We demonstrate the first erbium-doped fiber amplifier operating in a single, large-mode area, higher-order mode. A high-power, fundamental-mode, Raman fiber laser operating at 1480 nm was used as a pump source. Using a UV-written, long-period grating, both pump and 1564 nm signal were converted to the LP0,10 mode, which had an effective area of 2700 μm2 at 1550 nm. A maximum output power of 5.8 W at 1564 nm with more than 20 dB of gain in a 2.68 m long amplifier was obtained. The mode profile was undistorted at the highest output power.
©2010 Optical Society of America
The need to mitigate nonlinearities such as self-phase modulation, Brillouin scattering, and Raman scattering in high-power fiber lasers has led to an increase in the fiber effective area (Aeff). A number of strategies have been presented to achieve large-mode area (LMA) high power fiber lasers. One approach, the rod-type fiber, is to reduce the index contrast and make the fiber rigid . However the fiber must be held straight to avoid significant bend losses, which eliminates many of the advantages of the conventional fiber geometry. Another approach, taken by chirally coupled fibers , leakage channel fibers , and helically coiled cores , is to operate the fiber in the fundamental mode and introduce additional structures into the fiber to add differential loss to unwanted higher order modes. Fibers operating in the fundamental mode however suffer from bend induced reductions in Aeff, consequently leading to increased nonlinearities and offsetting the advantage of the LMA design, with the effect becoming more pronounced as Aeff is increased [5,6].
Recently a new approach to high power fiber lasers was introduced: intentionally operating in a single, large effective area, higher-order mode (HOM) of a specially designed fiber [7,8]. Higher-order modes have the advantage that they are less susceptible than the fundamental mode to bend induced area reduction . At the same time, compared to the fundamental mode, they are more resistant to nearest neighbor mode coupling, which scatters the LPM,N mode into the LPM ± 1,N and is the typically the dominant form of mode-coupling in multi-mode fibers . The resistance to nearest neighbor coupling occurs because the difference in effective index between the LP0,N and LP1,N modes increases with increasing N.
Amplification in a cladding-pumped, Yb-doped, higher-order mode amplifier has been demonstrated . In this work we demonstrate, for the first time, amplification in a higher-order-mode, erbium-doped fiber with an Aeff of 2700 μm2. ErYb fibers have high absorption but are difficult to fabricate with large-mode area due to constraints on core composition required for reasonable Yb-Er energy transfer. Furthermore, cladding pumping of an Yb-free, Er-doped fiber is difficult as the low absorption would require a long length of fiber. Instead we use a high power, single-mode (LP01) Raman fiber laser operating at 1480 nm as the pump source. Use of a single-mode pump allows both 1480 nm pump and 1564 nm signal to be converted to the same HOM using a broad-bandwidth, long-period grating (LPG), thus obtaining optimal spatial overlap between pump and signal, which allows for short amplifier lengths. We achieve a clean higher-order mode output with 5.8 W output power and more than 20 dB of gain in a 2.68 m long amplifier. The output power was limited by available pump power.
2. Higher-order-mode, Erbium-doped fiber
The index profile of the HOM, Er-doped fiber, shown in Fig. 1a , had a small inner core and a large outer core, similar in design to the original, passive, HOM fiber . The small inner core was designed such that the LP01 mode had a mode-field diameter of 9μm, allowing for effective excitation of the LP01 when the HOM fiber was spliced to single-mode fiber. In contrast, the higher-order modes expanded to occupy the outer core. The diameter of the outer core was chosen to provide an effective area of the LP0,10 mode of 2700 μm2 Both inner and outer core were doped with erbium with an absorption of approximately 30 dB/m at 1530 nm; a measurement of the absorption of the fiber is shown in Fig. 1b.
