WO2003102530A1

WO2003102530A1 - Multiple path interference measurement

Info

Publication number: WO2003102530A1
Application number: PCT/US2002/017294
Authority: WO
Inventors: Jerrold A. Cline; Donald C. Knowles, Jr.
Original assignee: Corning Incorporated
Priority date: 2002-05-30
Filing date: 2002-05-30
Publication date: 2003-12-11
Also published as: JP2005528605A; AU2002312223A1

Abstract

Disclosed is a method and apparatus for measuring multiple path interference in a part of an optical waveguide circuit. The apparatus includes a polarization scrambler to insure that the interfering signals traversing the alternative optical paths interfere with their respective polarization states aligned. The use of the polarization scrambler together with recordation of the maximum multiple path interference measured over a pre-selected time interval provides a value of multiple path interference most representative of the performance of the part of an optical waveguide circuit under test.

Description

Multiple Path Interference Measurement

Background of the Invention

[0001] 1. Field of the Invention

The invention is directed to a measurement apparatus and method for measuring multiple path interference of signals in a part of an optical waveguide circuit and particularly to such an apparatus that takes into account the polarization state of the signals.

[0002] 2. Technical Background

Multiple path interference (MPI) is noise caused by the interference of optical bit streams (optical signals) that travel at least two different optical path lengths in a part of an optical waveguide circuit. For example, the different modes in an optical waveguide fiber can constitute different path lengths for the optical bit streams, so that MPI can occur in a fiber that supports at least two modes. Alternative path lengths also can be traveled by light that has been scattered from an optical bit stream, either by double Rayleigh scattering or by multiple reflections at any of the fiber, planar, or free space devices that can be used as a part of an optical waveguide circuit. Such devices include fiber and planar waveguides as well as fiber, planar, or free space splices, connectors, filters, couplers, splitters, gratings, optical isolators, dynamic spectral analyzers, or other components that operate on optical signals. In double Rayleigh scattering, light back-scattered from the optical signal undergoes a second Rayleigh scattering, causing scattered light to propagate in the direction of the original optical signal. The original optical signal and double Rayleigh scattered light or light having undergone multiple reflections, traverse different optical path lengths and thus can interfere, resulting in beat noise and signal degraded.

[0003] In addition, the relative polarization states of the interfering electric fields determines the strength of their interaction and hence the intensity of noise in the signal due to MPI. To properly estimate impact of MPI on signal to noise ratio, the relative polarization of the electric fields traversing the multiple paths must be taken into account. [0004] Thus in order to properly design optical communications spans, there is a need to measure MPI induced in the optical waveguides and the components that together form an optical transmission path, especially those high performance optical transmission paths designed for use in high data rate or long distance communications systems. Because a desired level of MPI is typically in the range of -35 dB to -55 dB, the MPI measurement apparatus must be capable of measuring small amplitude signals and must be accurate and repeatable. (In this document, the power ratio dB refers to optical dB unless express reference is made to electrical dB. As is known in the art, 3 dB optical corresponds to 6 dB electrical.) Relative polarization state of the respective interfering electric fields must be taken into account.

[0005] The characterization of the maximum impact of MPI on a high performance transmission path is a crucial optical circuit design factor.

[0006] The present invention addresses the problem of making accurate, repeatable measurements of maximum MPI of optical signals propagating in a waveguide or in the devices included therein.

Summary of the Invention

[0007] One aspect of the present invention is an apparatus for measuring multiple path interference. The apparatus includes a light source for launching light into a part of an optical waveguide circuit. The light source is optically coupled to a first end of the part of an optical waveguide circuit. The light travels through the part of an optical waveguide circuit along at least two alternative paths of different optical path length. A detector at a second end of the part of an optical waveguide circuit converts the light to an electrical signal which is fed to an electrical spectrum analyzer. A polarization scrambler is optically coupled between the light source and the first end of the part of an optical waveguide circuit.

[0008] A polarization scrambler is a device that changes the polarization state of the launched light. An embodiment of a polarization scrambler includes an optical waveguide fiber wrapped around a frame. The frame can move in a manner that induces random bends in the fiber thus causing the fiber to have a birefringence that changes over time. [0009] In another embodiment, the polarization scrambler includes an optical waveguide fiber wrapped about a frame that can be deformed, i.e., changed in shape or size. The frame is deformed in a random way to induce random birefringence in the fiber.

