JP3093535B2 - 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) that 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 several tens mA.
This is necessary, and since the bias point of the laser (hereinafter also referred to as LD) is near the threshold value, the resonance frequency due to relaxation oscillation is small and is not suitable for high-frequency modulation of several Gbps or more. In addition, wavelength fluctuation during modulation is large, dispersion during long-distance transmission,
Crosstalk at the time of wavelength multiplex communication has become a problem.

On the other hand, the direct frequency modulation method using the injection current has a small amplitude of the modulation current of several mA, and the bias point of the LD is equal to or higher than the threshold value. 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.

However, there is a problem that the direct frequency modulation characteristics of the laser deteriorate at low frequencies of several MHz or less. For example, FIG. 16 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. FIG. 17 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, the phase also fluctuates, and when the frequency approaches DC (hereinafter also referred to as DC), a response of the opposite phase is exhibited. This is a matter of several M as a physical factor of 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 heat in the opposite phase having a cutoff at Hz and the effect of the change in the refractive index due to the carrier density in the same phase 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, FSK (Frequency shift keyin) for transmitting a digital signal as frequency modulation.
In the case of g), if the pulse width is less than several MHz, the waveform is converted and an error occurs in transmission. An example is shown in FIG. As shown in (a), the pulse becomes thin with a pulse width of 1 MHz, and as shown in (b), the pulse becomes out of phase at 100 kHz. 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 feedback control becomes difficult because the phase turns at a frequency where the wavelength fluctuation is several MHz or less.

In order to solve such a problem, several methods for improving a semiconductor laser device have been devised.
For example, as shown in FIG. 19, there is a three-electrode structure in which the effect of heat is suppressed by modulating the electrodes at both ends or the center electrode (YUZO YOSHIKUNI, et a).
l. , IEEE J. et al. Lightwave technology
ol. , Vol. LT-5, NO. 4, p. 516, A
pril, 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. FIG.
In a structure with a λ / 4 shift diffraction grating in the center as shown in FIG. 4 and the depth of the diffraction grating is deeper than the periphery, modulation of the center electrode causes the carrier effect to exhibit a reverse-phase response, There is also an example in which there is no phase rotation because the phase becomes the same as the effect, and the thermal effect can be reduced, so that the low-frequency characteristics can be improved (Ojimoto et al., Spring 1992 Preliminary Report, 30a-
SF-8). However, there are disadvantages that the fabrication is complicated, the yield is low, and the cost is high.

In order to improve the modulation efficiency and the low-pass characteristics of the phase difference with a simple device structure, the inventors of the present application have proposed the following semiconductor laser driving method.
This will be described briefly.

[0010] First, as shown in FIG.
Laser. The modulation characteristics of one electrode at this time are as shown in FIG. In this case, although the heat effect in the low band is smaller than that of the laser of FIG. 15, it can be seen that there is still a flat region only up to about several MHz. Therefore, low frequency (number
An inverted-phase signal synchronized with the above-mentioned modulation signal at a frequency of not more than 1 MHz is applied to the other electrode. Then, as shown in FIG. 3, the low-frequency characteristic can be extended to several kHz.

Although various means for generating such an inverted phase signal are conceivable, they can be easily realized because they need only be considered at a low frequency of DC to several MHz. For example, as shown in FIG. 21, 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 of several MHz through a bias T to each electrode through a bias T to generate a DC current and a DC current. What is necessary is just to superimpose. In this figure, the laser is represented as two diodes in parallel.

[0012]

As described above, a method of modulating both electrodes in opposite phases with a two-electrode DFB laser is effective, but it is necessary to optimize the ratio of the amplitudes of currents in opposite phases. is there.

The optimum amplitude ratio depends on the material, structure, and mounting form of the device, and it is necessary to determine the optimum current ratio after mounting each device. Also, the optimum current ratio may change depending on the ambient temperature, the DC bias current value of the laser, and the like. When the change exceeds the allowable change, readjustment is required each time.

