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We demonstrate an all-fiber tunable Tm/Ho-codoped laser operating in the 2 μm wavelength region. The wavelength tuning range of the Tm/Ho-codoped fiber laser (THFL) with 1-m length of Tm/Ho-codoped fiber (THDF) was from 1727 nm to 2030 nm. Efficient short wavelength operation and ultra-wide wavelength tuning range of 303 nm were both achieved. To the best of our knowledge, this is the broadest tuning range that has been reported for an all-fiber rare-earth-doped laser to date. By increasing the THDF length to 2 m, the obtainable wavelength of the THFL was further red-shifted to the range from 1768 nm to 2071 nm. The output power of the THFL was scaled up from 1810 nm to 2010 nm by using a stage of Tm/Ho-codoped fiber amplifier (THFA), which exhibited the maximum slope efficiency of 42.6% with output power of 408 mW at 1910 nm.
© 2014 Optical Society of America
1. Introduction
Fiber lasers in the eye-safe 2 μm region have attracted significant attentions due to a variety of applications such as atmospheric lidar measurement, remote sensing [1, 2], nonlinear frequency conversion [3] and medical surgery [4]. Broad tunable fiber lasers particularly for wavelength tunable fiber lasers in the 2 μm region have proved a significant impact in many diverse fields of science and technology [5]. The energy level transitions 3F4→3H6 from Tm-doped fibers and 5I7→5I8 from Ho-doped fibers are the most common fiber laser technology for realizing efficient emission in the 2 μm wavelength region [6–8].
There are many research results on wavelength tunable Tm-doped, Ho-doped and Tm/Ho-codoped fiber lasers in the past decades. By utilizing diffraction gratings and other free-space components, 2 μm fiber lasers with wide wavelength tuning range have been widely reported. By using an external diffraction grating, W. A. Clarkson et al demonstrated a Tm-doped fiber laser (TDFL) with a wide spectrum tunable from ~1860 nm to ~2090 nm [9]. Similarly, a tunable TDFL with wavelength tuning range from 1895 nm to 2109 nm was reported by C. Guo et al [10]. By utilizing two diffraction gratings and a rotating polygon mirror, M. Tokurakawa et al reported an ultra-wideband wavelength swept TDFL operating from 1750 nm to 2080 nm, which is believed to be the widest wavelength tuning range reported to date for a TDFL [11]. Pumped by a 1950 nm TDFL, a Ho-doped fiber laser with wavelength tuning range from 2043 nm to 2171 nm was demonstrated by N. Simakov et al [12]. By using a dichroic mirror and a diffraction grating, A. Hemming et al reported a THFL with a tuning range from 1920 nm to 2120 nm [13] and further demonstrated a tunable Tm/Ho-codoped aluminosilicate fiber laser with a wide spectrum operation from 1855 nm to 2135 nm [14]. However, all these lasers have relied upon external cavities incorporating free-space components, neither of which is ideal from a reliability point of review.
All-fiber lasers could provide tunable performance with a minimal sensitivity to environmental disturbances as well as high reliability. In recent years, all-fiber tunable lasers operating in the 2 μm wavelength region have been reported. By using a core-pumped tunable resonator configuration, J. M. Daniel et al reported an ultra-short wavelength operation of TDFL with a wavelength tuning range from 1660 nm to 1720 nm [15]. V. A. Kamynin et al reported an all-fiber Ho-doped laser tunable in the range of 2.045-2.1 μm by utilizing a compression of Bragg grating [16]. However, the tuning ranges of the TDFL and the Ho-doped laser were only 60 nm and 55 nm, respectively. By using different fiber Bragg gratings (FBGs) as wavelength selection elements, J. Li et al demonstrated a wideband tunable TDFL in the wavelength range from 1925 nm to 2200 nm, which is the reported longest wavelength from the Tm-doped fiber lasers (TDFLs) [17]. However, the wavelength range of the TDFL could not be continuously tuned. By using a fiberized grating-based tunable filter, Z. Li et al reported an all-fiber tunable Tm-doped laser operating at a wide spectrum from 1820 nm to 2075 nm [18] and further demonstrated an in-band diode-pumped Tm-doped fiber amplifier (TDFA) with a more than 240 nm wide window in the range of 1800-2050 nm seeded by a tunable laser source [19]. Since broadband amplifiers in the 2 μm wavelength region have proved very useful in the next-generation telecommunication networks, high performance low-noise TDFA designs have attracted growing interest and been reported in recent years [20].
