Lidar System Architectures and Circuits

Transcription

1 Inegraed Circuis for Communicaions Lidar Sysem Archiecures and Circuis Behnam Behroozpour, Phillip A. M. Sandborn, Ming C. Wu, and Bernhard E. Boser Absrac 3D imaging echnologies are applied in numerous areas, including self-driving cars, drones, and robos, and in advanced indusrial, medical, scienific, and consumer applicaions. 3D imaging is usually accomplished by finding he disance o muliple poins on an objec or in a scene, and hen creaing a poin cloud of hose range measuremens. Differen mehods can be used for he ranging. Some of hese mehods, such as sereovision, rely on processing 2D images. Oher echniques esimae he disance more direcly by measuring he round-rip delay of an ulrasonic or elecromagneic wave o he objec. Ulrasonic waves suffer large losses in air and canno reach disances beyond a few meers. Radars and lidars use elecromagneic waves in radio and opical specra, respecively. The shorer wavelenghs of he opical waves compared o he radio frequency waves ranslaes ino beer resoluion, and a more favorable choice for 3D imaging. The inegraion of lidars on elecronic and phoonic chips can lower heir cos, size, and power consumpion, making hem affordable and accessible o all he abovemenioned applicaions. This review aricle explains differen lidar aspecs and design choices, such as opical modulaion and deecion echniques, and poin cloud generaion by means of beam-seering or flashing an enire scene. Popular lidar archiecures and circuis are presened, and he superioriy of he FMCW lidar is discussed in erms of range resoluion, receiver sensiiviy, and compaibiliy wih emerging echnologies. A he end, an elecronic-phoonic inegraed circui for a micro-imaging FMCW lidar is presened as an example. Inroducion The dream of self-driving cars has finally become a realiy, bu no ye a commodiy. In addiion o legal barriers, many echnical issues remain o be solved before he seering wheel can be abandoned. Among hese echnical challenges is he refinemen of he 3D imaging and mapping ools used for objec recogniion, navigaion, and collision avoidance. The performance offered by processing 2D images, such as sereovision echniques, would no be sufficien for hese purposes, necessiaing he use of direc rangefinders based on ulrasonic, radar, and lidar echnologies. The propagaion of ulrasonic waves hrough air induces large losses ha preven he waves from reaching disances beyond a few meers, whereas radar and lidar waves can boh propagae across long disances. Radar is a well esablished ool ha can work even in poor weaher condiions such as heavy rain, snow, or fog. However, he shorer wavelengh and superior beam properies of he lighwaves used in lidar offer a more suiable choice for 3D imaging and poin cloud generaion. Unforunaely, curren lidar soluions are cosly, bulky, and power-hungry, or hey perform poorly. Researchers in his area are working o develop an inexpensive soluion ha offers he required performance wih reasonable size and power consumpion. In addiion o self-driving cars, numerous oher applicaions will benefi from an affordable 3D imaging echnology: Drones ha need 3D imaging for heir navigaion are becoming increasingly popular for use in surveillance, delivery of goods, aerial mapping, agriculure, consrucion, high-risk monioring, defense, and search and rescue missions. The number of personal and indusrial robos is projeced o surpass ens of millions during he nex decade, and 3D imaging could become a popular aid for heir conrol. There are counless oher applicaions in medical, scienific, indusrial, defense, and consumer areas ha would benefi from lidar-based rangefinders and 3D cameras. Specificaions including he operaing disance, range resoluion, accepable ambien ligh and inerferers levels, measuremen speed and frame rae, muli-arge deecion capabiliy, power consumpion, maximum permissible opical exposure, and oher parameers can vary significanly across differen applicaions. This aricle describes he basic lidar archiecure, followed by more deails on popular lidar schemes ha provide insigh ino he imporan design choices and rade-offs. Basic Lidar Archiecure Figure 1 illusraes he main componens of a ypical lidar, which, like radar, includes a ransmier and a receiver. The range R is measured based on he round-rip delay of ligh o he arge, : R = 1 2 c τ (1) where c is he speed of ligh in he medium beween he lidar and he arge (e.g., air). Based on his equaion, and because in mos cases he speed of ligh is known o a very good accuracy, he lidar-based range measuremen is equivalen o measuring he round-rip delay of lighwaves o he arge. This is achieved by modulaing he inensiy, phase, and/or frequency of he waveform of he ransmied ligh and measuring