1. Introduction

Due to the broadband spectral linewidth, high temporal stability and excellent beam quality, rare-earth-doped superfluorescent sources (SFSs) have many applications in various areas, including navigational-grade fiber optic gyroscopes (FOC) [1–3], low coherence interferometry (LCI) [4,5], and optical sensors and spectroscopy [6,7]. Since the power levels in the aforementioned applications are relatively low (normally in the range of several milliwatts to hundreds of milliwatts), most of the studies on SFSs were focused on optimizing the covering wavelength range to minimize the error induced by coherent effect [8,9], altering the emission wavelength ranges by using different rare-earth-doped media for extensive applications [10–15], and boosting the wavelength stability for high precision measurements [16–19]. However, along with the exploration of their great potentials in some other applications, such as pumping of Raman lasers [20], material processing and micromachining [21], generating high power widely tunable and/or narrow band sources for high power beam combining systems [22–26] and high power wavelength-division multiplexing applications, high power and high brightness SFSs have been strongly required and consequently become another hot spot recently.

Over the past decade, extensive efforts have been devoted to high power broadband SFSs based on different rare-earth-doped fibers. Traditionally, generating high power SFSs directly from single-stage architecture seems to be the most convenient approach. By using this technique, hundred-level output power had been realized by several groups in the 1 μm wavelength range [21,27,28], and about 16 W and 11 W output power were respectively achieved in 1.5 μm and 2 μm wavelength ranges [29,30]. However, due to the limitation of parasitic lasing induced by detrimental feedback from the output port, further power scaling is quite challenging by directly using single-stage SFS system. An alternative method to alleviate the influence of parasitic lasing effect is to use master oscillator power amplifier (MOPA) configuration, in which a low power SFS is amplified by multi-stage fiber amplifiers. By using MOPA structure, 1.01 kW and 122 W broadband SFSs have been demonstrated in the 1μm and 2 μm wavelength ranges quite recently [31,32].

Despite impressive results have been achieved, it should be noted that the polarization states of the high power SFSs above mentioned are all stochastic. In fact, in many direct or extendible applications, such as polarization beam combining for pumping of Raman lasers and SFS-seeded beam combining systems, except for efforts on power scaling, linear polarization and near single-mode operation should also be particularly considered. By far, the record power of linearly polarized SFS with broadband spectrum is just about 106 W in the 1μm wavelength range [33], which is much lower than that of stochastic polarization one. Besides, its structure includes some free space bulk-components. Output power of SFS with linear polarization in 1.5 μm wavelength range just stays at tens of milliwatts [34].

In this manuscript, we present a high power, linearly polarized (PER~19.1 dB), Yb-doped and all fiberized broadband SFS, which consists of a low power SFS seed and three stage all-fiber and polarization-maintained amplifiers. With limited 976 nm pump power, 1427 W output power is obtained with a slope efficiency of 80% and a M² factor of ~1.14. The wavelength range spans from 1050 to 1100 nm with a 3 dB band width (FWHM) of 11 nm. At maximal output power, the root-mean-square (RMS) and peak-vale (PV) values of the power fluctuation are respectively just 0.48% and within 3% during the observation time. Both the polarization extinction ratio (PER) and beam quality (M² factor) are degraded little during the power scaling process. To the best of our knowledge, this is the highest demonstration of all fiberized SFS with linear polarization and near-diffraction-limited beam quality.

