2412.16110v1.pdf

Performance limits of spectro-temporal unitary

transformations for coherent modulation

Callum Deakin, Xi Chen

Nokia Bell Labs, 600 Mountain Ave, Murray Hill, NJ, USA

Abstract: We analyse the performance limits of coherent modulation based on lossless

unitary transformations, demonstrating that they can achieve high (>30 dB) SINAD and

outperform conventional IQ modulators at equivalent transmitter laser powers. © 2024

The Author(s)

prohibitive for future transceivers with THz level bandwidths, such as highly parallel or frequency-comb-based

transceivers, where the available power on the photonic integrated circuit is limited.

An alternative modulation scheme based on multiple stages of alternating phase modulators and dispersive ele-

ments (Fig. 1(b)) has been proposed and demonstrated [1, 2], inspired by its spatial analog: the multi-planar light

converter (MPLC) [3]. Conceptually, the scheme achieves amplitude modulation by redistributing light temporally

rather than switching. This scheme is known as a spectro-temporal unitary transformation or all-pass modulation,

as the relation between the output target optical waveform (e.g. a Nyquist-pulse-shaped N-QAM waveform) and

the input CW light can be viewed as a unitary transformation on the spectro-temporal modes. Since it uses only

phase modulators and dispersive elements, it is theoretically lossless. It has been recently shown that this modu-

lation scheme can be operated in real time [4] and can modulate the different wavelengths of a frequency comb

distortion ratio (SINAD) is achievable if the bandwidth of the phase modulator is sufficient. We also reveal that

the digital-to-analog converter quantization noise can cause an nonlinear increase in the overall system noise and

distortion. Finally, by calculating the shot noise limited transmitter SINAD of the unitary transformation scheme,

we show that the phase modulation scheme can substantially outperform the lossy conventional approach at the

same transmitter laser power.

Fig. 1. (a) Conventional IQ modulation based on amplitude modulation, a(t). (b) Lossless spectro-temporal unitary transform

based arbitrary waveform modulation based on cascaded phase modulators φ(t) and dispersive elements H(ω).

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