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We have proposed and demonstrated a wideband and wavelength stable L-band erbium-doped superfluorescent fiber source (SFS) using an improved double-pass bi-directional (DP-BD) pumping configuration with a segment of un-pumped fiber. The effects of the fiber length and pump power arrangements on the output characteristics of the L-band SFS are examined. Simulations and experiments show that the output characteristics of the L-band SFS are significantly improved by using an un-pumped fiber in the DP-BD pumping configuration. This designed configuration allows a wavelength stable L-band SFS operation with a broadening bandwidth of 52.6nm, an enhanced pumping efficiency of 47.5%, and a mean wavelength of 1580.42nm.
©2007 Optical Society of America
1. Introduction
Superfluorescent fiber sources (SFS) using amplified spontaneous emission (ASE) from an erbium-doped fiber (EDF) have been a topic of continuing research because of their wide range of applications, from fiber optic gyroscopes (FOG), component testing sources to sliced spectrum sources for lower cost access networks and so on [1–3]. The conventional wavelength band (C-band) EDF SFSs have been researched with extreme detail and the double-pass backward (DPB) configuration has been demonstrated to offer the highest output power, better mean wavelength stability, and broader bandwidth for the C-band SFSs [4]. However, recent demand for immediate expansion of the fiber optic communication window has led to the development of the long wavelength band (L-band) sources. Therefore, more researches are focused on the L-band to increase the output power, spectral bandwidth and wavelength stability. In a recent study [5], Tsai et al found that the double-pass forward (DPF) configuration is the better one to implement a single-laser pumped L-band ASE source with a pumping-conversion efficiency of about 14%. In [6], a high output power L-band erbium-doped fiber ASE source using dual forward-pumping configuration that allows high pumping efficiency of 36.1% and bandwidth of 40.8nm was proposed. Tsai and Huang studied the double-pass bi-directional (DP-BD) pumping configuration to achieve a highest pumping efficiency L-band SFS of about 42% to the best of our knowledge [7, 8]. In [9], a low power L-band ASE seed source constructed in DPF configuration with an extra section of unpumped EDF was used for the Wall-level L-band SFS. On the other hand, the L-band SFS may be better than the C-band SFS for FOG applications since the L-band SFS can provide a larger bandwidth than that of the C-band SFS [8, 10, 11]. However, the bandwidth of the L-band SFS is always limited less than 43nm in those reports up to date [5–9]. While a broader bandwidth of SFS implies that a higher value of SNR would be obtained for the FOG application, therefore, further studies are still necessary on the wavelength stable L-band SFS with larger bandwidth.
In this paper, a new method is proposed to achieve a stable and wideband L-band SFS. The configuration is based on the DP-BD configuration while additional using a segment of un-pumped fiber between the reflector and the wavelength division multiplexing coupler. The effects of the fiber length and pump power arrangement on the output characteristics of the L-band SFS are examined. On the based of the optimized arrangement, the wide spectral bandwidth L-band SFS with pump power independent mean wavelength operation are experimentally obtained. The bandwidth of such an L-band SFS is significantly broadened over 10nm as compared with the conventional one in [7, 8]. The wavelength stable L-band SFS with such a large bandwidth is the best one reported up to date to the best of our knowledge.
2. Configuration of the L-band SFS
Figure 1 illustrates the suggested improved DP-BD configuration. The source consists of two sections of erbium-doped fiber (EDF), a 1480nm pumping laser diode (LD), two 1480/1590nm wavelength division multiplexers (WDM), a power splitter used to divide the pump power into two portions, a fiber loop mirror (FLM) used to reflect the ASE light to form into a double-pass configuration, and an optical isolator (ISO) at the output port. In the design, the section of EDF2 is bi-directional pumped by the LD through two WDMs, and the other one (EDF1) is un-pumped and arranged between the WDM1 and the FLM. We define the total length of EDF as L=L1+L2, where L1 and L2 refer to the first stage (EDF1) and the second stage (EDF2) lengths, respectively. The fiber length ratio of the EDF1 length to the total length is defined as RL=L1/L. Similarly, the pump ratio is defined as the forward pump power to the total pump power, i.e., Rp=P1/Ptotal. The EDF used in the numerical simulations is Lucent Technologies heavily doped LRL fiber (type number L12403) with a peak absorption of 27–33dB/m at 1530nm, mode field radius of 5.2µm, cutoff wavelength of 1100–1400nm, and numerical aperture of 0.25. From the Fig. 1 we can see that the L-band SFS is a conventional DP-BD configuration when the fiber length ratio is of 0.
