Narrow-linewidth Laser System Research Articles

Stable, narrow-linewidth laser system with a broad frequency tunability and a fast switching time

For a Rydberg atom-based sensor to change its sensing frequency, the wavelength of the Rydberg state excitation laser must be altered. The wavelength shifts required can be on the order of 10 nm. A fast-tunable narrow-linewidth laser with broadband tuning capability is required. Here, we present a demonstration of a laser system that can rapidly switch a coupling laser as much as 8 nm in less than 50 μs. The laser system comprises a frequency-stabilized continuous wave laser and an electro-optic frequency comb. A filter enables selection of individual comb lines. A high-speed electro-optic modulator is used to tune the selected comb line to a specific frequency, i.e., an atomic transition. Through Rydberg atom-based sensing experiments, we demonstrate frequency hopping between two Rydberg states and a fast switching time of 400 μs, which we show can be reduced to ∼50 μs with a ping-pong scheme. If updating the RF frequency is not required during frequency hopping, a 200 ns switching time can be achieved. These results showcase the potential of the laser system for advanced Rydberg atom-based radio frequency sensing applications, like communications and radar.

Fabrication of a Counter-Directional Pump/Signal Combiner with Built-In Mode Field Adapter

This article introduces a novel counter-directional pump/signal combiner with a built-in mode field adapter. This combiner offers additional functionality to address the problem of mode field mismatch between the gain fiber and delivery fiber, which occurs when using a large-core (50 μm) signal fiber to suppress stimulated Raman scattering. The combiner exhibits negligible degradation in beam quality as a result of implementing mode field matching and employing an effective feedback alignment technique. The beam quality of the mode field adapter integrated in the combiner is improved by 10%, compared to the conventional approach, thanks to the reduction in splicing points and the prevention of fusion loss caused by tapering-induced cladding diameter mismatch. The combiner demonstrates an average pump coupling efficiency of 98.5% and a temperature rise coefficient of less than 10 °C/kW without active cooling. Furthermore, an integrated device based on the combiner is fabricated to shorten the length of delivery fiber, therefore mitigating the effects of stimulated Raman scattering. All those techniques mentioned above are utilized in a narrow linewidth laser system, resulting in an output of approximately 5840 W with a nearly single-mode characteristic.

Atom Interferometry with Rb Blue Transitions.

We demonstrate a novel scheme for Raman-pulse and Bragg-pulse atom interferometry based on the 5S-6P blue transitions of ^{87}Rb that provides an increase by a factor ∼2 of the interferometer phase due to accelerations with respect to the commonly used infrared transition at 780nm. A narrow-linewidth laser system generating more than 1W of light in the 420-422nm range was developed for this purpose. Used as a cold-atom gravity gradiometer, our Raman interferometer attains a stability to differential acceleration measurements of 1×10^{-8} g at 1s and 2×10^{-10} g after 2000s of integration time. When operated on first-order Bragg transitions, the interferometer shows a stability of 6×10^{-8} g at 1s, averaging to 1×10^{-9} g after 2000s of integration time. The instrument sensitivity, currently limited by the noise due to spontaneous emission, can be further improved by increasing the laser power and the detuning from the atomic resonance. The present scheme is attractive for high-precision experiments as, in particular, for the determination of the Newtonian gravitational constant.

Yb-doped aluminophosphosilicate fiber amplifier with 244 pm linewidth and near diffraction limited beam quality for beam combination

Yb-doped aluminophosphosilicate fiber was fabricated by the combination system of chelate precursor doping technique and the modified chemical vapor deposition system. The technical details of the fabrication process were optimized to ensure the same elemental concentration ratio of Al/P. The capability of this fiber for the beam combining application was investigated based on a narrow linewidth fiber laser system. This fiber presented the output power of 1783 W with a slope efficiency of 83.7 % and a 20 dB root-mean-square linewidth of 0.244 nm with no nonlinear effects in the laser spectrum. Near-diffraction-limited beam quality was also observed with a M2 factor less than 1.27. In conclusion, this fiber presented an attractive laser performance for beam combination through the comprehensive optimization of fabrication technique and narrow linewidth laser system.

