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 ) 

Universidad Politécnica Madrid 

1 

“Characterization techniques... 

Scope and Goals of the tutorial 

1. Review of basic measurement techniques 

2. Extraction of device and material parameters 

3. Analysis of validity of extracted parameters 

4. Analysis of physical origin of main parameters 


2 

“Characterization techniques...

Outline 

Power-Current-Voltage measurements 

‣ CW and pulsed measurement set-ups 

‣ Parameter extraction 

‣ Cavity length dependence 

‣ Temperature dependence 

Spectral measurements 

Thermal resistance measurements 

Beam measurements 


3 


Power-Current-Voltage 

OPTICAL POWER (W) 

3.0 

2.5 

2.0 

1.5 

1.0 

0.5 

BA Laser 

920 nm 

2 mm x 100 µm 

0.0 

0.0 

0 1 2 3 4 5 6 

CURRENT (A) 

3.0 

2.5 

2.0 

1.5 

1.0 

0.5 

VOLTAGE (V) 

Parameters 

‣ Threshold current I th 

‣ Slope efficiency η slope 

‣ Series resistance R s 

‣ Diode voltage V 0 

‣ 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 

gth = Γ gmat 

(n 

th 

) = αin 

+ α 

m 

= αin 

+ 

1 

L 

Ln 

1 

( ) 

R 


5 


η 

out 

I =V 

th 

th 

act 

act 

LASER DIODE: simplified equations (II) 

P = η ( I − I ) ; I > 

slope 

q 

R( n th 

) 

2 

I = V q ( A n + B n + C n 

slope 

= η 

in 

hν 

⎡ α 

m 

q 

⎢ 

⎣α 

m 

+ αin 

th 

th 

⎤ 

⎥ 

⎦ 

Main assumptions: 

th 

I 

th 

3 

th 

) 

carrierdensity 

n th 

output 

0 0 

I th 

power 

• Homogeneity of carriers and photons 

• Isothermal conditions 

• No gain saturation 

⇒ Perfect carrier clamping at threshold 

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 

(thermopiles, pyroelectric, bolometers) 

• High responsivity and low noise 

• Fast response 

• Wavelength dependent 

• Low saturation power: use 

attenuators or integrating sphere 

• Flat wavelength 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 

50 Ω 

R S 

bias T 

CH2 

50 Ω 

Pulsed Current 

Source 

LD 

PD 

V CC 

OSCILLOSCOPE 


10 


PULSED P-I-V: Current Waveform 

Matching Impedance 

50 Ω Line Low ImpedanceLine 

Resistor R S 

Pulsed Voltage 

Source 

50 Ω 

50 Ω 

{ 

LD 

Pulsed Current 

Source 

LD 

i(t) 

PULSED VOLTAGE 

DRIVE 

PULSED CURRENT 

DRIVE 

R S 

= 40 Ω R S 

= 50 Ω R S 

= 100 Ω 


11 


PULSED P-I-V: Power measurement 

Power 

0 

0 

Time 

P peak 

 

Integrating P(t): 

P peak = T/τ 

Current 

Averaging within a 

window 

• Digital Oscilloscope 

• Box car integrator 

Power 

Window 


12 


PULSED P-I-V: Measurement conditions 

⎧ ⎛ t 

⎨1 

− exp 

⎜ − 

⎩ ⎝ τ 

th 

⎞ 

⎟ 

⎠⎭ ⎬⎫ 

P dis 

T QW 

R th C th 

T HS 

τ th = R th C th ∼ 1-10µs 

Measurement conditions to avoid self-heating: 

• Pulse width τ ON > τ th 

• Duty cycle τ ON / T ∼ 0.01- 1% 


13 


Threshold Current and Slope Efficiency 

Optical Power (W) 

dP/dI and d 2 P/dI 2 (a.u.) 

