Electroabsorption modulator, and fabricating method of the same

Details Electroabsorption modulator, and fabricating method of the same Electroabsorption modulator, and fabricating method of the same

2001-11-28

2003-08-05

Powell, William A. (Department: 1765)

Etching a substrate: processes

Forming or treating optical article

C359S298000, C359S315000, C438S718000, C438S737000

Reexamination Certificate

active

06602432

ABSTRACT:

CROSSREFERENCE TO RELATED APPLICATION
This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-368606, filed on Dec. 4, 2000; the entire contents of which are incorporated herein by reference.
BACKGROUND
1. Field of the Invention
The present invention relates to an electroabsorption modulator and a fabricating method thereof, in particular to an electroabsorption modulator appropriate for use in high speed modulation, 40 GHz or more, of laser light from a semiconductor laser, and a fabricating method thereof.
2. Description of the Related Art
Recently, as information communication demand in the Internet or the like increases, technology for high speed transmission of a larger volume of information to a distant place, not to mention to a trunk line, but also to a branch line, is demanded. As such technology, there is a large-capacity optical communication system.
In the large-capacity optical communication system, light emitted from a semiconductor laser is speedily modulated into digital signal; the modulated light is transmitted by means of the optical fiber; thereby high speed and distant transmission is realized. As an optical modulation method, it is general to directly modulate a semiconductor laser. However, the direct modulation of the semiconductor laser causes a relaxation oscillation in the semiconductor laser, thereby resulting in variation of wavelength due to chirping. When such light is transmitted through the optical fiber, there occurs a difference between the transfer times of the optical fiber, resulting in mode dispersion. As a result, communication distance is limited.
As a method for reducing the chirping at the direct modulation, there is a method in that light emitted from the semiconductor laser undergoes external modulation by means of an optical modulator. Among such optical modulators, an electroabsorption modulator (EA modulator) is in heavy usage. The electroabsorption modulator makes use of the quantum confinement Stark effect of a quantum well, and has advantages in that it may be mass-produced at relatively low costs, and may be driven at low voltages.
FIG. 13A
is a perspective view showing a rough configuration of an existing electroabsorption modulator, and
FIG. 13B
is a sectional view obtained by cutting along a C-D line in FIG.
13
A. In
FIG. 13A
, the electroabsorption modulator is provided with an optical modulation region MA of a length Lm and optical coupling regions CA, which are formed on both sides of the region MA; in the optical modulation region MA and the optical coupling regions CA, a mesa, which is formed in stripe in a light incident direction (A-B direction), and grooves MZ, which are formed on both sides of the mesa MS, are disposed. The length Lm of the optical modulation region MA may be set at, for instance, 100 &mgr;m.
In the optical modulation region MA, as shown in
FIG. 13B
, an n-InP cladding layer
62
is formed on an n-InP substrate
61
; and in the mesa MS portion, an optical absorption layer
63
, a p-InP cladding layer
64
, and a p-InGaAs contact layer
65
are formed. The optical absorption layer
63
has a multiple quantum well (MQW) structure, and may be formed by combining 14 pairs of, for instance, an InGaAsP quantum well layer of 1.5 nm and an InGaAsP barrier layer of 1.3 nm.
Furthermore, a silicon oxide film
68
is formed in the mesa MS and the grooves MZ, and resin
69
is filled in the grooves MZ. An n-side electrode
70
is formed on a back-face of the n-InP substrate
61
, and a p-side electrode
71
is formed on the mesa MS of the optical modulation region MA.
Meanwhile, in the optical coupling region CA, as shown in
FIG. 13A
, the n-InP cladding layer
62
is formed on the n-InP substrate
61
, and, in the mesa MS portion, an InGaAsP guide layer
66
and an n-InP cladding layer
67
are formed. Furthermore, the silicon oxide film
68
is formed in the portions of the mesa MS and the grooves MZ, and the resin
69
is filled in the grooves MZ. A composition of the InGaAsP guide layer
66
may be set so that a wavelength of, for instance, 1.1 &mgr;m may be obtained.
Light inputted in the optical coupling region CA is transferred through the InGaAsP guide layer
66
to the optical modulation region MA. Upon the light being transferred to the optical modulation region MA, the Stark effect is generated in the optical absorption layer
63
based on a voltage applied to the p-side electrode
71
, and an energy gap in the quantum well varies. When the energy gap varies, an optical absorption wavelength due to exciton varies, and transmittance of the laser light in the optical absorption layer
63
varies, thereby optical modulation is performed. The modulated light is emitted through the optical coupling region CA.
FIG.
14
A through
FIG. 16B
are diagrams showing a sequence of fabricating an existing electroabsorption modulator.
FIG. 14A
,
FIG. 14B
, and
FIG. 14D
are sectional views obtained by cutting along an A-B line in
FIG. 13A
, FIG.
15
B and
FIG. 16A
sectional views cut along a C-D line in
FIG. 13A
, and FIG.
15
C and
FIG. 16B
sectional views cut along an E-F line in FIG.
13
A.
In
FIG. 14A
, the n-InP cladding layer
62
, the optical absorption layer
63
, the p-InP cladding layer
64
, and the p-InGaAs contact layer
65
are successively grown on the n-InP substrate
61
, by means of MOCVD (metal-organic chemical vapor deposition).
Next, as shown in FIG.
14
B and
FIG. 14C
, a silicon oxide film
72
of a width Lm is formed on the p-InGaAs contact layer
65
, and etching, such as RIE, is performed with the silicon oxide film
72
as a mask, thereby the optical absorption layer
63
, the p-InP cladding layer
64
, and the p-InGaAs contact layer
65
in the optical coupling regions CA are removed.
Next, as shown in
FIG. 14D
, an InGaAsP guide layer
66
and an n-InP cladding layer
67
are selectively grown on the optical coupling regions CA, by performing deposition such as MOCVD with the silicon oxide film
72
as a mask. Then, a silicon oxide film
73
is deposited on an entire surface by means of CVD and so on. By depositing the InGaAsP guide layer
66
in the optical coupling region CA, the InGaAsP guide layer
66
and the optical absorption layer
63
may be allowed to optically couple.
Next, as shown in
FIG. 15A
,
FIG. 15B
, and
FIG. 15C
, the silicon oxide film
73
is patterned into stripes corresponding to the mesa MS and the grooves MZ. Then, by performing chemical etching with the patterned silicon oxide film
73
as a mask, the optical absorption layer
63
, the p-InP cladding layer
64
, and the p-InGaAs contact layer
65
in the grooves MZ of the optical modulation region MA are removed in mesa; and the InGaAsP guide layer
66
and the n-InP cladding layer
67
in the grooves MZ of the optical coupling region CA are removed in mesa. Thereby, the optical absorption layer
63
of the mesa MS is separated by the grooves MZ.
The length Lm of the optical modulation region MA may be set at, for instance, 100 &mgr;m; a width LS of the mesa MS at, for instance, 5 &mgr;m; and a width Lp of the optical absorption layer
63
at, for instance, 2 &mgr;m. Optical modulation is performed in the optical absorption layer
63
of the mesa MS corresponding to the stripe portion.
Next, as shown in FIG.
16
A and
FIG. 16B
, after removing the silicon oxide film
73
, the silicon oxide film
68
is deposited on an entire surface by means of CVD and so on, and the resin
69
is filled in the grooves MZ. Then, the p-side electrode
71
is formed on the mesa MS portion of the optical modulation region MA, and furthermore, a bonding pad
74
is formed. Thereafter, the n-InP substrate
61
is ground to substantially 100 &mgr;m, and the n-side electrode
70
is formed on a back-face of the n-InP substrate
61
.
A cut-off frequency of the optical modulator depends on element capacitance and element resistance. In order to allow the optical modulator to operate at high-speeds, the element capacitance is designed to be as small as

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