GB2497549A - Method and apparatus for use in passive q-switching - Google Patents

Method and apparatus for use in passive q-switching Download PDF

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Publication number
GB2497549A
GB2497549A GB201121379A GB201121379A GB2497549A GB 2497549 A GB2497549 A GB 2497549A GB 201121379 A GB201121379 A GB 201121379A GB 201121379 A GB201121379 A GB 201121379A GB 2497549 A GB2497549 A GB 2497549A
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Prior art keywords
text
switched
laser apparatus
emission
pump
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GB201121379D0 (en
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Stephen Lee
William Alexander
Trevor Cook
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Thales Holdings UK PLC
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Thales Holdings UK PLC
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Priority to GB201121379A priority Critical patent/GB2497549A/en
Publication of GB201121379D0 publication Critical patent/GB201121379D0/en
Priority to EP12810412.2A priority patent/EP2792034A1/en
Priority to PCT/GB2012/053103 priority patent/WO2013088139A1/en
Publication of GB2497549A publication Critical patent/GB2497549A/en
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
    • H01S3/11—Mode locking; Q-switching; Other giant-pulse techniques, e.g. cavity dumping
    • H01S3/1123—Q-switching
    • H01S3/113—Q-switching using intracavity saturable absorbers
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/0014—Monitoring arrangements not otherwise provided for
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/09—Processes or apparatus for excitation, e.g. pumping
    • H01S3/091—Processes or apparatus for excitation, e.g. pumping using optical pumping
    • H01S3/0912—Electronics or drivers for the pump source, i.e. details of drivers or circuitry specific for laser pumping
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
    • H01S3/11—Mode locking; Q-switching; Other giant-pulse techniques, e.g. cavity dumping
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
    • H01S3/13—Stabilisation of laser output parameters, e.g. frequency or amplitude
    • H01S3/131—Stabilisation of laser output parameters, e.g. frequency or amplitude by controlling the active medium, e.g. by controlling the processes or apparatus for excitation
    • H01S3/1312—Stabilisation of laser output parameters, e.g. frequency or amplitude by controlling the active medium, e.g. by controlling the processes or apparatus for excitation by controlling the optical pumping
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/09—Processes or apparatus for excitation, e.g. pumping
    • H01S3/091—Processes or apparatus for excitation, e.g. pumping using optical pumping
    • H01S3/094—Processes or apparatus for excitation, e.g. pumping using optical pumping by coherent light
    • H01S3/0941—Processes or apparatus for excitation, e.g. pumping using optical pumping by coherent light of a laser diode
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
    • H01S3/10038—Amplitude control
    • H01S3/10046—Pulse repetition rate control
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
    • H01S3/13—Stabilisation of laser output parameters, e.g. frequency or amplitude
    • H01S3/1305—Feedback control systems
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/14—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range characterised by the material used as the active medium
    • H01S3/16—Solid materials
    • H01S3/1601—Solid materials characterised by an active (lasing) ion
    • H01S3/1603—Solid materials characterised by an active (lasing) ion rare earth
    • H01S3/1611—Solid materials characterised by an active (lasing) ion rare earth neodymium
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/14—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range characterised by the material used as the active medium
    • H01S3/16—Solid materials
    • H01S3/163—Solid materials characterised by a crystal matrix
    • H01S3/164—Solid materials characterised by a crystal matrix garnet
    • H01S3/1643—YAG

Abstract

A method for use in controlling a passively Q-switched laser apparatus 120 comprising: triggering pumping of a passively Q-switched laser apparatus 120 at a pump trigger time, measuring an emission delay between the pump trigger time and a time of emission of a Q-switched optical pulse from the laser apparatus 120; using the measured emission delay and a target time for the emission of a further Q-switched optical pulse from the laser apparatus to determine a further pump trigger time. The method may be used for improving the timing of emission of Q switched optical pulses and may, in particular, serve to suppress pulse-to-pulse timing jitter in particular, though not exclusively, for laser target designation. Also disclosed are a controller 140 and a programmable controller 140, in communication with an optical detector 144 and the pump 130 (via pump driver 132) for performing the above method. Further disclosed is a passively Q-switched laser apparatus comprising: a pump 130 for pumping a passively Q-switched laser apparatus 120; an optical detector 144 and a controller 140, where the controller performs the above method.

Description

METHOD AND APPARATUS FOR USE IN PASSIVE Q-SWITCHING
FIELD
The present invention relates to a passively Q-switcbed laser apparatus and a method for use in controlling such a laser apparatus in particular, though not exclusively, for laser target designation. The present invention also concerns a controller and a program for a controller for implementation of the method.