5.E-02 

5.E-02 

4.E-02 

4.E-02 

3.E-02 

3.E-02 

2.E-02 

2.E-02 

1.E-02 

5.E-03 

0.E+00 

0.E+00 

0.1 0.15 0.2 0.25 0.3 0.35 0.4 

Current (A) 

d 2 P/dI 2 

50% 

I th 

dP/dI 

0.1 0.15 0.2 0.25 

Current (A) 

0.3 0.35 0.4 

0 

⎧≈ 

0 

P = ⎨ 

⎩η 

slope 

( I − I 

( I < I 

( I > I 

Parameter extraction 

• Linear fit I > I th 

• Double Slope Fit 

• First derivative (50% max.) 

• Second derivative (max.) 

th 

DIFFERENCES NOT 

IMPORTANT 

) 

th 

th 

) 

) 


14 


Series Resistance (I) 

VOLTAGE (W) 

2.5 

2.0 

1.5 

1.0 

0.5 

BA Laser 

920 nm 

2 mm x 100 µm 

R s 

= 66 mΩ 

V 0 

= 1.55 V 

EXPERIMENTAL 

LINEAR FIT 

LINEAR FIT (I > I th ) 

V = V 0 

+ I ⋅ 

R S 

ALTERNATIVE OPTION: 

IdV/dI vs I linear fit 

0.0 

0 1 2 3 4 5 6 

CURRENT (A) 

CW: V 0 

DECREASES WITH CURRENT!!! 

(INTERNAL TEMPERATURE) 


15 


Series Resistance (II) 

IdV/dI (V) 

0.40 

0.35 

0.30 

0.25 

0.20 

0.15 

0.10 

0.05 

R S 

= 0.95 Ω 

m = 1.6 

R S 

= 0.97 Ω 

R S 

= 0.94 Ω 

0.00 

0.00 0.05 0.10 0.15 0.20 0.25 0.30 

I th 

CURRENT (A) 

BA Laser 

808 nm 

300 x 100 µm 2 

2.0 

1.5 

1.0 

0.5 

0.0 

VOLTAGE (V) 

V 

= 

DERIVATIVE FIT 

mkT 

q 

⎛ 

Ln 

⎜ 

⎝ 

I 

I 

0 

⎞ 

⎟ + I ⋅ 

⎠ 

R S 

dV mkT 

I = + I ⋅ R ; I < 

dI q 

dV 

I = I ⋅ R ; I > 

dI 

S 

I th 

S 

I th 


16 


Wall Plug Efficiency 

OPTICAL POWER (W) 

3.0 

2.5 

2.0 

1.5 

1.0 

0.5 

0.0 

0 1 2 3 4 5 6 

CURRENT (A) 

BA Laser 

920 nm 

2 mm x 100 µm 

60 

50 

40 

30 

20 

10 

0 

WALL PLUG EFF. (%) 

WPE = 

P 

IV 

η 

WPE = 

out 

ext 

⋅ E 

ph 

/ q ⋅( 1− 

I / Ith 

) 

( V + I ⋅ R ) 

MAIN LOSSES 

• η ext < 1 

• V 0 > E ph /q 

• Series resistance 

• Threshold current 

0 

S 


17 


Cavity length dependence of Jth (I) 

J 

th 

= 

Ith 

L • W 

Threshold Current Density (A/cm 2 ) 

500 

450 

400 

350 

300 

250 

200 

150 

EXPERIMENTAL 

0 5 10 15 20 BA 25 lasers 30 35 40 

Inverse Cavity Lenght 808 (cm nm 

-1 ) 

How to define W? 

• BA: W ∼ Stripe Width 

• BA: W ∼ Stripe Width + 10 µm? 

• RW: W ∼ Ridge Width + 10 µm? 

(large error, non-unifomity) 

• Tapered laser: 

1 

< W > = ∫ + ? 

