Generation of optical carrier suppression millimeter-wave signal using one dual-parallel MZM to overcome chromatic dispersion

In this paper, we have proposed a novel approach to generate optical carrier suppression (OCS) mm-wave with signal only carried by one first-order sideband using a dual-parallel MZM in radio-over-fiber (RoF) system, and the transmission performance was also investigated. As the optical mm-wave signal is transmitted along the fiber, there is no time shift of the data symbols resulting from the group velocity dispersion in the fiber because the signal is only modulated on one sideband. The simulation results showed that the eye diagram keeps open and clear even when the optical mm-wave signals are transmitted over 96-km and the power penalty is about 1-dB after fiber transmission distance of 60-km, which is quite consistent with our theoretical analysis. Furthermore, the proposed OCS optical mm-wave generation approach is also used in a full-duplex RoF link based on wavelength reuse at the base station for the uplink due to another pure sideband without carrying data. The bidirectional 2.5-Gbit/s data is successfully transmitted over 40-km standard single mode fiber with less than 0.5-dB power penalty in the simulation. Both the unidirectional RoF downlink and the full-duplex RoF system have good performance.     

Generation and transmission of the double-sideband optical millimeter-wave with signal carried only by optical carrier

In this paper, we have proposed a new scheme to generate double-sideband (DSB) optical millimeter-wave (mm-wave) with signal carried only by optical carrier, and its transmission performance is investigated. The theoretical and numerical results show that the DSB optical mm-wave signal generated by our scheme suffers only from the fading effect, the time shift of the sidebands has little influence on its transmission performance. The signal demodulated from the harmonic photocurrent shows a clear and open eye diagram and has good performance even if the transmission distance is very close to but not just at the fading nodes. The theoretical results are in good agreement with our numerical simulation.     

The transmission performance degradation of the optical millimeter-wave signals by fiber chromatic dispersion

The influence of the fiber chromatic dispersion on double sideband (DSB), optical carrier suppression (OCS), and single sideband (SSB) optical mm-wave signals is investigated based on the Taylor expansion of the propagation constant and is verified by simulation. According to our theoretical results, the fading effect suppresses the signal power of the DSB optical mm-wave periodically in a cosine-like pattern, and it can be described by the zero-order Taylor expansion of the propagation constant. For the optical mm-wave with the signal modulated on two or more tones, the bit pulses of the mm-wave signal are distorted by the dispersion-inducing bit walk-off effect between tones, which is expressed by the first-order Taylor expansion of the propagation constant. Moreover, as the signal rate and the transmission distance are increased further, higher-order Taylor expansion of the propagation constant still degrades the optical mm-wave signal even if both the fading effect and the bit walk-off effect are eliminated completely. The distortion of the signal pulses of SSB optical mm-wave is derived based on the second-order Taylor expansion of the propagation constant. This degradation is verified by the simulation with the eye diagram evolution of the SSB optical mm-wave signal.     

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

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.     

An optical mm-wave generation scheme by frequency octupling using a nested MMI

A novel method of a filterless optical millimeter-wave (MMW) signal generation with frequency octupling via a nested multimode interference (MMI) coupler is proposed for Radio-over-fiber systems. By setting the DC bias voltage applied to the central arms of MMI-b and MMI-c accurately, the optical carrier can be completely suppressed. The OSSR can be as high as about 58 dB without optical filter and the radio frequency spurious suppression ratio (RFSSR) exceeds 32 dB, which is the best result as we know. Simulation results suggest that when the generated optical mm-wave signal is transmitted along the standard single-mode fiber, the eye diagram is still opened after being transmitted over a 50 km fiber.     

A full-duplex radio-over-fiber system using direct modulation laser to generate optical millimeter-wave and wavelength reuse for uplink connection

In this paper, we present a full-duplex radio-over-fiber system incorporating both optical millimeter-wave (mm-wave) generation and wavelength reuse for uplink connection. The optical double sidebands (DSB) signal is generated by using only one inexpensive broadband direct modulation laser (DML), to which a mixing RF signal is applied. An optical interleaver is then used to separate the first-order optical sidebands from the optical carrier of optical DSB signal. The separated first-order optical sidebands are beat to generate mm-wave signal that has double the frequency of the RF drive signal, while the separated optical carrier is reused as light source to remodulate uplink signal. Both detailed theoretical analysis and experiments to demonstrate the feasibility of the proposed system are presented. Experiment result shows that the bidirectional 2.5 Gb/s data can be successfully transmitted over 40 km standard single-mode fiber (SSMF) with less than 2 dB power penalty.     

