Abstract:
A method and apparatus for precision stabilization in optical communication systems, characterized by an optical tapped delay line which resolves multiple wavelength signals having extremely narrow wavelength spacing. The invention has particular utility in future DWDM systems having channel spacing at or below 25 GHz. Laser output wavelengths are alternatively or simultaneously locked, tuned or monitored depending upon the embodiments selected.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention pertains to the field of optical devices, and more particularly to precise wavelength control of laser sources for wavelength division multiplexing (WDM) communications systems.  
         BACKGROUND OF THE INVENTION  
         [0002]    Wavelength division multiplexing (WDM) systems communicate multiple signals through a single optical fiber by utilizing a different optical wavelength for each carrier signal. In the multiplexing process, an information signal is combined with a carrier signal and multiple such combined signals, called channels, are multiplexed into a single optical fiber for simultaneous transmission. Demultiplexing involves the separation of channels into individual data-carrying signals. The International Telecommunications Union has developed standards for WDM with predefined frequencies at channel spacings of 100 GHz (or 0.8 nm). By reducing the channel spacing, increased numbers of data-carrying channels may be added. Because of ever-increasing bandwidth requirements, telecommunications carriers need more channels of information and narrow channel spacings of 25 GHz and below are being intensely studied.  
           [0003]    For a number of reasons, practical systems utilizing 25 GHz and narrower spacing are developing slowly. In particular, maintaining increasingly narrower channel spacing demands extreme precision in the frequency stability from the source laser—a precision that is not reliably achievable. The wavelength of most lasers has a tendency to drift, and if the channel spacing is sufficiently close, crosstalk is introduced as the wavelength of one channel drifts closer to an adjacent channel. Factors such as equipment aging, device tolerances, power source fluctuations, and temperature changes all serve to complicate the problem.  
           [0004]    It is well known that the frequency stability of WDM systems is highly temperature dependent. Temperature changes cause variations in the optical devices that have a direct impact on optical properties, for example, by expanding or contracting a material to alter its physical dimensions or by changing the index of refraction of a material. The likely result is that the frequency of interest “drifts” relative to the target or detector with a corresponding degradation of the signal. Active compensation systems employ heater/coolers to maintain the components at a constant temperature. These devices effectively solve the problem of frequency drift, but at relatively high cost and with a loss of overall efficiency due to the power requirements.  
           [0005]    As a result of this problem, prior art solutions have been found that attempt to eliminate or minimize temperature-induced frequency drift. The various alternative devices to which this type of solution is applied are commonly referred to as wavelength references, wavelength lockers, or wavelength monitors. These devices vary in size, complexity and cost. Among the best performing wavelength lockers, from the standpoint of size, accuracy and cost, are those utilizing etalons.  
           [0006]    A well-known etalon-based optical device for performing wavelength locking is a Fabry-Perot etalon, an example of which is illustrated in FIG. 1. It includes two parallel partially reflective mirrors  20  and  21 . The mirrors are separated by a cavity  22 , which might be an air space or alternatively, a solid transparent material. Light from a spectrally broadband source, i.e., a laser, is input at plane  25 . In particular, a multi-spectral light ray input from point P 1  entering through the partially reflective mirror  20  at an angle θ undergoes multiple reflections between mirrors  20  and  21 . The emerging light rays  26 , having a common wavelength λ, interfere constructively along a circular locus P 2  in the output plane  27  where an appropriate detector might be positioned. The condition for constructive interference that relates a particular angle θ and a particular wavelength λ is given by the formula  
           2 d  cosθ= mλ   
           [0007]    where d is the separation of the reflecting surfaces and m is an integer known as the order parameter. The Fabry-Perot etalon thereby separates the component frequencies of the input light by using multiple beam reflections and interferences.  
           [0008]    When used in a wavelength locker, a portion of the modulated laser output beam is commonly split. One segment of the beam is routed directly to an output while a second segment first passes through the etalon before reaching a detector. Only the single wavelength λ exits the etalon and the device is designed to ensure that λ is the “lock” wavelength. Tuning is commonly achieved by physically rotating the etalon slightly during device fabrication. This position, and hence the lock wavelength of the device, is permanent once construction of the device is completed. Once the device is initially designed and calibrated, it defines a precise fixed relationship between the two signals provided to the detector. Variation of the relationship resulting from laser wavelength changes is monitored and the laser driver input is altered via a feedback loop to minimize detected differences. A more complete description of such a system may be found in the technical article entitled “Wavelength Lockers Make Fixed and Tunable Lasers Precise,”  WDM Solutions,  January 2002, p. 23.  
