Characterization techniques for high brightness laser diodes

Characterization techniques for high brightness laser diodes Ignacio Esquivias, José Manuel García Tijero, Helena Odriozola, Luis Borruel E.T.S.I.Telecomunicación, Univ. Politécnica de Madrid, Spain IN COLLABORATION WITH Nicolas Michel, Alcatel-Thales III-V Lab Bernd Sumpf, Ferdinand-Braun-Institut für Höchstfrequenztechnik Acknowledgements Julia Arias (Universidad Miguel Hernández, Elche, Spain ) 1 Scope and Goals of e tutorial 1. Review of basic measurement techniques. Extraction of device and material parameters 3. Analysis of validity of extracted parameters 4. Analysis of physical origin of main parameters

Outline Power-Current-Voltage measurements CW and pulsed measurement set-ups Parameter extraction Cavity leng dependence Temperature dependence Spectral measurements Thermal resistance measurements Beam measurements 3 Power-Current-Voltage OPTICAL POWER (W) 3..5. 1.5 1..5 BA Laser 9 nm mm x 1 µm.. 1 3 4 5 6 CURRENT (A) 3..5. 1.5 1..5 VOLTAGE (V) Parameters Threshold current I Slope efficiency η slope Series resistance R s Diode voltage V Wall-plug efficiency 4

LASER DIODE: simplified equations (I) x z y active region metal contact p- material n- material L W Threshold condition: gain losses g Γ gmat (n ) αin + α m αin + 1 L Ln 1 ( ) R 5 LASER DIODE: simplified equations (II) P η ( I I ) ; I > out I V act slope q R( n ) I carrierdensity n η I V q ( A n + B n + C n slope η in act hν α m q α m + αin Main assumptions: 3 ) output I power Homogeneity of carriers and photons Isoermal conditions No gain saturation Perfect carrier clamping at reshold I 6

CW P-I-V: Experimental set-up Current Source / Voltage meter LD Laser Holder Photodiode + Current meter (or Power meter) Integrating sphere 7 CW P-I-V: Power measurements LD Large Area Photodetector LD Small Area Photodiode Heat-sink I PD Heat-sink I PD Direct coupling Lens coupling LD Small Area Photodiode Heat-sink I PD Integrating Sphere 8

PHOTODIODES Photodetector Types THERMAL (ermopiles, pyroelectric, bolometers) High responsivity and low noise Fast response Waveleng dependent Low saturation power: use attenuators or integrating sphere Flat waveleng response (almost) High saturation power Poor sensitivity Slow response 9 PULSED P-I-V: example experimental set-up Why Pulsed P-I-V? AVOID SELF-HEATING CH1 Current Probe 5 Ω R S bias T CH 5 Ω Pulsed Current Source LD PD V CC OSCILLOSCOPE 1

PULSED P-I-V: Current Waveform Matching Impedance 5 Ω Line Low ImpedanceLine Resistor R S Pulsed Voltage Source 5 Ω 5 Ω { LD Pulsed Current Source LD i(t) PULSED VOLTAGE DRIVE PULSED CURRENT DRIVE R S 4 Ω R S 5 Ω R S 1 Ω 11 PULSED P-I-V: Power measurement Power Time P peak < P > Integrating P(t): P peak <P> T/τ Current Averaging wiin a window Digital Oscilloscope Box car integrator Power Window 1

PULSED P-I-V: Measurement conditions T QW t 1 exp τ P dis R C T HS τ R C 1-1µs Measurement conditions to avoid self-heating: Pulse wid τ ON << τ. (Typical. -.5 µs) Temperature rise Time t exp τ Pulse period T >> τ Duty cycle τ ON / T.1-1% 13 Threshold Current and Slope Efficiency Optical Power (W) dp/di and d P/dI (a.u.) 5.E- 5.E- 4.E- 4.E- 3.E- 3.E-.E-.E- 1.E- 5.E-3.E+.E+.1.15..5.3.35.4 Current (A) d P/dI 5% I dp/di.1.15..5 Current (A).3.35.4 P η slope ( I I ( I < I ( I > I Parameter extraction Linear fit I > I Double Slope Fit First derivative (5% max.) Second derivative (max.) DIFFERENCES NOT IMPORTANT ) ) ) 14