L L 

{ W ( z) 

10 µ m } 

dz 


18 



300 

150 

Cavity length dependence of J th (II) 

Plot in Log. scale 

450 EXPERIMENTAL 

LOG. FIT 

CHARACTERIZATION OF EPI-LAYER 

• Measure I th for different L 

• Plot J th vs 1/L 

J o = 148 A/cm 2 

Γ·G o = 82 cm -1 

0 5 10 15 20 25 30 35 40 

Inverse Cavity Lenght (cm -1 ) 

g mat 

( n) 

= G 

⎛ 1 ⎞ 

Ln ⎜ ⎟ 

1 R 

Ln( J th 

) = Ln( J ) + 

⎝ ⎠ 

0 

L Γ G 

J 

0 

= 

J 

th 

( L = ∞) 

0 

= 

n 

Ln( 

n 

J 

trans 

0 

) 

0 

αin 

+ 

Γ G 

0 


19 


Cavity length dependence of Jth (III) 


500 

450 

400 

350 

300 

250 

200 

150 

EXPERIMENTAL 

LINEAR FIT 

Plot in Lin. scale 

g mat 

J o = 116 A/cm 2 

0 5 10 15 20 25 30 35 40 


J 

0 

J 

th 

= J 

= J 

( L = ∞) 

dg 

( n) 

= ⋅( 

n − n0 

) 

dn 

o 

⎛ 1 ⎞ 

Ln ⎜ ⎟ 

1 R qdact 

+ 

⎝ ⎠ 

L Γ dg / dn τ 

= J 

trans 

n 

αin 

+ 

Γ dg / dn 

qd 

τ 

act 

n 


20 


Cavity length dependence of Jth (IV) 

Linear or log plot? 


500 

450 

400 

350 

300 

250 

200 

150 

100 

EXPERIMENTAL 

LINEAR FIT 

LOG. FIT 

0 5 10 15 20 25 30 35 40 


Lin: J o = 116 A/cm 2 

Log: J o = 148 A/cm 2 

•LOG. scale fit: SQW, low Γ 

•LIN. scale fit: MQW, high Γ 

DO NOT EXTRACT 

CONCLUSIONS FROM J 0 

, 

JUST COMPARE EPI- 

MATERIALS 


21 


Cavity length 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 

2.4 

2.2 

2.0 

1.8 

1.6 

η in 

= 0.95 

α in 

= 5.1 cm -1 

1.4 

EXPERIMENTAL 

1.2 

LINEAR FIT 

1.0 

0.00 0.05 0.10 0.15 0.20 0.25 

Cavity Length (cm) 

η 

η 

( η + η ) 

ext 

= 

slope1 

slope2 

−1 

−1 

ext = ηin 

⎢ 

⎡ α + in 

L 

1 

⎣ Ln 

q 

⋅ 

hυ 

⎤ 

( 1/ R) ⎥ ⎦ 


22 


Cavity length dependence of slope efficiency (II) 

Internal Efficiency and Internal Losses 

1 ) Extremely dependent on number of devices and device lengths 


31 devices 

Removing 2 devices 

2.4 

2.2 η in 

= 0.95 

2.4 

2.2 η in 

= 0.92 

2.0 α in 

= 5.1 cm 2.0 α in 

= 4.4 cm -1 

1.8 

1.8 

1.6 

1.6 

1.4 

1.4 

EXPERIMENTAL 

EXPERIMENTAL 

1.2 

1.2 

LINEAR FIT 

LINEAR FIT 

1.0 

0.00 0.05 0.10 0.15 0.20 0.25 

1.0 

0.00 0.05 0.10 0.15 0.20 0.25 





23 


Cavity length dependence of slope efficiency (III) 

η 

−1 

ext 

[ η ( n )] 

−1 

= 

in th 

Internal Efficiency and Internal Losses 

2 ) Based on many simple assumptions 

⎢ 

⎡ α + in 

1 

⎣ Ln 

( nth 

) L 

( ) ⎥ ⎤ 

1/ R ⎦ 

α 

in 

= α + α = α + σ 

scat 

fc 

scat 

fc 

n 

th 

η in and α in depend on carrier density and therefore on L 

L ↓ ⇒n th ↑⇒ α fc ↑ 

L ↓ ⇒n th ↑⇒ η in ↓ 

3) In CW, additional temperature effects 

DO NOT TRUST α in and η in !!!! , just indicative 


24 


Temperature dependence: T 0 and T 1 

Optical Power (W) 

