1.3 Applications of VCSELs
1.3.3 Sensor
Reflective optical sensors are used to sense the presence or absence of a distant object. Examples of reflective sensors used in a variety of industrial and consumer products include barcode scanners and proximity sensors. The packaging of optical reflective sensors can be quite compact, and in the case of some LED sources, can even be packaged in a single TO can. However, a significant disadvantage to these devices is the quantity of optical crosstalk that may degrade the signal-to-noise ratio (S/N) in the detector. Crosstalk results from the fact that LEDs emit from all surfaces and the emission subtends nearly 90°. Suppliers go to great lengths to isolate the LED and the detector by using a mechanical structure to separate the optoelectronic components. In addition, the LED optical output is not easily collimated or focused to a spot to increase the amount of reflected light from a distant object. By using the technical features of the VCSEL, integrating a phototransistor in the package, and
designing the optical element into the TO can lid, an effective reflective sensor can be developed. The advantages of the sensor include the ability to package the entire assembly in a single compact TO can, along with the focusing optics and a phototransistor. Depending on the application, a single-mode or multimode VCSEL can be used. In some cases, when coherence of the optical beam is desired, the single-mode VCSEL might be the best choice, but in other cases when total output power is more important, a multimode VCSEL might be more beneficial. For example, a multimode VCSEL can be mounted on the centerline of the lens and package, and the phototransistor mounted to the side of the VCSEL. In this configuration, the optimal signal is obtained by tilting the package with respect to the centerline of the TO. The optical system is made by including a melt-formed glass lens in the TO lid.
The lid can be designed to accept other lenses, and the height can be varied, which allows for the design of a wide variety of optical sensors. In addition to the reduced power consumption and single-package interface, the recurring theme in the application is the ability of the VCSEL sensor to provide higher S/N in environments where the LED sensor is not able to adequately perform. Other application areas include the sensing of diffuse reflective surfaces such as paper in a printing system, or low-reflectivity surfaces such as glasses or plastics. The small focal spot also has significant advantages in optical encoding applications such as barcode reading or positioning equipment. Figure 1.2 illustrates the VCSEL sensor module used in laser mouse application. The sensitivity and resolution of the laser mouse is 20 times high than the conventional LED mouse. In 2004, the VCSEL based biosensor also demonstrated by C. J. Chang-Hasnain et al. [18] [19]
Figure 1.1 Commercial 2.5 Gb/s VCSEL array for data communication application
Figure 1.2 A laser mouse module using VCSEL
References
[1] H. Soda, K. Iga and Y. Suematsu, “GaInAs/InP surface emitting injection lasers”, Jpn. J. Appl. Phys. v.18, 2329, (1979)
[2] K. Iga, Sishikawa, S. Ohkouchi and T. Nishimura, “Room temperature pulsed oscillation of GaAlAs/GaAs surface-emitting injection laser”, App. Phys. Lett.
v45, 348, (1984)
[3] F. Koyama, S. Kinoshita and K. Iga, “Room-temperature continuous wave lasing characteristics of GaAs vertical cavity surface-emitting lasers”, App.
Phys. Lett. V44, 221, (1989)
[4] G. M. Yang, M. H. MacDougal and P. D. Dapkus, “Ultralow threshold current vertical-cavity surface-emitting lasers obtained with slelective oxidation”, Electron. Lett., Vol. 13. pp. 886-888, 1995
[5] J. K. Guenter, J. A. Tantum, A. Clark. R. S. Penner, R. H. Johnson, R. A.
Hawthorne, J. R. Biard, Y. Liu, “Commericalisation of Honeywell’s VCSEL technology:further developments”, Proceedings of SPIE’s Optoelectronics 2001, Vol. 4286, pp. 1-14, 2001.
[6] J. S. Span, Y. S. Lin, C. F. Li, C. H. Chang, J. C. Wu, B. L. Lee, Y. H. Chuang, S.
L. Tu, C. C. Wu, “Commericailised VCSEL components fabricated at Truelight Corporation”, Proceedings of SPIE’s Optoelectronics 2001, Vol. 4286, pp.
15-21, 2001.
