Optical waveguides – With optical coupler – Particular coupling function
Reexamination Certificate
1998-08-04
2001-06-26
Henry, Jon (Department: 2872)
Optical waveguides
With optical coupler
Particular coupling function
C385S043000, C385S050000
Reexamination Certificate
active
06253003
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical coupling method and an optical coupling device for optically coupling two optical waveguide devices differing in spot size of light wave.
2. Description of the Related Art
For example, when a semiconductor optical device such as a semiconductor laser diode and a semiconductor switch is optically coupled to an optical waveguide device such as a single mode optical fiber, even if an end of the semiconductor optical device is directly butted against and coupled to (butt-jointed to) an end of the optical fiber, a coupling loss is caused in the butting portion due to the difference in the spot sizes of the light waves of the optical waveguides.
Typically, the spot size (mode diameter) of a light wave emitted from a semiconductor optical device is about 1 &mgr;m, while the spot size of a single mode optical fiber is about 5 &mgr;m. When these devices are butt-jointed to each other, the coupling loss amounts to about 10 dB.
Heretofore, a method for reducing loss due to coupling a semiconductor optical device to an optical fiber has generally employed the use of a lens to cause the spot sizes to coincide with each other.
However, when the devices are optically coupled by the use of the lens or the like, assembly is difficult because tolerance is tight for aligning a lens, which disadvantageously raises the cost of the module including the lens. More particularly, recently the use of modules incorporating the semiconductor laser diode is rapidly spreading from basic transmission systems to subscriber systems, LAN (local area network) systems, data link systems and the like, and these systems require a large number of inexpensive modules.
As described above, since difficulty in the assembly process is the main cause of the high cost of a module incorporating the semiconductor laser diode, it is desirable that the assembly process be facilitated by reducing the number of components and by passive alignment. Thus, various optical coupling devices for optical coupling and a laser light source on which the optical coupling device and the semiconductor laser diode are integrated have been developed.
FIG. 1
shows an example of a laser light source on which a conventional optical coupling device and a semiconductor laser diode are integrated and the relationship of the coupling loss to the device length. The optical coupling devices introduced in cited references A-F shown in
FIG. 1
are each provided with a tapered waveguide of hundreds of &mgr;m in length for converting the spot size. For converting from one guided mode to another guided mode, these devices are adapted so that the width of the waveguide is gradually narrowed (tapered). so as to thereby reduce coupling loss.
As shown in
FIGS. 2 and 3
, the conventional optical coupling device comprises: substrate
13
; first waveguide
11
which is a wide straight waveguide buried in substrate
13
; and second waveguide
12
in which the end in contact with first waveguide
11
is the same width as first waveguide
11
and that width is gradually narrowed toward the other end of minimum width. In these drawings, L denotes the length of second waveguide
12
, W
1
denotes the width of the widest portion, W
2
denotes the width of the narrowest portion, n
1
denotes the refractive index of substrate
13
, and nc (>n
1
) denotes the refractive index of first waveguide
11
and second waveguide
12
.
The above-described conventional optical coupling device has a problem in that an attempt to reduce the cost by shortening the device length increases the coupling loss as shown in FIG.
1
.
In some laser light sources on which the optical coupling device and the semiconductor laser diode are integrated, an optical coupling portion is also used as an active layer (for example, M. Kito et al., OECC '96 Technical Digest, PP. 574-575, 1996). In this case, the longer the tapered waveguide is at the same device length, the more the gain of the semiconductor laser diode is reduced. This caises a problem in that the threshold current or operating current of a laser diode in a high-temperature environment is increased.
That is, in order to reduce the cost of the module and prevent the operating current from increasing, it is necessary to shorten the above mentioned optical coupling device and to reduce the coupling loss.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide an optical coupling device which has less coupling loss and an optical coupling method which reduces the cost of a module by shortening the device length.
In order to achieve the above object of optically coupling two optical waveguide devices differing in the spot size of light wave, the optical coupling device converts the light wave into a light wave of a desired spot size using the interference between the radiation mode and the guided mode which occurs in the optical waveguide. One of two optical waveguide devices has a first optical waveguide and a second optical waveguide, the end of which is joined to the end of the first optical waveguide, whose width or thickness is different from the width or thickness of the first optical waveguide, and the length is set to the length at which a light wave of the desired spot size can be obtained.
In the optical coupling device as described above, the radiation mode and the guided mode are allowed to interfere with each other in the optical waveguide, whereby the field distribution in the radiation mode is varied depending on the position of the optical waveguide. The length of the optical waveguide can be appropriately selected to obtain the desired spot size. Moreover, even if the selected length of the optical waveguide is short, coupling loss can be reduced compared with the prior art.
The above and other objects, features, and advantages of the present invention will become apparent from the following description with references to the accompanying drawings which illustrate preferred embodiments of the present invention.
REFERENCES:
patent: 5261017 (1993-11-01), Melman et al.
patent: 6-174982 (1994-06-01), None
patent: 7-74396 (1995-03-01), None
patent: 8-37341 (1996-02-01), None
patent: 8-125279 (1996-05-01), None
patent: 8-171020 (1996-07-01), None
patent: 8-330671 (1996-12-01), None
patent: 10-90537 (1998-04-01), None
patent: 10-332966 (1998-12-01), None
Y. Itaya, et al., “Spot-size converted laser (SSC-LD)”, Proceedings by the Institute of Electronics, Information and Communication Engineers, SC-2-1, 1996, pp. 431-432.
M. Kito et al., “Narrow beam divergence of 1.3 um MQW lasers with tapered active strips”, First Optoelectronics and Communications Conference (OECC '96) Technical Digest, Jul. 1996, pp. 574-575.
Nakamura, et al., “High Efficiency 1/3 um Strained Multi-quantum Well Lasers Entirely Grown by MOVPE for Passive Optical Network Use” LEOS '96 Technical Digest, MA-2, 1996, pp. 8-9.
T. Yamamoto et al., “Low threshold current operation of 1.3 um narrow beam divergence tapered-thickness waveguide lasers”, Electronics Letters, Jan. 2, 1997, vol. 33, No. 1, pp. 55-56.
Aoki, et al., “Wide-Temperature-Range Operation of 1.3-um Beam Expander-Integrated Laser Diodes Grown by In-Plane Thickness Control MOVPE Using a Silicon Shadow Mask”, IEEE Photonics Technology Letters, vol. 8, No. 4, pp. 479-481, Apr. 1996.
Uda, et al., “Spot-size Expanded High Efficiency 1.3 um MQW Laser Diodes with Latterally Tapered Active Stripe”, IPRM '97 Technical Digest, pp. 657-660 (1997).
Henry Jon
McGinn & Gibb PLLC
NEC Corporation
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