VCSELs and Optical Interconnects

Transcription

1 VCSELs and Optical Interconnects Anders Larsson Chalmers University of Technology ADOPT Winter School on Optics and Photonics February 4-7, 6

2 Outline Part VCSEL basics - Physics and design - Static and dynamic properties - Applications Part Research highlights VCSELs for optical interconnects - Optical interconnects - High speed and energy efficient VCSELs - VCSEL arrays and integration

3 Semiconductor laser basics Current injection in a forward biased pin-junction population inversion optical gain through stimulated emission Band gap wavelength: Semiconductor Fabry-Perot laser: hc E g n = n +n n = p p = p +p E C electrons n-type E g p-type optical amplification through stimulated emission coherent output p-contact cleaved facet +V I cleaved facet coherent output E V p-type, larger band gap undoped, smaller band gap n-type, larger band gap insulating, larger band gap holes gain (active) region n-contact Gain medium (active region) = smaller band gap material Pump = current Optical feedback = cleaved facets (mirrors) optical field distribution holes active region Heterostructure confinement of carriers and photons (waveguide) Ey active region Optical confinement factor: Material gain: g Modal gain: E x, y dxdy y everywhere x, y dxdy electrons g m g

4 VCSEL vs. EEL Edge emitting laser (EEL) Vertical cavity surface emitting laser (VCSEL) Horizontal resonator Edge emitting (cleaving needed) Elliptical beam Large gain and optical mode volumes (~ µm 3 ) High current and high power (~ mw) operation Vertical resonator Surface emitting (on-wafer testing and screening) Circular beam Small gain and optical mode volumes (~ µm 3 ) Low current and low power (~ mw) operation Beam profiles EEL VCSEL

5 VCSEL building blocks vertical refractive index profile distributed Bragg reflector (DBR) active (gain) region embedded in pin-junction transverse current and optical confinement distributed Bragg reflector (DBR) transverse refractive index profile Vertical resonator created by two DBRs separated by the active region Current injection through doped DBRs Aperture for transverse current and optical confinement

6 absorption gain Active (gain) region Carrier injection Quasi-Fermi level separation energy Optical gain spectrum gain under full inversion (E Fn - E Fp = ) E C n-type p-type E E Fn I > I (E Fn -E Fp ) at I n = n +n p = p +p E g E g (E Fn -E Fp ) at I I E E Fp E V momentum absorption under thermal equilibrium (E Fn = E Fp = E F ) Thin active layer (QW) quantization of states modified density of states Strained (lattice mismatched) active layer modified density of states g max Lower transparency carrier density (n tr ), higher differential gain at low carrier density Bulk Quantum well (QW) Gain vs. carrier density: g n g n g n max n tr max g n ln n tr n = p Differential gain: dg max g d dg max n dn n g n tr,qw n tr,bulk

7 Reflection characteristics of an Al.9 Ga. As/Al. Ga.88 As DBR ( = 85 nm) Power reflectivity (%) Reflection phase (rad) Distributed Bragg reflectors N pairs of layers with different refractive index n and n Each layer is a quarter wavelength thick at the Bragg wavelength: d 4n / and d / 4n N.. n out n n n n n n n Reflections at all interfaces add in phase constructive interference large reflectivity at the Bragg wavelength N n out n n in n R max N n out n nin n The maximum reflectivity increases with the index contrast (n = n -n ) and the number of pairs (N) The spectral width (bandwidth) increases with the index contrast. R n n n n n n n in d d 9 N = 4 3 N = bandwidth (nm) (nm) = 85 nm

