One of the biggest challenges of silicon photonics is the efficient coupling of light between the sub-micron SiP waveguides and a standard optical fiber (SMF-28). We recently proposed a novel approach based on a spot-size converter (SSC) that fulfills this need. The SSC integrates a tapered silicon waveguide and a superimposed structure made of a plurality of rods of high index material, disposed in an array-like configuration and embedded in a cladding of lower index material. This superimposed structure defines a waveguide designed to provide an efficient adiabatic transfer, through evanescent coupling, to a 220 nm thick Si waveguide tapered down to a narrow tip on one side, while providing a large mode overlap to the optical fiber on the other side. An initial demonstration was made using a SSC fabricated with post-processing steps. Great coupling to a SMF-28 fiber with a loss of 0.6 dB was obtained for TEpolarized light at 1550 nm with minimum wavelength dependence. In this paper, SSCs designed for operation at 1310 and 1550 nm for TE/TM polarizations and entirely fabricated in a CMOS fab are presented.
TeraXion started silicon photonics activities aiming at developing building blocks for new products and customized
solutions. Passive and active devices have been developed including MMI couplers, power splitters, Bragg grating
filters, high responsivity photodetectors, high speed modulators and variable optical attenuators. Packaging solutions
including fiber attachment and hybrid integration using flip-chip were also developed. More specifically, a compact
packaged integrated coherent receiver has been realized. Good performances were obtained as demonstrated by our
system tests results showing transmission up to 4800 km with BER below hard FEC threshold. The package size is small
but still limited by the electrical interface. Migrating to more compact RF interface would allow realizing the full benefit
of this technology.
Sensing systems for defense and security operate in evermore demanding environments, increasingly leaving the comfort
zone of fiber laser technology. Efficient and rugged laser sources are required that maintain a high level performance
under large temperature excursions and sizable vibrations. This paper first presents a sample of defense and security
sensing applications requiring laser sources with a narrow emission spectrum. Laser specifications of interest for defense
and security sensing applications are reviewed. The effect of the laser frequency noise in interferometric sensing systems
is discussed and techniques implemented to reduce phase noise while maintaining the relative intensity noise
performance of these sources are reviewed. Developments towards the size reduction of acoustically isolated narrow-linewidth
semiconductor lasers are presented. The performance of a narrow-linewidth semiconductor laser subjected to
vibrations is characterized. Simulation results of interferometric sensing systems are also presented, taking into account
both the intensity and phase noise of the laser.
We review the improved performances of a narrow linewidth laser using negative electrical feedback obtained through
advances on narrowband FBG filters. Noteworthy, the tolerance of the laser to vibrations is significantly improved. As
an extension of this work, these narrow filters are proposed for filtering optical signals in RF photonics systems.
Frequency noise reduction of semiconductor lasers using electrical feedback from an optical frequency
discriminator is an efficient and simple approach to realize narrow linewidth lasers. These lasers are of great
interest for applications such as LIDAR, RF photonics and interferometric sensing. In this paper, we review
the technological choices made by TeraXion for the realization of its Narrow Linewidth Laser modules. The
method enables to decrease the linewidth of DFB lasers from several hundreds of kHz to a few kHz. We
present the work in progress to integrate such system into a miniature package and to incorporate advanced
functionalities such as multi-laser phase locking.
Chromatic dispersion (CD) in single-mode optical fiber distorts pulses and is a big obstacle against the upgrading of long-haul, dynamically routed wavelength division multiplexed (WDM) transmission systems at 10 Gbit/s and beyond. High-performance adaptive dispersion compensators are required as well as reliable low-cost hardware for the detection of residual CD. Targeting high-capacity metro systems, full-band 1.6-Tb/s (40x40Gbit/s) adaptive CD compensation is demonstrated in this experiment, using CSRZ-ASK and NRZ-DPSK modulation formats. A multichannel dispersion compensator, tunable in the range -700 to -1500 ps/nm, is automatically controlled by arrival time detection in one of the 40 transmitted WDM channels. Dispersion and its slope are tuned simultaneously by a thermal gradient of the grating-based compensator to match the parameters of standard single-mode fiber (SSMF) with lengths between 44 and 94 km.
Hongbin Zhang, A. Fauzi Abas, Ariya Hidayat, David Sandel, Suhas Bhandare, Frank Wüst, Biljana Milivojevic, Reinhold Noé, Martin Lapointe, Yves Painchaud, Martin Guy
The capacity limit of a thermally controlled fiber Bragg grating-based chromatic dispersion compensator, which was initially designed for 10 Gb/s operation, was investigated in a 40 Gb/s system. A CS-RZ DQPSK polarization division multiplex (PolDM) system was used as a testbed. An equivalent quasi error-free 5.94 Tb/s capacity was demonstrated when dispersion of up to 73.8 km of SSMF was compensated. The dispersion slope compensation was satisfactory for C-band operation. Additionally, it was found that the compensator introduced band-pass filtering behaviour, which reduced the compensator bandwidth as the dispersion setting was increased. It was also found that even after 41.5 km, there was around 2 dB penalty introduced to DQPSK system while 5 dB penalty to DQPDK-PolDM, referring to BER of 10-5.
