In-flight multiple field of view detector for supercooled airborne water droplets
An airborne multiple field-of-view water droplet sensor includes an illumination portion and a detection portion. The illumination portion includes a first optical beam emitter configured to output a light beam. The detection portion includes a kaleidoscope configured to channel a first portion of the backscattered light towards an inner reflective surface of a circle-to-line converter, a multiple field of view subsystem having at least a first detector configured to receive light reflected by the circle-to-line converter, and a single field-of-view subsystem configured to receive a second portion of the backscattered light, the second portion not having been reflected by the circle-to-line converter. The single field-of-view subsystem may include a dual channel circular polarization detector for distinguishing between liquid water droplets and ice crystals based on information in the single field-of-view.
1. An airborne multiple field of view water droplet sensor, comprising:
an illumination portion comprising:
a first optical beam emitter configured to output a light beam; and
a detection portion comprising:
a window configured to permit backscattered light from water droplets illuminated by said light beam to pass therethrough;
a first lens configured to focus said backscattered light after the backscattered light has passed through said window;
a kaleidoscope configured to channel a first portion of the backscattered light that has passed through the first lens, towards an inner reflective surface of a circle-to-line converter;
a multiple field of view subsystem comprising at least a first detector configured to receive light reflected by the circle-to-line converter; and
a single field of view subsystem configured to receive a second portion of the backscattered light that has passed through the first lens, the second portion not having been reflected by the circle-to-line converter.
2. The airborne multiple field-of-view water droplet sensor according to claim 1 , further comprising:
a fiber optic plate positioned between the inner reflective surface of the circle-to-line converter and the first detector, the fiber optic plate configured to transfer an optical image reflected by the circle-to-line converter.
3. The airborne multiple field-of-view water droplet sensor according to claim 1 , wherein the multiple field of view subsystem further comprises:
a second detector, the second detector also configured to receive light reflected by the circle-to-line converter, wherein:
the second detector is configured to detect reflected, backscattered light corresponding to field of view angles larger than those detected by the first detector.
4. The airborne multiple field-of-view water droplet sensor according to claim 3 , wherein each of the first and second detectors comprise a linear array of detector elements.
5. The airborne multiple field-of-view water droplet sensor according to claim 3 , further comprising:
a first set of optical elements positioned between a focal plane of the circle-to-line converter and the first detector, wherein the first set of optical elements amplifies small field of view reflections from the circle to line converter; and
a second set of optical elements positioned between the focal plane of the circle-to-line converter and the second detector.
6. The airborne multiple field-of-view water droplet sensor according to claim 5 , wherein the first set of optical elements comprises:
at least two lenses; and
a bandpass filter interposed between the two lenses.
7. The airborne multiple field-of-view water droplet sensor according to claim 1 , wherein:
the circle-to-line converter comprises a tapered area having a varying slope; and
a slope of an internal reflective surface of the tapered area proximate to the apex is under 1.0.
8. The airborne multiple field-of-view water droplet sensor according to claim 1 , wherein the single field of view subsystem comprises a beamstop.
9. The airborne multiple field-of-view water droplet sensor according to claim 1 , wherein the single field of view subsystem comprises a photodiode.
10. The airborne multiple field-of-view water droplet sensor according to claim 1 , further comprising:
a collimating lens interposed between the first lens and the single field of view subsystem; wherein:
a position of the collimating lens is axially adjustable along an optical axis of the sensor.
11. The airborne multiple field-of-view water droplet sensor according to claim 1 , wherein the first optical beam emitter comprises at least one from the group consisting of:
a light emitting diode whose output is linearly polarized; and
a laser.
12. The airborne multiple field-of-view water droplet sensor according to claim 11 , further comprising:
a reflector configured to reflect the light beam along an optical illumination axis of the sensor.
13. The airborne multiple field-of-view water droplet sensor according to claim 1 , further comprising:
a processor configured to receive signals output by the multiple field of view subsystem and said single field of view subsystem, and calculate at least one parameter reflective of a water droplet condition, based on said signals from the two subsystems.
14. The airborne multiple field-of-view water droplet sensor according to claim 1 , wherein the single field of view subsystem employs circularly polarized light to distinguish between water and ice crystals.
15. The airborne multiple field-of-view water droplet sensor according to claim 14 , wherein:
the illumination portion further comprises:
a first linear polarizer configured to receive the light beam and output a linearly polarized illuminating beam;
a first circular polarization element configured to receive the linearly polarized illuminating beam and output a circularly polarized illuminating beam; and
the detection portion further comprises:
a second circular polarization element configured to receive the circularly polarized backscattered light that has passed through the window and output linearly polarized backscattered light;
a first polarizing beam splitter configured to split the linearly polarized backscattered light into a first component linearly polarized backscattered light and a second component linearly polarized backscattered light;
a first component light detector configured to detect the first component linearly polarized backscattered light and output a first signal in response thereto;
a second component light detector configured to detect the second component linearly polarized backscattered light and output a second signal in response thereto; and
a processor configured to receive the first and second signals output by respective first and second component light detectors, and calculate at least one parameter reflective of an ice condition, based on said first and second signals.
16. The airborne multiple field-of-view water droplet sensor according to claim 15 , further comprising:
a second linear polarizer interposed between the first polarizing beam splitter and the first component light detector, the second linear polarizer configured to further linearly polarize the first component linearly polarized backscattered light, prior to detection by the first component light detector.
17. The airborne multiple field-of-view water droplet sensor according to claim 15 , wherein:
the first circular polarization element comprises a first quarter wave plate; and
the second circular polarization element comprises a second quarter wave plate.
18. The airborne multiple field-of-view water droplet sensor according to claim 15 , further comprising:
a reflector configured to reflect the circularly polarized illuminating beam along an optical illumination axis of the sensor;
a second linear polarizer interposed between the first polarizing beam splitter and the first component light detector, the second linear polarizer configured to further linearly polarize the first component linearly polarized backscattered light, prior to detection by the first component light detector;
a processor configured to receive the first and second signals output by respective first and second component light detectors, and calculate at least one parameter reflective of an ice condition, based on said first and second signals.
19. The airborne multiple field-of-view water droplet sensor according to claim 18 , further comprising:
a first lens interposed between the window and the second circular polarization element;
a second lens interposed between the first linear polarizer and the first component light detector;
a third lens interposed between the second linear polarizer and the second component light detector; and
a collimating lens interposed between the first lens and the second circular polarization element.
20. The airborne multiple field-of-view water droplet sensor according to claim 19 , wherein:
the second and third lenses are condenser lenses; and
a position of the collimating lens is axially adjustable along an optical axis of the sensor.