IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 30, NO. 11, JUNE 1, 2018
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Linearly Polarized Multi-Wavelength Fiber Laser Comb via Brillouin Random Lasing Oscillation Liang Zhang , Yuan Wang, Yanping Xu , Dapeng Zhou, Liang Chen, and Xiaoyi Bao
Abstract— A linearly polarized multi-wavelength Brillouin laser comb in the 1.5-µm telecom spectral window is established by cascading multiple Brillouin random lasing oscillations in a semi-open polarization maintaining fiber (PMF)-based composite cavity. Distributed Rayleigh scattering offers a broadband random reflector to build up efficient Brillouin lasing resonance along kilometer-long PMFs, yielding four orders of Stokes radiation with high degree of polarization (>99.67%) and polarization extinction ratio of >20 dB. Each Stokes component with kilohertz linewidth as well as over 40-dB optical signalto-noise ratio has been validated. Compared with single mode fiber-based laser design, the proposed PMF-based laser comb exhibits highly suppressed relative intensity noise, thanks to the immunity to external perturbation and random modes density reduction within polarization-matched Brillouin gain. Index Terms— Fiber lasers, Rayleigh scattering, Brillouin scattering, polarization-maintaining fibers.
I. I NTRODUCTION
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ULTI-WAVELENGTH laser comb, as a compact source with multiplexed narrow-linewidth wavelength components instead of integrating multiple sets of single-frequency lasers, are highly desirable in versatile applications such as wavelength-division-multiplexing (WDM) communication systems, sensor networks and precision spectroscopy [1]–[3]. Stimulated Brillouin scattering (SBS), originating from the acoustic phonons-coupled pump and Stokes lightwaves interaction, can be an effective approach to introduce a narrowband gain for multi-wavelength Brillouin fiber comb with an inherited ∼10 GHz channel spacing [4], [5]. To generate more Stokes channels, multi-wavelength Brillouin fiber lasers are constructed in a few km long fiber cavity with hybrid gain schemes such as Erbium doped fiber amplification or/and Raman gain [6]–[8]. However, the lasing emission suffers from nearly equal probability of the dense multi-longitudinal modes in ultra-long fiber cavity attributed by the equal phase cavity Manuscript received April 4, 2018; accepted April 15, 2018. Date of publication April 18, 2018; date of current version April 30, 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, D. Zhou, 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];
[email protected]). Y. Wang is with the Institute of Optoelectronic Technology, China Jiliang University, Hangzhou 310018, China (e-mail:
[email protected]). Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LPT.2018.2828096
modes, unless the mode suppression elements are introduced to remove multimode frequency, and hence the single-frequency operation for each Stokes channel is hardly achieved. Recently, distributed Rayleigh scattering derived from refractive index inhomogeneity in optical fiber has been implemented to provide one-dimensional random feedback for Brillouin lasing oscillation [9]–[11]. In particular, randomly distributed feedbacks remove multiple longitudinal modes by introducing variable phase delay via many spatial refractive index modulation in the spacing around optical wavelength, as a result, it destroyed the fixed phase correlation between different modes, i.e. the ground for multimode formation in a cavity laser [12]. Moreover, distributed Rayleigh feedbacks could naturally act as an ultra-broadband reflector with an arbitrary phase delay for different modes to eliminate any frequency selection in conventional cavity laser with equal cavity length associated modes, and hence it is favorable to multi-wavelength generation while each component remains at the single-mode operation. Taking advantages of hybrid gain mechanisms of Raman scattering, Er-doped amplification as well as Brillouin scattering, multi-wavelength random lasers have been intensively produced [13]–[16]. However, the optical signal-to-noise ratio (OSNR) of the multi-wavelength comb was inevitably deteriorated by amplified spontaneous emission within broadband Raman/Er-doped gain profile and hence limited to ∼20 dB whilst highly coherent single-frequency operation of each wavelength component would be hardly sustained. More recently, Brillouin random lasing resonance in tens of kilometers SMF-based random cavity has been employed to generate laser comb with remarkable elevation of OSNR (>40dB) and the light coherence extension for kHz-linewidth lasing emission [17], [18]. Nevertheless, the polarization state of both pump and Stokes randomly varies since the local birefringence along SMFs is strongly influenced by external perturbation, e.g., mechanical vibration, resulting in inefficient Brillouin gain as well as the intensity and frequency instability. Furthermore, the dense random mode structure in frame of tens of kilometer SMF-based random cavity aggravates gain competition, imposing multi-wavelength channels with higher intensity fluctuation. In this work, we report a linearly polarized multi-wavelength laser comb by activating cascaded Brillouin lasing oscillation in distributed Rayleigh scattering-based composite fiber cavity. Polarization-matched multiple Brillouin random lasing resonances synchronously deliver four orders of over kHz-linewidth Stokes emission at 41.5-dB OSNR and a high
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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 30, NO. 11, JUNE 1, 2018
Fig. 2. (a) Laser comb spectrum at 1mW incident pump power and 200mW EDFA power; (b) Power discrepancy among each Stokes components.
