Unfortunately, those slides don't provide much information on what they mean by "relay pathfinder" but the concept of using optical communication in space has been tested and is planned for several missions.
NASA has previously tested optical communications during a lunar mission. This was used as a demonstration of the concept for the (now not-so-) future missions.
Since you specifically asked about deep space missions, I'll focus on those, however I briefly want to mention the commercial interest. In the early 90's Motorola was considering optical comms for their intersatellite links (ISLs), but settled on radio links due to the complexity of optical comms. Now, according to an FCC filing, SpaceX will be using optical links between the satellites in their Starlink constellation.
In terms of deep space missions, there are currently no plans that I know of to completely replace the current DSN with optical communication. However, NASA does have plans to test out the system on the Psyche mission, a deep space mission to the asteroid belt. Additionally, NASA is testing their "Laser Communication Relay Demonstrator" which they hope to use for future Mars missions.
is it really an array of optical communications telescopes?
Not exactly. Some versions of the terrestrial version of optical communication use several transmitters and receivers to help account for atmospheric distortion. As far as I know, there are no plans to use an optical array as a phase array as you would for radio transmissions.
Also, what is that on the flatbed trailer?
My guess is that they were trying to show that the system would be mobile. That just looks like their receiver/transmitter design for their optical comms.
edit: For more information on NASA's ambitions to integrate optical communications systems into the DSN, read this paper: Toward a NASA Deep Space Optical Communications System.
Some more details:
From the JPL News item Deep Space Communications via Faraway Photons
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The DSOC project is developing key technologies that are being integrated into a deep space-worthy Flight Laser Transceiver (FLT), high-tech work that will advance this mode of communications to Technology Readiness Level (TRL) 6. Reaching a TRL 6 level equates to having technology that is a fully functional prototype or representational model.
As a "game changing" technology demonstration, DSOC is exactly that. NASA STMD's Game Changing Development Program funded the technology development phase of DSOC. The flight demonstration is jointly funded by STMD, the Technology Demonstration Mission (TDM) Program and NASA/ HEOMD/Space Communication and Navigation (SCaN).
Work on the laser package is based at NASA's Jet Propulsion Laboratory in Pasadena, California.
"Things are shaping up reasonably and we have a considerable amount of test activity going on," says Abhijit Biswas, DSOC Project Technologist in Flight Communications Systems at JPL. Delivery of DSOC for integration within the Psyche mission is expected in 2021 with the spacecraft launch to occur in the summer of 2022, he explains.
"Think of the DSOC flight laser transceiver onboard Psyche as a telescope," Biswas explains, able to receive and transmit laser light in precisely timed photon bursts.
DSOC architecture is based on transmitting a laser beacon from Earth to assist lineofsight stabilization to make possible the pointing back of a downlink laser beam. The laser onboard the Psyche spacecraft, Biswas says, is based on a master-oscillator power amplifier that uses optical fibers.
The laser beacon to DSOC will be transmitted from JPL's Table Mountain Facility located near the town of Wrightwood, California, in the Angeles National Forest. DSOC's beaming of data from space will be received at a large aperture ground telescope at Palomar Mountain Observatory in California, near San Diego.
Also see the 2018 SpaceOps Conference Best Paper award winner Toward a NASA Deep Space Optical Communications System that describes a fascinating solution where 8 meter optical telescopes for communications are built at low cost from segmented mirrors and integrated directly into 34 meter DSN dish beam waveguide antennas so that they take full advantage of sharing the pointing and targeting, and beaconing available from operating via microwaves and optical simultaneously!
