Measurement of ASE in an in-line EDFA

Transcription

1 Measurement of ASE in an in-line EDFA Amandeep kaur 1, Jagtar Singh 2 1 Student M.Tech.(ECE) Yadwindra College of Engg. 2 Associate Professor (ECE) Yadwindra College of Engg. Abstract- To achieve population inversion an EDFA uses stimulated emission process where incident photon and emitted photon have same phase. But, alongwith the stimulated emission that creates gain, the gain medium also produces spontaneous emission, which gives rise to the amplified spontaneous emission (ASE) spectrum of the amplifier []. In this paper we have considered the setup to measure amplified spontaneous emission in an in-line EDFA. The effect of pump power and doping radius is studied on ASE at pump wavelength of 1480nm. Also the ASE spectrum is observed with increased number of fiber spans. It has been observed that the ASE power increases with increase in pump power and EDFA length. It is further observed that increasing the doping radius reduces ASE power. The ASE spectrum peak is found to shift to longer wavelengths with increased number of fiber spans. The simulation has been done using optsim 5.2 software. Index Terms- Amplified Spontaneous Emission (ASE), Erbium Doped Fiber Amplifier ( EDFA), Noise Figure(NF) INTRODUCTION The transmission distance in any fiber optic communication system is limited by fiber losses. For long-haul systems, loss limitation was initially overcome by using optoelectronic repeaters in which the optical signal is first converted into an electrical current and then regenerated using a transmitter [1]. With growing transmission rates (WDM systems), electronic regeneration becomes more and more expensive. An alternative approach to loss management makes use of optical amplifiers, which amplify the optical signal directly without conversion to electrical domain. Optical amplifiers have really revolutionized the field of fiber optics communication. Optical amplifiers are in general bit rate transparent and can amplify signals at different wavelength simultaneously. ERBIUM DOPED FIBER AMPLIFIER An erbium doped fiber amplifier is an optical fiber, few meters in length, doped with erbium ions(er+3).the erbium ions absorb certain wavelength of light energy to excite atoms into higher energy states. The proportion of atoms excited into higher energy levels is called inversion level of erbium doped fiber. In this, population inversion is stronger due to large number of erbium ions that fall to lower level from upper levels. When optical information pass through such heavily populated erbium doped fiber, it would stimulate transition of erbium ions upper from level to lower level and 190

2 generating photons of same wavelength with direction and phase as input photon. EDFA consists of three basic components: length of erbium doped fiber, pump laser and wavelength selective coupler to combine the signal and pump wavelengths. Optimum value fiber length depends upon pump power, input signal power, amount of erbium doping and pumping wavelength. Efficient EDFA pumping is possible using semiconductor lasers WORKING PRINCIPLE To achieve optical amplification, population inversion is a necessity, which means the population of upper energy level has to be greater than that of lower energy level, i.e. N2> N1, where N1, N2 is population density of lower and upper state. This can be achieved by exciting electron into higher energy level by external source called pumping. When incident photon having energy E= hc/λ interacts with electron in upper energy state to cause it return to lower state with creation of second photon( h is Plank constant, c is velocity of light and λ is the wavelength of light), stimulated emission occurs. When incident photon and emitted photon are in same phase and release two more photons, light amplification occurs and continuation of process effectively creates avalanche multiplication. operating near 980 and 1480 nm wavelengths. EDFAs can be designed to operate in such a way that the pump and signal beams propagate in opposite directions (backward pumping) or same direction (forward pumping). In bidirectional configuration the amplifier is pumped in both the directions simultaneously by using lasers at two ends of fiber. Therefore amplified coherent emission is obtained. NOISE IN EDFA Erbium Doped Fiber Amplifiers are used as in-line amplifiers to compensate for power losses caused by fiber attenuation, connections, and signal distribution in networks. So the main requirement of this type of amplifier is stability over entire WDM bandwidth. Noise should be at the minimal level for an in-line amplifier. The fact that noise is inherent in all amplification systems based on atomic population inversion can be understood by the physical picture of amplified spontaneous emission. Besides the stimulated emission that creates gain, the gain medium also produces spontaneous emission, which gives rise to the amplified spontaneous emission (ASE) spectrum of the amplifier [4]. In EDFA, ASE is the dominant noise source. This spontaneous 191

