FAQ about OFDR

What is the principle of the wavelength swept light source?

The wavelength swept light source is an external cavity light source with Littman-type configuration using a MEMS scanning mirror. It outputs the selected single mode by using a diffraction grating and mechanical rotating mirror. Refer to our wavelength swept light source technical note for details.

Does the oscillation wavelength of the wavelength swept light source vary sinusoidally and does the measured interference signal also become sinusoidal?

When a light source wavelength is swept linearly over time, the interference beat signal varies sinusoidally. However, since our wavelength swept light source sweeps sinusoidally, the interference beat signal involves nonlinearities. Therefore, it is almost sinusoidal locally, while the overall sweep shows a shape where the frequency of the interference beat signal varies in proportion to the sweep speed. After the linearize processing removes the nonlinearity, the shape of interference beat signal becomes sinusoidal with a constant frequency.

What is the maximum measurable frequency at vibration measurement?

For the AQA5500P/AQA55000D with a swept frequency of 1250 Hz, the maximum measurable frequency is half the frequency, or 625 Hz.

What is the minimum measurable distance and does the required measurement time change at this time?

The minimum measurable distance depends on the focal length of the collimator lens. Meanwhile, measurement time depends on the swept frequency of the wavelength swept light source, and the measurement time for the same light source remains constant over distance.

How is coherence length measured?

The output power from the wavelength swept light source is split into two branches by an optical coupler, one of which has a fixed optical path length, and the other is recombined by connecting an optical fiber to change the difference between the two optical path lengths. Then, the distance at which the interference signal output becomes –3 dB is measured.

Is there any effect from speckle noise?

Although the spatial beam system causes fluctuations in the reflection peak intensity of the OFDR spectrum, the effect on measurement accuracy is negligible compared to an all-fiber system.

Do the Nyquist frequency limit and the coherence length affect the measurable distance?

The Nyquist frequency corresponds to half the sampling frequency, which means the maximum frequency in the FFT. Therefore, not only the coherence length but also the Nyquist frequency affect measurable distance. Refer to the wavelength swept light source technical note for details.

How is the measurement resolution defined?

The measurement resolution of FFT depends on the wavelength sweep width of the swept light source. The relationship between measurement resolution and wavelength sweep width is expressed by the following equation.
  Δz=λc^2 / (2Δλ)
where Δz is resolution, λc is sweep center wavelength, Δλ is wavelength sweep width.

Is it possible to measure transparent materials, such as glass?

An OFDR measures distance by capturing light reflected from the target. Even a transparent materials such as glass, if reflected light from the surface can be obtained, it is possible to measure the surface profile.