  A combination of optical feedback self-locking of a continuous-wave
distributed feedback diode laser to a V-shaped high finesse cavity, laser phase
modulation at a frequency equal to the free spectral range of the V-cavity and
detection of the transmitted laser beam at this high modulation frequency is
described for possible application in cavity-enhanced absorption
spectroscopy.In order to estimate an absorbance baseline noise of laser
intensity and frequency modulated light triplet passed through the V-cavity in
open air, a 1.5-cm long optical cell filled by C2H2 at low pressure was placed
behind the cavity output mirror. The performance of the setup was evaluated
from the experimental bandwidth normalised relative intensity noise on the
cavity output and the frequency modulation absorption signals induced by C2H2
absorption in the 1.5-cm cell. From these data we estimate that the
noise-equivalent absorption sensitivity of 2.1*10^-11 cm^-1 Hz^-1/2 by a factor
of 11.7 above a shot-noise limit can be achieved for C2H2 absorption spectra
extracted from the heterodyne beat signals recorded at the transmission maxima
intensity peaks of the successive TEM00 resonances.
