Abstract:
An integrated optical monitoring system comprises a hermetic package and an optical bench sealed within the package. An optical fiber pigtail enters the package via a feed-through to connect to and terminate above the bench. A tunable filter is connected to the top of the bench and filters an optical signal transmitted by the fiber pigtail. A detector, also connected to the bench, detects the filtered signal from the tunable filter. Thus, the entire system is integrated together, on a single bench within a preferably small package. This configuration makes the system useful as a subsystem, for example, in a larger system offering higher levels of functionality and optical signal processing capability.

Description:
RELATED APPLICATIONS  
       [0001]    This application is a Divisional of Ser. No. 09/648,413, filed Aug. 25, 2000 which claims the benefit of Provisional Application No. 60/186,800, filed Mar. 3, 2000 both of which are incorporated herein by this reference in its entirety. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    Wavelength division multiplexing (WDM) systems typically comprise multiple separately modulated laser diodes at the transmitter. These laser diodes are tuned to operate at different wavelengths. When combined in an optical fiber, the WDM optical signal comprises a corresponding number of spectrally separated channels. Along the transmission link, the channels are typically collectively amplified in gain fiber, such as erbium-doped fiber and/or regular fiber, in a Raman pumping scheme. At the receiving end, the channels are usually separated from each other using thin film filter systems, to thereby enable detection by separate photodiodes.  
           [0003]    The advantage of WDM systems is that the transmission capacity of a single fiber can be increased. Historically, only a single channel was transmitted in each optical fiber. In contrast, modern WDM systems contemplate hundreds or thousands of spectrally separated channels per fiber. This yields concomitant increases in the data rate capabilities of each fiber. Moreover, the cost per bit of data for WDM systems is typically less than comparable non-multiplexed systems. This is because any amplification system required along the link can essentially be shared by all of the separate channels transmitted in a single fiber link. With non-multiplexed systems, each channel/fiber would require its own amplification system.  
           [0004]    Nonetheless, there are challenges associated with implementing WDM systems. First, the transmitters and receivers are substantially more complex since, in addition to the laser diodes and receivers, additional optical components are required to combine the channels into, and separate out the channels from, the WDM optical signal. Moreover, there is the danger of channel drift where the channels loose their spectral separation and overlap each other. This interferes with channel separation and demodulation at the receiving end.  
         SUMMARY OF THE INVENTION  
         [0005]    In order to ensure that proper guard bands are maintained between adjacent channels and to also ensure that the carrier frequencies or wavelengths of the channels are correct both relative to other channels and relative to their wavelength assignments, optical monitoring systems are required in most WDM transmission systems. They are also useful in WDM channel routing systems, such as add/drop multiplexers and switches to ensure that the specific optical channels are being property controlled. Further, information concerning the relative and absolute powers in the optical channels is important as feedback to variable attenuators, for example.  
           [0006]    Historically, however, optical monitoring systems have been relatively large, complex systems. Their size and complexity, and resulting maintenance requirements, prevented them from being integrated into systems offering high levels of functionality such as cross-connect switches, amplifier systems, and integrated receivers, monitoring systems and transmitters, for example.  
           [0007]    The present invention concerns an optical monitoring system that is capable of being integrated into a small package to be used as a subsystem, or possibly even as a stand-alone system, in a WDM system, or other application requiring optical spectral monitoring.  
           [0008]    In general, according to one aspect, the invention features an integrated optical monitoring system. It comprises a hermetic package and an optical bench sealed within the package. An optical fiber pigtail enters the package via a feed-through to connect to and terminate above the bench. A tunable filter, connected to the top of the bench, filters an optical signal transmitted by the fiber pigtail. A detector, also connected to the bench, detects the filtered signal from the tunable filter. Thus, the entire system is integrated together, on a single bench within a preferably small package. This configuration makes the system useful as a subsystem, for example, in a larger system offering higher levels of functionality and optical signal processing capability.  
           [0009]    In the preferred embodiment, an isolator is also integrated onto the bench to prevent back reflections into the fiber pigtail.  
           [0010]    The preferred embodiment uses a reference signal source, also preferably integrated on the optical bench that generates a reference signal, which is filtered by the tunable filter. Such a reference signal enables absolute measurements of optical signal wavelength to ensure that each optical signal is broadcasted at the proper wavelength and to detect such problems as wavelength drift across all of these signals. As a result, the system is capable of detecting absolute frequency, in addition to ensuring that guard-bands are maintained between adjacent channels, for example.  
