JPH1126882A - Wavelength plate integrated type polarized wave independent semiconductor amplifier - Google Patents

Abstract

PROBLEM TO BE SOLVED: To solve polarized wave dependence of a semiconductor optical amplifier by setting the angle, made by a straight line connecting a first waveguide channel to a second waveguide channel and a straight line in parallel with a substrate, at a specified angle, and specifying the length of the second waveguide channel by a specified formula. SOLUTION: Under a condition where a TE polarized wave light including an electric field component in an x-direction is made incident, first, gain of TE polarized wave is provided in an optical amplifier region. When the incident light enters a λ/4 plate region, the light made incident on a region A enters a region B while holding the TE polarized wave. In the region B, in the major axis of a crystal (a line connecting two waveguide channels), the angle (θ) made by the direction orthogonal to the optical axis of the light of a first waveguide channel 21 (x-axis) and the axis connecting the first waveguide channel 21 and a second waveguide channel 22 is 45 deg.. By setting the length with a formula with two equivalent refractive indexes from the lowest order provided to an intrinsic waveguide mode in the region B as n1 and n2 (n1 >n2 ), polarized wave of the reflected light is rotated by 90 deg. at the entrance of the region B, and emitted as a TM polarized wave.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to optical communication, optical switching,
The present invention relates to a polarization independent semiconductor amplifier integrated with a wave plate used for optical information processing and the like.

[0002]

2. Description of the Related Art Considering the construction of a system using light such as optical communication, optical switching, and optical information processing, optical elements such as an optical fiber, an optical waveguide, an optical switch, an optical receiver, and an optical amplifier are indispensable. . These elements need to be made into practical parts by being combined with an optical fiber or by being integrated. The integration makes it possible to omit the optical axis alignment between the optical fiber and the semiconductor element, which is required with a precision of μm, so that the integration becomes robust and an increase in reliability can be expected.

By the way, in a normal optical fiber,
It does not have the function of maintaining the plane of polarization from the light source, and the signal light input to an optical functional element such as a semiconductor optical switch or a semiconductor optical amplifier changes its polarization state in accordance with a change in environment.

However, the waveguide structure of a semiconductor device as described above is not generally isotropic, and has a thickness of the order of sub-micron while having a width of several microns, and a switching characteristic of the waveguide layer. Alternatively, the amplification characteristics of the active layer differ depending on the polarization state. For this reason, there has been a problem that the output characteristic greatly varies depending on the polarization state of the input signal light (polarization dependency of gain).

[0005]

As a method of reducing the polarization dependence, it is conceivable to integrate a λ / 4 plate functional element at the output end of the semiconductor optical amplifier. Here, an anti-reflection film is added to the incident end, and a high-reflection film or the like is added to the output end to increase the reflectivity so as to be used as a reflective optical amplifier. At this time, the signal light reflected at the emission end passes through the λ / 4 plate again, and eventually operates as a λ / 2 plate, and the polarization plane can be rotated by 90 °. With such a configuration, there is no interference since the incident polarization and the exit polarization are different by 90 °. Therefore, the load on the anti-prevention film is reduced. As the optical element such as the wave plate, a bulk component is often used. A wavelength plate having a plate thickness at which the phase difference between two orthogonal linearly polarized components becomes π is λ /
Called two plates. There is a problem that the size of the λ / 2 plate is large and the matching with the semiconductor material is not good.

[0006]

A wave plate integrated type polarization independent semiconductor amplifier which solves the above problem has a buried waveguide at one end face of a semiconductor optical amplifier having a buried structure manufactured on a semiconductor substrate. The buried waveguide is composed of two first waveguides and a second waveguide, and the first waveguide (length: L ₁ ) is integrated with the active layer of the semiconductor optical amplifier. And a second waveguide (length: L ₂ , L ₂
≦ L ₁ ) is located parallel to the first waveguide, and forms an angle of 45 ° between a straight line connecting the centers of the two waveguides and a straight line parallel to the substrate.
° and two equivalent refractive indices n ₁ and n ₁ counted from the lowest order of the intrinsic waveguide mode of the two waveguides.
₂ (n ₁ > n ₂ ), the length L _{2 of the second} waveguide is defined by an equation.

(Equation 2)

In the above-mentioned polarization-independent semiconductor amplifier integrated with a wave plate, the shape of the first waveguide is optically isotropic.

In the above-mentioned polarization-independent semiconductor amplifier integrated with a wave plate, a current can be injected into at least one of the first waveguide and the second waveguide.

