Showing papers on "Optical Carrier transmission rates published in 2011"

Linearized electrooptic microwave downconversion using phase modulation and optical filtering.

Abstract: We propose and demonstrate an electrooptic technique for relaying microwave signals over an optical fiber and downconverting the microwave signal to an intermediate frequency at the receiver. The system uses electrooptic phase modulation in the transmitter to impose the microwave signal on an optical carrier followed by re-modulation with a microwave local oscillator at the receiver. We demonstrate that by subsequently suppressing the optical carrier using a notch filter, the resulting optical signal can be directly detected to obtain a downconverted microwave signal. We further show that by simply controlling the amplitude of the microwave local oscillator, the system can be linearized to third-order, yielding an improvement in the dynamic range.

Photonic Generation of Phase-Coded Microwave Signal With Large Frequency Tunability

Abstract: A photonic approach to realizing phase-coded microwave signal generation with large frequency tunability is proposed and demonstrated. Two coherent optical wavelengths are generated based on external modulation by biasing a Mach-Zehnder modulator (MZM) at the minimum transmission point to generate ±1 -order sidebands while suppressing the optical carrier. The two ±1-order sidebands are then sent to a fiber Sagnac interferometer (SI) incorporating an optical phase modulator (PM) and a broadband flat-top fiber Bragg grating (FBG), with one of the sidebands being phase modulated at the PM. A frequency tunable phase-coded microwave signal is generated by beating the two sidebands at a photodetector (PD). The proposed technique is experimentally investigated. The generation of a frequency tunable phase-coded microwave signal at 22 and 27 GHz is demonstrated.

Compensation of dispersion-induced power fading for highly linear radio-over-fiber link using carrier phase-shifted double sideband modulation

Abstract: A carrier phase-shifted (CPS) double sideband (DSB) modulation technique in radio-over-fiber (RoF) system is proposed and experimentally demonstrated. By tuning the biases in a single-drive dual parallel Mach–Zehnder modulator (SD-DPMZM), the optical carrier in the DSB spectrum acquires additional phase shift. The transmittance response of a dispersive RoF link is thus being controlled and shifted in the frequency domain. Experiments successfully turned the maximum transmission frequency to 10 GHz and 15 GHz for both 25 and 39 km fiber links. This is also a highly linear scheme, of which a spurious-free dynamic range (SFDR) of 111.3 dB·Hz2/3 is experimentally obtained.

All-optical link for direct comparison of distant optical clocks.

Abstract: We developed an all-optical link system for making remote comparisons of two distant ultra-stable optical clocks. An optical carrier transfer system based on a fiber interferometer was employed to compensate the phase noise accumulated during the propagation through a fiber link. Transfer stabilities of 2 × 10−15 at 1 second and 4 × 10−18 at 1000 seconds were achieved in a 90-km link. An active polarization control system was additionally introduced to maintain the transmitted light in an adequate polarization, and consequently, a stable and reliable comparison was accomplished. The instabilities of the all-optical link system, including those of the erbium doped fiber amplifiers (EDFAs) which are free from phase-noise compensation, were below 2 × 10−15 at 1 second and 7 × 10−17 at 1000 seconds. The system was available for the direct comparison of two distant 87Sr lattice clocks via an urban fiber link of 60 km. This technique will be essential for the measuring the reproducibility of optical frequency standards.

Widely tunable single-bandpass microwave photonic filter employing a non-sliced broadband optical source.

Abstract: We demonstrate a novel microwave photonic filter based on a non-coherent broadband optical source and the variable optical carrier time shift (VOCTS) method. Optical slicing which is essential conventionally is not employed in our scheme. Nevertheless, equivalent "electrical slicing" is performed by VOCTS, generating a passband free from the carrier-suppression effect. The baseband response is eliminated by using carrier-suppression or phase modulation. Single bandpass is also achieved due to the continuous-time sinusoidal impulse response. Detailed theoretical analyses are presented and agree with the experiments quite well.

