Patent Publication Number: US-2023155679-A1

Title: Temperature-independent Optical Link

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of provisional application No. 63/264,148 having a filing date of Nov. 16, 2021, and entitled, “Optical Link with Improved Optical Output Signal Control,” the entire contents of which are incorporated by reference. 
    
    
     FIELD OF THE DISCLOSURE 
     The disclosure relates to electronics and semiconductor testing and measurement equipment and methods of use. More specifically, the present disclosure relates to a temperature-independent optical link for converting a received electrical signal to an analog signal. The optical link can be housed in a probe head used in electrical signal testing and measurement. 
     BACKGROUND OF THE DISCLOSURE 
     In the field of electronics and of semiconductor testing and measurement applications, optical links for analog signal transmission are used to convert locally acquired electrical measurement signal to a remote processing unit, such as an oscilloscope. The electrical signal is converted to an optical signal and sent to the recipient via a fiber optic cable. The acquired measurement signals may for example represent voltage measurements. Inherent to the vast majority of test and measurement applications is the requirement of transmitting signals at the highest possible quality. Nevertheless, with traditionally used equipment and methods, errors are introduced to the signal during its conversion and transmission. 
     Output signals of electrical-to-optical transmitters are susceptible to fluctuations when exposed to changing environment conditions. The wavelength generated by an internal optical diode of a laser is temperature sensitive. 
     Achieving stable and accurate analog transmissions across an optical link, such as a fiber optic link, has a unique set of challenges. Known approaches include digitizing the analog electrical input signal and transmitting the digitized signal over the optical fiber link digitally, applying FM. Other schemes include modulating the signal before transmitting. Other methods involve direct amplitude modulation of the signal and then attempting to compensate for signal variations, like the laser slope-efficiency, over time and temperature with variable gain amplifiers and offset tracking algorithms. 
     Digitizing analog signals and FM or other modulation schemes are typically limited by their inherent bandwidth consumption which is to the detriment of the bandwidth available for measurement data transmission. Applying direct amplitude modulation of the signal and then trying to compensate for the variations may allow utilizing the full bandwidth capability of the electrical-to-optical transmitter; however, the compensation of the temperature-dependent variations in an electrical-to-optical transmitter are not complete and result in some residual errors of the measured signal. 
     According to another traditional approach, temperature sensitivity of lasers is managed by controlling the laser output power by a laser internal photodiode. Such control approaches assume that the amount of light entering the optical fiber corresponds to the amount of light entering the photodiode. The difference between the light entering the internal photodiode and the light being coupled into the fiber is also called front-back tracking ratio, meaning the ratio of the two different outputs of the laser. This allows the implementation of suitable algorithms to adjust laser drive conditions based on the detected change in laser output power change, detected by the photodiode. 
     Beside the necessity of complex sensor and software infrastructure, this approach has the disadvantage that only a small fraction of the many effects temperature fluctuations may have on the optical link are considered by the control. 
     Another challenge known in the field of optical links is the tracking between the back facet monitor internal photodiode in the laser and the fiber output power, meaning the laser coupling into the output fiber, fiber output power level, over temperature. The back facet monitor, the internal monitor photodiode, is used to setting the laser output bias point for constant output power, maintaining a stable DC/LF laser output signal and for reducing LF noise through a feedback loop. If this tracking mismatch error becomes significant over temperature, the operating point of the laser diode will change due to incorrect readout in the control circuit. Additionally, the DC/LF and HF gain will deviate from each other, and the result is poorly compensated waveform, LF comp. One of the main contributors of this tracking/alignment error is the terminal expansion of the mechanical package which changes in temperature which introduces optical alignment/coupling errors inside the laser package. 
     SUMMARY OF THE DISCLOSURE 
     What is needed is an optical link providing a constant and clean optical output signal in a test and measurement application without being subjected to bandwidth losses due to error signal modulation. The temperature-independent optical link for converting a received signal to an analog signal, the probe head comprising such optical link, and the method for converting a received signal to an analog signal allow to reliably measure high quality signals in different and transient temperature environments. 
     In one embodiment, the temperature-independent optical link comprises a temperature-controlled transmitter chamber housing an electrical-to-optical transmitter, the transmitter being configured to convert an electrical signal received from an electrical input connection to an analog signal and to transmit the converted analog signal to an optical output connection. The temperature-independent optical link further comprises a feedback-loop temperature control system for controlling the transmitter chamber to a set point temperature using a temperature control device and a temperature sensor. The transmitter chamber is configured to maintain the electrical-to-optical transmitter at the set point temperature. 
