High-stability Laser Research Articles

Construction of optical system for an atomic clock-beyond atomic fountain

We demonstrate the construction of the optical system in the atomic clock-beyond atomic fountain based on 87Rb atom. The optical system includes a high-stability laser system and an optical lattice. The high-stability laser system with the new scheme of frequency locking and shift is introduced in detail, which is an important laser source for laser cooling. The optimized frequency and intensity stability are achieved to 4 × 10–14τ−1/2 (τ is the averaging time) and 4 × 10–5τ−1/2, respectively, which are highly stable. On the basis of the conventional atomic fountain clock, the optical lattice is specially investigated along the direction of gravity and its characteristics are studied systematically. For the optimized and novel exploration, we predict the achievable stability of 3.6×10−14τ−1/2 and it has the potential to be improved to 3.6×10−15τ−1/2. The realizability of the construction due to the stabilized laser and optical lattice makes the beyond fountain promising candidate for the next-generation high performance microwave atomic clock.

Design of optically pumped cesium beam tube with hexapole magnetic system for longer lifetime and better SNR

In order to get a better tradeoff between the long lifetime and high performances of cesium beam atomic clocks, we proposed a design scheme of compact optically pumped cesium beam tube (OPCBT) based on hexapole magnetic focusing (HMF) system for the confinement of cesium. A practical optically pumped cesium beam tube without HMF system has been implemented and tested with high-stability laser by modulation transfer spectroscopy (MTS), and a signal-to-noise ratio (SNR) of 18,000 in 1 Hz bandwidth and Allan deviation of 2×10−12/τ were obtained. Based on this test, the simulation analysis results of the newly designed tube with HMF show that the predicted lifetime can reach over 20 years without degradation of the performances, and the expected frequency stability of the clock can be 6.5×10−13/τ with a lifetime of 10 years.

Compact 459-nm Cs Cell Optical Frequency Standard with 2.1×10−13/τ Short-Term Stability

We achieve a compact optical frequency standard with an extended cavity diode laser locked to the 459-nm $6{S}_{1/2}$-$7{P}_{1/2}$ transition of thermal ${}^{133}\mathrm{Cs}$ atoms in a $\ensuremath{\phi}10\phantom{\rule{0.1em}{0ex}}\mathrm{mm}\ifmmode\times\else\texttimes\fi{}50\phantom{\rule{0.1em}{0ex}}\mathrm{mm}$ glass cell, using modulation transfer spectroscopy (MTS). The self-estimated frequency stability of this laser is $1.4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}14}/\sqrt{\ensuremath{\tau}}$. With heterodyne measurement, we verify the linewidth-narrowing effect of MTS locking and measure the frequency stability of the locked laser. The linewidth of each laser is reduced from the free-running 69.6 to 10.3 kHz after MTS stabilization, by a factor of 6.75. The Allan deviation measured via beat detection is $2.1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}13}/\sqrt{\ensuremath{\tau}}$ for each MTS-stabilized laser. In addition, we measure the hyperfine structure of the $7{P}_{1/2}$ energy level based on the heterodyne measurements, and calculate the magnetic dipole constant A of the Cs $7{P}_{1/2}$ level to be 94.38(6) MHz, which agrees well with previous measurements. This compact optical frequency standard can also be used in other applications that require high-stability lasers, such as laser interferometry, laser cooling, geodesy, and so on.

Fundamental physics with a state-of-the-art optical clock in space

Recent advances in optical atomic clocks and optical time transfer have enabled new possibilities in precision metrology for both tests of fundamental physics and timing applications. Here we describe a space mission concept that would place a state-of-the-art optical atomic clock in an eccentric orbit around Earth. A high stability laser link would connect the relative time, range, and velocity of the orbiting spacecraft to earthbound stations. The primary goal for this mission would be to test the gravitational redshift, a classical test of general relativity, with a sensitivity 30 000 times beyond current limits. Additional science objectives include other tests of relativity, enhanced searches for dark matter and drifts in fundamental constants, and establishing a high accuracy international time/geodesic reference.

