Use Laser Speckle to Find the Beam Focus

Use Laser Speckle to Find the Beam Focus | Thorlabs Insights

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Күн бұрын

When a lens is mounted in a lens tube, optic mount, or cage plate, the exact position of the lens within the fixture may not be known, which can make it difficult to accurately predict the location of the beam focus and align the system. One approach to finding the beam focus with respect to the fixture uses laser speckle.
During this demonstration, laser speckle is used to find focal points provided by lenses mounted in individual lens tubes. After locating the focus of each mounted lens, the distance between the focal point and the external shoulder of each lens tube is measured.
The mounted lenses are then used to construct a Keplerian beam expander, using a design in which empty lens tubes are inserted between the two mounted lenses. The required total length of the lens tubes separating the two mounted lenses is estimated using the previously measured distances to the focal points. This approach avoids tedious trial-and-error that can result when the beam expander is constructed by first guessing the required total lens tube length, and then iteratively measuring beam quality and adjusting the length until achieving the desired result.
A transmissive glass diffuser is used during this demonstration to create the speckle pattern. The size of the speckle is largest when the ground surface of the diffuser is at the focus, where the beam size is smallest. It is recommended that the beam is incident on the ground face of the diffuser. This allows direct measurement of the distance to the focus, without needing to take the diffuser's optical thickness into account. After assembling the beam expander, a shearing interferometer is used to fine-tune the collimation of the expanded beam.
00:00 - Introduction
00:42 - View Beam Spot to Find Focus
01:53 - Speckle Size vs. Beam Diameter
02:56 - Diffuser Setup and Alignment
03:19 - Speckle Used to Find Focus
05:01 - Keplerian Beam Expander
05:33 - Building a 2X Beam Expander
08:46 - Check Beam Expansion
10:00 - Check Collimation with Shear Plate
Components used in this Demonstration Include:
- Optical Diffuser: www.thorlabs.com/navigation.c...
- DIY Adapter (Diffuser to Slip-Ring Mount), SM1RR Retaining Ring and SM1T1 Lens Tube Coupler: www.thorlabs.com/newgrouppage...
www.thorlabs.com/newgrouppage...
- Plano-Convex Lenses: www.thorlabs.com/navigation.c...
- SI100 Shearing Interferometer: www.thorlabs.com/newgrouppage...
- PL202 Compact Laser Module: www.thorlabs.com/newgrouppage...
- AD11F Adapter (Laser to Mount): www.thorlabs.com/newgrouppage...
- KM100T Threaded Kinematic Mount: www.thorlabs.com/newgrouppage...
- 1" Diameter Lens Tubes: www.thorlabs.com/newgrouppage...
- Adjustable 1" Diameter Lens Tubes: www.thorlabs.com/newgrouppage...
- SM1RC Slip-Ring Mount: www.thorlabs.com/newgrouppage...
- Optical Rail: www.thorlabs.com/newgrouppage...
- Optical Rail Carriers: www.thorlabs.com/newgrouppage...
- DIGC6 Digital Calipers: www.thorlabs.com/newgrouppage...
- R2 Post Collar: www.thorlabs.com/newgrouppage...
- EDU-VS1 Viewing Screen: www.thorlabs.com/newgrouppage...
- FSR3 Fiber Storage Reel: www.thorlabs.com/newgrouppage...
- SPW502 Retaining Ring Wrench: www.thorlabs.com/newgrouppage...
- BHM3 Ruler: www.thorlabs.com/newgrouppage...
- VC3C V-Groove Block: www.thorlabs.com/newgrouppage...
For more photonics how-to videos, visit www.thorlabs.com/newgrouppage...

Пікірлер: 11

@danieldahl 2 жыл бұрын

Amazing video as per usual. Thanks for the demo and tips.

@Youngster459 2 жыл бұрын

This is great, very useful. Something I can apply immediately!

@ohharrylovesme 2 жыл бұрын

Great explanation! Thank you for this! 👏

@chromarti 2 жыл бұрын

What was the "grit polish" number of the diffuser you used? Is it an important parameter for this application?

@thorlabs 2 жыл бұрын

We chose a diffuser polished with 1500 grit (Part Number: DG10-1500 www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=1132&pn=DG10-1500&YVI=23 ). In our demonstration, the source provided a collimated 635 nm laser beam with a 3 mm diameter. Lenses with 75 and 150 mm focal lengths provided focal spots estimated to be between 20 and 40 microns in diameter. We chose this diffuser because the resultant speckle was nicer in appearance, over a wider range of beam diameters, than the speckle provided by a diffuser polished with a coarser grit. There are a lot of variables to consider when selecting a diffuser. If you would like assistance choosing a diffuser for an application, please contact Tech Support (techsupport@thorlabs.com).

@onnobo7859 9 ай бұрын

really nice tutorial! but seems the placement direction of the diffuser shown in the scheme different from the real setup?

@thorlabs 9 ай бұрын

@onnobo7859 The orientation is the same in both. We were careful to make sure the beam was incident on the ground glass side, since it is so important to the accuracy of the measurement. Check out the view at 4:07, where it is easier to see that the ground glass face, and not the retaining ring, is flush with the front surface of the mount.

@thomaszhang3115 Жыл бұрын

would the distance between the white board and the lens be a factor in this demo?

@thorlabs Жыл бұрын

@Thomaszhang3115 The speckle pattern will change as the distance between the diffuser and the viewing screen changes, but using the technique demonstrated in this video does not require placing the viewing screen at a particular distance. However, adjusting the distance to the viewing screen might make it easier to see where the lens placement is optimized. This is because the average speckle size depends on the distance to the viewing screen. When the viewing screen is farther away, the average speckle size is larger.

@karthik_1001 Жыл бұрын

is this method affects the wavelength of light?

@thorlabs Жыл бұрын

The method also works for other wavelength of light. However, light of different wavelengths may focus at slightly different distances from the lens due to a lens effect called chromatic aberration. Achromatic lenses are designed to have reduced chromatic aberrations.