[0014]

In order to automatically control the optimum current amplitude ratio of the reverse-phase current, the present application uses a semiconductor laser having at least a first electrode and a second electrode as current injection electrodes. A frequency modulation method by directly injecting a modulation current, wherein a modulation current is injected into the first electrode when the frequency band of the modulation current is high, and a high frequency is applied to the first electrode when the frequency band is low. At the same time as injecting the modulation current, a modulation current having a phase opposite to that of the modulation current is injected into the second electrode, and the modulation currents having phases opposite to each other are injected into the first electrode and the second electrode. A method for driving a semiconductor laser, wherein the amplitude ratio is feedback-controlled by the optical output of the semiconductor laser, is proposed.

More specifically, the present application proposes a method of superimposing and injecting a low-frequency signal in which the effect of heat is dominant on a current to be injected into an electrode, and performing feedback control by a wobbling method. That is, two electrodes are modulated with opposite phases of the same amplitude by a low-frequency signal in which the effect of heat is dominant, for example, a sinusoidal current of about 100 Hz, and a part of the output signal of the laser is taken out.
Hz component, and modulates the laser.
Take the product with the 0 Hz sine wave signal. And that DC
When the components are extracted by a low-pass filter, the positive and negative values are determined by the degree of modulation due to the thermal effect in the two electrode regions.
, A negative DC output is obtained. Feedback modulation is performed based on this signal, and the modulation amplitude of one of the two electrodes is adjusted by an amplifier (attenuator).

[0016]

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 diagram for explaining a driving method according to a first embodiment of the present invention.

In FIG. 1, reference numeral 101 denotes an n-GaAs substrate;
02 is an n-AlGaAs (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 10 nm layers are alternately stacked in five layers, 104 is p-A
1GaAs (x = 0.15) Light guide layer, 105 is p
An AlGaAs (x = 0.45) cladding layer, 106
Is a p-AlGaAs (x = 0.07) contact layer,
107 is a p-AlGaAs (x = 0.4) buried layer, 108 is an n-AlGaAs (x = 0.4) buried layer, 109 is a p-electrode, and 110 is an n-electrode.
A pitch of 245 nm and a depth of 100 nm on the light guide layer
Is formed, and a non-reflective coating 111 is applied to one of the emission end faces. The p-electrode 109 and the contact layer 106 are divided into two near the center of the device as shown.

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 threshold value I ₁ + I ₂ = 30 mA (I ₁ , I ₂
> 10 mA) in the DFB mode. High-frequency current ΔI which is a modulation current only in I ₁ while oscillating
_{When 1} was superimposed and bias-modulated by the bias T ₁ which is AC coupling means, the modulation characteristic was as shown in FIG. It exhibits a flat modulation efficiency of about 0.6 GHz / mA over a modulation frequency of several MHz to several GHz,
It is understood that the modulation efficiency is increased 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.

Therefore, the compensation current ΔI ₂ was superimposed on I ₂ and flowed by the method as shown in FIG. The signal from the modulation power supply 409 is divided into two by a 1: 1 power divider 408.
Divided into one, gain 10 and cutoff frequency 10GH
z through an inverted wideband video amplifier 406 and a bias T402 with a low cutoff frequency of 1 kHz.
The laser 401 is driven as a current superposed on the DC current of the C power supply 405. Another output of the power divider is passed through a non-inverting operational amplifier 407 having a gain of g and a cutoff frequency of 10 MHz, and is similarly superimposed on the DC current of the DC power supply 405 by the bias T404 to produce a laser 4.
01. In this case, the output of the modulation power supply is variable, and the degree of modulation is adjusted by the power supply. Amplifier 40
The gain g of 7 is variable, and optimization is performed to suppress thermal effects in the low frequency range. The method will be described below.

A sine wave oscillator 410 outputs a signal of 5 kHz, one of which is input to a modulation power supply 409 and superimposed on the modulation signal. On the other hand, a part of the light output of the laser is taken out by a beam splitter 414 and transmitted through a frequency discriminator 415 such as a Fabry-Perot etalon to allow the photodetector 4
13 and converts the electric signal into an electric signal.
By extracting only the signal component near 5 kHz,
Mixing is performed by the balance modulator 411. The DC component of the mixing output is extracted by a low-pass filter 416, and the gain of the amplifier 407 is determined by the signal.
Is automatically controlled. The principle will be described with reference to FIG.

