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The cesium fountain primary frequency standard NIM5 has been developed at the National Institute of Metrology in China. The NIM5 loads atoms in an optical molasses from the background Cs vapor directly. Atoms are then cooled to a temperature of about 2 μK and launched to a height of 81 cm. The fringes of the Ramsey pattern have a width of 0.98 Hz. The NIM5 operates for more than 300 d a year, operating nearly continuously for 15 d at a time. By stabilizing the 9.19 GHz microwave frequency to the center of the central Ramsey fringe, a typical fractional frequency instability of 3 × 10−13 (τ/s)−1/2 is obtained when running at high atom density, and a combined uncertainty, including Type A and B uncertainties, is typically 1.6 × 10−15. Comparisons of data between NIM5 and 5 other fountain clocks were carried out in May 2013 via two-way satellite time and frequency transfer (TWSTFT), and the results show good agreement within the uncertainties. Six groups of NIM5 data from January to June 2014 have been published in Circular T 319 and 320.

An ultra-stable microwave down converted from the ultra-stable laser directly by an optical frequency comb (OFC) is built at the National Institute of Metrology China. The repetition rate (f r ) of OFC is stabilized by stabilizing the sum of the carrier envelope offset frequency (f ceo ) of OFC and the beating frequency between the ultra-stable laser (USL) and OFC. A current feedback with 30 kHz bandwidth and a piezoelectric transducer with 1 kHz bandwidth are applied for the frequency stabilization. The frequency jitter of the stabilized sum frequency is about 4 Hz. Though the counting of locked f r , the shifted frequency of the USL is about 2087 Hz/day. As the frequency jitter of the mixing frequency of f ceo and the beating frequency between this OFC and another USL is about 10Hz, the short time stability of f r is calculated to be about 5e-14. This ultra-stable microwave will be applied as the local oscillator for NIM5 Cs fountain to improve its shorter term instability.

We report the design of a new cesium fountain clock NIM6, which is under construction in NIM. Besides some improvements on the vacuum system, Ramsey cavity and microwave synthesizer to reduce the Type B uncertainty. Another major improvement on NIM6 is to collect more atoms from a MOT loading optical molasses and optical pumping to get a better signal to noise ratio at the detection. The atom distribution will be more uniform compared with a 2D MOT loading optical molasses, and the diameter of the cloud can be adjusted by the intensity and detuning of lights during the post cooling to keep the collisional-induced frequency shift low. The atom numbers can be further increased by a new de-pumping - optical pumping procedure to pump atoms to the |F=3, m F =0> clock state directly. With a new cryogenic sapphire oscillator (CSO) based frequency synthesizer, NIM6 is aiming to reach the quantum projection noise, thus leading to a reduced Type A uncertainty compared with NIM5.

A displacement self-sensing laser with a resolution of λ/8 was invented by us a few years ago. This simple laser device mainly consists of a half-intracavity He–Ne laser with a birefringence element in the cavity, outputting a beam with two orthogonally polarized components. One of its cavity mirrors can move along the optical axis of the laser to change the cavity length. When this mirror is pushed (or pulled) by an item to be measured, the polarization of the laser beam varies once every λ/8 variation of the cavity length. Though the displacement self-sensing laser has many outstanding functions, its measuring range was badly limited by inevitable misalignment resulted from sway of the cavity mirror in motion. The limited range prevents the sensor from going into actual application. This paper gives a solution to the problem, using a cat's eye retroreflector to replace the flat cavity mirror of the displacement self-sensing laser to construct an anti-misalignment cavity—cat's eye cavity (CEC). The misalignment loss in the CEC is theoretically analyzed and compared with that in a flat-concave cavity, and then the experiment to test the optical stability of CEC is carried out. Furthermore, the performance of the displacement self-sensing laser based on the CEC is also investigated. Results indicate that the misalignment tolerance of the CEC is much higher than that of the flat-concave cavity, i.e. the cavity stability is greatly improved. The measuring range of the displacement self-sensing laser can be up to 12 mm with a resolution of 79.097 nm and linearity better than 0.005%.

The caesium fountain primary frequency standard NIM5 has been developed at the National Institute of Metrology (NIM) of China. The NIM5 loads atoms in an optical molasses (OM) from the background Cs vapor directly. Atoms are then cooled to a temperature of about 2 μK and launched to a height of 81 cm. The fringes of the Ramsey pattern have a width of about 1 Hz. Stabilizing the 9.19 GHz microwave frequency to the centre of the central Ramsey fringe, a typical fractional frequency stability of 3×10 -13 (τ/s) -1/2 is obtained when running at high atom density, and a combined uncertainty, including Type A and B uncertainties is typically less than 1.6×10 -15 . The NIM5 operates for more than 300 days in a year with nearly continuous operation over 15 days. Comparisons of data among the NIM5 fountain clock and other 5 fountain frequency clocks were carried out in May, 2013 through Two-Way Satellite Time and Frequency Transfer (TWSTFT), also NIM5 was compared with UTC through UTC(NIM). The results demonstrated agreement within the stated uncertainties.

