Integrated Micro-Optics: an enabler for integrated photonics (Invited Paper)

Advanced integrated photonics components deliver and promises unprecedented functions with very compact format, mass manufacturability and lower power consumption than legacy systems. The components being developed for photon generation, manipulation, emission, reception and detection have complex geometries and very small dimensions, making their optical interfacing and overall assembly and packaging very demanding. Micro-optics improves dramatically these optical interfaces by improving the photon emission efficiency, detection efficiency and coupling efficiencies between integrated photonic devices as will be discussed in various cases. Stand-alone micro-optics components allowed dramatic cost reduction in various industries such as for light to fiber coupling, consumer electronics devices or automotive. A few examples of such stand alone micro-optics are presented, such as to miniaturize compact optical atomic clocks. However, integrating micro-optics together with integrated photonic devices offers further miniaturization, ease of assembly and unmatched performance and allow to address emerging needs in a variety of cases. Single‑photon detectors and imagers such as single‑photon avalanche diode (SPAD) arrays, silicon photomultipliers (SiPMs), and advanced CMOS image sensors are intrinsically limited by pixel fill factor, dead areas, and optical losses at the detector surface. However, they allow unmatched low light detection sensitivity, up to single photon detection capability, at room temperature. These limitations are particularly critical under photon‑starved conditions relevant to scientific instrumentation, quantum sensing, time‑resolved imaging, and super-resolution microscopy. Monolithically integrated microlens arrays provide an effective means to increase the effective photon collection area by concentrating incident light onto the active regions of the detector, boosting the quantum efficiency of these photon detectors. CSEM has developed a UV‑replication‑based microlens technology enabling the fabrication of a wide variety of microlens arrays with diameters ranging from a few micrometers to the millimeter scale. Microlenses can be fabricated from a variety of materials, including inorganic sol‑gels and organic polymers such as PMMA and polyurethane, allowing optimization with respect to optical transmission, mechanical stability, environmental robustness, and application‑specific constraints. The technology is compatible with wafers up to 8″ but as well down to individual chips down to 2 × 2 mm². The technology is compatible with aggressive environment such as space and cryostats. The impact of UV‑replicated microlenses has been evaluated on several classes of quantum photonic devices. For front‑illuminated SPAD imagers, cylindrical and square microlenses significantly increase the photon concentration factor, with measured values of approximately 2 to 9 in agreement with optical simulations. In low‑photon‑flux regimes, these microlenses enhance signal‑to‑noise ratio by increasing the number of detected photons without introducing additional electronic noise. Various optimization techniques are presented to further improve the imagers and detectors. In the context of high‑energy physics, round microlenses integrated on silicon photomultipliers developed for the upgrade of the Large Hadron Collider b detector at CERN achieve pixel fill factors exceeding 80% and enable approximately 15% more detected photons at room temperature as well as in cryostats. In a different direction, the rapid growth of data communications, cloud computing, and AI infrastructure is driving an increasing demand for high‑density optical interconnects capable of supporting large fiber counts while maintaining reliability in demanding environments. Conventional physical‑contact fiber connectors face significant challenges in such applications, including stringent sub‑micron alignment tolerances, high sensitivity to contamination, and reduced mechanical robustness as the number of fibers per connector increases. These limitations become particularly critical in telecom, datacom, defense, and industrial networks, where connectors must operate reliably over wide temperature ranges and under harsh environmental conditions. Expanded beam (EB) fiber connector technology offers an attractive alternative by eliminating direct fiber‑to‑fiber contact. By expanding, collimating, and refocusing the optical beam between mating fibers, EB connectors significantly relax alignment and cleanliness requirements. However, traditional EB implementations often rely on glass or polymer lenses that suffer from either high cost or strong thermal and environmental sensitivity, limiting scalability and stability. We have been developing a next‑generation multi‑fiber optical connector based on expanded beam technology using thermally compensated polymer microlens arrays fabricated and assembled at wafer level. The connector is developed by Zoharay in collaboration with CSEM, VTT, and TH‑Wildau, combining optical system design, advanced micro‑optical fabrication, material characterization, and precision bonding technologies. The core innovation lies in the use of polymer microlens doublets with matched thermo‑optic and thermo‑mechanical properties, enabling stable optical performance across a wide temperature range while retaining the cost and scalability advantages of polymer optics. The connector architecture consists of three main elements: a commercially available MT/MPO multi‑fiber ferrule, an optical insert incorporating the microlens arrays, and a mating interface ensuring repeatable alignment. The ferrule provides accurate fiber positioning through standard guiding holes and alignment pins, enabling compatibility with existing datacom ecosystems. Within the optical insert, paired polymer microlenses expand and collimate the beam, significantly reducing sensitivity to dust particles and lateral misalignment. Optical simulations and ray‑tracing optimization demonstrate a numerical aperture of approximately 0.22 and a coupling loss sensitivity below 0.3 dB for lateral misalignments of ±5 µm, while showing strongly reduced temperature dependence compared to conventional single‑lens EB designs. The resulting connector system targets multimode fiber arrays operating at 850 nm and supports high fiber densities while maintaining low insertion loss, high return loss, and robust environmental performance. Its compatibility with existing MPO‑based infrastructure makes it particularly attractive for large‑scale deployment in data centers, telecom networks, and harsh‑environment applications where conventional physical‑contact connectors reach their limits. In a third direction, micro-optics assembly on photonic integrated circuits (PIC) for waveguide array to fiber array unit (FAU) coupling is today a key enabler for advanced electronic and photonic packages, so called co-package optics. Co-package optics is widely evaluated as being key to reduce the energy consumption of datacenters while providing higher bandwidth inside each rack unit. Examples of implementation in the industry of micro.optics for PIC to FAU coupling are presented to review the current state of the art. Current limitations and expected future development in this strategic field are presented as well as the challenges to enable mass manufacturing of this micro-optics-enabled co-package optics systems.