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Fiber transmission systems with semiconductor optical amplifiers without in-line filtering
15 August 2002
Optics Communications 209 (2002) 309–319 www.elsevier.com/locate/optcom
Fiber transmission systems with semiconductor optical amplifiers without in-line filtering A. Shipulin a, G. Onishchukov a, D. Michaelis b,*, P. Riedel a, U. Peschel a, F. Lederer a a
b
Friedrich-Schiller-Universit€at Jena, Max-Wien-Platz 1, D-07743 Jena, Germany Fraunhofer Institut f€ur Angewandte Optik und Feinmechanik, Schillerstraße 1, 07745 Jena, Germany Received 14 February 2002; received in revised form 17 June 2002; accepted 18 June 2002
Abstract 10 Gbit/s RZ signal transmission at 1.3 lm in a fiber transmission system with in-line semiconductor optical amplifiers is investigated using a recirculating fiber loop setup. We found that under certain conditions the system operates without in-line filtering and with strongly saturated amplifiers, provided the signal is extracted from the amplified spontaneous emission by means of a narrow optical filter in front of the receiver. These results are compared with a transmission scheme with in-line filtering. Ó 2002 Elsevier Science B.V. All rights reserved.
1. Introduction Polarisation independent, high-gain, low noise semiconductor optical amplifiers (SOA) (bulk [1] and multiple quantum well, MQW, [2]) are attractive, cost-effective candidates for in-line amplifiers in optical fiber transmission lines. High bit rate transmissions with SOA have been studied [3– 8] and tested in field trials [9–12]. The investigations showed that both Return-to-Zero (RZ) and linear Non-Return-to-Zero (NRZ) transmission regimes can be realised using SOAs but the maximum propagation distance in SOA-based systems does not exceed 500 km [3–6,9–12]. Detailed nu-
*
Corresponding author. Tel.: +49-3641-657-660; fax: +493641-657-680. E-mail address: [email protected] (D. Michaelis).
merical calculations [7] revealed that the maximum propagation distance is limited by about 1000 km and 1500 km has been reached in a fiber loop setup after optimisation [8]. In spite of the fact that in-line optical filtering is a common method to improve the system performance, the most straight-line experiments at 1.3 lm were performed without in-line filtering at all [9] or with filters situated after every second amplifier only [10–12]. These schemes are of interest due to their simplicity, cost reduction and potential in wavelength division multiplexing (WDM) systems [13–16]. It has been shown that a transmission line with SOAs can be operated without in-line filters, provided the accumulated amplified spontaneous emission (ASE) does not saturate the in-line amplifiers. In this paper a SOA fiber transmission line without optical in-line filtering is considered. In
0030-4018/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 0 - 4 0 1 8 ( 0 2 ) 0 1 7 1 9 - 4
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contrast to the common approach we show that the transmission system operates well with amplifiers strongly saturated by ASE. The ASE and the pulse power dynamics, the Q-factor evolution, the red shift and the spectral walk off [17] are studied in detail for bulk and MQW amplifiers. The results are compared with the signal transmission with inline filtering. By means of a simple model the dynamics of the system is described analytically.
2. Experimental setup A standard recirculating fiber loop setup (see Fig. 1) was used to investigate the pulse propagation in long fiber transmission lines. Regular trains of 20-ps pulses at repetition rates of either 10, 5,
2.5, or 1.25 GHz were generated by an external cavity mode-locked semiconductor laser. Different bit patterns were created by an intensity modulator or by multiplexing of regular pulse trains of a lower repetition rate using a Mach–Zehnder fiber interferometer. A very long (20 ls) set of zero signals was simulated by filling the fiber loop only partially with pulses. The loop consisted of a standard single-mode optical fiber (loss 0.35 dB/ km, zero dispersion wavelength 1307 nm), an inline SOA, isolators and an optional optical bandpass filter (bandwidth 1 nm). Since the time gating of the injected signal pulse train and the loop cleaning was controlled by switching of the booster and the in-line SOA current, no acoustooptical modulators before and in the loop were necessary. Thus the excess loop losses were mini-
Fig. 1. Scheme of a re-circulating fiber loop setup.
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mised. The maximum length of fiber in the loop was about 32 km. This length was determined by the maximum allowed current of bulk amplifiers (250 mA). For this setup the corresponding optimum performance current for MQW amplifiers was about 200 mA. In each case the signal wavelength was at the red slope of the SOA gain profile, where the amplifiers show the best performance (low noise figure, high saturation power). In order to obtain the maximum propagation distance the system parameters as the in-line SOA gain, the state of the polarisation controllers in the loop, the polarisation and the power of the input signal, etc. were optimised. For the time-domain measurements a communication analyser HP 83480A (12.4-GHz bandwidth) with a lightwave converter HP 11982A (15-GHz bandwidth) followed by a lowpass electrical filter with 7-GHz bandwidth was used. The measured Q-factor in the presence of a strong pattern effect (inter-symbol interference), which is a typical feature of systems with cascaded SOA, does not correspond directly to the bit error rate (BER) that is a figure of merit of the system performance. But nevertheless we use the Q-factor measured separately for different pulses in a bit pattern as a rough estimation for the quality of the optical signal transmission because in that case the influence of the pattern effect is minimized. The optical spectra were measured by an optical spectrum analyser HP 70951B. The device was used in two different operating modes. In the first case it was triggered by a loop-timing generator to measure the dependence of the average optical spectrum on the transmission distance. To investigate the spectra of particular pulses in a bit pattern the analyser was also used as a tuneable optical filter before the lightwave converter. An additional SOA was placed in front of the analyser to compensate for the extra losses. The center wavelength was manually scanned to find the best quality of the desired part of the bit pattern on the screen of the communication analyser. The resolution bandwidth of the spectrum analyser was usually 0.2 nm and the accuracy of the wavelength measurements was about 20–30 pm.
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3. Results and discussion The system dynamics without in-line filtering differs essentially from that one with an in-line filter. Firstly, the broadband spontaneous radiation emitted by the SOA propagates without any suppression. This is in contrast to the case with inline filters where most part of the ASE is absorbed by the filter (typical ASE bandwidth: >50 nm, inline filter bandwidth: 1 nm) and the ASE accumulation is much slower. Secondly, as it will be shown later, the unsaturated SOA gain has to be significantly larger than the total losses of the elements between the amplifier nodes (mainly fiber losses). Thus there is a large net small signal gain, which leads to a fast growth of the ASE at the beginning of transmission line. But already after 5–6 amplifier nodes the in-line amplifiers are saturated. Fig. 2 shows the dynamics of the ASE with no injected signal (dashed curve) and of the total (ASE and signal) average power (solid curve) in the system. The evolution of the total average power with and without initially launched signal is practically identical, because the signal is only a small fraction of the total power. Therefore the inline SOAs are saturated mainly by the ASE. After reaching the saturation the total power remains constant along the whole propagation distance. To compare this scenario with the unsaturated
Fig. 2. Dynamics of the average power in the fiber loop without in-line filters: ASE together with signal (solid), ASE without input signal (dashed). Inset: Dynamics of the average power in the fiber loop with a 1 nm in-line filter, ASE together with signal (solid), ASE without input signal (dashed).
