1 A THz-TDS-based Measuremen Technique for Characerizing Picosecond Pulses Radiaed from Silicon Chips Peiyu Chen, Suden Member, IEEE, M. Mahdi Assefzadeh, Suden Member, IEEE, and Aydin Babakhani, Member, IEEE Absrac This paper presens a ime-domain opoelecronic picosecond pulse measuremen echnique, where radiaed pulses from a silicon chip are sampled by a laser-gaed phooconducive anenna (PCA). The PCA deecor provides a measuremen bandwidh up o 4.5THz. In his scheme, he radiaed picosecond pulses from a silicon chip are synchronized wih a femosecond laser source in an opical sampling sysem. In his echnique, waveform sampling is performed direcly a he PCA, miigaing measuremen complexiies associaed wih loss and disorion calibraions of waveguides, coaxial cables, and connecors. The repored cusom measuremen echnique is used o sample pulses as shor as 5.4ps. Index Terms picosecond pulse measuremens, opical sampling, silicon RFIC, eraherz opoelecronics, THz-TDS, ime-domain measuremens. D I. INTRODUCTION URING he pas decade, mm-wave and THz waves have been used in a variey of applicaions, such as biology and medical sciences [1], high-resoluion 3D imaging [2], non-desrucive evaluaion [3], and environmenal monioring [4]. The increasing curren gain cuoff frequency (f T) and maximum oscillaion frequency (f T) of ransisors in BiCMOS and CMOS processes, have resuled in silicon-based sources operaing in he THz regime. The majoriy of hese sources are based on narrowband Coninuous Wave (CW) signal generaion. Since hese sources are narrowband, hey can be characerized by waveguide probes or horn anennas coupled o recangular waveguides. These mehods are no suiable for broadband picosecond pulses ha span a frequency range from 3GHz o beyond 1THz. Recenly, silicon-based digial-o-impulse archiecures have been repored ha generae and radiae picosecond pulses [5], [6], [7], [8], [9]. These radiaors require a direc ime-domain sampler wih a measuremen bandwidh exceeding 1THz. Tradiionally, picosecond pulse generaion is done hrough Phooconducive Anennas (PCAs) riggered by femosecond laser pulses. Unforunaely, PCA sources require a bulky and expensive laser source. They also have a low repeiion rae (1MHz or below). Recenly, fully-elecronic silicon-based picosecond pulse radiaors are repored ha produce high-power broadband pulses in he THz regime wihou using any laser source [5], [6], [7], [8], [9]. These chips conver a low-frequency rigger signal o high-power picosecond pulses in he THz regime. They can also produce picosecond pulses wih a high repeiion rae of 5GHz. Alhough hese sources produce picosecond pulses, due o heir broadband naure, heir characerizaion remains exremely challenging. Convenionally, high-speed oscilloscopes are used o sample shor pulses bu heir bandwidh is limied o 1GHz [1], which is no high enough o characerize pulses wih several picosecond duraion in he THz regime. To be more precise, rise ime of oscilloscopes is an imporan performance meric for picosecond pulse measuremens. In an oscilloscope, he rise ime and he bandwidh are relaed by [11], [12], k BW = Rise Time (1% 9%) where BW is he bandwidh of oscilloscopes. k varies from.4 o.45 in oscilloscopes wih a bandwidh of larger han 1GHz. As repored in [12], oscilloscopes mus have sufficienly small rise ime o accuraely capure he deails of rapid ransiions as shown below, rise,oscilloscope rise,signal 5 where rise,oscilloscope is 1%-9% rise ime of oscilloscopes and rise,signal is ha of he signal of ineres. Therefore, o measure an impulse-like pulse wih a 1ps Full-Widh-a-Half-Maximum (FWHM), which has an approximae 5ps rise ime, oscilloscopes should have a rise ime less han 1ps. However, he highes bandwidh of off-he-shelf oscilloscopes is 1GHz, associaed wih 4.5ps rise ime (1%-9%) [1], which fails o mee he requiremen for measuring picosecond pulses. In addiion o he limied bandwidh, in he convenional oscilloscope-based echniques, he received pulse has o pass hrough an anenna, coaxial cables, and coaxial connecors before being sampled by he inernal elecronics of he (1) (2)
