Abstract:
An optical power control system for a semiconductor source spectroscopy system controls power fluctuations in the tunable signal from the spectroscopy system and thus improves the noise performance of the system. This general solution has advantages relative to other systems that simply detect reference power levels during the scan and then correct the detected signal after interaction with the sample by reducing the requirements for coordinating the operation of the sample detectors and power or reference detectors. The spectroscopy system comprises a semiconductor source and a tunable filter. The combination of the semiconductor source and tunable signal illuminate a sample with a tunable signal, being tunable over a scan band. The power control system comprises an amplitude detector system for detecting the power of the tunable optical signal and power control system for regulating the amplitude of the tunable optical signal in response to its detected power.

Description:
RELATED APPLICATIONS 
     This application is a continuation of copending U.S. application Ser. No. 10/953,046 filed on Sep. 29, 2004 and published as U.S. Publication No. U.S. 2006/0065834-A1, which is related to application Ser. Nos. 10/953,048, published as U.S. Publication No. U.S. 2006/0072633 A1, and 10/953,043, published as U.S. Publication No. U.S. 2006/0072632 A1, both filed on Sept. 29, 2004, by Flanders, et al., which are incorporated herein by this reference. 
    
    
     BACKGROUND OF THE INVENTION 
     Most spectroscopy systems fall into one of two categories. They can be tunable source systems that generate a tunable optical signal that is scanned over a scan band. A detector is then used to detect the tunable optical signal after interaction with the sample. The time response of the detector corresponds to the spectral response of the sample. Such systems are typically referred to as pre-dispersive. Alternatively, a tunable detector system can be used. In this case, a broadband signal is used to illuminate the sample. Then, a bandpass filter is tuned over the scan band such that a detector time response is used to resolve the sample&#39;s spectrum. Such systems are typically referred to as post-dispersive. 
     Among tunable source and tunable detector systems, tunable source systems have some advantages. They can have a better response for the same optical power transmitted to the sample. That is, tunable detector systems must illuminate the sample with a broadband power signal that covers the entire scan band. Sometimes, this can result in excessive sample heating. Also high power is generated at the optical source, most of it being unused, making the system inefficient. In contrast, at any given instant, tunable source systems only generate and illuminate the sample with a very narrow band power within the scan band. 
     Further, tunable source systems have advantages associated with detection efficiency. Relatively large detectors can be used to capture a larger fraction of the light that may have been scattered by the sample, since there is no need to capture light and then collimate the light for transmission through a tunable filter or to a grating and a detector array. 
     A number of general configurations are used for tunable source spectroscopy systems. The lasers have advantages in that very intense tunable optical signals can be generated. A different configuration uses the combination of a broadband source and a tunable passband filter, which generates the narrowband signal that illuminates the sample. 
     Historically, most tunable lasers were based on solid state or liquid dye gain media. While often powerful, these systems also have high power consumptions. Tunable semiconductor laser systems have the advantage of relying on small, efficient, and robust semiconductor sources. One configuration uses semiconductor optical amplifiers (SOAs) and microelectromechanical system (MEMS) Fabry-Perot tunable filters, as described in U.S. Pat. No. 6,339,603, by Flanders, et al., which is incorporated herein by this reference in its entirety. 
     In commercial examples of the broadband source/tunable filter source configuration, the tunable filter is an acousto-optic tunable filter (AOTF) and the broadband signal is generated by a diode array or tungsten-halogen bulb, for example. More recently, some of the present inventors have proposed a tunable source that combines edge-emitting, superluminescent light emitting diodes (SLEDs) and MEMS Fabry-Perot tunable filters to generate the tunable optical signal. See U.S. Pat. appl. Publication No. US 2005 -0083533 A1 published on Apr. 21, 2005, now U.S. Pat. No. 7,061,618, issued on Jun. 13, 2006, by Atia, et at., which is incorporated herein by this reference in its entirety. The MEMS device is highly stable and can handle high optical powers and can further be much smaller and more energy-efficient than typically large and expensive AOTFs. Moreover, the SLEDS can generate very intense broadband optical signals over large bandwidths, having a much greater spectral brightness than tungsten-halogen sources, for example. 