[0010] To facilitate measurement of multiple path interference at different wavelengths of launched light, the apparatus can include an optical switch or coupler. The optical switch or coupler has a plurality of input ports which are coupled respectively to one or more light sources having respective desired frequencies. The switch or coupler has at least one output port coupled to the polarization scrambler.

[0011] Another aspect of the invention is a method of measuring multiple path interference of a part of an optical waveguide circuit. In the method in accord with the invention, light is launched into a first end of the part of an optical waveguide circuit. The polarization state of the launched light is continuously changed. The light passed through the part of an optical waveguide circuit and is incident upon a detector that converts the optical signal to an electrical signal which is fed to an electric spectrum analyzer. The electrical spectrum analyzer measures the spectral power density of the detected signal. The multiple path interference is determined from the spectral power density.

[0012] In an embodiment of the method, the determination of multiple path interference is repeated for a pre-selected time period. The time period is preferably less than or equal to about 5 minutes, more preferably less than about 3 minutes, and most preferably about 2 minutes.

[0013] In another embodiment of the method in accord with the invention, the determination of multiple path interference is repeated not less than 500 times. A measurement of multiple path interference is typically completed in less than about 0.2 seconds so that 500 separate measurements can be made in 2 minutes or less.

[0014] In yet another embodiment of the method, the maximum value of multiple path interference measured during the pre-selected time interval or selected from a preselected number of measurements is recorded.

[0015] Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein, including the detailed description which follows, the claims, as well as the appended drawings.

[0016] It is to be understood that both the foregoing general description and the following detailed description are merely exemplary of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate various embodiments of the invention, and together with the description serve to explain the principles and operation of the invention.

Brief Description of the Drawings

[0017] Figure 1 is a schematic representation of the apparatus in accord with the invention.

[0018] Figure 2 is a representation of the spectral power density measured by the electrical spectrum analyzer.

[0019] Figure 3 is an optical circuit used to calibrate the measurement apparatus in accord with the invention.

[0020] Figure 4 is a chart comparing measured multiple path interference to expected value of multiple path interference.

Detailed Description of the Invention

[0021] Reference will now be made in detail to the present preferred embodiments of the multiple path interference measurement apparatus, an example of which is illustrated in Figure 1. In Figure 1, light source 2 is optically coupled through an optical coupler or switch 4 to the polarization scrambler 6. Optical coupler or switch 4 is an optional element of the invention useful in facilitating multiple path interference measurements at more than one wavelength. A measurement system using a single light source would typically directly couple light source 2 to polarization scrambler 6.

[0022] Polarization scrambler 6 is a device which changes, essentially continuously, the polarization state of light passing therethrough. Such devices are commercially available, for example as part number 11896A from Agilent Technologies Palo Alto, California, and will not be discussed in detail here. Briefly, continuous change of the polarization state of light is typically achieved by continuously changing the position or properties of one or more components that are effective in altering polarization state. For example, the geometrical properties and spatial disposition of an optical waveguide fiber determines the polarization state of light exiting the fiber. By continuously perturbing one or more of the geometrical properties or spatial disposition of the fiber, the polarization state of exiting light can be continuously changed. Thus a fiber or portion of a fiber can, for example, be wrapped about a frame and the frame rocked or vibrated to continuously change the spatial configuration, that is the number and nature of bends, of the fiber. The frame can include a number of separate spindles each of which holds a number of wraps of fiber. The spindles can be moved relative to each other to produce a random change in fiber spatial configuration. As an alternative, the fiber can be wrapped about a frame which is deformable, such as a piezo-electric spindle which changes shape under the action of an applied voltage. The polarization scramblers used in the apparatus in accord with the invention are so called because they effectively produce over an interval of time light having all possible polarization states.

[0023] The action of the polarization scrambler insures that, over an appropriate time interval, the polarization states of light traversing alternative paths in the part of an optical waveguide circuit under test will be aligned when they combine in a common path. Interference of light traversing the alternate paths produces maximum multiple path interference when the respective polarization states of the light are aligned, and conversly creates no interfernce when the polarization is orthogonal. Thus the measurement of multiple path interference using a system including a polarization scrambler provides a reading of the maximum impact multiple path interference can have on optical circuit performance. The multiple path interference measurement provided by the apparatus and method in accord with the invention thus gives a system designer the input needed to properly calculate the impact of MPI on optical circuit performance.