           [0011]    In the current embodiment, the reference signal source comprises a broadband source and an etalon. The etalon converts the broadband signal from a super luminescent LED (SLED), for example, into a signal with stable spectral characteristics.  
           [0012]    In other embodiments, two physically discrete tunable filter cavities are utilized. Typically, the cavity tuning is synchronized to obtain net signal transmission through both cavities.  
           [0013]    In general, according to another aspect, the invention is also characterized as a method for constructing an integrated optical monitoring system. This method comprises installing an optical bench in a hermetic package. A fiber pigtail is inserted through a fiber feed-through, into the package, and terminated on the optical bench. A tunable fiber is also installed on a top of the bench to filter an optical signal from the fiber pigtail. Finally, a detector is installed on the bench to detect the filtered optical signal from the tunable filter.  
           [0014]    The above and other features of the invention including various novel details of construction and combinations of parts, and other advantages, will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular method and device embodying the invention are shown by way of illustration and not as a limitation of the invention. The principles and features of this invention may be employed in various and numerous embodiments without departing from the scope of the invention. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    In the accompanying drawings, reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale; emphasis has instead been placed upon illustrating the principles of the invention. Of the drawings:  
         [0016]    [0016]FIG. 1 is a schematic, block diagram illustrating a first embodiment of the optical monitoring system with insets showing the spectral characteristics of the WDM signal and filter transfer function; according to the present invention;  
         [0017]    [0017]FIGS. 2A, 2B, and  2 C are spectral plots of exemplary WDM signals illustrating various problems that can be diagnosed with the optical channel monitoring system of the present invention;  
         [0018]    [0018]FIG. 3 is a more detailed block diagram illustrating the optical train of a first embodiment of the optical channel monitoring system of the present invention;  
         [0019]    [0019]FIG. 4 is a spectral plot illustrating the WDM system and reference signals of the first embodiment of the optical channel monitoring system of the present invention;  
         [0020]    [0020]FIG. 5 is a perspective view of the integrated optical channel monitoring system of the first embodiment of the present invention;  
         [0021]    [0021]FIG. 6 is a partial perspective view showing a hermetic package with its top removed and the optical bench installed inside the package;  
         [0022]    [0022]FIG. 7 is a schematic diagram showing an alternative implementation of a portion of the optical train surrounding the tunable filter in which the filter is arranged in a double pass configuration;  
         [0023]    [0023]FIG. 8 is a spectral plot of the filter&#39;s transfer function in a single and double pass configuration, according to the invention; and  
         [0024]    [0024]FIG. 9 is an optical train of an optical power monitor without the integrated reference signal source/detector. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0025]    [0025]FIG. 1 illustrates an optical system monitoring system  100 , which has been constructed according to the principles of the present invention.  
         [0026]    In the preferred or typical implementation, the system receives a WDM signal  14 , the spectral characteristics of which are illustrated by inset plot  10 . Specifically, plot  10  shows power as a function of wavelength. The WDM signal  14  comprises multiple channels or modulated carrier signals  12 . In the present scheme, these channels are distributed in two bands, typically termed the C-band, which stretches from 1530 to 1565 nm, and the L-band, which stretches from 1570-1605 nm.  
         [0027]    The WDM signal  14  enters the monitoring system  100 . According to the first embodiment, a wavelength reference signal  111  from a reference source  110  is added to the WDM signal  14 . The combined WDM and wavelength reference signal  14 / 111  is then filtered by a tunable filter  150 . Inset plot  114  illustrates an exemplary filter transfer function for the tunable filter  150 . The transmission peak  116  is variable based upon a control signal  120 , which is generated by the driver electronics  118  under control of the controller  128 . The driver electronics include includes a DC-DC power supply, a ramp generator, a thermo-electric cooler drive circuit, and LED driver.  
         [0028]    The combined optical signal  16 , which has been filtered by the tunable filter, includes both the filtered wavelength reference signal and the filtered WDM signal  14 . The filtered reference signal is then detected by a reference detector  122 , and the filtered WDM signal is detected by a signal detector system  124 . These detectors yield electronic signals that are received by post processing electronics  126 . A subsequent controller  128  performs analysis functions such as channel inventory.  
         [0029]    In the preferred embodiment, the post processing electronics  126  includes optical receiver circuits, the signal and wavelength reference and digital hardware, including an analog to digital converter.  