When the laser is modulated at 5 kHz, the frequency is modulated substantially only by the thermal effect, but there are three cases depending on the difference in the magnitude of the thermal effect between the two electrodes of the laser.
(A) when the frequency variation Delta] f ₁ is less than the frequency variation Delta] f ₂ that occurred in the heat effect of the [Delta] I ₂ occurs by the heat effect of the [Delta] I ₁ as the frequency variation of the light output is Delta] f ₁ and phase i.e. [Delta] I ₁ And the opposite phase. on the other hand,
In the opposite case as in (c), the phase becomes the same as ΔI ₁ . In addition, when they are almost equal as shown in (b), they cancel each other out and there is no frequency fluctuation. The frequency variable unit detects a positive slope of the frequency discriminator, taking the mixing between this output and the sine wave signal, [Delta] I ₁ and the negative-phase i.e. the sine wave signal and the phase of the signal as in (a) (△ If I ₁ is through inverting amplifier 406), then a positive DC output is obtained. Conversely, a negative DC output is obtained when the phase is opposite to the sine wave signal as shown in (c), and the mixing output is 0 when there is no frequency fluctuation as shown in (b). In the case of the output 0 as in the case of (b), the thermal effect in the two electrode regions cancels out to be optimal, and since it is detected which side is shifted from that point, the amplifier 407 is used.
Can be automatically controlled. Therefore, ΔI ₁ and ΔI ₁
It can always be optimally automatically controlling the amplitude ratio of I _2.

When frequency modulation is performed by such a driving method, low frequency characteristics can be greatly improved as shown in FIG.
A flat characteristic was obtained. At this time, the phase characteristic is also several kHz.
In-phase response was shown at z to 1 GHz. Thereby, in a DFB laser having a simple structure, a pulse width of several kHz to
Frequency modulation by a square wave signal up to several GHz, that is, FSK transmission has become possible. In the case of transmission, the sine wave component of 5 kHz used for control is removed on the receiving side.

In this embodiment, the wide-band 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.

In this embodiment, the frequency of the sine wave is 5 k
Hz is selected as a frequency low enough to be frequency-modulated only by the thermal effect and high enough not to be cut when passing through the bias T 1.

(Embodiment 2) FIG. 6 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, so as not to use the bias T having a low-frequency cut-off and to simultaneously reduce the size of the entire driving system. Things. The voltage-current converter can drive a current by a voltage signal. As shown in 603 and 604 in FIG. 6, a DC current source Ib and a modulation current source Ip are integrated in parallel in the driver IC, and a DCL is input by inputting an ECL level modulation signal.
A modulation current having a C offset current 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. EC
Output of modulation power supply 605 having L output
Input to 603 and 604. 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 603 takes out the in-phase output with respect to the modulation input, and the driver IC 604 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. The negative phase output is driven as a current I2 + ΔI2 of the laser 601 through a low-pass filter 602 having a cutoff of 10 MHz.
The in-phase output is the current I 1 of the laser 601 as it is.
Drive as + ΔI ₁ . The current ratio between ΔI2 and ΔI1 is optimized by the same method as in the first embodiment. That is, a sine wave signal of 100 Hz is superimposed, and the modulation current amplitude Ip of the driver IC is automatically controlled with the mixing output obtained in the same manner as in the first embodiment. The frequency of the sine wave signal is NR
To increase the continuity of the Z signal, the lower the better, the lower the cutoff of the bias T 1 as in the first embodiment. Here, the driver IC
As shown in FIG. 6, it is preferable that the laser is connected to the ground to draw a current so that high-speed operation can be achieved. Therefore, in this example, an element in which the conductivity type was reversed and a p substrate was used, and the electrode separating side was an n electrode was used.

By assembling such a drive system, 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 1, but in the present embodiment, there is no restriction of the low band. Therefore, the low-frequency characteristics can be further improved, and 100 Hz to several GH
The modulation characteristics could be flattened over z. By this means, when FSK transmission is performed, NRZ (No Return) with 3 Gbps and continuity of 2 ²⁰ −1 or more is used.
n to Zero) signal,
Very high-density transmission becomes possible.

(Embodiment 3) In a third embodiment of the present invention, the driving method of Embodiment 1 for improving the low-frequency characteristics of the laser is applied to narrowing of the 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. 7 shows the drive system.

The light emitted from the semiconductor DFB laser 701 passes through a frequency discriminator 710 to convert the fluctuation of the frequency of the light into a fluctuation of the light intensity, and the light detector 709 converts the fluctuation into an electric signal. Here, as a frequency discriminator,
Using a Fabry-Perot etalon having a free spectral range of 5 GHz and a finesse of 50, and using GaAs p as a photodetector.
An in photodiode was used. 5KH
z band elimination filter (trap circuit) 71
2 and superimposed on the injection current of the laser 701 in the same manner as in the first embodiment to perform feedback control.
The feedback rate is determined by the amplifier A1 706 and the amplifier A27.
07 and the optical power incident on the photodetector. Here, the gain of the amplifier A1 is set to 100,
2, the optical power is 10 μW to 100 μ
The optimum adjustment was made between W. In addition, the actual distance of the feedback system limits the high frequency band, so that the size is reduced to about 10 cm, and the frequency at which resonance occurs due to the time delay of feedback is kept away from 3 GHz.

In this control system, the gain of one amplifier is optimized by automatic control, as in the first embodiment.
A 5 kHz signal is input to both of the two amplifiers 706 and 707 by the sine wave oscillator 711, and the photodetector 7
09 is output to a bandpass filter 713 of 5 kHz.
, The signal is mixed with a sine wave by the balanced modulator 714, and the gain of the amplifier 707 is automatically controlled by this signal to cancel the thermal effect. The band elimination filter 712 is for removing a component of the oscillation wavelength in order to prevent a signal obtained by capturing the fluctuation of the oscillation wavelength due to the sine wave signal by the photodetector from being inputted as it is and causing oscillation. .

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

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

FIG. 8 shows an example in which an optical transmission system is assembled using a light source driven by this driving method. The light emitted from the light source 801 is coupled to the single mode optical fiber 802 and transmitted. On the receiving side, beat the laser 803 as a local oscillator oscillating at the same wavelength as the light source 801,
The light is detected by a photodetector 804 and detected by a delay detection method or the like. The oscillation wavelength of the laser 803 is calculated based on a signal obtained by detecting a beat signal with the photodetector 805 by using a PLL (Phase).
The oscillation wavelength is stabilized by the 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. 9 shows the wavelength variable characteristics. The bias current flowing through each electrode is equal, that is, the current I is maintained while maintaining the relation of I ₁ = I _{2.
When 1} is changed from 20 mA to 60 mA, the oscillation wavelength can be changed continuously by about 1 nm from 835 nm to 836 nm. In this embodiment, utilizing this fact,
After determining the central oscillation wavelength by controlling the bias current, the modulation as described in the first or second embodiment is performed, or the oscillation spectrum is narrowed as described in the third embodiment. It shows that you can do it.

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. 1
Reference numeral 001 denotes a light source for optical communication modulated by the FSK method according to the present invention. In this light source, the wavelength can be changed in the range of 1 nm as described above. In this embodiment, FS
The frequency shift amount of K was 5 GHz. In the device of this example, a frequency shift amount of 5 GHz was obtained by setting the modulation current amplitude to 8.3 mA. Therefore, in the case of wavelength multiplexing, even if they are arranged at intervals of about 10 GHz (up to 0.02 nm), no crosstalk is given to an adjacent channel. Therefore, when this light source device is used, 1
/0.02=wavelength multiplexing of about 50 channels is possible. Light emitted from this light source is coupled to a single mode fiber 1002 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 1003 and detected by a photodetector 1004. Here, the first embodiment is used as an optical filter.
The same structure as that of the DFB laser is used by biasing the current below the threshold value. By changing the current ratio between the two electrodes, the transmission wavelength can be changed by 1 nm while the transmission gain is kept constant at 20 dB. 10d of this filter
The transmission width of B down is about 10 GHz (~ 0.02 nm).
), And has characteristics sufficient for wavelength multiplexing at intervals of 0.02 nm as described above.

