WO2013079943A1

WO2013079943A1 - Stable thermal lens in a q- switched solid-state laser by pump light control

Info

Publication number: WO2013079943A1
Application number: PCT/GB2012/052946
Authority: WO
Inventors: Stephen Lee; William Alexander; Trevor Cook
Original assignee: Thales Holdings Uk Plc
Priority date: 2011-11-30
Filing date: 2012-11-29
Publication date: 2013-06-06
Also published as: GB201120668D0; GB2497108B; EP2786456A1; GB2497108A

Abstract

The invention relates to a pulsed laser apparatus comprising a rod- type solid-state gain medium (15) and at least one energy source for providing energy to the gain medium, wherein the apparatus is configured to selectively provide thermal compensation energy from the energy source to the gain medium when the thermal load of the gain medium and/or a pulse rate of the apparatus is below an optimum value. A transverse pumping scheme may be used with laser diode (35) pumping and being controlled by a pump diode driver (40). The resonator comprises further a HR mirror (30) and an output coupler (20) and a passive Q-switch (25). The curvature of the output coupler is chosen to realize a stable resonator for the gain medium (15) having a constant thermal lens. Between the pump pulses (60) resulting in lasing, additional pump pulses (70a-e) are provided with the laser being below threshold but keeping the thermal lens inside the gain medium constant.

Description

STABLE THERMAL LENS IN A Q - SWITCHED SOLID-STATE LASER BY PUMP LIGHT CONTROL

Field of the Invention

The present invention relates to a laser system that compensates for the effects of variable thermal lensing and an associated method of operation.

Background to the invention

Solid-state laser systems comprise a laser gain medium, such as Nd:YAG, provided within an optical resonator, wherein the laser gain medium may be supplied with pumping energy by an associated energy source, such as optical energy from a flash lamp and the like.

When the laser gain medium is pumped, a portion of the pumping energy is converted into heat within the gain medium. As a result of thermal conduction out of the gain medium towards a heat sink, a thermal gradient is created within the gain medium. Since the refractive index of the gain medium is dependent on the temperature, the thermal gradient can produce variations in the refractive index of the gain medium. These refractive index gradients may lead to optical distortions.

For example, the refractive index gradient can act in the same way as a virtual lens, and this effect is called thermal lensing. A laser resonator can be optimized for a static thermal lens, for example, through provision of a compensation lens or of curved mirror surfaces. However, the focal length of the thermal lens may vary as a result of changes in the thermal load experienced by the gain medium. This variation in the thermal lens may lead to changes in properties of the system, such as beam size, divergence and output energy.

In particular, in pulse pumped systems, the variation in thermal lens may lead to a change in beam and energy characteristics as the laser starts up and tends towards thermal equilibrium. Furthermore, changes in pulse rates may result in changes in the thermal loading experienced by the gain medium and thereby changes in the thermal lens. As such, a system that is optimised for a certain pulse rate may operate less effectively or not at all at other pulse rates.

Various techniques have been used in the art in order to address variable thermal lensing.

One method used in the art is to use a variable telescope within the laser resonator. The telescope may be varied to compensate for changes in the thermal lens. However, this solution adds extra components, cost and complexity to the system and may reduce its reliability.

Another method for reducing thermal lensing is to provide the gain medium in a zig-zag slab geometry, wherein the beam travels in a zig-zag path through the gain medium, undergoing total internal reflection at the side walls of the gain medium and wherein the end input/output faces are oriented at the Brewster angle such that the beam can pass through. However, the zig-zag slab geometry requires a very tightly toleranced and therefore expensive slab of gain medium. Furthermore, the mounting of the slab in such systems is critical and must be accurately done in order to prevent degredation of the beam. As a result, these systems are difficult and expensive to assemble.

It ts an aim of at least one embodiment of the present invention to provide an improved or at least alternative technique for compensating for variable thermal lensing.

Summary of Invention

According to a first aspect of the present invention is a pulsed laser apparatus comprising a gain medium and at least one energy source for providing energy to the gain medium, wherein the apparatus is configured to selectively provide thermal compensation energy from the energy source to the gain medium when the thermal load of the gain medium and/or a pulse rate of the apparatus is below an optimum value.

The energy source(s) may be configured to supply pump energy to the gain medium. The energy source(s) may comprise a diode, a diode laser, other laser sources, a flash lamp or the like.

