Invited Paper
2-2.7µm single frequency tunable Sb–based lasers operating in
CW at RT: Microcavity and External-cavity VCSELs, DFB
A. Garnachea , A. Ouvrarda , L. Ceruttia , D. Barata , A. Viceta , F. Gentya , Y. Rouillarda ,
D. Romaninib and E.A. Cerda-M´endezc
a
c
CEM2, CNRS UMR5507, Universit´e Montpellier 2, Montpellier, France;
b Laboratoire de Spectrom`
etrie Physique, CNRS UMR5588,
Universit´e Joseph Fourier - Grenoble, Saint Martin d’H`eres,France;
Instituto de Investigacion en Comunicacion Optica, Universidad Autonoma
de San Luis Potosi, San Luis Potosi, Mexico
ABSTRACT
DFB lasers, microcavity and External-cavity VCSELs exhibit narrow single-frequency operation and wide modehop-free tuning range, especially well adapted for gas spectroscopy application in the 2-2.7µm window. We will
present a review of the results achieved and a systematic comparison, with such Sb-based lasers emitting near
2.3µm. These sources operate in CW above 300K, with up to 5mW output power in a single transverse mode and
linear light polarization. Diode-pumped V(E)CSELs and electrically-pumped DFB lasers were designed, grown,
processed, and the spectral, spatial, thermal properties characterized. These sources are now being applied in
high sensitivity spectroscopy instruments for in-situ measurements.
Keywords: VCSEL, VECSEL, DFB, Type-I Sb-based Quantum-Well, Single Frequency, Gas analysis
1. INTRODUCTION
Mid-IR Sb-based semiconductor laser technology is maturing rapidly and is ﬁnding applications in areas such as
high resolution spectroscopy (trace gas analysis, in-situ water and carbon dioxide isotope ratio measurements),
medicine or free-space optical telecommunications for high power devices. Applications to trace gas measurement
by tunable diode laser absorption spectroscopy, or by more sensitive techniques such as Cavity Ring-Down
Spectroscopy1 (CRDS) or Optical-Feedback Cavity-Enhanced Absorption-Spectroscopy2 (OF-CEAS), require
reliable single frequency tunable devices with few mW of output power, preferably operating in the continuous
wave (CW) regime and at room temperature (RT). The 2.3µm atmospheric window gives access to many gaseous
species of interest such as CH4 , NH3 , CO, HF, without H2 O or CO2 interference, while near 2.7µm these molecules
have strong absorption lines suitable for isotope ratio measurement applications. In addition, low cost optics and
detectors are commercially available in this wavelength range: this is a real advantage compared to wavelengths
above 3.5µm where strong absorption lines are also present and quantum cascade lasers are becoming available
(but usually not operating in CW at RT). Thus there is a strong interest to develop high quality single frequency
tunable Sb-based laser operating in CW at RT in the 2-2.7µm range.3–6
Distributed-FeedBack (DFB) laser diodes, microcavity VCSELs (µc-VCSELs) and now External-cavity VCSELs (VECSELs),6, 7 deﬁned by a 1 /2 –VCSEL and an external concave mirror, exhibit single-frequency operation and wide mode-hop-free tuning range, especially well adapted for gas spectroscopy applications.2 DFB
laser devices became very recently commercially available in the 2-2.7µm range,8 but exhibit a highly divergent
beam far from the diﬀraction limit, unadapted for commercial optics. VECSEL devices have demonstrated
several advantages such as high beam quality at high output power, ultra narrow linewidth with high stability.7, 9 In this paper we will present a review of the results we achieved with such Sb-based lasers, operating
above RT in CW with both vertical7 and in-plane lasing10 conﬁgurations. 800nm diode-pumped V(E)CSELs
and electrically-pumped DFB lasers were designed, grown, processed, and fully characterized. A systematic
comparison of the spectral, spatial and thermal device properties will be presented. These devices are based on
InGaAsSb/AlGaAsSb type-I quantum-wells (QWs), grown by solid-source molecular beam epitaxy (MBE).
Further author information: (Send correspondence to A. Garnache)
A. Garnache: E-mail: [email protected], Telephone: +33 467 143 476
Semiconductor Lasers and Laser Dynamics II, edited by Daan Lenstra, Markus Pessa, Ian H. White,
Proc. of SPIE Vol. 6184, 61840N, (2006) · 0277-786X/06/$15 · doi: 10.1117/12.663448
Proc. of SPIE Vol. 6184 61840N-1
Figure 1. a) Record threshold current densities vs wavelength (2.05µm,5 2.38µm,3 2.60µm,11 3.04µm:4 ). b) Record
ηi vs wavelength (2.05µm,5 2.26 µm,12 2.38µm,13 2.5, 2.7 and 2.8µm14 ).
2. ANTIMONIDE-BASED DEVICES: DESIGN, GROWTH, TECHNOLOGIES
2.1. Design and properties of 2-2.7µm QW structures
To-date QW lasers have been successfully operated in the CW regime at RT in the range from 2.0 to 3.04µm.4, 5
The materials having led to the best results are without doubt found in the system GaInAsSb/AlGaAsSb
grown on a GaSb substrate. QWs are usually made of compressively strained GaInAsSb layers with thicknesses
and compositions ranging from 10 nm of Ga0.76 In0.24 As0.01 Sb0.99 (strain = 1.4%) at 2.05µm5 to 20 nm of
Ga0.5 In0.5 As0.23 Sb0.77 (strain = 1.5%) at 3.04µm.4 An interesting feature of these lasers in this wavelength
range is illustrated in Fig. 1. Fig. 1-a shows some of the best threshold current densities published by diﬀerent
groups working on QW laser diodes emitting in the mid-infrared. This ﬁgure calls for two comments. The
threshold current densities obtained near 2 µm (50A/cm2 at 2.05µm) can be compared to the lowest RT Jth
values for any QW lasers (the lowest one is to the best of our knowledge 44A/cm2 for a GaInAs SQW laser
emitting a 0.98µm15 ). Such a low value can be attributed ﬁrst to the small values of the masses of both electrons
and heavy holes at this wavelength (we estimate them to be 0.03 m0 and 0.04m0 respectively) resulting in a low
value of the radiative recombination current density. On the other hand, we can note a monotonous increase of the
threshold current densities with growing wavelength (Jth in the pulsed regime reaches 343A/cm2 at 3.04 µm4 ).