BACKGROUND
0-switching is a well known method for generating high-energy, short-duration laser pulses. 0-switching typically occurs as a consequence of the presence of variable attenuation within a laser cavity.
Active 0-switches are commonly used to control attenuation within a laser cavity for the production of reliable laser pulses. Although it is known to use mechanical devices for active 0-switching such as shutters, chopper wheels, spinning mirrors/prisms and the like inside a laser cavity, it is more common to use an intra-cavity modulator such as an intra-cavity acousto-optic or an electro-optic modulator.
However, such active Q-switches may be relatively expensive, complex and large, and may require relatively high driving voltages and relatively fast switching of the driving voltages.
Passive 0-switches are inherently cheaper, simpler and smaller than active 0- switches and require no control or drive requirements. However, known passive 0-switching methods produce more pulse-to-pulse timing jitter than active 0-switching methods. The pulse-to-pulse timing jitter of known passively 0-switched lasers may be at least partially attributable to the type of pump used for excitation of a laser gain medium. For example, many known passively 0-switched lasers utilise flashlamp pumps. Such pumps are, however, prone to random intensity fluctuations which result in variations in pump-rate arid, ultimately, in laser pulse-to-pulse timing jitter.
Regardless of the type of pump used for a passively 0-switched laser, er,viror.menthl factora may also contribute to the timing jitter. For exampie, changes in ambient temperature can translate into variations in laser efficiency through misalignment of laser optical components due to differential thermal expansion. Any change in thermal loading while the laser Fs firing, can couple into laser efficiency variations, for example, via thermal lensing effects or through misalignment of components. Thermal loading may, in particular, vary according to time elapsed from start-up of the laser from a cold condition or as a result of varying a laser pulse repetition rate in response to a varying user requirement. Any such changes in laser efficiency may couple into timing jitter.
As a result of such problems of known passive 0-switching methods, applications for which suppression of pulse-to-pulse timing jitter is critical, such as laser target designation, generally utilise active Q-switches at the expense of greater cost and size.
SUMMARY
According to a first aspect of the present invention there is provided a method for use in controlling a passively 0-switched laser apparatus, the method comprising: triggering pumping of a passively 0-switched laser apparatus at a pump trigger time; measuring an emission delay between the pump trigger time and a time of emission of a Q-switched optical pulse from the laser apparatus; and using the measured emission delay and a target time for the emission of a further 0-switched optical pulse from the laser apparatus to determine a further pump trigger time.
The method may comprise triggering pumping of the laser apparatus at the further pump trigger time for emission of the further 0-switched optical pulse.
Such a method may permit the emission of the further 0-switched optical pulse from the laser apparatus substantially at or around the target emission time. This may result in improved timing of emission of the further 0-switched optical pulse relative to known passive 0-switching methods. The method may, in particular, facilitate the use of passive 0-switching in applications where pulse-to-pulse timing is critical and where, traditionally, active 0-switching has previously been deemed necessary. Compared with active 0-switching, the method may enable the use of a simpler, more compact and lower cost 0-switched laser apparatus for such applications. Such a passively 0- switched laser apparatus may also have higher reliability than a known actively 0-switched laser apparatus.
Pumping of the passively Q-switched laser apparatus may comprise pumping a gain medium of the laser apparatus.
The measured emission delay for the Q-switched optical pulse from the laser apparatus may be used as an estimate of the emission delay for the further 0-switched optical purse.
The method may comprise determining the further pump trigger time by subtracting the measured emission delay from the target emission time.
The method may comprise determining the target emission time for the further Q-switched optical pulse based on a desired pulse-to-purse repetition rate.
The further 0-switched optical pulse may be the 0-switched optical pulse emitted by the laser apparatus which immediately follows the 0-switched optical pulse.
The laser apparatus may emit at least one intervening 0-switched optical pulse between the Q-switched optical pulse and the further 0-switched optical pulse.
The method may comprise: triggering pumping of the passively 0-switched laser apparatus at each of a plurality of pump trigger times; measuring an emission delay between each of the plurality of pump trigger times and a corresponding time of emission of a corresponding Q-switched optical pulse from the laser apparatus; and using the measured emission delays and a target time for the emission of a further 0-switched optical pulse from the laser apparatus to determine a further pump -trigger time.
