Diode lasers pulse shape

The factor k depends on the two-photon absorption cross section of the dye at the laser wavelength, on the pulse shape, and on the spatial energy distribution in the focus. Because the effective excitation depends on the reciprocal pulse width, not on its square, the prospects are not too bad for detecting diode-laser-excited two-photon fluorescence. It has in fact been demonstrated that two-photon excitation can be achieved even with CW lasers [228]. An experiment to demonstrate two-photon excitation by diode lasers is shown in Fig. 5.134. 

The TTS of conventional PMTs and miniature PMTs with metal channel dyn-odes can be measured with satisfactory accuracy using picosecond diode lasers. These lasers deliver pulses as short as 30 to 50 ps FWHM. However, the pulses may have a tail or a shoulder, especially at higher power. The diode driving conditions for clean pulse shape with minimum tail are usually not the same as for shortest FWHM. The TTS of MCP PMTs ean be reasonably measured only by a Ti Sapphire laser or a similar femtoseeond or picosecond laser system. 

The pulse shape obtained from diode lasers ehanges with the power. Typical pulse shapes are shown in Fig. 7.2. 

Fig. 7.2 Pulse shape of a 465-nm laser diode at different powers. Repetition rate 50 MHz, curves recorded by MCP-PMT and TCSPC, time scale 500 ps/div. Pulse width (FWHM), corrected for 30 ps TTS of MCP-PMT 100 ps, 50 ps, 46 ps, and 42 ps...

If a lens of short foeal length is used, e.g. a seeond laser diode collimator, magnification of the aberrations is avoided. Now the laser ean be coupled into a thin fibre. However, the NA is large, and so is the pulse dispersion. An example is shown in Fig. 7.23. Pulses from a 650 nm, 45 ps diode laser were sent through a 1 mm fibre of 2 m length. The pulse shape shown left is for an NA of 0.3, the right pulse shape is for an NA of < 0.1. 

Typical pulse shapes for LEDs driven by pulses of 3.6 ns FWHM from a Hewlett Packard HPlllOA pulse generator are shown in Fig. 7.88, left. Pulses from a 5 mW, 650 nm laser diode driven by 1 ns pulses from a HP8131A pulse generator are shown right. The detector was a Hamamatsu H5783P photosensor module. 

The electrons emitted by the photocathode are subsequently accelerated to 50 kV and focused on to a toroid-shaped anode. The anode is made of oxygen-free, high conductivity copper and is maintained at a high positive potential. The electron pulses interact with the copper anode forcing the emission of Cu-Ka x-ray photon pulses, which exit the vacuum chamber through a thin beryllium-foil window. A bend germanium crystal monochromator disperses and focuses the x-rays onto the sample. The duration of the x-ray pulses is measured by a Kentech x-ray streak camera fitted with a low density Csl photocathode. The pulse width of the x-rays at 50 kV anode-cathode potential difference is about 50 ps. This value is an upper limit for the width of the x-ray pulses because the transit time-spread of the streak camera has to be taken into consideration. A gold photocathode (100 A Au on 1000 A peiylene) is used to record the 266-nm excitation laser pulses. The intensity of the x-rays is 6.2 x 10 photons an r (per pulse), and is measured by means of a silicon diode array x-ray detector which has a known quantum efficiency of 0.79 for 8 kV photons. 

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