This behavior is in contradiction with the expected decrease of the radiative recombination current density (me
decreases down to 0.02 m0 at 3.04µm). Actually the increase of the threshold current density can be attributed
to another channel: the Auger recombination, which is known as a dominant mechanism for long wavelength
semiconductor lasers. Experimental determinations of the Auger coeﬃcient on bulk antimonides show that this
increase is of exponential type with decreasing bandgap energy (C = 2.3 × 10−28 cm6 s−1 at 0.54 eV/2.3 µm,16
10−26 cm6 s−1 at 0.35 eV/3.5µm,17 9.8 × 10−25 cm6 s−1 at 0.215 eV/5.8µm18 ).
Fig. 1-b shows some of the best internal quantum eﬃciencies ηi obtained by groups working on MIR QW
laser diodes. This ﬁgure exhibits a continuous decrease of ηi when going from 2.05µm (ηi = 95%5 ) to 2.8µm
(ηi = 35%14 ). The diﬀerence between these laser structures essentially resides in a decrease of hole conﬁnement
with growing wavelength, whereas electrons remain very well conﬁned (conﬁnement energy of 384 meV at 2.3µm,
e.g.). Indeed, we calculate a hole conﬁnement energy of 200 meV at 2.05µm and 40 meV at 2.8µm when barriers
made of Al0.25 Ga0.75 As0.02 Sb0.98 are taken into account. This reduction is due to the increased Arsenic content
in the QW. It is hence most probably hole leakage by thermionic emission which is responsible for the decrease of
ηi , and thus threshold, with growing wavelength.14 To summarize, GaInAsSb/AlGaAsSb lasers exhibit excellent
characteristics in the range from 2.0 to 2.7µm with threshold current densities below 200A/cm2 and ηi beyond
50%. On the other hand, designers willing to extend their wavelengths to 3.0µm and beyond are faced to a double
obstacle: Threshold densities rise exponentially and ηi decrease due to an increase of the Auger coeﬃcient and a
strong reduction of hole conﬁnement as the wavelength increases. Of course the latter obstacle can be bypassed
by increasing the Aluminum content in the barriers to a value of 0.30 (this is the solution chosen in4 ).
Proc. of SPIE Vol. 6184 61840N-2
Figure 2. a) Typical Sb-based 1/2-VCSEL structure design. b) Scanning-Electron-Microscopy (SEM) picture of the
structure. c) Reﬂectivity (Measured and calculated) and photoluminescence under high excitation of the 1/2 VCSEL
structure at 300K, showing a good matching between the bragg and the QW gain wavelengths.
2.2. MBE growth: QWs and n.i. doped 1/2-VCSEL structure for the 2-2.7 µm range.
GaInAsSb/AlGaAsSb lasers exhibiting the best performances have been grown by MBE. They are usually made
with sources containing Al, Ga, In, Be for p-type doping, Te for n-type doping and cracker sources generating
As2 and Sb2 ﬂuxes. Growth of laser structures is well mastered now in the range from 2.0 to 2.4µm where a
growth temperature (T◦ ) of 475◦ C has been identiﬁed as the T◦ leading to high crystal purity, thus the best
photoluminescence or quantum eﬃciency yield by Simanowski et al.19 and Cerutti et al..6 Near 2.3 µm, from
photoluminescence at very low excitation, we measured a QW non-radiative lifetime longer than 50 ns, better
than GaAs QWs. The measured defects assisted non-radiative rate is thus
2 × 107 s−1 .
Ad >
∼
(1)
On the other hand, special care must be taken when growing lasers for emission beyond 2.4µm. These laser
structures are associated with QWs with higher Indium contents (∼ 0.50) which are genrally located inside
the miscibility gap of the alloy. It then becomes necessary to carefully adjust the growth T◦ for each target
wavelength, and thus alloy composition. The usual route is to decrease the T◦ to avoid decomposition of the
unstable alloys. The n.i. doped QW active region has essentially the same design for both types of structure
we developed, i.e. electrically-pumped edge-emitting laser diode and optically-pumped 1/2-VCSEL. The main
diﬀerence lies in the QW number used: typically 1-3 for edge-emitters, and > 5 for VCSELs due to low vertical
gain. Here we will only give the design of the 1/2-VCSEL structure. Details on the growth of the edge emitting
structure used in this work for the 2-2.6 µm range can be found in.10, 11, 13
The growth part consists of a n.i. doped 1/2-VCSEL structure (Fig. 2) on a 500 µm thick undoped (100)
oriented GaSb substrate6 (p-type residual doping <
3 1016 cm−3 ). This structure was grown with a RIBER
∼
Compact 21E. The Bragg mirror is made of lattice matched quarter wave GaSb (no = 3.82 at 2.3 µm) and
AlAs0.08 Sb0.92 (no = 3.2) layers. Thanks to the large index ratio, from 1.22 to 1.19 in the 2 to 2.7µm range,
between these two alloys, a high reﬂectivity can be attained with a small number of pairs, similarly to GaAsbased bragg mirror. This is a great advantage compared to InP-based bragg mirror e.g., which needs typically
twice more pairs using quaternary alloys20 (long time growth, high thermal impedance and optical losses). In our
typical design, a 24.5 pairs Bragg mirror was designed (8.1 µm thick at 2.3 µm), allowing a compromise between
a high reﬂectivity - ranging from 99.94 % to 99.86 % between 2 to 2.7µm, with measured bulk absorption losses
Proc. of SPIE Vol. 6184 61840N-3
Figure 3. a) SEM picture of the DFB metal grating.10 b) V(E)CSELs diode-pumped by a low cost 100mW singletransverse-mode commercial 800nm GaAs laser-diode, linearly polarized. f1 : aspheric lens; f2 achromat lens. A piezoelectric is used for frequency tuning of the VECSEL; the external miroir radius of curvature is 4 or 15 mm.
of αi ∼ 3 cm−1 - and the thickness, in order to decrease the time growth and the high thermal resistance of this
stack. The gain region is formed by a 1(to 1.5) wavelength long microcavity, containing Al0.35 Ga0.65 As0.03 Sb0.97
barriers, with a bandgap of 1.05µm which allows eﬃcient optical pumping with a commercial GaAs laser diode.
