JP3210152B2 - Driving method and driving device for semiconductor laser, and optical communication method and optical communication system using the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention provides a driving method for performing modulation or wavelength stabilization when a semiconductor laser is used as a light source for optical communication.

[0002]

2. Description of the Related Art As a light source for optical communication, for example, a distributed feedback laser (DFB laser) which oscillates in a single longitudinal mode has been developed.

In the direct light intensity modulation system currently in practical use using this laser, the amplitude of the modulation current is required to be about several tens mA, and the bias point of the laser (hereinafter also referred to as LD) is near the threshold value. The resonance frequency due to vibration is small and not suitable for high-frequency modulation of several Gbps or more. Further, wavelength fluctuation at the time of modulation is large, and there are problems such as dispersion during long-distance transmission and crosstalk during wavelength multiplex communication.

On the other hand, in the method of directly modulating the frequency by the injection current, since the amplitude of the modulation current is as small as several mA and the bias point of the LD is equal to or higher than the threshold value, wide band modulation is possible and the wavelength fluctuation is reduced. Promising as a communication system for long-distance transmission or wavelength multiplex communication.

In order to apply this frequency modulation method to ultra-long distance transmission or high-density optical frequency multiplexing, when performing coherent optical communication, the spectral line width and oscillation wavelength of the light source must be highly stabilized. There is also a method of controlling it by electrical negative feedback, and in that case, the direct frequency modulation characteristic of the laser is applied. Such stabilization techniques are:
It is also applied in optical measurement.

[0006]

However, the problem is that the direct frequency modulation characteristics of the laser deteriorate at low frequencies of several MHz or less. For example, FIG. 15 shows the relationship between the modulation frequency and the modulation efficiency (frequency shift amount fluctuating with respect to a current of 1 mA) when a single-electrode DFB laser as shown in FIG. 14 is modulated with a sine wave current. FIG. 16 shows the relationship between the phase differences (the phase difference between the modulation current and the modulation optical signal). As shown in this figure, it can be seen that the modulation efficiency fluctuates at a frequency of several MHz or less, and the phase also fluctuates and, when approaching direct current (hereinafter also referred to as DC), shows a response in the opposite phase. This is a few MHZ as a physical factor in the direct frequency modulation of the laser.
This is because two effects, that is, the effect of the change in the refractive index due to the heat of the opposite phase having a cutoff at z and the effect of the change in the refractive index due to the carrier density of the same phase that is flat up to the resonance frequency are superimposed. In the low band, the effect of heat becomes dominant, and thus the flatness of the modulation characteristic is lost.

When such characteristics are exhibited, various problems occur. First, send a digital signal as frequency modulation
For FSK (Frequency shift keying), the pulse width is several MH
Below z, the waveform is converted, causing transmission errors. An example is shown in FIG. Pulse width 1MH as shown in (a)
The pulse is thin at z, and at 100 kHz, it becomes a pulse of opposite phase as shown in (b). Therefore, the low frequency band is restricted, and the degree of freedom of encoding is restricted. Also, when the spectral line width is narrowed by electrical negative feedback, negative frequency feedback control becomes difficult because the phase turns at a frequency of less than several MHz in wavelength fluctuation.

In order to solve such a problem, several methods for improving a semiconductor laser device have been devised.
For example, there is a three-electrode structure as shown in FIG. 18 in which the effect of heat is suppressed by modulating the electrodes at both ends or the center electrode (YUZO YOSHIKUNI, et al., IEEE J. Lightwav).
e technol., vol. LT-5, NO. 4, p. 516, April, 1987). However, in this case, the behavior changes depending on the value of the bias current of each electrode, and there is a variation between the elements, so that practical use is difficult. In a structure having a λ / 4 shift diffraction grating in the center as shown in FIG. 19 and the depth of the diffraction grating being deeper than the peripheral portion, the effect of the carrier can also be reversed by modulating the center electrode. In some cases, there is no phase rotation because it is in phase with the heat effect, and the heat effect can be reduced, thus improving the low-frequency characteristics (Mr. Motokoji, et al., Spring 1992 Spring Preliminary Report, 30a-SF- 8). However, there are disadvantages that the fabrication is complicated, the yield is low, and the cost is high.

