A novel quadruple-spaced multiwavelength Brillouin-erbium fiber laser

Multiwavelength Brillouin fiber lasers (MBFL) have widely gaining research interest due to their potential applications, such as optical communication [1], optical fiber sensing [2, 3], wavelength division multiplexing (WDM) [4, 5], spectrum analysis [6], microwave photonics [7, 8], and so on. Various MBFL and hybrid Brillouin Erbium [9], or Raman [10], or four-wave mixing effect [11], with different structures have been reported and stimulated Brillouin scattering (SBS) threshold [12], Brillouin Stokes quantity [13], and line-width [14] of them have been investigated. For the most MBFL, in general, the frequency spacing of the adjacent Stokes lines is around 10 GHz or 0.08 nm. The frequency downshift of the Stokes line is caused by a Doppler frequency shift that is introduced when the Brillouin pump (BP) is back-scattered by a moving acoustic wave [15]. Thus, the value of the Stokes frequency shift depends on the velocity of the acoustic wave in the optical fiber, and it is typically around 10 GHz for silica-based optical fibers in the 1550 nm region [16]. Using Brillouin fiber lasers could produce multi-wavelength output [17, 18]. However, the narrow frequency spacing can limit the practical usage of this MBFL. For instance, the multiwavelength laser used in a dense WDM (DWDM) system which has frequency spacing greater than 25 GHz.

To overcome the narrow frequency spacing of this kind MBFL, many laser configurations have been put forward to expand the frequency spacing to double-frequency spacing [19–22]. A dual-cavity structure was used to excite MBEFL with double-frequency spacing [19]. When the BP power was 4.8 mW and the 980 nm pump power was 92 mW, five odd-order Stokes lines and five even-order Stokes in a shared 0.49 km single-mode fiber (SMF) were excited in the two cavities, respectively. A double-Brillouin-frequency spaced MBEFL by circulating the odd-order Stokes in the Brillouin gain medium through a four-port circulator was demonstrated [20]. In 6.7 km SMF, at 0.5 mW BP power and 150 mW 1480 nm pump power, ten channels with 0.174 nm frequency spacing was obtained. The optical signal-to-noise ratio (OSNR) of the Stokes lines was around 30 dB, the wavelength tuning range was 8 nm. The literature [21] reported a C + L band MBEFL with 0.174 nm frequency spacing. In this work, six stable even-order stokes were generated in 10 km SMF and could be tuned over 80 nm by using a tunable filter. Using bi-directional amplification of different Stokes, 12 even-order Stokes and 13 odd-order Stokes with 0.172 nm wavelength spacing were obtained from two separate outputs in 9.4 km SMF when the BP power and the 980 nm pump power were 6 dBm and 20.8 dBm, respectively [22].

Recently, with the expansion of the 5G technology, millimeter-wave bands, such as 30 GHz and 40 GHz which were previously considered not suitable for cellular networks, will play an important role in the 5G communication [23]. Therefore, exploring the MBFL with frequency spacing no less than 30 GHz and their application in microwave signal generation has aroused researcher's considerable interest in past few years [24–27]. A tunable MBEFL with switchable frequency spacing between 10 GHz, 20 GHz and 30 GHz was demonstrated [24]. In this work, using three segments of SMF (9.4 km, 25.2 km, 25.2 km) embedded in the laser structure, 16 Brillouin-Stokes (BS) lines with 10 GHz spacing, seven stokes lines with 20 GHz and three Stokes lines with 30 GHz were generated by adjusting the BP power. By incorporating a single frequency spacing cavity (25 km SMF) and a double frequency spacing cavity (25 km SMF) into a ring structure, ten Stokes lines with 0.26 nm or 32.5 GHz frequency spacing were obtained when the BP power is 3 dBm and the 980 nm pump power were 27.8 dBm [25]. Using two segments of dispersion compensating fiber (both 10 km long), a three-port circulator and four-port circulator, demonstrated a 33 GHz frequency spaced MBEFL was demonstrated in [26]. In this work, six channels with 33 GHz frequency spacing were obtained when the BP power was 0 dBm and the 980 nm pump power was 22.3 dBm, the OSNR of the channels were larger than 40 dB. Another triple-frequency-spaced laser contained a single Brillouin frequency spacing cavity (20 km SMF) and a double Brillouin frequency spacing cavity (25 km SMF) was proposed in literature [27]. In the configuration, there were two high power erbium-ytterbium-doped fiber amplifiers (EYDFA). When the BP power, the EYDFA1 power and the EYDFA2 power were 5 dBm, 29.5 dBm and 28 dBm, respectively, ten stokes lines with 31.77 GHz frequency spacing were obtained at each output port of the three output ports in the configuration.

