October 1, 2014 / Vol. 39, No. 19 / OPTICS LETTERS
5713
Active control of gain saturation in fiber-optical parametric amplifier using stimulated Brillouin scattering Chaoran Huang, Xiaojie Guo, Xuelei Fu, Liang Wang, and Chester Shu* Department of Electronic Engineering and Center for Advanced Research in Photonics, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong, China *Corresponding author: [email protected] Received May 20, 2014; revised August 29, 2014; accepted September 2, 2014; posted September 4, 2014 (Doc. ID 212428); published September 29, 2014 We have demonstrated the use of stimulated Brillouin scattering (SBS) to control the gain saturation characteristics of fiber-optical parametric amplification (FOPA). The saturation can be dynamically deferred or expedited without changing the input FOPA pump and signal parameters. It is observed that the input signal power at saturation can be increased or decreased by up to 6 dB. Experiments are performed to further investigate the role of the SBS-induced nonlinear phase on the power transfer efficiency in the FOPA. © 2014 Optical Society of America OCIS codes: (060.4370) Nonlinear optics, fibers; (290.5900) Scattering, stimulated Brillouin; (190.4410) Nonlinear optics, parametric processes. http://dx.doi.org/10.1364/OL.39.005713
Fiber optical parametric amplifiers (FOPAs) are important for various applications such as low-noise amplification and all-optical signal processing [1]. A unique characteristic of FOPAs is the instantaneous gain saturation that depends strongly on the pump power [2]. With a strong pump, the amplifier saturates at relatively low input signal power. This will limit the maximum input for linear amplification purpose. Conversely, FOPA saturation can be utilized for signal regeneration. The saturation originates from pump depletion and phase mismatch among the interplaying fields [3]. To defer the saturation, Raman-assisted FOPA was recently proposed to compensate the depleting pump power via Raman amplification [4]. Quasi-phase-matching FOPA was also proposed to compensate the phase mismatch by modifying the relative phase periodically [5]. However, these techniques offer limited flexibility in operation. The different requirements of amplification and regeneration can hardly be satisfied by a single scheme. Noteworthy, stimulated Brillouin scattering (SBS) has previously been used for phase matching control in fourwave mixing (FWM) by introducing a local refractive index change [6]. The SBS gain and induced phase change have also been successfully decoupled [7]. Gain-transparent SBS has subsequently been applied to enlarge the wavelength conversion bandwidth and to arbitrarily reshape the gain profile of a FOPA [7,8]. The scheme enables dynamic control of phase matching via SBS. In [9], we demonstrated that FOPA saturation can be dynamically deferred or expedited by SBS-induced phase change. The input signal power required for saturation can also be increased or decreased by up to ∼6 dB. In this Letter, the extent of pump depletion is further compared to that of the conventional FOPA. The result shows that the SBS-induced nonlinear phase change can effectively control the power transfer from the pump to the signal by varying the total phase mismatch. The change of net gain is also investigated at different frequency detunings of the Brillouin pump and Stokes wave from the Brillouin frequency shift, corresponding to different induced phase changes. 0146-9592/14/195713-04$15.00/0
The principle of gain transparent SBS has been reported in [10]. If the input signal is within the Brillouin gain bandwidth, it will experience a complex gain expressed as [11]: gb δ
g0 g0 ; 1 − iδ 1–2iωSBS − ωS − ΩB ∕ΓB
(1)
where g0 is the peak gain, ωSBS − ωS is the frequency spacing between the Brillouin pump and signal, ΩB is the Brillouin frequency shift, δ represents the normalized detuning parameter, and ΓB is the gain bandwidth. The real part of gb δ is related to SBS amplification while the imaginary part is related to induced phase shift on the signal. Figure 1 illustrates the principle of operation. When both the Brillouin pump and Stokes wave propagate in the opposite direction to the input signal, the real parts of SBS gain and loss will cancel each other. The imaginary parts will however be added to double the total induced phase shift of the signal, thus modifying the phase mismatch in the FOPA. In this work, the Brillouin
Fig. 1. Control of gain saturation in fiber-optical parametric amplifier by SBS-induced nonlinear phase. vB : Brillouin frequency shift; f RF : frequency spacing between the Brillouin pump and signal, and between the signal and Stokes wave. © 2014 Optical Society of America
5714
OPTICS LETTERS / Vol. 39, No. 19 / October 1, 2014
pump amplifies the signal, whereas the Stokes wave depletes the signal. The total phase mismatch κ should include the additional phase mismatch term induced by SBS [3]: κ Δβ γ2P pump − P signal − P idler 2jImgb δP SBS ∕Aeff j;
(2)
where Δβ is the linear phase mismatch between FOPA pump, signal, and idler and γ2P pump − P signal − P idler is the nonlinear phase mismatch. The last term represents the SBS-induced phase mismatch that can be adjusted by tuning f RF , the frequency detuning between the Brillouin pump and the signal, and between the signal and the Brillouin Stokes wave. In the absence of the Brillouin pump and Stokes wave, the FOPA amplification factor can be expressed as [11]: Gs 1 γP pump ∕g2 sinh2 gL;
(3)
where g is the parametric gain that depends on both the pump power and the phase mismatch and is defined as: g
q γP pump 2 − κ∕22 .
