We demonstrate a tunable multi-wavelength Brillouin-Raman fiber laser with 20 GHz wavelength spacing. The setup is arranged in a linear cavity by employing 7.2 and 11 km dispersion compensating fibers (DCF) in addition to a 30 cm Bismuth-oxide erbium doped fiber. In this experiment, for the purpose of increasing the Stokes lines, it is necessary to optimize Raman pump power and Brillouin pump power together with its corresponding wavelengths. At the specific Brillouin pump wavelength, it is found that the longer length of 11 km DCF with optimized parameters results in larger number of Stokes combs and optical signal to noise ratios (OSNRs). In this case, a total of 195 Brillouin Stokes combs are produced across 28 nm bandwidth at Brillouin pump power of −2 dBm and Raman pump power of 1000 mW. In addition, all Brillouin Stokes signals exhibit an average OSNR of 26 dB.
©2013 Optical Society of America
Multi-wavelength laser sources with constant wavelength spacing have attracted considerable attention for their low costs and favorable applications in wavelength-division-multiplexing (WDM) systems . They are also important for optical sensor networks, optical component testing and microwave photonics in addition to spectroscopy [2–4]. The WDM system requires multi-wavelength laser sources with equal-wavelength spacing, high optical signal to noise ratio (OSNR), large number of channels, and high output uniformity over the channels spectra. However, it is a very demanding task to satisfy all these requirements. There have been different methods to generate multi-wavelength combs in fiber lasers by using nonlinear stimulated Brillouin scattering (SBS) effects in various gain media [5–8]. However, this type of Brillouin fiber laser (BFL) suffers from a limitation in output power due to the small coefficient of the Brillouin gain . Alternatively, although Brillouin-erbium fiber lasers (BEFL) have unique advantages such as large gain, high power conversion efficiency and lower threshold power, there are still a few drawbacks. These include power instability and restricted tuning range [7, 10]. Hence, a hybrid multi-wavelength Brillouin–Raman fiber laser (BRFL) has been investigated as an alternative choice of solution. A large number of lasing lines that support multi-wavelength operation at room temperature is one of the significant advantages offered. All the BRFL architectures with 0.08 nm (10 GHz) channel spacing as previously reported in [8, 11–14] concentrated on either a Fabry-Perot or ring cavities (or closed ring), in which the effect of Rayleigh scattering is significant. The generated multi-wavelength BRFLs have been demonstrated with up to 18 dB OSNR. However, researchers have found that it is difficult to demultiplex this narrow single spacing for practical applications in WDM and sensor networks. Therefore to address this problem, many efforts have been initiated to achieve relatively wider spacing. In the earlier work , a BRFL in a ring cavity with 22 GHz wavelength spacing is reported. This fiber laser produces only 16 lines with a 12 dB OSNR.
In the past, although a multi-wavelength BRFL with double wavelength spacing has been realized, the exploitation of high wavelength numbers with wider bandwidths and high OSNRs have been rarely demonstrated. Therefore in this paper, we demonstrate a double wavelength spacing (20 GHz) BRFL by using gain fibers based on DCFs. The setup is constructed in a simple linear cavity with the inclusion of a Bi-EDF. This allows us to analyze the role of this additional fiber on the OSNR quality. In addition, influences of varying several parameters on the number of Stokes signals generated have been studied. These include Raman pump (RPU) power, Brillouin pump (BP) power and its wavelengths together with different lengths of DCF. From the results obtained the combination of an 11 km DCF with a 30 cm Bi-EDF results in the generation of 195 Brillouin Stokes lines. The average OSNR is measured to be 26 dB across 28 nm bandwidths. To the best of our knowledge, this is the largest number of double wavelength spacing ever achieved in BRFLs with a high quality OSNR.
