JPH09160083A - Light transmission device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To make it possible to correct the refractive index dispersion of an optical fiber without lowering the S/N of converted light by setting the wavelength of pumping light at a wavelength exclusive the wavelength band of signal light and converted light. SOLUTION: The waveform of the signal light is increasingly distorted to the waveforms S1 to S2 by the dispersion of the refractive index of the first optical fiber F1 while the light propagates in the fiber. Since distortion occurs as the propagation speed on the long waveform side is higher and the propagation speed on the short waveform side is lower among the signals transmitting in the optical fiber F1. When the signal light consisting of the waveform S2 distorted in such a manner is made incident on a wavelength conversion element 2 consisting of QPM-DFG, the pumping light, the signal light and the converted light consisting of the inverted waveform S3 are obtd. in accordance with its characteristics. The signal light and the pumping light are removed therefrom by an optical filter 3 and only the converted light consisting of the waveform S3 is sent to the second optical fiber F2 via the optical amplifier 4.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical transmission device for correcting refractive index dispersion of an optical fiber when transmitting signal light through the optical fiber.

[0002]

2. Description of the Related Art FIG. 3 is a block diagram for explaining a conventional optical transmission device, and FIG. 4 is a diagram for explaining conventional wavelength conversion. That is, in the conventional optical transmission device 1 ′, in order to correct the refractive index dispersion of the first and second optical fibers F1 and F2,
A wavelength conversion element 2'comprising a four-wave mixer (FWM) at the midpoint between the first and second optical fibers F1 and F2 having the same length.
Is provided for converting the converted light whose wavelength has been converted by the optical filter 3 to an EDFA (Er Doping Fiber).
It is transmitted to the second optical fiber F2 via the optical amplifier 4 composed of an amplifier.

As shown in FIG. 2, a wavelength converter composed of an FWM is used.
The conversion element 2 ′ has a pump light (frequency ω _p)
Signal light (frequency ω_s), Converted light (frequency ω_c: Ω_c=
2ω _p−ω_s) Or converted light (frequency ω_c’: Ω_c’
= 2ω_s−ω_p) Is an element to convert. Take advantage of this property
The optical transmission device 1'uses the first optical fiber F1
The transmitted signal light is converted into light (frequency ω_c) Or
Converted light (frequency ω_c’) And through the optical filter 3
Converted light (frequency ω_c) Or converted light (frequency ω_c’)
I'm trying to take out one of them.

By the way, the waveform of the signal light transmitted through the first optical fiber F1 is distorted due to the refractive index dispersion as shown by S1 and S2 in FIG. This occurs because the propagation speed on the long wavelength side is high and the propagation speed on the short wavelength side is slow in the signal light transmitted in the first optical fiber F1.

When the signal light having such a distorted waveform S2 is incident on the wavelength conversion element 2'made of FWM, inverted converted light as shown by the waveform S3 'is generated based on the above characteristics. The inverted converted light is converted into the first optical fiber F.
By passing the second optical fiber F2 at a distance substantially the same as that of 1,
This time, the propagation speed on the long wavelength side of the converted light (corresponding to the short wavelength side of the signal light) is high, and the short wavelength side (corresponding to the long wavelength side of the signal light)
The propagation speed of the signal becomes slower and the distortion in the first optical fiber F1 can be canceled (see S4 ′).

[0006]

However, in such an optical transmission device, the frequency ω _p of the pump light in the wavelength conversion element composed of the FWM is the frequency ω _{s of the} signal light or the frequencies ω _c and ω _{c of the} converted light. Very close to (for example, the number n
Since the intensity of the pump light is stronger than that of the signal light and the converted light, it is difficult to completely remove the pump light, the signal light and one of the converted lights by the optical filter. As a result, the S / N of the converted light is lowered, and it becomes impossible to perform sufficient refractive index dispersion correction.

Further, when multi-channel signal transmission is performed, converted light corresponding to the frequency band of each signal light is generated, and it becomes more difficult to extract the converted light by the optical filter.

