CN1316767C - Method for reducing Brillonin scattering effect influence excited in optical transmission system - Google Patents

Abstract

The present invention discloses a method for reducing Brillonin scattering effect influence excited by an optical transmission system and a device. The optical transmission system comprises a transmitter. The transmitter comprises a continuous wave laser, DC bias, a photomodulator and a data signal thereof, wherein the transmitter also comprises a low frequency disturbing signal and a coupling capacitor. The low frequency disturbance signal can be applied to the DC bias point of a light carrier laser by the coupling capacitor, and the laser linewidth of the light carrier can be widened. Thereby, the excited Brillonin scattering effect threshold in optical fiber can be effectively enhanced, and the influence to a transmission system can be reduced. The method for reducing Brillonin scattering effect influence excited by an optical transmission system by adopting low frequency disturbance signals of the present invention has the characteristics of simple structure, easy realization, high stability, etc., and can not increase the system cost basically. Therefore, the present invention can be widely used for the optical transmission system.

Description

Reduce the method and the device of optical transmission system stimulated Brillouin scattering effects

Technical field

The present invention relates to a kind of minimizing optical transmission system stimulated Brillouin scattering (SBS, StimulatedBrillouin Scattering) method of effects, particularly a kind of implementation method and device that in the non-relay optical transmission system of long distance, reduces optical transmission system SBS effects.

Background technology

In the non-relay optical transmission system of long distance, need improve launched power as much as possible and come the compensated fiber loss, thus the Optical Signal To Noise Ratio of assurance receiving terminal.But, under higher launched power, stimulated Brillouin scattering effect in the optical fiber can be transferred to the luminous power of fl transmission light signal rear orientation light and phonon fields, thereby caused a kind of loss mechanism of flashlight, cause the receiving terminal signal to noise ratio to descend, seriously influenced the performance of optical transmission system.Several nonlinear effects of in SBS effect and the optical fiber other are as comparing from phase modulated (Self-Phase Modulation-SPM), cross-phase modulation nonlinear effects such as (Cross Phase Modulation-XPM), the SBS effect has minimum threshold power, and several milliwatts (mw) are generally only arranged.Produce for fear of the SBS effect, system's launched power can only be limited in below the SBS threshold value, and therefore, the SBS effect in the optical fiber has limited the raising of signal launched power widely.

In the prior art, can adopt the SPM effect of self channel, the XPM effect of monitoring and controlling channels to widen spectrum width, suppress the SBS effect, but this mode suppress limited in one's ability, and might cause the signal waveform distortion, effect is not good enough.The additive phase modulator that passes through that also has reduces the SBS effects, and effect is pretty good, but its cost is very high.The modulation format of the employing carrier suppressed that also has is widened spectrum width, but this mode need be changed original transmitter module configuration largely, realizes complicated difficulty, and cost is higher.

Summary of the invention

The object of the present invention is to provide the method and the device that reduce optical transmission system SBS effects in a kind of transmission system, effectively improve the threshold value of the SBS effect in the optical fiber, reduce its influence, and cost is lower transmission system.

For realizing purpose of the present invention, we provide a kind of method that reduces optical transmission system stimulated Brillouin scattering effects, it may further comprise the steps: the low-frequency excitation signal is applied on the direct current biasing point of light carrier laser, cause that light carrier laser linewidth widens the forcing frequency v of described low-frequency excitation signal _dShould satisfy C/2n _fL _Eff≤ v _d≤ v _L, wherein C is the light velocity, n _fBe optical fiber effective refractive index, L _EffBe optical fiber effective length, v _LFor the amplifier low frequency cuts off frequency.

The method of described minimizing optical transmission system stimulated Brillouin scattering effects, wherein said low-frequency excitation signal is a sine wave signal.

The method of described minimizing optical transmission system stimulated Brillouin scattering effects, wherein said low-frequency excitation signal is applied on the direct current biasing point of light carrier laser by coupling capacitance.

