A novel scheme to realize a low-cost and wideband frequency tunable optoelectronic oscillator based on a directly modulated distributed feedback (DFB) semiconductor laser is proposed and experimentally demonstrated. In the proposed scheme, neither an external modulator nor an electrical filter is used, and no more than 25 dB of the electrical loop gain is required due to the high modulation efficiency of the relaxation oscillation frequency of the DFB laser. Microwave signals with frequency coarsely tuned from 3.77 to 8.75 GHz are generated by changing the bias current and operation temperature of the DFB laser. The single sideband phase noise of the generated 6.97 GHz microwave signal is measured to be $-103.6\mathrm{dBc}/\mathrm{Hz}$ at 10 kHz offset.

In this Letter, the optical stealth transmission carried by super-continuum spectrum optical pulses generated in highly nonlinear fiber is proposed and experimentally demonstrated. In the proposed transmission scheme, super-continuum signals are reshaped in the spectral domain through a wavelength-selective switch and are temporally spread by a chromatic dispersion device to achieve the same noise-like characteristic as the noise in optical networks, so that in both the time domain and the spectral domain, the stealth signals are hidden in public channel. Our experimental results show that compared with existing schemes where stealth channels are carried by amplified spontaneous emission noise, super-continuum signal can increase the transmission performance and robustness.

Applying the quadrature amplitude modulation (QAM) format, quantum-noise randomized cipher (QNRC) systems hide the signal states in quantum phase noise and amplitude noise to prevent eavesdropping. In this paper, based on the traditional wire-tap channel model analysis method, the physical-layer security of QAM-QNRC system is investigated quantitatively under the metric of secrecy rate. The general expressions of secrecy rates of the data and key are derived separately. Furthermore, the maximum reachable secrecy rate of a QAM-QNRC system is put forward, under which the data and key are both safe in the view of mutual information evaluation. Finally, the variation trend of secrecy rate with various system parameters is discussed in detail. The simulation results show that we can obtain a higher secrecy rate by setting reasonable parameters, such as the level of ciphertext, mesoscopic signal power, and inner gain at the transmitter. Meanwhile, the security of the key is the main constraint of the maximum reachable secrecy rate.

Complementary encoder of stealth signal is proposed and demonstrated for coding, modulating and enhancing the privacy of optical stealth transmission. With complementary encoding, the stealth signal carried by amplified spontaneous emission (ASE) light keeps the same characteristic to ASE noise and can be concealed well under public channel. The experiment results demonstrate the feasibility of the scheme and show the stealth signal has the same impact on public channel in transmission performance, compared to the ASE noise.

An integrated single sideband (SSB) modulation method benefitting from the sideband amplification injection locking (SAIL) effect for radio-over-fiber (ROF) link is proposed and experimentally demonstrated, which is based on an integrated multi-section mutual injection (IMSMI) DFB laser. The IMSMI-DFB laser, which overcomes the drawbacks of using discrete devices, simplifies the whole structure. The SAIL effect, which is regulated by the input RF power and bias current of IMSMI-DFB laser, promises a good pathway for adjusting and controlling the modulation frequency, optical carrier to sideband ratio (OCSR) and sideband suppression ratio (SSR), meanwhile, provides high gain in the link. The characteristics of SAIL effect are investigated. The OCSR can be optimized to 0 dB, the SSR is over 26 dB and the transmission link gain improves over 20 dB from 16 GHz to 26 GHz. The ROF transmission experiments demonstrate that the power fading introduced by fiber dispersion is dramatically eliminated, and the error vector magnitudes (EVMs) of the received 20 MSymbol/s 32 QAM signals at frequencies from17 GHz to 22.3 GHz after 50 km fiber transmission are below 4.8%. This work provides an effective way to realizing integrated direct SSB modulation with outstanding features for ROF link.

We propose and experimentally demonstrate an optical stealth transmission system over a 200 GHz-grid wavelength-division multiplexing (WDM) network. The stealth signal is processed by spectral broadening, temporal spreading, and power equalizing. The public signal is suppressed by multiband notch filtering at the stealth channel receiver. The interaction between the public and stealth channels is investigated in terms of public-signal-to-stealth-signal ratio, data rate, notch-filter bandwidth, and public channel number. The stealth signal can transmit over 80 km single-mode fiber with no error. Our experimental results verify the feasibility of optical steganography used over the existing WDM-based optical network.