Fiber Optic Connector Housing

This invention concerns semiconductor laser mounted in a connector housing for illuminating the angled end of an optical fiber in a connector for minimized reflection interference. A connectorized photodiode is also used for receiving signals from an optical fiber.

This invention provides a low cost RF signal an electrical to optical connector housing for receiving a connector with an angled end of an optical fiber, such as a standard angled physical contact (APC) connector. An RF modulated distributed feedback semiconductor laser is mounted in the housing with a lens adjacent to the laser for directing light to an aperture plane in the housing. The laser and the lens are offset from the axis of the housing so that light from the laser passes through the aperture plane at an acute angle relative to the axis of the housing. Furthermore, the laser and lens are spaced so that the area of the aperture plane illuminated by the laser is larger than the end of an optical fiber. An APC connector can be mounted in the housing with the angled end of its optical fiber in the aperture plane for illumination by the laser.

These and other features and advantages of the invention will be appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein:

FIG. 1 illustrates in perspective an electrical to optical connector housing;

FIG. 2 is a schematic diagram of such an electrical to optical connector housing;

FIG. 3 is a longitudinal cross section through such a connector housing;

FIG. 4 is a transverse cross section through an optical isolator for use in the housing of FIG. 7; and

FIG. 5 is a schematic diagram of a photodiode in a connector housing.

An important feature for making a low cost reliable fiber optic RF signal distribution system comprises the laser and optical fiber connection. At least one end, and preferably both ends, of each optical fiber in the star configuration includes a conventional APC connector. A typical APC connector has a metal body surrounding a cylindrical ceramic plug through which the end of the optical fiber passes. A threaded nut on the APC connector removably fastens it to a connector housing. The details of the APC connector are not illustrated herein since they are conventional and well known in the art. The ceramic plug of such a connector is illustrated schematically in FIG. 2.

In an APC connector, the end of the optical fiber is flush with the outermost end of the ceramic plug. In conventional use of an APC connector, good optical coupling is provided between the end of the optical fiber and an adjacent similar fiber by physical contact between the ends of the ceramic plugs and hence the ends of the optical fibers. By avoiding an air gap between the fibers, there is good optical coupling. To assure that there is good physical contact between the ends of the optical fibers, the ends of the ceramic plugs are made with a very slight curvature instead of being plane. The curvature has a very large radius so that the end of the fiber is nearly flat. Furthermore, the center of curvature is offset from the axis of the fiber so that there is an acute angle (about 8.degree.) between the axis of the fiber and a radius to the center of curvature. All such connectors have a similar angle so that the contact between the ends of the optical fibers is angled relative to the fibers. This type of construction is the origin of the designation APC, "angled physical contact" connector.

An APC connector is one example of a connector useful in this invention. As will become apparent, other connectors with an angled end on the fiber may also be used. "Angled end" means an end plane which is not normal to the axis of the fiber.

Low cost in the fiber optic RF signal distribution system is achieved by employing a connector housing containing a laser and optical system for direct optical coupling between the laser and a connector with an angled end on the optical fiber that is plugged into the connector housing. Such a housing is illustrated in perspective in FIG. 1.

A typical connector housing for an APC connector has a metal body 41 with laterally extending flanges 42 for bolting the housing to the chassis of the RF signal distribution hub or an antenna station. The outer end of the body is threaded for receiving the nut on a standard APC connector. A slot 43 in the threaded end forms a key way for receiving a raised key on the body of an APC connector for correctly aligning the angled ceramic plug in the connector. A tight fitting split sleeve 44 in the body assures concentricity of the ceramic plug of the connector with the optical system in the connector housing. Such an arrangement for receiving a standard APC connector is conventional.

The connector housing may have a significantly different exterior. For example, instead of flanges for bolting to a housing, the connector half may have male threads for receiving a nut for fastening to a housing in the same manner as conventional panel mounting connector halves.

As illustrated in FIG. 3, in the connector housing provided in practice of this invention, there is a hollow insert 46 inserted into the body and laser welded to the body. The insert has a passage from its hollow interior to an end referred to herein as an aperture plane 48. A stop surface 49 is immediately adjacent to the perimeter of the passage at the aperture plane. The stop surface engages the end of the ceramic plug (not shown in FIG. 3) of a standard APC connector when it is inserted into the housing. The stop surface assures that the end of the optical fiber in the plug is located very close to the aperture plane. The split sleeve 44 engages an external surface on the insert 46 as well as the ceramic plug to keep the plug concentric with the insert.

