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Comparison of Focal Shifts in Achromatic and Aspheric FiberPorts
(-A Coated Versions Shown)

Features

• Five Degrees of Freedom Plus Rotational Adjustment
• Low Hysteresis (See Graphs Tab for More Information)
• FC/PC, FC/APC, and SMA Versions
  • FC/PC and FC/APC Versions Accept Both 2.1 mm Wide Key and 2.0 mm Narrow Key Connectors
  • SMA FiberPorts Are Designed for SMA905 Connectors
• Available with Either an Achromatic Doublet or an Aspheric Lens
• Suitable for Single Mode (SM), Multimode (MM), and Polarization-Maintaining (PM) Fiber
• AR Coating Options for Visible, NIR, and MIR Wavelength Ranges (See Selection Guide Tab for Details)

Thorlabs' compact, ultrastable FiberPort micropositioners provide an easy-to-use platform for coupling light into and out of optical fibers. Their compact size; repeatable, high-resolution alignment mechanism; high thermal stability*; and translation locking mechanisms (detailed in the Operation tab) make these FiberPorts an ideal solution for long- or short-term fiber coupling and collimation. Each FiberPort is factory-aligned for collimation at the wavelength specified in its respective mechanical drawing, which can be found by clicking on the red document icon () below. For information about the hysteresis and thermal stability, please see the Graphs tab.

Each FiberPort includes an achromatic doublet or aspheric lens with an effective focal length ranging from 2.0 mm to 18.4 mm. They are available with FC/PC, FC/APC, or SMA fiber bulkheads. For applications that are compatible with short effective focal lengths (≤7.5 mm), we offer FiberPorts with FC bulkheads that can be used with both FC/PC and FC/APC connectors. Their 5-axis adjustment combined with their short focal length leads to negligible off-axis sensitivity. For more details on how to select a FiberPort, please see the Selection Guide tab.

For a higher maximum theoretical coupling efficiency, we recommend using FiberPorts with our AR-coated single mode, multimode, or polarization-maintaining fiber optic patch cables for coupling and collimating light, as these patch cables reduce back reflections from high powered sources. For FiberPorts being used in the mid-infrared spectral region, we recommend our fluoride fiber patch cables.

Achromatic FiberPorts
Our achromatic FiberPorts collimate light over a large wavelength range with a very small focal length shift. This reduces the need for realignment if the wavelength of the source is changed. The focal length shift for both the achromatic and similar aspheric FiberPorts can be seen in the Selection Guide tab above. These FiberPorts are available with an effective focal length of 4.0 mm, 7.5 mm, 10 mm, or 15 mm. The EFL shift for each achromatic FiberPort can be viewed in the tables below. Additional information about achromatic doublet performance can be seen here.

Five Degrees of Freedom (Plus Bulkhead Rotation)
While holding the connector and fiber stationary, the built-in lens can be aligned with five degrees of freedom: linear alignment of the lens in X and Y, angular alignment for tip and tilt, and Z adjustment using the tip and tilt controls simultaneously. The travel range of the aspheric lens in the X and Y direction is ±0.7 mm with a resolution of 317 µm per revolution. The travel range in the Z direction is ±1.0 mm with resolution of 200 µm per revolution. In addition, the three flat-head screws on the front plate can be loosened to enable rotation of the bulkhead for PM fiber alignment. After alignment is complete, a locking setscrew on the side of the housing can be tightened to secure the X and Y position, as can the locking collars on the Zθ adjusters to lock the tip/tilt position. See the Operation tab for complete details on operating a FiberPort.

Mounting Options
FiberPorts contain four #2 counterbores that provide mechanical compatibility with our FiberBench accessories. We have also developed adapters for using FiberPorts with Ø1/2" posts, 30 mm cage systems, and HeNe lasers. Please see the FiberPort Mounts tab for more information.

Included Hardware
These FiberPorts include a dust cap, a 0-80 locking screw, a 0.028" hex key for the locking screw, a 0.050" hex key for the Zθ, X, and Y adjusters, and an SPW403 spanner wrench for the Zθ locking collars.

*Please note the maximum recommended temperature for operation is 80 °C.

