Optical Fiber Amplifiers-Review

Transcription

1 42 Optical Fiber Amplifiers-Review Mahmud Wasfi Senior Member IEEE, Canada Abstract: This paper reviews optical fiber amplifiers such as Erbium doped fiber amplifiers EDFAs, many types of Raman amplifiers RAs, Thulium doped fiber amplifiers, Tellurite-based fiber Raman amplifiers, semiconductor optical amplifiers and fiber optic parametric amplifiers, principle of operation and construction. As these amplifiers are used for optical fiber communication projects so we shall go through their main characteristics which are amplifier gain and span length, wavelength bandwidth and noise. Current repeater spacing and speed of EDFA is in the range of Km and 40 Gb/s while those for RA is in the range of Km and Gb/s. Each type of amplifier is suitable for certain application depending on these characteristics. In metro/access networks there are many add/drop locations resulting in high loss, coarse wavelength division multiplexing with about 20 nm separation between each channel as with this principle cheap multiplexing and laser transmitters are used. Keywords: Erbium-doped fiber amplifier, Raman amplifier, coarse wavelength division multiplexing. 1. Introduction As the optical signal moves along a standard single mode fiber SSMF, it gets attenuated along the fiber and if the data speed is high enough (> 10 Gb/s), it gets distorted due to chromatic and polarization dispersions. To counter attenuation optical fiber amplifiers OFA's are used. Introducing OFA's into the system causes additional problems such as amplified spontaneous emission ASE noise which accumulates as the number of OFA's which the signal goes through increases. The bandwidth of optical fibers is really great if the S-band (short nm), C-band (central nm), and L-band (long nm) are utilized efficiently. So optical fiber amplifiers must be designed to amplify the signal along the fiber, the more the gain, the more span distance between amplifiers as long as the signal is not distorted due to high optical power. To make use of this great bandwidth, dense wavelength division multiplexing DWDM is used, but each type of optical fiber amplifier has different bandwidth. Due to spectral dispersion in optical fibers, dispersion compensating fibers DCF's are commonly used to counter this effect even if one channel(wavelength) is used. For speeds of more than 10 Gb/s such as 40,80,160 and 320 Gb/s more elaborate type of spectral dispersion compensation is used. There are other factors that need to be taken into consideration such as polarization mode dispersion PMD, inter channel crosstalk due to simulated Raman scattering SRS between signals, pattern dependent and independent and inter channel cross-phase modulation XPM. These problems are common to all type of OFA's. Design of amplifiers depends on type application used such as long distance under the sea or terrestrial, short distance with a lot of add-drop locations such as in metro projects. 2. Brief Characteristics of each type of Optical amplifiers 2.1. EDFA In all existing optical fiber projects EDFAs are used, so we have to know what are their characteristics. A sample of commercially available types are the following each with their main specification [1]: (1) gain flattened for small signal SSG of db, saturated output power SOP of dbm, noise figure NF of 4.5 db and bandwidth BW of 35 nm (2) In-Line amplifier with SSG of 43 db, SOP of dbm, NF of 3.6 db and BW of 37 nm. Current repeater spacing is around Km for a speed of 10 Gb/s [2]. A typical system configuration can be as shown in Figure 1. It is made up of 80 Km SSMF, 12 Km DCF with one EDFA before and another after the DCF fiber to raise the gain. Table (1) shows fiber parameters. Figure 1. Common EDFA system with one span.

