Wavefront correction in two-photon microscopy with a multi-actuator adaptive lens

The group leaded by P. Artal at Murcia University has recently published an interesting paper related to adaptive optics using an adaptive lens. When working in a real scenario, imperfections in the optical elements you use or just the objects you want to image introduce optical aberrations in the pictures you obtain. Usually these aberrations reduce the quality of your images just a bit (introducing a bit of defocus or some astigmatism), but in the worst case scenario it may result in completely useless results.

In order to overcome this problem, usually liquid crystal spatial light modulators or deformable mirrors are used in optical systems to introduce phase corrections to the light going through the system, countering the phase of these aberrations and thus restoring the image quality. However, these systems present several problems. Even though both spatial light modulators and deformable mirrors can correct the problems I mentioned earlier, they work in a reflection configuration. This introduces additional complexity to the optical systems. Also, liquid crystal spatial light modulators are sensitive to polarization, usually have low reflectance values, and tend to be slow.

As a way to tackle those obstacles, the authors have used an adaptive lens in a two-photon microscope to perform the adaptive optics procedure. Adaptive lenses are being used more and more recently to perform aberration correction. In contrast to both spatial light modulators and deformable mirrors, they work in transmission and present very low losses. Moreover, they can introduce low and mid-order aberrations at refresh rates of almost 1 kHz. The working principle can be seen in this figure:

Adaptive_lens
Schematics of the working principle of an adaptive lens. The lens is formed by two thin glass layers, and a liquid in between. Each actuator is triggered by an electrical signal, which deforms the glass windows, generating different shapes and changing the phase of the wavefront passing through the lens. Figure extracted from Stefano Bonora et. al., “Wavefront correction and high-resolution in vivo OCT imaging with an objective integrated multi-actuator adaptive lens,” Opt. Express 23, 21931-21941 (2015)

In the paper, they show how this device can obtain results comparable to the traditional spatial light modulator approach, with the benefits mentioned before, in a multi-photon microscope.

Wavefront correction in two-photon microscopy with a multi-actuator adaptive lens

by Juan M. Bueno et al., at Optics Express

Abstract:

A multi-actuator adaptive lens (AL) was incorporated into a multi-photon (MP) microscope to improve the quality of images of thick samples. Through a hill-climbing procedure the AL corrected for the specimen-induced aberrations enhancing MP images. The final images hardly differed when two different metrics were used, although the sets of Zernike coefficients were not identical. The optimized MP images acquired with the AL were also compared with those obtained with a liquid-crystal-on-silicon spatial light modulator. Results have shown that both devices lead to similar images, which corroborates the usefulness of this AL for MP imaging.

results_bueno.png
Experimental results showing the improvement on the image obtained with the adaptive lens system. Figure 3 from the paper: Juan M. Bueno, et. al, “Wavefront correction in two-photon microscopy with a multi-actuator adaptive lens,” Opt. Express 26, 14278-14287 (2018)

 

Kilohertz binary phase modulator for pulsed laser sources using a digital micromirror device

People at Judkewitz lab tend to do really cool stuff. This time they have implemented a binary phase modulator using a DMD.

Kilohertz binary phase modulator for pulsed laser sources using a digital micromirror device,

M. Hoffmann et al, at Optics Letters

Abstract:

The controlled modulation of an optical wavefront is required for aberration correction, digital phase conjugation, or patterned photostimulation. For most of these applications, it is desirable to control the wavefront modulation at the highest rates possible. The digital micromirror device (DMD) presents a cost-effective solution to achieve high-speed modulation and often exceeds the speed of the more conventional liquid crystal spatial light modulator but is inherently an amplitude modulator. Furthermore, spatial dispersion caused by DMD diffraction complicates its use with pulsed laser sources, such as those used in nonlinear microscopy. Here we introduce a DMD-based optical design that overcomes these limitations and achieves dispersion-free high-speed binary phase modulation. We show that this phase modulation can be used to switch through binary phase patterns at the rate of 20 kHz in two-photon excitation fluorescence applications.

Controlling phase is of paramount interest in multiple optical scenarios. Doing it fast is very difficult, given that spatial light modulators that are really good at modulating phase precisely tend to be slow (~hundreds of Hz). On the other side, intensity modulators such as DMDs are very fast (~20 kHz), but they cannot directly modulate phase. There have been several workarounds with the general idea of using DMDs to modulate phase. I remember a very nice paper by A. Mosk, using groups of mirrors to codify the phase of a superpixel.

Here, they use the fact that DMDs reflect light in two different directions to introduce a phase shift with a moving mirror into one of the reflection directions, achieving binary phase distributions at kHz refresh rates.

 

 

Seems like we are getting closer and closer to get a high-efficiency method to modulate phase with DMD’s.