Introduction

Before selecting a FiberPort, it is crucial to consider:

• The wavelength of your source,
• The fiber type (single mode, multimode, or polarization maintaining),
• The fiber connector (FC/PC, FC/APC, or SMA), and
• The desired lens type (achromatic or aspheric).

Calculating the Effective Focal Length

Example:

• Wavelength: 633 nm
• Fiber: P1-630A-FC-2
• Collimated Beam Diameter Prior to Lens: Ø3 mm

The specifications for the P1-630A-FC-2, 633 nm, FC/PC single mode patch cable indicate that the 1/e² mode field diameter (MFD) is 4.3 μm at 633 nm. The MFD should equal the diffraction-limited spot size Ø_spot, which is given by the following equation:

Here, f is the focal length of the lens, λ is the wavelength of the input light, and D is the 1/e² diameter of collimated beam incident on the lens. Solving for the desired focal length of the collimating lens yields:

Thorlabs offers a large selection of FiberPorts. You'll note that the FiberPort with a focal length closest to 16 mm has a focal length of 15.3 mm (Item # PAF2P-15A), while also meeting the requirements for fiber connector type and antireflection coating range. This FiberPort also has a clear aperture that is larger than the collimated beam diameter. Therefore, this is the best option given the initial parameters (i.e., a P1-630A-FC-2 single mode fiber and a collimated beam diameter of 3 mm).

For optimum coupling, the spot size of the focused beam must be less than the MFD of the single mode fiber. As a result, if a FiberPort is not available that provides an exact match, then choose the FiberPort with a focal length that is shorter than the calculation above yields. Alternatively, if the clear aperture of the lens is large enough, the beam can be expanded before the lens, which has the result of reducing the spot size of the focused beam.

Please note that the NA values in the specification tables below are the numerical apertures of the lenses, not the required numerical aperture of the fiber you are using. As long as the lens NA is smaller than the NA of your fiber, you should be able to couple light. Please note that if your collimated beam diameter is smaller than the clear aperture of the lens you will need to recalculate the NA using the beam diameter. For best results, Thorlabs recommends using the equations above when choosing a FiberPort, or to use the calculator linked above.

Selecting a Lens

When selecting a FiberPort, it is essential to ensure that the lens included in the FiberPort is compatible with your setup. In particular, the wavelength of the laser source must fall within the AR coating range of the lens, and the spot radius and focal shift should perform in such a way that is beneficial to your system.

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AR Coatings

The lenses in Thorlabs’ FiberPorts use our -A (350 - 700 nm or 400 - 700 nm), -B (600 - 1050 nm or 650 - 1050 nm), -C (1050 - 1620 nm or 1050 - 1700 nm), -D (1800 - 2400 nm), or -E (2000 - 5000 nm) AR coatings. The plot to the right shows the typical per-surface reflectance of each AR coating. Each AR-coated lens is housed in a FiberPort package designed to be compatible with FC/PC-, FC/APC-, or SMA-terminated fibers. Care should be taken in selecting a FiberPort to make sure the correct fiber/connector/FiberPort combination is selected. AR-coating information and connector compatibility for each FiberPort are outlined in the tables below. Contact Tech Support for assistance.

Note: The C coating wavelength range specification was recently updated from 1050 - 1620 nm to 1050 - 1700 nm. All optics in stock currently have a C coating with a wavelength range of 1050 - 1700 nm. However, we are in the process of updating our documentation, and so some individual optics may still be presented with the old wavelength range on our website.

Aspheric vs. Achromatic FiberPort Performance

Thorlabs' achromatic FiberPorts utilize cemented doublets designed to minimize chromatic aberrations, making them ideal for collimating broadband light sources or sources with multiple discrete wavelengths. The small focal length shifts experienced by an achromatic doublet allow the FiberPort to be used over a broad wavelength range without the need for realignment (see the Graphs tab for more details).

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The performance of the achromatic doublet used in an achromatic FiberPort is compared to the aspheric lens used in a FiberPort with the same effective focal length, for a collimated beam focused onto a fiber. The lens is aligned to focus the light onto the fiber at 580 nm. Over a given wavelength range, the achromatic doublet produces a focused spot on the fiber, while the aspheric lens produces a series of wavelength dependent foci along its optical axis due to chromatic focal shift. Since the fiber's position was fixed at the focus of the aspheric lens, the spot size changed drastically with wavelength. However, appropriate adjustment of the fiber’s position as the wavelength is changed can minimize this effect.