2 43 amplifiers jointly rather than individually [5].A neural model was presented to design a RA taking into consideration the optimal pump power configuration to calculate on-off gain spectrum and NF [6].The calculated results agreed well with experimental results Thulium-Doped Fiber Amplifier Table 1. Fiber Parameters Figure 2 shows the quality function Q of Gb/s system under optimizes dispersion map using non-returnto-zero NRZ signal with 50 GHz channel spacing with deferent signal level of channel (6) as it is the worst channel with a wavelength of 1570 nm at the output of EDFA number one for a system of 1600 Km transmission (20 spans). An experiment was carried out using high Thulium concentration doping technique to shift the thulium-doped fiber amplifier TDFA gain band to the middle wavelength region of the S-band and using dual-wavelength pumping technique [7]. Figure 3 shows the gain and NF of TDFA with a fiber length of 13 m and a forward pump power ratio of 50 %. As can be seen a gain of 22 db and NF of less than 6 db was achieved with a band of nm, that is in the S-band, using high Thulium concentration of 6000 ppm, compared with 20 ppm of Erbium used in EDFA. The above band complements the conventional C band of EDFA's, and can be used effectively in dispersion shifted fibers DSF because we can avoid degradation due to four wave mixing FWM in the S band. Figure 2. Q factor of channel 6 for 80*10 Gb/s EDFA 2.2. Raman Amplifiers Raman scattering occurs in any silica glass which means if we inject an optical beam (pump) in an optical fiber, then a signal passing through that fiber will be amplified if its frequency is around the shifted frequency of the pump. This is called Stock shift, which is a round 13 GHz (equivalent to about 100 nm) from the pump propagating beam frequency assuming that its wavelength is 1450 nm [3]. That means the signal will be amplified if its wavelength is 1550 nm. Raman amplifiers are based on this phenomenon. There are two main Raman Amplifiers RAs distributed and discrete or lumped like EDFA's. In distributed types, amplification occurs all along the fiber between say two stations with the pump placed either near the transmitter in which case it is called forward pumping or near the receiver in which case it is called backward pumping. So in distributed RA, the fiber itself is acting as an amplifier which is of great advantage. What is exciting in RAs is the use of DCF to compensate for chromatic dispersion and loss [4]. This can be done by increasing the pump power. But if the number of cascaded amplifiers increases, then gain fluctuation might occur. One way to deal with this situation is to optimize the multispan Figure 3. Gain and NF spectrum of 13 m long TDF 2.4. Tellurite-Based Fiber Raman Amplifier It was reported that by using multi wavelength-pumped Raman Amplifier TFRA, a bandwidth of 160 nm ( ) was achieved but with dual peak profile and two bottoms [8]. But with three-stage hybrid FRA consisting of T-FRA with backward pump (P1) as first stage, DCF- RA with forward and backward pumps as a second stage, followed by a gain equalizing filter (or gain equalizer GEQ),and T-FRA with backward pump (P2) as the third stage. Pumps P1 and P2, pump two wavelengths each in order to achieve a wide, high gain and flat spectrum,

3 44 while the DCF-RA is bidirectionally pumped to lower its NF. parametric amplifiers [11]. But recently its advantage was recognized, so many experiments were conducted. It was found that if two laser pumps were injected into a conventional DSF which was used as a nonlinear medium, a gain of 37±1.5 db over 47 nm was obtained [12]. The effect of dispersion fluctuation on noise properties of FOPA's with use of highly non-linear HNL- DSF was studied, it was found that zero dispersion wavelength ZDWL variation became very significant with high gain of more than 25 db [13]. When the input signal power equals a power called the saturation power, the parametric gain will amplify the signal and idler waves equally [14]. The idler wave is produced in all parametric amplifiers PA with a frequency less than the pump by an amount equals exactly the frequency shift between the signal and that of the pump as shown in Figure 5. Figure 4. shows the gain characteristics of a TF (upper level) and three types of silica fibers (bottom level) Figure 4 Gain spectra of a hybrid FRA that has a couple of two-λ-pumped T-FRA stages and a two-λpumped DCF-RA stage between the T-FRA stages. Gain spectra of the FRA with and without the ideal GEQ. As can be seen the wavelength bandwidth obtained was 130 nm with net gain of 24 db and gain difference between on-off was only 17% and net NF was only 8.2 db. So it seems that hybrid FRA can be used as inline amplifier as well as post and preamplifier with wideband, flat high gain and low NF spectra Semiconductor Optical Amplifier Output-level control that accepts a wide range of input power and delivers constant output power is essential for in-line optical amplifiers, optical burst and packet systems and in all optical regeneration and reshaping (2R). Semiconductor optical amplifiers SOA's can meet this demand [9]. The reason for this is because SOA has short carrier lifetime of about several tenths to several hundreds of picoseconds compared to several hundred microseconds to several milliseconds in EDFA's. To control the output level of SOA's, external light injection can be used. It was found out that even if the level of the input signal changed by db at nm modulated at 10 Gb/s, the output level remained constant at + 10 dbm. This method of level control is used in photonic networks Fiber Optical Parametrical Amplifiers (FOPA) Optical parametric generation based on nonlinear optical principle was known for years [10], and also optical Figure 5. Amplified signal and idler after HNLF It was found that to maintain a high gain, either the pump power must be increased or the fiber length by the same ratio, as the gain increases exponentially with these two parameters. This is due to FWM. The effect of PMD and polarization dependent loss PDL was studied, it was found that both affected the quality of pump beams before entering the FOPA, resulting in production pf large changes in signal and idler powers at the output [15]. The effect was found to be very large with orthoganically polarized pumps, but negligible for copolarized pumps. So, by using polarizers just before the input end of the fiber, performance of dual-pump OFPA's can improve dramatically. 3. Level of Amplification, Span length, Speed and Bandwidth In general increase in amplifier gain, can increase the span length and do away with extra amplifiers on the way, on condition that the increase in power level will not cause noise or nonlinearity that affect performance. This principle was applicable in coaxial cable projects and is now applicable in optical fiber projects. As stated in (2.1) SSG o EDFA's can be in the range of 40 db with a span

