Image Credit: Konstantin Kolosov/Shutterstock.com
In the field of microscopy, many different processes and techniques have been developed over the past few centuries to help researchers reveal ever-more complex details of structures that exist beyond the limit of human eyesight. The first microscopes to be developed were very basic affairs, with only a minimal level of control mechanism built-in, but they could reveal details about the natural world never seen before.
More recently, with the development of more advanced optical microscopes, super-resolution microscopy techniques, including STORM and PALM, and technologies such as electron microscopes and atomic force microscopes, the level of detail revealed and control attainable has improved exponentially. One such method utilized to improve image acquisition in modern-day microscopy is beam shaping.
What is Beam Shaping?
In beam shaping, the phase or irradiance of the optical radiation is redistributed and controlled by the use of a purpose-designed optical element. This element contains a suitable lens or multiple lenses. There is no single method which is suitable for all applications.
When designing a beam shaping system, it is desirable to achieve a lossless result, where the energy of the input beam (Iin(x,y)) and the output beam (Iout(x,y)) is the same.
There are three factors to consider when designing an algorithm for a beam shaping system. These are:
- Scaling – How the difficulty of the problem is affected by wavelength, size of input and output beams, and focal length of the system.
- Smoothness – How the difficulty of the problem is affected by discontinuities in the distributions of the input and output irradiance.
- Coherence – How the difficulty of the problem is affected by the coherence width of the laser.
There are three main methods of beam shaping which are widely used in microscopy - beam aperturing, field mapping, and beam integrators.
The most basic form of beam shaping is by using the aperture to select a suitably flat portion of the beam once it is expanded, a method known as “beam aperturing.” The irradiance pattern in which results can then, with magnification, control the output beam’s size.
However, this method is basic and has its limitations as it can experience loss. Generally, it is not used in studies that require more specialized techniques.
Field Mapping of Refractive or Diffractive (DOE) Optics
Field mapping is one of the main beam-shaping techniques in use. This method transforms the input field into the desired field in a more controlled manner than beam aperturing, turning a single-mode Gaussian beam into one with uniform irradiance. The rays are bent in a plane, producing uniform distribution in the output plane. Effectively, field mappers can be made lossless. This approach is suitable for coherent single-mode laser beams.
Beam Integrators/Beam Homogenizers
In a beam integrator (also known as a beam homogenizer) a lenslet array breaks up the input beam into beamlets and superimposes them in the output plane with the primary lens. The output pattern is therefore composed of a group of diffraction patterns that are determined by the lenslet apparatus. Integrators are particularly suited to multimode lasers which have a relatively low degree of spatial coherence.
Again, much like field mappers, integrators/homogenizers can be operated so as to be, for all intents and purposes, lossless. However, integrators are not suitable for coherent beams, as undesirable interference effects occur when faced with this kind of beam.
Beam Shaping – A Useful, Versatile, but Difficult and Highly Specialized Technique
As the field of microscopy evolves in the modern laboratory, researchers and those developing these technologies are faced with designing beam shaping systems that are advantageous to the specific type of microscope. The techniques listed in this article are just three of the main ones that are utilized in beam shaping.
One recent microscopy technique that has been developed is selective plain illumination microscopy (SPIM) and a toolkit for beam shaping in this method has been developed, known as structured SPIM (SSPIM) which controls the diffraction pattern of an SLM (spatial light modulator) and is effective for both coherent and incoherent beams.
Overall, it is a very useful and versatile method that can be used by laboratories and researchers working in a variety of fields. However, obtaining desirable results with beam shaping requires highly specialized techniques and knowledge of the field.
References and Further Reading