Abstract:
An optically pumped solid-state laser in which an oscillator and optical amplifier are pumped by a common laser diode pump, with the pump beam first pumping the optical amplifier and a residual pump beam transmitted through the amplifier pumping the oscillator. Such an arrangement permits the use of high power laser diode arrays with poor focusability beams to be used as the laser pump to produce high energy laser pulses at least an order of magnitude greater than provided by the prior art and greatly expands the applications of such a solid-state laser.

Description:
BACKGROUND OF THE INVENTION 
     1) Field of the Invention 
     The invention relates to a solid state laser pumped by a laser diode pump. 
     2) Description of the Related Art 
     Many scientific, medical, printing, ranging and industrial applications of small lasers require the laser to be reliable and efficient with high peak to average power ratio capability, and to be able to emit near diffraction-limited short-duration pulses in a controlled manner. 
     In U.S. Pat. No. 4,710,940 to Sipes (1987) and in U.S. Pat. No. 4,739,507 to Byer et al (1988) miniature continuous wave (CW) Nd:YAG solid state laser oscillators end-pumped by a discrete laser diode pump are disclosed. In these miniature lasers the output of a laser diode pump, typically having a power of 0.1-1W, is imaged or focused to a size similar to that of the TEM 00  mode size of the solid state laser oscillator, that is to a focus which may be ˜50-200 μm in diameter. In this way, the TEM 00  mode (which gives the highest beam quality) is preferentially excited and caused to dominate the output of the solid state laser oscillator. Sipes teaches that, by concentrating the pump beam to a power density typically in the range 1-10 kW.cm −2  within an oscillator crystal, the laser oscillator can be very efficient. In a paper entitled, ‘Diode Laser End Pumped Neodymium Lasers: The Road to Higher Powers’, (Proc Tuneable Solid-State Laser Conf, Paper TuC6, p134-6, May 1989, publ. Optical Society of America), Fields et al report achieving a laser diode pump to solid state laser optical power efficiency of up to 61% with this type of miniature laser oscillator using Nd:YVO 4  as the oscillator material. U.S. Pat. No. 5,410,559 (1995) and U.S. Pat. 5,577,060 (1996) to Nighan et al teach that higher power performance can be achieved with larger lasers pumped at higher power where a oscillator is sufficiently long (typically 100 mm) to correspond to a large TEM 00  mode size in an oscillator crystal, and where care is taken to mitigate beam degrading effects caused by a severe thermal load in the crystal oscillator. 
     Pulsed output can be achieved from miniature diode pumped laser oscillators. Microsecond laser pulses, typically in the range 1-500 μs duration, may be achieved by using quasi-CW laser pump diodes ie diodes that can repetitively emit power for periods up to approximately 500 μs. Nanosecond duration laser pulses can be achieved by adding a controlled optical Q-switch to a laser oscillator. Pulses shorter than 1 ns may be achieved by adding instead a passive Q-switch. In U.S. Pat. No. 4,761,786 to Baer (1988) the use of a miniature acousto-optic modulator as the Q-switch in a CW pumped laser is taught to allow use of a short solid state laser oscillator and to provide fast optical pulse dynamics. Baer teaches the production of pulses in the range 10-50 ns duration and of 10-20 μJ energy from miniature oscillators using Nd:YAG and Nd:YLF as the laser crystal oscillators. In a paper entitled, Q-switching of a diode-pumped Nd:YVO 4  Laser Using a Quadrupole E-O Deflector (Appl Phys B, Vol 67, p267-70, 1998), Friel et al report operating a short laser oscillator (around 15 mm long) and the production of 10-20 μJ pulses of 1-2 ns duration and of the order of 10 kW peak power. If short pulses are required, and synchronisation is not important, a simple fast passive Q-switch (which can be very small) can be used and the oscillator made even shorter. This typically results in the generation of sub-nanosecond microjoule pulses at kHz repetition rate. In a review article entitled, “Q-Switched Microchip Lasers Find Real-World Application”. (Laser Focus World, August 1999, P129-36, PennWell Pub, USA), Zayhowski teaches that such lasers with a oscillator of only 0.75-1.5 mm length produce pulses of 0.2 ns duration and 141 μJ pulse energy. The average output power was up to 120 mW with a maximum 1W laser diode pump power. 
