Chapter I Introduction
1.2 Motivation
Vertical-cavity surface-emitting lasers (VCSELs) have attracted much attention in recent years. Singlemode VCSELs are needed for a number of applications, including high-speed laser printing, optical storage and long-wavelength telecommunications. For oxide-confined VCSELs, the current-confined aperture must be less than 3 μm in diameter to ensure stable singlemode operation [4]. However, the large resistance inherited from the small aperture limits the modulation bandwidth and degrades the high-speed performance. The lifetime of the oxide VCSEL also decreases proportionally as the diameter of the oxide aperture shrinks, even when the device is operated at a reduced current [4]. Methods reported to solve the problem include the increase of higher-order mode loss by surfacerelief etching [5] and hybrid oxide-implanted VCSELs [6, 7]. Recently, a two-dimensional photonic crystal (2-D PC)
structure formed on the VCSEL surface has been investigated as a control method of lateral mode. Singlemode output was realized from larger aperture photonic crystal VCSELs (PC-VCSELs).
The organization of this thesis is as following: In chapter 2, we discuss the theory of photonic crystal VCSEL, According to the effective index model, the number of guided mode of an proton-implanted aperture VCSEL is determined by V value. The measurement set up are described in chapter 3. The experimental results and discussions of PC-VCSEL are presented in chapter 4, 5 and 6. Finally, we give a conclusion of the thesis in chapter 7.
Reference
[1] S. F. Yu, “Analysis and design of vertical cavity surface emitting lasers”, Wiley-Interscience, 2003, pp.18
[2] H. Soda, K. lga, C. Kitahara, and Y. Suematsu, “GaInAsP/InP surface emitting injection lasers, “Japanese Journal of Applied Physics, vol.81, pp.2329-2330, 1979.
[3] K. lga, F. Koyama, and S. Kinoshita, “Surface-emitting semiconductor lasers, “IEEE Journal of Quantum Electronics, vol.24, pp.1845-1855, September 1988.
[4] Hawkins, B.M., Hawthorne III, R.A., Guenter, J.K., Tatum, J.A., and Biard, J.R.:
‘Reliability of various size oxide aperture VCSELs’.
Proc. 52nd Electronic Components and Technology Conf., 2002, pp. 540–550 [5] Haglund, A., Gustavsson, J.S., Vukusic, J., Modh, P., and Larsson, A.: ‘Single
fundamental mode output power exceeding 6 mW from VCSELs with a shallow surface relief’, IEEE Photonics Technol. Lett., 2004, 16, (2), pp. 368–370
[6] Hsueh, T.-H., Kuo, H.-C., Lai, F.-I., Laih, L.-H., and Wang, S.C.: ‘High-speed characteristics of large-area single-transverse-mode vertical-cavity surface-emitting lasers’, Electron. Lett., 2003, 39, (21), pp. 1519–1521
[7] Young, E.W., Choquette, K.D., Chuang, S.L., Geib, K.M., Fischer, A.J., and Allerman, A.A.: ‘Single-transverse-mode vertical-cavity lasers under continuous and pulsed operation’, IEEE Photonics Technol. Lett., 2001, 13, (9), pp. 927–929
Chapter II Theory
2.1 Gain-guiding mechanism
In the early development of a laser with a gain-guiding mechanism is based on a simple fabrication technique that is compatible with the existing technologies of facet emitting lasers [1]. A gain-guiding mechanism can be realized by forming a circular metal contact close to the active layer. The injection carrier concentration defines the gain region to confine the transverse mode. Figure 2-1 shows the schematic of a gain-guided VCSEL. The major advantage of this structure is easy to fabricate, but the transverse confinement of optical field and injection current density is weak so that the corresponding threshold current is high. The threshold current can be improved if the current leakage is minimized along the transverse direction. This can be easily achieved by ion implantation into the p-DBR (but should avoid damaging the active layer) to increase the electrical resistively [2]. Figure 2-2 shows the schematic of an ion implanted VCSEL. As is shown, the ion implanted region is defined selectively to control the flow of the injection current into the active layer. Unfortunately, this configuration of ion-implanted region has no control on the diffusion of carrier concentration along the transverse direction of the active layer. It is possible to apply ion implantation into the active layer, but this well increase the optical absorption loss (i.e., due to scattering of ions) of the device. Moreover, at high-power operation, higher-order transverse nodes can be excited because of the influence of thermal lensing and spatial hole burning of carrier concentration, which are the undesired characteristics of gain-guided VCSELs. The other problem is the electrical resistively of the DBRs, which may increase the heat generation inside the laser cavity.
