Oclaro DSDBR
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L5000VCJ Datasheet
C-band and InP/InGaAs technology: the laser diode vertical structure
The Phase element
The Semiconductor Optical Amplifier (SOA)
The Optical Shutter
Considerations on Gain, Phase and SOA
Bragg reflectors
Tunable gratings
The rear reflector
The front reflector
Overall Tuning
The total chip structure
Reliability issues for the chip
Points to be clarified
The Locker
M.Vanzi January 2012
C-band and InP/InGaAs technology: the laser diode vertical structure
The C-band ranges from 191 to 196 THz,
which corresponds to optical wavelengths (in vacuum) from 1570 to 1530 nm
and to photon energies from 0.79 to 0.81 eV.
The ternary compound In0.53Ga0.47As has its bandgap at 0.777 eV (at 300°K),
that leads its spectrum for spontaneous emission (Eg  Eg+2kT) to completely
embrace the C-band.
On the other side, this ternary compound is perfectly matched to the InP lattice,
which allows for growing In0.53Ga0.47As by epitaxy on an InP substrate.
Moreover, the higher bandgap of InP itself (1.27 eV at 300°K) makes InP perfectly
suitable for building the confinement layers of a laser diode with its active layer
made of that ternary compound.
Upper metal
p-cap layer
forward current
p-confinement
active layer
See my report: List of failure modes and
mechanisms in Laser Diodes part 1/2.
n-confinement
Appendix 2. Epitaxial rules for DH lasers
n-substrate
lower metal
The Phase element .1
The same structure may act as a phase element, provided:
Upper metal
forward current
•Light enters the element from outside at frequency n
•No reflection exists at entrance and exit sides
•A low, independent, forward current is allowed to flow
p-cap layer
p-confinement
InP
active layer
light
The injected current changes the charge density
The charge density changes the refractive index n
The phase change Df across the distance L is
Df  2n
L
0
 2n
InGaAs
InGaAs
n-confinement
InP
n-substrate
InP
nL
c
Low current= no gain= absorption= attenuation
lower metal
L
The Phase element .2
A gain and a phase element can then be combined within a single monolithic structure
phase current
Upper metal
p-cap layer
p-cap layer
p-confinement
p-confinement
active layer
n-confinement
n-confinement
n-substrate
n-substrate
lower metal
lower metal
gain
phase
mirror
forward current
Upper metal
active layer
mirror
forward current
laser current
The Phase element .3
metal
Upper
Upper
metal
layer
p-cap
p-cap
layer
p-confinement
p-confinement
mirror
n-confinement
n-confinement
mirror
mirror
layer
active
active
layer
n-substrate
n-substrate
metal
lower
lower
metal
Phase change is equivalent to a
change in the cavity length.
•Spectrum envelope unchanged
•Multimode operation survives
•Modes shift with phase change
•Mode spacing changes
The Phase element .4
Fine tuning of the Oclaro phase element spans a 50
GHz range, that is equivalent to the minimum
separation between C-band channels
30
20
10
GHz 0
-10
-20
-30
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5 7 7.5 8 8.5 9 9.5 10
Current mA
191 THz
196 THz
C-band channels
The Semiconductor Optical Amplifier (SOA)
The same structure may act as an Optical Amplifier, provided:
Upper metal
forward current
•Light enters the element from outside at frequency n
•No reflection exists at entrance and exit sides
•A high, independent, forward current is allowed to flow
p-cap layer
p-confinement
InP
active layer
light
High current= gain= no absorption= amplification
The SOA is not a laser itself only because of the
absence of its own resonant cavity
It does not affect modality: even multimodes are
equally amplified.
InGaAs
n-confinement
InP
n-substrate
InP
It is a pumped element as for fiber amplifiers
Anyway it also introduces a phase shift, because of
the current injection
InGaAs
lower metal
L
The Optical Shutter
As for the phase element, at low current the SOA attenuates light because of
optical absorption.
