OPTICAL AMPLIFIERS WITH A SIMPLE GAIN/OUTPUT CONTROL DEVICE

Abstract: An optical fiber amplifier (10) includes an optical system (15) and an electronic controller (40). The optical system includes: an optical gain medium (20); at least one pump source (22) coupled to the optical gain medium; an input port for an incoming signal; and an output port for an out-going signal. It also includes an input optical signal tap (28) coupled to a first optical detector (32); and an output signal tap (30) coupled to second optical detector (34). The controller (40) is provided with a user interface (55) that allows a user to control desired gain value; amplifier output power value or ASE noise compensation.

Description: OPTICAL AMPLIFIERS WITH A SIMPLE GAIN/OUTPUT CONTROL DEVICE CROSS-REFERENCED TO A RELATED APPLICATION Field of the Invention This invention relates to optical amplifiers and more specifically to automatic gain and output power control of Optical Amplifiers. Background In recent years optical amplifier modules have undergone considerable transformation. Increased demand for more data transfer resulted in development of wavelength division multiplexing (WDM) technology, which allows more data to be transmitted over one fiber by increased channel count (i.e., a larger number of narrower wavelength ranges within the same predetermined wavelength window). This WDM technology suffers from unwanted effects, such as a variation in output power when the input signal power is constant (for example, due to aging of the amplifier or due to stresses in the amplifier), and cross talk between different channels, for example, when the input signal is modulated at a low frequency. The low frequency is a frequency of up to 10 kHz. This low frequency modulation can be present, for example, due to the addition or dropping of some to the channels, or due to sudden loss of signal at certain wavelengths. These unwanted effects have a negative influence on the power transients (i.e., fluctuations of output optical signal power) of surviving channels, which results in a poor performance of the signal transmission, expressed in an increased bit error rate (BER). In order to minimize the unwanted output signal power fluctuation and the power transients due to the cross talk or other causes (such as fiber damage, adding or dropping of channels), it is common to introduce a mechanism for controlling either the output signal power or the gain of the optical fiber amplifier. Gain is the ratio of the optical signal output power to the optical signal input power. There are two known approaches for controlling output signal power or the gain of the optical fiber amplifier. The first approach, known as the electronic feedback/feed-forward approach, utilizes electronic circuitry to control power transients caused by the crosstalk produced in the optical fiber amplifier. More specifically, amplifier gain or power is controlled by analog tuning of the electronic components, for example by changes resistor's or capacitor's values. This approach allows the user, such as a communication company, to minimize power transients in any given optical amplifier by controlling either the amplifier gain or the amplifier output power, but not both. This approach also limits accuracy of gain control when signal power is small. Finally, this approach does not compensate for amplifier noise, such as ASE (amplified spontaneous emission). The second approach, known as the optical feedback control approach, utilizes only optical components to control power transients of the optical fiber amplifier. This approach is even less flexible than the all-electronic approach described above, because any change in power or gain control requirements requires the change in optical components. SUMMARY OF THE INVENTION. The optical fiber amplifier of the present invention is describerd in the appended claims. Embodiments of the present invention can provide an optical amplifier and a control technique that overcomes the difficulties associated with known optical amplifiers. It is an advantage of this optical amplifier that it has a flexible controller that provides a choice of different control parameters. It is also an advantage of this optical amplifier that it automatically suppresses power transients by fast control of the amplifier gain or the total output power. It is further an advantage of this optical amplifier that the controller minimizes the influence of amplifier ASE noise. For a more complete understanding of the invention, its objects and advantages refer to the following specification and to the accompanying drawings. Additional features and advantages of the invention are set forth in the detailed description, which follows. It should be understood that both the foregoing general description and the following detailed description are merely exemplary of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate various features and embodiments of the invention, and together with the description serve to explain the principles and operation of the invention. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 A illustrates schematically an optical fiber amplifier 10; Figure 1 B is shows a more detailed block diagram of a controller of the amplifier of Fig. 1A. Figures 2A-2D illustrate a close-loop Gain Control Mode performance of the amplifier of Figs. 1A and 1 B when input signal is constant. Figures 3A-3D illustrate a close-loop Gain Control Mode performance of the amplifier of Figs. 1A and 1 B when input signal drops. Figures 4A-4D illustrate a close-loop Power Control Mode performance of the amplifier illustrated in Figs. 1A and 1B. PREFERED EMBODIMENTS Figures 1A and 1 B illustrate an embodiment of an improved optical fiber amplifier 10. This optical fiber amplifier 10 includes an optical system 15 comprised of an optical gain medium 20, for example a rare-earth doped fiber and at least one pump source 22, such as a laser diode, driven by a pump drive unit 22A coupled via a coupler 22B to the optical gain medium 20. The optical system further includes an input port 24 for an optical signal entering the gain medium 20, an output port 26 for an out-going signal and two optical taps 28 and 30. The tap 28 is an input optical signal tap 28 and is connected to a first optical detector 32. The tap 28 is located either downstream of the input port 24, but in front of the gain medium 20, or, alternatively, may form a part of an input port 24 or the coupler 22B. The tap 30 is an output optical signal tap and is connected to a second optical detector 34. The tap 30 is located downstream of the gain medium 20 in front of the output port 26. Alternatively the tap 30 may form a part of the output port 26. In this embodiment the optical detectors 32 and 34 are photodiodes. With reference to Fig 1 B, in this embodiment, both input and output signal taps 28, 30 of the amplifier 10 have the same ratio α (α=0.02). This ratio α is defined as the optical signal power channeled into a tap divided by the total optical signal just before the tap. However, the input and output taps may be characterized by different α values. For example, if the total signal approaching the input tap is weak, a larger α ratio may be required by the input tap 28 in order to provide a better detection by the optical detector 32. Thus, predetermined portions of the total input optical signal power P ιn and of the total output power P out a e channeled into the taps 28, 30 according to their α ratios. The amplifier 10 may also include other optical components. These components are, for example, isolators, attenuators, light splitting couplers, optical multiplexers, demultiplexers and filters. The amplifier 10 further includes an electronic controller 40. It is the electronic controller 40 that controls the pump source 22 (for example by controlling drive current of the laser diode) by receiving information about optical power levels of the input and output signals. In this embodiment the electronic controller 40 includes input and output signal converters 42 and 43, which convert signals from the optical detectors 32 and 34, respectively, to electrical signals, and electrical signal amplifiers, such as transimpedance amplifiers 44, 45 that amplify the electrical signals provided by the optical detectors 32, 34. The electronic controller 40 further includes at least one analog to digital (A-to-D) converter 46 that converts amplified electrical signals to digital signals. The electronic controller 40 also includes at least one signal processing unit 48, such as a digital signal processing unit for processing the digital signals into a new set of digital signals and a digital to electrical signal converter 50, for converting the new set of digital signals to a new set of electrical signals. The level of pump power produced by the pump source 22 is determined by this new set of electrical signals. According to one embodiment of the present invention the signal processing unit 48 of the amplifier 10 is coupled to a user interface 55 that allows a user to chose and specify an amplifier control mode. The user, by specifying an appropriate control mode, commands the signal processing unit 48 to control changes in DC offset calibration, specifies a desired amplifier gain value; amplifier output power value, or optical noise (ASE) compensation. DC offset calibration is a process of compensation for constant error or noise signals (dark current, for example) introduced by the electronic or optical devices. This embodiment of the invention utilizes a feed-back loop to provide an automatic gain and output power control of optical fiber amplifier 10. A feed forward loop may be utilized in addition to the feedback loop to improve the power transients, if needed. The disclosed control method utilizes a unique control algorithm 100, described below. The algorithm 100 of this embodiment is based on the Proportional Plus Integral (PI) control law and is implemented in the digital signal processing unit 48 of the controller 40. Other control laws may also be utilized. Based on the user selected control mode specified through the user interface 55 and the data about optical power levels provided by the taps 28 and 30, the algorithm 100 controls the output power of the pump source 22 and, therefore, the amplifier output optical power P out . More specifically, the controller 40, through its signal processing unit 48 and the control algorithm 100, commands one or more pump drive units 22A to drive one or more pump