For example, the method may comprise determining the further pump trigger time by subtracting an average of the measured emission delays from the target emission time. The average of the measured emission delays may be less susceptible to any random intensity noise appearing on the individual Q-switched optical pulses and/or may be less susceptible to any measurement inaccuracies.
The method may comprise: triggering pumping of the passively 0-switched laser apparatus at each of a plurality of successive pump trigger times; measuring an emission delay between each of the plurality of successive pump trigger times and a corresponding time of emission of a corresponding 0-switched optical pulse from the laser apparatus; and using the measured emission delays and a target time for the emission of a further 0-switched optical pulse from the laser apparatus to determine a further pump trigger time.
The method may comprise determining the emission time of the 0-switched optical pulse from the timing of a feature of the 0-switched optical pulse. The particular feature of the 0-switched optical pulse used for determining the emission time may be selected according to the convenience with which the timing of the feature may be determined.
The method may comprise determining the emission time of the Q-switched optical pulse from the timing of a leading edge of the Q-switched optical pulse. The method may comprise determining the emission time of the 0-switched optical pulse as a time when an optFcal power of the Q-switched optical pulse rises through a predetermined threshold optical power level. The predetermined threshold optical power level may be defined as a proportion of a peak optical power of the Q-switched optical pulse. For example, the predetermined threshold optical power level may be 50% of a peak optical power of the 0-switched optical pulse. The method may comprise determining the emission time of the Q-switched optical pulse as a time corresponding to the maximum rate of increase of optical power of the Q-switched optical pulse.
The method may comprise determining the emission time of the Q-switched optical pulse from the timing of a trailing edge of the 0-switched optical pulse. The method may comprise determining the emission time of the 0-switched optical pulse as a time when an optical power of the 0-switched optical pulse falls through a predetermined threshold optical power level. The predetermined threshold optical power level may be defined as a proportion of a peak optical power of the Q-switched optical pulse. For example, the predetermined threshold optical power level may be 50% of a peak optical power of the Q-switched optical pulse. The method may comprise determining the emission time of the 0-switched optical pulse as a time corresponding to the maximum rate of decrease of optical power of the 0-switched optical pulse.
The method may comprise determining the emission time of the 0-switched optical pulse as a time corresponding to a peak in the optical power of the 0-switched optical pulse.
The method may comprise: detecting the 0-switched optical pulse so as to generate an electrical pulse which is generally proportional to the Q-switched optical pulse; and determining the emission time of the Q-switched optical pulse from the timing of a feature of the electrical pulse.
The emission time of the 0-switched optical pulse may be more easily determined from the timing of a feature of such an electrical pulse. For example, the method may comprise using an electronic timing method to measure the emission delay between triggering pumping and the timing of the feature of the electrical pulse.
The method may comprise optically pumping the laser apparatus. For example, the method may comprise coupling light from an optical pump into a gain medium of the laser apparatus for excitation of the gain medium.
The method may comprise using a diode pump to pump the laser apparatus.
The method may comprise using at least one of a laser diode, a light emitting diode or a super-luminescent light emitting diode to pump the laser apparatus.
The method may comprise electrically pumping the laser apparatus. For example, the method may comprise electrically pumping a gain medium of the laser apparatus for excitation of the gain medium.
The method may comprise using an electrical current source to pump the laser apparatus.
According to a second aspect of the present invention there is provided a controller for use in controlling a passively 0-switched laser apparatus, the laser apparatus comprising a pump for pumping the laser apparatus and an optical detector for detecting emission of a 0-switched optical pulse, and the controller being configured for communication with the pump and the optical detector and being configured so as to: trigger the pump at a pump trigger time; measure an emission delay between the pump trigger time and a time of emission of a 0-switched optical pulse from the laser apparatus; and use the measured emission delay and a target time of emission of a further 0-switched optical pulse from the laser apparatus to determine a further pump trigger time.
One or more of the optional features disclosed above in relation to the first aspect may apply alone or in any combination in relation to the second aspect.
According to a third aspect of the present invention there is provided a programmable controller for use in controlling a passively 0-switched laser apparatus, the laser apparatus comprising a pump for pumping the laser apparatus and an optical detector for detecting emission of a 0-switched optical pulse, and the controller being configured for communication with the pump and the optical detector and being programmed so as to: trigger the pump at a pump trigger time; measure an emission delay between the pump trigger time and a time of emission of a 0-switched optical pulse from the laser apparatus; and use the measured emission delay and a target time of emission of a further Q-switched optical pulse from the laser apparatus to determine a further pump trigger time.
