Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
2001-10-23
2003-09-02
Ip, Paul (Department: 2828)
Coherent light generators
Particular active media
Semiconductor
C372S046012
Reexamination Certificate
active
06614823
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor laser device for use in a semiconductor laser module suitable as an excitation light source for a Raman amplification system, and more particularly to a semiconductor laser device having a diffraction grating on a light reflection side.
2. Discussion of the Background
With the proliferation of multimedia features on the Internet in the recent years, there has arisen a demand for larger data transmission capacity for optical communication systems. Conventional optical communication systems transmitted data on a single optical fiber at a single wavelength of 1310 nm or 1550 nm, which have reduced light absorption properties for optical fibers. However, in order to increase the data transmission capacity of such single fiber systems, it was necessary to increase the number of optical fibers laid on a transmission route which resulted in an undesirable increase in costs.
In view of this, there has recently been developed wavelength division multiplexing (WDM) optical communications systems such as the dense wavelength division multiplexing (DWDM) system wherein a plurality of optical signals of different wavelengths can be transmitted simultaneously through a single optical fiber. These systems generally use an Erbium Doped Fiber Amplifier (EDFA) to amplify the data light signals as required for long transmission distances. WDM systems using EDFA initially operated in the 1550 nm band which is the operating band of the Erbium Doped fiber Amplifier and the band at which gain flattening can be easily achieved. While use of WDM communication systems using the EDFA has recently expanded to the small gain coefficient band of 1580 nm, there has nevertheless been an increasing interest in an optical amplifier that operates outside the EDFA band because the low loss band of an optical fiber is wider than a band that can be amplified by the EDFA; a Raman amplifier is one such optical amplifier.
In a Raman amplifier system, a strong pumping light beam is pumped into an optical transmission line carrying an optical data signal. As is known to one of ordinary skill in the art, a Raman scattering effect causes a gain for optical signals having a frequency approximately 13 THz smaller than the frequency of the pumping beam. Where the data signal on the optical transmission line has this longer wavelength, the data signal is amplified. Thus, unlike an EDFA where a gain wavelength band is determined by the energy level of an Erbium ion, a Raman amplifier has a gain wavelength band that is determined by a wavelength of the pumping beam and, therefore, can amplify an arbitrary wavelength band by selecting a pumping light wavelength. Consequently, light signals within the entire low loss band of an optical fiber can be amplified with the WDM communication system using the Raman amplifier and the number of channels of signal light beams can be increased as compared with the communication system using the EDFA.
Although the Raman amplifier amplifies signals over a wide wavelength band, the gain of a Raman amplifier is relatively small and, therefore, it is preferable to use a high output laser device as a pumping source. However, merely increasing the output power of a single mode pumping source leads to undesirable stimulated Brillouin scattering and increased noises at high peak power values. Therefore, the Raman amplifier requires a pumping source laser beam having a plurality of oscillating longitudinal modes. As seen in
FIGS. 15A and 15B
, stimulated Brillouin scattering has a threshold value P
th
at which the stimulated Brillouin scattering is generated. For a pumping source having a single longitudinal mode as in the oscillation wavelength spectrum of
FIG. 15A
, the high output requirement of a Raman amplifier, for example 300 mW, causes the peak output power of the single mode to be higher than P
th
thereby generating undesirable stimulated Brillouin scattering. On the other hand, a pumping source having multiple longitudinal modes distributes the output power over a plurality of modes each having a relatively low peak value. Therefore, as seen in
FIG. 15B
, a multiple longitudinal mode pumping source having the required 300 mW output power can be acquired within the threshold value P
th
thereby eliminating the stimulated Brillouin scattering problem and providing a larger Raman gain.
In addition to the multiple longitudinal modes required for a pump laser in a Raman amplification system, the present inventors have recognized that it is desirable that each of the longitudinal modes has substantially the same threshold gain in order to obtain the stable multi-mode oscillation.
In addition, because the amplification process in a Raman amplifier is quick to occur, when a pumping light intensity is unstable, a Raman gain is also unstable. These fluctuations in the Raman gain result in fluctuations in the intensity of an amplified signal which is undesirable for data communications. Therefore, in addition to providing multiple longitudinal modes, the pumping light source of a Raman amplifier must have relatively stable intensity.
Moreover, Raman amplification in the Raman amplifier occurs only for a component of signal light having the same polarization as a pumping light. That is, in the Raman amplification, since an amplification gain has dependency on a polarization, it is necessary to minimize an influence caused by the difference between a polarization of the signal light beam and that of a pumping light beam. While a backward pumping method causes no polarization problem because the difference in polarization state between the signal light and the counter-propagating pumping light is averaged during transmission, a forward pumping method has a strong dependency on a polarization of pumping light because the difference in polarization between the two co-propagating waves is preserved during transmission. Therefore, where a forward pumping method is used, the dependency of Raman gain on a polarization of pumping light must be minimized by polarization-multiplexing of pumping light beams, depolarization, and other techniques for minimizing the degree of polarization (DOP). In this regard it is known that the multiple longitudinal modes provided by the pumping light source help to provide this minimum degree of polarization.
FIG. 16
is a block diagram illustrating a configuration of the conventional Raman amplifier used in a WDM communication system. In
FIG. 16
, semiconductor laser modules
182
a
through
182
d
, include paired Fabry-Pérot type semiconductor light-emitting elements
180
a
through
180
d
having fiber gratings
181
a
through
181
d
respectively. The laser modules
182
a
and
182
b
output laser beams having the same wavelength via polarization maintaining fiber
71
to polarization-multiplexing coupler
61
a
. Similarly, the laser modules
182
c
and
182
d
output laser beams having the same wavelength via polarization maintaining fiber
71
to polarization-multiplexing coupler
61
b
. Each polarization maintaining fiber
71
constitutes a single thread optical fiber which has a fiber grating
181
a
-
181
d
inscribed on the fiber. The polarization-multiplexing couplers
61
a
and
61
b
respectively output the polarization-multiplexed laser beams to a WDM coupler
62
. These laser beams outputted from the polarization-multiplexing couplers
61
a
and
61
b
have different wavelengths.
The WDM coupler
62
multiplexes the laser beams outputted from the polarization-multiplexing couplers
61
a
and
61
b
, and outputs the multiplexed light beams as a pumping light beam to external isolator
60
, which outputs the beam to amplifying fiber
64
via WDM coupler
65
. Signal light beams to be amplified are input to amplifying fiber
64
from signal light inputting fiber
69
via polarization-independent isolator
63
. The amplified signal light beams are Raman-amplified by being multiplexed with the pumping light beams and input to a monitor light branching coupler
67
Funabashi Masaki
Tsukiji Naoki
Yoshida Junji
Ip Paul
Nguyen Tuan
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
The Furukawa Electric Co. Ltd.
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