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Abstract
We proposed and experimentally demonstrated a free-space optical stealth communication system that hides the stealth signal in wide-band spontaneous emission noise. Spontaneous emission light sources have been widely used for illuminations and has been recently deployed for short distance and indoor free-space optical communications, such as LiFi. Since free-space optical communication is a broadcasting network, the users’ privacy is exposed to eavesdropping attacks. In this paper, stealth communication is achieved by taking advantage of the existing properties of spontaneous emission light sources, random phase fluctuations, and protects users’ privacy in free-space communication networks. The keys to hide and recover the stealth signal are the optical delays at the transmitter and receiver. Only by matching the delay length with the pre-shared keys can the authorized receiver recover the stealth signal. Without the right key, the eavesdropper receives a constant power that is the same as illumination light sources and cannot detect the existence of the stealth signal.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Free-space optical communication provides data transmission with a large capacity and can be applied for the next generation (5G) wireless networks [1–3]. Since the optical signal is transmitted in free space, it can be easily accessed by both the authorized receiver and eavesdroppers. Digital encryption protects the signal from being decrypted, but the eavesdropper can still notice the existence of the communication channel by detecting the amplitude change in the time domain or the fingerprint in the frequency domain. In many cases, although the eavesdropper cannot read the data being transmitted, the detection of the existence of the communication already exposes users’ privacy to malicious attacks [4,5]. Therefore, stealth transmission of the signal is in urgent need for free-space optical communication to keep the signals undetectable by eavesdroppers and protect users’ privacy.
Wide-band spontaneous emission light source has been widely used in free space communications, not only because such the spontaneous emitter can be used for both illumination and communication [6,7], but also because of cost consideration for the applications in short-distance indoor communications [8]. The wide-band spontaneous emission light source has an intrinsic and unique property for hiding the signals. The large bandwidth is originated from the fast-changing phase noise of the light source, and the changing rate is in the order of a few THz to hundreds of THz, which is orders of magnitudes faster than the speed limit of optical detectors and analog-to-digital conversion circuits. A relatively narrow bandwidth signal is spread into the wide-band phase noise in the spontaneous emission through phase modulation, and the existence of the signal cannot be detected unless the noise is effectively canceled. The stealth communication has been tested in fiber links [9,10], and further protections of the stealth channel based on physical layer methods have been demonstrated, such as increasing the key space with another dimension by dispersion [11,12] and using phase mask to encrypt each stealth bit [13].
In this paper, we deploy the unique property of wide-band spontaneous emission light sources to protect the privacy of users’ information in free-space communication networks. Since spontaneous emission has been widely used in free-space communications, the introduction of the stealth channel does not incur extra power consumption. A free-space communication link is demonstrated with packaged stealth transmitter and receiver. The structures of both stealth transmitter and receiver are Mach-Zehnder interferometers [14]. Since the large bandwidth of the spontaneous emission corresponds to short coherence length, to recover the phase-modulated signal at the receiver and cancel the phase noise, the optical delays need to be precisely matched between the transmitter and receiver, which creates a large key space for hiding the stealth signal [15,16]. The synchronized optical delay can be changed based on the pre-shared keys, so even the eavesdroppers can scan the delay, they cannot follow the changes.
2. Experimental setup and principle
2.1 Experimental setup
The experimental setup of the system is shown in Fig. 1(a). We demonstrate a one-meter free-space transmission link between the transmitter and the receiver by using the wide-band amplified spontaneous emission (ASE) as the light source. The system uses a C-band ASE light source generated by ASE-C-M-N-50-2 by HJ Optronics, which generates wide-band spontaneous emission ranges from 1528nm to 1566nm. The spectrum of the ASE source is shown as the red line in Fig. 5. The structures of the stealth transmitter and the receiver are both fiber interferometers. The wavelength range of the wide-band source is from 1530 nm to 1570 nm and covers the spectrum range of 190 to 195 THz. The bandwidth of the noise Δf is 5 THz.
Fig. 1. (a) Experimental Setup (TD1 and TD2: tunable delays, PM: phase modulator, A1 and A2: Attenuator, PD: photo diode, EDFA: Erbium-doped fiber amplifier, Solid arrow: transmitting signal, Dashed arrow: jamming signal); (b), (c) Cancellation Process.