4 45 of Km. A speed of 40 channels each of 40 Gb/s (1.6 Tb/s) over a system of 800 Km (10 80 Km) under optimal precompensation ratio of 0.16 and inline dispersion compensation ratio of 1 and Gb/s with a span of 1600 Km was achieved for the first time [2]. An experiment was conducted in Germany using existing field optical fiber cable with a distance of 210 Km and third order distributed Raman amplification [16]. The pump used was of 4 W optical output level at a wavelength of 1276 nm (third order pump) which excited a wave at 1356 nm (second order) with an optical level of about 0 dbm, which in turn excited a first order wave of 1455 nm at an optical level of 10 dbm. This last first order wave acted as a pump for the signal of 1550 nm. Total amount of span attenuation was 67 db. The signal launched was at a level of 15 dbm of 170 Gb/s. The BER obtained at the end of the 210 Km was ³ with FEC, indicating error free transmission. The experiment was further extended to eight channels each of 170 Gb/s but to a distance of 165 Km with a total optical loss of 44 db and 300 GHz channel separation, and the result was a BER of 2 10 ³ with FEC, indicating error free transmission. In this experiment 10 Gb/s out of 170 Gb/s was used for error correction. So the total information transmitted was Gb/s or 1.28 Tb/s. The bandwidth of EDFAs as stated in 2.1 is in the range of 40 nm in the C-band. But in RAs, the bandwidth can be expanded greatly by using multiwavelength pumps [2]. An experiment was conducted using two stage configuration over a distance of 100 Km using SSMF, each channel with power level of -10 dbm with six pumps of wavelengths , , 1444, , 1473 and 1509 nm and of power levels in the range of 300 to 42 mw in the first stage and three pumps of wavelengths , and 1509 nm of power levels of 300 to 81 mw in the second stage, a bandwidth of 100 nm ( ) was achieved [5]. Figure 6 shows the net gain of the system. 4. Noises There are three optical impairments in EDFA systems [2]: (i)- ASE (ii) XPM (iii) Raman crosstalk pattern dependent an independent due to SRS between signal and signal. While in RA systems there are two more : (iv) Raman interference noise RIN between pump and signal (v) Multipath interferences MPI. Figure 6. Gain profile of a two stage 100nm In addition to the above optical impairments, there are electrical noises at the receiver after detection, these are : (a) Signal ASE beating noise (b) XPM-induced intensity noise (c) Pattern-dependent Raman-crosstalk intensity noise (d) Signal intensity transferred from forward Raman pumps (e) Signal MPI beating noise. Pattern independent Raman crosstalk ( item iii), does not generate electrical noise directly, but it introduces an extra loss at short wavelengths and extra gain at long wavelength channels. Noises due to optical impairments can be neglected as a first- order approximation. As for the five electrical noises, only (b) and (c) are statistically dependent. This is because XPM-induced intensity noise dominates in the high frequency region which is more than 500 MHz in SSMF and pattern dependent Ramancrosstalk-induced intensity noise dominates in the low frequency region that is less than 100 MHz in SSMF. Figure 7 shows the standard deviation (normalized to the signal level at bit '1') of signal-ase beat noise, XPMinduced intensity noise and pattern-dependent Ramancrosstalk-induced intensity noise due to signal-signal Raman interaction SSRI as a function of the signal channel 6 (worst channel) For Gb/s EDFA under optimized dispersion map. Figure 7. Normalized standard deviations of various noises of channel 6 for Gb/s EDFA system