For 2.3µm emission, the gain is provided by NQW = 5 (to 8) type-I LQW = 10 nm Ga0.65 In0.35 As0.1 Sb0.9 QWs,
grown at 470◦ C,6 separated by 20 nm barriers to inhibit the QW coupling. The QWs are located at the standing
wave antinode of the longitudinal cavity mode. The top part of the design is composed of a 2 (to 3) λ thick
(1440 to 2160nm) AlAs0.05 Sb0.95 layer in compressive strain. This layer acts as a heatspreader as well as a
barrier carrier conﬁnement layer. This composition allows to reduce the thermal resistivity relatively to the
lattice match AlAsSb.21 To ﬁnish, the heatspreader is capped by a 5nm GaSb layer to prevent from oxidation
the high Al-content layer. Details on the structure characterization can be found in.6
2.3. Device design and technology/processing
2.3.1. Electrically-pumped DFB laser
The post growth grating processing has been carried out by Nanoplus Nanosystems and Technologies GmbH,8
in the frame of the GLADIS EU Project. Details on the device design and DFB processing can be ﬁnd in.10
Brieﬂy, ridge waveguide structures have been deﬁned after the growth, using a Chlorine/Argon based electron
cyclotron resonance dry etching process. Ridge waveguides have been processed, from W = 5 to 3µm wide. For
stable single longitudinal mode emission, the depth of the ridges has been optimized for strong coupling between
the light wave and the metal grating.22 First order lateral DFB gratings have been exposed by electron beam
lithography beside the ridge (Fig. 3-a), with a period of 334 nm according to the eﬀective refractive index of the
structure and the wavelength. Next, a metal layer is evaporated and lifted-oﬀ. The grating is fabricated on both
sides of the ridge. A polymer is spin-coated for planarization of the ridge-structure. A p-contact metalization
followed by thinning and n-contact metalization ﬁnish the process. The back facet is dielectric coated to have a
∼ 80 % reﬂector. The chip is cleaved into single lasers having a length of Lc = 800µm and a width of 400µm.
Individual chips are epi-side-up mounted on standard TO 5.6 or TO 8 headers using a tin-lead solder and tested.
2.3.2. Diode-pumped µc-VCSEL and VECSEL
For these simple technology devices, no post-growth processing is required for carrier injection, as a low cost
low power commercial 800 nm GaAs laser diode is used for optical excitation (Fig. 3-b).6, 7 Thus the structure
is optimized optically, rather than electrically: reduction of optical losses and heating (mirror); no need for
interface grading; volume generation of carriers avoids problem of charge separation. With our design, eﬃcient
pump absorption is obtained in the barriers, as ∼ 75% of the pump power is absorbed in these layers - the
measured absorption coeﬃcient at 800 nm is αp ∼ 2.5µm−1 -. Note that for the target applications, i.e. gas
analysis, where the whole system volume is > 200 cm3 and weight is > 10 kg,2 a diode-pumped VCSEL device is
compact enough. The pump beam is focused with two standard optical lenses on a 2 wp = 10 − 40 µm diameter
spot (FWHM), depending on the device conﬁguration.7 For VECSEL, a large active area can be used for high
Proc. of SPIE Vol. 6184 61840N-4
power, still in single mode operation thanks to the external cavity stability, whereas for guided µc-VCSEL a
small one (wp < 10 µm typically here) is necessary to force single mode operation. The linearly polarized elliptic
pump beam (2.5 × 1) is focused on the chip with an incidence close to the Brewster angle, to have a quasi-circular
spot and reduce reﬂection (< 10%). The pump polarization is turned by 90 ◦ using a half-wave-plate.
The µc-VCSEL23 is formed by a 7 pairs dielectric mirror (TiO2 /SiO2 , ∼ 99% of reﬂectivity) evaporated on
the 1/2–VCSEL chip (Fig. 3-b). The VECSEL is formed by the bare 1/2–VCSEL, an air gap, and a commercial
concave glass-based dielectric mirror (99.4% of reﬂectivity, 4 or 15 mm cavity length). A top anti-reﬂection coating
is not necessarily require in the latter case, where the microcavity only acts as a spectral ﬁlter (for characterization
purpose, a Si3 N4 antireﬂection coating was evaporated on some samples). Note that the reﬂectivity of the output
coupler, as well as the number of QWs, have been optimized to ensure high external quantum eﬃciency without
large increase of the threshold density. Indeed, in the vertical gain conﬁguration, QWs are operated at high
carrier density - about twice the transparency here -, where non-radiative recombinations become dominant via
Auger process in long wavelength lasers, which in turns reduces the T0 of the device. To optimized the laser
parameters, we measured the modal gain per pass in the 1/2-VCSEL G 2 g ΓNQW LQW (conﬁnement factor
Γ = 1.67 with a 5 QW AR coated structure; material gain g in cm−1 ) at 293K, versus the incident pump intensity
Ip . This was carried out by using a L-shaped external cavity VCSEL and varying the intracavity losses with a
CaF2 window close to Brewster angle. A good ﬁt gave24
G(Ip ) = G0 × ln(Ip /Itr )
(2)
with G0 0.18%/QW and a transparency intensity Itr 45 W/cm2 /QW , equivalent to 30 A/cm2 /QW - ∼ 3×
lower than GaAs-based QWs -, in agreement with our theoretical study.25 We found out that a transmission
of 0.5-1 % for the output dielectric mirror is large enough compared to typically 0.15-0.26 % loss per pass in the
1/2-VCSEL (transmission plus bulk losses), evaluated in the 2-2.7 µm range. Obviously bulk losses increase for
doped structure via free-carrier absorption (∼ 5 − 10 cm−1 measured for n-doped 2 1018 cm−3 bragg mirror).