Fig. 2. Optical delay matching condition, when the delay in Paths 1 + 2 (a) matches with the delay in Paths 3 + 4 (c), the noise can be canceled, and the signal of interest can be recovered (b).
At the transmitter, an optical splitter is used to divide the light source into two paths. Path 1 contains a tunable delay (TD1) and a phase modulator (PM), with a 500 Mbps pseudorandom binary sequence (PRBS) modulated onto the light source. Path 3 contains an optical attenuator (A1). The tunable delay introduces optical delay differences between Path 1 and Path 3, which is the key to hide and recover the stealth signal. The optical attenuator matches the power attenuation from the phase modulator, so the signals in Paths 1 and 3 have the same amplitudes. The combined signal is then amplified by an erbium-doped fiber amplifier (EDFA) and sent into free-space through a fiber collimator.
At the receiver, the free-space signal is coupled into fiber with a lens collimator and is split by another optical splitter into two paths, both with polarization controllers. Path 2 contains an optical attenuator (A2), and Path 4 contains the second tunable delay (TD2). Paths 2 and 4 are combined with an optical combiner, the combined signal is amplified by another EDFA, and then received by a photodiode (PD). The photodiode converts the optical signal into the radio frequency signal and then the converted signal is observed by the Keysight oscilloscope DSOX3102T. The signal of interest tested is generated by Anritsu ME522A Error Rate Measuring Equipment Transmitter.
The transmitter and receiver are sensitive to temperature change and mechanical vibration, which can cause random phase changes to the signals. To minimize the effect of temperature, we set the system in a temperature-controlled environment (22 °C) with active feedbacks. To minimize the effect of vibration, both the transmitter and the receiver are packed and stabilized in vibration isolated platforms.
2.2 Cancellation principle
Figure 1(b) and 1(c) show the cancellation process. Both signal s(t) and noise n(t) are phase information of the optical waves. The central carrier frequency ωc = 2πfc, where fc = 192 THz. Since the phase of noise n(t) changes at the rate of 5 THz, the central frequency fc = 192 THz is expended to a bandwidth of 5THz. At the transmitter, Path 1 (red lines at the transmitter side of Fig. 1(b) and (c)) is delayed by introducing an extra length TD1 = c × t0, where c is the speed of light, and then add the phase-modulated signal s(t), so the waveform at the red path of the transmitter is $\cos [{{\omega_c}({t - {t_0}} )+ s(t )+ n({t - {t_0}} )} ]$; while Path 3 (blue lines at the transmitter side of Fig. 1(b) and (c)) remains the same, which only includes noise with waveform of $\cos [{{\omega_c}t\; + \; n(t )} ]$.The two paths are combined at the transmitter for long-distance transmission through a free space optical link.
At the receiver, the signals are split again for two paths, and delay is introduced to one of them. The splitter cannot differentiate the signals that went through the red and blue paths at the transmitter, so two possible combinations of signals are introduced at the receiver. Figure 1(b) is the first possible combination, where the red signal does not go through the second delay in Path 4 and remains the waveform $\cos [{{\omega_c}({t - {t_0}} )+ s(t )+ n({t - {t_0}} )} ]$; the blue signal goes through the second delay TD2 = c × t0, and has the waveform $\cos [{{\omega_c}({t - {t_0}} )\; + \; n({t - {t_0}} )} ]$. The combiner at the receiver sums the two waveforms together and get:
The signal is a differential phase-shift keying (DPSK) signal with values s(t) equals 0 or π, so $\textrm{cos}[{s(t )/2} ]$ has the values of 1 or 0. The detector functions as an envelope detector, and the signal output at the receiver is
The second possible combination is shown in Figs. 1(c), where the red signal goes through the second delay in Path 4, and has the waveform changed to $\cos [{{\omega_c}({t - 2{t_0}} )+ s({t - {t_0}} )+ n({t - 2{t_0}} )} ]$. The blue signal does not go through the second delay and remains the waveform $\cos [{{\omega_c}t\; + \; n(t )} ]$. The combiner at the receiver sums the two waveforms together and get:
