KEYWORDS: Fiber Bragg gratings, Near infrared, Diffraction, Optical alignment, Control systems, Modulation, Algorithm development, Process control, Space telescopes, Telescopes
The power of the next generation of telescopes that will rely largely on the combination of light-collecting area with excellent (ideally: diffraction limited) image quality. Therefore, the focus will heavily lean on adaptive optics and the near infrared wavelength regime. A severe limiting factor is the presence and strength of atmospheric OH emission lines in the NIR. OH suppression techniques involving fiber Bragg gratings (FBG) have been proposed, however as yet not fully demonstrated on sky. We are involved in the first generation FBG prototype development with partners in Australia, including the GNOSIS and PRAXIS on-sky experiments.
Since the supply of suitable multi-notch filters is no longer available from industry, we have made an effort at innoFSPEC Potsdam to build a specialized laboratory for the development and manufacture of 2nd generation FBGs for OH suppression.
Suppression of the strong NIR OH emission lines requires a single grating that reflects multiple wavelengths, spaced at non-periodic intervals, with flat-top profile and high suppression ratio. It has been shown that aperiodic fiber Bragg gratings (AFBGs) can provide such functions. However, the fabrication technology requires accurate optical alignment of several degrees of freedom as well as complex control of modulated beams to form a varying interference pattern. In our work, an algorithm is developed from the index profile of a multi-notch AFBG to the design of a complex phase-mask that can generate a matching UV diffraction pattern, which will in turn inscribe an single-mode fiber into the chosen AFBG. With such a phase mask, the fabrication of the AFBGs will be reduced to a simple UV-exposure process, i.e., the complex alignment and control processes of the interference pattern from modulated beams are avoided altogether. The resulting reliable and reproducible fabrication process will dramatically reduce of the cost of such filters. Packaging aspects for a complete sky emission filter system will also be discussed.
Compact yet highly functional optical components are desired in modern astronomical instruments targeted at low system cost and reduced maintenance complexity. Integrated photonic spectrometers based on planar lightwave circuits are attractive as the planar miniature device can provide high spectral resolution but also great robustness and flexibility in the design of spectrograph systems. Arrayed waveguide gratings (AWGs) have the potential to be adapted and optimized to function as compact spectrometers in astronomical spectrographs. In this work high-resolution AWGs based on low-loss silica waveguides have been designed, fabricated and characterized. The measured spectral resolution exceeds 104 with Δλ = 150 pm at 1548 nm. The insertion loss (including two times fiber-chip coupling) is merely 2.07 dB, amounting to a peak throughput of 62%. Adiabatic fiber taper is developed to bring down the mode field diameter of a standard single mode fiber to match the mode size of the designed waveguide, resulting in almost lossless coupling from the fiber to the waveguide. The free-spectrum range is 48 nm and the side-band suppression is 22 dB. The AWG is also polarization-insensitive. Rotating the linearly polarized input light by 180° results in a slight shift of the central wavelength ~ 30 pm. The excellent overall performance makes this AWG an ideal candidate as the key building block for the development of an integrated astronomical spectrograph module.
The fabrication of high performance gratings for distributed feedback (DFB) lasers by direct-write (DW) electron-beam
lithography (EBL) is presented. This paper starts with a short introduction of the grating theory and various types of
gratings commonly used in DFB lasers, laying out resolution requirements and other fabrication challenges. The
development and optimization process of the adopted EBL technology is then disclosed to address these challenges. In
the end, the state-of-the-art laser performance is demonstrated, validating the technology and also paving ways for more
advanced applications in the modern optical networks. We concentrate on grating fabrication technology of DFB lasers
for telecommunication applications as the technology has been continuously developed at Fraunhofer Heinrich Hertz
Institute (HHI) for more than two decades.
Hybrid photonic integration allows individual components to be developed at their best-suited material platforms without sacrificing the overall performance. In the past few years a polymer-enabled hybrid integration platform has been established, comprising 1) EO polymers for constructing low-complexity and low-cost Mach-Zehnder modulators (MZMs) with extremely high modulation bandwidth; 2) InP components for light sources, detectors, and high-speed electronics including MUX drivers and DEMUX circuits; 3) Ceramic (AIN) RF board that links the electronic signals within the package. On this platform, advanced optoelectronic modules have been demonstrated, including serial 100 Gb/s [1] and 2x100 Gb/s [2] optical transmitters, but also 400 Gb/s optoelectronic interfaces for intra-data center networks [3]. To expand the device functionalities to an unprecedented level and at the same time improve the integration compatibility with diversified active / passive photonic components, we have added a passive polymer-based photonic board (polyboard) as the 4th material system. This passive polyboard allows for low-cost fabrication of single-mode waveguide networks, enables fast and convenient integration of various thin-film elements (TFEs) to control the light polarization, and provides efficient thermo-optic elements (TOEs) for wavelength tuning, light amplitude regulation and light-path switching.
Recent progress on polymer-based photonic devices and hybrid photonic integration
technology using InP-based active components is presented. High performance thermo-optic
components, including compact polymer variable optical attenuators and switches are powerful
tools to regulate and control the light flow in the optical backbone. Polymer arrayed waveguide
gratings integrated with InP laser and detector arrays function as low-cost optical line terminals
(OLTs) in the WDM-PON network. External cavity tunable lasers combined with C/L band thinfilm
filter, on-chip U-groove and 45° mirrors construct a compact, bi-directional and color-less
optical network unit (ONU). A tunable laser integrated with VOAs, TFEs and two 90° hybrids
builds the optical front-end of a colorless, dual-polarization coherent receiver. Multicore polymer
waveguides and multi-step 45°mirrors are demonstrated as bridging devices between the spatialdivision-
multiplexing transmission technology using multi-core fibers and the conventional PLCbased
photonic platforms, appealing to the fast development of dense 3D photonic integration.