[0009]

According to the present invention, the activity
A drive method for frequency-modulating a semiconductor laser having at least a first electrode and a second electrode as electrodes for current injection into a layer by directly injecting a modulation current, wherein the frequency band of the modulation current is high. the the first electrode to inject <br/> modulation current in time, the second electrode modulation current and reverse phase
Without injection modulation current, at the same time when the low-frequency injecting Similarly modulation current in the case of high frequency to the first electrode, the modulation current of the reverse phase to the modulation current to the second electrode The above problem is solved without complicating the element structure by using a semiconductor laser driving method characterized by injection.

A more specific example will be described.
First, a DFB laser having a two-electrode structure is used as shown in FIG.
At this time, the modulation characteristics when current is injected into only one electrode are as shown in FIG. In this case, the thermal effect in the low frequency range is smaller than that of the laser of FIG.
It can be seen that there is only a flat area to the extent. Therefore,
An opposite-phase signal synchronized with the above-mentioned modulation signal at a low frequency (several tens of MHz or less) is applied to the other electrode. Although the magnitude varies depending on the material, structure, and mounting form of the element, an optimum value, that is, a state in which the phase difference between the phase of the current applied to one electrode and the phase of the output light is zero is maintained over a low range. Adjust as follows. Then, as shown in FIG. 3, the low-frequency characteristic can be extended to several kHz.

There are various means for generating such an inverted-phase signal, but it can be easily realized because it is sufficient to consider only a low frequency of DC to several tens of MHz. For example, as shown in FIG. 4, a signal is divided into two by a power divider, one is passed through a wideband inverting amplifier, and the other is passed through a non-inverting amplifier having a cut-off number of about 10 MHz. May be superimposed on the electrode of the laser by DC current and AC coupling. In this figure, the laser is represented as two diodes in parallel.

Alternatively, as shown in FIG. 5, a high-speed current driver chip may be used to drive each electrode through a low-pass filter with a cut-off number of 10 MHz while using the inverted output as it is and the non-inverted output as it is. As shown, the driver chip are those modulated current source I _p and DC bias current source I _b are integrated in parallel. In this case, since the bias T is not passed, there is no low-frequency cutoff of the modulation current, and it is possible to drive to DC. Further, it may be mounted on a small circuit board, and may be modularized into one box together with the laser.

[0013]

(Embodiment 1) FIG. 1 is a perspective view of a two-electrode DFB laser for realizing a driving method according to the present invention, and FIG. 4 is a view for explaining a driving method of a first embodiment according to the present invention. .

In FIG. 1, 101 is an n-GaAs substrate and 102 is n-AlGa
As (x = 0.45) cladding layer, 103 is an i-GaAs 6 nm well layer / i-AlGaAs (x = 0.22), a multiple quantum well active layer in which 5 layers of 10 nm are alternately stacked, 104 is p-AlGaAs (x = 0.15) ) Light guide layer,
105 is p-AlGaAs (x = 0.45) cladding layer, 106 is p-AlGaAs
(x = 0.07) contact layer, 107 is p-AlGaAs (x = 0.4) buried layer, 108 is n-AlGaAs (x = 0.4) buried layer, 109 is p
The electrode 110 is an n-electrode. 24 pitch on the light guide layer
A diffraction grating having a thickness of 5 nm and a depth of 100 nm is formed, and an anti-reflection coating 111 is applied to one of the emission end faces. p electrode 10
9 and the contact layer 106 are divided into two near the center of the element as shown in the figure.