To recover the phase signal hidden in the phase noise, two copies of the noise are sent (Fig. 2). One copy of the noise (Fig. 2(a)) is combined with the signals (Fig. 2(b)) and goes through Path 1 in Fig. 1. The other copy is pure noise (Fig. 2(c)) and goes through Path 3 in Fig. 1. The length difference between Paths 1 and 3 generates the optical delay between the two copies. The optical signal in the free space is always combined with the two copies of noise. Without the pre-shared key that indicates the optical delay used at the transmitter, the eavesdropper (red arrow in Fig. 1) can only see the phase noise. Whereas with the pre-shared key, the authorized receiver can match the delay, so the length of light path 1 + 2 is the same as the light path 3 + 4. Once the phase noise in Fig. 2 is canceled, the signal can be recovered by authorized receivers. In this experiment, at the transmitter, Path 1 is three meters longer than Path 3, and at the receiver, Path 2 is three meters shorter than Path 4. The optical delay difference can be dynamically adjusted based on the pre-shared key. There is no limit on the optical delay length to be applied in the system. Hundreds of kilometers of fiber can be used to generate the optical delay and the attenuation from the optical fiber can be compensated by optical amplifiers. Moreover, since the optical fiber is compact and has a diameter of 125 µm, a fiber spool with 100km optical fiber only consumes a space of 30.4 cm × 30.4 cm × 22.6 cm. Therefore, even the eavesdropper performs a brute force scan, the delay difference can always be changed before the eavesdropper finds the right key.
Fig. 4. (a) Received signal with optical delay lengths matched; (b) Received signal without optical delay lengths matched; (c) Received pure noise without signal.
Fig. 5. Spectral distribution for each transmission link: (Red line: original ASE output; Yellow line: Path 1 after going through tunable delay TD1 and phase modulator PM; Blue line: Path 3 after going through Attenuator A1; Green line: combined signal of Path 1 and Path 3; Black line: combined signal of Path 1 and Path 3 after EDFA amplification)
2.3 Key space analysis
The free-space optical stealth communication system can only recover the signal when the optical delay matching condition is met. The key space N is introduced to analyze the fit of the matching condition, which can be calculated by the ratio of the length of the optical delay D to the accuracy required by the matching condition ΔD in Eq. (4):
The length of the optical delay D can be adjusted either by adjusting the tunable delays or adding optical fibers. While the accuracy of the matching condition ΔD is determined by the bandwidth of the noise Δf. The relation between the accuracy of the matching condition ΔD and the bandwidth of the noise Δf is shown in Eq. (5), in which c stands for the speed of light, and equals 3×108 m/s.The key space can be increased by decreasing ΔD in Eq. (4), which can be achieved by increasing the bandwidth of noise Δf in Eq. (5), so the matching condition must achieve a more accurate value. Or key space can be increased by adding longer optical delay D. Depending on the applications, different optical fiber lengths, which consume different spaces, can be selected for the desired communication system.
3. Experimental results and analysis
3.1 Coherence length measurement
To measure the coherence length of the light source, we use the tunable delay (TD1) to scan one of the two paths. A photodiode was used to receive the optical signal and convert it into the electrical signal. If the length difference between Paths 1 + 2 and Paths 3 + 4 is larger than the coherence length ΔD, no interference will be detected, and the oscilloscope will only read a constant power. When the length difference between Paths 1 + 2 and Paths 3 + 4 is less than the coherence length ΔD, there will be interference between the two paths, and the oscilloscope can detect a power change. By using a constant speed to scan the tunable delay, constructive interference and destructive interference alternatively occur, and the detected optical power will vary as a time-dependent sinusoidal function. The envelope of the sinusoidal function, or in another word, the shape of the coherence peak (Fig. 3(b)), depends on the shape of the light source spectrum (Fig. 3(a)), and they are a Fourier Transform pair. In this experiment, an optical amplifier without gain flattening has been used to generate the noise, the peak around 1560nm with 20nm width contributes most of the power in the entire ASE spectrum The 40 nm range of the spontaneous emission spectrum corresponds to a coherence peak with full width at half-maximum of 1.02 ps in the unit of time or 306 µm in the unit of length (Fig. 3(b)).
