Reconfigurable Optical Delay Line Breaking the Bandwidth-Delay Limit

Introduction

Optical true-time delay lines (OTTDLs) are crucial components in various applications, including quantum photonics, optical communications, computing, and sensing. OTTDLs are used to delay photons for purposes such as implementing quantum gates and circuits, synchronizing data, buffering data, and realizing microwave photonics systems and optical beam-forming architectures. However, both resonant and non-resonant delay lines have limitations in the bandwidth-delay product [1-3]. This tutorial discusses a novel reconfigurable multistage Mach-Zehnder based integrated optical delay line that breaks the bandwidth-delay limit, enabling higher delays without narrowing the bandwidth.

Proposed Delay Line Architecture

The proposed device, shown in Figure 1, implements two cascaded Mach-Zehnder interferometers with four tunable couplers whose coupling ratios can be varied from 0 to 1. The two stages are connected in series, with the output of the second coupler of the first stage connected to the input of the first coupler of the second stage.

Figure 1: Schematic of the proposed delay line

Analysis of the Proposed Delay Line

The proposed delay line has two states of operation. In the first state, the delay line is used as a single-stage delay line, requiring only two couplers to tune, either in stage A or B. Figure 2 shows the normalized transmission and group delay spectra for different values of coupling coefficient K and different modes of coupler operation.

Figure 2: (a) Normalized transmission and (b) group delay spectrum for different values of coupling coefficient

To tune the normalized group delay from 0 to 2, the coupling ratio of stage A is set to 0, and the normalized group delay of stage B is tuned from 0 to 1. Then, the coupling ratio of stage A is set to 1, and the normalized group delay of stage B is tuned from 0 to 1. This enables normalized group delay tuning from 0 to 2 while maintaining the same spectral characteristics as a single-stage delay line.

Figure 3 shows the normalized linear group delay tuning from 0 to 2 around the operating frequency with the discussed operating condition. The bandwidth-delay product for 3 dB transmission bandwidth and 5% group delay bandwidth is also reported.

Figure 3: Right axis: Normalized group delay when stages A and B are tuned. Left axis: Bandwidth-delay product for different coupling coefficients, black line: transmission bandwidth, blue line: group delay bandwidth

In the second mode of operation, the phases of all four couplers are changed simultaneously, enabling operation of the delay line using a single controller. This mode provides a 27% higher transmission bandwidth than a single-stage delay line but at the cost of higher group delay dispersion [4].

Advantages and Applications

The proposed architecture overcomes the shortcomings of previous works [4, 5] and combines the advantages of both structures. It breaks the bandwidth-delay product, allowing higher delays without narrowing the bandwidth. The device has two operation states: one for high transmission bandwidth and the other for high group delay bandwidth with a flat group delay spectrum.

This makes the proposed delay line ideal for wideband applications such as microwave photonics beam steering, where multiple delay lines can be connected to form a beamforming circuit that electronically controls the radio beam direction.

Conclusion

The proposed continuously reconfigurable tunable true-time optical delay line based on Mach-Zehnder interferometers connected in series offers a unique solution that breaks the bandwidth-delay product limit. By enabling higher delays without sacrificing bandwidth, this delay line opens up new possibilities for various applications, particularly in the field of microwave photonics and beam steering.

References

[2] Waqas, T. Kaim Khani, and B. Corbett, "Reconfigurable Optical Delay Line Breaking the Bandwidth-Delay Limit," Tyndall National Institute, University College Cork, Cork, Ireland; Department of Telecommunication, Mehran University of Engineering and Technology, Jamshoro, Pakistan, 2024, pp. 1-6, doi: 979-8-3503-9404-7/24/$31.00 ©2024 IEEE.