Optical waveguides – With optical coupler – Particular coupling structure
Reexamination Certificate
2001-05-09
2004-07-06
Ullah, Akm Enayet (Department: 2874)
Optical waveguides
With optical coupler
Particular coupling structure
C385S088000
Reexamination Certificate
active
06760520
ABSTRACT:
TECHNICAL FIELD
The present invention relates to optical devices and the alignment thereof. More particularly, the present invention relates to a system and method for passively aligning and coupling optical devices with an external mode size transformer.
BACKGROUND OF THE INVENTION
Optical communications systems offer the potential for widespread delivery of broadband services to businesses and residences if the cost of optical termination can be reduced. An optical communications system can comprise an optical transmitter, a fiber optic transmission line, and an optical receiver. For typical telecommunication applications, the transmitter is comprised of a direct-modulated semiconductor laser diode
100
as illustrated in FIG.
1
. The transmission line is single-mode fiber and the receiver is a semiconductor photo-diode (not shown). In recent years the cost of single-mode fiber has dropped dramatically relative to that of metallic transmission media, but the cost of the opto-electronic devices used in the transmitters and receivers are still expensive relative to their electronic counterparts.
In addition to the cost of opto-electronic devices themselves, a time-consuming and costly step in the manufacture of laser transmitter modules involves the alignment of the fiber optic pigtail
110
(the short fiber optic lead that protrudes from the module package for optical interconnection) with the laser diode facet
115
(the point on the edge of the diode from which the laser beam emerges). For optimal transfer of laser energy from the facet
115
to the fiber core
120
(coupling efficiency), the tip
125
of the fiber pigtail
110
typically is precisely positioned with respect to the laser facet
115
prior to fastening and encapsulation in the module package. This is usually done using active alignment techniques, in which the laser diode
100
is powered up and the output optical power is monitored via the pigtail
110
while manipulating the pigtail tip
125
in up to five dimensions—transverse A, lateral B, longitudinal C, and axial pitch D and yaw E (FIG.
1
). This step distinguishes opto-electronic device packaging from semiconductor device packaging, and is considered the prime factor impeding further cost reduction in laser module production. Passive alignment techniques aim to eliminate this active alignment step in the packaging process.
Passive alignment of laser to fiber using purely mechanical means can be complicated by the extremely tight tolerance required to achieve adequate coupling efficiency. The reason for this tight tolerance is because of the small size of the optical mode of the laser (optical mode refers to the spatial distribution of electromagnetic energy of the fundamental mode propagating from the laser waveguided region through the core of the single-mode fiber). Typical semiconductor lasers have mode sizes of approximately 1 micron (&mgr;m) and emission angles of more than 30° in the transverse dimension; furthermore, the emission beam is an expanding elliptical cone
130
spreading more in the transverse direction than in the lateral direction. The fiber core
120
, on the other hand, supports a weakly guided optical mode that is typically 8-10 &mgr;m in size and circular in shape with a 6-7° acceptance angle. If the fiber pigtail
105
is simply butt-coupled to the laser
100
, the modal mismatch results in poor coupling efficiency. Coupling efficiency can be improved by fabricating a conical or spherical lens
135
directly onto the tip of the fiber; the lens alters the mode size and acceptance angle of the fiber
135
so as to better match that of the laser facet
115
. However, this increased coupling efficiency occurs at the expense of tighter alignment tolerance. The sub-micron tolerance needed to passively align a tensed fiber is tighter than can be reliably maintained with current automated assembly equipment.
There has been published investigation into the use of mode size transformers to address the current difficulties of passive alignment. A conventional mode size transformer (also called spot-size converter or mode expander) employs a specially tapered waveguide structure that increases the mode size of the laser to match that of the fiber. In doing so, larger alignment tolerances are possible with good coupling efficiencies, offering a way to lower packaging cost. Concurrently, the use of conventional micromachined silicon submount technology (also called silicon optical bench or silicon platform) for mechanical positioning of fibers with respect to other optical or opto-electronic devices is rapidly advancing for passive alignment and automated packaging of integrated optics (integrated optics refers to the combining of multiple optical elements within a single module by waveguiding light between them). A common and conventional silicon submount structure can comprise a V-groove having dimensions suitable for holding a fiber with precise lateral, transverse and axial position limited only by the +/−1 &mgr;m tolerance of fiber corto-cladding concentricity. Employing a mode size transformer that provides good laser-to-fiber coupling even when the laser and fiber are misaligned up to 1 micron from their ideal alignment, then, would permit the use of silicon submount technology for laser-to-fiber passive alignment This is the goal of the conventional published mode size transformer investigation.
Different conventional approaches have been investigated for monolithically integrating a mode size transformer into the semiconductor layers of a laser diode. A conventional expanded mode laser design can include a laser waveguided (active) region that is laterally tapered and built on top of a uniform weak passive waveguide layer. Adiabatic mode expansion between the active and passive layers of this structure can result in a significant reduction of the transverse emission angle of the laser, thus permitting much greater laser-to-fiber coupling efficiency. Three conventional different integrated mode transformer structures have been evaluated: one with a transversely tapered waveguide, one with a laterally tapered waveguide, and one comprising a small cross section of an active layer. Some conventional structures employ a shape in the laterally tapered active region that has been optimized. However, a disadvantage of the conventional integrated mode size transformer is the greater complexity and cost of processing the laser diode wafer to incorporate these waveguide structures.
A tapered waveguide structure external to the laser diode can also function as a mode size transformer. Tapered polymer waveguides designed to perform mode transformation between the laser facet and the optical fiber have been described in the conventional art. Such structures can include two laterally tapered waveguide layers stacked one on top of the other. One of the waveguide layers can form the input section that is optimized for coupling with the laser facet and confining the fundamental mode; the input section up-tapers laterally to a larger output section that is matched to the mode diameter of the fiber core. The other waveguide layer can laterally taper from zero width to the width of the first layer at the output section; the first layer can support mode expansion in the transverse dimension between the input section and the output section such that the output mode profile is approximately circular (i.e. matched to the fiber mode profile). One advantage of this conventional tapered waveguide structure is its ease of fabrication using polymer materials. However, polymer materials have not been widely accepted in telecommunications applications due to concern over long-term optical stability of polymers. Silica and glassy materials are typically preferred over polymers for use in optical waveguides intended for telecommunications because of their robust environmental stability.
Accordingly, there is a need in the art for a method and system for efficiently coupling one optical device to another. More specifically, there is also a need in the
Marple Eric T.
Medin Michael W.
Teralux Corporation
Ullah Akm Enayet
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