JPH05244096A - Optical transmission equipment and optical communication equipment - Google Patents

Abstract

PURPOSE:To attain the extension of transmission distance and the improvement of a code error rate by performing frequency modulation on the output light of a light source synchronizing with the on-bits of light in accordance with three kinds of bits detected by a detection circuit. CONSTITUTION:A frequency conversion circuit 16 drives a semiconductor laser 1 synchronizing with the on-bits of light in accordance with three kinds of bits; the rise bit, the fall bit, and the isolated bit of an information signal 3 detected by the detection circuit 7. The amplitude modulation of a drive current by a clock signal 4 generates frequency modulation on the output light 11 of the laser 1. A light modulator 2 is driven by the information signal 3, and transmits the light as a turned on and off optical signal 12 corresponding to the signal 3. At this time, phase shift matching between the signals 3 and 4 is performed by phase shifters 6 and 5 so as to apply the frequency modulation with three kinds of bits of the signal 3. The optical signal obtained in such a way is transmitted as the transmission signal of optical transmission equipment 13.

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 an optical communication device, and more particularly to an output light which is a continuous wave from a light source, and an optical on / off signal according to a binary information signal of an NRZ waveform. The present invention relates to an optical transmission device that converts an optical signal into an optical transmission signal and transmits the optical modulation signal, and an optical communication device using the same.

[0002]

2. Description of the Related Art In a digital optical communication system which has been put into practical use at present, a signal is transmitted through an optical fiber by turning on and off light of a semiconductor laser according to "0" and "1" of a digital signal. On the receiving side, the received optical signal is converted into a current by a photodiode to reproduce the signal. There are two factors that limit the transmission distance of this optical communication system.
The first factor is optical fiber loss, which can be overcome by optical amplifiers. The second factor is the distortion of the received waveform due to the wavelength dispersion of the optical fiber and the spread of the signal spectrum. This is a phenomenon in which the high-frequency component of the spectrum of the signal light takes a shorter time to propagate through the fiber (wavelength dispersion) than the low-frequency component, and a rectangular optical pulse is distorted on the transmitting side on the receiving side.

Particularly, as a transmitter, when a semiconductor laser is directly modulated with a signal current, the refractive index inside the laser changes and the oscillation wavelength changes according to the on / off of the modulation current.
This phenomenon is magnified by so-called chirping, resulting in waveform deterioration. As a transmission method in which this chirping does not occur, a method in which a semiconductor laser is continuously oscillated and its output light is converted into an on-off signal by an optical modulator is well known. However, even with this optical modulation method, when the transmission speed increases, waveform distortion occurs due to the spectrum spread of the signal itself and the dispersion of the fiber.

As a conventional technique for suppressing this waveform distortion, FIG.
An optical transmitter 13 shown in is known. Information signal 3 is N
The RZ-RZ conversion circuit 14 converts to an RZ waveform,
With this signal, the optical modulator 2 is driven to obtain the intensity-modulated optical signal 12. On the other hand, in the semiconductor laser 1, the phase of the clock signal 4 is adjusted by the phase shifter 6 so as to be synchronized with the information signal 3, the amplitude thereof is adjusted to an appropriate value by the attenuator (or amplifier) 10, and the constant current source 9 is supplied. The frequency-modulated light 11 is obtained by adding it to the bias current from and directly modulating with the added signal. The light 11 enters the optical modulator 2 and is intensity-modulated by the RZ-converted information signal. Light 1
In the wave number modulation of 1, the phase is set by the phase shifter 6 so that the first half of the RZ-converted bit is low-frequency modulation and the latter half is high-frequency modulation.

When the light 12 from the optical transmitter 13 is propagated through the optical fiber, the high frequency component light propagates in the optical fiber at a faster speed than the low frequency component light due to the wavelength dispersion of the optical fiber. In the light after propagating the length of the optical fiber, the frequency distribution of the light within one bit at the time of transmission (the first half of the bit is relatively low frequency, the second half of the bit is relatively high frequency) is inverted. That is, the first half of the bit has a relatively high frequency and the second half of the bit has a relatively low frequency. The optical fiber length at this time is when the above frequency modulation is not applied (in this case, the light of the high frequency component of the information signal spectrum is
It propagates through the optical fiber faster than the light of the low frequency component, and as a result, the waveform distortion occurs, it is longer than that of the optical fiber, and the purpose of extending the transmission distance is achieved. An example of an optical transmitter using this method is OFC 90, Post Deadline Paper, Pdy 8 (1990) (OF
C'90, Postdeadline Papers,
PD8 (1990)).

