Electroabsorption Modulated Laser Research Articles

Electro-absorption Modulated Lasers with High Immunity to Residual Facet Reflection by Using Lasers with Partially Corrugated Gratings

Residual facet reflection can significantly degrade the performance of electroabsorption modulated lasers (EMLs) operating at high data rates. This issue also complicates the fabrication and characterization of the highly demanded light sources for optical interconnects and transmission. It is desired to optimize the device structure to make EMLs robust and immune to residual facet reflection. EMLs with a partially corrugated-grating DFB section are designed and optimized to have much improved tolerance to the residual optical reflection from the modulator output facet. By designing the laser section with an appropriate grating length and linear gain coefficient, the EML can have good tolerance to residual facet reflection. The analysis indicates that 100% yield can be obtained with the optimal design. If the EML needs to operate over a wide ranges of gain coefficient and facet reflection, >70% of yield can still be obtained.

Experimental study of wideband in-band full-duplex communication based on optical self-interference cancellation.

In this paper, we experimentally demonstrate and study a wideband in-band full-duplex (IBFD) wireless communication system based on optical self-interference cancellation (SIC). The optical SIC performances based on antennas for broadband IBFD are firstly evaluated within high frequency bands (> 10GHz). In this system, two electro-absorption-modulated lasers (EMLs) and a balanced photo-detector (BPD) are employed to remove the wideband self-interference within received wireless signal. By theoretical derivation and experimental verification, the impact factors of SIC are analyzed, especially for non-flatness wireless channel case. Experimental results show more than 30-dB cancellation depth in 100-MHz bandwidth with employment of horn antennas. Besides, IBFD transmission performance based on OFDM signals for different bandwidth with 11.15-GHz center frequency is also demonstrated, and ~52.2- dB•Hz2/3 spurious-free dynamic range (SFDR) is obtained.

40-Gb/s TDM-PON Downstream Link With Low-Cost EML Transmitter and APD-Based Electrical Duobinary Receiver

We report a cost-effective 40-Gb/s time-division multiplexing passive optical network downstream link using three-level electrical duobinary modulation. The transmitter consists of a compact electroabsorption-modulated laser module. The receiver contains an avalanche photodiode packaged with a transimpedance amplifier, with digitally adjustable gain and bandwidth, followed by a duobinary decoder. Real-time eye diagram and bit-error rate (BER) measurements were performed. At a preforward error correction BER of $10^{-3}$ , a power budget of 23.6 dB in back-to-back conditions is demonstrated. For a dispersion range of −215 to 128 ps/nm, the power penalty does not exceed 3 dB.

EML-Based Vector Radio-Frequency Optical Signal Generation Adopting Frequency Doubling With Precoding

In this paper, an electroabsorption modulated laser (EML)-based vector radio-frequency (RF) optical signal generation scheme adopting frequency doubling and precoding is proposed and experimentally investigated. Compared with the expensive, large-size phase modulator, and intensity modulator, the EML is quite cost-efficient and much easier to integrate. Compared with the directly modulated laser, the EML has a larger bandwidth, which provides the possibility to generate huge bandwidth vector RF optical signal. We experimentally demonstrate the 4-GBd quadrature phase shift keying (QPSK) and 2-GBd 16-ary quadrature amplitude modulation (16 QAM) vector RF optical signal generation at 20 GHz, respectively. At the back-to-back case, the bit error rate (BER) can achieve the threshold of hard-decision feedforward error correction (HD-FEC) at 3.8 × 10−3. Moreover, 20-km single mode fiber-28 transmission causes no power penalty for the 20-GHz QPSK vector RF signal. To the best of our knowledge, this is the first time frequency doubling and precoding-based photonic vector RF signal generation adopting an EML has been demonstrated.

