  Highly coherent optical sources are a key element in several fields of
physics, in particular in time frequency metrology. Over the past decennia,
there has been particular efforts in developing such sources to the expense of
sophisticated laser systems and relatively smart electronics. We propose here a
new general principle of a self-adaptive oscillator where the intricate
operation of a 100-m-long active optical resonator and a standard semiconductor
laser offers a very high spectral purity and can be tailored to any wavelength.
Single frequency operation of this self-adaptive photonic oscillator is
achieved without any servo locking or stabilization electronics. Free running
operation leads to a Lorentzian linewidth of 40 mHz. The long-term drift of the
optical frequency in the free running regime is within 10 MHz over hours. This
principle applies to any wavelength attainable by laser diodes which opens
tremendous opportunities in particular in applications where atomic or
molecular transitions require precise wavelengths.
