Two-mirror Cavity Research Articles

Optical ring cavity for homogeneous quantum nondemolition measurement in atom interferometer

Quantum nondemolition (QND) measurement aided by high-finesse optical cavities is an important method for generating high-gain spin or momentum squeezed states, which can enhance the sensitivity of atom interferometers to surpass the standard quantum limit. Conventional two-mirror Fabry-Perot cavities have the drawback of a standing wave pattern, leading to inhomogeneous atom-light coupling and subsequent degradation of squeezing enhancement. In this study, we present a novel method for achieving homogeneous quantum nondemolition measurement using an optical ring cavity to generate momentum squeezed states in atom interferometers. We designed and demonstrated a high-finesse (<i>F</i> =2.4(1)×10<sup>4</sup>), high-vacuum compatible (1×10<sup>-10</sup> mbar) optical ring cavity that utilizes the properties of traveling wave fields to address the issue of inhomogeneous atom-light interaction. A strontium cold atomic ensemble was prepared and coupled into the cavity mode; the dispersive cavity phase shift caused by the atoms passing through was extracted through differential Pound-Drever-Hall measurement, enabling nondemolition measurement of the atom number. Experimental results indicate that, under a probe laser power of 20 µW, the dispersive phase shift of the ring cavity was measured to be 40 mrad. The effective number of atoms coupled into the cavity mode is around 1×10<sup>6</sup>. Verification of the consistency between the ring cavity dispersive phase shift and QND measurement theory was achieved by adjusting parameters such as matching the atomic position with the cavity mode and tuning the frequency of the probe laser. The optical ring cavity developed in this study provides a significant approach for generating spin or momentum squeezed states in atom interferometers, thus holding promise for enhancing their sensitivity and is expected to find wide applications in cavity-enhanced quantum precision measurements.

710 kW stable average power in a 45,000 finesse two-mirror optical cavity

Very high-average optical enhancement cavities (OECs) are being used both in fundamental and applied research. The most demanding applications require stable megawatt level average power of infrared picosecond pulses with repetition rates of several tens of MHz. Toward reaching this goal, we report on the achievement of 710 kW of stable average power in a two-mirror hemispherical optical enhancement cavity. This result further improves the state of the art. So far, in compact high-power systems, cavity geometry optimization has been driven by the need to limit the deformation of radii of curvatures due to thermal effects. Here we explicitly demonstrate that thermal lensing must be accounted for, too, and that it can be used to assess the absorption of coatings. Experimental observations are matched with a simple model of thermal effects in the mirror’s coatings. These results set a further stage for designing an optimized optical system for several applications where very high-average power enhancement cavities are expected to be operated.

Zigzag optical cavity for sensing and controlling torsional motion

Precision sensing and manipulation of milligram-scale mechanical oscillators has attracted growing interest in the fields of table-top explorations of gravity and tests of quantum mechanics at macroscopic scales. Torsional oscillators present an opportunity in this regard due to their remarked isolation from environmental noise. For torsional motion, an effective employment of optical cavities to enhance optomechanical interactions—as already established for linear oscillators—so far faced certain challenges. Here, we propose a concept for sensing and manipulating torsional motion, where exclusively the torsional rotations of a pendulum are mapped onto the path length of a single two-mirror optical cavity. The concept inherently alleviates many limitations of previous approaches. A proof-of-principle experiment is conducted with a rigidly controlled pendulum to explore the sensing aspects of the concept and to identify practical limitations in a potential state-of-the art setup. Based on this study, we anticipate development of precision torque sensors utilizing torsional pendulums that can support sensitivities below 10−19Nm/Hz, while the motion of the pendulums are dominated by quantum radiation pressure noise at sub-microwatts of incoming laser power. These developments will provide horizons for experiments at the interface of quantum mechanics and gravity. Published by the American Physical Society 2024

Optical characterization of high performance mirrors based on cavity ringdown time measurements with 6 degrees of freedom mirror positioning.

