Patent Description:
Fiber optics are used in a wide variety of applications. The use of optical fibers as a medium for transmission of digital data (including voice data) is becoming increasingly more common due to the high reliability and large bandwidth available with optical transmission systems. Fundamental to these systems are optical subassemblies (OSA) for transmitting and/or receiving optical signals. There is an on-going need to provide simplified platforms for OSAs that simplify optics and promote passive alignment while improving optical performance. The present invention fulfills this need among others. A prior art optical subassembly, on which the preamble of claim I is based. is disclosed in patent <CIT>. The subassembly includes a flexible wiring substrate with an aperture therethrough and wiring on one or both sides which is connected to: (i) a surface emitting laser on a first side of the substrate and aligned with the aperture; (ii) a driver integrated circuit; and (iii) a circuit board. A ferrule holder is connected to an opposite second side of the substrate and receives a ferrule containing a fiber which extends into the aperture.

According to the invention there is provided an optical subassembly according to claim <NUM>.

Referring to <FIG>, one embodiment of an optical subassembly (OSA) <NUM> of the present invention is shown. The OSA <NUM> comprises a fiber <NUM> having a first optical axis <NUM> and an interposer <NUM>. The interposer has first and second opposing (e.g. parallel) sides 101a. 101b, and defines an alignment aperture <NUM> extending from the first opposing side to the second opposing side. In one embodiment, the alignment aperture <NUM> receives the fiber and holds the fiber such that the first optical axis <NUM> is positioned perpendicular to the first and second opposing sides.

The interposer also defines traces <NUM> having first, second, and third contacts, 202a, 202c, 202b (see <FIG>). Referring to <FIG>, the first contact(s) 202a are configured for connection to the optical component, and, in one embodiment, are disposed about the perimeter <NUM> of the alignment aperture on the second opposing side. The second contact(s) 202c are configured for connection to a chip, and, in one embodiment, are disposed in the interior of the interposer. The third contact(s) 202b are configured for connection to a circuit board, and, in one embodiment, are disposed about the perimeter <NUM> of the interposer.

The OSA also comprises at least one optical component <NUM> mounted to the second opposing side and electrically connected to at least a portion of the first contacts. The optical component has a second optical axis <NUM> coincident with the first optical axis <NUM> of the fiber. The fiber <NUM> is directly coupled optically with the at least one optical component <NUM>. Referring to <FIG>, the OSA comprises further at least one chip <NUM> for operating the at least one optical component <NUM> (<NUM> in in <FIG>). The chip is mounted on either the first or second opposing side and is electrically connected to at least a portion of the second contacts. It should be understood that, although the second contact(s) 202c are depicted on the second opposing side of the interposer, if the chip were mounted on the first opposing side, then at least a portion of the second contacts would be disposed on the first opposing side. Additionally, in such an embodiment, the interposer may comprise vias for connecting the traces between the first and second opposing sides.

The OSA is described below in further detail and in connection with selected alternative embodiments.

An important element of the OSA of this disclosure is the interposer. An interposer functions as a substrate for optical, opto-electrical, and electrical components and provides interconnections to optically and/or electrically interconnect the optical/opto-electrical/electrical components. The interposer is rigid to support the optical and electrical components, and capable of being readily machined or etched. In one embodiment, the interposer comprises an insulating material to isolate electrical traces and contacts disposed thereon. In one embodiment, the interposer comprises a ceramic or glass. Alternatively, the interposer may comprise a semiconductor such as silicon. In one embodiment, the interposer comprises a material having essentially the same coefficient of expansion (COE) as the optical component and chip disposed thereon. (Silicon and ceramic have similar COEs. ) By matching the COE of the interposer to the components mounted thereon, the OSA is stable over a wide temperature range. This is particularly beneficial in applications in which sterilizing the OSA may be required such as in medical applications.

