Electronic Journal «Technical Acoustics» http://www.ejta.org 2010, 1 M. Arronte, E. Ortega, L. Ponce, E. de Posada, E. Rodriguez, T. Flores CICATA-IPN, Unidad Altamira, Carretera Tampico-Puerto Industrial Altamira, 89600, TAMPS, México, e-mail: arronte@yahoo.com Real-time monitoring of de-thorning process in Opunctia Nopalea by using a PILA technique Received 02.10.2009, published 04.02.2010 It is demonstrated that Photo-acoustic Induced by Laser Ablation (PILA) technique is suitable for the real time control and monitoring of the de-thorning process of nopal vegetable using pulsed laser. Two methods for detection of thorns are proposed; the first one uses the photo-acoustic amplitude and the second one is based in the harmonic composition of photo-acoustic signals. The latter detects clearly, thorns positions, differentiating them from other features that could exist in the nopal cortex. Harmonic composition of photo-acoustic signals also allows defining when thorns have been removed. Finally, it is obtained a characteristic pulse duration that, for the used energy range, maximizes thorns removal. Keywords: pulsed photo-acoustic, laser cleaning, laser ablation, photo-acoustic monitoring, laser-matter interaction. INTRODUCTION Recently, it has been demonstrated the possibility to carry out the de-thorning of Nopal using high-energy laser pulses [1]. This method relies on the strong absorption of the laser pulses by the thorns, both spine and glochids, to reach their ablation. Furthermore, it is possible to laser scan the surface of the Nopal, selectively eliminating the thorns without affecting the rest of the product, due to the low absorption of the laser beam in the cladode cortex. The latter add significant advantages to this method if compared with the traditional form used to remove thorns, which is basically carried out using knives. These advantages are:  The best use of the useful volume and the consequently reduction of waste. By using mechanic methods, the waste reach 30% of product [2], while using laser method only the spines are ablated.  The increase of the useful life of the product from one-two days to more than 15 days. In fact this advantage opens the possibility of long-term commercialization and export of opuntia in fresh form. Yet, an important aspect to take into account is the productivity of this method, which has to overcome the 40 kg/h reached by the traditional form. Two key factors for increasing the productivity of the de-thorning process are the precise identification of the thorns position over the cladode cortex and the detection of the exact moment when they are completely