Abstract: We develop a periodically-poled LiNbO3 (PPLN)
device for highly-efficient third-harmonic generation (THG), where
the THG efficiency is enhanced with a cavity. THG can usually be
produced via χ(3)-nonlinear materials by optical pumping with very
high pump-power. Instead, we here propose THG by moderate-power
pumping through a specially-designed PPLN device containing
only χ(2)-nonlinearity, where sum-frequency generation in the χ(2)
process is employed for the mixing of a pump beam and a
second-harmonic-generation (SHG) beam produced from the pump
beam. The cavity is designed to increase the SHG power with dichroic
mirrors attached to both ends of the device that perfectly reflect
the SHG beam back to the device and yet let the pump and THG
beams pass through the mirrors. This brings about a THG-power
enhancement because of THG power proportional to the enhanced
SHG power. We examine the THG-efficiency dependence on the
mirror reflectance and show that very high THG-efficiency is obtained
at moderate pump-power when compared with that of a cavity-free
PPLN device.
Abstract: The principle of all-silicon Raman lasers for an
output wavelength of 1.3 μm is presented, which employs
quasi-phase-matched structures and resonators to enhance the output
power. 1.3-μm laser beams for GE-PONs in FTTH systems generated
from a silicon device are very important because such a silicon device
can be monolithically integrated with the silicon planar lightwave
circuits (Si PLCs) used in the GE-PONs. This reduces the device
fabrication processes and time and also optical losses at the junctions
between optical waveguides of the Si PLCs and Si laser devices
when compared with 1.3-μm III-V semiconductor lasers set on the
Si PLCs employed at present. We show that the quasi-phase-matched
Si Raman laser with resonators can produce about 174 times larger
laser power at 1.3 μm (at maximum) than that without resonators
for a Si waveguide of Raman gain 20 cm/GW and optical loss 1.2
dB/cm, pumped at power 10 mW, where the length of the waveguide
is 3 mm and its cross-section is (1.5 μm)2.
Abstract: Electrical conduction in a quasi-one-dimensional
polycrystalline metallic ring with a long electron phase coherence
length realized at low temperature is investigated. In this situation, the
wave nature of electrons is important in the ring, where the electrical
current I can be induced by a vector potential that arises from a static
magnetic field applied perpendicularly to the ring’s area. It is shown
that if the average grain size of the polycrystalline ring becomes
large (or comparable to the Fermi wavelength), the electrical current
I increases to ~I0, where I0 is a current in a disorder-free ring. The
cause of this increasing effect is examined, and this takes place if the
electron localization length in the polycrystalline potential increases
with increasing grain size, which gives rise to coherent connection
of tails of a localized electron wave function in the ring and thus
provides highly coherent electrical conduction.
Abstract: The detection of environmental gases, 12CO2, 13CO2,
and CH4, using near-infrared semiconductor lasers with a short
laser path length is studied by means of wavelength-modulation
spectroscopy. The developed system is compact and has high
sensitivity enough to detect the absorption peaks of isotopic 13CO2
of a 3-% CO2 gas at 2 μm with a path length of 2.4 m, where
its peak size is two orders of magnitude smaller than that of the
ordinary 12CO2 peaks. In addition, the detection of 12CO2 peaks of
a 385-ppm (0.0385-%) CO2 gas in the air is made at 2 μm with a
path length of 1.4 m. Furthermore, in pursuing the detection of an
ancient environmental CH4 gas confined to a bubble in ice at the
polar regions, measurements of the absorption spectrum for a trace
gas of CH4 in a small area are attempted. For a 100-% CH4 gas
trapped in a ∼ 1 mm3 glass container, the absorption peaks of CH4
are obtained at 1.65 μm with a path length of 3 mm, and also the
gas pressure is extrapolated from the measured data.
Abstract: We have studied a method to widen the spectrum
of optical pulses that pass through an InGaAsP waveguide for
application to broadband optical communication. In particular, we
have investigated the competitive effect between spectral broadening
arising from nonlinear refraction (optical Kerr effect) and shrinking
due to two photon absorption in the InGaAsP waveguide with
χ(3) nonlinearity. The shrunk spectrum recovers broadening by
the enhancement effect of the nonlinear refractive index near the
bandgap of InGaAsP with a bandgap wavelength of 1490 nm. The
broadened spectral width at around 1525 nm (196.7 THz) becomes
10.7 times wider than that at around 1560 nm (192.3 THz) without
the enhancement effect, where amplified optical pulses with a pulse
width of ∼ 2 ps and a peak power of 10 W propagate through a
1-cm-long InGaAsP waveguide with a cross-section of 4 (μm)2.