JPH04269726A - Light level equalizing method - Google Patents

Abstract

PURPOSE:To improve the characteristics of a light wavelength multiplex transmission system or light frequency multiplex transmission system which uses optical amplifiers. CONSTITUTION:The optical amplifiers 1, 1... are provided on the transmission line from an optical multiplex signal transmitter 2 to an optical receiver 3 and a birefringence type light wavelength filter 4 is connected to the final stage among those optical amplifiers. Plural wavelength light beams transmitted from the optical multiplex signal transmitter 2 are transmitted to the optical amplifiers 1, 1... one after another. The multiplex light which differs in light level is passed through the birefringence type light wavelength filter 4 so as to flatten the inclination of the light level.

Description

Detailed Description of the Invention [0001] The present invention relates to an optical wavelength division multiplexing transmission system or an optical frequency division multiplexing transmission system using an optical amplifier.
The present invention relates to a light level equalization method that improves the characteristics thereof. [0002] Optical wavelength division multiplexing transmission system (or optical frequency division multiplexing transmission system) is a system that transmits multiple wavelength lights (or frequency lights) each carrying a signal through a single optical fiber. be. In optical wavelength division multiplexing transmission systems, optical amplifiers are useful components because they can amplify multiple wavelength lights at once. One of the problems when using an optical amplifier in an optical wavelength division multiplexing transmission system is that the wavelength characteristic of its gain is not flat. That is, since the signal gain differs depending on each signal wavelength, the optical level at the receiving end differs depending on the signal wavelength. This effect becomes greater as optical amplifiers are connected in multiple stages. The fact that the optical level differs depending on the signal wavelength is a major problem in the design of the receiving system. Therefore, an optical level equalization technique is needed to flatten the unbalanced optical level caused by the optical amplifier. As a method of compensating for the unbalance of the gain of an optical amplifier, an example of flattening the wavelength characteristics of an optical fiber amplifier using a grating type optical filter is reported in the following literature. "Literature" M. Tachibana,
R. I. Laming, P. R. Morkel, a.
ndD. N. Payne, “Gain-shaped
Eribium-doped fiber amplifier
fire with broad spectrum
“bandwidth”, Technical Diges
on Optical Amplifiers a
nd Their Applications, pp.
．． 44-47, 1990 [0005] FIG. 8 shows an example of compensation published in this document. The thick solid line is the gain wavelength characteristic of a single optical fiber amplifier, which is the subject of this report. The optical amplifier of this example has a 3 dB band peak of 4.5 nm near 1535 nm, and exhibits flat wavelength characteristics over a longer wavelength side of 20 nm. On the other hand, the wavelength characteristic when a fiber grating type optical filter having a blocking wavelength of 1535 nm is connected after the optical amplifier is shown by a thin solid line. Furthermore, the broken line shows the characteristics when the same filter is incorporated in the middle of the optical amplifier. A grating type optical filter exhibits a steep blocking characteristic at a specific wavelength, and has a transmission characteristic of transmitting other wavelengths with substantially no loss. Therefore, by connecting a grating type optical filter whose transmittance is set to be low in the vicinity of 1535 nm, the gain peak of the optical amplifier in the vicinity of 1535 nm is suppressed, and an overall flat wavelength characteristic is obtained. [0006] However, the conventional example has the following drawbacks. In other words, it is applicable only to optical amplifiers that have a peak at a certain wavelength and flat characteristics at other wavelengths. As mentioned above, grating-type optical filters exhibit steep blocking characteristics at specific wavelengths, and have transmission characteristics that transmit other wavelengths with almost no loss, so gain compensation is possible using this. , is limited to optical amplifiers having wavelength characteristics as shown by the thick solid line in FIG. In addition, it is generally difficult to arbitrarily control the transmission characteristics of grating-type optical filters, so it is possible to adjust the transmission characteristics when the optical level imbalance varies depending on the system due to multi-stage amplification or variations in the characteristics of each amplifier. difficult to do. The present invention effectively solves the above-mentioned problems and is applicable to optical multiplex transmission systems using general optical amplifiers, not limited to optical amplifiers with specific gain wavelength characteristics. The purpose of the present invention is to provide a light level equalization method that makes it possible to compensate for. Means for Solving the Problems The present invention provides an optical multiplexing transmission system using an optical amplifier, in which a birefringent optical wavelength filter whose transmission characteristics can be adjusted is connected in the transmission path, and the birefringent optical wavelength filter is It is characterized by flattening the light level of each channel at the receiving end by adjusting the transmission characteristics of the shaped optical wavelength filter. [Operation] According to the present invention, it is possible to flatten the optical level difference between each wavelength caused by the wavelength characteristics of an optical amplifier, and it is effective in optical wavelength division multiplexing transmission system or optical frequency division multiplexing transmission system. . Embodiment FIG. 1 is a block diagram of an optical multiplex transmission system for explaining an embodiment of the present invention. In the figure, reference numeral 2 is an optical multiplex signal transmitter, and reference numeral 3 is a receiver, and a plurality of optical amplifiers 1, 1, . . . are provided on the transmission path from the optical multiplex signal transmitter 1 to the receiver 2. It is being A birefringent optical wavelength filter 4 is connected to the last stage of these optical amplifiers 1, 1, . . . . Incidentally, if the birefringent optical wavelength filter 4 is not provided, the configuration will be a normal optical multiplex transmission system. Further, as the optical amplifier 1, an erbium-doped optical fiber amplifier is used. Next, using such an optical multiplex transmission system,
A method for equalizing light levels will be explained. A plurality of wavelength lights transmitted from the optical multiplex signal transmitting device 2 are transmitted to erbium-doped optical fiber amplifiers 1, 1, . . . connected in multiple stages.
are transmitted one after another. Generally, the erbium-doped optical fiber amplifier 1 does not have a flat gain wavelength characteristic, and therefore the optical level of each channel at the receiving end has different values. For example, an optical fiber amplifier 1 in which aluminum and erbium are co-doped into a silica base material exhibits gain wavelength characteristics as shown in FIG. When this is used as the optical amplifier 1 in the optical multiplexing transmission system shown in FIG. A maximum optical level difference of 6 dB occurs per stage. This level difference is not necessarily constant because the gain wavelength characteristics of the optical amplifier 1 differ depending on the pumping light and fiber length, and the optical level difference at the receiving end is added each time the number of stages of the optical amplifier 1 is increased. Therefore, the value depends on the number of stages of the optical amplifier and the characteristics of each stage. In many cases, within a certain wavelength range, the light level of each channel is considered to rise or fall with respect to the wavelength, as shown in FIG. however,
