Higher-order Laguerre-Gauss modes in (non-) planar four-mirror cavities for future gravitational wave detectors

Coherent angular signal amplification using an optical cavity

Precision angular sensing is an essential technology in physical experiments. Unlike length sensing with a laser beam, it has been thought that sensitivity to the angular motion cannot be enhanced with the help of an optical cavity. A method of angular signal amplification using an optical cavity, called the cavity-amplified angular sensor (CAAS), is proposed. By adjusting or compensating for the Gouy phase of the cavity, the electric field of the laser generated in proportion to the target rotation is coherently stacked in the proposed method. The advantage of this method over other angular sensors is its high sensitivity with the small sensing spot size. Three possible optical configurations are considered, of which two experimentally available ones are investigated. The angular signal amplification is demonstrated for both of them. Based on the theoretical calculation for a realistic model, the fundamental angular sensing noise level is expected to be as low as 10^-15rad/Hz^1/2, with a 1 mm laser beam size and 10 mW laser power.

Mode analyzer for known optical vortices from a spatial light modulator with collinear holography

The optical vortex has already found lots of applications in various domains. Among such applications, the precise and quantitative mode analysis of optical vortices is of great significance. In this work, we experimentally validate a simple method to analyze the mode of an already known optical field with collinear holography based on the phase-shifting technology. Further, we propose a ring interference strategy to improve the accuracy of mode analysis. In the proof-of-concept experiment, the complex amplitude is characterized, and the mode purity is well analyzed. This method has excellent accuracy and rapidity, which can be implemented in micro-manipulation, optical communication, and rotation speed measurement based on the rotating Doppler effect.

Performance manipulation of the squeezed coherent light source based on four-wave mixing

We present performance manipulation of the squeezed coherent light source based on four-wave mixing (FWM) in alkaline-earth atoms. We investigate the dynamic response of the system and the spectroscopic feature of lasing generated by resonantly enhanced wave-mixing in coherently prepared system. In this method, the spectral purity and stability of the wave-mixing lasing can be manipulated at will by choosing optimal laser parameters. We also analyze the effect of Langevin noise fluctuations on the system and the relative-intensity noise spectrum of the wave-mixing lasing is well below the standard quantum limit (down to -4.7 dB). This work opens new possibilities for alternative routes to laser stabilization and provides a promising path to realize precision metrology.

Adaptive optics enhanced sensitivity in Fabry-Pérot based photoacoustic tomography

All-optical ultrasound detection bears a number of unique advantages for photoacoustic tomography, including the ability for high resolution sampling of the acoustic field and its compatibility with a wide variety of other optical modalities. However, optical schemes based on miniaturized cavities are sensitive to optical aberrations as well as manufacturing-induced cavity imperfections which degrade sensor sensitivity and deteriorate photoacoustic image quality. Here we present an experimental method based on adaptive optics that is capable of enhancing the overall sensitivity of Fabry-Pérot based photoacoustic sensors. We experimentally observe clear improvements in photoacoustic signal detection as well as overall image quality after photoacoustic tomography reconstructions when applied to mammalian tissues in vivo.

Numerical analysis of LG3,3 second harmonic generation in comparison to the LG0,0 case

For coating Brownian thermal noise reduction in future gravitational wave detectors, it is proposed to use light in the helical Laguerre-Gaussian LG_3,3 mode instead of the currently used LG_0,0 mode. However, the simultaneous reduction of quantum noise would then require the efficient generation of squeezed vacuum states in the LG_3,3 mode. Current squeezed light generation techniques employ continuous-wave second harmonic generation (SHG). Here, we simulate the SHG for both modes numerically to derive first insights into the transferability of standard squeezed light generation techniques to the LG_3,3 mode. In the first part of this paper, we therefore theoretically discuss SHG in the case of a single undepleted pump mode, which, in general, excites a superposition of harmonic modes. Based on the differential equation for the harmonic field, we derive individual phase matching conditions and hence conversion efficiencies for the excited harmonic modes. In the second part, we analyse the numerical simulations of the LG_0,0 and LG_3,3 SHG in a single-pass, double-pass and cavity-enhanced configuration under the influence of the focusing, the different pump intensity distributions and the individual phase matching conditions. Our results predict that the LG_3,3 mode requires about 14 times the pump power of the LG_0,0 mode to achieve the same SHG conversion efficiency in an ideal, realistic cavity design and mainly generates the harmonic LG_6,6 mode.

Frequency-doubling of continuous laser light in Laguerre-Gaussian modes LG0,0 and LG3,3

For future generations of gravitational wave detectors, it is proposed to use the helical Laguerre-Gaussian LG_3,3 mode to reduce thermal noise, which limits the detector sensitivity. At the same time, this requires the efficient generation of squeezed vacuum states in the LG_3,3 mode for quantum noise reduction. Since this technique includes the process of second harmonic generation (SHG), we experimentally compare the conversion efficiency and harmonic output field of the LG_0,0 and LG_3,3 modes in a cavity-enhanced SHG using the same 7% doped MgO:LiNbO₃ crystal. Conversion efficiencies of 96% and 45% are achieved, respectively. The influence of mode mismatches and astigmatism is analyzed to estimate the ratio of the pump mode-dependent effective nonlinearities to be d_0,0/d_3,3∼5. Furthermore, we show that absorption loss in the crystal is more relevant for the LG_3,3 mode.

Transverse mode-switchable fiber laser based on a photonic lantern

We propose and experimentally demonstrate an intra-cavity transverse mode-switchable fiber laser based on a mode-selective photonic lantern and a few-mode Er-doped fiber amplifier. The six lowest-order LP modes can lase independently and are switchable by changing the input port of the photonic lantern. We measured the slope efficiency, mode intensity profile, and optical spectrum of each lasing mode. In addition, we demonstrate donut-shaped LP₁₁ and LP₂₁ modes using incoherent superposition and simultaneous lasing of the two degenerate modes.

Laser frequency instability of 2 × 10-16 by stabilizing to 30-cm-long Fabry-Pérot cavities at 578 nm

Laser light at 578 nm is frequency-stabilized to two independent 30-cm-long Fabry-Pérot cavities. To achieve a thermal-noise-limited cavity length stability, the geometry and support configuration of the Fabry-Pérot cavities are optimized. The fractional frequency instability of each cavity-stabilized laser system is 2 × 10^-16 at 1 s averaging time, approaching to the thermal-noise-induced length instability of the reference cavity. The most probable linewidth of each laser system is about 0.2 Hz, and the laser frequency noise at Fourier frequency of 1 Hz is 0.1 Hz/√Hz.

Thermal-noise-limited higher-order mode locking of a reference cavity

Higher-order mode locking has been proposed to reduce the thermal noise limit of reference cavities. By locking a laser to the HG₀₂ mode of a 10-cm long all ultra-low expansion (ULE) cavity and measuring its performance with the three-cornered-hat method among three independently stabilized lasers, we demonstrate a thermal-noise-limited performance of a fractional frequency instability of 4.9×10^-16. The results match the theoretical models with higher-order optical modes. The achieved laser instability improves the all ULE short cavity results to a new low level.

Spatial mode multiplexing using volume holographic gratings

We experimentally demonstrate spatial mode multiplexing of optical beams using multiplexed volume holographic gratings (MVGHs) formed in phenanthrenquinone-poly (methyl methacrylate) (PQ-PMMA) photopolymer. Multiple spatial modes of Laguerre-Gaussian (LG) beams are recorded at the same pupil area of a volume hologram resulting in MVHGs, for simultaneous reconstruction of spatial modes. In addition, a helical phase beam, a non-diffracting beam with conical phase profile, and a parabolic non-diffracting beam with cubic phase profile have also been simultaneously recorded and reconstructed from MVHGs. Utilizing Bragg wavelength degeneracy property of volume hologram these multiplexed modes are reconstructed at multiple wavelengths ranging from blue (450nm) to red (635). Due to combined effect of three-dimensional pupil, Bragg wavelength degeneracy, angular selectivity, together with spatial mode properties these, MVHGs can act as spatial mode filter with spectral filtering property. Advantages of volume holography in beam shaping are discussed. Multiple first diffraction orders with desired beam shapes obtained from the single optical element (i.e. a volume hologram with MVHGs) may find important applications in optical communication experiments, and in volume holographic imaging and microscopy. Experimental results show solid evidence that MVGHs in beam shaping provide a simple, compact, single element, and direct way to multiplex spatial modes.