Abstract:
An apparatus and method for thermally tuning an optical amplifier comprises an optical waveguide doped with a fluorescent material, a thermal device for either heating or cooling the optical waveguide, and a pump light for exciting the fluorescent material. The apparatus shapes, shifts, and/or flattens the gain curves of the doped optical amplifier. Thulium doped fiber is cooled to shift the gain curve into the C-band. Erbium doped fiber is heated to flatten the gain curve in the C-band and is cooled to shift the gain curve above the L-band. The apparatus similarly shapes the gain curves of other fluorescent materials. The thermal device comprises three types of optical cooling devices. The apparatus is a component in communications systems, lasers, medical lasers and the like. The method comprises either heating or cooling optical waveguides doped with fluorescent materials to achieve the desired shaping, shifting, and flattening of the gain curves.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to optical systems and, in particular, to apparatus and methods for tuning optical amplifiers and for cooling optical waveguides.  
           [0003]    2. Technical Background  
           [0004]    There is considerable interest in the field of optics, particularly relating to the area of telecommunication systems. Optical fibers are the transmission medium of choice for handling the large volume of voice, video, and data signals that are communicated over both long distances and local networks. Much of the interest in this area has been spurred by the significant increase in communications traffic, which is due, at least in part, to the Internet. Consequently, much interest is directed to increasing the data handling capacities of the optical fibers that comprise the communications networks. It is also of interest to increase the capacities of currently installed optical fibers and thereby eliminate the need for installing additional optical fiber cables. One way of increasing the capacity of optical fibers is to increase the number of channels or signals that are transmitted through each optical fiber. However, there are several technical difficulties with increasing the number of channels.  
           [0005]    One limitation is that only certain wavelengths of optical signals can be efficiently transmitted through an optical fiber. For a typical optical fiber, the most efficient wavelengths (i.e. those with low loss per unit length) are in the S-band, C-band and L-band which comprise wavelengths from approximately 1460 nm through 1630 nm. Signals having wavelengths outside of this range suffer increased signal loss and are therefore cannot be transmitted efficiently along optical fibers.  
           [0006]    Another limitation is related to amplifying the signals in the S, C, and L -bands when the signals are transmitted for long distances. Although modern optical fibers have very low loss per unit length, they nonetheless require periodic amplification of the transmitted signal to ensure accurate reception at a receiver. Initially, electronic amplifiers were used to amplify these optical signals. Electronic amplifiers converted the optical or light signals into electrical signals, amplified the electrical signals, and then retransmitted the signals as light signals. This was an expensive and inefficient way to amply optical signals.  
           [0007]    More recently, erbium doped optical amplifiers have revolutionized optical telecommunications by providing all optical high-gain, low-noise amplification over many channels without the need for the costly electronic repeaters. Optical amplifiers comprise a length of optical waveguide or fiber that is doped with a suitable fluorescent material such as erbium ions or thulium ions. A pump light source is injected into the doped fiber to excite the fluorescent ions. When a light signal is transmitted through the doped fiber, the excited ions release their energy at the same wavelength as the signal and thereby produce a highly amplified signal.  
           [0008]    Unfortunately, erbium doped fiber amplifiers are primarily useful only in the C-band and, to a lesser extent, in the L-band. As bandwidth demand increases, the need to amplify signals outside of the conventional erbium doped fiber amplifier (EDFA) range increases. In the quest to expand the bandwidth of optical amplifiers, the prior art has attempted to develop exotic doping materials to create an improved optical amplifier. The prior art has also experimented with various complex light pumping schemes to improve the performance of optical amplifiers.  
           [0009]    Yet another challenge confronting the industry are the increasingly tough specifications for amplifiers. Gain shaping, gain flattening, and gain tilt compensation are becoming more and more important as ripple and bandwidth specifications get tougher and wider.  
           [0010]    The continuing challenge is to find an apparatus and method to increase the bandwidth of optical amplifiers and shape and flatten the gain of amplifiers so that the transmission capacity of optical fibers can be increased.  
         SUMMARY OF THE INVENTION  
         [0011]    To address the problems discussed above, the invention discloses an apparatus and method for thermally tuning an optical amplifier which comprises: an optical wave guide doped with a fluorescent material, a thermal device for either heating or cooling the optical wave guide, and a pump light source for exciting the fluorescent material.  
