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about 112 dB . H z ~ ’ ~
The
. input power of the third order intercept point,
IIP3, is higher than 25 dBm. Fig. 3 shows the IIP3 and SFDR values of
the laser against the bias current of the gain section. Because there is
only about 1 dB difference of system total noise for bias currents from
50 mA to 120 mA, we uniformly use -160 dBm/Hz as the system
noise floor for simplification. The data indicate that there is an
optiinised bias current, around 55 i d , to realise the lowest distortion.
This results from the compromisc between the increased distortion at
low bias close to threshold and the decreased modulation response at
and
high bias current. The SFDR value remains ahovc I10 dB
the JIP3 above 22 dBm, up to I20 mA of bias.
-20O L
I
/
fundamental
U
Howevcr, for comparison, we still use -160 dBm/Hz to calculate the
SFDR values. The measurements show that the laser has relatively
uniform SFDR along the whole tuning rangc, with reductions from the
maximum value of no more than 10 dB . Hz”’. The IIP3 is always
above 20 dBm. Detuning of either the phase or mirror sections from
their optimum values for a specific channcl shows that the SGDBR
device is relatively insensitive to small detuning. There is both a noise
increase and a SFDR decrease at the mode hopping boundary, which is
due to the decrease in the output power and possible mode partition
noise.
Conclusion: The dynamic range properties of widely tunable
sampled-grating DBR lasers wcrc measured. At normal bias current,
the laser noise is limitcd by the photodiode shot noise, which is
-160 dBm/Hz. The highest SFDR value of 112 dB
is
obtained when only the gain section is biascd at 55 mA. At higher
bias currents and above 4 GHz, the SFDR drops by up to
10 dB . H z ” ~ . The lIP3 is always ahovc 20 dBm. The lasers show
uniform noise and distortion properties over the whole tuning r a n g of
50 nm and is relatively insensitive to mode detuning.
Acknowledgments; The authors would like to thank Agility Communications Inc. for supplying the laser device, This work was supported
by the DARPA RFLICS program via SPAWAR.
(c) IEE 2002
Electronics Lettem Online No: 20020120
Dol: IO. 1049/e1:20020120
-140
-120
-100
-80
-60
-20
-40
0
20
input RF power, dBm
Fig. 2 Two-tone measurements of’ SGDBR laser with gain seclion h i u
current of 60 mA
27 November 2001
H.X. Shi, D.A. Cohen, J. Barton, M. Majewski and L.A. Coldren
(University of’ California, Santa Barbara. CA 93106, U S A )
M.C. Larson and G.A. Fish (Agility Communications Ine., Sunla
Barhai-a, CA 93 I 17, USA)
References
I:
1
26
-112n
N
-
l o
T
m
2
cr-
3
U
U
0
-111
A
4
5
22
50
60
70
80
90
100
110
120
110
130
6
bias current of gain section, mA
Fig. 3 SFDR and i l P 3 values against bias current of’ gain section of
SGLlBK
The other three sections were disconncctcd
The same measurements were done on a few more SGDBR devices
for modulation frequencies from 0.1 GHz to 5 GHz. The gain section
was biased at 120 mA to reach the higher modulation fiequencies. They
showcd that the device has unifonn distortion at low frequencies, but at
frequcncies above 4 GHz, the dynamic range property of the laser gcts
worse, which is dne to the reduced modulation response. Nevertheless,
the reduction in SFDR is always less than 10 dB . H z ” ~and the IIP3 is
always above 20 dBm.
By modulating the lasers with one signal, thc sccond order harmonics
were also measured and a similar distortion phenomenon was observed.
The input power of the second order intcrcept point, IIP2, is always
higher than 20 dBm.
In the above section, we havc cxamined the noise and distortion of a
SCDBR lascr while biasing only the gain section. To investigate the
pcrformance over the tuning range, ten WDM channels from the ITU
standard covering a tuning range of 50 nm, were optimised through
wavelength mapping and analysed for noisc and distortion. The system
remains shot-noise limited over the entire tuning range [6]. A decreasc
of power at some channels, due to absorption loss induced by high
mirror currents, results in a few dBs decrease of the noise level.
