The present invention concerns an optical grating comprising a body defining a first external surface and a second external surface. The first external surface defines a periodic structure comprising a plurality of elongated protrusions and a plurality of elongated recesses or depressions. The periodic structure is configured to receive an incident first electromagnetic wave propagating in an input propagation direction and to transmit the first electromagnetic wave to the second external surface for transmission through the second external surface. The periodic structure is configured to reflect, back into the input propagation direction, an incident second electromagnetic wave propagating in an opposite direction. The first electromagnetic wave has a wavelength lower that of the second electromagnetic wave.