Cooling rate in diode laser bonding

Diode laser die bonding parameters were measured for the cases of slow cool and rapid cool die bonding processes. The thermal strain, solder composition and structure, thermal impedance, and bond strength of InP based diode lasers bonded to AlN chip carriers using pre-deposited Au-Sn solder were examined. Relative to the rapid cool process, the slow cool process was found on average: to induce greater strain in the laser chips; to exhibit a larger thermal impedance in the die bonds; to produce a rougher solder structure; and, to promote alloying of the solder material and chip carrier metallization.     

Extraction of bonding strain data in diode lasers from polarization-resolved photoluminescence measurements

A technique to determine strain in diode lasers by matching measured polarization-resolved photoluminescence to a finite element method model is described. The code is shown to be able to fit well a finite element model with degree of polarization measurements to extract strain information from diode lasers and predict strain values owing to die bonding. It is useful in characterizing die bonding processes and in detecting solder voids which can impact the reliability of devices. Using the code, it is demonstrated that there are differences in the die bonding strain from different die attachments. The bonding strain from InP chips die bonded to copper and silicon chip carriers using gold–tin solder are examined.     

纳米压印制作半导体激光器的分布反馈光栅

分布反馈(DFB)光栅的制作是半导体激光器芯片的关键工艺,通过纳米压印技术在InP基片表面涂覆的光刻胶上压印出DFB光栅图形,并分别通过湿法腐蚀和干法刻蚀技术将光栅图形转移到InP基片上。所制作的DFB光栅周期为240nm(对应于1 550nm波长的DFB激光器),光栅中间具有λ/4相移结构。采用纳米压印技术制作的DFB光栅相对于通常双光束干涉法制作的光栅具有更好的均匀性以及更低的线条粗糙度,而且解决了双光束干涉法无法制作非均匀光栅的技术难题。相对于电子束直写光刻法,采用纳米压印技术制作DFB光栅具有快速与低成本的优势。采用纳米压印技术在InP基片上成功制作具有相移结构的DFB光栅,为进一步进行低成本高性能的半导体激光器芯片的制作奠定了良好基础。     

The effect of die bonding distributed feedback lasers

The effect of die-bonding-induced strain on the output spectra of gain-coupled distributed feedback (DFB) diode lasers was investigated. Bonding strain affects the in situ diffraction grating in DFB lasers and manifests itself as changes in the output spectrum. It is shown in this paper that such changes are observed in DFB lasers by comparing the spectra before and after die bonding. In addition, it was also observed that upon die bonding the lasing mode can, in some cases, flip from one side of the stopband to the other. This flip was modeled using a probability-amplitude transfer-matrix DFB laser model that included a bonding-induced perturbation of the pitch of the Bragg grating along the length of the cavity. The nonuniform strain perturbation of the pitch of the grating was determined from a finite element method simulation of a die bonded laser chip and correlated well with the strain that was deduced from measurements of the degree of polarization of photoluminescence from bonded chips.     

Longitudinal mode behaviors of 1.5 µm range GaInAsP/InP distributed feedback lasers

Longitudinal mode behaviors of asymmetric structure distributed feedback buried heterostructure (DFB-BH) lasers are examined theoretically and experimentally. A 1.5 μm range GaInAsP/InP DFB-BH laser was fabricated by a three-step LPE growth process. We measured the stopband in the spectrum of the DFB laser. It was found that no resonance mode emission occurred in the gain spectrum and its spectrum was asymmetric with respect to the Bragg wavelength. Most of the lasing power concentrated on the DFB mode adjacent to the stop-band which was determined by the Bragg condition. The measured spectrum was explained by the calculated results of the coupled wave theory with external reflectors. The asymmetric spectrum was caused by the relative position of the cleaved facet on the corrugation grating. It was shown that the asymmetric structure DFB laser, which consisted of two end facets with different reflection coefficients, gives a stable single longitudinal mode. There was no mode jump up to 2.3 times threshold. At a modulation depth of 100 percent, the ratio of the highest nonlasing mode intensity to the lasing DFB mode was estimated to be -16.0 dB.     

