JPH1062731A - Optical phase modulator and light modulation device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical phase modulator and a light modulation device which control a light intensity passing through an optical waveguide or an optical phase by controlling a refractive index of multiplex quantum well layer composing an optical waveguide by an external applied voltage. SOLUTION: This optical phase modulator has a quantum well structure having a quantum well layer 3 consisting of one of InSb, InAs, InAsP, InSbP, or InAsSbP, and also, within a range of an applied voltage when the phase modulator operates, light absorption of the quantum well structure by transition of electron from a second quantum level of the valence band to heavy holes to a first quantum level of a conductive band is increased in the intensity as the applied voltage is increased in the absolute value, and the other light modulation device is such a device as the optical phase modulator and the phase modulator are additionally provided with an input means of optical signal consisting of TM polarized light.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to a method for controlling the refractive index of a multiple quantum well layer constituting an optical waveguide by an externally applied voltage.
The present invention relates to an optical phase modulator and an optical modulator that control the intensity or phase of light passing through an optical waveguide.

[0002]

2. Description of the Related Art In recent years, semiconductor multiple quantum wells (MQWs) and superlattice structures have appeared with the progress of compound semiconductor ultra-thin film forming technologies such as molecular beam epitaxy (MBE) and metal organic chemical vapor deposition (MOVPE). This makes it possible to remarkably improve the characteristics of optoelectronic devices as compared to bulk semiconductors. Of these, the electric field absorption effect of applying an electric field to the MQW structure to change its absorption coefficient or refractive index is much more remarkable than that of a bulk semiconductor, and a high-speed and low-voltage driven optical modulator is realized using this. Have been.

[0003] An optical modulator modulates an optical signal by:
The change of two kinds of physical quantities is used. Of these, the one that sets the wavelength (operating wavelength) in a region where the change in the absorption coefficient is small and uses the change in the refractive index there is called a phase modulator.
In this phase modulator, the applied voltage (electric field) is as low as possible.
It is necessary that a large change in the refractive index can be obtained at the same time, and that the change due to the absorption coefficient and its applied voltage is small.

If the refractive index is simply increased, the operating wavelength may be set in a region having a large absorption coefficient. However, in the case of a waveguide type optical modulator, light is changed before changing the phase of the light by changing the refractive index. It will be absorbed. Normally, the operating wavelength is set in a region where the absorption coefficient and its fluctuation can be neglected. Therefore, in order to obtain a change in the refractive index, it has been necessary to increase the applied voltage.

[0005]

Generally, as the wavelength is farther from the absorption region, the change in the refractive index becomes smaller. Therefore, in order to achieve high performance of the phase modulator, the absorption coefficient is small at the operating wavelength. We must find the answer to the seemingly contradictory question that the refractive index change will be larger.

SUMMARY OF THE INVENTION In view of the circumstances described above, it is an object of the present invention to provide a high-performance optical phase modulator and optical modulator capable of obtaining a large change in refractive index while sufficiently suppressing the absorption coefficient and its fluctuation. .

[0007]

According to the present invention, there is provided an optical phase modulator comprising InSb, InAs, and InAs.
A core has a quantum well structure having a quantum well layer made of any one of P, InSbP, and InAsSbP.

In the first optical phase modulator, as the absolute value of the applied voltage increases within the range of the voltage applied to the phase modulator during operation, the second quantum level of the valence band for heavy holes increases. Characterized in that the light absorption intensity of the quantum well structure due to the transition of electrons from the potential to the first quantum level in the conduction band increases.

On the other hand, an optical modulator according to the present invention is characterized by comprising the first and second optical phase modulators and means for inputting signal light composed of TM polarization to the phase modulator. .

[0010] In the current optical modulator, the operation mainly using the TE polarized light is mainly performed. FIG. 3 shows a plot of the voltage change of the absorption coefficient spectrum and the change of the absorption coefficient spectrum due to the voltage change when the TE-polarized light is incident on the modulator. As shown in FIG. 3, the three main exciton absorption peaks represent transitions from the heavy hole band (HH) and the light hole band (LH). If the absorption coefficient change Δα due to the voltage change ΔF is given as a function of the wavelength λ, the refractive index change Δn obtained at the wavelength λ ₀ will be determined by the function of the following equation (1) shown in “Equation 1”.

[0011]

(Equation 1)

Here, Ρ means taking the principal value of the integral. From FIG. 3, it can be seen that the contribution of exciton absorption (E ₁ −HH ₁ ) from the first quantum level of HH is large in TE polarized light, but the change of the absorption coefficient is the same in the positive and negative regions. Are offset by each other, and as apparent from the above equation (1), they do not contribute much to an increase in the change in the refractive index.

Accordingly, the present invention has focused on the operation by TM polarized light. FIG. 4 plots the voltage change of the absorption coefficient spectrum when the TM-polarized light is incident on the modulator and the change in the absorption coefficient spectrum due to the voltage change.

[0014] From FIG. 4, E ₁ -HH ₁ for TM polarized light.
Instead of exciton absorption from the second quantum level of HH (E
₁ -HH ₂ ) and the exciton absorption (E
₁ -LH ₁ ). here,
Of particular importance is that E ₁ -HH ₂ increases its absorption peak upon application of a voltage. This increase in the absorption peak is due to the band mixing effect, which is the interaction between heavy and light holes. As a result, as shown in FIG. 4, the increase in the peak of E ₁ −HH ₂ of the change in the absorption coefficient increases the positive absorption change, and as can be seen from the above equation (1), the change in the refractive index is the absorption coefficient. In this case, since the refractive index is given by the integral of the change, the change in the refractive index can be made larger than that of the TE polarized light. Further, as viewed from the operating wavelength, the peak of the E ₁ -HH ₂ is farther than the peak of the E ₁ -HH _1, the peak of the E ₁ -HH ₁ in TM polarization than that of the TE polarization 2-3 Order of magnitude smaller
Variation in absorption coefficient due to voltage application can be suppressed.