     In one embodiment, the transmitter chamber is made of a material having a high thermal conductivity. The transmitter chamber is a tube made of metal such as brass, aluminum, copper, steel, or metal alloys. The optical link has a thermally and electrically insulating material around the transmitter chamber. 
     In another embodiment, the temperature control device is a Peltier device connected to the electrical-to-optical transmitter. The temperature sensor is arranged inside the transmitter chamber and is thermally connected to the ETO output transmitter. The electrical-to-optical transmitter comprises a fiber coupled laser package having a laser diode section and a laser alignment section. The temperature-independent optical link is configured for converting a received electrical signal of high frequency to an optical signal. 
     In one embodiment, the temperature-independent optical link is housed in a probe head having a probe tip and a power supply. 
     In yet another embodiment, the probe head comprises an input buffer connected to the probe tip and the electrical input connection of the electrical-to-optical transmitter. 
     In a further embodiment, the electrical-to-optical transmitter is connected to the input buffer via a thermal damping device. 
     One method for converting a received electrical signal to an analog signal with the temperature-independent optical link comprises the steps of assembling an electrical-to-optical transmitter and a temperature sensor in the transmitter chamber and connecting the temperature control device to the electrical-to-optical transmitter and to the temperature sensor, applying an electrical signal to an electrical input connection, converting electrical signals into optical signals, maintaining a constant temperature in the transmitter chamber, and observing a constant optical power output for a given electrical input signal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings that are incorporated in and constitute a part of this specification illustrate several embodiments of the disclosure. Together with the description, they serve to explain the principles of the disclosure: 
         FIG.  1    illustrates a cross-section of an exemplary embodiment of an optical link in a side view. 
         FIG.  2    illustrates a cross-section of an exemplary embodiment of an electrical-to-optical transmitter of the optical link. 
         FIG.  3    illustrates a cross-section of an exemplary embodiment of probe head in a side view. 
         FIG.  4    illustrates a cross-section of an exemplary embodiment of probe head in a side view. 
         FIG.  5    illustrates a perspective view of an exemplary embodiment of a probe head. 
         FIG.  6    illustrates a cross-sectional side view of the probe head of  FIG.  5   . 
         FIG.  7    illustrates a cross-sectional top view of the probe head of  FIG.  5   . 
         FIG.  8    illustrates a cross-sectional rear view of the probe head of  FIG.  5   . 
         FIG.  9    illustrates an exemplary embodiment of a flow chart for controlling the electrical-to-optical transmitter using the optical link. 
         FIG.  10    illustrates an excerpt of another exemplary embodiment of a flow chart for controlling the electrical-to-optical transmitter using the optical link. 
     
    
    
     REFERENCE NUMERALS OF THE FIGS. 
       10  Optical link 
       12  Electrical input 
       14  Housing 
       14 A First half-shell of the housing 
       14 B Second half-shell of the housing 
       16  Input buffer 
       17  Fastener 
       18  Electrical input connection 
       20  Electrical-to-optical transmitter (“ETO transmitter”) 
       21  Laser diode section 
       22  Thermally and electrically insulating material 
       23  Laser alignment section 
       24  Temperature control device 
       25  Electrical-to-optical transmitter chamber 
       26  Temperature sensor 
       27  Fiber coupled laser package 
       28  Internal temperature controlled region 
       29  Header 
       30  Battery or power over fiber power converter 
       32  Screw-on cap 
       33  Thermal damping device 
       34  Laser diode 
       35  Hole in the thermal damping device 
       36  Laser beam 
       37  Lens 
       39  Glass capillary 
       40  Temperature control system 
       50  Optical output 
       52  Optical output connection 
       54  Bend limiter 
       100  Probe head 
     S 10 -S 50  Method steps 
     DETAILED DESCRIPTION 
     The present disclosure provides generally for an optical link for converting a received electrical signal to an analog signal, comprising a temperature-controlled transmitter chamber housing an electrical-to-optical transmitter (“ETO transmitter”), the transmitter being configured to convert an electrical signal received from an electrical input connection to an analog signal and to transmit the converted analog signal to an optical output connection. An ETO transmitter may include a laser diode section and a laser alignment section. The temperature-independent optical link further comprises a feedback-loop temperature control system for controlling the transmitter chamber to a set point temperature using a temperature control device and a temperature sensor, wherein the transmitter chamber is configured to maintain the ETO transmitter at the set point temperature. 