     FIG. 1 illustrates a prior-art diode end-pumped miniature solid-state laser  100  including a Q-switched Nd:YAG solid state oscillator  110  that emits an output beam  150  at a wavelength of 1064 nm. In such a sold-state laser, a pump beam  121  from a discrete laser diode pump  120  operating at a pump wavelength of 808 nm is focused or imaged by lenses  131 , 132  onto an end face  112  of a Nd:YAG oscillator crystal  111  so that energy from the pump beam  121  is absorbed in the oscillator crystal  111  by exciting Nd ions. The crystal is typically a few millimetres in diameter and a few millimetres long. Stimulated emission of laser light occurs when the excited Nd ions are de-energised and the resultant light resonates in the oscillator by repeated reflections from the front face  112  of the crystal and a partially reflecting external mirror  115  to produce an oscillator beam  114  a proportion of which forms the output beam  150 . To promote the reflections the crystal may have first and second high-damage-threshold dielectric coatings (not shown) applied to the face  112  illuminated by the pump diode and also to an opposed face  113  respectively. The first coating on face  112  is designed to transmit with low loss the laser diode pump beam  121  and to reflect the Nd:YAG oscillator beam  114 , and the second coating on face  113  to transmit with low loss the oscillator beam  114 . One or other or both of the reflecting surfaces comprising face  112  and that of mirror  115  may be curved to provide a stable oscillator. As illustrated, the laser oscillator  110  comprises in optical alignment, in addition to the miniature Nd:YAG crystal  111 , and the partially reflecting output mirror  115 , a miniature Q-switch  116  to allow the generation of laser pulses. The function of the Q-switch  116  is alternately to prevent and allow the oscillator  110  to resonate, so that while not resonating, increased energy is stored and on resonating, pulses of laser light are emitted. 
     When a quasi-CW pulsed pump diode  120  is used, the control of the diode and the Q-switch are synchronised. The laser output beam  150  exits the laser in the same direction as the pump diode beam  121  ie away from the diode  120 . The principles of operation are well known to the art, and are, for example, described in ‘Solid-State Laser Engineering’ by Koechner W, Springer Verlag, N.Y., Fifth Edition 1999, p363-370. 
     As taught by Sipes, the pump beam  121  at the crystal face  112  must be of a size similar to the TEM 00  mode of the oscillator and have a power density of the order of 1-10 kWcm −2  in the Nd:YAG crystal  111  to provide efficient operation. This restricts use of pump sources to discrete diodes. Since the work of Sipes, laser diodes have increased in CW power and diodes emitting 1-2W from a facet of approximately 100 μm×1 μm are commercially available. Thus the output beam  150  of this type of miniature laser is typically in the average power range up to several hundred milliwatts. 
     The above prior-art arrangements are now widely used fairly efficiently and controllably to produce microjoule pulses of some kilowatts power from solid state lasers. However, a significant disadvantage of their miniature design is that they are not scalable to much higher average power, or greater pulse energy or peak power. This is because: 
     i) the oscillator must be short (the shorter the better) to provide fast laser pulse dynamics and a short pulse, 
     ii) the pump power must be low to avoid induced thermal distortion degrading the oscillator beam quality, and 
     iii) for good beam quality, the pump beam must selectively pump the active crystal oscillator only in a small volume close to that of the TEM 00  mode. 
     As taught by Sipes, in a short oscillator, this last constraint requires high concentration of the pump beam for good performance. In particular, the constraints preclude use of higher power, larger area, laser diodes (or power diode arrays) because the output cannot be concentrated to a small spot corresponding to the TEM 00  mode of the laser oscillator. 
     Laser diode bars (or arrays of such bars) 10 mm long suitable for pumping solid state lasers are commercially available operating up to a quasi-CW pulsed power of 100W, and CW power of 40W or more (see for example OptoPower Inc, Tucson Ariz., USA product HO1-D040-mmm-CS data sheet). Schemes have been disclosed readily to concentrate the output of such laser diode bars to spots of typically 1-0.5 mm diameter, but not 100-200 μm diameter. Commercial devices are able to deliver 16W from a 20W-laser diode bar into a 0.6 mm spot with 0.37 numerical aperture (NA), but only of the order 1.2W power into a 100 μm spot. (See for example SDL Inc, San Jose, Calif. data sheets on products SDL-3460-P6 and SDL-2372-P3, respectively). 
     As a consequence of these limitations and others, it has not hitherto been possible to scale short pulse miniature laser diode pumped pulsed lasers to much higher pulse energy and average power. 
     It is an object of the invention to at least partially mitigate these disadvantages. 
     SUMMARY OF THE INVENTION 
     In accordance with a first aspect of this invention there is provided a solid state laser comprising an optical oscillator, at least one optical amplifier, a laser diode pump arranged to provide a pump beam common to both the optical oscillator and the at least one optical amplifier, wherein a portion of the pump beam from the laser diode pump is arranged to pass through the optical amplifier before pumping the optical oscillator. 
     Conveniently, the optical oscillator and the at least one amplifier are arranged to be end-pumped by the laser diode pump. 
     Advantageously, the portion of the pump beam is focused by focusing means onto an end face of the oscillator and the focusing means is provided with an optical path for an oscillator beam to pass from the oscillator to an end face of the amplifier substantially unaffected by the focusing means. 
     Conveniently, the focusing means comprises at least one lens and the optical path comprises an axial bore through the at least one lens. 
     Conveniently, first reflecting means are included to reflect some of the portion of the pump beam emerging from the amplifier back into the amplifier. 
     Preferably, the first reflecting means is a spherical mirror having a axial aperture for the passage of the portion of the pump beam focused by the focusing means onto the oscillator means. 
