An apparatus comprising: a source comprising a laser, wherein the laser has a short length unidirectional ring resonator cavity comprising a semiconductor optical amplifier gain medium and at least one arrangement which is a high-finesse wavelength tuning filter and which is in a resonator of the laser, wherein the wavelength tuning filter comprises a rotating polygon scanning mirror, a diffraction grating, a telescope comprising a set of lenses (260, 262) and wherein the wavelength tuning filter further comprises two end reflectors (280, 282), wherein the end reflectors are configured to generate four reflections on the polygon mirror via two double passes; wherein the arrangement is configured to, periodically and as a function of time, select at least one first electro-magnetic radiation based on a mean frequency of the at least one first electro-magnetic radiation, the periodic selection being performed at a first characteristic period, wherein the mean frequency varies linearly over time, characterized in that the apparatus further comprises: a time interleaving optical delay line and a booster optical amplifier, which are at an output port of the source, wherein the time interleaving optical delay line receives an output of the output port; the output having continuous wavelength sweeps which are separated from each other by idle portions; wherein the time interleaving optical delay line is configured to generate one or more delayed copies of the output and paste the delayed copies to the output and into the idle portions to fill up the idle portions; wherein the apparatus is configured to emit, using the output having the pasted copies, at least one second electromagnetic radiation that has a spectrum whose mean frequency changes periodically as a function of time with a second characteristic period, and wherein the first characteristic period is greater than the second characteristic period and the second characteristic period is 1/(N+1) times the first characteristic period, wherein N is a number of the copies. a source comprising a laser, wherein the laser has a short length unidirectional ring resonator cavity comprising a semiconductor optical amplifier gain medium and at least one arrangement which is a high-finesse wavelength tuning filter and which is in a resonator of the laser, wherein the wavelength tuning filter comprises a rotating polygon scanning mirror, a diffraction grating, a telescope comprising a set of lenses (260, 262) and wherein the wavelength tuning filter further comprises two end reflectors (280, 282), wherein the end reflectors are configured to generate four reflections on the polygon mirror via two double passes; wherein the arrangement is configured to, periodically and as a function of time, select at least one first electro-magnetic radiation based on a mean frequency of the at least one first electro-magnetic radiation, the periodic selection being performed at a first characteristic period, wherein the mean frequency varies linearly over time, characterized in that the apparatus further comprises: a time interleaving optical delay line and a booster optical amplifier, which are at an output port of the source, wherein the time interleaving optical delay line receives an output of the output port; the output having continuous wavelength sweeps which are separated from each other by idle portions; wherein the time interleaving optical delay line is configured to generate one or more delayed copies of the output and paste the delayed copies to the output and into the idle portions to fill up the idle portions; wherein the apparatus is configured to emit, using the output having the pasted copies, at least one second electromagnetic radiation that has a spectrum whose mean frequency changes periodically as a function of time with a second characteristic period, and wherein the first characteristic period is greater than the second characteristic period and the second characteristic period is 1/(N+1) times the first characteristic period, wherein N is a number of the copies. a time interleaving optical delay line and a booster optical amplifier, which are at an output port of the source, wherein the time interleaving optical delay line receives an output of the output port; the output having continuous wavelength sweeps which are separated from each other by idle portions; wherein the time interleaving optical delay line is configured to generate one or more delayed copies of the output and paste the delayed copies to the output and into the idle portions to fill up the idle portions; wherein the apparatus is configured to emit, using the output having the pasted copies, at least one second electromagnetic radiation that has a spectrum whose mean frequency changes periodically as a function of time with a second characteristic period, and wherein the first characteristic period is greater than the second characteristic period and the second characteristic period is 1/(N+1) times the first characteristic period, wherein N is a number of the copies.