     SUMMARY OF THE INVENTION 
     One drawback associated with semiconductor spectroscopy systems, however, is noise. Examples are shot noise, thermal noise, and relative intensity noise or RIN. Shot noise is generated by random fluctuations of current flowing through the detector due to the quantum nature of charge. Thermal noise results from electrons being freed in the detector due to thermal vibration and being indistinguishable from photoelectrons. RIN results from quantum fluctuations in the generation of light in the semiconductor cavity, being caused by optical interference between the signal and spontaneous emission within the cavity or changes in how the optical energy is instantaneously partitioned over the scan band. At higher optical powers, RIN dictates overall performance or signal-to-ratio (SNR), whereas at lower optical powers, shot noise usually tends to restrict the performance of the system. 
     RIN, however, is not unique to semiconductor sources. It can impact the performance of solid state laser spectroscopy systems as well as systems based on tungsten-halogen bulbs or standard diodes. But, it tends to be larger in semiconductor sources, such as optically filtered SLEDS and SOAs. Thus, while semiconductor sources provide advantages such as long life, high spectral brightness, size, and efficiency, on one hand, they tend to have somewhat worse inherent noise characteristics, on the other. 
     The present invention is directed to an optical power control system for a semiconductor source spectroscopy system. As such, this optical power control system can be used to control power fluctuations in the tunable signal from the spectroscopy system and thus improve the noise performance of the system. 
     This general solution has advantages relative to other systems that simply detect reference power levels during the scan and then correct the detected signal after interaction with the sample, since this solution reduces requirements for coordinating the operation of the sample detectors and power or reference detectors. 
     In general, according to one aspect, the invention features an optical power control system for a semiconductor source spectroscopy system. This spectroscopy system comprises a semiconductor source and a tunable filter. The combination of the semiconductor source and tunable filter illuminate a sample with a tunable signal, being tunable over a scan band. A detector is provided for detecting a sample signal generated by the interaction of the tunable signal with the sample. 
     The power control system comprises an amplitude detector system for detecting the power of the tunable optical signal and a power control system for regulating the amplitude of the tunable optical signal in response to its detected power. 
     In one embodiment, the semiconductor source comprises a light emitting diode generating a broadband signal that is converted to the tunable, narrowband signal by the tunable filter. In some implementations, standard light emitting diodes can be used. In other examples, superluminescent light emitting diodes are used. These are edge-emitting devices that can generate high power broadband signals. In fact, SLED&#39;s are currently preferred because of their high spectral brightness. 
     In another example, the semiconductor spectroscopy system comprises a rare-earth-doped gain fiber, such as an erbium-doped fiber, which is optically pumped by at least one semiconductor diode laser. 
     In some embodiments, the semiconductor source spectroscopy system comprises a laser cavity. In this case, the semiconductor source is a semiconductor optical amplifier. In some implementations, both facets of the SOA chip are coated to be anti-reflective. However, in other examples, a reflective SOA is used such that one end of the SOA defines one end of the laser cavity. The tunable filter is located within the laser cavity in order to provide a tunable laser configuration. In one implementation, the laser cavity is a linear cavity. However, in other cases, a ring laser configuration is used. 
     In the preferred embodiment, the tunable filter is a micro-electro mechanical (MEMS) filter. In one implementation, this MEMS filter comprises two thin film dielectric mirror structures providing for a low loss, high finesse system that can provide efficient, narrow bandwidth operation. 
     In the preferred embodiment, the amplitude detector system comprises a detector and a tap providing a portion of the tunable signal to the detector. In the current preferred embodiment, the semiconductor source, tunable optical filter, amplitude detector, and tap are attached to a common optical bench. It provides a small, highly integrated, highly stable, and highly mechanically robust system. In another embodiment, the optical tap and detector are in a separate package from the semiconductor-filter tunable light source. 
     The power control system then regulates the amplitude of the tunable optical signal in response to the amplitude detector. In one embodiment, the power control system regulates the amplitude of the tunable optical signal by controlling the power or current to the semiconductor source. However, in a different embodiment, a separate optical signal power regulator is provided to modulate the power of the tunable optical signal. This can be achieved with a semiconductor optical amplifier, for example. In the preferred implementation, however, a variable attenuator is used to dynamically attenuate the tunable signal in order to stabilize its output power across the scan band. Further, a combination of current control and optical signal power regulator are used in still other implementations. 
