Focus Collimated Light / Collimate Light from a Point Source

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Figure 1: The focal and optical axes of an OAP mirror do not coincide and are not parallel.

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Figure 3: When the collimated beam is not directed along the mirror's optical axis, the mirror does not provide a diffraction-limited spot. Instead, the focal region is spread out.

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Figure 2: When the collimated beam is parallel to the optical axis of a parabolic or OAP mirror, the light focuses to a diffraction-limited spot.

Parabolic and off-axis parabolic (OAP) mirrors will only provide the expected well-collimated beam or diffraction-limited focal spot when the correct beam type is incident along the proper axis. This due to the parabolic shape of these mirrors' reflective surfaces, which are not symmetric around their focal points.

Parabolic vs. Off-Axis Parabolic Mirrors
The reflective surface of an OAP mirror is a section of the parent parabola that is not centered on the parent's optical axis (Figure 1). A conventional parabolic mirror is illustrated in Figure 2.

The optical axis of an OAP mirror is parallel to, but displaced from the optical axis of the parent parabola. The focal point of the OAP mirror coincides with that of the parent parabola.

The focal axis of the OAP mirror passes through the focal point and the center of the OAP mirror. The focal and optical axes of an OAP mirror are not parallel. In contrast, these axes coincide for parabolic mirrors whose reflective surfaces are centered on optical axis of the parent parabola.

Focus Collimated Light
If a parabolic or OAP mirror is being used to focus a beam of collimated light to a diffraction-limited point, the light must be directed along the mirror's optical axis (Figures 1 and 2).

Collimated light that is not directed parallel to the optical axis will not focus to a unique point (Figure 3).

Thorlabs recommends against directing collimated light along the focal axis of OAP mirrors, or along any direction that is not parallel to the optical axis, since the light will not focus to a diffraction-limited spot.

Collimate Light from a Point Source
To obtain highly collimated light from a point source, the point source should be located at the mirror's focal point.

Light from a point source will be poorly collimated if the point source is placed along the OAP mirror's optical axis, or anywhere else that is not the focal point.

An OAP mirror can also be used to collimate a spherical wave, if its origin coincides with the focal point of the mirror.