Large tolerance fiber optic transmitter and receiver

An optical transmitter relaxes the tolerance between a source assembly and a fiber receptacle to facilitate passive alignment. The source assembly includes a light source and a lens. The lens is held at a fixed distance away from the light source using precise support structures typically formed by photolithographic processes. The fiber receptacle includes an optical element. The fiber receptacle is adapted to hold an optical fiber at a fixed distance from the optical element. The lens substantially collimates light from the light source into the form of collimated light. The optical element focuses the collimated light onto the aperture of the optical fiber.

This patent document is related to and hereby incorporates by reference the following co-filed U.S. patent application: Ser. No. 10/794,252, entitled “VCSEL with Integrated Lens,”.

BACKGROUND OF THE INVENTION

In a fiber optic system, a light source emits light pulses that travel through optical fibers to transmit data. The light source and the optical fiber must be accurately aligned to maximize the coupling efficiency. The coupling efficiency is a measurement of how much light transmitted by the light source is actually received by the optical fiber.

One of the methods used to achieve alignment between the light source and the optical fiber is known as active alignment. In active alignment, the light source is turned on while its aperture is aligned to the receiving end of the optical fiber. The light source and receiving end of the optical fiber are adjusted while the transmitting end of the optical fiber is monitored by a light detector. The light detector measures the amount of light passing through the optical fiber. When the light received is at its maximum, the light source and the optical fiber are at an optimal alignment, at which point the optical fiber and light source are fixed into place.

Active alignment is time consuming and therefore expensive. Thus, it is desirable to produce components that can be aligned in assembly without turning on the light source or using a light detector. Such a process is known as passive alignment.

Passive alignment has its own drawbacks. The apertures of the light source and the optical fibers are very small, and the focal lengths of the lenses impose their own strict requirements on the location of each component. For example,FIG. 1shows a prior art optical system51. The prior art optical system51includes a light source53, coupling optics55, and an optical fiber57. In conventional optical transmitters, the coupling optics55is a single unit having a first lens surface59and a second lens surface61. The first lens surface59has a focal length of F1. The second lens surface61has a focal length of F2. The coupling optics55receives light from the light source53and focuses it onto the optical fiber57. To achieve this, the optical axis of the light source53must be aligned with the optical axis of the first lens surface59, and the optical axis of the second lens surface61must be aligned with the optical axis of the optical fiber57. Furthermore, the light source53must be at a distance F1from the first lens surface59. Finally, the optical fiber57must also be at a distance F2from the second lens surface61.

The requirements of prior art optical system51leave very little tolerance during passive alignment. Consequently, expensive precision instruments are required to carefully measure, position, and place each component such that the light from the light source will be focused exactly on the target aperture of an optical fiber. Therefore, it is desirable to produce components that have greater tolerance so that passive alignment can be achieved with greater ease.

SUMMARY OF THE INVENTION

In a preferred embodiment, the present invention provides an optical transmitter with relaxed tolerances to facilitate passive alignment. The optical transmitter includes a source assembly and a fiber receptacle. The source assembly includes a light source and a lens. The lens is held at a fixed distance away from the light source using precise support structures typically formed by photolithographic processes. The fiber receptacle includes an optical element. The fiber receptacle is adapted to hold an optical fiber at a fixed distance from the optical element. The lens substantially collimates light from the light source into the form of collimated light. The optical element focuses the collimated light onto the aperture of the optical fiber.

This arrangement relaxes the tolerance between the source assembly and the fiber receptacle because a collimated beam produces a stable coupling efficiency throughout a substantial range of misalignment between the source assembly and the fiber receptacle.

Another embodiment of the present invention provides relaxed tolerances for passive alignment of an optical receiver. The optical receiver includes a fiber receptacle and a detector assembly. The fiber receptacle includes an optical element. The fiber receptacle is adapted to hold an optical fiber at a fixed distance from the optical element. The optical element substantially collimates light from the optical fiber into the form of collimated light. The detector assembly includes a lens and a light detector. The lens is held at a fixed distance away from the light detector using precise support structures typically formed by photolithographic processes. The lens focuses the collimated light onto the light detector.

Further features and advantages of the present invention, as well as the structure and operation of preferred embodiments of the present invention, are described in detail below with reference to the accompanying exemplary drawings. In the drawings, like reference numbers indicate identical or functionally similar elements.