(4)
modulation at a frequency f RF . The carrier suppression ratio is 25 dB. The frequency sidebands are amplified to act as the Brillouin pump and Stokes wave. The bandpass filters are used to filter out the amplified spontaneous emission noise after the EDFAs. The FOPA operates in the presence of backward-propagating Brillouin pump and Stokes wave. The HNLF has a length of 1 km, a nonlinear coefficient of 11.7∕W · km, a zero-dispersion wavelength of 1549 nm, a dispersion coefficient of 0.02 ps∕nm·km, a dispersion slope of 0.019 ps∕ nm2 ·km at ∼1550 nm, and a Brillouin frequency shift of 9.58 GHz. The FOPA pump and signals are monitored by an optical spectrum analyzer (OSA). Several polarization controllers (PC) are used in the setup to ensure the maximum performances of FWM and SBS. We first investigate the control of saturation by gaintransparent SBS at different FOPA pump powers. The FOPA pump is located at 1555 nm, and the power is first set at 26.5 dBm. The signal is located at the FOPA gain peak of 1545 nm. The power of the Brillouin pump and Stokes wave is fixed at 12 dBm, which maximizes the tuning range of the phase shift and meanwhile avoids the back-reflected SBS noise. Figure 3(a) shows the gain saturation behaviors. For a conventional FOPA, the maximum output power reaches 12.6 dBm at an input
The nonlinear phase mismatch term in Eq. (2) decreases with pump depletion and signal amplification along the fiber. When the SBS-induced phase mismatch term is positive, parametric power transfer is enhanced to defer the gain saturation. Conversely, a negative SBSinduced phase mismatch term will increase the total phase mismatch and expedite the saturation. Thus, one can dynamically control the saturation properties of FOPA to suit different purposes. The experimental setup is shown in Fig. 2. The output of a tunable laser (TL1) is spectrally broadened by phase modulation with 10 Gb/s 27 –1 pseudo-random binary sequence (PRBS) to increase the SBS threshold. It is then amplified to serve as the FOPA pump. Another tunable laser (TL2) is split into two branches. The upper branch serves as the FOPA signal. It is launched into the highly nonlinear fiber (HNLF) with the FOPA pump. The lower branch undergoes carrier-suppressed electro-optic
Fig. 2. Experimental setup on dynamic control of FOPA gain saturation. TL, tunable laser; PM, phase modulator; EDFA, erbium-doped fiber amplifier; EOM, electro-optic intensity modulator; BPF, bandpass filter; ISO, isolator; OSA, optical spectrum analyzer; HNLF, highly nonlinear fiber; PRBS, pseudorandom binary sequence.
Fig. 3. Output signal power against input signal power at (a) 26.5 dBm FOPA input pump, f RF 9.558 GHz and (b) 25.2 dBm FOPA input pump, f RF 9.675 GHz. Insets show the variation of signal power upon the application of gaintransparent SBS.
