We demonstrate a multiwavelength Brillouin-erbium fiber laser with double-Brillouin-frequency spacing. The wider channel spacing is realized by circulating the odd-order Stokes signals in the Brillouin gain medium through a four-port circulator. The circulated odd-order Stokes signals are amplified by the Brillouin gain and thus produce even-order Stokes signals at the output. These signals are then amplified by erbium gain block to form a ring-cavity laser. Ten channels with 0.174 nm spacing that are generated at 0.5 mW Brillouin pump power and 150 mW pump power at 1480 nm can be tuned from 1556 nm to 1564 nm. The minimum optical signal-to-noise ratio of the generated output channels is 30 dB with maximum power fluctuations of ±0.5 dB.
© 2011 OSA
Fiber lasers have attracted great attention due to its geometry that offers simple thermal management and a high degree of immunity from effects of heat loading, which are often too detrimental to conventional “bulk” solid-state lasers . In the design of fiber lasers, various types of optical fibers are employed as the gain media such as erbium-doped fiber, ytterbium-doped fiber, bismuth-oxide doped fiber and etc. Nonlinear optical effects inherent in a single mode fiber, namely stimulated Raman scattering , stimulated Brillouin scattering and Rayleigh scattering  are also utilized in order to assist the performance of fiber lasers.
A hybrid Brillouin-erbium fiber laser (BEFL) was first demonstrated by G. J. Cowle et. al.  which integrated two gain media in the design of a laser cavity. The erbium-doped fiber amplifier (EDFA) offers a linear gain for high power generation in the compensation of resonator loss. On the other hand, the Brillouin gain is provided by a section of optical fibers. In this case, the lasing wavelength generated at the Stokes-shifted frequency is determined from the injected Brillouin pump wavelength . In previously reported works, BEFLs were further investigated extensively in producing multiwavelength outputs [6–11]. These multiple outputs are constantly spaced by the Brillouin frequency vB, which depends on the fiber material. For silica-based fibers, this is approximately 10 GHz or 0.08 nm. The generation of multiple Stokes signals is realized from the cascaded Brillouin effect, in which low order Stokes signals are amplified by the EDFA to initiate higher order Stokes signals.
The multiwavelength fiber laser found its feasibilities in various applications that include wavelength division multiplexing (WDM) light sources  and sensor networks [13, 14]. However, the practical realization of multiwavelength BEFL in aforementioned applications is yet to be reported. The difficulty of channel demultiplexing from the narrow 10 GHz (~0.08 nm) wavelength spacing of BEFL limits its contributions in the system implementation. Therefore, researchers are investigating to expand the channel spacing between Stokes signals to ease the demultiplexing process. In order to achieve this objective, a comb fiber laser with channel spacing of both 10 and 20 GHz was reported in . It consists of two metal-coated fiber planar mirrors and a Sagnac reflector to discriminate the even-order Stokes waves and the odd-order Stokes waves. The oscillation of odd- and even-order Stokes waves are separated into two different cavities and each cavity consists of an EDFA. In contrast, M.K. Abd-Rahman et al. reported multiwavelength, bidirectional operation of a twin-cavity Brillouin/erbium fiber laser in . The configuration consists of two identical erbium-doped fiber ring lasers that share a common section of single mode fiber to produce interdependent bi-directional Stokes signals. The proposed laser structure can be viewed as two separate laser cavities, which can operate individually as a BEFL system. This configuration can produce channels with 20 GHz spacing, but it requires two identical EDF gain blocks to balance amplification in both cavities.
Recently, a bidirectional multiwavelength Brillouin fiber laser generation in a ring cavity was reported by M. R. Shirazi et al. in . Two outputs are obtained which consist of odd- and even-order Stokes signals accordingly. A directional coupler is used to form a Brillouin cavity by connecting both ends of the fiber spool. The 3-dB coupler introduces power division of the oscillating Stokes wave for each round trip and hence reduces the efficiency of cascaded Brillouin effect. Without the erbium gain in the cavity, a Brillouin pump as high as 14 dBm and a long single mode fiber of 25 km are needed to generate higher order Stokes signals. There are only four channels at each output with 20 GHz frequency spacing.
In this paper, we demonstrate a multiwavelength BEFL with wider channel spacing which is equal to the doubling of Brillouin Stokes-shifted frequency. The double-Brillouin-frequency shift is realized by incorporating a 4-port circulator to circulate and isolate the odd-order Stokes signals in the fiber as reported in . Ten output channels with 0.174 nm spaced signals are generated and can be tuned over 9 nm from 1556 nm to 1564 nm. With wider channel spacing, we hope that it can open up the possibilities to employ multiwavelength BEFL in diverse applications.
2. Experimental setup
Figure 1 shows the experimental setup of our proposed multiwavelength BEFL which is formed by a ring cavity and a double-Brillouin-frequency shifter (DBFS). The DBFS is the core structure of the design, which provides twice the Brillouin frequency down shifting within 20 GHz (subject to the material composition of optical fibers) every time the input signal is injected into it. This frequency shifter is constructed by incorporating a fiber-based 4-port circulator (Cir) and a spool of silica-based single mode fiber (SMF) as reported in . It is also known as the ring-cavity of the proposed multiwavelength BEFL structure.