ELECTRONICS LETTERS
14th February 2002
MASON: U.. FISH, G.A., BARTON, J., KAMAN, V, COLDKEN. L.A.,
DENRAARS, s.P., and BOWERS, J.: ‘Characteristics of sampled grating
DBR lasers with integrated scmiconductor optical amplifiers and
elechoabsorption modulators’. Proceedings of OFC2000. Baltimore,
MD, USA, 2000, Paper TuL6-1, pp. 193-195
FISH, G.A.: ‘Monolithic, widely-tunable DBK lasers’. Procecdings of
OFC2001, Anaheim, CA, USA, 2001, Paper TUBI
JAYARAM.4N, V, MATHUR, A., COLDREX, L.A., and DAPKUS, P.D.: ‘Theory,
design, and pcrformance of extended tuning range in sampled grating
DRR lasers’, IEEEJ Quantum Electron., 1993, 29, (6), pp. 1824-1834
FAN, J.C., LIJ. C.L., and KAZOVSKY. L.G.: ‘Dynamic range requirements for
microcellular personal communications systems using analog fiber-optic
links’, fEEE Trans. Microw. Theory k h . , 1997,45, (S), pp. 1390-1397
SAAVEDRk, A.A., RIGOLE, PJ., GOOBAK, E., SCHATZ, R., and NILSSOX, S . :
‘Relative intensity noise and linewidth measurements of a widely tunable
GCSR laser’, IEEE Photon. Technol. Lett., 1998, 10, (4), pp. 481483
SHI, H.X., COHEN. D., BARTON, I, MAIEWSKI, M., COLDREN, L.A.,
LARSON, M,, and FISH, G.A.: ‘Relative intensity noise measurements of a
widely-tunable sampled-grating DER laser’, to he published in
Photon. Technol. Lett.
Free-space optical transmission of
multimedia satellite data streams using
mid-infrared quantum cascade lasers
R. Martini, C. Bethea, E Capasso, C. Gmachl, R. Paiella,
E.A. Whittaker, H.Y. Hwang, D.L. Sivco, J.N. Baillargeon
and A.Y. Cho
Experimcnlal results for an optical fiee-space high-specd link using
dircct modulated mid-infrared (E. = 8.1 pm) quantum cascade lasers
are presented. A total of 800 digitally cncoded multimedia channels
were transmitted. The reliability of the system against weather
influence (fog)was experimentallycompared to that of a near-infrared
(2=0.85 rim) link.
Tnnlroductiont Bridging the so-called ‘last mile’ in tclecommunication
nctworks has revived interest in free-space optical (FSO) data transmission links. High bandwidth communication (2.5 Gbit/s) as well as
long distances (4 km) have already been demonstrated using fibre
Vol. 38 No. 4
181
components working in the 1.55 Itm wavelength regime [ 11. Nevertheless, frce-space links in thc mid-infrared (IR) spectrum seemed to
be more favourable as lower atmospheric transmission losses increase
the reliability of the system, especially under bad weather conditions
with low visibility.
Quantum cascade (QC) lasers arc now established as versatile
semiconductor light sources for this band and beyond (1. 3.524 pm) [2]. Gain-switching [3], modelocking [4], and high-speed
modulation without relaxation oscillations [5] of QC lasers have becn
demonstrated. These results are promising for high-speed telecommunication applications, especially high bandwidth free-space communication links. Mid-IR free-spacc links with QC lasers [6, 71 and their
high-speed digital modulation [8] werc recently invcstigatcd. In this
Letter we describe the first atmospheric transmission of complex data
(multimedia satcllite channels) with a mid-1R (A = 8.1 pin) QC laser
and contract the performance of this link to one operating in the near
infrared (A = 0.85 pm).
Experimentul setup: Fig. 1 a shows the optical setup of the transmission link. We used QC lasers grown by molecular beam cpitaxy in the
GaInAs/AllnAs material system, based on the so-called 'three-well
vertical' design of thc activc region [2]. In the following, we present
exemplary results from a 1.25 mm-long, 4.5 pm-wide deep etched
ridge laser (sample D2642BA) with an emission wavelength near
8.1 pm. The lasers were packaged and processed for high-frequency
modulation as described in [3]. The emission of the QC laser was
collimated using an f/3 ZnSe lens and then transmitted over an openair 100 m path to a retroreflector, mounted on another building of Bell
Laboratories in Murray Hill. The reflected light was collected using a
f/9 telescope with an aperture of 76 mm and focused onto a highspeed liquid nitrogen coolcd MCT detector (Sagem HgCdTc 01 1). To
compare the effect of the longer wavelength on the link quality and
stability, a second beam was included in the path, originating from an
0.85 pm diodc laser (10 mW output power) and detected with a
standard Si-detector. To ensure an identical beam path and easy
adjustment the optics for the outgoing beam and for the detection
were rigidly connected to the telescope.
