Patent Application
20140086579
The invention relates to optical communications networks, and more particularly, to an optical coupling system that provides optical feedback for use in monitoring optical output power levels in an optical transmitter module.
An optical transmitter (TX) module is a type of optical communications module used to transmit optical data signals over optical waveguides (e.g., optical fibers) of an optical communications network. An optical TX module includes input circuitry, a laser driver circuit, one or more laser diodes, and an optical coupling system. The input circuitry typically includes buffers and amplifiers for conditioning an input data signal, which is then provided to the laser driver circuit. The laser driver circuit receives the conditioned input data signal and produces electrical modulation and bias current signals, which are provided to the laser diodes to cause them to produce optical data signals having logic 1 and logic 0 intensity levels. The optical data signals are then directed by the optical coupling system onto the ends of respective transmit optical fibers held within a connector that mates with the optical transceiver module.
Optical TX modules often also include a closed loop optical output power feedback system that monitors and controls the modulation and/or bias currents of the laser diodes in such a way that the average optical output power levels of the laser diodes are maintained within a particular range. In closed loop optical output power feedback systems, the optical coupling system of the TX module couples a portion of the light produced by the laser diodes onto respective monitor photodiodes of the TX module. The monitor photodiodes produce electrical signals corresponding to the optical output power levels of the laser diodes. Electrical feedback circuitry of the feedback system receives the electrical signals produced by the monitor photodiodes and produces control signals that are then used to adjust the modulation and/or bias currents of the laser diodes such that their average optical output power levels are maintained at designed levels.
Many optical coupling systems currently in use in optical TX modules incorporate relatively elaborate optical features for providing optical feedback, such as gratings at the entrance surface for dividing the light beam and coated flat surface or angled surface at the exit surface for reflecting a portion of the divided light beam onto the optical feedback monitoring path. Manufacturing these types of optical features tends to be difficult and costly due to the complexity of the manufacturing processes. Diffractive optics systems are typically fabricated on a glass lens, which is a relatively expensive manufacturing process. Diffractive optics systems typically include a diffraction grating and collimating lens on the entrance surface for dividing the light beam produced by the laser diode and for collimating a portion of the divided light beam to be transmitted and a reflective surface on the exit surface for reflecting a portion of the divided light beam onto the monitor photodiode. Particular locations on the upper and or lower surfaces of the optical coupling system are sometimes coated with a light-absorbing material to reduce optical crosstalk to the laser diodes. Although diffractive optics could potentially be fabricated in plastic using a plastic molding process, such plastic molding technologies are not yet mature enough to fabricate all of the optical features that are needed to perform all of these functions.
The functions of dividing the optical beam and reducing optical crosstalk can also be accomplished by tilting the interface at which the optical beam output from the laser diode enters the optical coupling system. This is not an option, however, for cases where the monitor photodiode arrays are located on either side of the laser diode array due to difficulties that such a layout presents with designing and fabricating the pluggable optical connector that holds the ends of the optical fibers. In addition, tilting the interface makes alignment inspection more difficult to perform.
A need exists for an optical coupling system for use in an optical TX module for optical feedback monitoring that has high optical coupling efficiency, low optical crosstalk susceptibility, little or no polarization dependency, and that can be manufactured at relatively low costs.
The invention is directed to an optical coupling system for use in an optical communications module, an optical communications module that incorporates the optical coupling system, and methods for using an optical coupling system in an optical communications module. The optical coupling system is a unitary optical coupling system comprising an integrally-formed, unitary body having one or more collimating lenses and one or more focusing lenses formed on a first end of the body and having a diffraction grating formed on a second end of the body. The body is made of a material that is transparent to an operating wavelength of light. The diffraction grating has a diffractive pattern formed therein.
In accordance with one embodiment, the optical communications module comprises at least one laser diode that emits a diverging light beam, at least one monitor photodiode positioned near the laser diode, and an integrally-formed, unitary body having at least one collimating lens and at least one focusing lens formed on a first end of the body and having a diffraction grating formed on a second end of the body. The body is made of a material that is transparent to an operating wavelength of light. The diffraction grating has a diffractive pattern formed therein. The diverging light beam emitted by the laser diode is incident on the collimating lens and is collimated thereby. The collimated light beam is incident on the diffraction grating, which divides the collimated light beam into at least first and second collimated light beams. The first collimated light beam is transmitted through the diffraction grating out of the second end of the unitary body. The second collimated light beam is directed by the diffraction grating onto the focusing lens, which focuses the second collimated light beam onto the monitor photodiode.
