  LiteBIRD has been selected as JAXA's strategic large mission in the 2020s, to
observe the cosmic microwave background (CMB) $B$-mode polarization over the
full sky at large angular scales. The challenges of LiteBIRD are the wide
field-of-view (FoV) and broadband capabilities of millimeter-wave polarization
measurements, which are derived from the system requirements. The possible
paths of stray light increase with a wider FoV and the far sidelobe knowledge
of $-56$ dB is a challenging optical requirement. A crossed-Dragone
configuration was chosen for the low frequency telescope (LFT : 34--161 GHz),
one of LiteBIRD's onboard telescopes. It has a wide field-of-view ($18^\circ
\times 9^\circ$) with an aperture of 400 mm in diameter, corresponding to an
angular resolution of about 30 arcminutes around 100 GHz. The focal ratio f/3.0
and the crossing angle of the optical axes of 90$^\circ$ are chosen after an
extensive study of the stray light. The primary and secondary reflectors have
rectangular shapes with serrations to reduce the diffraction pattern from the
edges of the mirrors. The reflectors and structure are made of aluminum to
proportionally contract from warm down to the operating temperature at $5\,$K.
A 1/4 scaled model of the LFT has been developed to validate the wide
field-of-view design and to demonstrate the reduced far sidelobes. A
polarization modulation unit (PMU), realized with a half-wave plate (HWP) is
placed in front of the aperture stop, the entrance pupil of this system. A
large focal plane with approximately 1000 AlMn TES detectors and frequency
multiplexing SQUID amplifiers is cooled to 100 mK. The lens and sinuous
antennas have broadband capability. Performance specifications of the LFT and
an outline of the proposed verification plan are presented.
