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We demonstrate scaling of the effective area of higher-order mode, Er-doped fiber amplifiers. Two Er-doped higher-order mode fibers, one with 3800 μm2 Aeff in the LP0,11 mode, and one with 6000 μm2 effective area in the LP0,14 mode, are demonstrated. Output beam profiles show clean higher order modes, and S2 imaging measurements show low extraneous higher order mode content. CW and pulsed amplifier experiments are reported. Nanosecond pulses are amplified to 0.5 mJ pulse energy with 0.5 MW peak power.
©2012 Optical Society of America
1. Introduction
The drive to higher output powers and pulse energies in high-power fiber lasers brings with it a corresponding need to mitigate nonlinearities such as self-phase modulation, Brillouin scattering, and Raman scattering. Consequently there have been a number of strategies proposed to increase the fiber effective area (Aeff), in order to achieve large-mode area (LMA) high power fiber lasers. One approach, the rod-type fiber, is to scale up the core size, in order to strictly enforce single mode operation and make the fiber rigid to avoid macrobend losses associated with a weakly guiding core [1]. However the restriction that the fiber must be held straight eliminates many of the advantages of the conventional fiber geometry. Another approach, taken by chirally coupled fibers [2], leakage channel fibers [3], and helically coiled cores [4], is to use a core contrast that is not single moded, operate the fiber in the fundamental mode, and introduce additional structures into the fiber to add differential loss to unwanted higher order modes. However, all these approaches use the fundamental mode, which suffers 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, multi-mode fiber [7, 8]. Higher-order modes have the advantage that they are less susceptible than the fundamental mode to bend-induced area reduction [9]. 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 typically the dominant form of mode-coupling in multi-mode fibers [10]. 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.
There have been a number of recent demonstrations of fiber amplifiers and lasers based on higher-order modes [11–16]. Amplification in a cladding-pumped, Yb-doped, higher-order mode amplifier has been demonstrated with 200 W output power with a slope efficiency of 57% in the LP0,10 mode with 3300 μm2 Aeff [12]. A reconverting Yb-doped fiber amplifier with 52% slope efficiency with amplification in the higher-order mode and an output long-period grating to convert the higher-order mode back to the fundamental mode was also reported.
In addition to cladding-pumped Yb-doped HOM amplifiers, core-pumped Er-doped HOM amplifiers have been demonstrated. The HOM-Er amplifier is a core-pumped design, with both single mode pump source and signal propagating in the same higher order mode, allowing for maximum pump-signal overlap and thus short lengths of amplifier fiber. A high power, single mode, Raman fiber laser operating at 1480 nm [17, 18] is an effective pump source for core pumping Er-doped fiber amplifiers. Core pumping Er-doped fiber amplifiers helps keep the fiber short (length of few meters) and also allows for large-mode areas, both of which are important for mitigating nonlinearity in high pulse energy systems. Using the core-pumping architecture, CW amplification in an Er-doped HOM fiber in the LP0,10 with an effective area of 2700 μm2 was demonstrated [15]. Nanosecond pulse amplification in an HOM-Er fiber has also been demonstrated in the LP0,9 mode with an effective area of 2440 μm2 and the nonlinear properties of this amplifier compared to a conventional Er-doped LMA fiber with 800 μm2 Aeff [16]. Both amplifiers were limited by modulation instability. The nonlinearity of the HOM amplifier was found to be significantly lower than a conventional LMA amplifier, decreasing in proportion to the increase in Aeff. Furthermore new wavelengths generated through modulation instability were in the LP0,9 mode as well. A maximum pulse energy of 100 μJ was achieved in these experiments, corresponding to 100 kW peak power in the 1 ns pulse.
In addition to HOM-Er fiber amplifiers, there has been recent interest in other types of Er-doped fiber amplifiers for generating high-energy pulses at 1550 nm. Many high energy pulsed amplifier results in Er-doped fibers often rely on multi-mode operation [19–23].
