Intracavity Image Research Articles

Image enhancement in a miniature self-imaging degenerate optical cavity

Image-amplifying optical cavities offer an intriguing platform for nonlinear optical processing of two-dimensional signals, with potential applications in optical information processing and optically implemented artificial neural networks. Here, we analyze the performance of a self-imaging degenerate cavity via numerical simulations and show the existence of a cavity size-dependent minimum spread in the transverse mode resonances. This non-degeneracy, in turn, leads to an inherent trade-off between the amplification and the fidelity of the intracavity image as the cavity finesse changes. Our results point to a promising path to nonlinear image processing in a low-power, small-form-factor optical cavity.

Intracavity image upconversion system with fast and flexible electro-optic image gating based on polarization-frustrated phase-matching for range-gated applications.

We report on a new image gating mechanism for intracavity nonlinear image upconversion systems that uses sum-frequency mixing of an external infrared image and a pump laser beam. Fast and flexible time duration gating of the upconverted image is achieved through transient electro-optic frustration of the phase-matching condition in a nonlinear crystal placed inside the cavity of the pump beam. The phase-matching condition is controlled by altering the polarization state of the laser cavity beam without interrupting laser oscillation, using a Pockels cell in one arm of an L-folded standing-wave resonator. In this way, an external image shutter mechanism is added to an image upconverter system that allows for using low shutter-speed EMCCDs (Electron Multiplying CCD) in range-gated imaging systems across the whole IR and potentially in the THz range.

Intensity noise suppression of a high-power single-frequency CW laser by controlling the stimulated emission rate.

The intensity noise of a high-power single-frequency continuous-wave (CW) laser is very harmful for applications in precise measurements and quantum communication. By simply lengthening the length of a laser resonator to decrease the stimulated emission rate of the laser, the coupling strength of all noise sources in the resonant laser field will be reduced, and thus the intensity noise of the output laser will be prominently suppressed. Based on theoretical analyses of the laser noise spectra, we experimentally implement a low-noise high-power single-frequency laser with a 1050 mm long resonator. With the assistance of an intracavity imaging system and nonlinear second-harmonic generation, the amplitude of the resonant relaxation oscillation peak and the shot-noise level (SNL) cutoff frequency are successfully reduced to 8.6 dB/Hz above SNL and 1.0 MHz, respectively, under the output power of 16 W. The work provides an effective way to develop a high-quality laser with high output power and low intensity noise.

20 MHz forward-imaging single-element beam steering with an internal rotating variable-angle reflecting surface: Wire phantom and ex vivo pilot study

Feasibility is demonstrated for a forward-imaging beam steering system involving a single-element 20MHz angled-face acoustic transducer combined with an internal rotating variable-angle reflecting surface (VARS). Rotation of the VARS structure, for a fixed position of the transducer, generates a 2-D angular sector scan. If these VARS revolutions were to be accompanied by successive rotations of the single-element transducer, 3-D imaging would be achieved.In the design of this device, a single-element 20MHz PMN-PT press-focused angled-face transducer is focused on the circle of midpoints of a micro-machined VARS within the distal end of an endoscope. The 2-D imaging system was tested in water bath experiments with phantom wire structures at a depth of 10mm, and exhibited an axial resolution of 66μm and a lateral resolution of 520μm. Chirp coded excitation was used to enhance the signal-to-noise ratio, and to increase the depth of penetration. Images of an ex vivo cow eye were obtained. This VARS-based approach offers a novel forward-looking beam-steering method, which could be useful in intra-cavity imaging.

Prospective motion correction using tracking coils.

Intracavity imaging coils provide higher signal-to-noise than surface coils and have the potential to provide higher spatial resolution in shorter acquisition times. However, images from these coils suffer from physiologically induced motion artifacts, as both the anatomy and the coils move during image acquisition. We developed prospective motion-correction techniques for intracavity imaging using an array of tracking coils. The system had <50 ms latency between tracking and imaging, so that the images from the intracavity coil were acquired in a frame of reference defined by the tracking array rather than by the system's gradient coils. Two-dimensional gradient-recalled and three-dimensional electrocardiogram-gated inversion-recovery-fast-gradient-echo sequences were tested with prospective motion correction using ex vivo hearts placed on a moving platform simulating both respiratory and cardiac motion. Human abdominal tests were subsequently conducted. The tracking array provided a positional accuracy of 0.7 ± 0.5 mm, 0.6 ± 0.4 mm, and 0.1 ± 0.1 mm along the X, Y, and Z directions at a rate of 20 frames-per-second. The ex vivo and human experiments showed significant image quality improvements for both in-plane and through-plane motion correction, which although not performed in intracavity imaging, demonstrates the feasibility of implementing such a motion-correction system in a future design of combined tracking and intracavity coil.

Demonstration of improved beam quality in an image-rotating optical parametric oscillator

We performed laboratory and numerical modeling studies of an optical parametric oscillator with 90 degrees intracavity image rotation. We found that the signal beam was more symmetric than that from comparable cavities without image rotation, and it had low values of the beam quality factor, M(2) . Oscillator performance agreed well with our numerical model.

Photorefractive oscillation with intracavity image and multimode fiber

A photorefractive oscillation between two cascaded passive phase conjugate mirrors (2C-PPCM) with intracavity phase and amplitude modulation and multimode fibers is described. The two semilinear PPCM’s operate via four wave mixing in BaTiO3 crystals. A single beam pumps only one PPCM and generates all other self-starting beams of the two PPCM’s

Conversion of optical signals in a laser with an intracavity liquid-crystal spatial modulator

Image of amplification by a factor of ~ 105 and an increase in the contrast by a factor of 103 were achieved by intracavity image readout from spatial light modulators. Two types of photosemiconductor–liquid crystal modulators were used: 1) utilizing a cholesteric–nematic phase transition; 2) utilizing the electrooptic effect in a nematic liquid crystal. The image quality was at the diffraction limit both for a dye laser and a YAG:Nd3+ laser; the sharpness of the image boundaries and the contrast increased above the lasing threshold. When readout took place from an area of the diffraction-limit size in a free-running YAG:Nd3+ laser, the radiation energy was 0.8 × 10−3 J and the energy density reaching the modulator was ~ 1.4 J/cm2 (the energy of the recording light of the λ = 0.44 μ wavelength was ~0.4 × 10−8 J and the energy density in the recording stage was ~10−6J/cm2).

Spatial resolution of an intracavity image upconverter

An intracavity image upconversion system, using a proustite crystal and a flashlamp-pumped dye laser, is described. Experimental results regarding the spatial resolution of the system are also presented, and the conversion efficiency is discussed.