s–240 km Fiber Transmission Experiment

OPTICAL FIBER TECHNOLOGY ARTICLE NO.

2, 351]357 Ž1996.

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Repeatered Bidirectional 10 Gbrs]240 km Fiber Transmission Experiment J.-M. P. DELAVAUX, C. R. GILES, S. W. GRANLUND,

AND

C. D. CHEN

Lucent Technologies Inc., Bell Laboratories, Breinigs¨ ille, Pennsyl¨ ania 18031-9351 and Lucent Technologies Inc., Bell Laboratories, Crawford Hill, New Jersey 07733-0400 Received April 10, 1996

We report the design and performance evaluation of a novel circulator-based bidirectional optical amplifier. Using this amplifier as a mid-span repeater, we have demonstrated successful bit error rate (BER - 10 y13 ) transmission at 10 Gb / s over 240 km of dispersion shifted fiber in both directions for channel wavelengths at 1557.5 and 1559 nm. Q 1996 Academic Press, Inc.

INTRODUCTION

There has been a growing interest in bidirectional fiber transmission experiments. Among other advantages, bidirectional fiber transmissions can provide: Ž1. full duplex transmission over a single fiber, Ž2. capacity enhancement through wavelength dense multiplexing ŽWDM., and Ž3. network maintenance and surveillance via optical time domain reflectometry ŽOTDR.. Early repeaterless w1]3x and repeatered w4]8x bidirectional fiber transmission experiments have achieved successful results using single- or multiple-transmission wavelengths at modulation speeds up to 2.5 Gbrs. However, in these experiments the dominant Rayleigh backscattered ŽRBS. signal power was an issue that required careful narrow band filtering to prevent degradation of the signal to noise ratio at the receiver. More recently, the use of fiber reflection gratings together with optical circulators has allowed the efficient separation of Rayleigh backscattered signal from the transmitted signal and has led to the demonstration of practical unimpaired fiber transmission experiments w9]12x. In particular, we have reported w9, 10x successful bidirectional transmission at 10 Gbrs at the 1550-nm wavelength over both 150 km of dispersion shifted fiber ŽDSF. and 120 km of regular single-mode fiber ŽSMF.. In addition, we have also demonstrated w12x with the use of concatenated high-rejection, low-loss fiber gratings that the bidirectional transmission of up to six channels reduces four wave mixing ŽFWM. degradation while providing efficient bandwidth utilization.

In this paper, we describe the transmission performance of two 10 Gbrs 1550-nm repeatered channels over two spans of 120 km of DSF using a novel in line bidirectional erbium amplifier repeater ŽBEAR.. The proposed amplifier repeater design also relies on efficient reflection fiber gratings and optical circulators to provide high amplifier performance while ensuring low crosstalk between the transmission channels. Bit error measurements over 240 km of DSF showed no error floor below 10y1 3 bit error rate ŽBER. for both transmission directions, suggesting the potential increase in transmission capacity of existing unidirectional systems with repeatered bidirectional transmission systems. AMPLIFIER DESIGN AND PERFORMANCE

Figure 1 shows a schematic of the bidirectional erbium amplifier repeater. The amplifier design includes two four-port circulators that combine and separate the downstream Ž l1 . and the upstream Ž l 2 . wavelengths, respectively. The optical insertion loss and isolation for two consecutive circulator ports were less than 1 dB and greater than 70 dB, respectively. Prior to amplification by a two-stage amplifier, a reflection fiber grating Že.g., FG1. centered at the transmission wavelength Ž l1 . was placed in port 3 of the circulator at the input of the low-noise preamplifier stage Ž l 1 stage. in order to reflect the transmission wavelength and reject any Rayleigh backscattered signal power. All the fiber gratings had an insertion loss less than 1 dB, a 3-dB reflection bandwidth of 0.8 nm, and an isolation better than 50 dB. The erbium-doped active fiber lengths Ž l 1 and l 2 . of the tandem amplifier were chosen using a 30% ratio Ži.e., l 2 s 2 = l 1 . in order to provide a combined low-noise figure Ži.e., 3 dB. and maximum output power Ž) 12 dBm. for up to four-channel amplification with input signal power in the range y18 to y24 dBm. The two amplifier stages were separated with an optical isolator and centerpumped through a pair of wavelength multiplexers by one

351 1068-5200r96 $18.00 Copyright Q 1996 by Academic Press, Inc. All rights of reproduction in any form reserved.

