JP2003143076A - Light transmission circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a light transmission circuit which can obtain a high quality optical signal at a low cost in an analog optical transmission or the like, by reducing nonlinear distortion of an optical signal for communication itself. SOLUTION: In an optical signal generating part 1, the difference between a modulated electrical signal 11 and a feedback electric signal 12 is detected by a drive circuit 5, and a drive current proportional to the difference is injected into an LD 6, thereby generating an optical signal, which is transmitted as the optical signal 13 for transmission by an optical transmission line 2. A part of the optical signal 13 is branched as an optical signal 14 for monitoring and inputted in a monitoring part 4, in which the optical signal 14 is subjected to photoelectric conversion by a PD 7, amplified by an amplifier 8 and outputted as an output electric signal 15. A part of the electrical signal 15 is branched by a branching circuit 9, and inputted in the driving circuit 5 as the feedback electrical signal 12, and as a result, by making negative feedback applies to the circuit as a whole, the linearity of an output light of the LD 6 is improved and nonlinear distortion is reduced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a circuit for driving a laser diode with a low distortion to transmit an optical signal in an optical communication system, and an optical signal of a constant output from the laser diode externally modulated with a low distortion. The present invention relates to a circuit that transmits a signal.

[0002]

2. Description of the Related Art In optical communication, long distance transmission and high speed
In order to perform large-capacity transmission, a laser diode (hereinafter, abbreviated as LD) that has good coherence and high output is mainly used.

[0003]

From the above characteristics, the LD is suitable for baseband digital transmission and is widely used for optical communication. However, in general, the linearity of the optical output with respect to the electric input is poor and the analog signal is used. Is not suitable for use in optical transmission.

Conventionally, in order to use an LD for optical transmission of an analog signal, a special LD in which distortion generated in the LD itself is reduced by structural ingenuity, or a predistortion method is used in an electric stage of a transmitter / receiver, Since a non-linear distortion must be canceled by a feed-forward method or the like and it is necessary to adjust and minimize the distortion for each communication link, the device cost and the operation cost increase.

Further, a method is also used in which the output light intensity of the LD is fixed and is modulated by an electric signal using an external modulator. In this method, there is no chirping of the oscillation wavelength due to the direct modulation of the LD. Although it has the advantage of keeping the coherence of the light source high, the linearity of the external modulator is often worse than that of the LD, and in order to use it for optical transmission of analog signals, it is necessary to devise the same nonlinear distortion as above. there were.

The present invention has been made against such a background. By directly driving or externally modulating the LD on the transmitting side by the driving method of the present invention, the nonlinear distortion of the optical signal for communication itself is reduced, An object of the present invention is to realize an optical transmission circuit that can obtain a high-quality optical signal at low cost even in analog transmission.

[0007]

The present invention relates to a driving system for reducing non-linear distortion that occurs when an LD used as a light emitting element in optical communication is driven by an electric signal, and is a feature of the present invention. When a LD is driven by an electric signal in an optical transmission circuit, a part of the optical signal from the LD is taken out and photoelectrically converted by a light receiving element such as a photodiode (hereinafter abbreviated as PD) with less nonlinear distortion. Amplify and add a part of the output electric signal to the drive electric signal of the LD in anti-phase, and as a result, negative feedback is applied to the entire circuit to enhance the linearity of the output light of the LD and reduce nonlinear distortion. There is a place to let.

Further, in addition to the above, the present invention is a modulation system in which the output light of the LD is kept constant and the nonlinear distortion of the output light is reduced when the intensity of the output light is modulated using an external modulator. And, the feature of the present invention is that
When the external modulator is driven by an electric signal in the optical transmission circuit to externally modulate the output light of the LD, a part of the optical signal from the external modulator is taken out, and the light is received by the light receiving element such as PD having less nonlinear distortion. Linearity of the output light of the external modulator by amplifying after conversion, adding a part of the output electric signal to the drive electric signal of the external modulator in anti-phase, and consequently applying negative feedback to the entire circuit. , And reduce nonlinear distortion.

In the present invention, by introducing the effect of negative feedback in the LD direct drive system or the external modulation system as described above, the non-linear distortion of the optical signal itself is reduced, so that the analog signal which is easily affected by the non-linear distortion. As for the above, high-quality optical transmission can be realized at low cost only by electrical processing on the transmitting side.

Further, a light receiving element or an amplifying circuit used on the receiving side should have the same or equivalent characteristics as the light receiving element or the amplifying circuit used on the transmitting side for photoelectrically converting and amplifying a part of the optical signal. By using it, it is possible to further suppress the non-linear distortion of the final received electric signal.

