Circular Precision Pinholes, Molybdenum Foils

Precision Pinholes and Slits
Thorlabs offers precision pinholes with blackened stainless steel, gold-plated copper, tungsten, or molybdenum foils. Our pinholes with stainless steel foils are blackened on both sides for increased absorbance and are available from stock in circles from Ø1 µm to Ø9 mm and squares from 100 µm x 100 µm to 1 mm x 1 mm. Our stainless steel pinholes with a black PVD coating are vacuum compatible and available in 5 μm to 2 mm diameters. Our pinholes with gold-plated copper foils, plated with gold on one side and black PVD coated on the reverse, are available with pinhole diameters from 5 µm to 2 mm. High-power gold-plated copper foil pinholes are also available, with the gold-plated copper foil on the front face, and a PVD black coating on the rear. Our pinholes with tungsten foils are uncoated and available with pinhole diameters from 5 µm to 2 mm. Lastly, our pinholes with molybdenum foils have an absorptive polymer coating on the front sides and are available with pinhole diameters from 5 µm to 2 mm. We also offer slits in blackened stainless steel foils from stock with slit widths from 5 to 200 µm.

If you do not see what you need among our stock pinhole and slit offerings, it is also possible to special order pinholes and slits that are made with different foil materials, have different hole sizes and shapes, incorporate multiple holes in one foil, or provide different hole configurations. Please contact Tech Support to discuss your specific needs. For more information on the properties of the bulk materials from which the pinholes are fabricated, see the table below.

Material Properties
Depending on the application, it can be important to consider the material properties of the pinhole or slit. The material used to construct the aperture can have varying levels of melting point, density, and thermal conductivity, as detailed in the table below.

• Stainless steel pinholes and slits are blackened on both sides to increase absorbance. The material properties will be predominantly that of bulk stainless steel.
• Gold-plated copper pinholes have a thin coating of gold on one side of the bulk copper foil. With a beam incident on this side, reflectance will be dominated by the properties of the thin gold coating (77% @ 800 nm), while thermal properties will be predominantly copper-based.
• Molybdenum pinholes have an absorptive coating on the front side. The material properties will be predominantly that of bulk molybdenum.
• Damage threshold data refers to the bulk materials listed here only.

Reflectance
The reflectance of the foil material or coating affects performance in a variety of applications. Below is presented a reflectance graph for all the materials and coatings that are offered with our circular and square precision pinholes, as well as our mounted optical slits. The raw reflectance data can be found here.

It is important to note that the front of the gold-plated copper foil circular precision pinholes have a low-reflectance PVD black coating. The rear of these pinholes leaves the gold-plated copper foil bare. This also occurs on the molybdenum foil circular precision pinholes, which have a low-reflectance absorptive polymer coating on the front and the molybdenum foil is left bare on the back.

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 Figure 1: The beam circularization systems were placed in the area of the experimental setup highlighted by the yellow rectangle.

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Figure 4: Spatial Filter System

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Figure 3: Anamorphic Prism Pair System

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Figure 2: Cylindrical Lens Pair System

Comparison of Circularization Techniques for Elliptical Beams

Edge-emitting laser diodes emit elliptical beams as a consequence of the rectangular cross sections of their emission apertures. The component of the beam corresponding to the narrower dimension of the aperture has a greater divergence angle than the orthogonal beam component. As one component diverges more rapidly than the other, the beam shape is elliptical rather than circular. 

Elliptical beam shapes can be undesirable, as the spot size of the focused beam is larger than if the beam were circular, and as larger spot sizes have lower irradiances (power per area). Techniques for circularizing an elliptical beam include those based on a pair of cylindrical lenses, an anamorphic prism pair, or a spatial filter. This work investigated all three approaches. The characteristics of the circularized beams were evaluated by performing M² measurements, wavefront measurements, and measuring the transmitted power. 

While it was demonstrated that each circularization technique improves the circularity of the elliptical input beam, each technique was shown to provide a different balance of circularization, beam quality, and transmitted power. The results of this work, which are documented in this Lab Fact, indicate that an application's specific requirements will determine which is the best circularization technique to choose.

Experimental Design and Setup

The experimental setup is shown in Figure 1. The elliptically-shaped, collimated beam of a temperature-stabilized 670 nm laser diode was input to each of our circularization systems shown in Figures 2 through 4. Collimation results in a low-divergence beam, but it does not affect the beam shape. Each system was based on one of the following:

• LJ1874L2-A and LJ1638L1-A Plano-Convex Convex Cylindrical Lenses (Figure 2)
• PS873-A Unmounted Anamorphic Prism Pair (Figure 3)
• Previous Generation KT310 Spatial Filter System with P5S Ø5 µm Pinhole (Figure 4)

The beam circularization systems, shown to the right, were placed, one at a time, in the vacant spot in the setup highlighted by the yellow rectangle. With this arrangement, it was possible to use the same experimental conditions when evaluating each circularization technique, which allowed the performance of each to be directly compared with the others. This experimental constraint required the use of fixturing that was not optimally compact, as well as the use of an unmounted anamorphic prism pair, instead of a more convenient mounted and pre-aligned anamorphic prism pair.

The characteristics of the beams output by the different circularization systems were evaluated by making measurements using a power meter, a wavefront sensor, and an M² system. In the image of the experimental setup, all of these systems are shown on the right side of the table for illustrative purposes; they were used one at a time. The power meter was used to determine how much the beam circularization system attenuated the intensity of the input laser beam. The wavefront sensor provided a way to measure the aberrations of the output beam. The M² system measurement describes the resemblance of the output beam to a Gaussian beam. Ideally, the circularization systems would not attenuate or aberrate the laser beam, and they would output a perfectly Gaussian beam. 