2. Experimental setup

The experimental setup of the high power, linearly polarized, all fiberized broadband SFS based on MOPA configuration is shown in Fig. 1. The SFS seed is a homemade and co-propagated Yb-doped broadband one, which consists of a fiber pigtailed single mode laser diode (LD) with central wavelength of 974 nm, a broadband wavelength division multiplexing (WDM), about 21 m long polarization-maintained (PM) Yb-doped fiber with 6 μm core diameter and 125 μm cladding diameter, a 3 dB wideband and PM fiber coupler, and a PM fiber isolator (ISO1). Three cascaded all-fiber and PM Yb-doped amplifiers are used for further power scaling of the broadband SFS seed. As for the first stage amplifier (AMP-I in Fig. 1), 2.5 m long double-clad and PM Yb-doped gain fiber is employed, which has a core diameter of 10 um and a cladding diameter of 125 um. The cladding absorption coefficient of the gain fiber near 976 nm pumping wavelength is about 4.8 dB/m. The AMP-I is pumped by a multi-mode fiber pigtailed LD with maximal output power of ~9 W and central wavelength of ~974 nm via a (2 + 1) × 1 PM pump combiner (PM-PC). The output power of this amplifier is set to be about 0.5 W for the following amplification. In the second stage amplifier (AMP-II in Fig. 1), 1.5 m long highly doped (9 dB/m near 976 nm pumping wavelength), double-clad PM gain fiber is organized with a core diameter of 12 um and a cladding diameter of 125 um. The gain fiber in this amplifier stage is pumped by a wavelength-stabilized LD with central wavelength of 976 nm and maximal output power of 45 W via a (2 + 1) × 1 PM-PC. The output power in this stage is controlled to be 20 W power-level in the experiment. At the end facets of AMP-I and AMP-II, two high power PM isolators (ISO2 and ISO3) are respectively incorporated into the MOPA architecture to block off the backward powers from the following amplification. After AMP-II, a PM fiber tapper is used to diagnose the backward power in the period of power scaling of the main amplifier. The coupling ratios of the out port of the PM tapper to backward monitor port and signal port are 0.1% and 99.9%, respectively.

 

Fig. 1 The experimental setup of the high power, linearly polarized, all fiberized broadband SFS based on MOPA configuration.

Download Full Size | PDF

As for the main amplifier, six high power LDs (central wavelength of 976 nm, maximal output power of ~300 W) are injected into a (6 + 1) × 1 PM-PC to pump the gain fiber. The gain fiber in this stage is large mode area (LMA) and double clad PM Yb-doped fiber with a core diameter of 20 μm and an inner cladding diameter of 400 μm. Its cladding absorption coefficient is about 1.7 dB/m at 976 nm and 10 m long gain fiber is used to power extraction. The output end of the main amplifier is successively fused to a piece of 1 m long PM passive fiber and a high power fiber end-cap with~1.5 m long PM passive fiber for power delivering. About 0.5 m pump dump section is made in the 1 m long passive fiber for stripping out the residual pump and cladding light. The core and inner cladding diameters of the passive fibers remain the same as the gain fiber. The ultimately amplified superfluorescent light is collimated into free space by using a beam collimator.

3. Experimental results and discussions

In the experiment, the output power of the SFS seed is controlled to be about 30 mW, and its optical spectrum is shown in Fig. 2(a). The wavelength range of the SFS seed spans from 1028 to 1140 nm with a full width at half maximum (FWHM) of 25.6 nm, and its PER is measured to be 25 dB (99.7%) by using an angle-rotatable broadband Glan-Taylor prism. Firstly, the SFS seed is injected into the AMP-I for preliminary power scaling. After pre-amplification in AMP-I, the output power of the SFS seed is scaled to be 0.56 W at pump power of 2.2 W, and its PER is measured to be ~24 dB (99.6%). The emission spectrum at 0.56 W is shown in Fig. 2(b), which covers the wavelength range from 1030 to 1120 nm with central wavelength of 1051 nm and FWHM of 22 nm. Then, further power scaling of the super-fluorescence is performed in AMP-II. After AMP-II, the output power of the SFS is increased to 17.2 W at pump power of 32 W. The PER in this stage is measured to be ~20 dB (99%), and the output spectrum is shown in Fig. 2(c). Figure 2(c) shows that the wavelength range also covers from 1030 to 1120 nm after AMP-II. The FWHM and central wavelength are approximately estimated to be 24.9 nm and 1057 nm, respectively. From the previous results, it is shown that the spectral range and FWHM of the SFS seed are maintained well after the two pre-amplification processes. This is mainly attributed to the fact that short gain fibers are employed in the two pre-amplifiers (AMP-I and AMP-II).

 

Fig. 2 The output spectra of the SFS seed with figure (a), the first pre-amplifier (AMP-I) with figure (b), and the second pre-amplifier (AMP-II) with figure (c).

