IMPROVEMENTS TO LASERS

Abstract: A laser (2) includes a gain medium (10), a switch element (12) and a pulse controller (14). In one embodiment laser light of differing polarisations pass along respective paths and a pulsed laser output is generated via an electro-optical element. In another embodiment light of differing polarisations passes in differing directions through a cylical path. The invention can make use of a prism-shaped polariser having a polarisation selection face and two further faces. Yet further initial pulses can be controlled to reduce energy for example by progressively increasing the period or amplitude of successive pulses. As a result an efficient and high power laser apparatus is realised.

Description: IMPROVEMENTS TO LASERS The present invention relates to laser cavities, in particular electro-optical laser cavities including at least one electro-optical (EO) switch element, for cavity dumping a pulsed laser output of high average power, and a pulsed laser incorporating the same, as well as a polariser for use in such cavities. This invention further relates to the field of pulsed lasers. More particularly, this invention relates to a switched cavity for a pulsed laser incorporating an optical element that can be switched between a high loss state and a low loss state. Electro-optical laser cavities include a polarisation selective element, for example a polariser, and an electro-optical switch element which is operable to switch the cavity between low and high loss states, providing for hold off of the lasing action in the low loss (high Q) state and causing cavity dumping on switching to the high loss (low Q) state. The electro-optical element is operated, by the selective application of a voltage thereto, to alter the polarisation state of the laser light transmitted therethrough, with the degree of change in the polarisation state being related to the loss experienced by the cavity. Electro-optical elements have the particular benefit of being switchable at high rates to generate short laser output pulses, typically of the order of nanoseconds down to femtoseconds. Existing electro-optical laser cavities have been used to deliver laser output at low average powers, typically of the order of tens of watts as achieved by flash lamp pumping a gain medium. Flash lamp pumped laser cavities are not, however, suited to delivering laser output at higher average powers as significant heat is imparted to the gain medium, which heating leads to thermal aberrations in the optical elements, in particular the gain medium, and in turn causes the depolarisation of the laser light. This depolarisation leads to loss at the polarisation selective element, rendering the laser cavity inefficient in converting energy stored in the gain medium into output pulses of laser light. Diode pumped laser cavities also suffer from the same problem when driven to deliver laser output at higher average powers, typically about one hundred watts. Modifications have been proposed to existing laser cavities in an attempt to compensate for the deleterious effects of polarisation birefringence and allow use at higher average powers, see, for example, Op. Lett., 11, pages 360 to 362 (1986), Appl. Op., Vol. 32, No. 3 (1993) and IEEE J. Quan. Elec, Vol. QE-16, No. 4 (1980). However, these modifications have only been applied to flash lamp pumped laser cavities for delivering output powers of tens of watts. These modifications require the use of additional optical elements, in particular polarisation rotation optics, for correcting the polarisation birefringence. It is, however, very difficult in practice to achieve the required correction by utilising polarisation rotation, and also the introduction of additional optical elements reduces the power of the pulsed laser output. Another known arrangement is described in GB2037064. This document deals with correction of the effects of thermal birefringence in a laser rod including a quarter- wave plate allowing rotation of the plane of polarisation through 90 degrees after reflection. This arrangement, however, is an unnecessarily complex arrangement which has an undesirably low peak power and does not allow seeding of the arrangement from another source which in turn reduces the opportunities for a stable operation of the laser. A further arrangement is described in EP 0370620. According to this arrangement a laser medium exhibiting thermal birefringence is provided in a branched cavity and the output is taken from one of the branched arms in a Q-switching arrangement. This arrangement emits undesirably low peak powers. It is known to use acousto-optic elements within laser cavities to switch high average power laser cavities to and from high and low loss states in order to generate pulses of the laser radiation. Such acousto-optic switching elements suffer from limitations in the shortness of the laser output pulses that can be produced, which in turn may limit the maximum peak output power of the laser pulses. It is known when using acousto- optic switching elements that measures should be taken to suppress generation of an excessively high power first pulse when switching on the laser as this may cause damage to the laser system as a whole. This has been addressed by slowly ramping up the voltage applied to the acousto-optic switching element during startup. Measures that allow for a decrease in laser pulse duration without increasing the risk of system failure through generation of an excessively high energy laser pulse at startup are strongly advantageous. Known polarisers suffer from various problems. A polariser can be used to separate in angle and spatially, polarisations of an input laser beam. Polarisers are characterised in part by their threshold to damage. Polarisers suitable for high peak power applications are generally constructed from a plate of silica or glass, coated with a multilayer, thin film dielectric coating. The back surface of the plate is generally left uncoated. Such a design is optimum for the separation of polarisations for a beam travelling in a pre-designated direction through the plate. However in cavities where the beam travels in both directions through the polarising element, when the polarisation is rotated by a polarisation rotator, the beam is ejected from the cavity, however with a standard plate polariser, the beam also experiences a significant reflection from the uncoated surface of the plate polariser. This constitutes a mechanism of loss of energy from the main output beam. One known solution to this problem is to use either a cube polariser (or similar), consisting of two triangular (or similar) sections, or a plate polariser with a back surface coated anti-reflection (AR) for both p and s polarisations. Both of these options are however, complex. The AR coating for the plate polariser is more complex than standard coatings, and as such is both challenging to coating manufacturers and reduces the damage threshold of the device. It is thus an aim of the present invention to provide improved laser cavities for cavity dumping a pulsed laser output of high average power. It is a further aim of the present invention to provide an improved polariser for use in laser cavities. It is a particular aim of the present invention to provide a polariser which can withstand a high power laser and deliver a single output beam. Accordingly, the present invention provides a laser cavity for generating a pulsed laser output, comprising: gain medium for example a diode pumped gain medium including a light transmission axis; at least one first optical element disposed on one side of the gain medium to return laser light emitted from the one side of the gain medium back through the gain medium; a polarisation selective element disposed on the light transmission axis to the other side of the gain medium; at least one second optical element arranged to define a first optical path from the polarisation selective element along which laser light of one polarisation is in use directed; at least one third optical element arranged to define a second optical path from the polarisation selective element along which laser light of other polarisation is in use directed; and at least one electro-optical element disposed in