An Ytterbium-doped photonic crystal fibre laser is demonstrated with a 100 µm2 core area and single transverse mode with an output efficiency of 30%. Double-clad PCF laser structures are demonstrated with pump cladding NA greater than 0.8 and output power up to 3.9 W. Such lasers are potentially scalable to high power.
© 2003 Optical Society of America
Fibre lasers based on erbium- and ytterbium-doped silicate glass fibres have attracted much interest in recent years. Ytterbium in particular is capable of high efficiency and may be pumped directly by diode lasers at 915 or 980 nm. Fibre lasers have several advantages over bulk solid state lasers, amongst which is that lasers based on single-mode fibre have very good beam quality. This is particularly important for high power lasers as it is difficult to design high power bulk solid state lasers with single transverse mode output. However, at high power, the optical intensity within the small core of an optical fibre becomes very large and this can give rise to catastrophic bulk and surface fibre damage. High power lasers based on conventional Ytterbium doped step-index fibres have used relatively large mode areas to avoid these effects. A cladding-pumped 110 W fibre laser has been reported, using a fibre with a mode field diameter of 9.5 µm.
Photonic crystal fibre (PCF) is an optical fibre with an ordered array of microscopic air-holes running along its length. In such a fibre the guidance is determined by the geometry of the pattern of air-holes rather than by the material[2,3]. It has been shown that a pure silica PCF may be strictly single mode at all wavelengths or conversely may have a core of arbitrarily large diameter whilst remaining single mode. By introducing Yb3+-doped silica into a PCF we recently reported the first PCF laser. This laser had a very small core, but unusual dispersion properties. We have also reported the first large mode area PCF laser. In this paper we report a large mode area double-clad PCF laser, with a core diameter of 15 µm and a cladding numerical aperture of greater than 0.8, as well as output powers up to 3.9 W.
2. Large mode area fibre laser
PCF is generally fabricated from pure silica without dopants: the index contrast required for guidance is achieved by the presence of air-holes in the cladding rather than by using doped glasses of differing indices. Indeed, care must be taken that any doped regions within a PCF structure do not introduce index steps which would adversely affect the fibre waveguide characteristics. This is particularly important for large mode area (LMA) fibres, where the effective index step must be very small. One of the major advantages of LMA PCF is that there is no material index step to control – the small effective index step arises from a particular geometric size and arrangement of holes. One of the limitations on the fabrication of conventional LMA fibres is the control of small index steps in the glass manufacturing process. This is not significantly eased in doped PCF, indeed the problem may be worse: one must still fabricate a glass with a dopant and an index very close to pure silica, but the doped region, whilst having a large diameter, must be so close in index to the cladding glass that it does not form a waveguide itself, and the light is guided by the surrounding holes. The only alternative to reducing the index step is to reduce the transverse dimension of the doped region, which would result in a low absorption and gain.
There has been much research in fabrication of large mode area doped fibres by conventional MCVD techniques. Here we present an alternative, post-processing, method which places much less stringent requirements on the MCVD process. We have fabricated a solid rod with a small index step and quasi-uniform doping to form the core region of a LMA PCF laser by repeated stacking and drawing of Yb3+-doped and undoped silica. Fig. 1 shows an optical micrograph of this rod embedded in a PCF preform. Each of the 425 doped regions (the bright spots in Fig. 1) will have a diameter of less than 250 nm in the final fibre, which is too small to form a waveguide. The ensemble therefore forms an effective index medium. The area filling fraction of the doped glass is about 10%, reducing the effective index step from doped to undoped glass to of the order of 1 × 10-3 which is insufficient for strong guidance even with a 7.5 µm doped core diameter (V < 1.3). The composite doped rod was then stacked along with silica capillaries (see Fig. 1) to form a PCF in the usual manner[2,3,4].
The general principle of stacking and drawing solid doped glasses is a very flexible method for fabrication of engineered optical materials. Provided that the transverse average index (and/or active doping level) of the source rods is well known, then the average index, doping and homogeneity of the final structure is determined simply by geometrical stacking factors, which can be controlled to high accuracy. Furthermore, instead of stacking rods with high index along with pure silica, rods of index higher than silica could be stacked with rods of lower index, or a single type of rod with the correct average index, but with large index fluctuations, can be made into a uniform medium by this technique. Specific inhomogeneities, such as graded profiles, can also be implemented. By creating an optically homogeneous medium which is inhomogeneous on the nano-scale, different glass environments could also be created for different active dopants within the same optical medium.