Download Full Size | PDF

As for the final amplifier, the output power (P_l) and actual backward power (P_b) as a function of the absorbed 976 nm pump power are shown in Fig. 3(a). Considering that the coupling ratio of the out port of PM tapper to backward monitor port is 0.1%, the actual backward power of the final amplifier is calculated by multiplying 1000 to the collected power values of the Monitor (shown in Fig. 1). Figure 3 shows that pump-limited output power of 1427 W is achieved at pump power of 1805 W. The power scaling process is nearly linear and the slope efficiency is fitted to be ~80%. At maximal output power, the backward power of the final amplifier is just about 67 mW, which is well below the power handling threshold of the ISO3. Consequently, we believe that further power scaling can be expected by using higher power pump sources. The emission spectra of the final amplifier at different power levels are shown in Fig. 3(b). The blue line in Fig. 3(b) is the output spectrum of the injected superfluorescent light (17.2 W) when it transmitted through the final amplifier, which shows that the FWHM is narrowed to about 13 nm and the central wavelength is red shifted to 1072 nm. These spectrally-narrowed and red-shifted effects are attributed to the stronger reabsorption loss for shorter wavelength range in the final amplifier. At maximal output power, the FWHM of the final amplifier is further narrowed to 11 nm and the central wavelength is further red shifted to 1074 nm due to the reabsorption of shorter wavelength range. From the results of the output spectra of the two preamplifiers shown in Figs. 2(b) and 2(c), it is concluded that employing highly doped gain fiber with short length is a feasible method to mitigate the spectrally-narrowed and red-shifted effects.

 

Fig. 3 The output power and backward power as a function of the absorbed pump power with figure (a) and the emission spectra at different power levels with figure (b).

Download Full Size | PDF

As such a high power SFS, for its many existed and potential applications, some physics parameters should be concerned seriously during the power scaling process. These aspects mainly include the beam quality (normally characterized by M² factor), the PER, and the temporal stability of the output beam. In the experiment, the M² factor (measured by Laser Quality Monitor, from PRIMES Company) and the PER of the output superfluorescent light are carefully investigated at different power levels, which are shown in Fig. 4(a). From Fig. 4(a), it is shown that the PER of the amplified superfluorescent light fluctuates between 17.3 dB (98.2%) and 19.1 dB (98.8%) and the M² factor is well below 1.2 during the power scaling process. At maximal output power, the PER and M² factor are measured to be 19.1 dB (98.8%) and 1.14, respectively. Figure 4(b) shows the temporal stability of the MOPA structure at maximal output power, which is measured by using an oscilloscope with 1 GHz bandwidth and a fast InGaAs photo-detector with 5 GHz bandwidth. The root-mean-square (RMS) value for the long time operating stability (350 s) is calculated to be just 0.48% and the peak-vale (PV) value of the temporal fluctuation is within 3%. Besides, the measured signal in microsecond time domain (shown in the inset of Fig. 4(b)) also shows that no residual peaks and/or fluctuations exist in the nanosecond and microsecond time regimes, which denotes that the amplified superfluorescent light operates stably without the influence of relax-oscillating and self-pulsing effects.

 

Fig. 4 The M² factor and the PER of the amplified superfluorescent light at different power-level with figure (a) and the measured temporal stability results at maximal output power with figure (b).

Download Full Size | PDF

4. Conclusion

In conclusion, we demonstrate a high power broadband SFS based on an Yb-doped, all fiberized and polarization-maintained MOPA configuration, which is structured by using a low power SFS seed and three stage all-fiber and polarization-maintained amplifiers. With increasing of the 976 nm pump power, 1427 W output power is obtained with slope efficiency of 80%, PER of 19.1 dB, and M² factor of ~1.14. At maximal output power, the wavelength range of the amplified superfluorescent light spans from 1050 to 1100 nm with central wavelength of 1074 nm and 3 dB band width (FWHM) of 11 nm. By investigating the M² factor and PER at different power levels, we conclude that both the PER and M² factor are maintained well during the power scaling process. The ultimately amplified superfluorescent light operates stably without the influence of relax-oscillating and self-pulsing effects. For long time observation, the RMS and PV values of the power fluctuation are just 0.48% and within 3%, respectively. Further power scaling of this system is only limited by available pump sources. With more powerful pump sources, higher output power can be expected in the future.