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows a first embodiment of the optical transmission circuit of the present invention.
In the figure, 1 is an optical signal generator, 2 is an optical transmission line, 3 is an optical branch circuit, and 4 is a monitor.

The optical signal generator 1 generates an optical signal according to the difference between a modulated electric signal from the outside and a feedback electric signal described later, and detects the difference between the modulated electric signal and the feedback electric signal. The drive circuit 5 generates a drive electric signal proportional to the difference, and the LD 6 which is directly driven by the drive electric signal and outputs a modulated optical signal.

The optical branching circuit 3 branches a part of the optical signal output from the optical signal generator 1 to the optical transmission line 2 as a monitor optical signal.

The monitor section 4 converts the monitor optical signal into a feedback electric signal. The PD 7 converts the monitor optical signal into an electric signal, the amplifier 8 amplifies the output electric signal of the PD 7, and the amplifier 8. And a branch circuit 9 for branching a part of the output electric signal as a feedback electric signal.

In the above structure, the modulated electric signal 11 inputted to the optical signal generator 1 is inputted to one non-inverting input terminal of the driving circuit 5 and added with the feedback electric signal 12 inputted to the other inverting input terminal. That is, the difference is detected, converted into a drive current (drive electric signal) proportional to the intensity of the difference, and then injected into the LD 6. The LD 6 outputs a modulated optical signal according to this drive current.

This optical signal is output to the optical transmission line 2 and transmitted as a communication optical signal 13, a part of which is branched as a monitor optical signal 14 by the optical branch circuit 3 and the monitor unit 4 is provided.
Entered in.

In the monitor unit 4, the PD 7 photoelectrically converts the monitor optical signal 14, the amplifier 8 amplifies the optical signal, and the output electrical signal 15 is taken out. A part of the output electric signal 15 is branched by the branch circuit 9 and input to the drive circuit 5 of the optical signal generator 1 as the feedback electric signal 12.

Next, the principle of the present invention will be described with reference to FIG.

FIG. 2 is a schematic diagram of a circuit configuration for explaining the operation principle of the optical transmission circuit of the present invention. In the figure, 21 is an adder circuit, 22 is a phase inverting circuit, 23 is an E / O converter, Two
Reference numeral 4 is an optical branch circuit, 25 is an O / E converter, and 26 is a branch circuit.

Let the modulated electric signal 31 from the outside be i.
The modulated electric signal 31 [i] is added by the adder circuit 21 with the feedback electric signal 32 whose phase is inverted by the phase inverting circuit 22 so as to have the opposite phase to the modulated electric signal 31, and then input to the E / O converter. E / O converter 2 as signal 33
Enter in 3. It is assumed that the E / O converter input signal 33 is j, and the intensity of the output light of the E / O converter 23 is represented by L (j) as a function of this j.

The output light of the E / O converter 23 is branched by an optical branch circuit 24 into a main optical signal 35 output as a communication optical signal 34 and a monitor optical signal 36. The branching ratio of the main optical signal 35 and the monitor optical signal 36 is 1-α: α
(However, 0 <α <1).

The monitor optical signal 36 is input to the O / E converter 25 as an O / E converter input signal 37. This O / E converter input signal 37 is set to k, and the O / E converter 25
The output electric signal of is expressed as A (k) as a function of k.

The output electric signal of the O / E converter 25 is output from the branch circuit 26 as a monitor electric signal 39, which is output as an output electric signal 38, and the feedback electric signal 3.
It is branched to 2. The branching ratio of the monitor electric signal 39 and the feedback electric signal 32 is set to 1-β: β (where 0 <β ≦
1).

The intensity of the communication optical signal 34 is expressed as P (i) as a function of the modulated electric signal i, and the output electric signal 38 is expressed as B as a function of the modulated electric signal i.
It will be represented by (i).

Since a feedback electric signal is applied to the inverting input terminal of the driving circuit 5 in FIG. 1, the function of the adding circuit 21 and the function of the phase inverting circuit 22 are included in the driving circuit 5. There is. On the contrary, in FIG. 2, the voltage / current conversion function of the drive circuit 5 of FIG. 1 and the electro-optical conversion function of the LD 6 are shown as an E / O conversion unit 23.
The photoelectric conversion function of the PD 7 and the amplification function of the amplifier 8 in FIG. 1 are shown as an E / O conversion unit 25.

In the above circuit configuration, the E / O converter input signal j and the O / E converter input signal k are expressed by the following equations (1) and (2) j = i-βA (k) (1) ) K = αL (j) (2) Further, the communication optical signal P (i)
And the output electric signal B (i) is expressed by the following equations (3), (4) P (i) = (1-α) L (j) (3) B (i) = (1-β) A (k ) ... (4), respectively.

In order to obtain the linearity of (3) and (4), when each is differentiated by i, dP (i) / di = (1−α) · dL (j) / di (5) dB (i ) / Di = (1−β) · dA (k) / dk · dk / di From the equation (2), dB (i) / di = α (1−β) · dA (k) / dk · dL (j) / Di (6)

Where     dL (j) / di = dL (j) / dj · dj / di (7) However, from equation (1)     dj / di = 1-β · dA (k) / di               = 1-β · dA (k) / dk · dk / di               = 1-αβ · dA (k) / dk · dL (j) / di (8) Therefore, from equations (7) and (8)     dL (j) / di = dL (j) / dj / (1 + αβ · dA (k) / dk · d L (j) / dj) (9) Becomes

Substituting the equation (9) into the equations (5) and (6) and transforming them, dP (i) / di = (1-α) / [dA (k) / dk · {αβ + 1 / (dA (K) / dk · dL (j) / dj)} ... (10) dB (i) / di = α (1-β) / {αβ + 1 / (dA (k) / dk · dL (j) / dj )} (11) is obtained.

DL (j) / dj represents the change of the optical signal with respect to the change of the modulated electric signal of the E / O converter, and L (j) is approximately proportional to j in the normal LD operation range. Therefore, the value is approximately constant (the part depending on j acts as nonlinear distortion). dA (k) / dk is O /
The change in the output electric signal with respect to the change in the input optical signal of the E conversion unit is represented. The response A (k) of the O / E conversion unit is almost linear, and the amplification factor of the amplification circuit of the O / E conversion unit is If sufficiently high, dA (k) / dk · dL (j) / dj >> 1 (12) holds.

At this time, equations (10) and (11) are approximately dP (i) / di = (1-α) / (αβ · dA (k) / dk) ... (13) dB (i) / Di = (1-β) / β (14)

DP (i) / di and dB (i) / di are modulated electrical signals 31 input to the optical transmission circuit, respectively.
The state of changes in the communication optical signal 34 [P (i)] and the output electrical signal 38 [B (i)] with respect to [i] is shown. When the expression (12) is satisfied, the expression (14) is L (j)
It is a constant that depends only on the coefficient β that indicates the feedback ratio, not on A or k (A). At this time, B (i) becomes proportional to i, and the nonlinear effect almost disappears.

If the linearity of the O / E converter including the light receiving element and the amplifier is good, dA (k) / dk has a constant value that does not depend on k. In general, a light receiving element such as a pin photodiode is considerably excellent in linearity as compared with an LD or the like, and an amplifier having an appropriate linearity can realize an extremely good linearity. By using an O / E converter having good linearity in this way, dA (k) / dk can be made substantially constant.
At this time, the equation (13) can be regarded as a constant that does not depend on L (j) or A (k), and the nonlinear effect of the communication optical signal P (i) almost disappears.

Further, when the above conditions are satisfied, if the frequency response bands of the E / O converter 23 and the O / E converter 25 are wide, the equations (13) and (14) approximate the modulated electric signal i. Since it is a constant value that does not depend on the frequency, the frequency characteristics of the intensity of the communication optical signal 34, the amplitude of the output electric signal 38, and their phases can be flattened and widened.

By applying the feedback configuration of the present invention based on the principle described above and using a circuit having good linearity and a high signal amplification factor in the O / E conversion section (monitor section), a communication optical signal is obtained. The linearity of the output electric signal 34 and the output electric signal 38 with respect to the modulated electric signal 31 can be improved, and the occurrence of nonlinear distortion can be suppressed.

In addition to this, by using a circuit having the same or equivalent characteristics as the O / E converter 25 in the optical receiving circuit for receiving the signal optically transmitted from the communication optical signal 34, the equation (14) is obtained. Since the above holds for the output electric signal of the optical receiving circuit, even if the linearity of the O / E converter 25 itself is somewhat poor, it is possible to transmit the output electric signal with extremely small nonlinear distortion.

FIG. 3 shows a second embodiment of the optical transmission circuit according to the present invention, which is an embodiment using an external modulation method, in which the same components as those in the first embodiment are designated by the same reference numerals. Is represented by. That is, 2 is an optical transmission line, 3 is an optical branch circuit, 4 is a monitor unit, and 41 is an optical signal generation unit.

