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Fiber Coupling and Spatial Filter Systems

  • Free Space Fiber Launch System with Submicron XY Translator
  • Spatial Filter System Produces "Clean" Gaussian Beams


Free Space
Fiber Coupler


Spatial Filter System

Related Items

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Focusing Optics
Item # Description
C220TMD-A 350 - 700 nm, f = 11.0 mm, NA = 0.25 Aspheric Lens
C220TMD-B 600 - 1050 nm, f = 11.0 mm, NA = 0.25 Aspheric Lens
C220TMD-C 1050 - 1700 nm, f = 11.0 mm, NA = 0.25 Aspheric Lens


  • KT110 Fiber Coupler Accepts FC or SMA Fiber Cables
  • High-Precision Differential Adjusters Included with KT110 Provide Submicron Translation
  • KT310 Spatial Filter Directly Compatible with Ø9 mm Aspheric Lens Housings
  • Easy-to-Follow Instructions and Alignment Tools

Fiber Launch Systems

Thorlabs' KT110(/M) fiber launch system couples free-space laser beams into fiber optic cables. This system, which can be used with single or multimode fiber, is equipped with high-precision differential adjusters capable of submicron translation.

This fiber launch system only includes optomechanical components (see the Components tab for a complete list of components). The table to the right lists the recommended optics for focusing collimated light into the output fiber (KT110).

Spatial Filter Systems

Many applications, such as holography, require a beam with uniform intensity. The KT310 spatial filter is ideal for producing a clean, spatially uniform, Gaussian beam. The input of this system consists of a Z-axis translator, which can house a diffraction-limted aspheric lens. This aspheric lens focuses the beam through a pinhole, which can be mounted in the XY translator. The output consists of a Ø1" cage mount, which holds and centers a collimating optic.

This filter system only includes optomechanical components (see the Components tab for a complete list of components). The aspheric lens, collimating optics, and pinhole must be purchased separately. Please refer to the Tutorial tab for more information on choosing the appropriate optics and pinhole for your application.

Principles of Spatial Filters

For many applications, such as holography, spatial intensity variations in the laser beam are unacceptable. Our KT310 spatial filter system is ideal for producing a clean Gaussian beam.

Spatial Filter System Ray Diagram

Figure 1: Spatial Filter System

The input Gaussian beam has spatially varying intensity "noise". When a beam is focused by an aspheric lens, the input beam is transformed into a central Gaussian spot (on the optical axis) and side fringes, which represent the unwanted "noise" (see Figure 2 below). The radial position of the side fringes is proportional to the spatial frequency of the "noise".

Input Gaussian Beam

Figure 2

By centering a pinhole on a central Gaussian spot, the "clean" portion of the beam can pass while the "noise" fringes are blocked (see Figure 3 below).

Clean Gaussian Beam

Figure 3

The diffraction-limited spot size at the 99% contour is given by:

Diffraction-Limited Spot Size

where λ = wavelength, ƒ=focal length and r = input beam radius at the 1/e2 point.

Choosing the Correct Optics and Pinhole for Your Spatial Filter System

The correct optics and pinhole for your application depend on the input wavelength, source beam diameter, and desired exit beam diameter.

For example, suppose that you are using a 650 nm diode laser source that has a diameter (1/e2) of 1.2 mm and want your beam exiting the spatial filter system to be about 4.4 mm in diameter. Based on these parameters, the C560TME-B mounted aspheric lens would be an appropriate choice for the input side of spatial filter system because it is designed for use at 650 nm, and its clear aperture measures 5.1 mm, which is large enough to accommodate the entire diameter of the laser source.

The equation for diffraction limited spot size at the 99% contour is given above, and for this example, λ = (650 x 10-9 m), f = 13.86 mm for the C560TM-B, and r = 0.6 mm. Substitution yields

Spot Size Example

Diffraction-Limited Spot Size (650 nm source, Ø1.2 mm beam)

The pinhole should be chosen so that it is approximately 30% larger than D. If the pinhole is too small, the beam will be clipped, but if it is too large, more than the TEM00 mode will get through the pinhole. Therefore, for this example, the pinhole should ideally be 19.5 microns. Hence, we would recommend the mounted pinhole P20S, which has a pinhole size of 20 μm. Parameters that can be changed to alter the beam waist diameter, and thus the pinhole size required, include changing the input beam diameter and focal length of focusing lens. Decreasing the input beam diameter will increase the beam waist diameter. Using a longer focal length focusing lens will also increase the beam waist diameter.

