Coherent light generators – Optical fiber laser
Reexamination Certificate
1998-09-04
2001-07-17
Font, Frank G. (Department: 2877)
Coherent light generators
Optical fiber laser
C372S006000, C372S020000, C372S043010, C372S024000, C372S042000
Reexamination Certificate
active
06263002
ABSTRACT:
Vertical-cavity surface-emitting lasers (VCSELs) have recently received considerable attention because their unique structure offers several significant advantages over conventional edge-emitting lasers (see: K. Iga et al. (1988) “Surface emitting semiconductor lasers,” IEEE J. Quantum Electron. 24:1845; H. Soda et al. (1979) “GaInAsP/InP surface emitting injection lasers,” Jpn. J. Appl. Phys., 18:2329-2330; J. L. Jewell et al. (1989) “Low threshold electrically pumped vertical-cavity surface-emitting microlasers,” Electron. Lett., 25:1123-1124; J. L. Jewell (1991) “Vertical-cavity surface-emitting lasers: Design, growth, fabrication, characterization,” IEEE J. Quantum Electron. 27:1332-1346; A. Rosiewicz et al. (1997) “Optical pumping improves VCSEL performance” Laser Focus World (June) pp.133-136.) These advantages include (1) the possibility for monolithic integration with photonic circuits, (2) the ability to fabricate dense 2-dimensional laser arrays, (3) the convenience of full-scale on-wafer probe test before separation into chips, (4) robust single longitudinal mode operation due to the extremely short cavity between the bottom and top mirrors (reflective surfaces). The extreme short cavity of VCSELs, <2 &mgr;m in length including mirror thickness, gives a relatively large longitudinal mode spacing of >20,000 GHz. If the cavity length can be varied, these large mode spacings would allow continuous wavelength tuning without mode hopping. Single-frequency and tunable VCSEL are attractive for applications including among others: optical fiber communications, sensing, spectroscopy, data storage and retrieval, and display.
The output wavelength of VCSELs can presently be selected between about 400 to 1600 nm by appropriate choice of materials, thickness and doping of the quantum wells. Several wavelength regions are of particular interest for telecommunication applications (namely at &lgr;=0.8 &mgr;m, 1.3 &mgr;m, and 1.55 &mgr;m.) VCSELs employing optical pumping as well as electrical pumping are known in the art.
Most VCSEL development has been in the shorter wavelength region of 08-0.98 &mgr;m due to the ease of fabrication.
A common problem associated with VCSELs is multiple transverse mode lasing due to a lack of lateral field confinement. Furthermore, there have been several areas of technical difficulty in fabricating long-wavelength (1.3 &mgr;m and 1.55 &mgr;m) VCSELs: the fabrication of mirrors exhibiting both high reflectivity and high thermal conductivity; the reduction of the intrinsically higher nonradiative losses in the active materials at these wavelengths; and the efficiency of current pumping techniques.
The cavity of VCSELs are of the order of a few wavelengths long and typically involve quarter-wave mirror and gain regions with no lateral guiding. Very high plane-wave reflection coefficients can be obtained with practical semiconductor quarter-wave mirrors, but for beams of finite width, the reflection coefficient of a mirror with no lateral guiding, and hence the finesse of cavities that use such structures, will be limited by diffraction loss. Standard semiconductor Bragg mirrors made of quaternary materials require thick multi-layers to reach high-reflectivity (R>98%) for &lgr;=about 1.55 &mgr;m, which in turn introduce high diffraction losses. Low-loss dielectric mirrors using Si/SiO
2
and TiO
2
/SiO
2
have been tested as a solution to this problem, but they provide poor thermal conductivity.
Long wavelength lasing at 1.59 &mgr;m via optical pumping at 0.84 &mgr;m wavelength has been demonstrated by C. H. Lin et al. (1994) “Low threshold 1.59 &mgr;m vertical-cavity surface-emitting lasers with strain compensated multiple quantum wells,” Technical Digest, LEOS session SL4.2.
A semiconductor wafer-fusion technique used to fuse an AlAs/GaAs mirror to an InGaAsP active region has demonstrated 1.52 &mgr;m lasing under pulsed electrical pumping (D. I. Babic et al. (1994) “Double-fused 1.52 &mgr;m vertical-cavity lasers,” Technical Digest, LEOS, session PD1.4.) Development of electrical pumping via intracavity contacts has demonstrated 1.01 &mgr;m lasing with several mW of output power (J. W. Scott (1994) “High efficiency submilliamp vertical cavity lasers with intracavity contacts,” IEEE Photonics Tech. Lett. 6:678.) Intracavity pumping bypasses the inherently high electrical resistance through Bragg mirrors, to provide an approach for long-wavelength VCSEL development.
