Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
2001-02-28
2003-04-08
Leung, Quyen (Department: 2828)
Coherent light generators
Particular active media
Semiconductor
C372S046012, C372S023000, C438S022000
Reexamination Certificate
active
06546035
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a two-wavelength semiconductor laser diode array having a monolithic structure including two semiconductor laser diodes lazing at different wavelengths formed on a single substrate, and a method of fabricating the semiconductor laser diode array.
Appearance of a digital video disk (DVD) has advanced increase of the recording density of an optical disk, and an optical disk with mass storage of 8.5 GB has been realized these days. A general DVD reproducing system should reproduce data from not only a DVD but also a compact disk (CD), and is sometimes required to be capable of reproducing data from and recording data in a write-once CD (CD-R) currently rapidly spreading. As reproducing light for reproducing data from a DVD, a red laser beam of a wavelength of a 650 nm band is used, and as reproducing light for reproducing data from a CD or a CD-R, an infrared laser beam of a wavelength of a 780 nm band is used. Accordingly, a current DVD reproducing system includes two semiconductor laser chips, namely, a red semiconductor laser chip for generating the red laser beam and an infrared semiconductor laser chip for generating the infrared laser beam.
In accordance with development of compact information processing equipment such as a personal computer, it is necessary to make a DVD reproducing system more compact, and for this purpose, it is indispensable to realize a compact and thin optical pickup part. One means for realizing a compact and thin optical pickup part is simplification of an optical system. One method for simplifying an optical system is integration of a red semiconductor laser chip and an infrared semiconductor laser chip. As described above, a current DVD reproducing system includes two optical system components for the red semiconductor laser chip and the infrared semiconductor laser chip. Therefore, when the red and infrared semiconductor laser chips are integrated, the two optical system components can be shared, resulting in realizing a compact and thin optical pickup part.
As examples of the integration of a red semiconductor laser chip and an infrared semiconductor laser chip, a monolithic semiconductor laser diode array integrated on a single substrate is reported in Japanese Laid-Open Patent Publication No. 11-186651 (Conventional Example 1) and “The 60th Autumn Applied Physics Lecture Meeting”, 3a-ZC-10 (Conventional Example 2).
In the semiconductor laser diode array of Conventional Example 1, each of a red laser chip and an infrared laser chip includes, as a current block (confinement) layer for efficiently injecting a current into an active layer, GaAs with an energy gap equivalent to or smaller than the energy gap (band gap) of the active layer. In this manner, a complex refractive index waveguide structure is employed, in which a laser beam emitted from the active layer is absorbed so as to effectively confine generated light within a stripe-shaped region.
In the semiconductor laser diode employing the complex refractive index waveguide structure, however, generated light is absorbed by a current block layer of GaAs, and hence, it is very difficult to attain a self-oscillation characteristic and a high-temperature high-output-power characteristic required in an optical disk unit.
Alternatively, the semiconductor laser diode array of Conventional Example 2 has the so-called gain waveguide structure including no current block layer, and hence, light is never absorbed by a current block layer. The semiconductor laser diode having the gain waveguide structure, however, does not have the complex refractive index waveguide structure for effectively confining generated light. Therefore, in order to attain low noise required in an optical disk unit, it is necessary to provide means for suppressing interference between oscillation spectra by, for example, employing multiple modes for the oscillation spectra.
Furthermore, even when the multiple modes are employed for the oscillation spectra, the half bandwidth of each spectrum is so small that emitted light and return light of the emitted light to the semiconductor laser diode can easily interfere each other. Therefore, the relative noise intensity (RIN) cannot be lowered to −130 dB/Hz or less as is desired in an optical disk unit. Accordingly, the semiconductor laser diode array having the gain waveguide structure of Conventional Example 2 needs means for lowering the RIN by using a 1/4 &lgr; plate or the like, and hence, it is difficult to reduce the number of components included in the optical pickup part. In order to solve these problems, the semiconductor laser diode array should indispensably have a self-oscillation characteristic.
In addition, although the semiconductor laser diode array with the gain waveguide structure has a current confining function, it does not have a light confining function utilizing a refractive index distribution along a direction parallel to the principal plane of the active layer. Therefore, when it is operated at low output power of 10 mW or less in reproducing data from a DVD or a CD, a single lateral mode characteristic can be kept at room temperature, but a stable lateral mode characteristic is difficult to keep at a high temperature because carriers are so largely injected that gain can be more easily attained in a higher mode. Furthermore, since it does not have a light confining mechanism, the lateral mode characteristic is more difficult to stabilize when it is operated at high output power.
Moreover, since the optical system components are shared in the conventional monolithic two-wavelength laser diode array, the active layers of the respective laser diodes are preferably placed in the same position, namely, at the same height from the substrate surface. However, although the laser diode array is monolithic, the active layers of the respective laser diodes have different compositions, and hence, the active layers should be grown through different processes. Therefore, the heights of the active layers disadvantageously differ from each other.
SUMMARY OF THE INVENTION
An object of the invention is, for solving the aforementioned conventional problems, realizing a monolithic semiconductor laser diode array having a stable self-oscillation characteristic and definitely capable of operating at high output power and a high temperature in which a difference in the height between active layers is suppressed.
In order to achieve the object, in fabrication of a monolithic semiconductor laser diode array of this invention, a first laser diode showing laser action at a longer wavelength is formed priorly to a second laser diode showing laser action at a shorter wavelength. At this point, since a cladding layer formed closer to a substrate in the second laser diode may have a smaller thickness than a cladding layer formed closer to the substrate in the first laser diode, a buffer layer for improving the crystallinity of the second laser diode is formed and the buffer layer is provided with a height adjusting function.
Furthermore, both the first laser diode and the second laser diode are provided with a refractive index waveguide mechanism, so as to realize a stable lateral mode characteristic, and a current block layer of each laser diode is formed in a real refractive index waveguide structure, so as to realize a stable self-oscillation characteristic, a high output power operation and a high temperature operation.
Specifically, the semiconductor laser diode array of this invention comprises a first laser diode including a first cladding layer formed on a substrate from a first semiconductor of a first conductivity type, a first active layer formed on the first cladding layer from a second semiconductor and a second cladding layer formed on the first active layer from a third semiconductor of a second conductivity type; and a second laser diode including a third cladding layer formed from a fourth semiconductor of the first conductivity type on the substrate with a space from the first laser diode, a second activ
Imafuji Osamu
Yuri Masaaki
Leung Quyen
Matsushita Electric - Industrial Co., Ltd.
Nixon & Peabody LLP
Studebaker Donald R.
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