PCS-150 / PCI-200 High Speed Boxcar Modules - Becker & Hickl ...

Becker & Hickl GmbH PCSAPP.DOC
Kolonnenstr. 29
10829 Berlin
Tel. 030 / 787 56 32
Fax. 030 / 787 57 34
email: info@becker-hickl.de
http://www.becker-hickl.de
PCS-150 / PCI-200 High Speed Boxcar Modules
Recovery of Signals from Noise
Resolution down to 150 ps
Recording of High Speed Optical Signals
Luminescence Decay Measurements
Energy Measurement of ps and fs Pulses
Nonlinear Optical Absorption Measurements
Introduction
The PCS-150 and the PCI-200 are PC plug-in modules for signal recording by the sampling
or boxcar technique. They are intended for the measurement of repetitive signals with high
bandwidth and time resolution.
To record the signal, a sequential sampling technique is used. By using a high speed linear
gate, one sample is taken from the signal in each signal period. To record the signal as a
function of time, the sample point is shifted over the signal in small time increments.
To measure the signal value at a selected point of the signal, the sampling point can also be
fixed. In this case the signal voltage at the selected moment as a function of an externally
varied parameter is recorded.
In all modes the signal-to-noise ratio (SNR) the of the result can be enhanced by repeatedly
sampling each signal point and averaging the samples.
The PCS-150 / PCI-200 modules contain two signal channels which are controlled by a
common gate pulse. This enables the module to record two input signals at the same time. The
gate width of the PCS-150 is

Method of Signal Recording
Sampling
The principle of the sampling method is shown in the figure below.
In each signal period one sample is taken
from the signal. From period to period the
sample location is shifted by a small amount,
to sample a somewhat later point of the signal.
After n signal periods n samples have been
taken to recover the waveform of the signal. If
the sample point is shifted in sufficiently
small steps the signal bandwidth is
determined by the gate width only. Because
the processing time of the particular samples
does not affect the time resolution, the
sampling method provides a very high
bandwidth and an extremely high virtual
sample rate at a moderate hardware effort. In fact, the virtual sample rate and the signal
bandwidth can be 10 to 100 times higher than with digital oscilloscopes of comparable price.
To improve the signal to noise ratio the sampling method can be combined with signal
averaging. For that purpose, the procedure described above is repeated several times and the
obtained curves are averaged. Averaging of n curves improves the signal to noise ratio by a
factor of √n.
Boxcar
The boxcar method uses the same principle as the sampling method. It differs only in the
strategy of signal averaging. While the sampling method immediately proceeds to the next
signal point and averages the complete curves, the boxcar method averages the samples of
one signal point first and than proceeds to the
next point. The method is shown in the next
figure.
At each sample point several (in the figure
100) samples are taken and averaged with the
same delay setting. When the averaging for
this signal point is complete the delay is
increased. The SNR improvement is n1/2. The
practical difference compared to the sampling
method is the different effect of a possible
amplitude or time drift of the signal.
Boxcar with Fixed Delay
Signal
period:
1
2
3
4
5
6
7
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In some applications only one particular point of the signal rather than the complete
waveform is recorded. The signal value at this point is recorded as a function of the time or
any other externally variable parameter. For such applications the PCS-150 and PCI-200
modules can be operated in the 'Fixed Delay' mode. The principle is shown in the figure
below.
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10
Sample
Sampling Method
Signal
period:
1..100
101..200
201..300
301..400
401..500
501..600
601..700
701..800
801..900
901..1000
Sample
Signal
Signal
Boxcar method: 100 samples / step averaged
Result
Result
after 100 signal periods
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In the example shown the signal has a different
shape in the signal periods 3 to 8. This results
in a different sample value at the fixed sample
point. In the result the sample value as a
function of the signal period number is
displayed. The method can be combined with a
signal averaging technique in the same way as
the boxcar method.
Applications
Basic Measurement Setup
The PCS-150 and PCI-200 modules can be used in the same way as a normal sampling
oscilloscope. Thus, the modules need a trigger signal as a reference for the temporal location
of the signal. The trigger signal must appear at least 20 to 40 ns preceding the signal in order
to compensate the internal delay of the trigger circuit and the delay unit (see figure below).
If the signal has a repetition rate of more
than 50 MHz this delay is no problem. If
Signal
the trigger is too late, simply the next
Recorded Interval
Trigger
signal period is displayed. For signals
with low repetition rates, however,
trigger and signal must be in the correct
Delayed Signal
time relation. This can be achieved by
two different ways. The simplest way is
to delay the measured signals through
Internal Delay
Recorded Interval
delay lines by 20...40 ns. 4...8 m of high
quality 50 Ohm cable is required for that
purpose.
The second way is a delayed triggering of the experiment. This can be achieved by an
external delay generator or simply by delay cables. The signals from the experiment are fed
directly to the PCS-150 or PCI-200 and the external trigger input is connected to the output of
the generator that triggers the experiment. The second method avoids signal distortions by the
delay lines in the signal path.
