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We demonstrate the first polarization-maintaining, very-large-mode-area, Er-doped fiber amplifier with ~1100 μm2 effective area. The amplifier is core pumped by a Raman fiber laser and is used to generate single-frequency, one-microsecond, pulses with pulse energy of 541 μJ, peak power of 700 W, M2 of 1.1, and polarization extinction > 20 dB. The amplifier operates at 1572.3 nm, a wavelength useful for trace atmospheric CO2 detection.
© 2016 Optical Society of America
1. Introduction
The NASA ASCENDS (Active Sensing of CO2 Emissions over Nights, Days, and Seasons) mission is developing a fiber-laser, space-based LIDAR approach to CO2 sensing. The CO2 absorption line centered at 1572.335 nm has been chosen due to a confluence of several spectroscopic properties. It is relatively insensitive to temperature changes compared to other lines in the absorption band, free of absorption features from other atmospheric constituents, and has a convenient peak absorption amplitude that allows measurement of the full atmospheric column that optimizes SNR. It does not saturate, but is a large enough feature that it is easy to distinguish from background variations [1,2].
Fiber-based laser technology, in principle, has a number of advantages for space-based LIDAR systems, such as efficiency, weight, and robust, alignment-free operation. However, there are also challenges, as the measurement system requires low-repetition-rate (7.5 kHz), single-frequency, high-energy (> 500 μJ) pulses at a wavelength that is longer than is typical for high-energy Er-doped fiber amplifiers. Long wavelength operation requires correspondingly long amplifiers, and narrow-linewidth, high-energy pulses mean stimulated Brillouin scattering (SBS) will be a limiting factor. In addition, polarization-maintaining operation and diffraction-limited beam quality are essential.
There are a variety of reports of Er-doped fiber based sources of high-energy, narrow linewidth pulses in the 15xx wavelength range for LIDAR applications [3]. However, most work is at wavelengths closer to 1550 nm, too short for CO2 detection. For example, 1.1 kW peak power at 1545 nm in a 108 ns pulse was reported from Yb-free Er fiber [4]. A number of authors report high pulse energies and peak powers, but the work is based on multi-mode fiber and has poor M2 [5–8]. High aspect ratio, rectangular-core, Er-doped fibers produce very high pulse energies, but have not been demonstrated in an all-fiber format and the path to polarization maintaining operation is not clear [9]. Cladding-pumped, Yb-free Er fibers look promising, but to-date there is no polarization-maintaining demonstration [10]. A fiber laser for LIDAR using polarization-maintaining, commercial, off-the-shelf ErYb fiber has been demonstrated, but the relatively small effective area of the core made peak power scaling difficult [11,12].
Very-large mode area, (VLMA) Er-doped fiber amplifiers [13], core pumped by high-power 1480 nm, Raman fiber lasers [14], generate diffraction limited, high energy pulses at 1.5 micron wavelengths, and have applications in femtosecond fiber chirp-pulse amplifiers [15] and high-energy soliton generation [16], for example. They have been demonstrated with core diameters greater than 50 microns and effective areas greater than 1100 μm2. In general, the all-fiber, fusion-spliced architecture makes VLMA-Er fiber amplifiers ideal for the CO2 sensing application, except for the lack of polarization-maintaining demonstration to date.
Polarization maintaining operation is important for many LIDAR systems, but there have only been a few reports of PM, large-mode-area, Er-doped fibers. A 26 μm mode-field diameter (~530 μm2 Aeff) polarization-maintaining, Er-Yb doped, photonic-crystal fiber laser was previously demonstrated [17]. A multi-filamant fiber with 37 ErYb cores generated 940 W peak power with 1 MHz linewidth and an M2 of 1.3 at 1545 nm [18]. In addition this fiber had stress rods for polarization maintaining operation, but the polarization extinction ratio was relatively poor. A 25/250 μm ErYb fiber generated 1 kW peak power, 500 ns pulses at 1545 nm [19].
In this work, we demonstrate for the first time, a polarization maintaining, Er-doped VLMA amplifier with greater than 1000 μm2 effective area. We then use this amplifier to demonstrate high-energy, one-microsecond pulse amplification at 1572.3 nm. Single-frequency, 1572.3 nm, 1 μs pulses at 7.2 kHz repetition frequency were amplified to 700 W peak power with a pulse energy of 540 μJ. The polarization extinction ratio of the signal was better than 20 dB, and M2 = 1.1. We show that the PM-VLMA-Er amplifier is capable of meeting the optical requirements of the NASA ASCENDS mission.
2. Polarization-maintaining, very-large-mode-area, Er-doped fiber amplifier
A microscope image of the fabricated PM-VLMA Er fiber is shown in Fig. 1(a). The core diameter was approximately 50 μm, and erbium absorption at 1530 nm was 50 dB/m. A PANDA design was used to provide polarization-maintaining operation. The fiber was designed to have a birefringence beat length of 15.8 mm. The beat length was measured by measuring spectral interference caused by differential group delay between the polarization axes. The result of this measurement is shown in Fig. 1(b). The spectral fringe spacing was 7.3 nm for a 3 m length of fiber, corresponding to 14.1 mm birefringence beat length, close to the design target of 15.8 mm.
