Backscattering in silicon microring resonators: a quantitative analysis

Silicon microring resonators very often exhibit resonance splitting due to backscattering. This effect is hard to quantitatively and predicatively model. This paper presents a behavioral circuit model for microrings that quantitatively explains the wide variations in resonance splitting observed in experiments. The model is based on an in-depth analysis of the contributions to backscattering by both the waveguides and couplers. Backscattering transforms unidirectional microrings into bidirectional circuits by coupling the clockwise and counterclockwise circulating modes. In high-Q microrings, visible resonance splitting will be induced, but, due to the stochastic nature of backscattering, this splitting is different for each resonance. Our model, based on temporal coupled mode theory, and the associated fitting method, are both accurate and robust, and can also explain asymmetrically split resonances. The cause of asymmetric resonance splitting is identified as the backcoupling in the couplers. This is experimentally confirmed and its dependency on gap and coupling length is further analyzed. Moreover, the wide variation in resonance splitting of one spectrum is analyzed and successfully explained by our circuit model that incorporates most linear parasitic effects in the microring. This analysis uncovers multi-cavity interference within the microring as an important source of this variation

Spotlight on “Broadband on-chip near-infrared spectroscopy based on plasmonic grating filter array”

Broadband high-resolution spectroscopy: plasmonics makes it cheap on a chip. The development of portable, cost-effective solutions for on-site sensing requires spectroscopy systems that use neither expensive nor bulky instruments. On-chip spectrometers are good candidates due to their inherent low cost, but they typically suffer from a limited spectral resolution or operating bandwidth, or they need sophisticated optical coupling methods to couple light into sub-micron-scale waveguides. In this Optics Letters article, R. Li and coworkers effectively exploit the combination of subwavelength gratings with plasmonic nanostructures to realize an ultra-compact, broadband on-chip spectrometer with a very high resolution. The device integrates 28 individual subwavelength plasmonic grating filters in a footprint of less than 1.5 mm2, providing a spectral resolution as high as 10 nm over a near-infrared (IR) operation bandwidth of 270 nm. An accuracy comparable to that of conventional Fourier Transform IR spectroscopy is achieved thanks to a post-processing numerical method compensating for spurious side peaks in the transmission spectrum of each plasmonic filter. The optical transmission pattern of the entire filter array can be also acquired through a CCD camera, enabling the monitoring of all optical wavelengths simultaneously. Neither moving elements nor critical optical alignment systems are employed, thus improving the system reliability and simplifying measurement operations. The result is a spectrometer that is ultracompact, cost-effective, broad-band, high-resolution, reliable, robust and easy to use… definitely promising for future portable IR spectroscopy systems

Spotlight on "Second-harmonic generation of light at 245 nm in a lithium tetraborate whispering gallery resonator "