One or more of the optional features disclosed above in relation to the first aspect may apply alone or in any combination in relation to the third aspect.
According to a fourth aspect of the present invention there is provided a program for a programmable controller for use in controlling a passively Q-switched laser apparatus, the laser apparatus comprising a pump for pumping the laser apparatus and an optical detector for detecting emission of a Q-switched optical pulse, and the controller being configured for communication with the pump and the optical detector, wherein, when executed by the controller, the program causes the controller to: trigger the pump at a pump trigger time; measure an emission delay between the pump trigger time and a time of emission of a Q-switched optical purse from the laser apparatus; and use the measured emission delay and a target time of emission of a further Q-switched optical pulse from the laser apparatus to determine a further pump trigger time.
One or more of the optional features disclosed above in relation to the first aspect may apply alone or in any combination in relation to the fourth aspect.
According to a fifth aspect of the present invention there is provided a data carrier comprising a program according fourth aspect.
According to a sixth aspect of the present invention there is provided a passively Q-switched laser apparatus comprising: a pump configured for pumping a passively Q-switched laser apparatus; an optical detector for detecting emission of a Q-switched optical pulse; and a controller configured for communication with the pump and the optical detector, the controller being configured to: trigger operation of the pump ata pump trigger time; measure an emission delay between the pump trigger time and a time of emission of a Q-switched optical pulse from the laser apparatus; and use the measured emission delay and a target time of emission of a further Q-switched optical pulse from the laser apparatus to determine a further pump trigger time.
One or more of the optional features disclosed above in relation to the first aspect may apply alone or in any combination in relation to the sixth aspect.
The controller may be configured to trigger operation of the pump at the further pump trigger time for emission of the further 0-switched optical pulse.
The laser apparatus may comprise a gain medium.
The pump may be configured to pump the gain medium.
The pump may comprise an optical source.
The pump may comprise an optical diode source.
The pump may comprise at least one of a laser diode, a light emitting diode or a super-luminescent light emitting diode.
The pump may comprise an electrical pump.
The pump may comprise a current source.
The apparatus may comprise an optical arrangement configured to divert at least a portion of the optical power of the 0-switched optical pulse from a location inside and/or outside a cavity of the laser apparatus onto the optical detector.
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention will now be described by way of non-limiting example only with reference to the following figures of which: Figure 1 is a schematic of a prior art passively 0-switched laser apparatus; Figure 2 is a schematic of a passively 0-switched laser apparatus constituting an embodiment of the present invention; and Figure 3 is a schematic of the temporal variation of pump current and the corresponding optical pump power, laser output power and photodetector electrical signal of the passively 0-switched laser apparatus of Figure 2.
DETAILED DESCRIPTION OF THE DRAWINGS
With reference initially to Figure 1, there is provided a known passively 0-switched laser apparatus generally designated 10 comprising an optical cavity 20 defined between a high reflector 22 and an output coupler 24, the optical cavity 20 including an optical gain medium 26 and a passive Q-switch 28. The gain medium 26 is optically pumped by a flashlamp 30. The flashlamp 30 is driven by an electrical driver 32.
In use, the optical cavity 20 emits a passively 0-switched output beam 33 from the output coupler 24 according to variations in optical cavity gain and optical cavity loss as will now be described in more detail below. The electrical driver 32 drives the flashlamp 30 with a pulsed electrical signal represented by dotted line 34 in Figure 1 and the flashlamp 30 generates an optical pump pulse represented by solid line 36 in Figure 1.
The optical pump pulse 36 is coupled to the gain medium 26 so as to create a population inversion in the gain medium 26. The population inversion provides the optical cavity 20 with a gain which grows during the optical pump pulse 36 until the optical cavity gain exceeds the optical cavity loss such that the optical cavity 20 reaches threshold and emits stimulated radiation. The stimulated radiation saturates the passive 0-switch 28 causing a rapid fall in optical cavity loss. The population inversion in the gain medium 26 falls during stimulated emission until the optical cavity gain falls below the optical cavity loss once again thereby resulting in cessation of stimulated radiation, Such variations in the optical cavity gain and the optical cavity loss during a single optical pump pulse 36 result in the emission of a passively 0-switched optical output pulse at the output coupler 24.
It is also known to generate a stream of passively 0-switched laser pulses by operating the electrical driver 32 so as to generate a stream of electrical pulses 34 at the desired repetition rate. This causes the flashlamp 30 to generate a stream of optical pump pulses 36 at or around the desired repetition rate. Each optical pump pulse 36 results in the emission of a single passively 0-switched laser pulse as described abova However, the timing jitter between the 0-switched laser pulses may be unduly high and may preclude the use of the passively Q-switched laser apparatus for some applications.
