The device is a very fast vacuum photodiode to detect short laser pulses, type FD125, made by ITL Technology, UK. Vintage, on market in 70's, it consists of a vacuum tube (diode) made of a photocathode and a mesh anode, parallel and at short distance to each other, to minimize the time of flight of the emitted photoelectrons from the cathode to the anode. The photodiode is a current generator,i.e., it generates a current proportional to the light power received by the device. The design minimizes the risetime of the current generated by the photodiode, which is terminated into a coaxial 125 Ω connector. With a matched 125 Ω load, a peak voltage of 10 V - 20 V can be generated, which is proportional to the incoming laser power. With an appropriate 125 Ω characteristic impedance coaxial cable, it is connected to a real time, fast oscilloscope with a risetime of less than 0.35 ns (or a bandwidth more than 1 GHz), with an input impedance of 50 Ω - 125 Ω. The 70's oscilloscope model was Tektronix 519 oscilloscope. Besides the photodiode tube itself and the output connector, the device has a matching impedance mount and a high-voltage (HV) BNC connector. The latter connects to the high-voltage DC power supply to apply the voltage to the photodiode electrodes. The main characteristics of the device follows. Window diameter: 20 mm. Type of photocathode: S1, with spectral sensitivity 300 nm - 1200 nm (see last figure of its spectral sensitivity - initially intended and used to detect Nd:glass laser mode-locked pulses, at 1060 nm, can work well also in near UV and visible). Maximum supply voltage at HV BNC connector: 2 kV. Risetime of the generated pulse for a step voltage: 0.2 ns. Some additional info on its properties can be obtained from the paper of Smith et al."Impulse Rise and Fall Times of Biplanar Vacuum Photodiodes", Appl. Opt. vol. 12, pp. 1606-1610 (1973). The device was used occasionally in early (70') experiments for mode-locking Nd:glass lasers. Vintage device, the original cost was in the range of $2,000. Although today there are faster and smaller photodetectors, the device is still functional and useful for laser hobbyists, for educational purposes, or as a collectible item of laser history.
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