Finally, we need to choose the optic on the output side of the spatial filter so that the collimated beam's diameter is the desired 4.4 mm. To determine the correct focal length for the lens, consider the following diagram in Figure 4, which is not drawn to scale. From the triangle on the left-hand side, the angle is determined to be approximately 2.48o. Using this same angle for the triangle on the right-hand side, the focal length for the plano-convex lens should be approximately 50 mm.

Spatial Filter Diagram

Figure 4:
Beam Expansion Example

For this focal length, we recommend the LA1131-B plano-convex lens [with f = 50 mm at the design wavelength (λ = 633 nm), this is still a good approximation for f at the source wavelength (λ = 650 nm)].

Note: The beam expansion equals the focal length of the output side divided by the focal length of the input side.

For optimal performance, a large-diameter aspheric lens can be used in place of a plano-convex lens if the necessary focal length on the output side is 20 mm (see AL2520-A, AL2520-B, AL2520-C). These lenses are 25 mm in diameter and can be held in place using the supplied SM1RR Retaining Ring.

CP02 CP02/M SM1-Threaded 30 mm Cage Plate 1
CPA1 30 mm Cage System Alignment Plate 1
E09RMS Extended RMS to M9 x 0.5 Adapter 1
ER2 Cage Assembly Rod, 2" Long 4
ER3 Cage Assembly Rod, 3" Long 4
MA2 MA2/M Ø1.5" Post Mounting Adapter 1
P1.5 P30/M Ø1.5" Mounting Post 1
PB1 Mounting Post Base 1
SM1A3 Adapter with External SM1 Threads and
Internal RMS Threads
SM1D12 SM1 Lever-Actuated Iris Diaphragm 1
SM1FC FC/PC Fiber Adapter Plate 1
SM1RR SM1 Retaining Ring 3
SM1Z Z-Axis Translation Mount 1
SPW301 Spanner Wrench for a M9 x 0.5 Optics Housing 1
SPW801 Adjustable Spanner Wrench 1
ST1XY-D ST1XY-D/M XY Translator with Differential Drives 1
SM1SMA SMA Fiber Adapter Plate 1

CPA1 30 mm Cage System Alignment Plate 1
E09RMS Extended RMS to M9 x 0.5 Adapter 1
ER2 Cage Assembly Rod, 2" Long 8
MA2 MA2/M Ø1.5" Post Mounting Adapter 1
P2 P50/M Ø1.5" Mounting Post 1
PB1 Mounting Post Base 1
SM1L03 SM1 Lens Tube, 0.3" Thread Depth 1
SM1A3 Adapter with External SM1 Threads and
Internal RMS Threads
SM1RR SM1 Retaining Ring 3
SM1Z Z-Axis Translation Mount 1
SPT1 SPT1/M Coarse ±1 mm XY Slip Plate Positioner 1
SPW301 Spanner Wrench for a M9 x 0.5 Optics Housing 1
SPW801 Adjustable Spanner Wrench 1
ST1XY-A ST1XY-A/M XY Translator with 100 TPI Drives 1

Cage System Overview

The Cage Assembly System provides a convenient way to construct large optomechanical systems with an established line of precision-machined building blocks designed for high flexibility and accurate alignment.

16 mm, 30 mm, and 60 mm Cage System Standards

Thorlabs offers three standards defined by the center-to-center spacing of the cage assembly rods (see image below). The 16 mm cage, 30 mm cage, and 60 mm cage standards are designed to accomodate Ø1/2", Ø1", and Ø2" optics, respectively. Specialized cage plates that allow smaller optics to be directly inserted into our larger cage systems are also available. All locking setscrews used in these cage plates are standard 4-40 setscrews, which use a 0.05" hex key.

