r~3 HEWLETT ~r. PACKARD The Use ofgain-switched Vertical Cavity Surface-Emitting Laser for Electro-Optic Sampling Kok Wai Chang, Mike Tan, S. Y. Wang Koichiro Takeuchi* nstrument and Photonics Laboratory HPL-93-76 August, 1993 surface emitting laser, gain-switched semi-conductor lasers, electro-optic sampling, highspeed integrated circuit testing This paper describes the characteristics of a gainswitched vertical cavity surface-emitting laser (VCSEL) and its application as the optical pulse source for an electro-optic (EO) sampling system that employs a GaAs extemal electro-optic probe tip as the longitudinal electric field sensor to diagnose internal nodes of high-speed electronic devices. nternal Accession Date Only "Hewlett-Packard Laboratories-Japan, Kawaski-shi, Kanagawa, 213 Copyright Hewlett-Packard Company 1993
ntroduction: n order to satisfy the need for testing internal nodes of high speed-integrated circuits, Shinagawa and Nagatsuma [1-2] proposed and demonstrated an external electro-optic (EO) prober that employs a gain-switched ngaasp DFB laser diode at a wavelength of 1.3 urn as a sampling optical pulse source and a GaAs probe tip as the longitudinal fringing electric field sensor. n this letter we report for the first time the characterization of important parameters such as timing jitter, excess intensity noise and pulse width of a gain-switched vertical-eavity surface-emitting laser (VCSEL) for application as the optical pulse source of an external GaAs probe-tip-based EO sampling system. Characteristics of optical pulse: The VeSEL under investigation was a bottom-emitting laser constituted of 15 pairs of a p-type top DBR mirror, three multi-quantum well no.2gao.sas activeregions, and 18.5 pairs of an n type bottom/output DBR mirror. The laser was grown by gas-source molecular beam epitaxy. A 640 MHz comb generator with electrical pulse width of 50 ps was used to drive the VeSEL in order to produce short optical pulses. The optical pulse width was measured by a high-speed lightwave photodetector(with 34 GHz 3-dB bandwidth) and a 50 GHz sampling scope. Fig. la shows the optimum optical pulse width of 25 ps obtained by adjusting the drive conditions for a single spatial mode lasing of the VCSEL. The timing jitter of the optical pulse was measured to be 1.4 ps, as shown in Fig. 1b, by using the time histogram function of the sampling scope. Since the timing jitter is an order of magnitude smaller than the optical pulse width, the temporal resolution of the gain-switched VCSEL-based EO sampling system was limited by the optical sampling pulsewidth. The excess intensity noise of the gain-switched VeSEL was measured directly by a slow photodetector and a low-frequency RF spectrum analyzer to be -93 dbc/hz. This value is relatively larger than that of a gain-switched semiconductor Fabry-Perot laser with excess intensity noise value of -120 dbc/hz. n order to improve the sensitivity of the EO sampling system, a differen- 2
tial detection scheme [3] was employed to further suppress the high laser excess noise of the gainswitched VeSEL. Using the differential detection technique, the common mode excess noise of the gain-switched VeSEL was suppressed by as much as 25 db, and the corresponding laser excess noise was measured to be -118 dbc/hz. From the well known shot noise limited expression for the minimum detectable voltage [4] and the measured laser excess-noise data, an estimated theoretical value of 22.6 mv o_p / jii; was derived for an external probe with V1t of 50 kv and a photocurrent of 1uA. Electro-optic system configuration and performance: Fig. 2 shows the experimental setup for the gain-switched VeSEL-based EO sampling system. The external EO probe tip, used as a longitude electric field sensor, was fabricated from a 100 urn-thick GaAs wafer. An anti reflection (AR) coating was applied to the top surface of the GaAs probe to enable the optical beam to pass efficiently through the probe. A broadband high reflection coating with reflectivity of more than 0.95 from 950 nm to 1600 nm was deposited on the bottomsurfaceofthe GaAsprobe. Theincidentand reflected beams were separated by manipulation of their polarization [4] in such a way that the reflected beam produced a linear polarization inclined at 45 degrees to the axes of the polarizationbeamsplitterswhen therewas no applied field on the GaAs probe tip. The reflected beam polarization state became inclined at 45 + 0 degrees to the axes of the polarization beamsplitters when there was a longitudinal electric field overlapping the GaAs probe tip. The vertical component was directed by the first polarization beamsplitter into the first photodetector. The horizontal component was rotated by the Faraday rotator and the half wave plate to become vertically polarized and was then directed by the second polarization beamsplitter onto the second photodiode. A differential amplifier was then used to amplify the photocurrents from the two photodiodes and to suppress the common-mode laser excess noise. Fig. 3 shows the measured excess noise of the gain-switched VeSEL with and without differential detection. The top trace corresponds to the low-frequency laser excess noise associated with either the vertical or the horizontal component of the reflected laser beam. The lower trace shows 20-25 db suppression of laser excess noise by differential detection over the frequency range 3
