Fibre and waveguide lasers

Abstract

The guided-wave laser is almost as old as the laser itself. The first demonstration of laser action in glass made use of a multimode waveguide; a core rod encased in a cladding of lower refractive index so that confinement of the light would counteract the effect of the poor optical quality of the available glass, (Snitzer, 1961). However, although waveguiding has long been an essential feature of semiconductor diode lasers, in dielectric laser media by far the greatest research effort has gone into the development of bulk rods and slabs of high optical quality. Interest in the potential advantages of guided wave dielectric gain media was only quickened with the advent of high quality single mode optical waveguides, especially rare earth doped silica fibres, in which the propagation losses are so low that the benefits of optical confinement are fully realised. The most immediate advantage of the guided-wave geometry is that of reducing the cavity mode volume, and hence the pump power needed to reach threshold. Provided the guiding structure is designed so as to support only a single propagating mode at the gain wavelength, then the laser output will be spatially coherent, in a mode that is not affected, for example, by cavity misalignment. The guided-wave laser can be designed as a compact, stable, monolithic device, exploiting all the techniques of integrated optics, such as gratings, couplers, and modulators. Since the active region of a guided-wave laser is typically only a few µm in diameter, fabrication can involve a range of deposition techniques very different from those used to grow bulk media. The resulting gain medium may have a composition or dopant concentration not available to a bulk phase. On the other hand the advantages of a guided-wave structure are cancelled if propagation losses compete too effectively with the achievable gain. A further difficulty attending these optically pumped devices is the need to couple pump light into the waveguide core. The pump sources themselves must therefore emit spatially coherent beams, and expensive micropositioning techniques are required.
The literature on fibre and planar dielectric waveguide lasers is now so extensive that a review of this type cannot attempt to be comprehensive. My aim is rather to sketch the principles of guided-wave laser design and operation, and introduce a few selected devices of particular current interest. I shall pay particular attention to the role that guided-wave systems may play in the effort to develop compact and efficient sources emitting high power diffraction-limited beams. Diode-bar lasers emitting many tens of watts are now readily available, but it remains a challenge to convert the highly asymmetric and multimode output from such a device into a usable beam in a simple and efficient way. It is not at first sight obvious that a guided wave laser should be particularly suitable for high power operation. Waveguides are characteristically devices in which high core intensity accompanies low overall power, and scaling up the core area leads to multimode propagation and loss of spatial coherence. Recently, however, the technique of cladding pumping of fibre lasers has been found to be strikingly effective in overcoming these limitations.
It can be argued that planar waveguide structures are inherently highly compatible with high-power diode-bar pump lasers. Experimental investigation of such devices indicates that extremely compact sources, potentially able to handle 10 W or more of output, can be fabricated in this way. Control of the spatial mode is a central and difficult problem, and various technical approaches will be reviewed. Equally stringent is the necessity to couple pump radiation into the guide, whether longitudinally for greater efficiency, or transversely for a less divergent output and the possibility of power scaling. We shall see that with careful positioning this can be achieved without the use of any optical components whatsoever. If, moreover, the cladding-pumping principle is employed, then the position tolerances are significantly relaxed. Alternatively it may be possible to pump through the face of the device, and we shall review some practical schemes of this type.

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