Characterization techniques for high brightness laser diodes

Transcription

1 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

2 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) BA Laser 9 nm mm x 1 µm CURRENT (A) VOLTAGE (V) Parameters Threshold current I Slope efficiency η slope Series resistance R s Diode voltage V Wall-plug efficiency 4

3 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

4 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

5 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

6 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

7 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 Current (A) d P/dI 5% I dp/di Current (A) 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

8 Series Resistance (I) VOLTAGE (W) 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 CURRENT (A) CW: V DECREASES WITH CURRENT!!! (INTERNAL TEMPERATURE) 15 Series Resistance (II) IdV/dI (V) R S.95 Ω m 1.6 R S.97 Ω R S.94 Ω I CURRENT (A) BA Laser 88 nm 3 x 1 µm 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

9 Wall Plug Efficiency OPTICAL POWER (W) CURRENT (A) BA Laser 9 nm mm x 1 µm 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 ) EXPERIMENTAL BA 5 lasers 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

10 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 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 ) EXPERIMENTAL LINEAR FIT J o 116 A/cm 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

11 Cavity leng dependence of J (IV) Linear or log plot? Threshold Current Density (A/cm ) EXPERIMENTAL LINEAR FIT LOG. FIT 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 η in.95 α in 5.1 cm EXPERIMENTAL 1. LINEAR FIT Cavity Leng (cm) η η ( η + η ) ext slope1 slope 1 1 ext ηin α + in L 1 Ln q hυ ( 1/ R)

12 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 η in.95 α in 5.1 cm devices 1.4 EXPERIMENTAL 1. LINEAR FIT Cavity Leng (cm) Inverse External Efficiency Removing devices η in.9 α in 4.4 cm EXPERIMENTAL 1. LINEAR FIT 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

13 Temperature dependence: T and T 1 Optical Power (W) 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) BA lasers 88 nm CAVITY LENGTH (cm) DEPENDENCE ON TEMPERATURE RANGE LARGE DISPERSION 6

14 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) BA lasers 88 nm e T (C) MAIN EFFECTS: T η in (Increased leakage) T α in (Increased freecarrier absorption) 8

15 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) OPTICAL POWER (a.u.) WAVELENGHT (nm) WAVELENGTH (nm) Peak Waveleng: λ p Spectral wid: σ λ FWHM of spectral envelope 3

16 SPECTRA OF HIGH POWER LASERS Intensity (au) Intensity (au) I 7 ma 7 ma Intensity (au) 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 Waveleng (nm) 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 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

17 DEPENDENCE OF LASING PEAK ON TEMPERATURE Peak Waveleng (nm) L.3 mm L.6 mm L.1 mm.9 nm/k.6 nm/k.9 nm/k 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

18 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

19 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

20 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

21 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

22 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

23 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 θ1/e² Power θ 1/ θ ALTERNATIVES Rotating LD Using lenses and measuring beam profile 46

24 Example of fast axis FF I (u.a.) 1,,5 Depend on vertical waveguide structure Fourier transform of transverse mode profile, 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

25 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,, ,,8 Angle ( ), , // angle ( ), Angle ( ) 1,,8 NF I (u.a.),6,4 // I (u.a.),5 Intensité (u.a.),6,4,,, x(µm), x (µm), 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

26 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 (ISO : 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 % intensity / a.u % e -(x/σ Gauß ) 13.5 % edge translation position x / σ Gauß 5

27 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

28 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 position x / µm position x / µm angle θ / Tapered laser λ 88 nm, wids beam waist / µm far field / M L.75 mm, 1/e L RW 1 µm, R f.1%, T 5 C, P W. mom

29 Example of measurement of astigmatism Fast axis ( ) Half-wid at 1/e (µm) Slow axis (//) 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 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 ( LABSPHERE.Technical Document Library. ( NEWPORT ( Notes, Technical Notes ILX Lightwave.Application Notes, Technical Notes, And White Papers. International Engeeniering Consortium. Tutorials. AVTECH Application Notes (Pulsed measurements). Encyclopedia of Laser Physics and Technology (VIRTUAL LIBRARY). 58