1：Experimental Design and Method Selection:

The sensor is designed as a Mach-Zehnder interferometer (MZI) using a photonic crystal fibre (PCF) sandwiched between two single-mode fibres (SMFs) with waist-enlarged bitapers at the coupling points. The principle involves intermodal interference between core and cladding modes. Graphene is coated on the PCF surface to enhance sensitivity.

2：Sample Selection and Data Sources:

Sucrose solutions with concentrations ranging from 0 to 230 ppm (refractive indices from

3：3338 to 3376) were used as samples. The refractive indices were measured at room temperature (300 K). List of Experimental Equipment and Materials:

Equipment includes a fibre fusion splicer (Furukawa Electric Co. Ltd., S178C), PCF (YOFC, TIR-PCF), SMF (SMF-28), tube furnace (GSL 1300X), scanning electron microscope (TESCAN MIRA3), Raman spectrometer (LabRAM HR Evolution), amplified spontaneous emission source, and optical spectrum analyzer (Yokogawa AQ6370D). Materials include graphene, polymethyl methacrylate (PMMA), acetone, and sucrose solutions.

4：Experimental Procedures and Operational Workflow:

The SPS-MZI was fabricated by arc fusion splicing SMFs to PCF ends to form waist-enlarged tapers. Graphene/PMMA composite film was transferred to the PCF surface using chemical vapor deposition and copper dissolution, followed by PMMA removal with acetone and calcination at 300°C for 4 hours. The sensor was characterized using SEM and Raman spectroscopy. Transmission spectra were monitored with the OSA while immersing the sensor in sucrose solutions.

5：Data Analysis Methods:

Wavelength shifts of transmission dips were measured and fitted using linear regression to determine sensitivity and linearity. Raman spectra were analyzed to confirm graphene layer count based on G and 2D peak ratios.