WEBVTT 00:00:08.145 --> 00:00:15.132 It’s late, pitch dark, and a self-driving car winds down a narrow country road. 00:00:15.132 --> 00:00:18.724 Suddenly, three hazards appear at the same time. NOTE Paragraph 00:00:18.724 --> 00:00:20.846 What happens next? NOTE Paragraph 00:00:20.846 --> 00:00:24.043 Before it can navigate this onslaught of obstacles, 00:00:24.043 --> 00:00:26.083 the car has to detect them— 00:00:26.083 --> 00:00:29.846 gleaning enough information about their size, shape, and position, 00:00:29.846 --> 00:00:34.208 so that its control algorithms can plot the safest course. 00:00:34.208 --> 00:00:35.762 With no human at the wheel, 00:00:35.762 --> 00:00:40.547 the car needs smart eyes, sensors that’ll resolve these details— 00:00:40.547 --> 00:00:43.898 no matter the environment, weather, or how dark it is— 00:00:43.898 --> 00:00:45.920 all in a split-second. NOTE Paragraph 00:00:45.920 --> 00:00:50.159 That’s a tall order, but there’s a solution that partners two things: 00:00:50.159 --> 00:00:53.849 a special kind of laser-based probe called LIDAR, 00:00:53.849 --> 00:00:56.578 and a miniature version of the communications technology 00:00:56.578 --> 00:01:00.936 that keeps the internet humming, called integrated photonics. NOTE Paragraph 00:01:00.936 --> 00:01:06.006 To understand LIDAR, it helps to start with a related technology— radar. 00:01:06.006 --> 00:01:07.165 In aviation, 00:01:07.165 --> 00:01:11.866 radar antennas launch pulses of radio or microwaves at planes 00:01:11.866 --> 00:01:16.620 to learn their locations by timing how long the beams take to bounce back. 00:01:16.620 --> 00:01:18.593 That’s a limited way of seeing, though, 00:01:18.593 --> 00:01:22.679 because the large beam-size can’t visualize fine details. 00:01:22.679 --> 00:01:26.127 In contrast, a self-driving car’s LIDAR system, 00:01:26.127 --> 00:01:28.634 which stands for Light Detection and Ranging, 00:01:28.634 --> 00:01:32.190 uses a narrow invisible infrared laser. 00:01:32.190 --> 00:01:36.660 It can image features as small as the button on a pedestrian’s shirt 00:01:36.660 --> 00:01:38.123 across the street. 00:01:38.123 --> 00:01:42.483 But how do we determine the shape, or depth, of these features? NOTE Paragraph 00:01:42.483 --> 00:01:48.267 LIDAR fires a train of super-short laser pulses to give depth resolution. 00:01:48.267 --> 00:01:50.746 Take the moose on the country road. 00:01:50.746 --> 00:01:55.853 As the car drives by, one LIDAR pulse scatters off the base of its antlers, 00:01:55.853 --> 00:02:00.721 while the next may travel to the tip of one antler before bouncing back. 00:02:00.721 --> 00:02:04.278 Measuring how much longer the second pulse takes to return 00:02:04.278 --> 00:02:06.882 provides data about the antler’s shape. 00:02:06.882 --> 00:02:13.192 With a lot of short pulses, a LIDAR system quickly renders a detailed profile. NOTE Paragraph 00:02:13.192 --> 00:02:18.557 The most obvious way to create a pulse of light is to switch a laser on and off. 00:02:18.557 --> 00:02:23.428 But this makes a laser unstable and affects the precise timing of its pulses, 00:02:23.428 --> 00:02:25.669 which limits depth resolution. 00:02:25.669 --> 00:02:27.044 Better to leave it on, 00:02:27.044 --> 00:02:33.031 and use something else to periodically block the light reliably and rapidly. NOTE Paragraph 00:02:33.031 --> 00:02:35.987 That’s where integrated photonics come in. 00:02:35.987 --> 00:02:37.829 The digital data of the internet 00:02:37.829 --> 00:02:41.051 is carried by precision-timed pulses of light, 00:02:41.051 --> 00:02:44.473 some as short as a hundred picoseconds. 00:02:44.473 --> 00:02:49.104 One way to create these pulses is with a Mach-Zehnder modulator. 00:02:49.104 --> 00:02:52.865 This device takes advantage of a particular wave property, 00:02:52.865 --> 00:02:54.658 called interference. 00:02:54.658 --> 00:02:57.613 Imagine dropping pebbles into a pond: 00:02:57.613 --> 00:03:01.550 as the ripples spread and overlap, a pattern forms. 00:03:01.550 --> 00:03:05.464 In some places, wave peaks add up to become very large; 00:03:05.464 --> 00:03:08.450 in other places, they completely cancel out. 00:03:08.450 --> 00:03:11.517 The Mach-Zehnder modulator does something similar. 00:03:11.517 --> 00:03:17.292 It splits waves of light along two parallel arms and eventually rejoins them. 00:03:17.292 --> 00:03:20.784 If the light is slowed down and delayed in one arm, 00:03:20.784 --> 00:03:25.703 the waves recombine out of sync and cancel, blocking the light. 00:03:25.703 --> 00:03:28.335 By toggling this delay in one arm, 00:03:28.335 --> 00:03:33.606 the modulator acts like an on/off switch, emitting pulses of light. 00:03:33.606 --> 00:03:36.380 A light pulse lasting a hundred picoseconds 00:03:36.380 --> 00:03:39.790 leads to a depth resolution of a few centimeters, 00:03:39.790 --> 00:03:43.303 but tomorrow’s cars will need to see better than that. 00:03:43.303 --> 00:03:47.595 By pairing the modulator with a super- sensitive, fast-acting light detector, 00:03:47.595 --> 00:03:50.878 the resolution can be refined to a millimeter. 00:03:50.878 --> 00:03:52.781 That’s more than a hundred times better 00:03:52.781 --> 00:03:57.337 than what we can make out with 20/20 vision, from across a street. NOTE Paragraph 00:03:57.337 --> 00:04:02.925 The first generation of automobile LIDAR has relied on complex spinning assemblies 00:04:02.925 --> 00:04:05.777 that scan from rooftops or hoods. 00:04:05.777 --> 00:04:07.494 With integrated photonics, 00:04:07.494 --> 00:04:12.508 modulators and detectors are being shrunk to less than a tenth of a millimeter, 00:04:12.508 --> 00:04:17.837 and packed into tiny chips that’ll one day fit inside a car’s lights. 00:04:17.837 --> 00:04:21.806 These chips will also include a clever variation on the modulator 00:04:21.806 --> 00:04:27.275 to help do away with moving parts and scan at rapid speeds. NOTE Paragraph 00:04:27.275 --> 00:04:31.097 By slowing the light in a modulator arm only a tiny bit, 00:04:31.097 --> 00:04:36.208 this additional device will act more like a dimmer than an on/off switch. 00:04:36.208 --> 00:04:40.708 If an array of many such arms, each with a tiny controlled delay, 00:04:40.708 --> 00:04:44.786 is stacked in parallel, something novel can be designed: 00:04:44.786 --> 00:04:47.492 a steerable laser beam. NOTE Paragraph 00:04:47.492 --> 00:04:48.848 From their new vantage, 00:04:48.848 --> 00:04:52.248 these smart eyes will probe and see more thoroughly 00:04:52.248 --> 00:04:54.681 than anything nature could’ve imagined— 00:04:54.681 --> 00:04:57.544 and help navigate any number of obstacles. 00:04:57.544 --> 00:05:00.098 All without anyone breaking a sweat— 00:05:00.098 --> 00:05:03.988 except for maybe one disoriented moose.