We demonstrate a single-spacing, multi-wavelength Brillouin-Raman fiber laser utilizing an enhanced cavity of nonlinear amplifying loop mirror. In this structure, the optimization of multi-wavelength lasing is done with proper adjustments of coupling ratio and Brillouin pump power. When setting the Raman pump power to 300 mW, up to 28 channels with an average 17 dB optical signal-to-noise ratio are achieved. In this case, the Brillouin pump power is maintained at −2.6 dBm when the splitting ratio and Brillouin pump wavelength are fixed at 99/1 and 1555 nm, correspondingly. Our achievements present high numbers of Stokes channels with an acceptable optical signal-to-noise ratio at low pump power operation.
©2013 Optical Society of America
The fiber laser has been reported to be an attractive candidate to support dense wavelength division multiplexing systems because of its ability to generate multiple wavelengths from a coherent single wavelength light source. To realize these, various methods utilizing nonlinear optical effects such as stimulated Brillouin scattering (SBS) , four-wave mixing (FWM) , and nonlinear polarization rotation (NPR)  were employed. However, the implementation of FWM and NPR techniques to initiate multiple wavelengths introduces lower stability, high cost and complexity [4, 5]. Alternatively, a better solution is provided when utilizing SBS effects in various gain media [6–10]. Among these technologies, the Brillouin-Raman fiber laser (BRFL) has several advantages and potential applications [10–12]. These are such as wavelength stability, large gain bandwidth, and design simplicity. Other favorable benefits include spectral flatness and compatibility with fiber properties.
In the previously reported assessments, different BRFL structures have been proposed to attain multiple wavelengths with 10 or 20 GHz spacing through linear or ring cavity oscillators [10, 13]. In the scheme with 10 GHz spacing, the configuration was arranged in a linear cavity consisting of two high reflectivity mirrors. In this case, two Raman pump unit (RPU) with high power were used to enhance the Raman gain [11, 14]. The accomplishment of Brillouin Stokes (BS) signals is obtained relatively at the expense of a complex configuration with high cost. In addition, due to the spectral broadening effect at higher pump power, the quality of Stokes lines in terms of optical signal-to-noise ratio (OSNR) is deteriorated .
To address this problem, an alternative setup that increases the operational flexibility and functionality as well as offering a cost-saving solution is preferable. This is done by incorporating an enhanced nonlinear amplifying loop mirror (NALM) in the cavity . Another advantage is that this can also offer the best exploitation of amplitude switching and operation with low Raman pump power. Therefore in this paper, we propose and demonstrate experimentally the first work on a new configuration of BRFL that utilizes a NALM. By controlling the variable coupling ratio, Brillouin pump (BP) power and RPU power, its detail performances namely spectral combs and OSNRs are analyzed thoroughly. The main objective is to optimize these lasing attributes at low power levels to eliminate unnecessary spectral broadening effects that degrade OSNR performances. To date, this is the highest number of flat lasing lines with high qualities of OSNR and output peak power that are reported at only 300 mW Raman pump power.
2. Laser structure and principle of operation
The multi-wavelength BRFL layout is constructed by employing a NALM as illustrated in Fig. 1. Its main element of variable coupling is realized by incorporating a set of single-mode coupler that individually introduces different fixed coupling ratios, CR which include 1/99, 5/95, 20/80, 30/70, 50/50, 70/30, 80/20, 95/5, and 99/1. These couplers that have return and directivity losses of more than 55 dB are manufactured by Oplink Communications and they are represented by the VOC as illustrated in Fig. 1. The fiber loop is formed by incorporating a RPU, a wavelength selective coupler (WSC) and a 7.2 km long dispersion compensating fiber (DCF). This fiber behaves as a Brillouin-Raman gain medium with a nonlinear coefficient of 7.3 (Wkm)−1 and an effective area of 20 µm2. The WSC is used to combine the 1455 nm Raman pump unit to the nonlinear oscillator. In most cases, this weakly polarized pump source is set to a maximum of 300 mW power. In addition, the BP power is provided by a tunable laser source (TLS) that has a 200 kHz linewidh and a tuning range from 1520 nm to 1620 nm wavelengths. The TLS can also be tuned from −3 dBm to the maximum output power of 5 dBm. These are measured by connecting the equipment directly to a power meter. In this nonlinear cavity, the BP is connected to an optical circulator (C) that is spliced to one end of the VOC with a highly reflective mirror (M) at another fiber end. To investigate the lasing characteristics, the output terminal of the circulator is monitored by utilizing an optical spectrum analyzer (OSA), having a resolution bandwidth of 0.02 nm.