In a higher-order mode amplifier the choice of which mode order to operate in is constrained by the need for both large Aeff and high modal stability. In contrast to the cladding-pumped HOM-Yb amplifier, the core-pumped HOM-Er amplifier has the additional constraint of requiring both pump and signal to propagate in the same HOM, and consequently the need for broad-bandwidth mode-coupling further limits the available modes that can be used. Note that in certain applications such as ultra-short pulse propagation additional considerations such as dispersion and dispersion slope can also come into play . The difference in effective index between nearest neighbor modes at λ = 1564 nm as a function of their effective area is plotted as points in Fig. 2a for the LP0,1 through LP0,10 modes. The points are labeled with the mode order LP0,N. There is a trade-off between effective area and mode spacing. The LP0,1 mode has a large mode spacing, but a small effective area similar to single-mode fiber. In contrast, low order modes, such as the LP0,2 and LP0,3, have very large Aeff’s, but the mode spacing is decreased. As the mode order is increased though, a range of modes is reached where Aeff is still large (in the range of 3000 μm2) and the mode spacing increases. For comparison purposes the mode spacing for a conventional LP0,1 step index fiber (SIF) with V = 5 is also plotted in Fig. 2a (solid curve). Note that an SIF in which the core index is adjusted to achieve large mode area with V-number greater than 5 would not have substantially larger mode spacing, but lowering the V-number below 5 can dramatically decrease SIF mode spacing. Therefore with appropriate choice of mode order, mode spacing and stability can be increased in an HOM fiber compared to a best-case conventional LMA fiber. Figure 2b shows calculated intensity profiles at λ = 1564nm for the LP01 mode and the LP0,10. The LP0,1 and LP0,10 had calculated Aeff’s of 55 μm2 and 2700 μm2 respectively.
3. Broad-bandwidth, long-period gratings
A UV-written, LPG can be used to convert between modes in an HOM fiber by providing phase matching between modes. The typical bandwidth of an LPG is usually only a few nanometers as the phase matching curve has a strong wavelength dependence. However, using appropriate waveguide engineering the phase matching curve can be manipulated such that a single grating period can provide phase matching over a broad range of wavelengths . In doing so, LPGs with high conversion efficiency over a bandwidth greater than 100 nm have been demonstrated. Such a bandwidth is sufficient to convert both the 1480 nm pump as well as the 15xx signal to the same higher-order mode allowing for optimal pump/signal overlap in the HOM gain region.
The phase matching curves of the HOM-Er fiber were characterized by writing a series of LPG’s with different grating periods and measuring the transmitted spectra and output modes as a function of wavelength. The results of this measurement along with corresponding beam profiles are shown in Fig. 3 . Experimental measurements are shown as points, and phase matching curves calculated from the index profile are shown as lines. Phase matching curves for the LP0,7, LP0,8. LP0,9, and LP0,10 are shown. A small vertical offset between theoretical and experimental curves has been removed in this plot, but no other fitting was performed on the theoretical curves. It can be seen that as the mode-order increases the slope of the phase matching curve decreases, and the LP0,9 and LP0,10 modes achieve broad bandwidth operation for a single grating period. Given the desired operating wavelengths of 1480 nm pump and 1564 nm signal, the LP0,10 was chosen as the more appropriate mode. In addition, the LP0,10 mode also fulfils the desired area and stability requirements shown in Fig. 2a.
The transmission spectrum of a broad-bandwidth LPG that converted the LP0,1 mode to the LP0,10 mode is shown in Fig. 4 . The grating was characterized by splicing the output of the HOM fiber to SMF in order to measure residual light in the LP0,1 mode after the grating.
Experiments comparing residual fundamental mode power after the LPG to interferometric measurements of relative mode strength show that the long-period grating transmission spectrum is a good measure of relative mode power and conversion efficiency . The conversion efficiency of the LPG to the LP0,10 mode was higher than 20 dB over the entire wavelength range of interest, and reached higher than 30 dB at 1564 nm.