DETAILED DESCRIPTION

FIG. 2shows a high level diagram of a preferred embodiment made in accordance with the teachings of the present invention. An optical transmitter11includes a source assembly13and a fiber receptacle15. The source assembly13includes a light source17and a lens19. The lens19has a focal length F1. Although lens19is shown as a single component, it should be understood that multiple lenses or lens systems could be used. Using precise support structures to be disclosed below in further detail, the lens19is fixed at a distance F1away from the light source17, putting the light source17at the focal point of the first lens19. Light emitted by the light source17is substantially collimated by the lens19and emerges as collimated light20. In some applications, collimated light20may slightly deviate from being perfectly collimated to achieve optimal system performance.

The fiber receptacle15includes an optical element21. The fiber receptacle15is adapted for coupling to an optical fiber23. The optical element21has a focusing surface22that focuses collimated light20from the light source onto the aperture of the optical fiber23. The optical element21has a focal length F2. Although optical element21is shown as a single component, it should be understood that multiple optical elements or systems of optical elements could be used. The optical element21is fixed by the fiber receptacle15at a distance F2away from the optical fiber23, putting the optical fiber23at the focal point of the optical element21.

The source assembly13and the fiber receptacle15are aligned upon a Z-axis, which coincides with the axis of light propagation. The X- and Y- axis define a plane perpendicular to the Z-axis.

The present invention relaxes the tolerance between the source assembly13and the fiber receptacle15by tightening the alignment within the source assembly13itself. The light source17is precisely aligned to the focal point of the lens19within source assembly13, typically by using a support structure formed by photolithographic processes to position the lens19. The fiber receptacle15is also designed to align the optical fiber23to the focal point of the optical element21. Since the distances for the focal lengths of the lens19and the optical element21are already fixed, the distance between the source assembly13and the fiber receptacle15along the Z-axis is not as critical.

The tolerance between the source assembly13and the fiber receptacle15is further relaxed by collimating the light between the source assembly13and the fiber receptacle15. Since the light is collimated (into collimated light20), the alignment of the source assembly13to the fiber receptacle15within the XY-plane is not as critical, either. If the alignment is slightly off, only a small amount of light is lost.

FIGS. 3A and 3Bshow a preferred embodiment of the source assembly13.FIG. 3Ais a top view andFIG. 3Bis a cross-sectional view taken along the line B-B′ ofFIG. 3A. The light source in the preferred embodiment is a vertical cavity surface emitting laser (VCSEL)26, although other light sources, such as edge-emitting diodes and other lasers, may also be utilized. The VCSEL26is formed on a VCSEL substrate27using standard VCSEL manufacturing techniques. The VCSEL substrate27is formed from any suitable semiconductor material. A standoff31supports a ball lens33(not shown inFIG. 3A) in front of the light-emitting surface of VCSEL26. The standoff31is formed upon the VCSEL substrate27using standard photolithographic materials and methods. The formation of standoff31and the attachment of the ball lens33are described in co-filed U.S. patent application Ser. No. 10/794,252, entitled “VCSEL with Integrated Lens,”. The standoff31positions the ball lens33such that the VCSEL26aperture is located at the focal point of the ball lens33. The ball lens33may be spherical in shape. Since the VCSEL26is positioned at the focal point of the ball lens33, any light emitted from the VCSEL26and passing through the ball lens33emerges as collimated light20.

Due to the precise nature of photolithographic methods, the standoff31can be fabricated within a tight tolerance to achieve accurate positioning in the XY plane as well as the Z-direction. For example, current photolithographic methods are accurate to within 2-3 micrometers. As a result, the VCSEL26can be closely aligned to the focal point of the ball lens33. In an actual working embodiment, standard photolithographic processes were used to deposit polyimide onto the surface of the VCSEL26and etch the polyimide into a ring shape, creating the standoff31for supporting the ball lens33. The standoff31is not limited to the shape of a ring—a wide variety of shapes are acceptable for supporting the ball lens33. Further, a wide variety of other materials and methods are available and may be used to create the standoffs31.

FIG. 4illustrates a preferred embodiment of the fiber receptacle15, shown in a cross-sectional diagram. The fiber receptacle15includes an optical element21. The optical element21is typically a component that has the functionality of an optical lens (such as lens surface35). The optical element21is formed from plastic, glass or any other suitable optical grade material. The fiber receptacle15is mechanically adapted to hold a fiber connector22on an optical fiber23with a relatively tight tolerance. There are many kinds of standardized fiber connectors, such as FC, SC, ST, LC, MT-RJ and MTP connectors, any of which would be suitable for use. The mechanical adaptation may be an interlocking mechanism on fiber receptacle15for latching onto or mating to the fiber connector22. The optical fiber23is held at a fixed distance from the lens surface35, such that the aperture of the optical fiber23is located at the focal point of the lens surface35when the fiber connector22is mated to the fiber receptacle15. Collimated light20that passes through the lens surface35is focused onto the optical fiber23. Although the fiber receptacle15is shown with a straight body for light propagation along a straight path, it is possible to manufacture the fiber receptacle15with one or more turns and reflecting surfaces so as to bend the light20in the desired directions. The fiber may also be directly attached to the light source with adhesive during the assembly process, such as in a pigtailed transceiver design.