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operation regime in the case of in-line filtering, a 1nm in-line optical filter was inserted into the loop just behind the in-line SOA. The average power dynamics with (solid line) and without (dashed line) signal are presented in the inset of Fig. 2. Here the ASE power grows practically linearly and the in-line amplifiers are not saturated by ASE even at large distances. As it has been shown [7,8,13] in the case of in-line filtering the saturation of the SOA gain by the signal pulses cannot be stronger than few tenth of dB for an optimal transmission. In this case the system optimisation revealed that the gain has to be set very accurately (better than 0.1 dB) and in the optimum conditions there is a practically negligible amplification of the ASE and a decay of the pulse energy during propagation along the line [8]. In the saturated regime without in-line filtering the signal can be finally extracted from the strong ASE by an optical filter in front of the receiver. A sufficient signal-to-noise ratio for an error-free transmission can be achieved up to about 800 km propagation distance. Fig. 3 displays a ‘‘. . . 001100 . . .’’ 10 Gbit/s pulse train after 800 km
Fig. 3. Eye-diagrams of the 10 Gbit/s pattern ‘‘. . . 001100 . . .’’ after 800 km propagation distance in the fiber loop without inline filter, upper graph: without optical filter in front of the receiver, lower graph: with a 1 nm filter in front of the receiver.
propagation without (upper graph) and with (lower graph) an 1-nm optical filter in front of the receiver. In what follows we describe the transmission line by means of a simple analytical model that nevertheless gives an insight into the main peculiarities of the system under consideration. The dynamics of the amplifier gain G ¼ Pout ðtÞ=Pin ðtÞ ¼ ehðtÞ , where Pout ðtÞ is the output power and Pin ðtÞ is the input power, is given by a conventional rate equation [18]: dh h0 h Pin ðtÞ h ¼ ðe 1Þ: dt sr Es
ð1Þ
G0 ¼ eh0 is the unsaturated gain which is a direct function of the SOA pumping current. Es and sr denote the saturation energy and the gain recovery time. In the following we will term all physical quantities in front of the amplifier with the subscript ‘in’ and behind the amplifier with the subscript ‘out’. The total power in the loop consists of the ASE (PinASE ðtÞ) and the signal (Pinsignal ðtÞ) power: Pin ðtÞ ¼ PinASE ðtÞ þ Pinsignal ðtÞ:
ð2Þ
Because the signal wavelength is close to the point of the zero group velocity dispersion of the fiber the main effect of the dispersion is a fast temporal broadening of ASE spikes. Therefore the ASE can be considered as a quasi-CW radiation co-propagating with the signal pulses. Additionally the influence of the Kerr nonlinearity of the fiber can be neglected in comparison with the nonlinearity of the SOA. Thus the loss L (Pin ¼ Pout L) can be regarded as the main effect of the components (fiber, isolators, filter and tap couplers if any) between two amplifier nodes. Let us first consider the fiber loop with an inline filter. Experimental investigations showed that an optimal signal transmission is reached for a negligible amplification of the ASE, when G0 L 1. In that case the noise in the transmission system originates exclusively from the accumulation of the spontaneous radiation power DPSOA 1–5 mW at the amplifier output, which is added to the total power after each SOA passage. Therefore the ASE as a function of the roundtrip number k can be approximated by:
A. Shipulin et al. / Optics Communications 209 (2002) 309–319 ðASEÞ
Pin
ðkÞ kDPSOA
dmfilter L; dmSOA
ð3Þ
where dmfilter and dmSOA are the bandwidths of the in-line filer and of the gain spectrum of the SOA. Eq. (3) describes the linear growth of the ASE during propagation as it can be seen in the inset of Fig. 2. For our experimental situation the followsignal signal signal ASE ing relations hold: Pout > Pout , Eout Ein , signal Eout < Es , where Esignal and Es denote the signal energy and the saturation energy. Additionally the duration of a signal pulse (ssignal 20 ps) is much shorter than the gain recovery time (sr 200–400 ps). Hence the gain recovery during the pulse duration can be neglected. Taking into account these prerequisites the dynamics of the signal power can be estimated by means of Eq. (1): ! signal k Y Eout ðiÞ signal Pin ðkÞ 1 ð4Þ Pinsignal ð0Þ: 2E s i¼1 With the help of Eqs. (3) and (4) the optical signalto-noise ratio OSNR is obtained Qk signal 1 E ðiÞ=ð2E Þ s out dmSOA i¼1 OSNRðkÞ k dmsignal
Pinsignal ð0Þ ; DPSOA L
ð5Þ
where dmsignal is the signal bandwidth. The OSNR is directly connected with the Q-factor as Q ¼ ðOSNR 1ÞsqrtðdmASE =ð2df ÞÞ, where dmASE is the ASE bandwidth and df is the bandwidth of the receiver. As already mentioned in the case without in-line filtering the ASE reaches a stable, stationary level ASE ASE Pout ðkÞ ¼ Pout with respect to the roundtrip number k after several passages through the amASE plifier. For Pout PinASE this steady state is given by: ASE Pout ; Ps DPSOA ehstable L ¼ 1 ASE ; Pout
hstable h0
ð6Þ
where ehstable is the saturated gain and Ps is the SOA saturation power. The second formula of Eq. (6) shows that in the system without in-line filters not only the signal gain but even the ASE amplifica-
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tion per one round trip (Lehstable ) is always lower than 1. The stationary ASE level is provided by the regular addition of DPSOA to the total power after each amplifier node. This is in contrast to the dynamics of the ASE in a system with in-line filtering ASE – see Eq. (3). For Pout < Ps Eq. (6) can be easily solved and the steady state of ASE reads as: " Ps ASE Pout G0 L 1 2G0 L sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi# DPSOA 2 þ ð1 G0 LÞ þ 4G0 L : ð7Þ Ps A first-order approximation of Eq. (7) for small values of G0 L 1 results in more intuitive form for the ASE power sffiffiffiffiffiffiffiffiffiffiffiffiffi! pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Ps DPSOA ASE 1 DPSOA Ps þ Pout ðG0 L 1Þ: 2 Ps In the case of zero small signal net gain (G0 L 1 ¼ 0) the ASE power is determined by the spontaneous radiation powerpDP SOA and ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi the satuASE ration power Ps as Pout ¼ DPSOA Ps . The ASE power increases with an increasing small signal net gain (G0 L 1 > 0). The steady state of the ASE ASE Pout in the opposite case of a very strong saturaASE tion of the amplifier (Pout > Ps ) can be only obtained numerically by solving ASE expðPout =Ps ÞG0 L ¼ 1
DPSOA : ASE Pout
The signal pulses experience a gain, which is strongly saturated by the large amount of ASE (see Eq. (6)). Additionally the signal saturates the gain itself in the same manner like in Eq. (4). Therefore the dynamics of the signal power is given by
k DPSOA Pinsignal ðkÞ 1 ASE Pout ! signal k Y Eout ðiÞ 1 ð8Þ Pinsignal ð0Þ: 2E s i¼1 A comparison of Eqs. (4) and (8) shows that in the system without in-line filtering the signal pulses lose their energy during the propagation faster than in the case of in-line filtering. Because the
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gain saturation due to the ASE power is much stronger than that one caused by the signals the term ! signal k Y Eout ðiÞ 1 2Es i¼1 in Eq. (8) can be usually omitted. Furthermore the ASE relation DPSOA < Pout holds. In that case Eq. (8) can be approximated by
DPSOA Pinsignal ðkÞ exp ASE k Pinsignal ð0Þ; Pout i.e., the signals lose their power exponentially fast. In order to minimise the signal losses in the system ASE without in-line filtering the relation Pout DPSOA should hold. This relation implies that the SOA has to be saturated very strongly by the ASE, i.e., the amplifier driving current should be high enough to provide a high level of ASE during transmission. In previously reported experiments the authors tried to avoid this situation. There the ASE power remained at small values, the signal pulses were damped much stronger and shorter propagation distances were reached. Eq. (7) and Eq. (8) result in the optical signal-to-noise ratio OSNR in systems without in-line filters.
