Abstract:
Techniques and devices using a fiber cavity formed of two spaced fiber gratings to construct a laser frequency locker.

Description:
RELATED PATENT APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 60/149,004, filed Aug. 13, 1999. 
    
    
     ORIGIN OF THE INVENTION 
     The U.S. Government has certain rights in this invention pursuant to Grant No. ESC 9806922 awarded by the National Science Foundation. 
    
    
     BACKGROUND 
     This application relates to optical fiber devices and lasers. 
     The output frequency of a laser can drift or fluctuate due to various internal processes (e.g., shot noise and other fluctuations) or environmental factors (e.g., a change in temperature or vibrations). The frequency of a semiconductor laser, for example, can change with the electric driving current and temperature. Such variations in the laser frequency are undesirable in certain applications where the frequency stability is required. 
     For example, wavelength-division-multiplexing (WDM) has been used to expand the capacity of a fiber communication link by simultaneously transmitting different optical waves at different wavelengths. It is critically important to specify and standardize the wavelengths in WDM signals so that WDM devices, modules, and subsystems from different manufactures are compatible and can be integrated and deployed in commercial WDM networks. One commonly-used WDM wavelength standard is the International Telecommunication Union (ITU) standard, where the WDM wavelengths of different optical waves are required to match ITU grid frequencies. Hence, the laser transmitters for the different WDM channels need be stabilized against wavelength instability caused by either internal or external fluctuations. 
     Other applications, such as precision spectroscopic measurements and nonlinear optical processes, may also require frequency stabilization of lasers. 
     SUMMARY 
     This application includes techniques and devices that use optical fibers and fiber devices to stabilize the frequency of a laser. 
    
    
     DESCRIPTION OF DRAWINGS 
     FIG. 1 shows one embodiment of a laser frequency locker based on polarization mode dispersion in a fiber cavity formed of two sets of fiber gratings in a fiber. 
     FIG. 2A shows transmission spectrum of a fiber Fabry-Perot resonator with two spaced fiber gratings in absence of polarization mode dispersion, where a single transmission peak appears near 1535 nm within the reflective bandwidth from about 1534.5 nm to about 1535.6 nm. 
     FIG. 2B shows an example of two closed spaced transmission peaks within the reflective bandwidth of a fiber Fabry-Perot resonator caused by splitting a single transmission peak due to the polarization mode dispersion in the system shown in FIG.  1 . 
     FIG. 3 shows an exemplary error signal produced by an implementation of the system shown in FIG.  1 . 
     FIG. 4 shows measured laser frequency as a function of injection current to a diode laser with and without a frequency control shown in FIG.  1 . 
     FIG. 5 shows another example of a laser frequency locker based on two fiber cavities formed in two separate fibers. 
     Like reference symbols in the various drawings indicate like elements. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 shows one embodiment of a fiber-based frequency locker  100 . A laser  110  is optically coupled to a fiber  120  so that the output laser beam  112  is confined and guided within the fiber  120 . The fiber  112  is designed to be anisotropic so that the index of refraction for an optical polarization along a selected direction perpendicular to the fiber core is different from that for another optical polarization perpendicular to the selected direction. For example, a polarization-preserving fiber with such anisotropy in the index can be used as the fiber  120 . One way to achieve this anisotropy in the index of refraction is to mechanically stress the fiber  122  along the selected direction that is substantially perpendicular to the fiber axis. In another method, the fiber core is exposed to UV light to create the desired birefringence in the fiber core. Furthermore, a birefringent dielectric material may be used to form the fiber core. 
     The fiber  120  includes a segment  130  in which two sets of Bragg gratings  131  and  132  with substantially the same grating period are formed in the fiber and are spaced from each other by a gap  133 . Each fiber grating operates as a reflector to selectively reflect light at a Bragg wavelength that satisfies the Bragg phase matching condition and to transmit other spectral components. This Bragg wavelength is equal to twice of the product of the effective index of the refraction of the fiber and grating period. Since both gratings  131  and  132  have the same grating period, they are reflective at the same wavelength. The reflection of the gratings  131  and  132  is not limited to a single wavelength but rather has a reflection bandwidth where each grating is reflective to light at any wavelength within the bandwidth. The reflection bandwidth of each grating is a function of the grating strength which depends on the depth of the periodic modulation on the fiber&#39;s index of refraction and the number of periods in each grating. Hence, the two spaced gratings  131  and  132  form a Fabry-Perot cavity only for light at the Bragg wavelength. 