2. Experimental setup
The configuration of a multi-wavelength BRFL that comprises a tunable laser source (TLS) which acts as a Brillouin pump (BP) signal is depicted in Fig. 1. The TLS can be tuned over a tuning range of 100 nm (from 1520 to 1620 nm) with maximum power of 8 dBm. In addition, two optical isolators are used to reduce any back-reflection signals that can interrupt the stability of the lasers in the cavity and consequently to increase the overall OSNR . The Bi-EDF of 30 cm long is used as a linear gain medium. Its concentration doping is 6300 ppm with a nonlinear coefficient of 11(Wkm) −1. In fact, the advantage of employing this type of silica host is because of its doping capability with higher concentration of Er3+ of more than 3000 ppm . This can be realized without suffering ion quenching and clustering effects compared to the conventional SiO2 glass. For the nonlinear Brillouin–Raman gain medium, different lengths of 7.2 and 11 km dispersion compensating fibers (DCF) with the respective insertion losses of 5 dB and 7.3 dB, are utilized. Their nonlinear coefficient is 7.3 (Wkm)−1 and the effective area is 20 µm2 which justify its intrinsic properties of strong nonlinear effects. The pump source is provided by the 1455 nm Raman pump (RPU) laser with maximum power of 1 W. The 1480/1550 nm wavelength division multiplexing (WDM) coupler is used to multiplex the RPU power and BP signal seed. The DCF is primarily pumped by the RPU and its residual power is utilized to provide energy for amplification in the Bi-EDF. In this experiment, the implementation of forward pumping scheme offers better performances. These involve the generation of stronger SBS effects inside the DCF with 20 GHz wavelength spacing and also improvements in the noise figure to achieve a higher OSNR. An optical spectrum analyzer (OSA) with a 0.02 nm resolution is used to perform all measurements.
3. Results and discussions
The operation mechanism of multi-wavelength BRFL with 0.16 nm spacing (20 GHz) can be explained by interactions of stimulated Raman scattering (SRS), Rayleigh scattering (RS) and SBS. The 1455 nm RPU provides the required energy to initiate SRS effects inside the DCF. Firstly in the co-pumping direction, the BP signal from the TLS is injected to the cavity and encountering distributed Raman amplification along the fiber longitudinal dimension. Once the Brillouin threshold power is achieved, the first Brillouin Stokes signal propagates in the backward direction compared to that of the original BP propagation. When the threshold condition is satisfied, the becomes a new BP source to generate the second order BS signal in the forward direction. The terms “backward” and “forward” in these texts represent the direction of propagation with respect to that of BP signal. Simultaneously, is also backscattered elastically through Rayleigh scattering effects that act virtually as a distributed feedback mirror. These components and their subsequent odd-orders travelling in the forward direction are just slightly amplified due to the dependency of Raman gain to the pumping scheme . In contrast, the even-orders of BS combs grow well, thus experiencing saturation due to lower Brillouin scattering threshold power. The same process of generating higher order BS signals within the Raman amplification bandwidth continues to develop as long as the corresponding lower order BSs reach their SBS threshold condition. From this operating principle, double wavelength spacing is properly achieved.
From Fig. 1, in order to determine the remaining Raman pump power transmitted into the Bi-EDF, the evaluation is done at the end-facet of the DCFs. In this case, parameters such as the BP power and wavelength are set at 8 dBm and 1555 nm, correspondingly and the RPU power is fixed at 1000 mW. When utilizing an 11 km DCF, the remaining Raman pump power is measured to be around 62 mW. This value increases to 98 mW when replacing this fiber with a shorter strand of 7.2 km. These powers are strong enough to initiate further amplification inside the 30 cm long Bi-EDF, thus enhancing the OSNR. From the experiment also, the threshold powers required to create the first Brillouin Stokes line are 100 mW when employing the shorter gain media (7.2 km DCF + Bi-EDF) and 20 mW for the longer one (11 km + Bi-EDF). The underlying physics behind this phenomenon can be elucidated as follows. The higher Raman pump power consumption in the longer DCF implies more non-linear light interactions inside the fiber geometry. This induces a higher gain that is responsible for introducing a stronger SBS effect. As a consequence, the initiation of lower threshold operation is realized in this fiber scheme. Figure 2 presents an example of the whole multi-wavelength lasing spectra with the corresponding enlarged spectral profiles when maintaining the same pumping characteristics. When incorporating the shorter DCF, the lasing combs consist of 97 channels and the peak power for the Brillouin components is −12 dBm approximately as depicted in Figs. 2(a) and 2(b). In contrast, utilizing the longer DCF yields 103 Brillouin Stokes lines with −13 dBm peak power as manifested in Figs. 2(c) and 2(d). For both cases, the Rayleigh components that have lower peak power and OSNRs with respect to the higher orders Brillouin components are negligible. For clarifications, the total number of Stokes channels is counted by including all apparent signals that have almost equal spectral power with discrepancies of less than 3 dB. Accordingly, the same technique is applied when estimating the multi-wavelength bandwidth and OSNR. The former attribute relates to the total widths of the entire wavelength range that consist of multiple lasing lines. From the results obtained, it can be deduced that the implementation of longer DCF (11 km) leads to several benefits due to its higher gain property. These include a wider multi-wavelength bandwidth around 1 nm and higher number of Stokes combs. The bandwidth is estimated from the initial wavelength of 1555 nm and by ignoring the signals beyond 1569.5 nm for 7.2 km DCF and 1570.64 nm for 11 km DCF as they do not satisfy the 3 dB range of spectral power condition as demonstrated in Figs. 2(a) and 2(c). However instead of these favorable qualities, the longer DCF also suffers a higher cavity loss as manifested in the lower peak power of its Brillouin components. Therefore, there is a trade-off between cavity loss and SBS effects for optimization of multi-wavelength lasing performances.