[7] A. V. Krishnamoorthy, K. W. goossen, L. M. F. Chirovsky, R. G. Rozier, P.
Chandramani, S. P. Hui, J. Lopata, J. A. Walker, L. A. D’Asaro, “16x16 VCSEL array flip-chip bonded to CMOS VLSI circuit”, IEEE Photon. Technol. Lett., Vol. 12, pp. 1073-1075, 2000
[8] U. Eriksson, P. Evaldsson and K. Streubel. “A novel technology for monolithic integration of VCSELs and heterojunction bipolar transistors at 1.55um”, CLEO Pacific Rim ’97, Chiba, Japan, paper PD2.8, 1997
[9] K. Tai, G. Hasnain, J. D. wyn, R. J. Fisher, Y. H. Hang, B. Weir, J. Gamelin and A. Y. Cho, “90% coupling of top surface-emitting GaAs/AlGaAs quantum well laser output into 8 um diameter core silica fiber”, Electron. Lett., Vol. 26, pp.
1628-1629, 1990
[10] M. Y. Li, W. Yuen, G. S. Li and C. J. Chuang-Hasnain, “Top-emitting micromechanical VCSEL with a 31.6nm tuning range”, IEEE Photon. Technol.
Lett., Vol. 10, pp. 18-20, 1998
[11] U. Fiedler, G. Reiner, P. Schnitzer and K. J. Ebeling, “Top surface-emitting vertical-cavity laser diode for 10GB/s data transmission”, IEEE Photon.
Technol. Lett., Vol. 8, pp. 746-748, 1996
[12] M. W. Haney, M. P. Christensen, P. Milojkovie, J. Ekman, P. Chandramani, R.
Rozier, F. Kiamilev, Y. Liu and M. Hibbs-Brenner, “Multichip free-space global optical interconnection demonstration with integrated arrays of vertical-cavity surface-emitting lasers and photodetectors”, Appl. Opt., Vol. 38, pp. 6190-6200, 1999
[13] K. Goto, “Proposal for ultrahigh density optical disk system using a vertical cavity surface emitting laser array”, Jpn. J. Appl. Phys., Vol. 37, pp. 2274-2278, 1998
[14] R. L. Thornton, “Vertical-cavity lasers and their application to laser printing”, Proc. SPIE, Vol. 3003, pp. 112-119, 1997
[15] H. Martinsson, J. A. Vukusic, M. Grabherr, R. Michalzik, R. Jager, K. J. Ebeling and A. Larsson, “Transverse mode selection of large-area oxide-confined
vertical-cavity surface-emitting laser using a shallow surface relief”, IEEE Photon. Technol. Lett., Vol. 11, pp. 1536-1538, 1999
[16] http://grouper.iee.org/groups/802/3/ae/index.html
[17] D.V. Plant, M. B. Venditti, E. Laprise, J. Faucher, K. Razavi, M. Chateauneuf, A.
G. KirK, J. S. Ahearn, “256-channel bi-directional optical interconnect using VCSELs and photodiodes on CMOS”, J. Lightwave Techno. 19(8) 1093, 2001 [18] F. Mateus, M. C. Huang, Univ. of California/Berkeley; P. Li, B. Cunningham,
SRU Biosystems; C. J. Chang-Hasnain, “High sensitivity label-free biosensor using VCSEL” , Proceedings of SPIE’s BIOS 2004 vol. 5328-22, 2004.
[19] D. Kumar, H. Shao, Kevin Lear, "Dependence of vertical cavity surface emitting laser diodes with integrated micro-fluidic channels on fluid refractive index" Optical Information systems II in series Proceedings of SPIE, vol. 5557, Aug 2004
Chapter 2 VCSEL structure
In this chapter, we presented the fundamental structure of VCSEL, which includes Distributed Bragg Reflectors (DBR), Active layer, and electrical confinement. A VCSEL typically consists of an active layer, usually multi-quantum wells, sandwiched between two DBR mirrors, shown as Figure 2.1. Current confinement is a non-conductive region in the top DBR mirrors and to confine the injection carriers into the active layer.