The Brillouin pump (BP) power of the multiwavelength fiber laser is provided by a tunable laser source (TLS). It is directed to the cavity through a 4-port directional coupler (DC). A 1480 nm laser diode (LD) is coupled with a section of 21.5 m long erbium-doped fiber (EDF) via a wavelength selective coupler (WSC). This EDF gain block amplifies the incoming signal from the 90 percent port of DC. This amplified Brillouin signal is then injected into a 6.7 km long SMF which serves as the Brillouin gain medium through port 1 and 2 of the circulator.
The first-order Brillouin Stokes signal (BS1) is generated once the BP power exceeds its threshold and it propagates towards port 2 in the counter-direction to the BP signal. Then, BS1 is fed back to the 6.7 km SMF through port 3 of the circulator to complete a round trip. BS1 circulates in the cavity via counter-clockwise direction and its amplification provided by the BP. Once BS1 power goes beyond its threshold condition, the second-order Brillouin Stokes signal (BS2) is produced in the opposite propagation direction of BS1. Under this situation, BS2 propagates in the same direction as the BP’s traveling path. In this case, the 4-port circulator isolates the odd-order Brillouin Stokes signal to circulate within the SMF only. In addition, it also allows forward propagation of the incoming BP and its double-Brillouin-frequency shifted signal (BS2) from port 1 to port 4. This feature is very important for the formation of multiple wavelength lasers. Then these two signals (BP and BS2) pass through port 4 of the circulator is re-injected towards DC. The proposed laser structure enables the circulation of BS2 in a ring cavity that consists of an EDF that behaves as an amplification gain block. This new lasing wavelength then acts as the subsequent BP to generate higher even-order Stokes signals. The same process is repeated continuously until the lasing condition is terminated as the Stokes signal gain is less than its cavity loss. The output is measured from 10% port of DC by using an optical spectrum analyzer with the resolution bandwidth set at 0.015 nm.
3. Results and discussions
Firstly, the peak gain of the laser cavity is determined by disabling the BP injection. In this experiment, the laser operates as a conventional erbium-doped fiber laser (EDFL). Under this condition, the LD pump power is fixed to 100 mW and the lasing characteristics of the EDFL are recorded. The result is depicted in Fig. 2 that shows the existence of free-running cavity modes around 1561 nm. This measurement is critical because it creates an instability situation when low BP is injected into the laser. This leads to mode competition that limits the tuning capability of the proposed multiwavelength laser. In order to rectify this problem, the Stokes signal must be generated at which the same resonator would operate as a free-running EDFL without BP . It was experimentally proven that laser produced by BEFL operates in a single-longitudinal-mode and suffer mode hops only under environmental perturbations . In the reported work, the longitudinal mode beats were strongly suppressed when lasing occurs in a BEFL operation as compared to a free running EDFL operation. Hence, the BP should be launched into the BEFL at the coincident wavelength to the EDF peak gain to have an efficient generation of Brillouin Stokes signals. Under this condition, the generated Brillouin Stokes signal is able to suppress the EDFL operation in order to have a stable output .
The performance of the BEFL is studied for different levels of pumping power at 1480 nm LD when the BP is maintained at 4 mW. Figure 3 illustrates the generation of multiple channels when the wavelength spacing is twofold of the Brillouin-frequency shift, 2v B. When the 1480 nm pump power is set at 5 mW, the first channel (BS2) has just been initiated with its peak power is still lower than that of BS1 (odd-order). This first channel rises up from −35.4 dBm to 0.9 dBm when the LD pump power is set at 10 mW. Further increment in the LD pump power to 20 mW results in the generation of second channel (4th order Brillouin Stokes signal). Consequently, the third channel (6th order Brillouin Stokes signal) is recorded when the pump power is intensified to at 35 mW. In this experiment, all the desired channels are separated within 0.174 nm spacing. It is important to highlight that all the odd-order Brillouin Stokes signals (the lower peaks exist between desired channels) are measured at the output due to the Rayleigh scattering effect during their propagation along the 6.7 km SMF.
Figure 4 depicts the number of output channels generated by varying the BP power and 1480 nm pump power. These channels are counted by considering the signals with peak powers higher than −10 dBm only. In general, the output channels that can be generated depends on the optimization of gain media, which are the Brillouin and erbium gains inside the laser cavity [5, 11, 18]. The maximum of 10 channels are obtained at BP and 1480 nm pump powers of 0.27 mW and 150 mW, respectively. This indicates that the number of channels is a function of the total laser power . At constant BP power, it is shown that the number of channels increases with the increment of 1480 nm pump powers. This is owing to the expansion of EDFA gain that leads to higher circulating powers in the laser cavity. Therefore, the efficiency of Brillouin gain is also increased. However at a similar 1480 nm pump power, the number of channels reduces with the increment of BP powers. It is due to the fact that higher BP power reduces the EDFA gain (force the EDFA to operate in deep saturation regime), which results in the reduction of the lasing lines number .
