July 15, 2014 / Vol. 39, No. 14 / OPTICS LETTERS
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GHz bandwidth noise eater hybrid optical amplifier: design guidelines Gwennaël Danion,* François Bondu, Goulc’hen Loas, and Mehdi Alouini Institut de Physique de Rennes UMR 6251, CNRS, Université de Rennes 1, Campus de Beaulieu, Rennes 35042, France *Corresponding author: gwennael.danion@univ‑rennes1.fr Received February 27, 2014; revised June 12, 2014; accepted June 12, 2014; posted June 13, 2014 (Doc. ID 207284); published July 15, 2014 This Letter describes the design of an optical amplifier system optimized to reduce the relative intensity noise (RIN) of the input signal, and discloses its performance in terms of intensity noise reduction and bandwidth, without phase noise degradation. This polarization-maintaining amplifier is composed of an erbium-doped fiber amplifier (EDFA) cascaded with a semiconductor optical amplifier (SOA). The EDFA is sized to feed the SOA with a constant power corresponding to the optimal saturation level for noise reduction, through coherent population oscillations. When properly optimized, such an amplifier provides, simultaneously, 17 dB optical gain, 5.4 dB noise factor, and 20 dB reduction of the input-RIN across a 3 GHz bandwidth, without any electronics feedback loop. © 2014 Optical Society of America OCIS codes: (140.4480) Optical amplifiers; (270.1670) Coherent optical effects; (120.4570) Optical design of instruments; (250.5980) Semiconductor optical amplifiers. http://dx.doi.org/10.1364/OL.39.004239
Applications such as optical analog and coherent telecommunications, metrology, spectroscopy, and radar, require lasers with ultra-stable amplitude and phase. Even though solid-state lasers and fiber distributed feedback lasers are known to exhibit narrow spectral linewidths, they suffer from resonant intensity noise around their relaxation oscillation frequency from typically a few kHz to a few MHz. This excess noise can be suppressed by a servo-loop acting on the laser pump [1,2]. Another alternative is to deeply modify the laser dynamics so that it becomes inherently free from relaxation oscillations [3,4]. From a practical point of view, these latter techniques must be implemented when designing the laser. A reduction of the laser intensity noise outside the laser cavity is unavoidable when the laser is off-the-shelf, or when its design cannot afford additional improvements. For instance, this issue came up in one of our experiments, where a dual-frequency laser is to be stabilized on ultra-low expansion cavity of finesse 100,000 [5]. Indeed, this homemade laser is designed to provide, at the same time, high spectral purity, high long-term stability, dual-frequency operation, independent and wide tunability across 1 THz of the two frequencies, as well as voltage-controlled tunability; thus, the laser cavity design is too complex to cope with intracavity noise reduction. Amplitude noise reduction outside the laser cavity is, thus, unavoidable. External noise eaters are commonly used for this purpose, but their insertion leads to significant losses. Moreover, their bandwidth is limited to a few MHz due to the feedback electronics. For most common applications, an ideal noise eater would have a bandwidth of several tens of MHz, with zero insertion losses and even some gain. In this framework, it has been demonstrated that a semiconductor optical amplifier (SOA) might act, regarding an input signal, as a high-frequency bandpass filter, provided that it is operated in the saturated regime. Nevertheless, the use of SOAs is usually avoided because of their high noise factor (NF). One can then wonder whether a proper design of an optical amplification chain combining the advantages of erbiumdoped fiber amplifiers (EDFAs) and that of SOAs could 0146-9592/14/144239-04$15.00/0
