Classifications

• • H—ELECTRICITY
  • H03—ELECTRONIC CIRCUITRY
  • H03G—CONTROL OF AMPLIFICATION
  • H03G3/00—Gain control in amplifiers or frequency changers
  • H03G3/20—Automatic control
  • H03G3/30—Automatic control in amplifiers having semiconductor devices
  • H03G3/3084—Automatic control in amplifiers having semiconductor devices in receivers or transmitters for electromagnetic waves other than radiowaves, e.g. lightwaves
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04B—TRANSMISSION
  • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
  • H04B10/29—Repeaters
  • H04B10/291—Repeaters in which processing or amplification is carried out without conversion of the main signal from optical form
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04B—TRANSMISSION
  • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication

Definitions

• This invention relates to a method and apparatus for controlling a ratio of an output signal and an input signal of a device.
• the invention can be implemented using relatively simple control electronics and is suitable for a wide variety of applications including, for example, electronic control of electrical and optical amplifiers and attenuators.
• Conventional signal ratio control techniques typically involve monitoring an output signal or an input signal of a device relative to an external reference signal that is set in accordance with a predetermined target ratio.
• the device is controlled to operate at the target ratio based on an error signal representing a difference between the monitored signal and the reference signal.
• Another conventional technique involves monitoring both the output signal and the input signal of a device in order to determine a ratio of the two signals.
• the ratio thus determined is compared with an external reference signal, corresponding to the target ratio, in order to generate an error signal representing the difference between the determined ratio and the target ratio.
• the device is controlled to operate at the target ratio based on the error signal.
• Fig. 1 illustrates the conventional technique just described.
• the signal levels x and y may, for example, represent voltage, current, or power levels of the input and output signals.
• the ratio G of the output signal level to the input signal level may be represented as follows:
• the subtractor 14 subtracts the measured ratio y/x from the specified ratio Gsp to generate an error signal level E:
• the error E is supplied to a controller 16 which adjusts the device operation in order to bring the error E to zero.
• G becomes equal to GT as will be recognized from Equation (4).
• the present invention reflects a new and unique approach to the ratio control process that does not require an external reference signal or a division operation as in conventional techniques.
• the invention utilizes first and second signals having levels corresponding to the input signal level and the output signal level of the device, respectively.
• an error signal is generated for supply to a controller which, in turn, controls the device operation to bring the error signal level to zero.
• the ratio of the input and output signal levels of the device will match a predetermined target ratio.
• the first and second signals may be produced, respectively, using first and second amplifier units of known gains. When the error signal level is zero, the ratio of the input and output signal levels matches a predetermined target ratio based on the known gains of the first and second amplifier units.
• Fig. 2 is a diagram illustrating basic principles of implementation using first and second amplifier units as just described.
• an input-side amplifier unit 23 having a known gain Gj and an output-side amplifier unit 25 having a known gain G 0 are connected to the input and output, respectively, of a device 20 to be controlled (the connections having been omitted from the drawing for simplicity).
• the output-side amplifier unit 25 produces a signal having a level
• the present invention does not require an external reference signal representing the target input-output signal ratio and does not require a division to determine the actual operating ratio G. Instead, the control operation is based on a simple subtraction of signals having levels corresponding to the input and output signal levels. By avoiding the need for an external reference signal and a division operation, as are used in conventional techniques, the present invention offers the advantages of simplified control electronics and high control speeds at low cost.
• amplifier units to produce the signals to be subtracted may be preferred, as in the illustrative embodiments, due to advantages in accommodating devices having low input (and possibly also low output) signal levels, as well as devices that utilize non-electrical signals.
• transimpedance amplifier units may be employed in applications involving devices that utilize optical signals.
• the present invention enables ratio control of an input signal and an output signal of a device without the need for an external reference signal or a division operation.
• the invention provides a method of controlling a ratio of an output signal level from a device and an input signal level to the device, comprising: providing a first component connected to an input of the device to produce a first signal having a level corresponding to the input signal level; providing a second component connected to an output of the device to produce a second signal having a level corresponding to the output signal level; and adjusting the ratio based on a difference between the levels of the first and second signals.
• the device itself is an amplifier unit
• the first and second components are first and second amplifier units
• the controlled ratio is a gain of the amplifier unit.
• the device is a pumped fiber-optic amplifier unit, and the controlled ratio is an optical gain of the fiber-optic amplifier unit.
• the optical gain is adjusted by adjusting pump power of a pump laser of the fiber-optic amplifier unit.