     Advantageously, a polariser and a quarter wave plate are located between the focusing means and the optical amplifier such that a linearly polarised beam from the oscillator transmitted through the polariser and quarter wave plate is converted to circularly polarised light before entering the amplifier, and a beam emerging from the amplifier and passing a second time through the quarter wave plate is linearly polarised in a direction orthogonal to the original oscillator beam, to be reflected by the polariser out of the laser. 
     Conveniently, the optical amplifier has a truncated conical shape for relaying the portion of the pump beam onto the oscillator. 
     Preferably, second reflecting means are provided between the amplifier and the oscillator to reflect some of the portion of the pump beam back into the amplifier. 
     Advantageously, the optical oscillator and optical amplifier have a common optical axis. 
     Conveniently, the optical oscillator includes a Q-switch. 
     Preferably, active materials of the optical amplifier and the optical oscillator are rare earth doped crystals and in particular the active materials are selected from the group consisting of Nd:YAG, Nd:YVO 4  and Nd:YLF. 
     Alternatively, active materials of the optical amplifier and the optical oscillator are rare earth doped glasses. 
     Conveniently, the active material of the optical amplifier is selected to be different from the active material of the optical oscillator. 
     Advantageously, the active materials are Nd:YVO 4  and Nd:YAG. 
     Preferably, the laser diode pump is a CW laser. 
     Alternatively, the laser diode pump is a quasi-CW laser. 
     Conveniently, the laser is adapted to output high-power pulses. 
     Advantageously, the high-power pulses have an energy greater than 100 microjoules. 
     Conveniently, the laser is adapted to output pulses of less than 500 microsecond duration. 
     Preferably the output pulses are of less than 10 nanosecond duration. 
     Conveniently, the laser is adapted to produce an output beam substantially in a TEM 00  mode. 
     In accordance with a second aspect of this invention there is provided a method of producing high energy pulses from a solid state laser comprising the steps of: 
     a) providing an optical oscillator and at least one optical amplifier; 
     b) providing a laser diode pump common to the optical oscillator and the at least one optical amplifier; 
     c) end pumping the at least one optical amplifier with the laser diode pump to produce a pumped optical amplifier; 
     d) end pumping the optical oscillator with the laser diode pump to cause the optical oscillator to emit a laser beam; 
     e) amplifying the laser beam with the pumped optical amplifier to produce an amplified laser beam; and 
     f) outputting the amplified laser beam from the solid state laser. 
     The optical amplifier medium is typically in the form of a small rod with a polished barrel, but may be of other design. In a preferred embodiment, the optical amplifier is end pumped by a high power array of laser diodes using optics that efficiently couple the laser diode output beam into substantially the full numerical aperture of the amplifier. An aperture in a stop plate between the laser diode and optical amplifier is dimensioned such that a cone including most of the laser diode pump beam impinges within the numerical aperture (NA) of the amplifier, which guides and absorbs power from the laser diode beam so coupled. A substantial proportion of the residual pump power that is transmitted through the amplifier is coupled to a miniature laser oscillator crystal, typically by beam coupling optics. By this means, sufficient power is delivered in a small volume to enable the laser oscillator to be pumped and to operate in a low order mode, preferentially in the TEM 00  mode. 
     The laser oscillator is configured so that the output beam is emitted from the same end of the oscillator as that on which the incoming pump beam is incident. By this means, the emitted beam from the oscillator travels back through the amplifier in the contrary direction to the incoming pump beam and is amplified in so doing. Typically, the amplifier end faces (which may be planar or non-planar depending on the specific design) are anti-reflection coated at the output laser wavelength and the cylindrical barrel surface coated with an absorber at the output laser wavelength or other means used to mitigate parasitic oscillations and power loss. Conventional means are used to cool the amplifier via the barrel. A suitable choice of geometry and parameters allows the oscillator beam substantially to fill the amplifier aperture and efficiently extract stored energy absorbed from the pump beam. As a consequence, a high power pulsed output beam of high quality is emitted from the solid state laser. This beam is readily coupled out from the laser after amplification, eg by a dichroic mirror positioned between the diode pump source and the amplifier. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates an optical arrangement of a prior-art laser. 
     FIG. 2 illustrates an optical arrangement of a first preferred embodiment in accordance with the invention. 
     FIG. 3 illustrates an optical arrangement of a second embodiment in accordance with the invention. 
     FIG. 4 illustrates an optical arrangement of a third embodiment in accordance with the invention. 
     FIG. 5 illustrates an optical arrangement of a fourth embodiment in accordance with the invention. 
     FIG. 6 illustrates an optical arrangement of a fifth embodiment in accordance with the invention. 