         [0030]    Preferably, each detector operates in a differential detection scheme to minimize common-mode noise with gain-switching multiplexor to increase dynamic range. Gain switching is performed with a 4:1 multiplexor and several resistors. This configuration allows for different receiver sensitivities to be obtained via software command of the processor  128 . The advantage of doing this is to allow for an increased dynamic range. Each scan is performed several times at different gains and a recorded signal is combined in software.  
         [0031]    In the preferred embodiment, the analog to digital converter samples at 200 kilo-samples per second to one Megasamples per second. The controller  128  with the required RAM allows for the storage for samples and processing.  
         [0032]    According to the preferred embodiment, the optical channel monitoring system of FIG. 1 has a number of different modes of operation. In a basic mode, that is a single channel scan, an increasing ramp voltage is applied to the tunable filter  150 . This drives the changes in the size of the Fabry-Perot cavity of the filter  150  in a quasi-linear fashion. Because of the self calibration, the particular characteristics of the voltage ramp are not critical, since continuous calibration is performed by the inclusion of the out of band reference signal.  
         [0033]    [0033]FIGS. 2A, 2B, and  2 C illustrate different problems that can be characterized by the optical system monitoring system  100 . For example, in FIG. 2A, the relative strengths of the signals  12 , along with their absolute signal strengths relative to the noise floor  14 , are detectable. This information can be used as a control signal an upstream or downstream variable attenuator. As illustrated in FIG. 2B, inter-channel artifacts  16  are also detected. Finally, as illustrated in FIG. 2C, gain tilt problems, typically added by amplification systems, are also identifiable. Nonetheless, it should be understood that the present invention has applicability to many other applications where the spectral content of a signal is relevant.  
         [0034]    [0034]FIG. 3 shows the optical train of the optical channel monitoring system.  
         [0035]    The fiber  132  terminates above an optical bench  134 . The optical signal  14  is emitted out of the typically cleaved or cleaved-polished end-face of the fiber.  
         [0036]    The optical signal is typically diverging as it is emitted from the fiber&#39;s core. It is collimated by a first collimation lens  136 . Preferably, all lenses are formed utilizing mass-transport processes as described in U.S. Pat. No. 5,618,474, the teachings of which are incorporated herein by this reference in their entirety. The invention, however, is compatible with other types of microlenses such as those generated by diffractive, binary optics, gradient index processes, or refractive element replication, for example.  
         [0037]    A dichroic mirror  140  is used to add the reference signal  111  to the optical signal  14 . These dichroic mirrors or filters are typically referred to as WDM filters. In the illustrated implementation, the WDM filter  140  is reflective in a band surrounding 1300 nm, but transmissive in a band surrounding 1500 nm.  
         [0038]    In the illustrated embodiment, the 1300 nm reference signal is generated by a light emitting diode  142 . In one implementation, the light emitting diode is a super luminescent light emitting diode (SLED).  
         [0039]    The diverging beam from the LED is collimated by a second collimating lens  144 . An etalon  146  is used to convert the relatively wide-band signal from the SLED into a reference signal with stable spectral characteristics. More specifically, the etalon  146  functions as a Fabry-Perot filter with a 200 GigaHertz (GHz) free spectral range (FSR). This effectively converts the SLED&#39;s continuous, broadband spectrum into a signal with energy peaks every 200 GHz. These peaks are stable, particularly when the temperature of the system is controlled by a thermoelectric cooler or is otherwise stabilized.  
         [0040]    A fold mirror  145  redirects the reference signal to the WDM filter  140 . It should be noted, however, that this mirrors is not required, but is simply used to facilitate integration of the system on a compact bench.  
         [0041]    The combined optical signal  14 / 111  is transmitted through an isolator  138 . This component is used to prevent back-reflections from the subsequent optical components into the fiber  132 .  
         [0042]    A first focusing lens  148  is used to focus the collimated combined beam  14 / 111  onto a tunable filter  150 . After the tunable filter, the beam is recollimated by a third collimating lens  152 , and transmitted to a second dichroic/WDM filter  154 .  
         [0043]    The second WDM filter  154  functions to separate the filtered reference signal from the filtered optical signal in the filtered beam  16  from the tunable filter  150 . In the illustrated implementation, the second WDM filter  154  is reflective in a band around 1300 nm, but transmissive in a band around 1500 nm. As a result, the filtered reference signal is directed to the wavelength reference detector  122  for optical-electrical conversion.  
