skip to main content

Title: Ultrafast laser inscription of asymmetric integrated waveguide 3  dB couplers for astronomical K-band interferometry at the CHARA array

We present the fabrication and characterization of 3 dB asymmetric directional couplers for the astronomical K-band at wavelengths between 2.0 and 2.4 µm. The couplers were fabricated in commercial Infrasil silica glass using an ultrafast laser operating at 1030 nm. After optimizing the fabrication parameters, the insertion losses of straight single-mode waveguides were measured to be∼<#comment/>1.2±<#comment/>0.5dBacross the full K-band. We investigate the development of asymmetric 3 dB directional couplers by varying the coupler interaction lengths and by varying the width of one of the waveguide cores to detune the propagation constants of the coupled modes. In this manner, we demonstrate that ultrafast laser inscription is capable of fabricating asymmetric 3 dB directional couplers for future applications in K-band stellar interferometry. Finally, we demonstrate that our couplers exhibit an interferometric fringe contrast of><#comment/>90%<#comment/>. This technology paves the path for the development of a two-telescope K-band integrated optic beam combiner for interferometry to replace the existing beam combiner (MONA) in Jouvence of the Fiber Linked Unit for Recombination (JouFLU) at the Center for High Angular Resolution Astronomy (CHARA) telescope array.

; ; ; ; ; ; ; ; ; ; ; ; ;
Award ID(s):
1715788 1636624
Publication Date:
Journal Name:
Journal of the Optical Society of America B
Page Range or eLocation-ID:
Article No. 2455
0740-3224; JOBPDE
Optical Society of America
Sponsoring Org:
National Science Foundation
More Like this
  1. We study the relationship between the input phase delays and the output mode orders when using a pixel-array structure fed by multiple single-mode waveguides for tunable orbital-angular-momentum (OAM) beam generation. As an emitter of a free-space OAM beam, the designed structure introduces a transformation function that shapes and coherently combines multiple (e.g., four) equal-amplitude inputs, with thekth input carrying a phase delay of(k−<#comment/>1)Δ<#comment/>φ<#comment/>. The simulation results show that (1) the generated OAM order ℓ is dependent on the relative phase delayΔ<#comment/>φ<#comment/>; (2) the transformation function can be tailored by engineering the structure to support different tunable ranges (e.g., l={−<#comment/>1},{−<#comment/>1,+1},{−<#comment/>1,0,+1}, or{−<#comment/>2,−<#comment/>1,+1,+2}); and (3) multiple independent coaxial OAM beams can be generated by simultaneously feeding the structure with multiple independent beams, such that each beam has its ownΔ<#comment/>φ<#comment/>value for the four inputs. Moreover, there is a trade-off between the tunable range and the mode purity, bandwidth, and crosstalk, such that the increase of the tunable range leads to (a) decreased mode purity (from 91% to 75% formore »display='inline'>l=−<#comment/>1), (b) decreased 3 dB bandwidth of emission efficiency (from 285 nm forl={−<#comment/>1}to 122 nm forl={−<#comment/>2,−<#comment/>1,+1,+2}), and (c) increased crosstalk within the C-band (from−<#comment/>23.7to−<#comment/>13.2dBwhen the tunable range increases from 2 to 4).

    « less
  2. Integrated astrophotonic spectrometers are integrated variants of conventional free-space spectrometers that offer significantly reduced size, weight, and cost and immunity to alignment errors, and can be readily integrated with other astrophotonic instruments such as nulling interferometers. Current integrated dispersive astrophotonic spectrometers are one-dimensional devices such as arrayed waveguide gratings or planar echelle gratings. These devices have been limited to104resolving powers and<<#comment/>1000spectral bins due to having limited total optical delay paths and 1D detector array pixel densities. In this paper, we propose and demonstrate a high-resolution and compact astrophotonic serpentine integrated grating (SIG) spectrometer design based on a 2D dispersive serpentine optical phased array. The SIG device combines a 5.2 cm long folded delay line with grating couplers to create a large optical delay path along two dimensions in a compact integrated device footprint. Analogous to free-space crossed-dispersion high-resolution spectrometers, the SIG spectrometer maps spectral content to a 2D wavelength-beam-steered folded-raster emission pattern focused onto a 2D detector array. We demonstrate a SIG spectrometer with∼<#comment/>100kresolving power and∼<#comment/>6750spectral bins, which are approximately an order of magnitude higher than previous integrated photonic designs that operate over a wide bandwidth, in amore »display='inline'>0.4mm2footprint. We measure a Rayleigh resolution of1.93±<#comment/>0.07GHzand an operational bandwidth from 1540 nm to 1650 nm. Finally, we discuss refinements of the SIG spectrometer that improve its resolution, bandwidth, and throughput. These results show that SIG spectrometer technology provides a path towards miniaturized, high-resolution spectrometers for applications in astronomy and beyond.