2 Meal Conacs Femosecond Laser E THz () E THz () Semiconducor Subsrae Silicon Lens Probe Laser Pulse Probe Laser Pulse Conduciviy Response σ sub. (-) Fas Conduciviy Response δ sub. (-) THz Pulse Radiaion Fig. 1. Schemaic of a phooconducive anenna (PCA). oscilloscope. The loss and phase response of hese componens need o be calibraed in a broad frequency range so ha he disorion effecs on he picosecond pulses are de-embedded correcly. Therefore, he measuremen complexiy is significanly increased due o he broadband calibraions of numerous passive componens beween he anenna and he inernal sampler of he oscilloscope. This paper presens a novel ime-domain opoelecronic picosecond pulse measuremen echnique based on Teraherz Time-Domain Specroscopy (THz-TDS). The proposed mehod has a rise ime of abou 1fs [13], [14], associaed wih 4.5THz measuremen bandwidh. This inrinsic ulra-fas ransien response allows o accuraely measure picosecond pulses radiaed from a silicon chip. In he proposed measuremen echnique, received picosecond pulses are direcly sampled a a PCA deecor, miigaing measuremen complexiies associaed wih broadband calibraions of passive componens as in he convenional mehods based on high-speed oscilloscopes. This paper is oulined as follows. Secion II describes convenional phooconducive-based THz-TDS sysems, focusing on he measuremen principle and physical mechanism. Secion III repors he deails of he measuremen echnique and Secion IV discusses he measuremen resuls. Finally, Secion V concludes he paper. II. THZ-TDS SYSTEM In a THz-TDS sysem, phooconducive anennas are commonly used as THz sources and deecors. In his secion, firs, physical mechanisms of a PCA are described. Second, deails of he THz-TDS sysem used in his work, which is based on an Asynchronous Opical Sampling (ASOPS) scheme [15], are discussed. A. Phooconducive Anennas (PCAs) PCAs are he firs devices used o measure ransien elecric field of a THz pulse [16], [17]. In conras o bolomeric deecors, PCAs record boh phase and ampliude informaion of a THz pulse. Fig. 1 demonsraes a simple PCA, consising of Fig. 2. Sampling mechanism of a PCA deecor wih slow subsrae conduciviy response and fas subsrae conduciviy response. wo meal conacs evaporaed ono a semiconducor subsrae. When a PCA is used as a THz emier, he elecrodes are biased wih a consan DC volage. Then, a femosecond laser pulse shines a he gap beween he elecrodes and produces carriers in he semiconducor subsrae. This even generaes an ulra-fas ransien phoocurren, excies he anenna, and resuls in THz radiaion. A silicon lens is usually mouned on a PCA o increase he anenna gain. When a PCA is used as a THz deecor, wo elecrodes are conneced o he inpu of deecion and daa-acquisiion elecronics. In conras o a PCA THz emier, no DC bias is applied beween he elecrodes. In absence of a THz wave, when a femosecond laser pulse is shined a he gap beween he elecrodes, only phoo-carriers are produced and no curren is generaed. In presence of a THz wave, he elecric field of he THz wave drives he phoo-carriers and resuls in a non-zero curren. This curren is amplified and digiized by daa acquisiion unis following he PCA deecor. According o Ohm s law, in a semiconducor maerial, he generaed