Servo Electronics Research Articles

Stable and compact RF-to-optical link using lithium niobate on insulator waveguides

Optical frequency combs have become a very powerful tool in metrology and beyond, thanks to their ability to link radio frequencies with optical frequencies via a process known as self-referencing. Typical self-referencing is accomplished in two steps: the generation of an octave-spanning supercontinuum spectrum and the frequency-doubling of one part of that spectrum. Traditionally, these two steps have been performed by two separate optical components. With the advent of photonic integrated circuits, the combination of these two steps has become possible in a single small and monolithic chip. One photonic integrated circuit platform very well suited for on-chip self-referencing is lithium niobate on insulator—a platform characterized by high second and third order nonlinearities. Here, we show that combining a lithium niobate on insulator waveguide with a silicon photodiode results in a very compact and direct low-noise path toward self-referencing of mode-locked lasers. Using digital servo electronics, the resulting frequency comb is fully stabilized. Its high degree of stability is verified with an independent out-of-loop measurement and is quantified to be 6.8 mHz. Furthermore, we show that the spectrum generated inside the lithium niobate waveguide remains stable over many hours.

Note: Auto-relock system for a bow-tie cavity for second harmonic generation

This Note reports on the implementation of an automatic relocking system for a bow-tie cavity for second harmonic generation to produce an ultra-violet laser source. The system is based on a sample-and-hold technique for controlling the cavity length using simple servo electronics. Long-term stabilization of the cavity output power is successfully achieved, which makes this system suitable for designing stable atomic physics experiments.

Frequency stability of a femtosecond optical parametric oscillator frequency comb

Carrier-envelope-offset (CEO) and pulse-repetition frequencies of a Ti:sapphire-pumped femtosecond optical parametric oscillator were locked to uncertainties of 0.09 Hz and 0.16 mHz respectively, with the CEO beat signal linewidth being stabilized to 15 Hz (instrument limited). In-loop phase-noise power spectral density measurements showed a contribution of our servo electronics to the comb-line frequency uncertainty of up to 110 Hz. Complementary time-series data implied an in-loop comb instability of 2 x 10(-11) (1-s gate time), matching the Rb-stabilized reference used and verifying that dual servo-control of the CEO and repetition frequencies was effective in stabilizing the comb to at least this precision.

High Performance Small Optically Pumped Caesium Beam FrequencyStandard

An experiment of a high performance small optically pumped caesium (Cs)beam frequency standard is reported. An extended cavity diode laserworks as the probing laser, of which the frequency is stabilized by theZeeman modulation method. The running parameters of the frequencystandard are dynamically optimized via digital servo electronics. Theexperimental setup improves the frequency stability up to 1.8×10−12 at τ = 1 s and about 1.0×10−13 atτ = 105 s (Allan deviation).

Sub-ppm multi-gas photoacoustic sensor

A photoacoustic multi-gas sensor using tuneable laser diodes in the near-infrared region is reported. An optimized resonant configuration based on an acoustic longitudinal mode is described. Automatic tracking of the acoustic resonance frequency using a piezo-electric transducer and a servo electronics is demonstrated. Water vapour, methane and hydrogen chloride have been measured at sub-ppm level in different buffer gas mixtures. The importance of the system calibration in presence of several diluting gases is discussed. Finally, trace gas measurements have been assessed and detection limits (signal-to-noise ratio = 3) of 80 ppb at 1651.0 nm for CH 4, 24 ppb at 1368.6 nm for H 2O and 30 ppb at 1737.9 for HCl have been demonstrated.

Comparison of two wavelength reference laser systems at λ ≈ 515 nm, stabilized by different methods

The results of a comparison of two of the BIPM iodine-stabilized Ar+ laser systems at λ ≈ 515 nm are presented. One of these laser systems uses the traditional third-harmonic locking technique while the other is realized using high-frequency modulation techniques. The frequency stability found in terms of the relative Allan standard deviation was 1,3 × 10-12 τ-1/2 for τ < 100 s, with a minimum of 1,5 × 10-13 at about 100 s. This limit is believed to be set by imperfections in the servo electronics for the laser using the third harmonic technique. The frequency difference between the two lasers was measured for the a3 component of the P(13) 43-0 transition over a period of twelve days. The mean frequency difference was found to be 0,1 kHz with a sample standard deviation of 1 kHz. The influence on the laser frequency of the modulation index, iodine pressure and laser power are discussed.