Extended-Temperature Operation (−40 °C to +95 °C) of an EML TOSA Employing an Athermal Optical System

An electroabsorption-modulated laser (EML) transmitter optical sub-assembly (TOSA) employing an athermal optical system has been demonstrated for extended-temperature operation from −40 °C to 95 °C. The output power variation of the TOSA was held to <0.3 dB over this wide temperature range because of optical compensation using the thermo-optical properties of the plastic lens. A 0.8-dB improvement over the variation without this compensation was confirmed. Since the TOSA has sufficient electrical bandwidth for 20-Gb/s operation, clear eye openings with an extinction ratio of over 9.7 dB were obtained over the wide temperature range. The heat sink, the size of which was optimized for operation of the EML at 55 °C, enabled the TOSA to operate over the wide temperature range without thermal runaway. As a result, power consumption of <0.42 W was achieved over the temperature range.

25-Gb/s TDM Optical Link Using EMLs for Mobile Fronthaul Network of LTE-A System

We propose and demonstrate a common public radio interface (CPRI)-compliant 25.2923-Gb/s mobile fronthaul network for long term evolution-advanced (LTE-A) systems. In order to meet the CPRI specifications and the stringent latency requirement of the LTE-A fronthaul network cost-effectively, we use the synchronous time-division-multiplexing, forward-error correction, and electrical equalization techniques. To enhance the cost-effectiveness, we also use an electroabsorption-modulated laser and exploit the possibility of using simple modulation formats such as the ON–OFF keying, electrical duobinary, and four-level pulse amplitude modulation. The performance of the proposed network is evaluated experimentally in terms of the tolerances to fiber dispersion and optical reflection.

Flattened optical frequency-locked multi-carrier generation by cascading one EML and one phase modulator driven by different RF clocks

We propose a novel scheme for optical frequency-locked multi-carrier generation based on one electro-absorption modulated laser (EML) and one phase modulator (PM) in cascade driven by different sinusoidal radio-frequency (RF) clocks. The optimal operating zone for the cascaded EML and PM is found out based on theoretical analysis and numerical simulation. We experimentally demonstrate 25 optical subcarriers with frequency spacing of 12.5GHz and power difference less than 5dB can be generated based on the cascaded EML and PM operating in the optimal zone, which agrees well with the numerical simulation. We also experimentally demonstrate 28-Gbaud polarization division multiplexing quadrature phase shift keying (PDM-QPSK) modulated coherent optical transmission based on the cascaded EML and PM. The bit error ratio (BER) can be below the pre-forward-error-correction (pre-FEC) threshold of 3.8×10−3 after 80-km single-mode fiber-28 (SMF-28) transmission.

1.55 µm high speed low chirp electroabsorption modulated laser arrays based on SAG scheme.

We demonstrate a cost-effective 1.55 µm low chirp 4 × 25 Gbit/s electroabsorption modulated laser (EML) array with 0.8 nm channel spacing by varying ridge width of the lasers and using selective area growth (SAG) integration scheme. The devices for all the 4 channels within the EML array show uniform threshold currents around 18 mA and high SMSRs over 45 dB. The output optical power of each channel is about 9 mW at an injection current of 100 mA. The typical chirp value of single EML measured by a fiber resonance method varied from 2.2 to -4 as the bias voltage was increased from 0 V to 2.5 V. These results show that the EML array is a suitable light source for 100 Gbit/s optical transmissions.

Practical 12.5-Gb/s, 12.5-GHz spaced ultra-dense WDM PON.

We report a practical 12.5-Gb/s, 12.5-GHz-spaced ultra-dense wavelength-division-multiplexed passive optical network (UD-WDM PON). For the cost-effectiveness, we implement the downstream links by using electro-absorption modulated lasers (EMLs) in the 4-level pulse amplitude modulation (4PAM) format and PIN receivers, and the upstream links by using reflective semiconductor optical amplifiers (RSOAs) modulated in the quadrature phase-shift-keying (QPSK) format and low-cost self-homodyne receivers. To further enhance its cost-effectiveness, we also utilize an optical frequency comb generator, instead of a large number of wavelength-selected lasers, to provide the seed light for these colorless RSOAs. We optimize the operating conditions of the EMLs and RSOAs to maximize the power margins in the presence of the crosstalk arising from closely spaced neighboring channels and the inter-symbol interference (ISI) caused by the narrow passband of the cascaded arrayed-waveguide gratings (AWGs) as well as the limited modulation bandwidths of RSOAs. The experimental results show that we can secure the power margins of >2.5 dB for both upstream and downstream links of the proposed UD-WDM PON.