[0024] Continuing with Figure 1 , launched light is coupled from polarization scrambler 6 into part of an optical waveguide circuit 8. The part of an optical waveguide circuit can be any of the fiber, planar, or free space components (see above), used in optical waveguide communication systems, such as an amplifier, a switch, a coupler, a grating, the optical waveguide itself or any combination of these. Light traversing part of an optical waveguide circuit 8, over at least two alternative paths, is converted to an electrical signal by detector 10. The electrical signal is fed into electrical spectrum analyzer (ESA) 12 that measures the spectral power density of the electrical signal. An illustration of a typical spectral power density measurement is shown in Figure 2. Curve 14 represents the electrical power, measured in electrical dBm, versus frequency of a signal processed by electrical spectrum analyzer 12. The distribution of power over the frequency range extending from a selected lower frequency, f,, 16 to a selected higher frequency, f_u, 18 includes the effects of multiple path interference so that multiple path interference information can be extracted from the spectral power density as is described in detail below. The selection of lower and upper frequencies, 16 and 18 respectively, is arbitrary in the sense that the selection need only be consistent for both the measured data as well as the calculated spectral power density equation, in order that a comparison between experimental data and calculated results can be made.

[0025] The multiple path interference is determined from the spectral power distribution using an equation descriptive of the interference of electric fields. The equation that describes the power as a function of frequency, i.e., the spectral power of the signal received and analyzed by the ESA can be derived beginning with an equation representing the interference of electric fields delayed in time relative to each other; E(t) = E₀ cos[<yt + φ(t)] + aE₀ cos[<y(t + r₀ ) + φ{t + τ₀ )] , where Ε is electric field, ω is 2π times frequency f, t is time, τ₀ is the relative time delay between light traveling the first path as compared to the second path, φ is the phase angle, and α is the ratio of light (electric field amplitude) in the second path to light in the first path. The light traveling the two paths is rejoined at or before the detector where it is converted to a voltage. The electrical spectrum analyzer reads the spectral content of the voltage signal from the detector and provides an output in the form of electrical power corresponding to respective frequencies contained in the voltage signal. This is called the spectral power density of the electrical signal from which the multiple path interference can be extracted. The voltage signal is related to the electric field by the equation, V(t) = GxRx|Ε(t)|², where G is the gain in an electrical amplifier placed before the ESA and R is the responsitivity of the detector in units of volts/ampere. In accord with the calculation found in "Fiber Optic Test and Measurement" Derickson, HP Books, Section 5.3, 1998 and Quantum Phase Noise and Field Correlation in Single Frequency Semiconductor Laser Systems, Gallion et al., IEEE Journal of Quantum Electronics 20, 4, p.383 (1984), the spectral power density of the voltage signal can be written as,

^s, (/) = s.„_{W +} e-

where ε is multiple path interference, Δf is the line-width of the source laser, P is signal power incident on the detector in watts, and t is time, the remaining symbols retaining the meanings set forth above. In the limit where the product of Δf and τ₀, the relative delay time of the second path, becomes large compared to 1, light in the interfering paths is incoherent

4 and the expression for spectral power density reduces to S, = R εP The

ESA measures the power in electrical dBm passing through an electrical filter having bandwidth B. As is customary in the art, electrical dBm relates to milli-watts. Thus to bring the equation into a form compatible with the ESA measurement, the equation for S, must be multiplied by B and the constant 1000/50, 50 ohms being the value of the output resistor of the detector. Putting in these conversion factors as well as the gain G of the amplifier defined above, the equation as related to the ESA measurement becomes,

This equation can be solved

for ε using the log^"1 operator and integrating over the frequency range of the voltage signal, demarcated by the upper frequency f_u and the lower frequency f,, the definition of which is discussed above. The ESA samples the voltage in discrete frequency steps of 100,000 Hz, so the integral of S_l(ESA)(f) may be replaced by 100,000 times the sum over the i frequencies sampled. Solving the resulting equation for the multiple path interference gives, 50π∑ 100 S,_{( ESA )} ε =

4BG ²R²P² tan f^» - -ttaonn -' f ^ 1 Then MPI in dB is found from the

Af equation, MPI = lOlogf .