3.2 BER measurement
The time-domain measurement at the receiver shows that a clear eye diagram can be received by matching the optical delay (Fig. 4(a)). When the delay is not matched, there is no eye-opening (Fig. 4(b)), and the received signal is the same as pure noise without modulation (Fig. 4(c)). Figure 4(b) and (c) prove that the stealth signal is successfully hidden in the phase noise from the spontaneous emission, and the time domain measurement cannot detect the existence of the modulated signal.
Figure 5 shows the spectral distribution for each transmission link. The system uses C-band ASE as the light source which ranges between 1528 nm and 1566 nm. The red line shows the spectral distribution for the ASE source directly out of the light source module. The yellow line shows the spectral distribution of Path 1 when the signal of interest is converted by the optical phase modulator (PM). The blue line shows the spectral distribution of Path 3 when no signal of interest is transmitted. By comparing the yellow line and the blue line, the spectral distribution is exactly the same, this proves the transmission of the signal of interest cannot be detected in the spectral domain.
By comparing the spectrum of the combined signal of Path 1 and Path 3 (green line in Fig. 5) and the spectrum of the combined signal after EDFA amplified (black line in Fig. 5), the spectral distribution does not have an obvious change within C band apart from power amplification. Also, the spectral distribution for going through optical phase modulator (Path 1 in Fig. 1, yellow line in Fig. 5) and going through just attenuator (Path 3 in Fig. 1, blue line in Fig. 5) are the same before being combined with the optical combiner and amplified by EDFA, the extra ASE noises generated by EDFA affects equally for both Paths and will be canceled once matching condition is satisfied.
Since the received signal has a constant power when optical delay lengths are not matched, the system can be applied for LiFi. Without the key to recover the signal, any receivers or eavesdroppers in the free space can only receive a constant power with noise the same as the light source and cannot detect the existence of the stealth signal (solid arrows in Fig. 1). Even the eavesdropper tries to find the key by scanning the delay length to satisfy the matching condition, it takes seconds to minutes to perform the scan in meters, and the synchronized optical delay can be changed before the eavesdropper finds the matching condition.
Figure 6 shows the received BER after amplified by EDFA at different received power levels. A BER of 1.8×10−7 is achieved at the received power of 10.42 dBm. Figure 6(b) and (c) are the eye diagrams corresponding to points b and c in Fig. 6(a) and indicate BERs of 1.18×10−6 and 2.33×10−3, with received power of 7.63 dBm and −3.28 dBm respectively. Point b is close to, while still under the forward error correction (FEC) limit and can be reduced to be error-free with Reed-Solomon codes [17,18]. Figure 6 shows that the system can be operated in a wide range of power levels. Thus, when the system is applied for LiFi for both communication and illumination, the function of stealth communication can be achieved with different illumination power levels.
Fig. 6. (a) BER measurements versus received signal power after amplified by EDFA; (b) and (c) are the eye diagrams with received power of 7.63 dBm and −3.28 dBm respectively, corresponding to points b and c in (a).
3.3 Jamming attack analysis
The eavesdropping attack can be effectively defended by the phase noise. As a free-space communication system for short-distance indoor communications, another possible attack is the jamming attack. The jamming signal can be either from intentional attacks or unintentional interference caused by light sources with the overlapped spectrums. This section emulates the jamming attack (dashed arrow in Fig. 1(a)) by introducing an extra amount of spontaneous emission and measures the system performance when the jamming exists. The experimental results show that the BER increases exponentially with the power ratio between the jamming signal and the spontaneous emission that carries the signal of interest (Fig. 7(a)). This is because when the power of the jamming signal is comparable to the stealth signal (starting at point d in Fig. 7(a)), the jamming signal saturates the EDFA at the receiver, and most of the pumping power amplifies the jamming signal, instead of the signal of interest. Figures 7(b)–(e) correspond to points b–e in Fig. 7(a) and show the degradation of the eye diagram when the power of the jamming signal increases.
Fig. 7. (a) BER measurement with the ratio of jamming signal and signal of interest; (b) Eye diagram when there is no jamming signal; (c)-(e) Eye diagrams when the power ratios of the jamming signal to the spontaneous emission that carries the signal of interest are 0.15, 0.58, 1.24.
