Coherent light generators – Particular resonant cavity – Specified cavity component
Reexamination Certificate
2002-09-26
2004-06-01
Ip, Paul (Department: 2828)
Coherent light generators
Particular resonant cavity
Specified cavity component
C372S020000, C372S045013, C372S107000
Reexamination Certificate
active
06744805
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to semiconductor optoelectronic devices in general and, more particularly, to wavelength tunable surface emitting semiconductor lasers.
BACKGROUND OF THE INVENTION
Tunable vertical cavity surface emitting lasers (VCSEL's) have recently generated considerable interest in the art. This is because these devices show great promise not only for increasing bandwidth during wavelength division multiplexing (WDM) in fiber-optic communications, but also for use in switches, routers, highly compact spectroscopic interferometers, optical trans-receivers and numerous other applications.
For example, in a WDM system, a single tunable laser source can be used as a rapid back-up for disaster recovery. This is because the single tunable laser source can be quickly tuned to the desired wavelength if and when an existing laser source fails.
Tunable lasers also have wide applications in optical sensors. For example, in gas sensing applications, a tunable laser may be conveniently used to detect specific gases for environmental monitoring.
VCSEL's are extremely attractive for integrated optoelectronic circuits. For one thing, they operate at a single longitudinal mode with a circular aperture, thereby providing efficient coupling to fibers. In addition, they are compact, and can be monolithically fabricated in large, dense arrays on a wafer-scale.
As a fixed wavelength light source, VCSEL's have demonstrated limited application and functionality.
Some past effort has been directed towards achieving wavelength tuning in VCSEL's by introducing refractive index changes with (1) temperature (see, for example, Berger, P. R., Dutta, N. K., Choquette, K. D., Hasnain, G., and Chand, N., “Monolithically Peltier-cooled vertical-cavity surface-emitting lasers”, Applied Physics Letters, Vol. 59, No. 1, pp. 117-119, 1991; and Chang-Hasnain, C. J., Harbison, J. P., Zah, C. E., Florez, L. T., and Andreadakis, N. C., “Continuous wavelength tuning of two-electrode vertical cavity surface emitting lasers”, Electron. Lett., Vol. 27, No. 11, pp. 1002-1003, 1991); or (2) carrier injection (see, for example, Gmachi, C., Kock, A., Rosenberger, M., Gornik, E., Micovic, M., and Walker, J. F., “Frequency tuning of a double-heterojunction AlGaAs/GaAs-vertical-cavity surface-emitting laser by a serial integrated in-cavity modulator diode”, Applied Physics Letters, Vol. 62, No. 3, pp. 219-221, 1993).
Both of these techniques provide a tuning range of roughly 10 nm; however, this is still considerably short of the several tens of nanometer tuning range which is necessary for bandwidth-hungry WDM and dense WDM applications.
Wavelength tuning has also been achieved in edge emitting lasers by changing the cavity length, such as in external cavity laser systems, or by changing the refractive index along the cavity length, such as in DFB and DBR lasers. In external cavity lasers, tuning is achieved through mechanical rotation of external gratings and reflecting mirrors. Unfortunately, the tuning speed is slow and limited to the millisecond range. In DFB or DBR lasers, adjusting the refractive index to cover the whole EDFA range has permitted large-scale tuning on the order of 100 nm.
Adjustment of the refractive index may be achieved through heating, carrier injection and electro-optic effects. However, tuning often is quasi-continuous. To achieve the desired transmission wavelength, complicated electronics and computing algorithms must be integrated into the laser system. Additionally, device fabrication is complicated and involves numerous processing steps and selective epi-layer re-growth, thereby reducing yield and increasing costs considerably.
VCSEL's overcome the foregoing fabrication, performance and cost issues. As a result, VCSEL's are viable candidates for many real-world communications applications. VCSEL's are compatible with low-cost wafer level fabrication and characterization technologies. VCSEL's produce circularly-shaped, low-numerical-aperture output beams which may be easily coupled to fibers and other free space optics. The short cavity length of VCSEL's also ensures a single longitudinal lasing mode which is desirable for potential WDM or other wavelength addressing schemes.
