JPH0831646B2 - Optical frequency modulator - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an optical frequency modulator for generating frequency-modulated light used for optical fiber communication, optical measurement, and the like.

(Prior art) Optical frequency modulation (optical FSK) communication method, optical sensors that use frequency-modulated light, oscillation frequency control devices that control the optical frequency according to error signals, etc. The optical frequency of the laser
It is necessary to change it according to the so-called modulated signal.

Here, when the modulation signal is sinusoidal vibration of frequency f (modulation frequency), the frequency of the obtained light (modulated light) also changes sinusoidally at frequency f. This time,
Amplitude of frequency change of modulated light (frequency shift amount; hereinafter FM
The relationship between the phase of the modulation signal and the phase when the frequency of the modulated light changes sinusoidally (hereinafter referred to as the FM phase) can be grasped as a function of the modulation frequency f. Hereinafter, these are collectively referred to as frequency modulation characteristics.
These are referred to as FM amplitude characteristic and FM phase characteristic.

The semiconductor laser has an advantage that the oscillation frequency can be modulated by superimposing a modulation current on the injection current. In particular, distributed reflection semiconductor lasers (hereinafter referred to as DBR-LDs) and distributed feedback semiconductor lasers (hereinafter referred to as DFB-LDs) with a built-in diffraction grating provide stable single-mode operation even during frequency modulation by injection current. Can be oscillated (dynamic single-mode laser).

FIG. 2 is a block diagram showing a conventional optical frequency modulator using a DBR-LD (for example, IEEE Journal of Quanta
m Electronics, Vol.QE-23, No.6, pp.835-838,1987 SM
urata et.al., “Spectral Characteristics for a 1.5
μm DBR Laser with Frequency-Turning Region. ").

In FIG. 2, reference numeral 10 denotes a DBR-LD, which is an active layer region 11 which produces light by stimulated emission and an inactive layer region which forms a diffraction grating and which selectively reflects light by the diffraction grating.
12, an electrode 13 for injecting a current into the active layer region 11, and an electrode 14 for injecting a control current into the inactive layer region 12. 15 is an injection current supply device, which supplies the injection current Ia to the electrode 13 and, based on the modulation signal input to the modulation signal input terminal 16, superimposes the modulation current on the injection current Ia to obtain the value of the injection current Ia. Change. Reference numeral 17 is a control current supply device, which supplies the control current Ig to the electrode 14 and, based on the modulation signal input to the modulation signal input terminal 18, superimposes the modulation current on the control current Ig to change the value of the control current Ig. Change.

In such a configuration, by injecting the injection current Ia into the active layer region 11 via the electrode 13, the DBR-LD 10 oscillates to generate the output light Pout. On the other hand, when the control current Ig is injected into the inactive layer region 12 through the electrode 14, the refractive index of the inactive layer region 12 changes due to the plasma effect of carriers,
The oscillation frequency can be changed.

Here, by inputting the modulation signal to the modulation signal input terminal 16 or 18, the modulation current is superposed on the injection current Ia or the control current Ig, whereby the frequency-modulated optical signal is generated.
You can take out Pout.

FIGS. 3 and 4 are diagrams showing an example of measuring the optical frequency modulation characteristic of the optical frequency modulator of FIG. 2, and FIG. 3 shows the case where a modulation signal is input to the modulation signal input terminal 16 (injection). Electric current
FIG. 4 shows the characteristics when the modulation signal is input to the modulation signal input terminal 18 (the modulation current is superimposed on the control current Ig). Further, (a) of FIGS. 3 and 4 shows the FM amplitude characteristic, and (b) of both figures shows the FM phase characteristic. The horizontal axis corresponding to the modulation frequency is from 300 KHz to 3G.
Up to Hz is shown on a logarithmic scale.