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FIG. 1.

Bidirectional erbium-doped fiber amplifier repeater ŽBEAR..

single 90-mW, 980-nm laser diode. In this case, the 980-nm pump power was distributed by a 30% coupler ratio between the pre- and post-amplifier stages. An alternative design relied on an hybrid pumping, where both preamplifier stages were counterpumped by a shared 70-mW 980-nm pump through a 3-dB coupler, while both postamplifier stages were copumped equally with an 80-mW, 1480-nm laser pump via a 3-dB coupler. One distinctive advantage of the BEAR design is that not only does it allow the selection and allocation of the desired wavelengths assigned to each of the transmission directions, but it also provides an add and drop function for wavelength channels at the amplifier-repeater level. All the amplifier performances were obtained with with a pair of tunable wavelength external cavity laser sources together with a pair of three-port circulators to separate upstream and downstream signals. Noise, gain, and output power were measured using the optical spectrum analyzer method with 0.5-nm resolution bandwidth. Figure 2 shows a typical curves for the output power Ž Pout . and noise figure ŽNF. as a function of input signal Ž Pin . for both upstream Žsolid line, l 2 s 1559 nm. and downstream Ždashed line, l1 s 1557.5 nm. transmission wavelengths for the amplifier module BEARa1. For both transmissions directions the amplifier provides a small signal net gain greater than 29 dB, a noise figure value less than 7.5 dB, and a saturated output power greater than q12 dBm. In this plot, the amplifier performance at each wavelength Že.g., l1 . was measured, while the opposite wavelength Ž l2 . was set a constant input power of y20 dBm. Figure 3 illustrates the dependence of the upstream amplifier performance on four levels of input power levels for the downstream signal. The output power does not change while the noise figure curve exhibits a degradation of 0.2 dB for signal level below y30 dBm. The observed degradation comes from the amount of

Rayleigh backscattered amplified spontaneous emission ŽASE. of the downstream direction leaking through the reflection filter width of the upstream direction. Therefore, these results indicate no crosstalk occurs between the two amplified channel wavelengths. Three rack-mounted amplifier modules ŽBEARa1, a2, and a3. were tested under the same operating conditions and with the same pair of grating FG1 and FG2 . The measured results at an input signal Ž Pin . of y20 dBm are summarized in Table 1, and confirm the consistency in amplifier design and robustness for both amplified transmission directions Ži.e. A1 or A 2 .: The measured gain is greater than 29 dB and the noise remains below 7.5 dB. The noise figure values include the input connector loss, the circulator insertion loss, and the fiber grating loss, which account for a total of 3dB. In that respect, we believe that this loss contribution can be reduced by 2 dB

FIG. 2. Output power Ž Pout . and noise figure ŽNF. performance versus input signal power Ž Pin . of bidirectional amplifier ŽBEARa1. for downstream Ž l1 s 1557.5 nm. and upstream Ž l 2 s 1559 nm. transmission wavelengths.

