We have fabricated an ytterbium doped all-glass double-clad large mode area holey fiber. A highly efficient cladding pumped single transverse mode holey fiber laser has been demonstrated, allowing continuous-wave output powers in excess of 1W with efficiencies of more than 80%. Furthermore both Q-switched and mode-locked operation of the laser have been demonstrated.
©2001 Optical Society of America
Holey fiber (HF) technology allows the fabrication of fibers with remarkable optical properties, many of which are not obtainable using conventional doped fibers. In particular, small core HFs, which possess inherently high non-linearity and extraordinary dispersion characteristics, have been developed and both passive and active devices have recently been demonstrated [1,2]. In addition, it is also well known that HF technology offers an alternative route to the realization of large mode area (LMA) fibers . The development of LMA fibers is important for a wide range of practical applications most notably those requiring either the delivery or generation of high power optical beams. For many of these applications spatial mode quality is a critical issue and such fibers should preferably support just a single transverse mode. Relatively large moded LMA fibers can be made using conventional fiber doping techniques such as modified chemical vapour deposition (MCVD) simply by reducing the numerical aperture (NA) of the fiber and increasing the fiber core size such that the V-value of the fiber is kept below 2.405. In this way single mode guidance at the design wavelength can be ensured. However, the minimum NA and thus the maximum core size that can be reliably achieved is restricted by the accuracy of the control of the refractive index difference between the core and cladding, and in practice by the onset of excessive bend loss as the guidance mechanism is weakened.
Holey fiber technology is attractive for LMA fiber production for a number of reasons. Firstly, LMA HFs as well as offering large core sizes also offer other unique and valuable properties, most notably they can be single-moded at all wavelengths, (and are thus truly broadband). LMA HFs also have different bend-loss properties compared to conventional LMA fibers  and we have recently shown that the bend loss in an LMA-HFs is at least comparable in magnitude to that of conventional fiber with the same mode area at a given wavelength . By exploiting the intrinsic flexibility of these unusual structures, it may be possible to design LMA HF structures with substantially improved bend loss properties. Finally, HF technology provides an alternative, and potentially more accurate route, to controlling the index difference between core and cladding regions of the fiber. Moreover, the fabrication technique readily allows for the incorporation of high NA air-clad inner claddings within jacketed all-glass structures . The approach is thus of great interest in the context of high power, rare-earth doped (e.g. Yb3+, Nd3+), LMA devices. The use of such rare earth dopants in LMA HFs is however challenging. The presence of dopants (and associated co-dopants such as Germanium, Aluminium and Boron that are required to incorporate the rare earth ions at reasonable concentrations and to maintain laser efficiency) modifies the refractive index of the host glass. This affects the NA of the fiber, and can lead to the loss of some of the most attractive LMA HF features. Recently, an ytterbium LMA-HF was reported  in which the issue of the raised index of the doped glass was addressed via the introduction of hundreds of small doped core regions dispersed across the fiber core so as to reduce the effective volume average index of the core region. Despite this, the modal profile was found to be influenced substantially by the localized doped regions within the core, especially at high powers. Moreover, this approach limits the area of the total doped region, thereby restricting the pump absorption and limiting the potential for energy storage, a key issue for pulsed fiber laser systems. For these reasons, a new strategy is required to make active LMA HFs that are suitable for use as high power lasers and amplifiers.
In this paper, we demonstrate a method for realizing such a low NA LMA HF. In addition, we show that it is straightforward to adapt HF technology to achieve an all-glass double-clad structure, which is advantageous for the efficient use of low brightness pump sources. Furthermore, we demonstrate Q-switching and mode-locking operations using this fiber.
2. Development of doped single-mode holey fibres
2.1 Fabrication procedure
Typically HF preforms are created by stacking capillaries around a solid rod, which ultimately forms the core. To produce active HFs it is thus necessary to produce a doped core-rod, which can be achieved either by extracting the core region from a conventional fiber preform (by drilling and/or etching), or by synthesizing a doped glass directly. The former route has the advantage that it allows for accurate control of the refractive index within the rod, and for this reason we employed a conventional MCVD technique to produce the doped core. To reduce the effect of concentration quenching associated with the presence of ytterbium ions , aluminum co-doping is used (note that aluminum only slightly increases the core index when incorporated at the required concentration levels and that the Yb ions themselves have little effect on the index). We fabricated a conventional aluminosilicate ytterbium LMA fiber preform with an NA of 0.05 (Δn~8.5×10-4) at 632nm, as shown in Fig.1, whilst retaining the ytterbium concentration at a relatively high level (~3000ppm by weight). Note that this NA would be too low for practical application due to high bend loss sensitivity. Note that the substrate material used for the doped core rod (Heraeus: Suprasil F300) is the same material that is used to produce the capillary tubes in the holey fiber cladding.
Since our fabrication procedure is based on a capillary stacking technique, it is straightforward to introduce a low index outer-cladding region simply by inserting thinner walled capillaries into the preform stack around the relatively thicker walled capillaries used to define the inner holey cladding region. The preform is then drawn down to fiber dimensions on a standard fiber drawing tower. A Scanning Electron Microscope (SEM) photograph of a fiber produced in such a manner is shown in Fig.2. The inner holey cladding region of the fiber consists of 5 rings of small air holes and two rings of larger holes are used to define the outer cladding. The inner-cladding NA was measured to be 0.3~0.4 in a short piece (~10cm) of the fiber, although it is to be appreciated that from a device perspective the effective NA is likely to be slightly lower than this value, and length dependent . In the inner cladding region, the hole diameter d and hole-to-hole spacing Λ were 2.7µm and 9.7µm respectively (so d/Λ ~0.3), and the fiber outer diameter was 175µm. By choosing the position of the thin wall capillaries within the preform stack it is also possible to break the symmetry of the cladding so as to enhance the pump absorption. An offset core can also be used to accomplish this purpose and we offset the core from the center of this fiber by one ring of holes (see Fig.2). The mode field diameter (MFD) was measured to be 12.3µm at 1047nm using the knife-edge scanning technique described in . Since the diameter of the doped region is ~Λ, the overlap of the mode with the doped area is ~60%. Due to the relatively small MFD, we were able to impose a reasonable bend radius (~10cm) to further enhance the absorption of the pump light without inducing significant bend losses for the core mode.
4.2 Single-mode guidance criterion
Although rigorous numerical analysis is in general required to predict the fiber characteristics of holey fibers accurately (see for example ), the effective index model  can be used to roughly take into account the effect of the doped core on the guidance properties of the fiber. The V-value of the doped HF can be written as
where λ is the wavelength, ρ is the core radius (here taken to be Λ for simplicity), and Δn is the index perturbation introduced by the rare-earth dopant within the core. The cladding index ncl is the effective refractive index of the cladding region, and which is typically a strong function of wavelength and can be calculated by several methods (see , –). Finally, although the area ratio of the doped region to the entire core should strictly be taken into consideration, for ease of calculation we ignore this factor here. The dashed line in Fig.3 indicates the value of Δn (as a function of d/Λ) that results in V=2.405 at 1.06µm, and so any structures located beneath this line are single-mode at this wavelength.
Our objective here is to introduce a doped core region (with some associated Δn) while retaining single mode guidance (V<2.405) at the operating wavelength. From Eq. (1), the following condition can be used to define the regime where the holes in the HF cladding dominate the guidance induced by the core dopant(s):
and the structures that satisfy the condition in Eq.(2) lie under the solid line in Fig.3. The range of physical parameter ranges for Δn and d/Λ in which guidance is dominated by HF effects (and hence allow broadband SM operation) is the region beneath both of the curves shown in Fig.3. Owing to the wavelength of interest (λ~1.06µm) and the relatively modest value of Λ (10µm) considered here, the index perturbation due to the doped core can be as high as 10-3 using a structure with d/Λ~0.3. We note that in the fabricated fiber the actual doped area is only ~30% of the geometric core area (the hexagonal region defined by the innermost air holes). Thus Fig.3 somewhat underestimates the regime in which SM guidance is possible.
3. Laser experiments
A single transverse mode Ti:sapphire laser operating at 976nm, where the ytterbium absorption is maximum, was used as a pump source to examine a laser performance of the fiber. A conventional Fabry-Perot cavity configuration was used for the fiber laser, and the pump light was passed through a dichroic mirror and then free-space coupled into the fiber shown in Fig.2. The coupling efficiency of the pump light into this fiber was measured to be ~70% using an aspheric lens with a focal length of 20mm. The laser cavity was formed by the ~4% Fresnel reflection from the launch end of the fiber and a lens-coupled high reflector placed at the other end of the fiber.
The laser output power characteristics are plotted in Fig.4(a) for a 4.5m long fiber. Slope efficiencies as high as 82% were recorded (in terms of absorbed pump power), comparable to the best conventional ytterbium fiber lasers. As expected, the output beam was observed to be robustly single-mode. We observed that long lengths of fiber were required for significant absorption to occur relative to the lengths expected from the measured core absorption. This occurs because of the low NA of the inner core/cladding structure, which leads to a significant fraction of the launched light being located in the cladding. Hence this particular laser should be regarded as a hybrid core/cladding pumped fiber laser.
In addition, a truly cladding pumped laser was realized using a low brightness fiber coupled laser diode at 915nm (100µm core with an NA=0.22). This pump was coupled to the HF using a 1x telescope (f=17mm). The achieved coupling efficiency was 60%, somewhat lower than in the experiment described above. This is partially attributable to the fact that the overlap between the cladding modes (which are concentrated in the glass regions and not in the air holes) and the uniform illumination provided by the laser diode is degraded relative to solid inner cladding fiber. However, such a launch efficiency was more than adequate for our purposes. Again using a simple Fabry-Perot cavity with 4% Fresnel feedback we achieved average powers in excess of 1W using a 7.5m long fiber with a measured slope efficiency of 70% (see Fig.4(b)). This value indicates that any losses associated with the unconventional holey inner-cladding are small enough so as to not seriously affect the laser performance. This said an increase in the inner-cladding NA would most likely improve the efficiency of this device since we know from our earlier experiments that the core is capable of >80% efficiency. Note that as well as optically isolating the inner structure from the external environment the air cladding also thermally isolates the laser. Although it might be imagined that this could lead to thermal problems under high power operation no such problems were encountered in these experiments even at multi-Watt pump levels. (Note that in the past we have observed spontaneous fiber breakage in fiber laser constructed from small core, large air fill fraction, Yb doped fibers when operating at the 100 mW power level  and that we have attributed to localized thermal breakdown in the core region.)
Furthermore, by inserting first an acousto-optic modulator, and then replacing this with an acousto-optic tunable filter in the cavity, we have demonstrated that it was possible to both Q-switch, and mode-lock our cladding-pumped LMA-HF laser. In the Q-switching experiment, we obtained ~50µJ stable pulse at repetition rates of a few KHz. The output pulse duration was ~10ns, and the corresponding peak power was 5kW. Far higher average powers (approaching 1W) could be achieved at higher repetition rates, albeit with longer pulses and lower pulse energies). Although the maximum output pulse energy was limited by the relatively small doped area, and it should be possible to increase significantly the extracted pulse energy by further enlarging the doped core region, and mJ pulses should ultimately be possible. In the mode-locking experiments we obtained fundamental modelocking over a very broad wavelength tuning range in excess of 60nm (as shown in Fig.5.). The pulse duration was estimated to be of order ~100ps. An output power of more than 500mWwas achieved for a pump power of 1.33W. Ultimately, it should be possible to develop compact ~multi 10nJ femtosecond pulse sources operating at 1µm using LMA HF in conjunction with the well established Kerr nonlinearity based stretched pulse mode-locking technique.
We have described the fabrication of an efficient air-clad, Yb-doped LMA-HF and described the criteria required to achieve single-mode guidance in a doped LMA HF. We have performed preliminary core and cladding pumping experiments on lasers built using this fiber. The primary advantages of these forms of fiber relative to conventional polymer coated dual clad fibers are that they will ultimately allow for all-glass structures, with larger inner cladding NAs (at least >0.5) and good pump mode mixing. In addition, they offer single-mode guidance in cores that at least as large, (but most likely larger), as those than can be made in conventional fibres. In device terms, these features will translate to advantages including, amongst others, the possibility of higher coupled diode powers (for a given cladding dimension), shorter device lengths and extended tuning ranges. The main drawback is likely to be thermal management if truly high power (e.g. 100W ) operation is required. In conclusion, we believe that holey fiber technology provides a promising alternative route to achieving large mode active devices.
Tanya Monro and David Richardson acknowledge the support of Royal Society University Research Fellowships.
References and links
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