The FSK signal is detected using the transmission characteristics of the filter. As shown in FIG. 11, the transmission peak wavelength of the filter is changed to the mark frequency (“1” of the FSK signal) of a desired channel in the wavelength multiplexed signal, for example, CH2.
"), The space frequency (F
The “0” of the SK signal) is separated by the above-mentioned frequency shift amount of 5 GHz, so that the “1” after passing through the filter.
"1" and "0" can be detected with an extinction ratio of 0 dB.

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.

In addition to the present embodiment, the method of receiving a wavelength multiplexed signal may be a coherent system as described in the third embodiment.

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

(Embodiment 5) FIG. 12 shows a method of driving a light source for optical communication according to the present invention and an optical-electrical converter connected to each terminal when an optical communication system using the same is applied to a wavelength division multiplexing optical LAN system. FIG. 13 shows a configuration example of a unit (node), and FIG. 13 shows a configuration example of an optical LAN system using the node.

The optical signal is taken into the node by using the optical fiber 1201 connected to the outside as a medium,
As a result, a part of the light enters the receiving apparatus 1203 provided with 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 transmitting an optical signal from the node, the wavelength tunable DFB laser 1204 is driven by any of the methods of the first and second embodiments, and is modulated by the FSK method. Through the optical transmission line.

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

As a network of the optical LAN system,
FIG. 13 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 NORs, it is necessary to arrange optical amplifiers in series on a transmission line in order to decompensate light. Also, by connecting two nodes to each terminal and using two transmission paths, DQDB
Two-way transmission by this method is possible.

In such an optical network system, if the driving method of the device according to the present invention is used, the fourth embodiment can be used.
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 multiplexed CATV having a topology as shown in FIG. 14 can be constructed by the apparatus and the optical communication system according to the present invention. In the CATV center, the tunable laser is modulated by the driving method of the first or two embodiments 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,
Although it was difficult to use a DFB filter in such a system due to the effect of the dynamic wavelength fluctuation of the DFB laser, it has been made variable by the present invention.

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"). LN external modulator for bidirectional transmission in a system "OCS91-82, P.51), and by constructing a star network as shown in FIG.
The TV is variable, and the service can be enhanced.

[0050]

According to the present invention, the modulation band on the low frequency side when modulating the frequency of a semiconductor laser can be extended stably and largely regardless of changes in the mounting form and the use environment, and high-density transmission is possible. Becomes 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 diagram for explaining the principle of a driving method according to the present invention.

FIG. 6 is a block diagram illustrating a driving method according to a second embodiment of the present invention.

FIG. 7 is a block diagram illustrating a driving method for narrowing a spectral line width according to a third embodiment of the present invention.

FIG. 8 is a diagram illustrating a transmission system that performs coherent optical communication using the driving method according to the third embodiment.

FIG. 9 is a diagram showing a wavelength tunable characteristic of a distributed feedback semiconductor laser.

FIG. 10 is a diagram showing a transmission system when performing wavelength division multiplexing transmission according to a fourth embodiment.

FIG. 11 is a diagram illustrating a method of receiving a wavelength multiplexed signal.

FIG. 12 is a diagram illustrating a configuration example of an optical node according to a fifth embodiment;

FIG. 13 illustrates an optical LAN network.

FIG. 14 illustrates an optical CATV system.

FIG. 15 shows a single-electrode distributed feedback laser.

FIG. 16 shows a conventional frequency modulation characteristic.

FIG. 17 shows a conventional phase characteristic.

FIG. 18 is a diagram for explaining a conventional FSK response characteristic.

FIG. 19 is a diagram showing a conventional example of a three-electrode distributed feedback laser.

FIG. 20 is a diagram showing a conventional example of a distributed feedback laser having a non-uniform depth diffraction grating.

FIG. 21 is a diagram showing a conventional driving method when performing FSK modulation with a two-electrode DFB laser.

[Explanation of symbols]

101, 1041, 1051 Substrates 102, 105 Cladding layers 103, 1042, 1054 Active layers 104, 1043, 1053 Light guide layers 106 Contact layers 107, 108 Buried layers 109, 110, 1045, 1055 Electrodes 401, 601, 701, 1601 2 Electrode semiconductor lasers 402, 404, 702, 704, 1602,
1604 Bias T 403, 405, 703, 705, 1603,
1605 DC current sources 406, 407, 706, 707, 1606,
1607 Amplifiers 408, 708, 1608 Power dividers 409, 1609 Modulation power supplies 602, 406, 611, 715 Low-pass filters 412, 608, 713 Band-pass filters 712 Band-eliminate filters 410, 606, 711 Sine-wave oscillators 411, 607, 714 Modulator 603, 604 Laser driver IC 605 Modulation power supply 413, 609, 709, 804, 805,
1004 Photodetector 415, 610, 710 Frequency discriminator 801, 1001, 1204 Optical communication light source 802, 1002, 1201 Optical fiber 803 Local oscillation laser 806 PLL control circuit 1003 Wavelength variable optical filter 414, 1202, 1206 Optical splitter 1205 Optical isolator 1203 Receiving device 1044, 1052 Diffraction grating 1056 λ / 4 shift unit

────────────────────────────────────────────────── ─── Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI H04B 10/28 (56) References JP-A-4-137778 (JP, A) JP-A-3-295289 (JP, A) JP-A-61-290789 (JP, A) JP-A-3-174791 (JP, A) JP-A-2-248125 (JP, A) JP-A-63-204779 (JP, A) JP-A-2-201986 (JP JP, A) JP-A-5-267760 (JP, A) JP-A-64-73687 (JP, A) JP-A-2-234484 (JP, A) JP-A-61-290789 (JP, A) JP 5-95153 (JP, A) JP-A-2-268479 (JP, A) JP-A-2-268478 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) H01S 5 / 00-5/50 H04B 10/00

Claims (24)