A base 51 includes an upstanding sleeve 52 which fits over the outside of the body insert and is secured thereto by a plurality of spot welds or circumferential laser weld 53. As explained hereinafter, this fit between the sleeve and insert are used for adjustment during assembly of the connector housing.

A header 54 is secured in the base 51, typically by press fitting. Four metal pins 56 extend through glass seals in the header for making electrical connections to components inside the connector housing.

A lid 57 is hermetically sealed on the header to form a laser subassembly. A rigid post 58 within the laser subassembly supports a semiconductor laser 59. In the embodiment illustrated in FIG. 3, a photodiode 61 is cantilevered from another post and is directly behind the laser. Other electrical components are formed on a substrate 62 on the rigid post.

Typical electrical connections are illustrated schematically in FIG. 2. The laser is connected to a DC bias by an inductor 63 and is also connected to ground. An RF modulating signal is also connected to the laser by a resistor 64 and capacitor 66. The inductor, capacitor and resistor tend to isolate the RF signal and the DC bias from each other. The DC bias, RF modulating signal and electrical ground use three of the four pins through the header.

The fourth pin may be used for either a photodiode 61 as illustrated in FIG. 3 or a thermistor 67 (FIG. 2). Either of these additional components may be used as desired for a particular application. When a photodiode is used, a portion of light through the back face of the laser illuminates the photodiode. The photodiode output may be used in an external feedback circuit for adjusting the DC bias current for controlling laser performance. Alternatively, a thermistor which is mounted on the bottom of the laser subassembly monitors the temperature of the connector housing. The thermistor signal may be used in an external feedback for controlling temperature or adjusting the DC bias for compensating changes in laser performance as a function of temperature.

The front of the electrical to optical converter module has at least a center portion that is transparent and includes a lens 68. The lens and laser are aligned for appropriately illuminating the aperture plane with light from the laser. The correct positions of the laser and lens are determined during assembly of the laser subassembly. The sequence of assembly of the laser module into the connector housing fixes the least critical positions first and the more critical positions last.

The laser module is assembled first. A laser 59 is mounted on the post 58 in its correct position off of the axis of the header 54. The cover 57 is then hermetically sealed onto the header with the lens coaxial with the header. The header is then pressed into the base 51. A subassembly of insert 46, sleeve 52 and base with laser module is then loosely assembled. The insert and sleeve are welded together at the appropriate telescoped spacing for placing the focus of the light from the laser near, but not in, the aperture plane for the desired illumination of the aperture plane. Finally, the base is positioned so that the lens is in an appropriate position off the axis of the insert and the base is welded to the sleeve.

This insert assembly is then inserted into the body of the connector housing and spot welded in place. This position is not critical since the optical fiber is positioned relative to the insert 46 by the stop surface 49 and split sleeve 44. The angular orientation of the insert subassembly and the body 41 also has a loose tolerance and the position of the pins 56 relative to the slot 43 is adequate for alignment.

Although this sequence of aligning and welding the parts together is preferred, a different sequence could be used. Parts of the connector can be assembled with epoxy bonding or the like instead of welding.

As can be seen by the schematic illustration of FIG. 2, the laser 59 and lens 68 are not aligned with the axis of an optical fiber 69 extending through the ceramic plug 71 of a standard APC connector (indicated schematically). Instead, the laser and lens are offset at an angle that assures that light enters the end of the fiber in an axial direction after refraction at the angled end. It is for this reason that the laser, lens and interconnecting structures are carefully oriented relative to the key way of the connector housing which positions the connector and hence ceramic plug in its correct orientation.

The semiconductor laser used for the low cost RF signal distribution system is preferably a distributed feedback (DFB) laser. DFB lasers are conventional semiconductor devices. Many semiconductor lasers use cleaved output and back facets to obtain the feedback necessary for lasing. Feedback can also be obtained by periodic variations within the lasing cavity. Typically, this is produced by corrugating the interface between two adjacent cladding layers. The periodic ridges and grooves of the corrugated portion form a diffraction grating so that constructive interference due to the grating provides feedback distributed along at least a portion of the cavity length. As a result, the DFB laser has a single longitudinal mode output. The single output frequency (actually a narrow band of frequencies) is determined by the periodicity of the grating.

