HARLIE

Holographic Airborne Rotating Lidar Instrument Experiment

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The HARLIE transceiver is based on a volume phase holographic optical elements (HOE) made in dichromated gelatin (DCG) sandwiched between 2 layers of high quality float glass. It demonstrates the practical application of this technology to a compact scanning lidar system at 1064 nm wavelength. The HOE has the ability to withstand moderately high laser power and energy loading and is of sufficient optical quality for most direct detection systems,and overall efficiency rivaling conventional receivers. It's size and weight are approximately half of similar performing scanning systems using reflective optics.
Built as an atmospheric backscatter lidar system, HARLIE will also be used to test concepts for an airborne direct detection wind lidar that could one day be used for spaceborne applications. Referring to Figs. 1 and 2, it uses a 40 cm diameter by 1 cm thick Holographic Optical Element (HOE) as the receiver collecting and focusing aperture. It has a 45 degree diffraction angle and a 1 meter focus normal to its surface. It is continuously scanned up to 30 rpm, and can also operate in step and stare or static modes. Its 160µrad focal spot matches the 200 µm fiber optic field stop which delivers the light to the aft optics package. The aft optics contains a collimating lens, a 500 pm interference filter, focusing lens, a Geiger mode Avalanche Photo Diode, and measures 2.5 cm x 2.5 cm x 15 cm. The transmitter is a continuous diode pumped Q-switched Nd:YAG laser delivering 1 mJ pulses at a 5 KHz rep rate. The beam is expanded to 15 mm diameter before being transmitted through the center of the HOE, which also acts as the collimating lens of the beam expander, transmitting a 100µrad beam. The entire transmitter/receiver package can be placed within centimenters of an aircraft instrument window so that a 52 cm clear aperture window allows for an unobstructed view in all directions around the conical scan.

Picture showing HARLIE transceiver and electronics rack.

HARLIE transceiver (left) and electronics rack (right).

Mounted on its transportation dolly, HARLIE can operate on the ground in any of 8 elevation positions spaced 45° apart. In this figure the system is pointed up, so the HOE appears on top. It is mounted in a large ring ball bearing with a ring gear pressed into the inner race. It is belt driven by a DC servo motor with an overall gear ratio of 123:1. A 12 bit encoder on the motor shaft yields a 12.5 µrad resolution in the azimuth pointing position. The electronics rack contains the data system, the laser power supply and chiller, the scan motor controller and power supply, a GPS receiver, and an aircraft INS interface. The detector output is ping-ponged between a pair of 24 bit scalers to eliminate dead-time during the read-out cycle. A time history of backscatter profiles are displayed on the computer monitor in real-time as a false color image. Other salient technical specifications are listed below.

HARLIE Specifications

    Transceiver Assembly
  • Weight: 118 kg.
  • Overall dimensions (in cm, minus mounting rails): 56 w x 69 l x 102 h
  • Transmitter: diode pumped Nd:YAG, 1064nm wavelength, 1 mJ, 40 nsec pulse length, 5 kHz rep-rate, 100 µrad divergence
  • Receiver: 40 cm diameter, f/2.5 volume phase transmission HOE, 45° diffraction angle, effective collection area 1064 cm^2, 200 µrad field-of-view, 0.5 nm bandpass
  • Detector: Geiger mode Silicon Avalanche PhotoDiode

    Electronics Rack
  • Weight: 138 kg
  • Overall dimensions (cm): 56 w x 64 l x 127 h
  • Power requirements: 1000 W max. @110 Vac., 19 amps peak (2.2 kVA peak)

    Scanner
  • Scan Modes: Point and stare, 8 position step-stare, Continuous scan up to 30 rpm
  • Azimuth (scan) pointing resolution: 12.5 µrad

    Data System
  • Two ping-ponged 24 bit bin scalers
  • Range resolution: 30 m
  • Integration time: 100 msec

    Data Products
  • Real-Time Aerosol Backscatter Profiles: 20 m height resolution, 100 ms intervals (remote access also possible).
  • Boundary Layer Heights:±20 m, 1-scan, 1-minute, or other time intervals.
  • Entrainment Zone Thickness: ±20 m, 1-scan, 1 minute, or other time intervals.
  • BL spatial variability: (BL height versus azimuth and time) spatial resolution dependent on scan rate.
  • Cloud Coverage vs. Height: ±20 m averaged over 1 scan.
  • Cloud-tracked wind profiles: ±1.5 m/s averagedover 200 m altitude intervals and 15-30 min. time.

        SKYCAM Data Products

  • Visible imagery with 70 degree field of view, 2.5 frames/sec.  NTSC color video to VHS tape.  AVI format digital video files, spatial and temporal resolution adjustable.  
 

The following animations are best viewed using Quicktime Player Ver. 6

HOE exposure.mov (9 MB)

This graphical visualization illustrates the exposure geometry used in the manufacture of a Holographic Optical Element, or HOE, like the one used in the HARLIE system. The holographer sets up a series of optics that are used to manipulate the beam from a single mode laser, (Laser label appears) represented by the black cylinder with a green beam emanating from one end. A single mode laser is one that generates only a single frequency of light. The frequency of this mode must remain very stable during the course of the exposure of the holographic film plate, which could take many minutes. The film exposure must be done in a dark room so that the only light to expose the film is that from the laser.