     By providing a transmitter chamber comprising a temperature control device configured to keep the transmitter at a constant predetermined temperature, the entire ETO transmitter may be placed in an environment at constant temperature. In other words, the entire ETO transmitter may be shielded against temperature fluctuations by the transmitter chamber. This has the advantage that complex modulation, digitization, and front-back tracking ratio control approaches as previously discussed are rendered moot. This has the advantage that system complexity can be reduced significantly and bandwidth available for measurement data transmission can be maximized. 
     According to further embodiments, the optical link may further comprise a thermally and electrically insulating material around the transmitter chamber. The thermally and electrically insulating material may for example comprise expanded polystyrene, polyurethane (PU) foam, polyisocyanurate (PIR) foam, phenolic spray foam, or the like. Further, the temperature control device may comprise a temperature control system. Additionally, or alternatively, the temperature control device may comprise a Peltier device. Further, the temperature control device may comprise a temperature sensor, for example a thermistor or a thermocouple, and a temperature control system comprising a closed-loop or feedback-loop temperature control. 
     According to further embodiments, the ETO transmitter may comprise a fiber coupled laser package. The fiber coupled laser package may comprise, or consist of, a laser diode section and a laser alignment section. The laser diode section may comprise TO-can type package. The laser alignment section may comprise a sleeve, a glass capillary, and a proximal end of a fiber optic cable. 
     The coupled laser package may comprise a header, connecting the electrical input connection with the laser diode. The ETO transmitter may or may not comprise a cooled internal laser diode. The optical output connection may comprise a fiber optic cable and a bend limiter. The fiber optic cable may comprise a single mode fiber. Typically, the bend limiter and most of the fiber optic cable are not part of the ETO transmitter. 
     Using a temperature controlling device, such as a Peltier device (Thermo-Electric Cooler, TEC etc.), the ETO transmitter may be maintained at a constant temperature with a feedback-loop temperature control system. 
     External temperature fluctuations may influence the optical output signal to a great extent. As an example, a system having a front-back tracking ratio compensation, calibrated to 25° C., has been reported to have an error of 2% when operated at 27° C. In other words, this 2% error is attributed to the temperature expansion in the laser alignment section alone. 
     Since the entire ETO transmitter including the fiber alignment and coupling optics are inside the constant-temperature transmitter chamber, any temperature variations in performance can be greatly reduced or fully eliminated. By maintaining the ETO transmitter at a constant temperature, the threshold, I th , and slope efficiency (mW/mA, SE), tracking error (BFM vs Pout) of the ETO transmitter (i.e. laser) will remain constant over the external operating temperature range of the system. The ETO transmitter chamber, or transmitter chamber, temperature set point can be adjusted as needed to minimize the power requirement of the temperature controlling device at normal external operating temperature conditions and to fine-tune laser parameters. 
     Apart from changes in the external temperature, the internal temperature of the ETO transmitter can change due to self-heating. This will cause a drift in laser parameters and likely results in erroneous measurement results especially directly after power up. Such a laser parameter drift is also avoided with the described invention. 
     Since the gain of this type of analog electrical-to-optical transmission system depends on the slope efficiency of the laser, an additional benefit of controlling the transmitter chamber temperature is that controlling the temperature of the laser enables the ability to change the slope efficiency and therefore to control the electrical-to-optical gain of the system. This allows the system gain to be adjusted as needed, for instance to compensate for gain changes in other parts of the system. 
     A particularly suitable way to maintain a constant temperature is to enclose the complete system including the ETO transmitter and the temperature sensor, or just the ETO transmitter in the thermally isolated transmitter chamber. 
     The ETO transmitter may be an element converting an electrical signal into an optical output signal. The ETO transmitter may for example comprise a laser. 
     The transmitter chamber, the ETO transmitter chamber, may comprise a material having a high thermal conductivity. Optionally, the transmitter chamber may comprise a material having a high thermal conductivity and a high heat capacity. The transmitter chamber may further comprise another material having a high heat capacity. The transmitter chamber may be a tube made of metal, for example brass, aluminum, copper, steel, or metal alloys. The transmitter chamber may also serve as the thermal interface to the temperature controlling device. 