The bias current flowing to the electrode on the anti-reflection coating side is I ₁ , and the bias current flowing to the opposite side is I _2, and the DFB mode is performed with a threshold value I ₁ + I ₂ = 30 mA (I ₁ , I ₂ > 10 mA). Oscillated.
When the high-frequency current ΔI ₁ , which is a modulation current, is superimposed only on I ₁ and bias-modulated by the bias T serving as AC coupling means in the oscillating state, the modulation characteristics are as shown in FIG. 0.6 GHz / mA over modulation frequencies from several MHz to several GHz
Although the modulation efficiency is approximately flat, it can be seen that the modulation efficiency increases at a frequency of several MHz or less. This is because the effect of heat becomes dominant in low frequencies as described in the item of the problem. The phase characteristics were as shown in FIG.

[0016] Therefore, it flowed by superimposing a compensation current [Delta] I ₂ to I ₂ in such a way that in FIG. 1 from the modulation power supply 409
: 1 is divided into two by a power divider 408, and one is passed through an inverting wideband video amplifier 406 having a gain of 10 and a cutoff frequency of 10 GHz, and is superimposed on a direct current of a DC power supply 403 by a bias T402 having a low cutoff frequency of 1 kHz. To drive the laser 401 as a current. The other output of the power divider passes through a non-inverting operational amplifier 407 having a gain of 3 and a cutoff frequency of 10 MHz, and is similarly superimposed on a DC current of a DC power supply 405 by a bias T404 to be driven as a laser 401 current. In this case, the output of the modulation power supply is variable, and the degree of modulation is adjusted by the power supply.

In this embodiment, in order to optimize to effectively suppress the phase shift due to the thermal effect in the low frequency range,
The gain of the inverted wideband video amplifier 406 is fixed at 10,
The gain of the non-inverting operational amplifier 407 was changed. As a result, the gain of the non-inverting operational amplifier 407 was determined to be 3. Since this differs depending on the element structure, material, mounting form, etc., it is necessary to optimize each time.

When frequency modulation was performed by such a driving method, the low-frequency characteristics were greatly improved as shown in FIG. 3, and flat characteristics up to several kHz were obtained. At this time, the phase characteristics are also several kHz to 1G.
It showed in-phase response at Hz. This allows a simple structure DFB
In a laser, frequency modulation using a square wave signal with a pulse width of several kHz to several GHz, that is, FSK transmission, has become possible.

In this embodiment, the wideband video amplifier is of an inverting type and the operational amplifier is of a non-inverting type. This is because in general, the inverting type broadband amplifier provides better characteristics, and it does not matter if it is reversed depending on the required specifications.

(Embodiment 2) FIG. 5 is a diagram for explaining a driving method according to a second embodiment of the present invention. The elements used are
This is a two-electrode DFB laser almost the same as in the first embodiment.

In this embodiment, a laser driver IC is used as a voltage-to-current converter, which is a modulating coupling means, in order to reduce the size of the entire driving system without using a bias T having a low-frequency cutoff. Things. A voltage-current converter drives a current by a voltage signal. As shown in 503 and 504 in FIG. 5, a DC current source Ib and a modulation current source Ip are integrated in parallel, and a modulation current having a DC offset current is input by inputting an ECL level modulation signal. Can be driven. The degree of modulation and the amount of bias current can be controlled on the IC side.

Next, a specific driving method will be described. ECL
Output of modulation power supply 505 with output
To enter. When the modulation power supply is branched into two, if it is performed immediately before the two driver ICs, the power divider becomes unnecessary. The driver IC 503 takes out the in-phase output with respect to the modulation input, and the driver IC 504 takes out the in-phase output with respect to the modulation input. This may be driven by one driver IC having two in-phase outputs and two in-phase outputs.
Low-pass filter with cut-off 10MHz for reverse phase output 50
2 is driven as a current I ₂ + ΔI ₂ of the laser 501. The in-phase output is the current I ₁ + ΔI of the laser 501 as it is.
Drive as ₁ . Regarding the current ratio between ΔI ₁ and ΔI ₂ ,
Optimized at 10: 3 as in Example 1. Here, as shown in FIG. 5, it is desirable that the driver IC is configured to draw the current by connecting the laser to the ground, as shown in FIG. Therefore, in this embodiment, an element in which the conductivity type is reversed and a p-substrate is used, and the electrode separating side is an n-electrode is used.

