Self-heating control of edge emitting and vertical cavity surface emitting lasers

Self-heating leads to temperature rise of laser diode and limits the output power, efficiency and modulation bandwidth due to increased loss and decreased differential gain. The main heat sources in laser diode during continuous wave operation are Joule heating and free carrier absorption loss. To control device self-heating, the epi structure needs to be designed with low electrical resistance and low absorption loss, while the heat flux must spread out of the device efficiently. This dissertation presents the control of self-heating of both edge emitting laser diodes and vertical cavity surface emitting lasers (VCSELs). For the 980nm high power edge emitting laser, asymmetric waveguide is used for low free carrier absorption loss. The waveguide and cladding materials are optimized for high injection efficiency. BeO heatsink is applied to spread the heat efficiently. Injection efficiency of 71% and internal loss of 0.3 cm-1 have been achieved. A total output power of 9.3 W is measured from 0.5cm long device at 14.5A injection current. To further reduce the internal loss, the development of 980nm quantum dot active region is studied. Threshold current density as low as 59A/cm2 is reached. For the VCSELs, oxide-free structure is used to solve the self-heating problem of oxide VCSELs. Removing the oxide layer and using AlAs in the DBRs leads to record low thermal resistance. Optimization of the DBRs leads to low resistance and low free carrier absorption. Power conversion efficiency higher than 50% is achieved. To further reduce device voltage and heat generation, the development of intracavity contacts devices is introduced

Laser materials for the 0.67-microns to 2.5-microns range

Basic requirements for obtaining injection laser action in III-V semiconductors are discussed briefly. A detailed review is presented of materials suitable for lasers emitting at 0.67, 1.44, 1.93, and 2.5 microns. A general approach to the problem is presented, based on curves of materials properties published by Sasaki et al. It is also shown that these curves, although useful, may need correction in certain ranges. It is deduced that certain materials combinations, either proposed in the literature or actually tried, are not appropriate for double heterostructure lasers, because the refractive index of the cladding material is higher than the index of the active material, thus resulting in no waveguiding, and high threshold currents. Recommendations are made about the most promising approach to the achievement of laser action in the four wavelengths mentioned above

Effect of nonuniformity in temperature distribution on the performance of a stripe-geometry double-heterostructure laser

A theoretical model is developed to study the nonuniform temperature distribution in the laser cavity and its effect on the radiation pattern, threshold current, and emission spectrum of a gain-guiding stripe-geometry double-heterostructure laser. This model takes into account lateral current spreading, carrier out-diffusion, two dimensional heat diffusion, and the resultant gain and refractive index variation parallel to the junction planes. The effective dielectric constant method with parabolic approximation was used in solving the resulting two dimensional wave equation;Calculated results are generally in good agreement with existing experimental data. According to the model, lateral thermal guiding exists, in addition to gain-guiding, for stripe-geometry lasers under CW operation. This results in a reduction of lateral fundamental mode width and also affects the higher order mode intensity profiles. For instance, the intensity peaks of the first order lateral mode are shifted toward the center region of the stripe, thus increasing the mode gain. This may result in an optical nonlinearity at a lower radiation power level. Thermal guiding was found to be relatively important in thick active layer, narrow stripe, proton-bombarded lasers;The result also shows that the product of thermal resistance and threshold current at room temperature should be reduced to improve performance at elevated temperature. Reduction of active layer thickness up to the vicinity of 0.1 (mu)m decreases junction heating by lowering the threshold current without a substantial change in thermal resistance. For a GaAs/AlGaAs laser, a thin P-AlGaAs layer is also desirable for lower junction heating since the thickness of the layer effects thermal resistance of the device significantly. The effect of other laser dimensions, including stripe width and laser cavity length, was also included in the discussion of an optimal thermal design;The model is applicable to different stripe-geometry lasers as well as other III-V compound lasers such as the InGaAsP/InP device

Diode pumped Nd:YAG laser development

A low power Nd:YAG laser was constructed which employs GaAs injection lasers as a pump source. Power outputs of 125 mW TEM CW with the rod at 250 K and the pump at 180 K were achieved for 45 W input power to the pump source. Operation of the laser, with array and laser at a common heat sink temperature of 250 K, was inhibited by difficulties in constructing long-life GaAs LOC laser arrays. Tests verified pumping with output power of 20 to 30 mW with rod and pump at 250 K. Although life tests with single LOC GaAs diodes were somewhat encouraging (with single diodes operating as long as 9000 hours without degradation), failures of single diodes in arrays continue to occur, and 50 percent power is lost in a few hundred hours at 1 percent duty factor. Because of the large recent advances in the state of the art of CW room temperature AlGaAs diodes, their demonstrated lifetimes of greater than 5,000 hours, and their inherent advantages for this task, it is recommended that these sources be used for further CW YAG injection laser pumping work