Chromatic dispersion accumulated over a link of optical fiber causes transmission degradation for data rates of 10 Gbit/s and higher. The different technologies for compensating this effect are reviewed. They are compared based on their suitability for different applications. Characteristics such as system performance, tunability, insertion loss, bandwidth and size are discussed. Tunability is of increasing importance as the future networks become more and more agile and reconfigurable. Fine tuning is also a requirement at data rates of 40 Gbit/s and higher. In the context of WDM systems, operations in both single-channel and multi-channel regimes are discussed. Recent developments are presented and future trends are discussed.
Since the late 1990s, steady advances in wavelength division multiplexing (WDM) technology have provided better ways to increase the capacity of optical networks. Three significant trends become evident in long-haul transmission system, namely the continual increase in the number of dense WDM channels, the increase in data rates from 2.5 Gb/s to today's 10 Gb/s, to tomorrow's 40 Gb/s, and finally longer distances between electrical regeneration sites. These trends towards an increased optical network capacity are now clashing with chromatic dispersion. This paper will discuss the application of high-end FBGs to telecommunication systems, focusing on their performances with respect to chromatic dispersion. Two types of components will be discussed: low-dispersion FBG WDM filters and FBG dispersion compensators. High-quality ultra-low dispersion FBGs have been fabricated successfully and their key attributes will be discussed. Advanced applications of FBGs for chromatic dispersion compensation, such as broadband multi-channel dispersion and slope compensation, will be covered. In particular, FBG dispersion slope compensators can be used in conjunction with Dispersion Compensating Fiber (DCF) to fully manage the dispersion over a large number of WDM channels. The need for tunable dispersion compensation at 40 Gb/s transmission rates will be discussed. Experimental results will also be presented.
A technique for properly separating the scattering and absorption contributions in laser mammography is proposed. The technique is based on an empirical model obtained from a series of experiments performed on homogeneous scattering slabs containing a single inclusion. The scattering and absorption contributions are obtained by performing a Fit of an Inhomogeneous Diffusion Model (FIDM). The performance of this new technique is compared to that involving a curve fit of the solution of the diffusion model for a homogeneous slab. The FIDM technique allows a very good discrimination between scattering and absorption inclusions, better than that obtained with a curve fit of the homogeneous diffusion model. The mathematical expressions of the empirical model are extremely simple and allow for a fast calculation (about 1 second for computing two 441 pixel images compared to about 6 minutes with the previous technique). A perturbation analysis of the diffusion model will provide theoretical support to the FIDM technique and should allow its refinement. Although it has not been demonstrated that the separation between absorption and scattering is totally correct when applied to real breast scanning, the method associates some structures to absorption and some other to scattering, which could result in a better specificity of laser mammography.
A prototype for laser mammography based on a time-domain technique has been developed. The system uses a streak camera and a Titanium:sapphire laser which provides ultrashort pulses at a repetition rate of 80 MHz. A multi-port scanning head which includes optical fibers scans the breast in a point-by-point scanning procedure. Time-resolved transmission is measured at 15000 locations in 7 minutes. The breast is slightly compressed in both the cranio-caudal and the mediolateral projections. Amplitude calibration of the streak camera has been performed allowing for absolute measurement of time- resolved transmission. In addition to the shape of the time-resolved transmission, the absolute amplitude is relevant in properly evaluating the absorption and scattering coefficients. Promising results on solid phantoms and in vivo have been obtained. Both breasts of 10 volunteers have been scanned to date and a larger pilot study is planned in the near future. In addition to the usual time-gating processing, images of the scattering and absorption contributions are also extracted using an original data processing technique.
Propagation of light in a homogeneous scattering slab is conveniently modelled with a diffusion equation. This approach can be extended to a heterogeneous slab through a perturbation analysis. Within BornÕs approximation, the effect of an inclusion on the transmitted light is described by space-time integrals. Closed-form time integration is possible, which reduces the perturbation expressions to volume integrals over the inclusion. These can be useful to model small inclusions over which the integrand can be considered as constant. In the case of cubic inclusions with sides parallel and perpendicular to the boundaries of the surrounding slab, closed-form volume integration over the inclusion can be performed instead. Only time integrals are left, which reduces the numerical work. Numerical examples are presented. It is shown that inclusions with different volume and contrast with regards to the surrounding medium can produce the same effect on the transmitted light and are thus indistinguishable. The perturbation analysis has been used to assess the possibility of obtaining some longitudinal localization of an inclusion by using source beams and detectors of different sizes. Calculation results are also compared to experimental measurements to illustrate the validity of this analysis in the presence of small perturbations.
We present an efficient all-fiber intensity modulator designed for repetitive Q-switching of rare-earth doped fiber lasers. This device shows many advantages over standard bulk modulation technique like immunity to mechanical vibrations and lower excess losses. The principle of operation is simple and relies on the evanescent field coupling of the fiber mode to an overlay medium plate which is rapidly moved away from the surface of a standard polished coupler using a fast piezoelectric translator. A spliceless Q-switched erbium fiber laser incorporating this modulator was also tested.
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