If such a drive system is assembled, the entire system can be made very small and modularized into one box. Also,
In the first embodiment, the low band is limited by the band of the bias T,
In this embodiment, there is no restriction on the low frequency range. As a result, the low-frequency characteristics could be further improved, and the modulation characteristics could be made flat from 100 Hz to several GHz. This allows FSK
When transmitting, NRZ with 3Gbps and continuity of 2 ²⁰ -1 or more
(No Return Zero) signal, which enables very high-density transmission.

(Embodiment 3) In a third embodiment of the present invention, the driving method of Embodiment 1 for improving the low-frequency characteristics of a laser is applied to narrowing of an oscillation spectrum by electrical feedback. In the case where the period of the wavelength fluctuation is longer than a certain value, the narrowing by the general feedback cannot cope with the phase shift due to the thermal effect, but this embodiment can cope. FIG. 6 shows the drive system.

The light emitted from the semiconductor DFB laser 601 passes through a frequency discriminator 610 to convert the fluctuation of the frequency of the light into a fluctuation of the light intensity, and the light detector 609 converts the fluctuation into an electric signal. Here, a Fabry-Perot etalon having a free spectrum range of 5 GHz and a finesse of 50 was used as a frequency discriminator, and a GaAs pin photodiode was used as a photodetector. The electric signal is transmitted to the laser 601 in the same manner as in the first embodiment.
The feedback control is performed by superimposing the feedback control on the injection current. The feedback ratio can be adjusted by the gain of the amplifiers A ₁ 606 and A ₂ 607 and the optical power incident on the photodetector. Here, the gain of the amplifier A ₁ 100, the gain of the amplifier A ₂ and 30, the optical power is optimally adjusted between 10μW~100 μW.
In addition, the actual distance of the feedback system limited the high frequency band, so the size was reduced to about 10 cm, and the frequency at which resonance occurred due to the time delay of feedback was kept away from 3 GHz.

As a result, the spectral line width, which was about 15 MHz without feedback control, could be narrowed to about 1/100 up to about 200 kHz, which is a level usable as a light source for coherent optical communication.

In this state, by further superimposing the modulation current, FSK transmission can be performed by the driving system and the driving method of the first or second embodiment.

FIG. 7 shows an example in which an optical transmission system is assembled using a light source driven by this driving method. Light emitted from the light source 701 is coupled to a single mode optical fiber 702 and transmitted. On the receiving side, a beat with the laser 703 as a local oscillator oscillating at the same wavelength as the light source 701 is taken, detected by the photodetector 704, and detected by a delay detection method or the like. The oscillation wavelength of the laser 703 is based on the beat signal detected by the photodetector 705.
The oscillation wavelength is stabilized by the PLL (Phase Lock Loop) method.

(Embodiment 4) The fourth embodiment of the present invention utilizes the wavelength tunable characteristics of a laser to perform wavelength division multiplexing transmission. In the two-electrode DFB laser of the first embodiment, the oscillation wavelength can be changed by controlling the value of the current flowing through each electrode. FIG. 8 shows the wavelength tunable characteristics.
Shown in Make the bias current flowing through each electrode equal,
That is, when the current I ₁ is changed from 20 mA to 60 mA while maintaining the relationship of I ₁ = I _2, the oscillation wavelength becomes about 1 nm from 835 nm to 836 nm.
The wavelength can be changed continuously. In the present embodiment, utilizing this fact, the bias current is controlled to determine the central oscillation wavelength, and then the modulation is performed as described in the first or second embodiment, or as shown in the third embodiment. This indicates that it is possible to narrow the oscillation spectrum.