    « less
  3. Geometric diodes represent a relatively new class of diodes used in rectennas that rely on the asymmetry of a conducting thin film. Here, we numerically investigate a plasmonic analogue of geometric diodes to realize nanoscale optical asymmetric transmission. The device operates based on spatial symmetry breaking that relies on a unique property of surface plasmon polaritons (SPPs), namely, adiabatic nanofocusing. We show that the structure can realize on-chip asymmetric electromagnetic transmission with a total dimension of∼<#comment/>2µ<#comment/>m×<#comment/>6µ<#comment/>m. We demonstrate a signal contrast of 0.7 and an asymmetric optical transmission ratio of 4.77 dB. We investigate the origin of the asymmetric transmission and show that it is due mainly to asymmetric out-coupling of SPPs to far-field photons. We highlight the role of evanescent field coupling of SPPs in undermining the asymmetric transmission efficiency and show that by adjusting the plasmonic waveguide dimensions, a signal contrast of 0.94 and an asymmetric optical transmission ratio of 5.18 dB can be obtained. Our work presents a new paradigm for on-chip nanoscale asymmetric optical transmission utilizing the unique properties of SPPs.

  4. Electro-optic (EO) modulators rely on the interaction of optical and electrical signals with second-order nonlinear media. For the optical signal, this interaction can be strongly enhanced using dielectric slot–waveguide structures that exploit a field discontinuity at the interface between a high-index waveguide core and the low-index EO cladding. In contrast to this, the electrical signal is usually applied through conductive regions in the direct vicinity of the optical waveguide. To avoid excessive optical loss, the conductivity of these regions is maintained at a moderate level, thus leading to inherentRClimitations of the modulation bandwidth. In this paper, we show that these limitations can be overcome by extending the slot–waveguide concept to the modulating radio-frequency (RF) signal. Our device combines an RF slotline that relies onBaTiO3as a high-k dielectric material with a conventional silicon photonic slot waveguide and a highly efficient organic EO cladding material. In a proof-of-concept experiment, we demonstrate a 1 mm long Mach–Zehnder modulator that offers a 3 dB bandwidth of 76 GHz and a 6 dB bandwidth of 110 GHz along with a smallπ<#comment/>voltage of 1.3 V (Uπ<#comment/>L=1.3Vmm). Wemore »further demonstrate the viability of the device in a data-transmission experiment using four-state pulse-amplitude modulation (PAM4) at line rates up to 200 Gbit/s. Our first-generation devices leave vast room for further improvement and may open an attractive route towards highly efficient silicon photonic modulators that combine sub-1 mm device lengths with sub-1 V drive voltages and modulation bandwidths of more than 100 GHz.

    « less
  5. We experimentally demonstrate the utilization of adaptive optics (AO) to mitigate intra-group power coupling among linearly polarized (LP) modes in a graded-index few-mode fiber (GI FMF). Generally, in this fiber, the coupling between degenerate modes inside a modal group tends to be stronger than between modes belonging to different groups. In our approach, the coupling inside theLP11group can be represented by a combination of orbital-angular-momentum (OAM) modes, such that reducing power coupling in OAM set tends to indicate the capability to reduce the coupling inside theLP11group. We employ two output OAM modesl=+1andl=−<#comment/>1as resultant linear combinations of degenerateLP11aandLP11bmodes inside theLP11group of a∼<#comment/>0.6-kmGI FMF. The power coupling is mitigated by shaping the amplitude and phase of the distorted OAM modes. Each OAM mode carries an independent 20-, 40-, or 100-Gbit/s quadrature-phase-shift-keying data stream. We measure the transmission matrix (TM) in the OAM basis withinLP11group, which is a subset of the full LP TMmore »of the FMF-based system. An inverse TM is subsequently implemented before the receiver by a spatial light modulator to mitigate the intra-modal-group power coupling. With AO mitigation, the experimental results forl=+1andl=−<#comment/>1modes show, respectively, that (i) intra-modal-group crosstalk is reduced by><#comment/>5.8dBand><#comment/>5.6dBand (ii) near-error-free bit-error-rate performance is achieved with a penalty of∼<#comment/>0.6dBand∼<#comment/>3.8dB, respectively.

    « less