Fig. 1. Experimental setup of linearly polarized Brillouin random fiber laser comb.
degree linear polarization with >20dB polarization extinction ratio (PER). Efficient polarization-matched Brillouin gain along compact PMF-based semi-open cavity contributed significant suppression of the relative intensity noise (RIN) for each Stokes component. II. E XPERIMENTAL S ETUP AND P RINCIPLE Figure 1 depicts the experimental setup of the Brillouin frequency comb generation. A half-open fiber ring cavity was composed of Brillouin gain medium of a 2-km-long PMF (fiber loss of 0.3 dB/km, mode field diameter of 6.5 μm @1550 nm), two polarization-maintaining (PM) circulators (PM-CIRs 1 & 2) and a 50/50 PM optical coupler (PM-OC 2). Another 500-m PMFs are incorporated through the PM-CIR2. Here, two PM-CIRs prevent the Brillouin pump resonance in the ring cavity while the PM-CIR2 blocks the ring cavity feedback but only delivers the backward Rayleigh scattered Stokes light for random feedback. Cascading Brillouin pump scheme is achieved by a sub fiber loop which recombines one part of the generated Stokes waves from the PM-OC 2 and an external pump laser as a pump comb. After the PM-OC 1, an Erbium doped fiber amplifier (EDFA) is deployed to boost the optical power of the pump comb. A polarization beam splitter (PBS) is utilized to guarantee the linear polarization of the pump comb. By using two polarization controllers (PCs 1 & 2), the polarization state of the light can be adjusted so that the Brillouin pump comb can be aligned to the slow axis of the PBS with an optimized injection power. A PM isolator is inserted before the laser comb output to prevent any undesirable Fresnel reflection upon the fiber end surface which would introduce mirror-like longitudinal cavity modes. Consequently, coherent Stokes lasing oscillation can be aroused and sustained as SBS along the gain medium of PMF effectively amplifies the Rayleigh scattered Stokes seed in each roundtrip. Ultimately, the optical spectra are monitored by an optical spectrum analyzer (OSA) (AP2043B, Apex) while polarization properties of the laser emission are characterized by a polarimeter (IPM5300, Thorlabs). Compared to SMF, PMF is a good candidate for constructing Brillouin random laser since it offers a smaller effective mode field diameter for an enhanced Brillouin gain coefficient and stress-induced transverse asymmetric refractive index nonuniformity for stronger distributed Rayleigh scattering [11].