Series Resistance (I) VOLTAGE (W).5. 1.5 1..5 BA Laser 9 nm mm x 1 µm R s 66 mω V 1.55 V EXPERIMENTAL LINEAR FIT LINEAR FIT (I > I ) V V + I R S ALTERNATIVE OPTION: IdV/dI vs I linear fit. 1 3 4 5 6 CURRENT (A) CW: V DECREASES WITH CURRENT!!! (INTERNAL TEMPERATURE) 15 Series Resistance (II) IdV/dI (V).4.35.3.5..15.1.5 R S.95 Ω m 1.6 R S.97 Ω R S.94 Ω...5.1.15..5.3 I CURRENT (A) BA Laser 88 nm 3 x 1 µm. 1.5 1..5. VOLTAGE (V) V DERIVATIVE FIT mkt q Ln I I + I R S dv mkt I + I R ; I < di q dv I I R ; I > di S I S I 16

Wall Plug Efficiency OPTICAL POWER (W) 3..5. 1.5 1..5. 1 3 4 5 6 CURRENT (A) BA Laser 9 nm mm x 1 µm 6 5 4 3 1 WALL PLUG EFF. (%) WPE P IV η WPE out ext E ph / q ( 1 I / I ) ( V + I R ) MAIN LOSSES η ext < 1 V > E ph /q Series resistance Threshold current S 17 Cavity leng dependence of J (I) J I L W Threshold Current Density (A/cm ) 5 45 4 35 3 5 15 EXPERIMENTAL 5 1 15 BA 5 lasers 3 35 4 Inverse Cavity Lenght 88 (cm nm -1 ) How to define W? BA: W Stripe Wid BA: W Stripe Wid + 1 µm? RW: W Ridge Wid + 1 µm? (large error, non-unifomity) Tapered laser: 1 < W > +? L L { W ( z) 1 µ m } dz 18

Threshold Current Density (A/cm ) 3 15 Cavity leng dependence of J (II) Plot in Log. scale 45 EXPERIMENTAL LOG. FIT CHARACTERIZATION OF EPI-LAYER Measure I for different L Plot J vs 1/L J o 148 A/cm Γ G o 8 cm -1 5 1 15 5 3 35 4 Inverse Cavity Lenght (cm -1 ) g mat ( n) G 1 Ln 1 R Ln( J ) Ln( J ) + L Γ G J J ( L ) n Ln( n J trans ) αin + Γ G 19 Cavity leng dependence of J (III) Plot in Lin. scale Threshold Current Density (A/cm ) 5 45 4 35 3 5 15 EXPERIMENTAL LINEAR FIT J o 116 A/cm 5 1 15 5 3 35 4 Inverse Cavity Lenght (cm -1 ) J J J g mat J ( L ) dg ( n) ( n n ) dn o 1 Ln 1 R qdact + L Γ dg / dn τ J trans n αin + Γ dg / dn qd τ act n

Cavity leng dependence of J (IV) Linear or log plot? Threshold Current Density (A/cm ) 5 45 4 35 3 5 15 1 EXPERIMENTAL LINEAR FIT LOG. FIT 5 1 15 5 3 35 4 Inverse Cavity Lenght (cm -1 ) Lin: J o 116 A/cm Log: J o 148 A/cm LOG. scale fit: SQW, low Γ LIN. scale fit: MQW, high Γ DO NOT EXTRACT CONCLUSIONS FROM J, JUST COMPARE EPI- MATERIALS 1 Cavity leng dependence of slope efficiency (I) Measure Slope Efficiency for different L Plot η -1 ext vs L Extract Internal Efficiency η in and Internal Losses α in from linear fit Inverse External Efficiency.4.. 1.8 1.6 η in.95 α in 5.1 cm -1 1.4 EXPERIMENTAL 1. LINEAR FIT 1...5.1.15..5 Cavity Leng (cm) η η ( η + η ) ext slope1 slope 1 1 ext ηin α + in L 1 Ln q hυ ( 1/ R)

