Abstract:
A method and system for controlling an optical amplifier in an optical waveguide system to reduce the effects of noise, particularly due to amplified spontaneous emission. The method comprises determining the gain between an input and an output of the optical amplifier; determining a customized pulse train in accordance with the determined gain and a desired gain; and controlling a pump source of the optical amplifier in accordance with the customized pulse train. Determining the customized pulse train includes determining a desired pulse width as a function of the determined gain, the desired gain and the pulse width at the determined gain. Similarly, a desired pulse spacing is determined as a function of the determined gain, the desired gain and the pulse spacing at the determined gain. Controlling the pump source preferably includes driving the pump source in accordance with the customized pulse train.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates generally to optical amplifiers. More particularly, the present invention relates to Rare Earth Doped Fiber Amplifier (REDFA) systems.  
         BACKGROUND OF THE INVENTION  
         [0002]    Optical amplifiers are an essential component of optical systems. Signal loss and attenuation of signal strength are important considerations in designing an optical system whether that system serves a communications, computing, medical technology function, etc.  
           [0003]    Fiber optic technology is a well known optical technology and is used in a variety of communications networks. These networks often use long optical transmission lines, which are subject to attenuation of the optical signal. To compensate for this attenuated signal strength, optical amplifiers are used to boost the signal, thereby allowing long-haul transmission over greater distances. Optical amplifiers, such as REDFAs, are well known in the art. When the dopant ions in a fiber are energized to a condition of population inversion, the fiber will undergo emission in response to stimulation. In such a case the fiber acts as an amplifier. Such an amplifier will require a means for pumping (exciting) the dopant ions to the upper energy states, resulting in population inversion. The energy differential of the upper and lower energy bands of the fiber must correspond to the wavelengths of the optical input data in order to provide a corresponding gain band.  
           [0004]    A typical REDFA consists of an optical fiber doped with a few parts per million of the rare earth element (for example, erbium), a continuous wave current-driven laser pump diode, an optical data input port, an optical data output port, and a port for introduction of the pump signal. When the laser pump injects high energy photons into the optical fiber, some erbium ions within the fiber are excited (pumped) from a base state to a higher-energy-state. The erbium ions stay in the said higher-energy-state for several milliseconds. The input data stimulates the excited erbium ions to return from their heightened energy state, to their base state, thereby emitting photons of the same wavelength as the input data.  
           [0005]    Optical amplification is achieved when the number of photons emitted at a given wavelength exceeds (by several times) the number of input data photons at that same wavelength. For an Erbium Doped Fiber Amplifier (EDFA), this can occur at a number of wavelengths between 1530 nm and 1580 nm, known as “Conventional-Band or C-Band Amplification”. EDFAs can also be designed to provide amplification between 1580 nm and 1610 nm, which is known as “Long-Band or L-Band Amplification”.  
           [0006]    A single erbium doped fiber may carry several communication channels, where each channel is assigned to a different wavelength. Since the gain of the fiber amplifier is a function of the relative population of the energy bands, adding or dropping a channel can result in a temporary change in population. When the number of channels fluctuates within the fiber amplifier (in cases where one or more channels are dropped or added), the gain of the amplifier system fluctuates over time, before reaching the new steady state. The effect of adaptation to this change is referred to as transient response. In this situation, the amplifier system gain has to be adjusted in such a way that a uniform gain (flat gain over time) is maintained. This is known as “transient suppression”. Transient suppression is a common technique for reducing or mitigating transients in the amplitude of the optical signal.  
           [0007]    In an existing constant pump system, the driving current of the pump is a continuous wave. In cases where one or more channels are dropped, a constant gain is achieved by reducing the amplitude of the driving current of the pump laser diode. EDFA gain transients could result in fluctuations in optical data networks. Therefore, it is important to respond to the transients of EDFAs and reduce them. The transient suppression in the existing pump system is insufficient because the reaction time to transients is slow. This limitation confines the amplifier&#39;s ability to meet the system requirements, where gain must be regulated to maintain system performance.  
           [0008]    The use of continuous wave pumps, if not optimized, can create an undesired overpopulation of excited dopant ions in the said higher-energy-band; the unused excited ions will spontaneously decay to their ground states, producing amplified spontaneous emission (ASE) at various wavelengths. This will appear at the output of the amplifier as optical noise degrading the quality of the desired amplified signal.  
           [0009]    Another problem of traditional EDFAs is the overall power dissipated in the amplifier system as heat. System cooling and power requirements are important, especially for enclosed areas and remote applications. As integration density is increased, this power dissipation problem makes the implementation inefficient and costly.  