     In the drawings, like parts are designated by like reference numbers. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     First Embodiment 
     As illustrated in FIG. 2, a first embodiment of a laser  200  of the present invention comprises an optically aligned laser diode pump  220 , an optical amplifier  240  and an optical oscillator  210 . The optical oscillator may be any known optical oscillator and may, for example, have the same components as in the prior art of an optical Q-switch and external mirror. Located between the laser pump  220  and the optical amplifier  240  is an optical focusing system  230  and a dichroic mirror  260 , the mirror  260  being inclined at an angle to the optical axis A-A′ of the optical amplifier  240 , such that light  221  emitted by the laser pump  220  may be focused by the optical focusing system  230  through the dichroic mirror  260  onto a first end face  241  of the optical amplifier  240 . Between the optical amplifer  240  and the optical oscillator  210  are interposed focusing lenses  271 , 272  for focusing light from a second end face  242  of the laser amplifer  240  opposed to the first end face  241  onto a first end face  212  of the optical oscillator  210 . The first end face  212  of the optical oscillator  210  on which the light  243  from the optical amplifier  240  is incident is provided with a stop plate  217  having an aperture  218  to restrict the beam  243  to a central portion of the first end face  212 . The focusing lenses  271 ,  272  between the optical amplifier  240  and the optical oscillator  210  are each provided with an axial bore  273 ,  274  respectively, for the passage of light  214  returning from the optical oscillator  210  through the focusing lenses  271 ,  272  to the optical amplifer  240 . 
     The miniature oscillator  210  is different from the oscillator  110  of the prior-art illustrated in FIG. 1 in an essential respect. Dielectric coatings (not shown) deposited on the laser crystal (not shown) and mirror faces (not shown) are such that their reflectivities at the oscillator wavelength (1064 nm in the case of Nd:YAG as the laser material) are interchanged compared with the prior art devices. The dielectric coating on the first end face  212  of the oscillator crystal is made substantially 100% transmissive at the pump wavelength and partially reflecting at the laser wavelength. The coating on the second end face (or mirror, if used) is made substantially 100% reflecting at the laser wavelength. As a result, the oscillator beam  214  is emitted in the direction back towards the pump source  220  ie contrary to the direction in the prior-art device illustrated in FIG.  1 . The oscillator beam  214  passes freely through the small axial bores  273 ,  274  in the oscillator residual pump beam coupling optics  271 ,  272  before entering the amplifier  240 . 
     Both the miniature oscillator  210  and amplifier  240  are pumped with a beam  221 ,  222  from the single laser diode pump  220 , the diode beam  221  pumping the amplifier  240  first and pumping the oscillator  210  using a residual beam  222  transmitted through the amplifier  240 . An oscillator beam  214  emitted from the oscillator  210  is transmitted through and amplified by the optical amplifier  240  to form an amplified oscillator beam  243 . The amplified oscillator beam  243  is reflected out from the laser  200  by the thin dichroic dielectric mirror  260 , that is coated to substantially transmit the pump beam  221  and reflect the amplified beam  243  to form an output beam  250 . 
     The configuration  200  has the key advantage of enabling the coupling of the output  221  of the high power laser diode pump source  220  with poor beam quality to the laser oscillator  210 , whilst also giving high efficiency and simple construction. The invention provides for the majority of the pump beam  221  energy to be absorbed in the amplifier  240 , and a controlled amount of residual power transmitted through the amplifier  240  to pump the oscillator  210 . The ratio of the energy absorbed in the amplifier  240  and the oscillator  210  can be selected and is primarily determined by the geometry of the system, and the relative doping concentration of active ions in the amplifier  240  and oscillator  210  crystals. Under typical operating conditions, less than 1% of the active ions in an Nd laser amplifier are excited and, as a result, the amplifier absorption of the pump beam  221  in nearly constant. However, the ratio can be fine adjusted (if needed) via wavelength control of the diode source  220 , since the diode wavelength is temperature sensitive. The diode wavelength typically shifts ˜0.3 nm/° C. The range of wavelengths in the absorption spectrum of laser crystals is typically a few nanometres wide. 
     EXAMPLE 1 
     The first embodiment of the invention can be understood most easily by considering an example. Using Nd:YVO 4  as the material for the amplifier  240  and oscillator  210  typical parameters for the system may be calculated. 
     Nd:YVO 4  is a widely used, efficient laser material that can be pumped by laser diodes operating at ˜808 nm to emit laser radiation at a wavelength λ1064 nm. It is known that a laser diode pump beam  221  of &gt;1.5W focused into a spot of &lt;200 μm diameter on an oscillator  210  with an Nd:YVO 4  crystal can produce a high quality TEM 00  mode beam at 1064 nm of up to 1W average power and of the order 150 μm in diameter. 
     The TEM 00  mode half angle beam divergence (θ) for the output from the oscillator is given by: 
     
       
         θ˜λ/τ.ω o   
       
     
     where ω o  is the TEM 00  beam radius 
     Hence 
     
       
         θ˜5 mrad 
       
     
     For convenient fabrication, the Nd:YVO 4  amplifier  240  is selected to be of the order of 10 mm long and 0.6 mm in diameter. In this case, the TEM 00  beam needs to be close to 0.6 mm in diameter on exiting the amplifier for good power extraction from the amplifier  240 . In a case where the faces  241 ,  242  of the amplifier  240  are plane and normal to the optical axis A-A′, the above oscillator beam divergence implies that the oscillator to amplifier distance must be of the order of 50 mm. 
