Multiwavelength Brillouin-erbium fiber laser

with double-Brillouin-frequency spacing

Y. G. Shee,1,2

M. H. Al-Mansoori,3 A. Ismail,

1 S. Hitam,

1 and M. A. Mahdi

1*

1Wireless and Photonics Networks Research Center, Faculty of Engineering, Universiti Putra Malaysia,

43400 UPM Serdang, Selangor, Malaysia 2Department of Electrical and Electronic Engineering, Faculty of Engineering and Science,

Universiti Tunku Abdul Rahman, Jalan Genting Kelang, 53300 Setapak, Kuala Lumpur, Malaysia 3Faculty of Engineering, Sohar University, P.O Box 44, Sohar P.C. 311, Oman

*mdadzir@eng.upm.edu.my

Abstract: 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 Optical Society of America

OCIS codes: (140.3510) Lasers, fiber; (290.5900) Scattering, stimulated Brillouin; (190.4370)

Nonlinear optics, fibers; (060.2410) Fibers, erbium.

References and links

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8. M. H. Al-Mansoori, and M. A. Mahdi, “Tunable range enhancement of Brillouin-erbium fiber laser utilizing

Brillouin pump pre-amplification technique,” Opt. Express 16(11), 7649–7654 (2008). 9. M. H. Al-Mansoori, M. A. Mahdi, and M. Premaratne, “Novel multiwavelength L-band Brillouin-erbium fiber

laser utilizing double-pass Brillouin pump preamplified technique,” IEEE J. Sel. Top. Quantum Electron. 15(2),

415–421 (2009). 10. M. A. Mahdi, M. H. Al-Mansoori, and M. Premaratne, “Enhancement of multiwavelength generation in the L-

band by using a novel Brillouin-Erbium fiber laser with a passive EDF booster section,” Opt. Express 15(18),

11570–11575 (2007). 11. M. H. Al-Mansoori, and M. A. Mahdi, “Multiwavelength L-band Brillouin-erbium comb fiber laser utilizing

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#135986 - $15.00 USD Received 1 Oct 2010; revised 29 Dec 2010; accepted 3 Jan 2011; published 14 Jan 2011(C) 2011 OSA 31 January 2011 / Vol. 19, No. 3 / OPTICS EXPRESS 1699

14. G. Bolognini, M. A. Soto, and F. Di Pasquale, “Fiber-optic distributed sensor based on hybrid Raman and

Brillouin scattering employing multiwavelength Fabry–Pérot lasers,” IEEE Photon. Technol. Lett. 21(20), 1523–1525 (2009).

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laser with shared cavity of Sagnac reflector,” Opt. Commun. 201(4-6), 399–403 (2002). 16. M. R. Shirazi, M. Biglary, S. W. Harun, K. Thambiratnam, and H. Ahmad, “Bidirectional multiwavelength

Brillouin fiber laser generation in a ring cavity,” J. Opt. A, Pure Appl. Opt. 10(5), 055101 (2008).

17. Y. G. Shee, M. H. Al-Mansoori, A. Ismail, S. Hitam, and M. A. Mahdi, “Double Brillouin frequency shift through circulation of odd-order Stokes signal,” Appl. Opt. 49(20), 3956–3959 (2010).

18. G. J. Cowle, D. Y. Stepanov, and Y. T. Chieng, “Brillouin/erbium fiber lasers,” J. Lightwave Technol. 15(7),

1198–1204 (1997). 19. M. N. Mohd Nasir, Z. Yusoff, M. H. Al-Mansoori, H. A. Abdul Rashid, and P. K. Choudhury, “Widely tunable

multi-wavelength Brillouinerbium fiber laser utilizing low SBS threshold photonic crystal fiber,” Opt. Express

17(15), 12829–12834 (2009). 20. Z. Abd Rahman, M. H. Al-Mansoori, S. Hitam, A. F. Abas, M. H. Abu Bakar, and M. A. Mahdi, “Optimization

of Brillouin pump wavelength location on tunable multiwavelength BEFL,” Laser Phys. 19(11), 2110–2114

(2009).

1. Introduction

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 [1]. 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 [2], stimulated Brillouin scattering and

Rayleigh scattering [3] 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.

[4] 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 [5]. 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 [12] 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 [15]. 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 [7]. 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.

#135986 - $15.00 USD Received 1 Oct 2010; revised 29 Dec 2010; accepted 3 Jan 2011; published 14 Jan 2011(C) 2011 OSA 31 January 2011 / Vol. 19, No. 3 / OPTICS EXPRESS 1700

Recently, a bidirectional multiwavelength Brillouin fiber laser generation in a ring cavity

was reported by M. R. Shirazi et al. in [16]. 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 [17]. 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 [17].

It is also known as the ring-cavity of the proposed multiwavelength BEFL structure.

Fig. 1. Experimental setup of the multiwavelength BEFL.

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-

#135986 - $15.00 USD Received 1 Oct 2010; revised 29 Dec 2010; accepted 3 Jan 2011; published 14 Jan 2011(C) 2011 OSA 31 January 2011 / Vol. 19, No. 3 / OPTICS EXPRESS 1701

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 [5]. It was experimentally proven that laser produced by BEFL operates in a

single-longitudinal-mode and suffer mode hops only under environmental perturbations [5].

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

[18].

Fig. 2. Free running spectrum of erbium-doped fiber laser for the pump power of 100 mW at a

wavelength of 1480 nm with the absence of BP.

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, 2vB. 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

#135986 - $15.00 USD Received 1 Oct 2010; revised 29 Dec 2010; accepted 3 Jan 2011; published 14 Jan 2011(C) 2011 OSA 31 January 2011 / Vol. 19, No. 3 / OPTICS EXPRESS 1702

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.

Fig. 3. Generation of multiple channels at different 1480 nm pump powers (BP power =

4 mW).

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 [18]. 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 [11].

Fig. 4. Number of output channels against the variations in BP power and 1480 nm pump

power from 50 mW to 150 mW.

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

#135986 - $15.00 USD Received 1 Oct 2010; revised 29 Dec 2010; accepted 3 Jan 2011; published 14 Jan 2011(C) 2011 OSA 31 January 2011 / Vol. 19, No. 3 / OPTICS EXPRESS 1703

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 [18].

Fig. 5. (a) Tuning capability at 4 mW BP power and 75 mW pump power and (b) tuning

capability at 0.5 mW BP power and 150 mW pump power.

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 [19]. 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

[20].

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

#135986 - $15.00 USD Received 1 Oct 2010; revised 29 Dec 2010; accepted 3 Jan 2011; published 14 Jan 2011(C) 2011 OSA 31 January 2011 / Vol. 19, No. 3 / OPTICS EXPRESS 1704

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.

Fig. 6. Optical spectrum at 1 mW BP power and 100 mW LD pump power.

Fig. 7. Power stability of channels generated at 1 mW BP power and 100 mW LD pump

power.

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

#135986 - $15.00 USD Received 1 Oct 2010; revised 29 Dec 2010; accepted 3 Jan 2011; published 14 Jan 2011(C) 2011 OSA 31 January 2011 / Vol. 19, No. 3 / OPTICS EXPRESS 1705

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 [11]. 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].

Fig. 8. OSNR of the output against BP powers at 150 mW LD pump power.

4. Conclusion

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.

Acknowledgments

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).

#135986 - $15.00 USD Received 1 Oct 2010; revised 29 Dec 2010; accepted 3 Jan 2011; published 14 Jan 2011(C) 2011 OSA 31 January 2011 / Vol. 19, No. 3 / OPTICS EXPRESS 1706