We demonstrate for the first time a flat-top mode from a 50 µm-core Yb-doped leakage channel fiber (LCF). The flat intensity distribution leads to an effective mode area of ~1880 µm2 in the straight fiber, an over 50% increase comparing to that of regular LCF with the same core diameter. The flat-top mode was achieved by using a uniform Yb-doped silica glass in the core center with an index of ~2 × 10−4 lower than that of the silica background. The fiber was also tested in a laser configuration, demonstrating an optical-to-optical efficiency of ~77% at 1026 nm with respect to the pump at 975 nm.
© 2013 Optical Society of America
Despite significant developments in fiber laser technology in recent years, there are still great needs to scale powers in both CW and pulsed fiber lasers for use in a wide range of industrial, scientific and defense applications. Optical nonlinear effects, such as stimulated Brillouin scattering (SBS), stimulated Raman scattering (SRS), self-phase modulation (SPM) and four-wave-mixing (FWM) are some of the key limiting factors in power scaling. All these nonlinear effects can be mitigated by effective mode-area scaling of fibers while maintaining single-transverse-mode operation. In addition, large effective mode-area can also lead to a high pulse energy which is desired in many applications, due to an increase in stored energy in the amplification process.
A large number of approaches have been studied for scaling up the effective mode-area by increasing the fiber core diameter while mitigating the waveguide’s tendency to support an increasing number of modes at large core diameters. One approach is to use a waveguide with a reduced numerical aperture (NA), such as a photonic crystal fiber [1,2] and a triple-clad fiber . Another approach is to introduce high differential mode losses between the operating fundamental mode and higher-order modes, such as in conventional large-mode-area (LMA) fibers with tight coiling , chirally-coupled-core fibers , leakage channel fibers (LCF) [6,7] and more recently all-solid photonic bandgap fibers (ASPBF) [8–10]. Typically, the mode intensity profile in these fibers exhibits Gaussian-like structure with higher intensity at the center of fiber core. As a result, the effective mode-area is much reduced compared to the physical fiber core area. Thus, a flat-top mode with a uniform intensity distribution is more suitable for larger effective mode-area. It has been shown that a flat-top mode can increase the effective mode area by ~60% without having to increase core size [11–13]. The flat-top mode is also of benefit in marking and material processing applications.
In this work, we demonstrate the first flat-top mode in a 50 µm-core Yb-doped LCF fiber. This was achieved by using a 30 µm uniform Yb-doped area in the core center with an index very slightly below that of the background silica glass by 2 × 10−4. The resulting flat-top mode has a significantly increased effective mode area of ~1880 µm2 in the straight fiber, i.e. ~50% larger than that of a regular LCF with the same core . The flat-top mode in this work is the largest even demonstrated, with ~6 times the effective mode area of the record demonstrated previously . The fiber was tested in a laser configuration, showing a slope efficiency of ~93% at 1026 nm with respect to the absorbed pump power at 975 nm. Focused beam propagation was also measured and simulated.
The fiber used in this experiment has a similar design to the passive LCF previously reported . The cross section is shown in Fig. 1. The fiber core is 52 µm at its smallest dimension (flat-to-flat) and 60 µm at its largest dimension (corner-to-corner). The doped area (the dim area in in the center of Fig. 1) is 30 µm in diameter and is made of a highly uniform Yb-doped glass with an index very slightly below that of silica glass by 2 × 10−4. The two-layers of features in the cladding are made from fluorine-doped silica glass with a refractive index of 0.0155 below that of silica. The cladding diameter is ~420 µm and is coated with a low-index polymer coating (n = 1.375) to guide the pump light with a NA of 0.46. Pump absorption at 975 nm was measured to be 1.05 dB/m.
The first optical characterization of this fiber was to determine the refractive index difference between the Yb-doped active glass and silica background glass (∆n = nbackground−ncore). White light, after going through a long pass filter with a cut-off at 1 µm, was launched into the fiber core of a short fiber. The measured mode image and intensity distribution across the core (see the white line in the mode image) are shown in Fig. 2(a). Meanwhile, the guided fundamental mode was simulated for the fiber at 1050 nm with various ∆n from 0.25 × 10−4 to 3 × 10−4 with increments of 0.25 × 10−4. The mode is not strongly dependent on wavelength for this LCF. The boundaries used in simulation were extracted from the measured fiber cross-sectional image and represent the geometry of the real fiber. Figure 2(b) shows the mode pattern at ∆n = 2 × 10−4 with intensity distributions for all the simulated index differences. The asymmetry in the mode is from the slight asymmetry in the fabricated fiber. By comparing these intensity plots, the index difference between active glass and background silica glass is estimated to be ~2 × 10−4.
To investigate the impact of index difference ∆n to the effective mode area, we simulated the effective mode area of the fundamental mode in a straight fiber for various ∆n from zero to 4x10−4, as shown in Fig. 3. The effective mode area is increased to ~1880 µm2 at ∆n of 2x10−4, thanks to the uniform intensity distribution of the mode. This is ~50% increase compared to the 1200 µm2 of the Gaussian-like mode with the same core size at zero index difference. From Fig. 2(b), we also found that the beam profile has the best flatness at the index difference ∆n of 1.75x10−4 with a slightly decreased effective mode area as shown in Fig. 3. However, obtaining index accuracy on the order of 0.25x10−4 in fabrication is very difficult. The wavelength used in the simulation is 1050 nm.
