TECHNOLOGY EXPLAINER
LiDAR
— measuring the world by the round trip of light, with the wavelength set by the detector material
LiDAR is a sensor that shines laser light at an object, catches what bounces back and measures the distance. In one nanosecond, light covers only 15 cm of round trip. Light returning from an object 200 m away is a hundred times weaker than from one 20 m away. And what really decides whether a system uses 905 nm or 1550 nm is the semiconductor material that receives the light.
- What LiDAR is (the short version)
- Measuring method 1: ToF, counting the round-trip time
- Measuring method 2: FMCW, distance and speed together from a frequency shift
- Our calculation: returning light weakens with the square of distance
- 905 nm or 1550 nm
- A materials engineer's view (1): the wavelength is set by the detector material
- Light sources and detectors
- Steering the beam: rotation, MEMS, flash and optical phased arrays
- A materials engineer's view (2): what makes LiDAR with no moving parts hard
- Open problems and unconfirmed points
- Glossary / References / Claim-to-source audit
Sourced = stated in a standard, a peer-reviewed paper or a company's published material (link given)
Our calculation = a figure this article derived from assumptions it spells out
Not yet confirmed = a plan or outlook with no confirmed track record yet
Structural readings and materials-design interpretations are marked separately as commentary.
Experimental values from papers and company product specifications are different in kind and are kept apart, and claims such as "industry first" are treated as the company's own.
1. What LiDAR is (the short version)
LiDAR (Light Detection and Ranging) is a sensor that directs laser light at an object and measures distance (and speed) from the light that comes back. Sweep the light finely across many points and the three-dimensional shape of the surroundings is captured as a "point cloud" (our commentary).
- What it is for: an autonomous vehicle needs to detect the vehicles, pedestrians and other things around it, and safety concerns and the need for accurate estimation have led to LiDAR being introduced to complement camera- or radar-based perception, as the review by Li and Ibanez-Guzman (IEEE Signal Processing Magazine, 2020) explainsSourced
- How it measures: there are two methods: ToF, which measures the round-trip time of the light, and FMCW, which measures distance and speed from the "beat" of light whose frequency is swept continuouslySourced
- Wavelengths: mainly 850 to 950 nm (near infrared) and 1550 nm (short-wave infrared) are used, and the same paper says near-infrared systems such as 905 nm are still the mainstreamSourced
LiDAR repurposes optical technology from communications for measurement. 1550 nm light sources and optical fiber, coherent detection and silicon photonics link it to our explainers on semiconductor lasers, on coherent optical communications and DSP, and on silicon photonics. Photodetectors in general are covered in our photodetector explainer, and the temperature dependence of silicon's refractive index in our explainer on wavelength multiplexing and microrings (our own division of topics).
2. Measuring method 1: ToF, counting the round-trip time
The ToF (time of flight) method measures the time Δt between emitting light and its return, and obtains the distance as r = (1/2n) × c × Δt (c is the speed of light and n the refractive index of the medium, close to 1 in air)Sourced. The review says ToF systems prevail in today's automotive LiDAR market because of their simple structure and signal processing, but that the scope for extending their maximum range is limited by transmit power capped for eye safety, and that they can suffer interference from strong sunlight or light from other LiDARsSourced.
- Assumptions: refractive index of air taken as 1, speed of light c of about 300,000 km/s
- A round-trip time of 1 ns = a distance of about 15 cm Our calculation (c × 1 ns ÷ 2)
- The round trip to an object 200 m away takes about 1.33 µs
Assumptions and limits: a simple conversion that excludes delays in the detection circuit and signal processing. Resolving distance in 1 cm steps would require a time resolution of about 67 ps. The "15 cm intervals" at which Sony's SPAD sensor in Section 7 is said to measure correspond to time steps of exactly 1 ns (our commentary).
3. Measuring method 2: FMCW, distance and speed together from a frequency shift
The FMCW (frequency modulated continuous wave) method sends laser light whose frequency is swept continuously in a triangular or sawtooth pattern, and mixes the returning light with a reference kept back at the sensor (the local oscillator). This produces a frequency difference (the intermediate frequency) proportional to the round-trip timeSourced. The review describes this method as the optical version of FMCW radar and lists the following characteristicsSourced.