3 emission reduces the amplifier gain by consuming the photons that would otherwise be used for stimulated emission of the input signal. This ASE noise limits the optical signal-to-noise ratio (SNR) of a cascade of amplifiers and is quantified in the amplifier s noise figure (NF). This can be denoted as NF =2 n sp / n i in which n sp = N2 / (N2-N1) is the inversion parameter of the amplifier (i.e., the degree of population inversion, with N1 and N2 the fractional number of erbium atoms or carriers in the ground and excited states, respectively), and this is the input coupling loss. Both well-designed EDFAs and SOAs have inversion factors close to unity, but the fiber-chip coupling loss of the SOA puts it at a disadvantage. EDFA noise figures typically are 4 6 db, while SOA noise figures are usually 6 8 db [4]. LITERATURE SURVEY Stephen Z. Pinter presented a Simulink model for investigating erbium-doped fiber amplifier (EDFA) gain spectrum and ASE. They presented a method for simulating the gain spectrum and forward ASE of an EDFA using the Simulink model implemented by Novak and Gieske. The resulting EDFA model accurately represented EDFA gain dynamics and ASE. An approach to gain flattening was also discussed where a flat gain is achieved by manipulating the input pump power[7]. Hossein Sariri, Mohammad Mehdi Karkhanehchi, Ali Mohammadi, Fariburz Parandin presented a method for investigating EDFA dynamics using the tools MATLAB and Simulink. They investigated the effect of ASE on the gain modulation in EDFAs. It was shown that ASE has effect on predictions in the high gain/low saturation regime[8]. Sulaiman Wadi Harun, T. Subramaniam, Nizam Tamchek & Harith Ahmad presented the effect of injecting conventional band amplified spontaneous emission (C-band ASE) on the performance of long wavelength band erbium-doped fiber amplifier (L-band EDFA). It was shown that the injection of a small amount of ASE (attenuation of 20 db and above) improves the small signal gain with a negligible noise figure penalty compared to that of an amplifier without the ASE injection[9]. I.Roudas, N.Antoniades, R.E.Wagner, S.F.Habibi, T.E.Stern presented a theoretical study of the impact of filtered Amplified Spontaneous Emission noise and signal distortion on the performance of a cascade of Wavelength Add-Drop Multiplexers. In this study, they showed that the maximum allowable laser offset is approximately 30 GHz and the maximum laser misalignment tolerances must be smaller than 30 GHz[10]. M. A. 192

4 Mahdi, H. Ahmad proposed a novel method of enhancing gain in the long wavelength band erbium-doped fiber amplifier using the unwanted amplified spontaneous emission (ASE) self-pumping technique. A dual-stage amplifier is deployed, where the unwanted backward ASE from the first stage is fully utilized to pump the second stage amplifier. Flat-gain values in excess of 24 db and noise figures < 5 db were obtained from the proposed amplifier with a higher channel counts compared to the conventional amplifier[11]. Silvano Donati, Guido Giuliani formulated a new semi classical wave theory of the noise in optical amplifiers using a few quantum statements in a classical signal framework. They have shown that the optical amplifier can be modeled as an amplifying beam splitter and that the ASE comes from the amplification of vacuum fluctuations[12]. 10Gbps and 7 degrees pseudorandom sequence. Electrical generator is on-off ramp type with NRZ modulation format. It has rise and fall times of 40e-12 and ring filter type configuration. CW laser1 has a peak power of 1mW and operates at wavelength=1550nm in single mode with random intensity noise of - 150dB/Hz.External modulator1 is MZ type with on-off ratio=30db.an optical normalize is used to normalize the average power output of -40dBm. CW laser2 has a pump power of 0.1W and wavelength of 1480nm. It operates in single mode with random intensity noise (RIN) = - 150dB/Hz. The Write-Once/Read-Many model is a special model that is used in cases where connection branches into a repetition loop. SIMULATION SET UP TO MEASURE THE ASE NOISE The simulation setup to measure the ASE in an in-line EDFA is shown in fig 1.1. The data source is a PRBS generator with a bit rate of 10Gbps and baud rate of Fig.1.1Simulation setup to measure ASE The input signal is cached inside the model change the number of fiber spans. An and is read as many times as requested optical MUX is used to combine the inside the loop. The model has no signals from CW laser and input signal parameters. A repetition loop is used to through repetition loop. The MUX 193