Fig. 1 (a) Microscope image of the fabricated PM-VLMA Er-doped fiber. (b) Measurement of the birefringence beat length via spectral beating of the polarization axes. The fringes had a spacing of 7.3 nm for a 3 m length of fiber corresponding to a beat length of 14.1 mm. (c) Low-power, polarization-extinction-ratio measurement with a broad-band source with the PM-VLMA fiber pumped to transparency, showing P.E.R. > 30 dB.
Next, the polarization extinction ratio (P.E.R.) of the 3 m length of fiber was measured by launching a polarized Er-ASE source and pumping the fiber to transparency. The results of this measurement, plotted in Fig. 1(c), show that at low power, the P.E.R. of the fiber was > 30 dB.
Figure 2(a) shows a schematic of the PM-VLMA Er amplifier pumped by a 1480 nm, Raman fiber laser. The seed laser consisted of a 7.2 kHz pulse train at 1572.3 nm with 1 μs pulses. The pump laser was a Raman fiber laser producing up to 20 W output power at 1480 nm [14]. The unpolarized Raman fiber laser output was combined with the polarized seed laser via a polarization-maintaining, fused-fiber, wavelength-division multiplexer. The output of the fused-fiber WDM was then fusion spliced to the PM-VLMA Er-doped fiber. The output end of the PM-VLMA Er amplifier was terminated with an 800 μm long coreless fiber that was angle polished at 8 degrees. The output of the amplifier was collimated with an 11 mm focal length lens. A 1530 nm high-pass filter was used to transmit the signal and reject unabsorbed 1480 nm pump and residual short-wavelength Stokes lines from the Raman laser.
Fig. 2 (a) Schematic of the 1572.3 nm amplifier using polarization-maintaining very-large-mode-area Er-doped fiber pumped by a 1480 nm Raman fiber laser. (b) 7.2 kHz, 1 ms pulse train at 1572.3 nm. (c) Average output power vs. pump power for a 3.75 m long amplifier compared to a 3.25 m long amplifier.
To generate the seed laser pulse train, an external-cavity diode laser with linewidth of approximately 400 kHz was amplified, and modulated with an electro-optic modulator to produce a 500 kHz pulse train with 1 μs pulses. The function generator driving the electro-optic modulator provided some limited functionality for shaping the rising edge of the μs pulses. The pulses were then amplified again, before stepping down the pulse repetition rate to 7.2 kHz using an acousto-optic modulator (AOM). After the AOM a final pre-amplifier stage was used to boost the average power to a maximum of 30 mW. Additionally a band-pass filter at 1572.3 nm was included after the final pre-amplifier to reduce out-of-band ASE. The fibers in the seed laser system were non-polarization maintaining, so a polarization controller and in-line fiber polarizer were added after the 1572.3 nm band-pass filter, before launching the pulses into the PM-WDM, which had polarization maintaining fiber pigtails.
Output power vs. pump power for two different PM-VLMA fiber lengths of 3.75 m and 3.25 m is shown in Fig. 2(c). Because of the operating wavelength of 1572.3 nm, a longer fiber length was used than is the typical value of 2.5 to 2.7 m when operating at 1550 nm. While the longer fiber length provided higher slope efficiency, the shorter fiber length enabled higher average powers due to the higher threshold for the onset of stimulated Brillouin scattering. Details on the measurement of SBS are given below.
The optical spectrum at 3.5 W output power from the 3.25 m amplifier fiber is shown in Fig. 3(a). The measurement of P.E.R. at this output power is shown in Fig. 3(b). While the ASE was largely unpolarized, the polarization of the signal at 1572.3 nm was > 20 dB.
Fig. 3 (a) Optical spectrum (OSA resolution bandwidth = 2nm) and (b) P.E.R. measurement in the 3.25 m amplifier fiber at 3.5 W output power. The ASE was largely unpolarized, but the signal P.E.R. was > 20 dB.
When amplifying high-energy, long pulses in a fiber amplifier, shaping of the input pulses is critical to counter gain induced pulse steepening [20]. This effect is illustrated in Fig. 4 for the PM-VLMA Er amplifier. Figure 4 shows input seed pulses compared to output amplified pulses for square pulses compared to pulses with a rising edge. In Fig. 4(a), it is evident that the square pulses generated by the electro-optic modulator undergo some amount of steepening in the pre-amplifiers following the modulator. This steepening is increased dramatically in the PM-VLMA amplifier fiber, limiting the achievable pulse energy due to nonlinearities caused by the sharp leading peak. By pre-shaping the pulses, the gain-induced steepening can be counteracted, and as a result, for a given output peak power, the achievable pulse energy depends on the quality of the pre-shaping. For these experiments, the shaping was relatively limited due to the function generator used to drive the EOM. Pulse temporal waveforms were measured using a photodiode with 100 ps rise-time together with an HP 86100A sampling oscilloscope with 50 GHz bandwidth.
Fig. 4 (a) PM-VLMA input seed pulse and (b) output pulses comparing square pulse amplification to shaped pulse amplification.