           [0009]    The Fabry Perot etalon does not serve well as a WDM channelizer due to the difficulty in obtaining high optical throughput efficiency. If the input beam is divergent, e.g., the direct output of an optical fiber, then the output pattern for a given wavelength is a set of rings. Multiple wavelengths produce nested sets of concentric rings. It is difficult to collect this light efficiently and concentrate it at multiple detector points, or couple it to multiple output fibers, especially while maintaining the separation of wavelength components that the etalon has produced. If the input beam is collimated, e.g., the collimated output of an optical fiber, then the beam can be fanned over a narrow range of angles to produce only a single-order output (e.g., m=+1) for each wavelength of interest. This fanning makes it easy to concentrate the output light at multiple detector points or fibers, but there is inherently high loss.  
           [0010]    U.S. Pat. Nos. 5,428,700, 5,798,859 and 5,825,792 to Hall, Colbourne et al, and Villeneuve et al. respectively all reveal laser stabilization systems employing Fabry-Perot etalons of the type described above.  
           [0011]    U.S. Pat. No. 6,345,059 to Flanders describes a highly complex laser wavelength compensation system in which the tuned wavelength is maintained by controlling the optical length of the laser cavity. This disclosure states that the wavelength precision of this system is 0.1 nm accuracy, which equals 12.5 GHz channel spacing.  
           [0012]    U.S. Pat. No. 6,289,028 B1 to Munks, et al. discloses the simultaneous monitoring, stabilizing, tuning, and control of laser source wavelengths with the aid of an error feedback loop. A rotatable optical filter provides wavelength tuning by tilting the filter in accordance with feedback signals.  
           [0013]    An alternative type of wavelength locking is taught in PCT application IPO Number WO 01/35505 A1 to Sappey. A one or two-dimensional array of lasers at different spatial positions within an external resonating cavity illuminates a diffraction grating. Opposing the diffraction grating is either a mirror (in the one-dimensional case) or a second grating (in the two-dimensional case). Light fed back to the lasers causes the laser to lock to the wavelength of the feedback, resulting in each laser lasing as a discrete, well-controlled wavelength. Each channel of a WDM system requires its own stabilized laser.  
           [0014]    Significant channel spacing reductions in WDM systems will require substantial improvements in wavelength stability, with the corresponding precision ability to monitor, tune and lock those wavelengths as needed.  
         SUMMARY OF THE INVENTION  
         [0015]    The present invention, in a preferred embodiment, utilizes unique properties of an optical time delay line (OTDL) to, with high precision, monitor, tune and lock optical wavelengths. It permits passive mechanical compensation of output variations in the wavelength of a laser, due to thermal effects, equipment aging, power fluctuations or other causes. The unique OTDL construction permits a collimated multi-spectral beam to be separated into its constituent wavelengths and detected with high precision. Wavelength drift may be measured by comparison circuitry at the output and feedback signals are generated to retune the laser to correct for the unwanted drift. Advantages of the present invention, in a preferred embodiment, include the ability to tune the lock frequency at much better precision than currently known and the freedom from the necessity to introduce a radio frequency modulation to determine the error signal direction.  
           [0016]    It is an object of the invention, in a preferred embodiment, to permit measurement of laser lines of 1 pm (picometer) or narrower.  
           [0017]    It is also an object of the invention, in a preferred embodiment, to stabilize multiple laser sources with a single OTDL device having little sensitivity to temperature change.  
           [0018]    A wavelength stabilizer in accordance with a preferred embodiment of the invention would include a laser; an optical tapped delay line having an input for receiving a collimated beam of light from the laser and an output to which is provided multiple time-delayed output beams, the collimated beam comprising a plurality of predetermined wavelengths, the multiple time-delayed output beams being mutually phase-shifted as a function of the wavelengths of the collimated beam and being spatially distributed, whereby the collimated beam is channelized into constituent predetermined wavelengths; and, means connected to the output for detecting variations in the constituent wavelengths over time, and means for controlling the laser in accordance with the detected variations to return the collimated beams to their predetermined wavelengths.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]    The present invention, in a preferred embodiment, may be best understood when the detailed description below is read with reference to the attached drawings, in which:  
         [0020]    [0020]FIG. 1 illustrates an example of a prior art etalon commonly used in laser locker applications.  
