Abstract
An experimental study into the modal dynamics of a short cavity, fast frequency-swept laser is presented. This commercially available external cavity swept source is designed for use in optical coherence tomography (OCT) applications and displays a number of dynamic lasing regimes during the course of the wavelength sweep. Interferometric full electric field reconstruction is employed, allowing for measurement of the laser operation in a time-resolved, single-shot manner. Recovery of both the phase and intensity of the laser output across the entire sweep enables direct visualization of the laser instantaneous optical spectrum. The electric field reconstruction technique reveals the presence of multi-mode dynamics, including coherent mode-locked pulses. During the main part of the imaging sweep, the laser is found to operate in a second harmonic sliding frequency mode-locking regime. Examination of the modal evolution of this coherent regime reveals evidence of previously unobserved frequency switching dynamics.
Highlights
Swept frequency lasers are a class of laser whose output wavelength is modulated over a broad bandwidth at a high repetition rate in a continuous or quasi-continuous manner
Swept lasers are used in a variety of optical sensing techniques, including spectroscopy [1], thermometry [2], optical frequency domain reflectometry (OFDR) [3], fibre Bragg grating sensor arrays [4] and swept source optical coherence tomography (SS-OCT) [5]
This work has presented a comprehensive examination of the multi-mode dynamics present in a short cavity swept source laser
Summary
Swept frequency lasers are a class of laser whose output wavelength is modulated over a broad bandwidth at a high repetition rate in a continuous or quasi-continuous manner. We have demonstrated an interferometric self-delayed heterodyne technique based on a 3×3 passive fiber coupler which can reconstruct the phase of broadband swept sources [22, 23] Simultaneous measurement of both the phase and intensity of the output laser field allows for a time-resolved, single-shot measurement of the full sweep complex electric field. From the measured electronic spectrum, it is inferred that the cavity round-trip delay is twice the pulse repetition period (cavity delay of 0.75 ns or 22.5 cm roundtrip) This situation of harmonic sliding frequency mode-locking has been observed previously in other SFML devices [32], as well as very similar short cavity lasers [27, 33]
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