Abstract:
A system and method for achieving, while using a multimode diode laser and polarization-maintaining fibers, high signal-to-noise ratio in a magneto optical storage system. In particular, the system splits an incoming main light signal into two orthogonal polarization states, which then propagate over different distances before recombining. By pulsing the laser on and off at a high frequency and choosing an appropriate path difference for the polarization states, which is dependent upon the modulation frequency of the laser, the system eliminates first-order spectral polarization noise arising from a potential error in a key optical component.

Description:
RELATED APPLICATIONS 
     This application claims the benefit under 35 U.S.C. §119(e) to U.S. Provisional Application Ser. No. 60/081,227, Jeffrey P. Wilde, et al., filed on Apr. 9, 1998, entitled “Low-Noise Optical Storage System Based On An Optical Polarimetric Delay Line,” U.S Provisional Application 60/079,903 entitled “Optical Drive Utilizing Low Birefringence Fiber,” filed Mar. 30, 1998, U.S. Provisional Application 60/088,192 entitled “Laser Phase Noise Minimization In Optical Drive,” filed Jun. 5, 1998, U.S. Provisional Application 60/108,398 entitled “Optical Head Design Eliminating Fiber End Back Reflection” filed Nov. 13, 1998, U.S. Provisional Application 60/111,470 entitled “Optical Fiber Coupler Using A Spliced Polarization-Maintaining Fiber, filed Dec. 9, 1998, all of which are incorporated by reference herein in their entirety. This application also is related to U.S. application Ser. No. 08/745,095, Jeffrey P. Wilde, et al., filed on Nov. 7, 1996, entitled “Optical System and Method Using Optical Fibers For Storage And Retrieval Of Information,” which is incorporated by reference in its entirety herein. 
    
    
     BACKGROUND OF INVENTION 
     1. Field of Technology 
     The present invention generally relates to optical systems. More specifically, the present invention relates to noise reduction in the transmission of optical signals. 
     2. Description of Background of Invention 
     Conventional data storage systems utilize billions of magnetically recorded imprints (bits) on a platter (media) surface to store oppositely polarized (e.g., positive or negative) data bits. These complimentary magnetic dipoles (which are parallel to the disk surface) represent a logic state of either a ‘1’ or a ‘0’. Based upon the industry&#39;s current areal density (e.g., a few Gbits per in 2 ) growth rate, such conventional disk drives are approaching areal densities as high as 20 Gbits/in 2 , which results in potential problems associated with a superparamagnetic limit. In particular, this physical limit causes oppositely-polarized domains that reside in very close proximity to one another to degrade, thereby causing data corruption problems. 
     To avoid this potential technological hurdle, an alternative storage technology utilizing a magneto-optical (MO) storage system is used. Such MO storage systems are, in principle, able to attain areal densities beyond approximately 40 Gbits/in 2  without confronting the superparamagnetic limit. However, such an alternative technology results in the need to overcome new technological challenges, such as the effects of laser noise within the system. In particular, spectral polarization noise (SPN) comprising both mode partition noise (MPN) and laser phase noise must be minimized through careful optical system design. 
     For example, by propagating a multi-longitudinal mode laser light (e.g., from a Fabry-Perot diode laser) through a frequency selective polarization-maintaining (PM) fiber system, which contains slight unavoidable optical misalignment errors, SPN can develop, thereby increasing the likelihood of data corruption in a main light signal, which serves as the data conduit between an MO medium and a detection module. One partial solution for minimizing SPN is to utilize a single-mode (e.g., single-frequency) distributed feedback (DFB) laser, which does not generate these multiple modes within the system, thereby avoiding the effects of MPN. However, DFB lasers which operate in the red spectral range and at high power levels currently are not readily available on the commercial market. Although use of a DFB laser eliminates MPN, laser phase noise may still exit. In addition, since multimode laser diodes are considerably less expensive than DFB lasers, multimode lasers are the preferred type of laser source for MO storage systems. 
     What is needed is a system and method that utilizes a multimode diode laser and minimizes the effects of SPN within the MO storage system. 