One feature of one embodiment of the interposer is an alignment aperture to align the fiber such that the fiber's optical axis is precisely positioned in the interposer and essentially orthogonal/ perpendicular to the interposer. As used herein, the term essentially orthogonal/ perpendicular means about <NUM>°, not precisely <NUM>° but for example <NUM>° +/- <NUM>° or so. In one embodiment, the aperture is configured to hold the fiber in a precise position relative to the interposer, thereby facilitating passive alignment of the fiber with respect to the optical component. Passive alignment is generally preferred as it facilitates manufacturability (as opposed active alignment which requires energizing the optical components and then aligning optical components to optimize optical coupling).

The alignment aperture may have different embodiments. For example, in <FIG>, the interposer is shown with a V-groove 204a for aligning the optical fiber. V-grooves are well-known for providing precise alignment for optical elements having a round cross-section such as an optical fiber. Referring to <FIG>, an alternative embodiment is shown in which the alignment aperture is a borehole 204b. Although the V-groove and borehole are disclosed herein, it should be understood that other alignment aperture configurations can be used, including, for example, any polygon shape providing at least three points of contact with the fiber (e.g., a square aperture, hexagon aperture, etc.).

In another embodiment, the alignment aperture is configured as a ferrule receiver or as a receptacle to receive a plug containing the fiber. In one embodiment, the first opposing side of the interposer may comprise a structure for inter-engaging mechanically with the plug. Although not shown, such a configuration may involve a ferrule receiver <NUM> such as that shown in <FIG> (and described below) and a connector comprising a ferrule from which a fiber protrudes so as to be received in the alignment aperture. Alternatively, the aperture may be configured to receive a ferrule containing a fiber. Those of skill in the art will appreciate other suitable alignment aperture configurations in the light of this disclosure.

In addition to the alignment aperture, other alignment features may be used such as alignment holes/alignment pins for ferrules (e.g. MT ferrules) or for aligning components on/under the interposer as is known to those of skill in the art in the light of this disclosure.

Another feature of one embodiment of the invention is direct coupling between the optical fiber and the optical component. As used herein, direct coupling means no light bending between the optical axis of the fiber and the optical axis of the optical component. Accordingly, in a direct coupling, there are no intervening optics/reflective/refractive surfaces to change the direction of light propagation between the optical axis of the fiber and the optical axis of the optical component. In other words, unlike many conventional OSAs, the OSA of the present invention does not have reflective surfaces between the fiber and the optical component. Such an embodiment simplifies manufacturing and provides a more robust/high integrity optical path between optical components of different OSAs. For example, referring to <FIG>, a conventional interconnection <NUM> between a transmitter <NUM> and a receiver <NUM> is shown. In this conventional interconnection, light bending <NUM> is required to change the direction of light propagation from the optical components <NUM>, <NUM> of the OSAs to the fiber <NUM>. The optical axes <NUM>, <NUM> of the optical components <NUM>, <NUM> are orthogonal/perpendicular to the OSAs' circuit boards <NUM>, <NUM> and fiber <NUM>, requiring periscope optics or light bending <NUM> to turn the light. No such light bending is required in the interconnection <NUM> of the present invention. Here, the transmitter <NUM> and the receiver <NUM> have optical components <NUM>, <NUM> having an optical axis <NUM> which is essentially parallel to the circuit boards <NUM>, <NUM> and coincident with the optical axis of the optical fiber <NUM>.

The direct coupling between the fiber in the optical component may have different embodiments. For example, in one embodiment, the optical fiber is butt coupled to the optical component as shown in <FIG>. A butt coupled interface provides a high integrity/low loss optical coupling. In one embodiment, the butt coupled interface involves physically contacting the end face of the optical fiber with the optical component. In another embodiment, no physical contact is made between the fiber end face and the optical component, thereby defining an airgap therebetween, as shown, for example in <FIG>. In such an embodiment, it may be beneficial to use an antireflective coating to reduce Fresnel losses. In yet another embodiment, it may be beneficial to use an expanded beam coupling between the fiber and the optical component. For example, in one embodiment, a gradient-index (GRIN) lens is disposed between the fiber end face and the optical component. Alternately, a converging lens may be formed on the fiber end face or otherwise disposed near the end face for focusing light. Still other embodiments will be known to those of skill in the art in light of this disclosure.