- It measures distance and speed directly and at the same time (ToF estimates speed indirectly from successive measurements)
- Because it illuminates continuously, it needs less transmit power, making eye-safety requirements easier to meet
- It reduces interference from other light sources and strong sunlight
- But it needs a high-quality laser with a long coherence length
Using the review's formula fif = 4rB/(cT) (B: modulation bandwidth, T: waveform period) and v = fdλ/2Our calculation.
- Assumptions: B = 1 GHz, T = 10 µs and a distance r of 100 m are assumed
- Intermediate frequency fif = 4 × 100 × 10⁹ ÷ (3×10⁸ × 10⁻⁵) ≈ about 133 MHz
- Doppler frequency fd = 2v/λ for a relative speed of 30 m/s (108 km/h): about 38.7 MHz at 1550 nm
Assumptions and limits: B, T and r are assumptions made in this article, not values from real products. The optical frequency (about 193 THz at 1550 nm) has to be swept precisely and continuously over a range measured in GHz, which is part of the reason a "high-quality laser" is needed (our commentary).
4. Our calculation: returning light weakens with the square of distance
The review gives the received power of a pulsed LiDAR approximately as Pr = Ep × cηAr/(2r²) × β × TrSourced. Ep is the pulse energy, Ar the area of the receiving aperture, η the overall system efficiency, β a coefficient related to the target's reflectivity, and Tr the atmospheric transmission. The paper states that received power falls in inverse proportion to the square of distance, that simply raising transmit power is limited by the eye-safety standard IEC 60825, and that the overall system efficiency therefore has to be raised through optics, photodetectors and signal processingSourced. In fog, rain, dust and snow, particles in the air scatter and absorb the lightSourced.
The international eye-safety standard IEC 60825-1 (the edition checked for this article is the third, from 2014) applies to laser products emitting at wavelengths from 180 nm to 1 mm, and sets out a classification by degree of hazard and the safety information manufacturers must provideSourced. LiDAR's upper limit on transmit power is set within this framework (our commentary).
5. 905 nm or 1550 nm
The review says the choice of wavelength should weigh atmospheric windows, eye-safety requirements and cost together, and compares the two wavelength bandsSourced.
| Aspect | 850 to 950 nm (near infrared, e.g. 905 nm) | 1550 nm (short-wave infrared) |
|---|---|---|
| Maximum power allowed by eye-safety standards | — | Higher at 1550 nm, so longer range can be targeted |
| Detector | Mature silicon detectors can be used (inexpensive) | Expensive InGaAs detectors are needed, with lower efficiency than silicon |
| Absorption by water in the atmosphere | — | Stronger than at 850 to 950 nm |
| Availability of light sources | Both inexpensive diode lasers and powerful fiber lasers are readily available on the market | Same |
| Conclusion (paper) | Near-infrared LiDAR, such as 905 nm, is still the mainstream | |
All Sourced (Li and Ibanez-Guzman, IEEE Signal Processing Magazine 37(4), 2020 [Ref. 1]). "—" marks aspects for which the paper gives no comparison. The paper reflects the situation as of 2020.
6. A materials engineer's view (1): the wavelength is set by the detector material
The review says that selecting a photodetector is closely related to the choice of laser wavelength, explaining that silicon APDs are sensitive from the visible to the near infrared around 1000 nm, while InGaAs APDs are used at longer wavelengths up to 1700 nmSourced.
The reason is the semiconductor bandgap. The energy of a photon is inversely proportional to its wavelength: about 1.37 eV at 905 nm and about 0.80 eV at 1550 nmOur calculation (1239.84 ÷ wavelength in nm). Taking silicon's bandgap as about 1.12 eV, the longest wavelength it can absorb works out at about 1107 nm, so light at 1550 nm passes straight through silicon (our commentary). That is why choosing 1550 nm means needing a detector made of a compound semiconductor such as InGaAs.
