System and method for a packaging a monitor photodiode with a laser in an optical subassembly

A system and method of monitoring a laser output power and laser extinction ratio includes an optical subassembly physically placed between a point light source and a optical fiber device. The optical subassembly creates a convergent light beam by reflecting a collimated light beam from the point light source off an interior surface of a first side of the optical subassembly. The optical subassembly creates an incident ray of the convergent light beam by including on a second side of the optical subassembly a wedge-shaped air gap.

BACKGROUND

1. Technical Field

The system and method described herein relates to monitoring the average optical power output and the extinction ratio of a point light source.

2. Discussion of the Related Art

Optical telecommunication systems include the use of point light sources, e.g., lasers, to transmit information at high speeds through optical fibers. The threshold current and slope efficiency of typical point light sources vary due to age and changes in operating temperature. In order to control the average optical power output of the point light sources, photodetectors are placed in a control feedback loop to monitor the optical output of the laser. If the signal received by the photodiode should fall, for example, the electrical current supplied to the laser would be increased to compensate.

Point light source and monitor photodetector combinations may be mounted in a specially designed package which has a mounting base with insulated connector leads and a sealed cover. The cover may include a window of glass, or other transparent material over a central portion of the top such that the window is aligned with the emitting aperture of the point light source device. In some point light source and photodetector combinations, reflected light from the window of the glass is received by the photodetector. Because the light fluence or power in these systems is generally small and unfocused, large photodiodes are needed to gather enough light to provide a sufficient signal-to-noise ratio (SNR) to maintain the constant average optical output from the laser. Unfortunately, large area detectors have low electrical bandwidth, making them unsuitable for tracking the high speed modulation of the laser. Instead, they are limited to use as time-average power monitors.

Changes in the slope efficiency of the laser with temperature and age also affect the extinction ratio of the point light source output. The extinction ratio of a point light source is the optical power of the “one” state divided by the optical power of the zero state. In systems employing large area monitor photodetectors, the change in extinction ratio is generally ignored or corrected using a look-up table based on data obtained by characterizing lasers similar to those used in the system of interest. Alternatively, the superposition of a pilot tone, at a frequency within the bandwidth of the monitor photodetector, onto the data may be used to correct changes in the extinction ratio of the point light source. This approach, based on the principle that the amplitude of the received pilot tone is proportional to the amplitude of the data modulation, has the drawback of modulating the extinction ratio of the transmitted data as well, thereby introducing extra noise.

DETAILED DESCRIPTION

A present embodiment provides a system and method for packaging a monitor photodetector with a point light source in an optical communication system such that a portion of light from the point source is reflected and focused onto the monitor photodetector.FIG. 1illustrates an optical communications system according to an embodiment of the monitor photodetector packaged with a point source. The optical communication system2may include a point light source3, a photodetector4, a point light source driver5, an optical subassembly6, a receiving optical fiber device7, and an output port8.

The point light source3may be a laser. In one embodiment of the present invention, the point light source3may be a vertical cavity surface emitting laser (VCSEL). The point light source driver5may transmit an electrical signal, such as a current signal, to the point light source3to provide power for the point light source3. The point light source driver5and the point light source3may be placed in a closed feedback loop with a photodetector4to allow the photodetector4to monitor the average output power and the extinction ratio of the point light source3. The point light source driver5, the point light source3, and the photodetector4may all reside on a common substrate. In another embodiment, the point light source driver5may reside on a separate substrate from the point light source3and the photodetector4.

