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Optical Modulator With a Near-Linear Field Response
Abstract
The principle and characteristics of a linear optical modulator (LOM), which has a high field-response linearity, are discussed. A conventional Mach-Zehnder modulator (MZM) shows a nonlinear (sinusoidal) field response. This nonlinearity causes some problems in advanced digital coherent optical transmission systems that employ advanced multilevel modulation or pulse-shaping technologies. To solve this, we devised the LOM, which has a two-stage asymmetric lattice configuration with two MZMs driven with a pair of complementary voltage signals. The key idea of the LOM is to utilize the complementary output from the first MZM for generating the compensation signal. This enables us to improve the linearity without losing much optical power. Theoretical characteristics of the LOM are discussed in detail for the first time. To verify the LOM concept, we fabricated a linear in-phase-and-quadrature modulator (LIQM) with a hybrid configuration of silica planar lightwave circuits and a LiNbO3 chip with a high-speed phase-modulator array. Using the LIQM, we experimentally proved that the LOM provides a better loss-linearity trade-off than the conventional MZM does. The LIQM was also tested for 12.5-GBd 16-QAM modulation, and it generated a clear constellation where the nonlinearity of the MZM was compensated well.
... This configuration overcomes the nonlinearities and DAC/amplifier limitations by employing multiple I/Q modulators that are driven by binary electrical signals [78][79][80][81]. ...
... Those configurations require high-resolution DACs to cope with the non-linearity associated with the Mach-Zender modulator, as well as the driving power amplifiers in order to generate high quality symbol constellations and eye diagrams. While the optical hardware is simple, requiring only one I/Q modulator, the achievable transmission rate and signal quality are limited in practice by the speed and resolution of the DACs [78]. The optical domain configurations, on the other hand, overcome the non-linearity and DACs limitations by employing multiple QPSK modulators that are driven by binary electrical signals [78][79][80][81]. ...
... While the optical hardware is simple, requiring only one I/Q modulator, the achievable transmission rate and signal quality are limited in practice by the speed and resolution of the DACs [78]. The optical domain configurations, on the other hand, overcome the non-linearity and DACs limitations by employing multiple QPSK modulators that are driven by binary electrical signals [78][79][80][81]. However, those configurations are not easily scalable to higher-order modulation format and some would generate non-Gray coded symbol constellation [103], resulting in high error rate at the receiver. ...
This work presents a design for a binary driven optical square M-ary quadrature amplitude modulation (QAM) transmitter for high speed optical networks. The transmitter applies tandem quadrature phase shift keying (QPSK) modulators to eliminate the need for linear broadband amplifiers and high-resolution digital to analog converters (DACs), which are both required by conventional transmitters. The transmitter design could be scaled to any order of square M-ary QAM by simply adding more QPSK modulators in tandem. It also provides a Gray coded symbol constellation, insuring the lowest bit error rate possible during symbol recovery. We also provide the design for the coupling ratios of the optical couplers that take into account the insertion loss of the optical components, in order to generate a proper 16-QAM and 64-QAM symbol constellation with equally-spaced symbols. Additionally, we analyze the impact of coupling ratio errors as well as phase errors on the bit error rate (BER) performance and constellation diagrams. The performance is tested using the OptiSystem simulation at 50 Gbaud and under presence of additive white Gaussian noise (AWGN), which demonstrated high quality symbol constellation and a BER performance similar to theoretical expectations. For 16-QAM, a BER better than 10-4 and power penalty of about 2 dB are achieved for coupling ratio errors less than 10 %, or phase errors within ±7 degrees. The 64-QAM transmitter, on the other hand, demonstrated a BER better than 10-4 and power penalty of about 1 dB for coupling ratio errors less than 4%, or phase errors within ±2 degrees. Adviser: Lim Nguyen
... The third element to cost reduction is the search for creative solutions to address the issue of linearization of optical multilevel modulators which was not a problem before in the binary-level driven modulator [9][10][11][12]. Linearization of these modulators has become necessary, especially for 100/400 Gbit/s optical communication system. ...
... The basic idea of the design approach is to first convert and synthesize the RF multilevel signal from serial or parallel binary data sequence signals into a single analog multilevel electrical signal using Digital Analog Converter (DAC) or Arbitrary Waveform Generator (AWG) before it is injected into the optical MZ modulator as an RF drive signal. Thus, the role of the optical modulator is to insure that the electrical-to-optical conversion process is as linear as possible [9][10][11][12][13]. [34][35] that can generate 16-QAM constellations based on this approach. ...
... A significant challenge to coherent multilevel modulator-based on DP-IQ is the linearization of its output signal. Linearization is all about achieving low-distortion mapping of the electronic multilevel signal into the optical multilevel signal [9][10][11][12][13]. The critical source of the distortion is due to the nonlinear sinusoidal optical field response of each of the 4 sub-MZMs. ...
... These techniques, however, were proposed for direct detection and, therefore, there is no evidence reported so far on whether they can be appropriate for self-beating coherent systems. As recently outlined [14], this remains still an open question. ...
... We provide a discussion and a comparison of the results obtained using the two cases as well. Furthermore, we compare our approach with that based on a recently proposed near linear field response modulator (NLFRM) recently reported [14]. Finally, section 4 provides some conclusions and future directions of research. ...
... In fact, the topic of linearized electric field modulators has been scarcely addressed in the literature but is currently rising interest as coherent systems are becoming commonplace in optical fiber communications. A possible configuration for a near linear field response modulator (NLFRM) has been reported very recently in [14], which proposes a two-stage modulator configuration as shown in Fig. 4. The layout includes two MZMs, just as in the case of the DPMZM, so the overall device complexity is similar. The difference here is that the modulators are placed in tandem and there are two separate paths for signal propagation. ...
