JPH0750636A - Light transmitting device for light amplifying and repeating transmission - Google Patents

Abstract

PURPOSE:To make it difficult to generate the large loss and the degradation of an SN based on polarization direction dependency of the loss of optical parts in a light repeating transmission system composed of the multistage repeatings by a light amplifier. CONSTITUTION:Non-polarized state light 22 in a prescribed band is taken out from non-polarized state light from a light emitting diode 26 by an optical band-pass filter 28, an intensity modulation is performed for this non-polarized state light by a transmission pattern 24 in an optical modulator 23 and the light is defined as transmission signal light 25. An intensity modulation is performed for polarized light 34 from a laser diode 33 by the transmission pattern 24 in the optical modulator 23, the emission signal light is fluctuated in a polarized direction at higher speed than the transmission pattern 24 in a polarized modulator 36 and the signal is defined as transmission signal light 37.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical transmitter for injecting an optical signal into an optical amplification repeater transmission line constructed by arranging a large number of optical amplification repeaters in cascade.

[0002]

2. Description of the Related Art An optical amplifier repeater comprises an erbium-doped fiber that gives a gain to an optical signal and an optical component that gives a loss. Of these, the optical component usually has a slight polarization state dependency of the loss. In an optical amplification repeater transmission system in which a large number of optical amplification repeaters are connected in cascade, the influence of the difference in the loss of optical components depending on the polarization direction may be increased due to accumulation. Here, generally, in an optical component in which the loss amount depends on the polarization state, the polarization state in which the maximum value of the loss amount is given and the polarization state in which the minimum value is given are orthogonal to each other. Or later,
The traveling direction of the signal light is defined as the z-axis, and the two axes orthogonal to each other and the x-axis and the y-axis. Further, a polarization direction loss of the i-th optical component is a maximum a _i axis, a polarization direction loss is minimized and b _i axis. This is schematically shown in FIG. Here, the a _i axis, the b _i axis, and the z axis are orthogonal to each other. Note that the relationship between the a _i axis and the b _i axis and the x axis and the y axis is random.

In the optical amplification repeater transmission system, the polarization direction of the signal light is always in the a _i axis direction (1≤
and if i ≦ main signal is the number of light components transmitted through), in the case the polarization direction is always b _i axis, the total excess loss of signal light received from the light components differ greatly. For example, in an optical amplification repeater transmission system, when the number of optical components through which the main signal light passes between the transmitter and the receiver is 100 and the polarization state dependency of the loss of each optical component is 0.1 dB, The total excess loss can increase by 10 dB. Further, if the total excess loss increases, the SN ratio of the received signal also deteriorates excessively. Therefore, the SN ratio of the received signal also depends on the polarization state. further,
When the polarization state of the signal light fluctuates with time, the SN ratio of the received signal also fluctuates, which causes problems with stability and reliability.

[0004]

Due to the polarization state dependence of the loss of the repeater components of the optical amplification repeater transmission system, excessive loss of signal light and deterioration of the SN ratio of the received signal may occur. An object of the present invention is to provide an optical transmitter for optical amplification repeater transmission, which can reduce these and improve the stability and reliability of the optical amplification repeater transmission system.

[0005]

According to the invention of claim 1, a light source that emits light in a non-polarized state is used. According to the second aspect of the invention, a light source that emits polarized light is used as the light source, and the polarization state control means for changing the polarization state at a higher speed than the information signal is provided in series with the optical modulator.

[0006]

According to this structure, the electric field component of the signal light is always present in both the x-axis and the y-axis for each bit of the control signal for transmission. Therefore, even if the x-axis direction component (or the y-axis direction component) of the electric field of the signal light receives an excessive loss due to the polarization direction dependence of the loss of the optical component, the y-axis direction component (or the x-axis direction component). Can propagate to the optical receiver without suffering from excessive loss.

[0007]

FIG. 1A shows an embodiment of the invention of claim 1. The output light 22 in the non-polarized state from the non-polarized light source 21 is modulated by the transmission pattern (information signal) by the optical modulator 24, and the signal light 25 is transmitted in the non-polarized state. The non-polarized light source 21 is, for example, a block diagram. As shown in FIG. 1B, the output light 27 from the light emitting diode 26 is in a non-polarized state.
The light 22 narrowed in band by the optical bandpass filter 28 is also in the non-polarized state, and the electric field has both the x-axis component and the y-axis component. To be done.

Further, instead of the light emitting diode 26 in FIG. 1B, an optical amplifier circuit, for example, an erbium-doped optical amplifier circuit 31 having an optical non-reflection terminator 29 connected to the input side as shown in FIG. 1C may be used. The optical amplifier circuit 31 having no input light amplifies optical noise, and the optical amplifier circuit 31 emits broadband unpolarized light 32. FIG. 1D shows an embodiment of the invention of claim 2. The polarized light 34 from the light source 33 that emits polarized light is intensity-modulated by the optical modulator 23 by the transmission pattern 24. The polarization state controller 36 changes the polarization state of the signal light 35 emitted from the optical modulator 23 at a speed higher than that of the transmission pattern 24, and is transmitted as the signal light 37. As a light source of polarized light, for example, output light from a laser diode is usually in a completely polarized state, and this is used.

It is desirable to change the polarization direction by about 90 degrees in order to change the polarization state, but the effect can be obtained by changing the polarization direction by about 10 degrees or more. The rate of change may be at least as high as the bit rate of the transmission pattern. Furthermore, if the polarization direction is changed sufficiently fast and randomly,
The emitted signal light 37 is in a non-polarized state. FIG. 2A shows the embodiment of the invention of claim 2 more specifically. The signal light 35 intensity-modulated by the transmission pattern is bisected by the polarization beam splitter 38 to be signal lights 39 and 41. One of the signal lights 39 is incident on the optical phase modulator 42. The optical phase modulator 42 performs phase modulation on the signal light 39 at a speed equal to or higher than the bit rate of the transmission pattern 24, and the signal light 43 emitted from the optical phase modulator 42 is converted into the signal light 41 by the polarization beam splitter 44. The signal light 37 is combined and becomes a signal light 37 whose polarization state changes at high speed. Although not shown in the figure, it is preferable to insert an optical delay unit in the path of the signal light 41 so that the optical path length difference between the two paths between the two beam splitters 38 and 44 becomes zero.

FIG. 2B shows another example of the polarization state controller 36 shown in FIG. 1D. This polarization state controller is a polarization modulator using a uniaxial crystal, and the signal light 35 is transmitted to the electro-optic crystal 46 whose x-axis is the direction of the axis having a large electro-optic constant of the uniaxial crystal. The polarization components in both the axis and the y-axis are incident so that their electric powers are equal. In the electro-optic crystal 46, for example, 2 perpendicular to the x-axis is used so that the electric field strength in the x-axis direction can be changed.
The electrodes 47 and 48 are attached to the plane, and the two electrodes 47 and 4
By changing the potential difference between 8, the electric field strength in the x-axis direction can be changed. A change in the potential difference and the electro-optical effect cause a change in the phase of the signal light of the polarized waves in the x-axis direction and the y-axis direction. The magnitude of the phase change of the axially polarized signal light is different. Therefore, the polarization modulation can be applied to the signal light by changing the potential difference between the two electrodes 47 and 48 in this way. Here, the potential difference between the two electrodes 47 and 48 is changed at a speed higher than the bit rate of the transmission pattern 24. Then, the output optical signal 37 from this polarization modulator can change the polarization state at a high speed higher than the bit rate of the transmission pattern 24.

Instead of the polarization modulator shown in FIG. 2B, a configuration using a phase modulator using the electro-optical effect can be used. In this case, the polarization direction 49 of the signal light 35 is set so that the component in the polarization direction when used as a phase modulator is 1/2 of the incident signal light power. Also in the case of this configuration, the phase change amount of the input polarization direction signal component when used as a phase modulator due to the change of the phase modulation electric input,
Since the phase change amount of the signal component in the polarization direction orthogonal to that is different, polarization modulation can be applied. This configuration
Since the existing phase modulator can be used, there is no need to newly design and develop the modulator, and thus it can be used at low cost.

In the above description, the optical modulator 23 may be frequency modulation as well as intensity modulation. Further, in FIG. 1D, the polarization state controller 36 may be provided before the optical modulator 23.

[0013]

As described above, according to the optical transmitter of the present invention, the signal light incident on the optical amplification repeater transmission system is x.
Since there is an electric field component in both the axis and the y-axis, in the optical amplification repeater transmission system, excessive loss due to polarization direction dependency of repeater component parts and deterioration of the SN ratio are prevented, and stability and reliability are improved. Is possible.

[Brief description of drawings]

1A is a block diagram showing an embodiment of an optical transmitter for optical amplification relay transmission according to the invention of claim 1, B is a block diagram showing a specific example of a non-polarized light source 21, and C is a non-polarized light source 21. FIG. FIG. 11 is a block diagram showing a part of another example, and D is a block diagram showing an embodiment of an optical transmitter for optical amplification relay transmission according to the invention of claim 2.

FIG. 2A is a block diagram showing the embodiment of the invention of claim 2 in further detail; FIG. 2B is a perspective view showing another example of the polarization state controller 36 in FIG. 1D.

FIG. 3 is a diagram showing a relationship between an xy coordinate system of a transmission system and a polarization direction a _i that maximizes loss and a polarization direction b _i that minimizes loss.

Claims (2)

[Claims] 1. An optical amplification relay transmission optical transmitter for modulating and transmitting light from a light source according to an information signal, wherein an unpolarized light source is used as the light source. Optical transmitter. 2. An optical transmitter for optical amplification relay transmission, wherein light is optically modulated by an optical modulator with an information signal and transmitted from an optical source, wherein a light source for emitting polarized light is used as the light source, and the optical modulator. An optical transmitter for optical amplification relay transmission, which is provided in series with the polarization state control means for varying the polarization state at a higher speed than the information signal.