2.1.

Eye-Safe Operation

Significant efforts were made to produce an active short-wave infrared (SWIR) imaging system that is eye-safe to allow for imaging capability that could be fielded in nearly any scenario without major efforts to contain the laser transmittance path. A Quantel CFR 300 laser was modified to produce output at 1527 nm. A photo and diagram of the laser, as delivered, are shown in Fig. 1. The stock frequency doubling crystal was removed to provide access to the 1064-nm pump wavelength. An optical parametric amplifier (OPA) [potassium titanyle arsenate (KTA)] crystal was introduced by repurposing the frequency doubler’s housing for the KTA crystal. The $\frac{1}{4}$-waveplate and beam expanding optics were reconfigured to optimize the non-linear generation inside the KTA crystal. Following the KTA crystal are the addition of two dichroic mirrors that were setup to reflect the residual 1064-nm pump and the $3.5\text{-}\mu \mathrm{m}$ idler into a beam dump while allowing for the 1527-nm pulse to exit the laser housing. The reconfiguration for 1527-nm generation is shown in Fig. 2. Once the reconfiguration was complete, the KTA crystal orientation was optimized to maximize the laser output at 1527 nm via fine angle tuning of the crystal axis relative to the pump. The end result was a 1527-nm laser with $85\mathrm{mJ}/\text{pulse}$ with a pulse duration of $\approx 8\mathrm{ns}$. The outgoing laser pulse was sent through a beam expander that reduced the beam divergence to 3 mrad while increasing the beam size to $\approx 2\mathrm{cm}$. The beam divergence was measured in the lab over a path of $\sim 5\mathrm{m}$ by measuring the beam spot size. In addition, the 3-mrad divergence was confirmed during the field test in which the beam was found to fill the water tower (30-m diameter) at a 10-km range. Utilizing these parameters, the nominal ocular hazard distance is found to be 18 m using the ANSI Z136.1 standard.^6,7

Fig. 1

Quantel CFR 300 as delivered. Many parts were repurposed and reused to maintain the laser operation within the original housing.

Fig. 2

Quantel CFR 300—postmodification for operation at 1527 nm—resulting in $\approx 85\mathrm{mJ}$ at up to 28 Hz.

2.2.

Integrated System

The integrated laser transmitter and receiver are shown in Fig. 3. The Intevac LIVAR M506 camera ($640\times 512\text{-}13.4\mu \mathrm{m}$ pixel pitch) was mounted to an Orion StarBlast F/4, 450-mm focal length Newtonian telescope. This resulted in a $30\text{-}\mu \mathrm{rad}$ iFOV and a $0.98\mathrm{deg}\times 0.86\mathrm{deg}$ field of view. The laser divergence of 3 mrad was chosen to maintain power density for long-range operation while filling a significant portion of the field of view. The laser and receiver were arranged in a monostatic configuration through the addition of an outgoing turning mirror glued behind the secondary of the imaging telescope, as shown in Fig. 3. This decision was made to ensure that the system retained boresight alignment for all propagation distances. To eliminate any range errors due to the laser’s timing jitter, a high-speed photodiode was used as a master-sync to trigger the image acquisition for each laser pulse. The output of the photodiode response is used to trigger a Stanford Research Systems DG-535 that is utilized as the master delay clock, which then sends a transistor-transistor logic (TTL) pulse to trigger the camera’s exposure. Utilizing this setup, the camera exposure is then delayed until the return of the pulse from the range of interest—in our case, $\sim 10\mathrm{km}$ out and 10 km back from the water tower.

Fig. 3

SWIR active imaging integrated system. Laser path shown in red. The laser source was co-aligned with the collection optics, going out behind the secondary mirror of the imager.