Complementary Polarization-Diversity Coherent Receiver for Self-Coherent Homodyne Detection With Rapid Polarization Tracking

There has been renewed interest in coherent detection to provide high spectral efficiency transmission for short-reach optical interconnects. However, the expensive high-stable laser sources may preclude its potential application. To balance the high performance and low cost for short-reach optical networks, the self-coherent homodyne receiver has been investigated, where the dual-polarization (DP) signal and the remote local oscillator (LO) originating from the same laser source co-propagate over a duplex fiber, enabling a remarkable tolerance of laser linewidth. However, the fast evolution of the state of polarization (SOP) of remotely delivered LO becomes problematic for self-coherent homodyne detection systems. To combat the polarization wandering of the remote LO, the complicated adaptive/automatic polarization controller (APC) has been deployed with only up to a few hundred rad/s polarization tracking speed. In this paper, we propose a complementary polarization-diversity coherent receiver (C-PDCR) for self-coherent homodyne detection using remote LO. The proposed C-PDCR features rapid polarization tracking for remote LO utilizing electronic digital signal processing (DSP). The robustness of the proposed C-PDCR is verified and demonstrated with a 1.08-Tb/s line rate (net rate 769.4 Gb/s over 45-GHz electrical bandwidth) using DP PCS-256QAM signal under rapid polarization rotation rate up to 314 krad/s in the experiment, which is more than 3-orders improvement of magnitude compared to the prior reports.

High stability laser locking to an optical cavity using tilt locking.

This paper describes, to our knowledge, the first demonstration of high performance tilt locking, a method of stabilizing laser frequency to an optical reference cavity using a spatial-mode readout technique. The experiment utilized a traveling wave cavity with a finesse of approximately 10,000, housed in a thermally controlled vacuum chamber. The tilt locking method in a double pass configuration has promising performance in the 100 µHz-1 Hz band, including surpassing the Gravity Recovery and Climate Experiment (GRACE) Follow-On laser ranging interferometer requirement. Tilt locking offers a number of benefits such as high sensitivity, low cost, and simple implementation and therefore should be considered for future applications requiring high performance laser locking, such as future laser-based satellite geodesy missions and the Laser Interferometer Space Antenna.

High stability laser source for Taiji-1 satellite

A high stability Laser source at 1064 nm for Taiji-1 satellite is reported. The key component of the Laser source is a nonplanar ring oscillator (NPRO) solid laser with linewidth of 260 Hz. Frequency noise and power noise of Laser source are greatly improved by applying precision driving current control and temperature control. The frequency noise of 0.1 MHz/sqrt(Hz)@ 0.1 Hz and the power noise of 0.02%@0.1 Hz are obtained. Its working performances in-orbit are analyzed and compared with the ground test, showing negligible variation.

In-system optimization of a hologram for high-stability parallel laser processing.

A method for optimizing a computer-generated hologram (CGH) for high-stability laser processing is proposed. The CGH is optimized during laser processing; therefore, unpredicted dynamic changes in the laser processing system, in addition to its static imperfections, are automatically compensated for by exploiting the rewritable capability of the spatial light modulator. Consequently, the short-term and long-term stability are improved, which will contribute to the realization of high-speed, high-precision laser processing. A CGH that generated 36 parallel beams was continuously optimized, and the maximum uniformity reached 0.98, which is higher than reported in previous research. To the best of our knowledge, this is the first demonstration of gradual improvement of parallel laser processing with in-process optimization of the CGH. Furthermore, it was also demonstrated that the performance of the laser processing system against unexpected disturbances was improved.

Laser with 10−13 short-term instability for compact optically pumped cesium beam atomic clock

We realize a high-stability laser by modulation transfer spectroscopy and apply it to implement a high-performance compact optically pumped cesium beam atomic clock. Evaluated by the optical heterodyne method with two identical frequency-stabilized lasers, the frequency instability of the 852 nm laser directly referenced on thermal atoms is 2.6×10-13 at the averaging time of 5 s. Factors degrading the frequency stability of the laser are analyzed, and we will further control it to reduce the frequency noise of the laser. By comparing with a Hydrogen maser, the measured Allan deviation of the high-stability-laser-based cesium beam atomic clock is 2×10-12/τ, dropping to 1×10-14 in less than half a day of averaging time. To our knowledge, the Allan deviation of our cesium clock is better than that of any reported compact cesium beam atomic clocks at the averaging time of half-day. The high-performance atomic clock can promote the fields in metrology and timekeeping, and the high-stability laser additionally possesses great potential to be a compact optical frequency standard.