bring a straightforward solution to the problem of amplitude noise reduction in solid-state lasers. We detail in this Letter the design and optimization steps of a hybrid amplifier that simultaneously (i) raises the laser power to a constant value, (ii) reduces the relative intensity noise (RIN) within a GHz bandwidth, and (iii) does not add any noticeable close-to-the-carrier frequency noise. As already mentioned, in our application, the amplifier system has to meet several constraints [5]. More precisely, the Er/Yb:glass solid-state laser source has a RIN of −90 dB∕Hz at 100 kHz, corresponding to the relaxation oscillation frequency, and which must be reduced to −110 dB∕Hz. Furthermore, the laser output power of 1 mW must be amplified to 10 mW, at least. In addition, the amplifier total length should remain below 20 m to minimize the delay for the frequency stabilization loop, whose actuator is located upstream of the amplifier. Finally, a NF less than 7 dB is targeted to meet amplitude noise requirements far from the carrier. The basic idea here is to take advantage of coherent population oscillations (CPOs) in the SOA [6,7] to damp down the laser noise across a bandwidth of a few GHz and to use an EDFA amplifier specifically designed to bring the SOA in the optimal saturation regime for noise reduction, whatever the input laser optical power. In addition, the EDFA, which precedes the SOA, has to provide a NF close to the 3 dB quantum limit, so that the overall amplification chain exhibits a low NF, owing to Friis’s formula [8]: F F EDFA
F SOA − 1 ; GEDFA
(1)
where F EDFA and F SOA are the NF of the EDFA and the SOA, respectively, and GEDFA is the EDFA gain. The SOA under consideration is an InP/InGaAsP multiple quantum well layer single pass traveling wave amplifier that is commercially available (Thorlabs/ Covega BOA1004P). This SOA is chosen because of its high gain level of 25 dB and its high output saturation © 2014 Optical Society of America
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power of 15 dBm, which are both favorable for efficient noise reduction through gain saturation. Its NF in small signal condition is given to be 7 dB for an injection current of 600 mA. We first checked the achievable noise reduction bandwidth by characterizing the frequency response of the amplifier when operated under saturation. To this aim, a DFB laser, followed by a 20 GHz Mach– Zehnder modulator feeds the SOA. The output optical power is attenuated and detected with a 20 GHz InGaAs photodiode. The photodiode is connected to a vectorial network analyzer driving the Mach–Zehnder modulator. The frequency response of the optical link is then acquired with and without the SOA. After normalization, the frequency response of the SOA is recovered, as shown in Fig. 1(b). As expected, this plot shows a high-pass filter behavior leading to a reduction of about 15 dB of the modulated power over 3 GHz bandwidth. The frequency range over which this power reduction occurs is determined by the carrier’s lifetime in the SOA [9]. Obviously, this mechanism will occur as well in the EDFA. Nevertheless, it cannot be exploited for our purpose because the upper metastable level has a long lifetime of 10 ms, which leads to gain compression from DC to a few hundreds of Hz only. This is illustrated in the acquisition of Fig. 1(a), performed on the homemade EDFA that will be described later. The next step is to quantify the input optical power inducing a maximum noise reduction. Indeed, as already demonstrated [10], the output RIN is expected to decrease when increasing the mean optical power. However, increasing this power excessively might bleach the amplifier, resulting in the same output RIN level as the input RIN level. Consequently, the input power for optimal noise reduction has to be determined precisely. To this aim, the SOA is injected with an amplified single frequency solid-state Er/Yb:glass laser. When pumped at 980 nm with a pumping power 2.5 above threshold, this laser exhibits noise in excess at the relaxation oscillation frequency at 100 kHz (see Fig. 4). This laser is followed by a commercial EDFA and an optical attenuator to precisely adjust the power sent to the SOA. One then measures, at the output of the SOA, the RIN level at 100 kHz versus the input power. Figure 2 shows the RIN reduction for different input powers entering the SOA. For powers up to 6 mW, the RIN level decreases when increasing the input power. However, for higher input powers, the measured RIN reduction starts to saturate
Fig. 1. Frequency responses normalized to the gain for infinity frequency of (a) the EDFA and (b) the SOA.