[0006]

The prior art described in the above document uses a sinusoidal RZ bit waveform as a binary information signal. In addition, the transmission distance of the optical fiber is fixed to the distance where the time domain of the first half and the second half of the bit is inverted by chromatic dispersion.When it is longer or shorter than that, the waveform is distorted, and the bit error rate increases at the bit identification of the receiver Will occur.

In an actual optical communication system, an NRZ bit waveform is often used as a binary information signal, and the NRZ bit waveform may have to be used in relation to other communication equipment. As a binary information signal,
When the NRZ bit waveform is used and the above-mentioned conventional technique is used, the improvement effect is recognized, but it is not sufficient. It is not realistic to fix the transmission distance of the optical fiber to the ideal state, and in practice, it is often used at the ideal transmission distance with the least waveform distortion. In this case, if the transmission distance is shorter than the ideal transmission distance, the relatively high frequency light in the first half of the bit and the relatively low frequency light in the second half of the bit overlap at least partially. Since the bit waveform has a narrow width, an error is likely to occur in waveform identification. That is, it becomes difficult to obtain an ideal eye pattern.

In particular, when the NRZ waveform is used, when the ON bits continue, the bits other than the isolated bit, the rising 1 bit and the falling 1 bit (that is, the bit sandwiched between the rising and falling bits Even in (bit), unnecessary optical frequency modulation is performed, and the effect of removing waveform distortion due to dispersion is weakened. It is an object of the present invention to obtain an optical transmission signal which, when an NRZ waveform is used, removes the spectrum spread due to the signal and the received waveform distortion due to the dispersion of the optical fiber to extend the transmission distance and improve the code error rate. Is to be realized.

[0009]

In order to achieve the above-mentioned object, an optical transmitter of the present invention provides a light source for generating a continuous wave light and a light source for the light source according to a binary information signal having an NRZ waveform. In an optical transmitter including an ON-OFF signal and an ON-OFF signal, detecting rising bits, falling bits, and isolated bits (ON bits sandwiched between OFF bits before and after) in an NRZ waveform. When the above three kinds of bit signals are modulated by the detector and the optical modulator, the light from the light source is assumed to be in the first half of the bit time (from time t to t + T; where T is a 1-bit cycle). The part (from time t to t + T / 2) becomes low frequency, and the latter part (time t + T / 2 to t + T) becomes high frequency. The light from the light source is changed in frequency so that it becomes an intermediate frequency between the low frequency and the high frequency over the entire time. Which is configured by providing a modulation to optical frequency modulation circuit. In a preferred embodiment, the light source is composed of a semiconductor laser, and the frequency modulation circuit is composed of a circuit for controlling a drive current of the semiconductor laser.

[0010]

The light whose frequency is modulated so that the first half bit time region of the rising bit, the falling bit, and the isolated bit has a low frequency and the latter half bit time region has a high frequency, the binary light of NRZ is used. When modulated and transmitted with a signal, the high-frequency component light propagates through the optical fiber faster than the low-frequency component light due to the wavelength dispersion of the optical fiber. Therefore,
When the length of the optical fiber is lengthened, the light of the high frequency component in 1 bit overtakes the light of the low frequency component and is completely replaced by the length of a certain optical fiber, that is, the first half of 1 bit. Light of high frequency component is in the bit period, and light of low frequency component is in the latter half bit period. The length of this optical fiber is the ideal transmission distance.

In the present invention, the frequency modulation is not performed on the bits other than the above three types of bits, and the NRZ waveform is generated, so that no extra waveform distortion occurs. Further, even when the length of the optical fiber is shorter than the ideal transmission distance, the received waveforms of the bits other than the above three types of bits are not narrowed, so that the bit error rate is improved in the bit identification in the receiving device. To be done.

