A practically tunable multiwavelength Raman fiber laser based on few-mode fiber Bragg gratings without additional multichannel filters is investigated. We achieved high quality of the Raman laser output with a high extinction ratio of more than 45 dB. The number of lasing wavelengths can be adjusted by the properties of few-mode Bragg gratings with multiple resonant wavelengths. The lasing wavelength of Raman fiber laser can be readily controlled by the tension and compression strain, which correspond to the directional bending curvature change. The dynamic range of lasing wavelength change was more than 15 nm.
©2005 Optical Society of America
Multiwavelength fiber lasers have attracted great interest because of their potential application in optical fiber sensors, optical signal processing, sensor network multiplexing scheme, instrument testing, and so on [1–5]. They also have a variety of advantages like multiwavelength operation, simple structure, low cost, and insertion loss. In order to obtain the multiwavelength laser operation, versatile methods based on various gain media such as erbium-doped fiber amplifiers (EDFs) [1, 2], semiconductor optical amplifiers (SOAs) , and Raman amplifiers [4–6], have been investigated. EDF-based multiwavelength fiber lasers, however, have some limitation of the number of lasing wavelength unless the intrinsic characteristics of EDFA like homogenous broadening of erbium ions are suppressed. SOAs or Raman amplifiers are good candidates for realization of the stable multiwavelength fiber laser at room temperature. Compared with SOAs-based multiwavelength lasers, Raman-amplifiers-based multiwavelength fiber lasers have more flexibility of the lasing wavelength depending on properties of pump laser diodes [4–6]. To generate the multiwavelength output, multichannel filters are the important key component like Fabry-Perot filters , high birefringence fibers [2, 5], and cascaded long-period fiber gratings . However, their additional interposition into a laser cavity can degrade the overall performance due to the induction of insertion loss. Recently, few-mode-fiber-grating-based multiwavelength EDF lasers have been reported . However, it is difficult to obtain the stable multiwavelength laser output at room temperature unless the homogenous broadening of the EDF is suppressed by the cooling method of the EDF at cryogenic temperature or the frequency shift feedback method.
In this paper, we propose and experimentally demonstrate a flexibly tunable multiwavelength Raman fiber laser based on few-mode fiber Bragg gratings (FBGs) at room temperature without additional multichannel filters. Based on the symmetrical bending of the few-mode FBG, we could readily control the lasing wavelength of the multiwavelength Raman fiber laser depending on the direction of the bending curvature. The proposed tuning method is more effective to achieve the high tunability of the lasing wavelength compared with the other methods like the thermal tuning method . For the positive bending, the lasing wavelength shifted into the longer wavelength due to the tension strain. Contrarily, the negative bending induces the shift of the lasing wavelength into the shorter wavelength because of the induction of compression strain. The tunability of the lasing wavelength was 2.45 nm/m-1. The dynamic range was more than 15 nm. Compared with previous reports, the proposed multiwavelength Raman fiber laser scheme has a lot of advantages like stable operation at room temperature, lasing wavelength flexibility corresponding to a pumping wavelength, high extinction ratio, and so on.
2. Experiment and results
Figure 1 shows the experimental setup for the proposed multiwavelength Raman fiber laser. The Raman laser consists of a 50 km standard single mode fiber (SMF), a tunable chirped FBG, and few-mode FBG for a cavity mirror. The proposed multiwavelength Raman fiber laser is based on the characteristics of few-mode FBGs. Once considering the coupling mechanism in FBGs, it is clearly obvious that few-mode FBGs have multiple resonant wavelengths due to several guiding core modes in the few-mode fiber . Consequently, the multiwavelength Raman laser can be fabricated by using few-mode FBGs as a cavity mirror. For the few-mode fiber used in our experiments, it is the elliptic core fiber and the major and minor axes of the core are 12 and 6.6 μm, respectively. The cladding diameter is 125 μm. The relative index difference (Δ) is 1.3%. It has two air holes with the diameter of 20 ~ 25 μm. We utilized the KrF excimer with the pulse energy of 120 mJ/pulse to fabricate a few-mode FBG. Its length was 2 cm. In order to fabricate the Raman fiber laser, we utilized the length of 50 km SMF to get the sufficient Raman gain. Since the Raman gain medium has the inhomogeneous nature of gain broadening, we can generate simultaneous multiwavelength oscillations in this laser configuration. The Raman pump source consists of four laser diodes operating at 1425, 1435, 1455, 1465 nm, respectively. By combining all the pump wavelengths with a passive 14XX/C-band WDM coupler, a total pump power of up to 1 W could be launched into the SMF bypassing the chirped FBG. This level of pump was able to provide an on-off gain at a 1550 nm band within the SMF that was sufficient to compensate for the total cavity loss including background fiber loss.
