JPH03290614A - Optical modulator - Google Patents

Abstract

PURPOSE:To obtain an optical modulator which has a high extinction ratio and a small insertion loss by constituting a waveguide layer as a well layer where InAs and GaAs are laminated as one atom as the unit and a barrier layer of InA As or InP. CONSTITUTION:A waveguide layer 14 has a multiple quantum well structure which has a super-lattice, where InAs and GaAs are alternately laminated with one atomic layer one by one is used as a well layer 141 and has InAlAs or InP as a barrier layer 142, and an electric field is applied to this waveguide layer 14 by a p-side electrode 18 and an n-side electrode 19. Since the well layer 141 has the super-lattice where InAs and AlAs are alternately laminated with one atomic layer as the unit, local fluctuation of the composition does not occur, and the spectrum width of excitons is narrowed. Since the lattice oscillation in the lamination direction is suppressed in the super-lattice where the period is about one atomic layer, the coupling constant between excitons and the lattice oscillations is reduced to narrow the spectrum width of excitons. Therefore, a high extinction ratio and a small insertion loss are obtained.

Description

[Detailed description of the invention] [Industrial application field] The present invention relates to an optical modulator used for optical information processing and the like.

[Conventional technology]

In recent years, digital information processing that takes advantage of the high speed of light has attracted attention. This requires the development of logic elements that control light. One of these is an optical modulator that turns optical signals on and off electrically. Conventionally, as an optical modulator, the structure shown in Figure 4 or Electronics Letters (El
electronicsLetlers) Volume 21, 693
Wood, T. H., p., 1985).
It has been reported by et al. This optical modulator has a pin structure in which a quantum well layer 21 in which thin films of GaAs and GaAlAs are alternately laminated is sandwiched between cladding layers 22 and 23 made of GaAlAs of p-type and n-type conductivity, respectively. Light incident from the - side end face passes through the quantum well layer 21 and exits from the other end face. Application of an electric field reduces the effective bandgap, so switching is performed by utilizing the fact that the absorption coefficient of excitons in the quantum well for light on the low energy side increases.

[Invention or problem to be solved]

For light in the 1.55 μm wavelength band used for optical communication, In
GaAs/InP or InGaAs/I
n A ] A s multi-quantum well structure is used, but since these use a mixed crystal called GaAs in the well layer, the exciton spectrum is lower than when binary crystal GaAs is used in the well layer. The width is wide, and absorption loss is large even when no electric field is applied. Therefore, the extinction ratio of the optical modulator is small and the insertion loss is also large.An object of the present invention is to provide an optical modulator with a high extinction ratio and a small insertion loss by narrowing the spectral width of excitons. It is in.

[Means to solve the problem]

In the optical modulator of the present invention, a superlattice in which InAs and GaAs are alternately laminated one atomic layer each is used as a well layer, and InAl
It is characterized by having a multiple quantum well structure as a waveguide layer with As or InP as a barrier layer, and controlling transmission characteristics by applying an electric field to the multiple quantum well structure.

[Effect]

When a voltage is applied to the multiple quantum well structure serving as the waveguide layer, the energy of excitons in the multiple quantum well structure shifts to the lower energy side. For this reason, the number of absorption components 711) for light having slightly lower energy than the excitons increases, the transmittance of the optical modulator decreases, and the optical modulator changes from an on state to an off state. Regarding that, the refractive index changes. When no voltage is applied, the energy of the light is applied to the Fabry-Perot resonator, causing it to turn on. When voltage is applied, the refractive index changes, causing the resonance to shift and the transmittance to decrease, resulting in the off-state. becomes. The extinction ratio at this time can be written as 0N exp (△αd〉 opp. Here, △α is the change in the absorption coefficient of the waveguide layer. The absorption of the multi-quantum well structure, which is the waveguide layer, is It is desirable that the width of the exciton absorption spectrum of the multi-quantum well structure is narrow, and that the absorption of light at the wavelength used when no voltage is applied is 0. Even if the exciton absorption spectrum is wide, if the wavelength of the light used is far from the exciton absorption spectrum peak, the absorption when no voltage is applied can be ignored, or in that case, Even if a voltage is applied, the increase in the absorption coefficient is small and the extinction ratio becomes small, or the operating voltage required to obtain a sufficient extinction ratio becomes high.Furthermore, if there is absorption, the insertion loss also increases.