The optical subassembly6may be physically placed between the light point source3and the receiving optical fiber device7. In one embodiment of the present invention, the optical subassembly6may be composed of optically transparent plastic. In alternative embodiments, the optical subassembly may be made of a polycarbonate, such as LEXAN™, or a polyetherimide, such as ULTEM™. The optical subassembly6may assist in aligning a point light beam from the point light source3to the receiving optical fiber device7. In addition, the optical subassembly6may, where it is attached to the receiving optical fiber device7, include a wedge-shaped air gap to create a reflected incident ray of the point light beam. The incident ray of the point light beam may be transmitted back to a photodetector4. The incident ray of the point light beam may be focused by a lens and thereby received by the photodetector4at a relatively large fluence, allowing a small area photodetector4to simultaneously track changes in the extinction ratio as welt as changes in the average optical power over time. In one embodiment of the present invention, the lens used to focus the incident ray is the same as the lens used to focus the point light beam onto the core of the optical fiber device7.

The photodetector4may receive the incident ray of the light point beam producing a photocurrent that is proportional to incident optical power, i.e., watts. The photocurrent modulation amplitude and average value may provide feedback information to the point light source driver5which changes its output signal to the point light source3in response to the feedback information.

The light point beam received at the receiving optical fiber device7may be transmitted through the receiving optical fiber device7to an output port8. The output port8may be used to connect the receiving optical fiber device7to a transmission optical fiber device (not shown). In one embodiment of the present invention, the optical communication system2may be a packet switching device such as a network switch or a router, as illustrated in FIG.1. The packet switching device2may include a processor202, a physical interface card204, and a routing engine206. The physical interface card204may receive signals representing a plurality of packets at one of a plurality of input/output ports8210212214. The physical interface card204may forward the plurality of packets to a routing engine206. The routing engine206may receive the plurality of packets, decide the next step for the plurality of packets, and transmit the plurality of packets to a selected I/O port8of the plurality of I/O ports on the physical interface card204, wherein the selected port8of the plurality of I/O ports8210212214on the physical interface card204receives the plurality of packets. The selected port8of the plurality of I/O ports8210212214may utilize optical communication technologies so the electrical signals may need to be converted to optical signals. The point light source driver5may assist in converting electrical signals to optical signals by providing an output signal to the point light source representing the optical signals to transmit to represent the received plurality of packets. The point light source3may transmit a point light beam to the optical subassembly6, and the optical subassembly6may provide the incident light ray to the photodetector4to monitor the average output power and the extinction ratio of the point light source3. As discussed previously, the photodetector4may utilize the information from the incident ray to send average output power and extinction ratio data to the point light source driver5which controls the output of the point light source3.

FIG. 2illustrates an optical subassembly according to an embodiment of the present invention. The optical subassembly20may include a first side22, a second side24, and a third side26. The optical subassembly20may be composed of an optically transparent plastic, for example. A point light source10may transmit a collimated light beam12through the third side26of the optical subassembly20. An aspheric lens100may be attached to the exterior surface of the third side26of the optical subassembly20. The focal length of the lens100may be selected to produce an image spot with a numerical aperture matching the numerical aperture of an optical fiber core40. The chief ray of the collimated light beam12may enter the axis of symmetry of the aspheric lens100.

The collimated light beam12may travel through the aspheric lens100and third side26of the optical subassembly to an interior surface of a first side22of the optical subassembly20where it is completely reflected and becomes a convergent light beam14. The interior surface of the first side22of the optical subassembly20may be a total internal reflection (TIR) surface, which means the collimated light beam12is totally reflected to create the convergent light beam14. The convergent light beam may travel through a second side24of the optical subassembly20and into a receiving optical fiber device35. The receiving optical fiber device35may include an optical fiber core40and optical fiber cladding30. In the receiving optical fiber device35, the convergent light beam14may be transmitted only into the optical fiber core40. The optical fiber cladding30may enclose the optical fiber core40in relation to the optical subassembly20.

An alignment ferrule50may be used to precisely position the optical fiber device35at the focal point of the optical subassembly20. The alignment ferrule50may be attached to the optical subassembly20. Alternatively, the alignment ferrule50, the optical subassembly20, and the aspheric lens100may be manufactured as one part to minimize alignment inaccuracy. In this embodiment, the optical subassembly20, the alignment ferrule50, and the aspheric lens100may be made by an injection molding process using, for example, polycarbonate, polyolefin, or polyethylimide.