     Finally, in some implementations, a frequency reference system is further provided for detecting an instantaneous frequency of the tunable optical signal in order to further improve its performance and spectral accuracy. In one embodiment, for example, the amplitude detector system and frequency reference system are attached to a common optical bench. 
     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: 
         FIG. 1  is a perspective view of a tunable source semiconductor spectroscopy system with a wavelength and amplitude referencing system according to the present invention; 
         FIGS. 2A through 2D  illustrate a number of examples of the tunable semiconductor source for use with the present invention; 
         FIG. 3  is a plot of power (decibel scale) showing the gain spectrum of a semiconductor source as a function of wavelength; 
         FIG. 4  is a plot of transmission (decibel scale) showing the gain spectrum of the semiconductor source and the passband of the tunable filter; 
         FIG. 5A  is a plot of signal optical power as a function wavelength for a semiconductor source using the inventive spectral rolloff compensation and noise suppression; 
         FIG. 5B  is a plot of transmission noise as a function wavelength for a semiconductor source using the inventive noise suppression system; 
         FIG. 5C  is a plot of transmission noise as a function wavelength for a semiconductor source using the inventive noise suppression system; 
         FIG. 6  is a plot of noise as a function of wavelength showing the noise suppression gained from current control of the semiconductor chip; 
         FIG. 7A  is a circuit diagram showing an inventive analog drive circuit for the semiconductor source; 
         FIG. 7A-1  is a block diagram showing the operation of the inventive drive circuit; 
         FIG. 7B  is a circuit diagram showing an inventive analog drive circuit for the optical modulator; 
         FIG. 8  is a perspective view of a linear cavity tunable laser according to the present invention; 
         FIG. 9  is a perspective view of a linear cavity tunable laser according o the present invention; 
         FIG. 9A  illustrates the optical train between the SOA chip and the tilted Fabry Perot tunable filter, according to the invention; 
         FIG. 10  is a schematic view of a ring cavity tunable laser according o the present invention; 
         FIG. 11  is a perspective view of the ring cavity tunable laser according o the present invention; 
         FIG. 12  is a perspective view of a multi-chip tunable SLED system according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  shows a semiconductor source spectroscopy system  100 , which has been constructed according to the principles of the present invention. 
     Generally, the spectroscopy system  100  comprises a tunable semiconductor source  200 . This generates a tunable optical signal  210 . 
     In one example, the tunable signal  210  is transmitted to a frequency reference tap  310  that diverts a portion of the tunable optical signal  210 ′ to an optical frequency or wavelength reference  312 . In one example, this optical reference is a fixed cavity etalon that provides a number of spectral passbands located within and/or spectrally adjacent the scan band of the system  100 . Optionally, a post-amplifier tracking tunable optical filter is sometimes used to filter out or remove any optical noise contributed by the amplifier. 
     The signal  210 ′ that is transmitted through the optical reference  312  is then detected by a frequency reference detector  314 . The output of the frequency reference detector  314  is sent to an analog to digital converter  450  of a controller  410 . This enables the controller  410  to determine the instantaneous frequency of the tunable signal  210  to thereby provide frequency or wavelength calibration for the spectroscopy system  100 . 
     In some implementations, the tunable optical signal  210  is then amplified in an optical amplifier  316 . In one example, this optical amplifier can be a rare-earth doped amplifier, such as a fiber amplifier or waveguide amplifier. In a more preferable embodiment, a semiconductor optical amplifier is used to boost or increase the power of the tunable optical signal  210  while not significantly increasing the overall size of the system  100 . 
     The tunable optical signal  210  is then transmitted to an optical signal power regulator  318  of the power control system. This modulates the power of the tunable optical signal  210 . 
     In one example, the optical signal power regulator comprises an SOA that selectively amplifies or attenuates the tunable optical signal  210  by control of the SOA drive current. In this example, the optional SOA amplifier  316  would usually not be used. 