October 1, 2014 / Vol. 39, No. 19 / OPTICS LETTERS
signal power of −13 dBm. The output power drops quickly as the input signal further increases. In comparison, with gain-transparent SBS operated at f RF 9.558 GHz, the maximum output is increased by 1.2 dB and is obtained at a stronger input signal power of −7 dBm. It is important to ensure that the gain improvement is not caused by direct SBS amplification. We thus measure the power variation, i.e., the difference between the input and output powers, caused by the Brillouin pump and Stokes wave in the absence of FOPA pump. The result is displayed in the inset of Fig. 3(a). It is observed that the signal power remains unchanged until ∼6 dBm input. At 13 dBm, we observe ∼2 dB power drop due to imbalanced Brillouin gain and loss [10]. Therefore, the gain improvement from phase mismatch compensation may even be stronger if the SBS loss can be eliminated. Next, the FOPA pump power is changed to 25.2 dBm. The signal wavelength is chosen at the gain peak of the conventional FOPA at 1546.5 nm. To expedite the saturation, f RF is tuned to 9.675 GHz. As shown in Fig. 3(b), when the Brillouin pump and Stokes wave are applied to increase the phase mismatch, the FOPA saturates at −6 dBm signal input, which is ∼6 dB lower than that of the conventional FOPA. The maximum output becomes 9.11 dBm. The gain-transparent SBS operation is also verified. The power variation is relatively weak below 6 dBm input and reaches ∼2 dB at 13 dBm. We believe SBS-induced loss contributes little to the gain drop at input signal level beyond −10 dBm. The reason is that the FOPA gain grows exponentially with the fiber length, and the signal power reaches imbalanced Brillouin gain and loss only at very end of the fiber output. It’s worth mentioning that for both cases of deferring and expediting the saturation, f RF is carefully chosen within the gain transparent operation region to maximize the control effect. To further analyze the power transfer among the interplaying fields, the FOPA pump output power is measured and displayed in Fig. 4. Figure 4(a) shows the plots for the case when the FOPA gain saturation is deferred. In the linear gain region, the pump output shows only a slight reduction of less than 0.2 dB compared to that without SBS. The additional power consumption in the linear region accounts for the ∼0.5 dB signal power improvement. Near the gain peak of the conventional FOPA, the phase matching becomes worse. It is due to the change in the nonlinear phase mismatch along the fiber as a result of power transfer from the pump to the signal and idler. The SBS-induced nonlinear phase compensates the phase mismatch of the conventional FOPA and further enhances the FOPA gain even at the gain peak. In the saturation region, the pump output is ∼1 dB weaker than that of the conventional FOPA. The result indicates that the recovered phase matching sustains the gain which thus leads to a more severe pump depletion. Figure 4(b) shows the power plots when the FOPA saturation is expedited. The SBS-induced nonlinear phase suppresses power transfer from the pump to the signal. In the linear gain region, the pump output power is ∼0.1 dB slightly higher than that of the conventional FOPA. However, the power difference becomes larger than 1 dB when the FOPA begins to saturate. The output pump power comparison shows that the
5715
Fig. 4. Output signal and pump powers against input signal power at (a) 26.5 dBm FOPA input pump, f RF 9.558 GHz and (b) 25.2 dBm FOPA input pump, f RF 9.675 GHz.
SBS induced phase mismatch plays an important role in determining the power gain of the signal. By controlling the SBS induced nonlinear phase, κ as defined in Eq. (2) can be modified to enhance or suppress the power transfer from the pump to the signal. To dynamically adjust the nonlinear phase and hence the linear and saturated gains, one can simply tune the frequency spacing f RF . Figure 5 plots the change in FOPA gain due to the application of SBS at different frequency spacings f RF . The power of the 1555-nm FOPA pump is fixed at 25.2 dBm. The signal is located at 1546.5 nm, the gain peak of the conventional FOPA. The changes in gain at the linear and saturation regions are investigated separately. When the input signal power is set at −20 dBm, the application of SBS results in gain reduction for almost all values of f RF . It is worth noting that the frequency dependence of the gain change is not symmetric about the Brillouin frequency shift of 9.58 GHz. The asymmetry can be explained by imbalanced gain and loss introduced by the Brillouin pump and Stokes wave near the Brillouin gain peak. Away from the gain peak, the Brillouin gain and loss can nearly cancel out. The overall gain change is thus dominated by the influence of SBS-induced nonlinear phase, resulting in a dip at f RF 9.62 GHz and a peak at f RF 9.50 GHz. The effect of SBS is more pronounced in the saturation region. With an input signal power of −6 dBm, the FOPA gain is enhanced when f RF is lower than 9.57 GHz. The maximum gain change
5716
OPTICS LETTERS / Vol. 39, No. 19 / October 1, 2014
Fig. 5. Parametric gain change at different RF frequencies for both linear (−20 dBm input signal) and saturation (−6 dBm input signal) regions of the FOPA.