4. The HOM-Er amplifier
The experimental setup of the higher-order mode amplifier is shown in Fig. 5 . A narrow linewidth, external cavity laser was amplified to 50 mW and combined with a high-power, single mode Raman fiber laser at 1480 nm in a single-mode pump/signal combiner. The output of the pump/signal combiner was fusion spliced to the HOM-Er fiber. The length of amplifier fiber after the LPG was 2.68 m. The amplifier fiber was terminated with an angle cleave.
For amplification experiments the seed laser was tuned to 1564 nm, where the LPG conversion efficiency was maximized. In these experiments, the maximum available 1480nm pump power was approximately 16 W, although much higher powers from Raman lasers have been demonstrated . To measure the output spectrum the beam was imaged and in the image plane, a single-mode fiber (SMF) probe coupled to an optical spectrum analyzer was used to sample different portions of the beam and measure the spectrum, as shown in Fig. 5. By scanning the probe fiber across the beam, the beam profile at different wavelengths could be obtained. This setup is similar to the recent demonstrated S2 imaging measurement for analyzing mode content in multi-mode fibers . However, S2 imaging requires a broad-bandwidth optical source, so the full S2 data analysis was not performed here.
The spectrum measured at full power when the SMF probe was placed at the center of the beam is shown in Fig. 6a . Signal, unabsorbed pump, and residual Stokes orders from the Raman laser are visible at the output of the amplifier. While the residual Stokes orders appear relatively strong compared to the signal when the spectrum is sampled at the center of the beam, the Stokes orders do not interact with the LPG and remain in the LP01 mode, and so the residual Stokes orders are actually weaker than they appear in Fig. 6a. The beam profiles obtained using the scanning SMF probe for various output wavelengths are shown in Fig. 6b. The 1564 nm signal and unabsorbed 1480 nm pump wavelength are clearly in the LP0,10 mode and show excellent spatial overlap with each other. A calculation of the overlap integral of the electric fields obtained from the measured index profile of the fiber for the LP0,10 at 1480 nm and 1550 nm shows a spatial overlap of 99.4%. Only one of the representative Stokes orders, 1310 nm, is shown for clarity. All of the wavelengths other than pump and signal were observed to be in the LP0,1 mode.
Figure 7a shows the output power as a function of 1480 nm pump power launched into the HOM fiber. Total output power, 1564 nm signal power, 1480 nm pump power, and power from other wavelengths are plotted. The HOM amplifier provided a maximum of 20.6 dB of gain. The measured slope efficiency at 1564 nm was 43.2%, and the maximum output power was 5.8 W for 15.3 W of pump power. This slope efficiency compares well with that reported for conventional large-mode-area, Er-doped fibers pumped at 1480 nm. An amplifier using a conventional LMA Er-doped fiber with 30 dB/m absorption at 1530 nm and Aeff = 875 μm2 was recently reported to have a slope efficiency of 53.5% in a 5.75m long amplifier and 35.8% for a 4 m long amplifier [16,17].
The beam profiles obtained at the signal wavelength using the scanning SMF probe are plotted in Fig. 7b, showing virtually no change for 1 W of signal power out compared to the maximum signal power out of 5.8 W, confirming that the beam profile is undistorted, even with high gain from the amplifier. Note that small changes in alignment make quantifying mode content difficult in a simple line scan measurement, and in future work, full S2 measurements on the HOM amplifier are planned.
In conclusion, we have demonstrated the first higher-order-mode, erbium-doped fiber amplifier. A high-power, single-mode Raman laser at 1480 nm together with a broad-bandwidth, long-period grating enabled pump and signal to propagate together in the same higher-order mode, optimizing pump/signal overlap and allowing for a short amplifier length of 2.68 m. Over 20 dB of gain was demonstrated from the amplifier. The broad bandwidth available for mode-coupling also makes this system ideal for chirped-pulse amplification of ultrashort pulses.
Portions of this material are based upon work supported by Naval Sea Systems Command Contract No. N00024-09-C-4143 (Prime Contractor Raydiance, Inc.). Any opinions, findings and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of Naval Sea Systems Command.
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