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Optical Device Guide
Optical Device Guide

Benefits of Using OFDR /Swept Light Sources


What Is OFDR?

OFDR (Optical Frequency Domain Reflectometry) is an optical sensing method that utilizes the coherence of laser light. In recent years, it has been applied in various fields because it enables non-contact high-precision measurement of the distance from the position where laser light is emitted to an object to be measured.


Swept Light Sources Used in OFDR

Anritsu develops swept light sources that are optimal for OFDR. This page describes the principle and features of OFDR using swept light sources.

Technical Note; High-Coherence Wavelength Swept Light Source >


It is necessary to select the appropriate swept light source to use for OFDR according to the object to measure. When conducting OFDR, it is important to consider how the object will be measured, and what to focus on when measuring, and study the performance of the suitable swept light source.

Requirements for OFDR Performance Required for Swept Light Source
Increasing the resolution of distance Widening the wavelength sweep range
Speeding up the measurement time Speeding up the sweep repetition period
Widening the measurement range Increasing the coherence length
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Principle of OFDR

The figure below depicts a measurement block chart of the OFDR optical measurement method.

A measurement block chart of the OFDR optical measurement method

The light emitted from the swept light source branches to reference optical path LR and measurement optical path LM via an optical coupler. The light propagating measurement optical path LM strikes the object to measure via an optical circulator and a lens, is reflected, and then couples with reference optical path LR via the lens and optical circulator again. Since the wavelength is swept, the wavelength of the light that propagates reference optical path LR differs from the wavelength of the light that propagates measurement optical path LM. Therefore, the coupled light becomes interference light according to the optical path length difference between reference optical path LR and measurement optical path LM, and becomes the measurement signal (f BEAT) indicated with the above red waveform when converted to an electrical signal at a photo receiver.

If the optical path length difference is small (which means that the distance to the object to measure is short), the wavelength difference of the light that propagates each optical path becomes smaller. The measurement signal is depicted schematically in (a) in the figure below. Conversely, if the optical path length difference is large (which means that the distance to the object to measure is long), the wavelength difference becomes larger. The measurement signal is depicted schematically in (b) in the figure below.

Each optical path and the measurement signal

This means that the distance to the object to measure can be replaced with the frequency of the measurement signal, and enables the distance to the object to measure to be obtained by counting the number of waves in the measurement signal. The figure below is the result of performing FFT analysis on the measurement signal. The frequency is lower when the distance to the object to measure is short. Conversely, the frequency is higher when the distance to the object to measure is long.

The distance to the object to measure and the frequency
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Comparison of OFDR and Various Spatial Measurement Methods

The graph below compares various spatial measurement methods. It indicates the relationship between the distance to measure (measurement range) and optical resolution in distance, and shows how OFDR using a swept light source of Anritsu can obtain a good resolution with a wide measurement range.

For details on the OCT (Optical Coherence Tomography) in the graph, see the page on SLD light sources and the page on swept light sources.

Comparison of OFDR and Various Spatial Measurement Methods


Anritsu SLD (Super Luminescent Diode) >
Aritsu Swept Light Sources >
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