Patent Publication Number: US-11048017-B2

Title: Cooled optical apparatus, systems, and methods

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a U.S. national stage patent application of International Patent Application No. PCT/US2016/048954, filed on Aug. 26, 2016, the benefit of which is claimed and the disclosure of which is incorporated herein by reference in its entirety. 
     BACKGROUND 
     Understanding the structure and properties of geological formations can reduce the cost of drilling wells for oil and gas exploration. Measurements made in a borehole (i.e., downhole measurements) are typically performed to attain this understanding, to identify the composition and distribution of material that surrounds the measurement device downhole. Optical detectors are often used to perform these measurements. Optical detectors use fiber optic cables that have greater temperature capability, corrosion resistance and electromagnetic insensitivity as compared to some other types of energy conductors, such as wires or cables. However, optical detectors are still subject to various noise sources that can reduce accuracy and reliability of measurements taken with optical detectors. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of an optical detection system including an optical detector and a cooling system including a cold head for cooling the optical detector in accordance with various embodiments. 
         FIG. 2  is a block diagram of a single integrated optical chip including a group of optical detectors dedicated to distributed temperature sensing (DTS) and another group dedicated to distributed acoustic sensing (DAS) in accordance with various embodiments. 
         FIG. 3  is a block diagram of an optical detection system including an optical detector and a cooling system, without an intervening cold head, for cooling the optical detector in accordance with various embodiments. 
         FIG. 4  is a block diagram of an optical detection system including a switching mechanism to direct optical signals to an optical detector in accordance with various embodiments. 
         FIG. 5  is a flow chart of an example method of optical sensing in accordance with some embodiments. 
         FIG. 6  is a diagram of a wireline system embodiment. 
         FIG. 7  is a diagram of a drilling rig system embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Noise sources interfere with downhole measurement systems and can cause deterioration in signal-to-noise ratios (SNRs) of measurement signals. Thus, ongoing efforts are directed to reducing noise to improve signal-to-noise ratios in optical detection systems. For example, SNR can be increased by modifying certain parameters (e.g., resolution, fiber depth, and repetition rate). However, modification of these parameters can reduce the accuracy of the optical detection system. To address these concerns and others, systems, apparatuses, and methods described herein provide for cooling optical detectors to ultra-low temperatures (e.g., below 210 degrees Kelvin, below 70 degrees Kelvin or, in some embodiments, below 4 degrees Kelvin). This cooling can reduce or effectively eliminate thermal noise, thereby raising measurement signal SNRs without changing other parameters, in optical sensors for optical sensing systems. 
       FIG. 1  is a block diagram of an optical detection system  100 . An optical detection apparatus  101  of the optical detection system  100  includes an optical detector  102  for detecting light received through a fiber optic cable  104 . In some embodiments, the light includes wavelengths in a visible range of wavelengths. However, embodiments are not limited thereto and the light can include wavelengths in an infrared range of wavelengths and/or in an ultraviolet range of wavelengths. 
     In some embodiments, the optical detector  102  includes a low-light detector (LLD) or an extremely low-light detector (ELLD). Optical detectors, including LLDs and ELLDs, are available from Photon Spot Inc., of Monrovia, Calif., and from Princeton Instruments of Trenton, N.J. In some embodiments, the optical detector  102  includes a single-photon detector (SPD). In some embodiments, the optical detector  102  includes an avalanche photodiode. An example avalanche photodiode is the PGA-246-25 Single Photon Avalanche Diode available from Princeton Lightwave Inc. of Princeton, N.J. In some embodiments, the optical detector  102  includes carbon nanotubes or other nano structures. However, embodiments are not limited to these example optical detectors and other types of optical detectors can be used. For example, the optical detector  102  can include an integrated optical chip such as a silicon photonic resonator or a focal planar array detector, among other optical detector types. 
     In some embodiments, the integrated optical chip can support multiple SPDs with each detector (or group of detectors) dedicated to a specific sensing type. For example, as shown in  FIG. 2 , a single integrated chip  200  can include a group of optical detectors  202  dedicated to distributed temperature sensing (DTS) and another group  204  dedicated to distributed acoustic sensing (DAS). The DTS group can be further split into sub-groups of optical detectors that can sense scattered light (as opposed to incident light) components, including one group  208  of optical detectors for sensing an anti-Stokes signal and another group  206  of optical detectors for sensing the Stokes signal of scattered light. Likewise, the DAS group can be split into a set  212  of optical detectors that sense one interferometric output (the in-phase) and a set  214  that detects the other output (e.g., the quadrature). In some embodiments, the optical detector  102  can include additional optical elements to improve signal to noise ratios. These additional optical elements can include lenses, filters, mirrors, mixers, wavelength demultiplexers, and other elements. 
