Abstract

Wavelength-division multiplexing (WDM) systems require light sources with precisely determined optical frequencies. Such systems will contain complicated control circuits together with many lasers.¹ One altemative method is to use multi-wavelength light sources whose output frequencies are precisely separated. One possible candidate for such light sources are mode-locked lasers because their outputs contain many longitudinal modes whose frequencies are precisely locked to longitudinal mode spacing. Therefore, by combining appropriate optical filters, they can work as light sources with many different optical frequencies. Although there have been reports on multiwavelength light sources using mode-locked lasers, most of them use large size lasers such as solid state lasers² or Er³⁺-doped fiber ring lasers,³ which were complex. Moreover, as longitudinal mode spacing is much smaller than the desired channel spacing, it is difficult to extract one single mode from the adjusting modes. Therefore, the output of the light sources contains severa1 longitudinal modes that limit the transmission bit rate to the frequency separation of the modes. We report here a multiwavelength light source using semiconductor mode-locked lasers with the optical frequency spacing of 100 GHz. The schematic setup for the light source is illustrated in Fig. 1. In this light source, a mode-locked semiconductor laser was used for the original light source, which gave multimodes with constant frequency spacings (inset of Fig. 1). The laser had a monolithically integrated electroabsorption modulator.⁴ The pulse repetition rate (mode spacing) was set to 50 GHz by careful adjustment of the cavity length. The pulse width measured was about 3 ps, which was narrow enough to generate many longitudinal modes around the center frequency. The envelope of the lasing spectrum spread to about 10 nm. The output from the mode-locked laser was fed into silica-based arrayed waveguide grating filter (AWG) with the free spectral range of 100 GHz and 16 channels of input/output ports. The modes from the laser were demultiplexed to the output ports of the AWG in accordance with their wavelengths. Each output from the AWG is continuous wave because it contained only one mode, unlike that in other research paper.^1-3 Each output had the optical frequency spacing of 100 GHz (Fig. 2). The side-mode suppression ratio of eleven output channels (Chs. 4 ~ 14) was larger than 17 dB, which was limited by the bandwidth of one of the AWG's transmission peaks (30 GHz). These outputs were modulated with 5 Gbit/s 2⁷ – 1 NRZ signals using LiNbO₃ intensity modulators, and bit-error-rates were measured. The results are shown in Fig. 3. The power penalties of five mode channels (outputs from five ports for wavelengths around 1.545 μm; Chs. 8~12 in Fig. 2) were less than 1.5 dB compared with the measured bit-error-rate characteristic of a conventional DFB laser. In conclusion, light sources with precise optical frequency spacing were successfully demonstrated by extracting individual modes from mode-locked multimode spectra.

© 1996 Optical Society of America