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Achromatic Fiber Collimators, Adjustable Focus
C40FC-A Collimator Mounted on a Ø1/2" Post Using an SM1RC Slip Ring
C80APC-A (Front View)
80 mm Focal Length
C40SMA-B (Back View)
40 mm Focal Length
C20APC-C (Back View)
20 mm Focal Length
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The C80 collimators have external SM2 threading on the free-space end for attaching Ø2" lens tubes, while the C40 collimators have SM1 threading for attaching Ø1" lens tubes.
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In addition to external SM1 threading, the C20 collimators have internal SM05 threading on the free-space end for attaching Ø1/2" lens tubes.
Thorlabs' Achromatic Fiber Collimators with adjustable focus are designed with an effective focal length (EFL) of 20 mm, 40 mm, or 80 mm. Each collimation package is available with an AR coating for one of three wavelength ranges and an FC/PC, FC/APC, or SMA connector. The four-element, air-spaced-lens design produces superior beam quality (M2 close to 1) and less wavefront error when compared to aspheric lens collimators. The focus distance can be adjusted between infinity and the closest focusing distance using the red ring in the middle of the housing. These achromatic collimators can also be used for collimator-to-collimator coupling over long distances, which allows the free-space beam to be manipulated prior to entering the second collimator and can be useful in long-distance-communication applications.
For increased stability, the focus adjustment can be locked by tightening a nylon-tipped setscrew on the side of the housing using the included 0.05" (1.3 mm) hex key. The SM threads and fiber connector at either end of the collimator do not rotate when turning the focus adjustment ring, allowing the focus to be adjusted without disturbing any attached optics or optical fibers (see the Performance tab for more information).
To minimize losses caused by surface reflections, all the lenses in the collimator feature a broadband AR coating on both sides (see the Coatings tab for details). AR coatings for three different wavelength ranges are available from stock: 400 - 650 nm, 650 - 1050 nm, or 1050 - 1650 nm. For laser line (V) AR coatings centered at a specific wavelength, please contact Tech Support.
Collimation packages with ports for 2.2 mm wide key FC/PC or FC/APC connectors have tightly toleranced ceramic sleeves that provide excellent pointing repeatability, allowing the user to easily adjust the focus distance and replace the fiber without the need for large realignments. Please note that careful alignment is needed when mating a narrow key polarization maintaining fiber with the collimator's wide key receptacle.
The 40 mm EFL and 80 mm EFL achromatic fiber collimators are designed for use with large beam diameters. For additional large-beam collimators, please see our line of fixed focus, air-spaced doublet collimators. We also offer a variety of other collimators, including zoom fiber collimators, fixed collimators, and FiberPort adjustable collimation packages, which are well suited for use with a wide range of wavelengths. See the Collimator Guide tab for our complete line of collimation and coupling options.
Converging or Diverging
When collimating light, a shearing interferometer can be used to determine if a coherent beam of light is collimated. The interferometer is comprised of a wedged optical flat mounted at 45° and a diffuser plate with a ruled reference line down the middle.
The diffuser plate can then be used to view the interference fringes created by Fresnel reflections from the front and back surfaces of the optical flat (as shown in the images to the right). If the beam is collimated, the resulting fringe pattern will be parallel to the ruled reference line. In addition to the degree of collimation, the fringes will also be sensitive to spherical aberration, coma, and astigmatism.
When using these collimators as a free-space coupler, precise alignment is needed for good coupling efficiency. In general, six movement degrees of freedom are recommended, such as pairing a kinematic mount with an XYZ translation stage. For example, the 20 mm or 40 mm collimators can be mounted with a KM100T, KC1-T (KC1-T/M), or KS1T kinematic mount and an MT3 (MT3/M) translation stage. The 80 mm collimators can be mounted using the KM200T, KC2-T (KC2-T/M), or KS2T kinematic mounts with an MT3 (MT3/M) translation stage. The beam can be focused anywhere between the maximum waist distance (see the Divergence tab for details) and the closest focusing distance, enabling easy optimization of coupling efficiency. AR-coated single mode or polarization-maintaining patch cables can be used to further increase coupling efficiency and beam quality; these cables feature an AR coating on one connector end that improves return loss at the fiber-to-free-space interface when coupling.
