  Writing optical waveguides with femtosecond laser pulses provides the
capability of forming three-dimensional photonic circuits for manipulating
light fields in both linear and nonlinear manners. To fully explore this
potential, large depths of the buried waveguides in transparent substrates are
often desirable to facilitate achieving vertical integration of waveguides in a
multi-layer configuration, which, however, is hampered by rapidly degraded
axial resolution caused by optical aberration. Here, we show that with the
correction of the spherical aberration, polarization-independent waveguides can
be inscribed in a nonlinear optical crystal lithium niobate (LN) at depths up
to 1.4 mm, which is more than one order of magnitude deeper than the waveguides
written with aberration uncorrected femtosecond laser pulses. Our technique is
beneficial for applications ranging from miniaturized nonlinear light sources
to quantum information processing.