Next, a method of performing wavelength division multiplexing communication using the driving method according to the present embodiment will be described with reference to FIG. 901
Is a light source for optical communication modulated by the FSK method according to the present invention. With this light source, the wavelength can be changed in the range of 1 nm as described above. In this embodiment, the frequency shift amount of FSK is set to 5 GHz. In the device of this example, a frequency shift of 5 GHz was obtained by setting the modulation current amplitude to 8.3 mA. Therefore, in the case of wavelength multiplexing, 10 GHz (~ 0.02n
m) Even if they are arranged at intervals of about m), they do not give crosstalk to adjacent channels. Therefore, when this light source device is used, wavelength multiplexing of about 1 / 0.02 = 50 channels is possible. The light emitted from the light source is coupled to a single mode fiber 902 and transmitted. In the receiving apparatus, the signal light transmitted through the optical fiber is selectively demultiplexed into light having a desired wavelength by an optical filter 903 and detected by a photodetector 904. Here, an optical filter having the same structure as that of the DFB laser according to the first embodiment or the like is used by biasing a current below a threshold value. By changing the current ratio of the two electrodes, the transmission wavelength can be changed by 1 nm while the transmission gain is kept constant at 20 dB. This filter has a 10 dB down transmission width of about 10 GHz
(Up to 0.02 nm), which is sufficient for wavelength multiplexing at intervals of 0.02 nm as described above.

The FSK signal is detected by utilizing the transmission characteristics of the filter. As shown in FIG. 10, the transmission peak wavelength of the filter is set to a desired channel, for example, CH
When adjusted to the mark frequency of light 2 (corresponding to "1" of the FSK signal), the space frequency (corresponding to "0" of the FSK signal) is separated by the above-mentioned frequency deviation of 5 GHz, so that it passes through the filter. Later, with an extinction ratio of 10dB, "1", "
0 "can be detected.

Other optical filters, for example,
A Mach-Zehnder type, a fiber Fabry-Perot type, or the like described in the conventional example may be used. Although only one light source and one receiving device are described here, it is needless to say that several light sources or receiving devices may be connected by an optical coupler or the like for transmission.

The method of receiving a wavelength division multiplexed signal may be the coherent method described in the third embodiment, other than the present embodiment.

Although the GaAs laser has been described above, other materials such as an InP laser can be similarly realized.

(Embodiment 5) FIG. 11 shows a method for driving a light source for optical communication according to the present invention and an optical-electrical device connected to each terminal when an optical communication system using the same is applied to a wavelength division multiplexing optical LAN system. FIG. 12 shows a configuration example of a conversion unit (node), and FIG. 12 shows a configuration example of an optical LAN system using the node.

An optical signal is taken into the node by using the optical fiber 1001 connected to the outside as a medium,
As a result, a part of the light enters the receiving apparatus 1003 including the wavelength tunable optical filter as described in the fourth embodiment. With this receiving apparatus, only the optical signal of the desired wavelength is extracted and signal detection is performed by the method of the fourth embodiment. As the receiving method, the third embodiment
The coherent method as described above may be used. In that case,
A local oscillation laser is provided instead of an optical filter.

On the other hand, when an optical signal is transmitted from a node, the wavelength tunable DFB laser 1004 is driven by any of the methods of the first to third embodiments, and is modulated by the FSK method. Through the optical transmission line.

Further, by providing two or more plural tunable DFB lasers and plural tunable optical filters, the tunable range can be expanded.

As a network of the optical LAN system,
FIG. 12 shows a bus type in which nodes are connected in directions A and B, and a large number of networked terminals and centers can be installed. However, in order to connect a large number of nodes, it is necessary to arrange optical amplifiers in series on a transmission line in order to compensate for optical attenuation. Also, by connecting two nodes to each terminal and using two transmission paths, D
Two-way transmission by the QDB method or the like becomes possible.

In such an optical network system, if the driving method of the device according to the present invention is used, the fourth embodiment will be described.
As described above, a high-density wavelength division multiplexing optical transmission network with a multiplicity of 50 can be constructed.

As a network system, FIG.
It may be a loop type connecting A and B, a star type, or a composite form thereof.