We also simulated and compared the effective mode area of the flat-top mode and the Gaussian-like mode in the regular LCF under various coiling conditions, as shown in Fig. 4. The solid line represents the effective mode area of the flat-top mode in the active LCF fiber with the depressed refractive index active glass (∆n = 2x10−4) and the dashed line represents that of the Gaussian-like mode in the LCF fiber with uniform core index. As shown in the figure, the effective mode area of the flat-top mode reaches 1750 µm2 at 5m bending diameter, while the Gaussian-like mode reaches its maximum of 1190 µm2. In a straight fiber, the flat-top mode has an effective mode area of ~50% larger than the Gaussian-like mode of regular LCF. However, the effective mode area of the flat-top mode decreases very quickly with a reduction of coil diameter, and is smaller than Gaussian-like mode below 1m bending diameter. This strong bending sensitivity of the flat-top mode is a result of intensity concentration in the area just outside the doped glass with a relatively higher index in the coiled fiber (see inset in Fig. 4). Thus, the flat-top fiber is more suitable to be used in a straight configuration to benefit from large effective mode area. One possible approach is to leave a long straight section at the end of an amplifier, where the core-guided optical power is near the maximum and the effective mode area is also the largest. Thus the optical power density can be kept below the nonlinear threshold. Another possibility is to use a short straight length of fiber with high rare earth dopant concentration, in which bending of fiber is avoided.
Simulation was also performed on the waveguide confinement loss in this fiber for bending diameters from 0.6 m to 1.2 m, as shown in Fig. 5. The wavelength used in the simulation is 1050 nm and ∆n is 2x10−4. The line with circular dots represents the fundamental mode LP01. The loss is below 0.01 dB/m when the bending diameter is above 1.15 m with the tendency of deceasing at larger coil size. The two lines with square symbols represent the next set of high-order modes LP11. The losses are slightly above 1 dB/meter at 1.2 m bending diameter with remaining around 1 dB/m at larger coil sizes. The two lines with triangle symbols represent the second set of high-order modes LP21. The losses are above 10 dB/meter at 1.2 m bending diameter and larger at larger coil size. The two dashed lines represent the set average for two sets of high-order modes. The high-order suppressions in this flat-top-mode LCF are much lower comparing to regular LCF of the same core diameter .
We also tested the lasing efficiency for this core-index depressed active LCF fiber. A 6 meter long fiber was right-angle cleaved on both ends to create a 4%- and 4%-reflection laser cavity. The fiber was coiled at 1 meter diameter during the test. A multimode diode laser emitting at 975 nm with a 250 µm delivery fiber was used as pump source. Laser at 1026 nm was achieved and emitted from both ends of the fiber which were measured for calculation of the slope efficiency. The lasing slope efficiency with respect to the launched pump power was measured to be ~77%, as shown in Fig. 6. By subtracting the residual pump, the lasing slope efficiency with respect to the absorbed pump power for this fiber is ~93%, very close to the theoretical limit of 95% for the laser at 1026 nm with pump at 975 nm, obtained by considering only the quantum defect.
The output laser beam quality was also examined by tracing a focused beam propagation over 15 cm-long distance and comparing with the simulation. The output laser beam was first collimated by an aspheric lens and then focused by a spherical lens with 15 cm focal length. After the focusing lens, the beam profile was captured with a CCD camera in 2 mm steps along the propagation direction and analyzed for dimensions, as shown in Fig. 7. The triangle and square dot lines represent the measured beam waist in horizontal X and vertical Y directions. The results also show that the laser beam keeps a hexagonal shape with a nearly flat top over 6 cm-long distance down to 300 µm in diameter and then continues to contract as a Gaussian beam down to 100 µm in diameter. We also simulated the flat-top mode with the multi-Gaussian method which uses a sum of coherent Gaussian beams to create a flat-top mode . The simulated flat-top beam propagation is also plotted in Fig. 7 in the dashed lines, which fits reasonably well with the experimental result. The difference around the beam waist is due to the fine details of the mode around its edge, which is hard to accurately represent in the simulation. The mode images in Fig. 7 were taken along the propagation from experiment (left) and simulation (right), which also show that the experimental result is consistent with theoretical analysis of a coherent flat-top beam.
We have experimentally demonstrated a flat-top mode in a 50 µm-core LCF in an attempt to further increase effective mode area. As a result of the flat intensity distribution, the effective mode area in a straight fiber is increased to 1880 µm2 compared to 1200 µm2 of a conventional LCF. We have also demonstrated a very high slope efficiency of 93% in this fiber. Our simulation shows that the flat-top mode is very sensitive to bending and needs to be used in a straight configuration to take full benefit of the increased effective mode area. Our simulation further shows that the cost for the increased effective mode area is the compromised higher-order-modes suppressions of a LCF. The flat-top beam profile can be maintained over a long working distance, which makes it useful for some applications in marking and material processing.
This material is based upon work supported in part by the U. S. Army Research Laboratory and the U. S. Army Research Office under contract/grant number W911NF-12-1-0332 through a Joint Technology Office MRI program.
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