Enterprise AI Research Analysis

Numerical Modeling & Materials

The study employs the Finite Element Method (FEM) within the COMSOL 5.5 software environment to analyze the transmission characteristics of the proposed PCF SPR filter. The cross-section features a square-lattice design with three types of holes in the cladding. Key structural parameters include uniform square-lattice constant (Λ), cladding hole diameter (d₁), inner large hole diameter (d₂), elliptical hole long axis (a), short axis (b), and gold layer thickness (t).

To ensure calculation accuracy, a Perfect Matching Layer (PML) with a thickness of 2 µm is applied to absorb boundary scattering. The simulation assumes mixed polarized light incident vertically along the z-axis. The computational domain is discretized into multiple triangular elements, and iterative convergence curves confirm high accuracy.

The background material is silica glass, modeled by the Sellmeier equation, with parameters (A1-A3, B1-B3) provided and temperature restricted to room temperature (~20 °C). Gold is selected as the plasmonic material for exciting SPR, with deposition via chemical vapor deposition (CVD) into the inwalls of desired holes. Gold's permittivity is described by the Drude-Lorentz model, considering plasma frequency, damping frequency, and Lorentz oscillator parameters. The model operates under room temperature, assuming negligible temperature variation during SPR processes.

Performance metrics include Confinement Loss (α), which quantifies signal intensity loss, Normalized Output Power (NOP), evaluating relative changes in polarized signals, and Extinction Ratio (ER), a key metric for quantifying suppression of undesired polarization modes.

Dispersion & Resonance Dynamics

The proposed PCF SPR filter supports endless single-mode transmission within the 1.1–1.9 µm wavelength range. The study analyzed electric field distributions for various modes: x-pol and y-pol fundamental modes (1.1 µm), SPP1 mode (1.26 µm), SPP2 mode (1.5 µm), and two resonance modes (1.31 µm and 1.55 µm). These resonance modes indicate coupling between the core-guided x-polarized mode and the surface plasmon modes.

A critical finding is that the x-polarized mode interacts with two plasmonic modes, satisfying phase-matching conditions at 1.31 µm and 1.55 µm, thereby generating two distinct Surface Plasmon Resonance (SPR) peaks. This behavior is crucial for dual-window operation. In contrast, the y-polarized mode exhibits no SPR process within the investigated band.

At these resonance wavelengths, the confinement losses for the x-pol mode are significant: 103.0 dB/mm at 1.31 µm and 95.0 dB/mm at 1.55 µm. For the y-pol mode, confinement losses are much lower, only 0.2 dB/mm and 0.4 dB/mm, respectively. This results in high confinement loss ratios of 515 at 1.31 µm and 237.5 at 1.55 µm. This stark polarization-dependent difference in confinement loss forms the foundation for effective dual-window polarization filtering.

Structural Parameter Sensitivity

Fabrication errors can introduce slight deviations in PCF structural parameters, impacting filtering performance. The study analyzed these influences using the control variate method:

• Cladding hole diameter (d₁): Increasing d₁ gradually decreases the effective refractive index of the core mode, leading to a redshift in the phase-matching point. This affects both left-side (1.28-1.34 µm) and right-side (1.48-1.65 µm) resonance peak positions and confinement loss magnitudes.
• Pitch (Λ): PCF operating principles are highly sensitive to Λ. An increase in Λ causes a gradual redshift of the confinement loss curve, with minor shifts in peak wavelengths (e.g., 1.30-1.33 µm and 1.51-1.58 µm) and small changes in confinement loss values.
• Elliptical hole long axis (a): Increasing the long axis 'a' narrows the SPR channel, causing a slight reduction in the spacing between the two resonance peaks, bringing them closer together (e.g., left peak shifts 1.30-1.32 µm, right peak shifts 1.53-1.56 µm).
• Elliptical hole short axis (b): Variations in 'b' directly influence the core region size and reshape SPP modes, leading to a slightly greater shift in resonance peaks (e.g., left peak shifts 1.29-1.34 µm, right peak shifts 1.52-1.57 µm), bringing them closer.
• Gold layer thickness (t): The gold layer thickness (typically ~50 nm) is crucial for critical coupling and efficient energy transfer to plasmon waves. A slight blue shift is observed with increasing 't'. Reducing 't' (e.g., to 40 nm) enhances SPR excitation efficiency and resonance peak intensity (e.g., left peak: 116.4 dB/mm at 1.33 µm; right peak: 110.2 dB/mm at 1.56 µm).
• Plasmonic Material: While silver and copper can achieve similar polarization signal differences, gold exhibits superior chemical stability, making it a more suitable choice for long-term filter reliability, as silver and copper are prone to oxidation in direct contact with air.

Filtering Performance & System Integration

The length of the PCF is a critical factor influencing its filtering performance. The study analyzed lengths ranging from 0.5 mm to 3 mm. For all lengths, the x-polarized mode's Normalized Output Power (NOP) approaches 0, indicating strong attenuation, while the y-polarized mode's NOP remains near 1, signifying efficient transmission. As length increases, the y-pol NOP shows a slight downward trend.

The Extinction Ratio (ER) remarkably increases with PCF length. To balance compactness and filtering performance for dual communication windows, a length of 0.5 mm was selected. At this length, the filter achieves a maximum ER of -51.4 dB at 1.31 µm (O-band) and -47.3 dB at 1.55 µm (C-band). Furthermore, it delivers a broad operating bandwidth of >860 nm, covering the investigated range of 1.14–2 µm. This demonstrates simultaneous achievement of compact size, high extinction, and multi-wavelength selectivity.

The fabrication of such a device involves advanced techniques. The gold film can be deposited onto the microchannel's inner wall via Chemical Vapor Deposition (CVD) by introducing a precursor solution of gold ions and precisely controlling temperature and pressure. For system integration, the PCF is spliced to standard single-mode fibers (SMFs) to form an "SMF-PCF-SMF" structure using a high-precision optical fiber fusion splicer. The estimated splice losses are relatively low: ~2.22 dB at 1.31 µm and ~1.42 dB at 1.55 µm. Future strategies like multiple discharge or improved fusion techniques can further reduce these losses, enhancing coupling efficiency and mode field matching.