Symmetric 40-Gb/s TWDM-PON with 51-dB loss budget by using a single SOA as preamplifier, booster and format converter in ONU.

In this paper, we propose to use a semiconductor optical amplifier (SOA) in the optical network unit (ONU) to improve the loss budget in time and wavelength division multiplexed-passive optical network (TWDM-PON) systems. The SOA boosts the upstream signal to increase the output power of the electro-absorption modulated laser (EML) and simultaneously pre-amplifies the downstream signal for sensitivity improvement. The penalty caused by cross gain modulation (XGM) effect is negligible due to the low extinction ratio (ER) of upstream signal and the large wavelength difference between upstream and downstream links. In order to achieve a higher output power, the SOA is driven into its saturation region, where the self-phase modulation (SPM) effect converts the intensity into phase information and realizes on-off-keying (OOK) to phase-shifted-keying (PSK) format conversion. In this way, the pattern effect is eliminated, which releases the requirement of gain-clamping on SOA. To further improve the loss budget of upstream link, an Erbium doped fiber amplifier (EDFA) is used in the optical line terminal (OLT) to pre-amplify the received signal. For the downstream direction, directly modulated laser (DML) is used as the laser source. Taking advantage of its carrier-less characteristic, directly modulated signal shows high tolerance to fiber nonlinearity, which could support a downstream launch power as high as + 16 dBm per channel. In addition, the signal is pre-amplified by the SOA in ONU before being detected, so the sensitivity limitation for downstream link is also removed. As a result, a truly passive symmetric 40-Gb/s TWDM-PON was demonstrated, achieving a link loss budget of 51 dB.

The Dual-Electroabsorption Modulated Laser, a Flexible Solution for Amplified and Dispersion Uncompensated Networks Over Standard Fiber

We propose here a monolithically integrated dual RF access electroabsorption-modulated laser called D-EML for the generation of optical single-sideband signal for both nonreturn to zero and orthogonal frequency division multiplexing formats. The benefits of optical single-sideband over double sideband in terms of resilience against chromatic dispersion effects are presented for high-bit rate intensity modulation/direct detection transmissions over standard single mode fiber in the C-band. Both simulation and experimentation exhibit transmission distance enhancement in the case of dual modulation of the D-EML, i.e., in optical single-sideband configuration, making this optical source an interesting solution because of its low-cost, low-size, and reasonable-power consumption.

A 1.55 &lt;inline-formula&gt;&lt;tex-math&gt;$\mu$&lt;/tex-math&gt;&lt;/inline-formula&gt;m 40 Gbps EML TOSA Employing a Novel FPC Connection

We have developed a 1.55-μm 40 Gbps electro-absorption modulator laser (EML)-based transmitter optical subassembly (TOSA) using a novel flexible printed circuit (FPC). The return loss at the junctions of the printed circuit board and the FPC, and of the FPC and the ceramic feedthrough connection was held better than 20 dB at up to 40 GHz by a newly developed three-layer FPC. The TOSA was fabricated and demonstrated a mask margin of >16% and a path penalty of <0.63 dB for a 43 Gbps signal after 2.4-km SMF transmission over the entire case temperature range from −5° to 80 °C, demonstrating compliance with ITU-T G.693. These results are comparable to coaxial connector type EML modules. This TOSA is expected to be a strong candidate for 40 Gbps EML modules with excellent operating characteristics, economy, and a small footprint.

