JPH0614573B2 - Semiconductor laser - Google Patents

Description

The present invention relates to a buried hetero single longitudinal mode semiconductor laser,
Particularly, it relates to a semiconductor laser suitable for high-speed digital optical transmission.

“Prior Art” Originally, a semiconductor laser (LD) has a feature that high-speed modulation is easy, and research and practical use has been actively carried out as a light source for optical communication. The modulation for the semiconductor laser is generally a direct modulation by injection current modulation, and when the semiconductor laser is modulated by this, the intensity of the output light changes according to the modulation signal. In particular, even in the case of pulse modulation in which the linearity of the drive current-optical output (IL) characteristic of the semiconductor laser and the S / N ratio are loose, the following conditions are required. The extinction ratio is good, the amplitude and pulse width are small, the intersymbol interference is small, and the dynamic spectrum spread is small.

In the high-speed pulse transmission of 1 Gb / s or more, the dynamic spectrum broadening has been noted as a factor limiting the relay interval. In the 1.5 μm band, which is the minimum loss wavelength region of silica-based optical fibers, the dynamic spectrum spread is large, and the optical fiber dispersion value, which is one of the causes of the band limitation, is not negligible (10 ps / km / nm). This is because.
Even when a single longitudinal mode laser, which has been recently developed to reduce the dynamic spectrum spread, is used, the wavelength spread of 1.5 μm needs to be further compressed. Since there is a trade-off (incompatible) relationship between the extinction ratio and the dynamic spectrum spread, in order to suppress the spectrum spread in the current semiconductor laser structure, the deterioration of the extinction ratio must be accepted. There was a drawback that it did not happen.

Further, in the conventional direct pulse modulation, the injection current is modulated so that the extinction state corresponds to "0", so that the carrier density in the active layer relaxes with the carrier life time as a time constant during extinction. Since the carrier life time is about 1 ns or more, the carrier density at the rise of pulsed direct current depends on "0" and "1" of the code sequence before that, and the so-called pattern effect (intersymbol interference) cannot be ignored. There was a flaw.

"Means for Solving the Problems" According to the present invention, a belt-shaped active layer is formed on a substrate, and the width of the belt-shaped active layer is set to a diffusion length of a few carriers in the active layer or less, and First and second cladding layers, which are separated from each other by an insulating layer or a high resistance layer, are formed on both sides of the surface of the active layer opposite to the substrate, and the active layer is separately provided through the first and second cladding layers. It is designed so that current can be injected into it.

In this way, the temporal carrier density fluctuation, which is inevitable when the conventional injection current is directly pulse-modulated, is suppressed, so that the total injection current remains constant and the current path (cladding layer) injected into the active layer is kept constant. Two are provided, and the laser emission point can be moved according to the modulation signal by alternately passing a current through them, and this output light is transmitted through the movement of the laser emission point. By passing through a spatial filter with variable loss, it is possible to obtain a semiconductor laser capable of realizing optical pulse transmission having a good extinction ratio, a small pattern effect, and a narrow dynamic spectral width characteristic.

[Embodiment] FIG. 1 shows an embodiment of the present invention based on a buried heterojunction (BH) semiconductor laser, and is a sectional view perpendicular to the stripe. I _n G _{a A s} P active layer 2 is formed on a part of the P-I _n P substrate 1. Distributed feedback after forming the desired diffraction grating on the advance P-I _n P substrate 1 when taking the (DFB) semiconductor laser form in a direction perpendicular to the sheet of, on which is also grown a guide layer made of quaternary The active layer 2 may be grown.

In terms of cross-sectional shape, it is a DFB even in the Fabry-Perot type (FP type)
There is almost no difference in the (distributed feedback) type, so after that, FP
Only the (Fabry-Perot) type will be described, and the DFB type is not particularly mentioned.
Application to the mold is easy. The DBR (Distributed Bragg Reflection) type is a type of distributed feedback (DFB) type, in which a resonator is formed by diffraction gratings provided at both ends of the active layer 2 instead of the mirror surface by cleavage. Therefore, the cross-sectional shape in the lateral direction remains the same as that of the ordinary Fabry-Perot type.