Fig. 2. SOA RIN reduction measured at 100 kHz for different input powers.
as anticipated. Thus, for 10 mW input power, the SOA is highly saturated and, in addition to the 17 dBm it delivers, it also provides more than 20 dB RIN reduction. We have measured the SOA NF when the input power is set to 10 mW; that is, when the SOA operates in deeply saturated regime. It turned out that the NF is slightly degraded to 8.3 dB compared with the small signal NF of 7 dB. It is worthwhile to notice that, for a laser delivering 0 dBm, the RIN reduction would be only 4 dB at 100 kHz. To enable the SOA operating in the optimal noise reduction conditions whatever the laser power, we have designed an EDFA amplifier such that its output saturation power is 10 dBm and its small signal NF is close to the quantum limit of 3 dB. When placed in front of the SOA (see Fig. 3), this EDFA ensures simultaneous optimal power of 10 dBm at the input of the SOA, and a relatively low NF of the whole amplification chain. The EDFA includes polarization-maintaining elements (fibers and optical components) to comply with the polarization-sensitive SOA and because we intend to preserve the laser linear polarization at the output of the amplification chain. Obviously, a similar design would apply for a polarization-independent amplification chain. All fibered components are spliced with accuracy better than 1°. The gain medium is a short 1.25 m highly doped Er3 Panda-type fiber having a core diameter of 5 μm and a linear absorption of 28.8 dB∕m at 1530 nm. It is pumped at 980 nm with 40 mW in the forward configuration. As
Fig. 3. Setup of the GHz bandwidth noise eater hybrid amplifier. ISO, optical isolator; WDM, wavelength division multiplexer at 980 nm.
July 15, 2014 / Vol. 39, No. 14 / OPTICS LETTERS
sketched in Fig. 3, the pump diode is coupled to the doped fiber with a hybrid component associating a wavelength multiplexer (WDM) and a polarizing isolator. After the gain medium, another hybrid WDM/polarizing isolator enables both isolation of the first stage and rejection of the remaining 16 mW of pump power into a beam block. In these conditions, the minimum EDFA small signal gain is measured to be 20 dB on the wavelength band 1530–1560 nm, whereas its output saturation power is 10 dBm, as desired. For 1 mW input power the EDFA gain is compressed to 10 dB and its NF is obviously moderately degraded to 4.5 dB. The EDFA and SOA fibers are spliced to avoid any backreflection that could lead to RIN degradation [11]. When cascaded to the EDFA, the SOA input power is now 10 dBm leading to a saturated gain of 7 dB and output saturation power of 17 dBm. As already mentioned, under these conditions, its NF is measured to be 8.3 dB. Taking into account the WDM/isolator pigtails, the doped fiber and the SOA pigtails, the overall amplifying chain length is of about 9 m, which is much shorter than the upper limit of 20 m required for proper operation of an optical frequency stabilization loop, such as that reported in Ref. [5]. At offset frequencies larger than 10 GHz, the CPO mechanism becomes negligible for both amplifiers. Consequently, they can be considered as linear, so that the Friis formula [Eq. (1)] applies. Taking into account the measured values of gain and NF, one expects the NF of the whole amplifier chain to be 5.3 dB, which is lower than the SOA NF. This is confirmed experimentally, since we measure a NF of 5.4 dB for the whole amplification chain when the input power is set to 1 mW. It must be noted that the EDFA, being well-saturated for input powers in the mW range, always provides a constant 10 dBm output power, which feeds the SOA. In this condition, we benefit from a maximum RIN reduction and also from a constant output power of 17 dBm. The optical spectral range of the overall amplification chain is limited in our experiment by the EDFA, i.e., 1529–1561 nm. We then measured the effective RIN reduction with our solid-state laser, whose relaxation oscillation is at about 100 kHz. After the hybrid amplifier, we measured a RIN reduction of 20 dB, as shown in Fig. 4. One can notice that this noise reduction is broadband. According to Fig. 1(b), this is potentially valid up to 3 GHz. Nevertheless, the measurement has been performed up to 10 MHz, where it gets limited by the noise floor of our electrical detection setup. It is worthwhile to notice that, in this low frequency region, the proper SOA RIN is also subject to CPO and is, thus, well below −155 dB∕Hz [12]. For offset frequencies below 10 kHz, we found that the fluctuations of the EDFA pump power and of the SOA pump current play a nonnegligible role, so that the RIN reduction is less important. Nevertheless, this could be improved if required by using low noise drivers or batteries. We have finally verified that the amplitude noise reduction is not obtained at the cost of frequency noise increase. The frequency noise is analyzed using a self-heterodyne method with a fibered Mach–Zehnder interferometer [13]. One arm contains an acousto-optic modulator at 80 MHz, whereas the second arm includes a 700 m delay fiber. At the output, the 80 MHz beat note frequency is detected by a 16 GHz bandwidth InGaAs
4241
Fig. 4. RIN at input (laser RIN) and output of the amplifier chain. The peak corresponds to the excess noise lying at the relaxation oscillation frequency of the laser.
