JPH08186334A - Semiconductor laser device and optical transmitter using it - Google Patents

Abstract

PURPOSE: To obtain a semiconductor laser device which changes over the polarization state of the oscillation light of a semiconductor laser by adjusting a quantity of returned light to the semiconductor laser and to obtain an optical transmitter using it. CONSTITUTION: A semiconductor laser device is provided with a semiconductor laser 1, with an optical modulator 2, with a mirror 3 and with a control means which drives the semiconductor laser 1 and by which a signal corresponding to a modulation signal to be input from the outside is given to the optical modulator 2. In addition, the semiconductor laser device may be provided with an optical coupling means by which the semiconductor laser 1 and the optical modulator 2 are coupled optically and with an optical coupling means by which the optical modulator 2 and the mirror 3 are coupled optically.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor laser device in which the polarization of output light is switched between two components (for example, TE and TM) under external control, and an optical transmitter using the same. is there.

[0002]

2. Description of the Related Art Conventionally, as a semiconductor laser capable of switching the polarization state of output light between TE and TM, for example, JP-A-62-42593 and JP-A-2-159 are known.
No. 781 is available. Regarding the operation in which the polarization of the output light of the semiconductor laser is switched between TE and TM, the explanation given in Japanese Patent Laid-Open No. 2-159781 is easy to understand, and therefore, it will be explained here.

FIG. 11 shows the polarization states of output light as TE and T.
A configuration of a semiconductor laser that can be switched between M is shown. The laser shown in the figure is a distributed feedback semiconductor laser having a 3λ / 4 shift portion, and has an electrode structure capable of separately injecting current into a portion having a λ / 4 shift and a portion other than that. In FIG. 11, 110 is n-In
P substrate, 112 primary diffraction grating, 114 optical waveguide layer, 116 undoped InGaAs active layer (bandgap wavelength 1.55 μm, thickness 0.1 μm), 118 p-GaInAsP antimeltback layer, 119 p
-InP clad layer, 120 p ⁺ -InGaAsP cap layer, 122 p-InP layer, 123 n-InP
Layer, 124 is an undoped GaInAsP cap layer, 1
Reference numerals 24 and 128 are electrodes, 127 is an electrode for injecting a current into a region including a λ / 4 shift portion, and 129 is an antireflection film.

The operation of the distributed feedback semiconductor laser shown in FIG. 11 will be described with reference to FIG. FIG. 12 (a),
(B) and (c) show αL (mode gain α and resonator length L
And the wavelength (deviation from the Bragg wavelength) for TE mode and TM mode. In FIG. 12 (a), a current is injected into the central part of the resonator by using the electrode 127,
3λ / 4 condition in TM mode, but 3λ / 4 in TE mode
The amount is adjusted so as to be away from the / 4 condition (each mode is indicated by a circle). In this state, the TM gain oscillates because the required gain for the TM becomes small.
From this state, the amount of current injected into the electrode 127 is changed,
When the shift amount is changed, the TM mode threshold value increases and the TE mode threshold value decreases. And FIG.
As shown in FIG. 12B, the threshold value in both modes is almost one value, and then the TE mode threshold value becomes smaller as shown in FIG. 12C, and TE light oscillates.

By changing the amount of current injected into the electrode 127 as described above, the polarization direction of the output light is changed to TE and TM.
You can switch between

[0006]

However, in the above-mentioned conventional example, the phase shift amount is adjusted to adjust the polarization of the oscillated light to the TE level.
In order to select between TM and TM, the relationship between the wavelength and αL for each polarization state (broken line in FIG. 12) must be within a range in which polarization can be switched by adjusting the amount of phase shift. Therefore, in manufacturing the device, it was necessary to accurately match the width and thickness of the waveguide with the Bragg wavelength of the diffraction grating. Moreover, when stabilizing the operation and actually using it, a complicated control circuit was required.

An object of the first invention of the present application is to provide a semiconductor laser device capable of switching the polarization of output light with a configuration capable of controlling the amount of feedback light for a distributed feedback or distributed reflection type semiconductor laser. Is to provide. More specifically, it is characterized by including a semiconductor laser, an optical modulator, a mirror, and control means for driving the semiconductor laser and giving a signal corresponding to a modulation signal inputted from the outside to the optical modulator. It is a semiconductor laser device. Furthermore, an optical coupling means for optically coupling the semiconductor laser and the optical modulator,
An optical coupling means for optically coupling the light modulator and the mirror may be provided.

A second object of the present invention is to provide a configuration in which the amount of feedback light can be controlled more easily. More specifically, the semiconductor laser device further comprises a light intensity adjusting means, and the light intensity adjusting means is provided between the semiconductor laser and the light modulator or between the light modulator and the mirror. Is.

The third object of the present invention is the first object.
In addition to the object of the invention, it is an object of the present invention to provide a configuration capable of setting the positional relationship between a distributed feedback type semiconductor laser and feedback means with high accuracy. More specifically, the semiconductor laser device further comprises a phase adjusting means, and the phase adjusting means is provided between the semiconductor laser and the optical modulator or between the optical modulator and the mirror. .

A fourth object of the present invention is to provide a configuration in which the amount of feedback light and the phase of feedback light can be adjusted easily and more accurately. More specifically, the semiconductor laser device further includes light intensity adjusting means and phase adjusting means.

The object of the fifth invention relating to the present application is the first
Therefore, it is to facilitate the handling by making the configuration of the fourth invention into an integrated device. More specifically, the semiconductor laser device is characterized in that at least a portion excluding a control circuit is monolithically formed and integrated on a single semiconductor substrate.

A sixth object of the present invention is an optical transmitter using the above semiconductor laser device.

[0013]

In order to achieve the above object, the first invention of the present application is to provide a distributed feedback type semiconductor laser and an external mirror for returning a part of its output light. It is characterized in that it is provided and the amount of feedback light is adjusted by an optical modulator. In the above configuration, the polarization state (direction) of the output light can be switched by changing the feedback state from the outside with the optical modulator, and the optical modulation with an extinction ratio smaller than that when directly intensity-modulating with the optical modulator. Can be used.

The second aspect of the present invention is to provide means for adjusting the amount of feedback light in order to effectively switch the polarization state by feedback light. With such a structure, the polarization state of the output light of the semiconductor laser of the distributed feedback type or the like can be switched more easily.

According to the third aspect of the present invention, the phase of the feedback light can be adjusted by using the phase adjusting means, and the effect of the feedback light can be enhanced.

According to a fourth aspect of the present invention, by adjusting the intensity and phase of the return light by using the intensity adjusting means and the phase adjusting means, the effect of the return light is greater than that of each means. It becomes possible to raise.

A fifth aspect of the present invention is to simplify the adjustment of the positional relationship of each handling means by using the above-mentioned configuration and means as an integrated device.

A sixth invention according to the present application includes the above semiconductor laser device, a driving means for driving the semiconductor laser device, the semiconductor laser device and the driving means for converting an electric signal into an optical signal. The optical transmission device is composed of control means for controlling and polarization limiting means for transmitting either one of the two linearly polarized lights of the output light from the semiconductor laser device.

[0019]

[First Embodiment] FIG. 1 is a view best showing the features of the present invention. In FIG. 1, 1 is a distributed feedback semiconductor laser, 2 is an optical modulator, 3 is a reflecting mirror, and 4 is a distributed feedback semiconductor. It is the output light of the laser.

FIG. 4 is a three-dimensional view of FIG. The same members as those in FIG. 1 have the same reference numerals. That is, the distributed feedback semiconductor laser 1, the optical modulator 2, and the reflecting mirror 3. In the figure, 7 is a mount, 8 is an injection current (1) for driving the distributed feedback semiconductor laser 1, and 9 is a voltage for driving the optical modulator 2. The distributed feedback semiconductor laser 1, the optical modulator 2, and the reflecting mirror 3 are fixed on the mount 7 in a desired positional relationship.

A distributed feedback semiconductor laser 1 is shown in FIGS.
The structure of FIG. 6 is a sectional view taken along the line AA 'in FIG. In FIGS. 5 and 6, 21 is an n-type InP substrate, 22 is an n-type InP first cladding layer, and 23 is an n-type InGaAsP (bandgap wavelength 1.15 μm, thickness 0.1 μm). And the optical guide layer 24 is, for example, non-doped InGaAs
P (band gap wavelength 1.55 μm, thickness 0.1 μ
m) is an active layer, 25 is a second cladding layer made of p-type InP, 26 is a cap layer made of p-type InGaAs, 27 and 28 are first and second electrodes, and 29 is made of p-type InP. A first buried layer, 30 is a second buried layer made of, for example, n-type InP, 31 is a grating (having a period of about 240 nm, formed on the first cladding layer 22).
The depth is about 30 nm). The width of the active layer 24 was set to about 1 μm, and the cavity length was set to 500 μm. Output light having an oscillation wavelength of 1.55 μm was obtained with the distributed feedback semiconductor laser having this configuration.

FIG. 7 shows a configuration example of the optical modulator 2. Figure 7
In the figure, 40 is a substrate made of n-type InP, 41
Is n-type InGaAsP (bandgap wavelength 1.
A modulation layer made of 45 μm and a thickness of 0.3 μm, and 42 is, for example, n-type InGaAsP (bandgap wavelength 1.2 μm).
m, the thickness is 0.1 μm), 43 is a cladding layer (1) made of, for example, undoped InP (thickness 0.4 μm), 44 is a cladding layer (2) made of, for example, p-type InP, and 45 is , 46 is a contact layer made of p-type InGaAsP, 46 is a buried layer of high resistance made of Fe-doped InP, and 47 and 48 are electrodes. The width of the modulation layer 41 is about 5 μm. The optical modulator of this configuration is E
electronics Letters, Vo1.2
5, No. 2, pp. 88-89 (1989) and the like. By applying a voltage of -5V with the above configuration, it is possible to absorb about 20 dB of light.

Next, the reflector 3 will be described. Reflector 3
Any material may be used as long as it can reflect incident light to some extent, but here a cleavage plane obtained when the semiconductor crystal (substrate) is cleaved is used. Although not shown in FIG. 4, a metal film is formed on the surface of the reflector 3 to be fixed to the mount 7 similarly to the electrodes of the distributed feedback semiconductor laser 1 and the optical modulator 2 so that the metal film can be easily fixed to the mount 7. ing.

Next, the positional relationship between the distributed feedback semiconductor laser 1, the optical modulator 2 and the reflector 3 in this embodiment will be described. The output light of the distributed feedback semiconductor laser 1 is input to the optical modulator 2, the output light from the optical modulator 2 is partially or wholly reflected by the reflector 3, and is input to the optical modulator 2 again, and further distributed. Light is returned to the feedback type semiconductor laser 1.
The arrangement shown in FIG. 4 is the simplest configuration example. In order to enhance the optical coupling efficiency, an optical component such as a lens is used between the respective elements (between the distributed feedback semiconductor laser 1 and the optical modulator 2, or between the optical modulator 2 and the reflector 3). Good.

Next, the operation of this embodiment will be described. The distributed feedback semiconductor laser 1 is kept in a state where, for example, the TE polarized light and the TM polarized light compete with each other and the TE polarized light oscillates. Further, the distributed feedback semiconductor laser 1 is affected by the reflected light from the optical modulator 2 and the reflector 3 (the optical modulator 2 is in a transmissive state because no voltage is applied), and TM polarized light oscillates. To be In this state, a voltage is applied to the optical modulator 2 to reduce the amount of light reflected from the reflector 3. Then, until then, the TM-polarized oscillation light was output due to the effect of the reflected light, but the TE-polarized oscillation light is output because the reflected light amount is reduced. In this way, by changing the transmittance of the optical modulator 2, the polarization state of the output light of the distributed feedback semiconductor laser 1 can be switched between the TE polarization and the TM polarization.

It is also possible to perform an operation having a polarization state opposite to that of the above operation. That is, the distributed feedback semiconductor laser 1
However, by itself, it outputs TM polarized oscillation light,
The TE-polarized oscillation light is output in a state where the reflected light from the optical modulator 2 and the reflector 3 is present (however, the optical modulator 2 is in the transmissive state). Then, when a voltage is applied to the optical modulator 2 to reduce the transmittance, the optical modulator 2 is in an operating state in which it oscillates in TM polarization. .

[0027]

[Second Embodiment] FIG. 2 is a diagram showing a second embodiment of the present invention. In the figure, the same members as those in FIG. 1 are denoted by the same reference numerals. That is, the distributed feedback semiconductor laser 1, the optical modulator 2, the reflector 3, and the output light 4. Newly added is a light intensity adjusting means indicated by 5. Examples of the light intensity adjusting means 5 include an optical amplifier and an optical attenuator. In this embodiment, a semiconductor optical amplifier is used as the light intensity adjusting means 5. The semiconductor optical amplifier is devised so that laser oscillation does not occur even in a high current injection state by forming antireflection films on both end faces of the Fabry-Perot type semiconductor laser, and stimulated emission due to population inversion formed inside the active layer by current injection. It is used to amplify incident light.

Since the structure and operation of this embodiment are similar to those of the first embodiment, they are omitted here and the light intensity adjusting means 5 is omitted.
The effect of is described.

In the distributed feedback semiconductor laser 1, the polarization of the oscillation light is TE or TM depending on the amount of returned light. The optical modulator 2 has a considerable insertion loss, and when the coupling of the optical devices as shown in the first embodiment is rough, the amount of returned light is the output light 4 of the distributed feedback semiconductor laser 1.
There is a case where it does not become strong enough to switch the polarization state of.
Therefore, by introducing the light intensity adjusting means 5 as in this configuration (in this case, an optical amplifier), the amount of light is increased to such an extent that the polarization is switched, so that the configuration operates more stably than in the first embodiment. Becomes As described in the first embodiment, in the case where a component such as a lens is used in order to increase the optical coupling efficiency between the respective elements, the optical amplifier shown in this embodiment (second embodiment) is used. By using, it is possible to obtain more stable operation than when no lens is used. Light intensity adjusting means 5
The optical modulator 2 and the optical modulator 2 may be reversed in addition to the positional relationship shown in FIG.

[0030]

[Third Embodiment] FIG. 3 is a diagram showing a third embodiment of the present invention. In the figure, the same members as those in FIG. 1 are denoted by the same reference numerals. That is, the distributed feedback semiconductor laser 1, the optical modulator 2, the reflecting mirror 3, and the output light 4. The newly added member is the phase adjusting means indicated by 6. Phase adjusting means 6
Is for adjusting the phase of the light reflected by the reflector 3 when returning to the distributed feedback semiconductor laser 1, and its refractive index is changed by some external means. Here, a structure used for a distributed Bragg reflector semiconductor laser or the like is used as a semiconductor-based one.

Since the operation of this embodiment is similar to that of the first embodiment, it is omitted here and the effect of the phase adjusting means 6 will be described. The influence of the return light of the distributed feedback semiconductor laser 1 changes with the amount of light and the phase of the light of the return light. The phase adjusting means 6 shown in this embodiment has the effect of adjusting the phase of this light. In the first and second embodiments, it is necessary to adjust the phase of the return light by adjusting the positional relationship of each element, and to devise the operating points of the optical modulator 2 and the light intensity adjusting means 5 to make the adjustment. It was In the case of the configuration of the present embodiment, since the phase adjusting means 6 adjusts the phase, it is possible to use the best operating point as the operation of each element with respect to other elements.

[0032]

[Fourth Embodiment] FIG. 8 is a view showing a fourth embodiment of the present invention. In the figure, the same members as those in FIGS. 2 and 3 are given the same numbers. That is, the distributed feedback semiconductor laser 1,
Optical modulator 2, reflecting mirror 3, output light 4, light intensity adjusting means 5,
It is the phase adjusting means 6.

Since the operation of this embodiment is a combination of the second and third embodiments, its explanation is omitted here. By configuring as in this embodiment, it is possible to adjust both the intensity and the phase of the return light generated by the reflecting mirror 3, and the range of stable operation is expanded as compared with the first to third embodiments. effective.

[0034]

[Fifth Embodiment] FIG. 9 shows an integrated semiconductor device having the structure shown in the first embodiment. In the figure, 1
Reference numeral 0 denotes a semiconductor laser portion, which corresponds to the distributed feedback semiconductor laser 1 in FIG. 1, and 11 denotes a modulator portion, which corresponds to the optical modulator 2. Reference numeral 12 is a reflective film, which is made of, for example, SiO ₂ and gold film. A waveguide is connected between the semiconductor laser portion 10 and the modulator portion 11, but a groove 15 is formed in the upper clad layer for electrical isolation. Although not shown in FIG. 9, when an antireflection film is formed on the end face of the semiconductor laser portion 10, the output that can be taken out increases, the influence of the end face reflection is reduced, and stable operation is achieved. Since the operation of the device shown here is the same as that of the first embodiment, it is omitted here.

[0035]

[Sixth Embodiment] FIG. 10 shows a sixth embodiment of the present invention.
In FIG. 10, 50 is an optical transmitter, 51 is an element of the present invention (shown in the first to fifth embodiments), 52 is a control and drive circuit, 53 is a polarization limiting means and an optical coupling means, 5
4 is an optical fiber, 55 is an electric signal, 56 is an optical signal, 57
Is an optical output, and 58 is a control signal.

The operation of this embodiment will be described.
Electrical signal 5 representing a digital signal of "0" and "1"
5 is input to the optical transmitter 50, the control and drive circuit 5
2 drives the element 51 of the invention. That is, a control signal corresponding to a signal is input to the optical modulators 2 and 11, and an optical signal 57 whose polarization state is switched between TE and TM according to the signal is output. Polarization limiting means (eg, a polarizing plate, a polarization beam splitter) and an optical coupling means 53 are used for TE (or T
Only M) light is coupled into the optical fiber 54. As described above, the optical ASK signal corresponding to the electric signal can be transmitted.