         [0021]    [0021]FIG. 2 illustrates an example of a preferred optical tapped delay line suitable for use in accordance with the invention.  
         [0022]    [0022]FIG. 3 illustrates an example of an operational side view of an optical tapped delay line suitable for use in accordance with the invention.  
         [0023]    [0023]FIG. 4 illustrates an example of a laser wavelength locker in accordance with the teaching of the invention.  
         [0024]    [0024]FIG. 5 illustrates an example of a spectrum analyzer in accordance with the teaching of the invention configured as a spectrum analyzer.  
         [0025]    [0025]FIG. 6 illustrates an example of an embodiment of the invention configured as a Fabry-Perot resonator cavity.  
         [0026]    [0026]FIG. 7 illustrates an example of an embodiment of the invention utilizing a ring resonator cavity.  
         [0027]    [0027]FIG. 8 is a graph illustrating an example of the operation of measuring the drift of a laser line in a WDM system.  
         [0028]    [0028]FIG. 9 illustrates an example of a preferred embodiment of the invention.  
         [0029]    [0029]FIG. 10 illustrates an example of an alternative preferred embodiment of the invention.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0030]    [0030]FIGS. 2 and 3 illustrate an optical tapped delay line that has particular utility when incorporated into a preferred embodiment of the present invention. It is the subject of co-pending U.S. patent application Ser. No. 09/687,029, filed Oct. 13, 2000, which is incorporated herein by reference. With reference to FIG. 2, six collimated input beams  30   a - 30   f  enter a transparent plate  31 . The origin of the beams may be, for example, the collimated output of six optical fibers (not shown) where each fiber typically carries multiple wavelength channels. Referring to FIG. 3, the plate  31  has a first surface  32  that is provided with a coating  35  that is substantially 100% reflective. The plate  31  has a second surface  36  that is spaced from and opposed to the first surface  32 . The second surface  36  is provided with a coating  37  that is partially reflective.  
         [0031]    In the illustrated embodiment, transparent plate  31  separates the reflective surface coatings  35  and  37 . In alternative embodiments (not illustrated), the reflective surfaces may be separated by other transparent materials, including air, other gas, or empty space. The transparent plate may also be referred to as an optical cavity.  
         [0032]    [0032]FIG. 3 illustrates an example of an operational side view of the device shown in FIG. 2. The single input beam  30   f  illustrated in FIG. 3 corresponds to the input beam  30   f  illustrated as one of the multiple input beams  30   a - 30   f  in FIG. 2. Due to the perspective of FIG. 3, the other input beams  30   a - 30   e  are not illustrated. However, it will be understood that the other multiple input beams  30   a - 30   e  reside behind the input beam  30   f  in the view shown in FIG. 3, and that the device is capable of processing and channelizing all of the multiple input beams simultaneously. Referring to FIG. 3, the input beam  30   f  enters the cavity  31  as a collimated beam of light through a hole  33 , i.e., a section of plate  31  that is not covered by reflective coating  35 . This feature in particular distinguishes the OTDL from other prior are devices, such as the Fabry-Perot etalon illustrated in FIG. 1, in which light enters directly through the partially reflective coating  20 . While collimating the input beam  30   f  is necessary, focusing of the input beam is not required. After entering the cavity  31 , a portion of the collimated input beam exits the cavity at a first location or “tap”  40   a  as collimated output beam  41   a.  Another portion of the collimated input beam is partially reflected by the coating  37  and then totally reflected by the coating  35 . In other words, a portion of the beam “bounces” from the coating  37  to the coating  35  and then back. This reflected beam exits at a second location or tap  40   b  that is slightly displaced spatially from the first tap  40   a . As a result of the bounce, the distance traveled by the output beam  41   b  is slightly greater than the distance traveled by output beam  41   a.  The width of the optical cavity  31  between reflective surfaces  32  and  36  thereby introduces a slight time delay between adjacent taps. The reflective process is continued, producing multiple additional collimated output beams  41   a - f  exiting the cavity  31  at multiple tap locations  40   a - f.  The result is a series of output beams that are distributed in the y direction with a progressive time delay from beam to beam.  
         [0033]    The various beams remain substantially collimated throughout the reflective process. Divergence of the beams and interference among the beams is minimized. Numerous internal reflections within the cavity  31  may be achieved without substantial divergence or interference.  
         [0034]    In the embodiment shown in FIG. 2, the various output beams are then directed to an anamorphic optical system  42 ,  45  that is spaced from the optical cavity  31 . In the illustrated embodiment the anamorphic optical system comprises a cylinder lens  42  and a spherical lens  45 . The anamorphic optical system performs the functions of: 1) Fourier transformation of the output of the cavity  31  in the vertical dimension y, and 2) imaging of the output of the cavity  31  in the horizontal dimension x onto an output surface  46 . Although not illustrated in FIG. 2, it will be recognized that the optical system  42 ,  45  may have some form other than anamorphic as described above, depending on the particular application of the OTDL device. The functions performed may be, for example, Fourier transformation in both dimensions, partial or fractional Fourier transformation in one or both dimensions, imaging, or any combination of these functions.  
         [0035]    The output surface  46  illustrated in FIG. 2 is two-dimensional, with the vertical dimension corresponding to the wavelength of the light in the input beam. There are a wide variety of devices that might be positioned at the output surface  46 . For example, a detector array, a lenslet array, a light pipe array, a fiber optic bundle, an array of graded index (GRIN) lenses or any combination of the above may be positioned at the output surface  46 .  
         [0036]    [0036]FIG. 4 illustrates an example of a laser wavelength locker system in accordance with the teaching of the invention, in a preferred embodiment. A laser  50  provides a coherent beam  51  to a beam splitter  52 . Beam splitter  52  is designed to permit the majority of energy to pass directly through to output  55 , with a smaller quantity of the energy, perhaps 5%, being reflected as beam  56  to an OTDL  57  as illustrated in FIG. 2 and  3 . The output of OTDL  57  illuminates a suitable optical detector array  60 , such as a grid of photodetectors, which convert the received optical energy into electrical signals. The electrical signals are fed into a differential amplifier  61 , which provides control signals to a processor  62 , such as a computer. The output of laser  50  is determined and continuously adjusted according to temperature control signal  65  from temperature control  66  and signal  67  from current control  70 . A thermal sensor  71  continuously monitors the temperature of OTDL  57  and provides the temperature information to processor  62 .  
         [0037]    During stable operation, laser  50  provides coherent light having a constant wavelength to output  55  and OTDL  57 . OTDL similarly emits an unchanging light pattern onto the optical detector array  60 . The constant signals from both differential amplifier  61  and thermal sensor  71  received by processor  62  invoke no changes by temperature control  66  or current control  70  to alter the output of laser  50 .  
         [0038]    Any change in the wavelength of laser  50 , however, will alter the energy pattern incident on detector array  60 , and thereby the electrical inputs to differential amplifier  61 , due to the properties of the OTDL as explained above with respect to FIGS. 2 and 3. Processor  62  combines the new information from differential amplifier  61  and thermal sensor  71 , and provides information to temperature control  66  and current control  70  as appropriate, to return the output of laser  50  to the correct wavelength.  
         [0039]    In contrast to the prior art etalons such as that shown in FIG. 1 of the previously discussed Hall &#39;700 patent, the OTDL  57  in FIG. 4 provides the ability to resolve wavelength channel spacings as narrow as 1 pm and less. It is the unique features of OTDL  57  that permit the device of FIG. 4 to achieve significantly higher wavelength resolution through its dramatically greater sensitivity, ambiguity, separation and stability. The OTDL  57  of FIG. 4 may be configured in a number of ways for specific applications.  
         [0040]    [0040]FIG. 5 illustrates an example of an OTDL configured for spatially resolving the optical wavelength spectrum of an incoming optical signal. An incoming multi-frequency light beam  75  is directed into OTDL  76 . Lens  77  performs a Fourier transform on the multiple beamlets  80  emerging from OTDL  77 , which spatially separates the beam into its component wavelengths λ 1 , λ 2 , . . . λ n  at output plane  80 . In this configuration, the device functions as a spectrum analyzer.  