ransien curren densiy J () is he muliplicaion of ransien conduciviy response σ sub. () and ransien driving elecric field (). E THz The relaion can be expressed as J () = σ sub. ()E () THz (3) The conduciviy response of he semiconducor, σ sub. (), depends on he concenraion of phoo-carriers generaed by he femosecond pulse. As illusraed in Fig. 2, when he iming delay beween he received THz pulse and he femosecond laser pulse is, he generaed ransien curren, I(), will be expressed as I () = Aσ sub. ( )E () THz (4) where A is he cross-secion area of he conducing region in he semiconducor subsrae. If he semiconducor subsrae is grown a low emperaure or is ion-damaged, exhibiing very shor phoocarrier lifeimes [4],
3 Femosecond Laser PLL Common Frequency Reference PLL Femosecond Laser Pump Beam 1/f r1 1/f r2 Probe Beam + - V Daa Acquisiion Elecronics Fig. 3. Schemaic of a THz-TDS sysem wih ASOPS scheme. Sampling mechanism of ASOPS scheme. he deecion circui will be only closed as long as he PCA deecor is illuminaed by he femosecond laser pulse. As shown in Fig. 2, because he duraion of a femosecond laser pulse, e.g. ~5fs, is usually much shorer han ha of a THz pulse, Equaion (4) can be furher simplified using a Dirac dela funcion, I () = Kδ sub. ( )E () THz (5) where K is a consan number relaed wih he conduciviy response srengh and he cross-secion area A. Therefore, when he iming delay beween he received THz pulse and he femosecond laser pulse is, a each sampling, he colleced elecric charge Q() is + + Q() = I()d = Kδ sub. ( )E THz ()d = KE THz () 1/f r1 T=1/fr2-1/f r1 1/f r2 THz Pulse Radiaion PCA Deecor E THz () Probe Laser Pulse PCA Emier The sampled oupu, V ou (), is proporional o he colleced elecric charge Q(), which can be expressed as (6) V ou () Q() E THz () (7) Therefore, he shape of received ransien THz pulse can be obained by direcly ploing he sampled oupus wih differen iming delays. This condiion is valid in his work for measuring picosecond pulses. Even hough he above condiion is no valid for measuring <1fs THz pulses, he acual ime-domain waveform of THz pulses can sill be recovered if subsrae conduciviy response σ sub. () is characerized [18]. As shown in Equaion (3), σ sub. (), he conduciviy response of a PCA s semiconducor subsrae, deermines he bandwidh of he PCA deecor and how rapid ransiions can be capured accuraely. Wih a femosecond laser pulse exciaion, which can be considered as an insananeous exciaion, σ sub. () rises exponenially owards he seady-sae value wih a ime consan given by he carrier scaering ime [14]. A ypical value of he carrier scaering ime for a semiconducor is less han 1fs [13], which is much shorer han he sae-of-he-ar rise ime of off-he-shelf elecronic oscilloscopes (4.5ps). According o Equaion (1), he bandwidh of a ypical PCA THz deecor can be esimaed o be around 4.5THz. Apar from deecion mechanism, Signal-o-Noise Raio (SNR) is anoher concern for a PCA THz deecor. The inrinsic noise of a PCA mainly comes from he Johnson-Nyquis noise, which is relaed o he average resisance of he PCA. Subsrae maerials wih shorer phoocarrier lifeimes resul in PCAs wih a lower noise curren level. However, shorer lifeime subsrae maerials have lower phoocurren responsiviy, which reduces signal level as an expense. As a resul, o maximize PCA s SNR, i is imporan o opimize he subsrae maerial [19]. B. Asynchronous Opical Sampling (ASOPS) Scheme Fig. 3 shows a schemaic of a THz-TDS sysem, which is based on an Asynchronous Opical Sampling (ASOPS) scheme. In such a scheme, wo femosecond laser sources generae wo beams. One is called he pump beam, he oher is called he probe beam. These wo laser beams