Features

• AR Coated for the 650 - 1050 nm Range
• Positive Doublet Sizes: Ø5 mm, Ø6 mm, Ø6.35 mm,
  Ø8 mm, Ø1/2", Ø1", Ø30 mm, and Ø2"
• Negative Doublet Sizes: Ø1/2" and Ø1"
• 7.5 mm to 1000 mm Focal Lengths for Positive Doublets
• -20 mm to -100 mm Focal Lengths for Negative Doublets

Thorlabs' cemented Achromatic Doublets, which are designed for use in the near IR spectral region (650 - 1050 nm), are optimized at infinite conjugate ratios. With design wavelengths at 706.5 nm, 855.0 nm, and 1015 nm, these doublets are useful for controlling chromatic aberration; they are frequently used to achieve a diffraction-limited spot when using a monochromatic source like a laser.

Refer to the Application tab above for information about the superior performance of achromatic doublets compared to singlet lenses and the Measurement tab for examples of measurements that can be made by downloading the appropriate Zemax® file for the achromatic lens of interest. Zemax files can be found by clicking on the document icon next to the appropriate part number below.

For best performance, the side of the lens with the largest radius of curvature (flattest side) should face away from the collimated beam. When the engraved part number on the lens is oriented right side up, the flattest side of the lens is the bottom surface. Please see the diagram under the reference drawing link below for additional details.

A recommended fixed lens mount is listed in the footnote under each specification table below. Alternatively, choose a mount from our selection of fixed diameter lens mounts, self-centering adjustable lens mounts, or adjustable lens mounts. When choosing a lens mount, make sure the mount can accommodate the diameter and edge thickness specifications of the lens.

In the specification tables below a positive radius of curvature indicates that the surface is opening to the right when the lens is oriented as shown in the reference drawing while a negative radius of curvature indicates that the surface is opening to the left. Both the positive and negative lenses have an infinite conjugate ratio (i.e., if a diverging light source is placed one focal length away from the flatter side of the lens, the light rays emerging from the curved side will be collimated).

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Click to Download Reflectivity Data

Detailed information regarding each achromatic doublet can be found in the Zemax^® files included with the support documents for each doublet. Below are some examples of the measurement that can be made using the Zemax^® files.

Focal Shift vs. Wavelength

The graph below shows the paraxial focal shift as a function of wavelength for the AC254-200-B, which is a 200 mm focal length, Ø25.4 mm achromatic doublet AR coated for the 650 to 1050 nm range.

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Figure 1

Wavefront Error and Spot Size

Spherical doublet lenses have been corrected for various aberrations. One way of displaying the theoretical level of correction is through plots of wavefront error and ray traces to determine spot size. For example, in Figure 2, a plot of wavefront at the image plane reveals information regarding aberration correction by using the AC254-200-B. In this example, the wavefront error is theoretically on the order of 3/100 of a wave. This indicates that the optical path length difference (OPD) is extremely small for arrays going through the center of the lens and at nearly full aperture.

A ray trace for spot size at the image plane of the AC254-200-B is shown below in Figure 3. In this near IR achromatic doublet, the design wavelengths (706.5 nm, 855 nm and 1015 nm) have each been traced through the lens and are represented by different colors. The circle surrounding the distribution of ray intercepts represents the diameter of the Airy disk. If the spot is within the Airy disk, the lens is typically considered to be diffraction limited. Since the spot size is drawn using geometric ray tracing, spots much smaller than the Airy disk are not achievable due to diffraction.

Figure 2

Figure 3

Understanding Modulation Transfer Function, MTF

MTF image quality is an important characteristic of lenses. A common way to measure this is by using contrast. A plot of the modulation transfer function is used as both a theoretical and experimental description of image quality. The MTF of a lens describes its ability to transfer contrast from an object to an image at various resolution levels. Typically, a resolution target made up of black and white lines at various spacings is imaged and contrast can be measured. Contrast at 100% would consist of perfectly black and white lines. As the contrast diminishes, the distinction between lines begins to blur. A plot of MTF shows the percentage of contrast as the spacing between these lines decreases. The spacing between the lines at the object is usually represented as spatial frequency given in cycles/mm.

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Figure 4

The chart shows the theoretical MTF for our Ø25.4 mm, f=200 mm near IR achromatic doublet. The contrast is around 80% at a spatial frequency of about 20 cycles/mm. This represents 80% contrast at 0.05 mm spacings between lines. Theoretical MTF shows how well a design can perform if the optic was built exactly to the design dimensions. In reality, most optics fall short of the theoretical due to manufacturing tolerances.