Demonstration of a Hybrid Opto-Electronic Transmitter on SoI for Access Networks

We present a hybrid integrated transmitter comprised of an InP/InGaAsP electro-absorption modulated laser (EML) electrically driven by a BiCMOS SiGe electronic circuit on 4- <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\mu{\rm m}$</tex> </formula> SoI substrate. The EML includes an electro-absorption modulator integrated together with a distributed feedback laser and a spot-size expander. The modulator driver is fabricated using the SiGe BiCMOS approach with 2.5 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$V_{\rm p\hbox{-}p}$</tex></formula> nominal output swing. Both elements have been flip-chip mounted on a thick SoI board using 14- <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\mu{\rm m}$</tex></formula> gold-tin bumps and employing a high accuracy flip-chip bonder. The photonic chip has a footprint of 7 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${\rm mm}^{2}$</tex></formula> and consumes a power of 0.8 W. We test the device optically and electrically demonstrating error-free operation at 10 and 20 Gb/s.

A Compact Optoelectronic Oscillator Based on an Electroabsorption Modulated Laser

A novel method to miniaturize the optoelectronic oscillator (OEO) is proposed and demonstrated by replacing the laser source, intensity modulator, and photodetector with an electroabsorption modulated laser (EML), because the electroabsorption modulator (EAM) in the EML can perform simultaneously photodetection and intensity modulation. A 9.945-GHz microwave signal with a phase noise of -101.31 dBc/Hz at 10 kHz offset is experimentally generated. The dependence of the phase noise on the EAM bias voltage is investigated. The EML-based OEO features low cost, simple structure, and potentially high operational frequency, which can find applications in future communications, radars, navigation, and satellite systems.

Proposal and Analysis of Distributed Reflector-Laser Diode Integrated with an Electroabsorption Modulator

A novel integrated laser, that is, a distributed reflector laser diode integrated with an electroabsorption modulator, is proposed to improve the output efficiency, single-mode stability, and chirp. The proposed laser can be realized using the selective metalorganic vapor phase epitaxy technique (that is, control of the width of the insulating mask), and its fabrication process is almost the same as the conventional electroabsorption modulated laser (EML) process except for the asymmetric coupling coefficient structure along the cavity. For our analysis, an accurate time-domain transfer-matrix-based laser model is developed. Based on this model, we perform steady-state and large-signal analyses. The performances of the proposed laser, such as the output power, extinction ratio, and chirp, are compared with those of the EML. Under 10-Gbps NRZ modulation, we can obtain a 30% higher output power and about 50% lower chirp than the conventional EML. In particular, the simulation results show that the chirp provided by the proposed laser can appear to have a longer wavelength side at the leading edge of the pulse and a shorter wavelength side at the falling edge.

Physical Media Dependent Prototype for 10-Gigabit-Capable PON OLT

In this work, we study the physical layer solutions for 10-gigabit-capable passive optical networks (PONs), particularly for an optical link terminal (OLT) including a 10-Gbit/s electroabsorption modulated laser (EML) and a 2.5-Gbit/s burst mode receiver (BM-Rx) in a novel bidirectional optical subassembly (BOSA). As unique features, a bidirectional mini-flat package and a 9-pin TO package are developed for a 10-gigabit-capable PON OLT BOSA composed of a 1,577-nm EML and a 1,270-nm avalanche photodiode BM-Rx, including a single-chip burst mode integrated circuit that is integrated with a transimpedance and limiting amplifier. In the developed prototype, the 10-Gbit/s transmitter and 2.5-Gbit/s receiver characteristics are evaluated and compared with the physical media dependent (PMD) specifications in ITU-T G.987.2 for XG-PON1. By conducting the 10-Gbit/s downstream and 2.5-Gbit/s upstream transmission experiments, we verify that the developed 10-gigabit-capable PON PMD prototype can operate for extended network coverage of up to a 40-km fiber reach.

