Open Access
Add to BibSonomyAbstract
We demonstrate a broadband optical emission from Bi/Er co-doped fiber and a single 830nm laser diode pump. The ultra-broadband mechanism is studied and discussed in details based on a combination of experimental measurements, including luminescence, differential luminescence and ESA, on fiber samples of different Bi and Er concentrations. The Er co-doping in Bi doped fiber is found to be effective for broadband emission, by enhancing not only luminescence at C and L bands but also that at O and shorter wavelength bands. The luminescence intensity between 1100 and 1570nm is over −45dBm/5nm in single mode fiber using a few meters of Bi/Er co-doped fiber and offers a modest ~40dB dynamic range and a broad bandwidth of ~470nm for an OSA based spectral measurement.
©2013 Optical Society of America
1. Introduction
Broadband light sources have broad applications in the spectral measurements of optical fiber devices and systems. And super luminous diodes and rare earth doped fiber ASE sources are available and useful. However they have limited spectral bandwidth -normally smaller than 100nm. For optical fiber measurement requiring broader bandwidth, Xenon lamps are still the main light source. Their spectral intensities coupled in a single mode fiber are low, such as the commercialized white light source AQ4305 with a spectral intensity of −65dBm/nm from 700nm to 1700nm. The novel super continuum light source has the broadest spectrum and the stronger spectral intensity, from 400 to 1700 [1]. But its applications are still limited by its comparably higher price and its instability, especially when they are used for simple device measurements and sensing applications. So the cheap and stable fiber broadband sources are still needed to develop. Cr-doped and Bi-doped glass or silica material [2–6] show the broadband luminescence and of the great potential. To realize those in optical fiber form for optical fiber communication and fiber laser applications has attracted a lot of attentions recently [7–10]. The Bi doped fibers demonstrate a lot different characteristics comparing with the Bi doped bulk glass materials because the Bi emissions are the fabrication processing depended. The low Bi doped optical fiber [8–11] with low background loss has been developed for fiber lasers for optical communication and industrial applications. Bi doping in EDF [12] has also been reported for higher efficiency of EDFA. Recently we demonstrated a Bi/Er codoped fiber with an ultra-broadband luminescence covered from O to L bands when pumped by two pump sources of 532nm and 980nm in wavelengths [13]. Here we report our work for further development and better understanding toward broadband emission in Bi/Er codoped fiber. In our experiment broadband emission with appreciable spectral intensity -over −45dBm/5nm from 1100nm to 1570nm, is achieved using a short length of Bi/ Er codoped fiber and pumped by a single 830nm laser diode launching ~60mW power. We also experimentally studied the mechanisms for broadband emission through comprehensive luminescence measurements, including luminescence, differential luminescence and ESA, on fiber samples of different Bi and Er concentrations.
2. Bi/Er co-doped fiber and luminescence measurement
We fabricate a few Bi/Er co-doped fiber samples by in situ MCVD doping [14] with concentrations of [Bi2O3] ~X, [Er2O3] ~Y, [Al2O3] ~0.15, [P2O5] ~0.94, and [GeO2] ~12.9 mol %, respectively. X is from 0.1 to 0.01 mol % and Y is from 0.01 to 0 mol % for different fiber samples. The fibers have a numerical aperture NA ~0.19 and the core diameters are from 3.2µm to 6.0µm. The fiber of 3.2µm diameter has a cut-off wavelength λco~0.8 µm and is used to observe the luminescence when pumped by the 830nm laser diode. Shown in Fig. 1 is the experimental system using an optical spectral analyzer (OSA) to record the backward luminescence spectra of our Bi/Er co-doped fiber. An 830nm laser diode, connected to an 810/1310nm WDM coupler, offers a maximum ~60mW power lunched into the Bi/Er fiber. The Bi/Er co-doped fiber is spliced with lead-in single mode fiber with a splice loss of ~1dB because their mode fields are not matched. A power meter is used to monitor the left pump power and useful for the luminescence analysis later.
2.1 Broadband emission with single pump at 830nm
The luminescence spectrum emitted from a 3m long Bi/Er co-doped fiber under ~60mW pump power and measured by the OSA with a 5nm bandwidth is shown in Fig. 2(a). The red and green curves correspond to the directly measured and the true (corrected) emission spectra, respectively. The true spectrum is determined correctly from the directly measured by compensating the spectral transmission of the 810/1310nm WDM coupler. The true spectral intensity is over −45dBm from 1100nm to 1570nm and over −50dBm from 900nm to 1100nm. This intensity is over 10dB stronger than that of the normal white light source coupled in single mode fiber, such as Xenon lamps and some commercialized white light source with a single mode fiber output (AQ4305). The emission covers all optical fiber communication bands and is reasonably good for the spectral measurement of single mode fiber based devices, although the intensity remains significantly lower than the ASE and super luminescence sources that have the narrower bandwidth. Since the OSA spectral measurement limitation of ~-85dBm at the resolution of 5nm, we will have ~40dB dynamic measurement range for the broadband spectral measurement of kinds of fiber devices. With a lock-in based optical spectrum measurement system, a much better dynamic range can be achieved.
