Patent Publication Number: US-11662250-B2

Title: Wavelength reference device

Description:
FIELD OF THE DISCLOSURE 
     The present application relates to optical calibration and in particular to a wavelength reference device. 
     Embodiments of the present disclosure are particularly adapted for frequency calibration of optical devices. However, it will be appreciated that the disclosed subject matter is applicable in broader contexts and other applications. 
     DESCRIPTION OF RELATED ART 
     A wavelength reference device is an optical component or combination of components that provides an optical output with known frequency characteristics. Wavelength reference devices are typically used to measure the frequency error of optical spectrum measurement equipment such as spectrometers, optical spectrum analyzers (OSAs) and optical channel monitors (OCMs). 
     A wavelength reference device comprises an optical source, such as a light-emitting diode (LED) or super-luminescent light-emitting diode (SLED), and an optical reference filter, such as a gas absorber, notch filter(s), transmission filter(s) or Fabry-Perot etalon, which has frequency peaks (or notches) that are highly stable with temperature. 
     Conventional wavelength reference devices are application specific and each component has to be individually designed, assembled and tested. Typically the components are interconnected by fiber pigtails that have to be optically spliced together. Although these application specific devices can meet stringent frequency accuracy requirements, the cost of individual component assembly (e.g. mechanical packaging, collimating optics) and time to assemble the devices can be prohibitive on a large scale. Furthermore, significant effort is required to minimize temperature dependence of the filter and valuable space is required for fiber management to properly connect the components. In addition, it can be difficult to calibrate the wavelength reference device separate from the rest of the optical spectrum measurement equipment and the filter peak (notch) frequency can be very sensitive to alignment of collimated light. 
     Thus, the inventors have identified that current wavelength reference device designs do not adequately meet the needs of emerging applications in relation to low cost, small size, frequency accuracy and standalone calibration, particularly for OCMs. 
     Any discussion of the background art throughout the specification should in no way be considered as an admission that such art is widely known or forms part of common general knowledge in the field. 
     SUMMARY OF THE DISCLOSURE 
     In accordance with one aspect of the present disclosure, there is provided a wavelength reference device comprising: a housing defining an internal environment having a known temperature; a broadband optical source disposed within the housing and configured to emit an optical signal along an optical path, the optical signal having optical power within a wavelength band; an optical etalon disposed within the housing and positioned in the optical path to filter the optical signal to define a filtered optical signal that includes one or more reference spectral features having a known wavelength at the known temperature; and an optical output for outputting the filtered optical signal. 
     In accordance with another aspect of the present disclosure, the wavelength reference device disclosed above and described herein can be used in an apparatus to process signal input. The apparatus can comprise: an input of the apparatus receiving the signal input; a signal detection and processing module configured to detect and process the signal input; a module having the disclosed wavelength reference device disposed in optical communication with the input, the device being configured to produce a wavelength reference; and at least one controller in signal communication with at least the signal detection and processing module and the wavelength processing module, the controller configured to control the wavelength reference module and configured to calibrate the signal detection and processing module based on the produced wavelength reference. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Example embodiments of the disclosure will now be described, by way of example only, with reference to the accompanying drawings in which: 
         FIG.  1    is a schematic side view of a wavelength reference device according to a first embodiment; 
         FIG.  2    is an exemplary spectrum of an optical signal generated by a superluminescent diode; 
         FIG.  3    is an exemplary spectrum of the optical signal filtered by a Fabry-Perot etalon; 
         FIG.  4    is a schematic system diagram of an optical channel monitor device incorporating a wavelength reference device; 
         FIG.  5    is a schematic side view of a wavelength reference device according to a second embodiment; 
         FIG.  6    is a schematic side view of a wavelength reference device according to a third embodiment; 
         FIG.  7    is a schematic side view of a wavelength reference device according to a fourth embodiment 
         FIG.  8    is a schematic side view of a wavelength reference device according to a fifth embodiment 
         FIG.  9    is a schematic side view of a wavelength reference device according to a sixth embodiment; 
         FIG.  10    is a schematic side view of a wavelength reference device according to a seventh embodiment; and 
         FIG.  11    is a schematic side view of a wavelength reference device according to an eighth embodiment. 
     
