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Fig. 8.
Optical design schematics of the sample arm without (a) and with (b) the ETL. (c) Color photograph of the sample arm with the ETL. GS: galvanometric scanner, OBJ: objective, ETL: electrical tunable lens, and CM: collimator. Since the ETL is a refractive optical element and easy to align, switching between the two configurations without and with the ETL takes only a few minutes. (d) shows the simulation which reveals that the focus spot size becomes smaller as the focus is shifted away from the objective. The focus position is denoted by the distance from the bottom surface of the objective to the focal plane, the blue background denotes the focus shifting range