Rajiv Pathak - Academia.edu (original) (raw)
Papers by Rajiv Pathak
Developments in Nd-based lasers pumped on the 4I9/2-->4F3/2 transition have led to demands for in... more Developments in Nd-based lasers pumped on the 4I9/2-->4F3/2 transition have led to demands for increased power, brightness, and spectral stability from diode pump sources. We describe the development of fiber coupled diode pump sources that generate >120W of power from a 400μm, 0.22NA fiber at 88Xnm wavelengths. In order to maintain spectral purity at these high powers, we investigated the use of Volume Bragg Gratings to stabilize the wavelength of these multi-bar systems. A detailed study of the trade-offs between facet reflectivity and VBG reflectivity was conducted in order to determine an optimal combination that balances output power and locking range. In complement to the developments in 88Xnm pumping, recent interest in eye-safe fiber lasers have resulted in the development of Tm-doped fiber lasers pumped at 79X wavelengths. We describe the development of fiber coupled products with >80W from a 200μm, 0.22NA fiber, including the use of optimized bar geometries to improve fiber coupling efficiency.
State-of-the-art QCW solid-state lasers are demanding ever higher brightness from the pump source... more State-of-the-art QCW solid-state lasers are demanding ever higher brightness from the pump source-conduction cooled diode laser stacks. The intensity of a QCW vertical stack is limited by the peak power of each diode bar and the bar pitch. The minimum bar pitch of the existing laser diode stacks on the market is about 400um. In this paper, we present a unique vertical diode laser stack package design to achieve a bar pitch as low as 150um, which improves the intensity of the stack by nearly 3 times. Together with the state-of-art diode laser bar from Coherent, greater than 30kW/cm2 peak power density is achieved from the emitting area of the vertical stack. The p-n junction temperature of the diode bars in the device under QCW operation is modeled with FEA software, as well as measured in this research. Updated reliability results for these diode laser stacks are also reported.
Fiber lasers have made significant progress in terms of power output, beam quality and operationa... more Fiber lasers have made significant progress in terms of power output, beam quality and operational robustness over the past few years. Key to this progress has been advances in two technologies - fiber technology and 9xx nm diode laser pump technology based on single emitters. We present the operational characteristics of our new high brightness 9xx nm fiber laser pump sources based on diode laser bars and diode laser bar arrays and discuss the design trade offs involved for realization of devices focused on this application. These trade offs include achieving the lowest slow axis divergence while maintaining high wall plug efficiency and minimizing facet power density to maximize reliability.
The state-of-the-art beam quality from high-brightness, fiber-coupled diode laser modules has bee... more The state-of-the-art beam quality from high-brightness, fiber-coupled diode laser modules has been significantly improved in the last few years, with commercially available modules now rivaling the brightness of lamp-pumped Nd:YAG lasers. We report progress in the development of these systems for a variety of applications, such as material processing and pumping of solid state and fiber lasers. Experimental data and simulation results for wavelength stabilized outputs from 200 µm diameter fibers at 975 nm for power levels greater than 200 W will be presented. The enabling technology in these products is supported by key developments in tailored diode laser bars with low slow axis divergence, micro-optics, diode laser packaging, and modular architecture.
High power industrial fibre lasers are typically pumped by single emitter diodes, with pump power... more High power industrial fibre lasers are typically pumped by single emitter diodes, with pump power aggregation and the fibre laser cavity being achieved in a monolithic "all-fibre" architecture comprising fused fiber bundles, fiber Bragg grating reflectors and numerous splices. We adopted a very different approach based on end pumping of a stable multimode oscillator utilizing free space optics.
We present results from a survey of materials used for packaging semiconductor lasers, including ... more We present results from a survey of materials used for packaging semiconductor lasers, including Cu, CuW, BeO, diamond composite and other advanced materials. We present the results of residual bonding stress from various solders and consider the direct effects on wavelength and spectral width. We also provide data on the second order effects of threshold current and slow axis divergence. Additionally, we consider the heat spreading through different materials for a laser bar and present modeled and experimental data on the thermal performance. Finally, we consider the reliability under on-off life-testing and thermal cycling tests.
