3 January 2013
I have had a long standing interest in developing tunable and reconfigurable dual- and multi-wavelength fibre lasers (continuous-wave and mode-locked). Over the years, my colleagues and I have demonstrated various erbium-doped fibre lasers, semiconductor fibre lasers, and fibre-optic parametric oscillators with the narrowest wavelength spacing or the largest number of wavelengths reported. In general, multi-wavelength sources have numerous applications, especially in instrumentation (e.g., for component testing) as well as in fibre optic sensing. In particular, one of my colleagues/collaborators has recently demonstrated an approach for simultaneous and continuous multi-wavelength cavity ring down spectroscopy (CRDS). This work is aimed at developing multi-wavelength sources for use in this highly capable and novel sensing technique.
Tm-doped fluoride (ZBLAN) fibres can provide amplification at various wavelength ranges, in particular 810 nm, 1480 nm, 1900 nm, and 2300 nm. Indeed, a number of research groups have demonstrated various Tm:ZBLAN fibre lasers at these wavelengths. We have combined a variety of different approaches, including bidirectional upconversion pumping at 1064 nm, as well as a cascaded linear cavity design to implement for the first time a dual-wavelength Tm:ZBLAN fibre laser operating at 1480 nm (S-band) with a record narrow spacing of only 0.6 nm. Alternate wavelength operation and simultaneous dual-wavelength operation can be obtained by simple control of the pump powers.
A dual-wavelength laser with alternate wavelength switching capability may find several uses in sensing and communications. For example, it may be possible to develop a grating-based sensor system capable for discriminating temperature and strain. It is also possible to use such a laser for generating wavelength-encoded labels (headers) for packet-switched networks using out-of-band wavelengths (labels in the S-band and payloads in the conventional C-band). By using a different rare-earth dopant (e.g., Er rather than Tm), it is possible to extend operation from the S-band to mid-IR wavelengths, and such sensing platforms are perhaps some of the most promising applications of such a dual-wavelength laser.
This work is part of an on-going collaboration with several colleagues on developing mid-IR sensors and systems for chemical detection applications and involves several industrial partners, including IRphotonics, a leading manufacturer of fluoride (ZBLAN) fibre. As mentioned above, one of my colleagues has developed a novel simultaneous and continuous multi-wavelength CRDS system that allows for absorption measurements of microlitre sized samples in liquid detection volumes as small as 100 nL. While the current systems operate at near-IR wavelengths, we are very excited by the prospect of extending the work to mid-IR wavelengths where the absorption bands for many analytes are considerably stronger.
In addition to the work on fibre lasers and sensors, I am currently involved in several projects in fibre-optic communications. For example, we have been developing signal processing techniques to increase the capacity of incoherent and coherent optical orthogonal frequency division multiplexing transmission. We have also been testing low-cost and integrated approaches for packet header recognition and swapping in packet-switched networks. Finally, we have been fabricating and characterising waveguide devices based on silicon-on-insulator or CMOS compatible wide band-gap semiconductor nanowires for signal processing.
Over the years, there has been considerable research on developing mid-IR sources. Fibre-based configurations have numerous advantages and indeed, various lasers with high efficiency, output power, and beam quality have been demonstrated. However, a number of fibre lasers still involve bulk optic components or have limited reconfigurability/tunability. The continued development of advanced fibre-based mid-IR sources should enable a number of important sensing applications in the future.
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