Nonetheless, the attraction of this structure is its planar configuration, which guarantees the simplification in fabrication process and packaging so that low production costs can be maintained and the ion implantation can be executed in any semiconductor materials. Therefore, many manufacturers (Honeywell, etc.) manufactured products using ion implantation technology in their early-state development of VCSEL. The ion implantation technique has also been applied to
fabricate long-wavelength VCSELs [3, 4].
Figure 2-1 Schematic diagram of a gain-guiding VCSEL with circular electrode to confine injection carrier concentration into the active layer.
p-DBR
Implanted region
n-DBR Active region contact
substrate Emission aperture
p-DBR
Implanted region
n-DBR Active region contact
substrate Emission aperture
Figure 2-2 Schematic diagram of a gain-guided VCSEL ion implantation regions to confined injection carrier concentration into the active region.
2.2 Hybrid-guiding mechanism
In order to obtain high-power output, the gain area of the device needs to be increased, which increases the number of transverse modes. Furthermore, the wavelength separation and difference in optical losses between these transverse modes decrease. In the limit of very large emission apertures, there are many transverse modes with wavelengths and losses equal to the plane-wave limits. Combining high-power and single-mode operation is therefore very difficult as we discuss in the previous paragraph. The popular design used to obtain single-mode operation VCSEL are antiguiding-structure concept [5]. The main disadvantage of antiguiding structures is the fundamental mode also induces large radiation losses, and then increasing threshold and limiting maximum output power. Young et al proposed another approach using hybrid implant/oxide VCSELs that support single-mode operation [6]. This schematic structure of hybrid-guided approach is shown in Figure 2-3, which relies on modifying the overlap of the optical mode with the gain profile (also referred to as mode-selective gain) to generate high-power single-mode operation. However, the threshold current (Ith) of this VCSEL is rather high, about 5.8 mA. Jean-Francois P.
Seurin et al report this hybrid-confined structure as implant apertured index-guided VCSEL ( I2-VCSEL ) and demonstrated the results in Laser Focus World [7]. The main advantage of the I2-VCSEL is the independent control of the diameter of the etched gallium arsenide (GaAs) pillbox, which defines the index-guiding aperture, and the diameter of the ion implant, which defines the current aperture, by using different sets of masks. The structure is shown in Figure 2-3 (a). Photolithographic processing enables concise control of the diameters. Contrary to an oxide aperture, the implant aperture is invisible to the optical beam. The current aperture can therefore be made smaller than the index-guide aperture to reduce the overlap of the higher-order modes with the gain profile while at the same time keeping a sufficient fundamental mode/gain overlap. The concept of decoupling the high-order mode with gain is shown in Figure 2-3 (b). As the index-guide diameter is increased, the mode diameters will also increase and the overlap with the gain profile will decrease. Because higher-order modes have a relatively larger amount of their power spreading off center than does the
fundamental mode, they suffer more from this loss of overlap. Eventually, the modal threshold gains of higher-order modes will become too high and the fundamental mode will be the only one to laser.
(a) (b)
Figure 2-3 (a) The schematic structure of implant-aperture index-guided VCSEL (hybrid confinement), current confinement and photon confinement can be controlled independently. (b) The schematic of coupling relationship between transverse mode and gain spectrum.
2.3 V-parameter
Lateral-mode control of vertical-cavity surface-emitting lasers (VCSELs) is one of the key issues in realizing high performance optical communication systems, in which single-mode operation is necessary for long and short wavelength regions. High-power single-mode operation is also required for free-space data communication applications.
Recently, a two-dimensional photonic crystal (2D PC) structure formed on a VCSEL surface has been investigated as a lateral-mode control method. The most attractive feature of this structure is the enlargement of the emission area, thereby permitting higher power output. The large area can be realized because of strong wavelength dependence of the refractive index in the 2D photonic crystal structure, analogous to the situation in a
photonic crystal fiber. Although good single mode operation has been reported for a specific structure, the optimized design of 2-D photonic crystal structure was not clear, especially when considering a finite etching depth. Since the mode control mechanism utilized in this technology is the effective index control achieved by forming a 2D photonic crystal structure, a parameter representing this control must have a strong dependence, both theoretically and experimentally, of the effective index change in a VCSEL structure.