(Embodiment 6) A wavelength division multiplexing CATV having a topology as shown in FIG. 13 can be constructed by the apparatus and the optical communication system according to the present invention. In the CATV center, the wavelength tunable laser is modulated by the driving method of the first or second embodiment according to the present invention, and the driving method of the fourth embodiment is used as a wavelength multiplexed light source. A receiving apparatus provided with a wavelength tunable filter as in the fourth embodiment is used on the subscriber side as a receiver. Conventionally, D
Although it was difficult to use a DFB filter in such a system due to the influence of the dynamic wavelength fluctuation of the FB laser, the present invention has made it possible.

Further, a subscriber is provided with an external modulator, and a signal from the subscriber is received as reflected light from the modulator (a form of simplified bidirectional optical CATV, for example, Ishikawa, Furuta, "Optical CATV subscriber"). An LN external modulator for bidirectional transmission in a system "OCS91-82, p.51), and by constructing a star network as shown in FIG.
TV becomes possible, and the service can be enhanced.

[0044]

According to the present invention, the modulation band on the low frequency side when performing frequency modulation of a semiconductor laser can be greatly extended, and high-density transmission becomes possible. Further, since the band when the oscillation spectral line width is narrowed by the electric feedback method is widened, a narrower spectral line width can be easily obtained than before, and a light source suitable for coherent optical communication can be provided.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view of a part of a perspective view of a distributed feedback semiconductor laser to which a driving method according to the present invention is applied.

FIG. 2 is a diagram showing a modulation characteristic when only one of two electrodes is frequency-modulated.

FIG. 3 is a diagram showing a modulation characteristic when frequency modulation is performed by the driving method according to the present invention.

FIG. 4 is a block diagram illustrating a driving method according to a first embodiment of the present invention.

FIG. 5 is a block diagram illustrating a driving method according to a second embodiment of the present invention.

FIG. 6 is a block diagram illustrating a driving method for narrowing a spectral line width according to a third embodiment of the present invention.

FIG. 7 is a diagram illustrating a transmission system that performs coherent optical communication using the driving method according to the third embodiment.

FIG. 8 is a diagram showing a wavelength tunable characteristic of a distributed feedback semiconductor laser.

FIG. 9 is a diagram showing a transmission system when performing wavelength division multiplexing transmission according to a fourth embodiment.

FIG. 10 is a diagram illustrating a method of receiving a wavelength multiplexed signal.

FIG. 11 is a diagram illustrating a configuration example of an optical node according to a fifth embodiment;

FIG. 12 illustrates an optical LAN network.

FIG. 13 illustrates an optical CATV system.

FIG. 14 shows a single-electrode distributed feedback laser.

FIG. 15 is a diagram showing a conventional frequency modulation characteristic.

FIG. 16 shows a conventional phase characteristic.

FIG. 17 is a diagram for explaining a conventional FSK response characteristic.

FIG. 18 is a diagram showing a conventional example of a three-electrode distributed feedback laser.

FIG. 19 is a diagram showing a conventional example of a distributed feedback laser having a non-uniform depth diffraction grating.

[Explanation of symbols]

 101, 1041, 1051 Substrate 102, 105 Cladding layer 103, 1042, 1054 Active layer 104, 1043, 1053 Light guide layer 106 Contact layer 107, 108 Buried layer 109, 110, 1045, 1055 Electrode 401, 501, 601 Two-electrode semiconductor Laser 402, 404, 602, 604 Bias T 403, 405, 603, 605 DC current source 406, 407, 606, 607 Amplifier 408, 608 Power divider 409 Modulation power supply 502 Low-pass filter 503, 504 Laser driver IC 505 Modulation power supply 609, 704, 705, 904 Photodetector 610 Frequency discriminator 701, 901, 1004 Optical communication light source 702, 902, 1001 Optical fiber 703 Local oscillation laser 706 PLL control circuit 903 Wavelength tunable optical filter 1002, 1006 Optical splitter 1005 Optical isolator 1003 Receiver 1044, 1052 Diffraction grating 1056 λ / 4 shift unit

