Abstract:
A system and method for spectral conditioning an optical signal. An optical filter has an input for receiving an emitted optical signal and an output providing a filtered optical signal. The filter has a corresponding filter profile which includes a high wavelength skirt at an upper wavelength region of the filter profile. A laser is optically coupled to the optical filter input and emits the emitted optical signal. The laser is controllable to emit the optical signal at a wavelength proximate to the optical filter high wavelength skirt.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
         [0001]    n/a  
         STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
         [0002]    n/a  
         FIELD OF THE INVENTION  
         [0003]    The present invention relates to fiber optic communications, and in particular to an system and method for spectrally conditioning the output of a fiber optic directly modulated laser transmitter to reduce the effects of dispersion and increase the span length between network elements.  
         BACKGROUND OF THE INVENTION  
         [0004]    The proliferation of computing and networkable devices has created a need for increased bandwidth between locations, whether those locations are local, regional, national, or international. A technology extremely well suited to supporting high data rates over long distances is fiber optic communications. Typically, a fiber optic communication link includes a fiber optic transmitting device such as a laser, a fiber optic cable span, and a light receiving element. Fiber optic transmitters and receivers are typically quite extensive. As such, there is a desire to be able to increase the span length, i.e. increase the distance between network end points. However, the adverse effects of noise, attenuation and dispersion limit the distance between network elements. This impact is particularly seen as transmission rates increase, because as transmission rates increase, the sensitivity of the system to noise and dispersion also increases, effectively further limiting the span length as data rates increase. It is therefore desirable to have a method and system which increases dispersion-limited distance and permits the use of less expensive lasers in long distances.  
           [0005]    [0005]FIG. 1 is a graph generally showing light output as a function of current input into a laser. As shown in FIG. 1, there exists a knee  10  at which point the slope increases, i.e. light output increases at a greater rate for a given amount of current input into the laser than at points below the knee  10 . It is therefore desired to operate a laser at a point just above knee  10  such that for a small amount of current, light output increases in an amount sufficient for a receiver to be able to detect a light existence, i.e. “1 bit” condition from a light off, i.e. “0 bit” condition. In operation, the light signal level for a “0 bit” is just above the knee and the light signal level for a “1 bit” is at the rated power output of the laser. The ratio of the “on” to “off” light for a 1 and 0 is referred to as an extinction ratio. It is desired to have a large extinction ratio number.  
           [0006]    For directly modulated (“direct mod”) lasers, operating the last above knee  10  reduces the optical noise and signal distortion. However, the trade-off is extinction ratio which shows up as a sensitivity penalty at the receiver. As such, there is a trade-off between the distortions and noise caused by the high extinction ratio at or below knee  10  and a low extinction ratio receiver penalty. Further, operating a direct mod laser below the knee  10  results in unwanted noise, referred to as chirp.  
           [0007]    Section  2 - 2  in FIG. 1 corresponds to knee region  12  and is shown in exploded view in FIG. 2. As shown in FIG. 2, knee region  12  is sub-divided into six sub-regions labeled a, b, c, d, e, and f, respectively. As is shown in FIG. 2, the slope of each successive sub-region increases. The relationship between the increasing slope and sub-regions a-f is explained with reference to FIG. 3. FIG. 3 is a chart showing optical spectrum emitted for each bias environment depicted in FIG. 2. As shown in FIG. 3, the laser, when operating in sub segment f, has a high intensity about the laser wavelength p. This high intensity allows the receiver to clearly discern that a “1” has been transmitted. As sub-regions along the knee are traversed, the intensity decreases, and the spectrum of light emitted by the laser increases. The result is a dispersion in the energy transmitted by the transmitting laser as detected by the receiver, and further results in unwanted noise, i.e. spectral content far removed from point p. The resulting impact is that this unwanted noise, i.e. chirp, adversely impacts the transmission capabilities of the system.  
           [0008]    Another factor which limits span distance and which is exacerbated by the existence of chirp is fiber dispersion. The wider the spectral output of the laser, the more differentiation in the dispersion of the fiber at the receiver. In other words, the wider the spectrum at the transmitting end, the more penalty is paid at the receiving end. As shown in FIG. 4, there are three main types of dispersion known to those of skill in the art. Multi-path (multi-modal) dispersion is illustrated in fiber  14 . Chromatic dispersion is illustrated in fiber  16  and polarization mode dispersion is shown in fiber  18 . Multi-path dispersion and polarization mode dispersion are not directly relevant to the subject invention and their discussion is therefore omitted.  
