We have investigated the nonlinear optical response of low loss Si(0.6)Ge(0.4) / Si waveguides in the mid-infrared wavelength range from 3.25- 4.75μm using picosecond optical pulses. We observed and ...measured the three and four-photon absorption coefficients as well as the Kerr nonlinear refractive index. The dynamics of the spectral broadening suggests that, in addition to multiphoton absorption, the corresponding higher order nonlinear refractive phenomena also needs to be included when high optical pulse intensities are used at mid-infrared wavelengths in this material.
Context. Recently, an increasing number of scientific publications making use of images obtained with near-infrared long-baseline interferometry have been produced. The technique has reached, at ...last, a technical maturity level that opens new avenues for numerous astrophysical topics requiring milli-arc-second model-independent imaging. The Very Large Telescope Interferometer (VLTI) will soon be equipped with instruments able to combine between four and six telescopes. Aims. In the framework of the VLTI second generation instruments Gravity and VSI, we propose a new beam combining concept using integrated optics (IO) technologies with a novel ABCD-like fringe encoding scheme. Our goal is to demonstrate that IO-based combinations bring considerable advantages in terms of instrumental design and performance. We therefore aim at giving a full characterization of an IO beam combiner in order to establish its performance and check its compliance with the specifications of an imaging instrument. Methods. For this purpose, prototype IO beam combiners have been manufactured and laboratory measurements were made in the H band with a dedicated testbed, simulating a four-telescope interferometer. We studied the beam combiners through the analysis of throughput, instrumental visibilities, phases and closure phases in wide band as well as with spectral dispersion. Study of the polarization properties was also carried out. Results. We obtain competitive throughput (65%), high and stable instrumental contrasts (from 80% in wide band up to 100% ± 1% with spectral dispersion), stable but non-zero closure phases (e.g. 115° ± 2°) which we attribute to internal optical path differences (OPD) that can be calibrated. We validate a new static and an achromatic phase shifting IO function close to the nominal 90° value (e.g. 80° ± 1°). All these observables show limited chromaticity over the H band range. Conclusions. Our results demonstrate that such ABCD-like beam combiners are particularly well suited for interferometric combination of multiple beams to achieve aperture synthesis imaging. This opens the way to extending this technique to all near infrared wavelengths and in particular, the K band.
We characterize the nonlinear optical response of low loss Si(0.6)Ge(0.4) / Si waveguides in the mid-infrared between 3.3 μm and 4 μm using femtosecond optical pulses. We estimate the three and ...four-photon absorption coefficients as well as the Kerr nonlinear refractive index from the experimental measurements. The effect of multiphoton absorption on the optical nonlinear Kerr response is evaluated and the nonlinear figure of merit estimated providing some guidelines for designing nonlinear optical devices in the mid-IR. Finally, we compare the impact of free-carrier absorption at mid-infrared wavelengths versus near-infrared wavelengths for these ultra-short pulses.
We report a nonlinear signal processing system based on a SiGe waveguide suitable for high spectral efficiency data signals. Four-wave-mixing (FWM)-based wavelength conversion of 10-Gbaud ...16-Quadrature amplitude modulated (QAM) and 64-QAM signals is demonstrated with less than -10-dB conversion efficiency (CE), 36-dB idler optical signal-to-noise ratio (OSNR), negligible bit error ratio (BER) penalty and a 3-dB conversion bandwidth exceeding 30nm. The SiGe device was CW-pumped and operated in a passive scheme without giving rise to any two-photon absorption (TPA) effects.
Imaging the direct light signal from a faint exoplanet against the overwhelming glare of its host star presents one of the fundamental challenges to modern astronomical instrumentation. Achieving ...sufficient signal-to-noise ratio for detection by direct imaging is limited by three basic physical processes: aberration of the wavefronts (both instrumental and atmospheric), photon noise and detector noise. In this paper, we advance a novel optical setup which synthesizes the advantages of two different techniques: nulling interferometry to mitigate photon noise, and closure phase to combat optical aberrations. Our design, which employs technology from integrated optics and photonics, is intended to combine the advantageous aspects of both a coronagraph and a non-redundant interferometer inside a single optical device. We show that such an instrument would have a dynamic range limited by either (i) the readout noise (if perfect co-phasing), or (ii) the photon noise due to stellar flux leakage (in the case of imperfect nulling). This concept is optimal when the readout noise is not the main limitation, i.e. for space interferometry or for ground-based observations of bright stellar hosts (apparent magnitude brighter than 10).
Using the three-telescope IOTA interferometer on Mount Hopkins, we report results from the first near-infrared (l = 1.65 km) closure-phase survey of young stellar objects (YSOs). These closure phases ...allow us to unambiguously detect departures from centrosymmetry (i.e., skew) in the emission pattern from YSO disks on the scale of 64 mas, expected from generic "flared disk" models. Six of 14 targets showed small, yet statistically significant nonzero closure phases, with largest values from the young binary system MWC 361-A and the (pre main-sequence?) Be star HD 45677. Our observations are quite sensitive to the vertical structure of the inner disk, and we confront the predictions of the "puffed-up inner wall" models of Dullemond, Dominik, & Natta (DDN). Our data support disk models with curved inner rims because the expected emission appears symmetrically distributed around the star over a wide range of inclination angles. In contrast, our results are incompatible with the models possessing vertical inner walls because they predict extreme skewness (i.e., large closure phases) from the near-IR disk emission that is not seen in our data. In addition, we also present the discovery of mysterious H-band "halos" (65%-10% of light on scales 0."01-0."50) around a few objects, a preliminary "parametric imaging" study for HD 45677, and the first astrometric orbit for the young binary MWC 361-A.
Error-free transmission of 10 Gbit/s non-return to zero optical signals along 2.5 cm long silicon germanium waveguides at a wavelength of 1.98 μm is demonstrated. Bit error rate measurements confirm ...the absence of penalty during the transmission through a subwavelength of 1.3 and 2.2 μm wide waveguides.
We have measured nonzero closure phases for about 29% of our sample of 56 nearby asymptotic giant branch (AGB) stars, using the three-telescope Infrared Optical Telescope Array (IOTA) interferometer ...at near-infrared wavelengths (H band) and with angular resolutions in the range 5-10 mas. These nonzero closure phases can only be generated by asymmetric brightness distributions of the target stars or their surroundings. We discuss how these results were obtained and how they might be interpreted in terms of structures on or near the target stars. We also report measured angular sizes and hypothesize that most Mira stars would show detectable asymmetry if observed with adequate angular resolution.
Aims. Our objective is to prove that integrated optics (IO) is not only a good concept for astronomical interferometry but also a working technique with high performance. Methods. We used the ...commissioning data obtained with the dedicated K-band integrated optics two-telescope beam combiner that now replaces the fiber coupler MONA in the VLTI/VINCI instrument. We characterize the behavior of this IO device and compare its properties to other single mode beam combiner like the previously used MONA fiber coupler. Results. The IO combiner provides a high optical throughput, a contrast of 89% with a night-to-night stability of a few percent. Even if a dispersive phase is present, we show that it does not bias the measured Fourier visibility estimate. An upper limit of 5 x 10 super(-3) for the cross-talk between linear polarization states has been measured. We take advantage of the intrinsic contrast stability to test a new astronomical procedure for calibrating diameters of simple stars by simultaneously fitting the instrumental contrast and the apparent stellar diameters. This method reaches an accuracy with diameter errors on the order of previous ones but without the need of an already known calibrator. Conclusions. These results are an important step for IO, since they prove its maturity in an astronomical band where the technology has been specially developed for astronomical convenience. It paves the way to incoming imaging interferometer projects.
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