[Paper Review] Phase retrieval of programmable photonic integrated circuits based on an on-chip fractional-delay reference path

Xingyuan Xu, Guanghui Ren+6 more|arXiv (Cornell University)|Jul 29, 2022

Photonic and Optical Devices18 citations

TL;DR

This paper presents a novel phase retrieval method for programmable photonic integrated circuits (PICs) using an on-chip fractional-delay reference path. By leveraging only insertion loss measurements and a standard complex Fourier transform, the method uniquely and precisely identifies the impulse response of a 4-tap finite-impulse-response PIC, enabling accurate calibration without additional photodiodes or output ports.

ABSTRACT

Programmable photonic integrated circuits (PICs), offering diverse signal processing functions within a single chip, are promising solutions for applications ranging from optical communications to artificial intelligence. While the scale and complexity of programmable PICs is increasing, the characterization, and thus calibration, of them becomes increasingly challenging. Here we demonstrate a phase retrieval method for programmable PICs using an on-chip fractional-delay reference path. The impulse response of the chip can be uniquely and precisely identified from only the insertion loss using a standard complex Fourier transform. We demonstrate our approach experimentally with a 4-tap finite-impulse-response chip. The results match well with expectations and verifies our approach as effective for individually determining the taps' weights without the need for additional ports and photodiodes.

Motivation & Objective

• Address the growing challenge of characterizing and calibrating increasingly complex programmable photonic integrated circuits (PICs).
• Overcome the limitations of conventional phase retrieval methods that require multiple detection ports and complex measurement setups.
• Enable accurate, individual tap weight calibration in programmable PICs without adding extra hardware such as photodiodes or output ports.
• Develop a scalable and on-chip solution compatible with existing PIC fabrication processes for real-time or post-fabrication calibration.

Proposed method

• Integrate a fractional-delay reference path directly onto the photonic integrated circuit to create a coherent reference for phase retrieval.
• Measure the insertion loss of the PIC across a range of wavelengths or frequencies to obtain amplitude-only data.
• Apply a standard complex Fourier transform to the measured insertion loss to retrieve the full complex impulse response.
• Leverage the known phase relationship from the on-chip fractional-delay path to disambiguate the phase ambiguity inherent in intensity-only measurements.
• Use the unique phase reference to reconstruct the complex transfer function of the PIC with high precision.
• Validate the method on a 4-tap finite-impulse-response (FIR) PIC design to demonstrate accurate tap weight determination.

Experimental results

Research questions

• RQ1 Can a single on-chip fractional-delay reference path enable unambiguous phase retrieval in programmable PICs using only amplitude (insertion loss) measurements?
• RQ2 To what extent can the complex impulse response of a programmable PIC be reconstructed from intensity-only data when combined with a known phase reference?
• RQ3 Does the proposed method eliminate the need for additional photodiodes or output ports in the calibration of programmable PICs?
• RQ4 How accurately can individual tap weights in a finite-impulse-response PIC be retrieved using this approach?
• RQ5 Is the method scalable and compatible with existing photonic integrated circuit fabrication processes?

Key findings

• The method successfully retrieves the full complex impulse response of a 4-tap programmable PIC using only insertion loss measurements and a single on-chip reference path.
• The reconstructed impulse response matches theoretical expectations with high fidelity, confirming the accuracy of the phase retrieval process.
• The approach enables precise determination of individual tap weights without requiring additional photodiodes or output ports.
• The use of a standard complex Fourier transform on amplitude-only data, combined with the on-chip fractional-delay reference, resolves phase ambiguity effectively.
• Experimental results demonstrate that the method is robust and suitable for calibration of complex programmable PICs in practical settings.
• The technique is scalable and compatible with existing photonic integrated circuit platforms, enabling future on-chip calibration systems.