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IEEE International Conference on Nano/Micro Engineered and Molecular Systems

Fabrication and analysis optical microsphere cavity based on high Q erbium-doped

Shubin Yan, Xiaoqian Wang, Ma Kezhen, An-fu Zhang, Chenyang Xue, Wendong Zhang

Published Apr 1, 2013• 1 citation
Publisher

Abstract

Using CO2laser metal method process the top of erbium-doped tapered fiber, which formed by stretching a standard optical fiber (SMF-125) while heating it with a hydrogen flame. The top of tapered fiber formed erbium-doped microsphere cavity with higher spherical by its surface tension when heating is stopped. The size of erbium-doped microsphere is 21µm. The position of erbium-doped microsphere is accurately adjusted between the pump power and monochromatic spectrometer so that it can be easily excited by 980 nm and 1480 nm infrared light. Excited erbium ions produce energy level transition and a number of sharp transmission spectrums are observed around the center wavelength of 1550nm. It expanded the spectrum wide and proved the frequency selection role of micro-cavity. Further analysis of the experimental results, we obtained the quality factor Q=4.4×108through calculating the transmission spectrum which is consistent with the theoretical value largely.

DOI
10.1109/nems.2013.6559881

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By using Co2 laser melting to process erbium-doped fiber tip,a high spherical erbium-doped microsphere cavity is formed through surface tension.The size of erbium-doped microsphere is 21μm.By adjusting the location of erbium-doped microspheres precisely,using 980nm laser as the pumping source,the single mode erbium ion photon fluorescence spectra is obtainod,of which the central wavelength is around 1.55μm.It proves the frequency selection role of micro-cavity.Through further analysis of the spectrum,the Q of microsphere 108 is calculatecl,and the free spectral range in real measarment is basically consistent with that in theoretical analysis.

