We describe preliminary results of high-speed 2-dimensional Doppler OCT measurement using a unique optical
frequency domain imaging (OFDI) system. SSG-DBR laser is the light source from which wave number is swept
discretely over predetermined absolute values in equal wave number interval with constant output intensity.
Multi-sampling at each wave number enables us to reconstruct multiple images by 1 transversal scan, and the
2 dimensional flow image is reconstructed from the phase Doppler shift between 2 successive images. Maximum
detectable velocity is restricted by sampling rate, which is higher than A-scan flow has been carried out with this system adopting Kasai velocity estimator. The results predict
that the method is valid for stationary flow.
We demonstrate a single-mode and fast wavelength swept light source by using Superestrucuture grating
distributed Bragg reflector (SSG-DBR) lasers for use in optical frequency-domain reflectometry optical coherence
tomography. The SSG-DBR lasers provide single-mode operation resulting in high coherency. Response of the
wavelength tuning is very fast; several nanoseconds, but there was an unintentional wavelength drift resulting from a
thermal drift due to injecting tuning current. The dri1ft unfortunately requires long time to converge; more than a few
milliseconds. For suppressing the wavelength drift, we introduced Thermal Drift Compensation mesa (TDC) parallel to
the laser mesa with the spacing of 20 μm. By controlling TDC current to satisfy the total electric power injected into
both the laser mesa and the TDC mesa, the thermal drift can be suppressed.
In the present work, we fabricated 4 wavelength's kinds of SSG-DBR laser, which covers respective
wavelength band; S-band (1496-1529 nm), C-band (1529-1564 nm), L--band (1564-1601 nm), and L+-band (1601-1639).
We set the frequency channel of each laser with the spacing 6.25 GHz and 700 channels. The total frequency channel
number is 2800 channels (700 ch × 4 lasers). We simultaneously operated the 4 lasers with a time interval of 500
ns/channel. A wavelength tuning range of more than 140 nm was achieved within 350 μs. The output power was
controlled to be 10 mW for all channels. A single-mode, accurate, wide, and fast wavelength sweep was demonstrated
with the SSG-DBR lasers having TDC mesa structure for the first time.
We have been developing a unique discretely swept optical frequency domain imaging (OFDI) using superstructured-grating
distributed Bragg reflector (SSG-DBR) lasers. To increase resolution, four SSG-DBR lasers are being developed
to obtain spectral coverage of 160 nm in total. To increase speed of D-OFDI imaging, simultaneous scanning of multiple
sources with a parallel OFDI system and unique transversal scanning D-OFDI have been demonstrated. Introduction of
an optical amplifier can increase sensitivity beyond the conventional shot noise limit. A deep 12 mm depth range has
been demonstrated with the wavelength interval of 0.05 nm.
The superstructured-grating distributed-Bragg-reflector laser is a small (shorter than 1 mm in length) and
relatively cheap swept source for optical-frequency-domain- reflectometry optical coherence tomography (OFDR-OCT),
which practically enables use of multiple sources in a single OCT system. Simultaneous scanning of
multiple sources over different wavelength regions and at different wavelength values in the same wavelength
region enable improvement of the resolution and scanning speed, respectively. Those improvements have been
demonstrated using C-band and L-band SSG-DBR sources.
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