have slighly differen repeiion frequencies, f r1 and f r2, respecively. The frequency deuning is denoed as f r. The pump beam excies a PCA THz emier, which produces synchronized THz pulse radiaions. The probe pulse ravels o gae a PCA THz deecor in order o measure he THz pulse radiaion. The oupu of he PCA THz deecor is conneced o daa-acquisiion elecronics. The rigger signal required for daa acquisiion can be exraced from he pump and probe beams. As shown in Fig. 3, he repeiion frequency deuning beween he pump and probe beam enables he PCA THz deecor o sample he whole measuremen window, which is defined by he pulse-o-pulse spacing of THz radiaions. The scan rae is deermined solely by he repeiion frequency deuning f r, when i is much smaller han f r1 and f r2. The upper limi of f r is deermined by he bandwidh of deecion and dae acquisiion circuis, as well as he required iming resoluion [15]. Compared wih a convenional THz-TDS sysem, which usually incorporaes a mechanical ranslaion sage wih a reroreflecor mirror o shif iming delays [4], he ASOPS scheme has superior scan rae and eliminaes he noise
4 Oscilloscope Tekronix 414B-L Synchronizaion Circuiry RF Signal Generaor Keysigh E8257D 1MHz Locking Signal Silicon Chip Under Tes Trigger Advanes TAS75TS Measuremen Uni Fig. 4. Schemaic of he proposed measuremen echnique. E Pump induced by he mechanical movemens during he long iming delay. No maer which scheme is used, synchronizaion is he essence of ime-resolved THz measuremens. In he convenional THz-TDS schemes, a single femosecond laser beam passes hrough a beam splier, which resuls in wo synchronized pump and probe beams. The probe pulse is used o deec he THz pulse excied by he synchronized pump pulse. In he ASOPS scheme, pump and probe pulses are generaed by wo individual femosecond laser sources. Repeiion frequency flucuaions of each laser source generae a deviaion from he chosen scan rae f r, resuling in a iming jier and disorions in sampled waveforms, reducing SNR. Therefore, he repeiion frequencies of he wo femosecond laser sources are sabilized by wo PLLs ha share a common frequency reference. III. AN OPTOELECTRONIC MEASUREMENT TECHNIQUE FOR CHARACTERIZING PICOSECOND PULSES This secion describes an opoelecronic measuremen echnique for characerizing picosecond pulses based on an ASOPS THz-TDS sysem. In his secion, firs, deails of he repored measuremen echnique are described. Second, synchronizaion challenges beween elecrical and opical domains are discussed. A. An Overview of he Measuremen Technique As discussed in Secion II, a PCA THz deecor has superior bandwidh han curren off-he-shelf elecronic oscilloscopes. Therefore, a PCA deecor is used o measure picosecond pulses radiaed from a silicon chip. Since he PCA deecor works based on a pump-and-probe scheme, he pulses radiaed from he silicon chip under he es should be synchronized wih he femosecond laser associaed wih he PCA deecor. In his work, he laser sources and he PCA deecor (TAS123) in an Advanes TAS75TS THz-TDS sysem are used o sample he pulses radiaed from silicon chips. This D Parabolic Mirror Analysis Uni Parabolic Mirror THz Deecor Module (TAS123) measuremen seup has a bandwidh (SNR=1) of more han 4THz. The schemaic of he measuremen echnique is presened in Fig. 4. Advanes TAS75TS uilizes an ASOPS scheme and is equipped wih a measuremen uni. The measuremen uni provides an addiional femosecond laser oupu por, which is used for synchronizing he silicon chips under he es. The synchronizaion mehod will be discussed in secion III B. The measuremen uni ransfers he sampled waveform a he oupu of he PCA o an analysis uni, which performs