[0012]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram showing the configuration of an embodiment of an optical transmitter according to the present invention. In the following description, an embodiment will be described in which the ON / OFF of the binary information signal 3 corresponds to the ON / OFF of the output light 12 of the optical transmitter 13, respectively. The optical transmitter 13 of the present embodiment is a semiconductor laser 1 which is a light source that generates a continuous wave, and light that converts the output light of the semiconductor laser 1 into an on-off signal of light according to a binary information signal 3 of an NRZ waveform. A modulator 2, a detection circuit 7 for detecting three types of bits of a rising bit, a falling bit, and an isolated bit from the binary information signal 3, and a light of a light corresponding to the above three types of bits detected by the detection circuit 7. The frequency modulation circuit 16 drives the semiconductor laser 1 in synchronization with the ON bit.

The modulation circuit 16 includes a phase shifter 6 for shifting the phase of the clock signal 4, a gate 8 for allowing the output of the phase shifter 6 to pass when the above three types of bits are detected by the detection circuit 7, and a gate 8. Attenuator (or amplifier if necessary) 10 that adjusts the output of the device to a desired amplitude (described later), and attenuator (or amplifier) 1
Voltage-current converter 15 for converting the output of 0 into a current signal
And a bias current from the constant current source 9 is added to generate a drive current for the semiconductor laser 1.

Therefore, the drive current of the semiconductor laser 1 is modulated by the clock signal 4 only when the gate 8 is on. Since the frequency of the oscillation light of the semiconductor laser 1 changes almost in proportion to the change in the amplitude of the driving current of the semiconductor laser 1,
The amplitude modulation of the drive current by the clock signal 4 causes the output light 11 of the semiconductor laser 1 to be frequency-modulated. At this time 1
The phase shifter 6 adjusts the phase shift of the clock signal 4 so that the low frequency modulation is applied in the first half bit period of the bit time domain and the high frequency modulation is applied in the latter half bit period. It should be noted here that the output light 11 from the semiconductor laser 1 is continuously oscillated light, and its power is almost constant.

The optical modulator 2 is driven by the information signal 3, and according to the information signal 3, the optical signal 1 is turned on and off.
Send as 2. At this time, the phase shifter 6 and the phase shifter 5 are arranged so that the above frequency modulation is applied to the above-mentioned three kinds of bits, that is, the rising bit, the falling bit, and the isolated bit.
Thus, the phases of the information signal 3 and the clock signal 4 are matched. The optical signal 12 thus obtained is the optical transmitter 1.
The transmission light 12 of No. 3 is transmitted by a transmission medium such as an optical fiber.

FIGS. 3 and 4 are signal waveform diagrams showing the phase relationship of the signals of the respective parts of the embodiment shown in FIG. The operation of the embodiment of FIG. 1 will be described in detail with reference to the signal waveform diagram. FIG. 3A shows "010011101" as an example of the binary information signal 3 to be transmitted. In this example, the first 1, the second 1, the fourth 1, and the fifth “1” are the isolated bit, the rising bit, and the falling bit, respectively.
It is a rising or isolated bit. Therefore, gate 8
Is the first 1, the second 1, the fourth 1, the fifth 1 above
The signal which is opened at the time of the bit and is phase-shifted from the clock signal 4 ((b) of FIG. 3) passes through the gate 8. Therefore, the output signal of the gate 8 is as shown in FIG.

The output signal of the gate 8 is amplitude-adjusted by the attenuator 10, converted into a current by the voltage-current converter 15, and added with the bias current from the constant current source 9,
It becomes the drive current of the semiconductor laser 1. This semiconductor laser 1
The waveform of the driving current is shown in FIG. When the semiconductor laser 1 is driven by this drive current, the frequency of the light of the semiconductor laser 1 is modulated according to the current change. The electric field of the light 11 from the semiconductor laser 1 is schematically shown in FIG. FIG. 3 (d) and FIG. 4 (e)
As can be seen by comparing the above, when the driving current of the semiconductor laser 1 is the bias current Ib, the oscillation frequency of the laser 1 is f
It is 0 (Hz). When the drive current drops to Ib-Ip, the oscillation frequency of the laser 1 drops to f1 (Hz). That is, f1 has a lower frequency than f0. Conversely, when the drive current increases to Ib + Ip, the oscillation frequency of the laser 1 increases to f2 (Hz). That is, the frequency f2 is higher than the frequency f0. The phase of the phase shifter 6 is adjusted so that such frequency modulation is applied. Note that f0
-F1 = f2-f0.

