This paper is intended as a status report on high-speed detectors for the visible and near-infrared portion of the optical spectrum. Both vacuum and solid-state detectors are discussed, with the emphasis on those devices which can be used as direct (noncoherent) detectors of weak optical signals modulated at microwave frequencies. The best detectors for this application have internal current gain and in this regard the relevant properties and limitations of high-frequency secondary emission multiplication in vacuum tube devices and avalanche multiplication in p-n junctions are summarized.

High-speed photodetectors

PIN photodiodes and bulk photoconductors used as direct demodulators and as heterodyne mixers of coherent optical radiation are analyzed by taking the Fourier transform of the impulse current response produced by a single photoexcitation. This method allows the presentation of the fundamental physical processes in a manner which emphasizes the similarities rather than the differences between these two photodetectors. At microwave frequencies, the available power from the two devices is shown to have the same dependence upon frequency, and optical and circuit parameters. Further similarities are revealed by the noise analysis; in each case the limiting mechanisms are generation-recombination and thermal noise. The noise performance of the photodiode is found to be superior to that of the photoconductor at low light intensities. At high light levels, the noise performance of both devices is similar. The physical embodiment of these devices is such that the photodiode is generally a superior low-level detector (i.e., at light levels in the milliwatt range), whereas the photoconductor gives comparable or better results at high light levels (i.e., in the watt range). It is found, however, that the product of the bandwidth and fraction of the available power reaching the load is limited in the case of the photodiode because of the finite junction capacitance, but is essentially unbounded for the bulk photoconductor.

Solid-state photodetection: A comparison between photodiodes and photoconductors

A measurement of the signal‐to‐noise ratio and minimum detectable power for heterodyne detection of scattered radiation at 10.6 μm has been made. Using photoconductive Ge:Cu as the detector, the observed minimum detectable power, at a frequency of 70 kHz and in a bandwidth of 270 kHz, was 3.5 × 10−14 W. This corresponds to a minimum detectable power of 1.3 × 10−19 W in a 1‐Hz bandwidth, which is within a factor of 10 of the theoretically perfect photon counter.

OPTIMUM HETERODYNE DETECTION AT 10.6 μm IN PHOTOCONDUCTIVE Ge:Cu

Holography, speckle, and computers

Analyses and experiments have been carried out on heterodyne detection at 10.6 microns with the objective of obtaining IF bandwidth capability into the microwave region. UHF and microwave measurements on the quantum-noise-limited generation-recombination (G-R) noise spectrum of compensated copper-doped germanium photoconductive mixer elements, measured under operational conditions at 10.6 microns, have shown response to beyond 2 GHz. The experiments were carried out directly at 10.6 microns using a mixer geometry and circuit arrangement intended to yield large mixer conversion gain and IF bandwidth. Engineering design equations are given for noise equivalent power (NEP) and mixer conversion gain ( G ) in terms of such parameters as IF noise factor, carrier transit time, carrier lifetime, mixer resistance, local oscillator power, dc bias power, etc. An expression for quantum-noise factor (QF) is defined. Graphs are also presented showing the effect on NEP, G , and QF of various parameters, and the tradeoffs possible to achieve high-frequency IF capability. An alternative approach is presented in which mixer conversion gain is calculated directly from the mixer I-V characteristic in a manner analogous to microwave mixers.

5.2 - Infrared 10.6-micron heterodyne detection with gigahertz IF capability

Optical frequency mixing in bulk semiconductors

The mixing of the first four axial‐mode frequencies of a ruby laser in a bulk, room temperature, CdSe crystal to obtain microwave beat frequencies in the 1–5 Gc/sec range is described. A photoconductive mixing analysis based on a monomolecular recombination process is found to give excellent agreement with the experimental data. The beat frequency power was found to vary as the square of the dc bias voltage applied across the crystal, the square of the incident light power, and inversely as the square of the beat frequency. A minimum lifetime of 10−9 sec, a cross section of 1.3×10−16 cm2, a modulation index of 0.2, and a maximum beat frequency power of 0.2 mW were obtained in the experiments.

Photoconductive Mixing in CdSe Single Crystals

The problem of direct interband transitions in semiconducting crystals is considered in detail. The solution of the problem is shown to lead to a system of simultaneous coupled nonlinear differential equations. These equations do not distinguish between static and dynamic fields, and are obtained without recourse to any approximation procedures. They therefore apply for electromagnetic fields of arbitrary amplitude. It is shown, finally, that the linearization of these equations correctly predicts the fundamental optical absorption edge and the photoconductive rate equations which result in photomixing phenomena.

The theory of direct transitions in semiconductors

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Optical frequency mixing in bulk semiconductors

Photoconductive Mixing in CdSe Single Crystals

The theory of direct transitions in semiconductors