         [0044]    The filtered optical signal is transmitted to the signal detector system  124 . In the illustrated embodiment, the L- and C-bands are separated from each other by a third WDM filter  156 . This WDM filter  156  is reflective to the C-band and transmissive to the L-band. As a result, the C-band of the WDM signal is detected by a C-band photodiode  158 ; the L-band is transmitted through the WDM filter  156  to be detected independently by an L-band photodiode  160 . In other embodiments, more that two bands, such as three or four, are detected simultaneously by adding additional WDM filters and detectors.  
         [0045]    The FIG. 3 embodiment provides for out-of-band calibration. This yields the advantage that the calibration can occur simultaneously with wavelength monitoring. Specifically, one or more of the filter&#39;s modes are used for signal detection while another mode is used to simultaneously filter the calibration signal.  
         [0046]    In alternative embodiments, a similar stable source is used for in-band calibration. One downside to such embodiments, however, is the fact that complex post processing and/or time multiplexing functionality is required upstream of the detectors to switch between signal monitoring and signal calibration.  
         [0047]    In alternative embodiments, other LED sources are used, such as LED sources operating at approximately 1400 nm, such as an InGaAsP SLED.  
         [0048]    The salient features of the tunable filter  150  are its selectable free spectral range. In the preferred embodiment, the free spectral range is 20 nm&lt;FSR&lt;170 nm at 1550 nm wavelength. It preferably also has high finesse, i.e., greater than 3,000, and a compact size.  
         [0049]    In the preferred embodiment, the filter is as described in patent application Ser. No. 09/649,168, by Flanders, et al., entitled Tunable Fabry-Perot Filter, filed on an even date herewith, this application is incorporated herein by this reference.  
         [0050]    In the preferred embodiment, a 40 nm FSR is selected. This enables simultaneous scans of the C and L-bands, in addition to calibration relative to the reference band. Generally, to enable simultaneous scanning, the FSR of the filter must be greater than the bandwidth of at least one of the bands of interest so that successive modes of the filter can access both bands simultaneously. The FSR, however, must be less than the combined bandwidth of bands, again to enable simultaneous access. Generally, the FSR is determined by the length 1 of the Fabry-Perot cavity in the filter, FSR=21/c.  
         [0051]    This three-way simultaneous scanning reduces the total scan time while providing for simultaneous calibration. In other embodiments, the free spectral range of the tunable filter is increased to 57.5 nm to enable monitoring of the optical service channels that flank the C-and L-bands.  
         [0052]    In some implementations, a spatial mode aperture is used in conjunction with the tunable filter. Such intra-filter apertures are desirable when extra cavity mode control devices are not used. For example, in some other implementations, a length of single mode fiber follows the filter to attenuate higher order modes.  
         [0053]    [0053]FIG. 4 is a plot of power as a function of wavelength illustrating the spectral relationships between the active and passive optical components of FIG. 3 embodiment.  
         [0054]    Plot  210  illustrates the spectrum of the light emitted by the SLED  142 . As illustrated, it is a relatively broadband signal stretching from approximately 1250-1350 nm. The etalon, however, functions as a Fabry-Perot filter to convert the wideband output to a series of spikes spaced at 200 GHz centered around 1300 nm.  
         [0055]    Plot  214  illustrates the reflectance of the first WDM filter  140 . It is reflective in the 1300 nm range, but transmissive around the 1550 nm range. This allows the combination of the reference signal  111  and the optical signal  14  to produce the combined signal  14 / 111 .  
         [0056]    Plot  220  shows an exemplary optical signal  14 , comprising multiple energy spikes associated with each channel, stretching across the C and L-bands between approximately 1500 nm to over 1600 nm. Spectrally on either side of the channels are two optical service channels  222 ,  224 , which can be used to transmit additional channel information.  
         [0057]    Plot  216  is the reflectance curve of the third WDM filter  156 . It has a sharp transition between the C and L-bands to thereby separate the two bands so that they can be separately detected by the C-band photodiode  158  and the L-band photodiode  160 .  
         [0058]    [0058]FIG. 5 illustrates the integration of the optical channel monitoring system  100  on a single, miniature optical bench  134 . It also illustrates a second embodiment of the optical channel monitoring system, which does not have separate detectors for the C- and L-bands. Instead, a single detector  160  is used to detect the optical signal. This has the advantage of simplified construction, but negates any opportunity for simultaneous C- and L-band scanning. One implementation relies on an increased filter spectral range of about 115 nm or greater to scan the entire signal band of interest. In other implementations, the C/L band WDM filter  156  is installed in front of the detector  160  to provide for C or L band scanning only.  
