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The left image shows an RC02 collimator mounted into a KM05 Ø1/2" kinematic mount and the right image shows the RC02 mounted into a previous-generation Polaris^® Ø1" kinematic mount. Both kinematic mounts can be used to align the beam for fiber coupling.

Collimating Light from an Optical Fiber
(Can also be Used in Reverse for
Multimode Fiber)

Features

• Achromatic Design for Collimation Over the Mirror's Reflection Band
• Protected Silver Coating (450 nm - 20 µm) Offers High Reflection
• Great for Coupling Polychromatic Light into Multimode Fiber
• Surface Roughness: <100 Å (RMS)
• Ø7.5 mm, Ø11 mm, or Ø22 mm Clear Aperture
• Non-Magnetic Stainless Steel Housing

These Protected Silver Reflective Collimators are based on a 90° off-axis parabolic mirror. Metallic mirrors, unlike lenses, have a focal length that remains constant over a broad wavelength range. Due to this intrinsic property, parabolic mirror collimators do not need to be adjusted to accommodate various wavelengths of light, making them ideal for use with polychromatic light. Common applications include systems that utilize multiple wavelengths that need to be collimated, collimation/coupling in the IR, and coupling polychromatic light into large-core multimode fiber.

These protected silver reflective collimators offer excellent reflectivity within the 450 nm - 20 µm wavelength range; please see the Specs tab for more information. In multiple-wavelength systems operating in the visible and IR, these reflective collimators can be used to both couple polychromatic collimated light into large-core multimode fiber and to collimate polychromatic light emitted by optical fibers. When collimating light from a multimode patch cable, the fiber NA should be ≤0.40 (RC02), ≤0.30 (RC04), ≤0.15 (RC08), or ≤0.19 (RC12) to avoid clipping light on the housing. Note too that, in general, light from a multimode fiber cannot be well collimated. For more information on how to optimize coupling of light from an optical fiber, please see the Collimation Tutorial tab. 

The RC02, RC04, and RC08 reflective collimator housings are equipped with external SM05 (0.535"-40) threads while the RC12 collimator housing has external SM1 (1.035"-40) threads. Thus, these mounts are directly compatible with our SM05- or SM1-threaded optomechanics such as the LMR05(/M) or LMR1(/M) lens mounts, respectively. The back of the RC02 collimators are machined down to Ø1/2", enabling direct mounting into a Ø1/2" kinematic mount (see image to the right). Furthermore, the RC02, RC04, and RC08 may be directly mounted to Ø1" kinematic mounts by removing the knurled ring on the front of the housing, holding the housing behind the kinematic mount's Ø1" counterbore, and then rethreading the knurled ring over the back lip of the counterbore and onto the housing by using an SPW909 Spanner Wrench or an SPW801 Adjustable Spanner Wrench. The collimator can then be secured by tightening the mount's setscrew. The completed setup is shown in the image to the right.

Scattering from the mirror surface is minimal (~2% @ 633 nm) and results from the <100 Å RMS surface roughness achieved by the diamond turning process used to manufacture the off-axis parabolic mirror.

Thorlabs also offers a line of UV-Enhanced Aluminum Coating Reflective Collimators for the 250 - 450 nm wavelength range, Protected Silver Compact Collimators, and Protected Silver, Adjustable Focus Reflective Collimators.

Fiber Patch Cables for Reflective Collimators
Thorlabs offers single mode, polarization-maintaining, or multimode fiber optic patch cables for both coupling and collimating applications. If you cannot find the appropriate stock patch cable for your application, custom-connectorized patch cables with same-day shipping are also available.

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High-NA Fiber: The red line indicates the envelope of the beam that is clipped by the collimator housing. The dashed green line shows the portion of the beam that is collimated by the OAP mirror.

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Low-NA Fiber: The green line indicates the envelope of the beam as it diverges and is collimated by the OAP mirror.