Above-mentioned configurations are triple-Brillouin-frequency MBEFL. However, as far as we know, few works were reported on quadruple-frequency spaced MBFL [28–31]. Two same double-Brillouin-frequency shifters were cascaded and embedded in a ring cavity in literature [28]. Each frequency shifter consists of two three-port circulators and a SMF for 25 km. At the BP wavelength of 1550 nm, BP power of 8.3 dBm and the 980 nm pump power of 27.78 dBm, seven channels with quadruple-frequency spacing were obtained. The average OSNR of the Stokes lines exhibits 15 dB and the wavelength tuning range was 15 nm. In another ring cavity structure [29], through utilizing three coils (25 km, 20 km and 25 km) or four coils (25 km, 20 km, 25 km and 42 km) of SMF, the triple- or quadruple-frequency spaced MBEFL can be achieved, respectively. The generated triple-frequency spaced multiwavelength number was five and the generated quadruple-frequency spaced multiwavelength number was four. In [30], by adjusting the wavelength of the BP and the variable optical attenuator, a single longitudinal Stokes signal with quadruple wavelength spacing can be obtained. In this work, 40 GHz microwave signal was generated through beating the first-ordre BS and the fifth-order BS. Recently, a broadly tunable 40 GHz wavelength spaced MBEFL was investigated [31]. In this work, four stable Brillouin Stokes were obtained at 20 mW of the BP power and 225 mW of the 1480 nm pump power, respectively.

In this paper, we experimentally demonstrate a compact quadruple-frequency spaced MBEFL. In this fiber laser, we design a simple and compact quadruple-Brillouin frequency shifter by using two four-port optical circulators and two coils of identical SMF. Then a ring cavity is easy to obtain by using the quadruple-Brillouin frequency shifter. The two coils of identical (the same fiber length of about 20 km and the same Brillouin frequency of about 10.88 GHz) SMF act as Brillouin gain medium. The linear gain is provided by inserting two erbium doped fiber amplifier modules (or EDFA1 and EDFA2) in the ring cavity. In our experiments, there were eight stable Stokes lines with 43.5 GHz (0.348 nm) wavelength spacing can be realized under the low BP power of 5 mW under the BP wavelength of 1548.11 nm. The output multiwavelength spectra can be tuned continuously from 1546.12 nm to 1561.01 nm. The average OSNR of the output multiwavelength is about 27 dB and the maximum OSNR of the output channel is up to 37.1 dB.

The experimental setup and the principle of the proposed MBEFL are illustrated in figure 1. This MBEFL consists of two four-port circulators (Cir1 and Cir2), two SMFs (SMF1 and SMF2) with a length of 20 km, two erbium doped fiber amplifier modules (or EDFA1 module and EDFA2 module), a 3 dB four-port optical coupler (OC) and a tunable laser source (TLS). The TLS is used as the BP and the maximum output power is 15.5 mW, the tuning wavelength range is 1529.55 nm to 1561.01 nm. The same two erbium doped fiber amplifier modules are used as optical amplifiers. Each optical amplifier module consists of a 980 nm pump laser with an adjustable power range of 0 mW to 526 mW, a 5 m long erbium-doped fiber (EDF), and a wavelength division multiplexer (WDM). The two 980 nm pump lasers in the EDFA1 module and the EDFA2 module are named as pump 1 and pump 2, respectively. The output spectrum is observed by an optical spectrum analyzer (OSA) with 0.02 nm resolution.

