Chirp reduction of directly modulated semiconductor lasers at 10Gb/s by strong CW light injection

The influence of strong light injection on the reduction of the dynamical linewidth broadening of directly current-modulated semiconductor lasers at high bit rates is theoretically investigated and experimentally verified for 10 Gb/s NRZ pseudorandom modulation with a large current swing of 40 mA pp. Significant chirp reduction and single-mode operation are observed for bulk DFB, quantum well DFB lasers at 10 Gb/s and a weakly coupled bulk DFB laser at 8 Gb/s, so that an improvement of the transmission performance using standard monomode fibers in the 1.55 μm low-loss wavelength region can be achieved for all these laser types, where dispersion otherwise causes severe penalties for long-haul transmission. The properties of injection-locked bulk DFB and quantum well DFB lasers with respect to high bit rate modulation have been systematically studied by the use of the rate equation formalism. A dynamically stable locking range of more than 30 GHz under modulation has been found for both laser types with injection ratios higher than 0.5     

High-speed 1.5-μm compressively strained multi-quantum wellself-aligned constricted mesa DFB lasers

A great improvement in the high-speed characteristics for compressively strained multi-quantum-well (MQW) distributed-feedback (DFB) lasers with self-aligned constricted mesa structures is described. Negative wavelength detuning is an important factor in making possible the extraction of potential advantages for the compressively strained MQW DFB lasers. A 17-GHz bandwidth, which is the highest among the 1.5-μm MQW DFB lasers, is demonstrated. A wavelength chirp width of 0.42 nm at 10 Gb/s is obtained due to a reduced linewidth enhancement factor that has a magnitude of less than 2. Nonlinear damping K factor in a DFB laser with 45-nm negative detuning has drastically decreased to 0.13 ns, about half of that for unstrained MQW lasers. This is mainly due to an enhanced differential gain as large as 6.9×10 ^-12 m³/s. The estimated intrinsic maximum bandwidth is 68 GHz     

Enhanced wavelength tuning range in two-section complex-coupled DFBlasers by alternating gain and loss coupling

A novel tunable two-section complex-coupled distributed feedback (DFB) lasers were demonstrated which exhibited an enhanced wavelength tuning range beyond 7 nm by current injection only. By a simple master-and-slave type of injection current control, either in-phase gain-coupling or antiphase loss-coupling mechanism can be activated, in such a way that one of the two Bragg modes around DFB stopband will become the lasing mode, and its wavelength can be simply and independently tuned by corresponding injection current. More then 45 dB side-mode suppression ratio (SMSR) over entire tuning range were obtained. Good system performance for 375 km transmission with butt-coupled III-V Mach-Zehnder (MZ) modulator at 2.5 Gb/s over nondispersion-shifted fiber was also demonstrated     

Medium-density WDM system with Fabry-Perot laser diodes for subscriber loop applications

Medium-density wavelength division multiplexing (WDM) systems with approximately four to eight wavelengths can provide an evolution path from near-term fiber-to-the-curb systems to a broadband passive optical network. The authors demonstrate that multilongitudinal-mode Fabry-Perot laser diodes can be used in such a system, with four 622 Mb/s channels in the 1.3- mu m window. This may provide a lower-cost alternative to the use of DFB lasers for such applications. Despite a channel spacing of only 15 nm, significant wavelength misalignments of the laser diodes relative to the WDM passbands can be tolerated before filter-induced mode-partition-noise penalties are observed. The potential for extending the data rate to 1.25 Gb/s is demonstrated.<>     

Design and performance of externally modulated 1.5-μm lasertransmitter in the presence of chromatic dispersion

Key laser and modulator characteristics that impact the use of externally modulated lasers in the presence of chromatic dispersion, excluding effects due to fiber nonlinearities, are reviewed. After a brief consideration of transmission performance with directly modulated 1.5-μm DFB lasers which have limited application of up to 80 km at 2.5 Gb/s, the key design characteristics of externally modulated transmitters are discussed. Experimental results showing the effects of modulator chirp and laser linewidth at a 2.5-Gb/s transmission rate are presented. It is found that lasers with CW linewidth under 100 MHz have less than 2-dB dispersion penalty for 600 km of non-dispersion-shifted fiber. Lower dispersion penalties can be realized if the modulator chirp is tuned so as to narrow the transmitted pulses. Excellent modulator stability is demonstrated for 60 days of error- and degradation-free 2.5-Gb/s operation     

DFB laser spectra obtained by low-frequency current dither

The temporal behavior of the spectra of distributed-feedback (DFB) lasers is studied under high-bit-rate (2 Gb/s) modulation. The optical spectra are obtained by dithering the off-state laser bias current with a low-frequency (1 kHz) low-amplitude (2 mA) current. It is found that some lasers have side modes that emit only during the first ~125 ps of the pulse. During this short time interval, the intensity of the side mode emission is at least 10 dB stronger than that obtained from time averaged spectra. This technique is useful in understanding the high-speed dynamics of rapidly modulated DFB spectra     

Distributed feedback gain-coupled lasers based on InGaAs quantum-wire arrays

Fabrication and characterization of a current-injected InGaAs-GaAs quantum-wire gain-coupled distributed feedback (DFB) laser operating at 77 K at a wavelength of 923 nm are presented. Threshold current densities in broad area lasers were measured to be as low as 160 A/cm/sup 2/. The side-mode suppression ratio at twice threshold is 35 dB. A 4-/spl mu/m rib waveguide device has a threshold of 14 mA. The patterning process for the second-order DFB grating fabricated with deep UV holography and wet-chemical etching is described.     

