JPH0955706A - Optical transmitter - Google Patents

Abstract

(57) Abstract: A polarization scrambling method for an optical transmitter, and an object thereof is to provide an optical transmitter that performs polarization scrambling so that an optical signal with a small spectrum expansion can be received on the receiving side. . An optical signal that modulates linearly polarized light with a transmission data signal and maintains the linearly polarized state after the modulation,
In the optical transmitter that scrambles and outputs the linearly polarized state, a branching unit that branches the modulated optical signal into two.
First modulating means and, said branched other optical signal of the modulation signal having the predetermined frequency omega ₁ phase modulating and outputting the modulation signal having the branched _first predetermined frequency omega optical signal on one Is modulated by a signal shifted by a predetermined amount and is output, and a combining means is provided for adding and outputting the optical outputs of the first and second modulating means with their linear polarization axes orthogonal to each other. It consists of and.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical transmitter, and more particularly, in a multi-relay long-distance optical transmission system using an optical amplifier, polarization dependent loss of a transmission line, and further polarization dependent of an optical amplifier repeater. The present invention relates to a polarization scrambling method of an optical signal performed on the transmission side in order to reduce deterioration of the optical signal waveform on the reception side due to the influence of gain and the like.

The demand for increasing the repeater distance in a high-speed long-distance optical transmission system using an optical amplifier repeater is increasing more and more, and the interaction between the nonlinear effect and dispersion of the fiber and the cumulative spontaneous emission (ASE) of the repeater amplifier are increasing. However, it was found that factors such as polarization dispersion of the transmission line, polarization dependent loss, and polarization dependent gain of the optical amplification repeater cannot be ignored.
Countermeasures are beginning to be discussed.

[0003]

2. Description of the Related Art FIG. 7 is a block diagram of a first conventional optical transmitter. In the figure, linearly polarized CW light emitted from a laser oscillator (LD) 1 is input to an amplitude modulator (LiNbO ₃ (lithium niobate) modulator, etc.) 2 with its polarization direction aligned and transmitted by a transmission signal 6. Output after amplitude modulation. This output is input to a phase modulator (PM) (LiNbO ₃ modulator, etc.) 4 by rotating the plane of polarization by 45 degrees by a polarization controller 3.

Then, in the phase modulator 4, the linear polarization axis direction of the input light is 45 with respect to the modulation drive vector axis (for example, X axis) of the phase modulator 4 in a plane perpendicular to the propagation direction of the optical signal. It is considered that the electric field vector of the input light is evenly divided into the parallel component (X component) and the vertical component (Y component) because the angle is in degrees. This phase modulator 4 has a sine wave (πcos
ω ₁ t) or triangular wave (2 × sin ⁻¹ [cos (ω
₁ t)]) By adding the signal 5, only one polarization component (for example, X component) is phase-modulated and output.

As a result, the phase difference between the X component and the Y component changes in the period of ω ₁ in the range of −π to + π, and therefore FIG.
As shown in, the polarization state is upward linear polarization, counterclockwise elliptical polarization, counterclockwise circular polarization, counterclockwise elliptical polarization, sideways linear polarization, counterclockwise elliptical polarization, counterclockwise circular polarization, Left-handed elliptically polarized light, upward linearly polarized light, further right-handed elliptically polarized light, right-handed circularly polarized light, right-handed elliptically polarized light, sideways linearly polarized light, right-handed elliptically polarized light, right-handed circularly polarized light, right-handed circularly polarized light, right-handed circularly polarized light A cycle of elliptically polarized light and upward linearly polarized light occurs and is repeated with the same period of ω ₁ .

This is an example of the basic polarization scrambling output situation. One in which ω ₁ is faster than the speed of the transmission data signal is high-speed scramble, and _{one in} which ω ₁ is several tenths or less of the speed of the transmission data signal. Is called slow scrambling. These can be seen as changes in the polarization state over time at the optical signal transmission point, and the polarization state of the optical signal at each specific phase in the "1" portion of the output optical signal data is the ideal characteristic. It is invariable depending on the propagation on the optical transmission line when it is assumed to have, and it can be considered that the polarization state changes before and after the specific phase.