     Light at the Bragg wavelength in such a Fabry-Perot cavity  130  will be reflected by the gratings  131  and  132  to bounce back and forth to cause light interference. A constructive interference occurs to create a transmission peak at a resonance when the round-trip phase delay is 360 degrees or any multiple of 360 degrees. The optical transmission of the Fabry-Perot cavity  130  decays as the phase delay deviates from the resonance and becomes zero when the phase delay deviates from the resonance exactly by 180 degrees. As the phase delay continues to change, the optical transmission begins to increase and reaches the peak value when another resonance condition is met. This behavior is periodic with respect to the round-trip phase delay. This round-trip phase delay is essentially determined by the product of the effective index of refraction of the fiber and the gap  133  and a resonance peak occurs when the product is equals to one half of the wavelength. When expressed in frequency, the round-trip phase delay represents the frequency difference between two adjacent resonant peaks and is called the free spectral range (FSR) of the Fabry-Perot cavity. 
     The Fabry-Perot cavity  130  can be configured to have only a single transmission peak within the reflection bandwidth by making the reflection bandwidth of the fiber gratings  131  and  132  less than FSR. FIG. 2A shows the transmission spectrum of an exemplary fiber Fabry-Perot cavity where each fiber grating is designed to be reflective in the bandwidth from about 1534.5 nm to about 1535.6 nm. In absence of the polarization mode dispersion, a single transmission peak  200  appears in the reflective bandwidth when the frequency of input light matches one of the resonance peaks of the fiber Fabry-Perot cavity. 
     When the finesse of the fiber Fabry-Perot cavity  120  is relatively high, e.g., by increasing each grating&#39;s strength, the linewidth of each transmission peak is narrow and thus the resonance frequency of the transmission peak is a well-defined frequency. Therefore, this resonance frequency can be used as a frequency reference or marker. Since the transmission of the input light changes when the input frequency drifts relative to the resonance frequency, a variation in the transmission intensity of the input light can be used to indicate the frequency shift of the input light with respect to the resonance frequency. 
     Notably, the index of the refraction of the fiber  122  is anisotropic. For input light at a fixed input frequency, the optical polarization parallel to the selected direction experiences one value for the index of refraction and the optical polarization perpendicular to the selected direction experiences a different value for the index of refraction. These two different index values along two orthogonal directions create a polarization mode dispersion between two orthogonal polarizations and respectively produce two closely spaced resonance peaks, one for each polarization. The frequency spacing between the two peaks is determined by the index difference in the gap  133 . 
     FIG. 2B shows an example of two transmission peaks  210  and  220  near 1541.41 nm from the fiber cavity  130  formed of a polarization-preserving silicon glass fiber. The index difference between the two orthogonal polarizations makes the two polarizations to match the same resonance mode of the cavity at two different frequencies  210  and  220  that is about 2.18 GHz apart. 
     The system  100  is designed to use these two peaks along two orthogonal polarizations at the same resonance mode to create an error signal that indicates a variation in the laser frequency and to use a feedback mechanism to correct the laser frequency based on the error signal. 
     The fiber gratings  131  and  132  are designed so that their reflective bandwidth covers a spectral range in which the frequency of the laser  110  is to be stabilized. The polarization of the output laser beam  112  and the selected direction of the fiber  120  are oriented at about 45 degrees relative to each other so that the input laser power is essentially equally divided between two orthogonal polarizations, one parallel to the selected axis and another perpendicular to the selected axis. A polarization device  140  is coupled to the fiber  120  to receive the transmission from the fiber cavity  130 . Examples of such device  140  include, among others, a polarization beam splitter and a polarizing fiber coupler. In operation, the device  140  is oriented to separate the polarization along the selected direction to be a first output signal  141  from another polarization perpendicular to the selected direction which is a second output signal  142 . Two light detectors  151  and  152  are used to respectively receive and convert the signals  141  and  142  into electrical output signals. 
     An electronic circuit  160  is coupled to receive the output signals from the detectors  151  and  152 . One of the two signals  151  and  152  (e.g.,  152  which representing the peak  220  in FIG. 2) is inverted by the circuit  160  and is then added to another signal to produce a sum signal. The sum signal may be subsequently amplified for further processing. As illustrated, the circuit  160  may include an operational amplifier to perform the inverting, summing, and amplifying steps. 
     FIG. 3 shows one example of the above sum signal as a spectral dispersive line shape for a laser wavelength at about 1544.78 nm. The two peaks from the fiber Fabry-Perot cavity due to the anisotropy along two orthogonal polarizations are about 1.84 GHz apart from each other. When the polarization of the laser beam  112  is exactly 45 degrees with respect to the selected direction of the fiber  120 , the two peaks are essentially identical in spectral profile and strength but are spectrally shifted from each other. Hence, the sum signal goes to zero at the center frequency between the two peaks and becomes either positive or negative when the laser frequency deviates from that center frequency. This center frequency can be used as a frequency reference (f ref ) to which the laser frequency is locked. 