On the other hand, 905 nm is not an easy wavelength for silicon either. The closer a wavelength is to the bandgap, the weaker silicon's absorption and the deeper the light penetrates (our commentary). For its SPAD sensor for automotive LiDAR, Sony Semiconductor Solutions says it raised the absorption rate by forming an uneven structure on the light-incident surface to diffract the incoming light, achieving 24% photon detection efficiency at 905 nmSourced. A shape-based fix, fine surface texturing, makes up for the material's weak absorption (our commentary).
So the choice between 905 nm and 1550 nm is a choice between getting the most out of inexpensive, mature silicon detectors through clever design and using expensive compound-semiconductor detectors to gain eye-safety headroom. It can be read as the detector's material and cost, rather than the light source, deciding the wavelength (our commentary).
7. Light sources and detectors
Light sources
According to the review, the ToF method needs pulsed light, produced by a pulsed semiconductor laser or a fiber laserSourced.
| Light source | What the review says |
|---|---|
| Edge-emitting laser (EEL) | Long used in telecommunications. The beam is elliptical and needs beam-shaping optics |
| Vertical-cavity surface-emitting laser (VCSEL) | The beam is circular. Two-dimensional arrays are easy to make on one chip, which helps resolution. But output limits keep the range shorter |
| Automotive pulsed laser diode | A hybrid part combining a laser chip with a capacitor and a MOSFET. Its 905 nm output can be received by inexpensive silicon detectors. Repetition rate and peak power are limited, and cooling may be needed |
| Light source for flash | A stack of edge-emitting bars piled vertically. Heat dissipation is an issue and a heat sink is needed |
| Fiber laser | High output, useful at longer wavelengths. Light can be distributed to several points by fiber, and repetition rate and beam quality are excellent, but it is bulky and hard to integrate into a vehicle |
All Sourced (Li and Ibanez-Guzman [Ref. 1]).
Detectors
| Detector | What the review says |
|---|---|
| PIN photodiode | The basic form: reverse-biased, with absorbed photons producing a current |
| APD (avalanche photodiode) | Reverse voltage causes avalanche multiplication, giving an internal gain of about 100. Common in current LiDAR |
| SPAD | An APD able to detect single photons, with a gain of 10⁶. Can be made into arrays on one chip with CMOS technology |
| SiPM (silicon photomultiplier) | Many SPAD-and-quench-resistor pairs in parallel, so that photons can be counted |
All Sourced (Li and Ibanez-Guzman [Ref. 1]).
As an example of a SPAD turned into a product, Sony Semiconductor Solutions announced in September 2021 the commercialization of the IMX459, a stacked SPAD depth sensor for automotive LiDAR ("industry first" is the company's own claim)Sourced.
| Item | What the company announced |
|---|---|
| Structure | A back-illuminated SPAD pixel chip (top) and a logic chip with the ranging circuitry (bottom), stacked with Cu-Cu connections that link each pixel |
| Pixels | 10 µm square, about 100,000 effective pixels, type 1/2.9 (6.25 mm diagonal) |
| Photon detection efficiency at 905 nm | 24% (the uneven incident surface diffracts light to raise absorption) |
| Response | An active recharge circuit in each pixel gives a response time per photon of about 6 ns under normal conditions |
| Ranging | High-precision, high-speed ranging at 15 cm intervals from long to short distances |
All Sourced (Sony Semiconductor Solutions news release, 6 September 2021 [Ref. 6]). Product information as of the announcement.
8. Steering the beam: rotation, MEMS, flash and optical phased arrays
To measure the surroundings with a single beam, the light has to be swept (scanned). The review compares mechanical rotation, MEMS, flash and optical phased arrays (OPAs)Sourced.
Optical phased arrays (OPAs), which have "no moving parts", have been built with silicon photonics technology. The main peer-reviewed papers report the followingSourced.
| Paper | Content |
|---|---|
| Sun et al. (MIT), Nature 493, 195 (2013) | Integrated 64 × 64 (4,096) optical nanoantennas on a silicon chip of 576 µm × 576 µm, balancing the intensity and phase of every antenna to produce a designed far-field radiation pattern. Also showed dynamic beam steering with an 8 × 8 array |
| Poulton et al., Optics Letters 42, 4091 (2017) | Reported the first demonstration of coherent (FMCW) solid-state LiDAR using optical phased arrays in silicon photonics. Distance and speed measured together with triangular frequency modulation. Fabricated on a CMOS-compatible 300 mm wafer platform |
| Rogers et al., Nature 590, 256 (2021) | 3D imaging with a 512-pixel coherent detector array, exploiting monolithic integration of photonics and electronics; measured at 75 m with 3.1 mm precision using 4 mW of light. Says previous systems were limited to fewer than 20 pixels |
All Sourced (abstracts of each paper [Refs. 2, 3 and 4]). All are research demonstrations, not specifications of automotive products.