Switching the light on at any wavelength, easily and efficiently, is not an easy game. This is indeed one of the main issues people working in optics have to face, because a general-purpose recipe to build light sources does not exist. And realistically speaking, probably it will never be found. Light sources are like haute cuisine dishes, whose preparation needs to be accurately tailored to fulfil specific tastes and different requirements. The choice of material compounds is of primary importance, since high optical gain or large nonlinear response is required for light generation and manipulation at the desired wavelength. Yet, good ingredients are not always available. At certain wavelengths, no material seems to offer sufficient efficiency to generate light. In these cases, scientists are asked to operate like master chefs capable to transform apparently poor and flavourless ingredients into special food. For example, light generation is a challenging task in the short/mid ultraviolet range, where most optical materials exhibit poor transparency. A way to realize it exploits second-harmonic generation (SHG) in borate crystals, which are nonlinear materials with a transparency range that can spread deeply into the ultraviolet. Among these materials, lithium tetraborate (Li2B4O7) is particularly promising because the phase-matching wavelength (the wavelength at which SHG efficiency is maximum) is close to the blue emission line of argon-ion lasers (488 nm), around which many applications have been established; moreover, the UV wavelength of the harmonic light (244 nm) lies within the transparency window of the crystal. Nonetheless, the potential of Li2B4O7 as a crystal for SHG-mediated UV light generation has never been fully exploited due to its small nonlinear-optical coefficient. A classical powerful strategy to increase the conversion efficiency of weakly nonlinear materials makes use of resonant devices, such as whispering gallery resonators (WGRs), inside of which any nonlinear interaction can be dramatically boosted. Though simple on paper, high efficiency cavity-enhanced SHG in the UV range requires a master touch to be achieved in real life. This is indeed what Fürst and collaborators have done in their work. By exploiting resonant-enhanced SHG in an engineered WGR geometry, a conversion efficiency of more than 2% has been reached using 490-nm laser pump sources with a few milliwatts of output powers. To put it in context, this efficiency is over twenty times that of previous results, and at a thousand times lower pump power. No secret ingredients anywhere, just a careful optimization of the device design and fabrication technique. A millimetre-sized WGR was fabricated from a bulk Li2B4O7 single crystal that was mechanically machined to a spheroid and accurately polished to minimize surface roughness. In this way ultra-low loss was achieved in the WGR, that is comparable to thar of the bulk crystal (extinction coefficient of 0.1 m-1 at 490 nm), enabling to reach a quality factor as high as 2x108. Since the efficiency of cavity-enhanced SHG scales with the third power of the quality factor, it is easy to realize where such a high conversion efficiency comes from. To optimize the phase matching condition (that is strongly dependent on temperature), the WGR was thermally stabilized on a millikelvin scale to compensate against thermal instability caused by pump light absorption. A route to further improve the conversion efficiency is also identified, that exploits an active locking technique to reduce the effect of thermal instability, and the identification and exploitation of the whispering gallery modes with the smallest effective volume. Although the optical spectrum is becoming populated with many kinds of light sources, optics is still hungry for new solutions to realize more and more compact and efficient lasers. This work clearly shows that research has to be carried out on a two-fold path. On the one hand, looking for novel material compounds as new ingredients; on the other hand, developing novel device concepts and fabrication techniques to boost the efficiency of materials that are already in use. In other words, devices are the recipes to enhance the flavours of our ingredients. Let’s keep in mind that we have to make a good choice on both sides..

Spotlight on “Broadband Mid-Infrared Frequency Comb Generation in a Si3N4 Microresonator”

No more than four years ago, a broadband integrated frequency comb generator operating in the near-infrared range was highlighted in Spotlight on Optics. (https://www.osapublishing.org/spotlight/summary.cfm?id=220223). At that time, the realization of an entire device on a photonic chip set a first milestone towards what we called the “comb generator dream” of extremely compact, cheap and robust optical comb generators. A promising step forward compared to traditional technologies exploiting ultrafast mode-locked lasers, which definitely work very well, but suffer from bulkiness, cost and limited line-separation issues. Since then, optical comb generators have become even more attractive for many applications, such as optical clocks, precision frequency metrology, high-speed communications systems and optical waveform synthesizers. One of the next frontiers is now moving deeply into the mid-infrared range, where the advent of optical comb generators is expected to bring new tools for advanced precision spectroscopy, molecular structure investigation and gas sensing. Four years later, we are here to comment on another milestone in the optical comb generator route. The same material as before, namely silicon nitride (Si3N4). And the same research group, joining the teams of M. Lipson and A. Gaeta from Cornell University. Yet, much longer wavelengths now, well above 2 μm. And a completely new story begins. Unfortunately for people working in integrated optics, moving from a wavelength range to another is not a copy-and-paste process, even when dealing with passive devices. Not only can material properties change dramatically, but the behaviour of materials commonly used at certain wavelengths can be almost unknown at others, because of the lack of characterization instruments or even light sources. And this is the case of Si3N4, whose optical properties have been thoroughly studied at telecom wavelengths and below, but not in the mid-infrared range. For a reliable description of the optical properties of Si3N4 in the mid-infrared range, K. Luke and co-workers first derived a wavelength extended version of the Sellmeier equation. They achieved this by characterizing the refractive index and the absorption coefficient of the material over an ultra-broad wavelength range, spanning from the ultraviolet (193 nm) up to the far infrared (33 μm). Their results demonstrate that Si3N4 can provide strong enough anomalous dispersion to generate wide spectral combs in the near-infrared range, yet requiring an optical waveguide with a height (about 1 μm) well beyond the thicknesses typically limited by the intrinsic film stress. To overcome the mechanical stress limit, a technique previously developed by the same group was employed, which is based on crack isolation trenches realized before Si3N4 film deposition. Further, to prevent stress-induced wafer bowing, both sides of the wafers were processed. However, this was not enough. Loss was actually the key issue to address, because efficient resonator-based comb generators require very high Q-factors. In order to minimize absorption losses in the film, multiple annealing steps were performed during film deposition. These techniques allowed to increase the Q-factor of a microring resonator from a value of 55,000, that was measured on a reference single anneal device, to a record Q value of 1.0 × 106 at a wavelength of 2.6 μm, this being the highest Q ever achieved for on-chip resonators operating in this wavelength range. The authors claim also that the Q factor could be even improved with optimized etching process and improved anneal cycling during Si3N4 deposition. After reading this paper, one can’t help but to think about the next episode of the integrated comb generator saga. It is difficult to predict how the story will evolve, if even longer wavelengths will be covered with Si3N4 comb sources, or other materials will come into play. Let’s hope to have news soon, hopefully before the next four years

