Patent Publication Number: US-6906856-B1

Title: Early warning failure detection for a lasing semiconductor optical amplifier

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of, and claims the benefit of, U.S. patent application Ser. No. 10/029,676, now U.S. Pat. No. 6,707,600 entitled “Early Warning Failure Detection Within an Optical Network,” and filed Dec. 21, 2001, which, in turn, claims the benefit of U.S. Provisional Patent Application Ser. No. 60/274,470, entitled “Early Warning Failure Detection Within an Optical Network,” filed Mar. 9, 2001. Both of the above-referenced applications are incorporated herein in their respective entireties by this reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. The Field of the Invention 
     This invention generally relates to early warning failure detection for optical amplifiers. More particularly, it relates to early warning failure detection for lasing semiconductor optical amplifiers, such as vertical lasing semiconductor optical amplifiers (VLSOAs), where the failure detection is based on detecting a shift in wavelength of a ballast laser signal generated by the amplifier. 
     2. Related Technology 
     Optical amplifiers are a basic building block for many types of optical systems. For example, fiber optic communications systems transmit information over optical fibers. A typical communications system includes a transmitter, an optical fiber, and a receiver. The transmitter incorporates information to be communicated into an optical signal and transmits the optical signal via the optical fiber to the receiver. The receiver recovers the original information from the received optical signal. In these systems, phenomena such as fiber losses, losses due to insertion of components in the transmission path, and splitting of the optical signal may attenuate the optical signal and degrade the corresponding signal-to-noise ratio as the optical signal propagates through the communications system. Optical amplifiers are used to compensate for these attenuations. As another example, receivers typically operate properly only within a relatively narrow range of optical signal power levels; optical amplifiers may be used to boost an optical signal to the proper power range for the receiver. 
     It is generally beneficial to monitor optical amplifiers to ensure that they are operating correctly. For example, one factor in the efficient utilization of an optical network is the ability to detect and correct failures within the network. Monitoring of optical amplifiers in the network can help locate a point of failure. Early warning before failures occur would also be beneficial, as this can be used to prevent failures; optical amplifiers which are identified as subject to failure in the near future can be replaced before they actually fail. 
     One method typically used to monitor an optical amplifier is based on tapping a small portion of the amplified optical signal leaving the optical amplifier. If the strength of the tapped portion falls within a specified range, this is an indication that the optical amplifier is operating correctly (or at least outputting a signal). In contrast, if the tapped portion is unusually weak or non-existent, this suggests that the optical amplifier may have failed. However, this approach reduces the optical signal&#39;s strength since a portion of the optical signal is tapped for monitoring purposes. As optical networks expand and the number of amplifiers in a signal path increases, the cumulative effect of all of these tap losses can be significant. Another drawback to this approach is that it does not provide early warning of a future failure. 
     As a result, there is a need for a failure detection capability for optical amplifiers which does not introduce tap loss or other types of optical loss. There is also a need for a failure detection capability which provides early warning of failures. In the context of optical communications systems, early warning failure detection would allow re-routing of data traffic away from optical amplifiers before they fail. Additionally, early warnings provide more time for a network manager to replace a failed (or about to fail) optical amplifier. 
     BRIEF SUMMARY OF AN EXEMPLARY EMBODIMENT OF THE INVENTION 
     In accordance with the present invention, early warning failure detection is provided for an optical amplifier. The optical amplifier is based on a lasing semiconductor optical amplifier, which generates a ballast laser signal in addition to the amplified optical signal. The ballast laser signal exhibits a wavelength shift before failure and this wavelength shift is used as the basis for an early warning of future failure of the amplifier. 
     In one embodiment, the optical amplifier with early warning failure detection includes a lasing semiconductor optical amplifier coupled to a wavelength-sensitive detector. The lasing semiconductor optical amplifier includes a semiconductor gain medium, an amplifying path which traverses the semiconductor gain medium, a laser cavity which includes the semiconductor gain medium, and a pump input to the semiconductor gain medium. When the semiconductor gain medium is pumped above threshold for the laser cavity, the laser cavity generates a laser output (i.e., the ballast laser signal) which acts as a ballast for the amplification process. Early warning failure detection is based on detecting a wavelength shift in the ballast laser signal. The wavelength-sensitive detector receives the ballast laser signal for this purpose. 
     In one implementation, the lasing semiconductor optical amplifier is a vertical lasing semiconductor optical amplifier (VLSOA). In another aspect of the invention, early warning of failure is indicated by a shift to a longer wavelength. In yet another aspect, the wavelength-sensitive filter is implemented as an optical filter followed by a detector. For example, the pre-shift version of the ballast laser signal may fall in the stop band of the optical filter and the post-shift version in the pass band, or vice versa. In yet another variation, a VLSOA, optical filter and detector are implemented as layers of different materials stacked on a common substrate, thus yielding an integrated device. 
