The high speed shutter:
a crucial feature of high speed ICCD cameras.
The open and closed state of the high speed shutter is achieved by reversing the applied photocathode voltage.
Image intensifers enable high shutter speeds
Three voltages are applied to the image intensifier as shown in the drawings above. The voltage UC between photocathode and multichannel plate enables gating operation of the image intensifier. If the voltage UC between photocathode and multichannel plate is negative, the photoelectrons from the photocathode are accelarated towards the multi channel plate. This means that the shutter is open. If the voltage UC is positive, the photoelectrons are kept at the photoathode, thus the shutter is closed. This operation mode of the image intensifier is called gating and the voltage UC is called gating voltage. The ICCD camera itself is therefore also called gateable CCD camera.
Picosecond shutter speeds
An unique advantage of the gateable ICCD camera over all other kinds of cameras is the ability of ultrafast gating. Shutter speeds with exposure times of 5 to 10 nanoseonds (ns) are mostly standard with gateable ICCD cameras nowadys. Some cameras on the market yet offer sub-ns gating. The 4 Picos ICCD camera offers gating times of down to 200 picosecond (ps).
Measurement of ultra high speed shutter times
The question arises how this incredible fast gating times can be measured and characterized. Most of the ICCD camera manufacturer describe the shortest shutter speed with the Full Width at Half Maximum (FWHM) of the gaussian-like transmission curve. However, due to the behavior of the image intensifier in the gating mode a single gating rather looks like a rectangle with a sharp rising and falling edge. The Gaussian transmission curve is a result of the summation of numerous single measurements with a clearly higher Flat Top width. Furthermore, the opening time at 1% transmission is clearly higher as the proposed FWHM, as it is shown in the diagram below.
Comparison of Full Width at Half Maximum (FWHM) shutter time to the Flat Top high speed shutter gating time.
Therefore, all ICCD cameras from Stanford Computer Optics are characterized with Flat Top Width of a single gating event. Our unique Coaxially Coupling Technology (CCT) achieves outstanding rectangular shutter transmission characteristics down to the physical limits. The 1%-to-90% rise time as well as the 90%-to-1% fall time amount to 30ps each. As shown in the diagram (green curve), our shutter transmission yields 200ps at 90% opening hence only 260ps at 1% opening.
Reaching physical limits
Shutter speeds in the picoseconds regime are only able by the unique integrated system design. The photocathodes are coated with an in-house developed anti-iris grid and connected to the gating electronics by a dedicated coaxial waveguide. During the rise and fall times of 30ps the light travels only 9mm, which is exactly the half diameter of the image intensifier. Thus, our rectangular curve shape represents definitely the physical limits. During the 200ps flat top of the gating pulse the light travels 6cm which approximately is the distance between the gating electronics and CCD sensor.
Applications of ulftrafast shutter systems
Among a huge variety of ultrafast phenomenon applications, this unique rectangular shutter transmission enables range gating viewing with sub-millimeter spatial resolution. In addition, high speed ICCD cameras from Stanford Computer Optics are able to stationary maintain outstanding 10MHz repetition rate in multiple exposure mode. This unique feature makes them extremely valuable in a large variety of fluorescence microscopy applications like FLIM, FRET, FISH, TIRF and others. There the ICCD camera can be synchronously triggered to ultrafast laser excitation. Unlike to EMCCD cameras the readout noise of the ICCD camera is negligible compared to the already intensified light signal. There is no need for slow scan modi and the maximum frame rate is also applicable in difficult low light conditions and even in single photon counting mode.