In order to obtain the highest number of BS lines with a high OSNR, the optimization of injected BP power, BP wavelength and RPU power are carried out. All measurements are done by incorporating the hybrid gain media consisting of different DCF lengths. Fig. 3 indicates the effects of RPU and BP power on the number of output channels. In this case, the RPU power is increased from 700 mW to its maximum of 1000 mW. The BP wavelength is fixed at 1555 nm (Raman peak gain) and the BP power is varied from 8 dBm to −2 dBm. The results obtained demonstrate that the number of channels increase with the increments of RPU power at both lengths of DCF. This is due to the fact that higher pump power induces a higher Raman gain that cooperate with SBS effects for all cases of injected BP power as manifested in Fig. 3. Thus with the more energy transfer from the RPU pump to the signal, more BS lines are formed. By reducing the BP power from 8 to −2 dBm, the channel counts increase from 103 to 116 and 97 to 108 lines for 11 and 7.2 km DCF, respectively. This is attributed to the optimization of Raman gain at lower BP power where its gain saturation is fulfilled faster at higher BP values. In addition as clarified earlier, a higher number of lasing lines produced from the longer DCF is related to its inherent properties of lower threshold operation and higher nonlinear interactions.
Figure 4 depicts the relationship between BP wavelengths with the number of Stokes channels and multi-wavelength bandwidths. The RPU power is fixed at 1000 mW and the BP power is maintained at the optimum value of −2 dBm. From this figure, the bandwidth and number of channels are inversely proportional to the BP wavelength for both DCF lengths. With the increase of BP wavelengths in the C-band, the residual Raman gain bandwidth becomes narrower. This leads to the declining values on the number of channels although the spectral power remains the same for the entire BP wavelength range (see Figs. 6 and 7). When employing the shorter DCF, a maximum of 220 Stokes lines are attained. These are produced over 31 nm bandwidths when the BP wavelength is set at 1542 nm. In contrast for the longer DCF, maximum Stokes lines of 195 are achieved over 28 nm bandwidths. This occurs when the BP wavelength is set at 1545 nm. Various balances between gain and cavity loss in these two fiber lengths are found to be the main reasons for this discrepancy. However at the specific BP wavelength, the channel counts are higher for the longer fiber as it has a higher gain. Another important aspect is to analyze when the BP wavelength is detuned to the region of lower than 1542 and 1545 nm for 7.2 and 11 km DCF, respectively. In theory at a shorter wavelength span, higher energy is required for the attainment of wider multi-channel lasing bandwidth. This latter attribute is influenced by the wavelength dependencies on BP and SRS bandwidths. Higher Raman gain might broaden the SRS bandwidths. However, due to the accessibility of lower Raman gain at this band-edge, the build-up of self-lasing cavity modes cannot be efficiently suppressed which justifies the limitation in wavelength operation as shown in Fig. 4.
