Error-free 250 km transmission in standard fibre using compact 10 Gbit=s chirp-managed directly modulated lasers (CML) at 1550 nm D. Mahgerefteh, Y. Matsui, C. Liao, B. Johnson, D. Walker, X. Zheng, Z.F. Fan, K. McCallion and P. Tayebati 10 Gbit=s transmission over 250 km of standard fibre at 1550 nm with 4.8 dB penalty using a transmitter comprising a directly modulated DFB laser, a multi-cavity filter and a standard receiver, is demonstrated. Introduction: Many metro networks span up to 200 km; however, none of the existing transmitters can meet the requirements for long reach, low power consumption and small footprint, simultaneously. An electro-absorption modulator can offer small size and low power consumption; however, the reach is limited to 100 km at 10 Gbit=s owing to the broad spectrum associated with transient chirp [1]. The typical output power is limited to around 2 dBm and an option for the wavelength tunability is not usually available. Upgrading metro systems from 2.5 to 10 Gbit=s therefore currently calls for costly dispersion-compensating modules. In this Letter, we demonstrate a 250 km (4200 ps=nm) transmission in standard singlemode fibre (SSMF) at 1550 nm using a chirp- managed directly modulated (CML TM ), comprising a DFB laser and multi-cavity filter for extinction ratio (ER) enhancement and spectrum narrowing. The high dispersion tolerance is achieved by setting the adiabatic chirp of the DFB laser to establish a special phase correlation between the bits, and by vestigial sideband (VSB) filtering on the edge of the bandpass filter. The reach is comparable to the best reported performance for duo-binary transmission using LiNbO 3 external modu- lators [2], yet the CML fits in a 5.5  6.5  15.5 mm transmitter optical subassembly. Principle of operation: The CML is dispersion tolerant mainly because of simultaneous high ER AM and FM modulation. Consider a 1 0 1 bit sequence at 10 Gbit=s having a 5 GHz frequency modula- tion; i.e. one bits have 5 GHz higher frequency than 0 bits. Hence the phase of the carrier slips by 2p  5 GHz  100 ps ¼ p during the 0 bit making the second 1 bit p out of phase with the first 1 bit. In general, 1 bits separated by an odd number of 0 bits are p out of phase. Normally dispersion spreads the energy of the 1 bits into adjacent 0 bits and closes the eye. However, the p phase in the CML causes destructive interference in the middle of the 0 bits, keeping the eye open after fibre dispersion. This requires a high ER of 10–13 dB, which is provided through FM=AM conversion by the filter. The same phase correlation, first introduced in classical communication by Lender [3], is achieved in phase correlated optical duo-binary by use of pre-coding and low-biasing of a Mach-Zhender modulator [4]. The CML technique is to be distinguished from dispersion supported transmission (DST), which uses FM modulation with low ER AM, and relies on FM to AM conversion by fibre dispersion [5]. Coincidentally the optimum chirp for DST is 5 GHz for 200 km transmission, corresponding to a 1-bit delay after 200 km of standard fibre to increase amplitude modulation. However, DST does not enjoy the destructive interference, essential to CML, even with 5 GHz chirp, because of the significant energy in the 0 bits; the typical DST has an ER  2–3 dB. Therefore, DST does not produce a standard open eye, and requires a special receiver with a very low frequency filter before the decision circuit to distinguish the bits. Also, in contrast to the CML, the optimum chirp for DST increases significantly with decreasing distance. In the CML, the passage of the signal through the edge of the filter not only improves the ER but also produces a VSB effect, which adds blue transient chirp at the 1 to 0 and 0 to 1 transitions, further improving the eye opening after fibre dispersion. The VSB filtering also reduces the information bandwidth. Additional effects in CML are elimination of transient chirp both by use of high bias to the DFB laser, bandwidth limiting of the pulse by filter, and some negative dispersion from the filter. The detailed analysis of these combined effects will be presented elsewhere. Results: Fig. 1 shows a schematic of the CML, which consists of a high-speed DFB laser back end, and a filter=locker, separated by an isolator used to eliminate filter back reflection into the DFB. A tap directs  4% of the DFB power into a photodiode, PD1. A second photodiode, PD2, detects light reflected from the filter. The DFB wavelength is locked to the filter by adjusting laser temperature to keep PD2=PD1 constant. PD2 lens DFB chip isolator PD1 splitter filter Fig. 1 Schematic of chirp-managed directly modulated DFB laser (CML) The DFB was a buried heterostructure MQW having a 17 mA threshold, slope efficiency of 0.25 mW=mA, and FM efficiency of 0.23 GHz=mA. The laser was biased at 80 mA and modulated at 10 Gbit=s with 0.8 V peak-to-peak, generating an output with ER  1.1 dB. The adiabatic frequency excursion was  4 GHz. The ER after filter was increased to 11.5 dB by FM-AM conversion [6]. The filter was a two-cavity silica design with 50 GHz periodicity, a 3 dB bandwidth of 7.5 GHz, and a slope of 2.3 dB=GHz, at the operating point. The output power of the CML device was  2 dBm. The modulated spectrum width at the output of CML was only 15 GHz at the 20 dB point from the peak. This bandwidth reduction is the result of the phase correlation between the bits [3], but can also be understood qualitatively in terms of the relative phase of the AM and FM sidebands in the small-signal regime [7]. The system consisted of 100, 100 and 50 km of SSMF, with  24 dB gain EDFAs after each span. The input power to each fibre span was  0 dBm. A preamplified receiver with a pin detector (3 dB BW of 8 GHz) was used. To test tolerance of CML to negative dispersion, DC modules with 1350 and 2150 ps=nm of dispersion were used. Fig. 2 shows eye diagrams of the CML output and BT filtered eyes after each span, which are wide open up to 250 km. a b c d (25 ps/div.) Fig. 2 Eye diagrams of CML a Back-to-back optical eye diagram b BT filtered eye at 100 km of SSMF c BT filtered eye at 200 km of SSMF d BT filtered eye at 250 km of SSMF Fig. 3 shows the BER performance of CML at 10 Gbit=s for 2 31 1 PRBS. The OSNR into the pin was 35 dB=0.1 nm. The penalty was 0.2 dB at 100 km, 0.5 dB at 200 km and 4.8 dB at 250 km. The penalty for negative dispersion is 0.2 dB for 1350 ps=nm and 1.6 dB for 2150. The back-to-back sensitivity had only a 2 dB penalty compared to a LiNbO 3 transmitter with 13 dB ER and 23 ps rise and fall times. The slope of the sensitivity curve gradually changes with increasing received power because the dominant noise is thermal at lower powers (<32 dBm), and signal-ASE at higher powers. A relatively flat sensitivity is obtained over a wide range of dispersion from 2150 to 4200 ps=nm (Fig. 4). The shift in the penalty curve is due to the negative dispersion of the filter. ELECTRONICS LETTERS 28th April 2005 Vol. 41 No. 9