     The temperature sensor may be used to measure the internal controlled temperature or the temperature of the ETO transmitter package. The temperature sensor may comprise a passive temperature sensor that requires an external power source, for example a thermistor. Alternatively, or additionally, the temperature sensor may comprise an active temperature sensor that does not require an external power source, for example a thermocouple, in particular a type K-thermocouple comprising nickel-chromium positive electrodes as temperature-sensing components. 
     A temperature controlling device may be, or may comprise, an element having the ability to heat, cool, or both heat and cool, depending on the temperature needs of the ETO transmitter and the overall system. This element may for example be a Peltier device (Thermo-Electric Cooler, TEC). 
     A thermally and electrically insulating material may be a material of low thermal conductivity and may be used as a cladding of the transmitter chamber to achieve maximum heat resistance to the outside environment. It may also insulate the transmitter chamber electrically. It may surround the transmitter chamber as best as the design allows. Providing such a thermal cladding of insulating material has the advantage that transient temperature gradients can be flattened, resulting in a more uniform temperature distribution. 
     A temperature control system may be an element forming the closed loop thermal feedback control system by monitoring the temperature sensor and controlling the temperature controlling device to maintain a constant internal controlled temperature. The control loop for the laser may be a temperature-based feedback loop. 
     The temperature control system may be configured such that external temperature fluctuations are compensated by controlling the transmitter chamber to a target temperature, the set point. In an ideal case, the temperature of the transmitter chamber is equal to the set point and has a uniform temperature distribution across the entire surface of the transmitter chamber. Consequently, the entire environment of the laser, at least the ETO transmitter arranged inside the transmitter chamber, also has a uniform temperature distribution. 
     The set point may be an external property, set by design, set by a technician, or set by a separate control system, or control loop. The temperature control system may further be configured such that the set point is adjustable, for example gradually, dynamically, or incrementally. For example, the temperature control system can be configured to adjust the set point by increments of  1  degree. With such a temperature control system, a constant slope efficiency, constant backtracking, and a controlled gain can be achieved in the ETO transmitter. 
     By changing the set point, parameters of the laser can be fine-tuned. As an example, by increasing the set point in the temperature control system, a degraded slope efficiency of an older laser can be compensated. 
     An internal controlled temperature may be the controlled temperature inside the ETO transmitter chamber to maintain the ETO transmitter at a constant temperature. 
     An external environment temperature may be the outside temperature of the system that can fluctuate over time and that the system has no control over. 
     An ETO transmitter electrical connection, herein also referred to as electrical input connection, may comprise one or more electrical connections. These connections may include signal, bias, and monitoring connections. These connections may protrude past the insulating material into a zone subject to the external environment temperature. This end, these connections may have a small design and have a high thermal resistance to minimize the thermal path from the external environment temperature zone into the internal controlled temperature zone. 
     The optical output connection may be the optical output from the ETO transmitter element. 
     In the following sections, detailed descriptions of examples and methods of the disclosure will be given. The description of both preferred and alternative examples are exemplary only, and it is understood that to those skilled in the art that variations, modifications, and alterations may be apparent. It is therefore to be understood that the examples do not limit the broadness of the aspects of the underlying disclosure as defined by the claims. 
     DETAILED DESCRIPTIONS OF THE DRAWINGS 
     The adapter, also called probe head, is an active, compensated, single ended probe head suitable for high frequency electrical signal measurements and suitable for converting a received electrical signal to an analog signal. An electrical signal fed into a temperature-independent optical link embedded or housed in a probe head. The received electrical signal may for example be a high frequency electrical signal, for example having a frequency of 100 MHz or higher. High frequencies are greater than or equal to 100 MHz. It comprises the temperature-independent optical link according to the present disclosure and provides a constant and clean optical output signal in a test and measurement application without being subjected to bandwidth losses due to error signal modulation. Error signal modulation is the process of varying one or more properties of the error signal, which inherently leads to a higher bandwidth consumption. The higher the bandwidth needed for the error signal modulation, the lower the bandwidth available for the optical output signal. With the optical link according to the present disclosure, error signals can be drastically reduced by eliminating the error signal source, namely the temperature changes. Without an error signal, error signal modulation is moot, and the entire bandwidth is available for optical output signal transmission. 
     The conversion of a received electrical signal to an analog signal is agnostic of outside or inside temperature fluctuations. Hence, the conversion occurs at a constant temperature, which is the set point temperature. Accordingly, a user can utilize the exemplary probe head for various measurement applications under varying temperatures and at different measurement campaign intensities without observing temperature-change inflicted bandwidth losses. 