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This graph represents the focal length shift for the same setup described in the plot to the left. Both lenses have a similar focal length (F) at ~580 nm. Overall, the focal shift experienced by the aspheric lens in the range 475 nm - 655 nm (except ~580 nm) is approximately an order of magnitude larger than that of the achromatic doublet. For wavelengths less than 580 nm, the beam comes to a focus at a distance < F and longer wavelengths are focused at a distance > F.

Alignment Procedures

All FiberPorts contain a lens cell magnetically attached to a tilt plate that can be adjusted directly in the X and Y directions. Tip/Tilt adjustments along the optical axis are made by incremental adjustments of the Zθ adjusters, marked Zθ1, Zθ2, and Zθ3. These adjusters actuate the tilt plate. Translation in the Z-axis is made by uniformly turning the Zθ adjusters. We recommend using a low-power, visible alignment laser in all of the following adjustments.

The sections below provide recommendations on how to use your FiberPort based on your application. Regardless of application, the Pre-alignment section should be read first. Please note that your FiberPort comes collimated for a specific wavelength as provided in their mechanical drawings, which can be found below by clicking on the red document icon:. After pre-aligning your FiberPort, the Collimating with a FiberPort section provides instructions on how to use your FiberPort as a collimator, while the Coupling into Fiber via FiberPort section provides instructions on how to couple single mode or multimode fiber using your FiberPort.

Pre-alignment

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The tilt plate must be orthogonal to the beam axis for optimal alignment.

Before attempting to collimate or couple fiber with your FiberPort, it is crucial to ensure that the FiberPort is properly aligned in such a way that the tilt plate is orthogonal to the beam axis. If alignment is not done, coupling efficiencies will be lower than optimal, regardless of care taken later in the alignment process. If your FiberPort has been adjusted since purchase or requires collimation at a different wavelength, please follow the steps below to provide the best performance.

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Aligning the Tilt Plate

With the laser off, center the lens by eye in the tilt plate aperture by turning the X and Y adjustment screws. The tilt plate must be flat against the FiberPort body and orthogonal to the beam axis. This must be done fairly precisely, and can be achieved one of the following ways:

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Aligning the Tilt Plate on a FiberPort using a TPSM1 Magnetic Alignment Grid

Using Input Laser (Recommended)

1. Securely mount the FiberPort so it does not move during alignment.
2. Turn the Zθ adjusters counter-clockwise until it is clear that they are no longer translating the tilt plate¹.
3. Insert a visible fiber laser².
4. Aim the beam at an alignment screen³.
5. Turn each Zθ adjuster clockwise until the beam position is affected. Once each adjuster has just begun to affect position, the adjusters are in contact with the tilt plate. If done carefully, the tilt plate is still flush against the body of the FiberPort and is orthogonal to the optical axis.⁴

With No Input Laser

1. If you are familiar with FiberPorts or if you are using a FiberPort with a 2.0 - 5.0 µm coating², it is possible to align the tilt plate by feel alone.
2. Turn the Zθ adjusters counter-clockwise until it is clear that they are no longer translating the tilt plate¹. 
3. Carefully turn the Zθ adjusters clockwise until the resistance on the turn increases⁵.
4. The adjusters are contacting the tilt plate when resistance is first met. If done carefully, the tilt plate is still flush against the body of the FiberPort and orthogonal to the optical axis⁴.

Common Issues

1. I am unsure how far to unscrew the Zθ adjusters: Do not completely unscrew the Zθ adjusters.They should not be removed from the FiberPort.
2. I cannot see the beam exiting the FiberPort: FiberPorts with a 2.0 - 5.0 µm coating will not transmit visible light. Aligning these FiberPorts With No Input Laser, as detailed to the right, is recommended.
3. I am unsure how to tell if the adjustments are affecting the beam diameter: It is easiest to see the beam diameter change when using an alignment screen with sufficiently small gradations.
4. I do not know what the beam should look like: Since the Zθ adjusters are nearly completely unscrewed, the beam coming out of the FiberPort will be diverging. As a result, the farther downstream the screen is from the source, the easier the change will be to see. Additionally, ensure that the alignment screen that is being used has sufficiently fine gradations for your beam size.
5. I do not know if I have made the correct adjustments: It may take several attempts to get the exact location. Do not actuate past when first resistance is felt.