         [0041]    While FIG. 4 illustrates an example of an embodiment of the invention with the OTDL residing in a feedback loop external to the laser cavity, FIG. 6 illustrates an example of the invention configured as a fixed or tunable wavelength stabilizer residing effectively within the laser cavity. Partially reflective mirrors  81  and  82  define a laser cavity. A suitable lasing medium  85  such as a semiconductor is pumped by a suitable energy source  86  to generate an optical output beam  87 . Output beam  87  is processed by OTDL  90  and Fourier lens system  91  as previously described to illuminate mirror  82  at the focal plane of lens system  91 . Because mirror  82  is partially reflective, a portion of the light energy incident on the mirror will be reflected back through Fourier lens  91  and OTDL  90 , through lasing medium  85 , and reflected by mirror  81 . Because the OTDL spatially resolves different wavelengths of light, the vertical position of mirror  82  selects the wavelength that is allowed to resonate and lase within the cavity. As illustrated, the selected resonating wavelength is identified as λ 2.  Other wavelengths such as λ 1  and λ 3  are not reflected and therefore cannot resonate and lase. In a fixed wavelength stabilizer, the position of mirror  82  will be fixed. A tunable device results if mirror  82  is permitted to move vertically to enable selection of any one of the wavelengths λ 1 , λ 2 , λ 3 , . . . λ n .  
         [0042]    [0042]FIG. 7 illustrates an example of an alternative embodiment of the invention configured as a fixed or tunable wavelength stabilizer residing effectively within the laser cavity. A suitable lasing medium  95  such as a semiconductor is pumped by a flash tube or light emitting diode  96  to generate an optical output beam  97 . Output beam  97  is processed by OTDL  100  and Fourier lens system  101  as previously described to focus a plurality of discrete wavelengths λ 1 , λ 2 , . . . , λ n  on an opaque stop  102  at the focal plane of lens  101 . An aperture  103  in stop  102  is vertically positioned to permit a selected beam  104  having a selected wavelength, in this illustration λ 2 , to pass through stop  102  to partially reflecting mirror  106 . Mirrors  107 ,  110  and  111  reflect beam  104  back into lasing medium  95 . Because the OTDL spatially resolves different wavelengths of light, the vertical position of stop  102  selects the wavelength that is allowed to resonate and lase within the cavity. Other wavelengths such as λ 1  and λ 3  are not passed back through the lasing medium and therefore cannot resonate and lase. In a fixed wavelength stabilizer, the position of stop  102  will be fixed. A tunable device results if stop  102  is permitted to move vertically to enable selection of any one of the wavelengths λ 1 , λ 2 , λ 3 , . . . λ n .  
       OPERATION  
       [0043]    The operation of the instant invention, in a preferred embodiment, can be best understood with reference to FIG. 8, which is a graph illustrating the amplitude response of two detector channels ( 115 ,  116 ) set to center a wavelength at λ 1 . The response of one detector as a function of the laser wavelength is shown as curve  115 . The response of the adjacent detector is shown as  116 . When the laser is lasing at the desired wavelength, λ 1 , the response of both detectors is equal  120 . If the laser drifts down in wavelength then the response of one detector increases  121  and the other decreases  122 . Conversely, if the laser drifts upwards in wavelength, the detectors respond in an opposite sense  123 ,  124 . The control electronics can use this response difference, and its directional information, to control, “drive”, the laser back to its proper wavelength λ 1 .  
         [0044]    [0044]FIG. 9 illustrates an example of a preferred embodiment of the invention with one OTDL device simultaneously measuring wavelengths generated by four different lasers. Laser/modulators  140   a - 140   d  each provide a collimated output comprising a WDM information-carrying channel. A multiplexer  141  combines the four signals λ 1 , λ 2 , λ 3 , and λ 4  into a single WDM optical beam carried on an optical fiber  142 . A 95/5 beam splitter  143  divides the beam, with 95% of the energy passing on through the communication system and 5% directed through collimating lens  144 . An OTDL  145  receives the collimated beam  143  and spatially separates the four channels as previously described. Detectors  147  at focal plane  146  measure variations in the channel wavelengths as previously described with respect to FIG. 8. Suitable detectors include a photodetector array for electrical processing. Alternatively,  147  could be pairs of micromirrors or lenslets for coupling to a fiber for sending the information to another optical subsystem.  
         [0045]    [0045]FIG. 10 is another preferred embodiment of this invention, illustrating a laser feedback system in which the detector  150  is an array of very finely spaced detectors. The precision measurements across multiple locations permitted by this device allows for precise measurement of laser drift. These measurements may be sent to a processor  151 , which would provide feedback to the original lasers  140   a -  140   d  in accordance with well-known procedures.  
         [0046]    The invention is subject to numerous other arrangements that will be readily apparent to one skilled in the art. Accordingly, the preferred embodiments described and shown in the accompanying drawings are merely illustrative and are not to be interpreted as limiting the claims that follow.