Intracavity laser beam shaping has been achieved by adding intensity or phase filters to a Fabry-Perot resonator. Changing the output beam from one mode to another is a tedious process, requiring the replacing of custom optical elements, and careful realignment. The digital laser [1] is an innovation which allows the laser beam produced by a laser to be dynamically controlled by a computer. Essentially, one of the resonator mirrors is replaced by a spatial light modulator (SLM), which is a computer controlled, pixellated, liquid-crystal device. While the concept is the device is simple, the implementation revealed subtle properties of spatial light modulators and the liquid crystals contained in them. These properties had to be well understood before their undesirable characteristics could be overcome, allowing the laser to function as conceived in the design.
The beam from a laser resonator is determined by the optical elements it contains. Most commonly, these consist of two spherical mirrors, but phase- and amplitude-modulating elements can also be included to produce custom beams. For every custom beam new optics are required, and the resonator must be realigned, a process which can take several hours to days. The digital laser [1] is an innovation which allows the laser beam produced by a laser to be dynamically controlled by a computer. Essentially, one of the resonator mirrors is replaced by a spatial light modulator (SLM), which is a computercontrolled, pixellated, liquid-crystal device. While the concept is the device is simple, the implementation revealed subtle properties of spatial light modulators and the liquid crystals contained in them. These properties had to be well understood before their undesirable characteristics could be overcome, allowing the laser to function as conceived in the design.
In this paper we will outline our recent advances in all-digital control of light. Importantly, we will outline how to create a so-called “digital laser”, where a conventional laser mirror is replaced with a phase-only spatial light modulator. This allows the mirror properties to be dynamically changed by altering only the image sent to the device: on-demand laser modes. We demonstrate a myriad of laser beams that can be created from the same device without any realignment or additional custom optics.
In this paper we outline a simple laser cavity which produces customised on-demand digitally controlled laser modes by replacing the end-mirror of the cavity with an electrically addressed reflective phase-only spatial light modulator as a digital addressed holographic end-mirror. We show that on-demand digitally controlled laser modes are possible by changing the phase and amplitude of the computer generated hologram in a form of a grey-scale image on the holographic mirror. We demonstrate that customised digitally controlled laser modes can be generated on-demand by switching to several different spatial modes in real-time with the first the ‘digital laser’.
Porro prism lasers are insensitive to misalignment caused by, for example, shock and temperature variation, making
them useful in field applications, for example in target designation and range-finding systems. This property is a result of
the property of Porro prisms that they return a reflected beam parallel to the incident beam, regardless of any tilt on the
prism. These lasers are generally used in a marginally stable or unstable configuration for low divergence, but in the
stable configuration some interesting kaleidoscope modes can be modelled. In previous work on Porro prism resonators
we formulated an analytical method of determining which Porro angles resonate and result in petal output modes, as well
as the corresponding number of petals. This work has been verified using a numerical model as well as experimentally.
We have developed this work further and have investigated the losses associated with a range of Porro angles as well as
the effects of these losses on the resulting modes. We conclude by summarizing the design considerations for Porro
prism lasers.
Recent approaches to demonstrating adaptive optics and atmospheric turbulence have made use of spatial light
modulators (SLMs) as the active phase element. However, there are limitations in using SLMs as an accurate method of
simulating turbulence phase screens. In this work we investigate the limitation of laser beam shaping with a phase-only
spatial light modulator for the simulation of dynamic and pseudo-random turbulence in the laboratory. We find that
there are regimes where there are not sufficient pixels to resolve the phase. At the higher end of this range, at strong
turbulence levels, the zonal regions are tightly packed. This leads to two simultaneous effects: a phase screen with low
efficiency in some regions, and a modified turbulence structure due to the shifting of the zone peaks. These amplitude
and phase distortions have a deleterious effect on the accurate simulation of the turbulence. At the lower end of the
range, at weak turbulence, the phase change is too small to describe with sufficient grey scale levels, since the full 256
levels are associated with a full 2π phase shift. Further limitations include the frame rate of SLM for time-evolving
turbulence. We show experimental results demonstrating these limitations, and discuss the impact this has on
simulating turbulence with SLMs.
A simple model of a Porro prism laser resonator has been found to correctly predict the formation of the "petal" mode
patterns typical of these resonators. A geometrical analysis of the petals suggests that these petals are the lowest-order
modes of this type of resonator. Further use of the model reveals the formation of more complex beam patterns, and the
nature of these patterns is investigated. Also, the output of stable and unstable resonator modes is presented.
A model to describe the transverse field distribution of the output beam from porro prism resonators is proposed. The model allows the prediction of the output transverse field distribution by assuming that the main areas of loss are located at the apexes of the porro prisms. Experimental work on a particular system showed some interested correlations between the time domain behavior of the resonator and the transverse field output. These findings are presented and discussed.
Laser brightness is a parameter often used to compare high power laser beam delivery from various sources, and incorporates both the power contained in the particular mode, as well as the propagation of that mode through the beam quality factor, M2. In this study a cross Porro prism resonator is considered; crossed Porro prism resonators have been known for some time, but until recently have not been modeled as a complete physical optics system that allows the modal output to be determined as a function of the rotation angle of the prisms. In this paper we consider the diffraction losses as a function of the prism rotation angle relative to one another, and combine this with the propagation of the specific modes to determine the laser output brightness as a function of the prism orientation.
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