  Extending the cavity length of diode lasers with feedback from Bragg
structures and ring resonators is highly effective for obtaining ultra-narrow
laser linewidths. However, cavity length extension also decreases the
free-spectral range of the cavity. This reduces the wavelength range of
continuous laser tuning that can be achieved with a given phase shift of an
intracavity phase tuning element. We present a method that increases the range
of continuous tuning to that of a short equivalent laser cavity, while
maintaining the ultra-narrow linewidth of a long cavity. Using a
single-frequency hybrid integrated InP-Si3N4 diode laser with 120 nm coverage
around 1540 nm, with a maximum output of 24 mW and lowest intrinsic linewidth
of 2.2 kHz, we demonstrate a six-fold increased continuous and mode-hop-free
tuning range of 0.22 nm (28 GHz) as compared to the free-spectral range of the
laser cavity.
