JPS621332A - Optical fiber communication equipment - Google Patents

Abstract

PURPOSE:To expand remarkably the transmission distance by suppressing the generation of an induced Brillouin scattering light. CONSTITUTION:A signal light generation laser beam source 1 is subjected to a strength modulation by an electric pulse train generated from a drive circuit 2 and irradiates a multi-axis mode laser beam. The signal light pulse train irradiated from the light source 1 is connected to an optical fiber 3 by a lens 51. Then the optical fiber 3 is connected to an optical receiver 4 by a lens 52. In the said optical fiber communication equipment, the oscillated axial mode number N of the light source 1 and the signal light power Pin incident on the transmission line have the following relation; 2<=N<(1+(4T<2>-(DELTAt)<2>)<1/2>/M (lambda0).LDELTAlambda), Pin>P00, where P0 is a threshold input light power where an induced Brillouin scattering light is caused in the transmission line with single mode light incidence, DELTAt is a signal light pulse width, T is the period of optical pulse, lambda0 is the center wavelength of multi-modelight, L is a transmission distance and M(lambda) is scattering at a wavelength lambda.

Description

Detailed Description of the Invention (Industrial Field of Application) The present invention provides an optical fiber communication device, and more specifically, a fiber optic communication device that has a significantly longer transmission distance than conventional ones by suppressing the generation of stimulated Brillouin scattering light. The present invention relates to a distance optical fiber communication device.

(Prior technology) With the recent development of high-performance single-axis mode semiconductor lasers and low-loss, low-dispersion single-mode optical fibers, long-distance optical fiber communications with a transmission distance of about 200 km are now possible at the laboratory level. (8th International Conference on Optical Fiber Communications (OFC'85), 1985, lecture number WB
4). In such long-distance optical fiber communications, optical fibers with low loss values that are close to the theoretical limit are used, and there are limits to the improvement of the receiving sensitivity of optical receivers in the future. Therefore, in order to realize ultra-long-distance optical fiber communications, development of higher-power single-axis mode semiconductor lasers is underway to improve the power level of transmitted signal light.

(Problem to be solved by the invention) On the other hand, in a single mode optical fiber, since the core diameter is small, the energy density of the propagating signal light becomes extremely high, resulting in stimulated Raman scattering effects, induced bacterial photon mixing, etc. It is known that various nonlinear effects such as the stimulated Brillouin scattering effect and the stimulated Brillouin scattering effect are induced. (Proceedings, I.
Yi, Yi Yi (Proc.

IEEE), Volume 68, 1980, Page 1232)
. Among them, the stimulated Brillouin scattering effect has the largest gain coefficient, so it generally occurs at the lowest input signal light level. When this stimulated Brillouin scattering effect is induced, most of the signal light is converted to stimulated Brillouin scattering light and is backscattered in the direction of its input end, thereby limiting the power of the signal light that can be transmitted. In other words, no matter how high the power level of the signal light is made to enter the optical fiber, the power of the signal light that can reach the optical receiver cannot be increased, and the transmission distance cannot be extended. Furthermore, the generation of stimulated Brillouin scattered light causes level fluctuations in the received signal light. Therefore, in order to realize ultra-long distance optical fiber communication with a transmission distance of 200 km or more, it is necessary to sufficiently suppress the generation of stimulated Brillouin scattered light.

The present invention provides an optical fiber communication device that suppresses the generation of stimulated Brillouin scattering light that causes the various problems described above even when the input power level of signal light becomes high, and that allows longer transmission distances than conventional ones. It's about doing.

(Means for Solving the Problems) An optical fiber communication device of the present invention includes an optical transmitter having a laser light source for signal generation, a single mode optical fiber serving as a transmission path, and an optical receiver. In communication equipment,
When the signal light pulse output from the optical transmitter is a single-axis mode laser light, the signal light pulse input to the single-mode optical fiber in which stimulated Brillouin scattering light occurs is PQ, and the number of oscillation axis modes of the signal generation laser light source is PQ. is N, the input value of the signal light pulse to the single mode optical fiber is at least Po or more, and N is an integer of 2 or more, and the value of the adjacent signal after being transmitted through the single mode optical fiber is The generation of stimulated Brillouin scattered light in the single mode optical fiber is suppressed by selecting a number between optical pulse waveforms that does not cause code amount interference due to dispersion of the single mode optical fiber. Features.

