Tunable Active De-Multiplexer for Optical Frequency Combs

Abstract

Optical frequency comb (OFC) sources have recently attracted a lot of interest for their application as multi-carrier transmitters in all sectors of high capacity optical communication systems [1] including data-centre interconnects [2], access networks [3] and long-haul networks [4]. A major benefit of an OFC as a transmitter stems from the fixed frequency separation between the individual comb lines. This alleviates the need for large spectral guard bands between channels, thereby enhancing the spectral efficiency. Also, this fixed frequency separation can mitigate Kerr-based non-linear transmission effects, thereby enhancing the transmission reach [5].However, one of the major challenges in the adoption of OFCs, as multi-carrier transmitters in high-speed systems, entails the de-multiplexing of the individual comb tones. Recently, the ITU-T has extended its recommendations [6] to include the concept of a flexible grid. Moreover, next generation flexible networks may see the channel spacing granularity reduce from 12.5 to 6.25 GHz or even smaller [7]. Commercially available demultiplexers, such as arrayed waveguide gratings (AWG) and wavelength selective switches (WSS), are only compatible with traditional WDM systems using the ITU-T standard grid (channel spacing of 200, 100, 50 and 25 GHz). Hence, they don't offer adequate resolution to filter closely spaced (<; 25 GHz) lines and will struggle to achieve the stipulated channel isolation. In addition, they incur large insertion losses that are usually compensated with the aid of an optical amplifier. The addition of optical amplifiers will worsen the optical signal to noise ratio (OSNR) and thereby degrade overall performance/reach.Figure 1:(a) Block diagram of injection-locked demultiplexer; (b) Optical spectra of the demux output in red and overlaid comb spectrum in grey [7].One possible solution to de-multiplex (de-mux) densely spaced tones entails the use of an optical injection-locking technique [8]. A schematic of such a scheme (injection-locked de-multiplexer) is illustrated in Fig. 1(a). The OFC lines are injected into cavity of the semiconductor laser, referred to as the de-mux laser. Then, the demux laser is temperature tuned to match the wavelength of the desired comb tone to be de-multiplexed. This results in the injection locking of the demux laser by the required comb line. Since the modal power of the demux is much higher than that of the comb line, the selected tone is amplified relative to the unwanted lines. The optical spectrum of the demux output is depicted in Fig. 1(b) (superimposed over the injected comb).The talk will give an overview of the operational principle of the injection locked tunable demultiplexer. The effect of the injected comb line power (CLP) and the detuning frequency (Δf) on the comb line suppression ratio (CLSR) will also be discussed [8]. The CLSR is defined as the power difference between the amplified demuxed line and the unwanted (suppressed) comb line, as illustrated in Fig. 1(a). An experimental demonstration of the FSR tunability feature of the demux will be presented, with a tone spacing of 6.25, 12.5, and 15 GHz. Finally, the application of the demux as a gain-flattening filter and a modulator will be discussed to highlight the versatility of this technique.