Selecting a Collimator Based on Desired Output Beam Diameter

When using a reflective fiber collimator to collimate light from a fiber patch cable, selection of the appropriate mirror is often done based on the desired output beam diameter and the numerical aperture (NA) of the fiber. The fiber NA determines the divergence of the light emitted by the fiber prior to being collimated by the off-axis parabolic (OAP) mirror in the reflective collimator. The fiber NA is related to the divergence half angle of the fiber by the relationship:

where θ is the divergence half angle and n is the refractive index of the surrounding medium. The collimated beam diameter that results from a diverging light source incident on an OAP mirror is related to the reflected focal length (RFL) of the OAP and the fiber NA; it can be determined using the following equation using the small angle approximation:

where d is the beam diameter. Depending on how the NA of the fiber is specified, the above equation may result in a beam diameter specified at the 5% or 1% level rather than the 1/e² level. The theoretical divergence of the collimated beam can be calculated using the following equation:

where θ_c is the divergence angle of the beam after collimation and MFD is the mode field diameter. Because of the relatively low divergence after collimation, these reflective collimators are ideal for collimating the output from low-NA, single mode fiber patch cables. 

Although the beam can theoretically be expanded to a given beam diameter, there are two major limitations on collimation with multimode fibers. The large divergence angle of most multimode fibers (e.g., NA = 0.22, 0.39, or 0.50) will cause light to be clipped by the collimator housing before it reaches the OAP mirror (as shown in the image to the right). Therefore, when collimating light from a multimode patch cable, the fiber NA should be ≤0.40 (RC02), ≤0.30 (RC04), ≤0.15 (RC08), or ≤0.19 (RC12) to avoid clipping light on the housing. The divergence angle is also slightly affected by the fiber core diameter; the maximum NA allowed will decrease slightly as the core diameter increases. If the beam diameter is larger than the clear aperture of the reflective collimator, then the output beam will be clipped by the collimator housing. Both scenarios may result in reduced quality of the output beam.

The table below lists the output beam diameter as a function of the reflected focal length of the mirror and the NA aperture of common single mode and multimode fibers.

Insights into Off-Axis Parabolic Mirrors

Scroll down to read about the unique properties of off-axis parabolic (OAP) mirrors and how to take advantage of them:

• Why a Parabolic Mirror Instead of a Spherical Mirror?
• Benefit of an Off-Axis Parabolic Mirror
• Directionality of OAP-Mirror-Based Reflective Collimators

Click here for more insights into lab practices and equipment.

Why a Parabolic Mirror Instead of a Spherical Mirror?

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Figure 2: Spherical mirrors do not reflect all rays in a collimated beam through a single point. A selection of intersections in the focal volume are indicated by black dots.

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Figure 1: Parabolic mirrors have a single focal point for all rays in a collimated beam.

Parabolic mirrors perform better than spherical mirrors when collimating light emitted by a point source or focusing a collimated beam.

Focusing Collimated Light
Parabolic mirrors (Figure 1) focus all rays in an incoming, collimated light beam to a diffraction-limited spot. In contrast, concave spherical mirrors (Figure 2) concentrate incoming collimated light into a volume larger than a diffraction-limited spot. The size of the spherical mirror's focal volume can be reduced by decreasing the diameter of the incoming collimated beam.

Collimating Light from a Point Source
A point source emits light in all directions. When this highly divergent light source is placed at the focal point of a parabolic mirror, the output beam is highly collimated. If the point source were ideal, all reflected rays would be perfectly parallel with one another.

When a point source is placed within a spherical mirror's focal volume, the output beam is not as well collimated as the beam provided by a parabolic mirror. Different rays from the point source are not perfectly parallel after reflection from the spherical mirror, but two reflected rays will be more nearly parallel when they reflect from more closely spaced points on the spherical mirror's surface. Consequently, the quality of the collimated beam can be improved by reducing the area of the reflective surface. This is equivalent to limiting the angular range over which the source in the focal volume emits light. 

Choosing Between Parabolic and Spherical Mirrors
A parabolic mirror is not always the better choice. Beam diameter, cost constraints, space limitations, and performance requirements of an application all influence selection. Beam diameter is a factor, since the performance of these two mirrors is more similar when the beam diameter is smaller. Parabolic mirrors are more expensive, since their reflective profiles are more difficult to fabricate. Parabolic mirrors are also typically larger. Improved performance may or may not be more important than the difference in cost and physical size.

Date of Last Edit: Dec. 4, 2019

Benefit of an Off-Axis Parabolic Mirror

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Figure 4: An off-axis parabolic mirror can be thought of as a section of the larger parabolic shape. Both have the same focal point, but it is more accessible in the case of an OAP mirror.

