Advantage of OFDR/Wavelength Swept Light Source
About OFDR
Optical Frequency Domain Reflectometry (OFDR) is an optical measurement method using the coherence of laser light.
OFDR can perform remote, non-contact, and high-precision measurement in various fields.
Wavelength Swept Light Source for OFDR
Our company has developed a wavelength swept light source for OFDR. To learn more, refer to our Operation Principle
and OFDR Measurement Application Technical Note.
>
wavelength swept light source Technical Note Link
A suitable wavelength swept light source for OFDR should be selected according to the measurement objectives. When
performing OFDR, think about "how we want to measure the object" and "what should we focus on at measurement," and
consider the performance of a suitable wavelength sweep light source.
| Requirements for OFDR |
Required Wavelength Swept Light Source Performance |
| Higher distance resolution |
Wider wavelength sweep range |
| Faster measurement time |
Faster sweep repetition period |
| Wider measurement range |
Longer coherence length |
Principles of OFDR
The figure below shows the OFDR measurement block diagram.
The light output from the wavelength swept light source is split by an optical coupler into the reference optical
path LR and the measurement optical path LM. The light propagating in the measurement optical path LM is reflected
from the object under test back through the optical circulator and lens and is then combined with the reference
optical path LR. Since the wavelength is swept continuously, the wavelengths of the light propagating in the reference
optical path LR and the measurement optical path LM are different. Therefore, the combined interference light
corresponds to the difference in the lengths of the reference optical path LR and the measurement optical path LM. The
photodetector converts the light to an electrical measurement signal (fBeat) indicated by the red waveform in the
figure above.
When the difference in the optical path lengths is small (short distance to measured object), the wavelength
difference of the light propagating in each optical path is small, and the measured signal is like that in figure (a)
below. Conversely, when the difference in the optical path lengths is large (long distance to measured object), the
wavelength difference is large, and the measured signal is like that in (b).
In other words, the distance to the measured object is converted to the measurement-signal frequency and is obtained
by counting the number of waves in the measurement signal. The figure below shows the result of applying FFT analysis
to the measurement signal; the closer the measured object, the lower the frequency, and conversely, the farther the
distance, the higher the frequency.
Comparison of OFDR and Various Spatial Measurement Methods
The following figure shows the relationship between length measurement range and resolution for various spatial
measurement methods. OFDR using Anritsu's wavelength swept light source provides good resolution over a wide
measurement range. Refer to the SLD Light Source and Wavelength Swept Light Source website for details about OCT
(Optical Coherence Tomography) in the figure.
FM-CW: Frequency Modulated Continuous Wave Radar
TOF: Time-Of-Flight
OCT: Optical Coherence
Tomography
OFDR: Optical Frequency Domain Reflectometry
