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Laser DesignAskRP Photonics for working out an optimized laser design as discussed above. Powerful

numerical software is available for such purposes, e.g. for the design of resonators with

minimum sensitivity to misalignment and beam distortions.

The term design can have two different meanings. In some cases, it is meant to be a detailed

description of a device, including 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 some important aspects for 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.

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:

In Short

Details of the design of a laser product can have a strong impact on its performance, reliability,

flexibility, and manufacturing cost. It is essential to be aware of all the design goals, to know the

design properties required for reaching them, and to use a proper laser design (which is worked

out at the beginning) as a vital part of an efficient development process.

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, simplewavelength 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, avoidingthe 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.

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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, tuningissues, frequency stability, etc.), pump intensity

selection ofgain medium (e.g. a laser crystal) and pump source, suitable choice ofgeometry (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 oflaser resonator (e.g. as linear or ring laser, with monolithic or withdiscrete 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 oflaser mirrors and intracavity components for wavelengthtuning, generation of short pulses via mode locking, dispersion compensation, frequency

stabilization, etc.

mechanical housing, influencing mechanical stability, efficiency of cooling, temperaturedrifts, ease of maintenance, and safety issues

electronic equipment, e.g. for stabilizing the output power, controlling the laserwavelength, 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:

a detailed understanding of the requirements, which may include some understanding ofthe application

a detailed understanding of all the relevant physical effects, such as laser amplification,thermal lensing, resonator modes, laser noise, etc., and their interaction

the essential data, e.g. oflaser crystals the ability to reduce the complexity to a practical level without losing important details flexible software for calculations and simulations practical experience with lasers, enabling one to recognize typical problems, correctly

interpret experimental observations, etc.

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It is clear that software alone is by no means sufficient to work out good laser designs.

Role of a Design in a Development Project

It is common practice, but nevertheless generally not advisable, to consider a laser design as a

result of a development process which is largely based on trial and error. The design then plays aminor role, just summarizing the results of a lengthy process. In such cases, the design is often

not even properly documented, which 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 trivialit is very

advisable to attribute a vital role to the laser design:

The laser design is made in the office, not in the laboratory, and properly deals with allknown issues which can be or become relevant. Although this process can be much faster

and cheaper than a trial-and-error approach in the lab, it takes some considerable

discipline and of course requires a comprehensive expertise. A proper design is not just a set of ideas, but a very specific description, including e.g.

the list of required parts, a more or less 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.

The prototype is then fabricated according to the design, and not vice versa. This greatlyspeeds up the fabrication, thus making efficient use of costly laboratory resources.

For any future development of similar kind, the carefully worked out design will be avery 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 oflarge 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 appears 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.

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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.