     In the preferred embodiment, the optical signal regulator or modulator  318  is a variable optical signal attenuator. Preferably, it is a solid state, high-speed optical attenuator. One commercial example is available from Boston Applied Technologies, Inc. These are electro-optical, ceramic devices that are electrically modulated in order to control the level of the optical attenuation applied by the regulator  318  to the tunable signal  210 . 
     The power detector  322  detects the amplitude of the tunable optical signal from the optical signal regulator  318 . Specifically, a portion  210 ″ of the tunable optical signal  210  is preferably diverted by a power tap  320  to a power detector  322  of the power control system. The detected level of the tunable signal  210  is then provided to the controller  410 , and specifically its drive circuit  411 . 
     The remainder of the tunable optical signal  210  is sent through to the sample  10  and the sample detector  12 . In some examples, the output of this sample detector  12  is also provided to the controller  410 , which is then able to resolve the spectrum of the sample  10  by resolving the time response of the sample detector  12  into the spectral response of the sample  10 . 
     The drive circuit  411  of the controller  410  is used to regulate the power to the tunable semiconductor source  200  or the optical signal regulator  318 , or both. Specifically, the output of the power detector  322  provides a power feedback signal that the drive circuit  411  of the controller  410  uses in order to stabilize the level and reduce amplitude noise of the tunable signal  210  in spite of noise such as RIN noise or mode-hopping noise. The controller drive circuit  411  regulates the tunable semiconductor source  200  and/or optical signal attenuator  318  in order to stabilize or set the power in the tunable optical signal  210  over the scan band. 
       FIGS. 2A-2D  show a number of examples of the tunable semiconductor sources  200  that are used in embodiments of the inventive spectroscopy system  100 . 
     Specifically,  FIG. 2A  shows a first linear cavity laser embodiment ( 200 - 1 ) of the tunable semiconductor source  200 . This is generally analogous to the tunable laser described in incorporated U.S. Pat. No. 6,339,603. 
     Specifically, light is amplified in an SOA  610 . This light is filtered by an intracavity Fabry-Perot tunable filter  612 . In one embodiment, the Fabry-Perot tunable filter is manufactured as described in U.S. Pat. No. 6,608,711 or 6,373,632, which are incorporated herein by this reference. 
     Out-of-band reflections from the filter  612  are isolated from being amplified in the SOA  610  by a first isolation element  614  and a second isolation element  616 , on either side of the filter  612  in the optical train. In different implementations, these isolation elements  614 ,  616  are isolators or quarterwave plates. The laser cavity is defined by a first mirror  618  and a second mirror  620 . In some implementations, a reflective SOA  610  is used, which provides the reflectivity of the first mirror  618  at one end of the cavity. The tunable signal  210  is emitted through the second mirror  620  in one example. 
     In some embodiments, a portion of the laser cavity includes a length of optical fiber  615 . The second mirror  620  is then typically a discrete mirror or a fiber Bragg grating reflector that is formed in the fiber  615 . The advantage of using the hybrid freespace/fiber laser cavity is that the laser cavity can be made long, typically longer than 10 centimeters, and preferably long than 0.5 meters. The long cavity provides for tight longitudinal mode spacing to reduce mode hopping noise. 
     In a current embodiment, the SOA chip  610  is polarization anisotropic. Thus, polarization control is desired to stabilize its operation. As such, fiber  615  is polarization controlling fiber such a polarization maintaining or is fiber that only transmits or propagates a single polarization. 
       FIG. 2B  shows another implementation ( 200 - 2 ) of the linear cavity tunable laser functioning as the tunable semiconductor source  200 . Here, an SOA  610  is used in combination with a first mirror  618  and a second mirror  620 . The SOA  610  is isolated from the out of passband reflection of the Fabry-Perot tunable filter  612  by tilt isolation. Typically the angle a between the optical axis OAF of the filter  612  and the optical axis of the laser cavity OAC is less than 5 degrees, and preferably between 1 and 3 degrees. Currently, angle α is about 1.3 degrees. In this way, the system avoids the out-of-band reflections from being amplified in the SOA  610 . Preferably the tunable filter  612  has flat-flat mirror cavity to further improve isolation. 