FIG. 8 is a block diagram of a second conventional optical transmitter. In the figure, linearly polarized CW lights emitted from two laser oscillators (LDs) 7 and 9 having different wavelengths (respective frequencies are ω _P and ω _S ) are input to amplitude modulators 8 and 10, respectively. Then, the same transmission signal 6 is amplitude-modulated and output. Then, a signal obtained by rotating the polarization plane of the output of the one amplitude modulator 10 by π / 2 by the polarization controller 11 and the output optical signal of the other amplitude modulator 8 are added to the polarization combiner 12, and these polarizations are combined. Two optical signals having orthogonal wave vectors are added.

As a result, from the polarization combiner 12, the optical output whose output polarization state changes in the same manner as in FIG. 6 at the cycle of the frequency difference (ω _P −ω _S ) of the two laser oscillators 7 and 9. Is obtained.

[0009]

However, in the above-mentioned polarization scrambling method of the optical transmitter, in the case of the first conventional example, the S / N ratio is deteriorated due to the spread of the spectrum of the optical signal on the receiving side. In addition, there is a problem that the amount of waveform deterioration due to dispersion of the optical transmission line is large and is easily affected by the nonlinear effect of the optical transmission line.

The present invention has been made to solve the above-mentioned problems, and an object of the present invention is to provide an optical transmitter that performs polarization scrambling so that an optical signal with a small spectrum expansion can be received on the receiving side. .

[0011]

The above problems can be solved by the following constitution. That is, (Claim 1) and (Claim 2) For an optical signal that modulates linearly polarized light with a transmission data signal and maintains the linearly polarized state after the modulation, an optical signal that scrambles and outputs the linearly polarized state. In the transmitter, branching means for branching the modulated optical signal into two, first modulating means for modulating one of the branched optical signals with a modulation signal having a predetermined frequency ω ₁ and outputting the branched optical signal, and the branching means The other optical signal is
Second modulation means for modulating and outputting the phase of a modulation signal having ₁ by a predetermined amount and the optical outputs of the first and second modulation means are added with their respective linear polarization axes orthogonal to each other. And a multiplexing means for outputting.

[0012] Consequently, for example when the modulation performed by the first and second modulating means is an amplitude modulation, when using a signal of sin .omega ₁ t and cos .omega ₁ t as a signal having a predetermined frequency, multiplexing means Then, since the linear polarization axes of the both signals are made orthogonal to each other and are added, the optical signal is output from the multiplexing means as linearly polarized light while rotating on the polarization plane at the speed of ω ₁ .

Further, the optical signal power transmitted from the multiplexing means is constant because sin ² ω ₁ t + cos ² ω ₁ t = 1. (Claim 3) When the modulation performed by the first and second modulating means according to Claim 1 is amplitude modulation, | cos (ω ₁ t) | and | sin (ω ₁ t) are given as predetermined signals, respectively. )
By using |, the polarization state reciprocates between the polarization axes orthogonal to each other with a period of 2ω ₁ , and the driving conditions of the first and second modulation means (for example, the amplitude modulator) are relaxed. It

(Claim 4) When the modulation performed by the first and second modulating means according to claim 1 is phase modulation,
(Π / 2) cos (ω ₁ t) as a signal having a predetermined frequency in the _first modulating means, and (−π) as a signal obtained by shifting the phase of the signal in the second modulating means by a predetermined amount. /
2) Use cos (ω ₁ t).

As a result, the optical signal output from the combining means repeats a cycle of a series of polarization states such as linearly polarized light, elliptically polarized light and circularly polarized light at a cycle of ω ₁ . (Claim 5) The signals (π / 2) cos (ω ₁ t) and (−π / 2) cos (ω ₁ t) used in the first and second modulation means according to claim 4 are converted to sin ^{− 1} [cos (ω
₁ t)] and −sin ⁻¹ [cos (ω ₁ t)]. However, the range of the sinusoidal inverse function sin ⁻¹ is −π / 2 to π
/ 2.

This is a signal obtained by replacing the signal described in claim 4 with a triangular wave, and has the same operation and effect as in claim 4. (Claim 6) Two linearly polarized optical signals of different frequencies (ω _S , ω _P ) output from two light sources are modulated by the same transmission data signal, and these are multiplexed to combine the polarized light. In an optical transmitter that realizes wavefront scrambling, first polarization control means for converting the one modulated optical signal into a right-hand circularly polarized signal and outputting the converted signal, and the other modulated optical signal to the left-hand circular signal. Second polarization control means for converting and outputting the polarization signal is provided, and the outputs of the first and second polarization control means are added and output.

As a result, two optical signals modulated by the same transmission data signal (each frequency is ω _S , ω _P )
Are converted into a right-hand circularly polarized signal and a left-hand circularly polarized signal, respectively, and by adding them, the linearly polarized optical signal has a polarization vector at a speed corresponding to the frequency difference (ω _P − ω _s ). Will be output while rotating.

[0018]

1 is a block diagram of the first embodiment of the present invention. In the figure, laser diode (LD)
The linearly polarized CW light emitted from 1 is input to the amplitude modulator (LiNbO ₃ modulator, etc.) 2 with the polarization vector direction aligned, and the amplitude is modulated by the transmission signal 6 and output.
This is the same as the first conventional example (FIG. 7) described above.

In the first embodiment, the output of the amplitude modulator 2 is branched into two by the optical coupler 13 while maintaining the polarization vector direction, and is input to the amplitude modulators 16 and 18, respectively. Amplitude modulator
In 16, the output of the sine wave signal of the oscillator 15 (sinω ₁ t) to the delay circuit 14 [pi / 2 delayed signal (cosω ₁ t)
Is amplitude-modulated by, and the other amplitude modulator 18 is amplitude-modulated by the sine wave signal. An optical signal obtained by rotating the polarization plane of the output of the amplitude modulator 16 by π / 2 by the polarization controller 17 and an output optical signal of the amplitude modulator 18 are added by the optical coupler 19. This state is shown in FIG.

In FIG. 2, (1) is the optical output waveform of the amplitude modulator 2, and shows the case of "1110011" of the digital signal. Light of frequency ω ₀ exists in the “1” part. (2) and (3) are the optical output waveforms of the amplitude modulators 16 and 18, respectively. The part (b) of (2) is because cosω ₁ t is a negative value region (shown by the dotted line), Frequency of part b) ω
The phase of ₀ light is opposite to that of adjacent (a) and (c). Also for (3), the frequency ω ₀ of the part of (e)
The phase of the light is opposite to that of adjacent (d) and (f). (4) shows the optical output waveform of the optical coupler 19.

As a result, the output optical signal of the optical coupler 19 is
As shown in 3, the linear polarization remains ω ₁On the plane of polarization at the speed of
Is output while rotating. At this time, the polarization vector
The polarization vector axis direction is always along the time axis, ignoring the rotation.
When the complex amplitude of the electric field of is observed by tracing, the optical signal
No amplitude and phase distortion of
The point becomes a singular point for the phase condition of the optical signal.
Do not increase waveform deterioration due to dispersion characteristics of the transmission line.
Can be expected to be

The frequency ω ₁ of the modulation signal from the oscillator 15 is set to about 2 to 3 times the frequency of the transmission signal 6 that modulates the emitted light of the laser oscillator 1. That is, the polarization scrambling cycle is made to occur at least twice in one bit (in the case of digital signal) of the transmission signal.

On the other hand, since the modulation by the amplitude modulator 16 and 18 cos .omega ₁ t and sinω ₁ t, (sin ²
The optical signal power transmitted from the optical coupler 19 is constant (due to ω ₁ t + cos ² ω ₁ t = 1) (see (4) in FIG. 2).

Further, in FIG. 1, the amplitude modulators 16 and 18 are shown.
In the amplitude modulation signal | cosω ₁ t | a | sinω ₁ t | when the replace, polarization state because that will shuttle between X-polarized light and Y-polarized light with a period of 2 [omega _1, the amplitude modulator 16 and 18 Driving conditions are eased.

Next, a second embodiment of the present invention will be described. FIG. 4 is a block diagram of the second embodiment. In the figure, linearly polarized CW light emitted from a laser oscillator (LD) 1 is input to an amplitude modulator (LiNbO ₃ modulator, etc.) 2 with its polarization direction aligned and amplitude-modulated by a transmission signal 6. It is the same as the first embodiment until the output and the output is branched into two by the optical coupler 20.

In the second embodiment, the two-branched optical signal is input to the phase modulators 23 and 25, and one of the phase modulators 25 has a signal ((π / 2) cosω ₁ t from the oscillator 22. ), And in the other phase modulator 23, a signal ((−π / 2) cosω ₁ t) delayed by π by the delay circuit 21.
, Respectively, phase-modulates and outputs. Then, the polarization plane of the output of the phase modulator 23 is set to π / 2 by the polarization controller 24.
The optical coupler 26 adds the rotated signal and the output of the phase modulator 25.

As a result, the phase difference between the two polarization vectors separately phase-modulated and finally multiplexed by the optical coupler 26 should change in the range of -π to + π in the cycle of ω _1. The output status of polarization scrambling is also as shown in FIG. However, since the modulation drive range of each phase modulator is -π / 2 to π / 2, and the band expansion of the optical signal is suppressed, the waveform deterioration due to dispersion is alleviated as compared with the conventional example (FIG. 7). It

As a phase control signal output from the oscillator 22, instead of a sine wave, a triangular wave (2 sin ^-1 [cos
(Ω ₁ t)], where the range of the sin ⁻¹ function is −π / 2 to π /
2), the same effect as described above can be obtained.

Next, a third embodiment of the present invention will be described. As described above, in the second conventional example (FIG. 8), the polarization plane is scrambled from the optical transmitter by using the two laser oscillators 7 and 9 having different wavelengths (the respective frequencies are ω _P and ω _S ). The scramble system for outputting the generated optical signal has been described.

The third embodiment proposes another scrambling method using two laser oscillators having different wavelengths. FIG. 5 is a block diagram of the third embodiment. In the figure, linearly polarized CW lights emitted from two laser oscillators 7 and 9 having different wavelengths (respective frequencies are ω _P and ω _S ) are input to amplitude modulators 8 and 10, respectively, and the same. It is the same as the above-mentioned second conventional example until the amplitude modulation by the transmission signal 6 and the output.

In the present embodiment, the polarization plane of the output of one of the amplitude modulators 8 is rotated 135 degrees by the polarization controller 27, and is further passed through the λ / 4 plate 28 to be converted into a right circular polarization signal. . The polarization controller 29 is also used for the output of the other amplitude modulator 10.
Rotate the plane of polarization by 45 degrees with λ / 4 plate 30
It is converted into a left-hand circularly polarized signal by passing through.

When the right circular polarized signal and the left circular polarized signal are added by the optical coupler 31, these two wavelength components are set to have the same power, so that the output signal is the same as in the case of the first embodiment. , The frequency difference between two linearly polarized waves (ω
_It outputs while rotating on the plane of polarization at a speed equivalent to _P − ω _S ) (see Fig. 3).

At this time, the frequency difference (ω _P −ω _S ) between the two waves is set to about 2 to 3 times the frequency of the transmission signal 6. That is,
The polarization scrambling cycle is made to occur at least twice during one bit (in the case of digital signal) of the transmission signal.

[0034]

As described above, by using the optical transmitter according to the present invention, it becomes possible for the receiving side to receive an optical signal with a small spectrum expansion.

[Brief description of drawings]

FIG. 1 is a configuration diagram of a first embodiment of the present invention,

FIG. 2 is an explanatory diagram of the first embodiment,

FIG. 3 is a diagram showing a state of a polarization-scrambled optical signal vector in the first embodiment.