k dmSOA DPSOA OSNRðkÞ 1 ASE dmsignal Pout ! signal k Y Eout ðiÞ Pinsignal ð0Þ 1 : ð9Þ 2Es PinASE i¼1 In both cases (with and without in-line filtering) the OSNR can be improved by a decrease of the spontaneous radiation power DPSOA of the amplifier. This is the usual requirement of low noise figure of the amplifier. Let us now consider the conditions for an optimal system performance in the case without in-line filtering. On the one hand the higher the applied current the higher the ASE power in the line and the lower the losses for the signal pulses. But on the other hand the higher the ASE power the larger the amount of the noise, which disturbs the received signal. Hence there is an optimal interval for the gain saturation Dh ¼ h0 hstable in which the system operates most efficiently. As it
has been discussed above, the requirement of slow signal decay gives a lower condition for the gain ASE =Ps DPSOA =Ps . In order to saturation Dh ¼ Pout reach a good signal quality the ASE after the filter in front of the receiver should be much weaker than the signal. Hence the relation ASE Pout
dkf signal Pout dkASE
should hold, where dkASE is the bandwidth of the ASE spectrum and dkf is the bandwidth of the filter in front of the receiver. Finally one gets an upper and lower boundary for the optimal gain change in the system signal DPSOA dkASE Pout Dh : PS dkf Ps
ð10Þ
For typical values of dkASE P 20 nm, dkf signal 1 nm, Pout 5 mW, Ps 10 mW, L 0:01 the reasonable value of Dh (Dh 1–2) satisfies easily the upper limit condition (Dh 10) given by Eq. (10). In contrast to this the lower limit of Eq. (10) is much harder to satisfy using standard commercially available SOAs with DPSOA =Ps 0:5. But using SOAs with a shorter chip length or a decrease of the pumping current of a common SOA could decrease the parameter DPSOA =Ps to much more appropriate values. As already mentioned the last condition is similar to that one of low noise figure (Eq. (9)). Since the gain recovery time of the SOA (sr 200–400 ps) is comparable with the signal pulse distance, inter-symbol interference effects [20,21] are pronounced in both systems with and without in-line filtering. Let us assume a long set of zero bits (no signal pulses) before a signal pulse train. The first pulse of the train experiences a large, almost unsaturated gain and saturates the amplifier. The following pulses usually experience a smaller gain than the first one because of the incomplete gain recovery between two neighbouring pulses. Finally the leading pulse becomes more intense than the following ones (amplitude pattern effect). Although for our typical experimental conditions the signal pulse energy after the amplifier (0.1 pJ) is significantly smaller than the SOA saturation energy (2 pJ) a multiple passage of an irregular pulse train through the amplifier
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leads to a noticeable pattern effect as it could be seen in Fig. 3. For regular pulse trains the gain saturation leads to an effective reduction of the gain in comparison with that for low repetition rate pulses. The saturation of the SOA by a strong quasi-CW ASE and higher SOA driving currents in the system without in-line filters lead as well known to gain recovery time shortening. Therefore the pattern effect should be reduced in comparison with a transmission line with in-line filtering. Each change of the SOA gain is accompanied with a variation of the refractive index of the semiconductor. The effect is very useful for SOA applications in optical switching and wavelength conversion, but in transmission systems these index changes lead to a spectral red-shift of the pulses [17,20,21]. To estimate this effect we modify the model given in [18] to meet our experimental ASE situation (PASE Psignal , Es =Pout > Tsignal , where Tsignal is the signal pulse duration):
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in-line filtering are presented in Fig. 4 and Fig. 5. In the inset of Fig. 4 the signal power versus the propagation distance is displayed. For both bulk and MQW amplifiers the power decreases exponentially during propagation as it is predicted by Eq. (8). The decay rate of the signal pulses ASE is approximately 0.07 for the MQW DPSOA =Pout and 0.06 for the bulk amplifier. Because of the decrease of the signal power the signal quality measured by means of the Q-factor is reduced during the propagation too. The Q-factor reaches the critical value of Q ¼ 6 at a propagation distance of 800 km for bulk and 900 km for MQW amplifiers (see Fig. 4). Using the optical spectrum
signal bs Pout ðtÞ ; ð11Þ 4pEs where DmðtÞ is the frequency change of the signal pulse and bs is the Henry-factor. The average change of the central wavelength k0 of the signal after the kth roundtrip is given by:
DmðtÞ ¼
DkðkÞ ¼ c
k X signal Pout ðiÞ ;
ð12Þ
i¼1
where c¼
Fig. 4. Evolution of the Q-factor for a 10 GHz regular pulse train with a bulk (stars) and a MQW (circles) amplifier. Inset: Corresponding dynamics of the pulse peak power.
k20 bs ; 4pcEs
signal c is the velocity of the light, hPout ðiÞi is the signal power after the ith roundtrip. h i denotes the averaging over a single signal pulse. Due to the above mentioned amplitude pattern effect the power of the signal pulses may differ from each other. Thus these pulses (here termed with the indices 1 and 2) accumulate a different spectral red-shift k X signal 1 signal 2 dkðkÞ ¼ c Pout ðiÞ Pout ðiÞ : ð13Þ i¼1
dk is a spectral walk-off of the signal pulses 1 and 2 which can be regarded as a spectral pattern effect. The experimental results on the transmission of a regular 10-GHz pulse train in a system without
Fig. 5. Evolution of the red shift for a 10 GHz regular pulse train with a MQW (circles) and a bulk (stars) amplifier.
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analyser as a receiver filter the red-shift of the regular train of pulses is measured as a function of the propagation length. Fig. 5 shows these experimental results compared with theoretical estimations given by Eq. (12). Because of the exponential decrease of the signal power the increment of the red-shift reduces during the propagation. A typical Henry-factor for the bulk amplifiers is about bs 5 [18]. For the MQW amplifiers the Henryfactor may vary in a region around bs 3 depending on experimental conditions [19]. In our case the spectral shift for the MQW and the bulk amplifier is approximately the same (see Fig. 5). The performance optimisation of our system revealed that the pulse power for using MQW amplifiers has to be approximately twice higher than that one in the case of bulk devices (see the inset of Fig 4). It could be attributed to a 1.5 times shorter gain recovery time and a 4 times larger saturation power of the MQW amplifier. The system performance without in-line filtering depends strongly on the adjustment of polarisation controllers in the fiber loop. Additional formation of multiple peak spectra of ASE was observed where the peak positions strongly depend on the adjustment of the polarization controllers. This behaviour is due to the small but non-vanishing wavelength dependence of the fiber birefringence (polarisation mode dispersion) and the polarisation dependence of the SOA gain (less than 0.2 dB). During the propagation different wavelengths experience a different polarisation evolution and thus a different amplification. For an optimum system performance the position of the signal spectrum maximum should coincide with that one of the ASE. A slight deviation from the optimum results in a rather strong degradation of the quality of the signal pulses during propagation. The reason is that the ASE can adapt its polarisation much faster than the signal, which leads to a faster signal decay. The best performance could be obtained when not only the signal wavelength and signal polarisation experiences a maximum amplification but the ASE bandwidth is largest too. Thus it shows the importance of either using gain flattening filters or SOA with maximum gain wavelength close to that of the signal. Furthermore the minimisation of the polarisation
dependence of the gain as well as the polarisation dependent losses of other in-line elements will improve the system performance. It is worth to note that in real transmission lines the elements exhibit actually some fluctuations of their properties. This is the main difference to our loop setup where all roundtrips are identical. Due to an averaging process in real transmission experiments the multiple peak structure of the ASE spectrum could be smoothed but a detailed analysis of this problem is beyond the scope of the paper. Up to now we considered the transmission of regular pulse trains. Now let us investigate the dynamics of 10-Gbit/s irregular pulse trains. By means of the above described Mach-Zehnder optical multiplexer bit patterns like ‘‘. . . 000011110000 . . .’’ were generated. Fig. 8 shows the power evolution of the first and of the subsequent pulses. Actually the power levels of the second and third pulses are also slightly different but this difference is considerably smaller than that one between the first and second pulses. From practical point of view all pulses followed by the first one can be regarded as identical. According to Eq. (13) the first and subsequent pulses experience a different red shift of their central wavelengths. This spectral walk off as a function of the propagation distance is depicted in the inset in Fig. 6. The walk off seems to saturate during propagation.