he ime required for ha modulaion paern o appear back a he receiver. In he mos rivial case of inensiy modulaion, a shor ligh pulse Behnam Behroozpour is wih Bosch LLC; Phillip A. M. Sandborn, Ming C. Wu, and Bernhard E. Boser are wih he Universiy of California a Berkeley. The auhors explain differen lidar aspecs and design choices, such as opical modulaion and deecion echniques, and poin cloud generaion by means of beam-seering or flashing an enire scene. Popular lidar archiecures and circuis are presened, and he superioriy of he FMCW lidar is discussed in erms of range resoluion, receiver sensiiviy, and compaibiliy wih emerging echnologies. Digial Objec Idenifier: /MCOM IEEE Communicaions Magazine Ocober /17/$ IEEE 135

2 TX Laser modulaion and processing 3D camera Beam seering Laser TX opics Phoodiode (array) Beam seering uni (opional) opics TX ligh ligh Figure 1. Basic lidar-based 3D camera archiecure. is emied oward he arge, and he arrival ime of he pulse s echo a he receiver marks he disance. Lasers are he preferred source of ligh because of heir narrow specra and superior beam properies; furhermore, phase- and frequency-modulaion (PM and FM) lidars require he laser ligh s coherence. Lasers wih wavelenghs of 905, 1300, or 1550 nm, which are near he hree esablished elecommunicaions windows, are commonly used in lidar applicaions. To creae a 3D image, he ligh should be direced o all he poins in a desired field of view (FOV). This can be done by disribuing he ligh o he enire scene a once (flash lidar), by employing a beam-seering uni o scan he FOV, or by a combinaion of hese. In flash lidar, he differen poins in he FOV should be differeniaed in he receiver using proper imaging opics, similar o he lens-se of a phoographic camera. Over he years, many differen beam-seering echniques have been developed. Foremos among hese are mechanical moion of he ligh source [1]; deflecion of he ligh using a macroor micro-mechanical mirror [2]; opical-phased arrays (based on liquid crysals [3], MEMS mirrors [4], or silicon-phoonic unable phase elemens [5] and wavelengh uning [6]). Finally, in he receiver, he scaered ligh from he arge is colleced, and he delay in is modulaion paern vs. he source ligh is exraced and used for ranging. In a 3D camera based on flash lidar, he receiver has muliple pixels, and he ime of fligh should be measured separaely for each pixel. Imporan Performance Merics The mos imporan performance merics for a lidar-based 3D camera are is axial precision, laeral resoluion, FOV, frame rae, ransmi power in relaion o eye safey, maximum operaing range, sensiiviy o ambien ligh and inerferers, power consumpion, and cos. These merics are briefly discussed here. Axial Precision The erms axial or range precision refer o he sandard deviaion of muliple range measuremens performed for a arge a a fixed disance (s R ). This should no be confused wih range resoluion (dr). Range resoluion refers o he lidar s abiliy o resolve muliple closely spaced arges in he axial direcion. For example, when 3D imaging an organic issue, he emied ligh is refleced by he inerfaces beween he issue s differen layers. In his case, beer axial resoluion helps in deecing hinner issue layers, while beer axial precision improves he cerainy wih which he inerfaces beween hese layers can be locaed. The laer can be improved by averaging he resuls of muliple measuremens. For any ime-of-fligh ranging sysem based on eiher elecromagneic or ulrasonic waves, he range resoluion can be found using he following equaion [7]: δr = c 2B (2) where c is he velociy of he waves, and B is he bandwidh of he informaion hey carry. This means he informaion conen on he waves should vary fas enough ha he reflecions from wo arges separaed by dr can be meaningfully disinguished in he receiver. The ime difference beween he reflecions from wo such arges is d = 2dR/c, ranslaing o a bandwidh inversely proporional o his ime, or B = c/2dr. The speed of he waves in air for boh opical and radio frequency waves is equal o The bandwidh of radio frequency waves can reach ens of gigaherz, resuling in cenimeer-range resoluion; however, opical waves can have much larger bandwidh, enabling micromeer-range resoluion. Alhough he ranging precision is differen from he resoluion, heir values are no enirely independen. In [7], i has been shown ha sr 2 dr 2 / SNR; where sr 2 is he variance of he measured range, and SNR is he signal-o-noise raio of he received signal. In oher words, sharper changes in he informaion conen of he waves, resuling in smaller dr, as well as higher SNR can improve he ranging precision. FOV and Laeral Resoluion FOV is usually specified wih wo horizonal and verical angles around he axis perpendicular o he camera aperure wihin which