Figure 2.

Miniature objective and longitudinal focal shift. (a) Illustration of the customized miniature objective. A phase diffractive grating is sandwiched between two GRIN elements. The first GRIN element (~1/4 pitch) collimates the light from the DCF core, and the second GRIN element (<1/4 pitch) pre-focuses the light before it enters a high NA plano-convex lens for tight focusing. All the glass elements are encapsulated and fixed within a protective stainless-steel tube (1.0 mm inner diameter and 1.4 mm outer diameter). The fiber- and sample-side WDs are designed to be 200 μm in air and 200 μm in water, respectively. (b) Sample-side longitudinal focal shift measured over the wavelength range commonly involved in two-photon microscopy (see Supplementary Note S2 and Supplementary Fig. S2 for details of the measurement method).