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Surface emitting diode lasers
Françoise Lozes-Dupuy, Sophie Bonnefont, H. Martinot
To cite this version:
Françoise Lozes-Dupuy, Sophie Bonnefont, H. Martinot. Surface emitting diode lasers. Journal de Physique III, EDP Sciences, 1994, 4 (12), pp.2379-2389. �10.1051/jp3:1994284�. �jpa-00249270�
Classification Physic-s Abstracts
42.30 42.60 42.72 42.82
Surface
emitting
diode lasersF. Lozes-Dupuy, S. Bonnefont and H. Martinot
Laboratoire d'Analyse et d'Architecture des Systdmes du CNRS, 7 Avenue du Colonel Roche, 31077 Toulouse Cedex, France
(Receii,ed J4 February J994, accepted 9 September 1994)
Abstract. Surface-emitting diode lasers have recently made significant advances because of the emergence of a variety of applications such as optical interconnects, optical computing,
communications or diode-pumped solid-state lasers. These devices can be roughly grouped into three types based on their cavity configurations. The paper will examine the concepts, fabrication
techniques and operating characteristics associated with each type of laser. The performance of monolithic arrays of surface-emitting diode lasers will also be discussed.
1. Introduction.
Over the last years, surface-emitting semiconductor lasers, which emit from the planar surface of the wafer rather than from the cleaved end facets, have been extensively investigated. The
main advantages of these devices over the conventional edge emitting lasers lie in their spatial
and spectral output properties, manufacturability, testing and packaging at a lower cost, and the possibility of optoelectronic integration without a cleavage process. Because of their
suitability for integration into two-dimensional (2D) arrays, surface emitting lasers pave the way for a large variety of applications such as display and optical communication
technologies, data storage, optical interconnects between integrated circuits or within computers, free-space illumination, communication and ranging, nonlinear frequency conver- sion, optical pumping of solid-state lasers, etc.
The search for the optimum device configuration for a given application has led to three basic surface-emitting laser designs. The first approach utilizes a 45° etched mirror in a
horizontal (in-plane) cavity to deflect the beam into a direction normal to the wafer. These devices, which take advantage of the mature development and performance of conventional
edge-emitting lasers, are attractive in incoherent arrays for high-power applications such as pumping Nd : YAG lasers. An alternative method consists of replacing the mirrors by a waveguide region containing a second order grating to provide both the output-coupling
function and the feedback for laser oscillation. These grating surface emitting (GSE) lasers offer high power ouput, single wavelength operation and narrow beam divergence. Potentially they can therefore be envisaged for many coherent high-power applications such as satellite
@Les Editions de Physique 1994
communications or pumping. The third approach employs a vertical-cavity structure including
two highly reflective feedback mirrors on the top and bottom surfaces of the wafer. Given their unique topology, vertical-cavity surface-emitting lasers (VCSELS) can be fabricated into compact two-dimensional arrays for applications as diverse as optical interconnects, optical computing and optical communication.
2. In~plane surface«emitting lasers with 45° beam deflectors.
Various horizontal cavity surface-emitting diode lasers with 45° beam deflectors have been demonstrated. Surface emission is obtained by replacing the cleaved mirrors of conventional
edge-emitting lasers by dry etched vertical facets and external beam deflectors, or by internal total 45° reflecting deflectom, as shown in figure I. The performance of these surface-emitting
lasers depend on the quality of the etched mirrors. Dry etching techniques such as reactive ion
etching or ion beam etching are primarily used to fabricate the deflectors. In particular, they require a high degree of precision to arrive at an accurate etched angle without roughness of the mirror surface and to reach a level of performance comparable with that of cleaved lasers.
j__
~
Qal conventionallaser
P P
'--- --- '
~ ~
b) surface emittinglaser with e~emal deflectors
P P
~
/
'
c) surface emitting laser withfolding mirrors Fig. I. Schematic diagram of different laser cavities.
