Manufacturers go through a lot of trouble to design and build laser rods that can withstand the energy and power levels of their typical application environments. Despite all their precautions, there is a probability that some damage will occur to laser rods during normal operation.

Laser damage may be driven by adsorption or by dielectric breakdown. The type of damage that a laser medium is prone to will depend on material factors like specific heat, absorption coefficient, melting temperature, defects leading to scattering, geometric properties as well as properties of the laser beam. 

Damage can occur to the rods during operation in a few different ways:

The rod ends become contaminated and the laser beam damages the rod ends. 

In the laser industry, it is common to attach a laser rod tube to the end of a laser rod. This joint must be tightly sealed before the rods can be water-cooled as solid-state lasers usually are. Standard seal materials like 0-rings (made of elastomers) aren’t resistant to laser damage, but Teflon (PTFE) is. However, the seals must be properly installed. If there is a seal failure, the laser rod faces could get contaminated. Since the rod ends are exposed to concentrated laser beams, they are most vulnerable to damage. 

A focused beam is reflected back onto the rod face.

If the rod ends are accidentally chipped, it could damage the AR (anti-reflective) coating at the ends. Even improper handling could damage the coating and lead to contamination. Not only will the laser efficiency go down in this case, but the ends will act as reflectors for the focused laser beam. Excess light reflected off the laser faces can cause laser-induced damage in applications. Light reflecting backward through the system can create ghost images. These ghost images undermine the stability of the system by letting undesirable light into the laser cavity.

The rod barrel becomes contaminated from the dirty or stale coolant. 

In solid-state laser systems, one of the most common reasons for damage by users is an operation with contaminated coolant. Dirty or stale coolant can contaminate the rod barrel. This can lead to the coolant getting burned onto the rod barrel. 

To avoid this, cooling systems should be drained, cleaned and refilled at regular intervals. The recommended maintenance frequency is once a month. The cooling fluid should be replaced and filters should be replaced. The cooling fluid filter should also be inspected once a week for any sediments or color changes. A faulty filter can be a source of contamination, causing damage to the laser rods. 

The laser rod must also be monitored for signs of contamination such as a loss of power or an increase in current needed to reach the rollover point at the time of optimization. 

If the rod runs without coolant the rod can get fractured.

Running the laser system without coolant is also a common reason for laser rod damage by users. The temperature has a great impact on the performance of a laser system. Mid-power and high-power laser systems generate more energy, and as a result, higher heat. A laser cooling system helps to remove heat from the system and keeping the temperature at a tolerable level. 

The coolant is usually water or a solution of water and glycol. This solution transfers heat to and from the water chiller, with the help of a pumping system or reservoir. This water chiller prevents the laser system from overheating, allowing it to operate for long periods of time. 

Running the system without a coolant exposes your laser system to the risk of overheating. When heat isn’t moved from the sensitive areas of the laser system, you may see irregular results. If the heat isn’t removed fast enough from the rod, the host can overheat and distort the crystal’s optical quality or fracture. 

If too much current is applied to the drive, the rod can get fractured.

Laser systems are sensitive to excessive current levels, electrostatic discharge and current spikes. Too much current can start by reducing output power, shifting threshold current and changing the beam divergence. It can also affect the focus of the laser and ultimately lead to failure to lase. But in extreme cases, a laser can be catastrophically damaged by the excessive current to the drive. This can lead to fractures or pits and droplets in the laser surface. 

If the duty cycle is set too high the rod can get fractured.

A duty cycle is the percentage of time a laser is on in a given command signal. For instance, a 50% duty cycle means the laser fires half the cycle duration and it’s off in the other half. 

Pulsed lasers are duty cycle-limited, achieving short bursts of high peak power. Continuous wave lasers tend to have duty cycles of 100%, as they are continuously on. 

Setting the duty cycle of a laser too high in a laser system may fracture the rod. Duty cycles can be changed by changing pulse width. This characteristic is related to the allowable pumping power. Increasing the duty cycle lowers allowable pumping power. Correctly selecting pulse width, duty cycle and period is necessary to maintain maximum output laser power while avoiding laser rod fracture. 

The Bottom Line

In general, laser rods need to pass a standard QA process (MILL-STD-105D) where a random 20% of a batch is tested before approval. Lasers that pass the test are likely to be reliable under ideal conditions. 

But rods have “life expectancy” which is measured in working hours. Under ideal conditions and if they’re used as recommended, lasers can enjoy lifetimes of over 100,000 working hours. In order to optimize a laser through its lifetime, it’s important to understand laser damage caused through operations. Protecting laser systems from such damage will save potentially expensive repairs later. 

An understanding of laser damage is nearly not as common in labs as it should be. It is important to note that improper handling, dirty coolant (or no coolant), and a disregard for the laser’s material properties and duty cycles can all lead to laser damage.