Fig. 6. Dynamics of the signal peak power for the first (filled squares) and the subsequent pulses (empty squares) of a 10 GHz pattern ‘‘. . . 000011110000 . . .’’ in a fiber loop with a bulk amplifier. Inset: Corresponding evolution of the spectral walk off.
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But actually there is no physical mechanism, which may stabilise that effect as for the case with in-line filters [8,20,21]. Just due to the strong reduction of the pulse power the growth of the spectral walk off becomes negligible at large distances. For the fiber loop with the MQW amplifiers the results are qualitatively the same (see Fig. 7). Here the maximum value of the spectral walk off is about 0.06 nm whereas it is 0.15 nm using bulk devices. The difference can be explained by a different signal power and SOA parameters (Henry factor and saturation energy). Due to the spectral walk off the channel bandwidth usually suffers a 2-fold increase during propagation. The Q-factor is very sensitive to the center wavelength of the receiver filter. With a 0.2-nm filter in the optimum position for the detection of the first pulse, the second pulse is sometimes almost undetectable and vice versa. Hence for a successful detection of all pulses the filter must be adjusted at some middle position between the wavelengths of the first and the second signal pulses. Additionally the filter bandwidth has to be optimised too. Usually the signal-to-noise ratio for the first pulse is better than that one of the subsequent signals. Therefore the total Q-factor is mainly determined by these subsequent pulses. Fig. 8 shows the Q-factor of the irregular pulse train in the fiber loop with the MQW and the bulk
Fig. 7. Dynamics of the signal peak power for the first (filled squares) and the subsequent pulses (empty squares) in the 10 GHz pattern ‘‘. . . 000011110000 . . .’’ in a fiber loop with a MQW amplifier. Inset: Corresponding evolution of the red shift of the pulses.
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Fig. 8. Evolution of the Q-factor of the 10 GHz pattern ‘‘. . . 000011110000 . . .’’ in a fiber loop with a bulk (stars) and a MQW (circles) amplifier.
amplifier. In both cases the loop parameters are optimised for the longest propagation distance, but the detection schemes are the same. An errorfree propagation of about 600 km has been realised with bulk and up to approximately 800 km with MQW amplifiers. As it could be expected it is smaller than the distances obtained with regular pulse train transmission. In Fig. 9 one can see that the evolution of the Q-factor in a fiber loop with a 1-nm in-line filter is very similar to that one without in-line filters as it could be seen from Eqs. (5) and (9). Finally we optimise the bandwidth of the receiver filter. On the one hand the less the filter
Fig. 9. Comparison of the degradation of the Q-factor for a 10 GHz signal in a fiber loop with a 1-nm in-line filter (circles) and without an in-line filter (points).
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estimated using the Q-factor evolution. The degradation of the pulse quality measured by means of the Q-factor is similar to that one in a fiber loop with a 1-nm in-line filter. For effective performance the system has to be designed more carefully in the case without in-line filtering than with in-line filters. The use of SOA with a shorter gain medium, the application of gain flattening filters, the minimisation of the polarisation dependence of the gain and losses in the transmission line will lead to a further improvement of the system performance. Fig. 10. Q-factor of the 10 GHz pattern ‘‘. . . 000011110000 . . .’’ at a propagation distance of about 800 km in dependence on the bandwidth of the receiver filter.
bandwidth the lower the level of the detected ASE. From this point of view the minimum bandwidth of the receiver filter for a regular pulse train is determined by the signal bandwidth of about 0.2 nm. On the other hand the detection of the pulses of an irregular pulse train requires a much broader shape for the filter because of the above considered spectral walk off. Thus there is an optimal bandwidth for the receiver filter. For our experimental situation the optimal bandwidth is about 1 nm (see Fig. 10).
4. Conclusion A 10 Gbit/s RZ transmission system with cascaded semiconductor bulk and MQW amplifiers without in-line filters has been studied using an optical fiber loop setup. In contrast to previous experiments it has been shown that long transmission distances can be reached in such systems for a strong amplifier saturation. The typical system properties as the signal power and Q-factor evolution, the spectral red shift and the walk off are experimentally investigated. The results are described by means of simple theoretical model. The optimal bandwidth of the filter in front of the receiver is evaluated to be about 1 nm. The MQW amplifier exhibits a better performance than the bulk amplifier. An error-free propagation distances up to 800 km has been
References [1] C. Holtmann, P.A. Besse, T. Brenner, H. Melchior, Polarization independent bulk active region semiconductor optical amplifiers for 1.3 lm wavelengths, IEEE Photon. Technol. Lett. 8 (1996) 343–345. [2] P.J.A. Thijs, L.F. Tiemeijer, J.J.M. Binsma, T. Vandongen, Strained-layer InGaAs(P) quantum-well semiconductorlasers and semiconductor-laser amplifiers, Philips J. Res. 49 (1995) 187–224. [3] A. Shipulin, G. Onishchukov, P. Riedel, D. Michaelis, U. Peschel, F. Lederer, in: Optical Amplifiers and their Applications’97, Victoria, Canada, Techn. Digest TuD15, (1997) 185. [4] M. Settembre, M. Tamburrini, F. Matera, H. Haunstein, B. Teichmann, I. Gabitov, E. Laedke, K. Spatschek, S.K. Turitsyn, J.J.E. Reid, in: ECOC’97, Edinburgh, UK, Techn. Digest 1 (1997) 75. [5] P.I. Kuindersma, G.P.J.M. Cuijpers, J.J.E. Reid, G.N. van den Hoven, S. Walczyk, in: ECOC’97, Edinburgh, UK, Techn. Digest 1 (1997) 79. [6] A. Shipulin, G. Onishchukov, P. Riedel, D. Michaelis, U. Peschel, F. Lederer, Electron. Lett. 33 (1997) 507. [7] M. Settembre, F. Matera, V. H€agele, I. Gabitov, A.W. Mattheus, S. Turitsyn, J. Lightwave Technol. 15 (1997) 962. [8] G. Onishchukov, V. Lokhnygin, A. Shipulin, P. Riedel, Electron. Lett. 34 (1998) 1597. [9] P.I. Kuindersma, G.P.J.M. Cuijpers, J.G.L. Jennen, J.J.E. Reid, L.F. Tiermeier, H. de Waardt, A.J. Boot, in: ECOC’ 96, Oslo, Norway, Techn. Digest TuD 2.1 (1996) 165. [10] J.J.E. Reid, P.I. Kuindersma, G.P.J.M. Cuijpers, G.N. van den Hoven, S. Walczyk, B. Teichmann, C. Dorschky, R. Seitz, C. Schulien, L. Cucala, H. Gruhl, R. Leppla, A. Mattheus, in: ECOC’97, Edinburgh, UK, Techn. Digest 1 (1997) 83. [11] S. Reichel, J. Eckert, R. Leppla, R. Zengerle, A. Mattheus, L.C. Garcia, IEEE Photon. Technol. Lett. 10 (10) (1998) 1498–1500.
