Use Laser Speckle to Find the Beam Focus | Thorlabs Insights

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: https://www.thorlabs.com/navigation.c...
- DIY Adapter (Diffuser to Slip-Ring Mount), SM1RR Retaining Ring and SM1T1 Lens Tube Coupler: https://www.thorlabs.com/newgrouppage...
https://www.thorlabs.com/newgrouppage...
- Plano-Convex Lenses: https://www.thorlabs.com/navigation.c...
- SI100 Shearing Interferometer: https://www.thorlabs.com/newgrouppage...
- PL202 Compact Laser Module: https://www.thorlabs.com/newgrouppage...
- AD11F Adapter (Laser to Mount): https://www.thorlabs.com/newgrouppage...
- KM100T Threaded Kinematic Mount: https://www.thorlabs.com/newgrouppage...
- 1" Diameter Lens Tubes: https://www.thorlabs.com/newgrouppage...
- Adjustable 1" Diameter Lens Tubes: https://www.thorlabs.com/newgrouppage...
- SM1RC Slip-Ring Mount: https://www.thorlabs.com/newgrouppage...
- Optical Rail: https://www.thorlabs.com/newgrouppage...
- Optical Rail Carriers: https://www.thorlabs.com/newgrouppage...
- DIGC6 Digital Calipers: https://www.thorlabs.com/newgrouppage...
- R2 Post Collar: https://www.thorlabs.com/newgrouppage...
- EDU-VS1 Viewing Screen: https://www.thorlabs.com/newgrouppage...
- FSR3 Fiber Storage Reel: https://www.thorlabs.com/newgrouppage...
- SPW502 Retaining Ring Wrench: https://www.thorlabs.com/newgrouppage...
- BHM3 Ruler: https://www.thorlabs.com/newgrouppage...
- VC3C V-Groove Block: https://www.thorlabs.com/newgrouppage...

For more photonics how-to videos, visit https://www.thorlabs.com/newgrouppage...