The packaging of high power diode laser bars requires a high cooling efficiency and long-term sta... more The packaging of high power diode laser bars requires a high cooling efficiency and long-term stability. Due to the increasing output power of the diode laser bars the cooling performance of the packaging becomes more important. Nowadays micro channel heat sinks seem to be the most efficient cooling concept in regard to high power applications. The active area of the p-side down mounted laser bar is located directly above the micro channels. In other applications where conductive cooled heat sinks are used the bars are mounted on copper CS mount, CuW submount or high performance materials. All these packaging ideas use wire bonds or thin copper sheets as a n-contacts. The thermal advantage of these contacts can be neglected. N-contact cooling is typically used to achieve new records of optical output power in the labs. These studies analyze the properties of an additional n-contact cooling. The cooling performance of a package cooled on both sides can be improved by more than 20% when compared with typical wire bonds or metal sheets. Different packaging styles with metal sheets, heat spreaders (expansion matched) and active n-side cooling are investigated. The effect of n-side cooling with regards to the fill-factor and cavity length is analyzed also. The first part of this paper approaches the topic theoretically. Simulations are carried out and show the advantages and differences of different package styles in comparison to bar geometries variations. The second part of the studies characterizes and analyses fabricated samples made out of copper in view of cooling performance, handling, and induced stress. The results of different bar geometries and packaging styles are compared and guidelines for n-side cooling are developed.
European Journal of Vascular and Endovascular Surgery, 2006
Journal of Vascular and Interventional Radiology, 2007
Type II endoleak is defined as persistent blood flow and pressure within an aneurysmal sac after ... more Type II endoleak is defined as persistent blood flow and pressure within an aneurysmal sac after endovascular deployment of a stent graft from patent aortic branches. This paper describes the simultaneous deployment of an endoluminal graft, with limited extraperitoneal dissection of a collateral vessel and use of an ethylene vinyl alcohol copolymer, Onyx, to obliterate a large type II endoleak.
Developments in Nd-based lasers pumped on the 4I9/2-->4F3/2 transition have led to demands for in... more Developments in Nd-based lasers pumped on the 4I9/2-->4F3/2 transition have led to demands for increased power, brightness, and spectral stability from diode pump sources. We describe the development of fiber coupled diode pump sources that generate >120W of power from a 400μm, 0.22NA fiber at 88Xnm wavelengths. In order to maintain spectral purity at these high powers, we investigated the use of Volume Bragg Gratings to stabilize the wavelength of these multi-bar systems. A detailed study of the trade-offs between facet reflectivity and VBG reflectivity was conducted in order to determine an optimal combination that balances output power and locking range. In complement to the developments in 88Xnm pumping, recent interest in eye-safe fiber lasers have resulted in the development of Tm-doped fiber lasers pumped at 79X wavelengths. We describe the development of fiber coupled products with >80W from a 200μm, 0.22NA fiber, including the use of optimized bar geometries to improve fiber coupling efficiency.
State-of-the-art QCW solid-state lasers are demanding ever higher brightness from the pump source... more State-of-the-art QCW solid-state lasers are demanding ever higher brightness from the pump source-conduction cooled diode laser stacks. The intensity of a QCW vertical stack is limited by the peak power of each diode bar and the bar pitch. The minimum bar pitch of the existing laser diode stacks on the market is about 400um. In this paper, we present a unique vertical diode laser stack package design to achieve a bar pitch as low as 150um, which improves the intensity of the stack by nearly 3 times. Together with the state-of-art diode laser bar from Coherent, greater than 30kW/cm2 peak power density is achieved from the emitting area of the vertical stack. The p-n junction temperature of the diode bars in the device under QCW operation is modeled with FEA software, as well as measured in this research. Updated reliability results for these diode laser stacks are also reported.