9. A materials engineer's view (2): what makes LiDAR with no moving parts hard
The paper by Sun and colleagues sums up the promise and the difficulty of OPAs in a sentence. Because optical wavelengths are far shorter than radio wavelengths, large-scale integration is in prospect; but the short optical wavelength also imposes stringent requirements on fabrication, and as a result earlier demonstrations had been limited to one-dimensional or small two-dimensional arraysSourced.
An OPA works by aligning the phase of light from many antennas so that it adds up only in the intended direction. Phase is set by optical length divided by wavelength, so if waveguide width, thickness or refractive index differs even slightly from place to place, the phases drift, the beam blurs and light leaks in unwanted directions (our commentary). That is why lining up 4,096 antennas with balanced intensity and phase counted as an achievement.
Moreover, steering the beam by changing the phase requires changing the refractive index. As we saw in our explainer on wavelength multiplexing and microrings, silicon's refractive index changes with temperature (a thermo-optic coefficient of about 1.8×10⁻⁴ K⁻¹). In an OPA this is the means of steering the beam, and at the same time a cause of unintended pointing changes when the ambient temperature shifts (our commentary). That Poulton and colleagues describe simplifying control with cascaded phase shifters grouped into several sets also shows that handling many phases is the crux of implementationSourced.
In exchange for doing away with the "moving parts" of mechanical and MEMS systems, the uniformity of film thickness, line width and refractive index across the whole wafer, and temperature stability, come to decide performance. That can be read as the materials-and-process essence of solid-state LiDAR (our commentary).
10. Open problems and unconfirmed points
(1) Which method and wavelength will dominate is not settled
The review said that as of 2020 near-infrared systems such as 905 nm were the mainstreamSourced, but no settled primary source could be found at the time of writing on how far FMCW and optical phased arrays will spread in automotive useNot yet confirmed.
(2) The distance between paper demonstrations and products
Rogers and colleagues' "75 m with 3.1 mm precision at 4 mW" and Sun and colleagues' 4,096 antennas are research demonstrationsSourced. Whether the same performance holds under automotive temperature ranges, vibration and lifetime requirements has not been confirmed in this article.
(3) The edition of the eye-safety standard
The edition of IEC 60825-1 checked for this article is the third, from 2014Sourced. The specific permissible values for each wavelength are in the body of the standard (which is paid for), and this article does not give those numbers.
(4) What this article does not cover
The optical properties of LiDAR window and cover materials, countermeasures against dirt and raindrops, point-cloud processing algorithms, and comparisons of specific companies' products were left outside the scope of this article.
- LiDAR measures distance from the round-trip time of light (ToF) or from a frequency shift (FMCW)Sourced. A 1 ns round trip is about 15 cmOur calculation
- Returning light weakens with the square of distance, and transmit power is limited by eye-safety standardsSourced. At 200 m it is a hundredth of what it is at 20 mOur calculation
- 905 nm is received with silicon, 1550 nm with InGaAs. 1550 nm allows more power, but its detectors are expensive and water absorbs it more strongly; near infrared is the mainstreamSourced
- To receive 905 nm efficiently with silicon, a textured surface that diffracts the light has made it into a productSourced
- For OPAs with no moving parts, manufacturing uniformity and temperature stability decide performance (our commentary)
11. Glossary
- LiDAR
- Light Detection and Ranging. A sensor that measures distance with laser light.
- Point cloud
- The set of three-dimensional coordinates of the many points a LiDAR measures.
- ToF
- Time of flight. A method that obtains distance from the round-trip time of light.
- FMCW
- A method that uses light with a continuously swept frequency and obtains distance and speed from the beat with a reference.