     In another aspect of the invention, the efficiency with which an incoming pump current is converted into the ballast laser signal changes, typically decreasing, before failure and this change in conversion efficiency is used as the basis for an early warning of future failure of the amplifier. In one approach, the pump current is held constant and a decrease in the ballast laser signal then indicates a decrease in the conversion efficiency. In another approach, the pump current is adjusted so that the ballast laser signal is held constant. An increase in the amount of pump current required then indicates a decrease in the conversion efficiency. 
     The present invention is particularly advantageous because it provides early warning of future failure, thus allowing proactive steps to be taken before the actual failure of the optical amplifier. In addition, the early warning is provided without diverting a portion of the amplified signal. Thus, no tap loss is introduced. 
     Other aspects of the invention include methods based on the above and systems which include optical amplifiers with early warning failure detection capability. Examples of such systems include fiber optic communications systems, transmitters, receivers, and switching nodes. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments of the invention include other aspects which will be more readily apparent from the following detailed description of the invention and the appended claims, when taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a diagram of a vertical lasing semiconductor optical amplifier (VLSOA). 
         FIG. 2  is a flowchart showing the operation of a VLSOA used as an amplifier. 
         FIG. 3A  is a perspective view of one embodiment of a VLSOA. 
         FIG. 3B  is a detailed transverse cross-sectional view of one embodiment of a VLSOA. 
         FIG. 3C  is a longitudinal cross-sectional view of one embodiment of a VLSOA. 
         FIG. 4  is a functional block diagram of an optical amplifier with early warning failure detection. 
         FIG. 5  is a flow diagram illustrating operation of an optical amplifier with early warning failure detection. 
         FIGS. 6A-6D  are spectral diagrams illustrating early warning failure detection based on optical filtering. 
         FIGS. 7A-7C  are block diagrams of various embodiments of processing circuitry for early warning failure detection. 
         FIG. 8  is a perspective view of one implementation of an optical amplifier with early warning failure detection. 
         FIG. 9  is a perspective view of another implementation of an optical amplifier with early warning failure detection. 
         FIGS. 10-12  are diagrams of various fiber optic communications systems using optical amplifiers with early warning failure detection. 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS OF THE INVENTION 
       FIG. 1  is a diagram of a lasing semiconductor optical amplifier (lasing SOA)  100  in accordance with the present invention. The lasing SOA  100  has an amplifier input  112  and an amplifier output  114 . The lasing SOA  100  further includes a semiconductor gain medium  120 , with an amplifying path  130  coupled between the amplifier input  112  and the amplifier output  114  of the lasing SOA  100  and traversing the semiconductor gain medium  120 . The lasing SOA  100  further includes a laser cavity  140  including the semiconductor gain medium  120 , and a pump input  150  to the semiconductor gain medium  120 . Different geometries are possible. In a vertical lasing semiconductor optical amplifier (VLSOA), the laser cavity  140  is oriented vertically with respect to the amplifying path  130 . In transverse and longitudinal lasing SOAs, the laser cavity is oriented transversely or longitudinally (i.e., in-line), respectively, with respect to the amplifying path  130 . The pump input  150  is for receiving a pump to pump the semiconductor gain medium  120  above a lasing threshold for the laser cavity  140 . 
       FIG. 2  is a flow diagram illustrating operation of lasing SOA  100  when it is used as an amplifier. The lasing SOA  100  receives  210  an optical signal at its amplifier input  112 . The optical signal propagates  220  along the amplifying path  130 . The pump received at pump input  150  pumps  230  the semiconductor gain medium above a lasing threshold for the laser cavity  140 , thus generating a laser field. For reasons which will be apparent below, this lasing field shall be referred to as a ballast laser signal. It mayor may not be output from the lasing SOA  100  (e.g., it may be absorbed rather than output). When lasing occurs, the round-trip gain offsets the round trip losses for the laser cavity  140 . In other words, the gain of the semiconductor gain medium  120  is clamped to the gain value necessary to offset the round-trip losses. The optical signal is amplified  240  according to this gain value as it propagates along the amplifying path  130  (i.e., through the semiconductor gain medium  120 ). The amplified signal exits the lasing SOA  100  via the amplifier output  114 . 
     Note that the gain experienced by the optical signal as it propagates through the lasing SOA  100  is determined in part by the gain value of the semiconductor gain medium  120  (it is also determined, for example, by the length of the amplifying path  130 ) and this gain value, in turn, is determined primarily by the lasing threshold for the laser cavity  140 . In particular, the gain experienced by the optical signal as it propagates through the lasing SOA  100  is substantially independent of the amplitude of the optical signal. This is in direct contrast to the situation with non-lasing SOAs and overcomes the distortion and crosstalk disadvantages typical of non-lasing SOAs. 
