Laser design

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, important operation parameters, etc. In other cases, the term denotes the process leading to such a description. This article assumes the first mentioned meaning and discusses some important aspects for the design of laser devices, such as e.g. diode-pumped solid state lasers, or similar devices such as e.g. optical parametric oscillators. A separate article on laser development gives additional information, in particular on the role which a laser design plays within the process of laser development, and how this process can be optimized.

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., low 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 carefully work out 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

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

• general design parameters, e.g. resonator length (influencing compactness, tuning issues, frequency stability, etc.), pump intensity, etc.
• selection of 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, with monolithic or with discrete elements) and optimized resonator design, influencing aspects like 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 and 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.