Optical single sideband modulation with tunable optical carrier-to-sideband ratio using DP-BPSK modulator

An optical single sideband modulator without generating both the −2nd- and +2nd-order sidebands is presented. It is based on a parallel operation of two dual-drive Mach–Zehnder modulators (DDMZMs) where one is operated as a conventional single sideband (SSB) modulator and the other is operated as a single-drive Mach–Zehnder modulator (MZM) biased at the maximum transmission point. The 2nd-order sidebands generated by the SSB modulator and the maximum-biased MZM can be cancelled by controlling the sideband amplitude and phase. Canceling the 2nd-order sidebands eliminates the second-order harmonic component presented at the output of a long-reach fiber optic link. The new SSB modulator can be implemented using standard off-the-shelf components and has a wide bandwidth. Experimental results show >20 dB 2nd-order sidebands suppression for different modulation indexes and 22.6 dB reduction in the fiber optic link output second-order harmonic component, compared to using a conventional SSB modulator formed by a DDMZM and a 90° hybrid coupler. Results also demonstrate that the performance of the new SSB modulator is insensitive to the optical carrier frequency and the input RF signal frequency.

Comparing with the conventional double-sideband (DSB) modulation in communication system, single-sideband (SSB) modulation only demands half bandwidth of DSB in transmission. Two common ways are employed to implement SSB modulation by using optical filter (OF) or electrical 90° phase shift, respectively. However, the bandwidth of above methods is limited by characteristics of current OF and electrical phase shift. To overcome this problem, an ultra-wideband microwave photonic link based on SSB modulation is proposed and demonstrated. The radio frequency (RF) signal modulates a single-drive dual-parallel Mach-Zehnder modulator, and the SSB modulation is realized by combining an electrical 90° hybrid coupler and an optical bandpass filter. The experimental results indicate that the system can achieve SSB modulation for RF signal from 2 to 40 GHz. The proposed microwave photonic link provides an ultra-wideband approach based on SSB modulation for radio-over-fiber system.

As a promising solution for short-to-medium transmission systems, direct detection optical orthogonal frequency division multiplexing (DDO-OFDM) or discrete multi-tone (DMT) has been intensively investigated in last decade. Benefitting from the advantages of peak-to-average power (PAPR) reduction and signal-to-noise ratio (SNR) equalization, precoding techniques are widely applied to enhance the performance of DDO-OFDM systems. However, the conventional method of partitioning precoding sets limits the ability of precoding schemes to optimize the SNR variation and the allocation of modulation formats. Thus, the precoding transmission systems are hard to reach the capacity that traditional bit-power loading (BPL) techniques, like the Levin-Campello (LC) algorithm, can achieve. In this paper, we investigate the principle of SNR variation for precoded DDO-OFDM systems and theoretically demonstrate that the SNR equalization effect of precoding techniques is actually determined by the noise equalization process. Based on this fact, we propose an adaptively partitioned precoding (APP) algorithm to unlock the ability to control the SNR of each subcarrier. As demonstrated by the simulation and experimental results, the proposed APP algorithm achieves the transmission capacity as high as the LC algorithm and has nearly 1 dB PAPR reduction. Besides, the look-up table (LUT) operation ensures low complexity of the proposed APP algorithm compared with LC algorithm. To avoid severe chromatic dispersion (CD) induced spectral fading, single sideband (SSB) modulation is also implemented. We find that SSB modulation can reach the capacity of double sideband (DSB) modulation in optical back-to-back (OB2B) configuration by optimizing the modulation index. Therefore, the APP based SSB-DDO-OFDM scheme can sufficiently enhance the performance of cost-sensitive short-to-medium reach optical fiber communication systems.

