Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
2001-06-08
2003-06-17
Ip, Paul (Department: 2828)
Coherent light generators
Particular active media
Semiconductor
C372S075000
Reexamination Certificate
active
06580738
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor laser device which emits laser light having a wavelength of 0.7 to 1.2 &mgr;m.
2. Description of the Related Art
In many conventional semiconductor laser devices which emit laser light having a wavelength of 0.7 to 1.2 &mgr;m, a current confinement structure and an index-guided structure are provided in crystal layers constituting each semiconductor laser device so that each semiconductor laser device oscillates in a fundamental transverse mode.
For example, J. K. Wade et al. (“6.1 W continuous wave front-facet power from Al-free active-region (&lgr;=805 nm) diode lasers,” Applied Physics Letters, vol. 72, No. 1, 1998, pp.4-6) disclose a semiconductor laser device which emits light in the 805 nm band. The semiconductor laser device comprises an Al-free InGaAsP active layer, an InGaP optical waveguide layer, and InAlGaP cladding layers. In addition, in order to improve the characteristics in the high output power range, the semiconductor laser device includes a so-called large optical cavity (LOC) structure in which the thickness of the optical waveguide layer is increased so as to reduce the peak power density, and increase the maximum optical output power. However, when the optical power is maximized, currents generated by optical absorption in the vicinity of end faces generate heat, i.e., raise the temperature at the end faces. In addition, the raised temperature reduces the band gaps at the end faces, and therefore the optical absorption is further enhanced to damage the end face. That is, a vicious cycle is formed. This damage is the so-called catastrophic optical mirror damage (COMD). When the optical power reaches the COMD level, the optical output deteriorates with time. Further, the semiconductor laser device is likely to suddenly break down due to the COMD. Therefore, the above semiconductor laser device is not reliable when the semiconductor laser device operates with high output power.
Further, T. Fukunaga et al. (“Highly Reliable Operation of High-Power InGaAsP/InGaP/AlGaAs 0.8 &mgr;m Separate Confinement Heterostructure Lasers,” Japanese Journal of Applied Physics, vol. 34 (1995) L1175-L1177) disclose a semiconductor laser device which comprises an Al-free active layer, and emits light in the 0.8 &mgr;m band. In the semiconductor laser device, an n-type AlGaAs cladding layer, an intrinsic (i-type) InGaP optical waveguide layer, an InGaAsP quantum well active layer, an i-type InGaP optical waveguide layer, a p-type AlGaAs cladding layer, and a p-type GaAs cap layer are formed on an n-type GaAs substrate. However, the maximum optical output power of the semiconductor laser device is typically 1.8 W, i.e., low.
As explained above, the conventional semiconductor laser devices which emit laser light in the 0.8 &mgr;m band are not reliable when the semiconductor laser device operates with high output power since the catastrophic optical mirror damage or the like occurs.
SUMMARY OF THE INVENTION
The object of the present invention is to provide a semiconductor laser device which emits laser light having a wavelength in the range of 0.7 to 1.2 &mgr;m, and is reliable even when the semiconductor laser device operates with high output power.
According to the present invention, there is provided a first semiconductor laser device including: a GaAs substrate of a first conductive type; a lower cladding layer of the first conductive type, formed on the GaAs substrate; a lower optical waveguide layer made of InGaP of an undoped type or the first conductive type, and formed on the lower cladding layer; an active layer made of InGaAsP or InGaAs, and formed on the lower optical waveguide layer except for near-edge areas of the lower optical waveguide layer which are adjacent to opposite end faces of the semiconductor laser device, where the opposite end faces are perpendicular to a direction of laser light which oscillates in the semiconductor laser device; a first upper optical waveguide layer made of InGaP of an undoped type or a second conductive type, and formed on the active layer; a second upper optical waveguide layer made of InGaP of an undoped type or the second conductive type, and formed over the first upper optical waveguide layer and the near-edge areas of the lower optical waveguide layer; an upper cladding layer of the second conductive type, formed on the second upper optical waveguide layer; and a contact layer of the second conductive type, formed on the upper cladding layer.
Preferably, the semiconductor laser device according to the present invention may also have one or any possible combination of the following additional features (i) to (vi).
(i) In the semiconductor laser device, a ridge structure may be formed by removing more than one portion of the upper cladding layer and the contact layer, and a bottom of the ridge structure may have a width of 1.5 &mgr;m or more.
(ii) The semiconductor laser device may further include an additional layer made of InGaAlP of the first conductive type, and formed on the second upper optical waveguide layer other than a stripe area of the second upper optical waveguide layer so as to form a stripe groove realizing a current injection window, the upper cladding layer may be formed over the additional layer so as to fill the stripe groove, and a bottom of the stripe groove may have a width of 1.5 &mgr;m or more.
(iii) The active layer may be made of In
x1
Ga
1-x1
As
1-y1
P
y1
where 0≦x
1
≦0.3, 0≦y
1
≦0.5, and the product of the strain and the thickness of the active layer may be in a range of −0.15 to +0.15 nm.
The strain D of a layer formed on the GaAs substrate is defined as D=(c−c
s
)/c
s
, where c
s
and c are the lattice constants of the GaAs substrate and the layer formed on the GaAs substrate, respectively
(iv) The active layer may be a strained quantum well active layer, at least one barrier layer made of InGaP may be formed adjacent to the strained quantum well active layer, the at least one barrier layer may be oppositely strained to the strained quantum well active layer, and the sum of a first product and a second product may be in a range of −0.15 to +0.15 nm, where the first product is the product of the strain and the thickness of the active layer, and the second product is the product of the strain and the total thickness of the at least one barrier layer.
(v) Each of the lower cladding layer and the upper cladding layer may be made of Al
z1
Ga
1-z1
As, or In
x3
(Al
z3
Ga
1-z3
)
1-x
3
As
1-y3
P
y3
, where 0.55≦z
1
≦0.8, x
3
=0.49y
3
±0.01, 0<y
3
≦1, and 0<z
3
≦1.
(vi) Each of the lower optical waveguide layer and the first upper optical waveguide layers may be made of In
x2
Ga
1-x2
P, where x
2
=0.49±0.01.
According to the present invention, there is provided a second semiconductor laser device comprising: a GaAs substrate of a first conductive type; and a semiconductor layer formed on the GaAs substrate, the semiconductor layer including: a cladding layer of a first conductive type, formed on the GaAs substrate; a lower optical waveguide layer made of InGaP of the first conductive type or an undoped type, the lower optical wavelength layer being formed on the lower cladding layer; a compressive strain active layer made of InGaAsP or InGaAs, the compressive strain active layer being formed on the lower optical waveguide layer; an upper optical waveguide layer made of InGaP of a second conductive type or an undoped type, the upper optical waveguide layer being formed on the compressive strain active layer; and a cladding layer of the second conductive type.
Here, the second semiconductor laser device of the present invention is characterized in that: an InGaAsP lower barrier layer is provided between the lower optical waveguide layer and the compressive strain active layer, the InGaAsP lower barrier layer having a band gap larger than that of the compressive strain active layer; an InGaAsP upper barrier laye
Fuji Photo Film Co. , Ltd.
Ip Paul
Nguyen Dung
LandOfFree
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