3. LASER FUNDAMENTAL PROPERTIES AND CAVITY DESIGN
3.1. Thermal properties: high power CW operation and tuning limitations
The thermal resistance Rth has two main implications on the device properties: ﬁrst it limits the maximum T◦
operation achievable in CW regime, and thus the maximum output power, as the threshold increases with T◦
(T0 ). On the other hand, a high thermal impedance induces a large wavelength tuning rate of the gain or/and
cavity mode with CW excitation power of the semiconductor device: this is a proﬁtable property for spectroscopy
applications. For monolithic devices (”ﬁlled” cavity) like DFBs or µc-VCSELs, cavity mode frequency tuning is
achieved by modulating the excitation power via optical index modulation (∆n/∆T 5.5 10−5 /K or ∆νmode −12 GHz/K). A problem arises here, it is the unequal tuning speed with heat load of the peak gain and mode
in monolithic devices: gain tuning (∆νg /∆T −75 GHz/K) is typically six times faster, which provokes modehopping (Fig. 6). In contrast, in the VECSEL conﬁguration, ”ﬁlled” with air, the longitudinal mode tuning is
decoupled from the structure T◦ , and the mode frequency tuning is achieved here via cavity length modulation
(mechanically: piezo voltage modulation). However, the large gain tuning rate can be exploited for broad tuning,
but wihtout mode-hopping if gain and cavity mode are tuned in phase (Fig. 6). In the vertical conﬁguration,
the need for a thick multilayer stack as bragg mirror - with low thermal conductivity - and a small active area,
implies a high thermal impedance, larger by a factor 5-10 compared to the DFB conﬁguration. The tuning rate
is thus naturally larger in VCSELs. However the low gain value along the QW quantization axis implies high
QW excitation, thus a low T0 compared to DFBs, limiting the VCSEL CW operation. First we measured the
QW T◦ by measuring the device thermal resistance. This was carried out by measuring the laser wavelength
shift with T◦ and excitation power. Indeed, the thermal impedance between the QWs and the heatsink, average
over the modal gain volume, is given by :
∆λ −1 ∆λ TQW − Tsubstrate
(3)
×
Rth =
Pth
∆T
∆Pth
Note that for µc-VCSELs, this method will underestimate Rth , as the microcavity mode shift is related to the
average T◦ over the eﬀective cavity length (two mirrors plus QWs). Here we can assume that the copper heatsink
Proc. of SPIE Vol. 6184 61840N-5
κ (W/Km)
33
7
Material
GaSb
Al0.9 Ga0.1 As0.08 Sb0.92 Cladding21
AlAs0.08 Sb0.92 /GaSb Bragg mirror
Al0.35 Ga0.65 As0.03 Sb0.97 QW barriers21
AlAs0.05 Sb0.95 Heatspreader21
15
7.5
20
Table 1. Thermal conductivity κ (W/Km) at 300 K of the bulk Sb-based materials used in the structures.
ii
—
Figure 4. a) Semi-proﬁle (x,y) of the temperature distribution for an up-mounted DFB diode. Drive current 87 mA CW.
An increase of 13◦ C is calculated in the QWs. b) Semi-proﬁle (r,z) of the temperature distribution for the diode-pumped
1/2-VCSEL at 1W, 26 µm spot and 3 λ heatspreader. (Femlab simualtions).
has a weak contribution to Rth , as the substrate thickness is thick enough to support the whole heat dissipation
for up-mounted devices. We modelized the T◦ distribution in CW with a ﬁnite element method (Femlab) and
a three layer analythical model26 for VCSELs. For accuracy, thermal conductivities have to be known precisely
(Tab. 1). These simulations are however accurate enough to understand the thermal behaviour.
3.1.1. Electrically-pumped DFB laser
The typical value of the measured thermal resistance Rth of those Sb-based DFB structures is 115 K/W (Lc =
800 µm), an order of magnitude higher compared to 100 µm ridge broad area devices. The diodes are generally
epi side up mounted (n-doped side on the header). For estimation of Rth , we can assume that all the eletrical
power is converted into thermal power Pth . Under a drive current of 87mA, a typical optical power of 2.5 mW
was measured at 293K for an electrical power of 0.11W. We measured a T◦ increase of ∆T = 13◦ C between TQW
and Tsubstrate. The simulation of the T◦ distribution in the up-mounted laser is shown in Fig. 4-a. The heat load
is localized here √
in the active region of volume 0.8 × 4 × 800 µm3 for W = 4 µm ridge. The thermal impedance
scales with ∼ 1/ W × Lc . In contrary to up mounting, the down mounting (not shown) is more sensitive to the
quality of the thermal contact between the laser and the heat sink. In that case the T◦ increase is 6◦ C, thus the
wavelength tuning rate is divided by two.
Figure 5. a) Thermal impedance Rth of the diode-pumped 1/2-VCSEL computed for diﬀerent substrate thermal conductivities and diameters; experimental data are shown (analythical model26 ). b) Relative change in Rth of the 1/2-VCSEL
versus AlAsSb top heatspreader thickness for three pump diameters. Shadow area: thicknesses used in this work.
Proc. of SPIE Vol. 6184 61840N-6
δνL~5MHz
Air gap stable cavity VCSEL
µ -cavity VCSEL
Small circular beam - Guided
TEM 00 at low power
Polarisation switching
600
Feedback sensitive
SHB, ASE
Heavy Technology
Large circular beam
TEM 00 at high power
Linear polarisation
TEM 00
Gain
S
P
65GHz λ
Fixed or ∆(P,T)
Cavity modes
Gain
Mature
Technology
10-40GHz
2
P
10GHz
δν L <kHz VECSEL
x6
TEM 10
S
Simpler Technology
F IN E S SE
∆P(t)
µc-VCSEL
δνL~5MHz
λ
Edge emitting DL, DFB
External Cavity DL DFB
δνL~500kHz
Narrow ridge - Guided
TE00 Highly divergent elliptic beam, M 2>1
SHB, ASE, Feedback sensitive, Linear polarisation
mm
x6
Cavity modes
+ DFB Filter
Gain
cm
CAVITY LENGTH
50GHz
λ
∆Lc (t)
∆P (t)
λ
Figure 6. (left) Fundamental properties of semiconductor lasers: µc-VCSEL, VECSEL, DFB and External-cavity diode
laser. SHB: Spatial Hole Burning, ASE : Ampliﬁed Spontaneous Emission. (right) Spectral properties of optical cavities.
