Abstract:
Phase noise is at least partially cancelled for an interferometric system by using a delay/phase cross-correlation approach for two interferometers within the system. The cross-correlation approach may be used in measuring group delay of a device under test and includes determining the differences between the phase of the output of each interferometer at time t and the phase of the same output at the time t minus the delay of the other interferometer. In one embodiment, the first phase difference is the difference between the phase of a test interferometer output at time t and the phase of the test interferometer output at the time t offset by the known delay of a reference interferometer. The second phase difference is calculated using the same technique, but the time offset is a delay representative of the relative delay of two light propagations within the test interferometer. A noise-cancelled time series output that is indicative of group delay can then be generated by determining the difference between the first and second differences.

Description:
TECHNICAL FIELD 
   The invention relates generally to obtaining measurements for optical characteristics of a device under test and more particularly to canceling phase noise from measurements of group delay introduced by the device under test. 
   BACKGROUND ART 
   Techniques for testing or analyzing optical components are known. A “device” under test (DUT), such as a length of fiber optic cable, may be carefully tested for faults or may be analyzed to determine whether the device is suitable for use in a particular application. System components such as multiplexers, demultiplexers, cross connectors, and devices having fiber Bragg gratings may be separately tested before a system is assembled. 
   Optical testing may be performed using a heterodyne optical network analyzer. Such analyzers may be employed for measuring properties of optical components, such as group delay. “Group delay” is sometimes referred to as envelope delay, since it refers to the frequency-dependent delay of an envelope of frequencies, with the group delay for a particular frequency being the negative of the slope of the phase curve at that frequency. Typically, a heterodyne optical network analyzer includes two interferometers. An example of a heterodyne optical network analyzer  10  having two interferometers  12  and  14  is shown in  FIG. 1. A  tunable laser source (TLS)  16  generates a laser light beam that is split by a coupler  18 . The TLS is continuously tuned, or swept, between a start frequency and a stop frequency. By operation of the coupler  18 , a first portion of the coherent light from the TLS is directed to the DUT interferometer  12 , while a second portion is directed to the reference interferometer  14 . 
   The DUT interferometer  12  has a second coupler  22  that allows beam splitting between a first arm  24  and a second arm  26 . A mirror  28  is located at the end of the first arm and a DUT  20  is located near the reflective end of the second arm. The lengths of the two arms can differ, and the difference in the optical path length is represented in  FIG. 1  by L DUT . Since the DUT can be dispersive, the actual optical path length is a function of frequency. A detector  30  is positioned to measure the combination of the light reflected by the mirror  28  and the light reflected at the DUT  20 . Processing capability (not shown) is connected to the detector  30  to measure group delay of the DUT as a function of frequency. However, in order to very precisely measure the group delay, it is necessary to obtain knowledge of the frequency tuning of the TLS  16  as a function of time. The reference interferometer  14  is used for this purpose. 
   The structure of the reference interferometer  14  is similar to that of the DUT interferometer  12 , but a mirror  32  takes the place of the DUT  20 . A second detector  34  receives light energy that is reflected by the combination of the mirror  32  at the end of a third arm  36  and a mirror  38  at the end of a fourth arm  40 . As in the DUT interferometer, the lengths of these two arms can be different, and this difference in lengths is represented by L REF . The optical characteristics of the reference interferometer are fixed and known. 
   A potential problem occurs in the heterodyne optical network analyzer  10  when the path length difference (L DUT ) is sufficiently large that coherence effects become an issue. The frequency generated by the TLS  16  undesirably fluctuates in a random manner around its target frequency as it is tuned. The random fluctuations occur as a result of various quantum or stochastic effects. The random fluctuations of the frequency affect the frequency of the heterodyne interference signal measured by each detector  30  and  34 . When the group delay of the DUT  20  is calculated, the frequency fluctuations of the TLS  16  manifest themselves as noise in the group delay measurement. This ultimately limits the precision of the measurement process. This effect is referred to as “phase noise.” The phase noise on the measurement process increases as the path length mismatch for the two arms  24  and  26  of the DUT interferometer  12  increases, until the path length mismatch equals or exceeds the coherence length of the laser beam. 
   What is needed is a method and system for at least reducing the deleterious effects of phase noise in an interferometric system. 
   SUMMARY OF THE INVENTION 
   A reduction in the effects of phase noise introduced into an interferometric system is achieved by using a reference interferometer to “measure” the effects. A coherent light beam having both intentional frequency variations and undesired frequency fluctuations is divided into separate beam portions which are directed to the reference interferometer and a test interferometer. The reference interferometer has known optical delay characteristics and the test interferometer has known or estimated optical delay characteristics, allowing a delay/phase cross-correlation for each of the two interferometers. That is, delay information regarding one of the interferometers is used with phase information acquired from the other interferometer in the cancellation of phase noise effects. Typically, the method is used to eliminate the adverse effects of the phase noise within the test interferometer, but embodiments are contemplated in which the approach is used to offset phase noise effects in other components, such as a separate optical system in which a third portion of the coherent light beam is directed for other purposes. 
   The intentional variation of the light beam frequency is provided by operation of a tuned laser source that continuously sweeps through a frequency range. On the other hand, the undesired frequency fluctuations are random and occur as a result of quantum or other stochastic effects in the generation or manipulation of the light beam. These random fluctuations produce the phase noise effects. 
   In one embodiment, the cross-correlation approach includes determining the differences between the phase at the output of each interferometer at time t and the phase at the same output at the time t minus the delay of the other interferometer. That is, for each time t in a time series, a first phase difference is determined for the test interferometer and a second phase difference is determined for the reference interferometer. The first phase difference is the difference between the phase of the test output at the time t and the phase at the test output at the time t offset by the known delay of the reference interferometer. The second phase difference is the difference between the phase at the reference interferometer output at time t and the phase at the reference interferometer output at time t offset by a delay representative of the delay of the test interferometer. The representative delay may be a calculation of the mean of the delay, as determined using other techniques. Within this embodiment, the time series may be formed by determining the difference between the first and second difference. This double-difference technique provides an isolation of the random phase noise introduced by operation of the light beam source. 
   Typically, but not critically, the test interferometer includes a device under test (DUT) for which group delay is being measured. Thus, the phase noise is used to reduce or eliminate the adverse effects of such noise in the calculation of DUT group delay. The value of mean delay that is used in the determination of the second phase difference may be obtained using known techniques, such as optical frequency domain reflectometry (OFDR) or optical coherence domain reflectometry (OCDR). 
   An advantage of the invention is that more reliable determinations of the optical characteristics of a DUT can be achieved. Heterodyne optical network analyzers operate by splitting and then recombining a coherent light beam. When the split beams are recombined, the random frequency fluctuations of phase noise limit the precision of the measurement procedure. Thus, for a laser having a 100 kHz linewidth, the phase noise can be a limiting factor in measurement precision with only a few meters of delay introduced by a DUT in the interferometer. It follows that phase noise renders measurements of group delay and group velocity dispersion for particularly long DUTs, such as 10 km lengths of fiber, necessarily unreliable. However, the phase noise reduction of the invention allows high-delay devices to be analyzed. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram of a heterodyne optical network analyzer which may be used in measuring optical characteristics of a device under test. 
       FIG. 2  is a block diagram of a system for controlling phase noise in the analyzation of optical characteristics of a device under test, in accordance with the invention. 
   

