Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
1998-11-25
2001-01-16
Arroyo, Teresa M. (Department: 2881)
Coherent light generators
Particular active media
Semiconductor
C372S046012
Reexamination Certificate
active
06175582
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor laser device capable of operating at high efficiency and high output power, which is preferably used in the fields of communication, printing, laser medical treatment, laser beam machining and the like.
2. Description of the Related Art
For the purpose of enhancing an output power of a semiconductor laser, the applicant of the present application has proposed a semiconductor laser which is provided with carrier blocking layers having a wide band gap and a small thickness on both sides of an active layer, whereby the design freedom of a band gap of a cladding layer formed outside the carrier blocking layer is increased (WO 93/16513).
In such a configuration, the carrier blocking layer has a function of confining an injected carrier in the active layer efficiently and the carrier blocking layer is formed into a thin shape, so that light generated in the active layer can pass through the carrier blocking layer and leak out easily to an optical guide layer which is disposed outside. Therefore, it is possible to avoid catastrophic optical damage which occurs due to a localization of laser light on an emission facet of a semiconductor laser, and raise the breakdown level of a facet, with the result that an operation at high output power can be realized.
In order to fabricate a semiconductor laser device of higher efficiency, it is important to decrease losses, among which an inner loss depends on free carrier absorption to a large extent. In this free carrier absorption, a p-type layer is more involved than an n-type layer. In the case of using GaAs, for example, the free carrier absorption coefficient &agr;
fc
[cm−
1
] is expressed by formula (1) as shown below (see page 85, “Semiconductor Laser—basis and application—,” edited by Ryoichi ITO and Michiharu NAKAMURA):
&agr;
fc
=3×10
−18
·n+
7×10
−18
·p (1)
wherein n denotes the concentration of an n-type carrier and p denotes the concentration of a p-type carrier. It is apparent from formula (1) that the free carrier absorption coefficient afc is proportional to the concentration of carriers and the p-type layer is involved in free carrier absorption twice as much as the n-type layer or more.
In order to fabricate a semiconductor laser device of further higher efficiency and higher output power, it is important to limit the electric resistance and thermal resistance of the device to a low level. When the electric resistance is high, the energy conversion efficiency is decreased due to the generation of Joule's heat and the like. Moreover, since the temperature of the device rises, a threshold current is increased and an output power is decreased due to heat saturation.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a semiconductor laser device of high efficiency, by reducing free carrier absorption so as to limit inner losses to a low level.
It is another object of the invention to provide a semiconductor laser device of high efficiency and high output power, by reducing the electric resistance and the thermal resistance of the device.
The invention provides a semiconductor laser device including a plurality of sequentially formed layers, comprising:
a first cladding layer,
a first optical guide layer,
a first carrier block layer,
an active layer,
a second carrier blocking layer,
a second optical guide layer,
a second cladding layer,
band gaps of the first and second optical guide layers being wider than that of the active layer,
band gaps of the first and second cladding layers being wider than those of the first and second optical guide layers,
band gaps of the first and second carrier blocking layers being wider than those of the first and second optical guide layers,
one of the first and second cladding layers being of p-type, the other being of n-type,
wherein a refractive index of the p-type cladding layer is lower than that of the n-type cladding layer.
According to the invention, the p-type cladding layer is made so as to have a lower refractive index than that of the n-type cladding layer, whereby a wave guide mode will be pushed out to the side of the n-type cladding layer having a higher refractive index. Therefore, it is possible to reduce the distribution amount of light at the p-type cladding layer and the optical guide layer adjacent thereto which are involved in free carrier absorption more than the n-type layers, with the result that the efficiency of the laser device is enhanced with decrease in inner losses.
Further, in the invention it is preferable that a thickness of the p-type cladding layer is smaller than that of the n-type cladding layer.
According to the invention, the p-type cladding layer is thin, whereby the electric resistance of the p-type cladding layer itself is decreased. In general, the electric resistance of a p-type layer is higher than that of an n-type layer, and a cladding layer is likely to be the thickest of all the layers which constitute a semiconductor laser, so that the electric resistance of the p-type cladding layer makes up the largest portion of a total of electric resistance. Therefore, it is possible to largely reduce the total of electric resistance by reducing the electric resistance of the p-type cladding layer. Moreover, since the generation amount of Joule heat is also decreased, the temperature in the overall device is prevented from rising and the energy conversion efficiency is improved. In addition, the maximum output power, which is restricted by heat saturation, is also increased. Accordingly, a semiconductor laser of high efficiency and high output power can be implemented.
Further, the p-type cladding layer is thin, whereby the thermal resistance of the p-type cladding layer itself is also decreased. In a semiconductor laser device in general, respective layers are formed on an n-type substrate, and are usually adhered by junction down in order to enhance an efficiency of heat radiation, so that the layers formed on the side of the p-type layer are adhered on a mount. In a case where the p-type layers are adhered on the mount, heat generated in the active layer escapes to the mount through the p-type layer having a low thermal resistance, with the result that the heat is smoothly radiated and the temperature in the overall device can be prevented from rising.
Still further, the p-type cladding layer is formed into a thin shape, whereby the distance between the top face and the active layer is shortened. Therefore, in the case where a window or stripe structure is embedded by ion implantation or the like, an acceleration voltage on ion implantation is lowered, and damage to the laser device can be reduced. As a result, the semiconductor laser device is enhanced in reliability and output power.
REFERENCES:
patent: 3855607 (1974-12-01), Kressel et al.
patent: 4328469 (1982-05-01), Scifres et al.
patent: 5301202 (1994-04-01), Harder et al.
patent: 5764668 (1998-06-01), Ishizaka et al.
patent: 2-113586 (1990-04-01), None
patent: 7-231139 (1995-08-01), None
patent: WO 9316513 (1993-08-01), None
S. M. Sze, Physics and Technology, p. 36, 1985 (No Month Available).
G. P. Agrawal et al., Long-Wavelength Semiconductor Lasers, pp. 192-199, 1986 (No Month Available).
Ryoichi Ito and Michiharu Nakamura, “Gain of light and condition of laser oscillation”,Semiconductor Laser -basis and application-, p. 85, Apr. 25, 1989.
Fujimoto Tsuyoshi
Naito Yumi
Oeda Yasuo
Arroyo Teresa M.
Birch & Stewart Kolasch & Birch, LLP
Leung Quyen P.
Mitsui Chemicals Inc.
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