A new feature of the modern high-powered laser is the need to transmit various wavelengths through fiber optics. Fiber optics have emerged as the primary method for transmitting laser light due to its ease of setup and disconnection. Moreover, it safeguards end users from light exposure or eye contact, as the light is conveyed through an enclosed conduit.
“I would estimate 60 to 70% of our customers today are asking for the light to be delivered through fiber, and it is quickly becoming a requirement for university researchers and OEMs that integrate high powered lasers into a larger system,” said Dr. Mark Little, Technical and Scientific Marketing Consultant for Carlsbad, CA-based OPOTEK LLC, a global manufacturer of tunable lasers that can deliver wavelengths from deep UV to mid-infrared.
Despite the interest in fiber optics, Dr. Little says there remains a lack of comprehensive understanding regarding the potential utilization of fiber as a conduit for high-intensity laser light. “Some of our customers do not think it is even feasible to use fiber with the type of high power tunable lasers that we manufacture. Some users purchase conventional patch fibers and end up damaging it [due to the intensity/power of the light] at great cost.”
In fact, it is critical to select the right type of fiber material to transmit the light without damage, as well as have other design features that promote safety. With high powered lasers, this often means involving a custom fiber optic supplier that can recommend a specific solution optimized for the wavelength and intensity of the laser that will be utilized.
Fiber delivery
The OPO lasers that OPOTEK produces are the perfect example. Among the most flexible pulse-based lasers are Optical Parametric Oscillators (OPO) lasers that can be “tuned” to a wide spectrum of specific wavelengths. OPOs are utilized in sophisticated test and measurement applications such as mass spectrometry, photoacoustic imaging and spectroscopy.
Pulse-based OPO lasers deliver concentrated bursts of energy, measured in megawatts, in short durations measured in nanoseconds. According to industry safety regulations, OPO Lasers fall under Class 4 due to their capacity to cause significant eye and skin damage, as well as pose a fire risk.
One way to avoid these safety concerns is to transport the light through an enclosed conduit, in this case fiber. Fiber also resolves another issue common to OPOs and other high-power lasers, specifically the need to precisely align and focus the light from the laser source so it reaches its destination. This often involves a complex alignment process, and the light may need to be adjusted or redirected through multiple mirrors.
“Because OPO lasers were quite large in the past, moving and refocusing the laser so it hits the target took some time and adjustment,” explained Little. “Often this means adjusting mirrors so the light hits a target a few meters away, often at 45- or 90-degree angles to direct the light to the target.
“With fiber optic cable, you simply push one end into the port and the other on the output side and it is automatically aligned. Because it is inherently flexible, it can be moved, coiled or relocated easily.”
Despite the numerous advantages, the selection of the right type of fiber for use with high power lasers is critical.
Custom solutions are often required because certain wavelengths can trigger photochemical reactions in optical materials, changing their molecular structure or chemical composition and making them less effective. Prolonged exposure to high-intensity laser light can cause various forms of damage, including non-linear effects [unwanted wavelength generation], photodarkening, photobleaching and thermal damage. Protective coatings can also become compromised at specific wavelengths. For instance, UV light can cause photodegradation of coatings, reducing their protective properties.
Custom Fiber Solutions for High-Powered Lasers
To specify the ideal fiber for an application, OPOTEK works with its customers to define the parameters and then may partner with leading fiber suppliers like Armadillo SIA. Founded in 2018, the company specializes in silica optical fibers, specifically multi-mode, step index fiber in a variety of wavelengths, from the UV to the near infrared.
Armadillo SIA almost exclusively provides custom solutions with capabilities in fiber construction, multi fiber and bundles with custom active areas. Although the company holds a large inventory, it can also perform custom fiber draws to achieve specific core and cladding diameters.
The design of multi-mode step index optical fibers makes it well-suited for high-bandwidth applications over shorter distances, though there are limitations in terms of signal quality over long distances.
Multi-Mode fibers can carry multiple light modes simultaneously. In multi-mode fibers, the core is relatively large (typically 50 or 62.5 micrometers in diameter), allowing multiple light paths or modes to propagate through the core. In a step index optical fiber, the core has a uniform refractive index that is higher than that of the surrounding cladding. This contrast causes light to be confined within the core, reflecting off the core-cladding boundary and propagating through the fiber. A coating often serves as an outer layer to protect the fiber and provide additional strength.
Armadillo SIA has consulted with OPOTEK over the past decade to provide support and custom solutions for various projects. Ilya Rotenstein of Armadillo SIA estimates they have produced more than 20 different fiber design iterations for OPOTEK’s OPO lasers, ranging from intricate mapped bifurcated array bundles [which help make the light more uniform at the ends and minimizes hotspots] to single channel, several millimeter spot-to-spot bundle arrays, to simpler single fiber, large core cables.
“This is a developing industry with a lot of new requirements and capabilities needed from the fiber optic side,” said Rotenstein “Every time we get a new project, there seems to be some new higher power, larger broadband signal that needs to be transmitted through fiber.”