We develop, analyze and apply a linearization technique based on dual parallel Mach-Zehnder modulator to self-beating microwave photonics systems. The approach enables broadband low-distortion transmission and reception at expense of a moderate electrical power penalty yielding a small optical power penalty (<1 dB).
... Consequently, a single DP-MZM can be used to perform a similar operation [14]. However, without special linearization [15] measures the single DP-MZM arrangement is limited to small modulation indices 1 , 2 resulting in high insertion loss compared to the parallel DP-MZM solution. ...
... The power variation of the recovered signal at different phase shifts is found to be ∿1.8 dB maximum. This variation is the result of the nonlinear transmission function of the DP-MZM and may be corrected by predistortion of the in-phase and quadrature phase controls of the lower DP-MZM or by application of alternative methods [15] of the lower DP-MZM. The proposed circuit can be used up to the bandwidth of the phase modulators used to form the DP-MZM. ...
p>A means of applying an adjustable RF phase shift over a broad band of frequencies is a requirement of diverse application. Photonic solutions to the generation of RF phase shifts have receive significant attention for reasons of reduced cost, compactness and simplicity, yet the achievement of a phase shift extending beyond 360˚ range remains a challenge. The circuit architecture of a compact and broadband RF phase shifter with unbounded range based on two parallel DP-MZM architecture is presented and verified by simulation verification and emulated using off the shelf low frequency electronic components. Results demonstrate that the complex transmission of the phase shifter follows a trajectory that may encircle the origin an arbitrary number of times in either direction. The proposed architecture can be implemented using commercially available DP-QPSK modulator or can be integrated in any material platform that offers linear electro-optic phase modulators.
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... Consequently, a single DP-MZM can be used to perform a similar operation [14]. However, without special linearization [15] measures the single DP-MZM arrangement is limited to small modulation indices 1 , 2 resulting in high insertion loss compared to the parallel DP-MZM solution. ...
... The power variation of the recovered signal at different phase shifts is found to be ∿1.8 dB maximum. This variation is the result of the nonlinear transmission function of the DP-MZM and may be corrected by predistortion of the in-phase and quadrature phase controls of the lower DP-MZM or by application of alternative methods [15] of the lower DP-MZM. The proposed circuit can be used up to the bandwidth of the phase modulators used to form the DP-MZM. ...
p>A means of applying an adjustable RF phase shift over a broad band of frequencies is a requirement of diverse application. Photonic solutions to the generation of RF phase shifts have receive significant attention for reasons of reduced cost, compactness and simplicity, yet the achievement of a phase shift extending beyond 360˚ range remains a challenge. The circuit architecture of a compact and broadband RF phase shifter with unbounded range based on two parallel DP-MZM architecture is presented and verified by simulation verification and emulated using off the shelf low frequency electronic components. Results demonstrate that the complex transmission of the phase shifter follows a trajectory that may encircle the origin an arbitrary number of times in either direction. The proposed architecture can be implemented using commercially available DP-QPSK modulator or can be integrated in any material platform that offers linear electro-optic phase modulators.
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... The electro-optic modulator that maps the input signal onto the signal comb will have some degree of nonlinearity, depending on the modulator design and the driving conditions, that will cause harmonic distortion. Unlike the stochastic noise sources, this nonlinearity is deterministic and can be mitigated through highly linear modulator design or digital signal processing [23][24][25][26][27][28]. ...
... signal-signal beating) will be eliminated at the coherent receiver. We can therefore estimate the relative power of the harmonics to the fundamental by squaring and summing (24) to obtain the total harmonic distortion (THD) ...
Dual frequency combs are emerging as highly effective channelizers for radio frequency (RF) signal processing, showing versatile capabilities in various applications including Fourier signal mapping, analog-to-digital conversion and sub-sampling of sparse wideband signals. Although previous research has considered the impact of comb power and harmonic distortions in individual systems, a rigorous and comprehensive performance analysis is lacking, particularly regarding the impact of phase noise. This is especially important considering that phase noise power increases quadratically with comb line number. In this paper, we develop a theoretical model of a dual frequency comb channelizer and evaluate the signal to noise ratio limits and design challenges when deploying such systems in a high bandwidth signal processing context. We show that the performance of these dual comb based signal processors is limited by the relative phase noise between the two optical frequency combs, which to our knowledge has not been considered in previous literature. Our simulations verify the theoretical model and examine the stochastic noise contributions and harmonic distortion, followed by a broader discussion of the performance limits of dual frequency comb channelizers, which demonstrate the importance of minimizing the relative phase noise between the two frequency combs to achieve high signal-to-noise ratio signal processing.
... When the input voltage swing of null points is smaller than 2V π , the distortion can be avoided, although that results in a large optical loss. In general, digital arcsine pre-distortion or lookup table pre-distortion is used to weaken the distortion [20][21][22], but this reduces the effective resolution of the digital-to-analog converter (DAC) [22]. If the MZM is biased at the quadrature point, there is pure amplitude modulation to avoid carrier and phase recovery, but the linear region is smaller than operating at null points, and the OSNR sensitivity is relatively lower than operating at null points. ...
