Ultraviolet and Blue Picosecond Diode Laser Modules

Transcription

1 Becker & Hickl GmbH August 2004 Printer HP 4500 PS High Performance Photon Counting Tel. +49 / 30 / FAX +49 / 30 / info@becker-hickl.com BDL-375 BDL-405 BDL-475 Ultraviolet and Blue Picosecond Diode Laser Modules General The BDL-375, BDL-405 and BDL-475 are picosecond laser diode modules with an emission wavelengths in the range of nm, nm, and nm [1,2]. Pulse repetition rates of 20 MHz, 50 MHz, and 80 MHz can be selected. The maximum cw output power at 50 MHz is typically 0.2 to 0.8 mw for the BDL-375 and -475, and mw for the BDL-405. The maximum peak power of the pulses is approximately 125 mw for the BDL-375 and -475, and up to 500 mw for the BDL-405. The BDL laser modules have a TTL controlled shutdown input that can be used to switch the laser off and on within a time of 1 us. The shutdown input can be used to multiplex several lasers of different wavelength or to minimise sample exposure by activating the laser only during the measurement time interval [5,6]. The BDL-405 and BDL-375 lasers are operated from a simple +12 V power supply. Emission indicator LEDs and a key switch are in a box inserted in the power supply cable. The driving generator is incorporated in the laser module. The BDL laser modules are targeted at spectroscopy applications in combination with timecorrelated single photon counting (TCSPC) [3-8]. Their high repetition rate, multiplexing capability and exceptionally low RF radiation make the BDL laser modules an ideal choice for a wide variety of fluorescence lifetime, single molecule detection, lifetime microscopy, and fluorescence correlation applications. 1

2 Picosecond Operation of Laser Diodes Picosecond pulsing of laser diodes requires to drive extremely short current pulses trough the pn junction of the diode. Unfortunately commercial laser diodes are not designed for this kind of operation. Particularly, the junction capacitance C j and the lead inductance L l form an LC low pass filter that impedes a fast voltage rise across the diode junction. The situation is shown in the figure below. For low driving power the generator pulse initiates a damped sine-wave voltage across the diode junction. When the first positive peak reaches the forward conducting voltage of the diode, current starts to flow through the junction. As long as the laser threshold is not reached the light pulse is weak and broader than the current pulse. If the driving power is increased the first positive peak drives a substantial forward current through the diode junction. The dynamic impedance of the junction drops dramatically, preventing the voltage at the junction to increase much above the forward voltage. The current through the junction exceeds the laser threshold for a short fraction of the sine wave period, and a short light pulse is emitted. If the driving power is increased further the forward current pulse and consequently the light pulse becomes stronger. Because the dynamic resistance of the pn junction decreases the pulse width decreases. Eventually, the subsequent peaks of the sine wave start to drive a forward current through the diode junction resulting in a tail in the light pulse or afterpulses. Generator Rg Ll Laser Diode Vg Cj Ij Vj Vg Generator Voltage Vj Voltage accross Laser Diode Junction Ij Current through Laser Diode Junction Light Emission Junction voltage Vj and junction current Ij in a picosecond laser diode for different driving pulse amplitude Vg 2

3 The behaviour of the junction current explains why there is a relation between the pulse quality and the pulse power. Good pulse shapes can be obtained only at relatively low power. Taking a stronger diode does not help. It actually makes the situation worse because the junction capacitance is higher. Basically some improvement can be achieved by DC-biasing the laser diode in reverse direction and using a correspondingly higher driving pulse amplitude. However, laser diodes have a very low permissible reverse voltage which easily can be exceeded in the first negative half-period of the junction voltage. For blue laser diodes, which have a forward conducting voltage of 4 to 5 V, the driving amplitude is relatively high. This results in a correspondingly high reverse voltage in the negative half-period. To achieve safe operation at high pulse power the diodes are operated at zero or even positive (forward) bias. The influence of the diode bias is shown in the figure below. Generator Rg Ll Laser Diode Vg Cj Ij Vj Generator Voltage Vg V bias Vj Voltage accross Laser Diode Junction Bias 3 Bias 2 Bias 1 Ij Current through Laser Diode Junction Light Emission Junction voltage Vj and junction current Ij in a picosecond laser diode for different diode bias voltage It should be noted that the operating conditions of picosecond pulsed laser diodes are different from those of modulated laser diodes used in communication equipment. A modulated laser diode is always forward biased, and there is a continuous forward current through the laser diode. Consequently, the diode junction has a low dynamic impedance that shorts the junction capacitance. The speed of the diode is then determined mainly by the lead inductance and the generator impedance. Average Power and Peak Power The typical pulse width for a picosecond laser diode is of the order of 100 ps or less. For a repetition rate in the 20 to 80 MHz range the duty factor is of the order of 300. As shown in the figure below, the result is an relatively high peak power even for low average (cw) power. 3

