A large-mode area polarization maintaining single-mode ytterbium-doped fiber amplifier with distributed narrow passband filtering is demonstrated. The fiber passband is 40nm wide and centered at 1070nm for efficient filtering of both short- and long-wavelength amplified spontaneous emission as well as stimulated raman scattering and four-wave-mixing. The fiber shows reduced bend sensitivity, has a mode field diameter of 27μm and exhibits a slope efficiency of more than 65%.
©2009 Optical Society of America
Recent advancements in microstructured fiber designs for pulsed fiber amplifiers has increased the mode field area and made higher peak powers possible, by lowering the nonlinear threshold through large-core designs, while maintaining the much desired single-mode operation. This is achieved either through low core NA [1,2], differential mode loss  or resonant coupling of Higher-Order Modes (HOMs) . Still, the nonlinear effects set the upper limit for achievable peak power. For short pulses (<0.5ns) Self-Phase Modulation (SPM) induces spectral distortion, while for long pulses (>2ns) Stimulated Brillouin Scattering (SBS) seems to be the limiting effect for spectrally narrow sources. In the intermediate regime, with pulses around 1 ns, the limiting nonlinearity is speculated to be Stimulated Raman Scattering (SRS) and Four-Wave-Mixing (FWM) due to gain-induced phase matching [5,6]. Furthermore, as fibers are maturing other practical issues arises such as the need for compact fiber coils, beam delivery, suppression of Amplified Spontaneous Emission (ASE) and/or suppression of detrimental wavelengths generated through FWM, which can destroy the pumps since these wavelengths are not well isolated in standard pump reflectors. Compact coiling and low NA SM operation is often conflicting properties and higher core NA must be used to achieve sufficient form factor. ASE can be filtered out in multi-stage amplifiers using discrete filters but these have often low damage threshold. Distributed Spectral Filtering (DSF) of ASE is possible using photonic bandgap fibers and has been demonstrated to suppress long-wavelength ASE for lasing at 980nm  or suppression of short-wavelength ASE, which allows for efficient amplification at 1150-1200nm , but the bandgap structure may reduce the slope efficiency by guiding some of the pump light. This can be circumvented by using a hybrid Photonic Crystal Fiber (PCF) design , where a combination of air holes and high-index rods create a Total-Internal Reflection (TIR) guiding fiber having the spectral characteristics of photonic bandgap fibers. Recently, this design has been implemented in an all-solid version [10,11] for suppression of short-wavelength ASE. SRS can be a problem in delivery fibers for high-power systems but can for example be suppressed using a W-profile core . In this paper, a single-mode hybrid ytterbium-doped Large-Mode-Area (LMA) fiber amplifier is demonstrated, which utilizes anti-symmetric inclusions of high-index rods in the cladding region to create a narrow DSF functionality of only 40nm bandwidth. The DSF fiber filters out both short- and long-wavelength ASE as well as SRS/FWM and offers unique compact coiling properties compared to standard hexagonal LMA designs.
2. Symmetric and anti-symmetric passive DSF fibers
Three passive DSF fibers were fabricated (P1, P2 and P3) by stacking up a hexagonal air hole structure around a 7 cell silica core. On opposite side of the core, two rows of capillaries were replaced with high-index germanium rods (NA~0.29, 100/140, graded index), which creates a hybrid fiber that guides both by modified total internal reflection and the photonic bandgap effect. The rods provide a resonant path for the core light to couple to the cladding when the core and rod modes are phase matched. In P1, the diameter of both rows of rods is ~7.1μm, the pitch is 10.0μm and the relative hole diameter (d/Λ) is 0.15. The identical rods create two identical resonant structures, which provide confinement of the core light within specific wavelength ranges given by the resonant condition :9], but with a different core design.
In P2 and P3, the diameter of one row of rods is reduced relative to the other. The P2 fiber is shown on Fig. 1 . The pitch of the fibers P2 and P3 is 9.8µm and 10.0µm, respectively. The relative hole diameter is 0.14-0.15. The ratio between the smallest and largest rod diameter in P2 and P3 are 0.88 and 0.83, respectively. The anti-symmetric rod design creates two different resonant structures, which, if designed correctly, provides a method for tailoring the bandwidth of the transmission band by proper scaling of the rod dimensions. Scaling one of the resonant structures directly scales the resonant condition and allows for resonant coupling of light with wavelength λ1 through one resonant structure and λ2 through the second resonant structure and guides between λ1 and λ2.