           [0012]    It has been discovered that by changing or controlling the temperature of a doped optical amplifier, the gain curve of the fiber can be shifted or shaped over various wavelengths. Accordingly, the invention uses this discovery to shape, shift, and/or flatten the gain curves of doped optical amplifiers to achieve the desired results. For example, thulium doped optical fiber is cooled to shift the gain curve into longer, more useful wavelengths. Similarly, erbium doped optical fiber is cooled to shift the gain curve to longer wavelengths above the L-band. As yet another example, an erbium doped optical fiber is heated to flatten the gain curve in the C-band. The invention may also be used to shape the gain curves of other fluorescent materials used in optical amplifiers.  
           [0013]    In order to achieve the cooling of the optical amplifiers, three optical cooling devices are disclosed. The first optical cooling device comprises an optical fiber (cooling fiber) doped with an appropriate fluorescent refrigerant material and coupled with a pump light source. The cooling fiber is co-wound with an amplifying fiber and the cooling fiber is cooled as the pump light is activated. The amplifying fiber is cooled by the cooling fiber due to their close proximity.  
           [0014]    A second embodiment comprises doping the cladding of the optical amplifier fiber with a fluorescent refrigerant such as ytterbium. A pump light source is coupled with the cladding to cause the cladding to cool and thereby cause the core of the fiber amplifier to cool. An additional advantage of using this embodiment is that some fluorescent refrigerants, such as ytterbium, produce pump light as a by-product. This pump light may then be used to pump the fluorescent material in the core.  
           [0015]    A third embodiment comprises an optical fiber amplifier having two cores. A first core is doped with a fluorescent material suitable for amplifying an optical signal. The second core is doped with a fluorescent refrigerant material. When pump the is applied to the second core, the core cools and also causes the nearby first core to cool.  
           [0016]    The apparatus is useful in not only in telecommunication systems, but also in laser printers, laser marking, medical imaging, microsurgery, scientific research and development, and the like. It follows that the method of the invention comprises either heating or cooling an optical waveguide doped with fluorescent material to achieve desired shaping, shifting, and flattening of the gain curves.  
           [0017]    It is to be understood that the foregoing description is exemplary of the invention only and is intended to provide an overview for the understanding of the nature and character of the invention as it is defined by the claims. The accompanying drawings are included to provide a further understanding of the invention and are incorporated and constitute part of this specification. The drawings illustrate various features and embodiments of the invention which, together with their description serve to explain the principals and operation of the invention. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]    [0018]FIG. 1 illustrates a fluorescence curve for thulium which is representative of the gain curve of thulium;  
         [0019]    [0019]FIGS. 2 a  and  b  illustrates various Stark energy level transitions when thulium at different temperatures;  
         [0020]    [0020]FIG. 3 illustrates the population distribution among the nine Stark sublevels at various temperatures;  
         [0021]    [0021]FIG. 4 illustrates the quantum efficiency of thulium ions over temperature;  
         [0022]    [0022]FIG. 5 illustrates the fluorescence curve for erbium which is representative of the gain curve for erbium;  
         [0023]    [0023]FIG. 6 is a diagram of the gain of an exemplary amplifier, at 0° C. and 70° C. and pump laser diode operating at current of 105 mA and 110 mA;  
         [0024]    [0024]FIG. 7 is a diagram of amplifier&#39;s Gain Tilt at 0° C. and 70° C. of an exemplary amplifier with the pump laser diode operating at a current of 105 mA and 110 mA;  
         [0025]    [0025]FIG. 8 is a schematic diagram of the preferred embodiment of the invention;  
         [0026]    [0026]FIG. 9 is a schematic diagram of the invention heating an optical amplifier;  
         [0027]    [0027]FIG. 10 is a schematic diagram of the invention embodied in a fiber optic laser;  
         [0028]    [0028]FIG. 11 is a schematic diagram of the invention comprising a co-wound optical cooling waveguide;  
         [0029]    [0029]FIG. 12A is a schematic diagram of the invention comprising fluorescent refrigerant doped in the amplifying fiber;  
         [0030]    [0030]FIG. 12B is a cross section of an amplifying fiber doped with fluorescent refrigerant in the cladding;  
         [0031]    [0031]FIG. 12C is a cross section of an amplifying fiber having a second core doped with fluorescent refrigerant; 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0032]    The invention entails controlling the temperature of the optically active gain medium (e.g. the fluorescent material) in an optical amplifier to increase the performance and utility of the device. A brief description of the scientific discoveries that led to the invention is first presented.  