Systems and methods for providing an optical information transmission system

Abstract: The present invention describes systems and methods of providing optical information transmission systems. An exemplary embodiment of the present invention includes a precoder configured to differentially encode a binary data signal, a duobinary encoder configured to encode the differentially encoded binary data signal as a three-level duobinary signal, an electrical-to-optical conversion unit configured to convert the three-level duobinary signal into a two-level optical signal, and an optical upconversion unit configured to modulate the two-level optical signal onto a higher frequency optical carrier signal and transmit the modulated higher frequency optical carrier signal onto an optical transmission medium.

Long-reach radio-over-fiber signal distribution using single-sideband signal generated by a silicon-modulator

Abstract: The integration of passive optical network (PON) and radio-over-fiber (ROF) networks could provide broadband services for both fixed and mobile users in a single and low-cost platform. Combining the long-reach (LR)-PON (>100 km) and the LR-ROF can further reduce the cost by simplifying the network architecture, sharing the same optical components and extending the coverage of ROF network. However, the transmission and distribution of ROF signal in LR network is very challenging due to the chromatic dispersion generated periodic power fading and code time-shifting effects in the optical fiber. In this work, we propose and experimentally demonstrate a LR-ROF signal distribution using single-sideband (SSB)-ROF signal generated by a silicon ring-modulator. The silicon modulator is compact and has low power consumption. Besides, one unique feature of the silicon ring-modulator is that it only modulates the signal wavelength at the resonant null. This makes it very suitable for the generation of the SSB-ROF signal. Numerical comparison of the SSB-ROF with the double-sideband (DSB)-ROF and optical carrier suppress (OCS)-ROF signals; as well as the fabrication of the silicon ring-modulator will be discussed.

Ultra-Wideband Generation Based on Cascaded Mach–Zehnder Modulators

Abstract: A simple approach to generating ultra-wideband (UWB) pulses with a single optical carrier is proposed and demonstrated. The approach is realized by using a single laser diode (LD) and two cascaded Mach-Zehnder modulators (MZMs). In the experiment, the two MZMs are biased at opposite slopes of their modulation curves respectively and the time delay between them is variable. Negative monocycle pulses with a center frequency of 4.3 GHz and a fractional bandwidth of 165%, and positive monocycle pulses with a center frequency of 4.3 GHz and a fractional bandwidth of 188% are generated, respectively. The feasibility of high order UWB pulses generation with a single optical carrier is also discussed.

Optical Wireless OFDM System on FPGA: Study of LED Nonlinearity Effects

Abstract: Nonlinearities can drastically degrade the performance of OFDM (orthogonal frequency division multiplexing) based optical wireless (OW) communication systems using intensity modulation (IM) of the optical carrier. The light emitting diode (LED) transfer function distorts the signal amplitude and forces the lower signal peaks to be clipped at the LED turn-on voltage (TOV). Additionally, the upper signal peaks can result in optical output degradation. The induced distortion can be controlled by optimizing the bias point (BP) of the LED and/or backing-off the signal power modulating the LED. In this paper, the obtained experimental results using a hardware demonstrator for OW OFDM transmission based on field programmable gate array (FPGA) and off-the-shelf analog components are presented. The conducted measurements for the bit-error performance focus on determining the optimum BP and optimizing the OFDM signal amplitude to obtain best performance. In this context, the experimental bit-error ratio (BER) is obtained as a function of the LED BP and the RMS (root mean square) OFDM signal across the LED.