is obtained at 9.535 GHz. This gain enhancement region indicates a positive SBS-induced phase that allows more efficient power transfer from the FOPA pump to the signal. The gain suppression at f RF between 9.57 and 9.58 GHz (Brillouin frequency shift) can be attributed to net Brillouin loss of the signal. For f RF higher than 9.58 GHz, the SBS-induced phase shift is negative. The phase mismatch increases and further suppresses the parametric gain. A maximum gain suppression of 6.54 dB is obtained at f RF 9.64 GHz. The FOPA signal gain can be affected by direct amplification/attenuation from SBS, especially when f RF is close to the Brillouin frequency shift, ΩB . The gain imbalance in SBS can be alleviated by weakening the Brillouin pump and Stokes wave or by setting f RF away from ΩB . However, there is a tradeoff in the amount of SBS-induced nonlinear phase. It is worth mentioning that our scheme is applicable for data-modulated signal if a sufficiently strong FOPA pump is used without phase dithering. It is made possible by introducing a distributed strain along the HNLF or by adopting a HNLF with a higher SBS threshold [12,13]. Under this condition, the SBS-induced phase can be applied to the FOPA pump. Previously, the control of FWM conversion efficiency for datamodulated signal has been achieved [7]. In summary, we have demonstrated active control of saturation in a FOPA using gain-transparent SBS. By
tuning the SBS-induced phase mismatch, gain saturation can be flexibly deferred or expedited. The input signal power at saturation can be increased or decreased by up to 6 dB at FOPA pump powers of 26.5 and 25.2 dBm, respectively. A comparison of pump depletion is made with the conventional FOPA. The SBS-induced nonlinear phase can dynamically enhance or suppress power transfer from the pump to the signal by varying the total phase mismatch. Therefore, under the influence of SBS, the parametric gain can be sustained even in the presence of pump power depletion. Conversely, the gain can be suppressed in the absence of pump depletion. Hence, the SBS-assisted approach offers a versatile tool to enhance the performances of both linear amplification and optical regeneration in FOPAs. This work was supported by a General Research Fund (CUHK 416213). References 1. M. E. Marhic, Fiber Optical Parametric Amplifiers, Oscillators and Related Devices (Cambridge University, 2008). 2. P. Kylemark, H. Sunnerud, M. Karlsson, and P. A. Andrekson, J. Lightwave Technol. 24, 3471 (2006). 3. K. Inoue and T. Mukai, Opt. Lett. 26, 10 (2001). 4. X. Guo, X. Fu, and C. Shu, Opt. Lett. 38, 4405 (2013). 5. J. Kim, O. Boyraz, J. Lim, and M. Islam, J. Lightwave Technol. 19, 247 (2001). 6. E. Mateo, F. Yaman, and G. Li, Opt. Lett. 33, 488 (2008). 7. L. Wang and C. Shu, J. Lightwave Technol. 31, 1468 (2013). 8. L. Wang and C. Shu, IEEE Photon. Technol. Lett. 25, 1996 (2013). 9. C. Huang, X. Guo, X. Fu, L. Wang, and C. Shu, in Conference on Lasers and Electro-Optics (Optical Society of America, 2014), paper JW2A.73. 10. A. Loayssa and F. J. Lahoz, IEEE Photon. Technol. Lett. 18, 208 (2006). 11. G. P. Agrawal, Nonlinear Fiber Optics, 4th ed. (Academic, 2007). 12. J. M. Chavez Boggio, J. D. Marconi, and H. L. Fragnito, J. Lightwave Technol. 23, 3808 (2005). 13. L. Grüner-Nielsen, D. Jakobsen, S. Herstrøm, B. Pálsdóttir, S. Dasgupta, D. Richardson, C. Lundström, S. L. Olsson, and P. A. Andrekson, in European Conference on Optics Communications (Optical Society of America, 2012), paper We.1.F.1.