3.1.2. Diode-pumped µc-VCSEL and VECSEL
The typical value of the measured resistance Rth for those optically-pumped 1/2-VCSEL structures is 800 K/W for
a 26 µm pump spot (FWHM), shown in Fig. 5-a, which is twice the value of 980 nm GaAs based 1/2-VCSEL. The
fraction of heat power generated with incident pump power is Pth 0.7 Pinc in laser action, assuming ηi 90 %
and constant with T◦ . These values are in agreement with the experimental output power characteristic (see
Fig. 9-a). The simulation of the T◦ distribution in the up-mounted 1/2-VCSEL device is shown in Fig. 4-b.
The heat load is localized here in both the active region and bragg mirror layers, assuming an exponential pump
absorption proﬁle and a gaussian transverse proﬁle, of volume π222 /2×1/2.5 µm3 for wp = 22 µm pump radius
(at 1/e2 ). Fig. 5-a shows how Rth scales with the pump diameter, with a fair agreement theory/experiment.
Fig. 5-b shows how Rth changes with the top AlAsSb heatspreader thickness. The heatspreader critical thickness
ec , for which Rth is reduced to ∼ 1/2 is given by ec wp × (κQW /κHeatSpreader ). ec is ∼ 8 µm for a 13 µm spot
here. With our 1/2-VCSEL structures, Rth was reduced by 10 to 20 % thanks to the strain AlAsSb top layer,
but this solution shows its limits. For small diameter devices, another solution for the heatspreader material
(not shown) would be to use a lattice match (AlAs)(1) /(AlSb)(11) digital-alloy supperlattice,21 which has a high
parallel conductivity of κ ∼ 50 W/Km (κ⊥ ∼ 7.3 W/Km). For a 3 λ thick heatspreader, we calculated a 18 %
reduction of the impedance for a 26 µm spot, compared to AlAs0.05 Sb0.95 material. A solution to greatly reduce
the Rth of tunable VECSEL devices (shown in Fig. 5-a) would be to fabricate a reverse structure metal bonded
on a carrier substrate of high conductivity, like SiC e.g., and remove chimically the GaSb substrate. Top optical
bonding of a thick heatspreader window is not suited here as it would strongly limit laser tuning (parasitic
Fabry-Perot; requires a high quality AR coating) and polarization stability.
3.2. Optical cavity and guide: TE(M)00 beam, light polarization and spectral properties
To operate single frequency, i.e. in a single light state, ﬁrst of all the laser cavity has to oscillate on only one
transverse mode, usually the fundamental TE(M)00 mode, which puts strain on the cavity stability design (optical
guide section or length/mirror-radius of curvature for an external cavity). Secondly, only one polarization state
must be selected (via transition selection rules in the QWs or loss dichroism). Last, only one longitudinal cavity
mode has to be selected in the ∼ 4 T Hz gain bandwidth (via modal gain competition plus a ﬁlter usually).
Proc. of SPIE Vol. 6184 61840N-7
I
I
I
I
(hw)
Figure 7. (left) Intensity of the TE00 guided mode in the DFB (2D Femlab simulation x,y). Light is guided in the
active zone between the two cladding layer under a ridge of 4µm. The step index is ∼ 0.4 (⊥) and ∼ 0.02 (). (right)
Intensity (2D Femlab Simulation, circular symmetry) of guided modes, due to T◦ gradient induced index guiding, for the
µc-VCSEL, in agreement with the experimental mode waist. Shadow area: gain region.
Ampliﬁed-Spontaneous-Emission (ASE) and any non-linear mode/polarization coupling will generate non ideal
light state with reduced coherence (ﬁnite linewidth) and non-negligible side mode intensities.9, 24 For a guided
plan-plan monolithic laser cavity, like edge-emitting diodes or µc-VCSELs, ASE and non-linear mode coupling,
via longitudinal (DFB) or transverse (µc-VCSEL) Spatial-Hole-Burning (SHB), are strong: the devices usually
operate multimode, thus the side-mode-suppression-ratio (SMSR) is strongly reduced. In addition, due to the
guided nature of light here, the guide section has to be small enough (∼ few wavelengths, depending on the guiding
strength24 ) to force TE(M)00 operation, reducing the output power achievable. This is the reason why single
mode narrow-ridge edge emitters require a DFB-based narrowband spectral ﬁlter, and why a few wavelength
diameter strongly elliptical active area is needed for µc-VCSELs. This stress on the design disappear in the
VECSEL case, which takes advantage of the stability of an air-ﬁlled plan-concave cavity,24 where the active
area can be designed as large as required for high power operation still in TEM00 operation. We recently showed
that there is no SHB and very weak ASE in a 1-20 mm long linear cavity VECSEL.9 It thus allows stable highly
coherent single light state operation at high power without any obvious need of an intracvity ﬁlter: the gain
shape curvature is suﬃcient, via mode competition dynamics to select one mode after typically 1 ms,9 shorter
than acoustic/thermal perturbations. Any additional spectral ﬁlter will strengh the single frequency operation,
like for uncoated 1/2-VCSEL where the residual microcavity acts as a broad ﬁlter (∼ 700 GHz width). Fig. 6
shows the fundamental properties for the three devices and typical cavity mode features (longitudinal, transverse
TEMi,j and polarization s/p). It shows that the typical frequency gap between two light states is similar and
in the range 10-60 GHz for the three devices: thus only the intrinsic selectivity dynamics of the laser will play a
role for single frequency operation. It is thus of interest to know the photon lifetime: for DFBs and µc-VCSELs
it is typically 7 ps, to be compared with 4-12 ns for VECSELs. Thus monolithic devices are very sensitive to
optical feedback, whereas in contrary VECSEL does not exhibit relaxation oscillations and is very sensitive to
any weak selective losses in the cavity. VECSELs will then select a highly coherent light state of cavity.
