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Experimental optimization of high power Raman fiber lasers at 1495 nm using phosphosilicate fibers
Optics Communications 239 (2004) 137–145 www.elsevier.com/locate/optcom
Experimental optimization of high power Raman fiber lasers at 1495 nm using phosphosilicate fibers Z. Xiong b
a,*
, N. Moore a, Z.G. Li a, G.C. Lim a, D.M. Liu b, D.X. Huang
b
a Singapore Institute of Manufacturing Technology, 71 Nanyang Drive, Singapore 638075, Singapore Department of Optoelectronics Engineering, Huazhong University of Science and Technology, China 430074
Received 29 October 2003; received in revised form 29 March 2004; accepted 10 May 2004
Abstract We report in this paper on the experimental optimization of high power Raman fiber lasers at the second Stokes order of a phosphosilicate (P-doped) fiber doped with 13 mol% P2O5. The lasers were pumped with a 20 W Yb double clad fiber laser at 1070.75 nm and the cavity used two pairs of fiber Bragg gratings (FBGs). One pair is highly reflective at 1248 nm to fully cascade the pump wavelength to the first Stokes order, and the other includes a partial reflective (PR) FBG at 1495 nm to couple the laser out of the cavity. A total of 20 laser configurations were constructed and tested, consisting of four PR FBGs couplers with reflection from 4% to 45% and five fiber lengths from 200 to 1000 m. It is found that the optimal configuration is that with a fiber length of 300 m and a 33% PR FBG. The maximum power obtained from this laser is 6.66 W at 1495 nm with a threshold of 1 W and a slope efficiency of 39.18%. The highest slope efficiency achieved with pump power up to 18 W is 55.6% in the shortest fiber laser of 200 m without any PR FBG but a cleaved end. It is observed that the input–output characteristics are almost linear for all the configurations with feedback of PR FBGs P 15%, indicating the stimulated scattering to higher Stokes orders is negligible. Laser line width is also observed to become broader with increase in pump power, fiber length and the reflection of the PR FBGs. Ó 2004 Elsevier B.V. All rights reserved. Keywords: Raman fiber lasers; Optical fiber communications; Raman amplification; Phosphosilicate fiber; Stimulated Raman scattering
1. Introduction High power Raman fiber lasers (RFLs) are poised to play an important role in the next gener*
Corresponding author. Tel.: +67938502; fax: +67922779. E-mail address: [email protected] (Z. Xiong).
ation of optical telecommunication systems since they are one of the very few major pump sources for Raman fiber amplifiers (RFAs), which are being deployed in almost every new ultrabroad-band and long-haul fiber optic transmission system [1]. The available pumps for C-band RFAs at present are the intense laser beams at 14xx nm either from
0030-4018/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2004.05.010
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laser diodes or from RFLs. As a pump source, the RFLs have several advantages over the laser diodes. First of all, the power of the diode lasers launched into a single mode fiber is limited 300 mW. Ideally the overall pump power should be 800–1500 mW, which can be easily achieved with a RFL. These increasing output power levels are needed to provide the necessary gain margin for DWDM applications (such as gain flattening), as well as provide additional flexibility for amplifier designers. Secondly, amplification with both a wide-band and a flat-gain by use of multiple wavelengths [2,3] can be achieved within a single RFL because of its multi-wavelength generation feature associated with the stimulated Raman scattering of different Stokes orders [4] and various ions doped in fibers [5–7]. However, in the laser diode approach, multiple wavelengths require several diodes to be combined with wavelength multiplexers, which are costly and low efficient. Finally, the Raman wavelength shift is immune from the environmental conditions, while the wavelength of the laser diodes drifts with temperature. RFLs with high power of a few watts within single mode fibers have been realized [8,9] as a result of the successful development of three key components: high power double clad fiber (DCF) lasers [10], high quality fiber Bragg gratings (FBGs) [11], and high gain and low loss Raman fibers [12]. As feedback elements, FBGs with high reflectivity and a well matched band with the Raman scattering make it possible to construct highly efficient and compact Raman fiber lasers. The most powerful DCF lasers are those using fibers doped with Yb, which have a prominent lasing wavelength around 1100 nm. The conversion efficiency from this wavelength to the pump wavelength (for example, 1480 nm) for the C-band amplification strongly depends on the cascaded cavity and the Raman gain medium used. Therefore, a large wavelength shift in the RFLs is always preferable so that the number of cascading can be minimized. This allows the wavelength conversion efficiency to be enhanced, and the lasers can be more compact with fewer cascaded cavities. The fiber doped with phosphorus has been emerging as a promising and more efficient Raman gain medium [5,13–15] since the P2O5 bond vibration can create
a strong and large frequency shift of 40 THz, about three times as large as that of a germanosilicate fiber [6,16]. Due to this large increment in the Stokes order separation, the cascading process can be simplified significantly [17,18]. The conversion efficiency is also dependent on the extent of optimization on the laser configuration parameters, including the characteristics of the FBGs and the fiber length. This is evident from the evolution power along the Raman fiber. The steady state power in the Stokes lines is governed by a set of ordinary nonlinear differential equations, which are usually written as [9,19,20] F=B dP sn gn1 vsn ¼ Rn1 ðP Fsn1 þ P Bsn1 ÞP F=B sn dz vsnþ1 Aeff
gnR F F=B ðP þ P Bsnþ1 ÞP F=B sn asn P sn : Aneff snþ1
ð1Þ
It shows that the nth Stokes order line PsnF/B (either forward or backward) is excited by the (n 1)th-order, and contributes simultaneously to the (n + 1)th-order traveling in both directions. While it is not convenient to tailor the Raman fiber characteristics including the Raman gain coefficients gR, the effective mode areas Aeff and the absorption coefficients asn, the laser cavity configuration parameters play an important role in the experimental optimization of the laser performance. This is because the solution of Eq. (1) is also determined by its boundary conditions, i.e., the reflection characteristics of the FBGs at the start (z = 0) and end (z = L where L is the fiber length) points at different Stokes order wavelengths. By using various FBGs of different reflectivities with their central wavelengths well matched to the Stokes orders of interest and selecting suitable fiber length, one can optimize the laser performance. There are a few reports on the optimization of the high power Raman fiber lasers, both theoretically [9,19] and experimentally [8,9]. Ref. [19] reported on the theoretical optimization of Raman fiber lasers using germanosilicate fibers. It was predicted that the optimal fiber length for laser beams at 1240 (the second-order) and 1480 nm (the fifthorder) decreases with input pump power at 1117 nm, and the optimal feedback of the output coupler