     Referring now to  FIG.  1   , an exemplary embodiment of a temperature-independent optical link is shown in a cross-section through the optical link along a side view. 
     Referring now to  FIG.  1   , a schematic of an exemplary optical link according to an embodiment of the present disclosure is illustrated. The optical link  10  comprises several ETO transmitter electrical connections, herein also referred to as electrical input connections  18 , an optical output  50 , or optical output connection, and an ETO transmitter  20 . The optical link  10  further comprises an ETO transmitter chamber  25 , or transmitter chamber. The transmitter chamber comprises a temperature control device  24  which is configured to keep the ETO transmitter  20  at a constant predetermined temperature. The transmitter chamber  25  may be a cylindrical tube which fully incorporates, or houses, the ETO transmitter  20  on its inside. 
     The transmitter chamber  25  may comprise a material having a high thermal conductivity. Examples of preferred conductivities include conductivity higher than 100 W/m-K, but preferably equal to or higher than the thermal conductivity of brass (111 W/m-K), more preferably aluminum (205 W/m-K), or even more preferably copper (385 W/m-K). The material may also have a high heat capacity, having a specific heat capacity equal to or greater than brass (0.92 kJ/kg*K). Alternatively, or additionally, the transmitter chamber  25  may comprise another material having a high heat capacity. The transmitter chamber  25  may be a cylindrical tube made of brass. The purpose of the transmitter chamber  25  is to maintain all its inside components at a uniform temperature, which is achieved by its high heat capacity and high thermal conductivity. The transmitter chamber  25  may be configured such that it has a uniform temperature over its entire length, its entire inner area, and all components inside the transmitter chamber  25 . This keeps any heat-expansion-driven mechanical stress from the electrical-to-optical link  20  and allows uniform operation conditions for the ETO transmitter  20 . 
     The dimensions and the material to be used depend on the situation and the kind of ETO transmitter  20  used and can be identified by simple, straightforward experiments. 
     The optical link  10  may further comprise a temperature control system  40  which may be connected to the temperature control device  24 . The temperature control system  40  may comprise a feedback-loop temperature control. The temperature control system  40  may be an analog or digital control system. 
     The space between the ETO transmitter  20  and the transmitter chamber  25  is the internal temperature-controlled region  28 . This internal temperature-controlled region  28  may have thermally insulating properties to minimize heat fluxes from or to the ETO transmitter  20  across the internal temperature controlled region  28 . The internal temperature controlled region  28  may comprise air, a gas composition other than air, or even vacuum. According to an alternative embodiment, the internal temperature controlled region  28  may comprise a solid or a liquid. 
     The temperature control device  24  may have the ability to heat, cool, or both heat and cool the ETO transmitter  20 , or to neither cool nor heat the ETO transmitter  20 , depending on the control commands provided by the temperature control system  40 . The temperature control device  24  may for example comprise, or be a part of a Peltier device, a Thermo-Electric Cooler, TEC. The temperature control device  24  may further comprise a temperature sensor  26 . 
     The temperature sensor  26  may be a thermistor, or a thermocouple, suitable for measuring the internal controlled temperature or the temperature of the ETO transmitter  20 . The temperature sensor  26  may be thermally connected to the ETO transmitter  20  in such a way that a surface temperature thereof can be measured. The temperature sensor  26  may also be arranged inside the transmitter chamber  25 . 
     The ETO transmitter  20  may comprise a fiber coupled laser package, comprising a laser diode section  21  and a laser alignment section  23  (see  FIG.  2   ). 
     The optical link  10  may comprise thermally and electrically insulating material  22  around the ETO transmitter  20 . The thermally and electrically insulating material  22  may be provided as a cladding of the ETO transmitter  20 . The purpose of this insulating material is to achieve a maximum of thermal resistance. The thermally and electrically insulating material may comprise expanded polystyrene, polyurethane (PU) foam, polyisocyanurate (PIR) foam, or phenolic spray foam. The temperature zone outside the transmitter chamber  25  is hereinafter referred to as the external environment temperature zone. 
     The electrical input connection  18  may comprise several individual ETO transmitter electrical connections. These electrical input connections  18  may include signal, bias, and monitoring connections. These electrical input connection  18  may extend out of and past the thermally end electrically insulating material  22  into an external environment temperature zone. In order to minimize the thermal path created by the electrical input connection  18  extending through the thermally and electrically insulating material, the electrical input connections  18  may be designed small and with high thermal resistance. 