The width of the active layer 2 is set to be equal to or less than the minority carrier diffusion length in the active layer 2. Planar stripe type with a width of 6 μm
The I _n P-based semiconductor lasers and is Me Make experimentally that lateral spatial hole burning does not occur, diffusion length may be considered more 6 [mu] m, the width of the sub connexion active layer 2 is 6 [mu] m
It is smaller than the value larger than the degree.

On the left and right sides of the active layer 2, nI _n P cladding layers 3 and 4 are formed. After this is formed on the entire surface of the I _n P layer on the active layer 2,
The I _n P layer is selectively etched to obtain the cladding layers 3, 4
It is formed by dividing into. Thus, the cladding layers 3 and 4 are separated from each other by an insulating layer of air in this example. The same effect even if the high-resistance layer or P-I _n P layer is sandwiched in place of the insulating layer between the Kuratsudo layers 3 and 4 is obtained. The active layer 2 and the cladding layers 3 and 4
Is, P-I _n P substrate which are sequentially formed on the 1 _n-I n P layer 5, P
It is formed so as to be embedded in -I _n P layer 6, the n-I _n P layer 7 buried layer 8 made of. P- _InP P on the bottom of the substrate 1
The side electrode 9 is formed. Respectively on Kuratsudo layers 3 and 4 is extended over n-I _n P layer 7 is n-side electrodes 11 and 12 are formed respectively, P-side terminals 13, respectively P-side electrode 9, an n-side electrode 11, n The side terminals 14 and 15 are connected. Incidentally, in the structure shown in FIG. 1, the semiconductor of the conventional buried heterojunction (BH) structure except that the cladding layers are divided into 3 and 4 and the n-side electrodes for supplying current to these are divided into 11 and 12. It has the same structure as the laser.

Consider a mode of current injection in which a constant current is temporally switched to the n-side terminals 14 and 15 by a current switching circuit. For example, electrons flowing from the terminal 14 through the electrode 11 are injected from the cladding layer 3 into the active layer 2 from the surface opposite to the substrate 1 as shown by an arrow 16. The injected electrons diffuse in the active layer 2, but the cladding layer 3 near the injection region (left in the figure)
Since the side has a slightly higher electron density, it is more likely to oscillate, and the oscillated light intensity distribution also has an asymmetrical shape having a maximum value on the side of the cladding layer 3 (left).

When the electric current is switched and the electric current flows from the cladding layer 4 to the active layer 2 through the terminal 15 and the electrode 12, as shown by the arrow 17, the emission maximum point is from the center to the cladding layer 4.
Move to the (right) side.

Therefore, by switching to one of the injection current terminals 14 and 15 depending on the signal input "1" or "0", for example, when the light emitting point is on the cladding layer 3 (left) side from the center, "1", Digital pulse modulation that makes "0" when on the cladding layer 4 (right) side is possible. By inserting a kind of optical space filter that changes the passage loss depending on the position of the light emitting point between the light source whose light emitting point is modulated and the transmission line optical fiber, A light intensity signal corresponding to the position can be sent into the fiber.

In a normal coupling system that couples the light of a semiconductor laser that oscillates in the fundamental transverse mode to a single-mode fiber, the coupling efficiency changes significantly when the position of the emission point changes. The point modulation can be converted into a pulse-modulated optical signal in the optical fiber. Therefore, the transmission line following the single mode fiber as the spatial filter may be either single mode or multimode.

As a driving circuit according to the digital signal for the semiconductor laser shown in FIG. 1, for example, as shown in FIG.
P-side terminal 1 of the semiconductor laser 31 of the embodiment shown in FIG.
3 is connected to the positive terminal 32 of the power source, and the n-side terminals 14 and 15 are connected.
Are connected to the collectors of the transistors 33 and 34, respectively, and the emitters of the transistors 33 and 34 are connected to each other and connected to the negative terminal 36 of the power source through the constant current source 35, and the base of one of the transistors 33 is digitally connected. A signal input terminal 37 is connected, a terminal 38 for supplying a reference voltage is connected to the base of the other transistor 34, and a current switching circuit is constituted by the transistors 33 and 34. When the input digital signal is "1", the terminal 38 The voltage of the input terminal 37 becomes higher than the reference voltage, the constant current of the constant current source 35 flows through the transistor 33, the center of the light emitting point moves to the left side of the semiconductor laser 31, and the input digital signal is a logic "0". When the voltage becomes lower than the reference voltage of the terminal 38, the constant current of the constant current source 35 flows through the transistor 34, and the emission point of the semiconductor laser 31. Center moves to the right side.