A digital piezoelectric accelerometer with sigma-delta servo technique

This paper presents the design and performance of a direct digital accelerometer using oversampling sigma-delta servo electronics. The measuring system, also called an ‘electromechanical sigma-delta modulator’, is based on a piezoelectric measuring cell integrated inside the first stage of a second-order sigma-delta modulator. The piezoelectric measuring cell has a new structure in order to realize the acceleration sensing and the servo-loop summer. The active material used in an inexpensive and versatile piezoelectric polymer, polyvinylidene fluoride (PVDF). The accelerometer aims at a working range of ± 1 g and eight-bit resolution. It is suited for vibration measurement.

Diode-pumped, continuous-wave, single-mode, tunable Cr^3+:LiSrAlF_6 ring laser

We describe an all-solid-state, tunable, single-frequency, continuous-wave Cr(3+):LiSrAlF(6) ring laser. When pumped with 360 mW of power from a single laser diode the laser has tuning range from 814 to 892 nm and a free-running frequency stability relative to a reference cavity of 60 kHz rms. Further stabilization promises to be possible by use of standard servo electronics. We demonstrate the use of the laser by performing Dopplerfree saturation spectroscopy of the cesium D2 line.

Error analysis of the NIST optically pumped primary frequency standard

The major sources of systematic error in the NIST optically pumped primary frequency standard has been evaluated with an uncertainty of a few parts in 10/sup 14/. The cavity end-to-end phase shift is the only item which differed markedly from expectations based on the design and/or measurements made during assembly. The authors think the difference is attributable to the physical dimensions of the cavity and not to its asymmetry. The device will begin to function as the US primary frequency standard, even before planned extensions to the analysis presented and improvement to the laser and servo electronics allow evaluation at its full design accuracy of one part in 10/sup 14/. >

A secondary wobbling mechanism for a balloon-borne infrared telescope

A wobbling mechanism for a secondary mirror has been developed for a balloon-borne infrared telescope. Friction of the wobbling mechanism is negligibly small, and hence the wobbling mechanism is very reliable for the use in a severe environment at balloon altitudes. Motion is controlled by servo electronics, whose transfer function includes the second-order differential term of the error signal in order to improve the waveform. Good performance of the drive mechanism has been confirmed in two balloon flights in 1988 at an altitude of 31 km.

Active nutation control of spacecraft using gimballed momentum wheel

In this paper, mainly a control method using both spacecraft attitude information and wheel gimbal signals is described. At first, system formation and its equations of motion are described, where interaxis cross coupling is positively utilized. And then described are approximate analytical solutions of the equations, where the system is linearized and complex variables are used. If, sensor signals of both roll and yaw attitude angles of spacecraft are available in gimbal servo electronics, nutational motion of the spacecraft can be decayed very quickly. And the system eigen values can be solved in very simple forms. It is shown that these spacecraft attitude signals bring on by far effective results of the nutation damping compared to the open loop case where only gimbal-axis sensor signals are used. These results are discussed and it is pointed out that outer-rotor type momentum wheels using spiral grooved bearing or ball bearing may cause nutational motion of the spacecraft, i.e. negative nutational damping. Finally, the authors recommend inner-rotor type momentum wheel, in case of spiral grooved or ball bearing, from a stand point of spacecraft nutational motion. Computer simulation results are presented which verify the above-metioned analytical solutions.

Improved self-calibrating rubidium magnetometer accurate to 0.01 nT rms

Self-calibrating Rb87 magnetometers, designed for high-accuracy geomagnetic field measurements, have been improved to attain 0.008 nT rms absolute accuracy. Previous designs were limited to accuracies of 0.04 nT rms or less as a consequence of diurnal temperature drifts and high-frequency noise. We describe a modification to the magnetometer housing that eliminates the temperature drift. We also describe improvements in the 5-Hz servo and radio frequency electronics that lead to an increase in high-frequency stability.

Dye laser spectrometer for ultrahigh spectral resolution: design and performance

A dye laser spectrometer for ultrahigh spectral resolution is described. The laser frequency is stabilized to the side of a transmission fringe of an optical cavity by means of the usual differencing servo technique. With an intralaser-cavity AD(*)P phase modulator, driven by improved fast servo electronics, the linewidth of the jet stream dye laser was reduced to 1.8 kHz rms. With fast amplitude stabilization a 1.0-kHz line-width was observed. Good long-term stability and digital frequency scanning (with a step resolution of 1 kHz and a continuous tuning range of 900 MHz) are accomplished by transferring the long-term stability of an I(2)-stabilized He-Ne laser to the dye laser via a second optical cavity and an offset locked He-Ne laser. A drift rate of <1 kHz/min was obtained while using this dye laser spectrometer to investigate two-photon optical Ramsey fringes. A fringe width of the Ramsey features of 17 kHz has been observed, confirming for the first time the high resolution capability of two-photon optical Ramsey resonances.