Full-duplex radio-over-fiber system based on a modified single-side band modulation

A full-duplex radio-over-fiber system based on a modified single-sideband using external modulator is proposed and demonstrated. At the central station, a CW lightwave is intensity-modulated by a RF signal to generate a DSB signal. After the central carrier and the two first-order sidebands are separated by a FBG, the central carrier is modulated with a baseband data at 2.5 Gbit/s. Then, it is recombined with the un-modulated first-order sidebands to generate optical millimeter-wave by an optical coupler with a certain coupling coefficient and transmitted to the base station over single-mode fiber. The central carrier and one of the first-order sidebands are beaten to generate the mm-wave when they are detected by an optical receiver. Another first sideband is reused as carrier for uplink connection. The dispersion performance of the generated mm-wave is theoretically analyzed; one can see that the effect of dispersion and requirement of the optical power are reduced. The PIN-PD can avoid working in a high-DC saturation range which may distort the RF components and depress the responsibility of the detector. The stimulant results show that the system can reduce the effect of dispersion effectively, and immune the fading effect and the walking-off signals. It is suitable for a long distance transmission.     

Transmission performance of the optical mm-wave generated by double-sideband intensity-modulation

We have theoretically investigated the transmission of the optical mm-wave generated by the double-sideband (DSB) intensity-modulation. According to our analysis and simulation, the data signals, carried by DSB optical mm-wave after transmission along fiber, are degraded greatly because of the fading effect and the time shift of the codes caused by the chromatic dispersion in fiber, and these effects limit the transmission distance of the signals. Our theory is confirmed by the experimental results.     

Optical mm-wave generation by using external modulator based on optical carrier suppression

In this paper, we have theoretically investigated the transmission performance of the optical millimeter (mm)-wave generated by using an external modulator based on optical carrier suppression for the first time. According to our theory, the data signals carried by the optical mm-wave are transmitted in the dispersion fiber without fading but are degraded greatly because of the time shift of the code edges, which still limits the transmission distance. The experimental results agree well with our theory.     

64 GHz optical millimeter-wave generation by octupling 8 GHz local oscillator via a nested LiNbO3 modulator

A novel scheme to generate a 64 GHz optical millimeter (mm)-wave via a nested LiNbO₃ Mach-Zehnder modulator with an 8 GHz local oscillator is proposed and simulated. Since the frequency response of the modulator and the local oscillator frequency are greatly reduced, the bandwidth requirements of the optical and electrical components in the transmitter are significantly decreased. The simulation results show that the generated optical mm-wave signal maintains good performance even after being transmitted over 20 km standard single-mode fiber.     

The transmission performance of the single sideband optical millimeter-wave with BPSK signal in the duplex radio-over-fiber link

This paper has investigated the transmission performance of the single sideband (SSB) optical millimeter (mm)-wave with signal carried by the sideband in BPSK format in duplex radio-over-fiber (RoF) system theoretically and numerically. The SSB optical mm-wave signal is generated by a LiNbO₃ Mach–Zehnder modulator and there exists an optimal modulation index to generate the SSB optical mm-wave with a maximal RF photocurrent. The SSB optical mm-wave is much suitable for the duplex ROF link with the uplink lightwave recovered from the downlink because the optical carrier carries no signal. In such a duplex RoF link, although there are the spurs on the optical carrier, they have little influence on the downlink and the uplink signal even if the modulation index is large.     

Optical millimeter-wave generation and transmission system for 1.25 Gbit/s downstream link using a gain switched laser

In this paper, we propose a novel and cost effective system for optical millimeter-wave (mm-wave) generation and transmission of downstream data based on a gain switched laser (GSL). The GSL produces an optical comb spectrum that can be appropriately filtered to generate two optical sidebands spaced by more than 4 times the repetition rate of the GSL. These sidebands are modulated by baseband data and then transmitted via optical fiber to the remote antenna unit (RAU). At the RAU, the two sidebands are heterodyned using a photodetector to generate the electrical modulated mm-wave signal, before demodulation using self mixing. We demonstrate the distribution of 1.25 Gbit/s data OOK modulated onto a 60 GHz carrier, similar to that used in the IEEE 802.15.3c draft standard, over fiber lengths up to 62 km.     

