380A OPTI Intermediate Optics Lab 1: Semiconductor Light Sources
Tom Milster Professor, College of Optical Sciences, University of Arizona [e-mail]
Light Emitting Diode (LED)
!
Forward Bias
+
Cathode (n) Anode (p)
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Light Emitting Diode (LED)
!
Forward Bias
+
Cathode (n) Anode (p)
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LED Far-Field Pattern
! LED far-field patterns vary greatly, depending on the electrode layout and the type of lens in which the LED is encased. ! The LED you will be using in the lab has this characteristic:
Cotco LED LD-701CHR1-A5
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Light Emitting Diode (LED)
! Semiconductor that emits incoherent narrow-spectrum of light upon application of a current. ! An LED is a semiconductor diode with a structure called the p-n junction.
Spectra of blue, yellow-green and red LEDs FWHM is ~24-27 nm Blue, green and red LEDs
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Other LED Characteristics for This Lab
Cotco LED LD-701CHR1-A5
Emission area ~ 1mm2
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Inorganic semiconductor materials
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! aluminum gallium arsenide (AlGaAs) - red and infrared ! aluminum gallium phosphide (AlGaP) - green ! aluminum gallium indium phosphide (AlGaInP) - high-brightness orangered, orange, yellow, and green ! gallium arsenide phosphide (GaAsP) - red, orange-red, orange, and yellow ! gallium phosphide (GaP) - red, yellow and green ! gallium nitride (GaN) - green, pure green (or emerald green), and blue ! indium gallium nitride (InGaN) - near ultraviolet, bluish-green and blue ! silicon carbide (SiC) as substrate - blue carbide as substrate ! silicon (Si) as substrate - blue (under development) ! sapphire (Al2O3) as substrate - blue ! zinc selenide (ZnSe) - blue ! diamond (C) - ultraviolet ! aluminum nitride (AlN), aluminum gallium nitride (AlGaN) - near to far ultraviolet
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White LED efficiency
! Goal is to replace everything with LEDs
Cree produced 131 lumen/W 06/20/2006 true white
lumen = power weighted by luminosity function of human eye per unit of steradian
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emitted wavelength (blue and converter material to yellow)
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Fabry-Perot Laser Geometry
A Laser Requires: 1.) Gain Medium (Population Inversion in Depletion Region) 2.) Feedback (Facet Mirrors) 3.) Confinement (Index/Gain Guiding)
(Provides Carriers)
(Population Inversion)
(Mirrors for Feedback)
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Semiconductor Lasers: Spectral Output
Lasing starts to occur when gain (due to population inversion) is greater than greater than losses (due to absorption and mirror losses) of the cavity. P Below Threshold This is an OPTICAL POWER VS CURRENT (PI) curve, not a current vs voltage (IV) curve. Threshold Current PI Curve Linear Lasing Region (Above Threshold) Threshold) Linewidths of LDs (~10-4 nm) are much sharper than LEDs. The beam is also mostly coherent, due to stimulated emission. ~10-4 nm
P At Threshold Threshold
P
Power Spectra at Different Output Powers
v"
(frequency) (frequency)
OPTI380A - Lab 1: Semiconductor Light Sources
c !
(frequency)
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Semiconductor Lasers: Multi-Mode Operation
! Most commercial laser diodes have more than one dominant mode. ! Mode spacing is given spacing is given #$ Power Spectra at by: c Different #$ cavity " Temperatures 2nL
cavity
$
c = speed of light (3x108 m/sec) n = refractive index of cavity material
(~ 3.5)
#$ mode ! 10%4 nm (70MHz)
L = cavity length
Optical Spectrum Analyzers (OSAs) usually specify power in terms of dBm (decibel power ratio referenced to one milliwatt). Power in milliwatts is given by x
$
Increasing Temperature
P " 1010
where x is dBm
OPTI380A - Lab 1: Semiconductor Light Sources
$
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PI Curves for Different Semiconductor Light Sources
LED: Light Emitting Diode (Lowest power output) power output) LD: Laser Diode (Highest power output) SLD: Super Luminescent Diode (Higher power than LED)
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LD Package and Structure
LD beams are elliptically shaped, due to diffraction from the small emission area at the front facet.
(L)
(H)
(W)
Small emission area (~ 1&m by 3&m) The PIN photodiode provides a way to monitor output power (back facet).
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LD Polarization Ratio
(Sharp LD)
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Red LD Used in This Lab
(Full Spec Sheet on Website)
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Measuring Beam Divergence
Si Detector
Oscilloscope Function Generators Generators for AC Signal LD Power Supply Z Direction
LD Source LD Source LED Source Rotation Stage
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The Concept of Radiance
(Mostly from Wikipedia)
! Radiance is useful because it indicates how much of the power emitted by an emitting surface will be received by an optical system looking at the surface from some angle of view. In this case, the solid angle of interest is the solid angle subtended by the optical system's entrance pupil. Since the eye is an optical system radiance is good indicators of how bright eye is an optical system, radiance is a good indicators of how bright an object will appear.
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Characteristics of the Emission Areas
y x Laser diode (LD): Emission from small, pointlike area. Output beam is in phase as it area Output beam is in phase as it propagates. High power per unit area of the emission area. Spatially Coherent Source. (High radiance)
Output Facet
Output Facet
y x Light emitting diode (LED): Each point in the emission area is like small point source emission area is like a small point source, emitting in all directions. Point source emissions are not correlated. Beam phase is not well defined. Low power per unit emission area, but large area. Spatially incoherent source. (Low radiance)
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Measuring Relative Radiance
! Place Source 1 at the front focus of lens 1. ! Measure the power through the pinhole. ! Repeat for Source 2. ! Relative radiance is the ratio of the power radiance is the ratio of the power measurements.
Source Lens 1 Lens 2 Pinhole Detector
f1
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f2
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LD Collimation
! Numerical Aperture (NA) of the lens:
NA " sin 'marginal !
dlens 2f
! NA of the laser is approximately the sine of the divergence angle (can be different in x and y) ! Match NA of laser beam to NA of lens
Laser Divergence Angle ' marg
Laser
dlens
f
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LD Circularization
Single Prism
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