     The optical output connection, or optical output  50 , as shown in the embodiment  FIG.  1    may comprise a fiber optic cable, for example a single-mode fiber. The fiber optic cable may or may not be a cladded fiber-optic cable. 
     As can be seen from the illustration provided in  FIG.  1   , the ETO transmitter  20  comprises all components needed to convert electrical input signals into optical output signals. More specifically, only the electrical input connections  18  and the optical output connection  50  reach through the transmitter chamber  25  and the thermally and electrically insulating material  22 . 
     Turning now to  FIG.  2   , an exemplary ETO transmitter  20  is shown in a cross-sectional view. The ETO transmitter  20  may comprise a fiber coupled laser package as it is known from the state-of-the-art. Such an ETO transmitter  20  may be compatible with the optical link  10  according to the present disclosure. On the left-hand side, the ETO transmitter comprises electrical input connections  18 . On the right-hand side, the ETO transmitter  20  comprises an optical output connection  52 . The ETO transmitter  20  may comprise a fiber coupled laser package  27 . From left to right, the ETO transmitter  20  may comprise a header  29 , which connects the electrical input connections  18  to a laser diode  34 , the fiber coupled laser package  27  comprising a lens  37  receiving a laser beam  36  from the laser diode  34  and focusing the laser beam  3  onto a glass capillary  39 , as well as a bend limiter  54  and a fiber optic cable as optical output connection  52 . 
     The fiber coupled laser package  27  may comprise the laser diode section  21  and the laser alignment section  23 . The laser diode section  21  and the laser alignment section  23  may be considered the essential parts of the ETO transmitter  20  which are to be placed inside the transmitter chamber  25  shown in  FIG.  1   . In other words, the laser diode section  21  and the laser alignment section  23  may be prone to cause fluctuations in the analog optical output signal when subjected to thermally induced expansion or contraction. Therefore, at least those components are to be placed inside of the transmitter chamber  25 , which provides a constant-temperature environment. 
     Depending on their configuration, the header  29 , the bend limiter  54  and the parts of the fiber optic cable  52  may also be considered parts prone to cause fluctuations in the analog optical output signal when subjected to thermally induced expansion or contraction. In a preferred embodiment, the header  29 , the fiber optic cable  52  or the bend limiter  54  may also be part of the fiber coupled laser package  27 . 
     The laser diode section  21  may comprise a back facet monitor internal photodiode in the laser. The laser beam  36  generated by the laser diode may be detected by the photodiode and is subsequently guided to a lens provided in a lensed capsule. Exiting the lens  37 , the laser beam  36  is focused on a proximal end of a fiber optic cable provided in a glass capillary  39  which is held in place by the laser alignment section  23  provided in the form of a sleeve. 
     A change in temperature both from the outside or from the inside of the fiber coupled laser package  29  will lead to thermal expansion or contraction of the laser diode section  21  as well as the laser alignment section  23 . 
     In general, any temperature change inflicted to the ETO transmitter  20  changes at least the distances between the individual components on the inside of the ETO transmitter  20  relative to each other. Consequently, the optics of the transmitter change, causing the output power of the transmitter to fluctuate accordingly. This is avoided by placing the ETO transmitter inside the transmitter chamber  25  which is configured to keep the ETO transmitter  20  at a constant predetermined temperature. Keeping the ETO transmitter  20  at a constant predetermined temperature has the benefit that thermal expansion is not observed in the ETO transmitter  20  regardless of the fluctuations of the internal or external temperature. 
     If for example the ETO transmitter  20  comprises a fiber optic laser package  27 , at least of the laser diode section  21  and the laser alignment section  23  are such optical components which need to be placed within the transmitter chamber  25  to avoid analog optical output power fluctuations due to thermally induced expansion or contraction. 
     Referring now to  FIG.  3   , an exemplary embodiment of a probe head is shown in a cross-sectional view seen from the side. A probe head  100  may comprise a probe head tip  12  for electrical signal detection. The probe head tip  12  may be mounted to a housing  14  of the probe head. The housing  14  of the probe head  100  may comprise aluminum. The housing  14  may be a nickel-plated aluminum housing. The aluminum housing serves as heat sink to buffer transient temperature gradients between inside and outside of the housing. The housing  14  may further comprise a plastic covering for aesthetic purposes, but also for improved haptics and ergonomics, thermal, or electrical insulation of the housing  14 . 
     On the inside of the housing  14 , the probe head  100  may comprise an input buffer  16 , for example a printed circuit board, PCB or PCBA, electrically connected to the probe tip  12 . The probe head  10  may further comprise the optical link  10  shown in  FIG.  1   . Hence, the probe head  10  may comprise an ETO transmitter  20  which may be electrically connected to the input buffer  16  via an electrical input connection  18  on a proximal end of the ETO transmitter  20 . The electrical-to-optical-transmitter  20  may be arranged on the inside of an electrical-to-optical-transmitter chamber  25 . 
     The probe head  100  may further comprise a temperature control device  24  which is configured to keep the ETO transmitter at a constant predetermined temperature. The temperature control device  24  may be mounted to the inside of the housing  14  and may act as a support for the transmitter housing  25 . 
     The probe head  100  may further comprise a battery or power over fiber power converter  30  which may be placed in a dedicated compartment of the housing  14 . The probe head  10  may comprise a temperature control system  40  for the temperature control device  24 . The temperature control system  40  may comprise components for supplying power for controlling the temperature control device  24 . 
     The optical output  50  may be connected to a distal end of the ETO transmitter  20 . The probe head  100  may further comprise an optical output connection  52  which may be connected to the outside of the housing  14  via a bend limiter  54 . The optical output  50  may be guided on the inside of the optical output connection. 
     Turning now to  FIG.  4   , another exemplary embodiment of a probe head is shown in a cross-sectional view seen from the side. The embodiment shown in  FIG.  4    differs from the embodiment shown in  FIG.  3    in that the probe head  100  is an angled probe head. The housing  14  of the probe head  100  is configured such that the probe tip  12  and the optical output connection  52  are angled downwards. With an angled probe head  100 , handing and measurements may be improved further due to reduced space requirements and the ability to use shorter probe tip cables at reduced bending. In the illustration of  FIG.  4   , the screw-on cap  32  is illustrated detached from the housing  14 . 
     Turning now to  FIG.  5   , an exemplary probe head is shown in a perspective view. The illustrated probe head  100  is based on the probe head schematically shown in  FIG.  4   . The probe head  100  is an angled probe head and is shown in an assembled state. The housing  14  may comprise two complementary half-shells  14 A,  14 B which may be held together by fastener  17 . A fastener may be a screw, a nut and bolt, a pin, a nail, a male-female connector, or a snap. The screw-on cap  32  is shown in a screwed-on configuration. 
     Turning now to  FIG.  6   , the probe head  10  of  FIG.  5    is shown in a cross-sectional view seen from the side. The probe head  100  is based on the probe heads shown in  FIGS.  3 - 5    and the optical link  10  disclosed in  FIGS.  1 - 2   . The same principles, definitions, and explanations provided in the context of  FIGS.  1 - 5    also apply to the embodiment of  FIG.  6    where applicable. 
       FIG.  6    shows how the ETO transmitter  20  of the optical link  10  may be fully incorporated into the transmitter chamber  25 . The transmitter chamber  25  may be fully surrounded by the thermally and electrically insulating material  22 . The temperature control device is not shown in  FIG.  6    as it extends orthogonally to the depicted cross-section. 
     The transmitter chamber  25  may be thermally decoupled from the adjacent input buffer  16 , for example by a gap, void, or empty space between transmitter chamber  25  and input buffer  16 . 
     The probe head  100  may comprise a thermal damping device  33 . The thermal damping device  33  may be a part of the input buffer  33  protruding towards the electrical input connection  18 . The optical link  10  as disclosed in  FIG.  1    is also shown in the cross-section. The ETO transmitter  20  of the optical link  10  may be electrically connected to the input buffer via the thermal damping device  33 . The thermal damping device has low heat conducting properties to increase thermal isolation between the ETO transmitter  20  and the input buffer  16 . For example, the thermal damping device  33  may comprise holes  35 . As a result, heat conduction from the ETO transmitter  20  to the input buffer  16  is reduced. The thermal damping device  33  may further comprise or consist of a material having low thermal conductivity. 
