Title: Vertical Cavity Surface Emitting Lasers
1Vertical Cavity Surface Emitting Lasers
Ravi Kalyanam Oct 27, 2006
2What is Covered?
- Operating Principle of VCSELS
- Cavity Modes
- Axial Modes
- Transverse Modes
- Determination of Cavity Resonance
- Determination of Material Gain
- Obtaining Device Performance Parameters from
Experimental Data
Vertical Cavity Surface Emitting Lasers
3VCSEL Structure
- Optical cavity defined by 2 DBR mirrors
- Optical cavity typically one- ? thick
- Gain medium usually is MQW
- Cavity Resonance must overlap Gain Spectrum
Vertical Cavity Surface Emitting Lasers
4Operating Principles
- Carriers can be pumped, optically or electrically
to an excited state - An incoming photon stimulates de-excitation of
the electron - Modes supported by cavity survive reflections
within the cavity
Vertical Cavity Surface Emitting Lasers
5Threshold Condition
- Photons generated could be lost
- Intrinsically within the cavity (ai)
- At the mirrors on either side of the cavity (am)
- If gain equals the losses, we have threshold
condition, medium is ready to lase
Vertical Cavity Surface Emitting Lasers
6Longitudinal Modes
- Longitudinal modes are standing wave patterns
formed by waves confined in the cavity. - Only allowed modes which satisfy would
live, beause of constructive interference.
Cavity Modes
7Transverse Modes
- Transverse modes are the intensity patterns in a
plane perpendicular to the propagation - Described by a combination of Gaussian profile
and Laguerre polynomial.
Cavity Modes
8Reflectivity Measurement Technique
Determination of Cavity Resonance
9Cavity Resonance Spectrum
- Ratiometric Measurement Eliminates System
Spectral Response
Determination of Cavity Resonance
10LI Curve
- L-I curve directly gives
- Differential QE ?d
- Threshold Current Ith
- We also could get
- Intrinsic QE ?i
- Cavity loss ai
- Mirror Loss am
- Threshold Gain gth
Obtaining Device Parameters
11Extracting ?i and ai
- Perform L-I on lasers with different L
- Plot ?d vs. L
- Y-intercept is
- Slope gets us
Obtaining Device Parameters
12?gth vs. Jth Plot
- Use previous samples to calculate threshold
current density Jth - Plot modal gain vs. Jth
Obtaining Device Parameters
13VCSEL Parameters
- Obtain ?d and Jth from L-I curve.
- Obtain ?i and Jth from diagnostic lasers
- Use below relations for am and ai
Obtaining Device Parameters