As mentioned previously, the BEFL should operate at the wavelength which the EDFL generates free-running cavity modes, in this case around 1561 nm. However, the existence of these natural modes induces mode competition in the cavity that limits the tuning capability of the proposed fiber laser. In the next assessment, the output spectral tuning range at definite 1480 nm pump and BP powers are studied to evaluate the performance of our proposed BEFL. As shown by the inset in Fig. 5(a) , four channels are generated at 4 mW and 75 mW of BP and 1480 nm pump powers, respectively. All the channels can be tuned over 36 nm from 1542 nm to 1578 nm without the occurrence of spurious free-running EDFL cavity modes. It can be seen from this graph that the cavity modes at 1561 nm wavelength range tend to rise up when the BEFL operates away from this region. The EDF gain is not purely undergoes homogeneous broadening, thus both simultaneous BEFL as well as EDFL operation could sometimes occur if the BP wavelength is not close enough to the EDF gain peak .
On the other hand, for the corresponding BP and 1480 nm pump powers of 0.5 mW and 150 mW respectively, 10 channels are obtained. They can be tuned from 1556 nm to 1564 nm as illustrated in Fig. 5(b). However, these output channels cannot be tuned over larger wavelength range due to the fact that small Brillouin gain is not sufficient to suppress the free-running modes under EDFL operation. Based on these findings, the structure is incapable of producing high number of channels while maintaining wide tuning ranges thus limiting its practicality. This problem can be resolved by utilizing a spectral filtering technique as reported in . Unfortunately, this technique adds to the complexity of the laser design since the BP and the bandpass filter wavelengths must be optimized in order to obtain good lasers .
The laser power stability is analyzed when BP power and 1480 nm pump power are arranged at 1 mW and 100 mW, correspondingly. Eight output channels are observed as depicted in Fig. 6 where the spectrum is scanned every ten minutes during one hour period. It is found that the powers are stable except at Channel-8 as demonstrated in Fig. 7 . In this case, the peak power of Channel-8 varies between −6.8 dBm to −5.9 dBm. This is due to the fact that the Stokes signal at this channel is below its saturation level. In contrast, the middle channels (Channel 2-5) are stable with power variations within ±0.1 dB since they have reached their saturation powers, while the Channel-6 and Channel-7 have powers around the saturation level in which their peak powers swing within ±0.2 dB. In addition, for Channel-1, its power varies ±0.2 dB from its average level since it has the greatest influence from the BP power instabilities.
Figure 8 depicts the optical signal-to-noise ratio (OSNR) of the output channels when the 1480 nm pump power is set at 150 mW and BP power is varied from 0.5 mW to 4.0 mW. The OSNR is measured only for the channels with peak powers greater than −10 dBm. From Fig. 8, it can be seen that the lower order Stokes signals from Channel-1 to Channel-7 have good OSNR due to their higher amplitudes where their powers have been saturated by the Brillouin gain. In this case, larger BP powers lead to greater Brillouin gains that suppress the ASE from the EDF gain, which explains in better achievement of OSNR. The largest OSNR of 35.9 dB is obtained at Channel-1 when the BP power is set to 4 mW. At higher order channels (Channel-8 to Channel-10), the OSNR increases at low BP power and starts to decrease at higher BP powers. For Channel-8, this parameter rises from 28.9 dB (0.5 mW BP power) to 34.1 dB (1.6 mW BP power) before dropping to 31.9 dB when the BP power is further increased to 2.0 mW. The higher the BP power is, greater gain suppressions are induced, hence the higher order Stokes power diminished. In addition, lower BP powers are unable to compete with the laser cavity modes which reduce the quality of OSNR . For BP powers greater than 0.8 mW, all channels have OSNR above 30 dB which is comparable to the OSNR value of about 20 dB as published in [7, 16].
We have successfully demonstrated a multiwavelength Brillouin-erbium fiber laser that implies wider wavelength spacing, which is twice the Brillouin shift in the single mode fiber. The frequency spacing is doubled by keeping the odd-order Brillouin Stokes signals to circulate within the Brillouin gain medium in the ring-cavity structure formed by the 4-port circulator (DBFS). The even-order Brillouin Stokes signals generated from this structure are forced to oscillate in the ring-cavity laser that consists of erbium-doped fiber for amplifications. From the experiment, ten channels with 0.174 nm spacing are generated with tunabilities over 9 nm from 1556 nm to 1564 nm. All of these channels have their lasing peak power above −10 dBm. The attainment of wider spacing between channels opens up the potential for the realization of BEFLs as WDM light sources that are beneficial for wavelength demultiplexing. It also has the potential application as a technique to generate microwave/millimeter wave signals.
This work was partly supported by the Universiti Putra Malaysia, Ministry of Higher Education (research grant #05-01-09-0783RU), and the Ministry of Science, Technology and Innovation, Malaysia (Brain Gain Malaysia Program, research grant # MOSTI/BGM/R&D/19(3) and National Science Fellowship).
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