Standard Threads

The flexibility of our Cage Assembly System stems from well-defined mounting and thread standards designed to directly interface with a wide range of specialized products. The three most prevalent thread standards are our SM05 Series (0.535"-40 thread), SM1 Series (1.035"-40 thread), and SM2 Series (2.035"-40 thread), all of which were defined to house the industry's most common optic sizes. Essential building blocks, such as our popular lens tubes, directly interface to these standards.

An example of the standard cage plate measurements determining cage system compatibility.
Standard Cage System Measurements
Cage System 16 mm 30 mm 60 mm
Thread Series SM05 SM1 SM2
Rod to Rod Spacing 16 mm (0.63") 30 mm (1.18") 60 mm (2.36")
Total Length 25 mm (0.98") 41 mm (1.60") 71.1 mm (2.8")
Cage Components
Cage Rods 16 mm These rods are used to connect cage plates, optic mounts, and other components in the cage system. The SR Series Cage Rods are compatible with our 16 mm cage systems, while the 30 mm and 60 mm cage systems use ER Series Cage Rods.
30 mm
60 mm
Cage Plates 16 mm These serve as the basic building blocks for a cage system. They may have SM-threaded central bores, smooth bores sized for industry standard optics or to accommodate the outer profile of our SM Series Lens Tubes, or specialized bores for other components such as our FiberPorts.
30 mm
60 mm
Optic Mounts 16 mm Thorlabs offers fixed, kinematic, rotation, and translation mounts specifically designed for our Cage Systems.
30 mm
60 mm
Cage Cubes 16 mm These cubes are useful for housing larger optical components, such as prisms or mirrors, or optics that need to sit at an angle to the beam path, such as beamsplitters. Our cage cubes are available empty or with pre-mounted optics.
30 mm
60 mm
Post and Breadboard Mounts and Adapters Mounting options for cage systems can be found on our Cage System Construction pages. Cage Systems can be mounted either parallel or perpendicular to the table surface.
Size Adapters Cage System Size Adapters can be used to integrate components from different cage system and threading standards.
Specialized Components Thorlabs also produces specialized cage components, such as Filter Wheels, a HeNe Laser Mount, and a FiberPort Cage Plate Adapter, allowing a wide range of our products to be integrated into cage-mounted optical systems. Explore our Cage Systems Visual Navigation Guide to see the full range of Thorlabs' cage components.

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Posted Comments:
Posted Date:2016-09-25 21:39:59.157
I need to align a fiber optic to a beam under vacuum. When I pump down the chamber shifts. So I need to be able to actuate the fiber's position a small amount to get back my coupling efficiency. If I could remove the manual knobs and replace them with your vacuum-compatible motorized actuators that would solve my problem. Note the anodization isn't a big deal for my vacuum levels.
Posted Date:2016-09-29 10:47:23.0
Response from Jeremy at Thorlabs: You could replace the manual adjusters with the Z806V adjusters.
Posted Date:2016-06-10 17:51:00.23
Hi. I was looking for a cage system to couple light from free space to a nonlinear photonic crystal fiber (NL-PM-750). Is KT110 capable of this function? Because the nonlinear photonic crystal will be used for supercontinuum generation, I think the cage system needs to be mounted on a 3-axis stage. Will this be possible? Or do you have some recommendations on combining this cage system with a 3 axis stage in the supercontinuum generation experiment? Thanks for your help.
Posted Date:2016-06-14 02:35:08.0
Response from Bweh at Thorlabs USA: Yes that could be used if you have the proper adapters to mount the PCF fiber. We also have supercontinuum generation kits that may interest you here:
Posted Date:2013-02-01 00:53:58.73
Do you have a complete list of the components in KT110/M? I wonder if you have both SMA and FC adapters in the package? The sales told us only one of them is included. However, the picture in the website show both. Thanks.
Posted Date:2012-10-28 16:21:17.34
I have a very large (r~5mm), non-gaussian beam of CW 980nm light that is collimated from a 6 Watt diode. I want to clean up the beam as best I can before sending it through an AO and fiber. I think that the KT310 is what I need, but I'd like to use a long focal length asphere (e.g. 50mm), so that I can put my large beam in and still have a pretty large diffraction limit (~20microns). This way I can use the high power 25micron pinhole (P25C) and stay below the intensity damage threshold (any smaller and I think the intensity would be too high.) Will I be able to put a large asphere into the input mount, or is there another solution I should be pursuing? Best, Claire Thomas UC Berkeley Physics
Posted Date:2012-09-20 12:12:00.0
Response from Tim at Thorlabs to Stefan: It seems you may be viewing an Airy Disk which could indicate the focal length and pinhole are not appropriately matched to block the fringes. I will contact you to get more information on the components you are using.
Posted Date:2011-01-13 10:58:51.0
A response from Greg at Thorlabs to naveedbinqasim: I apologize for any issues you are having with the KT310. An Applications Engineer has been put in contact with you to help optimize the performance of your spatial filter.
Posted Date:2010-12-31 06:49:04.0
we bought spatial filter system KT-310 from you. but beam after passing through the pinhiole doesnt produce clean beam.i ve alligned KT-310 very precisely,but the output is distorted. Naveed Bin Qasim Research Assistant SSE Physics Lahore University of Management Sciences.
Posted Date:2007-11-26 09:40:51.0
In response to mthiels post, we have updated the web presentation to included recommended optics/pinhole parts for use with the KT310. In addition, there is now a "tutorial tab" that explains how to choose optics given an incident wavelength and beam diameter. Thank you for your feedback. we hope this additional information is helpful when it comes to choosing optics for use with your spatial filter.
Posted Date:2007-11-13 05:27:02.0
Regarding KT310/M: In the Katalog Vol18 there were some Recommended Parts (optics): 1. Lens Item #c220tme 2. Lens Item #la1131 3. Lens Item #p25s Now, where it is missing in the new catalog V19 and Web - customer (aspecially those who got no clue what optics to use) start discussions with tech support about optics. -So the recommended Parts would save some time for techies.

