Abstract:
A tunable optical filter system  10  has a reference source system  24  that is integrated with the tunable filter  22  on bench  14  and within hermetic package  12 . The reference source system  24  is temporally modulated to decrease interference or crosstalk into the scan of the optical signal  64  of interest. Specifically, a system controller  100  energizes the reference source during a reference scan in which the tunable filter  22  is scanned across a spectrum of the optical reference  66 . The controller  100 , however, lowers, such as simply decreasing or entirely cutting, power to the reference source system  24  during a signal scan, in which the tunable filter  22  is scanned across the optical signal&#39;s spectrum. In this way, interference during the signal scan from the reference source system is reduced.

Description:
BACKGROUND OF THE INVENTION 
     In wavelength division multiplexed (WDM) systems, tunable optical filter systems are typically used in monitoring applications. The filters, however, are also applicable to wavelength add/drop or routing devices, tunable receivers, and tunable sources, for example. Moreover, tunable optical filter systems have relevance to other applications. Remote trace chemical specie detection is one example. 
     In these applications, wavelength accuracy/stability is desirable. Many times discrimination of a few Gigahertz is necessary, with hundreds of Megahertz being desirable. 
     Achieving this level of wavelength stability through the design of the tunable filter can be challenging. It requires that the optical cavity length, in the case of a Fabry-Perot tunable filter, be controlled to picometers, for the infrared wavelengths, over the device&#39;s lifetime, which can be many years. Changes in optical cavity spacing, electrostatic cavity spacing (if used), and membrane spring constant can yield measurement shifts. Capacitive sensors typically do not have the stability required to compensate for these shifts. 
     As a result, many systems utilize reference sources that generate an optical reference signal with stable, known spectral features. The system&#39;s tunable filter is scanned across these spectral features, and the system uses the detected position of the features in a calibration for a subsequent scan of the signal. 
     SUMMARY OF THE INVENTION 
     The common approach for calibration using a reference source is to periodically switch in the reference source onto the optical fiber pigtail that transmits optical signals to the fiber-based or microelectromechanical system (MEMS) based filter, for example. This allows the system to calibrate the tunable optical filter against the spectral features of the reference source, and then switch to the optical signal for the scan of interest. 
     There are a number of problems or drawbacks associated with this scheme. First, the process of switching between the optical signal and the reference source can disturb the behavior of the tunable optical filter. This injects some uncertainty into the calibration. Moreover, the resulting system has a relatively low level of integration since the reference source system is not integrated with the tunable filter. Separate fiber pigtails are required between the system and each of the optical signal source and the reference source system. 
     An alternative approach is to integrate the reference source system with the tunable optical filter on a common optical bench using micro-optical bench technology, for example. This allows the reference source system, tunable optical filter, and possibly a detector system to be integrated together within a single, small hermetic package. 
     Experimentation, however, has demonstrated that a different set of problems can arise in these relatively highly integrated systems, especially when free space optical interconnects are used. Stray light can exist within the package that can be detected during the signal scan. This has the effect of raising the noise floor of the system, impacting system performance. 
     The present invention is directed to a tunable optical filter system. It has a reference source that is integrated with the tunable filter. According to the invention, this reference source is temporally modulated to decrease interference or crosstalk into the scan of the optical signal of interest. 
     In general, according to one aspect, the invention features a tunable optical filter system. The system comprises a package and a tunable filter, which is installed within the package. A reference source system is further commonly installed within the package. This reference source system generates an optical reference that is filtered by the tunable filter and typically used in its calibration. A detector system is further provided that detects the optical reference and an optical signal after being filtered by the tunable filter. 
     According to the invention, a system controller energizes the reference source during a reference scan in which the tunable filter is scanned across a spectrum of the optical reference. The controller, however, lowers, such as simply decreasing or entirely cutting, power to the reference source system during a signal scan, in which the tunable filter is scanned across the optical signal. In this way, interference during the signal scan from the reference source system is reduced. 
     In the present implementation, the package is a hermetic package. A butterfly configuration is shown. The tunable filter is a MEMS Fabry-Perot configuration, in the current implementation. The tunable filter can have a single resonant cavity or multiple resonant cavities depending on the required performance. 
