We report on the design of a polarization maintaining, double-clad, Yb doped photonic crystal fiber and demonstrate its lasing properties. The polarizing properties of the fiber rely on birefringence and differential loss introduced by an anisotropic hole structure. Due to a slight leak from the core to the inner cladding only ~80% of the output light is in the core mode. We have demonstrated 2.9W of output in this mode with a polarization ratio in excess of 200:1.
©2004 Optical Society of America
The development of air-silica microstructured or photonic crystal fiber  has led to the realisation of a range of remarkable optical components. The lower refractive index in the cladding caused by the presence of the air holes may lead to an endlessly single-mode fiber , significantly smaller core sizes than with conventional fibers , and an unprecedented control of the fiber dispersion properties . A particularly remarkable application is super-continuum generation , which relies on the freedom afforded by the microstructure to design a fiber with a small core size, and therefore a high non-linearity, and the zero-point for the dispersion in the visible to near-IR region. Using a very different design, it possible to realize a double-clad fiber, where a large, single mode core  is embedded in an air-silica photonic crystal inner cladding. The inner cladding is effectively suspended in a large number of sub-micron silica bridges effectively forming an air cladding . These fibers can have particularly high numerical apertures and are ideal, for instance, for delivery of high-power beams.
Active dopants may be introduced in the single-mode core in the center while a pump laser is guided by the inner cladding. Due to the distribution of the dissipated power throughout the length of the fiber this design has proven particularly successful for high-power operation of such lasers .
The photonic crystal fiber also lends itself to the introduction of a polarization maintaining structure. This has been demonstrated both through the introduction of stress on the core  and through the creation of form birefringence induced by a strong anisotropy in the hole structure [10, 11].
In this paper we report for the first time the introduction of a polarization maintaining hole structure in a double-clad, Yb doped photonic crystal fiber. A highly polarized laser output is demonstrated.
2. Polarization maintaining fiber
Microstructured fibers appear particularly well suited for tailoring of polarization properties due to their large number of free design parameters (including structure morphology, structure dimensions and core profile). For passive fibres, this was recently explored by Kubota et al. who demonstrated microstructured fibers with broadband single polarization properties . The design idea of our active fiber is to provide form birefringence as well as differential loss of the two polarization states within the emission wavelength range of Yb. Thereby stimulated emission preferably takes place for one polarization state – and a significant reduction or completely elimination of additional polarization control in PM fiber lasers and amplifiers is sought.
A first step in the development of our fibers was to fabricate a series of passive and active test fibers without air-clad. These were made to determine the optimum structural parameters for the final fiber, and the air-clad was eliminated in order to allow simpler characterization of the core properties (stripping of disturbing cladding light is more troublesome in double-clad fibres). The final air-clad fiber is shown in Fig. 1(a) and Fig. 1(b) shows a typical mode field distribution (here recorded for one of the passive fibers). The fiber is single-mode with an asymmetric mode field distribution.
The air-clad provides a high numerical aperture, NA, double-clad design without the use of polymer materials . The NA of the fiber is around 0.67 at a wavelength of 950 nm (with a slightly higher value expected at 980 nm). The inner cladding (pump guide) of the fiber has a diameter of 140 µm, a substantially circular shape and a typical loss level of less than 30 dB/km.
Within the inner cladding, the fiber comprises a microstructure with two types of air holes. Smaller holes are placed in layers around the core to provide light confinement hereto and two larger holes are placed in immediate proximity to the core. The smaller holes have pitch, Λ, and diameter, d, of 7.0 µm and 2.7 µm, respectively, and the larger holes have diameter 8.2 µm The larger holes enforce a substantially elliptical core shape – with approximate dimensions of 5 µm×11 µm. The core of the active fiber is doped with Yb, Ge, Al and F. The doped region of the core has an extent of approximately 3 µm×7 µm and an index increase, Δn, of about 4×10-4. Ge co-doping was chosen to allow Bragg gratings to be written into the fiber (demonstrated for one of the active, non-air-clad test fibers with similar core composition). The Yb, Ge and Al concentrations were kept relatively low in order to allow F co-doping to partly compensate their increasing effect on the core refractive index profile. The core absorption of the fiber is around 400 dB/m at 975 nm. The active fiber showed a birefringence of around 1.4×10-4 at a wavelength of 1100 nm and a differential loss on the order of 100 dB/km for the two polarization states.