pos-processing. These resuls can be displayed on he sysem monior. Since he analysis uni has a limied memory, he displayed ime-domain waveforms have a limied ime duraion. To solve his problem, a digial oscilloscope (Tekronix 414B-L) is conneced o he measuremen uni o read he real-ime sampled oupu of he received picosecond pulses. To characerize picosecond pulses radiaed from a silicon chip, a PCA THz deecor can be placed in he far-field region of he silicon chip, wihou uilizing parabolic mirrors. In his case, broadband radiaion paerns can be obained. If he overall gain of he PCA deecor and is following receiver pah is known, broadband EIRP can also be measured. However, o increase he SNR of he received signal, off-axis parabolic meal mirrors are used o focus picosecond pulses on he PCA deecor. Due o frequency-independen refleciviy of he parabolic mirrors, he pulse shape is no disored. In his measuremen, wo off-axis parabolic mirrors are used o guide he picosecond pulses radiaed from a silicon chip o he PCA (Fig. 4). In addiion o he enhanced measuremen bandwidh, he repored measuremen echnique offers a much simpler calibraion compared o convenional soluions based on high-speed oscilloscopes. This is because picosecond pulses are direcly sampled a he PCA. I eliminaes he need for broadband calibraions of waveguides, coaxial cables, and coaxial connecors ha connec anennas and oscilloscopes in he convenional soluions. B. Synchronizaion beween he Chip and he THz-TDS Sysem In he proposed measuremen echnique, he echnical challenge is o synchronize he silicon chip under he es wih he pump femosecond laser source in he ASOPS THz-TDS seup. To explain he synchronizaion mehod, i is necessary o briefly review he working principles of he silicon-based picosecond pulse radiaors [5], [9] ha are esed in his work. The pulse radiaing chips conver an elecrical inpu rigger signal o a radiaed wave ha is synchronized wih he rigger signal. Therefore, if he inpu rigger signal is synchronized wih he pump femosecond laser source, he radiaed picosecond pulses from he chips will be synchronized wih he PCA THz deecor of he THz-TDS sysem (Advanes TAS75TS). Fig. 5 shows he block diagram of he synchronizaion echnique. The oupu por of he pump laser source is aenuaed wih an opical aenuaor (Thorlabs OVA5-APC) before being convered o an elecrical rigger by a
5 Inpu Pump Femosecond Laser (5MHz) Opical Isolaor Divide-by-5 Frequency Divider (Hiie 438MS8G) Opical Aenuaor (Thorlabs OVA5) 5MHz Filer (Mini-Circuis SIF-5+) Phoodeecor (Thorlabs DET1CFC) Picosecond Pulse Labs 1/f r =1/(N 5MHz) T=1/f r2-1/f r1 =2fs E THz () 1/(N f r )=1/5MHz 1MHz Filer (Mini-Circuis SBP-1.7+) LNA (RF Bay LNA-25) 1MHz Filer (Mini-Circuis SBP-1.7+) 1MHz Locking Signal Oupu Probe Laser Pulse 1/f s =1/(5MHz+5Hz) Inpu Fig. 6. Valid sampling condiions of he proposed picosecond pulse measuremen echnique. Cusom-Designed Silicon Chip Oupu 2 2ns 1.5 1 3.5ns PCA THz Deecor (Advanes TAS123) Parabolic Meal Mirrors (Thorlabs MPD249-M1) V.5 2 4 6 8 1 Time (ns) (c) Fig. 5. Block diagram of he proposed synchronizaion echnique. The acual circui blocks for synchronizaion signal generaion. (c) Convered elecrical pulses from he pump femosecond laser. phoodeecor (Thorlabs DET1CFC). An opical isolaor proecs he laser by minimizing he power of he undesired reflecions and a bias-ee (Picosecond Pulse Labs) is used o provide a proper biasing condiion for he phoodeecor oupu. The generaed elecrical rigger is a 5MHz pulse rain wih a broadened pulse-widh due o he limied bandwidh of he phoodeecor, as shown in he Fig. 5(c). A 5MHz filer (Mini-Circuis SIF-5+) exracs a 5MHz signal and feeds i o a broadband