3.2.1. Electrically-pumped DFB laser
The small asymetric rectangular guide section (0.8 × 4 µm) forces to propagate only the TE00 transverse mode.10
A linear light polarization is thus perfectly deﬁned, parrallel to the layers. The beam waist (FWHM) in near ﬁeld
is calculated to be 0.8 µm vetically and 3.5 µm horizontally, in fair agreement with measured far ﬁeld distribution
(Tab. 2). The beam generated is thus strongly elliptic and divergent. The SMSR, limited by ASE,24 and the
maximum continuous tuning range are related to the coupling strengh between the guided mode and the DFB
metal grating.22 This narrowband ﬁlter produces a loss modulation between the strong mode and adjacent ones
of typically ∼ −3 cm−1 (to be compared to ∼ 20 cm−1 threshold modal gain).
3.2.2. Diode-pumped µc-VCSEL and VECSEL
For the diode-pumped VCSEL, we showed that the transverse modes are stabilized by an index guide, induced
by the radial T◦ gradient (thus index gradient) around the pump area in the 1/2-VCSEL in cw operation
(Fig. 7, right). Thus a high Rth , or pump power, induces a strong guiding, which in turns implies to reduce the
Proc. of SPIE Vol. 6184 61840N-8
I
I
I
GWLC0U
\I / IIEVI°°
I
1ET'°
/
-80
-20
-40
-30
0
L(hW)
LEVI3°
30
40
20
80
Figure 8. (left) Cavity stability of the plan–concave VECSEL induced by diﬀraction, w/o thermal lens in the 1/2-VCSEL
structure: beam waist on the chip versus the airgap length with external mirrors of Rc2 = 4 and 15 mm radius of curvature.
Rc1 : equivalent Rc of the thermal lens measured for 55 mW of pump. Inset: zoom around Lc = 15 mm (dot-line: pump
diameter), where laser action is possible over ∼ 50 µm, a value twice larger for Rc2 = 4 mm. (right) Intensity of stable
transverse modes for 40 µm waist VECSEL. Shadow area: gain region.
device diameter (below 20 µm typically) to force TEM00 operation with 1 mW output. The plan-plan cavity was
unstable, and no lasing observed, for pump pulses shorter than ∼ 3 µs (at 50 mW pump, 26 µm diameter). For the
VECSEL, the light is not guided, and the TEM00 mode is stabilized by free space diﬀraction in the stable planRc2 = 4/15 mm).
concave external cavity (Fig. 8), operating close to the stability limit of the ”cold” cavity (Lc <
∼
The beam waist, fonction of Lc and mirror radius of curvature Rc, can be thus as large as required, as the gain
area aperture can be adjusted, independently, to match the fundamental mode diameter. Note that in contrast
to guided beams, in this case all the transverse modes (of diﬀerent order) have the same waist: thus high order
modes extend further in space and are easy to suppress. We observed experimentally a cavity stability distortion
provokes by the thermally induced radial index gradient in the 1/2-VCSEL (Fig 8). The measured thermal lens,
power dependent, was equivalent to a concave mirror with Rc1 8.5 mm at 55 mW pump (26 µm spot). The
cavity stability shape ”jumps” for Rc1 < Rc2 (Fig 8). Also for both devices, due to residual (or intentional)
ellipticity in pump spot shape, we observed an Hermite-Gauss beam distribution showing circular symmetry
breakdown. The thermal lens generated in both devices could potentially damage the beam wavefront, thus
generate non-diﬀraction limited beam.
One of the drawbacks of the circular VCSEL geometry is that light polarization is not perfectly stable. At ﬁrst
order, the polarization states are circular. However, crystal birefringence along the [110] and [1¯10] axis breaks
the circular symmetry, and gives rise to a polarization mode degeneracy break. This leads to a rather large
frequency splitting for a ﬁlled µc-VCSEL (measured to be ∼10 GHz), but rather small for an air ﬁlled VECSEL
(∼10 MHz). Another channel for breakdown symmetry is the QW gain dichroism between these axis in the QW
plane. In our case, we measured a relative QW gain diﬀerence of few percents at threshold, stronger on the [110]
axis, which should stabilize a linear light polarization. This gain dichroism is ampliﬁed by using an elliptical spot
along [110]. For µc-VCSELs, this conﬁguration should also create an asymetric guide (breakdonw symmetry),
thus induce loss dichroism between orthogonal polarization states. However, polarization stabilization has been
clearly observed experimentally in this conﬁguration for VECSELs, but not for µc-VCSELs, which shows that
VECSELs are very selective as explained above. In contrast to VECSEL and DFB lasers, where the SMSR is
limited by adjacent longitudinal modes, for the µc-VCSEL, the SMSR is limited by the orthogonal polarization
extinction ratio and by the higher order transverse mode supression ratio. Surprisingly, we did not observed
polarization and TEM00 mode stabilization at high power or T◦ (i.e. for large gain red shift compared to the
microcavity mode frequency), as it is theoretically predicted and shown on Fig. 6 (right).
4. SINGLE FREQUENCY OPERATION NEAR 2.3 µM IN CW AT RT
For clarity, the parameters are summarized in Tab. 2. Details on this study can be found in7, 10
Proc. of SPIE Vol. 6184 61840N-9
Parameter (CW, 288 K)
Unit
Maximum Temperature Operation
Characteristic Temperature
Threshold Power|Current Density
Internal quantum eﬃciency
800 nm pump absorption eﬃciency
Bulk broadband optical losses αi
Single–Frequency Max. Output Power
Orthogonal polarization extinction ratio
Typical Beam Divergence , ⊥ (FWHM)
Beam Quality Factor M2
Cavity Free-Spectral-Range | Finesse
Orthogonal Polarization splitting
SMSR
Theoretical Laser Linewidth
Experimental ”Dynamic” Linewidth
K
K
kW |kA /cm2
%
%
cm−1
mW
dB
Max. Continuous Tuning Range
Total Tuning Range
Technology
GHz
THz
◦
–
GHz | –
GHz
dB
MHz
MHz
VECSEL
Lc =4/15mm
350
∼ 50
0.55|> 90
67
∼3
5.2
42
3.9
<1.2
37.5/10 | 650
∼ 10−2
>28
∼ 10−6
<< 20 10−3
high
stability
250/70
1.4
Prototype
µ-cavity VCSEL
26µm spot
320
–
3|> 90
67
∼3
1.5
30 - switching
5.6
∼1.5
23 103 | 550
∼ 10
>16
3
∼5
feedback
sensitive
42
0.6
Prototype
DFB
4 × 800 µm
370
110
-|1.3
30
–
∼ 10
10
>20
11.5x50
1.3x3.5
50 | 1.7
–
35
1.7
∼4
feedback
sensitive
400
∼ 0.7
Mature
> 5000h
Table 2. Comparison between the three Sb-basd laser device technologies (vertical and edge) developped at CEM2.
