1. Introduction

A fiber laser offers the capability to generate multiple closely and selectively fixed frequency spacing. It is a practical and potential source to support dense wavelength division multiplexing systems. Various techniques have been investigated for multi-wavelength generation including the integration of stimulated Brillouin scattering (SBS) [1]. This technique implemented in a hybrid type of laser source, mainly by involving Brillouin-erbium transition is firstly invented by Cowle and Stepanov [2,3]. In their laser configuration [3], the generation of six Stokes lines is realized through internal cascaded technique. The low order Stokes signals are amplified by the erbium-doped fiber amplifier (EDFA) and act as new pumping sources to initiate additional higher-orders wave. The frequency spacing varies from 9 to 12 GHz at 1550 nm region depending on the material compositions. This narrow frequency shift becomes a challenge for de-multiplexing and signals filtering process for optical communication system [4,5]. Therefore, the research focus in Brillouin-erbium fiber laser (BEFL) approaches is to further expanding the frequency spacing up to 20 GHz range. In this regards, different schemes have been demonstrated such as figure-of-eight configuration [6], micro-air gap cavity [7] and four-port circulator [8]. Other ways that support this development are by employing two metal-coated fiber planar mirrors and a Sagnac reflector [9] as well as toggling an optical switch [10].

In order to achieve wider multi-wavelength separations, the researchers further investigate the developments of triple frequency spacing [11–16] which is more likely to satisfy the demands for aforementioned applications. An early attempt by Qian et al. has successfully demonstrated two channels with a power discrepancy of 9 dB estimation [11]. Their experimental results also show that the laser can be tuned over 22 nm wavelength from 1547 to 1569 nm without any free-running cavity modes. However, these unwanted phenomena are observed once the Brilouin pump (BP) signal is set beyond this range. The slight improvement that produces up to four Stokes channels results in almost similar spectral flatness [12]. When utilizing two separate Brillouin frequency-shifting cavities, the accomplishment of ten channels is realized [13]. Despite of sacrificing the peak power discrepancy to nearly 13 dB, the widest tunability is achieved. This covers 45 nm span that begins at 1525 nm before ending at 1570 nm wavelength. However, when the BP signal is tuned further away from the erbium-doped fiber (EDF) peak gain, the onset of self-lasing modes is realized. Together in restricting the channel numbers, this also justifies the limitation in the maximum optical signal-to-noise ratio (OSNR) close to 25 dB compared to more than 42 dB attained in both previous reports [11,12]. Other following assessments by a few groups do not show drastic improvements either. For instance, six output channels are demonstrated but at the expense of deterioration in the power discrepancy of about 21 dB [14]. At the same time, the tunability also reduces to 19 nm range. The demonstrated laser utilizes shorter Brillouin gain media than those previously reported [11–13]. Another work shows nine channels by employing a modular structure [15]. Similar to those happen in [11,13,14], tuning away the BP wavelength from the center of optical amplifier peak gain results in the reduction of the output channel formation. Poor properties in channel flatness and average OSNR are assessed at 22.5 and 26 dB approximation, respectively. Both parameters are improved in the recent progress [16] where the peak power difference is rectified to 4 dB together with the upgrade in OSNR up to 45 to 50 dB-order. Three Stokes lines are maintained in the entire tunability coverage from 1530 to 1570 nm. Unlike the past experiments [11,13–15], the generation of Brillouin Stokes lines (BSLs) does not depend strictly on the control of BP wavelength with regards to the EDF peak gain.