基于全保偏光纤结构的高稳定性激光相位解调系统

基于全保偏光纤结构的高稳定性激光相位解调系统

Methodological and instrumental problems in high-precision in situ ellipsometry diagnostics of the mercury cadmium telluride layer composition in molecular beam epitaxy

Problems of high-precision in situ ellipsometry diagnostics of the composition of a mercury cadmium telluride (MCT) solid solution in the process of its growth using the molecular beam epitaxy are considered. The required precision was estimated for ellipsometry measurements aimed at determining the MCT composition with a permissible dispersion of ±0.003 mole fraction of CdTe. It has been revealed that for ellipsometers based on the static photometric scheme the instability of measurements is mainly caused by a random change in the directivity of the laser radiation. In combination with polarization nonuniformity over the area of the optical-section elements, this results in continuous drift of measured ellipsometric parameters. Based on these investigations, a high-stability laser ellipsometer has been designed. When used to monitor the in situ MCT layer growth by the molecular beam epitaxy, it allowed a decrease in the dispersion of the MCT composition by an order of magnitude from experiment to experiment and its precision to be maintained at a level of ±0.003 mole fractions of CdTe.

Technology for the next gravitational wave detectors

This paper reviews some of the key enabling technologies for advanced and future laser interferometer gravitational wave detectors, which must combine test masses with the lowest possible optical and acoustic losses, with high stability lasers and various techniques for suppressing noise. Sect. 1 of this paper presents a review of the acoustic properties of test masses. Sect. 2 reviews the technology of the amorphous dielectric coatings which are currently universally used for the mirrors in advanced laser interferometers, but for which lower acoustic loss would be very advantageous. In sect. 3 a new generation of crystalline optical coatings that offer a substantial reduction in thermal noise is reviewed. The optical properties of test masses are reviewed in sect. 4, with special focus on the properties of silicon, an important candidate material for future detectors. Sect. 5 of this paper presents the very low noise, high stability laser technology that underpins all advanced and next generation laser interferometers.

980 nm高稳定度激光泵浦源控制系统

980 nm高稳定度激光泵浦源控制系统

Experimental Study of Laser Amplifier Based on LDA Side-Pumping Nd:Glass

A high beam quality and high stability laser amplifier baseing on LDA side-pumping Nd-Glass which applied to amplify a nanosecond laser pulse source has been developed in the paper. In addition, Liquid crystal spatial light modulator is used in the amplifier for spatial beam shaping so as to improve the near-field uniformity. When the flat top pules with the pulse width of 3ns and energy of 500nJ are injected as the seed pulse, the average output energy of the laser amplifier is 100mJ. The modulation degree near field is less than 1.22:1, and the beam angle drifting of the far-field laser is less than 10 μrad.

Features of the operation of high-stability lasers after connection to an unmatched optical load

The results of investigation of the laser-interferometer electromagnetic system are presented by the example of the analysis of a three-mirror optical resonator as the simplest model of a laser connected to an unmatched load. Integral equations describing the electromagnetic field in a three-mirror resonator with cylindrical mirrors are analyzed. The results obtained are verified by the numerical simulation of the behavior of eigenfrequencies for several simplest designs of a three-mirror laser resonator.

Methods for Measurement and Verification of Purity of Iodine Cells for Laser Frequency Stabilization

We present an improved technique for detection of trace impurities in iodine-filled absorption cells for laser frequency stabilization. The results of purity investigation are compared to frequency shifts measured with a set of two iodine stabilized Nd:YAG lasers. The setup for direct fluorescence measurement with an Argon-ion laser operating at 502 nm wavelength is equipped with compensation for laser power and spectral instabilities. PTICAL FREQUENCY synthesis in the field of laser metrology is becoming mainstream in these days due to the great advances in femtosecond laser technology. Frequency locking of such comb generators may be done in the radiofrequency domain to atomic clocks extending their stability into the optical spectral range or in the reverse direction to high stability lasers and also to generate rf optical clocks. Nd:YAG stabilized lasers seem to be a suitable option thanks to the relative simplicity of the stabilization scheme and stabilities that can be achieved are not far from those of the rf cesium clocks. Stabilization on the basis of saturated absorption in molecular iodine may be called a traditional method and due to good signal-to-noise ratio at the 532 nm of frequency doubled Nd:YAG laser gives very good results. Good relative stability can be easily achieved with a low-noise laser source and properly designed stabilization scheme and control electronics but the absolute value of the optical frequency is given by center frequencies of hyperfine components of the iodine transitions. Only near-ideal purity of the iodine in the absorption cell can result in optical frequencies corresponding to theoretical values. We present a technology of iodine cell preparation that is a result of a long time development and verification by an independent method based on measurement of induced fluorescence and evaluation by the Stern-Volmer formula which has been used by the Bureau International des Poids et Mesures (BIPM) so there is a chance to compare our results (1), (2), (3). The level of induced fluorescence intensity here is limited by several relaxation processes such as ionization, predissociation and collisional quenching. The quenching - nonradiative transitions - can be caused by collisions with either iodine molecules or by collisions with molecules or atoms of impurities (foreign-gas quenching). The last mentioned process is the one that reflects the purity by reducing the lifetime of a state and can be evaluated by monitoring the spontaneous emission level from the irradiated cell. The main goal of this effort is to recognize the limits of the absolute precision of the optical frequency of iodine transitions and look for further improvements in the iodine cell manufacturing technology that may lead to even smaller frequency shifts. This technique was introduced into the metrology practice with relation to He-Ne iodine stabilized lasers. With the stability that can be reached with iodine stabilized frequency- doubled Nd:YAG lasers the chance to compare the purity investigation with measured frequency shifts could go even below the kHz level (4), (5), (6). 2. MEASUREMENT OF INDUCED FLUORESCENCE