4 Pp peak power Tpw Pp = Pa Tper Tpw Pa average power Tper Relation between peak power, average power, pulse width and pulse period The peak power for all ps diode lasers is far beyond the permissible steady state power for the used laser diodes. Due to the short pulse width this is tolerable to a certain extend. However, damage effects in laser diodes are extremely fast and highly nonlinear. For the BDL-405 and BDL-375 we recommend not to exceed an average power of 0.3 mw, 1 mw and 1.0 mw for 20 MHz, 50 MHz and 80 MHz respectively for a longer period of operation. Pulse Shape Pulse shapes for a BDL-405 laser for different average power at 50 MHz are shown in the figure below. 50 MHz, 0.4 mw FWHM = 127 ps 50 MHz, 0.8 mw FWHM = 84 ps 50 MHz, 1.2 mw FWHM = 62 ps 50 MHz, 1.6 mw FWHM = 52 ps Pulse shapes for a BDL-405 laser at 50 MHz. Recorded with Hamamatsu R3809U-50 MCP [8] and BH SPC-730 TCSPC module [5]. The curves were recorded with a Hamamatsu R3809U-52 MCP PMT and a bh SPC-730 TCSPC module. The R3809U-52 was operated at 3 kv yielding an instrument response function (IRF) of 30 ps fwhm. The pulse width continuously decreases with the output power. The best pulse shape for 80 MHz, 50 MHz, and 20 MHz is obtained at 1.6 mw, 1 mw, and 0.4 mw, respectively. Typical curves of the peak power and the pulse width are shown in the figure below. 4

5 600 mw 500 Peak Power 400 Pulse Width Peak Power Pulse Width 100 ps 300 fwhm mw average (CW equivalent) power Pulse width and peak power for a BDL-405 versus average power at 50 MHz repetition rate. Pulse width corrected for 30ps IRF width of detection system. The figure below shows the pulse shape for a BDL-475 laser for different average power at 50 MHz. 50 MHz, 100 µw FWHM = 100 ps 50 MHz, 250 µw FWHM = 50 ps 50 MHz, 400 µw FWHM = 46 ps 50 MHz, 600 µw FWHM = 45 ps Pulse shapes for a BDL-475 laser at 50 MHz. Recorded with Hamamatsu R3809U-50 MCP and BH SPC-730 TCSPC module. Typical curves of the peak power and the pulse width are shown in the figure below. 200 mw Peak Power 100 Pulse Width Peak Power 100 ps Pulse Width fwhm uw average (CW equivalent) power Pulse width and peak power for a BDL-475 versus average power at 50 MHz repetition rate. Pulse width corrected for 30ps IRF width of detection system. 0 5

