Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
1998-11-30
2003-05-06
Leung, Quyen (Department: 2828)
Coherent light generators
Particular active media
Semiconductor
C372S045013
Reexamination Certificate
active
06560260
ABSTRACT:
BACKGROUND OF THE INVENTION
(a) Field of the Invention
The present invention relates to a semiconductor laser diode, and more particularly, to a ridge waveguide semiconductor laser diode having a high output power and an excellent laser characteristic.
(b) Description of the Related Art
GaAs based quantum well laser diodes having an InGaAs strained quantum well active layer have been intensively researched and developed for a practical use. For realizing the above object, it is desired to increase the optical output of the laser diode. For instance, when a laser diode is used at a lasing wavelength of 0.98 &mgr;m as an exciting light source for an erbium-doped fiber for use in a 1.5 &mgr;m band optical fiber amplifier, or when a laser diode is used at a lasing wavelength of 1.02 &mgr;m as an exciting light source for a praseodymium-doped fiber for use in a 1.3 &mgr;m band optical fiber amplifier, the laser diode is required to have a high output power of about 200 mW.
Referring to
FIG. 1
, a typical GaAs based quantum well laser diode will be described as an example. A layer structure
10
of a conventional ridge waveguide GaAs laser diode includes, for instance, a buffer layer
14
having GaAs or AlGaAs based compound semiconductor layer, a lower cladding layer
16
, a first light confinement layer
18
, a second light confinement layer
20
, a first strained quantum well layer
22
, a barrier layer
24
, a second strained quantum well layer
26
, a third light confinement layer
28
, a fourth light confinement layer
30
, a first upper cladding layer
32
, an etch stop layer
34
, a second upper cladding layer
36
, and a cap layer
38
consecutively formed on a n-GaAs substrate
12
. The second upper cladding layer
36
and the cap layer
38
overlying the etch stop layer
34
have a mesa structure.
The chief factor for restricting the increase of the optical output from the GaAs based quantum well laser diode is catastrophic optical damage (COD) which signifies that the optical facet of the laser diode is damaged instantaneously. When the optical output increases to reach a specific value, the COD which is inherent in the GaAs based laser diode is generated to stop the function of the laser diode at the instant.
For preventing the generation of COD failure, for instance, a conventional wide mesa structure of about 4 &mgr;m is adopted to decrease the optical density in the active layers; however, in case of the wide mesa structure, a so-called spatial-hole-burning wherein the optical gain of the laser diode is uneven in the direction parallel to the axis of the active layers arises to thereby tend to generate a beam steering phenomenon.
The beam steering phenomenon is known in the art and means that the light beam moves in the direction parallel to the active layers, which causes the characteristic of the optical output with respect to injected current in the semiconductor laser diode to be non-linear to have a kink in the characteristic, thereby deteriorating the laser characteristic significantly. The non-linearity means that the external differentiated quantum efficiency “&eegr;” (=dL/d(I−I
TH
) does not remain as a constant, wherein “L” represents the optical output, “I” represents the injected current at the optical output of “L”, and “I
TH
” represents a threshold current. In a more extreme case, “&eegr;” sometimes becomes almost 0. Accordingly, signal conversion cannot be effected from the injected current due to the deteriorated laser characteristic. This is particularly critical when the laser diode is coupled to an optical fiber.
For preventing the generation of the problem beam steering phenomenon in a high output power range, it is necessary to consider complicated and various effects such as the waveguide mode control of the active layers and thus, the problem is not solved in the conventional GaAs quantum well laser diode.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a ridge waveguide semiconductor laser diode having a linear characteristic between the optical output and the injected current in a high optical output range.
The present invention provides a ridge waveguide semiconductor laser diode including a semiconductor substrate, at least one active layer overlying the semiconductor substrate, and at least one cladding layer having a ridge structure part and an underlying remaining part overlying substantially the entire surface of the active layer.
The remaining part has a thickness “D” defined by:
D≧W
×0.5,
wherein “W” is a width of a spot size having a strength of 1/e
2
for a near field pattern in the active layer in a direction perpendicular to the active layer, where “e” represents the base of the natural logarithm.
In accordance with the semiconductor laser diode of the present invention, due to the value for “D”, the characteristic of the optical output with respect to injected current caused by a beam steering phenomenon can be improved in terms of the kink, as a result of which an excellent linear characteristic can be obtained in a higher output power.
REFERENCES:
patent: 5394424 (1995-02-01), Ijichi et al.
patent: 6-037389 (1994-02-01), None
patent: 6-188513 (1994-07-01), None
patent: 7-030199 (1995-01-01), None
patent: 7-283484 (1995-10-01), None
patent: 09-307184 (1997-11-01), None
Savolainen et al, “High-Performance 980-nm Strained-Layer GalnAs-GalnAsP-GalnP Quantum-Well Lasers Grown by All Solid-Source, Molecular-Beam Epitaxy”, IEEE Photonics Technology Letters, vol. 8, No. 8, Aug. 1996, pp. 986-988.*
Asonen et al, “High-Power Operation of Aluminum-Free (lambda=0.98 um) Pump Laser for Erbium-Doped Fiber Amplifier”, IEEE Photonics Technology Letters, vol. 5, No. 6, Jun. 1993, pp. 589-591.
Ikegami Yoshikazu
Kasukawa Akihiko
Namegaya Takeshi
Ohkubo Michio
Coudert Brothers LLP
Leung Quyen
The Furukawa Electric Co. Ltd.
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