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ALL/RMB 3/8/04
Results on LED/Fibre tests #2.
Further measurements have been made of the signal amplitude that can be achieved by
shining a blue LED directly on to a harness of 200 fibres and illuminating a VPT with a single
fibre from the bundle.
To allow a measurement of the dependence of the output signal on the drive pulse parameters,
the avalanche pulse generator has been replaced with a standard pulse generator.
This unit provides pulses with a minimum width of 30 ns and maximum amplitude of 8 V into
25 . When the LED is driven under these conditions, the pulse height from the Stephenson
Amp is 200 mV. This is very similar to the output obtained when the LED is driven with the
APG.
The width of the light pulse from the LED has been measured with a pmt. A drive pulse
width of 30ns gives a pmt pulse width of 35 ns.
With the VPT, the pulse width out of the Stephenson Amp is 80 ns. (The figure of 100 ns
given in the last report is incorrect, and the estimate of the energy equivalence of the pulse
should be revised accordingly.)
Figure 1 shows the dependence of the VPT output on the amplitude of the LED drive pulse,
for a fixed pulse width of 30 ns. Above the threshold voltage, the light output increases
almost linearly with the amplitude of the drive pulse, up to the maximum of 8 V obtainable
from the PG, with 50 back-termination. Removing the back termination gives a 12.5 V
drive pulse and 270 mV from the Stephenson Amp. (The apparent flattening of the curve
between 8 V and 12.5 V may be spurious since the fibre was removed and re-inserted
between the measurements at 8 V and 12.5 V, thus even higher light output may be possible
with a bigger drive pulse.)
Using the expression from the previous report:
C = W/2.36 x H x (2) = 1.06 (W x H), with W = 80 ns and H = 5.4 mA,
gives Cout = 4.58x10-10 C.
Assuming g = 656 (to be checked),
Gives Cin = 7.0x10-13 C or 4.4x106 electrons.
This corresponds to 110 GeV.
A word of caution: in this test, the light is injected directly on to the photocathode. In CMS
the light will have to pass to and fro through the crystal and then fall within the sensitive area
of the photocathode (30% coverage). Thus the optical efficiency will be approximately 10
times less than in the test set-up and 110 GeV would equate to 10 GeV in CMS.
Figure 2 shows the dependence of the VPT output on the width of the LED drive pulse, for a
fixed amplitude of 8V. Over the range of pulse widths tested, the amplitude of the
Stephenson pulse increases linearly with driver pulse width. For drive pulses up to 40 ns
wide, the width of the Stephenson pulse is constant at 80 ns. For a drive pulse of 50 ns, the
Stephenson width is 82 ns, and for a drive width of 60 ns, it is 89 ns.
In the intended application in CMS, the useful upper limit on the LED pulse width is set by
the shaping time of the MGPA, which is 40 ns. Thus a drive pulse width of 30 ns, giving a
LED pulse width of 35 ns, is probably the maximum that can be used.
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ALL/RMB 3/8/04
Summary
It has been verified that the LED emission is sufficiently fast for it to be used as a test light
source in conjunction with an MGPA.
With a LED mounted directly on to an integrating sphere, pulses of approximately 10 GeV
energy equivalence would be expected in the CMS configuration.
A crucial question is the optical efficiency that can be achieved when the LED light is
injected into the integrating sphere through a fibre.
Next steps
- Verify the gain of this particular Stephenson Amp by implementing the charge injection
circuit.
- Try coupling the diode to a thick, unclad fibre, to establish the optical efficiency with
which light can be transported to an integrating sphere from a remotely mounted LED.
- Establish the saturation limit for light emission from the diode by using a larger drive
pulse.
VPT Output vs Drive Pulse
300.0
Output vs Amplitude
250.0
Output (mV)
200.0
150.0
100.0
50.0
0.0
0.0
Figure 1
2.0
4.0
6.0
8.0
Drive Pulse (V)
2
10.0
12.0
14.0
ALL/RMB 3/8/04
VPT Output vs Drive Pulse
400
Output vs Width
350
Output (mV)
300
250
200
150
100
50
0
0
Figure 2
10
20
40
30
Drive Pulse (ns)
3
50
60
70