An 800 nm band fiber Bragg grating sensing interrogation system using tilted FBG as the core wavelength division component is presented. A charge coupled device (CCD) linear array is put on the focal plane of the lens to detect the light. TFBG is used to tap light out of the fiber core to fiber cladding. The sensing wavelength is 795 to 830 nm, with accuracy of 20 pm and scan speed 100Hz. Using FBG sensor, we achieve the temperature sensitivity of 1.8 °C and strain sensitivity of 18 μ ɛ.

We are developing a new Cs fountain frequency standard in NIM, and shooting for the frequency uncertainty of a few parts in 1016. The new fountain will run in a lab with the temperature fluctuation of 0.3K, and no active temperature control system will be added outside the flight tube. Instead, an isothermal liner will be surrounded the flight tube. It is a special type of heat pipe, made of a thin layer of vacuum tube filled with pure water. The thermal conductivity of a glass isothermal liner is about 10 times higher than that of coppers. It also works like a low pass filter reducing the temperature fluctuations of the system. The advantage of using an isothermal liner is to make the temperature of the whole interrogation region more uniform and stable. With precision standard Pt thermometers measuring the temperatures of a few locations of the flight tube, the average temperature uncertainty is less than 50 mK. The frequency shift due to the blackbody radiation can be calculated, and the relative frequency shift due to black body radiation can reach down to 7.2E-17 of this new fountain clock. Key words: Fountain clock, blackbody radiation, heat pipe.

The ultra-stable laser can be converted to microwave signal with very high fidelity combining with the optical frequency comb, and the ultra-stable microwave down converted can be used as a local oscillator for Cs fountain clocks. To produce such a microwave source, we are developing a 1542 nm laser with a potential relative stability of 10 -15 in one second. In this paper, the ultra-stable laser system with some key parameters and the Pound-Drever-Hall (PDH) locking system are introduced. Furthermore, the method and procedure of coupling the laser into the cavity resonator is described and a coupling efficiency of better than 30% is achieved. Further experiments are carried out and final results will be reported in the future.

We report the generation of a 13dB 2nd order Bragg resonance in a conventionally UV inscribed 45° tilted fiber grating, showing strong polarization dependency and its application for singe polarization output of a fiber laser. © 2010 Optical Society of America.

In this paper, we report a simple fibre laser torsion sensor system using an intracavity tilted fibre grating as a torsion encoded loss filter. When the grating is subjected to twist, it induces loss to the cavity, thus affecting the laser oscillation build-up time. By measuring the build-up time, both twist direction and angle on the grating can be monitored. Using a low-cost photodiode and a two-channel digital oscilloscope, we have characterised the torsion sensing capability of this fibre laser system and obtained a torsion sensitivity of ~412µs/(rad/m) in the dynamic range from -150° to +150°.

A low-cost high-resolution wavelength-division-multiplexing (WDM) interrogation system operating around 800 nm region with operational bandwidth up to 60 nm and resolution of 12.7 pm utilizing a tilted fiber Bragg grating (TFBG) and a CCD-array detector has been implemented. The system has been evaluated for interrogating fiber Bragg grating based strain, temperature sensors, giving sensitivities of 0.59 pm/muepsiv and 5.6 pm/degC, which are in good agreement with previously reported values. Furthermore, the system has been utilized to detect the refractive index change of sample liquids, demonstrating a capability of measuring index change as small as 10-5. In addition, the vectorial expression of phase match condition and fabrication of TFBG have been discussed.

A multicore fibre (MCF) sensor to measure the radial deformation of a compliant cylinder under compression is presented. The sensor is connectorised and need not be permanently bonded to the test object. A differential measurement technique using FBGs written into the MCF makes the sensor temperature insensitive. FBG measurement of axial strain of a cylinder under compression is also reported.

We report a distinctive polarisation mode coupling behaviour of tilted fibre Bragg gratings (TFBGs) with tilted angle exceeding 45°. The ex-45° TFBGs exhibit pronounced polarisation mode splitting resulted from grating structure asymmetry induced birefringence. We have studied and analysed the property of ex-45° TFBGs under transverse load applied to their equivalent fast- and slow-axis. The results show that the coupling between the orthogonally polarised modes takes place only when the load is applied to its fast-axis, giving a prominent directional loading response. This transverse load related polarisation property may be exploitable for implementation of optical fibre vector sensors capable of measuring the magnitude and orientation of the applied transverse load.