Fiber lasers have made significant progress in terms of power output, beam quality and operationa... more Fiber lasers have made significant progress in terms of power output, beam quality and operational robustness over the past few years. Key to this progress has been advances in two technologies - fiber technology and 9xx nm diode laser pump technology based on single emitters. We present the operational characteristics of our new high brightness 9xx nm fiber laser pump sources based on diode laser bars and diode laser bar arrays and discuss the design trade offs involved for realization of devices focused on this application. These trade offs include achieving the lowest slow axis divergence while maintaining high wall plug efficiency and minimizing facet power density to maximize reliability.
The state-of-the-art beam quality from high-brightness, fiber-coupled diode laser modules has bee... more The state-of-the-art beam quality from high-brightness, fiber-coupled diode laser modules has been significantly improved in the last few years, with commercially available modules now rivaling the brightness of lamp-pumped Nd:YAG lasers. We report progress in the development of these systems for a variety of applications, such as material processing and pumping of solid state and fiber lasers. Experimental data and simulation results for wavelength stabilized outputs from 200 µm diameter fibers at 975 nm for power levels greater than 200 W will be presented. The enabling technology in these products is supported by key developments in tailored diode laser bars with low slow axis divergence, micro-optics, diode laser packaging, and modular architecture.
High power industrial fibre lasers are typically pumped by single emitter diodes, with pump power... more High power industrial fibre lasers are typically pumped by single emitter diodes, with pump power aggregation and the fibre laser cavity being achieved in a monolithic "all-fibre" architecture comprising fused fiber bundles, fiber Bragg grating reflectors and numerous splices. We adopted a very different approach based on end pumping of a stable multimode oscillator utilizing free space optics.
We present results from a survey of materials used for packaging semiconductor lasers, including ... more We present results from a survey of materials used for packaging semiconductor lasers, including Cu, CuW, BeO, diamond composite and other advanced materials. We present the results of residual bonding stress from various solders and consider the direct effects on wavelength and spectral width. We also provide data on the second order effects of threshold current and slow axis divergence. Additionally, we consider the heat spreading through different materials for a laser bar and present modeled and experimental data on the thermal performance. Finally, we consider the reliability under on-off life-testing and thermal cycling tests.
The packaging of high power diode laser bars requires a high cooling efficiency and long-term sta... more The packaging of high power diode laser bars requires a high cooling efficiency and long-term stability. Due to the increasing output power of the diode laser bars the cooling performance of the packaging becomes more important. Nowadays micro channel heat sinks seem to be the most efficient cooling concept in regard to high power applications. The active area of the p-side down mounted laser bar is located directly above the micro channels. In other applications where conductive cooled heat sinks are used the bars are mounted on copper CS mount, CuW submount or high performance materials. All these packaging ideas use wire bonds or thin copper sheets as a n-contacts. The thermal advantage of these contacts can be neglected. N-contact cooling is typically used to achieve new records of optical output power in the labs. These studies analyze the properties of an additional n-contact cooling. The cooling performance of a package cooled on both sides can be improved by more than 20% when compared with typical wire bonds or metal sheets. Different packaging styles with metal sheets, heat spreaders (expansion matched) and active n-side cooling are investigated. The effect of n-side cooling with regards to the fill-factor and cavity length is analyzed also. The first part of this paper approaches the topic theoretically. Simulations are carried out and show the advantages and differences of different package styles in comparison to bar geometries variations. The second part of the studies characterizes and analyses fabricated samples made out of copper in view of cooling performance, handling, and induced stress. The results of different bar geometries and packaging styles are compared and guidelines for n-side cooling are developed.
European Journal of Vascular and Endovascular Surgery, 2006
Journal of Vascular and Interventional Radiology, 2007
Type II endoleak is defined as persistent blood flow and pressure within an aneurysmal sac after ... more Type II endoleak is defined as persistent blood flow and pressure within an aneurysmal sac after endovascular deployment of a stent graft from patent aortic branches. This paper describes the simultaneous deployment of an endoluminal graft, with limited extraperitoneal dissection of a collateral vessel and use of an ethylene vinyl alcohol copolymer, Onyx, to obliterate a large type II endoleak.