FIG. 3illustrates a second side of an optical subassembly and the receiving optical fiber device according to an embodiment of the optical communication system. The second side24of the optical subassembly20may be aligned with the optical fiber device35in a manner to create an air gap70between a section of the second side24of the optical subassembly20and a section y of the optical fiber device35. Illustratively, the air gap may be a wedge-shaped air gap. For example, as illustrated inFIG. 3, the wedge-shaped air gap70may be of a length that is equal to the width of the optical fiber core and two sections x of the optical fiber cladding30.

The creation of the wedge-shaped air gap70may create an incident ray16of the convergent light beam14in accordance with Fresnel reflection based on an index of refraction mismatch between air and the plastic of which the optical subassembly20is composed. In one embodiment, the incident ray16of the convergent light beam14may be reflected almost 180 degrees relative to the convergent light beam14. The angle between the incident ray and the convergent light beam is dictated by the angle of the wedge-shaped air gap70.

FIG. 4illustrates a path of an incident ray of the convergent light beam and a path of a reflected light beam according to an embodiment of the optical communication system. The incident ray16of the convergent light beam14may be directed to the interior surface of the first side22of the optical subassembly20. Because the interior surface of the first side22of the optical subassembly20may be a TIR surface, the incident ray16of the reflected light beam may completely reflect off the interior surface of the first side22of the optical subassembly20and become a reflected light beam18, as illustrated in FIG.4.

The reflected light beam18may travel through a third side26of the optical subassembly20. The aspheric lens100may be attached to the exterior surface of the third side26of the optical subassembly20. The lens100may focus the reflected light beam18onto a photodetector80. The lens100may bring the reflected light beam18to a focus so that the reflected light beam18is brought to a relatively large fluence. The focus of the reflected light beam18may align with the receiving area of the photodetector80to maintain the relatively large fluence. Because the reflected light beam maintains a relatively large fluence, a small area photodetector80may be used. This configuration may allow the photodetector80to track the high speed modulation of the reflected light beam18. The photodetector80may receive the reflected light beam18and determine the average output power of and the extinction ratio of the reflected light beam18, which should be equivalent to the average output power and the extinction ratio of the collimated light beam12from the point light source10(see FIG.2). The photodetector80may provide this information to the point light source driver5(seeFIG. 1) in order to correct any changes that may have occurred in the point light source's output power or extinction ratio due to age or change in temperature operating characteristics.

FIG. 5illustrates a multi-channel wavelength division multiplexer with fiber optic input according to an embodiment of an optical communication system. The multi-channel wavelength division multiplexer100may utilize a plurality of thin film filters (TFFs)102,104,106, and108in a “zig-zag” scheme to perform channel separation. The multi-channel wavelength division multiplexer, or transmitter,100may include an optical subassembly120, a glass plate130, a plurality of TFFs102104106and108, a plurality of point light sources140(only one shown), a first aspheric lens150, a plurality of second aspheric lenses142144146and148, a plurality of focusing optical subassemblies152154156and158, and a plurality of photodetectors180(only one shown). To simplify description, only one point light source140and one photodetector180operation are illustrated. Light beams from point light source140may be collimated, re-directed into a zig-zag optical path, as illustrated byFIG. 5, and finally coupled into an optical fiber core180. In an embodiment, the point light source140may be located in a position below the optical subassembly120and the plurality of focusing optical subassemblies152154156and158.

The plurality of point light sources140(rest not shown) and the plurality of TFFs102104106and108may have non-overlapping passbands, with each passband centered at the emitting wavelength of the corresponding point light source140.

