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Lasers & Sources

Gas discharge in hollow-core fibers

Stable glow discharges achieved in waveguide fibers with inner diameters in the 50–250μm range may enable the fabrication of miniature fiber gas lasers.
25 March 2008, SPIE Newsroom. DOI: 10.1117/2.1200802.1071

The first waveguide helium-neon (HeNe) laser was reported by Smith in 1971.1 It operated at 0.633μm, and the laser light was guided in a hollow glass capillary tube of length 20cm with a 430μm inner diameter (id). Since then, few experiments have been reported on these lasers. In 1975, a waveguide HeNe laser operating at 3.39μm with a 510μm-id tube was demonstrated, followed by another one lasing at 632.8nm with a 490μm-id tube in 1976.2,3 In 1996, a waveguide HeNe operating at 632.8nm with a 1mm-id tube was announced.4 To date, Smith's first effort remains the smallest HeNe laser to appear in the literature. The lack of interest for small bore size tubes may be due to their experiencing greater waveguide loss according to conventional hollow waveguide theory,5 especially when the tube is subjected to bending.

Recent progress in hollow-core fibers (HCFs), however, is changing this picture. For example, fibers with a layer of higher index anti-resonant coating and hollow-core photonic bandgap (PBG) fibers with 2D ‘holey’ photonic crystal cladding now allow much lower waveguide losses, ranging from 0.02dB/m to 1dB/m for hollow core sizes of 5–250μm.6,7 PBG fiber losses do not increase significantly even when subjected to bending of a few centimeters or less. Efficient feedback configurations have also been proposed for the use of HCFs in laser cavities. They include placing high-reflection mirrors at the ends of the HCF,8,9 splicing solid-core fibers with in-fiber Bragg gratings, adding fiber loop mirrors at both ends of the HCF, and splicing the two arms of a fiber directional coupler to the HCF to form a ring cavity.

These developments show that it should be possible to construct miniature fiber HeNe gas lasers with the relatively small gain required for hollow-core tubes. These lasers are expected to have the same excellent monochromaticity as their bulkier counterparts with the additional advantage of full compatibility with optical fiber systems. They would be very attractive for precision measurements and instrumentation,10 in applications such as fiber ring laser gyros, and large pathlength difference fiber interferometers. They could also be used in fiber ring flow meters and methane gas detectors based on the 3.39μm HeNe line locked to a methane absorption line.

We recently succeeded in obtaining argon, helium, and carbon dioxide discharge in 250, 150, and 50μm-id HCFs. A stable glow discharge lasting at least several minutes was observed in these waveguides. Figure 1 shows the glow discharge achieved in one of our fibers filled with helium gas.

Figure 1. Side (a) and end (b) views of an HCF of 13.7cm length with a 150μm inner diameter during gas discharge.

We measured the current-voltage (I-V) characteristics of fibers of various size and length filled with gases at different pressures. The results are shown in Figure 2. In a 13.7cm-length fiber with 150μm−id, the I−V characteristics do not vary significantly when the hollow core is filled with different gases at the same pressure. Similar results were obtained for a fiber of 4.6cm length with 150μm-id. Due to the small size of the core, the composition of the fill gas is not able to significantly modify the I-V characteristics. When the length of the discharge tube increases, however, the I-V curve shifts upward, as shown in Figure 2.

Figure 2. Current-voltage (I-V) characteristics of 150μm-id tubes of different lengths and filled with different gases at the same pressure. He: Helium. CO(2): Carbon dioxide. Ar: Argon.

The effect of varying bore diameters on the I-V characteristics at comparable pressure can be seen in Figure 3. Although the 50μm-id tube has a shorter length and a slightly lower gas pressure, the voltage is still larger than that of the tubes with larger bore diameters. When the diameter of the hollow core is further reduced to less than 50μm, the voltage required for sustained glow discharge is considerably larger and the discharge becomes difficult. The smallest bore diameter tube in which we obtained gas discharge was a 20μm-id HCF. The breakdown voltage was approximately 26kV, and the sustained voltage was larger than 15kV.

Figure 3. Comparison of I-V characteristics for different bore diameters.

In summary, successful discharge in small bore diameter HCFs is a major prerequisite for the fabrication of fiber HeNe gas lasers. Further efforts, however, are still required to improve performance.

Wei Jin, Xin Shi, M. Süleyman Demokan
Department of Electrical Engineering
Hong Kong Polytechnic University
Hong Kong, China

Wei Jin is a professor. His current research interests are focused on microstructured optical fiber devices and sensors, optical fiber gas detectors, fiber laser gyroscopes, and smart structures.

Xinbing Wang
School of Optoelectronics Science and Engineering
Huazhong University of Science and Technology
Wuhan, China