JOM thanks the reviewers who served the journal in 2023.

Guest Editors Jeroen Missinne, Yanlu Li, Stefan Mohrdiek, and Padraic Morrissey introduce the Special Section on Packaging Challenges of Photonic Integrated Circuits.

Efficient packaging of fabricated photonic integrated circuits (PICs) has been a daunting task given the breadth of applications and skill required for scalable manufacturing. One particular challenge has been accurately assessing the polarization state at various points in a PIC during the test, assembly, and packaging process. Polarimetric monitoring is necessary for optimizing fiber alignment, for verifying the quality of PIC components and for polarization-related functional testing. We analyze and demonstrate small-footprint engineered scattering elements for polarization monitoring. We find that small scatterers placed above or below a Si or SiN waveguide provide the best polarization integrity in a way that preserves foundry compatibility. The polarization response of these elements along with proper placement provides an optical test point that can be utilized for optimized fiber coupling into waveguides.

Micro-transfer printing (μTP) has been widely used to integrate photonic components, such as lasers, modulators, photodetectors, micro-LEDs, on Si photonic platforms. There is a push toward the μTP of optical components in photonics packaging as it enables wafer-scale integration with high alignment accuracy. We demonstrate for the first time the μTP of thick optical components, such as micro-lenses, in the range of 250 to 1000 μm thickness. We explore the reliability of bonding such components using an ultraviolet (UV) curable epoxy and compare them with the current state of the art. The results show that the average shear strength of lenses bonded with InterVia is 19 MPa which is higher than currently used optical epoxies. Also, μTP process has no effect on the surface roughness and microstructure of lenses. Using our approach, we demonstrate how thick silicon and fused silica lenses can be integrated into photonic integrated circuits (PICs) using a tether-free process that is highly scalable and robust.

Co-packaged optics for high performance computing or other data center applications requires dense integration of silicon photonic integrated circuits (PICs) with electronic integrated circuits (EICs). This work discusses the impact of three-dimensional (3D) hybrid integration on the thermal performance of Si ring-based photonic devices in wavelength-division multiplexing PICs. A thermal finite element model of the EIC-PIC assembly is developed and calibrated with thermo-optic device measurements, before and after integration of an electrical driver on top of the PIC by means of microbump flip-chip bonding. Both measurements and simulations of the thermal tuning efficiency and crosstalk between silicon photonic devices show that the EIC can have a significant impact on the thermal performance of the integrated heaters in the PIC by acting as an undesired heat spreader. This heat spreading lowers the heater efficiency with 43.3% and increases the thermal crosstalk between the devices by up to 44.4% compared with a PIC-only case. Finally, it is shown that these negative thermal effects of 3D integration can largely be mitigated by a thermally aware design of the microbump array and the back-end-of-line interconnect, guided by the calibrated thermal simulation model.

With the increased interest in silicon photonics, integration and packaging technologies are essential to transforming photonic integrated circuits (PICs) into functional photonic systems. We describe in detail the process to obtain a fully packaged miniature photonic temperature sensor starting from bare PIC dies having Bragg grating sensors in a silicon waveguide. It is also shown that PICs fabricated via multiproject wafer services can show some variability, e.g., in the effective index, which has significant impact on the device functionality (Bragg wavelength) and optical interface (red-shifted grating coupler spectra at default coupling angles). To obtain a final sensor device that is as small as possible the PIC is interfaced from the back side using a 300 μm ball lens. Furthermore, this ensures that the top surface remains clear of any interfacing fibers. Based on this optical interfacing concept, we developed a solution for integrating a 1 mm × 1 mm sensor PIC with a single-mode fiber and packaging it in a 1.5 mm inner-diameter metal protective tube. The accurate position of the ball lens is ensured using a laser-fabricated fused silica precision holder. It is shown that the additional insertion loss caused by the ball lens interface is very limited. A packaged sensor was achieved by sequentially mounting the holder on a ceramic ferrule and then the PIC on the holder and finally gluing a protective metal tube surrounding the assembly, taking care that the PIC surface is flush with the end face of the tube. We demonstrated this concept by realizing a packaged phase-shifted silicon Bragg grating temperature sensor operating around 1550 nm, which could be read out in reflection using a commercial interrogator. A temperature sensitivity of 73 pm / ° C was found, and we demonstrated sensor functionality up to 180°C.

Managing the temperature of photonic chips within intricate electro-optic packages poses a notable challenge concerning the thermal crosstalk between the photonic chip, electronic chip, and the chip–fiber connection point. This is a multifaceted problem and requires packaging solutions that cannot only address high-performance thermal management but must also be scalable to high volumes. Glass has long been thought of as a suitable platform for next-generation photonic packaging due to its low thermal conductivity, which minimizes unwanted heat transfer between electronic and photonic components. Achieving proper thermal isolation between the chips and the chip–fiber interface necessitates a microscale thermal solution that guarantees accurate temperature regulation of the photonic circuitry without disrupting the optical coupling interface with the fiber array, due to the presence of epoxy used for fiber attachment. We propose a technique for the development of a substrate-integrated microthermoelectric cooler (SimTEC) for the effective temperature control of the electronic and photonic integrated devices. The proposed device uses glass substrate vias that are half-filled with p and n-type thermoelectric materials and the other half with copper. A COMSOL multiphysics model is developed to study the variations in the cooling performance of this SimTEC device based on changes in the via parameters. Interestingly, the maximum range of temperature gradient variation for SimTEC is 6 times greater compared to that of equivalent free-standing micro-TEC pillars. However, there are some challenges associated with implementing this method, as the temperature gradient (or cooling effect) achieved by SimTEC still falls short of that achieved by the free-standing micro-TEC pillars.