──────────────────────────────────────────────────続 き Continuation of the front page (51) Int.Cl. ⁷ Identification symbol FI H04B 10/28 (56) References JP-A-4-137778 (JP, A) JP-A-61-290789 (JP, A) JP-A-2-234484 (JP, A) JP-A-2-268479 (JP, A) JP-A-3-295289 (JP, A) JP-A-64-73687 (JP, A) JP-A-63-204779 ( JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) H01S 5/00-5/50 H04B 10/00

Claims (18)

(57) [Claims] 1. A semiconductor laser having at least a first electrode and a second electrode as electrodes for injecting current into an active layer ,
A driving method of a frequency modulated by injecting a modulation current directly, when the frequency band of the modulation current of high frequency
Injects modulation current to the first electrode, the second electrode
Without injecting a modulation current modulation current in opposite phase to, at the same time at the time of low-frequency <br/> injecting Similarly the modulation current and the time of the high frequency to the first electrode, the second A method for driving a semiconductor laser, wherein a modulation current having a phase opposite to that of the modulation current is injected into an electrode. 2. The method of driving a semiconductor laser according to claim 1, wherein the low-frequency band into which the opposite-phase modulation current is injected is about 10 MHz or less. 3. The modulation amplitude of the opposite-phase modulation current injected into the second electrode is adjusted so as not to cause a phase difference between the modulation signal injected into the first electrode and the modulated optical frequency modulation signal. 3. The method of driving a semiconductor laser according to claim 1, wherein the adjustment is performed in the following manner. 4. At least a first electrode as a current injection electrode.
A semiconductor laser drive device that frequency-modulates a semiconductor laser having an electrode and a second electrode by directly injecting a modulation current, wherein a means for dividing an output of a modulation power supply into two and a means for dividing the output into two. One of the outputs has a gain cutoff
After passing through a first amplifier of GHz or more, a DC component is cut and superimposed on a bias current and injected into the first electrode, and the other is divided into a second device having a gain cutoff frequency of about 10 MHz. Means for cutting the direct current component after passing through the amplifier, superimposing it on the bias current, and injecting it into the second electrode, wherein one of the first amplifier and the second amplifier is an inverting amplifier, Is a non-inverting amplifier, and the first
A driving ratio of the modulation amplitudes of two modulation currents having opposite phases to be injected into the laser by changing the gain ratio of the amplifier and the second amplifier. 5. At least a first electrode as a current injection electrode.
A semiconductor laser drive device that frequency-modulates a semiconductor laser having an electrode and a second electrode by directly injecting a modulation current, wherein a means for dividing an output of a modulation power supply into two and a means for dividing the output into two. A negative-phase output voltage-current converter for injecting one of the outputs together with the bias current into the first electrode, a common-mode output voltage-current converter for injecting the other one of the outputs together with the bias current into the second electrode, A low-pass filter having a cut-off frequency of about 10 MHz provided between one of the voltage-current converters and the semiconductor laser;
A semiconductor laser driving device characterized in that the ratio of the modulation amplitudes of the modulation currents of opposite phases to be injected into the laser can be adjusted by changing the ratio of the gains of the two voltage-current converters. 6. At least a first electrode as a current injection electrode.
A method of narrowing the oscillation spectrum line width of a semiconductor laser having a first electrode and a second electrode, and driving the semiconductor laser. The electric signal obtained by detecting the fluctuation of the oscillation wavelength of the output light of the laser is converted into the electric signal. When the frequency band of the signal is high frequency, a negative feedback current by the electric signal is injected into the first electrode, and when the frequency band is low, a negative feedback current by the electric signal is injected into the first electrode as in the case of the high frequency. And simultaneously injecting a current having a phase opposite to the negative feedback current due to the electric signal into the second electrode. 7. A low-frequency band for injecting a current having an opposite phase is about 10 MHz or less.
The driving method of the semiconductor laser according to the above. 8. The amplitude of the current of the opposite phase injected to the second electrode is adjusted so as not to cause a phase difference between the current injected to the first electrode and the fluctuation of the optical frequency due to the injection of the two currents. 8. The method of driving a semiconductor laser according to claim 6, wherein the method is adjusted. 9. At least a first electrode as a current injection electrode.