he disance can be measured. Laeral or angular resoluion of a 3D camera is a measure of is abiliy o disinguish wo adjacen poins in he FOV. Opical waves wih micromeer wavelengh can achieve laeral resoluions of 0.1 wih aperure sizes of only a few hundred micromeers (q l/d aperaure ) ha easily fis on a single chip. However, radio frequency waves wih frequencies near 100 GHz would require a 1-m aperure for he same resoluion, which is challenging o implemen in many applicaions. In a flash lidar, similar o a phoographic camera, he laeral resoluion and FOV are defined by he opical fron of he receiver and also he phoodeecor s array size. However, in a beam-seering lidar he properies of he emied laser beam, such as is divergence angle, side lobes, and scan range, have more significan effec on he FOV and laeral resoluion. FOV is of paricular imporance in 3D mapping for self-driving cars and drones, where a 360 view of he surroundings is ofen necessary. A he ime of his wriing, such a large FOV can be achieved eiher by mechanically moving a 3D camera wih a smaller FOV or by siching he oupus of muliple 3D cameras using compuer sofware. Emied Power and Eye Safey For lidar applicaions where a longer operaing range is imporan, a larger ransmi power is desired. However, he maximum ransmi power 136 IEEE Communicaions Magazine Ocober 2017

3 is ofen limied by eye safey regulaions. This is a greaer concern for lidars han radars because a coheren laser beam wih milliwas of power can cause serious damage o he human eye. The maximum permissible exposure (MPE) of a laser produc depends srongly on is wavelengh, beam diameer and divergence, beam moion, duraion of exposure for coninuous-wave operaions, and pulse widh and repeiion rae for pulsed operaions. As a resul, eye safey is an imporan deerminan in he selecion of such parameers when designing a lidar. Precision 1 m 1 mm 1 μm [13] [9] [8] Maximum Operaing Range Maximum operaing range is usually limied by he ransmi power level and he receiver sensiiviy. In a beam-seering lidar, he operaing range can be improved by reducing he beam divergence and is side-lobes. In all lidar caegories, a larger receive aperure can increase he amoun of colleced opical power and improve he operaing range. In long-range 3D cameras, beam-seering lidars are more commonly employed han flash lidars. This seems o be a sraighforward choice considering ha in a beam-seering lidar he enire laser power is focused on a single spo a one ime, creaing a sronger echo compared o he disribued ligh in a flash lidar; however, i mus be noed ha in a flash lidar, he parallel measuremen of all pixels allows a longer measuremen ime per pixel o achieve he same frame rae, which can be used o average he noise and reain he SNR o some exen. In he lidar ypes in which he modulaion is applied o he phase or frequency of he ligh, phase noise of he laser beam can also limi he maximum operaing range. Popular Lidar Archiecures The combinaion of choices available for he differen lidar blocks can resul in a wide variey of lidar archiecures. Among hese, pulsed, ampliude-modulaed coninuous-wave (AMCW) and frequency-modulaed coninuous-wave (FMCW) are he mos popular schemes, and hese are discussed in his secion. Figure 2 illusraes he range precision vs. maximum operaing disance for lidars presened since 1990, and he regimes in which each of hese lidar ypes have ofen been employed are indicaed by he shaded areas. Pulsed lidar can provide moderae precision over a wide window of ranges. This is hanks o he fac ha he nanosecond pulses used in hese lidars ofen have high insananeous peak power ha can reach far disances while mainaining low average power below he eye-safe limi. Furhermore, according o Eq. 2, he large bandwidh associaed wih shor pulses can enable high-precision range measuremens wih a relaive range error accepable even a shor disances. AMCW lidar can achieve precision similar o ha of he pulsed lidar bu only a moderae ranges; i is usually secondary parameers such as he fabricaion cos ha moivae he selecion of one or he oher in his regime of range and precision. AMCW lidars are no popular for long-range measuremens because hey ransmi coninuous opical power ha has o remain below he eye-safe limi a all imes; herefore, he eco signal a heir 1 nm 1 cm Range Figure 2. Precision vs. operaing range for academically published and indusrial lidars since receiver coming from far objecs is no as srong as i is in pulsed lidars. FMCW lidar is he only archiecure ha has been used o achieve sub-micromeer precision in muliple designs. This is enabled by direc modulaion and demodulaion of he signals in he opical domain wih much larger bandwidh han ha possible when using elecronic. There are also insances of using FMCW lidar