Fundamental of VCSEL - Introduction and overview

Chapter 1 Introduction and overview

1.2 Fundamental of VCSEL

▲ Densely packed and precisely arranged two-dimensional laser arrays

▲ Higher reliability

▲ Higher integration

The progress of the vertical cavity surface emitting laser in the late 1990s was very fast, and various applications to optoelectronics [3]. In 1992, VCSELs based on GaAs have been extensively studied some of the 0.98, 0.85 and 0.78 µm wavelength devices are now commercialized into optical system. VCSELs are currently to be the ideal laser source for fiber optic application.

1.2 Fundamental of VCSEL

The structure common to most VCSELs consists of two parallel reflectors which sandwich a thin active layer, is illustrated with Figure 1-2. The reflectivity necessary to reach the lasing threshold should normally be higher than 99.9%. Together with the optical cavity formation, the scheme for injecting electrons and holes effectively into the small volume of the active region is necessary for a current injection device.

The ultimate threshold current depends on how to make the active volume small and how well the optical field can be confined in the cavity to maximize the overlap with the active region. These confinement structures will be presented later.

There are some choices of the materials for semiconductor lasers in Figure 1-3.

The availability of substrates against lattice constant related to the systems is also shown. The problems are listed below that should be taken into account consideration for making VCSELs:

▲ Design of resonant cavity and mode-gain matching

▲ Multi-layered distributed Bragg reflectors (DBRs) to realize high-reflective mirrors

▲ Heat sinking for high temperature and high power operation In addition, the resistivity of material is crucial for high speed operation.

These are typical methods of current confinement schemes for the VCSEL

▲ Ring electrode type: This structure can limit the current flow in the vicinity of the ring electrode. The light output can be taken out from the center window.

This is easy to fabricate, but the current can not completely be confined to a small area due to diffusion.

▲ Proton-implant type: An insulating layer made by proton (H⁺) irradiation to limit the current spreading toward the surrounding area. The progress is rather simple and most commercialized devices are made by this method.

▲ Regrowth Buried-Heterostructure (BH) type: The mesa etching including active region form a wide-gap semiconductor to limit the current. The refractive index can be small in the surrounding region, resulting in forming an index-guiding structure. This is one of ideal structures in terms of current and optical confinement. The problem is that the necessary process is rather complicated.

▲ Air-post type: The circular or rectangular air post is used to achieve a current confinement. It is the simplest method of device fabrication, but non-radiative recombination at the outer wall may reduce the performance.

▲ Selective AlAs oxidation type: By oxidizing the AlAs layer to form an insulator.

By developing new process technology could achieve the better laser performance.

1.3 Development trend

The market forecast of 2002 global laser diode market analysis [4], indicated in Figure 1-5, appears that optical communication and storage have almost 84% market demand. Moreover, the market distinguished into wavelength, Figure 1-6, shows 780~980nm laser diode in chief. For telecommunication, development of Ethernet causes the needs of capacity increasing rapidly. Metro-area network (MAN) would use fiber optical communication replace traditional cable to satisfy high-capacity requirement. Local-area network (LAN) would expand fiber to the home (FTTH) which developed rapidly in Japan now. Primary transmission rate is developed almost up to 10Gbps suitable for OC-192 standard by ANSI [5]. Storage-area network (SAN), another major grownth sector, perhaps is the brightest spot in networking. These interconnections link large disk arrays to servers and other

that has become the standard. FC continues to get faster as the newer versions of the standard have increased rates to 1.0625 Mbits/s and 2.125 Gbits/s, making this system even more desirable today. Faster 4G and 10G systems are in the works.

VCSEL with advantage of wide-band and small-volume transmission capability is an appropriate laser source for those applications. Long wavelength VCSELs provide ultimate transmission capacity in long-distance; 850nm VCSELs are suited to short-distance communication such as LAN. VCSELs contain primary fiber optic communication channels, that is, 850/1310/1550nm which has less attenuation in fiber link, indicated in Figure 1-7. Beside of communication, the high-speed demand for optical storage is growth. International associate precisely define 650nm laser as beam recorder. VCSELs almost contain wide-band transmission application.

