Buried Heterostructure Distributed Feedback Research Articles

Experimental demonstration of directly modulated buried heterostructure DFB semiconductor laser based on reconstruction equivalent chirp technique

A directly modulated buried heterostructure distributed feedback (DFB) semiconductor laser based on the reconstruction equivalent chirp (REC) technique in the 1.3 μm wavelength domain is experimentally demonstrated. The packaged laser shows a good single longitudinal mode (SLM) property, with the side mode suppression ratio (SMSR) higher than 40 dB when the injection current is higher than 20 mA, as the threshold current is 10 mA. The direct modulation performance of the laser is also delightful, with the modulation bandwidth up to 10GHz at a low injection current (40mA), and the measured spurious free dynamic range (SFDR) up to 86 dB·Hz2/3.

Directly Modulated DFB Laser on SiO$_{\bf 2}$ /Si Substrate for Datacenter Networks

Reducing the operating energy of a distributed feedback (DFB) laser is a critical issue if we are to use the device as a directly modulated light source employing wavelength division multiplexing technologies in short-distance datacom networks. A membrane buried heterostructure (BH) DFB laser on a SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> layer is one candidate for reducing the operating energy because it provides a strong carrier and optical confinement in the active region. For low-cost fabrication, we have proposed and developed a fabrication procedure that employs the buried growth of an InP layer by using a directly bonded InP-based active layer on a SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /Si substrate, which enables us to use a large-scale Si wafer. To overcome the problem of the difference between the thermal expansion coefficients of Si, SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> , and InP, we have used a thin active layer (~250 nm) on a SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /Si substrate as a template for the epitaxial growth of a III-V compound semiconductor. A lateral current injection structure is essential for fabricating a device with a 250-nm-thick template. Our fabricated DFB laser with a 73-μm cavity length exhibits a threshold current of 0.9 mA for continuous operation at room temperature and achieves lasing at up to 100 °C. We have also demonstrated 171-fJ/bit operation with a 25.8-Gb/s NRZ signal. These results indicate that the BH DFB laser on a SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /Si substrate is highly suitable for use as a transmitter for datacom applications.

Directly modulated buried heterostructure DFB laser on SiO₂/Si substrate fabricated by regrowth of InP using bonded active layer.

We describe the growth of InP layer using an ultrathin III-V active layer that is directly bonded to SiO₂/Si substrate to fabricate a buried heterostructure (BH) laser. Using a 250-nm-thick bonded active layer, we succeeded in fabricating a BH distributed feedback (DFB) laser on SiO₂/Si substrate. The use of a lateral current injection structure is important for forming a p-i-n junction using bonded thin film. The fabricated DFB laser is directly modulated by a 25.8-Gbit/s NRZ signal at 50°C. These results indicate that our fabrication method is a promising way to fabricate high-efficiency lasers at a low cost.

Reduced Temperature Dependence of Lasing Wavelength in Membrane Buried Heterostructure DFB Lasers With Polymer Cladding Layers

An athermal operation of the lasing wavelength in a membrane buried heterostructure distributed feedback (BH-DFB) laser was demonstrated by adopting a polymer (benzocyclobutene) cladding layer which has a negative temperature coefficient of refractive index. Membrane BH-DFB lasers of core thickness 65 nm were operated under room-temperature continuous-wave conditions. The temperature dependence of the lasing wavelength was measured to be 2.45times10-2 nm/degC, which is about 20%-30% of that for conventional semiconductor lasers

Reliability of etched-mesa buried-heterostructure semiconductor lasers

This paper reviews the effects of metal-contact-type buried-heterostructure (BH) interfaces and substrate quality on the reliability of BH distributed-feedback (DFB) InGaAsP/InP lasers. For electrical contact, the significant improvement in device lifetime using Au/Ti/Pt/Au p-metallization over Au/Zn/Au will be shown. The temperature and current-density-acceleration factors of the diffusion of Au spiking during life-test aging are estimated. For BH interfaces, performance degradation related to damage on the mesa sidewall generated during the etching process and regrowth processes will be discussed. For the substrate, various failure mechanisms related to the formation and propagation of dark-spot defects (DSD) will be reviewed. The authors discussed various process remedies and show that a thicker buffer may lead to reliability-performance improvement with experimental results. This paper concludes with a brief description of the time-to-failure extrapolation methodology used.