Optimization of DFB lasers integrated with Franz-Keldysh absorptionmodulators

The influence of the residual facet reflectance on electroabsorption modulators monolithically integrated with DFB lasers is analyzed, for both index- and partly loss-coupled lasers. The optimum laser design is discussed from the standpoint of system requirements in long-distance 10 Gb/s transmission systems, requiring a minimum of chirp. The integrated device's performance will depend as much on the laser design as on the modulator. The DFB laser should be long (L>500 μm) with a coupling strength of about κ·L=2-3 and should have a high relaxation oscillation frequency. The residual reflectance should be less than 1·10^-4     

Repeated wavelength conversion of 10 Gb/s signals and convertedsignal gating using wavelength-tunable semiconductor lasers

Repeated wavelength conversion of 10 Gb/s pseudorandom nonreturn-to-zero signals is demonstrated using semiconductor lasers that can be tuned wavelength over a broad range. Error-free tunable wavelength conversion with a very low power penalty can be achieved in both first and second wavelength conversion over a broad wavelength range from 1.486-1.573 μm. The converted signal can be switched rapidly within this wavelength region. High-speed converted signal gating controlled by adjusting the bias current to the device active region is also demonstrated     

Integrated four-wavelength DFB laser array with 10 Gb/s speed and 5nm continuous tuning range

A 1.5 μm, four-wavelength DFB (distributed feedback) laser array operating at a speed of 10 Gb/s and with a continuous tuning range of 5 nm for each laser has been demonstrated. An adjacent channel electrical crosstalk penalty of 0.6 dB was measured, and the thermal tuning limitation was identified. Each laser could be modulated at a speed of 10 Gb/s with moderate electrical crosstalk penalty (~0.6 dB) from the adjacent laser. The high-speed performance was not degraded by thermal tuning up to 3.2 nm     

Dynamic and CW linewidth measurements of 1.55-μm InGaAs-InGaAsPmultiquantum well distributed feedback lasers

Measurement of the CW linewidth and frequency chirp as functions of modulation data rate and bias level for 1.55-μm InGaAsP multiquantum-well distributed feedback lasers grown by low-pressure MOCVD are presented. The results show that the CW linewidth of asymmetric facet-coated multiquantum-well DFB lasers can be as low as 2.0 MHz at 13.5 mW output power. The frequency chirp increases with modulation data rate and is significantly larger if the laser off-state is below threshold than if it is above threshold. The 20 dB down chirp widths are in the range of 1.9-5 Å for 40 mA peak-to-peak modulation current at 10 Gb/s under above-threshold bias     

FSK heterodyne transmission experiments at 560 Mbit/s and 1 Gbit/s

Optical frequency-shift-keying (FSK) signals are obtained from directly modulated distributed feedback (DFB) semiconductor lasers. Experimental studies of the direct frequency modulation (FM) characteristics of the DFB lasers show a nonuniform FM response due to the competing effects of thermal modulation of the laser active region and carrier density modulation. Equalization of the signal current to the laser is employed to produce a flat FM response from 30 kHz to 1 GHz. Optical FSK transmission and heterodyne detection experiments at 560-Mbit/s and 1-Gbit/s are conducted at a wavelength of 1497 nm. Receiver sensitivities of -39 dBm at 560 Mbit/s and -37 dBm at 1 Gbit/s are obtained. Transmission through 100 km of single-mode fiber at 1 Gbit/s is achieved with no degradation in receiver sensitivity.     

基于同一外延层结构的10Gb/s单片集成光发射模块

利用同一外延层集成工艺方法制作了10Gb/s电吸收调制器/分布反馈(DFB)半导体激光器单片集成光发射模块.在器件中引入增益耦合机制以提高单模成品率,并采用感应耦合等离子体干法刻蚀技术以降低调制器电容.集成器件阈值电流为12mA,在-2V偏置时的消光比为15dB,器件的小信号调制带宽超过10GHz.在10Gb/s调制速率下经过35km单模光纤传输后,误码率为10-12时的功率代价小于1dB.     

10Gb/s EML Module Based on Identical Epitaxial Layer Scheme

A 10Gb/s transmitter module containing an electroabsorption modulator monolithically integrated with a distributed feedback (DFB) semiconductor laser is fabricated using the identical epitaxial layer scheme.Gain-coupling mechanism is employed to improve the single mode yield of the DFB laser,while inductively coupled plasma dry etching technique is utilized to reduce the modulator capacitance.The integrated device exhibits a threshold current as low as 12mA and an extinction ratio over 15dB at －2V bias.The small signal modulation bandwidth is measured to be over 10GHz.The transmission experiment at 10Gb/s indicates a power penalty less than 1dB at a bit-error-rate of 1e－12 after transmission through 35km single mode fiber.     