for moderae and long-range applicaions wih a precision comparable o or beer han ha of pulsed and AMCW lidars. In he res of his aricle, he hree popular lidar caegories are discussed in more deail, and one insance in which inegraed circuis were effecively used o achieve a significan performance improvemen is presened for each ype. These examples are highlighed in Fig. 2. Pulsed Lidar In his ype of lidar (Fig. 3), he round-rip delay of a shor pulse of ligh o he arge is measured o find he arge s disance. Shorer pulse widhs are desired o increase he peak power while mainaining he average eye-safe exposure. Furhermore, from Eq. 2, i can be seen ha he axial resoluion of he lidar is improved by increasing he pulse bandwidh, which is equivalen o reducing is widh. Mos applicaions use pulses wih duraions from less han 1 ns o ens of nanoseconds. Alhough he name pulsed lidar is mainly descripive of he modulaion mehod in he ransmier, i also influences he receiver design. Single-phoon avalanche deecors (SPADs) are ofen employed in pulsed lidar receivers o improve heir sensiiviy and increase heir operaing disance. The high ineres in hese deecors has moivaed heir developmen in complemenary meal oxide semiconducor (CMOS)-compaible processes o reduce heir cos [8]. SPADs are essenially avalanche phoodiodes operaing in he reverse-biased mode slighly beyond heir breakdown volage. Because of he srong elecric field from he reverse-bias volage, he elecron-hole pairs generaed by phoon absorpion or hermal flucuaion are acceleraed o a level ha can rigger an avalanche process. A his poin, he elecronic around he SPAD mus 1 m 100 m Pulsed AMCW FMCW IEEE Communicaions Magazine Ocober

4 Maximum operaing range is usually limied by he ransmi power level and he receiver sensiiviy. In a beam-seering lidar, he operaing range can be improved by reducing he beam divergence and is side-lobes. In all lidar caegories, a larger receive aperure can increase he amoun of colleced opical power and improve he operaing range. TX Laser conrol TCSPC processing Powere supply [0, 5 V] SPAD array (a) 32x32 SPAD array 32-o-1 muliplexer (c) Digial oupu TX ligh ligh 250 m V p+ 32 channel decoder TX ligh inensiy ligh inensiy Phoon coun hisogram Time of fligh (b) (d) Time Time Time Figure 3. Pulsed lidar wih flash ligh disribuion presened in [8]: a) simplified archiecure; b) iming diagram; c) chip phoomicrograph; d) 3D image of a human face (in millimeers). reduce he reverse-bias volage o sop he avalanche and prepare he device for he nex deecion. The iming of he avalanche even can hen be recorded by he elecronic circuis o mark he arrival ime of he pulse echo o he receiver. The SPAD recovery ime can exend up o 100 ns and limi he measuremen rae. SPADs are suscepible o false deecions due o eiher he hermal noise of he deecor iself or phoons from he ambien ligh ha happen o be a he deecable wavelengh window. Therefore, SPAD receivers are ofen employed in a saisical archiecure where he arrival imes of muliple repeiive pulses, someimes recorded by many SPADs in parallel, are accumulaed in a hisogram. The recordings in a ime window of comparable duraion o he emied pulse widh have a higher chance of being par of he expeced signal. This fac is used o filer ou unwaned recordings and improve he measuremen precision. This echnique is referred o as ime-correlaed single-phoon couning (TCSPC), and has gained populariy in pulsed lidars and also in fluorescence lifeime measuremens. Pulsed lidars can operae in eiher flash or beam-seering modes. The laer is ofen he preferred choice for long-range applicaions. Among he beam-seering echnologies menioned in previous secions, silicon-phoonic phased arrays (SPPAs) are more popular because of heir compaibiliy wih fully inegraed chip-scale lidars. The foremos aracion of his echnology is ha i could provide solid-sae lidars wih no mechanical pars, aking advanage of he poenial high-volume and low-cos manufacuring achieved by oday s inegraed circui indusry. Recenly, here have been preliminary demonsraions of such echnologies, and srong growh in his direcion is expeced wihin he nex decade. However, he large peak power of he pulsed lidars combined wih he small effecive cross-secion of he silicon-phoonic waveguides can enhance undesirable nonlinear opical processes in he silicon. Therefore, pulsed lidars are no currenly preferred for use wih SPPAs, compared o coninuous-wave echniques such as AM- or FMCW. AMCW Lidar As wih pulsed schemes, AMCW lidars operae by modulaing he ligh s inensiy. However, he modulaion waveform does no include sharp pulses and carries much less frequency conen. Hence, AMCW lidars canno offer fine range resoluion wih muli-arge deecion capabiliy. Noneheless, he precision of he range measuremen