1.4 Applications of VCSELs

1.4.1 Data communication

Today, datacom moduels based on near-infrared VCSELs represent 95% of the VCSEL market, 80% of which are commercialized by a few companies: Agilent, Honeywell, Infineon, Furukawa and Zarlink. The remaining 20% are shared among numerous start-ups offering innovative designs. The market has recently exploded: it is evaluated to be worth USD 500 million at present, and is constantly growing due to the rapid deployment of Gigabit Ethernet and fiber channels. Most of today’s commercial datacom components are based on oxide 850 nm VCSELS. They are often package as single component or in

Figure 1-8 12 channel 2.5 Gb/s VCSEL array for short distance transmission applications.

parallel fiber modules of linear arrays, offering 4 to 12 channels at 2.6 GB/s per channel, which aggregate bandwidth up to 30 GB/s. The research and development efforts are focusing on the next generation of high speed VCSEL, and a number of groups have reported transmission at 10 GB/s or more for distances up to 300 m of MMF. Figure 1-8 shows a 12 channel 2.5 Gb/s VCSEL array for short distance transmission applications.

For long wavelength part, Metro and Access Networks are dominated by 1300nm and 1550 nm FP and DFB lsers up to now. A long wavelength VCSEL (LW-VCSEL) would be an ideal low-cost alternative to the DFB laser, particularly for the standard IEEE 802.3ae applications, which extend the existing Gigabit Ethernet into traditional SONET markets at OC-192 data rates. However, the performance specifications for such LW-VCSELs are challenging. For low-cost transceivers, they must operate over the 0 to 70 C temperature range for indoor applications and over the -40 to 85 C range for outdoor applications, without external temperature stabilization. The laser power launched into the single mode fiber must usually be more than 0.7 mW in order to support transmission distance of 10 km at 10 GB/s.

Despite intense research effort, the technology so far has not yet met these requirements.

1.4.2 Optical interconnect

The optical interconnect is considered to be inevitable in the computer technology. The performance of massively parallel computers is usually limited by the communication bottleneck between processors. Optic provides an effective mean to line these processors because of its high capacity, low crosstalk and attenuation, and the possibility to obtain three-dimensional architectures. Other potential applications include routers, switches and storage. The VCSEL is a strong candidate as the preferred optical light source for the emerging optical interconnect mass market, meeting the requirement of low cost, high density integration and low power dissipation. A 256-channel bi-directional optical interconnect using VCSELs and photodiodes on CMOS was demonstrated.

laboratories and corporations research in 10Gbps device, as Table1-1. In this paper, we focus on high-speed 850nm vertical cavity surface emitting lasers. First, we mention major VCSEL structure process, oxide and oxide-implantation, and explain high-speed test system in detail in chapter 2. In chapter 3, a simple introduction of phenomenon approach the rate equation is presented. The scattering coefficients are estimated by high-speed measurement, Transfer function is used in analyzing dynamic characteristics as relaxation frequency, damping rate. We measure static and dynamic characteristics of VCSELs and create an equivalent circuit for modeling in chapter 4. Finally, we give a brief conclusion in chapter 5.

Light output

Figure 1-2 A modal of vertical cavity surface emitting laser (VCSEL)

DBR DBR

Optical Confinement Current

Confinement Electrode: anode

Transverse Mode field

Active Layer D

Electrode: cathode

1.3~1.6

1.2~1.3

0.78~0.88

0.63~0.67

0.45~0.55

0.3~0.5

1.3 1.5

0.8 1.0 0.3 0.5

Wavelength (µm)

GalnAlN/GaAlN ZnSSe/ZnMgSSe GaAlInP/GaAs GaAlAs/GaAs GaInAs/GaAs GaInNAs/GaAs GaInAsP/InP AlGaInAs/InP

Figure 1-3 Material for VCSELs in wide spectral bands

Ring electrode Electrode

Active layer p-DBR

n-DBR

Substrate

H⁺ H⁺

Active layer p-DBR

n-DBR

Substrate

(a) Ring-electrode type (b) Proton-implant type

(d) Air-post type Substrate

n-DBR p-DBR Active layer Electrode

(c) Regrowth BH type Substrate

n-DBR Active layer Electrode

Oxide layer Electrode

Active layer p-DBR

n-DBR

Substrate Epitaxial regrowth

(e) Selective oxidation type

Figure 1-4 Structure for current confinement for VCSELs

Source: Laser Focus World, PIDA

Figure 1-5 Global consumption value of diode laser by the year 2002

Source: Laser Focus World, PIDA

Figure 1-6 Global consumption value of diode by the year 2002 (distinguished into wavelength)

Figure 1-7 Transmission spectrum for silica fiber

Parameter Symbol Material Wavelength Active layer thickness d 100 Ǻ ~ 0.1µm 80 Ǻ ~ 0.5µm