     SUMMARY OF INVENTION 
     Accordingly, the present invention overcomes the deficiencies of the prior art by providing a system and method that minimizes the first-order spectral polarization noise (SPN) by time shifting polarization components of a parasitic light signal away from a main light signal. In particular, a preferred embodiment of the system includes a multimode laser, a leaky beam splitter (LBS), a first half wave plate (HWP 1 ), a second half wave plate (HWP 2 ), a polarimetric delay line (PDL), a polarization-maintaining (PM) fiber, a first quarter wave plate (QWP 1 ), a second quarter wave plate (QWP 2 ) and a differential detection module. A parasitic light signal is generated by non-ideal properties of the optical system. 
     The multimode laser generates the main light signal, which is used as a read signal for carrying the current logic state from a specific location on the MO medium to the differential detection module. The laser is modulated on and off at a radio frequency, the particular value of which is determined by the optical path lengths associated with the PDL and the PM fiber. The PDL and the PM fiber are part of a continuous birefringent optical conduit for the propagation of the main light signal to and from the MO medium. 
     The HWP 1  and HWP 2  in conjunction with the QWP 1  alter the polarization of the main light signal to ensure that the first and second polarization components of the main light signal propagate along each delay path length of the PDL and each axis of the PM fiber. By propagating along one delay path length and axis on the forward path, and the opposing delay path length and axis on the return path from the MO medium, the two polarization components of the main light signal will have a net optical path difference of zero in the absence of an MO signal. In the presence of an MO signal, or magnetic Kerr effect, a small phase shift is introduced between the two polarization components of the main signal, making the net optical path difference slightly nonzero. To minimize SPN caused by retardation and/or orientation errors of QWP 1 , the PDL time shifts one half of the parasitic light signal ahead and the other half behind the main light signal so as to preclude coherent interaction between the parasitic and main optical pulse trains. 
     The LBS, which allows linearly polarized light to enter the PDL and the PM fiber on the forward path, reflects toward the differential detector on the return path, part of this polarized mode and most of the orthogonally polarized mode (generated by the magnetic Kerr effect) of the main signal. In addition, the LBS reflects a portion of the corresponding time-shifted parasitic signal toward the differential detection module. The QWP 2  modifies the phase between the two polarization components of the reflected main light signal to ensure that the logic state of the data signal carried by the main light signal is properly detected by the differential detection module. 
    
    
     BRIEF DESCRIPTION OF FIGURES 
     FIG. 1 illustrates an overall system of a preferred embodiment of the present invention. 
     FIG. 2 illustrates a mode partitioning of the output power of a multimode laser of a preferred embodiment of the present invention. 
     FIG. 3 illustrates a polarization-maintaining fiber of a preferred embodiment of the present invention. 
     FIG. 4 illustrates multi-longitudinal laser light (in this case two modes for simplicity) producing mode partition noise when the laser light propagates through a birefringent medium such as a PM fiber in a preferred embodiment of the present invention. 
     FIG. 5 illustrates the optical signal arriving at the detection module in the case of a well aligned quarter wave plate (QWP 1 ) in a preferred embodiment of the present invention. 
     FIG. 6 illustrates a parasitic light signal (arising from misalignment of QWP 1 ) time shifted away from the main light signal of a preferred embodiment of the present invention. 
     FIG. 7 illustrates a plot of multimode diode laser noise (SPN comprising both mode partition noise and phase noise) versus the path difference between two beams in a free-space interferometer. 
     FIGS.  8 ( a ) and  8 ( b ) illustrate the polarization states of the main light signal in the detection path before and after QWP 2 , respectively, with and without a logic state of a preferred embodiment of the present invention. 
     FIG. 9 illustrates a polarimetric delay line with a fiber collimator of an alternative embodiment of the present invention. 
     FIG. 10 illustrates an all-fiber version of the polarimetric delay line of an alternative embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Preferred embodiments of the present invention are now described with reference to Figures where like reference numbers indicate identical or functionally similar elements and the left most digit of each reference number corresponds to the Figure in which the reference number is first used. 
