High-power widely tunable Raman fiber laser

Summary form only given. Tunable Raman fiber lasers have attracted great interest owing to their high efficiency and reliability important for applications, such as optical fiber communications and sensing, spectroscopy, and instrument testing. Their tuning range is defined by the Raman gain bandwidth amounting to about 40 nm in telecom spectral range (~1550 nm) for conventional silica single mode fibers (SMF). To increase the range, highly nonlinear fibers which broaden pump spectrum may be incorporated in the cavity of Raman fiber lasers, see e.g. [1]. Another approach is to involve Rayleigh scattering forming random distributed feedback in a relatively long fiber resulting in prominent flattening of the tuning curve [2]. In this paper we report on combination of these two techniques in tunable Raman fiber lasers thus providing great improvement of their output characteristics. In the experiments we use highly-nonlinear True Wave fiber with zero-dispersion wavelength of 1457 nm. The pump wavelength of 1455 nm is close to that value, so the pump wave becomes spectrally broadened at propagation through the fiber (at high pump power even a supercontinuum can be generated in a long fiber [3]). In Fig.1a one of the studied experimental configurations is shown. Here a ring cavity with True Wave fiber (TWF) is pumped by 1455 nm laser via WDM coupler, a tunable filter is introduced in the cavity to select the wavelength. Output signal is measured through an output coupler. Linear cavities formed by loop mirror and normally or cleaved fiber ends were also explored. At large TWF length (>10 km) Rayleigh scattering (RS) becomes important [2] in both the ring and linear configurations of the cavity, at that pure RS-based random distributed feedback lasing is possible only for cleaved end linear configuration. The tuning curve for the ring cavity at 2W pumping is shown in the inset of Fig. 1a and corresponding output spectra are shown in Fig.1b: the laser is continuously - uned in the range 1520 - 1600 nm, i.e. 80 nm that is >2 times wider that that in [1,2]. At that, the power variations in this range are below 2 dB, while the maximum power is achieved around 1570 nm. Note that not only the generated power is changed at the tuning, but also the optical signal to noise ratio (OSNR) as well (Fig.1b). The maximum OSNR value exceeds 40 dB at 1551 nm and minimum OSNR is 25 dB at wavelength 1520 nm. The tuning range is limited by filter characteristics and may be further increased (especially in long-wavelength part). Maximum output power of >2 W has been achieved in linear configuration. Thus, a possibility to greatly increase a tuning range and output power of the Raman fiber laser by combining effects of highly-nonlinear fiber and Rayleigh-scattering based feedback in the cavity has been demonstrated. A detailed presentation of the experimental results in different configurations will be done.