3. CONCLUSION We have proposed and demonstrated an AMLFL based on the hybrid gain of a DSF and an EDF. A comparison of the output pulses of the fiber laser when the Raman pump is switched on or off is made, which shows that the supermode noise can be effectively suppressed by employing the Raman gain in the cavity of an AMLEDFL. Stable pulse output with the repetition of about 5 GHz and the pulse width of about 45 ps has been achieved based on the hybrid gain. ACKNOWLEDGMENT This work was partially supported by Natural Science Foundation of China under projects (NO. 60277018 and NO. 60407011) and Ministry of Education (China) under a grant supporting overseas Chinese. REFERENCES 1. S.G. Edirisinghe, A.S. Siddiqui, and X. Shan, Stabilised 10 GHz rationally mode-locked erbium fibre laser and its use in a 40 Gbit/s RZ data transmission system over 240 km of standard fibre, Optoelectron IEE Proc 147 (2000), 401– 406. 2. X. 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Mathew Microwave Tomography and Materials Research Laboratory, Cochin University of Science and Technology, Kochi 682 022, India; Corresponding author: ktma@cusat.ac.in Received 16 March 2007 ABSTRACT: Dielectric permittivity studies and 2-D microwave tomo- graphic imaging are performed on benign and malignant breast tissues to characterize these tumors at the ISM band. Significant difference in the permittivity values is obtained and a distinct characterization is done based on the nature of the permittivity profiles. Also, it is observed that benign tumors exhibit sharp images with well-defined borders, whereas malignant tumors show fuzzy and blurred boundaries. © 2007 Wiley Periodicals, Inc. Microwave Opt Technol Lett 49: 2341–2345, 2007; Published online in Wiley InterScience (www.interscience.wiley. com). DOI 10.1002/mop.22734 Key words: breast tumor; microwave tomographic imaging; dielectric permittivity 1. INTRODUCTION Breast cancer is a leading cause of death among women and its incidence is rising. Although curable especially when detected at early stages, breast cancer is expected to account for 28% of incident cancer cases and 20% of cancer deaths in women [1]. Mammography is currently not only the most sensitive method for detecting breast cancer, but also the most practical technique for screening and follow-up. However the X-ray mammography suf- fers many limitations like, missing of 15% of breast cancer, difficulty in imaging women with dense breasts, production of in-conclusive results, and difficulty in discriminating benign and malignant breast masses [2, 3]. Diagnosis often involves waiting for further imaging or biopsies. These limitations of X-ray mam- mography provide clear motivation for the development of a complementary breast imaging tool to assist in detection and diagnosis [4 – 6]. An ideal breast-screening tool should have low health risk, be sensitive to tumors, detect and discriminate breast tumors at an early stage, be noninvasive and simple to perform, be cost-effective and widely available, involve minimum discomfort, and provide easy to interpret and consistent results [7, 8]. An efficient characterization of benign and malignant breast tissues based on their reconstructed shape and permittivity profile Figure 1 Schematic diagram of the cavity resonator DOI 10.1002/mop MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 49, No. 10, October 2007 2341