Laser Development

This article discusses the process of designing laser devices, and is also applicable to related types of photonic devices, such as optical parametric oscillators or supercontinuum sources. It does not focus on specific technical details, but rather addresses the more general issue of designing an efficient development process.

A well-designed laser development process is important for both industrial lasers and scientific lasers. In the latter case, one may encounter more demanding performance requirements, while the requirements may be more relaxed in several other respects. For example, industrial lasers tend to have a stronger focus on reliability, which is generally required for industrial laser applications, such as laser material processing.

A Systematic Approach to Laser Development

Unsystematic approaches to laser development can fail for many reasons. A systematic approach, as described in the following, can be substantially much more efficient in terms of time and resources.

Assessing Requirements

The first step is to carefully assess what exactly is needed. It is important to include all of the requirements at this stage, as it can be much more expensive to introduce additional laser features later.

As an example, consider a pulsed laser for an application where precise pulse timing is critical. If this requirement is overlooked at the outset, the worst that can happen is that a technological approach is chosen that makes it very difficult to achieve the required timing accuracy. One may then be forced to either implement costly additional measures or change the technological approach at a later stage.

When a development team obtains just a short list of technical specifications from the management, that usually cannot be considered as complete. There may well be additional requirements which just have not yet been anticipated, and are revealed only within a careful dialog between technical experts and the management, also possibly involving potential customers.

For example, an ultrafast amplifier system may not only have to reach a certain pulse energy and pulse duration, but also have to ensure that no significant optical energy is delivered before the actual pulse: some applications can be severely affected by even very weak pre-pulses that may somehow precondition the targeted samples. Furthermore, certain types of amplifier systems have a tendency to cause such problems, and may require at least additional measures to keep them under control. An experienced person may detect such potential problems early, and that may be vital for the development project.

Suitability of Technology

The second step is to verify the overall appropriateness of the technology being considered, and perhaps to compare the chosen approach with competing options. This step is critical because mistakes made at this stage will affect the entire development process. An experienced laser engineer is therefore essential.

For example, there are many cases where a given laser could be realized either as a bulk laser or as a fiber laser. Both technologies have specific advantages and limitations, the relevance of which depends on the specific case.

Such a decision may depend not only on the requirements in the current case, but also on the expected requirements for further devices. It can be problematic to choose a technology that just meets current needs, but has insufficient potential for future needs; ease of upgrading to higher levels of performance may be important.

For example, very few technologies for high-power lasers are power-scalable in a meaningful sense. It is then often increasingly difficult to push the power limits any further. Power scalability will make such extensions much easier; there is then (by definition) a clear power scaling procedure that can be applied.

Working out a Prototype Design

The next step is to work out a sufficiently detailed laser design that can be used to build a prototype. This process involves quantitative checks of various issues before any lab work is done.

For example, if a passively mode-locked bulk laser is developed, the following aspects must be clarified:

• What is the required pump intensity and the laser gain (at different wavelengths) in the context of the threshold pump power? That determines details like the used pump optics.

• How does the pulse formation work? Understand the balance of chromatic dispersion and nonlinearity in the resonator, and the role of the saturable absorber (often a SESAM). Find suitable parameters like an appropriate modulation depth and the degree of absorber saturation.
• What is the expected alignment sensitivity of the laser resonator? Is it acceptable for a practical laser device, or does the resonator design need to be optimized further in that respect?

It is not the right approach to leave some of these questions open, so that problems are to be discovered only later when testing the prototype. For example, when the strength and robustness of pulse shaping in the laser turns out to be inadequate in experiments with the prototype laser, one will need to go back and study this in order to work out an improved prototype design. It is more efficient to clarify this issue beforehand, i.e., before ordering the parts and doing any lab testing.

Are Simulations Required?

In some cases, a question of interest (for example, on the robustness of pulse generation) can be answered to a sufficient extent by calculating a few key parameters and checking whether their values are within a reasonable range. For example, the total nonlinear phase shift per resonator round trip in a quasi-soliton-mode-locked bulk laser (which can be estimated easily) should be large enough to have sufficiently strong soliton pulse shaping effects, but also low enough to avoid certain instabilities.

However, such a simple check will have limited value for a mode-locked fiber laser, which often involves far more complex pulse shaping mechanisms. One will then need numerical simulations of the evolution of pulses (intracavity and over many round trips) with suitable laser modeling and simulation software. With that, one can quickly identify problems at the very early stage, and solve them quickly and with low cost. The resulting laser design should then safely avoid a number of foreseeable problems.

If the team does not have anybody who can perform that task, an external consultant will be needed. The expenses for that are generally lower than those by caused by additional iterations of the later testing and design refinements.

