JP6810671B2 - Semiconductor optical integrated device - Google Patents

Description

The present invention relates to a semiconductor optical integrated device in which an electric field absorption type (EA) light modulator is integrated on an InP substrate. More specifically, the present invention relates to a semiconductor optical integrated device consisting of a DFB laser, an EA modulator and a semiconductor optical amplifier (SOA).

Distributed feedback (DFB) lasers have excellent single wavelength properties, and are known to be monolithically integrated with an electric field absorption (EA) modulator on a single substrate. ing. This form of semiconductor optical integrated element (EA-DFB laser) is used as a light emitting device for long-distance transmission with a transmission distance of 40 km or more, and the signal light wavelength is mainly in the 1.55 μm band where the propagation loss of the optical fiber is small. Alternatively, a 1.3 μm band, which is not easily affected by the wavelength dispersion generated in the optical fiber, is used.

Then, in order to obtain an optical waveform capable of long-distance transmission with such an EA-DFB laser, the absolute value of the DC bias should be large. On the other hand, in order to obtain a high output, the absolute value of the DC bias applied to the EA modulator should be small. That is, the absolute value of the DC bias has a trade-off relationship. Therefore, Non-Patent Document 1 discloses that a semiconductor optical amplifier (SOA) is further integrated at the output end of the EA modulator in order to cancel the trade-off relationship. According to the disclosure of Non-Patent Document 1, by injecting a current into the SOA integrated at the output end of the EA modulator, the chirp value of the modulated light output from the EA modulator is converted and transmitted over a long distance. Is realized.

Further, Patent Document 1 discloses a semiconductor optical integrated device in which a DFB laser, an EA modulator, and an SOA are monolithically integrated on the same substrate in order to realize long-distance transmission.

Japanese Patent No. 5823920

Toshio Watanabe, 3 outsiders, “Chirp Control of an Optical Signal Using Phase Modulation in a Semiconductor Optical Amplifier”, Photonics Technology Letters, July 1998, vol.10, No.7, p.1027-1029. C. Sun et al., "Influence of residual facet reflection on the eye diagram performance of high-speed electroabsorption modulated lasers", Journal of lightwave technology, vol.27, no.15, pp.2970-2976, 2009.

However, in the case of a conventional semiconductor light integrated device, the modulated light reflected by the element end face at the output end may return to the DFB laser side and the laser light of the DFB laser may be disturbed.

In this regard, it has been reported that the EA-DFB laser has a problem that the wavelength of the modulated light is disturbed by the influence of the reflected light from the element end face at the output end (Non-Patent Document 2). Regarding this problem, Non-Patent Document 2 also reports that the wavelength of the modulated light becomes clear by improving the reflectance of the AR coating on the end face of the device.

Generally, in the case of an EA-DFB laser, the design target value of the reflectance of the element end face to which the AR coating is applied is 0.01% or less (-40 dB), so even if the reflectance of the AR coating is improved. There is a small amount of reflected light from the end face of the element.

Here, the SOA integrated EA-DFB laser (semiconductor optical integrated device) described above also has the same output end element end face as the EA-DFB laser, so that the reflected light from the element end surface cannot be completely eliminated. .. Moreover, if light is reflected by the element end face at the output end, it may be amplified by SOA and returned to the DFB laser. Assuming that the gain of SOA is 10 dB, the reflected light from the element end face at the output end is amplified by 10 dB and returned to the DFB laser. This can cause the laser beam of the DFB laser to be disturbed in the conventional semiconductor optical integrated device.

The present invention has been made under the above circumstances, and provides a semiconductor optical integrated device capable of obtaining good laser light in a configuration in which a DFB laser, an EA modulator, and an SOA are monolithically integrated. The purpose.

In order to achieve the above object, the semiconductor optical integrated element according to one embodiment of the present invention is mounted on the same substrate as the DFB laser, the EA modulator connected to the DFB laser, the DFB laser and the EA modulator. The SOA includes a monolithic integrated SOA connected to the emission end of the EA modulator, and the SOA is tilted along a first direction in plan view with respect to the optical axis direction of the emitted light of the DFB laser. having an optical waveguide formed Te, the optical waveguide has an inclined surface of the oblique shape in cross section, the inclination angle of the inclined surface gradually magnitude no longer toward the outgoing end face of the SOA, and when light propagating through the optical waveguide reaches the exit morphism end surface, and characterized in that it is set not return within the optical waveguide to the polarization of the light reflected by the outgoing end face in rotation To do.

The semiconductor optical integrated device of this, the length of the SOA, when the light propagating through the optical waveguide reaches the device end face of the output end, such that the state of polarization of the light is rotated 45 ° even as it is set not good.

In any of these semiconductor optical integrated device, each of the DFB laser and the SOA is not good even as the current from the same control terminal is configured to be implanted.