The operation principles of the NALM can be understood from its transmission and reflection characteristics where the theoretical description in this section is elaborated first. When an external narrow linewidth BP power () is injected to the circulator, it travels from port 1 to port 2 before moving to the VOC. Then after passing this device, it is divided into clockwise and counterclockwise signal power of and with the corresponding splitting ratio of . This is illustrated in Fig. 1 above where the RPU acts as a forward pumping source with respect to the Brillouin power, . In addition, it also serves as a backward pumping source in comparison to the Brillouin power, . After a cavity round trip, these interacting waves that encounter amplification in the DCF fiber acquire different gain values. These justify the generation of different power levels through nonlinear self-phase modulation that can be expressed as [17, 18],17], is the fiber effective area and is the operating wavelength. The loss-gain profiles along the fiber horizontal distance and that travel around the DCF fiber loop length are expressed as,18],Eq. (5) that include is the fiber loss coefficient at the signal wavelength and is that at the pump wavelength . In the next parameter, where is the Raman gain coefficient at the signal wavelength and is the injected Raman pump power.
From Eq. (1) above, it can be concluded that variations in RPU and BP power together with the coupling ratio result in different gain distributions and power intensities among these interacting signals. As a consequent, a nonlinear phase shift with a fluctuating behavior is induced. Once these signals ( and ) recombine interferometrically at the coupler, this phase shift determines the amount of emitting output power. These comprise that of the transmitted power, propagating along port 2 and that of the reflected power, accumulating at port 1 of the VOC. In the case that the fiber nonlinearity does not induce a significant phase shift between the beating spectra, the fiber loop acts as a mirror that reflects most of the BP input signal to port 1 of the VOC. On the other hand, the discrepancy in phase shifts that is induced by the fiber nonlinearity can be controlled to initiate the necessary amount of the transmitted and reflected power in the cavity. Therefore, the relationship between and needs to be examined thoroughly to confirm the right specifications required for the attainment of high numbers of lasing lines at low RPU power. This can be satisfied by exploiting the sensitivity of output power response to various splitting ratios implemented in this laser structure.
3. Experimental results and discussion
At the outset of this assessment, the best coupling ratio that is feasible for determining the right proportion between and is investigated when the RPU power is arranged at 300 mW. The tunable laser source is selected at 5 dBm output power and a wavelength of 1555 nm. This BP wavelength is fixed for the entire experiment as it corresponds to the Raman peak gain (RPG) when the first shift is initiated from the original RPU wavelength of 1455 nm . At this BP wavelength also, the energy transfer is capable to suppress the self-lasing cavity modes. Therefore its competition is reduced which can affect several lasing traits namely the number of channels, peak power and OSNR. To determine these, both and are measured when the mirror is removed temporarily from the setup. During experiment, the coupling ratio is varied by increasing the percentage at port 3 of the VOC gradually from 1% to 99% with random intervals as manifested in Fig. 2(a). Simultaneously, this implies the decline of the coupling ratio at port 4 from 99% down to 1% accordingly. From this figure, it can be inferred that 99/1 coupling ratio is the optimum value that leads to the highest generation up to 22 mW. Further reduction of power coupling to from 99% to 5% indicates a gradual decrease in . Then at of only 1%, a stronger decline to 11 mW is observed which might be due to the dynamics of phase difference that occurs among the interacting waves. In contrast to this tendency, the values do not show a similar power development as these reach minimum and maximum power at 99/1 and 70/30 couplers, correspondingly.
To confirm the role of both output power, analysis on the number of channels are carried out. The degree of flatness in the spectral envelope is defined by including all lasing lines that have discrepancies in peak power of less than 10 dB. Accordingly, the same method is implemented when estimating the average OSNR and number of channels. The signal quality involved within this range also indicates the absence of any turbulence wave. The results obtained is depicted in Fig. 2(b) when a mirror is employed in the laser structure. From this figure, when more BP power coupling is directed towards a slow enhancement in the lasing channels generated is observed from 8 to 23 lines. However when comparing the utilization of a 1/99 and 5/95 coupler, the result implies a smaller gap in Fig. 2(b) which contradicts to the bigger transmitted power discrepancy (17 – 11 mW) observed in Fig. 2(a). As at only 1/99 coupling ratio, it is believed that the reflected power might play the role to compensate the drastic decline in that of the transmitted one. Although all evaluations are taken from the reflected beams, its power trend does not influence substantially on the formation of more lasing channels. Thus, the results indicate that the optimization of is the specific feature that contributes mainly to the attainment of this objective. Since the SBS process requires sufficient energy transfer from the BP to its BS signal, accumulating more feedback as an additional BP source is the right option. In fact, this can be provided in the cavity when placing a reflective mirror at port 2, VOC when maximum transmitted power is achieved at 99/1 coupler. Once this is satisfied, more power can be reflected back by the mirror to induce a low SBS threshold that is responsible for attaining high numbers of lasing channels.
