Source: Wikipedia

The Basics of Phototubes in Photonics

Introduction to Phototubes

A phototube, also known as a photoelectric cell, is a photoemissive detector invented in 1893 by Julius Elster and Hans Geitel. It consists of a small glass tube containing electrodes where the external photoelectric effect is utilized. These tubes are either evacuated or filled with a gas under low pressure.

Operation of Phototubes

A typical phototube comprises a photosensitive cathode and an anode. When a voltage is applied to the electrodes, photoelectrons are swept from the cathode to the anode, generating a photocurrent. The quantum efficiency of a phototube determines its sensitivity to incident light, with values typically below 25%.

Quantum Efficiency and Tube Geometries

The quantum efficiency of a phototube depends on factors such as the photocathode material, tube glass, optical wavelength, and angle of incidence. Various cathode materials are available for different light wavelengths. Phototubes come in different geometries, such as “head-on” and “side-on” tubes, based on the direction of incident light.

Electronics and Gas-filled Phototubes

Electronics used with phototubes are similar to those for photodiodes but may require higher voltages. Some phototubes are filled with gas, like neon or argon, to amplify the responsivity through ionization. Gas-filled phototubes offer increased responsivity but have limitations related to voltage dependence and measurement bandwidth.

Photomultiplier Tubes and Advantages of Phototubes

Photomultiplier tubes are a specialized form of phototubes that provide high amplification of photocurrent. While solid-state devices like photodiodes have largely replaced phototubes, the latter offer advantages such as lower dark current, large photosensitive area, high stability, and suitability for UV applications.

Challenges and Considerations

Despite their advantages, phototubes have limitations, including lower quantum efficiency, sensitivity to vibrations, high operation voltage requirements, and limited photocurrent handling capacity. Care must be taken to prevent ambient light exposure and gas diffusion issues in certain environments.

Conclusion

In conclusion, phototubes have been instrumental in photonics applications for over a century, offering unique advantages and challenges compared to modern solid-state devices. Understanding the principles and operation of phototubes is essential for utilizing them effectively in various optical detection systems.

Source: Siliconvlsi
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