The slope differs depending on the number of stages of the optical amplifier and the characteristics of each stage. [0013] Such a difference in the optical level of each channel is undesirable in terms of the design of the receiving device, and it is necessary to flatten this difference. Moreover, it is desirable to be able to variably handle the difference in slope due to the number of stages of the optical amplifier 1 and the characteristics of each stage. In order to flatten the slope of this light level, in this embodiment, multiple lights with different light levels are transmitted to the optical amplifier 1.
, 1... is connected to the last stage of the birefringent optical wavelength filter 4.
Transmit to. [0015] Here, the birefringent optical wavelength filter will be explained. The birefringent optical wavelength filter has a structure as shown in FIG. 4, and basically includes polarization beam splitters 5, 6, 1/2 wavelength plates 7a, 7b, 9a, 9b, and birefringent medium 8a. ,8b,. The polarization beam splitter 5 divides input light having a general polarization state into two orthogonal linearly polarized components and outputs them. The output linearly polarized light passes through a 1/2 wavelength plate and is input into a birefringent medium. The birefringent medium is
It has two principal axes with different refractive indices. Here, the input linearly polarized light is input at a certain angle with respect to the principal axis of the birefringent medium, and furthermore, this angle is 1/2
It can be arbitrarily set using a wave plate. (A 1/2 wavelength plate is an optical element that, when linearly polarized light is input, outputs the linearly polarized light while changing its inclination angle while maintaining the linearly polarized state. Next, the input light to the birefringent medium is divided into two polarized components in the principal axis direction and propagates through the birefringent medium, each experiencing a different refractive index. Therefore, different propagation phases are added to these two polarized components. For example, assume that the relationship between the principal axis and the input linearly polarized light in the birefringent medium 8a is as shown in FIG. Here, x and y are the two main axis directions of the birefringent medium 8a,
θ represents the angle between the input light and the principal axis, and Ei represents the amplitude of the input light. At this time, the optical amplitude components Ex(0) and Ey(0) in the principal axis direction at the input end of the birefringent medium 8a are Ex(0)=E
icos (θ) Ey(0)=Eisin (θ). When each of these components passes through the birefringent index medium 8a, a propagation phase term is added to each optical electric field, and the optical electric fields Ex(L) and Ey(L) at the output end are as follows: Ex(L)=Eicos (θ )exp(jnx 2π
fL/c)Ey(L)=Eisin(θ)exp(j
ny 2πfL/c). Here, f is the optical frequency,
c is the speed of light, L is the length of the birefringent medium 8a, nx, ny
are the refractive indices in the x and y directions, respectively. The output from the birefringent medium 8a passes through a half-wave plate 9a and is input to the polarization beam splitter 6. Here, it is assumed that the polarization beam splitter 6 is set so that light having a polarization component tilted by θ with respect to the principal axis direction of the birefringent medium 8a is transmitted from the terminal a to the terminal c. This θ can be set using the 1/2 wavelength plate 9a. This relationship is assumed to be the same as that shown in FIG. Then, from terminal a to terminal c, Ex(L), Ey(L)
Of these, the component projected in the direction tilted θ from the principal axis is output, so the output light Ea from terminal a to terminal c
becomes as follows. Ea=Ex(L)cos(θ)+Ey(L)s
in(θ) = Ei{cos2(θ)e
xp(jnx 2πfL/c)
+sin2(
θ)exp(jny 2πfL/c)} This expression is for amplitude, and when converted to optical power, |Ea|2 = |Ei|2 {cos4(θ)
+sin4(θ)
+2cos2(θ)sin2(θ)cos(
Δn2πfL/c)}. However, Δn=nx −n
I set it as y. Looking at the above equation, the polarization beam splitter 6
It can be seen that the output has a cosine-like output characteristic with respect to the optical frequency. The period is expressed as c/ΔnL and can be freely set by adjusting the optical length of the birefringent medium. Further, the peak frequency can also be set by fine adjustment of L or Δn. Further, the maximum value is |Ei|2 and the minimum value is |Ei|2 cos2 (2θ), indicating that the maximum-minimum transmission ratio can be adjusted by θ. The output characteristics of the light that passes through the birefringent medium 8a and is output has been described above. The same applies to light passing through the birefringent medium 8a. In this case, by replacing θ in the above explanation with θ+π/4 and proceeding with the calculation, the same expression as the above equation can be obtained. However, at this time, the light output from terminal b of polarization beam splitter 6 to terminal c is
The output light from to terminal c has a polarization state that is orthogonal to that of the output light from terminal c. As explained above, in the birefringent optical wavelength filter having the configuration shown in FIG. The transmission ratio can be variably adjusted by adjusting the optical length of the birefringent medium and the input polarization angle. The optical length can be adjusted, for example, by applied voltage and temperature when an electro-optic crystal is used as the birefringent medium, and by pressure and temperature when a birefringent fiber is used. Note that the birefringent optical wavelength filter having the configuration shown in FIG. 4 is configured so that whatever polarization state the input light has, it passes through either the birefringent medium 8a or b.
It should be noted that there is no input polarization dependence. By transmitting the light through such a birefringent light wavelength filter 4, the light level of the multiplexed light is equalized. The slope portion of the cosine-shaped transmission characteristic of this birefringent optical wavelength filter is used to equalize the light level. Specifically, for example, assume that the optical level of multiplexed light that has been amplified in multiple stages is as shown in FIG. On the other hand, the transmission characteristics of the birefringent optical wavelength filter 4 are adjusted so as to be opposite to those shown in FIG. 6, as shown in FIG. By adjusting the peak wavelength and maximum/minimum transmission ratio of the birefringent optical wavelength filter 4, it is possible to obtain transmission characteristics opposite to the original light level for any level of inclination. It is. If the birefringent optical wavelength filter 4 having the characteristics shown in FIG. 7 is connected after the final stage optical amplifier 1, the light level of each channel transmitted through the birefringent optical wavelength filter 4 is flattened. This optical level equalization method adjusts the characteristics of the birefringent optical wavelength filter 4 even if the slope of the original optical level differs due to the number of stages of the optical amplifiers 1, 1... and the characteristics of each stage. This makes it possible to respond variably. In this embodiment, as shown in FIG.
Although a multiplex transmission system in which one birefringent optical wavelength filter 4 is inserted between the optical amplifier 1 of the final stage of light and the receiving device 3 is used, the present invention is not limited to this. An optical multiplexing transmission system is created in which birefringent optical wavelength filters 4, 4... are inserted for each amplifier 1, 1..., and using this, the optical amplifiers 1, 1...
The light level may be equalized every 1... [0024] As explained above, according to the present invention,
It is possible to flatten the optical level difference between each wavelength caused by the wavelength characteristics of the optical amplifier, and it is highly effective in optical wavelength division multiplexing transmission systems or optical frequency division multiplexing transmission systems.