5713
Active control of gain saturation in fiber-optical parametric amplifier using stimulated Brillouin scattering Chaoran Huang, Xiaojie Guo, Xuelei Fu, Liang Wang, and Chester Shu* Department of Electronic Engineering and Center for Advanced Research in Photonics, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong, China *Corresponding author: [email protected] Received May 20, 2014; revised August 29, 2014; accepted September 2, 2014; posted September 4, 2014 (Doc. ID 212428); published September 29, 2014 We have demonstrated the use of stimulated Brillouin scattering (SBS) to control the gain saturation characteristics of fiber-optical parametric amplification (FOPA). The saturation can be dynamically deferred or expedited without changing the input FOPA pump and signal parameters. It is observed that the input signal power at saturation can be increased or decreased by up to 6 dB. Experiments are performed to further investigate the role of the SBS-induced nonlinear phase on the power transfer efficiency in the FOPA. © 2014 Optical Society of America OCIS codes: (060.4370) Nonlinear optics, fibers; (290.5900) Scattering, stimulated Brillouin; (190.4410) Nonlinear optics, parametric processes. http://dx.doi.org/10.1364/OL.39.005713
Fiber optical parametric amplifiers (FOPAs) are important for various applications such as low-noise amplification and all-optical signal processing [1]. A unique characteristic of FOPAs is the instantaneous gain saturation that depends strongly on the pump power [2]. With a strong pump, the amplifier saturates at relatively low input signal power. This will limit the maximum input for linear amplification purpose. Conversely, FOPA saturation can be utilized for signal regeneration. The saturation originates from pump depletion and phase mismatch among the interplaying fields [3]. To defer the saturation, Raman-assisted FOPA was recently proposed to compensate the depleting pump power via Raman amplification [4]. Quasi-phase-matching FOPA was also proposed to compensate the phase mismatch by modifying the relative phase periodically [5]. However, these techniques offer limited flexibility in operation. The different requirements of amplification and regeneration can hardly be satisfied by a single scheme. Noteworthy, stimulated Brillouin scattering (SBS) has previously been used for phase matching control in fourwave mixing (FWM) by introducing a local refractive index change [6]. The SBS gain and induced phase change have also been successfully decoupled [7]. Gain-transparent SBS has subsequently been applied to enlarge the wavelength conversion bandwidth and to arbitrarily reshape the gain profile of a FOPA [7,8]. The scheme enables dynamic control of phase matching via SBS. In [9], we demonstrated that FOPA saturation can be dynamically deferred or expedited by SBS-induced phase change. The input signal power required for saturation can also be increased or decreased by up to ∼6 dB. In this Letter, the extent of pump depletion is further compared to that of the conventional FOPA. The result shows that the SBS-induced nonlinear phase change can effectively control the power transfer from the pump to the signal by varying the total phase mismatch. The change of net gain is also investigated at different frequency detunings of the Brillouin pump and Stokes wave from the Brillouin frequency shift, corresponding to different induced phase changes. 0146-9592/14/195713-04$15.00/0
The principle of gain transparent SBS has been reported in [10]. If the input signal is within the Brillouin gain bandwidth, it will experience a complex gain expressed as [11]: gb δ
g0 g0 ; 1 − iδ 1–2iωSBS − ωS − ΩB ∕ΓB
(1)
where g0 is the peak gain, ωSBS − ωS is the frequency spacing between the Brillouin pump and signal, ΩB is the Brillouin frequency shift, δ represents the normalized detuning parameter, and ΓB is the gain bandwidth. The real part of gb δ is related to SBS amplification while the imaginary part is related to induced phase shift on the signal. Figure 1 illustrates the principle of operation. When both the Brillouin pump and Stokes wave propagate in the opposite direction to the input signal, the real parts of SBS gain and loss will cancel each other. The imaginary parts will however be added to double the total induced phase shift of the signal, thus modifying the phase mismatch in the FOPA. In this work, the Brillouin
Fig. 1. Control of gain saturation in fiber-optical parametric amplifier by SBS-induced nonlinear phase. vB : Brillouin frequency shift; f RF : frequency spacing between the Brillouin pump and signal, and between the signal and Stokes wave. © 2014 Optical Society of America
5714
OPTICS LETTERS / Vol. 39, No. 19 / October 1, 2014
pump amplifies the signal, whereas the Stokes wave depletes the signal. The total phase mismatch κ should include the additional phase mismatch term induced by SBS [3]: κ Δβ γ2P pump − P signal − P idler 2jImgb δP SBS ∕Aeff j;
(2)
where Δβ is the linear phase mismatch between FOPA pump, signal, and idler and γ2P pump − P signal − P idler is the nonlinear phase mismatch. The last term represents the SBS-induced phase mismatch that can be adjusted by tuning f RF , the frequency detuning between the Brillouin pump and the signal, and between the signal and the Brillouin Stokes wave. In the absence of the Brillouin pump and Stokes wave, the FOPA amplification factor can be expressed as [11]: Gs 1 γP pump ∕g2 sinh2 gL;
(3)
where g is the parametric gain that depends on both the pump power and the phase mismatch and is defined as: g
q γP pump 2 − κ∕22 .
