there is higher refractive index material at the points of internal reflection. In an embodiment, the slab is surrounded by air and the slab has a refractive index greater than 1. In other embodiments, the slab may be contained, or be "sandwiched", between two other materials such as undoped laser crystals or of lower refractive index than the slab such as protective thin flims, transparent glasses, sapphire and/or diamond. Advantageously, these materials may provide more efficient heat extraction than air.

In an advantageous embodiment, the different trajectories through the slab are non-overlapping or only partially overlapping. In other words, the trajectories are not fully overlapping or collinear. Thus, a different region of the gain medium is exposed to the light beam on the second trajectory than was on the first trajectory, for example. That is, the volume of the gain medium on the first trajectory is not the same as the volume of gain medium on the second trajectory, for example.

Accordingly, in an embodiment, the first trajectory is different to the second trajectory. In another embodiment, the further trajectory is different to the first and/or second trajectory.

Embodiment shown utilises mirrors, however, the skilled person will understand that any reflector having the desired optical properties, including reflectivity, at the laser wavelength and the necessary laser-induced damage threshold may be equally suitable for redirecting the light beam back into the gain medium. The skilled person will also understand that the mirror is chosen and aligned based on the wavelength of the light beam, extent of expansion required, required shaping of the power density profile, and/or the geometry of the gain medium, for example.

There is therefore provided a mirror arranged to direct light through the slab laser gain medium on the first trajectory and direct the light beam back through the slab laser gain medium on a second trajectory. In other embodiments, the mirror is a reflective coating on the slab.

In an embodiment, the light beam may originate from a laser. Accordingly, in an embodiment, the light beam is a laser beam. The gain medium may be any optical medium or any laser gain medium which can be formed into a slab shape and be used to transmit the light beam by a series of internal reflections. For example, the slab gain medium may be Nd:YAG, Nd:YV0₄, Nd:Glass, Yb:YAG, EnYAG or any active gain medium made of crystal or glass material. Also, the gain medium may be a sandwich medium - such as undoped-doped-doped or sapphire-doped-sapphire or diamond-doped-diamond - particularly if the trajectories are along the width (second longest) or length (longest) direction of the sandwiched doped medium or multiple sandwiched doped medium.

The skilled person will understand that the shape of the laser beam can be defined by the relative angle and curvature of the mirrors used to define the different trajectories within the slab gain medium.

The skilled person will understand that the laser gain medium in accordance with the present disclosure may be pumped in any suitable fashion such as with diodes at the edge or through a lens duct, for example.

The invention is not restricted to the described embodiments but extends to the full scope of the appended claims.

Claims

A method of using a slab laser gain medium, the method comprising:

directing a light beam through the slab laser gain medium on a first trajectory 105b using a first mirror 109, wherein the first trajectory 105b comprises a zigzag trajectory; and

directing the light beam through the slab laser gain medium on a second trajectory 105d using the first mirror 109.

The method of claim 1 further comprising:

directing the light beam back through the slab laser gain medium on a further trajectory 105c using a second mirror 107.

The method of any preceding claim wherein the first mirror 109 and/or second mirror 107 comprises a cylindrical mirror having a convex mirror surface.

The method of claim 3 wherein the first mirror 109 and/or second mirror 107 comprises a phase plate.

The method of claim 1 wherein the second trajectory 105d comprises a zigzag trajectory and/or the method of claim 3 wherein the further trajectory 105c comprises a zigzag trajectory.

The method of any preceding claim wherein the zigzag trajectory of the first and/or second trajectory comprises a series of internal reflections between a first surface and a second surface of the slab laser gain medium.

The method of any preceding claim wherein the slab laser gain medium is sandwiched between two materials of lower refractive index.

8. The method of claim 7 wherein the materials of lower refractive index are at least one selected from the group comprising: undoped laser crystals, transparent glasses, sapphire and diamond.

9. The method of any preceding claim wherein the first trajectory is different to the second trajectory and/or the method of claim 2 wherein the first trajectory 105b and second trajectory 105d are different to the further trajectory 105c.

10. An optical arrangement for a slab laser gain medium, the optical arrangement comprising:

a slab laser gain medium;

a first mirror 109 arranged to direct light through the slab laser gain medium on a first trajectory 105b, wherein the first trajectory comprises a zigzag trajectory, and direct light through the slab laser gain medium on a second trajectory 105d.

11. The arrangement of claim 10 further comprising:

a second mirror 107 arranged to direct light through the slab laser gain medium on a further trajectory 105c.

12. The arrangement of claims 10 and 11 wherein the first mirror 109 and/or second mirror 107 comprises a cylindrical mirror having a convex mirror surface.

13. The arrangement of claim 12 wherein the first mirror 109 and/or second mirror 107

comprise a phase plate.

14. The optical arrangement of claim 10 wherein the second trajectory comprise a zigzag

trajectory and/or the optical arrangement of claim 11 wherein the third trajectory comprises a zigzag trajectory.

15. The optical arrangement of any proceeding apparatus claim wherein the zigzag trajectory of the first and/or second trajectory comprises a series of internal reflections between a first surface and a second surface of the slab laser gain medium.

16. The optical arrangement of any proceeding apparatus claim wherein the first trajectory is different to the second trajectory and/or the optical arrangement of claim 11 wherein the first trajectory 105b and second trajectory 105d are different to the further trajectory 105c.

17. The optical arrangement of any proceeding apparatus claim wherein the mirror 107 and/or mirror 107 is a reflective coating on the slab.

18. The optical arrangement of any proceeding apparatus claim wherein the light beam is a laser beam.

19. A method of using a slab laser gain medium or optical arrangement substantially as

hereinbefore described with reference to the accompanying drawings.