     FIG. 1 illustrates a low noise optical storage system  100  of a preferred embodiment of the present invention, which utilizes a flying optical head technology (not illustrated) (e.g., optically-assisted Winchester (OAW) disk drives, as discussed in U.S. application Ser. No. 08/745,095, Jeffrey P. Wilde, et al., entitled “Optical System and Method Using Optical Fibers For Storage And Retrieval Of Information”, which, as previously mentioned, is incorporated by reference in its entirety herein, to read polarized signals from a magneto-optical (MO) media. The system  100  includes a multimode laser  110 , a leaky beam splitter (LBS)  120 , a first half-wave plate (HWP 1 )  130 , a second half-wave plate (HWP 2 )  177 , a first quarter wave plate (QWP 1 )  185 , a second quarter wave plate (QWP 2 )  114 , a polarimetric delay line (PDL)  160 , a plurality of polarization-maintaining (PM) fibers  180 , a plurality of magneto-optical (MO) media  190  and a differential detection module  116 . To avoid unneeded complexity, only one PM fiber  180  and one MO medium  190  will be illustrated and primarily discussed. One skilled in the art will recognize that the same principles that apply to one PM fiber  180  and one MO medium  190  applies to the multiple PM fibers  180  and multiple MO medium  190 . In addition, one skilled in the art will recognize that the same principles, which apply to a preferred embodiment, also will apply to alternative embodiments, such as a fiber optic current sensor, where laser noise also must be minimized. 
     As illustrated in FIG. 2, the multimode laser  110 , which in a preferred embodiment is a Fabry-Perot (FP) laser diode, generates multiple pulsed longitudinal modes (main light signal), which carry the magneto-optical (MO) signal from the MO medium  190  to the differential detection module  116 , at a certain pulse frequency (e.g., typically 300-500 MHz). The LBS  120 , which is coupled to the multimode laser  110 , receives the main outgoing p-polarized light beam from the laser  110  and transmits most of this p-polarized light (e.g., approximately 80%) of the main light signal along a forward path toward the PDL  160  and the MO medium  190 . 
     In a preferred embodiment, the HWP 1   130 , which is coupled between the LBS  120  and a first end of the PDL  160 , rotates the main light polarization by approximately 45 degrees to ensure that upon entering the PDL  160  on the forward path, the main light signal will split into two relatively equal component signals with the first component signal maintained as a p-polarized light (p-wave) signal and the second component signal transformed as an s-polarized light (s-wave) signal. For illustrative purposes only, the p-wave will be presumed to propagate through a first polarizing beam splitter PBS 1   169  along a short delay path length L p  of the PDL  160  and the s-wave signal is presumed to be redirected by the PBS 1   169  to propagate along a long delay path length L s  of the PDL  160 . The p- and s-waves are then recombined and made co-propagating by a second polarizing beam splitter PBS 2   170 . The differing delay path lengths result in a forward path difference Δd forward (PDL)  of (L s −L p ), which corresponds to a delay time τ forward(PDL)  between the s-wave signal and the p-wave signal of: 
     
       
         (Δ d   forward (PDL) )/ c =( L   s   −L   p )/ c,   
       
     
     where c is the value for the speed of light in a vacuum. 
       
     The HWP 2   177 , which is coupled between a second end of the PDL  160  and a first end of the PM fiber  180 , aligns the p-wave signal and the s-wave signal, which exit the PDL  160 , with the birefringent (Δn B =n slow −n fast ) axes of the PM fiber  180  which are illustrated in FIG.  3 . In particular, the HWP 2   177  aligns the s-wave signal to propagate along the slow axis n slow  of the PM fiber  180  and the p-wave signal to propagate along the fast axis n fast  of the PM fiber  180 . In a preferred embodiment, the birefringence Δn B  is typically in the range from 10 −4  to 10 −3 . In alternative embodiments the s-wave and p-wave signals are aligned to propagate along the opposite fiber axes. 
     FIG. 4 illustrates, for a 2-mode laser, the manner in which mode partition noise (MPN) can arise in a preferred embodiment of the present invention. In particular, the figure illustrates two linearly polarized modes launched at 45 degrees into the PM fiber  180 . Each of the two modes splits into fast and slow polarization components inside the fiber  180 . Upon exiting the fiber each of the two longitudinal modes assumes a random polarization state that depends on the relative phase shift between the fast and slow polarization components. Because the two longitudinal modes are dynamically competing with one another for power as discussed with regard to FIG. 2, the net polarization state at the output of the fiber  180  fluctuates in time, producing polarization noise (e.g., mode partition noise). Upon passing through a polarizer, this polarization noise is converted into intensity noise. 