4.1. Threshold, Output power, High temperature operation and Polarization state
DFB laser diode, external-cavity and µc-VCSELs all exhibit CW single–frequency operation near 2.3 µm above
RT. For the VECSEL, the threshold incident pump intensity was 550 W/cm2 at 288 K, twice lower than GaAs–
based VCSELs, with a T0 50 K (Fig.9-a). This low threshold value - equivalent to 50 A/cm2 /QW, in fair
agreement with the theory25 - balances the low T0 (due to high QW excitation) and high Rth of the 1/2-VCSEL.
ηi was measured to be > 90 % here, in agreement with the theory. For the µc-VCSEL, the threshold density
was 3 kW/cm2 at 283 K (Fig.9-b), much higher due mainly to a detuning between the gain and the microcavity
mode. For the DFB, the threshold current density was 1.3 kA/cm2 (high compared to broad area devices13 )
with T0 =110 K (Fig.9-c), and a low ηi due to current spreading in QWs. With an optimized pump spot size
of 26µm (FWHM), the µc-VCSEL output was limited to 1.5 mW at 283 K in single frequency regime due to
transverse mulimode operation above, while the VECSEL and the DFB reached more than 5 mW. The DFB
operated up to 370 K thanks to its low Rth , but V(E)CSELs maximum T◦ operation (∼350 K) and output power
are still high here compared to InP-based VCSELs e.g.. The threshold values obtained here near 2.3 µm increase
drastically above 2.4 µm (Fig. 1-a), but cw lasing should still be reached at 2.7µm in VCSEL conﬁguration. Due
to the TEM circular geometry, several polarization mode hops, aligned along the [110] and [1¯10] crystal axes,
were observed for the µc-VCSEL. Thanks to its stable cavity, the polarization switching has been suppressed
in the VECSEL by using an elliptical pump spot along the [110] crystal axis (aspect ratio 1:2). From a direct
measurement, the extinction ratio between the linear polarization states for the µc-VCSELs and the VECSEL
were 30 dB and >42 dB, respectively. For the TE polarized DFB, the polarization is stable at >20 dB.
4.2. Transverse beam quality: far ﬁeld and M2
The transverse beam proﬁles - in far ﬁeld (θ) and near ﬁeld (w0 ) - were characterized using a Xenics extended InGaAs photodiode linear array (512 pixels - 25µm pitch). The beam quality factor M 2 = πw0 θ/λ (1 = diﬀraction
limit) was perform by imaging the near ﬁeld beam with two achromat lenses. Both V(E)CSEL devices exhibited
Proc. of SPIE Vol. 6184 61840N-10
Figure 9. CW output power versus incident pump power for: the VECSEL at a) 288 K (diﬀerential eﬃciency in cw
8 %), c) at diﬀerent temperatures (dash-line: simulation at 288 K); b) the µc-VCSEL at 283 K. d) Output power and
V(I) of the DFB laser versus current for diﬀerent heatsink temperatures (diﬀerential eﬃciency 11 %).
Figure 10. Transverse beam far ﬁeld distribution θ at the device output and beam quality factor M2 .
a circular single transverse mode beam distribution with a low divergence of 3-6◦ (FWHM) and a beam quality
factor M2 <1.2 close the diﬀraction limit for the VECSEL, and M2 ∼1.5 for the µc-VCSEL (Fig.10-a,b). Transverse multimode operation was observed for the µc-VCSEL at incident power higher than 40 mW, whereas the
VECSEL showed a TEM00 transverse beam distribution over the whole laser characteristic thanks to its stable
external cavity. The DFB beam has a strong divergence and is not diﬀraction limited (Fig.10-c). To collimate
the fast axis, a non-conventional lens with a numerical aperture higher than 0.8, is required. The quality factor
measured with a 0.5 NA Geltech aspheric lens was M2 =1.3 for the slow axis and 3.5 for the fast axis.
4.3. Spectral properties
4.3.1. SMSR, laser linewidth
We measured a SMSR>28 dB for the VECSEL, and >16 dB for the µc-VCSEL, both limited by the pedestal
of the apparatus function (Fig.11-a, b). The DFB exhibited a SMSR of 35 dB. We carried out laser linewidth
measurement (Fig.11-d) with a Scanning High Finesse Cavity (SHFC). The collimated beam was injected into
Proc. of SPIE Vol. 6184 61840N-11
Figure 11. Single–frequency spectrum of a) the VECSEL, b) the µc-VCSEL and c) the DFB laser. d) Laser linewidth
of devices versus the output power. The oblique lines show the quantum limits.
a 43 cm long high ﬁnesse passive cavity (20 kHz mode width).7 For the µc-VCSEL and the DFB, a two stage
isolator (-40 dB) was needed to supress optical feedback, whereas the VECSEL is much less sensitive due to
its long photon lifetime. In frequency tuning condition, we measured a 5 MHz linewidth for the µc-VCSEL (at
1 mW) and a 4 MHz one for the DFB (at 2.5 mW). The VECSEL linewidth was limited by the SHFC mode width.
By computing the VECSEL transmission through the SHFC,27 the experimental results was perfectly predicted.
