1：Experimental Design and Method Selection:

The study involves experimental investigation and modeling of the mode spacing dependence on external OFB delay lengths and strengths in single-section quantum dash optical frequency comb lasers.

2：Sample Selection and Data Sources:

Three lasers with lengths of 1 mm and 2 mm, corresponding to mode spacings of 40 GHz and 20 GHz, based on 6 and 9 layers of InAs/InGaAsP quantum dashes, are used.

3：List of Experimental Equipment and Materials:

The experimental setup includes a lensed single-mode fiber, a hybrid fiber-based and free-space OFB cavity, a variable optical attenuator, and a high-precision linear translation stage.

4：Experimental Procedures and Operational Workflow:

The optical path of the OFB cavity corresponds to macroscopic lengths ranging from 6.6 m to 73.9 m, selectable by single-mode fibers. Microscopic OFB delay is provided by a free-space retro-reflector. The OFB strength is controlled by a variable optical attenuator inside the OFB cavity.

5：6 m to 9 m, selectable by single-mode fibers. Microscopic OFB delay is provided by a free-space retro-reflector. The OFB strength is controlled by a variable optical attenuator inside the OFB cavity. Data Analysis Methods:

5. Data Analysis Methods: The radio-frequency beat notes are recorded for fixed gain currents, OFB strength, and macroscopic and microscopic lengths. Results are complemented by modeling using a stochastic time-domain model.