A. Shipulin et al. / Optics Communications 209 (2002) 309–319 [12] J.J.E. Reid, L. Cucala, M. Ferreira, M. Settembre, H. Haunstein, R.C.J. Smets, in: ECOC’98, Madrid, Spain, Techn. Digest WdD01 (1998). [13] L.H. Spiekman, J.M. Wiesenfeld, A.H. Gnauck, L.D. Garrett, G.N. Van Den Hoven, T. Van Dongen, M.J.H. Sander-Jochem, J.J.M. Binsma, IEEE Photon. Technol. Lett. 12 (6) (2000) 717–719. [14] L.H. Spiekman, J.M. Wiesenfeld, A.H. Gnauck, L.D. Garrett, G.N. Van Den Hoven, T. Van Dongen, M.J.H. Sander-Jochem, J.J.M. Binsma, IEEE Photon. Technol. Lett. 12 (8) (2000) 1082–1084. [15] L.H. Spiekman, A.H. Gnauck, J.M. Wiesenfeld, L.D. Garrett, Electron. Lett. 36 (12) (2000) 1046–1047.
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[16] H.K. Kim, S. Chandrasekhar, A. Srivastava, C.A. Burrus, L. Buhl, Electron. Lett. 37 (3) (2001) 185–187. [17] A. Shipulin, G. Onishchukov, P. Riedel, D. Michaelis, U. Peshel, F. Lederer, in: OFC’98, San Jose, USA, Techn. Digest ThO6 (1998) 323. [18] G.P. Agrawal, N. Anders Olsson, IEEE J. Quantum Electron. 25 (2) (1989) 297. [19] L.F. Tiemeijer, P.J.A. Thijs, T. Vandongen, J.J.M. Binsma, E.J. Jensen, IEEE Photon. Technol. Lett. 8 (7) (1996) 876– 878. [20] A. Mecozzi, Soliton transmission control with semiconductor amplifiers, Opt. Lett. 20 (15) (1995) 1616–1618. [21] S. Wabnitz, Opt. Lett. 20 (19) (1995) 1979–1981.
Optics Communications 209 (2002) 309–319 www.elsevier.com/locate/optcom
Fiber transmission systems with semiconductor optical amplifiers without in-line filtering A. Shipulin a, G. Onishchukov a, D. Michaelis b,*, P. Riedel a, U. Peschel a, F. Lederer a a
b
Friedrich-Schiller-Universit€at Jena, Max-Wien-Platz 1, D-07743 Jena, Germany Fraunhofer Institut f€ur Angewandte Optik und Feinmechanik, Schillerstraße 1, 07745 Jena, Germany Received 14 February 2002; received in revised form 17 June 2002; accepted 18 June 2002
Abstract 10 Gbit/s RZ signal transmission at 1.3 lm in a fiber transmission system with in-line semiconductor optical amplifiers is investigated using a recirculating fiber loop setup. We found that under certain conditions the system operates without in-line filtering and with strongly saturated amplifiers, provided the signal is extracted from the amplified spontaneous emission by means of a narrow optical filter in front of the receiver. These results are compared with a transmission scheme with in-line filtering. Ó 2002 Elsevier Science B.V. All rights reserved.
1. Introduction Polarisation independent, high-gain, low noise semiconductor optical amplifiers (SOA) (bulk [1] and multiple quantum well, MQW, [2]) are attractive, cost-effective candidates for in-line amplifiers in optical fiber transmission lines. High bit rate transmissions with SOA have been studied [3– 8] and tested in field trials [9–12]. The investigations showed that both Return-to-Zero (RZ) and linear Non-Return-to-Zero (NRZ) transmission regimes can be realised using SOAs but the maximum propagation distance in SOA-based systems does not exceed 500 km [3–6,9–12]. Detailed nu-
*
Corresponding author. Tel.: +49-3641-657-660; fax: +493641-657-680. E-mail address: [email protected] (D. Michaelis).
merical calculations [7] revealed that the maximum propagation distance is limited by about 1000 km and 1500 km has been reached in a fiber loop setup after optimisation [8]. In spite of the fact that in-line optical filtering is a common method to improve the system performance, the most straight-line experiments at 1.3 lm were performed without in-line filtering at all [9] or with filters situated after every second amplifier only [10–12]. These schemes are of interest due to their simplicity, cost reduction and potential in wavelength division multiplexing (WDM) systems [13–16]. It has been shown that a transmission line with SOAs can be operated without in-line filters, provided the accumulated amplified spontaneous emission (ASE) does not saturate the in-line amplifiers. In this paper a SOA fiber transmission line without optical in-line filtering is considered. In
0030-4018/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 0 - 4 0 1 8 ( 0 2 ) 0 1 7 1 9 - 4
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contrast to the common approach we show that the transmission system operates well with amplifiers strongly saturated by ASE. The ASE and the pulse power dynamics, the Q-factor evolution, the red shift and the spectral walk off [17] are studied in detail for bulk and MQW amplifiers. The results are compared with the signal transmission with inline filtering. By means of a simple model the dynamics of the system is described analytically.
2. Experimental setup A standard recirculating fiber loop setup (see Fig. 1) was used to investigate the pulse propagation in long fiber transmission lines. Regular trains of 20-ps pulses at repetition rates of either 10, 5,
2.5, or 1.25 GHz were generated by an external cavity mode-locked semiconductor laser. Different bit patterns were created by an intensity modulator or by multiplexing of regular pulse trains of a lower repetition rate using a Mach–Zehnder fiber interferometer. A very long (20 ls) set of zero signals was simulated by filling the fiber loop only partially with pulses. The loop consisted of a standard single-mode optical fiber (loss 0.35 dB/ km, zero dispersion wavelength 1307 nm), an inline SOA, isolators and an optional optical bandpass filter (bandwidth 1 nm). Since the time gating of the injected signal pulse train and the loop cleaning was controlled by switching of the booster and the in-line SOA current, no acoustooptical modulators before and in the loop were necessary. Thus the excess loop losses were mini-
Fig. 1. Scheme of a re-circulating fiber loop setup.
A. Shipulin et al. / Optics Communications 209 (2002) 309–319
mised. The maximum length of fiber in the loop was about 32 km. This length was determined by the maximum allowed current of bulk amplifiers (250 mA). For this setup the corresponding optimum performance current for MQW amplifiers was about 200 mA. In each case the signal wavelength was at the red slope of the SOA gain profile, where the amplifiers show the best performance (low noise figure, high saturation power). In order to obtain the maximum propagation distance the system parameters as the in-line SOA gain, the state of the polarisation controllers in the loop, the polarisation and the power of the input signal, etc. were optimised. For the time-domain measurements a communication analyser HP 83480A (12.4-GHz bandwidth) with a lightwave converter HP 11982A (15-GHz bandwidth) followed by a lowpass electrical filter with 7-GHz bandwidth was used. The measured Q-factor in the presence of a strong pattern effect (inter-symbol interference), which is a typical feature of systems with cascaded SOA, does not correspond directly to the bit error rate (BER) that is a figure of merit of the system performance. But nevertheless we use the Q-factor measured separately for different pulses in a bit pattern as a rough estimation for the quality of the optical signal transmission because in that case the influence of the pattern effect is minimized. The optical spectra were measured by an optical spectrum analyser HP 70951B. The device was used in two different operating modes. In the first case it was triggered by a loop-timing generator to measure the dependence of the average optical spectrum on the transmission distance. To investigate the spectra of particular pulses in a bit pattern the analyser was also used as a tuneable optical filter before the lightwave converter. An additional SOA was placed in front of the analyser to compensate for the extra losses. The center wavelength was manually scanned to find the best quality of the desired part of the bit pattern on the screen of the communication analyser. The resolution bandwidth of the spectrum analyser was usually 0.2 nm and the accuracy of the wavelength measurements was about 20–30 pm.