Meanwhile, linear polarization preservation of both pump and Stokes as aligned in one principal axis of the PMF manifests an efficient polarization-matched SBS interaction against ambient perturbations. Here, a compact (∼2km) PMF-based random cavity could dilute random mode density with the alleviation of gain competition for a stabilized multiple Brillouin random laser comb [19]. III. R ESULTS AND D ISCUSSIONS With the incident pump power of 1mW and the EDFA power of 200 mW, a Brillouin laser comb with cascaded Stokes components was simultaneously emitted. As shown in Fig. 2(a), up to 4 orders of Stokes laser lines were generated at the wavelengths of 1550.057 nm, 1550.140 nm, 1550.223 nm and 1550.306 nm, respectively, which corresponds to ∼0.083 nm Brillouin wavelength upshift. The wavelength shift of each Stokes component is less than 0.001nm over one hour, which is mainly caused by the incident pump wavelength drift and Brillouin frequency shift due to ambient temperature change. The 1-hour peak power fluctuations of the 1st to the 4th order Stokes were measured as ±0.1dB, ±0.2dB, ±0.3dB and ±0.6dB over one hour, mainly resulting from the gain competition of the pump comb within the homogeneous EDFA gain in the sub-fiber loop. Apparently, the power fluctuations of each order Stokes would be subsequently accumulated from its Brillouin pump. The OSNR of the laser emission was observed as high as 41.5 dB while the spectral noise floor remained below −65 dBm thanks to highly efficient noise suppression by narrowband backward Brillouin amplification during the lasing oscillation. Additionally, the sub-fiber loop topologically separates amplified Brillouin pump comb by the resonance cavity, essentially resulting in the elimination of spontaneous emission from the broadband Er-doped gain profile propagating counterclock-wise along the fiber cavity. The manipulation of the incident pump power plays an active role in the peak power equalization among each Stokes component of the laser comb. In our experiment, the power discrepancy of the laser comb (defined as the peak power difference among 4 orders of Stokes lines) was recorded as we gradually increased the incident pump power while the EDFA output power was fixed around 200 mW. As shown in Fig. 2(b), the measured power discrepancy, ranging from 2.3 dB to 20.0 dB, exhibits a strong dependence on the incident pump power. The minimum 2.3-dB power discrepancy of the laser comb appears with the incident pump power of 1.0 mW, originating from the equalized pump comb power from the EDFA amplification. On the other hand, the incident pump
ZHANG et al.: LINEARLY POLARIZED MULTI-WAVELENGTH FIBER LASER COMB
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Fig. 4. Degree of polarization evolution of the laser comb within 1000 millisecond. 1000 sequence of the measured SOP displayed on the Poincar´e sphere. (Black spots represent the polarization state of pump.)
Fig. 3. Linewidth measurement of each Stokes laser emission. (a)–(d) depict the beating RF spectra from the 1st to the 4th order of Stokes, respectively.
power with neither lower nor higher than 1.0 mW would impose the nonuniform power distribution among the pump comb, resulting in an inevitable enlargement of power discrepancy among Stokes components. Especially, the 4th Stokes peak power with incident pump power of > 1.5 mW dramatically dropped with over 10-dB reduction compared to the 1st Stokes and even its random lasing oscillation was faded. The coherence of each Stokes random laser emission was evaluated by the delayed self-heterodyne technique-based linewidth measurement. A fiber-based Mach-Zehnder interferometer was deployed by utilizing a 200-km delay fiber in one arm and an acousto-optic modulator (AOM) with 40-MHz carrier frequency shift in another arm. Each Stokes component of the laser comb was successively selected by a 3-GHz narrowband filter and then launched into the interferometer. Afterwards, the beating signal was captured by a photodetector (PDB130C, Thorlabs) and displayed by an electrical spectrum analyzer (FSW50, R&S). As shown in Fig. 3, the 20-dB linewidth of 4 orders Stokes were measured as 29.5 kHz, 32.5 kHz, 34.5 kHz, 38.5 kHz with a contrast over 40 dB. Hence, the corresponding 3-dB linewidth of each Stokes lines were calculated as 1.5 kHz, 1.6 kHz, 1.7 kHz and 1.9 kHz, respectively. Note that, no obvious beat signals of cavity feature (∼100 kHz for 2km cavity length) were detected beside the 40-MHz central frequency, indicating that longitudinal modes in cavity-based fiber laser has been eliminated by taking advantage of distributed Rayleigh random feedback. In Brillouin random lasing resonance, the generated Stokes seed from spontaneous Brillouin scattering has an identical SOP as that of the pump with up to 100 % degree of polarization (DOP) under a high pump power [20]. Meanwhile, the Rayleigh scattered portion also keeps the same SOP as the injecting light [21]. Due to the polarization pulling of SBS [22], the efficient Brillouin gain with linearly polarized pump light (PER>25dB) along 2km PMF guarantees the amplified Stokes with an identical SOP in each of the roundtrip lasing resonance, leading to a high-DOP random laser radiation. In Fig. 4, under the incident pump light with the DOP of 99.95 %, the laser comb remained a high averaging DOP (> 99.67%) as the 1st ∼ the 4th Stokes were gradually aroused with the EDFA power increasing from 50 mW to
Fig. 5. PER of each Stokes versus EDFA power. Inset is normalized transmitted power of each Stokes versus rotating angle of the polarizer.
