For bioimaging applications, commercial swept-sources currently provide enough power (tens of milliwatts) insuring good imaging condition without damaging the tissues. For industrial applications, more power is needed since the amount of light collected can be very low due to challenging measurement conditions or due to poor sample reflectivity. To address this challenge, we explore three different setups to externally amplify the output of a commercial swept-source: a booster semiconductor optical amplifier (BOA), an erbium-doped fiber amplifier (EDFA) and a combination of both. These external amplification setups allow the exploration of emerging OCT applications without the need to develop new hardware.
A CW kilowatt fiber laser numerical model has been developed taking into account intracavity stimulated Raman scattering (SRS). It uses the split-step Fourier method which is applied iteratively over several cavity round trips. The gain distribution is re-evaluated after each iteration with a standard CW model using an effective FBG reflectivity that quantifies the non-linear spectral leakage. This model explains why spectrally narrow output couplers produce more SRS than wider FBGs, as recently reported by other authors, and constitute a powerful tool to design optimized and innovative fiber components to push back the onset of SRS for a given fiber core diameter.
Ultrafast lasers are enabling precision machining of a wide variety of materials. However, the optimal laser parameters for proper material processing can differ greatly from one material to another. In order to cut high aspect-ratio features at high processing speeds the laser parameters such as pulse energy, repetition rate, and cutting speed need to be optimized. In particular, a shorter pulse duration plays an important role in reducing the thermal damage in the Heat-Affected Zones. In this paper we present a novel ps fiber laser whose electronically tunable parameters aim to facilitate the search for optimal processing parameters. The 20W 1064nm Yb fiber laser is based on a Master Oscillator Power Amplifier (MOPA) architecture with a repetition rate that can be tuned continuously from 120kHz to 120MHz. More importantly, the integration of three different pulse generators enables the pulse duration to be switched from 25ps to 50ps, or to any value within the 55ps to 2000ps range. By reducing the pulse duration from the ns-regime down to 25ps, the laser approaches the transition from the thermal processing regime to the ablation regime of most materials. Moreover, in this paper we demonstrate the synchronization of the pulses from two such MOPA lasers. This enables more elaborate multipulse processing schemes where the pulses of each laser can be set to different parameter values, such as an initial etching pulse followed by a thermal annealing pulse. It should be noted that all the laser parameters are controlled electronically with no moving parts, including the synchronization.
Mid-infrared lasers find interesting applications in laser-based countermeasure technologies, remote sensing,
maritime/terrestrial awareness and so on. However, the development of laser sources in this spectral region is limited.
We present here an alternative solution to the mid-infrared laser which is based on difference-frequency generation
(DFG) in a nonlinear crystal pumped by synchronized and tunable near-infrared fiber lasers that are commercially
available. This idea is not new and has been explored by other groups, but the latest innovations in near-infrared fiber
lasers have enabled the creation of fast-scanning picosecond fiber lasers. One such picosecond system is the
synchronized programmable laser from Genia Photonics that can combine two picosecond fiber laser systems in which
both output pulses are synchronized at the DFG crystal. The first laser was continuously tunable from 1525 nm to 1600
nm and one million different wavelengths can be scanned within one second. For the second fiber laser, its wavelength
was fixed at 1080 nm. In principle, the DFG in a PPLN crystal could produce a tunable mid-infrared source spanning
from 3.32 μm up to 3.7 μm. Other and wider tuning ranges are possible with different choices of pump wavelengths. For
the PPLN crystal used in this work, the DFG phase-matching window for a fixed temperature was 2.6 nm wide and was
broad enough for our 25 ps pulse train having a spectral width of 0.25 nm. The quantum efficiency achieved for the DFG
was 44% at the maximum power available.
We report here the successful realization of 25 millions wavelengths per second using an SOA based PL around 1565
nm at a 75 MHz repetition rate. The laser is simply composed of an SOA, a CFBG (10 ps/nm) with a 100 nm bandwidth,
an optical circulator, an EOM (intensity modulator), and an output coupler (20%). Pulse duration is around 45 ps and
OSNR of the pulse is around 35 dB at 1565 nm without sweeping. Tunable dispersion compensating module (TDCM)
was used to compress the chirped pulse output and 10 ps pulse duration was obtained at 1548 nm. Finally 25 megawavelengths
per second was realized with under 3 pulses per wavelength and 1024 discrete wavelengths. Linear k-space
sweeping function was enabled in the swept-source OCT (SS-OCT) system through graphical user interface (GUI).