Another feature of one embodiment of the present invention is the disposition of both the optical component and the electronic chip needed to operate the optical component on the interposer. As used herein, the optical component may be any known or later-developed component that can be optically coupled to an optical conduit as described below. The optical component may be for example: (a) an optoelectric device (OED), which is an electrical device that sources, detects and/or controls light (e.g., lasers, such as vertical cavity surface emitting laser (VCSEL), double channel, planar buried heterostructure (DC-PBH), buried crescent (BC), distributed feedback (DFB), distributed bragg reflector (DBR); light-emitting diodes (LEDs), such as surface emitting LED (SLED), edge emitting LED (ELED), super luminescent diode (SLD); photodiodes, such as P Intrinsic N (PIN) and avalanche photodiode (APD); photonics processor, such as, a complementary metal oxide semiconductor (CMOS) photonic processor, for receiving optical signals, processing the signals and transmitting responsive signals, electro-optical memory, electro-optical random-access memory (EO-RAM) or electro-optical dynamic random-access memory (EO-DRAM), and electro-optical logic chips for managing optical memory (EO-logic chips)); or (b) a hybrid device which does not convert optical energy to another form but which changes state in response to a control signal (e.g., switches, modulators, attenuators, and tunable filters). It should also be understood that the optical component may be a single discrete device or it may be assembled or integrated as an array of devices. In one embodiment, the optical component is a surface emitting light source. In one embodiment, the surface emitting light source is a VCSEL. In one embodiment, the optical component is photo sensitive. In one embodiment, the photo sensitive optical component is a photodiode.

The optical component works in conjunction with one or more electronic chips. A chip as used herein refers to any electronic/semiconductor chip needed to facilitate the function of the optical component. For example, if the optical component is a transmitter, then the chip may be a driver, or, if the optical component is a receiver, then the chip may be a transimpedance amplifier (TIA). The required chip for a given optical component is well known in the art will not be described here in detail.

As mentioned above, one feature of the claimed invention is disposing both the optical component and its associated chip on the interposer. That is, rather than disposing the chip on the circuit board and electrically connecting the chip with the optical component on the interposer as is done conventionally, here, in one embodiment, the transmitter/receiver chip is disposed on the interposer in close proximity to the optical component. Such a configuration has a number of important benefits. First, because the chip is in close proximity to the optical component, the traces between the chip and the optical component are very short which facilitates high speed operation by reducing impedance. Additionally, disposing the chip on the interposer eliminates the need to place it on the circuit board where space is typically limited due to the need to reduce circuit board size.

The placement of the chip(s) on the interposer may be configured in different ways. In one embodiment, the chip is disposed on the same side as the optical component, i.e. the second side. Such configuration has the benefit of simplicity since the optical component of the chip can be electrically connected via surface traces alone. Alternatively, the chip may be disposed on the first side of the interposer, i.e., on the opposite side of the optical component. Such a configuration has the benefit of utilizing space on the first opposing side of the interposer which may be important if space on the second opposing side of the interposer is limited. In this embodiment, vias may be required to connect the chip on the first opposing side to its respective optical component on the second opposing side.