Free-Space to Fiber Coupler

  • Accepts FC or SMA Fiber Cables
  • Accepts Microscope Objectives
  • High-Precision Differential Adjusters Provide Submicron Translation
  • Easy-to-Follow Instructions and Alignment Tools

The KT110 fiber launch system is designed to couple free-space laser beams into fiber optic cables terminated with either FC or SMA connectors. This fiber launch system is directly compatible with many of our diffraction-limited aspheres, which offer superior performance when compared to traditional microscope objectives. The C230TMD aspheric lens, which has an equivalent microscope maginification of 35X, is an ideal choice for most free-space coupling applications.

Note: The individual optomechanical parts used in the KT110 assembly are imperial or universal, while the parts used in the KT110/M assembly are metric or universal.

Based on your currency / country selection, your order will ship from Newton, New Jersey  
+1 Qty Docs Part Number - Imperial Price Available / Ships
KT110 Support Documentation
KT110Fiber Launch System: Free Space (Coupling Optic Sold Separately)
3-5 Days
+1 Qty Docs Part Number - Metric Price Available / Ships
KT110/M Support Documentation
KT110/MFiber Launch System: Free Space, Metric (Coupling Optic Sold Separately)

Spatial Filter System

  • Produces a Clean Gaussian Beam
  • Wide Assortment of Compatible Pinholes, Aspheric Lenses, and Collimating Lenses
  • Pre-Assembled System of Stock Catalog Components

The KT310 spatial filter is ideal for producing a "clean" Gaussian beam. The input side consists of a Z-translator that will hold a diffraction-limited aspheric lens to focus the laser through a pinhole. For easy adjustment, the pinhole should be mounted in the XY translator. Threaded holes on the output side are provided for mounting and centering a Ø1" collimating optic. Please see the Tutorial tab for more information on selecting the appropriate optics and pinhole for your application.

Note: The individual optomechanical parts used in the KT310 assembly are imperial or universal while the parts used in the KT310/M assembly are metric or universal.

Based on your currency / country selection, your order will ship from Newton, New Jersey  
+1 Qty Docs Part Number - Imperial Price Available / Ships
KT310 Support Documentation
KT310Spatial Filter System (Optics & Pinhole Sold Separately)
+1 Qty Docs Part Number - Metric Price Available / Ships
KT310/M Support Documentation
KT310/MSpatial Filter System, Metric (Optics & Pinhole Sold Separately)
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