     Referring again to  FIG. 1 , the optical detection apparatus  101  can include a housing  106  for enclosing the optical detector  102  and for providing an optical shield for the optical detector  102 . The housing  106  can include an aperture  108  for passage of the fiber optic cable  104 . However, embodiments are not limited thereto, and in some embodiments, a coupler can be mounted so that the fiber optic cable  104  terminates at a boundary of the housing  106 . The optical detection system  100  includes a light source  110 , separate from the optical detector  102  and the housing  106 , for producing light and providing light through the fiber optic cable  104 . In some embodiments, the light source  110  can provide light through one fiber optic cable  104  and light can be received by the optical detector  102  on a separate fiber optic cable  104 , although embodiments are not limited to any particular number of fiber optic cables  104  or combinations thereof. In some embodiments, the light source  110  can include additional optical components configured to provide or delivery the appropriate light to the system including filters, mixers, non-linear crystals, timing chips, wavelength multiplexers, etc. In some embodiments, the light source  110  can be downhole and in some embodiments, the light source  110  can be on the surface. In some embodiments, multiple light sources can be provided on the surface, down hole, or a combination thereof. 
     The optical detection apparatus  101  includes a cooling mechanism  112  having the housing  106  mounted thereto. The cooling mechanism  112  is configured to maintain the temperature of a light-sensitive region of the optical detector  102  within a temperature range below 210 degrees Kelvin. In some embodiments, the cooling mechanism  112  operates using liquid helium (He) or liquid nitrogen (N 2 ). In some embodiments, the cooling mechanism  112  maintains the temperature of the light-sensitive region of the optical detector  102  at a temperature at or below 80 degrees Kelvin. In some embodiments, the cooling mechanism  112  maintains the temperature of the light-sensitive region of the optical detector  102  at a temperature at or below 5 degrees Kelvin (e.g., when sealed helium systems are used). In some embodiments, the cooling mechanism  112  can be of one or more of a variety of configurations, including Dilution-Magnetic, Collins-Helium Liquefier, Joule-Thomson, Stirling-cycle cryocooler, self-regulated Joule-Thomson, Closed-Cycle Split-Type Stirling, Pulse Tube, a two-stage Gifford-McMahon cryogenic cooler or multi-stage Gifford-McMahon cryogenic cooler, or a cooler using magnetocaloric effect, by way of example. Lowering the temperature of the optical detector  102  improves the SNR of the optical detector  102  by decreasing dark current, by increasing sensitivity, and by reducing resistive loss by causing the optical detector  102  to enter a superconducting regime of operation. In some embodiments or configurations non-SPD optical detectors  102  will not enter a superconducting regime, while still having little to no thermal noise. 
     In some embodiments, the optical detection apparatus  101  includes a cold head  114  between the optical detector  102  and the cooling mechanism  112 . However, some embodiments do not include a cold head  114 .  FIG. 3  is a block diagram of an optical detection system  300  including an optical detector  302  and a cooling system  312 , without an intervening cold head, for cooling the optical detector  302  in accordance with various embodiments. As with the optical detection system  100 , the optical detection system  300  includes a light source  310 , and a fiber optic cable  304  coupled between the downhole unit  316  and the optical detector  302 . The optical detector  102  can be encased in a housing  306  with or without an aperture  308  for permitting passage of the fiber optic cable  304 . 
     Referring again to  FIG. 1 , in some embodiments, the housing  106  is mounted to the cooling mechanism  112  such that moisture is prevented from entering the housing. For example, the housing  106  can be mounted such that a vacuum seal is formed with the cooling mechanism  112  or the cold head  114 . The housing  106  can have a non-reflective inner surface. 
     The optical detection system  100  can further include a downhole unit  116  (e.g., a down hole tool or a down hole sensor) configured to provide an optical signal over the fiber optic cable  104 . The optical signal can be a pulsed signal originating from distributed sensing, or a continuous signal, among other signals. 