A two-dimensional (2D) photonic crystal structure formed on a VCSEL surface has been investigated as a control method of lateral mode. D. S. Song et al. first applied 2-D photonic crystal on oxide-confined VCSEL, and the schematic structure was shown in Figure 2-4 [3]. This concept was from the endless single-mode from photonic crystal fibers [4]. By introducing the single defect photonic crystal to the VCSEL, a waveguide is expected to be formed around the central defect region where the effective index is larger than the surrounding region. The effective index model [5, 6] is used to understand the VCSEL with lateral structural variation. According to the effective index model, in ref. [3], the number of guided mode of an oxide-confined VCSEL is determined by V parameter, which has the form [6]
2 where kcore,z , kclad,z , are longitudinal resonance wave vectors of core and cladding region, respectively, and hcore , hclad are transverse wave vector in the medium. d is a diameter of the core. The〈ε represents the dielectric constant weighted by the longitudinal standing 〉 wave. For the photonic crystal VCSEL shown in Figure 2-3, the resonance wavelengths would be different for central core region and rthe surrounding region. This situation is approximated as a simple step-index waveguide
Figure 2-4 Schematic of the 850 nm PC-VCSEL. Note that the first generation PC-VCSEL structure has no oxide current aperture. The oxide aperture is added to the second generation devices for current confinement.
surrounding region. This situation is approximated as a simple step-index waveguide as shown in Figure 2-4. With the aid of this model, the difference of effective indices in the two regions can be estimated by measuring the resonant wavelengths in the core and surrounding regions. The overall effects of the etch diameter, and pitch of the holes show up experimentally as the shift of the resonant wavelength. This may be written compactly as
core clad
core core
eff clad core
eff
n
n
, −/
,≈ Δ λ
−/ λ
Δ
(2-2)PC-VCSEL were Fabricated on selective oxide VCSEL, single-mode output with higher output power were realized from larger aperture [1]. However, those photonic crystal PC-VCSELs exhibit relatively high threshold current (Ith) due to large oxide confined apertures.
The 2D photonic crystal structure with finite etching depth incorporating a single
point or a seven-point defect is formed in the DBR. It is know that the normalized frequency or V-parameter is useful in evaluating the number of guided modes in cylindrical wave guides, an important example being step index optical fibers. The cutoff condition of the first higher mode leads to V
n
effn
cladn
coren
cladFigure 2-5 The Schematic of lateral effective index variation provided by the photonic crystal.
eff = 2.405, and thus a wave guide with Veff < 2.405 is considered to be single mode. In a photonic crystal VCSEL, the effective V-parameter can be expressed by
(2-3)
where λ is an operating wavelength, γ is an equivalent defect radius, neff is the effective index of the VCSEL cavity [9] without a photonic crystal structure present, Dn
is the refractive index reduction introduced by the photonic crystal structure, and r is
the hole depth dependence factor that accounts for finite etching depth of the photonic crystal holes in actual photonic crystal VCSELs. The r factor can be understood qualitatively as proportional to the spatial overlap between the photonic crystal structure. Thus, γ=0 for vanishing etching depth, γ=1 for infinite etching depth, and γ=0.5 for holes reaching the middle of the cavity. In the following discussion, the equivalent defect radii of a single point defect and seven-point defect structures are assumed to be Λ-d/2 and √3Λ-d/2, respectively, where L is lattice constant and d is the hole diameter of a circular hole of the 2-D photonic crystal structure. Since we need to investigate larger d/L ratios than those of PCFs [10], Veff is slightly modified from its appearance in Ref. 10, with the introduction of the Veff-parameter and the modified r in Eq. (2-3). The refractive index variation Dn can be obtained from the photonic band diagram of an out-of-plane propagation mode[11,12], it calculated by assuming that the photonic crystal structure is infinite both in lateral and vertical directions.
Reference
[1] S. F. Yu, “Analysis and design of vertical cavity surface emitting lasers”, Wiley-Interscience, pp.22, 2003.
[2] Y. J. Yang, T. C. Dziura, T. Bradin, S. C. Wang and R. Fernandez, “Continuouswave single transverse mode vertical cavity surface emitting lasers fabricated by Helium implantation and zinc diffusion,” IEE Electron. Lett., vol. 28, no. 3, pp. 274-275, 1992.
[3] J. Boucart, C.Starck, F. Gaborit, A. Plais, N. Bouche, E. Derouin, L. Goldstein, C.
