Towards a fully integrated optical gyroscope using whispering gallery modes resonators T. Amrane 1, J.-B. Jager 2, T. Jager 1, V. Calvo 2, J.-M. Leger 1 1 CEA, LETI, Grenoble, France. 2 CEA, INAC-SP2M / SiNaPS, Grenoble, France October 9th, 2014.
Outline 1. Introduction 2. The Whispering Gallery Modes Resonators design optimization for the gyroscope application 3. Development of a specific packaging to be able to rotate the device 4. The detection steps towards rotation rate measurements 5. Conclusion
Motivations Best performances of optical gyroscope (RLG, IFOG): Resolution < 0.001 /h Sensitive part size ~ 200 cm 3 IFOG, IXblue How to reach smaller dimensions with optical gyroscopes? Resonators with high Q D factor are requested to develop an IORG with high performances Tassadit AMRANE - ICSO2014 Tenerife, Spain. 3
A promising resonator for the development of an IORG The SiO 2 Whispering Gallery Modes Resonator (WGMR) on Si 100 µm Distinctive features : High Q (up to 10 8 10 9 already demonstrated) Integrated on silicon substrate (CMOS compatible) Our preliminary objectives: Resolution performances 1 /h Small size ~ 2 cm 3 Tassadit AMRANE - ICSO2014 Tenerife, Spain. 4
Transmission Transmission The WGMR and the Sagnac effect n air = 1 CW Ω n SiO2 = 1.45 Resonant modes localization CCW 100 µm n Si = 3.5 Stationary system Ω = 0 Rotating system Ω 0 CCW CW CCW CW Resonance modes frequencies Tassadit AMRANE - ICSO2014 Tenerife, Spain. 5
Outline 1. Introduction 2. The Whispering Gallery Modes Resonators design optimization for the gyroscope application 3. Development of a specific packaging to be able to rotate the Whispering Gallery Modes Resonators 4. The detection steps towards rotation rate measurements 5. Conclusion
The WGMR fabrication: the initial process flow 3 µm SiO 2 Si substrate 1) Thermal SiO 2 on Si substrate Photoresist 2) Photolithography step 3) SiO 2 wet etching 4) Silicon Reactive Ion Etching by SF 6 gas 5) Photoresist removing alcohol bath Tassadit AMRANE - ICSO2014 Tenerife, Spain. 7
Normalized transmission The resonator quality resulting from the initial process flow 0,98 0,96 100 µm 0,94 0,92 0,90 Q = 3.2 10 5 1556,30 1556,32 1556,34 1556,36 Wavelength (nm) The roughness at the edge impacts the Q factor 5 µm Tassadit AMRANE - ICSO2014 Tenerife, Spain. 8
The new process flow: the additional steps 3 µm SiO 2 Si substrate 1) Thermal SiO 2 on Si substrat 2) Dry oxidation (24h at 1000 C) remove the impurities in the SiO 2 layer Diffusion Q Photoresist 3) Photolithography step 4) Oxygen plasma and a post bake at 120 C enhance the photoresist pads definition 5) SiO 2 wet etching Interface roughness Q 6) Acid cleaning + oxygen plasma avoid particles contaminants on the resonator Diffusion sources Q 7) Silicon Reactive Ion Etching by SF 6 gas Tassadit AMRANE - ICSO2014 Tenerife, Spain. 9
Normalized transmission The resonator resulting from the new process flow 0,70 0,68 100 µm 0,64 0,62 0,60 Q = 4.2 10 6 1556,40 1556,41 1556,42 1556,43 Wavelength (nm) Smoother surface 5 µm Higher Q factor Tassadit AMRANE - ICSO2014 Tenerife, Spain. 10
Normalized transmission Normalized transmission The characterization results 5 µm 5 µm 0,98 0,70 0,96 0,68 0,94 0,92 Q = 3.2 10 5 0,64 0,62 Q = 4.2 10 6 0,90 1556,30 1556,32 1556,34 1556,36 Wavelength (nm) 0,60 1556,40 1556,41 1556,42 1556,43 Wavelength (nm) The Q factor is multiplied by ~ 10 Tassadit AMRANE - ICSO2014 Tenerife, Spain. 11
Normalized transmission Our current performances vs objectives 2 mm 0,95 0,94 0,93 5 mm diameter resonators Latest First results with updated process 0,92 0,91 0,90 Q = 6.5 10 6 1543,566 1543,568 1543,570 1543,572 Wavelength (nm) Objective Thicker SiO 2 layer with less impurities To further gain a factor of 10 Tassadit AMRANE - ICSO2014 Tenerife, Spain. 12
