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Laser Beam Shaping

Multi-Plane Light Conversion (MPLC): A unique technology

Discovered in 2010 in the Kastler Brossel Laboratory, a French research laboratory specializing in fundamental physics of quantum systems, Multi-Plane Light Conversion (MPLC) was originally meant for multimode quantum optics. It is based on the succession of transverse phase profiles, similar to highly complex lenses, separated by a specific propagation distance. And because it “is a very low loss process,” MPLC can combine and shape multiple light beams. The passive nature of this technology means it can be integrated in systems where reliability is critical.

The Multi-Plane Light Conversion (MPLC) main characteristics are: 

▪ Free-form beam shaping through succession of spatial phase profiles and propagation

▪ Passive beam shaping with no intrinsic loss

▪ Reflective implementation, can handle high power / energy

▪ Multiple beams can be shaped simultaneously

To learn more about MPLC, please watch the video below from CAILABS.

Using MPLC technology to improve ultrafast-laser based processes 

These laser material processes can benefit from MPLC by: 1.  increasing overall quality by shaping the light (free-form shaping, diffraction limited shaping, preserving the depth of focus)
2. increasing yield (optimal beam shaping, beam splitting, preserving the pulse duration
3. increasing the robustness (mode cleaning for beam stabilization, compatibility with industry standard setups)

MPLC vs. Adaptive Optics

Adaptive optics consists of reshaping the wavefront using a deformable mirror. Figure 1 shows a typical adaptive optics system. The incident signal is sent to the deformable mirror which corrects wavefront distortion. A fraction of the corrected signal is sent to a wavefront sensor to characterize its quality in real time. Finally, the other fraction of light is sent through the optical fiber to the telecommunication receiver. For this study, a wide-field photodiode was also used to measure the total intensity fluctuation of the wavefront in the focal plane. Diagram of a standard adaptive optics systemFigure 1: Diagram of a standard adaptive optics system Although its function is similar, the approach of the TILBA-R is different. Its objective is not to actively compensate for wavefront distortion, but rather to collect all the light and passively convert it into usable modes. A light wave can be decomposed into modes. In this case, we have chosen Hermite-Gauss modes. The Mutli-Plane Light Conversion technology (MPLC) on which the TILBA-R is based, then collects these various disruptive modes and demultiplexes them into as many single-mode fibers. The dynamics of turbulence still exist, but they are reflected in variations in phase and intensity of the modes, which themselves are fixed. After demultiplexing, these variations result in changes in phase and intensity of the respective signals, but in the single-mode fibers. This passive approach makes it possible to compensate for the phase shift in the fibers themselves rather than with mechanical elements, which are intrinsically limited in speed. Finally, it should be noted that if adaptive optics also use phase mode , such as Zernike polynomials in turbulence analysisMPLC works on the complex field, i.e. both the phase and the amplitude.
Figure 2: Comparison of AO and MPLC functionality. AO directly influences the wavefront by modifying the optical path to reform a Gaussian beam. The MPLC approach passively collects all light in the form of modes. These modes are converted into Gaussian modes and sent into single-mode fibers. 

Using Bessel Beams to Observe Microscopic Objects

Bessel beams are “needles” of light formed by the interference of a laser beam with a conical-phase (Fig. 1). They have many applications in both research and industry. In particular, they are used for machining transparent materials, such as glass or sapphire, and to make smartphone screens and watch glasses. Their properties also make them particularly attractive for viewing incredibly small structures and observing biological phenomena that are invisible to the naked eye.  Image2 Figure 1: Principle of Bessel beam generation   The conventional (Gaussian) focusing of a laser beam is not ideal and is even limited in some applications. Bessel beams have properties that are particularly suitable for microscopy and are increasingly used in new applications. They have been used for several years to improve laser microscopy techniques. So what are the benefits of Bessel beams for microscopy?
  • Depth of field
 
A Bessel beam has a depth of focus that is 10 to 100 times longer than a conventional focus(2) (Fig. 2). By using a Bessel beam to illuminate and image a sample, researchers can considerably increase the depth of field, making it possible to image a volume much faster, and thus observe live events taking place in a volume while resisting any small movements of the object being observed(3). Image3 Figure 2: Simulation of the energy distribution along the propagation axis of a Bessel beam (a) and a Gaussian beam (b)(2)  
  • Spatial resolution
 
It is important to be able to image a biological sample with a resolution smaller than the size of the living cells being studied. One of the advantages of Bessel beams is that they are not limited by the diffraction limit: the dimensions of their focus can be smaller than the wavelength used, so Bessel beams can have a width of the order of a hundred nanometers, providing excellent imaging resolution.
  • Damage to Samples
 
Observing biological tissues using a laser microscopy technique can disturb or even damage sensitive samples if they absorb the light energy emitted. Because of the extreme depth of focus of Bessel beams, the light energy is more “spread out” than for Gaussian beams. This allows samples to be imaged by exposing them to lower light intensity, thereby reducing any potential for damage(1).