In this paper, we report further scaling of the effective area of rare-earth doped, higher-order mode fiber amplifiers. We demonstrate amplification in two different core pumped Er-doped HOM fibers with effective areas of 3800 μm2 and 6000 μm2. These are, to the best of our knowledge, the largest effective area, higher-order mode amplifiers reported to date. S2 imaging measurements of the higher order mode content reveal low levels of extraneous mode content. Finally, we demonstrate pulse amplification in the 6000 μm2 Aeff fiber and achieve 1 nanosecond pulses with 0.5 mJ pulses and 0.5 MW peak power, a five times increase in peak power compared to the previous nanosecond pulse HOM amplifier result. This result is, to the best of our knowledge, the highest peak power achieved from a nanosecond, Er-doped fiber amplifier operating in a single, transverse mode.
2. Higher-order-mode, Er-doped, fiber amplifiers
Figure 1 shows a schematic of the index profile of a higher-order mode amplifier fiber, illustrating the HOM concept. A center peak in the index of refraction, referred to as the central core, guides the fundamental mode of the fiber, shown in red in Fig. 1. The higher order modes of interest in this work are the symmetric LP0,N modes, where N>1. The higher order mode, shown as a blue line in Fig. 1, resides in the large outer core region. The shaded area is doped with rare-earth ions such as erbium or ytterbium, to provide gain. Interestingly, the LP0,N higher-order modes of an optical fiber are truncated Bessel beams, and an HOM amplifier is an elegant and robust method for naturally generating high-power Bessel beams, which have intriguing properties such as diffraction-free propagation of the center spot and self-healing in the presence of obstructions [24].
Fig. 1 Index profile of a rare-earth doped, higher-order mode fiber. The LP0,1 mode is guided in the central core whereas the LP0,N higher-order modes reside in the outer core.
An important strength of the HOM platform is that the fundamental mode can be optimized for input coupling, while the HOM can be optimized for propagation. The fundamental mode designed to match well to the mode of single mode fiber, and thus readily lends itself to fusion splicing, aiding in the purity of the modal launch. Highly efficient conversion of the fundamental mode to a pure higher-order mode is accomplished with a UV-written long period grating, providing high modal specificity through phase matching and stable launch conditions.
A schematic of the Er-doped, HOM fiber amplifier is shown in Fig. 2 . A 15xx signal laser and high power 1480 nm Raman pump laser are coupled together using a single-mode fused fiber wavelength division multiplexer (WDM). The output of the WDM is fusion spliced to the HOM fiber, launching both pump and signal into the LP01 mode. An LPG provides high purity (> 99%) coupling to the desired LP0,N higher order mode, where amplification takes place. The output end of the HOM fiber is angle cleaved.
In the core-pumped configuration illustrated in Fig. 2, both pump and signal propagate in the same higher-order mode, which provides maximum pump/signal overlap, minimizes fiber length and helps suppress unwanted higher-order modes through differential spatial gain effects. Calculations of the overlap integral of the modes at 1480 nm and 1550 nm show spatial overlaps of better than 99%. 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 [25]. However, for a given fiber design, typically only one or two of the LP0,N modes meet the criteria of supporting both the 1480 nm pump and 1550 nm signal, which defines the higher-order mode that is used and consequently the fiber effective area in a core-pumped scheme [15]. A typical broad bandwidth LPG transmission curve is included in Fig. 2, with better than 20 dB conversion efficiency from LP0,1 to the HOM for both 1480 nm pump and 1550 nm signal. In a cladding pump amplifier, such as a Yb-doped HOM, the requirement on broad-bandwidth operation is relaxed as only the signal needs to be converted to the higher-order mode, and consequently, in a single fiber, a broad range of HOMs is available for use. In a cladding pumped amplifier, preferential amplification of only the desired signal HOM was observed with a pure modal launch (obtained via LPG mode conversion) and a sufficiently strong input signal [12].
Also shown in Fig. 2 is the typical diagnostic setup used in the amplifier experiments. An uncoated wedge reflected a calibrated portion of the main beam which was measured with power meters, an optical spectrum analyzer, phosphor-coated CCD camera for measuring the beam profile at 1550 nm, digital sampling oscilloscope and photodiode with 30 ps rise time, and an acousto-optic modulator (AOM) for measuring pulse extinction ratio. Long pass and short pass dielectric filters allowed for measuring the relative power in the unabsorbed pump, the signal, and residual Stokes wavelengths from the Raman laser pump [15, 18]. The main beam transmitted through the glass wedge was measured with a power meter.
