We investigate frequency comb generation in silicon nitride ring resonators by using a pump subject to a weak amplitude modulation. We show that a partial locking is obtained when the external modulation frequency differs from the resonator free-spectral-range by up to hundreds of MHz.
The generation of dissipative Kerr solitons is experimentally investigated in ring resonators with optical feedback. This new double-resonator geometry allows generating frequency combs with smooth solitonic spectral shape over much broader spectral bandwidths if compared with the standard ring resonator architecture. By using an amplitude modulated pump, the repetition rate of the generated frequency comb is locked to the external modulation and exhibits a stability comparable to the modulating radio frequency signal, i.e. the repetition rate linewidth is very narrow (20 Hz). Furthermore, the energy conversion efficiency (pump-to-frequency comb) can be up to 60%, being a record for microresonators.
Frequency combs in a Silicon-Nitride-Microring resonator with an ultra-stable repetition frequency of 28.55 GHz were generated by means of an amplitude modulated pump laser at 1568.8 nm and compared to numerical calculation based on a modified Lugiato-Lefever-Equation. The comb spectrum at a power level of -40 dB with respect to the pump line spans a wavelength range of 70 nm.
We here report on recent progress on astronomical optical frequency comb generation at innoFSPEC-Potsdam and
present preliminary test results using the fiber-fed Multi Unit Spectroscopic Explorer (MUSE) spectrograph. The
frequency comb is generated by propagating two free-running lasers at 1554.3 and 1558.9 nm through two dispersionoptimized
nonlinear fibers. The generated comb is centered at 1590 nm and comprises more than one hundred lines with
an optical-signal-to-noise ratio larger than 30 dB. A nonlinear crystal is used to frequency double the whole comb
spectrum, which is efficiently converted into the 800 nm spectral band. We evaluate first the wavelength stability using
an optical spectrum analyzer with 0.02 nm resolution and wavelength grid of 0.01 nm. After confirming the stability
within 0.01 nm, we compare the spectra of the astro-comb and the Ne and Hg calibration lamps: the astro-comb exhibits
a much larger number of lines than lamp calibration sources. A series of preliminary tests using a fiber-fed MUSE
spectrograph are subsequently carried out with the main goal of assessing the equidistancy of the comb lines. Using a
P3d data reduction software we determine the centroid and the width of each comb line (for each of the 400 fibers
feeding the spectrograph): equidistancy is confirmed with an absolute accuracy of 0.4 pm.
The innovation of optical frequency combs (OFCs) generated in passive mode-locked lasers has provided astronomy
with unprecedented accuracy for wavelength calibration in high-resolution spectroscopy in research areas such as the
discovery of exoplanets or the measurement of fundamental constants. The unique properties of OCFs, namely a highly
dense spectrum of uniformly spaced emission lines of nearly equal intensity over the nominal wavelength range, is not
only beneficial for high-resolution spectroscopy. Also in the low- to medium-resolution domain, the OFCs hold the
promise to revolutionise the calibration techniques. Here, we present a novel method for generation of OFCs. As
opposed to the mode-locked laser-based approach that can be complex, costly, and difficult to stabilise, we propose an
all optical fibre-based system that is simple, compact, stable, and low-cost. Our system consists of three optical fibres
where the first one is a conventional single-mode fibre, the second one is an erbium-doped fibre and the third one is a
highly nonlinear low-dispersion fibre. The system is pumped by two equally intense continuous-wave (CW) lasers. To be
able to control the quality and the bandwidth of the OFCs, it is crucial to understand how optical solitons arise out of the
initial modulated CW field in the first fibre. Here, we numerically investigate the pulse evolution in the first fibre using
the technique of the solitons radiation beat analysis. Having applied this technique, we realised that formation of higherorder
solitons is supported in the low-energy region, whereas, in the high-energy region, Kuznetsov-Ma solitons appear.
Dispersion engineering in integrated silicon nitride waveguides is numerically and experimentally investigated. We show that by modifying the transversal dimensions of the silicon nitride core, it is possible to have a good control of the chromatic dispersion. The inaccuracies due to typical fabrication process in PECD-SiXNY films shows that the dispersion uncertainty is in the order of 20 ps/nm-km at 1550 nm. Silicon nitride waveguides were then fabricated using the same PECVD process and the chromatic dispersion was measured using a low-coherence frequency domain interferometry technique. A comparison between measurements and simulations shows good agreement.
The engineering of the propagation constant in integrated silicon nitride waveguides is numerically investigated. We
compare several geometrical designs and show that fairly large chromatic dispersion control is obtained when the
transversal dimensions are modified.