     Since the slow axis of the PM fiber  180  has a refractive index, which is larger than the refractive index of the fast axis, the s-wave signal will propagate along the slow axis at a slower phase velocity. In addition, the slower phase velocity also corresponds to an optical path length F s , which is longer than the optical path length F p  of the fast axis. The relative optical path difference Δd forward fiber  between the slow axis and the fast axis can be expressed as (F s −F p ), which corresponds to a relative time delay, τ fiber (forward) , of (L fiber Δn B )/c (where L fiber  is the physical length of the PM fiber  180 , Δn B  is the birefrigence of the PM fiber  180  and c is the speed of light in a vacuum) between the p-wave and s-wave signals. This time delay, τ fiber(forward) , plus that of the PDL, τ PDL (forward) , corresponds to a net forward path phase difference of φ k =ω k  (τ forward (PDL) +τ forward (fiber) ) (where          ω   k     =       2                 π                 c       λ   k                              
     and λ k =wavelength of k th  laser mode) for the k th  laser mode, which in turn determines the polarization for the k th  laser mode at the fiber output. With each mode of the laser generally having a different polarization when exiting the second end of the PM fiber  180  in conjunction with the intensities of each mode fluctuating, the total PM fiber output polarization fluctuates at the second end of the PM fiber  180 , resulting in significant MPN. 
     Elimination of MPN and any accompanying laser phase noise can be achieved by reducing the optical path difference between the two polarization components of the main light signal to zero. In a preferred embodiment, such a reduction of the optical path difference is achieved by double passing the QWP 1   185 , thereby reorienting the polarization by 90 degrees for the return path through the PM fiber  180  and PDL  160 . In particular, the QWP 1   185  is aligned at 45° with respect to the fiber axes at the second end of PM fiber  180  so that the QWP 1   185  converts the two linearly polarized components, the s-wave signal and the p-wave signal, into left and right circularly polarized states. Upon reflection from the MO medium  190 , the sense of the two circular states is reversed (e.g., right polarization becomes left polarization and left polarization becomes right polarization). After passing through QWP 1  on the return path, the circular states are converted back to corresponding linear states with a 90° rotation. 
     Upon entering the second end of PM fiber  180  on the return path, the 90 degree polarization rotation of the main light signal results in a compensation of the optical path. For example, the first component of the main light signal, which originally propagated as a p-wave signal on the forward path along the short delay path length L p  of the PDL  160  and along the fast axis F p  of the PM fiber  180 , now propagates on the return path as an s-wave signal along the slow axis F s  of the PM fiber  180  and the long delay path length L s  of the PDL  160 . The original s-wave signal, which is now a p-wave signal, propagates on the return path along the fast axis F p  of the PM fiber  180  and the short delay path length L p  of the PDL  160 . By having each component of the main light signal propagate down one delay path length of the PDL  160  and one axis of the PM fiber  180  on the forward path and the opposite delay path length and fiber axis on the return path, the recombined main light signal, which exits the first end of the PDL  160 , does not experience a net optical path difference like the main light signal exiting the second end of the PM fiber  180 . This lack of a significant net optical path difference in the main light signal results in the minimization of the relative delay time between the two components of the main light signal, thereby avoiding the development of SPN. In particular, at the end of the round trip propagation through both the PDL  160  and the PM fiber  180 , the first component and the second component of the main light signal will have propagated the equal combined length of Lp+Fp+Fs+Ls and Ls+Fs+Fp+Lp, respectively. 
     FIGS.  8 ( a ) and ( b ) illustrate the nature of the return polarization state of the main light signal upon round-trip propagation through the system  100 . In FIG.  8 ( a ) with the absence of a Kerr effect, the polarization state reflecting off of the LBS  120  is identical to the input state, namely the p-wave signal. In FIG.  8 ( b ) after the p-wave signal passes through the QWP 2   114 , which is oriented at 45 degrees, this p-wave signal is converted into a circular polarized state which, due to the balance of this signal, produces a zero output signal in the differential detector  116 . 
     In the presence of a Kerr effect, FIG.  8 ( a ) illustrates that a small s-wave is produced that is either positive or negative 90 degrees out of phase with the p-wave depending on the sign (e.g., up or down) of the magnetism being probed at the MO disk  190 . After LBS  120 , FIG.  8 ( a ) illustrates that with the Kerr effect the main light signal has a slight elliptical polarization with either a right-hand (magnetism down) or left-hand (magnetism up) sense of rotation. After passing through QWP 2   114 , FIG.  8 ( b ) illustrates that these two states can be distinguished from one another by the differential detection  116  because the Kerr and non-Kerr light are brought back in phase and interfere to produce distinguishable difference signals. 
