JPS5853243A - Light transmitting system - Google Patents

Abstract

PURPOSE:To realize the transmission of a long distance with no intervention of a repeater for the carrier signal light which is identical with the stokes light, by providing a photocouplers at several areas at the middle of a light transmission path and putting together the pumping beams which are consecutively transmitted. CONSTITUTION:The stokes light of a light source 1 undergoes an optical modulation by the signal given from a signal source 2 and is condensed through a lesn 4 to be led into an optical fiber 7. The pumping beams are condensed through a lens 5 at several areas at the middle of the fiber 7 and then led into the fiber 7. Then the signal light and the pumping beams are put togethr. Then the energy of the pumping beam is absorbed in terms of a travelling wave by the Raman gain of the fiber 7. Thus the signal light can be amplified. In such a way, a long distance transmission is made possible through an optical fiber.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an optical transmission system that makes long-distance optical transmission possible using .

In conventional optical transmission systems, a signal is added to the current injected into the semiconductor laser to directly tune the semiconductor laser, or a semiconductor laser or solid-state laser is continuously oscillated to transmit the laser light iLiNbO2. Yani. The method used was to modulate the light using an external modulator such as a DP crystal, and then introduce the modulated output light into an optical fiber. In these methods, the optical power level at the output end of the optical fiber is determined by the loss of the optical fiber and the optical power level within the input end optical fiber, and the length of the optical fiber that can be installed is determined. In order to realize long-distance optical transmission, repeaters or optical amplifiers are required.

A repeater consists of a photodetector, a light source, and an electronic circuit. Inside the repeater, light is converted into an electrical signal and then converted back into light again, making it complex to implement.

Although optical amplifiers are considered essential for future optical transmission, practical optical amplifiers have not yet been realized. The semiconductor group V double heterostructure optical amplifier that has been studied in principle still has many problems, such as noise and difficulty in connecting the optical fiber and the optical amplifier.

The purpose of the present invention is to solve all of these problems and provide a completely new optical transmission system.To achieve this goal (2), the present invention utilizes nonlinear phenomena that occur within optical fibers to Long-distance optical transmission is realized by providing a wave-like optical amplification function.

The present invention will be described in detail below with reference to the drawings.

FIG. 1 shows a configuration example of an optical transmission system according to the present invention, where l is a light source corresponding to Stokes light, 2≠ and j are lenses, O is a 3 dB optical coupler or branch type combiner, and 7
and g is an optical fiber.

The power Pa(L) of Stokes light generated by stimulated Raman in an optical fiber is approximately expressed as Pa(L)
= Ps(o) eGPpI'/' (1)
It is expressed as Here, L is the effective length of the optical fiber and is expressed as r, = (/e'')/α (2). In addition, Ps (0) is the power of Stokes light at the input end of the optical fiber, and l is the power of the Stokes light at the input end of the optical fiber. α is the fiber length, α is the loss of the optical fiber, G is the stimulated Raman gain, Pp is the power of the pumping light, and A is the effective area of the core of the optical 7-eye fiber.

Here, as shown in FIG. 1, the signal changes to 1! The Stokes light propagated through the optical fiber 7 as a signal light is /
Suppose that it is emitted with a power of θμw (-X) dBm). As an ideal assumption, the connection loss and propagation loss between the optical fiber 7, the branch type combiner 6, and the optical fiber r become Pa(0)=/θμW. now,
Optical fiber loss α-0,/21an-' (θ:jan
/7I2+), il = 3θ/i, A=4'X/
θ cffl (7≠w core), G, , 0.92×/
θ cIIL/w (Y, 5asaki et al.: Ele
ctronics Letter, Vol, /
7. f'h I, pp, 3/j -3/A(
/911)), Ps(L) / Ps(O)
= /, 2 When Pp k is found, it becomes ppm ri&mw. That is, in order to obtain a power of 12 μw at the output end of the optical fiber t, the power of the pumping light is Am
W is a good thing. Since this value is an ideal case, it is considered that the necessary power of the pumping light is actually large. As described above, the signal light transmitted through the optical fiber 7 is weakened, but by introducing the pumping light, it is amplified again and transmitted through the optical fiber r.