one of the first and second optical paths and being operable to switch the polarity of the laser light transmitted therethrough such as to generate a pulsed laser output. For example first and second electro-optical elements may be disposed in the respective first and second optical paths. In another aspect the present invention provides a laser cavity for generating a pulsed laser output, comprising: gain medium for example a diode pumped gain medium including a light transmission axis; at least one optical element disposed on one side of the gain medium to return laser light emitted from the one side of the gain medium back through the gain medium; a polarisation selective element disposed on the light transmission axis to the other side of the gain medium; at least one further optical element arranged to define a cyclic optical path from the polarisation selective element along which laser light is in use directed in one of two directions depending on the polarisation of the laser light; and at least one electro-optical element disposed in the optical path and being operable to switch the polarity of the laser light transmitted therethrough such as to generate a pulsed laser output. Preferred aspects of the invention are further set out in the claims. Preferably the path between the gain medium and said first optical element is uninterrupted by polarisation altering elements. As a result the design is significantly simplified. Yet further preferably the electro-optical element is arranged to dump the cavity on operation. The cavity dumping arrangement allows an especially high peak power output. Preferably the first and second optical paths are detuned. Because the paths are of different lengths the effects of interference between the two arms are substantially mitigated, especially where the cavity is operated in the absence of seeding which improves the characteristics of the output beams in terms of spatial homogeneity and temporal and directional stability. The invention in this preferred form does not correct for thermal birefringence but instead provides a laser that has thermal birefringence in the gain medium for a polarisation switched application. This enables a greater peak power to be emitted from the laser, as well as enabling seeding of the laser with a pulsed or continuous wave laser source (regenerative amplification). This can lead to a more stable operation of the laser especially at high repetition rates. The omission of a polarisation altering element is particularly advantageous where the beam is many times (for example greater than ten times) transform limited. Preferably the pulsed laser output is output from the polarisation selective element. This arrangement allows cavity dumping and hence surprisingly high peak powers. In addition a single output pulse can be obtained even if the cavity is seeded by another short pulsed laser source. The invention also extends, to a polariser including a first polarisation separating optical surface, for example having a dielectric film coating, and second and third optical surfaces through which laser light is in use transmitted. The present invention also provides a laser cavity including the above-described polariser. The polariser is preferably prism-shaped, for example having a triangular cross- section. The triangular polariser enables the application of a simple dielectric anti- reflection coating on two surfaces, and a standard polarisation separating coating on the other. The two surfaces that have a simple AR coatings are aligned near normal, to the laser beam. The coating can be a single layer, and still reduce loss inducing reflections to a useable level. The polariser can be used in the various cavities described herein. The present invention also provides a laser cavity for generating a pulsed laser output, comprising: a gain medium including a light transmission axis; at least one optical element disposed on one side of the gain medium to return laser light emitted from the one side of the gain medium back through the gain medium; a polarisation selective element disposed on the light transmission axis to the other side of the gain medium; at least one further optical element arranged to define a cyclic optical path from the polarisation selective element along which laser light is in use directed in one of two directions depending on the polarisation of the laser light; and at least one electro- optical element formed of beta barium borate disposed in the optical path and being operable to switch the polarity of the laser light transmitted therethrough such as to generate a pulsed laser output. The present invention further provides a laser cavity for generating a pulsed laser output, comprising: a gain medium including a light transmission axis; at least one first optical element disposed on one side of the gain medium to return laser light emitted from the one side of the gain medium back through the gain medium; a polarisation selective element, for example the above described polariser, disposed on the light transmission axis to the other side of the gain medium; at least one second optical element arranged to define a first optical path from the polarisation selective element along which laser light of one polarisation is in use directed; at least one third optical element arranged to define a second optical path from the polarisation selective element along which laser light of other polarisation is in use directed; and at least one electro-optical element formed of beta barium borate disposed in one of the first and second optical paths and being operable to switch the polarity of the laser light transmitted therethrough such as to generate a pulsed laser output. Viewed from one aspect the present invention provides a pulsed laser comprising: a laser cavity; a laser gain medium within said laser cavity; an optical switch element operable to change a light loss level of said laser cavity and laser gain medium between a first loss state in which laser output is inhibited and second loss state in which laser output is not inhibited such that a laser pulse is generated; and a pulse controller operable to provide a control signal to said switch element to control repeated switching between said first loss state and said second loss state to trigger generation of a stream of laser pulses; wherein upon initiation of generation of laser pulses, said pulse controller generates control signals that progressively increase a period for which said laser cavity and laser gain medium are in said second loss state or progressively change said cavity light loss level of said second loss state towards a steady state second loss period such that laser pulse energy at startup is reduced compared to if said steady state second loss period or level was used immediately upon startup. By "steady state" is meant a regular, non-changing period. The invention recognises that suppression of a potentially damaging first pulse may be achieved by progressively increasing the period for which the low (second) loss state is adopted up to a steady state low loss period. In this way, the initial pulses produced are clipped such that the energy that they contain is reduced. In preferred embodiments the minimum period of the second loss state adopted at startup is too low to allow lasing to occur. In this way, a smooth and progressive startup of lasing may be aided. Whilst it will be appreciated that the technique of gradually increasing the second loss state period may be applied to different sorts of optical switch elements, including both acousto-optic and electro-optic switch elements, the invention is particularly well suited for use with electro-optic switch elements as these are able to change state at high speed and so achieve a controllable and repeatable clipping effect of the initial pulses. Whilst the optical switch elements could contain many different components, preferred embodiments are ones in which the optical switch element comprises one or more of at least one mirror, polarisation-modifying optics, focusing or diverging optics, beam-profiling optics, an electro-optic cell and an acousto-optic cell. These elements allow compensation for other characteristics of the laser system to be made whilst still