We demonstrate room-temperature operation of a spacing-adjustable multi-wavelength erbium-doped fiber laser using stimulated Brillouin scattering. With the use of an intra-cavity birefringent loop mirror filter, the laser output wavelengths can be well defined without applying an external Brillouin pump. By adjusting the comb spacing of the filter, a wide range of mode spacings can be obtained to support the oscillations of up to 49 output wavelengths.
©2006 Optical Society of America
Multi-wavelength laser sources are of great interest in optical sensing, optical spectroscopy, microwave signal processing, and high capacity WDM communications. The erbium-doped fiber (EDF) ring laser exhibits unique advantages for multi-wavelength generation including a large gain, a high saturation power, and a relatively low noise figure. However, as a result of the homogeneously broadened gain medium and unstable mode competition, multi-wavelength lasing is inhibited in an EDF ring laser at room temperature. Different approaches have been proposed to demonstrate multi-wavelength lasing including the immersion of the EDF in liquid-nitrogen [1, 2], the use of a frequency shifter in the cavity , the incorporation of a semiconductor optical amplifier for self-saturation , the use of polarization hole burning in overlapping fiber cavities , the introduction of four-wave mixing in a nonlinear fiber inside the ring , and the combination of Brillouin gain and EDF gain in the laser [7–9]. With Brillouin scattering, the spacing of the multi-wavelength source is determined by the Brillouin frequency shift and is fixed at around 9 - 12 GHz in the 1550 nm region for most fibers. The exact frequency shift depends on the velocity of the acoustic wave in the fiber. The acoustic velocity varies with the material and the structure of the fiber used in the experiment .
To increase the operational flexibility and functionality, it is desirable to demonstrate a multi-wavelength source with adjustable mode spacing. In this work, room-temperature operation of a multi-wavelength EDF laser has been demonstrated with a mode spacing ranging from 0.08 to 1.20 nm. Mode spacing adjustment is achieved by changing the comb spacing of an intra-cavity birefringent loop mirror filter (LMF). Multi-wavelength lasing is supported with the adoption of Brillouin scattering in the single mode fiber. The width of the gain band and the number of wavelengths are also adjustable by controlling the pump power of the EDF laser. The maximum number of output wavelengths is 49 and the power variation is below 3 dB. An output extinction ratio of up to 50 dB has been obtained.
2. Experimental setup and principle
Our multi-wavelength EDF ring laser is schematically illustrated in Fig. 1. The laser consists of an EDF amplifier, an optical circulator, a 2-km SMF, and a birefringent LMF.
The birefringent LMF shown inside the dotted box in Fig. 1 serves as a selective element to determine the output wavelengths. The optical reflection of the birefringent LMF is a periodic function of wavelength and the comb spacing is given by ∆λ = λ2/∆n·L , where ∆n and L are the birefringence and the length of the polarization-maintaining fiber, respectively. Those wavelengths that lie at the reflection peaks of the birefringent LMF are re-directed to the 2-km SMF. By adjusting the comb spacing of the LMF, wavelength-adjustable multi-wavelength output can be obtained from the erbium-doped fiber laser. In this approach, the birefringent LMF is used to determine the lasing wavelengths, while the multi-wavelength operation is supported by stimulated Brillouin scattering in the 2-km single mode fiber.
To understand the role of stimulated Brillouin scattering (SBS) in the EDF laser, a LMF with a comb spacing of 5.2 nm is initially used as the wavelength selective element inside the cavity. The relatively large comb spacing gives rise to broad transmission peaks and allow a clear observation of the oscillations at high orders of the Brillouin Stokes wavelengths. At the beginning, the 2-km SMF is removed from the setup. An unstable dual-wavelength output is obtained and the competition between the two wavelengths is very strong. After inserting the SMF, a stable dual-wavelength output is observed as shown in Fig. 2. Within the lasing linewidth of each wavelength, fine spectral components with a spacing of 0.088 nm are observed and are shown in the inset. The components originate from Brillouin Stokes shift of the lasing wavelength in the fiber and provide an evidence of the presence of SBS. The 0.088 nm spaced components within the band grow continuously to support the lasing of that wavelength. The growth of new components will sustain in the 2-km SMF as long as the spectral component under consideration is strong enough to create the next component. The LMF acts as a mirror for those wavelengths that are aligned with the broad reflection peaks. The lasing wavelengths are thus reflected back to the SMF to stimulate further growth of back-scattered SBS components. Since the 3-dB reflection peak is broad and is equal to half of the comb spacing, multiple high order SBS components can be supported.
The growth of SBS components can be explained with Fig. 3(a). The light that travels in the fiber cavity develops an oscillating mode from the amplified spontaneous noise (ASE). The wavelength that experiences the largest gain acts as the strongest Brillouin pump (BP) in the SMF and generates the 1st order Brillouin Stokes wave that travels backwards (from right to left in Fig. 3(a)). The 1st order backward Brillouin Stokes wave (BBS) creates another Brillouin Stokes wave propagating in the same direction as the BP. This becomes the 2nd order forward Brillouin Stokes (FBS). The 2nd order FBS will continue the growth by generating a 3rd order BBS. For the BP, when it reaches the LMF, it will be reflected back to the SMF and generates a 1st order FBS along its propagation. The above process will repeat to generate additional BBS and FBS components. Eventually, when the power of a component is too weak for the generation of the next Brillouin Stokes wave in the SMF, the growth will stop. It is interesting to note that the component at the long wavelength edge is relatively strong for the case shown in the inset of Fig.2. Nevertheless, further growth of higher order Stokes component cannot be supported owing to a sharp cut-off at the envelope of the spectrum. The abrupt profile observed at the long wavelength side is consistent with the result in Ref  when the EDF is pumped strongly to support a wider band of wavelengths. In contrast, earlier work on Brillouin/erbium multi-wavelength fiber lasers often shows a gradual decrease of power as the Stokes components grow towards the long wavelength side when an external Brillouin pump is used .
