Reading voxels in an optical storage medium using off-axis lighting
An off-axis lighting system for optical storage media comprises a Gaussian laser illumination source and illumination optics including collimating and cylindrical lenses. The system generates an illumination beam that propagates through the optical storage medium at angles different from the optical axis of a line scanning camera system. A collimating lens shapes the beam, adjusting characteristics such as beam waist diameter and location. A cylindrical lens further shapes the beam, creating an elliptical two-dimensional profile across its cross-section. The combination of off-axis propagation and elliptical profile restricts illumination to the voxels in the field of view of the line scanning camera at a specific layer in the optical storage medium. The off-axis lighting system enhances an ability of an optical read system to read voxels by improving the signal-to-noise ratio (SNR) of the voxel images captured by the line scanning camera used to decode symbols stored by the voxels.
1 . A method for optically reading data, the method comprising:
providing an optical storage medium comprising a three-dimensional transparent volume having a plurality of voxel storage layers arranged in a vertical stack along a z axis, each layer defining a distinct voxel layer, wherein each voxel storage layer contains voxels arranged in a plurality of one-dimensional (1D) arrays within an xy plane of the volume, the voxels encoding a plurality of corresponding symbols;
focusing a focal point of a microscopy objective of a line scanning camera on a predetermined layer of voxels of the plurality of voxel storage layers within the optical storage medium, wherein the microscopy objective is aligned with the line scanning camera along an optical axis;
imparting relative motion between the line scanning camera and the optical storage medium such that the line scanning camera sequentially captures images of voxels in a selected 1D array of voxels in the predetermined layer;
aligning a field of view (FOV) of the line scanning camera having a major axis, and a minor axis that is narrow relative to the major axis by aligning the major axis substantially with the selected 1D array of voxels being imaged;
generating an illumination beam at an illumination source to illuminate the voxels of the selected 1D array of voxels during the imaging;
directing the illumination beam to propagate along a path in the optical storage medium at an angle that is offset from the optical axis of the line scanning camera and the microscopy objective; and
receiving the captured voxel images at a decoder, the decoder being configured for decoding the imaged voxels to extract symbols or data.
2 . The method of claim 1 in which the illumination beam is arranged to illuminate a portion of the selected 1D array of voxels within a field of view (FOV) of the line scanning camera, the FOV having a major axis, and a minor axis that is narrow relative to the major axis, and in which the major axis is substantially aligned with the selected 1D array of voxels being imaged.
3 . The method of claim 1 in which the illumination beam has a cross-section with an elliptical shape having the major axis and the minor axis.
4 . The method of claim 1 further including controlling the line scanning camera and the relative motion between the line scanning camera and the optical storage medium to capture a plurality of voxel images in each frame of the sequential imaging.
5 . The method of claim 1 further including adjusting the focal point of the microscopy objective to a second predetermined layer of voxels of the plurality of voxel storage layers in the optical storage medium.
6 . The method of claim 5 further including imparting relative motion between the line scanning camera and the optical storage medium such that the line scanning camera sequentially images voxels in a second selected 1D array of voxels in the second predetermined layer to capture a second set of voxel images.
7 . An optical read system configured for reading data encoded in voxels positioned in sectors within layers in an optical storage medium, comprising:
a camera comprising a light-sensitive image sensor, the camera having an optical axis that is positioned relative to the optical storage medium such that a field of view (FOV) of the camera is capable of alignment with the layers of the optical storage medium;
a microscopy objective coupled to the camera having an adjustable depth of focus for controllably focusing on a selected layer of the layers of the optical storage medium, the microscopy objective being aligned along the optical axis of the camera;
a source of an illumination beam that propagates in the optical storage medium to selectively illuminate voxels in the selected layer of the optical storage medium within the FOV of the camera; and
illumination optics for shaping a cross-sectional profile of the illumination beam into an elliptical shape and positioning the illumination beam for entry at an oblique angle at a bottom surface of the optical storage medium.
8 . The optical read system of claim 7 in which the camera is a line scanning camera having the FOV characterized by a major axis and a minor axis, in which the minor axis is narrow relative to the major axis.
9 . The optical read system of claim 8 in which the camera further includes a shutter for controlling light exposure to the image sensor to capture images of illuminated voxels in the selected layer of the optical storage medium within the FOV of the camera.
10 . The optical read system of claim 9 in which the source comprises a laser providing an illumination beam with a wavelength of approximately 450 nm and the illumination optics provide a numeral aperture of 0.1 for the illumination beam such that a waist of the illumination beam is approximately 1.5 μm around an area of voxels being imaged.
11 . The optical read system of claim 9 further comprising a decoder for receiving image data from the camera and extracting respective symbol values from the imaged voxels.
12 . The optical read system of claim 9 further comprising a post-processor which applies machine learning for extracting respective symbol values from the imaged voxels.
13 . The optical read system of claim 7 further comprising one or more mechanical actuators for variably positioning or imparting relative motion to one or more of the optical storage medium, the camera, the microscopy objective, illumination beam source, or illumination optics.
14 . The optical read system of claim 7 in which the optical storage medium has a refractive index of approximately 1.5, and an angle of propagation of the illumination beam in the optical storage medium is approximately 40° from a normal projection at the bottom surface of the optical storage medium.
15 . The optical read system of claim 7 in which the oblique angle is approximately 75° from a normal projection at the bottom surface of the optical storage medium.
16 . An optical read drive for reading data from an optical storage medium containing a plurality of voxels arranged in layers, comprising:
a line scanning camera comprising an image sensor and shutter, the line scanning camera having a field of view (FOV);
a laser source configured to provide an illumination beam for illuminating voxels within the FOV of the line scanning camera in a selected layer of the layers of the optical storage medium;
a microscopy objective coupled to the line scanning camera and being controllably focused on the selected layer of voxels in the optical storage medium, the microscopy objective and the line scanning camera being aligned along a common optical axis that projects into the optical storage medium;
illumination optics coupled to the laser source for directing the illumination beam to propagate off-axis from the optical axis in the optical storage medium;
a motorized stage on which the optical storage medium is located, and
a controller coupled to the motorized stage, the microscopy objective, and the line scanning camera, the controller being configured to synchronize movement of the optical storage medium on the motorized stage with operation of the line scanning camera shutter and the microscopy objective.
17 . The optical read drive of claim 16 , wherein the motorized stage is configured for movably positioning voxels in the selected layer of the optical storage medium within the FOV of the line scanning camera to enable the line scanning camera to sequentially image selected voxels in the selected layer as the selected voxels are movably positioned.
18 . The optical read drive of claim 16 in which the illumination optics shape a cross-sectional profile of the illumination beam into an elliptical shape having major and minor axes and in which the FOV of the line scanning camera has major and minor axes, wherein the respective major and minor axes of the illumination beam and the FOV of the line scanning camera are substantially aligned.
19 . The optical read drive of claim 16 in which the microscopy objective is controllably focused using one of variable optical focus or relative motion imparted by one or more mechanical actuators between the microscopy objective and the optical storage medium.