10 Gb/s optical carrier distributed network with W-band (0.1 THz) short-reach wireless communication system

Millimeter-wave (mm-wave) operated in W-band (75 GHz–0.11 THz) is of particular interests, since this frequency band can carry signals at much higher data rates. We demonstrate a 10 Gb/s optical carrier-distributed network with the wireless communication system. The mm-wave signal at carrier frequency of 0.1 THz is generated by a high speed near-ballistic uni-traveling carrier photodiode (NBUTC-PD) based transmitter (Tx), which is optically excited by optical short pulses. The optical pulse source is produced from a self-developed photonic mm-wave waveform generator (PMWG), which allows spectral line-by-line pulse shaping. Hence these optical pulses have high tolerance to fiber chromatic dispersion. The W-band 10 Gb/s wireless data is transmitted and received via a pair of horn antennas. The received 10 Gb/s data is envelope-detected and then used to drive an optical modulator at the remote antenna unit (RAU) to produce the upstream signal sending back to the central office (CO). 20 km single mode fiber (SMF) error free transmission is achieved. Analysis about the optimum repetition rate of the optical pulse source and the transmission performance of the upstream signal are also performed and discussed.     

Influence of the modulation index of Mach-Zehnder modulator on RoF link with ASK millimeter-wave signal

In this paper, the influence of the modulation index of LiNbO₃ Mach-Zehnder modulator on the radio over fiber (RoF) link based on single sideband (SSB) optical millimeter (mm)-wave with ASK signal is theoretically and numerically investigated. Our investigation shows that there exists an optimal modulation index to generate the SSB optical mm-wave with a maximal RF photocurrent. Although the fiber dispersion distorts the code form and degrades the performance of the RF signal demodulated from the SSB optical mm-wave after fiber transmission, it does not cause the closure of the eye diagram. However, the influence of the fiber dispersion becomes more obvious as the modulation index increases. For the duplex RoF link with the optical carrier of the uplink recovered from the downlink, a larger modulation index of the downlink causes a worse crosstalk from the downlink to the uplink.     

一种改进的双边带调制产生光毫米波的方案

提出了应用于光纤无线通信系统中一种改进的双边带调制产生光毫米波方案。在中心站采用强度调制将射频信号调制到光载波上产生一个双边带信号,滤掉中心载波后,利用光交叉复用器把双边带信号的上下边带模分开,将数据速率为2.5Gbit/s的基带信号调制到下边带模后,再与未调信号耦合后产生光毫米波,然后通过单模光纤传输至基站,在基站经过光电转换后产生电毫米波信号。从理论上对该毫米波的色散特性进行了分析,发现毫米波的接收功率不会周期的衰减。同时通过实验研究表明,下行链路信号通过光纤能传输50km而不需要色散补偿,功率代价小于1dBm。因此,该方法产生的光毫米波能有效克服光纤色散引起的信号时延而导致的基带信号退化,适合于远距离传输。     

Characterization of Rayleigh backscattering performance of CS-SSB signal in carrier distributed passive optical network

Rayleigh backscattering (RB) is an intrinsic phenomenon when light propagating in fiber and its level is fixed by the fiber type and the network configuration used. Thus, RB noise would be one of the major impairments in a carrier distributed passive optical network (PON). Previously demonstrated carrier suppressed subcarrier-amplitude modulated phase shift keying (CSS-AMPSK) modulation may be useful to effectively mitigate the RB noise. However, this modulation is basically a carrier suppressed double sidebands modulation (CS-DSB) format, in which the same information is carried by the two sidebands with equal magnitude. This is not very power-efficient for PON. Here, we further extend our RB studies by using carrier suppressed single sideband non-return-to-zero (CS-SSB-NRZ) modulation. We experimentally characterize the CS-SSB-NRZ modulation when subjected to both contributions of RB: Carrier-RB and Signal-RB. Then, numerical simulations are performed, and the results are in good match with the experiment. Finally, upstream transmission experiment is performed showing the feasibility of using the CS-SSB-NRZ signal for the carrier distributed PON. The RB performance of the SSB-NRZ signals transmitted at different distances and driven at different radio-frequency (RF) f_s is also investigated.     