REPEATERED BIDIRECTIONAL 10 Gbrs]240 km FIBER TRANSMISSION EXPERIMENT

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TABLE 1 Gain and Noise Figure Performance at Both Signal Transmission Wavelengths ( l s = l 1 and l 2 ) for Three Rack-Mounted BEAR Modules

FIG. 3. Dependence of Pout and NF performance for the upstream transmission Ž l2 . on four levels of input signal Ž Pin . of the downstream transmission Ž l1 ..

with a better optimization of the circulator-fiber-grating assemblies. For practical purposes, each fiber grating had one extremity connected to the circulator via an FCrPC connector and the other extremity terminated with an angled FCrPC or with an optical isolator in order to avoid optical feedback reflection and to observe the nonreflected output spectrum. In order to test the detrimental effect of Rayleigh backscattered and stimulated Brillouin scattering ŽSBS.

power on the amplifier performance, 7.5 km of standard single-mode fiber was inserted in front of the amplifier under test. Under these conditions, the amplifier performance did not exhibit any change in performance for one transmission direction Že.g., the upstream transmission. in the presence of RBS or SBS power produced by the transmission in the opposite direction Že.g., the downstream transmission. for an input power of y4 dBm. Once more, these results confirmed the excellent rejection property of the fiber reflection gratings.

FIG. 4. BEAR a1 performance dependence on downstream Ž l1 . signal wavelength detuning with respect to grating center wavelength and fixed upstream wavelength Ž l2 s 1559 nm..

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Next, we measured the behavior of the amplifier performance versus detuning of the signal wavelength Ži.e., l1 . through the fiber grating ŽFG1 .. Figure 4 illustrates typically curves for six different wavelengths for the output power and noise figure against input power for the case of FG1 with a constant Pin of y20 dBm for the upstream channel wavelength Ži.e., l 2 .. As expected, these curves show that wavelength detuning from the grating center wavelength degrades the gain and the noise figure values, but without producing any instability in the operation of the amplifier repeater. Figure 5 describes the dependence of amplifier performance on wavelength detuning for both transmission directions with an input power of y20 dBm, corresponding to using the amplifier as an in-line repeater. The measurements yields a useful 0.6-nm flat gain and constant noise figure bandwidth for each transmission direction, with drastic degradation of amplifier performance on either side of this flat region due to the good rejection of the grating filter designs. FIBER TRANSMISSION EXPERIMENT RESULTS

The unrepeated transmission experiment setup is described in Fig. 6. The terminal optics at the end of transmission line included a three-port optical circulator that directed the transmitted and the received signals into the transmission fiber and the receiver, respectively. The amplifier repeater was sandwiched in betweeen with two equal spans of Ž; 120 km. dispersion shifted fiber ŽDSF1

FIG. 5.

and DSF2 .. The transmission fiber spans comprised several concatenated spools of dispersion shifted fiber having a mean wavelength Ž l 0 . of 1553 nm and a dispersion slope of 0.08 psrnm2rkm. The fiber span had lengths of 126 and 123 km with total span loss of 28.6 and 28 dB, respectively. Both transmitters consisted of externally modulated DFB lasers whose output power was boosted to a q15-dBm output power by an optical power amplifier ŽBA 1 or BA 2 .. At the output of the circulator, the launched power into the transmission fiber span was 13 dBm. No Brillouin scattering was observed. The Brillouin threshold in the DSF was q6 dBm for CW power. Optical filtering at the receiver discriminated the received signal from the Rayleigh backscattered of the near end transmitter signal. Figure 7 shows the spectra before and after temperature tuning of the fiber grating center wavelength. In this example, notice before filtering that the Rayleigh backscattered light from the near end transmitter Ž l1 s 1557.5 nm. was stronger in power than the received far end signal. After filtering, the optical signalto-noise ratio ŽSNR. was 32 dB and the crosstalk from the rejected signal was better than 42 dB with respect to the received signal. Figure 8 shows the bit error rate measurement curves for 240 km for both transmission channels. The figure insets are the received eye diagrams for both directions. Although the eye shapes are asymmetric due to fiber nonlinearities and residual dispersion, they reveal a large system eye margin. In case of the 1557.5-nm transmission channel, the BER curve shows a straight line down to a

Small signal gain ŽG. and noise figure ŽNF. dependence on signal wavelength detuning for both the downstream and upstream directions.

REPEATERED BIDIRECTIONAL 10 Gbrs]240 km FIBER TRANSMISSION EXPERIMENT

FIG. 6.

