Control of microsphere lasing wavelength using l =4-shifted distributed feedback resonator K. Sasagawa, Z. Yonezawa, J. Ohta and M. Nunoshita The oscillation wavelength of a microsphere laser is shown to be controllable using a l=4-shifted distributed feedback resonator fabri- cated on the surface of the microsphere. The lasing wavelength is in agreement with the Bragg wavelength of the grating estimated from the grating period and the effective index of the whispering gallery mode. Introduction: Microsphere resonators with whispering gallery modes (WGMs) have high quality factors and as such are of great interest in fields such as cavity quantum electrodynamics, laser optics, nonlinear optics and sensing [1]. Microsphere cavities doped with dyes or rare- earth ions exhibit laser oscillation with low threshold power due to the inherently high quality factor and small volume [2–7]. The wavelength of laser emission by these cavities is determined by the fluorescence and absorption spectrum of the dopant and the resonance condition of the sphere. For example, Nd-doped tellurite microsphere lasers exhibit single-mode operation only at around 1062 nm, which is near the gain peak of the Nd-doped tellurite glass, as shown in previous experiments by the present authors [6]. Nd ions in the tellurite glass, however, have a fluorescence band that extends from 1055 to 1080 nm, and single- mode operation within this band can be expected to be realised by controlling the oscillation wavelength. In this Letter, control of the oscillation wavelength of a microsphere laser is reported using a l=4- shifted distributed feedback (DFB) resonator fabricated on the surface of the sphere by focused ion beam (FIB) lithography. A DFB resonator without a phase-shift section has a stopband with a centre wavelength of l c ¼ 2n eff L, where n eff is the effective index of the waveguide, and L is the period of the grating. A l=4-shifted DFB resonator has a complementary passband at l c [8]. This characteristic renders l=4-shifted DFB resonators useful as cavities for singlemode laser oscillation and for use with optical filters with narrow linewidth. Fig. 1a shows a schematic diagram of the proposed microsphere laser system. The grating on the microsphere provides a periodic refractive- index change and acts as a l=4-shifted DFB resonator for WGMs. l/4 shifted section 5 m m a b Fig. 1 Schematic diagram of laser and SIM image of grating a Schematic diagram of microsphere laser with l=4-shifted DFB resonator and tapered optical fibre coupler b SIM image of l=4-shifted Bragg grating on microsphere Inset: entire microsphere Experiment: Nd-doped tellurite microspheres were used in the experiment, prepared as described previously [6]. The composition of the tellurite glass was 5Na 2 O–20ZnO–75TeO 2 –0.2wt%Nd 2 O 3 , which is a composition suitable for rare-earth-doped tellurite glass [9]. The refractive index of the glass was 1.98 at l ¼ 1050 nm. Microspheres of 50–200 mm in diameter were formed by heating and melting the end of a wire of the glass using a Kanthal wire heater. The Q value of the microsphere was estimated to be 10 5 . The l=4-shifted DFB resonators were created by fabricating gratings on the microspheres using an FIB system. To avoid charge-up of the dielectric microsphere during ion-beam irradiation, tungsten was depo- sited around the target area in advance. All gratings were fabricated under the same conditions. Scanning ion microscopy (SIM) images of the fabricated gratings (Fig. 1b) revealed that the gratings were 14.5 mm in length and had periods L of 278  5 nm. Tapered optical fibres were used for both coupling the pump laser with the microsphere resonators and extracting the output [4, 5, 10–12]. The tapered fibres were fabricated by chemically etching the cladding of single-mode optical fibres (SMF-28, Corning) [12]. A meniscus of 48% HF was applied to the fibre to etch the cladding down to the fibre core. The diameter of the tapered fibre waist was 3.8 mm. The microspheres were brought into contact with the tapered fibre during the experiment. The microspheres were pumped by a Ti:sapphire laser tuned to 800 nm (linewidth <5 GHz). The transmission of the tapered fibre at 800 nm was 8%. For efficient coupling of the pump light with the sphere, the linewidth of the light source should be narrower than that of the WGM. To ensure that this was the case in the experiments, the output spectrum was processed and monitored using an optical spec- trum analyser. Fig. 2 Emission spectra for microsphere with l=4-shifted DFB resonator a Spontaneous emission spectrum of microsphere (not to scale) —— below threshold pump power – – – bulkglass b Emission spectrum of microsphere in singlemode operation. Sphere diameter: 180 mm. Resolution: 0.1 nm Results and discussions: Fig. 2 shows the emission spectra for a microsphere with a l=4-shifted DFB resonator. The measured free spectral range (FSR) of the WGMs was 1.2 nm at 1060 nm, which corresponds to a diameter of 180 mm. Below the threshold pump power (Fig. 2a), the envelope of the output spectrum exhibits two peaks, at 1063 and 1072 nm. The former corresponds to the emission peak of the 4 F 3=2 ! 4 I 11=2 transition for Nd ions. The latter is not observed in the spectrum of bulk glass. At pump power slightly above the threshold (Fig. 2b), a sharp peak due to singlemode laser emission was observed at 1072.3 nm, differing from the fluorescence peak of the bulk sample. The threshold of the microsphere laser was less than 40 mW of pump power. ELECTRONICS LETTERS 11th December 2003 Vol. 39 No. 25