Rayleigh Noise Reduction in 10-Gb/s Carrier-Distributed WDM-PONs Using In-Band Optical Filtering

Abstract: To circumvent the challenging issue of Rayleigh noise reduction in wavelength-division-multiplexed passive optical network (WDM-PON), we provide an insight into the source of Rayleigh noise, and confirm that the suppression of carrier Rayleigh backscattering (RB) should be the primary target in the design of Rayleigh noise-resilient upstream receiver module for a transmission reach up to 60 km. Then we propose and demonstrate a novel scheme to effectively suppress the carrier RB in carrier-distributed WDM-PONs. By simply replacing the upstream modulation format of conventional on-off keying (OOK) with differential phase-shift keying (DPSK), the system tolerance to carrier RB is substantially enhanced by 19 dB, as the carrier RB can be considerably rejected by the notch filter-like destructive port of the delay-interferometer (DI) at the optical line terminal (OLT), which is used simultaneously to demodulate the upstream DPSK signal. The dependence of carrier RB suppression on DI's extinction ratio (ER) and optical carrier's line width is also theoretically analyzed. Experimental demonstration of 10-Gb/s upstream signal is achieved with less than 2.5-dB power penalty induced by Rayleigh noise after the transmission in 60-km single mode fiber, without using any amplifier in outside plant. The relation between system margin and the gain of optical network unit (ONU) is also studied.

Analysis of the carrier-suppressed single-sideband modulators used to mitigate Rayleigh backscattering in carrier-distributed PON.

Abstract: By using the carrier-suppressed single-sideband (CS-SSB) modulation, the Rayleigh backscattering (RB) experienced by the uplink signal can be effectively mitigated due to the reduction of the spectral overlap between the uplink signal and the distributed optical carrier. In this work, we first introduce the theoretical analysis of the CS-SSB generation using the dual-drive MZM (DD-MZM)-based and a dual-parallel MZM (DP-MZM)-based optical networking units (ONUs). Due to the different modulation mechanisms of the two CS-SSB modulations, the frequency components of the generated CS-SSB signals are also different. The transmission performance and the dispersion tolerance of the uplink signals generated by the two CS-SSB modulators are also analyzed and discussed.

True-time delay line with separate carrier tuning using dual-parallel MZM and stimulated Brillouin scattering-induced slow light

Abstract: We experimentally demonstrate a novel tunable true-time delay line with separate carrier tuning using dual-parallel Mach-Zehnder modulator and stimulated Brillouin scattering-induced slow light. The phase of the optical carrier can be continuously and precisely controlled by simply adjusting the dc bias of the dual-parallel Mach-Zehnder modulator. In addition, both the slow light and single-sideband modulation can be simultaneously achieved in the stimulated Brillouin scattering process with three types of configuration. Finally, the true-time delay technique is clearly verified by a two-tap incoherent microwave photonic filter as the free spectral range of the filter is changed.

Optical Comb Sources for High Dynamic-Range Single-Span Long-Haul Analog Optical Links

Abstract: We present a new technique for achieving high spurious-free dynamic range in long-haul analog optical links. Our technique utilizes an optical comb as the optical carrier in an externally intensity-modulated direct-detection link architecture. By distributing the optical carrier power over the comb (in contrast to concentrating it in a single continuous-wave laser, as in a conventional link), this technique circumvents the optical power limitations imposed by stimulated Brillouin scattering allowing a substantial increase in optical launch power. Increased launch power translates directly to decreased optical amplification requirements at the link terminus and results in a significantly lower noise floor than may be achieved in a conventional link architecture. To our knowledge, the dynamic range of SFDR=105.5 dB (1-Hz bandwidth, L=50 km link) is the highest reported to date for a single-span long-haul analog optical link.

Terabit transmission rate coherent light orthogonal frequency division multiplexing (OFDM) system based on optical comb

Abstract: The invention provides a feasible Terabit transmission rate coherent light orthogonal frequency division multiplexing system which is of simpler system structure, and comprises a transmitting end and a receiving end, wherein, the transmitting end comprises a transmitting end optical carrier generation module which comprises a transmitting end laser, an optical comb generator and an optical demultiplexer. In the system, on the basis of single light source, the optical comb generator is utilized to obtain light frequency combs with multiple wavelengths, which are used as optical carriers of base band OFDM signals, and mutually orthogonal OFDM frequency bands are generated on an optical domain, thus a plurality of radio frequency (RF) oscillation sources with complex structures are not need in the prior art. By utilizing the system, an OBM-COOFDM (orthogonal-band-multiplexed-coherent optical orthogonal frequency division multiplexing) optical signal is obtained under the condition of not using the RF oscillation sources, thus greatly reducing the complexity and cost of a transmitter; and the system has better feasibility of the OBM-COOFDM hardware, thus reducing the higher sampling rate requirements of the traditional COOFDM Terabit transceiver on a digital to analog converter (DAC) and an analog to digital converter (ADC).

