Abstract:
Integrated spectroscopy systems are disclosed. In some examples, integrated tunable detectors, using one or multiple Fabry-Perot tunable filters, are provided. Other examples use integrated tunable sources. The tunable source combines one or multiple diodes, such as superluminescent light emitting diodes (SLED), and a Fabry Perot tunable filter or etalon. The advantages associated with the use of the tunable etalon are that it can be small, relatively low power consumption device. For example, newer microelectrical mechanical system (MEMS) implementations of these devices make them the size of a chip. This increases their robustness and also their performance. In some examples, an isolator, amplifier, and/or reference system is further provided integrated.

Description:
RELATED APPLICATIONS  
       [0001]     This application is a Division of U.S. application Ser. No. 10/688,690 filed on Oct. 17, 2003, which is incorporated herein by reference in it entirety. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     Minimally, optical spectroscopy systems typically comprise a source for illuminating a target, such as a material sample, and a detector for detecting the light from the target. Further, some mechanism is required that enables the resolution of the spectrum of the light from target. This functionality is typically provided by a spectrally dispersive element.  
         [0003]     One strategy uses a combination of a broadband source, detector array, and grating dispersive element. The broadband source illuminates the target in the spectral scan band, and the signal from the target is spatially dispersed using the grating, and then detected by an array of detectors.  
         [0004]     The use of the grating, however, requires that the spectroscopy system designer make tradeoffs. In order to increase the spectral resolution of these systems, aperturing has to be applied to the light provided to the grating. As more spectral resolution is required, more light is required to be rejected by the narrowing spatial filter. This problem makes this strategy inappropriate for applications requiring a high degree of spectral precision combined with sensitivity.  
         [0005]     Another approach is to use a tunable narrowband source and a simple detector. A typical approach relies on a tunable laser, which is scanned over the scan band. By monitoring the magnitude of the tunable laser&#39;s signal at the detector, the spectrum of the sample is resolved. These systems have typically been complex and often had limited wavelength scanning ranges, however.  
         [0006]     Still another approach uses light emitting diodes (LEDs) and an acousto-optic modulator (AOM) tunable filter. One specific example combines multiple light emitting diodes (LEDs) in an array, each LED operating at a different wavelength. This yields a relatively uniform spectrum over a relatively large scan band. The light from the diodes is then sent through the AOM tunable filter, in order to create the tunable optical signal.  
         [0007]     The advantage of this system is the use of the robust LED array. This provides advantages over previous systems that used other broadband sources, such as incandescent lamps, which had limited operating lifetimes and high power consumption.  
         [0008]     While representing an advance over the previous technology, the disadvantages associated with this prior art system were related to the use of the AOMs, which are relatively large devices with concomitantly large power consumptions. Moreover, AOMs can also be highly temperature sensitive and prone to resonances that distort or alter the spectral behavior, since they combine a crystal with a radio frequency source, which establishes the standing wave in the crystal material to effect the spectral filtering.  
         [0009]     Grating-based spectrometers also tend to be large devices. The device packages must accommodate the spatially dispersed signal from the sample. Further, the interface between the grating and the detector array must also be highly mechanically stable. Moreover, these grating based systems can be expensive because of costs associated with the detector arrays or slow if mechanical scanning of the detector or grating is used.  
       SUMMARY OF THE INVENTION  
       [0010]     The drawbacks associated with the prior art spectrometers arise from the large size of the devices combined with the high cost to manufacture these devices combined with poor mechanical stability. These factors limit the deployment of spectrometers to applications that can justify the investment required to purchase these devices and further accommodate their physical size.  
         [0011]     Accordingly, the present invention is directed to an integrated spectrometer system. Specifically, it is directed to the integration of a tunable Fabry-Perot system with a source system and/or detector system. The use of the Fabry-Perot filter system allows for a high performance, low cost device. The integration of the filter system with the source system and/or detector system results in a device with a small footprint. Further, in the preferred embodiment, the filter system is based on microelectromechancial systems (MEMS), which yield a highly mechanically robust system.  
         [0012]     In general, according to one aspect, the invention features a spectroscopy system. The system comprises a source system for generating light to illuminate a target, such as a fiber grating or a material sample. A tunable Fabry-Perot filter system is provided for filtering light generated by the source. A detector system is provided for detecting light filtered by the tunable filter from the target. According to the invention, at least two of the source system, tunable Fabry-Perot filter system, and the detector system are integrated together.  