The performance of the polarization maintaining fiber is investigated using the setup shown in Fig. 2. Lengths of up to 30 m of the fiber were pumped by up to 10 W from a 980 nm diode laser coupled into a 200 µm diameter fiber with an NA of 0.2. This output was collimated with an 11 mm focal length lens and focused into the doped fiber with a 4.5 mm focal length lens with an NA of 0.55. Assuming this system does not introduce significant aberrations this would lead to a pump spot diameter in the region of 82 µm. A dichroic beam splitter was inserted in the collimated section between the two lenses at an angle of incidence of ~7°. At this angle the beam splitter has a transmission minimum for the pump laser while it is highly reflective for wavelengths in the range 1000–1100 nm. The laser operates without external components, i.e. with feedback only from the ~4% reflections from the fiber ends. However, for most measurements presented in this paper the reflection from the dichroic beam splitter is retro-reflected with a high-reflector in order to provide a strong feedback.
3.1 Output characteristics
The coupling efficiency of the pump light and the fiber pump absorption were determined by measuring the laser output power and remaining pump power for a range of length from 0.5 m to 30 m. The absorption is found to be 0.48 dB/m in good agreement with the design. A relatively low coupling efficiency of 55% is observed despite the fact that the pump spot on the fiber and its NA both are calculated to be lower than the respective values of the fiber cladding. This indicates a relatively low efficiency for pumping into a region of relatively large air holes. It should be noted that any light impinging on an air hole will refract into the inner cladding and travel across it at a sufficiently steep angle that it will refract out through the air cladding. However, for the present fiber and pump geometries the air holes constitute less than 30% of the pump spot and the low coupling efficiency may therefore partly be attributed to non-optimum coupling optics.
The output power from the laser as a function of coupled pump is shown in Fig. 3 for a 20 m length of fiber. The observed slope efficiency is 68%. Taking into account that ~11% of the coupled pump power is transmitted through the fiber, the internal quantum efficiency of the fiber laser is 77% to be compared with the 91% theoretical maximum given by the ratio of the laser and pump photon energies. It is, however, important to notice the spatial mode of the output from this particular fiber. The guiding properties of the core are not quite strong enough to ensure a single spatial mode output. Due to a limited number of holes surrounding the core, a fraction of the light leaks out into the cladding throughout the fiber and results in a component of the laser output in the cladding mode. This is discussed in more detail below.
The output from the core forms an elliptical beam with numerical apertures of 0.12 for the direction along the line joining the two large air holes near the core and 0.05 for the direction perpendicular to this. This corresponds to an elliptical mode in the fiber with waists (e-2 radii) of approximately 3 µ and 7 µ respectively.
3.2 Polarization analysis
The laser output is expected to be close to linearly polarized, so it is sufficient to analyze it with a half-wave plate followed by a linear polarizer. In examining the inherent polarizing properties of the polarization maintaining fiber, however, it is essential to distinguish between the laser output in the single-mode core and the laser output, that has leaked into the multi-mode cladding. The latter would be expected to be unpolarized. In order to polarization analyze only the light from the core, the laser output is imaged onto a CCD detector and measurements are made only over the core region. Care is taken to introduce sufficient attenuation of the light that this detector operates in its region of linear response to intensity.
The peak intensity in the core transmitted through a polarizing beam splitter cube is now determined as a function of the rotation angle of the half-wave plate. Data for a 20 m fiber operating with a total output power of 3.7 W is shown in Fig. 4. The polarization ratio, i.e. the ratio between the maximum and minimum power transmitted through the polarizer, is in excess of 200:1. No significant variation of this polarization ratio is observed with the laser operating power. The direction of the polarization is along the fiber axis with the smallest NA, i.e., the electric field vector is perpendicular to the axis joining the large air holes seen in Fig. 1.
