Geometry and design considerations

A schematic of the considered tunable RFL design is shown in Fig. 3.13. First of all, an all-fiber cascade of fixed-wavelength auxiliary resonators (ARs) formed by high-reflectivity FBGs at wavelengths λ_AR,i is formed. This is the principle of cascaded

Setup A

Setup B tunable

reflector FBGs for

auxiliary resonators atl

AR, i

{ {

Figure 3.13.: Two possible designs for tunable Raman fiber lasers with a bulk-optical tuning element. A: broadband reflector on one side. B: wavelength-selective reflectors on both sides.

resonators introduced for non-tunable RFLs in [GEM⁺94]. It is illustrated in Fig. 3.14.

The Stokes light generated by the pump laser in the first cavity (labeled “auxiliary res-onator 1” in Fig. 3.14) acts itself as the pump for the next cavity, labeled “auxiliary resonator 2”. This process is repeated until the desired output wavelength is reached.

Optical conversion efficiencies (from the pump wavelength to the output wavelength) greater than 50% can be achieved in such lasers [GEM⁺94]. Cascaded RFLs are also commercially available. (In Sect. 7.4, we will show that this concept can also be useful for silicon Raman lasers.)

Pump laser

Auxiliary Resonator 1

Auxiliary Resonator 2

Output

Resonator wavelength

Raman gain spectrum

Figure 3.14.: Illustration of the principle of cascaded Raman lasers. In this example, successive resonators are always placed in the Raman-gain maximum of the previous stage, which is actually not optimal if the cascade serves as the foundation for a tunable RFL, see discussion in the main text.

The cascade of auxiliary resonators (ARs) shown in Fig. 3.14 makes Raman gain available at all wavelengths in the range of interest, without introducing high losses. In our tunable RFL (see Fig. 3.13), an additional tunable resonator is formed by a bulk-optical tunable reflection grating on one side, while on the other side we can either place a broadband mirror (setup A) or another reflection grating (setup B). Setup A, inspired by [JLSA77], has the advantage of lower overall losses and easier alignment, but the problem of parasitic resonators is more critical (see below). Setup B was used in the

experiment published in [CKRB03], where actually the output beams of both fiber ends have been directed to the same reflection grating.

Parasitic resonators

The primary design goal is to avoid spectral gaps in the tunable range caused by com-petition of the tunable resonator (TR) with parasitic resonators (PRs). PRs are un-wanted resonators formed by reflecting elements such as fiber-end faces, FBGs of long-wavelength auxiliary resonators not needed for short tuned long-wavelengths, the broadband mirror in case of setup A, and Rayleigh backscattering. A PR is fatal if it starts to lase at a sufficiently high power level while depleting the TR, possibly stopping the TR to lase at all, and thus causing zero tuned output power. Whether a PR is critical depends not only on the setup, but also on the shape of the cascaded Raman gain spectrum and thus on the pump power. The behaviour described in this section was observed both experimentally [CKRB03] and in numerical simulations.

The most obvious PRs are ARs at wavelengths longer than the tuned wavelength.

These ARs are not required, because they do not provide Raman gain to the tuned wavelength. If the TR is spectrally close to one of the undesired ARs, the low-loss AR might start to lase at the expense of the relatively high-loss TR. Therefore, depending on the tuned wavelength, some of the long-wavelength ARs must be switched off. An AR can be switched off by detuning one of its FBGs by about one width of its reflectivity spectrum. In case of setup A, though, the broadband mirror on the left-hand side in Fig. 3.13 can still form a low-loss PR with the right-side FBG of the “switched-off” AR.

It turned out that, in addition, a reduction of the reflectivity of the FBG itself may be necessary in this case (which is not required for setup B).

We have observed that other broadband-reflecting elements such as fiber-end faces and even Rayleigh backscattering can also form PRs at wavelengths corresponding to the Raman gain maxima of other resonators lasing at shorter wavelengths. Using setup A, we observed this even when the right-side fiber end was immersed in an index-matched liquid, from which we conclude that Rayleigh backscattering can indeed be significant for the formation of PRs.

In setup B, even though there is no broadband mirror, the FBGs of switched-off ARs and the fiber-end faces can form PRs at the AR wavelengthsλ_AR,i. The problem of these fiber-end PRs is weakened when an AR does not spectrally coincide with the Raman gain maximum of its short-wavelength neighbour (this latter, non-optimal case is shown in Fig. 3.14). Instead, the ARs should be spaced closer together on the wavelength axis.

In any case, it is advisable to have tilted fiber-end faces so as to reduce their reflectivities.

Successive ARs must be placed spectrally closer to each other also in order to prevent the occurrence of tuned-wavelength intervals with too low cascaded Raman gain for the tunable resonator (due to the roughly triangular shape of the Raman gain curve, see Fig. 2.3).

In summary, the design of tunable RFLs according to our concept consists in choos-ing the optimum wavelengths λ_AR,i of the ARs (while minimising the total number of required ARs), determining at which wavelength the ARs must be switched on when sweeping through the tuning range, and deciding whether the AR FBGs need to be adjusted in their reflectivities (as opposed to be merely detuned).