2.1 Distributed Bragg Reflectors (DBR)
The DBR is a multi-pair layered structure, consists of repeating pairs of high and low refractive index layers. The thickness of both the high and the low refractive index materials is a quarter of the designed wavelength (λ/4). These DBR mirrors are used to generate the constructive reflection of light in cavity for increasing optical gain. The reflectivity of DBR mirrors must be higher than 99.9% to reach threshold gain for lasing. The Figure 2.2 shows DBR mirrors how to achieve the reflection. The
∆n is refractive index for each pair of high and low refractive index layers in Figure 2.2.
∆n=nhigh - nlow (2.1)
where nhigh and nlow denote the refractive index of the high and low refractive index layers, respectively. The DBR mirrors form a so-called stop-band in the vicinity of the Bragg wavelength by adding many pairs of high and low refractive index layers as
shown in Figure 2.3.
For an ideal DBR mirrors without any absorption, the reflectivity can reach any desired value [1] simply by adding pair of high and low refractive index layers. In a real DBR mirrors, the peak reflectivity is limited by the absorption in the mirror stack.
The amount of the optical loss in the DBR mirrors depends on the absorption of the material and the penetration depth of the light into the mirror stack. The light diffracted when the light penetrated the DBR mirror. The diffraction of light reduces the reflectivity of DBR mirror due to the diffraction occupies a finite area of the DBR mirror in VCSEL. The diffraction effect is not important for the Edge-Emitting Laser, but it leads to the increase of the DBR mirror stack, which increases the penetration depth of the light, to compensate the decrease of the reflectivity in each DBR mirror.
This effect therefore increases the importance of the lateral optical confinement in the VCSEL, which is described in the optical and electrical confinement section of this chapter. A DBR with high refractive index is advantageous to obtain wide stop bands, to reduce the penetration depth, DBR mirror stack and diffraction losses.
Another disadvantage for increasing DBR mirror stack is that the discontinuities in energy bands lead to a high series resistance in the VCSEL.
2.2 Active Layer
The active layer is a gain medium of light. In the VCSEL, the gain of the active layer is not enough to induce lasing. Therefore, the gain of the active layer needs to be compensated for inducing lasing by the DBR mirrors, which has a very high reflectivity. However, the material of the active layer still needs to produce a considerable gain. In general, the quantum wells (QW) are used as a material of active layer for the commercialized VCSELs. Figure 2.4 shows a typical structure of a
GaAs/AlGaAs single quantum well in the valence and conduction band. Figure 2.5 shows a typical multi-quantum wells structure. [2]
Electrons and holes are confined to the central region of QW in the conduction band and valence band, respectively. If the length of this region is made small enough, the confinement of the carriers in one direction will result in energy sub-bands. The following expression is the relation between the energy of the first sub-band and the QW width.
(2.2) where Eg is the bandgap of the material and mr is the reduced electron-hole pair mass and l is the width of the quantum well. The advantages of quantum well compared to a bulk material are that increased material gain for equivalent current density, and increased differential gain. The use of strained QW can further increase the differential gain and reduce the transparency carrier density. [3]
2.3 Optical and Electrical confinement
In the DBR section, we mentioned the diffraction of light in DBR mirrors reduces the reflectivity of DBR mirror. It is necessary to confine the electrical current and optical region in order to obtain high performance VCSEL. The Figure 2.6 shows five main types of electrical confinement and described below.
(a) Ring electrode type:
This structure restricts the electrical current in the vicinity of the ring electrode.
The light output is taken out from the central aperture of the ring electrode.
This structure is easy to fabricate, but the electrical current can not be confined completely in a small area due to the electrical current diffusion.
(b) Proton-implant type:
This structure usually uses proton (H+) to implant top DBR mirrors surrounding the desired optical region above active layer. The proton-implant builds a non-conductive region surrounding the desired optical region to confine the injection carriers into the opening of the electrical current aperture formed by the implanted area. Since proton-implant is a rather simple and low-cost technique and is simplified for further processing and packaging by the planarity, most commercialized VCSELs are fabricated by this scheme.
