Download ENE 623 - web page for staff

Lecture 7
Tunable Semiconductor Lasers
 What determines lasing frequency:
 Gain spectrum

A function of temperature.
 Optical length of cavity
 Mirror reflectance spectrum
 Any perturbation which affects refractive index and/or
lasing frequency.
Single frequency laser
 DFB and DBG lasers
 Tuning achieved by changing heat sink temperature.
 Tuning by changing bias current which affects the
number of carriers in tuning region.
4 nL
M
M 
4 M nL

 2M  ; M  integer
c
cM
2nL
Modulators
 Mach-Zehnder modulators (electro-optic modulators)
 Electro-absorption modulators
Phase Modulators
l
 
    ne3r33  V

g
Electrooptic Modulator
(A) Directional coupler geometry
(B) Mach-Zehnder configuration
Mach-Zehnder modulator
 Solve wave equation for mode field distribution &
propagation constant.
u ( x , y , z )  u ( x, y ) e  i  z
2 neff


neff  neff V  0   kV

where k = constant
Mach-Zehnder modulator
v
Pi
Po
 Thus, by applying V will cause a phase shift for
propagating mode.
Mach-Zehnder modulator
 By symmetry, equal amplitudes in 2 arms after passing
through the first branch.
Mach-Zehnder modulator
 For the second branch, output depends on relative
phases of combining waves:
 2 waves in phase.
 2 waves  rad out of phase
Mach-Zehnder modulator
 Wave amplitudes
Pout  A
2
out
Ain2 i1

e  ei2
4
2
Mach-Zehnder modulator
i1
e e
i2
2
  cos 1  cos 2    sin 1  sin 2 
2
 cos 2 1  cos 2 2  2 cos 1 cos 2
 sin 2 1  sin 2 2  2sin 1 sin 2
Pin
1  cos 1  2  
Pout 
2
2
Mach-Zehnder modulator
Pout = Pin 
1  2   2M  ; M  integer
Pout = 0 
1  2   0
1  0  cV
2  0  cV
1  2   2cV 
2 L

n
eff 1
 neff 2

Mach-Zehnder modulator
 V is a swiching voltage which give Pout -rad phase
difference.
 V is determined by material and electrode
configuration.
 V is different for dissimilar polarizations.
Pin
Pout 
2

V
1  cos  

 V



Diffused optical waveguides
 Diffused optical waveguides: Ti:LiNbO3 indiffused
waveguides.
 Waveguide modes (linearly polarized or ‘LP’):
 TE mode – light polarized in plane of substrate surface
 TM mode – light polarized normal to plane of substrate
surface.
Diffused optical waveguides
nTE ( x, y, z )  nsub / TE  nwg / TE ( x, y, z )  ne.o./ TE ( x, y, z )
nTM ( x, y, z )  nsub / TM  nwg / TM ( x, y, z )  ne.o./ TM ( x, y, z )
 Ti indiffused waveguides: Ti metal atoms cause
refractive index increase for both TE and TM waves.
 Proton exchanged waveguides: H atoms exchange with
Li atoms in lattice. Refractive index increases for only
one polarization; e.g, TE mode.
Diffused optical waveguides
 For digital transmission, different V could degrade
‘on-off radio’ or OOR. Ideally, we want OOR to be
close to infinity.
 Solutions for that are:
 Use polarized optical input.
 Use proton exchanged waveguides to eliminate TM modes (get Pout
only for TE mode).
Example
 Consider a Mach-Zehnder modulator with an electrode length of 2 cm
and electrode gap width g of 12 mm, such that
neff / TE  KTE E
neff / TM  KTM E
with E the applied electric field, assumed to be constant between the
electrodes, and KTE = 5.8 x 10-10 m/V and KTM = 2.0 x 10-10 m/V. What is
VTE and VTM ?
Note: neff = n0 + Δn in one arm and neff = n0 - Δn in the other arm.