Zero-crossing temperature of ultra-stable optical reference cavity measured by optical transition spectrum

In an experimental system of <sup>87</sup>Sr atomic optical lattice clock, the free-running 698 nm diode laser is locked in an ultra-stable optical reference cavity to obtain the ultra-stable narrow linewidth laser with good short-term frequency stability. The ultra-stable optical reference cavity, which is usually composed of glass material doped with titanium dioxide for ultra-low thermal expansion coefficient and two highly reflective fused quartz mirrors, is called ULE cavity. The cavity length is prone to being affected by mechanical vibration, temperature change, airflow, etc. The stability of the cavity length determines the stability of the final laser frequency. Near the room temperature, there exists a special temperature point for the ultra-low expansion glass material, at which temperature its thermal expansion coefficient becomes zero, which is called the zero-crossing temperature. At the zero-crossing temperature, the length of the ULE cavity is not sensitive to the temperature fluctuation, reaching a minimum value, and the laser locked to the ULE cavity has a minimum frequency drift. In order to reduce the influence of temperature on the laser frequency instability, the zero-crossing temperature of the ultra-stable optical reference cavity of 698 nm ultra-stable narrow linewidth laser system is measured by using the clock transition spectrum of the strontium atomic optical lattice clock. The frequency drift and frequency instability of the 698 nm ultra-stable narrow linewidth laser system at zero-crossing temperature are measured by using the change of the in-loop locked clock frequency of strontium atomic optical lattice clock. By scanning the atomic clock transition frequencies at different temperatures, the clock transition spectra at different temperatures are obtained. The second order polynomial fitting of the central frequency of the clock transition spectrum with the change curve of temperature is carried out, and the zero-crossing temperature of the 698 nm ultra-stable narrow linewidth laser system ULE cavity is measured to be 30.63 ℃. At the zero-crossing temperature, the 698 nm ultra-stable narrow linewidth laser frequency is used for in-loop locking of <sup>87</sup>Sr atomic optical lattice clock. The linear drift rate of the ULE cavity in the 698 nm ultra-stable narrow linewidth laser system is measured to be 0.15 Hz/s, and the frequency instability of the 698 nm ultra-stable narrow linewidth laser system is 1.6 × 10<sup>–15</sup> at an average time of 3.744 s. The determination of ULE cavity zero-crossing temperature for the 698 nm ultra-stable narrow linewidth laser system is of great significance in helping to not only improve the instability of the laser system, but also increase the instability of <sup>87</sup>Sr optical lattice clock system. In the future, we will improve the temperature control system of the ULE cavity in the 698 nm clock laser system, enhancing the temperature control accuracy of the ULE cavity and reducing the measurement error, thus achieving a more accurate zero-crossing temperature and further improving the frequency instability of the 698 nm ultra-stable narrow linewidth laser system.

A compact sub-hertz linewidth Fabry Perot cavity frequency stabilized laser for space application

The narrow linewidth laser which can operate reliably in space environment has important application value. This paper introduces the prototype of a miniaturized narrow linewidth laser for space application based on a self-developed high finesse optical reference cavity. The prototype performs beat frequency comparison measurement with another narrow linewidth laser system in the laboratory. The fractional frequency instability of a single system is 1.6×10- 15@1s achieved preliminarily, and the linewidth is 0.86 Hz.

Optical performance of large-area crystalline coatings.

