Direct-detection DWDM and information transmission using infrared acousto-optic femtosecond pulse shaping
Acousto-optic modulator (AOM)-based pulse shaping permits precise spectrum slicing, which is useful for dense wavelength division multiplexed (DWDM)-based architectures. In essence, this method uses microsecond-duration radio frequency pulses to completely control the spectra of femtosecond laser pulses, hence achieves dramatic temporal data compression. We demonstrated the AOM pulse shaper as the modulator and a CCD camera with 256 pixels as the receiver. The spectrum of a 200 fs Erbium Doped Fiber Laser (EDFL) pulse was dispersed across the AOM's aperture (FWHM was 35 nm) and was then modulated in a conventional pulse shaper. We tested 87 channels with channel-spacing of 0.41 nm using a 518-MHz modulator, and 120 channels with channel-spacing of 0.29 nm using a 148-MHz modulator (in each case a 0.1 nm guard band was used). Starting from the original pulses, this modulation creates time slots of 43 ps and 63.4 ps respectively. The equivalent speed of the transmission will be 2.0 Tb/s and 1.9 Tb/s in a highly multiplexed system. The spectral efficiencies achieved in this experiment were ∼46%, approaching the theoretical limit of 50% for On-Off Keying (OOK) modulation. A benchmark image was successfully sent over the test bed.