... When the input voltage swing of null points is smaller than 2V π , the distortion can be avoided, although that results in a large optical loss. In general, digital arcsine pre-distortion or lookup table pre-distortion is used to weaken the distortion [20][21][22], but this reduces the effective resolution of the digital-to-analog converter (DAC) [22]. If the MZM is biased at the quadrature point, there is pure amplitude modulation to avoid carrier and phase recovery, but the linear region is smaller than operating at null points, and the OSNR sensitivity is relatively lower than operating at null points. ...
A low-complexity and cost-efficient coherent detection-based 100 Gb/s or beyond passive optical network (PON) has attracted a lot of attention in recent years, as this technology offers high receiver sensitivity, colorless frequency selectivity, and linear detection enabling channel impairment compensation in the digital domain. We experimentally demonstrate the first single-wavelength 200 Gb/s coherent PON over 20 km downstream transmission with polarization division multiplexed four-level pulsed amplitude modulation (PDM-PAM-4) signals in the C-band. The intensity modulator replaces the costly in-phase/quadrature modulator, and hardware-efficient carrier recovery technologies are used for recovery of PAM-4 symbols. By using optimized Nyquist pulse shaping, the transceiver bandwidth can be reduced to within 50 GHz. A maximum power budget of 32.5 dB can be achieved for 200 Gb/s/λ PDM-PAM-4 at a bit error rate of over 20 km fiber transmission. The possibility of a 200 Gb/s/λ PON is investigated using intensity modulation and coherent detection for the first time to our knowledge.
... Second, there is a growing trend to employ multilevel advanced modulation formats (such as M-QAM) to achieve larger capacity, wider bandwidth, and higher spectral efficiency (SE) in digital coherent optical fiber communication systems. One key technology is the Digital-to-Analog Converter (DAC)-based multilevel optical transmitter that requires MZ-based linearized optical electric field modulator (LOFM) to perform the E-O conversion with high fidelity [2][3][4][5]. ...
... MZI-1 and MZI-2 have a 50:50 directional coupler, whereas the Ybranch combiner has a power split ratio of r/(1-r). It is worth noting that original implementation of Sin-Sin2 was first proposed and implemented as an active linear optical field modulator (LOFM) using 2×1 MZI arrangement for both MZI-1 and MZI-2 [3,4]. ...
... Prima facie, a linear treatment either restricts analysis to the initial linear oscillator regime in which the RF amplitude is small or requires a means of linearizing the modulator transfer function in the case of a fully established oscillation. The issue is not fundamental as there are variety of means of linearizing an MZM [31]. ...
An optoelectronic oscillator (OEO) is a time delay oscillator (TDO) that uses photonics technology to provide the long delay required to generate pristine microwave carriers. Parity-time (PT) symmetry concepts applied to an OEO offer the potential to achieve combined low phase noise and high sidemode suppression. A TDO composed of a pair of identical ring resonators coupled by a 2 × 2 coupler is modeled, and the coupler transmission matrix required for the oscillator to be − symmetric is derived. In a first configuration, the coupler is interpreted as the composition of a gain/loss block and a Mach-Zehnder interferometer (MZI) block. In practice, there are excess losses that must be compensated by a special dual amplifier with saturation characteristics compatible with PT−symmetry. The PT−symmetry phase transition determined by the gain/loss and the MZI differential phase parameters is found to be global and not local in its effect on modes. This is resolved by placing a short delay line within one arm of the MZI resulting in a frequency dependent and hence local mode-selective PT−symmetry phase transition. In addition, it is demonstrated that the first configuration may be transformed into a second but equivalent configuration as a cross-injection dual TDO with imbalanced delays. The local PT−symmetry phase transition may then be understood in terms of the Vernier effect. The cross-injection perspective facilitates the extension of the theory to a phase-only model of an established oscillation that inter alia provides an analytic prediction of the phase-noise spectral density. Advantageously, the second configuration enables the special dual amplifier to be replaced by a pair of matched but otherwise independent amplifiers. Thereby, the second configuration is amenable to practical implementation as a dual OEO using standard RF-photonic and RF-electronic components. The theory is validated by complex envelope model simulations using Simulink™ and phase model analytic results evaluated using MATLAB™. There is excellent agreement between the theoretical and simulation results.
... Prima facie, a linear treatment either restricts analysis to the initial linear oscillator regime in which the RF amplitude is small or requires a means of linearizing the modulator transfer function in the case of a fully established oscillation. The issue is not fundamental as there are variety of means of linearizing an MZM [31]. ...
An optoelectronic oscillator (OEO) is a time delay oscillator (TDO) that uses photonics technology to provide the long delay required to generate pristine microwave carriers. Parity-time (PT) symmetry concepts applied to an OEO offer the potential to achieve combined low phase noise and high sidemode suppression. A TDO composed of a pair of identical ring resonators coupled by a 2x2 coupler is modelled, and the coupler transmission matrix required for the oscillator to be PT- symmetric is derived. In a first configuration, the coupler is interpreted as the composition of a gain/loss block and a Mach-Zehnder interferometer (MZI) block. In practice, there are excess losses that must be compensated by a special dual amplifier with saturation characteristics compatible with PT- symmetry. The PT- symmetry phase transition determined by the gain/loss and the MZI differential phase parameters is found to be global and not local in its effect on modes. This is resolved by placing a short delay line within one arm of the MZI resulting in a frequency dependent and hence local mode-selective PT- symmetry phase transition. In addition, it is demonstrated that the first configuration may be transformed into a second but equivalent configuration as a cross-injection dual TDO with imbalanced delays. The local PT- symmetry phase transition may then be understood in terms of the Vernier effect. Advantageously, the second configuration enables the special dual amplifier to be replaced by a pair of matched but otherwise independent amplifiers. Thereby, the second configuration is amenable to practical implementation as a dual OEO using standard RF-photonic and RF-electronic components. The theory is validated by complex envelope model simulations using Simulink and phase model analytic results evaluated using MATLAB. There is excellent agreement between the theoretical and simulation results.