Figure 2 shows the Q-switch mode-locked light (wavelength/, 0611
m ) f Jj IgB O)I The excitation state of Stokes light when introduced into the vAD optical fiber is shown. wavelength/
The spectrum on the wavelength side longer than , 06 μm indicates the excitation of higher-order Stokes light, and the spectrum on the wavelength side shorter than i,ot μm indicates the excitation of anti-Stokes light. As can be seen, the spectrum width of Stokes light is wider than that of pumping light. This shows that the allowable wavelength range of the Stokes light and pumping light to be amplified is wide as shown in FIG. 1, that is, the wavelength controllability is not strict.

The relationship between the wavelength λp of the pumping light and the wavelength λB of the Stokes light in stimulated Raman oscillation is λS=λp/(/±mΔ
νλp ) (3). Here, m is the order of Stokes light, and (j) Δν is the gain width of Stokes light. For example, - next strike = 7.3 + 2 μm - Since the laser beam has zero dispersion of the quartz fiber, Iτ, and the quartz fiber has ΔC of 1110 cR, 2. =
/, 24Iμm. Therefore, since the difference in wavelength between the Stokes wave and the pumping light is large, if a filter is attached to the photodetector side, the pumping light and the noise contained therein can be removed. On the other hand, in IV group semiconductor optical amplifiers, the wavelength of the noise light is almost superimposed on the signal light, so that the noise light cannot be removed with commonly available filters.

The reason why the pumping light is continuously oscillated is because there is a difference in group velocity between the pumping light and the Stokes light, so that the pulsed Stokes light as the signal light is supplied with energy by the pumping light from anywhere within the optical fiber. This is to do so. In addition, the power of the pumping light should be large, but in order to avoid excitation of higher-order Stokes light,
Furthermore, when there is no signal light, the pumping light must be kept within a certain range so that the Stokes light is not excited by the pumping light alone.

As shown above, 3 dB optical couplers or branch type combiners t are placed at several locations in the optical transmission line (, <) as shown in Figure 1, and continuous wave pumping light is introduced. By merging them together, the carrier signal light, which is Stokes light, can be transmitted within the optical fiber over a long distance without passing through a repeater all along the way.

Note that the light sources 1 and 3 may be solid-state lasers, gas lasers, or semiconductor lasers, but semiconductor lasers are preferred from the viewpoint of longevity and wavelength tuning.

As explained above, according to the present invention, even if the power of the input signal light is small, the optical energy of the pumping light can be absorbed and amplified in the forward T wave manner within the gold optical fiber by the stimulated Raman gain, so that long-distance optical transmission becomes possible. Moreover,
The configuration of the optical transmission system of the present invention is extremely simple, and the light source for the pumping light can be driven by a DC voltage, so there is no complexity in the electrical circuit (therefore, it is convenient to place it in the middle of the optical transmission path, and it is also possible to use the Stokes optical Since there is a large difference in the wavelengths of the pumping light and the pumping light, it is easy to manufacture an optical filter.

[Brief explanation of the drawing]

Figure 1 is a block diagram showing an example of the optical transmission system of the present invention, and Figure 2 is a pumping light ωp of a YAG laser (wavelength 10tμm).
Each Stokes beam ω1 due to stimulated Raman excited in the optical fiber by . ω2. ... is a characteristic curve diagram showing the spectrum of a song. l...Light source corresponding to Stokes light, Co...Signal source, 3...Light source corresponding to pumping light, ≠, vessel, 7, g
...Optical fiber as a transmission line. Patent applicant Nippon Telegraph Corporation

Claims (1)

[Claims] Introducing Stokes light that undergoes stimulated Raman oscillation in the optical fiber transmission line as signal light to the mJ destination optical fiber transmission line,
A 3 dB optical coupler or a branch type combiner is disposed in the middle of the optical fiber transmission line, and the continuous oscillation pumping destination light for the stimulated Raman oscillation is combined with the Stokes light to fully amplify the Stokes light. An optical transmission method characterized in that the signal light is transmitted by.