laser sources 22 so as to increase or decrease optical power provided by the pump laser source 22. As stated above, this optical power is used for exciting the energy level of rare- earth ions (Erbium, for example) in the rare-earth doped amplifying fiber corresponding to the gain medium 20. Thus, the pump laser source 22 controls the amplifier by injecting the appropriate level of optical power at a specified wavelength to the optical gain medium fiber 20. The amplifier control may include the control of amplifier gain, output power, temperature, laser diode over-current and ASE (amplifier spontaneous emission). The end user is provided with a menu of control modes to choose from. The following is a more detailed description of the amplifier 10 and the algorithm 100. As discussed above, the optical signals channeled by the taps 28, 30 are detected by the detectors 32, 34 that provide electrical signals, such as current. The amplitude of the electrical signals provided by these detectors 32, 34 corresponds to the amplitude of the optical power incident on these detectors. Therefore, these electrical signals correspond to the total input optical signal power P m and total output power P out of the optical amplifier 10. The A-to-D converter 46 converts analog (i.e., electrical) signals, to a digital (i.e., numerical) representations of this signals, to be used by a typical computer or a processor. It is preferable that the A-to-D converter 46 has at least 12 bits of resolution in order to achieve a good dynamic range (i.e., greater than 30dB (1000:1)). The digital signal processing unit 48 of this embodiment takes discrete samples of digital data provided to it by the A-to-D converter 46 at a high frequency rate (i.e., at 1 MHz or higher sampling frequency) and the algorithm 100 of the digital signal processing unit 48 processes this data. The high sampling speed is needed to preserve the frequency characteristics of the analog power signal. If the sampling rate were low, part of the information about the signal would be lost. In order to process the control algorithm 100 at the high frequency rate, it is preferable that the digital signal processing unit 48 has enough speed and computational power to complete all control calculations and additional signal processing such as alarm processing and monitoring of problematic conditions such as, for example, low signal power, low output signal, loss of input signal, high temperature, low temperature, or laser diode over-current. More specifically, the input signal power, the output signal power (and optionally, temperature of the amplifier or its components, laser diode current, spectral characteristics of the output signal) and other parameters that require monitoring are periodically measured (approximately every 5μs or faster, and preferably every 1 μs or faster). The signal processing unit 48 may comprise a memory 48A containing a table with minimum and maximum acceptable values for these parameters. Monitoring software 48B of the digital signal processing unit compares the digital data corresponding to the actual conditions to the tabulated parameter values and, if the data conditions seems to be outside the acceptable range, raises an alarm flag within the signal processing unit 48 and sends a warning signal to a central monitoring location 60 by using some data bus. In response to the alarm flag the signal processing unit 48 may shut down the amplifier (in order to protect it from possible damage) by turning off the pumps, reduce the amount of current going to the laser diode or adjust the temperature of the amplifier or its individual elements by use of one or more temperature conditioner 65, such as a cooler or a resistive heater, for example. In addition, if the digital signal processing unit 48 detects the loss of input signal, the signal processing unit 48 may shut down the pumps, wait for signal to be restored and then turn on the pumps to activate the amplifier. This would avoid amplifying noise, in the absence of information carrying input signal. Finally, the temperature of different amplifier components, such as, for example, filters, gratings or couplers, may also be adjusted by the signal processing unit 48 and one or more heater/cooler drivers 65A that control the temperature provided by heaters/coolers 65 to provide dynamic tuning of the gain spectrum to compensate for aging of the amplifier, changed environmental conditions or other perturbations. The utilized heaters/coolers 65 may be coil heaters, laser pump heaters/coolers, or other devices, as needed. This embodiment utilizes a fixed-point digital signal processor (DSP) (integer arithmetic) as the signal processing unit 48 because of its high speed, low cost and small size. An example of such DSP processor is the Motorola 5630x series processor. It has 24-bit single-precision resolution and runs at high speeds (i.e., speeds of at least 100MHz). A DSP processor operating at 300MHz, recently announced by Motorola would provide more computing power, thus allowing for a more complex control algorithm and a more responsive amplifier. However, other signal processing units 48 may include, for example, a floating point DSP, a complex programmable logic device (CPLD), a field programmable