[0008]

SUMMARY OF THE INVENTION The present invention is an optical transmission device made to solve such problems. That is, the present invention is an optical transmission device that corrects the refractive index dispersion of an optical fiber, and receives a first optical fiber that transmits a predetermined signal light and a signal light that is transmitted by the first optical fiber. A converted wavelength light whose wavelength is inverted with respect to the wavelength of the signal light is generated by the incidence of the pump light having a wavelength outside the wavelength band of the signal light and the converted light, and the signal light, the converted light, and the pump light. The optical filter includes a second optical fiber that receives only the converted light and that transmits the converted light that passes through the optical filter at substantially the same distance as the first optical fiber.

In such an optical transmission device, since the wavelength of the pump light incident on the wavelength conversion element is apart from other than the wavelength band of the signal light and the converted light, the pump light is reliably removed by the optical filter. become able to. Further, it is possible to easily set the wavelength band of the signal light and the wavelength band of the converted light so as not to overlap with each other, so that even in the case of multi-channel signal transmission, only the converted light can be surely passed through the optical filter. Become.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of an optical transmission apparatus of the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram illustrating an embodiment of an optical transmission device of the present invention,
FIG. 2 is a diagram for explaining wavelength conversion. As shown in FIG. 1, the optical transmission device 1 according to the present embodiment includes a first optical fiber F1 that transmits a predetermined signal light and a QPM-DFG (Quasi Ph) arranged at an intermediate point of the optical fiber transmission line.
ass mate difference fr
from the wavelength conversion element 2 composed of the frequency generation, the optical filter 3 that passes only the converted light generated in the wavelength conversion element 2, the optical amplifier 4 composed of the EDFA, and the second optical fiber F2 for transmitting the converted light. It is configured.

As the optical fibers F1 and F2, for example, a single mode optical fiber for a wavelength of 1.55 μm is used.
As the wavelength conversion element 2 made of QPM-DFG, for example, an element in which a waveguide having a grating is formed in LiNbO ₃ (lithium niobate) is used. For example, pump light having a wavelength of 0.775 μm is incident on the wavelength conversion element 2 made of this QPM-DFG to generate converted light whose waveform is inverted with respect to the wavelength of the signal light.

As shown in FIG. 2, this wavelength conversion element 2
Frequency ω _{p of} pump light incident on (see FIG. 1)
Is a value far from the frequency band W of the signal light and the converted light. When the pump light (wavelength λ _p ) having such a frequency is incident on the wavelength conversion element 2 made of QPM-DFG, the signal light (wavelength λ) is satisfied when the following formulas 1 and 2 (QPM condition) are satisfied. _s ) converted light (wavelength λ _c )
Occurs.

[0013]

[Formula 1] ΔkΛ = 2π (1)

Δk = (2π / λ _p ) n _p − (2π / λ _s ) n _s − (2π / λ _c ) n _c (2) where Λ in the equation (1) is the wavelength conversion element 2 period of the grating provided on the waveguide, n _i in (2)
(I = p, s, c) is the refractive index at each wavelength.

In the optical transmission device 1 provided with such a wavelength conversion element 2, the wavelength conversion element 2 is based on the signal light (frequency ω _s1 , ω _s2 ) transmitted through the first optical fiber F1.
The converted light (frequency ω _c1 , ω _c2 ) is generated by, and only the converted light is reliably extracted via the optical filter 3. That is,
Since the wavelengths of the signal light and the pump light are distant from the converted light, the optical filter 3 can extract only the converted light from which the signal light and the pump light are completely removed.

Here, the correction of the refractive index dispersion when the signal light of the frequency ω _s is transmitted from the first optical fiber F1 will be described. As shown in FIG. 1, the waveform of this signal light is distorted into waveforms S1 and S2 due to its refractive index dispersion while being transmitted through the first optical fiber F1. This occurs because the propagation speed on the long wavelength side is high and the propagation speed on the short wavelength side is slow in the signal light transmitted in the first optical fiber F1.