The method of using an optical coupling system in an optical communications module to provide optical feedback comprises:
providing an optical communications module comprising at least one laser diode, at least one monitor photodiode, and the optical coupling system;
emitting a diverging light beam from the laser diode; with the collimating lens, collimating the diverging light beam into a collimated light beam;
with the diffraction grating, receiving the collimated light beam and dividing the collimated light beam into at least first and second collimated light beams, wherein the first collimated light beam passes through the diffraction grating and the second collimated light beam is directed by the diffraction grating onto the focusing lens; and
with the focusing lens, focusing the second collimated light beam onto the monitor photodiode.
These and other features and advantages of the invention will become apparent from the following description, drawings and claims.
In accordance with the invention, an optical coupling system is provided that includes a unitary, or integrally-formed, optical body having lenses formed on its lower end and a diffractive grating formed on its upper end. The unitary optical body is made of a material that is transparent to an operating wavelength of light. Some of the lenses are collimating lenses and some of the lenses are focusing lenses. Diverging light beams emitted by respective laser diodes of a parallel optical transmitter module are incident on the respective collimating lenses, which collimate the respective diverging light beams to produce respective collimated light beams. The respective collimated light beams are then incident on the diffractive grating. The diffractive grating divides each collimated beam into a first beam that is transmitted through the grating and a second beam that is reflected by the grating. Each reflected second beam is directed by the grating onto a respective focusing lens, which focuses the respective second beam onto the respective monitor photodiode of the transmitter module.
The diffractive grating is designed and manufactured to minimize the optical power that is contained in higher diffractive orders so that the optical power that is contained in the first beams, which are the 0 transmitted (0T) orders, and the optical power that is contained in the second beams, which are the −1 reflected (−1R) orders, is maximized while minimizing the amount of optical power that is contained in the light beams that are reflected back to the laser diodes, which are the 0 reflected (0R) orders.
In addition, the diffractive grating preferably is also designed and manufactured to minimize polarization dependence of the 0T orders. This latter feature reduces or eliminates polarization-dependent noise that may otherwise occur when a laser diode changes modes. Reducing or eliminating polarization-dependent noise minimizes mode-selective loss.
In accordance with illustrative embodiments, the optical coupling system is formed as a unitary, or integral, plastic part using existing plastic molding techniques. The plastic material may be, for example, polyetherimide (PEI). One moldable brand of PEI that is suitable for this purpose is ULTEM 1010 PEI, although many other PEIs and other plastics may also be suitable for this purpose. Making the optical coupling system as a unitary plastic part allows the optical coupling system to be manufactured at low cost with high yield while still achieving the optical goals of the invention.
The diffraction grating may have a relatively simple diffractive pattern formed in it, such as a sinusoidal pattern or a blaze pattern, although the invention is not limited to any particular type of diffractive pattern. Sinusoidal patterns and blaze patterns are advantageous in that they can be manufactured relatively easily in plastic using existing plastic molding techniques. During the process of designing the diffractive grating, the shape, pattern, period, and depth of the diffractive pattern are selected to achieve the aforementioned optical goals and to accommodate the layout requirements of the laser diodes and monitor photodiodes (e.g., the distance between the laser diodes and the monitor photodiodes and their relative positions).
The optical coupling system 30 has four collimating lenses 31 and four focusing lenses 32 on its lower end a diffractive grating 33 formed on its upper end. The diffractive grating 33 may be a sinusoidal or blaze diffractive grating, as described above with reference to
The second collimated light beams are directed by the diffractive grating 33 onto the respective focusing lenses 32 formed on the lower end of the optical coupling system 30. The lenses 32 focus the respective second collimated light beams onto respective monitor photodiodes 22. The monitor photodiodes 22 convert the respective second collimated light beams into respective electrical signals, which are then fed back to electrical circuitry (not shown for purposes of clarity) of the parallel optical transmitter module 20 where they are used to adjust the bias and/or modulation currents of the respective laser diodes 21. The manner in which monitoring photodiodes and electrical feedback signals are used for this purpose is known in the art, and therefore will not be described herein in the interest of brevity.