• the first and second amplifier units are transimpedance amplifier units which are connected by corresponding photodetectors to the input and output of the fiber-optic amplifier unit. (A transimpedance amplifier provides an output voltage signal proportional to an input current signal.)
• Each of the input signal and the output signal may be a composite signal constituted by a plurality of signals.
• the signal levels of the aforementioned first and second signals may correspond to the RMS signal levels of the composite input and output signals, respectively, and the controlled ratio may be an RMS optical gain.
• the present invention provides apparatus for implementing the above method.
• Yet another aspect of the invention provides a method of controlling a ratio of an output signal level from a device and an input signal level to the device, comprising, producing a first signal having a level corresponding to the input signal level, producing a second signal having a level corresponding to the output signal level, and adjusting the ratio based on a difference between the levels of the first and second signals
• Fig. 1 is a diagram for explaining a conventional ratio control technique.
• Fig. 2 is a diagram for explaining the control technique according to the present invention.
• Fig. 3 is a diagram illustrating a first apparatus according to the invention.
• Fig. 4 is a diagram illustrating a second apparatus according to the invention.
• Fig. 5 is a diagram illustrating a third apparatus according to the invention.
• Fig. 6 is a diagram illustrating a fourth apparatus according to the invention.
• Fig. 7 is a diagram of the control electronics employed in a test apparatus constructed according to Fig. 4.
• Fig. 8 is a flow diagram of the control operation in the test apparatus.
• Fig. 9 is an oscillograph showing temporal dynamics of an optical amplifier unit of the test apparatus without the ratio control circuitry in operation.
• Figs. 10 and 11 are oscillographs showing temporal dynamics of the optical amplifier with the ratio control circuitry in operation.
• FIG. 3 is a diagram illustrating a first apparatus 1 according to the present invention.
• Reference number 30 in the figure represents an electrical device operable with a controllable ratio G of an input signal and an output signal.
• device 30 is an adjustable-gain amplifier unit including a single amplifier.
• device 30 may be any electrical device that has a controllable ratio of its input and output signals (e.g., a multistage amplifier having cascaded amplifiers, a single or multi-stage attenuator, etc.).
• the input and output signals may be voltage or current signals, for example.
• a second amplifier unit 35 of known gain G 0 is connected to the output side of the controlled amplifier unit 30.
• the connections of amplifier units 33 and 35 to the input and output sides of amplifier unit 30 may be direct, as shown, or they may be indirect (e.g., via a current sensor) depending on the requirements of a given implementation.
• the output signals of the amplifier units 33 and 35 are supplied to a controller 36, which operates, as discussed in more detail below, to adjust the operation of the controlled amplifier unit 30 so as to control the ratio G.
• G is of course a gain of the amplifier unit 30.
• S, G,V,
• G, and G 0 represent the respective voltage gains of the input-side and output-side amplifier units 33 and 35, and G represents the voltage gain of the controlled amplifier unit 30.
• the controller 36 may utilize any suitable control algorithm for controlling the ratio G based on the error S er r- Proportional-integral (PI) or proportional- integral-derivative (PID) control algorithms may be preferred for optimal performance. Both digital and analog controllers may be used. Suitable algorithms for specific applications may be determined by conventional techniques for example, empirically and/or by computer simulation. For a more complete discussion of PI, PID, and other control techniques, see Koenig, D., Control and Analysis of noisysy Processes, Prentice Hall, 1991 (incorporated herein by reference).
• Fig. 4 illustrates a second apparatus 2 according to the present invention for controlling the optical gain (optical power gain) of an optical amplifier unit 40.
• the optical amplifier unit 40 is a single- stage (single-coil) pumped fiber-optic amplifier. Such amplifiers are well known in the art and so will not be discussed in detail herein.
• the amplifier unit includes a fiber-optic coil 41 doped with ions of a rare-earth element (e.g., erbium or praseodymium) and a wavelength- division-multiplex (WDM) optical coupler 42 that couples an input optical signal with "pump" light from a controller laser source 44.
• a rare-earth element e.g., erbium or praseodymium
• WDM wavelength- division-multiplex
• the optical input signal may be composed of a single optical signal at a predetermined wavelength, or it may be a composite signal composed of a plurality of optical signals at different predetermined wavelengths, as is typical in WDM fiber-optic communication networks.
• Light of the input signal stimulates the excited ions in the fiber coil 41 to emit additional light of the same wavelength(s), effectively amplifying the input optical signal.
• the optical power gain G of the amplifier unit 40 depends on the output power of the pump laser unit 44, and therefore can be controlled by adjusting the output power of the pump laser unit.
