Development, Production and Characterisation of Laser-Machined Micro-Optics

Lead Research Organisation: Heriot-Watt University
Department Name: Sch of Engineering and Physical Science

Abstract

Abstract: Micro-optic elements are widely used for corrective optics of high power laser diode arrays (HPLDAs). HPLDAs are use as the pump source for solid state lasers (SSLs) which are utilized in material processing applications. The continued development of these micro-optics is vital to the successful implementation of SSLs.

Introduction: Fabrication of micro-optics by CO2 laser ablation has allowed for rapid-prototyping of high quality, cost-efficient optics [1]. Further developments in design and fabrication of these optics is required for competitive advantage and to solve problems in optical design. In particular, the far field beam brightness of HPLDAs is reduced by diffraction, aberrations and other effects.To counter this, corrective optics for coupling HPLDAs to the gain medium of SSLs have been developed. Radiation from the diode aperture diverges quickly along one axis (fast axis) and slowly along another (slow axis). To remove this effect a cylindrical lens is introduced to collimate the fast axis, however this introduces aberrations into the slow axis and reduces the beam brightness. This is known as the "bow-tie effect" and is shown in figure 1b. The effect has been confirmed experimentally and currently limits beam brightness. Further development is required for its mitigation and removal.

Objectives:
-Research different corrective optics and ways of achieving fast axis collimation
-Experiment with different combinations of corrective optics to reduce unwanted effects
-Design, fabricate and implement further corrective optics

Methodology: Further characterisation of some unwanted effects may be necessary since some are not well documented. Ray tracing and diffractive optics will be used to model effects and corrective optics; however, experiment is needed as confirmation. The first phase of work will be a characterisation of the HPLDA beam if not done already, then characterisation of corrective optics to fully understand from where effects and aberrations arise, then alterations to the system can be modelled and tested experimentally. These phases will be independent from each other, however may be repeated. There is a risk that it may not be possible to remove effects, however, it may be useful to determine precisely why.

Conclusion: Project success would ultimately be represented by a significant improvement in HPLDA beam brightness and would contribute to the development of SSLs. However, if more understanding of these systems was achieved without fully resolving the problem of beam brightness then the project could still be considered successful to some degree.

Publications

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Studentship Projects

Project Reference Relationship Related To Start End Student Name
EP/R512539/1 01/10/2017 30/09/2021
2112222 Studentship EP/R512539/1 18/09/2017 17/09/2021 Paul Harrison
 
Description Wavefront detection has been achieved for the first time for a source with a wavelength of 2-micrometers which enables the fabrication of correction optics.
Exploitation Route The findings will produce better beam quality from 2-micronmeter sources making them more applicable for industrial, medical and defence applications.
Sectors Aerospace, Defence and Marine,Digital/Communication/Information Technologies (including Software),Electronics,Manufacturing, including Industrial Biotechology,Pharmaceuticals and Medical Biotechnology

 
Description The findings are to be integrated into a plan to develop a service to charactorise the wavefronts from 2-micrometer sources.
First Year Of Impact 2020
 
Title Empirical Modelling of Lens Fabrication Error 
Description An algorithm was created for correction of products commonly made at PowerPhotonic Ltd. The algorithm, which is empirically based, was based on a series of test parts which are representative of certain products commonly made at PowerPhotonic for commercial sale. The intention of the algorithm is to allow for the automatic correction of error associated with fabrication. Successful implementation of the algorithm reduces product development time and material cost, as well as increasing productivity. 
Type Of Material Computer model/algorithm 
Year Produced 2019 
Provided To Others? No  
Impact In the analysis of test parts a great deal of insight has been gained into the capability of PowerPhotonic's fabrication process. Geometrical factors and process conditions have been studied and served as a guide for product development, as well as making a significant contribution to the company's capability map. Success of explicit testing of the algorithm may result in huge gains for the company. 
 