[Operation] The λ / 2 plate has a function of rotating the output light polarization by 2θ from the incident light polarization, assuming that the angle between the incident light polarization and the crystal axis of the wavelength plate is θ. doing. If the plate thickness is set to be shifted by the phase difference π, the incident polarization is 9
Can rotate 0 °. According to the present invention, the function of rotating the plane of polarization of a λ / 2 plate based on exactly the same principle as when θ is set to 45 ° is made of a semiconductor, and integrated in a semiconductor optical amplifier. However, since the optical amplifier itself is used in the reflection type, it passes twice through the place of the wave plate. Therefore, as a wave plate, λ corresponding to half of the λ / 2 plate
A quarter plate is sufficient. The incident light passes through the wave plate region after passing through the amplification region, is reflected at the end face, and when passing through the wave plate portion again, the polarization is rotated by 90 °. Therefore, for any incident polarization, TE (lateral) polarization gain and TM
(Vertical) polarization gain can be obtained once,
As a whole, polarization independence can be achieved.

When the boundary between the amplifier region and the wave plate region is reached, there must be no phase difference between the waves in the two orthogonal directions (they must be linearly polarized). Therefore, the waveguide in the optical amplifier region must be optically isotropic.

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, preferred embodiments of the present invention will be described with reference to the drawings, but the present invention is not limited thereto.

<First Embodiment> FIG. 1 is a perspective view of a semiconductor polarization rotator according to a first embodiment of the present invention. 2A is an enlarged view of a region having the function of the wavelength plate in FIG. 1, FIG. 2B is a side sectional view thereof, and FIG. Here, reference numeral 11 denotes an InP substrate, 12 denotes an InP buried layer, 21 denotes an InGaAsP waveguide layer (first waveguide) having a band gap wavelength of 1.3 μm, and 22 denotes InP having a band gap wavelength of 1.1 μm.
Each of the GaAsP waveguide layers (second waveguides) is illustrated.

The principle of operation of the device will be briefly described. here,
As an amplification gain per one path of the semiconductor optical amplifier, TE
The (horizontal) polarization is G _TE , and the TM (vertical) polarization is G _TM . In a state where the TE polarized light having electric field component in x direction is incident, first get a gain of G _TE in the optical amplifier region. When the incident light enters the λ / 4 plate area, the light incident on the area A is TE
It enters the region B while maintaining the polarization. In this region B, as shown in FIG. 3, the main axis of the crystal (the line connecting the two waveguides) is orthogonal to the main axis (x-axis) of the light of the first waveguide 21 and the first waveguide 21 This means that the angle (θ) between the axis connecting the second waveguide 22 and the second waveguide 22 is shifted by 45 °.
The two equivalent refractive indices counted from the lowest order of the intrinsic waveguide mode in the region B are n ₁ and n ₂ ((n ₁ > n ₂ )).
By setting the length L to the following equation, light reflected on the basis of the above-described principle of the λ / 2 plate
The polarization is rotated by 90 ° at the entrance of the laser beam and is output as TM polarization.

(Equation 3)

Therefore, G _TE
Receiving a gain, receiving the gain G _TM in returning as TM polarized light. That is, the light is emitted with a gain of (G _TE + G _TM ) as a whole.

On the other hand, the same applies to the case where the TM polarization is incident. In other words, G _TM
, The polarization is rotated 90 ° in the wavelength plate region and TE
Receiving a gain of G _TE in returning the polarized light. That is,
The light is emitted with a gain of (G _TE + G _TM ) as a whole.

As described above, the same gain (TE gain and TM gain once each) is obtained in both incident states.
The polarization dependence can be eliminated. Actually, due to the problem of processing accuracy, a polarization rotation of 83 degrees was realized for incident light having a wavelength of 1.55 μm.

If the traveling speed of the wave propagating through the first waveguide is different between the x direction and the y direction, when the traveling wave reaches the boundary between the optical amplifier region and the λ / 4 plate region, Since there is a phase difference between the waves in the two directions, the waves do not become linearly polarized. Therefore, a desired effect cannot be obtained in the λ / 4 plate region. Therefore, the first waveguide 21 must be optically isotropic.

Here, the optical isotropy will be outlined. Let the wavelength of light and the length of the material in vacuum be denoted by λ and L, respectively. In a medium having a refractive index n, the optical wavelength and the optical length are λ / n and nL. Therefore, as shown in FIG. 4A, the material is not isotropic unless it has a square shape in a vacuum, but in the medium, as shown in FIG. 4B, different materials (n _C1 , n _C2 , n _C2 , n _C1 ) can realize a substantially isotropic situation. Specifically, the polarization dependence of the semiconductor optical amplifier occurs due to the difference in the confinement coefficient between the x direction and the y direction. Therefore, even if the active layer portion has a three-layer (LOC) configuration in the vertical direction even in a rectangular shape (horizontally long) due to the problem of fabrication accuracy, it is possible to make the polarization independent.

<Second Embodiment> FIG. 5 shows a second embodiment of the present invention.
FIG. 3 is a side cross-sectional view of the semiconductor polarization rotation element according to the embodiment. In contrast to the structure of FIG.
Is different from the first embodiment. By injecting a current using these two electrodes 31 and 32, the refractive index of the waveguide layers 21 and 22 can be changed, so that trimming becomes possible.