... Some of these techniques include the introduction of a dualwavelength of the light source [39]- [41] in the RoF link and an improvement in the EO modulator by using parallel and serial MZMs [42]- [48]. An alternative method uses different polarizations of optical signals [49]. ...
In this study, we perform an analytical investigation of the electro-optic (EO), Mach-Zehnder modulator (MZM) transmission boundary area with a nonlinear model. We propose a nonlinear model that represents the allowable RF input amplitude within the MZM transmission boundary area. We investigate the maximum RF input amplitude and confine its harmonics using nonlinear model theory, RoF simulation, and MZM characteristic experiments. The maximum RF input amplitude in the MZM transmission boundary area is a convenient technique that can confine harmonics without an optical amplifier, optical filter, or other optical devices in RoF links. We select several RF input amplitudes within the MZM boundary area and account for harmonic distortion in the investigation. The RF input amplitude is 0.8 times of the EO modulator switching voltages. The boundary areas show that near the switching voltage value, the harmonics-to-main RF ratio (HMR) is less than 10%. Furthermore, the maximum RF inputs are still appropriate for QPSK modulation. We evaluate the nonlinear model of these modulation signals in terms of the error vector magnitude and constellation diagram. After considering the optical fiber impairment, thermal noise, and harmonics distortion in the nonlinear model with the modulation signal of QPSK, the ACLR and EVMs satisfy the 3GPP specification. The nonlinear models are appropriate for a small or medium sized of radio-over-fiber links.
... In the case of a dominant non-linear influence on the signal, either Volterra series equalization [38][39][40] or pattern-dependent look-up table-based corrections [41,42] were demonstrated to increase the system performance. Furthermore, the nonlinear Mach-Zehnder modulator transfer function can, for instance, also be linearized by inversion of the latter [40,43] or by the design of a special modulator [44]. ...
An important challenge in optical communications is the generation of highest-quality waveforms with a Mach–Zehnder modulator with a limited electrical swing (Vpp). For this, we discuss, under limited Vpp, the influence of the waveform design on the root-mean-square amplitude, and thus, the optical signal quality. We discuss the influence of the pulse shape, clipping, and digital pre-distortion on the signal quality after the electrical-to-optical conversion. Our simulations and experiments, e.g., suggest that pre-distortion comes at the expense of electrical swing of the eye-opening and results in a lower optical signal-to-noise ratio (OSNR). Conversely, digital post-distortion provides operation with larger eye-openings, and therefore, provides an SNR increase of at least 0.5 dB. Furthermore, we find that increasing the roll-off factor increases the electrical swing of the eye-opening. However, there is negligible benefit of increasing the roll-off factor of square-root-raised-cosine pulse shaped signals beyond 0.4. The findings are of interest for single-channel intensity modulation and direct detection (IM/DD) links, as well as optical coherent communication links.
High-linearity electro-optic (EO) modulators play a crucial role in microwave photonics (MWP). Although various methods have been explored to enhance linearity in MWP links, they are often constrained by the intrinsic nonlinearity of modulator materials, the complexity of external control devices, the bulkiness of structures, and bandwidth limitations. In this study, we present an integrated thin-film lithium niobate (TFLN) linear Mach–Zehnder modulator (LMZM), showing, to our knowledge, a record-high spurious-free dynamic range (SFDR) of 121.7 dB · Hz 4/5 at 1 GHz with an optical power (OP) of 5.5 dBm into the photodetector (PD), based on a wide-bandwidth (>50 GHz ) dual-optical-mode (TE0 and TE1) co-modulated configuration with just one RF input. Additionally, compared to conventional MZMs (CMZMs), the LMZM exhibits a >10.6- dB enhancement in SFDR with an OP of >−8 dBm at 1 GHz, and maintains a 6.07-dB SFDR improvement even at 20 GHz with an OP of 0 dBm. The novel LMZM, featuring high linearity, wide bandwidth, structural simplicity, and high integration, holds significant potential as a key component in future large-scale and high-performance MWP integrated circuits.
The rapid advancements in machine learning have exacerbated the interconnect bottleneck inherent in binary logic‐based computing architectures. An interesting approach to tackle this problem involves increasing the information density per interconnect, i.e., by switching from a two‐valued to a multi‐valued logic (MVL) architecture. However, current MVL implementations offer limited overall performance and face challenges in scaling to process data signals with radix (number of logic levels) even just above 3. In this work, a novel concept is introduced for implementation of a highly scalable and fully passive inverter based on the frequency‐domain phase‐only linear manipulation of the input MVL data signal, which is encoded in the amplitude variations of an electromagnetic wave along the time axis. As a key advantage, this solution is entirely independent of the input radix. The proposed design is implemented using an optical fibre Bragg grating device. Inversion of quaternary signals is experimentally demonstrated, as well as binary and ternary signals, at a remarkable operation speed of 32 GBaud, with an estimated energy consumption of 24 fJ/bit. The proposed method is universal and can be applied to any system that supports transmission and detection of coherent waves, such as microwave, plasmonic, mechanical, or quantum.
