laser design (original) (raw)

Author: the photonics expert

Definition: the design of a laser device, or the process of working out such a design

Categories: laser devices and laser physics, methods

DOI: 10.61835/2yw [Cite the article](encyclopedia%5Fcite.html?article=laser design&doi=10.61835/2yw): BibTex plain textHTML Link to this page LinkedIn

Summary: This in-depth article explains

• how a detailed list of design goals must be worked out,
• which types of physical effects typically need to be considered,
• what is needed for successful laser design,
• what is the role of design in a development project,
• how laser designs are derived from older designs, extending their performance, and
• that occasional design reviews can be useful.

The term design can have two different meanings:

• In some cases, it is meant to be a detailed description of a device. It may include e.g. used parts, how they are put together, and important operation parameters.
• In other cases, the term denotes the process leading to such a description.

This article discusses both aspects in the context of the design of laser devices, such as diode-pumped solid-state lasers, or similar devices such as optical parametric oscillators. A separate article on laser development gives additional information. It explains that a laser design (rather than a laser) is effectively the end product of laser development.

A laser design in the sense of a recipe for building such lasers is obviously indispensable for later successfully building lasers. Furthermore, depending on how the design is generated, the fabrication and commissioning of a laser may be efficient and reliable or time-consuming, costly and full of surprises. Obviously, the design process should be such that it minimizes cost and required time as well as various risks. That is not trivial to achieve, however; the article gives useful guidelines for four how to make laser design efficient and reliable.

Defining the Design Goals

Before a design is made, the design goals must be carefully evaluated. These should include not only the central performance parameters such as output power and wavelength; many more details can be relevant:

• optimum performance, e.g. in terms of output power, power efficiency, beam quality, brightness, intensity and/or phase noise, long-term stability (e.g. of the output power or the optical frequency), timing jitter, etc.
• compact and convenient setup, ease of operation (e.g. simple turn-on procedure, simple wavelength tuning, no need for realignment)
• maximum flexibility (e.g. for changing operation parameters)
• reliability, low maintenance requirements, simple and cost-effective error analysis, maintenance and repair
• minimum sensitivity to vibrations, temperature changes, electromagnetic interference, aging of components
• low production cost, i.e., a small number of parts, simple alignment and testing, avoiding the use of parts which are expensive, sensitive, or difficult to obtain

It is certainly advisable to work out carefully the list of these requirements for the particular case before investing any significant resources in laser development because it can easily be much more expensive and time-consuming to introduce additional properties into an already existing device.

Important Aspects of Laser Designs

The properties of the designed laser device are largely determined by the design details, not only by the parts used. Some aspects are particularly important:

• general design parameters, such as resonator length (influencing compactness, tuning issues, frequency stability, etc.), pump intensity
• selection of the laser gain medium (e.g. a laser crystal) and pump source, suitable choice of geometry (e.g. rod or thin disk, side pumping or end pumping), doping concentration, crystal length, etc.
• pump setup (e.g. for diode-pumped lasers), influencing output power and beam quality, long-term stability, and the ease of exchanging pump diodes
• optimum type of laser resonator (e.g. as linear or ring laser, monolithic or with discrete elements) and optimized resonator design, influencing aspects such as the number of parts, the output power and beam quality, alignment tolerances, sensitivity to thermal lensing, mechanical stability and drifts
• selection and placement of laser mirrors and intracavity components for wavelength tuning, generation of short pulses via mode locking, dispersion compensation, frequency stabilization, etc.
• mechanical housing, influencing mechanical stability, efficiency of cooling, temperature drifts, ease of maintenance, and safety issues
• electronic equipment, e.g. for stabilizing the output power, controlling the laser wavelength, monitoring the status of pump diodes or temperatures, ensuring safe operation
• proper documentation, including a part list (possibly with suppliers), mechanical designs, alignment and testing procedures, design ideas, possibly optional extensions and limitations for modifying operation parameters

This list, which is certainly not yet complete, shows that proper laser designs are not a trivial matter, but are essential for achieving full customer satisfaction, cost efficiency, and flexibility for future developments.

What is Needed for Designing Lasers

Designing a laser is a challenging task. The following are definitely required:

In this video, we explain how simulations can make R & D more effective.

• a detailed understanding of the requirements, which may include some understanding of the application
• a detailed understanding of all the relevant laser physics effects, such as laser amplification, thermal lensing, resonator modes, laser noise, etc., and their interaction
• the essential data, e.g. of laser crystals
• the ability to reduce the complexity to a practical level without losing important details
• flexible software for calculations and simulations, in particular for reducing the resources required for development cycles
• practical experience with lasers, enabling one to recognize typical problems, correctly interpret experimental observations, etc.

Role of a Design in a Development Project

A laser design is the essential product of a development process. In addition, it plays a key role in the development. One starts with a first design idea and refines the design more and more until it is proven to work.

The resulting design should be carefully documented, as otherwise one creates a risk of losing a lot of potentially valuable information while saving only a minor amount of time at the moment.

In any non-trivial design project – and laser design projects are hardly ever trivial – it is very advisable to attribute a vital role to the laser design:

• It is normally not possible to directly arrive at a fully satisfactory design. Rather, one starts out with a simple design idea (or sometimes with an older design, see below) and develops it in a sequence of cycles involving testing, analysis and refinements. That is explained in the article on laser development.
• These cycles should be carried out as far as possible in the office rather than in the laboratory, because that way they will be far faster and cheaper. In simple cases, one can work with a set of calculations of key parameters, ensuring that all of them remain reasonable. Often, however, laser modeling and simulation is indispensable for sufficiently good testing.
• In the end, a design will basically always need to be tested on a physical prototype. As that stage is expensive, it should begin only after completing careful work in the office, so that multiple iterations in the lab are avoided.

A proper design is not just a set of ideas, but a very specific description, including e.g. the list of required parts, a detailed prescription on how to put them together wherever this is not trivial, and is ideally supplemented by a description of the underlying reasoning, a discussion of limitations, etc.

For any future development of similar kind, the carefully worked out design will be a very valuable input. If it does not exist, and particularly if in addition a vital person has left the company, future developments will be much less efficient.

Attempts to abbreviate this process carry the risk of obtaining reduced performance values and of large time delays due to unexpected technical problems. The later such problems are recognized, understood and solved, the larger can be the resulting damage.

Deriving Designs from Older Designs

In industrial development, it is common to derive some product design from an older design, rather than starting from scratch. Although this may appear to be very economical, there are significant risks, particularly in cases where the first design has not be properly worked out and documented in a process as described above. A central challenge is that modifying some detail of a laser design may easily have unexpected side effects, introducing new problems which then require additional measures, which again can have side effects.

For such reasons, starting with some initial design, which works e.g. with some lower than desired output power, can be helpful, but it still requires a detailed understanding of that design and its limitations. A proper design document for the initial design can make it easy to produce a whole family of designs, which differ in e.g. output power or pulse repetition rate.

Design Reviews

Under certain circumstances, it may be appropriate to make a review of an existing laser design. This can be the case, for example, when significant problems have occurred, or when the demands have increased and might be met with a revised design, rather than with a completely new one. If a properly documented design does not yet exist, it is high time to do this job; this process may already deliver important hints concerning what to improve.

More to Learn

Encyclopedia articles:

• resonator design
• lasers
• laser development
• laser modeling and simulation
• power scaling of lasers

Blog articles:

• The Photonics Spotlight 2006-07-05: “Laser Design: Side Product or the Basis of Laser Development?”

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