    
    
     DESCRIPTION OF THE DISCLOSURE 
     Referring initially to  FIG.  1   , there is illustrated an integrated wavelength reference device  100  comprising a housing  102  defining an internal environment  104  having a known temperature. The device is termed integrated as each of the components are integrated into a single package which provides for a standalone device. That is, the wavelength reference device  100  does not need to leverage components from the optical equipment being calibrated. 
     Housing  102  is preferably formed of a transistor outline (TO) package, such as a TO-46 package, and may be either cylindrical or rectilinear in profile. In addition to providing a sealed protective housing for internal components, the TO package provides for simple mounting of electrical components onto a TO header  106 , which forms a base of housing  102 . TO header  106  includes a plurality of internal electrical pins (not shown) for electrically mounting electrical components thereto and which are connected to external control pins  108  and  110 . Pins  108  and  110  are able to be connected to a controller such as a digital processor for powering and providing control signals to components of device  100 . 
     Device  100  includes a broadband optical source in the form of a superluminescent diode (SLED)  112  disposed within TO header  106  within housing  102  and configured to emit an optical signal  114  along an optical path through device  100  to an optical output. SLED  112  may be any suitable device providing a power spectral density of sufficient magnitude across the wavelength range of interest. For example, for the optical transmission C-band, power density between 1525 nm to 1570 nm may be preferable. In other embodiments, SLED  112  may be replaced with other types of broadband optical sources such as one or more LEDs or amplifier spontaneous emission (ASE) from an optical amplifier. 
     Referring to  FIG.  2   , there is illustrated an exemplary spectrum  150  of optical signal  114  generated by SLED  112 . Spectrum  150  has a spectral profile extending between 1525 nm and 1575 nm with a center wavelength around 1550 nm. 
     Referring again to  FIG.  1   , device  100  also includes an optical reference filter or optical etalon  116  disposed within housing  102  and positioned in the optical path to filter the optical signal  114  to define a filtered optical signal  118 . Filtered optical signal  118  includes one or more reference spectral features having a known wavelength at the known temperature in the form of one or more repeating spectral peaks of the etalon&#39;s resonant wavelengths. 
     The absolute wavelength of these spectral peaks are registered using a separate spectral measurement device such as an OCM or wave meter in an initial instrument calibration procedure. This calibration procedure is performed after the assembly of the wavelength reference component  100 . 
     Etalon  116  is preferably formed of a glass substrate having a pair of parallel disposed sides on which glass mirrors  120  and  122  are deposited. The glass substrate has a finite thickness such that mirrors  120  and  122  are separated by a fixed distance L. The glass substrate between mirrors  120  and  122  has a refractive index that is known to a high degree of accuracy. In some embodiments, etalon  116  may be formed of other materials other than glass. In some embodiments, etalon  116  may be formed of two parallel plates separated by an air gap. 
     In the illustrated embodiment, SLED  112  is positioned horizontally on TO header  106  to emit optical signal  114  horizontally. A turning mirror  124  is disposed on header  106  and angled at approximately 45 degrees to direct the horizontally propagating optical signal  114  vertically onto etalon  116 . Mirrors  120  and  122  of etalon  116  are disposed substantially horizontally such that optical signal  114  is incident perpendicularly onto an outer surface of mirror  120 . 
     Mirrors  120  and  122  of etalon  116  define a resonant cavity within which the optical signal  114  can resonate. Wavelengths that are an integral multiple of the mirror spacing L will resonate within etalon  116  and will dominate the power of the signal that passes through mirror  122 . These resonant wavelengths form the filtered optical signal  118 . 
     Referring now to  FIG.  3   , there is illustrated an exemplary spectrum  160  of the filtered optical signal  118 . The periodic peaks of the solid line indicate the resonant wavelengths of etalon  116  while the dashed line indicates the envelope defined by the spectrum  150  of SLED  112 . The peaks of the signal occur at known frequencies that are temporally stable at a given temperature. The spectral peaks are spaced apart by a constant spectral width called the “free spectral range”. The free spectral range is specific to an etalon and is defined by: 
     