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The red focusing ring provides smooth adjustment of the focusing distance.
Thorlabs' achromatic fiber collimators provide a fixed 20 mm, 40 mm, or 80 mm focal length while allowing the beam to be focused between the maximum waist distance (see the Divergence tab) and the closest focusing distance. This allows for simple optimization of coupling efficiency from free-space laser beam into a fiber.
To adjust the focusing distance, rotate the red section of the housing as shown in the image to the right; this will alter the focal point of the beam. There is a setscrew on one end for locking the focus which can be adjusted using the included 0.05" (1.3 mm) hex key.
Longitudinal Chromatic Aberration
The focal shift produced by these collimators when they are used as couplers is shown in the graphs below. The theoretical deviation of the focal plane location from the nominal focal length is plotted over the operating wavelength range. The calculation was performed for each coating type at the available focal lengths (20 mm, 40 mm, and 80 mm).
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Click Here for Theoretical Data
The chromatic focal shift for the 400 - 650 nm achromatic fiber collimators. The blue-shaded region indicates the specified operating wavelength range of the collimator.
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Click Here for Theoretical Data
The chromatic focal shift for the 650 - 1050 nm achromatic fiber collimators. The blue-shaded region indicates the specified operating wavelength range of the collimator.
The graphs below illustrate the theoretical beam diameter as a function of propagation distance for certain SM-coupled laser wavelengths using our achromatic fiber collimators adjusted for the minimum divergence. These graphs are valid for all achromatic collimators when using single mode fibers and show that the achromatic collimators exhibit minimal divergence over a long distance from the collimator.
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Click Here for Theoretical Data
The data is calculated for SMF-28-J9 single mode fiber.
Theoretical Approximation of the Divergence Angle
The 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 (SM) fibers, but will underestimate the divergence angle for multimode (MM) fibers since 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; both MFD and focal length must have the same units in this equation.
When the C40APC-A collimator is used with the P3-460B-FC-1 SM patch cable, MFD = 3.6 µm, f ≈ 40 mm, and λ = 543 nm, the divergence angle is:
θ ≈ (0.0036 mm / 40 mm) x (180 / 3.1416) ≈ 0.0051° or 0.089 mrad.
Theoretical Approximation of the Output Beam Diameter
The output beam diameter can be approximated from the equation below:
where λ is the wavelength of light being used, MFD is the mode field diameter, and f is the focal length of the collimator.
When the C80FC-C collimator (f = 80 mm) is used with the P1-SMF28E-FC-1 patch cable (MFD = 10.5 µm) at 1550 nm, the output beam diameter is:
d = 4 x 1550 nm x [80 mm / (3.1416 · 10.5 µm)] = 15.0 mm
Theoretical Approximation of the Maximum Waist Distance
The maximum waist distance, which is the furthest distance from the lens, 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.
When the C40FC-A collimator is used with the P3-460B-FC-1 SM patch cable, MFD = 3.6 µm, f = 40.0 mm, and λ = 543 nm, then the maximum waist distance is:
40 mm + (2 x (40 mm)2 x (543 nm) / (3.1416) x (3.6 µm)2) = 42.7 m.
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The picture above shows the image produced by the C40SMA-A Achromatic Fiber Collimator (left) and the RC08SMA-P01 Protected Silver Reflective Collimator (right) using a round-to-linear fiber bundle.
