Abstract
Emerging high-throughput optical imaging modalities, in particular those providing phase information, necessitate a demanding speed regime (e.g. megahertz sweep rate) for those conventional swept sources; while an effective solution is yet to be demonstrated. We demonstrate a stable breathing laser as inertia-free swept source (BLISS) operating at a wavelength sweep rate of 28 MHz, particularly for the ultrafast interferometric imaging modality at 1.0 μm. Leveraging a tunable dispersion compensation element inside the laser cavity, the wavelength sweep range of BLISS can be tuned from ~10 nm to ~63 nm. It exhibits a good intensity stability, which is quantified by the ratio of standard deviation to the mean of the pulse intensity, i.e. 1.6%. Its excellent wavelength repeatability, <0.05% per sweep, enables the single-shot imaging at an ultrafast line-scan rate without averaging. To showcase its potential applications, it is applied to the ultrafast (28-MHz line-scan rate) interferometric time-stretch (iTS) microscope to provide quantitative morphological information on a biological specimen at a lateral resolution of 1.2 μm. This fiber-based inertia-free swept source is demonstrated to be robust and broadband, and can be applied to other established imaging modalities, such as optical coherence tomography (OCT), of which an axial resolution better than 12 μm can be achieved.
Highlights
Ultrastable swept sources at an ultrafast sweep rate are the “holy grail” for a wide range of spectrally-encoded applications [1,2,3,4,5,6], especially the phase sensitive imaging
By leveraging a dispersive element, such as the optical fiber and linearly chirped fiber Bragg grating with large group-velocity dispersion (GVD), it spreads the wavelengths of the pulsed lightwave in the time domain
For a higher power efficiency and all-fiber design, grating-based dispersioncompensating module (DCM) can be replaced by dispersionengineered fibers, e.g. photonic crystal fiber (PCF) [36]
Summary
Ultrastable swept sources at an ultrafast sweep rate are the “holy grail” for a wide range of spectrally-encoded applications [1,2,3,4,5,6], especially the phase sensitive imaging. The sweep rate based on those methods, is largely limited to the kHz range, which is order of magnitude slower than the MHz regime required by ultrafast phenomena studies or high-throughput screening applications Techniques, such as time-multiplexing with buffer stages [11,12] and microelectromechanical systems (MEMS) [13,14], have recently been incorporated to enable MHz wavelength sweeping. Successfully applied in the areas such as time-stretch microscope [15], ultrafast spectroscope [17], and optical coherence tomography (OCT) [18,19,20,21,22], it is largely under-explored as ultrastable swept sources, in terms of sweeping bandwidth and stability In this regard, recently several works have devoted to the speed improvement of the swept source through optical time-stretch technique, in the wavelength windows of 800 nm [23,24] and 1550 nm [25,26]. By far the most developed fiber gain media in terms of efficiency and gain bandwidth has been devoted to this wavelength window [31], which makes it feasible to generate ultrafast high-power wideband swept source
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