Optical fiber sensors have been studied, developed, and already used in the industry for more than 50 years due to their multiplexing capabilities, lightweight design, compact form factors, and electromagnetic field immunity. The scientific community continuously studies new materials, schemes, and architectures aiming to improve existing technologies. Navigating through diverse sensor technologies, including interferometry, intensity variation, nonlinear effects, and grating-based sensors, fiber specklegram sensors (FSSs) emerge as promising alternatives due to their simplicity and low cost. This review paper, emphasizing the potential of FSSs, contributes insights to the present state and future prospects for FSSs, providing a holistic view of advancements propelling FSSs to new frontiers of innovation. Subsequent sections explore recent research, technological trends, and emerging applications, contributing to a deeper understanding of the intricacies shaping the future of FFS sensor technologies.

The averaged characteristics of the multimode fiber interferometer signals have been investigated, with the average being performed over ensemble, “long realization”, frequency, cross-section, and combination of these methods as well. The signals were produced at two wavelengths in few- and multi-mode regimes in the graded-index fiber. A practical example of the cross-section average is discussed in more details. The results would be useful for the multimode fiber interferometer utilization as a real-time high-accuracy sensor.

We investigated the gamma radiation response of fiber Bragg gratings (FBGs) inscribed in a few-mode polymer optical fiber. The fiber had a graded-index CYTOP core of 20 µm and XYLEX overclad of 250 µm in diameter. Four FBGs were exposed to gamma radiation during four irradiation sessions at a 5.3 kGy/h dose rate. The FBGs showed a linear Bragg wavelength shift with the received dose with a mean sensitivity of −3.95 pm/kGy at 43 °C. The increased temperature provides a rise in the sensitivity: it reached −10.6 pm/kGy at 58 °C. After irradiation, the FBGs showed partial recovery, which increased with the received dose. Furthermore, the FBG’s reflection power decreased with the dose. This attenuation is mainly due to insertion losses caused by the radiation induced attenuation in the CYTOP fiber. Linear response to the received dose makes CYTOP FBGs attractive for gamma radiation dosimetry. However, temperature dependence of the sensitivity should be compensated in practical applications.

We investigate the effect of γ-radiation on temperature (T) and relative humidity (RH) sensitivities of polymer perfluorinated fiber Bragg gratings (FBGs). To this aim, different γ-radiation doses (80, 120, 160, and 520 kGy) were applied to a set of FBGs. We show that irradiated FBGs demonstrate an RH sensitivity rise with the received dose: from 13.3 pm/%RH for a pristine FBG up to 56.8 pm/%RH for a 520-kGy dose at 30℃. In contrast, T sensitivity decreases with radiation dose with a subsequent change of sign from positive to negative. Therefore, by experimental interpolation, T sensitivity can be eliminated at around a 160-kGy dose. This opens the possibility of designing an RH sensor with enhanced sensitivity, which at the same time is insensitive to T.

An intermodal fiber interferometer with an optical frequency scanning light source is considered to attain two main goals: signal fading elimination and achieving a linear response to external fiber perturbations. It is demonstrated that the interferometric signal traces repeatedly generated by the laser frequency scans can be used to average the target signal by calculating their autocorrelation function. A correlation approach is studied for signal processing, and various correlation function modifications are proposed and theoretically analyzed to achieve complete signal reconstruction. In particular, the relation between the autocorrelation function and the averaged amplitude characteristic is demonstrated. The efficiency of the proposed interferometer’s scheme and the correlative signal processing are confirmed experimentally for the case of a sine-shaped fiber length modulation. We believe that the proposed method will be useful in real-time sensing applications.