1. Introduction

Microstructure fibers (MFs) used in this context as a generic term for holey and photonic crystal fibers show remarkable properties that are not attainable in conventionally-doped fibers. Interesting nonlinear properties are to be expected for MFs with small core areas. Meanwhile both active and passive devices have been demonstrated [1, 2, 3]. Alternatively, large-mode-area MFs [4] offer a wide range of practical applications, promising the generation of high-power beams with excellent mode quality, i.e. supporting preferably a single transverse mode. In conventional fibers, minimum values of the numerical aperture (NA), being synonymous with a maximum core size, pose restrictive demands on the refractive index difference between core and cladding, and more importantly, lead to excessive bend losses which prevent the use of such fibers. In principle, MFs can be designed and fabricated with improved bend losses [4]. A challenging task in MF technology is the incorporation of lasing dopants (Yb³⁺, Nd³⁺) into the core glass in order to make MF lasers. The dopants, along with the necessary co-dopants, especially germanium, give rise to an increase of the refractive index of the core glass which, in turn may affect the NA and may degrade attractive features of the MF. In this paper we describe a specially-designed Nd-doped MF laser (MFL) which retains its single mode character independent of pump strength along with the possibility of using a low-brightness laser diode for optical pumping.

2. The microstructure fiber

The preform [5] used for manufacturing lasing fibers has the following properties: It consists of an Nd³⁺-doped (Aluminum co-doped) silica rod (Nd₂O₃:1300 ppm) with an undoped silica cover. The diameter ratio of the entire rod to the doped core is 1.3 and the numerical aperture is 0.06, giving an index difference of ∆n = 1.22·10^-3 , which is an order of magnitude smaller compared to the value reported in [1]. The low doping concentration in the core makes this preform particularly useful for manufacturing a MF, but less suitable for a standard fiber. To work out the differences between the two types of fibers, a standard double- clad fiber and a MF were prepared from this preform to compare the relevant lasing properties. The standard double-clad fiber showed no lasing action mainly due to high (bend) losses. The lasing properties of the MF are reported below. The MF was fabricated by the stack and draw technique. The core has been placed slightly asymmetrically in the structure.

The scanning electron microscope picture of the manufactured fiber is shown in Fig. 1. [6].

 

Fig. 1 Scanning electron microscope image of the end of the microstructure fiber.

Core diameter: ~18 μm, diameter of the doped region: 13,8 μm, thickness of the undoped silica: 2 μm, hole diameter: 9 μm, hole to hole distance (pitch): 12 μm. Note the asymmetric position of the core.

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The radius r of the fiber core (consisting of doped and undoped material) was 8.5 μm. The radius of the doped region r_core was about 6.5 μm. The pitch Λ (hole-to-hole spacing) was about 12 μm and the mean hole diameter d was 9 μm. Therefore, d/Λ = 0.75. The total diameter of the MF was about 116 μm with a thick outer plastic jacket of lower (than silica) refractive index. The losses α measured at λ = 1060 nm, using the well established cut-back method, were in the range of α ≈ 8·10^-4 /cm, which is about 5 times better than the value reported in [1], but more than an order of magnitude worse compared to standard lasing fibers. The fiber length was 38 cm. Depending on the threshold overshoot, the diameters of the lowest order excited mode measured at λ = 1060 nm were approximately 11 μm (near threshold) and 15 μm (at 5 times threshold). Due to the stronger pump field high above threshold the outer portions of the active material are stronger pumped giving rise to a larger mode field diameter and a slight deformation of the mode profile.

To make sure that few-mode operation is predominantly due to the microstructured cladding surrounding the core, we estimated the modified refractive index ${{\text{n}}^{*}}_{\text{core}}$ of the core region by averaging the refractive index of the doped / undoped region, taking into account the area ratio of the doped part to the entire rod, giving ${{\text{n}}^{*}}_{\text{core}}$= 1.451. Measurement of the NA of the lasing fiber results in NA ≈ 0.07, from which we deduce n_clad = 1.4493. We adopt a condition cited in [2], ∆n < (${{\text{n}}^{*}}_{\text{core}}$ ² - ${{\text{n}}^2}_{\text{clad}}$) / 2·${{\text{n}}^{*}}_{\text{core}}$, which can be used to define the regime where the microstructure properties dominate light guidance in comparison to the core doping ∆n. This condition is fulfilled in our case.

3. Emission properties of the microstructure fiber laser

A pigtail-equipped diode laser emitting at λ = 805 nm was used as a pump source. This pumping wavelength guarantees optimum absorption efficiency. We used butt-coupled mirrors at the cleaved fiber endfaces. A dichroic mirror was used at the diode-coupling port. The out-coupling mirror has a 20% transmission at λ = 1060 nm. The pump radiation spot on the fiber has a diameter of about 75μm, which is large compared to the diameter of the doped fiber area. The ratio of the illuminated area to the doped core area is larger than 30. The NA of the focusing optics is 0.3. Guiding the pump light down the fiber is accomplished by the silica microstructure together with the silica jacket representing an inner cladding. The whole glass structure is embedded in a polymer (outer cladding) possessing a lower refractive index. At the end of the 38 cm long fiber we measured about 20 % of the input pumping intensity. Nevertheless, a substantial fraction of pump radiation is lost because the NAs of the optics and the fiber (defined by the microstructure and the annular silica jacket) are not matched. The laser output characteristics are plotted in Fig. 2a.

 

Fig. 2a The output characteristics of the MFL. The broken curve shows the output power for a bended fiber (bend radius: 5 cm).

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The influence of the bend losses is illustrated by the broken curve, for a bend radius of 5 cm. Roughly a doubling of losses occur. This dependence clearly illustrates how the bend loss influences the laser performance. Coiling the fiber to increase pump light absorption is not a method of choice for the type of fiber we have used. Therefore, effective absorption of the pump radiation over a short distance is of great importance. Fig. 2b shows a cut through the intensity distribution of the exited mode near threshold. Fig. 2c shows a cut through the intensity distribution at 5 times threshold.

 

Fig. 2b Cut through the near field at 1 mW output power

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Fig. 2c Cut through the near field at 10 mW output power

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In most cases a nearly-Gaussian-shaped intensity profile is retained up to the highest output powers. This invariance is an indication for the stability of this mode. An increase in width by a factor of two was found in going from low to high intensities, but no higher mode does appear.

An interesting aspect concerning the laser radiation distribution at laser threshold is shown in Fig. 3

 

Fig. 3 Near field intensity distribution (λ = 1060 nm) at the laser threshold.

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The laser light is found not only in the core but also in the silica strands. We believe that about two periods of holes are responsible for guiding the laser light generated in the core.

For the sake of completeness we present lasing spectra at two different power levels in Fig.4.

 

Fig. 4 Emission spectra at two different power levels. Upper graph: P_out = 0.5 mW, lower graph: P_out = 10 mW

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In going from threshold to 5 times threshold, a three fold broadening occurs.

4. Conclusion

We have described the successful realization of a single mode MFL at λ = 1060 nm. Optical pumping of the lasing ions was achieved by illuminating the cladding and the core. The Nd-doped microstructure fiber is cladding pumped in terms that the pump radiation illuminates substantial parts of the air-hole cladding. The absorption efficiency is quite good compared to standard double-clad fibers with non centric cores because the microstructure may be responsible for improved pump mode mixing. The effect of bending the fiber was demonstrated. The use of low-brightness, high-power diode lasers offer the possibility of achieving high output powers in the lowest-order mode of a large-mode-area fiber with short length.