     To meet the laser diode pump beam  221  requirements, an optimum solution is an optical arrangement  230  that is just able to focus the beam  221  into the full 0.6 mm amplifier aperture  241  as this provides the pump beam cone angle with the lowest possible NA. As noted above, prior-art shows that more than 75% of the power from a high power 10 mm long laser diode pump source can be readily focused into a 0.6 mm spot with 0.37NA. This allows at least 30W of power from a standard 40W laser diode pump source to be focused into the amplifier aperture  241  within a cone of 0.37NA. In this case the cone semi-angle (φ) is ˜21.7° or less. ie the optical arrangement  230  need to be of f/no ˜1.35, or faster. 
     Depending principally on the Nd active ion doping level in the Nd:YVO 4  amplifier  240  and oscillator  210  and the wavelength of the diode beam  221  emission, a proportion of the pump beam  221  is absorbed in the amplifier  240  whilst the beam is guided along and through the amplifier  240  towards the oscillator  210 . The guiding nature of the barrel of the amplifier  240  ensures that a residual emerging pump beam  222  is also within a 0.37NA cone, or somewhat less since the rays more angled to the amplifier axis A-A′ suffer a greater absorption path in traversing the length of the amplifier  240 . Thus the first of the coupling lens  271  of focal length (f 1 ) between the amplifier  240  and oscillator  210  is selected to be of f/no ˜1.35 to capture and efficiently collimate the residual pump beam  222 . The subsequent focusing lens  272  is selected to be of focal length (f 2 ) that is as fast as practicable so as to deliver a small intense pump beam  243  for exciting the oscillator  210 . Since small lenses with f/no ˜1.0 are readily available, f 2  is taken to be of this value in this example. Thus the spot size of the pump beam  243  at the oscillator  210  is of the order 0.6×1/1.35˜0.45 mm diameter. 
     A stop plate  217  having a circular aperture  218  of approximately 200 μm diameter is positioned in front of the oscillator  210  and transmits ˜20% of the power of the residual pump beam  243 . It follows that to deliver 1.5W pump power into the oscillator  210 , the power of the incoming beam  243  needs to be ˜7.5W. With 1.5W pump power, the oscillator  210  produces pulses of up to 50 μJ energy and up to 1W average power in a TEM 00  mode output beam  214 . 
     In considering the residual pump beam coupling optics  271 ,  272  in the example, it has been assumed that the axial holes  273 ,  274  transmitting the oscillator beam  214  do not significantly affect the collection efficiency of the lenses for the residual pump beam  222 . With an oscillator to amplifier distance of ˜50 mm, the coupling optics  271 ,  272  lenses typically have focal lengths f 1  and f 2  in the region of 20 mm and 15 mm, respectively, therefore the lens of focal length f 1  will be of the order 20 mm from the amplifier. Over this distance the 0.37NA residual pump beam  222  grows from 0.6 mm to ˜16 mm. ie a 1 mm axial hole through both lenses causes a loss of only ˜1% of the residual pump beam power, and represents a negligible loss. 
     With 30W of power from the laser diode pump source  220  entering the 0.6 mm diameter amplifier, the input pump power density (P) is: 
       P ˜30/π.(0.03) 2 ˜10 kWcm −2   
     On the basis that 7.5W residual pump power is required to be transmitted by the amplifier  240 , the amplifier absorption of the pump beam  221  is 22.5W ie 75%. In the case of a 10 mm long Nd:YVO 4  amplifier crystal, this corresponds to an absorption coefficient of ˜0.14 mm −1 . As noted above, a crystal with the appropriate Nd absorption characteristic can be easily pre-selected. 
     At 22.5W absorption in the amplifier  240 , the maximum small signal gain for the 1064 nm oscillator beam  214  is ˜25, giving the amplifier a gain of 15 for 20 μJ oscillator input pulses and ˜10 for 50 μJ input pulses. Therefore, in this example, the laser  200  illustrated in FIG. 2 produces TEM 00  output pulses of the order 0.5 μJ maximum energy. For CW diode pumping and operation at a 10-20 kHz Q-switch pulse rate, calculation indicates that this provides a system of the order of 5-10W average power. For optical pulses from the oscillator of nanosecond duration, the laser output pulses will be of peak power up to the order of 0.5 MW. 
     Second Embodiment 
     FIG. 3 illustrates a second embodiment  300  of the current invention. This is similar to the first embodiment  200  illustrated in FIG. 2, but includes an additional concave mirror  380  interposed between the optical amplifier  340  and the coupling lens  371  to raise efficiency further when using a high power diode pump beam  321  of large cone angle. The mirror  380 , of the order of 100% reflectivity at the pump wavelength, reflects a significant proportion  381  of the residual pump beam  322  back into the amplifier  340  where it can be further absorbed and add to the pumping of the amplifier  340 . This reflected radiation  381  would otherwise be lost and not contribute to pumping the amplifier  340 . 