There have been few reports on all-fiber tunable Tm/Ho-codoped lasers to date. In this paper, we report an all-fiber ultra-wideband Tm/Ho-codoped laser with up to 303 nm wavelength tuning range from 1727 nm to 2030 nm, which is the broadest tuning range that has been reported for an all-fiber rare-earth-doped laser to date. Efficient short wavelength operation was also achieved. The output power of the THFL was amplified from 1810 nm to 2010 nm by utilizing a stage of THFA, which exhibited the maximum slope efficiency of 42.6% with output power of 408 mW at 1910 nm.
2. Experimental setup of the THFL
Figure 1 depicts the experimental setup of the wavelength tunable THFL. The THFL was built in a ring configuration with a piece of THDF as gain medium, which was core-pumped by a 1 W 1550 nm fiber laser through a 1550/2000 nm wavelength division multiplexer (WDM). The THDF has a core/cladding diameter of 8/125 μm, an effective numerical aperture (NA) of 0.18, and an absorption coefficient of ~13 dB/m at 1550 nm. An isolator (ISO) was used to ensure unidirectional propagation of the signal light. A fiberized grating-based tunable filter was used to determine the operating wavelength of the laser. The tunable filter has a broadband spectral width in the range of 1700-2150 nm and can only be operated by hand. The 3 dB width of the tunable filter is ~1.7 nm. The output power of the THFL was extracted from an 80/20 coupler. In our experiment, a wavelength insensitive thermal power meter and an optical spectrum analyzer (OSA) with spectral measurement range from 1200 nm to 2400 nm were used to measure the output power and the output spectra, respectively.
3. Results and discussion
The Tm3+ and Ho3+ ions doped in THDF could exhibit a wide gain bandwidth covering from 1.7 μm to 2.1 μm [21, 22]. However Tm3+ ion is a three-level structure, which presents challenges for efficient laser emission at short wavelength band. Ho3+ ions doped in the THDF may absorb the long-wavelength Thulium-amplified spontaneous emission (ASE) which will benefit the short wavelength operation of the laser. To achieve the efficient short wavelength operation of the THFL, we utilized a core-pumped configuration allowing high excitation densities and 1-m length of THDF to minimize the reabsorption of short wavelength components.
Figure 2 illustrates the tunability of the THFL with 1-m length of THDF. Figure 2(a) presents the output spectra of the THFL at pump power of 1 W measured at 0.05 nm spectral resolution. Figure 2(b) plots the output power versus the operating wavelength of the THFL at pump powers of 618 mW, 818 mW and 1 W, respectively. With the increase of 1550 nm pump power, the tuning range of the THFL was blue-shifted. This behavior could be explained by the analysis below. With the decreasing of the operation wavelength, the THFL requires the higher pump power to achieve the threshold of lasing short wavelength light. The wavelength tuning ranges of the THFL at pump powers of 618 mW and 818 mW were 1757-2024 nm and 1741-2029 nm, respectively. Compared with the pump powers of 618 mW and 818 mW, the pump power of 1 W has achieved the lasing threshold of the shorter wavelength components. The tuning range of the THFL at pump power of 1 W therefore exhibited the shortest lasing wavelength of 1727 nm with the extended long wavelength end to 2030 nm. However, obvious power dips at 1930 nm and 1980 nm could be observed in Fig. 2(b). There will be a detailed analysis of the power dips in the following part of this paper.
