= Solution
Use <mode locking> to phase-lock many <longitudinal cavity modes>; their repeated constructive interference produces short <optical pulses>. A saturable absorber can select this pulsed operation. A $2\,\mathrm{MHz}$ fundamental repetition rate corresponds to a $500\,\mathrm{ns}$ round trip, or an optical round-trip length $c/R\simeq150\,\mathrm m$; a linear air-filled cavity would therefore be about $75\,\mathrm m$ long. A compact alternative is a faster mode-locked oscillator followed by pulse selection and, if required, amplification. Ordinary <Q-switching> alone generally does not provide $350\,\mathrm{fs}$ pulses.<Fundamentals of Photonics and Quantum Electronics lecture notes by Franz X. Kärtner>[Chapter 7, mode locking: phase-related resonator modes and ultrashort pulses]{ref}
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