     In this embodiment, a hybrid freespace-fiber cavity is used in some implementations to provide the long optical cavity/tight mode spacing characteristics by further including the fiber length  615 . 
     In a current embodiment, the SOA chip  610  is again polarization anisotropic. Thus, polarization control is desired to stabilize its operation. As such, fiber  615  is polarization controlling fiber such a polarization maintaining or is fiber that only transmits or propagates a single polarization. 
       FIG. 2C  shows another implementation ( 200 - 3 ) of the tunable semiconductor source. Here, a SLED  622  is used to generate a broadband signal that is then filtered by a Fabry-Perot tunable filter  612  in order to generate the tunable signal  210 . In other implementations, an intervening isolator  624  or tilt isolation is used to isolate the SLED  622  from he back reflection of the Fabry-Perot tunable filter  612 . Further, incorporated U.S. Pat. appl. Publication No. US 2005-0083533 A 1 published on Apr. 21, 2005, now U.S. Pat. No. 7,061,618, issued on Jun. 13, 2006 by Atia, et at., describes some other variants, which are used in still other embodiments, depending on whether increased scan band or power is required. 
     Finally,  FIG. 2D  shows another implementation ( 200 - 4 ) of the tunable semiconductor source  200 . This also combines an SOA  610  and a Fabry-Perot filter  612 . A ring cavity laser, however, is used. Specifically an optical fiber or bent beam path  626  is used to recirculate the light back through the SOA  610  for further amplification. 
       FIG. 3  is a plot of the power (arbitrary units, decibels) for a SLED or SOA as a function of wavelength in nanometers, which applies to the various embodiments of the semiconductor tunable source  200  in  FIGS. 2A-2D . This plot shows how the power output from the system is stabilized across the scan band. 
     Generally, the unmodulated SLED or SOA power spectrum will peak at some wavelength, here approximately 1680 nanometers (nm). This wavelength is usually dictated by the epitaxial structure of the devices along with the material system that is used. The power, however, can vary by greater than −40 dB over the desired scan band  510 , extending from about 1610 nm to 1770 nm, in this one example. This is due to the fact that the semiconductor gain medium does not generate all wavelengths within the scan band with equal efficiency. Moreover, the power in a narrow slice of the spectrum varies over time due to thermal noise and RIN. 
     The power to the tunable semiconductor source  200  and/or the optical signal regulator  318  are preferably controlled to stabilize the power of the tunable signal  210  transmitted to the sample from the semiconductor source  200  so that it is stable or at least known across the entire scan band of approximately 1610 to 1770 nm, in one exemplary scan band. In this way, the rolloff in the power spectrum is compensated. As a result, where an attenuator optical power regulator is used or the power to the SLED/SOA is modulated, the tunable optical signal power is attenuated or the power from the semiconductor source is lowered in order to achieve a stable or constant tunable signal power. However, the total power from the system  100  is lower than it would be otherwise be capable of generating, see area  511 . In this way, the present system can be used to stabilize the output power of the source due to spectral roll-off associated with limitations in the gain band of the semiconductor sources. 
       FIG. 4  is a plot of several tunable filter passbands  512  as a function of an exemplary SLED or SOA power spectrum  514  over a scan band  510  stretching from approximately 1340 to 1460 nm. This plot shows the relationship between the tunable passband of the Fabry-Perot tunable filter  612 , the semiconductor source gain spectrum  514 , and the operation of the inventive power control system. 
     As a result, without modulating the power of the tunable optical signal, the output power filling the passband  512  of the tunable filter  612  would peak at about 1400 nm. However, by modulating the power to the SLED  622  or SOA  610  or by controlling its attenuation using optical signal modulator  318 , the output power can be stabilized to the level  516 . Moreover, by modulating the tunable signal power at high speed, temporal amplitude fluctuations in the tunable signal  210  are mitigated or removed. This has the effect of increasing the SNR of the system and reducing the noise power over the scan band. 
       FIG. 5A  is a plot of the response of the power detector  322  as a function of the wavelength across the scan band of semiconductor spectroscopy system  100  showing the performance improvement provided by the present invention. 