The energy source may be operable to provide at least one laser firing pulse for generating optical (laser) output from the laser apparatus.

The energy source may be operable to provide thermal compensation energy to the gain medium in a non-lasing configuration in which substantially no optical (laser) output is produced by the apparatus, e.g. as a direct result of the thermal compensation energy. The energy source may be configured to provide the thermal compensation energy in one or more pulses. The energy source may be configured to provide the thermal compensation energy as a continuous wave. The energy source may be configured to provide the thermal compensation energy outwith the period in which laser firing pulses are produced. The energy source may be configured to produce thermal compensation energy between and/or before laser firing pulses.

The apparatus may comprise switching means for selectively switching the apparatus between lasing and non-lasing modes. The switching means may be configured to provide Q-switching. The switching means may comprise a passive Q switch. The passive Q switch may be switchable into a lasing (high Q) configuration when the intensity of light incident on the Q switch is above a lasing threshold.

The thermal compensation energy may be below the lasing threshold. The laser firing pulses may be above the lasing threshold.

The thermal compensation energy may be provided in pulses having substantially the same peak power as the laser firing pulses but a shorter duration than the laser firing pulses. The thermal compensation energy may be provided in pulses having a lower peak power than the laser firing pulses but a longer duration. The thermal compensation energy may be provided in a continuous wave having a lower peak power than the laser firing pulses.

The switching means may comprise an active Q switch. The provision of thermal compensation energy from the energy source may comprise applying the energy to the gain medium from the energy source whilst the apparatus is placed in a non-lasing mode through use of the switching means.

The apparatus may be configured to selectively apply thermal compensation energy from the energy source in a non-lasing configuration upon start-up of the apparatus and/or when a predetermined amount of time has elapsed since the system was last operated.

The amount of thermal compensation energy supplied may be variable. The amount of thermal compensation energy supplied may be dependent on the difference between a selected or current thermal load of the gain medium and the optimum thermal load of the gain medium. The amount of thermal compensation energy supplied may be dependent on the difference between the current pulse rate of the laser apparatus and the optimum pulse rate of the laser apparatus.

The pulse rate of the laser apparatus may comprise a pulse rate at which laser firing pulses are produced by the energy source and/or a pulse rate at which pulses of optical (laser) output is produced by the laser apparatus.

The apparatus may be configured to selectively apply energy from the energy source in a non-lasing configuration so as to control the thermal load of the gain medium to be constant. The optimum level of thermal energy of the gain medium may be a level of thermal energy of the gain medium for which static thermal lens compensation of the laser apparatus has been configured. The optimum pulse rate of the laser apparatus may be a pulse rate of the laser apparatus for which static thermal lens compensation of the laser apparatus has been configured. The optimum pulse rate of the laser apparatus and/or the optimum level of thermal energy of the gain medium may comprise a maximum pulse rate of the laser apparatus and/or a thermal energy of the gain medium associated with steady state operation of the laser apparatus at its maximum pulse rate.

The thermal compensation energy may be applied such that a time gap between the thermal compensation energy and a following laser firing pulse is shorter than a dissipation time associated with the gain medium for thermal energy to dissipate.

The system may be configured to access correlation data that correlates a pulse rate of laser apparatus with an amount of thermal compensation energy to be supplied by the energy source.

The system may be adapted such that the time gap between the thermal compensation energy and the laser firing pulse is greater than the fluorescence decay time of laser gain produced by the thermal compensation energy.

The system may be configured to produce a greater amount of thermal compensation energy before and/or for a predetermined time after start-up of the system. The system may be adapted to reduce the amount of thermal compensation energy provided with operating time after start-up until the system reaches steady-state operation.

According to a second aspect of the invention is a method for operating a pulsed laser system, wherein the laser system comprises a gain medium and at least one energy source for applying energy to the gain medium, and wherein the method comprises selectively applying thermal compensation energy to the gain medium when the thermal load of the gain medium and/or a pulse rate of the apparatus is below an optimum value.

According to a third aspect of the invention is a computer program product for implementing the method of the second aspect.

According to a fourth aspect of the invention is an apparatus when programmed with the computer program product according to the third aspect.

According to a fifth aspect of the invention is a controller for a pulsed laser system, wherein the laser system comprises a gain medium and at least one energy source for applying energy to the gain medium, and wherein the controller is configured to operate the at least one energy source to selectively apply thermal compensation energy to the gain medium when the thermal load of the gain medium and/or a pulse rate of the apparatus is below an optimum value.

