In a DWDM network, the passbands of each channel should ideally maintain a flat-top profile, rather than the more fundamental Gaussian shape. The main advantage is the wider flat top allows for less strict tolerances on laser linewidth and temperature control, letting the laser wavelength vary within the bandpass without creating unexpected loss or channel non-uniformity within the network. However, a standard flat-top AWG has higher intrinsic loss than a Gaussian AWG, so the trade-off for better network stability and uniformity, while using cost-effective laser sources, is usually higher flat-top AWG insertion loss.
With our patented MZI-AWG design, there is no need to have this trade-off. It uses a fixed Mach-Zehnder Interferometer (MZI) structure as a pre-filter, combined with an AWG, to provide a flat-top passband shape with no insertion loss penalty when compared to a Gaussian AWG.
The MZI is a waveguide structure that splits a single source into two waveguides and creates a fixed relative phase shift between the two signals. These are then recombined, creating a signal that has a beat frequency proportional to the length of the phase shift applied. The MZI pre-filter is designed to have a free spectral range (FSR) equal to the channel spacing of the AWG, giving a wavelength-versus-channel-number response that looks like a step function. Ultimately, this means that using an MZI-AWG allows for less strict tolerances, and therefore lower costs, on other components within the network, such as lasers and cooling systems. At the same time, it provides high channel uniformity and reliability over the full designed temperature range.