Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
2000-01-27
2002-12-31
Leung, Quyen (Department: 2828)
Coherent light generators
Particular active media
Semiconductor
C372S046012
Reexamination Certificate
active
06501776
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a temperature-insensitive semiconductor laser, such as a wavelength-stabilized semiconductor laser and a semiconductor laser having a high characteristic temperature, which can be preferably employed as a light source in a wavelength division multiplexing optical communication system and the like.
2. Related Background Art
In general, the oscillation wavelength of a semiconductor laser (a laser diode (LD)) is likely to vary depending on its ambient temperature. When the temperature changes, spacings between atoms and magnitudes of lattice vibrations vary in semiconductor crystals, and hence, the energy bandgap and refractive index thereof change. For this reason, the oscillation wavelength of the LD changes. Generally, the refractive index decreases as the bandgap increases.
A waveguide-type Fabry-Perot LD has a large number of resonance modes (longitudinal modes) in its gain spectrum, and oscillates at a wavelength of the resonance modes that is the closest wavelength to a wavelength at the gain peak. This resonance wavelength is proportional to an effective refractive index of a light waveguide when the effective refractive index is approximately uniform over the entire waveguide, while proportional to an optical length of its entire cavity when the effective refractive index varies along the waveguide. The gain-peak wavelength changes depending on the shape of its energy band structure. As a result, the oscillation wavelength of the Fabry-Perot LD depends on both the energy band structure and the refractive index (or the optical length of the cavity).
On the other hand, the oscillation wavelength of a distributed feedback semiconductor laser (DFB-LD) is determined by a pitch of its built-in diffraction grating and an effective refractive index of its light waveguide, provided that reflectivities at its end facets are negligibly low. In other words, its oscillation wavelength is independent of a wavelength at its gain peak. Thus, this oscillation wavelength is influenced only by the refractive index since the physical pitch is fixed. Naturally, no oscillation occurs when a difference between the gain-peak wavelength and the resonance wavelength is too large to obtain the oscillation threshold gain at the resonance wavelength. It does not, however, mean that the oscillation wavelength is under the influence of the wavelength at the gain peak.
Optical-fiber commmunication recently has been used in subscriber systems as well, as in trunk line systems as the information capacity has increased. The semiconductor laser of such subscriber systems is often used as a light source in hostile environments, such as a place where the temperature greatly changes, in contrast to that of the trunk line systems. Further, the demand for using such a semiconductor laser without any temperature controller has increased, because it reduces the cost.
Wavelength division multiplexing (WDM) transmission exists as a method for increasing the information transmission capacity. In WDM transmission, multiplexed wavelength signals must be precisely stabilized. Therefore, there is a need for a semiconductor laser, whose wavelength is stable even in a hostile atmosphere, such that the WDM transmission can be widely employed even in subscriber systems. Japanese Patent Application Laid-Open No. 9(1997)-219561 discloses such a wavelength-stabilized semiconductor laser.
In a device of this disclosure, its active layer and light guiding layer are composed of material whose bandgap (i.e., its refractive index) remains unchanged even when the temperature changes. Accordingly, where crystals are to be formed without any large difference between their lattice constants such that no inelastic strain is introduced therein, a combination of crystal materials must be selected from a narrow range since the lattice matching needs to be approximately attained between those crystals. Further, since laser light extends further to its substrate and cladding layer, it is impossible to maintain the effective refractive index of its light waveguide at a constant value when the temperature changes.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a temperature-insensitive semiconductor laser wherein one kind or a small number of kinds of materials constitute a layer structure, which is introduced to stabilize its cavity length or its effective refractive index of its waveguide, such that a change in its oscillation wavelength is satisfactorily small irrespective of a fluctuation in the temperature of the semiconductor laser.
Another object of the present invention is to provide a semiconductor laser wherein an overflow of carriers is suppressed against a fluctuation in the temperature of the semiconductor laser and its oscillation threshold current is thus stabilized.
The present invention is generally directed to a semiconductor laser which includes a substrate, semiconductor layers which are formed on the substrate and define a cavity including a waveguide with an active region, and which include at least one semiconductor layer whose refractive-index temperature coefficient is set at a non-positive or minute value to achieve at least one function of stabilizing a wavelength of the semiconductor laser and suppressing an overflow of carriers from the active region, and a driving unit for causing electron-hole recombination in the active region.
There are three configurations of the present invention, based on the above fundamental construction.
In accordance with a first configuration of the present invention, there is provided a semiconductor laser wherein the semiconductor layers formed on the substrate include at least one semiconductor layer whose temperature coefficient of the refractive-index (also referred to as a refractive-index temperature coefficient in this specification) is set at a negative value to achieve at least one of the functions of approximately maintaining an optical length of the cavity or an effective refractive index of the waveguide at a constant value against a change in the temperature of the semiconductor laser and suppressing an oscillation threshold current injected into the active region by the driving unit, such as a current-injecting unit, against a change in the temperature of the semiconductor laser.
The principle of the first configuration will be described in detail below. When a semiconductor layer with a negative refractive-index temperature coefficient is arranged in a laser-light existing region of the laser, both the above functions can be achieved. In contrast, when a semiconductor layer with a negative refractive-index temperature coefficient is arranged in a region of the laser where no laser light exists, only the above function of suppressing the oscillation threshold current injected into the active region against a change in the temperature of the laser can be achieved.
More specifically, the following specific structures are possible in the first configuration. The semiconductor layers formed on the substrate may include in the laser-light existing region a first semiconductor layer whose refractive-index temperature coefficient is set at a positive value and a second semiconductor layer whose refractive-index temperature coefficient is. set at a negative value to achieve the function of approximately maintaining the optical length of the cavity or the effective refractive index of the waveguide at a constant value against a change in the temperature of the laser. In this structure, the cavity length or the effective refractive index of the waveguide can be maintained approximately unchanged, and the oscillation wavelength can be hence stabilized.
The first semiconductor layer and the second semiconductor layer may be placed in an approximately parallel manner along a growth direction in the laser-light existing region. This relates to a structure wherein a compensation layer is disposed along the growth direction. The compensation layer will be described below in the
LandOfFree
Temperature-insensitive semiconductor laser does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Temperature-insensitive semiconductor laser, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Temperature-insensitive semiconductor laser will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2919645