T. Fremberg

Astronomical optical frequency comb generation and test in a fiber-fed MUSE spectrograph

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.

Astronomical optical frequency comb generation in nonlinear fibres and ring resonators: optimization studies

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.

Dispersion Engineering in Silicon Nitride

As part of ongoing efforts towards development of integrated optoelectronic platforms on a single chip, specifically integrated photonic spectrographs for Astronomy, we report numerical and experimental results from dispersion engineering in integrated silicon nitride waveguides.

Group velocity dispersion manipulation in integrated waveguides

The ability to arbitrarily control the chromatic dispersion in CMOS-compatible waveguides should strengthen the viability of this technology, particularly for nonlinear devices on a chip [1]. Here we report on a systematic investigation of group velocity dispersion engineering in channel and rib waveguides with a silicon nitride core (Si3N4). The dispersion control is done by including three cladding layers: the first two are thin (<;400nm) and are made of silica (SiO2) or Si3N4 with refractive indices that can be varied up to 3% with respect to an average value. All this is embedded in a silica cladding. Up to eight parameters can be tuned for dispersion optimization: height and width of the core, thickness and refractive index of the first two claddings, and the type of waveguide (rib or channel). The details of the waveguides under investigation are shown in the inset of Figure 1(a). We have two goals: 1) finding the flattest possible dispersion irrespective of its absolute value, and 2) finding the flattest and lowest dispersion. Figure 1 shows the results after optimizing the eight parameters for the rib and channel waveguide. The flattest dispersion (solid line) is found for the waveguide that includes a silica layer as the first cladding: over a bandwidth of 1000 nm (1350-2450nm) the dispersion is -68 ± 0.6 ps/nm-km. This result demonstrates that appropriate engineering in integrated waveguides produces flattened dispersion profiles comparable as those in photonic crystal fibres [2]. When the silica layer is not included, the flattest dispersion is anomalous (+45 ± 1.5 ps/nm-km) and spans over 700 nm (dotted line). If the goal is having flat and zero dispersion, again the structure with a silica cladding layer (dashed line) provides the best result (2 +/- 2 ps/nm-km over 900 nm). On the other hand the other structure provides flat and low dispersion over 500 nm (dot-dashed line). We analyzed what is the main requirement to have ultra-flat dispersion and we observed that the first silica layer allows for a large control of the dispersion flatness and its absolute value. On the other hand, having a rib waveguide or changing the layers refractive indices by a few % can flatten the dispersion, but will not allow for an arbitrary control. This indicates, that it is necessary to have a certain amount of refractive index contrast in order to modify at a great extent the dispersion.

Numerical investigation of propagation constant in silicon nitride waveguides with different refractive index profiles

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 waveguides and micro ring-resonators for astronomical optical frequency comb generation

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.

Silicon nitride waveguide with flattened chromatic dispersion

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.