Tunable 60 GHz Multiwavelength Brillouin Erbium Fiber Laser

Abstract

An experimental study of a tunable 60 GHz multiwavelength Brillouin erbium fiber laser is presented in this paper. Two unidirectional ring laser cavities and two pre-amplification laser cavities are used. In the first three cavities, a Brillouin gain medium is presented with a dispersion compensation fiber (DCF) spool, and a single-mode fiber (SMF) spool is used as a Brillouin gain medium in the fourth cavity. Three erbium amplifiers are utilized to supply enough gain to the generated Brillouin Stokes signal and to suppress cavity losses. For these three amplifiers, up to 450 mW (150 mW for each) of a 1480 nm pump power is used. In our proposed configuration, four sextuple Brillouin Stokes signals with a high power of 10 dBm and more than 55 dB as an optical signal-to-noise ratio are achieved. The obtained Brillouin Stokes signals can be tuned over 30 nm (1560–1590 nm) and can be easily used in dense wavelength division multiplexing in optics communication systems.

1. Introduction

Due to its advantages in many applications, such as waveguiding [1], spectroscopy [2,3], optical sensors [4,5,6,7], quantum computing, and spectroscopy [8], multiwavelength fiber laser sources have received much attention from many researchers. Different techniques have been proposed for producing multiwavelength fiber laser sources, such as four wave mixing (FWM) [9], nonlinear optical mirrors [10], and stimulated Brillouin scattering (SBS) [11,12]. Among these methods, SBS in optical fiber is considered the best option due to the ultra-slim linewidth [13]. In a multiwavelength Brillouin erbium fiber laser (MBEFL) cavity, a higher-order Brillouin Stokes signal is generated by an amplified lower-order Brillouin Stokes line [14,15]. However, the main drawback of the MBEFL’s source is the narrow frequency shift of the Brillouin Stokes lines (0.08 nm) [16,17], which makes the laser comb inapplicable in optical communication system applications, because it cannot be distinguished within the de-multiplexer in the receiver side. Many researchers have suggested double Brillouin frequency shifting of 0.17 nm [18,19,20,21,22,23]. To increase the Brillouin frequency shift, fewer researchers have suggested a triple Brillouin frequency shift of 0.25 nm to increase the possibility of using multiple laser sources in the de-multiplexing side [24,25,26,27,28,29,30].

A switchable Brillouin frequency shift was achieved via three single-mode fiber (SMF) spools [24]. Only two Brillouin Stokes lines with a low power of −3.5 dBm that could be tuned over 22 nm were recorded. A long SMF length of 50 km was utilized in two ring laser cavities to produce up to 10 Stokes lines with a triple frequency shift [25]. A wide tuning range of 35 nm was reported. However, a high pump power of 600 mW was utilized at the expense of a low Stokes power of −4 dBm. Switchable Brillouin frequency shift lines were produced using serial SMF spools in the MBEFL cavity [26]. However, to provide switchable frequency, these spools were manually added and removed within the cavity. In addition, a high pump power of 1.2 W was utilized. A MBEFL cavity with a triple Brillouin frequency shift was presented by [27]. Up to six Stokes lines were generated using two erbium amplifiers and 20 km of two dispersion compensation fiber (DCF) spools. A MBEFL cavity that utilizes two erbium-ytterbium amplifier sections was implemented by [28]. Up to nine Stokes lines with a triple frequency shift with a low signal power of −15 dBm were recorded at the central frequency of the self-lasing cavity modes (1565 nm). However, the laser tunability was limited to the free-running mode’s central frequency, and these modes emerged at the output laser comb once the frequency was shifted beyond 1565 nm. Previous literature [24,25,26,27,28] has shown self-lasing cavity modes’ competition, which affects the tunability of the laser cavity. Four triple Brillouin frequency lines that were tuned over 40 nm (1530–1570 nm) were produced by [29]. The tunability was limited only to the emission bandwidth of the erbium. However, a DCF spool with 12 km length was used, and for such a long length, both Brillouin gain saturation and high transmission losses occurred. Recently, a simple and widely tunable MBEFL with a triple frequency shift was presented by [30]. Six Brillouin Stokes lines that had a tunability of 50 nm (1530–1580 nm) were recorded at a relatively low pump power of 220 mW.