Zoom In Zoom Out Reset image size

The operating principle of the proposed MBEFL is described as follows: the 50% portion of the BP is injected to the OSA by the OC and another portion of the BP is pre-amplified by the EDFA1 via the OC and port 1, port 2 of the Cir1 before it arrives to the SMF1. The amplified the BP is injected into the right end of the SMF1. When the BP power exceeds the Brillouin threshold of the SMF1, the first-order stokes (BS1) shift 11 GHz (∼0.088 nm) down from the original the BP is initiated. The BS1 propagates in the backward direction against the BP and is amplified by the EDFA1. The amplified BS1 enters the right end of the SMF2 via the Cir2 from port1 to port2. The BS1 acts as the BP of the BS2. When the BS1 power exceeds the Brillouin threshold of the SMF2, the second-order Stokes (BS2) shift 11 GHz (∼0.088 nm) down from the BS1 will be produced. The BS2 propagates in the backward direction against the BS1 and arrives to the port1 of the Cir2 via the Cir1 port3 to port4. Then the BS2 is injected into the EDFA2 to amplify via port2 of the Cir2 before it arrives to the SMF2. The BS2 is launched into the left end of the SMF2 and acts as the BP of the BS3. When the BS2 power reaches the Brillouin threshold of the third-order Stokes (BS3), the BS3 with a wavelength spacing of 0.264 nm (∼33 GHz) will be excited. After that, the BS3 travels anticlockwise and is amplified by the EDFA2. The amplified BS3 arrives to the left end of the SMF1 via port2 to port3 of Cir2. The BS3 acts as the BP of the BS4. As the BS3 power reaches the Brillouin threshold of the fourth-order Stokes (BS4), the BS4 with a wavelength spacing of 0.352 nm (∼44 GHz) will be excited and propagates in the backward direction against the BS3. Therefore, the BS4 owns quadruple-frequency downshift compared to the initial BP. The generated BS4 is injected to the OC via port3 to port4 of the Cir2 and travels anticlockwise. The 50% portion of the BS4 is exacted by the OC for the investigation by using an OSA. And another portion is combined with the BP by the OC and acts as the BP pump of the next quadruple-frequency spaced Stokes, or the BS8. This cascaded SBS process in the Brillouin gain medium will be continuously carried out until the higher-order Stokes falls below the Brillouin threshold condition in the gain medium.

We have firstly investigated the relationship between the output performance of the MBEFL with the BP power, under the fixed powers of the pump 1 and the pump 2. Figure 2(a) shows the output spectra of the proposed fiber laser when the BP power is varied from 5 mW to 12 mW, meanwhile the BP wavelength, the pump 1 power, the pump 2 power were set at 1548.11 nm, 470 mW, 520 mW, respectively. Figure 2(b) illustrates the numbers of generated Stokes of the proposed fiber laser under the different BP power. As the BP power increases from 5 mW to 12 mW, the number of generated Stokes decreases from nine to six. It is due to the fact that higher BP power force the EDFA to operate in deep saturation regime so it reduces the EDF gain, which results in the reduction of the Stokes number. And the decrease in the number of Stokes may also be due to the high BP power that caused the anti-Stokes to be generated and strengthened. The generated multiwavelength spectrum under the BP power of 5 mW, the BP wavelength of 1548.11 nm, and the pump 1 power of 470 mW, the pump 2 power of 520 mW was shown in figure 3.

Zoom In Zoom Out Reset image size

Zoom In Zoom Out Reset image size

As shown in figure 3, a total of nine wavelengths from 1548.11 nm to 1550.878 nm can be produced and the peak power is about −2.42 dBm. The wavelength spacing between adjacent channels are 0.348 nm and determined by the Brillouin frequency shift of the two SMFs. The average OSNR of the first eight output channels is no less than 27 dB and the OSNR of the BP is up to 37.1 dB. This high OSNR of the BP will contribute to transfer the power to the higher-order BS. We can see that the first six output channels of output spectrum have excellent flatness from figure 3. The peak power discrepancy between BP and BS20 is only 2.6 dB and the discrepancy between BP and BS24 is 4.2 dB. This good flatness is due to the position of the erbium gain makes each order of stokes signals amplified once. The stability and power of BS32 are relatively low. If we use erbium-ytterbium-doped fiber and distributed Raman amplifiers in the experiment, we will get more stable wavelength channels and higher OSNR [32].

Then, we fixed the pump 1 power and changed the pump 2 power in our experiments, and investigated the output spectra of the proposed MBEFL with the pump 2 power. The measured results were shown in figures 4(a) and (b), which depict the output spectra and the numbers of generated Stokes of the MBEFL at the different pump 2 power with the fixation of the pump 1 power at 470 mW, the BP power at 5 mW, the BP wavelength at 1548.11 nm, respectively. We can find the numbers generated Stokes of the MBEFL from four to nine with the pump 2 power at 100 mW, 200 mW, 300 mW, 400 mW, 500 mW and 526 mW, respectively. This is due to the increase of the pump 2 power leads to higher circulating powers in the laser cavity which results in the efficiency of Brillouin gain also increases. Therefore the number of Stokes increases with the pump 2 power.

