By definition, a temporal photonic crystal (TPC) has a permittivity ε(t) that varies periodically with time. We prove that, in the long wavelength limit, a TPC is accurately mimicked by a dynamic transmission line (DTL) having a capacitance (inductance) per unit length equal to ε(t) (μ). Employing a DTL in the microwave region, we measured the photonic band structure, which results to display a genuine wave vector (k or β) gap, in very good agreement with our theoretical model and the equivalent TPC.

We present a novel concept of a modulated transmission line: a low-pass dynamic transmission line (DTL) whose capacitors are replaced by varactors that are externally modulated in tandem. The modulation voltages are periodic in time and identical in all the unit cells (and there is no pump wave). Accurate modeling leads to two bands, β ₁ (ω) and β ₂ (ω) for the propagation constant, separated by a gap Aβ. We have fabricated an eight-cell DTL and measured β _1,2 (ω) for a range of frequencies of the signal wave up to the modulation frequency f _M = 310 MHz, finding very good agreement between the experimental and theoretical results. These are also compared with an effective medium description, with the dynamic permittivity ε(t) being equal to the distributed capacitance C(t)/a at every instant t and the permeability μ equal to the distributed inductance L/a, where a is the size of the unit cell. There is good agreement for long wavelengths, βa ≲ 1, including the β-gap, which, for negligible resistive effects, is proportional to the modulation strength. Such gaps are characteristic of periodicity in time of a parameter and were predicted for “temporal photonic crystals.” We have also confirmed experimentally that for every frequency f , a harmonic of frequency f _M - f is excited, giving rise to beats when f ≅ f _M /2. We expect these experimental and theoretical results to open a new platform for useful applications.

This paper presents an axial strain laser sensing setup based on a core-offset Mach-Zehnder interferometer (MZI). The core-offset MZI was implemented by fusion splicing of three segments of single mode fiber and was used as wavelength selective filter and sensing head. The experimental results show that in the strain range from 0 to 333.333 με, the laser wavelength emission shifts linearly from 1554 to 1557 nm, achieving a strain sensitivity of 5.2332 pm µε⁻¹ and an R ² value of 0. 9988. The laser emission has a linewidth of 20 pm and a single-mode suppression ratio of 45 dB. Furthermore, the fiber laser offers output power stability at room temperature, compactness, and robustness.

A curvature sensing Ytterbium-doped fiber ring, using a Thin Core Fiber Modal Interferometer (TCFMI) was experimentally demonstrated in this work. The TCFMI was implemented by fusion splicing a segment of a thin-core fiber, with a small core diameter, between two single-mode fiber sections. The proposed TCFMI functioned as a wavelengthselective filter in the fiber ring laser cavity. It was optimized to achieve a Side Mode Suppression Ratio (SMSR) of 35 dB; the laser emission produced a sensor resolution of 0.37 nm. The experimental results also showed a high curvature sensitivity close to -38.26 nm/m-1 from 0 m-1 to 0.399 m-1 curvature range. Here, the laser sensor exhibited a bandwidth of 16 nm in the 1072 nm to 1056 nm wavelength range. The fiber ring laser is a lowcost alternative for curvature sensing applications.

We propose an all-fiber-optic multimode interferometer for measuring liquid refractive index. The interferometer was manufactured by using arc splice technique between standard single mode fiber and optical microfiber. A maximum refractive index sensitivity of 112dB/RIU is experimentally demonstrated. In addition, thermal effects exhibit low phase modulation (12pm/°C) with minimal intensity variations (-10.7*10-3dB/°C), as a result, cross-sensitivity around 0.05*10-6 RIU/ °C and 0.9 *10-6 RIU/ °C are obtained for intensity and phase modulation. The stability analysis shows minimal wavelength and power variations, 0.01nm and 0.27dB respectively, for constant temperature and refractive index. Spatial frequency analysis was explored as an alternative method to measure liquid refractive index avoiding thermal effects.