Abstract
We analyze the physics behind the newest generation of rapidly wavelength tunable sources for optical coherence tomography (OCT), retaining a single longitudinal cavity mode during operation without repeated build up of lasing. In this context, we theoretically investigate the currently existing concepts of rapidly wavelength-swept lasers based on tuning of the cavity length or refractive index, leading to an altered optical path length inside the resonator. Specifically, we consider vertical-cavity surface-emitting lasers (VCSELs) with microelectromechanical system (MEMS) mirrors as well as Fourier domain mode-locked (FDML) and Vernier-tuned distributed Bragg reflector (VT-DBR) lasers. Based on heuristic arguments and exact analytical solutions of Maxwell's equations for a fundamental laser resonator model, we show that adiabatic wavelength tuning is achieved, i.e., hopping between cavity modes associated with a repeated build up of lasing is avoided, and the photon number is conserved. As a consequence, no fundamental limit exists for the wavelength tuning speed, in principle enabling wide-range wavelength sweeps at arbitrary tuning speeds with narrow instantaneous linewidth.
Highlights
Over the last 10 years, rapidly and widely wavelength tunable lasers with sweep rates of several 10 kHz and relative wavelength tuning ranges of more than 10% have revolutionized optical imaging and sensing in many applications
In optical coherence tomography (OCT), especially in highly scattering tissue, such sources have triggered a dramatic increase in imaging speed [1,2,3,4,5,6]
The microelectromechanical system (MEMS)-vertical-cavity surface-emitting lasers (VCSELs), Fourier domain mode-locked (FDML) and Vernier-tuned distributed Bragg reflector (VT-DBR) lasers represent the generation OCT light sources, featuring several 100 kHz sweep repetition rate over wavelength ranges of ∼ 100 nm
Summary
Over the last 10 years, rapidly and widely wavelength tunable lasers with sweep rates of several 10 kHz and relative wavelength tuning ranges of more than 10% have revolutionized optical imaging and sensing in many applications. The most important impact of these sources is in biomedical imaging. In optical coherence tomography (OCT), especially in highly scattering tissue, such sources have triggered a dramatic increase in imaging speed [1,2,3,4,5,6]. SS-OCT can provide superior performance with respect to imaging speed [7], imaging range [8, 9], and sensitivity [10,11,12,13]. Other biomedical imaging applications include fast beam steering in endoscopes [14] and label free molecular imaging with Raman contrast [15]
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