OPTICAL VOICE AND DATA
Free Space Photomultiplier
Tubes
Technical Review Paper
Neil Gandhi
L03 Professor Koblasz and Professor James
Neil Gandhi
L03 Koblasz/James
Optical Voice and Data
Free Space Photomultiplier Tubes
Introduction
Free space optical (FSO) communication is quickly becoming a popular technology in data
networks all over the world. The technology consists of using light beams encoded with data to
communicate across distances without the need for a transmission medium such as a fiber optics cable.
Photomultiplier tubes (PMTs) detect the incoming light stream, which can then be decoded and used.
Improvements in accuracy and overall effectiveness of detecting incoming photons have been made in
recent years. This paper examines these improvements as well as commercial applications of
photomultiplier tubes and their basic operation.
Commercial Applications of Photomultiplier Tubes
One of the most important applications of photomultiplier tubes (PMTs) is in the use of
scintillators. Scintillators absorb incoming ionized particle and reemit the absorbed energy, causing a
small flash of light to occur. The photomultiplier tube reemits this energy in the form of electrical pulses
that can then be measured and information gathered. This technology is used in applications in medical
applications, physics, and in scanning luggage and containers. A new technique has been developed at the
Synchrotron Ultraviolet Radiation Facility (SURF III) at the National Institute of Standards and
Technology to determine the UV degradation of PMTs and better calibrate them. This technique involves
the use of synchrotron radiation to overcome the saturation that occurs with the use of high power beams
during normal operation [1].
In 2003 SensL, a company on the forefront of low light detector solutions released its first ever
alternative to standard PMTs. They created a solid state PMT known as the Silicon Photomultiplier
(SPM). This new detector has the efficiency of a PMT, while having a high gain, low voltage, small form
factor, high uniformity, and excellent scalability towards any application [2]. The SPM is made of a
parallel combination of Geiger mode Avalanche Photodiodes, which gave a fixed current pulse per
detected photon, allowing for an accurate reading the incoming phonon flux. These new devices are
quickly becoming the preferred detector in sensitive applications such as High Energy Physics, Radiation
Detection, PET/MRI, Spectroscopy and more as well as in environments in which PMTs cannot operate
[3].
There are a wide range of prices of PMTs depending on the application. According to Thorlabs a
PMT module is about $2,100 while normal photodiodes are much cheaper at $13 - $75 per unit [4].
Edmund Optics prices high quality PMTs between $790 to $1200 for various sensitivities and
wavelengths [5].
Underlying Technology of Photomultiplier Tubes
Photomultiplier tubes operate on the basic idea of detecting and measuring the flux of
light in the ultraviolet, visible, and near-infrared ranges of the electromagnetic spectrum. The
current detected is then multiplied 100 million times allowing for precision detection. PMTs rely
on two basic physical properties, the photoelectric effect and secondary emission. The
photoelectric effect causes electrons to be emitted from a material after the material absorbs
electromagnetic radiation. The secondary emission effect results when particles of a sufficient
amount of energy induce secondary particles, electrons in this case, to be emitted.
There have been recent advances in photomultiplier technology that have been needed for
more precision work. One improvement involves improving the light collected by the faceplate
by extending the area of the plate to be as large as or larger than the cross sectional area of the
PMT. Tapering the edge of the faceplate as a reflector of the light and avoiding obscuring the
optical path towards the photocathode also helps in increasing the overall light collected [6].
Another advance involves using multiple dynodes and a partitioning wall, which divides the
electron focusing area of the PMT in four. The series of dynode areas are able to effectively
multiply the electrons for the anodes to detect and thus increase the sensitivity of the overall
PMT [7]. Finally to improve the degradation rate of some PMTs, researchers have increased the
number of microchannel plates (MCPs) used. By using three MCPs, the average degradation
caused by excess ions was reduced from 35% (with 2 MCPs) to 4% [8].
Implementation of Photomultiplier Tubes
PMT devices are made of a vacuum tube which contains a photocathode with several
dynodes and an anode. Photons strike the photocathode, a window like material which causes
electrons to be produced and emitted by the photoelectric effect. From there the electrons are
focused by a series of dynodes which make use of secondary emission to multiply the amount of
electrons towards the anode, finally detecting the arrival of a photon at the photocathode [9].
These simple tubes can range anywhere from $790 for just the tubes [5] to over $2000 for more
elaborate enclosures [4].
PMTs generally make use of 1000 volts to 2000 volts of power in order to accelerate the
electrons through the array of dynodes, each at a higher positive voltage than the previous. The
overall current generated is amplified greatly by more than 100 dB. Because of their properties,
photomultiplier tubes must be shielded from ambient light to prevent the device from being
damaged by oversaturation. Since this shield is kept at high voltage, insulation is also required in
order to protect the user. Finally the photocathode itself can be made of different materials
depending on the type of light being detected. Light with wavelengths up to 200 nanometers
utilize Cs-I while InGaAs(Cs) can be used for wavelengths between 300 and 1040 nanometers.
References
[1 L. Hum, Ping-Shine Shaw, Zhigang Li, K.R. Lykke, and M.L. Bishop, "Ultraviolet degradation study of
] photomultiplier tubes at SURF III," in Chemical, Biological, Radiological, Nuclear, and Explosives
(CBRNE) Sensing X, Orlando, 2009, p. 730410 (8 pp.).
[2 SensL. (2009) SensL. [Online]. HYPERLINK "http://sensl.com/products/silicon-photomultipliers/"
] http://sensl.com/products/silicon-photomultipliers/
[3 SensL, "SPMMini High Gain APD," SPMMini datasheet 2009.
]
[4 ThorLabs. (2009) ThorLabs. [Online]. HYPERLINK
] "http://www.thorlabs.com/Navigation.cfm?Guide_ID=36"
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[5 Inc. Edmund Optics. (2009) Edmund Optics Worldwide. [Online]. HYPERLINK
] "http://www.edmundoptics.com/" http://www.edmundoptics.com/
[6 Charels M. Tomasetti and Scott C. Moulzolf, "Photomultiplier Tube with Improved Light Collection,"
] Hardware 7,141,926, November 28, 2006.
[7 Suenori Kimura, Takayuki Ohmura, Teruhiko Yamaguchi, and Masuo Ito, "Multi-anode type
] photomultiplier tube," Hardware 7,489,077, February 10, 2009.
[8 A.Yu. Barnyakov et al., "Photomultiplier tubes with three MCPs," Nuclear Instruments & Methods in
] Physics Research, Section A (Accelerators, Spectrometers, Detectors and Associated Equipment), vol.
598, no. 1, pp. 160-162, January 2009.
[9 Hamamatsu. (2006, February) Photomultiplier Tubes: Basics and Applications. [Online]. HYPERLINK
] "http://sales.hamamatsu.com/assets/applications/ETD/pmt_handbook/pmt_handbook_complete.p
df"
http://sales.hamamatsu.com/assets/applications/ETD/pmt_handbook/pmt_handbook_complete.pdf