In summary, the previous approaches of BEFL with triple Brillouin frequency spacing have been accomplished through cascaded SBS although in some cases free-running modes also appear [11,13–15]. These unwanted consequences bring a few drawbacks that include narrow tuning range, small number of channels, weak stability and poor channels flatness. In addition, due to spectral broadening at higher pump power, the attribute of Stokes lines with respect to OSNR deteriorates [13,15]. To date, no published work is available on this area that involves the assistance by distributed Raman amplifier (DRA) to mitigate these issues. Therefore, in this paper we employ this technique together in a setup that also consists of erbium-ytterbium-doped fiber amplifier (EYDFA). First, the assessment is carried out with EYDFA only before proceeding with the inclusion of DRA. From this evaluation, we prove that the additional incorporation of the latter pumping scheme alleviates the gain bandwidth limitation associated to homogeneous gain broadening in EYDFA. In this case, the power of lasing gain at different wavelengths is also enhanced. The fundamental principle is the success of completing 36 nm tunability that exceptionally shows no any sign of self-lasing modes. This supports the formation of up to 164 uniform Stokes lines with 0.25 nm wavelength spacing, spanning over 40 nm bandwidth which is tens of times better than those reported before in this field.

2. Experimental setup

The architecture of the multi-wavelength Brillouin erbium-ytterbium Raman fiber laser, consisting of double and single frequency shifters (DFS and SFS) that perform as Brillouin cascaded schemes is illustrated in Fig. 1. The external tunable laser source (TLS) serves as the BP that has a linewidth of 200 kHz and maximum output power of 7 dBm. In order to adjust the BP power accordingly, a variable optical attenuator (VOA) is connected next to this source. Then, a coupler with a splitting ratio of 50/50 is utilized to splice this unit to the laser arrangement. A linear gain block of EYDFA is used to boost both the BP signal and the generated Brillouin Stokes waves that relate closely to the DFS configuration. The DFS is formed by connecting the 7.2 km length of dispersion compensating fiber (DCF) to the four-port circulator (C1) in a ring cavity. This nonlinear Brillouin gain medium has dispersion and dispersion slope of −130 ps/nm/km and −0.438 ps/nm²/km at 1550 nm, respectively. At this wavelength also, the numerical aperture (NA) is 0.27 and the attenuation coefficient is about 0.45 dB/km.

 

Fig. 1 Schematic diagram of the hybrid multi-wavelength structure with EYDFA and DRA.

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Besides this, another linear random cavity that defines SFS comprises a three-port circulator (C2) and a 20 km length single-mode fiber (SMF) with nearly 10 GHz Brillouin frequency shift. The SMF bears an attenuation coefficient of 0.18 dB/km and a numerical aperture of 0.14, both at 1550 nm wavelength. This fiber acts as a hybrid nonlinear Brillouin and Raman gain medium over a wide wavelength range together with the support of random distributed Rayleigh scattering feedback. The role of this elastic scattering for building up Stokes line generation has been reported in [4,5]. The Raman pump (RP) source at 1455 nm wavelength is inserted into the SFS through a 1455/1550 nm wavelength selective coupler (WSC). It has 1 nm spectral bandwidth with a maximum 1 W output power level. In order to eliminate any Fresnel back-reflection that might harm the laser qualities, the end part of the SFS is terminated by utilizing an optical isolator (ISO). During experiment, all output spectra are monitored at one arm of the 3-dB coupler that is labeled as “OSA port” by using an optical spectrum analyzer (OSA) that has 0.02 nm bandwidth resolution. Ports A and B are employed as the monitoring point throughout the experiment.

3. Results and discussions

In reference to poor qualities of optical combs with triple wavelength shift reported in [11–16], the primary purposes of conducting this research assessment are to offer better solutions that provide improvements to numbers of Stokes waves and their corresponding bandwidth. These are attained with outstanding lasing stability by maintaining satisfactory OSNR and tunability. Before starting any analysis, the hybrid amplification feature is evaluated as illustrated in Fig. 2 when the BP signal is excluded but maintaining the EYDFA output power at its maximum of 950 mW. We maintain this value for the whole operation as this has been investigated to be the best parameter. From the red profile in Fig. 2 that is below −70 dBm power level, two peaks at 1535 and 1543 nm wavelengths relate to EYDFA transitions without the presence of RP power. When 1 W of this power is included, the amplification profile is elevated significantly to below −59 dBm power level as denoted by the blue line. In this case, other additional peaks at 1556 and 1566 nm emerge which correspond to the first-Stokes shift of the Raman amplifier [17]. The feedback in the SFS is mainly determined by Rayleigh backscattering from the 20 km length SMF piece. This is because the end-fiber termination by an isolator removes any source of back-reflection beam to the cavity. As a result, for over 50 nm scanning that starts from 1530 nm, no signature of self-lasing modes that might degrade SBS interactions is identified. This is the key point that deserves detail discussions of multiple lasing in our work. For clarification, these undesired modes lead to the requirement of critical BP wavelength alignment near EDF peak gain and restriction on the OSNR trait [13,15]. Other disadvantages include laser instability as well as hindering the tuning range and the formation of more BSLs. Thus, the observation confirms the correct implementation of the proposed laser design to minimize mode competition inflicted on Brillouin responses as most of these drawbacks are eliminated. In order to ascertain this, the next following assessments are conducted.