Rotenstein adds that engineers often have a different definition of what constitutes “high power” with lasers, which can range from hundreds of milliwatts to kilowatts.
“In our initial consultation, we determine the wavelengths of the light source, whether it is continuous wave or pulsed, as well as additional factors. Then it is our job to propose the most suitable fiber type for the application,” said Rotenstein.
To endure the high peak power of our tunable lasers, there are two distinct methods for configuring the delivery: single fiber strands and fiber bundles.
Single strand, large core fiber
Single strand, large core fibers refer to optical fibers with a relatively large core diameter compared to standard options. These fibers are designed to handle higher power levels and can offer better performance in certain applications, even when the laser source is low energy.
The core often measures tens of microns in diameter up to a couple of millimeters in diameter. Because the beam diameter is typically larger than the diameter of the fiber itself, it must be focused using a specialized lens to fit into the small input diameter of the fiber.
“With a single core fiber, for example, the fill factor is going to be a hundred percent, so all the photons are focused and collated into the fiber,” explained Rotenstein. “You’re going to have a hundred percent transmission more or less to end to end, depending on length because there are some DB per kilometer drop off.”
For applications from approximately 10-100 watts, Armadillo generally recommends a specialized high-power connector. Although there are several options, the SMA connector is often specified for use with OPOTEK’s Opolette UX models.
High-power SMA connectors are specifically designed for high-power fiber delivery. Designed to maintain performance while handling increased power levels without degrading or damaging the fiber or connector. Armadillo prefers high power freestanding grade SMA connectors that are better at handling any heat generated due to a unique air gap between the metal of the connector and the glass. This keeps the metal from potentially ablating and getting all over the fiber and burning it, as well as preventing heat of the mechanical adhesive or polymers from burning up.
Fiber Bundles
While a standard patch cable with a large core may be sufficient for many high-power laser applications, a larger active area for the light source and fiber bundle may be required. Fiber bundles are made up of many thin, flexible strands of optical fiber made of glass or plastic, which combined is capable of transmitting light signals over long distances with minimal loss.
The process involves thermally fusing hundreds of these small-diameter single fibers. The fusion occurs at high temperatures, causing the fibers to melt slightly and bond together without losing their individual light-transmitting capabilities. Once fused, the fibers form a cohesive, thick bundle that can be handled and utilized as a single entity, though it maintains the individual paths of the original fibers.
By combining multiple fibers, the bundle can carry more data or power than a single fiber. Multiple fibers to prevent damage and manage heat dissipation.
Fiber bundles are more robust and flexible, suitable for various applications where single fibers might be too fragile. A larger diameter fiber, you can have a bigger beam and therefore you’re spreading the energy out over a larger area which does less damage.
Armadillo is one of a few companies in the industry that provides bundles with fused ends.
“The heat of high-power lasers at this interface of the bundle can cause adhesives to heat and lead to fiber damage within the bundles,” explained Rotenstein. “To avoid potential damage, we propose fused ends.”
Fused ends create a smooth, continuous transition between fibers, which also helps minimize optical losses and reflections. This is particularly important in high-precision applications like lasers, where maintaining signal integrity is crucial.
“Fused end [fiber bundles] will act as a single core and it increases the transmission approximately 20% or more in terms of throughput,” said Rotenstein. “It also improves temperature tolerances, so you can push more power into the bundles. The coupling efficiency, throughput and fill factors all show considerable improvements over the more traditional bundles where the fibers are set and cured with epoxy.”
Dr. Little of OPOTEK says all the company’s high energy lasers now come standard with a port specifically designed to accept fiber bundles. Interlock micro switches on the port are also available, ensuring that when the fiber is removed the laser light immediately shuts off as a safety feature and to ensure the fiber is not damaged if it must be relocated. Moreover, fiber bundles can be configured with multiple output ‘arms’ with different shapes (e.g. line, square, circle, etc.) for the output connector.
Transmitting UV light
According to Dr. Little, OPO lasers can be designed to generate wavelengths down to 190 nm through multiple stages of optical conversion. However, solarization, or “photobleaching” of the fiber can occur due to prolonged exposure to UV or other forms of radiation. Solarization causes a gradual increase in the absorption of light, leading to a decrease in fiber performance.
The effects of solarization are even more pronounced in the “Deep UV” (Deep Ultraviolet) range, which generally refers to wavelengths below 210 nm.
To mitigate UV effects, fiber optics providers like Armadillo can apply special chemistry treatments and utilize unique optical materials to prevent light absorption and UV damage in Deep UV wavelengths.
Today’s high-powered lasers demand efficient and safe transmission methods, and fiber optics have emerged as a leading solution. Still, the key to success lies in selecting the right fiber material and custom solutions tailored to the unique requirements of each application to avoid damage and optimize performance. By leveraging advanced fiber technologies, OEMs can achieve precise alignment, enhanced flexibility and reliable performance, making high-powered lasers more accessible and effective for a wide range of applications.