According to the present invention, a good laser beam can be obtained in a configuration in which a DFB laser, an EA modulator, and an SOA are monolithically integrated.

It is a figure for demonstrating the outline of control of the semiconductor optical integrated element which concerns on one Embodiment of this invention. It is a figure for _{demonstrating} the relationship between I _op , I _DFB, and I _SOA in the semiconductor optical integrated device of this embodiment. It is a top view which shows the structural example of the semiconductor optical integrated element of this embodiment. It is a figure for demonstrating the cross section from a DFB laser to SOA in the propagation direction of the light propagating through an optical waveguide in the semiconductor optical integrated device of this embodiment. It is a schematic diagram for demonstrating the schematic cross section of SOA in AB of FIG. It is a top view which shows the structural example of the semiconductor optical integrated element of the modification 1. It is a schematic diagram for demonstrating the schematic cross section of SOA in AB of FIG.

Hereinafter, a semiconductor optical integrated device (hereinafter, simply referred to as “optical integrated element”) 100, which is an embodiment of the present invention, will be described. The optical integrated element of this embodiment is an EA-DFB laser.

[Outline of control of optical integrated element 100]
FIG. 1 is a diagram for explaining an outline of control of the optical integrated element 100 according to the present embodiment.

As shown in FIG. 1, the optical integration element 100 includes a DFB laser 11, an EA modulator 12, and a SOA 13 in order with respect to the light propagation direction, and these components 11 to 13 are single. It is integrally monolithically laminated on the semiconductor substrate.

In FIG. 1, the DFB laser 11 and the SOA 13 are controlled by the current value I _op injected from the same control terminal 14. At this time, assuming that the injection current to the DFB laser 11 is I _DFB and the injection current to the _SOA 13 is I _SOA , the current value I _op is given by I _op = I _DFB + I _SOA .

Generally, the value of I _op allowed in an optical transmission module equipped with an EA-DFB laser is 60 to 80 mA. From this viewpoint, even in the optical integrated device 100 of the present embodiment, the upper limit of I _op is preferably set to, for example, 80 mA.

The relationship between I _op , I _DFB, and I _SOA described above is shown in detail in FIG. 2, which will be described later. FIG. 2 is a diagram for explaining such a relationship. In FIG. 2, a DFB laser 11 having a general length of 450 μm is used.

As shown in FIG. 2, for example, when the SOA length is 50 μm, the SOA length is 1/9 of the length of the DFB laser (450 μm), so that most of the current value I _op is injected into the DFB laser. Laser.

On the other hand, as shown in FIG. 2, when the SOA length is 150 μm, the SOA length is 1/3 of the length of the DFB laser. Therefore, when I _op = 80 mA, an I _{DFB of} about 60 mA becomes the DFB laser. It is injected, and about 20 mA of I _SOA is injected into the SOA.

By adjusting the lengths of the DFB lasers 11 and SOA 13 in this way, the currents I _{DFB and} I _SOA injected into them can be adjusted.

For example, when the length of the DFB laser 11 is 450 μm, the I _op for obtaining the threshold current and SMSR (Sub-Mode Suppression Ratio) by driving the DFB laser 11 needs to be at least 60 mA. Therefore, the SOA length in the optical waveguide direction is preferably 150 μm or less.

Further, for example, when the DBR laser 1 is set to 300 μm, the I _op for obtaining the required SMSR can be reduced to about 40 mA. Therefore, it is possible to lengthen the _SOA 13 to increase the current I _SOA to the _SOA 13.

In this way, by changing the length of the SOA 13 so that the minimum required current can be applied to the DFB laser 11 having a predetermined length according to the balance (ratio) of the lengths of the DFB laser 11 and the SOA 13. Both stable single-mode operation and amplification of optical output can be achieved.

[Structure of optical integrated element 100]
Next, the configuration of the above-mentioned optical integrated element 100 will be described with reference to FIGS. 3 and 4. FIG. 3 is a diagram showing a configuration example of the optical integration element 100. FIG. 4 is a diagram for explaining a cross section from the DFB laser 11 to the SOA 13 in the propagation direction of the light propagating in the optical waveguide 5 in the optical integrated element 100. The material exemplified in relation to the description of the configuration of the optical integration device 100 is an example and can be freely changed.

As shown in FIG. 3, the optical integration element 100 includes a DFB laser 11, an EA modulator 12 connected to the DFB laser, and an SOA 13 connected to the emission end of the EA modulator 12.

In the DFB laser 11 and the EA modulator 12, the optical waveguide 5 is formed along the direction of the optical axis z of the emitted light of the DFB laser 11. On the other hand, in SOA 13, the optical waveguide 5 is formed so as to propagate in the first direction inclined with respect to the above-mentioned direction of the optical axis z. In the example of FIG. 3, the first direction is a direction inclined by θ1 (for example, 30 °) in the y direction with respect to the optical axis z direction.

