Abstract:
A method and apparatus for stabilizing the wavelength of a laser are disclosed. The invention provides a way to stabilize a laser for applications in dense wavelength division multiplexing (DWDM) systems where frequency spacing is crucial. The invention accomplishes laser stabilization by generating one or more optical paths which are passed through one or more filters to obtain one or more signals which are a function of frequency. Another optical path which does not contain a filter is generated to obtain a signal which is a function of power. The frequency signal(s) and the power reference signal are then converted from optical to electrical and from analog-to-digital. A microcontroller is then used to normalize one or more selected frequency paths with respect to the optical power path, process the signals via software code, and generate a signal which provides feedback to the laser for stabilization. By using a microcontroller; elements that lead to wavelength or frequency drift, or manufacturing component variations can be taken into account and the input signal to the laser can be adjusted accordingly.

Description:
RELATED APPLICATIONS 
     This application is a continuation-in-part of co-pending U.S. application Ser. No. 09/265,291 to Ackerman et al., entitled “Two Path Digital Wavelength Stabilization,” filed Mar. 9, 1999, having at least one common inventor, which is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a novel and useful method for stabilizing the wavelength of a laser source. 
     BACKGROUND OF THE INVENTION 
     Laser sources are widely used in wavelength division multiplexed systems. In wavelength division multiplexed systems, it is important that the wavelength used is very stable. Although lasers are inherently very stable, increased stabilization of a laser&#39;s wavelength becomes crucial as systems migrate to dense wavelength division multiplexing (DWDM) types. In DWDM systems, many wavelengths are placed on a single fiber to increase system capacity. Currently the spacing in DWDM systems between frequencies is around 100 GHz and can be handled by traditional laser stabilization methods. However, as technology moves toward frequency spacings of 25-50 GHz or less, increased stabilization will be required to prevent interference between wavelengths as the spacings become closer and closer. 
     Presently, to wavelength stabilize lasers, the wavelength or equivalently the optical frequency of a laser is compared to a stable reference element. One method is to use an optical filter as a reference element. The output of the laser is split and part of the beam is passed through an optical filter to create an optical signal which is a function of wavelength or frequency and optical power (hereinafter “the optical filtered path”). The optical filtered path is then processed, assuming that a change in signal amplitude corresponds to a change in frequency, and a signal is generated which is fed back to the laser to stabilize the laser&#39;s wavelength. However, a change in signal amplitude at the output of the filter could be the result of a change in the power output of the laser rather than a change in the laser&#39;s frequency. 
     Another method of stabilizing a laser is to pass a slightly diverging beam of light, obtained by splitting the output of the laser source, through a filter at different angles of inclination as shown in FIG.  1 . The two photo-detectors, P 1  and P 2 , act as apertures and capture a different portion of the light emitted by the divergent source. This produces two different spectral responses, offset in wavelength according to their angular difference with respect to the filter. Since P 1  captures a portion of the emitted light which passed through the filter at a higher tilt angle than that captured by P 2 , it&#39;s response will peak at a slightly lower wavelength than P 2  as depicted in FIG.  2 . The filter and alignment parameter are chosen so that the wavelength offset between the two responses is roughly equal to their effective bandwidths. The signals are then compared differentially to generate a signal which can be used to stabilize the wavelength of the laser by maintaining λ=λ 0 , low as further depicted in FIG.  2 . 
     In a stabilized system, wavelength or frequency drift can be introduced by the aging or temperature dependence of the laser itself, or by the aging or temperature dependence of the optical reference filter, the optical detectors, or the stabilization electronics. In addition, manufacturing variations of system components can result in varying wavelengths from system to system. Existing systems are unable to adequately compensate for the multitude of variables that can arise in a stabilization system when a very high level of stabilization is needed. 
     SUMMARY OF THE INVENTION 
     The present invention provides an improved method for stabilizing the wavelength of a laser source. The invention accomplishes this objective by using one or more optical filters, multiple optical paths, analog and digital conversion, and a microcontroller. 
