upon the beam exiting the substrate 12 results in a portion of the beam being split off at an angle an optical element may be provided at the output of the substrate 12 for operating on the primary beam also if the beam is not collimated when it enters the substrate 12 the substrate 12 may include an optical element at the input for collimating the beam further if the wavelength locker is used with a back facet the application beam is not needed the monitor beam split off from the primary beam reflects off the reflective portion 14 towards the partially reflecting layer 19 part of the light passes through the partially reflecting layer 19 to the patterned aperture 18 and then to the array of detectors 20 the rest of the beam is reflected from the partially reflecting layer 19 to the retro reflector 16 the retro reflector 16 reflects the beam back onto the partially reflecting film 19 where again part of the beam is passed through to the patterned aperture 18 and the array of detectors 20 use of a diffraction grating as the retro reflector 16 in such an arrangement is referred to as a littrow grating the original beam impinging on the aperture 18 and the beam reflected back to the aperture 18 by the retro reflector 16 interfere to create a periodic output typically sinusoidal on a detector 20 due to the phase shift between the two beams the aperture 18.