  We describe a compact and stable setup for detecting the optical second
harmonics, in which the incident plane rotates with respect to the sample. The
setup is composed of rotating Fresnel-rhomb optics and a femtosecond
ytterbium-doped fiber-laser source operating at the repetition frequency of 10
MHz. The setup including the laser source occupies an area of 1 m2 and is
stable so that the intensity fluctuation of the laser harmonics can be less
than 0.2 % for 4 h. We present the isotropic harmonic signal of a gold mirror
of 0.5 pW and demonstrate the integrity and sensitivity of the setup. We also
show the polarization-dependent six-fold pattern of the harmonics of a
few-layer WSe2, from which we infer the degree of local-field effects. Finally,
we describe the extendibility of the setup to investigate the samples in
various conditions such as cryogenic, strained, ultrafast non-equilibrium, and
high magnetic fields.