Silicon nitride ring resonators with diameter of 250 and 500 μm are fabricated and their spectral characteristics
investigated with the ultimate goal of optical frequency comb generation for astronomical spectrograph calibration. A
continuously tunable laser was used to evaluate the spectral characteristics (propagation losses and transmission
properties) of PECVD silicon nitride waveguides and ring-resonators. Losses were measured to be smaller than 0.75
dB/cm over the range between 1500 nm and 1620 nm. The transmission properties of the fabricated ring resonators were
assessed for the TE and TM modes, showing promise for the ultimate goal of astronomical optical frequency comb
generation.
We numerically investigated the possibility of generating high-quality ultra-short optical pulses with broad frequencycombs spectra in a system consisting of three optical fibres. In this system, the first fibre is a conventional single-mode fibre, the second one is erbium-doped, and the last one is a low-dispersion fibre. The system is pumped with a modulated sine-wave generated by two equally intense lasers with the wavelengths λ1and λ2 such that their central wavelength is at λc = (λ1 + λ2)/2 = 1531 nm. The modelling was performed using the generalised nonlinear Schrödinger equation which includes the Kerr and Raman effects, as well as the higher-order dispersion and gain. We took a close look at the pulse evolution in the first two stages and studied the pulse behaviour depending on the group-velocity dispersion and the nonlinear parameter of first fibre, as well as the initial laser frequency separation. For these parameters, the optimum lengths of fibre 1 and 2 were found that provide low-noise pulses. To characterise the pulse energy content, we introduced a figure of merit that was dependent on the group-velocity dispersion, the nonlinearity of fibre 1, and the laser separation.
A conventional Arrayed Waveguide Grating (AWG) has been modified, without output receiver waveguides, for nonconventional
applications such as Astrophotonics and spectroscopy sensing where the input signal can have information
over the entire band and a continuum of light/spectrum. The material system chosen for the AWG design is siliconnitride/
SiO2/Si (Si3N4-SiO2-Si) for its relatively high refractive index, which for a given channel spacing allowing a
more compact device than Silicon-on-Silica. Further, CMOS compatibility and the presence of high non-liner optical
coefficient would be an added advantage to design and fabricate densely integrated photonic sub-systems, such as
calibration source and AWG, for astrophotonics and spectroscopy. The proposed AWG utilizes a flat image plane
optimized for minimal aberration. An analytical calculation, based on Gaussian beam approximation, was used to
determine the optimal flat plane position where the non-uniformity in 1/e electric field widths is minimal. This plane can
be used as the dicing plane to re-image the entire output of the AWG onto a detector array to sample the entire spectrum.
Tailored AWG, with flat image-plane, designed to resolve 48 spectral channels with 0.4nm (50GHz) resolution and
adjacent channel cross-talk level within a 0.2nm window (ITU-grid) ~ -28dB. Calculated insertion loss non-uniformity is
close to 3dB. The foot-print of high index contrast (Δn=23%) IPS is ~ 12x8.5 mm2. The modelled mean spectral
resolving power, R, at the flat image-plane is ~ 7,600. The design principle could be utilised for devices using other
material systems with different parameters.
We here discuss recent progress on astronomical optical frequency comb generation at innoFSPEC-Potsdam. Two
different platforms (and approaches) are numerically and experimentally investigated targeting medium and low
resolution spectrographs at astronomical facilities in which innoFSPEC is currently involved. In the first approach, a
frequency comb is generated by propagating two lasers through three nonlinear stages – the first two stages serve for the
generation of low-noise ultra-short pulses, while the final stage is a low-dispersion highly-nonlinear fibre where the
pulses undergo strong spectral broadening. In our approach, the wavelength of one of the lasers can be tuned allowing
the comb line spacing being continuously varied during the calibration procedure – this tuning capability is expected to
improve the calibration accuracy since the CCD detector response can be fully scanned. The input power, the dispersion,
the nonlinear coefficient, and fibre lengths in the nonlinear stages are defined and optimized by solving the Generalized
Nonlinear Schrodinger Equation. Experimentally, we generate the 250GHz line-spacing frequency comb using two
narrow linewidth lasers that are adiabatically compressed in a standard fibre first and then in a double-clad Er/Yb doped
fibre. The spectral broadening finally takes place in a highly nonlinear fibre resulting in an astro-comb with 250
calibration lines (covering a bandwidth of 500 nm) with good spectral equalization.
In the second approach, we aim to generate optical frequency combs in dispersion-optimized silicon nitride ring
resonators. A technique for lowering and flattening the chromatic dispersion in silicon nitride waveguides with silica
cladding is proposed and demonstrated. By minimizing the waveguide dispersion in the resonator two goals are targeted:
enhancing the phase matching for non-linear interactions and producing equally spaced resonances. For this purpose,
instead of one cladding layer our design incorporates two layers with appropriate thicknesses. We demonstrate a nearly
zero dispersion (with +/- 4 ps/nm-km variation) over the spectral region from 1.4 to 2.3 microns.
The techniques reported here should open new avenues for the generation of compact astronomical frequency comb
sources on a chip or in nonlinear fibres.