     The optical detection system  100  can include more than one optical detector  102  (shown within the dashed box in  FIG. 1 ). At least one optical detector  102  can detect light received through the fiber optic cable  104  from more than one downhole unit  116 . The optical detection system  100  can include optical detectors  102  of more than one type. For example, some optical detectors  102  in the optical detection system  100  can be SPDs, avalanche photodiodes, integrated optical chips, etc. Some optical detectors  102  can be non-LLD detectors, or non-ELLD detectors (e.g., other than non-SPD detectors). Non-LLD detectors can include p-i-n (or PIN) photodiodes, phototransistors, photovoltaics, photoconductors. Downhole units  116  can include units configured to detect temperature. Other downhole units  116  can be coupled to the same optical detector  102  or to a group of optical detectors  102 , or in any other configuration. Downhole units  116  can detect other properties and perform other functions such as acoustic sensing, resistivity measurements, etc. Some optical detectors  102  may not be mounted on any cooling mechanism  112 , some optical detectors  102  may be mounted with other optical detectors  102  on a same cooling mechanism  112 , or the optical detection system  100  can include more than one cooling mechanism  112  arranged in any configuration with one or more optical detectors mounted thereto. 
     As shown in  FIG. 4 , an optical detection system  400  can further include a switching or splitting mechanism  402  to direct optical signals to an optical detector  404 ,  406  based on wavelength of the optical signal, power of the optical signal, polarization, or any other parameter or criterion. For example, high-power optical signals  403  can be routed to non-SPD optical detectors  404 , and away from SPDs  406  and low-powered optical signals  405  can be routed to SPD optical detectors  406 . This routing can be performed to prevent damage to SPD optical detectors  406  while still taking full advantage of LLD and ELLD capabilities of SPD optical detectors  406 . As will be appreciated, high-power optical signals  403  can cause saturation in SPDs  406 , leading to damage to the SPDs  406  or to inaccurate results. In some examples, saturation of SPDs  406  can occur with optical signal inputs having a power of about 100 microwatts, and damage can occur at about 10 milliwatts. The noise floor that can be detected by the SPD  406  can be at a level slightly below saturation level but is typically at least 20-30 dB. The saturation level and noise floors for non-SPD optical detectors  404  may be different from the saturation level and noise floors for SPDs  406 . The saturation levels and noise floors also may or may not overlap, and thus multiple types of detectors may be used that can cover the full power range required for system measurement. For at least these reasons, to measure a larger range of possible optical signals, SPD optical detectors  102  are used in a system with non-SPD optical detectors  102 . Switching mechanisms  402  can direct or reroute optical signals based on power level or other criteria, to take advantage of the different power ranges measurable by SPD optical detectors  102  versus non-SPD optical detectors  102 . 
     In addition to or instead of a switching mechanism  402 , the optical detection system  100  can include a coupling mechanism or other mechanism to split the light with optical couplers (with or without feedback). These mechanisms can be multi-stage (e.g., the light can be split in one stage, then split again in a second stage), and can split light based on power, wavelength, or phase. Processor or computation-based systems can also be used in some embodiments to dynamically direct or reroute light signals among any available optical path as power increases, or based on any other criteria. 
     Referring again to  FIG. 1 , the optical detection system  100  can further include a measuring device  118  (which may be part of a computer workstation) coupled to the optical detector  102  to obtain measurement data, with a display  120  to display a graphical representation of the measurement data. In some embodiments, some portions of the optical detection system  100  can be positioned at a surface of the Earth, while some portions to the optical detection system  100  can be placed downhole. When more than one optical detector  102  is used, for example, some of the optical detectors  102  can be placed downhole, and some can be placed at the surface. In some embodiments, one or more cooling mechanisms  112  can be placed downhole proximate one or more optical detectors  102  although power and geometry considerations should be taken into account with such configurations to provide power for cooling in an appropriately sized borehole. In some embodiments, the measuring device  118  and display  120  will be situated at the surface of the Earth, for example as shown in  FIGS. 6 and 7  described later herein. 
       FIG. 5  is a flow chart of an example method  500  of optical sensing in accordance with some embodiments. The example method  500  can be performed by the optical detection system  100  ( FIG. 1 ) or by components thereof. The example method  500  begins with operation  502  with coupling an optical sensing apparatus to a downhole unit  116  (e.g., a downhole sensing device) through a fiber optic cable  104 . The optical sensing apparatus can include at least one optical detector  102  within a housing  106 . However, embodiments are not limited to one optical detector  102 . The optical detector  102  can be cooled to a temperature below 210 degrees Kelvin by a cooling mechanism  112 . In some embodiments, the optical detector  102  can be cooled to a temperature below 5 degrees Kelvin. 