providing the desired suppression of initial startup pulses having an excessive energy. Niewed from yet another aspect the present invention provides a method of operating a pulsed laser having a laser cavity, a laser gain medium within said laser cavity, an optical switch element operable to change a light loss level of said laser cavity and laser gain medium between a first loss state in which laser output is inhibited and a second loss state in which laser output is not inhibited such that a laser pulse is generated, and a pulse controller operable to provide a control signal to said optical switch element to control repeated switching between said first loss state and said second loss state to trigger generation of a stream of laser pulses; said method comprising the step of: upon initiation of generation of laser pulses, generating control signals that progressively increase a period for which said laser cavity and laser gain medium are in said second loss state towards a steady state second loss period such that laser pulse energy at startup is reduced compared to if said steady state second loss period was used immediately upon startup. Niewed from one aspect the present invention provides a pulsed laser comprising: a laser cavity; a laser gain medium within said laser cavity; an electro-optic switch element operable to change a light loss level of said laser cavity and laser gain medium between a first loss state in which laser output is inhibited and second loss state in which laser output is not inhibited such that a laser pulse is generated; and a pulse controller operable to provide a control signal to said electro-optic switch element to control repeated switching between said first loss state and said second loss state to trigger generation of a stream of laser pulses; wherein upon initiation of generation of laser pulses, said pulse controller generates control signals that progressively change said cavity light loss level of said second loss state towards a steady state second loss level such that laser pulse energy at startup is reduced compared to if said steady state second loss level was used immediately upon startup. The invention uses an electro-optic switch element as compared with an acousto- switch element. Such an electro-optic switch element operates in a fundamentally different physical manner to bring about its switching and is able to produce much shorter laser pulses. However, in order to guard against excessively high energy pulses at startup, the invention provides a pulse controller that progressively decreases the low (second) loss state level from a startup low loss state level towards a steady state low loss state level in such a way that the laser pulse energy at startup is reduced to inhibit excessively high energy initial pulses whilst not imposing a limitation upon the energy level of the subsequent pulses that may be produced. A particularly safe way in which the laser can be started up is one in which the electro- optic switch element is controlled at startup to provide a light loss level that is too high even in the low loss state to allow lasing and then progressively to decrease this low loss state level such that lasing will start in a controlled manner. In addition to altering the loss level states between which the electro-optic switch element is moved upon startup, preferred embodiments also act to control the period for which the electro-optic switch element remains in the second loss state to gradually increase upon startup in a manner that suppresses generation of high energy initial pulses. In preferred embodiments the suppression of excessively high energy initial pulses is improved when the minimum initial duration of the second loss state at startup is sufficiently short to not allow a laser pulse to be generated. This low loss state duration can then be gradually increased so that lasing is commenced in a controlled manner. It will be appreciated that the electro-optic switch element could take many forms. However, preferred levels of control and matching with other characteristics of a high- power system may be achieved when the electro-optic switch element comprises one or more of at least one mirror, polarisation-modifying optics, focusing optics and beam-profiling optics. It will be appreciated that the pulse controller as well as serving to control laser startup to avoid system damage may also be operable to control the pulse frequency of the laser pulses that are generated. Niewed from another aspect the present invention provides a method of operating a pulsed laser having a laser cavity, a laser gain medium within said laser cavity, an electro-optic switch element operable to change a light loss level of said laser cavity and laser gain medium between a first loss state in which laser output is inhibited and a second loss state in which laser output is not inhibited such that a laser pulse is generated, and a pulse controller operable to provide a control signal to said electro- optic switch element to control repeated switching between said first loss state and said second loss state to trigger generation of a stream of laser pulses; said method comprising the step of: upon initiation of generation of laser pulses, generating control signals that, progressively change said cavity light loss level of said second loss state towards a steady state second loss level such that laser pulse energy at startup is reduced compared to if said steady state second loss level was used immediately upon startup. Preferred embodiments of the present invention will now be described hereinbelow by way of example only with reference to the accompanying drawings, in which: Figure 1 schematically illustrates a laser cavity in accordance with a first embodiment of the present invention; Figure 2 schematically illustrates a prism-shaped polariser in accordance with a preferred embodiment of the present invention; Figure 3 schematically illustrates a laser cavity in accordance with a second embodiment of the present invention; Figure 4 schematically illustrates a laser cavity in accordance with a third embodiment of the present invention; Figure 5 schematically illustrates a pulsed laser; Figure 6 schematically illustrates the generation of an excessively high energy laser pulse upon startup; Figures 1, 8 and 9 illustrate embodiments of the present invention by which control of the switching between a high loss state and a low loss state of the cavity during startup can be used to suppress generation of an excessively high energy initial pulse; and Figure 10 schematically illustrates an electro-optical switch element. Figure 1 illustrates a laser cavity in accordance with a first embodiment of the present invention for cavity dumping a pulsed laser output of high average power. The laser cavity comprises a diode pumped gain medium 1 having a light transmission axis extending therethrough. The laser cavity further comprises a first optical element 3, in this embodiment a high reflector, disposed on the light transmission axis to one side of the gain medium 1 to reflect laser light emitted from the one side of the gain medium 1 back through the gain medium 1. The laser cavity further comprises a polarisation selective element 5, in this embodiment a prism-shaped polariser including coated optical surfaces 5a, 5b, 5c, disposed on the light transmission axis to the other side of the gain medium 1. In this embodiment one, the first, optical surface 5 a faces the gain medium 1 and includes a thin dielectric film coating 7. The other, second and third optical surfaces 5b, 5c are each angled substantially normal to the laser light transmitted therethrough and include anti-reflective coatings 9, 9. In a preferred embodiment the bandwidth of the laser light is maintained such as to be sufficiently narrow that dispersion by the polariser 5 of differing wavelengths in different directions does not impact significantly on the laser operation or the spatial beam quality. In an alternative embodiment the polarisation selective element 5 could be a plane parallel plate polariser which includes a thin dielectric film coating on the optical surface facing the gain medium 1 and an anti-reflective coating on the other optical surface. Although a plane