Active layer area S 3 x 300 µm² 5x5µm²

Active volume V 60 µm³ 0.07µm³

Cavity length L 300 µm ~1 µm

Reflectivity Rm 0.3 0.99 – 0.999

Optical confinement ξ ~ 3 % ~ 4 % Optical confinement

(Transverse) ξt 3 – 5 % 50 – 80 %

Optical confinement

(Longitudinal) ξl 50 % 2 x 1% x 3 (3 QWs) Photon Life time τp4 ~ 1 ps ~ 1 ps

Relaxation Frequency

( Low Current Levels) fr < 5 GHz > 10 GHz

Table 1-1 Comparison of parameters between stripe laser and VCSEL

Chapter 2 Theory

The chapter begins by developing a reservoir model for a flow of charge into double-heterostructure active regions and its subsequent recombination.

Recombination mechanism is determined by electron-hole recombination at quantum well generates photons for light emission. For this we describe the phenomenological approach to VCSEL first.

The rate equations provide the most fundamental description of the laser in next section. It describes the time-evolution of carrier and photon densities in a laser cavity as a function of the pump rate, material gain and parameters associated with the material properties and laser construction. Dynamic characteristics of VCSEL are studied as rate equation, and it is produced for modulated current injection. Transfer function is relevant equation originated from rate equation which is explained next.

The last section is presented of V-parameter used in 2-D photonic crystal structure, the V-parameter evaluating the number of guided modes in cylindrical wave guides the rate equations for carriers and photons are found to be analogous to differential equations that describe the current and voltage in an RLC circuit.

Scattering parameter offers not only response as bandwidth but reflection coefficient which mirrored equivalent RLC circuit. Establishment of equivalent circuit is purposed to be found the limitation of bandwidth for VCSEL structure and making terminal impedance matching easily.

2.1 Recombination mechanism of VCSEL

The proposition considers the current injected into VCSEL, and suggests it is desirable to have all of it contributes to electrons and holes which recombine in the active region. Since the definitions of the active region and the internal quantum efficiency, ηi, are so critical to further analysis. Active region, evolved into lowest band-gap region, is where recombining carriers contribute to photon emission. Band diagram of active region, includes separate confinement hetero-structure (SCH)

band-gap region, illustrated in Figure 2-1. Internal quantum efficiency, ηi, is the fraction of terminal current that generates carriers in the active region. It is important to realize that includes all of the carriers that are injected into active region, not just carriers that recombine induce radiating at the desired transition energy.

The carrier density, n, in the active region is governed by a dynamic process. In fact, we could compare the process of a certain steady-state carrier density in the active region to that a reservoir analogy, which is being simultaneously filled and drained, in a certain water level. This is shown schematically in Figure 2-2(a). For the double heterostructure active region, the injected current provides a generation term and various radiative and nonradiative recombination processes as well as carrier leakage provides recombination term. Thus, rate equation is determined as

rec

gen R

dt G

dn = − (2-1)

where Ggen is the rate of injected electrons and Rrec is the rate of recombining electrons per unit volume in the active region. There are ηi I/q electrons per second being injected into the active region. V is the volume of the active region.

The recombination process is accompanied with spontaneous emission rate, Rsp, and a nonradiative recombination rate, Rnr, depicted in Figure 2-3. Carrier leakage rate, Rl, must be occurred at the transverse and/or lateral potential barrier are not sufficiently high. A net stimulated combination, Rst, is including both stimulated absorption and emission. That is, an increased injection results in an increased output, Rst, but no increase in carrier density as water level illustrated in Figure 2-2(b).

Total recombination rate is expressed as below

(2-2)

where the first three terms on the right refer to the natural carrier decay processes. It is common to describe the natural decay processes by a carrier lifetime, τ . In the absence of photon generation term, the rate equation for carrier decay is, dn/dt = n/τ , where n/τ = Rsp+ Rnr+ Rl, by comparison to Eq. (2-2). Thus, the carrier rate equation in equivalent be expressed as

i n Rst

The net stimulated recombination rate, Rst, in generating photons as well as effect of the resonant cavity in storing photons is investigated additionally. A rate equation for the photon density, np, which includes the photon generation and loss terms are constructed analogous to carrier rate equation. The main photon generation term above threshold is Rst. Every time an electron-hole pair is stimulated to recombine, another photon is generated. However, as indicated in Figure 2-4, the cavity volume occupied by photons, Vp, is usually larger than the active region volume occupied by electrons, V, the photon density generation rate will be (V/Vp)Rst

not just Rst. This electron-photon overlap factor, (V/Vp), is generally referred to as the confinement factor, Γ.