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Figure 3: The focal point of an on-axis parabolic mirror is close to the reflective surface, and typically surrounded by the reflective surface, which makes the focal point difficult to access.

One of the primary benefits of a concave parabolic mirror is its single focal point. All rays travelling parallel to the mirror's axis are reflected through this point. This is useful for a range of purposes, including imaging and manufacturing applications that require focusing laser light to a diffraction limited spot. 

There are a few negatives associated using with using conventional parabolic mirrors, which are symmetric around the focal point (Figure 3). One is that the sides of the mirror generally obstruct access to the focus. Another is that when the mirror is used to collimate a divergent light source, the housing of the light source blocks a portion of the collimated beam. In particular, light emitted at small angles with respect to the optical axis of the mirror is typically obstructed.

An off-axis parabolic (OAP) mirror (Figure 4) is one solution to this problem. The reflective surface of this mirror is parabolic in shape, but it is not symmetric around the focal point. The reflective surface of the OAP corresponds to a section of the parent parabola that is shifted away from the focal point. The section chosen depends on the desired angle and / or distance between the focal point and the center of the mirror. 

Date of Last Edit: Dec. 4, 2019

Directionality of OAP-Mirror-Based Reflective Collimators

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Figure 6: 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 5: 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.

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 5).

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 6). 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.

Date of Last Edit: Dec. 4, 2019

Fiber Collimator Selection Guide

Click on the collimator type or photo to view more information about each type of collimator.

