JPS6242245B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an equalization method for canceling delay time differences that occur when light with a spread wavelength spectrum propagates through an optical fiber with wavelength dispersion in optical fiber transmission.

The transmission band of single mode optical fibers is mainly limited by chromatic dispersion. Optical fiber has a wavelength of 1.1 to 1.6
Since the loss is low in μm, it is desirable to remove chromatic dispersion in this wavelength range. The wavelength at which wavelength dispersion becomes zero can be controlled to some extent by changing the core diameter and relative refractive index difference of the fiber. However, it is difficult to control this with high precision, and connection loss also increases. Conventionally, wavelength dispersion is equalized outside the optical fiber by using the demultiplexing characteristics of a diffraction grating or birefringence filter, dividing the wavelength range of the light source into several, and applying a different delay to each wavelength. After that, there was something to join. Since these devices are large-scale and have large insertion losses, the inventor first devised an equalizer that utilized the wavelength-selective reflection characteristics of the grating in the waveguide (Japanese Patent Application No. 1983-
67061, patent application No. 115840).

Figure 1 shows the structure of the equalizer. This equalizer consists of a core 1 and a cladding 2 that constitute a waveguide, and the waveguide 1 is provided with gratings 4, 5, and 6, and the periods of the gratings 4, 5, and 6 are equal to the wavelength λ ₁ , It is selected to selectively reflect λ ₂ and λ ₃ . Light 7 from a light source with mixed wavelengths λ ₁ , λ ₂ , λ ₃ is incident on one end of the waveguide 1 through an optical circulator 3 , and the light 7 with wavelengths λ ₁ , λ 2 , λ ₃ is input to one end of the waveguide 1 .
Each component of λ ₃ is reflected with a different delay depending on the position of the gratings 4, 5, and 6, returns to the optical circulator 3, and is guided to the outside as an output light 8. Therefore, by providing the gratings 4, 5, and 6 at positions that cancel the amount of delay determined by the wavelengths λ ₁ , λ ₂ , and λ ₃ of the light source and the chromatic dispersion value of the transmission optical fiber, the chromatic dispersion is equalized. be able to.

The various equalizers described above are designed assuming a light source with a fixed wavelength range and wavelength interval, and therefore have poor adaptability to the type of light source or changes over time. In particular, it was difficult to create a structure that could accommodate variations in the wavelength of the light source, and it was also difficult to make fine adjustments during installation.

In order to eliminate these drawbacks, this invention equalizes the delay time by inverting the spectrum of transmitted light, and in particular provides a wavelength dispersion equalization method that utilizes the stimulated Raman effect for spectrum inversion. .

FIG. 2 is a diagram showing an optical fiber transmission system for explaining the principle of the present invention. Transmission optical fibers 11 and 12 with lengths L ₁ and L ₂ are connected to a spectral inverter 13
are interconnected through. The optical fibers 11 and 12 are s ₁ and s ₂ in the considered wavelength range, respectively.
It has wavelength dispersion of (ps/Km・nm). Also, in order to simplify the explanation of the wavelength spread of the light source, λ ₁ , λ ₂ ,
It is represented by three wavelengths of _λ3 . For example, if a semiconductor laser is used as a light source, λ ₁ , λ ₂ , and λ ₃ are its longitudinal modes. The optical pulses 7 of λ ₁ , λ ₂ , λ ₃ that are simultaneously incident on the optical fiber 11 have a time difference for each λ ₁ , λ ₂ , λ ₃ after propagation, and in the case of S ₁ >0, the optical pulses 7 of λ ₁ , λ ₂ , λ They arrive in the order of ₃ . Next, the spectrum inverter 13 converts λ ₁ into λ' ₁ , λ ₂ into λ' ₂ , and λ ₃ into λ' ₃ . λ′ ₁ ,
Since the wavelength order of λ' ₂ and λ' ₃ is reversed, the chromatic dispersion S ₂ of the optical fiber 12 provides a delay in the direction of eliminating the time difference between the three wavelengths. In reality, the wavelength dispersion and length of optical fibers 11 and 12 are different. At this time, the time difference T that occurs after propagation through the optical fiber 12 is calculated as follows.

T=△λ(L _1s1 +L _2s2 ) (1) Here, {T: delay time difference between the lights of λ′ ₁ and λ′ ₂ △λ=λ′ ₁ −λ′ ₂ =λ ₂ −λ ₁ } Therefore, the wavelength The condition for completely equalizing the variance is L _1s1 =L _2s2 (2).

In this invention, the spectrum inverter 13 is realized by the stimulated Raman effect. FIG. 3 shows the configuration of the spectrum inverter 13 using stimulated Raman scattering. As the medium 14 that causes Raman scattering, a silica-based optical fiber is suitable because the Raman gain described below exists in a wide wavelength range and the medium length can be made long. Signal lights with wavelengths λ ₁ , λ ₂ , and λ ₃ are combined with excitation light of wavelength λ _p from a high-power laser 15 and a semi-transparent mirror 16 .
The mixture is mixed with the ion beam and introduced into one end of the medium 14. The high output laser 15 is, for example, a YAG laser (wavelength 1.06μ
m, 1.32 μm), E _r ⁸⁺ −C _a F ₂ laser (1.56 μm
m), an optical parametric oscillator (wavelength range including 1 to 2 μm), etc. can be used. The output light of the medium 14 is spectral-converted by the optical filter 17 and becomes λ'.
₁ , λ' ₂ , λ _' component light and other wavelength component lights. If the interval between the signal light, the excitation light, and the anti-Stokes light is selected to be about 0.02 μm, the optical filter 17 that separates them can be easily obtained by using a dielectric multilayer film filter. The semi-transparent mirror 16 also transmits 100% of the signal light and 100% of the excitation light.
By using a reflective filter, highly efficient mixing is achieved.