         [0013]     Specifically, in one embodiment, the source system and tunable Fabry-Perot system are integrated together on a common substrate, such as an optical bench, also sometimes called a submount. In another embodiment, the tunable Fabry-Perot filter system and the detector system are integrated together on a common substrate, such as an optical bench. Finally, in still another implementation, all three of the source system, tunable Fabry-Perot filter system, and the detector system are integrated together on a common bench, and possibly even in a common hermetic package.  
         [0014]     Temperature control is preferably provided for the system. Currently this is provided by a heater, which holds the temperature of the system above an ambient temperature, or a thermoelectric cooler. For example, the thermoelectric cooler is located between the bench and the package to control the temperature of the source system, tunable Fabry-Perot filter system, and/or the detector system. As a result, a single cooler is used to control the temperature of the filter and SLED chip, lowering power consumption, decreasing size, while increasing stability.  
         [0015]     In the preferred embodiment, the source system comprises a broadband source. This can be implemented using multiple, spectrally multiplexed diode chips. Preferably, superluminescent light-emitting diodes (SLEDs) are used. These devices have a number of advantages relative to other sources, such incandescent sources. Specifically, they have better spectral brightness, longer operating lifetimes, and a smaller form factor.  
         [0016]     In order to increase the spectral accuracy of the system, a tap can also be used to direct a part of the tunable signal to a detector. A spectral reference, such as a fixed etalon with multiple spectral transmission peaks is placed between the detector and the tap, in order to create a fringe pattern on the detector during the scan, thereby enabling monitoring of the wavelength of the tunable signal. An optical power tap can also be included to monitor the real time emitted optical power during the scan.  
         [0017]     The tunable Fabry-Perot filter system comprises single or multiple filters. In one example, multiple serial filters are used. In another embodiment, multiple parallel filters are used.  
         [0018]     In still further embodiments, multiple detectors can be used. These detectors can be responsive to different wavelengths or a calibration signal.  
         [0019]     In the preferred embodiment, in order to make the system small, compact and highly robust, a micro-electro-mechanical system (MEMS) Fabry-Perot tunable filter is used. These devices can achieve high spectral resolutions in a very small footprint.  
         [0020]     Finally, in the preferred embodiment, isolation is provided between the source system and the tunable Fabry-Perot filter system. This prevents back reflections from the filter into the source system that can disturb the operation of the source system. In one example, an isolator is installed on the optical bench between the SLED and the tunable filter. A quarter wave plate can also be used. This rotates the polarization of the returning light so that it is not amplified by the highly polarization anisotropic SLED gain medium. In another embodiment, the isolation is provided on the bench, with the tunable Fabry-Perot filter system and the detector system.  
         [0021]     The present invention is also directed to an integrated tunable source that combines a broadband source and a tunable filter, such as a tunable Fabry-Perot filter, although other tunable filters could be used in this configuration. Applications for this device extend beyond spectroscopy.  
         [0022]     Between the tunable filter and the light source, isolation is preferably provided. This stops back reflections from the tunable filter into the diode, which could impact its performance. Isolation can be achieved using a number of techniques. In one embodiment, a discrete isolator is used. In another embodiment, when a SLED is used as the source, a quarterwave plate is used between the SLED chip and the filter. Finally, a flat-flat cavity Fabry-Perot tunable filter is used in still another embodiment, with isolation being accomplished by tilting the filter relative to the SLED.  
         [0023]     A variety of other light sources can be used, including LEDs, doped fiber or waveguide amplified spontaneous emission sources, and thermal sources.  
         [0024]     According to still another aspect, the invention features a high power tunable source. This addresses one of the primary drawbacks associated with the use of a broadband source and tunable filter configuration, namely their usually low output power. Specifically, an optical amplifier is further added in order to increase the power of the tunable signal. As a result, power levels comparable to those attainable with tunable lasers can be achieved in this configuration.  
         [0025]     In a typical implementation, the amplifier is a semiconductor optical amplifier (SOA). In other examples, various types of fiber amplifiers are used, however.  
         [0026]     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.  