To further understand this effect, the laser operation is elucidated as follows. With the inclusion of Raman source, the input BP power that counter-propagate in the Brillouin-Raman gain medium (DCF) encounter Raman amplification. This results in the generation of clockwise power, and counter-clockwise power, that are clarified in Eq. (1). When the beginning SBS threshold is met, the two first-order BS signals propagate in opposite directions to each other and with the injected BP signal in the loop. As elucidated previously in Section 2, due to the resulting nonlinear phase shift that is induced by the self-phase modulation, these signals combine interferometrically at the coupler. These are then separated into two partially transmitted and reflected power of and . For clarification, these output power are the results of overlapping between multiple orders BS signals circulating inside the cavity. Due to the coupling ratio at 99/1, a larger portion of first-order BS beam emits from port 1 and that of a smaller fraction at port 2, VOC. With the employment of a mirror that increases the Raman gain, is reflected back into the fiber loop to serve as a new BP source. Consequently when the next threshold is fullfilled, an inverse phenomenon occurs where a larger portion of second-order BS signal accumulated as transmitted waves and that of a smaller fraction emitting as reflected waves. Simultaneously, the Rayleigh scattering effects of the corresponding beams are also initiated. These cascading effects continue to grow until the total gain in the laser cavity is less than the loss at the operating wavelengths. As a result, transmitted waves consist of even-orders attribute when those of odd-orders characterize the property of reflected waves as presented in Fig. 3. However due to reflective contribution of the mirror, multiple interference effects between the beating spectra lead to multi-wavelength operation with single-spacing (10 GHz) that also consist of the residual BP waves. However extracting higher order Brillouin–Stokes signals from port 1, VOC may be at the expense of their less output power. This disadvantage is not a main concern in this assessment yet since the number of channels generated is similar at both output ports. In the future attempts, improvements on these results can be done by taking the output signals from another arm of the laser structure.
Next, the effects of varying RPU power on the BS signal-noise level and its spectral evolution are investigated when the BP power is still at 5 dBm. For the entire assessments, the optimum value of CR is maintained at 99/1. From Fig. 4, the subsequent enhancement in RPU power from 200 to 300 mW yields 19 to 23 lasing lines. This development appears at the cost of a higher noise floor level. Despite of the growth in BS peak power, the average OSNR only shows a slight decrement from 17 to 16.5 dB. Thus, these excellent results indicate that the appropriate choice of low RPU power provides inadequate Raman gain for cavity modes amplification. Therefore, the laser cavity is not dominated mainly by the self-lasing cavity modes which can lead to spectral broadening effects . In contrast to this when the pump power is increased further up to 500 mW, the OSNR degrades severely. In this case, the lasing channels produced are only 10 lines with an OSNR of 8 dB. This phenomenon that retards the formation of evident channels is attributed to spectral broadening effects initiated by turbulent waves as also reported in . In summary, there is a trade-off in obtaining the maximum number of output channels with acceptable OSNRs when adjusting the RPU power.
Moreover, the relationship between BP power with the transmitted power, is analyzed when the BP power is varied from −2.6 dBm to 5 dBm. This is depicted in Fig. 5 when the RPU power is selected to be at 300 mW. From this figure, it can be deduced that the transmitted power is inversely proportional to the BP power. This is attributed to the optimization of Raman gain at lower BP power where its gain saturation is fulfilled faster at higher BP values within this range.
Once all these physical characteristics are understood, the same analysis is carried out when the BP power is changed to the optimum value of −2.6 dBm (refer to Fig. 5). The main aim is to further expanding the number of Stokes combs with an acceptable OSNR. Other experimental parameters that produce the best results as demonstrated in Fig. 4 (RPU = 300 mW, BP wavelength = 1555 nm, CR = 99/1) has never been changed. From the spectral envelope, 28 stable lasing lines are attained as shown in Fig. 6 which is slightly better than those obtained previously. In addition, the average OSNR of 17 dB is almost comparable to that achieved in Fig. 4 when comparing only at the same RPU power. For simplification, the red spectra in Fig. 4 is also attached in this figure to assist the comparison. Owing to the incorporation of a reflective mirror, the existence of turbulence wave for both spectra at the region higher than 1557 nm is not because of instability. However, it corresponds to the similar behavior in the free lasing mode that is shown in the figure (inset). This mode is also evaluated at the same pumping characteristics with the absence of BP signal.
A new multi-wavelength BRFL configuration that utilizes the effect of SBS in the NALM structure is demonstrated. The main objective of generating a high number of lasing lines with a reasonable OSNR at low RPU power has been successfully satisfied. This is attained by a proper adjustment of the coupling ratio to initiate more BS feedback responsible for the generation of low SBS threshold in the cavity. Other variables that assist this process are the optimization of BP power from 5 to −2.6 dBm when its corresponding wavelength is set at the Raman peak gain of 1555 nm. From the experiments, lasing lines from 23 to 28 has been achieved with outstanding OSNRs around 17 dB. The RPU power needed to accomplish these are only a few hundreds mW. These exceptional qualities allow many potential applications, namely in optical sensing and communications at low power operation.
This work was partly supported by the Graduate Research Fellowship Scheme under Universiti Putra Malaysia.
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