Fast Photodiode Research Articles

Differential photodiode detector for a shock tube laser schlieren system

A fast photodiode recording system is described for use in a shock tube laser schlieren system. This device is relatively inexpensive, has a superior signal-to-noise ratio, is insensitive to ripple in the light source, and provides a convenient calibration correction for long term variations in laser intensity. Measurements of both relaxation and dissociation behind shock waves have been carried out using this device.

Determination of laser line frequencies and vibrational-rotational constants of the 12C 18O 2, 13C 16O 2, and 13C 18O 2 isotopes from measurements of CW beat frequencies with fast HgCdTe photodiodes and microwave frequency counters

The frequencies of 12C 18O 2, 13C 16O 2, and 13C 18O 2 isotope lasers have been measured to better than 3 MHz (0.0001 cm −1) by comparing with a 12C 16O 2 reference laser. Heterodyne techniques were used to generate 139 difference frequencies in a liquid nitrogen cooled HgCdTe photodiode. Microwave frequency counter measurements of the difference frequencies were then used to calculate new values for the band centers and rotational constants of the rare isotopes. Procedures to achieve further improvements are discussed.

Time response of plastic scintillators

Abstract Time-response characteristics of eight commercial plastic scintillators have been studied. Samples were excited by 50 ps electron or bremsstrahlung pulses. Sampling techniques were used to record the signal from a fast vacuum photodiode. Effects of surface preparation on time response were studied. The observed fwhm (in ns) for the three fastest plastics was 1.5, 2.0, and 2.3 for NE111, Pilot U, and KL236 respectively. The effect of various optical filters was studied for NE111. With suitable filtration, a minimum fwhm of 1.15 ns was observed for NE111. Time response of NE102 samples was studied as a function of thickness. For thin samples, improvement in time response was observed.

Generation of reproducible giant pulses with an optically regenerative Q switch

In a new concept for an optically regenerative Q switch which is described, the oscillation of the laser with the cavity closed is sensed by a fast photodiode whose output signal opens the electro-optical Q switch. The threshold of the laser as a function of the modulator voltage is theoretically investigated; the empirical results obtained with a YAG:Nd3+ laser agree well with theory. The advantages offered by the optically regenerative Q switch are the easy controllability of the peak power of the giant pulses by way of the modulator voltage and the excellent reproducibility of the giant pulses. The experiments revealed the variations between pulses to be less than 3%. Thus the optically regenerative Q switch is practical for all giant-pulse lasers whose emission is required to be highly stable.

Absolute Calibration of a Laser System for Atmospheric Probing

In order to obtain quantitative data on the backscatter function from laser irradiance backscattered from the atmosphere, the ratio of power transmitted to power received must be accurately known. No absolute measurements of power, optical system transmittance, detector quantum efficiency, or electronic gain are necessarily required. The technique of measuring the power ratio by irradiating a smoked or painted target of known diffuse reflectance at a fixed range is used to calibrate a complete lidar system. The relative area of the output power pulse is monitored by a fast response photodiode, and the relative area of the returned pulse is also recorded after passing through a filter of known high optical density. It is essential to control the temperatures of the laser rod and receiver interference prefilter to ensure proper spectral matching. Field experience gained using this technique is described, and examples of calibration measurements and backscatter functions for smog and cirrus clouds are presented.

Optical sampling of subnanosecond light pulses

In this paper we shall describe a technique for the display of subnanosecond light pulses which is the optical analog of the technique used in electronic sampling oscilloscopes. The optical pulse waveform to be displayed is assumed to repeat itself with period T. A mode-locked laser provides a source of sampling pulses of a few picoseconds duration. The period of this laser is adjusted to be T + δT, where δT s is proportional to ∫ I 1 I 2 dt, where I 1 and I 2 are the intensities of the light signal beam and sampling beam, respectively. If the signal beam is slowly varying compared to the very brief sampling pulse, we have: Y s = constant × I 1 (t n ) where t n is the arrival time of the nth sampling pulse at the nonlinear crystal. Because of the slightly unequal periods, the sampling pulse scans the light signal in steps of δT seconds. Displays of the sum frequency signal Y s on a conventional oscilloscope therefore constitute a sampled display of the light signal. In an experiment to test the method, sampled displays of subnanosecond pulses generated by a mode-locked He-Ne laser (wavelength 0.633µ) were obtained. The source of sampling pulses, ∼4 psec in duration, was a mode-locked Nd:glass laser (wavelength 1.06µ). The two unfocussed beams were mixed in a KDP crystal and the sum frequency signal at 0.397µ was detected by a photomultiplier and displayed on an oscilloscope. The period T of the He-Ne laser pulses was 12.44 nanoseconds, and the sampling step δT was varied between 100 psec and 400 psec. Using this method the He-Ne laser pulse width was observed to vary between 700 psec and 900 psec depending on the He-Ne laser adjustment. These measurements were confirmed with a fast photodiode and an electronic sampling oscilloscope. The average He-Ne laser power was ∼1 mw. In the KDP crystal the sampling pulses converted red light into UV light with an efficiency of about 5%. Since with focussed beams an efficiency of better than 20% can be achieved, it appears possible to optically sample light signals with average powers much less than a milliwatt.