Device for narrowing the oscillation spectrum line width of a semiconductor laser having an electrode and a second electrode, means for extracting a part of the output light of the laser, and detecting fluctuations in the oscillation wavelength of the laser A frequency discriminator, a photodetector for detecting light passing through the frequency discriminator, a unit for dividing a signal obtained by the photodetector into two, and a gain cutoff for one of the two divided signals. After passing through the first amplifier over several GHz,
A DC component is cut and superimposed on a bias current, and the other is passed through a second amplifier having a gain cutoff frequency of about 10 MHz. Means for superimposing on a bias current and injecting the bias current into a second electrode, wherein one of the first amplifier and the second amplifier is an inverting amplifier and the other is a non-inverting amplifier; A driving ratio of the modulation amplitudes of two modulation currents having opposite phases to be injected into the laser by changing the gain ratio of the amplifier and the second amplifier. 10. A device for driving a semiconductor laser having at least a first electrode and a second electrode as current injection electrodes, by narrowing an oscillation spectrum line width of the semiconductor laser, wherein a part of output light of the laser is used. Extracting means, a frequency discriminator for detecting fluctuations in the oscillation wavelength of the laser, a photodetector for detecting light passing through the frequency discriminator, and means for dividing a signal obtained by the photodetector into two. A negative-phase output voltage-current converter for injecting one of the two divided signals into the first electrode together with the bias current, and a common-mode output voltage-current converter for injecting the other one into the second electrode together with the bias current Vessel and the 2
A low-pass filter with a cut-off frequency of about 10 MHz provided between one of the two voltage-current converters and changing the gain ratio of the two voltage-current converters, A driving device for a semiconductor laser, wherein a ratio of modulation amplitudes of modulation currents having opposite phases to be injected into a laser can be adjusted. 11. A driving method for driving a semiconductor laser according to claim 6, wherein two currents injected into the semiconductor laser further include modulation currents having opposite modulation phases, at least one of which has a modulation amplitude adjustable. A method for driving a semiconductor laser, comprising superimposing and frequency modulating. 12. An FSK (Frequency Shift Key) in which frequency modulation is performed by a digital signal.
13. The method of driving a semiconductor laser according to claim 11, wherein: 13. An optical communication method in which transmission light modulated by the driving method according to claim 12 is transmitted through an optical fiber, and a receiving side performs beat detection using a local oscillation laser to perform coherent communication. 14. The method of driving a semiconductor laser according to claim 1, wherein the frequency modulation is FSK modulated by a digital signal. 15. An optical communication method in which transmission light modulated by the driving method according to claim 12 is transmitted through an optical fiber and directly detected through a wavelength filter at a receiving side. 16. Wavelength division multiplexing in which a plurality of transmission light sources are connected, light of a plurality of wavelengths is modulated and transmitted on one optical fiber, and only a signal on the light of a desired wavelength is extracted on the receiving side. 16. The optical communication method for transmission, wherein the optical communication method for each wavelength is the method according to claim 13 or 15. 17. In an optical communication system in which a plurality of nodes are connected to one optical fiber and each of which modulates light of a plurality of wavelengths and performs wavelength division multiplexing transmission, each node is an optical communication system comprising a transmission unit and a reception unit. -An optical communication system comprising an electric conversion device and performing communication by the optical communication method according to claim 16. 18. A broadcasting center, wherein the broadcasting center and optical subscribers are connected by an optical fiber, and the broadcasting center drives the semiconductor laser according to the semiconductor laser driving method according to claim 1 to 3 to transmit a signal. An optical communication system in which an optical CATV is constructed by transmitting and detecting an optical beat using a local oscillation laser and receiving it, or by directly detecting and receiving through a wavelength filter.