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was higher for the higher order than that for the lower order. Karpov et al. [18] estimated a maximum power at the second Stokes order of the Pdoped fiber would be obtained at a much shorter fiber length than 1 km used by them, probably due to lack of higher pump power. Ref. [9] reported a maximum power of 2.24 W at 1484 nm realized experimentally in a 700 m P-doped fiber among other fiber lengths pumped by a 8 W fiber laser and a 15% output coupler. This was not consistent with their simulation results that predicted a higher power within a much shorter optimal fiber length. The reason was thought to be the mismatch of the FBG bandwidth from the Raman scattering spectrum. Furthermore, the optimization was performed with a fixed feedback of 15% for the output coupler. It is therefore that more detailed research work is needed to experimentally optimize the high power Raman fiber lasers. In this paper we report on the detailed investigation of high power Raman fiber lasers in the second-order Stokes line of the P-doped single mode fiber, with a view to shortening the gain fiber while remaining optimal operation. It is expected that the fiber shortening can reduce the RFL cost dramatically and increase the overall efficiency by reducing the fiber intrinsic loss that is linearly proportional to fiber length. To optimize the Raman fiber laser pumped with our existing Yb fiber laser of 20 W at 1070.75 nm, we constructed and tested 20 laser cavity configurations involving four partially reflective (PR) FBGs with different reflectivities acting as the output coupler and five fiber lengths for the gain medium. The maximum power we have obtained was greater than 6 W in the second Stokes order at 1495 nm, which was efficiently converted within a 300 m P-doped fiber with a pump power of 18 W.
2. Experimental details The experimental setup to perform the optimization of high power RFLs at 1495 nm is schematically shown in Fig. 1. It consists of an Yb fiber laser, a cascaded cavity of FBGs and the P2O5doped (phosphosilicate) fibers. The Yb doped DCF laser was used as the pump source with a
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maximum output power of 20 W centered at wavelength of 1070.75 nm with a bandwidth of 0.2 nm at low power levels to 1.5 nm at high power levels. The change in the central wavelength with output power is smaller than 0.05 nm. This pump light was delivered through a Flexcore 1060 fiber cable with a mode field diameter (MFD) of 7.0 lm. The Raman fiber was fabricated by the Fiber Optic Research Center of Russia with the specifications as detailed below. The doping concentration of P2O5 is 13 mol% in the core area, yielding a refractive index difference of 0.0108 between the core and the cladding, and a cut-off wavelength of 1030 nm. The losses are 1.6, 0.99 and 0.85 dB/ km at 1060 (pump wavelength), 1240 (the first Stokes order) and 1480 nm (the second Stokes or lasing line), respectively. The Raman gain was estimated to be around 5.6 dB/(W km) and the MFD was measured to be 6.3 lm, which is compatible with that of the pump delivery cable and also ensures that the splicing loss between the delivery cable and the Raman fiber is not significant. The FBGs were annealed after fabrication using H2 pre-loaded Flexcore 1060 fibers to minimize the insertion loss of splicing with the pump delivery cable and the Raman fibers. Their central Bragg wavelengths were experimentally determined by the Stokes lines of the Raman fibers, which are located at 1248 and 1495 nm for the first- and second-orders, respectively, pumped with the above Yb laser. To generate laser non-resonant oscillation in the second Stokes order line with high efficiency, we used one pair of highly reflective (HR) FBGs in the first Stokes line of 1248 nm at both ends of the Raman fiber so that the power of 1248 nm is maintained inside the cascaded cavity and experiences multi-pass scattering to be converted to 1495 nm. The laser resonator for 1495 nm was formed with an HR FBG as the rear mirror and a PR FBG as the output coupler, both centered at 1495 nm. All the FBGs have a central wavelength absolute accuracy of < ±0.2 nm and a reproducibility of < ±0.1 nm, in order to avoid any mismatch between the central wavelengths and the Stokes lines. The reflectivity of the HR FBGs is >99% with a broader bandwidth (HWFM) of 1.35 nm at both wavelengths 1248 and 1495 nm. To optimize the laser performance,
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Z. Xiong et al. / Optics Communications 239 (2004) 137–145 HR FBGs @ 1248 nm 1495nm
Delivery cable
Yb fiber laser (1070 nm) 20 W
Splices
Silica slide OSA
P-doped fiber
Power meter HR FBG PR FBG @1248 nm @1495nm Probe patchcord
Fig. 1. Experimental setup.
various output couplers (PR FBGs) with different reflection rates were tested, as shown in Fig. 2. In addition to the PR FBGs of 15%, 33% and 45% in reflection, the Fresnel reflection of 4% by the cleaved fiber end was also used as an output coupler. It can be seen from Fig. 2 the PR FBGs have a narrower HWFM of <0.2 nm, while the second Stokes scattering line has a much wider HWFM of 6.0 nm. It is therefore expected that these well matched wavelengths and bandwidths can minimize the environmental influence on the laser per-
15% 1.2
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formance due to the Bragg wavelength shift with temperature and stress/strain. To demonstrate the spectral match, we also display in Fig. 2 the spontaneous Raman scattering spectrum of the P-doped fiber pumped with the Yb fiber laser with a pair of HR FBGs at 1248 nm. The laser output power was measured with a power meter and the laser spectra were monitored with an optical spectrum analyzer (OSA, ANDO6317B). The spectral signal was sampled with an un-coated thin silica slide and picked up with a single mode fiber probe patchcord. The power of each individual line (1071, 1248 and 1495 nm) was calibrated based on the total power exited from the fiber end and the relative intensity in the OSA spectra. The experiments were carried out in five different lengths of cavity with respective fiber length of 200, 300, 500, 700, and 1000 m cut sequentially from a single phosphosilicate fiber. These fiber lengths, together with the above four output couplers, enabled us to construct and run the Raman fiber laser in 20 configurations, which could lead to a comprehensive optimization experimentally.
