Systems, methods, and apparatus for sensitive thermal imaging

Patent metadata

Field	Value
Publication number	US-9835885-B2
Application number	US-201514845941-A
Country	US
Kind code	B2
Filing date	Sep 4, 2015
Priority date	Sep 5, 2014
Publication date	Dec 5, 2017
Grant date	Dec 5, 2017

Abstract

The high-pixel-count uncooled thermal imaging arrays disclosed herein have liquid crystal (LC) microcavity transducers separate from the read-out integrated circuit (ROIC). The transducer converts incident infrared (IR) radiation in birefringence changes that can be measured with visible light. In other words, the system uses the temperature sensitivity of the LC birefringence to convert the IR scene to a visible image. Measurements on sample arrays indicate that the LC material quality is similar to that of bulk samples and has good noise performance. Additionally, high-fill-factor arrays on fused-silica substrates may be processed to enable optimization of conditions for greatly improved temperature sensitivity. An additional IR absorber layer may be integrated into the process to tune the structure for the infrared.

First claim

The invention claimed is:

1. A system for detecting incident infrared radiation, the system comprising:
a liquid crystal transducer comprising liquid crystal material having a birefringence Γ that varies as a function of the incident infrared radiation;
a visible light source, in optical communication with the liquid crystal transducer, to illuminate the liquid crystal transducer with a first beam of linearly polarized visible light so as to produce a beam of elliptically polarized visible light having a degree of ellipticity that varies in proportion to the birefringence of the liquid crystal material, the first beam of linearly polarized light having a polarization angle of 45° with respect to a fast axis of the liquid crystal transducer;
a quarter-wave plate, in optical communication with the liquid crystal transducer, to convert the beam of elliptically polarized visible light into a second beam of linearly polarized visible light, the quarter-wave plate having an optical axis aligned with the polarization angle of the first beam of linearly polarized light;
an analyzer, in optical communication with the quarter-wave plate, to polarize the second beam of linearly polarized visible light at an angle based on the birefringence Γ of the liquid crystal transducer; and
a detector array, in optical communication with the analyzer, to produce an electronic representation of the incident infrared radiation in response to detection of the second beam of linearly polarized visible light
wherein the analyzer is oriented at an angle based on (i) a change in intensity of the second beam of linearly polarized light as a function of a temperature change of the liquid crystal material induced by the incident infrared radiation and (ii) bend, twist, and/or splay noise associated with propagation of the first beam of linearly polarized light through the liquid crystal material.

2. The system of claim 1 , wherein the infrared radiation is within a wavelength range of about 2 μm to about 14 μm.

3. The system of claim 1 , wherein the system is configured to operate with a noise equivalent temperature difference (NETD) near room-temperature background limited noise performance (BLIP).

4. The system of claim 1 , wherein the liquid crystal transducer comprises:
a substrate;
an array of liquid crystal cells, each liquid crystal cell in the array of liquid crystal cells defining a respective sealed cavity containing a respective portion of the liquid crystal material; and
a plurality of thermal legs, each thermal leg in the plurality of thermal legs in physical contact with the substrate and with a respective liquid crystal cell in the array of liquid crystal cells to support the respective liquid crystal cell and to thermally isolate the respective liquid crystal cell from the substrate.

5. The system of claim 4 , wherein at least one respective sealed cavity has a width of about 2 μm to about 25 μm.

6. The system of claim 4 , wherein the sealed cavity in at least one liquid crystal cell in the array of liquid crystal cells is at least partially defined by an inorganic layer, the inorganic layer having a textured surface for aligning the liquid crystal material within the at least one liquid crystal cell.

7. The system of claim 1 , wherein the analyzer is oriented at an angle based at least in part on a signal-to-noise ratio of the electronic representation of the incident infrared radiation.

8. The system of claim 1 , further comprising:
a lens, in optical communication with the array of liquid crystal cells, to image the incident infrared radiation onto the liquid crystal transducer.