The method of described minimizing optical transmission system stimulated Brillouin scattering effects, wherein the low-frequency excitation signal frequency being arranged on amplifier response reaches outside the reception bandwidth of optical receiver, be used to reduce the additional eye pattern closure of low frequency modulations and eliminate the dispersion penalty that low frequency modulations causes, thereby effectively improve the transmission system performance, and described forcing frequency is 5KHz to 100KHz.

The method of described minimizing optical transmission system stimulated Brillouin scattering effects, the modulation depth of wherein said low-frequency excitation signal should satisfy m _d≤ 5%.

According to a further aspect in the invention, we provide a kind of device that reduces optical transmission system stimulated Brillouin scattering effects, described optical transmission system comprises a transmitter, described transmitter comprises: continuous-wave laser and direct current biasing point thereof, optical modulator, be added with data-signal on the described optical modulator, it is characterized in that also comprising a coupling capacitance and forcing frequency v _dSatisfy C/2n _fL _Eff≤ v _d≤ v _LThe low-frequency excitation signal, by described coupling capacitance described low-frequency excitation signal is applied on the direct current biasing point of light carrier laser; Wherein C is the light velocity, n _fBe optical fiber effective refractive index, L _EffBe optical fiber effective length, v _LFor the amplifier low frequency cuts off frequency.

The device of described minimizing optical transmission system stimulated Brillouin scattering effects, wherein said optical modulator are that electricity is inhaled external modulator or mach zhender external modulator.

The device of described minimizing optical transmission system stimulated Brillouin scattering effects, wherein said data-signal are nonreturn to zero code (NRZ) or NRZ (RZ).

The device of described minimizing optical transmission system stimulated Brillouin scattering effects, wherein said low-frequency excitation signal is a sine wave signal.

In sum, compare with the method for prior art, the method for employing low-frequency excitation signal of the present invention has simple in structure, easy realization, and therefore good stability and can not increase characteristics such as system cost substantially can be widely used in the optical transmission system.

Description of drawings

Fig. 1 is added with the transmitter module theory diagram of low-frequency excitation signal;

Fig. 2 disturbing signal is widened carrier wave spectral width schematic diagram.

Embodiment

Stimulated Brillouin scattering is a narrow band process, typically gains spectrum width in the silica fiber between 10MHz～100MHz, corresponding Stokes frequency shift amount 10GHz.

In digital communication, signal light-wave is general to be adopted and has 1/2 " 1 " pseudo-random code stream (PRBS) and it is carried out amplitude modulation(PAM) (Amplitude Modulation-AM), the threshold value P of SBS effect in optical fiber of modulated light signal _Th ^ASKBut approximate representation is:

$P_{\mathrm{th}}^{\mathrm{ASK}}=\frac{P_{\mathrm{th}}^{\mathrm{CW}}}{1-\frac{B}{2\mathrm{\Δ}{v}_B}(1-e^{-\mathrm{\ΔvB}/B})}---\left(1\right)$

P wherein _Th ^CWBe the threshold value of SBS effect in optical fiber of the continuous light (Continuous Wave-CW, i.e. light carrier) before the modulation, but approximate representation is:

$P_{\mathrm{th}}^{\mathrm{CW}}=21\frac{{\mathrm{KA}}_{\mathrm{eff}}}{{\mathrm{goL}}_{\mathrm{eff}}}\left(\frac{\sqrt{\mathrm{\Δ}{v}_B^2+\mathrm{\Δ}{v}_c^2}}{\mathrm{\ΔvB}}\right)---\left(2\right)$

