- Email: [email protected]
Elimination of low-frequency fluctuations of backscattered Rayleigh radiation from optical fiber with chaotic lasers
Optical Fiber Technology 17 (2011) 258–261
Contents lists available at ScienceDirect
Optical Fiber Technology www.elsevier.com/locate/yofte
Elimination of low-frequency fluctuations of backscattered Rayleigh radiation from optical fiber with chaotic lasers V.V. Spirin a,⇑, C.A. López-Mercado a, S.V. Miridonov a, L. Cardoza-Avendaño b, R.M. López-Gutiérrez b, C. Cruz-Hernández a a b
Scientific Research and Advanced Studies Center of Ensenada (CICESE), Carretera Ensenada-Tijuana No. 3918, Zona Playitas, 22860 Ensenada, B.C., Mexico Engineering Faculty, Baja California Autonomous University (UABC), Km 103 Carretera Tijuana-Ensenada, 22860 Ensenada, B.C., Mexico
a r t i c l e
i n f o
Article history: Received 17 November 2010 Revised 9 February 2011 Available online 21 March 2011 Keywords: Chaotic laser Rayleigh scattering Stimulated Brillouin scattering Optical fiber
a b s t r a c t We have experimentally demonstrated suppression of low-frequency fluctuations of backscattered Rayleigh radiation from optical fiber with chaotic single-longitudinal mode DFB and multi-longitudinal mode FP lasers subjected by incoherent optical feedback. Significant decreasing of Rayleigh power variations up to 15–20 dB for 10–1000 Hz frequency interval was recorded for both chaotic lasers. It was shown that chaotic DFB laser also efficiently suppresses stimulated Brillouin scattering in the test fiber. Ó 2011 Elsevier Inc. All rights reserved.
1. Introduction Parameters of backscattered Rayleigh radiation are very important for optical time domain reflectometry (OTDR) [1], optical frequency domain reflectometry (OFDR) [2,3], and transmission–reflection analysis (TRA) [4] systems. Unpredicted power fluctuations of Rayleigh radiation from optical fiber can significantly degrade the systems performance. For example, localization of loss induced perturbation with the TRA technique depends on correctness of Rayleigh mean power measurement [5]. Therefore, variations of Rayleigh power, especially within low-frequency range which are difficult to eradicate by averaging, can significantly diminish the accuracy of the localization with the TRA method. Backscattered Rayleigh power from the optical fiber becomes noisy because of coherent interference effects. Rayleigh intensity fluctuation which is sometimes called ‘‘Rayleigh coherent speckle noise’’ [1] is decreasing with decline of source coherence. However, the effect can take place even for the lasers with low coherence because any extremely weak reflections from a fiber setup can affect the radiation of a diode laser without built-in optical isolator. As well known extremely weak reflections or even Rayleigh backscattering itself is able to increase the short time coherence length of the diode laser that in turn leads to low-frequency variations of backscattered Rayleigh power [6].
⇑ Corresponding author. Fax: +52 646 1750554. E-mail address: [email protected] (V.V. Spirin). 1068-5200/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.yofte.2011.02.010
Chaotic lasers are very promising candidates for elimination of the low-frequency Rayleigh fluctuations from the optical fiber because they have low coherence length and are not sensitive to any additional weak reflections. Chaos research, especially in semiconductor laser systems, is developing rapidly and is expected to produce fruitful results not only for the fundamental research of chaos but also for applications [7–11]. Most of experimental attempts for chaotification of semiconductor lasers were based on the optical feedback systems due to their rich nonlinear dynamics [7]. In this paper, we demonstrate the elimination of the low-frequency variations of the backscattered Rayleigh radiation from the optical fiber with DFB and FP chaotic lasers subjected by strong incoherent optical feedback.
2. Experimental results and discussion The experimental setup for semiconductor lasers chaotification and backscattered Rayleigh power measurement is shown in Fig. 1. We study two fiber-pigtailed semiconductor lasers without buildin optical isolators, namely single-longitudinal mode DFB laser and multi-longitudinal mode FP laser. DFB laser emitted continuouswave (CW) light with power of 2 mW at a wavelength equal to 1552.6 nm with spectral linewidth about 0.01 nm. Multi-longitudinal FP laser emitted CW light within 1545–1547 nm interval with separation between modes roughly equal to 0.07 nm. Both lasers operate quite far from their thresholds. Operating currents exceed the threshold value by 70% in the DFB laser (IDFB = 1.7 Ith) and by 50% (IFP = 1.5 Ith) in the FP laser, respectively. The output power of
259
V.V. Spirin et al. / Optical Fiber Technology 17 (2011) 258–261
(a)
(b)
Fig. 1. Experimental setup.
