Technology for the bonding and packaging of high-power laser diode arrays is developed. The bonded and packaged laser diode arrays were used to pump a solid-state laser, where the peak output power of 25 kW with 80 ns of pulse duration at 10 kHz of repetition rate in Q-switched mode and 51.5 W CW power was achieved. The laser diode arrays in working conditions were mounted on the indigenously developed mechanical mounts. The mounted device is referred as bullet, where several laser didoes in a bullet work in a parallel combination. Subsequently, a die bonding procedure was used to bond 5 number of bullets on specially designed mounts in a series combination. Such a device assembly consisting of 5 bullets is referred as a diode rod. Three number of diode rods were mounted in a special cylindrical assembly such that they are connected in a series combination. A solid-state Nd:YAG rod was mounted at the center of the housing assembly for the realization of pumping process. The entire assembly is water cooled thus ensuring an efficient removal of the heat from the diode rods which is essential for its successful operation. The technology for bonding and packaging of laser diode arrays for the pumping of solid-state laser is now ready for the transfer.

Semiconductor laser diode devices convert electrical energy into optical energy of the specified wavelength with narrow spectral width at high intensity. The efficiency of this conversion process is generally more than 50%. The typical structures of semiconductor laser diode consist of several multilayers of AlGaAs/GaAs/InGaAs in the p-i-n configuration. Typical dimensions for the laser diode bar are 5 to 10 mm in width with a cavity length of 1 mm, and a thickness of 100 mm. Despite the high efficiency, the thermal management at high power operation in pulsed and continuous waves (CW) mode of the laser diodes is challenging. A thermally-effective packaging solution is being used and is continuously evolving to remove the excessive heat generated in the laser diode to its surroundings as quickly and uniformly as possible. Given the problem highlighted here, a cost-effective bonding and packaging solution is important because the bonding and packaging step usually dominates the final cost of the laser diode module. Hence, the development of laser diode bonding and packaging is not only a technological challenge but is a decisive step for real applications. The heat produced in the laser diode can be transported via the thermal process to the ambient environment by attaching a heat sink or heat spreader. Therefore, the laser diode must be die bonded to a package to ensure an efficient heat transfer via the low thermal interface. Typically, the heat generated in the active region of the laser diode has to flow through the entire substrate, both in length and width direction before reaching the heat sink. In addition, the thermal expansion coefficient (TEC) difference between the laser diode and the heat sink material is to be minimized to accommodate for mechanical stress generated in the device. Also, the solder material used for die bonding acts like an electrical interconnect, mechanical support, and heat dissipation medium. The bonding processes are optimized and executed in an indigenous system that has two sets of high-resolution digital microscopes along with a camera, and a uniform heating stage, with the presence of nitrogen gas, controlled by the programmable PID temperature controller, vacuum picker, etc. The uniformity of the heating stage is recorded by the thermal imager camera. Generally, copper heat sinks and diamond heat-spreaders are used for high-power linear diode array packaging. This is mainly due to their high thermal conductivity and suitable thermal expansion coefficient with the die and laser diode materials. Given this, we have developed a copper base heat sink of the specified geometry by the wire cutting machine, and subsequently, the holes were blocked by the silver-copper brazing. Subsequently, these heat sinks were gold coated for preventing oxide formation on their surface. The thermal energy profile in this module is simulated by taking care of several parameters. The simulation of high-power laser diode arrays on the copper-based heat sink with AlN heat spreader sheets and In-Sn solder materials are optimized under different levels of operation power by considering all the thermal resistance of the package. The simulation of high-power laser diode arrays on the copper-based heat sink with AlN heat spreader sheets and In-Sn solder materials is optimized. The electrical characteristics of diode modules were tested in a water-cooled configuration to check their suitability for pumping the solid-state laser media. Initially, all the laser diode modules are tested under pulse mode operations up to 10 A current. The typical operation pulse rate and their frequency is in the range of 10msec and 1kHz respectively. The emission characteristics of every laser diode element are checked by recording their emission wavelength and uniformity in all the elements. If the emission wavelength remains within the range of 800±1 nm then this module is used for pumping the Nd: YAG rod under the CW mode. Results confirm that the technology for bonding and packaging laser diode arrays for the pumping of solid-state laser is now ready for transfer. In conclusion, the technology for the laser diode arrays bonding and packaging is developed and their physical and optical characteristics are given below:

SPECIFICATION

Physical parameters and operating characteristics of the device

1	Size of the laser diode module	100 mm x 20 mm x 20 mm
2	Size of the solid-state laser	150 mm x 100 mm x 75 mm
3	Size of the entire unit	400 mm x 300mm x 200 mm
4	The emission wavelength of laser diode arrays	804 nm
5	Temperature stability	± 1.5 °C
6	The emission wavelength of solid-state laser	1064 nm
7	Running Capacity with one filling of LN2	1200-1300kms
8	The output power of solid-state laser	51.5 W
9	Peak Power of solid-state laser	25 kW
10	Pulse width	80 nsec
11	Pulse Repetition Rate	10kHz

The laser diode emission wavelength can be tuned at a rate of 0.3 nm/°C by varying the operating temperature. The output of the solid-state laser can be changed depending on the design of the laser diode pumping modules, the gain media, and the heat removal process

1. Self-reliance in bonding, packaging & testing technology of high power laser diode arrays for diode pumped solid state laser module and skill development.
2. High power laser diode arrays can be developed as per the need of the user-requirements in various R&D laboratories in DAE, which can be further extended to fulfil the requirements of various laboratories/industry across the nation.
3. Scaling up of fabrication facilities can help to reduce the cost of indigenous devices.
4. Complete fabrication technology can be transferred to an Indian industry for commercial production.
5. It opens a path for the development of tunable laser source for various wavelength range, for example vertical cavity surface emitting lasers and compact and efficient Terahertz sources and high power laser

The process developed are required in the following applications: Pump source for high-power solid-state lasers and high-power fiber lasers, polishing of diamonds, engravings in various materials, industrial applications in cutting and welding of metal sheets, medical applications, strategic applications in defence and nuclear sectors, and time resolved spectroscopy etc.

For the commercial production of bonding/packaging of laser diode arrays following facilities are required.

Floor area

1. A floor area of minimum size of 6 meter (w) x 12 meter (l) x 4 meter (h) is required to start the commercial production of bonding and packaging of laser diode arrays.
2. One clean working bench class 10 K of 2 meter (w) x 2 meter (l) size for bonding of devices.
3. Device testing stage for shorting of working laser diode arrays of 1 meter (w) x 2 meter (l) size
4. Characterization setup for testing individual modules of 1 meter (w) x 1.5 meter (l) size
5. Nitrogen reflow heater and bonding stage setup 2 meter (w) x 2 meter (l)
6. Assembling, packaging and testing of laser diode arrays modules and solid state laser setup of 2 meter (w) x 3 meter (l)
7. Chilled water unit of 1.5TR

Electrical connection

The workshop that will be used for commercial production of bonding and packaging of laser diode arrays must have an electrical connection of 1 kW power with 220 V AC, 5 A/15A, single phase electrical distribution box connections on each table.

Additional requirement

1. A small mechanical workshop, equipped with a bench vice, electrical drill machines, and basic machining/fitting too kit. 
2. The basic facilities related to optics like AR/HR coated mirrors, infra-red laser viewing cards, etc.  
3. The basic facilities related to electronics like digital multimeter, digital storage oscilloscope, soldering iron, wire cutter, wire stripper, etc. bonding and packaging materials must be available. 

MANPOWER

A minimum of three persons with the following qualifications are required to start the commercial production of bonding and packaging of laser diode arrays. Engineering In-charge (01): The candidate for the post of Engineering In-charge must have B.E./B.Tech., degree in Electronics/Instrumentation Engineering or M.Tech./M.Sc. degree in Optoelectronics.

Technicians B.1 Fitter (01): The Fitter must have an ITI certificate in Fitter Trade with 2 years of work experience in the field of fabrication of instrument cases.

Electronics (01): He must have an ITI certificate in Electronics Trade with 2 years of work experience in the field of wiring electronic instruments and PCB soldering.

The Electronic Technician can have either diploma in Electronics/Electrical/Instrumentation Engineering or an ITI certificate in Electronics/Electrical/Instrumentation with experience in the production of optoelectronic instruments.