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A laser pulse stretcher made from optical fibres
Nuclear Instruments and Methods in Physics Research A324 (1993) 14-18 North-Holland
NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH Section A
A laser pulse stretcher made from optical fibres David Hanna and Joseph W. Mitchell
Physics Department McGill University, Montreal, Canada
Received 1 April 1992 We describe a simple device which splits a laser beam into many parts, sends it down optical fibres of different lengths and then recombines it . This has the effect of generating several short pulses which are temporally separated. We present an application in which the series of short pulses is used to simulate a single long pulse of the same total energy. This is useful in avoiding saturation effects caused by high instantaneous currents in photomultipliers . 1. Introduction For many applications, one requires a short pulse of high intensity light of a specific wavelength . Lasers are the ideal solution for such problems and it is usually the case that shorter pulses are better. Thus the history of laser development has been marked by milestones in achieving ever faster devices. In this paper we discuss a situation where one wishes to increase the duration of a laser pulse and present a simple and flexible solution for doing so . Photomultiplier tubes (PMTs) are found in nearly every high energy physics experiment, used mainly in calorimetry and time-of-flight applications as well as for hodoscopes . It is standard practice to use laser light, delivered to the PMT by optical fibres, for calibration and monitoring . One can check timing stability and look for gain drifts and, with a series of optical filters, one can test the linearity of the PMTS and readout electronics. As experiments grow in size and complexity, more and more channels of optical fibres are needed for the increasingly large numbers of PMTs. For example in the uranium calorimeter of the ZEUS experiment [1-6] almost 12 000 PMTs are to be calibrated using light from a single laser. For linearity tests it is necessary to supply each of these with the light equivalent of energy depositions measured in the hundreds of GeV. One solution which was investigated involved the use of a small (- 100 wJ) nitrogen laser which produced subnanosecond pulses of ultraviolet (A = 337 nm) light. These were used to pump a dye laser module to make green (A = 500 nm) pulses which were several hundred picoseconds in duration . With these pulses sent along optical fibres which terminated just in front of the photocathodes of the PMTs it was
possible to simulate the highest energy showers one expects from HERA kinematics. This solution is unsatisfactory for the following reason. The ZEUS calorimeter is constructed of layers of depleted uranium and scintillator, read out by plates of wavelength shifter (WLS). The blue scintillator light from the scintillator is shifted to green by the WLS and routed to the PMTS . The light pulses reaching the PMTS have a complicated time structure which is a convolution of the decay time of the scintillator, the decay time of the wavelength shifter material and various transit times in the light guides (for example some of the light proceeds directly toward the PMT while a different fraction first journeys in the opposite direction and then reflects off the far end of the light guide). Because of these effects, the light pulses from the calorimeter are of the order of 10 ns in length . Since the dye laser pulses are so much shorter it is clear that, for an equal amount of integrated light, the instantaneous photocurrent will be much higher for laser pulses than for shower pulses. This is illustrated in fig. 1. One of the causes of PMT saturation is loading due to excessive instantaneous photocurrents. Thus to investigate problems it is inappropriate to use pulses which are radically different in time structure from those which one is trying to simulate . It is for this reason that we are forced to abandon the green dye laser in its simplest form . There are two alternatives which could be used instead. Since the largest component of the calorimeter pulse width is the decay time of the wavelength shifter, one can simulate pulses by firing the ultraviolet primary beam of the laser into a block of wavelength shifter plastic . The fluors in the wavelength shifter would then decay according to their characteristic time
0168-9002/93/$06 .00 © 1993 - Elsevier Science Publishers B.V . All rights reserved
15
D. Hanna, J. Mitchell / Laser pulse stretcher
c
0.2
U
-0 .2 -0 .4 -0 .6 L time
Fig. 1. Plots of current as a function of time for a long (a) pulse and a short (b) pulse. For the same integrated charge, the short pulse achieves a higher instantaneous current than the long pulse. This can give rise to different saturation characteristics for a given photomultiplier. constants. This would produce a green pulse of more appropriate width but at the cost of reduced light intensity. The reduction comes from the fact that the produced light is isotropic so only a small fraction (- 2%) can be captured by the optical fibres . (With a green dye laser, the conversion efficiency is - 15% but the capture efficiency is nearly 100% .) The deficit would have to made up for by using a more powerful laser which introduces new problems . Nitrogen lasers in the mJ energy range are expensive and require external gas supply and vacuum pumps. Working with a higher energy beam is also more dangerous and care must be taken that front-end optics (steering mirrors, optical filters and the wavelength shifter dye itself) are not damaged by the increased light levels . A variant of this last scenario is to use a blue dye laser to inject light into the fibres and let the blue to green conversion take place in the light guides (which are made of the WLS plastic over their whole length) just in front of the PMTs . Although more of the green light is captured, there is no gain in overall efficiency since the blue dye laser is only half as efficient as the green and blue light has a shorter attenuation length in the fibres . One would still need a mJ laser to supply the necessary light levels .