B bit rate in the formula; A _EffBe the effective core diameter area of optical fiber; Go is the gain coefficient of SBS effect in optical fiber; Δ vB is the spontaneous brillouin gain spectral width in the optical fiber; Δ vc is the CW spectral width; K is the polarization correlation factor; L _EffBeing the optical fiber effective length, when the optical fiber long enough, can approximate representation be L _Eff≈ 1/ α.L fiber lengths in the formula; α is the fibre loss coefficient.Can obtain as drawing a conclusion from above-mentioned two formulas: its SBS effect threshold value of signal after (1) modulation improves, and has for high bit-rate transmission systems $\frac{\mathrm{\ΔvB}}{B}\→0$ , then the SBS threshold value of modulation signal is approximately 2 times of CW light SBS effect threshold value; (2) the SBS effect in the optical fiber is mainly caused by the narrow carrier component in the modulation signal (CW light component), therefore can be by suitably increasing the carrier wave spectral width, when making it be far longer than the gain spectral width Delta vB of spontaneous brillouin effect in optical fiber, SBS effect threshold value in the optical fiber will improve greatly, thereby reduce the influence of this effect to systematic function.

The present invention is exactly according to above-mentioned analysis, on CW light direct current biasing point, add the small-signal low-frequency excitation, be generally sine wave, adopt square wave, triangular wave also can, but effect is good not as sinusoidal wave, make CW light spectral width suitably widen, effectively improved the SBS effect threshold value in the optical fiber, thereby allow the high-power fine transmission of going into.Suppose that the low-frequency sine signal that adds can be expressed as:

I(t)＝I _dsin(2πv _d*t+ ₀) (3)

When being applied to laser on by CW laser bias point this signal, be equivalent to carrier wave has been carried out direct low frequency intensity modulated, then the forcing frequency vd of low-frequency sine disturbing signal and modulation depth m _dCan influence inhibition effect and system transmissions performance to the SBS effect.

Modulation depth is determined: for modulation depth m _dCan be represented by the formula:

$m_d=\frac{I_d}{I_b-I_{\mathrm{th}}}---\left(4\right)$

I wherein _bAnd I _ThDc bias current and the threshold current thereof of representing the CW laser respectively.According to the directly modulated lasers characteristic, when directly modulating the CW laser with above-mentioned low-frequency sine signal, the low-frequency sine signal causes the cyclic variation of laser direct current biasing point, cause carrier concentration (being equivalent to the gain media refractive index n) the generating period variation in the dc laser device, then according to the relation of laser transmission center frequency values vc and gain media refractive index n:

$\mathrm{vc}\∝\frac{c}{2\mathrm{nL}}---\left(5\right)$

Wherein L, c are respectively the long and light velocity of laser chamber.Can be drawn by (5) formula, when not adding the disturbance signal, laser centre frequency vc is corresponding to fixing bias current I _b, and fixing laser linewidth Δ v is arranged.When adding disturbing signal, the laser centre frequency will be with the low-frequency sine modulation signal in the nearly periodic jitter of the attached vc of modified center frequency, causes that laser frequency warbles, and makes CW laser optical spectral line width broadening.Under the small-signal modulation case, CW laser optical spectrum peak place live width is mainly by the side-play amount decision of laser centre frequency with respect to vc, and this side-play amount final decision (is equivalent to modulation depth m in the low-frequency sine peak-to-peak value that applies _d).They have following relation:

vc±Δv∝I _bI _d (6)