Enlarging the Brillouin frequency spacing beyond 33 GHz is required in the demultiplexing process at the receiver side. However, only few studies have proposed a quadruple and quintuple Stokes channel (40 and 50 GHz) [26,31,32,33]. Four quadruple Brillouin Stokes lines with a low Stokes power of −2 dBm were produced by [26]. However, a long SMF length of 112 km and a high pump power of 1.1 W were utilized. Using such high values, the proposed setup may not be practical. Only two quadruple Stokes lines were generated [31]. However, a fiber Bragg grating (FBG) was used to extract the first-order quadruple Stokes signal, which added more cost and complexity to the cavity. A wide quadruple MBEFL cavity was suggested by [32]. Four quadruple Stokes lines at many output ports were generated. The generated Stokes lines had a a high Stokes power of 10 dBm and were tuned over 60 nm. Brillouin Stokes lines with a quintuple frequency shift (50 GHz) were proposed by [33]. Four quintuple channels with a maximum power of 10 dBm and a large optical signal-to-noise ratio (OSNR) of more than 50 dB were achieved. In the absence of the self-lasing cavity mode, the generated Stokes lines were tuned over a 40 nm from 1540–1580 nm.

In terms of real-time applications of optical communication systems, a quintuple Brillouin frequency shift (50 GHz) can achieve a data rate of about 200 Gb/s per single channel. Increasing the channel spacing to 60 GHz is required for a high data bit rate of 300 Gb/s per channel. To the best of our knowledge, no multiwavelength Brillouin erbium fiber laser with 60 GHz spacing has been produced. In this paper, experimental results on sextuple Stokes lines (60 GHz) are proposed. Four Brillouin Stokes lines with a sextuple frequency shift (0.48 nm) with a large OSNR of more than 50 dB and a high Stokes power of 10 dBm are achieved. The achieved signal can be tuned over 30 nm from 1560 to 1590 nm. Up to 62 channels with 60 GHz spacing can be implemented within the achieved 30 nm tuning range. In addition, most of the useful sextuple signals lie between 1570 and 1590 nm in the L-band region of DWDM communication systems. The wide tuning range was obtained due to two parameters: a high Brillouin pump (Bp) power and pre-amplification cavities that provide two factors, i.e., an increase in Brillouin gain and a reduction in the self-lasing cavity modes’ competition.

2. Experimental Setup

Figure 1 shows the experimental setup of the proposed laser configuration. A 3 dB optical coupler in addition to one four-port circulator (Cir2) as well as four three-port circulators (Cir1, Cir3, Cir4, and Cir5) are used in this design. The Brillouin gain medium is presented by three dispersion compensation fiber (DCF1, DCF2, and DCF3) spools and one single-mode fiber (SMF) spool. Three erbium-doped fiber amplifiers (EDFA1, EDFA2, and EDFA3) are employed to achieve a high Bp gain and to amplify the generated sextuple Stokes signals. EDFA1 was pumped by 150 mW using a 1480 nm pump unit, and both EDFA2 and EDFA3 were pumped by 300 mW through a 50:50 optical coupler using the same pump wavelength. The Bp signal was achieved via an external tunable source laser (TSL) with a maximum power of 14 dBm and a tuning range of 150 nm (1480–1630 nm). The residual Bp power and the generated Stokes signals were measured at the output side using an optical spectrum analyzer (OSA).

Firstly, TSL provides the Bp signal that is inserted into the first Brillouin gain medium (DCF) through a 3 dB coupler and Cir1. The first Brillouin frequency shift (10 GHz:0.08 nm) Stokes signal is generated when the Bp value is greater than the threshold power. The generated first Stokes signal is circulated in the first ring cavity via Cir1 and Cir2 to achieve the second (second Stokes) Brillouin frequency shift (20 GHz:0.16 nm) within the same Brillouin gain medium (DCF1). A high Bp power of 14 dBm (25 mW) is used to be sufficient enough to generate both the first and second Stokes signals. The second Stokes signal is directed by Cir2 to be amplified by EDFA1 and inserted into the second Brillouin gain medium (DCF2). In this stage, the third (third Stokes signal) Brillouin frequency shift (30 GHz:0.24 nm) is generated. Pre-amplification is used in the second laser cavity to enhance the subsequent Bp (2nd Stokes) and to improve the tuning range of the generated Stokes signal (3rd Stokes). The amplified third Stokes signal is forced to be inserted into DCF2 from the other end via Cir2 and Cir3. When the third Stokes signal is inserted, the fourth Brillouin frequency shift (40 GHz:0.32 nm) is achieved.

