Photon-number-resolving segmented detectors based on single-photon avalanche-photodiodes.

We investigate the feasibility and performance of photon-number-resolved photodetection employing single-photon avalanche photodiodes (SPADs) with low dark counts. While the main idea, to split n photons into m detection modes with a vanishing probability of more than one photon per mode, is not new, we investigate here a important variant of this situation where SPADs are side-coupled to the same waveguide rather than terminally coupled to a propagation tree. This prevents the nonideal SPAD quantum efficiency from contributing to photon loss. We propose a concrete SPAD segmented waveguide detector based on a vertical directional coupler design, and characterize its performance by evaluating the purities of Positive-Operator-Valued Measures (POVMs) in terms of number of SPADs, photon loss, dark counts, and electrical cross-talk.

Heterogeneous photodiodes on silicon nitride waveguides.

Heterogeneous integration through low-temperature die bonding is a promising technique to enable high-performance III-V photodetectors on the silicon nitride (Si3N4) photonic platform. Here we demonstrate InGaAs/InP modified uni-traveling carrier photodiodes on Si3N4 waveguides with 20 nA dark current, 20 GHz bandwidth, and record-high external (internal) responsivities of 0.8 A/W (0.94 A/W) and 0.33 A/W (0.83 A/W) at 1550 nm and 1064 nm, respectively. Open eye diagrams at 40 Gbit/s are demonstrated. Balanced photodiodes of this type reach 10 GHz bandwidth with over 40 dB common mode rejection ratio.

Segmented waveguide photodetector with 90% quantum efficiency.

We demonstrate a novel InGaAsP/InP segmented waveguide photodetector based on directional couplers. By matching the imaginary parts of the propagation constants of the even and odd modes, we designed a photodetector with 6 elements, each with an absorber volume of only 19 µm3 and a bandwidth of 15 GHz, that has an internal quantum efficiency (QE) of 90% at 1550 nm wavelength corresponding to 1.13 A/W.

Surface condensation sensor board for damp heat chamber.

In this paper, we demonstrate the use of a surface condensation sensor board for characterizing interior space in a damp heat chamber. The sensor board is approximately 18 in. × 12 in. in dimension. A total of 324 sense electrodes are designed on the board. The uniform gap between the sense electrodes is 250 µm throughout the surface area of the board. First, the surface leakage current of the sensor board is characterized with other commercially available humidity sensors. The relationship of leakage current to humidity is determined. Surface leakage current is spatially measured inside the chamber, and localized condensation spots are identified at 85 °C/85%RH stress condition. The main goal of this article is to characterize interior space and identify less-risky locations prior to qualification of semiconductor components, optical subassemblies, and optical modules.

High-sensitivity intravascular photoacoustic imaging of lipid-laden plaque with a collinear catheter design.

A highly sensitive catheter probe is critical to catheter-based intravascular photoacoustic imaging. Here, we present a photoacoustic catheter probe design on the basis of collinear alignment of the incident optical wave and the photoacoustically generated sound wave within a miniature catheter housing for the first time. Such collinear catheter design with an outer diameter of 1.6 mm provided highly efficient overlap between optical and acoustic waves over an imaging depth of >6 mm in D2O medium. Intravascular photoacoustic imaging of lipid-laden atherosclerotic plaque and perivascular fat was demonstrated, where a lab-built 500 Hz optical parametric oscillator outputting nanosecond optical pulses at a wavelength of 1.7 µm was used for overtone excitation of C-H bonds. In addition to intravascular imaging, the presented catheter design will benefit other photoacoustic applications such as needle-based intramuscular imaging.

High-speed intravascular photoacoustic imaging at 1.7 µm with a KTP-based OPO.

Lipid deposition inside the arterial wall is a hallmark of plaque vulnerability. Based on overtone absorption of C-H bonds, intravascular photoacoustic (IVPA) catheter is a promising technology for quantifying the amount of lipid and its spatial distribution inside the arterial wall. Thus far, the clinical translation of IVPA technology is limited by its slow imaging speed due to lack of a high-pulse-energy high-repetition-rate laser source for lipid-specific first overtone excitation at 1.7 µm. Here, we demonstrate a potassium titanyl phosphate (KTP)-based optical parametric oscillator with output pulse energy up to 2 mJ at a wavelength of 1724 nm and with a repetition rate of 500 Hz. Using this laser and a ring-shape transducer, IVPA imaging at speed of 1 frame per sec was demonstrated. Performance of the IVPA imaging system's resolution, sensitivity, and specificity were characterized by carbon fiber and a lipid-mimicking phantom. The clinical utility of this technology was further evaluated ex vivo in an excised atherosclerotic human femoral artery with comparison to histology.