Multiple-wavelength membrane BH-DFB laser arrays

We realized multiple-wavelength laser arrays based on a membrane buried-heterostructure distributed-feedback (BH-DFB) structure fabricated by electron-beam lithography (EBL), CH/sub 4//H/sub 2/ reactive-ion etching, and regrowth by organometallic vapor-phase epitaxy (OMVPE), and investigated their wavelength controllability using two different approaches, i.e., a modulation of the DFB grating period and that of the stripe width. Under optically pumped room-temperature continuous-wave (RT-CW) operation, a wavelength fluctuation for a fixed DFB grating period was found to be less than /spl plusmn/1.2 nm for all 75 samples prepared on the same wafer. A total wavelength span of 64 nm and an average channel spacing of 3.4 nm were obtained in the DFB period modulation range of 311.25 to 335.00 nm with 1.25-nm steps. An average channel spacing of 4.1 nm was also obtained in the stripe modulation range of 1.0-2.8 /spl mu/m with 0.2-/spl mu/m steps.

Optically pumped membrane BH-DFB lasers for low-threshold and single-mode operation

High optical confinement and high index coupling in 1.55 /spl mu/m-wavelength GaInAsP-InP membrane buried heterostructure distributed feedback (BH-DFB) lasers consisting of deeply etched single-quantum-well wire-like active regions were demonstrated under room-temperature (RT) continuous-wave (CW) optical pumping. A threshold pump power of 1.5 mW and a submode suppression ratio (SMSR) of 42 dB were obtained for a 142-nm-thick membrane with a cavity length of 120 /spl mu/m and a stripe width of 2 /spl mu/m. We have also realized membrane BH-DFB laser arrays by arranging the laser cavities (10 /spl mu/m spaced 15 elements with five different grating periods). A total wavelength span of 72 nm was achieved with a small lasing wavelength fluctuation up to 1.2 nm at RT-CW condition under optical pumping. From this value, membrane thickness fluctuation was estimated to be 0.4 nm. Threshold pump power of 3.4 mW and SMSR of 45 dB were achieved in a typical device.

Reliable Operation of GaInAsP/InP Distributed Feedback Laser with Wirelike Active Regions

A continuous-wave (CW) life test of 1550 nm wavelength buried-heterostructure distributed feedback (BH-DFB) lasers with wirelike active regions, fabricated by CH4/H2 reactive-ion-etching (RIE) and organometallic vapor phase epitaxy (OMVPE), was performed at room temperature. No degradations in lasing characteristics were observed after an aging time of 8200 h at a bias current of around 10 times the threshold value.

Sub-milliampere operation of 1.55 [micro sign]m wavelength high index-coupled buried heterostructure distributed feedback lasers

Sub-milliampere operation was realised in 1.55 µm wavelength high index-coupled buried heterostructure distributed feedback (BH-DFB) lasers consisting of deeply etched double-quantum-well wirelike active regions. A threshold current Ith as low as 0.7 mA and an external differential quantum efficiency of 23%/facet were obtained for a cavity length of 200 µm with cleaved facets and a stripe width of 2.3 µm.

The properties of MOVPE grown 1.3 μm DFB MQW lasers infilled with semi-insulating InP fabricated on semi-insulating substrates

