Time-domain optical coherence tomography at 2 $μ\mathrm{m}$ using GaSb-based broadband superluminescent diode

Technical Summary: Time-Domain Optical Coherence Tomography at 2 µm Using a GaSb-Based Broadband Superluminescent Diode

Problem Statement
While Optical Coherence Tomography (OCT) is well-established for biomedical imaging at 800 nm, 1060 nm, and 1300 nm, there is a growing need to extend these capabilities to non-biological materials, such as polymers, ceramics, and coatings. These materials often exhibit low water content and high scattering, where longer wavelengths (specifically the 2 µm region) offer advantages in penetration depth due to reduced scattering coefficients. However, current 2 µm OCT systems predominantly rely on supercontinuum (SC) sources. While SC sources offer broad bandwidths, they are often complex, bulky, expensive, and noisy, limiting their practicality for compact, scalable industrial applications. There is a lack of compact, electrically driven semiconductor light sources suitable for 2 µm time-domain OCT (TD-OCT) that can provide the necessary bandwidth and stability without the drawbacks of SC sources.

Methodology
The authors developed a fiber-based TD-OCT system operating in the 2 µm spectral region, utilizing a Gallium Antimonide (GaSb)-based Superluminescent Diode (SLD) as the light source.

• Light Source: The SLD features an active region with two InGaSb/GaSb quantum wells and a J-shaped ridge waveguide geometry. It was packaged in a standard 14-pin "butterfly" module with a single-mode fiber pigtail. The device was designed with a high-reflective (HR) coating on the straight facet and an anti-reflective (AR) coating on the curved facet to suppress cavity reflections and promote amplified spontaneous emission (ASE).
• System Configuration: The OCT setup employs a Michelson interferometer configuration using a 50:50 broadband fiber coupler. The system utilizes a balanced detection scheme with extended InGaAs PIN photodiodes to reduce common-mode noise. Scanning is achieved via XY galvo scanners in the sample arm and a translation stage for axial scanning.
• Characterization: The SLD's performance was characterized by measuring Light-Current (L-I) and Voltage-Current (V-I) curves, as well as emission spectra at various drive currents (80–400 mA) using an optical spectrum analyzer. Coherence measurements were performed by scanning the reference arm to determine the axial resolution.
• Imaging Experiments: The system was tested on non-biological samples, including a structured test object (a British five-pence coin) and a printed paper card ("UoK" text) covered with a thin layer of scattering paint.

Key Results

• Source Performance: The GaSb SLD demonstrated stable ASE operation at drive currents below 150 mA, maintaining a spectral ripple of less than 20%. At 150 mA, the source emitted approximately 2.5 mW of power with a Full-Width Half-Maximum (FWHM) bandwidth of ~80 nm centered near 2.1 µm. At higher currents (400 mA), the bandwidth increased to ~83 nm, but spectral modulation rose to ~50%, and the device approached lasing thresholds.
• Coherence and Resolution: The measured coherence envelope yielded an axial resolution of approximately 300 µm in air. This experimental resolution was lower than the theoretical limit (~23 µm) predicted by the source's optical bandwidth alone. The authors attribute this discrepancy to spectral modulation (ripple) and residual cavity effects within the SLD structure, which broaden the coherence envelope.
• Imaging Capability: The system successfully performed depth-resolved imaging of scattering paint-based coatings. It demonstrated the ability to visualize buried features, such as printed text beneath a paint layer, by leveraging coherence gating. The 2 µm wavelength facilitated penetration through the paint coating, revealing the interface between the paint and the substrate, as well as variations in paint thickness.

Key Contributions

• First Demonstration: This work represents the first demonstration of a TD-OCT system operating at 2 µm that utilizes a GaSb-based SLD.
• Compact Alternative: The study establishes the GaSb SLD as a viable, compact, and scalable alternative to supercontinuum sources for mid-infrared OCT. The source leverages mature semiconductor fabrication and standard packaging (butterfly modules), offering a pathway toward cost-effective and simplified OCT instrumentation.
• Non-Biological Imaging: The paper validates the utility of 2 µm OCT for imaging low-water-content, scattering materials, specifically demonstrating the capability to inspect layered coatings and subsurface structures in non-biological samples.

Significance and Claims
The authors claim that this work establishes the suitability of GaSb-based SLDs for compact mid-IR OCT instrumentation targeting non-biological materials. While the current axial resolution is limited by spectral ripple and packaging losses, the system successfully demonstrates the fundamental principles of coherence gating at 2 µm. The authors position this technology as a practical solution for non-destructive testing and materials diagnostics where the complexity and cost of supercontinuum sources are prohibitive. They note that further improvements in spectral smoothness (via optimized coatings) and the use of mid-IR fibers could enhance performance, but the current results already confirm the viability of this approach for industrial and scientific imaging scenarios.