       FIGS. 3A-3C  are a perspective view, transverse cross-section, and longitudinal cross-section, respectively, of one embodiment of a VLSOA  300  according to the present invention, with  FIG. 3B  showing the most detail. 
     Referring to FIG.  3 B and working from bottom to top in the vertical direction (i.e., working away from the substrate  302 ), VLSOA  300  includes a bottom mirror  308 , bottom cladding layer  305 , active region  304 , top cladding layer  307 , confinement layer  319 , and a top mirror  306 . The bottom cladding layer  305 , active region  304 , top cladding layer  307 , and confinement layer  319  are in electrical contact with each other and may be in direct physical contact as well. An optional delta doping layer  318  is located between the top cladding layer  307  and confinement layer  319 . The confinement layer  319  includes a confinement structure  309 , which forms aperture  315 . The VLSOA  300  also includes an electrical contact  310  located above the confinement structure  309 , and a second electrical contact  311  formed on the bottom side of substrate  302 . 
     Comparing to  FIG. 1 , the semiconductor gain medium  120  includes the active region  304  and the laser cavity  140  is formed primarily by the two mirrors  306  and  308  and the active region  304 . This embodiment is electrically pumped so the pump input  150  includes the electrical contacts  310 , 311 . 
     VLSOA  300  is a vertical lasing semiconductor optical amplifier since the laser cavity  340  is a vertical laser cavity. That is, it is oriented vertically with respect to the amplifying path  330  and substrate  302 . The ballast laser signal produced by the laser cavity  340  may be output through either end of the laser cavity (i.e., through top surface  320  and/or through the substrate  302 ). The VLSOA  300  preferably is long in the longitudinal direction, allowing for a long amplifying path  330  and, therefore, more amplification. The entire VLSOA  300  is an integral structure formed on a single substrate  302  and may be integrated with other optical elements. In most cases, optical elements which are coupled directly to VLSOA  300  will be coupled to the amplifying path  330  within the VLSOA. Depending on the manner of integration, the amplifier input  312  and amplifier output  314  may not exist as a distinct structure or facet but may simply be the boundary between the VLSOA  300  and other optical elements. Furthermore, although this disclosure discusses the VLSOA  300  primarily as a single device, the teachings herein apply equally to arrays of devices. 
     VLSOA  300  is a layered structure, meaning that it is made up of layers of different materials stacked on substrate  302 . This allows the VLSOA  300  to be fabricated using standard semiconductor fabrication techniques, preferably including organo-metallic vapor phase epitaxy (OMVPE) or organometallic chemical vapor deposition (OMCVD). Other common fabrication techniques include molecular beam epitaxy (MBE), liquid phase epitaxy (LPE), photolithography, e-beam evaporation, sputter deposition, wet and dry etching, wafer bonding, ion implantation, wet oxidation, and rapid thermal annealing, among others. 
     The optical signal amplified by the VLSOA  300  is confined in the vertical direction by index differences between bottom cladding  305 , active region  304 , and top cladding  307 , and to a lesser extent by index differences between the substrate  302 , bottom mirror  308 , confinement layer  319 , and top mirror  306 . Specifically, active region  304  has the higher index and therefore acts as a waveguide core with respect to cladding layers  305 , 307 . The optical signal is confined in the transverse direction by index differences between the confinement structure  309  and the resulting aperture  315 . Specifically, aperture  315  has a higher index of refraction than confinement structure  309 . As a result, the mode of the optical signal to be amplified is generally concentrated in dashed region  321 . The amplifying path  330  is through the active region  304  in the direction in/out of the plane of the paper with respect to FIG.  3 B. 
     The choice of materials system will depend in part on the wavelength of the optical signal to be amplified, which in turn will depend on the application. Wavelengths in the approximately 1.3-1.6 micron region are currently preferred for telecommunications applications, due to the spectral properties of optical fibers. The approximately 1.28-1.35 micron region is currently also preferred for data communications over single mode fiber, with the approximately 0.8-1.1 micron region being an alternate wavelength region. The term “optical” is meant to include all of these wavelength regions. In a preferred embodiment, the VLSOA  300  is optimized for the 1.55 micron window. 
     In one embodiment, the active region  304  includes a multiple quantum well (MQW) active region. MQW structures include several quantum wells and quantum wells have the advantage of enabling the formation of lasers with relatively low threshold currents. In alternate embodiments, the active region  304  may instead be based on a single quantum well or a double-heterostructure active region. The active region  304  may be based on various materials systems, including for example InAlGaAs on InP substrates, InAlGaAs on GaAs, InGaAsP on InP, GaInNAs on GaAs, InGaAs on ternary substrates, and GaAsSb on GaAs. Nitride material systems are also suitable. The materials for bottom and top cladding layers  305  and  307  will depend in part on the composition of active region  304 . 
