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JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 36, NO. 11, JUNE 1, 2018
Multi-Wavelength Brillouin Random Fiber Laser via Distributed Feedback From a Random Fiber Grating Liang Zhang, Yanping Xu, Ping Lu, Stephen Mihailov
Abstract—Multi-wavelength (MW) laser sources with high optical signal-to-noise ratios (OSNR) are of great interest for optical communications with ultrahigh data capacity as well as microwave and terahertz photonics. In this paper, we report the first demonstration of MW Brillouin random fiber laser in the telecom spectral window around 1.5 µm based on a unique random fiber grating (RFG). Random feedback by enhanced distributed Rayleigh scattering from the RFG basically enables a high-efficiency random lasing resonance of the Stokes wave via stimulated Brillouin scattering in optical fibers. A subfiber loop eventually delivers the cascading process for the high-efficiency generation of up to a 14-order Stokes’ comb with an ultrahigh OSNR of 41 dB and an optimized peak power discrepancy of 3.4 dB. Thanks to Rayleigh scattered random feedback from the RFG, single longitudinal mode operation of the Stokes laser emission is achieved. Index Terms—Brillouin scattering, fiber lasers, Rayleigh scattering.
I. INTRODUCTION ULTI-WAVELENGTH fiber lasers (MWFLs) have been drawing a great deal of interest in the past decade due to their versatile application in wavelength-division-multiplexing (WDM) systems, fiber sensor networks and microwave photonics [1]–[3]. Stable multi-wavelength laser emission has been demonstrated by sole or hybrid gain mechanisms such as erbium-doped fiber amplification [4], semiconductor optical amplifiers [5], Raman scattering [6], Brillouin scattering [7], [8], etc. Recently, random fiber lasers (RFLs) have attracted extensive attention owing to their application potentials in sensing and communication [9]. Random laser generation was enabled by the combination of the amplification given by stimulated Raman/Brillouin scattering or erbium doped amplification as well as the random distributed feedback derived from backward Rayleigh scattering in long fibers [10], [11]. Investigations have
M
Manuscript received August 11, 2017; revised December 30, 2017; accepted February 4, 2018. Date of publication February 12, 2018; date of current version March 5, 2018. This work was supported in part by the Natural Sciences and Engineering Research Council of Canada under Grant 06071/FGPIN/2015 and in part by the Canada Research Chair Program in Fiber Optics and Photonics. (Corresponding author: Liang Zhang.) L. Zhang, Y. Xu, L. Chen, and X. Bao are with the Department of Physics, University of Ottawa, Ottawa, ON K1N 6N5, Canada (e-mail:
[email protected];
[email protected];
[email protected];
[email protected]). P. Lu and S. Mihailov are with the National Research Council Canada, Ottawa, ON K1A 0R6, Canada (e-mail:
[email protected]; Stephen.
[email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JLT.2018.2805284
, Liang Chen
, and Xiaoyi Bao
been made on RFL features such as power scaling [12], lasing efficiency [13], [14], narrow linewidth [15], [16], singlelongitudinal-mode (SLM) operation [17], noise suppression [18], spectral tunability [19], [20], pulse generation [21], linearly polarized radiation [22], [23], etc. Multi-wavelength RFLs have been also implemented by cascading Raman scattering [24] or by incorporating a spectral selection element with a broadband inhomogeneous gain profile, e.g., a fiber loop mirror [25] and fiber Bragg grating array [26]. Particularly, stimulated Brillouin scattering (SBS) in optical fibers can be an effective approach to operate a frequency-comb random laser with a spacing of Brillouin frequency shift [27]–[29]. Although special fibers (e.g., large effective area fiber) have been utilized to suppress the spectral noise, the optical signal-to-noise ratio (OSNR) of the MWFL was still limited by ∼20 dB [30]. More recently, a high-OSNR multi-wavelength random fiber laser has been demonstrated [31], [32], in which six orders of Stokes laser emissions with over-40-dB OSNR were simultaneously generated by cascading SBS gain process with assistance of Rayleigh scattered distributed feedback. However, the long Rayleigh scattering fiber reduced the frequency spacing and increased the mode