1505
Fig. 2. Transmission spectra of the FBGs used in our experiments. The dashed line is the second Stokes order spontaneous Raman scattering spectrum of the P-doped fiber pumped with the Yb-fiber laser at 1071 nm.
3. Results and discussion Typical output characteristics of the Raman fiber laser are shown in Fig. 3. The spectrum shown
Z. Xiong et al. / Optics Communications 239 (2004) 137–145 1 0.9 0.8
Intensity (a.u.)
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Fig. 3. Typical spectral and input–output characteristics of the Raman fiber laser at 1495 nm. (a) is the spectrum of a 500 m Raman fiber laser with a 45% PR FBG coupler and 15 W pump power. (b) is the input–output characteristics for a 300 m Raman fiber laser with a 33% PR FBG coupler. The solid line in (b) is for total power, circles for 1495 nm, pluses for 1248 nm, and crosses for 1071 nm.
in Fig. 3(a) is for the laser with a 500 m P-doped fiber and a 45% PR FBG at 1495 nm, where three lines are observed at 1071, 1248 and 1495 nm, corresponding to the pump, the first and second Stokes orders, respectively. The pump input power was 15 W. We should note that the spectrum was acquired at a wavelength resolution of 2 nm for the OSA to cover the whole wavelength span from 1050 to 1550 nm. At this resolution, the peak intensity ratio of the three lines was 0.2:0.2:1 from 1071 to 1495 nm. With a higher resolution of 0.1 nm, this ratio changed to 0.26:0.30:1 as the lines were scanned individually at smaller spans. Further increase in the resolution made no difference
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to the intensity ratio. As the line width is usually different for different lines, the ratio in the total power of each line is different from the peak intensity ratio. The total power ratio of the three lines in the case of Fig. 3(a) becomes 0.085:0.095:1, indicating most of the power in the pump and the first Stokes line has converted to the second Stokes line. The input–output characteristics are described in Fig. 3(b) for the laser with a 300 m P-doped fiber and a 33% PR FBG at 1495 nm, where the pump power dependence of all output powers is plotted, including the total power and the powers of the individual lines. It is revealed that the power within the 1495 line, together with the residual power of the first Stokes order at 1248 nm, increases almost linearly with the pump, while the change in the residual power of the pump exiting from the output end is not significant. The threshold for the oscillation of the 1495 nm line is 1 W. Increasing linearly from this point, the power of the second Stokes order reached a maximum of 6.66 W at a pump level of 18 W, resulting in a slope efficiency of 39.18%. The maximum residual power of the first Stokes order was 0.53 W and the variation range of the residual power of the pump was from 0.32 to 0.51 W. It will be seen later that this configuration of the 300 m fiber with the 33% PR FBG turned out to be the best in term of laser performance with the present pump level among all the other configurations we constructed in this report. Fig. 4 summarizes the laser input–output characteristics of the 1495 nm line for all the configurations with each subplot for a single output coupler. An approximately linear relationship between the input and the output is clearly demonstrated for all the lasers using the PR FBGs as the output coupler. This probably predicts that the power capping by the stimulated scattering of SiO2 and P2O5 to other Stokes lines was not significant within the pump level we used. The linewidth dependence on the pump power of the 1495 nm band is shown in Fig. 5, with (a) for the 300 m fiber laser with different output couplers, and (b) for the lasers with a fixed output coupler (33% PR FBG) and different fiber lengths. In general, the linewidth increases with the pump power from below 0.5 nm at low powers to several
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(a) 4%
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Fig. 4. Detailed input–output characteristics of lasers with different fiber lengths. The fiber lengths are: 1000 m (dashed line), 700 m (crosses), 500 m (stars), 300 m (circles), and 200 m (pluses).
2.5
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Fig. 5. Change in linewidth at 1495 nm with the pump power in different laser configurations. (a) is for the 300 m laser with different couplers: 4% (circles), 15% (pluses), 33% (crosses), 45% (stars), and pump (dashed line) for comparison. (b) is for lasers with a 33% PR FBG and different fiber lengths: 1000 (circles and a dashed line), 700 (stars), 500 (crosses), 300 (pluses), and 200 m (circles with a solid line).
