A Volume Holographic Grating Diplexer (VHGD) is an optical element that diffracts the downstream 1490 nm signal while allowing the 1310 nm upstream signal to go through.Compared to thin film interference filter, the VHGD enables the 1310 nm transmitter laser (TX) and the receiver (RX) to be on the same side, either on the same plane or at different heights, thus enabling low cost packaging.