The present invention relates generally to optical communications devices and systems and relates more particularly to a low dispersion filter or interleaver for use in wavelength division multiplexing (WDM) and dense wavelength division multiplexing (DWDM) optical communication systems and the like.
Optical communication systems which utilize wavelength-division multiplexing (WDM) and dense wavelength division multiplexing (DWDM) technologies are well known. According to both wavelength-division multiplexing and dense wavelength-division multiplexing, a plurality of different wavelengths of light, typically infrared light, are transmitted via a single medium such as an optical fiber. Each wavelength corresponds to a separate channel and carries information generally independently with respect to the other channels. The plurality of wavelengths (and consequently the corresponding plurality of channels) are transmitted simultaneously without interference with one another, so as to substantially enhance the transmission bandwidth of the communication system. Thus, according to wavelength-division multiplexing and dense wavelength-division multiplexing technologies, a much greater amount of information may be transmitted than is possible utilizing a single wavelength optical communication system.
The individual channels of a wavelength-division multiplexed or dense wavelength-division multiplexed signal must be selected or separated from one another at a receiver in order to facilitate detection and demodulation thereof. This separation or demultiplexing process can be performed by an interleaver. A similar device facilitates multiplexing of the individual channels by a transmitter.
It is important that the interleaver separate the individual channels sufficiently so as to mitigate undesirable crosstalk therebetween. Crosstalk occurs when channels overlap, i.e., remain substantially unseparated, such that some portion of one or more non-selected channels remains in combination with a selected channel. As those skilled in the art will appreciate, such crosstalk interferes with the detection and/or demodulation process. Typically, the effects of crosstalk must be compensated for by undesirably increasing channel spacing and/or reducing the communication speed, so as to facilitate reliable detection/demodulation of the signal.
However, as channel usage inherently increases over time, the need for efficient utilization of available bandwidth becomes more important. Therefore, it is highly undesirable to reduce communication speed in order to compensate for the effects of crosstalk. Moreover, it is generally desirable to reduce channel spacing so as to facilitate the communication of a greater number of channels.
Filters are typically used within interleavers (and are also used in various other optical devices), so as to facilitate the separation of channels from one another in a wavelength-division multiplexing or dense wavelength division multiplexing system. Various characteristics of such filters contribute to the mitigation of crosstalk and thus to contribute reliable communications. For example, the ability of a filter to separate one optical channel from another or to separate one set of channels from another set of channels is dependent substantially upon width and depth of the filter""s stopband. Generally, the wider and deeper the stopband, the more effectively the filter rejects unwanted adjacent channels and thus the more effectively the filter mitigates crosstalk.
Further, the flatness and width of the filter""s passband is important. The flatness of the filter""s passband determines how much the signal is undesirably altered during the filtering process. A substantially flat passband is desired, so as to assure that minimal undesirable alteration of the signal occurs. The width of the passband determines how far from the ideal or nominal channel center frequency a signal can be and still be effectively selected. A wide passband is desirable because the nominal center frequency of a carrier which is utilized to define a communication channel is not perfectly stable, and therefore tends to drift over time. Further, the nominal center frequency of a filter passband likewise tends to drift over time. Although it is possible to construct a system wherein the center frequency of the communication channel and the center frequency of the filter are comparatively stable, it is generally impractical and undesirably expensive to do so.
Although having a wider filter passband is generally desirable, so as to facilitate the filtering of signals which have drifted somewhat from their nominal center wavelength, the use of such wider pass bands and the consequent accommodation of channel center wavelength drift does introduce the possibility for undesirably large dispersion being introduced into a filtered channel. Typically, the dispersion introduced by a birefringent filter or interleaver increases rapidly as the channel spacing is reduced and as a channel moves away from its nominal center wavelength, as discussed in detail below. Thus, as more channel wavelength error is tolerated in a birefringent filter or interleaver, greater dispersion values are likely to be introduced.
In order to construct a system wherein the center frequency of the communication channel and the center frequency of the filter are comparatively stable, it is necessary to provide precise control of the manufacturing processes involved. Since it is generally impractical and undesirably expensive to provide such precise control during manufacturing, the center frequency of communication channels and the center frequency of filters generally tend to mismatch with each other. Precise control of manufacturing processes is difficult because it involves the use of more stringent tolerances which inherently require more accurate manufacturing equipment and more time consuming procedures. The center frequency of the communication channel and the center frequency of the filter also tend to drift over time due to inevitable material and device degradation over time and also due to changes in the optical characteristics of optical components due to temperature changes. Therefore, it is important that the passband be wide enough so as to include a selected signal, even when both the carrier frequency of the selected signal and the center frequency of the passband are not precisely matched or aligned during manufacturing and have drifted substantially over time.