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3. Results and discussion The system dynamics without in-line filtering differs essentially from that one with an in-line filter. Firstly, the broadband spontaneous radiation emitted by the SOA propagates without any suppression. This is in contrast to the case with inline filters where most part of the ASE is absorbed by the filter (typical ASE bandwidth: >50 nm, inline filter bandwidth: 1 nm) and the ASE accumulation is much slower. Secondly, as it will be shown later, the unsaturated SOA gain has to be significantly larger than the total losses of the elements between the amplifier nodes (mainly fiber losses). Thus there is a large net small signal gain, which leads to a fast growth of the ASE at the beginning of transmission line. But already after 5–6 amplifier nodes the in-line amplifiers are saturated. Fig. 2 shows the dynamics of the ASE with no injected signal (dashed curve) and of the total (ASE and signal) average power (solid curve) in the system. The evolution of the total average power with and without initially launched signal is practically identical, because the signal is only a small fraction of the total power. Therefore the inline SOAs are saturated mainly by the ASE. After reaching the saturation the total power remains constant along the whole propagation distance. To compare this scenario with the unsaturated
Fig. 2. Dynamics of the average power in the fiber loop without in-line filters: ASE together with signal (solid), ASE without input signal (dashed). Inset: Dynamics of the average power in the fiber loop with a 1 nm in-line filter, ASE together with signal (solid), ASE without input signal (dashed).
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operation regime in the case of in-line filtering, a 1nm in-line optical filter was inserted into the loop just behind the in-line SOA. The average power dynamics with (solid line) and without (dashed line) signal are presented in the inset of Fig. 2. Here the ASE power grows practically linearly and the in-line amplifiers are not saturated by ASE even at large distances. As it has been shown [7,8,13] in the case of in-line filtering the saturation of the SOA gain by the signal pulses cannot be stronger than few tenth of dB for an optimal transmission. In this case the system optimisation revealed that the gain has to be set very accurately (better than 0.1 dB) and in the optimum conditions there is a practically negligible amplification of the ASE and a decay of the pulse energy during propagation along the line [8]. In the saturated regime without in-line filtering the signal can be finally extracted from the strong ASE by an optical filter in front of the receiver. A sufficient signal-to-noise ratio for an error-free transmission can be achieved up to about 800 km propagation distance. Fig. 3 displays a ‘‘. . . 001100 . . .’’ 10 Gbit/s pulse train after 800 km
Fig. 3. Eye-diagrams of the 10 Gbit/s pattern ‘‘. . . 001100 . . .’’ after 800 km propagation distance in the fiber loop without inline filter, upper graph: without optical filter in front of the receiver, lower graph: with a 1 nm filter in front of the receiver.
propagation without (upper graph) and with (lower graph) an 1-nm optical filter in front of the receiver. In what follows we describe the transmission line by means of a simple analytical model that nevertheless gives an insight into the main peculiarities of the system under consideration. The dynamics of the amplifier gain G ¼ Pout ðtÞ=Pin ðtÞ ¼ ehðtÞ , where Pout ðtÞ is the output power and Pin ðtÞ is the input power, is given by a conventional rate equation [18]: dh h0 h Pin ðtÞ h ¼ ðe 1Þ: dt sr Es
ð1Þ
G0 ¼ eh0 is the unsaturated gain which is a direct function of the SOA pumping current. Es and sr denote the saturation energy and the gain recovery time. In the following we will term all physical quantities in front of the amplifier with the subscript ‘in’ and behind the amplifier with the subscript ‘out’. The total power in the loop consists of the ASE (PinASE ðtÞ) and the signal (Pinsignal ðtÞ) power: Pin ðtÞ ¼ PinASE ðtÞ þ Pinsignal ðtÞ:
ð2Þ
Because the signal wavelength is close to the point of the zero group velocity dispersion of the fiber the main effect of the dispersion is a fast temporal broadening of ASE spikes. Therefore the ASE can be considered as a quasi-CW radiation co-propagating with the signal pulses. Additionally the influence of the Kerr nonlinearity of the fiber can be neglected in comparison with the nonlinearity of the SOA. Thus the loss L (Pin ¼ Pout L) can be regarded as the main effect of the components (fiber, isolators, filter and tap couplers if any) between two amplifier nodes. Let us first consider the fiber loop with an inline filter. Experimental investigations showed that an optimal signal transmission is reached for a negligible amplification of the ASE, when G0 L 1. In that case the noise in the transmission system originates exclusively from the accumulation of the spontaneous radiation power DPSOA 1–5 mW at the amplifier output, which is added to the total power after each SOA passage. Therefore the ASE as a function of the roundtrip number k can be approximated by:
A. Shipulin et al. / Optics Communications 209 (2002) 309–319 ðASEÞ
Pin
ðkÞ kDPSOA
dmfilter L; dmSOA
ð3Þ
where dmfilter and dmSOA are the bandwidths of the in-line filer and of the gain spectrum of the SOA. Eq. (3) describes the linear growth of the ASE during propagation as it can be seen in the inset of Fig. 2. For our experimental situation the followsignal signal signal ASE ing relations hold: Pout > Pout , Eout Ein , signal Eout < Es , where Esignal and Es denote the signal energy and the saturation energy. Additionally the duration of a signal pulse (ssignal 20 ps) is much shorter than the gain recovery time (sr 200–400 ps). Hence the gain recovery during the pulse duration can be neglected. Taking into account these prerequisites the dynamics of the signal power can be estimated by means of Eq. (1): ! signal k Y Eout ðiÞ signal Pin ðkÞ 1 ð4Þ Pinsignal ð0Þ: 2E s i¼1 With the help of Eqs. (3) and (4) the optical signalto-noise ratio OSNR is obtained Qk signal 1 E ðiÞ=ð2E Þ s out dmSOA i¼1 OSNRðkÞ k dmsignal
Pinsignal ð0Þ ; DPSOA L
ð5Þ
where dmsignal is the signal bandwidth. The OSNR is directly connected with the Q-factor as Q ¼ ðOSNR 1ÞsqrtðdmASE =ð2df ÞÞ, where dmASE is the ASE bandwidth and df is the bandwidth of the receiver. As already mentioned in the case without in-line filtering the ASE reaches a stable, stationary level ASE ASE Pout ðkÞ ¼ Pout with respect to the roundtrip number k after several passages through the amASE plifier. For Pout PinASE this steady state is given by: ASE Pout ; Ps DPSOA ehstable L ¼ 1 ASE ; Pout
hstable h0
ð6Þ
where ehstable is the saturated gain and Ps is the SOA saturation power. The second formula of Eq. (6) shows that in the system without in-line filters not only the signal gain but even the ASE amplifica-
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tion per one round trip (Lehstable ) is always lower than 1. The stationary ASE level is provided by the regular addition of DPSOA to the total power after each amplifier node. This is in contrast to the dynamics of the ASE in a system with in-line filtering ASE – see Eq. (3). For Pout < Ps Eq. (6) can be easily solved and the steady state of ASE reads as: " Ps ASE Pout G0 L 1 2G0 L sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi# DPSOA 2 þ ð1 G0 LÞ þ 4G0 L : ð7Þ Ps A first-order approximation of Eq. (7) for small values of G0 L 1 results in more intuitive form for the ASE power sffiffiffiffiffiffiffiffiffiffiffiffiffi! pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Ps DPSOA ASE 1 DPSOA Ps þ Pout ðG0 L 1Þ: 2 Ps In the case of zero small signal net gain (G0 L 1 ¼ 0) the ASE power is determined by the spontaneous radiation powerpDP SOA and ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi the satuASE ration power Ps as Pout ¼ DPSOA Ps . The ASE power increases with an increasing small signal net gain (G0 L 1 > 0). The steady state of the ASE ASE Pout in the opposite case of a very strong saturaASE tion of the amplifier (Pout > Ps ) can be only obtained numerically by solving ASE expðPout =Ps ÞG0 L ¼ 1