Common Alignment Setups

There are multiple ways to configure an alignment setup. A common method of alignment is to use two mirrors and two irises, as seen in the left and center images below. Alternatively, a FiberBench, seen in the image below and to the right, can provide a quick and compact way to utilize two FiberPorts in your system.

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A FiberBench provides a stable and quick way to integrate multiple FiberPorts into your system. A single-axis FB-51W FiberBench is seen above.

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A post-mounted alignment setup can be made with components that are common in many labs. Note: the previous generation BHM2 ruler is used in this setup.

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A cage system can simplify the alignment process by fixing all components along a common optical axis.

1. Ensure that the center of your mirrors, irises, and FiberPort are centered on the same optical axis. Our cage system and FiberBench systems can simplify this step.
2. If using an FC/PC or FC/APC connector, make sure that the key on your fiber connector is aligned with your FiberPort. If using polarization-maintaining fiber, the bulkhead of the FiberPort can be rotated to match the fiber's polarization. This should be done as early as possible in the alignment process. Please see the Operation tab for more information.
3. Align the mirror closest to the beam source with the iris it is closest to by placing a laser viewing card or aligment target directly after that iris. Partially close the iris until the beam spot begins to disappear. If one side of the beam is clipped first, adjust the mirror so that the beam is more centered with the iris. Align the other mirror and iris with each other in the same way.
4.  Repeat steps 2 and 3 until both mirrors and irises are aligned.

Collimating with a FiberPort

Step 1

Start with a pre-aligned FiberPort, as discussed in the Pre-alignment section above.

Step 2

Check for collimation by measuring the beam diameter close to and downstream from your source¹.

Step 3

Uniformly turn the Zθ adjusters clockwise by aligning your ball driver with the adjusters so that the number of rotations can be counted. Each adjuster must receive the same number of rotations to ensure the lens remains orthogonal to the beam axis and beam quality is preserved².

Step 4

After each set of adjustments, measure the beam diameter from the same locations in step 2. If the beam is converging, the lens is too far away from the fiber. If the beam is diverging, the lens is too close to the fiber. Adjust the Zθ adjusters according to the diagram to the right. Once the downstream beam diameter is approximately equal to the beam's diameter near the source, your beam is collimated.

Common Issues

1. I cannot tell if my beam is collimated: The further from the source you measure, the easier it is to view the difference in beam diameter.
2. Even though my X and Y position look correct, there is still an offset in the beam: Please ensure that care has been taken in pre-alignment. See the Pre-alignment section for more information.

Coupling into Fiber via FiberPort

When turning each adjuster by equal increments, the beam traces a triangular pattern of changing width. The maximum power is seen not at either end of each turn's travel range, but in the middle when the beam is closest to the fiber's core. The beam's path is shown with point of view of the fiber that is being coupled into. 

Principle

While translation in the X- and Y- directions can be directly made by using their respective adjusters, translation in the Z-direction must be made by incremental tip/tilt adjustments using the three Zθ adjusters. As a result, the beam's path as the adjusters are turned may not be intuitive. Equal rotations to each of the Zθ adjusters result in the beam spot tracing a triangle around the core of the fiber, as illustrated to the right. Once some measureable output exists, a typical alignment strategy would consist of turning each screw to maximize the output, and then continuing to turn slightly beyond maximum (about 95% of your local max). This must be done for each adjuster, and in the same order for each trio of adjustments¹. This method prevents the power from being stuck at a local maximum. The maximum seen in each passing will continue to increase until the lens-to-fiber spacing is optimized (spot size is minimized), and a global maximum is reached. This strategy has the effect of translating the beam in a triangle of decreasing width with each set of adjustments. 

Note that the typical maximum coupling efficiency is highly dependent on the system configuration and setup. Most configurations will achieve efficiencies between 60% and 80%. This efficiency can be improved upon by ensuring precise alignment with all system components, and verifying the compatibility of all components. Efficiencies below 50% may indicate significant component mismatch or alignment error.