can be less han a cenimeer, which is sufficien for many applicaions. AMCW lidars employ coninuous-wave or quasi-coninuous-wave laser diodes or LEDs on heir ransmier. The inensiy can be modulaed by varying he bias curren of he diode in he elecrical domain. The simpliciy of hese lidars makes hem an aracive choice for shor-range indoor applicaions such as gaming and roboics. To reduce he cos of he receiver chip, clever circui opologies similar o he radiional CMOS imaging pixels have been developed [9, 10]. A simplified circui schemaic and iming diagram of he pixel proposed in [9] are shown in Figs. 4a and 4b. The received ligh is deeced by a single phoodiode, bu he colleced charge is ransferred o wo separae nodes depending on 138 IEEE Communicaions Magazine Ocober 2017

5 Reurn ligh TG 1 TG 2 (a) Q1 Q2 C FD1 C FD2 BUF BUF V 1 V 2 TX ligh TG 1 TG 2 ligh T p T.o.F Q 2 Q 1 (b) FMCW lidars are fundamenally differen from he pulsed and AMCW schemes. Boh pulsed and AMCW lidars rely on modulaing he inensiy of he ligh. In he receivers of hese lidars, he phoons are ofen reaed as paricles wih he range informaion encoded in heir arrival imes. In conras, FMCW lidars rely on he wave properies of he ligh. 7.33mm Timing generaor Verical scanner Pixel array driver 336 x 252 CMOS TOF pixel array Oupu Noise canceller Horizonal scanner 8.66mm (c) (d) Figure 4. AMCW lidar employed in a CMOS imager for 3D deph measuremen in [9]: a) pixel opology; b) iming diagram; c) chip phoomicrograph; d) 3D image of scene (scale in meers). is ime of arrival. The charge ransfer is conrolled by he wo ransfer gaes, TG 1 and TG 2. For shor arge disances, mos of he charge generaed by he reurn ligh is ransferred o node Q 1. For longer disances, he reurn ligh experiences more delay, and herefore less charge ges ransferred o node Q 1 and more appears a node Q 2. Thus, he raio of he colleced charge a hese wo nodes is a measure for he ime of fligh. The conversion of he ime of fligh o charge renders his archiecure fully compaible wih he convenional CMOS RGB pixels ha ranslae he inensiy of he ambien ligh ino accumulaed charge. Furhermore, he raiomeric naure of he measuremen increases is robusness agains emperaure and process variaions and helps suppress he background ligh and environmenal disurbances. FMCW Lidar FMCW lidars are fundamenally differen from he pulsed and AMCW schemes. Boh pulsed and AMCW lidars rely on modulaing he inensiy of he ligh. In he receivers of hese lidars, he phoons are ofen reaed as paricles wih he range informaion encoded in heir arrival imes. In conras, FMCW lidars rely on he wave properies of he ligh. In hese lidars, he modulaion is applied o he frequency of he ligh field, and an inerferomeric deecion scheme is employed in heir receivers [11]. Therefore, he large frequency bandwidh in he opical domain becomes accessible and can be exploied o improve he lidar performance. Unlike pulsed or AMCW lidars, in he FMCW scheme, he inerferomeric down-conversion of he received signal in he opical domain eliminaes he need for wideband elecrical circuis. Therefore, mainsream CMOS elecronics can be used o achieve excepional range resoluion and precision. The basic archiecure of an FMCW lidar is shown in Fig. 5a. In his case, he frequency of he ligh emied from he ransmier is linearly modulaed vs. ime. The echo ligh reaches he receiver afer he round-rip delay d. For a saic arge wih negligible Doppler effec on he lighwaves, he delay beween he colleced ligh and he source causes a consan frequency difference f d beween hem, as shown in Fig. 5b. Wih he linear frequency modulaion, f d = g d is direcly proporional o d and hence he arge range. To measure f d, a branch of he source ligh is used as he local oscillaor (LO) and is combined wih he colleced ligh in a waveguide. The frequency difference beween he wo ligh componens ranslaes ino a periodic phase difference beween hem and causes an alernaing consrucive and desrucive inerference paern a he frequency f d. A phoodeecor is used o conver his paern ino a phoocurren. Measuremen of he phoocurren frequency enables range esimaion hrough he following: R = 1 2 c τ d = 1 2γ c f d (3) where g = (Df max )/T is he slope of he frequency modulaion vs. ime wih a uni of Herz per second. This equaion demonsraes ha he range precision depends on he measuremen precision of f d and also he precision wih which he modulaion slope g is conrolled or known. In addiion o finer range resoluion, he FMCW scheme can also offer much beer sensiiviy and robusness agains environmen- IEEE Communicaions Magazine Ocober

