Electro-Optics Technology, Inc.
3340 Parkland Ct. Traverse City, MI 49686 USA
(231)935-4044 ׀ (800)697-6782 ׀ sales@eotech.com ׀ www.eotech.com
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EOT reserves the right to inspect photodetectors returned under warranty to assess if the problem was
caused by a manufacturer defect. If EOT determines the problem is not due to a manufacturer defect,
repairs will be done at the customer’s expense. EOT will always provide a written quote for repair prior
to performing repairs at the customer’s expense.
VIII. Glossary of Terms
Amplifier: Provides a transimpedance gain throughout the photodiode’s bandwidth. The photodiode
current is converted to an output voltage.
Bandwidth: The range of frequencies from 0Hz (DC) to the frequency at which the amplitude decreases
by 3dB. Bandwidth and rise time can be approximately related by the equation:
Bandwidth ≈ 0.35/rise time for a Gaussian pulse input.
Bias Voltage: The photodiode’s junction capacitance can be modified by applying a reverse voltage.
The bias voltage reduces the junction capacitance, which causes the photodiode to have a faster
response.
Conversion Gain: The relative level of the optical input power that is amplified and converted into a
voltage output.
DC Block Capacitor: Prevents the DC voltage that is supplied through the amplifier output from
exiting the detector which would cause a large DC offset voltage. Therefore, the amplified detector is an
AC coupled device and will have a low cut-off frequency as well as a high cut-off frequency.
Decoupling Capacitor: Maintains bias voltage when fast pulses cause the battery voltage to reduce (this
would slow the response time of the photodiode); the capacitor allows the battery to recover to its initial
voltage. It also acts as a low-pass filter for external power supplies.
Noise Equivalent Power (NEP): A function of responsivity and dark current and is the minimum
optical power needed for an output signal to noise ratio of 1. Dark current is the current that flows
through a reverse biased photodiode even when light is not present, and is typically on the order of nA.
Shot noise (Ishot) is a source of noise generated in part by dark current; in the case of reversed biased
diodes it is the dominant contributor.
Photodiode: Converts photons into a photocurrent.
Resistor: Part of the low-pass filter at the photodiode cathode.
Responsivity: In amps per watt (A/W), responsivity is the current output of the photodiode for a given
input power, and is determined by the diode structure. Responsivity varies with wavelength and diode
material.
Rise time/Fall time: Rise Time is the time taken by a signal to change from a specified low value to a
specified high value. Fall Time is the time taken for the amplitude of a pulse to decrease from a
specified value to another specified value. A larger junction capacitance will slow the detector’s
response time.
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