3. CW Er-doped HOM amplifier with 3800 μm2 Aeff in the LP0,11 mode
HOM-Er fiber one had an outer core diameter of approximately 118 μm, fiber OD of 243 μm, and erbium absorption of approximately 30 dB/m at 1530 nm.The refractive index for this fiber, as well as the fiber presented in section 4, was similar to that shown in Fig. 1. Broadband LPGs were written that coupled both 1480 nm pump and 1550 nm signal from the fundamental mode to the LP0,11 mode, which had an effective area of approximately 3800 μm2, as calculated from the measured fiber index profile.
Amplifier results are shown in Fig. 3 , with the amplifier configured as shown in Fig. 2. The length of fiber in which the signal propagated in the HOM was approximately 4.5 m. The amplifier was tested with a CW signal at 1550 nm. Both high power (~1 W) and low power (0.04 W) were tested. With 1 W of input signal power, the slope efficiency at 1550 nm was 70%. When the launched signal power was decreased to 40 mW, the slope efficiency decreased to 41%. The decrease in slope efficiency for a low power seed was expected and is consistent with previous Er-doped HOM and conventional LMA fibers [15, 25] where output slope efficiency decreased for insufficient incident signal power. Measured output spectrum and seed laser spectrum are shown in Fig. 3(b) and the beam profile at 49.2 W is shown if Fig. 3(c). The optical spectra were measured with an Ando AQ-6315 B Optical Spectrum Analyzer with the resolution bandwidth set to 2 nm.
Fig. 3 Performance of the Er-doped amplifier based on the LP0,11 with 3800 μm2 effective area. (a) Output power vs. pump power. (b) Output spectrum at maximum power. Launched seed laser spectrum is also shown. (c) Beam profile at maximum output power.
The measured beam profile shows a clean LP0,11 output from the HOM amplifier at maximum power. However, in order to better quantify the amplifier output modal content, an S2 imaging measurement was performed. S2 imaging is a recently developed technique in which the spectral interference between different modes co-propagating in a multi-mode fiber is spatially resolved. The measurement can quantify the power level and differential group delays (DGD) of higher order modes with respect to the fundamental mode.
The original S2 measurement setup was implemented using a broadband source that was launched into a fiber under test [26, 27]. The output beam from the test fiber was imaged onto a single mode fiber which was scanned in position in the near-field image plane and coupled to an optical spectrum analyzer (OSA). An alternative setup for performing S2 imaging uses a tunable laser and CCD camera [21, 28, 29]. By measuring the beam profile as a function of wavelength, the same three dimensional set of data in (x,y,λ) is obtained as with the original S2 setup. A tunable laser with ~100 kHz linewidth and 1 pm wavelength step size was used as the optical source and an InGaAs camera was used to measure the beam profile at 1550 nm. By analyzing the spatial dependence of the optical spectrum in the Fourier domain, the beat notes between modes can be obtained and residual higher-order mode content can be quantified.
For these measurements, the tunable laser was amplified to approximately 50 mW in a single mode amplifier before being launched into a 4.5m long HOM-Er amplifier. The pump power to the HOM-Er amplifier was increased until the 1550 nm output power was approximately 2.5W. The tunable laser was tuned from 1555 nm to 1565 nm in steps of 0.01 nm. At each wavelength the beam profile was measured using the InGaAs camera.
Figure 4 shows the beam profile, mode beats as a function of DGD, and mode images and power relative to the fundamental mode, obtained from the S2 measurement. The beat note as a function of differential group delay is obtained by Fourier transforming the optical spectrum that corresponds to an individual camera pixel. The DGD plot in Fig. 4 is the sum of all the DGD plots from the individual pixels.
Fig. 4 S2 imaging measurement results from an HOM-Er fiber amplifier. Power in the mode relative to the LP0,11 mode is also shown.
The mode images and power relative to the fundamental mode were obtained through the S2 data analysis [20, 21]. The mode identification was made by counting the visible lobes in the mode images. The beam profile showed a clean LP0,11 mode. In addition to the residual LP0,1 mode there were other modes visible in the measurement results but the power contained in these modes was very weak. The LP1,11 was 25 dB weaker than the fundamental mode. Another mode, the LP6,8, was 30 dB weaker than the fundamental mode. Also visible in the DGD plot were spurious peaks caused by filters in the beam to remove unabsorbed pump light and attenuate the beam (the spurious peaks were determined by S2 measurements with SMF in the setup). These measurements show that the residual modal content other than the LP0,11 mode was very low and confirm the modal purity of the HOM amplifier. Furthermore, the mode shape at 2.5 W output power was very similar to that at 49 W output power, showing good modal purity over the range of amplifier output powers.
