360 research outputs found
Femtosecond optical parametric oscillator frequency combs for coherent pulse synthesis
Coherent pulse synthesis takes as its objective the piecewise assembly of a sequence of identical broadband pulses from two or more mutually-coherent sequences of narrowband pulses. The requirements for pulse synthesis are that the parent pulses share the same repetition frequency, are phase coherent and have low mutual timing jitter over the required observation time.
The work carried out in this thesis explored the requirements for broadband coherent pulse synthesis between the multiple visible outputs of a synchronously pumped femtosecond optical parametric oscillator. A femtosecond Ti:sapphire laser was characterised and used to pump a PPKTP-based OPO that produced a number of second-harmonic and sum-frequency mixing outputs across the visible region. Using a novel lock-to-zero CEO stabilisation technique, broadband phase coherence was established between all the pulses on the optical bench, producing the broadest zero-offset frequency comb to date. Employing a common optical path for all the pulses provided common-mode rejection of noise, ensuring less than 150 attoseconds of timing jitter between the pulses over a 1 second observation window. The parent pulses were compressed and their relative delays altered in a quasi-common path prism delay line, allowing pulse synthesis at a desired reference plane
Broadband phase coherence between an ultrafast laser and an OPO using lock-to-zero CEO stabilization
Multi-color carrier-envelope-phase stabilization for high-repetition-rate multi-pulse coherent synthesis
Using a zero-offset carrier-envelope locking technique, we have synthesized an octave-spanning composite frequency comb exhibiting 132-attosecond timing jitter between the constituent pulses over a one-second observation window. In the frequency domain, this composite comb has a modal structure and coherence which are indistinguishable from those of a comb that might be produced by a hypothetical single mode locked oscillator of equivalent bandwidth. The associated phase stability enables the participating multi-color pulse sequences to be coherently combined, representing an example of multi-pulse synthesis using a femtosecond oscillator
Observations of complex frequency comb structure in a harmonically-pumped femtosecond optical parametric oscillator
Continuous Ultraviolet to Blue-Green Astrocomb
The characterization of Earth-like exoplanets and precision tests of
cosmological models using next-generation telescopes such as the ELT will
demand precise calibration of astrophysical spectrographs in the visible
region, where stellar absorption lines are most abundant. Astrocombs--lasers
providing a broadband sequence of ultra-narrow, drift-free, regularly spaced
optical frequencies on a multi-GHz grid--promise an atomically-traceable,
versatile calibration scale, but their realization is challenging because of
the need for ultra-broadband frequency conversion of mode-locked infrared
lasers into the blue-green region. Here, we introduce a new concept achieving a
broad, continuous spectrum by combining second-harmonic generation and
sum-frequency-mixing in an aperiodically-poled MgO:PPLN waveguide to generate
gap-free 390-520 nm light from a 1 GHz Ti:sapphire laser frequency comb. We
lock a low-dispersion Fabry-Perot etalon to extract a sub-comb of bandwidth
from 392-472 nm with a spacing of 30 GHz, visualizing the thousands of
resulting comb modes on a high resolution cross-dispersion spectrograph.
Complementary experimental data and simulations demonstrate the effectiveness
of the approach for eliminating the spectral gaps present in
second-harmonic-only conversion, in which weaker fundamental frequencies are
suppressed by the quadratic \{chi}^((2)) nonlinearity. Requiring only ~100 pJ
pulse energies, our concept establishes a practical new route to broadband
UV-visible generation at GHz repetition rates.Comment: 14 pages; 4 figure
Discovery of Massive, Mostly Star-formation Quenched Galaxies with Extremely Large Lyman-alpha Equivalent Widths at z ~ 3
We report a discovery of 6 massive galaxies with both extremely large Lya
equivalent width and evolved stellar population at z ~ 3. These MAssive
Extremely STrong Lya emitting Objects (MAESTLOs) have been discovered in our
large-volume systematic survey for strong Lya emitters (LAEs) with twelve
optical intermediate-band data taken with Subaru/Suprime-Cam in the COSMOS
field. Based on the SED fitting analysis for these LAEs, it is found that these
MAESTLOs have (1) large rest-frame equivalent width of EW_0(Lya) ~ 100--300 A,
(2) M_star ~ 10^10.5--10^11.1 M_sun, and (3) relatively low specific star
formation rates of SFR/M_star ~ 0.03--1 Gyr^-1. Three of the 6 MAESTLOs have
extended Ly emission with a radius of several kpc although they show
very compact morphology in the HST/ACS images, which correspond to the
rest-frame UV continuum. Since the MAESTLOs do not show any evidence for AGNs,
the observed extended Lya emission is likely to be caused by star formation
process including the superwind activity. We suggest that this new class of
LAEs, MAESTLOs, provides a missing link from star-forming to passively evolving
galaxies at the peak era of the cosmic star-formation history.Comment: Accepted for publication in ApJ Letters on 15th July, 2015. 6 pages
including 3 figures and 2 table
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