Spotlight on “Accurate interchannel pitch control in graded index circular-core polymer parallel optical waveguide using the Mosquito method”

The successful story of fiber optics has demonstrated the superiority of optical communications over competing technologies in long-haul data transmissions. Now, short-range high-capacity data interconnects are envisioned as the next field where optics and photonics technologies are likely to play a revolutionary role. In this contest, energy consumption per bit is the key number, and photon transmission is potentially much less power hungry than electronic transmission. This is the reason why, in a near future, massive data exchanges between and inside supercomputers are expected to be carried by photons. A part of this new story has already become reality. Board-to-board interconnects in supercomputers are already performed though multimode fiber links, providing high-bandwidth-density wirings with lower power consumption than that required by electric cables. But we can definitely do more; we can try to bring photons closer and closer to the brain of supercomputers. The new frontier is chip-to-chip optical interconnects, that is photonics links providing high-capacity and energy saving communications between electronic chips. Although this research field has been under the lens for several years, a winner photonic technology has not emerged yet. And the race is getting faster and faster. Silicon photonics is considered one of the most promising technology candidates, because it enables the integration of fast modulators, wide-band routing and switching architectures, and photodetectors onto the same chip. In other words, all we need for realizing end-to-end optical links on a chip. Yet, nothing is for free, and silicon photonics has still to solve challenging issues related to strong sensitivity to fabrication tolerances, and to temperature and environmental fluctuations. Another option is given by the use of multimode polymer waveguide arrays, which are drawing much attention because they offer high-density wirings with very low-propagation loss, low interchannel crosstalk, high bandwidth, and high coupling efficiency with multimode fiber, VCSELs and photo detectors (PDs). The main issue with this technology is the development of a simple, low-cost and reliable process to fabricate cm-scale long waveguide arrays with inter-waveguide pitches of a few tens of μm. A very original and surprisingly effective solution has been proposed by Kinoshita and co-workers, who have developed the so-called Mosquito method. In this technique, UV curable silicone resins are employed for fabricating the core and the cladding of polymer optical waveguides. The core resin is dispensed into the cladding resin, deposited in a liquid phase on a substrate from a needle mounted on a syringe, whose position is controlled by a horizontally scanning robot. After UV curing and baking of the core and cladding resins, a circularly-shaped core waveguide with a graded-index profile is magically obtained. This method allows the simplification of the fabrication of polymer optical waveguides, since photomasks, large-scale UV exposure apparatus, and chemical etching processes are no longer needed. Further, graded index core waveguides exhibit better performance than step-index waveguides in terms of loss and mutual optical crosstalk. By optimizing the fabrication process, waveguide arrays with a circular core of 50 μm and a pitch as small as 62.5-μm have been realized in this work, with an impressive control of waveguide size, spacing and circularity. The suitability of the Mosquito method for high-bandwidth-density on-board and board-to-board optical interconnects is confirmed by the demonstration of 12 x 11.3 Gbps signal transmissions in an array of 12 waveguides without any signal deterioration. This successful result is a clear demonstration that technological processes must not be stuck in conventional schemes, because different ways of thinking are the primary key to the most significant advances. It is difficult to say at this point if the Mosquito method has the potential to compete with more consolidated photonic technologies, but it is always good to have alternative routes to follow. Let’s wait for more news from Kinoshita and co-workers