FIG. 4 is a configuration diagram of a second embodiment of the present invention,

FIG. 5 is a configuration diagram of a third embodiment of the present invention,

FIG. 6 is a diagram showing an example of a polarization-scrambled optical signal vector,

FIG. 7 is a configuration diagram of an optical transmitter of a first conventional example,

FIG. 8 is a configuration diagram of an optical transmitter of a second conventional example.

[Explanation of symbols]

 1 is a semiconductor laser (LD), 2 is an amplitude modulator, 3 is a polarization controller, 4 is a phase modulator, 5 is an oscillator, 6 is a transmission signal, 7 and 9 are semiconductor lasers (LD), 8 and 10 are Amplitude modulator, 11 is polarization controller, 12 is polarization combiner, 13 is optical coupler, 14 is delay circuit, 15 is oscillator, 16 and 18 are amplitude modulators, 17 is polarization controller, 19 and 20 Is an optical coupler, 21 is a delay circuit, 22 is an oscillator, 23 and 25 are phase modulators, 24 is a polarization controller, 26 is an optical coupler, 27 and 29 are polarization controllers, and 28 and 30 are λ / 4 plates. , 31 are optical couplers.

Continuation of the front page (51) Int.Cl. ⁶ Identification code Office reference number FI technical display location H04B 10/152 10/142 10/02 10/18

Claims (6)

[Claims] 1. An optical signal which modulates linearly polarized light with a transmission data signal and maintains the linearly polarized state after the modulation,
In an optical transmitter that scrambles and outputs the linearly polarized state, a splitting unit that splits the modulated optical signal into two, and one of the split optical signals is modulated by a modulation signal having a predetermined frequency ω _1. First modulating means for outputting the optical signal, and second modulating means for modulating and outputting the other branched optical signal by a signal obtained by shifting the phase of the modulated signal having the predetermined frequency ω ₁ by a predetermined amount. An optical transmitter, comprising: a combining unit that adds the optical outputs of the first and second modulating units with their respective linear polarization axes orthogonal to each other and outputs the added signals. 2. When the modulation performed by the first and second modulating means is amplitude modulation, the modulation signal having a predetermined frequency ω ₁ in the first and second modulating means is a cosine wave cos (ω ₁ t) and the sinusoidal wave sin (ω ₁ t) are set to a phase relationship, and the optical signals of the output optical signals of the first and second modulating means are set in a range in which the modulated signal has a negative value. The optical transmitter according to claim 1, wherein the first and second modulators are set so that the phase of the frequency ω ₀ is relatively deviated by π. 3. The first and second sets according to claim 2.
2. The cosine wave and the sine wave in the modulating means of claim 1 are replaced with the absolute values of the cosine wave and the sine wave.
The optical transmitter described in 1. 4. The first and second modulations according to claim 1.
When the modulation performed by the means is phase modulation, the signal having the predetermined frequency in the first modulating means is
(Π / 2) cos (ω ₁t), in the second modulating means
The phase of a signal with a specified frequency is shifted by a specified amount
The signal is (-π / 2) cos (ω₁t)
The optical transmitter according to claim 1. 5. The signals (π / 2) cos (ω ₁ t) and (−π / 2) cos (ω ₁ t) used in the first and second modulating means according to claim 4 are sin ^{−. 1} [cos (ω
₁ ) and -sin ^-1 [cos (ω ₁ t)], and the range of the sinusoidal inverse function sin ^-1 is -π / 2 to π / 2. Optical transmitter. 6. The two linearly polarized optical signals of different frequencies (ω _S , ω _P ) output from the two light sources are modulated by the same transmission data signal, and these are multiplexed to obtain a polarized wave. In an optical transmitter that realizes wavefront scrambling, first polarization control means for converting the one modulated optical signal into a right-hand circularly polarized signal and outputting the right-hand circularly polarized signal, and the other modulated optical signal to the left-hand circular signal. An optical transmitter having a second polarization control means for converting into a polarization signal and outputting the polarization signal and adding the outputs of the first and second polarization control means for output. .