Delphine Marris-Morini

High-performance waveguide-integrated germanium PIN photodiodes for optical communication applications

Silicon photonics integrated circuits development considerably spread in the last past years, and telecommunications and datacom applications are now clearly seen as its targets. With the increasing need of data rates, Si photonics components will have to offer very high speed as well as very low power consumption at lowest costs. The recent developments in photodetection have led to high speed and high responsivity waveguide integrated Ge photodetectors [1-3], with various configurations: butt coupling vs. evanescent coupling, vertical vs. lateral PIN junction. Nevertheless, Germanium absorption beyond 1550nm is limited, and long devices are needed, thus prohibiting Ge based photodiode use in the L-band (1565-1625) used in tele-communication. In this paper, we report on our latest development on very low dark current and high speed lateral PIN germanium photodetectors integrated with Si waveguides fabricated on 200mm and 300mm wafer size, for telecom and datacom applications .

Carrier-depletion-based optical modulator integrated in a lateral structure in a SOI waveguide

A high speed and low loss silicon optical modulator based on carrier depletion has been made. It is based on carrier depletion and consists of a p-doped slit embedded in the intrinsic region of a lateral pin diode. This device has advantages such as a low capacitance and low optical insertion loss. Experimental results are reported. Using a Mach-Zehnder interferometer with 4 mm-long phase shifter, contrast ratio of 14 dB has been obtained with insertion loss of 5 dB. A 3 dB-bandwidth of 10 GHz has been measured at λ = 1.55μm. Driving electrical power is evaluated. For a 5 mm-long active region, driving power is 100 mW at a frequency of 10 GHz. A large contribution of the dissipated power comes from the 50 Ω load at the end of the device. By integrating the modulator and its driver on CMOS chip, the load value could be varied and driving power could be reduced to a few tens of mW.

High-speed characteristics of strain-induced pockels effect in silicon (Conference Presentation)

With the fast growing demand of data, current chip-scale communication systems based on electrical links suffer rate limitations and high power consumptions to address these new requirements. In this context, Silicon Photonics has proven to be a viable alternative by replacing electronic links with optical ones while taking advantage of the well-established CMOS foundries techniques to reduce fabrication costs. However, silicon, in spite of being an excellent material to guide light, its centrosymmetry prevents second order nonlinear effects to exist, such as Pockels effect an electro-optic effect extensively used in high speed and low power consumption data transmission. Nevertheless, straining silicon by means of stressed thin films allows breaking the crystal symmetry and eventually enhancing Pockels effect. However the semiconductor nature of silicon makes the analysis of Pockels effect a challenging task because free carriers have a direct impact, through plasma dispersion effect, on its efficiency, which in turn complicates the estimation of the second order susceptibility necessary for further optimizations. However, this analysis is more relaxed working in high-speed regime because of the frequency limitation of free carriers-based modulation.nnIn this work, we report experimental results on the modulation characteristics based on Mach-Zehnder interferometers strained by silicon nitride. We demonstrated high speed Pockels-based optical modulation up to 25 GHz in the C-band.

Integrated SiN on SOI dual photonic devices for advanced datacom solutions

We report on the co-integration of an additional passive layer within a Silicon Photonic chip for advanced passive devices. Being a CMOS compatible material, Silicon Nitride (SiN) appears as an attractive candidate. With a moderate refractive index contrast compared to SOI, SiN based devices would be intrinsically much more tolerant to fabrication errors while keeping a reasonable footprint. In addition, its seven times lower thermo-optical coefficient, relatively to Silicon, could lead to thermal-tuning free components. The co-integration of SiN on SOI has been explored in ST 300mm R and D photonic platform DAPHNE and is presented in this paper. Surface roughness of the SiN films have been characterized through Atomic Force Microscopy (AFM) showing an RMS roughness below 2nm. The film thickness uniformity have been evaluated by ellipsometry revealing a three-sigma of 21nm. Statistical measurements have been performed on basic key building blocks such as SiN strip waveguide showing propagation loss below 0.7dB/cm and 40μm radius bends with losses below 0.02dB/90°. A compact Si-SiN transition taper was developed and statistically measured showing insertion losses below 0.17dB/transition on the whole O-band wavelength range. Moreover, advanced WDM devices such as wavelength-stabilized directional couplers (WSDC) have been developed.

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.

Advanced solutions in silicon photonics using traditional fabrication methods and materials of CMOS technologies (Conference Presentation)

Optical signal modulation is presently done using Si pn junctions which cause phase shifting due to Soref effect and, put in a Mach-Zehnder configuration, produce interference and generate amplitude modulation. The drawback of pn junctions is the relatively low phase shifting efficiency which consequently inflicts high power consumptions on the electrical driver. An alternative device to pn junctions was developed and consists of introducing capacitive structures within the optical waveguide. The proposed device has the same cross-section foot-print but is much shorter due to improved efficiencies. Typical pn-junctions can generate phase shifts of 60°/mm for the same implantation conditions. The device is made up of crystal Si, a thin SiO2 capacitor dielectric and poly-Si. Benchmarking the two phase shifters with respect to insertion losses, we observe that the proposed device is promising.nAnother material exhaustively used in CMOS technologies is Si3N4. In the data-communication bandwidths, the index contrast between Si3N4 (n = 1.95) and SiO2 (n=1.45) is smaller than that with Si (n = 3.5). Thus, nitride waveguides have lower optical mode confinements and are thus less sensitive to insertion losses caused by line edge roughness and wavelength shifting incurred by process variations. Moreover, the temperature induced index variations are 5 times les in Si3N4 than Si. Therefore, the use of nitride to fabricate devices in silicon photonics looks advantageous. However, high speed electro-optic devices are challenging in Si3N4. Consequently, a co-integration of both materials is essential. We developed a fabrication method and associated devices which allow to transfer the signal to and fro Si and Si3N4. We present some devices in each layer to illustrate the benefits.

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.

Silicon coupled cavities as a flexible platform for integrated nonlinear photonics

Multiply resonant silicon photonic devices based on three coupled nanobeam cavities are proposed engineer third order nonlinearities at wavelengths around λ=1.55μm. We show that varying the geometrical parameters allows a flexible tuning of linear properties of the system, especially the deviations between the cavity resonance wavelengths. Based on the linear regime results, the nonlinear properties of the system are studied using coupled mode expressions of the supermodes given by the Tight-Binding method for the calculation of the nonlinear integrals controlling the intensity of the third-order nonlinear effects of the photonic molecules. We geometrically control the self-phase modulation (SPM), the cross-phase modulation (XPM), and the degenerate four-wave mixing (DFWM) nonlinear coefficients of the three coupled nanobeam cavities and identify general trends for nonlinear applications such as optical switching and frequency conversion devices.