Figure 2, shows an embodiment of a passively 0-switched laser apparatus generally designated 110 comprising an optical cavity 120 defined between a high reflector 122 and an output coupler 124, the optical cavity 120 including an optical gain medium 126 and a passive 0-switch 128. The gain medium 126 is optically pumped by a pulsed laser diode pump 130. The laser diode pump 130 is driven by an electrical diode driver 132.
The laser apparatus 110 of Figure 2 is configured to generate a stream of passively 0-switched laser pulses having a desired repetition rate with a reduced or insignificant level of pulse-to-pulse timing jitter compared with the pulse-to-pulse timing jitter associated with the prior art laser apparatus 10 of Figure 1. The desired repetition rate will depend on the application and may be predetermined or user defined. One skilled in the art will also appreciate that the present invention is not limited to any particular choice of gain medium 126 or passive Q-switch 128. For example, in the embodiment shown in Figure 2, a suitable material for the gain medium 126 may be Nd:YAG, whilst a suitable material for the passive Q-switch 128 may be Cr:YAG. In other embodiments, different materials may be used for the gain medium 126 and the passive 0-switch 128.
The laser apparatus 110 further comprises a controller 140, an optical detector 144 and a partially reflecting mirror 146. The partially reflecting mirror 146 diverts a portion 147 of the light 133 emitted by the output coupler 124 onto the optical detector 144. The optical detector 144 is configured to provide an electrical signal 148 which is generally proportional to the optical power of portion 147. The optical detector 144 provides the electrical signal 148 to the controller 140. As indicated by the dotted line 150, the controller 140 is configured to control the electrical diode driver 132 according to the electrical signal 148.
In use, the controller 140 controls the electrical diode driver 132 so as to provide passively 0-switched pulses having the desired pulse repetition rate whilst suppressing or, at least reducing, pulse-to-pulse timing jitter. In particular, the controller 140 uses feedback from measurements performed during the emission of a passively 0-switched pulse to control the timing of the emission of a subsequent passively Q-switched pulse as will now be described with reference to Figure 3. It should be understood that Figure 3 is only intended to provide a schematic illustration of the temporal variation of pump current 134 and the corresponding temporal variation in optical pump power 136, optical power output 133 from the passively Q-switched laser apparatus 110 of Figure 2 and the electrical signal 148 provided by the optical detector 144, and that the actual temporal variations observed in practice may differ from those shown schematically in Figure 3.
At a time 7', the controller 140 sends an th electrical trigger signal to the diode driver 132, where i represents an integer index value. On receipt of the i electrical trigger signal, the diode driver 132 initiates an ith current pulse 134' to the laser diode pump 130. It will be appreciated that, as illustrated in Figure 3, there may be a delay between sending the ith electrical trigger signal to the diode driver 132 at time 7' and the leading edge of the ith current pulse 134'. However in some cases, this delay may be insignificant such that the leading edge of the current pulse 134 may occur at substantially the same instant 7' at which the ith electrical trigger signal is sent by the controller 140. The i°' current pulse 134 results in the output of an i"' optical pump pulse 136' from the laser diode pump 130. The i optical pump pulse 136' is coupled to the gain medium 126 which causes the gain of the gain medium 126 to increase until an 1th passively 0-switched pulse 133' is emitted from the optical cavity 120. The optical detector 144 generates an i1' electrical pulse 148 signal having an instantaneous value which is generally proportional to the power of the ith passively 0-switched pulse 133'. The controller 140 processes the ith electrical pulse 148' received from the optical detector 144 and determines a time of emission for the U1 passively Q-switched pulse 133'. 7 may be defined as the time corresponding to the leading edge of the ith electrical pulse 148 according to a predetermined definition of 7.
