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J. Eur. Opt. Society-Rapid Publ. 22, 56( 2026)
coupling was achieved using a single-mode( SM) polarization-maintaining( PM) 2 fiber aligned with a nanopositioner 3. A powermeter was used to measure the optical power, while a pick-off on a photodiode was measured to monitor laser power fluctuations during the measurements. The polarization state was set using a polarimeter 4( see Fig. 3a).
Experiments were performed with either a custom-made fiber array with PM fibers in a U-configuration, aligned to dedicated reference structures at the outer waveguide channels, or by coupling light through a single PM fiber of the array and collecting the output with a SM fiber 5 positioned behind the chip using a three-axis alignment system 6.
Optical measurements and characterization of the GC emission were performed using a commercial microscope system 7 equipped with a high-NA objective 8( 0.95 NA) and a CMOS camera 9. The PIC is mounted on a tip, tilt and rotation stage 10 to minimize tilt-induced angle errors. In addition, the grating efficiency was determined by summing the pixel intensities of the outcoupled light on the CMOS camera and converting them to optical power using a pre-established calibration factor. Prior to these measurements, a reference calibration was performed using a fibercoupled source to evaluate the objective transmission and to determine the conversion coefficient relating pixel intensity to optical power per exposure time.
2.3.2 Measurements
2.3.2.1 Propagation losses
Propagation losses were determined by measuring the transmitted power through waveguides of varying lengths arranged in a U-configuration. The exponential decay of transmitted power with length was fitted to extract the loss parameters, where the slope corresponds to the propagation loss and the intercept represents twice the combined incoupling and bend losses( see Fig. 3b). Measured propagation losses are( 2.91 ± 0.33) dB / cm for the TE mode and( 2.05 ± 0.18) dB / cm for the TM mode.
2.3.2.2 Taper losses
Ol-EBL taper losses have been characterized to be( 1.17 ± 0.07) dB per taper for TE and( 1.48 ± 0.04) dB per taper for TM by fitting the decay of the power transmission through waveguides with a different number of taper sections( see Fig. 3c). The OL-EBL taper losses are contributing to the total incoupling loss since they directly capture the loss introduced by the mix-and-match lithography approach.
2.3.2.3 Bend and bend-shift losses
Bend losses were determined by comparing waveguides containing bends with varying radii to a straight reference
2 PM780-HP. 3 PI P-616 NanoCubeÒ XYZ-Nanopositionierer. 4 SK010PA-NIR. 5 SM 780HP. 6 NanoTrakÒ Auto-Alignment Controller. 7 Olympus BXFM. 8 Olympus MPLAPON50X. 9 Hamamatsu ORCA-spark C11440-36U. 10 Thorlabs TTR001.
Fig. 3. a) Schematic illustration of the optical characterization setup. The red cone illustrates the beam emitted from the PIC. b) Cutback measurement to deduce the coupling and propagation losses for the TE and TM modes at a wavelength of 760 nm. Data points shown with reduced opacity were excluded from the fit. c) Characterization of the EBL – OL transition taper, showing the extracted taper loss. The inset displays the cascaded taper section.
waveguide. The extracted values include the intrinsic propagation loss within the curved sections. The total excess loss was then divided by the number of bends( 200) to obtain the bend loss per bend.
The corresponding bend-loss measurements are shown in Figures 4a and 4b for TE- and TM-polarized light, respectively. For both polarizations, the bend loss strongly depends on the bend radius and is significantly reduced by the bend-shift design. For TE polarization( Fig. 4a), bends with radii of 20, 40, and 60 lm exhibit losses below 0.06 dB per 90 ° bend. The lowest loss is observed for the 20 lm bend, with approximately 0.04 dB, while the corresponding bend-shift design further reduces the loss to below 0.03 dB. At larger radii, the loss remains nearly constant, indicating that radiation and mode mismatch losses are already well