0.10 

0.08 

0.06 

0.04 

0.02 

0.00 

15 ºC 

25 ºC 

35 ºC 

45 ºC 

55 ºC 

65 ºC 

75 ºC 

0 0.2 0.4 

Current (A) 

I 

EMPIRICAL 

EXPRESSIONS 

th 

⎛ T 

0 

exp T 

⎞ 

( T ) = I 

⎜ 

⎟ th 

⎝ 0 ⎠ 

⎛ T − 

( T ) = η 

⎜ 

⎟ slope0 

exp 

⎝ T ⎠ 

η 

slope 

1 

⎞ 


25 


Temperature dependence: T0 

140 

BA laser 

915 nm 

To (K) 

130 

120 

110 

100 

90 

80 

BA lasers 

808 nm 

0 0.05 0.1 0.15 0.2 0.25 

CAVITY LENGTH (cm) 

DEPENDENCE ON 

TEMPERATURE 

RANGE 

LARGE DISPERSION 


26 


T 0 : Physical origin 

MAIN EFFECTS: 

T ↑⇒n th ↑ (gain dependence on T) 

n th ↑⇒R (n th ) ↑ (increased recombination) 

Typical: Auger recomb. ⇒ T o ↓ 

g 

th 

( T ) Γ gmat[n 

th 

= ( T )] = α ( T ) + α 

in 

m 

I 

2 

3 

( T ) = V q [ A ( T ) n ( T ) + B( T ) n ( T ) C( T ) n ( T ) ] 

th act 

th 

th 

+ 

th 

ATENTION : 

Poor quality laser: high threshold (SRH recomb. or 

leakage current), but high T o 


27 


T 1 : Physical origin 

η 

slope 

( T ) 

= η ( T) 

in 

hν 

⎡ 

q 

⎢ 

⎣αm 

α ⎤ 

m 

+ α ( T ) 

⎥ 

in ⎦ 

ηext (W/A) 

0.55 

0.53 

0.51 

0.49 

0.47 

0.45 


808 nm 

e3 

10 20 30 40 50 60 70 80 

T (C) 

MAIN EFFECTS: 

T ↑⇒η in ↓ (Increased 

leakage) 

T ↑⇒α in ↑ (Increased freecarrier 

absorption) 


28 


SPECTRAL MEASUREMENTS 

EMISSION SPECTRA OF FP LASERS 

Gain 

cavity losses 

longitudinal 

modes 

carrier 

density 

0 

Wavelength (µm) 

lasing mode 

30-40 nm 

2 kL = 2mπ 

δλ ≈ 

2 

λ 

2Ln eff 

λ∼1 µm, 

L = 1 mm, 

δλ∼ 0.3 nm 

• Poor modal discrimination 

• Many modes excited 


29 


SPECTRAL PARAMETERS 

LOG. PLOT 

LINEAR PLOT 

OPTICAL POWER (dBm) 

0 

-20 

-40 

-60 

1270 1280 1290 1300 1310 1320 

OPTICAL POWER (a.u.) 

0.6 

0.5 

0.4 

0.3 

0.2 

0.1 

0.0 

1290 1300 

WAVELENGHT (nm) 

WAVELENGTH (nm) 

Peak Wavelength: λ p 

Spectral width: σ λ 

FWHM of spectral envelope 


30 


SPECTRA OF HIGH POWER LASERS 

Intensity (au) 


I = 70 mA 

70 mA 


964 965 966 967 968 969 970 

967.0 967.5 968.0 

Wavelength (nm) 


I = 200 mA 

I = mA 

200 mA 400 mA 


I = 100 100 mA 

• Changes in spectra 

with current (lateral 

modes) 

• Dependence of 

spectra on lateral 

position 

• Typical spectral 

width: 1 - 3 nm 

965.5 966.0 966.5 967.0 


968 970 972 


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. 0.75 m; 1200 lines/mm ⇒ 

0.03 nm @ 800 nm) 