An AR (Anti-Reflection) film is formed on the exit end surface 13A of the SOA 13, that is, the exit end surface 13A of the optical integration element 100.

Here, in general, in an optical integrated element, even when an oblique optical waveguide is provided and an AR film is formed on the element end face at the output end, the modulated light is reflected by the AR film to generate stray light. The laser light of the DFB laser can be disturbed.

From this point of view, in the optical integrated element 100 of the present embodiment, the optical waveguide 5 has an oblique shape to be described later so that the laser light of the DFB laser 11 is not disturbed by the stray light reflected by the AR film of the element end surface 13A at the output end. An inclined surface 5A is provided. The shape of the inclined surface 5A of the optical waveguide 5 is shown in a schematic schematic diagram in FIG. 5, which will be described later.

In FIG. 4, the optical integration element 100 includes an n-type InP substrate 102, and the DFB laser 11, the EA modulator 12, the SOA 13, and the receiver 14 are placed on the substrate 102 in order with respect to the optical waveguide direction. And are formed. Further, an n-type electrode 101 is provided on the back surface of the substrate 102.

The DFB laser 11 has an active layer 104 and a guide layer 105 laminated on the n-InP clad layer 103. The guide layer 105 includes a λ / 4 phase shift 105A and a diffraction grating 105B. The active layer 104 is formed of an InGaAlAs-based or InGaAsP-based material.

A p-InP clad layer 106 is formed on the guide layer 105, and a p-type electrode 107 is provided on the clad layer 106. The current I _DFB shown in FIG. 1 is injected into the electrode 107.

The EA modulator 12 has an absorption layer 108 laminated on the clad layer 103, a clad layer 106, and a p-type electrode 109. A bias voltage V _bi and a high frequency voltage RF for driving the EA modulator 12 are applied to the electrode 109 via the bias T200. As a result, the EA modulator 12 can modulate the light from the DFB laser 11.

The absorption layer 108 is made of InGaAlAs-based or InGaAsP-based material and has a quantum well structure.

The SOA 13 has an active layer 131 laminated on the clad layer 103, a guide layer 132, a clad layer 106, and a p-type electrode 133. The active layer 131 has the same composition as the active layer 104 of the DFB laser 11, and the guide layer 132 has the same composition as the guide layer 105 of the DFB laser 11. In this embodiment, the current I _SOA shown in FIG. 1 is injected into the electrode 133 of the _SOA 13. In this embodiment, for example, the emission wavelength of the DFB laser 11 and the SOA 13 at 25 ° C. is about 1.55 μm.

5A and 5B are schematic views for explaining a schematic cross section of SOA13, in which FIG. 5A is a cross section AB of FIG. 3, FIG. 5B is a cross section CD of FIG. 3, and FIG. The EF cross section is shown.

The cross-section example shown in FIG. 5A corresponds to, for example, the cross-section AB of FIG. 3 on the incident surface side of SOA13. In this cross section, the optical waveguide 5 of the SOA 13 does not have an inclined surface 5A, so that the SOA 13 is configured as follows. That is, in SOA 13, the active layer 131 is formed between the clad layers 103 and 106 provided on the substrate 102, and the p-type electrode 133 is formed on the clad layer 106 via the contact layer 134. To.

Then, in the CD cross section which is the intermediate point of the SOA 13 in FIG. 3, as shown in FIG. 5 (b), the optical waveguide 5 of the SOA 13 has an inclined surface 5A having an inclination angle of θ2.

In the present embodiment, the inclination angle θ2 of the inclined surface 5A is set so as to gradually increase toward the exit end surface 13A. In other words, the length of the inclined surface 5A is set so as to gradually increase toward the exit end surface 13A.

Therefore, in the EF cross section on the exit end surface 13A side of the SOA 13 in FIG. 3, the inclination angle θ2 of the inclined surface 5A is larger than that shown in FIG. 5 (b) as shown in FIG. 5 (c). Is set to be.

By configuring such an inclined surface 5A in the SOA 13, when the light propagating in the optical waveguide 5 of the SOA 13 reaches the exit end surface 13A, the polarization of the modulated light h incident on the SOA 13 is rotated by, for example, 45 °. Becomes possible. Therefore, it is possible to rotate the polarization of the light h reflected by the emission end surface 13A by 90 °, and even if the reflected light h is generated, it does not return to the optical waveguide 5 of the SOA 13, and the laser light of the DFB laser 11 is emitted. Not disturbed.

Here, the length of the SOA 13 is set so that the polarization of the light h is rotated by, for example, 45 ° when the light h propagating through the optical waveguide 5 of the SOA 13 reaches the emission end face 13A.