           [0009]    Chromatic dispersion, shown in fiber  16 , results from a characteristic in which different wavelengths of light travel at different velocities in a fiber optic cable. As a result, a wider spectral content results in a wider differentiation in arrival times of the light pulses, thereby causing intersymbol interference. For example, referring to fiber  16 , a pulse transmitted at a given point which has a non-narrow spectral content results in a portion of the spectral content arriving at point x in a given time t, while other spectral portions of the same transmission only travel to point y in time t.  
           [0010]    Eye diagrams  20   a ,  20   b , and  20   c  show the adverse effects of the various types of distortion along a fiber optic cable. These effects are shown by the decrease in eye  22   a ,  22   b , and  22   c  sizes along the distance of the fiber. The wider the eye, the easier it is for a receiver to detect the absence or presence of a bit. However, the longer the fiber, the more dispersion and the narrower the eye. Further, the shorter the bit period, the faster the effect impacts the receiver. As such, reducing the effects of dispersion along a fiber results in a wider eye, making reception easier. One way to accomplish this is by tightly controlling the transmission to, for example, reduce the effects of dispersion more effectively limiting the light spectrum transmitted by the laser. This can be accomplished by controlling chirp.  
           [0011]    Chirp controlling technologies are expensive and are presently addressed by electrical regeneration, dispersion-compensating modules, or by generating a clean pulse shape. Regeneration is inefficient, because it requires the addition of network components due to limiting span length. Dispersion-compensating modules waste optical power and often require the addition of optical amplifiers. A clean pulse shape can be generated, thereby controlling chip by using externally-modulated lasers. However, externally-modulated lasers are larger in size than their directly-modulated counterparts and are significantly more expensive. It is desirable to have an arrangement which controls chirp, thereby reducing the effects of fiber dispersion in a manner which allows the use of an inexpensive directly-modulated laser without the need for additional external components such as dispersion-compensating modules, light-regenerating devices, and the like.  
           [0012]    Standards such as those issued by the International Telecommunications Union (“ITU”) specify a grid which includes standard light wavelengths for different transmission bit rates. The grid sets forth center optical frequencies for a band pass filter mask inside of which the transmission frequencies must reside. This band pass filter mask becomes particularly important due to frequency drift experienced by lasers as they age as well as a change in the characteristics in filter/wavelength division multiplexing (“WDM”) coupling devices used to facilitate fiber optic communications.  
           [0013]    Because the wavelength of light emitted by a laser is a function of the temperature of the laser, prior art devices have attempted to control the emitted light wavelength by monitoring the temperature of the laser using a device such as thermistor and heating or cooling the laser, as necessary, to attempt to maintain a fixed frequency. In this manner, manufacturers have attempted to provide fiber optic transmission systems which remain within the ITU grid during the operating life of the laser. These methods have finite precision and do not take into account the aging characteristics of the filter. Accordingly, it is desirable to have a method and system which provides for transmission of a specific wavelength in order to comply with known standards in a manner which is accurate despite the changes in performance characteristics of the filter.  
         SUMMARY OF THE INVENTION  
         [0014]    The present invention advantageously provides a method and system which reduces the spectral output at a transmitter so that only the required spectrum is transmitted into the fiber optic cable. In order to reduce the spectral output, the present invention reduces chirp and locks the wavelength output of the emitted transmission laser light with respect to the filter edge. The effect is to minimize the dispersion penalty and allow an increase in network span. In addition, the present invention advantageously allows the use of inexpensive direct-mod lasers. Further, laser temperature control is not based on thermally-sensing laser temperature, but is instead based on the wavelength of the emitted transmission laser light.  
           [0015]    According to an aspect of the present invention, an optical spectral conditioning system has an optical filter and a laser. The optical filter has an input for receiving a first optical signal, an output which provides a filtered optical signal and a filter profile. The filter profile includes a high wavelength skirt at an upper wavelength region of the filter profile. The laser is optically coupled to the optical filter input and emits the first optical signal. The laser is controllable to emit the first optical signal at a wavelength proximate to the optical filter high wavelength skirt.  
           [0016]    According to another aspect, the present invention provides a method for spectrally conditioning an optical signal emitted by a laser and filtered by an optical filter optically coupled to the laser and having a characteristic filter profile including a high wavelength skirt at an upper wavelength region of the filter profile. The method includes controlling the laser to emit an optical signal at a wavelength proximate to the optical filter high wavelength skirt.  
           [0017]    According to still another aspect, the present invention provides an optical spectral conditioning system, in which an optical filter has an input for receiving an emitted optical signal, an output providing a filtered optical signal and a filter profile. The filter profile includes a high wavelength skirt at an upper wavelength region of the filter profile. A directly modulated laser is optically coupled to the optical filter input and emits the emitted optical signal. The directly modulated laser is operable to emit the emitted optical signal at a wavelength proximate to the optical filter high wavelength skirt.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]    A more complete understanding of the present invention, and the attendant advantages and features thereof, will be more readily understood by reference to the following detailed description when considered in conjunction with the accompanying drawings wherein:  
         [0019]    [0019]FIG. 1 is a prior art graph generally showing light output as a function of current input into a laser; and  
         [0020]    [0020]FIG. 2 is an enlarged view of section  2 - 2  in FIG. 1.  
         [0021]    [0021]FIG. 3 is a prior art chart showing optical spectrum emitted for each bias environment depicted in FIG. 2;  
         [0022]    [0022]FIG. 4 is a prior art diagram illustrating types of dispersion;  
         [0023]    [0023]FIG. 5 is an exemplary filter profile graph;  
         [0024]    [0024]FIG. 6 is a graph of simulated optical power versus optical frequency without implementing the present invention;  
         [0025]    [0025]FIG. 7 is a graph of simulated optical power versus optical frequency in which the wavelength is locked to the high wavelength side of the filter skirt;  
         [0026]    [0026]FIG. 8 is a diagram of an exemplary embodiment of a system constructed in accordance with the principles of the present invention;  
         [0027]    [0027]FIG. 9A is a graph of optical power and wavelength emitted by a laser constructed in accordance with the principles of the present invention; and  
         [0028]    [0028]FIG. 9B is graph of a resultant output of a filter which is provided to a fiber optic link constructed in accordance with the principles of the present invention.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0029]    To aid in the understanding of the present invention, the theory of operation is discussed first, followed by a description of an exemplary hardware embodiment which performs the inventive functions. FIG. 5 is a filter profile graph. The filter profile graph includes filter profile  24  and a plurality of superimposed intensity to wavelength traces  26 ,  28 , and  30  for a simulated 10 kilometer fiber optic cable,  40  kilometer fiber optic cable, and 80 kilometer fiber optic cable, respectively. As shown in FIG. 5, filter profile  24  represents a band pass filter whose center wavelength is approximately 1538.98 nanometers. Filter profile  24  may correspond to an ITU grid filter profile.  
         [0030]    Graph lines  26 ,  28 , and  30  represent measures of a receiver&#39;s performance, i.e. the ability to accurately recover a received signal. In the case of the specific graph lines shown in FIG. 5, a bit error rate of 10 −10  is assumed. As such, the smaller the intensity in the “y” axis, the better the ability of the receiver to recover the received signal. As can be seen in FIG. 5, for each simulated fiber length, the optimal receiver performance occurs toward the right skirt of filter profile  24  (the larger wavelengths). In the case of the example shown in FIG. 5, optimal receiver performance is exhibited within hatched region  32 . In other words, the optimum part of the filter is not at the center frequency or the lower frequency, but rather the optimum part of the filter is toward the right skirt. As such, locking the optical signal to the filter so that the transmit environment is working in hatched region  32  advantageously increases the dispersion limit of the network and allows for the recovery of the dispersion penalty paid when additional span length is implemented. FIGS. 6 and 7 are graphs illustrating the advantageous results provided by the present invention in reducing chirp when the optical wavelength is locked to the high wavelength side of the filter profile. FIGS. 6 and 7 each show optical power versus optical frequency relative to a particular frequency “x” which for example, can be a desired center laser frequency such as 194.77 terahertz. However, any center optical frequency can serve to illustrate the performance of the subject invention.  