Multichannel Arbitrary-Order Photonic Temporal Differentiator for Wavelength-Division-Multiplexed Signal Processing Using a Single Fiber Bragg Grating

Abstract: A multichannel photonic temporal differentiator implemented based on a single multichannel fiber Bragg grating (FBG) for wavelength-division-multiplexed (WDM) signal processing is proposed for the first time to our knowledge. The multichannel FBG is designed using the discrete layer peeling (DLP) algorithm together with the spatial sampling technique. Specifically, the DLP algorithm is used to design the spectral response of an individual channel, while the spatial sampling is employed to generate a multichannel response. The key feature of the proposed temporal differentiator is that WDM signals at multiple optical wavelengths can be simultaneously processed. Two sampling techniques, the phase-only and the amplitude-only, are employed. The use of the phase-only sampling technique to design a 45-channel first-order and second-order temporal differentiator is performed, and the use of the amplitude sampling technique to design a 3-channel first-order and second-order temporal differentiator is also performed. A proof-of-concept experiment is then carried out. A 3-channel first-order differentiator with a bandwidth of 33.75 GHz and a channel spacing of 100 GHz is fabricated. The use of the fabricated 3-channel FBG to perform first-order temporal differentiation of a 13.2-GHz Gaussian-like optical pulse with different optical carrier wavelength is demonstrated.

A full-duplex radio-over-fiber link with 12-tupling mm-wave generation and wavelength reuse for upstream signal

Abstract: A full-duplex radio-over-fiber (RoF) link with a novel scheme to generate 60 GHz mm-waves from a 5 GHz RF signal source is investigated. In the RoF downlink, the required frequency of the RF oscillator is reduced greatly. Since the optical carrier is not modulated by downstream data, part of it is reused to carry upstream data and the upstream data is transmitted to the central station using optical single-sideband modulation. In this way, a single wavelength is used for both downstream and upstream transmissions. Based on this scheme, a full-duplex RoF link is built and its transmission performance is analyzed. Theoretical analysis and numerical simulation show that the downstream signal cannot only eliminate code form distortion caused by time shift of the code edges, but also reduce the influence of the fading effect as the 60 GHz DSB optical mm-wave signal is transmitted along the fiber, and the upstream signal is immune to both fading effect and time shift of the code edges.

Secure orthogonal frequency multiplexed optical communications

Abstract: The invention provides a system and method for secure communication that involve encoding and transmitting an optical orthogonal frequency division multiplexed (OFDM) signal. Each subcarrier of an optical carrier in OFDM transmission is modulated with data individually, and a variety of data format are used, such as QPSK, OOK, QAM, etc. The data format of each subcarrier may change in time according to a predetermined pattern. An optical receiver uncovers the data transmitted via an optical link. It is based on a coherent optical receiver and a digital signal processing (DSP) unit. A key to the data mapping and change is transmitted via the same optical link or by a separate channel. In one embodiment, the key is transmitted using quantum encryption technique. Besides subcarrier modulation encoding, the system may provide additional layers of security: optical carrier frequency hopping and polarization scrambling.

Spatial Spectral Photonic Receiver for Direction Finding via Wideband Phase Sensitive Spectral Mapping

Abstract: An apparatus includes a single or dual output port, dual-drive Mach-Zehnder Interferometer configured to generate a first optical signal in one path, and to generate a second optical signal in a different path. The apparatus also includes an optical spectrum analyzer configured to receive output from at least one port of the dual-drive Mach-Zehnder Interferometer. A method includes causing radio frequency signals from two different antennae to modulate an optical carrier at a corresponding drive of a dual-drive Mach-Zehnder Interferometer, and causing output from at least one port of the Mach-Zehnder Interferometer to be directed to an optical spectrum analyzer. The method further comprises determining arrival angle at each of a plurality of frequencies in the radio frequency signals based on output from the optical spectrum analyzer.

