1,135 research outputs found
Boston College Environmental Center Summer Institute on Surtsey and Iceland
Studying geology, geochemistry, and biology of Iceland and Surtsey as examples of new and extreme environment
Stratigraphy of the East Flank of the Green Mountain Anticlinorium, Southern Vermont
Guidebook for field trips in Vermont: 64th annual meeting October 13, 14, 15, 1972 Burlington, Vermont: Trip B-
Soliton microcomb based spectral domain optical coherence tomography
Spectral domain optical coherence tomography (SD-OCT) is a widely used and
minimally invaive technique for bio-medical imaging [1]. SD-OCT typically
relies on the use of superluminescent diodes (SLD), which provide a low-noise
and broadband optical spectrum. Recent advances in photonic chipscale frequency
combs [2, 3] based on soliton formation in photonic integrated microresonators
provide an chipscale alternative illumination scheme for SD-OCT. Yet to date,
the use of such soliton microcombs in OCT has not yet been analyzed. Here we
explore the use of soliton microcombs in spectral domain OCT and show that, by
using photonic chipscale Si3N4 resonators in conjunction with 1300 nm pump
lasers, spectral bandwidths exceeding those of commercial SLDs are possible. We
demonstrate that the soliton states in microresonators exhibit a noise floor
that is ca. 3 dB lower than for the SLD at identical power, but can exhibit
significantly lower noise performance for powers at the milliWatt level. We
perform SD-OCT imaging on an ex vivo fixed mouse brain tissue using the soliton
microcomb, alongside an SLD for comparison, and demonstrate the principle
viability of soliton based SD-OCT. Importantly, we demonstrate that classical
amplitude noise of all soliton comb teeth are correlated, i.e. common mode, in
contrast to SLD or incoherent microcomb states [4], which should, in theory,
improve the image quality. Moreover, we demonstrate the potential for circular
ranging, i.e. optical sub-sampling [5, 6], due to the high coherence and
temporal periodicity of the soliton state. Taken together, our work indicates
the promising properties of soliton microcombs for SD-OCT
Widely Tunable, Low Linewidth, and High Power Laser Source using an Electro-Optic Comb and Injection-Locked Slave Laser Array
We propose a simple approach to implement a tunable, high power and narrow
linewidth laser source based on a series of highly coherent tones from an
electro-optic frequency comb and a set of 3 DFB slave lasers. We experimentally
demonstrate approximately 1.25 THz (10 nm) of tuning within the C-Band centered
at 192.9 THz (1555 nm). The output power is approximately 100 mW (20 dBm), with
a side band suppression ratio greater than 55 dB, and a linewidth below 400 Hz
across the full range of tunability. This approach is scalable and may be
extended to cover a significantly broader optical spectral range
Hybrid Integrated Photonics Using Bulk Acoustic Resonators
Microwave frequency acousto-optic modulation is realized by exciting high
overtone bulk acoustic wave resonances (HBAR resonances) in the photonic stack.
These confined mechanical stress waves transmit exhibit vertically
transmitting, high quality factor (Q) acoustic Fabry Perot resonances that
extend into the Gigahertz domain, and offer stress-optical interaction with the
optical modes of the microresonator. Although HBAR are ubiquitously used in
modern communication, and often exploited in superconducting circuits, this is
the first time they have been incorporated on a photonic circuit based chip.
The electro-acousto-optical interaction observed within the optical modes
exhibits high actuation linearity, low actuation power and negligible
crosstalk. Using the electro-acousto-optic interaction, fast optical resonance
tuning is achieved with sub-nanosecond transduction time. By removing the
silicon backreflection, broadband acoustic modulation at 4.1 and 8.7 GHz is
realized with a 3 dB bandwidth of 250 MHz each. The novel hybrid HBAR
nanophotonic platform demonstrated here, allowing on chip integration of
micron-scale acoustic and photonic resonators, can find immediate applications
in tunable microwave photonics, high bandwidth soliton microcomb stabilization,
compact opto-electronic oscillators, and in microwave to optical conversion
schemes. Moreover the hybrid platform allows implementation of momentum
biasing, which allows realization of on chip non-reciprocal devices such as
isolators or circulators and topological photonic bandstructures.Comment: 41 pages, 4 figure
The role of individual and social variables in task performance.
This paper reports on a data-based study in which we explored - as part of a larger-scale British-Hungarian research project - the effects of a number of affective and social variables on foreign language (L2) learners’ engagement in oral argumentative tasks. The assumption underlying the investigation was that students’ verbal behaviour in oral task situations is partly determined by a number of non-linguistic and non-cognitive factors whose examination may constitute a potentially fruitful extension of existing task-based research paradigms. The independent variables in the study included various aspects of L2 motivation and several factors characterizing the learner groups the participating students were members of (such as group cohesiveness and intermember relations), as well as the learners’ L2 proficiency and ‘willingness to communicate’ in their L1. The dependent variables involved objective measures of the students’ language output in two oral argumentative tasks (one in the learners’ L1, the other in their L2): the quantity of speech and the number of turns produced by the speakers. The results provide insights into the interrelationship of the multiple variables determining the learners’ task engagement, and suggest a multi-level construct whereby some independent variables only come into force when certain conditions have been met
A multifaceted approach to investigating pre-task planning effects on paired oral test performance
Despite the growing popularity of paired format speaking assessments, the effects of pre-task planning time on performance in these formats are not yet well understood. For example, some studies have revealed the benefits of planning but others have not. Using a multifaceted approach including analysis of the process of speaking performance, the aim of this paper is to investigate the effect of pre-task planning in a paired format. Data were collected from 32 students who carried out two decision-making tasks in pairs, under planned and unplanned conditions. The study used analyses of rating scores, discourse analytic measures, and conversation analysis (CA) of test-taker discourse to gain insight into co-constructing processes. A post-test questionnaire was also administered to understand the participants’ perceptions toward planned and unplanned interactions. The results from rating scores and discourse analytic measures revealed that planning had limited effect on performance, and analysis of the questionnaires did not indicate clear differences between the two conditions. CA, however, identified the possibility of a contrastive mode of discourse under the two planning conditions, raising concerns that planning might actually deprive test-takers of the chance to demonstrate their abilities to interact collaboratively
The effects of planning time on complexity, accuracy, fluency, and lexical variety in L2 descriptive writing
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