The use of optoelectronic integrated circuits (OEICs) is now emerging as a practical technology for a variety of applications, particularly in advanced telecommunications. OEICs consist of a range of devices such as lasers, waveguides, modulators, amplifiers, transistors, detectors, etc. fabricated on the same substrate. When a semi-insulating substrate is used, these devices can be electrically isolated by channel etching, resulting in a low capacitance structure with reduced electrical interference between the subcomponents. One of the devices which is particularly advantageous for this type of integration scheme is the distributed feedback (DFB) laser. The laser can be made to function more efficiently by minimizing the current flowing outside the active region. This can be achieved by surrounding the active region with semi-insulating iron doped InP. This work describes for the first time, the MOVPE growth, fabrication, and device characterization of 1.3 um buried heterostructure DFB MQW lasers, which combine the advantages of using both a semi-insulating substrate and a semi-insulating infill region in the same device structure. The potential advantage of this design scheme is improved OEIC performance as a result of, reduced capacitance and electrical crosstalk, enhanced laser output power, higher speed, increased efficiency, wider operating temperature and reduced threshold current. The laser active region consists of 8 x 140 A quantum wells of GalnAsP (λ = 1.3 μm) and 110 Abarriers of GalnAsP (λ= 1.07 μm). Single mode 1.3 urn devices of length 250 μm operating at room temperature produced threshold currents of 8 mA, efficiencies of up to 25%, output powers of 18 mW at 80 mA (pulsed), and a frequency response greater than 12GHz. The parasitic capacitance was estimated to be less than 3 pF.

InGaAs-GaAs-InGaP distributed feedback buried heterostructure strained quantum-well lasers for high-power operation at 0.98 mu m

Data are presented on device results from InGaAs-GaAs distributed feedback buried heterostructure (DFB BH) strained-quantum-well lasers with InGaP cladding layers. DFB BH lasers with a p-n InGaP current blocking junction entirely grown by a three-step MOVPE on GaAs substrates show a low laser threshold of 3.2 mA and a high output power of 41 mW with single-longitudinal-mode operation, both measured CW at RT. The monomode oscillation is obtained even at the injection current of 140 mA (44 times the laser threshold) with the side-mode suppression ratio of 35 dB and the temperature sensitivity of Bragg modes being 0.5 AA/ degrees C measured between 20 and 40 degrees C. >

High power 0.98 μm InGaAs-GaAs-InGaP distributed feedback buried heterostructure strained quantum well lasers

Data are presented on device results from InGaAs-GaAs-InGaP distributed feedback buried heterostructure strained quantum well lasers entirely grown by a three step MOVPE process showing a laser threshold of 2.9 mA and a maximum monomode output power of 58 mW, both measured CW at RT. The monomode oscillation is obtained even for an injection current of 150 mA (21 times the laser threshold) with a sidemode suppression ratio of 35 dB and the Bragg wavelength at 0.98 μm and this is, to the best of the authors' knowledge, the highest CW monomode output power ever obtained from GaAs-based DFB lasers.

Cross-sectional transmission electron microscopy parallel to the active stripe of degraded buried heterostructure distributed feedback laser devices

We demonstrate for the first time that cross-sectional transmission electron microscopy can be applied parallel to the active stripe of a distributed feedback buried heterostructure laser diode to identify the cause for degradation during reliability qualification. A dominant defect mechanism is found to be the generation of dislocation loops at or near the grating/waveguide-layer interface which subsequently propagate up through the multilayer structure. The vertical segments of the loops tend to be trapped onto the {111} planes to form a V-shaped configuration as revealed in the (011) cross section. A dislocation reaction mechanism is proposed to explain the observed dislocation configuration. Furthermore, complicated dislocation loops are grown out of the segments of dislocation threading through the active region forming &amp;lt;100≳ dark line defects revealed by the electroluminescence. The defect growth mechanism is believed to be a condensation of point defects induced by the nonradiative recombination assisted point defect migration process similar to that previously observed in degraded channeled substrate buried heterostructure lasers. The nucleation of dislocations at or near the grating/waveguide layer interface is consistent with a high interfacial strain observed in the degraded devices.

High power InGaAs-GaAs-InGaP distributed feedback buried heterostructure strained quantum well lasers grown by three step MOVPE

The first demonstration of distributed feedback InGa-AsGaAs buried heterostructure strained quantum well lasers with In0.49Ga0.51P cladding layers entirely grown by a three step MOVPE process is reported. Uncoated distributed feedback buried heterostructure lasers with a pn InGaP current blocking junction on a p+-GaAs substrate show a low laser threshold of 3.2 mA and a high output power of 45 mW both measured CW at RT.