[Brief explanation of the drawing]

FIG. 1 is an explanatory diagram of an embodiment of the present invention.

FIG. 2 is a gain wavelength characteristic of a silica base material erbium/aluminum co-doped fiber amplifier.

FIG. 3 is an optical level of an amplified multiplexed signal.

FIG. 4 is a basic configuration diagram of a birefringent optical wavelength filter.

FIG. 5 is a diagram showing the relationship between the principal axis direction of a birefringent medium and the polarization direction of input light in a birefringent optical wavelength filter.

FIG. 6 is a diagram for explaining optical level equalization by a birefringent optical wavelength filter.

FIG. 7 is a diagram for explaining optical level equalization by a birefringent optical wavelength filter.

FIG. 8 is a diagram showing a conventional example of an amplification gain compensation method.

[Explanation of symbols]

1 Erbium-doped optical fiber amplifier (optical amplifier) 2
Multiplexed signal optical transmitter (optical transmitter) 3 Optical receiver 4 Birefringent optical wavelength filters 5, 6 Polarization beam splitters 7a, 7b, 9a, 9b 1/2 wavelength plates 8a, 8b
birefringent medium

Claims (1)

[Claims] 1. In an optical wavelength division multiplexing transmission system or optical frequency division multiplexing transmission system using an optical amplifier on a transmission path from an optical transmitter to an optical receiver, a birefringent optical wavelength filter with variable transmission characteristics is provided on the transmission path. By arranging one or more birefringent optical wavelength filters to transmit multiple light, and adjusting the transmission characteristics of the birefringent optical wavelength filter, the optical level of each wavelength or frequency at the receiving end is smoothed. A light level equalization method characterized by: