Performance Investigations of InAs/InP Quantum-Dash Semiconductor Optical Amplifiers with Different Numbers of Dash Layers.

We present here a performance comparison of quantum-dash (Qdash) semiconductor amplifiers (SOAs) with three, five, eight, and twelve InAs dash layers grown on InP substrates. Other than the number of Qdash layers, the structures were identical. The eight-layer Qdash SOA gave the highest amplified spontaneous emission power (4.3 dBm) and chip gain (26.4 dB) at 1550 nm, with a 300 mA CW bias current and at 25 °C temperature, while SOAs with fewer Qdash layers (for example, three-layer Qdash SOA), had a wider ASE bandwidth (90 nm) and larger 3 dB gain saturated output power (18.2 dBm) in a shorter wavelength range. The noise figure (NF) of the SOAs increased nearly linearly with the number of Qdash layers. The longest gain peak wavelength of 1570 nm was observed for the 12-layer Qdash SOA. The most balanced performance was obtained with a five-layer Qdash SOA, with a 25.4 dB small-signal chip gain, 15.2 dBm 3 dB output saturated power, and 5.7 dB NF at 1532 nm, 300 mA and 25 °C. These results are better than those of quantum well SOAs reported in a recent review paper. The high performance of InAs/InP Qdash SOAs with different Qdash layers shown in this paper could be important for many applications with distinct requirements under uncooled scenarios.

Athermal echelle grating and tunable echelle grating demultiplexers using a Mach-Zehnder interferometer launch structure.

We present a comparative experimental study of three silicon photonic echelle grating demultiplexers that are integrated with a Mach-Zehnder interferometer (MZI) launch structure. By appropriate choice of the MZI configuration, the temperature induced shift of the demultiplexer channel wavelengths can be suppressed (athermal) or enhanced (super-thermal) or be controlled by an on-chip micro-heater. The latter two configurations allow the channel wavelengths to be actively tuned using lower power than possible by temperature tuning a conventional echelle demultiplexer. In the athermal configuration, the measured channel spectral shift is reduced to less than 10 pm/°C, compared to the 83 pm/°C shift for an unmodified echelle device. In super-thermal operation an enhanced channel temperature tuning rate of 170 pm/°C is achieved. Finally, by modulating the MZI phase with an on-chip heater, the demultiplexer channels can be actively tuned to correct for ambient temperature fluctuations up to 20 °C, using a drive current of less than 20 mA.

InAs/InP quantum dot mode-locked laser with an aggregate 12.544 Tbit/s transmission capacity.

Chip-scale optical frequency comb sources are ideal compact solutions to generate high speed optical pulses for applications in wavelength division multiplexing (WDM) and high-speed optical signal processing. Our previous studies have concentrated on the use of quantum dash based lasers, but here we present results from an InAs/InP quantum dot (QDot) C-band passively mode-locked laser (MLL) for frequency comb generation. By using this single-section QDot-MLL we demonstrate an aggregate line rate of 12.544 Tbit/s 16QAM data transmission capacity for both back-to-back (B2B) and over 100-km of standard single mode fiber (SSMF). This finding highlights the viability for InAs/InP QDot lasers to be used as a low-cost optical source for large-scale networks.

Photonic temperature and wavelength metrology by spectral pattern recognition.

Spectral pattern recognition is used to measure temperature and generate calibrated wavelength/frequency combs using a single silicon waveguide ring resonator. The ring generates two incommensurate interleaving TE and TM spectral combs that shift independently with temperature to create a spectral pattern that is unique at every temperature. Following an initial calibration, the ring temperature can be determined by recognizing the spectral resonance pattern, and as a consequence, the wavelength of every resonance is also known. Two methods of pattern-based temperature retrieval are presented. In the first method, the ring is locked to a previously determined temperature set-point defined by the coincidence of only two specific TE and TM cavity modes. Based on a prior calibration at the set-point, the ring temperature and hence all resonance wavelengths are then known and the resulting comb can be used as a wavelength calibration reference. In this configuration, all reference comb wavelengths have been reproduced within a 5 pm accuracy across an 80 nm range by using an on-chip micro-heater to tune the ring. For more general photonic thermometry, a spectral correlation algorithm is developed to recognize a resonance pattern across a 30 nm wide spectral window and thereby determine ring temperature continuously to 50 mK accuracy. The correlation method is extended to simultaneously determine temperature and to identify and correct for wavelength calibration errors in the interrogating light source. The temperature and comb wavelength accuracy is limited primarily by the linewidth of the ring resonances, with accuracy and resolution scaling with the ring quality factor.

Passively mode-locked quantum dash laser with an aggregate 5.376 Tbit/s PAM-4 transmission capacity.