     The example method  500  continues with operation  504  with the optical detector  102  receiving optical signals from the downhole sensing device over the fiber optic cable  104 . In some embodiments, the downhole sensing device includes an intrinsic fiber optic sensor. In other embodiments, the downhole sensing device comprises at least one fiber Bragg grating or some other reflector. In at least these embodiments, the example method  500  can further include providing an optical signal to the intrinsic fiber optic sensor and receiving a reflected or backscattered optical signal, responsive to providing the optical signal, that represents at least one downhole property. In embodiments, the backscattered signal can include Stokes and anti-Stokes components, or Raleigh components. 
     In embodiments, many optical signals can be multiplexed onto the fiber optic cable  104 . In at least these embodiments, the example method  500  can further include de-multiplexing the optical signals at a switching mechanism, and providing the de-multiplexed signals on at least two separate paths to at least two separate optical detectors  102 . 
     The example method  500  continues with operation  506  with the optical detection system  100  detecting at least one downhole property based on the optical signals. For example, the optical signal can be used to detect different properties of downhole structures, to provide optical analysis of fluid and material composition in a borehole or annulus, to perform geosteering, to determine values for porosity or composition of the borehole wall, etc. 
       FIG. 6  is a diagram of a wireline system  600  embodiment. The wireline system  600  can comprise portions of a wireline logging tool body  602  as part of a wireline logging operation. Thus,  FIG. 6  shows a well during wireline logging operations. In this case, a drilling platform  604  is equipped with a derrick  606  that supports a hoist  608 . 
     Drilling oil and gas wells is commonly carried out using a string of drill pipes connected together so as to form a drilling string that is lowered through a rotary table  610  into a wellbore or borehole  612 . Here it is assumed that the drilling string has been temporarily removed from the borehole  612  to allow a wireline logging tool body  602 , such as a probe or sonde, to be lowered by wireline or logging cable  614  into the borehole  612 . Typically, the wireline logging tool body  602  is lowered to the bottom of the region of interest and subsequently pulled upward at a substantially constant speed. 
     During the upward trip, at a series of depths instruments (e.g., downhole units  116  described above with reference to  FIG. 1 ) included in the wireline logging tool body  602  can be used to perform measurements on the subsurface geological formations adjacent the borehole  612  (and the wireline logging tool body  602 ). The measurement data can be communicated to a surface logging facility  616  for storage, processing, and analysis. The logging facility  616  can be provided with electronic equipment for various types of signal processing, which can be implemented by any one or more of the components of the optical detection system  100  ( FIG. 1 ). Similar formation evaluation data can be gathered and analyzed during drilling operations (e.g., during LWD operations, and by extension, sampling while drilling). 
     The wireline logging tool body  602  is suspended in the wellbore by a wireline cable  614  that connects the tool to a surface control unit (e.g., comprising a workstation  118 , which can also include a display  120 ). This wireline cable  614  can include (or perform functionalities of) the fiber optic cable  104  ( FIG. 1 ). The tool can be deployed in the borehole  612  on coiled tubing, jointed drill pipe, hard wired drill pipe, or any other suitable deployment technique. 
     In addition to wireline embodiments, example embodiments can also be implemented in drilling rig systems.  FIG. 7  illustrates a drilling rig system  700  embodiment. The system  700  can include a downhole unit  116  as part of a downhole drilling operation. 
     Referring to  FIG. 7 , it can be seen how a system  700  can also form a portion of a drilling rig  702  located at the surface  704  of a well  706 . The drilling rig  702  can provide support for a drill string  708 . The drill string  708  can operate to penetrate the rotary table  610  for drilling the borehole  612  through the subsurface formations  714 . The drill string  708  can include a Kelly  716 , drill pipe  718 , and a bottom hole assembly  720 , perhaps located at the lower portion of the drill pipe  718 . 