divide-by-5 frequency divider (Analog Devices 438MS8G). The undesired harmonic spurs are eliminaed by a 1MHz narrowband filer (Mini-Circuis SBP-1.7+). Afer applying a low-noise amplifier (RF Bay LNA-25) and anoher 1MHz narrowband filer, a clean 1MHz signal is obained wih enough power o synchronize a RF signal generaor (Keysigh E8257D), which provides he inpu rigger for he silicon chips. In he proposed measuremen mehod, he PCA deecor samples he received picosecond pulse radiaion a he rae, f s, which is given by ha of he probe femosecond laser. Here, f s is 5MHz+5Hz in he Advanes TAS75TS, where he repeiion rae of he THz pulse radiaion is 5MHz. This means Fig. 7. Measuremen seup for characerizing cusom-designed silicon chips. ha he effecive iming sep of sampling is (1/5MHz-1/5.5MHz)=2fs. I should be noed ha alhough he repeiion rae of he pump laser is 5MHz, he measuremen echnique allows us o rigger he chips wih any repeiion rae ha is a harmonic of 5MHz. For example, as shown in Fig. 6, if he repeiion rae of he radiaed pulse rain, f r, is an N h harmonic of 5MHz, he PCA deecor will sample one poin for every N received pulses, bu he acual iming sep of he sampled waveform is sill 2fs. In fac, in our measuremen, he repeiion rae of he radiaed picosecond pulses is larger han 1GHz. IV. MEASUREMENT RESULTS Fig. 7 presens he measuremen seup used for characerizing he cusom-designed silicon chips. As discussed in secion III, wo parabolic mirrors wih gold coaings (Thorlabs MPD249-M1) are used o focus radiaed picosecond pulses o he PCA deecor (Advanes TAS123). In his seup, he PCA deecor is mouned on a 3D posiioner for alignmen purposes. A 1MHz synchronizaion signal is generaed using he circui shown in Fig. 5 and is ime-domain waveform is shown in he Fig. 8. This waveform has a 7.7dBm peak power a 1MHz (Fig. 8). In he proposed echnique, he wo laser pulses, which are used o provide he rigger signal o he chips as well as he sampling signal o he PCA, respecively, are generaed almos a he
6 1 6 mv 5 mv 4 2 FWHM=5.8ps Averaging: 512-5 -2 Phase Noise (dbc/hz) -1 2 4 6 8 1 Time (ns) dbm -1-1 -11-12 -13-14 2-2 -4-6 -8 7.7dBm@1MHz Res BW: 47Hz Span: 1KHz -12 9.995 1 1.5 Frequency (MHz) 1MHz Synchronizaion Signal 1MHz Signal from Keysigh E8257D -127.8dBc/Hz @ 1KHz -128.8dBc/Hz @ 1KHz -15 1 2 1 3 1 4 1 5 1 6 Offse Frequency (Hz) (c) Fig. 8. Measured ime-domain waveform of he 1MHz synchronizaion signal. Measured frequency specrum of he 1MHz synchronizaion signal. (c) Measured phase noise of he 1MHz synchronizaion signal. same ime. Therefore, here is no need for long-erm sabiliy of he 1MHz synchronizaion signal. To measure he sabiliy of he generaed 1MHz signal, we have compared is phase noise wih ha of a sae-of-he-ar signal generaor (Keysigh 8257D) ha uses an inernal reference. This comparison is shown in Fig. 8(c). In his work, wo cusom-designed silicon picosecond pulse radiaors, [5], [9], are characerized using he measuremen seup shown in Fig. 7. The RF signal generaor (Keysigh E8257D) provides a sinusoidal rigger signal o he silicon chips. The chips radiae picosecond pulses wih he same repeiion rae of he inpu rigger. Design deails of chips are discussed in [5] and [9]. As discussed in secion III B, he repeiion rae of he picosecond pulses should be a harmonic of he sampling frequency (5MHz). In his work, he chips in [5] Normalized Specrum (db) -4 15 2 25 3 Time (ps) 1-1 -2-3 -4-5 57GHz 1dB BW 4GHz SNR=1 BW >175GHz Noise Floor -6 1 2 3 4 5 Frequency (GHz) Fig. 9 Measured ime-domain waveform and measured frequency specrum of he picosecond pulse radiaed from he chip [5]. and [9] are esed using 1GHz and 5GHz sinusoidal rigger