An instrument capable of measuring optical losses, transmission, and the radius of curvature of high reflectivity mirrors is presented. The measurement setup consists of two remote controlled hexapod systems with 6 degrees of freedom placed inside a vacuum enclosure. Mirror loss measurements are performed via the cavity ring-down time method using a linear resonant two-mirror Fabry-Perot cavity configuration. The use of high-precision positioning systems enables cavity loss mapping by transversely scanning the position of the cavity end mirror. Mirror surfaces of up to 30mm in diameter can be scanned, and the cavity length can be tuned by 120mm.

Secondary Raman and Brillouin mode suppression in two- and three-mirror-cavity diamond Raman lasers.

We report an investigation into secondary mode suppression in single longitudinal mode (SLM) 1240 nm diamond Raman lasers. For a three-mirror V-shape standing-wave cavity incorporating an intra-cavity LBO crystal to suppress secondary modes, we achieved stable SLM output with a maximum output power of 11.7 W and a slope efficiency 34.9%. We quantify the level of χ(2) coupling necessary to suppress secondary modes including those generated by stimulated Brillouin scattering (SBS). It is found that SBS-generated modes often coincide with higher-order spatial modes in the beam profile and can be suppressed using an intracavity aperture. Using numerical calculations, it is shown that the probability for such higher-order spatial modes is higher for an apertureless V-cavity than in two-mirror cavities due its contrasting longitudinal mode-structure.

Single-frequency lasing in a CW TEМ00 Nd:YVO4 disk laser with an M-type degenerate cavity and three-beam pumping

We demonstrate a continuous-wave TEM00 Nd:YVO4 disk laser with three-beam pumping and a two-mirror degenerate cavity configuration operating in a single-frequency regime without any intracavity wavelength selectors. The estimated lasing linewidth from the measurement was less than 200 MHz at the maximum output power of 1.35 W.

Preparation of continuously tunable orthogonal squeezed light filed corresponding to cesium D<sub>1</sub> line

The non-classical light resonance on the cesium D<sub>1</sub> (894.6 nm) line has important applications in solid-state quantum information networks due to its unique advantages. The cesium D<sub>1</sub> line has a simplified hyperfine structure and can be used to realize a light-atom interface. In our previous work, we demonstrated 2.8-dB quadrature squeezed vacuum light at cesium D<sub>1</sub> line in an optical parametric oscillator(OPO) with a periodically poled KTP(PPKTP) crystal. However, the squeezing level is relatively low, and the tunability that has practical significance for squeezed light has not been further investigated. Theoretically, the increase of the transmittance of output mirror and the decrease of the intra-cavity loss of the OPO can improve the squeezing level. Here, we use super-polished and optimal coating cavity mirrors to improve the nonlinear process in OPO. We prepare 447.3 nm blue light from 894.6 nm fundamental light by a second harmonic generation cavity (SHG). The SHG is a two-mirror standing-wave cavity with a PPKTP crystal as the nonlinear medium. The power of generated blue laser is 32 mW when the incident infrared power is 120 mW. Using the blue light to pump an OPO, we achieve quadrature squeezed vacuum light at cesium D<sub>1</sub> line. The OPO is a two-mirror standing-wave cavity with a PPKTP crystal. The threshold of OPO is reduced to 28 mW. The squeezing level of generated quadrature squeezed vacuum light is increased to 3.3 dB when the pump power is 15 mW. Taking into account the overall detection efficiency, the actual squeezing reaches 5.5 dB. We inject a weak signal beam into the OPO cavity to act as an optical parametric amplifier (OPA), and test the tunability of squeezzed light. The blue light and the squeezed light are tuned by using a low-frequency triangular wave signal to scan the Ti: sapphire laser. Gradually increasing the amplitude of the scanning triangle wave signal, the generated bright squeezed light can be continuously tuned over a range around 80 MHz without losing the stability of the whole system. The generated squeezed light offers the possibility for the efficient coupling between the non-classical source and solid medium in the process of quantum interface.

