Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
1999-09-15
2002-01-29
Leung, Quyen (Department: 2881)
Coherent light generators
Particular active media
Semiconductor
Reexamination Certificate
active
06343088
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a semiconductor laser module for emitting a laser beam, and more specifically to a semiconductor laser module for an erbium doped fiber amplifier (EDFA) pumping purpose.
BACKGROUND ART
Conventionally, in order to stabilize the wavelength of light emitted from an emitting device, optical feedback method is generally used such that the light emitted from a multimode emission laser is partially reflected back to the laser thereby make the emission wavelength of the laser constant.
For examples, a semiconductor distributed feed back (DFB) laser, a distributed bragg reflector (DBR) laser, etc. belong to this classification. The DFB laser is a laser in which a diffraction grating is formed in an active layer of a semiconductor laser device. The DBR laser is a laser in which a reflecting diffraction grating which is, unlike an active layer, transparent in connection with emitted light is formed in a semiconductor laser device along its waveguide portion made of a semi-conducting medium, so that light may be reflected (fed back) to an active layer.
Further, in recent years, a fiber bragg grating, that is, an optical fiber whose core has a refractive index varied in its axial direction so that the optical fiber may have a function of optical diffraction is developing rapidly, and lasers using various fiber bragg gratings are already disclosed. For example, Bulletin No. OPE97-1 of Society of Electronic Information and Communication discloses a “fiber grating external resonator type multiwavelength laser array” by Kato et al., and Bulletin No. OPE97-2 thereof discloses a “UV induced waveguide grating and application thereof to an integrated external resonator type laser” by Tanaka et al.
Further, Atsushi Hamakawa et al. have reported on a technique of using an FBG to stabilize the emission wavelength of a 1480 nm-band pump laser which is to be used as a light source for EDFA pumping, in the 2nd Optoelectronics & Communications Conference (OECC '97), Technical Digest, July 1997, Seoul, Korea (classification 9D2-5, pages 224 to 225).
However, the above mentioned emitting devices such as the DFB laser and the DBR laser have a single mode emission spectrum. They are used exclusively for communication and not suitable for amplifying an EDFA.
Further, the above mentioned external resonator type lasers disclosed by Kato et al. and Tanaka et al. have the following problems.
1) Those external resonator type lasers are signal mode emission lasers, and mode hop, that is, a shift of central wavelength of emission happens when operating temperature changes by several degrees [C]. Thus, the stability of emission wavelength under change of temperature is low.
2) In those external resonator type lasers, the distance between an emitting surface of a laser device and a diffraction grating for optical feedback, which constitute an external resonator, is short. The process of assembling a module is therefore troublesome and needs special means such as a lensed fiber, flat mounting and the like. In other words, it is difficult to form the above mentioned external resonator type lasers using a two-lens compound confocal coupling system as used in an ordinary Fabry-Perot laser diode or an ordinary pump laser diode.
3) Output power of those external resonator type lasers is low. Therefore, they are not suited to be a light source for exciting an EDFA.
Further, the above mentioned technique on which Atsushi Hamakawa et al. have reported has the following problems though it is suitable for exciting an EDFA:
1) It needs to use, as a diffraction grating, a special fiber bragg grating capable of reflecting light of two different wavelengths.
2) Mode hop due to change of operating temperature is relatively small, but there still happens mode hop of about 2.6 nm.
The present invention has been made in consideration of the above problems. The object of the present invention is to provide a semiconductor laser module which has high power, shows high stability of emission wavelength under change of temperature, and is suited to be a light source for exciting an EDFA.
DISCLOSURE OF THE INVENTION
In order to attain the above object, the present invention provides a semiconductor laser module comprising a semiconductor laser device having an emitting surface from which excited light is emitted and a reflecting surface opposite to the emitting surface, and an optical feedback medium for feeding most of optical power emitted from the emitting surface of the semiconductor laser device back to the semiconductor laser device and emitting part of the optical power, wherein the semiconductor laser device has a first multilayer coating formed on the emitting surface and having a reflectance of 10
−4
to 10% at the central wavelength of reflection, and the first multilayer coating has a reflection spectrum in the form of a curve having minimum values on both sides of the central wavelength of reflection.
In this semiconductor laser module according to the present invention, mode hop, that is, a shift of central wavelength of emission, caused by operating temperature of the semiconductor laser device itself is restrained.
Specifically, in an ordinary laser, light is reflected using a reflection reducing coating which does not have wavelength selectivity and has a flat reflection spectrum. For example, in a 1480 nm-band multimode emission semiconductor laser module, when the operating temperature changes within the range of 5 to 65° C., the central wavelength of emission spectrum shifts to the extent of about 30 nm at most.
In contrast thereto, in the semiconductor laser module of the present invention, the semiconductor laser device has, for example, a dielectric multilayer coating formed on its emission surface, and the dielectric multilayer coating has a reflection spectrum in the form of a curve having minimum values on both sides of the central wavelength of reflection and has a reflectance of 10
−4
to 10% at the central wavelength of reflection. Therefore, in the semiconductor laser module of the present invention, the extent of a shift of central wavelength of emission spectrum of the semiconductor device under change of operating temperature is restrained to be small, that is, several nanometers at most.
It is desirable that the first multilayer coating has a reflection spectrum which has a maximum value near the central wavelength.
Further, it is desirable that the optical feedback medium is an optical waveguide or an optical fiber which has a core, a diffraction grating is formed in the core along the optical axis thereof, and the optical feedback medium is arranged to face the semiconductor laser device leaving a space of at least 10 mm as measured from the diffraction grating to the emitting surface of the semiconductor laser device.
Further, it is desirable that the semiconductor laser device has a second multilayer coating formed on its reflecting surface, and the second multilayer coating does not have wavelength selectivity and has a reflectance of 90 to 98%, so that the semiconductor laser device may have a multimode emission spectrum.
Due to the above described features imparted by the present invention, the semiconductor laser module of the present invention has high power, shows higher stability of emission wavelength under change of temperature, and is suited to be a light source for EDFA pumping.
Here, an example will be taken in which the optical feedback medium is arranged to face the semiconductor device leaving a space of at least 10 mm as measured from the diffraction grating to the emitting surface of the semiconductor device. The semiconductor laser device has a multimode emission spectrum, and its device length is 800 &mgr;m. When the semiconductor laser module operates in a compound resonance mode, the compound resonator length is 800 &mgr;m+10 mm (strictly, the length required for reflection by the diffraction grating is included), and the mode distance &Dgr;&lgr;2 is about 0.03 nm.
Here, if the semiconductor laser device
Koyanagi Satoshi
Mugino Akira
Shimizu Takeo
Frishauf, Holtz Goodman, Langer & Chick, P.C.
Leung Quyen
The Furukawa Electric Co. Ltd.
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