FIELD

Embodiments described herein relate generally to a passive Q-switched laser apparatus and a method for controlling emission of optical pulses from the passive Q- switched laser apparatus.

BACKGROUND

It is known to use a laser rangefinder to determine a distance to a target. It is generally desirable that a laser rangefinder is efficient and portable. Laser rangefinders may use a single-shot time-of-flight method to determine distance to a target. Such laser rangefinders work by emitting a single laser pulse and, at the same time, starting a clock. The pulse is reflected from the target and received at the laser rangefinder. The clock is stopped on detection of the received pulse at the laser rangefinder and the time-of-flight of the laser pulse is converted into a distance using the speed of light. To achieve good distance resolution of the order of 30 cm, the laser pulse should be of the order of 1 ns in duration. To operate over long distances, the peak power of the laser pulse should be high, of the order of 100's of kW, to overcome attenuation through the atmosphere, low reflectance from the target, and divergence of the outgoing beam. Passive Q-switched solid-state lasers are capable of generating such optical pulses and are, therefore, widely used for compact laser rangefinders. In particular, laser rangefinders are known which incorporate passive Q-switched solid- state lasers operating in the eye-safe regions. For example, laser rangefinders are known which incorporate a gain medium comprising a glass which includes Erbium (Er).

The range of a laser rangefinder may be increased by increasing the peak power of the optical pulses and/or by increasing detection sensitivity. However, increasing the peak power may increase power consumption requiring a larger, heavier power source or requiring that the power source is replaced or recharged. Also, increasing the peak power may not be possible without exceeding eye-safe power levels or without exceeding the optical damage threshold of optically coated parts.

Increasing detection sensitivity may be achieved by increasing the receiver aperture of the laser rangefinder, but this may result in a larger, heavier apparatus. Laser rangefinders are also known which integrate multiple measurements of the time-of-flight values so as to increase sensitivity, where each time-of-flight value being measured using a different optical pulse. The sensitivity increases with the square root of the number of optical pulses used. Integration of this type is commonly adopted for direct-diode range finders since they are generally only capable of generating optical pulse energies of the order of ^ s compared with optical pulse energies of the order of mJ's generated using passive Q-switch solid-state lasers. Accordingly, it is common for direct diode laser rangefinders to integrate the range values determined using many tens or hundreds of optical pulses to achieve the sensitivity required. This may require a longer measurement time thereby preventing the use of a direct diode laser rangefinder where measurement time is critical, for example where a target is fast-moving or for covert operations. Eventually the emission of a large number of pulses may exceed the eye safety limit. Hence diode based systems may be limited in range or sensitivity compared to some solid-state lasers.

BRIEF DESCRIPTION OF THE DRAWINGS

A passive Q-switched laser apparatus and a method for controlling emission of optical pulses from the Q-switched laser apparatus will now be described by way of non-limiting example only with reference to the following drawings of which:

Figure 1 is a schematic of an integrating time-of-flight solid state laser rangefinder;

Figure 2 is a schematic illustration of the pump power and the corresponding optical gain and optical output power emitted by the laser rangefinder of Figure 1 as a function of time when the laser rangefinder is operated according to a first method;

Figure 3 is a schematic illustration of the pump power and the corresponding optical output power emitted by the laser rangefinder of Figure 1 as a function of time when the laser rangefinder is operated according to a second method; Figure 4 is a schematic illustration of the pump power and the corresponding optical output power emitted by the laser rangefinder of Figure 1 as a function of time when the laser rangefinder is operated according to a third method; and

Figure 5 is a schematic illustration of the pump power and the corresponding optical output power emitted by the laser rangefinder of Figure 1 as a function of time when the laser rangefinder is operated according to a fourth method.

DETAILED DESCRIPTION OF THE DRAWINGS

It should be understood that one or more of the features of any of the aspects or embodiments described herein may apply alone or in any combination in relation to any of the other aspects or embodiments.