As a result, the rotation accuracy can be improved as compared with the first embodiment, and a polarization rotation of 85 degrees with respect to the incident light having a wavelength of 1.55 μm is realized. In this case, the electrode may be limited to the region B only. It is also possible to provide an electrode in each region and perform electric separation between the regions. Further, it is also possible to partially provide an electrode so that current can be injected only into at least one of the waveguide layers 21 and 22.

Further, a superlattice structure can be used for the waveguide layer structure, or an SCH structure in which the waveguide layer portion is sandwiched by a composition having a band gap wavelength shorter than that of the waveguide layer can be adopted. is there.

In this embodiment, the InGaAsP material system on the InP substrate has been described. However, the present invention is not limited to this, and similar effects can be obtained by using other materials. it can. Further, a doping layer or the like is also optional.

The positional relationship between the two waveguides is as follows.
It does not matter which side is left, right, up or down. It is only important that the angle θ is 45 ° as shown in FIG.

Although the two waveguides having different compositions have been described as examples in the embodiment, it is also possible to adjust the dimensions by using the same waveguide.

Furthermore, a superlattice structure can be used for the waveguide layer structure, and an SCH structure in which the waveguide layer portion is sandwiched between compositions having a shorter band gap wavelength than the waveguide layer can be employed. It is.

In the second embodiment, the waveguide layer 22 or both the waveguide layer 21 and the waveguide layer 22 are set to have a bandgap wavelength.
If an InGaAsP waveguide layer of 1.55 μm band is used, it can be operated as an optical amplifier by current injection.

[0027]

As described above, in the semiconductor optical amplifier of the present invention, an embedded waveguide is integratedly connected to one end face of a semiconductor optical amplifier having an embedded structure formed on a semiconductor substrate. The embedded waveguide is composed of two waveguides (a first waveguide and a second waveguide), and the first waveguide (length: L ₁ ) is connected to the active layer of the semiconductor optical amplifier. Integrated second waveguide (length:
L ₂ , L ₂ ≦ L ₁ ) are positioned parallel to the first waveguide, and the angle between a straight line connecting the centers of the two waveguides and a straight line parallel to the substrate is set to 45 °. Assuming that two equivalent refractive indices are n ₁ and n ₂ (n ₁ > n ₂ ) counted from the lowest order of the intrinsic waveguide mode of the two waveguides, the length L _{2 of the second} waveguide is expressed by the following equation ( Characterized in that it is defined in 1),
Further, since an electrode structure capable of injecting current is provided in at least one of the first waveguide and the second waveguide, it is effective to eliminate the polarization dependence of the semiconductor optical amplifier. A simple polarization control device can be realized.

[Brief description of the drawings]

FIG. 1 is a perspective view of a semiconductor polarization rotator according to a first embodiment of the present invention.

2A is an enlarged view of a region having a gas phase of the wave plate of FIG. 1, and FIG. 2B is a side view thereof.

FIG. 3 is a sectional view in a region B.

FIG. 4 is an explanatory diagram related to optical isotropic.

FIG. 5 is a side sectional view of a semiconductor polarization rotating element according to a second embodiment of the present invention.

[Explanation of symbols]

Reference Signs List 1 active layer 11 InP substrate 12 InP buried layer 21 InGaAsP with band gap wavelength of 1.3 μm band
Layer (first waveguide) 22 InGaAsP with band gap wavelength of 1.3 μm band
Layer (second waveguide) 31, 32 Electrode

Continuation of front page (72) Inventor Toshio Ito 3-19-2 Nishishinjuku, Shinjuku-ku, Tokyo Nippon Telegraph and Telephone Corporation

Claims (3)

[Claims] A buried waveguide is integratedly connected to one end face of a semiconductor optical amplifier having a buried structure fabricated on a semiconductor substrate, wherein the buried waveguide is composed of two first waveguides and a second waveguide. The first waveguide (length: L ₁ ) is integrated with the active layer of the semiconductor optical amplifier, and the second waveguide (length: L ₁ )
₂ , L ₂ ≦ L ₁ ) is located parallel to the waveguide direction of the first waveguide, and the angle between the straight line connecting the centers of the two waveguides and the straight line parallel to the substrate is set to 45 °. If two equivalent refractive indices are n ₁ and n ₂ (n ₁ > n ₂ ) counted from the lowest order of the intrinsic waveguide mode of the two waveguides, the length L _{2 of the second} waveguide Is defined by the following expression: A wave plate integrated type polarization independent semiconductor amplifier. (Equation 1) 2. The polarization independent semiconductor amplifier according to claim 1, wherein the shape of the first waveguide is optically isotropic. 3. The polarization independent semiconductor amplifier according to claim 1, wherein a current can be injected into at least one of the first waveguide and the second waveguide.