The configuration of the optical component(s) on the interposer may vary. For example, in one embodiment, the interposer comprises just a transmit or receive optical component. In this embodiment, the OSA may be part of a dedicated transmitter or receiver. Alternatively, the interposer may comprise both transmit and receive optical components and the OSA may be part of a transceiver. In this embodiment, the optical components may be disposed separately on the interposer, or, in one embodiment, they may be disposed in series. For example, referring to <FIG> (showing an arrangement not forming part of the invention), optical components in series are shown. In this arrangement, the interposer <NUM> defines an alignment aperture through which optical fiber <NUM> is disposed. A transmitting optical component <NUM> is disposed adjacent the optical fiber <NUM>. A second optical component, for example, a receiving optical component <NUM>, may be disposed on the interposer such that its optical axis is coincident with that of the transmitting optical component <NUM>. In this particular arrangement, the receiving optical component <NUM> is behind the transmitting optical component <NUM>, and thus, the signal received by the receiving optical component <NUM> passes through the transmitting optical component <NUM>. This requires that the optical components be configured such that the transmitting optical component is essentially transparent to the received signal. Those of skill in the art will understand how to configure the transmitting optical component to be essentially transparent to the received signal in the light of this disclosure. (See, for example, <FIG> and associated text.

As shown in <FIG>, the receiving optical component <NUM> is mounted on a second interposer <NUM>. The second interposer <NUM> comprises traces/contacts <NUM>, <NUM> that electrically connect the receiving optical component <NUM> with the traces on the interposer <NUM>. Separate contacts <NUM> are provided for electrically connecting the transmitting optical component <NUM> with its respective chip. As shown, in this embodiment, a chip <NUM> is disposed on the substrate which can be configured to operate one or more of the optical components. The advantage of this configuration is that both the transmitting and receiving optical components are disposed on a single interposer and are coupled to a single fiber. The second planar interposer <NUM> may define an aperture <NUM> that may receive the transmitting optical component <NUM> which may be a surface emitting light source. The receiving optical component <NUM> may be a photo-detection chip.

Referring to <FIG>, the concept of the interposer of <FIG> is shown in an exploded view of OSA <NUM> similar to OSA <NUM>. As shown, a transmitting optical component <NUM> is disposed on interposer <NUM>. Traces <NUM> connect the transmitting optical component <NUM> with its associated chip <NUM>. The receiving optical component <NUM> is disposed over the transmitting optical component <NUM> and supported by the second interposer <NUM>. The second interposer <NUM> has an orifice <NUM> to accommodate/receive the transmitting optical component <NUM>. The second interposer <NUM> also has traces <NUM> and vias (not shown) which are configured to interface with traces <NUM> to connect the receiving optical component <NUM> to its associated chip <NUM>. Third contacts <NUM> are disposed along the perimeter of the interposer for contact with corresponding circuit board contacts.

The transceiver arrangement of OSA <NUM> simplifies installations. For example, referring to <FIG>, a schematic is shown in which the OSA <NUM> of <FIG> is connected to another OSA <NUM>' via optical fiber <NUM>. As mentioned above, such a connection does not require any additional optics/light bending to facilitate contact between optical connection between the two OSAs. Moreover, the OSAs <NUM>, <NUM>' facilitate bi-directional communication over a single fiber <NUM> according to a specific embodiment. For example, the receiving optical components <NUM>, <NUM>' of the OSAs <NUM>, <NUM>' are sensitive over a broad range, which also covers the wavelength of the transmitting optical components <NUM>, <NUM>'. The transmitting optical component <NUM>, <NUM>' transmit at different wavelengths, wherein the transmitting optical component <NUM> is transparent at the transmitting wavelength of the transmitting optical component <NUM>', and the transmitting optical component <NUM>' is transparent at the transmitting wavelength of the transmitting optical component <NUM>. For example, in one arrangement, the transmitting optical component is essentially transparent to received light having a wavelength around <NUM>. In this arrangement, the transmitted signal may have a significantly shorter wavelength, e.g., around <NUM>. This way, the receiving optical component <NUM> can receive the signal from the transmitting optical component <NUM>', even though the transmitting optical component <NUM> is right in front of it because the transmitting optical component <NUM> is transparent to the signal from the transmitting optical component <NUM> (and vice versa).

