Dynamic Dispersion Accumulation in Fiber Loops for Realizing Record-High Frequency Resolution or Ultra-Low Signal Sampling Rate in Dispersion-Based Photonics-Assisted Wideband Microwave Measurement Systems

Dispersion-based photonics-assisted microwave measurement systems provide immense potential for real-time analysis of wideband and dynamic signals. However, they face two critical challenges: a difficulty in achieving high frequency resolution over a wideband analysis bandwidth, and a reliance on large-bandwidth-and-high-sampling-rate oscilloscopes to capture the resulting ultra-narrow pulses. We introduce a dynamic dispersion accumulation technique to overcome these limitations. By circulating the optical signal in fiber loops containing a dispersion-compensating fiber, we achieve a high accumulated dispersion of -215700 ps/nm. This high dispersion relaxes the required chirp rate of the chirped optical signal, enabling two distinct advantages: When the analysis bandwidth is fixed, a lower chirp rate enables a longer temporal period, yielding a record-high frequency resolution of 27.9 MHz; When the temporal period is fixed, a lower chirp rate enables a smaller bandwidth, generating a wider pulse and thus relaxing pulse sampling requirements at the expense of analysis bandwidth. This sacrifice in analysis bandwidth can be compensated by a duty-cycle-enabling technique, which holds the potential to extend the analysis bandwidth beyond 100 GHz. This work breaks the performance and hardware limitations in dispersion-based systems, paving the way for high frequency resolution, wideband microwave measurement systems that are both real-time and cost-effective.

💡 Research Summary

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This paper addresses two fundamental limitations of dispersion‑based photonic microwave measurement systems: (1) the difficulty of achieving high frequency resolution across a wide analysis bandwidth, and (2) the reliance on ultra‑high‑bandwidth, high‑sampling‑rate oscilloscopes to capture the ultra‑narrow pulses produced by frequency‑to‑time mapping (FTTM). The authors propose a “dynamic dispersion accumulation” (DDA) technique that circulates a chirped optical signal repeatedly through a fiber loop containing dispersion‑compensating fiber (DCF). Each round adds the DCF’s dispersion (‑1685.7 ps/nm) to the signal, so after N circulations the total accumulated dispersion is D_total = N × (‑1685.7 ps/nm). In the experiment, 128 circulations yield a record‑high accumulated dispersion of approximately –215 700 ps/nm.

According to the pulse‑compression condition k = c · λ² / (2 · D_total) (k is the chirp rate), a larger D_total permits a much lower chirp rate for a given optical wavelength. The authors exploit this relationship in two complementary operating modes.

1. Fixed bandwidth, extended temporal period – By keeping the analysis bandwidth constant (e.g., 37 GHz) and using a long chirped optical pulse (64 ns), the reduced chirp rate (0.58 GHz/ns) yields a compressed pulse pair with a temporal separation that translates into a frequency resolution of 27.9 MHz. This is a record‑high resolution for a dispersion‑based system, surpassing the previous best of ~30 MHz obtained with a 450 000 ps/nm equivalent dispersion but with a much narrower bandwidth (0.53 GHz).

2. Fixed temporal period, reduced bandwidth – By fixing the temporal period (8 ns) and allowing a smaller bandwidth (4.63 GHz), the low chirp rate expands the compressed pulse width to 179.8 ps. This pulse can be captured with a modest 4.63 GHz oscilloscope, eliminating the need for expensive >100 GHz sampling equipment. Although the analysis bandwidth is reduced, the authors demonstrate that a duty‑cycle‑enabling technique can recover the lost bandwidth, potentially extending the usable range beyond 100 GHz.

The system architecture consists of a continuous‑wave laser, a Mach‑Zehnder modulator (MZM) for carrier‑suppressed double‑sideband (CS‑DSB) modulation of the signal‑under‑test (SUT), two optical switches (OS1, OS2) that gate the chirped pulse into the loop, a 50:50 coupler for injection/extraction, an erbium‑doped fiber amplifier (EDFA) to compensate loop loss, and the DCF inside the loop. Precise timing of OS1/OS2 controls the number of circulations, thus deterministically setting the accumulated dispersion.

Experimental results confirm the theory: after 128 circulations the accumulated dispersion reaches –215 770 ps/nm, enabling a required chirp rate of 0.58 GHz/ns for pulse compression. With an 8‑ns chirped signal (4.63 GHz bandwidth) a 179.8 ps compressed pulse is obtained, readily sampled by a 4.63 GHz oscilloscope. With a 64‑ns chirped signal (duty cycle ½) the system achieves a 27.9 MHz frequency resolution over a 37.03 GHz analysis bandwidth. The duty‑cycle technique further suggests that the effective bandwidth can be extended to >100 GHz while preserving the high resolution.

Key contributions of the work are: (i) introduction of a scalable, linear‑relationship‑based dispersion accumulation method using fiber loops; (ii) demonstration that high accumulated dispersion relaxes the chirp‑rate requirement, enabling two distinct performance trade‑offs (high resolution vs. low‑rate sampling); (iii) incorporation of a duty‑cycle‑enabling scheme to mitigate bandwidth loss; and (iv) experimental validation that surpasses prior state‑of‑the‑art dispersion‑based systems in both resolution and hardware simplicity.

The paper also discusses limitations and future directions. The free spectral range (FSR) of the loop is fixed by the loop length, which constrains the maximum unambiguous analysis bandwidth; tunable FSR or multi‑loop architectures could address this. Insertion loss and noise from the optical switches and EDFA affect signal‑to‑noise ratio and may limit dynamic range. Scaling to multi‑channel or parallel loop configurations could enable simultaneous wideband, multi‑tone analysis for complex radar or communication signals. Robustness to environmental variations (temperature, vibration) and automated calibration are identified as necessary steps toward practical deployment.

In summary, dynamic dispersion accumulation provides a powerful new lever for photonic microwave measurement systems, breaking the traditional trade‑off between frequency resolution, analysis bandwidth, and hardware cost. By leveraging high accumulated dispersion to lower the chirp rate, the technique achieves record‑high resolution (27.9 MHz) with modest bandwidth (4.63 GHz) or ultra‑low sampling requirements, while still offering pathways to >100 GHz analysis. This work opens a practical route toward real‑time, cost‑effective, ultra‑wideband microwave diagnostics for emerging applications such as cognitive radio, autonomous‑vehicle radar, and electronic warfare.