     Even though the system  100  theoretically can eliminate the relative time delay of the two components of the main light signal by just providing a net optical path difference of zero, unavoidable misalignment errors within the system  100  cause a parasitic light signal to be present which in turn affects the noise level of the detected signal. In particular, errors typically arise in individual optical components or in their respective alignment to one another. In a preferred embodiment, one component of particular interest is QWP 1   185 , which can reside on an optical recording head. Errors in both the retardation and alignment of QWP 1   185  are typically difficult to control due to the small physical size of this component (e.g., 0.090×0.20×1.0 mm 3 ). 
     The parasitic light signal is that portion of the light that propagates through the optical system and reaches the differential detection module with the two principal polarization components having experienced a non-zero optical path difference. In particular, each error in the thickness or the rotational alignment of the QWP 1   185  will produce parasitic light, which can in turn causes large first-order SPN (e.g., SPN due to one component error). In addition, second-order MPN (e.g., SPN due to a combination of two component errors) also may develop if the PDL  160  and the PM fiber  180  are misaligned in combination with an error in QWP 1 . In a preferred embodiment, the misalignment between the PM fiber  180  and the PDL  160  is avoided by grouping all of the PM fibers  180  into an array such that all of the axes of the PM fibers  180  are well aligned (e.g., with less than 1 degree of error) with respect to one another. An alternative embodiment for avoiding misalignment between the PM fibers  180  and the PDL  160  is to utilize a dynamic electrically controlled polarization rotator (e.g., by placing HWP 2   177  on an electrically controlled rotation stage or by replacing HWP 2   177  with a nematic liquid crystal cell used in combination with a quarter-wave plate) that can be used between the PDL  160  and the first end of the PM fiber  180  to provide active alignment for switching between PM fibers  180 . 
     The parasitic light signal, which is generated by a retardation or orientation error in QWP 1   185 , propagates on the return path along the same axis and path length as originally propagated along in the forward path. By failing to travel on the opposite axis and delay path length, the s- and p-components of the parasitic light signal experience a significant optical path length difference of 2(L s +F s )−2(L p +F p ) upon reaching the differential detection module, which in turn may produce SPN that degrades the system signal-to-noise ratio. In particular, the parasitic light signal on one axis will be time delayed and the parasitic light signal on the opposite axis will be time advanced relative to the main signal wave components (which both travel the same optical path). Generally speaking (e.g., when the laser is operated in a continuous wave fashion or under some arbitrary modulation condition), the parasitic waves will, in the detection system, overlap in time with one another and with the main light signal. As a result, these superimposed parasitic light signals and main light signal will interfere with each other. This interference can result in large amounts of SPN, causing the differential detection module  116  to have difficulty detecting the Kerr effect within the main light signal. Only if the laser diode is modulated in an on-off fashion at a proper frequency can the effects of SPN be overcome. When utilizing proper modulation, as disclosed herein, the parasitic light signal and the main light signal do not overlap in time, thereby eliminating the interference between the parasitic light signals and the main light signal, which in turn precludes the formation of SPN. More specifically, the PDL  160  creates a path length difference, which is sufficiently large when compared to those contributed only by a short piece of PM fiber  180 , to allow a reasonable modulation frequency to be able to be used to produce the necessary time separation. 
     With proper laser modulation (the details being subsequently provided), the PDL  160  eliminates first-order SPN due to a component error in QWP 1  by time shifting the parasitic light signals substantially away from each other and from the main light signal. In particular, the PDL  160  utilizes a physical one-way delay path length difference (L p −L s ) of approximately 0.2-0.5 meter. With such a small physical delay length, the PDL  160  is easy and inexpensive to implement. In addition, the PDL  160  minimizes the amount of both mode partition noise and laser phase noise present in the system  100  to enable the effects of laser noise to be minimized to approximately the shot-noise-limited performance level. 