(Function) As explained below, this configuration enables the oscillation of a signal generation laser light source that emits a signal light pulse train within a range where code amount interference does not occur between signal light pulses after transmission due to dispersion of the optical fiber. By increasing the number of longitudinal modes, the generation of stimulated Brillouin scattered light is suppressed even when the input power level of signal light is high.

The stimulated Brillouin scattering effect is the scattering effect of light by acoustic phonons. When the signal light is a single-axis mode laser beam, the optical fiber input average value Po of the signal light that generates stimulated Brillouin scattering light is approximately given by the following equation (
Applied Optic X (Appl, Opt,
) Volume 11, 1972, 2489 Hesi).

However, gB: peak gain coefficient of stimulated Brillouin scattering, α: transmission loss of optical fiber, l2! ! length, effective cross-sectional area of A2/l, K: is a constant that depends on the polarization conservation of u,
Usually, ni=2.

Here, when the signal light input is equal to or higher than the above-mentioned Po, stimulated Brillouin scattered light is significantly generated, so the equation (2) represents the upper limit of the optical fiber input value. On the other hand, when the signal light input is equal to or higher than Po, the amount of stimulated Brillouin scattered light generated is extremely small and can be ignored.

Next, when the signal light is N multi-axis mode laser beams, the optical fiber input value PN of the signal light at which stimulated Brillouin scattering light is generated is given by PH=N.Po...■. Formula 0 is an experimental formula found by the inventor through experiments. From this equation, by increasing N, the upper limit of the optical fiber input value can be greatly increased. Therefore, if the required optical fiber input value of the signal light required to transmit the desired distance is Pin, then Pu1 PN...
...■, and the generation of stimulated Brillouin scattered light can be eliminated. Here, as is clear from the purpose of the present invention, normally N is N≧2...
■Let it be.

By the way, optical fibers have a property that the propagation time of light differs depending on the wavelength, that is, there is dispersion, so if N is made too large, the pulse width of the signal light pulse will expand, which will eventually cause code amount interference and reduce the code error rate. It may not be possible to keep it. However, such a dispersion problem can be eliminated by setting N to the conditions described below.

The signal light pulse is N multi-axis mode laser beams with a wavelength interval Δλ, and the center wavelength is λ0 and the pulse width is Δ
t, and the repetition period is T (T≧Δt). The pulse width Δt out after such a signal light pulse propagates through an optical fiber with dispersion M (λ) at a large wavelength and length L
is approximately, Δt2=(Δt)2+(M(λo)・L4)+−(N
-1))2...■. ([Optical Fiber], Ohmsha (1981) p290).

Here, the condition that adjacent signal light pulses do not cause code amount interference at the fiber output end is Δtout<2T.
. . ■ (When Δtout=2T, the signals +1191 and +1O11 after receiving light cannot be distinguished at all).

Therefore, from this condition and equation 0, If so, the effect of dispersion can be almost eliminated.

In the end, together with the formula 0, the range of N is becomes.

Therefore, by selecting an appropriate value of N within the range that satisfies Equation 0, the pulse broadening of the signal light pulse and the code amount interference can be hardly caused, and moreover, as shown in Equation 0, stimulated Brillouin scattered light can be It is possible to suppress the occurrence of

(Example) Next, an optical fiber communication device of the present invention will be described in detail with reference to the drawings.

FIG. 1 shows an embodiment according to the present invention.

In FIG. 1, 1 is a laser light source for signal generation, 11 is a semiconductor laser element, 12 is an external mirror, 2 is a drive circuit for the semiconductor laser element, 3 is an optical fiber, 4 is an optical receiver, 51
．． 52 is a lens.

In this embodiment, the semiconductor laser device 11 is an InGaAsP/InP semiconductor laser device whose end face on the external mirror side is coated with anti-reflection coating so that the reflectance is 1% or less, and the external mirror 12 is a gold vapor deposited mirror with a reflectance of 100%. and
Both are external cavity type semiconductor lasers. The laser light source 1 for signal light generation having such a configuration is intensity-modulated by an electric Hals train of a 100 Mb/s, mark rate 1/2, non-return-to-zero (NRZ) signal generated from a drive circuit 2. Its average output is 30 mW, pulse width is 10 ns, center wavelength is 1.55 pm, and the number of axial modes is approximately 25.
It emits a multi-axis mode laser beam. Here, the axial mode spacing is approximately 2×10=nm (250 MHz).