The integrated system was tested in clear conditions to confirm reliable operation. As a first test, a range/depth map of the water tower was constructed by sequentially delaying the range-gated imager’s active exposure time over the laser pulse propagation. The SWIR imager was set with an exposure time of 100 ns corresponding to a 15-m exposure range and delayed in 20 ns (3 m) steps to effectively image different depths of the water tower. Figure 4(a) shows a visible image of the tower from close range. The water tower is $\sim 30\mathrm{m}$ in diameter with support struts evenly spaced around the base. Figure 4(b) shows the depth map created via the integrated range-gated SWIR imaging system from 10 km away. The application of incremental delays to the active time of the imager allowed for imaging of different range slices. The first range bin shows the front legs of the water tower. Subsequent increases in delays clearly reveal the side struts and the structure of the tower body. At the longest delay setting, the rear legs of the water tower are visible.

Fig. 4

(a) Water tower imaged locally from the ground $\approx 30\mathrm{m}$ in diameter. (b) Depth map of water tower as collected from the rooftop at JHU-APL at a 10-km range utilizing the range gated imager. The imager’s exposure time was set to 100 ns and stepped by 20 ns to image the water tower from front to back. The 0- to 15-m exposure shows primarily the front support struts, whereas the 12- to 27-m exposure shows the rear supports.

3.1.

Imagery Collection

The range-gated active imager was operated in August 2019 from a rooftop on JHU/APL’s campus, in Laurel, Maryland, to a water tower 10 km north–northeast. Figure 5 shows a satellite image of the 10-km path from APL’s campus to the water tower, including a close-up view inset; the water tower is 30 m in diameter. A test of the range-gated imagers weather penetration capability was performed during a rainstorm during which visibility to the tower was lost. Figure 6(a) shows a passive visible image and passive SWIR image [Fig. 6(b)] of the line-of-sight from the rooftop to the water tower during the time of the test. Although obscured by rain, the water tower is centered in the field of view of both images.

Fig. 5

Satellite image of the 10-km path from JHU/APL’s facility to the water tower. Inset: zoomed image of the water tower with the line-of-sight marked in yellow. The water tower has a diameter of 30 m.

Fig. 6

(a) Passive visible camera image and (b) passive SWIR camera image during time of test.

For rain penetration, a 500-ns exposure time was used, yielding a 75-m range bin—this exposure was delayed by the out and back time to the water tower of 67.2 ms (10,080 m each way). A time series of flash illuminated ($1\text{pulse}/\text{frame}$) frames operating at 20 Hz during a period of rain was collected and was postprocessed utilizing a 10-frame sliding window time average. A single-image frame can be seen in Fig. 7(a), and a typical 10-frame (0.5 s) sliding window average is shown in Fig. 7(b). Most notable is the significant increase in contrast as nearly the entire water tower becomes visible from the mitigation of rain effects and laser speckle.

Fig. 7

Active SWIR images during the rain penetration test. (a) A single frame (500-ns exposure) and (b) a typical 10-frame sliding window average.

3.2.

Atmospheric Attenuation and Path Transmittance via MODTRAN

In this section, the atmospheric conditions at the time of the test were analyzed to estimate the line-of-sight transmittance during the test. Ultimately, these characteristics allow for a quantification of performance gains enabled by active imaging, which is discussed in Sec. 3.3. First, the path-averaged rain rate was determined using NEXRAD data from Sterling, VA. This rain rate was subsequently used to generate one-way 10-km path transmittance from the range-gated imager to the water tower as a function of wavelength for different visibility levels. The NEXRAD derived rain rate was processed to yield a path averaged rain rate over the 10-km path to the water tower. Figure 8(a) shows the NEXRAD derived rain rate with the optical line-of-sight, overlaid in black. Figure 8(b) shows the rain rate as a function of distance along this line-of-sight. A path averaged rain rate of $0.83\mathrm{mm}/\mathrm{h}$ was calculated at the time of test.

Fig. 8

(a) NEXRAD derived rain rate (mm/h) during the time of the test. The bold black line is the LoS from the transmitter to the water tower. (b) Rain rate (mm/h) along the line-of-sight. A path weighted average rain rate of $0.83\mathrm{mm}/\mathrm{h}$ was calculated and used to seed Modtran.

