  Photonic integrated circuits based on erbium doped thin film lithium niobate
on insulator has attracted broad interests with insofar various waveguide
amplifiers and microlasers demonstrated. Wideband operation facilitated by the
broadband absorption and emission of erbium ions necessitates the functional
integration of wavelength filter and multiplexer on the same chip. Here a
low-loss wavelength division multiplexer at the resonant pumping and emission
wavelengths (~1480 nm and 1530~1560 nm) of erbium ions based on angled
multimode interferometer, is realized in the erbium doped thin film lithium
niobate on insulator fabricated by the photolithography assisted
chemomechanical etching technique. The minimum on-chip insertion losses of the
fabricated device are <0.7 dB for both wavelength ranges, and a 3-dB bandwidth
of >20 nm is measured at the telecom C-band. Besides, direct visualization of
the multimode interference pattern by the visible upconversion fluorescence of
erbium ions compares well with the simulated light propagation in the multimode
interferometer. Spectral tuning of the wavelength division multiplexer by
structural design is also demonstrated and discussed.
