Types and Basic Principles of Broadband Fiber Sources

Four general types of broadband fiber sources have been 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 this article. (Related products in Fiberstore: DWDM EDFA)

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Broadband Fiber Lasers

Although most continuous-wave (CW) fiber lasers produce 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 influenced by which one of these two processes dominates. Homogeneous mechanisms broaden the linewidth of the transitions between Stark levels in the same manner for all ions. On the other hand inhomogeneous broadening leads to a change in the distribution of the Stark levels that differs from ion to ion depending on the ion’s physical site within 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 in 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 greater for groups of ions that exhibit a stronger transition at that wavelength. Absorption is then site-specific. The medium behaves as if it were more strongly inhomogeneously broadened. This principle was studied in detail 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.

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Superfluorescent Fiber Sources

An SFS is made of an optically end-pumped rare earth doped fiber. The inverted ions produce a spontaneous emission, some of which is captured by the fiber core in both the forward direction (cotraveling with the pump) and the backward direction (against the pump). The forward and backward spontaneous photons are amplified as they travel along the fiber and produce amplified spontaneous emission (ASE), or superfluorescence, at the forward and backward output port, respectively.

Several SFS configurations are possible, each with its own characteristics, benefits, and disadvantages. The first one is the forward SFS (Fig. 1a). This device produces both a forward and a backward output, but only the former is used; namely, the output from the end opposite the pump input. This is a single-pass device: the ASE travels only once through the fiber. In general, for the output power to be sizable the fiber must be pumped hard to exhibit a high gain. Consequently, if reflections into the fiber are allowed to occur from both ends, in particular Fresnel reflections at the fiber ends, this device 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 need be, reflections from the pump optics can also be reduced by placing an optical isolator on the pump input arm.

Another single-pass configuration is the backward SFS (see Fig. 1b). The signal that is used is now the backward ASE, which comes out at the pump input end. The pump is filtered out from the output by a WDM fiber coupler that couples minimally at the pump wavelength (ideally 0%), but strongly over the band- width of the ASE (ideally 100%) (or vice versa). One advantage of the backward SFS is that its sensitivity to feedback is lower (especially if the fiber is very long, as required for high efficiency) than that of a forward SFS. However, it is often used with an isolator placed at the output to reduce the sensitivity of its mean wavelength to changes in feedback levels.

A third configuration is the double-pass SFS (see Fig. 1c). A high reflector at the ASE wavelength is added (e.g., at the pump input port), to propagate the backward ASE through the fiber a second time. This configuration produces only a forward output. Alter- natively, the reflector can be placed 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 experience a higher gain than in a single-pass SFS (by as much as a factor of 2). Thus, the threshold of a double-pass SFS is concomitantly lower, and its pump power requirement is reduced. Also, the length of fiber that maximizes its efficiency is shorter than for a forward SFS. The main disadvantage of the double-pass SFS is that the high reflector exacerbates the spectrum susceptibility to external feedback from the system to which light is coupled, which means that a higher- extinction isolator is required. In the double-pass backward SFS it is also generally required to reduce reflection from the pump optics by placing an isolator between the pump source and the WDM coupler. (Related products in Fiberstore: DWDM Filter)

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The fourth and last SFS configuration is the fiber amplifier source (FAS; see Fig. 1d). It was originally designed for the FOG. It is a backward SFS without an isolator, so that the signal returning from the FOG can travel through the doped fiber and be ampli- fied before reaching the detector. The FAS acts as both a source and an amplifier. Thus, it increases the detected signal power, which reduces electronic noise in the detection and simplifies electronic processing. This configuration offers the same potential benefits for applications other than the FOG.

Other Types of Broadband Fiber Sources

The two other types of broadband fiber sources have received much less attention. The first one is the SLD–EDFA tandem source, in which the broadband output of an SLD is amplified by an EDFA. Its main benefit is that, because the EDFA is seeded by an external signal, it has 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, in particular of its thermal stability (as the seed source is strongly temperature-sensitive).(Related products in Fiberstore: CATV EDFA)

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The last type of broadband fiber source is the wavelength-swept fiber laser (WSFL). It is a fiber laser with an acousto-optic (AO) modulator incorporated in the cavity. For a given acoustic frequency and a given alignment of the two cavity reflectors, the Bragg condition is satisfied only at a specific wavelength, and the laser oscillates at this lower-loss wavelength. When the acoustic frequency is changed, the laser wavelength also changes. If the acoustic frequency is scanned slowly enough to allow buildup of the laser field in 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 is 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 can 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 with an Er-doped fiber and a bulk AO modulator. An all-fiber version could be constructed using existing all-fiber components.

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