JPH0550875B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a semiconductor laser wavelength stabilizing device that stabilizes the oscillation frequency.

(Prior Art and its Problems) In recent years, with the remarkable improvement in the characteristics of semiconductor lasers and optical fibers, long-distance and high-speed optical fiber communication systems using semiconductor lasers and single-mode optical fibers are being considered. However, when using the 1.5 μm wavelength range where the transmission loss of the optical fiber is the lowest, the chromatic dispersion is large in that wavelength range, so even if the oscillation axis mode of the semiconductor laser is only one, when modulating the optical signal, The variation in wavelength (churping) that occurs limits the transmission distance or transmission speed, and is one of the obstacles to achieving longer distances and higher speeds. Therefore, in order to perform long-distance, high-speed transmission, a semiconductor laser with stable oscillation frequency and less chirping during modulation has been desired.

Conventionally, semiconductor lasers have been described in the IEEE Journal of Lightwave Technology (IEEE Journal of Lightwave Technology) published in 1983.
Journal of Lightwave Technology) No. LT-
The i.e.
There is a semiconductor laser published in a paper by I. Mito et al. As shown in FIG. layer 41
Electrodes 416 and 417 are placed on the active layer 415 sandwiched between
In some cases, a current is injected into the active layer 415, and light is emitted and amplified in the active layer 415 to obtain laser light. However, in such a semiconductor laser, when the number of carrier electrons injected into the active layer 415 is varied in order to modulate the intensity of laser oscillation light, dispersion of refractive index related to free electron plasma effect and interband transition of electrons occurs. There was a drawback that the optical resonator length varied due to the change in the curve, and as a result, the oscillation frequency varied.

(Objective of the Invention) The object of the present invention is to eliminate the above-mentioned drawbacks and to stabilize the wavelength of a semiconductor laser in which the oscillation frequency does not change even if the number of carrier electrons injected into the active layer is varied in order to modulate the intensity of the laser oscillation light. The goal is to provide equipment.

(Structure of the Invention) A semiconductor laser wavelength stabilizing device of the present invention includes an optical waveguide layer that includes an active layer and is divided into a first optical waveguide section and a second optical waveguide section, and the first optical waveguide section. a first electrode for injecting current into the first electrode;
a second optical waveguide section that is connected to the optical waveguide section and whose refractive index can be controlled by an injected current, and a second electrode that injects a control current that controls the refractive index of the second optical waveguide section. and the semiconductor laser generated by applying a modulation signal current to the first electrode and simultaneously injecting the modulation signal current into the active layer for modulating the optical output intensity from the semiconductor laser device. The structure includes an electric circuit that injects a control current into the second electrode through a peaking circuit to cancel out fluctuations in the oscillation wavelength of the element.

(Operation/Principle of the Present Invention) With this configuration, fluctuations in oscillation frequency due to direct intensity modulation that occur in conventional semiconductor lasers can be suppressed to a small level. That is, by varying the number of carrier electrons injected into the active layer through the first electrode in order to modulate the intensity of the laser oscillation light, the refractive index related to the free electron plasma effect and the interband transition of electrons can be improved. By compensating for variations in the optical cavity length caused by changes in the dispersion curve by increasing or decreasing the number of carrier electrons injected from the second electrode, the optical cavity length can be kept constant throughout the cavity. As a result, chirping of the oscillation frequency is suppressed.

In addition, for intensity modulation of the laser oscillation light, the first
When injecting a control current into the second electrode to compensate for the refractive index change caused in the first optical waveguide section by the modulation current injected into the second electrode and keep the optical resonator length constant, Since the DC level of the control current is below the oscillation threshold level, the cutoff frequency of the frequency response of the carrier fluctuation due to the control current applied to the second electrode is equal to the frequency of the carrier fluctuation with respect to the modulation current applied to the first electrode. Although it is smaller than the response cutoff frequency, in this configuration the second
Since the electrical circuit for applying the control current to the second electrode is provided with a peaking circuit, it is possible to compensate up to a frequency higher than the cut-off frequency on the second electrode side.

(Example) Hereinafter, the present invention will be described in detail with reference to the drawings.

FIG. 1 is a block diagram of a first embodiment according to the present invention, and FIG. 2 is a perspective view including a cross section of a semiconductor laser device used in the first embodiment according to the present invention.

Moreover, FIG. 3 is an explanatory diagram for explaining one embodiment according to the present invention.

Now, in FIG. 1, 101 is a modulation signal power supply, 102 is a branch circuit, 103 is a polarity inversion circuit,
104 is a peaking circuit, 105 is a first DC power supply, 106 is a second DC power supply, 107 is a first mixing circuit, 108 is a second mixing circuit, 109 is a semiconductor laser element used in this example, 110 is a common terminal connected to the common electrode 111, 112 is a first terminal connected to the first electrode 113, 114 is a second terminal connected to the second electrode 115, and 116 is an active layer of the semiconductor laser element 109. be.