(c)
the DFB or FP laser is transmitted through a polarization controller (PC) and divided between two optical arms by a 90/10 coupler. Polarization controller allows aligning the back-reflected polarization with an initial laser polarization in order to maximize the feedback efficiency [12]. The main part of the radiation passes the Grin Fiber Collimator (GFC) with an antireflection coating and is reflected back by a metallic mirror. The reflected power is continuously monitored by a power meter (see Fig. 1). The power reflected back to the laser cavity was controlled by tilting the mirror. As we have found, this technique allows adjusting the feedback strength without a change of the polarization state of the reflected light. The feedback strength is defined here as the ratio between the reflected back power (PR) and output power of the laser (P0). The mirror tilting varies the optical feedback strength PR/P0 from 0% to 40%. The coherence length specified by datasheets for both free-running lasers is significantly shorter than two way distance between the laser and the mirror which is equal to 6 meters, approximately. Therefore, the optical feedback is effectively incoherent for both lasers. The external optical feedback destabilizes the semiconductor laser and induces chaotic oscillations of the output laser power. The chaotic radiation is passed through an optical isolator (OI), amplified by an optical amplifier (EDFA) and launched into a 5km length optical fiber through an optical circulator (OC). Transmitted through the optical fiber signal is measured by a 5-GHz photodetector (PD-1) and recorded by a spectrum analyzer with a bandwidth of 3 GHz. Back-reflected Rayleigh radiation power was analyzed with a 1-GHz photodetector (PD-2) and by an optical spectrum analyzer. Fig. 2 shows RF power spectrum of the DFB laser radiation transmitted through the optical fiber. For the feedback with the strength exceeding 3% RF spectra have a flat profile without strong
Fig. 3. RF power spectra of Rayleigh backscattered power for DFB laser: (a) without external feedback, (b) s – 40% feedback strength, and (c) d – noise floor without input optical signal.
peaks up to 3 GHz. This form of the RF spectrum is usually attributed to the presence of chaotic oscillations in the DFB laser [7]. Fig. 3 presents the RF spectrum of the Rayleigh backscattered power within the frequency range of 10–4000 Hz. In the chaotic regime with the feedback strength more than 3% significant reduction of fluctuations in Rayleigh signal was recorded. For low frequencies below 1 kHz, the spectral power density of the backscattered signal in the chaotic regime decreases by 15–20 dB from that one for the free-running DFB laser without an additional feedback. Power spectrum for the chaotic laser almost coincides with a noise floor level recorded without the optical signal. As shown in Fig. 4, with free-running DFB laser the most of the fluctuations in the Rayleigh signal are concentrated below 50 kHz. Elimination of this noise by averaging in many applications can occupy an enormous time. Use of chaotic laser can effectively solve this problem and improve the accuracy of measurements, for example, in TRA-based fiber-optic sensors [4,5]. Throughout the measurement of the power variations of Rayleigh scattering, an optical spectrum of the backscattered light was continuously monitored with an optical spectrum analyzer (see Fig. 1) to maintain the optical power below a threshold of a stimulated Brillouin scattering (SBS) in the test fiber. However, we have found an important feature of the laser chaotification: besides the decrease of the low-frequency noise in the Rayleigh backscattered signal, it also efficiently suppresses SBS in the test fiber. Indeed, for our free-running DFB laser without any external feed-
−85
−50 −90
Rayleigh Power, dBm
Transmitted Power, dBm
(a) −60
−70
−80
−90
(a)
−100 (b)
−110
(b)
(d)
−120
−100
(c)
0.5
1
1.5
2
2.5
3
20
40
60
80
100
Frequency, KHz
Frequency, GHz Fig. 2. (a) RF power spectrum of transmitted radiation of DFB laser with 40% feedback strength and (b) d – noise floor without input optical signal.