In the pre-amplification cavity, EDFA2 is used as a pre-amplifier to amplify the generated fourth Stokes signal (the subsequent Bp), which is inserted into DCF3 via Cir4. The generated Stokes signal (fifth Stokes signal (50 GHz:0.4 nm)) is also amplified by EDFA2 for more gain to be sufficient enough to generate the higher-order Stokes, signal since another cavity is used, and the generated Stokes signal cannot be inserted at the other end of DCF3. The amplified fifth Stokes signal is inserted into the SMF spool for generating the sextuple Stokes signal (sixth Stokes, 60 GHz:0.48 nm). Finally, the generated sixth Stokes signal is amplified by EDFA3 for the next sextuple Stokes generation. The generation of the sextuple Stokes signal will continue until the total gain inside the cavities is equal to the total losses. Two open-edge cavities (DCF3 and SMF) are used to enhance the tuning range by extracting the unwanted self-lasing cavity modes that increase competition with the Brillouin gain out of the cavity. In addition, the optical signal-to-noise ratio is improved with this kind of geometrical cavity.

3. Results and Discussions

Figure 2 shows the free-running modes of the proposed laser cavity. These modes were recorded in the absence of the Bp signal. An optical span of 120 nm (1500–1620 nm) and a maximum pump power value of 450 mW were used. An erbium gain peak bandwidth of 10 nm from 1570 to 1580 nm was obtained. Measuring these modes is important to select the suitable Bp signal wavelength that can obtain a maximum erbium gain level.

The threshold power for the generated sextuple Stokes signals is illustrated in Figure 3. At a fixed Bp power of 14 dBm (25 mW) and a wavelength of 1565 nm, the threshold values of the 6th, 12th, and 18th Stokes signals were recorded. The wavelength of the Bp signal was selected at the maximum gain of the self-lasing cavity modes, as shown in Figure 2. The sixth Stokes signal (first sextuple Stokes) was generated at a 130 mW pump power (the power was divided equally to all EDFAs). The generated sixth Stokes signal is represented as the subsequent Bp power to generate the second sextuple Stokes signal (12th Stokes), and it became sufficient enough with increasing the pump power further. As the pump power reached 130 mW, the 12th Stokes signals appeared. To generate the third-order sextuple Stokes signal (18th Stokes), the pump power was increased to 410 mW. At such a high pump power, the second-order sextuple Stokes signal reached the threshold value of the higher-order sextuple Stokes signal.

At the central wavelength of the self-lasing cavity modes (1570 nm) and at a high Bp power of 25 mW, the generated sextuple Stokes signal’s spectrum was as illustrated in Figure 4. Referring to Figure 1, a high Bp power is needed to be forceful enough for generating the first and the second Stokes signals in the absence of erbium gain. The erbium amplifiers (EDFA1, EDFA2, and EDFA3) were chosen to be of the same type, so that the generated Stokes signals (third, fourth, and fifth) can experience the same gain within the laser cavity. At the optimum Bp power and wavelength and at the optimum pump power of 450 mW, four sextuple Stokes signals (Bp, 6th, 12th, and 18th) were obtained. For all generated Stokes signals, a fixed Brillouin frequency shift of 0.48 nm (60 GHz), high signal power values, and a high optical signal-to-noise ratio (OSNR) of more than 56 dB were recorded.

Figure 5 shows the laser tunability of the generated sextuple Stokes signals at the maximum powers of Bp and a pump power of 25 and 450 mW, respectively. The tuning range was measured in the absence of the free-running modes at the output laser comb. The recorded tuning range was recorded within the range of 30 nm from 1560 to 1590 nm. Three parameters were behind such a wide tuning range: a high Bp power, the use of pre-amplification techniques for the generated Stokes signals, and a reduction in the competition between the Brillouin Stokes signals and the free-running mode by extracting this mode out of the cavity in DCF3 and SMF loops.

Figure 6 shows the effect of the pump power on the number of generated Stokes signals and the tuning range of the laser cavity. It is clear that with the increase in the pump power value, the number of generated Stokes signals increased linearly with different slopes and was saturated at a pump power value of 540 mW at four Stokes signals. At 150 mW, the first sextuple Stokes signal appeared (sixth Stokes signal). With the increment in the pump power, no higher-order sextuple Stokes signal (12th Stokes signal) was generated until a pump power value of 350 mW. In other words, there was no change in the number of generated Stokes within the range of 150 to 350 mW. Increasing the pump power further, the third sextuple Stokes (18th Stokes signal) was effective at a pump power value of 450 mW. On other hand, the tuning range also decreased linearly for all pump power ranges. Therefore, the pump power at which the Stokes signal number was saturated was chosen as 450 mW.

