On-chip optical pulse shaping using cascaded co-directional couplers

Abstract

Picosecond-resolution optical pulse shaping (OPS) is desired for many important applications in ultrahigh-bit-rate optical fiber communications and ultrafast optical signal processing. Commercially available optical pulse shapers operate based on the well-established spatial-domain processing approach, allowing programmable synthesis of arbitrary waveforms with resolutions better than 100 fs. However, the need for high-quality bulk-optics components makes the implementation of this method relatively complex. Similar OPS principles have been implemented using compact on-chip arrayed diffraction gratings (ADGs), but ADGs are challenging to fabricate, yet they suffer from limited spectral resolution. All-fiber and integrated-waveguide grating structures have been considered as another solution for OPS due to their mature fabrication process and low loss (particularly in fiber-based schemes). However, grating devices have proven challenging to fabricate in integrated-waveguide configurations and it would be also difficult to add reconfigurability in these structures. To overcome the aforementioned limitations, we will review here recent work on a novel design based on a structure of cascaded co-directional couplers. In particular, our new design exploits the fact that under relatively weak-coupling conditions, the ‘discrete’ amplitude and phase ‘apodization’ profile of the structure can be directly mapped into the output temporal response of the device, see scheme in Fig. 1. We refer to this approach as discrete space-to-time mapping (D-STM). The proposed design is based on forward coupling between a main-waveguide and a bus-waveguide, where the coupling is controlled in a discrete fashion through standard co-directional couplers (‘amplitude apodization’). On the other hand, phase tuning can be done by adjusting the relative path-length different between sections of bus and main-waveguide at each stage of the device. The devices obtained through this new design method are notably simpler to fabricate than their Bragg grating-based counterparts, while potentially enabling reconfigurability through well-established mechanisms. Using D-STM, we have successfully re-shaped (sub-)picosecond Gaussian-like pulses into several practically relevant pulse shapes, including a 1.25 ps (FWHM) long flat-top pulse, an 8-bit, 200 G-baud 16-quadrature amplitude modulation (QAM) bit sequence and a 70 ps (FW) long high quality flat-top pulse. The power spectral responses (PSRs) presented in Fig. 2, have been measured using an optical vector analyzer (O-VA). Besides, direct time-domain characterization of the output synthesized waveforms has been carried out using Fourier-transform spectral interferometry (FTSI). As shown in Fig. 2(f) a combination of single-mode waveguide (SMW) and multi-mode waveguide (MMW) has been used to reduce the loss and phase-noise for the case of long-duration OPS devices.