Narrow Linewidth Lasers in Biomedical Imaging and Diagnostics

Enabling High-Resolution Optical Coherence Tomography (OCT)

Narrow linewidth lasers are the cornerstone of modern Optical Coherence Tomography (OCT), a non-invasive imaging technique that provides cross-sectional views of biological tissues. The principle behind OCT is low-coherence interferometry, where the imaging depth resolution is inversely proportional to the spectral bandwidth of the light source. Here, a narrow linewidth translates to a longer coherence length, which is useful for specific applications. However, swept-source OCT (SS-OCT), a dominant high-speed modality, utilizes lasers that rapidly tune across a broad bandwidth while maintaining a narrow instantaneous linewidth. This combination enables micron-scale resolution at impressive imaging depths, allowing ophthalmologists to visualize individual retinal layers and cardiologists to assess coronary artery plaques with incredible clarity, all in real-time.

Revolutionizing Flowmetry and Microscopy

Beyond structural imaging, the coherence properties of narrow linewidth lasers are harnessed for functional diagnostics. In techniques like Laser Speckle Contrast Imaging (LSCI) and Doppler OCT, the interference of coherent light scattered by moving red blood cells generates measurable signals. A stable, narrow linewidth is critical for achieving high contrast and sensitivity in measuring blood flow dynamics, crucial for monitoring cerebral perfusion, skin graft viability, and microvascular diseases. In advanced microscopy, such as coherent Raman scattering (e.g., SRS, CARS) microscopy, two synchronized narrow linewidth lasers are used to excite specific molecular vibrations. The precision of these lasers allows for label-free, chemical-specific imaging of lipids, proteins, and drugs within living cells and tissues, opening new frontiers in metabolic research and histopathology.

Driving Precision in Spectroscopy and Sensing

Diagnostics often rely on detecting subtle biochemical signatures. Narrow linewidth lasers excel in absorption and fluorescence spectroscopy due to their precise targeting of molecular absorption lines with minimal spectral noise. This enables breath analyzers to detect trace biomarkers for diseases like asthma or cancer with high specificity. In flow cytometry, they provide brighter, more consistent fluorescence excitation, improving the detection of rare cell populations. Furthermore, emerging photoacoustic imaging techniques combine optical absorption and ultrasound detection, where tunable narrow linewidth lasers can scan across wavelengths to map the distribution of specific chromophores like hemoglobin or melanin deep within tissue, offering powerful diagnostic contrast without ionizing radiation.