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We present a widely tunable narrowband superfluorescent source near 2 μm employing a monolithic Tm-doped fiber amplifier (TDFA), and the output power exceeds 250 W. A broadband superfluorescent source with a narrowband tunable band pass filter was used as the seed source. The spectra of the seed source can be tuned in a range of ~1930-2030 nm with full width at half maximum (FWHM) of ~1.7 nm. The Tm-doped fiber amplifier scales up the power of the seed source to a level of more than 250 W with a tuning range of ~35 nm (1966-2001 nm) and a FWHM of ~1.5-2.0 nm, and the slope efficiency is about 0.50. The output power is limited by the available pump power, and the tuning range is limited by the amplifier spontaneous emission at other wavelengths. Higher output power can be achieved if launching more pump power into the amplifier, and the tuning range can be further improved by optimizing the parameters of the TDFA. To the best of our knowledge, this is the first demonstration on a widely tunable narrowband superfluorescent source at 2 μm with average output power exceeding 250 W.
© 2015 Optical Society of America
1. Introduction
Laser sources at 2 μm based on Tm-doped fiber (TDF) have the broadest emission bandwidth of all rare-earth doped fibers, and cover the window of absorption lines of numerous important atmospheric gases and liquid water [1, 2]. Thus, Tm-doped fiber lasers (TDFLs) can be widely applied in various situations such as eye safe LIDAR, remote sensing, laser communication, nonlinear frequency conversion, and material processing [2–7]. The development of TDFLs flourishes in recent years and the output power reaches 1 kW [8]. However, superfluorescent sources, also known as amplified spontaneous emission (ASE) sources, based on TDF have grabbed less attention during the last decades. TDF based superfluorescent sources, in fact, possess some special characters, including short coherence length, high temporal stability, and good beam quality, which enable superfluorescent sources to be significant light sources in applications of low coherence interferometry, fiber sensors, spectroscopy, and medical imaging [9, 10]. Furthermore, high power superfluorescent sources based on TDF can be high brightness light sources, which may be employed to replace high power TDFLs in many high intensity applications (industry processing, for example).
There are only several demonstrations on superfluorescent sources based on TDF in the past years [9–16]. Oh et al. reported the observation of broadband superfluorescent emission in silicate based TDF in 1994 [11]. Tsang et al. demonstrated superfluorescent sources based on TDF and Tm-Ho codoped fiber (THDF) with bandwidths (full width at half maximum, FWHM) of 20-50 nm in 2006 [12]. In 2012, Halder et al. presented a superfluorescent source with Bismuth–Thulium codoped fiber, and the FWHM covers from 1817 nm to 1984 nm [15]. Kuan et al. reported a superfluorescent source employing Tm-doped tungsten tellurite glass double-cladding fiber with a FWHM of ~45-140 nm in 2013 [16]. Honzatko et al. demonstrated a THDF based superfluorescent source with the maximum spectral width (~20 dB) of ~800 nm employing the combination of forward and backward ASE in 2014 [10]. However, the aforementioned reports can provide superfluorescent sources with output power at μW or mW levels, and superfluorescent sources with higher output power might be needed before they are used in various practical applications. Thus, Shen et al. demonstrated a high power superfluorescent source with a FWHM of ~36 nm and an output power of 11 W in 2008 [13]. Since higher output power of single-stage superfluorescent sources will be limited by the parasitic lasing due to residual feedback from the fiber end facets, all-fiber power amplifier can be employed to further scale up the power, as the conventional TDFLs do. Liu et al. reported a 122 W wideband superfluorescent source with a FWHM of 25 nm and a 120 W narrowband superfluorescent source with a FWHM of 1.2 nm in 2014 [9]. In many applications, narrowband superfluorescent sources based on TDF may be more suitable compared with broadband ones, and they can be much more attractive if the central wavelength can be widely tuned [17–21]. The only tunable narrowband superfluorescent source based on TDF near 2 μm, to the best of our knowledge, is reported by Liu et al., and can be tuned in a range of 2.2 nm with output power of 120 W [9].
In this paper, we present the first demonstration, as far as we know, on a high power widely tunable narrowband superfluorescent source based on Tm-doped fiber amplifier (TDFA) configuration. The tuning range is about 35 nm with a FWHM of ~1.5-2.0 nm, and the output power can exceed 250 W at all the tuning wavelengths. The amplifier’s slope efficiency is about 0.50, and the optical signal to noise ratio (OSNR) is ~30-40 dB.