Fig. 4. RF power spectra of Rayleigh backscattered power for DFB laser: (a) without external feedback, (b) 0.4% feedback strength, (c) s – 40% feedback strength, and (d) d – noise floor without input optical signal.
260
V.V. Spirin et al. / Optical Fiber Technology 17 (2011) 258–261
(a)
−40
−90
Transmitted Power, dBm
Backscattered Power, dBm
−85
−95 −100 (b)
−105 −110 0
(c)
5
10
15
20
25
−50 −60
(a) −70 −80 −90
30
(b)
Frequency, MHz −100
0.5
Fig. 5. RF backscattered power spectra for 40 mW input power: (a) without external feedback, with Brillouin scattering, (b) s – 40% feedback strength, suppressed SBS, and (c) d – noise floor without input optical signal.
1
1.5
2
2.5
3
Frequency, GHz Fig. 6. (a) RF power spectrum of transmitted radiation of FP laser with 40% feedback strength and (b) d – noise floor without input optical signal.
Dm p 1þ Dm B where Dmp is the chaotic pump laser linewidth and DmB is full width at half maximum of the Brillouin gain spectrum which is approximately equal to 40 MHz for silica-based fiber [17]. Experimentally measured SBS threshold with the chaotic DFB laser (for the feedback strength greater than 3%) at least 10–15 times exceeds the threshold found with the free-running laser. Therefore, the chaotic DFB laser efficiently suppresses Rayleigh backscattered power variations and also restrains SBS in the test fiber. Qualitatively the same results for suppression of the low-frequency variations of the Rayleigh backscattered radiation were obtained for the chaotic Fabry Perot laser without built-in optical isolator. Fig. 6 presents RF power spectrum of the multi-longitudinal mode FP laser with the strong external feedback. For the feedback strength equal to 40% the RF power spectral density is uniform up to 3 GHz and by more than 30 dB exceeds the spectral density when the external feedback is absent. The plane-shape RF spectrum attributed to the chaotic oscillations is registered for the FP laser with the feedback stronger than 4% that is comparable with 3% value for the DFB laser. Contrary to the DFB laser, for the multimode FP laser relatively large oscillations were detected in the spectral power density. Fig. 7 presents the RF power spectrum of the Rayleigh backscattered power for the FP laser within the frequency interval 10– 4000 Hz. The spectral power density of this signal with the chaotic laser decreases by 15–20 dB in comparison with the free-running FP laser and almost coincides with the noise floor of the measurement system. The SBS threshold was never reached in our experiments with the chaotic multi-longitudinal FP laser. However, noticeable power
−75 −80 (a)
Rayleigh Power, dBm
back the stable SBS from 5-km length test fiber was detected for the input power above 8–10 mW. Fig. 5a presents the RF power spectrum of backscattered light for the input power of 40 mW which is significantly greater than SBS threshold. The fluctuations of the backscattered optical power with Brillouin scattering occupy the frequency range up to 25 MHz (see Fig. 5a). However, the power spectrum of the backscattered signal for the chaotic laser with 40% feedback strength almost coincides with the noise floor level (see Fig. 5b). It means that chaotic DFB laser efficiently restrains SBS in the test fiber. As well known, stimulated Brillouin, Raman and Rayleigh backscattering processes can demonstrate complicated cooperative behavior in optical fiber [13–15]. On the other hand, chaotic oscillations of the laser radiation cause widening of the laser linewidth and increase the Brillouin threshold [16] by factor:
−90 (b)
−100
−110
(c)
−120 (d)
−130
1000
2000
3000
4000
Frequency, Hz Fig. 7. RF power spectra of Rayleigh backscattered power for FP laser: (a) and (b) without external feedback, (c) s – 40% feedback strength, and (d) d – noise floor without input optical signal.
variations (up to 10 dB) between the longitudinal modes due to weak randomly altering reflections potentially may lead to nearly single-longitudinal mode operation of the FP laser and therefore, reduce the SBS threshold. Thus, more detailed analysis for understanding of the SBS influence on the backscattered power variations with the FP laser is required.