The screen captures to the right and left are actual measurements taken using a USAF 1951 resolution chart as the object.

For the target selected, the contrast measured 82.3%.

Achromatic Doublet Lenses have far superior optical performance to Singlet Lenses. Whether your application has demanding imaging requirements or laser beam manipulation needs, these doublets should be considered.

Achieve a Tighter Focus

The figures below show a comparison of a plano-convex singlet focusing a 633 nm laser beam and an achromatic doublet focusing the same laser beam. The spot (circle of least confusion) from the doublet is 4.2 times smaller than the singlet spot size.

Superior Off Axis Performance

Achromatic Doublet lenses have a much reduced sensitivity to centration of the lenses on the beam axis.

The figures below show two 50.0 mm focal length lenses, one plano-convex and the other an achromatic doublet. Both are Ø25.4 mm lenses with a Ø3 mm beam through the optical axis and one offset by 8.0 mm. Lateral and transverse aberrations are greatly reduced by the achromatic doublet.

Nearly Constant Focal Length Across a Wide Range of Wavelengths

When using a white light source with a singlet lens, the focal point and circle of least confusion are blurred by chromatic aberration. Chromatic aberration is due to the variation of refractive index with respect to wavelength. In an achromatic doublet this effect is somewhat compensated for by using glasses of two different refractive indexes to cancel these aberrations.

The figures below show the effect on focal length for a number of different wavelengths of light through an achromatic doublet and a plano-convex singlet. The figures also shows how the circle of least confusion for white light is reduced by using an achromatic doublet.

Laser Induced Damage Threshold Tutorial

This tutorial 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.).

Testing Method

Thorlabs' LIDT testing is done in compliance with ISO/DIS11254 specifications. A standard 1-on-1 testing regime is performed to test the damage threshold.

First, a low-power/energy beam is directed to the optic under test. The optic is exposed in 10 locations to this laser beam for a set duration of time (CW) or number of pulses (prf specified). After exposure, the optic is examined by a microscope (~100X magnification) for any visible damage. The number of locations that are damaged at a particular power/energy level is recorded. Next, the power/energy is either increased or decreased and the optic is exposed at 10 new locations. This process is repeated until damage is observed. The damage threshold is then assigned to be the highest power/energy that the optic can withstand without causing damage. A histogram such as that below represents the testing of one BB1-E02 mirror.

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 it is only representative of one coating run and that Thorlabs' specified damage thresholds 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) [1]. Pulsed lasers with pulse lengths longer than 1 µs can be treated as CW lasers for LIDT discussions. Additionally, when pulse lengths are between 1 ns and 1 µs, LIDT can occur either because of absorption or a dielectric breakdown (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 large 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:

1. Wavelength of your laser
2. Linear power density of your beam (total power divided by 1/e² spot size)
3. Beam diameter of your beam (1/e²)
4. Approximate intensity profile of your beam (e.g., Gaussian)

The power density of your beam should be calculated in terms of W/cm. The graph to the right shows why the linear power density provides the best metric for long pulse and CW sources. Under these conditions, linear power density scales independently of spot size; 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 consider hotspots in the beam or other nonuniform 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 1/e² 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.

Pulsed Lasers

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 pulse lengths that our specified LIDT values are relevant for.

Pulses shorter than 10^-11 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 [2]. In contrast, pulses between 10^-9 s and 10^-6 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:

1. Wavelength of your laser
2. Energy density of your beam (total energy divided by 1/e² area)
3. Pulse length of your laser
4. Pulse repetition frequency (prf) of your laser
5. Beam diameter of your laser (1/e² )
6. Approximate intensity profile of your beam (e.g., Gaussian)

The energy density of your beam should be calculated in terms of J/cm². The graph to the right shows why the energy density provides the best metric for short pulse sources. Under these conditions, energy density scales independently of spot size, 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 power 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 [3]. 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 [4]. 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^-11 s and 10^-9 s. For pulses between 10^-9 s and 10^-6 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.

[1] R. M. Wood, Optics and Laser Tech. 29, 517 (1997).
[2] Roger M. Wood, Laser-Induced Damage of Optical Materials (Institute of Physics Publishing, Philadelphia, PA, 2003).
[3] C. W. Carr et al., Phys. Rev. Lett. 91, 127402 (2003).
[4] N. Bloembergen, Appl. Opt. 12, 661 (1973).