Features analogous to those described in relation to the first aspect of invention are also applicable to each of the other aspects of invention.

Advantages of these embodiments are set out hereafter, and further details and features of each of these embodiments are defined in the accompanying dependent claims and elsewhere in the following detailed description.

It will be appreciated that features described in relation to any of the above aspects of invention may also optionally be applicable to any other aspect of invention. Furthermore, it will also be appreciated that method features analogous to any described apparatus features are intended to fall within the scope of the disclosure and vice versa. Brief Description of the Drawings

At least one embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 is a schematic illustration of a laser apparatus according to the invention;

Figure 2 shows an energy profile generated by the pump diodes of the apparatus of Figure 1 ;

Figure 3 shows an alternative energy profile generated by the pump diodes of the apparatus of Figure 1 ;

Figure 4 shows an alternative energy profile generated by the pump diodes of the apparatus of Figure 1 ;

Figure 5 shows an alternative energy profile generated by the pump diodes of the apparatus of Figure ; and

Figure 6 shows an alternative energy profile generated by the pump diodes of the apparatus of Figure 1.

Detailed Description of Preferred Embodiments

Throughout the following description, identical reference numerals will be used to identify like parts.

Figure 1 shows a laser system 5 or apparatus according to an embodiment of the invention. The laser system 5 comprises a solid-state diode side pumped laser 10 comprising a rod of laser gain medium 5 provided within a switched resonator formed by an output coupler 20, a passive Q switch 25 and a high reflectivity mirror 30.

One or more pump diodes 35 are positioned adjacent to the laser gain medium 15 and are operable to provide optical pump energy in the form of light pulses to the laser gain medium 15. The pump diodes 35 are operated by a pump diode driver 40. The pump diode driver 40 provides high current electrical pulses 50 to the pump diodes 35 according to waveforms generated by a waveform generator 45. In this case, the waveform generator 45 may be comprised in or controllable by a system controller 55. The light pulses produced by the pump diodes 35 correspond to the high current pulses 50 generated by the pump diode driver 40 such that the light pulses supplied by the pump diodes 35 to the laser gain medium 15 are controllable by varying the waveform generated by the waveform generator 45. For example, the pulse rate, peak power and duration of the light pulses provided by the pump diodes 35 to the laser gain medium 15 can be varied.

The pump diodes 35 are operable to provide laser firing pump pulses 60 (see

Figures 2 to 6) above a threshold pump power and at a pulse rate sufficient to achieve lasing and result in corresponding pulses of optical (laser) output 65 from the laser 10. The optical (laser) output 65 is produced when the intensity of light incident on the passive Q switch 25 exceeds a threshold and the Q switch 25 switches to a high Q configuration, thereby bringing the laser 10 into a resonant condition.

A portion of the pumping energy is converted into heat within the gain medium 15. As a result of the heat supplied to the gain medium 15 by the optical pump pulses generated by the pump diodes 35 in combination with thermal conduction out of the gain medium 15, a thermal gradient is created in the gain medium 15. Since the refractive index of the gain medium 15 is dependent on the temperature, the thermal gradient can produce variations in the refractive index of the gain medium 15, which leads to optical distortions. The distortions have the effect of a virtual lens (a so called thermal lens). The focal length of the thermal lens varies with the thermal energy of the gain medium 15, which in turn is dependent on the energy supplied by the pump diodes 35.

The radius of curvature of the output coupler 20 and of the reflector 30 are configured to compensate for a static thermal lens that results when the laser 10 is in a steady state operating condition corresponding to a constant pulse rate of optical pump / output pulses 60. The thermal energy of the gain medium 15 associated with the steady state operating condition for which the static thermal lens is configured may be considered to be an optimum or compensated energy and the associated pulse rate of the system 5 for the steady state operating condition can be considered to be an optimum or compensated pulse rate for which the effects of thermal lensing are minimised. Preferably, the output coupler and reflectors are configured to compensate for a static thermal lens that results when the laser system 5 is being operated at its maximum pulse rate.

However, the effect of the static thermal lens compensation provided by the physical selection and arrangement of the system components does not fully compensate for thermal lensing effects when the system is being operated outwith the pulse rate for which the thermal lens compensation of the system 5 is configured. Nor can it fully compensate for thermal lensing at start up, i.e. before the system 5 reaches steady state operation.