In the device of FIG. 2, when the injection current Ia is modulated (FIG. 3), at a low modulation frequency, when the injection current Ia increases due to the effect of heat, frequency modulation is performed in the direction in which the oscillation frequency changes to the lower side. This is done (this FM phase is defined as π, and this is hereinafter referred to as redshift frequency modulation).
However, since the effect of heat has a large time constant, the FM amplitude based on this principle decreases as the modulation frequency increases.

On the other hand, there is also frequency modulation based on the principle that the injection current Ia increases, the number of carriers increases, the refractive index of the active layer region 11 decreases, and the oscillation frequency changes to the higher side. The frequency modulation due to the effect of the carrier is in the opposite direction (FM phase zero) to the change of the oscillation frequency due to the effect of heat described above, and is referred to as frequency modulation of blue shift.

Therefore, as shown in FIG. 3 (b), the FM phase characteristic is inverted as the modulation frequency is increased. Further, in general, since the DBR-LD 10 does not have a built-in diffraction grating in the active layer region 11, the FM amplitude due to the effect of carriers is small and the FM amplitude characteristic is flat as shown in FIG. 3 (a). I won't.

On the other hand, when the control current Ig is modulated (FIG. 4), the region where the modulated current is injected is the inactive layer region 12, so that the FM amplitude due to the effect of carriers is larger than that due to the effect of heat. It is possible to obtain a flat FM amplitude characteristic as shown in FIG. 4A and a blue shift FM phase characteristic without phase rotation as shown in FIG. 4B.

In this case, since the non-active layer region 12 is modulated,
The modulation band is limited by the carrier lifetime (1 to 3 ns) based on spontaneous emission, and the 6 dB bandwidth remains at 2 to 300 MHz.

FIG. 5 is a block diagram showing a conventional optical frequency modulator using a 3-electrode DFB-LD (for example, IEEE / OSA Jou.
rnal of Lightwave Technology, Vol.LT-5, No.4, pp.516
−522,1987, Y. Yoshikuni et.al., “Multielectrode Dis
tributed Feedback Laser for Pure Frequency Modulat
ion and Chirping Suppressed Amplitunde Modulatio
In FIG. 5, reference numeral 20 denotes a DFB-LD, which is an active layer region 21 in which a diffraction grating is formed and an electrode for current injection into the active layer region 21.
It has 22,23,24. 25 is an injection current supply device, which is an electrode
The injection current Ia1 is supplied to 23 and the modulation signal input terminal 26
The modulation current is superimposed on the injection current Ia1 on the basis of the modulation signal input to, and the value of the injection current Ia1 is changed. Similarly, 2
7 is an injection current supply device, which supplies the injection current Ia2 to the electrodes 22 and 24, and based on the modulation signal input to the modulation signal input terminal 28, superimposes the modulation current on the injection current Ia2, Change the value.

In such a configuration, the injection current is injected through the electrodes 22, 23, 24 divided into three into the active layer region 21 containing the diffraction grating.
By injecting Ia1 and Ia2, the DFB-LD 20 oscillates to generate the output light Pout.

Here, by appropriately selecting the ratio and magnitude of the injection currents Ia1 and Ia2 and inputting the modulation signal to the modulation signal input terminal 26 or 28, the injection current Ia1 or the injection current Ia2 is obtained.
A modulation current is superposed on, and thereby the frequency-modulated light Pout can be taken out.

FIG. 6 is a diagram showing the optical frequency modulation characteristic of the optical frequency modulator of FIG. 5, where (a) of FIG. 6 shows the FM amplitude characteristic and (b) of FIG. 6 shows the FM phase characteristic. The horizontal axis corresponding to the modulation frequency indicates from 100 Hz to 1 GHz on a logarithmic scale.

In the device of FIG. 5, if the ratio and magnitude of the injection currents Ia1 and Ia2 to the divided electrodes 22, 23, 24 can be optimally selected, by inputting the modulation signal to the modulation signal input terminal 26 or 28, As shown in Fig. 6, there is a flat FM amplitude characteristic and a blue shift or red shift without phase rotation.
FM phase characteristics can be obtained.