3. Simulations and experiments
We first use the commercial amplifier simulation package OASIX [12] to optimize the parameters of this configuration. It is believed that the simulation software is accurate for presenting the same results as those obtained by experiments [8, 11]. The total EDF length L is optimized according to spectrum flatness in the case of RL=0, and Rp=1. The effective FLM reflectivity is selected to be 90% and the pump power is set to 100mW in the simulation. The output spectra and bandwidth of the DPF L-band SFS at various EDF lengths indicate that for 100mW pump power, 19m is the optimal length to obtain a largest bandwidth i.e. a flattest L-band spectrum. Therefore, the total EDF length is fixed at 19 m for the suggested L-band SFS in the following simulations and experiments.
Then, the effects of the fiber length ratio R L on the output characteristics of the suggested L-band SFS are examined. The pump power is fixed at 100mW and the pumping ratio is fixed at 0.5. The spectra of the L-band SFS with different R L are given in Fig. 2. Curves (a) to (d) correspond to R L=0 (L1=0, L2=19m), 0.368 (L1=7, L2=12m), 0.632 (L1=12, L2=7m), and 0.842 (L1=16, L2=3m), respectively. The variations of the mean wavelength, bandwidth, and output power versus R L are illustrated in Fig. 3. As apparent from Fig. 2 and Fig. 3, when R L is low, R L has little influence on the spectral shape. The mean wavelength and the spectral bandwidth remain almost unchanged expect a gradually increase of the output power. With R L further increasing larger than 0.5, the spectral intensity increases obviously in the short-wavelength range and decreases in the long-wavelength range gradually, which results in the mean wavelength shifting toward shorter wavelengths and the bandwidth increasing to a maximum value when R L is adjusted to around 0.632. If R L is very high (e.g. R L=0.842), the SFS is no longer an L-band fiber source, the spectral range shifts to the C-band with a decreasing of the bandwidth and output power. A significative comparison between the proposed improved SFS configuration and the conventional DP-BD pumping configuration should be indicated. With the same components and fiber length, the proposed configuration has obvious merits for a 13.1nm (from 43.6nm to 56.7nm) broader bandwidth and 6.1% (from 54.6% to 60.7%) higher conversion efficiency only by changing R L from 0 to a proper value of 0.632.
The reason for the spectral characteristics on the RL shown in Fig. 2 can be illustrated as follows. The DP-BD configuration with a segment of un-pumped fiber between the reflector and the wavelength division multiplexing coupler can be seen as an L-band ASE seed source with a bi-directional pumped erbium doped fiber amplifier. Therefore, the output of the SFS includes two components: the amplified L-band seed light and the residual ASE of EDF2 in the output port. The wavelength ranges and the proportion of these two components determine the spectral shape and wavelength range of the SFS. When the RL is very low, the effect of the un-pumped fiber is weak. In this case, the spectral characteristics of proposed SFS have little changes as compared to the conventional DP-BD. With the increase of RL, the amplified L-band seed light has gradually considerable power to compete with the ASE of EDF2. At the same time, the wavelength range of the ASE of EDF2 moves to the long-wavelength edge of the C-band due to the shorter EDF2. Therefore, the combination of the amplified L-band seed light and the residual ASE of EDF2 results in the L-band output broadening to the edge of C-band, thus a broader bandwidth is obtained. The results show that RL=0.632 is the optimal fiber length ratio to achieve a flat L-band spectrum with maximal bandwidth, under the pump power of 100mW. When the RL is very high, the EDF2 is pumped with a high inversion level, it will play little amplification to the L-band seed light. Thus, the L-band light is low and the output will be a C-band spectrum.
