Types and Basic Principles of Broadband Fiber Sources
Four general types of broadband fiber sources happen to be investigated: namely, resonant fiber lasers, superfluorescent fiber sources, wavelength-swept fiber lasers, and sources involving an SLD and an Er-doped fiber amplifier (EDFA). Their principles are briefly reviewed in the following paragraphs. (Related products in Fiberstore: DWDM EDFA)
Broadband Fiber Lasers
Although most continuous-wave (CW) fiber lasers create a narrow emission, under proper conditions they can be operated as a broadband source. The laser transitions of triply ionized rare earths are broadened by both homogeneous and inhomogeneous processes. The spectral properties of a fiber laser are strongly affected by which one of the processes dominates. Homogeneous mechanisms broaden the linewidth of the transitions between Stark levels very much the same for all ions. However inhomogeneous broadening results in a change in the distribution from the Stark levels that is different from ion to ion with respect to the ion's physical site inside the host. When a dopant is pumped near the center of one of its absorption bands, pump photons have a high probability of being absorbed by one of the several Stark transitions of every ion within the material. All ions have roughly equal probability of absorbing (i.e., the medium behaves quasi homogeneously). However, if the dopant is pumped in the tails of the band, the probability of absorption is bigger for groups of ions that exhibit a stronger transition at that wavelength. Absorption will be site-specific. The medium behaves as if it were more strongly inhomogeneously broadened. This principle was studied at length in Nd-doped fibers.
Based on this effect, a fiber laser can be made to produce a broadband emission, provided the fiber dopant is at least partly inhomogeneously broadened and pumped on the edge of an absorption band. This principle was demonstrated with an Nd-doped fiber laser.
Superfluorescent Fiber Sources
An SFS consists of an optically end-pumped rare earth doped fiber. The inverted ions create a spontaneous emission, some of which is captured by the fiber core in both the forward direction (cotraveling using the pump) and the backward direction (from the pump). The forward and backward spontaneous photons are amplified because they travel across the fiber and produce amplified spontaneous emission (ASE), or superfluorescence, in the forward and backward output port, respectively.
Several SFS configurations are possible, each using its own characteristics, benefits, and downsides. The first one is the forward SFS (Fig. 1a). This device produces both a forward along with a backward output, only the former can be used; namely, the output from the end opposite the pump input. This is a single-pass device: the ASE travels just once through the fiber. In general, for the output power to be sizable the fiber should be pumped difficult to exhibit a high gain. Consequently, if reflections into the fiber are allowed to occur from both ends, in particular Fresnel reflections in the fiber ends, this product will become a laser and emit an undesirably narrow spectrum. To avoid this effect the fiber ends are usually polished at an angle (typically 7 degrees or greater). If necessary, reflections from the pump optics can also be reduced by putting an optical isolator around the pump input arm.
Another single-pass configuration may be the backward SFS (see Fig. 1b). The signal that is used is now the backward ASE, which comes out in the pump input end. The pump is filtered out from the output by a WDM fiber coupler that couples minimally in the pump wavelength (ideally 0%), but strongly over the band- width of the ASE (ideally 100%) (or the other way around). One advantage of the backward SFS is that its sensitivity to feedback is lower (especially if the fiber is extremely long, as required for high efficiency) compared to a forward SFS. However, it is usually used with an isolator placed in the output to reduce the sensitivity of its mean wavelength to alterations in feedback levels.
A third configuration may be the double-pass SFS (see Fig. 1c). A higher reflector at the ASE wavelength is added (e.g., at the pump input port), to propagate the backward ASE with the fiber again. This configuration produces only a forward output. Alternatively, the reflector can be put at the other end of the doped fiber to produce a backward output only. The primary advantage of the double-pass configurations is that the signal photons travel through the fiber twice and notice a higher gain compared to a single-pass SFS (up to a factor of two). Thus, the threshold of a double-pass SFS is concomitantly lower, and it is pump power requirement is reduced. Also, the length of fiber that maximizes its efficiency is shorter than for a forward SFS. The primary disadvantage of the double-pass SFS would be that the high reflector exacerbates the spectrum susceptibility to external feedback from the system that light is coupled, meaning a higher- extinction isolator is needed. In the double-pass backward SFS it is also generally required to reduce reflection in the pump optics by putting an isolator between your pump source and the WDM coupler. (Related products in Fiberstore: DWDM Filter)
The fourth and last SFS configuration may be the fiber amplifier source (FAS; see Fig. 1d). It was originally created for the FOG. It is a backward SFS with no isolator, so that the signal returning from the FOG can traverse the doped fiber and become ampli- fied before reaching the detector. The FAS serves as both a source and an amplifier. Thus, it increases the detected signal power, which reduces electronic noise within the detection and simplifies electronic processing. This configuration provides the same potential benefits for applications other than the FOG.
Other Types of Broadband Fiber Sources
The two other kinds of broadband fiber sources have obtained much less attention. The first is the SLD-EDFA tandem source, where the broadband output of an SLD is amplified by an EDFA. Its main benefit is the fact that, because the EDFA is seeded by an external signal, it features a lower threshold than a single-pass SFS, which is seeded by spontaneous photons. An SLD-EDFA tandem source with an output signal of 20 mW and a bandwidth of 21 nm was demonstrated using 60 mW of pump power. Further studies of this interesting source are warranted, particularly of its thermal stability (as the seed source is strongly temperature-sensitive).(Related products in Fiberstore: CATV EDFA)
The last type of broadband fiber source may be the wavelength-swept fiber laser (WSFL). It's a fiber laser with an acousto-optic (AO) modulator incorporated within the cavity. For a given acoustic frequency along with a given alignment of these two cavity reflectors, the Bragg condition is satisfied only at a specific wavelength, and also the laser oscillates at this lower-loss wavelength. When the acoustic frequency is modified, the laser wavelength also changes. If the acoustic frequency is scanned slowly enough to allow buildup of the laser field within the resonator at each acoustic frequency before moving on to the next one, the wavelength is swept continuously across the gain curve. This produces emission that's broad over a long time scale compared with the inverse of the sweeping rate. The WSFL works equally well with homogeneously and inhomogeneously broadened transitions. Also, because the photons are recirculated, its threshold could be lower than that of an SFS. Because the laser frequency is continuously shifted at each round-trip through the resonator, this source may also be less sensitive to feedback than other sources. This principle was demonstrated by having an Er-doped fiber along with a bulk AO modulator. An all-fiber version could be constructed using existing all-fiber components.
Article source: http://www.fiber-optic-solutions.com/