For single-element diodes using external deflectors, the highest output powers for
continuous-wave (CW) operation have been in the 200 mW range on single quantum-well
graded-index separate-confinement heterostructure (SQW-GRINSCH) GaAlAs/GaAs devices [I]. By employing GalnP/GalnAlP materials, similar devices, emitting in the visible range at 635 nm have been developed for applications such as optical recording, barcode scanners, laser printers. Pulsed power of 170 mW has already been obtained [2]. Furthermore, several 2D arrays have been fabricated to increase the level of the ouput power. An array with two
parabolic mirrors has been obtained from a SQW-GRINSCH GaAlAs/GaAs device and
operated under quasi continuous wave (QCW) conditions up to 60 W at 70 A corresponding to
a power density of 600W/cm~ [3]. A CW output power of 37 W, limited by the current
available has recently been demonstrated by mounting a similar device on an adequate
microchannel heatsink, corresponding to a CW power density as high as 148 W/cm~ [4]. By etching the substrate before epitaxy in order to form the 45° mirrors and by microcleaving the vertical facets of the laser cavity, Groussin et al. [5] have reported high QCW power operation
up to 100 W with a monolithic array of 120 laser diodes. The compact planar association of
10 arrays on the same submount demonstrated a record QCW power of 000 W, with a beam
divergence less than 20°.
The second approach to surface emission with 45° mirrors makes use of a beam deflection inside a folded cavity. Several structures have been designed with one high-reflectivity
vertical-etched facet and one internal 45° mirror. High output CW power can be achieved by using a junction-down mounting and light emission through the substrate. 1.4 W CW output
power in a junction down configuration i,ia etched substrate has been achieved from a broad
area contact SQW-GRINSCH GaAlAs/GaAs laser [6]. The use of strained layer GalnAs
quantum wells emitting at l ~m with transparent GaAs substrate has obviated the need for removal of all or part of the substrate and has allowed 80 mW to be obtained per element of CW single mode output power and SOWCW total power from a monolithic 2-D
I cm~ array [7]. Similar output CW power density has been achieved from a lnGaAs/GaAs
SQW-GRINSCH laser with two folding mirrors [8].
3. Grating surface-emitting lasers.
The basic grating surface-emitting laser, shown in figure 2, consists of a gain section between two second-order gratings that provide wavelength selective feedback for laser oscillation in the second order and couple out the laser radiation perpendicular to the surface in the first
order. In zeroth order, the gratings can also transmit light to another photonic component or to
adjacent gain regions to achieve phase-locking in the longitudinal direction. Coupling losses
are very low because the epitaxial structure is based on a common quantum-well waveguide through the gain and the grating regions [9-11].
2nd.order grating
quantum-well active region
Fig. 2. Schematic of a grating surface-emitting laser.
Radiating waves are generated into both the substrate and the superstrate. However, high-
reflect coatings over the grating regions and antireflect coating on the substrate have been used to provide outcoupled light through the substrate and enhance efficiency. For wavelengths greater than 0.9 ~m, strained GaInAs quantum wells are used and the GaAs substrate is
transparent ; at wavelengths lower than 0.9 ~m. GSE lasers were grown on transparent AlGaAs substrates to avoid optical losses in the substrate. The efficiency can be also improved by growing a Bragg reflector into the substrate.
GSE lasers have demonstrated stable single-mode operating with narrow linewidth and
nawow-divergence electronically steerable output beams. A unique feature of GSE lasers is the
use of longitudinal coupling between two adjacent emitters to form coherent linear arrays,
alternating gain and grating regions. Arrays of mutual injected emitters exhibit a narrower spectral linewidth than that of a single emitter device and CW single mode longitudinal operation with a linewidth as low as 300 kHz was reported on a three element array [12]. Of
course, arrays of GSE lasers can be high-power sources and pulsed output power as high as
16 W with a differential efficiency of 50 fb was reported on a linear array of ten emitters [13].
However, multielement arrays suffer from a lack of coherence of the output radiation, because the device behaviour is very sensitive to structure inhomogeneities or variations in operating
conditions. Monolithic 2D coherent laser arrays have also been fabricated, by coupling linear arrays in the lateral direction using either evanescent field overlap or Y-branch coupling. 2D GSE arrays have operated continuously to more than 3 W, pulsed to more than 30 W, with a differential efficiency of 20-45 % per surface, linewidths in the 40- loo MHz range. Because of their large aperture, these devices have very narrow far fields, with beam divergences of about 1° x 0.01° [14].