Distributed feedback InGaAsP/InP lasers with window region emitting at 1.5 µm range

Distributed feedback (DFB) InGaAsP/InP lasers with a window region formed at an end of the corrugated DFB region were made in order to overcome the problems inherent in the previous structures of DFB lasers with cleaved, sawed, etched, or AR-coated facets, or with an unexcited corrugation region. The window structure DFB lasers showed linear current versus output (I-L) curves, in contrast to those with a hysteresis or a kink appearing in a DFB laser with an unexcited region. Suppression of Fabry-Perot (F-P) resonances due to the two facets were sufficient enough to keep a single longitudinal mode property by DFB up to high excitation level. CW operation up to 65°C was achieved at the 1.5 μm wavelength range. Axial modes concerning the corrugated resonator were measured at about the threshold current. A stop band of a DFB laser was clearly observed with two dominant modes and much smaller submodes, which almost agreed with the axial modes predicted from a basic DFB theory.     

Reliability in InGaAsP/InP buried heterostructure 1.3 µm lasers

A study was conducted of aging-induced Sn whisker growth at the surface of Au-Sn bonding solder layers around a laser chip, as well as metallurgical reactions, especially in p-side down lasers, at the interface of the solder and laser crystal just below the active layer. These phenomena cause electrical shorts in InGaAsP/InP laser diodes, which occur suddenly in devices operated for long periods without any previous symptoms having appeared in their aging characteristics. To completely eliminate such failures, a novel assembling method in which chips were mounted p-side up on semiinsulating SiC submounts using Pb-Sn solder, was applied to InGaAsP/InP buried heterostructure (BH) lasers emitting at 1.3 μm. BH lasers assembled by this method do not suffer any shorting failure even after 8000 h operation under 60°C and 5 mW/facet output conditions. The small degradation rates obtained, for example, 3 percent/kh (median) at 60°C, certify the reliability of these improved lasers. As long as the stripe width of the active layer was optimized to be in the range of1.5-2.5 mum necessary for obtaining kink-free light output versus current properties, no detrimental changes in laser characteristics, including transverse and longitudinal modes and dynamic output response, were observed in aged lasers. In this paper, the long-term degradation modes observed are presented, and possible causes are discussed.     

Surface emitting InGaAsP/InP distributed feedback laser diode at 1.53 mu m with monolithic integrated microlens

Monolithic integration of a distributed feedback (DFB) surface-emitting laser diode with a microlens is demonstrated. The transverse and longitudinal cross-sectional views of the laser diode are illustrated. The microlens and a DFB laser structure are located on opposite sides of an n-InP substrate. 11 mA minimum continuous wave (CW) threshold current and 5 mW CW emission perpendicular to the InP substrate are achieved at room temperature using a chemically etched 45 degrees mirror. Single mode emission at 1.53 mu m is obtained. The integrated microlens, etched by ion beam and coated with aluminum oxide, provides optical beam collimation and an ultralow laser mode reflectivity of <10/sup -4/.<>     

1.5 µm range InGaAsP/InP distributed feedback lasers

Lasing characteristics of InGaAsP/InP distributed feedback (DFB) lasers in the 1.5 μm range were studied theoretically and experimentally. Wave propagation in five-layer DFB waveguides were analyzed to estimate the effect of the structural parameters on threshold conditions. A brief consideration on designing a low threshold laser and its lasing wavelength was made. DFB buried heterostructure lasers with fundamental grating emitting at 1.53 μm were prepared by liquid phase epitaxial techniques. CW operation was confirmed in the temperature rangeof -20° to 58°C, and a CW threshold current was as low as 50 mA at room temperature. A stable single longitudinal mode operation was observed both in dc condition and in modulated condition by a pseudorandom pulse current at 500 Mbits/s. No significant increase in the threshold current was observed after 1400 h continuous CW operation at 20°C.     