- Local oscillator
- In coherent detection, the reference light kept back at the sensor to mix with the returning light.
- Doppler frequency
- The shift in the frequency of returning light caused by the target's motion. Proportional to speed.
- IEC 60825-1
- The international standard for the safety of laser products. It sets classes by degree of hazard.
- Bandgap
- The minimum energy a semiconductor needs to absorb light and generate electrons. Longer wavelengths are not absorbed.
- InGaAs
- Indium gallium arsenide, a compound semiconductor used to detect light in the 1550 nm band.
- APD
- Avalanche photodiode. A detector that multiplies the signal about 100 times through avalanche multiplication.
- SPAD
- Single-photon avalanche diode. Can detect a single photon, and can be made into arrays in CMOS.
- SiPM
- A detector made of many SPADs in parallel, able to count photons.
- Photon detection efficiency
- The fraction of incident photons that are detected.
- VCSEL
- Vertical-cavity surface-emitting laser. Emits light perpendicular to the chip surface and is easy to make into two-dimensional arrays.
- MEMS mirror
- A tiny movable mirror made on a chip, steering light on one or two axes.
- Flash LiDAR
- A method that illuminates the whole field of view at once and measures every pixel simultaneously.
- Optical phased array (OPA)
- A technology that steers a beam without moving parts by controlling the phase of many optical antennas.
12. References (primary sources)
- Y. Li, J. Ibanez-Guzman "Lidar for Autonomous Driving: The Principles, Challenges, and Trends for Automotive Lidar and Perception Systems", IEEE Signal Processing Magazine 37(4), 50–61 (2020) (full text available as arXiv:2004.08467) https://doi.org/10.1109/MSP.2020.2973615
- J. Sun et al. (MIT) "Large-scale nanophotonic phased array", Nature 493, 195–199 (2013) https://doi.org/10.1038/nature11727
- C. V. Poulton et al. "Coherent solid-state LIDAR with silicon photonic optical phased arrays", Optics Letters 42(20), 4091–4094 (2017) https://doi.org/10.1364/OL.42.004091
- C. Rogers et al. "A universal 3D imaging sensor on a silicon photonics platform", Nature 590, 256–261 (2021) https://doi.org/10.1038/s41586-021-03259-y
- IEC "IEC 60825-1:2014 Safety of laser products - Part 1: Equipment classification and requirements", Edition 3.0 https://webstore.iec.ch/en/publication/3587
- Sony Semiconductor Solutions "Industry first: stacked SPAD depth sensor for automotive LiDAR commercialized", 6 September 2021 (in Japanese) https://www.sony-semicon.com/ja/news/2021/2021090601.html
- Y. Li, J. Ibanez-Guzman (arXiv version of Reference 1) "Lidar for Autonomous Driving: The principles, challenges, and trends for automotive lidar and perception systems" https://arxiv.org/abs/2004.08467
13. Claim-to-source audit
| Claim in the text | Basis | Label |
|---|---|---|
| That LiDAR was introduced to complement cameras and radar for safety and accurate estimation. The ToF range equation, that ToF prevails in the automotive market, the transmit power limit, and interference from sunlight and other LiDARs. The FMCW principle (triangular and sawtooth waves, local oscillator, intermediate frequency f_if = 4rB/(cT), v = f_dλ/2), simultaneous measurement of distance and speed, reduced interference, and the need for a laser with a long coherence length. The received-power formula and inverse-square dependence on distance, the IEC 60825 limit, and scattering and absorption in fog, rain and so on. The aspects of wavelength choice; that permissible power is higher at 1550 nm; that InGaAs is expensive and less efficient; stronger water absorption; that near infrared is still the mainstream. That detector choice is tied to wavelength, with Si APDs to about 1000 nm and InGaAs APDs to 1700 nm. Light sources (EEL, VCSEL, hybrid pulsed LD, heat dissipation in flash stacks, fiber lasers). Detectors (PIN, APD gain of about 100, SPAD gain of 10⁶, SiPM). Scanning methods (mechanical, MEMS on one or two axes as near-solid-state, flash under 100 m, OPA as true solid-state) | Reference 1 https://doi.org/10.1109/MSP.2020.2973615 | Sourced |
| That the statements above were checked in the full text of Reference 1 (arXiv version) | Reference 7 https://arxiv.org/abs/2004.08467 | Sourced |