More recently, a monolithic master oscillator/power amplifier (MOPA) approach has been
proposed to achieve coherent output power of more than W in a single-lobed diffraction- limited output beam. Several GSE structures have been operated as monolithically integrated
MOPAS by using the grating to couple a distributed Bragg reflector (DBR) or a distributed feedback (DFB) laser to either a single amplifier or a chain of amplifiers and grating output couplers [15, 16]. The period of the grating output couplers is detuned from the reflection
bandwith to prevent undesirable feedback to the amplifier or to the master oscillator. The emission from the output coupler therefore radiates approximately 10° from the normal.
Unfortunately, the far-field of cascaded GSE amplifier arrays degrades at high output power
even when the current is controlled in each amplifier in order to maintain a single-mode,
diffraction-limited, far-field emission. To date, the most promising MOPA cunfigurations are
based on a laser oscillator monolithically coupled to an active grating surface emitting amplifier [17] or to a single flared optical amplifier (an output power in excess of 2 W CW was reported on an edge-emitting device [18]).
DBR ring oscillators were recently proposed as another key to obtaining high coherent power level. These structures are based on a large output aperture delimited by orthogonal gratings which select a single longitudinal and transverse mode of operation. Single frequency operation with high differential efficiency and narrow diffraction-limited output beams have yet been reported [19] and further cavity designs could optimize the manner in which surface
emission is obtained.
Second-order circular gratings have also been used to fabricate circular-grating distributed- feedback (CG-DFB) and circular-grating distributed-Bragg-reflector (CG-DBR) surface-emit- ting lasers. It has been shown that these lasers feature a circular symmetric surface emission, a
narrow wavelength spectrum and an emission power in excess of loo mW [20, 21]. They
could be very promising for 2D arrays.
4. Vertical-cavity surface-emitting lasers.
Vertical-cavity surface-emitting lasers (VCSELS) have orthogonal cavities relative to those of conventional edge-emitting diode lasers. This laser structure pattem offers three advantages.
Due to its short cavity length, VCSEL can inherently operate in a single longitudinal mode.
The output beam can be circular with a low divergence angle. The active volume of VCSELS
can be designed in such a way that high packing density and low threshold current can be obtained.
Because of the short gain length in a VCSEL, a high mirror reflectivity (~ 0.95) is required
to reach the threshold. Various techniques have been used to achieve this high reflectivity, including metal mirrors, dielectric distributed Bragg reflectors (DBR) mirrors and semiconduc-
tor DBR mirrors. On the other hand, two main VCSEL types can be distinguished :
ii the double heterojunction VCSEL, a structure made up of a few microns thick, coated with mirrors that are usually dielectric ;
it) the quantum well (QW) VCSEL using higher finesse cavities and thinner active material that generally are all-epitaxially grown.
The schematic generic structure of a GaAs double heterojunction VCSEL [22] is shown in
figure 3. The length of the cavity between two SiO~/TiO~ mirrors is 5 ~m and current confining layers define a buried circular 7 ~m diameter mesa. These devices exhibit a 28 mA threshold
current, a lo fl differential quantum efficiency and a CW output power of about I mW,
limited by heat dissipation. Lower threshold currents have been obtained by means of a buried heterostructure coated with an AlGaAs multilayer reflector and a Silsio~ dielectric mirror [23].
The threshold current lies in the range 3 to 9 mA for a device diameter between 2.5 and 5 ~m, but the cavity size reduction causes an increase of the electric and thermal resistance.
Substrate
Light
SC~/Tio~
multilayer
Act'»~ ~~~'°~
sjo~mo2/Si°2/AU rr'rr°r
Fig. 3. Schematic structure of GaAs double heterostructure VCSEL (after Koyama [21]).
In the long wavelength range (1.3-1.55 ~m) various types of devices based on GaInASP active layer and InP cladding layers grown by Liquid Phase Epitaxy [26], [29], Metalorganic
Chemical Vapor Deposition [24, 25, 28], Chemical Beam Epitaxy [27], have been
demonstrated. Threshold currents as low as 3 mA have been obtained at 77 K [25, 26]. Room temperature pulsed oscillation has been achieved [24, 28, 29]. However mirror reflectivity
must still be improved and too high leakage current could be decreased by a buried
heterostructure for example [29], in order to obtain a low room temperature threshold current
density.