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•Jan 1, 2013•0 citations
Using CO2laser metal method process the top of erbium-doped tapered fiber, which formed by stretching a standard optical fiber (SMF-125) while heating it with a hydrogen flame. The top of tapered fiber formed erbium-doped microsphere cavity with higher spherical by its surface tension when heating is stopped. The size of erbium-doped microsphere is 21µm. The position of erbium-doped microsphere is accurately adjusted between the pump power and monochromatic spectrometer so that it can be easily excited by 980 nm and 1480 nm infrared light. Excited erbium ions produce energy level transition and a number of sharp transmission spectrums are observed around the center wavelength of 1550nm. It expanded the spectrum wide and proved the frequency selection role of micro-cavity. Further analysis of the experimental results, we obtained the quality factor Q=4.4×108through calculating the transmission spectrum which is consistent with the theoretical value largely.
By using Co2 laser melting to process erbium-doped fiber tip,a high spherical erbium-doped microsphere cavity is formed through surface tension.The size of erbium-doped microsphere is 21μm.By adjusting the location of erbium-doped microspheres precisely,using 980nm laser as the pumping source,the single mode erbium ion photon fluorescence spectra is obtainod,of which the central wavelength is around 1.55μm.It proves the frequency selection role of micro-cavity.Through further analysis of the spectrum,the Q of microsphere 108 is calculatecl,and the free spectral range in real measarment is basically consistent with that in theoretical analysis.
A dielectric microsphere may be considered as an artificial photonic atom. The electromagnetic modes in the microsphere are completely assigned with indexes similar to those used to characterize simple atomic systems. Microsphere modes, as well as microring or microdisk, trap light in circular orbits near the sphere surface and strongly modify the phase of the interacting light, resulting in very steep dispersions. Pulse propagation experiments were performed in a system consisting of a fiber taper coupled with a silica glass microsphere, and it was shown that the light velocity in the system can be largely changed through the control of the coupling strength. Slow light was also demonstrated in a system in which two ultrahigh-Q silica microspheres of different diameters were coupled in tandem to a fiber taper demonstrating classical analog of electromagnetically induced transparency in atomic system. Here, we employed interferometric techniques based on the Mach-Zehnder interferometer for examining extremely steep dispersions induced by whispering gallery modes with a quality factor on the order of Q 1⁄4 10. Such a narrow resonance in the dielectric microsphere is comparable to that in cooled atomic systems. We successfully observed both normal and anomalous dispersions at the resonant frequency, depending on the coupling strength between the sphere and the fiber. Figure 1 shows our experimental system based on an interferometric setup. We used the second harmonics of a single mode ring cavity Nd3þ yttrium aluminum garnet (Nd3þ:YAG) laser with a line width of 1 kHz at 532 nm. The laser frequency was tuned using thermal control of the ring cavity length, over a mode-hop-free range of greater than 10GHz. The reference arm was empty, while the sample arm contained the microsphere-fiber system. Silica microspheres were fabricated from standard telecommunication optical fiber. The end of the etched fiber was fused using a CO2 laser and the microsphere formed due to the surface tension. The experiments were performed with a microsphere 60 mm in diameter. The fiber taper was also fabricated from the same telecommunication fiber. The typical fiber waist diameter at the coupling region was 0.5 mm. A large number of spectrally overlapping resonance modes exist within a microsphere. The fiber taper method is essential for coupling the incident light to a specific whispering gallery mode selectively and efficiently. The microsphere was attached to a translation stage controlled by a piezo actuator. The relative position between the sphere and the fiber were carefully adjusted so that we could observe spectrally isolated whispering gallery modes with proper coupling strength. The optical path length and dispersion were equalized in each arm as completely as possible. The fringe pattern was captured using a charge-coupled device (CCD) camera for each frequency increment. The observed interference patterns appeared as concentric circles due to the interference between the plain reference wave and the spherical wave from the sample arm. A least square fit for each fringe pattern was performed to extract the phase information in the transmitted light. Figures 2(a) and 2(b) show examples of transmission spectra through the microsphere fiber taper coupled system. These spectra were taken in two different whispering gallery modes at different laser frequencies in the mode-hop-free laser tuning range. The transmission dips appear when the incident laser frequencies are resonant at specific whispering gallery modes in the microsphere. The two whispering gallery modes shown in Figs. 2(a) and 2(b) are denoted as WG1 and WG2, respectively. The transmission minimum was Tmin 1⁄4 0:12, and the resonance width was 1⁄4 BS
We present experimental results on an all-optical, thermally-assisted technique for broad range tuning of microsphere cavity resonance modes to arbitrary probe wavelengths. An erbium:ytterbium co-doped phosphate glass (Schott IOG-2) microsphere is pumped at 978 nm via the supporting stem and the heat generated by absorption of the pump light expands the cavity and changes the refractive index. This is a robust tuning method that decouples the pump from the probe and allows fine tuning of the microsphere's whispering gallery modes. Pump/probe experiments were performed to demonstrate thermo-optical tuning to specific probe wavelengths, including the 5 S 1/2 F= 3 to 5 P 3/2 F'= 4 laser cooling transition of rubidium-85. This is of particular interest for cavity quantum electrodynamics (QED)-type experiments, while the broad tuning range achievable is useful for integrated photonic devices, including sensors and modulators.
