Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
1998-04-22
2001-04-10
Chan, Jason (Department: 2633)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S199200, C359S199200, C359S199200, C359S199200
Reexamination Certificate
active
06215572
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical transmission technique in an optical communication system and more particularly, to a wavelength division multiplexing optical transmission apparatus which modulates light outputs of a plurality of light emitting elements whose optical wavelengths are shifted mutually slightly with individual electric signals to obtain light signals, and which combines these light signals into a single light transmission signal to be transmitted onto a single optical fiber cable.
2. Description of the Related Art
Active researches have been conducted on increase in the transmission capacity of an optical fiber communication system. An optical transmitter in an optical communication transmission system generally inputs a digital electric signal to a light intensity modulator to modulate light intensity of a continuous wave (CW) light signal received from a laser light source and to transmit the modulated light signal onto a transmission line. In order to increase the transmission capacity, it is necessary to perform the light intensity modulation at high speed. However, when the above object is tried to be realized only by improvement in driving frequency bands of IC and optical device, its limit is expected. To avoid this limitation, various methods have been suggested. Among them, in particular, a wavelength division multiplexing (WDM) system using a plurality of light sources having respective different oscillation wavelengths is drawing attention as a promising means in future. One of such prior art wavelength division multiplexing systems will be explained below.
FIG. 1
shows an example of arrangement of a prior art wavelength division multiplexing system. In the arrangement, n light sources
11
to
11
n
have different oscillation wavelengths of &lgr;
1
to &lgr;
n
, respectively. To realize stabilization in oscillation wavelengths of the light sources, temperature control of a laser chip and fine adjustment of laser resonators in their intervals are carried out. Light intensity modulators
21
to
2
n
are provided which correspond to light signals of light sources
11
to
11
n
, respectively, to obtain data-modulated output light signals. All of these output light signals are combined by optical coupler
3
, amplified by optical amplifier
5
, and then sent onto a transmission line.
In the wavelength division multiplexing system, the intervals of the oscillation wavelengths of the respective light sources becomes one of important factors. When the light signals received from the respective light sources are different in light powers, a light receiver cannot transmit the light signals correctly. To avoid this, it is necessary to set the light signals within a wavelength region where the amplification factor of the optical amplifiers of the light transmitters is substantially constant; therefore the region of the light signal wavelengths is limited. For the purpose of realizing a wavelength division multiplexing system of a large capacity, it is only required to transmit a multiplicity of light signals having different wavelengths included in the limited light signal wavelength region. However, when the wavelength intervals are too narrow, a nonlinear effect within an optical fiber involves noise generation and a transmission distance must be inevitably set to be short. Further, even when an accurate optical filter is used in the light receiver, it becomes difficult to identify the wavelengths of the light signals, thus disabling the data reception.
For the purpose of make equal the light receiving powers of the respective light signals in the light receiver, there is a pre-emphasis method for providing previously-different powers for the signal light sources to overcome the unequal gain among the optical amplifiers. In this case, it becomes possible to transmit more light signals by enlarging the light signal wavelength range, the wavelength intervals being kept constant. However, this also involves another problem that the unequal gain of the optical amplifier caused by fluctuations of the oscillation wavelength unfavorably affects its output power or that, when the light receiver employs a fixed-type optical filter, this causes deterioration of signal receiving characteristics.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a stable wavelength division multiplexing optical transmission apparatus which, in order to solve the above problems in the prior art, can realize additional stabilization of oscillation wavelengths of light of a plurality of light sources while avoiding overlapping of the wavelengths of the light from the light sources.
In accordance with an aspect of the present invention, the above object is attained by providing a wavelength division multiplexing optical transmission apparatus for modulating light emitted from a plurality of light sources having different oscillation wavelengths with individual electric signals with respect to light intensity, combining these light signals into a single combined light signal and then transmitting the combined light signal onto a transmission line, wherein one of these light sources is used as a reference light source, a voltage value corresponding to the wavelength of the reference light source is always monitored to control the oscillation wavelength of the reference light source to be always constant.
That is, the combined light signal is sent to an optical coupler and then partly branched therefrom into an optical band pass filter. The optical band pass filter has a function of determining a wavelength of a transmitted light signal according to the driving voltage value of a driving electric signal externally supplied thereto, and is able to pass therethrough light signals each having a single wavelength from the combined light signal received from the optical coupler in a predetermined wavelength order of all the wavelengths according to the driving voltage value of the periodically-varying driving electric signal. Next, an optical detector, when received the transmitted light signal (of the single wavelength) from the optical band pass filter, photoelectrically converts the received light signal to a detection pulse and outputs the detection pulse. A driving voltage identifying circuit receives the detection pulse from the optical detector and the external driving electric signal, detects the detection voltage value of the driving electric signal then inputted each time the driving voltage identifying circuit receives the detection pulse, holds and sends the detection voltage value of one specific order for each period of the driving electric signal, and resets the detection voltage value. A control circuit receives the detection voltage value received from the driving voltage identifying circuit for each period and controls an oscillation wavelength of one of the light sources corresponding to a reference voltage value to be constant, one of the detection voltage values for the first period being used as the reference voltage value.
The driving voltage identifying circuit finds a difference between two detection voltage values of the sequentially-detected light signals corresponding to the adjacent wavelengths and externally issues an alarm when the difference is equal to or smaller than a predetermined set value.
In the aspect of the present invention, the driving electric signal is a sawtooth wave voltage signal which periodically varies between 0 and maximum to be associated with all the wavelengths of the light signals of the light sources, a detection voltage value of predetermined order received for the first period is held as the reference voltage value, the first-received detection voltage value corresponding to the reference light source is monitored for the subsequent periods to minimize a deviation from the reference voltage value, thereby controlling the wavelengths of the light sources to be constant.
REFERENCES:
patent: 5680246 (1997-10-01), Takahashi et al.
patent: 5
Chan Jason
NEC Corporation
Ostrolenk Faber Gerb & Soffen, LLP
Sedighian M. R.
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