-
a
MCT-det.
a wavelength of 0.85 pm had comparable losses, which are thcreforc
attributed to beam spreading and losses in the optical elements.
A typical example of the transmitted data stream is shown in Fig. 2 .
The modulation in the frequency region from 900 MHz to 1.45 GHz
contains the digitally encoded information (QPSK-code: q ud drature
phase shift keying) consisting of 800 television channels and 100
radio channels. Owing to the limited bandwidth of the detector, the
channels in the higher frequency rcgion wcrc detccted with a 10 dB
higher loss, rcducing the number of actually dccodable channels to
650. The link power margin is 7 dB, corresponding to a receivcd
power of 0.125 mW below which the receiver becomes unstable. As an
example of the quality of the link, the inset of Fig. 2 displays a
screenshot of a transmitted television picture showing one advertising
page from the digital satellite provider.
-
I
I
I
-100'
1.o
I
1.2
1.4
frequency,GHz
Fig. 2 Example o j lransmitted data stream
.__
signal
__
at output
of LNR (transmitted)
signal aftcr transmission uvcr rrec-spacc link (reccived)
Inset: television screenshot
-
To evaluate the advantage of the longer wavelength relative to the
collinearly propagating near-infrared beam (0.85 pm) the intensity of
the latter was monitored in parallel. For typical weather conditions
including sunshine, strong rain as well as thunderstorm, no differenccs
in sensitivity wcre obscrved. Neverlhclcss, a strongly pronounced
dcviation was obscrved during a deiise fog situation, with nearly zero
visibility. Fig. 3 shows the temporal evolution of the detected DC
intensities for both laser links, starting at a very dense fog situation in
the early morning of 8 January 2001. As the fog lifted slowly at around
3.15 a.m. thc QC laser link rcgained transmission much more quickly
than the near-IR link. The QC laser link had reached transmission of
nearly 70% of its optiinal value, when the intensity of the near-IR link
was still below the detection limit. As a result, at around 4.00 a.m., the
mid-IR telcvision link became stable again, whereas the near-1R link
was still unstable almost for another hour.
Fig. 1 Schematic diagrams
a Optical setup of traiismissioii link
h Electrical setup of QC laser and detection systcm
1 .O
Fig. 16 shows the electrical setup of the QC laser and of the detection
system. The sigma1 was rcceived from a satellite dish using a low noisc
block (LNB) down converter-module. This high-frcquency signal
(750 MHz-1.45 GHzj was combined with a DC current to drive the
QC laser continuously above its threshold. The modulated laser radiation was transmitted over the total distance of -200 m before it was
detected. The DC component was split off with a bias-Tee and used as
monitor of the reccived laser intensity. The high-frequency p"rt was
amplified and fcd into a spectrum analyser as well as into a standard
satellite set-top-box connected to a TV monitor.
Link perfiJ:fi,mnzunce:
Under typical QC laser operating conditions
(500 mA DC current at a temperature of 25 K) the link could be run
continuously and stably for at least 5 h. Owing to the beamsplittcr and
the multiple optical elemcnts in the outgoing bcam path only 7.5 mW
of the initial 25 mW output power were actually used for the transmission. About 10% of the original intensity (0.75 mW) could still be
detected under good weather conditions after the transmission and the
collecting telescope optics. The simultaneously transmitted beam with
182
2
c
ul
9
-
1
ul
C
._
v)
0.1
3.21
3.50
4.19
4.48
time (a.m.)
Fig. 3 Comparison of received intensities of mid-IR and near-IR link
against time
Fog came at around 2 a.m. and progressively dissipated during measurement
Inset: Logarithmic plot or ratio of two curvcs in main graph
ELECTRONICS LETTERS
14th February 2002
Vol. 38 No. 4
In the inset to Fig. 3, we plot thc logarithm of the ratio of the signal
strcngths of the two links against time. This is a measure of the
difference in their optical losses and pcaks at a value of 25 kin-’,
which is 250 times larger than the calculated value betwccn wavelength
of 8 and 1.3 pm for a condition of typical haze (visibility 10 !un) [9].