     Turning now to  FIG.  7   , the probe head of  FIG.  6    is shown in a cross-section seen from above. In this cross-section view, the temperature control device  24  may be seen. The optical link  10  as disclosed in  FIG.  1    is also shown in the cross-section. The temperature control device  24  may extend from an inner surface of the housing  14  through the thermally and electrically insulating material  22  to the transmitter chamber  25 . The thermally and electrically insulating material  22  covers the transmitter chamber  25  almost entirely and is interrupted only by the temperature control device  24  to the side, by the optical output  50  to the rear, and by the input buffer  16  to the front. It may also be seen from  FIG.  7    that the temperature sensor  26  may be arranged on the inside of the transmitter chamber  25 . The wires of the temperature sensor  26  may be wrapped around the temperature control device  24  and led to the temperature control system  50 . 
     Turning now to  FIG.  8   , the probe head of  FIGS.  6  and  7    is now shown in a cross-section seen from behind. The optical link  10  as disclosed in  FIG.  1    is also shown in the cross-section. The temperature control device  24  may extend from an inner surface of the housing  14  through the thermally and electrically insulating material  22  to the transmitter chamber  25 . 
     Turning now to  FIG.  9   , a flow chart of an exemplary method for converting a received electrical signal to an analog signal with the temperature-independent optical link according to the present disclosure is shown. The method may be understood as a method for controlling the output power of an ETO transmitter. Alternatively, or additionally, the method may be suitable for converting a received electrical signal to an analog signal with probe head according to the present disclosure. The method may be suitable for controlling the output power of an ETO transmitter using a probe head according to the present disclosure. 
     The method may comprise the steps of assembling S 10  an ETO transmitter and a temperature sensor in the transmitter chamber, and connecting the temperature control device to the ETO transmitter and to the temperature sensor, applying S 20  an electrical signal to the output input connection, converting S 30  electrical signals into optical output signals, maintaining S 40  a constant temperature in the transmitter chamber, and observing S 50  a constant optical power output for a given electrical input signal. 
     A given electrical input signal may be a constant electrical signal, for example a high frequency signal at a constant frequency, time period, and amplitude. What is meant is that the output optical power remains constant for the same electrical input signal, even if a temperature change outside the ETO transmitter occurs. 
     Turning now to  FIG.  10   , an excerpt of another exemplary embodiment of a flow chart for controlling the ETO transmitter using the optical link is illustrated. The shown excerpt illustrates sub steps of step S 40  shown in  FIG.  9   . Hence, the step of maintaining S 40  a constant temperature may comprise the sub steps of retrieving S 42  a current temperature, retrieving S 44  a temperature deviation by comparing the retrieved current temperature against a set point temperature, and generating S 46  a temperature control output to compensate the temperature deviation. A closed-loop control may be established whereby no external inputs are needed to control the temperature to maintain a constant temperature in the transmitter chamber. The method steps may be repeated continuously, and additional signal filter steps may be implemented. The method may further comprise the steps of generating a laser beam and shooting the laser beam into an end of a fiber optic cable. The method may further comprise a step of electrical or optical signal modulation. 
     In preferred methods, the temperature sensor device provides the current temperature. The set point temperature is set by design, by a technician, or by an algorithm. Temperature deviation is determined by calculating the difference between the retrieved current temperature and the set point temperature. This may be achieved using the temperature control system, which may also generate the temperature control output to compensate the temperature deviation. The temperature control output is fed to the temperature control device. 
     Conclusion 
     A number of embodiments of the present disclosure have been described. While this specification contains many specific implementation details, these details should not be construed as limitations on the scope of any disclosures or of what may be claimed. 
     Certain features that are described in this specification in the context of separate embodiments can also be implemented in combination in a single embodiment. Conversely, various features that are described in the context of a single embodiment can also be implemented in combination in multiple embodiments separately or in any suitable sub-combination. Moreover, although features may be described above as acting in certain combinations and even initially claimed as such, one or more features from a claimed combination can in some cases be excised from the combination, and the claimed combination may be directed to a sub-combination or variation of a sub-combination. 
     Similarly, while operations are depicted in the drawings in a particular order, this should not be understood as requiring that such operations be performed in the particular order shown or in sequential order, or that all illustrated operations be performed, to achieve desirable results. In certain circumstances, multitasking and parallel processing may be advantageous. 
     Thus, particular embodiments of the subject matter have been described. Other embodiments are within the scope of the following claims. In some cases, the actions recited in the claims can be performed in a different order and still achieve desirable results. It will be understood that various modifications may be made without departing from the spirit and scope of the claimed disclosure.