Frequency Stabilization Scheme Research Articles

Frequency stabilization using infinite impulse response filtering for SSFP fMRI at 3T

The steady-state free precession (SSFP) method has been shown to exhibit strong potential for distortion-free functional magnetic resonance imaging (fMRI). One major challenge of SSFP fMRI is that the frequency band corresponding to the highest functional sensitivity is extremely narrow, leading to substantial loss of functional contrast in the presence of magnetic field drifts. In this study we propose a frequency stabilization scheme whereby an RF pulse with small flip angle is applied before each image scan, and the initial phase of the free induction decay (FID) signals is extracted to reflect temporal field drifts. A simple infinite impulse response (IIR) filter is further employed to obtain a low-pass-filtered estimate of the central reference frequency for the upcoming scan. Experimental results suggest that the proposed scheme can stabilize the frequency settings in accordance with field drifts, with oscillation amplitudes of <0.5 Hz. Phantom studies showed that both slow drifts and fast fluctuations were prominently reduced, resulting in less than 5% signal variations. Visual fMRI at submillimeter in-plane resolution further demonstrated 15% activation signals that were nicely registered in the microvessels within the sulci. It is concluded that the IIR-filtered frequency stabilization is an effective technique for achieving reliable SSFP fMR images at high field strengths.

High-power frequency-stabilized laser for laser cooling of metastable helium at 389 nm

A high-power, frequency-stabilized laser for cooling of metastable helium atoms using the 2S13→3P23 transition at 389 nm has been developed. The 389 nm light is generated by frequency doubling of a titanium:sapphire laser in an external enhancement cavity containing a lithium–triborate nonlinear crystal. With a maximum conversion efficiency of 75%, 1 W of useful 389 nm power is produced out of 2 W at 778 nm. While being stabilized to the 2S13→3P23 transition, the 389 nm frequency is tunable over ±150MHz with respect to the field-free atomic resonance frequency. This is accomplished by Zeeman tuning of the absorption lines used in the frequency-stabilization scheme. The setup for saturated absorption spectroscopy in an rf discharge cell, used to stabilize the 389 nm laser to the atomic transition, is described in detail.

Performance of a 1200 m long suspended Fabry–Perot cavity

Using one arm of the Michelson interferometer and the power recycling mirror of the interferometric gravitational wave detector GEO 600, we created a Fabry–Perot cavity with a length of 1200 m. The main purpose of this experiment was to gather first experience with the main optics, its suspensions and the corresponding control systems. The residual displacement of a main mirror is about 150 nm rms. By stabilizing the length of the 1200 m long cavity to the pre-stabilized laser beam, we achieved an error point frequency noise of 100 μHz Hz−1/2 at 100 Hz Fourier frequency. In addition we demonstrated the reliable performance of all included subsystems by several 10-hour-periods of continuous stable operation. Thus the full frequency stabilization scheme for GEO 600 was successfully tested.

Phase-coherent multilevel two-photon transitions in cold Rb atoms: Ultrahigh-resolution spectroscopy via frequency-stabilized femtosecond laser

A two-photon transition in cold Rb atoms will be probed with a phase-coherent wide bandwidth femtosecond laser comb. Frequency domain analysis yields a high-resolution picture where phase coherence among various transition pathways through different intermediate states produces interference effects on the resonantly enhanced transition probability. This result is supported by the time domain Ramsey interference effect. The two-photon transition spectrum is analyzed in terms of the pulse repetition rate and carrier frequency offset, leading to a cold-atom-based frequency stabilization scheme for both degrees of freedom of the femtosecond laser.

Diode laser frequency-noise suppression by &gt;50 dB by use of electro-optic parametric master oscillators

We demonstrate a novel frequency-stabilization scheme for laser diodes that is capable of linewidth reduction by more than 5 orders of magnitude. In this master-slave scheme the diode laser emission is frequency-offset phase locked to the emission of an electro-optic parametric oscillator (EOPO), and the laser diode is simultaneously used as the EOPO pump source. As a result the initial frequency fluctuations of the pump are reduced to the intrinsic noise level of the EOPO, which can be extremely small. We demonstrate that subhertz linewidths of the beat notes of the signals of two of these systems can be readily achieved if acoustic perturbations are suppressed.