The “deliverable” of this design phase is a clearly written design report, containing not only all the parameters of the proposed prototype, but also a clear description of the reasoning behind the design, and possibly the likely limitations, some warnings, the list of required parts, and so on.

Building and Testing the Prototype

Once the prototype design is in place, the necessary parts can be ordered and the prototype can be built according to the detailed specifications in the laser design. Obviously, it would be inefficient to begin this phase without a proper prototype design, for example, because additional components may need to be ordered during the lab testing phase (causing additional costs and delays), and because various types of problems would be more difficult and costly to identify and resolve.

Testing of the prototype may still reveal some unforeseen problems, depending on the level of care and experience applied in the design phase. The design should be revised accordingly after the sources of problems and appropriate solutions have been identified.

Optimizing a design with further changes based on experimental tests in a trial-and-error process is often very time-consuming. For example, redesigning a laser resonator often implies substantial changes. In addition, experimental results are often inconclusive or even misleading because measured performance values can be affected by poorly controlled influences such as non-ideal alignment, dirty components, or temperature changes. For these reasons, the effects of possible design changes can be evaluated much more efficiently and reliably on a computer using appropriate laser simulation software.

Finalizing a Product Design

For a variety of reasons, additional steps are often required to transform the prototype design into a final product design:

• Sometimes performance requirements change during development, or additional requirements are discovered.
• It may be discovered that some tolerances (e.g., concerning alignment) are still too tight for the final product.
• In general, industrial laser designs must be subject to stricter requirements than a prototype, e.g. concerning aspects like efficient production, sufficient robustness for practical applications up to harsh conditions, and overall convenience for the customers.
• You may want to minimize production costs by using fewer or cheaper components.

Again, one will need to evaluate the detailed effects of various design changes, which is often more time-consuming and less conclusive when done with experimental tests rather than a laser simulation model.

Internal Documentation

The entire design process should be carefully documented. For example, such a design report should include the following:

• the initial specifications and further detailed specifications developed during the design phase
• all relevant considerations in the design phase, e.g. what possible adverse effects have been considered, how they have been estimated, and what measures have been taken to deal with them
• a detailed description of the prototype design, and later the final design, including instructions on how to build it
• a comprehensive test procedure to be applied to the built devices

Such documentation will be very valuable in several situations – for example

• for analysis of problems discovered later and for working out necessary revisions
• for the next design tasks, where earlier mistakes can be avoided
• for the manufacturing and testing of lasers to be delivered to customers.

A common mistake is to view the laser design document as a byproduct of the development process, rather than its foundation. This easily leads to inefficient iterative steps in the lab, which can consume much more time and resources than the process of working out a detailed design at an earlier stage. Therefore, careful documentation must accompany the entire project from start to finish.

User Manual

Unlike the design document, the user manual can be written after all the prototype testing and refinement. Here it is an advantage to already have all the practical experience with the device. However, it is important that the author of a laser manual is able to fully recognize the needs of the end users who have less detailed knowledge of the technical details of the device. For the laser engineer, this aspect can be somewhat challenging, and it can be very beneficial to have it reviewed by people who are not as familiar with the details.

Advantages of a Structured Development Strategy

The benefits of this structured approach are many:

• Most importantly, the costly and time-consuming laboratory work can be done in the most efficient way. Compared to a trial-and-error approach in the lab, it is easier to identify problems, their causes, and appropriate solutions on paper and/or on a computer because this process is not limited by available diagnostics, access to internal parts of the device, or available alternative optical components.
• As a result, the laser product can be brought to market sooner, generating revenue earlier and positioning the company in the marketplace.
• The well-documented laser design helps the development team learn more about the technical details and is a very useful input for any further development – even if it is done by other people.
• Another benefit of a well-planned design process is that fewer parts are ordered that are not ultimately used.

More to Learn

Encyclopedia articles:

• laser design
• industrial lasers
• scientific lasers

Blog articles:

• The Photonics Spotlight 2006-07-05: “Laser Design: Side Product or the Basis of Laser Development?”
• The Photonics Spotlight 2006-12-09: “The Laser Industry – High Tech or Low Tech?”
• The Photonics Spotlight 2007-03-16: “Time To Market and the Economics of Laser Development – or How to Cause Great Financial Damage without Spending Money”
• The Photonics Spotlight 2007-11-26: “Solving Laser Problems Step by Step”
• The Photonics Spotlight 2008-02-14: “How Laser Development Can Go Wrong”
• The Photonics Spotlight 2009-06-29: “Doing Things Properly: It's the Economy, Stupid!”
• The Photonics Spotlight 2010-07-12: “Laser Development: Get an Expert Early on!”