Step 3

Choose an sequence to make your adjustments, and keep to that sequence¹ . Turn each Zθ adjuster clockwise to maximize the output, then continue to turn slightly beyond local maxima (to ~95% of your local maximum). If turning an adjuster clockwise decreases output, skip that adjuster for that round of adjustments and repeat. Once the local maxima values begin to decrease, reverse the direction, and turn each adjuster to maximize the output, and not beyond.

Step 2

Insert a multimode (MM) fiber that is compatible with your system² into the FiberPort and a power meter. Maximize the X and Y alignment by observing where the intensity peaks for each position. Once a maximum is reach, these adjusters should not be changed unless SM fiber is later used.

Step 1

Start with a pre-aligned FiberPort, as discussed in the Pre-alignment section above. 

Step 6 (Optional)

If desired, the adjusters can be locked via the locking collars by the use of the included SPW403 spanner wrench. Additionally, the lens cell can be locked in place by installing the included 0-80 screw located in the 4 o'clock position of the front face of the FiberPort.

Single Mode Fiber Only

Step 5

Repeat Step 3. The adjustments will need to be smaller, as the system will be more sensitive. If adjustment of all screws in either direction lowers the output, the beam spot may be centered on the fiber core, but be improperly focused. Turn each adjuster a small amount (1/16^th turn) in the same direction, then maximize each Zθ adjuster. If the new maximum is lower than the previous, turn each Zθ adjuster a small amount in the other direction and maximize. Repeat until absolute maximum is found.

Step 4

If coupling into a SM fiber, exchange the MM fiber with a SM fiber. The intensity measured by the power meter will likely drop significantly. 

Common Issues

1. I do not know what order to turn the adjusters: For instance, if you chose to adjust Zθ3, then Zθ1, then Zθ2, ensure you continue in this order for each set of adjustments.
2. I do not know what fiber is most compatible with my system: Please see the Selection Guide tab for more information

Theoretical Approximation of the Divergence Angle

The full-angle beam divergence listed in the specifications tables is the theoretically-calculated value associated with the fiber collimator. This divergence angle is easy to approximate theoretically using the formula below as long as the light emerging from the fiber has a Gaussian intensity profile. Consequently, the formula works well for single mode fibers, but it will underestimate the divergence angle for multimode (MM) fibers since the light emerging from an MM fiber has a non-Gaussian intensity profile.

The Full Divergence Angle (in degrees) is given by

where MFD is the mode field diameter and f is the focal length of the collimator. (Note: MFD and f must have the same units in this equation).

Example:

When the PAF2-A4A collimator is used with a single mode fiber such as the 460HP such that MFD = 3.5 µm and f ≈ 4.0 mm, the divergence angle is

θ ≈ (0.0035 mm / 4.0 mm)*(180/3.1416) ≈ 0.050° or 0.875 mrad.

Theoretical Approximation of the Output Beam Diameter

The output beam diameter can be approximated from 

where λ is the wavelength of light being used, MFD is the mode field diameter, and f is the focal length of the collimator.

Example:

When the PAF2-A4A collimator (f = 4.0 mm) is used with the 460HP fiber (MFD = 3.5 µm) and 450 nm light, the output beam diameter is

(4)(450 nm)[4.0 mm / (π · 3.5 µm)] = 0.65 mm

Theoretical Approximation of the Maximum Waist Distance

The maximum waist distance, which is the furthest distance from the lens the waist can be located in order to maintain collimation, may be approximated by:

where f is the focal length of the collimator, λ is the wavelength of light used, and MFD is the mode field diameter.

Example:

When the PAF2-A4A collimator is used with a single mode fiber such as the 460HP such that MFD = 3.5 µm, f ≈ 4.0 mm, and λ = 450 nm, then the maximum waist distance is

4 mm + (2 (4 mm)² (450 nm) / (3.1416) (3.5 µm)²) = 378 mm.

Insights into Optical Fiber

Scroll down to read about:

• When does NA provide a good estimate of the fiber's acceptance angle?
• Why is MFD an important coupling parameter for single mode fibers?
• Does NA provide a good estimate of beam divergence from a single mode fiber?

Click here for more insights into lab practices and equipment.

When does NA provide a good estimate of the fiber's acceptance angle?

