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Efficient operation and characterization of distributed feedback Er3+-doped fiber lasers via the 520-nm transition band
W.H.Loh, S.D.Butterworth, W.A.Clarkson, B.N.Samson, J.P.de Sandro
Single-frequency distributed feedback (DFB) and distributed Bragg reflector (DBR) Er3+-fiber lasers are attractive for applications in optical communications and sensor systems. However, as the cavity lengths need to be short (a few cm) for robust single-mode operation, pump absorption is low, limiting laser output powers to < 1mW. Higher laser output powers are clearly desirable, e.g., for CATV applications. Previous ways suggested for dealing with this issue include redirecting the unabsorbed pump-to-power fiber amplifier , intracavity pumping , or increasing the 980-nm absorption with co-doped Yb3+:Er3+ phosphosilicate fibers .
We present a new approach: efficient laser operation and high output powers are achievable simply by pumping in the 520-nm transition instead of 980 nm. While 980 nm pumping is conventionally considered to be an excellent choice for erbium-doped fiber amplifiers (EDFAs), this is less clear for erbium-doped fiber lasers (EDFLs), e.g., long-wavelength (resonant) pumping of EDFLs can improve their stability . For short-cavity single-frequency EDFLs, the very large absorption cross-section of the 520-nm transition is attractive, as it should enable considerable improvements in the efficiency and output power to be achieved with lasers fabricated in conventional erbium-doped fibers. With compact diode-pumped all-solid-state green sources (e.g., micro-chip lasers) now commercially available, and rapid progress being made in the field of green laser diodes, this approach to pumping single-frequency DFB and DBR fiber lasers appears promising.
In the experimental demonstration, a 10-cm-long DFB laser was fabricated in an Er3+-doped fibre (N.A. 0.17, λcutoff 930 nm, small-signal 980-nm absorption 10 dB/m). A diode-pumped frequency-doubled Nd:YLF laser was used as the pump source. The performance of the laser pumped at 524 nm is shown in Fig. 1. Output powers of 17 mW and a slope efficiency of 10% is achieved, a ten-fold improvement over that obtainable with conventional 980-nm pumping.
Furthermore, we show that valuable information about the intensity profile in DFB lasers can also be extracted via the 520 nm green transition, by pumping the DFB laser at 980 nm and monitoring the green fluorescence along the laser length. The nonuniform intensity profiles in DFB lasers have long been of great interest, as it is well known that they can give rise to severe spatial hole burning, affecting single-mode stability . Consideration of the erbium laser rate equations for 980-nm pumping, taking into account pump ESA from the 4I11/2 level, show that the green fluorescence should be a good indicator of the lasing intensity. Figure 2 shows the green fluorescence distribution obtained by scanning along the central 3 cm portion of a 5-cm-long quarterwave phase-shifted DFB Er3+-fiber laser. The distributions exhibit a sharp peak near the center of the laser where the λ/4-phase shift is located, as expected from theoretical models of discrete phase-shifted DFB lasers . The use of this green fluorescence technique for mapping out the profiles in DFB fiber lasers should be very useful in the study and optimisation of more complex DFB laser structures.
Figure. 1. Comparison of the lasing characteristic of a 10-cm-long DFB Er3+-fiber laser for
(a) 524-nm pumping, and (b) 980-nm pumping.
Figure. 2. Distribution profile of the green fluorescence intensity along a DFB fiber laser
for different 980-nm pump powers: 74 mW, 110 mW and 167 mW.
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