   DETAILED DESCRIPTION 
   With reference to  FIG. 2 , a heterodyne optical network analyzer  42  having phase noise cancellation is shown as being used for measurements of group delay, as indicated by a group delay module  44 . However, the analyzer may be used for other measurements relevant to optical characteristics of a device under test (DUT), such as measurements of group velocity, transmissivity, reflectivity and chromatic dispersion. Moreover, the phase noise cancellation may be quantified using the techniques to be described below, but the quantifications may be applied in other systems in which beam portions are separately conducted for comparison purposes. 
   The analyzer  42  is shown as including components that are the functional equivalents of components of FIG.  1 . The coherent light beam that is generated by the TLS  16  is split by a coupler  18  into beam portions that are separately directed to the test interferometer  12  and the reference interferometer  14 . The interferometers  12  and  14  may be structurally identical to the ones shown in  FIG. 1 , but the detectors  30  and  34  are shown as being separated from the interferometers in FIG.  2 . The interferometers of  FIG. 2  need not be identical to the interferometers of FIG.  1 . In addition to the conventional Michelson and Mach-Zehnder configurations, the invention may be used with other interferometer architectures. 
   In the embodiment of  FIG. 2 , the test interferometer  12  includes the capability of being attached to a DUT. For example, the DUT may be a length of fiber, a multiplexer, a demultiplexer, or a cross connector. The optical characteristics of the DUT will affect the characteristics of the light that reaches the test detector. However, there may be embodiments in which the optical characteristics of the test interferometer remain fixed, in the same manner as the reference interferometer  14 . 
   As is known in the art, the TLS  16  generates swept-frequency light that is split by the coupler  18  and directed into the two interferometers  12  and  14 . Each detector  30  and  34  may be a photoreceiver that measures an intensity I as a function of time t, where
 
