Semiconductor device manufacturing: process – Making device or circuit emissive of nonelectrical signal – Including integrally formed optical element
Reexamination Certificate
1999-09-22
2001-05-29
Pham, Long (Department: 2823)
Semiconductor device manufacturing: process
Making device or circuit emissive of nonelectrical signal
Including integrally formed optical element
C438S022000, C438S032000, C438S040000, C438S042000, C438S681000, C438S942000, C372S045013, C372S046012, C372S050121
Reexamination Certificate
active
06238943
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical semiconductor device and a method of manufacturing the same and more particularly to an optical semiconductor device having a high output semiconductor laser used for a light source of an information processing unit such as an optical printer and an optical disk unit, a semiconductor laser amplifier used for light amplification, an optical active element such as a semiconductor laser used in a transmitter of an optical communication apparatus or a photodiode used in a receiver thereof and an optical waveguide, and a method of manufacturing the same.
2. Description of the Prior Art
With the advance of semiconductor communication technique, the production technique of a semiconductor laser is also improved, and researches in integration of a semiconductor laser with another optical semiconductor element are being made extensively in recent years. For example, an apparatus in which a DFB (distributed feedback) laser and a light modulator are integrated and an apparatus in which a DBR (distributed Bragg reflector) laser and a mode transducer (a beam size converter) are integrated are available.
A mode transducer is a mechanism for narrowing an output beam of a semiconductor laser originally having an output angle as wide as 30 to 40 degrees, and for facilitating optical coupling in case a semiconductor laser and an optical fiber are formed into a module.
In a semiconductor laser, the more intense the optical confinement of an optical waveguide is, i.e., the smaller a light spot diameter is, the smaller an oscillation threshold becomes, and a luminous efficiency is improved. As the light spot diameter gets smaller, however, coupling with the optical fiber becomes more difficult.
Further, a semiconductor laser requiring a high output such as a laser for exciting a fiber type optical amplifier using an optical fiber doped with erbium or a semiconductor laser for writing information in a optical disk has such a problem that an optical power density rises at a laser end face and damage of the end face is liable to be produced. Furthermore, a semiconductor laser amplifier has such a drawback that the optical output is saturated easily when beam confinement is intense.
Those photodiodes that have a rapid speed of response and a high quantum efficiency are required, and furthermore, those that can be formed into a thin shape and in that electrical wiring is easy are demanded.
An end face incidence waveguide type photodiode is available as a photodiode which meets such a requirement. In this waveguide type photodiode, the more intense the beam confinement is, the shorter the waveguide is made. With this, internal pn junction capacity thereof is reduced, thus making high-speed response possible. Moreover, reactive components of light absorption such as free carrier absorption of a cladding layer and the quantum efficiency is increased, thus improving sensitivity.
Under such circumstances, a semiconductor laser, a semiconductor laser amplifier and a photodiode having an optical waveguide in which beam confinement is intense inside and beam confinement is weak at an end face are demanded in the fields of optical communication and optical information processing.
So, a waveguide for converting an optical beam diameter composed of a semiconductor has been proposed as shown in
FIG. 1A
to FIG.
1
C. The semiconductor waveguide shown in
FIG. 1A
is disclosed in [1] the Institute of Electronic Information Communications in Japan, National Autumn Meeting 1992, Lecture Number C-201 for example. The semiconductor waveguide shown in
FIGS. 1B and 1C
has been proposed in [2] the Institute of Electronic Information Communications in Japan, National Autumn Meeting 1992, Lecture Number C-202 for instance. In [3] T. L. KOCH et al., IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 2, NO. 2, 1990, the semiconductor waveguide is proposed in a mode transducer integrated Fabry-Perot semiconductor laser (hereinafter referred to also as an FP-LD) having a waveguide for converting an optical beam system.
An optical beam diameter converting waveguide shown in
FIG. 1A
has an InGaAsP core layer
3
surrounded by an InP substrate
1
and an InP cladding layer
2
at top and bottom and left and right.
The lateral width of the InGaAsP core layer
3
is wide at one end face and gets narrower as getting near another end face, and the optical beam diameter spreads in a lateral direction on the side where the width is narrowed in accordance with the variation of the width. The thickness of the InGaAsP core layer
3
is uniform, and the plane pattern thereof is formed by lithography technique using an exposure mask.
Thus, the semiconductor waveguide is effective for converting an optical beam diameter in a lateral direction, but the optical beam diameter in a longitudinal direction (a thickness direction of the layer) is not converted.
Now, in an optical waveguide using a semiconductor, since the optical beam diameter in the thickness direction of the core layer is generally smaller than that in the width direction thereof, conversion of the optical beam diameter in the longitudinal direction is important for the improvement of the optical coupling efficiency with an optical fiber, an optical semiconductor element or the like. Since the optical beam diameter in the thickness direction of the layer is not converted in the semiconductor waveguide for converting the optical beam diameter shown in
FIG. 1
, a significant effect cannot be expected for the improvement of the optical coupling efficiency between the semiconductor optical waveguide and an optical fiber or the like.
In order to increase the coupling efficiency, it is conceivable to form the semiconductor waveguide for converting the optical beam diameter thereof and the waveguide-shaped photodiode into an integral construction. Since the thickness and the composition of these core layers become uniform in the optical axis direction, however, the end face of the semiconductor waveguide also becomes a light absorption region and the optical loss is increased. Furthermore, since a PN junction is exposed at the end face, a dark current is increased. When the dark current is more or less increased, there is a problem that a signal-to-noise ratio is deteriorated in case very high sensitivity is required.
As against the above, the semiconductor wave-guide for converting the optical beam diameter shown in
FIGS. 1A and 1B
has a construction that the optical beam diameter is converted in the thickness direction of the film.
This semiconductor waveguide has a first InP cladding layer
5
laminated on an InP substrate
4
, a multi-quantum well (MQW) layer
6
composed of an InP well and an InAgAs barrier formed thereon and a second InP cladding layer
7
formed on the MQW layer
6
. Further, an InGaAsP core layer
8
is formed in the MQW layer
6
, and the film thickness at one end thereof is thinned in the MQW layer
6
. Besides, the MQW layer
6
serves as a cladding layer for the core layer
8
and as a core layer for the first and the second InP cladding layers
5
and
7
. The core layer
8
is formed both in a gain region
9
and a mode conversion region
10
. Further, the core layer
8
in the mode conversion region
10
is formed in a tapered shape in the thickness direction, and gets thinner as becoming more distant from the gain region
9
.
The light advancing in such a semiconductor waveguide is confined in the MQW layer
6
and further confined more intensely in the cladding layer
7
. The optical beam diameter is converted at a portion where the film thickness of the core layer
8
is changed. Further, since the light excited in the MQW layer
6
is confined more weakly in the mode conversion region
10
than in the gain region
9
, a near field pattern at a taper bottom end portion is spread. As a result, a far field pattern which is a diffracted pattern of the near field pattern is contracted. Accordingly, an output angle of a beam emitted f
Ekawa Mitsuru
Fujii Takuya
Kobayashi Hirohiko
Ogita Shouichi
Okazaki Nirou
Armstrong Westerman Hattori McLeland & Naughton LLP
Fujitsu Limited
Pham Long
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