5 46 Figure 8 shows a similar type of noises of hut-skipped Raman system of Gb/s. As can be seen ASE noise and intrachannel nonlinear effects are the dominant sources of optical impairment. Also we can see that the relative impact of pattern-dependent Raman crosstalk is smaller than in Figure 8, this is because pattern-dependent Raman crosstalk depends only on the low frequency components of the signal which is smaller than 100 MHz in SSMF because of large signal-pump walk off. with a net gain of 10 db was developed [19]. In this experiment a bandwidth of 140 nm was achieved with signal spacing of 20 nm. Because of this large spacing, no effect was caused by modulated laser diodes DML's which were used in this experiment, they were modulated directly and there was no need to be cooled, they were cheap. In general CWDM is simpler and cheaper than DWDM as the the probability of interference between channels is much less. Another experiment was conducted in which triple band S,C and L bands was achieved by using double-pass Erbium-doped silica amplifier with an embedded DCF module [20] using seven channels within nm, spaced 20 nm, each of 2.5 Gb/s with a distance of 122 Km. References Figure 8. Normalized standard deviation of various noises for Gb/s Raman system It has been shown that using copumping in association with counter pumping causes transfer of RIN from pump to signal [17]. This is due to PMD in the fiber and nonlinear gain. Dispersion fluctuation has great influence on noise in FOPA's [13] and that ZDWL variations become more significant in high-gain (>25 db). One way to prevent accumulation of noise is to use multiple quantum well saturable absorbers SA's [18]. A SA is a device exhibiting optical nonlinearity such that its attenuation decreases in response to high optical powers due to excitonic absorption bleaching in multiple quantum well materials. So, when used in a return-to-zero transmission system, it can prevent accumulation of noise on the zeros. Using MQW SA's can prevent impairment arising from low level radiation between pulses. In fact MQW SA's can be used as regenerators in 10 Gb/s DSF soliton and in WDM soliton systems by using DSF. This is because it prevents accumulation of ASE on the zeros. 5. Coarse Wavelength Division Multiplexing Sometimes in metro/access networks there is no need for DWDM, for this reason coarse wavelength division multiplexing CWDM is used as it is cheaper and simpler. Because of many splices and connectors, attenuation is high in CWDM with many add-drop locations. An experiment was conducted in which lumped Raman amplifiers LRA's with HNLF with low loss were used. In this experiment four and eight channel CWDM system [1] Bar Code EDFA from Internet. [2] X. zhou, M.Birk "Performance comparison of an 80- Km-per-span EDFA system and a 160-Km hut skipped all-raman sytem over standard single-mode fiber", J.Lightw. Technol., vol. 24, no. 3, pp , Mar [3] K.Rottwitt, J.H.Povlsen "Analysing the fundamental properties of Raman amplifiers in optical fibers", J.Lightw. Technol.,vol. 23, no. 11, pp , Nov [4] J.H.Lee, Y.M.Chang, Y.G.Han, H.Chung, S.H.Kim, S.B.Lee "A detailed experimental study on singlepump Raman/EDFA hybrid amplifiers: static, dynamic, and system performance comparison", J.Lightw. Technol., vol. 23, no. 11, pp , Nov [5] J.Chen, X.Liu, C.Lu, Y.Wang, Z.Li "Design of multidtage gain-flattened fiber Raman amplifiers", J.Lightw. Technol., vol. 24, no. 2, pp , Feb [6] J.Zhou, J.Chen, X.Li, G.Wu, Y.Wang, W.Jiang "Robust, compact, and flexible heural model for a fiber Raman Amplifier", J.Lightw. Technol., vol. 24, no. 6, pp , Jun [7] S.Aozasa, H.Masuda, M.Shimizu "S-band Thuliumdoped fiber amplifier employing high Thulium concentration doping technique", J.Lightw. Technol., vol. 24, no. 10, pp , Oct [8] H.Masuda, A.Mori, K.Shikano, M.Shimizu "Design and spectral characteristics of gain-flattened Tellurite-Based fiber Raman amplifiers", J.Lightw. Technol., vol. 24, no. 1, pp , Jan [9] K.Morito "Output-level control of semiconductor optical amplifier by external light injection", J.Lightw. Technol., vol. 23, no. 12, pp , Dec [10] Wikipedia-Encyclopedia.

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