Liviu Neagu

Extra long imaging range swept source optical coherence tomography using re-circulation loops

One of the main drawbacks of the swept source optical coherence tomography (SS-OCT) is its limited axial range. A novel interferometer configuration is proposed, equipped in each arm with an adjustable path length ring. By compensating the losses in the rings using semiconductor optical amplifiers, multiple paths A-scans can be obtained which when combined axially, can lead to an extremely long overall axial range. The effect of the re-circulation loops is equivalent with extending the coherence length of the swept source. In this way, the axial imaging range in SS-OCT can be pushed well beyond the limit imposed by the coherence length of the laser, to exceed in principle many centimeters.

Combined Neodymium–Ytterbium-Doped ASE Fiber-Optic Source for Optical Coherence Tomography Applications

Optical coherence tomography (OCT) imaging at the 1060-nm region proved to be a successful alternative in ophthalmology not only for resolving intraretinal layers, but also for enabling sufficient penetration to monitor the subretinal vasculature in the choroid when compared to most commonly used OCT imaging systems at the 800-nm region. To encourage further clinical research at this particular wavelength, we have developed a compact fiber-optic source based on amplified spontaneous emission (ASE) centered at 1060 nm with 70-nm spectral bandwidth at full-width at half-maximum and output power 20 mW. Our approach is based on a combination of slightly shifted ASE emission spectra from a combination of Neodymium- and Ytterbium-doped fibers. Spectral shaping and power optimization have been achieved using in-fiber filtering schemes. We have tested the performance of the source in an OCT system optimized for this wavelength.

Multiple-depth en face optical coherence tomography using active recirculation loops.

We present a novel low-coherence interferometer configuration, equipped in each arm with an adjustable optical path length ring. By compensating for the losses in the rings using semiconductor optical amplifiers, interference of low-coherence light after traversing the two rings 18 times is obtained. This configuration is employed to demonstrate simultaneous en face optical coherence tomography imaging at five different depths in a Drosophila melanogaster fly.

Experimental Method to Find the Optimum Excitation Waveform to Quench Mechanical Resonances of Fabry–Pérot Tunable Filters Used in Swept Sources

We report experimental evidence of improving the nonlinearity of conventional wavelength swept laser sources based on a fiber Fabry-Perot tunable filter as a wavelength-selective element. Our solution is based on applying a nonsinusoidal, synthesized waveform to the tunable filter that can be identified experimentally. A significant improvement in the optical coherence tomography image quality has been obtained without any software recalibration method.

Automated test station for laser-induced damage threshold measurements according to ISO 21254-1,2,3,4 standards

An automated test station to measure the laser-induced damage threshold (LIDT) according to ISO 21254-1,2,3,4:2011 standards is presented. The laser is a single longitudinal mode, 500 mJ, 6 ns, Q-switched, 10 Hz, linearly polarized, 1064 nm laser, with 2-nd and 3-rd harmonic capabilities. The machine is able to operate the S-on-1 test (S = 500), or the Type 2 endurance (durability) test. The main blocks of the station are described, emphasizing some original solutions. Preliminary results of LIDT measurements using the S-on-1 test on several coatings and on uncoated fused silica substrates with various degrees of roughness are also presented.

Characterization of a fibre optic swept laser source at 1 μm for optical coherence tomography imaging systems

We report the development of a swept wavelength laser at 1 micron based on a linear cavity fibre configuration with an intra-cavity half symmetrical confocal Fabry-Perot tunable filter and a semiconductor optical amplifier as a gain medium. The performances of the source in terms of parameters like: sweep repetition rate (1-20 kHz), center wavelength (1065 nm), wavelength scanning range (max. 50nm), instantaneous line-width (<0.1nm) and a boosted output power of around 40 mW are demonstrated. The new source tested on an OCT system is exhibiting sufficient linearity in wave-number (k-space) at 1 kHz repetition rate; therefore no k-trigger, or wavelength rescaling process was needed.

20-mW 70-nm bandwidth ASE fibre optic source at 1060-nm wavelength region for optical coherence tomography

Optical coherence tomography (OCT) imaging at 1060 nm region proved to be a successful alternative in ophthalmology not only for resolving intraretinal layers, but also for enabling sufficient penetration to monitor the sub-retinal vasculature in the choroids when compared to most commonly used OCT imaging systems at 800 nm region. To encourage further clinical research at this particular wavelength, we have developed a compact fiber optic source based on amplified spontaneous emission (ASE) centered at ~1060 nm with ~70 nm spectral bandwidth at full-width half maximum (FWHM) and output power >20 mW. Our approach is based on a combination of slightly shifted ASE emission spectra from a combination of two rare-earth doped fibers (Ytterbium and Neodymium). Spectral shaping and power optimization have been achieved using in-fiber filtering solutions. We have tested the performances of the source in an OCT system optimized for this wavelength.

Investigations of OCT imaging performance using a unique source providing several spectral wavebands

The authors report investigations into the suitability of a broadband supercontinuum fiber laser (SCFL) for use in Optical Coherence Tomography (OCT). The supercontinuum of light extending from 400 nm to 1800 nm can be used selectively in several spectral wavebands from 600 nm to 1700 nm in order to characterize the performance of single mode (SM) fiber OCT systems through spectral and auto-correlation measurements, dispersion measurements and image acquisition. Spectral selection and tailoring is made possible through a combination of bandpass optical filters. In addition, for the first time, given the optical bandwidth available, we perform evaluation of effective noise bandwidths which take into consideration the spectral behavior of the optical splitter in the balanced detection receiver.

Generating multiple depth en-face images in optical coherence tomography

In this manuscript, a novel low coherence interferometer configuration is presented, equipped in each arm with an adjustable optical path length ring. By compensating the losses in the two rings using semiconductor optical amplifiers, interference of low coherence light after traversing the two rings 18 times is obtained. This configuration can successfully be employed to produce simultaneous en-face OCT images from different depths.

Real-time adaptive optimization of laser induced nano ripples by laser pulse shaping

We propose a control technique for laser induced sub- micron ripples on titanium and silicon using femtosecond laser pulse shaping. This is based on a real-time observation method of nano ripples by diffraction of UV laser beam and programmable pulse temporal design. The feedback diffraction signal provided information of ripples’ period, area, direction, and arrangement uniformity. By using a genetic algorithm optimization, ripples formation was optimized for their period tuning ability and their uniformity. The diffraction signals were validated with scanning electron microscope (SEM) images. At the generation wavelength of 800 nm and depending on the pulse form, ripples on titanium show periods from 610 nm to 680 nm, and ripples on silicon has periods from 710 – 770 nm. Laser pulse energy affects optimization due to transient energy deposit on material with pulse form effects in the threshold fluence and ripple areas.