CRITERIOS DE EVALUACIÓN
UNIDAD 8. LOS VECINOS SANEN AL PRINCIPÍN CONTENIDOS
To obtain high output power from semiconductor lasers, efficient thermal management of the devices is required. Generally, high power operation of lasers is obtained at high injection currents, which lead to production of large heat flux in the device. To remove heat efficiently from the lasers, design solutions such as buried heterostructure designs could be used. The lasers performance is strongly dependent on the thermal management of the devices. Devices operating at 1.55 μm have worst temperature performance due to variety of reasons including the Auger recombination, which is a dominant non-radiative recombination mechanism in long-wavelength semiconductor lasers. The heat flux is generated in the active region of the laser which, flow through the cladding layers, semiconductor substrate, solder layers, sub- mount and then finally absorbed by the heat sink [5]. An efficient external thermal management is thus quite important for devices heat dissipation. Laser diodes are commonly mounted on a high thermal conductivity copper (Cu) heat sink and the temperature of the heat sink is actively controlled and monitored. The most critical part of thermal management of semiconductor lasers is how they are mounted to its heat sink. Commonly, laser bars are manually mounted epitaxial-side-up on brass sub-mounts using conductive epoxy. The brass sub-mounts take the generated heat away from the device and pass it to the Cu sink thus reducing the thermal impedance and temperature of the device. The thermoelectric cooler
Chapter 7 Mode Locked Lasers with Integrated Tapered SOAs: Results
- 140 - (TEC) connected to the Cu heat sink dissipate the heat of the laser and a thermister placed in the Cu heat sink monitors and control the temperature of the heat sink. The length and the width of the brass sub-mounts need to be carefully designed. If the width of the sub-mount is shorter than that of a laser bar, the heat dissipation from the overhanging regions of the lasers bars will be insufficient and may degrade the device performance. The soldering interface and the sub-mounts should provide sufficient mechanical stability, low electrical resistance and effective heat sinking to the devices. Different sub-mounts materials such as silicon (Si), copper (Cu), Diamond and Aluminium Nitride (AlN) has been widely used for mounting of laser bars. Comparing to other options, Cu is cost effective with high thermal conductivity (~393W/mK at RT). The use of Cu sub-mounts is not a preferable choice due to its co- efficient of thermal expansion (CTE) (Cu ~ 17 x 106/K) mismatch to that of InP ( CTE of InP~ 4.5 x 10-6) [6]. Such a large CTE mismatch leads to stress in the lasers structures, which reduce the devices lifetime and degrade the performance. The soldering material used to mount laser bars to the sub-mounts is an important component. Normally, devices are soldered to Cu using indium (In) as a solder. The In requires aggressive pre-processing such as acid etching before soldering to allow flux-free soldering. Lasers mounting on the Cu heat sinks using In solder and flux reduces the devices reliability and reproducibility [6]. Other solders such as SnPb and Pb-free which are mostly used, exhibits low stress but insufficient bonding strength and low creep resistance. Low creep resistance of the solders causes reduction in the fiber coupling efficiency of the devices. To obtain reliable device performance with efficient heat sinking, the bonding stress and the number of voids in the bonding should be minimized. Uniform solder interface, reduced solder voids, greater creep resistance and excellent mechanical and thermal properties could be obtained using Gold-Tin (AuSn) eutectic solder [7]. The AuSn eutectic solder has a melting point of 283°C and is composed of 80 % Au and 20 % Sn. Further, AuSn has the advantage of low oxidation rate which allows fluxless soldering. Since AuSn is a hard solder with high melting point (283°C), a bonding substrate material (sub-mount) with CTE matched between the laser and the sub-mount material is necessary to reduce the bonding stress. Furthermore, the sub-mount material should provide high thermal conductivity to allow efficient thermal management. Due to these demands, Aluminium nitride (AlN) was selected as sub-mount material. The AlN sub-mounts has high thermal conductivity (~180W/mK) and CTE value of 4.5 x 10-6/K, which is perfectly matched to that of the InP [8]. Further, the AlN is an electrical insulator and allows the formation of electrical interconnects on the sub-mounts. Properties of AlN, such as high purity, superior
Chapter 7 Mode Locked Lasers with Integrated Tapered SOAs: Results
- 141 - micro-structural and chemical uniformity leads to very consistent properties [9]. Different metal coatings could be applied to the AlN sub-mounts for reliable soldering and wire-bonding purposes. The metal layers are deposited to full-fill three general requirements: (1) surface adhesion, (2) diffusion barrier, which should provide stable bond with the contact layer and non-reactive to Sn and (3) cap layer, which is used to prevent oxidation of the surface prior to reflow. Generally, for the ceramic sub-mounts, Ti/Pt/Au (adhesion/barrier/cap) are used to full-fill the above mentioned requirements.
