  Dual comb spectroscopy (DCS) is a broadband technique offering high
resolution and fast data acquisition. We describe a hybrid dual comb
spectrometer comprising a broadband commercial fiber laser system offering a
wide range of sample interrogation, and an actively mode locked semiconductor
laser (MLL) having a widely tunable, relatively narrow spectrum. The mutual
coherence over 100 seconds has been realized between the two combs. We employed
the DCS system to characterize the absorption spectrum of rubidium atoms at 313
K with a high signal to noise ratio. The broadband laser is directly locked on
a high finesse cavity, providing long-term stability, while the semiconductor
laser is locked to it. To characterize the absolute stability of the DCS
system, the linewidth of the MLL comb line is measured and shown to reduce from
880 kHz to $17$ kHz when the system is fully locked. The long-term stability
was measured to be $5 \times 10^{-12}$ at $1$ second and $5 \times 10^{-14}$ at
$350$ seconds. The measured timing jitter of the MLL is ten times smaller due
to the overall locking. In addition, we have addressed the effect of dispersion
on the locking quality, which is significant for broadband comb lasers.