     Examples of top and bottom mirrors  306  and  308  include Bragg reflectors and non-Bragg reflectors such as metallic mirrors. Bottom mirror  308  in  FIG. 3  is shown as a Bragg reflector. Top mirror  306  is depicted as a hybrid mirror, consisting of a Bragg reflector  317  followed by a metallic mirror  313 . Bragg reflectors may be fabricated using various materials systems, including for example, alternating layers of GaAs and AlAs, SiO 2  and TiO 2 , InAlGaAs and InAlAs, InGaAsP and InP, AlGaAsSb and AlAsSb or GaAs and AlGaAs. Gold is one material suitable for metallic mirrors. The electrical contacts  310 , 311  are metals that form an ohmic contact with the semiconductor material. Commonly used metals include titanium, platinum, nickel, germanium, gold, palladium, and aluminum. In this embodiment, the laser cavity is electrically pumped by injecting a pump current via the electrical contacts  310 ,  311  into the active region  304 . In particular, contact  310  is a p-type contact to inject holes into active region  304 , and contact  311  is an n-type contact to inject electrons into active region  304 . Contact  310  is located above the semiconductor structure (i.e., above confinement layer  319  and the semiconductor part of Bragg reflector  317 , if any) and below the dielectric part of Bragg reflector  317 , if any. For simplicity, in  FIG. 3 , contact  310  is shown located between the confinement layer  319  and Bragg reflector  317 , which would be the case if Bragg reflector  317  were entirely dielectric. VLSOA  300  may have a number of isolated electrical contacts  310  to allow for independent pumping within the amplifier. This is advantageous because VLSOA  300  is long in the longitudinal direction and independent pumping allows, for example, different voltages to be maintained at different points along the VLSOA. Alternately, the contacts  310  may be doped to have a finite resistance or may be separated by finite resistances, rather than electrically isolated. 
     Confinement structure  309  is formed by wet oxidizing the confinement layer  319 . The confinement structure  309  has a lower index of refraction than aperture  315 . Hence, the effective cross-sectional size of laser cavity  340  is determined in part by aperture  315 . In other words, the confinement structure  309  provides lateral confinement of the optical mode of laser cavity  340 . In this embodiment, the confinement structure  309  also has a lower conductivity than aperture  315 . Thus, pump current injected through electrical contact  310  will be channeled through aperture  315 , increasing the spatial overlap with optical signal  321 . In other words, the confinement structure  309  also provides electrical confinement of the pump current. 
     As lasing SOAs approach failure, there is a shift in the wavelength of the ballast laser signal. The remainder of this disclosure shall be described in the context of VLSOAs and a shift to a longer wavelength but it is to be understood that the invention is not limited to this scenario. It is equally applicable to other types of lasing SOAs, including both the transverse and longitudinal geometries, and to cases in which the shift is to a shorter wavelength. 
     In certain cases, the cause of the wavelength shift may be an increase in temperature within the VLSOA. In other words, as the VLSOA begins to fail, its temperature increases, thus causing the wavelength shift. Regardless of the cause, the wavelength shift may be used as an early warning device to identify VLSOAs just prior to their actual failure. The wavelength shift typically accelerates with time as the failure mode of the VLSOA becomes more pronounced. Small wavelength shifts (e.g., of 0.1 nm) may be detected significantly in advance of failures (possibly even months before the actual failure) and much larger wavelength shifts can occur minutes or even seconds before the failure. 
       FIG. 4  is a functional block diagram of an optical amplifier  400  with early warning failure detection. The optical amplifier  400  includes a VLSOA  405  and a wavelength-sensitive detector  470 . The wavelength-sensitive detector  470  is coupled to receive the ballast laser signal  425  from VLSOA  405 . The wavelength-sensitive detector  470  detects the wavelength shift in the ballast laser signal  425 , thus permitting early warning of a future failure of the VLSOA  405 . 
     In the implementation shown in  FIG. 4 , the wavelength-sensitive detector  470  includes an optical filter  430  coupled to a detector  440 . The optical filter  430  has a spectral response which varies with wavelength. In particular, the spectral response is selected in order to detect the shift in wavelength. Examples of optical filters include thin-film resonant cavity filters, thin-film multicavity filters, and arrayed waveguide grating type filters. The optical filter  430  may be integrated with VLSOA  405 , directly coupled to VLSOA  405  or implemented as components which are discrete from VLSOA  405 . Examples of detectors  440  include PIN diodes and avalanche photodetectors. Examples of wavelength-sensitive detectors  470  which do not consist of an optical filter  430  combined with a detector  440  are detectors whose inherent spectral sensitivity is suitable for detecting the wavelength shift. 
     In alternate embodiments, early warning of a future failure is based on observing the ballast laser signal  425 , but not on detecting a wavelength shift. For example, the ballast laser signal  425  can be monitored for a change in the efficiency of converting the incoming pump current to the ballast laser signal  425 . Assume for the moment that the amplitude of the incoming optical signal is constant or that there is no incoming optical signal so that variations in the amplitude of the ballast laser signal are not caused by variations in the incoming optical signal. In one implementation, efficiency is monitored by using a constant pump current. A decrease in the ballast laser signal  425  then indicates a decrease in the conversion efficiency from pump current to ballast laser signal  425 . This, in turn, is often an early indication of future failure. Alternately, the pump current can be adjusted so that the ballast laser signal  425  has a constant strength. An increase in the amount of pump current required means there has been a decrease in the conversion efficiency, again signaling possible future failure of the device. 
     The optical amplifier  400  of  FIG. 4  can be straightforwardly adapted to implement this type of early warning. For example, the optical filter  430  may be removed. If a constant pump current is applied, then the detector output  445  serves as an early warning signal. If a constant ballast laser signal  425  is maintained, then the pump current (or an indication of the amount of pump current required to maintain the constant strength ballast laser signal  425 ) serves as the early warning signal. As with the wavelength-sensitive approach, the detector  440  can be integrated on chip with the VLSOA, placed inside a common package with the VLSOA, and/or coupled to the VLSOA via free space optics, beam splitters, mirrors, filters, guided wave optics, fibers, etc. As with the wavelength-sensitive approach, although the above example uses a VLSOA, this approach also applies to lasing SOAs which utilize other geometries (including transverse and longitudinal geometries). 
     Returning now to the wavelength-sensitive case,  FIG. 5  is a flow diagram illustrating operation of amplifier  400 . The VLSOA  405  is pumped  510  above a lasing threshold for the VLSOA. As a result, the VLSOA  405  generates a ballast laser signal  425 . VLSOA  405  also has an amplifier input  412  and an amplifier output  414 . An optical signal enters the VLSOA  405  via amplifier input  412 , where it is amplified and transmitted via amplifier output  414 . The ballast laser signal  425  acts as a ballast with respect to the amplification process, thus gain-clamping the VLSOA. When the VLSOA  405  is functioning properly, the wavelength of the ballast laser signal  425  falls within some normal operating range. When the VLSOA  405  is approaching failure, the wavelength of the ballast laser signal  425  experiences a pre-failure shift. The wavelength-sensitive detector  470  monitors  520  the ballast laser signal for the pre-failure shift. 
       FIGS. 6A-6D  are spectral diagrams illustrating early warning failure detection based on optical filtering. These examples use the filter  430 —detector  440  implementation shown in FIG.  4 . In other words, as a special case of step  520 , the optical filter  430  filters  522  the ballast laser signal  425 . The detector  440  monitors  524  the amplitude of the filtered signal  435 . 
     For convenience, the symbol λ is used to denote a normal operating wavelength and λ+δ to denote a shifted wavelength. The use of the variable δ is not meant to imply that the shift in wavelength occurs as a discrete jump. In other words, it is not meant to imply that the wavelength jumps from λ to λ+δ prior to failure. Typically, but not always, the wavelength varies continuously instead, at some point reaching and then passing λ+δ. Thus, the quantity δ can be thought of as a threshold for early warning failure detection. Positive δ indicates a shift to longer wavelengths and negative δ indicates a shift to shorter wavelengths. In one embodiment, the operating wavelength λ falls in the range of approximately 1.3-1.6 micron (i.e., the wavelength range currently used for telecommunications) and the threshold δ is in the range of 1-15 nm. 
     In  FIGS. 6A-6D , the spectrum of the ballast laser signal during normal operation lies in the vicinity of λ and is shown by curve  610 . The spectrum after the wavelength shift to λ+δ is shown by curve  620 . The spectral response of the optical filter  430  is shown by curves  630 A-D, respectively. 
     In  FIGS. 6A and 6B , the post-shift wavelength λ+δ lies in the pass band of the optical filter  430 ; whereas the operating wavelength λ lies in the stop band. In  FIG. 6A , the optical filter  430  is a bandpass filter with one edge located between wavelengths λ and λ+δ and the other edge located at a wavelength longer than λ+δ. In  FIG. 6B , the optical filter  430  is a lowpass filter with edge located between wavelengths λ and λ+δ. 