Growth and characterization of high yield, reliable, high-power, high-speed, InP/InGaAsP capped mesa buried heterostructure distributed feedback (CMBH-DFB) lasers

The capped-mesa buried-heterostructure distributed-feedback (CMBH-DFB) laser structure requires three epitaxial growths and is designed to allow good control of the width of the active layer using straightforward chemical etching techniques. The base structure, which contains the active layer, was fabricated using a variety of epitaxial techniques: liquid-phase epitaxy, hydride vapor-phase epitaxy (VPE) and metalorganic vapor-phase epitaxy (MOVPE). The final cap growth was done using hydride VPE. High yields of low-threshold high-power DFB lasers were produced from a number of wafers at emission wavelengths of 1.3 and 1.55 mu m.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

High-power, high-speed 1.3-μm semi-insulating-blocked distributed-feedback lasers

We describe a high-performance 1.3-μm InGaAsP/InP distributed feedback buried heterostructure laser which is compatible with all-vapor-phase growth technology. Current confinement is provided by metalorganic vapor-phase growth of semi-insulating InP blocking layers. The laser has a small-signal bandwidth of 12.5 GHz, a large signal digital capability at 16 Gb/s, and maintains single-frequency operation to output powers as high as 23 mW.

Complete single longitudinal mode oscillation in a GaAlAs/GaAs distributed feedback laser with a modulated stripe width structure fabricated using reactive ion etching

A modulated stripe width structure (MSW) has been applied to a GaAlAs/GaAs distributed feedback buried heterostructure laser to obtain complete single longitudinal mode oscillation. An MSW device having a gradual modulation scheme and antireflecting films on the cleaved facets has been fabricated using reactive ion etching (RIE). Principal results include realization of a reactive ion etched second-order grating with the grooves as deep as 0.15 μm after regrowth and a modulated stripe having extremely fine definition made possible by RIE. The device had a spectrum which agreed markedly well with an analytical result. The effective functioning of the MSW structure has thereby been confirmed experimentally.

Low-threshold operation of 1.5μm buried-heterostructure DFB lasers grown entirely by low-pressure MOVPE

Buried-heterostructure (BH) distributed feedback (DFB) lasers have been fabricated by a three-step low-pressure MOVPE process. The CW threshold currents were as low as 15mA and a high differential efficiency of 0.21 W/A was obtained. The laser operated in a stable single longitudinal mode over a wide range of driving currents and under highspeed direct modulation.

Spectral linewidth estimation of a 1.5 µm range InGaAsP/InP distributed feedback laser

Spectral linewidth of a 1.5 μm range distributed feedback buried heterostructure (DFB-BH) laser in CW operation is estimated theoretically and experimentally. Considering the equivalent mirror facet loss coefficient and the confinement factor in the active layer, etc., we modified the conventional formula for the spectral linewidth of single-mode semiconductor lasers and presented a formula for the linewidth of DFB lasers. Furthermore, power-dependent linewidth measurements of a 1.5 μm range InGaAsP/InP DFB-BH laser with a window region were carried out using Fabry-Perot interferometers. The linewidth was observed to increase linearly with inverse output power. The measured result was explained by the calculated result through the modified formula. The full width at half maximum was estimated to be 50 MHz at an output power of 1 mW.

InGaAsP/InP distributed feedback buried heterostructure lasers with both facets cleaved structure

Feasibilities of InGaAsP/InP distributed feedback buried heterostructure lasers in the 1.3 and 1.5 μm wavelength regions with two cleaved facets are shown. Theoretical and experimental examinations show that the kL values ranging between 1 and 2 are sufficient for high-performance operation with the stable single longitudinal mode in a temperature range wider than 100 degrees. Distribution of lasing characteristics is investigated for 140 randomly chosen LD's from two wafers. Approximately 65 percent of the devices are found to be operable in the single longitudinal mode. A CW single longitudinal mode operation power of 20 mW at 25°C for a 1.5 μm device, and that of 40 mW (60 mW with AR coating) at 25°C for a 1.3 μm device with low threshold current are achieved. These characteristics are attained by both first- and second-order grating devices fabricated by the PH <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> addition LPE technique. The results of the aging test at 70°C and 110 mA in the automatic current control condition are also presented.