We deduced a VECSEL linewidth at least an order of magnitude below 20 kHz. To ﬁnish, we calculated7 the
theoretical quantum limit for the laser linewidths by using the modiﬁed Schawlow–Townes equation24 (Henry
factor α 3). They are in relatively good agreement with the experiment for the µc-VCSEL and the DFB (see
Table 2), but jitter may explain the experimental broadening. Whereas the theory gives ∼1 Hz (at 1 mW) for
the VECSEL, in practice acoustic/thermal noise should limit the linewidth to the kHz level.28
4.3.2. Continuous frequency tuning and total wavelength span
For laser absorption spectroscopy application, the mode–hop free frequency tuning range is a very important
parameter. To characterize the continuous tunability range, we measured the transmission through a 2.34 GHz
free spectral range Germanium Fabry-Perot etalon. We also acquired the transmission through a 1.8 cm long
saturated methane cell (ﬁtted using the HITRAN-96 database), which are shown on Fig. 12 (top). For the
V(E)CSEL, the frequency tuning rate of the mode with cavity length Lc was –33 GHz/µm for Lc = 4 mm (pump
power and T◦ were ﬁxed). Here the gain/cavity mode detuning limited the continuous tuning to ∼250 GHz,
already 7× cavity FSR (Fig.12-a) without any ﬁlter. This was possible thanks to threshold change induced gain
tuning, as the laser beam waist scales with Lc . The cavity mode tuning rate with heatsink T◦ was -12 GHz/K
for the µc-VCSEL and the DFB, and the mode tuning rate with pump power/current was -4.8 GHz/mW and
-2.2 GHz/mA respectively. We measured a typical mode–hop free–tuning range of 45 GHz for the µc-VCSEL,
limited by polarization state switching (Fig.12-b). The DFB laser showed ∼400 GHz of continuous tuning range
(Fig.12-b) thanks the the selective grating, but limited by the gain/ﬁlter mode detuning. The maximum tuning
1kHz for VECSELs, ∼ 100 kHz for µc-VCSELs, and ∼ 1 M Hz for DFBs. To end,
speed was measured to be <
∼
we show that VECSELs allow an overall emission range of 1.4 THz, with mode hops (Fig.12), by varying the T◦
and pump power, as the large gain tuning rate can be exploited here (-75GHz/K and ∼-30 GHz/mW).
Proc. of SPIE Vol. 6184 61840N-12
Figure 12. (top) Continuous frequency tuning for a) the VECSEL (wavelength= piezo-voltage(t)), b) the µc-VCSEL
(wavelength= pump power(t)) and c) the DFB (wavelength= current(t)). (bottom) Broadband VECSEL tuning (by mode
hops) by varying the pump power and heatsink temperature. Gray solid lines: HITRAN-96 database.
5. CONCLUSION
In this paper, we presented a review and a systematic comparison between three diﬀerent type of Sb-based
laser devices for the 2-2.7 µm range: diode–pumped microcavity and external–cavity VCSELs, and electrically–
pumped DFB laser. For the three laser sources, tunable single–frequency operation was demonstrated above RT
in CW with >1 mW output at 2.3-2.4µm. Thus those devices are well suited for laser spectroscopy applications,
such as trace gas analysis, as their linewidth is well below the doppler width of a gas. The very low beam quality
of the DFB laser, in contrast to VCSELs, does not allow to use commercial low cost optics for beam shaping,
which is a severe limitation. This needs to be improved by a new waveguide design. However Sb-based VCSELs
are still laboratory prototypes, in contrast to Sb-based DFB technology which is already mature for the market.8
On contrary, V(E)CSEL sources show a high beam quality adapted to commercial low cost optics, well suited
for long collimation range or tight focusing, and high quality mode matching in a external optical system,1 e.g..
The drawbacks of the µc-VCSEL are its low power, the polarization instability and a rather limited continuous
frequency tunability. The last two can be improve by designing very small strongly elliptic active area laser
(< 10 µm diameter), to increase the Rth , but to the detriment of the output power.
This comparative study shows that monolithic devices need more complex and expensive technological stages
to get the required speciﬁcations, than the ”simple” external-cavity VCSEL conﬁguration presented here. Indeed
this new vertical cavity design ”boosts” the coherence of the emission: circular close to diﬀraction limit beam at
high output power, with low divergence of few degrees, ultra narrow linewidth at the kHz level with high SMSR,
broad continuous tunability close to the THz level, low sensitivity to optical feedback, and good polarization
stability. For low power applications (<1 mW), work is in progress at CEM2 to developp electrically-pumped
Sb-based 1/2-VCSEL devices. Work is also in progress to developp ”robust” VECSEL with shorter external
cavity, suitable non standard packaging, and to tune both modal gain peak (pump modulation) and cavity mode
(piezo voltage) synchronously to strength and extend the continuous tuning above THz level.
Proc. of SPIE Vol. 6184 61840N-13
Figure 13. (Top) OF-CEAS spectra from DFB laser, obtained by joining 4 current scans (60-100
mA) taken at diﬀerent
√
temperatures from 4 to 11◦ C (80 ms scan). The noise level is ∼ 2 10−8 /cm (or 2 10−10 /cm/ Hz). Cavity ring-down time
on the baseline (subtracted) was ∼ 6.5 µs. (Bottom) HITRAN2004 simulation.
The ultra-narrow VECSEL linewidth renders it well suited for the injection of a high ﬁnesse cavity, like
in CRDS.1 Sb-based DFB sources are now applied in high sensitivity spectroscopy instruments for in-situ
measurements at LSP laboratory, Grenoble, using the OF-CEAS techniques29 (Fig. 13). Work is in progress to
apply both type of VCSEL sources to CRDS1 or/and OF-CEAS instruments at LSP.
ACKNOWLEDGMENTS
This work was supported in part by the GLADIS (Gas Laser Analysis by Infrared Spectroscopy) EU project (IST
2001-35178), by a cooperation ECOS Nord/ANUIES (M01 P02), by OLDHAM and RIBER groups, and by the
R´egion Languedoc Roussillon. The authors would like to thank their colleague M. Garcia from Thales Research
Technology France for the fabrication of the dielectric Bragg mirror, in the frame of the GLADIS project.
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