Spotlight on “Effect of Injection Current and Temperature on Signal Strength in a Laser Diode Optical Feedback Interferometer”

Backreflections from the external world are among the worst enemies of lasers. Most people with some knowledge in optics are well aware of that. Optical feedback can induce fluctuations in the output power, lasing frequency drifts, bifurcation phenomena, mode hopping and ultimately chaos. In other words, under strong feedback conditions a laser can behave very differently from what a laser is expected to do. Yet, in some circumstances, your worst enemy can even become your best friend... Each time a system suffers from strong sensitivity to some external agents, we can think of exploiting this vulnerability for sensing. Backreflections from objects to be monitored are indeed at the basis of what is commonly known as optical feedback interferometry (OFI), which is one of the most widely employed techniques in sensing applications, for instance in measurements of displacement, velocity, and vibration. In OFI, two possible approaches can be used to access information on the target. We can do it optically, by measuring the variations induced by the optical feedback on the intensity of the light emitted by the rear facet of the laser diode; or electrically, by measuring the voltage variations induced across the laser terminals. In both cases, to achieve high sensitivity, it is fundamental to work with the maximum signal-to-noise ratio. It is not surprising that both optical and electrical OFI signals depend on the laser structure and on the operation parameters, such as the laser bias current. What is far less obvious is that the dependence of the OFI signal strength versus injection current can be dramatically different for the optical and electrical signals. Some observations of this tricky behaviour were reported in the literature, but a clear understanding of this phenomenon was still missing... until the work by Al Roumy and co-workers. These researchers have succeeded at developing a simple model that provides a clear explanation of the dependence of the OFI signal on laser diode injection current and temperature. Compared to previous studies, the key used in their model is to include a realistic dependence of the laser slope efficiency on the injection current and temperature. Nothing more than that, yet extraordinarily effective. In fact, their model nicely shows why the optical signal strength increases with injection current, while the electrical signal is at a maximum just above the laser threshold, and subsequently decreases at higher injection current. Moreover, the same model provides also a clear explanation of the more pronounced decrease in the optical signal with temperature, compared to the electrical signal. As a main result, golden rules to select the optimum injection current to maximise OFI sensitivity are provided for both optical and electrical read-out configurations. The biasing strategy is indeed radically different for the two schemes, the first exhibiting better sensitivity at higher bias current, the second having the optimal injection current close to threshold. This study was limited to single-mode laser structures, but the authors are confident on its extension to multiple transverse or longitudinal mode operation. Finally, note that there is also lesson to be learned from the approach itself followed in this work. The model was derived from the Lang and Kobayashi equations, which were proposed more than thirty years ago to study the effects of weak optical feedback on semiconductor laser properties. The results achieved by Al Roumy and coworkers were somewhat hidden inside these equations, but nobody had been able to unveil them before. This demonstrates that new stories may come from well-known models, so we must never believe that old models have already told us everything they can

Light Dependence of Silicon Photonic Waveguides

In OPN’s December “Optics in 2015” review of interesting research conducted in the previous year, Stefano Grillanda and Francesco Morichetti explore the crucial impact of surface effects in the behavior of light in nanoscale optoelectronic waveguides, such as those in integrated photonic chips—creating a metal-like “skin” of conductivity on the surface of the waveguid

Light-induced metal-like surface of silicon photonic waveguides

The surface of a material may exhibit physical phenomena that do not occur in the bulk of the material itself. For this reason, the behaviour of nanoscale devices is expected to be conditioned, or even dominated, by the nature of their surface. Here, we show that in silicon photonic nanowaveguides, massive surface carrier generation is induced by light travelling in the waveguide, because of natural surface-state absorption at the core/cladding interface. At the typical light intensity used in linear applications, this effect makes the surface of the waveguide behave as a metal-like frame. A twofold impact is observed on the waveguide performance: the surface electric conductivity dominates over that of bulk silicon and an additional optical absorption mechanism arises, that we named surface free-carrier absorption. These results, applying to generic semiconductor photonic technologies, unveil the real picture of optical nanowaveguides that needs to be considered in the design of any integrated optoelectronic device