Still other arrangements are possible, for example, the chip is integrated with the optical component. In such an arrangement, it should be understood that there would not be any traces between the optical component and the chip as shown in <FIG>.

The fiber's integration into the OSA of the present invention may have different arrangements. According to the invention, referring to <FIG>, a fiber stub is shown disposed in the borehole of the interposer. Such an embodiment facilitates manufacturing as discussed in more detail with respect to <FIG>.

In another embodiment, the interposer comprises a ferrule-receiving fixture disposed on the first opposing side to receive a connector <NUM>. In one embodiment, the ferrule-receiving fixture <NUM>, such as a fiber alignment sleeve, has an axis coincident with the first optical axis and being configured to receive a ferrule <NUM> containing a terminated fiber <NUM> such that the terminated fiber optically couples with the fiber stub in interposer <NUM>.

In the arrangement shown in <FIG>, rather than a fiber stub, the alignment aperture may be configured to receive a longer length of fiber or even be configured as a connector to receive a plug. For example, referring to <FIG>, the interposer does not have a fiber stub but instead has a longer length of a fiber that is disposed in the interposer and extends beyond the side of the interposer. In still another arrangement, the fiber is terminated in a ferrule, which is then disposed in the interposer. In yet another arrangement, multiple fibers may be disposed in the interposer for a multifiber connection to the optical component(s). In this regard, although a single-fiber ferrule is shown in <FIG>, it should be understood that a multi-fiber ferrule, such as the MT ferrule, may be used in the interposer of the present invention according to other embodiments.

In one arrangement, one end of the optical fiber extends from the first opposing side 101a freely. In other words, although one end of the optical fiber may be held in a ferrule or borehole, the other end extends freely from the interposer allowing it to be bundled/routed as need be. For example, referring to <FIG>, the fiber <NUM> extends freely from the first opposing side of the interposer, allowing the fiber to be routed or terminated as need be. For example, in the arrangement of <FIG>, the fiber <NUM> from OSA <NUM> is terminated in another OSA <NUM> as described below.

The interposer of the present invention facilitates a variety of different OSA packaging configurations. First, because the optical component(s) and associated chips are disposed on an interposer and are not distributed between an interposer and a circuit board (as is traditionally done), the interposer of the present invention tends to be more modular, affording greater flexibility in manufacturing and packaging configurations. For example, the interposer may be disposed essentially orthogonal/perpendicular to a circuit board or parallel to the circuit board, depending on the application. As mentioned above, in one embodiment, the interposer comprises contacts along the perimeter of the interposer to facilitate connection to the circuit board. Although locating the second contacts along the perimeter of the interposer is preferred as it provides a convenient connection location to the circuit board, it should be understood that other embodiments exist. For example, island type connectors can be used to connect the interposer to a circuit board.

According to specific embodiments of the invention, the OSA may be embodied as a plug as or it may be integrated in a motherboard or backplane connector assembly. For example, referring to <FIG>, one embodiment of a OSA plug <NUM> is shown. In this embodiment, the interposer <NUM> is disposed essentially orthogonal/ perpendicular on a printed circuit board <NUM>. The fiber <NUM> extends from the plug such that its optical axis is essentially parallel with the printed circuit board. As described above with respect to <FIG>, such an embodiment simplifies connections between OSAs, thereby minimizing additional components and signal losses. Referring back to <FIG>, in this embodiment, the plug also comprises a potting structure <NUM> to contain the potting material which, in one embodiment, is applied over the interposer. In this embodiment, a soft overmold <NUM> is also applied to the assembly to facilitate handing and protect the circuit board. In this embodiment, a plug interface <NUM> is provided on the bottom of the plug. Referring to <FIG> and <FIG>, according to another embodiment, a plug assembly <NUM> similar to the plug <NUM> is shown. Here, the plug <NUM> contains the interposer <NUM> of the present invention with the fiber <NUM> extending essentially parallel to the circuit board as described above. On the bottom of the plug <NUM> are contacts 441a that are configured for connection to corresponding contacts 441b of a socket <NUM> as shown in <FIG>. The socket <NUM> comprises circuit board contacts <NUM> for contacting corresponding contacts on a circuit board. <FIG> shows the bottom side of plug <NUM> and the various traces and contacts for interfacing with the socket <NUM>. It should be understood that this plug embodiment is just one embodiment of many packages in which the interposer may be used.