This paper presents an InAs/InP quantum dash (QD) C-band passively mode-locked laser (MLL) with a channel spacing of 34.224 GHz. By using this QD-MLL we demonstrate an aggregate 5.376 Tbit/s PAM-4 data transmission capacity both for back-to-back (B2B) and over 25-km of standard single mode fiber (SSMF). This represents the first demonstration of QD-MLL acting as error-free operation at an aggregate data transmission capacity of 5.376 Tbit/s for some filtered individual channels. This finding highlights the viability for InAs/InP QD lasers to be used as a low-cost optical source for data center networks.

On-chip Fourier-transform spectrometers and machine learning: a new route to smart photonic sensors.

Miniaturized silicon photonics spectrometers capable of detecting specific absorption features have great potential for mass market applications in medicine, environmental monitoring, and hazard detection. However, state-of-the-art silicon spectrometers are limited by fabrication imperfections and environmental conditions, especially temperature variations, since uncontrolled temperature drifts of only 0.1°C distort the retrieved spectrum precluding the detection and classification of the absorption features. Here we present a new strategy that exploits the robustness of machine learning algorithms to signal imperfections, enabling recognition of specific absorption features in a wide range of environmental conditions. We combine on-chip spatial heterodyne Fourier-transform spectrometers and supervised learning to classify different input spectra in the presence of fabrication errors, without temperature stabilization or monitoring. We experimentally show the differentiation of four different input spectra under an uncontrolled 10°C range of temperatures, about $ 100\times $100× increase in operational range, with a success rate up to 82.5% using state-of-the-art support vector machines and artificial neural networks.

Empirical model for the temperature dependence of silicon refractive index from O to C band based on waveguide measurements.

An accurate model for the silicon refractive index including its temperature and wavelength dependence is critically important for many disciplines of science and technology. Currently, such a model for temperatures above 22°C in the optical communication bands is not available. The temperature dependence in the spectral response of integrated echelle grating filters made in silicon-on-insulator is solely determined by the optical properties of the slab waveguide, making it largely immune to dimensional uncertainties. This feature renders the echelle filters a reliable tool to evaluate the thermo-optic properties of silicon. Here we investigate the temperature dependence of silicon echelle filters for the wavelength range of both O and C bands, measured between 22°C to 80°C. We show that if a constant thermo-optic coefficient of silicon is assumed for each band, as is common in the literature, the predictions show an underestimate of up to 10% in the temperature-induced channel wavelength shift. We propose and assess a model of silicon refractive index that encompasses both the wavelength and temperature dependence of its thermo-optic coefficients. We start from literature data for bulk silicon and further refine the model using the echelle filter measurement results. This model is validated through accurate predictions of device channel wavelengths and their temperature dependence, including the quadratic term, over a wide wavelength and temperature range. This work also demonstrates a new high-precision method for characterizing the optical properties of a variety of materials.

Demonstration of a compressive-sensing Fourier-transform on-chip spectrometer.

We demonstrate compressive-sensing (CS) spectroscopy in a planar-waveguide Fourier-transform spectrometer (FTS) device. The spectrometer is implemented as an array of Mach-Zehnder interferometers (MZIs) integrated on a photonic chip. The signal from a set of MZIs is composed of an undersampled discrete Fourier interferogram, which we invert using l1-norm minimization to retrieve a sparse input spectrum. To implement this technique, we use a subwavelength-engineered spatial heterodyne FTS on a chip composed of 32 independent MZIs. We demonstrate the retrieval of three sparse input signals by collecting data from restricted sets (8 and 14) of MZIs and applying common CS reconstruction techniques to this data. We show that this retrieval maintains the full resolution and bandwidth of the original device, despite a sampling factor as low as one-fourth of a conventional (non-compressive) design.

Broadband polarization independent nanophotonic coupler for silicon waveguides with ultra-high efficiency.

Coupling of light to and from integrated optical circuits has been recognized as a major practical challenge since the early years of photonics. The coupling is particularly difficult for high index contrast waveguides such as silicon-on-insulator, since the cross-sectional area of silicon wire waveguides is more than two orders of magnitude smaller than that of a standard single-mode fiber. Here, we experimentally demonstrate unprecedented control over the light coupling between the optical fiber and silicon chip by constructing the nanophotonic coupler with ultra-high coupling efficiency simultaneously for both transverse electric and transverse magnetic polarizations. We specifically demonstrate a subwavelength refractive index engineered nanostructure to mitigate loss and wavelength resonances by suppressing diffraction effects, enabling a coupling efficiency over 92% (0.32 dB) and polarization independent operation for a broad spectral range exceeding 100 nm.

Optical fiber interferometer array for scanless Fourier-transform spectroscopy.