Although the SBS frequency shift is determined by the SMF characteristic and is fixed at 0.088 nm, a multi-wavelength source with a mode spacing of 0.080 nm can be obtained with the introduction of a LMF at that comb spacing. As illustrated in Fig. 3(b), the dotted curve shows the reflection characteristic of the LMF and the solid curve shows the envelope of the SBS components. Although the SBS linewidth is smaller than 20 MHz for a well defined pump at a given wavelength, the linewidth observed in our experiment is much wider since the pump can cover the whole spectral region within the reflection band of the LMF. The 3-dB linewidth of an individual lasing component is larger than 0.02 nm in our work. In Fig. 3(b), consider the left-most component to be the one that initiates the SBS process. The 1st order Brillouin Stokes wave is generated at 0.088 nm away from the BP wavelength. Combining the spectral response of the LMF and the SBS gain, a new band of components will be reflected. After several round-trip propagations in the cavity, the components will evolve and become centered at point A since it is the spectral position with the lowest cavity loss within the band. These components will in turn generate a finite band of 2nd order Stokes wave centered at point B (0.088 nm away from point A) and travels in the opposite direction. Again, the band of components is regulated by the frequency comb of the LMF. The above process will continue and eventually result in a multi-wavelength source at 0.080 nm spacing. Owing to the narrow spacing between adjacent wavelength components, it is believed that four-wave mixing also plays a role in the generation and stabilization of the multi-wavelength output.
3. Results and discussion
The EDF laser is first operated without using the 2-km SMF. As expected, multi-wavelength lasing cannot be supported since SBS does not take place inside the cavity. An unstable optical spectrum is obtained as shown in Fig. 4.
Next, we include the 2-km SMF in the setup to observe the Brillouin scattering effect. A 1.5-m standard SMF is first used in place of the PMF (shown in Fig. 1) in the LMF. The LMF now acts as an ideal optical reflector such that all the incident wavelengths are redirected to the 2-km SMF to strengthen the Brillouin scattering. As shown in Fig. 5(a), a multi-wavelength output with a mode spacing of 0.088 nm that corresponds to the Brillouin shift is obtained. Since there is no external Brillouin pump and no wavelength-selective element is used to define the exact positions of the reflecting wavelengths, the output extinction ratio between the selected and the non-selected components is relatively low and is less than 10 dB.
To increase the extinction ratio and to match the mode spacing with the ITU grid, a 100-m PMF with a birefringence of 3×10-4 is then used in the LMF that provides a comb spacing of 0.080 nm. As shown in Fig. 5(b), the multi-wavelength components that are spaced at 0.080 nm show an improved extinction ratio of up to 20 dB. The lasing bandwidth is almost doubled. The output contains 40 wavelengths with a power variation of less than 3 dB. The lasing to non-lasing modes suppression ratio is about 25 dB.
The width of the gain band and the number of output wavelengths can be controlled by adjusting the pump power of the EDFA. We vary the EDFA output power over the range of 13.7 to 17.9 dBm and observe that the number of lasing wavelengths increases from 30 to 49, as shown in Fig. 6. When the power is below 13.7 dBm, the multi-wavelength source becomes unstable. The instability occurs because the optical power is below the Brillouin threshold. In our experiment, to obtain a multi-wavelength source at 0 dBm, the EDFA input power is 10.2 dBm while the EDFA output power is 16.0 dBm.
To demonstrate adjustment of the mode spacing, birefringent LMFs with different comb spacings have been used in the setup. Different PMF lengths at 50.0, 13.4, 10.0, 8.0, and 6.7 m are used inside the LMF to produce comb spacings of 0.16, 0.60, 0.80, 1.00, and 1.20 nm, respectively. The corresponding optical spectra of the multi-wavelength output are shown in Fig. 7. Compared with Fig. 6, the center lasing wavelength is slightly red shifted by 2 nm owing to the difference in polarization setting of the laser cavity. The output extinction ratio in each of the cases can reach as high as 50 dB. The amplitude variation among the lasing wavelengths is less than 3 dB. A zoom-in view of an individual component is shown in the inset of Fig. 7. The optical spectrum is measured by an optical spectrum analyzer with a resolution of 0.01 nm (~1.25 GHz at 1560 nm). The figure is plotted in the linear scale that clearly shows the 0.088 nm components.
In principle, our approach can be extended for use with spacing-adjustable comb filters  for efficient generation of multi-wavelength outputs with different spacings without replacing the PMF. When the comb spacing increases, the linewidth of each lasing component also increases owing to the wider reflection peaks of the LMF. To reduce the linewidth, a 2nd order LMF that exhibits a narrower pass band can be used as a replacement for the conventional LMF. To eliminate the side modes of the multi-wavelength source, one can use a WDM coupler or a diffraction grating together with a spatial filter to block the undesired components . Also, fast tuning of the multi-wavelength output is possible by controlling the birefringence of the LMF through electrical or optical means [14–15].
A spacing-adjustable multi-wavelength laser source has been proposed and demonstrated. The EDF laser contains a birefringent loop mirror filter to define the mode spacing and a 2-km SMF to support stimulated Brillouin scattering. Our approach eliminates the use of an external Brillouin pump while an extinction ratio of over 20 dB can be achieved. Multi-wavelength outputs with spectral spacing ranging from 0.08 to 1.20 nm have been successfully produced.
The authors would like to thank Dr. T. K. Liang for a fruitful discussion of the work. This project is supported by the Research Grants Council of Hong Kong under Grants CUHK 4369/02E and 4157/05E.
References and links
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