C.-H. Jiang 1 , Jin-Wei Shi 1 *, J.-L. Yen 2 , K.-M. Chen 2,3 , and Ying-Jay Yang 2
1
Department of Electrical Engineering, National Central University
Taoyuan 320, TAIWAN, R.O.C.
Tel: +886-3-4227151 ext. 34528
FAX:+886-3-4255830
Email: jwshi@ee.ncu.edu.tw
2 Department of Electrical Engineering, National Taiwan University,
Taipei 106, TAIWAN, R.O.C.
3
Institute of Physics, Academia Sinica, Taipei 115, TAIWAN, R.O.C.
Abstract
In this paper, we demonstrate the single-mode and low divergence angle performance of vertical-cavity surface-emitting lasers (VCSEL) with a ring shaped light-emitting aperture, which is defined by the Zn-diffusion technique. As compared to the control VCSEL with different shape of light-emitting apertures
(circular plate vs. ring) at the wavelength of 850nm, the demonstrated device can suppress its higher order transverse mode more effectively. Under 2mW output power with single-mode operation, much narrower divergence angle (~5 ∘ vs. ~10 ∘ ) and lower differential resistance (~70

vs. ~200

) than the reported values of typical single-mode VCSELs have been achieved.
Index Terms —vertical-cavity surface-emitting lasers, single mode, far field distribution, Zn diffusion, single-lobed
* Corresponding author, Tel: +886-3-4227151 ext. 34528, FAX:+886-3-4255830, Email: jwshi@ee.ncu.edu.tw
1

Vertical-cavity surface-emitting lasers (VCSELs) [1] have attracted lots of attention due to their advantages, such as, low-threshold current, one- and two- dimensional array formation [2,3], and inexpensive device fabrication. High output power of several milli-watts with an optical single-mode or single-lobed output of
VCSEL is much desired for several applications, such as free space optical interconnects [4], laser printing, and airborne light detecting and ranging (LIDAR) systems [5]. Unfortunately, due to the relatively large transverse dimension of high power VCSEL, it will exhibit several spatial transverse modes and lase at multiple wavelengths. On the other hand, the single-mode VCSELs usually suffer from the problems of low optical power [6] and large differential resistances [7,8]. Several methods have been developed to demonstrate VCSELs with single-mode output and high optical power, such as anti-resonant reflecting optical waveguide structure [9], surface relief structure [10], and the combination of implant and oxide apertures [11].
However, in order to implement these structures, complex regrowth process or precisely control of the etching depth of top-mirrors are necessary. Furthermore, the divergence angles of output optical beams from the reported single-mode VCSELs are usually very large (>10 ∘ ) [6-11]. Large divergence angle of output optical beam will not only increase the difficulty in beam-collimation and collecting the optical power, which play important roles in the application of free-space optical interconnects, but also degrade the spatial resolution of LIDAR system.
In this paper, we demonstrate a VCSEL structure with ring-shaped light-emitting area [12], which can achieve stable single-mode performance without utilizing the complex etching and re-growth processes. With a top ring-shaped partially disordered mirror formed by selective Zn diffusion [6,13,14] through the top
2

distributive Bragg reflector (DBR) and an ion-implanted current-confined aperture, the demonstrated device can achieve single-mode performance under continuous wave (CW) operation with 2mW output power. Under such operation condition, this device also exhibits small divergence angles and low differential resistances.
In order to characterize the performances of this novel structure, two types of devices were fabricated. Fig.1 (a) and (b) shows the top view and cross-sectional view of these two devices. Both devices share the same epi-layer structure, which is consisted of a heavily carbon doped p-type GaAs cap layer, a 30-pair p-type and a
20-pair n-type Al x
Ga
1-x
As/AlAs distributed Bragg reflectors. Three GaAs/AlGaAs quantum wells sandwiched as active region, which has a center wavelength at 850nm.
Device A and B both have the same circular current-confined aperture with 17μm diameter, which is defined by ion implantation. As shown in Fig. 1(a), the only difference between A and B is that the device B has a circular shape of optical absorption aperture, which is defined by Zn-diffusion [6,13,14], inside the center of circular light-emitting aperture. The diameter of light absorption and emitting apertures is 5μm and 15μm, respectively. Regarding with device A, it can be treated as a regular VCSEL with a circular shape of light-emitting aperture.
The fabrication processes of these two devices can be described as follows: First a Si
3
N
4
mask was formed by photolithography on the sample to define the ring shape, then the masked sample with a Zn
2
As
3
source was sealed in an evacuated quartz ampoule and heated to 650 ℃ for 15min. The resulted in a Zn diffused region with a diffusion depth (~1 μ m) outside the Si
3
N
4
masked area which forms a higher order mode absorber. After the Zn diffusion, using a photoresist layer as a mask, an ion implantation was conducted on the Zn-diffused sample to form a current-confined aperture. After that, a Ti/Au film with an opening window and an Au/Ge/Ni film
3