3. Conclusion We have experimentally demonstrated suppression of the low-frequency variations of the backscattered Rayleigh power from the optical fiber with chaotic single-longitudinal mode DFB and multi-longitudinal mode FP lasers subjected by incoherent optical feedback. Significant decreasing of the Rayleigh power variations up to 15–20 dB inside 10–1000 Hz frequency interval was recorded for both chaotic lasers. Additionally, it was shown that stimulated Brillouin scattering is also suppressed with chaotic DFB laser.
Acknowledgments This work was supported by the CONACYT, México under Research Grants J49593-Y, P50051-Y, and SEMARNAT-23770.
V.V. Spirin et al. / Optical Fiber Technology 17 (2011) 258–261
References [1] W.V. Sorin, Optical reflectometry for component characterization, in: D. Derickson (Ed.), Fiber Optic Test and Measurement, Prentice-Hall, PTR, 1998, pp. 383–433. [2] S.G. Pierce, A. MacLean, B. Culshaw, Optical frequency-domain reflectometry for microbend sensor demodulation, Appl. Opt. 39 (2000) 4569–4581. [3] R.M. López, V.V. Spirin, M.G. Shlyagin, S.V. Miridonov, G. Beltrán, E.A. Kuzin, A. Márquez, Coherent optical frequency domain reflectometry for interrogation of bend-based fiber optic hydrocarbon sensors, Opt. Fiber Technol. 28 (2004) 79–90. [4] V.V. Spirin, F.J. Mendieta, S.V. Miridonov, M.G. Shlyagin, A.A. Chtcherbakov, P.L. Swart, Localization of a loss-inducing perturbation with variable accuracy along a test fiber using transmission–reflection analysis, IEEE Photon. Technol. Lett. 16 (2004) 569–571. [5] V.V. Spirin, Autonomous measurement system for localization of loss-induced perturbation based on transmission–reflection analysis, in: Advances in Measurement Systems, In-Tech, 2010, pp. 81–104. [6] P. Gysel, R.K. Staubli, Spectral properties of Rayleigh backscattered light from single-mode fibers caused by a modulated probe signal, J. Lightw. Technol. 8 (1990) 1792–1798. [7] J. Othsubo, Semiconductor Laser, Stability, Instability and Chaos, second ed., enlarged ed., Springer-Verlag, Berlin Heidelberg, 2008. [8] Y. Wang, B. Wang, A. Wang, Chaotic correlation optical time domain reflectometer utilizing laser diode, IEEE Photon. Technol. Lett. 20 (2008) 1636–1638.
261
[9] R.M. López-Gutiérrez, C. Posadas-Castillo, D. López-Mancilla, C. CruzHernández, Communicating via robust synchronization of chaotic lasers, Chaos Solit. Fract. 42 (2009) 277–285. [10] C. Posadas-Castillo, R.M. López-Gutiérrez, C. Cruz-Hernández, Synchronization of chaotic solid-state Nd:YAG lasers: application to secure communication, Commun. Nonlin. Sci. Numer. Simul. 13 (2008) 1655–1667. [11] P. Li, Y.C. Wang, J.Z. Zhang, All-optical fast random number generator, Opt. Exp. 18 (2010) 20360–20369. [12] A. Wang, Y. Wang, H. He, Enhancing the bandwidth of the optical chaotic signal generated by a semiconductor laser with optical feedback, IEEE Photon. Technol. Lett. 20 (2008) 1633–1635. [13] S.K. Turitsyn, S.A. Babin, A.E. El-Taher, P. Harper, D.V. Churkin, S.I. Kablukov, J.D. Ania-Castañón, V. Karalekas, E.V. Podivilov, Random distributed feedback fibre laser, Nat. Photon. 4 (2010) 231–235. [14] A.A. Fotiadi, Random lasers: an incoherent fibre laser, Nat. Photon. 4 (2010) 204–205. [15] A.A. Fotiadi, R.V. Kiyan, Cooperative stimulated Brillouin and Rayleigh backscattering process in optical fiber, Opt. Lett. 23 (1998) 805–1807. [16] E. Lichtman, A.A. Friesem, R.G. Waarts, H.H. Yaffe, Stimulated Brillouin scattering excited by two pump waves in single-mode fibers, J. Opt. Soc. Am. B 4 (1987) 1397–1403. [17] M. Niklés, L. Thévenaz, P.A. Robert, Brillouin gain spectrum characterization in single-mode optical fibers, J. Lightw. Technol. 15 (1997) 1842–1851.
