An optical wavelength division multiplexer/demultiplexer, comprising: a receptacle having a collimate lens and configured to receive an inlet light or receive a combined beam from a reflector and transmit an outlet light; a substrate having a first and a second side; said reflector is mounted to the second side of the substrate and configured to reflect the inlet light or the combined beam at an angle; a multiplexer/demultiplexer block coupled to the second side of the substrate and configured to: when acting as a demultiplexer block, receive the inlet light from the reflector, and separate the inlet light into first multiple wavelengths, or, when acting as a multiplexer block, receive second multiple wavelengths from a folding prism, the second multiple wavelengths being intended for transmission, and to combine the second multiple wavelengths into the combined beam; and a focal lens array coupled to the first side of the substrate opposite to the folding prism, wherein the folding prism is coupled to the second side of the substrate, wherein the focal lens array is configured to receive and focus the first multiple wavelengths or configured to receive and focus the second multiple wavelengths intended for transmission, characterized in that the folding prism is a stepped folding prism having a plurality of faces (654, 655, 656, 657) that together form a stepped surface of the stepped folding prism, a second surface, and a third surface, wherein each face (654, 655, 656, 657) has a surface at a same step angle θ and is further disposed at an angle Φ with respect to the vertical of the third surface, wherein the folding prism is coupled to the substrate and configured to: receive the first multiple wavelengths from the multiplexer/demultiplexer block through the plurality of faces (654, 655, 656, 657) to refract them and to reflect them at the second surface due to total internal reflection, so that the light then travels through the third surface into the focal lens array when the optical wavelength division multiplexer/demultiplexer acts as an optical wavelength division demultiplexer, or, receive the second multiple wavelengths from the focal lens array through the third surface to refract them and to reflect them at the second surface due to total internal reflection, so that the light then travels through the plurality of faces (654, 655, 656, 657) into the multiplexer/demultiplexer block when the optical wavelength division multiplexer/demultiplexer acts as an optical wavelength division multiplexer. a receptacle having a collimate lens and configured to receive an inlet light or receive a combined beam from a reflector and transmit an outlet light; a substrate having a first and a second side; said reflector is mounted to the second side of the substrate and configured to reflect the inlet light or the combined beam at an angle; a multiplexer/demultiplexer block coupled to the second side of the substrate and configured to: when acting as a demultiplexer block, receive the inlet light from the reflector, and separate the inlet light into first multiple wavelengths, or, when acting as a multiplexer block, receive second multiple wavelengths from a folding prism, the second multiple wavelengths being intended for transmission, and to combine the second multiple wavelengths into the combined beam; and a focal lens array coupled to the first side of the substrate opposite to the folding prism, wherein the folding prism is coupled to the second side of the substrate, wherein the focal lens array is configured to receive and focus the first multiple wavelengths or configured to receive and focus the second multiple wavelengths intended for transmission, characterized in that the folding prism is a stepped folding prism having a plurality of faces (654, 655, 656, 657) that together form a stepped surface of the stepped folding prism, a second surface, and a third surface, wherein each face (654, 655, 656, 657) has a surface at a same step angle θ and is further disposed at an angle Φ with respect to the vertical of the third surface, wherein the folding prism is coupled to the substrate and configured to: receive the first multiple wavelengths from the multiplexer/demultiplexer block through the plurality of faces (654, 655, 656, 657) to refract them and to reflect them at the second surface due to total internal reflection, so that the light then travels through the third surface into the focal lens array when the optical wavelength division multiplexer/demultiplexer acts as an optical wavelength division demultiplexer, or, receive the second multiple wavelengths from the focal lens array through the third surface to refract them and to reflect them at the second surface due to total internal reflection, so that the light then travels through the plurality of faces (654, 655, 656, 657) into the multiplexer/demultiplexer block when the optical wavelength division multiplexer/demultiplexer acts as an optical wavelength division multiplexer.