8 N a - N d ( 8 cm -3 ) Aluminum content (-) Hole concentration ( 8 cm -3 ) Normalized field squared Distributed Bragg reflectors (cont d) Field penetration and phase of reflection are represented by a hard mirror placed at a distance L eff from the start of the DBR For highly reflecting DBRs: L eff B 4 n where: B = Bragg wavelength Hard mirror L eff Semiconductor DBRs have graded composition interfaces and are modulation doped to reduce resistance and optical loss (free carrier absorption) R R Al.9 Ga. As/Al. Ga.88 As p-dbr % Al.5 8 cm -3 modulation doping 4. graded interfaces.8 % Al Position (nm) Position (nm)

9 The vertical resonator longitudinal optical field DBR active region L eff L active effective resonator length DBR L eff z y Longitudinal optical confinement factor: long z Ey QWs Ey entire resonator dz z dz Effective optical length of the resonator: Lopt ndbr Leff nactive Lactive ndbr L eff where in the DBRs n n n L opt very small (~ one wavelength) large longitudinal mode spacing only one longitudinal mode under the gain spectrum single longitudinal mode laser! Small longitudinal optical confinement factor (-%), thin gain region (QWs) high DBR reflectivities needed (close to %) Field antinodes (maxima) strong interaction with the gain region (multiple QWs), resonant gain enhancement Field nodes (minima) weak interaction with the medium (regions with high optical loss)

10 Transverse current and optical confinement GaAs-based VCSELs (67 nm) p-contact p-semiconductor DBR fundamental mode LP LP LP active region oxide aperture n-semiconductor DBR LP LP3 LP z n-substrate d ox = 6 µm y n eff n eff d ox n eff n-contact The number of transverse modes depends on the aperture size and the effective index difference Multiple transverse modes multiple emission wavelengths (multimode VCSEL) Single transverse mode (LP) single emission wavelength (single mode VCSEL) Transverse optical confinement factor: trans The transverse confinement factors are large (>95%) Ey everywhere Ey within aperture x, y dxdy x, y dxdy Multimode VCSEL

11 Transverse current and optical confinement (cont d) InP-based VCSELs (3 nm), GaSb-based VCSELs ( 35 nm) buried tunnel junction (n + /p + ) active region dielectric DBR blocking pn-junction n-contact (intra-cavity) n-type material p-type material n-semiconductor DBR n-substrate n-contact n eff n eff d BTJ n eff

12 power voltage Power (mw), Voltage (V) Threshold current, slope efficiency and power efficiency From optical resonator simulations: Transmission loss through top DBR ( tr,top ) Transmission loss through bottom DBR ( tr,bottom ) Absorption loss in DBRs ( abs ) Material gain at threshold (g th ) free carrier absorption gain balances loss Threshold carrier density (QWs): g n gth / g g ln nth ntr e n tr defect recombination spontaneous emission Auger recombination Spontaneous recombination rate at threshold: R sp 3 n An B n C n th th th th (no stimulated emission) Threshold current (injection balances recombination): ii qv th act R n act I R n sp th th qv i sp th where V act N QW L QW d 4 External differential quantum efficiency: Slope efficiency (W/A): P SE I hc q d i i tr,top tr,top tr,top tr,bottom tr,top tr,bottom abs abs P d ox = µm Power efficiency: p P VI i = internal quantum efficiency N QW = number of QWs L QW = QW thickness d = aperture diameter V I th I P I 5 5 Current (ma) current

13 Threshold current (ma) Detuning and temperature dependence Emission wavelength set by the resonance wavelength Resonance wavelength and gain spectrum redshift with temperature Gain peak shifts ~3 times faster than resonance Small temperature variation of the threshold current with proper detuning T = RT gain spectrum resonance.5 detuning.5 T > T T 3 > T Temperature (C)

14 VCSEL dynamics damping Intrinsic small signal modulation response: Z parasitics pad chip intrinsic laser H int ( f ) p( f i a f r resonance ) const frequency f r ; fr f f j damping rate v s ip C p R m i c Cm i a p Resonance frequency and D-factor: f D I r I th ; D v i qv a g ( g / n ) Optical confinement () Differential gain (g/n) Carrier transport () Photon lifetime ( p ) Damping rate and K-factor: Active volume (V a ) Gain compression () r K f ; K 4 p v g ( g / n ) I I I 3 Damping limited modulation banwidth: f 3dB, damping K I < I < I 3 < I 4 I 4