In recent decades, the demand for computational power has surged, particularly with the rapid expansion of artificial intelligence (AI). As we navigate the post-Moore's law era, the limitations of traditional electrical digital computing, including process bottlenecks and power consumption issues, are propelling the search for alternative computing paradigms. Among various emerging technologies, integrated photonics stands out as a promising solution for next-generation high-performance computing, thanks to the inherent advantages of light, such as low latency, high bandwidth, and unique multiplexing techniques. Furthermore, the progress in photonic integrated circuits (PICs), which are equipped with abundant photoelectronic components, positions photonic-electronic integrated circuits as a viable solution for high-performance computing and hardware AI accelerators. In this review, we survey recent advancements in both PIC-based digital and analog computing for AI, exploring the principal benefits and obstacles of implementation. Additionally, we propose a comprehensive analysis of photonic AI from the perspectives of hardware implementation, accelerator architecture, and software-hardware co-design. In the end, acknowledging the existing challenges, we underscore potential strategies for overcoming these issues and offer insights into the future drivers for optical computing.
b>The circuit architecture of a compact and broadband RF phase shifter is presented along with simulation verification.</b
b>The circuit architecture of a compact and broadband RF phase shifter is presented along with simulation verification.</b
Silicon-organic hybrid integrated devices have emerging applications ranging from high-speed optical interconnects to photonic electromagnetic-field sensors. Silicon slot photonic crystal waveguides (PCWs) filled with electro-optic (EO) polymers combine the slow-light effect in PCWs with the high polarizability of EO polymers, which promises the realization of high-performance optical modulators. In this paper, a high-speed, power-efficient, low-dispersion, and compact optical modulator based on an EO polymer filled silicon slot PCW is presented. Lattice-shifted PCWs are utilized to engineer the photonic band diagram and thus enable an 8 nm-wide low-dispersion spectrum range, which is over an order of magnitude wider than that in modulators based on non-band-engineered PCWs and ring-resonators. A small voltage-length product of V π × L = 0.282 V × mm measured at 100 KHz is achieved by slow-light enhancement, corresponding to an unprecedented record-high effective in-device EO coefficient (r 33) of 1230 pm/V among silicon-organic hybrid modulators. Excluding the slow-light effect, the actual in-device r 33 is estimated to be 98 pm/V. By engineering the RC time constant via silicon doping and also utilizing a backside gate technique, the 3-dB modulation bandwidth of the device is measured to be 15 GHz. In addition, the RF power consumption of the modulator is estimated to be 24 mW at 10 GHz, and the estimated energy consumption for potential digital modulations is approximately 94.4 fJ/bit at 10 Gb/s.
The architecture and performances of a multilevel driver for pulse amplitude modulation (PAM) formats, designed and fabricated in 0.7 μm InP double-heterojunction bipolar transistor technology, are reported. The driver part is based on a power-DAC architecture which is integrated with the multiplexing stage composed of three 2:1 selectors. Up to 100 GS/s operation was validated and PAM-2, -4, -8 signals with high amplitude were measured. In particular, PAM-4 at 84 GBd and PAM-8 at 64 GBd operation was demonstrated with, respectively, a 3.7 and 4 Vpp differential output signal. This compact driver circuit is characterised by the highest merit factor in terms of high amplitude and the transmission capacity for an electronically generated multilevel signal.
We generated a 1-Tb/s single-carrier PDM-16QAM signal using a single optical modulator with external PDM emulation. To achieve the high symbol rate of 125 Gbaud, we used high-speed InP MUX-DAC modules, each consisting of six 2:1 MUXs and a 6-bit DAC in a single package, and an integrated optical modulator, which contains a generator of two orthogonal CSRZ pulse trains for spectrally efficient OTDM followed by IQ modulators for each tributary. The signal was received and demodulated without using any time-domain demultiplexing. Transmission over 80-km SSMF was demonstrated.
Digital signal pre-processing at the transmitter is a key enabler for next-generation optical transceivers. One of the major challenges faced by these transponders is the limited resolution and -3 dB electrical bandwidth of the digital-to-analog converters (DAC), severely limiting the transmission performance. On the other hand, these spectrally efficient flexible transport networks are extremely sensitive to nonlinear channel impairments, essentially limiting the maximum system reach. In this paper, we employ a simple digital pre-emphasis (DPE) algorithm to mitigate DAC-induced signal distortions, and further report on the impact of DPE algorithm on nonlinear transmission performance. We demonstrate, both numerically and experimentally, that DPE not only counteracts DAC low-pass response, allowing for higher baud rate transmission, but also enables better nonlinear channel tolerance. In particular, we first establish the maximum transmittable baud rates in back-to-back configuration for polarization multiplexed m-state quadrature amplitude modulation (PM-mQAM) formats, allowing up to 50, 46 and 44 Gbaud transmission for PM-4QAM, PM-8QAM, and PM-16QAM, respectively assuming typical DAC specifications of 16 GHz -3 dB bandwidth and 5.5 effective number of bits. Furthermore, we show that the performance improvements from DPE increase with increasing modulation order and decreasing span count, with maximum improvements of up to ~2.2 dB. In a conventional standard single mode fiber transmission link with lumped amplification, employing DPE, we report maximum reach of up to ~10 000, ~5000, and ~3000 km, for PM-4QAM, PM-8QAM, and PM-16QAM, respectively. Moreover, at pre-FEC bit-error-rate, relative reach improvements from DPE are found to be up to 21%, 25%, and 27%, respectively. Finally, we show that DPE gains are constant beyond a minimum required channel spacing, with minimum spacing requirements of 1.1 × baud rate, 1.15 × baud rate, and 1.2 × b- ud rate for PM-4QAM→PM-16QAM, respectively.
