HAL Id: jpa-00249269
https://hal.archives-ouvertes.fr/jpa-00249269
Submitted on 1 Jan 1994
HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.
L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.
Novel application of optically pumped vertical cavity surface emitting lasers : fast amplifying optical switch
R. Raj
To cite this version:
R. Raj. Novel application of optically pumped vertical cavity surface emitting lasers : fast am- plifying optical switch. Journal de Physique III, EDP Sciences, 1994, 4 (12), pp.2371-2378.
�10.1051/jp3:1994283�. �jpa-00249269�
Classification
Physic-s Abstracts 42.90
Novel application of optically pumped vertical cavity surface
emitting lasers : fast amplifying optical switch
R. Raj
Centre National d'Etudes des Tdldcommunications, Paris B, Laboratoire de Bagneux, B-P. 107, 92225 Bagneux, France
(Received 4 March J994, accepted 30 June J994)
Abstract. An amplification of 30 dB of an extemal beam by a photopumped vertical cavity surface emitting laser structure is obtained. The temporal response of 20 ps points to the use of the
structure as a fast amplifying switch, capable of attaining several GHz frequency.
A major advance in the processing of optical signals in recent years has been due to the
development of high quality epitaxial Fabry«Pdrot structures with multiple quantum wells active medium sandwiched between Bragg reflector mirrors. Vertical cavity structures have an
advantage over conventional guided-wave structures for bidimensional optical processing of information due to their geometry : small size and vertical access. A wide range of innovative
use has been made of these structures such as laser emission (VCSEL : vertical cavity surface
emitting laser) [1, 2], bistability in optical dtalons [3] and modulation using asymmetric Fabry- Pdrot structures [4]. The cascading of these different functions still faces the problem of losses.
We have recently demonstrated the feasibility of a fast amplifying optical switch [5] with optical access perpendicular to the substrate which might provide this missing link for
achieving active optical interconnects [6, 7). In order to show it's affiliation to VCSEL'S, this
switch has been baptised Vertical cavity Amplifying Photonic Switch (VCAPS). Since the
VCAPS share much of the same technology as VCSEL'S an outline of the recent developments
of the latter will be made in the next section.
VCSEL.
These lasers were originally proposed by Iga [8] in 1979. A major breakthrough occurred years later when Jewell demonstrated arrays of I mA-2 mA threshold devices [9]. Since then several groups have been working in the field and the progress has been tremendous. Continuous-wave (CW) output powers above 100 mW in multi-transverse mode oscillation [10] have been
obtained, limited only by thermal effects. Differential quantum efficiencies between 20 and 50 percent are quite common leading to total electrical-to-optical conversion efficiencies of 10 fb for the best devices. Power levels as high as I W have been obtained under pulsei
§Les Editions de Physique 1994
2372 JOURNAL DE PHYSIQUE III N° 12
excitation [11]. Device heating places a serious constraint on device performance. Recent
development of low resistive graded Bragg reflectors [12-14] and the introduction of dielectric mirrors [15, 16] have greatly reduced the threshold voltage to less than 2 V, resulting in
improved thermal behaviour.
Progress has reached a stage where most of the current work is on the fine tuning of the characteristics of the structure. The key issues are increasing wall plug efficiency [17], control of the transverse mode [18, 19], fabrication of extremely high reflecting mirrors in order to
compensate for the short gain region. The drive towards all optical fibre communication systems has stimulated a number of groups into working on room temperature operation of long wavelength devices.
VCSELS operating in the long wavelength optical fibre transmission windows (1.3 ~m and 1_55 ~m) are in their infancy. The development in this wavelength region has not been as fast
mainly because stringent requirements for low loss Bragg reflectors, efficient current
confinement, high optical gain, and excellent thermal characteristics are hard to fulfil simultaneously. But CW lasing operation has been achieved up to temperature of 14 °C [20.
21] by reducing series resistance of the mirrors.