This paper presents a unique and novel picosecond laser source that offers complete tailoring of the wavelength sweep
and that benefits swept-source optical coherence tomography (SS-OCT) applications. Along with the advantages of a
fiber-based architecture, the source is a fully programmable, electronically controlled actively mode-locked laser capable
of rapidly tuning the wavelength and pulse characteristics. Furthermore, several sweep modes and configurations are
available which can be defined by range, with linear sweeps in wavelength or k-space, or by arbitrary wavelengths. The
source design is discussed and its use in SS-OCT with a prototype using a semiconductor optical amplifier as a gain
medium is illustrated.
We present a programmable picosecond fiber laser delivering a single pulse at two wavelengths, one in the C band, the
other in the L band. The difference between those wavelengths is tunable over 75 nm or up to 9.4THz. The laser thus
yield tunable synchronized wavelength ideal for the nonlinear generation of frequencies in the THz or pump-probe
experiments such as CARS/SRS in the fingerprint spectral region.
We describe theoretical and experimental investigations on the spectral and temporal control of an actively
mode-locked erbium-doped fiber laser equipped with a highly dispersive cavity. The laser design is based on a
unidirectional ring cavity in which a pair of diffraction gratings is inserted. A direct outcome of the dispersion
due to the diffraction gratings resides in the fact that the duration of a complete roundtrip in the laser cavity
becomes sensitively dependent upon laser wavelength. Tuning of laser emission is then achieved by controlling
the modulation frequency of the waveform applied to the loss modulator that produces mode-locked operation.
Such a fiber laser enables the generation of picosecond pulses with a rapid tuning over a large bandwidth. We also
incorporated a Gires-Tournois interferometer (GTI) in the laser setup in order to investigate how perturbations
such as group delay ripple affect the temporal shape of the laser pulses and their spectral content, as well as
the stability of the selected laser wavelength. Variation of pulse duration between 40 to100 picoseconds and
continuous tuning of laser wavelength will be described.
A rapidly tunable, electronically controlled, pulse duration adjustable, arbitrarily programmable wavelength, picosecond
mode-locked fiber laser is presented. The laser is tunable over 80 nm with sweeping frequency over 10 million
wavelengths per second. The user can select from a preset linear sweep in either wavelength or optical frequency (kspace)
or a custom (user-defined) sweep. Pulse duration is adjustable over tens of picoseconds with nearly Fourier
limited linewidth. The laser can be harmonically mode-locked over 1 GHz. The average power is again fully
programmable and is at least 50 mW, Watt level is possible with a high power amplifier. The output is a single mode
polarization maintaining fiber. The laser possesses several external triggers, such as one trigger per optical pulse, one
delayed trigger per optical pulse to synchronize with the experiments, one at the beginning when the laser is ready to
sweep to start the data acquisition and one for each consecutive sweep, and finally one trigger for each wavelength
change. Such a laser is so versatile that it can be used for medical imaging, material machining and nonlinear optics. It
proves also a valuable research tool since all the parameters are adjustable.
We report theoretical and experimental investigations on the spectral and temporal control of a mode-locked
fiber laser using a chirped fiber Bragg grating and a loss modulator in either a undirectionnal ring cavity or a
standing-wave cavity. The fiber laser generates picosecond pulses with a rapid tuning over a large bandwidth.
Tuning is achieved by controlling the frequency of the applied modulation waveform. The adjustement of pulse
duration between 40 - 500 ps and the rapid tuning from 1513 nm to 1588 nm are described.