         [0033]    [0033]FIG. 1 illustrates the fluorescence spectrum of the thulium Tm 3+  as a function of temperature in 50 degree Kelvin increments. Since the Tm 3+  emission is a four level transition, the fluorescence curve is indicative of the gain curve of a thulium-doped amplifier. At normal room temperature, much of the gain curve is in the undesirable spectrum below 1500 nm. However, as the temperature is decreased, the fluorescence intensity on the red side (i.e. longer wavelengths) of the spectrum, in the desirable 1500 nm region, increases at the expense of the useless fluorescence at 1400 nm. The gain shape also changes considerably with temperature and this is used to decrease gain ripple. This can be seen as the relatively flat portion of the gain curve between about 1460 nm and 1510 nm. Since this temperature dependent characteristic is observed in thulium and erbium it is also predicted that similar advantageous gain curve changes will be present in other fluorescent materials and particularly other rare earth elements.  
         [0034]    Rare earth energy levels are split into 2J+1 Stark sublevels, so the Tm 3+3 H 4 → 3 F 4  transition at 1460 nm has 9 sublevels in the initial state and 9 sublevels in the terminal state. Thus the 1460 nm transition is actually made up of 81 individual transitions and is why this emission is so broad and non-gaussian. The population distribution among the nine sublevels is dependent on temperature and is proportional to e − Δ E/kT  where ΔE is the energy difference between the lowest Stark level and the level of interest, k is Boltzmann&#39;s constant, and T is absolute temperature. FIGS. 2A and 2B show the relative populations of the sublevels and the transitions between the Stark levels. FIG. 2A illustrates the 81 possible transitions at room temperature. Conversely, FIG. 2B illustrates the four transitions possible at cold 10 degrees Kelvin. The seven small dashes are representative of the Stark levels, but each of these levels are essentially empty at low temperatures.  
         [0035]    Referring to FIG. 3 there is illustrated the relative population distribution of the thulium  3 H 4  Stark levels at various temperatures. At low temperatures (e.g., 10 degrees Kelvin) the population is entirely confined to the two lowest lying Stark components and as the temperature increases there is enough thermal energy to distribute the population to the higher lying Stark components. Thus as the temperature is decreased, the population among the initial level shifts to the lowest lying states resulting in a red shifting of the fluorescence since the ions are starting in a lower energy state. Likewise, the absorption bands will blue shift to higher energies as the temperature is decreased since the ions are starting from lower Stark levels making the absorption energy gap effectively larger.  
         [0036]    Turning to FIG. 4, the increase in the Tm 3+  fluorescence lifetime of the 1460 nm transitions is illustrated as the temperature is decreased. Non-radiative decay mechanisms which reduce the radiative efficiency of rare earth (RE) ions are less probable at lower temperatures, so the quantum efficiency increases as the temperature is decreased. Thus it is advantageous to cool an amplifier for increased efficiency as well as wavelength tuning and shaping.  
         [0037]    These characteristics are not limited to Tm 3+ . It is predicted that many, if not all, optically active ions and molecules will exhibit these same or similar thermal trends. For example, FIG. 5 shows the fluorescence spectrum of an erbium ion (Er 3+ ) doped fiber as a function of temperature. As the fiber is cooled from high temperature, the fluorescence red shifts toward longer wavelengths. This is advantageous for an L-band amplifier which normally would operate on the weaker erbium fluorescence in the 1580 to 1610 nm region. Cooling the erbium fiber, for example to −200 degrees C., can therefore produce approximately a 10-20% gain advantage over higher temperature fiber. In particular, the gain advantage in the region of 1600 to 1630 and especially 1610 nm to 1630 nm is particularly useful in extending the bandwidth of the erbium doped fiber amplifier (EDFA).  
         [0038]    While cooling is advantageous in the L-band, heating is advantageous for gain flattening in the C-band. As shown in FIG. 5, as the fiber is heated to 800 degrees C., the fluorescence intensity variation in the C-band (approximately 1520 nm to 1560 nm) is diminished. However, the changes in absorption should also be taken into account since this is a 3-level system and ground state absorption will also be important in determining the gain shape.  