nm at high powers, and the linewidth change with the pump power is not linear. There are points of the input power at which the linewidth tends to increase sharply. As observed in Fig. 5(a), the linewidths for different couplers remain similar at
low powers and increases differently at high powers. The linewidth of the laser with a larger reflection of the FBG increases sharply at a smaller pump power and reaches to a larger value. For example, the laser with a 45% PR FBG starts to increase in linewidth from 0.4 nm at 9 W to 1.03 nm at 10 W and reaches to 2.44 nm at 18 W, while that with a 15% PR FBG, these corresponding values are 0.43 nm at 12 W, 0.99 nm at 15 W and 1.17 nm at 18 W, respectively. Note that the linewidth with the cleaved end (4% Fresnel reflection) only as the output coupler behaves slightly differently from the above tendency. It is clearly seen that its linewidth is located between those of the lasers with 15% and 33% FBGs. This can be considered as a result of the broadband feature of the Fresnel reflection. This sharp increasing (or linewidth hoping) is attributed to the emergence of a multi-peak feature in the pump laser when the power is higher than 9 W. If the contribution of the side peaks to the linewidth were not taken into account, the linewidth or that of the central peak would remain small below 0.5 nm for all the pump powers. Another main factor to affect the linewidth is the length of the Raman gain medium. As seen from Fig. 5(b), longer Raman fiber usually leads to larger linewidth, for instance, the linewidth at 15 W pump power changed from 0.40 nm in the 200 m laser to 6.42 nm in the 1000 m laser. This is believed due to the nonlinear feature of the Raman scattering, which involves a wavelength shift with energy equal to the energy separation of the adjacent Raman stokes orders, or the optical phonon energy induced by bond vibration. Stimulated Raman radiation occurs only when the pump laser is very intense and, more importantly, it can itself also play the role as a pump for the stimulated Raman radiation of the next orders [21]. In the case where the Stokes photons are so dense, new (stimulated) Stokes photons are more likely to be generated. In this way, the power scaling of the RFLs at a specified wavelength is limited if the fiber laser parameters are not selected properly, such as the fiber is too long. This is the case observed in our Raman fiber lasers with long fibers, where the intensity in the central part tends to be scattered to other Stokes lines while the intensity in the sides experience more amplification along the fiber. As
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a result, the spectrum of the longer fiber lasers likely becomes broader than that of the shorter ones. To find out the best performance of all the laser configurations, we extract the data in Fig. 4 to resummarize in Fig. 6, where the maximum power is plotted against the fiber length. The input pump power was 18 W for all the cases except the 1 km lasers, as in this case the maximum power we could apply was 15 W, further increase in the pump power could damage our Yb laser due to strong back propagating light of other wavelengths. But this does not affect our analysis made here since the power of the 1 km lasers at 15 W pump power was much lower than that of all the other configurations. It can be seen that the largest power of 6.66 W was delivered from the 300 m laser with a 33% PR FBG as the coupler, and the lasers with the Fresnel reflection as the coupler have lowest powers in all fiber lengths. At 200 m, laser powers with PR FBGs just slightly below the their counterparts of 300 m, but much better than those of longer (>300 m) fibers. At 500 m, higher reflection from the PR FBGs results in better laser performance, while at 700 m the laser with the 15% PR FBG has the best performance. From Fig. 6, it can also be seen that the optimal fiber length for the Raman fiber and the pump power used in the experiments was 300 m
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regardless of the PR FBG reflectivity. This was similar to that reported for Raman fiber lasers at the first Stokes order [22]: the fiber length is more critical than reflection levels of the FBGs in determining the laser performance. Because it is the fiber length that controls the laser gain and loss including attenuation and spontaneous Raman scattering, while the PR FBG reflectivity determines only the ratio between the powers inside and outside the laser cavity. We should also note here that, pumped by higher powers, the optimal fiber length is much shorter than that reported previously [8]. This agrees well with the theoretical prediction [19] that it is possible to build lasers with shorter fiber lengths. The shortening in the expensive fiber from the usually >700 to 300 m can lead to a significant reduction in component costs. The calculation results for the threshold and the slope efficiency of the lasers are shown in Fig. 7. Both (a) and (b) demonstrate that the fiber lasers with shorter fiber lengths have larger lasing thresholds and higher slope efficiencies, and higher reflection of the PR FBGs leads to smaller lasing threshold. The slope efficiency is in general much smaller than the quantum limit of 71.6%. This can be attributed to the following two major factors: the power leakage from the HR FBGs at 1248 nm and the relatively large background loss 10
7
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Fig. 6. Maximum output power at 1495 nm obtained in various laser configurations with the couplers: Fresnel reflection (stars), 15% (pluses), 33% (circles), and 45% (crosses) PR FBGs.
200 400 600 800 1000 Fiber length (m)
20
200 400 600 800 1000 Fiber length (m)
Fig. 7. Dependence of (a) threshold and (b) slope efficiency on the fiber length with different PR FBGs: Fresnel reflection (stars), 15% (pluses), 33% (circles), and 45% (crosses).
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at both wavelengths 1248 and 1495 nm. The highest slope efficiency achieved was 55.6% in the shortest fiber laser of 200 m with a cleaved end. Note that with this configuration, the threshold of 9.6 W is much higher than other cases. The thresholds with higher (33% and 45%) PR FBGs remain low and do not change much from each other, and the change in the slope efficiency does not vary significantly for different PR FBGs from 15% to 45%. It is further noticed that within these cases (excluding the case of the Fresnel reflection), the configuration of 300 m/33% has a largest slope efficiency of 39.18%.
4. Conclusion In conclusion, we have demonstrated high power Raman fiber lasers at 1495 nm, the second Stokes order of phosphosilicate fibers containing 13 mol% P2O5 with an intrinsic loss of 0.85 dB/ km at the lasing wavelength. Experimental optimization has been performed by constructing and testing up to 20 cavity configurations with different output couplers and fiber lengths. The reflection of the PR FBGs as the output coupler was in the range from 4% to 45%, and the fiber length was from 200 to 1000 m. Among them, the configuration with a 300 m fiber and a 33% PR FBG delivered the highest power of 6.66 W with a slope efficiency of 39.18%. The fiber shortening from the previously reported 700–1000 m without a sacrifice in performance led to a significant drop in the total cost, to which the specialty Raman fiber contribute a significant part. The slope efficiency has also been boosted due to the reduction in the intrinsic loss caused by the fiber length. The highest slope efficiency achieved was 55.6% in the 200 m fiber laser, which is however still far below the quantum limit of wavelength conversion 71.6% from 1070.75 to 1495 nm due to the power leakage from the HR FBGs and the relatively large intrinsic loss of the fiber. The residual power in the pump line and the first Stokes order (the power leakage) was not negligible, particularly in the shorter fiber lasers where 10% or more in the total residual power can be usually observed. The laser linewidth has been found to become broader with
increase in pump power, fiber length and the reflectivity of the PR FBGs. In most of the cases, the output power was approximately linearly proportional to the input power, indicating that the power capping due to the stimulated scattering to the Stokes orders of the SiO2 bond vibration or the higher Stokes orders of the P2O5 bond vibration was not significant within the pump level we used.