The degree of increase in the peak of E ₁ -HH ₂ is better for a material having a larger effective mass of heavy holes and light holes in a direction perpendicular to the quantum well plane in the well layer material. Current quantum well crystal growth techniques are based on Group III and V compound semiconductor materials. From theoretical calculations, III
Among the group V semiconductor materials, compound semiconductors having a remarkable effective mass difference between heavy holes and light holes are InP, InAs, and InP.
It was found to be a ternary compound or quaternary compound obtained from Sb or a combination of these binary compounds. Specifically, it is preferable to select the well layer material from any one of InSb, InAs, InAsP, InSbP, and InAsSbP.

On the basis of the present invention obtained based on the above-mentioned findings, an unprecedented change in refractive index can be achieved while suppressing an increase in absorption fluctuation.

[0017]

Embodiments of the present invention will be described below.

FIG. 1 is a schematic diagram of a phase light modulator according to an embodiment of the present invention. As shown in FIG. 1, an n-InP substrate 1
An n-InP cladding layer 2 and a non-doped InSbA
A multiple quantum well layer 3 made of sP / InP, a p-InP cladding layer 4, and a p-InGaAs layer 5 are sequentially stacked. An N-side electrode 6 is provided on the back surface of the n-InP substrate 1.
On the other hand, a P-side electrode 7 is provided on the p-InGaAs layer 5 side.

A lead 8 is connected to the P-side electrode 7 for connection to voltage applying means (not shown) for applying a voltage corresponding to the modulation signal. Incident light 9 composed of TM polarized light from a light source (laser) not shown is emitted as modulated light 10.

Here, the quantum well layer 3 is made of InSbA.
It has a multiple quantum well structure using sP and InP as a quantum well layer and a barrier layer, respectively, and is manufactured by a crystal growth method such as a molecular beam epitaxial growth method or a metal organic chemical vapor deposition method.

FIG. 2 shows the voltage dependence of the refractive index change by a solid line using the phase modulator having the structure of FIG. 1 according to the present invention. For comparison, typical characteristics of the conventional structure are also shown.

As shown in FIG. 2, from the comparison between the two, the characteristics of the structure of the present invention show that the change in the refractive index rapidly increases compared to the conventional characteristics, and the change in the refractive index at the same voltage is smaller than that of the conventional structure. It was found that a high value of 2 to 3 times or more could be obtained.
This is a result of the band mixing effect being amplified by the semiconductor material used in the present invention as described above.

However, if the voltage exceeds a certain threshold, the difference in effective mass between heavy holes and light holes is canceled out by the change in the band structure due to the electric field, so that the band mixing effect is no longer effective, and the refractive index is no longer effective. The voltage dependence of the change has a peak structure as shown in the figure.

There are various means for obtaining the incident light 9 composed of TM polarized light. For example, the output light of the semiconductor laser may be guided to the optical modulator shown in FIG. 1 by a polarization maintaining fiber, and the output end of the fiber may be rotated so that the incident light becomes TM polarized light. By combining such means with the optical modulator shown in FIG. 1, it is possible to easily assemble an optical modulator that efficiently modulates the phase of incident light.

[0025]

As described above, according to the present invention, InSb, InAs, InAs
When the quantum well layer is formed of any one of AsP, InSbP, and InAsSbP, the change in the voltage of the absorption peak from the valence band second level in TM polarized light can be made large. It is possible to provide a high-performance phase modulator and an optical modulation device that surpass modulation operations.

[Brief description of the drawings]

FIG. 1 is a schematic diagram of an optical modulator according to an embodiment of the present invention.

FIG. 2 is a comparison diagram of the voltage dependence of the change in the refractive index of the optical modulator according to the embodiment of the present invention and the voltage dependence of the change in the refractive index in the conventional structure.

FIG. 3 is a wavelength spectrum diagram of a change in absorption coefficient corresponding to a voltage change in an absorption coefficient spectrum in TE polarized light.

FIG. 4 is a wavelength spectrum diagram of a change in absorption coefficient spectrum and a corresponding change in absorption coefficient spectrum in TM polarized light.

[Explanation of symbols]

Reference Signs List 1 1 n-InP substrate 2 n-InP cladding layer 3 non-doped InSbAsP / InP multiple quantum well layer 4 p-InP cladding layer 5 p-InGaAs layer 6 N-side electrode 7 P-side electrode 8 Lead wire 9 incident light 10 modulated light

Claims (3)

[Claims] 1. InSb, InAs, InAsP, In
An optical phase modulator comprising a core having a quantum well structure having a quantum well layer made of any one of SbP and InAsSbP. 2. The optical phase modulator according to claim 1, wherein within a range of a voltage applied when the phase modulator operates,
As the absolute value of the applied voltage increases, the intensity of light absorption of the quantum well structure due to the transition of electrons from the second quantum level in the valence band to the first quantum level in the conduction band for heavy holes increases. An optical phase modulator characterized by the above-mentioned. 3. The optical phase modulator according to claim 1, wherein:
Means for inputting signal light composed of TM polarized light to the phase modulator.