We have also addressed the effects of pump ratio Rp on the output characteristics for the suggested L-band SFS. Fig. 4 illustrates the output power, bandwidth and mean-wavelength against Rp with the total pump power is fixed at 100mW and the fiber length ratio is fixed at 0.632. Obviously, the SFS becomes a single pumped backward configuration that generate a C-band spectrum when the Rp=0. While when the Rp=1, the SFS becomes a single pumped forward configuration with an un-pumped fiber that generate an L-band spectrum. Output of the SFS simply includes two components when bi-directional pumping synchronously: the amplified L-band seed light and the residual ASE of EDF2 in the output port. The L-band ASE generated by the forward pump is amplified by the C-band ASE generated by the backward pump. Therefore, the output spectrum shifts gradually from C-band to L-band gradually with the increasing of forward pump power, i.e. Rp from 0 to 1. It is important to note that there is an optimal Rp to obtain a broadest bandwidth of L-band SFS. The largest bandwidth of 56.7nm is obtained when the Rp=0.5.
Figure 5 illustrates the calculated mean wavelength as a function of the total pump power with fiber ratio RL=0.632 and pump ratio of 0.5. The dependence of mean wavelength on total pump power of the SFS in a pump power range of 160–190mW can be more clearly seen in the inset of Fig. 5. The result shows that the pump power independent mean wavelength operation with ∂λ/∂P=0 is able to exist for the SFS configuration in a large pump power range from 172–184mW, corresponding to a mean wavelength of 1580.87nm. It is well known that the accuracy of rotation detection of the FOG is determined by the stability of scale factor, which depends on the mean wavelength stability of its light source [1]. On the other hand, broad bandwidth and high output power are the other two desirable characteristics of the light source used in FOG since a broader bandwidth implies that a higher value of SNR would be obtained for the FOG. Therefore, these characteristics with the pump power independent mean wavelength operation, broad bandwidth and high output power of the proposed L-band SFS make it be very useful in a high precision FOG applications.
Fig. 6. Measured mean wavelength versus pump power for L1=12, L2=7m with R P=0.5. Inset: Measured SFS spectrum for a pump power of 185mW.
The characteristics of the optimized improved DP-BP L-band SFS were experimentally measured. Fig. 6 illustrates the mean wavelength as a function of the total pump power with L1=12, L2=7m and fixed pump ratio of 0.5. The output spectrum was measured using an Advantest optical spectrum analyzer (OSA) that divided the spectrum into 1000 discrete points, the mean wavelength and bandwidth of the spectrum is computed by the equations in ref.[1]. The experimental results are in good agreement with the simulations except little discrepancies since the components insertion loss are not considered in the simulation process. The stable mean wavelength operation was observed when the pump power up to 185mW. The inset of Fig. 6 is the output spectrum measured under pump power of 185mW, corresponding to a mean wavelength of 1580.42nm, a spectral bandwidth of 52.6nm, and an output power of 88mW, i.e. a pumping efficiency of 47.5%. To the best of our knowledge, the characteristics of the proposed wavelength stable L-band SFS is the best one reported up to date.
4. Conclusion
We have presented a mean wavelength stability L-band SFS with high pumping efficiency and wide bandwidth by using an improved DP-BD configuration. Bi-directional pumping configuration can have a high efficiency and by use of an un-pumped segment of fiber in front of the WDM to take full use of the backward ASE, the spectral bandwidth is obviously broadened and the pumping efficiency is also slightly enhanced. Simulation results show that a flat L-band SFS spectrum with maximal bandwidth can be obtained simply by means of optimizing the fiber length ratio of the un-pumped fiber to the total fiber length and the pump ratio. The wavelength stable L-band SFS with a broad bandwidth of 52.6nm, a high pumping efficiency of 47.5%, and the mean wavelength of 1580.42nm has been experimentally obtained. It is believable that the suggested L-band SFS with a high pumping efficiency, broad bandwidth and stable mean wavelength will make it widely applicable to WDM system, fiber optic gyroscopes and fiber sensor systems etc.
Acknowledgments
This work was supported by the Natural Science Foundation of Fujian Provincial of China under Grant No.2006J0243, the Key Scientific and Technical Innovation Project of Xiamen University under Grant No.K70007, the Key Laboratory of Opto-Electronic Science and Technology for Medicine (Fujian Normal University), Ministry of Education under Grant No.JYG0511, and the Program for New Century Excellent Talents in Fujian Province University.
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