Passive Cavity Surface–Emitting Lasers: Option of

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Passive Cavity Surface–Emitting Lasers:
Option of Temperature–Insensitive
Lasing for Uncooled Dense Wavelength
Division Multiplexing Systems
Vitaly Shchukin
VI Systems GmbH, Berlin, Germany
Photonics West 2016, 9766–08, San Francisco, February 17, 2016
Photonics West 2016, 9766–08. San Francisco, February 17, 2016
Cooperation
Vitaly Shchukin, Nikolay Ledentsov
VI Systems GmbH
Hardenbergstr. 7, Berlin 10623, Germany
Thomas Slight, Wyn Meredith
Compound Semiconductor Technologies
Hamilton G72 OBN, Scotland, UK
Nikita Gordeev, Yury Shernyakov, Alexey Nadtochy,
Alexey Payusov, Mikhail Maximov, Sergey Blokhin,
Alexey Blokhin, Yury Zadiranov, Nikolay Maleev,
Viktor Ustinov
A.F. Ioffe Physical Technical Institute
St. Peterburg 194021, Russia
Kent Choquette
Electrical and Computer Engineering Department
University of Illinois
Urbana, IL 61801, USA
Outline
 Introduction
 Passive cavity surface–emitting laser
 Prototyped passive cavity device
 Refractive index temperature coefficients of dielectric
materials
 Modeling temperature insensitive lasing wavelength
 Passive cavity edge–emitting laser
 Summary
Conventional Lasers: Active Medium
inside the Cavity
Conventional VCSEL
Conventional edge-emitter
Active medium in the waveguide
between the cladding layers
(evanescent mirrors)
Active medium in the cavity between DBRs
 Conventional laser: active medium in a cavity surrounded by mirrors
 Resonant cavity or DFB grating are used for wavelength stabilization
Motivation
 DFB lasers and VCSELs are used for low–cost wavelength–
stabilized operation
 As the refractive index changes upon temperature uncooled
wavelength division multiplexing systems are impossible
 Interference filters and feedback loops are used for wavelength
stabilization
 These concepts are not applicable in low–cost data
communication and silicon photonics
 Can we solve the problem?
Outline
 Introduction
materials
 Summary
Passive cavity surface–emitting laser:
Example design
 Active medium shifted from the cavity
 Active medium placed in a DBR
 Passive dielectric cavity and DBRs
 Confinement factors are comparable
 Epitaxial thickness is reduced
Advantages of Passive Cavity Design
 A broad variety of materials for dielectric cavity:
 Variety of thermal coefficients dn/dT (>0, =0, <0)
 Possibility to obtain temperature–insensitive lasing wavelength
 True photonic crystals penetrating to the cavity possible
 Can be applied in DFB edge–emitters if
the waveguide is made of dielectric material
Prototyping of Passive Cavity
Surface–Emitting Laser
Field intensity in
the gain region
 The concept of the gain medium in the DBR is prototyped and shown to work
despite reducing of the optical confinement factor by a factor of 3 as compared
to conventional GaAs–GaAlAs–based VCSEL
Aiming at Control of Thermal Shift of
Lasing Wavelength
 Wavelength of 850 nm VCSEL shifts upon temperature at a rate ~0.07 nm/K
 Industrial VCSELs must operate at all temperatures between –20oC and +85oC
 In a WDM system neighboring channels must be sufficiently separated in
wavelength (4 channels with 30 nm spacing are under standardisation)
 If thermal shift of the wavelength is eliminated, an uncooled WDM system with
significantly smaller spacing (and larger number of channels) becomes feasible
 Idea:
Compensate thermal shift
in semicondcutors by using
dielectrics with dn/dT<0
Refractive Index of Dielectric:
Positive and Negative Thermal Shifts
Lorentz–Lorenz formula
Molecular
Molecular
denisty
polarizability
n 2  1  T     , T 