     In accordance with the present invention, a laser generates a signal which is carried by a fiber optic cable. Two separate paths are created from the fiber optic cable via photocouplers. The first path is an optical filtered path which passes through an optical filter. The second path is a power reference path used for normalization. Since the optical filtered path contains an optical filter, it provides a signal the power of which is a function of wavelength as well as the optical power output of the laser. The power reference path is unfiltered so as to provide a signal the power of which is a function only of the optical power output of the laser. A change in the output power of the optical filtered path should primarily indicate a frequency change of the laser. However, the change may be due to a change in the optical power of the laser. By normalizing the optical filtered path to the power reference path, the change in power in the optical filtered path that is due to frequency change rather than laser output power change can be isolated and used to stabilize the frequency of the laser source. 
     More than one optical filtered paths containing one or more optical filters may be used in place of the single optical filtered path. The microcontroller utilizes one or more of the available optical filtered paths for processing. This arrangement allows the microcontroller to choose a desirable optical filtered path signal or to combine the signals from two or more optical filtered paths to achieve a desirable signal. By allowing the microcontroller to choose a desirable signal or to combine signals to achieve a desirable signal, the effects of undesirable optical filtered path signals can be minimized. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of a two-path wavelength stabilization system in accordance with the prior art; 
     FIG. 2 is a graph depicting signal amplitude vs. wavelength of the signals at the photo-detectors, P 1  and P 2 , in the circuit of FIG. 1; 
     FIG. 3 is a block diagram of a two-path wavelength stabilization system in accordance with the present invention; 
     FIG. 3A is a graph of the output intensity of an etalon versus frequency, normalized to a reference amplitude; 
     FIG. 3B is a block diagram of a multi-path wavelength stabilization system in accordance with the present invention; 
     FIG. 4 is a circuit diagram of an exemplary pre-amplifier and current-to-voltage converter for use in the circuit of FIG. 3; 
     FIG. 5 is a graph of the voltage level in the optical filtered path prior to analog-to-digital conversion in accordance with the present invention; 
     FIG. 6 is a graph of the voltage level in the power reference path prior to analog-to-digital conversion in accordance with the present invention; 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring more specifically to the drawings, in FIG. 3, an embodiment of a multiple path wavelength stabilization system is depicted. FIG. 3 illustrates the components of a two path wavelength stabilization system  10  which include: a laser source  12 , an optical fiber  14 , photo couplers  16  and  18 , an optical filter  20 , photo detectors  22  and  24 , current-to-voltage converters  28  and  32 , amplifiers  34  and  36 , analog-to-digital converters  38  and  40 , microcontroller  50 , and digital-to-analog converter  49 . The components connected together, as depicted in FIG. 3, provide increased stabilization for a laser to be used in dense wavelength division multiplexing (DWDM) systems or similar systems where very stable laser frequencies are required. The output  13  from either the front face or the back face of the laser  12  produces a signal having a power P L  which is placed on the fiber optic cable  14 . The initial signal on the fiber optic cable is then used to create two independent paths, the optical filtered path  101  and the power reference path  102 . The optical filtered path  101  and the power reference path  102  are created by placing photo-couplers  16  and  18 , respectively, on the fiber optic cable  14  carrying the signal from the laser  12 . The optical filtered path  101  is passed through an optical filter  20  to obtain a signal which is, at least partially, a function of wavelength or frequency, and becomes a reference element for frequency stabilization. The power reference path  102  does not pass through the optical filter  20  and provides a signal which is a function solely of the laser&#39;s optical power PL, and is eventually used for normalizing the optical filtered path  101 . 
     The optical filtered path  101  and the power reference path  102  are then processed to provide suitable signals for the microcontroller  50 . Separately, each path passes through a photo-detector  22  or  24 , current-to-voltage converter  28  or  32 , amplifier  34  or  36 , and analog-to-digital converter  38  or  40 . 
     The photo-detectors  22  and  24 , transform the optical signal from each path into an electrical signal which is required as an input for electrical circuits. The photo-detectors  22  and  24  produce an electrical current which is a function of the optical signal strength. The conversion or responsivity of the photo-detectors  22  and  24  is, for example, roughly  1  ampere of electrical current for each watt of optical power. Assuming the optical power into the photo-detectors  22  and  24  is 1 μW, the initial electric current out of the photo-detectors  22  and  24  is in the neighborhood of 1 μA. 
     The current-to-voltage converters  28  and  32 , convert the output of the photo-detectors  22  and  24  from a signal represented by a current to one represented by a voltage and a provide some pre-amplification. The conversion of the signal from current to voltage and the signal&#39;s pre-amplification is combined as depicted in FIG.  4 . In FIG. 4, a current signal, i in , is amplified and transformed into a voltage signal, v out . The amplification and current to voltage transformation is accomplished by a transimpedance amplifier  60  created by using an inverting amplifier  62  with resistor  64  in a feedback loop. If a 100 kΩ resistor is used for feedback resistor  64 , the output voltage v out  will be approximately the input current, 10 −6 A, times the feedback resistance, 100 kΩ, or about 0.1 V. 
     The amplifiers  34  and  36 , provide additional gain to the signal to condition the signal for the analog-to-digital converters  38  and  40 . If the amplifiers  34  and  36  provide a gain of  10 , the signals will be approximately 1 V as they enter the analog-to-digital converters. FIGS. 5 and 6 depict the signals on the optical filtered path and the power reference path, respectively, prior to entering the analog-to-digital converters  38  and  40 . As can be seen in the figures, at this point, the signals are DC voltages carrying some noise with the voltage of the optically filtered path  101  slightly lower than the voltage of the unfiltered path  102 . This example assumes that the components in the two paths are matched (which, of course, is not a requirement). 
     The analog-to-digital converters  38  and  40  convert the input analog signals to digital signals. The resultant digital signals  42  and  44  are in a form which can be processed and manipulated by the microcontroller  50 . 
     The digital signals  42  and  44  are then processed by the microcontroller  50 , which produces the output signal  48 . The microcontroller  50  numerically divides the optical filtered path digital signal  42  by the power reference path digital signal  44  to normalize the optical filtered path digital signal  42 , whereby a digital value which is a function solely of the laser&#39;s wavelength is derived. The microcontroller can then use the digital value representing the laser&#39;s wavelength to generate signal  48 . Signal  48  is then converted from digital to analog by digital-to-analog converter  49  to produce a laser adjustment signal  51  which can be used for adjusting the wavelength of the laser  12 . The processing by microcontroller  50  can be accomplished by any of the following types of apparatus: microprocessor, processor, digital signal processor, computer, state machine, or essentially any digital processing circuit. 
     The signal  51  can be in any form desired for controlling the frequency of the laser  12  and can be modified by changes in the microcontroller&#39;s software code via remote input  46 . The signal  51  generated through the digital-to-analog converter  49  by the microcontroller  50  may be a current for adjusting the temperature of a thermoelectric cooler on which the laser  12  is mounted, or the microcontroller  50  may generate other appropriate signals either with or without digital to analog conversion depending on the method used to modify the frequency of the laser  12 . 
     In a preferred embodiment, optical filter  20  is an etalon. However, optical filter  20  may be any device which produces a measurable output that varies based on the frequency of an optical input. An etalon is a piece of partially reflective glass which produces an interference pattern when light containing many different frequencies passes through the etalon. FIG. 3A depicts the output amplitude  310  of an etalon versus frequency, normalized to a reference amplitude  300 . The interference pattern  310  created by the etalon is characterized by a plurality of peaks  320 A and  320 B, and a plurality of cusps  330 A,  330 B, and  330 C. When light within a narrow frequency range is passed through the etalon, light frequencies which correspond to a peak  320 A-B will exhibit a higher intensity than light frequencies which correspond to a cusp  330 A-C. 
     In accordance with the present invention, a small input frequency change should produce an output amplitude change which can be detected by the associated circuitry in the path. Desirable operating frequencies are located between the peaks  320 A-B and the cusps  330 A-C, such as at location  350 A or  350 B, in order to maximize amplitude change as a function of frequency change. Operating at the peaks  320 A-B is undesirable because small changes in frequency may cause a sign change in the slope of the output or may result in an amplitude which is above the desired operating range. Operating in the cusps  330 A-C is undesirable because changes in frequency may produce very little change in the output, such as between points  360  and  370 . In the preferred embodiment, the output amplitude  310  of the etalon is normalized to a reference amplitude  300 , such that the etalon output amplitude  310  straddles the reference amplitude  300 . In this arrangement, if the desired operating frequency occurs at location  350 A, a drop in frequency from location  350 A will result in an increase in the etalon output amplitude, indicating to the microcontroller  50  that a change in frequency has occurred so that the microcontroller  50  can adjust the laser  12 . Similar processing would occur for an increase in frequency from location  350 A. 
     If the desired operating frequency of the laser  12  corresponds to a cusp  330 A-C, small changes in the output frequency of the laser  12  may be undetectable by the associated circuitry in the path. Therefore, in order to stabilize a laser  12  operating at a frequency that corresponds to a cusp  330 A-C of the etalon, one or more etalon which exhibit interference patterns in which the peaks and cusps don&#39;t correspond to the peaks  320 A-B and cusps  330 A-C of the original etalon may be incorporate into the stabilization system  10  so that the microcontroller  50  can choose the etalon having the best amplitude to frequency change ratio for the desired frequency. 
     The additional etalon may be incorporated into the stabilization system  10  as depicted in FIG. 3B, which illustrates an alternative embodiment of a multiple path wavelength stabilization system  10 A. FIG. 3B is identical to FIG. 3 with the exception that an additional optical filtered path  101 A is included. All of the components of optical filtered path  101 A are similar to the corresponding components in optical filtered path  101 . The additional optical filtered path  101 A provides an additional reference path for use by microcontroller  50 . The additional optical filtered path lOlA may be used to accommodate manufacturing variations in optical filters  20  and  20 A. For example microcontroller  50  could base calculations on either optical filtered path  101  or  101 A or microcontroller  50  could use a combination of the optical filtered paths  101  and  101 A, based on the characteristics of the optical filtered paths  101  and  101 A. Flexibility in the manipulation of optical filtered paths  101  and  101 A can be incorporated with software in microcontroller  50 . In addition to optical filtered paths  101  and  101 A, additional optical filtered paths may be added without departing from the spirit of the present invention. 
     The additional optical filtered path  101 A is developed and processed in a manner similar to optical filtered path  101 . The output  13  from either the front face or the back face of the laser  12  produces a signal having a power P L  which is placed on the fiber optic cable  14 . The initial signal on the fiber optic cable is then used to create the optical filtered path  101 A. The optical filtered path  101 A is created by placing photo-coupler  16 A on the fiber optic cable  14  carrying the signal from the laser  12 . The optical filtered path  101 A is passed through an optical filter  20 A to obtain a signal which is, at least partially, a function of wavelength or frequency, and becomes a potential reference element for frequency stabilization. Optical filter  20 A may be physically separate from optical filter  20 , or optical filter  20 A and optical filter  20  may be different portions of a multiple step or graduated optical filter, such as a two step etalon. In addition, more optical filters may be added or many different portions of a multiple step optical filter may be used to filter additional optical filtered paths without departing from the spirit of the present invention. 
     The optical filtered path  101 A is then processed to provide a suitable signal for the microcontroller  50 . The optical filtered path  101 A passes through a photo-detector  22 A, current-to-voltage converter  28 A, amplifier  34 A, and analog-to-digital converter  38 A. The photo-detector  22 A, transform the optical signal into an electrical signal which is required as an input for electrical circuits. The photo-detector  22 A produces an electrical current which is a function of the optical signal strength. The current-to-voltage converter  28 A converts the output of the photo-detector  22 A from a signal represented by a current to one represented by a voltage and provide some pre-amplification. The amplifier  34 A, provides gain to the signal to condition the signal for the analog-to-digital converter  38 A. The analog-to-digital converter  38 A converts the input analog signal to a digital signal. The resultant digital signal  42 A is in a form which can be processed and manipulated by the microcontroller  50 . 