Further, FIG. 3 shows the side where the laser light thus emitted is coupled to the optical fiber by using an optical filter having a different passage loss depending on the light emitting position. When the center of the emission point of the laser light from the semiconductor laser 31 is on the left (clad layer 3) side, as shown by the solid line 41, the single mode optical fiber 4
An emission beam is made incident on the core 43 at the incident end face of No. 2 by the coupling lens system 44 and is transmitted into the optical fiber 42. However, when the center of the light emitting point is on the right (clad layer 4) side, the light emitting beam is incident on the cladding 46 of the optical fiber 42 by the coupling lens system 44 as shown by the dotted line 45, and is rapidly attenuated in the optical fiber 42. Not transmitted.

Although the above example is an embodiment in which an element for switching the current is provided outside the semiconductor laser, it is also possible to form this inside the semiconductor laser chip. Therefore, either an FET or a bipolar transistor can be used as the active element. FIG. 4 shows a case where an FET is incorporated, and the same reference numerals are given to the portions corresponding to those in FIG.
The left and right n-side electrodes 11 and 12 are short-circuited to make the source terminal 18
It is said that Which corresponds to the drain is n-I _n P Kuratsudo layer near right above the active layer 2. In the FIG. 2 for separation in the left and right Kuratsudo layers 3, 4 as described above, Aru selectively ion-implanted in the n-I _n P layer shows an example of forming a means pursuant high resistance region 19. Furthermore the buried P-I _n P layer 6 is set to a low concentration, to separate the P-I _n P layer 6 up and down by a ^{_{P + -I n G a A s}} P layer 21, the P ⁺ -I _n G _{a A} embedded P-I _n P on _s P layer 21
By removing a part of each of the cladding layers 3 and 4 of the layer 6 on the opposite side,
Put ^{_{P + -I n G a A s}} P layer 21 left electrodes 22 and 23 on. Terminals 24 and 25 are attached to the electrodes 22 and 23, respectively. Furthermore, by ion implantation, P ⁺ regions 2 as shown by the dotted lines on both sides of the mesa
6, 27 are formed on the left and right respectively. These areas 2
6,27 act as a gate.

In order to operate this composite element, when the terminals 13 and 18 are forward biased to positive and negative, respectively, a signal is input to the terminal 24, and a reference voltage is applied to the terminal 25, either one of the left and right cladding layers 3 and 4 is applied. The channels open and the other opens, allowing current to be injected into the active layer 2 from the open cladding layer (3 or 4) of the channel. As a result, the same effect as that of the embodiment shown in FIG. 1 can be obtained.

The current path switch can also be realized by integrating a bipolar transistor rather than a FET in the semiconductor laser. An example thereof is shown in FIG. Allowed to grow ^{_{P + -I n G a A s}} P base layers 28 and 29 divided into upper and lower each of these in the left and right Kuratsudo layers 3 and 4 in place of the gate in Figure 4, the base layer 28, 29 the connecting respectively the left and right of the ^{_{P + -I n G a a s}} P layer 21. Left and right terminals 24, 25
Alternately, a base current is flown alternately to turn on one transistor to form a current path, that is, a current can be injected into the active layer 2 from the cladding layers 3 or 4.

A configuration in which the P-n polarity is inverted in the above-described embodiment is also possible, but there are some differences in the manufacturing process. However, this problem can be solved at present when BH-LD can be manufactured on both P substrate and n substrate. Further, in the above description, the description of the buffer layer that is first grown on the substrate is omitted. This is because the composition of the buffer layer is basically the same as that of the substrate, and thus the buffer layer may be provided or omitted. Further, the thickness of the active layer 2 may be exactly the same as the structure of the current BH-LD, and therefore it is thinner than 1 μm and thicker than 0.1 μm. Therefore, the thickness direction of the active layer 2 is a value sufficiently smaller than the carrier diffusion length.

