Optical integrated circuit substrate and optical module

Details Optical integrated circuit substrate and optical module Optical integrated circuit substrate and optical module

2001-10-30

2004-02-17

Healy, Brian (Department: 2874)

Optical waveguides

Integrated optical circuit

C385S129000, C385S131000, C385S141000

Reexamination Certificate

active

06694069

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical integrated circuit substrate having an optical waveguide and a thin-film optical element which are integrated on the same substrate, more particularly to an optical integrated circuit substrate which is, like a WDM (Wavelength Division Multiplex) optical module substrate, suitably used for the case where a plurality of thin-film optical elements and other devices need to be mounted on the same substrate and in which miniaturization of the substrate, improvement of productivity, and enhancement of optical transmitting/receiving efficiency are achieved by integrating together an optical waveguide and a thin-film optical element on the same substrate.
The invention also relates to an optical module, which is used for an optical signal transmission system, having a semiconductor light-emitting element, an optical waveguide, and a monitoring semiconductor light-receiving element.
2. Description of the Related Art
In recent years, research and development have been under way on an optical element which lends itself to improvement of capability and productivity of an optical transmission module, and also on a technique for mounting an optical element with higher density, higher accuracy, and higher optical connection efficiency.
For example, “Thin-Film Multimaterial Optoelectronic Integrated Circuits” carried in “IEEE Transactions on Components, Packaging, and Manufacturing Technology, part B, Vol. 19, No. 1, February 1996” deals with a technique whereby an optical light-receiving element is grown epitaxially on a semiconductor substrate and thereafter only the resultant epitaxial layer is isolated therefrom so as to form a thin-film optical light-receiving element to be mounted on another mounting substrate. According to this technique, a thin-film optical element made of various materials can be mounted on a mounting substrate with higher density and higher accuracy.
Moreover, as an example of optical element mounting techniques, an optical integrated circuit substrate proposed in Japanese Unexamined Patent Publication JP-A 7-128531 (1995) is shown in section in FIG.
8
. In
FIG. 8
, the optical integrated circuit substrate includes: a substrate
31
; an optical waveguide
32
having a lower clad layer
34
, a core layer
35
, and an upper clad layer
36
; and a surface light-receiving element
37
, built as a thin-film optical element, disposed on the substrate
31
such that its light-receiving surface is covered with the lower clad layer
34
. In this construction, an electromagnetic field of light propagating around the core layer
35
is spread out over the lower clad layer
34
. This makes possible optical connection with the surface light-receiving element
37
.
On the other hand, in the case where optical connection is established between the optical waveguide formed on the substrate and the thin-film optical element embedded in the optical waveguide, the following problem arises.
The thin-film optical element is composed solely of an epitaxial layer and thus has a thickness of no greater than several &mgr;m. Moreover, in a typical single-mode optical waveguide, difference in specific refractive index between the cladding and the core falls in a range of 0.2 to 1.5%, and the core has a thickness of about 4 to 8 &mgr;m. Here, to minimize the interaction between the substrate and light to be transmitted, the thickness of the lower cladding needs to be made more than 1.5 times as large as that of the core, more specifically, the lower cladding needs to have a thickness of about 6 to 12 &mgr;m. Meanwhile, to bring sufficiently high efficiency to the optical connection between the optical waveguide and the thin-film optical element arranged therebelow, the lower cladding of the optical waveguide needs to be made thin enough to reduce the distance between the core and the thin-film optical element.
Conventionally, after a thin-film optical element is formed or arranged on a substrate surface, an optical waveguide is formed thereon by coating. Accordingly, in the case where, after the thin-film optical element
37
is disposed on the substrate
31
, an optical waveguide is formed thereon so as to achieve optical connection, as shown in
FIG. 8
, it is necessary to provide a curve or bend portion
38
in the optical waveguide
32
so that, of the lower clad layer
34
, one part located above the thin-film optical element
37
is made thin, and the other part free of the thin-film optical element
37
is made thick. In this case, if the curve portion
38
has an unduly large curvature, the interaction between the substrate
31
and transmitted light occurs over a wider area in the vicinity of the thin-film optical element
37
, which results in significant light transmission losses. By contrast, if the curve portion
38
has an unduly small curvature, although the interaction between the substrate
31
and transmitted light is prevented from occurring over a wide area in the vicinity of the thin-film optical element
37
, transmitted light radiates over the curve portion
38
, which results in significant light transmission losses and occurrence of stray light which causes cross talk.
JP-A 7-128531 further proposes, as Practical example 3, a construction fabricated in the following manner. A semiconductor layer of substantial height is used as a base substrate so that an active layer or a light absorbing layer acting as a thin-film optical element is located at a considerable distance from the substrate. Then, an optical waveguide is formed thereon by coating. In this construction, however, the core of the optical waveguide is significantly bent in the vicinity of the optical element. This causes radiation losses of light in the bend portion and also causes scattering losses of light in the optical element portion. Another problem with this construction is that, in forming an optical waveguide by coating, because of stepped configuration created due to the arrangement of the optical element, the process accuracy of the core of the optical waveguide is deteriorated, or it is difficult to form the coated or bend portion around the optical element into desired shape. This makes it impossible to obtain satisfactory performance capability as intended.
Moreover, for an optical signal transmission system, an optical module is used that includes: an optical waveguide formed on a substrate; a semiconductor light-emitting element arranged so as to be optically connected to the optical waveguide; and a monitoring semiconductor light-receiving element for detecting intensity of light which is emitted from the semiconductor light-emitting element and transmitted through the optical waveguide. The monitoring semiconductor light-receiving element serves to stabilize optical output from the semiconductor light-emitting element by monitoring the intensity of the light emitted from the semiconductor light-emitting element and then providing feedback for a driving circuit of the semiconductor light-emitting element.
Shown in
FIG. 9
as a plane figure is a conventional optical module proposed in Japanese Unexamined Patent Publication JP-A 11-38279 (1999).
In the optical module shown in
FIG. 9
, on a substrate
41
is formed an optical waveguide
46
and mounted a semiconductor light-emitting element
47
(a laser diode is used here). The laser diode has two excitation ends composed of semiconductor cleavage planes. In addition, on the substrate
41
is mounted a monitoring semiconductor light-receiving element
42
opposed to one of the excitation ends. In this construction, backward light emitted from the semiconductor light-emitting element
47
(light to be monitored) is monitored by the semiconductor light-receiving element
42
, and the output of the semiconductor light-emitting element
47
is so controlled as to be kept constant by an optical output level stabilizing circuit (not shown).
However, the above-described conventional optical module having the semiconductor light-emitting element
47

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