Specifications

Typical Experiment
70
increase the pulse height to 100 mV, one amplifier
in the SR445A preamp provides a gain of 5 with a
350 MHz bandwidth. The discriminator threshold
is set to 20 mV. When viewed with a 300 MHz
oscilloscope, it is apparent that this threshold
setting will count the majority of output pulses,
but will not count anode rings or amplifier noise.
The green LED is flashed at a 60 Hz rate for about
500 µs. During this time, population integrates in
the upper level of the laser transition. Spontaneous
decays from the upper level are counted by the
photon counter during a 100 µs gate. The intensity
of the LED is adjusted for a count rate of about 10
MHz at the peak of the decay curve. This
corresponds to counting about 1000 photons in
each gate, or 60,000 photons per realtime second.
The gate is scanned over 10 ms in 100 µs steps in
order to measure the lifetime of the state.
INSTRUMENT CONFIGURATION
The photon counter's gate generator is triggered
by the same pulse which flashes the LED. The
instrument is operated in the 'A FOR PRESET T'
mode. The A-counter counts the photons which
occur during the 100 µs gate. The T-counter is
preset to count 10 triggers, so data points will be
sent to the computer six times per second at the 60
Hz trigger rate. After each data point is taken, the
photon counter advances the A-gate by 100 µs,
resets the A-counter, and starts the next count
interval.
Data may also be taken without a computer, using
an X-Y chart recorder to record the data. The chart
recorder is interfaced to the SR400 via two D/A
outputs. The front panel D/A output has been set
to output a voltage proportional to the value in the
A-counter. ( The D/A may also be set to output a
value proportional to the Log of Counter A.)
PORT1, a rear panel BNC output, has been set to
take 0.1 V steps after each data point to provide a
linear ramp to drive the X-axis of the chart
recorder. Since the gate delay is also stepped after
each data point, the chart recorder will plot the
count rate vs. gate delay, showing the exponential
decay of population from the excited state.
COMPUTER DATA ACQUISITION AND
REDUCTION
The SR465, a data acquisition and control program
for the SR400 photon counter, greatly expands the
capabilities of the instrument. Intended for IBM
PC's and compatible computers, the program sets
INSTRUMENT CONFIGURATION
MODE:
A GATE:
LEVELS:
A FOR PRESET T
A = INPUT 1
T = TRIG
T SET = 1E1
N PERIODS = 100
AT N=STOP DWELL=2E-3
D/A = A
D/A RANGE = #####432#
DISPLAY = HOLD
A GATE=SCAN ∆=100.0 µs
A DELAY = 0.000 µs
A WIDTH = 100.0 µs
TRIG SLOPE = RISE
TRIG LVL = +1.000 V
A DISC SLOPE = FALL
A DISC LVL = -20.0 mV
PORT1 = SCAN ∆=+0.10
PORT1 LVL = +0.00 V
Counter A for preset interval
Counter A to count input #1
Counter T to count triggers
Counter T preset to 10 counts
Data points in scan = 100
Single scan, 2 ms dwell
D/A output shows Counter A
D/A full scale is 9,999 counts
Display shows final count value
Scan A’s gate in 100 ms steps
First point’s gate delay is 0
Gate width set to 100 ms
Trigger on rising edge of LED
Trigger threshold set to +1.00
A’s disc on falling edge
A’s disc set to -20 mV
Rear panel BNC takes 0.1 V steps
Rear panel BNC starts at 0.0 V