Accessible Tuning Range Of Direct Intensity Modulated Three-section DBR Lasers

A variety of optical network architectures utilize wavelength division multiplexing (WDM) to access the bandwidth of single-mode optical fiber and provide network functions such as routing, switching and service segregation. Distributed Bragg reflector (DBR) lasers are well suited to applications which require tuning to a few wavelengths, as the distinct longitudinal modes obtained by changing the Bragg section current can be utilized 111-[2]. In this paper, the portion of the CW mode tuning curve which provides reliable performance using direct modulation of the active section is determined experimentally for WDM systems based on tunable fiber Fabry-Perot optical filters. The results are useful for determining procedures and requirements for the characterization of individual lasers in WDM systems. Fig. 1 illustrates the CW tuning curve for a three-section DBR laser (GEC-Marconi LD5231) as a function of the Bragg section current for an active section current of 50 mA and a phase section current of 0 mA. The discontinuous tuning range is 8.3 nm for a 50 mA change in the Bragg section current. The dependence of the CW mode suppression ratio (MSR) on the Bragg section current is shown in Fig. 2. Within the central region of individual mode tuning curves, the MSR is typically greater than 32 dBc. In the vicinity of a mode jump, the MSR degrades and becomes a ratio between adjacent longitudinal modes [3]. The active section of the DBR laser was modulated using a 1 Gb/s, 215 1 PRBS NRZ signal. The modulated tuning curve is virtually identical t o the CW tuning curve. The modulated MSR is illustrated in Fig. 3 as a function of the Bragg section current. Within the central regions of the mode tuning curves, the modulated MSR is within 2 dB of the CW MSR. However, in the vicinity of a mode jump, the modulated MSR typically degrades to less than 2 dBc. The bit error ratio (BER) was measured using a fiber Fabry-Perot optical filter with a bandwidth of 14 GHz to perform the wavelength division demultiplexing function required in a practical WDM system. The filtered optical signal was detected by a p-i-n/FET front-end, amplified, and lowpass filtered. The dependence of the BER on the Bragg section current is shown in Fig. 4. Two curves are associated with each individual mode tuning curve (except for the modes corresponding to Bragg section currents of 0 mA and 50 mA). The two curves illustrate the beginning and end of reliable performance (BER < lO'O) as the Bragg section current spans a mode tuning curve. To clarify the presentation of the results, alternate modes are represented by dashed lines with square symbols and solid lines with circular symbols. The BER changes by almost five orders of magnitude for a change in the Bragg section current of 0.1 mA to 0.5 mA depending on the longitudinal mode. The portions of individual CW mode tuning curves which provide reliable performance under direct modulation correspond quite strongly to modulated MSR values exceeding 29 dBc, and range from 0.0 for the modes centered a t 2.3 mA and 4.6 mA to 0.6 for the modes centered at 24 mA and 32 mA. The interval of the Bragg section current which yields BER < is not centered on the CW mode tuning curve, but rather coincides with the portion of the tuning curve corresponding