Figure 2 shows the transmission spectra of P1 and P3 (symmetric and anti-symmetric). The P1 fiber, with two identical resonant structures, exhibits wide transmission bands in accordance with Eq. (1), whereas P3 exhibits more narrow transmission bands caused by the presence of two different resonant structures.
Figure 3 shows the transmission spectra of P1-P3 from 950nm-1300nm. The spectral width of the transmission band has been reduced from ~200nm in P1 to only ~50nm in P3. Figure 3 also shows that a new long-wavelength band appears around 1150nm-1170m as the transmission band is made narrower. This is due to the presence of additional bandgaps found in hybrid structures .
Figure 4 shows the near field at 1064nm wavelength of the P2 fiber. The mode field diameter (1/e2) was measured (using a CCD camera) to 27μm.
Figure 5 shows simulated mode profiles of the guided mode of P3 near the short- and long- wavelength passband edge of the transmission passband. At wavelengths around the short-wavelength edge, the mode starts to leak out of the core through the largest resonant structure (left), while the mode leaks out through the smallest structure at the long-wavelength edge (right).
Theoretically, a straight 7 cell hexagonal LMA structure with an index-matched core having d/Λ = 0.15 supports several modes at 1064nm wavelength . But, when using resonant effects, HOMs can leak out of the core through resonant coupling when the effective index of the HOM is equal to the effective index of a rod mode. Figure 6 shows simulated near fields of HOMs at the same wavelengths as used for the fundamental mode simulation in Fig. 5 (left). Since the HOMs becomes phase matched with the rod modes of the largest resonant structure at longer wavelengths than the fundamental mode, HOMs will resonantly couple out of the core close to the short-wavelength edge of the passband.
This effect is clearly very sensitive to both pitch and hole size and was observed in the P3 fiber. Near the short-wavelength passband edge the HOMs couples to the largest structure (defining the short-wavelength edge) and leak out through resonant coupling. Figure 7 shows a series of near field images (at 1064nm) of the fiber output when the input is scanned in the x and y direction. It was not possible to excite any HOMs in a 2 meter section of straight fiber. Although this effect was not observed in the P1 fiber, no HOMs could be excited in a slightly bend 1 meter section, having a minimal local bend radius of 37.5cm, indicating that the HOMs are very leaky and no special coiling or launch effort need to be taken to operate the fiber in the fundamental mode
The group birefringence was measured using the fixed analyzer technique, where white light is launched at a polarization 45° to the fiber axis and the output light is collected through a polarizer angled at −45°. Figure 8 shows the transmission spectrum, the polarization beat spectrum and the calculated group birefringence for the P2 fiber. The birefringence is >1e-4 from 1030nm-1050nm. Figure 9 shows the polarization crosstalk for P2 and is measured to >20dB (noise limited) in the entire passband from 1030nm-1110nm (measured on 2 meter fiber coiled to 15cm radius) and indicate good polarization maintaining behavior.
Bend characteristics of bandgap fibers are quit different than index-guiding fibers, due to the formation of bands of modes both above and below the core index. Long-wavelength light will, therefore, leak out through the inside of the bend, while short-wavelength light will leak through the outside of the bend .
Since the fibers P2 and P3 are anti-symmetric, the largest and smallest resonant structure defines the short- and long- wavelength edge, respectively. Coiling the fiber with control of the two resonant structures in the coil provides unique coiling characteristics. Figures 10 -12 show transmission spectra of the fiber when the fiber is coiled with the smallest rods oriented at the outside of the bend, inside of the bend and orthogonal to the bend plane, respectively. 2 meter of fiber was used and coiled in a single turn in a given coil diameter.