       
         
           
             
               Δλ 
               ⁡ 
               
                 ( 
                 FSR 
                 ) 
               
             
             = 
             
               
                 λ 
                 2 
               
               nL 
             
           
         
       
     
     Where λ is the wavelength of light incident onto the etalon (optical signal  114 ), n is the refractive index of the media within the cavity of the etalon and L is the length of the cavity (distance between mirrors  120  and  122 ). The media between mirrors  120  and  122  is glass in the illustrated embodiment but may be air or other materials having a known refractive index in other embodiments. Given this known formula, the wavelength of each spectral peak of the filtered optical signal  118  can be established by a calibration process using a wave meter or OSA and used as reference spectral features to reference and calibrate optical devices such as an OCM. As the temperature of the etalon changes, the refractive index changes, which affects the FSR. This is visible as a wavelength shift of the peaks, which can be measured. Typical glass etalons have temperature dependence of approximately 1.5 GHz/° C. This temperature dependency is accounted for in system  100  as described below. 
     The parameters of the filtered optical signal  118  may be defined during manufacture to suit a corresponding application. The FSR of the spectrum is determined primarily by the width of etalon  116  and the material used to define the cavity (e.g. glass having a refractive index of about 1.5). The FSR is chosen such that a plurality of wavelength peaks (e.g. 10 or more) are present across the desired spectrum to be referenced as each spectral peak represents a sample point of known wavelength to characterize an optical spectrum. By way of example, in a telecommunications application, across spectrum of 4-5 THz, an FSR between 100 GHz and 200 GHz may be chosen to provide 20-50 reference spectral peaks of known wavelength. 
     The width of each spectral peak (typically characterized by the Full Width at Half Maximum—FWHM) can also be controlled to a degree by the reflectivities of mirrors  120  and  122 . Typically both mirrors will be highly reflecting having a reflectivity of greater than 50%. However, higher reflective mirrors (e.g. greater than 90% reflectivity) will produce narrower spectral peaks and higher contrast ratio in filtered optical signal  118 , thereby providing more accurate wavelength resolution. But, as a trade-off, higher reflective mirrors will increase the insertion loss and therefore result in smaller peaks. 
     The FSR, spectral width and contrast ratio are key parameters that can be set during etalon manufacture to suit a specific application of device  100 . By way of example, one suitable device may provide a FWHM spectral width of less than 5 GHz, a contrast ratio of at least 10 dB and an FSR or around 100 GHz. 
     For efficient packaging of device  100 , etalon  116  is positioned above SLED  112  and turning mirror  124 , and held in place by support struts  123  and  125 . However, this need not be the case and different orientations and configuration of SLED  112  and other components is possible. 
     Although the outer surfaces of mirrors  120  and  122  are illustrated as being parallel to the other surfaces in the package (e.g. base  106  and window  126 ), in some embodiments, the outer surfaces of mirrors  120  and  122  are slightly angled by a fraction of a degree to reduce an interference pattern resulting from reflections off other surfaces. 
     The filtered optical signal  118  is directed through a transparent window  126  in an upper region of housing  102 . Window  126  forms an optical output for outputting filtered optical signal  118  from device  100 . Transparent window  126  is preferably formed of glass material that is highly transparent at the wavelength of the broadband optical source  112 . 
     Filtered optical signal  118  is typically coupled to a fiber collimator  128  for coupling the signal to the system that is designed to utilize this wavelength reference, such as an OCM. In some embodiments, window  126  or housing  102  includes a coupling structure (not shown), such as a fiber connector, to connect a fiber to device  100 . In some embodiments, transparent window  126  includes a lensing structure (not shown) to focus, partially focus, collimate or partially collimate the filtered optical signal  118  to more efficiently couple it into the fiber collimator  128 . 
     SLEDs and other broadband sources typically have a wide divergence (up to 10&#39;s of degrees) and collimating/focusing lenses or mirrors can help confine the light for more efficient coupling to a pigtailed fiber. In some embodiments, collimator  128  may be formed integrally with housing  102  and sold as a single package with device  100  and optionally a length (pigtail) of optical fiber. In some embodiments (not illustrated), device  100  includes a connector for connecting an optical fiber or collimator  128  to housing  102  adjacent the transparent window  126 . 