The AlN sub-mounts and AuSn soldering provides efficient heat sinking and offers benefits of high scalability and reliability. In this work, AlN sub-mounts with pre-deposited top face covered with Ti/Pt/AuSn/Au (100nm/60nm/3000nm/50nm) and bottom face covered with
Ti/Pt/Au/Ti/Pt/AuSn/Au (100nm/200nm/1000nm/40nm/60nm/3000nm/50nm) were used to get
better heat sinking of the devices. The laser devices were epi-layer-up soldered to the top face of the AlN sub-mount using Cammax EDB-80 die bonder and mounted on the Cu heat sink using silicon thermo-conductive paste. The schematic of a device mounted on the AlN sub- mount using AuSn soldering, and placed on Cu heat sink is shown in Figure 7.3. The Thermo- conductive paste applied in the interface between the ceramic sub-mount and the Cu heat sink improves the thermal conductivity between the device and the heat sink.
Figure 7.3: Schematic of a device mounted on AlN sub-mount using AuSn soldering.
7.3.1 Performance of Device Mounted on AlN Sub-mounts Using AuSn
Solder
To compare the heat sinking performance of the AlN sub-mounts and the brass sub-mounts, 1.7 mm long ridge waveguide lasers were mounted p-side up on AlN sub-mounts using AuSn soldering and brass sub-mounts using conductive epoxy. In order to assess the optical and electrical performance of the devices mounted on both different schemes, the L-I and V-I curves were recorded under CW current conditions. Devices were placed on a temperature
Chapter 7 Mode Locked Lasers with Integrated Tapered SOAs: Results
- 142 - controlled Cu heat sink and thermo-conductive paste was applied to the interface between the sub-mounts and heat sink to improve the thermal conduction.
Figure 7.4: L-I-V comparison of 1.7 mm long ridge waveguide lasers mounted on AlN and brass sub-mounts.
Figure 7.4 shows the L-I-V curves of the ridge waveguide lasers mounted on AlN sub-mounts (indicated by red colour) and brass sub-mounts (indicated by black colour). From the L-I curves, it is clear that the threshold current for the devices in both the schemes is around the same. The devices mounted on the AlN sub-mounts exhibits higher slope efficiency. For the devices mounted on the brass sub-mounts, the output power roll-over occurs at an output power of 78 mW, while for the devices mounted on the AlN sub-mounts using AuSn soldering, the power roll-over occurs at an output power of around 100 mW. The power roll-over in the semiconductor lasers is mainly due to the device self heating under the CW current operation. The self heating of the device increases optical losses and non-radiative recombination, which enhances the power roll-over of the devices at lower injection currents [10, 11]. The 28 % increase in the power roll-over value of the devices mounted on the AlN sub-mounts is due to the lower junction temperature caused by the increased thermal conductivity of the AlN sub- mounts. The V-I curve of the devices mounted on AlN sub-mounts shows slightly lower device resistance, which is likely to be due to the increased electrical conductivity of the AuSn soldering as compared to the conductive epoxy.
7.4 MLLs with Integrated Tapered SOAs: Device Layout
The schematic of a MLL monolithically integrated with a tapered SOA is shown in Figure 7.5. The integrated device consists of four sections, i.e. gain, SA, DBR and a tapered SOA section.
Chapter 7 Mode Locked Lasers with Integrated Tapered SOAs: Results
- 143 - The tapered SOAs were monolithically integrated at the output of the DBR-MLLs in a similar way of to the devices discussed in Chapter 6.
Figure 7.5: Schematic of a DBR-MLL with monolithically integrated tapered SOA.
The total length of the device was 2200 µm with a 30 µm long SA section, 1020 µm long gain section, 150 µm long DBR section and 955 µm long tapered SOA section. Similar to the devices discussed in Chapter 6, the length of the SA was 3 % of the total cavity length. The DBR section consists of 150 µm long 3rd order surface etched gratings with the gratings period of 734 nm. Tapered SOAs with taper angles of 2° and 6° respectively, were integrated with the DBR-MLLs for comparing their performance and beam quality under CW current operation. The output facet of a 955 μm long 2°-tapered SOA was 36 µm wide, whereas that for a 955 μm long 6°-tapered SOA was 105 µm wide. To reduce the facet back reflection into the waveguide, the output waveguide of the SOA was tilted at 10° to the facet. A curved waveguide was required to connect straight (laser section) and the 10° tilted waveguide (SOA section). The radius of the curved waveguide was chosen to be 800 µm, which is significantly larger than the critical bend radius (~ 350 μm) as shown in Chapter 6 (Figure 6.7). Further, this curve waveguide also acts as a spatial mode filter, which cut off the higher order modes in the waveguide [12].