The light 11 thus frequency-modulated is converted by the optical modulator 2 into an optical signal 12 whose intensity is turned on / off according to the binary information signal 3. The electric field of the optical signal 12 is schematically shown in the waveform diagram of FIG. In the first half-bit period of the three types of bits (the rising bit, the falling bit, and the isolated bit) whose intensities are turned on as described above, the light is of low frequency (frequency f1) and the latter half of the bits. The period is high frequency light. The phase shifter 5 is adjusted so as to realize such a phase relationship. Bits other than the three types of bits whose intensity is on,
That is, in the example of FIG. 3, the frequency of light is f0 in the third bit corresponding to 1. The optical signal 12 realized in this way is transmitted as the transmission light 12 of the optical transmission device 13 through an optical fiber or the like as a transmission medium. Note that FIG.
(A), (c), (d) of FIG. 4 and ticks marked on the time axis of (f) of FIG. 4 are for indicating a 1-bit time interval.

Detection circuit 7 for detecting the three types of bits
Is composed of a conventionally known circuit. For example, the information signal 3 itself and a digital circuit that takes the exclusive OR of the information signal 3 delayed by 1 bit, or an analog circuit that performs the full-wave rectification of the output by differentiating the information signal 3 To be done. It goes without saying that the gate 8 can be realized by a simple switch.

Next, the means for adjusting the amplitude of the clock signal 4 passing through the gate 8 by the attenuator (or amplifier if necessary) 10 will be described. Now, consider a case where the longest transmission distance is realized without causing waveform distortion. The transmission distance is L (unit is km). The chromatic dispersion value at the central wavelength Q (unit: μm) of the transmission line optical fiber used is D (unit: ps / km / nm). Note that the frequency f0 of the light is f0 = c ÷ Q × 10exp
It is given in 6. Here, c represents the speed of light (unit is m / sec), and 10exp6 represents 10 6th power. The transmission rate, that is, the bit rate is B (the unit is Gb
It / s). FIG. 5 shows an example of chromatic dispersion characteristics of an optical fiber. Considering the characteristics of FIG. 5, in the above-mentioned three kinds of bits after transmission, the light of the high frequency component (frequency f2) occupies the first half bit of the bit due to the wavelength dispersion of the optical fiber, and the low frequency component (frequency f1) occupies the latter half bit of the bit.

FIG. 6 schematically shows the electric field of light at the output end of the optical fiber of the transmission line, that is, at the receiving section, as learned from FIG. As can be seen from a comparison between FIG. 6 and FIG. 4 (f) of the electric field of the light 12 of the transmitter, the light of the high frequency component (frequency f2) that occupied the first half bit period in the above-mentioned three types of bits is the light. The chromatic dispersion of the fiber occupies the latter half bit period of the bit. When this is expressed by an equation, c / f0 / f0 * (f2-f1) * D * L = 1E (-6) / 2 / B. Here, 1E () represents a power of 10. The left side of this equation expresses the propagation time difference between the light of the high frequency component and the light of the low frequency component due to the wavelength dispersion of the fiber, and the above equation expresses that the propagation time difference of the light is equal to half of one bit time. There is. Using this formula, when the distance L of the optical fiber, the chromatic dispersion value D, the oscillation center wavelength Q of the laser 1 and the bit rate B are determined, the required frequency modulation (f2-f1) can be obtained.

Here, the frequency modulation efficiency of the semiconductor laser 1 is assumed to be F (unit: Hz / A). Then, the amplitude value of the modulation current (Ip in (d) of FIG. 3) is Ip = (f2-f1) / 2
÷ Calculated by F. The conversion coefficient of the voltage-current converter 15 in FIG. 1 is W (unit is A / V), the attenuation rate of the attenuator 10 is V,
Assuming that the amplitude of the clock signal 4 is V0 (see (b) of FIG. 3), the attenuation rate V is obtained by V = Ip ÷ V0 ÷ W.

The above explanation is based on the assumption that
ON / OFF of the value information signal 3 is output light 1 of the optical transmitter 13.
There is a one-to-one correspondence with ON and OFF of 2. Here, it is assumed that the ON / OFF of the binary information signal 3 corresponds to the OFF / ON of the output light 12 of the optical transmitter 13, respectively. Also in this case, if the exclusive OR is used for the detection circuit 7, the above-mentioned 3 bits, that is, the output light 1 of the optical modulator 2 is output.
A rising bit, a falling bit, and an isolated bit can be detected among the bits for which 2 is turned on. The subsequent operation is exactly the same as that described above.