photodiode, and then demodulated with respect to I and Q quadratures. The phase is then reconstructed, and its spectrum converted into laser frequency noise. In Fig. 5, the black line represents the frequency noise of our laser before the amplifier, whereas the gray line represents the frequency noise at the output of the hybrid amplifier. We note that the frequency noises are almost identical, except at the relaxation oscillation frequency of the laser, where a tiny peak is observed. This residual frequency noise in excess is probably induced by amplitude-to-phase conversion in the SOA [14]. In conclusion, we report here, step by step, the realization of a 2-GHz bandwidth noise eater hybrid amplifier. This amplifier provides 17 dB gain and is optimized to damp by a 20 dB factor the amplitude excess noise of the input source over a bandwidth of 2 GHz. It mainly consists of a carefully designed EDFA cascaded to a commercial SOA, so that the EDFA output saturation power fits the SOA input power for which a maximum noise reduction is achieved through coherent population oscillations. This optimized design makes it possible to operate the SOA in the optimal noise reduction condition for a significantly large power range at the input of the
Fig. 5. Frequency noise at input and output of the hybrid optical amplifier system.
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EDFA (typically, −10 dBm up to 5 dBm). Owing to the low NF of the EDFA, this hybrid amplifier exhibits an acceptable NF of 5.4 dB. Being developed for amplifying solid-state lasers while reducing their amplitude noise in excess, in particular at the relaxation oscillation frequency, we have checked that this hybrid amplifier does not bring any significant degradation of the frequency noise of the amplified signal. The characteristics reported here are obtained in the “passive” mode operation. The SOA injection current can be handled for further amplitude stabilization or for more sophisticated control of the noise. It also provides a simple way to tag the optical carrier at a given frequency from DC to a few GHz, if required. Passive and active mode operation are currently implemented in the framework of a broader project, aimed at generating THz frequency references through photo-mixing dual-frequency lasers locked to ultra-low-expansion high-finesse cavities [5]. The authors acknowledge C. Hamel, L. Frein, and S. Bouhier for their technical support. This work is supported by the Agence Nationale de la Recherche (OSMOTUS ANR-2011-BS03-010-01) and by Région Bretagne. References 1. S. Taccheo, G. De Geronimo, P. Laporta, and O. Svelto, Opt. Lett. 21, 1747 (1996).