     The fiber cavity  130  can be designed to set the frequency reference at a desired value by controlling, e.g., anisotropy in the index of the fiber  120 , the structure of the gratings  131  and  132  and the gap  133 . For WDM applications, for example, the frequency reference may be one of the WDM wavelengths specified by the ITU standard. 
     Referring back to FIG. 1, an error signal  170  may be a voltage or current signal from the circuit  160  that represents the sign and magnitude of the sum signal. The error signal  170  is then fed back to the laser  110  to form a servo control feedback loop. The servo loop, in response to the error signal  170 , operates to adjust the frequency of the laser  110  towards the frequency reference so that the magnitude of the error signal  170  is reduced within an acceptable range and hence the laser frequency stays “locked” at the frequency reference. The sign of the error signal  170  dictates the direction of the correction to the laser frequency and the magnitude determines the amount of the correction to the laser frequency. 
     The laser  110  in general may be any tunable laser whose output frequency can be controlled by a signal  116  from a frequency control unit  114 . For example, the laser  110  may include a semiconductor laser, a fiber laser with either a linear or ring cavity, or a laser based on a semiconductor or fiber laser. The frequency control unit  114  may be an internal part of the laser  110  or a device external to the laser  110 . For example, if the laser  110  is a semiconductor diode laser, the frequency control  114  may be the driving circuit and the control signal  116  may be the driving current to the diode. Alternatively, the frequency control  114  may be a temperature controller that controls the temperature of the diode. If the laser  110  includes a fiber as part of the optical path, the frequency control unit  114  may include a fiber stretcher that controls the fiber length and hence the laser frequency. 
     FIG. 4 shows measured laser wavelength as a function of the injection current to a laser diode with and without the servo control as shown in FIG.  1 . Curve  410  shows the measured data when the frequency servo control is in operation. In comparison, curve  420  shows the measured data when the frequency servo control is turned off. The measurements indicate that the servo control improves the frequency stability of the diode laser by one order of magnitude. Higher stability can be obtained with higher servo loop gain. 
     FIG. 5 shows another frequency servo control system  500  by using two fiber cavities  130 A and  130 B in two separate fibers  120 A and  120 B, respectively. A fiber coupler  510  may be used to divide the laser beam  112  into two equal beams and respectively couple the two beams into the fibers  120 A and  120 B. Unlike the fiber  120  used in the single fiber system  100 , the two fibers  120 A and  120 B are not required to have two different indices for two orthogonal polarizations to create the closely spaced resonance peaks to match the same resonance cavity mode due to polarization mode dispersion. Instead, the two fiber cavities  130 A and  130 B are designed so that their resonance frequencies for the same order of the resonance are slightly shifted from each other by a desired amount. This may be achieved by, for example, configuring their gaps between the two gratings, or the grating periods, or both, to be different. In one implementation, the two fibers  120 A and  120 B may be placed under slightly different tensions to achieve the frequency shift between the two peaks. 
     The gap between the two fiber gratings in each fiber cavity is set sufficiently small that the FSR of the cavity is greater than the reflective bandwidth. Hence, only a single transmission resonance peak can appear within the reflective bandwidth for each fiber cavity. Like the polarization dispersion in the system  100  of FIG. 1, the center frequency between the two different transmission peaks from the two fiber cavities  130 A and  130 B can be used as a frequency reference to lock the laser  110 . Detectors  151  and  152  are used to receive the signals for the two transmission peaks and the circuit  160  is used to invert one of the signals to produce the sum signal  170  as a feedback control. 
     In both systems  100  in FIG. 1 and 500 in FIG. 5, the frequency reference may be made adjustable so that the laser frequency, while being locked to the reference, can be tunable. For example, the two shifted resonance peaks in both systems may be shifted to change the center frequency. As long as the servo control operates properly, the laser  110  can be adjusted in response to the signal  116  to make the laser frequency to follow the center frequency. 
     In the system  100  in FIG. 1, this may be achieved by changing the grating spacing and the gap  133  between the two fiber gratings  131  and  132  so that both peaks shift by the same amount toward the same direction. A fiber stretcher may be engaged to the fiber  120  at the position of the cavity  130  to implement the tuning. In the system  500  in FIG. 5, one or both fibers  120 A and  120 B may be stretched to shift the center frequency between the two resonance peaks. 
     A number of embodiments have been described. Nevertheless, it will be understood that various modifications and enhancements may be made without departing from the spirit and scope of the following claims.