Power scaling and frequency stabilization of an injection-locked Nd:YAG rod laser

The power scaling and frequency stabilization of a high-power, injection-locked, arc-lamp-pumped Nd:YAG laser at 1064 nm are discussed theoretically and experimentally. Thermal lensing and induced birefringence at high pump powers are modeled, and the effectiveness of the model for compensating thermal lensing is demonstrated with four different laser heads. Two distinct active frequency-stabilization schemes for injection-locked lasers are also compared theoretically and experimentally. These efforts yield a 24-W, linearly polarized, continuous-wave, TEM00 output with a spectral linewidth of 1.5 Hz measured by heterodyne detection.

Laser frequency stabilization by means of optical self-heterodyne beat-frequency control

We propose and demonstrate a novel frequency-stabilization scheme that provides linewidth narrowing and at the same time is compatible with high-speed frequency agility. The method relies on sensing and control of a heterodyne beat signal derived from a fiber interferometer and functions in the absence of a fixed reference frequency. Our demonstration utilizes a short-external-cavity diode laser equipped with an intracavity electro-optic crystal for frequency correction. The stabilization method is shown to suppress impressed laser frequency modulation by nearly 2 orders of magnitude. Without further modification of our scheme we also demonstrate laser frequency tuning control.

Laser frequency stabilization using selective reflection spectroscopy

line. We discuss the performance of this novel frequency stabilization scheme and analyze its dependence on various system parameters. The technique is compared to the commonly used saturation spectroscopic techniques.

CO/sub 2/ laser stabilization to 0.1-Hz level using external electrooptic modulation

We have developed a frequency stabilization scheme for CO/sub 2/ lasers using only external modulation via an electrooptic modulator (EOM). One of the two laser sidebands which are generated by the EOM and frequency-modulated is set in resonance with a Fabry-Perot cavity, itself filled with OsO/sub 4/ as an absorber. The saturation signal of an OsO/sub 4/ line detected in transmission of the Fabry-Perot cavity is used for stabilization. We obtained a stability of 0.1 Hz (/spl Delta//spl nu///spl nu/=3.5 10/sup -15/) on a 100-s time scale, and a reproducibility up to 10 Hz with the strongest OsO/sub 4/ reference lines. These results largely improve the performance of our previous setup for which modulation was applied through piezoelectric transducers. Further, the stabilized laser is not frequency-modulated and is easily tunable.

Decentralized frequency stabilization scheme for a dense OFDM system involving simple filters and an absolute reference

A frequency control scheme is introduced where within each transmitter its frequency is referenced to a silica based optical frequency discriminator, which in addition is locked to an absolute reference frequency. An absolute stability of /spl plusmn/1 GHz was achieved. As frequency-determining elements, coupler-based Michelson interferometers or coated gradient-index (GRIN) lenses were used. The described set-up is employed in an OFDM based optical crossconnect-demonstrator (RACE 2065; COBRA).

Continuously tunable single-frequency erbium ring fiber laser

We report on a frequency-stabilized narrow-linewidth continuous-fiber erbium-doped ring fiber laser in which the operating wavelength is defined by an overlay filter. The oscillating frequency is stabilized by a side-of-fringe locking technique, and longitudinal mode hops are inhibited by phase-sensitive detection of the signal rejected from an intracavity all-fiber étalon. With this frequency-stabilization scheme resolution-limited linewidths of 68 kHz with a root mean square frequency jitter of 10 kHz are measured. The oscillating frequency is continuously tunable over 3 GHz.

Frequency stabilization of tunable DBR laser using multiwavelength light injection locking technique

This paper proposes a frequency stabilization scheme for tunable three-section DBR laser diodes (3S-DBR LD) that use multiwavelength light injection locking. The oscillating wavelength of the SS-DBR LD is discretely switched between cavity modes when the injection current into DBR section is changed, and locked to one wavelength of the multiwavelength light injected from the DBR section facet under the injection locking condition. The light injection properties of the capture range and the relationships of relaxation oscillation versus input power and detuning are investigated experimentally. Injection locking on the multistate wavelength of a tunable DBR LD is performed using a two wavelength multiplexed light. As a result, we demonstrate 1 GHz capture range and more than 26 dB rejection ratio for the multiplexed injected light.

81% Conversion efficiency in frequency-stable continuous-wave parametric oscillation

We have studied doubly resonant parametric oscillation in the regime of maximum conversion efficiency. A monolithic MgO:LiNbO3 resonator with a threshold of 28 mW, pumped by a frequency-doubled Nd:YAG laser, was employed. With an active frequency-stabilization scheme, single-mode operation without mode hops at 105 mW combined signal and idler output power was achieved at a pump power four times above threshold, with a conversion efficiency of 81%. Excellent agreement between theory and experiment was found for the dependence of efficiency and depletion on pump power. The efficiency and the nearly complete pump depletion of 94% were limited only by imperfect mode matching of the pump wave and by the small cavity losses.