This property is used enabling even reverse bias of the SOA, that turns itself
into a highly absorbing element, that is an optical shutter
Considerations on Gain, Phase and SOA.1
Two slides about… wrong solutions
Considerations on Gain, Phase and SOA.2
A hypothetic structure
phase current
laser current
Upper metal
Upper metal
p-cap layer
p-cap layer
p-cap layer
p-confinement
p-confinement
p-confinement
active layer
active layer
n-confinement
n-confinement
n-confinement
n-substrate
n-substrate
n-substrate
lower metal
lower metal
lower metal
Amplifier
/shutter
Such a structure is:
gain
tunable and gain controllable,
BUT
multimodal and with SOA affecting phase control
active layer
phase
mirror
forward current
Upper metal
forward current
mirror
forward current
SOA current
Considerations on Gain, Phase and SOA.3
Another hypothetic structure
laser current
phase current
Upper metal
p-cap layer
p-cap layer
p-cap layer
p-confinement
p-confinement
p-confinement
active layer
n-confinement
n-confinement
n-substrate
n-substrate
n-substrate
lower metal
lower metal
lower metal
amplifier
gain
mirror
n-confinement
active layer
mirror
active layer
Upper metal
forward current
Upper metal
forwardcurrent
forward current
SOA current
phase
Such a structure would avoid SOA interference on phase control
BUT is not feasible in monolithic technology.
And remains multimodal
Considerations on Gain, Phase and SOA.4
For tunability across the whole C-band one needs:
1)
2)
3)
4)
5)
Single mode selection
Tunability 100 times wider than the range of the phase element
Cavity resonance
Gain flattening, if tuning affects gain
Monitoring of power and frequency
The first two requirements ask for tunable gratings
Bragg reflectors .1
They are made of corrugated layers, with proper diffraction index, in the
vicinity of the active layer.
Apart from corrugation, the structure is the same as for the gain element
ng
light
waveguide
tail
na
nc
Moving wavefront
nc
The “tails” of the optical wave, extending outside the active layer,
sense the corrugation as an effective modulation of the refractive
index along the waveguide
Effective index along the waveguide
Bragg reflectors .2
Any index variation acts on the propagating wave as an
impedance variation for an electric signal.
Reflected waves are originated at any corrugation point.
They can interfere positively (strong reflection) or negatively (null
reflection), depending on .
The shape of the corrugation defines the reflection function R as a function of .
The reflection function is related to the Fourier transform of the grating function
Proper patterning of the grating allows for “R() engineering”
Tunable gratings .1
Tuning current
As for the phase element, when a given
forward current is fed across the structure,
the overall refractive index of the
waveguide is multiplied by a constant
factor.
ng
na
nc
That is equivalent to change =0/n
That is, in turn, equivalent to stretch the
grating, and then to change the reflection
function R()
In order to span the whole C-band, the tuning should change  by some 3%.
This is excessive.
nc
Tunable gratings .2
phase
rear Bragg reflector
6a
a
Assuming a similar width of the ridge, the contact area on the rear Bragg reflector is about 6 times the area
of the phase element.
On the other side, the maximum current fed into the latter is 60 mA, while in the former is 10.
This means a similar maximum injected density of charges, and then a similar tuning range.
30
1549
1548
20
Lasing Wavelength (nm)
1547
10
1546
1545
GHz 0
1544
1543
-10
1542
-20
1541
1540
0
10
20
30
Rear Current (mA)
40
50
60
-30
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5 7 7.5 8 8.5 9 9.5 10
Current mA
The rear reflector .1
In order to allow for full C-band coverage, the Oclaro rear Bragg reflector is designed
to produce 7 peaks (supermodes), spaced of some 6-7 nm.
Tuning will allow any wavelength in the C-band to be centered by at least one peak.
S band
C band
L band
Reflection coefficient
0.15
0.1
0.05
0
1.5
1.52
1.54
1.56
Wavelength ( m)
1.58
1.6
The rear reflector .2
Total shift of the “comb reflection”
1549
1548
Lasing Wavelength (nm)
1547
1546
1545
1544
1543
1542
1541
1540
0
10
20
30
40
50
60
Rear Current (mA)
shift of a single peak
It remains only to select the
supermode, that is the
specific peak
The front reflector .1
The front reflector is made of a chirped grating.
