IEEE SENSORS JOURNAL 1 Characterization Measurements Methodology and Instrumental Set-up Optimization for new SiPM Detectors - Part II: Optical Tests Giovanni Bonanno, Davide Marano*, Massimilano Belluso, Sergio Billotta, Alessandro Grillo, Salvatore Garozzo, Giuseppe Romeo, Maria Cristina Timpanaro, Abstract—A comprehensive and in-depth characterization pro- cedure for obtaining very accurate measurements on silicon photo- multiplier detectors (SiPMs) is here described. A large amount of optical tests are systematically carried out and discussed in terms of the most important SiPM performance parameters; in particu- lar, an accurate estimation of the photon detection efficiency (PDE) in the 350-900nm wavelength spectral range and in steps of 10nm is achieved, based on the single-photon counting technique, with substraction of the dark noise contribution and avoiding the addi- tional noise sources of cross-talk and afterpulsing. Some recently produced detectors are analyzed and their relevant electro-optical parameters are evaluated in order to demonstrate the effectiveness and efficacy of the adopted characterization procedure and data- handling protocols in assessing the overall SiPM performance, re- gardless of the specific device tested. Tests repeatibility is carefully verified and all the evaluated parameter trends are proved to be compatible with the physics theory of the SiPM device. Index Terms—Electro-optical characterizations, precision mea- surements, silicon photomultipliers, solid-state detectors. I. I NTRODUCTION I N line with the growing evolution of astrophysics, medical imaging and nuclear science, the realization of optical solid- state sensors has been given a continuously rising emphasis dur- ing the recent years and is gaining a significant and widespread research interest within the scientific community. Silicon Photo-Multiplier detectors (SiPMs), also referred to as Multi-Pixel Photon Counters (MPPCs), are a relatively new category of semiconductor device addressing the challenge of detecting, timing and quantifying low-light signals down to the single-photon level. SiPM sensors operate at low bias voltages and feature a high multiplication ratio, fast dynamic response, excellent time resolution, high photon detection efficiency and wide spectral response range; as a consequence, they are capa- ble of delivering the high-performance level required for photon counting applications and thus have a strong inherent potential for replacing traditional phototube detectors. The number of perspective applications exploiting the advan- tages of SiPMs is rapidly increasing over time, and remarkable research studies and technological development in this class of detectors have been extensively undertaken by a rising number of companies and institutions [1]–[21]. Manuscript received: November 6, 2013; revised: April 22, 2014. The authors are with INAF (Istituto Nazionale di Astrofisica), Osservatorio Astrofisico di Catania, Via S. Sofia 78, I-95123 Catania, Italy (e-mails: [gio- vanni.bonanno, davide.marano, massimiliano.belluso, sergio.billotta, alessan- dro.grillo, salvatore.garozzo, giuseppe.romeo, mc.timpanaro]@oact.inaf.it). * Corresponding author. SiPM detectors promise to fulfill a wide set of requirements coming from a large number of emerging contexts, and several silicon foundries primarily driven by the physical and medical fields are currently investing in future development and innova- tion. Recently, new SiPM detectors with enhanced overall fea- tures have been produced by the world leading manufacturers, and further performance improvements are shortly foreseen. For a particular application field, an appropriate experimental set-up and a well-defined characterization procedure for SiPM sensors is important to explain how accurate measurements and systematic data-handling procedures for the evaluation of their electro-optical parameters can be profitably exploited to obtain a reliable qualification of the detector performance. The systematic procedures pursued to derive the major SiPM performace parameters, along with the extensive analyses and measurements performed on different devices, have allowed for a standardization of the adopted testing methodology, in order to be profitably applicable to every kind of solid-state detectors produced by all manufacturing industries. The present part of the manuscript is focussed on the optical characterization of SiPM detectors, in terms of dark count rate assessment, cross-talk analysis and estimation, optical dynamic range tests and measurements, and photon detection efficiency evaluation, and represents the natural and subsequent comple- ment of the work performed in the previous part, devoted to the initial SiPM electrical analysis and characterization. It should be remarked that all PDE measurements carried out and presented are treated so as to reduce as much as possible the contributions of cross-talk and afterpulsing, allowing a reliable determination of the detector optical performance. II. SET- UP ENVIRONMENT The experimental equipment set-up exploited for the optical characterizations of solid-state detectors is one of the available facilities at the Catania astrophysical Observatory Laboratory for Detectors (COLD). It is a long time since COLD laboratory is concerned with detectors characterization [10]. In the recent past various photon counting devices have been characterized, such as SPADs, SPAD arrays, and first generation SiPMs [10]– [15]. A detailed description of the initial optical equipment can be found in [12]. For each detector an appropriate instrumental apparatus, tailored for the specific device, has been employed. The optical apparatus of the characterization facility has been slightly modified from the original realization. In the following, a brief description of the novel testing envi- ronment engaged for the optical characterization is addressed.