Si-rich silicon nitride for nonlinear signal processing applications

We report the demonstration of an integrated silicon-rich silicon nitride wavelength converter based on the Bragg scattering intermodal four-wave mixing process. This broadband wavelength converter incorporates on-chip mode conversion, multiplexing and de-multiplexing functionalities. The system allows for broadband signal conversion with a 3dB bandwidth exceeding 70 nm.


INTRODUCTION
Modern optical transmission systems are evolving to meet the ever-increasing global demand for bandwidth, essential for handling the growing volume of data transmission worldwide.A key strategy to address this challenge involves exploring under-utilized frequency windows of the electromagnetic spectrum, specifically the L and U bands (1565 -1675 nm).These advancements are crucial for systems that need to efficiently manipulate wavelengths, with a focus on flexibility and high capacity.Recent efforts in optical fiber communication have highlighted the potential of the L-(1565 -1625 nm) and U-bands (1625 -1675 nm) beyond the conventional C-band (1530 -1565 nm).The core technology driving these developments relies on optical devices that utilize third-order nonlinearities, enabling the manipulation of wavelength components through processes like four-wave mixing (FWM).
A notable area of innovation has been in the realm of intermodal FWM (IM-FWM), where distinct spatial modes within the same waveguide are utilized.This approach allows for more versatile waveguide dispersion engineering and allows, in principle, to convert and generate wavelength components over a large bandwidth, enhancing the flexibility and efficiency of wavelength converters.Our work demonstrates the effectiveness of this method using a silicon-rich silicon nitride (SRSN) wavelength converter based on Bragg scattering (BS)-IM-FWM. 1, 2

EXPERIMENTAL RESULTS
The device is designed for efficient wavelength conversion using the BS-IM-FWM process in a multimode waveguide.The key to its efficiency is maintaining momentum and energy conservation among the interacting waves.This is achieved when the inverse group velocities (IGV) of one mode is the frequency-shifted copy of the IGV of the other mode.This allows for the phase-matching condition to be satisfied even with a significant pump wavelength detuning. 2In our design, two pump waves are coupled into the fundamental transverse electric mode (T E 00 ) of the multimode waveguide, while a signal is placed in a higher-order TE mode (T E 10 ).This setup generates an idler component in the T E 10 mode.The device is built on a SRSN platform, 3,4 featuring a layer of SRSN on thermal silicon dioxide.Our device can efficiently operate from the C to the U band, using the first two TE modes of a waveguide with properly designed dimensions.It includes inverted taper-based edge couplers for signal coupling, a multimode interference (MMI) coupler combined with a phase shifter and a sinusoidal-profile Y-branch for mode conversion and multiplexing.This arrangement ensures that the signal is converted to the desired T E 10 mode, while the pumps remain in their original T E 00 mode.At the output, a similar setup converts and de-multiplexes the modes, allowing the idler and seed signal to be extracted from one output port while directing residual pumps to another.This design eliminates the need for external filtering of the pump waves from the signals.
The optical pumps (P1 and P2) were created using two tunable lasers and two Erbium-doped fibre amplifiers (EDFAs), coupled together and sent to the device.A third source generated the seeding signal (S) sent to a different port.At the output, signals and idlers were directed to an optical spectrum analyzer via an optical switch.The device used fibre arrays with lensed fibres for light coupling.Initial tests showed a fibre-to-fibre loss of about 5 dB and around 20 dB of crosstalk.Nonlinear measurements involved launching a total pump power of 27.6 dBm into the waveguide.In the first set of measurements, the wavelength of the second pump P2 was varied to study the pump-to-pump detuning bandwidth (BW) of the BS-IM-FWM process.The results showed a small decrease in conversion efficiency (CE) for the red-shifted idler and a narrower BW for the blue-shifted idler, as expected from numerical simulations.A 3dB BW of 72 nm was measured for the red-shifted idler.The second set of measurements assessed the signal-detuning BW.The pumps were set at specific wavelengths (1540 and 1542 nm for P1 and P2, respectively), and the signal wavelength was varied.The results indicated no CE reduction even with significant signal detuning (+-50 nm) around the best phase-matched signal wavelength (1600 nm).However, when the pump-to-pump detuning was altered (1540 and 1570 nm for P1 and P2, respectively), the phase matching condition was not maintained across all wavelengths, resulting in a 3dB BW of 25 nm centered around the best phase-matched signal wavelength (1600 nm).The maximum CE recorded was -41 dB, limited mainly by the waveguide's length and material losses.

CONCLUSIONS
This paper highlights the development of an advanced, fully-integrated wavelength converter using BS-IM-FWM on a SRSN platform.It features on-chip mode conversion, multiplexing, and demultiplexing functionalities with a 3dB bandwidth exceeding 70 nm, the widest ever reported for such a system based on intermodal FWM nonlinearities.