     In device  100 , the optical path between SLED  112  and etalon  116  is fixed in space with no moving components. This fixed optical path provides for a very stable frequency output from device  100 . 
     Knowledge of the temperature of internal environment  104  is important as the transmission spectrum of etalon  116  is temperature dependent. As such, the position of the spectral peaks in filtered optical signal  118  will vary depending on temperature. This temperature dependence of device  100  is calibrated initially by measuring the spectral peaks of filtered optical signal  118  across using an OSA or wave meter across a range of temperatures. In order to have knowledge of the temperature of internal environment  104  during operation of device  100 , the temperature may be passively sensed using one or more temperature sensors, or the temperature may be actively set using a temperature control device such as a thermoelectric cooler (TEC) having heating and/or cooling capability (or other active heating or cooling device). Any passive or active temperature devices should be capable of operating within the operating temperature range of telecommunications equipment. For example, the devices should be operable within −5° C. to 70° C. 
     As noted, operation of device  100  depends on temperature. As disclosed in more detail below, a temperature component provides temperature control and/or temperature information for the operation of device  100 . In this way, temperature can be controlled and/or known during use, so a suitable wavelength reference can be provided. As disclosed below, such a temperature component can include a thermistor in the housing  112 , a deposited thermistor on the reference filter  116 , a heating element in/on the housing  112 , a deposited heating element on the reference filter  116 , or any combination of these. 
     In device  100  of  FIG.  1   , a temperature sensor in the form of a thermistor  130  is mounted to TO header  106  within housing  102  and configured to sense the temperature of internal environment  104 . Although other types of thermal sensor may be used, thermistors have the benefit of simplicity and low cost. This sensed temperature is fed to an external controller (not shown) of the broader optical instrument being referenced (e.g. OCM) that is connected to device  100  via pins  108  and  110 . The external controller has a database or lookup table of the wavelength-temperature dependence of device  100  and uses the temperature information to determine the wavelengths of the spectral peaks in filtered optical signal  118 . This simple passive temperature sensing avoids the cost associated with a thermoelectric controller. 
     In operation, device  100  is incorporated into a broader optical measurement instrument, such as an OCM, as a module of that instrument. Referring to  FIG.  4   , there is illustrated a system diagram of an OCM  200  which incorporates wavelength reference device  100 . OCM  200  can be similar to that disclosed in U.S. Pat. No. 9,628,174, which is incorporated herein by reference. 
     OCM  200  is configured to receive an incoming wavelength division multiplexed optical signal  202 . Both WDM signal  202  and filtered optical signal  118  from device  100 , which represents a wavelength reference signal, are coupled to an input or optical switch module  204 , which is capable of switching the signals to be passed to a detection and processing module (i.e., a primary OCM scanning and processing module  206 ). Scanning and processing module  206  performs the primary spectral monitoring of the WDM channel spectrum. A controller  208  performs controlling functions of the OCM  200 , including controlling switch  204  to switch between WDM signal  202  and wavelength reference signal  118 , monitoring the temperature of internal environment  102  of device  100  to calibrate the spectral peaks of signal  118 , setting the temperature of internal environment  102  (if active temperature control is included). To perform the various functions, controller  208  includes drivers for components like thermistors, TECs and the like. Controller  208  may represent an internal controller of the OCM  200  itself or a separate controller specific to the wavelength reference device. 
     As disclosed herein, wavelength reference device  100  is preferably a modular component having its integrate package of elements. This allows device  100  to be assembled, tested, and calibrated on its own independent of OCM  200  and then readily integrated directly into the circuitry and the optical path of OCM  200 . 
     A number of variations to a wavelength reference device according to the present disclosure may be implemented and these are outlined below with reference to  FIGS.  5  to  11   . In subsequent embodiments, corresponding features of device  100  are designated with the same reference numerals for clarity. 
     Referring to  FIG.  5   , there is illustrated a second embodiment wavelength reference device  300 . In device  300 , passive temperature sensing is performed by a thermistor  302  that is deposited or mounted directly onto upper mirror  122  of the etalon  116 . 