(4)
modulation at a frequency f RF . The carrier suppression ratio is 25 dB. The frequency sidebands are amplified to act as the Brillouin pump and Stokes wave. The bandpass filters are used to filter out the amplified spontaneous emission noise after the EDFAs. The FOPA operates in the presence of backward-propagating Brillouin pump and Stokes wave. The HNLF has a length of 1 km, a nonlinear coefficient of 11.7∕W · km, a zero-dispersion wavelength of 1549 nm, a dispersion coefficient of 0.02 ps∕nm·km, a dispersion slope of 0.019 ps∕ nm2 ·km at ∼1550 nm, and a Brillouin frequency shift of 9.58 GHz. The FOPA pump and signals are monitored by an optical spectrum analyzer (OSA). Several polarization controllers (PC) are used in the setup to ensure the maximum performances of FWM and SBS. We first investigate the control of saturation by gaintransparent SBS at different FOPA pump powers. The FOPA pump is located at 1555 nm, and the power is first set at 26.5 dBm. The signal is located at the FOPA gain peak of 1545 nm. The power of the Brillouin pump and Stokes wave is fixed at 12 dBm, which maximizes the tuning range of the phase shift and meanwhile avoids the back-reflected SBS noise. Figure 3(a) shows the gain saturation behaviors. For a conventional FOPA, the maximum output power reaches 12.6 dBm at an input
The nonlinear phase mismatch term in Eq. (2) decreases with pump depletion and signal amplification along the fiber. When the SBS-induced phase mismatch term is positive, parametric power transfer is enhanced to defer the gain saturation. Conversely, a negative SBSinduced phase mismatch term will increase the total phase mismatch and expedite the saturation. Thus, one can dynamically control the saturation properties of FOPA to suit different purposes. The experimental setup is shown in Fig. 2. The output of a tunable laser (TL1) is spectrally broadened by phase modulation with 10 Gb/s 27 –1 pseudo-random binary sequence (PRBS) to increase the SBS threshold. It is then amplified to serve as the FOPA pump. Another tunable laser (TL2) is split into two branches. The upper branch serves as the FOPA signal. It is launched into the highly nonlinear fiber (HNLF) with the FOPA pump. The lower branch undergoes carrier-suppressed electro-optic
Fig. 2. Experimental setup on dynamic control of FOPA gain saturation. TL, tunable laser; PM, phase modulator; EDFA, erbium-doped fiber amplifier; EOM, electro-optic intensity modulator; BPF, bandpass filter; ISO, isolator; OSA, optical spectrum analyzer; HNLF, highly nonlinear fiber; PRBS, pseudorandom binary sequence.
Fig. 3. Output signal power against input signal power at (a) 26.5 dBm FOPA input pump, f RF 9.558 GHz and (b) 25.2 dBm FOPA input pump, f RF 9.675 GHz. Insets show the variation of signal power upon the application of gaintransparent SBS.
