Thorlabs GIF50C GRIN Fiber: Advanced Photonics

• Thorlabs GIF50C GRIN fiber is a graded-index multimode fiber with a 50 μm core and a quadratic refractive index profile that enables self-imaging and controlled nonlinear dynamics.
• Its design supports invariant self-imaging, efficient Kerr self-cleaning, polarization control, and spatiotemporal soliton formation, yielding high-brightness and stable output.
• Precise characterization of its gradient index constant underpins reliable applications in ultrafast microscopy, beam shaping, quantum photonics, and advanced imaging systems.

The Thorlabs GIF50C GRIN fiber is a commercially available graded-index multimode optical fiber widely utilized in advanced photonics applications. Its defining characteristic is a refractive index that varies quadratically in the radial direction, conferring robust self-imaging properties and enabling unique nonlinear and spatiotemporal dynamics across the telecommunication and near-infrared wavelengths. This fiber is a prototypical component in nonlinear optics, ultrafast fiber-based microscopy, beam shaping, polarization control, and quantum photonics, among other domains.

1. Physical Structure and Index Profile

The GIF50C fiber is structured with a 50 μm diameter core surrounded by a cladding, and features a numerical aperture (NA) of approximately 0.2. The core refractive index follows the canonical quadratic law: $n(r) = n_0 \left(1 - \frac{g^2}{2}\, r^2 \right)$ where $n_0$ is the maximum refractive index at the fiber axis, $r$ is the radial position, and $g$ is the gradient index constant.

Experimental measurements at 780 nm yield $$g = 0.0057~\mu\textrm{m}^{-1} \pm 0.0001~\mu\textrm{m}^{-1}$$ and at 1550 nm, $$g = 0.0055~\mu\textrm{m}^{-1} \pm 0.0001~\mu\textrm{m}^{-1}$$, demonstrating minimal wavelength dependence (Leonard et al., 17 Sep 2025). The parameter $g$ directly governs beam focusing dynamics, self-imaging period, and modal propagation constants within the fiber.

2. Self-Imaging and Propagation Dynamics

A fundamental property of GRIN fibers such as the GIF50C is spatial self-imaging, wherein a localized input field repeatedly revives along the propagation axis with a characteristic period determined by the core radius $\rho$ and the core-cladding refractive index contrast $\Delta$: $z_s = \frac{\pi \rho}{\sqrt{2\Delta}}$ The self-imaging period is invariant with input power, a result established analytically via moment methods and verified experimentally by side-scattering and fluorescence imaging techniques even in the presence of substantial Kerr nonlinearity (Hansson et al., 2020). However, the amplitude of beam-width oscillations—namely the periodic spatial focusing—shows strong intensity dependence, giving rise, in nonlinear regimes, to localized compression (self-focusing) or, at high enough powers, beam splitting and collapse phenomena.

These dynamics are underpinned by the interplay between diffraction, the parabolic index confinement, and intensity-induced self-phase modulation. The exact periodicity is a central parameter in the engineering of devices such as saturable absorbers, supercontinuum sources, and mode-locked fiber lasers.

3. Nonlinear Effects: Kerr Self-Cleaning and Soliton Attractor Formation

When subject to high peak powers (several kW), GIF50C-like GRIN fibers exhibit pronounced Kerr beam self-cleaning: an initial speckled multimode field nonlinearly evolves toward a robust bell-shaped profile dominated by the fundamental mode (Krupa et al., 2018). This process is accompanied by:

• An increase of the degree of linear polarization (DOLP) by up to a factor of 2.5 at the self-cleaning threshold (~2.5–3 kW).
• Nonlinear polarization rotation, with the polarization azimuth ψ varying by over 15° as power increases.

These polarization dynamics are captured via the Stokes parameter formalism: $$\textrm{DOLP} = \frac{\sqrt{S_1^2 + S_2^2}}{S_0} \,,\quad \tan(2\psi) = \frac{S_2}{S_1}$$

Importantly, self-cleaning enables the fiber to deliver a high-brightness, nearly single-mode output, critical for coherent beam delivery and nonlinear photonics.

Further, under ultrafast pulse propagation ($<$200 fs), robust spatiotemporal solitons can form and propagate over long distances. Experiments and simulations show an irreversible "self-cleaning" of initially multimode femtosecond solitons into singlemode attractors (LP01), governed by: $$N^2 = \frac{L_D}{L_{NL}},\qquad N = \sqrt{\frac{n_2 T_0 E_1}{\lambda |\beta_2 (\lambda)| w_e^2}}$$ where $N\to1$ at the attractor, $w_e$ is the effective beam waist, and $E_1$ is pulse energy (Zitelli et al., 2021). This dynamical attractor phenomenon supports long-range, stable, diffraction-limited propagation—even from initially highly multimodal excitations.