Fig. 2 has provided the schematic diagram of CW laser linewidth with low-frequency sine modulation signal broadening.From the whole time, the spectrum of the CW laser behind the broadening is exactly the whole envelope behind the broadening of spectral lines among Fig. 2, and the effective line width of CW laser is exactly the envelope spectrum width.For low frequency sinusoidal perturbation signal peak peak value, i.e. modulation depth m _dSize, mainly frequency/current response and the SBS effect gain spectrum width by semiconductor laser determines.For the semiconductor laser of present employing, frequency/electric current (Δ v/ Δ I) response is 0.008nm/mA, i.e. 1GHz/mA.The front was said, the gain spectrum width 10MHz～100MHz of SBS effect in optical fiber, therefore need only the CW laser linewidth to medium frequency both sides broadening ± 1GHz, just be enough to the SBS effect threshold value in the optical fiber is brought up to more than the 18dBm, be enough to satisfy the launched power requirement of existing communication system, corresponding modulation depth md～about 3%.Certainly, modulation depth m _dSlightly larger, the CW spectral width will be wide more, and it is strong more then to a certain extent the SBS effect to be suppressed ability.But, modulation depth m _dCan not be too big, because added the small-signal low-frequency excitation, disturbing signal will be superimposed upon the back and be driven by data-signal on the light eye pattern of optical modulator generation, can make the level"1" thickening of eye pattern on the one hand, cause eye opening to reduce; On the other hand, the serious broadening of CW spectrum can bring dispersion penalty and bring unnecessary negative effect to system to transmission system, therefore for existing transmission system, modulation depth m _dGenerally should be at m _dBe advisable in≤5% the scope.

Forcing frequency is determined: for low-frequency sine forcing frequency vd, in order to make the SBS effect in whole optical fiber effective length scope, can both experience bigger CW spectral line width, thereby effectively suppress the SBS effect in the optical fiber, then should guarantee the transmission time that the low-frequency sine disturbing signal cycle transmits in optical fiber less than light signal at least, therefore there is a lower limit in low-frequency excitation frequency vd, should satisfy following relation:

$\mathrm{vd}\≥\frac{c}{{2n}_f{L}_{\mathrm{eff}}}---\left(7\right)$

N wherein _fBe the optical fiber effective refractive index.For existing general single mode fiber, its loss factor representative value α～0.2dB, so forcing frequency vd at least should be more than or equal to 5KHz.For existing erbium-doped fiber amplifier EDFA or Raman (Raman) amplifier, the typical response time 0.1ms of EDFA～several ms magnitudes, when amplifier is in degree of depth saturation condition, its response time will shorten, and representative value is 10us.Then for greater than the intensity-modulated signal in 10KHz～100KHz scope, the amplifier that works in the saturation region can't respond, to the undistorted amplification of signal.The front was said, low frequency sinusoidal perturbation signal to CW with laser modulation after, to cause the level"1" thickening of light signal eye pattern, if forcing frequency vd is during greater than 10KHz～100KHz, amplifier in the transmission link will amplify the low frequency intensity-modulated signal, make the light signal eye opening further reduce, thereby influence the transmission system performance.Therefore also there is a upper limit in the low-frequency excitation frequency, promptly should be in the amplifier low frequency and cut off below the frequency vL, and then the low-frequency excitation signal frequency should satisfy following condition:

$\frac{c}{{2n}_f{L}_{\mathrm{eff}}}\≤\mathrm{vd}\≤\mathrm{vL}---\left(8\right)$

According to above-mentioned analysis, the low-frequency sine disturbing signal generally selects 5KHz～100KHz magnitude to be advisable.In this frequency range, not outside the reception bandwidth of optical receiver, therefore help eliminating the dispersion penalty that low frequency modulations causes.

Fig. 1 has provided the transmitter module theory diagram that is added with the low-frequency excitation signal, it (can be electric absorption (EA) external modulator by CW laser 101 and direct current biasing point 102 thereof, optical modulator 103, also can be mach zhender (MZ) external modulator) and data-signal 104 (can be the NRZ sign indicating number, also can be the data-signal of RZ sign indicating number form) and the being used for low-frequency excitation signal 105 and the coupling capacitance 106 of widening CW light spectrum form.Low-frequency excitation signal 105 is worth in the electric current 102 partially by the direct current that coupling capacitance 106 is coupled to CW laser 101, CW laser 101 is carried out the direct intensity modulated of small-signal, make CW laser 101 output spectrum live widths be widened to greater than the SBS effect gain bandwidth in the optical fiber, then the output of the CW behind broadening light is sent into the optical modulator 103 of back, drive through data-signal 104 again and produce the light signal that has the low-frequency excitation signal, send in the optical fiber link and transmit.