According to an aspect or an embodiment there is provided a method for controlling emission of optical pulses from a passive Q-switched laser apparatus.

The method may comprise pumping a gain medium of the laser apparatus for emission of a plurality of Q-switched optical pulses from the laser apparatus.

The method may comprise pumping the gain medium so as to maintain an optical gain provided by the gain medium above an unpumped optical gain level for the duration of a time period from emission of one Q-switched optical pulse to emission of a subsequent Q-switched optical pulse.

The method may comprise varying a rate of pumping of the gain medium between emission of the one Q-switched optical pulse and emission of the subsequent Q-switched optical pulse so as to control the temporal separation between the one Q- switched optical pulse and the subsequent Q-switched optical pulse.

The method may comprise pumping the gain medium so as to vary a rate of increase of optical gain provided by the gain medium between emission of the one Q- switched optical pulse and emission of the subsequent Q-switched optical pulse. The subsequent Q-switched optical pulse may be the next successive Q-switched optical pulse emitted by the laser apparatus after the emission of the one Q-switched optical pulse.

The one Q-switched optical pulse may be the first Q-switched optical pulse emitted by the laser apparatus after commencement of pumping of the gain medium. The method may comprise controlling pumping of the gain medium so as to provide a desired temporal separation between successive Q-switched optical pulses.

The method may comprise determining the desired temporal separation required for a desired range so as to avoid cross-talk between reflections of successive Q-switched optical pulses from a target.

The method may comprise selecting the desired temporal separation to be equal to the minimum temporal separation required for the desired range so as to avoid cross-talk between reflections of successive Q-switched optical pulses from a target.

The method may comprise:

pumping the gain medium at a first rate until the laser apparatus emits a first Q- switched optical pulse; and then

pumping the gain medium at a second rate.

The method may comprise selecting the second rate to be different to the first rate.

The method may comprise selecting the second rate to be less than the first rate.

The method may comprise selecting the second rate so as to provide a desired temporal separation between successive Q-switched optical pulses.

The method may comprise pumping the gain medium continuously for the duration of the time period extending from emission of the one Q-switched optical pulse to emission of the subsequent Q-switched optical pulse.

The method may comprise pumping the gain medium discontinuously between emission of the one Q-switched optical pulse and emission of the subsequent Q- switched optical pulse.

The method may comprise pumping the gain medium continuously after commencement of pumping of the gain medium until the laser apparatus emits a first Q-switched optical pulse.

The method may comprise pumping the gain medium discontinuously after emission of a first Q-switched optical pulse after commencement of pumping of the gain medium.

The method may comprise using a pulse-width modulation (PWM) technique to vary a rate of pumping of the gain medium.

The method may comprise varying a rate of pumping of the gain medium according to a PWM waveform. The method may comprise selecting a mark-to-space ratio of the PWM waveform according to a desired temporal separation between successive Q-switched optical pulses.

The method may comprise using a pulse-width modulation (PWM) technique to vary a rate of pumping of the gain medium after emission of a first Q-switched optical pulse after commencement of pumping of the gain medium.

The method may comprise:

measuring the temporal separation between two successive Q-switched optical pulses; and

controlling pumping of the gain medium according to the measured temporal separation.

The method may comprise ceasing pumping of the gain medium after the laser apparatus has emitted a predetermined number of Q-switched optical pulses.

The method may comprise ceasing pumping of the gain medium after emission of 2 - 20 Q-switched optical pulses, after emission of 2 - 10 Q-switched optical pulses or after emission of 4 Q-switched optical pulses.

The method may comprise determining a time of emission of each Q-switched optical pulse from the passive Q-switched laser apparatus.

The method may comprise determining a time of arrival of each Q-switched optical pulse at the passive Q-switched laser apparatus after reflection from a target.

The method may comprise determining a time of flight for each Q-switched optical pulse from the time of emission and the time of arrival.

The method may comprise determining a range to the target from the times of flight for the plurality of Q-switched optical pulses and the speed of light.

The method may comprise determining an average time of flight or a weighted average time of flight from the times of flight for the plurality of Q-switched optical pulses.

The method may comprise determining a range to the target from the average time of flight or the weighted average time of flight and the speed of light.

The method may comprise measuring an amplitude, energy or power of each

Q-switched optical pulse received by the Q-switched laser apparatus after reflection from a target.

The method may comprise determining an average amplitude, energy or power from the amplitudes, energies or powers of the plurality of received Q-switched optical pulses. The method may comprise optically pumping the gain medium.

The method may comprise electrically pumping the gain medium.

According to an aspect or an embodiment there is provided a passive Q- switched laser apparatus for the emission of a plurality of Q-switched optical pulses.

The passive Q-switched laser apparatus may comprise a gain medium.

The passive Q-switched laser apparatus may comprise a pump for pumping the gain medium.

The passive Q-switched laser apparatus may comprise a controller for controlling the pump so as to maintain an optical gain provided by the gain medium above an unpumped optical gain level for the duration of a time period from emission of one Q-switched optical pulse to emission of the a subsequent Q-switched optical pulse; and so as to vary a rate of pumping of the gain medium between emission of the one Q-switched optical pulse and the subsequent Q-switched optical pulse so as to control the temporal separation between the one Q-switched optical pulse and the subsequent Q-switched optical pulse.

The controller may be configured to control the pump so as to vary a rate of increase of optical gain provided by the gain medium between emission of the one Q- switched optical pulse and emission of the subsequent Q-switched optical pulse. The gain medium may comprise a solid state gain medium.

The gain medium may comprise glass doped with Erbium (Er).

The gain medium may comprise a semiconductor material.

The pump may comprise an optical pump.

The pump may comprise at least one of a laser diode and a light emitting diode

(LED).

The pump may comprise an electrical pump.

The pump may comprise a current source.

According to an aspect or an embodiment there is provided a laser rangefinder comprising the Q-switched laser apparatus.

According to an aspect or an embodiment there is provided a remote optical sensing apparatus comprising the passive Q-switched laser apparatus.

According to an aspect or an embodiment there is provided a laser scanner apparatus comprising the passive Q-switched laser apparatus.

According to an aspect or an embodiment there is provided a LIDAR apparatus comprising the Q-switched laser apparatus. According to an aspect or an embodiment there is provided a method for controlling emission of a plurality of optical pulses from a passive Q-switched laser apparatus, comprising:

pumping a gain medium of the passive Q-switched laser apparatus so as to vary a rate of increase of optical gain provided by the gain medium between emission of one Q-switched optical pulse and emission of a subsequent Q-switched optical pulse for control of a temporal separation between the one Q-switched optical pulse and the subsequent Q-switched optical pulse whilst maintaining the optical gain above an unpumped optical gain level for the duration of a time period from emission of the one Q-switched optical pulse to emission of the subsequent Q-switched optical pulse.

According to an aspect or an embodiment there is provided a passive Q- switched laser apparatus for the emission of a plurality of Q-switched optical pulses, the laser apparatus comprising:

a gain medium;

a pump for pumping the gain medium; and

a controller for controlling the pump so as to vary a rate of increase of optical gain provided by the gain medium between emission of one Q-switched optical pulse and emission of a subsequent Q-switched optical pulse for control of a temporal separation between the one Q-switched optical pulse and the subsequent Q-switched optical pulse whilst maintaining the optical gain provided by the gain medium above an unpumped optical gain level for the duration of a time period from emission of the one Q-switched optical pulse to emission of the subsequent Q-switched optical pulse.

Figure 1 is a schematic of an integrating time-of-flight solid state laser rangefinder 102 including an optical emitter generally designated 104 for emitting an optical output beam 105, an optical receiver 106 for receiving an optical input beam 107, and a controller generally designated 108. The emitter 104 includes a pump laser diode 1 10, a passive Q-switched laser generally designated 1 12 and a photodiode 1 13 for monitoring optical power generated by the passive Q-switched laser 1 12. The passive Q-switched laser 1 12 includes a solid-state gain medium in the form laser glass gain medium 1 14, a saturable absorber 1 16. The pump laser diode 1 10 is optically coupled to the gain medium 1 14. One of ordinary skill in the art will appreciate that the emitter 104 may include one or more additional optical elements which are not explicitly shown in Figure 1 for directing, coupling and/or conditioning an optical beam. In particular, the passive Q-switched laser 1 12 may include one or more mirrors or prisms or the like (not shown) for defining an optical cavity of the passive Q-switched laser 1 12.

The emitter 104 includes a beam splitter 1 18. The beam splitter 1 18 is configured to tap a proportion of the optical power generated by the passive Q- switched laser 1 12. The tapped beam 1 19 is optically coupled to the photodiode 1 13. One of ordinary skill in the art will also appreciate that the beam splitter 1 18 may tap a proportion of the optical power within the optical cavity of the passive Q-switched laser 1 12 or a proportion of the optical power output from the optical cavity of the passive Q- switched laser 1 12. One of ordinary skill in the art will also appreciate that the beam splitter 1 18 may not be a separate dedicated component, but that the light detected by the photodiode 1 13 may be light which is directed, for example reflected or scattered, from any of the components within, or external to, the optical cavity of the passive Q- switched laser 1 12.

The controller 108 includes a processor 120, a laser diode driver 122, a timer 124, and a buffer 126. As described in more detail below, the controller 108 calculates and outputs a range 132 to a display (not shown) on receipt of a range command 130 from a user interface (not shown).

In use, on receipt of a range command 130, the processor 120 communicates with the laser diode driver 122 causing the laser diode driver 122 to supply a first pulse of electrical current to the pump laser diode 1 10. The pump laser diode 1 10 emits a first optical pump pulse 140a shown in the uppermost trace of Figure 2. The optical pump pulse 140a is coupled to the gain medium 1 14 of the passive Q-switched laser 1 12 causing the optical gain provided by the gain medium 1 14 to increase along a first rising edge or curve 143a as shown in the middle trace of Figure 2. The saturable absorber 1 16 introduces an optical loss which, at first, prevents a laser pulse being emitted. Once the optical gain reaches the threshold gain level G1 required to overcome the optical losses in the passive Q-switched laser 1 12, the saturable absorber 1 16 becomes largely transparent and the Q-switched laser 1 12 emits a first Q-switched optical pulse 105a as shown in the lowermost trace of Figure 2. After the emission of the first Q-switched optical pulse 105a, the saturable absorber 1 16 returns to its absorbing state and hence the optical gain within the optical cavity begins to fall along a first falling edge or curve 144a as shown in the middle trace of Figure 2.

On detection of emission of the first Q-switched optical pulse 105a by the photodiode 1 13, the timer 124 starts, the processor 120 increments a pulse count index by one and shuts down the laser diode driver 122 causing the power of the first optical pump pulse 140a to return to zero and resulting in the first optical gain profile 142a shown in the middle trace of Figure 2.

The optical receiver 106 subsequently detects a first input optical pulse (not shown) corresponding to a reflection of the first output optical pulse 105a from a target (not shown). The timer 124 stops when the optical receiver 106 detects the first input optical pulse (not shown). The resulting time-of-flight for the first Q-switched optical pulse 105a is stored to the buffer 126 and the process is repeated for subsequent Q- switched optical pulses 105b, 105c and 105d, the processor 120 incrementing the pulse count index by one after emission of each Q-switched optical pulse 105b, 105c and 105d, and the corresponding time-of-flight for each Q-switched optical pulse 105b, 105c and 105d being stored to the buffer 126. Once the pulse count index has reached a predetermined final pulse count N, for example N = 4 as shown in Figure 2, the processor 120 calculates an average of the times-of-flight stored in the buffer 126. The processor 120 then calculates the range 132 from the average time-of-flight and a speed of light.

As shown in the middle trace of Figure 2, the gain medium 1 14 must be pumped to the gain level G1 for the emission of each of the Q-switched optical pulses 105a, 105b, 105c, 105d. For a laser glass gain medium 1 14, a typical duration of each optical pump pulse 140a, 140b, 140c, 140d required for emission of a Q-switched optical pulse 105a, 105b, 105c, 105d is between 1 and 3 ms. Accordingly, the minimum separation T1 of successive Q-switched optical pulses may be of the order of 1 - 3 ms. Thus, the time taken to perform a range measurement associated with the method illustrated with reference to Figure 2 is of the order of N times T1 .

Figure 3 illustrates an alternative method of operating the integrating time-of- flight solid state laser rangefinder 102 of Figure 1 . On receipt of a range command 130, the processor 120 communicates with the laser diode driver 122 causing the laser diode driver 122 to supply a pulse of electrical current to the pump laser diode 1 10. The pump laser diode 1 10 emits an optical pump pulse 240 shown in the uppermost trace of Figure 3. The optical pump pulse 240 is coupled to the gain medium 1 14 of the passive Q-switched laser 1 12 resulting in an increase in the optical gain 242 along a first rising edge or curve 243a as shown in the middle trace of Figure 3. Once the optical gain 242 reaches the threshold gain level G1 required to overcome the optical losses in the optical cavity of the passive Q-switched laser 1 12, a first passive Q- switched optical pulse 205a is emitted from the Q-switched laser 1 12 as shown in the lowermost trace of Figure 3 and the optical gain 242 falls along a first falling edge or curve 244a to a lower gain level G2 as shown in the middle trace of Figure 3

On detection of the emission of the first Q-switched optical pulse 205a by the photodiode 1 13, the timer 124 starts and the processor 120 increments a pulse count index by one. The optical receiver 106 subsequently detects a first input optical pulse (not shown) corresponding to a reflection of the first output optical pulse 205a from a target (not shown). The timer 124 stops when the optical receiver 106 detects the first input optical pulse (not shown). The resulting time-of-flight for the first Q-switched optical pulse 205a is stored to the buffer 126.

In contrast to the method described with reference to Figure 2, however, the processor 120 maintains the electrical current provided by the laser diode driver 122 after the emission of the first Q-switched optical pulse 205a thereby maintaining the power of the optical pump pulse 240 after the emission of the first Q-switched optical pulse 205a. This maintains the optical gain 242 above an unpumped optical gain level after emission of the first Q-switched optical pulse 205a. More specifically, this results in recovery of the optical gain 242 along a second rising edge or curve 243b until the optical gain 242 reaches the threshold gain level G1 again and a second Q-switched optical pulse 205b is emitted. Emission of the second Q-switched optical pulse 205b causes the optical gain 242 to fall along a second falling edge or curve 244b back to the lower gain level G2. The process is repeated for subsequent Q-switched optical pulses 205c and 205d, the processor incrementing the pulse count index by one after emission of each of the subsequent Q-switched optical pulses 205b, 205c and 205d and the corresponding time-of-flight calculated for each subsequent Q-switched optical pulse 205b, 205c and 205d is stored to the buffer 126. Once the pulse count index has reached a predetermined final pulse count N, for example N = 4 as shown in Figure 3, the processor 120 shuts down the laser diode driver 122 causing the power of the optical pump pulse 240 to return to zero. The processor 120 calculates an average of the times-of-flight stored in the buffer 126 and then calculates and outputs the range 132 from the average time-of-flight and the speed of light.

Relative to the method described with reference to Figure 2, the method described with reference to Figure 3 results in the emission of many passive Q- switched optical pulses with a much reduced temporal separation T2 between successive Q-switched optical pulses. As such, the method described with reference to Figure 3 significantly reduces the time taken for a range measurement relative to the method described with reference to Figure 2. The method described with reference to Figure 3 also involves the use of a single optical pump pulse 240 for the emission of many passive Q-switched optical pulses. Accordingly, the method described with reference to Figure 3 may also significantly reduce power consumption relative to the method described with reference to Figure 2. Such a method may also enhance measurement sensitivity relative to a one-shot range measurement method without unduly increasing power consumption.

Figure 4 illustrates a further alternative method of operating the integrating time- of-flight passive Q-switched solid state laser rangefinder 102 of Figure 1 . The method of Figure 4 only differs from the method of Figure 3 in that the processor 120 controls the laser diode driver 122 so as to provide a greater initial electrical current to the pump laser diode 1 10 during an initial pumping period 341 a before emission of a first Q- switched optical pulse 305a resulting in a greater initial optical pump power P1 during the initial pumping period 341 a as shown in the uppermost trace in Figure 4. The processor 120 subsequently controls the laser diode driver 122 so as to reduce the electrical current supplied to the pump laser diode 1 10 during a subsequent pumping period 341 b after emission of the first Q-switched optical pulse 305a resulting in a reduced optical pump power P2 during the subsequent pumping period 341 b as shown in the uppermost trace in Figure 4. The resulting optical pump power profile 340 of the pump laser diode 1 10 is shown in the uppermost trace of Figure 4. As a consequence of the optical pump power profile 340, the optical gain profile 342 shown in the middle trace of Figure 4 increases along a first rising edge or curve 343a before emission of the first Q-switched optical pulse 305a at a rate which is faster than the subsequent recovery of the optical gain profile 342 along rising edges or curves 343b, 343c and 343d after the emission of the Q-switched optical pulses 305a, 305b, 305c respectively shown in the lowermost trace of Figure 4.

The greater optical pump power P1 is selected so as to minimise the time taken to obtain the threshold gain level G1 required for emission of the first Q-switched optical pulse 305a, whereas the smaller optical pump power P2 is selected independently of P1 so as to provide the minimum separation T3 between successive Q-switched optical pulses required to avoid cross-talk for a given range. For example, for a 20 km range, successive Q-switched optical pulses should be separated by a minimum ranging window T3 of approximately 133 με so as to avoid the possibility that a reflection of a later Q-switched optical pulse from a nearer target arrives at the receiver 106 around the same time as a reflection of an earlier Q-switched optical pulse from a more distant target. The method of Figure 4 may significantly reduce the range measurement time relative to the method described with reference to Figure 2. Such a method may also significantly reduce power consumption relative to the method described with reference to Figure 2. In addition, for the reasons described in more detail below, the method of Figure 4 may reduce power consumption compared to the method of Figure 3. In the method of Figure 3, if the pump laser diode 1 10 were operated at its maximum rating P1 , successive Q-switched pulses may be emitted too close in time such that a subsequent Q-switched pulse may enter within the ranging window of a previous Q- switched pulse resulting in cross-talk. Consequently, when using the method of Figure 3, it may be necessary to reduce the pump power level in order to prevent cross-talk. In contrast, in the method of Figure 4, the pump laser diode 1 10 is operated at its maximum rating P1 up to the time of emission of the first Q-switched optical pulse 305a, and then the pump laser diode 1 10 is operated at a reduced pump output power P2, in order to prevent cross-talk. The method described with reference to Figure 4 may also enhance measurement sensitivity relative to a one-shot range measurement method without unduly increasing power consumption and for no increase in size or weight of the laser rangefinder 102.