parallel plate polariser could be used as the polarisation selective element 5, the prism-shaped polariser is clearly advantageous over the plane parallel plate polariser. Available information confirms that, as compared to a standard plane parallel plate polariser with an anti-reflective coating on the back surface, the damage threshold to laser peak power density can be increased by about 20 %, and, in comparison to a standard plane parallel plate polariser without an anti-reflective coating on the back surface, the useful laser power transmitted is increased by about 18 %. The laser cavity further comprises a second optical element 11, in this embodiment a high reflector, arranged to reflect laser light to or from the second optical surface 5b of the polariser 5. The laser cavity further comprises a third optical element 13, in this embodiment a high reflector, arranged to reflect laser light to or from both the second optical element 11 and the first optical surface 5a of the polariser 5. In this embodiment the polariser 5 and the first and second optical elements 11, 13 define a triangular optical path about which laser light is cyclically directed in the high Q state. It should be understood, however, that other optical path shapes could be employed. For example, a square optical path could be defined by the inclusion of a further optical element. The laser cavity further comprises an electro-optical switch element 15 located on the optical path between the second and third optical elements 11, 13 and being operable to switch, that is, rotate, the polarisation of the laser light transmitted therethrough through 90 degrees. In this embodiment the electro-optical element 15 is formed of beta barium borate (BBO) which exhibits less thermal distortion and also enables higher switching rates as compared to the conventional materials of KH 2 PO 4 (KDP) and KD 2 PO 4 (KD*P). The laser cavity further comprises a seed light source 17 operable in one mode of operation to seed the gain medium 1 with laser light, either as a pulse or as a continuous wave (CW). In this embodiment the laser cavity can be seeded through the polariser 5. In other embodiments the laser cavity could be seeded through or off any of the optical elements. In another mode of operation the gain medium 1 is driven from the noise therein, referred to as spontaneous decay. Operation of the laser cavity in cavity dumping a pulsed laser output of high average power will now be described hereinbelow. In one mode of use, the electro-optical element 15 is switched such that the laser cavity is in the high Q state, that is, the laser light is trapped in the cavity, and the flux of the laser light in the cavity increases from noise. At or near the peak of the laser flux, the electro-optical element 15 is switched, in a time period less than or of the order of the time period of the cavity round trip, such that the cavity is in the low Q state and a pulse of laser light is dumped from the cavity. In another mode of use, the electro-optical element 15 is switched such that the laser cavity is in the high Q state and the cavity is seeded by the seed light source 17. At or near the peak of the flux of the laser light in the laser cavity, the electro-optical element 15 is switched, in a time period less than or of the order of the time period of the cavity round trip, such that the cavity is in the low Q state and a pulse of laser light is dumped from the cavity. The advantages of seeding the cavity are to improve the spatial beam quality, deliver an output pulse of shorter duration, and provide control of the output pulse profile in time by enabling control of the seed input pulse profile in time. Figure 3 schematically illustrates a laser cavity in accordance with a second embodiment of the present invention for cavity dumping a pulsed laser output of high average power. The laser cavity comprises a diode pumped gain medium 1 having a light transmission axis extending therethrough. The laser cavity further comprises a first optical element 3, in this embodiment a high reflector, disposed on the light transmission axis to one side of the gain medium 1 to reflect laser light emitted from the one side of the gain medium 1 back through the gain medium 1. The laser cavity further comprises a polarisation selective element 5, in this embodiment a prism-shaped polariser including coated optical surfaces 5a, 5b, 5c, disposed on the light transmission axis to the other side of the gain medium 1. In this embodiment one, the first, optical surface 5 a faces the gain medium 1 and includes a thin dielectric film coating 7. The other, second and third optical surfaces 5b, 5 c are each angled substantially normal to the laser light transmitted therethrough and include anti-reflective coatings 9, 9. In a preferred embodiment the bandwidth of the laser light is maintained such as to be sufficiently narrow that dispersion by the polariser 5 of differing wavelengths in different directions does not impact significantly on the laser operation or the spatial beam quality. In an alternative embodiment the polarisation selective element 5 could be a plane parallel plate polariser which includes a thin dielectric film coating on the optical surface facing the gain medium 1 and an anti-reflective coating on the other optical surface. Although a plane parallel plate polariser could be used as the polarisation selective element 5, the prism-shaped polariser has clear advantages over the plane parallel plate polariser. As mentioned hereinabove, available information has confirmed that, as compared to a standard plane parallel plate polariser with an anti-reflective coating on the back surface, the damage threshold to laser peak power density can be increased by about 20 %, and, in comparison to a standard plane parallel plate polariser without an anti- reflective coating on the back surface, the useful laser power transmitted is increased by about 18 %. The laser cavity further comprises a second optical element 11, in this embodiment a high reflector, arranged to reflect laser light back to the second optical surface 5b of the polariser 5. The laser cavity further comprises a third optical element 13, in this embodiment a high reflector, arranged to reflect laser light back to the first optical surface 5 a of the polariser 5. In this embodiment the polariser 5 and respective ones of the second and third optical elements 11, 13 define separate optical paths as ones of a fork. In this embodiment the second and third optical elements 11, 13 are arranged such that the path lengths of the optical paths differ i.e. are de-tuned for example by the order of a few centimetres. The advantage of arranging the optical paths to be of different length is that interference between the laser light of different polarity travelling through each optical path is avoided, leading to improved output spatial quality and temporal and direction stability. The path lengths can be closely matched in laser cavities predisposed to higher output spatial quality, for example with less thermal loading in the gain medium 1, without immediate loss of spatial quality. In another embodiment the path lengths could be the same. The laser cavity further comprises first and second electro-optical switch elements 15, 16 located in respective ones of the optical paths between the second and third optical elements 11, 13 and the polariser 5 and each being operable to switch, that is, rotate, the polarisation of the laser light transmitted therethrough through 90 degrees. In this embodiment the electro-optical elements 15, 16 are each formed of beta barium borate (BBO) which, as mentioned hereinabove, exhibits less thermal distortion and also enables higher switching rates as compared to the conventional materials of KH 2 PO 4 (KDP) and KH 2 PO 4 (KD*P). The laser cavity further comprises a seed light source 17 operable in one mode of operation to seed the gain medium 1 with laser light, either as a pulse or as a continuous wave (CW). In this embodiment the laser cavity can be seeded through the polariser 5. In other embodiments the laser cavity could be seeded through or off any of the optical elements. In another mode of operation the gain medium 1 is driven from the noise