When the signal light having such a distorted waveform S2 is incident on the wavelength conversion element 2 made of QPM-DFG, it is composed of the pump light, the signal light and its inverted waveform S3 based on the characteristics described above. Converted light is obtained. That is, the signal light comprising the frequency omega _s1, omega _s2 as shown in FIG. 2, the frequency omega by the pump light composed of frequency omega _p
c1 (ω _c1 = ω _p − ω _s1 ) and frequency ω _c2 (ω _c2 = ω _p − ω
_s2 ), the waveform S2 shown in FIG. 1 becomes a waveform S3 inverted by the wavelength conversion element 2.

Then, of the signal light, the pump light and the converted light, the signal light and the pump light are removed by the optical filter 3 and only the converted light having the waveform S3 passes through the optical amplifier 4 to the second optical fiber F2. Will be sent.

As shown in FIG. 2, the frequency of the pump light ω _p
Has a value farther than the frequency band W between the signal light and the converted light, so that the optical filter 3 can easily remove the pump light. Further, even when the signal light having the frequency ω _s1 and the signal light having the frequency ω _s2 are transmitted in multiple channels, the frequency band W _s of the signal light and the corresponding frequency bands W _c of the converted light are Since they do not overlap, the optical filter 3 can easily remove the signal light.

That is, it becomes possible to reliably pass only the converted light through the optical filter 3. The converted light having the waveform S3 in which unnecessary light is not mixed is transmitted to the second optical fiber F2. At this time, since the distance between the second optical fiber F2 and the first optical fiber F1 is almost equal, the propagation speed on the long wavelength side of the converted light (corresponding to the short wavelength side of the signal light) is high, and the short wavelength side (signal light is (Corresponding to the long wavelength side of 1) becomes slower, and a signal having a waveform S4 that cancels the distortion in the first optical fiber F1 can be obtained.

Therefore, since only the converted light having the waveform S3 obtained by inverting the waveform S2 of the signal light is reliably transmitted to the second optical fiber F2, the signal having the waveform S4 with the distortion canceled is transmitted through the transmission line. The refractive index dispersion of the optical fiber is accurately corrected.

In the above embodiment, LiNbO _{3 is used.}
The example using the QPM-DFG wavelength conversion element 2 made of (lithium niobate) has been described.
The same applies when an element such as O ₄ is used. Further, the applicable wavelengths of the first optical fiber F1 and the second optical fiber F2 and the wavelength of the pump light input to the wavelength conversion element 2 shown in the embodiment are examples, and the present invention is not limited thereto. .

[0022]

As described above, the optical transmission device of the present invention has the following effects. That is, in the optical transmission device of the present invention, by setting the wavelength of the pump light to a wavelength other than the wavelength band of the signal light and the converted light, the pump light and the signal light are removed by the optical filter and only the converted light is reliably transmitted. become able to. This makes it possible to correct the refractive index dispersion of the optical fiber without reducing the S / N of the converted light. Further, even when the signal light is transmitted through multiple channels, it is possible to surely obtain the converted light corresponding to each channel, and it is possible to correct the refractive index dispersion without lowering the S / N.

[Brief description of the drawings]

FIG. 1 is a block diagram illustrating an embodiment of an optical transmission device of the present invention.

FIG. 2 is a diagram illustrating wavelength conversion of the present invention.

FIG. 3 is a block diagram illustrating a conventional optical transmission device.

FIG. 4 is a diagram illustrating conventional wavelength conversion.

[Description of Reference Signs] 1 optical transmission device 2 wavelength conversion element 3 optical filter 4 optical amplifier F1 first optical fiber F2 second optical fiber

Claims (1)

[Claims] 1. An optical transmission device for correcting the refractive index dispersion of an optical fiber, comprising: a first optical fiber for transmitting a predetermined signal light; and a signal light transmitted by the first optical fiber. A wavelength conversion element that receives the converted light having a waveform inverted with respect to the wavelength of the signal light when the pump light having a wavelength outside the wavelength band of the signal light and the converted light is generated, the signal light, and the conversion light. An optical filter that receives light and the pump light and passes only the converted light of the pump light, and a second optical fiber that transmits the converted light that passes through the optical filter at substantially the same distance as the first optical fiber. An optical transmission device comprising: