WO2005084397A2

WO2005084397A2 - Transmitter and receiver optical sub-assemblies with optical limiting elements

Info

Publication number: WO2005084397A2
Application number: PCT/US2005/007341
Authority: WO
Inventors: Lewis B. Aronson
Original assignee: Finisar Corporation
Priority date: 2004-03-03
Filing date: 2005-03-03
Publication date: 2005-09-15
Also published as: JP2007525722A; EP1721372A2; KR20060116860A; WO2005084397A3; KR100818688B1

Abstract

A transmitter optical subassembly (104) includes an optical emitter (130) and a fiber receptacle (122) within which an optical fiber (108) is received. An optical limiting element (134) is positioned between the optical emitter (103) and the fiber receptacle (122). When an optical signal is emitted from the optical emitter, the optical signal passes through the optical limiting (134) element before the optical signal reaches the fiber receptacle (122) and is received the optical fiber. The optical limiting element (134) has a property such that if the power of the optical signal entering the optical limiting element (134) exceeds a predetermined limit, the power of the optical signal is optically attenuated so that the power of the optical signal exiting the optical limiting element remains below a predetermined limit.

Description

TRANSMITTER AND RECEIVER OPTICAL SUB-ASSEMBLIES WITH OPTICAL LIMITING ELEMENTS BACKGROUND OF THE INVENTION The Field of the Invention The present invention relates generally to the field of optical transceivers. More particularly, embodiments of the present invention relate to eye safety requirements for optical signals transmitted from optical transceivers. Related Technology Laser signals are widely employed in a variety of different technologies and applications. For example, lasers have been widely used in military contexts as range finders, as target designators, and in guidance systems. Lasers are also widely incorporated into communication systems for high-speed data transfer. The practical uses of lasers, as well as the physical properties of different lasers, vary greatly. While some lasers emit relatively low power signals, other lasers may emit signals of much higher power. In many instances, sensitive equipment, as well as human eyes, may be severely damaged by exposure to high power laser signals. In order to protect human eyes from damaging laser signals, eye safety requirements have been developed to guide manufacturers of optical transmission devices. One set of eye safety requirements, Class 1 eye safety requirements, provide guidelines for safely transmitting laser signals in environments where unprotected eyes may be exposed to those laser signals. Class 1 eye safety limits incorporate limits on laser power and exposure time. Thus, the power of a laser signal may be high for a short period of time, or lower for a long period of time, and still conform to Class 1 eye safety requirements. Class 1 eye safety requirements apply to the emission of laser signals in applications such as optical transceivers. For fiber optic transceivers, Class 1 eye safety requirements apply under all conditions, including all reasonable single fault conditions, which are defined as reasonable failures of a single component or connection within a transceiver. In order to comply with eye safety requirements, transceivers are generally designed to ensure eye safety in one of two ways. First, the transceiver may be fundamentally safe because the maximum power the transceiver can emit may be less that the eye safety limit. This is often the case with transceivers incorporating longer wavelength lasers that operate in the range of 1310 - 1550 nm. Second, for cases where the laser signal emitted from the transceiver may not be fundamentally safe, as, for example, with transceivers using lasers which transmit signals with wavelengths in the 850 nm range, the eye safety limit is ensured by redundant electrical circuits that monitor either the laser current, or, more directly, monitor the laser output power through a monitor photodiode. While eye safety systems based on electrical circuitry are useful for keeping the power of laser signals within the eye safety limits, such eye safety systems may become complex and can increase cost, complicate production, and affect performance of optical transceivers. Eye safety systems based on electrical circuitry include redundancies to ensure that the optical transceiver will continue to function in the event of the failure of a single electrical component or connection within the electrical circuitry used in the transceiver. These electrical circuitry systems generally serve to cut off the bias current to the laser when a fault is detected, and therefore often consist of two transistors in series with the laser element. However, because the series components can reduce the electrical headroom within transceivers, thereby limiting transceiver performance, configuring transistors in series within transceivers may be impractical and inefficient. Another example of redundant circuitry used to detect or compensate for single point failures involves the use of monitor photodiodes. In an eye safety system incorporating a monitor photodiode, the output of the monitor photodiode is monitored, and when the output exceeds a preset level, the laser bias current is restricted. In such an eye safety system, failure of either the monitor photodiode or the connection to the monitor photodiode must be detected because many systems use the monitor photodiode in a feedback loop to maintain the optical output power in a desired range. If the monitor photodiode, or the connection to the monitor photodiode, fails, the feedback loop will tend to drive the bias current to the maximum level, which in many systems would cause the output power level to exceed eye safety limits. Thus, a redundant system is needed to detect failures of the monitor photodiode, or the circuitry connection to the monitor photodiode, and shut down the laser independently. If all possible failure modes are to be addressed, the total eye safety circuitry can become complex, inefficient, and expensive. Moreover, it is possible to find reasonable single fault failures which will not be detected by a typical eye safety circuit. Furthermore, design of shortwave optical transceivers is often complicated by the fact that the desired normal operating power is often quite close to the eye safety limit, thereby making design of a system to reliably distinguish between normal and unsafe levels challenging, hi fact, the standards for acceptable output power are often defined by a minimum value and a maximum which corresponds to the eye safety limit. The desire to have the largest output power range for high manufacturing yield tends to make the problem of eye safety control more difficult. SUMMARY OF AN EXEMPLARY EMBODIMENT OF THE INVENTION These and other problems relating to controlling high-powered optical signals are overcome by embodiments of the present invention which are generally directed to incorporation of an optical limiting material into a transmitter optical subassembly (TOSA) in order to control the optical signal output power and facilitate conformance with eye safety requirements. One such TOSA includes an optical emitter and a fiber receptacle that receives an optical fiber. In order to limit the optical output and thereby facilitate eye safety standard compliance, an optical limiting material is positioned between the optical emitter and the fiber receptacle. An optical limiting material that will limit power to the appropriate level without materially affecting output power under normal conditions is selected. ■■ When the optical signal is transmitted from the optical emitter to the optical fiber received into the fiber receptacle, the optical limiting material attenuates any signal with a power that exceeds eye safety requirements. BRIEF DESCRIPTION OF THE DRAWINGS To further clarify the above and other aspects of embodiments of the present invention, a more particular description of the invention will be rendered by reference to specific embodiments thereof which are illustrated in the appended drawings. These drawings depict only typical embodiments of the invention and are therefore not to . be considered limiting of its scope. In addition, the drawings are not drawn to scale. The. invention will be described and explained with additional specificity and detail through the use of the accompanying drawings in which: Figure 1 is a cutaway view of an exemplary optical transceiver module wherein optical limiting materials and elements are employed; Figure 2 is a cutaway view of an embodiment of a TOSA with an optical limiting material bonded directly to the emitting surface of the optical emitter; Figure 3 is a graph of the output power of an optical signal transmitted from an optical limiting material as a function of the input power of an optical signal entering the optical limiting material; and Figure 4 is a flow diagram showing a process for limiting the power of an optical signal transmitted in a TOSA. Figure 5 is a cutaway view of an exemplary ROSA incorporating optical limiting materials; Figure 6 is a cutaway view of an exemplary ROSA incorporating optical limiting materials on the detecting surface of a detector element; . , Figure 7 is a cutaway view of an exemplary ROSA incorporating optical limiting materials in conjunction with a physical contact element; Figure 8 is a cutaway view of an exemplary ROSA incorporating optical limiting materials and a discrete lens; Figure 9 is a cutaway view of an exemplary ROSA incorporating optical limiting materials positioned within a lens holder; ,. Figure 10 is a graphical representation of output power of an optical signal exiting an optical limiting material as a function of input power of an optical signal entering an optical limiting material; and Figure 11 is a flow diagram showing a method for optically attenuating optical signals. DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS Exemplary embodiments of the present invention are concerned with optical transceivers that meet the relevant eye safety requirements by incorporating materials known as optical limiting materials into the design of transmitter optical subassembhes ("TOSA"s) and receiver optical subassemblies ("ROSA"s). Optical limiting elements composed of optical limiting materials serve to optically attenuate an optical signal when the optical signal power level exceeds a predetermined threshold. In an exemplary TOSA, a signal is transmitted from the optical emitter, and the signal travels to a fiber receptacle where the optical signal is received by an optical fiber. In order to ensure that the power of the optical signal received into the optical fiber from the optical emitter does not exceed a predetermined limit, such as an eye safety limit, embodiments of the present invention include one or more optical limiting elements between the optical emitter and the optical fiber. The optical limiting elements effectively limit the power of the transmitted optical signal by attenuating an optical signal having a power level above a desired threshold. By limiting the power of optical signals transmitted from the TOSA, optical limiting materials ensure that optical signals transmitted from the TOSA do not exceed eye safety limits and damage unprotected eyes. I. Exemplary Operating Environments With reference now to Figure 1, an exemplary optical transceiver module 100 is shown. In addition to a housing 102, transceiver module 100 includes TOSA 104 and receiver optical subassembly (ROSA) 106 substantially enclosed within housing 102. TOSA 104 receives optical fiber 108 while ROSA 106 receives optical fiber 110. Each of TOSA 104 and ROSA 106 connects to printed circuit board (PCB) 112 through first electrical contact elements 114 of the PCB 112. First electrical contact elements 114 connect to signal traces 116 and circuitry of PCB 112 (not shown), where such circuitry may include a post- amplifier, laser driver, and related circuits, and the signal traces then connect with second electrical contact elements 118. Second electrical contact elements 118 connect to external and/or internal electrical components (not shown). Optical transceiver module 100 receives and transmits optical signals. When an optical signal is received into optical transceiver module 100, the optical signal enters ROSA 106 through optical fiber 110. After entering ROSA 106, the optical signal is converted from an optical signal to an electrical signal. The electrical signal is transmitted from ROSA 106 to PCB 112 through first electrical contact elements 114. The electrical signal then travels through signal traces 116 and circuitry of PCB 112 to second electrical contact elements 118, and then on to external and/or internal electrical components. In addition to receiving electrical signals from ROSA 106, external and/or' internal electrical components may transmit electrical signals to TOSA 104. In one example, an electrical signal is transmitted from external components to second electrical contact elements 118 and travels through PCB 112 along signal traces 116 that are connected with circuitry such as a laser driver for example. Ultimately, the electrical signal, which may be processed by the circuitry, reaches TOSA 104 where the electrical signal is converted to an optical signal. The optical signal is then transmitted from TOSA 104 through optical fiber 108. With more particular attention now to exemplary TOSA 104, TOSA 104 includes housing 120. In one embodiment of the invention, housing 120 is implemented as a single molded plastic element. Housing 120 may also be configured of alternative materials such as glass, for example. In addition, housing 120 defines fiber receptacle 122 that receives optical fiber 108. A fiber stop 124 located at the end of fiber receptacle 122 serves to limit the distance to which optical fiber 108 can be inserted into the housing 120 of TOSA 104. ■ • ; , The housing 120 of the TOSA 104 is further configured to engage an enclosure 126. - The enclosure 126 is exemplarily implemented as a TO can and includes a window 128. An optical emitter 130 is hermetically sealed by the enclosure 126 and is positioned within the enclosure 126 so that optical signals from the optical emitter 130 pass through the window > 128. In one embodiment, the optical emitter 128 is a laser, examples of which include, but are not limited to, vertical cavity surface-emitting lasers ("NCSEL"), Fabry-Perot ("FP") . lasers and distributed feedback ("DFB") lasers. The laser or other optical emitter employed in the TOSA 104 may be selected to generate optical signals at a particular desired wavelength. In one exemplary embodiment, the laser emits optical signals having wavelengths of about 850 nm. In other exemplary embodiments, the laser emit optical signal having wavelengths of about 1310 nm, or. about 1550 nm. With continuing attention to Figure 1, a lens 132 is positioned within TOSA 104 to focus an optical signal from optical emitter 128 into optical fiber 108. Additionally, an . optical limiting element 134 is positioned within TOSA 104 between optical fiber 108 and optical emitter 128. One or more optical limiting element(s) 134 may be positioned at any location, or at multiple locations, between optical fiber 108 and optical emitter 128. In the particular example of Figure 1, the optical limiting element 134 positioned on window 130 of enclosure 126. Various materials may be used in the construction of optical limiting elements such as optical limiting element 134. Examples of such material include, but are not limited to, glass, transparent glass gels, polymers, and semiconducting polymers blended with materials having non-linear optical properties such as, for example, fullerenes. In other embodiments of the invention, the optical limiting element may be constructed with a polymer, or a polymer mixed with a dopant. In operation, when an electrical signal is received by TOSA 104 from PCB 112, the electrical signal is converted to an optical signal which is then emitted from optical emitter 128. The optical signal travels from optical emitter 128 through window 130 of enclosure 126 to optical limiting element 134. If the power of the optical signal transmitted to optical limiting element 134 is below a predetermined limit, the optical signal remains substantially unchanged as the optical signal travels through optical limiting element 134. If, however, the power of the optical signal is above a predetermined limit, the optical limiting element 134 optically attenuates the power of the optical signal. Thus, the optical limiting element ensures that the power of the optical signal transmitted from TOSA 104 does not exceed a predetermined limit. In one embodiment of the invention, the predetermined limit corresponds to eye safety requirements. In one embodiment of the invention the eye safety requirements are Class 1 eye safety requirements. < II. Exemplary TOSAs with Optical Limiting Materials Directing attention now to Figure 2, an exemplary TOSA 200 is shown. TOSA 200 includes housing 202 which defines fiber receptacle 204 and an associated fiber stop 206; An optical fiber 208 is received into fiber receptacle 204 and extends to fiber stop 206. The housing 202 also includes , an integral lens 210 and is configured to attach to enclosure 212. A window 214 of enclosure 212 is positioned so that an optical emitter 21 hermetically sealed within enclosure.212 is able to transmit optical signals through the window 214 and into the optical fiber 208. The TOSA 200 also includes optical limiting element 218 located, in this- exemplary embodiment, on the emitting surface of optical emitter 216. In general, the optical limiting element serves to attenuate, if necessary, the power of optical signals from optical emitter 216. Although the optical limiting element 218 shown in Figure 2 is placed directly on the emitting surface of optical emitter 216, in other exemplary embodiments, the optical limiting element may be placed at any position, or at multiple positions, between optical emitter 216 and fiber receptacle 204. Furthermore, optical limiting materials may be incorporated into other components of the TOSA 200, such as, for example, the lens 210 and/or the fiber stop 208. Although some exemplary embodiments provide for the use of a discrete optical limiting element, such as optical limiting element 218 for example, the optical limiting functionality associated with the TOSA can be implemented in other Ways as well. For example, an alternative embodiment of the TOSA includes a housing, or portion thereof, configured of a material which is doped with an optical limiter compound, such as, for example, a two-photon absorption dye. Any other suitable doping materials may alternatively be employed however. In such an exemplary embodiment, the housing is configured so that an optical signal emitted from an optical emitter passes through a portion of the housing before entering the optical fiber. In this exemplary implementation then, no separate optical limiting element is provided, and the optical limiting functionality is implemented by the TOSA housing itself. One example of such an implementation is similar in configuration to TOSA 200 disclosed in Figure 2, except that no separate optical limiting element 218 is provided. III. Exemplary TOSAs with Optical Limiting Materials With attention now to Figure 5, a cut-away view of ROSA 500 is shown. In this exemplary embodiment, the housing 502 of the ROSA 500 is a single plastic molded element. Housing 502 may be fabricated of any number of different materials including, for example, plastic, glass, or any other optically suitable material. As indicated in Figure 5, the housing 502 of ROSA 500 engages an enclosure 504, exemplarily implemented as a TO-can, within which a detector element 506 is disposed and hermetically sealed. In one embodiment of the invention, detector element 506 is an avalanche photodiode detector (APD). Various other types of detector elements may alternatively be employed however. Examples of such detector elements include, but are not limited to, a PIN photodiode. As well, a lens 508 of the enclosure 504 is configured and positioned to focus an optical signal onto detector element 506. In the illustrated embodiment, the lens 508 is integral with the enclosure 504, but comprises a discrete component in some alternative embodiments. With continuing attention to the exemplary ROSA 500 of Figure 5, the housing 502 defines a fiber receptacle 510 within which an optical fiber 512 is received. The fiber receptacle 510 communicates with a fiber stop 514 that serves to limit the extent to which the optical fiber 512 can be inserted into the housing 502 of the ROSA 500: Similar to the exemplary arrangement disclosed in Figure 1, optical limiting material 516 is positioned in ROSA 500 between optical fiber 512 and detector element 506 to attenuate, if necessary, the power of optical signals received into the ROSA 500 from optical fiber 512, so as to prevent damage to the detector element 506. hi the exemplary embodiment illustrated in Figure 5, the optical limiting element 516 takes the form of a block of material that is attached to the housing 502 so as to be positioned between the fiber stop 514 and the lens 508. As noted elsewhere herein however, this arrangement is exemplary only, and the configuration and positioning of the optical limiting element 516 may be modified as necessary. In operation, when an optical signal is received into ROSA 500, the optical signal is , transmitted from optical fiber 512 through fiber stop 514 and optical limiting element 516. If the power of the optical signal received at the optical limiting element 516 from the optical fiber 512 is below a predetermined limit, the optical signal passes through optical limiting element 516 without attenuation. • » • On the other hand, if the power of the optical signal received at the optical limiting element 516 from the optical fiber 512 is above a predetermined limit, optical limiting element 516 optically attenuates the power of the optical signal to the extent necessary to ensure that the optical power of the optical signal at the detector element 506 is within acceptable limits. The detector element 506 then detects the optical signal and converts the received optical signal to an electrical signal, m this way, the optical signal that ultimately reaches optical detector 506 has been optically attenuated, if necessary, to a level which is below an optical overload limit, damage threshold, and/or other predetermined limit(s) associated with optical detector 506. As noted earlier herein, the ρosition(s) of the optical limiting element(s) within a ROSA can be varied. With attention now to Figure 6, details are provided concerning a ROSA 600 where the optical limiting element is positioned adjacent to the detector element. As indicated in Figure 6, the exemplary ROSA 600 includes a housing 602 configured to engage an enclosure 604. A detector element 606 is positioned within the enclosure 604 and is arranged to receive optical signals from a lens 608, where the lens 608 is implemented as an integral portion of the enclosure 604. hi this exemplary embodiment, optical limiting element 610 is provided that is positioned directly on the detecting surface of optical detector 606. With continuing reference to Figure 6, the housing 602 of the ROSA 600 defines a fiber receptacle 612 within which is received an optical fiber 614. A fiber stop 616 is defined by the housing 602 at one end of the fiber receptacle 612 so as to limit the depth to which optical fiber 614 can be inserted into the housing 602 of the ROSA 600. When optical fiber 614 receives an optical signal, the optical signal travels from optical fiber 614 through fiber stop 616 to lens 608. The lens 608 then focuses the optical signal and passes the optical signal to the optical limiting element 610, located on the detecting surface of detector element 606. In general, and as described above in connection with the discussion of Figure 1 for example, the optical limiting element 610 attenuates, to the extent necessary, the power of the optical signal transmitted to the detector element 606, so as to prevent damage to, or other problems with, the detector element 606. The extent to which such attenuation is implemented, if at all, may be defined, for example, by an optical overload limit, or a damage threshold of the detector element 606. More generally, the extent to which optical attenuation is implemented by the optical limiting element 610 can be determined with reference to any of a variety of thresholds and limits. Directing attention now to Figure 7, details are provided concerning yet another alternative embodiment of the invention. In this instance, a ROSA 700 is disclosed that includes a housing 702 attached to an enclosure 704. Similar to other exemplary embodiments disclosure herein, a detector element 706 is disposed within the enclosure 704 and hermetically sealed therein by the cooperation of the enclosure 704, and a lens 708. In this embodiment, the lens 708 serves to focus an incoming optical signal onto the detector element 706. As well, the housing 702 of ROSA 700 defines a fiber receptacle 710 which receives an optical fiber 712. In contrast with other exemplary embodiments disclosed herein, the exemplary embodiment disclosed in Figure 7 further includes a physical contact element 714 positioned to contact the fiber receptacle 710 in order to limit reflection within ROSA 700. In addition, physical contact element 714 ensures that optical fiber 712 does not extend beyond the end of fiber receptacle 710. In one embodiment of the invention, physical contact element 714 is glass, but embodiments of the invention are not limited to glass and may include plastic or other suitable materials. In this embodiment then, optical limiting element 716 is positioned in ROSA 700 between the physical contact element 714 and the detector element 706, and the optical limiting element is in contact with the physical contact element 714. In operation, an optical signal is received into ROSA 700 through optical fiber 712. The optical signal travels through physical contact element 714 and into the optical limiting element 716. If the power of the optical signal transmitted into the optical limiting element 716 is below a predetermined limit, the optical signal remains substantially unchanged as the optical signal passes through the optical limiting element 716. However, if the power of the optical signal entering the optical limiting element 716 exceeds a predetermined limit, the optical limiting element 716 optically attenuates the power of the incoming optical signal so that the power of the optical signal exiting the optical limiting element 716 is below the predetermined limit. Thus, optical limiting element 716 ensures that the power of the optical signal ultimately received by detector element 706 is below the predetermined limit. Directing attention now to Figure 8, a cut-away view of an alternative ROSA, denoted generally at 800 and incorporating optical limiting elements and a discrete lens, is shown. As shown in the Figure, a housing 802 of ROSA 800 engages enclosure 804, and a detector element 806 is disposed within the enclosure 804. The enclosure 804 includes a window 808 positioned proximate the detector element 806 to allow an optical signal to pass through enclosure 804 to the detector element 806. This alternative embodiment differs from other exemplary embodiments in that the optical limiting material 810 is located on the window 808 of enclosure 804, in order to limit the power of optical signals transmitted to the detector element 806. In addition, the lens arrangement employed in ROSA 800 likewise differs from that of some other embodiments. In particular, a discrete lens 812 is provided that is held in place by lens holder 814, and positioned within ROSA 800 to focus an optical signal onto detector element 806. A physical contact element 816 is interposed between the lens holder 814 and fiber receptacle 818 and, in at least some embodiments, the physical contact element contacts one or both of the lens holder 814 and fiber receptacle 818. In operation, an optical signal received into ROSA 800 is initially transmitted from optical fiber 820 through physical contact element 816 to lens 812. Lens 812 focuses the optical signal, which is then transmitted through optical limiting element 810 located on the window 808 of enclosure 804. If the power of the optical signal transmitted to optical limiting element 810 is below a predetermined limit, the optical signal remains substantially unchanged as the optical signal passes through optical limiting element 8 0. However, if the power of the optical signal transmitted to optical limiting element 810 is above a predetermined limit, the optical limiting element .810 optically attenuates the optical signal, thus ensuring that the optical signal detected by detector element 806 does not exceed the predetermined limit. Directing attention now to Figure 9, a further exemplary embodiment of the invention is disclosed. Similar to other exemplary embodiments disclosed herein, Figure 9 shows a cut-away view of a ROSA 900 that includes a housing 902 and enclosure 904 engaged with each other. A detector element 906 is provided that is disposed within the enclosure 904. In addition to engaging enclosure 904, the housing 902 defines fiber receptacle 908 within which is disposed an optical fiber 910. The ROSA 900 further includes a physical contact element 912 and lens holder 914. Optical limiting element 914 is positioned in the lens holder 916 between optical fiber 910 and detector element 906, and the lens holder additionally retains one or more lenses 918, and/or other optical components. When an optical signal is received into ROSA 900, the optical signal enters ROSA 900 through optical fiber 910. The optical signal then travels through physical contact element 912 and optical limiting element 914. If the power of the optical signal transmitted to optical limiting element 914 is below a predetermined limit, the optical signal will remain substantially unchanged as the optical signal travels through optical limiting element 914. However, if the power of the optical signal exceeds the predetermined limit, the optical limiting element 914 optically attenuates the power of the optical signal to ensure that the power of the optical signal detected by detector element 914 is below the predetermined limit. After passing through optical limiting element 914, the optical signal continues through lens 918, where the optical signal is focused onto detector element 906. The optical signal is then received and converted to an electrical signal by detector element 906. IV. Exemplary Optical Limiting Materials As is evident from this disclosure, the disclosed optical limiting elements are exemplary structural embodiments of a means for optically attenuating the power of an optical signal. However, the scope of the invention is not limited to the exemplary types and arrangements of the exemplary optical limiting materials disclosed herein. Rather, any other structure(s) of comparable functionality may likewise be employed. As suggested earlier herein, optical limiting materials are materials having non-linear optical properties, for at least some optical power ranges, whereby transmissivity through the optical limiting material is relatively high for low powered optical signals and transmissivity decreases to a relatively low level if the power of the optical signal exceeds a predetermined upper limit. In general, the optical limiting material absorbs, if necessary, a portion of the energy of the optical signal that enters the optical limiting material, h this way, the power of optical signals that exit the optical limiting material is kept at or below a predetermined limit. As suggested elsewhere herein, optical limiting materials have characteristic response times for attenuating optical signals that exceed a given power threshold. As discussed below, the particular desired response time may vary from one application and/or device to another. The response time of a particular optical limiting material is closely tied to standards, such as eye safety requirements, that provide guidelines for the permissible power level of an optical signal, as well as for the permissible maximum time that an eye may be exposed to the optical signal. In particular, the response time of an optical limiting material is related to the permissible time that an eye can be exposed, without harm, to an optical signal having a certain power. The response time varies from one optical limiting material to another and is typically selected with reference to eye safety standards, for example, and the anticipated power of the optical signals that will be involved. Exemplary response times are measured in time lengths anywhere from hundreds of seconds to micro-seconds. In one exemplary embodiment, the response time of the optical limiting material is in the range of about 100 μs to about 100ms. Because eye safety levels are a function of the total time the eye is exposed to a given power level, time limits for exposure to relatively low powered optical signals may be significantly higher than time limits for exposure to relatively high powered optical signals. In addition, if an optical limiting material is able to respond to a relatively high-powered optical signal relatively quickly, the output power of the optical signal could remain relatively high for a relatively longer period of time without exceeding eye safety limits. Another parameter of optical limiting materials relates to the effect, on the optical limiting materials, of optical signals whose power is above a predetermined limit. In particular, the response of optical limiting materials to optical signals transmitted by the optical emitter of the TOSA may be reversible or irreversible. Although high powered optical signals are attenuated by both reversible and irreversible optical limiting materials, the transmissivity of the reversible optical limiting materials returns to relatively high levels when the power of the optical signal decreases below the predetermined power limit, hi contrast, irreversible optical limiting materials are unable to return to high levels of optical signal transmissivity once an optical signal exceeds the power threshold. Reversible optical materials can further be divided into at least two categories. Materials in the first category are largely absorptive, and may also be known as two-photon absorption materials. Materials in the second category are largely refractive. Although refractive optical limiting materials are suitable for use in optical systems having a strongly converging or diverging optical beam, refractive and/or absorptive optical limiting materials are useful in exemplary embodiments of the present invention. As noted above, an optical limiting material is selected for use in exemplary embodiments of the invention based on specific properties of the optical: limiting material,, examples of which include response time, transmissivity and reversibility. Another consideration in the selection of a particular optical limiting material and/or arrangement of. optical limiting material relates to the wavelength(s) of the associated optical signal. For example, if the wavelength of the optical signal emitted by the laser of the TOSA is about 850 nm, an optical limiting material having a limiting power in the range of about -3 dBm to about -1.3 dBm could be selected to provide the desired optical attenuation functionality. The upper bound of the optical signal power that can be transmitted through such an optical limiting material, and the maximum eye safety limit for optical signal power, is about -2 dBm. These power limits refer to the power of the optical signal received at the optical fiber. Of course, such power limits are exemplary only and are not intended to limit the scope of the invention. The optical limiting material may be selected for use with optical signals of other wavelengths as well. For example, some embodiments of the invention employ lasers that emit signals of 1310 nm, and 1550 nm, respectively. This wavelength information would thus inform the selection of particular optical limiting material(s). In this particular example, response times for optical limiting materials selected for use in TOSAs transmitting signals in at 1310 nm or 1550 nm can be relatively longer than response times for optical limiting materials selected for use in TOSAs transmitting signals in the 850 nm range, without exceeding eye safety requirements. Thus, embodiments of the invention employ a variety of different optical emitters, transmitting at various wavelengths. The optical limiting material can be effectively employed with a variety of optical signals generated by devices such as, but not limited to, FP lasers, DFB lasers, and VCSELs. As well, optical limiting materials are suited for use with various types of detectors, examples of which include . avalanche photodiodes ("APD"), and P-I-N photodiodes. Similarly, the particular optical limiting material to be employed in a given situation is typically selected with respect to the particular wavelength, or range of wavelengths, that the optical limiting material is expected to encounter. Examples of such wavelengths include, but are not limited to, 1310 nm and 1550 nm. The foregoing are examples only however and the scope of the invention should not be construed to be limited to any particular device, configuration or operating wavelength(s). It was noted earlier herein that one damage mechanism for detector elements is thermal in nature. Therefore, in order to protect a detector element from damage due to receipt of excess optical power which could raise the temperature of the detector element or other optical transceiver components to unacceptable levels, exemplary optical limiting materials limit the time the detector is exposed to the high powered optical signal to a period significantly shorter than the time period necessary to raise the temperature beyond a critical point. For optical power levels of about 6-20 dBm or 4-100mW for example, the time scale is in the range of microseconds, or about 10^"δ - 10^"3 seconds. Therefore, optical limiting materials incorporated into some embodiments of the invention are characterized by response times of about 10^"6 to about 10^"3 seconds. However, the response times of optical limiting materials used in the present invention will vary and are not limited to any particular times. Another parameter of optical limiting materials relates to the effect, on the optical limiting materials, of optical signals whose power is above a predetermined limit. In particular, the response of optical limiting materials to optical power signals received into the ROSA may be reversible or irreversible. Although high powered optical signals are attenuated by both reversible and irreversible optical limiting materials, the transmissivity of the reversible optical limiting materials returns to relatively high levels when the power of the optical signal decreases below the predetermined power limit. In contrast, irreversible optical limiting materials are unable to return to high levels of optical signal transmissivity once an optical signal exceeds the power threshold. Reversible optical materials can further be divided into at least two categories. Materials in the first category are largely absorptive, and may also be known as two-photon absorption materials. Materials in the second category are largely refractive. Although

^• refractive optical limiting materials are suitable for use in optical systems having a strongly converging or diverging optical beam, refractive and/or absorptive optical limiting materials are useful in exemplary embodiments of the present invention. As noted above, embodiments of the present invention incorporate optical limiting materials, examples of which include absorptive or refractive materials, into ROSAs to improve the optical overload limit, and to increase the damage threshold for detector elements. The use of optical limiting materials with suitable response times and energy absorption helps ensure that the optical power reaching detector element is maintained below the optical overload limit, or other predetermined limit, of the detector element for a large range of optical input powers. Further, incorporation of optical limiting materials into a ROSA can be performed without adverse impact to the overall structure and design of the associated transceiver. Various aspects of embodiments of the invention can be modified as necessary to allow the use of particular arrangements and materials and/or to achieve desired effects. For example, the optical limiting function can be implemented in a ROSA where the distance between the detector element and the optical fiber is about 1 mm or less. In particular, more options are possible if the optical limiting material has the mechanical characteristics of glass, which is likely for limiters based on glass doped with appropriate absorbers. For softer materials, a substrate made of glass can be used to support the material. Furthermore, as another example of such an aspect, some embodiments of the present invention use optical limiting materials that provide little or no optical attenuation effect until the power of the optical signals input to the optical limiting material reaches a level within the range of +3 to +10 dBm, and most typically + 6 dBm, as determined by the damage threshold of the detector element. If the optical limiting material is designed to extend the normal operating range of the receiver, then the threshold at which optical attenuation begins falls within the range of -6 to +3 dBm, where this threshold is chosen based on the saturation level of the detector. Since the nature of the response of the optical limiting material is based on optical intensity, measured in terms of power per unit area, the properties of the optical limiting material may make it desirable to place the optical limiting material in a portion of the optical path with a suitable optical signal cross section. Conversely, the location in the optical path may be chosen to tune the effective power limit of the ROSA or transceiver. The optical limiting material is also chosen to have a response time which is shorter than the time at which the optical detector may become damaged by the power of the optical signal. In general then, various parameters concerning the optical limiting materials may be adjusted as necessary to suit a particular application. Examples of such parameters include, but are not limited to, transmissivity, reversibility, response time, range of limiting power, positioning of the optical limiting materials, and energy absorption. As the foregoing makes clear, various parameters concerning the optical limiting materials may be adjusted as necessary to suit a particular application. Examples of such parameters include, but are not limited to, transmissivity, reversibility, response time, range of limiting power, positioning of the optical limiting materials, wavelength of the optical emitter, and energy absorption. V. Performance of Optical Limiting Materials With attention now to Figure 3, a graphical representation of output power from the optical limiting material used in a TOSA as a function of the input power to the optical limiting material shows the optical limiting properties of optical limiting materials. Optical limiting materials used in exemplary embodiments of the present invention are selected so that the transmissivity of the optical limiting material corresponds with a specified power threshold, as shown in Figure 3 by line 302. In one embodiment of the invention, the power threshold is the damage threshold of the detector element. In another embodiment of the invention, the power threshold is the optical overload limit of the detector element. When the input power of the optical signal received into the optical limiting material is below the power threshold, as shown by line AB, the output power is substantially equal to the input power. Thus, the transmissivity through the optical limiting material is approximately 100%. As the input power reaches an input power threshold, indicated at point "B," that corresponds with an output power that is a predetermined level below the power threshold, line 304, the optical limiting material . attenuates the power of the optical signal, thereby preventing the power of the output optical signal from exceeding the power threshold, notwithstanding any further increase in the input power beyond the-input power threshold "B." That is, increases in the input power to the optical limiting material beyond point "B" do not result in significant changes in the output power from the optical limiting material, due to the attenuation properties of the optical limiting material. Correspondingly, the transmissivity response of the optical limiting material flattens and may approach zero for increases in input optical power beyond the input power threshold. In embodiments of the invention where the optical limiting material is reversible, the transmissivity of the optical signal returns to a level near 100% when the input power of the optical signal drops below the input power threshold indicated at "B." With attention now to Figure 10, a graphical representation of output power from the optical limiting material used in a ROSA as a function of the input power. to the optical limiting material shows the optical limiting properties of optical limiting materials. Optical limiting materials used in exemplary embodiments of the present invention are selected so that the transmissivity of the optical limiting material corresponds with a specified power threshold, as shown in Figure 10 by line 1002. In one embodiment of the invention, the power threshold is the damage threshold of the detector element. In another embodiment of the invention, the power threshold is the optical overload limit of the detector element. When the input power of the optical signal received into the optical limiting material is below the power threshold, as shown by line AB, the output power is substantially equal to the input power. Thus, the transmissivity through the optical limiting material is approximately 100%. That is, there is a 1:1 ratio, or unity, between output optical power and input optical power. As the input power reaches an input power threshold, indicated at point "B," that corresponds with an output power that is a predetermined level below the power threshold, the optical limiting material attenuates the power of the optical signal, thereby preventing the power of the output optical signal from exceeding the power threshold, notwithstanding any further increase in the input power beyond the input power threshold "B." That is, for at least a predetermined range of input powers, increases in the input power to the optical limiting material beyond point "B" do not result in significant, changes in the output power from the optical limiting material, due to the attenuation properties of the optical limiting material. Correspondingly, the transmissivity response of the optical limiting material flattens and may approach zero for increases in input optical power beyond the input power threshold.

VI. Methods for Optical Attenuation With attention now to Figure 4, a method for processing an optical signal within a TOSA is shown. At stage 402 an electrical signal is converted to an optical signal. The optical signal is then emitted, by a laser for example, as shown at stage 404. If the input power of the optical signal exceeds the input power threshold, the power of the optical signal is optically attenuated, as shown at stage 406 so that the power of the output optical signal remains below a predefined power limit. At stage 408, the attenuated optical signal is transmitted. The power of the optical signal transmitted is thereby maintained at a power below the eye safety limit, or at some other predetermined threshold. Embodiments of the present invention maintain optical signal output power levels within a desired limit by incorporating optical limiting materials into components such as

TOSAs. By placing an optical limiting material between an optical emitter, such as a laser, and an optical fiber, the power of the optical signal which is ultimately transmitted into the optical fiber is prevented from exceeding a desired limit. With attention now to Figure 11, a method 1100 for attenuating the power of an optical signal exceeding a predetermined limit is shown. At stage 1102, an optical signal is received. If the power of the received optical signal exceeds a predetermined limit, the power is optically attenuated as shown at stage 1104. In this way, no input optical signal having a power over a predetermined limit is received at the detector element. At stage 1106 of the method 1100, the attenuated optical signal is focused. The attenuated optical signal is then detected, as shown at stage 1108. Finally, at stage 1110, the attenuated optical signal is converted to an electrical signal. The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within. their scope.

Claims

CLAIMS What is claimed is: 1. A transmitter optical subassembly comprising: an enclosure within which an optical emitter is disposed; a housing attached to the enclosure and defining a fiber receptacle configured so that when an optical fiber is received in the fiber receptacle, an optical signal from the optical emitter passes into the optical fiber; and an optical limiting element positioned between the optical emitter and the fiber receptacle, wherein the optical limiting element has a property such that when power of the optical signal entering the optical limiting element falls within a predetermined range, power of the optical signal leaving the optical limiting element remains at or below a predetermined limit.

2. The transmitter optical subassembly of claim 1, wherein the optical emitter is one of: a vertical cavity surface emitting laser (VCSEL); a Fabry-Perot laser; or, a distributed feedback (DFB) laser.

3. The transmitter optical subassembly as recited in claim 1, wherein the emission wavelength of the optical emitter is one of: about 850nm; about 1310nm; or, about

1550nm.

4. The transmitter optical subassembly as recited in claim 1, wherein the optical limiting element is one of: absorptive; or, refractive.

5. The transmitter optical subassembly as recited in claim 1, wherein the optical limiting element is located on an emitting surface of the optical emitter.

6. The optical transmitter assembly as recited in claim 1, wherein the optical limiting element is one of: reversible; or, irreversible.

7. The optical transmitter assembly as recited in claim 1, wherein the predetermined limit is an eye safety limit.

8. The optical transmitter assembly as recited in claim 1, wherein a transmissivity of the optical limiting element is substantially linear in a first range of input optical powers, and substantially non-linear in a second range of input optical powers.

9. The optical transmitter assembly as recited in claim 1, further comprising a lens interposed between the fiber receptacle and the optical emitter.

10. A receiver optical subassembly comprising: an enclosure within which a detector element is disposed; a housing attached to the enclosure and defining a fiber receptacle configured so that when an optical fiber is received in the fiber receptacle, signals from the optical fiber are directed into the detector element; and an optical limiting element positioned between the detector element and the fiber receptacle, wherein the optical limiting element has a property such that when power of the optical signal entering the optical limiting element falls within a predetermined range, power of the optical signal leaving the optical limiting element remains at or below a predetermined limit.

11. The receiver optical subassembly as recited in claim 1, wherein the optical limiting element is one of: absorptive; or, refractive.

12. The receiver optical subassembly as recited in claim 1, wherein the detector element comprises one of: an APD; or, a P-I-N photodiode.

13. The A receiver optical subassembly as recited in claim 1, wherein the enclosure includes a window to which the optical limiting element is attached.

14. The receiver optical subassembly as recited in claim 1, wherein the predetermined limit is one of: an optical overload limit; or, a damage threshold.

15. The receiver optical subassembly as recited in claim 1, wherein the enclosure comprises a TO-can.

16. The receiver optical subassembly as recited in claim 1, wherein the optical limiting element is incorporated into a physical contact element positioned adjacent to the fiber receptacle.

17. The receiver optical subassembly as recited in claim 1, further comprising a physical contact element to which the optical limiting element is bonded, the physical contact element being interposed between the fiber receptacle and the detector element.

18. The receiver optical subassembly as recited in claim 1, wherein the optical limiting element is positioned on the detector element.

19. The receiver optical subassembly as recited in claim 1, wherein a transmissivity of the optical limiting element is substantially linear in a first range of input optical powers, and substantially non-linear in a second range of input optical powers.

20. The receiver optical subassembly as recited in claim 1, further comprising a lens interposed between the fiber receptacle and the detector element.

21. The receiver optical subassembly as recited in claim 1, wherein the property of the optical limiting element is such that the optical limiting element has a transmissivity that is substantially unity until the power of the optical signal entering the optical limiting element falls within one of the following ranges: about -6 dBm to about +3dBm; or, about +3 dBm to about +10 dBm.

22. An optical transceiver module comprising: a printed circuit board; a receiver optical subassembly connected to the printed circuit board; a transmitter optical subassembly connected to the printed circuit board, and comprising: an optical emitter; a fiber receptacle configured so that when an optical fiber is received therein signals from the optical emitter pass into the optical fiber; and means for optically attenuating an optical signal that is generated by the optical emitter; and a housing that substantially encloses the printed circuit board, the transmitter optical subassembly, and the receiver optical subassembly.

23. An optical transceiver module as recited in claim 22, wherein the receiver optical subassembly comprises: a detector element; a fiber receptacle configured so that when an optical fiber is received therein signals from the optical fiber pass into the detector element; and means for optically attenuating an optical signal.