We review the recent progresses on the nonlinear perturbation pre-distortion including the modified nonlinear model for performance improvement and nonlinear terms combination for complexity reduction. With Nyquist nonlinear model, the performance shows even more improvement.
We show that joint digital companding/predistortion for modulator nonlinearity mitigation improves OSNR headroom of coherent optical OFDM systems by up to 2.2 dB (4.4 dB) compared to conventional (no) predistortion.
We examine analog-to-digital and digital-to-analog converters (ADCs and DACs), as well as digital signal processing (DSP) functions for optical coherent modems.
An analytical model of a super linear optical modulator with high spurious-free-dynamic-range (SFDR>130dB) is presented and analyzed. The linear modulator is referred to as IMPACC which stands for Interferometric Modulator with Phase-modulating And Cavity-modulating Components. The modulator is based on a unique combination of a RF-driven phase-modulator (PM) and a ring resonator (RR) within a Mach–Zehnder interferometer (MZI) configuration. Our analysis shows that our design can achieve SFDR values which are ~20dB higher than the standard MZI modulator and 3–5dB from the Ring Assisted Mach-Zehnder Interferometer (RAMZI) modulator. Both PM and RR in the IMPACC are simultaneously driven by a RF signal of the same frequency, but not necessarily the same amplitudes. The analytical model shows that the combination of these two optical elements, with the proper choice of RF-driving and device parameters, can lead to four important and compelling consequences. First, it offers a wholistic and elegant model in which the standard MZI modulator and the RAMZI modulator are just special cases of IMPACC. Second, the model offers an excellent parameter optimization methodology for fast parameter (internal and/or external) selection and performance evaluation. Third, it provides additional degree of control through the introduction of an external control parameter, the RF power split ratio (F). Lastly, it demonstrates one unique feature of IMPACC such as adaptive SFDR characteristics, where manufacturing tolerances in the transmission coefficient (τ) of the RR can be compensated with proper adjustments of the external parameter of the power split ratio (F).
We demonstrate the first integrated realization of a new modulation format-optical DQPSK. Transmission experiments using an integrated encoder demonstrate tolerance to chromatic dispersion, polarization mode dispersion, self phase modulation and low OSNR.
Optical networks are undergoing significant changes, fueled by the exponential growth of traffic due to multimedia services and by the increased uncertainty in predicting the sources of this traffic due to the ever changing models of content providers over the Internet. The change has already begun: simple on-off modulation of signals, which was adequate for bit rates up to 10 Gb/s, has given way to much more sophisticated modulation schemes for 100 Gb/s and beyond. The next bottleneck is the 10-year-old division of the optical spectrum into a fixed ¿wavelength grid,¿ which will no longer work for 400 Gb/s and above, heralding the need for a more flexible grid. Once both transceivers and switches become flexible, a whole new elastic optical networking paradigm is born. In this article we describe the drivers, building blocks, architecture, and enabling technologies for this new paradigm, as well as early standardization efforts.
Design and fabrication improvements to InP optical IQ modulators resulted in a reduced Vπ of 1.5V while maintaining 40GHz bandwidth and loss <7.5dB over C-band. Dual polarization QPSK constellations using 3Vpp drive are presented.
This paper reviews our recent study on a linear optical modulator (LOM). The LOM has a highly linear field response, which is suitable for digital coherent optical communications systems where advanced multilevel transmission signals are generated in the electrical domain using high-speed digital-to-analog converters (DACs). A twostage lattice optical configuration enables us to compensate for the nonlinearity (sinusoidal nature) of the response of a conventional Mach-Zehnder modulator (MZM), which is an obstacle to achieving low-loss and low-distortion electro-optic conversion of the multilevel signals. We experimentally proved that the LOM has an advantage over the MZM in terms of the trade-off between the linearity and intrinsic optical-power loss.
Optical modulators encode electrical signals to the optical domain and thus constitute a key element in high-capacity communication links. Ideally, they should feature operation at the highest speed with the least power consumption on the smallest footprint, and at low cost. Unfortunately, current technologies fall short of these criteria. Recently, plasmonics has emerged as a solution offering compact and fast devices. Yet, practical implementations have turned out to be rather elusive. Here, we introduce a 70 GHz all-plasmonic Mach-Zehnder modulator that fits into a silicon waveguide of 10μm length. This dramatic reduction in size by more than two orders of magnitude compared with photonic Mach-Zehnder modulators results in a low energy consumption of 25 fJ per bit up to the highest speeds. The technology suggests a cheap co-integration with electronics.
We present silicon photonic integrated circuits (PICs) based coherent optical transmitters and receivers for high-speed long-distance fiber optical transmission. High-degree photonic integration is achieved by monolithically integrating silicon electrooptic modulators, germanium photo detectors, silicon nitride-assisted on-chip polarization rotators, thermal phase shifters, and various passive silicon optical devices on a single wafer platform. We demonstrate the use of these PICs for modulating and detecting 112-Gb/s polarization-division-multiplexed quadrature phase-shift keying (PDM-QPSK) and 224-Gb/s PDM 16-ary quadrature amplitude modulation (PDM-16-QAM) signals. Transmission and coherent detection of a 112-Gb/s PDM-QPSK signal over 2560-km standard single-mode fiber is also demonstrated. The high-degree photonic integration for silicon PICs promises small-form-factor and low-power-consumption transceivers for future coherent systems that demand high cost efficiency and energy efficiency.