(Problems to be solved by the invention) In the optical FSK communication system, the optical frequency modulator has a flat FM amplitude characteristic and an FM phase characteristic, and further, those characteristics are maintained until a high modulation frequency, that is, It is important to have a wide modulation band. If the former is not satisfied, the transmission characteristics deteriorate, and if the latter is not satisfied, the information transmission rate is controlled. In addition, when applying the optical FSK communication method to the optical frequency division multiplexing (optical FDM) method,
At the same time, it is desirable to be able to change the steady value of the oscillation frequency over a wide range (optical frequency tuning).

However, in the device using the DBR-LD10 described above, by injecting the control current Ig into the inactive layer region 12, the optical frequency can be easily tuned, but the optical frequency modulation is flat and wideband. You cannot get the characteristics.

On the other hand, in the device using the three-electrode DFB-LD20, if the bias conditions for the divided electrodes 22, 23, 24 can be optimally set, it is possible to obtain a good optical frequency modulation characteristic in a flat and wide band. You can However, such an optimum bias condition is complicated by many factors derived from the fact that the diffraction grating is incorporated in the active layer region 21 into which the current in which the modulation current is superimposed is injected. It is difficult to set the laser systematically.

In addition, if the bias condition is changed to tune the oscillation frequency, the optical frequency modulation characteristic may deteriorate.

The present invention has been made in view of such circumstances,
The purpose is to have a good FM frequency characteristic that the FM amplitude characteristic is flat and that the FM phase characteristic has no phase rotation over a wide frequency band, and that the oscillation frequency can be easily tuned. An object is to provide an optical frequency modulator.

(Means for Solving the Problem) In order to achieve the above object, the present invention includes a distributed reflection type semiconductor laser having an active layer region and a non-active layer region including a distributed reflection layer region having a built-in diffraction grating. In the optical frequency modulator, an injection current supply device that supplies a part of the injection current and a modulated injection current that supplies a modulated injection current that is modulated by superimposing a modulation current for optical frequency modulation on the remaining injection current A current supply device, at least one current injection electrode for injecting the injection current into a part of the active layer region, and an injection current into the remaining part of the active layer region different from the injection region of the injection current. At least one modulated current injection electrode for injecting the modulated injection current so that the density thereof is lower than the injection current density of the current injection electrode.

(Operation) According to the present invention, the injection current supply device and the modulation injection current supply device are provided to the respective electrodes for current injection and modulated current injection which are divided so that the current density injected into the active layer region becomes non-uniform. When an injection current is supplied by, the non-uniform carrier distribution occurs in the active layer region. At this time, when the modulated injection current supply device supplies the modulated injection current that is modulated by superimposing the modulation current to the modulated current injection electrode, due to the effect of the carrier generated at that portion, the oscillation frequency in the blue shift direction is generated. Tends to change (FM amplitude in this case is
(It is generally small as shown in FIG. 3 (a)).

On the other hand, by injecting the modulated injection current on which the modulation current is superimposed into the active layer region, intensity modulation is also induced,
The photon number density in the active layer region changes. Along with this, the carrier density of the active layer region into which the current without the modulation current injected is also changed. When the modulation current increases and the light intensity increases, the carrier density of the region into which the current without the modulation current injected is decreased, and the oscillation frequency tends to change in the direction of red shift.

The FM amplitude due to this effect can be increased by supplying the modulated injection current on which the modulation current is superimposed to the modulated current injection electrode as described above. Therefore,
The FM phase characteristic due to the carrier effect can be red-shifted in phase with the thermal effect, and a flat FM amplitude characteristic is realized. It is the carriers in the active layer region that are responsible for this effect, whose lifetime is equivalently short due to the strong stimulated emission and can be modulated up to high frequencies.

Further, since the distributed reflection layer region is included, stable single-mode oscillation is maintained even when the current is nonuniformly injected and frequency modulation is applied. Further, a control current is injected into the non-active layer region, and the oscillation frequency is tuned without affecting the optical frequency modulation characteristic.