Direct demodulation of a 10.6µm FM signal with an NH&amp;lt;inf&amp;gt;2&amp;lt;/inf&amp;gt;D stark cell

A new technique for direct demodulation of a 10.6-μm FM signal using a Stark absorption cell is described. A Stark tunable absorption line in NH 2 D is used to track the time-varying frequency of the incoming FM laser signal using a closed servo loop. The demodulated FM signal is proportional to the feedback voltage applied to the Stark cell. Demodulation at frequencies up to 5 kHz has been demonstrated, the rate being limited only by the bandwidth of the available servo electronics.

Test Evaluation of a High Performance Pulse Rebalanced Gyro for Strapdown IMU Applications

Recent mission requirements in the strapdown Inertial Measurement Unit (IMU) field have emphasized the need for inertial sensors which combine the capability of providing precision attitude reference with long mission life. The single-axis hydrodynamic-spin-bearing gyroscope operated with pulse-rebalance servo electronics is a prime candidate sensor for these missions. This paper provides a summary of recent test results and analytical evaluation for a high-performance instrument of this type. The sensor parameters and performance test results summarized in this paper are based on an extensive evaluation program of the 64 PM RIG gas-bearing gyro. The 64 PM RIG design base is the 64 AC RIG (25 IRIG), which was initially developed by MIT, with the primary difference between the two instruments being the use of a permanent magnet DC torquer (rather than an AC torquer) in the 64 PM RIG. The 64 AC RIG was originally developed and produced in large quantities for an inertially-stabilized platform used in a missile application and has since been used as an inertial sensor on several spacecraft programs (OSO-7 and ATS-6). A single-degree-of-freedom gyro, the 64 PM RIG utilizes a hydrodynamic gas bearing, a microsyn pickoff, a magnetic suspension, and a permanent-magnet torquer, and is fully floated to minimize error torques about its sensitive axes. Sensor drift data is summarized which demonstrates the bias variability over a 15 hour period to be less than 0.001 deg/hr. The specific strapdown IMU performance areas which are addressed include sensor scale factor linearity and stability, the noise output of the sensor as characterized by its power spectral density, the attitude reference error buildup as a function of the interval between bias updates, and performance of the sensor in the dynamic environment typically encountered during vehicle maneuvers. The pulse-rebalance technique implemented is binary (forced-limit-cycle), and uses quantized pulse-width modulation of the gyro torquing signal. A dual-scale design of the pulse-rebalanced sensor has been implemented and evaluated. The pulse-rebalanced sensor has a demonstrated capability of maintaining an attitude reference within 1.0 arc-second for time periods exceeding 30 minutes. Test data demonstrating scale factor non-linearity of less than 40 PPM and scale factor stability below a PPM/day is presented. The 64 PM RIG sensor is being incorporated into two strapdown IMUs which will provide the primary attitude reference for the International Ultraviolet Explorer (IUE) and the High Energy Astronomy Observatory (HEAO) spacecrafts. The six-sensor IUE/IMU and the modular two-sensor HEAO/IMU are discussed from the packaging (size, weight, power) and functional viewpoints. Included is a brief description of the hybridized pulse-rebalance servo electronics used for the two programs.

Micro Compass for Small Boat Navigation

The rapid growth of a vast variety of small boats with wide cruising ranges has created an increased demand for improved navigation capability utilizing miniaturized, low cost, low power equipment having a high degree of performance. At the heart of this system is the ship's compass. Magnetic compasses are subject to large uncertainties. Gyro compasses have been too large, too costly and have required too much power. A new gyro compass concept utilizing the principles of an inertial quality gyroscopic component has been conceived that can provide a marine compass meeting the above requirements. Combining the mechanism of the classic pendulous compass with an automatic correction technique leads to a unit of outstanding simplicity and miniaturization. This approach eliminates the need for conventional servo electronics which are inherent in modern, precision gyrocompasses. A conceptual description of the compass is introduced and a brief discussion of computer simulations of its operating characteristics.