           [0010]    It is known to modify the pump characteristics to create a signaling channel between stages. One approach uses non-continuous pumping to supervise any serious failure within the span and inform the receiving terminal of any failure when detected. The approach mentioned deliberately focuses on depletion of the reservoir of excited atoms, so as to impose an additional signal on the payload being amplified. The supervisory information is transferred by providing a distinct modulation frequency on the pumping source for each repeater stage. This was an improvement over the prior art&#39;s reliability of the optical transmission system by providing for communication of possible failure information without significantly disrupting the main signal. However, the supervisory modulation signal: does not directly contribute to any improvement in amplification efficiency; is focused on alarm detection rather than performance optimization; varies between stages rather than at a stage; and is non-responsive to gain.  
           [0011]    Accordingly, there still exists a need for an optical amplifier that utilizes a laser pump whereby the pumping situation is optimized to reduce the power consumption and dissipation without degrading the performance (gain, noise level, etc.) of the amplifier system in conditions where continuous streams of high bit-rate digital pulses require amplification.  
         SUMMARY OF THE INVENTION  
         [0012]    It is an object of the present invention to obviate or mitigate at least one disadvantage of previous optical amplifier laser pumps. Accordingly, it is an object of the present invention to provide a laser pump used in an optical amplifier, whereby the power consumption and dissipation of the overall amplifier system is reduced.  
           [0013]    In a first aspect, the present invention provides a method for controlling an optical amplifier in an optical waveguide system. The method comprises determining an operating characteristics at an input and an output of the optical amplifier; determining a customized pulse train in accordance with the determined characteristic and a desired characteristic; and controlling a pump source of the optical amplifier in accordance with the customized pulse train.  
           [0014]    In presently preferred embodiments, the determined characteristic is the gain between the input and the output, or the output power. Determination of the gain includes splitting off optical input data and optical output data from the input and the output, respectively. Determining the customized pulse train can include determining a desired pulse width as a function of the determined characteristic, the desired characteristic and the pulse width at the determined characteristic. Similarly, a desired pulse spacing or amplitude can be determined as a function of the determined characteristic, the desired characteristic and the pulse spacing, or amplitude, at the determined characteristic. Controlling the pump source preferably includes driving the pump source in accordance with the customized pulse train.  
           [0015]    In a further aspect, the present invention provides an optical amplifier control system. The control system comprises a gain controller for determining the gain between an input and an output of an optical amplifier, and for determining a customized pulse train in accordance with the determined gain and a desired gain. A driver, connected to the gain controller, receives the customized pulse train and drives a pump source of the optical amplifier in accordance with the customized pulse train. Preferably, a messaging unit enables remote communication with the gain controller.  
           [0016]    In a further aspect, the above optical amplifier control system can be integrated with an optical waveguide amplifier and a pump source to provide an optical amplifier system.  
           [0017]    Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying Figures. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]    Embodiments of the present invention will now be described, by way of example only, with reference to the attached Figures, wherein:  
         [0019]    [0019]FIG. 1 is a block diagram of an erbium-doped fiber amplifier system of the present invention;  
         [0020]    [0020]FIG. 2 is a graphical illustration of the pump driving current signal of the present invention;  
         [0021]    [0021]FIG. 3 is a graphical illustration of the fiber input power;  
         [0022]    [0022]FIG. 4 is a graphical illustration of the fiber output power uncorrected as per prior art;  
         [0023]    [0023]FIG. 5 is a graphical illustration of a customized pulse train and average current of the present invention; and  
         [0024]    [0024]FIG. 6 is a graphical illustration of the fiber input power vs. fiber output power corrected as per the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0025]    A typical optical amplifier system of the present invention consists of an optical input data stream, a laser pump source and driver; control circuitry and sensors used for gain control. The input data coupled with the pump signal is applied to the optical amplifier waveguide (erbium-doped fiber in the preferred embodiment) to excite the (erbium) ions in the amplifier. A small percentage of the light is tapped from the input and output of the amplifier to measure the input and output power from the amplifier. This input and output power measurement is applied to the gain control circuitry to control the driver. The driver provides the driving current to the pump source whereby the corresponding pump power is applied to the waveguide to achieve optical amplification.  
         [0026]    The invention takes advantage of the long time constant of the upper energy state of the rare-earth doping atoms in the embodiment. Pumping power is supplied discontinuously so that just sufficient excited atoms are available for amplification, but not so many that a significant number will decay spontaneously creating unwanted optical noise. This enables the user to reduce the power dissipation of an optical amplifier system by applying a pulsed drive current to the pump. With Pulse Width Modulation the power transistors operate within the most efficient points of operation: saturation and cutoff. Resistance within the switching transistors is either very high or very low, and the low on-resistance of MOSFETs helps to reduce the power consumed in the PWM supply for an equivalent amount of power supplied to the pump laser. By adjusting the drive current&#39;s pulse width and/or amplitude in response to the output of the gain controller circuit, a pump driver that requires minimum power is implemented without compromising the optimal output power of the pump signal that is applied to the optical amplifier waveguide. Other advantages and effects will become apparent in the ensuing description of the invention.  