Cavity leng dependence of slope efficiency (II) Internal Efficiency and Internal Losses 1 ) Extremely dependent on number of devices and device lengs Inverse External Efficiency.4.. 1.8 1.6 η in.95 α in 5.1 cm -1 31 devices 1.4 EXPERIMENTAL 1. LINEAR FIT 1...5.1.15..5 Cavity Leng (cm) Inverse External Efficiency.4.. 1.8 1.6 Removing devices η in.9 α in 4.4 cm -1 1.4 EXPERIMENTAL 1. LINEAR FIT 1...5.1.15..5 Cavity Leng (cm) 3 Cavity leng dependence of slope efficiency (III) η 1 ext [ η ( n )] 1 in Internal Efficiency and Internal Losses ) Based on many simple assumptions α + in 1 Ln ( n ) L ( ) 1/ R α in α + α α + σ scat fc scat fc n η in and α in depend on carrier density and erefore on L L n α fc L n η in 3) In CW, additional temperature effects DO NOT TRUST α in and η in!!!!, just indicative 4

Temperature dependence: T and T 1 Optical Power (W).1.8.6.4.. 15 ºC 5 ºC 35 ºC 45 ºC 55 ºC 65 ºC 75 ºC..4 Current (A) I EMPIRICAL EXPRESSIONS T exp T ( T ) I T ( T ) η slope exp T η slope 1 5 Temperature dependence: T 14 BA laser 915 nm To (K) 13 1 11 1 9 8 BA lasers 88 nm.5.1.15..5 CAVITY LENGTH (cm) DEPENDENCE ON TEMPERATURE RANGE LARGE DISPERSION 6

T : Physical origin MAIN EFFECTS: T n (gain dependence on T) n R (n ) (increased recombination) Typical: Auger recomb. T o g ( T ) Γ gmat[n ( T )] α ( T ) + α in m I 3 ( T ) V q [ A ( T ) n ( T ) + B( T ) n ( T ) C( T ) n ( T ) ] act + ATENTION : Poor quality laser: high reshold (SRH recomb. or leakage current), but high T o 7 T 1 : Physical origin η slope ( T ) η ( T) in hν q αm α m + α ( T ) in ηext (W/A).55.53.51.49.47.45 BA lasers 88 nm e3 1 3 4 5 6 7 8 T (C) MAIN EFFECTS: T η in (Increased leakage) T α in (Increased freecarrier absorption) 8

SPECTRAL MEASUREMENTS EMISSION SPECTRA OF FP LASERS Gain cavity losses longitudinal modes carrier density Waveleng (µm) lasing mode 3-4 nm kl mπ δλ λ Ln eff λ 1 µm, L 1 mm, δλ.3 nm Poor modal discrimination Many modes excited 9 SPECTRAL PARAMETERS LOG. PLOT LINEAR PLOT OPTICAL POWER (dbm) - -4-6 17 18 19 13 131 13 OPTICAL POWER (a.u.).6.5.4.3..1. 19 13 WAVELENGHT (nm) WAVELENGTH (nm) Peak Waveleng: λ p Spectral wid: σ λ FWHM of spectral envelope 3

SPECTRA OF HIGH POWER LASERS Intensity (au) Intensity (au) I 7 ma 7 ma Intensity (au) 964 965 966 967 968 969 97 967. 967.5 968. Waveleng (nm) Waveleng (nm) I ma I ma ma 4 ma Intensity (au) I 1 1 ma Changes in spectra wi current (lateral modes) Dependence of spectra on lateral position Typical spectral wid: 1-3 nm 965.5 966. 966.5 967. Waveleng (nm) 968 97 97 Waveleng (nm) Index Guided Tapered Laser 975 nm nm 31 INSTRUMENTATION FOR SPECTRAL MEASUREMENTS Microscope Objective LD slit Grating Monochromator PD Grating Monochromator: Difficult coupling Very high spectral resolution (i.e..75 m; 1 lines/mm.3 nm @ 8 nm) LD Heat-sink Integrating Sphere Optical Spectrum Analyzer FO input Very good dynamic range Typical resolution:.1-.5 nm Optical Fiber Optical Spectrum Analizer 3

DEPENDENCE OF LASING PEAK ON TEMPERATURE Peak Waveleng (nm) 85 8 815 81 85 8 795 L.3 mm L.6 mm L.1 mm.9 nm/k.6 nm/k.9 nm/k 4 6 8 Temperature (ºC) Pulsed Measurements BA lasers 88 nm Red shift: band gap vs T Blue shift: band filling Typical: ~.3 nm/k 33 DEPENDENCE OF LASING PEAK ON CURRENT 4 ma 35 ma 3 ma 5 ma PULSED: Negligible dependence CW: Temperature dependence (red shift) I T E g λ p Mode hopping ma 16 ma I 14 ma 34

