Optical Modulation Index Research Articles

Capacity-achieving symbol distributions for directly modulated laser and direct detection systems.

We investigate the capacity-achieving symbol distributions for directly modulated laser (DML) and direct-detection (DD) systems utilizing probabilistic constellation shaped pulse amplitude modulation formats. DML-DD systems utilize a bias tee to feed the DC bias current and AC-coupled modulation signals. Also, an electrical amplifier is commonly utilized to drive the laser. Thus, most DML-DD systems are subject to the constraints of the average optical power and peak electrical amplitude. We compute the channel capacity of the DML-DD systems under these constraints by using the Blahut-Arimoto algorithm to obtain the capacity-achieving symbol distributions. We also carry out experimental demonstration to verify our computation results. We show that the probabilistic constellation shaping (PCS) technique marginally increases the capacity of DML-DD system when the optical modulation index (OMI) is less than 1. However, the PCS technique allows us to increase the OMI larger than 1 without clipping distortions. As a result, we can increase the capacity of DML-DD system by using the PCS technique in comparison with the uniformly distributed signals.

Comparing the Performance of OFDM and OCDM-based Visible Light Communications: Numerical and Experimental Analysis

Numerical and experimental analysis of visible light communication (VLC) systems based on orthogonal chirp division multiplexing (OCDM) and orthogonal frequency division multiplexing (OFDM) are presented in this paper. Unlike most of the related publications, in this work we numerically compared OCDM and OFDM based VLC systems with the same equalization process, i.e., employing an one tap equalizer (OTE) in both systems. Simulation results in line-of-sight VLC channels show that both systems have the same performance when baseband multicarrier signals are used. The impact of the optical modulation index and the bias current of a previously characterized light-emitting diode were also evaluated. Nevertheless, the experimental demonstrations, in which bandpass signals were used due to the bandpass characteristic of the exploited setup, show that OFDM-based system outperforms the VLC links with OCDM signals, when the OTE equalizer is used in both systems. The performance of the OCDM-based system was improved by using a zero forcing frequency domain equalizer.

Performance evaluation of a simplified power-domain NOMA for visible light communications.

The performance of a visible light communication (VLC) system based on power-domain nonorthogonal multiple access (PD-NOMA) is experimentally evaluated in this paper. The simplicity of the adopted nonorthogonal scheme is provided by the fixed power allocation method at the transmitter and the single one-tap equalization executed before the successive interference cancellation at the receiver. The experimental results proved the successful transmission of the PD-NOMA scheme with three users in VLC links of up to 2.5m, after a proper choice of the optical modulation index. All users achieved error vector magnitude (EVM) performances below forward error correction limits in all evaluated transmission distances. At 2.5m, the user with the best performance reaches an E V M=2.3%.

Performance Analysis Of Optical Modulation Techniques Based On Reduction Of Second Order Harmonics And Chromatic Dispersion on OFDM Signals

ABSTRACT This work explains the theoretical and mathematical investigations of the transmission performance of an optical Single Sideband (SSB) modulation technique generated by a Mach Zehnder Modulator (MZM) with a 90° and 120° phase shift using hybrid coupler (Modified SSB) in addition to the Vestigial Sideband technique (VSB). The problem addressed in this paper is the effect of non-linearity of the transmitter system and the chromatic dispersion in single mode fiber due to the presence of second- and higher order harmonics. The non-linearity of the transmitter generates second and higher order harmonics, which affects the system performance leading to reduction in the data rate and increased distortion. The transmission performance of SSB, M-SSB and VSB is evaluated based on the extinction ratio of the MZM, different transmission lengths of the fiber, optical signal modulation index and the receiver sensitivity. A mathematical model has been developed for all the systems representing the modulation techniques till the second order. The baseband modulation consists of an RF signal of frequency 18 GHz and an Orthogonal Frequency Division Multiplexing (OFDM) signal with 4-QAM modulation technique converted to optical signal, using the MZM and transmitted over a SMF of distance of 75 Km. Our results show that the M-SSB technique proves to be more immune toward transmitter nonlinearity and chromatic dispersion as compared to its counterparts. In addition to this, it improves the receiver sensitivity by 6.56 dBm. The results generated by software simulation are in tandem with the mathematical model.