Simultaneous modulation and transmission of 10-Gb/s baseband and 60-GHz microwave signals in a radio-over-fiber system

A novel radio-over-fiber (ROF) scheme simultaneously to modulate and transmit 10-Gb/s baseband and 60-GHz microwave signals on a single wavelength using single-sideband (SSB) modulation technique is proposed and verified by simulation. The scheme is based on a Dual-Parallel Mach-Zehnder Modulator (DPMZM). The scheme can carry the microwave signals on either the upper sideband (USB) or the lower sideband (LSB) only and the baseband signals on the other sideband. The optical 60-GHz millimeter wave (mm-wave) carrier is generated by means of subcarrier-multiplexing (SCM) technique to carry 155-Mb/s baseband signals while 10-Gb/s baseband signals are imposed on the original optical carrier via SSB modulation. The signals with dual services are successfully transmitted over 50-km single-mode fiber.     

A novel frequency sextupling scheme for optical mm-wave generation utilizing an integrated dual-parallel Mach-Zehnder modulator

A novel scheme is proposed for frequency sextupling mm-wave generation based on a laser and an integrated dual-parallel Mach-Zehnder modulator (MZM) without optical filter. Theoretical analysis is presented to suppress the undesired optical sidebands for the high quality generation of frequency sextupling mm-wave signal. The performance of the proposed scheme is evaluated by simulations. Utilizing the integrated MZM consisted of two sub-MZMs with extinction ratio of 30 dB, the optical sideband suppression ratio (OSSR) is as high as 29.9 dB and the radio frequency spurious suppression ratio (RFSSR) exceeds 24 dB without any optical or electrical filter. The impact of the nonideal RF driven voltage and phase difference of RF driven signal applied to two sub-MZMs of the integrated MZM on OSSR and RFSSR is discussed and analyzed. After transmission over fiber, the generated optical mm-wave signal demonstrates good performance. Furthermore, the performance of two cases for the proposed scheme is also compared.     

Frequency sextupling technique using two cascaded dual-electrode Mach–Zehnder modulators interleaved with Gaussian optical band-pass filter

A frequency sextupling technique for the generation of millimeter-wave (mm-wave) is proposed and investigated. The proposed technique is comprised of two cascaded dual-electrode Mach–Zehnder modulators (MZMs), which are interleaved with Gaussian optical band-pass filter (GOBF). The first MZM, biased at minimum transmission, is only used for optical carrier suppression modulation, and the second MZM, biased at maximum transmission, is used for both even-order optical harmonic generation and data signal modulation. The GOBF between two MZMs is used to suppress the high-order optical harmonics beyond the first-order optical harmonics. On the basis of theoretical analysis and simulated demonstrations, it is concluded that with the use of an RF signal at 10 GHz, which carries the data signal and drives the MZMs, an mm-wave signal at 60 GHz can be obtained. The simulation results show that the proposed sextuple leads to a 7 dB improvement in receiver sensitivity in comparison with the modulation technique, i.e. using two cascaded dual-electrode MZMs without GOBF. Furthermore, the eye diagrams show that the quality of generated mm-wave signal is satisfactory. The proposed technique is verified by experiments.     

Optical demultiplexing of millimeter-wave subcarriers for wireless channel distribution employing dual wavelength FBGs

An optical mm-wave demultiplexer is presented. Double sideband modulation with suppressed optical carrier and filtering properties of dual overwritten fiber Bragg gratings are the fundamentals for optical demultiplexing of mm-wave radio-on-fiber signals: using a single optical carrier, Millimeter-wave signals of 20 and 40 GHz frequencies carrying independent data are created, transmitted over fiber, demultiplexed and wireless distributed to be detected and data recovered in a mobile unit. Double sideband modulation with suppressed optical carrier yields no power penalty due to chromatic dispersion, while the filtering properties of the dual overwritten fiber Bragg gratings allow less than −40 dB electrical power interchannel leakage. Independent 2.5 Gb/s On-Off-keyed data have been successfully transmitted through the 20 and 40 GHz channels.