4. Photonic Applications and Imaging

Due to its modal and temporal control, the Thorlabs GIF50C is central to several advanced imaging methodologies:

Lensless Two-Photon Micro-Endoscopy

Lensless micro-endoscopy leverages the self-imaging and modal properties to achieve robust light focusing and scanning at the distal end, even through bent fibers, with all instrumentation confined to the proximal end. Spatial light modulators (SLM) are used to sculpt the input wavefront; optimization (often genetic-algorithm-based) maximizes a nonlinear two-photon fluorescence signal detected at the proximal facet (Rosen et al., 2015).

By exploiting the fiber's self-imaging and the nonlinear scaling of two-photon emission, the focus is controlled and localized at the distal end. The detected fluorescence pattern at the proximal end encodes the position of the focus, enabling scanning without direct distal access. The use of GIF50C fiber (core diameter 50 μm, NA 0.2) has demonstrated robust, bend-insensitive operation for lensless, high-resolution, depth-resolved biological imaging.

Ultra-Thin Rigid Endoscope

Point-scanning two-photon endoscopes have been constructed from a few centimeters of graded-index MMF (core diameter 50–62.5 μm, cladding diameter 125 μm). The transmission matrix $H$ is experimentally measured, permitting pre-compensation of the input mode phases and achievement of diffraction-limited focal spots for 3D imaging (Sivankutty et al., 2015). GRIN fiber’s minimized group velocity spread (e.g., 8.58 ps/m) ensures femtosecond-pulse preservation and thus high-resolution, optically sectioned images. The ultra-thin design—probe diameters as small as 125 μm—enables minimally invasive deep-tissue imaging.

5. Quantum and Nonlinear Optical Device Integration

The GIF50C supports high-efficiency device architectures that depend on precise beam shaping, focusing, and mode conversion:

Quantum Photonic Sources

Double-GRIN lens assemblies, based on GIF50C-like fibers, enable collection and mode-matching of single-photon emission from quantum dot nanowires. Mode overlap, beam waist, and numerical aperture can be precisely tailored (e.g., experimental NA ≈ 0.40, waist ≈ 0.80 μm at 950 nm), producing up to 35% source-fiber collection and 10% overall collection efficiency, and achieving single-photon purity $g^{(2)}(0)\approx0.015$ (Northeast et al., 2021). Permanent, room-temperature alignment leads to robust “plug and play” integration.

Supercontinuum and Nonlinear Frequency Conversion

In the context of geometric parametric instability (GPI), the ability to spatially and spectrally resolve sidebands at the output of a GRIN fiber supports efficient generation and amplification of narrowband sources, which can be subsequently broadened into the mid-infrared supercontinuum (1.7–3.4 μm) (Leventoux et al., 2021). The spatial-spectral mapping technique enables direct selection and amplification of Stokes sidebands, determined by the fiber’s self-imaging period and modal dispersion.

6. Metrology, Characterization, and Device Design

Precise knowledge of the GIF50C’s gradient index constant is essential for device engineering. The measurement methodology—based on Gaussian beam profiling using a custom-built profiler and modeling the fiber’s ABCD matrix—enables extraction of $g$ with $<2\%$ uncertainty (Leonard et al., 17 Sep 2025). The ABCD matrix for a GRIN section of length $l$ is: $$M_g = \begin{bmatrix} \cos(g l) & \frac{1}{g}\sin(g l) \ -g\sin(g l) & \cos(g l) \end{bmatrix}$$ These parameterizations inform the design of GRIN-based fiber components (e.g., lenses, Fabry–Perot cavities, beam shapers), ensuring accurate modeling of beam waist evolution, coupling efficiency, and periodic imaging characteristics. Consistency in measured $g$ across wavelength (780 nm: $0.0057~\mu\textrm{m}^{-1}$; 1550 nm: $0.0055~\mu\textrm{m}^{-1}$) and agreement with prior reports confirm the reproducibility and reliability of the GIF50C for high-precision photonic systems.

7. Summary and Implications

The Thorlabs GIF50C GRIN fiber exemplifies a high-quality, large-core, parabolic-index multimode optical fiber with well-characterized self-imaging, nonlinear, and modal dynamics. Key features include:

• Invariant self-imaging period with respect to input power
• Robust Kerr self-cleaning and polarization dynamics under high intensity
• Formation of spatiotemporal soliton attractors, enabling single-mode output after long propagation
• Suitability for advanced imaging, beam delivery, quantum fiber coupling, and nonlinear frequency conversion
• Precisely measured and nearly wavelength-independent gradient index constant, facilitating reliable device modeling

The GIF50C’s optical properties constitute the foundational physics for diverse applications in fiber-based photonics, providing a replicable standard for experimental and applied research in multimode nonlinear fiber optics.