2. Experimental setup
The setup of the tunable narrowband superfluorescent source seed is shown in Fig. 1. Two home-made 1550 nm fiber lasers were used as the pump lasers, and the pump lights were launched into the gain fiber via two 1550/2000 nm wideband wavelength division multiplexers (WDMs) at both ends of the gain fiber. The gain fiber was a length of 8 m THDF with a core diameter of 9 μm and a clad diameter of 125 μm. A 1 × 2 fiber coupler was fusion spliced at one end of the superfluorescent source to generate single end output. A polarization-insensitive isolator (ISO) was used at the other end of the superfluorescent source to reduce the feedback from the fiber end facet. The output broadband superfluorescent light from the ISO was launched into a manual tunable band pass filter (TBPF) with bandwidth of ~2.8 nm to generate the widely tunable narrowband superfluorescent source seed.
Fig. 1 Schematic sketch of the superfluorescent source seed. WDM: wavelength division multiplexer; THDF: Tm-Ho codoped fiber; ISO: isolator; TBPF: tunable band pass filter.
The superfluorescent source seed was amplified via a two-stage pre-amplifier, as depicts in Fig. 2. In the first stage of the pre-amplifier, a 1550 nm fiber laser, a WDM, a length of 5 m single cladding TDF (core diameter of 9 μm and clad diameter of 125 μm), and a polarization-insensitive ISO were employed. A 95/5 fiber coupler was used to extract 5% of the signal light for monitoring. The second stage of the pre-amplifier was composed of two 793 nm multimode laser diodes (LDs), a (2 + 1) × 1 signal-pump combiner, a piece of 8 m double cladding TDF (DC TDF), and a high power polarization-insensitive ISO. The cladding absorption efficiency of the DC TDF at 793 nm was about 3 dB/m.
Fig. 2 Schematic sketch of the pre-amplifier of the superfluorescent source seed. WDM: wavelength division multiplexer; TDF: Tm-doped fiber; ISO: isolator; LD: laser diode; DC TDF: double cladding Tm-doped fiber.
The main amplifier is shown in Fig. 3. Six commercial 793 nm multimode LDs were used as the pump lasers, which can provide a total pump power of more than 500 W. A (6 + 1) × 1 signal-pump combiner with matched fibers was employed to launch the signal light and the pump light into the gain fiber, the coupling efficiencies can be more than 95%. The gain fiber was 3 m DC TDF with a core diameter of 25 μm. The cladding absorption efficiency of the DC TDF at 793 nm was about 9 dB/m. The unabsorbed pump light was dumped out at the fusion spliced joint between the DC TDF and a piece of 0.5 m matched passive fiber with high refractivity gel. The output end of the passive fiber was angle cleaved with an angle of 8 degrees to reduce the unwanted feedback from the fiber end facet. All the gain fiber and the corresponding fusion spliced joints were placed on a water-cooled conductive heat sink to remove the waste heat and protect the fiber system. The output spectrum was analyzed by an optical spectrum analyzer (OSA) with a resolution of 0.05 nm.
3. Experimental results and analysis
The superfluorescent source seed without TBPF was investigated firstly, and the results are shown in Fig. 4. Since the main amplifier’s ASE band in this experiment was tested to be mainly located near 1980 nm, the wavelength of the seed source should cover the amplifier’s ASE band to achieve a better amplifying performance. Initially, the gain fiber used in the seed source was a piece of 5 m single cladding TDF, and the wavelength ranges ~1860-1930 nm (case 1). In order to move the wavelength to a longer range, the TDF was replaced with a piece of 8 m THDF, and the wavelength range covers from ~1900 nm to ~2050 nm (case 2). The main reason of the result is that, longer gain fiber may enhance the reabsorption of the spontaneous emission and result in the superfluorescent source’s wavelength being red-shifted. The threshold of the superfluorescent source seed is about 1.25 W due to the much long gain fiber, and the output power of the broadband superfluorescent source seed is about 290 mW.
Fig. 4 Output power of the superfluorescent source seed without TBPF. Inset: spectra at different cases.
When the TBPF was used, the broadband superfluorescent source was filtered to a tunable narrowband superfluorescent source, as shown in Fig. 5. The tuning spectra ranges from ~1930 nm to ~2030 nm with a FWHM of about 1.7 nm. The OSNR can be as high as ~40 dB, and the lowest one can also reach 20 dB. The different OSNRs in Fig. 5 can be attributed to the inhomogeneous gain of the ASE in the active fiber at various wavelengths, which can be seen in the inset of Fig. 4 (case 2). Thus, when the central wavelength of the TBPF is at the peak of the ASE gain curve (~1975 nm), the OSNR can be as high as ~40 dB, and the OSNR is lower when the central wavelength moves to the tails of the gain curve at both ends (~1930 nm or ~2030 nm). The maximum output power of the tunable narrowband superfluorescent source is ~5 mW.