The generation of a broadband optical frequency comb with 80 GHz spacing by propagation of a sinusoidal wave
through three dispersion-optimized nonlinear stages is numerically investigated. The input power, the dispersion, the
nonlinear coefficient, and lengths are optimized for the first two stages for the generation of low-noise ultra-short pulses.
The final stage is a low-dispersion highly-nonlinear fibre where the ultra-short pulses undergo self-phase modulation for
strong spectral broadening. The modeling is performed using a Generalized Nonlinear Schrodinger Equation
incorporating Kerr and Raman nonlinearities, self-steepening, high-order dispersion and gain.
In the proposed approach the sinusoidal input field is pre-compressed in the first fibre section. This is shown to be
necessary to keep the soliton order below ten to minimize the noise build-up during adiabatic pulse compression, when
the pulses are subsequently amplified in the next fibre section (rare-earth-doped-fibre with anomalous dispersion). We
demonstrate that there is an optimum balance between dispersion, input power and nonlinearities, in order to have
adiabatic pulse compression. It is shown that the intensity noise grows exponentially as the pulses start to be compressed
in the amplifying fibre. Eventually, the noise decreases and reaches a minimum when the pulses are maximally
compressed. A train of 70 fs pulses with up to 3.45 kW peak power and negligible noise is generated in our simulations,
which can be spectrally broadened in a highly-nonlinear fibre. The main drawback of this compression technique is the
small fibre length tolerance where noise is negligible (smaller than 10 cm for erbium-doped fibre length of 15 m). We
finally investigate how the frequency comb characteristics are modified by incorporating an optical feedback. We show
that frequency combs appropriate for calibration of astronomical spectrographs can be improved by using this technique.
A conventional Arrayed Waveguide Grating (AWG) has been tailored for non-conventional applications such as Astro-Photonics, Life-science and spectroscopy where the input signal can have information over the full continuum of
light/spectrum, compared to discrete optical channels in optical communication systems. The material system chosen for
the AWG design is silicon-nitride/SiO2/Si (Si3N4-SiO2-Si) for it's relatively high refractive index, which for a given
channel spacing allowing a more compact device than Silicon-on-Silica. While existing conventional AWGs cannot be
utilized in spectroscopy when the input is a continuum, due to the fixed output waveguides where the centre wavelength
λc and therefore rest of the wavelength channels have been assigned to predetermined output waveguides, the device
under development has no output waveguides permitting to utilize the entire-image plane of the output star-coupler. The
output of the AWG can then be re-imaged onto a detector array to sample the entire output spectrum, such as the 2-D
infrared arrays used in astronomy. The designed AWG can resolve up to 40 spectral channels with wavelength spacing
0.4nm (50GHz), adjacent channel cross-talk level < -25dB at the ITU grid (25GHz) and non-uniformity ~ 2.5dB. The
modeled mean spectral resolving power, R, at the flat image-plane is ~ 12,200.
A technique for flattening the chromatic dispersion in silicon nitride waveguides with silica cladding is proposed and
numerically investigated. By modifying the transversal dimensions of the silicon nitride core and by adding several
cladding layers with appropriate refractive indices and thicknesses, we demonstrate dispersion flattening over large
spectral bandwidths in the near infrared. We analyze several cladding refractive index profiles that could be realistically
fabricated by using existing materials and doping procedures.
We show that cladding engineering allows for much more dispersion control (and flattening) in comparison with
optimizing only the core transversal dimensions. For the latter case it is demonstrated that while the zero dispersion
wavelength can be shifted to a great extent, the effect of the cross-section adjustment in the flatness is very limited. In
sharp contrast, by adding two cladding layers and decreased refractive index values, the dispersion ripple can be strongly
reduced. By further adding one more layer and by adjusting their refractive indices it is possible to obtain nearly constant
chromatic dispersion (only +/- 3 ps/nm-km variation) over the spectral region from 1.8 to 2.4 microns. In our
calculations, the analyzed change in the silica or silicon nitride refractive index is up to +/-3%. Our technique should
open new avenues for the demonstration of high-performance nonlinear devices on a chip. Furthermore highly dispersive
integrated photonic components can be envisaged for slow light applications and integrated photonics spectrographs.
Supercontinuum white light sources (SCLS) are intense, spatially coherent laser sources with a very broad and flat
spectral energy distribution which have very quickly found ubiquitous use in optical laboratories. As photonics is now
providing more and more applications for astronomical instrumentation, the possible use of SCLS as a calibration light
source for spectroscopy has been tested. A standard industrial SCLS was coupled to the calibration unit of the PMAS
integral field spectrophotometer and compared directly to the PMAS standard tungsten filament lamp that is normally
used for calibration exposures. We report on comparative measurements concerning flux, spectral energy distribution,
and temporal stability.
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