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 “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

Spotlight on “All-silicon monolithic Mach-Zehnder interferometer as a refractive index and biochemical sensor”

Imagine on the palm of your hand a small portable device for bio-chemical sensing. Now, someone tells you that the embedded detection apparatus exploits optical interferometry. Since you do not see any optical input/output, you guess that the whole optical system is well hidden somewhere inside, with light being internally generated by some optical source and sent to end-of-line photodetectors through suitable optical arrangements. If curiosity is so strong as to make you open the package, you would probably discover that in the optical system there are no components such as lenses, prisms and collimators. Quite reasonable indeed, because free-space bulk optics would hardly match the accuracy, reliability, size, and cost issues required by portable systems. What you would see instead is a small photonic chip, where every optical component is monolithically built on board. How far is current reality from this picture? Actually, the suitability of integrated optics for high-sensitivity multi-analyte label-free sensing does not sound like anything new today. Yet, as long as cost effective solutions are not available, its potential will remain substantially untapped. So far, chip re-usability and the need for inexpensive packaging tools are among the main issues that have hindered the full exploitation of photonic technologies in commercial point-of-need portable analytical devices. In particular, a constant matter of concern in integrated optics is how to couple light to optical waveguides in a way that is effective, reliable, and cost effective. Direct integration of optical sources onto the photonic chip would probably be the best solution to the problem. Unfortunately, conventional interferometric sensing schemes, based for instance on microring or Mach-Zehnder interferometers (MZIs), require the use of monochromatic laser sources, whose monolithic integration is still an open issue. If only we could use a broadband source, everything would become much easier... This is indeed what K. Misiakos and co-workers have demonstrated in their work, where they realized the first silicon device hosting complete on chip interferometry for multianalyte label-free biosensing. The key element they developed is a photonically engineered MZI interferometer that can accommodate the light from a broadband light emitting diode (LED), yet providing the same functionality as a conventional MZI device employing a monochromatic light source. This behaviour is achieved by suitably optimizing the waveguide design of both (sensing and reference) arms of the MZI, in such a way that the relative phase difference between the light propagating through the two arms is almost wavelength independent within the 200 nm bandwidth of the LED. The broadband functionality of the MZI dramatically relaxes the complexity of the optical source integration, because a LED can be directly realized in silicon, for instance by biasing an avalanche p/n diode beyond its breakdown voltage. The full interferometric system includes ten LED sources, each coupled to a sensing cell (integrated MZIs), which share a single detector and provide a limit of detection of 10−5 refractive index units. Multianalyte detection is achieved by spotting the sensing arm of each MZI with appropriate probe molecules and by time-multiplexing the optical sources (that is, biasing one LED at a time) so that all the ten interferometers can be sequentially interrogated. Having everything integrated on a silicon chip carries also additional benefits in terms of cost, foundry service availability and ability to integrate mainstream readout electronics. The photonic chip is then equipped with a fluidic chamber and a removable electrical probe head assuring small size, chip re-usability and inexpensive packaging. After reading this work, we realize that bringing photonic technologies into portable analytical devices is today truly a viable option. It is not unrealistic to imagine that, in the near future, we will keep in our pocket optical sensing systems, enabling point-of-care biochemical analyses, rapid and inexpensive diagnostics, and at-home self monitoring. Probably smaller and cheaper than our smartphone, or even inside it..

Spotlight on “Observation of ~20-ns group delay in a low-loss apodized fiber Bragg grating”