Acknowledgement The authors thank K.M. Teh and Abdul Kassim Shafi for technical assistance. This work was financially supported by Agency for Science, Technology and Research (A*STAR).
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Experimental optimization of high power Raman fiber lasers at 1495 nm using phosphosilicate fibers Z. Xiong b
a,*
, N. Moore a, Z.G. Li a, G.C. Lim a, D.M. Liu b, D.X. Huang
b
a Singapore Institute of Manufacturing Technology, 71 Nanyang Drive, Singapore 638075, Singapore Department of Optoelectronics Engineering, Huazhong University of Science and Technology, China 430074
Received 29 October 2003; received in revised form 29 March 2004; accepted 10 May 2004
Abstract We report in this paper on the experimental optimization of high power Raman fiber lasers at the second Stokes order of a phosphosilicate (P-doped) fiber doped with 13 mol% P2O5. The lasers were pumped with a 20 W Yb double clad fiber laser at 1070.75 nm and the cavity used two pairs of fiber Bragg gratings (FBGs). One pair is highly reflective at 1248 nm to fully cascade the pump wavelength to the first Stokes order, and the other includes a partial reflective (PR) FBG at 1495 nm to couple the laser out of the cavity. A total of 20 laser configurations were constructed and tested, consisting of four PR FBGs couplers with reflection from 4% to 45% and five fiber lengths from 200 to 1000 m. It is found that the optimal configuration is that with a fiber length of 300 m and a 33% PR FBG. The maximum power obtained from this laser is 6.66 W at 1495 nm with a threshold of 1 W and a slope efficiency of 39.18%. The highest slope efficiency achieved with pump power up to 18 W is 55.6% in the shortest fiber laser of 200 m without any PR FBG but a cleaved end. It is observed that the input–output characteristics are almost linear for all the configurations with feedback of PR FBGs P 15%, indicating the stimulated scattering to higher Stokes orders is negligible. Laser line width is also observed to become broader with increase in pump power, fiber length and the reflection of the PR FBGs. Ó 2004 Elsevier B.V. All rights reserved. Keywords: Raman fiber lasers; Optical fiber communications; Raman amplification; Phosphosilicate fiber; Stimulated Raman scattering
1. Introduction High power Raman fiber lasers (RFLs) are poised to play an important role in the next gener*
Corresponding author. Tel.: +67938502; fax: +67922779. E-mail address: [email protected] (Z. Xiong).
ation of optical telecommunication systems since they are one of the very few major pump sources for Raman fiber amplifiers (RFAs), which are being deployed in almost every new ultrabroad-band and long-haul fiber optic transmission system [1]. The available pumps for C-band RFAs at present are the intense laser beams at 14xx nm either from
0030-4018/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2004.05.010
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laser diodes or from RFLs. As a pump source, the RFLs have several advantages over the laser diodes. First of all, the power of the diode lasers launched into a single mode fiber is limited 300 mW. Ideally the overall pump power should be 800–1500 mW, which can be easily achieved with a RFL. These increasing output power levels are needed to provide the necessary gain margin for DWDM applications (such as gain flattening), as well as provide additional flexibility for amplifier designers. Secondly, amplification with both a wide-band and a flat-gain by use of multiple wavelengths [2,3] can be achieved within a single RFL because of its multi-wavelength generation feature associated with the stimulated Raman scattering of different Stokes orders [4] and various ions doped in fibers [5–7]. However, in the laser diode approach, multiple wavelengths require several diodes to be combined with wavelength multiplexers, which are costly and low efficient. Finally, the Raman wavelength shift is immune from the environmental conditions, while the wavelength of the laser diodes drifts with temperature. RFLs with high power of a few watts within single mode fibers have been realized [8,9] as a result of the successful development of three key components: high power double clad fiber (DCF) lasers [10], high quality fiber Bragg gratings (FBGs) [11], and high gain and low loss Raman fibers [12]. As feedback elements, FBGs with high reflectivity and a well matched band with the Raman scattering make it possible to construct highly efficient and compact Raman fiber lasers. The most powerful DCF lasers are those using fibers doped with Yb, which have a prominent lasing wavelength around 1100 nm. The conversion efficiency from this wavelength to the pump wavelength (for example, 1480 nm) for the C-band amplification strongly depends on the cascaded cavity and the Raman gain medium used. Therefore, a large wavelength shift in the RFLs is always preferable so that the number of cascading can be minimized. This allows the wavelength conversion efficiency to be enhanced, and the lasers can be more compact with fewer cascaded cavities. The fiber doped with phosphorus has been emerging as a promising and more efficient Raman gain medium [5,13–15] since the P2O5 bond vibration can create
a strong and large frequency shift of 40 THz, about three times as large as that of a germanosilicate fiber [6,16]. Due to this large increment in the Stokes order separation, the cascading process can be simplified significantly [17,18]. The conversion efficiency is also dependent on the extent of optimization on the laser configuration parameters, including the characteristics of the FBGs and the fiber length. This is evident from the evolution power along the Raman fiber. The steady state power in the Stokes lines is governed by a set of ordinary nonlinear differential equations, which are usually written as [9,19,20] F=B dP sn gn1 vsn ¼ Rn1 ðP Fsn1 þ P Bsn1 ÞP F=B sn dz vsnþ1 Aeff
gnR F F=B ðP þ P Bsnþ1 ÞP F=B sn asn P sn : Aneff snþ1
ð1Þ
It shows that the nth Stokes order line PsnF/B (either forward or backward) is excited by the (n 1)th-order, and contributes simultaneously to the (n + 1)th-order traveling in both directions. While it is not convenient to tailor the Raman fiber characteristics including the Raman gain coefficients gR, the effective mode areas Aeff and the absorption coefficients asn, the laser cavity configuration parameters play an important role in the experimental optimization of the laser performance. This is because the solution of Eq. (1) is also determined by its boundary conditions, i.e., the reflection characteristics of the FBGs at the start (z = 0) and end (z = L where L is the fiber length) points at different Stokes order wavelengths. By using various FBGs of different reflectivities with their central wavelengths well matched to the Stokes orders of interest and selecting suitable fiber length, one can optimize the laser performance. There are a few reports on the optimization of the high power Raman fiber lasers, both theoretically [9,19] and experimentally [8,9]. Ref. [19] reported on the theoretical optimization of Raman fiber lasers using germanosilicate fibers. It was predicted that the optimal fiber length for laser beams at 1240 (the second-order) and 1480 nm (the fifthorder) decreases with input pump power at 1117 nm, and the optimal feedback of the output coupler