 

Fig. 2 Distributed amplification spectra measured at OSA port with and without DRA (EYDFA output power = 950 mW and BP power is switched “off”).

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Firstly, the BP signal is injected to the DFS scheme after having amplification by the EYDFA gain block. Without using the RP power, the Brillouin pumping traits are selected randomly at −10 dBm and wavelength at 1550 nm just to show the modes of operation conveniently. Once the threshold in the DCF is met, the backward first-order Brillouin Stokes line (S1) with 10 GHz downshifted frequency is produced. The S1 radiates towards port 2 to port 3 of the C1 in the anti-clockwise direction. The beam returns back to the DCF to complete a round trip and likewise, the attainment of its threshold yields the second-order Brillouin Stokes line (S2) in the clockwise direction. The circulation of S1 is restricted within the DCF strand although a small part is backscattered towards the main cavity. In an ideal condition, only the clockwise propagation of the residual BP signal and S2 travel from port 3 to port 4 of C1. A bigger portion of S2 enters C2 and acts as the first seed pump wave in the SFS scheme. Once its threshold is fulfilled in the SMF, 10 GHz frequency downshifted wave is initiated which results in the formation of third-order Brillouin Stokes line (S3). The isolator eliminates the remaining S2 from the SFS arrangement and the light output from port 3 of C2 is dominated primarily by S3 owing to the high SBS conversion efficiency in the long length of SMF. The relative intensity of the Ryleigh backscattering portions of BP signal and S2 are very small by comparing to S3 as monitored at OSA port. At the completion of the first round-trip, the BP and S3 demonstrate triple wavelength separation as illustrated in Fig. 3 where we concentrate only on the green profile that corresponds to the EYDFA power of our interest. The latter beam behaves as a new pump wave that is redistributed back to the laser cavity. This cascaded generation with three times shifting continues to develop in the same process where additional Stokes-orders lines of S6, and S9 are formed. Even though that the EYDFA is driven at its maximum power of 950 mW, the next-order of S12 cannot be fully initiated as the corresponding preceding threshold is not satisfied. Therefore, the need for incorporating the DRA for further advancements in the expected results is well justified.

 

Fig. 3 Wavelength interval of 0.25 nm at various EYDFA output power (BP wavelength = 1550 nm, BP power = −10 dBm and RP power is set at “off” state).

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By maintaining the same BP wavelength and EYDFA power, we study the impact of BP power adjustment on the channels progress. In this case, all lasing lines that are inside the description of 10 dB discrepancy between the maximum to minimum peak power level are counted. The TLS is scaled-up from −27 to 6 dBm under two conditions of RP power setting. Firstly, without this device, only a maximum of 4 lines are formed as illustrated by the red line in Fig. 4(a). However, the insertion of 1 W RP power yields more than 37 channels at below −9 dBm power scope as represented by the blue line in the same figure. The attainments at both conditions are self-described by the amplification features represented in Fig. 2. The significant achievement in the second case is also partially assisted by the inhomogeneous nature of Raman broadening that favors stronger SBS formations over that of weaker gain competition by the oscillating modes. In order to make-up the right decision, we focus at the segment of lesser or equal −20 dBm power where the variation in Stokes waves indicates very narrow margin from 83 to 85 channels. Visually, better OSNR is observed in the black spectral envelope (38 dB) compared to that initiated in the red profile (30 dB) as manifested in Fig. 4(b). This verifies the selection of −20 dBm BP power in order to complete more succeeding investigations where choosing precisely the right RP power and BP wavelength to maximize the development of BSLs are the main aims. The relationship between number of channels and OSNR against BP wavelength is studied further by maintaining the BP power at −20 dBm. In this experiment, the selected RP power is reduced slightly from 1000 to 900 mW which is found to produce better outputs. These relate to the maximum OSNR (38.5 dB) where the number of Stokes waves is maintained at 83 counts above this power level as manifested in Fig. 5.