Time-resolved spectra of high-current pulsed argon lasers

The time-resolved laser spectrum of a pulsed argon discharge has been examined in the visible region with a fast response photodiode and with an image converter camera in the streak mode. At 10 mtorr pressure of argon, laser action takes place on up to eight lines between 4579 and 5145 Å, seven of these lines being <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4p \rightarrow 4s</tex> transitions and one a <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4p' \rightarrow 3d</tex> transition. With peak discharge currents of 50 to 1500 amperes (400 to 12000 amperes-cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> ) and duration of about 2 μs, laser action takes place both during the current pulse and in the afterglow. The primary laser output occurs during the first half of the current pulse, while the current is increasing, and then laser action ceases until the current has ceased to flow. In the afterglow, laser action recommences and lasts for up to 60 μs. The effects of variation of gas pressure, applied potential difference, and current pulse length on the threshold conditions, intensities, and duration of the lines are examined.

Photoelectricity in solids

Theoretical and experimental aspects of internal photoeffects in solids are discussed. The theory underlying the generation and recombination of carriers and their movement by diffusion and under the influence of applied electric and/or magnetic fields is indicated.Photovoltaic, photoconductive and photoelectromagnetic effects are considered and the operation of p-n junctions as solar batteries or fast photodiodes is discussed. Some applications of solid state photoeffects - particularly for infra-red detectors - are outlined.

Some applications of semiconductor counters in high energy physics

The small size of semiconductor counters are a considerable limitation for their application to detect high energy particles. However, in experiments on accelerators there is a certain number of special problems where semiconductor counters can be rather effectively used. In particular, good spectrometric properties, non-sensitivity to the magnetic field and the compactness of the semiconductor counter allow to use it for identifying slow protons and deutrenos in elastic pp and pd scatterings. In our case the target was bombarded with the 10 GeV internal proton beam of the proton synchrotron of the Joint Institute for Nuclear Research. Semiconductor counters operating in the photodiode regime were applied to measure particle fluxes in other investigations carried out on the synchrocyclotron. Optical characteristics of lithium-drifted silicon counters of the surface-barrier type were studied. Being rather perfect fast photodiodes, the counters can be used also for determination of the spark coordinates in a spark chamber.

A Recording Sampling System for Measuring Laser Energy

An apparatus has been designed and built which simultaneously measures the energy incident on a biological system which is being exposed to a laser beam. The advantages of the direct reading system are: (1) no attention is required of the laser operator, (2) a permanent record is produced, (3) true integration of the pulse train is accomplished, (4) high inherent accuracy, (5) in conjunction with a fast oscilloscope, it reproduces pulse waveforms with minimum distortion, and (6) calibration is simple and direct. Basically the system employs a fast high-current capability photodiode, a high-quality integrating capacitor, an emission-limited pump diode with a servo-motor amplifier combination for dark current balance. The photodiode cathode surface is S-1 so that both ruby and neodymium wavelengths 694.3 mmicro and 1060 mmicro, respectively, may be measured. Reproducibility is better than 3% when compared with a blackbody receiver (cone radiometer). A Tektronix 585A oscilloscope and P-80 cathode follower probe fitted with a 50-Omega load resistor can be plugged into the photodiode housing in place of the integrating capacitor for waveform observation and power-level measurements. Calibration on an absolute basis is easily accomplished by simultaneous comparison with a calibrated blackbody receiver (cone radiometer).