Fiber optic interferometers are employed to measure different physical magnitudes such as temperature, strain, pressure or refractive index. In this post we will explain the basics of interferometry and the different types of existing interferometers.
The basic operation of an interferometer consists in splitting an incident light beam into two parts, the reference beam and the sensing beam, where the latter is affected by a variable of interest that we want to measure. Then, the two beams are recombined together to create an interference pattern that enables to recover information about the desired variable [1,2].
An odd number of half wavelengths for the optical path difference (OPD) travelled by the beams corresponds to a destructive point of the interference pattern whereas an even number of half wavelengths corresponds to a constructive point. These optical paths can correspond to a single optical fiber with two or more different optical fiber modes or to separate optical fibers [2]. The coherence length of the employed light source has to be larger than the OPD in order to produce the interference, so high coherence light sources, such as lasers, are commonly used in interferometry.
Different configurations have exploited optical fibers for the fabrication of interferometers as it is described in the next paragraphs:
- Michelson interferometer: it is one on the most common and simplest types of interferometers. It employs a single beam splitter (coupler) for separating and recombining the beams. In the conventional Michelson interferometer, the reference beam is reflected by a fixed mirror, while the sensing beam is reflected by a moving mirror. The interference pattern enables to recover the displacement of this mirror, as each interference fringe corresponds to a λ/2 displacement, where λ is the wavelength of the used light source [3].
- Fabry-Perot interferometer: it is an optical cavity made from two parallel mirrors. Depending on the position of the detector, the interferometer is based on either light transmission or reflection [2]. Other classification distinguishes between intrinsic Fabry-Perot interferometers, when the reflectors are within an optical fiber and extrinsic ones, when they are not [1]. The transmitted and reflected spectrums vary with the cavity length, the refractive index of the medium and the mirrors reflectivity. The variable of interest affects one of these parameters, therefore modifying the spectrum. A major application of these interferometers is associated to the creation of Fabry-Perot cavity lasers.
- Sagnac interferometer: here, the input beam is split into two counterpropagating beams with equal intensity that travel along a ring path. If the Sagnac interferometer is rotated, there is a relative phase shift between the beams, known as Sagnac effect. The interference spectrum depends on the angular frequency of the setup. This interferometer can be used for monitoring currents, acoustic waves, strain and temperature. The fiber-optic gyroscope (FOG, see our post) is based on the Sagnac effect [2].
- Mach-Zehnder interferometer: in this configuration the light beam is initially divided into two parts that travel respectively through the reference and the sensing arm, and they are then recombined. As opposed to the Michelson interferometer, the optical paths are only traversed once. The interferometer has two outputs, where two photodetectors are placed. The intensities measured by both detectors enable to measure the phase shift caused by the variation in the sensing arm induced by temperature, strain or refractive index changes, among others. This interferometer can be implemented in integrated optics, therefore protecting the device, reducing its size and enabling its use in other applications such as biosensing [4].
- Modal interferometer: it is based on the difference between the propagation speed of two different modes and it is employed in the case of photonic crystal fibers (PCFs, see our post) [2].
- Moiré interferometer: it is based on a fringe pattern that can be obtained by overlaying two or more gratings at different angles or by properly arranging several polarization maintaining fibers (PMFs) [2].
- White light interferometry: in general, the light sources that are employed in interferometry have a high coherence length, such as lasers. However, there are structures, such as nanocavities on the tip of an optical fiber (see our post) were broadband light sources (LEDs, SLEDs) can produce an interference pattern, in which is known as white light or broadband light source interferometry [2].
In conclusion, interferometers benefit from fiber optic inherent advantages, including their fast response to external or internal variations, their immunity to electromagnetic interferences, their low power consumption or the possibility of wavelength multiplexing [5]; in order to develop sensors that monitor position, pressure, vibration, temperature, strain, current, magnetic field, etc. in different disciplines including chemistry, medicine, geophysics or the automobile industry.
Written by J.J. Imas
Bibliography
[5] López-Higuera, J.M. Handbook of Optical Fibre Sensing Technology; Wiley, 2002