The plots show that both the passband edges as well as the overall loss is relative insensitive to coil diameter if the smallest structure is located at the outside of the bend (Fig. 10). If the fiber is coiled with the smallest rods oriented inside the bend, both passband edges becomes very sensitive to coil diameter and provides a mean for controlling the width of the passband by controlling the coil diameter (Fig. 11 ). But, in both coiling configurations, the transmission loss is considerably lower than for a standard 7 cell LMA structure, which is represented by Fig. 12, and the rods facilitates more compact coiling than possible with a standard 7 cell LMA structure. The opposite was concluded in , where a single cell all-solid structure showed less bend sensitivity when coiled with the rods orthogonal to the bend plane i.e. bended in the TIR plane. This was attributed to a larger index-spacing between the guided mode and the TIR cladding modes than to the rod modes. The TIR spacing was calculated to ~3e-3 in the middle of the passband, while the mode-spacing to the rods modes was only ~2e-3. In the 7 cell LMA structure, the TIR spacing is only ~3e-4 i.e. on order of magnitude smaller than in the single cell case and coiling in the TIR plane, therefore, shows higher bend loss. The small TIR spacing is fundamental for low NA LMA fibers and controlling the orientation of resonant structures in the coil (both symmetric and anti-symmetric designs) may, therefore, provide a mean for achieving more compact coiling of single-mode fibers having large mode field area.
3. Anti-symmetric ytterbium-doped DSF fiber
An ytterbium-doped anti-symmetric DSF fiber (A1) with a transmission passband similar to the P3 fiber was fabricated. The fiber had similar dimensions as the passive fiber but further included a 191μm pump air-clad, which yielded a pump absorption of 8-10dB/m at 976nm. The fiber had an outer diameter of 425μm and was coated with a high-temperature acrylate coating. 7 meter of fiber was coiled and oriented with the down-scaled rods at the outside of the bend in a 40cm coil diameter. The fiber was backward-pumped using seven combined 105μm/0.15NA 915nm single-emitter pump diodes and the co-pump propagating core ASE spectrum was recorded and shown on Fig. 13 .
The ASE spectrum, which reflects the transmission passband, is only 40nm wide and centered at 1070nm. Lower levels of modulated ASE from 1110 to 1190nm is also observed. The fiber efficiently suppresses ASE below 1050nm and above 1090nm as well as SRS at ~1114nm. The P3 fiber suppresses the SRS gain peak at 1114nm with >20dB for a 2 meter device length. The fiber was seeded with a 1064nm Continuous Wave (CW) signal and a slope efficiency >65% was measured (Fig. 14 ). Figure 15 shows the near field image of the fiber without any gain. The refractive index of the ytterbium-doped core was slightly higher than that of silica. HOMs could therefore be excited in the active fiber but fundamental mode operation was easy obtainable. A slightly down-doped core or smaller air holes would alleviate this.
To further demonstrate the ASE filtering, the fiber was seeded with a pre-amplified gain-switched diode delivering 5ns, 50 kHz gaussian pulses with 35mW average power. The signal spectrum contained ASE from the preamplifier and nothing was done to remove this. The seed spectrum is shown on Fig. 16 together with the amplified spectrum at 5W average output power. The pulse shape showed no degradation up to 9W output power (~25dB gain) where an ASE peak at ~1090nm decreased the signal/ASE ratio to below 20dB and no amplification above this level was attempted. The signal was amplified with 65% slope efficiency, which is shown on Fig. 17 . Figure 18 shows the input pulse and the output pulse at 8W output power and no degradation is observed.
The use of spatially confined resonant structures, such as the anti-symmetric design, in LMA fibers offers a high degree of flexibility in designing both the width and center wavelength of the transmission passband in fibers with distributed spectral filtering for suppression of ASE, SRS and/or FWM in amplifier systems. The design provides a mean for designing both passive and rare-earth doped fibers exhibiting spectral filtering, polarization-maintaining properties, higher-order mode suppression and compact coiling characteristics. These features can be implemented in standard air/silica hexagonal cladding structures (as here), in fluoride/silica cladding structures or in conventional step-index structures for efficient filtering or/and providing compact coiling. The features can be scaled to other wavelength regions of the emission band for amplification at for example 1100-1200nm as well as scaled to other core and cladding dimensions having higher or lower mode field area as well as higher or lower pump absorption.
References and links
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