MEASUREMENT OF THERMAL RESISTANCE I CW Heat-sink T QW R T P dis ( T T ) QW HS ( VI Pout ) T HS MEASUREMENT PROCEDURE 1. Measure λ p vs T HS in pulsed conditions at fixed I. Measure P-I-V and λ p vs I (CW) 3. Calculate T QW from λ p (CW) 4. Calculate R from T QW and P-I-V 35 Electro-optical characterisation - Scheme Temperature Controller 15 C T 75 C Accuracy T < ±.5 K Test Chamber Wi Minibar Calibrated Integrating Sphere Fibre Current Controller Profile LDC 365 I < 65 A Accuracy I < ± 1 ma Det. Power Meter Spectrum Analyser λ.1 nm Data Acquisition Desktop - PC 36

Outline Power-Current-Voltage measurements Spectral measurements Thermal resistance measurements Beam measurements Beam propagation and beam parameters Far-field measurements Near field measurements M measurements 37 Beam Propagation IDEAL GAUSSIAN BEAM Waist d W W(z) θ θ hw z W(z) W λ(z z) 1 π(w ) + beam spot size z W(z) W λz π θ hw W λ π θ d 4λ π DIFRACTION LIMIT 38

Beam Propagation ARBITRARY BEAM How to define e wid? How to characterize e propagation? 39 Beam Propagation ARBITRARY BEAM Waist d w W(z) θ θ hw z W x (z) W x M xλ(z z 1+ π(wx ) x ) ONLY VALID IF W x (z) AND W y (z) ARE DEFINED AS SECOND MOMENT WIDTHS W (z) y W y M yλ(z z 1+ π(wy) y ) W x σ x dσ x 4 σ x W y d y σ y σ 4 σ y 4

Beam Propagation en, in bo x and y directions z W(z) W M λz π π W (z) W λ z M M : - BEAM PROPAGATION FACTOR (Prof. Siegman) - BEAM PROPAGATION RATIO (ISO 11146:5) - TIMES-DIFRACTION-LIMIT-FACTOR (ISO 11146:1999) BUT NOT: BEAM QUALITY FACTOR (Prof. Siegman) θ hw W M λ π θ d σ M 4λ π 41 Second Moment beam wid Intensity distribution: E ( x, y, z) Beam wid: d σx ( z) 4 σ ( z) x First Moment; i.e. Mean Value x x E ( x, y, z) dx dy E ( x, y, z) dx dy Second Moments; i.e. Standard Deviation σ x σ x ( z) ( x x) E ( x, y, z) dx dy E ( x, y, z) dx dy 4

Why M? d θ Beam Product Parameter BPP 4 σ M Invariant in geometrical optics λ π Brightness B P[W] A [cm²] Ω[srad] P λ M M B x y Power and M define e Brightness of a source 43 Gaussian beam: M (1/e ) d (z) d ( z) 4 ( z) 1/e σx σ x wi d e full wid at 1/e 1/e Arbitrary beam: we could define M (1/e ) M (1/ e ) θ d π 1/ e 1/ e BUT THEN THE PROPAGATION DO NOT FOLLOW THE HYPERBOLIC LAW 4λ d 1/e (z) d 1/e M λ(z z 1/e 1+ π(d / ) 1/e ) 44

Spatial emission of laser diodes Fast axis (y) Near Field Laser diode W x θ y Optical cavity θ x Slow axis Far-field Fast axis (y): z oy facet; M y 1; θ y Slow axis (x): z ox ; M x; w x (θ x ) 45 LD Divergent beam FF measurements θ Rotating Photodiode z a n g l e. - -1 1 θ1/e² Power..4.6.8 1. θ 1/ θ ALTERNATIVES Rotating LD Using lenses and measuring beam profile 46

Example of fast axis FF I (u.a.) 1,,5 Depend on vertical waveguide structure Fourier transform of transverse mode profile, -6-3 3 6 Angle ( ) 47 Microscope Objective NF measurements z Laser diode CCD camera Beam analysis software ALTERNATIVES Moving slit Moving pin-hole Near field Scanning (Fiber tip) 48