6 The mixing gain amplifies he signal before is deecion in he phoodiode, reducing he elecrical noise of he deecor referred back o he opical domain. Furhermore, he phase and frequency coherence of he received signal and he LO is necessary o creae he inerference paern, rendering he coheren receiver more selecive agains he ambien ligh. TX Freq. modulaion & processing V crl TL Tunable laser V crl FM ligh Ligh field FM ligh emier Phoodiode (c) Beam seering (a) Fixed-frequency laser Beam seering uni LO ligh ligh Combiner Splier TX opics Fixed-frequency laser RF signal Chirped RF sig. 0 EOM I Q Opical 2 phase shifer EOM (e) TX ligh ligh Opical frequency Elecro-opical modulaor (EOM) FM ligh 1 FM ligh Combiner Chirped RF signal Time (d) Opical specral densiy f f f mod d Opical specral 1 f max (b) LO f d γ 0 1 f f mod Figure 5. FMCW lidar: a) archiecure; b) waveforms. FM ligh generaion using: c) unable laser; d) elecro-opic modulaor; e) I/Q modulaor. al disurbances compared o he pulsed and AMCW lidars because of he FMCW s coheren deecion scheme. The inerference paern of he colleced beam and he LO in he coheren receiver is similar o he mixing of he wo signals in an elecrical receiver. The mixing gain amplifies he signal before is deecion in he phoodiode, reducing he elecrical noise of he deecor referred back o he opical domain. Furhermore, he phase and frequency coherence of he received signal and he LO is necessary o creae he inerference paern, rendering he coheren receiver more selecive agains he ambien ligh. I was previously menioned ha a consan oupu opical power is desirable for he silicon-phoonic-based beam-seering echniques. Therefore, unlike he pulsed archiecure, where he large peak power consrains is use in SPPAbased lidars, he fixed ligh inensiy of he FMCW scheme can become increasingly popular as he growing accessibiliy of SPPAs makes hem a mainsream choice for beam-seering lidars. As illusraed in Figs. 5c and 5d, a unable laser (TL) or an elecro-opic modulaor (EOM) can be used o modulae he ligh s frequency. Tunable lasers are similar o elecrical volage-conrolled oscillaors (VCOs), bu heir oupu is an opical wave raher han an elecrical signal. Elecro-opic modulaors can be viewed as elecrical mixers ha accep one opical and one elecrical signal as heir inpus and oupu an opical signal ha is he mix of he wo inpus. The frequency of he oupu opical signal can be uned by employing a frequency-chirped elecrical signal a he modulaor inpu. As wih elecrical mixers, he elecro-opic modulaors also creae wo sidebands in he opical specrum, as shown in Fig. 5d. In such cases, a coheren receiver capable of deecing boh in-phase and quadraure (I/Q) opical fields can be used o exrac he arge range. An alernaive mehod is o use an I/Q elecro-opic modulaor o suppress he carrier and creae a single-side-band frequency shif in he emied ligh [12], as shown in Fig. 5e. Alhough boh of he aforemenioned frequency modulaion echniques are heoreically equivalen, here are some pracical differences ha migh make one or he oher more suiable for a paricular applicaion. The main difference beween he wo mehods is ha when using a unable laser, he frequency uning happens purely in he opical domain, whereas wih an elecro-opic modulaor, he frequency uning is generaed in he elecrical domain and used o modulae he frequency of he ligh in anoher sep. The modulaion bandwidh of a unable laser can reach beyond 10 THz, which is no achievable by elecro-opical modulaion. Therefore, archiecures based on unable lasers are more suiable for applicaions where deep sub-millimeer resoluion is necessary. The possibiliy of varying a unable laser s frequency by a large amoun and a a fas rae 140 IEEE Communicaions Magazine Ocober 2017

7 f ref ±1 V LF Loop filer PFD k/s V crl Phoograph of he gear (a) TL τ MZI f MZI Deph (mm) MZI FM ligh for ranging Measuremen σ: 11 m 0.18 m CMOS Si-Phoonic chip TIA & EO-PLL Digial blocks Swiching inegraor (b) PD MZI Inpu couplers 3 mm The coherence range of widely unable laser diodes can be as small as a few millimeers, whereas for a fixed-frequency laser employed in an FMCW lidar wih elecro-opic modulaor, he coherence range can reach up o hundreds of meers. This makes he laer a more suiable opion for longrange applicaions. Y-displacemen (mm) (c) X-displacemen (mm) Figure 6. Inegraed elecro-opical PLL for precision FM ligh generaion a) archiecure; b) chip picure and phoomicrograph; c) phoograph of a gear and is 3D microimage from he FMCW lidar. makes is wavelengh more sensiive o noise and environmenal disurbances such as emperaure variaion; hence, widely unable lasers ofen suffer from larger phase noise. This phase noise can be oleraed as long as he arge range and relaed delay beween he received ligh from he arge and he LO are sufficienly small ha he majoriy of heir phase noise is correlaed and cancels ou in he coheren deecion process. However, for long-range lidars, he phase noise