In order to compensate for these effects, when the thermal energy of the gain medium 15 is below the compensated energy and/or the operating pulse rate of the system 5 is below the compensated pulse rate, the system 5 shown in Figure 1 is configured to selectively generate additional energy using the pump diodes 35 in order to raise the thermal energy of the gain medium 15 to, or at least closer to, the compensated energy. This additional energy acts as thermal compensation energy 70 (see Figures 2 to 6), which is additional energy supplied to the gain medium 15 in order to compensate for a reduction in thermal energy supplied by the laser firing pump pulses 60 when the system 5 is being operated below the compensated pulse rate or when the system 5 is being started up and the gain medium 15 hasn't yet achieved the compensated energy associated with steady state operation of the system 5.

For example, the pump diodes 35 may be operable to provide laser firing pulses 60 having an optical energy (i.e. a pulse duration, intensity and/or puise rate) sufficient to result in generation of optical (laser) output 65 from the system 5 at a desired output pulse rate. In addition to the laser firing pulses 60, the pump diodes 35 are also operable to selectively provide thermal compensation energy 70 before and/or between the laser firing pulses when the thermal energy of the gain medium 15 and/or the pulse rate of the system 5 is below the compensated energy / compensated pulse rate. Optionally, the thermal compensation energy 70 can be supplied in pulses 70a, 70b, 70c or as a continuous wave 70d.

!n embodiments of the invention, the thermal compensation energy 70 is provided when the system 5 is in a configuration in which optical (laser) output 65 is not produced by the laser 10. In the passively switched system 5 of Figure 1 , the thermal compensation pulses are provided at an energy level below a lasing or pulsing threshold of the system 5 (i.e. below the level required to switch the Q-switch 25 into the high Q / resonant configuration).

In other embodiments that are actively switched, the thermal compensation energy 70 can be applied when the optical switch 25 is placed in a configuration in which oscillation and thus lasing of the system 5 is prevented (i.e. in the low Q configuration).

When the system 5 is run at an output pulse rate that is below the compensated pulse rate, the average thermal energy deposited by the laser firing pulses 60 is less than would be the case if the system 5 is operated at the compensated pulse rate. The thermal compensation pulses 70 are selected and configured to provide thermal energy corresponding to the difference in the thermal energy provided by the pump diodes 35 at the selected/desired pulse rate and the thermal energy provided by the pump diodes 35 at the compensated (i.e. in this example maximum) pulse rate. In this way, the thermal energy provided to the gain medium is generally constant and corresponds to the thermal energy provided when the system 5 is operating at the compensated pulse rate, which in turn corresponds to the state for which the system 5 is configured to minimise thermal lensing and other optical effects.

For example, the system controller 50 may be configured to store and/or access correlation data from a look up table, database or the like, the correlation data providing details of the thermal compensation energy 70 to be applied for a given operational pulse rate of the system 5. Alternatively, the system controller 50 may be configured to store or access data representative of the compensated energy or compensated pulse rate and calculate the additional thermal compensation energy 70 required based on the selected or current operating pulse rate. As a result, an appropriate amount of thermal compensation energy can be selectively applied to the gain medium 15 in addition to the thermal energy supplied by the laser firing pulses 60 when the operating pulse rate of the laser system 5 is below the compensated pulse rate, such that the thermal lens of the system 5 remains essentially static, regardless of the operating pulse rate of the system 5.

The controller may be configured to perform an initialisation procedure upon start-up of the system or when a predetermined period of time has elapsed since energy was last supplied to the gain medium 15 by the pump diodes 35. The initialisation procedure may comprise providing thermal compensation energy from the pump diodes 35 to the gain medium 15 when the system 5 is in a configuration in which optical (laser) output is not produced by the laser system 5, i.e. the thermal compensation energy is provided at a level below a lasing or pulsing threshold of the passively switched system 5 or is applied when the optical switch 25 is in a configuration in which oscillation and thus lasing of the system is prevented in an actively switch system. In this way, the system can be brought to a steady state condition faster and thermal lensing effects minimised upon start-up or after a period of inactivity.

In certain embodiments of the invention, the initialisation procedure comprises providing a higher amount of thermal compensation energy 70 before the first laser firing pulse and/or between the initial laser firing pulses after start-up, with the amount of thermal compensation energy 70 being reduced as the system 5 approaches steady state operating conditions and thermal equilibrium, in this way, a larger amount of thermal compensation energy may be provided initially, in order to minimise delays in bringing the system 5 into thermal equilibrium but can be reduced as the system 5 approaches its optimum steady state operational conditions.

