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Optical fibers in the ribbon geometry have the potential to reach powers well above the maximum anticipated power of a circular core fiber. In this paper we report the first doped silica high order mode ribbon fiber oscillator, with multimode power above 40 W with 71% slope efficiency and power in a single high order mode above 5 W with 44% slope efficiency.
©2013 Optical Society of America
1. Introduction
The average power of fiber lasers has increased dramatically in recent years [1]. Up to 10 kW fiber lasers are now commercially available [2–4], but diffraction-limited high power lasers in the region of 10s of kW to greater than 100 kW are needed for defense, manufacturing and future science applications. A balance of thermal lensing, and Stimulated Brillouin Scattering (SBS) for narrowband amplifiers and Stimulated Raman Scattering (SRS) for broadband amplifiers is likely to limit the average power of circular core fiber amplifiers to 2 kW (narrowband) or 36 kW (broadband).
Beam combining in free space can potentially break these single aperture power limits on circular core fibers, however, any beam combining approach would benefit from increased power in each unit cell [5–11].
A ribbon fiber, which has a rectangular core, operating in a high order mode can overcome these obstacles by increasing mode area without becoming thermal lens limited and without the on-axis intensity peak associated with circular high order modes. Conversion between the fundamental mode and high order modes of circular core fibers has been reported [12]. High order circular modes suffer from high peak power on axis while high order ribbon fiber modes exhibit modes in which the lobes are of nearly equal intensity. High order ribbon fiber modes can also be converted to a fundamental Gaussian mode with high efficiency for applications in which this is necessary [13,14].
Unlike the single core approach presented here, others have investigated multi-core ribbon fibers. In one such approach, a series of widely separated active cores are used which remain mutually incoherent [15]. The advantage of the approach presented here is that it can exhibit perfect beam quality (ideal v.s. actual mode overlap = 1) if operated in a pure single mode, unlike the case of incoherent cores with weak cross-talk.
There has also been work in moderately coupled cores which exhibit supermodes if well coupled and if de-phasing does not occur [16]. In the approach reported in this manuscript, there is no need to couple cores in a form of coherent beam combining, because there is only a single core with a ribbon like shape.
In another approach, a single core ribbon fiber is reported in which the fiber is very weakly guided in the wide dimension. This lack of guiding is used to select the fundamental mode which is their preferred mode, unlike in this paper which has a guided core and is intended to operate in a single high order mode [17–19]. An active ribbon fiber operating in a gain-selected high order mode has been previously demonstrated [20], but refractive index non-uniformity across the core created modal impurities, and its relatively high absorption, low melting-temperature phosphate-based glass limited its power to less than 1 W. Selective mode excitation in ribbon-core fibers [14] and efficient conversion from a high-order ribbon mode to a TEM00 beam [13] has already been demonstrated. Single mode oscillation in a multimode fiber by use of an intra-cavity spatial filter has previously been reported, but to the knowledge of the authors, not with an elongated core ribbon fiber [21]. In contrast to the previous work, this work is based on silica photonic crystal fibers. We have already reported amplification of a single high order mode in a silica based ribbon fiber amplifier [22]. Here we report the first multi-watt ribbon fiber oscillator, both multi-mode and single-mode, fabricated in doped silica via a stack and draw technique. This oscillator could be scaled to high power or used as a seed for a ribbon fiber amplifier.
We will first report a multimode ribbon fiber oscillator with greater than 40 W output power, and greater than 70% slope efficiency. Next we report high order mode oscillation with 5 W output power, and 44% slope efficiency. The air clad, ytterbium doped, ribbon fiber used for these experiments is shown in Fig. 1.
Fig. 1 Air clad rare-earth doped ribbon fiber. The central ribbon core has a higher refractive index than its surroundings, and thus preferentially guides light (as intended).