(Embodiment) FIG. 1 is a configuration diagram showing a first embodiment of an optical frequency modulator according to the present invention. In FIG. 1, 30 is DBR-L
In D (distributed reflection type semiconductor laser), an active layer region 31 having a length of 300 μm for generating light by stimulated emission and a diffraction grating are formed, and a non-active layer region (distributed reflection) for selectively reflecting light by the diffraction grating Layer region) 32, a current injection electrode 33a having a length set to 200 μm for injecting an injection current Ia1 described later into the active layer region 31, and a modulated injection current IA described below is injected into the active layer region 31. The length for doing is 100 μm
The modulated current injecting electrode 33b set to 1 and the control current injecting electrode 34 for injecting a control current Ig, which will be described later, into the inactive layer region 32.

Reference numeral 35 is an injection current supply device, which supplies an injection current of 100 mA to the electrode 33a.
Supply Ia1. 36 is a modulated injection current supply device, which is used for the electrode 3
The injection current Ia2 of 20 mA is supplied to 3b, and the injection current Ia2 is supplied based on the modulation signal input to the modulation signal input terminal 37.
The modulated injection current IA which is modulated by superimposing the modulation current on 2 is supplied to the current 33b. 38 is a control current supply device, which is an electrode 34
The control current Ig is supplied to the control current Ig and the modulation current is superimposed on the control current Ig based on the tuning signal input to the tuning signal input terminal 39 to change the value of the control current Ig.

The DBR-LD30 is, for example, a butt joint type DBR-LD (Electronics Lette) manufactured by MOVPE method.
rs, Vol.24, No.24, pp.1481-1483, 1988, Y. Tohmori et a
l., “NARROW LINEWIDTH AND LOW CHIRPING CHARACTERIS
TICS IN HIGH POWER OPERATING BUTT-JOINTED DBR LAS
ERS GROWN BY MOVPE. ")).

 Next, the operation of the above configuration will be described.

In the active layer region 31, an injection current Ia1 of 100 mA is injected by the injection current supply device 35 through the electrode 33a, and based on the modulation signal input to the modulation signal input terminal 37 by the modulated injection current supply device 36, The modulated injection current IA, which is modulated by superimposing the modulation current on the injection current Ia2 of 20 mA, is injected. This causes a non-uniform carrier distribution in the active layer region 31, and in the basin where the modulated injection current IA is injected,
The oscillation frequency tends to change toward the blue shift due to the effect of the carrier.

In addition, the active layer region 31 into which the modulated injection current IA is injected
Then, intensity modulation is also induced to change the photon density, and the carrier density of the active layer region 31 into which the injection current Ia1 is injected also changes accordingly. Here, when the modulation current is increased to increase the light intensity, the carrier density of the active layer region 31 into which the injection current Ia1 is injected decreases, and the oscillation frequency tends to change in the direction of red shift.

Thus, of the two divided electrodes 33a and 33b, the injection current Ia1 is supplied to the electrode 33a and the modulated injection current IA is supplied to 33b,
The FM amplitude can be increased by increasing the imbalance in the current injection density into the active layer region 31. Therefore,
The FM phase characteristic due to the carrier effect can be red-shifted in the same phase as the thermal effect, and the flat FM phase characteristic is realized.

In addition, carriers in the active layer region 31 are involved in the above modulation, and the lifetime thereof is equivalently short due to the effect of strong stimulated emission. Therefore, modulation can be applied up to a high frequency region.

Further, based on the tuning signal input to the tuning signal input terminal 39, the control current Ig on which the modulation current is superimposed is supplied to the electrode 34 and injected into the inactive layer region 32, thereby affecting the optical frequency characteristic. It is possible to tune the oscillation frequency without the need.

FIG. 7 shows an optical frequency measuring method for canceling the influence of optical intensity modulation secondary to the modulating signal of the apparatus shown in FIG. 1 (Iwashita, “Optical Frequency Modulation Characteristic Measuring Device”, Japanese Patent Application No. 63-
FIG. 10 is a diagram showing optical frequency modulation characteristics measured using No. 092783). 7 (a) shows the FM amplitude characteristic, and FIG. 7 (b) shows the FM phase characteristic, and the horizontal axis corresponding to the modulation frequency is shown on a logarithmic scale from 300 KHz to 3 GHz.