The rotation angle of the polarization described above is not limited to 45 °, and can be changed as long as the reflected light d from the exit end surface 13A does not return to the optical waveguide 5 of the SOA 13.

As described above, in the optical integrated element 100 of the present embodiment, the optical waveguide 5 of the SOA 13 has an inclined surface 5A having an oblique shape in a cross-sectional view. Here, the inclination angle θ2 of the inclined surface 5A gradually increases toward the exit end surface 13A, which is the element end surface of the output end. For this reason, in the SOA 13, the polarization of the light h propagating in the optical waveguide 5 rotates, so that even if the light h is reflected by the exit end surface 13A of the SOA 13, the reflected light d is inside the optical waveguide 5 of the SOA 13. It does not return (does not return to the DFB laser 11 side), and the laser beam of the DFB laser 11 is not disturbed. Therefore, good laser light can be obtained.

Next, a modification of the optical integration element 100 of the present embodiment will be described.

(Change example 1)
In the above embodiment, the case where the width of the optical waveguide 5 of the SOA 13 is constant has been described, but in order to further suppress the light reflection on the inclined surface 5A, the width of the optical waveguide 5 of the SOA 13 gradually increases toward the exit end surface 13A. It may be widened to.

For example, FIG. 6 is a top view showing an optical integrated element 100A similar to FIG. 3, exemplifying a configuration in which the optical waveguide 5 of the SOA 13 is gradually widened toward the exit end surface 13A. In FIG. 6, unless otherwise specified, the reference numerals and the like used in the description of FIG. 3 are used as they are.

7A and 7B are schematic views for explaining a schematic cross section of SOA13 shown in FIG. 6, in which FIG. 7A is a cross section AB of FIG. 6, FIG. 7B is a cross section CD of FIG. ) Indicates the EF cross section of FIG.
As shown in FIGS. 7 (a) to 7 (c), in the optical integration element 100A of the first modification, the inclined surface 5A of the SOA 13 gradually becomes longer as in the case of the one shown in FIG. 3, but the absorption layer 131 The lengths a, b, and c of the lower surface are set so that a <b <c. Even in this case, the same effect as that of the above embodiment (because the polarization of the light h propagating in the optical waveguide 5 of the SOA 13 rotates, even if the light h is reflected by the exit end surface 13A of the SOA 13, the reflection thereof. The light d does not return to the inside of the optical waveguide 5 of the SOA 13, and the laser light of the DFB laser 11 is not disturbed), and the light reflection on the inclined surface 5A can be further suppressed.

(Change example 2)
In the above embodiment, the embodiment in which the optical integration element 100 is mounted on the optical transmission module is not mentioned, but such an optical transmission module may be configured.

(Change example 3)
In the above, the case where the current is injected into each of the DFB laser 11 and the SOA 13 from the same control terminal 14 has been described with reference to FIG. 1, but the current is injected into each of the DFB laser 11 and the SOA 13 from different control terminals. You may try to do it. In this case, the currents I _DFB and I _SOA are injected into the p-type electrodes 107 and 133 of the DFB laser and the SOA from their respective control terminals.

(Change example 4)
In the above, the case of oscillating at a wavelength of 1.55 μm has been described, but the same effect as that of the above embodiment can be obtained even if other wavelengths are applied. For example, even in the case of oscillating in the 1.3 μm band, the crystal composition of each component 11, 12, and 13 of the optical integration element 100 for optical communication can be changed and applied.

5 Optical Waveguide 5A Inclined Surface 11 DFB Laser 12 EA Modulator 13 SOA
14 Control terminal 100 Semiconductor optical integrated device 101 p-type electrode 102 Substrate 103, 106 Clad layer 104 Active layer 105 Guide layer 108, 132 Absorption layer

Claims (3)

With DFB laser
With the EA modulator connected to the DFB laser,
Monolithically integrated on the same substrate as the DFB laser and the EA modulator, the E
Includes SOA connected to the exit end of the A modulator,
The SOA has an optical waveguide formed so as to be inclined along a first direction in a plan view with respect to the optical axis direction of the emitted light of the DFB laser.
The optical waveguide has an inclined surface having an oblique shape in a cross-sectional view.
Inclination angle of the inclined surface, when the SOA of toward the exit end face gradually magnitude no longer, the light and propagating through the optical waveguide reaches the morphism edge out the, of the light reflected by the outgoing end face A semiconductor optical integrated device characterized in that the polarization is set so as not to return to the optical waveguide in a rotating state. The length of the SOA is characterized in that when the light propagating through the optical waveguide reaches the element end face of the output end, the polarization of the light is rotated by 45 °. The semiconductor optical integrated device according to claim 1. The semiconductor optical integrated device according to claim 1 or 2, wherein each of the DFB laser and the SOA is configured so that a current is injected from the same control terminal.

Images