A modulated signal current generated by a modulated signal power supply 101 is branched into two currents at a ratio of 10:1 in a branch circuit 102. That is, the bit rate generated by the modulation signal power supply 101 is 1 Gb/s, and the pulse current value is 33 mA.
In the branch circuit 102, a pulse current of 30 mA is branched to the first mixing circuit 107, and a pulse current of 3 mA is branched to the second mixing circuit 108 via the polarity inversion circuit 103 and the peaking circuit 104. It branches into a pulse current. 1st
The 30 mA pulse modulated current branched to the mixing circuit 107 is mixed with the 23 mA DC bias current supplied from the first DC power supply 105 in the first mixing circuit 107, and then connected to the semiconductor through the first terminal 112. It is injected into the laser element 109. Here, the DC bias current of 23 mA has a value slightly below the oscillation threshold of the semiconductor laser element 109 of 25 mA. On the other hand, the 3 mA pulse current flowing toward the second mixing circuit 108 is reversed in polarity by the polarity reversing circuit 103, and after the high-frequency current is emphasized by the peaking circuit 104, it is supplied from the second DC power supply 106. After being mixed with the 3 mA DC bias current in the second mixing circuit 108, the second terminal 114
As a control current through the semiconductor laser element 109
injected into.

Next, the semiconductor laser device used in this example will be explained using FIG. 2.

In FIG. 2, 200 is an n-type InP substrate, 2
01 is a non-doped InGaAsP active layer, 202
203 is a p-type InGaAsP guide layer, 203 is a p-type InP cladding layer 204, which is formed at the interface between the substrate 200 and the active layer 201, and is a diffraction grating for oscillation frequency selection. 205 is a non-doped InGaAsP first block. 206 is a second block layer of p-type InGaAsSP, 207 is a third block layer of p-type InP, 208 is a fourth block layer of p-type InP, 20
9 is the fifth block layer of n-type InP, 210 is the p-type
11 in Fig. 1 consists of an InP buried layer, 211 a p-type InGaAsP cap layer, and 212 an Au-Ge layer.
1, 213 is the first electrode made of Au-Zu and shown in 113 in FIG. 1, 214 is the common electrode shown in FIG.
The second electrode shown at 115 in FIG. 1 is made of Au-Zn, 215 is a common terminal connected to the common electrode, 216 is a first terminal connected to the first electrode 213, and 217 is the second electrode. The active layer 201 and the guide layer 202 form an optical waveguide layer. Here, the active layer 201 has a thickness of about 0.1 μm and a composition band gap of 1.3 μm in wavelength, and the guide layer 202 has a thickness of about 0.2 μm and a composition band gap of 1.3 μm in wavelength.
It is 1.2 μm. The stripe width of the active layer 201 and the guide layer 202 is about 1.5 μm, the stripe length is about 300 μm, and the lengths of the first electrode 213 and the second electrode 214 in the active layer stripe direction are about 240 μm and about 50 μm, respectively. , the gap between both electrodes is approximately
It is 10μm. Further, the length of the portion where the diffraction grating 204 is formed is about 240 μm from the first cleavage plane 218 along the active layer stripe direction,
This approximately corresponds to the current injection region of the first electrode.
The period of this diffraction grating 204 is about 3900 Å, which corresponds to the band gap of the composition of the active layer 201.

In the above configuration, when a current is applied to the active layer 201 through the first electrode 213 and the common electrode 212, the resonator formed by the diffraction grating 204 and the second cleavage plane 219 causes laser oscillation.
At this time, in order to modulate the intensity of the laser oscillation light,
When a modulation signal is applied to the first electrode 213, the density of carrier electrons in the region directly under the first electrode 213 where current is injected in the active layer 201 changes, resulting in a free electron plasma effect and an electron band. A change in the refractive index occurs due to a change in the dispersion curve of the refractive index related to the transition between the two regions, and the optical length of the region directly under the first electrode 213 in the active layer 201 changes. However, since a control current is injected into the second electrode 214 that causes a change in the refractive index to compensate for the change in the optical resonator length caused by the modulation signal, the resonator length is kept constant as a whole. drooping The fact that the optical resonator length is kept constant as a whole even when a modulation current is applied will be explained in more detail with reference to FIG.

A DC bias current of 23 mA was applied to the first electrode 213 and a bit rate of 1 Gb/s as shown in Fig. 3a.
When a pulse modulation current Δi ₁ with a pulse current value of 33 mA is applied, the carrier density in the first optical waveguide section becomes slightly rounded at the rise and fall of the relative change amount ΔN ₁ , as shown in Figure 3b. It changes as it arises.