Fig. 2 (a) The broadband emission of Bi/Er co-doped fiber (X~0.02, Y~0.01), (b) the relationship between the total emission power and the pump power, (c) the stability of the whole emission spectrum.
Figure 2(b) shows the relationship between the pump power and total emission power over the whole spectrum in the range of 900nm to 1600nm. We measured the spectrum every 5min over an hour period and found that the standard deviation in the emission power of the broadband spectrum is <0.2dB as shown in Fig. 2(c). This shows its good stability, which is important for the spectral measurement application. The careful observation and analysis of such broadband spectra is needed and this is to be carried out in the following with more detail emission observation.
2.2 The excitation-emission characteristics
We experimentally investigated the low energy levels and the excitation-emission spectroscopy of our Bi/Er codoped fiber, with a similar method used in [11]. In our experiment, two Agilent tunable lasers with a combined tuning wavelength range from 1260nm to 1495nm together with an Agilent OSA are used for the excitation-emission study. The two tunable laser systems (Agilent tunable laser measurement system 8164B/81600B-130: 1260nm to 1375nm, low-SSE and 8164B/81600B-140: 1370nm to 1495nm, low-SSE) are providing a low and constant power (~70μW) lunched into the doped fiber. The low pump power is to minimize possible up-conversion related emission. The excitation-emission spectrogram of a typical Bi/Er codoped fiber is shown in Fig. 3(a). There are two obvious emission bands with the central wavelengths at 1420nm and 1530nm, respectively. The two bands should be related to the Bi and Er related color centers.
Fig. 3 The excitation-emission characteristics of a Bi/Er doped fiber sample. (a) the excitation-emission spectrogram; (b) the emission spectra of 830nm pump with different pump power; (c) the incremental emission versus the increase of the 830nm pump power; (d) the emission progressing at different wavelengths versus the 830nm pump power.
We have also investigated the luminescence spectral change of a short section (<10cm) of our Bi/Er co-doped fiber under an 830nm laser diode pump with varying pumping powers. The results are shown in Fig. 3(b) and 3(d). We show in Fig. 3(c) the increased emissions of 3 cases when the pump power is increased from: (1) 0mW to 1mw, (2) 10mW to 15mW, and (3) 55mw to 60mw. It is obvious that the emission band at ~1420nm appears and saturated early while the emission band between 900nm and 1200nm appears and saturates later, shown in Fig. 3(c) and 3(d).
2.3 The role of Er codoping on broadband emission
We draw a few fiber samples with varying Bi and Er concentrations in order to know the effect of Bi and Er for the whole spectrum. The emission from short fiber samples (length<10cm), pumped by an 830nm laser diode of 60mw power, have been obtained and shown in Fig. 4(a). Here we choose the length of the fiber sample is shorter than 10cm to make sure that the fiber is excited fully and reduce the re-absorption of the fiber sample itself. The emission band at 1530nm is obviously enhanced when the concentration of Er ions are increased. That is easy to understand because the Er ions can give the emission at 1530nm when pumped by the 830nm laser. So the emission at 1530nm is stronger when the Er doped concentration is higher. However it can also be seen from both Figs. 4(b) and 4(c) that the emission around 1200nm increases when the Er related emission at 1530nm is increased. Further we could easily find that the emission band is the narrowest and the emission between 1000nm and 1300nm is a lot less when there is no obvious Er emission band at 1530nm. An absorption spectrum of the Bi doped fiber sample without co-doping Er is shown in Fig. 4(b) and there is no obvious Er absorption peak compared with that of the Bi/Er co-doped fiber, shown in our previous paper [12]. It means that the emission between 1000nm and 1300nm is also Er dependent. The similar effect has been observed at Er/Bi co-doped germanate glasses by Peng et. al [6]. It may be explained as the energy transfer effect, since Er has no emission at the band between 1000nm and 1300nm.
Fig. 4 (a) The relative luminescence intensity Bi/Er codoped fibers with varying compositions. Here the highest Bi-doping: X~0.05, Y<0.005 mol%; the lowest Bi-doping: X~0.01, Y~0 mol%; the highest Er doping: X~0.02, Y~0.01 mol%; Please note that, for comparison purpose, the intensity is normalized to the luminescence from 1cm fiber sample. (b) The typical absorption spectrum of Bi doped fiber without codoping Er; (c) The 830nm pump absorption of the fiber samples.
