LD module

Details LD module LD module

2001-04-06

2003-05-06

Lee, John D. (Department: 2874)

Optical waveguides

With disengagable mechanical connector

Optical fiber to a nonfiber optical device connector

C385S033000

Reexamination Certificate

active

06558048

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an LD (laser diode) module, in particular, a low-cost, small-sized and high-performance LD module which is utilized in optical communication systems.
This application claims the priority of Japanese Patent Application No.2000-147867 filed on May 19, 2000 which is incorporated herein by reference.
2. Description of Related Art
FIG. 1
shows a prior art LD module
1
which is sealed in a vertical metal case. This is one of the most prevalent LD modules at present. The transmitting (LD) module
1
has a metallic package. A circular metal stem
2
has a pole
3
erected near the center. An LD (laser diode)
4
is bonded on a side of the pole
3
. A top incidence type PD
5
is mounted with a light receiving surface upward at the center of the stem
2
. A cylindrical metal cap
6
is welded on the stem
2
for covering the LD
4
and the PD
5
. The cap
6
has a top opening
7
for guiding the LD light outward.
A metallic cylindrical lens holder
8
is welded outside of the cap
6
upon the stem
2
for supporting a lens
9
just above the LD
4
. A conical ferrule holder
10
is further welded upon the lens holder
8
for maintaining a ferrule
12
at an axial hole. The ferrule
12
holds an end of an optical fiber
11
. The LD
4
emits light from both ends in both directions. The LD light has a wide aperture with wide diverging angles. Front light emitted from the front end is converged and guided by the lens
9
to the end of the fiber
11
. The front light is signal light. The ends of the fiber and the ferrule are ground slantingly for prohibiting the light reflected at the end from returning to the LD and inducing instability of the LD oscillation. Such a metal packaged three dimensional LD module is still in prevalent, influential use at present. The metal case can hermetically seal the LD chips or the PD chip completely. The metal package prevents corrosive water or oxygen from invading into the package. The hermetic seal protects the module from oxidization or corrosion of the chips, the patterns or the wires. The metal case brings about a long lifetime and high reliability to the module. The metal package is a standardized one which reduces the part cost. The prior LD module of
FIG. 1
is still an excellent one.
Further, prevalence of the optical networks strenuously requires lower-cost and smaller-size optoelectronic modules than the metal packaged modules. An improvement of LD modules is a purpose of the present invention. The Inventors are interested in the structure of LDs (laser diodes). Conventional LD modules have commonplace, cheap LD chips having a uniform emission stripe with a constant width w and a constant thickness d extending in the longitudinal direction (z-direction). The emission stripe means a spatially-restricted, active layer which makes laser light by the stimulated emission induced by an electric current injection.
FIG. 2
shows a schematic structure of a conventional LD chip for showing only the active layer (stripe) clearly in a perspective view. This is a simple Fabbry-Perrow laser diode. The LD generates light power by inducing light by the injected current, amplifying the light in-phase by the repetitions of propagation in the stripe and reflections at the ends (mirrors) and leaking a part of the amplified light out of the (half-mirror) end. Some sophisticated LDs have light waveguides with a periodical saw-teeth shape upon the stripes for selecting the wavelength more strictly. The shape of the stripe itself is the uniform stripe same as the cheap one of FIG.
2
.
For example, an LD emitting 1.3 &mgr;m wavelength light has a 300 &mgr;m length, a 300 &mgr;m width and a 100 &mgr;m thickness. The active layer (emission stripe) of the LD chips has e.g., a 1.2 &mgr;m width, a 0.2 &mgr;m thickness and a 300 &mgr;m length (cavity length). The active layer is otherwise called a “cavity”, a “stripe”, an “emission stripe” or a “resonator”. All the terms signify the same matter. In the prior LD chip in
FIG. 2
, the width and the thickness of the active layer are constant in the full length. The length of the stripe determines the wavelength of the emitted light. The width and the thickness determine the aperture of the light. The LD light disperses in a wide angle. The aperture angle of the light emitted from the stripe is about 30 degrees to 40 degrees. The wide beam spread is one of the drawbacks of the current LDs. The LD wide beam dispersion sometimes induces difficulties.
An optical fiber consists of a core and a cladding. The core has a 10 &mgr;m diameter in a single-mode fiber for the 1.3 &mgr;m band light. The cladding has a 125 &mgr;m diameter. The LD active layer (w=1.2 &mgr;m, d=0.2 &mgr;m) is smaller than the fiber core (10 &mgr;m) in section. But almost all of the LD light escapes from the fiber and dissipates in vain due to the wide dispersion (30° to 40°) of the LD light, even if the fiber is brought into direct contact with the LD without gap. A lens, therefore, is indispensable for the coupling of an LD to a fiber.
FIG. 1
denotes the lens
9
of converging the dispersing LD light into the end of the fiber
11
for joining the fiber
11
to the LD
4
on an optimum condition. The LD module
1
utilizes a spherical lens having convex spherical curves on both surfaces as converging optics. Sophisticated LD modules employ aspherical lenses for enhancing the coupling efficiency further. Inexpensive LD modules use a ball lens for facilitating the assembly and reducing the cost. In any cases, the three-dimensional structure LD modules require converging optics.
The solid-structured LD module of
FIG. 1
is encapsulated in a metal package of a 5.6 mm outer diameter. The LD
4
is a common cheap chip with a uniform stripe as denoted in FIG.
2
. An active layer
14
is shown in a chip
13
in a perspective view. As described before, the inexpensive commonplace LD chips have the uniform active layer having a constant breadth w and a constant thickness d. The uniform stripe forgives the light dispersing in a wide aperture. LD modules require converging optics for coupling the LD to the fiber. The converging optics is a ball lens, a spherical lens or an aspherical lens. Low-power, inexpensive modules adopt ball lenses for joining the LD to the fiber.
A ball lens is inexpensive but has large aberration. The aberration prevents the ball lens from heightening the coupling efficiency up to the maximum. Conventional ball lenses allow the LD driving current of about 30 mA to produce an output power of 0.2 mW to 0.5 mW at another fiber end. Thus, the ball lens convergence optics are mainly employed for low-power, low-cost, low-speed and short-range optical communication networks.
On the contrary, high-power, long-range optical communication networks require aspherical lenses for the convergence optics. Aspherical lenses can suppress all kinds of aberration to low levels. The use of the aspherical lens enables the same 30 mA LD driving current to make a 1 mW to 2 mW output power at another fiber end. The coupling efficiency is enhanced several times by the aspherical lens. Aspherical lenses, however, are expensive. The aspherical lenses are used in mainstream fiber cable lines or equipment in central stations.
The ball lens optics and the aspherical lens optics are chosen by the criterion whether the module should be a low-cost, low-power one or a high-cost, high-power one. One purpose of the present invention is to provide a low-cost, high-power, small-size, high-value added LD module. Another purpose of the present invention is to provide an inexpensive ball-lens converging LD module with as high power as the expensive aspherical lens converging LD module. A further purpose of the present invention is to provide an inexpensive ball lens LD loaded module with higher coupling efficiency than the conventional ball lens loaded LD module.
Preparatory descriptions are required for showing the idea of the present invention. A novel type of laser diode has been pro

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