This technology is confirmed in our experiment, adds the sine wave signal of 10KHz on CW laser direct current biasing, just more than the 18dBm that the SBS effect threshold value in the optical fiber can be improved, is enough to satisfy present optical fiber telecommunications system and goes into preceding power requirement.

The present invention is suitable for cable TV CATV, SDH (Synchronous Digital Hierarchy) SDH and dense wave division multipurpose DWDM optical transmission system, the data signal modulation form can be nonreturn to zero code (NRZ) or NRZ (RZ), optical transmission system can comprise and contain erbium-doped fiber amplifier EDFA or raman amplifier, transmission link by G.652 655 653 Transmission Fibers form.

In sum, the present invention describes the preferred embodiments of the present invention in conjunction with the accompanying drawings in detail, however description of the invention, describe in detail and above-mentioned accompanying drawing be not be used for limiting of the present invention.To those skilled in the art, can carry out various corresponding modification and can not exceed the spirit and scope of the present invention under instruction of the present invention, therefore this variation should be included within claim of the present invention and the equivalent scope thereof.

Claims (9)

1. method that reduces optical transmission system stimulated Brillouin scattering effects, it is characterized in that may further comprise the steps: the low-frequency excitation signal is applied on the direct current biasing point of light carrier laser, cause that light carrier laser linewidth widens the forcing frequency v of described low-frequency excitation signal _dShould satisfy C/2n _fL _Eff≤ v _d≤ v _L, wherein C is the light velocity, n _fBe optical fiber effective refractive index, L _EffBe optical fiber effective length, v _LFor the amplifier low frequency cuts off frequency.

2. the method for minimizing optical transmission system stimulated Brillouin scattering effects as claimed in claim 1 is characterized in that described low-frequency excitation signal is a sine wave signal.

3. the method for minimizing optical transmission system stimulated Brillouin scattering effects as claimed in claim 1 is characterized in that described low-frequency excitation signal is applied on the direct current biasing point of light carrier laser by coupling capacitance.

4. the method for minimizing optical transmission system stimulated Brillouin scattering effects as claimed in claim 1, it is characterized in that the low-frequency excitation signal frequency is arranged on amplifier response to be reached outside the reception bandwidth of optical receiver, be used to reduce the additional eye pattern closure of low frequency modulations and eliminate the dispersion penalty that low frequency modulations causes, thereby effectively improve the transmission system performance, and described forcing frequency is 5KHz to 100KHz.

5. the method for minimizing optical transmission system stimulated Brillouin scattering effects as claimed in claim 1 is characterized in that the modulation depth of described low-frequency excitation signal should satisfy m _d≤ 5%.

6, a kind of device that reduces optical transmission system stimulated Brillouin scattering effects, described optical transmission system comprises a transmitter, described transmitter comprises: continuous-wave laser and direct current biasing point thereof, optical modulator, be added with data-signal on the described optical modulator, it is characterized in that also comprising a coupling capacitance and forcing frequency v _dSatisfy C/2n _fL _Eff≤ v _d≤ v _LThe low-frequency excitation signal, by described coupling capacitance described low-frequency excitation signal is applied on the direct current biasing point of light carrier laser; Wherein C is the light velocity, n _fBe optical fiber effective refractive index, L _EffBe optical fiber effective length, v _LFor the amplifier low frequency cuts off frequency.

7. the device of minimizing optical transmission system stimulated Brillouin scattering effects as claimed in claim 6 is characterized in that described optical modulator is electricity suction external modulator or mach zhender external modulator.

8. the device of minimizing optical transmission system stimulated Brillouin scattering effects as claimed in claim 6 is characterized in that described data-signal is nonreturn to zero code (NRZ) or NRZ (RZ).

9. the device of minimizing optical transmission system stimulated Brillouin scattering effects as claimed in claim 6 is characterized in that described low-frequency excitation signal is a sine wave signal.