53 research outputs found
Using ultra-short pulses to determine particle size and density distributions
We analyze the time dependent response of strongly scattering media (SSM) to
ultra-short pulses of light. A random walk technique is used to model the
optical scattering of ultra-short pulses of light propagating through media
with random shapes and various packing densities. The pulse spreading was found
to be strongly dependent on the average particle size, particle size
distribution, and the packing fraction. We also show that the intensity as a
function of time-delay can be used to analyze the particle size distribution
and packing fraction of an optically thick sample independently of the presence
of absorption features. Finally, we propose an all new way to measure the shape
of ultra-short pulses that have propagated through a SSM.Comment: 15 pages, 29 figures, accepted for publication in Optics Express will
update with full reference when it is availabl
Surface acoustic waves for acousto-optic modulation in buried silicon nitride waveguides
We theoretically investigate the use of Rayleigh surface acoustic waves
(SAWs) for refractive index modulation in optical waveguides consisting of
amorphous dielectrics. Considering low-loss SiN waveguides with a
standard core cross section of 4.40.03 m size, buried 8 m
deep in a SiO cladding we compare surface acoustic wave generation in
various different geometries via a piezo-active, lead zirconate titanate film
placed on top of the surface and driven via an interdigitized transducer (IDT).
Using numerical solutions of the acoustic and optical wave equations, we
determine the strain distribution of the SAW under resonant excitation. From
the overlap of the acoustic strain field with the optical mode field we
calculate and maximize the attainable amplitude of index modulation in the
waveguide. For the example of a near-infrared wavelength of 840 nm, a maximum
shift in relative effective refractive index of 0.7x10 was obtained for
TE polarized light, using an IDT period of 30 - 35 m, a film thickness of
2.5 - 3.5 m, and an IDT voltage of 10 V. For these parameters, the
resonant frequency is in the range 70 - 85 MHz. The maximum shift increases to
1.2x10, with a corresponding resonant frequency of 87 MHz, when the
height of the cladding above the core is reduced to 3 m. The relative
index change is about 300-times higher than in previous work based on
non-resonant proximity piezo-actuation, and the modulation frequency is about
200-times higher. Exploiting the maximum relative index change of
1.210 in a low-loss balanced Mach-Zehnder modulator should allow
full-contrast modulation in devices as short as 120 m (half-wave voltage
length product = 0.24 Vcm).Comment: 19 pages, 8 figure
A gain-coefficient switched Alexandrite laser
We report on a gain-coefficient switched Alexandrite laser. An electro-optic
modulator is used to switch between high and low gain states by making use of
the polarization dependent gain of Alexandrite. In gain-coefficient switched
mode, the laser produces 85 ns pulses with a pulse energy of 240 mJ at a
repetition rate of 5 Hz.Comment: 8 pages, 5 figure
On-Chip Phase-Shift Induced Control of Supercontinuum Generation in a Dual-Core SiN Waveguide
We investigate on-chip spectral control of supercontinuum generation, taking
advantage of the additional spatial degree of freedom in strongly-coupled
dual-core waveguides. Using numerical integration of the multi-mode generalized
nonlinear Schr\"odinger equation, we show that, with proper waveguide
cross-section design, selective excitation of supermodes can vary the
dispersion to its extremes, i.e., all-normal or anomalous dispersion can be
selected via phase shifting in a Mach-Zehnder input circuit. The resulting
control allows to provide vastly different supercontinuum spectra with the same
waveguide circuit
Ring resonator enhanced mode-hop-free wavelength tuning of an integrated extended-cavity laser
Extending the cavity length of diode lasers with feedback from Bragg
structures and ring resonators is highly effective for obtaining ultra-narrow
laser linewidths. However, cavity length extension also decreases the
free-spectral range of the cavity. This reduces the wavelength range of
continuous laser tuning that can be achieved with a given phase shift of an
intracavity phase tuning element. We present a method that increases the range
of continuous tuning to that of a short equivalent laser cavity, while
maintaining the ultra-narrow linewidth of a long cavity. Using a
single-frequency hybrid integrated InP-Si3N4 diode laser with 120 nm coverage
around 1540 nm, with a maximum output of 24 mW and lowest intrinsic linewidth