16×10Gb/s symmetric WDM-FOFDM-PON realization with colorless ONUs

Abstract: A novel symmetric WDM-PON scheme with colorless ONU is proposed. The baseband 4-ASK Fast-OFDM signal is upconverted by an intermediate frequency carrier, reserving a frequency gap between the FOFDM signal and the optical carrier. After distributing different wavelengths to corresponding ONU by AWG, periodic BPFs are employed to extract the optical carriers for upstream transmission, achieving colorless ONUs. A WDM-PON system with 16 colorless ONUs is established, and 10-Gb/s symmetric transmission for each ONU is also realized. Experiment shows that the system tolerance to the intrachannel crosstalk is greatly improved when the crosstalk signal locates at relatively higher frequency band.

Wavelength Assignment in Multi-Carrier Distributed Optical Ring Networks With Wavelength Reuse

Abstract: This paper investigates the problem of wavelength assignment in wavelength reusable multi-carrier distributed (WRMD) wavelength-division-multiplexing (WDM) ring networks. In conventional WDM ring networks, each edge node (EN) has its own light sources, and optical channels, called lightpaths, are established by using optical carriers generated from laser diodes (LDs) at the source EN. However, such networks will suffer from the need for complicated wavelength management (e.g., monitoring LDs, avoiding wavelength collision) in the future since each EN requires a large number of LDs to deal with the exponential increase in traffic. On the other hand, a WRMD ring network overcomes this problem. In this network, lightpaths between source and destination ENs are established by using carriers generated from a centralized multi-carrier light source. Moreover, the carrier regeneration technique is applied for the purpose of reducing the number of wavelengths used for lightpath establishment. Although optical carrier regeneration reduces the number of wavelengths, the quality of the regenerated carrier is slightly degraded after carrier regeneration. Therefore, in the WRMD network, the allowable number of carrier regenerations per wavelength must be limited in order to avoid communication error. This paper formulates the wavelength assignment problem, minimizing the number of wavelengths needed to establish all requested lightpaths, as the vertex coloring problem, and then an integer linear programming (ILP) solution is provided. Since ILP problems are non-deterministic polynomial-time- (NP-) complete, a heuristic algorithm is developed. Numerical results indicate that our developed algorithm performs well in our test cases. It is observed that one and two carrier regenerations per wavelength reduce the number of wavelengths for lightpath establishment by approximately 50% and 60%, respectively, compared to that without carrier regeneration. The results also show that regenerating carriers more than two times per wavelength has little effect on the required number of wavelengths regardless of the number of ENs.

Circuit-level transient simulation of configurable ring resonators using physical models

Abstract: This paper presents a methodology for the incorporation of actively configurable interference-based optical elements into a circuit-level optoelectronic simulator. The self-consistent optoelectronic simulator is based on modified nodal analysis. The paper uses ring-resonator-based devices as examples of configurable devices. Construction of compact models of these devices from optical scattering and the waveguide elements using fundamental principles is presented in detail. In the results section, accuracy of the compact model of the ring resonator is first confirmed for static devices for steady-state and transient conditions. Last, the devices are placed in a complex optical circuit and used to modulate an optical carrier and select a particular channel from a multichannel optical signal. The modelling framework proposed proved to be robust and efficient for transient simulation of configurable elements in a self-consistent optoelectronic simulation engine.

All-optical Data Frequency Multiplexing on Single-Wavelength Carrier Light by Sequentially Provided Cross-Phase Modulation in Fiber

Abstract: A novel configuration for an all-optical bus-topology data-multiplexing system is proposed in which multichannel signals are sequentially multiplexed by frequency-division multiplexing (FDM) on a single-wavelength optical carrier by cross-phase modulation (XPM) in nonlinear fibers (NLFs) with subcarrier-modulated optical data signals. By using highly NLFs for XPM generators and providing amplitude-modulated subcarrier-multiplexed optical signals generated with directly modulated lasers or optical beat signals at multiple local connection points, we successfully demonstrate sequential all-optical FDM of data signals on a single-wavelength optical carrier followed by error-free data transmission through a standard fiber.