Slowing the light down to its lowest limit is a challenge that in recent years has attracted the attention of a large number of researchers. And not only because of pure scientific curiosity, but mostly because slow light can really bring benefits in many applications, such as sensing, optical communications, and nonlinear optics. In fact, light speed reduction is inherently associated with a large delay, or equivalently with a high group index experienced by the propagating field, that results in a strongly enhanced efficiency of any light-matter interactions. But (unfortunately there is always a “but”), if we want to make slow light really exploitable, we need to keep in mind two things: first, to be of practical use and not merely a wonderful lab experiment, slow light has to be generated and controlled in an easy and reliable way. Second, regardless of the physical phenomenon we exploit, when we dream of slow light, loss is the worst nightmare we can have. In other words, the ideal slow-light medium should be simple to fabricate and manipulate, low cost and (almost) loss free. Probably, reading these lines, most of you are now thinking of the same medium... optical fibers! And indeed this would be a very good choice, because, considering the requirements above, it would be very hard to find anything better. Also the generation of slow light in optical fibers is quite easy, at least in principle, because one of the most established fiber optic components, the fiber Bragg grating (FBG), is itself a slow-light device. At wavelengths just outside the bandgap, a uniform FBG acts as an interferometer: here, constructive interference occurs producing transmission peaks where the light experiences a large group delay. Moreover, suitable optimized designs for apodized and π-shifted FBGs, as well as grating pairs arranged in Fabry-Perot cavities, can be used to realize high Q-factor resonators with a high group index. Unfortunately conventional UV-writing techniques employed to realize FBGs are not loss-free. Typical FBG loss coefficients range around 1 m-1 (i.e. nearly four orders of magnitude higher than in the bare optical fiber) for a refractive index modulation of about 10-3, this loss being the main limiting factor to the maximum group index that can be achieved in FBGs. In their work, G. Skolianos and co-workers demonstrate that the loss of a FBG can be dramatically reduced using a femtosecond laser technique instead of conventional UV-writing. With this technique, a loss coefficient as low as 0.12 m-1 was achieved, with no reductions of the refractive index modulation. Such a small loss enabled them to realize FBGs with a group delay of about 19.5 ns, corresponding to a record group index of 292 (or a Q-factor of 1.5×107). Actually this achievement was not only the result of technology improvements in the FBG writing, but also of an optimized design of the FBG refractive index profile. In fact, a strong Gaussian-like apodization was employed, leading to the creation of Fabry-Perot interference effects associated with much stronger slow-light resonances than in uniform FBGs of comparable index modulation. This combined technology and design optimization was the enabling key for a 4-fold increase of the group index compared to the previous record value. But nothing is for free and a narrow bandwidth (as small as 0.1 pm in the reported experiment) is the price to be paid for slow-light. From this work we learn that we must never make the error of considering well-know devices, such as FBGs, scientifically obsolete. Not only is the Devil in the details, but sometimes also some treasures are there. And in this work, both in design and technology optimization, details enabled to pull the rabbit from the hat

Abdominal-Pelvic Actinomycosis Mimicking Malignant Neoplasm

Abdominal-pelvic actinomycosis is often mistaken for other conditions, presenting a preoperative diagnostic challenge. In a 46-year-old female, computed tomography showed an abdominal-pelvic retroperitoneal mass extending from the lower pole of the right kidney to the lower pelvis. The patient had a 3-year history of intrauterine device. The mass appeared to involve the ascending colon, cecum, distal ileum, right Fallopian tube and ovary, and ureter anteriorly and the psoas muscle posteriorly. The resection of retroperitoneal mass, distal ileum appendicectomy, right hemicolectomy, and right salpingo-oophorectomy was performed. The postoperative period was uneventful. Penicillin therapy was given for six months without any complication. The retroperitoneal mass measured 4.5 × 3.5 × 3 cm, surrounded adjacent organs and histologically showed inflammatory granulomatous tissue, agglomeration of filaments, and sulfur granules of Actinomyces, with positive reaction with periodic acid Schiff. Right tubo-ovarian abscess was present. Abdominalpelvic actinomycosis should always be considered in patients with a pelvic mass especially in ones using intrauterine device

On-Chip OSNR Monitoring with Silicon Photonics Transparent Detector

Non-invasive integrated detectors, named contactless integrated photonic probe (CLIPP), are employed to demonstrate on-chip noise-independent power monitoring of optical channels and in-band optical signal to noise ratio (OSNR) measurement. The proposed technique is based on a two-step lock-in demodulation of optical signals that are suitably labeled with low-modulation-index labels. We demonstrate OSNR measurement from 8 up to 27 dB/0.1 nm on 10-Gb/s ON-OFF keying signals with a power level ranging from -25 up to -15 dBm. This approach provides a promising tool for the monitoring of channels in reconfigurable optical networks with flexible channel allocation strategy, where the small channel separation makes the measurement of the in-band OSNR challenging