of he wo ligh componens becomes uncorrelaed and he specrum of he inerference signal widens, dropping he power in is fundamenal one. The arge range a which he power in he fundamenal one drops o half of is maximum expeced value is called he coherence range. This is a measure of he FMCW lidar s maximum operaing range. The coherence range of widely unable laser diodes can be as small as a few millimeers, whereas for a fixed-frequency laser employed in an FMCW lidar wih elecro-opic modulaor, he coherence range can reach up o hundreds of meers. This makes he laer a more suiable opion for longrange applicaions where a few millimeers of resoluion is sufficien and wide opical uning is no needed. Elecronic-Phoonic Inegraed Circui for FMCW Lidar As wih a VCO, he frequency of a unable laser can be conrolled in a feedback archiecure [13], as illusraed in Fig. 6a. This is achieved by measuring he modulaion slope and adjusing i by he laser conrol signal V crl [14]. The modulaion slope is measured using a Mach-Zehnder inerferomeer (MZI), he operaion of which is very similar o he FMCW range measuremen echnique, excep he unknown round-rip delay o he arge is replaced wih a known fixedlengh waveguide. Consequenly, he inerference frequency generaed a he oupu of he MZI is proporional o g and he waveguide delay: f MZI = g MZI. Because he waveguide delay is fixed, any flucuaions in f MZI can be inerpreed as variaion in g. A phase locked loop (PLL) circui can be used o measure hese flucuaions agains a reference frequency f ref, and he flucuaions can be suppressed by adjusing he laser conrol signal V crl o ensure ha g = f ref / MZI. Because he linear modulaion canno coninue indefiniely, a hyseresis comparaor observes he level of and reverses is slope (o generae up/down ramps) whenever i crosses some predefined boundaries. The conrol loop for he laser modulaion is implemened on a heerogeneously inegraed elecronic-phoonic chip sack as described in [13]. The MZI and he phoodeecor are fabricaed on a silicon-phoonic chip, and he elecronic circuis are designed in a 0.18 m CMOS process. The wo dies are inegraed using hrough-silicon-vias (TSVs) o make a single chip-sack as shown in he phoograph. This elecronic-phoonic inegraed circui modulaes he frequency of a discree unable laser wih high precision and repeaabiliy. The oupu ligh of he laser is used o creae a 3D image of a gear placed a a 40-cm disance from he source, a a rae of 10 kp/s wih 11-m range precision and 250-m laeral resoluion. The 3D image reconsruced from his measuremen is shown in Fig. 6c. While he objecive of his paricular work was o achieve a fine range precision, he design IEEE Communicaions Magazine Ocober

8 As phoonic devices become more accessible hrough monolihic CMOS processes or heerogeneously inegraed silicon-phoonic and CMOS plaforms, more flavors in FMCW ransmi and receive archiecures will lead o fully inegraed nex-generaion lidars ha can be designed and opimized for a wide range of applicaions. rade-offs explained herein can also be used o guide he developmen of FMCW lidars ha are more suiable for long-range applicaions wih lower range resoluion [15]. Conclusion Accurae deecion of he surrounding environmen is of he umos imporance o he successful operaion of auonomous machines such as self-driving cars, drones, and indoor robos. Among differen sensory sysems, 3D cameras have proven o be an essenial aid for such machines, providing precise dimensions of and disances o objecs in heir viciniy. Among differen 3D imaging echniques, he fine volumeric resoluion and long operaional disance of lidar-based soluions have significanly surpassed hose of oher echniques. Many differen lidar archiecures have been invesigaed over he las several decades. Among hem, FMCW lidars provide he fines resoluion for shor-range applicaions. Because of heir coheren deecion scheme, hey can also deec he lowes reurning ligh levels from disan arges a he fundamenal sho noise limi. In addiion, he consan opical power level a heir oupu is compaible wih he emerging silicon-phoonic-based opical phased arrays for beam seering, which canno easily accommodae he large peak power of a pulsed lidar. This is paricularly imporan, because he high cos of he curren beam seering soluions is one of he major challenges in developing inexpensive longrange lidars, and silicon-phoonic phased-array is one of he mos promising echnologies ha can solve his issue. These characerisics have made he FMCW lidars an increasingly aracive choice for applicaions from hose in advanced medical and scienific fields o self-driving cars and drones. As phoonic devices become more accessible hrough monolihic CMOS processes or heerogeneously inegraed silicon-phoonic and CMOS plaforms, more flavors in FMCW