     In a preferred embodiment, the PDL  160  comprises a first polarization beam splitter (PBS 1 )  169 , a second polarization beam splitter (PBS 2 )  170 , a first mirror  165  and a second mirror  175 . Prior to the main light signal entering the PDL  160  on the forward path, the HWP 1   130  orients the light signal by 45 degrees to ensure that the PBS 1   169 , which is coupled to the HWP 1   130 , receives and splits the main light signal into two equal-amplitude components with the transmitted component, a p-wave signal, and the reflected component, an s-wave signal. The PDL  160  then allows the s-wave signal to propagate along the optical path length L s  by redirecting the s-wave signal to reflect off of both the first mirror  165  and the second mirror  175 . The p-wave signal propagates along the delay path length L p  by direct transmission through PBS 1   169  and PBS 2   170 . The PBS 2   170  receives both the s- and p-wave signals and recombines them back into the main light signal. The PBS 2   170  receives both the s- and p-waves and directs both of them through the HWP 2   177  (or an equivalent polarization rotator) into one of the set of PM fibers  180 , which effectively acts as an extension of the PDL  160 . The alignment between the PDL  160  and the PM fiber  180  by the HWP 2   177  results in the s- and p-waves leaving the PDL  160  and entering onto the fast and slow fiber axes of the PM fiber  180 , respectively (or vice versa). The resulting optical path difference Ad between the p-wave signal and the s-wave signal components in the forward path is therefore: 
     
       
         Δ d =( L   s   +F   s )−( L   p   +F   p )=( L   s   −L   p )+Δ nL   fiber , 
       
     
     where Δn is the fiber birefringence and L fiber  is the PM fiber length. In a preferred embodiment, with L p =10 cm, L s =50 cm, Δn=10 −3 , and L fiber =100 cm, the forward path difference Δd would equal 40.1 cm. 
     In an alternative embodiment, the HWP 2   177  can be eliminated if the axes of the PDL  160  are mechanically aligned with sufficient precision (e.g., less than one degree) with the axes of each PM fiber  180 . In an additional alternative embodiment, this system  100  also could be constructed from one long piece of PM fiber  180 , but the corresponding length would be approximately 401 meters. The PDL  160  of a preferred embodiment, however, offers a more compact and less expensive implementation. 
     The main light, after leaving the PM fiber  180  in the forward path, passes through QWP 1   185 , reflects off of the MO disk  190  and once again goes through QWP 1   185 . Double passing QWP 1   185  converts the outgoing s-wave signal into a p-wave signal on the return trip and the outgoing p-wave signal into an s-wave signal. To the extent that QWP 1   185  has errors in either its retardation (e.g., its phase shift departs from 90 degrees) or its 45 degree orientation, parasitic waves will exist. These parasitic waves correspond to that portion of the outgoing light that is not properly converted by QWP 1   185 , namely outgoing s-waves that return as s-waves and outgoing p-waves that return as p-waves. Each polarization component of the main light signal, therefore, propagates along both delay path lengths of the PDL  160  while each component of the parasitic light signal only travels along one of the two optical paths lengths. In this way the two polarization components of the parasitic light signal will experience a round-trip time shift relative to the main light signal and themselves. 
     For example, as illustrated in FIG. 6, upon exiting the first end of the PDL  160  on the return path, both the s- and p-wave components of the main light signal  610  have propagated a total optical length of L p +F p +F s +L s . Since the parasitic light signal propagates along the same fiber length and delay path length on both the forward and return paths, a p-wave parasitic light signal  630  would propagate a shorter net optical length of 2F p +2L p  and an s-wave parasitic light signal  620  would propagate the longer net optical length of 2F s +2L s . This difference in the net optical length between the two components of the parasitic light signal and the main light signal results in the p-wave parasitic light signal  630  being time shifted ahead of the main light signal and the s-wave parasitic light signal  620  time shifted behind the main light signal. 
     To ensure approximately complete time-separation of both the s-wave and p-wave parasitic light signals from the main light signal, the multimode laser  110  is pulsed on and off at a duty factor of approximately 33%. Such a duty factor ensures that each of the three time separated pulses (e.g., the s-wave and p-wave signals of the parasitic signal and the main light signal), which are of the same temporal width, each will separately monopolize approximately ⅓ of the laser pulse period. Failure to utilize such a duty factor causes the components of the parasitic light signal to overlap with other light signals. For example, in a continuous wave laser environment, the parasitic light signal components would temporally overlap (e.g., overlap in time) with each other as well as with the main light signal, thereby generating SPN effects in the detection channel. 
