Gas Sample Cell Based on Photonic Bandgap Fiber
By splicing a single-mode fiber to each end of a gas-filled, hollow-core, photonic bandgap fiber, researchers at the University of Bath in the UK have created a gas cell with unprecedented interaction efficiency between the sample and light. They have demonstrated efficient stimulated Raman scattering in the cell and have built what they believe is the first self-contained, fiber-based frequency stabilization system. The realization of such tiny gas cells may lead to ultraminiature photonic components for spectroscopy, telecommunications, medicine and other applications.
Figure 1. The periodic array of airholes in the cladding of a photonic bandgap fiber prevents light from propagating there, much as the periodic crystal structure in a semiconductor creates a bandgap where electrons cannot exist. Images ©Nature Publishing Group.
In a photonic bandgap fiber, light is guided in the core not by total internal reflection, but by the existence of a photonic bandgap surrounding the core. Thus, the refractive index of the core need not be greater than that of the cladding. The bandgap in the cladding is created by a periodicity in the refractive index, achieved with a system of airholes adjacent to the core (Figure 1). The delicate structure of the airholes makes it difficult to splice other fibers to the photonic bandgap fiber, however, because the microstructure can collapse or become contaminated.
Figure 2. A micrograph of the photonic bandgap fiber cleaved at the junction of the splice with a single-mode fiber reveals a concavity, apparently caused by surface tension that induces the airhole microstructure to shrink on itself when softened during the splicing process.
The researchers fabricated good splices, with loss of only 1 or 2 dB, by purging the volume where the splice was made with argon gas and using a commercial splicer. They calculate that the best possible splices between the two types of fiber would have losses of 0.6 to 0.8 dB -- approximately 0.15 dB resulting from the difference in refractive index between the cores, and another 0.4 to 0.6 dB from mode mismatch. They attribute the difference between an ideal splice and their best splices to a recess formed at the end of the photonic bandgap fiber during the splicing process (Figure 2).
Figure 3. The frequency-locking system monitored the transmission through an acetylene-filled photonic bandgap fiber sample cell and generated an error signal that adjusted the laser resonator's length.
The researchers fabricated the tiny gas cells by splicing a single-mode fiber to one end of the photonic bandgap fiber, evacuating the hollow-core fiber and backfilling it with the gas, and then splicing a second single-mode fiber to the other end. They point out that such a small and rugged device could readily be combined with commercially available fiber components such as lasers, modulators, isolators, fiber Bragg gratings and multiplexers to create numerous practical devices.
Figure 4. When the control loop in Figure 3 was closed, the rms frequency fluctuations of the external-cavity laser diode were limited to ~310 kHz.
In a 5-m-long photonic bandgap fiber filled with hydrogen at ~6 bar, the researchers observed efficient stimulated Raman scattering with 1047-nm pulses whose peak power was only 100 to 180 W. Normally, stimulated Raman scattering in a gas requires laser peak powers in the megawatt range or higher. The efficiency was a direct result of the high optical intensity in the holey fiber -- where light propagated in a single transverse mode -- and the long interaction length.
In a second demonstration of the potential of the gas cells, the researchers used an absorption line in an acetylene-filled cell to lock the frequency of an external-cavity laser diode operating at the telecom wavelength of 1530.43 nm (Figure 3). When the loop was closed, the laser exhibited a maximum rms frequency deviation of ~310 kHz (Figure 4).
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