         [0039]    When signal channels entering an optical amplifier are either added or dropped, the overall inversion of the optically active ions is changed, giving rise to a change in gain as illustrated FIG. 6. The absolute gain and gain tilt increase when the inversion increases and decrease when inversion decreases. To compensate for this gain tilt, when channels are added or dropped, the inversion of the amplifier must be changed by increasing or decreasing the pump power to restore the gain shape. However this can change the absolute gain of the amplifier which then must be compensated by a variable optical attenuator or VOA. VOA&#39;s are mechanical deices that intentionally waste optical power VOAs have a slow response time and add expense to the cost of the amplifier. However, the gain tilt problem can be solved by using temperature control to compensate for gain variations (including gain tilt) caused by added or dropped channels. This is illustrated in FIG. 7 and is described below.  
         [0040]    Consider an amplifier operating at 70° with 110 mA of pump power. (The gain ripple would be compensated by a filter to make the gain from 1530 to 1560 flat). The pump current is then decreased by 5% to 105 mA to simulate the adding of a channel, resulting in decreased gain and tilt (both undesirable). FIG. 6 shows that by cooling the amplifier fiber to 0° C. the gain is partially restored and the gain tilt or gain ripple is reduced. FIG. 7 shows the differences in the gain as a result of these changes. The 5% power fluctuation cause a 0.9 dB drop in the gain and 0.6 dB of ripple (variation). However by reducing the temperature, the ripple is decreased by a factor of 2 to 0.3 dB.  
         [0041]    Referring now to FIG. 8 there is illustrated a schematic diagram of a preferred embodiment of the invention as a thulium doped fiber amplifier (TmDFA)  60 . A light signal  61   a  is coupled into fiber  62  from light signal source  61 . Signal source  61  is typically a telecommunications network or some component thereof. Light signal  61   a  preferably has wavelengths in the range of 1460 nm to about 1540 nm and more preferably in the range of the S-band or 1485 nm to 1525 nm.  
         [0042]    Light signal  61   a  is transmitted through fiber  62  to amplifying fiber  63 . The symbol indicating amplifying fiber  63  is a loop since the doped optical fibers are typically coiled due to space limitations. Amplifying fiber  63  is preferably doped with either erbium or thulium, however the invention may be practiced with most any fluorescent material including the family of rare earth elements such as: cerium (Ce), praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), ytterbium (Yb).  
         [0043]    In another aspect of the invention the amplifying fiber  63  may be a planar waveguide or, in general, any type of optical waveguide.  
         [0044]    Pump light source  64  provides pump light  64   a  for exciting the erbium ions in amplifying fiber  63 . Light source  64  is preferably a semiconductor laser operating at about 980 nm or 1480 nm wavelengths. However, various other sources of pump light may also be used. Pump light  64   a  is coupled to fiber  62  via optical coupler  62   a.    
         [0045]    Cooling of the amplifying fiber  63  is accomplished by cooling device  65 , which is preferably a thermal electric (TE) cooler. Amplifying fiber  63  is wound or coiled around the TE cooler  65 . In an alternate embodiment, the amplifying fiber  63  is wound around a thermally conducting material, such as aluminum, which is thermally coupled to the TE cooler  65 . TE coolers are known in the art. Recent advances in TE coolers useful to the instant invention. Cooling may be accomplished by virtually any cooling means, including air cooling and liquid cooling. Another cooling option is optical or fluorescent cooling which is described in detail below.  
         [0046]    The cooling of amplifier fiber  63  may be regulated via a temperature sensor  66  in communication with a controller  67 , which regulates cooling device  65 . Temperature sensor is preferably a thermocouple device that generates a temperature signal  66   a  indicative of the temperature of amplifying fiber  63 . Controller  67  is a conventional digital control unit programmed to implement a suitable feedback control algorithm.  
         [0047]    The desired temperature of the amplifying fiber  63  will vary depending on the requirements and specifications of a particular application. Generally the fiber  63  should be cooled below 270 degrees Kelvin. Preferably the fiber  63  is cooled to about 200 degrees Kelvin. More preferably the fiber  63  is cooled to about 150 degrees Kelvin and most preferably the fiber  63  is cooled to about 100 degrees Kelvin. It is also preferred to cool the fiber  63  to about 50 degrees Kelvin.  
