Photonic Integrated Circuits (PIC) are best known for their important role in the telecommunication sector, e.g. high speed communication devices in data centers. However, PIC also hold the promise for innovation in sectors like life science, medicine, sensing, automotive etc. The past two decades have seen efforts of utilizing PIC to enhance the performance of instrumentation for astronomical telescopes, perhaps the most spectacular example being the integrated optics beam combiner for the interferometer GRAVITY at the ESO Very Large Telescope. This instrument has enabled observations of the supermassive black hole in the center of the Milky Way at unprecedented angular resolution, eventually leading to the Nobel Price for Physics in 2020. Several groups worldwide are actively engaged in the emerging field of astrophotonics research, amongst them the innoFSPEC Center in Potsdam, Germany. We present results for a number of applications developed at innoFSPEC, notably PIC for integrated photonic spectrographs on the basis of arrayed waveguide gratings and the PAWS demonstrator (Potsdam Arrayed Waveguide Spectrograph), PIC-based ring resonators in astronomical frequency combs for precision wavelength calibration, discrete beam combiners (DBC) for large astronomical interferometers, as well as aperiodic fiber Bragg gratings for complex astronomical filters and their possible derivatives in PIC.
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.
The accurate characterization of the field at the output of the optical fibres is of relevance for precision spectroscopy in astronomy. The modal effects of the fibre translate to the illumination of the pupil in the spectrograph and impact on the resulting point spread function (PSF). A Model is presented that is based on the Eigenmode Expansion Method (EEM) that calculates the output field from a given fibre for different manipulations of the input field. The fibre design and modes calculation are done via the commercially available Rsoft-FemSIM software. We developed a Python script to apply the EEM. Results are shown for different configuration parameters, such as spatial and angular displacements of the input field, spot size and propagation length variations, different transverse fibre geometries and different wavelengths. This work is part of the phase A study of the fibre system for MOSAIC, a proposed multi-object spectrograph for the European Extremely Large Telescope (ELT-MOS).
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.
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.
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.
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.
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 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.
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