Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
2002-03-08
2004-01-27
Ip, Paul (Department: 2828)
Coherent light generators
Particular active media
Semiconductor
C372S045013
Reexamination Certificate
active
06683898
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to vertical cavity surface emitting lasers and more particularly to the control of transverse laser modes using a photonic band gap structure having a light aperture.
BACKGROUND OF THE INVENTION
The optical radiation emitted from lasers may have different spatial distributions. If a laser emits optical radiation that contains only the fundamental transverse mode, the radiation is a narrow laser beam with a Gaussian-distributed cross-section, most intense in the centre and less intense at the edges. In contrast, laser radiation having a higher transversal mode displays bright and dark spots across a cross-section of the radiation. The transverse electromagnetic modes are conventionally designated as TEM
00
, TEM
01
, TEM
10
, TEM
11
, etc. where TEM
00
is the fundamental transverse mode and the others are higher transverse modes.
Radiation with higher transverse modes is normally undesirable because it is difficult to couple such radiation into optical fibres and to focus it for free-space beam forming. In addition, higher transverse mode radiation travels at somewhat slower speed in an optical fibre than the fundamental transverse mode radiation, thereby creating mode dispersion, i.e., broadening of an optical pulse as it travels in an optical fibre.
Vertical Cavity Surface Emitting Lasers (VCSELs) are semiconductor lasers having an optical cavity formed by mirrors which are parallel to a substrate on which the laser is formed. Thus, the optical cavity of a VCSEL is perpendicular to the substrate; and optical radiation is emitted from the VCSEL in a direction normal to the substrate. VCSELs are typically layered structures where the cavity mirrors are formed as stacks of distributed Bragg reflectors (DBR) around an active semiconductor layer.
VCSELs have many advantages over conventional edge emitting semiconductor lasers. For example, VCSELs can be made extremely small; VCSELs can easily be made into arrays that contain a large number of VCSELs; VCSELs can be tested at an early stage during manufacturing (on-wafer testing) which is an important parameter in the cheap production of VCSELs; VCSELs can be subject to very fast modulations compared to lasers in the same price-range.
The DBRs are either grown epitaxially with the rest of the structure, or deposited at a later stage. In the first case the mirrors are made from semiconducting material, whereas in the latter case the mirrors are made from dielectrics. The gain medium of the VCSEL is formed by providing an electrical current to the active layer. Typically, a small, micrometer sized current aperture is fabricated near the active layer to define a transverse extent of the gain region. When the mirrors are made from semiconducting material, the current injection to the active layer can happen through the mirrors by properly doping the semiconducting material. Dielectric mirrors cannot conduct current, and a lateral charge injection to the active layer is provided by electrical contacts.
The current aperture controls both the transverse extent of the gain region and the transverse mode lasing of the laser. A critical VCSEL design issue is related to the current aperture which laterally concentrates the injected carriers to provide large enough gain to overcome cavity losses and achieve lasing. The transverse dimension of the current aperture also determines the amount of power which can efficiently be coupled to lasing modes and thereby ultimately determines the obtainable output power of the laser. It is often necessary to have a current aperture which is large in relation to the transverse dimensions of the transverse modes in order to obtain a reasonable power output. As a result, the VCSEL lases in several transversal modes already at medium powers of a few mW. A typical circular shaped VCSEL with a current aperture diameter greater than 10 &mgr;m, emits TEM
00
mode radiation only at low currents. At higher currents, lasing in the higher transverse modes sets in.
U.S. Pat. No. 5,317,587 relates to a method of manufacturing VCSELs. The method uses dielectric current confinement in addition to transparent metal contacts and a mesa-shaped area to separately control the.injected current distribution and the optical mode.
Also, VCSELs typically emit radiation having uncontrolled directions of polarisation. In many applications (e.g. magneto-optical disks, optical communication applications etc.), lasers having predetermined directions of polarisation are highly desirable. Further, adjacent VCSELs in a VCSEL array have a tendency to couple with each other in an uncontrolled manner. In some instances, this results in unwanted beam cross sections.
U.S. Pat. No. 5,412,680 provides a method for controlling the polarisation and the lasing mode of VCSELs by using strained semiconductor layers with a preferred direction of conductivity.
JP 10 284 806 and JP 11 186 657 provide a VCSEL with a two-dimensional photonic band gap (PBG) structure in a plane parallel with the cavity mirrors. The PBG structure restricts spontaneous emission from the gain region so as to decrease cavity losses.
The article
Enhanced coupling to vertical radiation using a two-dimensional photonic crystal in a semiconductor light-emitting diode
by Erchak et al. (Applied Physics Letters, 78, 563, 2001), describes the use of PBG structures in the surface of light emitting diodes to enhance the light extraction efficiency up to six times.
W. D. Zhou et al., Electronics Letters, Vol. 36, no. 28, 1541 (2000) describes the experimental work with a surface emitting optical device. The device is made from the GaAs/AlGaAs system and consist of a bottom distributed Bragg mirror of n-type, an undoped lambda cavity with two embedded InGaAs quantum wells for optical gain, and a top p-type layer. The structure has been structured with a regular array of holes penetrating the top p-type layer, the lambda cavity with the embedded gain material, and a part of the bottom Bragg-mirror. The etching depth is in total 0.8 microns. The regular structure is intended to have a photonic band gap effect and have a defect defined by leaving out one of the holes. The structure is lasing on a mode defined by the defect by observing light emission concentrated in a region including the defect. The maximum output power from the device is reported to be 14.4 &mgr;W.
In the described structure it is necessary to etch through the top and the gain region to get sufficient overlap with the field. The requirement of etching through the gain material restricts the total area of gain material within the defect area and reduces the resulting output power. The authors achieve 14.4 &mgr;W. At least two orders of magnitude more power are needed to be useful in practice.
SUMMARY OF THE INVENTION
Separating the current confinement and the mode control can result in VCSEL with considerably increased power output and better beam properties, e.g. in the form of single mode operation. In the prior art, such separation has been performed by simply introducing large losses for higher order modes e.g. by providing a lower reflectivity for higher order modes as compared to the fundamental mode. However, since energy can be coupled between modes, this results in large energy losses leading to a low efficiency and low power output. The present invention provides a separation of the confinement of the gain region and the mode control, without the disadvantages of the prior art.
In a first aspect, the present invention provides a VCSEL comprising:
a semiconductor material layer having a gain region adapted to generate light and to emit the generated light,
first and second at least substantially parallel mirrors forming a laser cavity comprising the gain region and at least one spacer layer being positioned between the gain region and the first and/or the second mirror, at least one of the mirrors being partially transparent to the generated light so as to allow the light generated in the gain region to be emitted through said at least one mirror, the laser cav
Birkedal Dan
Østergaard John Erland
Alight Technologies A/S
Ip Paul
Nguyen Tuan
LandOfFree
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