     EXAMPLE 2 
     To help clarify the advantages of this embodiment  300 , a second example is considered. Again, a laser  300  using Nd:YVO 4  as the oscillator  310  and amplifier  340  crystals is considered. The material has refractive indices of ˜2.0 and ˜2.2 in the ordinary (a-axes) and extra-ordinary (c-axis) crystal directions, respectively. As a result of these high refractive indices, a pump beam  321  with a cone angle much greater than 21.7° corresponding to a NA of 0.37 can be focused by an optical arrangement  330  into the amplifier aperture  341  and guided within the amplifier  340 . This is practically important, because laser diode pump arrays  320  of power greater than 40W CW and 100W quasi-CW are readily available but have even worse beam quality than the lower power devices  120 ,  220  so far considered. As a result, the pump beam  321  from a high power diode array  320  is collected efficiently only by using a faster optical arrangement  330  which produces a larger pump beam cone angle φ to achieve a spot size compatible with the amplifier aperture  341 . 
     In the case of a poorer beam quality pump source  320  with a large cone angle φ, the residual pump power  322  transmitted through the amplifier  340  will also exit with a large cone angle, similar to that of the input cone φ. In this case, a much-reduced fraction of the power will be within the required cone angle for collection by the residual pump beam coupling optics  371 .  372  for efficiently pumping the oscillator  310 . For example, use of a 100W laser diode pump source would result in approximately 20W in the residual pump beam, of which only ˜5-10% is useful and needed efficiently to pump the oscillator. In the embodiment  300  illustrated in FIG. 3, the mirror  380  has an axial aperture  382  and is centred on the amplifier to oscillator optical axis A-A′. The aperture  382  freely allows transmission of the oscillator beam  314  but only partially transmits the residual pump beam  322 . The radius of curvature of the mirror  380 , the mirror position, and the mirror aperture  382  diameter are chosen to reflect the non-useful part  381  of the residual pump beam  322  back into the amplifier  340  to assist the amplifier pumping, thereby substantially raising the laser efficiency. In this example, the pump power absorbed in the amplifier  340  is increased from less than 60W to more than 75W due to the additional pumping facilitated by the mirror  380 . 
     As a further method of enhancing the laser pumping efficiency, a stop plate  317  containing a small aperture  318  positioned and aligned in front of the oscillator  310  can be of optical quality and designed to be of the order of 100% reflectivity at the pump beam wavelength. Thus, with suitable alignment, pump power falling on the stop plate  317  (which power would otherwise be wasted) is back-reflected and a substantial proportion coupled back into the amplifier aperture to assist the pumping thereof. 
     Third Embodiment 
     FIG. 4 illustrates a third embodiment  400  of the current invention wherein an amplifier  440  is double passed by an oscillator beam  414  further to improve the power extraction and pulse energy, particularly for lower gain, longer storage time laser media such as Nd:YAG or Nd:YLF. In the case that the laser amplifier  440  and oscillator  410  comprise (say) Nd:YAG crystals, which are not birefringent in their conventional orientation, a simple arrangement of a defining polariser  491  and quarter waveplate  492  can be used to extract the double passed beam  417  after amplification. However, those skilled in the art will recognise that strongly birefringent laser materials such as Nd:YVO 4  and Nd:YLF, with different gains in different crystal directions, can equally well be used if, for example, the quarter waveplate  492  is replaced by a Faraday Rotator giving 45° rotation of the plane of polarisation of the oscillator beam  414 ,  417  each time the oscillator beam passes through it. 
     Referring to FIG. 4, a polarised 1064 nm beam  414  from the oscillator  410 , transmitted by the polariser  491  and quarter waveplate  492 , is converted to circularly polarised light before first entering the amplifier  440 . A mirror  493  positioned and aligned between a laser diode pump optical arrangement  430  and the amplifier  440  is dielectrically coated to transmit substantially 100% of the pump beam  421 , and to reflect back ˜100% of the 1064 nm laser beam  414 ; returning the latter  414  back through the amplifier  440  and the quarter waveplate  492  to the polariser  491 . On passing this second time through the quarter waveplate  492 , the circularly polarised beam becomes linearly polarised in a direction orthogonal to the original input beam  414 , and is substantially 100% reflected out of the laser  400  by the polariser  491 . Thus the oscillator beam  417 , having double-passed through the amplifier  440  exits from the laser  400  as an amplified output beam  450 . 
     EXAMPLE 3 
     An example helps quantify the advantages of this embodiment. Consider an oscillator beam  414  of ˜150 μm diameter and freely diverging with ˜5 mrad divergence, using Nd:YAG as the laser material with the amplifier  440  chosen to be of the order 1.0 mm diameter and 10 mm long, and to have plane faces normal to the amplifier optical axis A-A′. In this case, the double-passed beam  417 ,from the oscillator  410  is arranged to be of the order of 1.0 mm diameter, and to fill the amplifier aperture  442  after the double pass. This occurs when the oscillator  410  to amplifier  440  distance is of the order of 60 mm. (If for some reason this distance is not convenient, it can easily be adjusted by choosing to make one or other of the amplifier faces  441 ,  442  curved.) 