An ultra-broadband 6 bit digital-to-analogue converter (DAC) has been designed and fabricated in InP-HBT technology. The DAC IC includes six 2:1 MUXs and thus operates with a half-rate clock. The DAC module is equipped with 1 mm connectors for its analogue output to ensure broadband characteristics. The measured analogue output bandwidth (BW) is >40 GHz. Four-level pulse amplitude modulation signals with clear eye openings were measured at symbol rates of up to 75 GBd. This MUX-DAC module has the widest BW of any previously reported DACs and is suitable for high-symbol-rate optical transmitters.
A compact 224-Gb/s DP-16QAM InP-based modulator module including linear driver ICs and polarization multiplexing micro-optics is demonstrated. A power dissipation is 3.2 W with compatible performance with LiNbO3-based modulator in back-to-back operation.
We generate a 72-GBd single-carrier 64-QAM signal using high-speed digital- to-analog converters. We obtain a record line rate of 864Gb/s on a single wavelength and demonstrate 5-channel WDM transmission over 400 km of fiber.
128-Gb/s DP-QPSK is realized using silicon IQ modulator monolithically integrated with partial-rib polarization rotator under +/-3.25-Vpp push-pull RF driving condition. Low passive insertion loss 12-13 dB is achieved over C band.
The nonlinear (sinusoidal) response of a conventional Mach-Zehnder modulator is an obstacle to achieving low-loss and low-distortion electro-optic signal conversion in a DAC-based optical transmitter. Our linear optical modulator solves this problem.
We report on the experimental demonstration of a GaAs IQ modulator. The device consists of two “nested” Mach–Zehnder modulators for the inphase and quadrature component and is operated at a symbol rate of 25 GBd. Using QPSK, 16QAM, 32QAM and 64QAM, data rates of up to 150 Gbit/s were encoded on a single carrier in one polarization. The individual Mach–Zehnder modulators, and hence, the IQ-modulator have an electro-optic 3 dB bandwidth of 27 GHz and a 6 dB bandwidth larger than 35 GHz. The extinction ratio of the Mach–Zehnder exceeds 20 dB. The devices exhibit small footprint of 2 mm × 40 mm and can be integrated on large-area GaAs wafers using high-yield fabrication processes while providing performance similar to established lithium niobate devices.
We show that the linearity of a silicon electro-optic modulator can be improved by optimizing the embedded diode structure. Optimized designs of silicon modulators can give 5.9 dB improvement in SFDR over conventional LiNbO3 modulators.
We report 2427km, 112Gb/s DP-QPSK optical transmission using the smallest Silicon Photonic modulator and lowest power CMOS MZM driver. BER characterization of this device demonstrates comparable performance relative to a commercial LiNbO3 modulator.
Silica/LiNbO3 hybrid integration enables us to realize compact and low-loss optical modulators for advanced formats. We propose a hybrid modulator design to mitigate the difference between the thermal expansion of a silicon substrate and LiNbO3, and report the evaluation results.
We devised an optical modulator with a high field-response linearity, which is suitable for a DAC-based multilevel coherent transmitter. An advantage over a conventional MZM in terms of the linearity-loss tradeoff was demonstrated.
Organic materials combined with strongly guiding silicon waveguides open the route to highly efficient electro-optical devices. Modulators based on the so-called silicon-organic hybrid (SOH) platform have only recently shown frequency responses up to 100 GHz, high-speed operation beyond 112 Gbit/s with fJ/bit power consumption. In this paper, we review the SOH platform and discuss important devices such as Mach-Zehnder and IQ-modulators based on the linear electro-optic effect. We further show liquid-crystal phase-shifters with a voltage-length product as low as V
π
L = 0.06 V·mm and sub-μW power consumption as required for slow optical switching or tuning optical filters and devices.
We propose nonlinear tolerant single carrier frequency-division-multiplexing (SC-FDM) signal enhanced by digital pilot-tone for future high speed Ethernet transport like 400 G Ethernet. First, we discuss system configuration and the wavelength-division-multiplexed (WDM) transmission of SC-FDM signals employing polarization-division-multiplexed (PDM) 64-ary quadrature amplitude modulation (64-QAM). Next, we describe the long-haul transmission characteristics of 50 GHz-spaced 538 Gb/s × 7 ch WDM signals. We compare digital back-propagation (DBP) and digital pilot-tone for nonlinearity compensation and experimentally show that digital pilot-tone can effectively compensate the phase noise induced by inter-channel nonlinear effects with less computational complexity than DBP. Then we discuss a high-capacity transmission experiment employing 548 Gb/s PDM-64QAM SC-FDM; 102.3 Tb/s (224 × 548 Gb/s) C- and extended L-band WDM transmission is demonstrated over 240 km (3 × 80 km) of pure-silica-core fiber (PSCF) with all-Raman amplification. Thanks to the high nonlinear tolerance enhanced by pilot-tone, we can employ the 80 km repeater spacing used in conventional terrestrial systems. Assuming 20% forward error correction (FEC) overhead, a spectral efficiency of 9.1 b/s/Hz is achieved.
We experimentally characterize the performance of 100-GHz electro-optical polymer Mach-Zehnder modulators in both dual-drive and single-drive versions using broadband modulation. For the dual-drive version, we measure bit-error rate (BER) at 80 Gbit/s for differential phase-shift keying (DPSK) and >; 90 Gbit/s for on-off keying (OOK). The eye diagrams of 100 Gbit/s OOK and S21 measurement of up to 110-GHz further indicate a response bandwidth of >;100 GHz. Tunable-chirp operation is also demonstrated by changing the phase and amplitude of each driving signal. For the single-drive version, a BER of 10-9 is achieved for both 100-Gbit/s OOK and DPSK signals without optical or electrical equalization. The single-drive version shows a 7-dB bandwidth of >; 110-GHz and a chirp factor of as low as -0.02.
