Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
2001-02-07
2003-07-22
Ip, Paul (Department: 2828)
Coherent light generators
Particular active media
Semiconductor
C372S024000, C372S050121, C372S103000
Reexamination Certificate
active
06597715
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor laser that outputs a laser beam utilizing self-coupled effect, an optical head, an optical disk apparatus and the manufacturing method of the semiconductor laser.
2. Description of the Related Art
As for an optical disk apparatus, the density and the capacity of an optical disk have been increased from a compact disc (CD) to a digital versatile disc (DVD), and further increase of the capacity is desired because of the improvement of the performance of a computer and the precision of a display.
The enhancement of the recording density of an optical disk is basically restricted by the diameter of a light spot formed on a recording medium. As a measure of acquiring an optical spot below the diffraction limit of a laser beam, a method of providing a small aperture on a light spot position of a transparent condensing medium and utilizing near-field light that leaks out from the small aperture is drawing attention. However, as efficiency for light utilization by this method is very small, the intensity of near-field light that leaks out is low and smaller recording mark than a recording mark (approximately 0.1 &mgr;m) obtained by condensing by a conventional type lens has not been obtained yet.
As a measure of solving this problem, a method of recording and reproducing utilizing the self-coupled effect (SCOOP effect) of a semiconductor laser is proposed. That is, it is a method of forming a small aperture on a spot position on the output face of the semiconductor laser and using a laser beam leaked out from the small aperture for recording and reproduction, and particularly in reproduction a reflected light from a recording medium is re-incident into the resonator of the laser via the small aperture and is electrically or optically detected the modulation of the oscillated state of the laser. According to this method, as the sensitivity is high, reproduction is enabled even if re-incident light is weak.
Conventional type optical heads using this method are disclosed in “Tech. Dig. ISOM/ODS 1999, ThC-1 (1999) p. 352” (hereinafter called Document I) by A. Partovi and in “Jpn. J. Appl. Phys. 38 (1999) Pt. 2, No. 11B, p. L1327” (hereinafter called Document II) by S. Shinada, for example.
FIG. 14
shows the conventional type optical head disclosed in Document I. As for the optical head
1
, an edge emitting semiconductor laser
2
is arranged at the rear end
11
a
of a flying slider
11
. As for the edge emitting semiconductor laser
2
, a high-reflective multilayer film
10
a
and a low-reflective multilayer film
10
b
respectively including a resonator having the oscillation wavelength of 980 nm are respectively arranged on the rear end face and the front end face of an oscillation area
8
, and a metallic shade
4
having a small aperture
5
formed by etching using a focused ion beam (FIB) of Ga ions is arranged on the surface of the low-reflective multilayer film
10
b
. In such a configuration, recording and reproduction are performed by radiating a laser beam
6
of minute size leaked out from the small aperture
5
to a phase change-type recording medium
7
a
of an optical disk
7
. In reproduction, information is reproduced by making reflected light from the recording medium
7
a
incident into the resonator of the semiconductor laser
2
via the small aperture
5
and inducing the self-coupled effect, that is, electrically or optically detecting the modulation by re-incident light of the semiconductor laser
2
. The recording density can be enhanced by using the laser beam
6
made minute by the small aperture
5
for recording and reproduction.
FIG. 15
shows a conventional type semiconductor laser disclosed in Document II. The semiconductor laser
2
is a vertical cavity surface emitting laser
2
made of a semiconductor crystal of AlGaAs and oscillated at the wavelength of 850 nm, and high-reflective multilayer film
10
a
, a p-type AlAs layer
33
, a p-type spacer layer
34
, high-reflective multilayer film
10
c
having partial transmission and a phase compensation layer
35
are sequentially formed on a substrate
11
made of GaAs and a metallic shade
4
wherein a small aperture
6
is formed over an oscillation area
8
by etching using a focused ion beam is arranged on the output surface
3
of the semiconductor laser
2
. The high-reflective multilayer film
10
a
and the high-reflective multilayer film
10
c
respectively of a resonator are respectively made up by alternately laminating a GaAs layer and an AlGaAs layer respectively having the thickness equivalent to a quarter wavelength. A mirror for the resonator on the output side is made up of the high-reflective multilayer film
10
c
and the metallic shade
4
. Also, as reflection on the metallic shade
4
inverts the phase, the phase compensation layer
35
having the thickness in which optical path length is a quarter wavelength and made of AlGaAs is arranged between the high-reflective multilayer film
10
c
and the metallic shade
4
so that the reflection can be intensified. The recording density can be enhanced by using a laser beam
6
made minute by the small aperture
5
for recording and reproduction.
In the meantime, for a semiconductor laser having configuration different from that of the two conventional examples though it is a semiconductor laser that emits a laser beam utilizing self-coupled effect, there is the one disclosed on p. 27 of the 73rd Minute Optics Workshop Document (September, 1999) for example.
FIG. 18
show the semiconductor laser. The semiconductor laser
2
is a vertical cavity surface emitting laser and is provided with a beam-condensing body
61
in the shape of a pyramid, called as Total Reflection Tip, provided to the output surface of the laser
2
and made of semiconductor material, a conical central metallic body
66
provided to the end of the beam-condensing body
61
, a minute coaxial body
65
configured by a carbon nanotube formed at the end of the central metallic body
66
and a metallic film
63
formed via a dielectric layer
62
around the beam-condensing body
61
, the central metallic body
66
and the minute coaxial body
65
. According to this configuration, as shown in
FIG. 19
, propagation light (in TEM00 mode) of minute size is obtained from the minute coaxial body
65
.
However, according to the conventional type semiconductor laser shown in
FIGS. 14 and 15
, as an air gap is formed between the semiconductor laser and a recording medium and corresponds to the thickness of the metallic shade provided to the output surface of a laser beam, the output power is rapidly decreased more than quantity in inverse proportion to the area of the aperture even if the size of the small aperture is reduced to enhance the recording density, consequently the recording density cannot be enhanced.
That is, in the case of a simple aperture, when the diameter of the aperture is equal to or below a half of the wavelength, cutoff will be occurred as same as a wave guide of a microwave. The aperture becomes narrower, a laser beam which can pass the aperture will be decreased exponentially. Also, in that case, though a laser beam intervenes in the vicinity of an interface mainly as near-field light, the broadening width is approximately the aperture size. In case the aperture width is 100 nm, the depth and the aperture width are substantially equal as shown in
FIG. 16A
, the intensity of near-field light decreases in the direction of the normal line of the aperture exponentially as shown in
FIG. 16B and a
laser beam hardly reaches outside the surface
4
a
of the metallic shade
4
. Hereby, when the aperture size is reduced as described above, power is rapidly decreased.
FIG. 17
shows relationship between the aperture size and the optical output power. As a recordable optical recording medium, a phase change-type recording medium mainly consisted of GeSbTe, for example, and a magneto-optic recording medium mainly made of FeTbCo, for example, are known and both require t
Fuji 'Xerox Co., Ltd.
Oliff & Berridg,e PLC
Rodriguez Armando
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