The path averaged rain rate was used to produce a one-way 10-km path transmittance from MODTRAN. MODTRAN is the moderate resolution transmittance model originally developed by the Air Force Research Lab (AFRL) to determine path transmittance and radiance for optical propagation.⁸ A mid-latitude summer MODTRAN model was seeded using the path averaged rain rate of $0.83\mathrm{mm}/\mathrm{h}$ along with an associated visibility that weights the aerosol loss along the path. The calculated transmittance curve, for the 10-km path to the water tower, as a function of wavelength is shown in Fig. 9.

Fig. 9

One-way 10-km path transmittance from the laser source to the water tower. Top black curve is the transmittance solely due to the rain at 0.83 mm/h. The four color curves are the results of MODTRAN with the rain rate fixed and different visibilities. The dashed-red vertical line is the operational laser wavelength.

The path transmittance, assuming no aerosol loss, was calculated by determining the extinction due to rain:^9,10

3.3.

Relative Range Improvement

In the previous section, performance analysis was driven by atmospheric data. In this section, the image data were also processed to determine the contrast ratio and relative gain established by active imaging. In combination with the path extinction derived in the previous section, we utilize the normalized contrast metric to determine an equivalent visibility range for the system. This allows for a direct measure of range improvement enabled by active imaging, following an approach simliar to Christnacher et al.¹¹ To calculate the true range improvement, we need to determine the normalized contrast, e.g., the range-gated contrast in bad weather scaled to the clear day maximum contrast for the same time period. This was determined by imaging the water tower on a clear afternoon [Fig. 10(a] with optimal lighting conditions, and the contrast was found to be $\approx 0.22$. The range-gated contrast during the rainstorm of the water tower was calculated for the images shown in Figs. 10(b) and (c). The mean of the pixel values for the background are calculated from the red box, and the signal is calculated for the black box on the water tower. Figure 10(b) shows a single frame from the range-gated imager (single laser pulse), Fig. 10(c) shows a 10-frame average (10 laser pulses). The contrast is calculated as follows:

Fig. 10

(a) Clear day image of water tower used to calculate contrast for normalization metric; (b) single frame from range gated imager through rain; and (c) 10-frame average from range gated imager. Black boxes represent the regions where the background and signal were determined for the contrast calculation. The red box was used for the background and the black box on the tower was utilized for the signal.

The normalized contrast from the imager ($0.032/0.22=0.145$) is used to calculate an equivalent path visibility to determine the relative range improvement factor. The equivalent visibility was determined for three transmittance values at 1527 nm, determined from Modtran in Fig. 9 utilizing Eq. (4) and summarized in Table 1. Given the most probable atmospheric extinction of $0.512{\mathrm{km}}^{-1}$ from Modtran atmospheric analysis, we see a range improvement factor of $\sim 2.6$. Table 1 shows that the range of our improvement factor is between 2.5 and 3.1, given the results of Modtran modeling:

Table 1

Contrast and relative range improvement for range gated imager in rain. The transmittance to extinction calculation is performed via Eq. (2) and the contrast to equivalent visibility is calculated via Eq. (4).

We can similarly plot the normalized contrast as a function of attenuation lengths (${\sigma }_{\mathrm{ext}}\times \text{range}$) and compare with the Beer–Lambert law. The Beer–Lambert law describes the constrast of a passive imager system as a function of attenuation lengths. Figure 11 shows the Beer–Lambert law normalized contrast as a function of attenuation lengths in green. The data from our test are shown as red dots. Looking at the plot, one can see that a normalized contrast of 0.145 should only be achievable at $\sim 2$ attenuation lengths, whereas our active range gated system is capable of the same contrast at $\approx 5$ to 6 attenuation lengths. Taking the ratio of the Beer–Lambert law attenuation lengths at a normalized contrast of 0.145 to the three plotted points (0.145 contrast at 4.8, 5.1, and 5.99 atten. lengths) results in the same values shown in Table 1 as the range improvement factor.

Fig. 11

Normalized contrast as a function of attenuation lengths for Beer–Lambert law evaluated at $\lambda =1527\mathrm{nm}$ and calculated normalized contrast of the range-gated imager. Normalized contrast of the imager was found to be 0.145 and is plotted for a 10-km range for each of the extinction values in Table 1. Inset: linear scale ($\text{attenuation length}={\sigma }_{\mathrm{ext}}\times \text{range}$).