<FIG>, shows a method for preparing the interposer in <FIG> is shown. In step <NUM>, an insulating substrate such as a ceramic wafer is provided. In step <NUM>, the alignment aperture is defined in the wafer. In this particular embodiment, the alignment aperture is a borehole which may for example be drilled through the wafer. In step <NUM>, a fiber stub is inserted in the borehole. In step <NUM>, a fritt ring is disposed between the fiber and the borehole to center the fiber in the borehole. In step <NUM>, the wafer is heated such that the fritt ring melts and flows into the borehole between the fiber and the wafer thereby holding the fiber in place. Next, in step <NUM>, the fiber is polished on both sides to be essentially flush with the first and second opposing sides of the wafer as shown. It should be noted that for some embodiments of the interposer, such as shown in <FIG>, this step <NUM> could be omitted where the fiber stub of <FIG> is not applicable.

In step <NUM>, trace, contacts, and other features are deposited on the either side of the wafer as shown. It should be noted that, in this deposition step, not only are traces/contacts for the optical components/chips deposited, but also, in this embodiment, a connection for the female connector <NUM> is defined. Having these critical elements defined in the same deposition process is not only efficient, but also improves precision by avoiding tolerance buildup which can result from multiple deposition steps. In step <NUM>, the optical components/and associated chip are disposed on the contacts on the interposer, and a ferrule-receiving structure is added to the opposite side of the interposer. It should be understood, that this is only one embodiment of preparing interposer of the present invention. Those of skill in the art will appreciate many variations are possible within the scope of the claims.

Claim 1:
An optical subassembly (<NUM>) comprising:
an interposer (<NUM>) having first (101a) and second (101b) opposing sides and defining an alignment aperture (<NUM>) extending from said first opposing side (101a) to said second opposing side (101b), said interposer (<NUM>) defining traces (<NUM>) having first, second, and third contacts (202a, 202c, 202b), said first contacts (202a) being configured for electrical connection to at least one optical component (<NUM>, <NUM>), said second contacts (202c) being configured for electrical connection to at least one chip (<NUM>), and said third contacts (202b) being configured for electrical connection to a circuit board;
at least one fiber (<NUM>) disposed at least partially in said alignment aperture (<NUM>) and having a first optical axis (<NUM>), said fiber (<NUM>) being held such that said first optical axis (<NUM>) is positioned essentially orthogonal to said first (101a) and second (101b) opposing sides;
said at least one optical component (<NUM>, <NUM>) mounted to said second opposing side (101b) and being electrically connected to at least a portion of said first contacts (202a), said at least one optical component (<NUM>, <NUM>) having a second optical axis (<NUM>) coincident with said first optical axis (<NUM>);
said at least one chip (<NUM>) for operating said at least one optical component (<NUM>, <NUM>), said at least one chip (<NUM>) being mounted on said first (101a) or second (101b) opposing side and electrically connected to at least a portion of said second contacts (202c); and
said circuit board configured to receive said interposer (<NUM>) such that said interposer (<NUM>) is essentially orthogonal to said circuit board, said circuit board being electrically connected to at least a portion of said third contacts (202b),
said fiber (<NUM>) being a fiber stub (<NUM>);
characterized in that:
(i) said interposer (<NUM>) is rigid and planar;
(ii) said fiber stub (<NUM>) is polished to be flush with said first (101a) and second (101b) opposing sides.