Similarly, short wavelength lasers for the visible spectrum are limited by mirror imperfec-
tions and high series resistance. Pulsed operation of VCSELS with emission wavelengths
below 700nm have been reported [22]. Concurrently, an intense effort is made in the
fabrication and functioning of arrays. The surface emitting geometry is ideal for realizing two- dimensional arrays of optical devices including sources, switches, amplifiers and « smart »
pixels when integrated with electronic devices. Considerable advances have been made in vertical cavity array technology in recent years. An independently addressable two-dimension- al array containing 8 x 18 individual elements has operated over 000 hours without failure or
degradation [23]. However, the performance of these devices still falls some way short of
requirements, particularly in the control of transverse optical modes. There is a two-fold interest in the development in this domain, namely for the increase in parallel interconnection
density while maintaining low crosswalk between devices as well as for the lateral confinement of modes.
Microcavity.
To approach ideal devices in which there are only a few allowed optical modes the size of the laser cavity must be of the order of the wavelength, I-e- the laser must have a microcavity. In an optical microcavity, the interaction of light with the active material is modified, leading to the
possibility of optimising the performance of optoelectronic devices. Microcavities are known to alter the spontaneous emission distribution in lasers and thus to offer a means of tailoring the
interaction of light with the gain medium. By reducing the lateral dimension of the lasers it is
possible to control the transverse mode too but surface recombination, optical scattering from
the sidewalls, current confinement and series resistance must all be well controlled. The spontaneous emission coupling efficiency is important, since spontaneous emission acts as a
seed for stimulated emission, and hence determines the threshold. For this a new approach is
iecessary. research is directed in two directions control of spontaneous emission [24] and mode tailoring by photonic bandgap engineering.
Slusher at Bell Labs has experimentally demonstrated an interesting design, the whispering gallery mode microlaser. It has a threshold pump power of about 50 ~V when optically pumped at 77 K and a 0.95 mA threshold when pumped electrically at room temperatures [25].
Baba and Iga have made calculations for dielectric post microcavities. Their theoretical estimates for these lasers indicate that they can operate at sub ~W power, and that a substantial fraction (~10 fl) of the spontaneous emission can be coupled to the lasing mode [26].
In order to realize the ultimate performance of semiconductor devices, a full control of spontaneous emission in the frequency domain as well as in all spatial directions may be needed. In 1984 Yablonovitch suggested that three-dimensional periodic dielectric structures
would realize a forbidden energy band for electromagnetic radiation I-e- an optical
bandgap [27]. In such « crystals », the existence of a gap has been experimentally verified at microwave frequencies. The remarkable property of this structure is that it can be fabricated from a semiconductor wafer with conventional lithographic and etching techniques.
New applications : vertical cavity amplifying photonic switches (VCAPS).
There is an exciting new application in which much of the progress in VCEL'S may be
profitably used : the VCAPS. The feasibility of using vertical cavity structures for amplifying optical signals by stimulated emission in a photopumped active layer has recently been demonstrated by us [5]. A gain of 30 dB with a response time of ?0 ps is now possible in a GaAs based structure operating in the 0.8-1.0 ~m region.
The gain processes in the vertical cavity structure have been probed using both spectral and temporal optical characterization techniques. The light source is an optical parametric
generator (OPG) consisting of a pair of LiIO~ crystals, pumped by a frequency doubled
Nd :YAG Q-switched mode-locked laser. The parametric light has a pulse duration of 20 ps and its wavelength is continuously tunable between 0.7 ~m and 2 ~m by angle tuning the
crystals. The energy of the pulses is I ~j and their spectral width I nm. There are two sets of such independently tuneable beams.
The vertical cavity Fabry-Pdrot structure, designed to be used in the reflection mode was
grown by MOCVD. The structure is composed basically of 130 GaAs/Alo~Gao7As multiple
quantum wells with a nominal thickness of 10 nm/10 nm sandwiched between a pair of Bragg
reflectors. Alternate AM layers of AlAs and AlojGao~As constitute the Bragg reflector
mirrors. There are 14 periods for the back mirror and 7 periods for the front one giving reflectivity's of 0.97 and 0.91 respectively.
The sample was characterized spectrally by means of a linear reflectivity measurement
performed using a test beam which was scanned between 780 nm and 880 nm stop band of the sample. The nonlinear spectra for exploring the amplification regime was obtained by monitoring the intensity of the reflected test beam while pumping the active medium by means
of a strong beam. The pump beam and the test beam were focused by a single lens to a spot radius of 30 ~m and were incident on the sample at angles of 15° & 20° (Fig. I).