therein, referred to as spontaneous decay. Operation of the laser cavity in cavity dumping a pulsed laser output of high average power will now be described hereinbelow. In one mode of use, the electro-optical elements 15, 16 are switched such that the laser cavity is in the high Q state, that is, the laser light is trapped in the cavity, and the flux of the laser light in the cavity increases from noise. At or near the peak of the laser flux, the electro-optical elements 15, 16 are switched, in a time period less than or of the order of the time period of the cavity round trip, such that the cavity is in the low Q state and a pulse of laser light is dumped from the cavity. In another mode of use, the electro-optical elements 15, 16 are switched such that the laser cavity is in the high Q state and the cavity is seeded by the seed light source 17. At or near the peak of the flux of the laser light in the cavity, the electro-optical elements 15, 16 are switched, in a time period less than or of the order of the time period of the cavity round trip, such that the cavity is in the low Q state and a pulse of laser light is emitted from the cavity. As mentioned hereinabove, the advantages of seeding the cavity are to improve the spatial beam quality, deliver an output pulse of shorter duration, and provide control of the output pulse profile in time by enabling control of the seed input pulse profile in time. Figure 4 illustrates a laser cavity in accordance with a third embodiment of the present invention for cavity dumping a pulsed laser output of high average power. The laser cavity of this embodiment is very similar to the laser cavity of the above- described second embodiment, and thus, in order to avoid unnecessary duplication of description, only the differences will be described in detail. This embodiment differs only in that the second and third optical elements 11, 13 comprise stigmatism imparting optical elements, in this embodiment astigmatic mirrors, and that the laser cavity further comprises fourth and fifth, stigmatism imparting optical elements 19, 21, in this embodiment astigmatic lenses, disposed in respective ones of the optical paths between the polariser 5 and the respective electro- optical element 15, 16. In the context of the present invention, it is meant by stigmatism imparting that a stigmatism is imparted to non-stigmatic light and the stigmatism of stigmatic light is altered. In other embodiments the stigmatism imparting optical elements could comprise ones of cylindrical or tilted lenses and mirrors. Operation of the laser cavity is the same' as for the above-described second embodiment. By separately compensating for the differing astigmatism imparted by the gain medium 1 to the horizontal and vertical polarisations, a laser output is achieved which is free of astigmatism and does not require the use of two matched gain media and an optical rotator, such as a polarisation rotator formed preferably of quartz. The latter compensation technique is generally considered to be incomplete when applied to real laser cavities in relying on the gain media having identical lensing characteristics and the optical ray passing the separate gain media in exactly the same manner. As will be appreciated, these conditions are difficult to meet in real laser cavities. Figure 5 schematically illustrates a pulsed laser 2 having a laser cavity 4. The laser cavity 4 is bounded by high reflectors 6 and 8. (The high reflector 8 could be replaced with a partial reflector in certain designs to stop the cavity flux becoming too high.) A pumped laser gain medium 10 is disposed within the laser cavity 4. An electro-optic switch element 12 is also disposed within the laser cavity 4. The electro-optic switch element 12 also includes a polarisation element which can be used to direct radiation out of the laser cavity 4 when it is desired to "dump" the cavity. A pulse controller 14 coupled to the electro-optic switching element 12 serves to switch the electro-optic switching element 12 between a high loss state and a low loss state. In the case of the use of an acousto-optic switch element (which creates a diffraction grating from internal acoustic waves that diffracts the radiation so causing a loss) in place of the electro-optic switch element 12, a partial reflector would replace the high reflector 8 to provide an exit for radiation from the laser cavity 4. In steady state operation, the high loss state of the electro-optic switch element 12 is such that lasing within the laser cavity 4 is inhibited whilst the low loss state of the electro-optic switch element 12 is such that lasing within the laser cavity 4 will occur. The rapid response of the electro-optic switch element 12 in moving between these two different loss states enables very short laser pulses to be generated. In the case of the use of the electro-optic switching element 12, highly reflecting mirror 6 and 8 are used at both ends of the cavity and there is the added advantage of being able to seed and dump the cavity to produce shorter pulses. If built up from laser noise, there is a constant laser flux everywhere in the laser cavity 4 until the electro-optic switch element is switched, when the cavity flux can be dumped (output) in a round trip time, hence creating an output pulse. If seeded, a pulse goes around in the cavity until one chooses to dump it out, which may enable an even shorter laser pulse to be produced. Figure 6 schematically illustrates the possibility of laser damage that can occur with an uncontrolled startup. In the example illustrated, the first laser pulse 16 that is generated may have a peak intensity/power that is sufficient to produce damage to the system as a whole. This excessively high power level of the first pulse may be the result of a greater population inversion within the laser gain medium 10 at startup than in the steady start or other characteristics of the pulsed laser system 2 that vary at startup compared to the steady state. As shown in the example, subsequent laser pulses generated are below the level at which damage may occur and eventually settle to a quasi-constant level as the pulsed laser system 2 equalises to its steady state condition. Figure 7 illustrates a first technique by which the pulse controller 14 can modify the control pulses (control voltages, which may be negative) supplied to the electro-optic switch element 12 in a manner that reduces the likelihood of generation of an excessively high energy initial pulse. As shown in the lower graph, the loss level associated with the electro-optic switch element 12 is switched between a high loss level HL and a low loss level beneath this. The high loss level HL is above a lasing threshold such that lasing is inhibited by the degree of loss within the laser cavity 4 when the electro-optic switch 12 has this high loss level HL. At startup, the first low loss level state adopted has a level L 0 beneath the high loss level HL. This first low loss level state is insufficiently low to allow lasing to occur. Subsequent pulsed low loss level states adopted decrease the low loss state level progressively through the lasing threshold such that lasing gradually becomes sustainable until a steady state low loss level state L 3 is reached. In the example illustrated, the progressive decrease in the low loss level is shown as occurring relatively rapidly whereas in practice this may take place over many more laser pulse cycles. Figure 8 illustrates an alternative way in which the pulse controller 14 may generate control pulses that suppress the generation of an excessively high energy initial pulse. In this embodiment the period for which the control pulse replaces the electro-optic switch element 12 in the low loss state is progressively increased from a period t 0 to a steady state period t 3 . The initial low loss state period t 0 is insufficient to allow lasing to occur. As this period gradually increases, laser flux starts to build up, but is clipped in comparison to what would be their normal duration by return of the laser cavity 4 to a high loss state using the electro-optic switch 12 prior to the energy stored in the laser gain medium 10 being depleted by the generation of the laser pulse. Again, the increase in the low loss