Zhuozhao Luo

We present a polarization-maintaining photonic crystal fiber (PM-PCF), featuring a hybrid lattice structure that simultaneously achieves broadband single-mode guidance and enhanced birefringence through coordinated geometric and cross-optical-mode modulation. The fiber structure incorporates a hybrid triangular air-hole arrangement and a slightly elliptical core, effectively enhancing birefringence while enabling endlessly single-mode operation. Experimental results reveal a group birefringence of 5.6 × 10 ⁻⁵ at 1550 nm, the suppression of polarization mode coupling with an h -parameter below 1.2 × 10 ⁻⁴ m ⁻¹ , and broadband single-mode operation across 600 to 1600 nm. This fiber combines high PM capability with broadband single-mode performance, offering potential applications in optical communication, high-power laser systems, and fiber optic sensing.

We demonstrate that by using a cascaded configuration of five solid-core photonic crystal fiber (PCF) samples with progressively decreasing core diameters, ultraviolet light with wavelengths as short as ∼300 nm can be generated in a supercontinuum (SC) set-up. With a nanosecond laser as the pump light, the modulation instability effect leads to the generation of multiple optical solitons in the first PCF sample which has a close-to-zero dispersion value at the pump wavelength (∼1064 nm). The following PCF samples with decreasing core diameters enhance the waveguide nonlinearity, and at the same time provide continuously-shorter phase-matching wavelengths for dispersive-wave emission, thereby pushing the short-wavelength edge of the SC spectrum into the deep ultraviolet spectral region. While the PCF splicing technique ensures the compactness of this SC set-up, the generated SC spectrum, spanning ∼350 nm to ∼2000 nm with a flat spectral profile, may be applied in fluorescence microscopy and biochemical imaging.

Maintaining high fundamental mode purity in low‐loss single‐ring hollow‐core photonic crystal fiber remains a key challenge. Here a double‐clad anti‐resonant‐reflecting design is reported that, through modal filtering, provides high suppression of higher‐order modes while maintaining a low‐loss fundamental mode. The structure consists of a ring of six thin‐walled capillaries fused to the inner wall of a thicker‐walled capillary that is in turn fused at a single point to the silica outer cladding. Greater than 40 dB m⁻¹ suppression of higher order modes over a 6 nm wide window whose central wavelength can be tuned by adjusting the gas pressure is observed. The novel design paves the way to practical applications of such fibers in high‐quality laser beam delivery and low‐noise optical‐signal transmission.

Photonic crystal fibers (PCFs) with various micro to nano-scale features are of intensive interest for fiber sensing. Flexible fiber structures enable the selective control of fiber properties, such as transmission modes, dispersion tailoring, trigger of multiple nonlinear effects, and so on. In this paper, three pieces of few-mode PCFs (FM-PCFs) are designed and fabricated. Guided modes in each fiber are theoretically analyzed and experimentally observed through careful excitation. Later one of the PCFs (PCF1) is used as a fiber interferometer, based on the analyses of intermodal couplings/interferences of all PCFs. The intermodal interferences between linearly polarized (LP) core modes LP01/LP11 of PCF1 are experimentally investigated, using selective mode excitation technique. The fringe contrast and intensity of the interferences can be precisely manipulated by varying the relative energies of the LP01 and LP11 modes, as the basis for the intermodal interferometer. Different from most previous studies, the interferences are mainly limited to the two core modes, which intrinsically reduces the cross-influence from temperature variations, when this fiber is used for strain sensing. Experimentally, a strain sensor using this FM-PCF based Mach-Zehnder (MZ) interferometer, is demonstrated with a restrained temperature response. This work provides significant guidance for the construction of FM-PCF based fiber sensors with simple fabrication procedure and precisely controlled interference, and most critically, with reduced cross-sensitivity between temperature and strain.

A highly-sensitive vibration frequency and acceleration sensor is demonstrated, using a liquid-filled photonic crystal fiber. The designed fiber has a unique structure that a central air-bore of microscale was introduced during fiber fabrication, for the purpose of enhanced sensitivity to mechanical perturbations. The transmission properties of this novel fiber waveguide can therefore, be significantly altered when the fiber is filled with liquids, making it a sensitive carrier to vibrations. This sensor performs stably and reliably in measuring vibration frequencies of extended range from 10 Hz to 20 kHz, as well as the detection of dual-frequency signals, damped vibrations and acceleration measurements. Extremely weak vibrations, such as finger movement and human breathing can be monitored in real-time, as a crucial feature of this sensor. The vibration mechanism is further discussed in terms of enhanced sensitivity, when the fiber is filled with liquids. This sensor possesses advantageous properties such as compact structure and enhanced sensitivity, which show promising potentials in versatile vibration signal monitoring.

A Fabry-Perot interferometric sensor for temperature measurement was fabricated based on a silica glass solid-core photonic crystal fiber with a central air-bore. By splicing a stub of photonic crystal fiber to a standard single-mode fiber, an intrinsic Fabry-Perot cavity was formed inside the photonic crystal fiber. Sensing experiment results show that the sensor can work stably for a consecutive 24 h under temperatures up to 1100 °C, and the short-term operation temperature can reach as high as 1200 °C (<30 min). In the measurement range of 300–1200 °C, the temperature sensitivity of the peak wavelength shift can reach as high as 15.61 pm/°C, with a linearity of 99.76%. The presented interferometric sensor is compact in size and possesses advantages such as an extended working range and high sensitivity, showing promising application prospects.