Electrical measurements of the noise figure of a fiberoptical parametric amplifier are presented and compared with optical measurements. The transfer of pump noise by four-wave mixing was clearly demonstrated. A numerical model was developed to simulate the transfer of pump noise and validated by these measurements. Using this model, we determine, for practical systems, a minimum required pump optical signal-to-noise ratio of 65 dB

Bayart, D.

Durécu-Legrand, A.

Lantz, E.

Maillotte, H.

Mussot, Arnaud

Simonneau, C.

Sylvestre, T.

HAL Descartes

Impact of pump OSNR on noise figure for fiber-optical parametric amplifiers

Hal-Diderot

HAL - Université de Franche-Comté

1178 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 17, NO. 6, JUNE 2005Impact of Pump OSNR on Noise Figure forFiber-Optical Parametric AmplifiersA. Durécu-Legrand, C. Simonneau, D. Bayart, Member, IEEE, A. Mussot, T. Sylvestre, E. Lantz, and H. MaillotteAbstract—Electrical measurements of the noise figure of a fiber-optical parametric amplifier are presented and compared withoptical measurements. The transfer of pump noise by four-wavemixing was clearly demonstrated. A numerical model was de-veloped to simulate the transfer of pump noise and validated bythese measurements. Using this model, we determine, for practicalsystems, a minimum required pump optical signal-to-noise ratioof 65 dB.Index Terms—Noise figure (NF), optical amplifiers, optical fibercommunication, optical Kerr effect, parametric amplifiers.I. INTRODUCTIONWITH THE recent advances, it is now well-known thatthe fiber-optical parametric amplifiers (FOPAs) succeedin providing broad-band and high gain amplification. Very fewstudies are presently focusing on the noise performance. First,Blows and French showed that, in the case of a parametricpump constituted by a continuous source amplified by anerbium-doped fiber amplifier (EDFA), the output noise level ofthe FOPA is considerably reduced when the pump is filteredso as to remove any broad-band amplified spontaneous emis-sion (ASE) noise present in the amplification band [1]. Morerecently, it was proved that rather than an optical measurement,an electrical measurement of the noise figure (NF) of theFOPAs was required to take into account other sources of noisethan ASE [2]. Kylemark et al. numerically found that the NFis degraded by the remaining ASE under the pump peak [3].In this letter, we present an electrical measurement method ofthe NF. With this method, we experimentally demonstrate theimpact of the pump optical signal-to-noise ratio (OSNR) onthe FOPA NF. Finally, we show that a pump OSNR as high as65 dB is required to keep the NF to the quantum limit with achannel input power of 21 dBm.II. MEASUREMENT METHOD OF THE NOISE FIGUREThe NF of an optical amplifier can be evaluated either opti-cally from the measurements of the input and output OSNRs,or electrically. However, optically, only broad-band noise, likeASE, is detected because of the resolution of optical spectrumManuscript received October 21, 2004; revised January 20, 2005.A. Durécu-Legrand, C. Simonneau, and D. Bayart are with the Researchand Innovation Department, Alcatel, Marcoussis F-91460, France (e-mail:anne.durecu@alcatel.fr).A. Mussot, T. Sylvestre, E. Lantz, and H. Maillotte are with the Départe-ment d’Optique P.M. Duffieux, Institut FEMTO-ST,U.M.R CNRS/Universitéde Franche, Besançon Cedex 25030, France.Digital Object Identifier 10.1109/LPT.2005.846559analyzers (OSAs). An electrical measurement gives a more ac-curate estimation of the NF. The electrical NF is determinedwith a relative-intensity noise (RIN) analyzer and by the RINsubtraction technique [4](1)where is the input signal power, and are,respectively, the RIN measured at the amplifier input and output.The measured value of RIN is due to different types of noise:The actual RIN of the signal source but also the beating noisebetween the signal and any other optical noise(2)(3)In these expressions, is the amplifier gain, an inputnoise, and noise generated in the amplifier responsibleof the NF. The notation means that only the polarization com-ponent of noise, parallel to the signal one, beats the signal.In order to validate the electrical method, an experiment wasset up to perform simultaneously optical and electrical measure-ments of noise in the well-known case of an EDFA. Output noiseis only constituted by broad-band ASE which can be easily de-tected either optically or electrically. The output of the amplifieris divided by a coupler. On one port, an OSA measured the gainand the OSNR. On the other port, an optical filter with a 3-dBbandwidth of 0.2 nm selects the signal wavelength and a RINanalyzer measures the output RIN at a particular frequency. TheRIN analyzer is calibrated and subtracts the thermal noise andthe shot noise from the total noise. To determine the