state period illustrated in Figure 4 takes place over many fewer laser pulse cycles than may in practice be utilised. Figure 9 illustrates a third embodiment in which a combination of the techniques of Figures 7 and 8 is used. In this embodiment both the level of the low loss state is progressively decreased and the duration of the low loss state progressively increased in a manner whereby lasing is initially not sustainable and then progressively becomes sustainable in a manner in which the maximum pulse energy is not sufficient to render damage to the laser system 2. Figure 10 schematically illustrates an example electro-optic switching element 12 in more detail. This electro-optic switch incorporates an electro-optic cell 18, containing a material that exhibits the Pockels effect (such as KDP, LiNO 3 or BBO), and a polarisation selective element 20. The electro-optic switch switches the polarisation of light passing through it in dependence upon a control voltage applied by the pulse controller 14. An electro-optic cell can switch slowly (order of many round trips), in which case it can be used in a similar way to an acousto-optic switch. There is still an advantage of an electro-optic switch though, in that it can hold off greater cavity gain from lasing. If switched fast, electro-optic cells enable short pulse production (a few nanoseconds is possible), even for high average power laser emissions. They also enable seeding. Seeding may give the advantage of improved spatial beam quality as well as a shorter output pulse. It will be appreciated that in the embodiments of Figures 4 and 5, the electro-optic switch 12 could be replaced by an acousto-optic switch using the same control techniques to stop damaging pulses on start up. In one modification of the laser cavity of the above-described first embodiment, the single electro-optical element 15 could be replaced by two electro-optical elements separated by an optical rotator, typically a polarisation rotator preferably formed of quartz. This arrangement compensates for the polarisation birefringence within electro-optical elements and also partially within the gain medium 1. In one modification of the laser cavity of the above-described second embodiment, one of the electro-optical elements 15, 16 could be omitted from one optical path. In operation, the laser light of the polarity which travels through the one optical path including no electro-optical element is eventually depolarised sufficiently by the gain medium 1 as to travel through the other optical path and be controlled by the electro- optical element in that other path either to remain in the laser cavity or be switched out. This modified laser cavity offers the particular advantage that the laser output is of a single polarisation state. A disadvantage is that the output pulse may be longer in duration.

Claims: CLAIMS 1. A laser cavity for generating a pulsed laser output, comprising: a diode pumped gain medium including a light transmission axis; at least one first optical element disposed on one side of the gain medium to return laser light emitted from the one side of the gain medium back through the gain medium; a polarisation selective element disposed on the light transmission axis to the other side of the gain medium; at least one second optical element arranged to define a first optical path from the polarisation selective element along which laser light of one polarisation is in use directed; at least one third optical element arranged to define a second optical path from the polarisation selective element along which laser light of other polarisation is in use directed; and at least one electro-optical element disposed in one of the first and second optical paths and being operable to switch the polarity of the laser light transmitted therethrough such as to generate a pulsed laser output. 2. The laser cavity of claim 1, further comprising at least one further electro- optical element disposed in the other of the first and second optical paths and being operable in conjunction with the at least one electro-optical element to switch the polarity of the laser light transmitted therethrough. 3. A laser cavity for generating a pulsed laser output, comprising: a gain medium including a light transmission axis; at least one first optical element disposed on one side of the gain medium to return laser light emitted from the one side of the gain medium back through the gain medium; a polarisation selective element disposed on the light transmission axis to the other side of the gain medium; at least one second optical element arranged to define a first optical path from the polarisation selective element along which laser light of one polarisation is in use directed; at least one third optical element arranged to define a second optical path from the polarisation selective element along which laser light of other polarisation is in use directed; and first and second electro-optical elements disposed in the respective ones of the first and second optical paths and being operable to switch the polarity of the laser light transmitted therethrough such as to generate a pulsed laser output. 4. The laser cavity of any of claims 2 to 3, wherein at least one of the first and second or further electro-optical elements is formed of beta barium borate. 5. A laser cavity for generating a pulsed laser output, comprising: a gain medium including a light transmission axis; at least one first optical element disposed on one side of the gain medium to return laser light emitted from the one side of the gain medium back through the gain medium; a polarisation selective element disposed on the light transmission axis to the other side of the gain medium; at least one second optical element arranged to define a first optical path from the polarisation selective element along which laser light of one polarisation is in use directed; at least one third optical element arranged to define a second optical path from the polarisation selective element along which laser light of other polarisation is in use directed; and at least one electro-optical element formed of beta barium borate disposed in r one of the first and second optical paths and being operable to switch the polarity of the laser light transmitted therethrough such as to generate a pulsed laser output. 6. The laser cavity of claim 1, 3 or 5 wherein the at least one first optical element comprises a reflective element. 7. The laser cavity of claim 1, 3 or 5 or 6, wherein the polarisation selective element comprises a polariser. 8. The laser cavity of claim 7, wherein the polariser comprises a plane parallel- plate polariser. 9. The laser cavity of claim 8, wherein the polariser includes an anti-reflective coating on the back surface thereof. 10. The laser cavity of claim 7, wherein the polariser comprises a prism-shaped polariser. 11. The laser cavity of claim 10, wherein the polariser includes a first optical surface which includes a dielectric film coating and is in optical communication with the other side of the gain medium and the at least one third optical element, a second optical surface which is in optical communication with the at least one second optical element, and a third optical surface which is the surface through which laser output pulses are in use dumped. 12. The laser cavity of claim 11, wherein the second and third optical surfaces of the polariser are arranged such that the laser light transmitted therethrough is not more than 30 degrees from a direction normal to the respective optical surface. 13. The laser cavity of claim 12, wherein the second and third optical surfaces of the polariser are arranged such that the laser light transmitted therethrough is not more than 10 degrees from a direction normal to the respective optical surface. 14. The laser cavity of claim 13, wherein the second and third optical surfaces of the polariser are arranged such that the laser light transmitted therethrough is substantially normal to the respective optical surface. 15. The laser cavity of any of claims 11 to 14, wherein the second and third optical surfaces of the polariser include an anti-reflective coating. 