2. M. Alouini, B. Benazet, M. Vallet, M. Brunel, P. Di Bin, F. Bretenaker, A. Le Floch, and P. Thony, IEEE Photon. Technol. Lett. 13, 367 (2001). 3. G. Baili, M. Alouini, D. Dolfi, F. Bretenaker, I. Sagnes, and A. Garnache, Opt. Lett. 32, 650 (2007). 4. A. El Amili, G. Kervella, and M. Alouini, Opt. Express 21, 8773 (2013). 5. G. Danion, G. Loas, L. Frein, C. Hamel, A. Carre, S. Bouhier, M. Vallet, M. Brunel, A. Rolland, M. Alouini, F. Bondu, F. Cleva, J. P. Coulon, M. Merzougui, A. Brillet, A. Beck, G. Ducournau, M. Zaknoune, C. Coinon, X. Wallart, E. Peytavit, T. Akalin, J. F. Lampin, G. Pillet, L. Morvan, G. Baili, and J. Bourderionnet, in European Frequency and Time Forum and International Frequency Control Symposium (EFTF/IFC) (2013), pp. 40–42. 6. R. Boula-Picard, M. Alouini, J. Lopez, N. Vodjdani, and J. C. Simon, J. Lightwave Technol. 23, 2420 (2005). 7. M. Shtaif and G. Eisenstein, Opt. Lett. 21, 1851 (1996). 8. H. T. Friis, Proc. IRE 32, 419 (1944). 9. P. Berger, M. Alouini, J. Bourderionnet, F. Bretenaker, and D. Dolfi, C. R. Phys. 10, 991 (2009). 10. A. D. McCoy, L. B. Fu, M. Ibsen, B. C. Thomsen, and D. J. Richardson, Electron. Lett. 40, 107 (2004). 11. M. Kobayashi, T. Ishihara, and M. Gotoh, IEEE Photon. Technol. Lett. 5, 925 (1993). 12. P. Berger, J. Bourderionnet, F. Bretenaker, D. Dolfi, and M. Alouini, Opt. Express 19, 21180 (2011). 13. L. Richter, H. Andelberg, M. Kruger, and P. McGrath, IEEE J. Quantum Electron. 22, 2070 (1986). 14. C. H. Henry, J. Lightwave Technol. 4, 288 (1986).
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GHz bandwidth noise eater hybrid optical amplifier: design guidelines Gwennaël Danion,* François Bondu, Goulc’hen Loas, and Mehdi Alouini Institut de Physique de Rennes UMR 6251, CNRS, Université de Rennes 1, Campus de Beaulieu, Rennes 35042, France *Corresponding author: gwennael.danion@univ‑rennes1.fr Received February 27, 2014; revised June 12, 2014; accepted June 12, 2014; posted June 13, 2014 (Doc. ID 207284); published July 15, 2014 This Letter describes the design of an optical amplifier system optimized to reduce the relative intensity noise (RIN) of the input signal, and discloses its performance in terms of intensity noise reduction and bandwidth, without phase noise degradation. This polarization-maintaining amplifier is composed of an erbium-doped fiber amplifier (EDFA) cascaded with a semiconductor optical amplifier (SOA). The EDFA is sized to feed the SOA with a constant power corresponding to the optimal saturation level for noise reduction, through coherent population oscillations. When properly optimized, such an amplifier provides, simultaneously, 17 dB optical gain, 5.4 dB noise factor, and 20 dB reduction of the input-RIN across a 3 GHz bandwidth, without any electronics feedback loop. © 2014 Optical Society of America OCIS codes: (140.4480) Optical amplifiers; (270.1670) Coherent optical effects; (120.4570) Optical design of instruments; (250.5980) Semiconductor optical amplifiers. http://dx.doi.org/10.1364/OL.39.004239
Applications such as optical analog and coherent telecommunications, metrology, spectroscopy, and radar, require lasers with ultra-stable amplitude and phase. Even though solid-state lasers and fiber distributed feedback lasers are known to exhibit narrow spectral linewidths, they suffer from resonant intensity noise around their relaxation oscillation frequency from typically a few kHz to a few MHz. This excess noise can be suppressed by a servo-loop acting on the laser pump [1,2]. Another alternative is to deeply modify the laser dynamics so that it becomes inherently free from relaxation oscillations [3,4]. From a practical point of view, these latter techniques must be implemented when designing the laser. A reduction of the laser intensity noise outside the laser cavity is unavoidable when the laser is off-the-shelf, or when its design cannot afford additional improvements. For instance, this issue came up in one of our experiments, where a dual-frequency laser is to be stabilized on ultra-low expansion cavity of finesse 100,000 [5]. Indeed, this homemade laser is designed to provide, at the same time, high spectral purity, high long-term stability, dual-frequency operation, independent and wide tunability across 1 THz of the two frequencies, as well as voltage-controlled tunability; thus, the laser cavity design is too complex to cope with intracavity noise reduction. Amplitude noise reduction outside the laser cavity is, thus, unavoidable. External noise eaters are commonly used for this purpose, but their insertion leads to significant losses. Moreover, their bandwidth is limited to a few MHz due to the feedback electronics. For most common applications, an ideal noise eater would have a bandwidth of several tens of MHz, with zero insertion losses and even some gain. In this framework, it has been demonstrated that a semiconductor optical amplifier (SOA) might act, regarding an input signal, as a high-frequency bandpass filter, provided that it is operated in the saturated regime. Nevertheless, the use of SOAs is usually avoided because of their high noise factor (NF). One can then wonder whether a proper design of an optical amplification chain combining the advantages of erbiumdoped fiber amplifiers (EDFAs) and that of SOAs could 0146-9592/14/144239-04$15.00/0
