Click to Enlarge 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 POLARIS-K1 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)
Achromatic Design for Collimation Over the Mirror's Reflection Band
Great for Coupling Polychromatic Light into Multimode Fiber
Surface Roughness: <100 Å (RMS)
Ø7.5 mm, Ø11 mm, or Ø22 mm Clear Aperture
Nonmagnetic 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, a parabolic mirror collimator does not need to be adjusted to accommodate various wavelengths of light, making them ideal for use with polychromatic light. The limitations of the achievable surface roughness of the diamond turning process and tooling marks will cause a minimal (~2% @ 633 nm) amount of scattering off of the mirrored surface. 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 Specstab 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. Note that, in general, light from a multimode fiber cannot be well collimated. When collimating light from a multimode patch cable, the fiber NA should be <0.40 (RC02), <0.36 (RC04), <0.167 (RC08), or <0.216 (RC12) to avoid clipping light on the housing. For more information on how to optimize coupling of light from an optical fiber, please see the Collimation Tutorialtab.
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 or LMR1, 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 and rethreading it over the back lip of the Ø1" counterbore 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.
Pointing Errorb (FC/PC- and FC/APC-Connectorized Models)
Approximate beam divergence. Divergence is based on the MFD of the fiber. 0.02° was achieved using S460-HP fiber at 543 nm.
Pointing error is not guaranteed for the SMA connectors due to the metal ferrule.
FC/PC and FC/APC versions use wide key connectors.
Approximate, based on 0.13 NA fiber
Effective Focal Length
Parent Focal Length: Off-axis parabolic mirrors can be made individually or cut from an on-axis parent parabolic mirror. When an off-axis parabolic mirror is cut from a parent on-axis parabolic mirror, we have a parent focal length (PFL) spec that arises. The PFL is the EFL of the parent mirror. Note that the EFL specified in the table above is that of the off-axis parabolic mirror. See the drawing to the right for more information.
Click to Enlarge 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.
Click to Enlarge 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, assuming the surrounding medium is air (n = 1). 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 θc is the divergence angle of the beam after collimation. 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), <0.36 (RC04), <0.167 (RC08), or <0.216 (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.
Calculated Beam Diameter for Given Fiber NA and RFL
0.13 NA, Single Mode
0.22 NA, Multimode
0.39 NA, Multimode
7 mm RFL (Item # Prefix RC02)
15 mm RFL (Item # Prefix RC04)
33 mm RFL (Item # Prefix RC08)
50.8 mm RFL (Item # Prefix RC12)
Due to the large NA of these fibers, the output beam will be clipped by the collimator housing.
These output beam diameters are larger than the clear aperture of the reflective collimator.
Fiber Collimator Selection Guide
Click on the collimator type or photo to view more information about each type of collimator.
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.
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.
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.
Thorlabs' High-NA Achromatic Collimators pair a meniscus lens with an achromatic doublet for high performance across the visible spectrum with low spherical aberration. Designed for use with high-NA multimode fiber, these collimators are ideal for Optogenetics and Fiber Photometry applications.
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 reflective collimators are ideal for single-mode fiber.
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.
These collimators are designed to connect onto the end of an FC/PC or FC/APC connector and contain an AR-coated aspheric lens. The distance between the aspheric lens and the tip of the FC-terminated fiber can be adjusted to compensate for focal length changes or to recollimate the beam at the wavelength and distance of interest.
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.
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.
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.
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 six wavelengths: 532 nm, 830 nm, 1030 nm, 1064 nm, 1310 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 slow 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.
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.
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.
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?
Click to Enlarge 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.
Click to Enlarge 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
Click to Enlarge 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.
Click to Enlarge 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
Click to Enlarge 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.
Click to Enlarge 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.