PCS-150 / PCI-200 High Speed Boxcar Modules

Becker & Hickl GmbH Kolonnenstr. 29 10829 Berlin Tel. 030 / 787 56 32 Fax. 030 / 787 57 34 email: info@becker-hickl.de http://www.becker-hickl.de PCSAPP.DOC PCS-150 / PCI-200 High Speed Boxcar Modules Recovery of s from Noise Resolution down to 150 ps Recording of High Speed Optical s 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 <150 ps (typically 120 ps), the gate width of the PCI-200 can be selected from 2 ns to 50 ns. To record high speed optical signals, a wide variety of detectors is available. Depending on the required speed and sensitivity, pin and avalanche photodiode modules, PMT modules and energy proportional pin photodiode modules with fj sensitivity can be connected directly to the PCS-150 and the PCI-200. The trigger pulse can be either derived from the signal of the channel A, or an external trigger pulse can be used. For accurate triggering on optical pulses, an optical 'constant fraction' trigger module is available. All functions are controlled by the software delivered with the module. The software controls the measurement procedure, provides the selection of operation modes and measurement parameters and performs the display, evaluation and storage of the results. 1

Method of 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. period: 1 2 3 4 5 6 7 8 9 10 Sample Sampling Method period: 1..100 101..200 201..300 301..400 401..500 501..600 601..700 701..800 801..900 901..1000 Sample Boxcar method: 100 samples / step averaged Result Result after 100 signal periods Boxcar with Fixed Delay 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. 2

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. period: 1 2 3 4 5 6 7 8 Sample 9 10 Boxcar measurement with fixed delay Result 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 the trigger is too late, simply the next Recorded Interval signal period is displayed. For signals with low repetition rates, however, trigger and signal must be in the correct Delayed 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. Delay Delay Experiment PCS Source Experiment PCS Delaying the Delaying the Experiment 3

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 A PCS-150 Measurement of a noisy signal 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 PMT output signal for a low level light signal PMT signal recorded without and with 4096 accumulations per point 4

Fluorescence Lifetime Measurements In the figure below a simple setup for the measurement of fluorescence decay functions is shown. N2 Laser P1 P2 C M F1 F2 F3 OCF-400 PDM-400 PDM-400 APM-400 APM-400 Del1 Del2 Trg A B PC with PCS-150 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 ' 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. 5

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. Prism Pulsed Laser P1 Dye Laser Optical Delay M2 Probe Beam Filter D1 PDM-400 M1 Pump Beam P2 Sample P3 D2 PDI-400 D3 PDI-400 PCI-200 Channel A Channel B Transient Absorption Measurement 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. 6

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 C1 C2 Filter D1 D2 PDI-400 PDI-400 D3 PDM-400 M PCI-200 Measurement of non-linear absorption Channel B Channel A 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 %. 7

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 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. 8