         [0027]     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  
       [0028]     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:  
         [0029]      FIGS. 1A and 1B  illustrate embodiments of the integrated spectroscopy system according to the present invention;  
         [0030]      FIG. 2  is a perspective view showing a tunable source, according to the present invention, and including a detector system for detecting the tunable signal from the sample;  
         [0031]      FIG. 3A  is a perspective view of the MEMS Fabry Perot tunable filter, used in embodiments of the present invention, which is compatible with tombstone mounting on the optical bench;  
         [0032]      FIG. 3B  is an exploded view of the inventive Fabry Perot tunable filter;  
         [0033]      FIG. 4  is a perspective view showing an amplified tunable source, according to the present invention, in a hermetic package;  
         [0034]      FIG. 5  is a perspective view showing a reference detector embodiment of a tunable source, according to the present invention, in a hermetic package;  
         [0035]      FIG. 6  is a block diagram of an embodiment of the tunable source with both a wavelength and power reference detector;  
         [0036]      FIG. 7  is a perspective view showing still another embodiment of a tunable source, according to the present invention, using two SLED chips;  
         [0037]      FIG. 8  is a perspective view showing a further embodiment of a tunable source, according to the present invention using multiple SLED chips and tunable filters;  
         [0038]      FIG. 9  is a perspective view showing another embodiment of a tunable source, according to the present invention, using parallel filters and multiple SLED sources;  
         [0039]      FIG. 10  is a perspective view showing a further embodiment of a tunable source, according to the present invention, using serial filters;  
         [0040]      FIG. 11  is a plot of wavelength as a function of transmission showing the relationship between the free spectral ranges of the serial filters, in one embodiment;  
         [0041]      FIG. 12A  is a perspective view of a tunable detector spectroscopy system, according to the present invention;  
         [0042]      FIG. 12B  is a perspective view of another embodiment of a tunable detector spectroscopy system, according to the present invention;  
         [0043]      FIG. 13  is a perspective view of still another embodiment of a tunable detector spectroscopy system, according to the present invention, using multiple parallel filters; and  
         [0044]      FIGS. 14 and 15  show two fully integrated spectroscopy systems in which a tunable source and a detector system are integrated on the same bench. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0045]      FIGS. 1A and 1B  illustrate an integrated spectroscopy system  1 , which has been constructed according to the principles of the present invention.  
         [0046]     Specifically,  FIG. 1A  shows two alternative integration configurations.  
         [0047]     According to configuration  1 , a source system  100  is provided. This is a broadband source, which generates light  40  for illuminating a target, such as sample S- 1  or a fiber grating, for example. This target selectively absorbs and/or scatters the light from the source system  100 . The transmitted light is then received by a tunable Fabry-Perot filter system  200 . This functions as a narrow band tunable spectral filter. It tunes its passband over the scan band within the spectral band of the source system  100 . As a result, it resolves the spectrum of the target S- 1  into a time response. This time-resolved signal is then detected by detector system  300 .  
         [0048]     According to the integration provided by this configuration  1 , the tunable Fabry-Perot filter system  200  and the detector system  300  are integrated together. Specifically, in the preferred embodiment, the tunable Fabry-Perot filter system  200  and the detector system  300  are installed on a common bench B- 1 . Moreover, in the current embodiment, the tunable Fabry-Perot filter system  200  and the detector system  300  are integrated together on the common bench B- 1  in a common hermetic package.  
         [0049]     The integration of the Fabry-Perot filter system  200 , with the detector system  300  on the common bench B- 1 , yields the tunable detector  20  which is characteristic of the configuration  1  integration.  
         [0050]      FIG. 1A  also illustrates a second configuration, configuration  2  integration. In this second configuration, the source system  100  and the tunable Fabry-Perot system  200  are integrated together. In the preferred embodiment, they are installed together on a common bench B- 2 . Further, in the current implementation, the source system  100  and the tunable Fabry-Perot filter system are integrated together on the common bench B- 2  and installed in a common hermetic package to yield a tunable source  10 . This tunable source  10  generates a tunable signal  30 , which is used to illuminate a target, located in this second configuration at position S- 2 . The target either scatters or absorbs spectral components of the tunable signal as it is scanned across the scan band. This allows the detector system  300  to resolve the time varying signal as the spectral response of the target S- 2 .  
         [0051]      FIG. 1B  illustrates a fully integrated system according to still another embodiment. Here, the source system  100 , the tunable Fabry-Perot filter system  200 , and the detector system  300  are integrated together. Specifically, in the preferred embodiment, they are integrated together and installed on a common bench B. This bench B is preferably located in a hermetic package.  
         [0052]     Depending on whether the tunable source system  100 , the Fabry-Perot filter system  200 , and detector system  300 , are combined as a tunable source  10  or tunable detector  20 , the target is located either in position S- 1  or S- 2 . Specifically, in the implementation of a tunable source  10  with the source system  100  and the tunable Fabry-Perot filter system  200  functioning to create a tunable signal, the tunable signal  30  is coupled outside of the hermetic package and off of the bench B to the target S- 2 , in the case of configuration  2 . Alternatively, if the tunable Fabry-Perot filter system  200  and the detector system  300  function to yield the tunable detector  20 , then the broadband signal  40  from the source system  100  is coupled off of the bench and the outside of the hermetic package to the target S- 1  in the case of the first configuration.  
         [0053]      FIG. 2  illustrates a first embodiment of the tunable source  10 . Specifically, in this embodiment, the bench B- 2  holds the tunable Fabry-Perot filter system  200  and the source system  100  on a common planar surface. The generated tunable signal  30  is coupled off of the bench B- 2  by an optical fiber  102 . In the preferred embodiment, this optical fiber  102  is a single transverse mode fiber. This has advantages in that it renders the tunable signal  30  very stable, even in the event of mechanical shock to the single mode fiber  102 .  
         [0054]     The source system  100  is implemented using, in this embodiment, a superluminescent light emitting diode (SLED)  110 . The diode  110  is installed on a submount  112 . The submount  112  is, in turn, installed on the bench B- 2 . In the preferred embodiment, the SLED chip  110  is solder bonded to the submount  112 , which further includes metallizations  114  to facilitate wire bonding to provide electrical power to the SLED chip  110 . Further, the submount  112  is solder bonded to the bench B- 2 , which in turn, has metallizations  115  to enable formation of the solder bonds.  
         [0055]     These SLEDs are relatively new, commercially-available devices and are sold by Covega Corporation, for example, (product numbers SLED 1003, 1005, 1006). These devices are currently available in wavelength ranges from 1,200 nanometers (nm) to 1,700 nm from a variety of vendors. They are waveguide chip devices with long gain mediums similar to semiconductor optical amplifiers. An important characteristic is their high spectral brightness.  
         [0056]     The broadband signal  40  that is generated by the SLED chip is collimated by a first lens component  114 . This lens component  114  comprises a lens substrate  117 , which is mounted onto a deformable mounting structure  118 . The deformable mounting structure is preferably as those structures described in U.S. Pat. No. 6,559,464 B1 to Flanders, et al., which is incorporated herein in its entirety by this reference. The alignment structure system allows for post installation alignment by mechanical deformation of the mounting structure  118  of the lens substrate  117 .  
         [0057]     The collimated light from the first lens component  114  in the preferred embodiment is coupled through an isolation system, such as an isolator  120  or quarterwave plate. The beam from the isolator is then collimated by a second lens element  122  and coupled into the Fabry-Perot tunable filter system  200 . The isolator system prevents all back reflections or back reflections that have a polarization that is aligned with the gain polarization of the SLED chip  110 . These reflections arise from the Fabry-Perot filter system  200 . This isolation promotes the stability in the operation of the SLED chip  110 .  
         [0058]     In the preferred embodiment, the tunable filter system  200  is implemented as a MEMS tunable Fabry-Perot filter  116 . This allows for single transverse mode spectral filtering of the broadband light  40  from the SLED chip  110 , yielding the tunable signal  30 . Tunable signal  30  is coupled into the endface  104  of the single mode optical fiber  102 . In the current embodiment, the endface  104  of the optical fiber  102  is held in alignment with the MEMS tunable filter  116 , via a fiber mounting structure  106 . Again, this allows for post installation alignment of the fiber endface  104  to maximize coupling of the tunable signal  30 , into the single mode fiber  102 . The fiber  102  transmits the tunable signal  30  to target S- 2  and then, the response is detected by detector system  300 .  
         [0059]     Depending on the embodiment, the Fabry-Perot filter  116  has either a curved-flat cavity or a flat-flat cavity. The curved flat cavity increases angular tolerance between the two mirrors of the Fabry-Perot filter. The flat-flat cavity provides better single mode operation. Moreover, there is the option to avoid the necessity for discrete isolators or waveplates by angle isolating the filter for the source system  100 .  
