This paper discusses a dispersion effect in a grating-based time-domain delay line that is different from the second- or higher-order dispersion in a grating-based Fourier-domain delay line. When the lateral broadening of the beam profile after grating dispersion exceeds the collection aperture of the reference fiber, the peripheral spectrum is decoupled by the fiber. The loss of reference spectral bandwidth by this geometric-beam broadening thus degrades the axial resolution. The polarizing-beam reflector used in the Fourier-domain delay line for suppression of lateral beam walk-off is implemented in this grating-based time-domain delay line to minimize geometric-beam broadening. Theoretical analysis and experiments are given to validate the axial resolution improvement after geometric-beam broadening is minimized. In vitro and in vivo imaging results are presented to demonstrate the improvement. It is also shown that geometric-beam broadening may exist in other optical coherence tomography reference arm configurations.
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