We report a spatial heterodyne Fourier-transform spectrometer implemented with an array of optical fiber interferometers. This configuration generates a wavelength-dependent stationary interferogram from which the input spectrum is retrieved in a single shot without scanning elements. Furthermore, fabrication and experimental deviations from the ideal behavior of the device are corrected by spectral inversion algorithms. The spectral resolution of our system can be readily scaled up by incorporating longer optical fiber delays, providing a pathway toward surpassing current spectroscopy resolution limits.

High-resolution Fourier-transform spectrometer chip with microphotonic silicon spiral waveguides.

We report a stationary Fourier-transform spectrometer chip implemented in silicon microphotonic waveguides. The device comprises an array of 32 Mach-Zehnder interferometers (MZIs) with linearly increasing optical path delays between the MZI arms across the array. The optical delays are achieved by using Si-wire waveguides arranged in tightly coiled spirals with a compact device footprint of 12 mm2. Spectral retrieval is demonstrated in a single measurement of the stationary spatial interferogram formed at the output waveguides of the array, with a wavelength resolution of 40 pm within a free spectral range of 0.75 nm. The phase and amplitude errors arising from fabrication imperfections are compensated using a transformation matrix spectral retrieval algorithm.

Simultaneous retrieval of fluidic refractive index and surface adsorbed molecular film thickness using silicon wire waveguide biosensors.

For silicon wire based ring resonator biosensors, we investigate the simultaneous retrieval of changes in the fluidic refractive index ∆n(c) and surface adsorbed molecular film thickness ∆d(F). This can be achieved by monitoring the resonance shifts of the sensors operating in the TE and TM polarizations at the same time. Although this procedure is straightforward in principle, significant retrieval errors can be introduced due to deviations in the sensor waveguide cross-sections from their nominal values in the range commonly encountered for silicon photonic wire devices. We propose a method of determining the fabricated waveguide size using the group indices derived from measured free spectral range (FSR) of the resonators. We further demonstrate that using experimentally measured group index values, the waveguide size can be determined to accuracies of ± 2 nm in width and ± 1 nm in height. By using this procedure, ∆n(c) and ∆d(F) can be obtained to a precision of within 10% of the true values using optically measurable parameters, improving the retrieval accuracy by more than 3 times.

Ultracompact polarization converter with a dual subwavelength trench built in a silicon-on-insulator waveguide.

The design and fabrication of an ultracompact silicon-on-insulator polarization converter is reported. The polarization conversion with an extinction ratio of 16 dB is achieved for a conversion length of only 10 µm. Polarization rotation is achieved by inducing a vertical asymmetry by forming in the waveguide core two subwavelength trenches of different depths. By taking advantage of the calibrated reactive ion etch lag, the two depths are implemented using a single mask and etching process. The measured converter loss is -0.7 dB and the 3 dB bandwidth is 26 nm.

Archimedean spiral cavity ring resonators in silicon as ultra-compact optical comb filters.

We present an ultra-compact comb filter using an add-drop ring resonator with an Archimedean spiral cavity. The cavity consists of two interleaved spiral branches which are connected in the center using arcs of circle of a radius that causes minimum bend loss. We describe the design procedure and examine the physical parameters governing the resonator performance. As an example, we demonstrate experimentally a comb filter with a 25 GHz channel spacing made of silicon photonic wires and only occupies an area of 80 x 90 microm(2), approximately a 70 fold size reduction compared to a racetrack resonator. The filter transmission is free of spurious reflections, attesting to the smooth transition between different sections of the resonator cavity. Over a 40 channel wavelength span, the filter exhibits a quality factor Q > 35,000, extinction ratios > 10 dB, and an excellent power uniformity with variations < 0.5 dB for both the through and drop ports.

Compact and low power thermo-optic switch using folded silicon waveguides.

By exploiting the small bend radius achievable using high-index-contrast silicon photonic wire waveguides, we demonstrate a new low power thermo-optic switch arranged in a dense, double spiral geometry. Such a design permits the waveguide length to be extended for increased phase shift, without the need for increased heated volume. This provides an effective means to reduce the power consumption of thermo-optic switches, as well as a compact geometry desirable for the development of switch arrays. A low switching power of 6.5 mW was obtained for a spiral-path Mach-Zehnder interferometer device having a 10% - 90% rise time of 14 micros. The switching power is shown to be reduced by more than 5 times compared to a Mach-Zehnder interferometer employing a conventional straight waveguide geometry.

Unlocking the potential of thiaheterohelicenes: chemical synthesis as the key.

The possibility of combining the electronic properties of oligothiophenes with potential chiroptical properties has fueled research in the area of thiaheterohelicenes. Recent reports that these molecules also exhibit fascinating interactions with biologically important macromolecules place further emphasis on the need for new synthetic methods to access thiaheterohelicenes. This review highlights the synthetic methods currently being used to prepare thiaheterohelicenes and discusses the role that chemical synthesis plays in the exploration of the properties of these helically chiral molecules.