Type		Description
Fixed FC, APC, or SMA Fiber Collimators		These fiber collimation packages are pre-aligned to collimate light from an FC/PC-, FC/APC-, or SMA-terminated fiber. Each collimation package is factory aligned to provide diffraction-limited performance for wavelengths ranging from 405 nm to 4.55 µm. Although it is possible to use the collimator at detuned wavelengths, they will only perform optimally at the design wavelength due to chromatic aberration, which causes the effective focal length of the aspheric lens to have a wavelength dependence.
Air-Spaced Doublet, Large Beam Collimators		For large beam diameters (Ø5.3 - Ø8.5 mm), Thorlabs offers FC/APC, FC/PC, and SMA air-spaced doublet collimators. These collimation packages are pre-aligned at the factory to collimate a laser beam propagating from the tip of an FC or SMA-terminated fiber and provide diffraction-limited performance at the design wavelength.
Triplet Collimators		Thorlabs' High Quality Triplet Fiber Collimation packages use air-spaced triplet lenses that offer superior beam quality performance when compared to aspheric lens collimators. The benefits of the low-aberration triplet design include an M2 term closer to 1 (Gaussian), less divergence, and less wavefront error.
Achromatic Collimators for Multimode Fiber		Thorlabs' High-NA Achromatic Collimators pair a meniscus lens with an achromatic doublet for high performance across the visible to near-infrared spectrum with low spherical aberration. Designed for use with high-NA multimode fiber, these collimators are ideal for Optogenetics and Fiber Photometry applications.
Reflective Collimators		Thorlabs' metallic-coated Reflective Collimators are based on a 90° off-axis parabolic mirror. Mirrors, unlike lenses, have a focal length that remains constant over a broad wavelength range. Due to this intrinsic property, a parabolic mirror collimator does not need to be adjusted to accommodate various wavelengths of light, making them ideal for use with polychromatic light. Our fixed reflective collimators are recommended for collimating single and multimode fiber and coupling into multimode fiber. These collimators are available with UV-enhanced aluminum or protected silver reflective coatings and with FC/PC, FC/APC, or SMA connector compatibility.
Compact Reflective Collimators		Thorlabs' Compact Reflective Collimators incorporate a 90° off-axis parabolic mirror with a protected silver coating. Because the focal length is independent of wavelength for an off-axis parabolic mirror, they are ideal for use with polychromatic light. Our fixed reflective collimators are recommended for collimating single and multimode fiber and coupling into multimode fiber. These collimators are directly compatible with our 16 mm cage system. They are available with FC/PC, FC/APC, or SMA connector inputs.
Adjustable Reflective Collimators		Thorlabs' Adjustable Focus Reflective Collimators are based on a 90° off-axis parabolic (OAP) mirror with a protected silver coating. The adjustable fiber-to-OAP distance, combined with the OAP having a constant focal length across wavelengths, makes these collimators ideal for optimizing collimation or coupling of polychromatic light with single mode or multimode fiber. These adjustable collimators have a 15 mm reflected focal length and are available with FC/PC, FC/APC, or SMA connectors.
FiberPorts		These compact, ultra-stable FiberPort micropositioners provide an easy-to-use, stable platform for coupling light into and out of FC/PC, FC/APC, or SMA terminated optical fibers. It can be used with single mode, multimode, or PM fibers and can be mounted onto a post, stage, platform, or laser. The built-in aspheric or achromatic lens is available with five different AR coatings and has five degrees of alignment adjustment (3 translational and 2 pitch). The compact size and long-term alignment stability make the FiberPort an ideal solution for fiber coupling, collimation, or incorporation into OEM systems.
Adjustable Fiber Collimators		These collimators are designed to connect onto the end of an FC/PC, FC/APC, or SMA connector and contain an AR-coated aspheric lens. The distance between the aspheric lens and the tip of the fiber can be adjusted to compensate for focal length changes or to recollimate the beam at the wavelength and distance of interest.
Achromatic Fiber Collimators with Adjustable Focus		Thorlabs' Achromatic Fiber Collimators with Adjustable Focus are designed with an effective focal length (EFL) of 20 mm, 40 mm, or 80 mm, have optical elements broadband AR coated for one of three wavelength ranges, and are available with FC/PC, FC/APC, or SMA905 connectors. A four-element, air-spaced lens design produces superior beam quality (M2 close to 1) and less wavefront error when compared to aspheric lens collimators. These collimators can be used for free-space coupling into a fiber, collimation of output from a fiber, or in pairs for collimator-to-collimator coupling over long distances, which allows the beam to be manipulated prior to entering the second collimator.
Zoom Fiber Collimators		These collimators provide a variable focal length between 6 and 18 mm, while maintaining the collimation of the beam. As a result, the size of the beam can be changed without altering the collimation. This universal device saves time previously spent searching for the best suited fixed fiber collimator and has a very broad range of applications. They are offered with FC/PC, FC/APC, or SMA905 connectors with three different antireflection wavelength ranges to choose from.
Single Mode Pigtailed Collimators		Our single mode pigtailed collimators come with one meter of fiber, consist of an AR-coated aspheric lens pre-aligned with respect to a fiber, and are collimated at one of eight wavelengths: 532 nm, 633 nm, 780 nm, 850 nm, 1030 nm, 1064 nm, 1310 nm, or 1550 nm. Although it is possible to use the collimator at any wavelength within the coating range, the coupling loss will increase as the wavelength is detuned from the design wavelength.
Polarization Maintaining Pigtailed Collimators		Our polarization maintaining pigtailed collimators come with one meter of fiber, consist of an AR-coated aspheric lens pre-aligned with respect to a fiber, and are collimated at one of five wavelengths: 633 nm, 780 nm, 980 nm, 1064 nm, or 1550 nm. Custom wavelengths and connectors are available as well. A line is engraved along the outside of the housing that is parallel to the fast axis. As such, it can be used as a reference when polarized light is launched accordingly. Although it is possible to use the collimator at any wavelength within the coating range, the coupling loss will increase as the wavelength is detuned from the design wavelength.
GRIN Fiber Collimators		Thorlabs offers gradient index (GRIN) fiber collimators that are aligned at a variety of wavelengths from 630 to 1550 nm and have either FC terminated, APC terminated, or unterminated fibers. Our GRIN collimators feature a Ø1.8 mm clear aperture, are AR-coated to ensure low back reflection into the fiber, and are coupled to standard single mode or graded-index multimode fibers.
GRIN Lenses		These graded-index (GRIN) lenses are AR coated for applications at 630, 830, 1060, 1300, or 1560 nm that require light to propagate through one fiber, then through a free-space optical system, and finally back into another fiber. They are also useful for coupling light from laser diodes into fibers, coupling the output of a fiber into a detector, or collimating laser light. Our GRIN lenses are designed to be used with our Pigtailed Glass Ferrules and GRIN/Ferrule sleeves.