(Begin naming optics in order) The first optic the laser beam encounters is a beam splitter, so called because it divides the beam into two by means of a partially reflecting, partially transmitting coating applied to one surface. Lenses are used to spread the beams out, making them diverge into cones of light. A parabolic mirror is used to recollimate one beam, so that it is no longer diverging but traveling in a cylinder that grows in diameter only very slowly with distance. Other, flat mirrors are used to redirect the beams without changing their divergence properties. A holographic film plate consisting of a thin holographic film, about 10 thousandths of a millimeter thick, applied to one side of a transparent glass substrate, is placed where the two beams will intersect.

(Begin beam motion) The portion of the beam that is transmitted through the beamsplitter is spread out using a lens. This beam fills the surface of the parabolic mirror, which collimates the light upon reflection into the direction of the film plate. This beam is called the reference beam and will have wave fronts that are flat and perpendicular to its direction of travel.

The portion of the beam that is reflected from the beamsplitter (begin second beam) is directed using the two flat mirrors through a lens that spreads the beam out in the direction of the film. Because this beam is spreading rapidly, it will have spherical wave fronts that appear to emanate from the focus of the lens. This beam is called the object beam. Where the two beams intersect their waves will interfere with each other, creating a standing wave pattern of light and dark fringes. Called interference fringes, they are similar to the patterns of standing water waves that are generated on the water surface when you tap or vibrate a glass or bowl filled with water. Standing waves are motionless, despite the fact that two sets of moving waves are used to generate them. (Close-up of film appears) The film is placed at the intersection of the beams, and the interference pattern exposes the HOE. The fringes are 3-dimensional, consisting of alternating bright and dark curved surfaces. Bright fringes occur when the colliding wavefronts from the two beams are in phase, that is, when their “peaks” are aligned. The dark fringes occur where the peaks from one beam meet the valleys of the other, thus canceling out each other’s light energy. Here we see a close-up of a simulated fringe pattern in the holographic film. The actual fringes are microscopic in size, separated by about a half of the wavelength of light, less than one thousandth of a millimeter. The energy in the bright fringes is absorbed by a chemical dye in the film, causing the film material to polymerize at those locations. This means that molecular cross-links are established that locally harden the film and increase its refractive index, which affects the way light will propagate through the film. The film now has a permanent record of the fringe pattern created by the two beams. Instead of consisting of light and dark regions, they now consist of hardened and unhardened regions in the clear film. Invisible to the naked eye, their effect on light becomes apparent when the HOE is properly illuminated.

After the exposure is complete, the film is chemically processed to remove the remaining absorbing dye, and sealed by gluing the cover glass and substrate together with the film in between. The fringe pattern recorded in the film has all of the information needed to recreate either one of the two beams by illuminating the film with the other.

 

HOE Playback.mov   (40 MB)

This animation sequence illustrates how a transmission HOE is used in the HARLIE instrument. A laser transmitter directs its light through a lens to diverge its beam. The beam is then directed using flat mirrors through the center of the HOE. This beam acts like the original object beam used to expose the HOE. The fringes in the HOE will redirect most of this light into a beam that acts like the original reference beam used to expose the HOE. That is, it will now change direction to correspond to the direction of travel of the original reference beam and assume collimated behavior, no longer rapidly spreading out with distance. Notice that the transmitted laser beam only illuminates a small portion of the middle of the HOE. This allows us to use the rest of the HOE to act as a receiver telescope. The transmitted laser beam in HARLIE consists of a series of light pulses, each about 5 meters in length. As each pulse propagates through the atmosphere, some of the light is scattered by the molecules and dust in the air. A very small portion of this scattered light is heading back in the direction of HARLIE, where it again passes through the HOE. The next pulse is not launched until the previous pulse has left the atmosphere and the backscatter from it has ceased. This takes about 200 millionths of a second.

Because the light scattered by the atmosphere arrives from large distances, as far as twenty kilometers or more, and the HOE collects such a small portion of it, it appears to have flat wavefronts, a property of the original collimated reference beam in the HOE exposure setup. However, this light is traveling in the opposite direction with respect to the HOE, which we call the conjugate of the original reference beam. The fringe pattern in the HOE acts on the atmospheric backscatter to recreate the conjugate of the original object beam. Therefore the light will be directed to a point where the lens focus was for the object beam during exposure. This focal spot is very small, about 150 microns in diameter. It is small enough that we can place an optical fiber to “catch” the light and direct it to other optics for filtering and detecting elsewhere in the HARLIE instrument. Note that the central portion of the HOE used to transmit the laser is not available to focus backscattered light because the small mirror used to steer the transmitted beam will block the light.

To scan, HARLIE rotates the HOE by means of a belt drive and ring gear around the perimeter of the HOE. Since the HOE was manufactured with a 45-degree angle between the reference beam and the film, HARLIE transmits the laser and receives atmospheric backscatter only at this angle. Therefore, when we rotate the HOE it makes a conical scan of the sky.

harlie_trailer.mov (6.5 MB)

utah.mov (64.7MB, Quicktime Movie)(taken March 10 at Space Dynamics Lab)

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