The superior perfonnance of the QC laser link compared to the near-IR
link can readily be understood from the wavclcngth dependence of
Raylcigh- and Mie-scattering. The particular shapc of the curve in the
inset of Fig. 3 is relatcd to the size and distribution of water droplets in
the air and changcs with fog density and structure over timc. The mid111 link is much less affected by these fluctuations owing to thc
considcrably longer wavelength. This effect can also bc seen from the
smaller intensity fluctuations of the QC laser link ovcr time (see
particularly at around 4.20 a.m.).
Conclusion: We demonstrated that QC lasers can be used to transmit
complex data streams through the atmosphere and with clearly grcater
reliability than near-IR links under conditions of poor visibility.
Acknowledgments: The authors wish to thank A.M. Sergcnt and
E. Chaban for technical assistance, T. Katsufuji and S.-W. Chcong
for the growth of bulk Geo.25Seo.75,
and \i.-K. Chen, J.E. Johnson and
L. Ketelsen for the loan of certain components used in this experiment. The work performed at Bell Laboratories, Lucent Technologies,
was partly supported by Darpa/US Army Research Office undcr
contract DAADl9-00-C-0096. Stcvcns Institute of Technology
acknowledges support fiom the US Department of Energy under
Contract DB-FG08-99NV13656 and the US Army CBCOM under
Contract DAAB07-98-D-A759.
IEE 2002
Electronics Letters Online No: 20020122
DOT: 10. 1049/eI:20020122
26 November 2001
R. Martini, C. Bethea, E Capasso, C. Gmachi, H.Y. Wang, D.L. Sivco,
J.N. Baillargcon and A.Y. Cho (Bell Laboratories, Lucent Technologies, 600 Moiintuin Avenue, MurrciJ:Hill, NJ 07974, USA)
E.A. Whittaker (Department of Phy.yics and Engineering, Stevens
Institute of lechnology, Hoboken W 07030, USA)
R. Paiclla (Agere Systems, 600 Mountain Avertiit,
0 79 74, USA
iMLirrc1.v
Hill,AY
R. Martini: Also at Departmcnt of Physics and Engineering, Stevens
Institute of Technology, Hoboken, NJ 07030, USA.
References
1
2
3
4
5
6
7
8
9
er al.: ‘2.4 km rrcc-space optical communication
1550 nm transmission link operating at 2.5 Gb/s - experimental
results’ in KOREVAAR, F,.J. (Ed.): ‘Optical wireless coimnunications’
Proc. SPIE, 1998,3552, pp. 2 9 4 0
CAPASSO, k, et ai.:‘New frontiers in quantum cascade lascrs and
applications’, IEEE 1 Sel. Top. Quunhim Eleclron., 2000, 6 , pp. 931947 (and references therein)
PAIELLA,R., et a/.:
‘Generation and detection of high-spcedpulses ofmidinfrared radiation with intcrsubband semiconductor lasers and dctectors’,
IEEE Photonics Technol. Lett., 2000, 12, pp. 780 782
P;\IELLA, R.; et al.: ‘Sclf-mode-locking in quantum cascadc lasers with
giant ultrafast optical nonlinearitics’,%ience. 2000, 290, pp. 1739-1742
PAIELLA, R., el al.: ‘High-frequency modulation without the relaxation
oscillationresonance in quantum cascade lasers’, Appl. Phys. Lett., 2001,
19, pp. 2526-2528
MARTWI, R., et ul.: ‘High-speed modulation and free-space optical
audio/video transmission using quantum cascade lasers’, &cfmn.
Lett., 2001, 37, pp. 1 1 1-1 12
BLASER, s., et a/.: ‘Free-space optical data link using Peltier-cooled
quantum cascade laser’, Electron. Lett., 2001, 37, pp. 778-780
ivlAKrh-I, R., el al.: ‘High-speed digital data transmission using midinfrared quantum cascade lasers’, Electron. Lett., 2001, 37, pp. 12901292
ZCJEV; VE.: ‘Laser-lighttransmission through the atmosphere’, in HINKLET,
ED. (ed.): Laser monitoring of thc atmosphere’ (Springer, Heidelberg,
1976)
SZAJOWSKI. P.F.:
ELECTRONICS LETTERS
14th February 2002
High-brightness 735 nm tapered diode
lasers
B. Sumpf, R. Hulsewede, G. Erbert, C. Dzionk, .I.Fricke,
A. Knauer, W. Pittroff, P. Ressel, J. Sebastian, H. Wenzel and
G. Trankle
High hrightncss 735 nm single emitter tapered diode lasers were
manucactored and analysed. A beam propagation factor M 2 sniallcr
than 1.4 is achieved up to an output power of 2 R!