Dynamic and CW linewidth measurements of buried heterostructuredistributed feedback lasers

Measurements of frequency chirp as a function of modulation current, data rate, and bias level for the etched-mesa buried heterostructure distributed feedback (EMBH-DFB) laser are presented. The results show that the chirp increases with increasing modulation current and is significantly larger if the laser off-state is below threshold than if it is above-threshold. The 20-dB down chirp widths are in the range of 4 to 6 Å for 40 mA of modulation current at 5 Gb//s under above threshold bias. Using the EMBH-DFB laser, a system experiment over 22-km fiber at 16 Gb/s has been carried out which shows less than 0.5 dB dispersion power penalty. The continuous-wave (CW) linewidths of the asymmetric facet-coated DFB lasers are in the range of 10 to 40 MHz at 6 mW of output power. This wide range arises principally from a variation of the phase of the grating at the high-reflectivity coated facet     

InAs-InAlGaAs quantum dot DFB lasers based on InP [001]

The InAs-InAlGaAs quantum dot (QD) lasers with the InAlGaAs-InAlAs material system were fabricated on distributed feedback (DFB) grating structures on InP [001]. The single-mode operation of InAs-InAlGaAs QD DFB lasers in continuous-wave mode was successfully achieved at the emission wavelength of 1.564 /spl mu/m at room temperature. This is the first observation on the InP-based QD lasers operating around the emission wavelength window of 1.55 /spl mu/m. The threshold current density of the InAs-InAlGaAs QD DFB laser with a cavity length of 1 mm and a ridge width of 3 /spl mu/m, in which one of the cleaved facets was coated with 95% high-reflection, was 1.23 kA/cm/sup 2/ (176 A/cm/sup 2/ for single QD layer). The sidemode suppression ratio value of the QD DFB laser was as high as 42 dB at the driving current of 100 mA.     

基于直接调制和外调制的高速半导体激光光源

直接调制和外调制的半导体激光光源在现代光纤通信系统中有着重要的应用。首先介绍了应用于10 Gb/s接入网系统的直接调制AlGaInAs多量子阱DFB激光器。由于AlGaInAs量子阱的导带不连续性较大,因此基于该材料的半导体激光器具有良好的温度特性,其特征温度达到了88 K。同时,该直接调制激光器的3 dB小信号调制响应带宽超过15 GHz。随后介绍面向40 Gb/s干线传输系统的高速DFB激光器/EA调制器集成光源。该集成光源采用同一外延层集成方案,并采用Al2O3高速微波热沉进行了管芯级封装,在3 V反向偏压下获得大于13 dB的静态消光比,3 dB小信号调制带宽超过40 GHz。     

3.125-Gb/s modulation up to 70/spl deg/C using 1.3-/spl mu/m VCSELs fabricated with localized wafer fusion for 10GBASE LX4 applications

The development of the 10GBASE-LX4 communication standard for aggregated 10-Gb/s rates feeds the need for low-cost laser sources in the 1275-1350-nm wavelength range operating at modulation rates of 3.125 Gb/s. We present comprehensive characterization of wafer fused vertical-cavity surface-emitting lasers with characteristics that meet the IEEE802.3ae specification for 10GBASE-LX4. These include output power greater than 1.5 mW up to 80/spl deg/C, wavelength around 1340 nm, single-mode emission and modulation at 3.125 Gb/s, and wide open eyes with rise and fall times below 100 ps up to 70/spl deg/C.     

53% wallplug efficiency 975nm distributed feedback broad area laser

100 mum-stripe, 2 mm-long, DFB diode lasers with narrow spectral widths of 2.3 Aring FWHM were achieved at a CW power of 5 W. The centre wavelength was locked at the Bragg condition and shifts at a rate of 0.065 nm K^-1. 1 mm-long DFB laser showed a record high 53% wallplug efficiency for grating-stabilised semiconductor lasers at 25degC heatsink temperature and 2 W CW output power     

Monolithic multiple wavelength surface emitting laser arrays

A two-dimensional (2-D) surface emitting laser array emitting 140 unique, nonredundant, uniformly separated, single-mode wavelengths in the 980-nm regime is described. The wavelength separation between neighboring lasers is as small as 0.3 nm. A large total wavelength span of 43 nm was obtained without compromising the performance of the lasers. All 140 lasers have nearly the same threshold currents, voltages, and resistances. The techniques used are generic and can be readily extended to both longer and shorter wavelength lasers. The authors also report the first wavelength division multiplexing system experiment using part of this laser array. A BER (bit-error ratio) of 10 ^-9 at 155 Mb/s was obtained with simultaneous operation of four lasers at a wavelength separation of 1.5 nm. Negligible optical and electrical crosstalk was observed between the lasers