of 2.2 kHz, we demonstrate a six-fold increased continuous and mode-hop-free
tuning range of 0.22 nm (28 GHz) as compared to the free-spectral range of the
laser cavity.Comment: 12 pages, 7 figure
High-purity microwave generation using a dual-frequency hybrid integrated semiconductor-dielectric waveguide laser
We present an integrated semiconductor-dielectric hybrid dual-frequency laser
operating in the 1.5 m wavelength range for microwave and terahertz (THz)
generation. Generating a microwave beat frequency near 11 GHz, we observe a
record-narrow intrinsic linewidth as low as about 2 kHz. This is realized by
hybrid integration of a single diode amplifier based on indium phosphide (InP)
with a long, low-loss silicon nitride (SiN) feedback circuit to extend
the cavity photon lifetime, resulting in a cavity optical roundtrip length of
about 30 cm on a chip. Simultaneous lasing at two frequencies is enabled by
introducing an external control parameter for balancing the feedback from two
tunable, frequency-selective Vernier mirrors on the SiN chip. Each
frequency can be tuned with a wavelength coverage of about 80 nm, potentially
allowing for the generation of a broad range of frequencies in the microwave
range up to the THz range
High confinement, high yield Si3N4 waveguides for nonlinear optical application
In this paper we present a novel fabrication technique for silicon nitride
(Si3N4) waveguides with a thickness of up to 900 nm, which are suitable for
nonlinear optical applications. The fabrication method is based on etching
trenches in thermally oxidized silicon and filling the trenches with Si3N4.
Using this technique no stress-induced cracks in the Si3N4 layer were observed
resulting in a high yield of devices on the wafer. The propagation losses of
the obtained waveguides were measured to be as low as 0.4 dB/cm at a wavelength
of around 1550 nm.Comment: 10 pages, 4 figure
Ultra-narrow linewidth hybrid integrated semiconductor laser
We demonstrate a hybrid integrated and widely tunable diode laser with an
intrinsic linewidth as narrow as 40 Hz, achieved with a single roundtrip
through a low-loss feedback circuit that extends the cavity length to 0.5 meter
on a chip. Employing solely dielectrics for single-roundtrip, single-mode
resolved feedback filtering enables linewidth narrowing with increasing laser
power, without limitations through nonlinear loss. We achieve single-frequency
oscillation with up to 23 mW fiber coupled output power, 70-nm wide spectral
coverage in the 1.55 m wavelength range with 3 mW output, and obtain more
than 60 dB side mode suppression. Such properties and options for further
linewidth narrowing render the approach of high interest for direct integration
in photonic circuits serving microwave photonics, coherent communications,
sensing and metrology with highest resolution.Comment: 13 pages, and 11 figure
A hybrid-integrated diode laser in the visible spectral range
Generating visible light with wide tunability and high coherence based on
photonic integrated circuits is of high interest for applications in
biophotonics, precision metrology and quantum technology. Here we present the
first demonstration of a hybrid-integrated diode laser in the visible spectral
range. Using an AlGaInP optical amplifier coupled to a low-loss Si3N4 feedback
circuit based on microring resonators, we obtain a spectral coverage of 10.8 nm
around 684.4 nm wavelength with up to 4.8 mW output power. The measured
intrinsic linewidth is 2.30.2 kHz.Comment: 8 pages, 6 figure
Ellipsometry with an undetermined polarization state
We show that, under the right conditions, one can make highly accurate
polarization-based measurements without knowing the absolute polarization state
of the probing light field. It is shown that light, passed through a randomly
varying birefringent material has a well-defined orbit on the Poincare sphere,
which we term a generalized polarization state, that is preserved. Changes to
the generalized polarization state can then be used in place of the absolute
polarization states that make up the generalized state, to measure the change
in polarization due to a sample under investigation. We illustrate the
usefulness of this analysis approach by demonstrating fiber-based ellipsometry,
where the polarization state of the probe light is unknown, and, yet, the
ellipsometric angles of the investigated sample ( and ) are
obtained with an accuracy comparable to that of conventional ellipsometry
instruments by measuring changes to the generalized polarization state.Comment: 6 pages, 4 figures, 1 tabl
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