The linear spectrum is displayed in figure 2. The Fabry-Pdrot modes in the transparent and absorption regions of the active medium are displayed in the linear spectrum. As expected, the
narrow reflectivity dip at 875 nm is situated in the transparent region while the broad dips at
804nm and 825 nm are in the absorption region of the active medium. There is a less
pronounced fairly narrow dip which is in the Urbach's tail. The free spectral range is 21 nm and, and as is evident from the spectrum, is not uniform over the entire spectral range due to
dispersion of the refractive index.
Figure 3 depicts the nonlinear spectrum obtained with a pump intensity of 2.14 mJ/cm~. The pump wavelength is 8?snm where only the active medium is absorbent. The effective
coupling of the pump into the active medium is about 10 fl. It is seen that the dip at 855 nm blue shifts to 847 nm and switches into a peak with a reflectivity greater than one. The reflected
test beam undergoes strong optical amplification when the gain due to stimulated emission sets in the active layers of the cavity. The amplification is resonantly enhanced by the optical microcavity, which is responsible for multipass enhancement of the optical gain. The amplification takes place over 2.75 nm.
2374 JOURNAL DE PHYSIQUE III N° 12
AMPLIFIED TEST
PUMP
I
Bragg reflectors 7 periods MQW
active medium Bragg reflectors
periods
Fig. 1. Sample and amplification configuration.
~
I £
'a H
~
~
o
~
780 800 820 840 860 880
840 844 848 852
Wavelength (nm) WAVE LENGTH (nm)
Fig. 2. Fig. 3.
Fig. 2. Linear spectrum obtained in the absence optical excitation.
Fig. 3. Amplification for a pumping intensity of 2.14 mJ/cm2 with the pump wavelength at 825 nm.
The temporal response of the system was explored by introducing a variable optical delay
between the pump and the test beams and detecting the test beam. In figure 4 the pump
wavelength is 825 nm and its intensity is varied between 0.2 mJ/cm2 and 2.14 mJ/cm~, and the test-beam wavelength is at 847 nm where the amplified reflection is maximum in figure 3.
As long as the pumping intensity is below the lasing threshold it is seen that the amplification
(a)
fly 1=226pJ/cm~
~
z
<
(b)
f 1=536pJ/cm~
z
<
(c) fir
~ l=2.14mJ/cm~
o 50 ioo i
DELAY (ps)
Fig. 4. Temporal response for different pumping intensities, sho/ng the duration of amplification as
measured by a 20 ps test pulse at 847 nm and pump at 825 nm.
falls off fairly slowly. The amplification reaches its peak with less and less delay as the pump
intensity increases. We see two important features in figure 4c (1) that the amplification of the test beam lasts for only 20 ps, the duration of the pump pulse and (2) that there is a sharp fall off in the signal. By sending the amplified test beam and laser emission simultaneously into a streak camera it was verified that the amplification and the emission takes place during the
pumping time. Thus, for the pumping energies used in this experiment, the laser emission occurring within the pumping pulse width depletes the system, responsible for the sharp fall off in the signal in figure 4c and the depletion is such that the amplification lasts only for 20 ps.
In the plane wave approximation using the standard concepts of multibeam interference, the transmission T and reflectivity R of a Fabry-Pdrot of length L, with front and back mirrors with
reflectivity's R~ and R~ containing a material with gain coefficient g is given by :
~ itrJnsm,fled (~ ~f) (~ ~b) ~~~
~j~
i~nc~den' ( R~)~ + 4R~ Sirl~ #
2376 JOURNAL DE PHYSIQUE III N° 12
j~ reflected
(~f ~a )~/~f + 4 R~ Slfl~ ~
Iinc~dent (I R~)~ + 4 R Sirl~ # ~~~
where R~
=
fiexp (gL).