density with more number of the modes to compete the finite Brillouin gain, which limited the number of the Stokes components. In order to increase the Stokes order, there is a tradeoff between maximum gain (limited by the gain fiber length) with minimized loss and the effective gain for each cavity mode. This can be realized by replacing tens of kilometers Rayleigh fiber with a few centimeters of the indexchange enhanced fiber via inscription of the random feedback media such as a disordered Bragg grating [33] and a random fiber grating [34], [35], which provides two advantages: reduce the cavity mode density via the compact feedback length and decrease the cavity loss due to the high Rayleigh scattering coefficient. In this work, we demonstrate a multi-wavelength Brillouin random fiber laser (MW-BRFL) based on a random fiber grating (RFG). The random Rayleigh scattered feedback is effectively achieved by the RFG that is formed by multiple reflections from 25000 randomly spaced refractive-index-modulated spots distributed over 2.5-cm length of single mode fiber (SMF), which has reduced the cavity mode spacing significantly comparing with over a few kilometers of Rayleigh scattering fiber, and enhanced feedback strength to reduce cavity loss for the random laser with a high Q-factor. As a result, multiple Stokes lines achieve random lasing resonance with sub-kHz linewidth and the SLM operation. The OSNR of laser comb remains as high
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ZHANG et al.: MULTI-WAVELENGTH BRILLOUIN RANDOM FIBER LASER VIA DISTRIBUTED FEEDBACK FROM A RANDOM FIBER GRATING
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Fig. 1. Experimental setup for RFG-based MW-BRFL. (EDFA—Erbium doped fiber amplifier, OC—optical coupler, PC—polarization controller, CIR— optical circulator.)
as 41 dB while the peak powers of the Stokes lines can be equalized with a minimum power discrepancy of 3.4 dB. Furthermore, the impact of the utilization of gain fibers was experimentally investigated for further optimization. II. EXPERIMENTAL SETUP AND PRINCIPLE Fig. 1 depicts the experimental setup of RFG-based MWBRFL which comprises a main fiber ring cavity incorporating by a sub-fiber loop. In the main fiber ring cavity, the Brillouin gain medium, which consists of a 25-km length of SMF, is connected to the cavity by two circulators (CIRs 1 & 2) and a 50/50 optical coupler (OC 2). The RFG was incorporated through the CIR 2 in order to introduce the distributed random feedback for the Stokes lasing generation within the main fiber cavity. An isolator was placed at the end of the RFG before the laser output to block any undesirable Fresnel reflection upon the fiber end surface. The lasing Stokes lines were split by the OC 2 in the main fiber ring cavity and then recombined with the input pump laser (Rock Module, NP Photonics) by another 50/50 optical coupler (OC 1) in the sub fiber loop. An Erbium doped fiber amplifier (EDFA) was used to boost the optical power before the injection into the main fiber cavity. Two polarization controllers (PCs 1 & 2) were utilized to adjust the polarization state of the light. In our approach, the RFG was utilized in order to provide the distributed random feedback with enhanced Rayleigh scattering efficiency for the random lasing resonance. The random index variation in the core of SMF28 fiber was induced by tight focusing of femtosecond infrared (fs-IR) pulses from an ultrafast Ti:sapphire regenerative amplifier through into the fiber with an optical microscope objective. The fs-IR laser (Spitfire, Coherent) operates at the wavelength of 800 nm with a repetition rate of 100 Hz and pulse duration of 80 fs. A beam reducer consisting of one plano-convex lens and one plano-concave lens was used to reduce the laser beam width to match the open aperture of the objective (50X/0.6, Nikon). The fiber was mounted on an air bearing stage (Aerotech) that moves at the speed of 100 μm/second. The objective was mounted on a piezo stage (NanoMax-TS, Melles Griot) that is dithered in pseudo random fashion at a frequency of 100 Hz along the fiber axis with
Fig. 2. (a) Illustration of randomly distributed reflectors along the fiber core of the RFG; (b) Spectral reflection of the 2.5-cm RFG and 12-km SMF; (c) input and reflected trace and (d) corresponding intensity probability function.