     Both of these examples function similarly. A properly functioning VLSOA  405  generates a ballast laser signal  425  with spectrum  610 . Since the spectrum  610  falls primarily in the stop band of the optical filter  430 , it is significantly attenuated. A weak optical signal  435  is received by detector  440 , which then outputs a correspondingly weak electrical signal  445 . Comparatively, when the VLSOA  405  begins to fail, the spectrum of ballast laser signal  425  shifts. Curve  620  shows the spectrum after it has shifted by an amount δ. As a result of this wavelength shift, more of the spectrum falls in the pass band of the optical filter  430 . Detector  440  converts the stronger optical signal  435  into a stronger electrical signal  445 . Thus, as the VLSOA  405  approaches failure, the electrical signal  445  produced by detector  440  increases. 
     In  FIGS. 6C-6D , the opposite approach is taken. The post-shift wavelength λ+δ lies in the stop band of the optical filter; whereas the operating wavelength λ lies in the pass band. In  FIG. 6C , the optical filter  430  is a bandpass filter with one edge located between wavelengths λ and λ+δ and the other edge located at a wavelength shorter than λ. In  FIG. 6D , the optical filter  430  is a highpass filter with edge located between wavelengths λ and λ+δ. 
     In these two examples, a properly functioning VLSOA  405  generates a ballast laser signal  425  with a spectrum  610  which falls primarily in the pass band of the optical filter  430 . However, when the VLSOA  405  begins to fail, the spectrum of the ballast laser signal  425  shifts towards the stop band. As a result of this wavelength shift, the optical signal  435  received by detector  440  falls in strength. Thus, as the VLSOA  405  approaches failure, the electrical signal  445  produced by detector  440  decreases. 
       FIGS. 7A-7C  are block diagrams of various types of processing circuitry suitable for analyzing the early warning electrical signals  445  generated by optical amplifier  400 . The processing circuitry basically monitors the signal for an increase/decrease which would indicate a future failure of VLSOA  405 . The following examples are discussed assuming that an increase in the electrical signal  445  indicates an early warning for VLSOA failure, but the same principles apply to the case of a decrease also. 
     In  FIG. 7A , the optical amplifier  400  is coupled to the following elements in series: a comparator  710 , processor  720  and early warning failure indicator  730 . Examples of comparator  710  include analog circuitry, digital circuitry (assuming an A/O conversion), and comparison functionality implemented in software. Examples of processor  720  include general purpose processors, DSP processors, micro controllers, and logic circuitry (e.g., as a stand-alone chip or integrated as a part of a larger chip). Examples of early warning failure indicator  730  include light emitting diodes, display icons on computer screens, and the activation of messages or software routines. In  FIG. 7A , the comparator  710  compares the incoming early warning signal  445  to a reference threshold  715 . The threshold may be either static or dynamic (e.g., adaptive). When the threshold is exceeded, the comparator  710  signals detection of a shift in wavelength, for example by toggling its output. The processor  720  then activates the early warning failure indicator  730 . 
     In  FIG. 7B , there are two optical amplifiers  400 A and  400 B coupled to comparator  710 . The early warning signals  445 A and  445 B from the optical amplifiers  400  are compared by comparator  710 . If the two signals  445  are approximately equal, then it is assumed that both VLSOAs  405  are functioning properly. However, if the two signals  445  differ by too much, then comparator  710  signals the processor  720 , which activates the early warning failure indicator  730 . 
     In  FIG. 7C , the processor  720  implements the comparison function. The early warning signal  445  is converted from analog to digital form by A/D converter  712  and then received by processor  720 , which implements the logic necessary to determine whether to activate the early warning failure indicator  730 . 
       FIG. 8  is a perspective view of one implementation of an optical amplifier. In this embodiment, a VLSOA  405 , optical filter  430  and detector  440  are integrated on a common substrate. The VLSOA  405  has the structure shown in  FIG. 3 , although not as much detail is shown in  FIG. 8  for purposes of clarity. The VLSOA  405  outputs the ballast laser signal  425  through its top surface  320 . Some or all of this ballast laser signal  425  enters the optical filter  430 . 
     The optical filter  430  is implemented as a Fabry-Perot filter integrated directly above the top surface  320  of the VLSOA  405 . The Fabry-Perot filter  430  includes two mirrors  820  and  830  separated by an optical cavity  825 . In this example, the mirrors  820  and  830  are InP/InGaAsP Bragg mirrors. The cavity  825  is formed from typical materials such as InP, InGaAsP or InGaAs, and it typically has an optical path length which is an integer number of half wavelengths. Examples of other materials suitable for use in cavity  825 , mirror  820  and mirror  830  include other semiconductor materials (e.g., InP/InGaAs, GaAs/AlGas, AlInGaAs, AlN, InGaAsN, GaN, Si, and amorphous-Si), dielectric materials (e.g., SiO2, MgO and A1203) and polymer materials. The ballast laser signal  425  emitted through the VLSOA top surface  320  enters the first mirror  820  at a right angle to its surface. The light resonates within the cavity  825 , causing only the resonant wavelengths to add in phase. The length of the cavity  825  determines the resonant wavelengths. The resonant wavelengths are transmitted through the second mirror  830  to the detector  440 . 