The optical coupling system 30 is formed as a unitary, or integral, part. Although the invention is not limited with respect to the material of which the optical coupling system 30 is made, one of the advantages of the invention is that the unitary part 30 can be made of molded plastic relatively inexpensively using known plastic molding technologies. Making the unitary part 30 of molded plastic greatly reduces the cost of the optical coupling system while still allowing the aforementioned optical goals to be achieved. For this reason, the unitary optical coupling system 30 is preferably, but not necessarily, made of a molded plastic material.
It should be noted that although the optical coupling system 30 has been described with reference to being used in an optical transmitter module 20 that is mechanically coupled to a separate optical connector 40 that holds ends of four optical fibers 41, the optical transmitter module 20, the optical coupling system 30 and the optical connector 40 may be parts of a single module that terminates an end of an active optical cable. The optical connector 40 is shown as being a separate part for illustrative purposes.
When the optical coupling system 50 is implemented in a twelve-channel parallel optical transmitter module (not shown for purposes of clarity), twelve diverging light beams emitted from twelve VCSELs (not shown) are incident upon the respective collimating lenses 52. The collimating lenses 52 collimate the diverging beams into respective collimated beams, which are then incident on the diffractive grating 51. The diffractive grating 51 divides each collimated beam into first and second collimated beams. The first collimated beams pass out of the optical coupling system 50 and are received by an optics system (not shown) of a connector module (not shown) that holds the ends of twelve optical fibers (not shown). The optics system of the connector module directs the first collimated beams into the ends of the respective optical fibers. The second collimated beams are directed onto the respective focusing lenses 53, which focus the second collimated beams onto the respective monitor photodiodes.
The optical transmitter module 60 includes an optical coupling system 70 that has four collimating lenses 71 and four focusing lenses 72 on its lower end a diffractive grating 73 formed on its upper end. In
Each of the laser diodes 71 emits a diverging beam of light that is incident on a respective one of the collimating lenses 71. Each of the collimating lenses 71 collimates the respective diverging light beam into a respective collimated light beam. The respective collimated light beams are then incident on the diffractive grating 73. The diffractive grating 73 divides each of the collimated light beams into first and second collimated light beams, where the first collimated light beam corresponding to the 0T order and the second collimated light beam corresponding to the −1R order. The first collimated light beams pass out of the optical coupling system 70 and are received in the optical connector 80. The first collimated light beams are directed by an optics system (not shown) of the optical connector 80 onto respective focusing lenses 82. The respective focusing lenses 82 focus the first collimated light beams onto respective ends of respective optical fibers 81.
The second collimated light beams associated with the odd-numbered channels are directed by the diffractive grating 73 onto the focusing lenses 72a formed on the lower end of the optical coupling system 70. The second collimated light beams associated with the even-numbered channels are directed by the diffractive grating 73 onto the focusing lenses 72b formed on the lower end of the optical coupling system 70. The focusing lenses 72a focus the respective second collimated light beams onto respective monitor photodiodes 62a Likewise, the focusing lenses 72b focus the respective second collimated light beams onto respective monitor photodiodes 62b. The monitor photodiodes 62a and 62b convert the respective second collimated light beams into respective electrical signals, which are then fed back to electrical circuitry (not shown for purposes of clarity) of the parallel optical transmitter module 60 where they are used to adjust the bias and/or modulation currents of the respective laser diodes 61.
The optical coupling system 70 is formed as a unitary, or integral, part. Although the invention is not limited with respect to the material of which the optical coupling system 70 is made, one of the advantages of the invention is that the unitary part 70 can be made of molded plastic relatively inexpensively using known plastic molding technologies. Making the unitary part 70 of molded plastic greatly reduces the cost of the optical coupling system while still allowing the aforementioned optical goals to be achieved. For this reason, the unitary optical coupling system 70 is preferably, but not necessarily, made of a molded plastic material. Persons of skill in the art will understand, in view of the description being provided herein, the manner in which various diffractive gratings may be designed and manufactured in various other materials (e.g., glass) to achieve the aforementioned optical goals.
It should be noted that although the optical coupling system 70 has been described with reference to being used in an optical transmitter module 60 that is mechanically coupled to a separate optical connector 80 that holds ends of four optical fibers 81, the optical transmitter module 60, the optical coupling system 70 and the optical connector 80 may be parts of a single module that terminates an end of an active optical cable. The optical connector 80 is shown as being a separate part for illustrative purposes.
