1：Experimental Design and Method Selection:

The experiment involves designing a cascaded MZI structure using HCFs and MMFs to exploit the Vernier effect for gas pressure sensing. Theoretical models for phase difference and free spectral range are used.

2：Sample Selection and Data Sources:

Two samples with different HCF lengths (840 μm and 946 μm for sample 1; 938 μm and 803 μm for sample 2) are fabricated. Data is collected using an optical spectrum analyzer.

3：List of Experimental Equipment and Materials:

Equipment includes a Super luminescent Light Emitting Diode (SLED), optical spectrum analyzer (OSA) AQ6370, pressure pump ConST162, digital pressure meter ConST-211, fusion splicer Fujikura FSM-100P, femtosecond laser, objective lens Sigama Koki, and CCD camera. Materials include single-mode fiber SMF-28, multimode fiber Nufern MM-S105/125-22A, and hollow core fiber.

4：Experimental Procedures and Operational Workflow:

Fabricate the cascaded MZI structure by fusion splicing, use femtosecond laser to open one HCF, connect the sensor to SLED and OSA, place it in a gas chamber, vary pressure from 0 to

5：8 MPa, and measure spectrum shifts. Temperature response is tested in a heating chamber from 30°C to 100°C. Data Analysis Methods:

Spectrum envelope is extracted using curve fitting and sinusoidal fitting methods; wavelength shifts are monitored and sensitivities calculated.