Almost following the relative change amount ΔN ₁ in the carrier density, the refractive index of the first optical waveguide portion changes by the relative change amount ΔN ₁ as shown in FIG. 3o. The amount of change Δn ₁ is about 10 ^-6 . Therefore, as a control current, a control current Δi ₂ of about 3 mA including peaking at the rise and fall as shown by the solid line in FIG. 3d is applied to the second electrode 215 together with a DC bias current of about 3 mA. In this case, the polarity of the control current Δi ₂ is opposite to that of the modulation signal current shown in FIG. 3a, and the DC bias current density applied to the second electrode 215 is set below the oscillation threshold density. At this time, the carrier density in the second optical waveguide section changes with almost the same rise and fall times as in FIG. 3b, as shown by the solid line in FIG. 3e, and
It changes in such a way that the increase and decrease is opposite to that of . Therefore, the refractive index Δn ₂ of the second optical waveguide section increases or decreases in the opposite direction to the change in the refractive index of the first optical waveguide section shown in FIG. 3o, as shown by the solid line in FIG. 3f. and change with almost the same rise and fall times. Therefore, the change in the optical resonator length due to the refractive index change caused by the modulation signal current in the first optical waveguide section is canceled by the refractive index change caused by the control current in the second optical waveguide section, The resonator length is kept constant overall.

Here, the changes shown by broken lines in FIGS. 3 d to 3 f are the case where the peaking circuit 104 shown in FIG. The rise of the refractive index change at
Because of the difference in fall time, a sufficient compensation effect cannot be obtained.

The peaking circuit is a well-known parallel circuit of a resistor and a capacitor, and outputs spikes at points where a step-like change in a step-like pulse input signal occurs. Its circuit and characteristics are for example
The Bell System Technical Journal, published in 1975.
Technical Journal) Volume 54, No. 1, Page 53~
T.P. Lee listed on page 68
Familiar with the paper by LEE).

The present invention has been explained above with reference to the drawings. In this embodiment, the modulation signal current and the control current are obtained by branching the current from the same modulation signal power supply using a branch circuit, but the power supply may be separate. good. however,
In that case, it is necessary to synchronize the signal generation at both power supplies. Furthermore, although the length of the first electrode of the semiconductor laser device used in this example was approximately 240 μm, and the length of the second electrode was approximately 50 μm, the lengths are not limited to these. Also, its total length
It is not limited to 300 μm. Further, in this embodiment, a diffraction grating was provided in the semiconductor laser element to ensure single-axis mode oscillation, but the diffraction grating may be omitted if single-axis mode oscillation can be maintained.

Here, the features of this embodiment are that since both the first electrode and the second electrode are biased in the forward direction, it is relatively easy to maintain isolation between the two electrodes; The semiconductor laser used in the example is easy to manufacture because it can be manufactured using the same manufacturing process as the commonly used distributed feedback type or plug reflection type semiconductor laser except for the electrode structure. This makes it easy to obtain single-axis mode oscillation.

(Effects of the Invention) Finally, the effects of the present invention include that the laser oscillation frequency is extremely stable and that the types of optical systems that can utilize semiconductor lasers are further expanded.

[Brief explanation of drawings]

FIG. 1 is a configuration diagram of an embodiment according to the present invention, and FIG.
The figure is a perspective view including a cross section of a semiconductor laser element used in an embodiment of the present invention, FIG. 3 is an explanatory diagram for explaining an embodiment of the present invention, and FIG. 4 is a diagram illustrating a conventional semiconductor laser. FIG. In FIG. 1, 101...modulation signal power supply, 1
02... Branch circuit, 103... Polarity inversion circuit, 1
04... Peaking circuit, 105... First DC power supply, 106... Second DC power supply, 107... First mixing circuit, 108... Second mixing circuit, 10
9... Semiconductor laser element used in this example, 11
0... Common terminal, 111... Common electrode, 112...
...first terminal, 113...first electrode, 114...
...Second terminal, 115...Second electrode, 116...
. . . is an active layer of a semiconductor laser device, and in FIG. 2, 200 . . . substrate, 201 .
2... Guide layer, 203... Clad layer, 204
... Diffraction grating, 205 ... First block layer, 2
06...Second block layer, 207...Third block layer, 208...Fourth block layer, 209
...Fifth block layer, 210...Embedding layer,
211...Cap layer, 212...Common electrode, 2
13...first electrode, 214...second electrode, 2
15... Common terminal, 216... First terminal, 21
7...Second terminal, 218...First cleavage plane, 2
19...second cleavage plane, 220,221...groove,
In Fig. 4, 411...cleavage plane, 412...
Cleavage plane, 413...p-type semiconductor layer, 414...n
type semiconductor layer, 415... active layer, 416, 417
...It's an electrode. Further, in FIG. 2, the shaded portion is a cross-sectional portion.

Claims (1)

[Claims] 1 a first optical waveguide section including an active layer; a first electrode for injecting a current into the first optical waveguide section; a second optical waveguide section that can control the
a semiconductor laser device comprising a second electrode for injecting a control current for controlling the refractive index of the second optical waveguide section; and a modulation signal current for modulating the optical output intensity from the semiconductor laser device. an electric circuit that simultaneously applies a control current to the second electrode that cancels fluctuations in the oscillation wavelength of the semiconductor laser device caused by injecting the modulation signal current into the active layer; A semiconductor laser wavelength stabilizing device, further comprising a peaking circuit in a portion of the electric circuit that applies a control current to the second electrode.