6 Trigger Skew The trigger pulse is derived directly from the output of the laser diode driver. It therefore appears almost simultaneously with the light pulse. For measurements with the bh SPC modules, please use a trigger cable that is 1 m to 3 m longer that the detector cable. This compensates for the delay in the detector and places the stop pulse behind the end of the recorded time interval [5]. The trigger delay does not change appreciably for different output power and for different repetition rate. The shift in the light pulse with the power referred to the trigger pulse is shown in the figure below. The repetition rate is 50 MHz, the power varies from 0.3 mw to 1.3 mw. Shift of the light pulse with the output power referred to the trigger pulse. Left to right: Power 0.3 mw, 0.5 mw, 0.8 mw and 1.3 mw On-Off Behaviour The BDL lasers can be switched on and off by applying TTL/CMOS signal to pin 7 of the sub-d connector. TTL Low or connecting the pin to GND switches the laser off. When TTL Low is applied the optical output and the trigger output shut down almost instantaneously. With TTL High the laser resumes normal operation within 1 us. The switching behaviour is shown in the figure below. 40 µs TTL high pulses were applied to the /laser on input, and the sequence was accumulated for 10 5 pulses in the Scan Sync Out mode of an SPC-730 TCSPC module. The photons were detected by an R3809U-52 MCP-PMT. Each curve of the sequence represents an interval of 1 µs. On-off behaviour of the BDL lasers. 40 µs TTL high pulses were applied to the /laser on input. Each curve of the sequence represents an interval of 1 µs. SPC-730 TCSPC module with R3809U MCP-PMT. 6

7 Operating the BDL-375, BDL-405 and BDL-475 laser modules Adjusting the Collimator The collimator can be adjusted laterally by loosening the four screws shown in the figure below. Collimator screws turn loose to shift collimator laterally Turn collimator to adjust focus Focus adjustment is done by turning the collimator lens assembly in its mounting thread. Note: Please do not loosen the plastic screw left and right of the collimator. The screws hold the peltier cooling assembly inside the laser. Loosening them can cause thermal damage to the laser diode. The standard focal length of the collimator lens is 8 mm. This gives a relatively large beam diameter, as can be seen from the figure above (parallel beam 1 m from laser). The long focal length results in a low divergence of the parallel beam. By changing the adjustment of the collimator lens of the laser, the beam can be directly focused into a spot as close as 10 cm from the laser, or a divergent beam can be obtained. The elliptical beam profile of the laser diode itself results in noncircular cross section of the beam. Nevertheless, the beam can be focused into a near-diffraction limited spot. If a more circular beam cross section is needed, e.g. for efficient illumination of a microscope lens, a anamorphotic lens or prism pair should be used to correct the shape. Typical applications of different beam configurations are shown below. A parallel beam is used for focusing on distant objects, and for focusing by a microscope objective lens designed for infinite conjugate foci, i.e. for microscopes with a tube lens. A convergent beam can be used to focus directly into a small spot or to compress the beam diameter by a single concave lens. A divergent beam can be useful for beam expansion, and for focusing by a microscope objective lens designed for use at conjugate foci, i.e. for microscopes without a tube lens. Laser Laser Lens Laser Lens Laser Focusing on distant objects Microscope lens Beam compression by a single lens Laser Cuvette Beam expansion by a single lens Laser Microscope lens Focusing through a microscope lens (corrected for infinite conjugate ratio) Focusing into short distance Focusing through a microscope lens (corrected for finite conjugate ratio) Beam patterns for different collimator adjustment and typical applications The range of beam parameters that can typically be obtained is shown in the table below. 7

8 Collimator Adjustment Focus distance Divergence Beam cross-section Increase of beam mradian at collimator output diameter over 1m Convergent beam min. 10 cm max. 45 x x 1.8 mm Parallel beam - < x 1.8 mm < 0.1 mm Divergent beam min. -10 cm max. -45 x x 1.8 mm Laser safety regulations demand for a special adjustment tool and a beam stop for class 3B lasers. Please do not make any collimator adjustments on operating class 3B lasers with tools other than specified. Note: Please do not loosen the plastic screws left and right of the collimator. The screws hold the heat sink of the peltier cooling assembly inside the laser. Loosening them can cause thermal damage to the laser diode and the peltier cooler. Caution: Light emitted by the device may be harmful to the human eye. Please pay attention to safety rules when operating the devices. Avoid exposure to the laser beam, and not look into the collimated beam. For wavelengths exceeding 650 nm the light intensity may be much higher than it appears to the eye. In particular, laser safety forbids adjusting the collimator of class 3B lasers when the lasers are switched on unless a special protecting tool is used. Please see section Laser Safety. Input and Output Signals The rear side of the BDL laser is shown in the figure below. Power & Control Trigger Out Laser On Cooling High Cooling Active Power Bias Power and Control Inputs 1 not connected 9 Laser diode bias test output 2 Frequency 20 MHz V Power supply input 3 Frequency 50 MHz 11 not connected 4 Frequency 80 MHz 12 External bias input 5 GND 13 Temperature test output 6 not connected 14 not connected 7 /Laser Off 15 GND 8 Pulser supply test output 8

