2.2. NIR2D reader

A portable reader device was designed to couple with the NIR2D. The reader device is comprised of 3D printed assembly parts, InGaAs amplified photodetector with power adapter (PDF10C, Thorlabs), focusing optics (F230SMA-C, AD11F, SM1L05-P5, and LA4052-C, Thorlabs), long-pass filter (FEL1400, Thorlabs), data acquisition (DAQ) board with 12-bit analog-to-digital converter (USB-204, Measurement Computing), and a 40 × 40 mm² cooling fan (NF-A4xlO-5V, Noctua).

Differential voltage is measured by connecting the positive and negative output from the InGaAs photodetector to two analog inputs in the DAQ board. To keep the photodetector’s temperature close to that of the patient room, which is regulated between 21 °C–23 °C, a 22 × 22 mm² heat sink (HS-BK-10, Gadgeter) is attached to the back of the photodetector and air is blown through the reader device using the cooling fan. This configuration is necessary since the photodetector is sensitive to temperature changes and outputs reliable signals when operated within its calibrated conditions (23 °C ± 5 °C).

The entire experimental apparatus is portable due to the compact form factor, with an exterior dimension of 180 × 140 × 90 mm³ and has a SMA optomechanical port on the side wall to connect with the optical fiber. A schematic diagram and photograph of the assembled device is shown in figure 1.

(a) Schematic diagram and (b) photograph of the NIR2D assembly.