Description Collaborative Work With Heriot Watt University To Develop a Wavefront Sensor for Measurements of 2-micrometer Wavelength Sources and The Development of a Novel Stable-Unstable Hybrid Laser Resonator 
Organisation Heriot-Watt University
Country United Kingdom 
Sector Academic/University 
PI Contribution Manufacture of toroidal and cylindrical mirrors for a novel stable-unstable hybrid laser resonator (a quotation has been issued). Equipment has been loaned to Heriot Watt. Software provided which contributed to the PhD thesis of a member of Daniel Esser's group at Heriot Watt. Draft drawings made which contributed to the master thesis of a member of Daniel Esser's group at Heriot Watt.
Collaborator Contribution Clean room facility, office space, lab equipment and a locker.
Impact Development of a wavefront sensor which measures sources at a wavelength of 2-micrometer.
Start Year 2018
 
Title A Linear Algebra Based Computation for Calculating Large Numbers of Refracted and Reflected Rays 
Description This model allows the user to easily calculate the directions of a large number of optical refracted and reflected rays from a surface. The user defines the initial rays in the form of normalised vectors at the surface and then passes the rays into a simple function which then calculates their directions after refraction and reflection by the surface (this is sometimes referred to as optical caustics). 
Type Of Technology Physical Model/Kit 
Year Produced 2018 
Impact The model has been used in the PhD work of a member (Daniel Morris) of Daniel Esser's group at Heriot Watt. It was possible to analyse a total internal reflecting laser resonator using the model and this will now form part of Daniel Morris's PhD thesis. 
 
Title A Wavefront Sensor for Sources at 2 Micrometer Wavelength 
Description A wavefront sensor for sources at 2 micrometer wavelength was developed at Heriot Watt university with Daniel Esser's group. The purpose of the sensor is to measure the wavefront of collimated diode sources which are often used in pumping applications of solid state lasers as well as other applications. These sources are fabricated in bars or stacks and suffer from fabrication errors such as smile and other effects which degrade the beam quality. Using an appropriate wavefront sensor one can make a phase plate which reduces those degrading effects. Relatively recently, these sources have become more numerous in the 2 micrometer wavelength region due to a growing number of applications at that wavelength. The wavefront sensor developed here is the first such instrument to operate at 2 micrometer wavelength and using the data it obtains together with the fabrication techniques developed at by a commercial partner, a correction phase plate can be made to improve the quality of collimated beam. Ten-fold improvements have been achieved in applications at other wavelengths, which equates to a huge improvement in the efficiency of these sources and is often critical for applications. This wavefront sensor is based on the technology presented in the link provided. 
Type Of Technology Detection Devices 
Year Produced 2020 
Impact Recently, the first ever wavefront measurements of a 2-micron source have been made using this instrument. The wavefront sensor is being developed together with a commercial partner and is intended to become part of a future service for wavefront corrections at 2 micrometer wavelength. The wavefront sensor currently being used to provide corrections to prototype diode source being for another commercial partner. 
URL http://www.powerphotonic.com/company/news/unique-high-power-wavefront-sensor
 
Title Diffractive Optics Propagation for Beam Shaping Elements 
Description A Matlab based implementation of diffractive (sometimes called physical or wave) optics propagation was developed and is in use by PowerPhotonic Ltd. The primary use is to propagate a laser beam profile (i.e. intensity and phase) to the focus of a lens, to an intermediate point or through focus. This allows the user to calculate quantities such as beam size and power in the bucket values from beam shaping optics both in focus and out-of focus without extensive functional testing. This is often required to integrate beam shaping elements into a larger optical system. Optical modeling packages such as Zemax can perform these calculations. However, Zemax acts as a black box for this purpose and it has been difficult to obtain consistent and reliable results, therefore this model was developed and shows far superior results. 
Type Of Technology Physical Model/Kit 
Year Produced 2018 
Impact This model has been used in determining beam size at intermediate points in an optical system as part of the analysis of a beam shaping element made at PowerPhotonic for a customer. Another notable use is in the analysis of axicons.