9 Pin 2,3,4, Frequency Frequency select pins. The laser works with the specified frequency if the corresponding pin is at TTL/CMOS High or open. Tie the pins of the unused frequencies into GND or use the Frequency switch in the laser switch box inserted in the power supply cable. Pin 5, Ground Reference pin for all signals and power supply - pin. Pin 7, /Laser Off Connecting this pin to TTL/CMOS Low or GND switches the laser off. The laser beam is shut down and the trigger output becomes inactive. After disconnecting the pin from GND or switching to TTL/CMOS high the laser resumes normal operation within 1 us. Leave the pin open if you want the laser to run continuously. The /Laser Off signal is used to switch off the laser during the beam flyback in laser scanning microscopes. Please notice that the laser does not deliver trigger pulses when it is switched off by /Laser Off = low. For a connected bh TCSPC modules this is no problem. However, if the /Laser Off signal is pulsed at a high rate the SPC module will display a SYNC rate lower than the actual value. Pin 8, Pulser supply test output Pin 8 is a test output. It delivers the internal power supply voltage of the driving generator. Depending on the power, the voltage at pin 8 is between +5V and +6.9V. Do not connect this pin. Pin 9, Laser diode bias test output Pin 9 is a test output. It delivers the diode bias voltage. Please note that the diode is forward biased, i.e. the power increases with increasing bias voltage. Do not connect this pin. Pin 10, Power supply input Pin 10 is the power supply input. The nominal power supply voltage is +12 V. The laser works in a range of +9V to +15 V. The power supply current may vary between about 300 ma and 1.5 A. Most of the input power is used for temperature stabilisation of the laser diode. Therefore the power supply current varies with the temperature and with the time after switch-on. Important note: For reasons of laser safety the BDL laser modules must be operated with the power supply cable delivered with the modules and the box containing the switch box and the emission indicators. Pin 12, External bias input Pin 12 is an external diode bias input. An input voltage in the range of -10 V to +10 V changes the bias of the laser diode. Positive inputs increase the diode bias and increase the output power. The external bias voltage is added to the bias voltage set by the diode bias potentiometer. Pin 13, Temperature test output This pin delivers an output from the temperature regulation amplifier. The voltage is typically 1 V to 1.5 V after switch on and drops to 100 mv to 200 mv after the laser diode temperature has stabilised. Trigger Output The trigger pulse is derived directly from the output of the laser diode driver. It therefore appears almost simultaneously with the light pulse. For measurements with the bh SPC modules, please use a trigger cable that is 1 m to 3 m longer that the detector cable. This compensates for the delay in the detector and places the stop pulse behind the end of the recorded time interval. 9

