An optical source comprising: I) a laser cavity section disposed between a first optical reflector and a second optical reflector, the laser cavity section comprising: A. an optical gain section; and, B. an optical phase control section in optical communication with the gain section, and configured to be able to change the longitudinal mode frequency of the optical source; and, II) an optical filter external to and in optical communication with the laser cavity section; wherein: at least one of the first or second optical reflectors is a partial optical reflector; and, the optical filter is configured to receive laser light from one of the said at least one partial optical reflectors and filter the said received light; and, the optical source is configured to input filtered light into one of the said at least one partial optical reflectors; and characterized in that: the optical filter comprises an optical thin film filter comprising a passband filter response; and, the optical source is configured to change the central wavelength of the pass-band response of the optical filter by changing the angle of incidence that the output laser light subtends with the optical thin film filter. I) a laser cavity section disposed between a first optical reflector and a second optical reflector, the laser cavity section comprising: A. an optical gain section; and, B. an optical phase control section in optical communication with the gain section, and configured to be able to change the longitudinal mode frequency of the optical source; and, A. an optical gain section; and, B. an optical phase control section in optical communication with the gain section, and configured to be able to change the longitudinal mode frequency of the optical source; II) an optical filter external to and in optical communication with the laser cavity section; wherein: at least one of the first or second optical reflectors is a partial optical reflector; and, the optical filter is configured to receive laser light from one of the said at least one partial optical reflectors and filter the said received light; and, the optical source is configured to input filtered light into one of the said at least one partial optical reflectors; and characterized in that: the optical filter comprises an optical thin film filter comprising a passband filter response; and, the optical source is configured to change the central wavelength of the pass-band response of the optical filter by changing the angle of incidence that the output laser light subtends with the optical thin film filter. at least one of the first or second optical reflectors is a partial optical reflector; and, the optical filter is configured to receive laser light from one of the said at least one partial optical reflectors and filter the said received light; and, the optical source is configured to input filtered light into one of the said at least one partial optical reflectors; and characterized in that: the optical filter comprises an optical thin film filter comprising a passband filter response; and, the optical source is configured to change the central wavelength of the pass-band response of the optical filter by changing the angle of incidence that the output laser light subtends with the optical thin film filter. the optical filter comprises an optical thin film filter comprising a passband filter response; and, the optical source is configured to change the central wavelength of the pass-band response of the optical filter by changing the angle of incidence that the output laser light subtends with the optical thin film filter.