We report an accelerometer based upon a simple fibre cantilever constructed from a short length of multicore fibre(MCF) containing fibre Bragg gratings (FBGs). Two-axis measurement is demonstrated up to 3 kHz.

We report the implementation of a low-cost high-resolution WDM interrogation system operating around 800nm region with bandwidth up to 60 nm and resolution of 13 pm by utilising a tilted fibre Bragg grating as an out-coupling device and a CCD-array detector. The system has been evaluated for interrogating fibre Bragg grating based strain, temperature sensors, giving sensitivities of 0.6 pm/µe and 5.6 pm/°C which are in good agreement with previously reported values. Furthermore, the system has been utilised to detect refractive index change of sample liquids, demonstrating a capability of measuring index change as small as 10-5.

An 800nm band fiber Bragg grating sensing interrogation system using TFBG as the core wavelength division component is presented. A charge coupled device (CCD) linear array is put on the focal plane of the lens to detect the light. TFBG is used to tap light out of the fiber core to fiber cladding. The sensing wavelength is 795 to 830nm, with accuracy of 20pm and scan speed 100Hz. Using FBG sensor, we achieve the temperature sensitivity as 1.8°C and strain sensitivity as 18μe.

We report for the first time the fabrication of long-period gratings in near-single-mode GeO2glass fiber by 800nm-femtosecond-laser inscription, revealing grating resonances strongly polarisation dependent and their temperature sensitivities varying significantly from 96pm/°C to 11pm/°C.

We report an implementation of optical fibre sensors based on fibre Bragg gratings with excessively tilted (>45°) structures, showing distinctive polarisation characteristics, desirable low thermal-cross-sensitivity and enhanced responsivity to surrounding-medium-refractive-index.

A transversal-load sensor based on the local pressure-induced refractive index change in a chirped fiber Bragg grating (CFBG) is proposed. The local pressure induced refractive index change in the touch point can generate a main transmission peak and several subpeaks on the long wavelength side of the reflection band of the CFBG. The difference of the wavelength shifts for the main transmission peak and the first subpeak is used to measure transversal-load with temperature compensation capability.

For the first time, Fiber Bragg grating (FBG) structures have been inscribed in single-core passive germanate and three-core passive and active tellurite glass fibers using 800 nm femtosecond (fs) laser and phase mask technique. With fs peak power intensity in the order of 10(11)W/cm(2), the FBG spectra with 2nd and 3rd order resonances at 1540 and 1033 nm in the germanate glass fiber and 2nd order resonances at approximately 1694 and approximately 1677 nm with strengths up to 14 dB in all three cores in the tellurite fiber were observed. Thermal responsivities of the FBGs made in these mid-IR glass fibers were characterized, showing average temperature responsivity approximately 20 pm/ degrees C. Strain responsivities of the FBGs in germanate glass fiber were measured to be 1.219 pm/microepsilon.

We report a distinctive polarization mode coupling behaviour of tilted fibre Bragg gratings (TFBGs) with a tilted angle exceeding 45°. The ex-45° TFBGs exhibit pronounced polarization mode splitting resulted from the birefringence induced by the grating structure asymmetry. We have fabricated TFBGs with a tilted structure at 81° and studied their properties under transverse load applied to their equivalent fast and slow axes. The results show that the light coupling to the orthogonally polarized modes of the 81°-TFBGs changes only when the load is applied to their slow axis, giving a prominent directional loading response. For the view of real applications, we further investigated the possibility of interrogating such a TFBG-based load sensor using low-cost and compact-size single wavelength source and power detector. The experimental results clearly show that the 81°-TFBGs plus the proposed power-measurement interrogation scheme may be developed to an optical fibre vector sensor system capable of not just measuring the magnitude but also recognizing the direction of the applied transverse load. Using such an 81°-TFBG based load sensor, a load change as small as 1.6 × 10-2 g may be detected by employing a standard photodiode detector.

The novel principle of laser feedback displacement sensors is presented and demonstrated. A birefringent dual-frequency laser is used as the light source, which outputs two polarization lights, o light and e light. The two polarization lights are divided by a Wollaston prism and detected separately by two photoelectric detectors. When the feedback mirror is moved along the laser axis, the intensities of both the o light and the e light change periodically. And an increase of o light intensity always accompanies a decrease of e light intensity. If a threshold intensity for the output light is appropriately given by the signal processing circuits, the remaining intensity curves in each period shows four different polar- ization states: e light, e light and o light, o light, and no light. Each change of polarization state corresponds to a l/8 displacement of the feedback mirror. We judge the movement direction by the difference of the order of the polarization states that appear. The resolution of the system is 0.079 mm. The possibility of employing the fold feedback cavity is discussed, and its resolution is believed to reach 0.0395 mm. © 2003