 I ( t )= I   arm1   +I   arm2 +2( I   arm1   I   arm2  cos Φ( t ))  Eq. 1
 
That is, the measured intensity is a function of the intensities of the light from the two arms and is a function of the phase of the light at time t. For group delay measurements, Φ(t) is an important component of Eq. 1.
 
   The phase Φ TI (t) measured by the test interferometer  12  at time t is 
                       ⁢         Φ   TI     ⁡     (   t   )       =       ⁢       2   ⁢     π   ⁡     [       v   0     +     γ   ⁢           ⁢   t     -       γ   2     ⁢     τ   TI       +     χ   ⁡     (     t   -       τ   TI     2       )         ]       ⁢     τ   TI       +                       ⁢       ϕ   ⁡     (   t   )       -     ϕ   ⁡     (     t   -     τ   TI       )                     =       ⁢         ω   ⁡     (     t   -       τ   TI     2       )       ⁢     τ   TI       +     ϕ   ⁡     (   t   )       -     ϕ   ⁡     (     t   -     τ   TI       )                     =       ⁢         ω   TI     ⁢     τ   TI       +     ϕ   ⁡     (   t   )       -     ϕ   ⁡     (     t   -     τ   TI       )                       Eq   .           ⁢   2             
 
where the subscript “TI” indicates that the variable is associated with the test interferometer, ω(t) is the radian frequency produced by the TLS  16 , ν O  is the initial frequency of the swept laser light, γ is the rate of the linear sweep in units of Hz/second, χ(t) represents the nonlinear components of the frequency sweep, φ(t) represents the random phase evolution associated with the finite coherence of the TLS  16 , and τ TI  is the delay introduced by the DUT. For a dispersive DUT, τ TI  can vary with frequency. In fact, the optical path length mismatch, L DUT , is proportional to τ TI . The reference interferometer has no dispersive elements, and consequently, the corresponding delay in the reference interferometer,  96   RI , is assumed to be constant. By analogy to Eq. 2, the phase of the reference interferometer, Φ RI , at time t can be determined to be
 
Φ RI ( t )=ω RI τ RI +φ( t )−φ( t−τ   RI )  Eq. 2.1
 
where “RI” indicates that the variable is associated with the reference interferometer. The optical radian frequency ω(t) produced by the TLS is swept in time and can be written as
 
ω( t )=2π[ν 0   +γt +χ( t )]  Eq. 3
 
   At least with regard to this description of the invention, the TLS  16  is modeled as a quasi-monochromatic light source, where the light waves E generated by the TLS satisfy
 
 E ( t )= E   o   e   iω(t)t+φ(t)   Eq. 4
 
When the random phase evolution (φ) at time t is approximately the same as the random phase evolution at the time t offset by τ TI  (as will occur when τ TI  is very short compared to the coherence time of the TLS), the group delay τ g  of the device under test can be obtained from 
                 τ   g     ≡       ⅆ     Φ   TI         ⅆ   ω         =           ⅆ     Φ   TI         ⅆ   t           ⅆ     ω   TI         ⅆ   t         =       τ   TI     +       ω   TI     ⁢       ∂     τ   TI         ∂     ω   TI                       Eq   .           ⁢   5             
 
However, when τ TI  becomes larger, the phase noise terms begin to induce significant errors that ultimately are so large as to render the measurement of the group delay unreliable.
 