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Figure 1: Rays incident at angles ≤θ_max will be captured by the cores of multimode fiber, since these rays experience total internal reflection at the interface between core and cladding. 

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Figure 2: The behavior of the ray at the boundary between the core and cladding, which depends on their refractive indices, determines whether the ray incident on the end face is coupled into the core. The equation for NA can be found using geometry and the two equations noted at the top of this figure. 

Numerical aperture (NA) provides a good estimate of the maximum acceptance angle for most multimode fibers, as shown in Figure 1. This relationship should not be used for single mode fibers. 

NA and Acceptance Angle
Incident light is modeled as rays to obtain the relationship between NA and the maximum acceptance angle (θ_max ), which describes the fiber's ability to gather light from off-axis sources. The equation at the top of Figure 1 can be used to determine whether rays traced from different light sources will be coupled into the fiber's core. 

Rays with an angle of incidence ≤θ_max  are totally internally reflected (TIR) at the boundary between the fiber's core and cladding. As these rays propagate down the fiber, they remain trapped in the core.

Rays with angles of incidence larger than θ_max  refract at the interface between core and cladding, and this light is eventually lost from the fiber. 

Geometry Defines the Relationship
The relationship among NA, θ_max , and the refractive indices of the core and cladding, n_core  and n_clad , respectively, can be found using the geometry diagrammed in Figure 2. This geometry illustrates the most extreme conditions under which TIR will occur at the boundary between the core and cladding.

The equations at the top of Figure 2 are expressions of Snell's law and describe the rays' behavior at both interfaces. Note that the simplification sin(90°) = 1 has been used. Only the indices of the core and cladding limit the value of θ_max .

Angles of Incidence and Fiber Modes
When the angle of incidence is ≤θ_max , the incident light ray is coupled into one of the multimode fiber's guided modes. Generally speaking, the lower the angle of incidence, the lower the order of the excited fiber mode. Lower-order modes concentrate most of their intensity near the center of the core. The lowest order mode is excited by rays incident normally on the end face.

Single Mode Fibers are Different
In the case of single mode fibers, the ray model in Figure 2 is not useful, and the calculated NA (acceptance angle) does not equal the maximum angle of incidence or describe the fiber's light gathering ability.

Single mode fibers have only one guided mode, the lowest order mode, which is excited by rays with 0° angles of incidence. However, calculating the NA results in a nonzero value. The ray model also does not accurately predict the divergence angles of the light beams successfully coupled into and emitted from single mode fibers. The beam divergence occurs due to diffraction effects, which are not taken into account by the ray model but can be described using the wave optics model. The Gaussian beam propagation model can be used to calculate beam divergence with high accuracy.

Date of Last Edit: Jan. 20, 2020

Why is MFD an important coupling parameter for single mode fibers?

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Figure 3 For maximum coupling efficiency into single mode fibers, the light should be an on-axis Gaussian beam with its waist located at the fiber's end face, and the waist diameter should equal the MFD. The beam output by the fiber also resembles a Gaussian with these characteristics. In the case of single mode fibers, the ray optics model and NA are inadequate for determining coupling conditions. The mode intensity (I ) profile across the radius ( ρ ) is illustrated.

As light propagates down a single mode fiber, the beam maintains a cross sectional profile that is nearly Gaussian in shape. The mode field diameter (MFD) describes the width of this intensity profile. The better an incident beam matches this intensity profile, the larger the fraction of light coupled into the fiber. An incident Gaussian beam with a beam waist equal to the MFD can achieve particularly high coupling efficiency.

Using the MFD as the beam waist in the Gaussian beam propagation model can provide highly accurate incident beam parameters, as well as the output beam's divergence.

Determining Coupling Requirements
A benefit of optical fibers is that light carried by the fibers' guided mode(s) does not spread out radially and is minimally attenuated as it propagates. Coupling light into one of a fiber's guided modes requires matching the characteristics of the incident light to those of the mode. Light that is not coupled into a guided mode radiates out of the fiber and is lost. This light is said to leak out of the fiber. 

Single mode fibers have one guided mode, and wave optics analysis reveals the mode to be described by a Bessel function. The amplitude profiles of Gaussian and Bessel functions closely resemble one another, which is convenient since using a Gaussian function as a substitute simplifies the modeling the fiber's mode while providing accurate results (Kowalevicz). 