CW TEM00 spectral narrow band Nd:YVO4 disk laser with two-mirror degenerate cavity configuration

The output parameters of a disk Nd:YVO4 laser with a degenerate two-mirror cavity and one-, two- and three-point diode pumping are studied without the use of intracavity spectral selectors. The TEM00 lasing contained two equal-intensity frequency components with a spectral width less than 6 × 10−3 cm−1 (<180 MHz) full width at half maximum (FWHM) and a distance between them of 5 × 10−2 cm−1 (1.5 GHz). The explanation of the nature of the spectrally selective properties of the degenerate resonator is proposed and experimentally substantiated.

Frequency stabilization of a quantum cascade laser by weak resonant feedback from a Fabry-Perot cavity.

Frequency-stabilized mid-infrared lasers are valuable tools for precision molecular spectroscopy. However, their implementation remains limited by complicated stabilization schemes. Here we achieve optical self-locking of a quantum cascade laser to the resonant leak-out field of a highly mode-matched two-mirror cavity. The result is a simple approach to achieving stable frequencies from high-powered mid-infrared lasers. For short time scales (<0.1ms), we report a linewidth reduction factor of 3×10-6 to a linewidth of 12 Hz. Furthermore, we demonstrate two-photon cavity-enhanced absorption spectroscopy of an N2O overtone transition near a wavelength of 4.53 µm.

Higher-order Hermite-Gauss modes for gravitational waves detection

As part of the research on thermal noise reduction in gravitational-wave detectors, we experimentally demonstrate the conversion of a fundamental ${\mathrm{TEM}}_{00}$ laser mode at 1064 nm to higher-order Hermite-Gaussian modes (HG) of arbitrary order via a commercially available liquid crystal spatial light modulator. We particularly studied the ${\mathrm{HG}}_{5,5}/{\mathrm{HG}}_{10,10}/{\mathrm{HG}}_{15,15}$ modes. A two-mirror plano-spherical cavity filters the higher-order modes spatially. We analyze the cleaned modes via a three-mirror diagnosis cavity and measure a mode purity of $96/93/78%$ and a conversion efficiency of $6.6%/3.7%/1.7%$, respectively. A full set of simulations and mathematical proofs are also presented which shows that (i) Hermite-Gauss modes resonate in a two-mirror cavity provided mirrors are properly angled with respect to the impinging mode, and (ii) Hermite-Gauss modes resonate in triangular cavities. Hence, higher-order Hermite-Gauss modes are compatible with ground-based gravitational-wave detectors' architecture and can be employed for the mitigation of mirror thermal noise for the third generation Einstein Telescope or Cosmic Explorer.

Passively Q-switched Nd:GSGG laser at 936.3 nm

An efficient continuous-wave (CW) and pulse Nd:GSGG laser at 936.3 nm is reported with the help of a simple and compact two-mirror cavity. The CW power at 936.3 nm is 2421 mW at a pump power of 10.1 W with a conversion efficiency of 23.7 %. In the pulse regime, a passively Q-switched quasi-three energy Nd:GSGG at 936.3 nm is firstly achieved. The maximum peak power reaches 2.29 kW with an output power of 1089 mW and the narrowest pulse width of 15.3 ns.

High-Order Vortex Generation From CW and Passively Q-Switched Pr:YLF Visible Lasers

We report on the high-order transverse-mode generation in a diode-pumped Pr:YLF visible laser operating at 639 nm. Using a two-mirror linear cavity and operating the laser in continuous-wave regime, tunable Hermite–Gaussian modes HG0,m have been directly generated with m varying from 0 up to 120 by off-axis pumping and misaligning the laser resonator. In the same way, but using a V-shaped cavity and adding a Co:ASL saturable absorber, high-order HG0,m modes were also generated in Q-switched regime with m varying up to 12. The achieved passively Q-switched laser has a pulse width of about 149 ns at a repetition rate of 91 kHz, which leads to a pulse energy of about 2.5 $\mu \text{J}$ and a pulse peak power of 16.9 W. In both cases, intracavity HG0,m modes were converted into the corresponding Laguerre–Gaussian modes LG0,m via a pair of cylindrical lenses placed outside the cavity and the helical nature of the wavefronts were checked by using a specially designed Fizeau interferometer. Such tunable and directly generated high-order laser transverse modes obtained in continuous-wave as well as in Q-switched regimes are well suited for various modern applications, such as optical manipulation and 3D material processing.

Cavity-enhanced Thomson scattering measurements of electron density and temperature in a hollow cathode discharge.

A cavity-enhanced Thomson scattering (CETS) diagnostic has been developed to perform electron density and temperature measurements in low-density weakly ionized discharges. The diagnostic approach is based on generating a high-power beam in an optical build-up cavity and using the beam as a light source for Thomson scattering from plasma housed within the cavity. In our setup, a high-power (∼5 W) fiber laser at 1064nm allows an intra-cavity power of 11.7kW in a two-mirror cavity for measurements in the plume of a BaO hollow cathode discharge. A study of plasma density and temperature was performed at various operating conditions. Electron densities and temperatures in the range of ∼1012 cm-3 and ∼3 eV were measured, respectively. The high signal-to-noise ratio (SNR) of the present measurements (SNR=1100) suggests the ability to measure significantly lower density plasmas in the range of ∼3×109 to 3×1010 cm-3, thereby extending current laser Thomson scattering diagnostic capabilities.

Increased atom-cavity coupling and stability using a parabolic ring cavity

Optical cavities are one of the best ways to increase atom-light coupling and will be a key ingredient for future quantum technologies that rely on light–matter interfaces. We demonstrate that traveling-wave ‘ring’ cavities can achieve small mode waists w (i.e. large atom-cavity coupling) while maintaining stability to fluctuations and large mirror separation. Additionally, our ring cavity design can achieve arbitrary transverse-mode spacing simultaneously with the large mode-waist reductions. Following these principles, we build a parabolic atom-ring cavity system that achieves strong collective coupling NC = 15(1) between N = 103 Rb atoms and a ring cavity with a single-atom cooperativity C that is significantly larger than what could be achieved with a near-confocal two-mirror cavity with the same mirror separation and finesse. By using parabolic mirrors, we eliminate astigmatism–which can otherwise preclude stable operation–and increase optical access to the atoms. Cavities based on these principles, with enhanced coupling and large mirror separation, will be particularly useful for achieving strong coupling with ions, Rydberg atoms, or other strongly interacting particles, which often have undesirable interactions with nearby surfaces.

TERAHERTZ KLYNOTRON WITH AN AZIMUTHAL COMB

According to theoretical and experimental research of orotrons, which are diffraction radiation oscillators with spatially developed electromagnetic systems, usually being excited by non-relativistic spatially developed electron beams, this class of devices can be successfully used in the practical applications for THz frequency range. One of the major problems to be solved when the efficiency of such sources increases with the shortening of their operation wavelength is how to ensure a high utilization factor of all partial layers of a highly perveance spatially developed electron beam in the electron-wave interaction zone. This can be achieved by increasing the microwave field strength in the energy exchange zone between electrons and electromagnetic waves. The klynotronic effect has been proven to be an efficient way of increasing the utilization factor of all partial cross-sectional layers of a spatially developed electron beam. In this study, we will discuss the mathematically simulated energy characteristics of electron-wave interaction in a radial electron flow orotron with an underlying azimuthal comb serving as a diffraction grating mounted on the useful surface of a flat mirror in a two-mirror open cavity. The results of mathematical simulation and optimization of the energy characteristics of electron-wave interaction in the klynoorotron of the terahertz range made us believe that, due to the klynotronic effect, the electronic efficiency can be increased up to 20% in oscillators of this design.