         [0048]    Light signal  61   a  is transmitted through amplifying fiber  63  where the signal is amplified manyfold and the amplified signal is output on output fiber  68 .  
         [0049]    Another aspect of the invention is illustrated in FIG. 9 where a heating device  71  heats the amplifying fiber  63 . As discussed above in relation to FIG. 5, the gain curve of an erbium-doped optical fiber will flatten as the temperature increases. The signal source  61 , pump light source  64 , temperature sensor  66 , and controller  67  are identical to the components in FIG. 8.  
         [0050]    Amplifying fiber  63  is preferably heated above 200 degrees C., and more preferably heated to above 400 degrees C., and still more preferably heated above 600 degrees C., and most preferably heated to about 800 degrees C. Heating device  71  is a resistive heating element but may also be any other type of heating device including air heating, radiant heating, inductive heating, or the like.  
         [0051]    A particularly useful application for the invention is in fiber lasers, an exemplary schematic of which is illustrated in FIG. 10. Pump light  64  injects pump light into amplifying fiber  63 . A light signal  61   a  may also be injected by light signal source  61 . The light signal is amplified by amplifying fiber  63  and output to grating  81  which allows a portion of the light to pass and reflecting a portion of the light back into amplifier fiber  63  as is commonly known. A problem with fiber lasers is that it is difficult to obtain laser light of various wavelengths.  
         [0052]    The invention solves this problem my enabling the amplifying fiber  63  to be tuned to a desired wavelength by controlling the temperature. Using the invention, previously difficult to generate laser wavelengths are easily generated. This schematic is shown with a thermal device  82  to indicate that the amplifying fiber  63  may be either heated or cooled to achieve the desired result. In other respects the components function the same as the components in FIGS. 8 and 9.  
         [0053]    An innovative apparatus useful to cool an amplifying fiber or waveguide is illustrated in FIG. 11. Using the principles of optical cooling a device is constructed to cool the amplifying fiber  63 . Recent advances in optical cooling make this method and apparatus viable.  
         [0054]    To describe briefly, an optical cooling device comprises a light waveguide doped with an appropriate fluorescent refrigerant and pumped with an suitable light wavelength. The absorption of thermal energy presumably produces the cooling effect. In FIG. 11, a cooling optical fiber  91  (dashed line) is doped with such a fluorescent refrigerant and co-wound with the amplifying fiber  63 . The cooling pump  92  launches pump light into cooling optical fiber  91 . As fiber  91  cools the co-wound amplifying fiber is also cooled. A temperature sensor and controller may be added to control the cooling process as was done in FIG. 8.  
         [0055]    In another aspect of the invention, the fluorescent refrigerant is doped into the same optical waveguide as the amplifying fiber  63 . This is accomplished in at least two ways. First, as illustrated in FIG. 12B, the fluorescent refrigerant  101  is doped into the cladding or outer layer  103  of amplifying fiber  63 . Light signals  61   a  are transmitted via core  102  while light is pumped through the cladding  103  causing the cladding  103  to cool. As the cladding  103  is cooled, the core  102  is also cooled.  
         [0056]    Secondly, as illustrated in FIG. 12C, the amplifying fiber  63  comprises a second core  105  doped with fluorescent refrigerant. Pump light is injected through the second core  105  causing it to cool and consequently cooling the central core  102 .  
         [0057]    [0057]FIG. 12 illustrates a schematic diagram of these two embodiments. The amplifying fiber  63  and pump light source  64  are identical to those in FIG. 8. Cooling pump  92  is added to inject pump light into either the cladding or second core of amplifying fiber  63  via optical coupling  62   b.    
         [0058]    The method of the invention follows from the description of the apparatus. A light waveguide is provided that is doped with a fluorescent material suitable for being excited by pump light and amplifying a light signal. A pump light source is provided and optically coupled to the light waveguide providing a suitable pump light to excite the fluorescent material. A thermal device, such as a heating device, a cooling device, or optical cooler is thermally coupled to the light waveguide. Finally, a temperature sensor senses the temperature of the waveguide and communicates a temperature signal to a controller, which in turn controls the thermal device to achieve a desired waveguide temperature.  
         [0059]    It will become apparent to those skilled in the art that various modifications to the preferred embodiment of the invention as described herein can be made without departing from the spirit or scope of the invention as defined by the appended claims.