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was higher for the higher order than that for the lower order. Karpov et al. [18] estimated a maximum power at the second Stokes order of the Pdoped fiber would be obtained at a much shorter fiber length than 1 km used by them, probably due to lack of higher pump power. Ref. [9] reported a maximum power of 2.24 W at 1484 nm realized experimentally in a 700 m P-doped fiber among other fiber lengths pumped by a 8 W fiber laser and a 15% output coupler. This was not consistent with their simulation results that predicted a higher power within a much shorter optimal fiber length. The reason was thought to be the mismatch of the FBG bandwidth from the Raman scattering spectrum. Furthermore, the optimization was performed with a fixed feedback of 15% for the output coupler. It is therefore that more detailed research work is needed to experimentally optimize the high power Raman fiber lasers. In this paper we report on the detailed investigation of high power Raman fiber lasers in the second-order Stokes line of the P-doped single mode fiber, with a view to shortening the gain fiber while remaining optimal operation. It is expected that the fiber shortening can reduce the RFL cost dramatically and increase the overall efficiency by reducing the fiber intrinsic loss that is linearly proportional to fiber length. To optimize the Raman fiber laser pumped with our existing Yb fiber laser of 20 W at 1070.75 nm, we constructed and tested 20 laser cavity configurations involving four partially reflective (PR) FBGs with different reflectivities acting as the output coupler and five fiber lengths for the gain medium. The maximum power we have obtained was greater than 6 W in the second Stokes order at 1495 nm, which was efficiently converted within a 300 m P-doped fiber with a pump power of 18 W.
2. Experimental details The experimental setup to perform the optimization of high power RFLs at 1495 nm is schematically shown in Fig. 1. It consists of an Yb fiber laser, a cascaded cavity of FBGs and the P2O5doped (phosphosilicate) fibers. The Yb doped DCF laser was used as the pump source with a
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maximum output power of 20 W centered at wavelength of 1070.75 nm with a bandwidth of 0.2 nm at low power levels to 1.5 nm at high power levels. The change in the central wavelength with output power is smaller than 0.05 nm. This pump light was delivered through a Flexcore 1060 fiber cable with a mode field diameter (MFD) of 7.0 lm. The Raman fiber was fabricated by the Fiber Optic Research Center of Russia with the specifications as detailed below. The doping concentration of P2O5 is 13 mol% in the core area, yielding a refractive index difference of 0.0108 between the core and the cladding, and a cut-off wavelength of 1030 nm. The losses are 1.6, 0.99 and 0.85 dB/ km at 1060 (pump wavelength), 1240 (the first Stokes order) and 1480 nm (the second Stokes or lasing line), respectively. The Raman gain was estimated to be around 5.6 dB/(W km) and the MFD was measured to be 6.3 lm, which is compatible with that of the pump delivery cable and also ensures that the splicing loss between the delivery cable and the Raman fiber is not significant. The FBGs were annealed after fabrication using H2 pre-loaded Flexcore 1060 fibers to minimize the insertion loss of splicing with the pump delivery cable and the Raman fibers. Their central Bragg wavelengths were experimentally determined by the Stokes lines of the Raman fibers, which are located at 1248 and 1495 nm for the first- and second-orders, respectively, pumped with the above Yb laser. To generate laser non-resonant oscillation in the second Stokes order line with high efficiency, we used one pair of highly reflective (HR) FBGs in the first Stokes line of 1248 nm at both ends of the Raman fiber so that the power of 1248 nm is maintained inside the cascaded cavity and experiences multi-pass scattering to be converted to 1495 nm. The laser resonator for 1495 nm was formed with an HR FBG as the rear mirror and a PR FBG as the output coupler, both centered at 1495 nm. All the FBGs have a central wavelength absolute accuracy of < ±0.2 nm and a reproducibility of < ±0.1 nm, in order to avoid any mismatch between the central wavelengths and the Stokes lines. The reflectivity of the HR FBGs is >99% with a broader bandwidth (HWFM) of 1.35 nm at both wavelengths 1248 and 1495 nm. To optimize the laser performance,
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Z. Xiong et al. / Optics Communications 239 (2004) 137–145 HR FBGs @ 1248 nm 1495nm
Delivery cable
Yb fiber laser (1070 nm) 20 W
Splices
Silica slide OSA
P-doped fiber
Power meter HR FBG PR FBG @1248 nm @1495nm Probe patchcord
Fig. 1. Experimental setup.
various output couplers (PR FBGs) with different reflection rates were tested, as shown in Fig. 2. In addition to the PR FBGs of 15%, 33% and 45% in reflection, the Fresnel reflection of 4% by the cleaved fiber end was also used as an output coupler. It can be seen from Fig. 2 the PR FBGs have a narrower HWFM of <0.2 nm, while the second Stokes scattering line has a much wider HWFM of 6.0 nm. It is therefore expected that these well matched wavelengths and bandwidths can minimize the environmental influence on the laser per-
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formance due to the Bragg wavelength shift with temperature and stress/strain. To demonstrate the spectral match, we also display in Fig. 2 the spontaneous Raman scattering spectrum of the P-doped fiber pumped with the Yb fiber laser with a pair of HR FBGs at 1248 nm. The laser output power was measured with a power meter and the laser spectra were monitored with an optical spectrum analyzer (OSA, ANDO6317B). The spectral signal was sampled with an un-coated thin silica slide and picked up with a single mode fiber probe patchcord. The power of each individual line (1071, 1248 and 1495 nm) was calibrated based on the total power exited from the fiber end and the relative intensity in the OSA spectra. The experiments were carried out in five different lengths of cavity with respective fiber length of 200, 300, 500, 700, and 1000 m cut sequentially from a single phosphosilicate fiber. These fiber lengths, together with the above four output couplers, enabled us to construct and run the Raman fiber laser in 20 configurations, which could lead to a comprehensive optimization experimentally.
