BACKGROUND

Quantum cascade lasers provide a tunable mid-infrared (MIR) light source that can be used for spectroscopic measurements and images. Many chemical components of interest have molecular vibrations that are excited in the mid-IR region of the optical spectrum, which spans wavelengths between 5 to 25 microns. Hence, measuring the absorption of MIR light at various locations on a sample can provide useful information about the chemistry of the sample.

SUMMARY

The present invention includes an apparatus for generating a MIR image of a specimen and a method for imaging a specimen in the MIR. In one embodiment, the apparatus includes a MIR imaging system having a MIR laser that generates a first light beam, and a stage adapted to carry a specimen to be scanned. An optical assembly focuses the first light beam to a point on the specimen, and a first light detector measures a first intensity of light leaving the point on the specimen. A stage actuator assembly causes the specimen to move relative to the point in two dimensions. A controller forms a MIR image from the first intensity. The optical assembly includes a scanning assembly having a focusing lens that focuses the first light beam to the point and a mirror that moves in a first direction relative to the stage such that the focusing lens maintains a fixed distance between the focusing lens and the stage, the stage moving in a direction orthogonal to the first direction.

In one aspect of the invention, the first light detector measures light reflected from the specimen. In another aspect of the invention, the first light detector measures light transmitted by the specimen.

In a still further aspect of the invention, a beam intensity detector measures an intensity of the first light beam. In embodiments in which the MIR laser is a pulsed light source, the controller sums measured intensities from the reflected light detector only during periods in which the measured beam intensity is greater than a first threshold. In one aspect of the invention, the controller determines a ratio of the measured intensity of the first light beam and the measured beam intensity to form the MIR image.

In another aspect of the invention, the apparatus includes a visible light imaging station that displays an image of the specimen generated by illuminating the specimen with light in a visual range of light wavelengths. The controller is configured to receive input from a user indicating a region in the visible image that is to be scanned by the MIR imaging system to generate a corresponding MIR image.

In a still further aspect of the invention, the MIR laser generates light in a range of wavelengths that is controlled by the controller; the controller is configured to receive input from a user indicating a feature of interest in the MIR image, and the controller measures light leaving the features at different wavelengths from the feature of interest to generate a spectrum characterizing the feature of interest.

In another embodiment of the invention, the MIR imaging system includes a pulsed MIR laser that generates a first light beam, and a stage adapted to carry a specimen to be scanned. An optical assembly focuses the first light beam to a point on the specimen, and a first light detector measures a first intensity of light leaving the point on the specimen. The stage actuator assembly causes the specimen to move relative to the point in two dimensions and a beam intensity detector measures an intensity of the first light beam. A controller forms a MIR image from the first intensity. The controller sums measured intensities from the reflected light detector only during periods in which the measured beam intensity is greater than a first threshold.

In one aspect of the invention, the controller determines a ratio of the measured intensity of the first light beam and the measured beam intensity to form the MIR image. In another aspect of the invention, the controller only sums measured intensities associated with pulses that generate an intensity greater than a predetermined threshold in the beam intensity detector.

In a still further aspect of the invention, the beam intensity detector includes a directional attenuator that blocks light not traveling in a predetermined direction.