We present experimental results on an all-optical, thermally-assisted technique for broad range tuning of microsphere cavity resonance modes to arbitrary probe wavelengths. An erbium:ytterbium co-doped phosphate glass (Schott IOG-2) microsphere is pumped at 978 nm via the supporting stem and the heat generated by absorption of the pump light expands the cavity and changes the refractive index. This is a robust tuning method that decouples the pump from the probe and allows fine tuning of the microsphere's whispering gallery modes. Pump/probe experiments were performed to demonstrate thermo-optical tuning to specific probe wavelengths, including the5S1/2F= 3 to5P3/2F'= 4 laser cooling transition of rubidium-85. This is of particular interest for cavity quantum electrodynamics (QED)-type experiments, while the broad tuning range achievable is useful for integrated photonic devices, including sensors and modulators.
Erbium is a rare-earth metal that, when incorporated in a solid, can emit light at a wavelength of 1.5 μm. It plays a key role in current day telecommunication technology as the principle ingredient of optical fiber amplifiers. In this thesis the control of the Er spontaneous emission in three different types of microphotonic materials is described. Part I of this thesis focuses on the effect of a metallo-dielectric interface on the spontaneous emission of optical emitters in silica glass. It is shown that Er ions near a Ag interface can couple to surface plasmons (SPs) via a near-field interaction. By coupling SPs out into the far field, large changes in the Er photoluminescence emission distribution, spectra, and polarization can be observed. The excitation of SPs also results in an increase of the Er photoluminescence decay rate. The observed decay rates are in good agreement with calculations based on a classical dipole oscillator model. From the change in photoluminescence decay rate of Si nanocrystals near a Ag interface it is shown that Si nanocrystals can efficiently excite SPs and have an internal quantum efficiency of 77 %. Part II focuses on the effect of a microcavity on the spontaneous emission of Er and describes how ion implantation can be used to dope dielectric microresonators with optically active Er ions. The fabrication and characterization of an Er ion-implanted silica microsphere resonator is described that shows lasing at 1.5 μm when pumped above its lasing threshold. Ion implantation is also used to dope toroidal microcavities on a Si chip with Er. The microtoroids are doped by either pre-implantation into the SiO2 base material, or by post-implantation in a fully fabricated microtoroid. The optical activation of Er ions in the microtoroid is investigated and Er lasing at 1.5 μm is observed for both types of microcavities with the lowest threshold (4.5 μW) for the pre-implanted microtoroids. Part III describes the fabrication of an Er-doped Si-inverse opal photonic crystal. These photonic crystals can potentially have a photonic bandgap that can fully inhibit the spontaneous emission of on optical emitter. Fabrication criteria are derived for such a photonic crystal, based on the lattice parameter, filling fraction, and Si refractive index. In the opal photonic crystal composed of both Si and SiO2 we show that Er ions can be selectively excited in both the Si and SiO2 part of the photonic crystal by changing the excitation wavelength and/or the measurement temperature.
In this thesis, the evanescent field sensing techniques of tapered optical nanofibres and microspherical resonators are investigated. This includes evanescent field spectroscopy of a silica nanofibre in a rubidium vapour; thermo-optical tuning of Er:Yb co-doped phosphate glass microspheres; optomechanical properties of microspherical pendulums; and the fabrication and characterisation of borosilicate microbubble resonators. Doppler-broadened and sub-Doppler absorption spectroscopic techniques are performed around the D2 transition (780.24 nm) of rubidium using the evanescent field produced at the waist of a tapered nanofibre with input probe powers as low as 55 nW. Doppler-broadened Zeeman shifts and a preliminary dichroic atomic vapour laser lock (DAVLL) line shape are also observed via the nanofibre waist with an applied magnetic field of 60 G. This device has the potential for laser frequency stabilisation while also studying the effects of atom-surface interactions. A non-invasive thermo-optical tuning technique of Er:Yb co-doped microspheres to specific arbitrary wavelengths is demonstrated particularly to 1294 nm and the 5S1/2F=3 to 5P3/2Fʹ=4 laser cooling transition of 85Rb. Reversible tuning ranges of up to 474 GHz and on resonance cavity timescales on the order of 100 s are reported. This procedure has prospective applications for sensing a variety of atomic or molecular species in a cavity quantum electrodynamics (QED) experiments. The mechanical characteristics of a silica microsphere pendulum with a relatively low spring constant of 10-4 Nm-1 are explored. A novel method of frequency sweeping the motion of the pendulum to determine its natural resonance frequencies while overriding its sensitivity to environmental noise is proposed. An estimated force of 0.25 N is required to actuate the pendulum by a displacement of (1-2) μm. It is suggested that this is of sufficient magnitude to be experienced between two evanescently coupled microspheres (photonic molecule) and enable spatial trapping of the micropendulum. Finally, single-input borosilicate microbubble resonators with diameters <100 μm are fabricated using a CO2 laser. Optical whispering gallery mode spectra are observed via evanescent coupling with a tapered fibre. A red-shift of (4-22) GHz of the resonance modes is detected when the hollow cavity was filled with nano-filtered water. A polarisation conversion effect, with an efficiency of 10%, is observed when the diameter of the coupling tapered fibre waist is varied. This effect is also achieved by simply varying the polarisation of the input light in the tapered fibre where the efficiency is optimised to 92%. Thus, the microbubble device acts as a reversible band-pass to band-stop optical filter for cavity-QED, integrated solid-state and semiconductor circuit applications.