15 Modulation response [db] VCSEL dynamics thermal effects and parasitics Increasing current increasing temperature (self heating) saturation of photon density and resonance frequency thermally limited bandwidth f r D(T ) I I th (T ) ; D v i qv a g ( g / n ) f r,max f r,max v g g S n p max Major heat sources: DBR resistance Free-carrier absorption Increasing frequency modulation current shunted past active region parasitics limited bandwidth H( f ) p( f i a ) i v a s const f r f f r f j j f f p 5 with thermal effects intrinsic response Major parasitics: intrinsic response parastics DBR resistance Capacitance over oxide layer (GaAs) or reverse biased pn-junction (InP) with thermal effects and parasitics Frequency [GHz]

16 output power VCSEL dynamics large signal modulation Digital modulation: biasing above threshold + current pulses ( and ) bias current modulated output power rise time ringing fall time 9% on-level (binary ) optical pulse ringing % off-level (binary ) I th current current pulse bias current modulation current time Eye-diagram VCSEL operating at Gbit/s

17 Lane rate (Gbit/s) VCSEL applications datacom Lane rate up to 5-8 Gbit/s (Ethernet, Fiber Channel) Aggregate capacity up to 4 Gbit/s (6 x 5 Gbit/s) in parallel fibers Reach typically - m Multimode optical fiber (5 µm core) Wavelength: 85 nm (GaAs-based VCSELs) Facebook Datacenters Ethernet Fiber Channel Infiniband Corning Pluggable tranceiver Consumer electronics Active optical cable IBM High performance computing Mid-board optical module TE Connectivity TE Connectivity TE Connectivity. 5 5 Year rack - rack shelve - shelve Multimode optical fiber board - board module - module Polymer optical waveguide Distance (m)

18 VCSEL applications sensing Major applications: - Optical navigation (computer mouse, finger navigation modules, guesture recognition, ) - Spectroscopy (e.g. gas sensing and analysis) - Optical encoding (digital encoding of e.g. linear and rotary positions) - Many sensing applications require spectral purity, coherence and/or focusing capabilites single-mode and polarization stable VCSELs It is expected that a smartphone will contain up to ~ VCSELs!

19 VCSEL applications high power D integration of s of VCSELs enables > W low brightness VCSEL arrays Beam profile of a W VCSEL array Major applications: - Surface treatment (heating) - Illumination (night vision) - Laser pumping Princeton Optronics VCSEL array top view 8 W VCSEL array D array of VCSEL arrays Philips Photonics 4 W VCSEL emitter Lasertel 9.6 kw VCSEL module

20 Lane rate (Gbit/s) Optical interconnects developments Higher capacity and bandwidth density (SDM, WDM) - Lane rates: 5-8 Gbit/s 5, 56, 64, Gbit/s - Aggregate capacity: 4 Gbit/s (6 x 5 Gbit/s) multi-tbit/s Datacenters Higher efficiency - s of pj/bit pj/bit High operating temperature (85 C, at least) Longer and shorter reach - Datacenters are growing in size (~ 3 m) - Migration closer to the onboard circuits (~ -3 m) Corning Facebook Pluggable tranceiver Consumer electronics Active optical cable IBM High performance computing Mid-board optical module Ethernet Fiber Channel Infiniband TE Connectivity TE Connectivity VCSEL-based optical interconnects TE Connectivity rack - rack shelve - shelve Multimode optical fiber. 5 5 Year board - board module - module Distance (m) Polymer optical waveguide

21 Optical interconnects developments (cont d) Higher speed optoelectronics and electronics + electronic compensation + multilevel modulation higher lane rates Spatial and wavelength division multiplexing (SDM, WDM) higher aggregate capacity and higher bandwidth density Single fiber Single core Single wavelength ~ Gbit/s IBM VI Systems Multiple fibers Single core Single wavelength ~ Tbit/s TE Connectivity Multiple fibers Multiple cores Single wavelength ~ Tbit/s Multiple fibers Single core Multiple wavelengths 3 4 Multiple fibers Multiple cores Multiple wavelengths ~ Tbit/s

22 High speed 85 nm VCSELs Speed optimized with respect to damping, thermal effects and parasitics Modulation bandwidth = 8 5 C and 85 C Data rates up to 47 (57) Gbit/s with limiting (linear) optical receiver short cavity, reduced photon lifetime (reduced damping) low resistance, low optical loss DBRs strained InGaAs/AlGaAs QWs BCB multiple oxide layers high thermal conductance n-dbr undoped GaAs substrate 4 µm 7 µm 4 µm 7 µm 4G 5C 4G 85 C 47G 5C

23 Impact of damping on VCSEL dynamics D v i qv a g ( g / n ) Modulation bandwidth: trade-off between resonance frequency and damping rate Eye quality and BER: trade-off between rise/fall times, modulation efficiency and jitter K 4 p v g ( g / n ) K =.35 ns K =.3 ns K =. ns K =.4 ns 6.4 ps 5.3 ps 3.3 ps. ps. ps K =.4 ns K =.3 ns K =. ns K =. ns 3.3 ps 5.3 ps Gbit/s 6.4 ps 4 Gbit/s = best receiver sensitivity Highest bandwidth (K =. ns)

24 log (BER) Transmitter integration equalization Driver and receiver circuits with two-tap feed forward equalization (FFE) IBM SiGe BiCMOS 8HP (3 nm) 6 GHz VCSEL (Chalmers), and 3+ GHz photodiode (Sumitomo) Error-free transmission over MMF up to 7 Gbit/s at 5 C and 5 Gbit/s at 9 C Driver VCSEL 56 Gbit/s 6 Gbit/s 64 Gbit/s Gbit/s, 7 m 5G 3G 4G 5G 6G 64G 68G 7G Gbit/s, 7 m 6 Gbit/s, 7 m 64 Gbit/s, 57 m OMA (dbm) 7 Gbit/s, 7 m

25 Onboard polymer waveguide interconnect Polymer waveguide embedded in backplanes and circuit boards to interconnect boards and modules 5 GHz high power/high slope efficiency VCSEL 4 Gbit/s NRZ and 56 Gbit/s PAM-4 transmission over m polymer waveguide Record speed-distance products (4 and 56 Gbit/s m).4 85 nm 5 Gbit/s 36 Gbit/s 4 Gbit/s 56 Gbit/s PAM-4 Back-to-back Waveguide

26 Output power (mw) Relative intensity (db) Voltage (V) Modulation response (db) log(ber) Speed and efficiency enhancement by strong confinement Half-wavelength optical resonator, oxide apertures close to and on either side of the active region Modulation bandwidth = 3 GHz Transmission at 5-5 Gbit/s with energy dissipation < fj/bit short cavity, reduced photon lifetime (reduced damping) low resistance, low optical loss DBRs strained InGaAs/AlGaAs QWs BCB multiple oxide layers high thermal conductance n-dbr µm.5.5 ma Wavelength (nm) Bias current (ma) 3 undoped GaAs substrate µm ma -6.7 ma ma.5 ma Frequency (GHz) Gbit/s.3 ma 85 fj/bit 4 Gbit/s.8 ma 78 fj/bit 5 Gbit/s.5 ma 95 fj/bit Received optical power (dbm)

27 Transverse and polarization mode control Surface micro/nano structures for transverse and polarization mode control Multi-mW single mode output power db orthogonal polarization suppression Oxide aperture 6 µm Mode filter 3 µm Sub-wavelength surface grating nm grating period