16. The laser cavity of any of claims 1, 3 or 5 to 15, further comprising at least one further electro-optical element disposed in the other of the first and second optical paths and being operable in conjunction with the at least one electro-optical element to switch the polarity of the laser light transmitted therethrough. 17. The laser cavity of any of claim 16, wherein the at least one further electro- optical element is formed of beta barium borate. 18. The laser cavity of any of claims 1, 2, 3, or 5 to 17, wherein at least one of the first and second optical paths includes at least one stigmatism imparting optical element. 19. The laser cavity of claim 18, wherein at least one of the at least one second and at least one third optical elements is a stigmatism imparting optical element. 20. The laser cavity of claim 18, wherein both the first and second optical paths include at least one stigmatism imparting optical element. 21. The laser cavity of claim 20, wherein the at least one second and at least one third optical elements are stigmatism imparting optical elements. 22. The laser cavity of any of claims 1 to 3 or 5 to 21, wherein the at least one second optical element comprises a reflective element. 23. The laser cavity of any of claims 1 to 3 or 5 to 23, wherein the at least one third optical element comprises a reflective element. 24. The laser cavity of any of claims 1 to 3 or 5 to 23, further comprising a seed light source for seeding the cavity with laser light. 25. The laser cavity of any of claims 1 to 3 or 5 to 24, wherein the gain medium is a diode pumped gain medium. 26. The laser cavity of any preceding claim in which the path between the gain medium and said first optical element is uninterrupted by polarisation altering elements. 27. The laser cavity of any preceding claim in which the electro-optical element is arranged to dump the cavity on operation. 28. The laser cavity of any preceding claim in which the first and second optical paths are de-tuned. 29. The laser cavity of any preceding claims in which the pulsed laser output is output from the polarisation selective element. 30. A laser cavity for generating a pulsed laser output, comprising: a diode pumped gain medium including a light transmission axis; at least one optical element disposed on one side of the gain medium to return laser light emitted from the one side of the gain medium back through the gain medium; a polarisation selective element disposed on the light transmission axis to the other side of the gain medium; at least one further optical element arranged to define a cyclic optical path from the polarisation selective element along which laser light is in use directed in one of the directions depending on the polarisation of the laser light; and at least one electro-optical element disposed in the optical path and being operable to switch the polarity of the laser light transmitted therethrough such as to generate a pulsed laser output. 31. The laser cavity of claim 30, wherein the polariser includes a first optical surface which includes a dielectric film coating and is in optical communication with the other side of the gain medium, a second optical surface which is in optical communication with the at least one further optical element, and a third optical surface which is the surface through which laser output pulses are in use dumped. 32. A laser cavity for generating a pulsed laser output, comprising: a gain medium including a light transmission axis; at least one optical element disposed on one side of the gain medium to return laser light emitted from the one side of the gain medium back through the gain medium; a polarisation selective element disposed on the light transmission axis to the other side of the gain medium; at least one further optical element arranged to define a cyclic optical path from the polarisation selective element along which laser light is in use directed in one of the two directions depending on the polarisation of the laser light; and at least one electro-optical element formed of beta barium borate disposed in the optical path and being operable to switch the polarity of the laser light transmitted therethrough such as to generate a pulsed laser output. 33. The laser cavity of claim 32, wherein the polariser comprises a prism-shaped polariser and preferably includes a first optical surface which includes a dielectric film coating and is in optical communication with the other side of the gain medium, a second optical surface which is in optical communication with the at least one further optical element, and a third optical surface which is the surface through which laser output pulses are in use dumped. 34. The laser cavity of any of claims 30 to 33, wherein the at least one further optical element comprises first and second further optical elements, with the first further optical element reflecting laser light to or from the second optical surface of the polariser and the second further optical element reflecting laser light to or from the first optical surface of the polariser. 35. The laser cavity of claim 34, wherein the at least one electro-optical element is disposed between the first and second further optical elements. 36. The laser cavity of any of claims 30 to 33, comprising first and second electro- optical elements and an optical rotator disposed therebetween. 37. The laser cavity of any of claims 30 to 33, further comprising a seed light source for seeding the cavity with laser light. 38. The laser cavity of any of claims 30 to 33, wherein the gain medium is a diode pumped gain medium. 39. The laser cavity of any of claims 30 to 38 further including the features of any of claims 1 to 29. 40. A polariser including a first polarisation separating optical surface and second and third optical surfaces through which laser light is in use transmitted. 41. The polariser of claim 40, wherein the second and third optical surfaces are arranged such that the laser light transmitted therethrough is not more than 30 degrees from a direction normal to the respective optical surface. 42. The polariser of claim 41, wherein the second and third optical surfaces are arranged such that the laser light transmitted therethrough is not more than 10 degrees from a direction normal to the respective optical surface. 43. The polariser of claim 42, wherein the second and third optical surfaces are arranged such that the laser light transmitted tlierethrough is substantially normal to the respective optical surface. 44. The polariser of any of claims 4 to 43, wherein the second and third optical surfaces include an anti-reflective coating. 45. A laser cavity including the polariser of any of claims 40 to 44. 46. A laser cavity for generating a pulsed laser output, comprising: a gain medium including a light transmission axis; at least one optical element disposed on one side of the gain medium to return laser light emitted from the one side of the gain medium back through the gain medium; the polariser of any of claims 40 to 44 disposed on the light transmission axis to the other side of the gain medium; at least one further optical element arranged to define a cyclic optical path from the polariser along which laser light is in use directed in one of two directions depending on the polarisation of the laser light; and at least one electro-optical element disposed in the optical path and being operable to switch the polarity of the laser light transmitted therethrough such as to generate a pulsed laser output. 47. The laser cavity of claim 46 further including the features of any one of claims 1 to 39. 48. A laser cavity for generating a pulsed laser output, comprising: a gain medium including a light transmission axis; at least one first optical element disposed on one side of the gain medium to return laser light emitted from the one side of the gain medium back through the gain medium; the polariser of any of claims 40 to 44 disposed on the light transmission axis to the other side of the gain medium; at least one second optical element arranged to define a first optical path from the polarisation selective element along which laser light of one polarisation is in use directed; at least one third optical element arranged to define a second optical path from the polarisation selective element along which laser light of other polarisation is in use directed; and at least one electro-optical element disposed in one of the first and second optical paths and being operable to switch the polarity of the laser light transmitted therethrough such as to generate a pulsed laser output. 