Diffraction compensated compact wavelength division multiplexing devices

A method of processing light in a wavelength division multiplexing device, comprising:

• transmitting, by a common collimator of a common port, an optical beam including a plurality of optical signals, wherein each optical signal of the plurality of optical signals is associated with a different wavelength range;
• receiving the optical beam at a first filter of a plurality of filters, wherein the plurality of filters defines an optical path for the optical beam, and each filter of the plurality of filters includes an interface having a filter radius of curvature;
• transmitting one of the plurality of optical signals through the interface of the first filter;
• reflecting a remaining portion of the optical beam from the interface of the first filter toward a second filter of the plurality of filters;
• wherein transmitting the optical beam by the common collimator and reflecting the remaining portion of the optical beam by the first filter cause the remaining portion of the optical beam to have:
• a beam waist located in the optical path midway between the first filter and the second filter, and
• a wavefront radius of curvature at the second filter that matches the filter radius of curvature of the second filter;
• wherein the first filter is spaced from the second filter by a filter separation optical distance measured along the optical path, and
• wherein either:
• the common collimator has a working distance equal to zero and is located in the optical path at a common collimator optical distance from the first filter that is equal to half the filter separation optical distance, or
• the common collimator has a working distance equal to half the filter separation optical distance and is located in the optical path at a common collimator optical distance from the first filter that is equal to the filter separation optical distance.