(c) Regrowth Buried-Heterostructure (BH) type:
This structure uses mesa etching to etch from top to bottom DBR mirrors including the active layer. After mesa etching, a wide-gap semiconductor is formed to restrict the electrical current in the VCSEL. The refractive index will be small in the surrounding optical region, leads to an index-guiding structure is formed. This is one of ideal structures in terms of the electrical current and the optical confinement. But the essential processes are rather complex for fabricating this structure.
(d) Air-post type:
A circular or rectangular air post is used to achieve the electrical current confinement. This structure is the simplest to fabricate, but non-radiative recombination at the outer wall may degrade the performance of VCSEL.
(e) Selective AlGaAs oxidation type:
A high Al content AlGaAs layer is made by oxidation process in the DBR mirrors. The Al-oxide formed a non-conductive layer during the oxidation process. This non-conductive layer has a lower refractive index than AlGaAs, acts as both an electrical current aperture and an optical waveguide. This effect leads to high conversion efficiency for converting electrical current to
optical output. By carefully monitoring the speed of the oxidation, the current aperture as small as a few µm can be reached, creating a waveguide that supports only the fundamental optical mode. The disadvantage of this structure is that the reproducibility is more difficult than the proton-implant type. Since the oxidation of the high Al content AlGaAs layer is very sensitive to material and process parameters, the speed of oxidation is highly dependent on the Al concentration and the temperature of the furnace in which the oxidation takes place. [4]
Figure 2.1 A typical structure of the VCSEL
Figure 2.2 How to achieve the reflection of light by DBR mirrors
Figure 2.3 Stop band effect in DBR mirrors
Figure 2.4 A GaAs/AlGaAS quantum well
Figure 2.5 A typical multi-quantum wells structure
(a) Ring electrode type (b) Proton implant type
(c) Regrowth Buried-Heterostructure type (d) Air-post type
(e) Selective AlAs oxidation
Figure 2.6 The electrical confinement schemes
Reference
[1] G.M. Yang, M.H. MacDougal and P.D. Dapkus, “Ultralow threshold current vertical-cavity surface-emitting lasers obtained with selective oxidation”, Electron. Lett., Vol. 31, pp. 886-888, 1995.
[2] Rainer Michalzik and Karl Joachim Ebeling, “Operating Principles of VCSELs”, University of Ulm, Optoelectronics Department, D-89069 Ulm, Germany.
[3] L.A. Coldren and S.W. Corzine, “Diode Lasers and Photonic integrated Circuits”, John Wiley & Sons, Chapter 4, 1995.
[4] K.D. Choquette, K.M. Geib, C.I.H. Ashby, R.D. Twesten, O.Blum, H.Q. Hou, D.M. Follstaedt, B.E. Hammons, D. Mathes and R.Hull, “Advances in selective wet oxidation of AlGaAs alloys”, IEEE J. Select. Topics Quantum Electron., Vol.3, pp. 916-926, 1997.
Chapter 3
VCSEL dynamics
For digital communication, the maximum bit-rate, which the information can be sent, depends on how fast the laser can be modulated. The constantly increasing demand for bandwidth requires faster lasers. Therefore, It is important to understand the mechanisms occurring in a VCSEL under modulation, to identify the limiting factors and ultimately to minimize them. This chapter presented the small signal analysis based on the rate equations. In this chapter we presented the small signal analysis first. Second, we presented three main factors of bandwidth-limitation, damping, thermal and parasitic effects. Third we presented a part of S-parameters, which are used in our experiment.
3.1 Small signal analysis
Figure 3.1 shows a small sinusoidal perturbation above threshold current (Ith). In the small signal analysis with time dependence e jωt is assumed to initiate only small changes in the steady state values of carrier and photon densities. The Equations 3.1 and 3.2 expresses the rates of change of carriers and photons numbers, which are rate equations.
(3.1)
where (dN/dt) states that the temporal increase of the number of carriers in the cavity. N is the number of carriers in the cavity, Iin is the injection rate of carriers, Isp is the rate of carriers dissipated through spontaneous recombination, and the RstS is the stimulated emission.