bring a straightforward solution to the problem of amplitude noise reduction in solid-state lasers. We detail in this Letter the design and optimization steps of a hybrid amplifier that simultaneously (i) raises the laser power to a constant value, (ii) reduces the relative intensity noise (RIN) within a GHz bandwidth, and (iii) does not add any noticeable close-to-the-carrier frequency noise. As already mentioned, in our application, the amplifier system has to meet several constraints [5]. More precisely, the Er/Yb:glass solid-state laser source has a RIN of −90 dB∕Hz at 100 kHz, corresponding to the relaxation oscillation frequency, and which must be reduced to −110 dB∕Hz. Furthermore, the laser output power of 1 mW must be amplified to 10 mW, at least. In addition, the amplifier total length should remain below 20 m to minimize the delay for the frequency stabilization loop, whose actuator is located upstream of the amplifier. Finally, a NF less than 7 dB is targeted to meet amplitude noise requirements far from the carrier. The basic idea here is to take advantage of coherent population oscillations (CPOs) in the SOA [6,7] to damp down the laser noise across a bandwidth of a few GHz and to use an EDFA amplifier specifically designed to bring the SOA in the optimal saturation regime for noise reduction, whatever the input laser optical power. In addition, the EDFA, which precedes the SOA, has to provide a NF close to the 3 dB quantum limit, so that the overall amplification chain exhibits a low NF, owing to Friis’s formula [8]: F F EDFA
F SOA − 1 ; GEDFA
(1)
where F EDFA and F SOA are the NF of the EDFA and the SOA, respectively, and GEDFA is the EDFA gain. The SOA under consideration is an InP/InGaAsP multiple quantum well layer single pass traveling wave amplifier that is commercially available (Thorlabs/ Covega BOA1004P). This SOA is chosen because of its high gain level of 25 dB and its high output saturation © 2014 Optical Society of America
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power of 15 dBm, which are both favorable for efficient noise reduction through gain saturation. Its NF in small signal condition is given to be 7 dB for an injection current of 600 mA. We first checked the achievable noise reduction bandwidth by characterizing the frequency response of the amplifier when operated under saturation. To this aim, a DFB laser, followed by a 20 GHz Mach– Zehnder modulator feeds the SOA. The output optical power is attenuated and detected with a 20 GHz InGaAs photodiode. The photodiode is connected to a vectorial network analyzer driving the Mach–Zehnder modulator. The frequency response of the optical link is then acquired with and without the SOA. After normalization, the frequency response of the SOA is recovered, as shown in Fig. 1(b). As expected, this plot shows a high-pass filter behavior leading to a reduction of about 15 dB of the modulated power over 3 GHz bandwidth. The frequency range over which this power reduction occurs is determined by the carrier’s lifetime in the SOA [9]. Obviously, this mechanism will occur as well in the EDFA. Nevertheless, it cannot be exploited for our purpose because the upper metastable level has a long lifetime of 10 ms, which leads to gain compression from DC to a few hundreds of Hz only. This is illustrated in the acquisition of Fig. 1(a), performed on the homemade EDFA that will be described later. The next step is to quantify the input optical power inducing a maximum noise reduction. Indeed, as already demonstrated [10], the output RIN is expected to decrease when increasing the mean optical power. However, increasing this power excessively might bleach the amplifier, resulting in the same output RIN level as the input RIN level. Consequently, the input power for optimal noise reduction has to be determined precisely. To this aim, the SOA is injected with an amplified single frequency solid-state Er/Yb:glass laser. When pumped at 980 nm with a pumping power 2.5 above threshold, this laser exhibits noise in excess at the relaxation oscillation frequency at 100 kHz (see Fig. 4). This laser is followed by a commercial EDFA and an optical attenuator to precisely adjust the power sent to the SOA. One then measures, at the output of the SOA, the RIN level at 100 kHz versus the input power. Figure 2 shows the RIN reduction for different input powers entering the SOA. For powers up to 6 mW, the RIN level decreases when increasing the input power. However, for higher input powers, the measured RIN reduction starts to saturate
Fig. 1. Frequency responses normalized to the gain for infinity frequency of (a) the EDFA and (b) the SOA.
