# Wideband Mixed Signal Separation Based on Photonic Signal Processing
## 3 Stealth Communication
### Related Work  
Experiments have been demonstrated to transmit 500 Mbps-5 Gbps binary signals in optical noise with bandwidths up to 5000 GHz [3, 4, 12, 71, 72, 73, 74]. The noise and signals are mixed with nonlinear functions, and the signal is recovered with physical level methods before ADC. The experimental setup is shown in Figure 5. The signal carrier is amplified spontaneous emission (ASE) noise from optical amplifiers and covers the spectrum range of 192 THz to 197 THz (Figure 6a), or in terms of wavelength, 1520 nm-1560 nm. The bandwidth of the noise is 197 THz \(-\) 192 THz \(=\) 5 THz, which means the phase of the noise changes 5 trillion times per second. The bandwidth of the fastest receiver that is currently available is 100 GHz, which is only 1/500 of the bandwidth of the noise. To recover the signal and cancel the noise by authorized users, a matching condition must be satisfied between the transmitter and receiver. The cancellation can only be performed in an analog way because none of the currently available receiving devices can perform ADC at the sampling frequency of 5 THz. The basic structures of the stealth transmitter and receiver are two fiber interferometers. The solid lines in Figure 5 are optical fibers. The optical delays in both interferometers (D1 and D2 in Figure 5) must be precisely matched to cancel the noise. The stealth signal \(s(t)\) is a binary sequence with a bit rate of 500 Mbps, and noise \(n(t)\) is a random analog signal with bandwidth 5 THz. Both stealth signal and noise are phase information. \(\omega_{c}=2\pi f_{c}\) is the carrier frequency, where \(f_{c}=194.5\) THz. At the transmitter, the noise is split into two paths: one path (Path 1 at the transmitter side of optical delay is added by introducing an extra length \(D1=c\times t_{0}\), where \(c\) is the speed of light. The stealth signal is modulated onto Path 1 after the delay. The other path (Path 2) only includes noise terms. The two paths are combined at the transmitter for long-distance transmission through a fiber link. Fiber length ranges from 25 km to 240 km has been demonstrated [72, 73, 75]. At the receiver, the signals are split again for two paths and another delay is introduced to one of them. The splitter cannot differentiate the signals that went through Paths 1 and 2 at the transmitter, so there are two possible combinations at the receiver. In the first case, Path 1 goes through the second delay D2. In this case, the noise in both Path 3 (noise with signal) and Path 4 (only noise) has the same delay \(t_{0}\), and can be canceled at the receiver. In the second possible combination, where Path 1 goes through the second delay D2 (Path 4). In this case, the noise in Path 1 goes through doubled delay \(2t_{0}\), while the noise in Path 2 goes through zero delay. The noise cannot be canceled and affect the signal-to-noise ratio of the system. The private key in the stealth system is optical delays in both interferometers. Two copies of the noise (interference signal) are sent into the stealth system, one copy of the noise is combined with the signals of interest (Path 1 in Figure 5), the other pure noise goes through Path 2. They are combined by an optical combiner and sent through an optical fiber link. At the receiver end, the mixed signal is split by an optical splitter and sent through Paths 3 and 4. The stealth system can only recover the signal of interest at the receiver end once the optical delays in Path 1 + 3 matches Path 2 + 4. In this case, the noise set through the system cancels with the inverse of the noise, and the signal of interest isrecovered. For any eavesdroppers, if they do not have the correct optical delays (private key), the noise and the inverse of the noise do not match and cannot be cancelled, so they will only receive the mixed noise and cannot recover the signal of interest hidden in it. The stealth system can be designed by changing the optical delays in Path 1, 2, 3, 4 using optical tuneable delays or adding extra optical fibers.