Diverse types of lasers, such as nanosecond, pulsed, and excimer, have been considered for various applications in the photovoltaic industry, including edge isolation, edge deletion, drilling for back contact, cutting of Si-wafer, and patterning of crystalline solar cells. High power lasers, with high stability and high efficiency in addition to high beam quality, are needed now more than ever.

A possible solution to this demand for technology comes in the form of the unique design of the disk laser, which offers power scalability and high beam quality.

Understanding the Disk Laser

Figure 1. Pump cavity of a disk laser
The concept of the disk laser is based on the use of a disk medium with a large surface-area-to-volume ratio. The thin disk is cooled through the high-reflective, coated back side of the crystal, which generates a one dimensional heat flow. Therefore, the thermal gradient is parallel to the laser propagation and the internal thermal lensing is effectively eliminated. This allows true scalability of the laser output power. Diode laser light is used to pump the thin disk. Typically a beam parameter product of about 500mm mrad is desired to pump a high average power thin disk laser. Therefore, fiber coupled laser diodes are possible, as well as homogenized laser diode arrays. In a real world application, the latter approach is preferred because of the much-reduced cost per watt of pump power. With a simple light path arrangement, the pump light is guided up to 20 times through the disk, which ensures the high efficiency of the disk laser. Figure 1 illustrates the design of a disk cavity, where the disk is mounted in the center and pumped by the diode laser light.

Figure 2. Microscopic picture of an ablated area done with square fiber.
Today industrial continuous disk lasers are available with an output of up to 5.5 kW from one disk and with an optical efficiency of above 60%. If additional power is necessary, up to four cavities can be used in one resonator. The output power of the laser device increases to a maximum of 16 kW without changing the beam quality.

This thin disk technology, due to its high beam quality and high scalable power, not only allows for cw, but also for high power nanosecond and picosecond lasers. For pulsed operation, various technologies can be applied, ranging from q-switched or cavity dumping oscillators to mode-locked Master-Oscillator-Power-Amplifier (MOPA) lasers. Due to the high upper state lifetime of Yb:YAG and the relatively small gain in the thin disk laser, the typical pulse duration of a simple q-switched disk laser is in the order of one microsecond. Through employing a cavity dumped configuration, a wider range of pulse durations is accessible. Hence, the cavity dumped TruMicro 7050 provides tunable pulse width between 25 and 700 nanoseconds and a repetition rate of up to 100 kHz. Using a pockels cell inside the cavity and ejecting the pulses through a thin film polarizer, the laser is able to produce an average output power of 750 watts with a beam quality that allows a core diameter of 100 um or larger in the delivery fiber.

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