fibres and each fraction will propagate with a characteristic transit time . At the far end of the bundle, the fibres are grouped together and the light is recombined spatially into a single beam. Mixing bars before and after the fibre bundle ensure, by means of total internal reflection, that the beam is well scrambled so as to have a relatively flat intensity profile. This means that the portion going down each fibre in the bundle will be approximately the same. The exit mixing bar ensures that every part of the final beam will contain portions from each of the fibres. The output of this stretcher system for a narrow input pulse of amplitude Vo is a series of N output pulses with amplitude VO IN which are spaced by increments of At where At is the incremental difference in transit time between one fibre and the next longest in the series . This is illustrated schematically in fig. 3. For our tests, we have chosen a single value of A t (i .e . fibres with lengths which are all integer multiples of some fundamental length) but this is not essential. By changing the number of fibres of given lengths one could synthesize pulses of arbitrary shape.
2. The pulse stretcher In view of the difficulties just outlined we decided to attempt to avoid using blue light and somehow produce green pulses of a width similar to that of pulses caused by electromagnetic showers in the calorimeter. The solution that we arrived at is sketched in fig. 2. The main component is a bundle of optical fibres in which the fibres have different lengths. Light which is injected into the bundle will be split among the various
Fig. 2. Schematic illustration of the pulse stretcher concept. Optical fibres of different lengths are bundled together at both ends allowing a beam of light to be injected and ex tracted. A single pulse of light which is injected at one end emerges as a train of pulses at the other.
16
D. Hanna, J. Mitchell / Laser pulse stretcher
We have confined our initial investigations to two stretchers designed to produce trains of equal amplitude pulses . The first stretcher, called from here on the short stretcher, is a bundle of 80 fibres containing fibres with eight different lengths. There are 10 fibres of length 0.5 m, ten with length 1 .0 m and so on at increments of 0.5 m up to 4 .0 m. Thus the maximum length difference within the bundle is 3.5 m, corresponding to a time of about 15 ns . These fibres have been randomly grouped together at each end and glued into an endplate made of PVC plastic with a square hole cut in it . After the glue had cured the ends of the fibres were cut and polished . The use of uniform increments in fibre length allows a neat construction where all but the shortest fibres can have their extra length coiled into loops of identical diameters which can then be bundled using cable ties . We used 80 fibres since that is the number which have to be fed in the ZEUS system . (These primary fibres supply bundles of secondary fibres further "downstream"). The fibres have a 200 Wm core diameter and the outside diameter, which includes the cladding, is 230 Wm . This means that the 80 fibres can be packed into a square of less than 2.5 mm on a side . This is the cross section of the mixing bars used at the entry and exit of the system . The transmission efficiency can be calculated as the active area of the fibres divided by the area of the mixing bar. It works out to - 40% . The other stretcher was identical to the short one in all respects except that it had groups of five fibres at each length, at 0.5, 1.0, 1 .5 and so on up to 8.0 m. Thus the dispersion introduced is - 30 ns . This stretcher is called the long stretcher.
time
Fig. 3. Illustration of how the pulse of fig. l b would appear after travelling through a pulse stretcher made of four fibres with four regularly spaced lengths .
mixing . Upon exiting the same mixing bar (or the downstream one in the case of the stretched data), the beam entered the fanout which consists of a tight bundle of fibres at one end and an array of commercially available (SMA) connectors at the other end. Two fibres were connected to this array, one leading to the "test" phototube and the other to the "monitor" phototube. For each of the four variations described above, a series of runs were taken under different conditions . Each run consisted of 500 laser pulses . During the runs, the variable filter was run through its range just over two times thus providing a continuous scan of laser intensities. This was achieved by advancing the filter by 1 .8° with every pulse of the laser, using a stepper motor.
3. Tests of the system To see whether this idea would work as planned, we performed a series of bench tests. We compared the saturation behaviour of a phototube when illuminated by the following; - green light produced by illuminating a mixing bar made from green wavelength shifting plastic with light from a blue dye laser, - green light directly from a green dye laser which first passed through a clear mixing bar, - green laser light which passed through the short stretcher, - green laser light which passed through the long stretcher. The setup is shown in fig. 4. Light from the laser passed through a circular variable filter and short focal length (25 mm) converging lens before hitting the first mixing bar. The lens helps to maximize the number of reflections in the mixing bar and thereby improve the
Fig. 4. Test setup used to measure the effects of the pulse stretchers . Light from a dye laser system is attenuated by a variable filter wheel and injected into the stretcher via a mixing bar which ensures a relatively even distribution of light among all the fibres in the bundle. The light emerging from the different lengths of fibre is recombined using a second mixing bar at the exit of the stretcher and is then fanned out to phototubes . PMT 1 was used as a monitor while PMT 2 was the test phototube.
D. Hanna, J. Mitchell / Laserpulse stretcher
The high voltage supplying the monitor PMT was kept constant for all runs but the high voltage for the test PMT was increased by 100 V for each new run. Thus the two PMTS started out in a region where both were linear but the test PMT became nonlinear as its voltage was increased. The signal from the test PMT was attentuated before entering the ADC to keep it in range. The data from these runs can be expressed most effectively as a series of four plots, each corresponding to a different dye or stretcher variation . These are shown in fig. 5. In each plot there are several curves,
4000
17
the average pulse height in the test PMT vs the pulse height seen in the monitor PMT. Each curve corresponds to a different high voltage (and therefore gain) setting on the test PMT. The curves in fig. 5 have been adjusted to take into account unavoidable changes in the optics as the test system was changed from one configuration to another. For example, the sharing of light between the fibres leading to the test and monitor PMTs was not precisely the same for each configuration. But at low voltages where both PMTs are linear, P1 = aP2 where a is a slope which will change if the light sharing between the
4000 t
2000
2000
0
40~
F===_
0
80
p1 vs p2
4000
2000
2000
0
40
80 p1 vs p2
40
80
120
p1 vs p2
4000
0
0
120 pl vs p2
Fig. 5. Results from different test configurations : (a) No stretcher but with blue light from the dye laser exciting a mixing bar made of green wavelength shifter plastic placed at the position of the second mixing bar in fig. 4. The green light hitting the phototubes has a time structure given by the decay properties of the wavelength shifter . This configuration most closely simulates the pulses arising from showers in a calorimeter. (b) No stretcher and with green light from the dye laser injected directly into the second mixing bar shown in fig. 4. (c) Green light passing through the short stretcher. (d) Green light passing through the long stretcher. In all plots, the response of PMT 2 as a function of the response of PMT 1 is shown. The different curves in each plot correspond to different values of high voltage on PMT 2. (The same set of high voltage values is used for all plots.) It is clear that the unstretched green light results in much more serious saturation effects than the blue light. The green light passing through the short stretcher produces saturation effects very similar to those caused by the blue light while the long stretcher overcorrects .
18
D. Hanna, J. Mitchell / Laser pulse stretcher
test and monitor PMTS does . Thus we use aP2 rather than P2 as a monitor. This means that the lowest voltage point for each configuration will result in a line of unit slope in a plot of P1 vs aP2. As the voltage on the test PMT is raised, the slope of P1 vs P2 will increase since the gain is rising . If there were no saturation effects the curves would remain straight and would all behave identically, independent of the configuration . However the increased high voltage results in larger currents in the dynode chain and saturation becomes apparent, manifesting itself as a turnover of the curves at high light levels . As predicted in the discussion above, the unstretched green laser light suffers the most from this effect since most of the photons hitting the photocathode arrive within a nanosecond or less. The green light produced by the decay of WLS fluors arrives at the photocathode over a longer time period and produces less saturation . This is also true for light from the two stretcher setups . It appears that the short stretcher simulates the behaviour of the natural WLS decay rather well and the longer version over compensates . This is expected since in the limit of infinitely long, zero amplitude pulses, one would never experience saturation effects. 4. Conclusions From the preceding, one can conclude that it is relatively simple to "slow down" a laser pulse using a fibre optic bundle . This method has several advantages. It is inexpensive, easy to fabricate and very stable . It offers a high efficiency, limited primarily by
the core-to-cladding ratio of the fibres and packing density of the bundle . Finally, it is flexible . As stated earlier, an arbitrary pulse shape can be built up by appropriate choice of the fibre lengths. In addition, by integrating two groups of fibres, each with a different mean length one can simulate double pulses (or with several mean lengths, multiple pulses). This can be useful for studying multi-hit capabilities of certain detectors or electronics .
Acknowledgements We would like to thank Robert Nowac and David Binding of McGill University for help in fabricating the stretchers . This work is supported by the National Sciences and Engineering Research Council of Canada .
References [1] ZEUS, A detector for HERA, Letter of Intent, DESY (June 1985). [2] The ZEUS Detector, Technical Proposal, DESY (March 1986). [3] The ZEUS Detector, Status Report 1987, DESY (September 1987). [4] The ZEUS Detector, Status Report 1989, DESY (March 1989). [5] A. Andresen et al., Nucl . Instr. and Meth . A290 (1990) 95 [6] A. Andresen et al ., Nucl . Instr . and Meth. A309 (1991) 101 .
NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH Section A
A laser pulse stretcher made from optical fibres David Hanna and Joseph W. Mitchell
Physics Department McGill University, Montreal, Canada
Received 1 April 1992 We describe a simple device which splits a laser beam into many parts, sends it down optical fibres of different lengths and then recombines it . This has the effect of generating several short pulses which are temporally separated. We present an application in which the series of short pulses is used to simulate a single long pulse of the same total energy. This is useful in avoiding saturation effects caused by high instantaneous currents in photomultipliers . 1. Introduction For many applications, one requires a short pulse of high intensity light of a specific wavelength . Lasers are the ideal solution for such problems and it is usually the case that shorter pulses are better. Thus the history of laser development has been marked by milestones in achieving ever faster devices. In this paper we discuss a situation where one wishes to increase the duration of a laser pulse and present a simple and flexible solution for doing so . Photomultiplier tubes (PMTs) are found in nearly every high energy physics experiment, used mainly in calorimetry and time-of-flight applications as well as for hodoscopes . It is standard practice to use laser light, delivered to the PMT by optical fibres, for calibration and monitoring . One can check timing stability and look for gain drifts and, with a series of optical filters, one can test the linearity of the PMTS and readout electronics. As experiments grow in size and complexity, more and more channels of optical fibres are needed for the increasingly large numbers of PMTs. For example in the uranium calorimeter of the ZEUS experiment [1-6] almost 12 000 PMTs are to be calibrated using light from a single laser. For linearity tests it is necessary to supply each of these with the light equivalent of energy depositions measured in the hundreds of GeV. One solution which was investigated involved the use of a small (- 100 wJ) nitrogen laser which produced subnanosecond pulses of ultraviolet (A = 337 nm) light. These were used to pump a dye laser module to make green (A = 500 nm) pulses which were several hundred picoseconds in duration . With these pulses sent along optical fibres which terminated just in front of the photocathodes of the PMTs it was
