Optical fiber is drawn from a dual-head 3D printer fabricated preform made of two optically transparent plastics with a high-index core ($\mathrm{NA}\sim 0.25$, $V>60$). The asymmetry observed in the fiber arises from asymmetry in the 3D printing process. The highly multimode optical fiber has losses measured by cut-back as low as $\alpha \sim 0.44\mathrm{dB}/\mathrm{cm}$ in the near IR.

The temperature distribution within extrusion nozzles of three low-cost desktop 3D printers is characterized using fiber Bragg gratings (FBGs) to assess their compatibility as micro-furnaces for optical fiber and taper production. These profiles show remarkably consistent distributions suitable for direct drawing of optical fiber. As proof of principle, coreless optical fibers ($\phi =30\mathrm{\mu m}$) made from fluorinated acrylonitrile butadiene styrene (ABS) and polyethylene terephthalate glycol (PETG) are drawn. Cutback measurements demonstrate propagation losses as low as $\alpha =0.26\mathrm{dB}/\mathrm{cm}$, which are comparable with standard optical fiber losses with some room for improvement. This work points toward direct optical fiber manufacture of any material from 3D printers.

We demonstrate a novel compact optical magnetic field sensor composed of a single-mode microfiber sagnac loop interferometer (SMSLI) and ferrofluid (FF). The refractive index of FF cladding in the twist area of SMSLI is changed with the externally applied magnetic field, which makes the dip-wavelength of the SMSLI transmission spectra shift. The experimental results show that the transmission spectrum of the proposed sensor shifts regularly with the change of externally applied magnetic field intensity. When the magnetic field intensity increased from 0 to 600 Oe, the dip-wavelength of transmission spectrum shifted nearly 200 pm. In addition, a minimal detectable magnetic field strength of the sensor responding to the 50-Hz alternating magnetic field was demonstrated to be ~3 Oe experimentally.

The monitoring of acoustic waves is essential for a variety of applications, ranging from photoacoustic imaging to industrial non-destructive evaluation and monitoring. Optical acoustic sensors have many advantages over electrical counterparts such as passivity and immunity to electromagnetic interference, and can be quite compact with all-fiber structures. A common approach is to optically detect the acoustically induced mechanical movement of a cantilever or a reflective membrane. However, sensors based on moving mechanical parts have limitations, because they are influenced by the mechanical properties of the structures involved that behave as a coupled spring-mass system. Here a new type of optical acoustic sensing concept based on a spherical microcavity fiber Fizeau interferometer is described. The sensor is fabricated by inserting a section of single-mode fiber into a spherical microcavity to form a Fizeau interferometer. Thanks to the elasto-optic effect, the sound pressure modifies the refractive index of the microcavity; then a corresponding change in the interferometric spectrum can be observed. The sensor was successfully used to detect acoustic waves under the whole audible region, from 20 Hz to 20 kHz. The experimental results show that the sensitivity can be improved via altering the length of microcavity. The structure is membrane-free, and the sensitivity will not be limited by these size restrictions which makes the technology an interesting alternative to conventional transducers for acoustic wave measurement.

A novel dual parallel structured fiber-optic Fabry-Perot interferometers (FPIs) for ultrasensitive hydrogen ( ${{\text{H}}_{2}}$ ) sensing using the Vernier effect is proposed and demonstrated. Leveraging the high mechanical strength and chemical stability of hexagonal boron nitride (hBN), a nanoscale hydrogen-sensitive thin film of palladium (Pd)-decorated hBN is prepared and integrated with optical fiber to form a probe-type FPI hydrogen sensor. To further enhance the sensitivity, dual FPIs with similar but different cavity lengths are connected in parallel to exploit the Vernier effect, resulting in a significant amplification factor. The sensitivity of dual parallel structured FPIs based on the Vernier effect is $-$ 14.19 nm/%, which is 9.27 times higher than that of the individual FPI within the ${{\text{H}}_{2}}$ concentration range of 0 $ \sim$ 1%. Moreover, as the dual FPIs possess similar temperature coefficients, the Vernier effect reduces the impact of ambient temperature on the sensor, yielding a temperature coefficient as low as $ \sim$ 46 pm/ ${}^\circ \text{C}$ . Additionally, the sensor offers several advantages, including good repeatability, fast response time ( $ \sim$ 31.3 s), compact structure, easy preparation, and low cost, making it a practical solution for detecting trace amounts of hydrogen.