Fiber lasers have recently received a lot of attention after the dramatic increase in output power achieved from single fibers. In particular, Ytterbium doped fibers offer a very low quantum defect and a very broad emission between 1 and 1.1 μm. Triggered by the progress in high-brightness pump diodes and the availability of large-mode-area (LMA) gain fibers, several fiber lasers with output powers in the 1kW range from a single fiber have been demonstrated [1-4]. While these demonstrations typically employ a length of gain fiber pumped via free-space coupling and free space optics as the high reflector, there are fewer reports of integrated all-fiber laser cavities, e.g. [4]. The availability of high-power fiber-optic components and the assembly thereof is therefore crucial for making this technology accessible for a variety of applications. Fiber lasers and amplifiers are very attractive light sources for applications requiring high power as well as excellent beam quality, because they are much less susceptible to thermo-optic distortions than conventional solid-state lasers. A transform-limited beam quality (M2=1) is possible even at kW level output power. Another advantage is the excellent overlap between the signal light and the pump absorption achievable in properly designed fibers. This allows a very efficient operation and up to 80% of optical conversion efficiency have been demonstrated based on the launched pump power [2]. Once assembled, fiber-optic modules do not require alignment and are therefore inherently robust. The tight confinement of the laser light combined with the long interaction length in fibers also makes them prime candidates for high gain systems.
Fiber lasers have shown extraordinary progress in power level, reaching the kilowatt range. These results were achieved with large mode area fibers pumped with high power laser diodes coupled with bulk-optics. To enable the commercial development of these high power fiber lasers, we have demonstrated several All-Fiber components, which replace the bulk-optic interface in the present laser configurations. These components include multimode fused fiber bundle combiners with or without signal fiber feed-through, Bragg gratings and mode field adaptors. The multimode fibers are used to couple several fiber pigtailed pump diodes to a double-clad fiber. Such combiners may contain a signal fiber to provide an input or output for the core modes of the double-clad fiber. Mode field adaptors perform fundamental mode matching between different core fibers. Bragg gratings are used as reflectors for the laser cavity. These components exhibit low-loss and high power handling of 200 Watts has been demonstrated. They enable the design of true high power single-mode All-Fiber lasers that will be small, rugged and reliable.
We report results from a systematic study of the linear refractive
index of thin films made of As-S-Se glasses which are part of
the chalcogenide family. We have studied eight different
compositions. The refractive index are measured by the mean of a
grating coupling experiment. The measurements are performed around
1.5μm for both annealed and non-annealed glasses. We observe
that annealing the samples increases their refractive index. We
also note that the increase of Selenium concentration increases
the refractive index and the decrease of Arsenic concentration
decreases the refractive index.
The creation of microchannels in a photosensitive material, the arsenic trisulphide As2S3, is reported. It is shown that microchannels are created through the process of self-writing and are very sensitive to the photosensitivity of the material, the quality of the incident wavefront and the
light intensity. The very large photosensitivity of As2S3 allows for the self-written waveguide to become much smaller than the incident beam. It can indeed be as small as 1 micron wide. We present a numerical analysis based on the nonlinear Schroedinger equation that accounts well for the
diversity of the microchannels experimentally observed. It is also shown that the microchannels can actually guide light efficiently.
In an effort to develop an improved medium for optical communication, chalcogenide glasses are being investigated for waveguide and integrated optical components. These glasses are attractive for integrated optics applications due to their good infrared transmission and high nonlinear Kerr effects. The fact that these glasses can be fabricated in thin films and optical fiber forms constitute a major advantage for future high-speed optical devices applications. However, to advance these novel characteristics, it is crucial to identify the structure/property relationship in the glass, in both bulk and film materials. Rutherford Backscattering Spectroscopy (RBS) is an analytical tool that gives very useful information regarding compositional and structural analysis of the films, as well as a precise measurement of the film's layer thickness. Results obtained showed no apparent variation in composition and small (less than 10%) density variation in single layer As2S3 films. Multilayer films, which thickness were measured using SEM images, displayed compositional and density modifications associated with the annealing process. The same calculations were conducted after almost a year from the previous measurements to study changes induced due to film aging. Stoichiometric and thickness modifications, caused by aging, were observed in unannealed structures. No apparent changes were detected in annealed films. Waveguide Raman Spectroscopy was used as a complementary tool to identify the molecular features responsible for the changes.