     As for example 1 of the first embodiment  200  illustrated in FIG. 2, the pump power required for the oscillator  410  is of the order of ˜1.5W in a spot &lt;200 μt in diameter. Although the amplifier aperture  441  in this example of the third embodiment  400  is larger by ˜67% than example 1 of the first embodiment  200 , the pump beam  421  will have a reciprocal 67% smaller cone angle φ. The selection of a shorter value of the focal length f 2  for lens  474 , results in a similar efficiency for pumping the oscillator  410  to that in example 1. 
     Using again the value of 30W pump power entering the amplifier  440 , the maximum power density (P) at the amplifier is: 
     
       
           P ˜4 kWcm −2 . 
       
     
     On the basis that 7.5W residual pump power is required to be transmitted by the Nd:YAG amplifier, the amplifier absorption of the pump beam is 22.5W ie 75%. Also again, for the 10mm long amplifier crystal, the absorption coefficient is ˜0.14 mm −1 , which can be achieved by a proper choice of the Nd:YAG crystal doping and pump wavelength. 
     At 22.5W absorption in the amplifier, the maximum double pass small signal gain for the 1064nm Nd:YAG oscillator beam is again ˜25, giving the amplifier a gain of 22 for 20 μJ oscillator input pulses and ˜20 for 50 μJ input pulses. As a consequence, the laser  400  produces TEM 00  output pulses of the order of 1.0 mJ maximum energy, and up to 5-10W average power under CW diode pumping and at a Q-switch pulse rate in the range 5-10 kHz. Where the pulses from the oscillator  410  are of nanosecond duration, pulses of peak output power up to the order of ˜1 MW are achieved from the laser  400 . 
     Fourth Embodiment 
     FIG. 5 illustrates a fourth embodiment  500  in which residual pump beam coupling optics between the amplifier  540  and oscillator  510  are eliminated. The amplifier  540  has a truncated cone geometry with both end faces  541 ,  542  normal to the amplifier optical axis A-A′. An aperture of the amplifier at the end face  542  adjacent to the oscillator  510  is of a size similar to that required for the oscillator pump beam. An aperture of the amplifier at an end face  541  nearer to the pump source  520  is larger so as to allow easy coupling in of the pump power. As in the previous embodiments described in relation to FIGS. 2-4, the pump power is guided and transmitted within the amplifier  540  and a proportion absorbed therein. The residual power transmitted through the amplifier  540  is used to pump the oscillator  510  which, for convenience, is located in close proximity to the amplifier  540 . To prevent the residual pump power  581  that exits the amplifier at large angles reaching the oscillator  510  and pumping outside the required mode volume, a suitable stop plate  517  having a small aperture  518  is positioned between the amplifier  540  and oscillator  510 . 
     To ensure efficient filling of the amplifier  540  by the oscillator beam  514 , the smaller optical face  542  of the amplifier  540  is concave to the oscillator beam so that the beam  514  within the amplifier  540  substantially fills the increasing amplifier diameter as the beam propagates. At the amplifier exit, the optical face  541  is plane, or conveniently, can be of suitably convex radius if a substantially collimated output is required. As in the case for the embodiments of FIGS. 2 and 3, a simple optical device such as a suitable dichroic mirror  560  provides for the amplified beam  543  to be efficiently extracted from the system as the laser output  550 . 
     EXAMPLE 4 
     As for the previous embodiments, a simple example illustrates the advantages of this embodiment  500 . Consider a laser medium of Nd:YVO 4  and 40W as the power of the pump source  520 . The amplifier  540  is 10 mm long, and of the order 0.6 mm and 0.2 mm diameter at its two end faces  541  and  542 , respectively. Using a laser diode pump beam  521  of cone semi-angle Φ˜21.7°, calculation shows that the cone semi-angle becomes ˜10° after entering the amplifier, increases to ˜30° by the time the beam  514  has propagated to the smaller end  542  of the amplifier  540 , and increases up to ˜90° on exiting the amplifier. The proportion of this residual pump power entering the oscillator  510  via the aperture  518  of the order of 200 μm in diameter, is determined by the amplifier to aperture distance. Where the distance is similar to the aperture size, the fraction will be ˜20%, and (again) approximately 7.5W of residual power will be enough to provide up to 1.5W to pump the oscillator  510 . 
     For the oscillator beam  514  to fill the amplifier aperture, the radius of the concave input face  542  will be of the order of 3 mm. If a collimated amplified output beam is chosen, the radius of the amplifier exit face  541  will be convex and of the order of 8 mm. As is well known in the art, the chosen values may also be selected to include compensation for thermal lensing in the amplifier  540  attributable to power absorbed from the pump beam  521 . 
     To obtain of the order of 7.5W residual pump power, the amplifier absorption of the pump beam  521  is again 22.5W ie 75%, and, with a proper choice of the Nd:YVO 4  crystal doping and pump wavelength, the laser  500  performance will be similar to that for the embodiment  200  illustrated in FIG.  2 . 
     Fifth Embodiment 
     The embodiment illustrated in FIG. 6 includes a modification of that shown in FIG. 5 to recover a significant proportion of the pump power  681  transmitted by the amplifier  640 , that would otherwise be wasted. FIG. 6 shows the essential detail illustrating how this can be achieved. 