     According to the implementation, the optical reference, which is generated by the optical reference system, comprises stable or known spectral features. The calibration of the tunable filter is made against these spectral features. In one example, the stable spectral features are generated using a light source, e.g., broadband, and a filter that generates the optical reference from the emission of the light source. The combination of a super luminescent light emitting diode (SLED) and fixed Fabry-Perot etalon has been tested. 
     Depending on the implementation, the detector system can comprise single or multiple detectors. In a multidetector configuration, a first detector can detect the optical reference and a second detector can detect the optical signal. Path separation can be achieved using a dichroic filter. Different orders of the filter are preferably used to scan the optical reference and the optical signal. In an alternative implementation, the detector system comprises a detector that detects both the optical reference and the optical signal. 
     In one embodiment, the controller ramps the tuning voltage to the tunable filter to perform the reference scan either before or after the signal scan. In the preferred embodiment, however, the reference scan is performed both before and after the signal scan to enable a two point calibration on either side of the optical signal&#39;s spectrum. These tuning voltage ramps can be increasing or decreasing, linear or non-linear, ramps. Finally, in one implementation, the controller entirely removes power to the reference source during the signal scan. Alternatively, however, the controller can simply reduce, but not entirely turn-off, power to the reference source system. This later approach can mitigate shifts due to thermal transients, for example. 
     In general, according to another aspect, the invention features a method for controlling a tunable optical filter system. This method comprises driving a tunable optical filter to scan over an optical reference spectrum and an optical signal spectrum. A reference source system is controlled to generate the optical reference, while the tunable filter is scanning the optical reference spectrum. While the tunable filter is scanned over the optical signal spectrum, the power to the optical reference system is lowered. 
     The step of driving the tunable filter can comprise ramping the drive voltage to scan the optical reference spectrum and then ramping the drive voltage to scan the optical signal spectrum. In the scan of the optical reference spectrum, the optical signal can be blocked from reaching the tunable filter. 
     In a preferred embodiment, the step of driving the tunable filter comprises scanning the optical reference spectrum before and after the scan of the optical signal spectrum. 
    
    
     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 
     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: 
     FIGS. 1A and 1B are perspective views of two integrated reference source/tunable optical filter systems to which the principles of the present invention are applicable; 
     FIGS. 2A and 2B are block diagrams of optical filter systems illustrating modalities for reference source stray light interference; 
     FIGS. 3A through 3C are plots of spectral power (dBm) as a function wavelength (nanometers) illustrating the spectral relationship between the optical reference and communications optical signal with plots of reference source state as a function of time from scan start (arbitrary units), according to the present invention; 
     FIGS. 4A through 4B are plots of spectral power (dBm) as a function wavelength (nanometers) illustrating the spectral relationship between the optical reference and communications optical signal according to another embodiment with plots of the reference source state as a function of time from scan start (arbitrary units); 
     FIGS. 5A through 5B are plots of spectral power (dBm) as a function wavelength (nanometers) illustrating the spectral relationship between the optical reference and communications optical signal according to still another embodiment with plots of reference source state as a function of time from scan start (arbitrary units); and 
     FIGS. 6A and 6B are block diagrams illustrating the operation of a tunable optical filter system during a reference scan and during a signal scan according to an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIGS. 1A and 1B show tunable optical filter systems, to which the principles of the present invention are applicable. 
     Specifically, with reference to FIG. 1A, the tunable optical filter system generally comprises a package  12  and an optical bench  14 , which is sometimes referred to as a submount. The bench  14  is installed in the package, and specifically on a thermoelectric (TE) cooler  16 , which is located between the bench  14  and the package  12 , in the specific illustrated embodiment. 
     The package  12 , in this illustrated example, is a butterfly package. The package&#39;s lid  18  is shown cut-away in the illustration. 
     The optical system, which is installed on the top surface of the bench  14 , generally comprises a detector system  20 , a tunable filter system  22 , a reference source system  24 , and a signal source  26 . 
     In more detail, the optical signal to be monitored is transmitted to the system  10  via a fiber pigtail  28 , in the illustrated example. This pigtail  28  terminates at an endface  30  that is secured above the bench  14  using a fiber mounting structure  32  in the illustrated implementation. The optical signal passes through a first lens optical component  33  and a second lens optical component  34 , which launches the optical signal into a tunable filter optical component  36 . A MEMS implementation of the tunable filter is shown. The filtered signal passes through a third lens optical component  38  and is then detected by an optical signal detector  40 . 