Recent progress of low-loss silicon nitride waveguide in polymer is presented. The fabrication technology requires only low temperature (<200°C) processes and standard photolithography. These waveguides feature a large geometric aspect ratio, resulting in a strong waveguide birefringence. For the TM mode an average propagation loss of ~ 0.72 dB/cm is measured, while for the TE mode the value is ~ 0.96 dB/cm. Also demonstrated are uniform Bragg-grating filters and sampled grating filters. Since the waveguide modes are weakly guided and the majority of light field distributes in the polymer cladding, various optical properties of the polymer materials can be exploited. A thermally tunable waveguide Bragg grating is thus demonstrated, with wavelength tuning range above 57 nm for the TM mode and 49 nm for the TE mode, at a tuning power of ~ 220 mW.
We perform the thermal and optical simulations of silicon nitride / polymer hybrid waveguides with different heating
schemes by finite element method. Both the top and buried microheaters are adopted to realize tuning function by the
thermo-optic effect. We find the buried microheater is more energy-efficient than the top microheater in creating a
uniformed temperature environment in the waveguide region. On the other hand, the top electrode tends to create a
strong temperature gradient through the waveguide, which in turn distorts the optical mode. This distortion, however, is
different for TE and TM modes. This thermally induced birefringence effect is thoroughly investigated in this paper.
In this work, a direct DQPSK receiver was fabricated, which comprises a polymer waveguide based delay-line
interferometer (DLI); a polymer based optical hybrid, and two monolithic pairs of > 25 GHz bandwidth photodiodes that
are vertically coupled to the polymer planar lightwave circuit (PLC) via integrated 45° mirrors. The common mode
rejection ratio (CMRR) is used to characterize the performance of coherent receivers, by indicating the electrical power
balance between the balanced detectors. However, the standard CMRR can only be measured when the PDs can be
illuminated separately. Also, the standard CMRR does not take into account the errors in the relative phases of the
receiver outputs. We introduce an adapted CMRR to characterize the direct receiver, which takes into account the
unequal responsivities of the PDs, the uneven split of the input power by the DLI and hybrid, the phase error and the
extinction ratio of the DLI and hybrid.
We experimentally demonstrate all-optical signal processing functions using silicon microring resonators with a
450×250-nm cross section. These results include slow-light delay of phase-modulated data and microwave
photonic signal, wavelength conversion/multicasting, format conversions, optical differentiation, and concentric
micro-ring resonators with deeper notches for label-free bio-sensing applications.
We propose and experimentally demonstrate all optical format conversion from non-return-to-zero (NRZ) format to
frequency-shift-keying (FSK) format based on free carrier dispersion effect in a silicon mode-split microring resonator.
The injection of the high-power NRZ signal generates free carriers leading to the blue shift of the spectrum when a '1'
comes. Therefore, there is a selective filtering for the two probes with certain separation located at different position of
the split mode according to the information carried by the NRZ signal. Then the NRZ signal is converted to the FSK
format. The microring resonator features ultra-compact size with a radius of 10 μm thus is suitable for integration with
silicon-on-insulator (SOI) based optical and electronic devices. The split mode can provide large and variable frequency
deviation for the FSK signal. 1 Gb/s NRZ signal is successfully converted to FSK format with a frequency deviation of
40 GHz, which can find application for interconnection between a metropolitan area networks (MAN) and a passive
optical network (PON) system.
In this work, we have analyzed, fabricated and demonstrated concentric micro-ring resonators in silicon-on-insulator (SOI) structure for enhanced transmission notches. The operation principles of the concentric ring resonators are studied by time-domain coupled-mode theory. Directional coupling between concentric rings offers another freedom in designing deep notch optical filters and ultra-sensitive biosensors. The finite-difference-time-domain (FDTD) simulations have shown the improvement of the notch depth, evenly distributed mode field and the effect of the resonance shift. The device is demonstrated in silicon-on-insulator structure. Transmission notch depth improvement of ~ 15dB is demonstrated for the 21-20.02-μm-radius double-ring structure comparing with the single 21-μm-radius ring.
We experimentally demonstrate optically tunable buffer in a
nano-scale silicon microring resonator with a 20-μm
radius. The delay-tuning mechanism is based on the red shift of the resonance induced by the thermal nonlinear
effect. We use a non-return-to-zero (NRZ) pseudo random bit sequence (PRBS) signal with different data rates as
the probe signal, and investigate its delay performance under different pump powers.
A series of microcavities in 2D hexagonal lattice photonic crystal slabs (PCS’s) are studied. A combination of FDTD techniques and Pade approximation with Baker's algorithm is used to accurately determine the resonant frequencies and quality factors of the cavity modes simultaneously. Q factor larger than 106 is obtained for a one missing hole cavity. Another cavity with smaller and simpler design keeps Q factor larger than 105. Microcavities in silicon-on-insulator-type (SOI-type) PCS’s are also studied. Simulations show that Q factors of cavities in SOI-type PCS’s are much smaller than those in the membrane PCS’s. Deep etching of air holes is still required to obtain relatively high Q cavities in SOI-type PCS’s.
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