We fabricated an ultra-compact InP-based DP-QPSK modulator by the hybrid integration of an InP twin-IQ modulator and a PLC polarization multiplexing circuit for the first time. 112-Gb/s DP-QPSK modulation is successfully demonstrated.
We show that the linearity of a silicon electro-optic modulator can be improved by optimizing the embedded diode structure. Optimized designs of silicon modulators can give 5.9 dB improvement in SFDR over conventional LiNbO(3) modulators.
Orthogonal frequency-division multiplexing (OFDM) has a high peak-to-average ratio (PAR), which can result in low optical power efficiency when modulated through a Mach-Zehnder (MZ) modulator. In addition, the nonlinear characteristic of the MZ can cause significant distortion on the OFDM signal, leading to in-band intermodulation products between subcarriers. We show that a quadrature MZ with digital predistortion and hard clipping is able to overcome the previous impairments. We consider quantization noise and compute the minimum number of bits required in the digital-to-analog converter (D/A). Finally, we discuss a dual-drive MZ as a simpler alternative for the OFDM modulator, but our results show that it requires a higher oversampling ratio to achieve the same performance as the quadrature MZ.
We propose a highly linear radio-over-fiber system with low intermodulation distortion (IMD) using a single-drive dual-parallel Mach-Zehnder modulator (SD-DPMZM). The optical carrier is modulated in one of the two parallel modulators, while remaining unmodulated in the other one. There exists optimal working points for SD-DPMZM that makes its two kinds of origins of third-order IMD (IMD3) have opposite phase and equal intensity, and cancel each other; hence the output IMD3 is suppressed dramatically. Experimental results show that the proposed scheme can achieve a spurious-free dynamic range of up to 122.9 dB · Hz2/3, which is in agreement with the theoretical analysis. It is about 20 dB more than a conventional MZM. The error vector magnitude of the proposed scheme, for a 16-QAM 10-MSym/s signal centered at 4 GHz, is 1.94%.
Polymeric APC-CPW electrooptic modulators incorporating an inverted-rib waveguide structure were fabricated and tested for the first time. The inverted structure greatly simplifies fabrication procedures, and additionally improves propagation loss performance. Mach-Zehnder electrooptic modulators fabricated using this structure exhibited 6 dB of fiber-to-fiber insertion loss for 2.3-cm length and 6 V of driving voltage for a one-arm micro-strip electrode of 1.5-cm interaction length, at a wavelength of 1.55 /spl mu/m.
We demonstrated a linearized Y-fed directional coupler modulator in Z-cut LiNbO/sub 3/. The proof-of-concept device employed two electrode sections of opposite polarity whose lengths were chosen based on the results of our previous theoretical modeling. A highly linear modulation characteristic and improved tolerance to fabrication errors were achieved with a simple design, in excellent agreement with the theory. The linearity exceeded 90% over a wavelength range of 45 nm, centered around 1530 nm, with a maximum linearity of 97% at 1510 nm and a maximum modulation depth of 98%. By comparison, the linearity of conventional modulators is typically around 70%. Thus the device was proven to have a high linearity with a wide usable spectral range and relaxed fabrication tolerances.
A novel linearization concept to reduce the distortion products from integrated-optic modulators is proposed and analyzed. The proposed linearization concept could be used on any type of modulator with an odd transfer function. A dual parallel Mach-Zehnder modulator was linearized, and the results are compared to those obtained using a conventional linearization concept. The proposed concept gives a 40% higher modulation index compared to a concept based on reduction of the cubic nonlinearity of the modulator.< >
An electrooptic modulator which offers high linearity and low intermodulation distortion (IMD) is proposed. The modulation characteristics, harmonic and intermodulation distortions, are calculated. In the two-tone test with an optical modulation depth of 16% for each channel, the third-order IMD is expected to be as low as -95 dB. Such a linearized electrooptic modulator used in conjunction with diode-pumped solid-state lasers can be expected to outperform directly modulated injection lasers for CATV transmission systems.< >
An intersymbol interference (ISI)-suppressed optical multilevel modulation technique that is applicable to a wide range of binary and multilevel signaling is proposed. It employs binary phase-shift keying modulations that are generated by Mach-Zehnder intensity modulators as basic building blocks, and complex multilevel modulations are synthesized using interferometric addition and tandem modulations. Its feasibility and ISI suppression effect are verified in various binary and multilevel signal synthesis schemes using numerical simulations. Furthermore, the generation of ISI-suppressed zero-chirp binary and quaternary amplitude-shift keying modulations is experimentally demonstrated. Finally, its applicability to complex optical multilevel signaling is shown in the generation of a 40-Gb/s 16-level amplitude- and phase-shift keying signal, which results in 3-dB sensitivity improvement compared with the one using a conventional four-level electrical driving signal.
A general formula is given that expresses frequency chirping in
some types of external intensity modulators, such as the loss modulator,
directional-coupler-type modulator, Mach-Zehnder interferometry-type
modulator, and total-internal-reflection-type modulator. The chirping
phenomenon treated is caused by the phase modulation due to an
accompanied refractive index change. It is uniquely expressed in terms
of an α-parameter that contributes to frequency chirping in the
same manner as in the direct modulation of a semiconductor laser. In
addition, the transmission bandwidth of a single-mode fiber system using
an external modulator is discussed and compared with the results
obtained utilizing direct laser modulation
Compact LiNbO3 optical modulator for Polarization-division-multiplexing RZ-DQPSK
- shiraishi