Tunable random fiber laser

An optical fiber is treated as a natural one-dimensional random system where lasing is possible due to a combination of Rayleigh scattering by refractive index inhomogeneities and distributed amplification through the Raman effect. We present such a random fiber laser that is tunable over a broad wavelength range with uniquely flat output power and high efficiency, which outperforms traditional lasers of the same category. Outstanding characteristics defined by deep underlying physics and the simplicity of the scheme make the demonstrated laser a very attractive light source both for fundamental science and practical applications.

Figure 1

Schematics of the tunable RDFB fiber laser. Photons propagating in both directions are scattered obeying Rayleigh's law. Amplification is achieved by coupling two equal-power, 1455-nm pump waves into the center, while the tunable bandpass filter in the middle sets the wavelength (filter transmission spectrum is in the inset); the fiber length was chosen to give maximum output power.

Figure 2

RDFB fiber laser tuning characteristics. (a) Tuning range in linear scale for different total pump power $P$ = 2$P_{\mathit{in}}$ = 2.5 W (diamonds), 3.5 W (squares), 4.5 W (triangles), 6 W (circles); at $P$ > 5 W the variation of the laser output power is ∼3% (0.1 dB) for all wavelengths in the range 1535–1570 nm. (b) Spectra at different filter wavelengths are shown at $P$ = 6 W. For wavelengths away from the Raman gain maximum (gain spectrum nearly corresponds to amplified background noise with two maxima at 1555 and 1565 nm and 10 dB gain variation), lasing at ∼1555 nm appears [12].

Figure 3

RDFB fiber laser power in comparison. Left output power (at 1560 nm) as a function of the total input pump power $P=2P_{\mathit{in}}$ (at 1455 nm). For random fiber laser (diamonds) the output power is $>1.1$ W, demonstrating efficiency of ∼30% for total power from both ends (see inset). For 4% cavity and loop-mirror cavity (squares and triangles, respectively) the threshold is lower but maximum power is lower as well. The scheme with loop mirror has only one output end so its total power is two times lower than in symmetric schemes. Inset: RDFB laser efficiency in experiment (points) and theory (line).

Figure 4

Tunable RDFB laser spectrum. Spectra for wavelength 1562.5 nm at different total pump power. With increasing power, the top part of the spectrum remains almost constant while exponential wings grow. Inset: Radio frequency spectra. For random fiber laser no mode beatings are seen but for a conventional fiber Raman laser a clear mode structure is observed.