Figure 7 shows the output spectrum of the generated sextuple Stokes signals at different Bp wavelengths (1560, 1565, 1570, 1580, 1585, and 1590 nm), which lies within the tuning range bandwidth. For all selected Bp wavelengths, the maximum power of Bp (25 mW) and pump power (450 mW) were used. Figure 7a,f at Bp wavelengths of 1560 and 1590 nm, respectively, show three sextuple Stokes signals only as compared to four sextuple Stokes signals generated at the other Bp wavelengths. The reason for this can be explained easily in that these two wavelengths lie at the edge of the free-running modes of the erbium gain, whereas the other Bp wavelengths (1565 to 1585 nm) lie at the central wavelength of these modes. For all generated Stokes signals, a low gain difference between the Bp and the sextuple Stokes signals, high Stokes signal powers, and a high OSNR of more than 55 dB were recorded. The spectrums of these sextuple Stokes signals prove the applicability of such Stokes signals in dense wavelength division multiplexing system applications. In addition, our conducted experimental results are compared with the previous different Brillouin frequency shifts in a multiwavelength fiber laser.

Table 1. Summary of different Brillouin frequency shifts in multiwavelength fiber laser with our work.

Ref. No.	No. of Stokes	Tuning Range (nm)	Channel Spacing GHz	Description
[24]	2	22	30	Three cascade-long single-mode fiber Manual change in cavity Low output Stokes power of −3.5 dBm
[25]	10	35	30	Long SMF length of 50 km High pump power of 600 mW Low Stokes power of −4 dBm
[26]	4	not covered	30	70 km SMF length 1.2 W pump power
[28]	9	only at the center wavelength	30	Two sections of erbium-ytterbium-doped fiber amplifier (EYDFA) Long SMF spool with length of 45 km
[29]	3	40	30	Long DCF spool with length of 12 km added an additional loss inside the cavity
[26]	3	not covered	40	Four cascade SMFs for long length of 112 km High EDF pump power of 1.1 W
[31]	1	not covered	40	High-finesse ring filter Two cascaded double Brillouin frequency-spacing cavities
[32]	4	60	40	Two ring cavities Two DCF (6 km) for each one Two EDF (5 m)
[33]	4	40	50	Two ring cavities One open-end cavity by DCF Three DCF (6, 6, and 3 km) Pre-amplification
our work	4	30	60	Two ring cavities Two open-end cavities with DCF and SMF Three DCF (6, 6, and 3 km) Three EDF (10 m)

shows a summary of previous published works and our presented work.

Figure 8a,b shows the peak power stability of the generated Stokes signals (Bp, S6, S12, and S18) within a short (one hour: five minutes each) and long (12 h: one hour each) period, respectively. Stability was recorded at the maximum pump powers of the pump and Bp for these two periods. It is clear that S6 and S12 were stable within these periods. The reason for this can be attributed to the gain saturation of both S6 and S12, which makes them stable. On the other hand, a fluctuation of almost 2 dB from 6.5 to 8.5 dBm was observed within the two periods for S18, since this Stokes signal did not reach the saturation level.

4. Conclusions

The experimental results of a tunable 60 GHz multiwavelength Brillouin erbium fiber laser are successfully demonstrated in this paper. Four sextuple Stokes channels are presented and tuned over 30 nm from 1560 to 1590 nm. Three important parameters were incorporated to achieve such a wide tuning range: a high Bp power, a pre-amplification technique, and extraction of the self-lasing cavity modes outside the cavity using an open-end cavity for DCF3 and SMF loops. Extracting these modes can effectively reduce the competition between the Brillouin Stokes and the free-running modes and subsequently enhance the laser tunability. The advantage of the proposed design is a wide tuning range, high Stokes signal powers, a high OSNR, an excellent output spectrum at different wavelengths within the tuning range bandwidth, and a rigid Brillouin frequency shift of 60 GHz, which shows an opportunity for use in a DWDM communication system. Up to 62 sextuple channels can be used within the achieved tuning range and high data bit rate of about 300 Gb/s per individual sextuple Stokes signal. In addition, a large part of the tuning lies in the long band (L-band) optical communication region (1570–1590); therefore, most of the useful channel is out of the C-band region, which is fully utilized in an optical communication system.