Fortin, D. Carpentier, P. Salet, F. Brillouet and J. Jacquet, “1mW CW-RT monolithic VCSEL at 1.55μm,” IEEE Photon. Technol. Lett., Vol. 11, pp.629-631, no.6, 1999.
[4] Y. Qian, Z. H. Zhu, Y. H. Lo, D. L. Huffaker, D. G. Deppe, H. Q. Hou, B. E.
Hammons, W. Lin and Y. K. Tu, “Low-threshold proton implanted 1.3μm vertical cavity top surface emitting lasers with dielectric and wafer bonded GaAs-AlAs bragg mirrors,” IEEE Photon. Technol. Lett., vol. 9, no.7, pp.866-868, 1997.
[5] J. W. Scott, B. J. Thibeault, D. B. Young, L. A. Coldren and F. H. Peters, “High efficiency submilliamp vertical cavity lasers with intracavity contacts,” IEEE Photon. Technol. Lett., vol. 6, no. 6, pp.678-680, 1994.
[6] E. W. Young, K. D. Choquette, S. L. Chuang, K. M. Geib, A. J. Fischer and A. A.
Allerman, “Single-transverse-mode vertical-cavity lasers under continuous and pulsed operation,” IEEE Photon. Technol. Lett., vol. 13 pp. 927–929, 2001
[7] J.-F. P. Seurin, S. L. Chuang, L. M. F. Chirovsky, and K. D.
Choquette, “NovelVCSEL designs deliver high single-mode output power,” Laser Focus World, vol. 38, issue 5, 2002.
[8] T. Honda, T. Shirasawa, N. Mochida, A. lnoue, A. Mastsutani, T. Sakaguchi, F.
Koyama, and K. lga, “Design and fabrication processes consideration of GaN-based surface emitting lasers“, Trans. IEICE, J81-C-II, pp.97-104,1998.
[9] G..P.Agrawal, Fiber-Optic Communication System~Wiley, New York, 1997.
[10] R.Brand, “10 gigabit Ethernet interconnection with wide area networks,” 10GEA, March 2003.
[11] A.A. Maradudin and A.R. McGurn, J. Mod. Opt. 41, 275~1994.
[12] J.D. Joannopoulos, R.D. Meade, and J.N. Winn, Photonic Crystal-Molding the Flow of Light~Princeton University Press, Princeton, NJ, 1995.
Chapter III Measurement setup
3.1 Probe station and spectrum measurement system
Probe station system was essential instrument for basic characteristics measurement such as I-L (current versus light output), I-V (current versus voltage). We are scheming out the probe station system, illustrated in Figure 3-1, contained probe station, current source, and power-meter module. Keithley 238 as current source supplies continuous current for diode laser and receives relative voltage synchronously. Laser output power is measured by Newport power-meter module (model 1835C). With these data, we could plot the trend of L-I-V curves associated with computer. The light output was detected by a calibrated an integrating sphere with Si photodiode. For accuracy power measurement, an integration sphere was used to pick up whole light output from PC-VCSEL.
For basic measurement, PC- VCSEL device was placed on platform of probe station and injected bias current with microprobe. We could observe threshold condition, slope efficiency, turn-on voltage and differential resistance as L-I-V curves information by sweeping bias current injection. Distribution of transverse mode power is metered as near-field pattern. Near-field pattern (NFP) is still obtained by specific CCD and traces out results with computer. Beam-view analyzer software used in taking NFP. We could obtain NFP image under threshold current, as spontaneous emission, to define oxide aperture size.
Emission spectrum was measured by Advantest optical spectrum analyzer (OSA).
We served a multi-mode fiber bundle on probe close to emission aperture in focus for taking spectra. OSA had small spectrum resolution as 0.1nm for accurately measuring PC-VCSEL lasing spectrum. The spectrum measurement system was combined with probe station as showed in Figure 3-1.