The optical fiber 3 has an effective core diameter of 8 pm and a length of 300 mm.
km, transmission loss at wavelength 1.55 pm 0.20 dB/k
m, a single mode silica fiber with a dispersion of 15 ps/nm-km, and the optical receiver 4 is an InGaAs fiber with a receiving diameter of 50 pmΦ.
・Avalanche photodiode (InGaAs-APD
), and lenses 51 and 52 use cell 7 lens lenses.

The signal light pulse train with an average power of about 80 mW emitted from the signal generation laser light source 1 is processed by the lens 51.
Approximately 22 mW is coupled into the optical fiber 3.

Here, in the optical fiber used in this example, the value of Po calculated from equation (2) is approximately 4 mW in average power. Therefore, in this embodiment, since N=25, P20=1
00 mW, and the above-mentioned optical fiber input value of 22 mW satisfies the condition that stimulated Brillouin scattering light does not occur: Pin<100 mW. Further, in this embodiment, the condition that the signal light pulse hardly spreads due to the dispersion of the optical fiber is N<2151 according to the equation 0, and N=25 satisfies this condition as well.

Next, the performance of this device will be explained. As mentioned above,
The fiber input average power of the signal light pulse is +13.
It is 4 dBm (approximately 22 mW). The total loss of the optical 7 fiber is 60 dB. Therefore, the average power of the signal light received by the receiver was -46.6 dBm. On the other hand, the reception sensitivity of this optical receiver at 100 Mb/s was -50 dBm with an error rate of 10-9, so the margin during 300 km transmission was 3.4 dB.

On the other hand, when a single-axis mode semiconductor laser is used as a laser light source for signal generation, no matter what signal light power is incident on the optical fiber, it is received by the optical receiver due to the generation of stimulated Brillouin scattering light. The average power of the signal light was -54 dBm at most, and an error rate of less than 10-9 could not be obtained.

Although the optical fiber communication device according to the present invention has been described above using one embodiment, the present invention is not limited to this embodiment, and several modifications can be made.

For example, in this embodiment, the number of axial modes of the signal generating laser light source is set to N=25, but it may be any natural number as long as it satisfies the condition of the above-mentioned formula 00.

The laser light source for signal generation was an external cavity type semiconductor laser with a wavelength of 1.55 pm;
Other wavelength ranges such as the 3pm band may be used, or Nd:Y
Other laser light sources such as an AG laser may also be used. Further, the optical fiber may have any core diameter, length, etc., and is not limited to this embodiment. Also,
It goes without saying that a germanium avant-garde photodiode (Ge-APD), a pIN-FET, or the like may be used as the optical receiver.

(Effects of the present invention) As explained above, in the optical fiber communication device according to the present invention, the oscillation of the laser light source for signal generation is performed within the range where code amount interference does not occur between signal light pulses due to dispersion of the single mode optical fiber. Since the number of axial modes is increased to suppress the generation of stimulated Brillouin scattered light even when the input level of the signal light is high, there is an advantage that the transmission distance can be longer than that of the conventional method.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of an embodiment according to the present invention. In the figure, 1... Laser light source for signal generation, 11... Semiconductor laser element, 12... External mirror, 20. ...Semiconductor laser element drive circuit, 3...
- Optical fiber, 4... Optical receiver, 51.52... Luns. Representative patent attorney F ,, -'=≧Nosoff:/

Claims (1)

[Scope of Claim] An optical fiber communication device comprising an optical transmitter having a laser light source for generating an optical signal, a transmission path made of a single mode optical fiber, and an optical receiver, wherein the oscillation axis mode of the light source The number N and the signal light power Pin entering the transmission line are: ▲ There are mathematical formulas, chemical formulas, tables, etc. ▼ Pin > P However, P_0: Threshold input at which stimulated Brillouin scattered light occurs in the transmission line when single mode light is incident Optical power Δt: signal light pulse width, T: repetition period of optical pulse, λ_0: center wavelength of multimode light, L: transmission distance, M(λ
): An optical fiber communication device comprising an optical transmitter whose output is determined to satisfy a fraction of the input wavelength.