Edge-emitting laser diodes also emit astigmatic beams, and it can be desirable to force the displaced focal points of the orthogonal beam components to overlap. Of the three circularization techniques investigated in this work, only the cylindrical lens pair can also compensate for astigmatism. The displacement between the focal spots of the orthogonal beam components were measured for each circularization technique. In the case of the cylindrical lens pair, their configuration was tuned to minimize the astigmatism in the laser beam. The astigmatism was reported as a normalized quantity.

Experimental Results

The experimental results are summarized in the following table, in which the green cells identify the best result in each category. Each circularization approach has its benefits. The best circularization technique for an application is determined by the system’s requirements for beam quality, transmitted optical power, and setup constraints. 

Spatial filtering significantly improved the circularity and quality of the beam, but the beam had low transmitted power. The cylindrical lens pair provided a well-circularized beam and balanced circularization and beam quality with transmitted power. In addition, the cylindrical lens pair compensated for much of the beam's astigmatism. The circularity of the beam provided by the anamorphic prism pair compared well to that of the cylindrical lens pair. The beam output from the prisms had better M² values and less wavefront error than the cylindrical lenses, but the transmitted power was lower. 

Method	Beam Intensity Profile	Circularitya	M2 Values	RMS Wavefront	Transmitted Power	Normalized  Astigmatismb
Collimated Source Output (No Circularization Technique)	Click to Enlarge Scale in Microns	0.36	X Axis: 1.28 Y Axis: 1.63	0.17	Not Applicable	0.67
Cylindrical Lens Pair	Click to Enlarge Scale in Microns	0.84	X Axis: 1.90 Y Axis: 1.93	0.30	91%	0.06
Anamorphic Prism Pair	Click to Enlarge Scale in Microns	0.82	X Axis: 1.60 Y Axis: 1.46	0.16	80%	1.25
Spatial Filter	Click to Enlarge Scale in Microns	0.93	X Axis: 1.05 Y Axis: 1.10	0.10	34%	0.36

• Circularity=d_minor/d_major, where d_minor and d_major are minor and major diameters of fitted ellipse (1/e intensity) and Circularity = 1 indicates a perfectly circular beam.
• Normalized astigmatism is the difference in the waist positions of the two orthogonal components of the beam, divided by the Raleigh length of the beam component with the smaller waist.

Components used in each circularization system were chosen to allow the same experimental setup be used for all experiments. This had the desired effect of allowing the results of all circularization techniques to be directly compared; however, optimizing the setup for a circularization technique could have improved its performance. The mounts used for the collimating lens and the anamorphic prism pair enabled easy manipulation and integration into this experimental system. It is possible that using smaller mounts would improve results by allowing the members of each pair to be more precisely positioned with respect to one another. In addition, using made-to-order cylindrical lenses with customized focal lengths may have improved the results of the cylindrical lens pair circularization system. All results may have been affected by the use of the beam profiler software algorithm to determine the beam radii used in the circularity calculation.

Additional Information

Some information describing selection and configuration procedures for several components used in this experimental work can be accessed by clicking the following hyperlinks: 

• Mounting Laser Diodes
• Driving a Laser Diode
• Selecting a Collimating Lens
• Aspheric Lenses
• Spatial Filters

Apertures Selection Guide
Aperture Type	Representative Image (Click to Enlarge)	Description	Aperture Sizes Available from Stocka
Single Precision Pinholesa		Circular Pinholes in Stainless Steel Foils	Ø1 µm to Ø9 mm
		Circular Pinholes in Stainless Steel Foils, Vacuum Compatible	Ø5 µm to Ø2 mm
		Circular Pinholes in Gold-Plated Copper Foils	Ø5 µm to Ø2 mm
		Circular Pinholes in Gold-Plated Copper Foils with High-Power Housings	Ø5 µm to Ø500 µm
		Circular Pinholes in Tungsten Foils	Ø5 µm to Ø2 mm
		Circular Pinholes in Molybdenum Foils	Ø5 µm to Ø2 mm
		Square Pinholes in Stainless Steel Foils	100 to 1000 µm Square
Slitsa		Slits in Stainless Steel Foils	3 mm Slit Lengths: 5 to 500 µm Widths 10 mm Slit Lengths: 20 to 500 µm Widths
		Double Slits in Stainless Steel Foils	3 mm Slit Lengths with 40, 50, or 100 µm Widths, Spacing of 3X or 6X the Slit Width
Half-Apertures		Mounted, Half-Aperture Foils	Half-Apertures for Knife-Edge Scan Measurements
Annular Apertures		Annular Aperture Obstruction Targets on Quartz Substrates with Chrome Masks	Ø1 mm Apertures with ε Ratiosb from 0.05 to 0.85 Ø2 mm Aperture with ε Ratiob of 0.85
Pinhole Wheels		Manual, Mounted, Chrome-Plated Fused Silica Disks with Lithographically Etched Pinholes	Each Disk has 16 Pinholes from Ø25 µm to Ø2 mm and Four Annular Apertures (Ø100 µm Hole, 50 µm Obstruction)
		Motorized Pinhole Wheels with Chrome-Plated Glass Disks with Lithographically Etched Pinholes	Each Disk has 16 Pinholes from Ø25 µm to Ø2 mm and Four Annular Apertures (Ø100 µm Hole, 50 µm Obstruction)
Pinhole Kits		Stainless Steel Precision Pinhole Kits	Kits of Ten Circular Pinholes in Stainless Steel Foils Covering Ø5 µm to Ø9 mm

• Precision pinholes and slits can be special ordered with different aperture sizes, foil materials, shapes, and hole distributions than those offered from stock. Please contact Tech Support with inquiries.
• Ratio of the Obstruction Diameter to the Pinhole Diameter