ransmi and receive archiecures will lead o fully inegraed nex-generaion lidars ha can be designed and opimized for a wide range of applicaions. Acknowledgmen The auhors would like o hank Dr. Niels Quack and Dr. Tae-Joon Seok, former members of he lidar research eam a he Universiy of California a Berkeley, and also Dr. Mahdi Kashmiri and Dr. Ken Wojciechowski from Rober Bosch LLC Research and Technology Cener in Palo Alo for heir criical feedback on his aricle. The auhors would also like o hank DARPA s Diverse Accessible Heerogeneous Inegraion Program as he main funding agency for his research; he TSMC Universiy Shule Program for CMOS chip fabricaion; he Marvell Nanofabricaion Laboraory a he Universiy of California a Berkeley for he fabricaion of he silicon-phoonic chips; and he Berkeley Sensor and Acuaor Cener and he UC Berkeley Swarm Lab for heir coninuous suppor. References [1] R. Halerman and M. Bruch, Velodyne HDL-64E LIDAR for Unmanned Surface Vehicle Obsacle Deecion, Proc. SPIE Defense, Securiy, and Sensing, In l. Sociey for Opics and Phoonics, Orlando, FL, [2] E. S. Cameron, R. P. Szumski and J. K. Wes, Lidar Scanning Sysem, U.S. Paen , 9 Apr [3] D. P. Resler e al., High-Efficiency Liquid-Crysal Opical Phased-Array Beam Seering, Opics Leers, vol. 21, no. 9, 1996, pp [4] B.-W. Yoo e al., A Opical Phased Array Using Polysilicon Sub-Wavelengh High-Conras-Graing Mirrors, Opics Express, vol. 22, no. 16, 2014, pp. 19, [5] J. Sun e al., Large-Scale Nanophoonic Phased Array, Naure, vol. 493, no. 7431, 2013, pp [6] K. Van Acoleyen e al., Off-Chip Beam Seering wih a One-Dimensional Opical Phased Array On Silicon-On-Insulaor, Opics Leers, vol. 34, no. 9, 2009, pp [7] P. M. Woodward, Probabiliy and Informaion Theory, wih Applicaions o Radar, Pergamon Press, [8] C. Niclass e al., Design and Characerizaion of a CMOS 3-D Image, J. Solid-Sae Circuis, vol. 40, no. 9, 2005, pp [9] S. Kawahio e al., A CMOS Time-of-Fligh Range Image Sensor wih Gaes-on-Field-Oxide Srucure, IEEE Sensors J., vol. 7, no. 12, 2007, pp [10] W. Kim e al., A 1.5Mpixel RGBZ CMOS Image Sensor for Simulaneous Color and Range Image Capure, Proc. In l. Solid-Sae Circuis Conf., San Francisco, CA, [11] D. Uam and B. Culshaw, Precision Time Domain Reflecomery in Opical Fiber Sysems Using a Frequency Modulaed Coninuous Wave Ranging Technique, J. Lighwave Technology, vol. 3, no. 5, 1985, pp [12] P. A. Sandborn e al., Dual-Sideband Linear FMCW Lidar wih Homodyne Deecion for Applicaion in 3D Imaging, Proc. Conf. Lasers and Elecro-Opics, San Jose, CA, [13] B. Behroozpour e al., Elecronic-Phoonic Inegraed Circui for 3D Microimaging, IEEE J. Solid-Sae Circuis, vol. 52, no. 1, 2017, pp [14] N. Sayan e al., Precise Conrol of Broadband Frequency Chirps Using Opoelecronic Feedback, Opics Express, vol. 17, no. 18, 2009, pp [15] G. N. Pearson e al., Chirp-Pulse-Compression Three-Dimensional Lidar Imager wih Fiber Opics, Applied Opics, vol. 44, no. 2, 2005, pp Biographies Behnam Behroozpour [M 16] received his B.Sc. degree from Sharif Universiy of Technology, Tehran, Iran, in 2010, his M.Sc. degree from he Universiy of Twene, Enschede, he Neherlands, in 2012, and his Ph.D. degree from he Universiy of California a Berkeley in He is currenly a research engineer wih Bosch LLC, Palo Alo, California. His research ineress include elecronic and phoonic inegraed circuis, MEMS, LIDAR, and 3D imaging echnologies. Phillip A.M. Sandborn received his B.Sc. degree in elecrical engineering and mahemaics from he Universiy of Maryland, College Park, in He is currenly pursuing a Ph.D. degree in elecrical engineering wih he Universiy of California a Berkeley. His curren research ineress include 3D imaging using LIDAR, opical phase-locked loops, opical packaging, and noise-limied performance of opical sysems. Ming C. Wu [F 02] received his Ph.D. degree from he Universiy of California a Berkeley in He is currenly a Norel Disinguished Professor of Elecrical Engineering and Compuer Sciences wih he Universiy of California a Berkeley. He was a Packard Foundaion Fellow from 1992 o He received he 2007 Paul F. Forman Engineering Excellence Award from he Opical Sociey of America and he 2016 William Sreifer Award from he IEEE Phoonics Sociey. Bernhard E. Boser [F 03] received his Ph.D. degree from Sanford Universiy, California, in In 1992, he joined he Faculy of he EECS Deparmen, Universiy of California a Berkeley. He is also a co-founder of SiTime, Sana Clara, California, and Chirp Microsysems, Berkeley, California. He served as he Presiden of he Solid-Sae Circuis Sociey and on he Program Commiees of ISSCC, VLSI Symposium, and Transducers. He also served as he Edior-in-Chief of he IEEE Journal of Solid-Sae Circuis. 142 IEEE Communicaions Magazine Ocober 2017