DPSOA : ASE Pout
The signal pulses experience a gain, which is strongly saturated by the large amount of ASE (see Eq. (6)). Additionally the signal saturates the gain itself in the same manner like in Eq. (4). Therefore the dynamics of the signal power is given by
k DPSOA Pinsignal ðkÞ 1 ASE Pout ! signal k Y Eout ðiÞ 1 ð8Þ Pinsignal ð0Þ: 2E s i¼1 A comparison of Eqs. (4) and (8) shows that in the system without in-line filtering the signal pulses lose their energy during the propagation faster than in the case of in-line filtering. Because the
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gain saturation due to the ASE power is much stronger than that one caused by the signals the term ! signal k Y Eout ðiÞ 1 2Es i¼1 in Eq. (8) can be usually omitted. Furthermore the ASE relation DPSOA < Pout holds. In that case Eq. (8) can be approximated by
DPSOA Pinsignal ðkÞ exp ASE k Pinsignal ð0Þ; Pout i.e., the signals lose their power exponentially fast. In order to minimise the signal losses in the system ASE without in-line filtering the relation Pout DPSOA should hold. This relation implies that the SOA has to be saturated very strongly by the ASE, i.e., the amplifier driving current should be high enough to provide a high level of ASE during transmission. In previously reported experiments the authors tried to avoid this situation. There the ASE power remained at small values, the signal pulses were damped much stronger and shorter propagation distances were reached. Eq. (7) and Eq. (8) result in the optical signal-to-noise ratio OSNR in systems without in-line filters.
k dmSOA DPSOA OSNRðkÞ 1 ASE dmsignal Pout ! signal k Y Eout ðiÞ Pinsignal ð0Þ 1 : ð9Þ 2Es PinASE i¼1 In both cases (with and without in-line filtering) the OSNR can be improved by a decrease of the spontaneous radiation power DPSOA of the amplifier. This is the usual requirement of low noise figure of the amplifier. Let us now consider the conditions for an optimal system performance in the case without in-line filtering. On the one hand the higher the applied current the higher the ASE power in the line and the lower the losses for the signal pulses. But on the other hand the higher the ASE power the larger the amount of the noise, which disturbs the received signal. Hence there is an optimal interval for the gain saturation Dh ¼ h0 hstable in which the system operates most efficiently. As it
has been discussed above, the requirement of slow signal decay gives a lower condition for the gain ASE =Ps DPSOA =Ps . In order to saturation Dh ¼ Pout reach a good signal quality the ASE after the filter in front of the receiver should be much weaker than the signal. Hence the relation ASE Pout
dkf signal Pout dkASE
should hold, where dkASE is the bandwidth of the ASE spectrum and dkf is the bandwidth of the filter in front of the receiver. Finally one gets an upper and lower boundary for the optimal gain change in the system signal DPSOA dkASE Pout Dh : PS dkf Ps
ð10Þ
For typical values of dkASE P 20 nm, dkf signal 1 nm, Pout 5 mW, Ps 10 mW, L 0:01 the reasonable value of Dh (Dh 1–2) satisfies easily the upper limit condition (Dh 10) given by Eq. (10). In contrast to this the lower limit of Eq. (10) is much harder to satisfy using standard commercially available SOAs with DPSOA =Ps 0:5. But using SOAs with a shorter chip length or a decrease of the pumping current of a common SOA could decrease the parameter DPSOA =Ps to much more appropriate values. As already mentioned the last condition is similar to that one of low noise figure (Eq. (9)). Since the gain recovery time of the SOA (sr 200–400 ps) is comparable with the signal pulse distance, inter-symbol interference effects [20,21] are pronounced in both systems with and without in-line filtering. Let us assume a long set of zero bits (no signal pulses) before a signal pulse train. The first pulse of the train experiences a large, almost unsaturated gain and saturates the amplifier. The following pulses usually experience a smaller gain than the first one because of the incomplete gain recovery between two neighbouring pulses. Finally the leading pulse becomes more intense than the following ones (amplitude pattern effect). Although for our typical experimental conditions the signal pulse energy after the amplifier (0.1 pJ) is significantly smaller than the SOA saturation energy (2 pJ) a multiple passage of an irregular pulse train through the amplifier
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leads to a noticeable pattern effect as it could be seen in Fig. 3. For regular pulse trains the gain saturation leads to an effective reduction of the gain in comparison with that for low repetition rate pulses. The saturation of the SOA by a strong quasi-CW ASE and higher SOA driving currents in the system without in-line filters lead as well known to gain recovery time shortening. Therefore the pattern effect should be reduced in comparison with a transmission line with in-line filtering. Each change of the SOA gain is accompanied with a variation of the refractive index of the semiconductor. The effect is very useful for SOA applications in optical switching and wavelength conversion, but in transmission systems these index changes lead to a spectral red-shift of the pulses [17,20,21]. To estimate this effect we modify the model given in [18] to meet our experimental ASE situation (PASE Psignal , Es =Pout > Tsignal , where Tsignal is the signal pulse duration):
315
in-line filtering are presented in Fig. 4 and Fig. 5. In the inset of Fig. 4 the signal power versus the propagation distance is displayed. For both bulk and MQW amplifiers the power decreases exponentially during propagation as it is predicted by Eq. (8). The decay rate of the signal pulses ASE is approximately 0.07 for the MQW DPSOA =Pout and 0.06 for the bulk amplifier. Because of the decrease of the signal power the signal quality measured by means of the Q-factor is reduced during the propagation too. The Q-factor reaches the critical value of Q ¼ 6 at a propagation distance of 800 km for bulk and 900 km for MQW amplifiers (see Fig. 4). Using the optical spectrum
signal bs Pout ðtÞ ; ð11Þ 4pEs where DmðtÞ is the frequency change of the signal pulse and bs is the Henry-factor. The average change of the central wavelength k0 of the signal after the kth roundtrip is given by:
DmðtÞ ¼
DkðkÞ ¼ c
k X signal Pout ðiÞ ;
ð12Þ
i¼1
where c¼
Fig. 4. Evolution of the Q-factor for a 10 GHz regular pulse train with a bulk (stars) and a MQW (circles) amplifier. Inset: Corresponding dynamics of the pulse peak power.
k20 bs ; 4pcEs
signal c is the velocity of the light, hPout ðiÞi is the signal power after the ith roundtrip. h i denotes the averaging over a single signal pulse. Due to the above mentioned amplitude pattern effect the power of the signal pulses may differ from each other. Thus these pulses (here termed with the indices 1 and 2) accumulate a different spectral red-shift k X signal 1 signal 2 dkðkÞ ¼ c Pout ðiÞ Pout ðiÞ : ð13Þ i¼1
dk is a spectral walk-off of the signal pulses 1 and 2 which can be regarded as a spectral pattern effect. The experimental results on the transmission of a regular 10-GHz pulse train in a system without
Fig. 5. Evolution of the red shift for a 10 GHz regular pulse train with a MQW (circles) and a bulk (stars) amplifier.