The optical signal generator 41 generates an optical signal according to the difference between a modulated electric signal from the outside and a feedback electric signal which will be described later, and detects the difference between the modulated electric signal and the feedback electric signal. A drive circuit 42 that generates a drive electric signal proportional to the difference, an LD 43 that receives a constant drive current from a drive current source (not shown) and emits CW with a constant light intensity, and a light from the LD 43 into a drive electric signal. It is composed of an external modulator 44 which modulates in accordance with the above.

In the above configuration, the modulated electric signal 51 input to the optical signal generator 41 is input to one non-inverting input terminal of the drive circuit 42 and added with the feedback electrical signal 52 input to the other inverting input terminal. That is, the difference is detected, converted into a drive electric signal proportional to the intensity of the difference, and then input to the external modulator 44. The external modulator 44 receives the CW from the LD 43 according to the drive electric signal.
Intensity modulation of light.

The modulated optical signal is output to the optical transmission line 2 and transmitted as a communication optical signal 53, a part of which is branched by the optical branching circuit 3 as a monitoring optical signal 54 to the monitor section 4. Is entered.

In the monitor section 4, the PD 7 photo-electrically converts the monitor optical signal 54, amplifies it by the amplifier 8, and extracts it as an output electric signal 55. A part of the output electric signal 55 is branched by the branch circuit 9 and input to the drive circuit 5 of the optical signal generator 1 as the feedback electric signal 52.

In general, the modulation characteristic of an external modulator is inferior in linearity to a modulated electric signal.
In the case of a Zender (MZ) type modulator, its modulation characteristic curve is represented by a sine wave, and a large second-order distortion occurs, which is not suitable for analog optical communication.

However, according to the present invention, the non-linear distortion generated in the external modulator by the negative feedback can be reduced by the same principle as in the case of the LD direct drive system described above, and the distortion is simple and the distortion is low. An optical signal for communication can be obtained.

[0044]

As described above, according to the present invention,
It is possible to significantly reduce the non-linear distortion of the output optical signal by simply adding a monitor unit and a feedback circuit to the LD drive circuit and external modulation circuit used in ordinary optical communication, and to reduce the optical output intensity and The frequency characteristic of the phase can be improved.

Further, by using an optical receiving circuit having the same or equivalent characteristics as the monitor unit on the receiving side, even if the linearity of the monitor unit or the optical receiving circuit is not good, the optical receiving circuit As a result, it is possible to obtain a received signal with low distortion as to the output electric signal from the.

[Brief description of drawings]

FIG. 1 is a configuration diagram showing a first embodiment of an optical transmission circuit of the present invention.

FIG. 2 is a schematic diagram of a circuit configuration for explaining the operation principle of the optical transmission circuit of the present invention.

FIG. 3 is a configuration diagram showing a second embodiment of an optical transmission circuit of the present invention.

[Explanation of symbols]

1, 41: optical signal generator, 2: optical transmission line, 3, 24: optical branch circuit, 4: monitor unit, 5, 42: drive circuit, 6, 4
3: LD, 7: PD, 8: amplifier, 9, 26: branch circuit, 11, 31, 51: modulated electric signal, 12, 32, 5
2: Feedback electric signal, 13, 34, 53: Optical signal for communication, 14, 36, 54: Optical signal for monitor, 15,
38, 55: output electric signal, 21: adder circuit, 22: phase inverting circuit, 23: E / O converter, 25: O / E converter, 27: feedback circuit, 33: E / O converter input signal, 35: main signal light, 37: O / E converter input signal, 39: monitor electric signal, 44: external modulator.

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ Identification code FI theme code (reference) H04B 10/26 10/28

Claims (4)

[Claims] 1. An optical signal generator for generating an optical signal corresponding to a modulated electric signal and outputting the optical signal to an optical transmission line, the optical signal generating section generating an optical signal according to a difference between the modulated electric signal and a feedback electric signal. And an optical branching unit that branches a part of the optical signal as a monitoring optical signal, and a monitoring unit that converts the monitoring optical signal into a feedback electric signal. 2. The optical signal generator detects a difference between the modulated electric signal and the feedback electric signal, and generates a drive electric signal proportional to the difference, and outputs an optical signal modulated by the drive electric signal. 2. The optical transmission circuit according to claim 1, further comprising: 3. The optical transmitter circuit according to claim 2, wherein a laser diode directly driven by the drive electric signal is used as the means for outputting the optical signal modulated by the drive electric signal. 4. A laser diode that emits light with a constant light intensity, and an external modulator that modulates the light from the laser diode according to the drive electric signal, as means for outputting an optical signal modulated by the drive electric signal. 3. The method according to claim 2, wherein
The optical transmission circuit described.