Characteristics of the Airborne UV DIAL System

 

 The airborne UV DIAL (Differential Absorption Lidar) system consists of two frequency-doubled ND:YAG lasers that are used to pump two high efficiency dye lasers which are frequency-doubled into the UV to produce the DIAL on-line (289 nm) and off-line (300 nm) wavelengths.  The residual 1064-nm beams from the frequency-doubling process of the Nd:YAG lasers are transmitted along with the residual visible beams from the frequency-doubling process of the dye lasers.  In total there are four wavelengths (288, 300, 600, and 1064 nm) transmitted simultaneously into the atmosphere below and above the aircraft for lidar backscatter measurements.  The DIAL on and off wavelengths are produced in sequential laser pulses with a time separation of 300 ms.  This close spacing ensures that the same atmospheric scattering volume is sampled at both wavelengths during the DIAL measurement.  The four laser beams are transmitted collinearly with the receiver telescope through 40-cm diameter fused silica windows in the top and bottom of the aircraft.  The receiver system consists of two 35-cm diameter Cassegrain telescopes (one pointed in the zenith and the other in the nadir) with optics to direct the received signals through narrowband optical filters and onto detectors.  The detectors include gateable photomultiplier tubes for the UV and 600-nm returns, and avalanche photodiodes (APD) for the 1064-nm returns.  Four different detectors can be accommodated simultaneously in each direction.  The transmitter and receiver characteristics for the UV DIAL system are summarized in Table 1, and Figure 1 shows a block diagram of the system.  The measurement capabilities of the airborne UV DIAL system are given in Table 2.

 

Table 1. Airborne UV DIAL System Characteristics

 

Transmitter:

     Pump lasers (2)                                                                                 Continuum PL-9030

Pulse separation, ms                                                                                              300

Pulse energy at 532, mJ                                                                                        800

Pulse length, ns                                                                                                        6

Repetition rate, Hz                                                                                                  30

Transmitted laser energy at 1064 nm, mJ                                                                  400   

             Dye lasers (2 Lasers)                                                                      Continuum ND-6000

             Dye laser output energy, fundamental, each laser,  mJ                                          220    

             Doubled fundamental (UV laser energy), each laser, mJ                              30 on /off

             UV laser linewidth, pm                                                                                        <4

             Transmitted UV laser energy, each direction, mJ                                    15 on & off

             Transmitted fundamental energy,  each direction, mJ                                             50

 

Receiver:                                                                   Wavelength Regions

                                                                289-300 nm       578-600 nm         1064 nm

Efficiency to detector,3 %                                31                     40                     31

Detector quantum efficiency, %                  21 (PMT)          8 (PMT)          40 (APD)

Total receiver efficiency, %                             6.5                    3.2                   12.4

Receiver field-of-view (selectable), mrad         <1.5                  ≤1.5                  ≤1.5

Physical p arameters

     Total weight (lbs)                                                                                      3825

     Dimensions of lasers and laser support structure (L x W x H)             234” x 40” x 43”

(telescope included in length, but not height)

     Power requirements (kW)                                                                           30

                                                                                                                                                                                   

1 Near 600 nm; 2 Near 300 nm;

3Includes filter transmission for daytime operation.

 

[Side-view configuration of the airborne UV DIAL system on the DC-8 aircraft.]

Figure 1.   Side-view configuration of the airborne UV DIAL system on the DC-8 aircraft.

 

 

 

Table 2. Ozone and aerosol measurement parameters for airborne UV DIAL system.

 

Vertical resolution

Aerosol vertical averaging interval:                     60 m

            Ozone vertical averaging interval:                       300 m

 

Horizontal resolution

            Aerosol horizontal averaging interval:                 2 seconds or approximately 470 m
            Ozone horizontal averaging interval:                   300 s or approximately 70 km

 

Measurement accuracy and precision

            Aerosols:           Accuracy and precision:   1%
            Ozone:              Accuracy: 10% or 2 ppbv, whichever is larger
                                    Precision:  better than 5% or 1 ppbv, whichever is larger

 

Measurement range

            Aerosols:          Near surface to 750 m below aircraft and 750 m above aircraft to

                                                10-15 km above aircraft
                         Ozone:             300 m above surface to 900  m below aircraft and 900 m above
                                                 Aircraft to 10-15 km above aircraft