NF, levelsof noise are first measured at the input of the amplifier then atthe output. A fair agreement was found between both electricaland optical NFs; the discrepancy between both is about 0.35 dB.These measurement techniques can be applied on a parametricamplifier provided we take into account the particularity of theone-pump FOPA, which is to produce polarized ASE contraryto EDFAs as pointed out in [6]. This assertion was experimen-tally checked out with the following experiment. At the outputof the FOPA, a filter selected 0.2 nm in the signal band at themaximum of gain. No signal was injected in the FOPA and the1041-1135/$20.00 © 2005 IEEEDURÉCU-LEGRAND et al.: IMPACT OF PUMP OSNR ON NF FOR FOPAs 1179Fig. 1. Experimental setup for the measurement of the NF of the FOPA.pump was filtered so that only ASE was detected. A polariza-tion controller placed in front of a polarizer enabled to measurea polarization extinction ratio of 18 dB with an OSA. It corre-sponds to a degree of polarization of 98% so we can state thatthe ASE of the FOPA is fully polarized.At high gain, the standard definition of NF is NF, where is the population inversion factor. ForFOPA, the ASE is polarized soand the optical NF is related to the total ASE power asfollows:(4)whereas, in the case of an EDFA, the ASE is not polarized so. Therefore, the optical NF isrelated to the total ASE power as follows:(5)III. PARAMETRIC AMPLIFIER UNDER TESTThe FOPA under test is depicted in Fig. 1. The pump is firstphase-modulated by a pseudorandom bit sequence of 3.5 Gb/sto increase the threshold power of the stimulated Brillouin scat-tering. Then it is successively amplified by two EDFAs with afiltering of the ASE in between. Finally, a circulator and a reflec-tive fiber Bragg grating (Filter 2) reduce the level of ASE fromthe EDFAs, as proposed in [1]. A variable attenuator, placed infront of the first EDFA, is used to vary the OSNR of the pump,i.e., the level of the residual ASE at the pump wavelengthnm. The pump power remains constant to 640 mW. Theamplifying fiber used in this FOPA is a highly nonlinear dis-persion-shifted fiber ( and nm). Onthe signal path, a polarization controller allows adjusting thesignal polarization to maximize the gain and an isolator protectsthe input from occasional reflections. The signal is amplifiedby an EDFA and its power is controlled with a variable attenu-ator; it is coupled to the pump through the fiber Bragg grating.At the output of the highly nonlinear fiber, a circulator and afiber Bragg grating (similar to Filter 2 of the pump) separatethe amplification spectral band from the pump. Electrical mea-surements are performed at the frequency 68 MHz, where thepump RIN is below 155 dB/Hz. FOPAs are generally pumpedby a low power source, amplified by successive EDFAs. Firstthe pump has to be filtered to suppress ASE produced by theEDFAs in the amplification spectral band of the FOPA. But evenwith the narrowest filtering centered at the pump wavelength,ASE from the EDFAs remains in the bandwidth of the filter atthe pump wavelength. When this noise is injected in the para-metric amplifier all together with the pump and the signal, it isFig. 2. Input pump spectrum (res = 0:05 nm).Fig. 3. Example of noise transferred from the pump pedestal (represented inFig. 2) to the signal wavelength (res = 0:05 nm).reproduced at the signal wavelength through a four-wave mixing(FWM) process between the signal, the pump, and this pumpnoise. To obtain Fig. 2, we intentionally degrade the FOPA per-formances by reducing the pump OSNR. In Fig. 3, the transferof the pump noise to the signal can be observed on the OSA (res-olution set to 0.05 nm) as an additional noise, very narrow, lo-cated around the signal wavelength. Flat broad-band parametricASE can be seen as well, at a much lower level. With a betterpump OSNR, transfer of pump noise at the signal wavelengthcan be no longer observed experimentally on the OSA so that itis not taken into account in the measurement of OSNRs.IV. RESULTS AND DISCUSSIONSAs the noise observed on the OSA is the result of an FWMproduct between the pump, the signal, and ASE at the pumpwavelength, its level is proportional to the signal power and tothe level of ASE, i.e., to the inverse of the OSNR of the pump.These trends have been experimentally verified. The OSNRof the pump and the signal power are controlled with vari-able attenuators (see Fig. 2). First for a signal wavelength of1595 nm, the output noise has been measured optically andelectrically while the input signal power was increased from35 to 5 dBm, the pump OSNR remained constant as well asthe net gain (19.1 dB). This measurement was carried out forthree different pump OSNRs. Then at the same signal wave-length, the output noise was also measured versus the pump1180 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 