Self- coherent optical OFDM: An interesting alternative to direct or coherent detection

Abstract: Recently, coherent optical orthogonal frequency division multiplexing (CO-OFDM) has been considered for the next generation 400-Gb/s long haul data transmission. For long haul transmission systems coherent detection is beneficial compared to direct detection as it provides higher sensitivity. Though CO-OFDM offers a virtually unlimited tolerance against linear impairments, phase noise represents major challenge that must be compensated for. Thus, phase noise compensation (PNC) technique must be applied in the digital signal processing (DSP), which is already limited in speed and processing capabilities. Conversely, self coherent optical (SCO-) OFDM is an interesting alternative which does not require any PNC in the DSP. Furthermore, no local oscillator (LO) is required at the receiver. In this paper, an overview of the recently seen self coherent techniques will be given. Generally, self coherent detection is realized by sending an optical carrier along with the data signal like in direct detection systems. At the receiver, however, the optical carrier is extracted from the signal with the use of filters with very narrow bandwidth and used as an LO using heterodyne or homodyne detection. The performance of such a system depends on the filter bandwidth that is used to extract the carrier.

Wavelength Tunability of All-Optical Clock-Recovery Based on Quantum-Dash Mode-Locked Laser Diode Under Injection of a 40-Gb/s NRZ Data Stream

Abstract: Wavelength tunability of an all-optical clock recovery operation based on a quantum-dash mode-locked Fabry-Perot laser diode is experimentally investigated. Synchronization of the device to the injection of 40-Gb/s nonreturn-to-zero (NRZ) incoming data is assessed by analyzing both the carrier-to-noise ratio and the linewidth of the 40-GHz beat-tones measured at the mode-locked laser output. Under optical injection, beat-tone linewidths below 10 Hz are measured. Recovered clock pulses featuring a width of 1.6 ps are obtained irrespective of the wavelength detuning between the laser spectra and the optical carrier of the incoming data stream.

Frequency-Tunable Microwave Generation Based on Time-Delayed Optical Combs

Abstract: A novel approach to generating a frequency-tunable microwave signal based on time-delayed optical combs is proposed and demonstrated. The fundamental principle is to generate multiple optical combs with identical comb profile, but with each optical comb carried by an optical carrier at a different wavelength. If the optical carriers are spaced with an identical wavelength spacing, the optical combs will be time delayed with an identical time delay after passing through a dispersive fiber. By applying these optical combs to a photodetector, microwave comb lines at the fundamental-order and higher order harmonic frequencies will be generated. For a well-designed time-delay structure, however, the desired microwave harmonic will have the highest output due to constructive interference, while the other harmonics will be suppressed. An analysis is performed, which is verified by a proof-of-concept experiment. A microwave signal that is tunable from 16.8 to 27 GHz is generated. The performance of the generated signal in terms of stability and phase noise is also evaluated.

Orthogonal frequency division multiple access (OFDMA) passive optical network transmission system

Abstract: The invention relates to an orthogonal frequency division multiple access (OFDMA) passive optical network transmission system, belonging to the technical field of optical communication. The OFDMA passive optical network transmission system comprises an optical link terminal, a feed-in optical fiber, a far-end node, a plurality of distributed optical fibers and a plurality of optical network units, wherein the optical link terminal is connected with one end of the feed-in optical fiber and outputs downlink data signals and uplink optical carrier signals; the other end of the feed-in optical fiber is connected with the far-end node and transmits the downlink data signals and the uplink optical carrier signals; two ends of the distributed optical fibers are respectively connected with the far-end node and the optical network units and transmit the downlink data signals and the uplink optical carrier signals; the optical network units process the downlink data signals and separate out theuplink optical carrier signals for modulating the uplink data signals and outputting the modulated uplink data signals through the distributed optical fibers to the far-end node and the optical link terminal; and the optical link terminal receives the uplink data signals for carrying out coherent demodulation. In the invention, the passivation of the optical network units is realized and the complexity and cost of the optical network units are reduced.