     In the illustrated implementation, each of the lens and tunable filter optical components comprises the optical element and a mounting structure that is used to secure the optical element to the bench, while enabling post installation alignment. 
     Turning to the path of the optical reference, the emission from a reference light source  42 , such as a broadband source, e.g., a SLED, passes through reference lens optical component  44  to a fixed filter  46 , which, in the present implementation, is a Fabry-Perot etalon. It converts the broadband spectrum of the SLED  42  into a series of spectral peaks, corresponding to the various orders of the etalon, thereby producing the stable spectral features of the optical reference. 
     The optical reference is then reflected by fold mirror  48  to a dichroic or WDM filter  50 , which is tuned to be reflective at the wavelength of the optical reference, but transmissive within the band of the optical signal. Thus, the optical reference is similarly directed to the optical filter system  22 . 
     At the detector system  20 , a dichroic filter  52  reflects the optical reference to a reference detector  54 . 
     FIG. 1B shows an operationally similar tunable optical filter system  10 , for the purposes of the present invention. Reference numerals have been used for functionally equivalent parts. The differential between the two designs lies in the design of the detector system  20 . This second embodiment utilizes only a single detector  40 ,  54  that detects both the optical reference and the optical signal. In this illustration, the package is not shown for clarity. 
     FIGS. 2A and 2B illustrate a stray light interference problem that can arise with the systems illustrated in FIGS. 1A and 1B. Stray light from the reference source system  24  can be reflected off the walls of the package  12 . This stray light  60  can reach the signal detector  40 . As a result, the optical signal detector  40  is thereby responsive both to the filtered optical signal  62  and to the stray interfering light  60  from the reference source system  24 . This has the effect of decreasing the system&#39;s performance by raising the noise floor, for example. This effect occurs whether separate reference and signal detectors are used as shown in FIG. 2A or a common detector is used as illustrated in FIG. 2B since the detectors will not discriminate relative to wavelength. 
     Interestingly, another source of stray light in the package  12  comes from the optical signal source  26 , but this stray light source is less of a problem. 
     That is, in the typical WDM system, the WDM optical signal  64  has multiple populated wavelength or channels. One of these channels may correspond to the filter&#39;s instantaneous passband and thus propagate through the filter and be detected. The other channels, however, will be reflected since Fabry-Perot filters, for example, reflect light that is outside of their passband. The optical signal detector  40  will thus be responsive both to the filtered optical signal  62  and to any stray light that propagates through the optical train and is rejected by the Fabry-Perot tunable filter  22 . 
     Generally, however, stray, optical signal light does not dramatically impact performance since the level of this stray light tends to decrease with decreases in the level of the optical signal  64 . Thus, when the system  10  is detecting a low power optical signal  64 , any stray light in the package from this optical signal  64  will generally be lower, thereby lowering the noise floor as the signal power decreases. Stray signal light will become more of a problem when there are high levels of power tilt between the channels in the WDM signal  64 . In most optical systems, however, this type of tilt is minimized by design to prevent interchannel interference. 
     According to the invention, the system  10  has a controller  100  that controls both the operation of the tunable filter  22  via a driver  110  and the light source  42  of the optical reference system  24 . They are commonly controlled, i.e., synchronized, so that the power to the light source  42  is applied during the reference scan, when the tunable filter&#39;s passband is scanning the spectral features of the optical reference  66 , but decreased or completely removed when the tunable filter is scanning across the spectrum of the optical signal  64 , i.e., signal scan. 
     FIGS. 3A through 3C are plots of spectral power as a function of wavelength and reference source state as a function of time from scan start. The plots illustrate the spectral relationship between the optical signal  64  and the optical reference  66 . Specifically, in the specific example of FIGS. 3A-3C, the optical reference  66  is spectrally divided into high frequency portion  66 A that exists below the wavelength range of the optical signal  64  and the low frequency portion  66 B that exists above the wavelength range of the optical signal  64 . 