4. Er-doped HOM amplifier with 6000 μm2 Aeff in the LP0,14 mode
4.1 CW amplification in the LP0,14 mode
HOM-Er fiber two had an outer core diameter of approximately 148 μm, fiber outer diameter of 256 μm, and erbium absorption of approximately 30 dB/m at 1530 nm. Broadband LPGs were written that coupled both 1480 nm pump and 1550 nm signal from the fundamental mode to the LP0,14 mode, which had an effective area of approximately 6000 μm2, as calculated from the measured fiber index profile.
CW amplification in a 4.5m length of fiber, shown in Fig. 5 , was very similar to the 3800 μm2 fiber. With 1 W of input signal power, the slope efficiency was 66% and the maximum output power at the signal wavelength was 54.5 W. The slightly lower slope efficiency for this amplifier could be attributable to differences such as different cleave quality or different splice loss at the input to the amplifier.
Fig. 5 CW performance of an Er-doped HOM amplifier operating in the LP0,14 mode with 6000 μm2 effective area.
4.2 Nanosecond pulse amplification in the 6000 μm2 HOM fiber
For pulsed amplification experiments, a tunable, pulsed seed source was developed with flexibility in wavelength, pulse repetition frequency, and pulse width (Fig. 6(a) ). A tunable narrow linewidth external cavity laser (few 100 kHz linewidth) was amplified and modulated at 1 MHz repletion frequency with dual electro-optic modulators (EOMs) for high extinction ratio pulse carving. Measurement of the linewidth of the seed source after amplification was limited by the OSA resolution bandwidth of 0.05 nm. For the HOM amplifier experiments, pulses with 1 ns width were used. After modulation the pulses were amplified, filtered to remove ASE and amplified again. An acousto-optic modulator (AOM) was used to select pulses and reduce the repetition frequency. Finally, a large mode area (LMA) Er-doped fiber with 900 μm2 effective area [30] pumped by a 5W, 1480 nm Raman laser served as a final amplification stage. The output of the LMA amplifier was fusion spliced to SMF with a mode matched splice. At 500 kHz pulse repetition frequency a maximum of 1.2 W average power was available with a peak power of 2.4 kW in 1 ns pulses.
Fig. 6 (a) Schematic of the pulsed seed source used in the amplifier experiments. (b) Measured pulse extinction ratio at the output of the seed source as a function of pulse repetition frequency.
In low duty-cycle pulsed fiber amplifiers, power in between the pulses in the seed laser (due to leakage in the modulators, for example) can be amplified and account for a substantial amount of total output power [31]. For these systems, an AOM can be used to measure the pulse extinction ratio, i.e. the ratio of power in the pulse to the power in between pulses. For the pulsed seed source in Fig. 6(a), a second AOM was used to measure the pulse extinction ratio as a function of repetition frequency. The result of this measurement is plotted in Fig. 6(b). The pulse extinction ratio remained better than 20 dB for pulse repetition frequencies above 10 kHz, but dropped rapidly for lower frequencies.
The seed source was then launched into the HOM amplifier where it was converted to the LP0,14 mode with effective area of 6000 μm2, as shown in Fig. 2. The pulse extinction ratio measurement was repeated for the output of the HOM amplifier as a function of output signal power and repetition frequency. The result of this measurement is shown in Fig. 7 . As the output power was increased, the amount of power in between pulses also increased, with the pulse extinction ratio impairment being more significant for low pulse repetition frequencies. Therefore for a low duty cycle system it is critical to take the pulse extinction ratio into account when calculating pulse energy from the measured average power. All pulse energy and signal power measurements reported for the pulse HOM amplifier in this work were scaled by the measured pulse extinction ratio.
Fig. 7 Pulse extinction ratio measured at the output of the HOM amplifier as a function of pulse repetition frequency and output power at the signal wavelength.