Thorlabs' SMA-terminated achromatic fiber collimators are excellent for use with multimode fibers. To demonstrate this feature, an achromatic collimator and a reflective collimator are used to collimate light from a halogen light source with the use of a BFL105LS02 Round-to-Linear Multimode Fiber Bundle. The collimator images the fiber’s output onto a viewing screen placed approximately 0.35 m from the output, which is the closest focus distance for the C40SMA-A collimator.
The image to the right demonstrates the C40SMA-A Achromatic Fiber Collimator’s excellent off-axis image performance for large multimode fiber cores, showing all 7 fiber cores in sharp, distinguishable focus. By contrast, the RC08SMA-P01 Reflective Collimator demonstrates significant off-axis aberrations (i.e. coma, astigmatism and field curvature), when used with the same round-to-linear multimode fiber bundle. These aberrations are the result of the off-axis parabola principle.
The graphs below illustrate the theoretical imaged core diameter as a function of focusing distance using our SMA-terminated achromatic fiber collimators. The size of the image produced can be approximated with the following equation:
The graphs below show the reflectance with respect to the wavelength of the AR coatings used on the 8 lens surfaces in our achromatic fiber collimators (two surfaces per lens). The blue shaded region indicates the wavelength range specified for each coating. The table below details which AR coating is used with each collimator.
Damage Threshold Data for Thorlabs' Achromatic Fiber Collimators
The specifications to the right are measured data for Thorlabs' Achromatic Fiber Collimators.
Laser Induced Damage Threshold Tutorial
The following is a general overview of how laser induced damage thresholds are measured and how the values may be utilized in determining the appropriateness of an optic for a given application. When choosing optics, it is important to understand the Laser Induced Damage Threshold (LIDT) of the optics being used. The LIDT for an optic greatly depends on the type of laser you are using. Continuous wave (CW) lasers typically cause damage from thermal effects (absorption either in the coating or in the substrate). Pulsed lasers, on the other hand, often strip electrons from the lattice structure of an optic before causing thermal damage. Note that the guideline presented here assumes room temperature operation and optics in new condition (i.e., within scratch-dig spec, surface free of contamination, etc.). Because dust or other particles on the surface of an optic can cause damage at lower thresholds, we recommend keeping surfaces clean and free of debris. For more information on cleaning optics, please see our Optics Cleaning tutorial.
Thorlabs' LIDT testing is done in compliance with ISO/DIS 11254 and ISO 21254 specifications.
The photograph above is a protected aluminum-coated mirror after LIDT testing. In this particular test, it handled 0.43 J/cm2 (1064 nm, 10 ns pulse, 10 Hz, Ø1.000 mm) before damage.
According to the test, the damage threshold of the mirror was 2.00 J/cm2 (532 nm, 10 ns pulse, 10 Hz, Ø0.803 mm). Please keep in mind that these tests are performed on clean optics, as dirt and contamination can significantly lower the damage threshold of a component. While the test results are only representative of one coating run, Thorlabs specifies damage threshold values that account for coating variances.
Continuous Wave and Long-Pulse Lasers
When an optic is damaged by a continuous wave (CW) laser, it is usually due to the melting of the surface as a result of absorbing the laser's energy or damage to the optical coating (antireflection) . Pulsed lasers with pulse lengths longer than 1 µs can be treated as CW lasers for LIDT discussions.
When pulse lengths are between 1 ns and 1 µs, laser-induced damage can occur either because of absorption or a dielectric breakdown (therefore, a user must check both CW and pulsed LIDT). Absorption is either due to an intrinsic property of the optic or due to surface irregularities; thus LIDT values are only valid for optics meeting or exceeding the surface quality specifications given by a manufacturer. While many optics can handle high power CW lasers, cemented (e.g., achromatic doublets) or highly absorptive (e.g., ND filters) optics tend to have lower CW damage thresholds. These lower thresholds are due to absorption or scattering in the cement or metal coating.
Pulsed lasers with high pulse repetition frequencies (PRF) may behave similarly to CW beams. Unfortunately, this is highly dependent on factors such as absorption and thermal diffusivity, so there is no reliable method for determining when a high PRF laser will damage an optic due to thermal effects. For beams with a high PRF both the average and peak powers must be compared to the equivalent CW power. Additionally, for highly transparent materials, there is little to no drop in the LIDT with increasing PRF.
In order to use the specified CW damage threshold of an optic, it is necessary to know the following:
Thorlabs expresses LIDT for CW lasers as a linear power density measured in W/cm. In this regime, the LIDT given as a linear power density can be applied to any beam diameter; one does not need to compute an adjusted LIDT to adjust for changes in spot size, as demonstrated by the graph to the right. Average linear power density can be calculated using the equation below.
The calculation above assumes a uniform beam intensity profile. You must now consider hotspots in the beam or other non-uniform intensity profiles and roughly calculate a maximum power density. For reference, a Gaussian beam typically has a maximum power density that is twice that of the uniform beam (see lower right).
Now compare the maximum power density to that which is specified as the LIDT for the optic. If the optic was tested at a wavelength other than your operating wavelength, the damage threshold must be scaled appropriately. A good rule of thumb is that the damage threshold has a linear relationship with wavelength such that as you move to shorter wavelengths, the damage threshold decreases (i.e., a LIDT of 10 W/cm at 1310 nm scales to 5 W/cm at 655 nm):
While this rule of thumb provides a general trend, it is not a quantitative analysis of LIDT vs wavelength. In CW applications, for instance, damage scales more strongly with absorption in the coating and substrate, which does not necessarily scale well with wavelength. While the above procedure provides a good rule of thumb for LIDT values, please contact Tech Support if your wavelength is different from the specified LIDT wavelength. If your power density is less than the adjusted LIDT of the optic, then the optic should work for your application.
Please note that we have a buffer built in between the specified damage thresholds online and the tests which we have done, which accommodates variation between batches. Upon request, we can provide individual test information and a testing certificate. The damage analysis will be carried out on a similar optic (customer's optic will not be damaged). Testing may result in additional costs or lead times. Contact Tech Support for more information.
As previously stated, pulsed lasers typically induce a different type of damage to the optic than CW lasers. Pulsed lasers often do not heat the optic enough to damage it; instead, pulsed lasers produce strong electric fields capable of inducing dielectric breakdown in the material. Unfortunately, it can be very difficult to compare the LIDT specification of an optic to your laser. There are multiple regimes in which a pulsed laser can damage an optic and this is based on the laser's pulse length. The highlighted columns in the table below outline the relevant pulse lengths for our specified LIDT values.
Pulses shorter than 10-9 s cannot be compared to our specified LIDT values with much reliability. In this ultra-short-pulse regime various mechanics, such as multiphoton-avalanche ionization, take over as the predominate damage mechanism . In contrast, pulses between 10-7 s and 10-4 s may cause damage to an optic either because of dielectric breakdown or thermal effects. This means that both CW and pulsed damage thresholds must be compared to the laser beam to determine whether the optic is suitable for your application.
When comparing an LIDT specified for a pulsed laser to your laser, it is essential to know the following:
The energy density of your beam should be calculated in terms of J/cm2. The graph to the right shows why expressing the LIDT as an energy density provides the best metric for short pulse sources. In this regime, the LIDT given as an energy density can be applied to any beam diameter; one does not need to compute an adjusted LIDT to adjust for changes in spot size. This calculation assumes a uniform beam intensity profile. You must now adjust this energy density to account for hotspots or other nonuniform intensity profiles and roughly calculate a maximum energy density. For reference a Gaussian beam typically has a maximum energy density that is twice that of the 1/e2 beam.
Now compare the maximum energy density to that which is specified as the LIDT for the optic. If the optic was tested at a wavelength other than your operating wavelength, the damage threshold must be scaled appropriately . A good rule of thumb is that the damage threshold has an inverse square root relationship with wavelength such that as you move to shorter wavelengths, the damage threshold decreases (i.e., a LIDT of 1 J/cm2 at 1064 nm scales to 0.7 J/cm2 at 532 nm):
You now have a wavelength-adjusted energy density, which you will use in the following step.