2. Ytterbium doped, air clad, ribbon fiber
The fiber used in this experiment was fabricated via a photonic crystal stack and draw process. Thirteen ytterbium-doped silica rods were stacked within a larger stack of un-doped silica rods to form a raised index ribbon core visible in Fig. 1. The Yb-doped silica rods were fabricated by Heraeus Tenevo; they are co-doped with 0.05 mol.% Yb203, and 1.0 mol.% Al203, with the manufacturer’s goal of 250 dB/m absorption at 976 nm. The Yb-doped silica had a refractive index of 2.54x10−3 above that of the pure silica giving a core NA of 0.086. The pump cladding was formed with a single circular row of silica capillaries which were expanded with a few kPa of air during the draw to form a 0.3 NA air-cladding. Air-clad fibers with NA > 0.45 have been reported in the literature [23], so we are confident our future fibers could achieve higher NA. The core is approximately 8.3 x 107.8 µm, while the air cladding has an inner diameter of 167 µm. The fibers outer dimension is 245 µm. The mode area within the doped core is approximately 600 µm2, and varies slightly depending on mode number. The normalized frequency in the narrow dimension is calculated to be 4.35, which is not quite single mode. To be completely single mode in the narrow dimension, the core height would need to be less than 5.99 µm. There is a greater discussion on fiber modes in § 3.2.
The fiber had a tendency to break with thermal stress, such as the stress induced by quickly turning the pump power up or down. We believe this is caused by the relatively thin outer wall of the fiber, and possibly the thin long webbing which connects the outer wall with the cladding. Other air-clad fibers have been reported without these thermal stress issues; so we are confident that this can be overcome in future fibers.
3. Ribbon fiber oscillator
3.1 Multimode
A multimode oscillator can be formed by flat cleaving one end of the fiber and providing a Fourier transform reflection on the other. Figure 2 shows the experimental setup with the optional mode selection filters which will be discussed later.
Fig. 2 Ribbon fiber oscillator experimental setup with optional mode selection. The cross-section of the spatial filter (top left) is designed such that only the two far field lobes of the desired mode can pass the filter
The Jenoptik JOLD-75-FC-11 fiber coupled diode stack at 915 nm was used as a pump. The pump delivery fiber has a core of 105 µm and an effective NA of 0.15 (the NA of the light exiting the fiber). The dichroic beam splitters used to separate the output beam from the pump are Semrock FF875-Di01 long pass filters, and can be angularly tuned to reflect the 915 nm pump light while transmitting longer wavelengths. The fiber coupling lenses FC-L1, FC-L2, and FC-L3 were 4.5 mm, 8 mm, and 6.2 mm in focal length respectively. For multimode operation, only one additional lens was needed to focus the output onto a 100% mirror at the beams Fourier plane.
The output diagnostics illustrated in Fig. 3 consists of measurements of power, spectrum, and the intensity in the near and far fields. A pair of lenses was used to create conjugate Fourier planes, and beam samplers direct the beam toward a camera placed in the near-field and far-field planes to record the intensity pattern. A removable mirror was used to direct the beam to an integrating sphere from which the spectrum was measured. The power meter was placed beyond the anti-reflection coated lenses, and power was only measured when all beam samplers, and removable mirrors were not in the beam path.
The manufacturer of the Yb-doped glass rods reports that the core is doped to achieve 250 dB/m at 976 nm, which translates into approximately 83 dB/m at 915 nm. The core mode/cladding area overlap is approximately 0.029. In this case the cladding absorption should be 2.4 dB/m, and the fiber length should be 8.4 m to absorb 20 dB of pump light in the absence of strong signal. However, the ideal fiber length was found to be 6-6.5 m through a cut-back optimization which suggests the existence of stimulated emission, some ASE absorption, scattering losses, and/or bending losses which are not accounted for in the simplified small signal absorption calculation.