     The detector  440  is integrated directly above the Fabry-Perot filter  430 . In the example of  FIG. 8 , the detector  440  is a PIN detector. From bottom to top, it includes a bottom cladding layer  840  which is either n or p doped, an undoped or intrinsic absorbing layer  845 , and a top cladding layer  850  which has the opposite doping as the bottom cladding layer  840 . Electrical contacts  842  and  852  are made to the bottom and top cladding layers  840  and  850 , respectively. In the example shown in  FIG. 8 , the top and bottom cladding layers  840 - 850  are InP and the absorbing layer  845  is InGaAs. The filtered ballast laser signal is absorbed in the lower bandgap intrinsic layer  845  and electron-hole pairs are generated. The built-in field surrounding the p-i-n junction sweeps out the holes to the higher bandgap p-region and the electrons to the higher band gap n-region and thus generates a current between the two electrical contacts  842 ,  852 . The built-in field can be enhanced by reverse biasing the p-i-n junction. The generated current serves as the early warning signal  445 . If the filter  430  is electrically conductive, the top electrical contact  310  to the VLSOA  405  (see  FIG. 3B ) and the bottom electrical contact  842  to the detector  440  may be implemented as a single contact. 
       FIG. 9  is a perspective view of another implementation of an optical amplifier, in which the VLSOA  405 , optical filter  430  and detector  440  are discrete components. The amplifier input and output of the VLSOA  405  are coupled to fiber pigtails  912  and  914 . A lens  920  collects some or all of the ballast laser signal  425  generated by VLSOA  405  and deposits it onto the detector  440 . An aperture  910  placed over the top surface  320  of VLSOA  405  defines the extent of the ballast laser signal  425  which is collected by the lens  920 . The optical filter  430  is located in the optical path between the VLSOA  405  and the detector  440 . The discrete elements are held in position using conventional opto-mechanical packaging techniques. 
       FIGS. 10-12  are diagrams of example fiber optic communications systems which use optical amplifiers with early warning failure detection.  FIG. 10  depicts a transmitter system,  FIG. 11  depicts a receiver system, and  FIG. 12  depicts a node for a fiber optic network. These are examples of various components in a fiber optic communications system. One task in overall network management is the detection and correction of device failures within the network. These failures may result in a large amount of data being lost. As a result, a network manager must respond quickly to minimize the amount of lost data. In the event of a failure, the network manager typically identifies the source of the failure, routes data traffic around the failure, and repairs the failure. Early warning before the occurrence of a failure allows the network manager to take proactive steps instead. For example, he may re-route traffic and then replace the optical amplifier when it is not handling live data. This avoids loss of data and also gives the network manager more time and flexibility as to when to replace the optical amplifier. 
     Beginning with  FIG. 10 , transmitter system  1000  includes input ports  1002 A-N, an output port  1004 , and an optical transmitter  1050  coupling the input ports  1002  to the output port  1004 . In this particular example, the optical transmitter  1050  further includes two electrical multiplexers  1052 A and  1052 B, two electro-optic modulators  1054 A and  1054 B, two lasers  1056 A and  1056 B, and a wavelength division multiplexer  1058 . Each electrical multiplexer couples some of the input ports  1002  to the electrical input of one of the electro-optic modulators  1054 . The lasers  1056  provide the optical input to the modulator  1054 . The outputs of the two modulators  1054  are coupled to the wavelength division multiplexer  1058 , which is coupled to the output port  1004 . 
     The optical transmitter system  1000  operates as follows. In general, each input port  1002  receives data, which is combined by the optical transmitter  1050  and output as an optical signal via the output port  1004 . In this particular implementation, each electrical multiplexer  1052  combines some of the incoming data signals into an electrical signal which drives the corresponding modulator  1054 . Each modulator  1054  impresses the data onto the laser signal produced by the corresponding laser  1056 . The two lasers  1056  operate at different wavelengths. The wavelength division multiplexer  1058  combines the two modulated optical signals into a single optical signal, which is transmitted via output port  1004 . 
     Optical transmitter system  1000  also includes optical amplifiers  400 , which are located wherever amplification is beneficial. The exact locations will depend on the overall system design. The exact locations will depend on the overall system design. The optical amplifier symbols  400  shown in  FIG. 10  show examples of where an optical amplifier may be located, but they do not imply that there must be an optical amplifier at every location shown. In  FIG. 10 , optical amplifiers  400  are shown between lasers  1056  and modulators  1054 , in order to amplify the laser signals generated by the laser  1056 . They are located between the modulators  1054  and the wavelength division multiplexer  1058 , in order to amplify the single-wavelength modulated optical signals produced by the modulators  1054 . An optical amplifier  400  is also located after the wavelength division multiplexer  1058 , in order to amplify the multi-wavelength modulated optical signal. The optical amplifiers described previously are suitable for use in the optical transmitter system  1000 . The optical amplifiers  400  include early warning failure detection and generate early warning signals  445 . These signals  445  are routed to management system  1040 , which monitors the status of the optical amplifiers  400  and takes appropriate actions when an early warning signal  445  indicates future failure. 