In obtaining these results it was essential to rotate the input end of the fiber such that the laser polarization was either pure s- or pure p-polarization relative to the tilted dichroic beam splitter in the input end. A mixed state of polarization on this dielectric coating would lead to rotation of the polarization of the reflected light and hence a mixing of the polarization states of the fiber.
With the highly polarized core we have an easy way of estimating the fraction of light, that has leaked into the cladding mode. Assuming this light is unpolarized half of it will be transmitted through a polarizer set to block the light from the core. For the polarizer in the opposite position we detect the other half the light in the cladding and all the core light. For the 20 m long fiber we typically measured a ratio of 9.5:1 indicating that 82% of the light was carried by the core. A closer analysis of the polarization of the laser light emitted from the cladding indicates an up to 30% asymmetry in favor of the core polarization decreasing the core fraction of light to 79% corresponding to 2.9W of highly polarized single-mode output. It is somewhat surprising that the laser light propagating in the highly multi-mode inner cladding has this ‘memory’ of the core polarization.
3.3 Operation without external feedback
As mentioned above the laser may also operate without external feedback, i.e., with just the two 4% reflections from the fiber ends. The main difference in characteristics between the two configurations is that the laser tends to self-pulse when operated without feedback and with fiber lengths longer than about 6 m. Self-pulsing results from nonlinear effects due to the long fiber length, small core size and relatively high power (similar effects reported by Hideur et al. ). This prevents us from using the CCD detector for monitoring the core polarization ratio. However, the more robust detection of the ratio of the two polarizations of the total laser output yields the same ratio of 9.5:1 from the output end of a 20 m fiber indicating that the polarization in the core has not deteriorated significantly despite the significant reduction in circulating power. Assuming that the coupling from core to cladding is unchanged in this configuration an easily detectable change in the ratio to 8.5:1 would correspond to a core polarization ratio of approximately 50:1.
With the feedback blocked we now have access to the output from the incoupling end of the fiber. As the laser gain is higher in this end coupling into the cladding mode can be expected to be less significant. Indeed, the polarization ratio for the total output from this end has gone up to 15:1 corresponding to more than 86% of the light propagating in the core in the end, where the gain is highest.
The output powers measured from the two ends are also shown as a function of the coupled pump power in Fig. 3. Again we observe an asymmetry between the ends. The power from the incoupling end is approximately 20% higher than from the traditional output coupling end. However, it is worth noting that the sum of the two outputs show virtually the same slope efficiency as observed in the configuration with external feedback but with a slightly higher threshold.
3.4 Dependencies on fiber length
As already mentioned fiber length from 0.5 m to 30 m were investigated. Fig. 5 shows the length dependence of the output power and the remaining pump light, which was the basis for the determination of the fiber absorption mentioned above. Across this range the lasing wavelength varies from ~1020 nm at the short lengths to ~1080 nm at 20 m and above.
The polarization of the total laser output shows a significant variation with fiber length. On the one hand the polarization of the light in the core is a manifestation of a differential loss to the cladding of the two polarization states, while on the other the finite loss of both states results in the output propagating in the cladding. Hence, as the fiber length is reduced we can expect both a reduction of the light in the cladding and a reduction in the polarization ratio of the core. The latter, observed as explained above by imaging the fiber output onto a CCD detector, shows a polarization ratio of better than 50:1 for fiber length of down to 8 m. As shown in Fig. 5 the output power drops significantly for shorter length than this. The polarization measurements show a corresponding drop in core polarization ratio to only 3.4:1 for a fiber length of 2 m.
We have demonstrated, to the best of our knowledge for the first time, that a highly polarized output can be generated from a rare-earth doped photonic crystal fiber by the introduction of an asymmetric air hole structure. The polarizing properties of the fiber are a result of birefringence introduced by this asymmetry and a differential loss of the two states of polarization. The detailed fiber design is not yet fully optimized, so a small residual loss for the lasing polarization was observed. This resulted in only approximately 80% of the light being emitted in the mode of the core. However, this light was highly polarized with a polarization ratio in excess of 200:1 for the fiber lengths providing the most efficient operation.
The authors wish to acknowledge valuable discussions with Nigel Langford. The work was supported by NKR Research and the NKT Academy.
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