FIG. 7 is a block diagram showing the configuration of another embodiment of the optical transmitter according to the present invention. In this embodiment, instead of the clock signal 4 used in the embodiment shown in FIG.
An oscillator 17 that oscillates at a frequency substantially equal to the bit rate of the information signal 3 is provided, and the output signal of the oscillator 17 is used for frequency modulation of the semiconductor laser 1. Since the other components are substantially the same as those of the embodiment shown in FIG. 1, the same components and functional parts are designated by the same numbers as in FIG.

FIG. 8 is a block diagram showing still another embodiment of the optical transmission device according to the present invention. In this embodiment, an optical transmission device 13 and an optical reception device 18 are connected by an optical fiber 19 as an optical signal transmission line to form an optical transmission device.
The transmission light 12 from the optical transmission device 13 is transmitted through the optical fiber 19 and converted into an electric signal by the optical reception device 18. The optical transmitter 13 is the same as the embodiment shown in FIG. 1 or 6. The specific configurations of the semiconductor laser 1 and the optical modulator 2 of the present invention are not limited, and naturally include a case where they are monolithically integrated.

[0026]

According to the present invention, an optical signal having an NRZ waveform can be transmitted over a long distance through an optical fiber having a large wavelength dispersion. Compared with the optical transmitter of the present invention which does not use the optical transmitter, that is, the optical transmitter which does not perform frequency modulation, the transmission distance giving 1 dB of deterioration of the reception sensitivity due to wavelength dispersion is almost doubled by the present invention.

[Brief description of drawings]

FIG. 1 is a block diagram showing the configuration of an embodiment of an optical transmitter according to the present invention.

FIG. 2 is a configuration diagram of a conventional optical transmitter.

FIG. 3 is a signal waveform diagram showing a phase relationship of signals of respective parts of the embodiment shown in FIG.

FIG. 4 is a signal waveform diagram showing a phase relationship of signals of respective parts of the embodiment shown in FIG.

FIG. 5 is a diagram showing chromatic dispersion characteristics of an optical fiber.

FIG. 6 shows light at an output end of an optical fiber which is a transmission path.

FIG. 7 is a block diagram showing the configuration of another embodiment of the optical transmitter according to the present invention.

FIG. 8 is a block diagram showing still another embodiment of the optical transmission device according to the present invention.

[Explanation of symbols]

1. ． ． Semiconductor laser 11. ． ． Output light of the semiconductor laser 1, 2. ． ． Light modulator,
12. ． ． Output light of the optical modulator 2. ． ． Information signal, 13. ． ． Optical transmitter, 4. ． ． Clock signal, 14. ． ．
NRZ-RZ conversion circuit, 5, 6. ． ． Phase shifter,
15. ． ． Voltage-current converter, 7. ． ． Detection circuit, 16. ． ． Modulation circuit, 8. ． ． Gate, 17. ． ． Oscillator, 9. ． ．
Constant current source, 18. ． ． Optical receiver, 1
0. ． ． Attenuator, 19. ． ． Optical fiber for transmission line.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification code Office reference number FI technical display location H04B 10/06

Claims (5)

[Claims] 1. An optical transmitter comprising a light source for generating continuous wave light and an optical modulator for converting the light from the light source into an on / off signal of light according to a binary information signal of an NRZ waveform. A detection circuit for detecting three types of bits of the binary information signal, a rising bit, a falling bit, and an isolated bit, and a light-on bit corresponding to the three types of bits detected by the detection circuit. An optical transmitter, comprising: a frequency modulation circuit that synchronously frequency-modulates the output light of the light source. 2. The optical transmitter according to claim 1, wherein the frequency modulation circuit is composed of a circuit that performs frequency modulation so as to have a low frequency and a high frequency, respectively, with respect to an optical frequency of a bit other than the three types of bits. An optical transmission device characterized by the above. 3. The optical transmitter according to claim 1 or 2, wherein a semiconductor laser is used as the light source. 4. The optical transmitter according to claim 1, 2 or 3, wherein the modulator is formed of a modulation circuit for directly modulating the light of the continuous wave. .. 5. The optical transmitter according to claim 1, 2, 3, or 4, an optical fiber for transmitting an optical signal from the optical transmitter, and an optical signal from the optical fiber. And an optical receiver of the above.