         [0060]      FIG. 3A  is a close up view of the tunable filter  116 . The tunable filter  116  comprises a MEMS die  410 . This has a number of wire bond locations  412  for making electrical connection to the MEMS die  410 . A MEMS die  410  provides the moveable mirror portion or component of the tunable filter. A fixed mirror portion or component  414  is bonded to the MEMS die  410  in order to define the Fabry Perot cavity. In one embodiment, the fixed mirror component provides the flat mirror and the MEMS die  410  provides the curved mirror.  
         [0061]     In the preferred embodiment, the tunable filter  116  is “tombstone” mounted onto the bench B, B- 1 , B- 2 . Specifically, the fixed mirror substrate  414  extends down below the bottom of the MEMS die  416  by a distance L. Specifically, the fixed mirror substrate has a bottom surface  418  that serves as a foot that is bonded to the bench. Preferably, a layer of solder  420  is used to attach the fixed mirror substrate  414  to the bench B. In the preferred embodiment, the distance L is approximately 1-10 micrometers.  
         [0062]      FIG. 3B  is an exploded view of the tunable filter  116 . This shows the fixed mirror substrate  414  disconnected from the MEMS die  410 . Flexures  421  define a MEMS membrane  423 . The deflectable membrane  423  holds the mirror layer  424  of the tunable mirror and covers a depression  425  formed in the membrane  423  that forms the curved mirror of one embodiment. Metallization pads  426  are provided on the MEMS die  410  in order to solder attach the fixed mirror substrate  414  to the MEMS die  410 .  
         [0063]     The general construction of this tunable filter is described in, for example, U.S. patent application Ser. No. 09/734,420, filed on Dec. 11, 2000 (now Publication No. U.S. 2002-0018385). This application is incorporated herein, in its entirety by this reference.  
         [0064]      FIG. 4  illustrates another embodiment of the tunable source  10 . In this embodiment, the broadband signal generated by the SLED chip  110  is again coupled through a first lens component  114  to an isolator  120 . A second lens component  122  is further provided for coupling the broadband signal into the filter  116  of the Fabry-Perot filter system  200 .  
         [0065]     Then, a third lens component  126  is provided to couple the tunable optical signal  30  into a semiconductor optical amplifier  128 . In the preferred embodiment, this semiconductor optical amplifier chip  128  is installed on an amplifier sub-mount  130 , which is installed on the bench B- 2 . The amplified tunable optical signal generated by the semiconductor optical amplifier chip  128  is then coupled into the endface  104  of the optical fiber  102  to be coupled out of the hermetic package  132 . This allows the tunable signal  30  to be coupled, in an amplified state, to the target S- 2  followed by detection by the detector system  300 .  
         [0066]     In some other embodiments additional isolators are located between the fiber endface  104  and the amplifier chip  128  and between the amplifier chip  128  and the third lens component  126 .  
         [0067]     In the preferred embodiment, the hermetic package  132  is a standard telecommunications hermetic package. Specifically, it comprises a standard butterfly package. The lid  136  is shown cut away to illustrate the internal components. Further, the optical bench B- 2  is preferably installed on a thermoelectric cooler  134 , which enables a controlled environmental temperature to stabilize the operation of the SLED chip  110  and the tunable Fabry-Perot filter system  200 .  
         [0068]     Electrical leads  138  are further provided to transmit electrical signals to the pads  146  on the inside of the hermetic package  132 . Wire bond are made between pads  146  and the active components such as the SLED chip  110 , MEMS tunable filter  116 , and SOA  128 .  
         [0069]     The  FIG. 4  embodiment has the advantage that the tunable signal  30  received from the tunable Fabry-Perot filter system  200  is amplified to further increase the dynamic range and the signal-to-noise ratio of the spectroscopy system.  
         [0070]     In the embodiment of  FIG. 5 , the tunable source  10  also combines a SLED chip  110 , a first lens component  114 , isolator  120 , and a second lens component  122 . This launches the broadband signal  40  from the SLED chip into the tunable filter system  200 . A third lens component  126  is further provided. This collimates the beam. A splitter, however, comprising a partially reflective substrate  149 , provides a portion of the tunable signal  30  to a detector  140 . This detector  140  can be used to monitor the magnitude or power in the tunable signal  30 . In another embodiment, a reference substrate  148  is installed between the detector  140  and the tap  149 . This reference substrate  148  provides stable spectral features. In one embodiment, this is provided by a fixed etalon substrate. A controller monitoring the output of the detector  140  compares the tunable signal to the spectral features of the reference substrate  148  to thereby resolve the instantaneous wavelength of the tunable signal  30 .  