As is apparent from FIG. 7, the optical frequency modulation characteristics of the conventional device using the DBR-LD in which the electrode for injecting current into the active layer region, which is described with reference to FIGS. 3 and 4, is not divided. Compared with, in the device of FIG. 1, a flat FM phase characteristic without phase rotation (red shift) and a relatively flat
FM amplitude characteristics are obtained, and a wider modulation band (6 dB bandwidth about 1 GHz) is obtained compared to the case where the inactive layer region 32 is modulated (Fig. 4, 6 dB bandwidth 2 to 300 MHz). There is.

It should be noted that this modulation band is limited by the effect that the high-frequency modulation current superimposed on the injection current is attenuated by the capacitance of the electrodes and the inductance of the power supply line of the actually manufactured device. -It is possible to further widen the modulation band by improving the power supply section.

Further, at this time, as described above, by inputting a signal to the tuning signal input terminal 39 to change the control current Ig, the oscillation frequency can be independently adjusted without affecting the optical frequency modulation characteristic shown in FIG. It was possible to tune, but even the prototype device was able to tune continuously at approximately 30 GHz.

Even if the roles of the electrodes 33a and 33b are exchanged, a device having the same advantages can be constructed. Further, even if the current injection electrode into the active layer region 31 is divided into three or more, a similar device can be constructed by superimposing the modulation current on one or a plurality of electrodes having a small injection current.

FIG. 8 is a block diagram showing a second embodiment of the optical frequency modulator according to the present invention. The difference between the second embodiment and the first embodiment is that the inactive layer region 41 is divided into a distributed reflection layer region 41a containing a diffraction grating and a phase matching region 41b not containing a diffraction grating. Using the DBR-LD40, and supplied from the control current supply device 38 via the control current injection electrodes 42, 43, the control current Ig1, Ig2 divided by resistors R1, R2 at an appropriate ratio are distributed reflection layer regions. 41a and the phase matching region 41b, respectively.

With such a configuration, a mode jump that occurs when only the control current Ig2 is changed (corresponding to the configuration of FIG. 1) is injected into the phase matching region 41b with the control current Ig1 so that the oscillation phase condition is changed. It can be suppressed by the adjusted effect and the oscillation frequency can be changed over a wide range without mode jump (eg, Nikkei Electronics, 1987.6.15, (no, 423), pp.149-161, Kobayashi et al.,
"Continuously change the wavelength of the semiconductor laser").

On the other hand, the effect of dividing the current injection electrode into the active layer region 31 is similar to that of the first embodiment, and good optical frequency modulation characteristics are obtained also in the second embodiment. To be That is, an optical frequency modulator that can tune the oscillation frequency over a wide range and that has good optical frequency modulation characteristics is realized.

In FIG. 8, the phase matching region 41b is arranged between the active layer region 31 and the distributed Bragg reflector region 41a. However, even if it is provided on the left side of the active layer region 31, the optical frequency modulation having the same effect is obtained. The device can be configured.

FIG. 9 is a block diagram showing a third embodiment of the optical frequency modulation device according to the present invention. The difference between the third embodiment and the first embodiment is that in the first embodiment, as shown in FIG. 1, the DBR- which has a semi-transparent mirror structure in which the left end in the drawing is a wall opening is shown. While the LD30 is used, in the third embodiment, the DBR-LD50 having the same inactive layer region (distributed reflection layer region) 51 as the right end is used at the left end, and the DBR-LD50 from the control current supply device 38 is used. The control current Ig is configured to be injected into the inactive layer regions 32 and 51 via the control current injection electrodes 34 and 52.