Thermal conductivity of synthetic garnet laser crystals

The thermal conductivities of nine different synthetic garnet laser crystals at various temperatures, range from 273 to 393 K have been investigated by instantaneous measurement method. The results show that the thermal conductivity of each crystal decreases exponentially with the temperature increasing. It is notable that, different host crystals, such as YAG, GGG, and GSGG have different thermal conductivity, which is attributed to the crucial influence of crystal structure and composition on the absolute value of their thermal conductivity. Moreover, with respect to the same host crystals, the impurity scattering also results in the change of their thermal conductivities. This is because that a higher concentration of doped ions leads to a more phonon scattering modes, which results in a shorter mean free path of the phonons and a lower thermal conductivity. In addition, different host crystals have various dependences of thermal conductivity on dopant concentration. This works provides reliable and useful information for designing high power, high quality, and high stability laser devices.

Topical Papers on Microchip Lasers and Applications. Characteristics of Blue and Green Solid-State Lasers Using MgO-LiNbO3 Periodic Domain Inverted Bulk SHG Crystal.

A quasi-phasematched second harmonic generation device with a periodically poled crystal is one of the most attractive methods for realizing high-quality visible lasers. We have recently newly developed a blue and green laser diode-pumped intracavity frequency doubling solid-state lasers through the use of periodically poled bulk MgO-LiNbO3. The periodically poled MgO-LiNbO3 with uniform periodicity was successfully obtained by a novel corona discharge method. The larger temperature-acceptance bandwidth of the periodically poled bulk MgO-LiNbO3 made it possible to realize the simultaneous temperature control of the laser diode and of the solid-state laser cavity. In addition, the stable single axial mode operation in the solid-state laser was accomplished by using an etalon. As a result, our new blue and green solid-state laser is compact, has high-stability laser power, low noise operation, and high beam quality. By using these blue and green solid-state lasers, a new digital photo-printing system and a new fluorescent image analyzer were realized.

Laser investigations and quantum theory

Modern high-stability lasers are extremely precise measuring instruments. It is not surprising that laser investigations often deal with the most essential problems of quantum theory. In the present paper, two close questions are discussed concerning laser physics: the reason for random photocounts in photodetectors under the action of high-coherence light and the reason for the smallness of quantum-mechanical wave packets of macroscopic bodies. Conventionally, these phenomena are explained on the basis ofa priori statistical (stochastic) approaches. The paper shows that such an explanation is internally contradictory in the context of modern laser measurements. It is also shown that these phenomena have natural explanations in terms of physics—the first, on the basis of the instability and stochastic character of electron plasma in electronic devices, and the second, on the basis of the gravitational self-focusing of wave packets for macroscopic bodies. Thus, it is shown thata priori statistical explanation of some phenomena is not physically justified, and by abandoning it one can get deep insight into them.

Modulated laser source for monitoring the stability of detectors employing photomultipliers

Light from an argon high stability laser is used together with an acoustic-optic modulator to provide constant energy calibration pulses to the photomultiplier tubes in an experimental setup. This solution is proposed as the most appropriate when many (up to one/two hundred) of PMTs must be monitored and controlled for gain stability. In our case, gain changes should be limited to 1% which means a still better reproducibility for both the intensity and the width of the calibrator light pulses. In this paper we motivate our choice, present a simple crystal controlled pulse generator which assures the required long-term width stability and give the results obtained in several experimental runs.