Scientific laser users have long relied on state-of-the-art performance (e.g., higher peak power, shorter pulse duration, wider wavelength tuning) to achieve groundbreaking results. Unfortunately, this high performance has often been emphasized at the expense of ease-of-use and reliability. Recently, however, this paradigm has dramatically changed, and some of the latest scientific lasers — including complex ultrafast amplifiers — now deliver both cutting-edge performance and exceptional reliability. This advance is sometimes referred to as “The Industrial Revolution in Ultrafast Science.”

Designing Reliability – Back to the Future

Figure 1. Conceptual representation of vibration and temperature profiles for HALT/HASS test in relation to operating and non-operating environmental condition for a given product.

While 100% reliability is the goal of many manufacturers, in reality, some failures will occur with any new product. There are two primary reasons for this. First are inherent weaknesses in the way the product and/or its key components are designed and engineered. Second are manufacturing errors. These can be systematic problems, or just a combination of random statistical deviations in various production processes. Engineers have long recognized these problems and sought methods to address them. In some industries, like aviation or medical equipment, in particular, the focus has been to identify and eliminate potential failures, before they occur.

The most direct approach to identifying failure sources is simply to put products out in the field and wait for them to fail. Needless to say, this isn’t a particularly effective tactic for companies concerned about their customers or their own reputations.

Another — and just as impractical — approach would be to manufacture products in their final form, and rather than delivering them to customers, continue to operate them under standard operating conditions and track their failures — a process that could take years before acquiring any statistical significance.

Figure 2. To enable high-density HALT/HASS testing, Coherent invested in a large (Qualmark Typhoon) chamber capable of accommodating even a complete ultrafast amplifier weighing hundreds of pounds.

But what if there were a way to put products out in the field long enough for all potential design and production weaknesses to reveal themselves, and then to travel back in time and redesign the product so that these weaknesses are eliminated beforehand? While actual time travel isn’t possible, in essence, it’s the underlying concept behind a set of protocols called Highly Accelerated Life Testing and Highly Accelerated Stress Screening (HALT/HASS).

HALT/HASS have proven themselves extremely effective in numerous other industries where failure is simply unacceptable. These accelerated testing methods serve to effectively compress time by orders of magnitude. This makes them particularly useful in cases where waiting for customer feedback is not a practical option because of low production volumes or other factors.

HALT – Highly Accelerated Lifetime Testing

HALT is a method where simultaneous stressors, typically but not solely temperature and vibration, are applied during the product design phase to greatly accelerate normal aging. To provide ample design margin, the HALT conditions should be more extreme than the specified operating and non-operating limits. HALT deliberately drives a product to failure in order to identify, and then eliminate, its weak points. Iterative cycles of HALT testing, re-design, and further HALT testing serve to eliminate any weakness or potential failure mechanism in the final design. The resulting final product will then reliably withstand stresses beyond its specified level.

HASS – Highly Accelerated Stress Screening

Figure 3. Some HALT results. Comparison of an earlier version (top) of optical mounts with the final version (bottom) used in a femtosecond laser manufactured at Coherent. The final version is much more resistant to externally induced vibration and temperature change cycles.

HASS is used to screen production units for any manufacturing weaknesses or errors. Once the overall design for the product has been “frozen,” but before starting the standard production cycle, an appropriate HASS test protocol is defined and implemented. These stresses are not as high as during the HALT phase because the goal is to reveal workmanship and material issues, without compromising the lifetime and performance of each tested unit (Figure 1).

At first, HASS is performed on each manufactured unit, verifying that there are only negligible variations in performances before and after the HASS test cycle. The analysis of failed units determines the weakness and provides paths to correct them, be it a material quality issue or a workmanship error. Once a sustainable, high pass rate is achieved, HASS is then performed on a statistically significant sample of production units throughout the lifetime of the product.

Practical HALT/HASS

Implementation and Immersion Because HALT/HASS had never previously been used in the laser industry, we had to first determine how best to implement HALT/HASS at Coherent. We partnered with Qualmark, a recognized expert in HALT/HASS, with a long history of successfully providing solutions in closely related industries, such as consumer electronics, medical, and defense. Moreover, Qualmark is the chosen HALT/HASS resource for several leading companies in civil avionics, an industry with understandably high-reliability targets.

Very early on it became clear that Coherent would have to invest in HALT/HASS equipment in-house, for two reasons. First, experience from multiple industries confirms that HALT/HASS is most successful when it is an integral part of the design and manufacturing process. Occasional, out-sourced testing at third-party facilities will not deliver the same results as high-density testing and screening. Outsourcing also adds time and cost to any iteration design cycles.