   Therefore, the invention uses the reference interferometer  14  to “measure” the phase noise to allow cancellation of its effects. In  FIG. 2 , a first differencing module  46 , a second differencing module  48 , and a phase noise cancellation module  50  are used to enable phase noise cancellation for the measurements that occur at the group delay module  44 . Typically, the operations of these modules are executed in programming (software modules), but specific hardware circuitry can be dedicated to enabling the operations. That is, the term “module” should be interpreted herein as including programming, circuitry or a combination of programming and circuitry. The differencing modules and the phase noise cancellation module cooperate to provide a double-difference time series Z(t) where
 
 Z ( t )=Φ TI ( t )−Φ TI ( t−τ   RI )−[Φ RI ( t )−Φ RI ( t−τ   10 )]  Eq. 6
 
In Eq. 6, the first phase measure (Φ TI (t)) is determined from the test output  52 , while the second phase measurement is the phase at time t offset by the delay imposed within the reference interferometer  14 . This offset delay is represented by component  54  in FIG.  2 . The third measure of phase within Eq. 6 is determined from the reference output  56  from the detector  34 , while the last phase measurement is the phase with the additional offset τ 10 . As will be explained more fully immediately below, the offset, τ 10 , is based upon an approximation of the delay of the test interferometer (e.g., the mean of τ TI ) In  FIG. 2 , the delay offset component  58  is used by the second differencing module  48  to generate the fourth phase measurement. In its simplest form, the phase noise cancellation component  50  merely determines the difference between the two phase differences computed by the modules  46  and  48 . That is, the phase noise cancellation component  50  generates the double-difference time series Z(t) of Eq. 6.
 
   The offset delay, τ 10 , imposed by the component  58  represents the delay at the test interferometer  12 . The imposed offset may be a constant that is assumed to be approximately equal to the mean of the test interferometer delay. For optimal results, the offset should be sufficiently close to the test interferometer delay such that for all of the measured frequencies, ω(t−τ TI )−ω(t−τ 10 )≅0. The value for the offset can be obtained using known techniques, such as those used in OTDR or OFDR. Rather than a constant, the offset may vary with laser beam frequency, so that, like the actual test interferometer delay, the offset is a function of frequency (which is a function of time during the sweep of the TLS  16  through the frequency range). 
   The first of the four phase measures of Eq. 6 can be replaced with Eq. 2. Similarly, the third phase measure can be replaced with Eq. 2.1, as can the phase measures having the offsets, yielding the time series, Z(t) as 
                 (         ω   TI     ⁢     τ   TI       +     ϕ   ⁡     (   t   )       -     ϕ   ⁡     (     t   -     τ   TI       )         )       ︸       Φ   TI     ⁡     (   t   )           -       (         (       ω   TI     ⁡     (     t   -     τ   RI       )       )     ⁢     (       τ   TI     ⁡     (     t   -     τ   RI       )       )       +     ϕ   ⁡     (     t   -     τ   RI       )       -     ϕ   ⁡     (     t   -     τ   TI     -     τ   RI       )         )       ︸       Φ   TI     ⁡     (     t   -     τ   RI       )           -             [     (         ω   RI     ⁢     τ   RI       +     ϕ   ⁡     (   t   )       -           ⁢     ϕ   ⁡     (     t   -     τ   RI       )         )         ︸       Φ   RI     ⁡     (   t   )             -           (         (       ω   RI     ⁡     (     t   -     τ   10       )       )     ⁢     τ   RI       +     ϕ   ⁡     (     t   -     τ   10       )       -     ϕ   ⁡     (     t   -     τ   10     -     τ   RI       )         )     ]     ︸         Φ   RI     ⁡     (     t   -     τ   TI       )                 Eq   .           ⁢   7             
 
Assuming that φ(t−τ TI ) is approximately equal to φ(t−τ 10 ), the phase noise components in Eq. 7 cancel. Consequently,
 
 Z ( t )=ω TI τ TI −(ω TI ( t−τ   RI ))(τ TI ( t−τ   RI ))−[ω RI τ RI −ω RI ( t−τ   10 )τ RI ]  Eq. 8
 
In Eq. 2, since 
       ω   ⁢     (     t   -       τ   TI     2       )         
 
is generally equal to ω TI , 
               Z   ⁡     (   t   )       =       ⁢         ω   ⁡     (     t   -       τ   TI     2       )       ⁢     τ   TI       -       (     ω   ⁡     (     t   -     τ   RI     -       τ   TI     2       )       )     ⁢           ⁢     (       τ   TI     ⁡     (     t   -     τ   RI       )       )       -   ⁠   ⁢     [           ⁢         ω   ⁡     (     t   -       τ   RI     2       )       ⁢     τ   RI       -       ω   ⁡     (     t   -     τ   10     -       τ   RI     2       )       ⁢     τ   RI         ]               Eq   .           ⁢   9             
 