2.4 The role of Bi concentration on broadband emission
To look into the relationship between the broadband emission and the Bi-doped concentration, we studied several fiber samples with different Bi concentrations and their emissions with ~60mw power input from the 830nm laser diode pump are shown in Fig. 4(a). As seen in Fig. 4(a), the bandwidth is not significantly affected by the Bi concentration. However stronger emission at 1420nm can be observed for a fiber with higher Bi concentration and, more importantly, stronger emission at 1200nm can be observed for a fiber with higher Er codoping. The stronger emission at 1200nm for Bi/Er co-doped fiber with higher Er codoping suggests that Er codoping helps not only the C-band and L-band but also O-band emissions. The typical absorption spectrum of the Bi-doped fiber sample without Er ions is shown in Fig. 4(b). Here the absorption bands at 600nm, 800nm and 1400nm are similar to those of Bi doped silica fiber as shown in Fig. 1(b) in [11]. It is clear that 830nm could be a good pumping wavelength for Bi related color centers. Some differences in details could result from different material composition and fabrication process. There is no Er related absorption peak compared with that of Bi/Er co-doped fibers, as shown in our previous Bi/Er co-doped fiber [12]. The higher Bi concentration also introduces higher absorptions, including both small-signal absorption and saturation absorption, of the 830nm pump as shown in Fig. 4(c). The higher small-signal absorption for the higher doped fiber is reasonable because the small-signal absorption is directly proportional to the product of the concentration and absorption cross-section of Bi. The higher saturation absorption for higher Bi concentration under the 830nm pump could be linked to ESA and other Bi related saturation absorptions. The existence of ESA has been observed from the up-conversion luminescence at the visible wavelength that could be seen by eyes in a dark room.
We cannot give the accurate unsaturated absorption value due to the limited pump power. However, it has been confirmed in [9] that higher unsaturable absorption occurs in the fiber with higher Bi concentration. The higher unsaturable absorption usually means poorer emission efficiency which should be avoided when the high efficient amplifiers or lasers are targeted and this is the reason why most Bi-doped fibers have low Bi concentrations.
We measured the pump absorption using the cutback method and using the power meter as shown in the setup in Fig. 1. We observe the on-off gain (including ESA) of our fiber samples at wavelength range from 900nm to 1600nm based on mono-chromator system, a white light source and an 830nm pump, similar to the scheme in [15], and the results are shown in Fig. 5. The negative on-off gain means that the cross-section of ESA is dominant over the sum of the cross-sections of ground state absorption and emission [15]. The fiber with the higher Bi concentration gives the wider and stronger ESA band. The fibre with the highest Bi doping has the widest ESA band (930nm~1310nm) of 380nm. The emission in the band with ESA, compared with that in the band without ESA, would become relatively lower when pump power or fiber length is increased. This particular behavior of ESA could effectively reduce emission bandwidth when we render higher pump or longer active fiber length to increase overall emission intensity. Hence the existence of ESA in the band of interest is not desirable for broadband emission applications.
Fig. 5 Observation of on-off gain of the Bi/Er codoped fiber samples with different Bi concentrations.
3. Discussions
We believe more than one Bi related color center should be taken into consideration in order to explain the ultra-broadband emission observations here. The main energy level, corresponding to 1420nm emission band, is confirmed by the tunable laser based emission spectra in Fig. 3(a). The Bi-Si color center (BAC-Si) [11] is the reasonable explanation for this emission band because of the 85 mol% concentration of SiO2 of our fiber. Our Bi/Er co-doped fibers could give two emission bands, at 1200nm and 1530nm when pumped by a 980nm laser diode as shown in our previous paper [13]. The emission at 1200nm could be explained by the Al-Si-Bi or P-Si-Bi color centers, shown as Fig. 3(a) and 4(a) in [11]. The stronger emission at 1200nm in Bi/Er co-doped fiber, compared with that of Bi fiber with little or no Er co-doping, may be explained by the energy transfer between Bi and Er [6]. We also have observed the up-conversion emission at visible wavelengths when the 830nm pump power is increased to a few mWs. This upconversion could be contributed by the ESA of both Er and Bi. At least it is well-know the ESA of Er could introduce the green light (532nm) when pumped by 830nm. In addition, stronger up-conversion emission from the Bi/Er fiber with higher Bi concentration is observed.
4. Conclusion
In conclusion, we fabricated Bi/Er codoped fibers and achieved an appreciable ultra-broadband emission (from 1000nm to 1570nm) by a single pump at 830nm. Two roles of Er codoping, introducing emission at C + L band and enhancing the emission at the band of 1200nm, are considered experimentally evident when comparing the luminescence results from the Bi/Er codoped fibers with different Er concentrations. We also observe that the width and intensity of the ESA band at the band of 1200nm are increased when increasing the Bi concentration, which point out that the lower Bi concentration should be chosen to optimize the broadband emission further.