Preferably, the thermal compensation energy is applied into the same volume of gain medium 15 and with the same distribution as that produced by a laser firing pump pulse 60, so that the corresponding thermal gradients are the same. In addition, it is preferable that the thermal diffusion time for energy to dissipate away from the initial distribution is long relative to the time between pulses provided by the pump diodes 35. In this way, the thermal energy provided by the thermal compensation pulse 70 remains throughout the operation of the system and does not decay before the following laser firing pump pulse 60 is triggered.

The thermal compensation energy 70 provided by the pump diodes will produce laser gain in the gain medium 15, which decays with a characteristic fluorescence decay time. Preferably, a gap is left between any pulses providing thermal compensation energy 70 and the laser firing pulses 60, wherein the gap is longer than the fluorescence decay time of the laser gain produced by the thermal compensation energy 70. In this way, the pulse energy required for the laser firing pulse 60 remains constant, regardless of any thermal compensation 70 that is carried out.

Various operating schemes for providing thermal compensation energy 70 are envisaged. Figures 2 to 5 show the optical output of the pump diodes 35 over time according to examples of such operating schemes for use with the passively switched system of Figure 1. In the operating scheme shown in Figure 2, the pump diodes 35 are configured to provide thermal compensation energy 70 by producing a single thermal compensation pulse 70a per cycle between the laser firing pulses 60 and having the same peak power as the laser firing pulses 60. As can be seen from Figure 2, the thermal compensation pulses 70a are of shorter duration than the laser firing pu!ses 60 so that the energy provided by the thermal compensation pulses 70a is below the lasing threshold, and no optical output is produced by these additional pulses 70a.

In a further example of a suitable pulse scheme, as shown in Figure 3, multiple thermal compensation pulses 70b, 70b may be provided between laser firing pulses 60. In this case, each thermal compensation pulse 70b has a peak power equivalent to the laser firing pulse 60 but a duration that is shorter, such that the energy provided is below the lasing threshold. For example, as the pulse rate of the system 5 drops, the operating scheme may be switched between that of Figure 2 in which only a single thermal compensation pulse 70a is provided between each laser firing pulse 60 and that of Figure 3 in which two thermal compensation pulses 70b, 70b are provided between each laser firing pulse. It will be appreciated that as the pulse rate drops further, additional thermal compensation pulses may be provided between each laser firing pulses 60.

Another example of a pulse scheme according to the invention is shown in Figure 4. In this example, the thermal compensation pulses 70c have a peak power that is lower than that of the laser firing pulses 60. In this case, the duration of the thermal compensation pulses 70c may be longer than the laser firing pulses 60, within the limitation that the energy provided by each thermal compensation pulse 70c is below the lasing threshold.

In a further example, the thermal compensation energy is provided as a continuous wave 70d between laser firing pulses 60, as shown in Figure 5. In this case, the continuous wave 70d providing the thermal compensation energy has a peak power that is sufficiently lower than the laser firing pulses 60 such that the energy supplied by the continuous wave 70d is below the lasing threshold and the continuous wave 70d does not directly result in any optical (laser) output from the system.

Operating schemes analogous to those illustrated in Figures 2 to 5 may also be used as part of the initialisation procedure. For example, a single pulse of thermal compensation energy 70a, similar to those shown in Figure 2, or multiple pulses of thermal compensation energy 70b, similar to those shown in Figure 3, may be applied before the first laser firing pulse 60 is generated, wherein the peak power of the thermal compensation pulses 70a, 70b is the same as that of the laser firing pulse 60, but the pulse duration shorter, such that the energy provided by the thermal compensation pulse(s) 70a, 70b is below the lasing threshold. Similarly, a thermal compensation pulse 70c having a lower peak power than the laser firing pulses 60 but a longer duration may be provided before the first laser firing pulse or a continuous wave 70d having a peak power below the threshold required to active the Q-switch 25 may be provided before and up until the first laser firing pulse 60 is generated. In this way, the system 5 can be more quickly brought into a condition where thermal lensing effects that may otherwise affect the initial laser pulses upon start-up or after a period of inactivity are minimised.