     In this embodiment, the direct temperature of etalon  116  may be measured, improving the accuracy of temperature calibration of the etalon spectral response and therefore wavelength of the spectral peaks in filtered optical signal  118 . Furthermore, no standalone thermistor device is required, thereby reducing the number of overall components in the device. 
     In some embodiments, temperature control may also be actively provided by a temperature control device, such as a thermoelectric controller, a thermoelectric heater/cooler (TEC), or the like. Referring now to  FIG.  6   , there is illustrated a third embodiment wavelength reference device  400 , which includes an active temperature control device in the form of a thermal source  402 . This thermal source may be any controlled heating or cooling device and is controlled by a separate controller (e.g. controller  208  of  FIG.  4   ) and may have either heating or cooling capability, or both heating and cooling capability. 
     Device  400  also includes a separate thermistor  130  for sensing the temperature within environment  104 . However, in some embodiments, thermal source  402  includes an internal thermistor or other temperature sensor thereby avoiding the need for separate thermistor  130 . Thermal source  402  is mounted within housing  102  directly onto TO header  106  for powering by electrical pins  108  and  110 . Thermal source  402  forms a base upon which other components such as SLED  112 , etalon  116 , turning mirror  124  and thermistor  130 . In this manner, setting the temperature of thermal source  402  provides for directly setting the temperature of all components above. 
     Together with an external controller (not shown), thermistor  130  and thermal source  402  provide for a complete temperature control loop in which the temperature of internal environment  104  can be set. In particular, external controller  208  of  FIG.  4    can be configured to receive a temperature signal from thermistor  130  and, in response, send a control signal to thermal source  402  to switch on/off or increase/reduce the thermal output of thermal source  402  to adjust the temperature of environment  104 . Where thermal source  402  includes temperature sensing capability, this feedback loop may be implemented directly by thermal source  402  in response to control signals from controller  208 . The temperature control may be based on user-specified or other predefined temperature values for environment  104  which are conducive to efficient operation and accurate wavelength referencing. 
     Referring now to  FIG.  7   , there is illustrated a fourth embodiment wavelength reference device  500 . In device  500 , a thermal source  502  is mounted directly onto upper mirror  122  of etalon  116 , but located so that the optical path of filtered optical signal  118  is not blocked. 
     In this embodiment, the direct temperature of etalon  116  may be actively set by the thermal output of thermal source  502 , improving the accuracy of temperature calibration of the etalon spectral response and therefore wavelength of the spectral peaks in filtered optical signal  118 . The direct mounting of thermal source  502  onto etalon  116  also removes the need for passive sensing temperature by a separate thermistor or temperature sensor. However, thermal source  502  may also incorporate an internal thermistor or other temperature sensor to sense the temperature of etalon  116  and provide feedback to the external controller. 
     In some embodiments, the temperature control may be performed from outside housing  102 . Referring now to  FIG.  8   , there is illustrated a fifth embodiment wavelength reference device  600  in which housing  102  is substantially surrounded by a temperature controlled device such as a thermoelectric controlled coating  602 . Coating  602  may be mounted directly to an outside of housing  102  and controlled by an external controller (not shown) to provide a controlled thermal output to device  600 . The temperature of internal environment  104  may still be sensed by thermistor  130 . Through accurate temperature control, the temperature of internal environment  104  can be accurately set and the wavelength peaks of filtered optical signal  118  accurately known. As shown, a temperature sensor in the form of a thermistor  130  can be mounted in the internal environment  104  to measure the internal temperature according to the purposes disclosed herein. As illustrated, coating  602  is prohibited from entirely covering transparent window  126  so that the optical path of filtered optical signal  118  is not blocked. In some embodiments, coating  602  takes the form of a thermal blanket that can be wrapped around housing  102 . 
