A Single Reflective Collimator Port is Designed for Collimated Light

Directionality of OAP-Mirror-Based Reflective Collimators 

The two ports on Thorlabs' reflective collimators are not interchangeable. One port accepts an optical fiber connector and requires the highly divergent light of a point source. The other port is designed solely for collimated, free-space light (Figure 1).

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Figure 2: The reflective element of the collimator is an off-axis parabolic mirror. The mirror's substrate is highlighed in red. The shape of the reflective surface is a segment of the parabolic curve displaced from the vertex. The focal points of the parent parabola and the OAP mirror coincide.

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Figure 1: Thorlabs offers reflective collimators that include a port for an optical fiber connector and a port for free space, collimated light that propagates parallel to the optical axis.

Free Space Port
Light input to this port should be collimated and directed parallel to the optical axis. Diverging light from a fiber end face, a laser diode, or other source should not be input. This light would not be collimated at the fiber connector port or coupled into the fiber connected to the fiber port.

Optical Fiber Connector Port
This port aligns the fiber's end face with the focal point of the mirror. Since the fiber's end face approximates a point source placed at the focal point, a collimated beam is output from the free-space port. The alignment of the fiber end face with the focal point is also the reason that all light input to the free-space light port should be collimated and directed parallel to the optical axis.

Source of Directionality
The collimator's directionality is a consequence of using a non-rotationally symmetric, off-axis parabolic (OAP) mirror as the reflective element (Figure 2). The cut-away view illustrates that the fiber's end face is positioned at the focal point of the parent parabola, which is also the focal point of the OAP mirror.