gate array (FPGA), a microprocessor, a microcontroller or a combination thereof. To increase computational power of the controller 40, a plurality of signal processing units 48 may also be utilized. Let's denote the digital outputs from A-to-D converters 46 as P ou t (k) and Pι N (k), where P ou t(k) represents a discrete value of the scaled total output power signal P out (t), and PiN(k) is a discrete value of the scaled total input power Pin(t). In this embodiment the output and input powers are scaled in order to correctly represent them within the available numeric range. The values Pouτ(k) and Pj n (k) are represented in at least 12-bit resolution from the Electronic component side 39 and when they enter the Digital Processor side 41 they are zero padded (by adding zeros in front or behind the 12 digit numeral to create a numeral represented by more digits) to higher resolution (24 bits for the above mentioned Motorola DSP 5630x processors). From this point on the digital control signal u(k) is calculated by using some of standard control laws. As stated above, in this embodiment we use the proportional plus integral (PI) controller in the form: ι{t) = K p e{ή+ K, ) e{τ)dτ (1 ) where u(t) represents current or power that controls the laser pump; K p is a proportional constant of the PI controller; Kj is an integral constant of the PI controller; e(t ) represents error signal, i.e. the difference between the desired value for gain or output power of the amplifier, given as the setpoint G sp or P sp , respectively. When the amplifier is operating in either gain or power mode the error signal is: for gain control f° r power control The equation (1 ) has to be converted to a discrete form with the sampling interval of h seconds, since it is implemented in digitally by the digital signal processor (DSP) 48. A discrete transfer function in Z space of equation (1) obtained by bi-linear transform is where U(ζ '1 ) is the complex form of the control function in frequency domain. The above equation (3) can be represented in a difference equation form u{k) = u(k - l) + (κ p + K,h)e{k)- K p e{k - ϊ) (4) where the variable k denotes the current sampling instant, i.e. t = kh , where h is a current sampling interval. The algorithm given below describes the implementation of the Gain/Output power control with PI controller. Many other controller algorithms can also be used. The following is a description of the exemplary algorithm 100. The algorithm 100 may include the following steps: 1. Choosing the control mode (Gain or Power or constant pump power) 2. Setting the sampling interval h and the controller parameters K p and K t . A typical sampling interval is about 1 usec and the choice of controller parameters depends on the type of the amplifier. 3. If the control mode is Gain, setting the following reference values: G.„ ≠ O and = 0. Go to 5. 4. If the control mode is Power, setting the following reference values: G sp = and P ψ ≠ 0. 5. At sampling time t : (i) Converting the analog values for input and output optical power to digital form by AD converter (ii) Multiplying the input signal P m (k) by the gain set point G φ . (iii) Calculating the error signal e(k) e(k) = P 5p - e (k) , where e'(k)=G S pPι N (t)-Pouτ(t) ; and (iv) Calculating the control signal u(k) as a function of the error signal u(k) = u{k - l)+ (K p + K i h)e{k)- K p e{k - l) . 6. Transforming the control signal to analog form u(k) → u(t) with D-to-A converter. This signal controls the pump laser by converting the electric current to optical power P p (t) . 7. Waiting until the end of sampling interval h and then seting t + 1 → t and going back to step 5. It is noted that the step of obtaining value for input and output power electronic circuitry DC off set is usually done only once, during the amplifier manufacturing process. Illustrations of the Controller Performance Figures 2A-D, 3A-D, and 4A-D illustrate the closed-loop performance of the controller 40. (By closed-loop we mean that the control algorithm is in place and provides feed-back control.) More specifically, Figures 2A-2D and 3A-3D indicate the performance of the amplifier 10 in the Gain Control Mode, while Figures. 4A-4D illustrate the performance of the amplifier 10 in the Output Power Control Mode. Figures. 2A, 2B and 2C illustrate the behavior of input signal P s , output signal P o {t), and the pump laser control signal P p {t), respectively. Fig. 2D shows the change of the set point gain G ψ and the resultant change in actual gain G(t) . Figures 2A-2D illustrate that while input power P in remains constant, when the user specified value for G sp changes, the optical power P p (t) supplied by the pump laser source 22 changes in order to change the actual gain G(t) of the amplifier 10. Figure 2C also shows that the output power P out of the amplifier 10 changed in response to the change in pump power P p {t). Figure 2D indicates that in this embodiment the gain value G(t) reached its specified gain value level in 3.5x10 "3 seconds. Figures 3A-3D are similar to Figures 2A-2D, the only difference being that the input signal R. changes to simulate a drop of some input channels (See Figure 3A), while the setpoint gain G sp remains constant at 20 dB (see Figure 3D). Figure 3D