Coherent light generators – Particular resonant cavity – Specified cavity component
Reexamination Certificate
2000-09-27
2002-01-01
Davie, James W. (Department: 2881)
Coherent light generators
Particular resonant cavity
Specified cavity component
C372S020000, C372S032000
Reexamination Certificate
active
06335944
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a semiconductor laser module for emitting laser light, and more specifically, to a semiconductor laser module for exciting an Er
3+
, Al
3+
doped fiber amplifier (EDFA).
BACKGROUND ART
Conventionally, the wavelength of light emitted from a light emitting device is stabilized by using means that optically feeds back light from a multi-mode oscillation laser by any means, thereby making the oscillation wavelength mode of the laser single.
Lasers of this type include, for example, a distributed feedback (DFB) laser, in which a diffraction grating is formed on an active layer of a semiconductor laser device, a distributed Bragg reflector (DBR) laser, in which a reflective diffraction grating having a transparent reflection characteristic for emitted light with a wavelength different from the Bragg wavelength is formed along the longitudinal direction of a waveguide portion that is formed of a semiconductor medium of a semiconductor laser device, whereby the light is reflected on (or fed back to) an active layer, etc.
With the recent rapid progress of a fiber Bragg grating (FBG), in which an optical fiber is given a light diffraction function by changing the refractive index in a core in the axial direction, various techniques for lasers with FBG have been disclosed. For example, “Multi-wavelength Bragg fiber-grating semiconductor laser array” is described in the Technical Report of IEICE (the Institute of Electronics, Information and Communication Engineers) OPE 97-1 by Kato et al., and “UV written waveguide grating and its application to integrated external-cavity laser” is described in OPE97-2 by Tanaka et al.
For a light source for EDFA excitation, moreover, a technique for stabilizing the wavelength of a 1,480 nm band pump laser by means of a fiber Bragg grating (FBG) is reported in 2
nd
Optoelectronics & Communications Conference (OECC '97) Technical Digest, July 1997, Seoul, Korea by Atsushi Hamakawa et al. (Draft Collection 9D2-5, pp. 224-225, “1,480 nm pump fiber-grating external-cavity laser with two fiber gratings”).
The DFB and DBR lasers described in the aforementioned documents, however, have a single-mode oscillation spectrum. They are used for a dedicated light source for optical communication. Therefore, these lasers are not suited for EDFA amplification.
The external-cavity lasers described in the Technical Report by Kato et al. and Tanaka et al. have the following problems.
(1) The aforesaid external-cavity lasers are single-mode oscillation lasers, in which mode hopping is caused such that the center wavelength of the oscillation mode changes when the working temperature changes by several degrees ° C., so that the stability of the oscillation wavelength to resist temperature change is poor.
(2) The aforesaid external-cavity lasers are hard to manufacture, since the reflectance of an anti-reflection film on its front end face should be lowered extremely. In many cases, moreover, the external-cavity lasers are constructed so that laser beams are emitted at an angle to the cleavage plane of the laser in order to lower the reflectance of the anti-reflection film equivalently. According to the external-cavity lasers constructed in this manner, therefore, the coupling efficiency of the laser beams coupled to an optical fiber is lowered, and it is very hard to fix the optical fiber with center alignment.
(3) The aforesaid external-cavity lasers are designed so that the space between the emission surface of a laser device that constitutes an external cavity and a diffraction grating for optical feedback is narrow. Accordingly, assembling a module requires complicated processes, and therefore, use of special means such as plane mounting.
(4) Output power of the aforesaid external-cavity lasers is low. Therefore, they are unfit for use as light sources for EDFA excitation.
Although the technique reported by Atsushi Hamakawa et al. is suited for the excitation of a high-power EDFA of 1,480 nm wavelength that uses an InGaAsP/InP-based semiconductor film, it has the following problems.
(1) The aforesaid technique requires use of a special fiber Bragg grating that reflects two wavelengths.
(2) According to the aforesaid technique, the variation of the oscillation wavelength attributable to change of the working temperature, although small, is about 2.6 nm.
(3) The aforesaid technique cannot be applied to a GaAs/AlGaAs-based semiconductor laser with the same construction. The InGaAsP/InP semiconductor laser is not subject to degradation that is attributable to catastrophic optical damage (COD), and the mode field pattern of its emitted light is circular. On the other hand, the GaAs/AlGaAs-based semiconductor laser is subject to the COD-induced degradation and its mode field pattern is elliptic, so that the efficiency of mode coupling with an optical fiber based on a two-lens system is very poor.
(4) In a GaAs/AlGaAs-based multi-mode oscillation semiconductor laser having random wavelength characteristics without gain ripples, the oscillation wavelength can be stabilized by modularization such that the fiber Bragg grating (FBG) is coupled to the laser. In this semiconductor laser, the state of polarization of the optical fiber continually changes depending on the bent state of the externally attached FBG, stress acting thereon, distortion, etc. In a module that uses this semiconductor laser, therefore, a light touch on the optical fiber inevitably changes the value of the monitor output current by a figure, although the optical output and the monitor output current are apparently stable. Thus, the operating characteristics of the semiconductor laser module of this type considerably changes depending on the state of polarization, fiber shape, etc. of the optical fiber.
(5) In the semiconductor laser module coupled with the fiber Bragg grating, furthermore, mode competition between light oscillated in a mode of the FBG and light oscillated in the Fabry Perot (FP) mode causes fluctuation of the optical output power on the front end face of the semiconductor laser, which is attributable to mode hopping, and fluctuation of the optical output power on the rear end face that is used to monitor the optical output.
In this case, in particular, power fluctuation on the monitor side of the rear end face of the semiconductor laser (optical output on the rear end face side is converted into current by means of a photodiode in the semiconductor laser module) or fluctuation of the monitor output current is more susceptible than that on the side of the front end face of the semiconductor laser.
The semiconductor laser uses the monitor output current to control the optical output automatically. If mode hopping or mode competition occurs in the semiconductor laser, in this case, the monitor output current changes rectangularly or in the shape of spikes with time, so that automatic control is impossible. If the monitor output current of the semiconductor laser is reduced rectangularly, moreover, high current flows into the semiconductor laser in order to maintain the constant optical output, possibly damaging the semiconductor laser and arousing a great problem on the operational reliability.
The present invention has been contrived in consideration of these circumstances, and its object is to provide a semiconductor laser module, which enjoys high oscillation wavelength stability against operating current injected into a semiconductor laser and temperature change, and is suited for use as a light source for EDFA excitation or a high-output, low-noise light source.
DISCLOSURE OF THE INVENTION
A semiconductor laser module according to the present invention, which includes the following means, can restrict fluctuation of optical outputs or fluctuation of monitor output current, which is caused in case of mode hopping or mode competition, within a practically negligible range, in consideration of the relation between a reflection center wavelength &lgr;BG of a Bragg grating and a gain peak wavelength &lgr;LD(I) for an operating curr
Mugino Akira
Shimizu Takeo
Davie James W.
The Furukawa Elctric Co., LTD
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