Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
2001-03-09
2004-08-17
Ip, Paul (Department: 2828)
Coherent light generators
Particular active media
Semiconductor
C372S044010, C372S045013, C372S046012, C372S049010, C372S050121
Reexamination Certificate
active
06778573
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor laser device which oscillates in a fundamental transverse mode.
2. Description of the Related Art
Practical optical output power of high-quality narrow-stripe semiconductor laser devices oscillating in a fundamental transverse mode is increasing year by year. Recently, the practical optical output power of fundamental-transverse-mode semiconductor laser devices is remarkably increasing. In particular, the practical optical output power of 0.98 &mgr;m band fundamental-transverse-mode semiconductor laser devices which are used for exciting an Er doped optical fiber amplifier is 250 mW or more. In addition, the practical optical output power of 0.78 &mgr;m band and 0.65 &mgr;m band fundamental-transverse-mode semiconductor laser devices are also increasing, where the 0.78 &mgr;m band and 0.65 &mgr;m band fundamental-transverse-mode semiconductor laser devices are used in recordable CDs (compact disks) and recordable DVDs (compact disks), respectively. The increase in the practical optical output power is also important in various applications in the fields of image recording. Further, semiconductor laser devices having higher output power are required in digital drive printers, such as laser thermal printers, using sensitized material having relatively low sensitivity.
In the above applications, stability of the fundamental transverse mode is required as well as the high output power and high reliability of the semiconductor laser devices. However, the output power of the fundamental-transverse-mode semiconductor laser devices has an upper limit which is determined by the two factors.
The first factor is decrease in the reliability due to facet degradation caused by high optical density at the light-exit end facet. The facet degradation can be greatly reduced by high-quality end-facet coating or use of an unabsorbent end-facet window structure. The high-quality end-facet coating is disclosed in IEEE Journal of Selected Topics in Quantum Electronics, Vol. 5, p. 832 (1999). The use of the unabsorbent end-facet window structure is disclosed in IEEE Journal of Quantum Electronics, Vol. QE-15, p. 775 (1979), IEEE Journal of Selected Topics in Quantum Electronics, Vol. 5, p. 817 (1999), Japanese Journal of Applied Physics, Vol. 30, p. L904 (1991), and Electronics Letters, Vol. 34, p. 1663 (1998).
The second factor is stability of the fundamental mode. In high-power semiconductor laser devices having output power in excess of 200 mW, the stripe width W and the equivalent-refractive-index difference &Dgr;n in the direction parallel to the active layer (i.e., a difference in the equivalent refractive index between the area of the active region under the stripe and the other area) in the index-guided structure are designed so that higher transverse modes are cut off.
FIG. 5
shows a primary mode cut-off condition in the case where the oscillation wavelength is 1,060 nm. In the hatched area under the curve indicated in
FIG. 5
, the semiconductor laser device can stably oscillate in the fundamental mode. For example, when the stripe width W is 2.5 micrometers, and the equivalent-refractive-index difference &Dgr;n is 4×10
−3
, the semiconductor laser device can stably oscillate in the fundamental transverse mode. However, when the output power of the actual semiconductor laser devices is increased to hundreds of milliwatts, the optical output-current characteristic deviates from a straight line, and the radiation pattern is deformed. This is the upper limit of the stable fundamental transverse mode. Since the carrier density of the active layers of semiconductor laser devices is about 10
18
cm
−3
, the refractive index is decreases by about 2×10
−3
due to the plasma effect, and spatial hole burning of the carrier density is caused by the increase in the optical output power. Accordingly, the refractive index irregularly varies in the direction of the extent of the active layer. Therefore, although the higher modes do not occur, the laser beam is shifted in the horizontal direction, i.e., a so-called beam steering occurs. Thus, the fundamental transverse mode becomes unstable.
In addition, high-output-power semiconductor laser devices usually include a so-called SCH (separate confinement heterostructure) having a quantum well. As reported in Applied Physics Letters, Vol. 75, p. 1839 (1999), it is known that the reliability of the semiconductor laser devices in a high-output-power operation increases when the thickness of the optical waveguide layer is increased, since the optical density at the light-exit end face is reduced by the increase in the thickness of the optical waveguide layer. However, when the thickness of the optical waveguide layer is increased, the amount of light penetrated into the cladding layer is decreased. Therefore, it is not easy to realize a difference in the refractive index from the active layer. Further, the present inventors have found that the transverse modes become unstable when the thickness of the optical waveguide layer is great. Since the distance between the active layer and the current confinement layer is increased by the thickness of the optical waveguide layer, carriers widely spread in the direction parallel to the active layer when the thickness of the optical waveguide layer is great. Thus, the transverse modes become unstable.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a semiconductor laser device which can stably oscillate in a fundamental transverse mode even when the output power is high.
According to the present invention, there is provided a semiconductor laser device having an index-guided structure and oscillating in a fundamental mode, comprising: a lower cladding layer; a lower optical waveguide layer formed above the lower cladding layer; a quantum well layer formed above the lower optical waveguide layer; an upper optical waveguide layer formed above the quantum well layer; and a current confinement structure formed above the upper optical waveguide layer. The upper optical waveguide layer has a first thickness smaller than a second thickness of the lower optical waveguide layer. Further, additional semiconductor layers may be formed between the above layers.
In other words, the semiconductor laser device according to the present invention comprises an SCH structure in which a quantum well layer is sandwiched between optical waveguide layers, and the quantum well layer is not located at the center of the SCH structure. That is, the SCH structure is not symmetric about the quantum well layer, which is located relatively near to the current confinement layer. According to this arrangement of the SCH structure, the current injected into the semiconductor laser device does not widely spread, and therefore the semiconductor laser device according to the present invention can maintain a stable oscillation in a fundamental transverse mode even when the output power is high.
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 (viii).
(i) Barrier layers may be formed between the quantum well layer and the upper and lower optical waveguide layers.
(ii) The sum of the first and second thicknesses may be 0.5 micrometers or greater. In this case, the semiconductor laser device can oscillate with higher output power.
(iii) The distance between the bottom of the current confinement structure and the upper surface of the quantum well layer may be smaller than 0.25 micrometers. The bottom of the current confinement structure determines the stripe width of the current confinement structure. When the index-guided structure is realized by a ridge structure, the bottom of the current confinement structure is the bottom of the ridge portion. When the index-guided structure is realized by an internal stripe structure, the bottom of the current confinement structure is the bottom of
Fukunaga Toshiaki
Hayakawa Toshiro
Ip Paul
Rodriguez Armando
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