A generic QW VCSEL is shown in figure 4. The structure consists of an n-doped AlAs- GaAs Bragg reflector deposited on n+-GaAs substrate by Molecular Beam Epitaxy or Metalorganic Chemical Vapor Deposition, an interlayer with the active quantum well zone and
an upper p-doped Bragg reflector also in AlAs-GaAs. The Bragg reflectors contain
)Top
emissionSurface contact
° IJ4n GaAS
Topmirror
~4~ AjAs
o
QW gain region Spacerregion
Spacerregion
o
Bottommirror o
)Light
at 850 nmSurface contact
pdopedDBR reflector
. H+implant
GaAs QW gain region
n
aAssubstrate
With the achievement of a low threshold current VCSEL, the fabrication of two- dimensional aways has also intensified. 32 x 32 arrays with a GaInAs strained quantum well active layer, arranged in a matrix addressing architecture, have been demonstrated [48].
Independently addressable top emitting 8 x 18 arrays with a multiquantum well gain region
have been produced with average threshold current of 4.2 mA [49].
On the other hand a significant amount of work has been done on integrated optoelectronic logic elements using VCSEL. Monolithic and integrated versions of two-dimensional active
optical logic devices based on the surface emitting laser and other optical switches, as photodetectors, heterojunction phototransistors or photothyristors, have been reported by
several groups [50-52], demonstrating the basic optical logic functions, latching and
bistability.
5. Conclusion.
Over the last years, major advances in surface-emitting lasers have been recorded. Monolithic 2D surface-emitting arrays of diode lasers with 45° beam deflectors have demonstrated more
than 100 W/cm~ CW power densities and peak power densities of 1000 W/cm~ with pulse-
widths enabling pumping of a variety of solid-state laser materials. Devices based on a MOPA
configuration are promising for producing greater than I W of coherent power output in a
stable single-lobed diffraction limited beam. Progress is still being made in the performance of VCSELS. Threshold currents in the range 0.7-3 mA, output powers in the range 0.5-1 mW are currently obtained for elementary devices I W output power has even been reached in the
case of high power devices. Higher power, higher electrical power-conversion efficiency, higher speed, higher reliability, multiple new wavelengths are the near future likely significant improvements over today devices. Progress toward strained-layer quantum-well lasers, quantum-wire lasers, new designs and advances in technology are encouraging for further
development of surface-emitting lasers for numerous and new applications with potentially large markets.
References
[1] Ou S. S., Jansen M., Yang J. J. and Sergant M., High power cw operation of GaAs/GaAlAs
surface-emitting lasers mounted in the junction-up configuration, Appl. Phys. Lent. 59 (1991) 1037-1039.
[2] Ou S. S., Yang J. J. and Jansen M., 635-nm GaInP/GaAIInP surface-emitting laser diodes. Pioc.
CLEO'93 11 (1993) 163-164.
[3] Donnely J. P.. Goodhue W. D., Bailey R. J., Lincoln G. A., Wang C. A. and Johnson G. D., High quantum efficiency monolithic arrays of surface-emitting A1GaAs diode lasers with dry-etched vertical facets and parabolic deflecting mirrors, Appl. Phys. Left. 61 (1992) 1487-1489.
[4] Donnely J. P., Goodhue W. D., Wang C. A., Bailey R. J., Lincoln G. A., Johnson G. D., Missaggia L. J, and Walpole J. N., CW operation of monolithic arrays of surface-emitting
AlGaAs diode lasers with dry-etched vertical facets and parabolic deflecting mirrors, IEEE Photon. Technol Lett. 5 (1993) 1146-1149.
[5] Groussin B., Pitard F., Parent A. and Carriere C., 1000 W QCW output power from surface
emitting GaAs/AlGaAs laser diode arrays, Elec.fi.on. Lent. 29 (1993) 370-372.
[6] Jansen M., Ou S. S., Yang J. J., Sergant M., Hess C., Tu C., Alvarez F. and Bobitch H., High
power ill W CW), reliable, surface-emitting laser diodes with ail-etched mirrors. Pioc.
CLEO'93 11 (1993) 398-399.
j7] Nam D. W., Waarts R. G.. Welch D. F., Major J. S., Jr. and Scifre~ D. R.. Operating
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