• the optical amplifier unit 40 may incorporate a gain-flattening filter so that the individual wavelength components of the composite signal experience equal gain. Otherwise, the optical power gain will be the RMS gain for the collective wavelength components.
• the optical amplifier unit 40 may also include optical isolators (not shown) at its input and output sides.
• a first transimpedance amplifier (TIA) unit 43 preferably including a single amplification stage, has its input connected to an input side of the optical amplifier by way of a photodetector 47 (e.g., photodiode) and an optical tap 48.
• the optical tap functions to couple a small portion of the incoming optical signal from an input fiber I of the apparatus to a monitor output 48b which is connected to photodetector 47.
• the remainder of the incoming optical signal propagates via a main output 48a of the tap to the input of the optical amplifier unit 40.
• a second TIA unit 45 has its input connected to an output side of the optical amplifier unit 40 via a corresponding photodetector 47' and associated optical tap 48'.
• This optical tap operates to couple a small portion of the output light from the optical amplifier unit to a monitor output 48b' which is connected to photodetector 47'. The remainder of the output light propagates via a main output 48a' of the tap 48' to an output fiber O of the apparatus.
• the coupling ratios of the optical taps 48, 48' may be the same or different. Also, they are not restricted to any particular limit. But, it is generally preferred to use coupling ratios which substantially preserve the input and output signal powers of the apparatus for example, a coupling ratio of at least 90/10 (meaning that 10% of the light is coupled to the monitor output with 90% of the light propagating to the main output).
• the photodetectors 47, 47' convert the light received from taps 48, 48' into electrical current signals having levels proportional to the amounts of light received via monitor outputs 48b, 48b'.
• the current signals are thus proportional to the optical power levels of the input and output optical signals of the optical amplifier 40.
• the TIA units 43 and 45 produce output voltage signals proportional to their input current signals, and these output signals are supplied to a controller 46.
• the respective output voltage signals Si and S 0 of the TIA units 43 and 45 may represented as follows:
• G, and G 0 respectively represent the transimpedance gains of the input-side TIA unit 43 and the output-side TIA unit 45
• P, and P 0 respectively represent the input optical signal power level and the output optical signal power level
• G represents the optical power gain of the optical amplifier unit
• Ci and C 2 represent proportionality constants dependent upon the coupling ratios of the taps 48, 48' and the responsivities of the photodetectors 47, 47'.
• the controller subtracts one of the two voltage signal levels from the other to obtain an error voltage level S err :
• the controller 46 Based on the error voltage, the controller 46 generates a control signal to adjust the operation of the pump laser unit 44, and thereby adjust the optical gain of the amplifier unit 40, to bring S err to zero and maintain that condition.
• the pump laser unit may incorporate a conventional voltage- controlled pump current controller, and the control signal from controller 46 may be a voltage established based on a PI or PID control algorithm using the error signal level S er r- Again, the control algorithm may be implemented by digital or analog circuitry as desired in a particular application.
• the target gain is simply a predetermined value based on the known gains of the input-side and output- side amplifier units 43, 45. No external reference signal representing the target gain is required. Nor is a division operation to determine the actual operating gain G.
• Fig. 5 illustrates another embodiment 3 of the invention, in this case for controlling a multi-stage pumped fiber-optic amplifier unit 40'.
• the arrangement in Fig. 5 is generally similar to that in Fig. 4, except that the amplifier unit includes a plurality of amplification stages, each including a doped fiber coil 41 , WDM coupler 42, and pump laser unit 44 as previously described in connection with Fig. 4.
• the number of amplification stages is two, but a greater number of stages may be used.
• the amplifier coils of the different stages (which may be pumped the same but provide the same or different gains depending upon the particular application) are connected in series, as shown.
• Each stage may also include a gain-flattening filter (not shown).
• the controller in Fig. 4 the controller in Fig.
• the controller Based on the error thus obtained, the controller generates control signals for adjusting the outputs of the pump laser units and thereby adjusts the optical gain of the optical amplifier unit 40' to bring the error S err to zero and to maintain that state. In that state, the optical gain of the amplifier unit 40' will equal the target gain, which is based on the known gains of the input-side and output-side TIA units, as previously explained.
• the pump laser units may receive identical control signals, even though the two amplification stages may or may not be identical. Of course, in a case where the amplification stages differ and do not provide equal contributions to the overall gain, they can be controlled differently, based on their respective gain contributions.
• Fig. 6 shows another embodiment 4 in which two individually controlled optical amplifier units 40 are connected in series to form a multi-stage amplifier unit.
• This arrangement allows for more precise control of the overall optical gain than the arrangement of Fig. 5, since each amplification stage is individually monitored and controlled.