To defend against the jamming attack, a spectrum hopping method can be used. Since the spontaneous emission has a wide spectrum, the stealth signal can be carried by part of the spectrum and changes its band during the attacks. The spectrum of the signal carrier and the envelope of the coherence peak is a Fourier Transform pair. A narrower spectrum corresponds to a larger coherence length, which leads to a smaller key space.
4. Application
One of the applications of free-space optical stealth communication is smart sensor networks, which can be demonstrated in Fig. 8. The data transmits from the server to the node, which includes both the public data and the stealth data, is in the free space optical communication channel. The public data includes the control and synchronization signal to the sensor node, and the stealth data provide a key for the sensor node to encrypt the uploading signal. The sensor nodes collect information including temperature, humidity, variation, tension, and upload the collected information through a wireless channel. This hybrid and asymmetric design take into consideration the power consumption of optical free-space communication. The sensor node is usually powered by the battery and is not feasible to transmit a strong optical signal over a long distance. The server, which collects data from sensor nodes, is powered by cable and can send optical signals with large average power.
Since the physical space for the optical delay in the sensor node is limited, the expected delay length range is 10–100m for optical fibers. This can be compensated by using signal carriers with wider bandwidth (Eq. (4) and Eq. (5)). Since the uploading signal from the sensor node is the information collected by the sensors, time division multiplexing can be used to have multiple sensor nodes share the stealth channel capacity. The carrier signal, which is the wideband analog noise, can cover the entire bandwidth allocated to the sensor network to maximize the key space.
To avoid interference from other unwanted visible light sources and differentiate the communication light beam from the aircraft warning lights, the communication light beam is highly directional, and the wavelength is chosen to be in the near-infrared range. A single optical transmitter at a server station distributes the key signals through the stealth channel to multiple sensor nodes with the time-division multiplexing technique. The highly directional light also protects the stealth channel from eavesdropping attacks, where the eavesdropper can be easily identified by measuring the change of received power.
The problems regarding the free space loss, divergence angle, atmospheric turbulence introduced by the environment variation occur for most free-space optical transmission. FSO communication systems for high-speed transmissions have already been highly studied and proven work [19,20], and weather and environmental effects on FSO communication links have been analyzed [21]. A recent study demonstrated free-space optical transmission between two 5G airship floating base stations worked for a total transmission distance of 12 km under atmospheric channel perturbation using laser tracking and targeting techniques [22].
5. Conclusion and outlook
We proposed and experimentally demonstrated a free-space optical stealth communication system that hides the signal of interest in phase noise of wide-band spontaneous emissions. The wide-band property of the light source requires a strict matching condition of optical delays at the transmitter and receiver, which creates a large key space for the stealth channel. Since spontaneous emission light sources, such as LEDs, are widely used for indoor illuminations, the stealth channel can be introduced by using the existing light source without consuming extra power. A data rate of 500 Mbps in a one-meter free-space communication link is demonstrated. The proposed system can be used with the wireless communication network to accelerate the data rate and enhance the users’ privacy. A BER of 1.8×10−7 is achieved and can be reduced to be error-free with FEC. The system can be operated under a wide range of power levels, so the stealth channel can be introduced with different power levels of the light source for indoor illumination and smart sensor networks.
In terms of future work, white light-emitting diode (LED), which is widely used for illumination, can be used as the light source of the stealth communication system. The phase of the light from a white LED is random noise and covers the frequency range of 375 THz–750 THz with the bandwidth Δf of 375 THz. Compared with the experimental result, which uses analog noise with 5THz bandwidth, and the accuracy of the matching condition is 306 µm, the bandwidth of visible light range is 75 times wider than the bandwidth of 5 THz. Accordingly, the accuracy of the matching condition is 75 times smaller than 306 µm, which is 4 µm. This means if the same optical delay is applied, key space N is enlarged by 75 times. With a spectrum range about 75 times larger than that of the light source used in the current experimental system, the use of white LED will further decrease the coherence length and thus increase the difficulty for eavesdroppers to find the matching condition and detect the existence of the stealth channel.
Funding
New Jersey Health Foundation (PC 77-21); National Science Foundation (2128608).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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