Using the doped rod shown in Fig. 1 two different fibres were drawn (Fig. 2) with slightly different air-hole sizes but both with a core diameter (the shortest distance across the solid core between opposite air holes) of 15 µm and an overall fibre diameter of 120 µm. Guided modes of the fibres were studied using a full vector numerical model based on the plane-wave method. The index profile modelled included the actual hole shapes of the fibres, together with a representation of the doped region by a circular index step of 0.0011, 7.5 µm diameter, in the centre of the solid core. Fibre LMA-A, with smaller holes (ratio of hole diameter, d, to pitch, Λ, d/Λ=0.3, Fig. 2a), is strictly single mode, whilst fibre LMA-B, with larger holes (d/Λ = 0.55, Fig. 2b), supports second order modes in the wavelength range of interest. The central portion of the single mode of LMA-A, down to at least the 20% intensity contour is very close to a gaussian, with a mode field diameter of 11.3 µm. The effective area of the mode, calculated numerically from the entire profile is 100 µm2. A critical bend radius of approximately 1.5 cm was measured for fibre LMA-A at a wavelength of 1 µm.
A length of fibre was pumped longitudinally by a cw Ti:sapphire laser operating at 915 nm. A laser cavity was formed using the perpendicularly cleaved input face of the fibre as the output coupler and a high reflectance (HR) dielectric mirror at the other end of the fibre. The HR mirror was either butt-coupled to a perpendicular cleave at the end of the fibre or an antireflection coated aspheric lens was used to couple light to and from the mirror. The fibre laser output travelling back along the pump beam was split off with a dichroic mirror.
Using 1.5 m of the single mode fibre LMA-A a maximum output power of 260 mW was obtained, at an efficiency of 25%, which was the same with either a butt-coupled or lens-coupled HR mirror. Threshold pump power was less than 300 mW. The output was observed to be in a single mode with a clear hexagonal structure imposed by the hexagonal array of air-holes in the cladding. As indicated by the measured critical bend radius for this fibre, the output power was severely affected if the fibre was bent with a small radius but was unaffected by general handling of the fibre. The numerical aperture of the fibre mode was measured to be 0.11 at a wavelength of 1 µm.
3. High NA Double-clad PCF laser
It has previously been postulated that an outer cladding with high air-filling fraction would provide a low effective index, and thus a high NA when coupled with a solid or PCF inner cladding [6,7,8]. However even with air filling fractions of up to 65% the measured NA was low – less than 0.5 even for relatively short lengths of less than 1m [7,8]. Here we have implemented double-clad PCFs with measured NA as high as 0.8 at 1 µm for fibre lengths of 1 m, which is maintained above 0.75 for lengths of 20–50 m. This is achieved by suspending the inner cladding in air by webs of silica which are substantially narrower than the wavelength of light (Figs 3, 4).
The large mode area PCF structures described above have been combined with high NA structures into a cladding-pumped fibre laser. A further advantage of the PCF fabrication technique is that the core can easily be placed at any lattice point within the inner cladding. In order to maximise the coupling of pump light in the cladding to the absorption in the fibre core it is in many cases advantageous to place the core close to the edge of the inner cladding, whilst making sure that the guided mode is not disturbed by the interface between the inner and outer cladding regions. Here the core was placed three periods in from one corner of the hexagonal inner cladding. Two fibres of this type are shown in Figs 3 and 4. The doped core was the same as that used in the single clad fibre lasers LMA-A and LMA-B, and again the core diameter was 15 µm. When pumped in the single mode core in the same type of laser cavity as used for fibres LMA-A and LMA-B, laser performance was similar.