10 The trigger pulse is positive and has a pulse width of about 1 ns. The amplitude depends on the power and is about 100 to 200 mv. If you use the BDL lasers with a bh SPC module, please use the A-PPI adapter to invert the trigger pulse. Power and Diode Bias Adjust There are two potentiometers at the back of the laser module. The Power adjust changes the operating voltage of the driving generator. The Bias adjust changes the bias voltage of the laser diode. Higher voltage and higher bias give higher output power. The best pulse shape is obtained with minimum bias and moderate power. We recommend to turn the Bias adjust screw until the power is at minimum and then to adjust the desired power and pulse shape with the Power screw. Please see also section Picosecond Operation of Laser Diodes. LED Indicators on the Laser The left LED flashes when the power of the laser is on and the /Laser Off signal is high or unconnected. The right LED is on when the cooling of the laser diode is active. It may turn off after some time of operation when the diode has been cooled down and almost no cooling power is required to hold it at constant temperature. The red LED in the middle turns on when the cooling power is at maximum. It should turn off after some minutes of operation. Power Supply Cable for the Class 3R and Class 3B Lasers The BDL laser modules are operated from a simple +12V wall-mounted AC-DC-adapter power supply. For reasons of laser safety the Laser Switch Box shown below is inserted in the power supply cable from the AC-DC adapter to the laser. The switch box contains a key switch, several parallel power on or emission indicator LEDs of different colour, and one LED that indicates that the laser is switched off via the /laser off signal. The /laser off signal and an analog power control signal can be connected to SMA connectors at the back of the switch box. The 15 pin connector at the laser side can be used as a remote interlock connector. The connector can be pulled off or plugged in at any time without causing damage to the laser. Remote Interlock Connector SMA connectors: Repetition Rate Switch Shutdown 1) Analog Power Control +12V in from wall-mounted AC-DC power adapter 15 Pin Sub D Connector connected to laser module 9 Pin Sub-D Connector Key Switch 1) Analog power control changes the power only within the range specified for the laser Laser switch box with key switch, emission indicator LEDs and control inputs 10

11 Laser Safety The BDL-375 is a class 3B, the BDL-405 and BDL-475 are 3R laser products. The laser class is indicated on the laser by an explanatory label, see figure below. BDL-375 (UV) BDL-405 and BDL-475 (visible) The laser aperture is marked with the aperturere label and the hazard warning label shown below. Moreover, each laser has a manufacturer label. The label specifies the type, the wavelength and repetition rate, and the certification of the laser. An example for the BDL-405 laser is shown below. The position of the labels on the laser modules is shown in the figures below. Manufacturer label (white, left) and explanatory label (yellow, right) Aperture label (on top of the laser) and laser hazard labels (left and right of the aperture) Caution: Laser safety regulations forbid the user to open the housing of the laser, or to do any maintenance or service operations at or inside the laser. Use of controls or adjustments or performance of procedures other than specified herein may result in hazardous radiation exposure or damage to the laser module. If the collimator of a class 3B laser has to be adjusted, the laser has to be turned off during adjustment, or a special adjustment tool has to be used. Please contact bh. Moreover, do not look into the laser beam through lenses, binoculars, magnifiers, camera finders, telescopes, or other optical elements that may focus the light into your eye. When using the lasers in combination with a microscope make sure that the beam path to the eyepieces is blocked when the laser is on. 11

12 Application to Fluorescence Lifetime Spectroscopy Fluorescence Lifetime Experiments A simple setup for general fluorescence lifetime experiments is shown below. +12V from AC/DC Adapter Trigger to SPC A-PPI Adapter BDL-405 Laser +12V to PMH SPC-630, 730 or 830 TCSPC Module Fluorescence Sample Cell PMH-100 PMT Module Detector Signal Electronic components and system connections for fluorescence lifetime experiments The electronic setup consists of the BDL-405 laser module, an SPC-630, -730 or -830 TCSPC module and a PMH-100 detector module [5]. An A-PPI adapter is used to connect the trigger output of the laser to the SYNC input of the SPC module. The PMH-100 gets its power supply from the SPC module. With an optical setup consisting of a few lenses and filters the system can be used for highsensitivity fluorescence lifetime experiments. Please note that blue and NUV laser diodes emit a substantional amount of background light in a wide wavelegnth range. It is therefore important to place a cleaning filter in the excitation light path. Laser Scanning Microscopy and Related Applications In combination with a confocal microscope, fluorescence correlation measurements [11-14] and fluorescence lifetime imaging [15-18] are possible. A typical setup for fluorescence lifetime imaging with a confocal laser scanning microscope is shown below. Blue and NUV laser diodes have been proved to be applicable to laser scanning microscopy and ophthalmologic imaging [18-20]. 12