2
f ( x)
0
0
x
It is made
of a continuously varying pitch, that
produces aA broad,
uniform
1
reflection across the wole C-band.
100
Reflection coefficient
0.15
rear Bragg reflector
0.1
front reflector
0.05
0
1.5
1.52
1.54
1.56
Wavelength ( m)
1.58
1.6
When two adjacent contacts are fed by current, the effective local pitch is
modified. This changes the reflection function, depleting some reflected
wavelengths and enhancing others. A broad peak forms, able to select a
single supermode
The front reflector .2
Tuning
8 metal contacts, operated in pairs
tuned front reflector
Reflection coefficient
0.15
rear Bragg reflector
0.1
front reflector
0.05
0
1.5
1.52
1.54
1.56
Wavelength ( m)
1.58
1.6
Overall Tuning
Continuous tuning
Supermode selection
The total chip structure
2
Even
4
6
8
ISO
IGain
Phase
IRear
r
A
I
AR
AR
1
3
5
Odd
7
Front reflectors 7 + 8 = Short λ
The total structure is then full integrated
into a single monolithic element
Front reflectors 1 + 2 = Long λ
Reliability issues for the chip
Advantages:
•No movable parts
•Full internal cavity (no interfaces in open air)
•Only one thermal control needed
•No local mirrors (no COD)
Disadvantages:
•Many corrugated epitaxial interfaces: risk of defect growth
•Rather high operating currents for rear reflector, coupled with corrugation
•Absorbing elements: need for a SOA (further current)
Points to be clarified
1. Vertical structure (TEM required)
2. Details of gratings (very many FIB-TEM required, in several locations)
3. Material analysis
1. The bent ridge in the SOA sections calls for a laterally confined optical guide.
A BH solution is expected (see List of Failures part 1)
4
2.1 The rear grating, in order to give a comb reflection, is expected to be a
sampled structure
g( x)
0
0
x
40
2.2 The front grating, in order to give a wide flat reflection, is said to be a
linearly chirped structure. This should be verified.
2
f ( x)
0
0
x
A  1
100
The Locker .1
Beamsplitter
The locker elements are slightly rotated in
order to avoid unwanted resonance between
back reflections.
PD-2
PD -1
Transmit
Primary beam
Transmit( )
Split
Reflection( )
Photodiode (Rx)
The Etalon has a transmission function T given by
(see my document: The Double Etalon …)
Where
c
n0 
 100 GHz
nd
that implies nd 3 mm
The reflection function R is its complement to unit:
Their ratio is
T (n ) 
R(n )
n 
2

 F sin  2 
T (n )
 n0 
P
Etalon
Photodiode (Tx)
1
 n 
1  F sin 2  2 
 n0 
 n 
F sin 2  2 
 n0 
R(n ) 
 n 
1  F sin 2  2 
 n0 
It is useful to note that the beam reflected by the Etalon undergoes another splitting (dashed line) when crossing the splitter. This
means that PD2 does not read the full reflection of the Etalon.
Anyway, being the splitter weakly reflecting, in order to save power in the primary beam, or the attenuation at PD2 is neglected, or
is compensated by upscaling the PD2 reading.
The Locker .2
The Oclaro documents indicate the ratio R/T as determining the frequency, and the sum R+T
as monitoring the total power.
About power, it is clear that the sum R+T is proportional to the intensity of the transmitted primary beam.
About frequency, Oclaro plots the difference T-R instead of the ratio R/T.
The following graph plots everything. A value F=2 has been assumed in order to fit the original drawing.
2
Ch 49
Ch 50
T n 
Rn
T  n  R  n 
Rn
T n 
 0.4
195800
n
196050
In any case, the 100GHz channels result perfectly tuned at the maxima and minima of the four curves
The utility of the ratio R/T is that tunes the 50GHz channels exactly at midway of the descending or
ascending nearly linear parts of the curve (where the intensity read by the two photodiodes is equal).