Contents lists available at ScienceDirect
Optical Fiber Technology www.elsevier.com/locate/yofte
Elimination of low-frequency fluctuations of backscattered Rayleigh radiation from optical fiber with chaotic lasers V.V. Spirin a,⇑, C.A. López-Mercado a, S.V. Miridonov a, L. Cardoza-Avendaño b, R.M. López-Gutiérrez b, C. Cruz-Hernández a a b
Scientific Research and Advanced Studies Center of Ensenada (CICESE), Carretera Ensenada-Tijuana No. 3918, Zona Playitas, 22860 Ensenada, B.C., Mexico Engineering Faculty, Baja California Autonomous University (UABC), Km 103 Carretera Tijuana-Ensenada, 22860 Ensenada, B.C., Mexico
a r t i c l e
i n f o
Article history: Received 17 November 2010 Revised 9 February 2011 Available online 21 March 2011 Keywords: Chaotic laser Rayleigh scattering Stimulated Brillouin scattering Optical fiber
a b s t r a c t We have experimentally demonstrated suppression of low-frequency fluctuations of backscattered Rayleigh radiation from optical fiber with chaotic single-longitudinal mode DFB and multi-longitudinal mode FP lasers subjected by incoherent optical feedback. Significant decreasing of Rayleigh power variations up to 15–20 dB for 10–1000 Hz frequency interval was recorded for both chaotic lasers. It was shown that chaotic DFB laser also efficiently suppresses stimulated Brillouin scattering in the test fiber. Ó 2011 Elsevier Inc. All rights reserved.
1. Introduction Parameters of backscattered Rayleigh radiation are very important for optical time domain reflectometry (OTDR) [1], optical frequency domain reflectometry (OFDR) [2,3], and transmission–reflection analysis (TRA) [4] systems. Unpredicted power fluctuations of Rayleigh radiation from optical fiber can significantly degrade the systems performance. For example, localization of loss induced perturbation with the TRA technique depends on correctness of Rayleigh mean power measurement [5]. Therefore, variations of Rayleigh power, especially within low-frequency range which are difficult to eradicate by averaging, can significantly diminish the accuracy of the localization with the TRA method. Backscattered Rayleigh power from the optical fiber becomes noisy because of coherent interference effects. Rayleigh intensity fluctuation which is sometimes called ‘‘Rayleigh coherent speckle noise’’ [1] is decreasing with decline of source coherence. However, the effect can take place even for the lasers with low coherence because any extremely weak reflections from a fiber setup can affect the radiation of a diode laser without built-in optical isolator. As well known extremely weak reflections or even Rayleigh backscattering itself is able to increase the short time coherence length of the diode laser that in turn leads to low-frequency variations of backscattered Rayleigh power [6].
⇑ Corresponding author. Fax: +52 646 1750554. E-mail address: [email protected] (V.V. Spirin). 1068-5200/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.yofte.2011.02.010
Chaotic lasers are very promising candidates for elimination of the low-frequency Rayleigh fluctuations from the optical fiber because they have low coherence length and are not sensitive to any additional weak reflections. Chaos research, especially in semiconductor laser systems, is developing rapidly and is expected to produce fruitful results not only for the fundamental research of chaos but also for applications [7–11]. Most of experimental attempts for chaotification of semiconductor lasers were based on the optical feedback systems due to their rich nonlinear dynamics [7]. In this paper, we demonstrate the elimination of the low-frequency variations of the backscattered Rayleigh radiation from the optical fiber with DFB and FP chaotic lasers subjected by strong incoherent optical feedback.
2. Experimental results and discussion The experimental setup for semiconductor lasers chaotification and backscattered Rayleigh power measurement is shown in Fig. 1. We study two fiber-pigtailed semiconductor lasers without buildin optical isolators, namely single-longitudinal mode DFB laser and multi-longitudinal mode FP laser. DFB laser emitted continuouswave (CW) light with power of 2 mW at a wavelength equal to 1552.6 nm with spectral linewidth about 0.01 nm. Multi-longitudinal FP laser emitted CW light within 1545–1547 nm interval with separation between modes roughly equal to 0.07 nm. Both lasers operate quite far from their thresholds. Operating currents exceed the threshold value by 70% in the DFB laser (IDFB = 1.7 Ith) and by 50% (IFP = 1.5 Ith) in the FP laser, respectively. The output power of
259
V.V. Spirin et al. / Optical Fiber Technology 17 (2011) 258–261
(a)
(b)
Fig. 1. Experimental setup.
