  We demonstrate an external cavity laser formed by combining a silicon nitride
photonic integrated circuit with a reflective semiconductor optical amplifier.
The laser uses an alignment tolerant edge coupler formed by a multi-mode
waveguide splitter right at the edge of the silicon nitride chip that relaxes
the required alignment to the III-V gain chip and equally splits the power
among its two output waveguides. Both the ground and first order mode are
excited in the coupler and reach the quadrature condition at the waveguide
junction, ensuring equal power to be coupled to both. Two high-quality-factor
ring resonators arranged in Vernier configuration close a Sagnac loop between
the two waveguides. In addition to wideband frequency tuning, they result in a
longer effective cavity length. The alignment tolerant coupler increases the
alignment tolerance in the two directions parallel to the chip surface by a
factor 3 relative to conventional edge couplers, making it ideal for gain chip
integration via pick-and-place technology. Lasing is maintained in a
misalignment range of $\pm$6 $\mu$m in the direction along the edge of the
chip. A Lorentzian laser linewidth of 42 kHz is achieved.