         [0031]    [0031]FIG. 6 shows simulated optical power versus optical frequency without implementing the present invention, i.e. without locking the wavelength to the high wavelength skirt of the profile. As is shown in FIG. 6, graph region  34  shows a significant amount of chirp in the 8 to 15 gigahertz region. Conversely, FIG. 7 shows a simulation of the same exemplary system in which the wavelength is locked to the high wavelength side of the filter skirt. As is seen in FIG. 7, no chirp is exhibited. The performance increase shown in FIG. 7 allows a larger fiber optic span length because the reduction of chirp minimizes adverse effects of dispersion.  
         [0032]    An exemplary embodiment of a system constructed in accordance with the principles of the present invention is shown in FIG. 8 and designated generally as  36 . System  36  includes laser  38  coupled to optical filter/wavelength division multiplexor (“WDM”)  40  via optical link  42 . Laser  38  is also optically coupled to back facet monitor (“BFM”)  44  via optical link  46 . Tap  48  diverts a small amount of optical power from the output of filter  40  and is optically coupled to monitor pin  50  via optical tap link  52 . Fiber optic cable  54  provides an optical link to other devices, such as a transmitter at the remote end of the link (not shown). Thermolelectric cooler (“TEC”)  56  is thermally coupled to laser  38 . Because the wavelength of light emitted by a laser can be controlled and adjusted by varying the temperature of the laser, TEC  56  can be used to control and adjust the temperature thereby controlling and adjusting the resultant light wavelength emitted by laser  38 . TEC  56  can be any suitable thermal electric cooler such as a Peltier device.  
         [0033]    The thin lines in FIG. 8 represent electrical signal lines which electrically couple BFM  46 , monitor pin  50 , and TEC  56  to comparator  58 . As is explained below in detail, by comparing the wavelength emitted by laser  38  using BFM  44  with the actual filtered wavelength observed by monitor pin  50 , TEC  56  can be controlled to cause laser  38  to operate in the high wavelength side of the filter skirt. In other words, by dithering TEC  56 , the wavelength can be locked to the high wavelength side of the filter wall. In the case of the subject invention, dither is a small electric current applied to TEC  54  to adjust the wavelength of laser  38 . In this manner, electrical current can be translated into a corresponding wavelength.  
         [0034]    Laser  38  is preferably a directly-modulated laser. Use of a directly-modulated laser advantageously reduces component and system costs. Filter  40  can be a thermally-sensitive or athermal device. Although filter  40  is shown as having only a single input from optical link  42 , it is understood that filter  40  supports other optical link inputs which are wavelength division multiplexed and output to fiber optic cable  54 . Of note, because the present invention locks the optical wavelength from laser  38  to the filter wall, thermally-sensitive devices can be used. Reference to thermally-sensitive devices with respect to filter  40  refers to temperature dependency in which the filtering characteristics of the device vary as the temperature changes. Also, because most DWDM systems already include a filter such as filter  40 , the present invention advantageously allows the use of these existing filters.  
         [0035]    Tap  48  is any optical tap suitable for deferring a small portion of the optical power to an output tap. BFM  44  measures the amount of light coming out of the back facet of laser  38 . As is understood by those of skill in the art, measuring the amount of light emitted by the back facet of laser  38  allows a calculation which yields the amount of light coming out of the front facet of lasers  38 , i.e. the amount of light input into optical link  42 . BFM  44  includes an output which electrically corresponds to the measured power. It is this output which is input into comparator  58 . BFM  44  is typically co-packaged in the laser package.  
         [0036]    As with BFM  44 , monitor pin  50  is used to detect a quantity of light and represent that detected quantity electrically. The electrical output of monitor pin  48  is provided as an input to comparator  58 .  