         [0027]    In addition to the power consumption/dissipation advantage of this invention, the present invention may make it possible to reduce the ASE. This affects the optimization of the signal to noise ratio of the amplifier system and as a result enhances the quality of the desired amplified signal.  
         [0028]    Additionally, the present invention may substantially facilitate transient suppression. In the invention, the power output of the laser pump is pulsed in the duration where the gain response of the amplifier is non-uniform. This enhances the performance of the amplifier where the number of wavelength channels may fluctuate and enable transient signals to be reduced. In this manner the present invention may reduce saturation. Further, pulsed operation will reduce high temperatures and ameliorate ageing in some laser pumps.  
         [0029]    The present invention is applicable to any situation where optical amplifiers can be used, such as: optical networks for long haul transcontinental, intercontinental &amp; transoceanic (point-to-point) networks, Wide Area Networks (WAN), Metropolitan Area Networks (MAN), and Local Area Networks (LAN), biophotonics (including medical technology), printing technology, optical imaging, fiber sensor detection, optical computing, etc.  
         [0030]    The invention will be described in more detail and in relation to the Figures and Drawings included in this disclosure. In the illustrated embodiments, an EDFA is used. However, those skilled in the art will appreciate that other types of optical amplifiers, rare earth doped or otherwise fabricated, and geometry&#39;s other than fiber (e.g. deposited or delineated waveguides) can be used while remaining within the scope of the invention.  
         [0031]    [0031]FIG. 1 is a block diagram of the preferred embodiment of the present invention showing an optical amplifier system. This diagram is a simplified representation and only includes the elements, which are necessary for the purpose of illustrating the invention.  
         [0032]    The block diagram illustrates an EDFA system  100  of the present invention with input and output data splitters  104 A and  104 B respectively, a wavelength division multiplexer (isolator)  108 , an erbium doped fiber  103 , electro-optical transducers (photodiodes in this embodiment)  110  and  111 , a controller  105 , a current source,  106 , and a pump source  107  used to pump the amplifier.  
         [0033]    The optical input data path  101 , the pump optical signal path  102 , and the erbium-doped fiber  103  are connected to the combiner  108 . The optical input data path  101  is also connected to the splitter  104 A, along with the input source  112  and photodiode  111 . The erbium-doped fiber  103  is connected to the splitter  104 B along with photodiode  110  and the optical output data path  113 . Both photodiodes  110  and  111  connect (electrically) to a controller  105 . A current source  106  connects the controller  105  to the laser pump  107 . The laser pump is the source of the pump optical signal path 102 .  
         [0034]    A percentage of the light is tapped from the input source  112  of the amplifier by splitter  104 A to measure the input power to the amplifier. The tapped light is received by the input monitor photodiode  111 . Similarly, a small amount of light is tapped from the output of the amplifier by splitter  104 B, to measure the output power from the amplifier. Light is tapped from the output of the amplifier and received by the output monitor photodiode  110 . The tapped light received by the respective photodiodes  111  and  110  provides the respective input and output amplifier power measurements to the controller  105 . The controller  105  is comprised of circuitry that generates the signal needed to regulate the current source  106  in response to the said input and output amplifier power measurements. Accordingly, the driver  106  generates the corresponding driving current signal (Ip)  200 , applied to the pump source  107 . The current signal  200  is a pulsing signal whereby the pulse rate, amplitude and offset are varied in response to the controller to provide the driving current to the pump source  107 . The coupler  108  combines the input data path  101  with the pump light. The coupled signals are passed through the erbium doped fiber  103  to pump the erbium ions into population inversion and to concurrently induce stimulated emissions in the amplifier. The corresponding amplified optical output data is transmitted by the output path  113  coupled through the splitter  104 B.  
         [0035]    The advantages of the invention described are enhanced when there is capability to communicate remotely with the controller, allowing data to be collected from the controller or changes of pump conditions to be implemented in conjunction with other system needs. Apparatus for communicating these messaging signals will be obvious to those skilled in the art. Such apparatus is represented by the messaging unit, 120 , of FIG. 1.  
         [0036]    A similar apparatus may incorporate additional pumps to better service the fiber on the basis of wavelength, polarization, or fiber connection location. Those skilled in the art will understand that such a similar system involving more than one pump falls within the scope of the invention.  
         [0037]    [0037]FIG. 2 is a detailed illustration of the pump driving current signal (Ip)  200  of driver  106  in FIG. 1 of the present invention. The pump driving current  200  is operated at an adjustable DC-offset current  201 . The driving current&#39;s amplitude  202 , and pulse rate (pulse width  203  and/or separation  204 ) are adjusted by the controller circuitry  105 . The object is to minimize the electrical power required by the driver to efficiently provide the desired current to the pump source without altering the optimal output power of the pump signal that is applied to the EDFA.  
