Terahertz (THz) quantum cascade lasers (QCLs) are compact sources of radiation in the 1–5 THz range with significant
potential for applications in sensing and imaging. Laser feedback interferometry (LFI) with THz QCLs is a technique
utilizing the sensitivity of the QCL to the radiation reflected back into the laser cavity from an external target. We will
discuss modelling techniques and explore the applications of LFI in biological tissue imaging and will show that the
confocal nature of the QCL in LFI systems, with their innate capacity for depth sectioning, makes them suitable for skin
diagnostics with the well-known advantages of more conventional confocal microscopes. A demonstration of
discrimination of neoplasia from healthy tissue using a THz, LFI-based system in the context of melanoma is presented
using a transgenic mouse model.
We propose a compact, self-aligned, low-cost, and versatile infrared diffuse-reflectance laser imaging system using a laser feedback interferometry technique with possible applications in in vivo biological tissue imaging and skin cancer detection. We examine the proposed technique experimentally using a three-layer agar skin phantom. A cylindrical region with a scattering rate lower than that of the surrounding normal tissue was used as a model for a non-melanoma skin tumour. The same structure was implemented in a Monte Carlo computational model. The experimental results agree well with the Monte Carlo simulations validating the theoretical basis of the technique. Results prove the applicability of the proposed technique for biological tissue imaging, with the capability of depth sectioning and a penetration depth of well over 1.2 mm into the skin phantom.
In this paper, we introduce the self-mixing phenomenon in terahertz quantum cascade lasers (THz QCLs) and present
recent advancements in the development of coherent THz imaging and sensing systems that exploit the self-mixing effect.
We describe an imaging method which utilises the interferometric nature of optical feedback in a THz QCL to employ it
as a homodyning transceiver. This results in a highly sensitive and compact scheme. Due to the inherently low penetration
depth of THz radiation in hydrated biological tissue, imaging of superficial skin is an ideal application for this technique.
We present results for imaging of excised skin tissue, showing high-contrast between different tissue types and pathologies.
The conventional self-mixing sensing systems employ a detection scheme utilizing the photocurrent from an integrated photodiode. This work reports on an alternative way of implementing a Vertical-Cavity Surface-Emitting Laser (VCSEL) based self-mixing sensor using the laser junction voltage as the source of the self-mixing signal. We show that the same information can be obtained with only minor changes to the extraction circuitry leading to potential cost saving
with reductions in component costs and complexity. The theoretical linkage between voltage and photocurrent within the self-mixing model is presented. Experiments using both photo current and voltage detection were carried out and the results obtained show good agreement with the theory. Similar error trends for both detection regimes were observed.