This study focuses on the photomechanical behavior of step-index optical fibers with a polydimethylsiloxane (PDMS) core and fluoropolymer claddings. The PDMS hosts dopant graphene nanoplatelets, which are heated when illuminated by visible light. Sample fibers were fabricated by drawing the doped resin into thin fluoropolymer tubing before the resin set. The individual motions of the core and cladding were observed to be coupled, which resulted in a relatively small photomechanical effect. Adding a castor oil lubricant between the core and cladding significantly increased the core’s photomechanical strain. The large strain of the lubricated cores exhibited both irreversible and reversible photomechanical motion. Cycling the pump beam resulted in a fully reversible photomechanical piston that can be modeled by a linear response function to the incident pump beam.

To enable optical biopsy in clinical applications, it is essential to miniaturize fiber-optic two-photon endomicroscopy (TPEM). This study used theoretical modeling and experimental measurements on a 1-mm-outer-diameter piezoelectric ceramic tube (PZT) fiber scanner for TPEM. After determining resonant modes, the effects of the driving voltage, PZT length, PZT inner diameter, fiber cantilever length, and fiber eccentricity on the fiber’s first- and second-order resonant characteristics were investigated. A 2.7-mm endomicroscopic probe was also integrated, and its two-photon imaging capability was validated using ex-vivo mouse heart and brain tissues. This study’s findings contribute to the advancement of compact nonlinear endomicroscopy.

Holographic optical elements (HOEs) have the potential to enable more compact, versatile, and lightweight optical designs, but many challenges remain. Volume HOEs have the advantage of high diffraction efficiency, but they present both chromatic selectivity and chromatic dispersion, which impact their use with wide spectrum light sources. Single-color light emitting diode (LED) sources have a narrow spectrum that reduces these issues and this makes them better suited for use with volume HOEs. However, the LED source size must be taken into consideration for compact volume HOE-LED systems. To investigate the design limits for compact HOE-LED systems, a theoretical and experimental study was carried out on the effects of an extended source on the HOE output for different holographic lenses, with focal lengths from 25-100 mm. The lenses were recorded in a commercially available photopolymer [Bayfol HX200], and their diffraction efficiency was characterized across the lens aperture by measuring the Bragg angular selectivity curve at each location. Offset point sources were used to experimentally study the effects of a non-point source on the HOEs, and the system was modeled using Matlab and Zemax.

We present a magnetic position sensor for scanning fiber endoscopes to address their inherent short- and long-term position instability, which is a major hurdle before their widespread clinical deployment. The position sensor uses a ring-shaped micro-magnet at the tip of the fiber cantilever, producing a dynamic magnetic field as the scanner resonates. A miniaturized three-dimensional Hall sensor accommodated within the endoscopic probe housing measures the magnetic field vector, which is then mapped directly to the beam position using closed-form calibration curves empirically obtained through a one-time calibration step using a position sensitive detector. By integrating the sensor into an OCT-endomicroscope recently developed in our group, we demonstrate an average position resolution of 20 μm over a field-of-view of 2.1 mm field-of-view and distortion-free OCT images recorded with various scan parameters. We also discuss how sub-pixel (e.g., better than half the diffraction-limited spot size) position resolution can be attained with the new sensing scheme.

Optical spectroscopy is a well-suited technique for the nondestructive and real-time maturation monitoring of fruits and vegetables. Although many commercial spectroscopy systems exist, including some for portable use in the field, a significant gap in agricultural monitoring is the ability to continuously measure the condition of fruits in the field over longer periods of time. To this end, we present here a fully integrated, flexible microspectrometer consisting of multiple light sources and multiple broadband photodiodes for the spectral evaluation of grape maturation in the field. To enable the microspectrometer design, a customized grape berry model generated from the optical properties, primarily light scattering, of grape berries was developed. The microspectrometer was fabricated using a scalable fabrication process based on a spin-coated, flexible polyimide substrate. Experiments were conducted both in a controlled laboratory environment as well as during the grape maturation period in the vineyard and these demonstrated that the spectral properties of grapes at different maturation stages can be accurately measured. Using suitable chemometric models, the amount of total soluble solids in °Br, which is the most important factor for the maturation estimation of grapes, was determined from the microspectrometer data.

Vertical cavity surface-emitting (VCSEL) arrays offer an attractive platform to develop a photonic Ising computer due to their scalability and compact physical size. Ising interactions can be encoded between VCSELs through mutual optical injection locking, with the polarity of the interaction determined by the presence or absence of a half-wave plate in the optical path, and the bit itself represented by polarization state. The performance of this approach is investigated computationally by extending the spin-flip model to describe a system of mutually injection locked VCSELs for 2-, 3-, and 4-bit Ising problems. Numerical simulations demonstrate that the modeled system solves the given Ising problems significantly better than chance, with critical parameters in the model identified as crucial for achieving an unbiased Ising solver. The quantum well gain anisotropy parameter as well as the ratio of phase anisotropy to decay rate of the local carrier number causes the system to favor particular Ising configurations over others, but this may not prohibit the system from reaching the ground state.

Erratum corrects errors in Table 2.