Fiber optical transmission system, transmitter and receiver for DQPSK modulated signals and method

Abstract: A transmitter (3) for generating a DQPSK-modulated optical signal, comprising: a splitter (7) for dividing an optical carrier signal into a first and second branch (8a, 8b), a first and second Mach-Zehnder interferometer (9, 10) in the first and second branch (8a, 8b), respectively, a phase shifter (11) in one of the branches (8b) generating a nominal phase shift of a/2, and a combiner (7') for combining the optical output signals of the two branches (8a, 8b). The transmitter (3) has a feedback circuit (12) generating at least a first and second bias signal (15.1 to 15.3) for adjusting a biasof at least the first and second Mach-Zehnder interferometers (9, 10), the feedback circuit (12) comprising: a detector for generating at least a first and second feedback signal from a sample signalextracted from the optical signal after the combiner (7'), and for each bias signal: a local oscillator generating an auxiliary signal modulating the bias signal (15.1 to 15.3) at a pre-defined frequency (f1, f2, f3), a lock-in detector determining a phase difference between the feedback signal and the auxiliary signal, and a bias circuit for generating the bias signal (15.1 to 15.3) from an output signal of the lock-in detector, wherein the pre-defined frequencies of the auxiliary signals (f1, f2, f3) are different from one another (f1 f2 f3) and preferably no integer multiples of each other.

RoF transport systems with OSNR enhanced multi-band optical carrier generator.

Abstract: A combination of an economic multi-band optical carrier generator and a novel optical signal to noise ratio (OSNR) enhancement circuit is proposed and demonstrated for radio over fiber (RoF) transport systems. Different from normal RoF transport systems which a central station (CS) needs multiple dedicate wavelength laser diodes (LDs) to support various base stations (BSs), the proposed scheme can employ a single LD to provide multiple optical carriers for various BSs. To verify this scheme, 8 coherent optical carriers are firstly generated using a single LD and a local oscillator (LO). Subsequently, their OSNR values are optimized by the developed OSNR enhancement circuit. An up to 15 dB OSNR enlargement in those optical carriers is experimentally achieved. To demonstrate the practice of the proposal, a pair of those optical carriers is employed to experimentally achieve frequency up-conversion process in a RoF transport system. Clear eye diagram and error free transmission reveal that with a proper carrier selector the proposed scheme can be employed to support multiple RoF transmissions. Furthermore, this proposal also presents a high possibility to achieve 60 GHz RoF transmission using a 10 GHz LO, a LD and a low frequency external modulator.

An All-Optical Carrier Recovery Scheme for Access Networks With Simple ASK Modulation

Abstract: We theoretically investigate and experimentally demonstrate a scheme for all-optical carrier recovery in loopback access networks that avoids orthogonal or complex modulation formats for the downstream or upstream signals. The applied technique is based on a passive resonating circuit that is capable of recovering the optical carrier of the amplitude-shift-keyed downstream signal for remodulation with a reflective modulator as upstream transmitter enabling full-duplex 10 Gb/s operation. The scheme is compared with alternative pattern suppression techniques based on optical gain saturation and electro-optical feed-forward injection for the stringent requirements of next-generation access networks, namely, an extended loss budget and high upstream data rates. Operation at downstream modulation depths of ~3 dB is reported with the feed-forward approach, while higher modulation depths of up to 9 dB are demonstrated with the all-optical carrier recovery technique, for which the dependence on longer sequences of consecutive identical bits is investigated. Finally, the feasibility of the all-optical downstream cancelation technique for optical access networks is evaluated in a wavelength division multiplexed passive optical network, showing full-duplex transmission with margins of at least 9 dB.