We demonstrate an approach to achieve a widely tunable photonic-assisted microwave phase shifter (MPS) based on single polarization multiplexing electro-optic modulator (PM-EOM). The PM-EOM can generate orthogonally polarized and phase coherent optical signals. A phase shifted microwave signal is generated by combining the orthogonally polarized signals and generating the signal in a photodetector. A polarization controller (PC) continuously tunes phase of the recovered microwave signals and ensure its magnitude is constant. Through joint adjusting of the wavelength plates inside the PC before a polarizer, phase of the recovered microwave signal in a frequency range of 1–18 GHz can be fully tuned over 360°. Phase deviation is less than 1.5°. The corresponding magnitude almost remains unchanged. Magnitude deviation is less than 2 dB. Phase drift is less than 4° within 120 s. The motivation of this work lies in the fact that the previously reported MPSs using EOMs are hard to be realized with single modulator without special devices or an optical bandpass filter. The photonic-assisted MPS can be expanded into a phase shifter array with a help of a 1 × N optical coupler, which can be applied in optically controlled beamforming.

This paper analyzes the influence of amplitude and phase error of different path on single sideband modulator (SSB) which can be used as an upconverter. Unwanted sideband suppression expression and error variables equation for maximum output sideband suppression are also deduced. The simulation curve shows the variation of sideband suppression with the amplitude and phase error of the components of SSB is given. Simulation results show that components of the amplitude and phase of the highly consistent or inconsistent amplitude and phase which meet the relationship can achieve higher sideband suppression for the SSB. The completed modulator used as an upper sideband upconverter shows −11 dB typical conversion gain with 22‐dB sideband suppression. © 2008 Wiley Periodicals, Inc. Microwave Opt Technol Lett 50: 2590–2594, 2008; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.23721

We present, characterize, and demonstrate a simple, and novel, continuous-time band-folding microwave photonic link. Implemented with a single dual-drive Mach-Zehnder modulator (DDMZM) acting as 2 parallel phase modulators, the link effects an 8:1 bandwidth folding to enable direct sampling and digitization of an entire band. Two microwave tones modulated on one arm along with their 3rd-order intermodulation (IMD) products serve as the spectral comb that exists only in the desired band, and the microwave spectrum of interest (SOI) is modulated onto the other arm. Parallel implementation adds, rather than multiplies, the tone and signal together, eliminating many cross-modulation products and spurs that would otherwise complicate and limit the foldable bandwidth. The link can operate with no optical filters if biased or with no bias if an optical bandpass filter is used.

Photonics-assisted frequency conversion based on cascaded modulators is an effective way to satisfy the requirements of distributed antenna array applications. A filter-free photonic image-rejection down-converter with immunity to dispersion-induced power fading is proposed and demonstrated. The radio frequency (RF) signal from the distributed antenna unit is injected into a dual-drive Mach–Zehnder modulator (DDMZM) through an electrical 90° hybrid coupler (HC), and a single-sideband (SSB) signal is realized. It is then transmitted to the center office by an optical fiber link. At the center office, the modulated RF signal is injected into a dual-polarization dual-parallel MZM (DP-DPMZM) after a polarization controller (PC). Afterwards, an LO signal remodulates it through the Y-DPMZM, and carrier-suppressed SSB modulation is fulfilled via another electrical 90° HC. After the power splitting, quadrature I and Q IF signals can be obtained by a photonics-based continuously adjustable phase shifter. Through a combination of the I/Q IF signals with a low-frequency electrical 90° HC, a photonics image-rejection down-converter based on phase cancellation is realized. Moreover, no optical filter is required; thus, the image-rejection down-converter can work with a large bandwidth. The proposed filter-free photonic image-rejection down-converter is theoretically analyzed and experimentally verified.

A new optical technique for microwave single sideband modulation is reported. It uses metal-semiconductor-metal Schottky photodiodes formed in a GaAs/Al<SUB>0.3</SUB>Ga<SUB>0.7</SUB>As materials system to detect microwave in-phase and quadrature signals on optical carriers. Modulation of the photodetector bias voltages results in a single sideband modulation of the microwave signal, rf and undesired-sideband suppression of 36 dB and 27 dB, respectively, were achieved. The optical wavelength was 850 nm, and the bandwidth of the photodetectors is greater than or equal to 29 GHz.

