A method of controlling optical feedback in an optical system having a radiation source optically coupled via mode matching optics with a resonant optical cavity having a configuration of a V-shaped cavity having three cavity mirrors (5, 6, 7), one of which is a cavity coupling mirror, the resonant optical cavity having a plurality of optical resonance cavity modes, wherein the radiation source emits a beam of continuous wave radiation and is capable of being scanned whereby a mean optical frequency of the beam of continuous wave radiation is adjustable over a range of frequencies, wherein the radiation source is responsive to optical feedback radiation emerging from the resonant optical cavity and impinging on the radiation source, and wherein the mode matching optics couples the beam of continuous wave radiation to the resonant optical cavity via the cavity coupling mirror, and wherein the cavity coupling mirror is arranged at an angle with respect to the beam, the method comprising aligning the radiation source and the mode matching optics to control an optical feedback radiation fed back from the resonant optical cavity to the radiation source, wherein a mode fill ratio of cavity modes is reduced relative to a maximum mode fill ratio, wherein for the maximum mode-fill ratio the beam of continuous wave radiation is coupled with a fundamental cavity spatial mode, and wherein the aligning the radiation source and the mode matching optics includes: aligning or setting a relative distance between the radiation source and a component of the mode matching optics to achieve the maximum mode fill ratio, and thereafter adjusting the relative distance between the radiation source and the component of the mode matching optics to attain the reduced mode fill ratio and control the optical feedback radiation fed back from the resonant optical cavity to the radiation source. aligning the radiation source and the mode matching optics to control an optical feedback radiation fed back from the resonant optical cavity to the radiation source, wherein a mode fill ratio of cavity modes is reduced relative to a maximum mode fill ratio, wherein for the maximum mode-fill ratio the beam of continuous wave radiation is coupled with a fundamental cavity spatial mode, and wherein the aligning the radiation source and the mode matching optics includes: aligning or setting a relative distance between the radiation source and a component of the mode matching optics to achieve the maximum mode fill ratio, and thereafter adjusting the relative distance between the radiation source and the component of the mode matching optics to attain the reduced mode fill ratio and control the optical feedback radiation fed back from the resonant optical cavity to the radiation source. aligning or setting a relative distance between the radiation source and a component of the mode matching optics to achieve the maximum mode fill ratio, and thereafter adjusting the relative distance between the radiation source and the component of the mode matching optics to attain the reduced mode fill ratio and control the optical feedback radiation fed back from the resonant optical cavity to the radiation source.