1505
Fig. 2. Transmission spectra of the FBGs used in our experiments. The dashed line is the second Stokes order spontaneous Raman scattering spectrum of the P-doped fiber pumped with the Yb-fiber laser at 1071 nm.
3. Results and discussion Typical output characteristics of the Raman fiber laser are shown in Fig. 3. The spectrum shown
Z. Xiong et al. / Optics Communications 239 (2004) 137–145 1 0.9 0.8
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Fig. 3. Typical spectral and input–output characteristics of the Raman fiber laser at 1495 nm. (a) is the spectrum of a 500 m Raman fiber laser with a 45% PR FBG coupler and 15 W pump power. (b) is the input–output characteristics for a 300 m Raman fiber laser with a 33% PR FBG coupler. The solid line in (b) is for total power, circles for 1495 nm, pluses for 1248 nm, and crosses for 1071 nm.
in Fig. 3(a) is for the laser with a 500 m P-doped fiber and a 45% PR FBG at 1495 nm, where three lines are observed at 1071, 1248 and 1495 nm, corresponding to the pump, the first and second Stokes orders, respectively. The pump input power was 15 W. We should note that the spectrum was acquired at a wavelength resolution of 2 nm for the OSA to cover the whole wavelength span from 1050 to 1550 nm. At this resolution, the peak intensity ratio of the three lines was 0.2:0.2:1 from 1071 to 1495 nm. With a higher resolution of 0.1 nm, this ratio changed to 0.26:0.30:1 as the lines were scanned individually at smaller spans. Further increase in the resolution made no difference
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to the intensity ratio. As the line width is usually different for different lines, the ratio in the total power of each line is different from the peak intensity ratio. The total power ratio of the three lines in the case of Fig. 3(a) becomes 0.085:0.095:1, indicating most of the power in the pump and the first Stokes line has converted to the second Stokes line. The input–output characteristics are described in Fig. 3(b) for the laser with a 300 m P-doped fiber and a 33% PR FBG at 1495 nm, where the pump power dependence of all output powers is plotted, including the total power and the powers of the individual lines. It is revealed that the power within the 1495 line, together with the residual power of the first Stokes order at 1248 nm, increases almost linearly with the pump, while the change in the residual power of the pump exiting from the output end is not significant. The threshold for the oscillation of the 1495 nm line is 1 W. Increasing linearly from this point, the power of the second Stokes order reached a maximum of 6.66 W at a pump level of 18 W, resulting in a slope efficiency of 39.18%. The maximum residual power of the first Stokes order was 0.53 W and the variation range of the residual power of the pump was from 0.32 to 0.51 W. It will be seen later that this configuration of the 300 m fiber with the 33% PR FBG turned out to be the best in term of laser performance with the present pump level among all the other configurations we constructed in this report. Fig. 4 summarizes the laser input–output characteristics of the 1495 nm line for all the configurations with each subplot for a single output coupler. An approximately linear relationship between the input and the output is clearly demonstrated for all the lasers using the PR FBGs as the output coupler. This probably predicts that the power capping by the stimulated scattering of SiO2 and P2O5 to other Stokes lines was not significant within the pump level we used. The linewidth dependence on the pump power of the 1495 nm band is shown in Fig. 5, with (a) for the 300 m fiber laser with different output couplers, and (b) for the lasers with a fixed output coupler (33% PR FBG) and different fiber lengths. In general, the linewidth increases with the pump power from below 0.5 nm at low powers to several
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(a) 4%
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Fig. 4. Detailed input–output characteristics of lasers with different fiber lengths. The fiber lengths are: 1000 m (dashed line), 700 m (crosses), 500 m (stars), 300 m (circles), and 200 m (pluses).
2.5
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Fig. 5. Change in linewidth at 1495 nm with the pump power in different laser configurations. (a) is for the 300 m laser with different couplers: 4% (circles), 15% (pluses), 33% (crosses), 45% (stars), and pump (dashed line) for comparison. (b) is for lasers with a 33% PR FBG and different fiber lengths: 1000 (circles and a dashed line), 700 (stars), 500 (crosses), 300 (pluses), and 200 m (circles with a solid line).