(c)
the DFB or FP laser is transmitted through a polarization controller (PC) and divided between two optical arms by a 90/10 coupler. Polarization controller allows aligning the back-reflected polarization with an initial laser polarization in order to maximize the feedback efficiency [12]. The main part of the radiation passes the Grin Fiber Collimator (GFC) with an antireflection coating and is reflected back by a metallic mirror. The reflected power is continuously monitored by a power meter (see Fig. 1). The power reflected back to the laser cavity was controlled by tilting the mirror. As we have found, this technique allows adjusting the feedback strength without a change of the polarization state of the reflected light. The feedback strength is defined here as the ratio between the reflected back power (PR) and output power of the laser (P0). The mirror tilting varies the optical feedback strength PR/P0 from 0% to 40%. The coherence length specified by datasheets for both free-running lasers is significantly shorter than two way distance between the laser and the mirror which is equal to 6 meters, approximately. Therefore, the optical feedback is effectively incoherent for both lasers. The external optical feedback destabilizes the semiconductor laser and induces chaotic oscillations of the output laser power. The chaotic radiation is passed through an optical isolator (OI), amplified by an optical amplifier (EDFA) and launched into a 5km length optical fiber through an optical circulator (OC). Transmitted through the optical fiber signal is measured by a 5-GHz photodetector (PD-1) and recorded by a spectrum analyzer with a bandwidth of 3 GHz. Back-reflected Rayleigh radiation power was analyzed with a 1-GHz photodetector (PD-2) and by an optical spectrum analyzer. Fig. 2 shows RF power spectrum of the DFB laser radiation transmitted through the optical fiber. For the feedback with the strength exceeding 3% RF spectra have a flat profile without strong
Fig. 3. RF power spectra of Rayleigh backscattered power for DFB laser: (a) without external feedback, (b) s – 40% feedback strength, and (c) d – noise floor without input optical signal.
peaks up to 3 GHz. This form of the RF spectrum is usually attributed to the presence of chaotic oscillations in the DFB laser [7]. Fig. 3 presents the RF spectrum of the Rayleigh backscattered power within the frequency range of 10–4000 Hz. In the chaotic regime with the feedback strength more than 3% significant reduction of fluctuations in Rayleigh signal was recorded. For low frequencies below 1 kHz, the spectral power density of the backscattered signal in the chaotic regime decreases by 15–20 dB from that one for the free-running DFB laser without an additional feedback. Power spectrum for the chaotic laser almost coincides with a noise floor level recorded without the optical signal. As shown in Fig. 4, with free-running DFB laser the most of the fluctuations in the Rayleigh signal are concentrated below 50 kHz. Elimination of this noise by averaging in many applications can occupy an enormous time. Use of chaotic laser can effectively solve this problem and improve the accuracy of measurements, for example, in TRA-based fiber-optic sensors [4,5]. Throughout the measurement of the power variations of Rayleigh scattering, an optical spectrum of the backscattered light was continuously monitored with an optical spectrum analyzer (see Fig. 1) to maintain the optical power below a threshold of a stimulated Brillouin scattering (SBS) in the test fiber. However, we have found an important feature of the laser chaotification: besides the decrease of the low-frequency noise in the Rayleigh backscattered signal, it also efficiently suppresses SBS in the test fiber. Indeed, for our free-running DFB laser without any external feed-
−85
−50 −90
Rayleigh Power, dBm
Transmitted Power, dBm
(a) −60
−70
−80
−90
(a)
−100 (b)
−110
(b)
(d)
−120
−100
(c)
0.5
1
1.5
2
2.5
3
20
40
60
80
100
Frequency, KHz
Frequency, GHz Fig. 2. (a) RF power spectrum of transmitted radiation of DFB laser with 40% feedback strength and (b) d – noise floor without input optical signal.