17, NO. 6, JUNE 2005Fig. 4. FOPA NF as a function of signal power for three different pumpOSNRs. (Case of NF . Symbols: Eexperiments. Lines: Simulations).Fig. 5. FOPA NF as a function of pump OSNR for three different signalpowers. (Case of NF . Symbols: Experiments. Lines: Simulations).OSNR in the case of three input signal powers. The resultingNFs are presented in Figs. 4 and 5. In these figures, the elec-trical and optical NF are represented; for the optical one, themeasurements takes into account only parametric ASE. As canbe seen, the electrical measurement detects an additional noise,which depends on the input signal power and more particularlyon the level of the pump OSNR. This noise gets negligiblecompared to parametric ASE when the signal power is low andthe pump OSNR high. In this case, electrical NF meets opticalNF to 4.4 dB. It is the first time to our knowledge that a realnoise floor is electrically measured and that this noise floorcorresponds to the optical measurement. This 4.4-dB limit ismainly due to relatively high coupling and distributed losses(respectively, about 0.8 and 0.3 dB) and noise produced by theRaman effect [5], [6]. Moreover, we numerically simulatedthe transfer of pump noise toward the signal through FWMby solving the scalar nonlinear Schrödinger equation with astandard split-step Fourier method. The pump and its noiseare modeled by a monochromatic continuous wave, and inthe spectral domain, we add a square of random noise cen-tered at the pump wavelength, with a bandwidth of 0.2 nmand which average level corresponds to the required OSNR.The signal is also a monochromatic continuous wave. Opticalwaves are all considered in the same state of polarization. Thesimulated pump OSNR is higher than the experimental oneby 3 dB because pump noise in the orthogonal polarization isnot accounted for. At the output of the amplifier, we calculatethe level of noise produced around the signal wavelength.The model does not simulate the Raman effect, leading to aminimum NF of 3.8 dB. In experiments, as the pump powerremains constant, Raman noise is also constant. A constantnoise power was, therefore, added to account for the Ramannoise and to achieve the experimental 4.4-dB NF. In Figs. 4and 5, the simulated results are represented by the lines andthe experimental ones by the points. A fair agreement is foundbetween simulations and experimental results, so the numericalmodel can be used to define a pump OSNR specification. Inpractical systems, amplifiers provide a total output power of20 dBm, which corresponds, for 128 channels, to a channelinput power of 21 dBm. According to our model, a 4.4-dBNF is achieved provided the pump OSNR is higher than 65 dB(0.1 nm), in this case, the pump noise is negligible comparedto parametric ASE.V. CONCLUSIONAn electrical measurement method of the NF of an opticalamplifier has been implemented. The results of this measure-ment were first compared to a standard optical measurementand validated in the case of an EDFA. When applied to anFOPA, it allowed us to experimentally quantify, for the firsttime, the noise limitations caused by the pedestal noise presentat the pump wavelength. The FOPA NF was measured versusthe pump OSNR and as a function of the signal power. A nu-merical model was developed to simulate the transfer of pumpnoise and validated by these measurements. Using this model,we determine, for practical systems, a minimum required pumpOSNR (0.1 nm) of 65 dB.REFERENCES[1] J. L. Blows and S. E. French, “Low-noise-figure parametric amplifierwith a continuous-wave frequency-modulated pump,” Opt. Lett., vol. 27,pp. 491–493, 2002.[2] P. Kylemark, T. Torounidis, P. O. Hedekvist, H. Sunnerud, and P. A. An-drekson, “Noise figure characterization of fiber optical parametric am-plifiers,” in 29th Eur. Conf. Optical Communication, Rimini, Italy, 2003,Paper We.1.6.4.[3] P. Kylemark, P. O. Hedekvist, H. Sunnerud, M. Karlsson, and P. A. An-drekson, “Noise characteristics of fiber optical parametric amplifiers,” J.Lightw. Technol., vol. 22, no. 2, pp. 409–416, Feb. 2004.[4] F. W. Willems, J. C. van der Plaats, C. Hentschel, and E. Leckel, “Opticalamplifier noise figure determination by signal RIN subtraction,” in Tech.Dig. Symp. Optical Fiber Measurements, Sep. 1994, pp. 7–9.[5] P. L. Voss and P. Kumar, “Raman-noise-induced noise-figure limit for(3) parametric amplifiers,” Opt. Lett., vol. 29, no. 5, pp. 445–447,2004.[6] R. Tang, P. L. Voss, J. Lasri, P. Devgan, and P. Kumar, “Noise-figurelimit of fiber-optical parametric amplifiers and wavelength converters:Experimental investigation,” Opt. Lett., vol. 29, pp. 2372–2374, 2004.

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Impact of pump OSNR on noise figure for fiber-optical parametric amplifiers

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HAL - Université de Franche-Comté