     These data were taken from the embodiment that used the modulation of the optical signal attenuator  318  in order to improve the noise performance of the system. 
     Specifically, plot  413  shows the response at the sample detector  12  with the attenuator  318  not being driven, i.e., in a transmissive state. No sample  10  is present for this experiment. The spectrum generally peaks around 1,700 nm, which is the center of the gain band of the semiconductor source  610 . Further, the power exhibits a high degree of variability across the scan band  510 . 
     In contrast, data  412  illustrate the response at the sample detector  322  with the attenuator  318  being driven by drive circuit  411  in order to flatten the response. Specifically, the response is generally flat across an entire scan band from approximately 1620 to 1780 nm, showing only a small degree of ripple, which is believed to be attributable to polarization sensitivity in one of the taps. Most importantly, the high frequency variability has been reduced, improving the noise performance of the system. 
     Of note is the fact that the power output from the system is generally lower when the attenuator  318  is in operation. This is because it achieves the spectral flatness and stability by selectively attenuating the tunable optical signal  210 . 
       FIG. 5B  is a plot of transmission as a function of wavelength across the scan band. Here again, the response with the attenuator not operating is shown by data  414 . However, when the attenuator is being driven to stabilize the output, the transmission  416  stabilizes around the normalized value of 1. 
       FIG. 6  is a plot of the response of the sample detector  12  as a function of the wavelength across the scan band of semiconductor spectroscopy system  100  showing the performance improvement provided by the present invention by using current control to the semiconductor SOA chip  610  or SLED chip  622  in order to improve the noise performance of the system. 
     Specifically, plot  550  shows the normalized transmission response at the sample detector  12  in absorbance units (absorbance =Log 10 (transmission −1 )). The spectral noise has been substantially reduced. In the illustrated example, the SNR was greater than 44000 RMS. However, the noise performance did degrade in regions  552  and  554 , possibly due to a broadening of the tunable filter&#39;s passband in these spectral regions. 
       FIG. 7A  is a circuit diagram showing the semiconductor source drive circuit  411 . This is used in the embodiments that control the current to the SLED  622  or SOA  610  in order to achieve a power-stable tunable signal  210 . 
     The circuit comprises a transimpedance amplifier  670 . This element is not always necessary. In the current embodiment, it is used for common mode rejection of noise sources, such as 60 hertz interference. 
     The voltage across the power detector photodiode  322  regulates the input to an integrating amplifier  672 . Specifically, integrating amplifier  672  is used to control the loop bandwidth. By changing capacitor C  1 , the bandwidth response of the circuit can be adjusted. 
     Sub-circuit  674  is used to control the biasing of the photodiode  322 . Capacitor C 2  (reference numeral  676 ) controls the frequency response of the circuit. 
     In the illustrated embodiment, two power transistors  678  and  680  are used to control the power to the anode of the SLED  622  or SOA  610 . 
       FIG. 7A-1  is a block diagram illustrating the operation of the drive circuit  411 . Specifically detector  322  provides the feedback signal to drive circuit  411 . The drive circuit signal is then summed with a current set point signal  413 . This is the power control signal to the SOA or SLED  610 ,  622 . 
       FIG. 7B  is a circuit diagram showing the drive circuit  411  for the modulator/attenuator  318 . This circuit similarly has a trans-impedance amplifier  670  to assist in common mode rejection. The power detector photodiode  322  provides the input to an integrating amplifier  686 . Capacitor C 4  ( 688 ) is used to control the loop bandwidth of the amplifier  686 . Specifically, by increasing the size of the capacitor C 4 , the bandwidth is decreased. A second amplifier  690  is used to provide the gain to the input terminal of the attenuator  318 . Variable resistor  692  is used to control the gain of the second amplifier  690 . 
     One of the advantages of the present invention is that its optical train can be implemented in a small-robust unit, possibly being integrated on a single optical bench. The following illustrates a number of different embodiments showing various levels of integration. 
       FIG. 8  is a perspective view of one embodiment of the tunable laser linear cavity configuration  200 - 1  illustrated in block diagram form in  FIG. 2A . Specifically, the tunable laser  200 - 1  is integrated on a common bench  800 . Specifically, the SOA chip  610  is installed on a submount  822 , which holds the chip  610  off of the bench  800 . The back facet  618  of the chip  610  functions as the back reflector  618  of this linear cavity laser. 