We demonstrated optical frequency sweep using optical single sideband modulation technique, where the frequency shift range was 2 GHz-10 GHz. Undesired sidebands due to electric phase errors can be compensated by synchronized bias control. (2 pages)

An ultrafast optical frequency sweeping technique for narrow linewidth lasers is reported. This technique exploits the large frequency modulation bandwidth of a wideband voltage controlled oscillator (VCO) and a high speed electro-optic dual parallel Mach-Zehnder modulator (DPMZM) which works on the state of carrier suppressed single sideband modulation(CS-SSB). Optical frequency sweeping of a narrow linewidth fiber laser with 3.85 GHz sweeping range and 80 GHz/μs tuning speed is demonstrated, which is an extremely high tuning speed for frequency sweeping of narrow linewidth lasers. In addition, injection locking technique is adopted to improve the sweeper's low optical power output and small side-mode suppression ratio (SMSR).

Experimental demonstration of tunable optical single sideband modulation and 1.5 Gb/s RoF downlink using stimulated Brillouin scattering

In this article, we demonstrate the use of a network analysis optical frequency domain reflectometer (NA-OFDR) for distributed vibration sensing at a spatial resolution of ≈3 cm and an acquisition rate up to ≈80 Hz. The system makes use of a continuous-wave (CW) laser, whose amplitude is modulated using a vector network analyzer. The latter is also used to demodulate the amplitude and phase of the beat signal obtained by coherently mixing the backscattered light with a local oscillator. Two configurations are considered, based on either double sideband or single sideband modulation of the probe light. The single sideband modulation is shown to possess superior performance in terms of localization capabilities of the applied perturbation.

Frequency-swept lasers are used in many important applications, such as the high-precision optical spectrum analyzer, the high-resolution laser spectroscopy, the frequency-modulated continuous wave (FMCW) lidar, the optical frequency-domain imaging (OFDI), the optical time-domain reflectometry (OTDR) and the optical frequency-domain reflectometry (OFDR), the optical communications and microwave photonics, and fundamental scientific researches, as introduced in Sect. 7.1. The method of direct current modulation of LD is introduced in Sect. 7.2; the sweeping linearity and its characterization are discussed. Section 7.3 describes methods by using intra-cavity modulators and beam deflectors inside the ECDL cavity and the related electro-optic materials and devices. Section 7.4 introduces methods by using external modulations placed in the path out of laser cavity, and the related acousto-optic modulators and electro-optic modulators, especially the single side band modulator.

In order to generate optical Ultra-Wideband (UWB) pulse train with a switchable shape, a novel method based on a Polarization Modulator (PolM), a Polarization Controller (PC) and a delay line, is proposed and experimentally demonstrated. Experimental results show that when a Gaussian pulse is applied to the PolM, under different phase shifts introduced by the PC, a gauss or gauss doublet pulse will be generated at the output of the Polarization Beam Splitter (PBS). If the two-path signals after PBS are properly delayed, Gaussian monocycles or third-order gauss pulses will be generated at the output of the Polarization Beam Combiner (PBC).

We have designed, assembled and tested a phase-array antenna using fiber Bragg gratings in the highly dispersive transmission region as our tunable true-time delay (TTD) generators. The TTD generator is designed by cascading 29 identical fiber gratings and 1&times;2 fiber splitter pairs. Tapping from each fiber splitter allows us to steer our RF microwave beam from a 30&times;4-element antenna by tuning the wavelength of a laser. The 10Ghz RF signal is superimposed upon a laser beam by means of a LiNbO₃ modulator. However, with a conventional modulator, the optical frequency spectrum of the modulated beam consists of two sidebands on opposite sides of the optical carrier; all three of which may experience very different time delays due to dispersion. This may have detrimental effects on time-delay sensitive processes such as antenna beamforming. Therefore, we studied the use of single-sideband versus double-sideband modulators in the transmitters of photonic phased array antenna. We focus in particular on the effect of the different spectral profiles of single and double sideband modulators on beamforming when using the fiber Bragg gratings as TTD generators. With very high dispersion in our fiber Bragg gratings close to the band edge, the absolute propagation times are different for each sideband and the optical carrier. Therefore double sideband modulated signals will generate two sets of separate delays in the same microwave signal which causes beam deterioration and increased side-lobes. We demonstrated this theoretically and verified experimentally by comparing the antenna patterns generated by single-side-band and by double sideband modulators.