The phase shift experienced by the field upon completion of half a round trip through the
cavity is
where ~~ and ~~ are the additional phase shifts due to the penetration in the mirrors, is the wavelength of the test beam, n is the refractive index of the medium which govems the
cavity resonance and o is the internal angle of incidence. In equation (2), R is measured and all other parameters are known, except the gain, which may thence be calculated. Typically for an observed gain of 30 dB we obtain a value of 200 cm-' for the gain coefficient. It is obvious from the expression for reflectivity that, the dependence of ~ on the incident angle
6 and the large beam divergence angle &6 of the incident beam due to tight focusing,
contribute to the large spectral width of 1.15 THz over which amplification takes place. The Gaussian distribution of intensity in the pump pulse is an additional mechanism which chirps
the amplified pulse thereby widening it. Experiments are underway to determine the relative contribution by measurements resolving spectro-temporally the amplified signal using a streak
camera coupled to a spectrometer. The expression for reflectivity describes qualitatively the
experimental observation the onset of saturation, the blue shift and the gain observed. For a
gradually varying pumping intensity between 42 and 515 ~J/cm~ the gain peak shift is
depicted in figure 5. The intensity dependence of the reflectivity comes through the refractive index change in the phase term in (3). The theoretical curves are plotted taking into account the
carrier-induced refractive index changes and they agree very well with the experimental
observation. Thus the amplification configuration can be used to obtain a direct measurement of the carrier induced refractive index change [28].
Thus VCAPS presents several attractive features for device applications. By combining amplification with the fast response of the system it is possible to use it for optical switching
with amplification with negligible insertion loss. The nature of the cavity offers the possibility
of functioning at oblique angles for the incident light beam. VCAPS are compatible with angle multiplexed signals. The divergence of the beam can be fairly high, which means that this system may be used with ease for image amplification. The geometry of the structure makes it
a perfectly adapted amplifier for the vertical cavity laser arrays [29] emitting different
wavelengths.
Vertical cavity technology has the potential to produce a range of extremely attractive
optoelectronic devices for a variety of applications in optical fibre communications, optical interconnects, parallel optical signal processing. Discrete amplifiers based on this technology
will be low cost, reliable, efficient, integrable and easily packaged sources capable of high speed operation. They will be of vital importance to the long term aims in the microelectronics
arena. Its potential applications include enabling optoelectronic technologies and materials,
high performance interconnect systems and advanced packaging. Among the anticipated applications are, improved dynamic performance giving a simple route to higher bit-rates, the
potential to act as a simple resonant amplifier in wavelength division multiplex optical fibre systems and the capacity to be used as a switch or routing element in wavelength division
switching. VCAPS are potentially attractive as cascadable elements in a high density
interconnect fabric. They offer optical switching with gain at high bit rates with the prospect of
la) .
~ "
fi
~
i~ ,'
d
,,-~,
i# ,"' ",,
16)
j ~
i ~~~
~ 181
b " 155
2 '. 104
$ ; 42
Lu 0
# ,~~ "
~
840 844 848
WAVELENGTH(nm)
Fig. 5. Theoretical (al and experimental (b) curves of resonant gain for varying pumping intensities.
low noise and low crosstalk operation in two-dimensional arrays. The possibility of integration
with other optoelectronic components in large two-dimensional arrays of functionality is also
an exciting prospect.
References
[1] Ogura M., Hata T., Yao T., Distributed feedback surface emitting laser diode with multilayered heretostructure, Jpn J. Appl. Phys. Lent. 23 (1984) L512.
[2] Jewell J., Mccall S. L., Lee Y. H., Scherer A., Gossard A. C., English J. H., Lasing characteristics of GaAs microresonators. Appl. Phys. Lent. 54 (1989) 1400.
[3] Sfez B. G., Oudar J. L., Michel J. C., Kuszelewicz R., Azoulay R., Extemal beam switching in monolithic bistable GaAs quantum well dtalons, Appl. Phys. Left. 57 (1990) 1849.
[4] Whitehead M. and Parry G., High-contrast reflection modulation at normal incidence in asymmetric multiple quantum well Fabry-Pdrot structure, Electron. Left. 25 (1989) 566.
[5] Raj R., Levenson J. A., Oudar J. L., Bensoussan M.. Vertical microcavity optical amplifying switch, Electron Lett. 29 (1993) 167.
[6] Fice M. J., Goodwin A. R., Thomson G. H. B., Whiteway J. E. A., realization of monolithically integrated single frequency MQW laser and booster amplifier at 1.5 ~m wavelength, Electron.
Lent. 27 (1991) 2307.
[7] Numai T., Ogura I., Kosaka H.. Sugimoto M., Tashiro Y., Kasahara K., Optical self-routing
switch using vertical to surface transmission electrophotonic devices with transmission light amplification function. Electron. Lent. 27 (1991) 605.