Characterizing and modeling backscattering in silicon microring resonators

This paper was published in OPTICS EXPRESS and is made available as an electronic reprint with the permission of OSA. The paper can be found at the following URL on the OSA website: http://dx.doi.org/10.1364/OE.19.024980. Systematic or multiple reproduction or distribution to multiple locations via electronic or other means is prohibited and is subject to penalties under lawWe present an experimental technique to characterize back-scattering in silicon microring resonators, together with a simple analytical model that reproduces the experimental results. The model can extract all the key parameters of an add-drop-type resonator, which are the loss, both coupling coefficients and backscattering. We show that the backscattering effect strongly affects the resonance shape, and that consecutive resonances of the same ring can have very different backscattering parameters. © 2011 Optical Society of America.The authors acknowledge financial support from the Spanish Ministry of Science and Innovation through contract SINADEC (TEC2008-06333). Joaquin Matres is supported by the Formacion de Personal Investigador grant program of the Universidad Politecnica de Valencia.Ballesteros García, G.; Matres Abril, J.; Martí Sendra, J.; Oton Nieto, CJ. (2011). Characterizing and modeling backscattering in silicon microring resonators. Optics Express. 19(25):24980-24985. https://doi.org/10.1364/OE.19.024980S24980249851925De Vos, K., Bartolozzi, I., Schacht, E., Bienstman, P., & Baets, R. (2007). Silicon-on-Insulator microring resonator for sensitive and label-free biosensing. Optics Express, 15(12), 7610. doi:10.1364/oe.15.007610Almeida, V. R., Barrios, C. A., Panepucci, R. R., & Lipson, M. (2004). All-optical control of light on a silicon chip. Nature, 431(7012), 1081-1084. doi:10.1038/nature02921Dumon, P., Bogaerts, W., Wiaux, V., Wouters, J., Beckx, S., Van Campenhout, J., … Baets, R. (2004). Low-Loss SOI Photonic Wires and Ring Resonators Fabricated With Deep UV Lithography. IEEE Photonics Technology Letters, 16(5), 1328-1330. doi:10.1109/lpt.2004.826025Morichetti, F., Canciamilla, A., Martinelli, M., Samarelli, A., De La Rue, R. M., Sorel, M., & Melloni, A. (2010). Coherent backscattering in optical microring resonators. Applied Physics Letters, 96(8), 081112. doi:10.1063/1.3330894Little, B. E., Laine, J.-P., & Chu, S. T. (1997). Surface-roughness-induced contradirectional coupling in ring and disk resonators. Optics Letters, 22(1), 4. doi:10.1364/ol.22.000004Kippenberg, T. J., Spillane, S. M., & Vahala, K. J. (2002). Modal coupling in traveling-wave resonators. Optics Letters, 27(19), 1669. doi:10.1364/ol.27.001669Zhang, Z., Dainese, M., Wosinski, L., & Qiu, M. (2008). Resonance-splitting and enhanced notch depth in SOI ring resonators with mutual mode coupling. Optics Express, 16(7), 4621. doi:10.1364/oe.16.004621Morichetti, F., Canciamilla, A., Ferrari, C., Torregiani, M., Melloni, A., & Martinelli, M. (2010). Roughness Induced Backscattering in Optical Silicon Waveguides. Physical Review Letters, 104(3). doi:10.1103/physrevlett.104.033902Little, B. E., Chu, S. T., Haus, H. A., Foresi, J., & Laine, J.-P. (1997). Microring resonator channel dropping filters. Journal of Lightwave Technology, 15(6), 998-1005. doi:10.1109/50.588673Morichetti, F., Canciamilla, A., & Melloni, A. (2010). Statistics of backscattering in optical waveguides. Optics Letters, 35(11), 1777. doi:10.1364/ol.35.001777McKinnon, W. R., Xu, D. X., Storey, C., Post, E., Densmore, A., Delâge, A., … Janz, S. (2009). Extracting coupling and loss coefficients from a ring resonator. Optics Express, 17(21), 18971. doi:10.1364/oe.17.01897