Photonic-assisted microwave frequency measurement towards cognitive radio

Cognitive radio frequency (RF) systems can be seen through an OODA (observe, orient, decide and act) loop, and electronic warfare (EW) can be used in some of these stages, namely the observe and the act. Instantaneous microwave frequency measurement (IFM) systems can contribute to cognitive RF technology taking the real world to the data world. In an OODA loop, IFM can be arranged as in the observe stage. Here we have proposed an IFM device that allows the adjustment of the directional couplers coupling coefficients, optical fiber lengths, modulation index and modulation formats, which makes the system tunable. The analytical model result shows good agreement with the simulation, achieving frequency differences of 130 MHz and 210 MHz for the lowest and the highest amplitude comparison function values, respectively.

Hierarchical modulation PAM4 with digital Nyquist pulse-shaped for flexible multi-ONU provisioning in NG-TDM PON

Abstract In this paper, we study the performance of digital Nyquist-shaped and hierarchical modulation four-level pulse amplitude modulation (N-HM-PAM4), which is one of the most cost-effective modulations, in the context of the intensity-modulation and direct-detection (IM/DD) enabled the next generation time division multiplexed passive optical network (NG-TDM PON). In addition, by considering the power fading (which is incurred by the chromatic dispersion (CD)) and linear and nonlinear inter-symbol interference (ISI) (which is incurred by CD and square-law photo-detection) during the transmission, vestigial side band (VSB) optical equalization, overlap frequency-domain equalization (OFDE), and iterative nonlinear ISI digital domain compensation are applied in the system to improve both bandwidth efficiency and receiver sensitivity. Simulation results show how different parameters of the N-HM-PAM4 based NG-TDM PON (i.e., optical modulation index (OMI), iterative cancellation number, power ratio, and the distance between the optical line terminal (OLT) and its optical network units (ONUs)) affect the performance of the system, which is defined as the bit error ratio (BER) and received optical power (ROP) of the ONU.

Closed-Form Expression for BER of CE-OFDM in Optical Intensity-Modulated Direct-Detection Systems

A closed-form expression for the bit-error-rate (BER) of constant envelope orthogonal frequency division multiplexing (CE-OFDM) in intensity-modulated direct-detection optical systems is provided in this letter. To the best of our knowledge, this letter is the first that provides an expression of the BER that considers the impact of electrical phase modulation index, signal bandwidth, optical modulation index, and Mach–Zehnder modulator bias point. Simulation and experimental results are included to validate the accuracy of the expression, subject to an analysis of the impact of electrical phase modulation index and the level of subcarrier mapping. With the analytical expression, we are able to reduce the computation time in design and parameter optimization of optical systems based in the CE-OFDM.

Investigation of optical power budget for expanding indoor-mobile coverage in 5G indoor distributed antenna system (DAS) supporting 32 FAs transmission

We experimentally present the maximum optical power budget in the analog IFoF-based indoor distributed antenna system (DAS) link supporting 32 FAs transmission of 5G wireless signals. For maximum optical power budget, we derive the maximum launched optical power and the optimum optical modulation index (OMI)/frequency assignment (FA) based on laser over-modulation. To investigate the effect, we analyze the impacts of nonlinear distortion of the single-mode-fiber (SMF) on the Tx side, and influence of noise and distortions of the analog IFoF-based indoor DAS link. Also, we include the avalanche photodiode (APD) as an optical receiver and the erbium-doped fiber amplifier (EDFA) as an optical power booster in mmWave 5G based indoor DAS link to meet the transmission performance at low received optical power. It is experimentally observed that the optical power budget is improved with using the laser over-modulation by 23 dB in analog IFoF based 5G indoor DAS link for 32 FAs x 100 MHz 5G wireless signals.

Linearization of multiband OFDM RoF system employing frequency-separated model based digital pre-distortion

To mitigate the nonlinear distortion of a multiband orthogonal frequency division multiplexing (OFDM) radio over fiber (RoF) system, a frequency-separated model-based digital pre-distortion (DPD) technique is proposed and experimentally demonstrated herein. Frequency-separated model DPD, when used as a pre-equalizer, mitigates nonlinearity in a transmitter. The nonlinear characteristic of an RoF system depends on the frequency band. Therefore, the proposed equalizer separates the input by frequency band and mitigates nonlinearities. A least mean square algorithm is used to estimate the variables of the proposed equalizer. Using the proposed DPD technique, performance enhancement was verified experimentally relative to the adjacent carrier leakage ratio (ACLR). The proposed technique obtained ACLR values of 27 to 30 dB. The results demonstrate that signal performance improved in terms of error vector magnitude from 6.3% to 3.5% at an optical modulation index value of 63%.

Increasing the Spectral Efficiency of DDO-CE-OFDM Systems by Multi-Objective Optimization

A multi-objective optimization was implemented to increase the spectral efficiency of direct-detection optical orthogonal frequency division multiplexing (OFDM) systems with constant-envelope (CE) signals. Based on a genetic algorithm, the optimization procedure evaluates the impact of fiber injection power in the reduction of the guard band between the optical carrier and the CE-OFDM signals, optimizing important parameters such as electrical phase modulation and optical modulation indices of the denominated DDO-CE-OFDM system. Simulation results show that a guard-band reduction around 40% was achieved in a 4 Gbps optimized DDO-CE-OFDM system with 16-QAM subcarrier mapping and -7 dBm fiber input power. Results obtained in an experimental proof-of-concept conducted to validate the optimization procedure show that this reduction can reach 66% according to a power penalty of ≈2 dB demanded by the inherent spectral broadening of CE-OFDM signals, after propagation over 40 km of standard single-mode fiber.

Theoretical CSPR Analysis and Performance Comparison for Four Single-Sideband Modulation Schemes With Kramers-Kronig Receiver

Recently, high-speed optical transmission using single-side-band (SSB) modulation with Kramers-Kronig (KK) scheme has gained great attention due to its high spectral efficiency (SE) and capability of electronic chromatic dispersion compensation (CDC), which can be widely employed in optical metro networks as a promising transmission scheme. To our knowledge, the carrier-signal power ratio (CSPR) is the key parameter to SSB-based systems and the optimum CSPRs can vary distinctly in different SSB modulation schemes. In this paper, the SSB-based KK systems with four kinds of SSB signal generation schemes are investigated, i.e. optical carrier-assisted SSB (OCA-SSB), virtual carrier-assisted SSB (VCA-SSB), SSB subcarrier modulation (SSB SCM) with IQ modulator (IQM) and with dual-drive Mach-Zehnder modulator (DDMZM) respectively. Here, the detailed theoretical derivations of these modulation processing are given and the theoretical relationship between device parameters and the CSPR has been further studied for the different SSB schemes. In order to verify those theoretical analyses, a 224-Gb/s direct detection (DD) transmission with SSB 16-ary quadrature amplitude modulation (16-QAM) signal has been demonstrated based on four SSB generation methods. The results show that theoretical derivations are closely consist with numerical analysis, such as $V_{bias}$ in the scheme of SSB SCM with IQM and optical modulation index (OMI) in SSB SCM with DDMZM. Therefore, our research can provide a theoretical guidance of appropriate device parameter configuration so as to achieve the optimum performance for SSB systems.

Power budget improvement of multi-IFoF based analog indoor-distributed antenna system (DAS) with laser over-modulation

Abstract We demonstrate the improvement of the optical power budget in multi-intermediate frequency over fiber (IFoF) based indoor-distributed antenna system (DAS) with 48 IF carriers of LTE-A signal using laser over-modulation. The laser over-modulation can improve the optical power budget by increasing the input RF power to DFB-LD until a limit of nonlinear distortion such as CTB and clipping, when the received optical power is very low. To compensate the decrease of optical power budget in analog optical link, we employ the laser over-modulation, and derive the optimum optical modulation index (OMI)/ch to enhance link budget. To investigate the effect of laser over-modulation, we analyze the noise characteristics associated with the carrier-to-noise ratio (CNR) depending on the received optical power, and compare three different optical receivers (Rx) most often used: i) PIN-photodiode (PD), ii) APD, iii) PIN-PD following an EDFA (EDFA-PIN), respectively. Using the laser over-modulation, it is experimentally observed to achieve optical power budget of 25 dB for PIN-PD, 31 dB for APD, and 33 dB for EDFA-PIN in multi-IFoF based indoor-DAS for 48 x 20 MHz LTE-A signals transmission, respectively.