9. A method for thermal imaging of a scene, the method comprising:
imaging long-wave infrared (LWIR) radiation representative of the scene onto at least one liquid crystal transducer so as to vary a birefringence of the at least one liquid crystal cell by an amount proportional to an intensity of the LWIR radiation;
illuminating the at least one liquid crystal cell with a first beam of linearly polarized visible light so as to produce a beam of elliptically polarized visible light having a degree of ellipticity that varies by an amount proportional to the birefringence of the at least one liquid crystal cell, the first beam of linearly polarized light having a polarization angle of 45° with respect to a fast axis of the liquid crystal transducer;
transmitting the beam of elliptically polarized visible light through a quarter-wave plate so as to produce a second beam of linearly polarized visible light having an orientation angle proportional to the birefringence of the at least one liquid crystal cell, the quarter-wave plate having an optical axis aligned with the polarization angle of the first beam of linearly polarized light;
transmitting the second beam of linearly polarized visible light through a linear polarizer to polarize the second beam of linearly polarized visible at an angle based on the birefringence Γ of the liquid crystal transducer; and
detecting the second beam of linearly polarized visible light to form an electronic representation of the scene proportional to the intensity of the LWIR radiation,
wherein the linear polarizer is oriented at an angle based on (i) a change in intensity of the second beam of linearly polarized light as a function of a temperature change of the at least one liquid crystal cell induced by the LWIR radiation and (ii) bend, twist, and/or splay noise associated with propagation of the first beam of linearly polarized light through the at least one liquid crystal cell.

10. The method of claim 9 , wherein transmitting the beam of elliptically polarized visible light through the quarter-wave plate reduces effects of liquid crystal noise.

11. The method of claim 9 , wherein transmitting the second beam of linearly polarized visible light through the linear polarizer comprises orienting the linear polarizer at an angle of based on a signal-to-noise ratio of the electronic representation of the scene.

12. A method for thermal imaging of a scene, the method comprising: imaging long-wave infrared (LWIR) radiation representative of the scene onto at least one liquid crystal transducer so as to vary a birefringence of the at least one liquid crystal transducer by an amount proportional to an intensity of the LWIR radiation;
illuminating the at least one liquid crystal transducer with polarized light so as to vary a polarization state of the polarized light by an amount proportional to the birefringence of the at least one liquid crystal transducer;
transmitting the polarized light through a quarter-wave plate so as to transform the polarization state of the polarized light to a linear polarization state having a rotation angle proportional to the birefringence of the at least one liquid crystal transducer;
transmitting the polarized light through a linear polarizer to reduce an amplitude of the polarized light by an amount proportional to the birefringence of the at least one liquid crystal transducer; and
detecting the polarized light transmitted through the linear polarizer to form a representation of the scene proportional to the intensity of the LWIR radiation,
wherein the linear polarizer is oriented at an angle based on (i) a change in intensity of the polarized light as a function of a temperature change of the liquid crystal transducer induced by the LWIR radiation and (ii) bend, twist, and/or splay noise associated with propagation of the polarized light through the liquid crystal transducer.

Assignees

• Reich Robert K
• Clark Harry R
• Bozler Carl O
• Berry Shaun R
• Muldavin Jeremy B
• Massachusetts Inst Technology

Inventors

• Reich Robert K
• Clark Harry R
• Bozler Carl O
• Berry Shaun R
• Muldavin Jeremy B

Classifications

• G02F2001/133638

  Physics · mapped topic

• G01J5/58

  using absorption; using extinction effect · CPC title

• H04N5/33

  Transforming infrared radiation (cameras or camera modules for generating image signals from infrared radiation H04N23/20; circuitry of SSIS for transforming infrared radiation into image signals H04N25/20) · CPC title

• G02F2203/11

  involving infrared radiation · CPC title

• G02F1/1337

  Surface-induced orientation of the liquid crystal molecules, e.g. by alignment layers · CPC title