13 b&h SPC-830 TCSPC Imaging Module PMH ps R3809U <50ps Detector Fibre Output start stop +12V BDL-405 Scanning Head Scanning Microscope Zeiss LSM 510 Leica SP 2 Biorad Radiance, etc. Pixel Clock, Line Clock, Frame Clock Microscope Control Box Fluorescence lifetime imaging with confocal laser scanning microscope The required power in the focus of the microscope objective is less than 50 µw. However, in scanning microscopes the back aperture of the objective is over-illuminated to obtain a uniform intensity distribution over the whole aperture. This gives maximum spatial resolution for a given numerical aperture of the objective, but wastes most of the laser power. If the BDL lasers are used at a scanning microscope the beam geometry should be checked and, if necessary, a beam expander optics be used to obtain the optimum beam diameter. With an available power of the order of 500 µw a reasonable compromise between the power in the focal plane and the spatial resolution can be achieved. Multiplexing BDL Laser Modules Multiplexing of several laser modules can be used to record fluorescence decay curves or lifetime images quasi-simultaneously for several excitation wavelengths. The principle is shown in the figure below. Lasers multiplexed via /shautdown signals SPC module records individual curves for both lasers 100 Hz to 10 khz TRG Out BDL /off power combiner SYNC TRG Out /off BDL Routing bh TCSPC module Multiplexing BDL laser modules Several laser modules are switched on sequentially by controlling the /shutdown inputs in a way that only one laser is active at a time. On the detection side the routing capability of the BH TCSPC modules is used. Simultaneously with the switching of the lasers the control logic sends a routing signal to the TCSPC module that directs the recorded photons into different memory blocks. Laser multiplexing can be used in conjunction with multi-detector operation [4-8]. The synchronisation signal for the TCSPC module is generated via a passive power combiner (a reversed power splitter). The TCSPC module delivers separate waveforms (fluorescence decay curves, time-of-flight distributions, etc.) for the different lasers. Due to the short switch-on time of the laser multiplexing rates in the 10 to 100 khz range can be obtained. 13

14 Compared to a pulse-by-pulse multiplexing the pulse group multiplexing technique shown above has the benefit that the effective stop rate of the TCSPC module is not reduced. Because the maximum count rate of the TCSPC technique is proportional to the effective TAC stop rate pulse group multiplexing allows to run the experiment at higher count rate. Please see [5,6] for details. 14

15 Specification Optical BDL-375 BDL-405 BDL-475 Repetition Rate MHz, selectable Wavelength typical, nm Wavelength, min - max, nm Pulse Width (FWHM, Power 0.5 mw, 50 MHz) 60 to 90 ps Peak Power, max, mw 1) Optical Power 2) mw, 20 MHz 0.1 to to 0.3 (Average CW power, mw, 50 MHz 0.2 to to 0.8 adjustable) mw, 80 MHz 0.3 to to1.0 Stability of Repetition Rate ± 100 ppm Pulse-to Pulse Jitter < 10 ps Power and pulse shape stabilisation after Laser on signal 1 µs Power and pulse shape stabilisation after switch-on 3 min Trigger Output Pulse Amplitude Pulse Width Output Impedance Connector Delay from Trigger to Optical Pulse Jitter between Trigger and Optical Pulse Control Inputs Frequency 20 MHz 3) Frequency 50 MHz 3) Frequency 80 MHz 3) /Laser Off 3) External Bias Input Power Supply Power Supply Voltage Power Supply Current 4) Mechanical Data Dimensions Mounting Thread Maximum Values all lasers +100 mv (peak) into 50 Ω 1 ns 50 Ω SMA < 500 ps < 10 ps all lasers TTL / CMOS high TTL / CMOS high TTL / CMOS high TTL / CMOS low analog input, -10 V to + 10V all lasers + 9 V to +12 V 300 ma to 1 A all lasers 160 mm x 90 mm x 60 mm two M6 holes all lasers Power Supply Voltage 0 V to +15 V Voltage at Digital Control Inputs -2 V to +7 V Voltage at Ext. Bias Input -12 V to + 12 V Ambient Temperature 0 C to 30 C 5) 1) Typical values, sample tested. Depends on pulse width and selected power. 2) Recommended power adjust range. Lower power gives broader pulses, higher power gives ringing in pulse shape. Power levels above the given range can be selected, but are not guaranteed and may impair the lifetime of the laser diode. 3) All inputs have 10 kω pull-up resistors. Open input is equivalent to logic high. 4) Dependent on ambient temperature. Cooling current changes due to temperature regulation of laser diode 5) Operation below 13 C may result in unstable power or extended warm-up time. Caution: Light emitted by the device may be harmful to the human eye. Please pay attention to safety rules when operating the devices. Do not look into the collimated laser beam. 15