Figure 5 illustrates a yet further alternative method of operating the integrating time-of-flight solid state laser rangefinder 102 of Figure 1 . The method of Figure 5 is similar to the method of Figure 4 in that the method of Figure 5 may be used for the emission of a plurality of Q-switched optical pulses 405a, 405b, 405c, 405d shown in the lowermost trace of Figure 5 at essentially the same times as the plurality of Q- switched optical pulses 305a, 305b, 305c, 305d shown in the lowermost trace of Figure 4. Like the method of Figure 4, the processor 120 controls the laser diode driver 122 so as to supply a continuous current to the pump laser diode 1 10 so as to operate the pump laser diode 1 10 at its maximum rating P1 during a first time period 441 a before the emission of the first Q-switched pulse 405a to maximise laser efficiency. This results in an optical gain profile 442 shown in the middle trace of Figure 5 having a first rising edge or curve 443a which is the same or similar to the first rising edge or curve 343a in the optical gain profile 342 obtained using the method described with reference to Figure 4.

However, in contrast to the method of Figure 4, in the method of Figure 5, the processor 120 subsequently controls the laser diode driver 122 so as to provide a pulse width modulation (PWM) electrical current waveform to the pump laser diode 1 10 during a subsequent pumping period 441 b after emission of the first Q-switched optical pulse 405a. This provides a PWM variation in the optical pump power during the subsequent pumping period 441 b as shown in the uppermost trace of Figure 5. The resulting optical gain profile 442 is shown in the middle trace of Figure 5. The optical gain profile 442 is generally similar to the optical gain profile 342 obtained using the method described with reference to Figure 4. However, one of ordinary skill in the art will understand that the evolution of the optical gain 442 may not be piecewise linear during the subsequent pumping period 441 b as illustrated in the middle trace of Figure 5, but may exhibit some ripple along the different rising edges or curves 443b, 443c and 443d and the different falling edges or curves 444a, 444b and 444c as a consequence of the use of the PWM variation in the optical pump power during the subsequent pumping period 441 b shown in the uppermost trace of Figure 5. The processor 120 controls the laser diode driver 122 so as to provide a PWM electrical current waveform during the subsequent pumping period 441 b with a mark-to-space ratio selected so as to provide a rate of increase of optical gain along rising edges or curves 443b, 443c and 443d required to provide the desired temporal separation T3 between the successive Q-switched optical pulses 405a, 405b, 405c and 405d. Modulating the optical pump in this way may allow a simplified or more efficient design for the laser diode driver 122 compared to that required to implement the method described with reference to Figure 4.

One skilled in the art will appreciate that the controller 120 may be configured differently, for example programmed differently, so as to implement the methods of operating the passive Q-switched laser 1 12 described above with reference to Figures 2 to 5.

One skilled in the art will also appreciate that various modifications may be made to passive Q-switched laser 1 12 and the associated methods of operating the passive Q-switched laser 1 12 described above. For example, the method may comprise driving the pump laser diode 1 10 with an electrical current pulse having a profile which is selected so as to provide an optical pump pulse having a different power profile to any of those shown in Figures 2 to 5. The electrical current profile may be selected so as to provide the optical pump pulse with any power profile which maintains the optical gain above an unpumped optical gain level between the emission of successive Q-switch optical pulses.

The method may comprise measuring the temporal separation of two successive Q-switch optical pulses and controlling pumping of the gain medium according to the measured temporal separation. Such a method may allow the actual temporal separation of two successive Q-switch optical pulses to be monitored and, if necessary, to allow the temporal separation of subsequent successive Q-switch optical pulses to be adjusted in real time to minimise cross-talk.

Although the methods described with reference to Figures 1 - 5 have been described in the context of operating a laser rangefinder, each method may be used to generate a plurality of Q-switch optical pulses and to receive reflections of the Q-switch optical pulses from a target for other technical applications. For example, the method may comprise measuring an amplitude, energy or power of each Q-switch optical pulse received by the Q-switch laser apparatus after reflection from a target and determining an average amplitude, energy or power from the amplitudes, energies or powers of the plurality of received Q-switch optical pulses. Such a method may be used for remote optical sensing, laser scanning and/or LIDAR imaging.

While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the invention. Indeed, the novel apparatus and methods described herein may be embodied in a variety of other forms. Furthermore, various omissions, substitutions and changes in the form of the apparatus and methods described herein may be made without departing from the spirit of the invention. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the invention.