     The bottom hole assembly  720  can include drill collars  722 , a downhole tool  724 , and a drill bit  726 . The drill bit  726  can operate to create the borehole  612  by penetrating the surface  704  and the subsurface formations  615 . The downhole tool  724  can comprise any of a number of different types of tools including MWD tools, LWD tools, and others. In some examples, fiber optic cable  104  will be spliced, rerouted, coupled, guided, or otherwise modified to maintain connections at each drill collar  722  and at each position along the drill string  708 . In some embodiments, a fiber optic connector can be provided at each drill collar  722  or other joint or position downhole. In some embodiments, the fiber optic cable  114  can be placed inside a steel casing as shown in  FIG. 7 , outside a casing, inside or outside of a production tube, inside or outside of coiled tubing, on a wireline cable, or in any other placement, configuration, or combination thereof. In some embodiments, downhole units  116  can be placed in a repeater configuration or in an amplifier configuration to improve signal strength at the surface. In yet other embodiments, fiber can be deployed continuously through a drill string using a dart, a torpedo, a reel, a feedthrough, or some other deployment device. 
     During drilling operations, the drill string  708  (perhaps including the Kelly  716 , the drill pipe  718 , and the bottom hole assembly  720 ) can be rotated by the rotary table  610 . Although not shown, in addition to, or alternatively, the bottom hole assembly  720  can also be rotated by a motor (e.g., a mud motor) that is located downhole. The drill collars  722  can be used to add weight to the drill bit  726 . The drill collars  722  can also operate to stiffen the bottom hole assembly  720 , allowing the bottom hole assembly  720  to transfer the added weight to the drill bit  726 , and in turn, to assist the drill bit  726  in penetrating the surface  704  and subsurface formations  714 . 
     Thus, it can be seen that in some embodiments, the systems  600 ,  700  can include a drill collar  722 , a downhole tool  724 , and/or a wireline logging tool body  602  to house one or more downhole units, similar to or identical to the downhole units  116  providing information over the fiber optic cable  104  and illustrated in  FIGS. 1 and 2 . 
     Thus, for the purposes of this document, the term “housing” when used to address tools below the surface (e.g., downhole), can include any one or more of a drill collar  722 , a downhole tool  724 , or a wireline logging tool body  602  (all having an outer wall, to enclose or attach to magnetometers, sensors, fluid sampling devices, pressure measurement devices, transmitters, receivers, acquisition and processing logic, and data acquisition systems). The tool  724  can comprise a downhole tool, such as an LWD tool or MWD tool. The wireline logging tool body  602  can comprise a wireline logging tool, including a probe or sonde, for example, coupled to a logging cable  614 . Many embodiments can thus be realized. 
     Thus, a system  600 ,  700  can comprise a downhole tool body, such as a wireline logging tool body  602  or a downhole tool  724  (e.g., an LWD or MWD tool body), and fiber optic cable  104  to provide signaling to the optical detection system  100  or to components thereof (e.g., an optical detector  102 ) as described above. 
     In various embodiments, a non-transitory machine-readable storage device can comprise instructions stored thereon, which, when performed by a machine, cause the machine to perform operations, the operations comprising one or more features similar to or identical to features of methods and techniques described herein. A machine-readable storage device, herein, is a physical device that stores data represented by physical structure within the device. Examples of machine-readable storage devices can include, but are not limited to, memory in the form of read only memory (ROM), random access memory (RAM), a magnetic disk storage device, an optical storage device, a flash memory, and other electronic, magnetic, or optical memory devices, including combinations thereof. These can be provided in integrated chips that include optical detectors  102 , or in other surface computer systems for taking measurements or analyzing measurements as part of the optical detection system  100  ( FIG. 1 ). 
     The physical structure of such instructions can be operated on by one or more processors. Executing instructions determined by these physical structures can cause the optical detection system  100  or components thereof to perform operations according to methods described herein. The instructions can include instructions to cause associated data or other data to be stored in a memory. 
     The wireline logging tool body  602  ( FIG. 6 ) can include or otherwise be utilized in conjunction with any number of measurement tools such as resistivity tools, seismic tools, acoustic tools, temperature sensors, porosity sensors and others. In one embodiment, the wireline logging tool body  602  is equipped with transmission equipment to communicate ultimately to a surface processing unit of a surface logging facility  616  ( FIG. 6 ). Such transmission equipment can take any desired form, and different transmission media and methods can be used. Examples of connections include wired, fiber optic, wireless connections and memory based systems. 
     Any of the above components, for example components of the optical detection system  100 , can all be characterized as “modules” herein. Such modules can include hardware circuitry, and/or a processor and/or memory circuits, software program modules and objects, and/or firmware, and combinations thereof, as desired by the architect of the optical detection system  100  as appropriate for particular implementations of various embodiments. For example, in some embodiments, such modules can be included in an apparatus and/or system operation simulation package, such as a software electrical signal simulation package, a power usage and distribution simulation package, a power/heat dissipation simulation package, and/or a combination of software and hardware used to simulate the operation of various potential embodiments. 
