The optical analog of the Vernier effect applied to fiber interferometers is a recent tool to enhance the sensitivity and resolution of optical fiber sensors. This effect relies on the overlap between the signals of two interferometers with slightly detuned interference frequencies. The Vernier envelope modulation generated at the output spectrum presents magnified sensing capabilities (i.e., magnified wavelength shift) compared to that of the individual sensing interferometers that constitute the system, leading to a new generation of highly sensitive fiber sensing devices. This review analyses the recent advances and developments of the optical Vernier effect from a fiber sensing point‐of‐view. Initially, the fundamentals of the effect are introduced, followed by an extensive review on the state‐of‐the‐art, presenting all the different configurations and types of fiber sensing interferometers used to introduce the optical Vernier effect. This paper also includes an overview of the complex case of enhanced Vernier effect and the introduction of harmonics to the effect.

The use of sensors in the real world is on the rise, providing information on medical diagnostics for healthcare and improving quality of life. Optical fiber sensors, as a result of their unique properties (small dimensions, capability of multiplexing, chemical inertness, and immunity to electromagnetic fields) have found wide applications, ranging from structural health monitoring to biomedical and point-of-care instrumentation. Furthermore, these sensors usually have good linearity, rapid response for real-time monitoring, and high sensitivity to external perturbations. Optical fiber sensors, thus, present several features that make them extremely attractive for a wide variety of applications, especially biomedical applications. This paper reviews achievements in the area of temperature optical fiber sensors, different configurations of the sensors reported over the last five years, and application of this technology in biomedical applications.

In the coming years, in vivo biomedical and biomechanical applications will benefit from a wide range of fiber-optic sensor (FOS) turnkey systems for sensing and measuring almost any physical quantity. These systems have four basic components: the light source, the optical fiber (OF), the sensor element, and the light detector. The light source provides the electromagnetic radiation whose energy is transmitted through the OF to the sensor element, in general, under the principle of total internal reflection. The FOS or transducer is the light modulator, i.e., the entity that causes a light property to change (e.g., amplitude or optical power, phase, polarization, and wavelength or optical frequency) under the influence of a certain physical quantity. Thus a physical quantity (e.g., pressure) can change the physical properties of the sensing element, which, in turn, leads to a change in the light properties. The light detector is necessary to read and analyze a light property variation. Since the four light properties can be considered in most circumstances independent parameters, they offer a wide range of solutions to sense several physical quantities.

The optical Vernier effect magnifies the sensing capabilities of an interferometer, allowing for unprecedented sensitivities and resolutions to be achieved. Just like a caliper uses two different scales to achieve higher resolution measurements, the optical Vernier effect is based on the overlap in the responses of two interferometers with slightly detuned interference signals. Here, we present a novel approach in detail, which introduces optical harmonics to the Vernier effect through Fabry–Perot interferometers, where the two interferometers can have very different frequencies in the interferometric pattern. We demonstrate not only a considerable enhancement compared to current methods, but also better control of the sensitivity magnification factor, which scales up with the order of the harmonics, allowing us to surpass the limits of the conventional Vernier effect as used today. In addition, this novel concept opens also new ways of dimensioning the sensing structures, together with improved fabrication tolerances.