Precise Grating Profile Evaluation for DFB Lasers Using an Optical Metrology

A precise grating profile evaluation method for distributed-feedback (DFB) laser diodes was achieved using a simple optical metrology. Spectroscopic scatterometry, an optical-wavelength light-diffraction technique, is emerging as a fast, accurate, and nondestructive means of monitoring profiles. A photoresist grating pattern on an InP substrate and an etched InP grating with periods of 0.2 and 0.24 mum were successfully evaluated by using unpolarized simple optical metrology. A precise grating profile evaluation technique with an accuracy of nanometers enabled us to obtain an accurate coupling coefficient (kappaL) for DFB laser diodes. DFB laser diodes with well-controlled kappaL are very promising for practical use in cost-effective optical networks.     

Linewidth enhancement factor of 1.3 mu m InGaAsP/InP strained-layer multiple-quantum-well DFB lasers

The linewidth enhancement factor alpha in a 1.3 mu m InGaAsP/InP strained-layer multiple-quantum-well (SL-MQW) distributed feedback (DFB) laser has been evaluated from the relation between the frequency and intensity modulation indexes, and the spontaneous emission spectra below threshold current. It is demonstrated that the measured alpha -parameter of a 1.3 mu m SL-MQW DFB laser is about two, and is much smaller than that in a conventional bulk DFB laser. From the resonance frequency dependence on the output power, it is concluded that this reduction of the alpha -parameter originates in the increased differential gain. The reduction of wavelength chirping, as a result the low alpha -parameter, was experimentally confirmed for the SL-MQW DFB laser.<>     

Suppression of longitudinal spatial hole-burning effect inλ/4-shifted DFB lasers by nonuniform current distribution

The mode and the output properties of an asymmetric λ/4-shifted DFB (distributed-feedback) laser with a distributed injection current density along the cavity are investigated theoretically and experimentally. Coupled wave equations that describe the longitudinal spatial hole-burning phenomenon due to both distributions of a stimulated recombination carrier density and an injection current density are developed. The calculations show that an appropriate distribution of the injection current similar to the intensity profile can be effectively suppress the longitudinal spatial hole-burning effect from the aspect of threshold gai difference Δα_th. A 1.55-μm InGaAsP/InP asymmetric λ/4-shifted DFB laser was fabricated with three divided electrodes. Improvement of the linearity of the output characteristics, decrease of threshold current, stability of the single-mode property, and narrowing of the spectral linewidth by injection current and distribution along the cavity were observed     

A tractable large-signal dynamic model-application to stronglycoupled distributed feedback lasers

The modulation characteristics of DFB semiconductor lasers have been studied using a transfer matrix method combined with an appropriate rate equation analysis. The model takes into account longitudinal mode spatial hole burning, as well as the nonuniform current injection resulting from the axially varying Fermi voltage, and can be used for the efficient simulation of static, small-signal, and large-signal dynamic properties. The program is applied to the interpretation of experimental data from a strongly coupled InGaAsP/InP DFB laser. The experimental high-frequency properties of this device are well described by the simulations     

A complete small-signal equivalent circuit model of cooled butterfly-type 2.5 Gbps DFB laser modules and its application to improve high frequency characteristics

A small-signal equivalent circuit model of 2.5 Gbps DFB laser modules with butterfly-type dual-in-line packages has been proposed and verified using extracted parameters. Parameters related to the equivalent circuit have been extracted from measured S parameters using the modified two-port black box model. This model includes small-signal equivalent circuits of components used for 2.5 Gbps DFB laser modules such as DFB laser, coplanar waveguides, matching resistor, bonding wires, and thermoelectric cooler (TEC). From this equivalent circuit modeling, we show that calculated frequency characteristics of DFB lasers on submount and complete DFB laser modules are similar to their measured frequency characteristics, respectively. Based on this equivalent circuit model, we propose and demonstrate a method that can improve frequency characteristics of 2.5 Gbps DFB laser modules through both experiments and simulations.     

