Introduction to Laser Spatial Filtering

In a variety of applications, the diameter of the laser beam needs to be increased with homogenous or smoothly varying beam intensity over that larger diameter. The optical devices that can be used for manipulating laser beams, such as laser beam expanders and spatial filters, are covered in this article.

The Spatial Filter

In many laser experiments, the laser beam traverses through optical components like prisms, lenses and beam splitters. Defects on and within these components can be detrimental to the beam profile. In addition, the particulates present in the air through which the beam propagation takes place can cause further beam degradation. In such cases, the nominal beam form needs to be homogenous, or at least as smoothly varying as possible. It can be ensured by cleaning up the contaminated laser beam using a spatial filter (Figure 1).

Beam Expander System with spatial filtering (a), and without (b).

Figure 1. Beam Expander System with spatial filtering (a), and without (b).

The simplest form of a spatial filter consists of a well corrected positive lens, generally a microscope objective, and a pinhole positioned at the focus of that lens. This setup facilitates accurate focusing of all collimated light at the pinhole. Other light, which has emanated at points of contamination, for instance a scratch on a mirror in the experimental setup, is focused prior to or after the pinhole, and so does not traverse through it. The pinhole filters out that energy from the system.

Selection of a Pinhole and Objective Lens

For efficient spatial filtering, the best combination of pinhole and microscope objective needs to be selected for the laser to be manipulated. In selecting the proper combination, the first step is to determine the laser beam diameter at the spatial filter input. The next step is to estimate the 1/e2 diameter of the laser beam’s focused spot.

The final step involves comparison of the spot sizes x 1.5 with the available pinhole diameters. Any spot size x 1.5 that is nearly equal to a pinhole diameter provides a proper lens-pinhole combination. Nevertheless, the selection of the best combination relies on the application of the filtered beam.

Freely Diverging Beam

The filtered laser beam may be used in its freely diverging form exiting from the pinhole of the spatial filter assembly. Figure 2 shows the calculation of the cone angle of this freely diverging beam. The required cone angle also plays a role in the selection of a lens-pinhole combination. Figure 3 shows the smooth illumination of an object using standard Oriel components. Here, the irradiance of the object will have a smooth Gaussian profile characteristic of the laser output.

Calculation of the cone angle of a freely diverging beam exiting the 15221 Spatial Filter Assembly.

Figure 2. Calculation of the cone angle of a freely diverging beam exiting the 15221 Spatial Filter Assembly.

Using standard Oriel components to smoothly illuminate an object.

Figure 3. Using standard Oriel components to smoothly illuminate an object.

Laser Beam Expander with Spatial Filter

A basic 17.8X laser beam expander equipped with a spatial filter is depicted in Figure 4. An input beam with a diameter of 1.28mm, and a divergence of 1.2mrad, is focused by a 9.2mm microscope objective, and then spatially filtered by transmitting through a 10µm diameter pinhole. The diverging beam is gathered and collimated by a well corrected 160mm focal length doublet.

15281 Keplerian type Laser Beam Expander with Spatial Filter.

Figure 4. 15281 Keplerian type Laser Beam Expander with Spatial Filter.

The resulting beam has a diameter of 22.8mm, with a divergence of 0.068mrad. The introduction of a beam expander lowered the rate of beam divergence and increased the beam diameter, both by a factor of 17.8X.

Beam Expander without Spatial Filter

For using a laser beam expander devoid of a spatial filter, the Galilean telescope type can be selected. The Galilean type lacks an internal focus, an issue for higher power pulsed lasers. Figure 5 delineates a 20X Beam Expander with basic characteristics identical to those of the design illustrated in Figure 4, but with improved performance at a lower cost due to its simplified form.

15610 Galilean type Laser Beam Expander.

Figure 5. 15610 Galilean type Laser Beam Expander.

The focal length of the negative input lens is -8mm. The use of a 160mm focal length output lens provides the overall beam expander with magnification of 20X. This factor is applicable to the increase of output beam diameter, and the reduction in beam divergence. As these lenses are made of fused silica, the beam expander can be used with UV, VIS, and NIR lasers.

About Oriel Instruments

Oriel Instruments, a Newport Corporation brand, was founded in 1969 and quickly gained a reputation as an innovative supplier of products for the making and measuring of light. Today, the Oriel brand represents leading instruments, such as light sources covering a broad range, from UV to IR, pulsed or continuous, and low to high power.

Oriel also offers monochromators and spectrographs, as well as flexible FT-IR spectrometers, which make it easy for users across many industries to build instruments for specific applications. Oriel is also a leader in the area of Photovoltaics with its offering of solar simulators, that allow you to simulate hours of solar radiation in minutes. Oriel continues to bring innovative products and solutions to Newport customers around the world.

This information has been sourced, reviewed and adapted from materials provided by Oriel Instruments.

For more information on this source, please visit Oriel Instruments.

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