Fig. 4. RF power spectra of Rayleigh backscattered power for DFB laser: (a) without external feedback, (b) 0.4% feedback strength, (c) s – 40% feedback strength, and (d) d – noise floor without input optical signal.
260
V.V. Spirin et al. / Optical Fiber Technology 17 (2011) 258–261
(a)
−40
−90
Transmitted Power, dBm
Backscattered Power, dBm
−85
−95 −100 (b)
−105 −110 0
(c)
5
10
15
20
25
−50 −60
(a) −70 −80 −90
30
(b)
Frequency, MHz −100
0.5
Fig. 5. RF backscattered power spectra for 40 mW input power: (a) without external feedback, with Brillouin scattering, (b) s – 40% feedback strength, suppressed SBS, and (c) d – noise floor without input optical signal.
1
1.5
2
2.5
3
Frequency, GHz Fig. 6. (a) RF power spectrum of transmitted radiation of FP laser with 40% feedback strength and (b) d – noise floor without input optical signal.
Dm p 1þ Dm B where Dmp is the chaotic pump laser linewidth and DmB is full width at half maximum of the Brillouin gain spectrum which is approximately equal to 40 MHz for silica-based fiber [17]. Experimentally measured SBS threshold with the chaotic DFB laser (for the feedback strength greater than 3%) at least 10–15 times exceeds the threshold found with the free-running laser. Therefore, the chaotic DFB laser efficiently suppresses Rayleigh backscattered power variations and also restrains SBS in the test fiber. Qualitatively the same results for suppression of the low-frequency variations of the Rayleigh backscattered radiation were obtained for the chaotic Fabry Perot laser without built-in optical isolator. Fig. 6 presents RF power spectrum of the multi-longitudinal mode FP laser with the strong external feedback. For the feedback strength equal to 40% the RF power spectral density is uniform up to 3 GHz and by more than 30 dB exceeds the spectral density when the external feedback is absent. The plane-shape RF spectrum attributed to the chaotic oscillations is registered for the FP laser with the feedback stronger than 4% that is comparable with 3% value for the DFB laser. Contrary to the DFB laser, for the multimode FP laser relatively large oscillations were detected in the spectral power density. Fig. 7 presents the RF power spectrum of the Rayleigh backscattered power for the FP laser within the frequency interval 10– 4000 Hz. The spectral power density of this signal with the chaotic laser decreases by 15–20 dB in comparison with the free-running FP laser and almost coincides with the noise floor of the measurement system. The SBS threshold was never reached in our experiments with the chaotic multi-longitudinal FP laser. However, noticeable power
−75 −80 (a)
Rayleigh Power, dBm
back the stable SBS from 5-km length test fiber was detected for the input power above 8–10 mW. Fig. 5a presents the RF power spectrum of backscattered light for the input power of 40 mW which is significantly greater than SBS threshold. The fluctuations of the backscattered optical power with Brillouin scattering occupy the frequency range up to 25 MHz (see Fig. 5a). However, the power spectrum of the backscattered signal for the chaotic laser with 40% feedback strength almost coincides with the noise floor level (see Fig. 5b). It means that chaotic DFB laser efficiently restrains SBS in the test fiber. As well known, stimulated Brillouin, Raman and Rayleigh backscattering processes can demonstrate complicated cooperative behavior in optical fiber [13–15]. On the other hand, chaotic oscillations of the laser radiation cause widening of the laser linewidth and increase the Brillouin threshold [16] by factor:
−90 (b)
−100
−110
(c)
−120 (d)
−130
1000
2000
3000
4000
Frequency, Hz Fig. 7. RF power spectra of Rayleigh backscattered power for FP laser: (a) and (b) without external feedback, (c) s – 40% feedback strength, and (d) d – noise floor without input optical signal.
variations (up to 10 dB) between the longitudinal modes due to weak randomly altering reflections potentially may lead to nearly single-longitudinal mode operation of the FP laser and therefore, reduce the SBS threshold. Thus, more detailed analysis for understanding of the SBS influence on the backscattered power variations with the FP laser is required.
3. Conclusion We have experimentally demonstrated suppression of the low-frequency variations of the backscattered Rayleigh power from the optical fiber with chaotic single-longitudinal mode DFB and multi-longitudinal mode FP lasers subjected by incoherent optical feedback. Significant decreasing of the Rayleigh power variations up to 15–20 dB inside 10–1000 Hz frequency interval was recorded for both chaotic lasers. Additionally, it was shown that stimulated Brillouin scattering is also suppressed with chaotic DFB laser.