As described above, since the optical ASK signal is output from the optical transmitter 50 of the present embodiment, the present embodiment is an optical transmitter of an optical communication system (including an optical LAN, etc.) that uses the optical ASK signal. It can be used as an optical transmission part in an optical transceiver.

[0038]

As described above, according to the first invention of the present application, by controlling the amount of feedback light to the semiconductor laser of the distributed feedback type or the like from the outside, the output light that is easier to handle than before. Semiconductor laser that can switch the polarization state of
The device can be provided.

According to the second invention of the present application, the first
It is possible to adjust the amount of feedback light according to the invention, and it is possible to more easily select an operating point suitable for each element.

According to the third invention of the present application, the phase of the amount of feedback light can be easily adjusted.

According to the fourth invention of the present application, a more stable operating point can be easily selected by simultaneously using the second and third inventions.

According to the fifth invention of the present application, the integration facilitates the setting of the positional relationship of each means and simplifies the handling.

[Brief description of drawings]

FIG. 1 is a diagram showing a configuration according to a first exemplary embodiment of the present invention.

FIG. 2 is a diagram showing a configuration according to a second exemplary embodiment of the present invention.

FIG. 3 is a diagram showing a configuration according to a third exemplary embodiment of the present invention.

FIG. 4 is a perspective view of the configuration of FIG.

FIG. 5 is a diagram showing a configuration of a distributed feedback semiconductor laser used in an example of the present invention.

6 is a cross-sectional configuration diagram taken along the line AA ′ in FIG.

FIG. 7 is a diagram showing a configuration of an optical modulator used in an example of the present invention.

FIG. 8 is a diagram showing a configuration according to a fourth exemplary embodiment of the present invention.

FIG. 9 is a perspective view of a device having an integrated device according to the first embodiment of the present invention.

FIG. 10 is a configuration diagram when the semiconductor laser device of the present invention is used in an optical transmitter.

FIG. 11 is a diagram showing a configuration of a conventional distributed feedback semiconductor laser capable of switching the polarization state of output light.

FIG. 12 is a diagram for explaining the operation of the element shown in FIG.

[Explanation of symbols]

 1 Semiconductor Laser 2 Optical Modulator 3 Reflector 4 Output Light 5 Light Intensity Adjusting Means 6 Phase Adjusting Means 7 Mount 8 Injection Current 9 Voltage 10 Semiconductor Laser Part 11 Optical Modulator Part 12 Reflective Film 21, 40 Substrate 22, 25, 43 , 44 Clad layer 23 Guide layer 24 Active layer 26 Cap layer 27, 28, 47, 48 Electrode 29, 30, 46 Buried layer 31 Grating 41 Modulation layer 42 Buffer layer 45 Contact layer 50 Optical transmitter 51 Semiconductor laser device 52 Control and Drive circuit 53 Polarization limiting means and optical coupling system 54 Optical fiber 55 Electric signal 56 Optical signal 58 Control signal

Claims (8)

[Claims] 1. A semiconductor laser, an optical modulator, a mirror, and a control means for driving the semiconductor laser and providing a signal corresponding to a modulation signal input from the outside to the optical modulator. Semiconductor laser device. 2. An optical coupling means for optically coupling the semiconductor laser and the optical modulator, and an optical coupling means for optically coupling the optical modulator and the mirror. The semiconductor laser device according to claim 1. 3. The semiconductor laser device according to claim 1, wherein the semiconductor laser is a distributed feedback semiconductor laser or a distributed reflection semiconductor laser. 4. The light intensity adjusting means is further provided, and the light intensity adjusting means is provided between the semiconductor laser and the light modulator or between the light modulator and the mirror. 1. The semiconductor laser device according to 1. 5. The semiconductor laser device according to claim 4, further comprising a phase adjusting unit. 6. The method according to claim 1, further comprising a phase adjusting means, wherein the phase adjusting means is provided between the semiconductor laser and the optical modulator or between the optical modulator and the mirror. Semiconductor laser device. 7. The semiconductor laser device according to claim 1, wherein at least a portion excluding a control circuit is monolithically formed and integrated on one semiconductor substrate. 8. A semiconductor laser device according to claim 1, drive means for driving the semiconductor laser device, and control means for controlling the semiconductor laser device and the drive means for converting an electric signal into an optical signal. An optical transmitter comprising a polarization limiting means for transmitting either one of two linearly polarized lights of the output light from the semiconductor laser device.