Experiment
Delay
Trigger
Signal
Signal
period:
1
2
3
4
5
6
7
8
Signal
Delay
Sample
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Boxcar measurement with fixed delay
Signal
PCS Trigger
Experiment
PCS
Source
Delaying the Signal Delaying the Experiment Trigger
Trigger
Result
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Measurement of noisy signals
The next figure shows an example for the measurement of a noisy signal. The investigated
device (experiment) is excited by a pulse generator. The experiment delivers a noisy output
signal on each input pulse. Due to the noise the signal from the experiment cannot be used for
triggering the PCS-150 / PCI-200. Therefore, the module is triggered externally by the same
pulse generator that triggers the experiment. To compensate the internal delay of the PCS-150
or PCI-200, the experiment trigger is delayed in relation to the PCS-150 / PCI-200 trigger.
Pulse Generator
Experiment
Signal A
Trigger
Measurement of a noisy signal
PCS-150
The figure above shows an example of the noise suppression which is achieved by signal
averaging. By averaging 4096 samples per delay step the signal-to-noise ratio improved by a
factor of 64.
Measurements with PMTs
Photomultiplier tubes (PMTs) are used to record low level
light signals with a resolution down to 1ns (FWHM). The
typical gain of a PMT is in the order of 10 5 to 10 7 . With this
gain one single photon yields an output pulse from 16 uA to
1.6 mA or 0.8 mV to 80 mV at a load resistor of 50 Ω.
Thus, only a few photons can be detected within the linear
range of the PMT and the signal is very noisy. Often the
PMT output signal consists only of random current pulses
due to the detection of the individual photons of the light
signal (Figure right).
Any attempt to improve the signal by additional amplifiers
or by increasing the gain of the PMT in this situation results
in increased noise or decreased dynamic range. There is
only one remedy: To detect more photons either by
decreasing the PMT gain while increasing the light
intensity or by averaging many periods of the signal. In the
figure right a typical example is shown. A fast PMT was
illuminated with a light pulse of 0.3 ns FWHM from a laser
diode. The intensity was reduced to keep the PMT signal
within the linear range of the output current. The upper
recording shows the noise due to the sampling of the
random single photon pulses of the PMT. By averaging
4096 signal periods the number of photons is increased
accordingly, and the shape of the PMT response is shown
clearly.
Light Pulse
PMT Output Signal
PMT output signal for a low level light signal
PMT signal recorded without and with 4096
accumulations per point
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Fluorescence Lifetime Measurements
In the figure below a simple setup for the measurement of fluorescence decay functions is
shown.
N2 Laser
PC with PCS-150
Trg
P1 P2 C
M
F1 F2 F3
OCF-400 PDM-400 PDM-400
APM-400 APM-400
A
Del1 Del2
Flourescence Decay Measurement with the PCS-150
The nitrogen laser generates light pulses with less than 1ns duration and a repetition rate of
10...100Hz. The light pulses excite the sample cell C. The fluorescence light from the sample
cell is detected by a photodiode module PDM-400 or APM-400. The signal is fed to channel
B of the PCS-150. P1 and P2 are glass plates which reflect a part of the laser radiation to the a
second photodiode module and to the 'Optical Constant Fraction Trigger' OCF-400. The
signal from OCF-400 is used as a trigger for the PCS-150. The signal from the second PDM-
400 (or APM-400) is used to record the shape of the excitation pulse. The filters F1..F3 are
provided to adjust the signal amplitudes, and the monochromator M selects an appropriate
wavelength of the fluorescence light.
The apparatus shown records fluorescence decay functions, time resolved fluorescence
spectra or multiple decay curves at different wavelengths i.e. the complete wavelength-time
behaviour of the fluorescence. For recording decay functions the sampling or boxcar mode is
used. To suppress noise and amplitude fluctuations of the laser pulses 'Samples averaged' is
chosen as high as possible with regard to the measuring time. The measurement delivers the
decay function at the selected wavelength and the shape of the exciting laser pulse. A typical
result is shown in the figure below.
B
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Recording of time resolved spectra is achieved in the 'Fixed Delay' mode of the PCS-150. The
sample point is set to the desired point of the decay function. Instead of the sample point the
wavelength of the monochromator is scanned during the measurement, thus recording of the
fluorescence intensity at the selected time as a function of the wavelength.
The operation mode 'Block Increment' can be used to obtain the entire wavelength-time
dependence of the fluorescence. In this mode the PCS / PCI performs subsequent
measurements of the input waveform using the sampling or boxcar method. The curves are
stored in different memory blocks. By scanning the wavelength, full information about the
fluorescence behaviour of the sample is obtained.
Transient Absorption Measurements
In the figure below a simple arrangement for transient absorption measurements is shown.