Whilst the above embodiments shown in Figures 2 to 5 are particularly suited to use in a passively switched system, they are also applicable to actively switched systems. It will be appreciated that in an actively switched system, the switching is directly controllable such that a much greater degree of freedom in applying the thermal compensation energy 70 is available. In particular, whilst the Q-switch 25 is set such that the Q-factor is low, the thermal compensation energy 70 may be applied by the pump diodes 35 with any peak power and/or duration that would result in the thermal energy of the gain medium 15 being raised to the compensated energy.

In the embodiments illustrated by Figures 2 to 5, the thermal compensation energy 70 is advantageously provided by the pump diodes 35 such that no optical (laser) output is produced by the system 5 as a direct result of the thermal compensation pulses 70. However, this need not necessarily be the case.

For example, as shown in Figure 6, the laser firing pulses 60 provided by the pump diodes may be selectively extended in duration by an amount 70e required to provide the difference between the thermal energy provided to the gain medium 15 by the laser firing pulses 60 at the compensated (maximum) pulse rate and the thermal energy that would be provided to the gain medium 15 by the same laser firing pulses 60 if applied at the selected or current pulse rate of the system 5.

With the above operating schemes, the system 5 utilises the pump diodes 35 to selectively provide extra thermal compensation energy 70 in situations where the thermal energy supplied to the gain medium 15 by the laser firing pulses 60 is below the compensated energy, such that the thermal compensation energy 70 supplied by the pump diodes 35 corresponds to the difference in thermal energy deposited by the lasing firing pulses 60 produced by the pump diodes 35 when the system is operating at a pulse rate for which the static thermal lens compensation of the system is configured, which is preferably the maximum pulse rate, and the thermal energy deposited by the laser firing pulses 60 produced by the pump diodes 35 when the system is operating at the selected or current pulse rate.

In this way, when the laser 10 is operating at its maximum operating pulse rate, i.e. it is pumped with laser firing pulses 60 at the maximum rate, then no extra thermal compensation energy 70 is provided. However, at lower pulse rates, additional thermal compensation energy is provided by the pump diodes 35. The amount of thermal compensation energy 70 provided may be increased as the operating pulse rate of the laser 10 decreases, for example by increasing the number of thermal compensation pulses 70 provided between laser firing pulses 60 or before the first laser firing pulse and/or by increasing the duration and/or peak power of the thermal compensation energy 70. In this way, the thermal load of the gain medium 15, and thus the thermal lens, may be made constant, regardless of the pulse rate at which the system 5 is operated.

The above system 5 allows thermal lensing compensation to be provided for a wide range of applications without impacting the output from the system 5 or the operation being performed. Furthermore, the thermal lens compensation is provided in a simple, cheap and robust arrangement.

By providing a system 5 that provides the thermal lens compensation described above in an initialisation procedure before a first laser firing pulse is generated, the system 5 may be brought into thermal equilibrium faster, and may allow the first pulse to have the same properties as later pulses, or allow steady state operation to be achieved quicker. In this way, start-up problems such as missed pulses, low energy pulses and variations in beam qualities may be minimised or avoided.

An example of the above system in use is hereafter described in more detail. In this example, the apparatus 5 shown in Figure 1 is optimised to operate at 10 Hz and therefore, in the absence of additional thermal compensation energy being provided, is generally not operable to emit pulses until the apparatus 5 has reached a steady state, the thermal lens is constant and the optical power density equivalent to that supplied by laser firing pulses 60 at 10 Hz is incident on the passive Q Switch 25. If the pulse repetition rate of the laser firing pulses 60 is reduced below 10 Hz, then optical (laser) output of the system 5 will be inhibited.

The apparatus 5 was operated according to a variety of pulse schemes and the results shown in Table 1 below. The data provided in Table 1 indicates the repetition rate/frequency (PRF) at which laser firing pulses 60 are provided, the pulse width of the laser firing pulses 60 (LFP PW), the number of thermal compensation pulses 70 provided per laser firing pulse 60 (No of TC pulses), the pulse width of the thermal compensation pulses 70 (TC PW) and the minimum and maximum energy outputs (O/P energy minimum and O/P energy maximum respectively).

In a first pulse scheme, the pump diodes 35 were operated to provide 500 laser firing pulses 60 at a rate of 10Hz and no thermal compensation energy 70 was provided. As can be seen from Table 1 , of the 500 laser firing pulse provided, only 492 resulted in optical output from the laser 10. The initial 8 laser firing pulses did not result in optical (laser) output as the system 5 had stil! to achieve optimal steady state thermal condition and the system performance was degraded to such an extent that the first 8 output laser pulses were not produced.