(57) [Claims] At least a first electrode as a current injection electrode is provided.
A method of frequency-modulating a semiconductor laser having an electrode and a second electrode by directly injecting a modulation current, wherein the modulation current is injected into the first electrode when the frequency band of the modulation current is high. At the time of low frequency, a modulation current is injected into the first electrode in the same manner as at the time of high frequency, and at the same time, a modulation current having a phase opposite to that of the modulation current is injected into the second electrode. And a feedback control of a ratio of amplitudes of modulation currents having opposite phases to be injected into the electrodes by an optical output of the semiconductor laser. 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 feedback control according to claim 1, wherein the phase difference between the modulation signal injected into the first electrode and the modulated optical frequency modulation signal is controlled to be constant over a wide band. A method for driving a semiconductor laser. 4. A method of performing feedback control, wherein a sine wave signal having a frequency lower than the modulation frequency is superimposed on both of the modulation currents having opposite phases, and fluctuation of the optical frequency of the semiconductor laser due to the sine wave signal is optically controlled. Converted into intensity fluctuations and detected, the product of the detected signal and the sine wave signal is taken, frequency components lower than the sine wave signal are taken out of the product signal, and 4. The semiconductor laser driving method according to claim 1, wherein the method is a method of controlling at least one modulation amplitude of the out-of-phase modulation currents. 5. At least a first electrode as a current injection electrode.
A semiconductor laser having a first electrode and a second electrode, which is frequency-modulated by directly injecting a modulation current, comprising: a modulation signal; and a sine wave signal having a frequency lower than at least the frequency of the modulation signal. Is superimposed on one of the same two signals with a gain cutoff of several GHz.
Means for cutting the direct current component, superimposing it on the bias current and injecting it into the first electrode after passing through the first amplifier,
Means for passing the other one through a second amplifier having a lower gain cutoff frequency than the first amplifier, cutting a DC component, superimposing the DC component on a bias current, and injecting the bias current into a second electrode; Means for extracting a part of the optical output, an optical frequency discriminator for converting the fluctuation of the optical frequency of the optical output into a fluctuation of the light intensity, and a photodetector for detecting the optical output through the optical frequency discriminator. A band-pass filter that extracts only a frequency component near the frequency of the sine wave signal among the electric signals obtained by the photodetector, A low-pass filter for extracting a frequency component lower than the sine wave from the electric signal obtained by taking the product; and adjusting a gain ratio between the first amplifier and the second amplifier based on the output of the low-pass filter. Having means to A one is an inverting amplifier of the first amplifier and the second amplifier, a semiconductor laser drive apparatus, wherein the remaining one is a non-inverting amplifier. 6. The cut-off frequency of the second amplifier is 1
6. The driving device for a semiconductor laser according to claim 5, wherein the driving frequency is about 0 MHz. 7. At least a first electrode as a current injection electrode.
A semiconductor laser having a first electrode and a second electrode, which is frequency-modulated by directly injecting a modulation current, comprising: a modulation signal; and a sine wave signal having a frequency lower than at least the frequency of the modulation signal. And a voltage-current converter having an opposite-phase output for injecting one of the same two signals with the bias current into the first electrode and a voltage having an in-phase output for injecting the other one with the bias current into the second electrode. A current converter, a low-pass filter provided between one of the two voltage-current converters and the semiconductor laser, and a unit for extracting a part of the optical output of the semiconductor laser;
An optical frequency discriminator for converting the fluctuation of the optical frequency of the optical output into a fluctuation of the light intensity, a photodetector for detecting the optical output through the optical frequency discriminator, and an electric signal obtained by the photodetector A band-pass filter for extracting only a component near the frequency of the sine wave signal; a means for obtaining a product of the electric signal passing through the band-pass filter and the sine wave signal; A low-pass filter for extracting a frequency component lower than the wave, and means for adjusting a gain ratio between the voltage-current converter having the opposite-phase output and the voltage-current converter having the same-phase output based on the output of the low-pass filter. A driving device for a semiconductor laser, comprising: 8. The semiconductor laser drive according to claim 7, wherein a cutoff frequency of a low-pass filter provided between one of the two voltage-current converters and the semiconductor laser is about 10 MHz. apparatus. 9. 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. At the same time, a current having a phase opposite to that of the negative feedback current due to the electric signal is injected into the second electrode, and the ratio of the currents having phases opposite to each other injected to the first electrode and the second electrode is determined by the light of the semiconductor laser. A method for driving a semiconductor laser, wherein feedback control is performed by an output. 10. The method of driving a semiconductor laser according to claim 9, wherein the low-frequency band in which the opposite-phase current is injected is about 10 MHz or less. 11. The feedback control according to claim 8, wherein the phase difference between the current injected into the first electrode and the fluctuation of the optical frequency due to the two current injections is constant over a wide band. A method of driving a semiconductor laser, wherein the method is feedback control for controlling the temperature of the semiconductor laser. 12. A feedback control method comprising superimposing a sine wave signal having a frequency lower than the current frequency on both of the currents having opposite phases to each other, and suppressing fluctuations in the optical frequency of the semiconductor laser due to the sine wave signal to light intensity. Of the fluctuation, and the product of the detected signal and the sine wave signal is calculated.