5 ASE power(dbm/hz) ISSN: operates in multiband mode and has no losses. The EDFA is unidirectional with meta-stable life of 10ms and 1550nm wavelength. A nonlinear fiber with 81km length and 0.25dB/km loss is used as transmission medium. The optical filter has Gaussian shape with 1550nm center wavelength. The receiver employs an APD with a quantam efficiency of 0.8. Measurements are done with the help of spectrum analyzer, eye diagram analyzer and property map SIMULATION RESULTS In this section we present the results of simulations carried out using optsim software. We have studied the variation of ASE power with increasing EDFA length at different pump powers and doping radius. The variation in ASE power with EDFA length for different pump powers at pump wavelength of 1480 nm, is shown in fig4.3. It is observed that, as the EDFA length is varied from 1m to 10m there is an increase in ASE power. It is further noticed that at higher pump powers the value of ASE power is high. At pump power value of 1mW, maximum value ASE is observed to be -158dBm/Hz, at pump value of 10mW maximum value of ASE is -118dBm/Hz, at pump value of 100mW maximum value of ASE is - 100dBm/Hz. So, in order to minimize ASE noise the pump power supplied to the amplifier should be just enough to achieve population inversion Pump power=1mw Pump power=10mw Pump power=100mw EDFA length(m) Fig1.2 ASE power Vs EDFA length Further the variation of ASE power with doping radius of amplifier is studied. The doping radius is varied from 0.5µm to 4µm. It is observed that ASE power decreases with increase in doping radius. Although, it is desirable to have large doping radius for decreasing the ASE power, but at the same time signal gain increases as doping radius decreases, because the signal light does not suffer from additional absorption. That is, the Er(3+) ions do not exist in the area where the pump power is small. So there is a 194

6 ASEpower(dBm/Hz) ISSN: trade-off between high gain and low ASE while deciding on the doping radius Doping Radius(um) Fig. 1.3 ASE Vs Doping Radius Further, the ASE spectrum is observed by increasing the number of spans of nonlinear fiber (and amplifier). It is observed that initially the ASE spectrum is most strongly peaked near 1530 nm, but as the number of spans increases, spectral peak gradually shifts to longer wavelengths or shorter frequencies. The ASE spectra peak is at 1532nm after first span, after fourth span ASE is peaked at 1533nm, ASE spectra peak after eighth span is 1558nm and that after the twelfth span the ASE peak is at is 1560nm. This shift in ASE spectra peak towards longer wavelengths with increased number of fiber spans is depicted in figure below. Fig 4.1(a) shows the ASE spectra after the first span, (b) shows the spectra after the fourth span, (c) shows the spectra after the eighth span and (d) shows the spectra after the twelfth fiber span. Fig 1.4(a) ASE spectra after 1 st span Fig 1.4(b).ASE spectra after 4 th span 195