For cladding pumped lasers an end-pumped configuration was used. The pump laser was a fibre coupled diode array delivering 30 W at 911 nm from a fibre bundle 840 µm diameter with an NA of 0.12. This output was matched to the 150 µm diameter inner cladding of the high NA double-clad PCF using a pair of antireflection coated aspheric lenses (O1, O2 New Focus ×10 (ƒ= 15.4 mm, NA=0.16, working distance 14 mm) and ×60 (ƒ=2.8 mm, NA=0.65, working distance 1.6 mm) respectively) with a separation of 20 mm. The lens pair gave a transmission of 87% and a nominal spot size of 140 µm at a nominal NA of 0.12 × 60 / 10=0.7. This NA is higher than the specified acceptance angle of the output lens and will introduce aberrations which reduce the coupling efficiency into a laser fibre. The dopant level in the core of this prototype double-clad fibre was rather low for cladding pumping, as it was designed for single-clad lasers (as fibres LMA-A and LMA-B), so the absorption of the pump light into the core is approximately 0.16 dB/m (measured by the excess loss at 915 nm over the background for white-light transmission). The laser cavity was formed either by Fresnel reflection from perpendicularly cleaved end facets of the fibre, or by inserting a 1 mm thick dichroic mirror (>95% transmission at 915 nm, >99.5% reflectance at 1040 nm) butt-coupled to the input facet of the fibre. A thin mirror is required in order to fit within the short, 1.6 mm, working distance of lens O1. With this mirror in place the pump launch efficiency was unaffected, except for the small reflectance of the mirror coating at the pump wavelength. The laser output was separated from transmitted pump light using a long wave pass dichroic filter (<1% transmission at 911 nm, 95% transmission at 1040 nm). A second identical filter was used, when required, to check that the residual pump power was indeed negligible compared to the laser power.
The initial double-clad fibre (Fig. 3) unfortunately showed an unusually high background attenuation in the cladding. The excess loss obviously severely limits the laser efficiency by reducing the amount of pump power usefully absorbed in the core. However laser action was readily observed in the laser cavity formed by the reflections from the cleaved end faces of the fibre, with a single-mode output.
For high power operation a second series of double-clad fibres was fabricated with low background loss. This fibre is shown in Fig. 4. The interstitial holes in the inner cladding reduce the effective core diameter to 10 µm, and the core is multimode, however all other aspects of the fibre are unaffected. The only significant structural change from the initial double-clad fibre is that the jacket thickness is reduced, giving an overall fibre diameter of 280 µm, instead of 500 µm. This smaller diameter allows the fibre to be ruggedised by applying a polymer coating. The measured background attenuation over 50 m of this fibre is 15 dB/km at 1080 nm, and the Yb3+ absorption at 915 nm is 0.15 dB/m (Fig. 5).
Several lengths of fibre were tested, from 10 to 50 m in length, pumped at 915 nm using the 30 W fibre pigtailed semiconductor source. The launch efficiency from the pump pigtail output to the PCF was around 67%, which includes some loss in the coupling lens pair. Best results were obtained for a 17 m length of fibre. Using no mirrors, just the 4% Fresnel reflections from either fibre end face, laser oscillation was observed to occur for 0.9 W of launched power. When a high reflectivity mirror was butted to the input end of the fibre an output power of 3.6 W was achieved for 20 W launched. By using another mirror at the output to reflect the residual pump power back in to the fibre (R>99% at 915 nm, T=90% at 1035 nm) the laser output was boosted to 3.9 W for the same launched power. These are the highest ever reported output powers for a double clad PCF laser, and correspond to slope efficiencies of 18% and 21% with respect to launched pump power (Fig. 6). We have achieved a maximum slope efficiency of 31% with respect to absorbed power. These efficiencies are comparable to the single-clad fibres LMA-A and B, fabricated using the same doped core. We attribute the moderate efficiency to defects in the stacked core structure, as the maximum laser efficiency has been similar for this same core in many different laser structures. In the final double-clad laser there are occasional scattering centres which only appear above the laser threshold, indicating defects in the core. We would expect that a more carefully fabricated core, whether with a uniform or structured doping, would eliminate this laser cavity loss and allow operation at higher power and with higher efficiency. There is also no practical barrier to removing the interstitial holes (as in Fig. 3) to ensure a single-mode output profile.
A 3.9 W cladding pumped PCF laser has been demonstrated which has the potential for scaling to much higher output powers. The application of a stack-and-draw fabrication process to the production of a doped core enables us to realise quasi-uniform and customisable refractive index profiles, without some of the limitations of direct use of MCVD. PCF stack and draw techniques also enable fabrication of double-clad structures in which the inner cladding has an NA greater than 0.8. We have incorporated both types of structure into a high-NA cladding pumped fibre laser, using a low-brightness high-power diode array as a pump source.
The authors would like to thank L.F. Michaille and T.J. Shepherd of QinetiQ Ltd, UK for helpful and stimulating discussions. W. J. Wadsworth is a Royal Society University Research Fellow. This research was partly funded by QinetiQ Ltd (formerly DERA), within the UK Ministry of Defence Corporate Research Programme.
References and links
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