     It should also be understood that the apparatus and systems of various embodiments can be used in applications other than for logging operations, and thus, various embodiments are not to be so limited. The illustrations of optical detection systems  100  are intended to provide a general understanding of the structure of various embodiments, and they are not intended to serve as a complete description of all the elements and features of apparatus and systems that might make use of the structures described herein. 
     Applications that can include the novel apparatus and systems of various embodiments include electronic circuitry used in high-speed computers, communication and signal processing circuitry, modems, processor modules, embedded processors, data switches, and application-specific modules. Some embodiments include a number of methods. 
     It should be noted that the methods described herein do not have to be executed in the order described, or in any particular order. Moreover, various activities described with respect to the methods identified herein can be executed in iterative, serial, or parallel fashion. Information, including parameters, commands, operands, and other data, can be sent and received in the form of one or more carrier waves. 
     Upon reading and comprehending the content of this disclosure, one of ordinary skill in the art will understand the manner in which a software program can be launched from a computer-readable medium in a computer-based system to execute the functions defined in the software program. One of ordinary skill in the art will further understand the various programming languages that can be employed to create one or more software programs designed to implement and perform the methods disclosed herein. For example, the programs can be structured in an object-orientated format using an object-oriented language such as Java or C#. In another example, the programs can be structured in a procedure-orientated format using a procedural language, such as assembly or C. The software components can communicate using any of a number of mechanisms well known to those skilled in the art, such as application program interfaces or interprocess communication techniques, including remote procedure calls. The teachings of various embodiments are not limited to any particular programming language or environment. Thus, other embodiments can be realized. 
     In summary, using the apparatus, systems, and methods disclosed herein can provide more accurate measurements by optical detection apparatuses through removal or reduction of noise sources including thermal noise sources. These advantages can significantly enhance the value of the services provided by an operation/exploration company, while at the same time controlling time-related costs. 
     The accompanying drawings that form a part hereof, show by way of illustration, and not of limitation, specific embodiments in which the subject matter can be practiced. The embodiments illustrated are described in sufficient detail to enable those skilled in the art to practice the teachings disclosed herein. Other embodiments can be utilized and derived therefrom, such that structural and logical substitutions and changes can be made without departing from the scope of this disclosure. This Detailed Description, therefore, is not to be taken in a limiting sense, and the scope of various embodiments is defined only by the appended claims, along with the full range of equivalents to which such claims are entitled. 
     Such embodiments of the inventive subject matter can be referred to herein, individually and/or collectively, by the term “invention” merely for convenience and without intending to voluntarily limit the scope of this application to any single invention or inventive concept if more than one is in fact disclosed. Thus, although specific embodiments have been illustrated and described herein, it should be appreciated that any arrangement calculated to achieve the same purpose can be substituted for the specific embodiments shown. This disclosure is intended to cover any and all adaptations or variations of various embodiments. Combinations of the above embodiments, and other embodiments not specifically described herein, will be apparent to those of skill in the art upon reviewing the above description. 
     Various examples include: 
     Example 1 is an apparatus or device (e.g., an optical detection apparatus) comprising: an optical detector for detecting light received through a fiber optic cable; a housing for enclosing the optical detector and to optically shield the optical detector, the housing including an aperture for passage of the fiber optic cable; a light source, separate from the optical detector and the housing, for providing light through the fiber optic cable; and a cooling mechanism having the housing mounted thereto, the cooling mechanism configured to maintain the temperature of a light-sensitive region of the optical detector within a temperature range below 210 degrees Kelvin. 
     In Example 2, the subject matter of Example 1 can optionally include wherein the cooling mechanism operates using liquid helium (He) or liquid nitrogen (N 2 .). 
     In Example 3, the subject matter of Example 2 can optionally include wherein the cooling mechanism maintains the temperature of the light-sensitive region of the optical detector at a temperature at or below 80 degrees Kelvin. 
     In Example 4, the subject matter of Example 3 can optionally include wherein the cooling mechanism maintains the temperature of the light-sensitive region of the optical detector at a temperature at or below 5 degrees Kelvin. 