CW (continuous-wave) operation of VCSEL lasers at ~1.5 &mgr;m has been reported at room temperature by D. I. Babic et al. (1995) “Room temperature continuous-wave operation of 1.54 &mgr;m vertical-cavity lasers.” IEEE Photonics Technol. Lett., 7:1225 and at 64° C. by N. M. Margalite et al. (1997) “64° C. Continuous wave operation of 1.5 &mgr;m vertical-cavity laser,” IEEE J. Select. Topics Quantum Electron. 1(2): 359. Recent VCSEL research incorporating dielectric and semiconductor wafer-bonded Bragg mirrors show promise for the development of room temperature CW operation of a 1.3 &mgr;m VCSEL (Y. Qian et al. (1997) “Low-threshold proton-implanted 1.3 &mgr;m vertical-cavity top-surface-emitting lasers with dielectric and wafer-bonded GaAs—AlAs Bragg Mirrors,” IEEE Photonics Technol. Lett, 2:866-868.)
Tuning of VCSELs has been accomplished by tuning the cavity length in several ways. N. Yokouchi et al. (1992) “40 Å Continuous tuning of a GaInAsP/InP vertical cavity surface emitting laser using an external mirror,” IEEE Photonics Technol. Lett. 4:701-703 reports a VSCEL with an external Si/SiO
2
dielectric mirror directly butt-coupled to one side of the SEL controllable to 10 Å. A tuning range of 4 nm (40 Å) was achieved under CW operation across the 1.44 &mgr;m wavelength region at liquid nitrogen temperatures by physically moving the external mirror. The tuning range was reportedly limited because of misalignment between the mirror and the chip.
T. Wipiejewski et al. (1993) “Tunable extremely low-threshold vertical-cavity laser diodes,” IEEE Photonics Technol. Lett., 5:889-892 reports 8-&mgr;m active diameter VCSEL device with threshold current of 650 &mgr;A. A tuning range of 2.2 nm is reported for a similar low threshold 12-&mgr;m active diameter VCSEL device. Wavelength tuning in this VCSEL were achieved using thermal effects to vary the optical cavity length. The tuning range in such devices is limited by the maximum possible change of the refractive index in the laser. A maximum 10-nm tuning range has been obtained using such thermal effects (L. Fan et al. (1994) “10.1 nm range continuous wavelength-tunable vertical cavity surface-emitting laser,” Electron. Lett. 30:1409-1410 and L Fan et al. (1994) “8×6 Wavelength tunable vertical-cavity surface-emitting arrays,” Proc. LEOS, Boston Mass. paper SL5.3.)
M. Y. Li et al. (1998) “Top-emitting micromechanical VCSEL with a 31.6 nm tuning range,” IEEE Photon. Technol. Lett. 10:18-20 reports continuous tuning over 31.6 nm in a VCSEL structured with a micromachined gap formed with a cantilevered arm between the top and bottom mirrors of the VCSEL. Tuning was achieved by varying the gap by application of a 0-5.7 V to the arm to create electrostatic attraction toward the substrate. E. C. Vail et al. (1995) OSA Topical Digest for CLEO'95/QELS (Baltimore, Md.), QPD12-2 (see: J. Hecht (1995) “Wider tunability and lower thresholds improve VCSELs” Laser Focus World) had earlier reported a 15-nm tuning range in a similarly structured 960 nm VSCEL.
M. C. Larson et al.(1996) “Wide and continuous wavelength tuning in a VCSEL using a micromachined deformable-membrane mirror,” Appl. Phys. Lett. 68:891-893 and M. C. Larson et al. (1995) IEEE Photonics Technol. Lett. 7:382-384 (see also: K. Lewotsky (1996) “Deformable mirror tunes VCSEL over 15 nm,” Laser Focus World April p.32) relate to VCSEL structures within microinterferometers constructed with a suspended deformable-membrane top mirror. Cavities were tuned by application of a 0-14 V bias to give a 32-nm tuning range at the 920 nm center wavelength,
Hsu Kevin
Miller Calvin M.
Flores Ruiz Delma R.
Font Frank G.
Greenlee Winner & Sullivan, P.C.
Micron Optics Inc.
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