1. Field of the Invention
The present invention relates to a light emitting device and a light emitting device module containing such light emitting device. The present invention is successfully applicable to a semiconductor laser device such as an excitation light source for an optical fiber amplifier. It is also applicable to light emitting devices such as an LED and a super-luminescent diode.
2. Description of the Related Art
Remarkable progress has been made in recent technologies for optical information processing and optical communication. For example, to accomplish bi-directional communication on an optical fiber network at a high-speed and in a large capacity communication such as that in imaging transmission, a signal amplifier flexibly adaptable to the transmission system thereof, as well as a large-capacity optical fiber transmission path, is indispensable. A rare earth ion doped optical fiber amplifier such as Er3+ doped fiber amplifier (EDFA) is one of the typical examples under competitive investigations in the fields concerned. Thus there is a large expectation to the development of an excellent semiconductor laser for an excitation light source, which is indispensable as an EDFA component.
The excitation light source may, in principle, have three possible oscillation wavelengths of 800 nm, 980 nm and 1480 nm. It is known from the aspect of performance of the amplifier that the excitation at 980 nm is the best, taking the gain and noise-figure into account. A laser diode having a wavelength in the 980-nm band is materialized generally by employing a GaAs substrate and an InGaAs active layer stacked thereon, in which contradictory requirements for high output and long life must be satisfied. Because there is also a requirement for an SHG (secondary harmonic generation) light source in a wavelength range around 980 nm, more specifically, a range from 890 to 1150 nm, development of a laser diode showing excellent properties in various applications is desirable.
In the field of information processing, recent trends prefer shorter wavelength of a semiconductor laser diode in order to achieve high density storage. In particular, a recent blue laser diode makes remarkable progress. This is exemplified as a GaN-base material layer grown on a substrate made of AlOx or the like, which gains improved reliability and is under further investigations.
Critical characteristics required for an exemplary laser diode fabricated on a GaAs substrate for 980-nm-band emission, beside the foregoing high output and high reliability, include stability in light output, linearity in the current-light output characteristics, and stability in the oscillation wavelength. A reason why the stability in the light output is a matter of greatest importance relates to a fact that fluctuation in the degree of excitation of Er3+ doped fiber directly results in fluctuation in the gain of the optical amplifier per se. An excellent linearity in the current-light output characteristics is necessary for controlling the laser diode in order to ensure a constant output even under fluctuation in the external environment such as temperature. Another importance resides in the stability in the oscillation wavelength, because an absorption band of Er+ doped fiber is narrow.
It is, however, quite usual for a semiconductor laser diode to cause fluctuation in the intensity noise, or fluctuation in light output, which is ascribable to fluctuation in the longitudinal-mode. Such observation is known as a mode-hopping noise, and is reported by a number of literatures. An example of the substrate showing absorption at the oscillation wavelength can be found in a 780-nm-band laser diode having an AlGaAs active layer on a GaAs substrate (Journal of Quantum Electronics, Vol. 21, No. 8, p. 1264–1270, August 1985). As is described in the literature, a device having a plurality of competitive longitudinal-modes tends to cause hopping of the oscillation wavelength between the modes relatively close to each other, for example, between adjacent Fabry-Perot modes defined by a cavity length. The above literature describes that the mode-hopping occurs in a width of 0.2 to 0.3 nm, which causes the intensity noise of a semiconductor laser diode since different oscillation wavelengths give different gains. This is observed as fluctuation (non-linearity) in the current-light output characteristics of the laser diode even when the waveguide is designed to oscillate in a single transverse-mode.
Another problem is also pointed out in that the mode-hopping is associated with hysteresis, and the oscillation wavelength will differ depending on the temperature history or injected current history even when the device is operated at a constant light output mode.
In addition to such mode-hopping, a larger intensity noise having a large wavelength spacing as compared with that in the previously reported laser diodes is observed when the substrate is transparent at the wavelength of the light emitted from the active layer (which is typical for a laser diode emitting light around 980 nm in which a GaAs substrate is transparent to light emitted from an InGaAs active layer). The reason is as follows (Journal of Quantum Electronics, Vol. 33, No. 10, p. 1801–1809, October 1997).
Light leaking downward from the active layer is guided within the substrate rather than being absorbed by such substrate. This is because the substrate has a refractive index greater than that of a clad layer contained in the device structure, and is transparent at the emission wavelength. Hence, a substrate-mode will appear. In particular, a higher-mode thereof can readily couple with a general Fabry-Perot mode guided through a laser structure fabricated on the substrate. Because the substrate generally has a thickness of 100 to 150 μm, intensity modulation with approx. 2 to 3 nm mode-spacing is observed in the spectrum. That is, longitudinal-modes which can oscillate relatively easily appear at a spacing of 2 to 3 nm and show mode-competition.
When the longitudinal-mode spacing becomes larger, the laser gains grow further apart. The mode-competition hardly occurs between the modes extremely distant from each other by as far as 10 nm. On the contrary, a mode spacing 2 to 3 nm is not so large enough to forbid the mode-competition, while causing a relatively large difference in the gains, and is causative of extremely large intensity noise as compared with the mode-hopping between the adjacent Fabry-Perot modes. That is, the competition of the longitudinal-mode and spectral-intensity-modulation due to the mode propagating in a transparent substrate which has a larger refractive index than the cladding layers, which is referred to as “substrate-mode” hereinafter, will be more responsible for the intensity noise than the general mode-hopping between Fabry-Perot modes observed for the substrate showing absorption at the emission wavelength.
Stability in the wavelength is another critical issue when the device is to be applied, for example, to an excitation light source for an EDFA. Another problem arises in that such a laser diode, affected by the competition of the longitudinal-mode and spectral-intensity-modulation due to the resonant mode coupling between the ordinary mode in a laser waveguide (laser-mode) and substrate-mode, can exhibit only a limited effect of wavelength stabilization even when coupled with an external cavity such as a grating fiber used for stabilizing the wavelength.
Based on consideration on the foregoing problems in the conventional technology, an object of the present invention is to suppress the competition of the longitudinal mode and spectral-intensity-modulation due to the substrate mode, which are observed for the case the substrate is transparent at the emission wavelength, to thereby provide a light emission device excellent in linearity of the current-light output characteristics, and to thereby improve the coupling characteristics with an external cavity. It is also an object of the present invention to provide a module with a light emitting device capable of operating in a stable manner over a wide range of temperature and output.