Birefringent filters for use in wavelength-division multiplexing and dense wavelength-division multiplexing communication systems are well known. Such birefringent filters are used to select or deselect optical signals according to the channel wavelengths thereof. However, contemporary birefringent filters tend to suffer from deficiencies caused by inherent carrier and passband instability due to manufacturing difficulties and due to drifting over time, as discussed above. That is, the passband of a contemporary birefringent filter is not as flat or as wide as is necessary for optimal performance. Further, the stopbands of such contemporary birefringent filters are not as deep or as wide as is necessary for optimal performance. Third, it typically has large dispersion which would introduce significant signal distortion. Therefore, it is desirable to optimize such birefringent filters in a manner which enhances the width of the passband, makes the passband more flat, and which also widens and deepens the stopband. It is desirable to provide a birefringent filter whereby the width of the stopband is roughly equal to the width of the passband, so as to facilitate the efficient separation of equally spaced channels in a wavelength-division multiplexing or dense wavelength-division multiplexing communication system. Further, it is desirable to provide a birefringent filter which possess zero or extremely low dispersion.
Such birefringent filters typically comprise a plurality of birefringent elements placed end-to-end between two polarization selection devices, so as to define a contemporary Solc-type optical filter.
Referring now to FIG. 1, a typical layout of a Solc-type filter is shown. This layout is common to Solc-type filters. This filter comprises an input polarization selection device (e.g., polarizer) 11, an output polarization selection device 12, and a birefringent element assembly disposed generally intermediate the input polarization selection device 11 and the output polarization selection device 12. The polarization axis of the input polarization selection device 11 and the output polarization selection device 12 are typically parallel to one another.
According to contemporary practice, the birefringent element assembly 13 of such a Solc-type filter comprises three birefringent elements or crystals. A first birefringent crystal 15 has a length of L. A second birefringent crystal 16 has a length of 2L. A third birefringent crystal 17 has a length of 2L.
Although such contemporary Solc-type filters are generally suitable for some applications in optical communications, such contemporary Solc-type filters suffer from inherent deficiencies which detract from their overall effectiveness. Such contemporary Solc-type filters are birefringent filters which suffer from high dispersion when the actual channel wavelength is not at the nominal channel center wavelength.
As those skilled in the art will appreciate, dispersion is the non-linear phase response of an optical device or system wherein light of different wavelengths is spread or dispersed, such that the phase relationship among the different wavelengths varies undesirably as the light passes through the device or system. Such dispersion undesirably distorts optical signals, such as those used in optical communication systems.
The nonlinear phase response or dispersion of WDM and DWDM devices is responsible for signal distortion which results in undesired limitations on channel capability. That is, such dispersion undesirably limits the useable bandwidth of a channel, such as that of a fiber optic communication system. Such undesirable limitation of the bandwidth of a channel in a fiber optic communication system inherently reduces the bit rate of digital data transmitted thereby.
Contemporary interleavers have dispersion versus wavelength curves which have zero dispersion value at a particular wavelength, such as at nominal channel center wavelength. The dispersion versus wavelength curve of such contemporary interleavers departs drastically from this zero dispersion value as the wavelength moves away from the nominal channel center wavelength. Thus, small deviations in channel center wavelength can result in undesirably large dispersion values being realized.
Since, as discussed in detail above, it is extremely difficult, if not impossible, to maintain a channel center wavelength precisely at its nominal value, such channel center wavelengths do vary, thereby resulting in undesirably large dispersion values.
The problem of comparatively small differences between actual channel center wavelength and the nominal value thereof causing undesirably large dispersion values can be mitigated by constructing an interleaver having either a dispersion versus wavelength curve which has a value of approximately zero for all wavelengths, or alternatively, by constructing an interleaver having a dispersion versus wavelength curve which does not deviate substantially from a zero dispersion value at least for those wavelengths to which the actual channel center wavelength is likely to drift.
An optical interleaver is one type of comb filter which is commonly used in optical communications systems. Such interleavers have the potential for substantially enhancing performance in future optical communications networks by substantially enhancing bandwidth thereof. Common contemporary interleavers provide channel spacings of 200 GHz and 100 GHz. 50 GHz interleavers are just beginning to emerge in the marketplace. Further reduction of optical channel spacing to 25 GHz, 12.5 GHz and beyond presents substantial technical challenges.
As channel spacing is decreased below 50 GHz, significant and undesirable dispersion appears and can dramatically degrade optical signal quality, particularly in high bit rate optical communication systems. Thus, there is substantial need for techniques and apparatus which mitigate or suppress the dispersion introduced by an interleaver in an optical communication system. More generally, there also exists a similar need for techniques and apparatus which compensate for dispersion in various other devices, such as those commonly used in WDM/DWDM communication systems.
The present invention comprises techniques and apparatus which mitigate undesirable interleaver dispersion. The present invention also provides techniques and apparatus which compensate for dispersion from various different optical devices in an optical communication system.
More particularly, the present invention comprises a zero or low dispersion birefringent filter or interleaver assembly having a first interleaver and a second interleaver. The second interleaver is configured so as to provide a dispersion vs. wavelength curve wherein each dispersion value thereof is approximately opposite in value to a dispersion value at the same wavelength for the first interleaver, so as to mitigate dispersion in the interleaver assembly. In this manner, the dispersion of an interleaver substantially cancels out the dispersion of the other interleaver. In a similar manner, a single interleaver may be utilized to substantially mitigate dispersion in various other optical components in an optical communication system or the like.
These, as well as other advantages of the present invention, will be more apparent from the following description and drawings. It is understood that changes in the specific structure shown and described may be made within the scope of the claims without departing from the spirit of the invention.