Beam diameter is also important to know when comparing damage thresholds. While the LIDT, when expressed in units of J/cm², scales independently of spot size; large beam sizes are more likely to illuminate a larger number of defects which can lead to greater variances in the LIDT . For data presented here, a <1 mm beam size was used to measure the LIDT. For beams sizes greater than 5 mm, the LIDT (J/cm2) will not scale independently of beam diameter due to the larger size beam exposing more defects.
The pulse length must now be compensated for. The longer the pulse duration, the more energy the optic can handle. For pulse widths between 1 - 100 ns, an approximation is as follows:
Use this formula to calculate the Adjusted LIDT for an optic based on your pulse length. If your maximum energy density is less than this adjusted LIDT maximum energy density, then the optic should be suitable for your application. Keep in mind that this calculation is only used for pulses between 10-9 s and 10-7 s. For pulses between 10-7 s and 10-4 s, the CW LIDT must also be checked before deeming the optic appropriate for your application.
Please note that we have a buffer built in between the specified damage thresholds online and the tests which we have done, which accommodates variation between batches. Upon request, we can provide individual test information and a testing certificate. Contact Tech Support for more information.
 R. M. Wood, Optics and Laser Tech. 29, 517 (1998).
In order to illustrate the process of determining whether a given laser system will damage an optic, a number of example calculations of laser induced damage threshold are given below. For assistance with performing similar calculations, we provide a spreadsheet calculator that can be downloaded by clicking the button to the right. To use the calculator, enter the specified LIDT value of the optic under consideration and the relevant parameters of your laser system in the green boxes. The spreadsheet will then calculate a linear power density for CW and pulsed systems, as well as an energy density value for pulsed systems. These values are used to calculate adjusted, scaled LIDT values for the optics based on accepted scaling laws. This calculator assumes a Gaussian beam profile, so a correction factor must be introduced for other beam shapes (uniform, etc.). The LIDT scaling laws are determined from empirical relationships; their accuracy is not guaranteed. Remember that absorption by optics or coatings can significantly reduce LIDT in some spectral regions. These LIDT values are not valid for ultrashort pulses less than one nanosecond in duration.
A Gaussian beam profile has about twice the maximum intensity of a uniform beam profile.
CW Laser Example
However, the maximum power density of a Gaussian beam is about twice the maximum power density of a uniform beam, as shown in the graph to the right. Therefore, a more accurate determination of the maximum linear power density of the system is 1 W/cm.
An AC127-030-C achromatic doublet lens has a specified CW LIDT of 350 W/cm, as tested at 1550 nm. CW damage threshold values typically scale directly with the wavelength of the laser source, so this yields an adjusted LIDT value:
The adjusted LIDT value of 350 W/cm x (1319 nm / 1550 nm) = 298 W/cm is significantly higher than the calculated maximum linear power density of the laser system, so it would be safe to use this doublet lens for this application.
Pulsed Nanosecond Laser Example: Scaling for Different Pulse Durations
As described above, the maximum energy density of a Gaussian beam is about twice the average energy density. So, the maximum energy density of this beam is ~0.7 J/cm2.
The energy density of the beam can be compared to the LIDT values of 1 J/cm2 and 3.5 J/cm2 for a BB1-E01 broadband dielectric mirror and an NB1-K08 Nd:YAG laser line mirror, respectively. Both of these LIDT values, while measured at 355 nm, were determined with a 10 ns pulsed laser at 10 Hz. Therefore, an adjustment must be applied for the shorter pulse duration of the system under consideration. As described on the previous tab, LIDT values in the nanosecond pulse regime scale with the square root of the laser pulse duration:
This adjustment factor results in LIDT values of 0.45 J/cm2 for the BB1-E01 broadband mirror and 1.6 J/cm2 for the Nd:YAG laser line mirror, which are to be compared with the 0.7 J/cm2 maximum energy density of the beam. While the broadband mirror would likely be damaged by the laser, the more specialized laser line mirror is appropriate for use with this system.