High slope efficiency and low noise characteristics intapered-active-stripe DFB lasers with narrow beam divergence

We report high slope efficiency and low noise characteristics in a distributed feedback (DFB) laser lasing at 1.3 μm with narrow beam divergence by employment of a laterally tapered active stripe over the whole cavity. This tapered structure is designed to realize narrow beam divergence, low threshold current, and high longitudinal mode selectivity. The fabricated tapered-active-stripe DFB lasers demonstrated of 9.20×13.40 and a record slope efficiency, for a narrow beam DFB laser, of 0.59 mW/mA. The temperature characteristics from -10°C to 85°C shows high output power at high temperature, stable single longitudinal mode oscillation and stable far-field patterns. Furthermore, for the first time ever, a low relative intensity noise characteristic of under -155 dB/Hz has been realized when butt-coupling into a single-mode fiber     

Low threshold operation of 1.55 μm GaInAsP/InP DFB-BH LDsentirely grown by MOVPE on InP gratings

1.55 μm GaInAsP/InP DFB-BH LDs on corrugated InP substrates were fabricated by only two-stage MOVPE including burying layer growth. The 9 mA minimum threshold current was achieved with both facets cleaved, which the authors believe is the lowest among MOVPE grown DFB LDs with InP grating. Up to 20 mW maximum output power and 0.21 W/A differential quantum efficiency were also attained under single longitudinal mode operation     

AlGaInAs/InP应变补偿多量子阱激光器

为了优化在长距离光纤通讯系统中采用的1.31μm波长的量子阱激光器,对AlGaInAs/InP材料的有源区应变补偿的量子阱激光器进行了设计研究。采用应变补偿的方法,根据克龙尼克-潘纳模型理论计算出量子阱的能带结构,设计出有源区由1.12%的压应变AlGaInAs阱层和0.4%的张应变AlGaInAs垒层构成。使用ALDS软件对所设计出的器件进行了建模仿真,对其进行了阈值分析和稳态分析。结果表明,在室温25℃下,该激光器具有9mA的低阈值电流和0.4W/A较高的单面斜率效率;在势垒层采用与势阱层应变相反的适当应变,可以降低生长过程中的平均应变量,保证有源区良好的生长,改善量子阱结构的能带结构,提高对载流子的限制能力,降低阈值电流,提高饱和功率,改善器件的性能。     

Properties of loss-coupled distributed feedback laser arrays forwavelength division multiplexing systems

The characteristics of loss-coupled distributed feedback (DFB) semiconductor laser arrays are investigated both theoretically and experimentally. Using simulations based on a transfer matrix method, the strong influence of the residual facet reflectivity on the singlemode yield and the statistical fluctuation of the emission wavelength for as-cleaved and AR/HR coated loss-coupled DFB lasers is pointed out and compared to purely index-coupled λ/4 phase-shifted devices. Experimental results and the fabrication techniques are given for loss-coupled 1.55 μm InGaAs/InGaAlAs/InP DFB laser arrays with four channels and integrated striped thin-film heaters, which were successfully used for fine tuning the channel spacings     

Performance comparison of gain-coupled and index-coupled DFBsemiconductor lasers

Comprehensive numerical simulations with the transmission-line laser model (TLLM) are used to compare the behavior of gain-coupled DFB lasers with index-coupled DFB lasers fabricated from identical materials. These simulations compare slope efficiency, threshold current, spectra, small-signal modulation bandwidth, maximum-intrinsic modulation bandwidth, large-signal transient response and chirp, relative-intensity-noise (RIN) spectra, and feedback sensitivity for coherence collapse. In most cases gain-coupled lasers with additional index coupling have better performance than index-coupled lasers for a given material. However, high-coupling factor index-coupled lasers do have lower threshold currents, lower RIN levels, and lower sensitivity to external feedback than gain-coupled lasers, although spatial hole burning in these devices can be disadvantageous     

Tailoring characteristics of DFB lasers with low- and high-reflection facets by using sampled gratings

Tailorability of distributed-feedback (DFB) laser characteristics by sampled grating has been pursued for DFB lasers with low- and high-reflection facets. This is based on the finding that there exists close correlation between the threshold gain difference and the axial photon distribution of the main and side modes. An effective-index transfer matrix method is used. This finding enables us to tailor DFB laser characteristics, and shows the futility of an attempt to relieve longitudinal spatial hole burning by making similar the axial photon distribution of the main and side modes. The threshold gain difference is shown to be improved by more than 15% by simple changing the arrangement of the sampled regions.