Pulsed Laser
M1
P1
Dye Laser
Pump
Beam
Optical
Delay
PCI-200
M2
P2
Sample
P3
Probe
Beam
Filter
Channel A
Channel B
Trigger
Transient Absorption Measurement
D1
PDM-400
D2
PDI-400
D3
PDI-400
The output of a high power pulsed laser (i.e. N2 laser, excimer laser or frequency multiplied
diode laser pumped YAG) is divided into two parts. One part is used to pump the sample, the
other part pumps a dye laser which generates a light pulse of the appropriate wavelength to
probe the absorption of the excited molecules in the sample. The detector D1 is a fast PDM-
400 photodiode module which generates a trigger pulse for the PCI-200 Boxcar Module. The
absorption in the sample is measured by the detectors D2 and D3. D1 and D2 are PDI-400
integrating photodiode modules and deliver energy proportional output pulses of some 100ns
duration. The amplitudes of these pulses are recorded by the two signal channels of the
PCI-200 Boxcar module. The PCI-200 is run in the ‘Fixed Delay’ mode. Thus, it records a
curve consisting of subsequent averages over a selectable number of D2 and D3 intensity
values. If the optical delay is continuously changed during the measurement and the quotient
A/B is displayed the result shows the decay of the abasorption of the excited state species in
the sample.
Prism
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Nonlinear Optical Absorption Measurements
Another example for the fixed delay mode and the wide gate width of the PCI-200 is given in
the next figure. The shown setup is used for the measurement of the intensity-dependence of
the light absorption in organic dyes.
A high power pulsed laser (i.e. nitrogen laser or pulsed dye laser) generates short pulses
(< 1ns) with high energy (1mJ). The intensity is controlled by a suitable optical attenuator.
The beam is split into two parts by the glass plate P2. The main part of the light is focused
into the sample cell C1. The other part is fed through the reference cell C2. Both light signals
are fed through a filter to the Detectors D1 and D2. D1 and D2 are PDI-400 integrating
photodiode modules and deliver energy proportional output pulses of some 100ns duration.
These pulses are recorded by the two signal channels of the PCI-200. The trigger pulse for the
PCI-200 is generated by the photodiode PD3. Due to the long duration of the signal pulses,
delay lines in the signal path are not required. The gate width and the delay of the PCI-200
are set to sample a signal portion near the peak of the input pulses.
The main problem in non-linear optical absorption measurements is, that an absorption
accuracy of better than one percent over several orders of magnitude of the intensity is
required. To reach the required absorption accuracy, the shown setup uses a second signal
path trough a reference cell and the detector module D3. By using a common replaceable
filter for both channels the signal intensity can be held inside the useful input voltage range of
the PCI-200 without degrading the accuracy of the measured absorption values.
Pulsed Laser
P1
optical
Attenuator
P2
D3
PDM-400
Trigger
PCI-200
Measurement of non-linear absorption
M
Channel A
C1
C2
Channel B
Filter
D1
D2
PDI-400
PDI-400
The measurement delivers pairs of signal values from which the intensity and the ratio of
small signal and large signal absorption can be derived. By referring the A value (large signal
absorption) to the B value (intensity and small signal absorption) the influence of the laser
instability and the error of the optical attenuator do not appear in the measured absorption
values. The apparatus is able to measure absorption variations as small as 1 %.
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Accessories
DCA Series Preamplifiers
These DC coupled preamplifiers have an excellent input offset
stability and a bandwidth of up to 400 MHz. Inverting and
noninverting versions with gains up to 10 (20 dB) are available.
ACA Series Preamlifiers
The ACA preamplifiers are AC coupled and have a bandwidth up to
2 GHz and a gain up to 70 (37 dB).
PDM-400 Photodiode Modules
The PDM-400 is a PIN photodiode module with 400 ps FWHM and
a spectral range from 330 to 1000 nm. The modules do not require a
special power supply, the operating voltage is taken from a connector
at the PCS / PCI module.
APM-400 Avalanche Photodiode Modules
The APM-400 is an avalanche photodiode module with an internal
gain up to 100. Different versions with detector areas from 0.03 to
7 mm 2 are available. Depending on the detector area, the speed is
from 0.32 to 3 ns FWHM.
PDI-400 Integrating Photodiode Modules
The PDI-400 is an integrating detector for pulsed light
signals in the fJ range. The PDI-400 includes a high
performance photodiode, a low noise charge sensitive
amplifier and an active high pass filter. Due to filtering, most
of the amplifier noise and low frequency background signals
are rejected and the PDI-400 is insensitive to roomlight. Its
high sensitivity, low noise and wide dynamic range makes it
extremely useful in all applications where accurate and
reproducable measurements of light pulse energies are
essential.
OCF-400 Optical Constant Fraction Trigger
The OCF-400 is used to derive electrical trigger pulses from optical
pulses with variable amplitude. Due to the constant fraction trigger
principle the trigger point is widely independent of the pulse
amplitude. Compared to a simple photodiode, the OCF-400 offers
negligible influence of the light pulse energy on the trigger delay. It
is used for measurements with Nitrogen Lasers or Dye Lasers with
unstable pulse energy.
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