FIG. 5illustrates an alternate embodiment for the source assembly13, shown in diagrammatic form. Here, a lens71is suspended over the VCSEL26by the standoff31. Lens71can be a diffractive or a refractive lens. Again, since the standoff31is formed using photolithographic techniques, the lens71can be positioned with great precision such that the VCSEL26is positioned at the focal point of the lens71, collimating the light from the light source. The formation of standoff31and the lens71are described in co-filed U.S. patent application Ser. No. 10/794,252, entitled “VCSEL with Integrated Lens,”.

FIG. 6illustrates a second alternate embodiment for the source assembly13, shown in diagrammatic form. The VCSEL shown here is a bottom-emitting VCSEL that can be flip-chip bonded to a header29. The header29can be a printed circuit board or another semiconductor substrate. The header29may include auxiliary circuitry such as a driver for the VCSEL. No standoff or supporting structure is required in this embodiment because a lens81is formed directly on the surface of a VCSEL substrate29. Lens81is formed using photolithographic processes. Typically, a layer of photopolymer is deposited onto the surface of the substrate29. The photopolymer is etched to create the desired shape of the lens81. The lens81can be formed directly from the substrate material as well, by performing an additional etching process to etch the shape of the lens into the VCSEL substrate27itself.

FIG. 7illustrates an alternate embodiment for an optical receiver90in an optical system, shown in diagrammatic form. The optical receiver90includes a fiber receptacle91and a detector assembly93. The fiber receptacle91includes an optical element97that is typically a component that has the functionality of an optical lens, such as lens surface99. The lens surface99has a focal length of F3. The optical element97is formed from plastic, glass or any other suitable optical grade material. Although the fiber receptacle91is shown with a straight path for light propagation, it is possible to manufacture fiber receptacle91with one or more turns and reflecting surfaces so as to bend the light in the direction desired.

The fiber receptacle91is mechanically adapted to hold a fiber connector94on an optical fiber95with a relatively tight tolerance. The mechanical adaptation may be an interlocking mechanism on the fiber receptacle91for latching onto or mating to a fiber connector94. The optical fiber95is held at a fixed distance F3away from the lens surface99, such that the aperture of the optical fiber95is located at the focal point of the lens surface99. Light emitting from the optical fiber95is collimated by the lens surface99and emerges as collimated light103.

Any of the source assemblies shown inFIGS. 3,5, or6can also be used as the detector assembly93by replacing the VCSEL27with a light detector105. The light detector105can be a photodiode, a phototransistor, or any other device that is responsive to incident light. The detector assembly93enables high speed applications, since the active area of detector105can be very small in this application, on the order of 10-30 micrometers in diameter.

The disclosed embodiments of the present invention can also be easily adapted to parallel optic applications. In an alternate embodiment for a parallel transmitter, a source assembly includes an array of light sources producing a light array, and an array of lenses positioned over the array of light sources to collimate the light array. A fiber receptacle includes an array of optical elements for receiving the collimated light array. The array of light sources may all be formed on a single die, but this results in a lower manufacturing yield since a larger die has a greater probability of having a defect. Manufacturing yields are improved by separating the light sources into separate dies that are subsequently packaged into an array.

In the prior art conventional designs, the parallel sources were subject to tight tolerances because it was difficult to align an array of light sources with an array of lenses. The present invention relaxes these tolerances because each light source can be individually formed with its own integrated lens, thus foregoing the alignment of a light source array to a lens array altogether.

In an alternate embodiment for a parallel receiver, a fiber receptacle includes an array of optical elements for collimating an array of light from an array of optical fibers. A detector assembly includes an array of light detectors, and an array of lenses positioned over the array of light detectors to receive and focus the collimated light array. The array of light detectors may be formed on a single die, or the light detectors may be separated into separate dies and subsequently packaged into an array.

Although the present invention has been described in detail with reference to particular preferred embodiments, persons possessing ordinary skill in the art to which this invention pertains will appreciate that various modifications and enhancements may be made without departing from the spirit and scope of the claims that follow.