nm at high powers, and the linewidth change with the pump power is not linear. There are points of the input power at which the linewidth tends to increase sharply. As observed in Fig. 5(a), the linewidths for different couplers remain similar at
low powers and increases differently at high powers. The linewidth of the laser with a larger reflection of the FBG increases sharply at a smaller pump power and reaches to a larger value. For example, the laser with a 45% PR FBG starts to increase in linewidth from 0.4 nm at 9 W to 1.03 nm at 10 W and reaches to 2.44 nm at 18 W, while that with a 15% PR FBG, these corresponding values are 0.43 nm at 12 W, 0.99 nm at 15 W and 1.17 nm at 18 W, respectively. Note that the linewidth with the cleaved end (4% Fresnel reflection) only as the output coupler behaves slightly differently from the above tendency. It is clearly seen that its linewidth is located between those of the lasers with 15% and 33% FBGs. This can be considered as a result of the broadband feature of the Fresnel reflection. This sharp increasing (or linewidth hoping) is attributed to the emergence of a multi-peak feature in the pump laser when the power is higher than 9 W. If the contribution of the side peaks to the linewidth were not taken into account, the linewidth or that of the central peak would remain small below 0.5 nm for all the pump powers. Another main factor to affect the linewidth is the length of the Raman gain medium. As seen from Fig. 5(b), longer Raman fiber usually leads to larger linewidth, for instance, the linewidth at 15 W pump power changed from 0.40 nm in the 200 m laser to 6.42 nm in the 1000 m laser. This is believed due to the nonlinear feature of the Raman scattering, which involves a wavelength shift with energy equal to the energy separation of the adjacent Raman stokes orders, or the optical phonon energy induced by bond vibration. Stimulated Raman radiation occurs only when the pump laser is very intense and, more importantly, it can itself also play the role as a pump for the stimulated Raman radiation of the next orders [21]. In the case where the Stokes photons are so dense, new (stimulated) Stokes photons are more likely to be generated. In this way, the power scaling of the RFLs at a specified wavelength is limited if the fiber laser parameters are not selected properly, such as the fiber is too long. This is the case observed in our Raman fiber lasers with long fibers, where the intensity in the central part tends to be scattered to other Stokes lines while the intensity in the sides experience more amplification along the fiber. As
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a result, the spectrum of the longer fiber lasers likely becomes broader than that of the shorter ones. To find out the best performance of all the laser configurations, we extract the data in Fig. 4 to resummarize in Fig. 6, where the maximum power is plotted against the fiber length. The input pump power was 18 W for all the cases except the 1 km lasers, as in this case the maximum power we could apply was 15 W, further increase in the pump power could damage our Yb laser due to strong back propagating light of other wavelengths. But this does not affect our analysis made here since the power of the 1 km lasers at 15 W pump power was much lower than that of all the other configurations. It can be seen that the largest power of 6.66 W was delivered from the 300 m laser with a 33% PR FBG as the coupler, and the lasers with the Fresnel reflection as the coupler have lowest powers in all fiber lengths. At 200 m, laser powers with PR FBGs just slightly below the their counterparts of 300 m, but much better than those of longer (>300 m) fibers. At 500 m, higher reflection from the PR FBGs results in better laser performance, while at 700 m the laser with the 15% PR FBG has the best performance. From Fig. 6, it can also be seen that the optimal fiber length for the Raman fiber and the pump power used in the experiments was 300 m
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regardless of the PR FBG reflectivity. This was similar to that reported for Raman fiber lasers at the first Stokes order [22]: the fiber length is more critical than reflection levels of the FBGs in determining the laser performance. Because it is the fiber length that controls the laser gain and loss including attenuation and spontaneous Raman scattering, while the PR FBG reflectivity determines only the ratio between the powers inside and outside the laser cavity. We should also note here that, pumped by higher powers, the optimal fiber length is much shorter than that reported previously [8]. This agrees well with the theoretical prediction [19] that it is possible to build lasers with shorter fiber lengths. The shortening in the expensive fiber from the usually >700 to 300 m can lead to a significant reduction in component costs. The calculation results for the threshold and the slope efficiency of the lasers are shown in Fig. 7. Both (a) and (b) demonstrate that the fiber lasers with shorter fiber lengths have larger lasing thresholds and higher slope efficiencies, and higher reflection of the PR FBGs leads to smaller lasing threshold. The slope efficiency is in general much smaller than the quantum limit of 71.6%. This can be attributed to the following two major factors: the power leakage from the HR FBGs at 1248 nm and the relatively large background loss 10
7
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Fig. 6. Maximum output power at 1495 nm obtained in various laser configurations with the couplers: Fresnel reflection (stars), 15% (pluses), 33% (circles), and 45% (crosses) PR FBGs.
200 400 600 800 1000 Fiber length (m)
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200 400 600 800 1000 Fiber length (m)
Fig. 7. Dependence of (a) threshold and (b) slope efficiency on the fiber length with different PR FBGs: Fresnel reflection (stars), 15% (pluses), 33% (circles), and 45% (crosses).
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at both wavelengths 1248 and 1495 nm. The highest slope efficiency achieved was 55.6% in the shortest fiber laser of 200 m with a cleaved end. Note that with this configuration, the threshold of 9.6 W is much higher than other cases. The thresholds with higher (33% and 45%) PR FBGs remain low and do not change much from each other, and the change in the slope efficiency does not vary significantly for different PR FBGs from 15% to 45%. It is further noticed that within these cases (excluding the case of the Fresnel reflection), the configuration of 300 m/33% has a largest slope efficiency of 39.18%.
4. Conclusion In conclusion, we have demonstrated high power Raman fiber lasers at 1495 nm, the second Stokes order of phosphosilicate fibers containing 13 mol% P2O5 with an intrinsic loss of 0.85 dB/ km at the lasing wavelength. Experimental optimization has been performed by constructing and testing up to 20 cavity configurations with different output couplers and fiber lengths. The reflection of the PR FBGs as the output coupler was in the range from 4% to 45%, and the fiber length was from 200 to 1000 m. Among them, the configuration with a 300 m fiber and a 33% PR FBG delivered the highest power of 6.66 W with a slope efficiency of 39.18%. The fiber shortening from the previously reported 700–1000 m without a sacrifice in performance led to a significant drop in the total cost, to which the specialty Raman fiber contribute a significant part. The slope efficiency has also been boosted due to the reduction in the intrinsic loss caused by the fiber length. The highest slope efficiency achieved was 55.6% in the 200 m fiber laser, which is however still far below the quantum limit of wavelength conversion 71.6% from 1070.75 to 1495 nm due to the power leakage from the HR FBGs and the relatively large intrinsic loss of the fiber. The residual power in the pump line and the first Stokes order (the power leakage) was not negligible, particularly in the shorter fiber lasers where 10% or more in the total residual power can be usually observed. The laser linewidth has been found to become broader with
increase in pump power, fiber length and the reflectivity of the PR FBGs. In most of the cases, the output power was approximately linearly proportional to the input power, indicating that the power capping due to the stimulated scattering to the Stokes orders of the SiO2 bond vibration or the higher Stokes orders of the P2O5 bond vibration was not significant within the pump level we used.
Acknowledgement The authors thank K.M. Teh and Abdul Kassim Shafi for technical assistance. This work was financially supported by Agency for Science, Technology and Research (A*STAR).
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