possible to simulate the highest energy showers one expects from HERA kinematics. This solution is unsatisfactory for the following reason. The ZEUS calorimeter is constructed of layers of depleted uranium and scintillator, read out by plates of wavelength shifter (WLS). The blue scintillator light from the scintillator is shifted to green by the WLS and routed to the PMTS . The light pulses reaching the PMTS have a complicated time structure which is a convolution of the decay time of the scintillator, the decay time of the wavelength shifter material and various transit times in the light guides (for example some of the light proceeds directly toward the PMT while a different fraction first journeys in the opposite direction and then reflects off the far end of the light guide). Because of these effects, the light pulses from the calorimeter are of the order of 10 ns in length . Since the dye laser pulses are so much shorter it is clear that, for an equal amount of integrated light, the instantaneous photocurrent will be much higher for laser pulses than for shower pulses. This is illustrated in fig. 1. One of the causes of PMT saturation is loading due to excessive instantaneous photocurrents. Thus to investigate problems it is inappropriate to use pulses which are radically different in time structure from those which one is trying to simulate . It is for this reason that we are forced to abandon the green dye laser in its simplest form . There are two alternatives which could be used instead. Since the largest component of the calorimeter pulse width is the decay time of the wavelength shifter, one can simulate pulses by firing the ultraviolet primary beam of the laser into a block of wavelength shifter plastic . The fluors in the wavelength shifter would then decay according to their characteristic time
0168-9002/93/$06 .00 © 1993 - Elsevier Science Publishers B.V . All rights reserved
15
D. Hanna, J. Mitchell / Laser pulse stretcher
c
0.2
U
-0 .2 -0 .4 -0 .6 L time
Fig. 1. Plots of current as a function of time for a long (a) pulse and a short (b) pulse. For the same integrated charge, the short pulse achieves a higher instantaneous current than the long pulse. This can give rise to different saturation characteristics for a given photomultiplier. constants. This would produce a green pulse of more appropriate width but at the cost of reduced light intensity. The reduction comes from the fact that the produced light is isotropic so only a small fraction (- 2%) can be captured by the optical fibres . (With a green dye laser, the conversion efficiency is - 15% but the capture efficiency is nearly 100% .) The deficit would have to made up for by using a more powerful laser which introduces new problems . Nitrogen lasers in the mJ energy range are expensive and require external gas supply and vacuum pumps. Working with a higher energy beam is also more dangerous and care must be taken that front-end optics (steering mirrors, optical filters and the wavelength shifter dye itself) are not damaged by the increased light levels . A variant of this last scenario is to use a blue dye laser to inject light into the fibres and let the blue to green conversion take place in the light guides (which are made of the WLS plastic over their whole length) just in front of the PMTs . Although more of the green light is captured, there is no gain in overall efficiency since the blue dye laser is only half as efficient as the green and blue light has a shorter attenuation length in the fibres . One would still need a mJ laser to supply the necessary light levels .
fibres and each fraction will propagate with a characteristic transit time . At the far end of the bundle, the fibres are grouped together and the light is recombined spatially into a single beam. Mixing bars before and after the fibre bundle ensure, by means of total internal reflection, that the beam is well scrambled so as to have a relatively flat intensity profile. This means that the portion going down each fibre in the bundle will be approximately the same. The exit mixing bar ensures that every part of the final beam will contain portions from each of the fibres. The output of this stretcher system for a narrow input pulse of amplitude Vo is a series of N output pulses with amplitude VO IN which are spaced by increments of At where At is the incremental difference in transit time between one fibre and the next longest in the series . This is illustrated schematically in fig. 3. For our tests, we have chosen a single value of A t (i .e . fibres with lengths which are all integer multiples of some fundamental length) but this is not essential. By changing the number of fibres of given lengths one could synthesize pulses of arbitrary shape.