     The chip&#39;s waveguide  619  generates light that is collimated by a first lens component  820 . Specifically, this lens component comprises a lens substrate S that is installed on the bench  800  via a mounting structure M. 
     The collimated light from the first lens component  820  passes through a first isolator  614  or quarter waveplate to a second lens component  810 . This second lens unit  810  focuses the light to launch it into the MEMS Fabry-Perot tunable filter  612 . Light in the passband, exiting from the tunable filter  612 , is recollimated by a third lens component  812  to pass through the second isolator  616  to a focusing lens component  822 . 
     An endface  816  of the fiber  818  collects the light. A reflector  626  is provided on the fiber to function as the output end of the laser cavity. This fiber  818  is then fiber coupled to the attenuator  318 , if used. A fiber tap  320  directs a portion of the tunable optical signal from the attenuator  318  to the power detector  322 . 
     Note that in other embodiments, the attenuator  318 , tap  320 , and detector  322  are integrated in common on the bench  800  with the other components of the tunable laser. 
       FIG. 9  is a perspective view of one embodiment of the tunable laser linear cavity configuration  200 - 2  illustrated in block diagram form in  FIG. 2B . Specifically, the tunable laser  200 - 2  is integrated on a common bench  800 . Specifically, the SOA chip  610  is installed on a submount  822 , which holds the chip  610  off of the bench  800 . The back facet  618  of the chip  610  functions as the back reflector  618  of this linear cavity laser. 
     The chip&#39;s waveguide  619  generates light that is collimated by a first lens component  820 . Specifically, this lens component comprises a lens substrate S that is installed on the bench  800  via a mounting structure M 
     The collimated light from the first lens component  820  is launched into the MEMS Fabry-Perot tunable filter  612 . Light in the filter passband, exiting from the tunable filter  612 , is recollimated by a third lens component  812  to enter endface  816  of the fiber  818 . The filter optical axis is tilted such that reflected light outside the filter passband is not coupled back to the semiconductor chip waveguide. A reflector  626  is provided on the fiber to function as the output end of the laser cavity. This fiber  818  is then fiber coupled to the attenuator  318 , if used. A fiber tap  320  directs a portion of the tunable optical signal from the attenuator  318  to the power detector  322 . 
     Note that in other embodiments, the attenuator  318 , tap  320 , and detector  322  are integrated in common on the bench  800  with the other components of the tunable laser. 
       FIG. 9A  illustrates one embodiment of the optical train between the front facet of the SOA  610  and the tunable filter  612 . Specifically, the beam displacement δx enables the decoupling of the back reflection from the filter  612  and the SOA  610 . Here is it is less than about 50 micrometers, and preferably less than 20 micrometer, specifically about 10 micrometers. In the example, the distance between the chip front facet ff and the filter  612  is less than about 10 millimeters. The focal length of the first lens component is less than 0.5 millimeters, specifically about 200 micrometers, of specified at 220 micrometers. 
       FIG. 10  is a block diagram showing one configuration of the ring laser illustrated in  FIG. 2D . Here again, the components are installed on an optical bench  800 . Light is generated in the SOA chip  612  and collimated by a first lens  820  to pass through an isolator  616  to a second lens  810  that launches the light into the MEMS Fabry-Perot tunable filter  612 . The light exiting from the tunable filter  612  is then recollimated by a third lens  812 . A portion of the light exits from the tunable laser cavity as the tunable signal  210 . The remaining light, however, is returned to recirculate through the cavity. 
     In one implementation, this is achieved by a reflector or prism  632  that is attached onto the bench  800 . In other implementations, the light is recirculated through the cavity using a length of polarization maintaining (pm) single mode fiber  635 . The second embodiment has advantages in that the length of the optical cavity can be controlled. Specifically, the cavity can be made longer to increase the longitudinal modal density. 
     In any event, the light returns along path  638 , which can be above the bench  800  or provided by pm fiber length. Returning light is returned by a prism  634  through a second isolator  614  and a lens  632  to be reamplified in the SOA  612 . 