Next, the effect of BP wavelengths on OSNRs for four types of setups is investigated. These are similar to that illustrated in Fig. 1, but the arrangements of hybrid gain media in the blue-dashed box are varied. The first and second arrangements utilize two different DCF lengths with the absence of Bi-EDF. On the other hand, those of the third and fourth include the Bi-EDF in the experimental layout that also consists of these two different DCF lengths. The results obtained are illustrated in Fig. 5 where the RPU power is fixed at 1000 mW and the BP power at −2 dBm. Due to the formation of high number BS lines, the average values of OSNR is presented by comparing the peak power of the Brillouin components with the noise floor level for each output channel as shown in Fig. 2. In general, the OSNR increases as a function of BP wavelength tunability as manifested in Fig. 5. In fact from the previous observations, at shorter BP wavelengths more Stokes lines are generated [Fig. 4] with the same spectral power. By taking into account that the same amount of energy is shared among the higher number of Stokes lines, the energy experienced by each line becomes lesser. This behavior is reflected by the average OSNR degradation since the available energy is insufficient to suppress the free running modes that result in the noise floor increment. Nevertheless, the improvement of OSNR by extending the DCF length towards 11 km can be explained due to its higher Raman gain property that yields stronger nonlinear effects (SBS and SRS). This ascertains the capability of suppressing the build-up of oscillating modes that are associated to Raman peak gain. Moreover when the Bi-EDF is included in the setup, the OSNR quality is further improved up to 1dB due to the suppression of noise floor level. This is possible owing to the existence of an additional amplification scheme, which is fully analogous to the amplifier setup . As a result, in the setup that includes a combination of 11 km DCF and 30 cm Bi-EDF, average OSNRs are 26, 28, and 30 dB for the BP wavelengths of 1545, 1555 and 1565 nm, respectively. However for the shorter fiber combination, the OSNR is reduced by 1 dB at the corresponding BP wavelengths.
At the optimized value of RPU power, BP power and wavelengths, as well as the DCF length, the multi-wavelength BRFL can be tuned continuously from 1542 nm to 1570 nm. The tuning spectra as a function of BP wavelengths is shown in Fig. 6 when the RPU and BP powers are fixed at 1000 mW and −2 dBm, respectively. The corresponding magnified output spectra are shown clearly in Fig. 7 at a selective BP wavelength of 1555 nm.
From Fig. 6, the longer gain media exhibits a tuning range of 25 nm from wavelengths of 1545 nm to 1570 nm against 28 nm for the shorter media. The reduction of tuning range for the longer combination is due to its higher SRS effects that leads to stronger self-lasing cavity modes at the Raman peak gain around 1555 nm . Therefore, the generation of multiple BS lines is not able to suppress the existence of these unwanted modes that limits its tunability. In spite of this, the longer fiber combination can generate higher number of Stokes combs and OSNR compared to that of the shorter combination. The results demonstrate a good performance of multi-wavelength BRFL on the operating wavelengths, OSNR, and output power.
5. Conclusion and outlook
We have demonstrated a hybrid multi-wavelength BRFL with 20 GHz spacing in the linear cavity arrangement. The number of Stokes lines and OSNR generated is strongly dependent on various physical parameters such as RPU power, BP power and its wavelengths. Other important parameters include lengths of DCF, as well as the presence of Bi-EDF as an amplification medium. When employing an 11 km DCF with the inclusion of Bi-EDF, a total of 195 Stokes lines are achieved across 28 nm bandwidth with an average OSNR of 26 dB. In addition, 220 Stokes lines across 31 nm bandwidth are attained with an average OSNR of 24 dB when a shorter strand of fiber combination is utilized. The maximum output tuning range is from 1542 nm to 1570 nm by adjusting the appropriate BP wavelengths. The experimental results demonstrate excellent performances of BRFL on both the operating wavelengths and OSNR. In the future attempts, a larger number of lasing lines and a higher quality of OSNR can be produced with the optimization of DCF lengths. Due to wider emission characteristics of the Bi-EDF in the L band, the tuning range of BRFL can also be extended further by using a longer pump wavelength. In conclusion, we believe that this simple multi-wavelength BRFL has potential applications in various areas that cover from optical communications, optical testing and measurement, to microwave photonic systems.
This work was partly supported by the Graduate Research Fellowship Scheme under Universiti Putra Malaysia.
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