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analyser as a receiver filter the red-shift of the regular train of pulses is measured as a function of the propagation length. Fig. 5 shows these experimental results compared with theoretical estimations given by Eq. (12). Because of the exponential decrease of the signal power the increment of the red-shift reduces during the propagation. A typical Henry-factor for the bulk amplifiers is about bs 5 [18]. For the MQW amplifiers the Henryfactor may vary in a region around bs 3 depending on experimental conditions [19]. In our case the spectral shift for the MQW and the bulk amplifier is approximately the same (see Fig. 5). The performance optimisation of our system revealed that the pulse power for using MQW amplifiers has to be approximately twice higher than that one in the case of bulk devices (see the inset of Fig 4). It could be attributed to a 1.5 times shorter gain recovery time and a 4 times larger saturation power of the MQW amplifier. The system performance without in-line filtering depends strongly on the adjustment of polarisation controllers in the fiber loop. Additional formation of multiple peak spectra of ASE was observed where the peak positions strongly depend on the adjustment of the polarization controllers. This behaviour is due to the small but non-vanishing wavelength dependence of the fiber birefringence (polarisation mode dispersion) and the polarisation dependence of the SOA gain (less than 0.2 dB). During the propagation different wavelengths experience a different polarisation evolution and thus a different amplification. For an optimum system performance the position of the signal spectrum maximum should coincide with that one of the ASE. A slight deviation from the optimum results in a rather strong degradation of the quality of the signal pulses during propagation. The reason is that the ASE can adapt its polarisation much faster than the signal, which leads to a faster signal decay. The best performance could be obtained when not only the signal wavelength and signal polarisation experiences a maximum amplification but the ASE bandwidth is largest too. Thus it shows the importance of either using gain flattening filters or SOA with maximum gain wavelength close to that of the signal. Furthermore the minimisation of the polarisation
dependence of the gain as well as the polarisation dependent losses of other in-line elements will improve the system performance. It is worth to note that in real transmission lines the elements exhibit actually some fluctuations of their properties. This is the main difference to our loop setup where all roundtrips are identical. Due to an averaging process in real transmission experiments the multiple peak structure of the ASE spectrum could be smoothed but a detailed analysis of this problem is beyond the scope of the paper. Up to now we considered the transmission of regular pulse trains. Now let us investigate the dynamics of 10-Gbit/s irregular pulse trains. By means of the above described Mach-Zehnder optical multiplexer bit patterns like ‘‘. . . 000011110000 . . .’’ were generated. Fig. 8 shows the power evolution of the first and of the subsequent pulses. Actually the power levels of the second and third pulses are also slightly different but this difference is considerably smaller than that one between the first and second pulses. From practical point of view all pulses followed by the first one can be regarded as identical. According to Eq. (13) the first and subsequent pulses experience a different red shift of their central wavelengths. This spectral walk off as a function of the propagation distance is depicted in the inset in Fig. 6. The walk off seems to saturate during propagation.
Fig. 6. Dynamics of the signal peak power for the first (filled squares) and the subsequent pulses (empty squares) of a 10 GHz pattern ‘‘. . . 000011110000 . . .’’ in a fiber loop with a bulk amplifier. Inset: Corresponding evolution of the spectral walk off.
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But actually there is no physical mechanism, which may stabilise that effect as for the case with in-line filters [8,20,21]. Just due to the strong reduction of the pulse power the growth of the spectral walk off becomes negligible at large distances. For the fiber loop with the MQW amplifiers the results are qualitatively the same (see Fig. 7). Here the maximum value of the spectral walk off is about 0.06 nm whereas it is 0.15 nm using bulk devices. The difference can be explained by a different signal power and SOA parameters (Henry factor and saturation energy). Due to the spectral walk off the channel bandwidth usually suffers a 2-fold increase during propagation. The Q-factor is very sensitive to the center wavelength of the receiver filter. With a 0.2-nm filter in the optimum position for the detection of the first pulse, the second pulse is sometimes almost undetectable and vice versa. Hence for a successful detection of all pulses the filter must be adjusted at some middle position between the wavelengths of the first and the second signal pulses. Additionally the filter bandwidth has to be optimised too. Usually the signal-to-noise ratio for the first pulse is better than that one of the subsequent signals. Therefore the total Q-factor is mainly determined by these subsequent pulses. Fig. 8 shows the Q-factor of the irregular pulse train in the fiber loop with the MQW and the bulk
Fig. 7. Dynamics of the signal peak power for the first (filled squares) and the subsequent pulses (empty squares) in the 10 GHz pattern ‘‘. . . 000011110000 . . .’’ in a fiber loop with a MQW amplifier. Inset: Corresponding evolution of the red shift of the pulses.
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Fig. 8. Evolution of the Q-factor of the 10 GHz pattern ‘‘. . . 000011110000 . . .’’ in a fiber loop with a bulk (stars) and a MQW (circles) amplifier.
amplifier. In both cases the loop parameters are optimised for the longest propagation distance, but the detection schemes are the same. An errorfree propagation of about 600 km has been realised with bulk and up to approximately 800 km with MQW amplifiers. As it could be expected it is smaller than the distances obtained with regular pulse train transmission. In Fig. 9 one can see that the evolution of the Q-factor in a fiber loop with a 1-nm in-line filter is very similar to that one without in-line filters as it could be seen from Eqs. (5) and (9). Finally we optimise the bandwidth of the receiver filter. On the one hand the less the filter
Fig. 9. Comparison of the degradation of the Q-factor for a 10 GHz signal in a fiber loop with a 1-nm in-line filter (circles) and without an in-line filter (points).
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estimated using the Q-factor evolution. The degradation of the pulse quality measured by means of the Q-factor is similar to that one in a fiber loop with a 1-nm in-line filter. For effective performance the system has to be designed more carefully in the case without in-line filtering than with in-line filters. The use of SOA with a shorter gain medium, the application of gain flattening filters, the minimisation of the polarisation dependence of the gain and losses in the transmission line will lead to a further improvement of the system performance. Fig. 10. Q-factor of the 10 GHz pattern ‘‘. . . 000011110000 . . .’’ at a propagation distance of about 800 km in dependence on the bandwidth of the receiver filter.
bandwidth the lower the level of the detected ASE. From this point of view the minimum bandwidth of the receiver filter for a regular pulse train is determined by the signal bandwidth of about 0.2 nm. On the other hand the detection of the pulses of an irregular pulse train requires a much broader shape for the filter because of the above considered spectral walk off. Thus there is an optimal bandwidth for the receiver filter. For our experimental situation the optimal bandwidth is about 1 nm (see Fig. 10).
4. Conclusion A 10 Gbit/s RZ transmission system with cascaded semiconductor bulk and MQW amplifiers without in-line filters has been studied using an optical fiber loop setup. In contrast to previous experiments it has been shown that long transmission distances can be reached in such systems for a strong amplifier saturation. The typical system properties as the signal power and Q-factor evolution, the spectral red shift and the walk off are experimentally investigated. The results are described by means of simple theoretical model. The optimal bandwidth of the filter in front of the receiver is evaluated to be about 1 nm. The MQW amplifier exhibits a better performance than the bulk amplifier. An error-free propagation distances up to 800 km has been
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