The HRI [High Rate Imager] is a multipurpose flexible gated imaging system. It can be used in gated modes down to around 300ps and RF gain modulation modes to 1GHz. Many laser sources are mode locked at frequencies around 100MHz. Experiments may involve illuminating a sample and looking at the emitted at a specific time later for a short integration time, i.e. to time resolve the light at some later time. Often the peak power light output from such a laser is not sufficient to perform experiments with regular gated imagers. In order to overcome this, users extract a pulse from the mode locked train and amplify it to sufficient power so that imaging is possible. This is typically done at repetition rates up to 10kHz and generally the average power from the amplified pulse train is no higher than that in the original mode locked laser. This process is difficult and expensive. A pulse from the mode locked train has to be "picked" and amplified, often in a regenerative amplifier and then extracted from the amplifier possibly with some cleaning up of the pulse to remove pre or post pulse reflections. In addition it may be difficult to retain a narrow pulse width in such a system.
For experiments which last for less time than the interpulse period of a mode locked laser, this problem is transformed into simplicity by the HRI. The HRI can be gated down to 300 ps at rates up to 110MHz, consequently it can be used directly on the mode locked laser pulse train. No amplifier is needed and the signal is integrated onto the output phosphor of the HRI.
The cathode of the HRI is standard and similar to that that would be used in a regular gated imager. Also light detection in the cathode is a single photon process so no signal to noise is lost by this technique. It all comes down to average power.
The HRI is not a cheap instrument, but when compared with the cost of a standard gated imager plus a laser amplifier it offers a much lower cost alternative.
The HRI is not a solution to all problems. It may be that signal to noise is not limited purely by average laser power but rather by the presence of some other light source. In this case the peak power of an amplified system will still out perform the technique outlined above. However, the HRI can do this as well. So one need only consider the extra cost of the HRI over a regular gated system with the possible advantages of high rate image collection.
Note that the HRI does not offer output frame rates of 110MHz. The output frame rate is limited by the recording system and the integration time decay time of the output phosphor.
The HRI does not include, as standard, a recording output. The user may fit there own CCD system or buy a complete package from our German agents who market this as the Picostar.
The standard GOI gated intensifier that Kentech has marketed for many years is capable of gating down to 80ps and specials below 50ps.
We have recently successfully integrated the GOI and HRI into a single device. When in fast gate mode (GOI) the repetition rate can be up to 10kHz, whilst the HRI mode is available for use down to 300ps.
The GOI applies a larger voltage to the cathode of the intensifier and so at the same gate width will offer somewhat better spatial resolution, albeit with a reduced repetition rate.