Recently, the chalcogenide glasses (ChG) have attracted much attention in the field of optical
communication and integrated optics. High transparency in the infrared spectral region, low phonon
energy, high nonlinear properties, and high photosensitivity at near band-gap (Eg 2.35 eV,
a 1-2 x iO cm1 at 5 14 nm) are important characteristics of these glasses. In particular, the
photosensitive effects, among them photodarkening and giant photoexpansion (2-5%) [1], have been
extensively studied and several holographic elements, such as microlenses, diffraction and Bragg
gratings, and channel waveguides have been realized in fiber, bulk, and thin film forms of these
materials [2-4]. In this work, we report for the first time to our knowledge, the observation and study of
the strong polarization dependent photoinduced surface relief gratings in As2S3 thin films. A model to
describe the observed phenomena is also presented.
Semiconductors below the half band gap exhibit higher bond- electronic, off-resonant nonlinearities compare to silica fibers. They also exhibit low scattering losses, negligible two-photon absorption, and no linear absorption. Furthermore, due to the mature fabrication technology many complicated device designs can be easily fabricated. Thus, this material system offers unique opportunity to observe and test novel nonlinear phenomena and devices. In this paper, we will review the nonlinear optical properties of AlGaAs waveguides near the half band gap.
Chalcogenide glasses (ChG) based on As, S and Se are transparent in the infrared and have found applications in bulk, planar and fiber waveguide optical components. Due to their recent use in planar channel waveguide devices, a study to assess how structural variations imposed by processing conditions (film deposition) lead to changes in linear and nonlinear optical properties, is ongoing in our group. High resolution, near infrared (NIR) ((lambda) equals 840 nm) Raman spectroscopy has been employed to characterize changes in bonding between bulk glass specimens and glass in planar form. To obtain spectroscopic and spatially resolved information on chemical bonding, a microscope attachment has been constructed and is characterized as to its spatial resolution. Measurements are presented on single layer films prepared using processing and illumination conditions such as those used in fabricating waveguide components. These data are discussed in comparison to spectra obtained on bulk glass materials.
The photolithographic and photosensitive fabrication processes of infrared sulfide chalcogenide glasses waveguides are presented. Integrated optical devices such as photoinduced waveguides and directional couplers are investigated and the propagation characteristics of the waveguides are reported.
We report on the measurements of the nonlinear susceptibility of ZnSe at 783 nm with 110 fs pulses of various intensities. We point out limitations for the measurement of (chi) (5) nonlinear susceptibility using Z- scan technique. We also present a new technique using self- imaging (SIZ-scan) which could provide more sensitive measurements of nonlinear refractive effects.
This paper presents a summary of some of the research activities conducted at the Centre d'optique, photonique et laser from Universite Laval that find applications in the field of optical communications. The subjects considered are: absolute frequency control, compensation of dispersion and nonlinear effects within dense wavelength division multiplexed systems, writing of in-fiber Bragg gratings, optical fast frequency-hopped code division multiple access, gain dynamics in Erbium doped fiber amplifiers, short pulsed emission at high repetition rates, dual wavelength operation of semiconductor lasers, applications of Fourier-transform spectrometers and holographic interconnections.
In this paper we present two different applications of ion implantation in chalcogenide glasses: rare earth doping and channel waveguide fabrication. The luminescence of a neodymium-implanted arsenic tri-sulfide waveguide at 1083 nm is reported. The most efficient pump wavelength is determined to be 818 nm. The dopant distribution following ion implantation is predicted by molecular dynamic simulation and measured by Rutherford Backscattering Spectrometry. This observation of luminescence from rare- earth ion implantation into chalcogenide glass suggest that this technique can be useful for rare-earth doped devices. A study of neodymium luminescence peak power as a function of dopant concentration is reported. The second application of ion implantation is in the fabrication of channel waveguides by helium implantation.
A novel switch is introduced which has the capability of interconnecting 1 input channel to N output channels in a single device, without crosstalk. It is based on the unique properties of spatial solitons which propagate without diffracting in space and create index channels which can be used to guide signals at the same or different frequencies. Angular scanning of the soliton channels is achieved by chirping the phase of the input wavefront. Some properties of the switch and the initial demonstration of soliton scanning in an AlGaAs planar waveguide at 1550 nm are discussed.
Semiconductors and in particular AlGaAs operated at photon energies below half the band gap have proven over the last few years to be optimum materials for studying nonlinear guided phenomena, including ultrafast all-optical switching. Here we report experimental results on a range of characterization measurements and implementations of all-optical switching devices.
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