Control the Transverse mode of Vertical Cavity Surface Emitting Lasers
by Anti-Reflection Coating
S. W. Chiou, C. P. Lee
Department of Electronics Engineering
National Chiao Tung University
H. P. Yang, C. P. Sung
Opto-Electronics and Systems Laboratories
Industrial Technology Research Institute
Hsinchu, Taiwan
Abstract
We have successfully developed a selective surface coating technique to control the modal behavior of the
ion-implanted vertical cavity surface emitting laser. Using selective deposition of germanium coating by lift-off process, we could spatially control the threshold gain condition of the VCSEL to support the single transverse mode. The threshold current is 7 mA and single transverse mode operation is maintained up to 1 mW. The method is simple and nondestructive compared to other techniques.
Keywords: Key words: VCSEL, single-mode, proton implant, and AR coating
Introduction
Vertical cavity surface emitting laser (VCSEL) has emerged as an attractive light source for next generation fiber
communication system because of its inherent two-dimensional configuration and circular output beam shape [1-3].
However, the relative wide output aperture of a typical VCSEL leads to higher-order transverse modes which degrade its
ability for high speed and long distance data communication. Therefore, it is important to achieve stable and single
transverse mode operation of VCSEL.
Several attempts have been made to obtain the stable single transverse mode. The most popular way is to use an oxide-confined aperture to make the device small enough to support only the fundamental mode [4]. This approach has the disadvantage of increased series resistance and reduced output power because of the reduced current aperture. Recently, another technique, which utilizes the surface relief etching, was developed and was effective to suppress the higher order modes [5]. This technique, which relies on the precise control on the etched thickness of the top ALAs layers in the DBR, allows only fundamental mode oscillation by keeping the higher order modes off-resonance. But since the etching was done on part of the top DBR, it may cause the reliability problem. In this paper, we describe a new technique, to obtain the single transverse mode operation. The mode was selected by selectively controlling the thickness of an additional germanium coating on the surface. We could spatially control the threshold gain of the VCSEL to support only the single fundamental
mode.
Design and Fabrication
The principle behind the mode selection lies on the spatially dependent cavity loss introduced by selective
deposition or removal of an additional coating on the surface. Since the length of a VCSEL cavity is two order of magnitude
shorter than an edge-emitting laser, the mirror reflectivity of the DBRs required to achieve threshold gain is very high
(-99.99%). Additional material coating over the DBR would render the mirror reflectivity to a lower value so that the lasing operation can no longer be sustained.
Fig. 1 shows the dependence of the peak reflectivity of a DBR with 20 pairs of A1As/AlGaAs on the refractive
index of a surface AR-coating layer. Taking Ti02 (n2.45) for example, the peak reflectivity of the DBR drops from
at 850nm. With such a high refraction index of5.2, the peak reflectivity ofthe DBR can be reduced down to 96%. Thus, we could easily change the threshold gain condition ofthe VCSEL by additional mirror coating. By selective deposition of this
additional coating, the mirror loss, or the threshold gain required, can be spatially varied. In other words, by selective
control ofthe opening size ofAR coating, we can spatially change the threshold condition ofVCSEL and therefore maintain the stable fundamental transverse mode at all range of driving current.
The devices investigated are 850 nm VCSELs with a proton implanted-isolated area with a diameter of 20 rim. After proton implantation, a Ti/Pt/Au ring contact with 1 5-rim inner diameter was deposited for p-side contact. And the AuGe/Ni/Au n-type contact was evaporated on the backside. Then a 1/4 X thick germanium film was deposited on the top surface with a 5 tm diameter opening at the center defined by photolithography and lift-off. The device was then bonded for
test. Fig. 2 shows the SEM photograph of our fabricated VCSEL. The cross ection of the device is also shown
schematically in the figure.
Results
To demonstrate the operation principle of our single-mode VCSEL, we have compared several devices with
different kinds of surface coating. First, VCSELs with different coatings and without coating were fabricated and measured as reference. Then, our newly designed VCSELs with 5 tm opening ofGe coating were investigated.
Fig. 3 shows the light versus current (L-I) characteristics ofVCSEL with a Ti02 coating, a Ge coating and without
coating. The typical thresioId currert of l5-jtm-implanted VCSEL without surface coating is 5 mA and the external quantum efficiency is 0.25 W/A. For the VCSEL with Ti02 coating, the threshold current increases to 14 mA and the
quantum efficiency increases to 0.5 W/A. The increased threshold current and quantum efficiency is due to the reduced reflectivity ofthe top DBR. For the VCSEL with a Ge coating, the threshold current further increases to 21 mA while the quantum efficiency remains the same as the VCSEL without surface coating. The rather lower quantum efficiency is due to the absorption ofthe Ge coating.
Fig. 4 shows the L-I curve of the VCSEL with a 5 tm opening in the Ge coating. The threshold current is 7 mA. This slightly increase in threshold current is due to the current crowding effect in the relatively large proton defmed aperture [6]. The small lasing window at the center of the aperture is located at where the injection current is less efficient, causing slight increasing in threshold current. Fig. 5(a) shows the lasing spectra ofthe laser at different driving currents. We can see that the single mode operation is obtained as long as the current is below 12 mA, which is 1 .7 times of the threshold current. Between 12 mA and 17 mA, the laser has two modes. Above 17 mA, the single mode operation returns and it remains till the laser stop operation due to heating effect. The mode change can be also seen in the L-I curve shown in Fig. 4. The two kinks marked on the curve indicate the onset and the end of the two modes operation. The higher order mode is due to the
non-uniform carrier depletion at the lasing condition (spatial hole burning). Fig. 5(b) shows the spectra of an original
VCSEL without Ge coating. Multi-mode operation is observed at all levels of the driving current. From Fig.4 and Fig.5, we can see that using selective Ge coating, we can achieve fundamental transverse mode operation in our VCSELs up to 13 times ofthe threshold current with a maximum output power of 1 mW.
Fig. 6 shows the magnified spectrum of a VCSEL driven at 11 mA. The mode suppression ratio is over 30 dB. The rather lower symmetric side peaks are not from higher order modes but due to the resonance-cavity-enhanced spontaneous emission from the area that is covered by Ge coating [7].
cause destructive damage to the DBR mirror. This technique should be useful for the realization of low resistance single mode VCSELs with high output power and extended lifetime.
Acknowledgement:
This work is supported by the National Science Council under contract NSC 89-2215-E-009-013 and the Lee-MTI center ofthe National Chiao Tung University.
Figure caption:
Figure 1. The peak reflectivity versus the refractive index of a surface AR-coating of a 20 pairs of A1As/A1GaAs DBR for
850 nm VCSEL.
Figure 2. The SEM photograph ofthe top laser facet with a Ge AR-coating and a 5.tm window.
Figure 3. The light versus current (L-I) characteristics for the VCSELs with Ti02, Ge coating films, and without
AR-coating.
Figure 4. The L-I curve ofa VCSEL with a Ge AR-coating and a 5j.tmwindow.
Figure 5(a). The spectra ofthe VCSELs with a Ge AR-coating and a 5j.tmwindow measured at 7 mA, 9mA, 1 imA, 15 mA, 20 mA
5(b). The spectra of a VCSEL without surface AR-coating measured at 5mA,
7
mA,9
mA,15mA,20 mA. Figure 6. The magnified spectrum ofa VCSEL with a Ge AR-coating and a 5 .tm window measured at the 11 mA.Reference:
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