     In  FIG. 11 , receiver system  1100  includes an input port  1104 , a number of output ports  1102 A-N, and an optical receiver  1150  coupling the input port  1104  to the output ports  1102 . In this particular example, the optical receiver  1150  includes a wavelength division demultiplexer  1158 , two detectors  1154 A and  1154 B, and two electrical demultiplexers  1152 A and  1152 B. The wavelength division demultiplexer  1158  couples the input port  1104  to the detectors  1154 . Each electrical demultiplexer  1152  couples one of the detectors  1154  to the output ports  1102 . 
     The optical receiver system  1100  generally implements the reverse functionality of optical transmitter system  1000 . The input port  1102  receives an optical signal which contains modulated data at multiple wavelengths (two wavelengths in this example). The wavelength division demultiplexer  1158  separates the wavelengths, feeding one to each detector  1154 . The detectors  1154  recover electrical signals from the incoming modulated optical signals. These electrical signals are further split by electrical demultiplexers  1152  and then output at ports  1102 . 
     Optical receiver system  1100  also includes optical amplifiers  400 , which are located wherever amplification is necessary. In  FIG. 11 , an optical amplifier  400  is shown before wavelength division demultiplexer  1158 , in order to amplify the incoming multi-wavelength modulated optical signal. Optical amplifiers  400  are also shown between wavelength division demultiplexer  1158  and detectors  1154 , in order to amplify the single-wavelength modulated optical signals produced by the wavelength division demultiplexer  1158 . As with  FIG. 10 , the optical amplifiers  400  in  FIG. 11  include early warning failure detection and generate early warning signals  445 . These signals  445  are routed to management system  1140 , which monitors the status of the optical amplifiers  400  and takes appropriate actions when an early warning signal  445  indicates future failure. 
       FIG. 12  depicts a node  1200  for a fiber optic network. The node includes a high-speed input port  1214 , a high-speed output port  1204 , low-speed input ports  1202  and low-speed output ports  1212 . The ports are coupled to each other by an add-drop multiplexer  1250 . In this example, the add-drop multiplexer  1250  includes a wavelength division demultiplexer  1252 , an optical switch  1254  and a wavelength division multiplexer  1256 . The wavelength division demultiplexer  1252  is coupled between the high-speed input port  1214  and the optical switch  1254 . The wavelength division multiplexer  1256  is coupled between the optical switch  1254  and the high-speed output port  1204 . The low speed ports  1202 , 1212  are also coupled to the optical switch  1254 . 
     The high-speed ports  1204 , 1214  handle optical signals which contain multiple channels of data. In this example, each channel is located at a different wavelength. The low-speed ports  1202 , 1212  handle single channels of data. The wavelength division demultiplexer  1252  splits the incoming multi-channel signal into its constituent channels, which then enter the optical switch  1254 . The wavelength division multiplexer  1256  combines channels from the optical switch  1254  into a single, multi-channel optical signal. The optical switch  1254  routes the various channels between the various ports. 
     As with systems  1000  and  1100 , node  1200  also include optical amplifiers  400 , which may be located in many different places. The optical amplifiers  400  include early warning failure detection and generate early warning signals  445 . These signals  445  are routed to management system  1240 , which monitors the status of the optical amplifiers  400  and takes appropriate actions when an early warning signal  445  indicates future failure. 
       FIGS. 10-12  depict specific implementations of a transmitter, receiver and switching node, but these are intended only as examples. In addition, early warning failure detection can also be used in other types of transmitters, receivers and switching nodes; in other components of fiber communication systems; as well as in other applications which utilize optical amplifiers. For example, optical amplifiers are often used to amplify signals traveling long distances through fibers. The early warning failure detection described above can be used in these systems also. 
     As another example, the early warning failure detection described above can also be used during the manufacture of optical devices, including testing of devices. For example, early warning failure detection can be used to implement wafer scale testing of devices which include VLSOAs. That is, these devices can be tested while still in wafer form, without having to first dice or cleave the wafer or singulate the devices. The failure detection mechanism can be used to identify already failed or about to fail devices on the wafer, which can then be discarded during subsequent processing. 
     Although the invention has been described in considerable detail with reference to certain preferred embodiments thereof, other embodiments will be apparent. Therefore, the scope of the appended claims should not be limited to the description of the preferred embodiments contained herein.