 

Fig. 4 (a) Stokes lines count with and without DRA as a function of BP power and (b) the corresponding spectra at specific BP power (EYDFA output power = 950 mW, RP power = 1000 mW and BP wavelength = 1550 nm).

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Fig. 5 Lasing properties against RP power (EYDFA output power = 950 mW, BP power = −20 dBm and BP wavelength = 1550 nm).

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Figure 6 depicts a few examples with all clean-cut spectra at definite BP wavelengths of 1540, 1555 and 1565 nm that comprise 119, 60 and 22 channels respectively. By including this data, the relevant graph for the 36 nm wavelength tunability that starts from 1532 nm is presented in Fig. 7. From this figure, it is found that the inverse proportionality between Stokes lines count and BP wavelength is realized. With the increment of BP wavelength, the residual Raman gain bandwidth becomes narrower which justifies this observation. Detuning at shorter or longer wavelengths implies the accessibility of lower Raman gain at this band-edge (see blue curve in Fig. 2) which elucidates the restriction on the wavelength operation. Nevertheless, to compete that of 40 nm tuning accomplished in [16], a broader gain can be provided by utilizing multiple wavelengths RP sources or a combination of different kinds of amplifiers [18,19]. Similar to [16], no reliance of BP wavelength selection on the EYDFA peak gain is involved. In contrast, this type of preference characterizes earlier reports [11,13–15] in order to acquire higher numbers of waves. This deficient increases the critical need for sufficient BP power to discriminate the existing free-running cavity modes. The mode competition caused by these unwanted effects result in the reduction of tuning capability of BEFL from 19 to 22 nm [11,14]. In Fig. 7 also, the average OSNR is characterized by comparing the peak power of the Brillouin components with the noise floor level for each output channel. The same technical definition as illustrated in Figs. 4 and 6 apply. This attribute enhances slightly from 36 to 41 dB level versus wavelength. As the Stokes lines decrease, most energy is consumed to initiate higher peak power level in relation to the noise floor which explains this attainment. Both growths agree well with those behaviors assessed in other findings that utilize a Brillouin-Raman fiber laser (see Figs. 4 and 5 in [20]).

 

Fig. 6 Optical envelopes at selected BP wavelengths of (a) 1540 nm, (b) 1555 nm and (c) 1565 nm (BP power = −20 dBm, RP power = 900 mW and EYDFA output power = 950 mW).

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Fig. 7 Stokes lines count and their corresponding average OSNR as a function of BP wavelength that relate to Fig. 6 (BP power = −20 dBm, RP power = 900 mW and EYDFA output power = 950 mW).

