Wavelength monitoring device and its adjusting method, and...

Coherent light generators – Particular beam control device – Optical output stabilization

Reexamination Certificate

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Reexamination Certificate

active

06567437

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to a wavelength monitoring device for monitoring the wavelength of a laser light, its adjusting method, a wavelength stabilizing light source for emitting a laser light of a stable wavelength and also a transmission system provided with a plurality of such wavelength stabilizing light sources, and more particularly to an industrial technique which is applicable to the method of wavelength division multiplexing transmission (hereinafter referred to just as “WDM”).
2. Description of the Related Art
In the recent trend of optical communications, various transmission methods have been investigated to cope with a vastly increasing amount of information to be transmitted. Among those transmission methods, the above-mentioned WDM method has been developed for increasing transmission capacity by multiplexing various optical signals of different wavelengths and transmitting the thus multiplexed optical signals concurrently. However, even though a plurality of optical signals of different wavelengths are multiplexed and transmitted concurrently, no wavelength outside the band in which it is amplifiable by an amplifier can be used, and thus, in order to multiplex various optical signals and transmit concurrently the thus multiplexed light signals, it will be the most significant requirement to reduce the wavelength of individual optical signals, and also reduce the intervals between the wavelength of individual optical signals. In order to meet this requirement, a technique for monitoring the wavelength of optical signals of a narrow band and stabilizing the thus monitored wavelength in high precision has been required.
(First Prior Art)
FIG. 1
is a schematic diagram showing the wavelength monitoring device according to a first prior art as described in “Convention of Communication Society of 1998”, the transaction of the Institute of Electronics, Communication, and Information Engineers (B-10-180). In the figure, reference numeral
101
denotes a first beam splitter for separating an incident light, numeral
102
denotes a second beam splitter for separating the input light passed through the first beam splitter
101
,
103
denotes a first photo-diode (hereinafter referred to just as “PD”) for receiving one part of the incident light separated by the first beam splitter
101
,
104
denotes a second PD for receiving one part of the input light separated by the second beam splitter
102
,
105
denotes a first Fabry-Perrot Etalon filter (hereinafter referred to just as “FP Etalon Filter”) disposed between the first beam splitter
101
and the first PD
103
,
106
denotes a second FP Etalon filter disposed between the second beam splitter
102
and the second PD
104
. Here, the first FP Etalon filter
105
and the second FP Etalon filter
106
have different wavelength transmission characteristics.
The operation of the device is explained below.
One part of the incident light separated in the first beam splitter
101
is transmitted through the first FP Etalon filter
105
, and is received by the first PD
103
thereafter. Similarly, one part of the input light separated in the second beam splitter
102
is transmitted through the second FP Etalon Filter
106
, and is received by the second PD
104
thereafter.
Since the first Etalon filter
105
and the second Etalon filter
106
have different wavelength transmission characteristics from each other, the signals emit from these first and second PDS
103
and
104
show respectively different wave characteristics from each other. For this reason, as to the difference between the output signal from the first PD
103
and that from the second PD
104
, the strength of the difference signal therebetween becomes zero if the input light is of the wavelength at which the signal strength of the light transmitted through the first Etalon filter
105
and that transmitted through the second Etalon filter
106
are made equal to each other. Due to this, if the wavelength at which the strength of the difference signal becomes zero is made a reference wavelength, the degree of a change in the wavelength of the incident light varied from the reference wavelength can be represented by the strength of the difference signal having either a positive or a negative symbol.
(Second Prior Art)
FIG. 2
is a schematic diagram showing the wavelength monitoring device according to the second prior art, which is disclosed in the U.S. Pat. No. 5,825,792. In the figure, reference numeral
111
denotes a DFB (Distributed Feed Back) semiconductor laser, numeral
112
denotes an optical lens for adjusting the width of the beam spot emitted from the DFB semiconductor laser
111
, numerals
113
and
114
denote respectively a first and a second PDs for receiving lights emitted from the semiconductor laser
111
and transmitted through the optical lens
112
, numeral
115
denotes an FP Etalon filter disposed between the optical lens
112
and both the first and second PDs
113
and
114
, numeral
116
denotes a subtractor for obtaining the difference between the signal output from the first PD
113
and that output from the second PD
114
, and feeds the thus obtained value back to the DFB semiconductor laser
111
. The first and the second PDs are disposed separately from each other within the light transmitted through the FP Etalon filter
115
, and are fixed to a common base
117
. The FP Etalon filter
115
is disposed in an inclined manner with respect to the optical axis.
The operation of the device is explained below.
The width of the beam spot of the light emitted from the DFB semiconductor laser
111
is adjusted at the optical lens
112
, transmitted through the FP Etalon filter
115
, and finally received by the first and second PDs
113
and
114
.
Since the FP Etalon filter
115
is disposed in an inclined manner with respect to the optical axis, an incident angle against the FP Etalon filter
115
varies depending on the position of the incident light beam, and the wavelength transmission characteristic also varies in accordance with the thus varied incident angle. For this reason, the signals output from the first PD
113
and the second PD
114
disposed separately from each other within the light beam transmitted through the FP Etalon filter
115
have respectively different wavelength characteristics. In other words, FP Etalon filters having different wavelength transmission characteristics from each other are not required, but only one FP Etalon filter is required for obtaining two signals having different wavelength transmission characteristics from each other. Due to this, when the first and the PD filters
113
and
114
are disposed in such a manner that the strength of the incident lights thereto are made equal at the reference wavelength &lgr;
0
, the strength of the difference signal from the subtractor
116
becomes 0 at the reference wavelength &lgr;
0
, so that the change in the wavelength of the emitted light from the DFB semiconductor laser
111
varied from the reference wavelenth can be represented by the strength of the difference signal having either a positive or a negative symbol.
The difference signal is fed back to the DFB semiconductor laser
111
, and the wavelength of the light emitted from the DFB semiconductor laser
111
is thus stabilized.
Since the wavelength monitoring device according to the first prior art is configured as mentioned before, wherein two beam splitters, two PDs, and two FP Etalon filters are provided, there has been a problem that the number of components is increased, and thus the total size of the device is thereby increased.
In addition to this, since two beam splitters are used, three light propagating directions are generated, and thus the alignment thereof is quite difficult.
Still further, since the wavelength transmission characteristics of these two FP Etalon filters used therein vary due to a temperature change, there has been such a problem that the wavelength at which the strength of the differen

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