     During operation, the controller  100  controls the voltage driver  110  to execute a voltage ramp to drive the tunable filter  22 . This results in the passband  70  of tunable filter  22  being scanned, in one example, first across the high frequency portion  66 A of the optical reference as illustrated in FIG. 3A, then across the spectrum of the optical signal  64  as illustrated in FIG.  3 B. Finally, the passband  70  of the tunable filter  22  is scanned across the low frequency portion  66 B of the optical reference as illustrated in FIG.  3 C. 
     According to this operation, the system  10  converts the scanned spectrum into a time series that is detected by the detector system  20  and analyzed by the controller  100 . 
     According to the invention, time series nature of this spectral readout is used to minimize interference from the reference source system  24  during the signal scan. 
     Specifically, as illustrated FIG. 3A, the state of the reference source is on or high such that it is emitting light to generate the optical reference  66  during the reference scan, when the passband  70  of the tunable filter  22  is being scanned across the high frequency portion  66 A of the optical reference  66 . 
     In contrast, during the signal scan as illustrated in FIG. 3B, the reference source system  24 , and specifically SLED  42 , is de-energized or placed in a lower or low power state during the signal scan, when the passband  70  is within the spectral range of the optical signal  64 . In one implementation, the reference source system is turned-off or switched to a non-emission or low emission state 2 milliseconds after the beginning of the scan (scan start) by the controller  100 . 
     Finally, as illustrated in FIG. 3C, during the second portion of the reference scan, the reference source system is again switched on or into a high power state to enable detection of the low frequency portion  66 B of the optical reference  66 . This switch occurs about 4.6 milliseconds after scan start. 
     Providing a scan of the optical reference  66  spectrally above and below the optical signal has advantages in that it enables two-point calibration or curve fitting based on optical reference&#39;s spectral features that are spectrally above and below the optical signal  64 . 
     FIGS. 4A and 4B illustrate another embodiment in which the optical reference  66  is located spectrally only on one side of the optical signal  64 . In the specific example, the passband  70  is scanned across the optical reference  66 , illustrated in FIG. 4A, and then across the optical signal  64  as illustrated in FIG. 4B, with the reference source system being in a lower power state during the signal scan. This implementation provides decreased complexity regarding some of the fixed dichroic filters in the system, but enables only a single-sided calibration. 
     FIGS. 5A and 5B illustrate still another implementation of the present invention, which further decreases the complexity of the fixed filter material within the system  10 . Specifically, in this example, the wavelength band of the optical reference and the optical signal are overlapping. It relies on the switching the optical reference and the optical signal. 
     Specifically, as illustrated in FIG. 5A, the passband  70  of the tunable filter system  22  is first scanned across the optical reference  66  in a reference scan. During this scan, the reference source system state is on or in a high emission state. During this scan, a beam switch or shutter  150  is activated, in some embodiments, to prevent the optical signal  64  from reaching the tunable filter, especially if only a single detector is used. 
     Then, as illustrated in FIG. 5B, the reference source state is switched to be de-energized or in a low light emission state and the shutter  150  removed from the beam path of the optical signal to thereby enable the signal scan. 
     FIGS. 6A and 6B illustrate the optical system configuration corresponding to the operation described with reference to FIGS. 5A and 5B. 
     Specifically, during the signal scan, the system  10  is in a state as illustrated in FIG.  6 A. The SLED  42  is in a low power state and not producing the optical reference, or a low or very low intensity optical reference, and the beam switch/shutter  150  is in a transmissive state to thereby enable the optical signal  66  to reach the tunable filter  22  and thereby be detected by the detector  40 ,  54 . 
     FIG. 6B shows the state of the optical system  10  during the reference scan. Specifically, in this state, the shutter  150  blocks the transmission of the optical signal  64  to the tunable filter  22 , but enables the optical reference  66  that is generated by the now energized reference source system  24  to be first filtered by the tunable filter  22  and then detected by the detector  40 ,  54 . The advantage of this system is that it requires no WDM filters in some examples, but requires the addition of the beam shutter/switching element  150 . 
     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. For example, in some implementations, the spectrum of the optical reference and the optical signal may not be adjacent but instead displaces by a free spectral range of the tunable filter. In this case, different orders of the filter are used to scan the optical reference and the optical signal.