49. The laser cavity of 48 further including the features of any one of claims 1 to 39. 50. A pulsed laser comprising: a laser cavity; a laser gain medium within said laser cavity; an optical switch element operable to change a light loss level of said laser cavity and laser gain medium between a first loss state in which lasing is inhibited and second loss state in which lasing is not inhibited such that a laser pulse is generated; and a pulse controller operable to provide a control signal to said optical switch element to control repeated switching between said first loss state and said second loss state to trigger generation of a stream of laser pulses; wherein upon initiation of generation of laser pulses, said pulse controller generates control signals that progressively increase a period for which said laser cavity and laser gain medium are in said second loss state towards a steady state second loss period such that laser pulse energy at startup is reduced compared to if said steady state second loss period was used immediately upon startup. 51. A pulsed laser as claimed in claim 50, wherein upon startup a minimum period for which said laser cavity and laser gain medium are in said second loss state is too low to allow lasing. 52. A pulsed laser as claimed in any one of the preceding claims, wherein said optical switch element comprises an electro-optic cell or an acousto-optic cell. 53. A pulsed laser as claimed in any one of the preceding claims, wherein said optical switch element comprises at least one of : at least one mirror, polarisation-modifying optics; focusing optics or diverging optics; and/or beam-profiling optics. 54. A pulsed laser as claimed in any one of the preceding claims, wherein said control signal is a periodic signal with a control signal frequency matching a pulse frequency at which said laser pulses are generated. 55. A pulsed laser as claimed in any one of the preceding claims, wherein said optical switch element is an electro-optic switch element. 56. A method of operating a pulsed laser having a laser cavity, a laser gain medium within said laser cavity, an optical switch element operable to change a light loss level of said laser cavity and laser gain medium between a first loss state in which lasing is inhibited and second loss state in which lasing is not inhibited such that a laser pulse is generated, and a pulse controller operable to provide a control signal to said optical switch element to control repeated switching between said first loss state and said second loss state to trigger generation of a stream of laser pulses; said metliod comprising the step of: upon initiation of generation of laser pulses, generating control signals that progressively increase a period for which said laser cavity and laser gain medium are in said second loss state towards a steady state second loss period such that laser pulse energy at startup is reduced compared to if said steady state second loss period was used immediately upon startup. 57. A method as claimed in claim 56, wherein upon startup a minimum period for which said laser cavity and laser gain medium are in said second loss state is too low to allow lasing. 58. A method as claimed in any one of claims 56 and 57, wherein said optical switch element is an electro-optic switch element. 59. A pulsed laser comprising : a laser cavity; a laser gain medium within said laser cavity; an electro-optic switch element operable to change a light loss level of said laser cavity and laser gain medium between a first loss state in which lasing is inhibited and second loss state in which lasing is not inhibited such that a laser pulse is generated; and a pulse controller operable to provide a control signal to said electro-optic switch element to control repeated switching between said first loss state and said second loss state to trigger generation of a stream of laser pulses; wherein upon initiation of generation of laser pulses, said pulse controller generates control signals that progressively change said cavity light loss level of said second loss state towards a steady state second loss level such that laser pulse energy at startup is reduced compared to if said steady state second loss level was used immediately upon startup. _ 60. A pulsed laser as claimed in claim 59, wherein upon startup a minimum cavity light loss level is too high to allow lasing. 61. A pulsed laser as claimed in any one of claims 59 and 60, wherein upon initiation of generation of laser pulses, said pulse controller generates control signals that progressively increase a period for which said laser cavity and laser gain medium are in said second loss state towards a steady state second loss period such that laser pulse energy at startup is reduced compared to if said steady state second loss period was used immediately upon startup. 62. A pulsed laser as claimed in claim 3, wherein upon startup a minimum period for which said laser cavity and laser gain medium are in said second loss state is too low to allow lasing. 63. A pulsed laser as claimed in any one of the preceding claims, wherein said electro-optic switch element comprises at least one of: an electro-optic cell; at least one mirror; polarisation-modifying optics; focusing optics or diverging optics; and/or beam-profiling optics. 64. A pulsed laser as claimed in any one of the preceding claims, wherein said control signal is a periodic signal with a control signal frequency matching a pulse frequency at which said laser pulses are generated. 65. A method of operating a pulsed laser having a laser cavity, a laser gain medium within said laser cavity, an electro-optic switch element operable to change a light loss level of said laser cavity and laser gain medium between a first loss state in which lasing is inhibited and a second loss state in which lasing is not inhibited such that a laser pulse is generated, and a pulse controller operable to provide a control signal to said electro-optic switch element to control repeated switching between said first loss state and said second loss state to trigger generation of a stream of laser pulses; said method comprising the step of: upon initiation of generation of laser pulses, generating control signals that progressively change said cavity light loss level of said second loss state towards a steady state second loss level such that laser pulse energy at startup is reduced compared to if said steady state second loss level was used immediately upon startup. 66. A method as claimed in claim 65, wherein upon startup a minimum cavity light loss level is too high to allow lasing. 67. A method as claimed in any one of claims 65 and 66, wherein upon initiation of generation of laser pulses, generating control signals that progressively increase a period for which said laser cavity and laser gain medium are in said second loss state towards a steady state second loss period such that laser pulse energy at startup is reduced compared to if said steady state second loss period was used immediately upon startup. 68. A method as claimed in claim 67, wherein upon startup a minimum period for which said laser cavity and laser gain medium are in said second loss state is too low to allow lasing. 69. A pulsed laser comprising a laser cavity, a laser gain medium within said laser cavity and an optical switch element operable to switch said cavity and gain medium between a first loss state in which lasing is inhibited and a second loss state in which lasing occurs; and a pulse controller operable to control switcliing between said first and second loss states so as to progressively increase laser energy in the second loss state.

Inventors: ALLWOOD DANIEL A, KLUG DAVID R, MERCER IAN P, PALMER JASON, ALLWOOD, DANIEL, A., KLUG, DAVID, R., MERCER, IAN, P., PALMER, JASON
Applicants: ALLWOOD DANIEL A, KLUG DAVID R, MERCER IAN P, PALMER JASON, POWERLASE LTD, ALLWOOD, DANIEL, A., KLUG, DAVID, R., MERCER, IAN, P., PALMER, JASON, POWERLASE LIMITED
Application year: 2001
Main classification: H01S3
IPC codes: H01S3