October 1, 2014 / Vol. 39, No. 19 / OPTICS LETTERS
signal power of −13 dBm. The output power drops quickly as the input signal further increases. In comparison, with gain-transparent SBS operated at f RF 9.558 GHz, the maximum output is increased by 1.2 dB and is obtained at a stronger input signal power of −7 dBm. It is important to ensure that the gain improvement is not caused by direct SBS amplification. We thus measure the power variation, i.e., the difference between the input and output powers, caused by the Brillouin pump and Stokes wave in the absence of FOPA pump. The result is displayed in the inset of Fig. 3(a). It is observed that the signal power remains unchanged until ∼6 dBm input. At 13 dBm, we observe ∼2 dB power drop due to imbalanced Brillouin gain and loss [10]. Therefore, the gain improvement from phase mismatch compensation may even be stronger if the SBS loss can be eliminated. Next, the FOPA pump power is changed to 25.2 dBm. The signal wavelength is chosen at the gain peak of the conventional FOPA at 1546.5 nm. To expedite the saturation, f RF is tuned to 9.675 GHz. As shown in Fig. 3(b), when the Brillouin pump and Stokes wave are applied to increase the phase mismatch, the FOPA saturates at −6 dBm signal input, which is ∼6 dB lower than that of the conventional FOPA. The maximum output becomes 9.11 dBm. The gain-transparent SBS operation is also verified. The power variation is relatively weak below 6 dBm input and reaches ∼2 dB at 13 dBm. We believe SBS-induced loss contributes little to the gain drop at input signal level beyond −10 dBm. The reason is that the FOPA gain grows exponentially with the fiber length, and the signal power reaches imbalanced Brillouin gain and loss only at very end of the fiber output. It’s worth mentioning that for both cases of deferring and expediting the saturation, f RF is carefully chosen within the gain transparent operation region to maximize the control effect. To further analyze the power transfer among the interplaying fields, the FOPA pump output power is measured and displayed in Fig. 4. Figure 4(a) shows the plots for the case when the FOPA gain saturation is deferred. In the linear gain region, the pump output shows only a slight reduction of less than 0.2 dB compared to that without SBS. The additional power consumption in the linear region accounts for the ∼0.5 dB signal power improvement. Near the gain peak of the conventional FOPA, the phase matching becomes worse. It is due to the change in the nonlinear phase mismatch along the fiber as a result of power transfer from the pump to the signal and idler. The SBS-induced nonlinear phase compensates the phase mismatch of the conventional FOPA and further enhances the FOPA gain even at the gain peak. In the saturation region, the pump output is ∼1 dB weaker than that of the conventional FOPA. The result indicates that the recovered phase matching sustains the gain which thus leads to a more severe pump depletion. Figure 4(b) shows the power plots when the FOPA saturation is expedited. The SBS-induced nonlinear phase suppresses power transfer from the pump to the signal. In the linear gain region, the pump output power is ∼0.1 dB slightly higher than that of the conventional FOPA. However, the power difference becomes larger than 1 dB when the FOPA begins to saturate. The output pump power comparison shows that the
5715
Fig. 4. Output signal and pump powers against input signal power at (a) 26.5 dBm FOPA input pump, f RF 9.558 GHz and (b) 25.2 dBm FOPA input pump, f RF 9.675 GHz.
SBS induced phase mismatch plays an important role in determining the power gain of the signal. By controlling the SBS induced nonlinear phase, κ as defined in Eq. (2) can be modified to enhance or suppress the power transfer from the pump to the signal. To dynamically adjust the nonlinear phase and hence the linear and saturated gains, one can simply tune the frequency spacing f RF . Figure 5 plots the change in FOPA gain due to the application of SBS at different frequency spacings f RF . The power of the 1555-nm FOPA pump is fixed at 25.2 dBm. The signal is located at 1546.5 nm, the gain peak of the conventional FOPA. The changes in gain at the linear and saturation regions are investigated separately. When the input signal power is set at −20 dBm, the application of SBS results in gain reduction for almost all values of f RF . It is worth noting that the frequency dependence of the gain change is not symmetric about the Brillouin frequency shift of 9.58 GHz. The asymmetry can be explained by imbalanced gain and loss introduced by the Brillouin pump and Stokes wave near the Brillouin gain peak. Away from the gain peak, the Brillouin gain and loss can nearly cancel out. The overall gain change is thus dominated by the influence of SBS-induced nonlinear phase, resulting in a dip at f RF 9.62 GHz and a peak at f RF 9.50 GHz. The effect of SBS is more pronounced in the saturation region. With an input signal power of −6 dBm, the FOPA gain is enhanced when f RF is lower than 9.57 GHz. The maximum gain change
5716
OPTICS LETTERS / Vol. 39, No. 19 / October 1, 2014
Fig. 5. Parametric gain change at different RF frequencies for both linear (−20 dBm input signal) and saturation (−6 dBm input signal) regions of the FOPA.