[Advantages of the Invention] In the semiconductor laser of the present invention, the width of the active layer 2 is equal to or less than the diffusion length of the minority carrier in the active layer.
The change in the carrier density in the active layer 2 in the width direction of the active layer is small, and if the total amount of injected carriers (injection current) is constant, the carrier density is almost constant in the active layer 2 regardless of the injection position. Is kept constant. Therefore, active layer 2
Since the refractive index of is also kept almost constant, the dynamic spectrum spread due to the movement of the light emitting point can be suppressed to a small value.

Further, the tailing of the pulse, which has been a problem in the conventional intensity modulation, can be significantly reduced, and the pattern effect can be reduced.
That is, since the space signal in the conventional intensity modulation corresponds to the extinction state by stopping the current injection, once the laser oscillation stops, the carrier density in the active layer is several n.
This is because the carrier life time on the order of s is relaxed as a time constant. However, in the semiconductor laser according to the configuration of the present invention, the light emitting spot can be moved without stopping the oscillation to obtain the modulation signal, so that the problem due to the carrier life time in the conventional intensity modulation can be eliminated.

Note that the extinction ratio of the optical pulse in the optical fiber when the center of the light emitting point is moved in this way will be roughly calculated. As shown in Fig. 6, when an ideal circular spot moves on the single-mode fiber entrance surface by twice its spot size (half width of e ^-2 value), the coupling efficiency to the fiber is about 35d.
B decreases. That is, an optical pulse with an extinction ratio of 35 dB is obtained. Since the light intensity decreases inversely exponentially with respect to the distance from the center thereof, the larger the center distance between the left and right light emitting spots 41 and 42, the better the extinction ratio can be obtained. Note 4 The P-I _n P buried layer 6 low concentration in view P ^- -I _n P
The reason is to localize the depletion layer at the time of reverse bias around the gate to enable high speed modulation.

[Brief description of drawings]

FIG. 1 is a sectional view showing an example of a semiconductor laser according to the present invention, FIG. 2 is a view showing an example of a semiconductor laser drive circuit, FIG. 3 is a view showing an example of coupling a semiconductor laser and an optical fiber,
FIG. 4 is a sectional view showing an example of a semiconductor laser of the present invention in which a driving n-channel FET is integrated, FIG. 5 is a sectional view showing an example of a semiconductor laser of the present invention in which a driving npn transistor is integrated, and FIG. It is explanatory drawing of spot movement. 1: P-I _n P _{substrate, 2: I n G a A} s P active layer, 3: _n-I n P left Kuratsudo layer, 4: _n-I n P right Kuratsudo layer, 5: n-I
_n P buried layer, 6: P-I _n P buried layer, 7: n-I _n P buried layer, 9: P-side electrode, 11: n-side left electrode, 12: n-side right electrode, 19: by ion implantation High resistance region formed, 21: P ⁺ −
I _n G _{a A s} P gate voltage supply buried layer, 22: left gate electrode, 23: right gate electrode, 26: P ⁺ -I _n P left gate,
27: P ⁺ -I _n P right _{^{gate, 28: P + -I n G}} a A s P left _{^{platform, 29: P + -I n G}} a A s P right base.

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Kiyoji Nakagawa 1, 2356 Take, Yokosuka City, Kanagawa Prefecture Yokosuka Electro-Communications Research Laboratories, Nippon Telegraph and Telephone Public Corporation (56) Reference JP-A-54-123886 (JP, A) Sho 59-84592 (JP, A)

Claims (1)

[Claims] 1. A semiconductor laser in which a strip-shaped active layer is formed on a substrate, and the width of the active layer is about the fractional carrier diffusion length in the active layer or less, and a surface of the strip-shaped active layer opposite to the substrate. First and second clad layers, which are separated from each other by an insulating layer or a high resistance layer, are formed on both upper sides, and a current is separately injected into the active layer through the first and second clad layers. Embedded heterojunction single longitudinal mode semiconductor laser.