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Once all necessary pumping criteria have been determined, the measurement done in Fig. 7 when the BP signal is specifically chosen at 1532 nm is depicted in Fig. 8(a). From this figure, the flatness value is represented by the peak power discrepancy amongst the cascaded Stokes lines in a given multi-wavelength bandwidth. At 3-dB span, this is evaluated as 33.7 nm that implies 140 BSLs. However, by considering the 10-dB spectral flatness difference, 164 BSLs are formed over 40 nm bandwidth. The average OSNR is 36 dB and the average peak power per channel is about −18 dBm as magnified in Fig. 8(b). All these specifications are summarized in Table 1 together with the same type of competitors in this area. It should be highlighted again that equivalent to those plotted in Figs. 6 and 7, the signal profile confirms the absence of spurious cavity mode noises at the bottom of its level. This has been predicted earlier from the combined amplified spontaneous emission of both amplifiers (see Fig. 2). For comparison, although lower than 10 dB flatness can be attained in [11,12,16], less than 5 Stokes waves are developed. The improvement in channels formation from 6 to 10 numbers degrades the peak power discrepancy from 13 to 23 dB order [13–15]. The 45 nm and 30 nm tunabilities in [13] and [15] come together with self-lasing modes that reduce the OSNR in the vicinity of 26 dB. In addition, the 40 nm tuning in [16] without these detrimental signs is recompensated by only 3 BSLs. These validate the tolerable traits of tunability and OSNR in this literature as listed in Table 1.

 

Fig. 8 (a) Triple wavelength spacing signal at optimum conditions and (b) its corresponding enlarged view (BP wavelength = 1532 nm, BP power = −20 dBm, RP power = 900 mW, EYDFA output power = 950 mW).

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Table 1. Progresses in the triple Brillouin frequency spacing in multi-wavelength fiber lasers.

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In addition to the uniform power distribution over the specified wavelength range, stability tests for the signal shown in Fig. 8 are other preconditions for WDM sources implementation. Temporal evaluations at every one minute interval over 45-minute duration are carried out. In order to present the findings in a meaningful discussion, only three lasing wavelengths at 1550.15, 15550.40 and 1550.65 nm are considered as depicted in Fig. 9. The spectral patterns of the spectrogram shown in both Figs. 9(a) and 9(b) are almost constant throughout the observation time. The variation of lasing wavelength is within $\pm$9.2 pm for all three signals as illustrated in Fig. 9(c). The peak power for the selected Stokes lines signifies only a maximum of 0.3 dB discrepancy as portrayed in Fig. 9(d). In addition, the total output power of the entire optical comb is also quantified by using an optical power meter. The outstanding fluctuations within 0.74 dB limitation that correspond from the minima of 4.94 dBm to the maxima of 5.68 dBm is shown in Fig. 9(d), blue line. In comparison to other triple Brillouin frequency multi-wavelength lasers that yield 3 to 10 channels, the stability of 1.0 to 4.5 dB are reported [12,13,15,16]. The superior completions in this work owe to stable characteristics of the BP signal under controlled environment from temperature and strain relations. This is further supported by the boosting mechanism of the hybrid amplification media consisting of EYDFA and Raman amplifier. Both units are set at almost their optimum operation (see Figs. 3 and 5) which saturates the generated Stokes lines, thus explaining this excellent stability and flatness. In fact, the role of self-stabilization by gain saturation has been very well explained in [21].

 

Fig. 9 Stability assessments that relate to Fig. 8 as represented by spectrogram of selected Stokes lines in (a) 3D and, (b) 2D views, (c) wavelength variation against time and (d) their associated temporal power changes.

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4. Conclusion

In summary, the main objective of advancing multiple channels with triple Brillouin frequency spacing in a multi-wavelength fiber laser assisted by a DRA has been successfully completed. The DRA is employed to upscale the power of higher order Stokes waves in a relatively broader gain bandwidth and to relieve the disadvantageous of homogenous broadening in an EYDFA. The laser configuration is built by including two frequency shift cavities namely DFS and SFS that serve as the main scheme responsible to realize the intended wavelength spacing. The main aspect behind multiple Rayleigh scattering feedback in the SFS is to prevent any light circulation in the cavity that might result in unnecessary self-lasing modes. This is the key factor that supports more influential build-up of SBS lines with remarkably 0.74 dB total power stability by concerning the number of Stokes waves. From the experiment, up to 164 BSLs with an acceptable average OSNR of 36 dB are obtained at 10 dB peak power discrepancy. This extends from 1532 to 1568 nm that implies 40 nm wavelength range where the limitation is associated to the accessible gain profile of the embedded amplifiers.