Figure 4 shows the output laser power vs. coupled pump power giving a slope efficiency of 71%, and a maximum output power of greater than 40 W. Figure 5 shows the spectrum of the output centered at 1035 nm, with a 3dB bandwidth of ~3 nm. The beam quality of the multimode beam was measured to be (M2)x = 15 in the multimode wide dimension, and (M2)y = 1.6 in the narrow dimension which supports more than one mode. Figure 6 shows the beam waist profiles and M2 fit for both dimensions.
Fig. 4 Output power v.s. coupled pump power for a multimode ribbon fiber oscillator showing 71% slope efficiency.
3.2 Single mode
Most free space applications require a pure TEM00 mode, but higher order modes are more stable than low order modes due to greater effective index separation between neighboring modes. We have reported that ribbon fiber amplifiers can amplify a high order mode without losing modal purity through amplification, provided it is seeded by that mode. Converting a free space TEM00 mode to a ribbon mode to seed amplification can be accomplished [13,14,22], but it is not necessary if the seed oscillator oscillates in the desired high order mode. Therefore, it is desirable for a ribbon fiber laser to oscillate in a single high order mode for either conversion to the fundamental mode outside the cavity or as a single mode seed for a ribbon fiber amplifier.
The modes of a ribbon fiber are similar to those of a slab waveguide. The near field intensity of a 100% pure 8-lobed mode of a hypothetical ribbon fiber is shown in Fig. 7. The near field consists of a series of equal intensity lobes with alternating 0-π phase shift which closely resemble the cosine wave intensity patter of two plane waves interfering. The number of near field lobes scales with mode number. The far field, shown in Fig. 8, consists of two primary lobes at discrete angles with a series of smaller lobes between them. The number of smaller lobes also scales with mode number. The two primary far field lobes contain approximately 88% of the total power.
Fig. 7 Near field intensity pattern of a 7-lobed mode of a hypothetical ribbon fiber with dimensions of 100x10 µm and NA = 0.05. Note: the phase of each lobe alternates between 0 and π.
Fig. 8 Far field intensity pattern of a 7-lobed mode of a hypothetical ribbon fiber with dimensions of 100x10 µm and NA = 0.05.
The near field output of the oscillator is a superposition of all oscillating transverse modes, and so it is difficult to select only one with a spatial filter. However, in the far field, the primary lobes of individual modes are spatially separated, and a simple one dimensional amplitude mask can be placed in the far field such that the two primary lobes of the desired mode are transmitted and all other modes are blocked. This mask provides intra-cavity selection of a single high order ribbon fiber mode. A baseboard-profile measuring tool from a hardware store was used to provide an adjustable 1-D mask to select a high order mode of the ribbon fiber shown in Fig. 1. The mask must be placed in a far field plane in the external cavity formed by a set of lenses and a 100% mirror. Figure 2 shows the experimental setup with the optional mode selection optics and mask to the left of the fiber. The fiber coupling lens, FC-1, was 4.5 mm, followed by a 35 mm lens, 750 mm lens, 150 mm lens and a 100% mirror to complete the cavity. All lenses were placed at one focal length from the Fourier planes. The far field plane following the 750 mm lens was magnified to be 21 times larger than the far field directly behind the fiber coupling lens, FC-1. This allowed the use of a course amplitude mask to select the high order mode.
Alternative masking methods, such as an additional mask in the near field may add to purity. It has also been demonstrated that careful placement of the active cores can provide modal selection inside the fiber, but this was not done for this demonstration [20,24].
Figure 9 shows the output power v.s. coupled pump power for single high order mode operation with 44% slope efficiency up to 5 W before the onset of multimode oscillation. Above 5 W, a slow degradation of modal purity is observed with increasing power.