Magnetic resonance on single molecules in an external magnetic field. The Zeeman effect of a single electron spin and determination of the orientation of individual molecules

The interaction of a single electron spin with an external magnetic field is presented. The broadening and shift in the electron spin resonance lineshape is discussed on the basis of classical ensemble theories. Due to a novel frequency stabilization scheme we were able to perform time consuming investigations at different orientations of the external magnetic field with respect to the molecule. Two different orientations of the molecules in the sample have been found which coincide in their relative orientation with two sites of matrix molecules. The orientational disorder of the molecules belonging to one of these sites is below 8°.

Frequency Stabilization Based on Attenuated Fiber Loop Resonator (AFLR)

A new device ― Attenuated Fiber Loop Resonator (AFLR) ― for frequency stabilization is proposed. The AFLR simply includes a variable intensity attenuator in a fiber loop which can be tuned to achieve the best frequency stabilization condition. We derive the transmission function of AFLR by improving the matrix model which now includes intrinsic fiber loss in the fiber loop and coupling loss in the fiber coupler. The relation between coupling coefficient and loop transmission is also analyzed. Two new laser diode frequency stabilization schemes using AFLR are proposed and the optimum operating conditions are obtained. Simulation results also show that the coupling loss which degrades the transmission response of AFLR is detrimental to frequency stabilization

A frequency stabilization arrangement for dense wavelength division multiplexing systems

A frequency stabilization scheme using a tunable twin-guide (TTG) laser diode and a Bragg grating filter on a passive waveguide is proposed. This arrangement is fully compatible with the technologies of monolithic integrated optics. The concept was tested in a hybrid assembly. The frequency drift at laser temperature was 820 MHz/ degrees C and the tuning rate at output power was 310 MHz/mW.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Accurate and simple optical frequency stabilization for a coherent multichannel system

Previously published optical frequency stabilization schemes needed development of a fair amount of extra hardware. The authors use a personal computer and a standard heterodyne receiver to perform optical frequency stabilization with a 40-channel capability and only +or-80 MHz channel separation drift. This eliminates a large part of the overhead costs of frequency stabilization in coherent optical multichannel systems.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

A novel IF frequency stabilization scheme for coherent optical FDM systems

An IF frequency stabilization scheme for coherent multichannel systems was demonstrated. The transmitter laser diodes are frequency-modulated with sinusoidal signals for channel spacing stabilization. A frequency-selective IF servo-system consisting of a phase-locked loop and a synchronous filter was used to make the local oscillator track the additional sinusoidal modulation on the transmitter frequency so that the modulation amplitude in the IF domain was greatly reduced. In the demonstration experiment with frequency-shift-keying (FSK) transmission, it was reduced to 5 MHz/sub p-p/ and no system performance degradation due to this residual modulation was observed. The circuit also gives a channel identification signal. >

Frequency stabilization of semiconductor lasers for applications in coherent communication systems

The stabilization of the mean frequency of a single-mode semiconductor laser used under modulation in a FSK heterodyne transmission system is achieved by a novel method. The frequency stabilization scheme locks the laser to one of the resonance frequencies of a Fabry-Perot interferometer (FPI). The laser is weakly modulated at a frequency higher than any signal frequencies. Light reflected from the FPI is detected and synchronously demodulated for feedback and current control of the laser. A laser modulated by a pseudorandom sequence with a rate of 140 Mbit/s and an optical frequency deviation of 280 MHz is stabilized with this method against variations in operating temperature and bias current. The mean optical frequency is maintained within ± 10 MHz for hours. Analysis and experiments confirm that the laser frequency remains stable even when the laser linewidth changes under weak optical feedback.

Temporal coherence properties of picosecond pulses generated by GaAlAs semiconductor lasers for directly modulated and frequency stabilized optical communication systems

The temporal coherence properties of picosecond pulses from GaAlAs semiconductor lasers have been studied by measuring first- and second-order autocorrelation functions with a Michelson interferometer. The time-bandwidth product and chirp have been evaluated for several different laser structures and frequency stabilization schemes. Time-bandwidth products from 1.2 for a CSP laser to 3.6 for a TJS laser were obtained with operating conditions representative of high-speed communication systems. Computer simulations based on the rate equations have been used to model the temporal coherence during the pulse and they show good agreement with our measurements. The results show that it is important to select the appropriate laser structure and stabilization scheme in applications where the temporal coherence is significant for system performance.