28 High speed VCSEL for extended reach Integrated mode filter for single mode emission Reduced impact of chromatic dispersion with mode filter Extended reach: 5 Gbit/s over 3 m MMF, Gbit/s over m MMF RMS =.5 nm VCSEL with integrated mode filter Oxide aperture 6 µm Mode filter 3 µm 9 GHz bandwidth without mode filter RMS =.8 nm 6 Gbit/s, m (6 Gbit/s km) 5 Gbit/s, 3 m (3.5 Gbit/s km) Gbit/s, m (4 Gbit/s km)

29 Multilevel modulation over VCSEL-MMF link Increased capacity through improved spectral efficiency Pulse amplitude modulation (PAM) low complexity, low power consumption, good receiver sensitivity PAM-n modulation (n levels, log (n) bits/level) 3 Gbaud PAM-4 (6 Gbit/s) transmission over a GHz link (no equalization, no FEC) 4 Gbaud PAM-4 (8 Gbit/s ) with equalization and FEC (7 Gbit/s) Gbaud PAM-8 (6 Gbit/s) with FEC (56 Gbit/s) 3 Gbaud PAM-4 (6 Gbit/s) 4 Gbaud PAM-4 (8 Gbit/s) Gbaud PAM-8 (6 Gbit/s) PAM signal generator VCSEL MMF VOA GHz optical receiver error analyzer Back-to-back 5/ m OM4 fiber

30 VCSEL arrays for multicore fiber interconnects 6 µm Multicore, multimode optical fiber for increased capacity and bandwidth density 6-channel circular VCSEL array 4 Gbit/s/channel up to 8 C, 4 Gbit/s aggregate capacity No crosstalk penalty 39 µm Power (mw) Ch Ch Ch 3 Ch 4 Ch 5 Ch 6 5 C 85 C 5 C Voltage (V) Ch Ch 4 Gbit/s, 5 C 4 Gbit/s, 85 C 85 C Ch Current (ma) Ch 4 Ch5 5 C 85 C Ch6 Ch4 5 C 85 C Ch 5 Ch Ch Ch3 Ch 6

31 Integrated transmitters for WDM optical interconnects Integration platform - Short wavelength compatible (high speed and high efficiency GaAs-based optoelectronics) - Wavelength multiplexing, drive electronics Monolithic multi-wavelength VCSEL array - Wavelength setting in a post-growth process - Hybrid (flip-chip) or heterogeneous integration - In-plane emission multi-wavelength VCSEL array 4 3 wavelength multiplexer,, 3, 4 multi-channel driver silicon nitride waveguide PIC Heterogeneous integration: half III-V VCSEL bonding interface silicon silicon SiN HCG and waveguide on Si dielectric DBR and SiN grating/waveguide on Si

32 Multi-wavelength high-contrast grating VCSEL array High contrast grating (HCG) for wavelength setting in a planar post-growth fabrication process High, broadband and mode/polarization selective reflectivity Phase of reflection (resonance wavelength) controlled by grating parameters (duty cycle, period) 4 channels with ~5 nm channel spacing at 98 nm Single transverse and polarization mode HCG (GaAs) mirror active (gain) region distributed Bragg reflector (DBR) oxide apertures 3 4

33 Power (mw) Voltage (V) log (BER) Si-integrated short wavelength VCSEL with a hybrid cavity GaAs-based half-vcsel attached to a dielectric DBR on Si using adhesive BCB bonding Hybrid III-V/Si cavity Sub-mA threshold current, high slope efficiency Gbit/s modulation n-contact p-contact half III-V VCSEL oxide aperture bonding interface AlGaAs DBR dielectric DBR on Si Si substrate SiO /Ta O 5 DBR Si substrate bonding interface Wavelength (nm) µm 5G G G 5G 3 µm 5 µm 7 µm 9 µm Current (ma) -8 G - G Received optical power (dbm)