a maximum displacement of 2.5 μm. In order to increase the cross-sectional volume of the index change produced by a single femtosecond laser pulse and to better match it to the fiber core diameter thus enhancing higher mode coupling efficiency, a cylindrical lens with focal length of 1 meter was placed 1 meter away from the fiber. A plane of induced index change with a diameter of ∼10 μm across the fiber core was produced. As shown in Fig. 2(a), a total of N = 25000 index modified planes with random spatial interval Δ from 0 to 3.5 μm were introduced along the 2.5-cm RFG. In Fig. 2(b), the spectral reflection of the RFG was validated as −30 dB over a broadband range from 1540 nm to 1560 nm, exhibiting ∼5 dB enhancement compared to that of 12-km SMF, despite the fluctuation of spectral reflection due to the interference from the interferometers along the RFG. Compared to Bragg grating-based random reflector [33], the broadband RFG with a weak wavelength dependence is favorable to the multi-wavelength operation. The statistics of randomly distributed reflection of the RFG were characterized by launching a cw laser beam with Gaussian intensity probability density function (PDF). As shown in Fig. 2(c) and (d), with respect to Rayleigh scattered intensity trace from 12-km SMF with the PDF of a negative exponential function [36], the scattered intensity distribution from the RFG exhibits a gamma distributionlike PDF due to reduced number of the scattering centers [37]. A gamma distribution is related to the beta distribution and arises
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naturally in processes of Poisson distribution. The finite number of the group modes induced by the index modulation with birefringence makes it deviate from symmetry Poisson distribution. Compared with nanometer spaced Rayleigh scattering centers in SMF, the RFGs with the large separation between the modulation index spots bring groups of the distinct modes to form interference patterns as shown in Fig. 2(b), as well as randomly varied intensity fluctuation in Fig. 2(c). Initially originating from the spontaneous Brillouin scattering, the Stokes wave was generated at an upshift from pump wavelength and then efficiently amplified by the SBS process in the main fiber cavity. Instead of a conventional mirror reflection, the RFG delivered randomly distributed feedback which was injected back to the gain medium for the feedback. Coherent lasing resonance at each order of Stokes wave occurs as its injected pump power reaches the lasing threshold. Considering the total M roundtrips in the open cavity, the j-th Stokes lasing output amplitude Ej could be theoretically modeled as [26], Ej =
M
Esp
m =1
×
N k =1
m u =1
exp −αtot
2π −i L λj
2π Gu ,k Au ,k exp −i 2zu ,k , λj
Fig. 3. Optical spectra of laser output at the input pump power of 0.25 mW and the EDFA power of 190.55 mW: (a) without sub-fiber loop; (b) with sub-fiber loop.
(1)
where Esp is the spontaneous Brillouin scattering; αtot is the total loss including the fiber loss and insertion loss of optical components in the main cavity; λj is the j-th Stokes wavelength and L is the Brillouin gain fiber length; Gu ,k = exp(gB Pp,j Lef f ) is the Brillouin gain for the backscattered j-th Stokes wave from k-th scattering center in u-th roundtrip; Au ,k and zu ,k are the backscattering coefficient and position of the k-th scattering center, respectively. The total number N of effective scattering centers along the RFG is 25000. Note that, the polarization mismatch and the gain saturation are omitted for simplicity. With increasing the EDFA power, the generated j-th Stokes line could be recurred and amplified in the sub-fiber loop as the subsequent pump for the (j + 1)-th Stokes generation. Consequently, multiwavelength random laser emissions with a Brillouin shift spacing can be efficiently generated through cascading the Brillouin lasing resonances and then monitored by an optical spectrum analyzer (OSA, Apex AP2043B). In our setup, the sub-fiber loop plays a key role in cascading the Stokes lasing emission in order to generate the Brillouin frequency comb while the subsequent Stokes line was generated by the lasing resonance in the main fiber cavity. In this way, the Brillouin pump source was topologically separated by the lasing cavity, which essentially guarantees the high performance in terms of the OSNR by suppressing the resonance of spontaneous emission from the EDFA. III. EXPERIMENTAL RESULTS A. Laser Spectra and Optical Signal-to-Noise Ratio With the input pump power of 0.25 mW, the output spectra of the RFG-based MW-BRFL were monitored as the EDFA power increased to 190.55 mW. As the sub-fiber loop was