Variation of the length of a Fabry-Perot cavity has been shown to be a viable technique for accomplishing wavelength tuning in VCSEL's without affecting the laser gain medium. This can be achieved in surface emitting devices by the provision of a top mirror that can be translated relative to the bottom mirror by the application of an electrostatic field. By the selective application of an electrostatic voltage to the movable mirror, the cavity length, and hence the lasing wavelength, may be tuned continuously. The ability to tune over the entire gain region, without mode hopping, is a significant benefit.
This technique has been implemented in tunable Fabry-Perot devices such as (1) filters (see, for example, Larson, M. C., Pezeshki, B., and Harris, J. S., “Vertical coupled-cavity microinterferometer on GaAs with deformable-membrane top mirror”, IEEE Photonics Technology Letters, Vol. 7, pp. 382-384, 1995; and Tran, A. T. T. T., Lo, Y. H., Zhu, Z. H., Haronian, D., and Mozdy, E., “Surface Micromachined Fabry-Perot Tunable Filter”, IEEE Photonics Technology Letters, Vol. 8, No. 3, pp. 393-395, 1996); (2) light emitting diodes (see, for example, Larson, M. C., and Harris, J. S., “Broadly-tunable resonant-cavity light emission”, Applied Physics Letters, Vol. 67, No. 5, pp. 590-592, 1995); and (3) VCSEL's (see, for example, Wu, M. S., Vail, E. E., Li, G. S., Yuen, W., and Chang-Hasnain, C. J., “Tunable micromachined vertical-cavity surface emitting laser”, Electronic Letters, Vol. 31, No. 4, pp. 1671-1672, 1995; and Larson, M. C., Massengale, A. R., and Harris, J. S., “Continuously tunable micromachined vertical-cavity surface emitting laser with 18 nm wavelength range”, Electronic Letters, Vol. 32, No. 19, pp. 330-332, 1996).
For VCSEL's to qualify for use in telecommunications applications, single-mode operation is essential. Achieving single-mode operation is difficult in conventional VCSEL's with flat DBR structures where the large lateral dimension of the device allows excitation of higher order spatial modes. Typically, obtaining single, fundamental spatial mode operation in a conventional VCSEL is achieved by decreasing the dimensions of the current injection area of the device, index guiding by lateral oxidation, etched mesa formation, or re-growth. These techniques are difficult to implement in more complicated structures such as microelectromechanically tunable VCSEL's.
Providing uniform current injection significantly improves the ability to achieve single-mode laser operation.
One technique for achieving uniform current injection is to provide doped cladding layers that urge the charge toward the aperture. Another technique is to provide a barrier layer on the cladding layer.
SOME ASPECTS OF THE PRESENT INVENTION
This patent application claims benefit of pending prior U.S. patent application Ser. No. 09/105,399, filed Jun. 26, 1998 by Parviz Tayebati et al. for MICROELECTROMECHANICALLY TUNABLE, CONFOCAL, VERTICAL CAVITY SURFACE EMITTING LASER AND FABRY PEROT FILTER, which document is hereby incorporated herein by reference.
This patent application also claims benefit of pending prior U.S. Provisional Patent Application Serial No. 60/146,396, filed Jul. 30, 1999 by Peidong Wang et al. for TUNABLE MICROELECTROMECHANICAL VCSEL WITH HALF-SYMMETRIC CAVITY, which document is also incorporated herein by reference.
The present invention addresses the single mode operation issues in this novel, microelectromechanically (MEM) tunable, half-symmetric, vertical cavity surface emitting laser (VCSEL).
The present invention also includes another innovation for producing, via micromachining, a half-symmetric cavity VCSEL that comprises a tunable cavity formed between a set of planar DBR's and a set of curved DBR's.
Tayebati Parviz
Vakhshoori Daryoosh
Wang Peidong
Ip Paul
Menefee James
Steubing McGuinness & Manaras LLP
LandOfFree
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