7 Fig 1.4(d) ASE spectra after 8 th span CONCLUSION From the above discussion it is concluded that, the ASE power in an EDFA increases with increase in pump power. So the pump power provided to an EDFA should be just enough to ensure population inversion. Further, ASE power decreases with increase in doping radius, but at the same time increase in doping radius gain decreases so there is a trade-off between increasing gain and decreasing ASE while deciding upon doping radius. It is also observed that as the number of fiber spans increases in an optical link the ASE spectrum peak shifts to longer wavelengths or shorter frequencies. REFERENCES 1. Agarwal G.P. "Fiber Optic communication systems,"3 rd ed. John Wiley, New Delhi, Gerd Keiser, Optical Fiber Communication, McGraw-Hill Higher Education, 2000 pp. 8-12, 35-37, , Agrawal G.P., "Nonlinear fiber Optics," 3rd ed. Academic Press, San Diego, Ca, Donald R. Zimmerman and Leo H. Spiekman, Amplifiers for the Fig.1.4(c) ASE spectrum after 12 th span Masses: EDFA, EDWA, and SOA 196

8 Amplets for Metro and Access Applications Journal Of Lightwave Technology, Vol. 22, No. 1, January P. C. Becker, N. A. Olsson and J. R. Simpson, Erbium-Doped Fiber Amplifiers: Fundamentals and Technology, Academic Press, New York, Rsoft, Optsim user guide and application notes, Rsoft Design Group, Inc., Stephen Z. Pinter, EDFA Simulink Model For Analyzing Gain Spectrum And ASE Hossein Sariri, Mohammad mehdi Karkhanehchi, Ali Mohammadi, Fariburz Parandin, A Dynamic Simulink Model For Erbium- Doped Fiber Amplifiers Overmodulation in Presence of Amplified Spontaneous Emission Effect, J. Basic. Appl. Sci. Res., 2(3) , Sulaiman Wadi Harun, T. Subramaniam, Nizam Tamchek& Harith Ahmad, Gain And Noise Figure Performances of L-Band EDFA with an Injection of C-Band ASE, Jurnal Teknologi, 40(D) Jun. 2004: 9 16 Universiti Teknologi Malaysia. 10. I.Roudas, N. Antoniades, R.E. Wagner, S.F. Habiby and T.E. Stern, Influence of filtered ASE noise and optical filter shape on the performance of a WADM cascade ECOC 97, September 1997, Conference Publication no. 448, c IEE, M.A. Mahdi, H. Ahmad, Gain enhanced L-band Er3+ doped fiber amplifier utilizing unwanted backward ASE, IEEE Photon. Technol. Lett., vol. 13, pp , Oct Silvano Donati, Guido Giuliani, Noise in an optical amplifier: Formulation of a new semiclassical model IEEE journal of quantam electronics, vol. 33, no. 9, september

9 Electromagnetic Waves and Genetic Algorithms. Jagtar Singh Sivia was born in 1976 at Bathinda, Punjab, India. He received Bachelor of Engineering Degree in Electrical and Electronics Communication Engineering from Punjab Technical University Jalandhar,Punjab, India in 1999 and Mater of Technology Degree in Electronics Communication Engineering from Punjab Technical University Jalandhar, Punjab, India in 2005.Currently, he is pursuing Ph.D. Degree in the area of antenna systems from Sant Longowal Institute of Engineering and Technology, Longowal, Sangrur, Punjab,India. He is associate Professor in Department of Punjabi University at Yadawindra college of Engineering Talwandi Sabo in Electronics Communication Engineering Section, Bathinda, and Punjab, India. He has published more than 30 papers in various international Journals and referred conferences. His main research interests are in Neural Networks, Antenna Systems Engineering, He is a member of the Institution Engineers (India), Indian Society of Technical Education (India) and International Association of Engineers (IAENG) Amandeep kaur was born in 1978 at Bathinda, Punjab, India. She received her bachelor of Technology Degree in electrical and electronics communication engineering from Punjan Technical University Jallandhar, Punjab, India in She is a student of M.Tech. in the Department of Punjabi University at Yadawindra College of Engineering Talwandi Sabo in Electronics and Communication Section,Bathinda,Punjab,India. 198