A DFB laser is preferred in the low cost transceiver system since it has the lowest noise of semiconductor lasers. A DFB laser, however, is more sensitive to external reflections than a Fabry-Perot laser. The optical system in the connector housing is therefore designed to minimize reflections. This design includes non-axial alignment of the laser and lens and "loose coupling" of the light from the laser to the optical fiber.

An angled end on the optical fiber such as provided in an APC connector is employed with the DFB laser so that illumination from the laser is not normal to the end of the ceramic plug and fiber. Thus, specular reflections from the end of the plug and fiber cannot reenter the laser.

The spacings of the laser, lens and aperture plane are fixed during assembly of the connector housing so that light from the laser is deliberately not focused on the aperture plane. The alignment is set so that about three to five percent of the light from the laser actually enters the core of the fiber. This is a result of two effects. One is that with the defocusing of the illumination at the aperture plane, the wave fronts encountering the end of the fiber are somewhat spherical instead of flat. Only the "flat" component of the spherical wavefront successfully traverses the fiber. Secondly, the area of the ceramic plug illuminated is larger than the area of the end of the fiber core and only light that illuminates the end of the fiber core can enter it.

Another way of loosely coupling light from the laser into the fiber would be to have the light from the laser illuminate the angled end of the optical fiber at an angle that is not optimum for refraction into the fiber in the axial direction. Flooding the end of the fiber with light that is not focused at the aperture plane, but which is at the correct angle for refraction into the fiber, is preferable since the proportion of light coupled into the fiber remains substantially unchanged if the fiber is displaced laterally within the illuminated area. Thus, manufacturing and positioning tolerances are significantly relaxed as compared with a device where the laser light is focused on the core of the fiber.

Since only about three to five percent of the light from the DFB laser enters the optical fiber, any reflections or backscattering from the fiber which could reenter the laser have extremely low power levels relative to the power level of the laser. Such loose coupling of the illumination thereby avoids reflections from the fiber that would destabilize the DFB laser.

As an alternative to loose coupling of the laser output and the optical fiber input, one may employ an optical isolator and couple a higher proportion of the laser light into the fiber. Very small dimension and low cost optical isolators are becoming available. Such an optical isolator can be placed in a passage 47 in the insert 46 between the lens and aperture plane of the connector.

Such an optical isolator is illustrated in FIG. 4 and comprises a sandwich of a plane polarizer 72, polarization rotator 73 and another polarizer 74, positioned within a rare earth magnet 76. Each polarizer comprises a thin sheet of glass including a surface layer containing silver dipoles tuned to the wavelength of the light from the laser and stretch oriented for high quality polarization of the light beam. Such a polarizing material is available from Corning as Polacor. The rotator is a garnet type crystal sandwiched between the polarizers. The polarizers are set at 45.degree. with respect to each other in the optical isolator. Other polarizers may also be used.

A light beam from the laser is plane polarized by the first polarizer. The rotator then rotates the plane of polarization 45.degree. so that it is aligned with the second polarizer and readily passes therethrough. Any light reflected back toward the optical isolator passes through the first polarizer and is again rotated 45.degree. by the rotator. This places the plane of polarization of the reflected light 90.degree. from the polarization of the original light beam. When this rotated reflected light beam encounters the second polarizer, its passage is prevented.

It is important to use an angled end on the optical fiber, such as in an APC connector, for the connectorized laser module. When two APC connector halves are connected together, there is essentially no air gap between the ends of the fibers and there is good optical coupling. In addition, there is very low reflection from the fiber connection. When there is an "open" connector where a fiber end has a glass-air interface, reflected light will be coupled back into the fiber if the fiber has an end that is normal to the axis of the fiber. In this case, light that is backscattered from within the fiber or reflected from the far end will be-reflected from the near end and interfere with forward travelling light. That is avoided with an angled end on the fiber. The end of a fiber in the connector described herein has a glass-air interface and reflections within the fiber are inhibited by using an angled end face.

It is also desirable to employ an angled end face such as provided by an APC connector half at the output end of an optical fiber. In a low cost system it is also desirable to avoid having both halves of a connector in use at the output end of the fiber. Instead, one may use a fiber-air interface of an APC connector in combination with a connectorized photodiode. Such an arrangement is illustrated schematically in FIG. 5. Structurally the connector arrangement is similar to what has already been illustrated in FIGS. 1 and 3 and the details are omitted from FIG. 5.

