Ravinder Chutani

Coherent population trapping resonances in Cs–Ne vapor microcells for miniature clocks applications

We report the characterization of dark line resonances observed in Cs vapor microcells filled with a unique neon (Ne) buffer gas. The impact on the coherent population trapping (CPT) resonance of some critical external parameters such as laser intensity, cell temperature, and microwave power is studied. We show the suppression of the first-order light shift by proper choice of the microwave power. The temperature dependence of the Cs ground state hyperfine resonance frequency is shown to be canceled in the 77–80 °C range for various Ne buffer gas pressures. The necessity to adjust the Ne buffer gas pressure or the cell dimensions to optimize the CPT signal height at the frequency inversion temperature is pointed out. Based on such Cs–Ne microcells, we preliminary demonstrate a 852 nm vertical cavity surface emitted laser (VCSEL)-modulated based CPT atomic clock exhibiting a short term fractional frequency instability σy(τ)=1.5×10−10τ−1/2 until 30 s. These results, similar to those published in the literat...

Effects of getters on hermetically sealed micromachined cesium–neon cells for atomic clocks

The wafer-level integration technique of PageWafer??(SAES Getters? solution for getter film integration into wafer to wafer bonded devices) has been tested in hermetically sealed miniature glass-Si-glass cells filled with Cs and Ne, e.g. for microelectromechanical systems (MEMS) atomic clock applications. Getter effects on the cell atmosphere are analyzed by quadruple mass spectroscopy and coherent population trapping (CPT) spectroscopy. The quadruple mass spectroscopy revealed that the residual gases (H2, O2, N2?and CO2) that are attributed to anodic bonding process are drastically reduced by the getter films while desirable gases such as Ne seem to remain unaffected. The impurity pressure in the getter-integrated cells was measured to be less than 4???10?2?mbar, i.e. pressure 50?times lower than the one measured in the cells without getter (2 mbar). Consequently, the atmosphere of the getter-integrated cells is much more pure than that of the getter-free cells. CPT signals obtained from the getter-integrated cells are stable and are, in addition, similar to each other within a cell batch, suggesting the strong potential of applications of this getter film and especially for its wafer-level integration to MEMS atomic clocks and magnetometers.

Laser light routing in an elongated micromachined vapor cell with diffraction gratings for atomic clock applications.

This paper reports on an original architecture of microfabricated alkali vapor cell designed for miniature atomic clocks. The cell combines diffraction gratings with anisotropically etched single-crystalline silicon sidewalls to route a normally-incident beam in a cavity oriented along the substrate plane. Gratings have been specifically designed to diffract circularly polarized light in the first order, the latter having an angle of diffraction matching the (111) sidewalls orientation. Then, the length of the cavity where light interacts with alkali atoms can be extended. We demonstrate that a longer cell allows to reduce the beam diameter, while preserving the clock performances. As the cavity depth and the beam diameter are reduced, collimation can be performed in a tighter space. This solution relaxes the constraints on the device packaging and is suitable for wafer-level assembly. Several cells have been fabricated and characterized in a clock setup using coherent population trapping spectroscopy. The measured signals exhibit null power linewidths down to 2.23 kHz and high transmission contrasts up to 17%. A high contrast-to-linewidth ratio is found at a linewidth of 4.17 kHz and a contrast of 5.2% in a 7-mm-long cell despite a beam diameter reduced to 600 μm.

Characterization of Cs vapor cell coated with octadecyltrichlorosilane using coherent population trapping spectroscopy

We report the realization and characterization using coherent population trapping (CPT) spectroscopy of an octadecyltrichlorosilane (OTS)-coated centimeter-scale Cs vapor cell. The dual-structure of the resonance lineshape, with presence of a narrow structure line at the top of a Doppler-broadened structure, is clearly observed. The linewidth of the narrow resonance is compared to the linewidth of an evacuated Cs cell and of a buffer gas Cs cell of similar size. The Cs-OTS adsorption energy is measured to be (0.42 ± 0.03) eV, leading to a clock frequency shift rate of 2.7 × 10−9/K in fractional unit. A hyperfine population lifetime, T1, and a microwave coherence lifetime, T2, of 1.6 and 0.5 ms are reported, corresponding to about 37 and 12 useful bounces, respectively. Atomic-motion induced Ramsey narrowing of dark resonances is observed in Cs-OTS cells by reducing the optical beam diameter. Ramsey CPT fringes are detected using a pulsed CPT interrogation scheme. Potential applications of the Cs-OTS cell ...