12. The semiconductor according to claim 9, wherein a frequency component lower than the sine wave signal is extracted from the signal obtained by taking the product, and a ratio of the currents having opposite phases is controlled by the extracted signal. Laser driving method. 13. An apparatus for driving a semiconductor laser having at least a first electrode and a second electrode as current injection electrodes to narrow the oscillation spectrum line width of the semiconductor laser, wherein two sine wave signals having the same phase are provided. Is passed through a first amplifier having a gain cut-off of several GHz or more, and then a DC component is cut, superimposed on a bias current, and injected into a first electrode. Cutoff frequency is first
A second amplifier lower than the amplifier, a DC component is cut, superimposed on a bias current, and injected into a second electrode; and a part for extracting a part of output light of the semiconductor laser; A frequency discriminator that detects fluctuations in the oscillation wavelength of the laser, a photodetector that detects light that has passed through the frequency discriminator, and a signal obtained by removing the frequency component of the sine wave from the signal obtained by the photodetector. Means for superimposing on each of the two sine wave signals and inputting the signals to a first amplifier and a second amplifier, and a band-pass for extracting only a component near the frequency of the sine wave signal from the electric signal obtained by the photodetector A filter, means for taking a product of the electric signal having passed through the band-pass filter and the sine wave signal, and a low-pass filter for taking out a frequency component lower than the sine wave signal in the electric signal obtained by taking the product; Said low Means for adjusting the ratio of the gain of the first amplifier to the gain of the second amplifier based on the output of the pass filter. One of the first and second amplifiers is an inverting amplifier, and the other is an inverting amplifier. A driving device for a semiconductor laser, one of which is a non-inverting amplifier. 14. The cut-off frequency of the second amplifier is:
14. The driving device for a semiconductor laser according to claim 13, wherein the driving frequency is about 10 MHz. 15. A device for driving a semiconductor laser having at least a first electrode and a second electrode as current injection electrodes, with the oscillation spectrum line width being narrowed, wherein two sine wave signals having the same phase are provided. A negative-phase output voltage-current converter for injecting one of them into the first electrode together with a bias current;
A voltage-current converter having an in-phase output for injecting the remaining one with the bias current into the second electrode, a unit for extracting a part of the output light of the semiconductor laser, and a frequency discriminator for detecting fluctuation of the oscillation wavelength of the laser A photodetector that detects light passing through the frequency discriminator, and a signal obtained by removing the frequency component of the sine wave from the signal obtained by the photodetector, and superimposing the signal on the two sine wave signals. Means for inputting to the first amplifier and the second amplifier, a low-pass filter provided between one of the two voltage-current converters and the semiconductor laser, and an electric signal obtained by the photodetector A band-pass filter for extracting only a component near the frequency of the sine wave signal; a means for obtaining a product of the electric signal passing through the band-pass filter and the sine wave signal; A low-pass filter for extracting a frequency component lower than the signal; and means for changing a gain ratio between the voltage-current converter having the opposite-phase output and the voltage-current converter having the same-phase output based on the output of the low-pass filter. A driving device for a semiconductor laser, comprising: 16. The semiconductor laser drive according to claim 15, wherein a cut-off frequency of a low-pass filter provided between one of the two voltage-current converters and the semiconductor laser is about 10 MHz. apparatus. 17. When driving a semiconductor laser by the driving method according to any one of claims 9 to 11, two currents injected into the semiconductor laser are applied to the first electrode and the second electrode in the semiconductor laser driving method according to claim 1 to 4. A method for driving a semiconductor laser, comprising superimposing currents to be injected into two electrodes and frequency-modulating the currents. 18. An FSK (Frequency Shift Key) in which frequency modulation is performed by a digital signal.
18. The method of driving a semiconductor laser according to claim 17, wherein: 19. An optical communication method in which the transmission light modulated by the driving method according to claim 18 is transmitted through an optical fiber, and the receiving side performs beat detection using a local oscillation laser to perform coherent communication. 20. The method of driving a semiconductor laser according to claim 1, wherein the frequency modulation is FSK modulated by a digital signal. 21. An optical communication method in which transmission light modulated by the driving method according to claim 18 or 20 is transmitted through an optical fiber and directly detected at a receiving side through a wavelength filter. 22. 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. 22. An optical communication method for transmission, wherein an optical communication method for each wavelength is the method according to claim 19 or 21. 23. In an optical communication system in which a plurality of nodes are connected to one optical fiber, and light of a plurality of wavelengths is modulated and wavelength-division multiplexed and transmitted, each node comprises an optical 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 22. 24. 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 and outputs 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.