The non-repetitive transient weak signal exists in diverse fields, such as astronomy, spectroscopy, and biology, and it is difficult to be perceived and analyzed due to its short duration, large instantaneous bandwidth, unexpected arrival time, and low power. Over the past years, photonic technology has witnessed remarkable progress in the wideband and weak signal loss in the noise. For example, the signal stands out from the noise by using the Talbot effect to redistribute the energy of repetitive waveforms into fewer replicas for noiseless intensity amplification and avoiding digital post-processing. However, it is only valid for repetitive waveforms. A spectral cloning receiver based on coherent dual optical frequency combs (OFCs) is proposed to detect a random non-repetitive signal hidden in the noise. In this proposal, the spectrum of the received signal is sliced into a series of sub-bands (or channels) due to the difference in the free spectrum ranges (FSRs) of two combs. Then, coherent in-phase summation of these sub-bands can lead to a signal-to-noise ratio (SNR) increase (10lg S) linearly proportional to the cloning count or the available sub-bands, which is associated with the effective comb line of the coherent dual OFCs (S). In order to achieve a large SNR improvement, considerable comb lines of the OFCs must be required. However, generating massive comb lines not only needs high input radio frequencies (RFs) and laser powers but also needs superior optoelectronic hardware, which makes the system complex and costly. In addition, the comb channel number is also constrained by the available wavelength range. Therefore, a novel method using a single comb-based scheme for deep denoising in covert wireless communication is proposed. However, it also faces the problem that the system is difficult to further increase the number of comb lines. At this time, a method is urgently needed to effectively solve the problems of the above methods.

To this end, a novel photonic approach to enhance the sensing sensitivity of non-repetitive transient weak signals in a noisy background is proposed. Based on a coherent dual OFCs-assisted spectral cloning receiver, thetransient signal is firstly multicast by N-line signal OFCs and then split equally into M copies, while the local OFC is replicated to multiple replicas by multiple acousto-optic frequency shifters (AOFSs). Therefore, the spectrum of the received transient signal is decomposed into N×M sub-bands. Subsequently, different spectral slices are extracted and down-converted to the same intermediate frequency by an I/Q demodulator. Finally, all the sub-band signals are processed in digital signal processing (DSP) and positively superimposed in the frequency domain. Since the phase characteristics of the spectrum in each sub-channel are in-phase, but the noise frequency spectrum is random, the collinear superposition of the signal spectral vector will be realized, while the noise is randomly superimposed, which leads to a multiplied SNR enhancement of N × M for the transient signal sensing.

A spectral cloning receiving scheme based on the dual coherent OFCs with nine comb lines is used to perceive a transient signal. The performance of the system is characterized by an 18-GHz wide pulse at various noise levels. As the scatter diagram shows, the separation degree of signal cloud and noise cloud is greater than that of the single channel after 18 channels are superimposed in phase with one AOFS used (N=9 and M=2). Compared with that of a single channel, the detection sensitivity increases by 3 dB after spectral cloning, which is consistent with the theoretical value (Fig. 5). The local OFC and signal OFC are replicated 1, 2, and 3 times, and the detection gain is improved by 3.0 dB, 4.5 dB, and 5.9 dB, reaching to 12.63 dB, 14.04 dB, and 15.43 dB, respectively, which accord well with the theoretical value (Fig. 7). In this proposal, the complexity of the OFC generation module and the spectral coverage do not increase basically, but a multiplied SNR rise is achieved. In addition, the splitting loss induced by the optical couplers (OCs) will cause a faint power at the detectors so that the system cannot detect the optical signals. Although Erbium-doped fiber amplifiers (EDFAs) can compensate for the loss, the spontaneous emission noise introduced by EDFAs will greatly influence the detection gain of the system. Therefore, it is necessary to select the appropriate splitting number according to the actual link characteristics, such as EDFA's noise and detector sensitivity.

A simplified and low-cost spectral cloning receiver using multiple AOFSs for large SNR enhancement is proposed and demonstrated. Based on the dual OFC channelization scheme, multiple AOFSs and OCs are used to replicate the local OFC and modulated signal so that the spectrum of the received transient signal is divided into multiple sub-bands. After the collinear superposition of the sub-band signal spectrum and the random superposition of the noise spectrum, the detection sensitivity of the transient weak signal is greatly improved. The dual-OFCs spectral cloning receiver with nine lines is used to detect the sub-noise transient signal with a bandwidth of 18 GHz. The detection sensitivity gain is increased to 12.63 dB, 14.04 dB, and 15.43 dB under varying noise power conditions by replicating the local OFC 1, 2, and 3 times. Compared with the previous scheme, the proposed scheme requires fewer optical frequency comb lines and a smaller available wavelength range to achieve the same SNR improvement under the available sensitivity of the receiving unit.