| That 64 × 64 (4,096) optical nanoantennas were integrated in 576 µm × 576 µm with balanced intensity and phase. Dynamic beam steering with 8 × 8. That the short optical wavelength imposes stringent requirements on fabrication, and that earlier demonstrations were limited to one-dimensional or small two-dimensional arrays | Reference 2 https://doi.org/10.1038/nature11727 | Sourced |
| The reported first demonstration of coherent solid-state LiDAR with OPAs in silicon photonics, simultaneous distance and speed measurement with triangular modulation, simplified control with cascaded phase shifters in several groups, and fabrication on a CMOS-compatible 300 mm wafer platform | Reference 3 https://doi.org/10.1364/OL.42.004091 | Sourced |
| The 512-pixel coherent detector array, monolithic integration, 3.1 mm precision at 75 m with 4 mW, and previous systems of fewer than 20 pixels | Reference 4 https://doi.org/10.1038/s41586-021-03259-y | Sourced |
| The title of IEC 60825-1:2014 (Edition 3), its application from 180 nm to 1 mm, classification and provision of safety information | Reference 5 https://webstore.iec.ch/en/publication/3587 | Sourced |
| The announcement of IMX459 commercialization (6 September 2021; "industry first" is the company's claim), Cu-Cu stacking of back-illuminated SPAD pixels on a logic chip, 10 µm square pixels, about 100,000 effective pixels, type 1/2.9, 24% photon detection efficiency at 905 nm through diffraction by an uneven incident surface, a response of about 6 ns, and ranging at 15 cm intervals | Reference 6 https://www.sony-semicon.com/ja/news/2021/2021090601.html | Sourced |
| 1 ns = about 15 cm; about 1.33 µs round trip at 200 m; about 67 ps per 1 cm. FMCW intermediate frequency of about 133 MHz and Doppler of about 38.7 MHz. Relative values by distance (100, 16, 4, 1). Photon energies (about 1.37 eV and about 0.80 eV) and the Si absorption edge of about 1107 nm | Our calculation. A refractive index of 1 for air, B = 1 GHz, T = 10 µs, r = 100 m, v = 30 m/s and a Si bandgap of 1.12 eV are this article's assumptions. Atmospheric and reflectance differences are not included | Our calculation |
| The spread of FMCW and OPAs in automotive use; which method and wavelength will become mainstream | No settled primary source could be confirmed at the time of writing in this article. Stated as an outlook | Not yet confirmed |
| The reading that detector material and cost decide the wavelength. The general explanation that Si absorption weakens near the bandgap. The reading that surface texturing compensates for weak material absorption. The relationship of OPA phase errors to manufacturing uniformity and temperature stability, and the framing of the thermo-optic effect as both the steering mechanism and a source of error. Matching 15 cm intervals to 1 ns. The four-way grouping of scanning methods | This article's commentary based on published content and general physics. Not views expressed by the companies or authors | Commentary |
| Specific permissible power for each wavelength, properties of window and cover materials, and reproducibility of paper results under automotive conditions | Not stated, because the body of the standard is paid for and no primary source could be confirmed within the scope of this article | Commentary |
| That Figs. 1 and 5 are explanatory drawings and Figs. 2 and 3 drawings that include our calculation, and that the hero image and Fig. 4 are AI-generated images | Our note | Commentary |
Last updated 26 September 2026. Sources are limited to primary material (peer-reviewed papers, the official page of an international standard, and a company's official news release); market estimates from research firms are not used. Experimental values from papers and company product specifications are kept distinct, and claims such as "industry first" are treated as the company's own. Specific permissible power for each wavelength, and how far FMCW and OPAs have spread in automotive use, are not stated because they could not be confirmed in published primary sources. All figures are for explanation. Figs. 1 and 5 are vector drawings, Figs. 2 and 3 are vector drawings that include our calculation, and the hero image and Fig. 4 are AI-generated images; none of them shows real measurement data, equipment or products.