A point light source140may be positioned beneath a second aspheric lens148. InFIG. 5, only one light beam (solid line), one point light source140, one reflected light beam (dotted lines), and one photodetector180are shown for clarity. The light beam emitted from the point light source140may pass through its corresponding second aspheric lens148. A focusing subassembly158corresponding to the second aspheric lens148redirects the collimated light beam into the glass plate130. The plurality of TFFs102104106and106may be attached to a bottom surface of the glass plate130as illustrated in FIG.5. The light beam may pass through the corresponding TFF108because the emitting wavelength of the point light source140corresponds to the passband of TFF108. The rest of the plurality of point light sources pass through the remaining corresponding TFFs102104106108, respectively, in a similar fashion. Because the plurality of TFFs102104106108have non-overlapping passbands, different wavelength components are extracted from the plurality of collimated light beams from the plurality of point light sources. Inside the glass plate130, the collimated light beam from point light source140(solid line) may travel in a “zig-zag” optical path while being reflected by the HR coated surface132and the remaining TFFs102104and106. The remaining TFFs102104and106may not allow the collimated light beam from point light source140to pass because the emitting wavelength of the point light source140does not correspond to the passband of the remaining TFFs102104and106. After leaving the glass plate130, the collimated light beam may be coupled into an optical subassembly120through the first aspheric lens150.

The collimated light beam may be directed through the aspheric lens150and through a third side200of the optical subassembly120to an interior surface of a first side204of the optical subassembly120. The interior side of the first side204may be a TIR surface. The collimated light beam may reflect off the first side204and become a convergent light beam which is directed off the first side204of the optical subassembly120through a second side202of the optical subassembly120and into the optical fiber core190. The optical subassembly120may be configured so that where the second side202of the optical subassembly connects to the optical fiber device190an air gap may exist. (not shown). In one embodiment, a wedge-shaped air gap creates an incident ray (dotted line) of the convergent light beam in accordance with Fresnel reflection based on an index of refraction mismatch between air and the plastic of which the optical subassembly120is composed. The incident ray of the convergent light beam reflects back towards the interior surface of the first side204. The interior surface of the first side204reflects the incident ray of the convergent light beam. The reflected light beam may pass through the third side200of the optical subassembly120and the first aspheric lens150to the glass plate130. The reflected light beam may travel in a “zig-zag” pattern alternately reflecting off the HR coated surface132and the plurality of TFFs102104and106(whose passband does not equal the emitting wavelength of the point light source140which originally generated the reflected light beam). If the reflected light beam is of the wavelength that is allowed to pass through the corresponding TFF, in this case TFF108, the reflected light beam (dotted line) travels through the associated TFF108, the associated focusing optical subassembly158, and the associated second aspheric lens148. The associated aspheric lens148focuses the reflected light beam onto the viewing portion of the photodetector180, which monitors the output power and the extinction ration of the point light source140.

FIG. 6shows a complete transceiver module according to an embodiment of the present invention. The complete transceiver module includes a dual fiber optic connector610, an injection-molded optical assembly650, a printed circuit board (PCB)630, and a metal shield for minimizing electromagnetic interference (not shown). Optical fibers10a,10bare connected to the dual fiber connector610. One optical fiber in the dual connector610is for the receiver, and the other one is for the transmitter. As described, the optical assembly650is preferably a one-piece injection-molded optical subassembly with a connector housing600. The dual fiber connector610slides into the connector housing600. The PCB630is aligned with the one-piece injection-molded optical subassembly in the optical assembly650. On the PCB630, there are laser sources, photodetectors, chips for processing electrical signals, other circuitry, etc. To aid the alignment, a ledge structure is provided in a plane parallel to the plane tangential to, and passing through, the apex of the aspheric lenses of the collimating and optical subassemblies in the optical assembly650. The ledge structure allows the PCB630to be inserted and to be parallel to the aspheric lenses within a few microns of tolerance.

While the description above refers to particular embodiments of the present invention, it will be understood that many modifications may be made without departing from the spirit thereof. The accompanying claims are intended to cover such modifications as would fall within the true scope and spirit of embodiments of the present invention. The presently disclosed embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims, rather than the foregoing description, and all changes that come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.