Photonic crystal fibers (PCFs) have been extensively used for optical communications and sensing applications for the last two decades. In contrast to the conventional optical fibers that rely on differing doping levels in their core and cladding, PCFs guide light through subtle refractive index variations.
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This is achieved by confining light within a microscopic and periodically arranged set of air holes. This unique characteristic results in a wavelength-dependent cladding index, emphasizing the wavelength-scale periodic microstructure along the PCFs' length. They are, therefore, often referred to as micro-structured fibers.
Which PCF Configuration Is Preferred for High-Speed Optical Systems?
Single-mode and multimode optical fibers are the most popular types of optical fibers that are used as per the application requirements. Single-mode PCF fibers generally exhibit a relatively low dispersion coefficient. A primary limitation of the single-mode fibers is the occurrence of chromatic dispersion. In contrast, multimode fibers face challenges from waveguide, material, and chromatic dispersion. The choice of optimum configuration and type of fiber can ultimately lead to much better light and signal transmission.
A study published in the Journal of Optical Communication has outlined different configurations of PCFs as essential solutions for optimizing data rate transmission through light signals. The proposed fibers—hexagonal photonic crystal fiber (HPCF), octagonal photonic crystal fiber (OPCF), and elliptical photonic crystal fiber (E-PCF)—were utilized in the optical system during the experiment.
The researchers examined key parameters such as the dispersion coefficient, variations in pulse broadening, and data rate transmission for these fibers. This analysis was conducted under similar fiber lengths and a consistent number of quantization levels, employing pulse code modulation (PCM).
The relationship between the total dispersion parameter factor coefficient and variations in the number of quantization levels revealed that OPCF exhibited superior performance compared to HPCF and EPCF. The use of OPCF reduced the dispersion coefficient compared to other PCFs, showcasing its effectiveness. OPCF also outperformed in terms of variations in data rates or bit rate transmission.
OPCF demonstrated the best performance among the proposed PCFs, contributing to the enhancement of optical fiber system performance. In specific instances, data rates reached approximately 320 Gb/s for OPCF with a transmission propagation range of 1 km. Bit rate transmission also reached around 20 Gb/s for OPCF with a transmission propagation range of 10 km. Ultimately, the researchers recommended OPCF for high-speed light transmission.
A Study of the Microstructure of PCF
PCF optical fibers play a pivotal role in long-distance telecommunications, serving as a crucial element for the seamless integration of the Internet of Things (IoT) and the packet transmission requirements of the Big Data era. Photonic crystal fiber-based sensors have gained widespread use in scientific instrumentation and various industries. To comprehend their functionality, it is essential to explore the microstructure deformation of a photonic crystal fiber and the subsequent modifications in its transmitting properties.
A recent article published in Silicon presents the findings of a study focused on studying the microstructure of a hexagonal PCF using a localized compression test. A 5 cm-long photonic crystal fiber was employed to transmit a steady-intensity light signal generated by an infrared LED. A press was utilized to manage the direction of localized compression.
The photonic crystal fiber, complete with coating and jacket, underwent quasi-static compression at various angles using the press. At each angle, the relative intensity of the transmitted signal was measured. This measurement represents the ratio between the signal's intensity with the fiber experiencing localized compression and the signal's intensity when the fiber is uncompressed.
To understand the relationship between the direction of localized compression and the corresponding changes in relative intensity of the transmitted signal within the fiber, graphs were generated, depicting the relative intensity against the angular position of the press.
Each curve, linked to a specific compression force, exhibited a discernible pattern of local maxima and minima. Higher compression forces yielded more pronounced alterations in the relative intensity. Experimental findings affirmed that the relative intensity displayed a periodic behavior concerning the angular position of the press.
Improvement in Performance of PCF Using Polarization-Dependent Optical Fillers
Many existing optical fiber components are bigger, hindering the progress of innovative, compact, in-fiber optical devices. Thus, it is necessary to incorporate new materials and nanostructures into fiber components to improve processing and transmission capabilities, introduce novel functionalities, and enhance compactness. Metasurfaces, comprising arrays of subwavelength elements designed to control the phase and amplitude of transmitted, reflected, and scattered light, offer distinctive methods for advanced light manipulation.
In a recent article published in Nanophotonics, researchers conducted experimental demonstrations of ultra-compact in-fiber polarization-dependent optical filters at the end face of polarization-maintaining photonic crystal fibers (PM-PCFs). The metasurface-optical fiber filter consisted of periodic negative cross-typed metallic nanostructures with orthogonal slits.
The observed transmission showed high polarization and wavelength dependence, achieving a transmission efficiency of approximately 70 % in the telecommunication wavelength. This effect was achieved by introducing light into two orthogonal linear polarization states of the fiber. The operational wavelength of the metasurface filter could be extensively controlled by nano-engineering the metasurface's geometry.
This research introduces a new approach to developing nanoscale in-fiber devices, including in-fiber polarization- and wavelength-dependent filters, polarizers, and metalens. These devices hold promise for emerging applications in optical fiber imaging and sensing.
How Is 3D Printing Used for New Types of PCFs?
PCFs, often referred to as holey fibers, are optical fibers consisting of a single material. These fibers incorporate an array of microscopic longitudinal hollow channels that facilitate the guidance of light. Optical waveguides, designed based on PCF structures, can be used as small-scale components as the foundation for constructing complex miniaturized devices that perform advanced photonic operations.
Precise control over the longitudinal variation of the PCF geometry opens avenues for developing innovative optical devices. Employing 3D printing to create centimeter-scale PCFs proves to be an effective approach, enhancing design flexibility.
A paper published in Optica demonstrated the application of high-resolution 3D printing for in-situ single-step fabrication of stacked ultrashort photonic crystal fiber (PCF)-like segments with diverse geometries. This innovation enabled the creation of all-fiber integrated devices capable of performing intricate optical operations within sub-mm lengths.
This novel approach completely circumvented the conventional drawing process, which is fraught with limitations and drawbacks. Instead, it offered unparalleled design flexibility and precision in managing transverse and longitudinal PCF geometry.
This marks the pioneering case study of fabricating miniaturized complex structures composed of stacked segments (featuring PCF designs) or rapid longitudinal tapers and precisely controlled lateral offsets.
More from AZoOptics: Properties and Applications of Photonic Crystals
References and Further Reading
Amiri, I. et. al. (2023). Different Photonic Crystal Fibers Configurations with the Key Solutions for the Optimization of Data Rates Transmission. Journal of Optical Communications. doi.org/10.1515/joc-2019-0100
Sánchez, A. et al. (2023). Study on the Microstructure of a Photonic Crystal Fiber using the Elasto-Optical Effect. Silicon. doi.org/10.1007/s12633-023-02472-w
Ghimire, I. et al. (2022). Polarization-dependent photonic crystal fiber optical filters enabled by asymmetric metasurfaces. Nanophotonics. doi.org/10.1515/nanoph-2022-0001
Bertoncini, A., & Liberale, C. (2020). 3D printed waveguides based on photonic crystal fiber designs for complex fiber-end photonic devices. Optica. doi.org/10.1364/OPTICA.397281
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