Second, for maximum effectiveness, HALT/HASS has to be fully embraced, from the component level up to the complete product. Coherent is a vertically integrated company where all critical components and sub-systems are manufactured in-house. Therefore, full implementation was clearly going to require significant, ongoing HALT/HASS activity — far more than could ever be realistically outsourced.
Years of experience with industrial lasers had taught us that optical alignment issues were the leading cause of laser unreliability, followed by optics failure, and then electronics failure. To screen for these failure mechanisms, Qualmark recommended the purchase of one of its high-performance Typhoon testing chambers. These are capable of accommodating even a complete ultrafast amplifier, and can subject it to extraordinary thermal swings accompanied by digitally controlled vibrations in three dimensions (Figure 2). With a temperature range from minus 100°C to plus 200°C, the entire chamber (even when filled with lasers) can be driven through a 100°C oscillation in less than two minutes. Additionally, the vibration platform can handle lasers or components totaling hundreds of pounds in weight. This platform is supported by 24 separate pneumatic actuators, enabling programmable and/or random three-dimensional vibrational sequences with accelerations up to 70 g.
Sample HALT/HASS Results
The following examples demonstrate just a few of the results that have been achieved by applying HALT/HASS protocols from the component through the system level.
High-Stability Optical Mounts. Figure 3 illustrates the successful use of HALT methodology in the design of optics mounts for a femtosecond laser. In the first design iteration, about 60 mounts were tested, and a significant population showed misalignment after extreme temperature and vibration testing. Moreover, the performance of this 60-unit sample covers a wide statistical range. As a result of these experimental findings, the mounts were redesigned. Repeated testing after each design cycle quantified the stability statistically. Figure 3 also shows the much improved stability and consistency from HALT tests of the final design iteration.
One-Box Ultrafast Oscillators. There has always been an accepted trade-off between short pulse width and reliability in ultrafast lasers. In fact, users report that ultrafast lasers from some manufacturers need optics cleaning and adjustment on a weekly interval or less. Figure 4 shows the end result on the first generation of broadband ultrafast oscillators, designed from the ground up using the HALT/HASS approach. This is a 12,500- hour run on the model Vitara-S, operated at constant diode pump power (i.e., without any light feedback loop), which indicates the absence of any degradation in the optical losses of the resonator components.
Integrated Femtosecond Amplifiers. Surely no commercial laser system is more challenging, in terms of stability and reliability, than a femtosecond amplifier. Moreover, today, many applications prefer a turnkey, one-box system with simple on/off functionality and reliability to pump various types of nonlinear processes (like parametric amplification or terahertz generation).
Figure 5 shows the HASS protocol used to test this first generation of ultra-fast amplifiers designed and manufactured completely using HALT. As can be seen in the figure, this protocol combines different intensity bursts of vibration, together with extreme temperature transients of 50 °C in 2-3 minutes. The system must show no significant change in output parameters at the end of these tests or it is rejected for shipping.
The end result for the laser user? As described in a recent case study, university researchers are successfully using one of these lasers to conduct two-dimensional spectroscopy in experiments requiring up to 48 hours of uninterrupted data acquisition.
The quality, consistency, and throughput of experimental data are essential contributors to a team’s or individual investigator’s success. Laser performance and reliability directly impact these, for example, by delaying the acquisition of data needed for a conference paper or time-critical grant proposal. In the case of tenure track, this data can even make or break a scientist’s academic career. While scientific laser users have long accepted a tradeoff in performance versus reliability, the implementation of design techniques already used in industrial lasers, together with HALT/HASS protocols, have now eliminated this compromise.
Leveraging Industrial Laser Experience
Coherent’s success with implementing HALT/HASS with scientific lasers was also facilitated by our experience in manufacturing industrial lasers, where the economic impact of downtime and higher product volumes had already served to drive high reliability. In fact, while HALT/HASS provides a method for improving product design and manufacturing methods, it also needs a good starting point. Our scientific laser team was able to draw on the expertise of engineers working with Coherent industrial lasers, already well-versed in good engineering design practices and knowledgeable in materials and production methods needed for long, maintenance-free lifetimes.
A typical example is understanding and avoiding cavity contamination to ensure long optics lifetimes, without frequent cleaning. For example, some of the degradation effects of materials subject to intense nanosecond UV laser pulses with high average power are similar to the ones resulting from femtosecond near-IR pulses where multi-photon effects play a role. Although a nanosecond UV industrial laser may be quite different from a femtosecond laser for a spectroscopy lab, the challenges in controlling degradation mechanisms may be surprisingly similar.