[0026] To test the effectiveness of the apparatus in accord with the invention, a test circuit having known multiple path interference was constructed as shown schematically in Figure 3. Points A and B in the optical circuit of Figure 3 are optically connected at the corresponding points A and B in the apparatus shown in Figure 1. Point A is located immediately after polarization scrambler 6 and point B is located immediately before detector 10. The first light path of the circuit of Figure 3 is the optical waveguide fiber 24 extending between connection points A and B. The launched light is divided at optical splitter 26 so that a pre-selected portion of the launched light, for example 10 % of the launched light, is directed into the second path that includes a length of fiber 20 and a variable attenuator 22. The variable attenuator is a standard component that can be purchased for example from JDS, Inc., part designation VA6B. Decreasing the amount of attenuation due to the variable attenuator 22 serves to increase the multiple path interference and conversely so that the amount of multiple path interference can be effectively set to a particular one value in a range of possible values. The length of fiber 20 determines whether the multiple path interference is incoherent or coherent. When the relative time delay in the second path exceeds the coherence time of the source, the multiple path interference is incoherent and conversely. A fiber length of about 1.3 km of SMF-28™, available from Coming Inc., provides incoherent multiple path interference while a fiber length of about 10 m of SMF-28™ provides coherent multiple path interference.

[0027] The ratio of selected multiple path interference to measured multiple path interference is shown in Figure 4. The expected result is curve 28 having a slope of 1. Points 29 show the ratios of the measured multiple path interference values, taken in this example for the case of coherent multiple path interference. In accord with the measurement method of the invention, the maximum value of multiple path interference determined over a measurement time interval of about 2 minutes was recorded for each setting of variable attenuator 22. The difference between measured and expected ratios does not exceed 2 dB, which demonstrates the accuracy of the measurement made in accord with the invention.

[0028] It will be apparent to those skilled in the art that various modifications and variations of the present invention can be made without departing from the spirit and scope of the invention. Thus, it is intended that the present invention include the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.

Claims

We Claim:

1. An apparatus for measuring multiple path interference in a part of an optical waveguide circuit comprising: a light source optically coupled to a first end of said part of an optical waveguide fiber circuit; a detector optically coupled to a second end of said part of an optical waveguide circuit; an electrical spectrum analyzer electrically coupled to said detector; and, a polarization scrambler optically coupled between said light source and the first end of said part of an optical waveguide circuit.

2. The apparatus of claim 1 wherein the polarization scrambler comprises an optical waveguide fiber wrapped about a movable frame.

3. The apparatus of claim 1 wherein the polarization scrambler comprises an optical waveguide fiber wrapped about a deformable frame.

4. The apparatus of claim 1 wherein the optical coupling between said light source and said polarization scrambler includes a switch having a plurality of input ports and at least one output port, an output port being optically coupled to the polarization scrambler and said light source optically coupled to one of the input ports.

5. The apparatus of claim 4 further including a plurality of light sources each having a characteristic wavelength optically coupled to a respective input ports of the plurality of input ports.

6. A method of measuring multiple path interference in a part of an optical waveguide circuit comprising the steps: a) launching light into a first end of said part of the optical waveguide fiber circuit; b) during said launching step, continuously changing the polarization state of the light; c) at a second end of said part of the optical waveguide fiber circuit, detecting light which has passed through said part of the optical waveguide to provide an electrical detected signal; d) measuring the spectral power density of the electrical detected signal; and, e) determining the multiple path interference using the spectral power density.

7. The method of claim 6 wherein steps a) through e) are repeated continually for a preselected time period.

8. The method of claim 6 wherein steps a) through e) are repeated not less than 500 times.

9. The method of either one of claims 7 or 8 wherein steps a) through e) are repeated for a time period less than or equal to 5 minutes.

10. The method of claim 9 wherein the time period is less than 3 minutes.

11. The method of claim 9 wherein the time period is about 2 minutes.

12. The method of either one of claims 7 or 8 further including the step f) recording the maximum value of multiple path interference determined.