Fig. 2 Tunability of the THFL with 1-m length of THDF at different pump powers. (a) Output spectra of the THFL operating at different wavelengths at pump power of 1 W. From left to right, these are 1727 nm, 1760 nm, 1810 nm, 1860 nm, 1910 nm, 1960 nm, 2010 nm and 2030 nm, respectively. The OSA resolution was set at 0.05 nm. (b) Output power versus the operating wavelength of the THFL at pump powers of 618 mW, 818 mW and 1 W, respectively.
Figure 3 illustrates the tunability of the THFL at pump power of 1 W with different THDF lengths. Figure 3(a) presents the output spectra of the THFL with 2-m length of THDF measured at 0.05 nm spectral resolution. Figure 3(b) plots the output power versus the operating wavelength of the THFL with different THDF lengths of 1 m, 2 m and 3 m, respectively. As can be seen in Fig. 3(b), the tuning range of the THFL was red-shifted with the increase of the THDF length. The THFL with 1-m length of THDF exhibited the shortest lasing wavelength of 1727 nm, whereas the 2-m-long THDF pushes the longest wavelength to 2071 nm. The long wavelength edge of the THFL at 2071 nm was limited by the weakened gain coefficient of the THDF above 2071 nm. The wavelength tuning ranges of the THFL with THDF lengths of 1 m, 2 m and 3 m were 1727-2030 nm, 1768-2071 nm and 1801-2069 nm, respectively. This behavior is due to the fact that Tm3+ ion is a three-level structure resulting in the reabsorption of short wavelength components with the increase of THDF length. This illustrates that the tuning range of the THFL can be adjusted by optimizing the THDF length for operation in the wavelength region desired. Besides the power dips at 1930 nm and 1980 nm, obvious power dips at 2040 nm in the output power of the THFL with THDF length of 2 m and 3 m also could be observed in Fig. 3(b).
Fig. 3 Tunability of the THFL at pump power of 1 W with different THDF lengths. (a) Output spectra of the THFL with 2-m length of THDF operating at different wavelengths. From left to right, these are 1768 nm, 1800 nm, 1850 nm, 1900 nm, 1950 nm, 2000 nm, 2050 nm and 2071 nm, respectively. The OSA resolution was set at 0.05 nm. (b) Output power versus the operating wavelength of the THFL with different THDF lengths of 1 m, 2 m and 3 m, respectively.
To explore the reason why there existed dips in the output power of the THFL at 1930 nm, 1980 nm and 2040 nm, it is necessary to measure the operation characteristics of the four components in a wide spectral range. So we constructed a supercontinuum (SC) source with a spectral range from 1550 nm to 2100 nm to measure the spectral transmission characteristics of the WDM, the ISO, the coupler and the tunable filter, respectively.
The output spectra of the SC, the WDM, the ISO and the coupler measured are shown in Fig. 4(a). Compared with the spectrum of the input SC, the output spectra of the WDM, the ISO and the coupler were slightly broadened as a result of fiber nonlinearities. The 1800-1950 nm fine spectral structures in Fig. 4(a) were due to the gases absorption in our laboratory environment. There were not dips at 1930 nm, 1980 nm and 2040 nm in the output spectra of the WDM, the ISO and the coupler, which will not lead to the dips in the output power of the THFL. The 1920-1940 nm, 1970-2000 nm and 2030-2050 nm output spectra of the SC and the tunable filter measured are shown in Figs. 4(b)–4(d), respectively. Obvious dips observed in the output spectra of the tunable filter resulted from the large insertion loss, which led to high lasing thresholds of the THFL at 1930 nm, 1980 nm and 2040 nm. Therefore,the THFL exhibited power dips with the same pump power in Figs. 2(b) and 3(b).
Fig. 4 (a) Output spectra of the SC, the WDM, the ISO and the coupler, respectively. Output spectra of the SC and the tunable filter in the wavelength range: (b) 1920-1940 nm; (c) 1970-2000 nm; (d) 2030-2050 nm. The OSA resolution was set at 0.05 nm.