Fig. 9 Ribbon fiber oscillator operating in a single high order mode. Output power v.s. coupled pump power showing a slope efficiency of 44%
This mode, TE11,0, was chosen because it was the most experimentally stable mode (in the scalar approximation, the effective indices and mode profiles of the TE and TM modes are degenerate, and polarization is ignored). The fiber should support 15 modes which are zero order modes in the narrow dimension (1xN lobes), and 6 additional modes which are first order modes in the narrow dimension (2xN lobes). The chosen mode is not the highest order mode, but an upper middle mode. Higher order modes are predicted to be the most stable due to increased effective index difference between neighbor modes, but the highest order modes can be lossy. Despite supporting multiple modes in the narrow dimension, selecting a high order mode with the one dimensional spatial filter provides adequate suppression of the first order high order modes in the narrow dimension.
Higher power in a single mode should be possible as well as improved cavities and slope efficiency, but have not yet been fully explored. Figure 10 shows the near and far field of the oscillating mode for a series of different power levels showing a nearly unchanged modal profile. The first image at 65 mW is just above threshold, and the final image at 5.1 W begins to show the modal impurities rising in the far field. The small amount of power in a polluting mode, as can be seen in the far field profile, begins to grow slightly as the power is increased, but does not overtake the initial mode. Further experiments could better isolate the single higher order mode with better amplitude masks, and improved pointing stability of the beam on the mode selection side of the oscillator. The thermal stresses, observed through breaking of the air cladding, may also contribute to mode pollution, but improved thermal management, and improved fiber designs should mitigate this effect.
Fig. 10 Near field (top) and far field (bottom) of a single high order mode ribbon fiber oscillator. The modal purity is nearly unchanged from threshold (0. 65 W) to 5 W
The near field profile of a high order ribbon mode in Fig. 10 does not look like the ideal ribbon modes of a slab waveguide as the intensity varies from lobe to lobe. Other modes of this fiber show similar intensity variations as does the composite multimode near field profile. Modal impurities can contribute to this effect; however, the relative stability of the near and far field profiles as the power is increased suggests that this is mostly a single pure mode with a small amount of modal impurities. The variation in intensity between the lobes is likely due to non-uniformity of the refractive index and/or doping concentration of the active Ytterbium rods. This behavior has been observed in another multi-rod ytterbium laser [15]. Currently, our method of determining modal purity by applying the Gerchberg-Saxton phase retrieval algorithm requires prior knowledge of the fiber modes [13,22,25]. The modal purity is not calculated in this case because the actual modes of this fiber do not match the expected modes. In future research, a method of calculating modal purity which does not require prior knowledge of the modes should be employed such as the C2 or S2 method [26].
A solution to this issue of near field lobe uniformity could be to index match the ytterbium rods to the surrounding glass and provide guiding in the core via photonic crystal air holes which has demonstrated high order mode uniformity in the near field [22].
4. Summary and conclusions
Ribbon fiber lasers and amplifiers show great potential in increasing the theoretically limited power that can be achieved with circular core fiber lasers and amplifiers. Circular core fiber amplifiers are likely to be limited to 2 kW (narrowband), or 36 kW (broadband) by a combination of thermal lensing and stimulated Brillouin scattering (narrowband), or stimulated Raman scattering (broadband). In this paper we report the first multi-watt high order mode ribbon fiber oscillator with a fiber fabricated out of doped silica in a stack and draw technique. Laser output of 40 W with 71% slope efficiency was observed in multimode operation. Single high order mode lasing was demonstrated up to 5 W with 44% slope efficiency.
The active fiber results displayed less than ideal modal profiles in which each lobe of the near field was not of equal magnitude. This is likely due to inconsistencies in the doping concentrations and refractive index of the core rods. For this reason, future ribbon core fibers should be guided via the photonic crystal structure with the active core rods index matched to the surrounding glass. Although there was no modal selectivity in the fiber, the external far field amplitude mask selected a high order mode, and the mode remained pure until approximately 5 W at which point the impurities began to grow. For this reason, future ribbon core fibers should also have active core rods that are selectively placed in the core in a designed such that it would have high mode selective gain as was previously proposed [20,24].
Acknowledgments
This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. (IM release # LLNL-JRNL-635640).
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