As is well known, the wavelength characteristic of the Raman gain of an optical fiber is approximately 0.3 μm around the wavelength λ _p of the pumping light P.
It is becoming more and more widespread. When the signal light S is injected into the longer wavelength side than the pumping light, the signal light S itself is amplified (Stokes light amplification), and at the same time anti-
Stokes light AS occurs. Figure 4a shows the wavelength characteristics of Raman gain. Figure 4b shows the pumping light P and the signal light S.
The wavelength relationship between this and anti-Stokes optical AS is shown. light
AS originally occurs at a position where the signal light S is inverted with respect to the pumping light P on the optical frequency axis. Therefore, light P,
If the wavelengths of S and AS are λ _p , λ _s , and λ _As , then λ _p
=2/(1/λ _s +1/λ _As ). Since we are now dealing with the case where these wavelengths are close to each other, it can be assumed that the lights S, P, and AS are equally spaced on the wavelength axis. Therefore, as in the case of Fig. 3, the wavelength λ is used as the signal light S.
When the components of wavelengths ₁ , λ ₂ and λ ₃ are incident simultaneously, the wavelength order is switched and components of wavelengths λ' ₃ , λ' ₂ and λ' ₁ are generated as anti-Stokes light AS. The relationship between these wavelengths is shown in FIG. λ′ ₃ , λ′ ₂ , λ′
Wavelength inversion can be performed by newly using component ₁ as signal light for transmission.

As a numerical example, the longitudinal mode of the semiconductor laser of the light source is 1.60, 1.598, 1.596, 1.594, 1.592, 1.590 μm.
If the wavelength dispersion of the transmission optical fiber is 20 ps/Km・nm, then after propagating 20 km through the first transmission optical fiber 11, the oscillation time is 4 ns.
This causes a time difference. If this is input into an optical fiber with a length of about 5 to 50 m, which is a Raman medium, and light with a wavelength of 1.55 μm from an optical parametric oscillator is simultaneously input as excitation light, wavelengths of 1.50, 1.502, 1.504,
Generates anti-Stokes light of 1.506 and 1.508 μm.
If the length of the second transmission fiber 12 having a chromatic dispersion of 20 ps/Km·nm at a wavelength near 1.5 μm is 20 km, the delay time difference becomes zero after propagation through the second optical fiber.

In this invention, medium 1 with large Raman gain
4 or by increasing the pumping light intensity, anti-Stokes light stronger than the injected signal light can be obtained, so it is possible to amplify and equalize the optical signal at the same time.

As explained above, since the present invention is an equalization method that inverts the wavelength spectrum, it is only necessary to insert the same device at one location even for fibers with different transmission distances and wavelength dispersions. Furthermore, since Raman gain has a wide wavelength spread, the property of spectrum inversion remains unchanged even if the wavelength of the light source changes, and the equalization ability does not change. Furthermore, since a medium with gain is used, there is an advantage that amplification can be performed at the same time.

[Brief explanation of the drawing]

Figure 1 is a plan view showing a delay equalizer using a conventional grating waveguide, Figure 2 is a diagram showing an optical fiber transmission system explaining the principle of this invention, and Figure 3 is spectrum inversion in Figure 2. Figure 4 shows the Raman gain of the optical fiber and the wavelength relationship between pumping light P, signal light S and anti-Stokes light AS, and Figure 5 shows the wavelengths of signal light and anti-Stokes light. FIG. 3 is a diagram showing an inversion relationship. 11, 12: Transmission optical fiber, 13: Spectral inverter, 14: Raman medium fiber, 1
5: High power laser as excitation light source, 16: Semi-transparent mirror, 17: Optical filter.

Claims (1)

[Scope of Claims] 1. In a transmission system in which light having a wavelength width around a center wavelength λ _a propagates through a transmission optical fiber and equalizes delay time differences for each wavelength caused by wavelength dispersion of the optical fiber, The length of the optical fiber before a certain point P is L ₁ , the chromatic dispersion at wavelength λ _a of the optical fiber before P is s ₁ , and the length after P is
L ₂ , when the chromatic dispersion at the wavelength λ _b of the optical fiber after P is s ₂ , at a point P that substantially satisfies L _1s1 = L _2s2 , the center wavelength at which a delay time difference has occurred due to the fiber before P The optical signal of λ _a ,
It enters the Raman medium excited by excitation light with a wavelength λ _p = 2/(1/λ _a + 1/λ _b ) (where λ _a ≠ λ _b ), which is further away from the wavelength width, and its stimulated Raman effect The spectrum of the optical signal is inverted converted to a wavelength range symmetric with respect to λ _p , and the inverted anti-Stokes light with the center wavelength λ _b is used as a new signal light, and λ An optical fiber chromatic dispersion equalization method characterized in that components _a and λ _p are removed and transmitted to the optical fiber after P.