We describe the design and characterization of solid core large mode area bandgap fibers exhibiting low propagation loss and low bend loss. The fibers have been prepared by modified chemical vapor deposition process. The bandgap guidance obtained thanks to a 3-bilayer periodic cladding is assisted by a very slight index step (5.10-4) in the solid core. The propagation loss reaches a few dB/km and is found to be close to material loss.
©2006 Optical Society of America
In high power fiber lasers, nonlinear effects severely alter the spatial and spectral beam qualities. To increase their threshold of appearance, Large Mode Area Fibers (LMAFs) are currently used. The increase in the core diameter has also the advantage to increase the doped area and thus the available output power. Step-index LMAFs are limited in the singlemode regime (necessary to reach diffraction-limited output beams) to core diameter D ~ 25 μm at λ = 1.08 μm corresponding to the emission wavelength of Yb3+-doped fiber lasers . In this case, curvature loss severely limits packaging and implies the use of almost straight fibers, making the laser somewhat cumbersome. Endlessly singlemode photonic crystal fibers (PCF) are also employed . The core diameter can be increased to 40 μm at the cost of a slightly multimode propagation. High-order mode filtering obtained by appropriate curvature allows to reach singlemodedness and very good spatial beam quality (M2<1.1) . Simpler holey fiber consisting of a solid core surrounded by a single ring of large holes has also proved to be a good candidate .
Solid core photonic bandgap fibers such as Bragg fibers  or low-index step PCFs [6,7] can also be used. The first case is of particular interest since the whole preform is made by Modified Chemical Vapor Deposition (MCVD) process only. The proposed Bragg fiber  was composed of high- and low-index concentric layers surrounding a low index broad (D ~ 34 μm) core. The high-index layers were Ge-doped so as to reach an index difference ∆n = 5.10-3. A thin fluorine doped outer layer was added to decrease curvature loss. Singlemode operation in a 4-m-long sample was observed. Inaccuracy of the refractive index profile (RIP) together with the small index difference in the cladding layers have led to relatively high propagation loss (~1 dB/m).
Reducing the propagation loss is of primary interest since in future rare-earth-doped LMA Bragg fiber laser or amplifier applications background loss will severely limit the net gain. Moreover practical use of these fibers could greatly take advantage of lowered bend loss. In this paper we demonstrate an improved design of LMA bandgap fiber with low confinement loss and report on characterization of fabricated fibers. We show that LMA fiber with low bend-sensitivity can be realized with widespread MCVD technique.
2. Design considerations
An example of considered index profile is shown in Fig. 1(a). The fiber consists of a low-index core surrounded by high- and low-index layers with index contrast ∆n. In bandgap fibers, confinement loss can be limited by increasing the number of cladding layers or increasing ∆n. Confinement loss in large core (D = 22 λ) bandgap fiber has been computed versus wavelength and plotted in Fig. 1(b) for values of ∆n achievable by MCVD. The maximum index contrast considered is 50.10-3, implying validity of the weak guidance approximation. Thus computations were carried out using a standard one-dimensional solver for the scalar wave equation. Confinement loss was compared to material loss computed taking into account a pure-silica core .
Bragg fiber proposed is the low-∆n cylindrical version of antireflecting resonant optical waveguide (ARROW) proposed by Duguay et al. . In such waveguides, provided the wavelength is much smaller than the cladding periodicity (See Fig. 1(a)), the spectral properties of the core mode can be deduced from the thickness (d) and ∆n of the first high-index ring. Low-loss spectral ranges are separated by large attenuation peaks attributed to cutoff of the ring modes . Due to the cylindrical symmetry of the proposed fiber only modes with zero azimuthal order have to be considered. Thus the band edges coincide with the cutoff wavelengths (λc 0,n) of the LP0,n modes of the ring where n > 0 is the radial order. λc 0,n can be solved for using the dispersion relation of the annular waveguide . For an ideal fiber with ∆n = 30.10-3 and d = 1.07 μm λc 01 → ∞, λc 02 = 0.606 μm and λc 03 = 0.301 μm. The band edge located at 0.6 μm is clearly seen in Fig. 1(b).
As demonstrated earlier , ∆n = 5.10-3 leads to propagation loss around 1 dB/m. Since the loss decreases when increasing ∆n, a large index contrast (∆n > 0.03) is necessary to reach confinement loss lower than material loss (~2 dB/km at λ = 0.8 μm). A second improvement can be done by slightly raising the core index. An example of this improved design is shown in Fig. 2(a). Index difference in the core is ∆ncore.
First the intensity distribution of the Bragg mode has been computed and plotted in Fig. 2(a). No major change has been brought compared to the distribution plotted in Fig. 1(a). Then the curve of effective index (ne) of the Bragg mode has been plotted in Fig. 2(b) versus the wavelength and compared to pure-silica cladding index and doped (∆n = 3.10-4) core index. It is worth to note that ne is comprised between those two indices for λ < 0.85 μm indicating total-internal-reflection guidance mechanism occurs for this spectral range. By looking at the electric field distribution for λ = 0.8μm, magnified in Fig. 2(c) for the purpose of clarity, one can observe the canceling at the low-n to high-n transitions which is a feature of ARROWs . Hence, the index-raised core implies a dual photonic bandgap and total internal reflection wave guidance mechanism to occur at short wavelengths. The confinement loss at λ = 0.8 μm has been computed and plotted in Fig. 2(d) for various ∆ncore. Calculations show that an index step in the core as small as ∆ncore = 3.10-4 allows to decrease confinement loss around 0.01 dB/km.
Accurately manufacturing such a positive index step is difficult by the MCVD process. On the other hand, the low-index layers may be F-doped so as to decrease the refractive index. Two preforms were prepared with ∆n = 15.10-3 and an effective ∆ncore = +5.10-4, values than can be readily obtained by MCVD process. The higher ∆ncore was chosen to ensure lower confinement loss. The number of layers’ pairs is still equal to 3. An outer F-doped layer was added to decrease bend loss.
Fibers were drawn with outer diameter (OD) equal to 125 μm, 170 μm and 220 μm so as to operate at 0.8 μm, 1.064 μm and 1.55 μm wavelengths, respectively. The measured RIP of the 125-μm-OD fibers are shown in Fig. 3.
In the very first manufactured fiber  the index transitions were very smooth. On the contrary in the new fiber the index transitions are steep. Moreover thicknesses fairly coincide with those deduced from the modeling stage. Note the RIPs are slightly different. In Fig. 3(b) the high-n layers are more Ge-doped. This should imply lower loss.
First, the modal behavior has been studied in the proposed structure. To do so, the beam from a singlemode fiber at short wavelengths (cut-off wavelength λc = 0.77 μm) has been travelled in front of the 125-μm-OD fiber along two orthogonal directions. This experiment was carried out for 2 pieces of fiber with lengths L1 = 1.2 m and L2 = 30 m drawn from the same preform. Near field intensity patterns have been plotted in Fig. 4 for centered and shifted excitations.
It is worth to note that the near field patterns of the short piece of fiber, obtained with shifted excitation, exhibit non Gaussian shape. This indicates a multimode behavior. On the other hand, whatever the launching conditions into the longest sample, the pattern in the core remains circular, indicating the 30-m-long fiber is singlemode. Obviously, when launching is strongly shifted, modes of the first high-index ring are excited. When propagation is perturbed (e.g. the fiber is bent with 2-cm curvature radius), the LP1,1 mode is clearly observed in the short sample whereas the field pattern at the output of the longest sample remains circular. This experiment confirms the singlemode behavior of the 30-m-long piece of LMA bandgap fiber. Higher-order mode filtering through large loss discrimination is a feature of leaky waveguides such as bandgap fibers and makes possible the fabrication of very large mode area bandgap waveguides as anticipated by Duguay et al. . Here, this results in vanishing of LP1,1 mode after a sufficient propagation length. This behavior was referred to as asymptotically single-mode propagation by Johnson et al. . Experimental and computed radial intensity distributions are shown in Fig. 3(a) and found in excellent agreement. The effective area of the fundamental mode has been computed to 140 μm2 at 833 nm.
This operation has been made for the various fiber samples and the same conclusions were drawn. The fibers are asymptotically singlemode at their operating wavelengths.
4.2 Spectral attenuation
The attenuation spectrum of the 30-m-long piece of 125-μm-OD fiber has been measured by the cut-back technique and plotted in Fig. 5(a).
The material loss spectrum is also reported. Broadband transmission has been observed from 0.7 μm to 1 μm. The minimal loss (~ 3 dB/km) occurs at a wavelength of 0.79 μm. The uncertainty on the measured value arises from the short length used. The loss spectrum of a 125-μm-OD fiber drawn from the other preform has been accurately measured using a 100-m-long sample. Results have been plotted in Fig. 5(b). The minimal attenuation reaches 2.8 dB/km at 0.81 μm. This value is very close to the material loss (2 dB/km). In these original structures, the confinement loss has been reduced to ~1 dB/km.
In the 170-μm-OD (resp. 220-μm-OD) sample, the minimal attenuation and the mode area have been measured around 10 dB/km (resp. 9 dB/km) and 270 μm2 (resp. 500 μm2) at 1 μm (resp. 1.5 μm), respectively.
For all the fabricated fibers, multiple bands of very high (~100 dB/km) attenuation appear in the spectrum. To better understand this phenomenon, the near field intensity pattern has been observed in a large attenuation peak (See Fig. 5(a)) for the 125-μm-OD fiber. The Gaussian mode is coupled to the LP8,1 mode of the annular waveguide consisting of the first high-index ring surrounded by the core and the first low-index ring. This F-doped layer is of finite dimension and the outer pure silica cladding was also considered. Extension of this layer is supposed to be infinite. Such discrete couplings have already been observed in dual concentric core fibers and used to realize spectral filters . In the proposed fiber, the modes’ parities differ and coupling should not occur. Let us point out the slight distortion of the LP8,1 mode intensity distribution. The localization of the field in the first quadrant may be attributed to bending during the experiment . Hence the overlap between the Bragg mode and the odd mode of the ring is not zero and coupling occurs. Such couplings are also a feature of hollow-core photonic crystal fibers, in which they allow for an exacerbation of nonlinear effects in silica . We have then computed the effective index curves of the Bragg mode and of some of the high-order modes of the annular guide alone versus the wavelength (See Fig. 6(b)). These curves cross at specified phase-matching wavelengths. Thanks to longitudinal fluctuations of the RIP or bending, the modes couple around the phase-matching wavelengths.
One other important fact, pointed out in Fig. 6(b), is that the effective index of the Gaussian mode is smaller than the index of silica constituting the outer cladding whatever the wavelength is. Then the ring modes and the supermodes radiate outside the fiber (they are cutoff) and lead to large attenuation peaks. As shown in Table 1 the peak attenuation wavelengths correspond to the computed cut-off wavelengths and not to the phase-matching wavelengths.
Cut-off is due to the F-doped inner cladding associated to the outer pure-silica cladding. To avoid these leaky couplings, an optimized fiber should not incorporate negative index step.
4.3 Bend loss
Bend loss has been measured in the 170-μm-outer diameter core fiber at a wavelength of 1.064 μm . For the purposes of comparison, a singlemode step-index fiber has also been prepared by MCVD process. The cut-off wavelength of the SIF has been measured to 1003 nm. The mode field radius is 9.5 μm at 1.064 μm, equal to that of the bandgap fiber. Results have been plotted in Fig. 7. Refractive index profile of SIF is shown in in-set. The bandgap fiber appears to be less bend-sensitive than the step-index fiber. For instance, for a bend-radius equal to 8 cm, bend loss has been measured to 0.065 dB/m in the bandgap fibre and 1.16 dB/m in the SIF. The bend loss has also been computed in LMA photonic crystal fiber with hole-to-hole spacing Λ = 15 μm, hole diameter d = 6.6 μm (d/Λ = 0.44 for singlemode operation) and 8 rings of holes. The mode field radius is equal to 9.5 μm. Hence the PCF properties may be compared to those of bandgap and step-index fibers. Computations using the simple technique reported in  showed the PCF is less bend-sensitive than the SIF but more susceptible to curvature than the bandgap fiber.
An improved fiber design associating total internal reflection and bandgap guidance mechanisms has been proposed to dramatically reduce confinement loss in large mode area fiber. Optimized fibers have been prepared by MCVD process. A loss reduction by 3 orders of magnitude has been demonstrated compared to the very first fabricated structure. No preconditioning is necessary to filter out high-order modes, provided the fiber is a few meter long. Couplings to cut-off modes of the first annular guide lead to high attenuation peaks. The minimal attenuation wavelength has been adjusted to 1.064 μm by properly scaling the fiber outer diameter. The bandgap fiber has been found to be much less bend-sensitive than step-index fibers with the same mode field diameter.
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