Optical Modulation Enhancement Through CW Injection Locking of a Sinusoidally Modulated Fabry-Perot Laser Diode

Abstract This paper reports an interesting phenomenon of optical modulation enhancement produced by CW optical sideband injection locking of a directly modulated Fabry-Perot Laser Diode (FPLD) lasing at 1557.1 nm wavelength, the modulation being sinusoidal in nature and effected at 300 kHz and 3 MHz. This investigation is carried out for direct modulation of the FPLD effected in the RF region at 300 kHz and 3 MHz in contradistinction to earlier studies being conducted with the modulation in the microwave frequency band. A nonlinear analysis has been carried out which corroborates experimental observation on modulation enhancement. At low level of optical Intensity Modulation (IM) index, not exceeding 20 %, typical modulation enhancement of 2 % at 300 kHz and 11 % at 3 MHz modulation frequency have been observed. Larger modulation enhancement is expected at higher values of IM indices of the slave FPLD and for medium to high power laser diodes. This circuit can have direct application in Wavelength Division Multiplexed-Passive Optical Network (WDM-PON).

Code reservation enabled PAPR reduction of digital CDM based channel aggregation for mobile fronthaul.

A code reservation technique is proposed to reduce the peak-to-average power ratio (PAPR) of digital code-division multiplexing (CDM) based channel aggregation for mobile fronthaul. We numerically investigate the relationship between the PAPR and the number of aggregated channels during the CDM based channel aggregation, and experimentally verify the transmission performance with the code reservation technique. The PAPR of aggregated signal, which is composed of 48x20MHz Long Term Evolution (LTE) signal mapped with 64 quadrature amplitude modulation (QAM), is reduced with the help of code reservation technique. The PAPR reduction enables larger optical modulation index (OMI) per channel at the linear operation region of a directly modulated laser (DML), leading to the optical signal-to-noise ratio (OSNR) improvement. After the transmission over 10km standard single mode fiber (SSMF), the receiver sensitivity can be improved by 4dB owing to the PAPR reduction.

Optical single side-band Nyquist PAM-4 transmission using dual-drive MZM modulation and direct detection.

We present the design and optimization of the optical single side-band (SSB) Nyquist four-level pulse amplitude modulation (PAM-4) transmission using dual-drive Mach-Zehnder modulator (DDMZM)modulation and direct detection (DD), aiming at the C-band cost-effective, high-speed and long-distance transmission. At the transmitter, the laser line width should be small to avoid the phase noise to amplitude noise conversion and equalization-enhanced phase noise due to the large chromatic dispersion (CD). The optical SSB signal is generated after optimizing the optical modulation index (OMI) and hence the minimum phase condition which is required by the Kramers-Kronig (KK) receiver can also be satisfied. At the receiver, a simple AC-coupled photodiode (PD) is used and a virtual carrier is added for the KK operation to alleviate the signal-to-signal beating interference (SSBI).A Volterra filter (VF) is cascaded for remaining nonlinearities mitigation. When the fiber nonlinearity becomes significant, we elect to use an optical band-pass filter with offset filtering. It can suppress the simulated Brillouin scattering and the conjugated distortion by filtering out the imaging frequency components. With our design and optimization, we achieve single-channel, single polarization 102.4-Gb/s Nyquist PAM-4 over 800-km standard single-mode fiber (SSMF).

RoF-Based Mobile Fronthaul Networks Implemented by Using DML and EML for 5G Wireless Communication Systems

We investigate the feasibility of implementing a mobile fronthaul network (MFN) based on the radio-over-fiber (RoF) technology for the fifth generation wireless communication systems cost effectively by using either directly modulated lasers (DMLs) or electroabsorption modulated lasers (EMLs) operating in the 1.55- μ m window. The results show that the performance of the RoF-based MFN implemented by using DMLs is primarily limited by the composite second-order (CSO) distortions arising from the interplay between the DML's adiabatic chirp and fiber's chromatic dispersion. Thus, when we implement the RoF-based MFN by using the currently available commercial DML, its reach could be limited to ∼5 km due to the CSO distortions. To increase this reach, we can utilize the DML having a very small adiabatic chirp (or the optical dispersion compensation or CSO cancelation technique). On the other hand, when we implement the RoF-based MFN by using EML, its performance is limited mostly by the relatively poor linearity and low output power of EML rather than the chirp. For example, if we increase the optical modulation index to compensate for the low output power of EML, the distortions caused by the EML's nonlinear transfer curve can also be increased. Thus, for the use in the RoF-based MFN, it would be highly desirable to increase the output power of EML as much as possible. For a demonstration, we successfully transmit 24 100-MHz-bandwidth filtered orthogonal-frequency-division-multiplexing signals over 20 km of the standard single-mode fiber by using an EML transmitter with 7-dBm output power.