         [0059]    Specifically, the fiber  132  is terminated on the bench  134  at a mounting and alignment structure  252 . This mounting and alignment structure  252  holds the fiber in proximity to the first collimating lens  136  held on its own mounting and alignment structure  254 .  
         [0060]    In the reference signal optical train, the SLED  142  generates the broadband beam, which is focused by the second collimating lens  144  held on mounting and alignment structure  256 . This collimates the beam to pass through the etalon  146  installed on the bench  134 . The reference beam generated by the etalon is reflected by fold mirror  145  to the first WDM filter  140 . As a result, the combined beam  14 / 111  is transmitted to the isolator  138 , which is installed directly on the bench  134  in the illustrated implementation.  
         [0061]    After the isolator, a focusing lens  148  held on mounting and alignment structure  258  focuses the combined beam onto the tunable filter  150 , which is held on the filter mounting and alignment structure  258 . The beam from the filter  150  is re-collimated by a third collimating lens  152  held on mounting and alignment structure  260 . This beam is then separated into the reference beam and the optical signal by a second WDM filter  154 . The reference signal is detected by detector  122 . The filtered optical signal is transmitted through the second WDM filter  154  to the signal photodiode  160 .  
         [0062]    Also shown is the installation of the thermistor  270 , which is used by the controller to control the package&#39;s thermoelectric cooler  
         [0063]    [0063]FIG. 6 illustrates the installation of the optical bench  134  into a hermetic package  300 . The thermoelectric cooler  310  is installed under the bench  134 . The optical fiber  132  passes through an optical fiber feed through  312  to terminate on the optical bench  134 . In the figure, the hermetic package  300  has its top removed. Preferably, this is a standard 0.75×0.5 inch butterfly hermetic package.  
         [0064]    [0064]FIG. 7 is a block diagram illustrating the configuration of the tunable filter according to a third embodiment of the present invention. In this embodiment, the tunable filter  150  in FIG. 3, for example, is replaced with the illustrated system.  
         [0065]    Specifically, the combined optical signal/reference signal  14 / 111  from the isolator  138  is sent through a polarization scrambler  410 . This yields an unpolarized signal, of which 50% passes through polarization beam splitter  412 . The transmitted signal is indicated by reference numeral  411 . The polarization scrambler ensures that the incoming beam has a uniform distribution of polarization states so that the polarization beam splitter always passes exactly 50% of the light. Without the scrambler, the incoming beam could have had its polarization state either parallel or perpendicular to the polarization beam splitter, or an intermediate state, meaning that the transmitted beam would have varied between 0 and 100%.  
         [0066]    The optical signal essentially passes through two, series, synchronized Fabry-Perot filter cavities  416 . This is accomplished by sending the signal to the right, in FIG. 7, through the tunable filter  150 , reflecting the signal with a Faraday mirror  414  and then sending the signal back through the Fabry-Perot cavity  416  a second time. The Faraday mirror  414  has the effect of rotating the polarization of the beam  411  by 90 degrees.  
         [0067]    The signal with the rotated polarization is separated by the polarization beam splitter  412  and is output as signal  16 . This combined and twice-filtered signal is sent to a single detector, a detector system, or L-band, C-band, and reference signal photodetectors  122 ,  158 ,  160 , depending on the implementation/embodiment.  
         [0068]    [0068]FIG. 8 illustrates in increased wavelength selectivity obtained by the double pass or two filter cavity arrangement. The transfer function a single pass filter is illustrated by plot  510 —whereas in the double-pass configuration, much steeper transfer function is achieved as illustrated by plot  512 .  
         [0069]    The double-pass or two filter cavity configuration has the advantage of also de-emphasizing any side lobes in the filter&#39;s transfer function.  
         [0070]    [0070]FIG. 9 shows the optical train according to still another embodiment of the present invention. This configuration is termed an optical power monitoring system. The reference signal is not present. C-band and L-band photodiodes  158 ,  160 , however, are provided. This is useful when the relative spacing of the optical channels  12  is important, but not necessarily the absolute wavelengths of those optical channels  12  in the optical signal  14 .  
         [0071]    While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.