16 References [1] S. Nakamura, S.F.Chichibu, Introduction to nitride semiconductor blue lasers and light emitting diodes. Taylor & Francis (2000) [2] S. Nakamura, M.Senoh, S. Nagahama, N. Iwasa, T. Matsushita, T. Mukai, InGaN/GaN/AlGaN-based LEDs and laser diodes. MRS Internet Journal of Nitride Semiconductor Research, 4s1, 1.1. (1999) [3] D.V. O Connor, D. Phillips, Time Correlated Single Photon Counting, Academic Press, London 1984 [4] W. Becker, A. Bergmann, H. Wabnitz, D. Grosenick, A. Liebert, High count rate multichannel TCSPC for optical tomography. Proc. SPIE 4431(2001) [5] SPC-134 through SPC-730 TCSPC Modules, Operating Manual and TCSPC Compendium. Becker & Hickl GmbH, [6] Wolfgang Becker, Axel Bergmann, Controlling TCSPC Experiments. [7] Routing Modules for Time-Correlated Single Photon Counting, Becker & Hickl GmbH, [8] 16 Channel Detector Head for Time-Correlated Single Photon Counting, Becker & Hickl GmbH, [9] R3809U MCP-PMT, Hamamatsu data sheet, [10] Wolfgang Becker, Axel Bergmann, Detectors for High-Speed Photon Counting. [11] R. Müller, C. Zander, M. Sauer, M. Deimel, D.-S. Ko, S. Siebert, J. Arden-Jacob, G. Deltau, N.J. Marx, K.H. Drexhage, J. Wolfrum, Time-resolved identification of single molecules in solution with a pulsed semiconductor diode laser. Chem. Phys. Lett. 262 (1996) [12] R. Kühnemuth, C.A.M. Seidel, Principles of single molecule multiparameter fluorescence spectroscopy. Single Molecules 2 (2001) [13] C. Zander, K.H. Drexhage, K.-T. Han, J. Wolfrum, M. Sauer, Single-molecule counting and identification in a microcapillary. Chem. Phys. Lett.286 (1998) [14] W. Becker, H. Hickl, C. Zander, K.H. Drexhage, M. Sauer, S. Siebert, J. Wolfrum, Time-resolved detection and identification of single analyte molecules in microcapillaries by time-correlated single photon counting. Rev. Sci. Instrum. 70 (1999) [15] TCSPC Laser Scanning Microscopy - Upgrading laser scanning microscopes with the SPC-730 TCSPC lifetime imaging module, Becker & Hickl GmbH, [16] Wolfgang Becker, Axel Bergmann, Georg Weiss, Lifetime Imaging with the Zeiss LSM-510. Proc. SPIE 4620 (2002) [17] W. Becker, A. Bergmann, K. Koenig, U. Tirlapur, Picosecond fluorescence lifetime microscopy by TCSPC imaging. Proc. SPIE 4262 (2001) [18] A. Rück, F. Dolp, C. Happ, R. Steiner, M. Beil, Fluorescence lifetime imaging (FLIM) using ps pulsed diode lasers in laser scanning microscopy. Proc. SPIE (2003) [19] D. Schweitzer, A. Kolb, M. Hammer, E. Thamm, Tau-mapping of the autofluorescence of the human ocular fundus. Proc. SPIE Vol. 4164, [20] D. Schweitzer, A. Kolb, M. Hammer, E. Thamm, Basic investigations for 2-dimensional time-resolved fluorescence measurements at the fundus. Int. Ophthalmol. 23 (2001)