         [0071]     In still other embodiments, instead of a reference substrate, a gas cell is used as the spectral reference for calibrating the scan of the tunable filter  116 . Also two splitters can be included to provide simultaneous spectral and power references.  
         [0072]     The tunable signal, which is not coupled to the detector  140  by the tap  149  is launched by a fourth lens component  147  into the fiber endface  104  of the optical fiber  102 .  
         [0073]      FIG. 6  illustrates the general operation provided by a controller  150  of the tunable source  10 . Specifically, the controller  150  is used to control the power or current supplied to the SLED chip  110 . Its broadband signal  40  is coupled to the isolator  120 . The controller also controls the tunable pass band of the tunable filter system  200  to generate the tunable signal  30 .  
         [0074]     In the case of monitoring the frequency of the tunable signal, a first tap  149  couples a portion of the tunable signal to a spectral reference  148 , which in the illustrated embodiment, is a fixed etalon. This allows the detector  140  to detect the wavelength of the tunable signal  30  during the scan.  
         [0075]     In the preferred embodiment, a power detector  154  is also provided. This is added to the optical train using second tap  152 , which again couples the portion of the tunable signal  30  to a power detector  154 . The controller  150  controls and monitors the wavelength detector  140  and the power detector  154  to determine both the wavelength and the power in the tunable signal  30 .  
         [0076]      FIG. 7  illustrates another embodiment of the tunable source  10 . This embodiment is used either to increase the power or the spectral width of the scan band of the tunable source  10 . Specifically, multiple SLED chips, and specifically two SLED chips  110 A and  110 B, are installed together on the optical bench B- 2 . In the illustrated embodiment, the SLED chips  110 A and  110 B are installed on a common sub-mount  112 , which is in turn, bonded to the bench B- 2 .  
         [0077]     Two first lens components  118 A,  118 B are provided to couple the broadband signals from their respective SLED chips  110 A,  110 B and collimate those beams. A combination of a fold mirror  156  and a combiner  158  are provided to combine the broadband signals from each of these SLED chips  110 A,  110 B into a single broadband signal, which is coupled through the isolator  120 .  
         [0078]     The beam from the isolator  120  is then focused by a second lens component  122  into the tunable filter  200 . A third lens component  126  then couples the tunable signal into the optical fiber  102  via the endface  104 .  
         [0079]     In the high power version of the  FIG. 7  embodiment, a polarization rotator, such as a quarterwave plate  160  is provided in the beam path of one of the SLED chips  110 A,  110 B. In the illustrated embodiment, this polarization rotator  160  is provided in the beam path of the second SLED chip  110 . This rotates the polarization of the light from the second SLED chip  110 B by 90°. Then, the combiner  158  is a polarization combiner that is transmissive to the polarization of the light from the first SLED chip  110 A, but reflective to the polarization of light from the second SLED chip  110 B. As a result, the beams from each of the SLED chips  110 A,  110 B are merged into a common broadband signal with increased power.  
         [0080]     In a second implementation of the  FIG. 7  embodiment, the SLED chips  110 A,  110 B operate at different spectral bands. Specifically, SLED chip  110 A generates light in a scan band A and SLED chip  110 B generates light in an adjacent but different scan band B. The combiner  158  is a wavelength division multiplex combiner that is configured to be transmissive to the band of light generated by the SLED chip  110 A, but reflective to light in the band generated by SLED chip  110 B. As a result, the combined signal generated together by the SLED chip  110 A,  110 B has a broader scanband then could be generated by each of the SLED chips individually. This allows for increased bandwidth in the tunable signal  30  that is generated by the tunable source  10 .  
         [0081]      FIG. 8  illustrates another embodiment of the tunable source  10 . This embodiment uses a tunable filter system  200 , which includes an array of tunable filters  116  and broadband light sources in order to increase the spectral width of the scanband. Typically, and in the illustrated embodiment, an array of five SLED chips  110  are mounted in common on the bench B- 2 . The light from each of these SLED chips  110  is collimated by respective first lens components  118 . Specifically, there is a separate lens component  118  for each of these SLED chips  110 . Separate isolators  120  are then provided for the broadband signals from each of the SLED chips  110 .  
         [0082]     An array of second lens components  122  is further provided to couple the broadband signal into an array of tunable filters  200 . Specifically, separate Fabry-Perot tunable filters  116  are used to filter the signal from each of the respective SLED chips  110 . Finally, an array of third lens components  126  is used to re-collimate the beam from the tunable Fabry-Perot filters  116  of the tunable filter system  200 .  