LD 

Heat-sink 

Integrating 

Sphere 

Optical Spectrum 

Analyzer 

• FO input 

• Very good dynamic range 

• Typical resolution: 0.1-0.5 nm 

Optical Fiber 

Optical Spectrum 

Analizer 


32 


DEPENDENCE OF LASING PEAK ON TEMPERATURE 

Peak Wavelength (nm) 

825 

820 

815 

810 

805 

800 

795 

L = 0.3 mm 

L = 0.6 mm 

L = 2.1 mm 

0.29 nm/K 

0.26 nm/K 

0.29 nm/K 

0 20 40 60 80 

Temperature (ºC) 

Pulsed Measurements 


808 nm 

• Red shift: band gap vs T 

• Blue shift: band filling 

Typical: ~ 0.3 nm/K 


33 


DEPENDENCE OF LASING PEAK ON CURRENT 

40 mA 

35 mA 

30 mA 

25 mA 

PULSED: 

• Negligible dependence 

CW: 

• Temperature dependence (red 

shift) 

I ↑⇒T ↑⇒E g ↓⇒λ p ↑↑ 

• Mode hopping 

20 mA 

16 mA 

I = 14 mA 


34 


MEASUREMENT OF THERMAL RESISTANCE 

Heat-sink 

T QW 

R 

th 

= 

∆T 

P 

dis 

= 

I CW 

( ) 

T 

− T 

QW HS 

( VI − Pout 

) 

T HS 

MEASUREMENT PROCEDURE 

1. Measure λ p vs T HS in pulsed conditions at fixed I 

2. Measure P-I-V and λ p vs I (CW) 

3. Calculate T QW from λ p (CW) 

4. Calculate R th from T QW and P-I-V 


35 


Electro-optical characterisation - Scheme 

Temperature 

Controller 

15°C ≤ T ≤ 75°C 

Accuracy ∆T < ± 0.5 K 

Test 

Chamber 

With 

Minibar 

Calibrated 

Integrating 

Sphere 

Fibre 

Current Controller 

Profile – LDC 3065 

I < 65 A 

Accuracy ∆I < ± 1 mA 

Det. 

Power 

Meter 

Spectrum 

Analyser 

∆λ ≤ 0.1 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 2 measurements 


37 


Beam Propagation 

IDEAL GAUSSIAN BEAM 

Waist d 0 =2 W 0 

W(z) 

θ = 2θ hw 

z 

W(z) = 

W 

0 

⎛ λ(z − z0) 

1 

2 

π(W 

0 

) ⎟ ⎞ 

+ 

⎜ 

⎝ ⎠ 

2 

beam spot size 

z → 

∞ 

W(z) ⋅ W 

0 ≅ 

λz 

π 

θ hw 

⋅ 

W 

0 

= 

λ 

π 

θ ⋅ 

d 

0 

= 

4λ 

π 

DIFRACTION LIMIT 


38 



ARBITRARY BEAM 

How to define the width ? 

How to characterize the propagation ? 


39 



ARBITRARY BEAM 

Waist d 0 =2 w 0 

W(z) 

θ = 2θ hw 

z 

W 

x 

(z) = 

W 

0x 

2 

⎛ M 

xλ(z 

− z 

1+ 

⎜ 

⎝ π(W0x 

) 

0x 

2 

) 

⎟ ⎞ 

⎠ 

2 

ONLY VALID IF W x (z) AND 

W y (z) ARE DEFINED AS 

SECOND MOMENT 

WIDTHS 

W (z) = 

y 

W 

0y 

2 

⎛ M 

yλ(z 

− z 

1+ 

⎜ 

⎝ π(W0y) 

0y 

2 

) ⎞ 

⎟ 

⎠ 

2 

W x 

≡ 2⋅σ x 

dσ x 

= 4⋅σ x 

W y 

d y 

≡ 2⋅σ 

y 

σ 

= 4⋅σ 

y 


40 



then, in both x and y directions 

z → 

∞ 

W(z) ⋅ W 

0 ≅ 

M 

2 

λz 

π 

π ⋅ W (z) ⋅ W 

λ ⋅ z 

2 

0 

M ≡ 

M 2 : - BEAM PROPAGATION FACTOR (Prof. Siegman) 