To achieve a tunable multiwavelength Raman fiber laser, we used symmetric bending method as shown in Fig. 1(b). The bending-based tuning method is much effective due to the high tunability compared with the thermal one . The few-mode FBG was attached onto the flexible metal plate, which the length and thickness (d) are 7 cm and 0.3 mm, respectively. The flexible metal plate is made of the spring steel with the high resistance against fatigue and corrosion.
The middle of the metal plate is defined as the neutral axis (the dashed line) that retains its original length even if the metal plate is bent. A distance between the central axis of the FBG and the neutral axis of the metal plate is given as d/2. As shown in Fig. 1, when the flexible metal plate is bent ( the case of A), the region above the neutral axis will experience compression while the region below the neutral axis is elongated. Therefore, the compression strain can be included across the whole length of the few-mode FBG. Contrarily, in the case of B, the tension strain can be included along the few-mode FBG since the region above the neutral axis will be stretched. Therefore, the compression and tension strain can be created corresponding to the bending direction of the metal plate. For the positive bending, the resonant wavelength of the few-mode FBG shifts into the longer wavelength due to the tension stress. However, the resonant wavelength of the grating moves onto the shorter wavelength when the negative bending is applied to the metal plate . The lasing wavelength shift (Δλ) can be estimated  as
where λp is the lasing wavelength of the Raman fiber laser, ρ is the photo-elastic coefficient, and ε (=d/2R, d and R is the thickness and the bending curvature radius of the metal plate) is the strain induced by the bending of the fiber. Figure 2 shows the reflection spectrum of the few-mode FBG when the bending curvature changes. The few-mode FBG has three resonant wavelengths corresponding to the number of guiding mode in the few-mode fiber. Their center wavelengths are 1550.88, 1554.38, 1557.92 nm, respectively. Each of lasing wavelengths shifted into the shorter and longer wavelength depending on the compression and tension strain, respectively, as shown in Fig. 2.
The output spectra of the multiwavelength Raman laser using the few-mode FBG were shown in Fig. 3. In general, the few-mode FBG has strong polarization dependence, and the multiple resonance wavelengths have different polarization states . Therefore, we could stabilize the cavity state and the number of the lasing lines by adjusting the state of the polarization controller. Three lasing channels could be obtained because of three resonant wavelengths of the few-mode FBG as seen in Fig. 2. It is clearly evident that the number of lasing channels can be extremely increased once a number of few-mode FBGs are concatenated in serial form. High quality of independent lasing outputs could be obtained with a high extinction ratio of more than 45 dB. The peak difference of each channel was less than 0.5 dB. The bandwidth of each channel was less than ~0.12 nm. Once the polarization state within the laser cavity is stabilized, the three lasing wavelength is readily achievable.
Figure 3 shows the output spectra of the Raman fiber laser as the bending curvature along the few-mode FBG changes. For the positive bending of the few-mode FBG, the three lasing wavelengths shifted into the longer wavelength because of the tension strain. Contrarily, the compression strain, which was caused by the negative bending, induced the shift of three lasing wavelengths into the shorter wavelength. Figure 4 shows the lasing wavelength shift as a function of the bending curvature change. The similar tunability of 2.45 nm/m-1 was observed in the bending curvature range from 0.5 to 3.5 m-1 for both tension and compression strain due to the symmetric mechanical characteristics of silica under stress. The dynamic range was more than 15 nm.
3. Discussion and conclusion
We have experimentally demonstrated a practically tunable multiwavelength Raman laser based on a few-mode FBG at room temperature. Based on the symmetrical bending of the few-mode FBG, the lasing wavelength of the multiwavelength Raman fiber laser could be controlled effectively depending on the direction of the bending curvature change. For the positive bending, the lasing wavelength shifted into the longer wavelength due to the tension strain. Contrarily, the negative bending induces the shift of the lasing wavelength into the shorter wavelength because of the compression strain. The dynamic range was more than 15 nm. High quality of the multiwavelength Raman fiber laser output with a high extinction ration of more than 45 dB was also achieved. The number of lasing wavelengths can be adjusted corresponding to the properties of few-mode Bragg gratings with multiple resonant wavelengths. Once several few-mode FBGs are cascaded in the end of the Raman laser cavity, the number of lasing channels can be also increased. Since the proposed all FBGs-based multiwavelength Raman fiber is very useful for applications to optical communication systems as well as long-distance remote sensing systems.
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