     In Example 5, the subject matter of Example 4 can optionally include wherein the cooling mechanism includes at least one of a Pulse Tube, a Gifford-McMahon cryogenic cooler, a Stirling cryocooler, or a cooler using magnetocaloric effect. 
     In Example 6, the subject matter of Example 1 can optionally include wherein the housing is mounted to the cooling mechanism such that moisture is prevented from entering the housing. 
     In Example 7, the subject matter of Example 1 can optionally include wherein the optical detector includes a single-photon detector. 
     In Example 8, the subject matter of Example 1 can optionally include wherein the optical detector includes an avalanche photodiode. 
     In Example 9, the subject matter of Example 1 can optionally include a cold head between the cooling mechanism and the housing, and wherein the housing is mounted on the cold head. 
     In Example 10, the subject matter of Example 1 can optionally include wherein the light includes wavelengths in an infrared range of wavelengths. 
     In Example 11, the subject matter of Example 10 can optionally include wherein the light includes wavelengths in a visible range of wavelengths. 
     In Example 12, the subject matter of Example 10 can optionally include wherein the light includes wavelengths in an ultraviolet range of wavelengths. 
     In Example 13, the subject matter of Example 1 can optionally include wherein the housing has a non-reflective inner surface. 
     Example 14 is a system or a detection system (e.g., an optical detection system) comprising: a downhole unit configured to provide an optical signal over a fiber optic cable; a light source configured to produce light and to provide the light through the fiber optic cable to the downhole unit; an optical detector for detecting light received through a fiber optic cable from the downhole unit; a housing for enclosing the optical detector; and a cooling mechanism having the housing mounted thereto and to maintain the temperature of the optical detector within a temperature range below 80 degrees Kelvin. 
     In Example 15, the subject matter of Example 14 can optionally include a plurality of optical detectors, wherein at least one optical detector is to detect light received through a fiber optic cable from a downhole unit configured to detect temperature. 
     In Example 16, the subject matter of Example 15 can optionally include a plurality of optical detectors, wherein at least one optical detector is to detect light received through a fiber optic cable from a plurality of downhole units, at least one of the plurality of downhole units configured to detect temperature, and at least one of the plurality of downhole units configured to detect properties other than temperature. 
     In Example 17, the subject matter of Example 14 can optionally include at least one fiber optic sensor comprising at least one fiber Bragg grating. 
     In Example 18, the subject matter of Example 14 can optionally include a plurality of optical detectors, and one or more cooling mechanisms, wherein at least one of the plurality of optical detectors is not mounted on a cryocooling mechanism; and a switching mechanism to direct an optical signal to an optical detector of the plurality of optical detectors based on one of wavelength of the optical signal and power of the optical signal. 
     In Example 19, the subject matter of Example 18 can optionally include wherein the light source, and at least one of the plurality of optical detectors, are located at a surface of the Earth. 
     In Example 20, the subject matter of Example 14 can optionally include a measuring device coupled to the optical detector to provide measurement data associated with the light detected by the optical detector; and a display to display a graphical representation of the measurement data. 
     Example 21 is a method of optical sensing including: coupling an optical sensing apparatus to a downhole sensing device through a fiber optic cable, the optical sensing apparatus including at least one optical detector within a housing, the at least one optical detector being cooled to a temperature below 210 degrees Kelvin by a cooling mechanism; receiving optical signals from the downhole sensing device over the fiber optic cable; and detecting at least one downhole property based on the received optical signals. 
     In Example 22, the subject matter of Example 21 can optionally include wherein the downhole sensing device includes an intrinsic fiber optic sensor and wherein the method further comprises: providing an optical signal to the intrinsic fiber optic sensor; and receiving a reflected or backscattered optical signal, responsive to providing the optical signal, that represents at least one downhole property. 
     In Example 23, the subject matter of Example 21 can optionally include wherein the optical signals are multiplexed onto the fiber optic cable; and wherein the method further includes: de-multiplexing the optical signals at a switching mechanism to generate de-multiplexed signals; and providing the de-multiplexed signals on at least two separate paths to at least two separate detectors of a plurality of detectors that includes the at least one optical detector. 
     Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that any arrangement that is calculated to achieve the same purpose can be substituted for the specific embodiments shown. Various embodiments use permutations or combinations of embodiments described herein. It is to be understood that the above description is intended to be illustrative, and not restrictive, and that the phraseology or terminology employed herein is for the purpose of description. Combinations of the above embodiments and other embodiments will be apparent to those of ordinary skill in the art upon studying the above description.