Acknowledgments
Authors thank the support by international science linkages (ISL) project (CG130013) from the department of industry, Innovation, Science and Research (DIISR), Australia. An Australian Research council (ARC) LIEF grant helped to fund the national fiber facility at UNSW. Authors thank for the support by National Science foundation projects (60907034, 11178010 61077063, and LBH-Z10195), Harbin Science foundation (2011RFLXG004), Fundamental Research Funds of the Central University, China and the China Postdoctoral Science Foundation funded project (20100480965).
References and links
1. J. M. Dudley, G. Genty, and S. Coen, “Supercontinuum generation in photonic crystal fiber,” Rev. Mod. Phys. 78(4), 1135–1184 (2006). [CrossRef]
2. J. C. Chen, Y. S. Lin, C. N. Tsai, K. Y. Huang, C. C. Lai, W. Z. Su, R. C. Shr, F. J. Kao, T. Y. Chang, and S. L. Huang, “400-nm-bandwidth emission from a Cr-doped glass fiber,” IEEE Photon. Technol. Lett. 19(8), 595–597 (2007). [CrossRef]
3. Y. Fujimoto and M. Nakatsuka, “Infrared luminescence from bismuth-doped silica glass,” Jpn. J. Appl. Phys. 40(Part 2, No. 3B), L279–L281 (2001). [CrossRef]
4. Y. Kuwada, Y. Fujimoto, and M. Nakatsuka, “Ultra-wideband light emission from Bismuth and Erbium doped silica,” Jpn. J. Appl. Phys. 46(4A), 1531–1532 (2007). [CrossRef]
5. M. Y. Peng, J. R. Qiu, D. Chen, X. Meng, and C. Zhu, “Broadband infrared luminescence from Li2O-Al2O3-ZnO-SiO2 glasses doped with Bi2O3,” Opt. Express 13(18), 6892–6898 (2005). [CrossRef] [PubMed]
6. M. Peng, N. Zhang, L. Wondraczek, J. Qiu, Z. Yang, and Q. Zhang, “Ultrabroad NIR luminescence and energy transfer in Bi and Er/Bi co-doped germanate glasses,” Opt. Express 19(21), 20799–20807 (2011). [CrossRef] [PubMed]
7. H. Y. Tam, W. H. Chung, B. O. Guan, H. L. Liu, P. K. A. Wai, and N. Sugimoto, “Development of Bi2O3 – based erbium-doped fibers,” Proc. SPIE 5644, 259–269 (2005). [CrossRef]
8. V. V. Dvoyrin, V. M. Mashinsky, L. I. Bulatov, I. A. Bufetov, A. V. Shubin, M. A. Melkumov, E. F. Kustov, E. M. Dianov, A. A. Umnikov, V. F. Khopin, M. V. Yashkov, and A. N. Guryanov, “Bismuth-doped-glass optical fibers--a new active medium for lasers and amplifiers,” Opt. Lett. 31(20), 2966–2968 (2006). [CrossRef] [PubMed]
9. M. P. Kalita, S. Yoo, and J. Sahu, “Bismuth doped fiber laser and study of unsaturable loss and pump induced absorption in laser performance,” Opt. Express 16(25), 21032–21038 (2008). [CrossRef] [PubMed]
10. I. A. Bufetov and E. M. Dianov, “Bi-doped fiber lasers,” Laser Phys. Lett. 6(7), 487–504 (2009). [CrossRef]
11. S. V. Firstov, V. F. Khopin, I. A. Bufetov, E. G. Firstova, A. N. Guryanov, and E. M. Dianov, “Combined excitation-emission spectroscopy of bismuth active centers in optical fibers,” Opt. Express 19(20), 19551–19561 (2011). [CrossRef] [PubMed]
12. Y. Luo, J. Wen, J. Zhang, J. Canning, and G. D. Peng, “Bismuth and erbium codoped optical fiber with ultrabroadband luminescence across O-, E-, S-, C-, and L-bands,” Opt. Lett. 37(16), 3447–3449 (2012). [CrossRef] [PubMed]
13. Y. Q. Qiu, X. Y. Dong, and C. L. Zhao, “Spectral characteristics of the erbium-bismuth co-doped silica fibers and its application in single frequency fiber laser,” Laser Phys. 20(6), 1418–1424 (2010). [CrossRef]
14. A. S. Webb, A. J. Boyland, R. J. Standish, S. Yoo, J. K. Sahu, and D. N. Payne, “MCVD in-situ solution doping process for the fabrication of complex design large core rare-earth doped fibers,” J. Non-Cryst. Solids 356(18-19), 848–851 (2010). [CrossRef]
15. S. Yoo, M. P. Kalita, J. Nilsson, and J. Sahu, “Excited state absorption measurement in the 900-1250 nm wavelength range for bismuth-doped silicate fibers,” Opt. Lett. 34(4), 530–532 (2009). [CrossRef] [PubMed]