Deep Wet-Etched Silicon Cavities for Micro-Optical Sensors: Influence of Masking on

In this paper, we investigate the influence of different masking parameters onto the surface quality of the {111} sidewalls in order to generate specifically deep cavities by wet-anisotropic-etching of bulk silicon, for optical sensors using cavity sidewalls as reflectors. Mask alignment with crystal planes prior to wet-etching is found to be essential in order to avoid the appearance of additional planes during long etching. Mask deposition processes have been also evaluated. Among the different employed mask materials, Cr/Au gives the best results. It is then shown that cavities as deep as 1 mm with low roughness sidewalls can be simply produced with potassium hydroxide solution with periodic piranha cleaning.

\{111\}

We report on the microfabrication and packaging of Cs-vapor cells filled with Ne or Ar for micro atomic clocks (MACs) based on the coherent population trapping (CPT). We present a two-step anodic bonding process to control the cell atmosphere for long-term operation of MACs. Furthermore, the results of thermal analysis of a cell embedded in the physical package are shown in order to discuss the optimal design of the package for the thermal management of the MAC.

Sidewalls Surface Quality

This paper reports on a compact table-top Cs clock based on coherent population trapping (CPT) with advanced frequency stability performance. The heart of the clock is a single buffer gas Cs-Ne microfabricated cell. Using a distributed feedback (DFB) laser resonant with the Cs D1 line, the contrast of the CPT signal is found to be maximized around 80°C, a value for which the temperature dependence of the Cs clock frequency is canceled. Advanced techniques are implemented to actively stabilize the clock operation on a zero-light-shift point. The clock frequency stability is measured to be 3.8 × 10-11 at 1 s and well below 10-11 until 50 000 s. These results demonstrate the possibility to develop high-performance chip-scale atomic clocks using vapor cells containing a single buffer gas.

Microfabrication and thermal behavior of miniature cesium-vapor cells for atomic clock operations

In this paper, we present the construction and preliminary experimental results of a MOEMS fiber-based integrated probe for endoscopic optical imaging of stomach tissue using a Swept-Source Optical Coherence Tomography (SSOCT). The probe consists of a Mirau micro-interferometer, combined with a GRIN lens collimator and a micromirror scanner. We describe the building blocks of the probe, especially the monolithically integrated Mirau mirointerferometer, fabricated by wafer-level vertical stacking and anodic bonding of Si/glass components, and the electrothermal 2-axis MEMS microscanner allowing large swept angles (up to ±22°) at high frequencies (> kHz) for low driving voltages (<20 V). The results of probe characterization, performed in a designed SS-OCT system, have confirmed proper operation of the probe. The B-scan images were obtained for central wavelength of λc = 840 nm, swept range of Dλ = 60 nm and A-scan frequency of fA= 110 kHz. The axial resolution of the probe is equal to 5.2 μm (determined by applied swept source), whereas the lateral resolution, measured by use of USAF test pattern, is 9.8 μm.

A high-performance frequency stability compact CPT clock based on a Cs-Ne microcell

This study concerns the dynamic behaviour of microfabricated Room-TemperatureVulcanizing (RTV) silicone elastomers based Hybrid Membranes (HMs) when subjected to different operating temperatures and after aging. The micro fabricated HMs structures aimed at being implemented in micromachines, such as for instance a micro-Stirling engine. The hybrid membranes consist of a silicon spiral structure embedded with a thin layer of an elastomeric RTV-silicone. The different process temperatures are consistent with clean room process, such as anodic bonding. In addition to an experimental study, Finite Element Model (FEM) numerical simulations are presented. The simulations include the location of the stress concentration points during a solicitation and membrane dynamic frequency, which are consistent with the experimental test results.

The SS-OCT endomicroscopy probe based on MOEMS Mirau micro-interferometer for early stomach cancer detection

We report techniques based on cesium dispensers to fill microfabricated vapor cells for miniature atomic clocks applications. Two main types of dispensers are considered, which take the form of pills or paste. Whereas the pill contains a large quantity of Cs, the paste is more compatible with wafer-level fabrication. In both cases, the main advantage is the release of the cesium vapor only once the cell has been sealed, ensuring an optimal cell fabrication. Activation is thus performed through local heating of the dispenser with a laser. Cells using both dispenser types are shown to be compatible with atomic clocks requirements, namely a clock frequency instability below 1 × 10−11 at one day integration time.