Abstract. A supercontinuum is generated for the Ørst time and studied in twin tapered Øbres with a micron waist upon pumping by femtosecond pulses. The supercontinuum is obtained in the range from 460 to 1070 nm at the 28 dB level with the degree of polarisation up to 97 %. The polarisation and coherent properties of the supercontinuum are studied experimentally. Keywords: tapered Øbre, supercontinuum generation, femtosecond pulses. 1. Introduction Progress in the technology of manufacturing optical Øbres with special properties (varied-dispersion Øbres, small-core- diameter Øbres, holey Øbres, etc.) stimulates the studies of nonlinear effects appearing during the propagation of ultrashort light pulses in the Øbres. One of the examples of qualitatively new recent results obtained by several research groups [1 º 4] using new Øbres, in particular, small- core-diameter ( 2 mm) Øbres is the generation of a supercontinuum in the region between 400 and 1600 nm upon pumping the Øbres by comparatively low-power (pulse energy  1 nJ) femtosecond Ti : sapphire and Cr : for- sterite lasers [5, 6]. Earlier, a supercontinuum was generated upon pumping usual Øbres and other media by an order of magnitude higher-power light pulses [7]. The lowering of the peak power of pump pulses generating a supercontinuum in new optical Øbres opens up the outlook for the development of more efØcient radiation sources (converters) with unique properties. These expectations are connected to a great extent with small-diameter Øbres, whose core has a diameter of a few micrometres and partially or completely borders air. Such Øbres (hereafter, microØbres) can be manufactured based on holey Øbres or tapered Øbres with a low-diameter waist. The wavelength of the group-velocity zero dispersion in microØbres is shifted to the visible spectral region and the effective mode area in them is small, which provides an increase in nonlinear refraction. Except the supercontinuum generation, a number of nonlinear processes were observed in such microØbres such as the self-shift of the soliton carrier frequency [8 º 11], the third-harmonic generation, etc. In this paper, we obtained for the Ørst time and studied experimentally the generation a polarised supercontinuum in silica Øbres of a new type º twin microØbres with a quasi- elliptic waist cross section, pumped by a femtosecond Ti : sapphire laser. 2. Experimental Twin microØbres were manufactured from standard SMF- 28 Corning Øbres. At the initial moment of Øbre drawing, a mechanical contact between two Øbres was provided over the length of a few centimetres. The drawing was performed in the Œame of a hydrogen torch in several stages using the technology described in Ref. [4]. The diameter of Øbres was reduced at each stage and the Øbres were simultaneously welded together. A twin microØbre manufactured this way had a long waist (of length up to 14 cm) with a quasi- elliptic cross section of micron diameter and two input and two output ends representing the undrawn pieces of an SMF-28 Øbre of length 5 º 40 cm. We failed to measure the waist diameter because of its smallness; however the cross section of a twin microØbre at one of the stages of Øbre drawing was photographed. Figure 1a shows the photo- graph of the cross section of a twin microØbre illuminated from one of its input ends. The calculated dimensions of the cross section of the waist of a twin microØbre were four times smaller than these presented in Fig. 1a. The assumed Ønal cross section of the waist is shown in Fig. 1b. The size of the minor axis of the quasi-elliptic proØle is d  23 mm, and that of the major axis is D  1:7d. The pump radiation was coupled into the core (of diameter 8 mm) of one of the input ends and then propagated along the tapering part of the Øbre and its microwaist that bordered ambient air. After propagation through the microwaist and the expanding part of the Øbre, the pump radiation propagated along the cores of the two output ends of the undrawn Øbre. The lengths of the tapering and expanding parts of the Øbre were 2.5 cm each and the microwaist length was from 6 to 14 cm in different Øbres. Thus, the pump radiation was coupled into and coupled out of the microwaist through standard pieces of SMF-28 Øbres. The scheme of the experimental setup used for studying manufactured Øbres is shown in Fig. 2. We used in our experiments a femtosecond Ti : sapphire laser pumped by a cw argon laser. The output pulse duration was 60 fs, the S.M. Kobtsev, S.V. Kukarin, N.V. Fateev Novosibirsk State University, ul. Pirogova 2, 630090 Novosibirsk, Russia; e-mail: kobtsev@lab.nsu.ru Received 27 May 2003; revision received 11 July 2003 Kvantovaya Elektronika 33 (12) 1085 º 1088 (2003) Translated by M.N. Sapozhnikov PACS numbers: 42.81.Bm; 42.55.Tv FIBRES DOI:10.1070/QE2003v033n12ABEH002558 Generation of a polarised supercontinuum in small-diameter quasi-elliptic Øbres S.M. Kobtsev, S.V. Kukarin, N.V. Fateev 379/513 º MB º 28/ix-07 º SVERKA º 4 ÒÑ˛ÑÔ ˝ÑˇÒ. å 3 Quantum Electronics 33 (12) 1085 º 1088 (2003) ß2003 Kvantovaya Elektronika and Turpion Ltd