         [0083]     For channel  1 , C- 1 , a fold mirror  156  is used to redirect the beam from the SLED chip  110 . The WDM filter  160  is used to combine the broadband signal from the SLED chip  110  of channel C- 2  with the signal from channel C- 1 . Specifically, the filter  160  is reflective to the wavelength range generated by the SLED chip  110  of channel C- 2 , but transmissive to the wavelength range of light generated by the SLED chip  110  of channel C- 1 .  
         [0084]     In a similar vein, WDM filter  162  is reflective to the signal band generated by the SLED chip  110  of channel C- 3 , but transmissive to the bands generated by SLED chips  110  of channels C- 1  and C- 2 . WDM filter  164  is reflective to the light generated by SLED chip  110  of channel C- 4 , but transmissive to the bands generated by the SLED chips  110  of channels C- 1 , C- 2 , and C- 3 . Finally, WDM filter  158  is reflective to all of the SLED chips, but the SLED chip  110  of channel C- 5 . As a result, the light from the array of SLED chips is combined into a single broad band tunable signal  30 .  
         [0085]     A first tap  149  is provided to reflect a portion of the light through the etalon  148  to be detected by the wavelength detector  140 . Then, another portion is reflected by tap  152  to the power detector  154 . The remaining tunable signal is coupled by the fourth lens component  106  into the optical fiber  102  via the endface  104 .  
         [0086]     The  FIG. 8  embodiment can operate according to a number of different modes via a controller  150 . Specifically, in one example, only one of the SLED chips in channels C- 1  to C- 5  is operating at any given moment in time. As a result, the tunable signal  30  has only a single spectral peak. The full scan band is achieved by sequentially energizing the SLED chip of each channel C- 1  to C- 5 . This tunable signal is scanned over the entire scan band covered by the SLED chips of channels C- 1  to C- 5  turning on the SLED chips in series, or sequentially.  
         [0087]     In another mode, each of the SLED chips is operated simultaneously. As a result, the tunable signal has spectral peaks in each of the scan bands, covered by each of the SLED chips  110  simultaneously. This system results in a more complex detector system  300 , which must demultiplex the separate scan bands from each of the SLED chips  110  from each of the channels at the detector. Specifically, in one embodiment, five (5) detectors are used with a front-end wavelength demultiplexor.  
         [0088]      FIG. 9  shows an embodiment of the tunable source  10  that has both increased power and increased scanning range over a single SLED. It comprises two subcomponents, which are configured as illustrated in  FIG. 7  embodiment. Specifically, each channel C- 1 , C- 2  has two SLED chips  110 A,  110 B that are polarization combined. The output is isolated by an isolator  120  and then filtered by a tunable filter  116  for each channel C- 1 , C- 2 . The signals from the two channels are then wavelength multiplexed using a combination of a fold mirror  170  and a dichroic or WDM filter  172 . Specifically, the dichroic mirror  172  is transmissive to the scan band of the SLED chips  110 A,  110 B of channel C- 1 , but reflective to the SLED chips  110 A,  110 B of channel C- 2 .  
         [0089]     In order to improve the manufacturing yield of the  FIG. 9  embodiment, in one implementation, each of the channels C- 1  and C- 2  are fabricated on separate sub-benches SB- 1  and SB- 2 . The sub-benches SB- 1 , SB- 2  are then bonded to each other or to a common bench in order to yield the  FIG. 9  embodiment.  
         [0090]      FIG. 10  illustrates still another embodiment, which covers a wide spectral band. Specifically, it includes five SLED chips  110 A to  110 E. A series of first lens optical components  118  are used to collimate the beams from each of the SLED chips  110 A- 110 E. In the present embodiment, the SLED chips  110 A to  110 E, each operate over different spectral bands. They are then wavelength combined using a combination of fold mirrors and filters  156 ,  160 ,  162 ,  164  and  158 , as discussed with reference to the  FIG. 8  embodiment.  
         [0091]     The  FIG. 10  embodiment further includes, preferably, two isolators  120 A,  120 B. These isolate respective tunable filters  116 A,  116 B. Lens components  180 ,  182 ,  184 , and  186  are used to couple the optical signal generated by the SLEDS  110 A- 110 E, through the first tunable filter  116 A and the second tunable filter  116 B of the tunable filter system  200 , and then, through the wavelength tap  149  and the power tap  152  to the endface  104  of the optical fiber  102 .  