Photon loss occurs within the cavity due to optical absorption and scattering out of the mode, and it also occurs at the output coupling mirror where a portion of the resonant mode is usefully coupled to some output medium. These losses could characterize by a photon lifetime, τ p. The photon rate equation takes the form

where β is the spontaneous emission factor. For uniform coupling to all modes, β is just the number of spontaneous emission coupled into specific mode in the bandwidth of all spontaneous emission.

Rst represents the photon stimulated net electron-hole recombination which generates more photons. This is a gain process for photon. An increased photon is proportional to an increased injection carriers overfill the reservoir, which is critical condition to generate stimulated emission, shown in Figure 2-3. The proportion is defined as gain coefficient, g(n). That is,

(2-5) to laser oscillation condition with regenerated amplifier approach due to multiple reflection, as shown in Appendix (1). ntr is a transparency carrier density, and vg is the

group velocity of the mode of interest including both material and waveguide dispersion. vg still can be expressed as C/nr, where C is 3x10⁸ m/s and nr is index of cavity medium. Thus Rst is replaced with g(n)np, and now the carrier and photon density rate equations can be written as

Equations (2-6) and (2-7) are two coupled equations that can be solved for the steady-state and dynamic responses of a diode laser in next section.

2.2 Transfer function

Under small signal modulation, the carrier and photon density rate equation, Eq.

(2-6) and (2-7), are used to calculate relaxation resonance frequency and its relationship to laser modulation bandwidth.

Consider the application of an above-threshold DC current, I0, carried with a small AC current, Im, to a diode laser. Illustration is, shown in Figure 2-5, under basic L-I characteristics (Light output power versus current). The small modulation signal with some possible harmonics of the drive frequency, ω. Small signal approximation, assumes Im<<I0 bias and spontaneous emission term, β , is neglected, is expressed as

Before applying these to Eq. (2-6) and (2-7), the rate equation is rewritten for the gain. Assumption under DC current is sufficiently above threshold that the spontaneous emission can be neglected. Without loss of generality, we suppose full overlap between the active region and photon field, Γ =1; furthermore, internal quantum efficiency, η_i, is neglected. That is,

substitute Eq. (2-8) into Eq. (2-9)

)

for this, it is similarly expressed modulation terms as

The small signal terms in frequency domain of carrier and photon are given by

substitute into Eq. (2-11) and (2-12), the equations become

Carrier modulation term in frequency domain is simplified as

Photon modulation term in frequency domain is simplified as response of two arranged equations as below

where

……Damping constant (decay rate)

With the Eq. (2-15) and (2-16) we observe the coupling between the small signal photon, npm, and carrier, nm. Small signal carrier injection induces photon achieved oscillation. This phenomenon produces a natural resonance in the laser cavity which shows up the output power of the laser in response to sudden changes in the input current. The natural frequency of oscillation associated with this mutual dependence between nm and npm. Modulation response is expanded the small signal modulation relationship to steady-state. From Eq. (2-15) and (2-16), the modulation response is denoted as

The general behavior of M(ω) is shown in Figure 2-6. Modulation bandwidth is determined as cutoff frequency, fc, which is the position with half response written as

( )

²¹

for ωr2Ω² << (ωc2-ωr2)², the cutoff frequency, ωc, is approximated to 3 ωr.

Transfer function, H(ω), is the identical term in Eq. (2-11) and (2-12) respectively obtained with Cramer’s rule [3]. It is similar to modulation response, M(ω), describing the response of the laser intensity to small variations in the drive current through the active region. That is,

π γ similar to Eq. (2-18), and C is a constant. Accounting for additional extrinsic limitations due to carrier transport and parasitic elements related to the laser structure results in an extra pole in the small signal modulation transfer function

⎟⎟

where fp is the cutoff frequency of the low pass filter characterizing the extrinsic limitations. It is crucial for microwave applications that the modulation bandwidth of the VCSEL is sufficiently large so that efficient modulation is achieved as the modulation frequency.

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 (2-D PhC) 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 2-D

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 refractive index control achieved by forming a 2-D photonic crystal structure, a parameter representing this control must have a strong dependence on the etching depth. We have investigated the etching depth dependence, both theoretically and experimentally, of the effective index change in a VCSEL structure.

The 2-D photonic crystal structure with finite etching depth incorporating a single point or a seven-point defect is formed in the top DBR. It is known 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 Veff = 2.405, and thus a wave guide with Veff < 2.405 is considered to be single mode. In a photonic crystal

在文檔中 850nm高速和光子晶體面射型雷射之特性量測與分析 (頁 13-0)