Outline 1. Introduction 2. The Whispering Gallery Modes Resonators design optimization for the gyroscope application 3. Development of a specific packaging to be able to rotate the Whispering Gallery Modes Resonators 4. The detection steps towards rotation rate measurements 5. Conclusion
The initial coupling system The spectral response depends on the gap distance between the resonator and the suspended tapered fiber Evanescent coupling g = 2 µm g = 1 µm gap distance Any vibration will impact the measurements It is not possible to rotate the device Tassadit AMRANE - ICSO2014 Tenerife, Spain. 14
The coupling system adaptation for the gyroscope application The unpackaged coupling system Open environment The tapered fiber is suspended and dissociated from the resonator The optical properties are not stable when the device is moving Encapsulation in a low refractive index polymer Polymer Fix the gap distance Keep the light confinement The device can be rotated without modifying the coupling characteristics Tassadit AMRANE - ICSO2014 Tenerife, Spain. 15
Normalized transmission The influence of the encapsulation Polymer MY132 A, n = 1.32 n = 1.32 instead of 1 Stronger coupling Slight decrease of the Q factor (~40%) Wavelength (nm) The device can now be characterized in rotation in a proof of concept test setup Tassadit AMRANE - ICSO2014 Tenerife, Spain. 16
Outline 1. Introduction 2. The Whispering Gallery Modes Resonators design optimization for the gyroscope application 3. Development of a specific packaging to be able to rotate the Whispering Gallery Modes Resonators 4. The detection steps towards rotation rate measurements 5. Conclusion
Towards Sagnac detection measurements 1. Obtain identical resonant modes characteristics for CW and CCW lightwaves Implement a setup to obtain simultaneous and identical CW and CCW spectral responses 2. Follow the resonance frequencies evolution resulting from the rotation (Sagnac shift) Lock-on CW and CCW resonances frequencies 3. Define and test a closed-loop system architecture Define compensation of all the noise sources Define final measurement protocole Tassadit AMRANE - ICSO2014 Tenerife, Spain. 18
Identical CW and CCW mode characteristics is mandatory 5 mm 3 µm Tapered fiber WGMR Identical CW and CCW lightwaves spectral responses characteristics have been simultaneously obtained Tassadit AMRANE - ICSO2014 Tenerife, Spain. 19
Towards lock-on the CW and CCW resonance frequencies SG Laser Oscilloscope The positioning stage The sample LIA PMs Tassadit AMRANE - ICSO2014 Tenerife, Spain. 20
Transmitted signal (V) Towards lock-on the CW and CCW resonance frequencies f m = 10 MHz V pp = 2.45 V Resonant peaks CW Error signals CCW D = 5 mm Q = 1.8 10 6 The CW and CCW resonances error signals have been obtained: Locking on CW and CCW resonances is now possible and relative Sagnac frequency shift measurement resulting from a rotation can be achieved ΔV Δf The detection sensitivity ΔV/Δf: 5 60 Hz/ /s Tassadit AMRANE - ICSO2014 Tenerife, Spain. 21
Summary Towards using SiO 2 WGMR in IORG, we have successfully : Adapted WGMR characteristics to the application constraints, i.e. increased the Q D product with and upgraded process flow Initial results First results with upgraded process Current results with upgraded process Final objective QxD 4.10 2 1,3.10 4 7.10 4 > 10 6 Ω SN 5x10 4 /h 125 /h 25 /h < 1 /h We are currently at the same order of magnitude as the state of the art for IORG but with a smaller sensitive part (d=5 mm vs 2.5 cm for H. Ma and 4 cm for M. Lei) Developped a dedicated packaging to be able to rotate the device for a proof of concept demonstration Implemented the first steps for a dedicated setup to measure the Sagnac shift Tassadit AMRANE - ICSO2014 Tenerife, Spain. 22
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