Fig. 2. SOA RIN reduction measured at 100 kHz for different input powers.
as anticipated. Thus, for 10 mW input power, the SOA is highly saturated and, in addition to the 17 dBm it delivers, it also provides more than 20 dB RIN reduction. We have measured the SOA NF when the input power is set to 10 mW; that is, when the SOA operates in deeply saturated regime. It turned out that the NF is slightly degraded to 8.3 dB compared with the small signal NF of 7 dB. It is worthwhile to notice that, for a laser delivering 0 dBm, the RIN reduction would be only 4 dB at 100 kHz. To enable the SOA operating in the optimal noise reduction conditions whatever the laser power, we have designed an EDFA amplifier such that its output saturation power is 10 dBm and its small signal NF is close to the quantum limit of 3 dB. When placed in front of the SOA (see Fig. 3), this EDFA ensures simultaneous optimal power of 10 dBm at the input of the SOA, and a relatively low NF of the whole amplification chain. The EDFA includes polarization-maintaining elements (fibers and optical components) to comply with the polarization-sensitive SOA and because we intend to preserve the laser linear polarization at the output of the amplification chain. Obviously, a similar design would apply for a polarization-independent amplification chain. All fibered components are spliced with accuracy better than 1°. The gain medium is a short 1.25 m highly doped Er3 Panda-type fiber having a core diameter of 5 μm and a linear absorption of 28.8 dB∕m at 1530 nm. It is pumped at 980 nm with 40 mW in the forward configuration. As
Fig. 3. Setup of the GHz bandwidth noise eater hybrid amplifier. ISO, optical isolator; WDM, wavelength division multiplexer at 980 nm.
July 15, 2014 / Vol. 39, No. 14 / OPTICS LETTERS
sketched in Fig. 3, the pump diode is coupled to the doped fiber with a hybrid component associating a wavelength multiplexer (WDM) and a polarizing isolator. After the gain medium, another hybrid WDM/polarizing isolator enables both isolation of the first stage and rejection of the remaining 16 mW of pump power into a beam block. In these conditions, the minimum EDFA small signal gain is measured to be 20 dB on the wavelength band 1530–1560 nm, whereas its output saturation power is 10 dBm, as desired. For 1 mW input power the EDFA gain is compressed to 10 dB and its NF is obviously moderately degraded to 4.5 dB. The EDFA and SOA fibers are spliced to avoid any backreflection that could lead to RIN degradation [11]. When cascaded to the EDFA, the SOA input power is now 10 dBm leading to a saturated gain of 7 dB and output saturation power of 17 dBm. As already mentioned, under these conditions, its NF is measured to be 8.3 dB. Taking into account the WDM/isolator pigtails, the doped fiber and the SOA pigtails, the overall amplifying chain length is of about 9 m, which is much shorter than the upper limit of 20 m required for proper operation of an optical frequency stabilization loop, such as that reported in Ref. [5]. At offset frequencies larger than 10 GHz, the CPO mechanism becomes negligible for both amplifiers. Consequently, they can be considered as linear, so that the Friis formula [Eq. (1)] applies. Taking into account the measured values of gain and NF, one expects the NF of the whole amplifier chain to be 5.3 dB, which is lower than the SOA NF. This is confirmed experimentally, since we measure a NF of 5.4 dB for the whole amplification chain when the input power is set to 1 mW. It must be noted that the EDFA, being well-saturated for input powers in the mW range, always provides a constant 10 dBm output power, which feeds the SOA. In this condition, we benefit from a maximum RIN reduction and also from a constant output power of 17 dBm. The optical spectral range of the overall amplification chain is limited in our experiment by the EDFA, i.e., 1529–1561 nm. We then measured the effective RIN reduction with our solid-state laser, whose relaxation oscillation is at about 100 kHz. After the hybrid amplifier, we measured a RIN reduction of 