The tunable narrowband superfluorescent source seed light was then launched into the pre-amplifier as shown in Fig. 2, and the power was amplified to about 100 mW and 5 W at the first and second stages of the pre-amplifier, respectively. The coupler in Fig. 2 monitored the pre-amplified power and spectrum of the tunable narrowband superfluorescent source seed. The main amplifier’s output power is depicts in Fig. 6. Five wavelengths were selected to verify the MOPA’s performance: 1966 nm, 1970 nm, 1980 nm, 1990 nm and 2001 nm. It can be seen that the output power data at different wavelengths increase almost linearly as the pump power increases, and the slope efficiencies are all about 0.50. The similar efficiencies indicate that gain band of the main-amplifier well covers the tuning range of the narrowband superfluorescent source seed, thus the stored energy in the gain fiber is effectively abstracted in the amplification process, The output spectrum at the maximum output power level shows small ripples, as shown in the inset of Fig. 6, which can be attributed to the interference between the high-order modes. In fact, the core diameter of the main-amplifier is 25 μm, although which means the fiber is a few-mode fiber for light at 2 μm, and the beam quality of the superfluorescent source can be well maintained, there still exist a few high-order modes in the main-amplifier. Thus, the interference between the modes may result in small ripples in the spectra.
The maximum output power data of the five illustrational wavelengths are shown in Fig. 7. All the output powers exceed 250 W with pump powers of around 510 W, and the minimum output power is 251.4 W at 1970 nm, the maximum output power is 264.5 W at 1980 nm. The results indicate that the widely tunable narrowband superfluorescent source based on all-fiber TDFA can provide more than 250 W output power at any wavelengths in the range of 1966-2001 nm. The differences of the intensity of the spectra can be attributed to the different measuring circumstance of the OSA in the experiment. The FWHMs of the five wavelengths are about 1.5-2.0 nm. The differences between the FWHMs may be induced by the different ripples at the peak of the spectra. Thus, the FWHMs can be further improved to be more uniform by filtering the high-order modes of the TDFA with various methods, such as placing the gain fiber in coils with smaller diameter and then dumping out the high-order modes in the fiber’s inner cladding.
The tuning range of the TDFA is mainly limited by the ASE of the amplifiers, as shown in Fig. 8. At the maximum output power, the spectrum of 1966 nm narrowband superfluorescent source shows slight ASE near 1980 nm, thus it is not robust and efficiency to amplify shorter tuning wavelengths (less than 1966 nm) of the narrowband superfluorescent source seed. On the other hand, the spectrum of the narrowband superfluorescent source seed at 2001 nm after the first stage of the pre-amplifier shows ASE near 1860 nm, which degrades the OSNR of the seed. So the amplification of longer tuning wavelengths is also limited in this system. An effective solution is to optimize the initial ASE bands of each stages of the amplifier, and make the ASE bands of each amplifying stages overlap together exactly, which will not only extend the tuning range of the narrowband superfluorescent source TDFA, but also improve the efficiency and performance of the system. The improvement will be our future endeavors. However, this widely tunable narrowband superfluorescent source based on TDFA has the widest tuning range (35 nm) and the highest output power (>250 W) as far as we know, which will be meaningful in the development and application of 2 μm superfluorescent source based on TDF.
The temporal domain characteristic of the tunable narrowband superfluorescent source after the main amplifier was measured by a high speed photodetector with a bandwidth of 200 MHz and a digital oscilloscope with a bandwidth of 500 MHz. The data redrawn in Fig. 9 show that there is no relaxation oscillation or self-pulsing in the output of the narrowband superfluorescent source amplifier, which indicates that the tunable narrowband superfluorescent source TDFA’s output is rather stable. Furthermore, the power fluctuation of the TDFA is less than 1%, thus the TDFA is robust and reliable.
Fig. 9 Oscilloscope trace of the tunable narrowband superfluorescent source after the main amplifier.
4. Conclusion
In conclusion, we present a high power and widely tunable narrowband superfluorescent source employing all-fiber TDFA configuration. The narrowband superfluorescent source can be tuning in a range of 1966-2001 nm with FWHMs of ~1.5-2.0 nm. The output power at wavelengths between 1966 and 2001 nm can exceed 250 W with slope efficiencies of ~0.50. The output power is limited by the available pump power, and the tuning range is limited by the ASEs in the amplifier. The temporal output of the tunable narrowband superfluorescent source TDFA is stable and robust. The performance of the superfluorescent source TDFA can be further improved by optimizing the amplifier’s parameters.
Acknowledgment
This work was supported by the Innovation Foundation for Graduates of National University of Defense Technology (Grant No. B130704), National Natural Science Foundation of China (Grant No. 61322505) and Program for New Century Excellent Talents in University. The authors would like to thank PhD candidate Ke Yin for his support of the TBPF.
References and links
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