200 mW, although the fluctuation of the DOP was raised from 0.13 % to 0.86 % by boosting EDFA power. As shown in the inset of Fig. 4, the laser comb radiated at the identical SOP as the incident pump which was aligned to the slow axis of the PMF. The PER of each linearly polarized Stokes emission was evaluated by power transmission through a rotatable polarizer. As the EDFA power increased from 50 mW to 350 mW, the 1st –4th Stokes laser emission subsequently appeared. By gradually changing the polarizer rotation angle, the generated laser comb passed through a polarizer and then a narrowband filter (3GHz bandwidth) to assess the PER of each Stokes component. Thanks to a high-degree linear polarization preservation at slow axis of PMF as well as a strong SBS pulling effect [22], a high PER of around 25 dB was achieved in each order of Stokes lasing emission, as shown in Fig. 5. It signifies that the Brillouin frequency comb can be generated in a good linear polarization operation, albeit with up to 3-dB reduction as the 2nd and 4th Stokes lines operate around its random lasing threshold. In the inset of Fig. 5, power transmissions of each Stokes identically depend on the rotation angle of the polarizer with a fit of I = I0 cos2 θ . To characterize the RIN of the laser comb, the temporal trace of each Stokes laser emission was recorded by a photodetector (PDB130C, Thorlabs) and an oscilloscope (DS081204B, Agilent). The RINs of a commercial NP Photonics fiber laser and the 25km-SMF-based Brillouin random laser comb were measured for comparison. In Fig.6, the NP fiber laser exhibits a low RIN of < −130dB/Hz with feedback controlled RIN suppression. Due to ambient perturbations and the gain competition among different Stokes components, the multiwavelength random laser combs show a higher intensity noise.
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Fig. 6.
IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 30, NO. 11, JUNE 1, 2018
RIN comparison of each Stokes component in the laser comb.
Even though, the proposed PMF-based multi-wavelength comb exhibits a significant intensity stabilization with respect to the SMF-based laser comb by suppressing the instability from ambient mechanical vibration in the low frequency domain. Due to the polarization-sensitive SBS amplification, ambient mechanical vibration and temperature drift (bandwidth up to 102 Hz) would disturb the local birefringence along SMFs and degenerate the stability of the SBS interaction. However, the proposed all-PM laser comb with slow axis alignment of pump and Stokes light provides polarization-matched SBS interaction with the immunity to external disturbance, giving rise to a RIN suppression of 10-20dB in frequency domain of less than 102 Hz. Moreover, efficient polarization-matched gain in 2-km PMFs enable a more compact random laser cavity instead of tens-of-kilometer gain fiber length, resulting in an enlarged mode spacing in Rayleigh scattering-based random cavity and thus a significant reduction of random mode density [11]. Consequently, mode hopping-induced intensity noise in high frequency domain (103 ∼105Hz) turns out to be highly suppressed by 30-40 dB in each Stokes laser emission of PMF-based laser comb with respect to that of SMF-based laser comb. It should be noted that pump comb instability imposed by the Er-doped fiber amplification in the sub fiber loop could be alternatively optimized with inhomogeneous gain of semiconductor optical amplifier [23]. IV. C ONCLUSION To conclude, a linearly polarized multi-wavelength laser comb by Brillouin random lasing resonance in polarizationmaintaining fibers was experimentally demonstrated. With assistance of distributed Rayleigh scattering random feedback, up to 4 orders of Stokes waves were simultaneously emitting as coherent random lasing oscillation with an ultra-high OSNR of ∼41.5 dB as well as kHz narrow linewidth. Prominent intensity noise improvement was achieved by taking advantage of polarization-matched SBS interaction and random mode density suppression in PMF-based random fiber cavity, paving the way for versatile potentials in fiber communications, precision metrology, microwave/terahertz photonics and fiber sensing. R EFERENCES [1] S. Diaz, D. Leandro, and M. Lopez-Amo, “Stable multiwavelength erbium fiber ring laser with optical feedback for remote sensing,” J. Lightw. Technol., vol. 33, no. 12, pp. 2439–2444, Jun. 15, 2015.
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