2. The pulse stretcher In view of the difficulties just outlined we decided to attempt to avoid using blue light and somehow produce green pulses of a width similar to that of pulses caused by electromagnetic showers in the calorimeter. The solution that we arrived at is sketched in fig. 2. The main component is a bundle of optical fibres in which the fibres have different lengths. Light which is injected into the bundle will be split among the various
Fig. 2. Schematic illustration of the pulse stretcher concept. Optical fibres of different lengths are bundled together at both ends allowing a beam of light to be injected and ex tracted. A single pulse of light which is injected at one end emerges as a train of pulses at the other.
16
D. Hanna, J. Mitchell / Laser pulse stretcher
We have confined our initial investigations to two stretchers designed to produce trains of equal amplitude pulses . The first stretcher, called from here on the short stretcher, is a bundle of 80 fibres containing fibres with eight different lengths. There are 10 fibres of length 0.5 m, ten with length 1 .0 m and so on at increments of 0.5 m up to 4 .0 m. Thus the maximum length difference within the bundle is 3.5 m, corresponding to a time of about 15 ns . These fibres have been randomly grouped together at each end and glued into an endplate made of PVC plastic with a square hole cut in it . After the glue had cured the ends of the fibres were cut and polished . The use of uniform increments in fibre length allows a neat construction where all but the shortest fibres can have their extra length coiled into loops of identical diameters which can then be bundled using cable ties . We used 80 fibres since that is the number which have to be fed in the ZEUS system . (These primary fibres supply bundles of secondary fibres further "downstream"). The fibres have a 200 Wm core diameter and the outside diameter, which includes the cladding, is 230 Wm . This means that the 80 fibres can be packed into a square of less than 2.5 mm on a side . This is the cross section of the mixing bars used at the entry and exit of the system . The transmission efficiency can be calculated as the active area of the fibres divided by the area of the mixing bar. It works out to - 40% . The other stretcher was identical to the short one in all respects except that it had groups of five fibres at each length, at 0.5, 1.0, 1 .5 and so on up to 8.0 m. Thus the dispersion introduced is - 30 ns . This stretcher is called the long stretcher.
time
Fig. 3. Illustration of how the pulse of fig. l b would appear after travelling through a pulse stretcher made of four fibres with four regularly spaced lengths .
mixing . Upon exiting the same mixing bar (or the downstream one in the case of the stretched data), the beam entered the fanout which consists of a tight bundle of fibres at one end and an array of commercially available (SMA) connectors at the other end. Two fibres were connected to this array, one leading to the "test" phototube and the other to the "monitor" phototube. For each of the four variations described above, a series of runs were taken under different conditions . Each run consisted of 500 laser pulses . During the runs, the variable filter was run through its range just over two times thus providing a continuous scan of laser intensities. This was achieved by advancing the filter by 1 .8° with every pulse of the laser, using a stepper motor.
3. Tests of the system To see whether this idea would work as planned, we performed a series of bench tests. We compared the saturation behaviour of a phototube when illuminated by the following; - green light produced by illuminating a mixing bar made from green wavelength shifting plastic with light from a blue dye laser, - green light directly from a green dye laser which first passed through a clear mixing bar, - green laser light which passed through the short stretcher, - green laser light which passed through the long stretcher. The setup is shown in fig. 4. Light from the laser passed through a circular variable filter and short focal length (25 mm) converging lens before hitting the first mixing bar. The lens helps to maximize the number of reflections in the mixing bar and thereby improve the
Fig. 4. Test setup used to measure the effects of the pulse stretchers . Light from a dye laser system is attenuated by a variable filter wheel and injected into the stretcher via a mixing bar which ensures a relatively even distribution of light among all the fibres in the bundle. The light emerging from the different lengths of fibre is recombined using a second mixing bar at the exit of the stretcher and is then fanned out to phototubes . PMT 1 was used as a monitor while PMT 2 was the test phototube.
