  We demonstrate a robust low-loss optical interface by tiling passive (i.e.,
without doping of active ions) thin film lithium niobate (TFLN) and active
(i.e., doped with rare earth ions) TFLN substrates for monolithic integration
of passive/active lithium niobate photonics. The tiled substrates composed of
both active and passive areas allow to pattern the mask of the integrated
active passive photonic device at once using a single continuous
photolithography process. The interface loss of tiled substrate is measured as
low as 0.26 dB. Thanks to the stability provided by this approach, a
four-channel waveguide amplifier is realized in a straightforward manner, which
shows a net gain of ~5 dB at 1550-nm wavelength and that of ~8 dB at 1530-nm
wavelength for each channel. The robust low-loss optical interface for
passive/active photonic integration will facilitate large-scale high
performance photonic devices which require on-chip light sources and
amplifiers.