3.2 Far field pattern measurement system
Distribution of divergence angle is metered as far-field pattern. We used a long work distance objective (20X,Nikon), fixed in a triple-divide translation stage, to pick up the laser output from VCSEL. Light was received by CCD (coherent 4800). Far-field pattern (FFP) is still obtained by specific CCD and traces out results with computer. Beam-view analyzer software used in taking FFP. We could obtain FFP image under threshold, to define divergence angle illustrated with Figure 3-2
Power meter
Figure 3-1 Probe station measurement instrument setup
CCD
Objective Keithley 238 driver
Power supply Spectrum analyzer
Computer
Integrating sphere PD
Prober
+ –
電流源 物鏡
CCD
光譜儀
功率計 近場圖案
工程試算軟 體(Labview)
optical fiber
光場分佈 繪圖軟體
probe station
偵測頭
Figure 3-2 Far field pattern measurement system
Chapter IV Characteristics of singlemode proton-implanted photonic crystal VCSEL
4.1 Introduction
Vertical-cavity surface-emitting lasers (VCSELs) have attracted much attention in recent years. High-power, singlemode operation is desired for a number of applications, including high-speed laser printing, optical storage and long-wavelength telecommunications. Recently, a two-dimensional photonic crystal (2-D PC) structure formed on a VCSEL surface has been investigated as a lateral-mode control method. The most attractive feature of this structure is the enlargement of the emission area, thereby permitting higher power output. It is critical that PC VCSELs obtain single transverse-mode operation and a small far-field angle for efficient coupling into singlemode fiber. In this study, we designed a low threshold current (Ith), high-power, single-lateral-mode, low divergent angle operation VCSEL by employing proton implantation combined with a single-point defect photonic crystal index guiding layer.
The large area can be realized because of the specifically tailored refractive index induced by the 2D PC structure, analogous to the situation in a PC fiber (PCF). [1] To understand the VCSEL with lateral structure variation, the effective index model is used.
According to the effective index model, the number of guided mode of a proton-implanted aperture VCSEL is determined by V parameter which it has the form:
where neq is the equivalent refractive index of the photonic crystal region surrounding the central defect (determined from band diagram analysis and etching depth dependence), nm is the unmodified index of the defect region, Λ is the lattice constant, λ is the free-space wavelength [2][3]. If Veff is less than 2.405, the structure is considered to be
singlemode. In general, an increase in diameter of hole with a given lattice constant or an increase in the size of the central defect will increase the likelihood of laser operation in multiple higher order transverse modes, rather than the lowest order single transverse mode operating point.
4.2 Device fabrication
The epitaxial layers of the PC-VCSELs wafer structure were grown on the n+-GaAs substrate by a metal organic chemical vapor deposition (MOCVD) system.
The bottom distributed Bragg reflector (DBR) consists of a 30.5-pair n-type (Si-doped) quarter-wave stack of Al0.12Ga0.88As/AlAs. The top DBR consists of 22-period p-doped (carbon-doped) Al0.12Ga0.88As/AlA quarter-wave stack. Above that, a heavily-doped p-type GaAs contact layer. The graded-index separate-confinement heterostructure (GRINSCH) GaAs/AlGaAs active region has an undoped three-quantum-well (3 QWs) GaAs/Al0.3Ga0.7As, a lower linearly-graded undoped Al Gax 1-xAs (x = 0.6Æ0.3) waveguide layer and an upper linearly-graded undoped-Al Gax 1-xAs (x=0.3Æ0.6) waveguide layer. The proton (H+)-implanted VCSEL was fabricated before combination with photonic crystal holes. Firstly was then, the p-contact ring with an inner diameter of 24 to 46 µm was deposited on the top of p-contact layer and n-contact was deposited on the bottom of n+-GaAs substrate. The device was annealed in thermal annealing system at 430 under N℃ 2 ambient for 30 sec. The current
The bottom distributed Bragg reflector (DBR) consists of a 30.5-pair n-type (Si-doped) quarter-wave stack of Al0.12Ga0.88As/AlAs. The top DBR consists of 22-period p-doped (carbon-doped) Al0.12Ga0.88As/AlA quarter-wave stack. Above that, a heavily-doped p-type GaAs contact layer. The graded-index separate-confinement heterostructure (GRINSCH) GaAs/AlGaAs active region has an undoped three-quantum-well (3 QWs) GaAs/Al0.3Ga0.7As, a lower linearly-graded undoped Al Gax 1-xAs (x = 0.6Æ0.3) waveguide layer and an upper linearly-graded undoped-Al Gax 1-xAs (x=0.3Æ0.6) waveguide layer. The proton (H+)-implanted VCSEL was fabricated before combination with photonic crystal holes. Firstly was then, the p-contact ring with an inner diameter of 24 to 46 µm was deposited on the top of p-contact layer and n-contact was deposited on the bottom of n+-GaAs substrate. The device was annealed in thermal annealing system at 430 under N℃ 2 ambient for 30 sec. The current