may, for example, be defined as the time when the i' electrical pulse 148' rises through a predetermined reference level. 7 may be defined as the time when the ith electrical pulse 148' rises through a level equivalent to 50% of the peak signal value of the th electrical pulse 148' or the time corresponding to a maximum rate of increase in signal value of the ith electrical pulse 1481. It will be understood, however, that the exact definition of is not essential and that, for the purposes of controlling passive Q-switching of the laser apparatus 110, it is only important that a consistent definition of is used. It should also be understood that the ith passively 0-switched pulse 133' is subject to pulse-to-pulse timing jitter and that the emission time of the ith passively Q-switched pulse 133' is shown in Figure 3 as being offset relative to the ideal or target time of emission of the ith passively 0-switched pulse 133 (i.e. the time of emission of the jth passively 0-switched pulse 133' corresponding to zero pulse-to-pulse timing jitter) given by: = T° + AT qiargel qiarget q Equation I where is a target time of emission for the 01h passively Q-switched pulse (not shown) and A2 is a desired pulse4o-pulse temporal separation or period corresponding to a desired pulse repetition rate of 1/Al; Having determined the emission time i for the passively Q-switched pulse 133', the controller 140 determines an emission delay Si; for the ith passively Q-switched pulse 133' defined as the time elapsed between emission of the th electrical trigger signal at T,' and the emission time for the ith passively Q-switched pulse 133' according to the relation: Si; Equation 2 The controller 140 subsequently uses the emission delay AT4 of the passively Q-switched purse 133' as an estimate for the emission delay q+1) of a future (1+1)th passively Q-switched pulse j3301) to ensure that the emission of the (i+l)th passively Q-switched pulse l331) takes place at, or as close as possible to, a target time of emission for the (i+l)th passively Q-switched pulse 1330+1) given by: cte' n7L +(i+1)LITq Equation 3 More specifically, the controller 140 determines a time 7" for triggering the diode driver 132 for the initiation of the emission process for the (i+l)th passively Q-switched pulse 133Q'l) according to the relation: T(+1) - -AT' F -qiarge; (q Equation 4 Having determined t1 from Equation 4, the controller 140 sends an electrical trigger signal to the diode driver 132 at time to initiate the (i+l)" current pulse 1340+1).
The (i÷l)th current pulse 1340+1) results in the output of an (iil)th optical pump pulse 136°" from the laser diode pump 130. The (j÷1)th optical pump pulse 13&' is coupled to the gain medium 126 which causes the output of an (i+lf' passively 0-switched pulse 13t1 from the optical cavity 120 at or close to the target emission time 710+1) required to provide the desired pulse repetition rate of 1/i\7..
One skilled in art will appreciate that various modifications of the apparatus and the methods described above are possible without departing from the scope of the present invention. The controller 140 may, for example, comprise a comparator, a timer and a processor. The comparator may be configured to receive the electrical signal 148 from the photodetector 144 and to compare the electrical signal 148 to a reference level input. The timer may be configured to receive a trigger signal from the processor and an output signal from the comparator. In use, at time 27, the processor may simultaneously send an ith electrical trigger signal to the diode driver 132 and to the timer to initiate an elapsed time measurement. The comparator may receive the ith electrical pulse 148 and compare it to the reference level input. When the it" electrical pulse 148' rises through the reference level, the comparator output signal may cause the timer to finish the elapsed time measurement. Subsequently, the timer may provide the measured elapsed time to the processor which treats the elapsed time as the emission delay /XT4 and calculates the time 7 for triggering the diode driver 132 for the initiation of the emission process for the (i+l)th passively 0-switched pulse 133+1) according to Equation 4.
Rather than using a laser diode pump 130, an alternative optical pump source may be used such as a light emitting diode (LED), a superluminescent light emitting diode (SLED), a flashlamp, or the like. In other embodiments, the gain medium 126 may be electrically pumped. In such electrically pumped embodiments, the gain medium 126 may, for example, be electrically pumped directly by an electrical driver.
In addition to, or as an alternative to detecting a fraction 147 of the optical power 133 output by the passively Q-switched laser apparatus 110 using the partial reflector 146 and photodetector 144, a fraction of the optical power may be diverted or tapped from the laser cavity 120 and detected using a further photodetector (not shown) configured for communication with the controller 140.
The controller 140 may determine the emission delay A7 between triggering the diode driver 132 and any feature of the electrical pulse 148' received from the S optical detector 144. For example, the controller 140 may determine the emission delay LtST, between triggering the diode driver 132 and a peak or a falling edge of the electrical pulse 146'.