illustrates that the gain G(t) drops quickly when input signal P s drops, but the controller 40 brings it back to its setpoint value in about 0.5x10 "3 seconds. Figure 3C illustrates that this is achieved through a fast increase in the optical pump power P p (t) supplied by the pump laser source 22. controller 40 in the Output Power Control Mode. Figure 4D illustrates that the setpoint value of output signal P changes from 20mW to 40mW at t=2.5ms. At this time the controller 40 increases the pump power P out (t=2.5ms) to about 70mw (see Figure 4C) and thus drives the amplifier output power P mt it) from 20 to 40mW in less than 0.5ms. As a result, the error signal e(k), illustrated in Figure 4B, drops down to zero. Figure 4A illustrates that the input signal P s (t) drops at t = 4ms . This corresponds to a drop in output power P 0 and the corresponding increase in the error signal e(k). Figures 4C and 4D illustrate that the disturbance in output power caused by change of input signal P s (t) is quickly eliminated by increase in the pump power Pp. An improved optical amplifier and a simple new method for automatic electronic control of optical amplifiers have been described. The improved method utilizes the capabilities of a digital processor and simple control algorithm to achieve (1) gain control mode or (2) output power control mode. It has a capability of setting the reference values for gain or output power and flexibility of choice of the control algorithm. The improved amplifier can utilize more complex control laws than the classical proportional plus integral controller subject to the need and the digital signal processor speed. This control method is suitable for use in the communication systems where the remote control of the device is required. Accordingly, it will be apparent to those skilled in the art that various modifications and adaptations can be made to the present invention without departing from the spirit and scope of this invention. It is intended that the present invention cover the modifications and adaptations of this invention as defined by the appended claims and their equivalents.

Claims: I claim: 1. An optical fiber amplifier comprising: (A) an optical system including: (i) an optical gain medium; (ii) at least one pump source coupled to the optical gain medium; (iii) an input port for an in-coming signal; (iv) an output port for an out-going signal; (v) an input optical signal tap coupled to a first optical detector; (vi) an output optical signal tap coupled to a second optical detector; and (B) an electronic controller adapted to utilize inputs received from said detectors to control optical power provided by said pump source, said controller including a programmed unit and a user interface that allows a user to chose to control at least one of the following available functions: desired amplifier gain value, amplifier output power value, DC off-set calibration, and ASE noise compensation. 2. The optical amplifier according to claim 1 , wherein said electronic controller includes a signal processing unit provided with a software control algorithm that computes a value corresponding to the required optical power to be provided by said pump source, so as to provide either a specific gain or a specific output power by said amplifier. 3. The optical amplifier according to claim 2 further including an additional signal processing unit. 4. An optical fiber amplifier comprising: (A) an optical system including: (i) an optical gain medium; (ii) at least one pump source coupled to the optical gain medium and providing optical power to said medium; (iii) an input port accepting an input signal and coupling said input signal into said optical gain medium; (iv) an output port providing an out-going signal; (v) an input optical signal tap channeling a portion of said input signal toward a first optical detector; and (vi) an output optical signal tap channeling a portion of said out-going signal toward a second optical detector; and (B) an electronic controller including: (i) input and output signal converters, which convert signals from said first and second optical detectors to electrical signals, (ii ) an electronic to digital signal converter, converting said electrical signals; (iii) at least one digital signal processing unit processing said digital signals into a new set of digital signals; (iv) a digital signal to an electrical signal converter, converting said new digital signals to new electrical signals; wherein said pump power is driven by said new electrical signals; said controller further including a user interface that allows a user to change DC offset calibration; specify desired amplifier gain value; amplifier output power value; and optical noise compensation. 5. An optical fiber amplifier according to claim 4, wherein said electronic controller is adapted to provide alarm processing and temperature control. 6.An optical fiber amplifer according to claim 1 , wherein user interface is located at a remote location from said optical system and said programmed unit.

Inventors: GERRISH KEVIN S, LELIC MUHIDIN, GERRISH, KEVIN, S., LELIC, MUHIDIN
Applicants: CORNING INC, CORNING INCORPORATED
Application year: 2001
Main classification: H01S
IPC codes: H01S3, H04B10, H04J14