Temporal integration is a basic function that has a wide range of applications in signal processing systems. In contrast to electrical integrators that are subject to the electronic bandwidth bottleneck, photonic techniques offer distinctive advantages such as the broad bandwidth, strong immunity to electromagnetic interference, and low loss [1-3], thus holding great promise to address the limitations of their electrical counterparts.
Extensive effort has been made to achieve photonic integrators (see Table 1 for comparison of existing photonic integrators), such as those based on gratings [4-6], and micro-ring resonators (MRRs) [7-9]. These approaches achieve optical signal integration with a time resolution as fast as 8 ps [7] and a large time-bandwidth product with high-Q resonant structures.
However, these approaches still face limitations. Many are not reconfigurable in terms of the temporal resolution and the length of the integration window, preventing processing of RF signals with different bandwidths and varying integration time windows. In addition, many approaches process optical signals, instead of the RF signals directly, in which case they require electro-optical interfaces that limit their performance.
Approaches to photonic integrators based on transversal structures offer high reconfigurability and accuracy owing to the parallel scheme where each path can be controlled independently [10-12]. By tailoring the progressive delay step, RF signal integration with a reconfigurable operation bandwidth can be achieved [10-12]. Yet these integrators are still limited by the number of channels. To increase the number of wavelength channels, discrete laser arrays or electro-optical comb sources can be employed, although these approaches have a trade-off between the number of wavelengths and system complexity, ultimately leading to a limited number of channels and time-bandwidth product.
Recently, [13-19] a novel multi-wavelength source—integrated microcombs— has attracted great interest in RF photonic systems [20, 21]. Microcombs arise from optical parametric oscillation in ultra-high-Q monolithic MRRs and offer many advantages over traditional multi-wavelength sources, including a much higher number of wavelengths and a greatly reduced footprint and complexity for the system. A wide range of RF applications have been demonstrated based on microcombs, such as RF true time delays [22, 23], transversal signal processors [24-29], frequency conversion [30], phase-encoded signal generators [31], and channelizers [32, 33] and Hilbert transforms [34].
In this paper, we demonstrate a highly reconfigurable photonic RF integrator using an integrated soliton crystal micro-comb source [35, 36] with a low comb spacing of 49GHz. The input RF signal is multicast onto the flattened microcomb lines and progressively delayed via dispersion, and then summed upon detection to achieve temporal integration. The large number of wavelengths - up to 81 - offered by the microcomb enable a large integration time window of ~6.8 ns with a time resolution as fast as ~84 ps. A comb shaping system is developed to compensate for the non-flat spectral output of the soliton crystal microcomb. We successfully test the system on a range of different input signals. The experimental results match well with theory, verifying the performance and feasibility of our approach to achieving photonic RF integration with a large time window and potentially lower cost and footprint.
Table 1. Parameters of existing photonic RF integrators
Method
|
Reconfigurability
|
Time window
|
Time resolution
|
Bandwidth
|
Time-bandwidth product
|
Fiber Bragg grating [4]
|
×
|
N.A.
|
6ps
|
N.A.
|
N.A.
|
Active fiber Bragg grating [5]
|
×
|
~5 ns
|
50 ps
|
20 GHz
|
100
|
apodized uniform period fiber Bragg grating [6]
|
×
|
60 ps
|
2.5 ps
|
400 GHz
|
24
|
Passive high-Q MRR [7]
|
×
|
800 ps
|
8 ps
|
200 GHz
|
100
|
InP-InGaAsP MRR [8]
|
×
|
6331 ps
|
< 54 ps
|
>15 GHz
|
>117
|
Passive MRR [9]
|
×
|
12.5 ps
|
1.9 ps
|
400 GHz
|
6.6
|
Incoherent light source [10]
|
√
|
200 ps
|
7 ps
|
>18 GHz
|
28.5
|
Multi-wavelength fiber laser [11]
|
√
|
1.24 ns
|
~27 ps
|
36.8 GHz
|
~45.9
|
Incoherent light source and fiber Bragg grating [12]
|
√
|
7 ns
|
344 ps
|
2.9 GHz
|
20.3
|
Microcomb (this work)
|
√
|
6.8 ns
|
84 ps
|
11.9 GHz
|
81
|