Wavelength multiplexing light source

Details Wavelength multiplexing light source Wavelength multiplexing light source

1999-03-04

2001-08-28

Arroyo, Teresa M. (Department: 2881)

Coherent light generators

Particular beam control device

Tuning

C372S018000, C372S043010, C372S075000, C372S097000

Reexamination Certificate

active

06282214

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a wavelength multiplexing light source used in an optical wavelength division multiplexing (WDM) transmission system or the like.
In the existing final optical fiber communication system, a WDM transmission method by which a large capacity of information is put into a single optical fiber for the transmission has become mainstream.
This is achieved by a wavelength multiplexing light source which realizes a large capacity of optical transmission, as shown in
FIG. 12
, by carrying information on laser lights respectively having different wavelengths &lgr;
1
, &lgr;
2
, &lgr;
3
, . . . etc. and outputted from semiconductor lasers LD
1
, LD
2
, LD
3
, . . . etc. as light sources for modulators MOD
1
, MOD
2
, MOD
3
, . . . etc., and by coupling the laser lights at an optical coupler OS. Accordingly, without forcibly increasing a modulation rate (bit rate: b/s) of each of the light sources (channels) LD
1
, LD
2
, LD
3
, . . . etc., a large capacity of transmission (several hundred gigabits/second) can be realized comparatively easily.
On the other hand, in an advanced informationalized society, there is no limits in a demand for an enlargement of capacity and therefore it is the present situation that makers all over the world have been competing desperately for the developments. In order to realize a further enlargement of capacity by e.g. the WDM method, it is necessary only to increase the number of wavelength (channel) to be transmitted. For this reason, such an idea can be hit that as many wavelengths as possible are transmitted by narrowing a wavelength interval between the light sources.
However, in the existing optical fiber communication system, an Er-doped optical fiber amplifier having an amplification band in a wavelength of 1.55 &mgr;m has been put into practice, to which band all optical signals have to be confined. Accordingly, it is required to narrow the wavelength interval of each light source and arrange the same in a high density within the wavelength band of 1.55 &mgr;m.
Presently, the wavelength interval standardized by the International Telecommunication Union is 100 GHz (≈0.8nm) in frequency, and is scheduled to be narrowed up to 50GHz. This is only 0.03% of the oscillation frequency (1.55 &mgr;m ≈193.55THz) of a semiconductor laser as a light source, requiring an oscillation wavelength (frequency) control with an extremely high stability.
2. Description of the Related Art
To control such an oscillation wavelength, a conventional stabilization has been attempted under a temperature control of a light source and an injection current control. A general arrangement for such a stabilization is shown in
FIG. 13. A
part of the output light of a semiconductor laser LD is divided by a beam splitter BS, and its oscillation wavelength is monitored by an optical device MD. After the oscillation wavelength is converted into an electrical signal by a photo detector PD and an electrical signal processing is then performed to a deviation from a reference wavelength at a signal processor SP, current generated by a Peltie element PE combined with the semiconductor laser LD is fed back and controlled by a temperature controller TC.
The control accuracy for the oscillation wavelength by such a wavelength controller depends on; (1) the stability of the device which monitors the oscillation wavelength; (2) the accuracy of the electrical signal processing; and (3) the stability of the temperature controller. Furthermore, these controls are required to be executed per each light source. If such a high density wavelength multiplexing art is further advanced in the future, the number of the wavelength controller has to be increased accordingly and the control accuracy has to be improved, a very difficult situation being expected.
SUMMARY OF THE INVENTION
It is accordingly an object of the present invention to provide a light source which enables a wavelength multiplexing without enhancing the number of a wavelength controller and a control accuracy.
As a method for controlling an oscillation wavelength (frequency) of a semiconductor laser, a phenomenon which is generally called an “injection locking” is known. This method is based on that a coherent light is injected into the semiconductor laser from the outside thereof to lock the oscillation wavelength of the laser. A general arrangement of such an injection locking is shown in FIG.
14
.
Namely, an output light from a semiconductor laser (master laser: oscillation frequency f
M
) ML of single longitudinal mode whose narrow line width and oscillation frequency (wavelength) are stably controlled is injected into another semiconductor laser (slave laser: oscillation frequency f
S
) SL of the same mode having the oscillation wavelength adjacent to that of the master laser through an optical isolator OI. As a result, an oscillation frequency of the slave laser SL is pulled in that of the master laser ML, thereby stabilizing the oscillation frequency, narrowing the line width, and suppressing the side mode if the slave laser SL lases in multi-longitudinal mode. It is to be noted that generally for the occurrence of the pull-in the oscillation frequencies of two lasers must be within a close range (within several hundred MHz-several GHz) which is called a “pull-in range”.
In a wavelength multiplexing light source according to the present invention, it is paid attention to the fact that a modulation side band of an arbitrary frequency is generated in the master laser with the light injection, and that by pulling the oscillation frequency of the slave laser which is within the “pull-in range” of the side band component in the same side band these two lasers having an oscillation frequency interval (|fM-fS |) out of the “pull-in range” generate the injection locking.
Hereinafter, the principle (claim
1
) of the present invention will be described referring to FIG.
1
. As shown in
FIG. 1A
, at least three semiconductor lasers LD
1
-LD
3
oscillated in the single longitudinal mode are used as light sources. An output light {circumflex over (
1
)} of a first semiconductor laser LD
1
(oscillation frequency:&ohgr;
1
) as an injection laser is injected in one way into an active layer of a second semiconductor laser LD
2
(oscillation frequency:&ohgr;
2
) as the master laser through a first one-way optical injection means.
At this time, because of a non-linear characteristic in the semiconductor laser LD
2
, as shown in
FIG. 1B
, a light whose frequency is &ohgr;
FWM
=2&ohgr;
2
−&ohgr;
1
which corresponds to the modulation side band of the semiconductor laser LD
2
is generated. This phenomenon is generally known as a Four Wave Mixing (hereinafter abbreviated as FWM), and can be easily generated in a wide range whose frequency interval |&ohgr;
2
−&ohgr;
1
| is from several GHz to several THz.
Next, an output light {circumflex over (
2
)} of the semiconductor laser LD
2
is injected in one way into a third semiconductor laser LD
3
(oscillation frequency:&ohgr;
3
) as the slave laser through a second one-way optical injection means. At this time, if the difference between the frequencies &ohgr;
3
and &ohgr;
FWM
is within the pull-in range of not much more than several GHz, the injection locking takes place so that as shown in the spectrum of the injection light {circumflex over (
2
)}, the frequency &ohgr;
3
is pulled in the frequency &ohgr;
FWM
. Accordingly, the oscillation frequency of each semiconductor laser can be isolated out of the “pull-in range” at a normal injection locking without the modulation side band.
As a result, the oscillation frequency difference (&ohgr;
3
−&ohgr;
2
)=(&ohgr;
FWM−
&ohgr;
2
) between the semiconductor laser LD
3
and the semiconductor laser LD
2
is equal to the frequency interval (&ohgr;
2
−&ohgr;
1
) as shown in the spectrum of an injection light {circumflex over (
3
)}. Accordingly, if only the oscillation frequencies of the semiconduc

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