Figures 5(a)–5(c) show the output power of the THFL operating at 1810 nm, 1860 nm, 1910 nm, 1960 nm and 2010 nm as a function of the launched pump power for the THDF lengths of 1 m, 2 m and 3 m, respectively. With the increase of the THDF length, the thresholds of the THFL operating at 1810 nm, 1860 nm, 1910 nm, 1960 nm and 2010 nm all increased. As a result of the weakened gain and increasing reabsorption of short wavelength components, the threshold of the THFL operating at 1810 nm among the five wavelengths exhibited the highest increment speed with the increase of the THDF length.
Fig. 5 Output power of the THFL operating at 1810 nm, 1860 nm, 1910 nm, 1960 nm and 2010 nm as a function of the launched pump power for the THDF lengths of (a) 1 m, (b) 2 m and (c) 3 m, respectively. (d) Slope efficiency of the THFL as a function of the operating wavelength for the THDF lengths of 1 m, 2 m and 3 m, respectively.
Figure 5(d) shows the slope efficiency of the THFL as a function of the operating wavelength for the THDF lengths of 1 m, 2 m and 3 m, respectively. It is observed that the slope efficiencies of the THFL with THDF lengths of 1 m, 2 m and 3 m operating at 1910 nm among the five wavelengths exhibited the maximum. When operating in the wavelength region around 1810 nm, the THFL with THDF length of 1 m exhibited the maximum slope efficiency. The THFL with THDF length of 2 m exhibited the maximum slope efficiency in the spectral region from 1860 nm to 1960 nm. When operating in the wavelength region around 2010 nm, the THFL with THDF length of 3 m exhibited the maximum slope efficiency. This was due to the increasing reabsorption of short wavelength components with the increase of THDF length.
4. Experimental setup and results of the THFA
To confirm the power amplification ability of the THFL in a wide wavelength region, we utilized a stage of THFA to scale up the output power of the THFL with 1-m length of THDF. The experimental setup of the THFA is illustrated in Fig. 6. The THFA consists of a 2-m-long 8/125 μm THDF which was pumped by a 982 mW 1570 nm fiber laser through a 1570/2000 nm WDM. The THDF has an effective NA of 0.18 and an absorption coefficient of ~20 dB/m at 1570 nm. The output end of the THDF was angled cleaved to prevent the light from being reflected back into the system.
The output power of the THFL was amplified by a stage of THFA from 1810 nm to 2010 nm. The output power of the THFA as a function of the launched pump power operating at 1810 nm, 1860 nm, 1910 nm, 1960 nm and 2010 nm is shown in Fig. 7(a). The maximum slope efficiency of the THFA was 42.6% with output power of 408 mW at 1910 nm. The output spectra of the THFA operating at different wavelengths are shown in Fig. 7(b). More than 40 dB ASE suppression ratio of the THFA output spectra was achieved at 1810 nm, 1860 nm, 1910 nm, 1960 nm and 2010 nm.
Fig. 7 (a) Output power of the THFA as a function of the launched pump power operating at 1810 nm, 1860 nm, 1910 nm, 1960 nm and 2010 nm. (b) Output spectra of the THFA operating at different wavelengths. From left to right, these are 1810 nm, 1860 nm, 1910 nm, 1960 nm and 2010 nm, respectively. The OSA resolution was set at 0.05 nm.
5. Conclusions
We presented an all-fiber Tm/Ho-codoped laser incorporating 1-m length of THDF with an ultra-wideband 303 nm tuning range from 1727 nm to 2030 nm. To the best of our knowledge, this is the broadest tuning range that has been reported for an all-fiber rare-earth-doped laser to date. Efficient short wavelength operation was also achieved. By increasing the THDF length to 2 m, the operation wavelength of the THFL could be red-shifted to the range from 1768 nm to 2071 nm. The output power of the THFL was amplified from 1810 nm to 2010 nm by a stage of THFA, which exhibited the maximum slope efficiency of 42.6% with output power of 408 mW at 1910 nm. Such a compact structure laser with an ultra-wide wavelength tuning range should have many applications in optical sensing, medical area and so on.
Acknowledgments
This work was supported by projects of National Natural Science Foundation of China under Grant No. 61235008 and Hunan Provincial Natural Science Foundation of China under Grant No. 14JJ3001.
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