Even in such a configuration, by inputting the tuning signal to the tuning signal input terminal 39, the first
The oscillation frequency can be tuned in the same manner as in the above embodiment. Further, since the equivalent cavity length is lengthened by the inactive layer region 51 provided at the left end, it is possible to obtain the output light Pout having a small phase noise, that is, a narrow spectral line width.

On the other hand, the effect of dividing the current injecting electrode into the active layer region 31 can be similar to that of the first embodiment, and good optical frequency modulation characteristics can be obtained also in the third embodiment. To be That is, an optical frequency modulator having a narrow spectral line width and good optical frequency modulation characteristics is realized.

In addition, except the portion of the non-active layer region 51 of FIG.
It is also possible to realize the same configuration as that of the above-described embodiment, or further, to arrange the phase matching region between the non-active layer region 51 and the active layer region 31 at the left end.

(Effects of the Invention) As described above, according to the present invention, there is provided an optical frequency modulator having both a flat optical frequency modulation characteristic having a wide modulation frequency band and no phase rotation and a wide frequency tuning band. Can be easily obtained. Therefore, in the optical FSK communication system, the oscillation frequency on the transmitter side can be easily adjusted, the specifications for the oscillation frequency value at the time of laser manufacturing are reduced, and a system using the optical frequency multiplex communication system is used. This has the advantage of increasing the degree of freedom in design when configuring the.

Furthermore, when negative feedback control is performed by using the error signal detected by the interferometer or the beat signal with other light to control the oscillation frequency or the spectral line width, the error due to the inversion of the FM phase characteristic is caused. There is also the advantage that stability does not occur and the design of the feedback electric circuit becomes easy.

[Brief description of drawings]

FIG. 1 is a block diagram showing a first embodiment of an optical frequency modulator according to the present invention, FIG. 2 is a block diagram of a conventional optical frequency modulator using a distributed Bragg reflector (DBR) semiconductor laser, and FIG. FIGS. 4 and 5 are diagrams showing the measurement results of the optical frequency modulation characteristics of the conventional device shown in FIG. 2, and FIG. 5 is a configuration diagram of a conventional optical frequency modulation device using a distributed feedback (DFB) semiconductor laser, Sixth
FIG. 7 is a diagram showing the optical frequency modulation characteristic of the conventional device shown in FIG. 5, and FIG. 7 is a measurement result of the optical frequency modulation characteristic of the device according to the first embodiment of the present invention shown in FIG. Figure, 8th
FIG. 9 is a configuration diagram showing a second embodiment of the optical frequency modulation device according to the present invention, and FIG. 9 is a configuration diagram showing a third embodiment of the optical frequency modulation device according to the present invention. In the figure, 30, 40, 50 ... Distributed reflection type semiconductor laser (DBR-L
D), 31 ... Active layer region, 32,41,51 ... Inactive layer region, 3
3a ... Current injection electrode, 33b ... Modulated current injection electrode, 34,42,43,52 ... Control current injection electrode, 35 ... Injection current supply device, 36 ... Modulated injection current supply device , 38 ……
Control current supply device.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Kunishige Oe 1-1-6 Uchisaiwaicho, Chiyoda-ku, Tokyo Nihon Telegraph and Telephone Corporation (56) Reference JP-A-2-238686 (JP, A) Kaihei 1-223791 (JP, A) JP-A-1-175277 (JP, A)

Claims (1)

[Claims] 1. An optical frequency modulator provided with a distributed Bragg reflector semiconductor laser having an active layer region and a non-active layer region including a distributed Bragg reflector layer region having a built-in diffraction grating, and supplying a part of an injection current. An injection current supply device, a modulated injection current supply device that supplies a modulated injection current that is modulated by superimposing a modulation current for optical frequency modulation on the remaining injection current, and the injection into a part of the active layer region. At least one current injection electrode for injecting a current, and the injection current density in the remaining part of the active layer region different from the injection current injection region may be lower than the injection current density by the current injection electrode. An optical frequency modulator according to claim 1, further comprising at least one modulated current injection electrode for injecting the modulated injection current.