Rather than estimating the emission delay for the (j÷1)th passively Q-switched pulse from the emission delay t?sT4 for the immediately preceding ith pulse, the controller 140 may estimate the emission delay (e.g. A1)) for the (i+lf' passively 0-switched pulse using an emission delay A7$, a7_2), sT4'3... of an earlier passively Q-switched pulse and determine the time 1' for triggering the diode driver 132 for initiation of the emission process for the (1+1)th passively Q-switched pulse 1330+1) from any one of the relations: -q'(i+l) -I -qIIIrgCt tq I T(1) -T&t1) -I -qtarge( Iq I T(+1) = T'0 - 1 qtorgel Iq I Equation 5 The controller 140 may estimate the emission delay AT4 for the (i+l)tI passively Q-switched pulse from a function f of the emission delays of a plurality of preceding passively 0-switched pulses and determine the time +1) for triggering the diode driver 132 for the initiation of the emission process for the (i+l)th passively 0-switched pulse 1 33U from: -r(t+1) -(ATe AT' I -qlOrgeI J Iq, tq Y*' Equation 6 f(ATiaT1)) may, for example, be an average of the emission delays M, and j-(zxuzT@-)=j.(t.T' +Air)) Equation 7 Although the foregoing description of the operation of the laser apparatus 110 of Figure 2 with reference to Figure 3 only refers to the emission of a single passively 0-switched laser pulse per optical pump pulse, it will be appreciated that, depending on the duration and/or the intensity of the optical pump pulse, the laser apparatus 110 may be operated so that more than one passively 0-switched laser pulse may be emitted per optical pump pulse.
Although, in use, the photodetector 144 of the laser apparatus 110 of Figure 2 generates electrical pulses 148', 1480 which are generally proportional to the power of the corresponding passively 0-switched pulses 133, 13t", it will be understood that1 depending on the speed of response of the photodetector 144, the electrical pulses 148, 14&+1) may not be exactly proportional to the optical power of the passively 0-switched pulses 133', but may be delayed or may be of a different shape to the passively 0-switched pulses 133', l331). This does not, however, affect the passive 0-switching method described above.

Claims (1)

  1. <claim-text>CLAIMS1. A method for use in controlling a passively 0-switched laser apparatus, the method comprising: triggering pumping of a passively 0-switched laser apparatus at a pump trigger time; measuring an emission delay between the pump trigger time and a time of emission of a 0-switched optical pulse from the laser apparatus; and using the measured emission delay and a target time for the emission of a further 0-switched optical pulse from the laser apparatus to determine a further pump trigger time.</claim-text> <claim-text>2. A method according to claim 1 comprising triggering pumping of the laser apparatus at the further pump trigger time so as to initiate an emission process for the further Q-switched optical pulse.</claim-text> <claim-text>3. A method according to claim 1 or 2, comprising: determining the further pump trigger time by subtracting the measured emission delay from the target emission time.</claim-text> <claim-text>4. A method according to any preceding claim, comprising: determining the target emission time for the further 0-switched optical pulse based on a desired pulse-to-pulse repetition rate.</claim-text> <claim-text>5. A method according to any preceding claim, wherein the further 0-switched optical pulse is the Q-switched optical pulse emitted by the laser apparatus which immediately follows the Q-switched optical pulse.</claim-text> <claim-text>6. A method according to any of claims 1 to 4, wherein the laser apparatus emits at least one intervening Q-switched optical pulse between the 0-switched optical pulse and the further 0-switched optical pulse.</claim-text> <claim-text>7. A method according to any preceding claim, comprising: triggering pumping of the passively Q-switched laser apparatus at each of a plurality of pump trigger times; measuring an emission delay between each of the plurality of pump trigger times and a corresponding time of emission of a corresponding Q-switched optical pulse from the laser apparatus; and using the measured emission delays and a target time for the emission of a further 0-switched optical pulse from the laser apparatus to determine a further pump trigger time.</claim-text> <claim-text>8. A method according to claim 7, comprising: determining the further pump trigger time by subtracting an average of the measured emission delays from the target emission time.</claim-text> <claim-text>9. A method according to any preceding claim, comprising: determining the emission time of a 0-switched optical pulse from the timing of a feature of the 0-switched optical pulse.</claim-text> <claim-text>10. A method according to any preceding claim, comprising: determining the emission time of a 0-switched optical pulse from the timing of a leading edge of the 0-switched optical pulse.</claim-text> <claim-text>11. A method according to claim 10, comprising: determining the emissrori time of the 0-switched optical pulse as a time when an optical power of the 0-switched optical pulse rises through a predetermined threshold optical power level.</claim-text> <claim-text>12. A method according to claim 10 or 11, wherein the predetermined threshold optical power level is a defined proportion of a peak optical power of the 0-switched optical pulse.</claim-text> <claim-text>13. A method according to claim 10, comprising: determining the emission time of the 0-switched optical pulse as a time corresponding to the maximum rate of increase of optical power of the 0-switched optical pulse.