         [0092]     The use of the five SLED chips  110  increases the effective scan band of the tunable source  10 . In the preferred embodiment, the tandem tunable filters have free spectral ranges FSR as illustrated in  FIG. 11 .  
         [0093]     Specifically, filter  1   116 A, and filter  2   116 B have different free spectral ranges. As a result, they function in a vernier configuration. This addresses limitations in the free spectral range of the tunable filters individually.  
         [0094]     Typically, if a single tunable filter was used, its free spectral range would have to be at least as wide as the total scan band of the broad band signals generated by the sources. In the illustrated embodiment, the tunable filters are combined to increase the free spectral range of the tunable filter system, since the peak transmissivity, through both tunable filters  116 A- 116 B, only arises at wavelengths where the passbands of the two filters  116 A- 166 B are coincident.  
         [0095]      FIGS. 12A and 12B  show tunable detector systems  20 , to which the principles of the present invention are also applicable.  
         [0096]     Specifically, with reference to  FIG. 12A , the tunable detector system generally comprises a package  132  and an optical bench B- 1 , which is sometimes referred to as a submount. The bench B- 1  is installed in the package  132 , and specifically on a thermoelectric (TE) cooler  134 , which is located between the bench B- 1  and the package  132 , in the specific illustrated embodiment.  
         [0097]     The package  132 , in this illustrated example, is a butterfly package. The package&#39;s lid  136  is shown cut-away in the illustration.  
         [0098]     The tunable detector optical system, which is installed on the top surface of the bench B- 1 , generally comprises the detector system  300 , the tunable filter system  200 , and an optional reference source system  24 .  
         [0099]     In more detail, the optical signal from the target S- 1  to be monitored is transmitted to the system via a fiber pigtail  310 , in the illustrated example. This pigtail  310  terminates at an endface  312  that is secured above the bench B- 1  using a fiber mounting structure  314  in the illustrated implementation. The optical signal passes through a first lens optical component  316 , which collimates the beam to pass through an isolator  320 . A second lens optical component  318  launches the optical signal into the tunable filter system  200 . A MEMS implementation of the tunable filter is shown. The filtered signal passes through a third lens optical component  322  and is then detected by an optical signal detector  324 .  
         [0100]     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 most installation alignment.  
         [0101]     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 fixed etalon. It converts the broadband spectrum of the SLED  42  into a series of spectral peaks, corresponding to the various orders of the etalon transmission, thereby producing the stable spectral features of the optical reference.  
         [0102]     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  200 .  
         [0103]     At the detector system  20 , a dichroic filter  52  reflects the optical reference to a reference detector  54 .  
         [0104]      FIG. 12B  shows an operationally similar tunable optical filter system  20 , 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  300 . This second embodiment utilizes only a single detector  324 ,  54  that detects both the optical reference and the optical signal. In this illustration, the package is not shown for clarity.  
         [0105]      FIG. 13  shows another embodiment of the tunable detector  20 . The signal from the target is transmitted to the detector  20  via fiber  310 . A first lens component  316  collimates the light from the fiber. Second lens components  318  couple the light into the tunable filters  116 A,  116 B of the filter system  200 . Third lens components  322  focus the light to the detector system  300 .  
         [0106]     This version uses two tunable filters  116 A,  116 B, each filtering a portion of the scan band. Corresponding detectors  334 A,  334 B detect the transmitted signal from each filter.  
         [0107]     The spectrum is divided into two subbands by WDM filter  360 , which reflects half of the spectral scan band to the second filter  116 B via fold mirror  362 . The other half of the spectrum is transmitted through the WDM filter  360  to tunable filter  116 A.  
         [0108]      FIG. 14  shows a single bench fully integrated system according to still another embodiment. It generally operates as described relative to the  FIG. 8  embodiment. Specifically, it uses a series of SLEDs in five channels, yielding a tunable source  10 , to generate a wide band tunable signal  30 . The detector system  300  is integrated on the same bench B and the tunable source. Specifically, light returning from the target in fiber  310  is coupled to detector  334  using lens component  322 .  
         [0109]      FIG. 15  shows another single bench fully integrated system according to still another embodiment. Here, the light to and from the target is carried in the same fiber  102 ,  310 . The tap substrate  152  is used to direct outgoing light  30  to the power detector  154  and light returning from the target to the detector  334 .  
         [0110]     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.