Given their excellent optical and mechanical properties, substrate-transferred crystalline coatings are an exciting alternative to amorphous multilayers for applications in precision interferometry. The high mechanical quality factor of these single-crystal interference coatings reduces the limiting thermal noise in precision optical instruments such as reference cavities for narrow-linewidth laser systems and interferometric gravitational wave detectors. In this manuscript, we explore the optical performance of GaAs/AlGaAs crystalline coatings transferred to 50.8-mm (2-inch) diameter fused silica and sapphire substrates. We present results for the transmission, scattering, absorption, and surface quality of these prototype samples including the defect density and micro-roughness. These novel coatings exhibit optical performance on par with state-of-the-art dielectric structures, encouraging further work focused on the fabrication of larger optics using this technique.

Self-reliance and independently developed high-finesse spherical ultrastable optical reference cavity

Ultra-stable reference cavity with high finesse is a crucial component in a narrow-linewidth laser system which is widely used in time and frequency metrology, the test of Lorentz invariance, and measure of gravitational wave. In this paper, we report the recent progress of the self-made spherical reference cavity, aiming at the future space application. The main function of cavity is the reference of ultra-stable laser, which is the local reference oscillation source of space optical clock. The diameter of the designed spherical cavity spacer made of ultra-low expansion glass is 80 mm, and the cavity length is 78 mm, flat-concave mirrors configuration, and the radius of the concave mirror is 0.5 m. The support structure is designed to have two 3.9 mm-radius spherical groves located at the poles of the sphere along the diameter direction (defined as support axis), and a 53 angle between the support axis and the optical axis. The mechanic vibration sensitivities of the cavity along and perpendicular to the optical axis are both calculated by finite element analysis method to be below 110-10/g. Five-axis linkage CNC machining sphere forming technology is applied to S80 mm spherical surface processing with spherical contour degree up to 0.02. After a three-stage surface polishing processes, the fused silicamirror substratessurface roughness is measured to be less than 0.2 nm (rms). Implementing double ion beam sputtering technique for mirror coating, the reflection of the coating achieves a reflectivity of 99.999% and a loss of 4 ppm for 698 nm laser. The coating surface roughness is measured to be 0.3 nm (rms). The cavity spacer and the mirror are bonded by dried optical contact. In order to improve the thermal noise characteristics of the cavity, an ultra low expansion ring is contacted optically to the outer surface of the mirror. The cavity is characterized by ring-down spectroscopy, and the finesse is around 195000. With the help of a home-made 698 nm ultra narrow line-width laser, the cavity line-width is measured to be 9.8 kHz by sweeping cavity method. A 698 nm semiconductor laser is locked to this spherical cavity by PDH technology, and the cavity loss is measured to be5 ppm.

Thermal analysis of optical reference cavities for low sensitivity to environmental temperature fluctuations.

The temperature stability of optical reference cavities is significant in state-of-the-art ultra-stable narrow-linewidth laser systems. In this paper, the thermal time constant and thermal sensitivity of reference cavities are analyzed when reference cavities respond to environmental perturbations via heat transfer of thermal conduction and thermal radiation separately. The analysis as well as simulation results indicate that a reference cavity enclosed in multiple layers of thermal shields with larger mass, higher thermal capacity and lower emissivity is found to have a larger thermal time constant and thus a smaller sensitivity to environmental temperature perturbations. The design of thermal shields for reference cavities may vary according to experimentally achievable temperature stability and the coefficient of thermal expansion of reference cavities. A temperature fluctuation-induced length instability of reference cavities as low as 6 × 10(-16) on a day timescale can be achieved if a two-layer thermal shield is inserted between a cavity with the coefficient of thermal expansion of 1 × 10(-10) /K and an outer vacuum chamber with temperature fluctuation amplitude of 1 mK and period of 24 hours.

Narrow Line Cooling of 88Sr Atoms in the Magneto-optical Trap for Precision Frequency Standard

We report on our progress toward the realization of a Strontium optical lattice clock, which is under development at VNIIFTRI as a part of GLONASS program. We’ve prepared the narrow line width laser system for secondary cooling of 88Sr atoms which allows us to reach atom cloud temperature below 3μK after second cooling stage.