signals, respecively. Fig. 9 demonsraes he characerizaion resuls of he chip repored in [5]. The ime-domain waveform of he capured pulse is shown in Fig. 9. This chip radiaes pulses wih a Full-Widh-a-Half-Maximum (FWHM) of 5.8ps. In order o increase SNR, an averaging of 512 is used. The frequency specrum of he picosecond pulse radiaion is obained by performing DFT on he recorded ime-domain waveform (Fig. 9). The 5.8ps pulse radiaion has a peak frequency of 57GHz. Is 1dB bandwidh is 4GHz, and is SNR=1 bandwidh is more han 175GHz. Fig. 1 shows he characerizaion resuls of he chip repored in [9]. This chip consiss of a 4 2 on-chip array of impulse radiaors. In his experimen, he iming of radiaion for each elemen is conrolled using programmable delay generaors o perform coheren spaial combining of impulses in space, by delaying he rigger of each radiaing elemen. The measured combined ime-domain signal is shown in Fig. 1, and an FWHM of 5.4ps is measured. In addiion, by conrolling he curren swich bias node in he elecronic chip [9], he ampliude of he radiaion is modulaed. Ampliude modulaion resuls are ploed in Fig. 1. Finally, a comparison beween he proposed measuremen echnique and a convenional mehod ha uses a
7 TABLE I COMPARISON BETWEEN THE PROPOSED MEASUREMENT TECHNIQUE AND THE CONVENTIONAL METHOD BASED ON AN STATE-OF-THE-ART OSCILLOSCOPE Proposed Measuremen Technique Convenional Mehods Sampling Mehod Opoelecronic sampling using a PCA deecor Ulra-fas sampling using a real-ime oscilloscope Measuremen Bandwidh 4.5THz 1GHz 1 Rise Time 1fs 2 3.5ps (2%-8%) 1 Broadband Calibraion Requiremen PCA deecors Coaxial cables, connecors, sampler Real-Time Sampling Rae 5MS/s 24GS/s 1 Effecive Timing Sep of Sampling 2fs 3 4ps 1 Based on he fases commercial oscilloscope, Teledyne Technologies LabMaser 1 Zi-A High Bandwidh Modular Oscilloscopes 2GHz-1GHz [1]. 2 This is a ypical number of a PCA deecor, as discussed in secion II A. 3 This is esimaed by calculaing he difference beween 1/5MHz and 1/5.5MHz. In pracice, his number is limied by he jier of he measuremen seup and he number of averaging used. Vmeas (mv) 25 2 15 1-1 -55-15 -2 Zoomed -25 8 9 1 11 12 13 14 Time (ps) 25 Peak Ampliude (mv) 2 15 1 5 1. 1.1 1.2 1.3 1.4 1.5 1.6 V 2 (V) FWHM = 5.4ps Ampliude Modulaion Fig. 1 Measured ime-domain waveform of he picosecond pulse radiaed from he chip [9]. Measured pulse ampliude modulaion of he chip [9]. V. CONCLUSIONS In his work, a novel opoelecronic measuremen echnique is demonsraed for characerizing picosecond pulses radiaed from silicon chips. The mehod uses a PCA deecor and is based on a pump-and-probe sampling scheme, which is provided by an Advanes THz-TDS sysem (TAS75TS). The uilized PCA deecor has an SNR=1 bandwidh up o 4THz, which is more han 4 imes higher han ha of he bes off-he-shelf elecronic oscilloscopes (1GHz). In addiion, since he THz pulse is sampled a he PCA, here is no need for complex broadband calibraions of waveguides, coaxial cables and connecors used in convenional mehods ha are based on oscilloscopes. In he repored scheme, he radiaed pulses from he silicon chips need o be synchronized wih he pump laser of he THz-TDS sysem. Based on his scheme, wo cusom-designed silicon-based picosecond pulse radiaors are characerized. ACKNOWLEDGMENT Auhors would like o hank Dr. Eiji Kao from Advanes Corporaion, Dr. Tim Noe from Rice Universiy for echnical suppor, and all members in Rice Inegraed Sysems and Circuis (RISC) lab for valuable discussions. This work is funded by W. H. Keck Foundaions. sae-of-he-ar off-he-shelf oscilloscope is summarized in Table 1.
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