355

Bidirectional transmission setup.

10y1 4 BER level at a receiver sensitivity of y13 dBm. No degradation of receiver sensitivity was observed compared to the back-to-back unidirectional measurement. This demonstration of system stability was possible because of the use of robust rack-mounted transmitter, receiver, and amplifier modules. In the case of the 1559-nm transmission channel, the BER measurement also shows straight line parallel but shifted by 1 dB with respect to the 1557.5-nm results. In this case, demonstration of BER lower than 10y1 3 was

not possible since there was no automatic bias control of circuit of the bench top modulator]transmitter arrangement used for this transmission direction. Indeed, part of the 1-dB sensitivity penalty may have its origin in the nonoptimized operating conditions at the transmitter end. Overall, these results indicate steady performance and no BER floor above a 10y1 3 BER level. One important consideration in system implementation is the system stability with respect to wavelength detuning generated over the lifetime of the laser or caused by

FIG. 7. Received spectrum before and after temperature-tuned filtering of Rayleigh backscattered downstream signal by fiber reflection grating filter FG1.

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km of DSF. The inset displays the optical SNR and the receiver penalty at 10y1 0 BER. Over the range 1557.3 to 1557.9 nm Žor 0.6 nm bandwidth., the SNR remains above y31 dB, which should provide the condition for error-free transmission at 10 Gbrs. In contrast, the operating range for negligible receiver penalty Ži.e. 0.1 dB penalty. restricts the operating wavelength from 1557.5 to 1557.8 nm Žor 0.3 nm bandwidth.. This reduction in the useful operating bandwidth corresponds to the broadening of the laser spectrum by the baseband information. Nevertheless, these results indicate that the bidirectional system continues to operate with a good stability even in the presence of wavelength detuning. CONCLUSION

FIG. 8. Bidirectional transmission bit error curves through 240 km of dispersion-shifted fiber ŽDSF. at 10 Gbrs, with insets showing received eye patterns for both transmission directions.

change in the environmental operating conditions Že.g., temperature change.. In that respect, we have investigated the dependence of BER results on the detuning of the downstream signal wavelength from its initial 1557.5nm value. Figure 9 shows the measured BER curves for six downstream wavelengths for transmission through 240

We have proposed a novel bidirectional erbium amplifier repeater whose design used both narrow-band fiber reflection grating and optical circulators. This amplifier yielded a combination of performance not available in conventional design: high signal gain Ž) 29 dB., low noise figure Ž- 7.5 dB., and high saturated output ower Ž) 12.5 dBm.. In addition, this efficient amplifier provides two attractive advantages: Ž1. arbitrary selection and allocation of closely set transmission wavelengths Že.g., 1.5 nm. with no wavelength crosstalk and Ž2. inherent capability for an add-and-drop-channels function at the amplifier repeater level.

FIG. 9. Dependence of BER transmission for the downstream direction on wavelength detuning Ž l1 .. Inset shows the optical signal-to-noise ratio ŽSNR. and sensitivity penalty at BER of 10y1 0 versus signal Ž l1 . wavelength detuning.

REPEATERED BIDIRECTIONAL 10 Gbrs]240 km FIBER TRANSMISSION EXPERIMENT

A rack mounted version of the BEAR was successfully tested in a repeatered bidirectional 10 Gbrs 240-km fiber transmission system experiment. Straight bit error rate curves with no floor below 10y1 3 were demonstrated for both downstream Ži.e., 1557.5-nm. nd upstream Ži.e., 1559nm. transmission channels. The use of the same repeatered system configuration should enable the increase transmission capacity of existing systems with long-distance WDM bidirectional transmission systems.

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ACKNOWLEDGMENTS w7x We thank the 10 Gbrs group at the Solid State Technology Center, Breinigsville; K. Ogawa for his support; G. Badulak for technical assistance, and V. Mizrahi for providing the fiber gratings used in this experiment.

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