     The digital signal  42 A is then processed by the microcontroller  50  along with digital signals  42  and  44 , to produces the output signal  48 . The microcontroller  50  numerically manipulates optical filtered path digital signals  42  and  42 A and the power reference path digital signal  44  to derive a digital value which is a function solely of the laser&#39;s wavelength. By using two optical filtered paths  42  and  42 A, the effect of an undesirable value on one of the optical filtered paths can be accommodated by the microcontroller  50 , and thereby negated. For example, microcontroller  50  may combine optical filtered paths  101  and  101 A and use the power reference path to normalize the combined paths. Alternatively, microcontroller  50  could choose one of the two optical filtered paths and use the power reference path to normalize the chosen path. Various microcontroller  50  numerical manipulations for achieving a digital value which is a function solely of the laser&#39;s wavelength will be readily apparent to those skilled in the art. In addition, microcontroller  50  could base calculations on one or more of many optical filtered paths if additional optical filtered paths are incorporated into a wavelength stabilization system. 
     The microcontroller  50  can then use the digital value representing the laser&#39;s e wavelength to generate signal  48 . Signal  48  is then converted from digital to analog by digital-to-analog converter  49  to produce a laser adjustment signal  51  which can be used for adjusting the wavelength of the laser  12 . 
     The present invention teaches a multiple path digital wavelength stabilization method to achieve a level of wavelength stabilization that is impractical or impossible via analog means. For example, improved stabilization can be achieved by identifying small variations in the laser&#39;s wavelength. Small wavelength variations can be masked by noise in the laser  12  and stabilization circuitry  10 . In order to increase the signal to noise ratio, the normalized signal can be integrated over a period of time, with improved signal to noise ratios resulting from longer integration periods. Traditional analog systems are constrained by an RC (resistance and capacitance) time constant. In order to obtain long integration times, such as a month, a capacitor the size of a trash can would be required. By using microcontroller  50 , the signals can be sampled over a period of minutes, days, months, or even years, depending on the amount of time required to obtain a desirable signal to noise ratio. The microcontroller can accomplish long integration times by storing signal values in memory or keeping a running total of averages digitally. 
     Additionally, the digital approach to wavelength stabilization allows for flexibility in choosing system components. Different types of filters with varying characteristics can be used for optical filters  20  and  20 A by modifying software in the microcontroller  50 , without changing other system components. This allows for using inexpensive filters or incorporating new filter designs into stabilization circuits  10  and  10 A. Also, photo-detectors  22 ,  22 A, and  24 , current-to-voltage converters  28 ,  28 A, and  32 , and amplifiers  34 ,  34 A, and  35  can be chosen based on availability or cost with variations in their respective signal levels accommodated by software in the microcontroller  50 . For example, if the optical filtered path digital signal  42  was twice as big as the power reference path digital signal  44 , due to mismatched components, the microcontroller  50  could divide the optical filtered path  42  by two or multiply the power reference path  44  by two. Attempting system modifications such as this, although readily achievable with a microcontroller, would require almost completely redesigning a circuit to accomplish in an analog system. 
     Further, this method of wavelength stabilization allows for the use of components with high levels of manufacturing variations, permitting the use of less expensive components. Variations in system components can be accommodated by changing software code in the microcontroller  50 , either at the factory when the laser&#39;s frequency is originally set, or via remote input  46  at a later date. The microcontroller  50  software can numerically account for amplifier component variations resulting in digital signal levels that are too high or too low, filters with varying wavelength characteristics, and other types of system variations. Attempting similar flexibility in an analog system would require exhaustive design considerations. 
     The wavelength stabilization systems according to the present invention offers vast improvements over traditional stabilization systems. As stated above, long integration times, which were previously impractical because of unrealizable component values, flexibility in choosing system components, and accommodation of manufacturing variations in the optical filters and other components, are all easily achievable utilizing digital stabilization in accordance with the present invention. The flexibility gained by using the new stabilization system is due to the ability to program the microcontroller  50  to perform many different functions on the digital inputs with software using mathematical equations, versus attempting to use analog circuit components to accomplish the same type of functions in an analog system. In addition, the remote input  46  can be used to modify software code in the microcontroller  50 . For example, various control algorithms or normalization methods can be used or changed at will via code changes in the microcontroller  50  via remote input  46 . 
     Having thus described a few particular embodiments of the invention, various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications and improvements as are made obvious by this disclosure are intended to be part of this description though not expressly stated herein, and are intended to be within the spirit and scope of the invention. Accordingly, the foregoing description is by way of example only, and not limiting. The invention is limited only as defined in the following claims and equivalents thereto.