Figure 3 illustrates the single mode fiber's mode intensity cross section, which the incident light must match in order to couple into the guided mode. The intensity (I ) profile is a near-Gaussian function of radial distance ( ρ ). The MFD, which is constant along the fiber's length, is the width measured at an intensity equal to the product of e^-2 and the peak intensity. The MFD encloses ~86% of the beam's power.

Since lasers emitting only the lowest-order transverse mode provide Gaussian beams, this laser light can be efficiently coupled into single mode fibers.

Coupling Light into the Single Mode Fiber
To efficiently couple light into the core of a single-mode fiber, the waist of the incident Gaussian beam should be located at the fiber's end face. The intensity profile of the beam's waist should overlap and match the characteristics of the mode intensity cross section. The required incident beam parameters can be calculated using the fiber's MFD with the Gaussian beam propagation model.

The coupling efficiency will be reduced if the beam waist is a different diameter than the MFD, the cross-sectional profile of the beam is distorted or shifted with respect to the modal spot at the end face, and / or if the light is not directed along the fiber's axis.

References
Kowalevicz A and Bucholtz F, "Beam Divergence from an SMF-28 Optical Fiber (NRL/MR/5650--06-8996)." Naval Research Laboratory, 2006.

Date of Last Edit: Feb. 28, 2020

Why is MFD an important coupling parameter for single mode fibers?

Significant error can result when the numerical aperture (NA) is used to estimate the cone of light emitted from, or that can be coupled into, a single mode fiber. A better estimate is obtained using the Gaussian beam propagation model to calculate the divergence angle. This model allows the divergence angle to be calculated for whatever beam spot size best suits the application.

Since the mode field diameter (MFD) specified for single mode optical fibers encloses ~86% of the beam power, this definition of spot size is often appropriate when collimating light from and focusing light into a single mode fiber. In this case, to a first approximation and when measured in the far field,

,

(1)

is the divergence or acceptance angle (θ_SM ), in radians. This is half the full angular extent of the beam, it is wavelength () dependent, and the beam's waist diameter has been set equal to the fiber's MFD (Kowalevicz).

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		Rayleigh Range:
		Beam Radius at Distance z:
Figure 4: These curves illustrate the consequence of using NA to calculate the divergence (θSM ) of light output from a single mode fiber. Significant error in beam spot diameter can be avoided by using the Gaussian beam propagation model.  This plot models a beam from SM980-5.8-125. The values used for NA and MFD were 0.13 and 6.4 µm, respectively. The operating wavelength was 980 nm, and the Rayleigh range was 32.8 µm.

Gaussian Beam Approach
Although a diverging cone of light is emitted from the end face of a single mode optical fiber, this light does not behave as multiple rays travelling at different angles to the fiber's axis.

Instead, this light resembles and can be modeled as a single Gaussian beam. The emitted light propagates similarly to a Gaussian beam since the guided fiber mode that carried the light has near-Gaussian characteristics.

The divergence angle of a Gaussian beam can differ substantially from the angle calculated by assuming the light behaves as rays. Using the ray model, the divergence angle would equal sin^-1(NA). However, the relationship between NA and divergence angle is only valid for highly multimode fibers.

Figure 4 illustrates that using the NA to estimate the divergence angle can result in significant error. In this case, the divergence angle was needed for a point on the circle enclosing 86% of the beam's optical power. The intensity of a point on this circle is a factor of 1/e² lower than the peak intensity.

The equations to the right of the plot in Figure 4 were used to accurately model the divergence of the beam emitted from the single mode fiber's end face. The values used to complete the calculations, including the fiber's MFD, NA, and operating wavelength are given in the figure's caption. This rate of beam divergence assumes a beam size defined by the 1/e² radius, is nonlinear for distances z < z_R, and is approximately linear in the far field (z >> z_R).

The angles noted on the plot were calculated from each curve's respective slope. When the far field approximation given by Equation (1) is used, the calculated divergence angle is 0.098 radians (5.61°).

References
Kowalevicz A and Bucholtz F, "Beam Divergence from an SMF-28 Optical Fiber (NRL/MR/5650--06-8996)." Naval Research Laboratory, 2006.

Date of Last Edit: Feb. 28, 2020
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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.