Abstract
We investigate the origin of high frequency noise in Fourier domain mode locked (FDML) lasers and present an extremely well dispersion compensated setup which virtually eliminates intensity noise and dramatically improves coherence properties. We show optical coherence tomography (OCT) imaging at 3.2 MHz A-scan rate and demonstrate the positive impact of the described improvements on the image quality. Especially in highly scattering samples, at specular reflections and for strong signals at large depth, the noise in optical coherence tomography images is significantly reduced. We also describe a simple model that suggests a passive physical stabilizing mechanism that leads to an automatic compensation of remaining cavity dispersion in FDML lasers.
Highlights
We investigate the origin of high frequency noise in Fourier domain mode locked (FDML) lasers and present an extremely well dispersion compensated setup which virtually eliminates intensity noise and dramatically improves coherence properties
To reach an intuitive physical understanding of the dynamics and more so about the reason for why the holes initially exhibit very deep intensity modulation, we suggest the following simple model: We consider the light roundtrip through the FDML laser cavity starting from the exit facet of the FFP, propagating through the cavity and ending at the input facet (Fig. 10(a))
Theoretical considerations 5.1 Phase jumps We suggest that the holes we see in the output intensity of the FDML laser occur when the field of light reaching the input facet of the Fabry-Pérot filter has a significantly different phase than the light stored inside the Fabry-Pérot filter
Summary
FDML lasers are narrow band, high-speed optical frequency swept sources [1] that have proven their value in many different applications such as optical coherence tomography (OCT) [2,3,4,5,6,7,8], stimulated Raman spectroscopy [9], picosecond pulse generation [10], and sensing [11,12,13,14,15,16,17,18]. Kraetschmer and Sanders pointed out that an ultra-stable mode locked behavior centered around a very narrow range at the laser’s zero dispersion wavelength exists when the cavity is very well dispersion compensated [33]. This range can be identified by the CW tuning steps described in [27]. This sweet spot operation would be extremely interesting for biomedical imaging, spectroscopy and sensing Since this sweet spot was observed around the zero-dispersion wavelength of the FDML laser, additional dispersion compensation might extend this range. For OCT imaging a buffer stage, booster [35], interferometer and detection unit described in an earlier publication [22] were used
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