Electrical constant envelope signals for nonlinearity mitigation in coherent-detection orthogonal frequency-division multiplexing systems

A theoretical analysis and a numerical investigation of 3-dB peak-to-average power ratio electrical constant envelope over orthogonal frequency-division multiplexing (OFDM) signals in coherent-detection optical (CO) systems are proposed. A 100-Gb/s optical system is studied to increase the nonlinear tolerance (NLT) to both Mach–Zehnder modulation and optical propagation impairments. Simulation results show that, with 16- and 64-QAM subcarrier modulations, the proposed system outperforms a conventional CO-OFDM system, if an optical modulation index OMI=2.5 and an electrical phase modulation index 2πh=3 are simultaneously adopted. The achieved NLT denoted by a performance gain of 24 dB is attractive after propagation through 1200 km of dispersion-uncompensated standard single-mode fiber, especially for 10-dBm fiber input power and despite the intrinsic bandwidth enlargement of PMs.

Assessment of the DC Bias to Mitigate the Clipping Noise in DCO-OFDM, ACO-OFDM; and Non-linear Distortion of DFB Laser Transmitted through Dispersive Single Mode Fibers in IM/DD Systems

An analytical investigation on the impact of the DC bias on the clipping noise and the laser non-linearity in Asymmetrically Clipped Optical Orthogonal Frequency Division Multiplexing (ACO-OFDM) and Direct Current--Biased Optical OFDM (DCO-OFDM) is performed. Clipping noise in the OFDM system is introduced either by transmitter non-linearity or due to the low power sensitivity of the receiver. However the clipping noise in the DCO-OFDM is also dependent upon the DC bias applied to make the bipolar OFDM signal unipolar. The addition of this extra DC bias increases the average signal power of the DCO-OFDM signal. The non-linearity of the DFB laser increases with the optical modulation index, which gives rise to intermodulation and harmonic distortions at the receiver. The OFDM signal when transmitted through the Single Mode Fiber (SMF) suffers from group--velocity dispersion (GVD) which limits its performance in the Passive Optical Networks (PON). In this paper, an assessment of the DC bias to reduce the clipping noise in the two systems under consideration is performed. A practical laser model is considered and analysis is performed to estimate the average laser drive power to maintain its non-linearities within a tolerable region for both the OFDM systems. A simulation is performed to transmit 5 and 8 Gb/s (ACO and DCO) OFDM signal over 120 km SMF where the effect of GVD is taken into consideration. An optimum OFDM drive signal power is calculated to maintain the laser transmitter in its linear region such that the effect of second order harmonic distortions (HD2) and third order intermodulation distortion (IMD3) is low. The SNR requirement of DCO-OFDM being higher than the ACO-OFDM leads to more clipping noise, HD2 and IMD3.

Calculation of Receiver Sensitivities in (Orthogonal) Subcarrier Multiplexing Microwave-Optical Links

Microwave-based all-analogue (orthogonal) subcarrier multiplexing (SCM) permits a direct processing of baseband data at Gbit/s while achieving low power consumption, low latency, low cost, and tolerance to dispersion. A key figure of merit in any SCM link is the sensitivity in the receiver, which depends on the transmitter, the link and the receiver. By analysing the impact of the nonlinearities of an optical IQ modulator in the presence of optical noise, sensitivities are mathematically estimated as a function of the optical modulation index (OMI) at the transmitter. The results are verified with simulations achieving a good agreement with the mathematical model. The theoretical model provided can be employed as a tool to predict the best achievable sensitivities and the optimum OMI in broadband SCM and orthogonal SCM links.

Nonlinear distortion evaluation in a directly modulated distributed feedback laser diode-based fiber-optic cable television transport system

This study evaluated a directly modulated distributed feedback (DFB) laser diode (LD) for cable TV systems with respect to carrier-to-nonlinear distortion of LDs. The second-order distortion-to-carrier ratio is found to be proportional to that of the second-order coefficient-to-first-order coefficient of the DFB laser diode driving current and to the optical modulation index (OMI). Furthermore, the third-order distortion-to-carrier ratio is proportional to that of the third-order coefficient-to-first-order coefficient of the DFB laser diode driving current, and to the OMI2.