Scheme PRF / Hz LFP PW No of TC TC PW / No. of O/P O/P

Number / ms pulses ms Shots energy energy min max

1 10 1.32 0 Na 492 3.41 3.71

2 9 1.32 0 Na 468 3.31 3.64

3 9 1.32 1 0.15 488 3.39 3.73

4 8 1.32 0 Na 0 - -

5 8 1.32 1 0.32 490 3.37 3.71

6 6 1.32 1 0.87 494 3.42 3.78

7 4 1.32 2 1.0 498 3.4 3.78

8 3 1.32 4 0.77 493 3.31 3.67

9 2 1.32 6 0.87 500 3.38 3.72

Table 1 in a second pulse scheme, 500 laser firing pulses 60 were provided at a rate of 9 Hz and no thermal compensation energy 70 was provided. As expected, the number of missing output pulses increased from 8 to 32, as indicated in Table 1.

In a third pulse scheme, 500 laser firing pulses 60 were also provided at a rate of 9 Hz, as in the second pulse scheme. However, a single thermal compensation energy pulse 70 of 0.15ms duration and the same peak power as the laser firing pulses 60 was introduced before every laser firing pulse 60. As can be seen from Table 1 , the number of missing output pulses significantly reduced from 32 with the second pulse scheme to 12. This would indicate that the time taken for the system 5 to achieve an operable condition was reduced significantly by the introduction of thermal compensation pulses 70.

In a fourth pulse scheme, the pulse rate at which laser firing pulses 60 were generated was reduced to 8Hz and no thermal compensation energy 70 was provided. In this case, the laser 10 did not emit any optical output over the whole 500 shot sequence.

In a fifth pulse scheme, the 500 laser firing pulses 60 were provided at a rate of 8Hz, as in the fourth pulse scheme, but one longer duration thermal compensation pulse 70 of 0.32ms and having the same peak power as the laser firing pulses 60 was provided between every laser firing pulse 60. In this case, the laser output increased to 490 shots, which is roughly comparable to the 10 Hz level provided in the first pulse scheme.

In a sixth exemplary pulse scheme, the pulse rate of laser firing pulses 60 was decreased to 6Hz and the pulse width of the thermal compensation pulse 70 was increased to 0.87ms. It can be seen from Table 1 that the number of output laser shots achieved actually exceeded the case for the first pulse scheme

In the seventh through to the ninth exemplary pulse schemes, the pulse rate at which laser firing pulses 60 were provided was successively dropped until it reached only 2Hz for the ninth exemplary pulse scheme. Multiple thermal compensation pulses were required for each of these lower pulse rate schemes in order to prevent the energy provided by the thermal compensation pulses 70 from exceeding the pump threshold. However, the data achieved by use of these pulse schemes, as indicated in Table 1 , shows that if a suitable number and pulse width of thermal compensation pulses for the selected pulse rate is used, it is possible to achieve an output for every laser firing puise 60 generated by the pump diodes 35 over the range of pulse rates covered by this experiment.

In view of the above, it has been shown that for a laser 10 that stops operating at 8Hz when operated conventionally and without any thermal compensation, providing extra thermal compensation energy 70 in the form of non-lasing thermal compensation pulses from the pump source 35 can achieve operation down to 2Hz and that this could be employed to produce a stable resonator over a much wider range of operating frequencies.

It should also be noted that whilst the accompanying claims set out particular combinations of features described herein, the scope of the present invention is not limited to the particular combinations hereafter claimed, but instead extends to encompass any combination of features or embodiments herein disclosed irrespective of whether or not that particular combination has been specifically enumerated in the accompanying claims at this time.

Claims

A pulsed laser apparatus comprising a gain medium and at least one energy source for providing energy to the gain medium, wherein the apparatus is configured to selectively provide thermal compensation energy from the energy source to the gain medium when the thermal load of the gain medium and/or a pulse rate of the apparatus is below an optimum value.

The pulsed laser apparatus according to claim 1 , wherein the energy source is operable to provide thermal compensation energy to the gain medium in a non-lasing configuration in which substantially no optical (laser) output is produced by the apparatus.