InGaAs/InP or I
nGaAs/I nA I As are known, but since these use a mixed crystal called InGaAs in the well layer, they are binary crystals due to fluctuations in exciton energy caused by local fluctuations in the composition. The spectral width of excitons becomes wider than when GaAs is used for the well layer. In the quantum well structure used in the present invention, the well layer is made of InAs and A
Since lAs is formed into a superlattice in which one atomic layer is alternately stacked, such local fluctuations in composition do not occur, and the spectral width of excitons is narrow. Furthermore, in a superlattice with a period of about one atomic layer, lattice vibration in the stacking direction is suppressed, so
The coupling constant between excitons and lattice vibrations decreases, the width of the exciton spectrum narrows, and due to the above effects, the absorption of the quantum well structure when no voltage is applied decreases, and the extinction ratio, Insertion loss is improved.

Embodiment] FIG. 1 is a configuration diagram showing an embodiment of the present invention. A 5X film with a thickness of 1°Onm is placed on the Sn-doped InP substrate 11.
Buffer layer 12 of InP doped with 1017cra ~ 3Si, thickness 21 layers m and 5 X 1017 cm -'S
A cladding layer 13 of InP doped with i is sequentially laminated by the hydride VPE method. Next, as shown in Figure 2,
In, As by ALE method.

A multi-quantum well is formed by laminating 40 layers of a superlattice well layer 141 in which Ga and As are alternately deposited for 14 cycles, and a barrier layer 142 in which In and P are alternately deposited in 1 atom for 20 cycles. The waveguide layer 14 consists of a structure. The energy band structure of the waveguide layer 14 at this time is as shown in FIG. The well layer 141 has a thickness of 7 nm, and the barrier layer 142 has a thickness of 10 m. The active layer 14 is not doped. Furthermore, a diffusion prevention layer 1 made of non-doped InGaAsP with a thickness of 50 nm and a bandgap wavelength of 1.3 μm.
5. Thickness 1 doped with Zn 5 x 1017 cm-3
Cladding layer 16 with μm InP, Zn 5×1
018arm-3 doped p-In with thickness O, 1 μm
Contact layers 17 of GaAsP are sequentially stacked, and Ti
/A p-side electrode 18 is formed. Leaving a striped waveguide portion with a width of 5 μm and an electrode pad with a diameter of 100 μm, the other portions are etched and removed up to the substrate 11. The length of the waveguide is 200 μm. After mirror polishing the substrate 11 to a thickness of 100 μm, the n-side electrode 19 is formed.

In this example, the width of exciton absorption is reduced, and the absorption coefficient for light 15 m e V away from the exciton absorption peak is reduced from about 600 aa-1 to 100 cm-1.
' became. Therefore, when an electric field of 105 V/cm is applied, the extinction ratio goes from 6 dB to 20 dB, and the insertion loss is 10 dB.
B to 3 dB, which is a significant improvement over the conventional method.

〔Effect of the invention〕

As described above in detail, the effects of the present invention can be summarized in that an optical modulator with a high extinction ratio and a small insertion loss can be obtained by narrowing the spectral width of excitons.

[Brief explanation of drawings]

Fig. 1 is a block diagram of an embodiment of the present invention, Fig. 2 is an enlarged view of the waveguide layer portion of Fig. 1, and Fig. 3 is an energy band structure diagram of a multiple quantum well structure that is the active layer of the present invention. , FIG. 4 is a block diagram showing an example of a conventional technique. In the figure, 11 is a substrate, 12 is a buffer layer, 13 is a cladding layer, 14 is a waveguide layer, 15 is a diffusion prevention layer, 16 is a cladding layer, 17 is a contact layer, 18 is a p-side electrode, 19 is an n-side electrode It is. Further, 141 is a well layer, and 142 is a barrier layer.

Claims (1)

[Claims] It has a multi-quantum well structure in which a superlattice in which one atomic layer of InAs and GaAs is alternately stacked as a well layer and InAlAs or InP as a barrier layer is used as a waveguide layer, and is provided with means for applying an electric field to the multi-quantum well structure. An optical modulator characterized by: