lectures 11 and 12: lasers, leds and photodiodes ecen...

Lectures 11 and 12: Lasers, LEDs and Photodiodes ECEN 4606, Fall 2010, Prof. Gopinath

Professor J. GopinathUniversity of Colorado at Boulder

ECEN 4606, Undergraduate Optics Lab

ECEN 4606Undergraduate Optics Lab

Lasers, LEDs and Photodiodes

Professor J. Gopinath



Administrative Details

• No prelab or lab reports this week• Averages

- Lab #6 <av>= 86, σ = 10- Prelab #7 <av>= 16, σ = 2.5

• Lab reports- Most of problems are in analysis- If your score is not what you’d like, focus on analysis

section first!• Handouts

- Lecture notes- Polarization practice for final

• Final project- Need to get it approved by Nov. 17- Prelab #13 (due Wed. Dec. 1, should contain diagram of experimental setup, list of parts needed, and goal of experiment – see revised schedule!

• Office hours- My office hours this week will be 11 – noon Wed., but Friday office hours are cancelled- Dongxue’s office hours will be 11 – 12:30 pm on Friday (not the usual Wed. ones)



Lasers vs. lampsIncandescent lampLaser

Efficiency

Wall-plug efficiency is optical powerout to electrical power in. As high as60% for diode lasers, 30% for diode-pumped solid states.

Luminous efficiency is power in visiblespectrum / electrical power in. As lowas 2% for incandescent bulbs. Filterscan reduce spectral width butdramatically reduce efficiency.

Brightness

[ ][ ]( ) [ ]( )

2

2

2220 radm

W

0

=

=

Mn

MwL

wn

λφ

πφ

πλ

= 1012 [W/m2/Sr] for a perfect 1 W laserat λ = 1 um in air.

[ ][ ]( ) [ ]( )Sr4mareafilament

W2 π

φ=L

= 105 [W/m2/Sr] for a bulb with a 1 mm2

filament emitting 1 WProf. McLeod, CU-Boulder



Indirect vs. Direct Absorption

Fox: Optical Properties of Materials



Modes of photodiode operation

Prof. McLeod, CU-Boulder

Open circuit•aka “Photovoltaic”•Solar cells•Low dark current•Slow response

Short circuit

Reversed biased• Drift field increase speed• Lower capacitance• Larger sensitive area

Larger R allows for:1. More sensitivity2. Smaller range3. Smaller

bandwidth

νη

hPei

ieii

P

PTK

e

rsB

≡

−

−= 1

v

P

P

P



Pn and PIN diodes

• pn: Response time faster than photoconductor (due to E) but limited by diffusion which may be as large as carrier recombination lifetime.

• pin: Increased depletion layer width gives:• larger capture area• decreased capacitance (faster response)

pn:

pin:

1. e- & h+ recombine and do not contribute

to current2. e- & h+ may diffuse

into depletion region3. e- & h+ drift under E

field

Depletion

E

-+

n p i D

iffus

ion

Diff

usio

n

1 32

Depletion

E

-+

n p Diff

usio

n

Diff

usio

n

21 3



Photodiode responsivity

• Only respond to photons with energy above bandgap

• If energies are much, much greater than bandgap, will all be absorbed in bulk and there is no current!

www.thorlabs.comwww.udt.com

http://www.thorlabs.com/�



Dark Current

Sharp PD412PI

id vs temperature at VR = 10 V id vs bias at 25 oC

Typical dark current for Si photodiode

Prof. McLeod, CU-Boulder

lectures 11 and 12: lasers, leds and photodiodes ecen...

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