     As for the second embodiment  300  illustrated in FIG. 3, a concave mirror  617  having a small aperture  618  adjacent to the oscillator  610  comprises an optical surface  619  of the order of 100% reflectivity at the pump beam wavelength. The miniature mirror  617  has an axial aperture  618  and is centred on the amplifier  640  to oscillator  610  optical axis A-A′. The aperture  618  allows free transmission of the oscillator beam  614  but only the central portion of the residual pump beam  681 , which is of large cone angle, is transmitted. The mirror radius of curvature, the mirror position, and the mirror aperture diameter are chosen to reflect the otherwise non-useful part of the residual pump beam back into the amplifier aperture  642  to assist the amplifier pumping, thereby substantially raising the overall laser efficiency. 
     Since the pump beam  681  exiting the amplifier  640  is highly divergent so that without the mirror  617  a majority of the beam would be lost, the embodiment illustrated by FIG. 6 is very energy efficient. 
     Since all the components in all the embodiments can be small, the whole laser can be engineered into a very compact, rugged and reliable assembly. 
     The laser can be operated either with a CW or a quasi-CW pump source, and also with the oscillator Q-switch disabled or removed. Thus the system can be used to generate a CW laser output beam, a beam comprising single or repetitive long (microsecond duration) pulses, as well as the nanosecond pulses illustrated by the embodiments above. The system is particularly attractive for generating either high brightness microsecond or nanosecond pulses. Some further laser materials have substantially the same laser emission wavelength, and also absorb pump power at substantially the same wavelength as those described. Thus the invention is not restricted to the use of the same laser material for the oscillator as for the amplifier. ‘Handbook of Laser Wavelengths’, Weber MJ, CRC Press NY (1999) lists such materials and their associated laser wavelengths. Both Nd:YAG and Nd:YVO 4  have laser emission wavelengths at around 1064.1 nm. The transitions giving rise to these wavelengths in both materials have linewidths of the order 0.5 nm. Both materials can be pumped by a laser diode beam with a wavelength in the region of 800-810 nm. Thus Nd:YAG and Nd:YVO 4  is an example of a pair of different materials which can be used together in the laser, another example is Nd:YAG and Nd:Glass. 
     Operation of a laser using a different laser material in the oscillator from that in the amplifier can improve performance optimisation. For the example of Nd:YAG and Nd:YVO 4 , the latter crystal material is currently difficult to grow in large pieces (greater than 10-20 mm length) of laser optical quality, and thus a Nd:YVO 4  amplifier is restricted to this size. In some cases, Nd:YVO 4  has characteristics that make it less suitable than Nd:YAG for amplifying laser pulses, but more suitable for use in the oscillator. In such cases, it is beneficial to use an Nd:YAG amplifier to amplify Nd:YVO 4  oscillator pulses. An example is where Nd:YVO 4  is preferred in the oscillator to generate the shortest pulses because of its high gain and low energy storage characteristics, and Nd:YAG is preferred as the amplifier to give high energy output pulses because of its lower gain and higher energy storage characteristics. 
     The foregoing examples, given to help illustrate typical performance that might be achieved by the preferred embodiments, relate to operation of lasers with nanosecond oscillator pulses. However, numerous further examples are possible, and those given are in no way intended to limit the scope of the invention. Equally, much shorter or longer duration optical pulses can be used. For example, operation of a laser with an oscillator producing optical pulses in the picosecond duration range (10 −12  to 10 −9 s) would generate similar gains in pulse energy and peak power, and average output power. 
     It will be apparent to those skilled in the art, that much higher power laser system scaling, within the scope of the invention and beyond that illustrated, is possible with the increasing availability of suitable higher power laser diodes (both CW and quasi-CW power), and with proper choice of system design parameters, including use, for example, of more than one amplifier, use of an amplifier or amplifiers with significantly larger dimensions and suitable choice of coupling optics and radius of optical amplifier faces to compensate for increased optical lensing effects introduced by the greater thermal load in the laser materials. 
     It will be apparent to those skilled in the art that alternative optical arrangements exist for system layouts using the invention. For example reflective or diffractive elements rather than refractive elements to manipulate the beams, mirrors to de-couple the amplifier axis from that of the oscillator to provide a folded system, or more than one amplifier, may be used. 
     Similarly, an optical fibre may be provided to deliver pump energy from a laser diode array to the optical amplifier. In addition, more than one laser diode pump array may be provided. 
     It will also be apparent to those skilled in the art that, since the laser can generate pulses of high peak and average power and very high brightness, the laser will be a highly efficient and convenient source for generating pulses at other wavelengths (in the visible, UV and IR) by harmonic conversion in non-linear crystals, the pumping of parametric oscillators or the use of Raman shifting devices, for example. 
     Changes and modifications in the specifically described and illustrated embodiments can be carried out without departing from the spirit and scope of the invention. 
     The foregoing descriptions of the preferred embodiments of the invention have been presented for the purposes of illustration and description only. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obviously, many modifications and variations will be apparent to practitioners skilled in the art. It is intended that the scope of the invention be defined by the following claims and their equivalents.