Using the shaped input pulses, SBS from the PM-VLMA amplifier was then characterized (Fig. 5). The backward propagating optical spectrum is illustrated in Fig. 5(a) showing the backward ASE, as well as the Rayleigh scattered 1572.3 nm peak. Most of the backward propagating power was contained in ASE. To monitor for the onset of SBS, the amplitude of the 1572.3 nm peak was monitored in the optical spectrum analyzer. The amplitude of this peak as a function of 1480 nm pump power and launch seed power is shown in Fig. 5(b). For seed powers greater than 12 mW, there is an increase in the slope of the 1572.3 nm peak at high pump powers, indicating the onset of SBS. The amplifier output power was not increased beyond the point at which the slope of the backward 1572.3 nm was observed to increase. In contrast, for a seed power of 5.6 mW, the increase in backward 1572.3 nm was not observed and SBS was not the limiting factor. Rather, the limitation was sporadic lasing of the backward ASE, which occurred at a pump power of approximately 13 W.
Fig. 5 (a) Backward propagating spectrum showing ASE and the 1572 nm Rayleigh scattered peak. (b) Amplitude of the 1572 nm backward-propagating peak as a function of pump power and launched seed power.
Pulsed performance vs. seed power for the 3.25 m long amplifier is shown in Fig. 6. Output power vs. pump power as a function of seed power is given in Fig. 6(a). Slope efficiency increases with increasing seed power, but the SBS threshold decreases, limiting achievable pulse energy at higher seed powers. As discussed above, at 5.4 mW seed power, the output power was limited by backward lasing of the 1550 nm ASE.
Fig. 6 Amplifier performance vs. seed power for the 3.25 m long amplifier. (a) Output power vs. pump power and launched seed power. (b) Pulse profile for maximum pulse energy vs. seed power. (c) Maximum pulse energy and (d) maximum peak power vs. seed power.
The temporal profile of the pulses for different seed power at maximum output power are shown in Fig. 6(b). The maximum pulse energy and maximum peak power vs. seed power are plotted in Figs. 6(c) and 6(d), respectively. The maximum pulse energy was 540 μJ and the peak power was 700 W, for a seed power of 12 mW. Although some pre-shaping of the pulse was utilized by adjusting the leading edge of the seed pulse, as discussed above, the output pulses still displayed a relatively large peak, as evidenced in Fig. 6(b). This peak in turn led to high peak powers, limiting the pulse energy. We expect that with better pulse shaping, significantly higher pulse energy could be obtained for the same peak power and level of SBS. For example, a 1 μs square pulse with 700 W peak power would have 700 μJ pulse energy.
Because of the low rep-rate of the pulse train, it is necessary to measure the fraction of total output power that is contained in the pulse (the pulse extinction ratio). For this experiment, we used the setup shown in Fig. 7(a). The output of the PM-VLMA amplifier was coupled into an acousto-optic modulator. The output of the modulator was split between a power meter and an optical spectrum analyzer. The AOM was used to select a time-slot corresponding to the pulse, or the time in-between pulses.
Fig. 7 (a) Setup for measuring fraction of output power in the pulse. (b) Optical spectrum in the pulse, compared to spectrum in-between pulses. (c) Fraction of power in the pulse, vs. output power.
The spectrum measured in the pulse, compared to the spectrum in-between pulses is plotted in Fig. 7(b). From this result it is clear that the ASE builds up in-between pulses, and the OSNR of the optical spectrum is a good measure of the pulse extinction. Alternatively, we measured the pulse extinction ratio directly with the power meter after the AOM. We calculated the pulse extinction both from the OSNR of the optical spectrum and directly from the power meter measurement. The two results agreed very well. The average of the two measurement techniques is shown in Fig. 7(c). For approximately 3.8 W of output power, 97% of the power was contained in the pulse, showing high purity of the pulse train, in spite of the low repetition rate.
Finally, the beam profile and M2 were measured at maximum output power (Fig. 8). M2 was measured with a commercial device based on a rotating slit (Thorlabs M2 measurement system). The M2 measurement was made with a CW seed laser, as the M2 measurement device did not operate with low rep-rate pulse lasers. We did not observe any significant beam changes when switching from CW to pulsed operation. Furthermore, S2 imaging measurements were made to quantify the residual higher-order mode content in the amplifier [21]. Residual higher-order mode content was found to be very low, with approximately 3% to 4% of the output power contained in the LP11 mode.
3. Conclusions
In conclusion, to the best of our knowledge, we have presented the first demonstration of a polarization-maintaining, very-large mode area, Er-doped fiber with effective area of 1100 μm2. Using this fiber we demonstrated amplification of single-frequency, 1 μs pulses in a 7.2 kHz pulse train at 1572 nm. 540 μJ pulses with 700 W peak power were achieved, ultimately limited by SBS. With further improvements in pre-shaping of the input pulses, we expect increases in output pulse energy. The output pulses had a polarization extinction ratio of > 20 dB, a diffraction limited beam with M2 < 1.2 and 97% of the output power contained in the signal pulse. The optical performance demonstrated with this PM-VLMA Er amplifier therefore meets the requirements of the NASA ASCENDS mission for CO2 sensing.
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