Lithographic Vertical-cavity Surface-emitting Lasers

Remarkable improvements in vertical-cavity surface-emitting lasers (VCSELs) have been made by the introduction of mode- and current-confining oxide optical aperture now used commercially. However, the oxide aperture blocks heat flow inside the device, causing a larger thermal resistance, and the internal strain caused by the oxide can degrade device reliability, also the diffusion process used for the oxide formation can limit device uniformity and scalability. Oxide-free lithographic VCSELs are introduced to overcome these device limitations, with both the mode and current confined within the lithographically defined intracavity mesa, scaling and mass production of small size device could be possible. The 3 μm diameter lithographic VCSEL shows a threshold current of 260 μA, differential quantum efficiency of 60% and maximum output power density of 65 kW/cm2 , and shows single-mode singlepolarization operation with side-mode-suppression-ratio over 25 dB at output power up to 1 mW. The device also shows reliable operation during 1000 hours stress test with high injection current density of 142 kA/cm2 . The lithographic VCSELs have much lower thermal resistance than oxide-confined VCSELs due to elimination of the oxide aperture. The improved thermal property allows the device to have wide operating temperature range of up to 190 °C heat sink temperature, high output power density especially in small device, high rollover current density and high rollover cavity temperature. Research is still underway to reduce the operating voltage of lithographic VCSELs for high wall plug efficiency, and the voltage of 6 µm device at injection current density of 10 kA/cm2 is reduces to 1.83 V with optimized mesa and DBR mirror iv structure. The lithographic VCSELS are promising to become the next generation VCSEL technology

High-performance III-V quantum structures and devices grown on Si substrates

III-V material laser monolithically grown on silicon (Si) substrate is urgently required to achieve low-cost and high-yield Si photonics. Due to the material dissimilarity between III-V component and Si, however, several challenges, such as dislocations and antiphase domains, remain to be solved during the epitaxial growth. In this regard, quantum dot (QD) laser diodes have been demonstrated with impressive characteristics of temperature insensitive, low power consumption and defects tolerance, and thus QD material is regards as an ideal material for laser directly grown on Si substrate. In this thesis, both QD laser diodes with 1.3 µm wavelength and quantum dot cascade laser with mid-infrared wavelength have been investigated. To understand the unique advantages of QD material, the comparison of QD and quantum well (QW) materials and devices grown on Si substrate is carried out in chapter 3. Based on identical fabrication and growth conditions, Si-based QW devices are unable to operate at room temperature, while the room-temperature Si-based QD is obtained with threshold current density of 160 A/cm2 and single-facet output power of >100 mW under continuous wave (c.w.) injection current driving. Besides, Si-based QD laser also shows remarkable temperature stability which the c.w. operation temperature reaches 66 ℃. The results point out that QD material has great potential in monolithic growth of III-V on Si for silicon photonics. Then, a novel approach of all-MBE grown QD laser on Si substrate is reported in chapter 4, with the optimization of buffer layer. The all-MBE grown QD laser on on-axis Si substrate with maximum operation temperature of 130 oC is achieved by utilizing thin Germanium (Ge) buffer. The mid-infrared silicon photonics has wide applications and market, but the lack of Si-based mid-infrared laser is a subsistent problem. Because the bandgap of conventional QW and QD materials is impossible to match the wavelength in mid-infrared range (3 µm to 20 µm), the Si-based quantum cascade laser (QCL) devices is regarded as an effective method to meet the requirement. Therefore, the high-performance QCL is firstly explored in chapter 5, and then, several methods in fabrication process are researched to enhance the performance for QCL devices. After the optimization of structure design and development of fabrication process, the InP-based QCL shows impressive properties with 600 mW emission power and over 100℃ operation temperature under c.w. mode. Following the previous work on Si-based QD laser, the quantum dot cascade laser (QDCL) is expected as a suitable solution for Si-based QCL devices. With the continuous improvement in structure design, the QDCL with multilayer QDs shows comparable performance, compared with conventional QCL devices. It is noted that the QDCL generates both TE and TM modes output, which is a breakthrough towards surface emitting QCL because the common QW-based QCL has only-TM emission in principle. Finally, the Si-based QCL is attempted with different structure design based on the pervious results

Coherently coupled photonic-crystal surface-emitting laser array

The realization of a 1 × 2 coherently coupled photonic crystal surface emitting laser array is reported. New routes to power scaling are discussed and the electronic control of coherence is demonstrated