A gas/fiber laser hybrid
A team of researchers from the University of Bath in the UK is working on hybrid laser concepts that build bridges between gas, solid-state and fiber laser technology.
The full potential of photonic crystal fibers (PCFs) can hardly be fathomed, even 20 years after they were invented. Professor Philip Russell had the brilliant idea in the mid-1990s: Optical fibers could be drawn from bundled glass pipes instead of from glass rods. That means dozens of longitudinal cavities are then formed around either a solid or a hollow core. They keep the light trapped in the core and—unlike in a pure glass fiber—it cannot propagate in the space. The light is reflected back from the core’s walls and cast back millions of times over onto its narrow path. The absorption losses are minimal. That makes PCFs highly suited for applications where high performance requirements are demanded. Moreover, they can be modified almost as desired—by selectively embedding metal wires, doped glass fibers or filling the cavities with fluids or gases.
Russel is now Director at the Max Planck Institute for the Science of Light in Erlangen. He laid the foundations for PCF technology at the University of Bath in England, where a group of researchers has now developed an innovative fiber gas laser based on hollow-core PCFs. The hybrid laser system combines the described advantages of PCFs with the narrow linewidth and high power output of gas lasers. The team led by Professor Jonathan Knight and Dr. William Wadsworth fills the cavities of a PCF drawn from silica glass at a pressure of 0.3 mbar with acetylene (C2H2), whose molecules have a 3.16 µm mid-infrared emission. In contrast to what is usual with gas lasers, the system is not pumped electrically, but with a 1.53-µm diode designed for the telecommunications industry. Its 40 nanosecond pulsed light and the gas interact in a 10 meter long, gas-filled amplifier fiber. In pulsed mode, a more than 100 meter long hollow-core PCF as the feedback fiber and dichroic mirrors round out the ring resonator structure. The mirrors have high transmission at 1.53 μm, but reflect infrared light at 3.16 µm. To ensure that oscillation occurs, the pump frequency and cycle length are precisely coordinated. Alternatively, operation as a continuous beam laser is possible, but then with a 3 m long feedback fiber.
Still at the research stage
To date, the power of the hybrid laser is in the low milliwatt (mW) range. “We used an unamplified diode with less than 20 mW and an output coupler with 7 percent reflectivity,” says Wadsworth. The use of powerful pump diodes and more efficient couplers means the laser power is easy to scale. The key components are, in any case, the rugged silica glass hollow-core fibers, whose losses are below 30 decibels a kilometer even in the tough mid-IR range. The British researchers are considering using alternative gases to tap further wavelength ranges. “We assume that we can advance into wavelength ranges up to 5 µm with the new design of the fiber gas laser,” adds Wadsworth. The flexible and compact system could help solve the problem of high absorption losses of conventional fibers in the mid-infrared range. If scaling and tapping of new wavelength ranges are successful, the fiber gas laser is likely to be of interest to users in materials processing, manufacturing and research. “We’re already cooperating with various manufacturers in the field of hollow-core fibers,” states Wadsworth. “Now we’re eager to see how the industry responds to our new laser concept.”
“Exciting,” says Dr. Mo Zoheidi, Chief Technology Officer of OBERON GmbH Fiber Technologies, about the research by the British team. The expert has conducted research and development in the field of fiber optics for almost 20 years and has held posts at leading fiber manufacturers and in science and academia. However, he notes that there is still a long way to go before the concept can be marketed—especially in the field of medicine, where lengthy and extensive approval processes have to be undergone. OBERON is currently pursuing different concepts: “Apart from therapy, a new field is opening up for our fibers in diagnostics,” he explains. For example, to detect leukocytes in blood by means of Raman spectroscopy or fluorescence microscopy or to distinguish tumors in their early stage from healthy tissue. Zoheidi’s reply is typical for the fiber sector. The diversity of applications is virtually unlimited—and this diversity promotes the constant new and further development of optical fibers. An end to this dynamism is not in sight.
Source image: Max-Planck-Institute for the science of light, Erlangen