disconnected, the 1st Stokes lasing emission at the wavelength of 1550.235 nm was generated with high OSNR of 54 dB, as shown in Fig. 3(a). Without the sub-fiber loop, high order Stokes cannot be produced due to the unidirectional propagation in the main cavity. Although the even-order Stokes could be generated by SBS under the high power 1st Stokes, its lasing resonance would be blocked by two CIRs in the ring cavity. By connecting the sub-fiber loop, the generated Stokes lines can be injected back into the main fiber ring cavity which acts as the Brillouin pump for subsequent order Stokes generation. Consequently, the Brillouin frequency comb with the wavelength spacing of 0.087 nm was emitted. As shown in Fig. 3(b), up to 14 orders of Stokes laser lines were generated with high OSNR of 41 dB when the input pump power and EDFA power were 0.25 mW and 190.55 mW, respectively. Note that, higher order Stokes and anti-Stokes lines were also observed at two sides of the generated laser comb, which attributes to four wave mixing effect among the multiple Stokes lines propagating along the 25-km SMF. The peak power discrepancy among 14 Stokes lines was measured as 3.4 dB. Note that, multiple Stokes lines were generated with the noise floor of −67 dBm which is the minimum detectable limitation of the OSA. Apparently, the effective suppression of spontaneous emission from EDFA has been validated. B. Optical Power Evolution In order to investigate the optical power evolution, each order of the Stokes laser emission was selected by a tunable bandpass filter and monitored by a power meter as one gradually increases the EDFA power. The input pump power was fixed at 0.3 mW. In Fig. 4, the 1st Stokes lasing output appears as the pump injection power was boosted by the EDFA above 3.6 mW. High order Stokes lines were successively created by increasing the EDFA power. Meanwhile, the lasing efficiency of each order
ZHANG et al.: MULTI-WAVELENGTH BRILLOUIN RANDOM FIBER LASER VIA DISTRIBUTED FEEDBACK FROM A RANDOM FIBER GRATING
Fig. 4. power.
Optical power of multiple Stokes laser lines as a function of the EDFA
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Fig. 6. Linewidth measurement of each Stokes component in the laser comb with different total Stokes lines number.
D. Linewidth
Fig. 5. Power discrepancy from 1st to 8th order Stokes and corresponding total Stokes lines number under different input pump powers. The EDFA power was fixed at 190.55 mW.
Stokes was gradually reduced as subsequent order Stokes lines were generated. Finally, the optical powers of the 1st to 11th Stokes tended to remain at saturated power level as the EDFA power reached 190.55 mW. C. Power Discrepancy In the MW-BRFL, the power discrepancy among the Stokes lines has been shown to have a strong dependence on the input pump power [32]. Proper adjustment of the input pump power could provide an effective approach to equalize the peak power of each of the Stokes orders. Herein, the investigation of the input pump power on high order Stokes generation as well as its power discrepancy has been implemented. Fig. 5 illustrates the number of the generated Stokes lines by varying the input pump power and the corresponding power difference among eight Stokes lines (from the 1st to the 8th order Stokes). As the EDFA power was set as 190.55 mW, the laser emission could deliver more Stokes lines at relatively low input pump power while only eight orders of Stokes lines were generated as the input pump power was increased to 5.62 mW. The reason is that the input pump laser at relatively high power would obtain most of the power amplification from the EDFA whilst the power of other orders of Stokes lines after the EDFA were accordingly decreased, leading to inefficient generation of higher order of Stokes. On the other hand, the power discrepancy among eight orders of Stokes lines was measured as the lowest value of 0.7 dB at the input power of 0.25 mW that is comparable to the saturated power of the 1st order Stokes as shown in Fig. 4. When the input pump power was above 0.25 mW, the observed power discrepancy exhibited a monotonously increasing trend versus the input pump power.
The linewidth of each Stokes line in the multi-wavelength laser was characterized by using delayed self-heterodyne technique. A narrow-band filter was utilized to select each Stokes line into an unbalanced Mach-Zehnder interferometer with 100-km delay fibers as well as a 40-MHz frequency shifter by an acousto-optic modulator. As shown in Fig. 6, each Stokes component in the laser comb exhibits a sub-kHz ultra-narrow linewidth (