     With the forward path difference (Δd forward(PDL) ) in the PDL  160  in a preferred embodiment set in a manner that ensures that the one-way delay time (τ forward(PDL) =Δd forward(PDL) /c) is approximately one-third of the laser modulation period (T laser ), the path difference Δd forward(PDL)  of the two path lengths is c(T laser )/3. By pulsing the multimode laser  110  on and off in a preferred embodiment at a high frequency (e.g., approximately 100-1000 MHz) and designing the PDL  160  to have such an appropriate frequency-dependent delay path difference          Δ                   d     forward        (   PDL   )           =           cT   laser     /   3                   Δ                   d     forward        (   PDL   )           =       cT   laser     3                              
     (e.g., 10-70 cm), the PDL  160  effectively minimizes the effects of the parasitic light signal in the system  100 , thereby minimizing the SPN that otherwise corrupts the main light signal. For example, as illustrated in FIG. 7, when the laser modulation frequency of an illustrative embodiment is 450 MHz, a path difference Δd forward(PDL)  of approximately 30 cm is required. More specifically, for small path differences (e.g., less than about 0.1 m), the noise is dominated by mode partition effects and therefore shows significant structure with noise minima occurring at coherence peaks. The lowest noise region of the illustrative embodiment as illustrated in FIG. 7 occurs at a path difference of approximately 0.33 m, at which point the two light signals exiting the interferometer do not overlap in time. As the path difference increases beyond 0.33 m, the noise again increases and is dominated by laser phase noise effects. Such an illustrative example demonstrates the principle utilized in various embodiments of the present invention. 
     Even though a preferred embodiment focuses upon a free-space version of PDL  160  having propagation paths with a refractive index approximately equal to air (i.e., n=1), alternative embodiments of the present invention achieve the same necessary frequency-dependent optical path differences by utilizing guided-wave propagation paths with refractive indices greater than 1. For example, FIG. 9 illustrates a first alternative embodiment of the PDL  160  comprising the PBS 1   169 , the PBS 2   170  and a fiber collimator  910 . The fiber collimator  910  enables the system  100  to be more compact and manufacturable. In particular, the fiber collimator  910  comprises a first GRIN lens  920  (e.g., with a 0.25 pitch), a second GRIN lens  930  (e.g., with a 0.25 pitch) and a fiber  940 , which has a refractive index of approximately 1.5. The fiber collimator  910  accepts and outputs a collimated free-space beam of the appropriate diameter. The fiber  940  in this embodiment of the PDL  160  can be a polarizing fiber (e.g., PZ fiber), PM fiber, or a suitably routed low-birefringence (Lo-Bi) fiber. The primary constraint for an alternative embodiment is that the fiber  940  must propagate a single linear polarization state (e.g., s-wave signal) with a high extinction ratio. 
     FIG. 10 illustrates a second alternative embodiment of the system  100  with an all-fiber version of the PDL  160 . In particular, this PDL  160  comprises polarization beam splitters  1010  within the fiber  1045 . The long path length L s  is provided by fiber  940  while the small path length L p  of the PDL  160  is provided by fiber  1045 . To make the device compatible with collimated free-space operation, the first GRIN lens  920  (e.g., with a 0.25 pitch) and the second GRIN lens  930  (e.g., with a 0.25 pitch), respectively, are placed at the input and the output of the fiber  940 . The all-fiber PDL approach depicted in FIG. 10 eases difficulty of alignment of the components compared to the free-space version of FIG.  1  and the hybrid approach of FIG. 9; however, the all-fiber version requires high-performance PM splitters  1010 . Furthermore, to avoid reflections from the end surfaces of the fiber  940 , which could increase the SPN in the system  100 , the ends of the fiber  940  are angle-cleaved and carefully aligned with the GRIN lenses  920  and  930  that have been similarly angle polished. When compared to straight-cleaved fibers  940 , angle-cleaved fibers  940  generally result in the forward coupling efficiency for the system  100  to be as high as 70-80%. End face reflections from straight-cleaved fiber  940  also can be eliminated by using index-matching epoxy between the GRIN lenses  920  and  930  and the fiber ends. 
     The foregoing description of the preferred embodiments has been presented for purposes of illustration and description. It is not intended to be exhaustive nor to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. Preferred embodiments were chosen and described to best explain the principles of the present invention and its practical application to thereby enable others skilled in the art to best utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims and their equivalents.