were deposited on the top and bottom of the sample to form a p- and n-type electrode respectively.
Fig.2 shows the light output versus current (L-I) characteristics of the two fabricated devices operated at room temperature. The light output power was measured by a calibrated Newport silicon p-i-n photodetector. We can clearly see that both devices exhibit almost similar threshold current and L-I curves. According to the measurement results, we can conclude that the optical absorption aperture in our demonstrated structure will not degrade the performance of optical power and threshold current significantly. We have measured the output optical spectrum of both devices under different current operation. Device B reveals single-mode performance when the amount of injected current is below 5mA, which corresponds to 2mW output power. On the other hand, device A has very poor performance of optical mode-stability. It exhibits multi-mode behaviors when the amount of current is just over threshold (2.8mA). The insets of Fig. 2 show the measured spectrums of two devices under the same current operation (~5mA). Single-mode spectrum of device B has been observed, and its side-mode suppression ratio is about 30dB.
Compared with the previous work on Zn-diffused [6] and oxide-confined [8] VCSELs with single-mode output and traditional circular light-emitting aperture, our demonstrated structure has significant improvement in output power (2mW vs.
0.8mW) and differential resistance (70

vs. 400

), respectively.
The measured far field profile in the x-direction of device A and B under different current operations are showed in Figure 3 (a) and (b), respectively. The measured far-field patterns exhibit almost similar shapes and divergence angles in both X and Y axes. For the case of device A, its far-field pattern and divergence angle, which is defined by the full width half maximum (FWHM) of far-field patterns,
4

are influenced strongly by the amount of injected current. The Gaussian mode arises initially at threshold current, higher order spatial modes with two main peaks appear with increasing the injected current. The phenomenon is mainly due to spatial hole burning effect [15,16]. With respect to device B, its far-field pattern exhibits single-lobed performance with a much higher directivity (narrower divergence angle) even when the output optical power is up to 7.1mW with an injected current of 8mA.
Table.1 lists the measured divergence angles in the x and y directions of both devices at various drive-current levels. Device B obviously has much more stable single-lobed performance and narrower divergence angle compared with device A.
The demonstrated sing-lobed with high output power (7.1mW) performance is comparable to the performance of 2-dimensional (2-D) array of VCSELs [2,17], which is designed for high power and single-lobed with narrow divergence angle performance. However, its far-field usually suffers from the problem of multi-lobed distribution [2,17].
Under single-mode operation (current < 5mA), the obtained divergence angle
(less than 5 ∘ ) is much less than the reported values of previous works on single-mode VCSELs [6-11], which usually have a divergence angle larger than 10 ∘ .
The excellent performance of divergence angle is originated from that our device has much larger area of light-emitting and current-confined apertures than the area of typical single-mode VCSEL. We can use the following equations to roughly approximate the divergence angle of single-mode VCSEL with circular light-emitting aperture [18]:
θ  λ/d (1)
Where θ is the divergence angle, λ is the operation wavelength and d is the diameter of a circular aperture. For the case of single-mode VCSELs, which usually have small
5

diameters of light-emitting aperture (less than 5μm), the typical divergence angle of the FWHM is larger than 8°.
For the case of our demonstrated ring-shaped VCSEL, which has an aperture with outer diameter of ~15μm, the calculated ideal divergence angle is almost independent of the width of ring aperture and slightly less than 2° [12].
This value is much smaller than the measured divergence angle (~5°) as shown in
Table 1. The differences between theoretical and measured divergence angles can be possible attributed to that just small part of the area in the ring-shaped light-emitting aperture lase coherently. Most of the light-emitting area can’t lase in-phase and coherently. 2-D VCSELs array usually suffer from the similar problems. The 2-D
VCSEL array usually can’t lase coherently with fundamental single supermode [2,17].
In conclusion, we have fabricated and investigated the characteristics of VCSEL with ring-shaped light-emitting aperture. The optical and electrical aperture of demonstrated device is defined by Zn-diffusion and ion-implantation techniques, respectively. The newly designed structure is presented to have single-mode output with high optical power and much narrower divergence angle than the reported values of single-mode VCSELs. The measured far-field patterns also reveals excellent single-lobed with high power performance, which is suitable for the applications of
LIDRA and free-space optical interconnect. According to the divergences angles of measured far-field patterns, we can conclude that the light-emitting area in the ring-shaped aperture exhibits partially coherent-lasing phenomenon. VCSEL with improved performance of mode-stability, power, and divergence angle, can be expected by increasing the coherence of ring-shaped light-emitting aperture. This work is sponsored by National Science Council of Taiwan under grant number NSC
92-2218-E-008-011-, 93-2215-E-008-022-, and 93-2215-E-002-026
6

Figure Captions:
Figure 1. The top view of device A and B (a) and cross-sectional view of device B (b).
Device A is the conventional broad-area VCSEL and device B is VCSEL with the ring-shaped light-emitting aperture.
Figure 2. The characteristics of optical output power versus biasing current of device
A and B. The insets show the emission spectra of device A and B under the injected current of 5mA. The unit between two adjacent grids of y-axis in the insets is about 6dB.
Figure 3. The measured far field patterns in x-direction of device A (a) and device B
(b) at different current levels. Square: 4mA, Circle: 6mA, Triangle: 8mA
7

Table Captions:
Table 1. The measured divergence angles of device A and B under different level of injected currents. The values in the column of “average” are defined by the mathematical average of measured divergence angles in X and Y directions.
Current
4mA
6mA
8mA
Divergence angle of device A
X-axis
7.1
°
7.5
°
9.6
°
Y-axis
6.6
°
6.6
°
8.4
°
Divergence angle of device B
Average angle
6.85
°
7.05
°
9
°
X-axis
5
°
5
°
6
°
Y-axis
4.3
°
5
°
5.7
°
Average angle
4.65
°
5
°
5.85
°
8

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