from khz to 500 khz. A test electrical signal generated by a 640 MHz comb generator was used to compare the EO prober with a conventional electrical sampling oscilloscope. Fig. 4 shows the EO sampling spectrum of the comb generator measured by the external probe with (lower trace) and without (upper trace) differential detection. The comb generator was driven by a 640.0064 MHz sine wave. From this data one can see that the differential detection scheme suppresses the common-mode excess intensity noise and allows the detection ofthe EO sampling signal at a multiple of 6.4 khz. A fast digitizing oscilloscope was used as a signal averager to further improve the signal-to-noise ratio. The trigger signal for the digitizing oscilloscope was provided by another low frequency synthesizer that was phase locked to the 640 MHz and 640.0064 MHz RF synthesizers. The measured waveform is shown in Fig. 5a; it is similiar to the waveform (Fig. 5b) measured by a conventional electrical sampling oscilloscope. Finally, the minimum detectable voltage measured by the equivalent sampling method [2-3] and differential detection technique was measured to be 20 mv o_p / jii; for the gain-switched VeSEL- based EO sampling system. This measured value is very close to the theoretical estimated value of 22.6mV o_p / jii; as discussed earlier, and is also an order of magitude larger than 2.5 mv o_p / jii; of the gain-switched ngaasp DFB laser-based EO sampling system [2]. The reason for the larger value of the minimum detectable voltage in the gain-switched VeSELbased EO system is that the VeSEL used for this study becomes multi-spatial mode at higher bias voltage and starts generating multiple pulses within one electrical excitation period;therefore, the bias conditions must be adjusted to allow only one spatial mode to lase with less output power and thus higher excess-intensity noise. n order to reduce the minimum detectable voltage, a single spatial mode VeSEL with higher output power and less excess intensity noise should be developed for EO sampling applications. Conclusion: n conclusion, an EO sampling system using gain-switched VeSEL and a GaAs probe tip has 4
been developed. The system has a optical probe pulsewidth limited to the temporal resolution of 25 ps and a minimum detectable voltage of 20 mv o_p jii;. To further improve system performance, the development of high-power and low-noise single mode VeSEL is in progress. 5
References: 1. Shinagawa, M., and Nagatsuma, T. : "Electro-optic sampling using an externalgaas probe tip", Electron. Lett, Vol. 26, 1990, pp. 1341-1342. 2. Shinagawa, M., and Nagatsuma, T. : "A laser-diode-based picosecond electro-optic proberfor high-speed LSls ", EEE Trans. nstrum. Meas., Vol. 41, No.3, June 1992, pp. 375-380. 3. Nagatsurna, T.,Yaita, M., and Shinagawa, M. : "Externalelectro-optic sampling using poled polymers",japan J. Appl. Phys., Vol. 31, 1992, pp. 1373-1375. 4. Weingarten, K. J., Rodwell, M.J.W., and Bloom, D.M. : "PicosecondOptical Sampling of GaAs ntegrated Circuits", EEE J. Quantum Electron., QE-24, 1988, pp. 198-220. 6
... ;> "'0- ~ o ),/"\.1' \ -f ~ ---------..---1\. --------------- : --; : _. --... -!!. _.. J 20 ps div Fig. la. Optimum optical pulse width of gain-switched VeSEL. Sigma = laps ;> :.a- ~ o ) 20 ps div Fig. b. Timing jittermeasured by time histogram of digitizing oscilloscope. 7
VCSEL DC biased Comb Gene. Al2 Digital Scope FR HP8662 Synthesizer Al4 Al2 GaAs Probe Tip HP 8662 Synthesizer Comb Gene. 50 ohm test line 50 ohm Fig. 2. Schematic for electro-optic sampling system based on gain-switched VLSEL. 8
REF = -60 db dbm/hz RBW: 1 KHz ~--...--...-... 'Ot--+--+--+--+--+--+--t-~+-+--- Start 1,000.0 Hz Stop 500,000 Hz Fig.3. ntensity noise spectrum of gain-switched VeSEL with (lower trace) and without (upper trace) differential detection. REF =-45 dbm RBW: 100Hz ". nj L~.l.u.. _.... L..J... "'''' w... > ;a...h1++-+ft"t""':+--+--+-+----f--+-+---1 ++ +ff+-lh+---1--+---+--+--+--f -~H- 10...,... tlvtlllt*l'4'... iltf\f1 q....h ow' '"....,.,. T '1. Start 1,000.0 Hz Stop 200,000 Hz Fig.4. EO sampling spectrum of the comb generator measured by external probe with (lower trace) and without (upper trace) differential detection. 9
.b,;" ""'... ~" ~L.N4. e., /1" \.. "'. :N ~.. y ~ Time (200 ps div) Fig. 5a Waveform of a 640 MHz comb generator signal measured by VeSEL-based external EO sampling system.. : \!f\! "" ~:' 1 ~ 1 ~ : L:.:.:.: U ~ 1': ' v Time (200 psdiv) Fig. 5b. Wavefonn of a 640 MHz comb generator signal measured by a conventional sampling oscilloscope... 10