The primary difference between the optical coupling system 100 shown in
When the optical coupling system 100 is implemented in a twelve-channel parallel optical transmitter module, twelve diverging light beams emitted from twelve VCSELs (not shown) are incident upon the respective collimating lenses 102. The collimating lenses 102 collimate the diverging beams into respective collimated beams, which are then incident on the diffractive grating 101. The diffractive grating 101 divides each collimated beam into first and second collimated beams. The first collimated beams (the 0T orders) pass out of the optical coupling system 100 and are received by an optical coupling system (not shown) of a connector module (not shown) that holds the ends of twelve optical fibers (not shown). The optical coupling system of the connector module directs the first collimated beams into the ends of the respective optical fibers.
The second collimated light beams (the −1R orders) associated with the odd-numbered channels are directed by the diffractive grating 101 onto the focusing lenses 103a formed on the lower end of the optical coupling system 100. The focusing lenses 103a focus the respective second collimated light beams onto respective monitor photodiodes (not shown for purposes of clarity). The second collimated light beams (the −1R orders) associated with the even-numbered channels are directed by the diffractive grating 101 onto the focusing lenses 103b formed on the lower end of the optical coupling system 100. The focusing lenses 103b focus the respective second collimated light beams onto respective monitor photodiodes (not shown for purposes of clarity). The monitor photodiodes convert the respective second collimated light beams into respective electrical signals, which are then fed back to electrical circuitry (not shown for purposes of clarity) of the parallel optical transmitter module where they are used to adjust the bias and/or modulation currents of the respective laser diodes.
The optical coupling system 100 is formed as a unitary, or integral, part. Although the invention is not limited with respect to the material of which the optical coupling system 100 is made, one of the advantages of the invention is that the unitary part 100 can be made of molded plastic relatively inexpensively using known plastic molding technologies. Making the unitary part 100 of molded plastic greatly reduces the cost of the optical coupling system while still allowing the aforementioned optical goals to be achieved. For this reason, the unitary optical coupling system 100 is preferably, but not necessarily, made of a molded plastic material. As indicated above, persons of skill in the art will understand, in view of the description being provided herein, the manner in which various diffractive gratings may be designed and manufactured to achieve the aforementioned optical goals.
As indicated above, the diffractive grating preferably is also designed and manufactured to minimize polarization dependence of the 0T orders, which, in turn, reduces or eliminates polarization-dependent noise that may otherwise occur when the mode composition of the laser diode emission changes. Reducing or eliminating polarization-dependent noise minimizes mode-selective loss. This is accomplished by ensuring that the difference between the 0T order for the transverse electric (TE) mode and the 0T order for the transverse magnetic (TM) mode is small. With reference again to
The same is true for the diffractive grating that uses the blaze diffractive pattern shown in
From the above description of the illustrative embodiments, it can be seen that an optical coupling system can be made as a unitary part at relatively low cost while still achieving the aforementioned optical goals of: (1) minimizing the optical power that is contained in higher diffractive orders so that the optical power that is contained in the 0T and the −1R orders is maximized (i.e., the light to be coupled into the end of the optical fiber and the light that is to be used for feedback monitoring); (2) minimizing the amount of optical power that is contained in the 0R orders (i.e., the light that might otherwise be coupled into the aperture of the laser diode as optical crosstalk); and (3) minimizing polarization dependence of the 0T orders in order to minimize polarization-dependent loss. It should be noted that these optical goals are achieved without the need for using reflective coatings for reflecting the light to be used for feedback monitoring and without the need for using light-absorbing materials to reduce or eliminate optical crosstalk in the laser diode.
It should be noted that the invention has been described with reference to a few illustrative embodiments for the purpose of demonstrating the principles and concepts of the invention. It will be understood by persons of skill in the art, in view of the description provided herein, that the invention is not limited to these illustrative embodiments and that many variations can be made to the illustrative embodiments without deviating from the scope of the invention. For example, while the diffractive grating has been described for exemplary purposes as a sinusoidal or blaze diffractive grating, persons of skill in the art will understand the manner in which the principles and concepts of the invention may be applied to achieve other types of diffractive gratings having other diffractive patterns that achieve the aforementioned optical goals. Also, while the optical coupling system has been described with reference to being used in a parallel optical transmitter module, it may be used in a parallel optical transceiver module or in other types of parallel and non-parallel (i.e., single channel) optical communications modules.