     Referring now to  FIG.  9   , there is illustrated a sixth embodiment wavelength reference device  700 . Device  700  includes a collimating lens  702  disposed in the optical path between SLED  112  and etalon  116 . Lens  702  is adapted to collimate or partially collimate the diverging beam of optical signal  114  emitted by SLED so that the rays are primarily directed perpendicularly onto etalon  116 . This perpendicular or normal incidence produces sharper spectral peaks in the filtered optical signal, thereby improving the accuracy of the wavelength referencing. It will be appreciated that equivalent collimation may be performed by other optical elements or combinations of elements such as curved mirrors. In some embodiments, collimating lens  702  collimates or partially collimates optical signal  114  and transparent window  126  includes focusing power to focus the collimated filtered optical signal  118  into a coupled optical fiber. 
     Referring now to  FIG.  10   , there is illustrated a seventh embodiment wavelength reference device  800 . In device  800 , SLED  112  is mounted to housing  102  in a vertical configuration. In particular, SLED  112  is mounted vertically to a mount  802  so that the optical signal  114  is emitted substantially vertically. This design provides for a simple linear optical path from SLED  112  through etalon  116  and transparent window  126  to collimator  128 . In some embodiments, device  800  also includes a collimating lens disposed in the optical path adjacent the broadband optical source  112 . 
     Referring now to  FIG.  11   , there is illustrated an eighth embodiment wavelength reference device  900 . In device  900 , etalon  116  is integral with transparent window  126 . That is, the internal and external sides of transparent window  126  respectively include etalon mirrors  120  and  122  formed thereon. This embodiment reduces the need for a separate etalon device, thereby reducing the number of components in the package and simplifying the assembly process. 
     It will be appreciated that combinations of the elements of the separate embodiments described above may be implemented. By way of example, a device having a vertically mounted SLED may be used in combination with a thermal source mounted to the TO header within the housing and having an etalon that is integrated within the transparent window of the housing. Therefore, the present disclosure is intended to encompass such combinations and modifications. 
     The above described wavelength reference devices can be produced from readily available components while providing high wavelength accuracy. In particular, the device can be integrated into a standard TO package such as a TO-46 package, providing a small spatial footprint when integrated into an optical instrument. The output of the package can include a window or lens, which may be one of several different designs. For example, the output can be a ball lens, a flat window, or an integrated lens. As noted above, the output of the package may include a fiber collimator for collimating the optical signal to a fiber for use in additional components of a system or apparatus. The device can be formed of a small number of components, which reduces the overall cost of manufacture. The device incorporates a fixed optical path, which provides for high frequency stability and a simple calibration process. 
     The device has a modular design with a single functional block and single optical output. This can support calibration and testing in isolation to other OCM components, thereby simplifying OCM calibration/testing and increasing yield. 
     Interpretation 
     Reference throughout this specification to the term “frequency” in a relative sense such as a “frequency range”, “frequency spectrum”, “change in frequency”, “frequency error” or the like is intended to be synonymous with “wavelength” as they are related by the constant relationship: speed of light=frequency×wavelength. The term “infrared” is used throughout the description and specification. Within the scope of this specification, infrared refers to the general infrared area of the electromagnetic spectrum which includes near infrared, infrared and far infrared frequencies or light waves. 
     Unless specifically stated otherwise, as apparent from the following discussions, it is appreciated that throughout the specification discussions utilizing terms such as “processing,” “computing,” “calculating,” “determining”, analyzing” or the like, refer to the action and/or processes of a computer or computing system, or similar electronic computing device, that manipulate and/or transform data represented as physical, such as electronic, quantities into other data similarly represented as physical quantities. In a similar manner, the term “controller” or “processor” may refer to any device or portion of a device that processes electronic data, e.g., from registers and/or memory to transform that electronic data into other electronic data that, e.g., may be stored in registers and/or memory. A “computer” or a “computing machine” or a “computing platform” may include one or more processors. 
     The foregoing description of preferred and other embodiments is not intended to limit or restrict the scope or applicability of the inventive concepts conceived of by the Applicants. It will be appreciated with the benefit of the present disclosure that features described above in accordance with any embodiment or aspect of the disclosed subject matter can be utilized, either alone or in combination, with any other described feature, in any other embodiment or aspect of the disclosed subject matter.