At this output end of the fiber optic cable an optical fiber 169 extends through the ceramic plug 171 of a standard APC connector 81 indicated only as a dashed outline in the drawing. The APC connector is mated with a connectorized photodiode, the housing 82 of which is also indicated schematically in FIG. 5. This connector housing includes a lens 168 positioned close enough to an aperture plane 83 of the housing that light from the optical fiber encounters the lens despite being refracted at an angle from the axis of the fiber because of the angled end on the fiber. Preferably the lens is large enough or positioned close enough to the end of the fiber that the light is incident on a central portion of the lens where optical aberrations are minimized. The lens refocuses the light from the fiber toward the optical axis of the connector.

A photodiode 84 is located on the optical axis in or near the focal plane of the lens. Light from the fiber refracted by the lens is therefore directed onto the photodiode. Precise focusing on the photodiode or exact positioning on the optical axis are not critical. All that is required is that a maximum amount of the light from the fiber actually illuminates the photodiode.

This lack of criticality can be appreciated by recognizing that a connectorized photodiode may be employed without a focusing lens between the end of the fiber and the photodiode. All that would be needed is that the photodiode have a large enough area to receive most or all of the light from the fiber even though the light is off axis. It is preferred, however, to employ a lens instead of a large area photodiode since the capacitance of the photodiode increases with increasing area and such capacitance can limit the high frequency response of the photodiode.

It will be noticed that in a connector arrangement as illustrated in FIG. 5 it is not necessary to angularly orient the APC connector relative to the housing of the connectorized photodiode. A large enough lens can readily be provided on axis for illumination by the light from the fiber regardless of its angular orientation.

As an alternative arrangement, one may provide for angular orientation between the APC connector and the housing of the connectorized photodiode. In such an embodiment a small area photodiode may be placed off axis at a location where it will be illuminated by light from the fiber. Such an off axis photodiode can be a relatively small size for adequate high frequency response.

Thus, a suitable low cost fiber optic signal transmission system may comprise a connectorized laser module as illustrated in FIGS. 1 to 3, an optical fiber having an angled end at each end such as by having an APC connector at each end, and a connectorized photodiode as illustrated in FIG. 5. Thus, each end of the optical fiber has a fiber-air interface, but since both ends are angled relative to the axis of the fiber, reflections do not pose any problems. Such an arrangement is desirable for a number of reasons. Such a fiber optic cable is not "handed". In other words, either end of the cable can be the input or the output. Surplus connector halves can be avoided by reason of the connectorized laser module and connectorized photodiode. The system is low cost since low cost components are used. Installation is easy and does not require highly skilled labor.

Although described for particular utility in an RF signal distribution system, it will be apparent that a connector housing with built-in DFB laser is useful in a number of other applications where a modulated semiconductor laser illuminates an optical fiber. Previously, low reflection laser modules have been constructed which have an optical fiber output. This optical fiber is connected to another optical fiber for long transmission distances by means of an APC connector or the like. Regardless of the excellent transmission characteristics of APC connectors, there is the cost of a pair of such connectors. In effect, applicants have provided a laser module with an output for direct coupling to an optical fiber connector instead of an optical fiber output. This permits connecting the optical fiber directly to the module instead of requiring an additional connector half.

Although described as an RF signal distribution system for communications outside of a system, it will be apparent that a cellular telephone system is merely exemplary of the applications. Such an optical fiber wireless signal distribution system may be used in mobile radio systems for vehicles moving around a warehouse, for example. Because of the broad bandwidth available in a short range system of this sort, it is ideally suited for mobile personal communications. Such a system may also be used as a broadband wireless local area network with or without communication outside of the local area network. Such a system may simply be used as a wireless PBX which permits communication with anyone within range, regardless of their location. Other wireless digital terminal equipment systems will be apparent.

Although the connectors for a fiber optic data transmission system have been described as standard APC connectors, it will be apparent that these are merely exemplary of connectors that may have an angled end on the optical fiber. Thus, in a standard APC connector the end of the fiber is not only angled relative to the axis of the fiber, the ceramic plug and fiber end have a slight curvature so that the ends of adjacent fibers are sure to be in physical contact. Such curvature is not important in practice of this invention since the ends of the fiber have a fiber-air interface instead of a fiber-fiber interface. A connector with a simple flat diagonal end is equivalent for practice of this invention. A standard APC connector with curvature may, however, be useful so that cables may be standardized and used with conventional connectors as well.

For such reasons, it will be apparent that many modifications and variations may be made and that this invention may be practiced otherwise than as specifically described.