- BEAM PROPAGATION RATIO (ISO 11146:2005) 

- TIMES-DIFRACTION-LIMIT-FACTOR (ISO 11146:1999) 

BUT NOT: BEAM QUALITY FACTOR (Prof. Siegman) 

θ hw 

W 

0 

= 

M 

2 

λ 

π 

θ 

d 

σ 0 

= 

M 

2 

4λ 

π 


41 


Second Moment beam width 

• Intensity distribution: 

E ( x, 

y, 

z) 

• Beam width: 

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 

2 

x 

( z) 

= 

∫∫ 

( x − x) 

∫∫ 

2 

⋅E 

( x, 

y, 

z) 

dx dy 

E ( x, 

y, 

z) 

dx dy 


42 


Why M 2 ? 

d ⋅θ 

Beam Product Parameter BPP = = 

4 

σ 0 2 

M 

Invariant in geometrical optics 

λ 

π 

Brightness 

B = 

P[W] 

A [cm²] × Ω[srad] 

P 

= λ 

2 2 

⋅M 

⋅M 

B 

2 

x y 

Power and M 2 define the 

Brightness of a source 


43 


• Gaussian beam: 

M 2 (1/e 2 ) 

d 2 

(z) = d ( z) 

4 ( z) 

1/e σx 

= ⋅σ 

x 

with d the full width at 1/e 2 

1/e 

2 

• Arbitrary beam: we could define M 2 (1/e 2 ) 

2 

M (1/ e ) = θ 2 d 2 

2 π 

1/ e 

01/ e 

BUT THEN THE PROPAGATION DO NOT FOLLOW THE HYPERBOLIC LAW 

4λ 

d 

1/e 

2 

2 

(z) 

≠ 

d 

01/e 

2 

2 

2 

⎛ M 2λ(z 

− z 

1/e 

1+ 

⎜ 

π(d 2 

⎝ / 2) 

01/e 

0 

2 

) ⎞ 

⎟ 

⎠ 

2 


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 2 y∼ 1; θ y 

• Slow axis (x): z ox ; M 2 x; w 0x (θ x ) 


45 


LD 

Divergent beam 

FF measurements 

θ 

Rotating 

Photodiode 

z 

a 

n 

g 

l 

e 

0.0 

-20 

-10 

0 

10 θ1/e² 

Power 

0.2 0.4 0.6 0.8 1.0 

θ 1/2 

θ 

20 

ALTERNATIVES 

• Rotating LD 

• Using lenses and 

measuring beam profile 


46 


Example of fast axis FF 

I (u.a.) 

1,0 

0,5 

• Depend on vertical 

waveguide structure 

• Fourier transform of 

transverse mode profile 

0,0 

-60 -30 0 30 60 

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; 0.3 W 

Tapered laser; 0.6 W 

1,0 

1,0 

1,0 

0,8 

0,8 

0,8 

FF 

I (u.a.) 

0,6 

0,4 

0,2 

// 

I (u.a.) 

0,6 

0,4 

0,2 

// 

Intensité (u.a.) 

0,6 

0,4 

0,2 

0,0 

-15 -10 -5 0 5 10 15 

1,0 

0,8 

Angle (°) 

0,0 

-25 -20 -15 -10 -5 0 5 10 15 20 25 

1,0 

// 

angle (°) 

0,0 

-10 -5 0 5 10 

Angle (°) 

1,0 

0,8 

NF 

I (u.a.) 

0,6 

0,4 

// 

I (u.a.) 

0,5 

Intensité (u.a.) 