         [0038]    By reducing the power consumed in the said current driver, the power consumption and dissipation of the overall amplifier system is reduced. The adjustment of pulse rate and amplitude differs from traditional methods, in which a continuous current is switched on and off to control system gain. It is not necessary for the pump drive pulses to be regularly spaced in time. Because of the characteristics of the Er energy levels in silica fiber, it is best if the time separation between drive pulses does not exceed the relaxation lifetime (˜ms) of the excited Er level.  
         [0039]    In the present invention transients are reduced and power levels controlled by adjusting the amplitude  202  and pulse width ( 203 ) and separation ( 204 ) of the driving current. For example, in instances where rapid changes of gain are required within the erbium-doped fiber, the driving current can be optimized so as to maintain constant the gain over time within the amplifier system.  
         [0040]    Ideally, the controller  105  would expand separation  204  in the case of higher gain than desired. Conversely, the controller  105  would compress the separation  204  in the case of lower gain than desired. Generally the separation may conform to the formula: 
           S=S   0   +k ( G−G   d ) 
         [0041]    Where S is the separation  204 , S 0  is the separation in the case of desired gain (G d ) and G is the gain detected by the controller  105 . The coefficient k relates the severity of the response. Note that a system could incorporate maximum and minimum separation  204  without diverging from the essence of the invention. Similarly a minimum divergence on G may before observed before altering the separation.  
         [0042]    Similar to the separation  204 , the pulse-width  203  may be controlled. Generally the pulsewidth  203  will conform to: 
           P=P   0   −m ( G−G   d ) 
         [0043]    Where P is the pulse-width  203 , P 0  is the pulse-width in the case of desired gain (G d ) and G is the gain detected by the controller  105 . The coefficient m relates the severity of the response. Note that a system could incorporate maximum and minimum separation  204  without diverging from the essence of the invention. Similarly, a minimum divergence on G may before observed before altering the separation. Either of the aforementioned adjustments may be used independently, together or in conjunction with a similar amplitude formula. This does not diverge from the scope of the invention. Those skilled in the art will understand simple variations may be made to the formulae above and still remain within the essence of the invention.  
         [0044]    By virtue of the flexibility provided by the digital controller to alter pulse-width  203 , separation  204 , and amplitude  202 , it is easy for those skilled in the art to program a customized pulse train for the drive current of the pump. Referring to FIG. 3, we see the input power  305  associated with a fiber that, for example, experienced an increase in the number of channels, and a corresponding increase  310  in input power  305 . Referring to FIG. 4, the output power  315 , responds latently  320  to the input power. This latent response results in low gain. Similarly in the case of dropping channels, temporarily high gain results. Referring to FIG. 6, a more responsive output power  325  is shown with respect to input power  305 .  
         [0045]    Referring to FIG. 5 we see a separation-varying customized pulse train  330 , resulting in an average output current  340 . This customized pulse train is shown for the channel-adding scenario described in FIGS.  3 - 5 . The pulses arrive at an initial rate  345  corresponding to supplying a number of channels. The pulses arrive at a final rate  350  corresponding to supplying a final increased number of channels. During the transition  355  the pulse separation varies in a manner related to the discrepancy in gain as described above. In this case the separation alone is responsive, but similarly the pulse width  203  or amplitude  202  may be so, or some combination thereof.  
         [0046]    This pulse train  330  will effectively pump the atoms in the fiber as required to generate the necessary output waveform from the amplifier, even in the face of abrupt changes at the input of the amplifier. In cases where a sudden decrease in current might be needed, the rate of pulses applied can be slowed down, or conversely accelerated when an increased current is needed. The delivery of this energy in pulsed form can provide power saving benefits, and also enables the pumping to be ideally adapted to the gain required, without excess pumping and associated noise generation.  
         [0047]    The pump driving current  200  provides current to the pump source  107  efficiently, so as to avoid overpopulation of excited dopant ions in the higher-energy-band of the fiber. Therefore, reducing ASE (which is a result of unused excited ions in the said higher-energy-band) improves the signal to noise ratio of the amplifier system.  
         [0048]    It should be further understood by those skilled in the art that certain variations and modifications can be made to this rare-earth-doped fiber amplifier system. For example, another waveguide geometry besides fiber may suffice, another rare earth dopant may be compatible, or another type of optical amplifier may be employed. It may be advantageous to incorporate these techniques with a fiber laser. None of these variations deviates from the original scope of the invention.  
         [0049]    The above-described embodiments of the present invention are intended to be examples only. Alterations, modifications and variations may be effected to the particular embodiments by those of skill in the art without departing from the scope of the invention, which is defined solely by the claims appended hereto.