Abstract:
A servo system includes multiple servo channels being driven by a common error signal. Each channel has a controller that receives an error signal and provides a drive signal to a driver. The servo channels are arranged serially, with a drive signal from one controller forming the error signal for a downstream controller. As a result, the downstream controller does not attempt to correct for phase error directly, but instead attempts to keep the upstream driver at or near its operational midpoint. The servo channels can be arranged in order of decreasing controller bandwidth, from fastest to slowest. In contrast with a parallel configuration, in which servo channels all simultaneously receive a common error signal, the serial configuration can allow each controller to use its full bandwidth, can eliminate crosstalk between servo channels, and can prevent saturation of upstream drive signals.

Description:
TECHNICAL FIELD 
       [0001]    Embodiments pertain generally to servo systems. Some embodiments pertain to configuring a servo system that controls a physical quantity by two or more mechanisms that have different bandwidths, speeds, and/or dynamic ranges. 
       BACKGROUND 
       [0002]    In a pulsed laser, light inside the laser circulates along an optical path. In many cases, a repetition rate or repetition frequency of the pulsed laser depends on the optical path length of the optical path. If the optical path length decreases or increases away from a specified value, the repetition rate of the pulsed laser can increase or decrease away from a specified value, which is undesirable. 
         [0003]    Accordingly, there exists a need for improving the systems and methods for controlling the optical path length and/or the repetition frequency. 
       SUMMARY 
       [0004]    There are systems in which a servo system monitors and controls a particular physical quantity. For instance, a mode-locked laser can use a servo system to monitor and control an optical path length within the laser, which can stabilize the laser repetition frequency. 
         [0005]    In some of these systems, two or more mechanisms can simultaneously control the physical quantity, where the mechanisms can have different bandwidths, speeds, and/or dynamic ranges. For instance, in the mode-locked laser, both an electro-optic modulator and a piezo-electric transducer can simultaneously vary the optical path length. The electro-optic modulator has a relatively large bandwidth, and can therefore vary the optical path length very quickly, but has a limited dynamic range. The piezo-electric transducer has a relatively small bandwidth, and therefore cannot vary the optical path length as quickly as the electro-optic modulator, but has a larger dynamic range than the electro-optic modulator. Using both an electro-optic modulator and a piezo-electric transducer, simultaneously, to control the optical path length can, in principle, provide servo control with a relatively fast speed and with a relatively large dynamic range. 
         [0006]    In practice, it is difficult to use multiple mechanisms to control the same physical quantity. For the example of the mode-locked laser, if both the electro-optic modulator and the piezo-electric transducer are controlled by the same phase error signal, both make uncoordinated corrections to the optical path length in parallel, and can conflict with each other. To avoid such conflicts, the electro-optic modulator and the piezo-electric transducer can be detuned with respect to each other, which can be inefficient. 
         [0007]    A serial servo system includes multiple servo channels being driven by a common error signal. Each channel has a controller that receives an error signal and provides a drive signal to a driver. The servo channels are arranged serially, with a drive signal from one controller forming the error signal for a downstream controller. As a result, the downstream controller does not attempt to correct for phase error directly, but instead attempts to keep the upstream driver at or near its operational midpoint. The servo channels can be arranged in order of decreasing controller bandwidth, from fastest to slowest. 
         [0008]    In contrast with a parallel configuration, in which servo channels all simultaneously receive a common error signal, the serial configuration can allow each controller to use its full bandwidth, can eliminate crosstalk between servo channels, and can prevent saturation of upstream drive signals. In some examples, the serial servo system can be used to control an optical path length in a mode-locked pulsed laser, which can stabilize the laser repetition frequency. 
         [0009]    This summary is intended to provide an overview of subject matter of the present patent application. It is not intended to provide an exclusive or exhaustive explanation of the invention. The Detailed Description is included to provide further information about the present patent application. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    In the drawings, which are not necessarily drawn to scale, like numerals may describe similar components in different views. Like numerals having different letter suffixes may represent different instances of similar components. The drawings illustrate generally, by way of example, but not by way of limitation, various embodiments discussed in the present document. 
           [0011]      FIG. 1  is a schematic drawing of an example of a serial servo system for monitoring and controlling a physical property, in accordance with some embodiments. 
           [0012]      FIG. 2  is a schematic drawing of an example of a serial servo system for monitoring and controlling an optical path length, in accordance with some embodiments. 
           [0013]      FIG. 3  is a schematic drawing of an example of a stabilized pulsed laser, in which an electro-optic modulator, a piezo-electric transducer, and a heater control an optical path length, in accordance with some embodiments. 
           [0014]      FIG. 4  is a flow chart of an example of a method of operation of a serial servo system, in accordance with some embodiments. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]      FIG. 1  is a schematic drawing of an example of a serial servo system  100  for monitoring and controlling a physical property  102 . 
         [0016]    A error signal  104  monitors the physical property  102 , and is configured to vary in response to variation of the physical property  102 . In most cases, the error signal  104  passes through zero when the physical property  102  has a specified value. The error signal  104  is negative when the physical property  102  is on one side of the specified value, and is positive when the physical property  102  is on an opposite side of the specified value. In some examples, the error signal  104  is monotonic, with respect to the physical property  102 . In other examples, the error signal  104  can be periodic, with respect to the physical property  102 , and the servo system  100  can operate on one period of the periodic signal. 
         [0017]    A first controller  106  is configured to receive the error signal  104  and generate a first drive signal  108  in response to the error signal  104 . A first driver  110  is configured to receive the first drive signal  108  and adjust the physical property  102  in response to the first drive signal  108 . The first driver  110  drives a first mechanism  112  to adjust the physical property  102 . The first driver  110  and first mechanism  112  have a first bandwidth, which can be related to the speed at which the first driver  110  and first mechanism  112  can respond. The greater the bandwidth, the faster the driver can control the physical property  102 . 
         [0018]    A second controller  114  is configured to receive the first drive signal  108  and generate a second drive signal  116  in response to the first drive signal  108 . A second driver  118  is configured to receive the second drive signal  116  and adjust the physical property  102  in response to the second drive signal  116 . The second driver  118  drives a second mechanism  120  to adjust the physical property  102 . The second driver  118  and second mechanism  120  have a second bandwidth less than the first bandwidth, so that the second mechanism  120  responds more slowly than the first mechanism  112 . 
         [0019]    An optional third controller  122  is configured to receive the second drive signal  116  and generate a third drive signal  124  in response to the second drive signal  116 . An optional third driver  126  is configured to receive the third drive signal  124  and adjust the physical property  102  in response to the third drive signal  124 . The third driver  126  has a third bandwidth less than the second bandwidth. The third driver  126  drives a third mechanism  128  to adjust the physical property  102 , so that the third mechanism  128  responds more slowly than the second mechanism  120 . The third controller  122  can also generate an optional additional error signal (not shown in  FIG. 1 ), which can drive a downstream controller, or can be used to measure a fidelity of the serial servo system  100 . 
         [0020]    In the example of  FIG. 1 , the physical property  102 , the error signal  104 , the first mechanism  112 , the second mechanism  120 , and the third mechanism  128  are not part of the serial servo system  100 . In other examples, any or all of these elements can be part of the serial servo system  100 . 
         [0021]      FIG. 2  provides a tangible example for the schematic framework of  FIG. 1 .  FIG. 2  is a schematic drawing of an example of a serial servo system  200  for stabilizing a pulsed laser by monitoring and controlling an optical path length  202  within the laser, and consequently stabilizing a repetition frequency of the pulsed laser. The laser can be mode-locked. 
         [0022]    A phase error signal  204  monitors the optical path length  202 , and is configured to vary in response to variation of the optical path length  202 . In some examples, the phase error signal  204  crosses zero when a repetition frequency of the comb is exactly equal to a reference RF synthesizer. The phase error signal  204  can be positive (or negative) when the repetition frequency has a higher frequency than the reference synthesizer, and can be negative (or positive) when the repetition frequency has a lower frequency than the reference synthesizer. The absolute sign of the phase error depends on the sign of the gain and the implementation of the phase error signal (switching inputs on phase detector introduces a minus sign). 
         [0023]    An electro-optic modulator (EOM) controller  206  is configured to receive the phase error signal  204  and generate an EOM drive signal  208  in response to the phase error signal  204 . An EOM driver  210  is configured to receive the EOM drive signal  208  and adjust the optical path length  202  in response to the EOM drive signal  208 . The EOM driver  210  drives an EOM  212  to adjust the optical path length  202 . The EOM driver  210  and EOM  212  have an EOM bandwidth, which can be on the order of 10 5  Hz. The EOM  212  can adjust the optical path length  202  relatively quickly, but has a relatively small dynamic range, or stroke. 
         [0024]    A piezo-electric transducer (PZT) controller  214  is configured to receive the EOM drive signal  208  and generate a PZT drive signal  216  in response to the EOM drive signal  208 . A PZT driver  218  is configured to receive the PZT drive signal  216  and adjust the optical path length  202  in response to the PZT drive signal  216 . The PZT driver  218  drives a PZT  220  to adjust the optical path length  202 . The PZT driver  218  and PZT  220  have a PZT bandwidth, which can be on the order of 10 1  Hz. The PZT bandwidth is less than the EOM bandwidth. The PZT  220  responds more slowly than the EOM  212  and has a greater stroke than the EOM  212 . 
         [0025]    An optional temperature controller  222  is configured to receive the PZT drive signal  216  and generate a temperature drive signal  224  in response to PZT drive signal  216 . An optional temperature driver  226  is configured to receive the temperature drive signal  224  and adjust the optical path length  202  in response to the temperature drive signal  224 . The temperature driver  226  drives a heater  228  to adjust the optical path length  202 . The temperature driver  226  and heater  228  have a heater bandwidth that is less than the PZT bandwidth. The heater  228  responds more slowly than the PZT  220  and has a greater stroke than the PZT  220 . 
         [0026]    The EOM controller  206 , PZT controller  214 , and temperature controller  222  are configured serially, so that a drive signal from one controller is used as an error signal to drive a downstream controller. For instance, the PZT controller  214  does not respond to the phase error signal  204 , but instead responds to the EOM drive signal  208 . Responding to the EOM drive signal  208  can help ensure that the EOM  212  operates at or near an operational midpoint of its stroke. In some examples, it can be advantageous for the EOM  212  to operate at or near the operational midpoint of its stroke. Such operation can prevent or reduce saturation of the EOM drive signal  208 . Addition of a temperature servo loop can help keep the PZT  220  at or near an operational midpoint of its stroke as well, which can also be advantageous. In some examples, it may be desirable to keep the EOM  212  at or near a specified setpoint that is offset from an operational midpoint of its stroke. 
         [0027]      FIG. 3  is a schematic drawing of an example of a stabilized pulsed laser  330  that includes an example of a serial servo system  300 . Elements numbered between  300  and  328  are similar in structure and function to corresponding elements  200  through  228  in  FIG. 2 . 
         [0028]    The stabilized pulsed laser  330  in  FIG. 3  is used to form a stable microwave signal. The laser  330  is passively mode-locked, with a frequency spectrum that resembles a comb, with peaks about every 100 MHz. In the time domain, the output of the laser  330  also resembles a comb, with pulses about every 10 nsec. The pulses are split off from a closed optical path and are directed onto a photodetector to form the microwave signal. 
         [0029]    The closed optical path in the pulsed laser  330  extends counterclockwise in  FIG. 3 , from a coupler  332 , to a wave plate  334 , to a polarizing beam splitter  336 , to an electro-optic modulator  312 , to another wave plate  338 , to another coupler  340 , through a length of single mode fiber  342  that includes a portion of erbium-doped fiber  344  to provide gain, through a wavelength division multiplexer and isolator  346  that couples in a 980 nm pump beam, and back to the coupler  332 . 
         [0030]    The polarizing beam splitter  336  splits off a portion of the beam from the optical path, and directs the beam to a coupler  348  and into a fiber, through an isolator  350 , to a photodetector  352 . The photodetector  352  converts the incident pulsed optical signal to a pulsed electrical signal. The pulsed electrical signal is directed to a digital phase detector  354  that produces a phase error signal  304 , and to an arbitrary waveform generator  356  that can function as an RF synthesizer. 
         [0031]    The phase error signal  304  is directed as input to the serial servo system  300 . The serial servo system  300  controls the optical path length of the closed optical path in three different locations—at the EOM  312 , the PZT  320 , and the heater  328 . All three of the EOM  312 , the PZT  320 , and the heater  328  can lengthen or contract the optical path length, and all three operate simultaneously, as described above. 
         [0032]      FIG. 4  is a flow chart of an example of a method  400  for monitoring and controlling a physical property. The physical property can be an optical path length  202 , or another suitable physical property. The method  400  can be used with the serial servo systems  100 ,  200 ,  300 , as described above, or with other suitable servo systems. 
         [0033]    At  402 , method  400  receives an error signal configured to vary in response to variation of the physical property. The error signal can be a phase error signal  204 ,  304 , or can be another suitable error signal. At  404 , method  400  produces a first drive signal in response to the error signal. The first drive signal can be an EOM drive signal  208 , or can be another suitable drive signal. At  406 , method  400  adjusts the physical property with a first mechanism in response to the first drive signal. The first mechanism can be an EOM  212 ,  312 , or can be another suitable mechanism. At  408 , method produces a second drive signal in response to the first drive signal. The second drive signal can be a PZT drive signal  216 , or can be another suitable drive signal. At  410 , method  400  adjusts the physical property with a second mechanism, different from the first mechanism, in response to the second drive signal. The second mechanism can be a PZT  220 , or can be another suitable mechanism. 
         [0034]    The servo system can include a computer system that includes hardware, firmware and software. Examples may also be implemented as instructions stored on a computer-readable storage device, which may be read and executed by at least one processor to perform the operations described herein. A computer-readable storage device may include any non-transitory mechanism for storing information in a form readable by a machine (e.g., a computer). For example, a computer-readable storage device may include read-only memory (ROM), random-access memory (RAM), magnetic disk storage media, optical storage media, flash-memory devices, and other storage devices and media. In some examples, computer systems can include one or more processors, optionally connected to a network, and may be configured with instructions stored on a computer-readable storage device.