Pulsed Nanosecond Laser Example: Scaling for Different Wavelengths
This scaling gives adjusted LIDT values of 0.08 J/cm2 for the reflective filter and 14 J/cm2 for the absorptive filter. In this case, the absorptive filter is the best choice in order to avoid optical damage.
Pulsed Microsecond Laser Example
If this relatively long-pulse laser emits a Gaussian 12.7 mm diameter beam (1/e2) at 980 nm, then the resulting output has a linear power density of 5.9 W/cm and an energy density of 1.2 x 10-4 J/cm2 per pulse. This can be compared to the LIDT values for a WPQ10E-980 polymer zero-order quarter-wave plate, which are 5 W/cm for CW radiation at 810 nm and 5 J/cm2 for a 10 ns pulse at 810 nm. As before, the CW LIDT of the optic scales linearly with the laser wavelength, resulting in an adjusted CW value of 6 W/cm at 980 nm. On the other hand, the pulsed LIDT scales with the square root of the laser wavelength and the square root of the pulse duration, resulting in an adjusted value of 55 J/cm2 for a 1 µs pulse at 980 nm. The pulsed LIDT of the optic is significantly greater than the energy density of the laser pulse, so individual pulses will not damage the wave plate. However, the large average linear power density of the laser system may cause thermal damage to the optic, much like a high-power CW beam.
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Old Collimator Packaging
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New Collimator Packaging
Smart Pack Goals
Thorlabs' Smart Pack Initiative is aimed at minimizing waste while providing adequate protection for our products. By eliminating any unnecessary packaging, implementing design changes, and utilizing eco-friendly materials, this initiative seeks to reduce the environmental impact of our product packaging.
As we move through our product line, we will indicate re-engineered, eco-friendly packaging with our Smart Pack logo, which can be seen in the image to the right.
Insights into Beam Characterization
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Click here for more insights into lab practices and equipment.
Beam Size Measurement Using a Chopper Wheel
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Figure 2: The blade traces an arc length of Rθ through the center of the beam and has a angular rotation rate of f. The chopper wheel shown is MC1F2.
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Figure 1: An approximate measurement of beam size can be found using the illustrated setup. As the blade of the chopper wheel passes through the beam, an S-curve is traced out on the oscilloscope.
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Figure 4: The diameter of a Gaussian beam is often given in terms of the 1/e2 full width.
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Figure 3: Rise time (tr ) of the intensity signal is typically measured between the 10% and 90% points on the curve. The rise time depends on the wheel's rotation rate and the beam diameter.
Camera and scanning-slit beam profilers are tools for characterizing beam size and shape, but these instruments cannot provide an accurate measurement if the beam size is too small or the wavelength is outside of the operating range.
A;chopper wheel, photodetector, and oscilloscope can provide an approximate measurement of the beam size (Figure 4). As the rotating chopper wheel's blade passes through the beam, an S-shaped trace is displayed on the oscilloscope.
When the blade sweeps through the angle θ , the rise or fall time of the S-curve is proportional to the size of the beam along the direction of the blade's travel (Figure 5). A point on the blade located a distance R from the center of the wheel sweeps through an arc length (Rθ ) that is approximately equal to the size of the beam along this direction.
To make this beam size measurement, the combined response of the detector and oscilloscope should be much faster than the signal's rate of change.
Example: S-Curve with Rising Edge
has a factor of 0.64 to account for measuring rise time between the 10% and 90% intensity points.
Date of Last Edit: Jan. 13, 2020
Fiber Collimator Selection Guide
Click on the collimator type or photo to view more information about each type of collimator.