Vladyslav Vakarin

Ge-rich graded-index Si_1-xGex waveguides with broadband tight mode confinement and flat anomalous dispersion for nonlinear mid-infrared photonics

This work explores the use of Ge-rich graded-index Si1-xGex rib waveguides as building blocks to develop integrated nonlinear optical devices for broadband operation in the mid-IR. The vertical Ge gradient concentration in the waveguide core renders unique properties to the guided optical mode, providing tight mode confinement over a broadband mid-IR wavelength range from λ = 3 µm to 8 µm. Additionally, the gradual vertical confinement pulls the optical mode upwards in the waveguide core, overlapping with the Ge-rich area where the nonlinear refractive index is larger. Moreover, the Ge-rich graded-index Si1-xGex waveguides allow efficient tailoring of the chromatic dispersion curves, achieving flat anomalous dispersion for the quasi-TM optical mode with D ≤ 14 ps/nm/km over a ~1.4 octave span while retaining an optimum third-order nonlinear parameter, γeff. These results confirm the potential of Ge-rich graded-index Si1-xGex waveguides as an attractive platform to develop mid-IR nonlinear approaches requiring broadband dispersion engineering.

O-band quantum-confined Stark effect optical modulator from Ge/Si0.15Ge0.85 quantum wells by well thickness tuning

We report an O-band optical modulator from a Ge/Si0.15Ge0.85 multiple quantum well (MQW). Strong O-band optical modulation in devices commonly operating within E-band wavelength range can be achieved by simply decreasing the quantum well thickness. Both spectral photocurrent and optical transmission studies are performed to evaluate material characteristics and device performance from a surface-illuminated diode and a waveguide modulator, respectively. These results demonstrate the potential of using Ge/Si0.15Ge0.85 MQWs for the realization of future on-chip wavelength-division multiplexing systems with optical modulators operating at different wavelengths over a wide spectral range.

Giant electro-optic effect in Ge/SiGe coupled quantum wells

Silicon-based photonics is now considered as the photonic platform for the next generation of on-chip communications. However, the development of compact and low power consumption optical modulators is still challenging. Here we report a giant electro-optic effect in Ge/SiGe coupled quantum wells. This promising effect is based on an anomalous quantum-confined Stark effect due to the separate confinement of electrons and holes in the Ge/SiGe coupled quantum wells. This phenomenon can be exploited to strongly enhance optical modulator performance with respect to the standard approaches developed so far in silicon photonics. We have measured a refractive index variation up to 2.3 × 10−3 under a bias voltage of 1.5 V, with an associated modulation efficiency VπLπ of 0.046 V cm. This demonstration paves the way for the development of efficient and high-speed phase modulators based on the Ge/SiGe material system.

Graded SiGe waveguides with broadband low-loss propagation in the mid infrared

Mid-infrared (mid-IR) silicon photonics is expected to lead key advances in different areas including spectroscopy, remote sensing, nonlinear optics or free-space communications, among others. Still, the inherent limitations of the silicon-on-insulator (SOI) technology, namely the early mid-IR absorption of silicon oxide and silicon at λ~3.6 µm and at λ ~8.5 µm respectively, remain the main stumbling blocks that prevent this platform to fully exploit the mid-IR spectrum (λ ~2-20 µm). Here, we propose using a compact Ge-rich graded-index Si1-xGex platform to overcome this constraint. A flat propagation loss characteristic as low as 2-3 dB/cm over a wavelength span from λ = 5.5 µm to 8.5 µm is demonstrated in Ge-rich Si1-xGex waveguides of only 6 µm thick. The comparison of three different waveguides design with different vertical index profiles demonstrates the benefit of reducing the fraction of the guided mode that overlaps with the Si substrate to obtain such flat low loss behavior. Such Ge-rich Si1-xGex platforms may open the route towards the implementation of mid-IR photonic integrated circuits with low-loss beyond the Si multi-phonon absorption band onset, hence truly exploiting the full Ge transparency window up to λ ~15 µm.

Nonlinear Properties of Ge-rich Si 1−x Ge x Materials with Different Ge Concentrations

Silicon photonics is a large volume and large scale integration platform for applications from long-haul optical telecommunications to intra-chip interconnects. Extension to the mid-IR wavelength range is now largely investigated, mainly driven by absorption spectroscopy applications. Germanium (Ge) is particularly compelling as it has a broad transparency window up to 15 µm and a much higher third-order nonlinear coefficient than silicon which is very promising for the demonstration of efficient non-linear optics based active devices. Si1−xGex alloys have been recently studied due to their ability to fine-tune the bandgap and refractive index. The material nonlinearities are very sensitive to any modification of the energy bands, so Si1−xGex alloys are particularly interesting for nonlinear device engineering. We report on the first third order nonlinear experimental characterization of Ge-rich Si1−xGex waveguides, with Ge concentrations x ranging from 0.7 to 0.9. The characterization performed at 1580 nm is compared with theoretical models and a discussion about the prediction of the nonlinear properties in the mid-IR is introduced. These results will provide helpful insights to assist the design of nonlinear integrated optical based devices in both the near- and mid-IR wavelength ranges.