     In one embodiment, the prism  634  is mounted to the bench  800  using a piezoelectric actuator unit  636 . This allows for the location of the prism  634  to be moved in the direction of the X-axis, thereby controlling the length of the optical cavity and therefore the spectral location of the longitudinal modes of the ring laser  100 - 4 . 
       FIG. 11  is a perspective view of the ring laser source  200 - 4  illustrated in  FIG. 2D . The components are installed on optical bench  800 . Light is generated in the SOA chip  612  on submount  822 . Its light is collimated by a first lens  810  to pass through an isolating device, such as a quarter wave plate  616 , to a second lens  820  that launches the light into the MEMS Fabry-Perot tunable filter  612 . The light exiting from the tunable filter  612  is then recollimated by a third lens  812 . A portion of the light exits from the tunable laser cavity as the tunable signal  210  using tap or splitter  910  and fold mirror  912  and focusing lens  914 . The remaining light, however, is returned to recirculate through the cavity via lens  916  and polarization-controlling or pm fiber length  638 . 
     The light returns to lens  918 , second isolator  614  and a lens  632  to be reamplified in the SOA  612 . 
       FIG. 12  illustrates another embodiment of the tunable source  10 , which is based on source  200 - 3  of  FIG. 2C . 
     This embodiment uses a tunable filter system  900 , which includes an array of tunable filters  612  and broadband light sources  622  in order to increase the spectral width of the scanband. Typically, and in the illustrated embodiment, an array of five SLED chips  622  is mounted in common on the bench  800 . The light from each of these SLED chips  110  is collimated by respective first lens components  918 . Specifically, there is a separate lens component  918  for each of these SLED chips  622 . Separate isolators  920  are then provided for the broadband signals from each of the SLED chips  622 . 
     An array of second lens components  922  is further provided to couple the broadband signal into an array  900  of tunable filters  612 . Specifically, separate Fabry-Perot tunable filters  612  are used to filter the signal from each of the respective SLED chips  622 . Finally, an array of third lens components  926  is used to re-collimate the beam from the tunable Fabry-Perot filters  612  of the tunable filter system  900 . 
     For channel  1 , C- 1 , a fold mirror  956  is used to redirect the beam from the SLED chip  622 . The WDM filter  960  is used to combine the broadband signal from the SLED chip  622  of channel C- 2  with the signal from channel C- 1 . Specifically, the filter  960  is reflective to the wavelength range generated by the SLED chip  622  of channel C- 2 , but transmissive to the wavelength range of light generated by the SLED chip  622  of channel C- 1 . 
     WDM filter  962  is reflective to the signal band generated by the SLED chip  622  of channel C- 3 , but transmissive to the bands generated by SLED chips  622  of channels C- 1  and C- 2 . WDM filter  964  is reflective to the light generated by SLED chip  622  of channel C- 4 , but transmissive to the bands generated by the SLED chips  622  of channels C- 1 , C- 2 , and C- 3 . Finally, WDM filter  958  is reflective to all of the SLED chips, but the SLED chip  622 of channel C- 5 . As a result, the light from the array of SLED chips is combined into a single tunable signal  210 . 
     A first tap  310  is provided to reflect a portion of the light through the etalon  312  to be detected by the wavelength detector  314 . Then, another portion is reflected by tap  318  to the power detector  322 . The remaining tunable signal  210  is coupled by the fourth lens component  906  into the optical fiber  818  via the endface  816 . 
     The  FIG. 12  embodiment can operate according to a number of different modes via a controller  410 . 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  210  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. 
     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  622  simultaneously. This system results in a more complex detector system, which must demultiplex the separate scan bands from each of the SLED chips  622  from each of the channels at the detector. Specifically, in one embodiment, five (5) detectors are used with a front-end wavelength demultiplexor. 
       FIG. 12  further shows a single bench fully integrated system according to still another embodiment. The sample detector system  12  is integrated on the same bench  800  and the tunable source. Specifically, light returning from the sample  10  in fiber  310  is coupled to sample detector chip  334  using lens component  322 . 
     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.