By: Sami T. Hendow

Multiwave Photonics

Pulsed fiber lasers are being used extensively in microstructuring of materials.  Various reports indicate that the temporal shape and duration of the pulse are important for increasing the material removal rates and cutting speed.  In particular, the leading edge, which is often observed in pulsed fiber lasers, is suggested to enhance the process of cutting and material ablation.

In this report, a pulsed MOPA fiber laser is used to demonstrate that significant improvement in ablation quality and cutting speeds of hard materials, such as silicon, can be obtained if the pulse shape, pulse energy, and peak power are controlled and optimized.

An important capability of the MOPA laser used is the freedom to change one of the pulse parameters without affecting the others.  This enables us to optimize the process for the application.  For example, we can adjust the pulse peak power, while maintaining pulse energy to suite the application, or we can adjust repetition frequency without affecting the shape of the pulse, up to the maximum average power for the laser.

Optimization of the ablation process

The leading edge is observed to enhance material removal and to speed up wafer cutting speeds.  However, this is seen to be important only when the pulse has sufficient tail energy.  This later segment of the pulse contributes to heating of the surface of the material, which in turn is coupled with the leading edge to enhance material ablation.  Alternately, a short pulse with high peak power and no tail, leads only to ablation of a thin layer.  This is often attributed to the formation of highly ionized plasma which prevents the remainder of the pulse from reaching the material.

To facilitate the study, the MOPA laser is operated in two separate modes of operation; (1) constant pulse energy as the pulse width is adjusted; and (2) constant output peak power as pulse width and repetition frequency are changed.  In all cases, the system is operated with an average power of 10W at the work piece.

The MOPA system is configured to operate with pulses ranging from 10ns to 250ns, and with pulse repetition frequencies from single shot to 750kHz.  Pulse energy can be up to 0.5mJ, and peak power of up to 10kW.  On target, these translate to the high fluence levels of over 150J/cm2, or over 3GW/cm2 peak power levels.

Data is presented that demonstrates a factor of two improvements in cutting time and material removal if pulse peak power, energy and shape are controlled and optimized.  Other data is presented for scribing and percussion drilling of mono and multi-crystalline silicon, aluminium-coated silicon and copper.

Fig.1. To optimize the process of machining, the pulsed MOPA laser is operated in two configurations: (A) pulses of equal energy but different widths and peak power, and (B) pulses of different widths but with the same peak power.

The above brief overview was extracted from its original abstract and paper presented at The International Congress on Applications of Lasers & Electro-Optics (ICALEO) in Orlando, FL. To order a copy of the complete proceedings from this conference click here

Paper M209