The pulsed laser apparatus according to claim 1 or claim 2, wherein the energy source is configured to generate at least one laser firing pulse for generating optical laser output from the laser apparatus and the energy source is configured to provide the thermal compensation energy outwith the period in which laser firing pulses are produced.

The pulsed laser apparatus according to any of the preceding claims, wherein the apparatus comprises switching means for selectively switching the apparatus between lasing and non-lasing modes, wherein the switching means optionally comprises a passive Q-switch that is switchable into a lasing configuration when the intensity of light incident on the Q switch is above a lasing threshold.

The pulsed laser apparatus according to claim 4, wherein the thermal compensation energy is below the lasing threshold and the laser firing pulses are above the lasing threshold.

The pulsed laser apparatus according to any preceding claim, wherein the thermal compensation energy is provided in pulses, wherein the pulses of thermal compensation energy have either substantially the same peak power as the laser firing pulses but a shorter duration than the laser firing pulses; or lower peak power than the laser firing pulses but a longer duration.

The pulsed laser apparatus according to any of claims 1 to 5, wherein the thermal compensation energy is provided in a continuous wave having a lower peak power than the laser firing pulses.

The pulsed laser apparatus according to any of claims 4, 6 or 7, wherein the switching means comprises an active Q switch and application of energy from the energy source in a non-lasing configuration may comprise applying the energy to the gain medium from the energy source whilst the apparatus is placed in the non-lasing mode through use of the switching means.

The pulsed laser apparatus according to any preceding claim, wherein the apparatus is configured to selectively apply thermal compensation energy from the energy source in a non-lasing configuration upon start-up of the apparatus and/or when a predetermined amount of time has elapsed since the system was last operated.

The pulsed laser apparatus according to any preceding claim, wherein the amount of thermal compensation energy supplied is dependent on the difference between a selected or current thermal load of the gain medium and the optimum thermal load of the gain medium and/or the difference between the current pulse rate of the laser apparatus and the optimum pulse rate of the laser apparatus.

The pulsed laser apparatus according to any preceding claim, wherein the apparatus is configured to selectively apply thermal compensation energy from the energy source so as to control the thermal load of the gain medium to be constant.

The pulsed laser apparatus according to any preceding claim, wherein the optimum level of thermal energy of the gain medium is a level of thermal energy of the gain medium for which static thermal lens compensation of the laser apparatus has been configured and/or the optimum pulse rate of the laser apparatus is a pulse rate of the laser apparatus for which static thermal lens compensation of the laser apparatus has been configured.

The pulsed laser apparatus according to any preceding claim, wherein the optimum pulse rate of the laser apparatus and/or the optimum level of thermal energy of the gain medium may comprise a maximum pulse rate of the laser apparatus and/or a thermal energy of the gain medium associated with steady state operation of the laser apparatus at its maximum pulse rate.

The pulsed laser apparatus according to any preceding claim, wherein the thermal compensation energy is applied such that a time gap between the thermal compensation energy and a following laser firing pulse is shorter than a dissipation time associated with the gain medium for thermal energy to dissipate.

The pulsed laser apparatus according to any preceding claim, wherein the system is adapted such that a time gap between the thermal compensation energy and a following laser firing pulse is greater than the fluorescence decay time of laser gain produced by the thermal compensation energy.

The pulsed laser apparatus according to any preceding claim, wherein the system is configured to access correlation data that correlates a pulse rate of the laser apparatus with an amount of thermal compensation energy to be supplied by the energy source.

The pulsed laser apparatus according to any preceding claim, wherein the system is configured to produce a greater amount of thermal compensation energy before and/or for a predetermined time after start-up of the system and/or reduce the amount of thermal compensation energy provided with operating time after start-up until the system reaches steady-state operation.

A method for operating a pulsed laser system, wherein the laser system comprises a gain medium and at least one energy source for applying energy to the gain medium, and wherein the method comprises selectively applying thermal compensation energy to the gain medium when the thermal load of the gain medium and/or a pulse rate of the apparatus is below an optimum value.

19. A computer program product for implementing the method of claim 18.

20. An apparatus when programmed with the computer program product of claim 19.

21. A controller for a pulsed laser system, wherein the laser system comprises a gain medium and at least one energy source for applying energy to the gain medium, and wherein the controller is configured to operate the at least one energy source to selectively apply thermal compensation energy to the gain medium when the thermal load of the gain medium and/or a pulse rate of the apparatus is below an optimum value.