Fast Photodiodes

The photocurrent generates a signal voltage Vg = Up = RL ph across the load resistor Rl which is proportional to the absorbed radiation power over a large intensity range of several decades (Fig.4.84b). From the circuit diagram in Fig.4.85 with the capacitance Cg of the semiconductor and its series and parallel resistances R. quency limit [4.103] 

For photon energies hu close to the band gap, the absorption coefficient decreases, see (4.121). The penetration depth of the radiation, and with it the volume from which carriers have to be collected, becomes large. This increases the collection time and makes the diode slow. 

Very fast response times can be reached by using the photoeffect at the metal-semiconductor boundary known as a Schottky barrier [4.106]. Because of the different work functions and of the metal and the semiconductor, electrons can tunnel from the material with low to that with high t (Fig. 4.87) causing a space-charge layer and a potential barrier 

For measurements of optical frequencies, ultrafast Metal-Insulator-Metal (MIM) diodes have been developed [4.107], which can be operated up 

These MIM diodes can also be used as mixing elements at optical frequencies. If the beams of two lasers with the frequencies f and f2 are focussed onto the junction between a nickel surface and the sharp tip of a tungsten wire the MIM diode acts as rectifier and antenna, and generates a signal with the difference frequency Difference frequencies up into 

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In the typical setup, excitation light is provided by a pulsed (e.g., nanosecond) laser (emitting in the visible range, e.g., at 532 nm, if Mb is investigated), while the probe is delivered by a continuous-wave (cw) laser. The two beams are spatially overlapped in the sample, and the temporal changes in the optical properties (such as optical absorption or frequency shift) that follow the passage of the pump pulse are registered by a detector with short response time (relative to time scale of the processes monitored), such as a fast photodiode. 

Oscillogram of a short laser pulse seen by a fast photodiode. Horizontal axis, time in 50 ps/division... 

A small part of the infrared light was frequency doubled in a KNbC>3 crystal to 486 nm and combined with the blue light of the dye laser that excites the IS —2S transition. A fast photodiode is used to observe the frequency difference v(2S — AS/AD) — 1/4 v(lS — 2S). Fitting the 2S —4S and 2S — AD line profiles with a theoretical model calculated by Garreau et al. [17,18,19] and correcting for some systematic effects, the ground state Lamb shift could be determined with an accuracy of 1.3 parts in 105, one order of magnitude more precise than in previous measurements [21,22]. 

The first equation is realized at the LKB while the second one is carried out at the LPTF. A first titanium-sapphire laser excites the hydrogen transition. A laser diode (power of 50 mW) is injected by the LD/Rb standard and frequency doubled in a LiBsOs (LBO) crystal placed in a ring cavity. The generated UV beam is frequency compared to the frequency sum (made also in a LBO crystal) of the 750 and 809 nm radiations produced by a second titanium-sapphire laser and a laser diode. A part of the 809 nm source is sent via one fiber to the LPTF. There, a 809 nm local laser diode is phase locked to the one at LKB. A frequency sum of this 809 nm laser diode and of an intermediate CO2 laser in an AgGaS2 crystal produces a wave at 750 nm. This wave is used to phase lock, with a frequency shift S, a laser diode at 750 nm which is sent back to the LKB by the second optical fiber. This 750 nm laser diode is frequency shifted by lyfCOo) + S with respect to the one at 809 nm. In such a way, the two equations are simultaneously satisfied and all the frequency countings are performed in the LKB. Finally, the residual difference between the two titanium-sapphire lasers is measured with a fast photodiode or a Schottky diode. 

Figure 1 Apparatus of Oxford experiment [6]. LI, L2 tunable dye lasers. UV ultra violet radiation (243 nm). RF radiofrequency dissociation of flowing molecular hydrogen. PI signal photomultiplier (Lyman-a detector). P2 photomultiplier for cavity locking and signal normalisation. SI cavity length servo-control. C conrouter. AOM acousto-optic modulator. T heated quartz cell containing tellurium. S2 laser frequency servo-control. D fast photodiode...

Fig. 7.13 Schematic representation of the test arrangement for the determination of the detonation velocity using the optical fiber/fast photodiode/osciiioscope technique.

Figures 7.12 and 7.13 show the experimental set-up for using a high-speed streak camera or a fast photodiode/osciiioscope for the determination of the detonation velocity.

As discussed above, a convenient way of measuring the detonation velocity is to convert the light signal using a fast photodiode that has a rising time of about 10 ns into an electric signal that may be recorded by either a fast oscilloscope (Fig. 7.13) or a multi-channel analyzer (Fig. 7.14). 

FIG. 12.5 Setup for Nd YAG and He-Cd laser experiments I, Nd YAG laser 2,4,1064 ran mirtors 3, glass plate 5, BBO crystal 6,7, half-wave QJ2) plates 8,9,35S nm mirror pair 10, polarizer 11. color filter 12, 13, lenses 14, thin film sample 15, fast photodiode 16, iris pinhole 17,21, neutral densi filters 18, PMT 19, pump damper 20, He-Cd laser and 22, mirror. 

The total eost of a TCSPC oseilloseope system is no higher than that of an optical oscilloscope consisting of a fast photodiode and a fast oscilloscope. However, the sensitivity is many orders of magnitude greater. Moreover, the detection area of a PMT is much larger than that of an ultrafast photodiode, so alignment is no longer an issue. 

Figure 5.40. Measured frequency responses erf several PMTs and a fast photodiode 0>D). Data ire from Refs. KB and 104 and litenluie...

Fig. 6.11 Measured pulses of a mode-locked argon laser at A = 488 nm (a) monitored with a fast photodiode and a sampling oscilloscope (500 ps/div). The small oscillations after the pulse are cable reflections (b) the attenuated scattered laser light was detected by a photomultiplier (single-photon counting) and stored in a multichannel analyzer. The time resolution is limited by the pulse rise times of the photodiode and photomultiplier, respectively [656]...

The technical realization of this synchronized system is shown in Fig. 6.16a. The frequency Vs = /27t of the ultrasonic wave is chosen as an integer multiple v =q c I Id of the mode-locking frequency. A fast photodiode, which detects the mode-locked optical pulses, provides the trigger signal for the RF generator for the ultrasonic wave. This allows the adjustment of the phase of the ultrasonic wave in such a way that the arrival time of the mode-locked pulse in the cavity dumper coincides with its maximum extraction efficiency. During the ultrasonic pulse only one mode-locked pulse is extracted. The extraction repetition frequency Vq = (cjld jk can be chosen between 1 Hz to 4 MHz by selecting the repetition rate of the ultrasonic pulses [669]. 

For sufficiently low difference frequencies (Av = Acojln <10 Hz) fast photodiodes or photomultipliers can be used for detecting Aco. The two laser beams are superimposed onto the active area of the detector. The output signal S of the photodetector is proportional to the incident intensity, averaged over the time constant r of the detector. For Aco In/r < co = (coi- - (02)12 we obtain the time-averaged output signal... 

Fast photodiodes are always operated at a reverse bias voltage U <0, where the saturated reverse current is of the dark diode is small (Fig. 4.85a). From (4.128) we obtain, with [Qxp(eU/kT) 1] for the total diode current,... 

If the optical fibres/fast photodiode/oscilloscope technique is used, an oscillogram similar to the one in Figure 4.16 is obtained. By further treatment of the oscillogram, the detonation velocity is obtained as a ratio of distance traveled and corresponding time interval. 

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