Nonlinearity mitigation for high-speed optical OFDM transmitters using digital pre-distortion

Optical orthogonal frequency-division multiplexing (OOFDM) signal is sensitive to nonlinear distortions induced by optical modulators. We propose and experimentally demonstrate a digital pre-distortion (DPD) algorithm to linearize the optical modulators including electro-absorption modulated lasers (EML) and Mach-Zehnder modulators (MZM) used in high-speed OOFDM transmitters. By using an adaptive DPD algorithm with a learning structure, the inverse transfer function of a modulator, which is based on a polynomial model, has been obtained. In the experiment, the performance improvements with and without considering the memory effects of the DPD model are illustrated. The two typical kinds of high-speed OOFDM signals with a bit rate up to 30-Gb/s have been implemented experimentally. The results show that the nonlinear distortion induced by optical modulators can be compensated by using the DPD algorithm to substantially improve the optical modulation index.

Colorless Optical Transmitter for Upstream WDM PON Based on Wavelength Conversion

We propose and demonstrate a novel colorless optical transmitter based on all-optical wavelength conversion using a reflective semiconductor optical amplifier (RSOA) for upstream transmission in wavelength-division-multiplexed passive optical networks. The proposed colorless optical transmitter for the optical network unit is composed of an electro-absorption modulated laser (EML), an optical coupler, and an RSOA. Through cross-gain modulation in the RSOA, the upstream data from the EML pump light are imposed onto a continuous-wave probe light provided from the central office (CO). An optical delay interferometer at the CO tailors the chirp of the upstream signal to improve the bandwidth of the system and dispersion tolerance. The proposed upstream optical transmitter is based on the fast gain recovery of the RSOA governed by carrier-carrier scattering and carrier-phonon interactions. Thus, it can potentially operate at >; 10 Gb/s. Two separate wavelength bands are allocated, one for the pump signals and the other for the probe signals. Therefore, the proposed transmitter operates in a colorless manner since the EML can have any arbitrary wavelength within the pump band. We demonstrate the transmission of a 10.7-Gb/s upstream signal generated by the proposed scheme in a single-fiber loopback-configured network. The conditions for colorless operation of the proposed transmitter are investigated through experiment.

ElectroAbsorption Modulated Laser Integrated with a Semiconductor Optical Amplifier for 100-km 10.3 Gb/s Dispersion-Penalty-Free Transmission

We report, for the first time, a transmission experiment over standard fiber at 1.55 μ m using an electroabsorption modulated laser (EML) integrated with a semiconductor optical amplifier (SOA). We show clearly that negative pre-chirped signals can be obtained by adjusting both modulator and amplifier driving parameters. Negative chirp operation is evaluated through small-signal chirp measurement as well as through time-resolved dynamic chirp measurement. Signal chirp control at emission enhances transmission and increases optical reach. Thanks to the SOA gain and chirp compensation for modulator loss and chirp, dispersion-penalty-free with clear open eye diagrams over 100 km of optical fiber at 10.3 Gb/s is achieved. A transmission up to 125 km is also obtained. This novel fully integrated EML-SOA is very promising as a high-performance, compact and low-cost optical transmitter for the future access-metropolitan network convergence.

Characterization of passive optical components with ultra-fast speed and high-resolution based on DD-OFDM

The passive optical components with very fine structures in wavelength domain are very sensitive to the mechanical vibrations or thermal fluctuations. If the measurement speed is lower than the temperature and mechanical fluctuation, we cannot measure the dynamic characteristics of the optical components. In this paper, we propose and demonstrate a novel method with ultra-fast measurement speed and high-resolution based on optical channel estimation using direct-detected orthogonal frequency division multiplexing (DD-OFDM) signal, which can be used to measure the dynamic characteristics and fine structure of the passive optical components. In experiment, by using fast Fourier transform (FFT) and a low-cost electro-absorption modulated laser (EML), we can achieve the transfer function characteristics with 3.9 MHz resolution. Compared with the optical channel estimation using coherent OFDM signal reported before, the proposed measurement technique is cost-effective.