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Air-Spaced Doublet Collimators - FC/PC, FC/APC, & SMA
780 nm, Focal Length: 36.01 mm
2.0 µm, Focal Length: 37.52 mm
543 nm, Focal Length: 34.74 mm
The F810 Series Fiber Collimation Packages are pre-aligned to collimate a laser beam propagating from the tip of an FC/PC, FC/APC, or SMA terminated fiber with diffraction-limited performance at the design wavelength. Since the F810 Series fiber collimators do not have any movable parts, they are compact and not susceptible to misalignment. Due to chromatic aberration, the effective focal length (EFL) of the doublet lens is wavelength dependent. As a result, these collimators will only perform optimally at the design wavelength. The doublet lens, which features an AR coating that minimizes surface reflections, is factory aligned for each design wavelength so that it is one focal length away from a fiber tip inserted into the collimator. Additionally, the receptacles for the FC/APC versions are angled so that light exiting the fiber enters the collimator perpendicular to the focal plane.
We recommend using the F810APC and F810FC collimation packages with our AR-coated single mode fiber optic patch cables. These cables feature an antireflective coating on one fiber end for increased transmission and improved return loss at the fiber-to-free-space interface. F810SMA fiber collimation packages are optimized for single mode fibers and are compatible with our SMA-terminated hybrid single mode fiber optic patch cables. Alternatively, our large selection of standard fiber patch cables can also be used.
Theoretical Approximation of the Divergence
The divergence angle listed in the specifications tables below is the theoretical full-angle divergence when using the fiber collimator at its design wavelength with the listed fiber. Simulations of the theoretical divergence of the F810 collimators at wavelengths other than the design wavelengths are shown below. Similarly, the beam diameter as a function of propogration distance was simulated for each of our F810 collimators at the design wavelength, assuming input from the design fiber and a Gaussian intensity profile.
The graphs below show the reflectance with respect to wavelength of the AR coatings used on the lens surfaces in our F810 series collimators. The blue shaded region indicates the wavelength range specified for each coating. The table below details the AR coating designations with their corresponding wavelength ranges and average reflectance. Transmission for the N-SF6 lens material is also provided.
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 a multimode 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).
When the F220FC-A collimator (f ≈ 11.0 mm; not exact since the design wavelength is 543 nm) is used to collimate 515 nm light emerging from a 460HP fiber (MFD = 3.5 µm), the divergence angle is approximately given by
θ ≈ (0.0035 mm / 11.0 mm) x (180 / pi) = 0.018°.
When the beam divergence angle was measured for the F220FC-A collimator, a 460HP fiber was used with 543 nm light. The result was a divergence angle of 0.018°.
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. (Note: MFD and f must have the same units in this equation).
When the F240FC-1550 collimator (f = 8.18 mm) is used with the P1-SMF28E-FC-1 patch cable (MFD = 10.4 µm) and 1550 nm light, the output beam diameter is
d ≈ (4)(0.00155 mm)[8.18 mm / (pi · 0.0104 mm)] = 1.55 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. (Note: MFD and f must have the same units in this equation).
When the F220FC-532 collimator (f = 10.9 mm) is used with the P1-460B-FC-1 patch cable (MFD ≈ 4.0 µm; calculated approximate value) and 532 nm light, then the maximum waist distance is approximately
zmax ≈ 10.9 mm + [2 · (10.9 mm)2 · 0.000532 mm] / [pi · (0.004 mm)2] = 2526 mm.
Damage Threshold Data for Thorlabs' Air-Spaced Doublet Collimators
The specifications to the right are measured data for a selection of Thorlabs' air-spaced doublet collimators. Damage threshold specifications are constant for these collimators, regardless of the connector type.
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/DIS11254 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).
Fiber Collimator Selection Guide
Click on the collimator type or photo to view more information about each type of collimator.