Example of slow axis FF and NF BA; 1 W RW;.3 W Tapered laser;.6 W 1, 1, 1,,8,8,8 FF I (u.a.),6,4, // I (u.a.),6,4, // Intensité (u.a.),6,4,, -15-1 -5 5 1 15 1,,8 Angle ( ), -5 - -15-1 -5 5 1 15 5 1, // angle ( ), -1-5 5 1 Angle ( ) 1,,8 NF I (u.a.),6,4 // I (u.a.),5 Intensité (u.a.),6,4,,, 4 8 1 16 4 x(µm), 8 3 36 4 44 x (µm), 5 75 1 15 15 175 x (µm) 49 M Measurements: ISO11146:5 The test is based on e measurement of e cross-sectional power density distribution at a number of axial locations along e beam propagation axis The second moment beam wids d σx (z) and d σy (z) are determined Hyperbolic fit of d σ (z) to: d (z) a + bz + cz σ d σ (z) b z a beam waist location z z d 1 c σ 4 ac b beam wid at waist d σ M π 8 λ 4ac b 5

Laser diodes: additional lens M Measurements: ISO11146:5 w' At least 1 different z positions shall be taken (half of em beyond two Rayleigh lengs) Background correction procedures shall be applied to determine e beam wids Alternative meods for beam wid measurements (ISO11146-3: 4): Variable aperture meod Moving knife meod Moving slit meod 51 Measurement principles Knife edge Knife edge moved rough e beam profile Analysis: e.g. beam dimensions from 16 % and 84 % of e intensity integrals σ Gauß knife edge detector 1..8 84.1 % intensity / a.u..6.4.. 15.9 % e -(x/σ Gauß ) 13.5 % edge translation -3 - -1 1 3 position x / σ Gauß 5

Measurement principles Moving slit Moving slit moved rough e beam profile Analysis: e.g. beam dimensions from 13.5 % of e intensity profiles slit detector Moving slit: slit translation 53 Meod of e moving slit FBH set-up f1 Near field wlaser dmess f L1 L Slit LD f 1 f 1 +f f PD resolution: w µm uncertainty: ca. 6 % Far field L1 Θ Laser f f 1 d f mess 3 Slit resolution: LD f 1 f 1 +f f 3 PD θ.8 uncertainty: ca. 9% 54

Meod of e moving slit FBH set-up Set-up x-y-ztranslation stage.3µm LD Slit µm PD Step wid 3 µm 9 µm 3 points I(t) 5 A t Pulse 1 ms L1 Near field: L f f 1 41 Boxcar Integrator Far field: f f 1.4 PC 55 Typical beam profiles (FBH) near field (facet) beam waist far field intensity / a.u. intensity / a.u. intensity / a.u. - -1 1 position x / µm -3 - -1 1 3 position x / µm - -1 1 angle θ / Tapered laser λ 88 nm, wids beam waist / µm far field / M L.75 mm, 1/e 5.9 13.6 1.3 L RW 1 µm, R f.1%, T 5 C, P W. mom. 6.5 16.3 7.3 56

Example of measurement of astigmatism Fast axis ( ) Half-wid at 1/e (µm) 5 4 3 1 Slow axis (//) -5-5 5 5 Position of e lens (µm) Position of e lens for waist at 1/e in e slow axis: x // in e fast axis: x Astigmatim x // - x Waist in e slow axis Waist in e fast axis 57 Some Reference Material Professor Anony E. Siegman Web Pages http://www.stanford.edu/%7esiegman/ Laser beam quality tutorial An annotated bibliography of references on e definition and measurement of "laser beam quality" and e "M-squared" parameter. MELLES GRIOT, tecnical documents (http://www.mellesgriot.com/) LABSPHERE.Technical Document Library. (http://www.labsphere.com/tecdocs.aspx) NEWPORT (http://www.newport.com/)application Notes, Technical Notes ILX Lightwave.Application Notes, Technical Notes, And White Papers. http://www.ilxlightwave.com/navpgs/app-tech-notes-white-papers.html International Engeeniering Consortium. Tutorials. http://www.iec.org/online/tutorials/ AVTECH Application Notes (Pulsed measurements). http://www.avtechpulse.com/appnote/ Encyclopedia of Laser Physics and Technology (VIRTUAL LIBRARY). http://www.rp-photonics.com/encyclopedia.html 58