The results of amplification of 1 ns pulses are shown in Fig. 8 . Output signal power versus pump power and pulse repetition frequency is shown in Fig. 8(a). For 500 kHz and 100 kHz pulse trains, the amplifier length was 4.5 m. For the 10 kHz pulse train, the amplifier length was cut to 4 m the limit the nonlinearity as much as possible. For HOM fiber lengths shorter than 4 m, the output power dropped off rapidly. The launch power was primarily limited by nonlinearity in the SMF pigtail between the seed laser (after the LMA pre-amp) and the LPG in the HOM amplifier. For the 100 kHz pulse train the average power of the seed input was approximately 50 mW, and for the 10 kHz pulse train the average power of the seed input was approximately 10 mW. The output pulse energy is shown in Fig. 8(b). Both average power and pulse measurements were calibrated to factor in the measured pulse extinction ratio. A maximum energy of 0.5 mJ per pulse was achieved for 10 kHz pulse repetition frequency, corresponding to 0.5 MW of peak power in the 1 ns pulses.
Fig. 8 (a) Average signal power as a function of pulse repetition frequency. (b) Energy per pulse as a function of pulse repetition frequency and pump power
For pulsed Er-doped fiber amplifiers the limiting nonlinearity is self-phase modulation which leads to modulation instability in fiber amplifiers with anomalous dispersion [16, 32]. Figure 9(a) shows the output spectra for 0.2 mJ pulses for 10 kHz and 100 kHz pulse trains. The modulation instability is visible in the side bands of the signal, however at 0.2 mJ per pulse the side bands were 10 dB lower than the previous demonstration of nanosecond pulse amplification to 0.1 mJ per pulse in a 2440 μm2 effective area HOM-Er amplifier. In addition the similar level of nonlinearity for similar pulse energies at different pulse repetition frequencies confirms that the pulse extinction ratio was correctly calibrated.
Fig. 9 Optical spectra for the pulsed amplifier. (a) Spectra for 0.2 mJ pulses for 10 kHz and 100 kHz pulse trains. (b) Spectra for 10 kHz pulse train for 0.2 mJ and 0.5 mJ pulses.
As the pulse energy was increased the level of nonlinearity also increased. Figure 9(b) shows the optical spectra for 0.2 mJ per pulse and 0.5 mJ per pulse in a 10 kHz pulse train. As the pulse energy was increased further above 0.5 mJ per pulse, the spectrum broadened substantially, leading to continuum generation and visible spectral components in the green.
Finally, Fig. 10 compares the amplified pulses at 10 kHz repetition frequency to the seed pulse launched into the HOM fiber. The pulses in Fig. 10 have been offset horizontally for clarity. At 0.4 mJ pulse energy, there was some steepening observed, but otherwise minimal distortion to the pulses. However, above approximately 0.45 mJ noise in the pulse appeared due to the increasing modulation instability.
Fig. 10 Amplified nanosecond pulses from the higher-order mode amplifier. The pulse from the seed laser is compared to output pulses at 0.4 mJ and 0.5 mJ per pulse in a 10 kHz pulse train. Pulses have been offset horizontally for clarity.
6. Conclusions
In conclusion, we have demonstrated scaling of the effective area of higher-order mode, erbium doped fiber amplifiers to effective areas as large as 6000 μm2 in the LP0,14 mode. The combination of ultra-large effective areas and core-pumping using a high-power Raman fiber laser is ideal for low-nonlinearity, Er-doped pulse amplifiers in the eye-safe wavelength range of 1550 nm. The residual higher-order mode content was measured using S2 imaging and found to be very low, with most higher order modes being more than 25 dB weaker than the primary mode. Using the HOM amplifier more than 50 W CW power was achieved at 1550 nm. In a 500 kHz pulse train 40 W average power was achieved, and in a 10 kHz pulse train, 0.5 mJ pulse energy with 0.5 MW peak power was obtained directly from the amplifier in a 1 ns pulse. This peak power is, to the best of our knowledge, the highest peak power obtained from a single transverse mode from an Er-doped fiber laser. The single transverse mode together with low residual mode content mean that, although the output beam from an HOM fiber is structured, it also has high spatial coherence and a stable beam profile, allowing the HOM beam to be reshaped to a Gaussian beam using bulk-optic approach [33]. Currently we see no impediment to scaling the HOM amplifier to even larger effective areas.
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