D. Hanna, J. Mitchell / Laserpulse stretcher
The high voltage supplying the monitor PMT was kept constant for all runs but the high voltage for the test PMT was increased by 100 V for each new run. Thus the two PMTS started out in a region where both were linear but the test PMT became nonlinear as its voltage was increased. The signal from the test PMT was attentuated before entering the ADC to keep it in range. The data from these runs can be expressed most effectively as a series of four plots, each corresponding to a different dye or stretcher variation . These are shown in fig. 5. In each plot there are several curves,
4000
17
the average pulse height in the test PMT vs the pulse height seen in the monitor PMT. Each curve corresponds to a different high voltage (and therefore gain) setting on the test PMT. The curves in fig. 5 have been adjusted to take into account unavoidable changes in the optics as the test system was changed from one configuration to another. For example, the sharing of light between the fibres leading to the test and monitor PMTs was not precisely the same for each configuration. But at low voltages where both PMTs are linear, P1 = aP2 where a is a slope which will change if the light sharing between the
4000 t
2000
2000
0
40~
F===_
0
80
p1 vs p2
4000
2000
2000
0
40
80 p1 vs p2
40
80
120
p1 vs p2
4000
0
0
120 pl vs p2
Fig. 5. Results from different test configurations : (a) No stretcher but with blue light from the dye laser exciting a mixing bar made of green wavelength shifter plastic placed at the position of the second mixing bar in fig. 4. The green light hitting the phototubes has a time structure given by the decay properties of the wavelength shifter . This configuration most closely simulates the pulses arising from showers in a calorimeter. (b) No stretcher and with green light from the dye laser injected directly into the second mixing bar shown in fig. 4. (c) Green light passing through the short stretcher. (d) Green light passing through the long stretcher. In all plots, the response of PMT 2 as a function of the response of PMT 1 is shown. The different curves in each plot correspond to different values of high voltage on PMT 2. (The same set of high voltage values is used for all plots.) It is clear that the unstretched green light results in much more serious saturation effects than the blue light. The green light passing through the short stretcher produces saturation effects very similar to those caused by the blue light while the long stretcher overcorrects .
18
D. Hanna, J. Mitchell / Laser pulse stretcher
test and monitor PMTS does . Thus we use aP2 rather than P2 as a monitor. This means that the lowest voltage point for each configuration will result in a line of unit slope in a plot of P1 vs aP2. As the voltage on the test PMT is raised, the slope of P1 vs P2 will increase since the gain is rising . If there were no saturation effects the curves would remain straight and would all behave identically, independent of the configuration . However the increased high voltage results in larger currents in the dynode chain and saturation becomes apparent, manifesting itself as a turnover of the curves at high light levels . As predicted in the discussion above, the unstretched green laser light suffers the most from this effect since most of the photons hitting the photocathode arrive within a nanosecond or less. The green light produced by the decay of WLS fluors arrives at the photocathode over a longer time period and produces less saturation . This is also true for light from the two stretcher setups . It appears that the short stretcher simulates the behaviour of the natural WLS decay rather well and the longer version over compensates . This is expected since in the limit of infinitely long, zero amplitude pulses, one would never experience saturation effects. 4. Conclusions From the preceding, one can conclude that it is relatively simple to "slow down" a laser pulse using a fibre optic bundle . This method has several advantages. It is inexpensive, easy to fabricate and very stable . It offers a high efficiency, limited primarily by
the core-to-cladding ratio of the fibres and packing density of the bundle . Finally, it is flexible . As stated earlier, an arbitrary pulse shape can be built up by appropriate choice of the fibre lengths. In addition, by integrating two groups of fibres, each with a different mean length one can simulate double pulses (or with several mean lengths, multiple pulses). This can be useful for studying multi-hit capabilities of certain detectors or electronics .
Acknowledgements We would like to thank Robert Nowac and David Binding of McGill University for help in fabricating the stretchers . This work is supported by the National Sciences and Engineering Research Council of Canada .
References [1] ZEUS, A detector for HERA, Letter of Intent, DESY (June 1985). [2] The ZEUS Detector, Technical Proposal, DESY (March 1986). [3] The ZEUS Detector, Status Report 1987, DESY (September 1987). [4] The ZEUS Detector, Status Report 1989, DESY (March 1989). [5] A. Andresen et al., Nucl . Instr. and Meth . A290 (1990) 95 [6] A. Andresen et al ., Nucl . Instr . and Meth. A309 (1991) 101 .