Adaptive autofocus technique for speckle control in optical feedback interferometry

• A new automated method of speckle control in OFI is presented.
• No external mechanical setup is required for the control.
• SNR rather than the amplitude of the OFI signal is used as feedback.
• Errors in OFI long range displacement measurements have been reduced by 8.8%.
• Errors in OFI velocity measurements have also been reduced for long motions.

An adaptive autofocus technique for the control of speckle effect in optical feedback interferometry (OFI) is discussed. The beam spot size effect over the OFI signal is presented, justifying the proposed approach. An automated setup for the control of speckle effect using a liquid lens with electro-optical focusing is demonstrated. The spot size is modified according to the signal to noise ratio (SNR) bounds selected for the captured OFI signal, therefore avoiding signal fading caused by speckle effect and possible chaotic behavior because of strong feedback. The quality and shape of the acquired OFI signal when the spot size change takes place shows a transient oscillation without any additional effects over the OFI SNR and the OFI shape. Experimental examples are provided proving the effectiveness of the approach for speckle control in displacement and velocity measurements within a movement range of 20 mm.