         [0037]    Comparator  58  is any electrical device capable of receiving an electrical signal from monitor pin  50  and BFM  44 , comparing the two signals and creating an output signal provided to dither TEC  56 . As such, comparator  58  compares the output of BFM  44  to determine what signal should be output by laser  38  with the output from monitor pin  50  and outputs the compared result to TEC  56  to adjust the temperature of laser  38  such that the output of laser  38  provides a wavelength at the filter wall. An advantageous result is that if the filter characteristics move, the laser moves with it. Because comparator  58  is comparing a signal based on the output of filter  40 , any drift or degradation of filter  40  based on temperature or aging is taken into account when deriving the output signal to control TEC  56 . In other words, any drift in the filter skirt is taken into account by comparator  58 . The iterative comparative feedback process therefore advantageously establishes, detects, and maintains operation at the filter wall. It is also contemplated that the present invention can be implemented without BFM  44  by instead comparing the signal from tap  48  with the input to TEC  56 .  
         [0038]    Exemplary results of the system shown in FIG. 8 are described with reference to FIGS. 9A and 9B, both of which are graphs showing optical power versus wavelength. FIG. 9A represents a graph of optical power and wavelength emitted by laser  38 . As can be seen, there is a significant positive chirp component at wavelengths greater than desired optimal emitted wavelength shown as peak  58 . This chirp region is shown within area  60 . Filter profile  62  is superimposed on the output graph for laser  38 . Filter profile  62  corresponds to the profile of filter/WDM  40 . Filter profile  62  is a band pass filter.  
         [0039]    The resultant output of filter  40  which is provided to fiber optic link  54  is shown in FIG. 9B. As can be seen, by locking the wavelength output of laser  38  to the other wavelength portion of the filter profile for filter  40 , area  60  is not within the band pass region of the filter and is thereby filtered out. The result is a refined optical power peak  58  with significantly minimized chirp present on the outgoing signal.  
         [0040]    As is discussed above, a traditional implementation using a direct mod laser and filter which must comply with a standard such as the ITU standard implies that the designers must make sure that the laser wavelength does not wander and also that the filter characteristics do not wander or account for any wander such as filter wander based on aging. In the case of the present invention, a thermal electric cooler is used to make sure that the laser does not wander with respect to the filter. In other words, even if the filter characteristics wander, the laser output is changed to correspond to the amount of filter wander such that the laser wavelength always operates at the higher wavelength side of the filter skirt. In other words, the present invention operates at the edge of the filter so the aging characteristics of the filter do not need to be controlled nor even specified in order to account for filter wander based on aging or temperature changes.  
         [0041]    Put another way, in the prior art, the designer had to know the filter characteristics because they are a reference which dictated laser performance. As such, many designers used athermal filters because their characteristics were more predictable and allowed for system design which could be accurately predicted to stay within a particular range, i.e. comply with known standards. Although use of athermal filters is possible with the present invention, it is not necessary because laser output is tied to the changes in filter characteristics. The present invention, therefore, makes system design easier, because by locking the wavelength to the filter one need only control the filter specifications to make sure that the filter will operate within the desired specifications, and not the filter and laser. The result is that the present invention obviates the need to use wavelength lockers to lock laser output to a particular wavelength.  
         [0042]    The present invention advantageously provides a method and system which allows the spectral output of a transmitting laser to be conditioned to reduce chirp and wander by locking the output of the direct mod laser to the higher wavelength region of its corresponding filter. Two advantages result. First, is that the present invention reduces chirp. Second, the present invention reduces system sensitivity to changes in individual component characteristics due to thermal changes and aging.  
         [0043]    The present invention advantageously allows the use of relatively inexpensive direct mod lasers. The present invention is operable using thermally-sensitive filters and athermally-sensitive filters, although athermally-sensitive filters are preferred if all other specifications such as cost are equal. However, the present invention advantageously allows the use of thermally-sensitive filters as compared with expensive athermally-sensitive filters to decrease noise output by the transmitting system and thereby increase the span length possible between transmitting and receiving fiber optic elements.  
         [0044]    Although an embodiment of the present invention is described with the use of directly modulated lasers, it is understood that the present invention can also be implemented using other modulation types such as externally modulated lasers.  
         [0045]    It will be appreciated by persons skilled in the art that the present invention is not limited to what has been particularly shown and described herein above. In addition, unless mention was made above to the contrary, it should be noted that all of the accompanying drawings are not to scale. A variety of modifications and variations are possible in light of the above teachings without departing from the scope and spirit of the invention, which is limited only by the following claims.