Zoom In Zoom Out Reset image size

And then, in the experiments, we changed the pump 1 power, the pump 2 power and fixed the BP wavelength to 1548.11 nm and the BP power to 5 mW. The measured experimental results are shown in figures 5(a)–(d). As can be seen in figure 5(a), the BS4 is stable excited when the pump 1 power and pump 2 power are 70.4 mW and 36.3 mW, respectively. The pump 1 power and pump 2 power increased to 141 mW, 205 mW, the BS8 and the BS12 are also stable excited and the record spectra are illustrated in figure 5(b). Figure 5(c) depicts that six stable wavelength channels are obtained when the pump 1 power and pump 2 power are tuned to 273.9 mW, 326.4 mW. Eight stable wavelength channels are produced when the pump 1 power and pump 2 power are 470 mW and 520 mW, respectively, shown in figure 5(d). This is also because higher circulating powers in the laser cavity leads to the number of Stokes increase.

Zoom In Zoom Out Reset image size

At last, the wavelength tunability and peak power stability of the proposed MBEFL was measured. We set the BP power as 5 mW, the pump 1 power as 470 mW and the pump 2 power as 520 mW. By tuning the BP wavelength, the output multiwavelength with 0.348 nm wavelength spacing can be tuned continuously within the wavelength range about 15 nm from 1546.12 nm to 1561.01 nm. The tuning process within the range from 1546.12 nm to 1561.01 nm at a 5 nm step is shown in figure 6. In the tuning process, the self-lasing cavity modes are efficiently suppressed and each output channel keeps a stable OSNR.

Zoom In Zoom Out Reset image size

Additionally, the stabilities of the output channels are very important and essentially to be investigated. We repeatedly scanned and recorded the output spectra at every 10 min interval in an hour by the OSA and plotted the measured output spectra in figure 7. Figure 7(a) shows the scanned output spectra in an hour. As can be seen from figure 7(a), the output spectra and the peak power of each channel have good repeatability. The peak power stability of the output eight channels are plotted in figure 7(b). The maximum peak power fluctuations of the output multiwavelength channels are 0.066 dB, 0.201 dB, 0.288 dB, 0.347 dB, 0.419 dB, 0.768 dB, 0.793 dB, 1.368 dB and 3.879 dB respectively. High power fluctuation was observed for BS32 due to this signal does not reach its saturation level thus it became highly affected by the gain fluctuation inside the cavity. We also find the power fluctuations increase with the increasing of the Stokes orders. This because the higher-order stokes channels are stimulated by the former-order Stokes. The power fluctuations can be accumulated and this accumulation can downgrade the stability of the high-order Stokes channels [23].

Zoom In Zoom Out Reset image size

In summary, a compact quadruple-Brillouin-frequency spaced multiwavelength fiber laser has been proposed and experimentally demonstrated. We obtained an optimized output multiwavelength spectra by adjusting the BP wavelength, power and 980 nm pumps' power. Eight stable wavelengths with quadruple-Brillouin-frequency spacing are produced under a low BP power in our experiments. The output channels are rigidly separated by 0.348 nm and can be tuned from 1544.92 nm to 1561.01 nm without using tunable optical filter. The average OSNR of the output wavelengths were higher than 27 dB. Additionally, the optimization of parameters of EDFA pump power and the locations of the optical amplifiers in the optical circuit, BP power and Brillouin fiber type are critical for achieving more Stokes lines with higher OSNR.

This work is supported by National Key R&D Program of China (2019YFB2203903, 2017YFB0405501); National Natural Science Foundation of China (62075047,61965006, 61735009); Guangxi Key R&D Program under Grant (AB18221033); Guangxi Natural Science Foundation (2018GXNSFAA281272, 2018GXNSFAA294099, 2019GXNSFBA24505, 2020GXNSFAA238040); Guangxi Key Laboratory of Optoelectronic Information Processing Foundation (GD18103); Guangxi Key Laboratory of Automatic Detection Technology and Instrument Foundation (YQ19108).