0,6 

0,4 

0,2 

0,2 

0,0 

0 40 80 120 160 200 240 

x(µm) 

0,0 

28 32 36 40 44 

x (µm) 

0,0 

50 75 100 125 150 175 200 

x (µm) 


49 


M 2 Measurements: ISO11146:2005 

• The test is based on the measurement of the cross-sectional 

power density distribution at a number of axial locations along 

the beam propagation axis 

• The second moment beam widths d σx 

(z) and d σy 

(z) are 

determined 

2 

• Hyperbolic fit of d σ (z) to: d (z) = a + bz + cz 

σ 

d σ 

(z) 

− b 

z 0 

= 

a 

beam waist location 

z 0 

z 

d 

1 

2 c 

σ 0 

= 4 

ac − b 

2 

beam width at waist 

d σ0 

M 

2 

π 

= 

8 λ 

4ac − b 

2 


50 


• Laser diodes: additional lens 

M 2 Measurements: ISO11146:2005 

w' 2 

• At least 10 different z positions shall be taken (half of them 

beyond two Rayleigh lengths) 

• Background correction procedures shall be applied to 

determine the beam widths 

• Alternative methods for beam width measurements (ISO11146- 

3: 2004): 

‣ Variable aperture method 

‣ Moving knife method 

‣ Moving slit method 


51 


Measurement principles – Knife edge 

• Knife edge moved through the beam profile 

Analysis: 

• e.g. beam dimensions from 16 % and 84 % of the 

intensity integrals 

σ Gauß 

knife edge 

detector 

1.0 

0.8 

84.1 % 

intensity / a.u. 

0.6 

0.4 

0.2 

0.0 

15.9 % 

e -2(x/σ Gauß )2 

13.5 % 

edge translation -3 -2 -1 0 1 2 3 

position x / σ Gauß 


52 


Measurement principles – Moving slit 

• Moving slit moved through the beam profile 

Analysis: 

• e.g. beam dimensions from 13.5 % of the intensity profiles 

slit detector 

Moving slit: 

slit translation 


53 


Method of the moving slit – FBH – set-up 

f1 

Near field wLaser 

= dmess 

f2 

L1 L2 Slit 

LD 

f 1 

f 1 +f 2 f 2 

PD 

resolution: 

∆w = 2 µm 

uncertainty: 

ca. 6 % 

Far field 

L1 

Θ 

Laser 

= 

f 

f 

2 

1 

d 

⋅ 

f 

mess 

3 

Slit 

resolution: 

LD 

f 1 f 1 +f 2 f 3 

PD 

∆θ = 0.08 ° 

uncertainty: 

ca. 9% 


54 


Method of the moving slit – FBH – set-up 

Set-up 

x-y-ztranslation 

stage 

∆ = 0.3µm 

LD 

Slit 20 µm 

PD 

Step width 

3 µm – 90 µm 

300 points 

I(t) ≤ 50 A 

t Pulse 

= 1 ms 

L1 

Near field: 

L2 

f 

f 

1 = 

2 

41 

Boxcar 

Integrator 

Far field: 

f 

f 

2 = 

1 

0.024 

PC 


55 


Typical beam profiles (FBH) 

near field (facet) beam waist far field 




-200 -100 0 100 200 

position x / µm 

-30 -20 -10 0 10 20 30 

position x / µm 

-20 -10 0 10 20 

angle θ / ° 

Tapered laser λ = 808 nm, 

widths 

beam waist / µm 

far field / ° 

M 2 

L = 2.75 mm, 

1/e 2 5.9 

13.6 

1.3 

L RW = 1000 µm, R f = 0.1%, 

T = 25°C, P = 2 W 2. mom. 

26.5 

16.3 

7.3 


56 


Example of measurement of astigmatism 

Fast axis (⊥) 

Half-width at 1/e 2 (µm) 

50 

40 

30 

20 

10 

Slow 

0 

axis (//) 

-500 -250 0 250 500 

Position of the lens (µm) 

Position of the lens 

for waist at 1/e 2 

in the slow axis: 

x 0 

// 

in the fast axis: 

x 0 

⊥ 

Astigmatim = 

x 0 

// - x 0 

⊥ 

Waist in the 

slow axis 

Waist in the 

fast axis 


57 


Some Reference Material 

 

 

 

 

 

 

 

 

Professor Anthony E. Siegman Web Pages 

http://www.stanford.edu/%7Esiegman/ 

• Laser beam quality tutorial 

• An annotated bibliography of references on the definition and 

measurement of "laser beam quality" and the "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

Characterization techniques for high brightness laser diodes

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