Fig. 1: Concept of a photonic-electronic arbitrary waveform generator (PE-AWG) relying on quadrature multiplexing and an active optical phase stabilization in a phase-locked loop (PLL).

a Visionary illustration of a chip-scale hybrid integrated PE-AWG comprising photonic integrated circuits (PICs) and electronic integrated circuits (EICs) intended to give an idea of what a miniaturized PE-AWG system could look like. Two phase-locked optical tones at frequencies f₁ and f₂ are derived from a continuous-wave (CW) laser. After coupling to the main PIC, the light emitted by the CW laser is split into two portions. One portion is frequency-shifted (FS) by B = f₂ − f₁ and acts as a carrier for an optical IQ waveform; the other one serves as a local oscillator (LO) tone for heterodyne down-conversion in a high-bandwidth balanced photodetector (BPD1). The drive signals for the IQ modulator (IQM) are provided by two time-synchronized DAC modules with bandwidth B each. A feedback loop (turquoise) in a phase-stabilized coherent combiner (PSCC) compensates the unwanted phase deviation Δϕ between the optical IQ waveform and the LO tone. The insets show spectra at positions ① to ③ in Subfigure (a) and illustrate the basic principle of quadrature multiplexing in the frequency domain, where the tilde denotes the Fourier transforms of the respective time-domain quantities. In these spectral plots, dashed lines refer to spectral components at negative frequencies that are simply the complex conjugate of their positive-frequency counterparts. ① Real-valued electric drive signal as fed to the I and Q ports of the IQM. ② Optical IQ signal covering a bandwidth of 2B along with the LO tone located at the edge of the signal spectrum. ③ Real-valued waveform with bandwidth 2B obtained at the output of BPD1 after coherent down-conversion. b Histogram of a PAM2 signal at the PE-AWG output for various phase offsets Δϕ. A non-zero phase offset introduces severe distortions to the generated electrical waveform, and the histogram deteriorates to the point of being unrecognizable as a PAM2 signal. c Digital synthesis of the drive signals \(\Re \{\underline{u}(t)\}\) and \(\Im \{\underline{u}(t)\}\) for the IQM. A discrete-time version of the target waveform s_n covering a bandwidth 2B is generated, Inset Ⓐ. Next, its Hilbert transform, Inset Ⓑ, is added as an imaginary part, leading to an elimination of the negative-frequency components, Inset Ⓒ. The analytic signal \({\underline{A}}_{s,n}\) is then frequency-shifted to be centered around zero frequency, Inset Ⓓ, and the real and imaginary parts of the resulting digital signal \({\underline{u}}_{n}\) are fed to the two DAC modules with bandwidth B, Inset Ⓔ. d Vision of an ultra-broadband PE-AWG that combines the idea of quadrature multiplexing with the generation of broadband optical waveforms by feedback-stabilized stitching of spectrally sliced tributary signals as illustrated in the Insets Ⓐ to Ⓒ³⁰. An illustrative example with N = 4 tributaries is sketched. Each of the phase-stabilized coherent combiners (PSCC) consists of an optical hybrid (OH) and a PLL for phase stabilization. For the PSCCs with a single output only, the output labeled ‘I⁻’ in (a) is unused.