</claim-text> <claim-text>14. A method according to any of claims ito 9, comprising: 17 / determining the emission time of a Q-switched optical pulse from the timing of a trailing edge of the 0-switched optical pulse.</claim-text> <claim-text>15. A method according to claim 14, comprising: determining the emission time of the 0-switched optical pulse as a time when an optical power of the Q-switched optical pulse falls through a predetermined threshold optical power level.</claim-text> <claim-text>16. A method according to claim 14 or 15, wherein the predetermined threshold optical power level is a defined proportion of a peak optical power of the Q-switched optical pulse.</claim-text> <claim-text>17. A method according to claim 14, comprising: determining the emission time of the 0-switched optical pulse as a time corresponding to the maximum rate of decrease of optical power of the 0-switched optical pulse.</claim-text> <claim-text>18. A method according to any of claims ito 9, comprising: determining the emission time of a 0-switched optical pulse as a time corresponding to a peak in the optical power of the 0-switched optical pulse.</claim-text> <claim-text>19. A method according to any preceding claim, comprising: detecting a 0-switched optical pulse so as to generate an electrical pulse which is generally proportional to the 0-switched optical pulse; and determining the emission time of the 0-switched optical pulse from the timing of a feature of the electrical pulse.</claim-text> <claim-text>20. A method according to any preceding claim, comprising: optically pumping the laser apparatus.</claim-text> <claim-text>21. A method according to any preceding claim, comprising: using a diode pump to pump the laser apparatus.</claim-text> <claim-text>22. A method according to any preceding claim, comprising: using at least one of a laser diode, a light emitting diode or a super-luminescent light emitting diode to pump the laser apparatus.</claim-text> <claim-text>23. A method according to any of claims ito 19, comprising: electrically pumping the laser apparatus.</claim-text> <claim-text>24. A method according to claim 23, comprising: using an electrical current source to pump the laser apparatus.</claim-text> <claim-text>25. A controller for use in controlling a passively 0-switched laser apparatus, the laser apparatus comprising a pump for pumping the laser apparatus and an optical detector for detecting emission of a 0-switched optical pulse, and the controller being configured for communication with the pump and the optical detector and being configured so as to implement the method according to any of claims 1 to 24.</claim-text> <claim-text>26. A programmable controller for use in controlling a passively 0-switched laser apparatus, the laser apparatus comprising a pump for pumping the laser apparatus and an optical detector for detecting emission of a 0-switched optical pulse, and the controller being configured for communication with the pump and the optical detector and being programmed so as to implement the method according to any of claims 1 to 24.</claim-text> <claim-text>27. A program which, when executed on the controller of claim 26, implements the method according to any of claims I to 24.</claim-text> <claim-text>26. A data carrier comprising a program according to claim 27.</claim-text> <claim-text>29. A passively 0-switched laser apparatus comprising: a pump configured for pumping a passively Q-switched laser apparatus; an optical detector for detecting emission of a 0-switched optical pulse; and a controller configured for communication with the pump and the optical detector, the controller being configured to: trigger the pump at a pump trigger time; measure an emission delay between the pump trigger time and a time of emission of a 0-switched optical pulse from the laser apparatus; and use the measured emission delay and a target time for the emission of a further 0-switched optical pulse from the laser apparatus to determine a further pump trigger time.</claim-text> <claim-text>30. A passively 0-switched laser apparatus according to claim 29, wherein the controller is configured to trigger the pump at the further pump trigger time so as to initiate an emission process for the further 0-switched optical pulse.</claim-text> <claim-text>31. A passively 0-switched laser apparatus according to claim 29 or 30, wherein the pump comprises an optical source.</claim-text> <claim-text>32. A passively Q-switched laser apparatus according to any of claims 29 to 31, wherein the pump comprises a diode source.</claim-text> <claim-text>33. A passively 0-switched laser apparatus according to any of claims 29 to 32, wherein the pump comprises at least one of a laser diode, a light emitting diode or a super-luminescent light emitting diode.</claim-text> <claim-text>34. A passively 0-switched laser apparatus according to claim 29 or 30, wherein the pump comprises an electrical pump.</claim-text> <claim-text>35. A passively Q-switched laser apparatus according to claim 34, wherein the pump comprises a current source.</claim-text> <claim-text>36. A passively 0-switched laser apparatus according to any of claims 29 to 35, comprising an optical arrangement configured to divert at least a portion of the optical power of the 0-switched optical pulse from a location inside and/or outside a cavity of the laser apparatus onto the optical detector.</claim-text>
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