is obtained at 9.535 GHz. This gain enhancement region indicates a positive SBS-induced phase that allows more efficient power transfer from the FOPA pump to the signal. The gain suppression at f RF between 9.57 and 9.58 GHz (Brillouin frequency shift) can be attributed to net Brillouin loss of the signal. For f RF higher than 9.58 GHz, the SBS-induced phase shift is negative. The phase mismatch increases and further suppresses the parametric gain. A maximum gain suppression of 6.54 dB is obtained at f RF 9.64 GHz. The FOPA signal gain can be affected by direct amplification/attenuation from SBS, especially when f RF is close to the Brillouin frequency shift, ΩB . The gain imbalance in SBS can be alleviated by weakening the Brillouin pump and Stokes wave or by setting f RF away from ΩB . However, there is a tradeoff in the amount of SBS-induced nonlinear phase. It is worth mentioning that our scheme is applicable for data-modulated signal if a sufficiently strong FOPA pump is used without phase dithering. It is made possible by introducing a distributed strain along the HNLF or by adopting a HNLF with a higher SBS threshold [12,13]. Under this condition, the SBS-induced phase can be applied to the FOPA pump. Previously, the control of FWM conversion efficiency for datamodulated signal has been achieved [7]. In summary, we have demonstrated active control of saturation in a FOPA using gain-transparent SBS. By
tuning the SBS-induced phase mismatch, gain saturation can be flexibly deferred or expedited. The input signal power at saturation can be increased or decreased by up to 6 dB at FOPA pump powers of 26.5 and 25.2 dBm, respectively. A comparison of pump depletion is made with the conventional FOPA. The SBS-induced nonlinear phase can dynamically enhance or suppress power transfer from the pump to the signal by varying the total phase mismatch. Therefore, under the influence of SBS, the parametric gain can be sustained even in the presence of pump power depletion. Conversely, the gain can be suppressed in the absence of pump depletion. Hence, the SBS-assisted approach offers a versatile tool to enhance the performances of both linear amplification and optical regeneration in FOPAs. This work was supported by a General Research Fund (CUHK 416213). References 1. M. E. Marhic, Fiber Optical Parametric Amplifiers, Oscillators and Related Devices (Cambridge University, 2008). 2. P. Kylemark, H. Sunnerud, M. Karlsson, and P. A. Andrekson, J. Lightwave Technol. 24, 3471 (2006). 3. K. Inoue and T. Mukai, Opt. Lett. 26, 10 (2001). 4. X. Guo, X. Fu, and C. Shu, Opt. Lett. 38, 4405 (2013). 5. J. Kim, O. Boyraz, J. Lim, and M. Islam, J. Lightwave Technol. 19, 247 (2001). 6. E. Mateo, F. Yaman, and G. Li, Opt. Lett. 33, 488 (2008). 7. L. Wang and C. Shu, J. Lightwave Technol. 31, 1468 (2013). 8. L. Wang and C. Shu, IEEE Photon. Technol. Lett. 25, 1996 (2013). 9. C. Huang, X. Guo, X. Fu, L. Wang, and C. Shu, in Conference on Lasers and Electro-Optics (Optical Society of America, 2014), paper JW2A.73. 10. A. Loayssa and F. J. Lahoz, IEEE Photon. Technol. Lett. 18, 208 (2006). 11. G. P. Agrawal, Nonlinear Fiber Optics, 4th ed. (Academic, 2007). 12. J. M. Chavez Boggio, J. D. Marconi, and H. L. Fragnito, J. Lightwave Technol. 23, 3808 (2005). 13. L. Grüner-Nielsen, D. Jakobsen, S. Herstrøm, B. Pálsdóttir, S. Dasgupta, D. Richardson, C. Lundström, S. L. Olsson, and P. A. Andrekson, in European Conference on Optics Communications (Optical Society of America, 2012), paper We.1.F.1.