20 dB, as shown in Fig. 4. One can notice that this noise reduction is broadband. According to Fig. 1(b), this is potentially valid up to 3 GHz. Nevertheless, the measurement has been performed up to 10 MHz, where it gets limited by the noise floor of our electrical detection setup. It is worthwhile to notice that, in this low frequency region, the proper SOA RIN is also subject to CPO and is, thus, well below −155 dB∕Hz [12]. For offset frequencies below 10 kHz, we found that the fluctuations of the EDFA pump power and of the SOA pump current play a nonnegligible role, so that the RIN reduction is less important. Nevertheless, this could be improved if required by using low noise drivers or batteries. We have finally verified that the amplitude noise reduction is not obtained at the cost of frequency noise increase. The frequency noise is analyzed using a self-heterodyne method with a fibered Mach–Zehnder interferometer [13]. One arm contains an acousto-optic modulator at 80 MHz, whereas the second arm includes a 700 m delay fiber. At the output, the 80 MHz beat note frequency is detected by a 16 GHz bandwidth InGaAs
4241
Fig. 4. RIN at input (laser RIN) and output of the amplifier chain. The peak corresponds to the excess noise lying at the relaxation oscillation frequency of the laser.
photodiode, and then demodulated with respect to I and Q quadratures. The phase is then reconstructed, and its spectrum converted into laser frequency noise. In Fig. 5, the black line represents the frequency noise of our laser before the amplifier, whereas the gray line represents the frequency noise at the output of the hybrid amplifier. We note that the frequency noises are almost identical, except at the relaxation oscillation frequency of the laser, where a tiny peak is observed. This residual frequency noise in excess is probably induced by amplitude-to-phase conversion in the SOA [14]. In conclusion, we report here, step by step, the realization of a 2-GHz bandwidth noise eater hybrid amplifier. This amplifier provides 17 dB gain and is optimized to damp by a 20 dB factor the amplitude excess noise of the input source over a bandwidth of 2 GHz. It mainly consists of a carefully designed EDFA cascaded to a commercial SOA, so that the EDFA output saturation power fits the SOA input power for which a maximum noise reduction is achieved through coherent population oscillations. This optimized design makes it possible to operate the SOA in the optimal noise reduction condition for a significantly large power range at the input of the
Fig. 5. Frequency noise at input and output of the hybrid optical amplifier system.
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OPTICS LETTERS / Vol. 39, No. 14 / July 15, 2014
EDFA (typically, −10 dBm up to 5 dBm). Owing to the low NF of the EDFA, this hybrid amplifier exhibits an acceptable NF of 5.4 dB. Being developed for amplifying solid-state lasers while reducing their amplitude noise in excess, in particular at the relaxation oscillation frequency, we have checked that this hybrid amplifier does not bring any significant degradation of the frequency noise of the amplified signal. The characteristics reported here are obtained in the “passive” mode operation. The SOA injection current can be handled for further amplitude stabilization or for more sophisticated control of the noise. It also provides a simple way to tag the optical carrier at a given frequency from DC to a few GHz, if required. Passive and active mode operation are currently implemented in the framework of a broader project, aimed at generating THz frequency references through photo-mixing dual-frequency lasers locked to ultra-low-expansion high-finesse cavities [5]. The authors acknowledge C. Hamel, L. Frein, and S. Bouhier for their technical support. This work is supported by the Agence Nationale de la Recherche (OSMOTUS ANR-2011-BS03-010-01) and by Région Bretagne. References 1. S. Taccheo, G. De Geronimo, P. Laporta, and O. Svelto, Opt. Lett. 21, 1747 (1996).
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