• the output-side TIA unit 45 for the first (left) amplification stage and the input-side TIA unit 43 for the second (right) amplification stage may have their inputs connected in common to a photodetector 47' (47).
• Each amplification stage is separately controlled in the manner described in connection with Fig. 4.
• the two amplification stages are the same in Fig. 6, as are their respective control systems, this need not be the case in practice.
• the two amplification stages may differ, as may their control systems (e.g., the TIA unit pairs may be selected to provide different target gains).
• An apparatus as described in connection with Fig. 4 was constructed and tested to examine its temporal dynamics.
• the single coil amplifier unit was composed of 13.7 m of conventional erbium-doped fiber, two optical isolators at the input and output, a 1550/980 nm WDM optical coupler, and a standard 976 nm grating stabilized pump laser unit including a voltage-controlled pump current source capable of sub-microsecond ( ⁇ s) response time.
• the input signal to the controlled amplifier unit consisted of two signals: a -10 dBm continuous wave (CW) 1555 nm signal, and a 0 dBm, 500 Hz on/off modulated square-wave signal at 1553 nm.
• CW continuous wave
• 0 dBm 500 Hz on/off modulated square-wave signal
• the TIA units were constructed from respective operational amplifiers (frequency response at least 10 MHz), resistors, and capacitors to have transimpedance gains of 10,000 for the input-side TIA unit and 681 for the output-side TIA unit. These values were determined to be suitable based on the particular physical parameters of the system, such as splice losses, characteristics of the pump laser, characteristics of the photodetectors, etc.
• the TIA units were connected to the input and output sides of the optical amplifier unit via 90/10 optical couplers and substantially identical photodetectors (InGaAs PIN photodiodes with at least 10 MHz frequency response).
• the amount of light coupled to the output-side photodetector is 1/10 of the light output from the optical amplifier unit.
• the input and output fibers of the apparatus were standard SMF-28 optical fiber. Optical connections between components of the apparatus were also made with SMF-28 optical fiber. The signal on the output fiber was measured with a 125 MHz photoreceiver and a 500 MHz digital oscilloscope.
• a digital, microprocessor-based control system including:
• Fig. 7 is a block diagram showing the arrangement of the control system 100 and its connections to the TIA units and laser pump current controller (pump drive circuitry).
• the two A/D converters 101 and 102 are used to digitize the electrical output voltages from the input-side and output-side TIA units 43 and 45, respectively.
• the PAL 103 performs a fast hardware subtraction of the digitized amplifier outputs from the A/D converters.
• the subtraction result, representing the error S er r, is used by the microprocessor 104 to execute a PI control algorithm, and the resulting calculated control signal is then output via the D/A converter 105 to the pump drive circuitry of the laser unit 44.
• Fig. 8 is a flow diagram of the control process.
• the PAL reads the digitized TIA outputs (Si, S 0 ) from A D converters 101 , 102.
• the microprocessor 104 calculates a control move M(n) for adjusting the pump laser output to bring the error S err to 0.
• step S4 the value S err (n-1) for use in the next iteration is set to the current error S err (n).
• Fig. 9 shows the traces of the square-wave input signal and the resulting output signal.
• the relative scales of the two signal traces were set so that the traces could be superposed, as shown, for easier comparison.
• a comparison of the signal traces reveals that the amplifier output suffers severe distortion due to the slow gain dynamics associated with erbium ions in glass.
• Fig. 10 shows the temporal dynamics with the gain control system turned on. As seen in Fig. 10, the optical output signal has a square-wave shape with low distortion.
• the details of the turn-on transient are shown in Fig. 11.
• the initial rapid increase in the output signal is provided by the initial inversion (inversion refers to the population of erbium ions in the excited state).
• the pumping rate is insufficient to accommodate the increased signal power.
• the gain control circuit increases the pump power to the maximum allowed value.
• the amplifier responds with a temporally increasing gain until the desired gain is achieved.
• the pump power is then decreased to the value required for steady-state operation.
• the total time that is required to correct the gain is approximately 25 ⁇ s.
• the response speed of the gain control system can, of course, be increased if more pump power is available during the turn-on transient. Additionally, the response time can be reduced by using faster electronics, e.g., an analog control circuit instead of a microprocessor based circuit.

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• Engineering & Computer Science (AREA)
• Computer Networks & Wireless Communication (AREA)
• Signal Processing (AREA)
• Lasers (AREA)
• Control Of Amplification And Gain Control (AREA)
• Optical Communication System (AREA)

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• 1999
  • 1999-12-14 AU AU20533/00A patent/AU2053300A/en not_active Abandoned
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