2
n 2
3 0
Competeing trends:
 Density decreases upon temperature: thermal expansion
 Polarizability increases upon temperature
Examples known for
dn
0
dT
BaO–B2O3 glass
ThBr–ThI mixture (KRS–5)
Lithotec CaF2, etc.
 Need to explore conventional dielectrics applied for laser coatings
Test Resonant Dielectric Structures
 Two resonant dielectric structures based on conventional
SiO2/TiO2 dielectrics designed and deposited to measure
optical power reflectance spectra versus temperature
Resonant Structure with SiO2 Cavity:
Optical Spectra
Cross–section SEM
Resonant Structure with SiO2 Cavity:
Temperature Shift of Optical Spectra
Shift of dip position upon temperature
 Small positive shift ~0.005 nm/K, about 10 times smaller than
in III–V semiconductors
Resonant Structure with TiO2 Cavity:
Optical Spectra
Resonant Structure with TiO2 Cavity:
Temperature Shift of Optical Spectra
Shift of dip position upon temperature
 Significant negative shift ~ – 0.02 nm/K !!!
 Also demonstrated – 0.03 nm/K for SiO2/Ta2O5 structure at 630 nm
Modeling of Resonant Dip Thermal Shift
 1dip 
d 1dip  1dip  dn1  1dip  dn2  1dip 





 d11  
 d2 2
dT
 n1  dT  n2  dT  d1 
 d 2 
 2dip 
d 2dip  2dip  dn1  2dip  dn2  2dip 





 d11  
 d2 2
dT
 n1  dT  n2  dT  d1 
 d 2 
SiO2
TiO2
Index change in
Index change in
Thermal expansion Thermal expansion
material 1
material 2
of material 1
of material 2
dn1
 2.110 5 K 1
dT
dn2
 9.2  10 5 K 1
dT
 Fitted values used for modeling of passive cavity laser
Modeling of Passive Cavity
Surface–Emitting Laser
Refractive Index and Optical Field Profiles
Zero Thermal Shift
 Zero thermal shift of the resonant wavelength feasible
 Device fabrication in progress
Passive Cavity Surface–Emitting Laser
as Enabler
 Control temperature shift of the lasing wavelength
 Achieve on purpose d/dT >0, =0, <0
 Achieve temperature–insensitive lasing wavelength
 Reduce the channel spacing in WDM systems
Outline
 Introduction
materials
 Summary
Passive Cavity Edge–Emitter.
Tilted Wave Laser
 Thin active WG coupled to a thick passive WG
 Passive WG can be made of semiconductor or
dielectric material
 Prototyping as semiconductor TWL:
 Both substrate and epilayer possible as passive
WG
 Passive WG controls the optical mode shape and
far–field profile
Mode Engineering. Control over Far Field
 Tilt angle of the lobes can
be tuned by changing the
passive and active layer
thicknesses and
compositions
 By reaching /2 condition
for the active and passive
cavities duo-lobe pattern
converts to a single lobe
 When small leaky angles
in the second cavity are
chosen single lobe
emission can generate an
narrow beam
Tuning the Lobes
 Option: extended active
GaAs cavity
 Reduced angle between
the beams
High Power per Facet Length




90% differential efficiency
26 µm waveguide. Possible replacement of VECSEL
>40 W per 50 µm facet
>8.4 kW/cm at no facet passivation
Outline
 Introduction
materials
 Summary
Summary
 Passive cavity surface–emitting laser with dielectric cavity and dielectric top DBR
enables ultimate control over thermal shift of the lasing wavelength
 On purpose d/dT >0, =0, and <0 can be achieved
 Design is modeled allowing temperature–insensitive lasing wavelength
 Approach allows significant reduction of the spectral spacing between
channels in WDM systems enabling a larger number of channels for the
same spectral range
 Passive cavity edge–emitting laser allows ultra–narrow laser beams from ultra–
thick facets and high efficiency
 Direction of the beams as well as transition from double beam to single beam
lasing can be controlled by design of the coupled waveguide
 Dielectric passive waveguide possible
Thank You!
Visit us at Booth 4147 - Hall D
This project has received funding from the European Union’s Horizon 2020 research and innovation
programme under grant agreement No 666866.

Passive Cavity Surface–Emitting Lasers: Option of

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