Photonic Bandgap Fiber Makes Unique Gas Sensor

Breck Hitz

Many approaches to making gas sensors from optical fibers have been explored during the past 20 years, and scientists at Virginia Polytechnic Institute in Blacksburg recently developed a novel gas sensor based on a random-hole photonic crystal fiber. Now a collaboration of experimenters at Helsinki University of Technology in Otaniemi, Finland; the Danish Institute of Fundamental Metrology in Lyngby, Denmark; Technical University of Denmark, also in Lyngby; and Crystal Fibre A/S in Birkerød, Denmark, has reported a gas sensor based on a photonic bandgap fiber.

Figure 1. The photonic bandgap fiber had a hollow core that could be filled with gas.

Photonic crystal and photonic bandgap fibers are both members of the family of microstructured fibers or, as they are known colloquially, holey fibers. But there is a fundamental difference between the two. In a photonic crystal fiber, the effective refractive index of the cladding is lowered by a series of airholes running the length of the fiber, and light is guided in the core by total internal reflection.

In a photonic bandgap fiber, however, the periodicity of the airholes in the cladding creates a region in which photons of given energy cannot exist, so light is guided in the core because it cannot exist in the cladding. The physics of a photonic bandgap fiber is analogous to that of a semiconductor, in which the crystal periodicity creates a region where electrons of given energy cannot exist.

Figure 2. The photonic bandgap in the fiber's cladding guided light from ~1400 to ~1600 nm.

To make their gas sensor, the scientists in Finland and Denmark first fabricated a hollow-core, photonic bandgap fiber for the 1500-nm spectral range (Figures 1 and 2). They spliced one end of the bandgap fiber to a conventional single-mode fiber from an optical source (either a laser or an LED) and placed the other end in a V-groove in a vacuum chamber. Fifty microns away from the end in the V-groove, they placed a multimode fiber leading to a detector (Figure 3). The separation allowed gas to flow into or out of the hollow core of the bandgap fiber while still allowing for efficient optical coupling between the fibers.

Figure 3. A 50-µm gap between the photonic bandgap fiber and the multimode fiber allowed gas to flow into and out of the hollow core of the former.

To test the sensor, they filled a 1-m length of the fiber with acetylene to a pressure of 10 mbar and illuminated it with a tunable laser, scanning the laser in 1-pm steps from 1524 to 1538 nm. They observed the expected spectrum of acetylene in that spectral range, with a signal-to-noise ratio better than 20 dB (Figure 4, left).

To compare the fiber sensor with a more traditional approach, they made a similar measurement of acetylene in a 1-m gas cell, employing the same laser (Figure 4, right). The slightly inferior signal-to-noise with the fiber is the result of vibrations and reflections from the end of the fiber.

Figure 4. Although the spectrum of acetylene taken with the fiber sensor (left) had slightly poorer signal-to-noise than the spectrum taken in a conventional gas cell (right), the fiber is a much smaller, more rugged and less expensive sensor.

They believe that the signal-to-noise ratio may be improved in later experiments. However, even without improvement, a slight decrease in signal quality would be a small price to pay in many applications for the inherent advantages of the fiber-based sensor: small size, ruggedness and low cost.