  Photonic-crystal surface-emitting lasers (PCSELs) are capable of single-mode,
high-power lasing over a large resonator area owing to two-dimensional
resonance at a singularity point of the photonic band structure. Since the
number of photons in the lasing mode in PCSELs are much larger than those in
conventional semiconductor lasers, PCSELs are in principle suitable for
coherent operation with a narrow spectral linewidth. In this paper, we
numerically and experimentally investigate intrinsic spectral linewidths of
1-mm-diameter PCSELs under continuous-wave (CW) operation, and we demonstrate
CW operation with 1-kHz-class intrinsic linewidths and 5-W-class output power.
