100 kHz axial scan rate swept-wavelength OCT using sampled grating distributed Bragg reflector lasers

Abstract

Fast wavelength tunable sampled grating distributed Bragg reflector (SG-DBR) lasers are used to generate fast, linear, continuous wavelength sweeps. High resolution wavelength sweeps in excess of 45 nm are demonstrated at a 100 kHz repetition rate. The front mirror, back mirror and phase segment tuning segments can be modulated at very fast rates, which allows for very fast wavelength ramp rates. This sweep is generated through three time synchronized current versus time waveforms applied to the back mirror, front mirror and phase sections of the laser. The sweep consists of fifty separate mode-hop-free tuning segments which are stitched together to form a near continuous wavelength ramp. The stitching points require a maximum of 60 ns for amplitude, wavelength, and thermal settling time to allow the laser to equilibrate. Wavelength tuning non-linearities, output power wavelength dependency, and wavelength discontinuities are defects in the wavelength sweep that result from properties of the wavelength tuning mechanism as well as limitations of the signal generators that produce the time varying bias currents. A Michelson Interferometer is used to examine the effects of these defects for optical coherence tomography (OCT). The OCT measurements demonstrate spectral broadening of the source and interference signal reduction as the penetration depth increases. However, these effects are not very severe for delay differences less than 2 mm even without correction for sweep nonlinearities.

Discretely swept optical coherence tomography system using super-structure grating distributed Bragg reflector lasers at 1561-1639nm

“…Using SSG-DBR lasers as the sources, removal of stitching noises is required as reported by O'connor et al [6] using SG-DBR lasers similar to SSG-DBR lasers. Also seamless switching from L+-band laser to L-band laser is required.…”

“…After an abrupt change of the injection currents, the laser becomes unstable and requires certain length of time to stabilize. Associated with such unstable operation of the laser, stitching noises appear in the interference signal [6]. In our case, because sweep was stepwise, we repeated the same injection currents value at the stitching point until the laser output became stable.…”

“…The currents must be abruptly changed at a few frequencies [6]. After an abrupt change of the injection currents, the laser becomes unstable and requires certain length of time to stabilize.…”

All-semiconductor high-speed akinetic swept-source for OCT

“…In essence, a VT-DBR swept laser is a random-access tunable laser, able to transition between operating points (wavelength/frequency, operating power, or other optical characteristic) in nanoseconds or hundreds of picoseconds. The VT-BDR swept laser can easily tune over relatively wide wavelength bands at rates of hundreds of thousands to a million sweeps per second 11 . Figure 2 shows a cross-section of the semiconductor laser chip design.…”