The passive optical network based on wavelength division multiple access (WDM-PON) can provide almost unlimited bandwidth to the subscribers with protocol transparency. To realize WDM-PON, the development of a low cost WDM source is key issue. The wavelength-locked Fabry-Perot laser diode (FP-LD) was proposed as a low cost WDM source. By injecting spectrum sliced BLS (Broadband Light Source), the FP-LD is forced to oscillate in a quasi-single mode. Thus, the BLS is one of the basic building block for the proposed light source.
In this paper, we proposed a new Broadband Light Source based on a chirped FP-LD by direct modulation. The direct modulation causes modulation of refractive index and it causes frequency chirp of the output optical field. As a result, spectral broadening is induced and it decreases the coherent time of the light output that is necessary to realize BLS.
Since the output of the proposed BLS is a train of pulses, it is not sufficient to use a BLS for wavelength locked FP LDs. To improve the characteristics of the BLS, we proposed the time division multiplexing with time delay (The pulse train which forms a BLS was divided by two and multiplexed in time after time delay.). The delay time by the length of delay line is determined by the correlation time of the mode partition noise in the modulated FP LD. We found experimentally that the required delay line is longer than 88 cm.
We demonstrate error free transmission of 3-channel 155.52 Mb/s WDM signals with wavelength locked FP LDs based on the proposed BLS. The channel spacing is 100 GHz and the transmission distance is 20 km of standard single mode fiber. There is no bit error rate degradation induced by crosstalk. The proposed BLS requires about 6 dB higher injection power compared with the ASE based BLS.