(3.2)
The Equation 3.2 is derived from the wave equation. [1] (dS/dt) states that the temporal increase of the number of photons. S is the net increase of the number of
photons, Rsp is the rate of the generated photons through spontaneous emission, RstS is stimulated emission, γi is the internal loss rate of the photons due to the absorption of photons through scattering and free carrier absorption. The γm is the loss rate of the photons in DBR mirrors. The output coupling is represented by the term γmS. The term Rsp is the amount of spontaneous emission that is resonant with the cavity and within the same longitudinal mode as the coherent light. The spontaneous recombination current Isp
describes the recombination of the electrons in the conduction band with holes in the valence band.
In the small signal analysis, the Equations 3.1 and 3.2 can be linearized by neglecting the quadratic and higher power of ∆S and ∆N, and can be written in the following matrix form [2]
Rst is the stimulated emission rate while the spontaneous emission is neglected above threshold. The subscripts S and N denote the partial derivatives with respect to the photon and carrier numbers and the subscript 0 denotes the steady state solutions of the rate equations. By solving ∆N and ∆S from the above equation system, the following solutions are obtained:
(3.4)
(3.5) where Q0(ω) = - ω2 + jω(A + D0) + (B0C + AD0) (3.6)
The term (B0C+AD0) represents the resonance frequency ωR2, and (A+D0) governs the damping γ. Therefore the modulation intrinsic response of a laser is
(3.7)
In addition to the two complex conjugated poles of the intrinsic response, the measured response often contains a third real pole. This parasitic-like roll-off is due to the electrical capacitance of the metallic contact and the high conductive layers connected to the metallic contact. However, slow diffusion limited carriers transport through undoped layers can also cause parasitic-like effects due to the charge pile.
From above discussion, the theoretical transfer function describing the modulation response of a laser can be written a function of the frequency f:
(3.8) γm
where C1 = 4π 2C fP ——— (3.9)
γm + γi
C1 is a coefficient that depends on the coupling efficiency and the quantum efficiency. fr
and γ are the resonance frequency and damping rate of the relaxation peak, fP is the parasitic-like roll-off frequency.
The angular resonance frequency can be written as follows:
(3.10)
where vg is the group velocity, a is the differential gain, Np is the photon density (ΓS/V) and τP is the photon lifetime in the cavity. This equation shows the importance of the differential gain in the modulation response of a laser.
The damping rate of the resonance peak, γ, can be written
(3.11)
where ε is the gain compression factor and τ∆N is the spontaneous differential lifetime.
The first two terms can be interpreted as the inverse differential stimulated carrier lifetime and the third term is the excess damping associated with gain compression. The damping rate of the resonance peak is also affected by spontaneous emission rate, carrier transport effects and spatial hole burning effects but they are neglected here for simplicity.
Using Equation 3.10 the damping rate γ can then be expressed as function of the resonance frequency fr
γ = Kfγ2 + γ0 (3.12)
1 ε where K = 4π [τ + —— ] and γ = —— P 0
vga τ∆N
K is called the K-factor and it describes the damping of the response for high bias currents. γ0 is the damping factor offset and determines the laser bandwidth at threshold.
These two parameters can be extracted from the result of the RF measurement which is described in the chapter 5. Because the damping increases in proportion to fr2, the response becomes flatness while the laser is driven at high current. At some points, the damping is large enough that drops the response below the 3dB cut-off at frequencies smaller than fr. As a result, there is a maximum bandwidth that can be achieved.
By taking into account the steady state solution of the rate equations for the photon density, and assuming that all injected carriers are converted to photons above threshold current, Ith. The photon density, Np, can be written:
(3.13)
where q is the electron charge, Γ is the confinement factor and V is the active region volume. I and Ith are measured in Amperes.
By plugging Equation 3.13 into Equation 3.10, we can then obtained
(3.14)
where D is called the D-factor and has been defined as
(3.15)
3.2 Bandwidth limitations
At high relaxation frequencies the linear relation between γ and fr2 implies a maximum possible bandwidth, the damping limit given by [3]
(3.16)
The modulation bandwidth of the laser may be limited by thermal effects that gives a
The modulation bandwidth of the laser may be limited by thermal effects that gives a