Radiant energy – Photocells; circuits and apparatus – Optical or pre-photocell system
Reexamination Certificate
2000-06-15
2002-04-23
Allen, Stephone B. (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Optical or pre-photocell system
C250S226000, C356S464000
Reexamination Certificate
active
06376830
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to measuring the transfer function of optical devices. More specifically, the invention is a system and method for measuring the transfer function associated with single-port guided wave device or the transfer function matrix of a multi-port guided wave device, e.g., Bragg gratings, couplers, etc.
2. Description of the Related Art
A variety of guided wave devices (e.g., Bragg gratings, directional couplers, isolators, amplitude modulators, amplifiers, wave division multiplexers, etc.) are used in the world's telecommunication network. In order to understand and predict how these devices will affect an incoming (light) signal, it is necessary to characterize the impulse response (i.e., know the transfer function or transfer function matrix) of these devices. Current systems/methodologies for measuring such transfer functions are very expensive, slow, and/or subject to unsatisfactory levels of error.
One commercially available impulse response characterization system is shown in
FIG. 1
where light from a tunable laser
10
undergoes amplitude modulation at
12
as controlled by a fixed frequency oscillator
14
. The modulated light is directed (by an optical coupler
16
) down an optical fiber
18
to a device under test (DUT)
20
such as a Bragg grating. The light will at least partially reflect off DUT
20
and pass back through optical coupler
16
where it is directed to a detector
22
. The (modulated) reflection signal at detector
22
is recovered with some phase shift as measured by a vector volt meter
24
. Due to the nature of DUT
20
, the delay experienced by the reflected signal is a function of the center wavelength of tunable laser
10
. The delay as a function of this center wavelength (i.e., dispersion measurement) is used to characterize DUT
20
. However, this system is expensive and can take twenty minutes to test a single device.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide a system for measuring transfer functions of guided wave devices.
Another object of the present invention is to provide a simple and inexpensive system for measuring transfer functions of a variety of guided wave devices.
Still another object of the present invention is to reduce the time required to measure transfer functions associated with guided wave devices.
Other objects and advantages of the present invention will become more obvious hereinafter in the specification and drawings.
In accordance with the present invention, a method and system are provided for measuring the N×N scalar transfer function elements for an N-port guided wave device. The device has a maximum effective path length L
D
and an effective index n. Optical energy of a selected wavelength is generated at a source and directed along N reference optical paths having N reference path lengths L
Ri
, i=1 to N. Each reference optical path terminates in one of N detectors such that N reference signals are produced at the N detectors. The reference signals are indicative of amplitude, phase and frequency of the optical energy carried along the N reference optical paths. The optical energy from the source is also directed to the N-ports of the guided wave device and then on to each of the N detectors such that N measurement optical paths are defined between the source and each of the N detectors. A portion of the optical energy is modified in terms of at least one of the amplitude and phase to produce N modified signals at each of the N detectors. At each of the N detectors, each of the N modified signals is combined with a corresponding one of the N reference signals to produce N combined signals at each of the corresponding N detectors. A total of N
2
measurement signals are generated by the N detectors. Each of the N
2
measurement signals is sampled at a wave number increment &Dgr;k so that N
2
sampled signals are produced. In the present invention, it is required to define N measurement path lengths L
Mj
, j=1 to N, of the N measurement optical paths associated with each of the N detectors such that
|L
Ri
−L
Mj
|>N*L
D|
j=
1,N |
i=1,N′
&LeftBracketingBar;
L
Ri
-
L
Mj
&RightBracketingBar;
+
L
D
<
π
2
n
Δ
k
&RightBracketingBar;
j
=
1
,
N
&RightBracketingBar;
i
=
1
,
N
,
and such that the N combined signals at each of the N detectors are spatially separated from one another in the time domain. The N×N transfer function elements are generated using the N
2
sampled signals.
REFERENCES:
patent: 4828389 (1989-05-01), Gubbins et al.
patent: 5563705 (1996-10-01), Sanders
Volanthen, M. Et al., “low coherence technique to characterise reflectivity and time delay as a function of wavelength within a long fibre grating”Electronics Letters,GB, IEE Stevenage, vol. 32, No. 8, Apr. 11, 1996, pp. 757-788.
Takada K., et al., “Optical low coherence method for characterizing silica-based arrayed-waveguide grating multiplexers”,J. Lightwave Technology,Vo. 14, No. 7, Jul. 1, 1996, pp. 1677-1689.
Cohen, L. G., “Comparison of single-mode fiber dispersion measurement techniques”,J. Lightwave Technology,vol. LT-3, No. 5, Oct. 1985, pp. 958-966.
Okoshi, T., et al., “Measuring the complex frequency response of multimode optical fibers”,Applied Optics,vol. 20, No. 15, Apr. 15, 1981, pp. 1414-1417.
M. M. Ohn et al, “Measurement of Fiber Grating Properties Using an Interferometric and Fourier-transform-based Technique”, Conference on Optical Fiber Communications, IEEE/Lasers and Electro-Optics Society, pp. 154-155, (Feb. 16, 1997).
U. Glombitza and E. Brinkmeyer, “Coherent Frequency-Domain Reflectometry for Characterization of Single-Mode Integrated-Optical Waveguides”, Journal of Lightwave Technology, vol. 11 (No. 8), pp. 1377-1384, (Aug. 1, 1993).
Erdogan Turan
Froggatt Mark E.
Allen Stephone B.
Hammerle Kurt G.
The United States of America as represented by the Administrator
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