Introduction: There is increasing dcinand for high brightness diode
lasers in the spectral range 715-780 nm. Examples of applications are
photodynamic therapy (PDT) and pumping of solid-statc lasers. In
addition to high ontput power, high brightness is required. This
corresponds to the demand for nearly diffraction-limited b e a m with
a small beam propagation factor M2.
Broad area (BA) diodc lascrs for this spectral region reach maximum
output powers of scvcral watts based on AlGaAs or hA1GaAs quantuni
wells (QWs) [l-31 and Al-free InGaAsP QWs [4]. Tensile-strained
GaAsP QWs embedded in AlGaAs were applied by our group for the
manufacturing of reliable diode lasers near 735 nm with degradation
rates below 5 x IO-’ h at 2 W output power from a 100 pin stripc over
2000 h [SI.
Broad arca devices with a stripe width of about 100 pm suffer from
poor beam quality. Typical hcam divergences ( l/e2-values) are at least
10 times larger than the diffraction limit, Le. M 2 > 10. A possiblc
solution to ovcrcome this limitation is the use of tapered lasers
consisting of an index-guided straight scction and a gain-guided tapered
section. For the wavelength range 980-1550 nm the approach has been
successfully realised [6-91.
In this Letter we present tapered lasers optimised for the wavelcngth
range around 735 nm. Details of the structure, as well as the lightcurrent charactcristic, bcam quality and spectral properties are reported.
’
Lasev structure: The laser strncturc is similar to that presented in [ 5 ] .
The epitaxial layers were grown by low prcssure MOVPB on (100)
n-CdAS substrates. Thc active GaAso.67Po.33
QW with a thickness of
9 nni is embedded in AI0.65Gao.35A~
waveguide and Alo.7DGao.;oAs
cladding laycrs. The layer sequence is completed by a highly doped
IJ-GaAs contact layer.
The tapered laser consists of an index-guided straight scction and a
gain-guided tapered section. The index guiding is achieved by a ridge
waveguide (RW) formed by reactive ion etching and dcpositing of an
insulator (AI2O3)on the etched surface. The ridge width was chosen to
be WKw= 3 Iim. In the tapered section, the contact layer outsidc of the
p-clcctrode is removed by wet chemical etching to reduce currcnt
spreading. The metallisation on the p-side contact was formed by
evaporating a Ti-Pt-Au niultilaycr and by electro-plating a thick Au
layer. After thinning and n-metallisation thc wafer was clcaved to obtain
a total cavity length of L = 2.5 mn.
The front facet w-as antireflection coated (R, = l%), thc rcar facet was
high-reflection coated (Rr2 94%). Thc lasers were mountedp-side (episide) down cin CuW submounts. All devices were soldered with AuSn
using a procedure also applied for BA lasers [ 5 ] . The n-side was
contacted by wire bonding.
To kecp the processing of the lasers as simple as possible, no cavityspoiling grooves for transverse-mode filtering were used since they would
requirc an additional etch step and an additional planarisation for epi-sidc
down mounting. Instcad, the length LKwof the 1<W section and the full
angle (pTR of the tapcrcd scction were carefully optimised. The highest
brighmcss was obtained for values Llcw= 1000 Fm and qTR= 6 .
Results: A typical powcr-voltage-current characteristic is shown in
Fig. 1. The threshold currcnt is 500 mA; the slope efficiency has a
value of -1 .0 W/A slightly above threshold. Comparing these values
with those of a BA laser made from the same epitaxial material having
a stripe width of 100 pm; thc threshold current is comparable but the
slope efficiency of the tapercd lascr is only -83% owing to the
additional radiation losscs caused by the tapered cavity. Nevertheless,
the conversion efficiency for the tapcrcd laser reaches almost 45%) at
1 W. A maximum output powcr of 3.3 W was obtained at an injection
current of 5 A.
Vol. 38 No. 4
-
183
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