where the broadening parameter τ -1 c is the cavity loss rate, and τ c is the cavity lifetime (which is usually reported as the cavity quality factor Q = ω c τ c ). The typical value of τ c for the VSC experiments is τ c ≈ 100 -1000 fs. See Fig. (violet curve) for an example of J eff (ω). A detailed derivation of Eq. 6 can be found in Ref. 9.