   In addition to the output  60  of the phase noise cancellation module  50 , the group delay module  44  receives an output  62  of a tuning detector  64 . The tuning detector  64  is a module which is conventional to heterodyne optical network analyzers and is used to detect the frequency sweep of the TLS  16 . The operations of the tuning detector  64  and the group delay module  44  are most likely carried out in software. That is, the operations are not executed using circuitry that is separate from other components of the system  42  of FIG.  2 . There are a number of approximations that may be used to simplify Eq. 9 in the group delay module  44  of FIG.  2 . 
   The simplification approximations are appropriate when  6  changes linearly on time scales of τ RI  or τ TI . A first appropriate approximation is 
               ω   ⁡     (     t   -     τ   RI     -       τ   TI     2       )       =       ⁢       ω   ⁡     (     t   -       τ   RI     2       )       -       (         τ   RI     2     +       τ   TI     2       )     ⁢       ∂   ω       ∂   t       ⁢     (     t   -       τ   RI     2       )                 Eq   .           ⁢   10             
 
Another simplification is 
                 τ   TI     ⁡     (     t   -     τ   RI       )       =         τ   TI     ⁡     (     t   -       τ   RI     2       )       -         τ   RI     2     ⁢       ∂     τ   TI         ∂   t       ⁢     (     t   -       τ   RI     2       )                 Eq   .           ⁢   11             
 
Using the approximations of Eqs. 10 and 11, we see 
               Z   ⁡     (   t   )       =       τ   RI     ⁢     {           ∂     ω   ⁡     (     t   -       τ   RI     2       )           ∂   t       ⁡     [         τ   TI     ⁡     (     t   -       τ   RI     2       )       -     τ   10     -         τ   TI     2     ⁢       ∂       τ   TI     ⁡     (     t   -       τ   RI     2       )           ∂   t           ]       +       ω   ⁡     (     t   -       τ   RI     2       )       ⁢       ∂       τ   TI     ⁡     (     t   -       τ   RI     2       )           ∂   t           }               Eq   .           ⁢   12             
 
Referring to the third phase measure Φ RI (t) in Eq. 7, it can then be determined that 
                 ⅆ       Φ   RI     ⁡     (   t   )           ⅆ   t       =         ∂     ω   ⁡     (     t   -       τ   RI     2       )           ∂   t       ⁢     τ   RI               Eq   .           ⁢   13             
 
With this information, the group delay (τ g ) can be recovered from 
                         Z   ⁡     (   t   )         ⅆ     Φ   RI           ⅆ   t       =       ⁢         τ   TI     ⁡     (     t   -       τ   RI     2       )       -     τ   10     -                     ⁢           τ   TI     2     ⁢       ∂   ω       ∂   t       ⁢     (     t   -       τ   RI     2       )     ⁢       ∂     τ   TI         ∂   ω       ⁢     (     t   -       τ   RI     2       )       +       ω   ⁡     (     t   -       τ   RI     2       )       ⁢       ∂     τ   TI         ∂   ω       ⁢     (       t   -     τ   RI       2     )                     =       ⁢         τ   g     ⁡     (     t   -       τ   RI     2       )       -     τ   10                     Eq   .           ⁢   14             
 
The group delay recovery is possible since the term 
           τ   TI     2     ⁢       ∂   ω       ∂   t       ⁢     (     t   -       τ   RI     2       )         
 
has been determined to be so small that the portion of the equation in which it is a multiplicand can be disregarded without significantly affecting the process. Moreover, since only the relative group delay is typically of importance, the constant term, τ 10 , does not interfere with the measurement, so that it can be disregarded or numerically removed.
 
   From the foregoing it is also possible to determine the relationship between ω and t. With this relationship, a resampling of 
         τ   g     ⁢     (     t   -       τ   RI     2       )         
 
results in 
               τ   g     ⁡     (     ω   ⁡     (     t   -       τ   RI     2       )       )             Eq   .           ⁢   15             
 
   Using these techniques, the group delay can be recovered substantially independently of any adverse effects of phase noise introduced by the TLS  16  of FIG.  2 . The technique may be used to measure group delay and/or group velocity dispersion of devices under test, where phase noise would otherwise be a problem, such as in the testing of fibers having lengths longer than 1 km. The phase noise cancellation by using the two interferometers may also be used in other applications.