1.96-μm Tm:YAG Ceramic Laser

We have demonstrated an efficient diode-end-pumped continuous-wave Tm:YAG ceramic laser at 1962 nm using a compact two-mirror cavity. The laser oscillation at 1962 nm was realized by increasing the cavity loss at 2016 nm to limit oscillation with the strongest laser gain. For comparison, two different output couplers were used to build single wavelength lasers operating at 1962 and 2016 nm. A laser output power of 2.06 W at 1962 nm was achieved for an absorbed pump power of 6.59 W with the laser ceramic temperature maintained at 15 °C. The corresponding slope efficiency and conversion efficiency were calculated to be 37.8% and 31.3%, respectively. In contrast, the maximum output power at 2016 nm was approximately 3.47 W under the same conditions. The 1962 nm laser has potential application in the analysis of CO2 and HBr molecular gases.

Fundamental limitations of cavity-assisted atom interferometry

Atom interferometers employing optical cavities to enhance the beam splitter pulses promise significant advances in science and technology, notably for future gravitational wave detectors. Long cavities, on the scale of hundreds of meters, have been proposed in experiments aiming to observe gravitational waves with frequencies below 1 Hz, where laser interferometers, such as LIGO, have poor sensitivity. Alternatively, short cavities have also been proposed for enhancing the sensitivity of more portable atom interferometers. We explore the fundamental limitations of two-mirror cavities for atomic beam splitting, and establish upper bounds on the temperature of the atomic ensemble as a function of cavity length and three design parameters: the cavity g-factor, the bandwidth, and the optical suppression factor of the first and second order spatial modes. A lower bound to the cavity bandwidth is found which avoids elongation of the interaction time and maximizes power enhancement. An upper limit to cavity length is found for symmetric two-mirror cavities, restricting the practicality of long baseline detectors. For shorter cavities, an upper limit on the beam size was derived from the geometrical stability of the cavity. These findings aim to aid the design of current and future cavity-assisted atom interferometers.

Application of Modified Bragg Structures in High-Power Submillimeter Cyclotron Autoresonance Masers

We perform theoretical analysis and numerical simulation of cyclotron autoresonance masers, in which we propose using electrodynamic systems in the form of hybrid two-mirror cavities based on modified and conventional Bragg reflectors. It is shown that a stable regime of narrow-band generation with a multimegawatt power level can be achieved at a frequency of about 300 GHz in the described scheme of a generator based on a near-axis, moderately relativistic electron beam with an accelerating voltage of 500 kV and a current of 10 A, where the transverse size (diameter) of the interaction space is about 5 wavelengths and the guiding magnetic field is about 5 T.

Continuous-wave laser operation of diode-pumped Tm-doped Gd3Ga5O12 crystal

We report on a diode-pumped Tm:Gd3Ga5O12 (GGG) laser at 2004 nm operated in continuous-wave mode with two-mirror linear cavity configuration. The maximum output power reaches 0.58 W with laser threshold absorbed pump power of about 0.39 W and overall slope efficiency of about 18.4%, which is believed to be the highest output power for Tm:GGG laser up to now. The Tm:GGG laser shows obvious thermally induced saturation of the output power, which indicated that power and efficiency scaling could be furtherly realized by more efficient thermal removal of the laser crystal.

Single- and dual-wavelength lasers of diode-pumped Nd:LuYAG mixed crystal on various 4F3/2 → 4I13/2 Stark-level transitions

A diode-end-pumped Nd:LuYAG mixed crystal laser in the 1.3 μm spectral domain using a simple and compact two-mirror laser cavity is reported for the first time to our knowledge. In the free-running laser operation, we obtain a simultaneous dual-wavelength laser at about 1319 and 1338 nm with a maximum output power of 2.92 W and a slope efficiency of about 26.7%. By inserting an undoped YAG etalon into the laser cavity, the two 1319 and 1338 nm lasers can be operated singly with the maximum output powers of 1.36 and 1.73 W. Moreover, three simultaneous dual-wavelength lasers at 1334.67 and 1340.93 nm, 1319.15 and 1354.57 nm, and 1335.54 and 1354.94 nm can also be yielded with maximum output powers of 1.02, 0.66, and 0.47 W as well as slope efficiencies of 12.3%, 8.9%, and 6.9%, respectively. These dual-wavelength laser sources provide potential THz wave generation by beat frequency.