Acknowledgments This work was supported by the CONACYT, México under Research Grants J49593-Y, P50051-Y, and SEMARNAT-23770.
V.V. Spirin et al. / Optical Fiber Technology 17 (2011) 258–261
References [1] W.V. Sorin, Optical reflectometry for component characterization, in: D. Derickson (Ed.), Fiber Optic Test and Measurement, Prentice-Hall, PTR, 1998, pp. 383–433. [2] S.G. Pierce, A. MacLean, B. Culshaw, Optical frequency-domain reflectometry for microbend sensor demodulation, Appl. Opt. 39 (2000) 4569–4581. [3] R.M. López, V.V. Spirin, M.G. Shlyagin, S.V. Miridonov, G. Beltrán, E.A. Kuzin, A. Márquez, Coherent optical frequency domain reflectometry for interrogation of bend-based fiber optic hydrocarbon sensors, Opt. Fiber Technol. 28 (2004) 79–90. [4] V.V. Spirin, F.J. Mendieta, S.V. Miridonov, M.G. Shlyagin, A.A. Chtcherbakov, P.L. Swart, Localization of a loss-inducing perturbation with variable accuracy along a test fiber using transmission–reflection analysis, IEEE Photon. Technol. Lett. 16 (2004) 569–571. [5] V.V. Spirin, Autonomous measurement system for localization of loss-induced perturbation based on transmission–reflection analysis, in: Advances in Measurement Systems, In-Tech, 2010, pp. 81–104. [6] P. Gysel, R.K. Staubli, Spectral properties of Rayleigh backscattered light from single-mode fibers caused by a modulated probe signal, J. Lightw. Technol. 8 (1990) 1792–1798. [7] J. Othsubo, Semiconductor Laser, Stability, Instability and Chaos, second ed., enlarged ed., Springer-Verlag, Berlin Heidelberg, 2008. [8] Y. Wang, B. Wang, A. Wang, Chaotic correlation optical time domain reflectometer utilizing laser diode, IEEE Photon. Technol. Lett. 20 (2008) 1636–1638.
261
[9] R.M. López-Gutiérrez, C. Posadas-Castillo, D. López-Mancilla, C. CruzHernández, Communicating via robust synchronization of chaotic lasers, Chaos Solit. Fract. 42 (2009) 277–285. [10] C. Posadas-Castillo, R.M. López-Gutiérrez, C. Cruz-Hernández, Synchronization of chaotic solid-state Nd:YAG lasers: application to secure communication, Commun. Nonlin. Sci. Numer. Simul. 13 (2008) 1655–1667. [11] P. Li, Y.C. Wang, J.Z. Zhang, All-optical fast random number generator, Opt. Exp. 18 (2010) 20360–20369. [12] A. Wang, Y. Wang, H. He, Enhancing the bandwidth of the optical chaotic signal generated by a semiconductor laser with optical feedback, IEEE Photon. Technol. Lett. 20 (2008) 1633–1635. [13] S.K. Turitsyn, S.A. Babin, A.E. El-Taher, P. Harper, D.V. Churkin, S.I. Kablukov, J.D. Ania-Castañón, V. Karalekas, E.V. Podivilov, Random distributed feedback fibre laser, Nat. Photon. 4 (2010) 231–235. [14] A.A. Fotiadi, Random lasers: an incoherent fibre laser, Nat. Photon. 4 (2010) 204–205. [15] A.A. Fotiadi, R.V. Kiyan, Cooperative stimulated Brillouin and Rayleigh backscattering process in optical fiber, Opt. Lett. 23 (1998) 805–1807. [16] E. Lichtman, A.A. Friesem, R.G. Waarts, H.H. Yaffe, Stimulated Brillouin scattering excited by two pump waves in single-mode fibers, J. Opt. Soc. Am. B 4 (1987) 1397–1403. [17] M. Niklés, L. Thévenaz, P.A. Robert, Brillouin gain spectrum characterization in single-mode optical fibers, J. Lightw. Technol. 15 (1997) 1842–1851.