Bismuth Doped Fiber for Filtering Applications

Optical filters have numerous applications in various optical systems due to their wavelength based discriminative transmission losses. These applications include detection of an emitted weak signal while avoiding strong excitation signal in any spectroscopy system, laser beam diagnostics, and tailoring the spectrum of a broadband light source etc in different sectors such as astrophysics, chemical analysis, imaging systems, communication, aerospace, and optics labs etc. A fiber based filter is highly desirable as it can be easily spliced to other fibers leading to an integrated optical systemthanks to the low splice losses. Several fiber based filters have been demonstrated based on birefringence [1], tapering [2], dispersion [3], and absorption [4-5] etc. The most important characteristic of a filter is a steep absorption edge with a low insertion loss, therefore it must offer a very high absorption in attenuated band compared to transmission band. The rare-earth absorption based fiber filters can offer very high absorptions with steep edge. Moreover, they are superior in quality than others as they can afford to handle much higher power thanks to the long length of the fiber. A first demonstration of fiber filter based on holmium doped silica fiber was demonstrated by Farries et al. [4]. In this paper, we propose for the first time a bismuth (Bi) doped silica fiber as a long pass filter with a cut-off wavelength of ~1050 nm. Bi doped silica fibers are emerging as an attractive gain medium for lasers and amplifiers in wavelength range of 1100 nm to 1500 nm [5-6]. However, their use for filtering application unlike holmium remains unexplored. Bi doping has been investigated along with different co-dopant such as AhO
₃
and P
₂
O
₅
in silica and germania hosts [5-6]. The crucial role of doping concentration of Bi, codopants (such as AhO
₃
or P
₂
O
₅
), and the host matrix (SiO
₂
or GeO
₂
) in the absorption spectrum of the fiber has been shown in [5-6]. The steepness of the curve and order of discriminative losses in forbidden and transmission band decides the suitability of fiber for filtering application. Recently, we fabricated a series of bismuth and alumina doped silica fibers using optimized MCVD process in conjunction with solution doping technique [7]. A doping level of up to 5mol % of alumina was achieved with a core diameter of 8 μm with a 100 μm cladding diameter and the relative index difference between core and cladding is 0.008 [7]. Figure 1(a) shows the measured absorption spectra of two Bi-Al doped fibers (different Bi doping). Fiber 366 (red curve in Fig. 1(a)) has a broad absorption till ~775 nm with an additional peak at ~980 nm. Fiber 367 (black curve in Fig. 1(b)) offers a very sharp edge between ~640 nm and ~735 nm. Further, fiber366 was tested at high power level. A high power supercontinuum source (output power >1W, bandwidth 600 nm to 2400 nm) was used for further characterization. Figure 1 (b) shows the measured spectrum of the output of supercontinuum light. A second spectrum shown in figure 1(b) was obtained by coupling the output of the SC source to a 54 m long Bi-Al doped fiber. The spectrum clearly shows the complete suppression of light before 1050 nm. This clearly shows the potential of Bi doped fiber for filtering applications.