A narrow linewidth diode laser at 243 nm

The two-photon spectroscopy of 1S-2S transition in atomic hydrogen needs a narrow linewidth laser at the wavelength of 243 nm. In order to reduce the linewidth to several tens hertz level, a free operation CW ECDL 972 nm laser has been locked to a high fineness ultralow expansion reference cavity by using the Pound-Drever-Hall technique. And the part of 972 nm laser output is set into the tapered amplifier and the two enhanced doubling frequency stages to obtain the output of purple light at 243 nm. It is estimated that such a narrow linewidth laser system at 243 nm can be used well in the detection of the 1S-2S transition of hydrogen.

Narrow linewidth laser system realized by linewidth transfer using a fiber-based frequency comb for the magneto-optical trapping of strontium

A narrow linewidth diode laser system at 689 nm is realized by phase-locking an extended cavity diode laser to one tooth of a narrow linewidth optical frequency comb. The optical frequency comb is phase-locked to a narrow linewidth laser at 1064 nm, which is frequency stabilized to a high-finesse optical cavity. We demonstrate the magneto-optical trapping of Sr using an intercombination transition with the developed laser system.

Stable Narrow Linewidth 689 nm Diode Laser for the Second Stage Cooling and Trapping of Strontium Atoms

We report stable narrow linewidth laser systems based on self-developed Littman configuration external cavity diode lasers (ECDLs). The frequency of the ECDL is stabilized to a high fineness ultralow-expansion glass reference cavity with the Pound-Drever-Hall technique. By heterodyne beating of two identical systems, we conclude that the linewidth of each ECDL is reduced to lower than 150 Hz and its frequency stability reaches 4.3 × 10-14 at an averaging time of 1s, the averaged long-term frequency drift is less than 0.2 Hz/s over 30 h measurement time.

Dual-wavelength narrow-linewidth light source with ultranarrow wavelength spacing based on the pump-induced thermal effects in an Er-Yb-codoped distributed-Bragg-reflector fiber laser

A dual-wavelength narrow-linewidth laser system, with ultranarrow wavelength spacing, is proposed and demonstrated. The operation principle of the laser system is based on the pump-induced thermal effects of the Er-Yb-doped fiber distributed Bragg reflector (DBR) laser. We designed and constructed a common Er-Yb-doped DBR fiber laser with proper parameters. With the pump power of the DBR laser increasing, the Bragg wavelengths and the bandwidths of the two gratings will change unequally. When the laser is pumped with 45 mW, we obtain stable dual-wavelength lasing with a 3-dB bandwidth of 0.014 nm and a wavelength spacing of 0.055 nm.

Tunable high-power narrow-linewidth semiconductor laser based on an external-cavity tapered amplifier

A high-power narrow-linewidth laser system based on a tapered semiconductor optical amplifier in external cavity is demonstrated. The external cavity laser system uses a new tapered amplifier with a super-large optical-cavity (SLOC) design that leads to improved performance of the external cavity diode lasers. The laser system is tunable over a 29 nm range centered at 802 nm. As high as 1.95 W output power is obtained at 803.84 nm, and an output power above 1.5 W is achieved from 793 to 812 nm at operating current of 3.0 A. The emission linewidth is below 0.004 nm and the beam quality factor M2 is below 1.3 over the 29 nm tunable range. As an example of application, the laser system is used as a pump source for the generation of 405 nm blue light by single-pass frequency doubling in a periodically poled KTiOPO4. An output power of 24 mW at 405 nm, corresponding to a conversion efficiency of 0.83%/W is attained.

Simple and low cost experimental arrangement for continuous tunable narrow-band dye laser

The conversion of a commercial dye laser into a narrow-linewidth laser system, tunable over a large frequency range, is described. Its experimental details as well as its application to high resolution laser spectroscopy in molecular beams are reported. The laser linewidth in different configurations was determined. The major advantages of the system described include its low cost and simplicity in construction and performance.