7.5 µm wavelength fiber-chip grating couplers for Ge-rich SiGe photonics integrated circuits

The mid infrared (MIR) region, which ranges from 2 μm to 20 μm, has attracted a lot of interest, particularly for novel applications in medical diagnosis, astronomy, chemical and biological sensing or security, to name a few. Most recently, Germanium-rich Silicon Germanium (Ge-rich SiGe) has emerged as a promising waveguide platform to realize complex mid-IR photonic integrated circuits. The Ge-rich SiGe graded buffer benefits from a wide transparency window, strong 3rd order nonlinearity, and the compatibility with mature large-scale fabrication processes, which in turn, paves the way for the development of mid-IR photonic devices that afford improved on-chip functionalities, altogether with compact footprints and cost-effective production. Albeit, low-loss waveguides and wideband Mach-Zehnder interferometers (MZIs) have been recently successfully demonstrated at mid-IR wavelengths, the coupling of light between external access ports, typically optical fibers, and integrated circuits remains challenging. Surface grating couplers provide technologically attractive scenario for light coupling, since they allow flexible placement on the chip, thereby enabling automatic testing of fabricated devices on a wafer-scale, preferred for large-volume developments. In this work, we report two designs for surface grating couplers implemented on the Ge-rich SiGe graded buffer. The grating couplers are designed for transverse electric (TE) and transverse magnetic (TM) polarizations, respectively, both operating at 7.5 μm wavelength. In particular, the TE-designed grating coupler with an inverse taper excitation arrangement yields a coupling efficiency of 6.3% (-12 dB), a 1-dB bandwidth of 300 nm, and reduced back-reflection less than 1%. Furthermore, the TM-designed grating coupler with a conventional taper injection stage predicts an improved coupling performance up to 11% (-9.6 dB), with a 1-dB bandwidth of 310 nm, and only 1% back-reflection. These results open up the way for the realization of complex and multifunctional photonics integrated circuits on Ge-rich SiGe platform with operation at midIR wavelengths.

Low loss grating coupled optical interfaces for large volume fabrication with deep ultraviolet optical lithography

Optical input/output interfaces between silicon-on-insulator (SOI) waveguides and optical fibers, allowing robust, costeffective and low-loss coupling of light, are fundamental functional elements in the library of silicon photonic devices. Surface grating couplers are particularly desirable as they allow wafer-scale device testing, yield improved alignment tolerances, and are compatible with state-of-the-art integration and packaging technologies. While several factors jointly contribute to the coupler performance, the grating directionality is a critical parameter for high-efficiency fiber-chip coupling. To address this issue, conventional coupler designs typically call upon comparatively complex architectures to improve light coupling efficiency. Increasing the intrinsic directionality of the grating by exploiting the blazing effects is another promising solution. In this paper, we report on our recent advances in development of low-loss grating couplers that afford excellent directionality, close to the theoretical limit of 100%. In particular, we demonstrate, by theory and experiments, several implementations of blazed grating couplers with layout features that are compatible with deepultraviolet (deep-UV) optical lithography. Devices can be advantageously implemented on various photonic platforms, including industry-specific and the offerings of publicly accessible foundries. The first experimental realizations of uniform deep-UV-compatible couplers yield losses of -2.7 dB at 1.55-µm and a 3-dB bandwidth of 62 nm. A subwavelength-index-engineered impedance matching transition is used to reduce back-reflections down to -20 dB.

Mode converters based on periodically perturbed waveguides for mode division multiplexing

Bandwidth demands in optical communication systems are growing steadily and making Wavelength Division Multiplexing (WDM) reach its limit. New multiplexing techniques are required in order to fulfill future bandwidth demands in next generation optical communications. Mode Division Multiplexing (MDM) has been recently proposed as good solution to increase aggregate bandwidth by multiplexing on the spatial domain. In this work we discuss the propositions of ultra-compact mode converters based on periodically perturbed waveguides. A corrugation (perturbation) is periodically inserted on one side of the waveguide. Each time the fundamental mode propagates through a perturbation a part of the incident light is transferred to the second mode. Around 5 periods are only needed to achieve complete power transfer, enabling for ultra-compact devices. Insertion loss below 0.5 dB and extinction ratio higher than 13 dB in the C-Band have been evaluated in a device with a total length of only 12 μm.

Ge-rich graded-index Si1-xGex devices for Mid-IR integrated photonics

Mid-infrared (mid-IR) silicon photonics is becoming a prominent research with remarkable potential in several applications such as in early medical diagnosis, safe communications, imaging, food safety and many more. In the quest for the best material platform to develop new photonic systems, Si and Ge depart with a notable advantage over other materials due to the high processing maturity accomplished during the last part of the 20th century through the deployment of the CMOS technology. From an optical viewpoint, combining Si with Ge to obtain SiGe alloys with controlled stoichiometry is also of interest for the photonic community since permits to increase the effective refractive index and the nonlinear parameter, providing a fascinating playground to exploit nonlinear effects. Furthermore, using Ge-rich SiGe gives access to a range of deep mid-IR wavelengths otherwise inaccessible (λ ~2-20 μm). In this paper, we explore for the first time the limits of this approach by measuring the spectral loss characteristic over a broadband wavelength range spanning from λ = 5.5 μm to 8.5 μm. Three different SiGe waveguide platforms are compared, each one showing higher compactness than the preceding through the engineering of the vertical Ge profile, giving rise to different confinement characteristics to the propagating modes. A flat propagation loss characteristic of 2-3 dB/cm over the entire wavelength span is demonstrated in Ge-rich graded-index SiGe waveguides of only 6 μm thick. Also, the role of the overlap fraction of the confined optical mode with the Si-rich area at the bottom side of the epitaxial SiGe waveguide is put in perspective, revealing a lossy characteristic compared to the other designs were the optical mode is located in the Ge-rich area at the top of the waveguide uniquely. These Ge-rich graded-index SiGe waveguides may pave the way towards a new generation of photonic integrated circuits operating at deep mid-IR wavelengths.

Ge-rich SiGe photonic-integrated circuits for mid-IR spectroscopy

Recent works towards the development of Ge-rich SiGe photonic integrated circuits will be presented, such as the demonstration of low-loss waveguides and ultra-wideband Mach Zehnder interferometer from 5.5 to 8.6 μm wavelength, as well as the first steps towards the realization of efficient wideband optical sources.