Optical waveguides – Having nonlinear property
Reexamination Certificate
2001-12-12
2004-06-15
Lee, John D. (Department: 2874)
Optical waveguides
Having nonlinear property
C385S015000, C385S027000
Reexamination Certificate
active
06751385
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method and device for measuring the waveform of an optical signal.
2. Description of the Related Art
In the field of optical fiber communication, the modulation rate of a signal continues to increase year after year. Recently, it has been seriously examined to use a signal speed of 100 Gb/s or more much higher than an electrical band by applying a time division multiplexing technique to multiplex an RZ signal of about 10 Gb/s. In researching and developing a technique related to such an ultrahigh-speed signal, a highly stable waveform measuring device having a time resolution of picoseconds to subpicoseconds is indispensable. In particular, the observation of an eye pattern as an overlaid display of signals is important from the viewpoint of application to communication.
As a device for measuring the eye pattern, a sampling oscilloscope is known. The sampling oscilloscope is a device using a signal sampling method to extract instantaneous voltage components, i.e., samples, of a periodic input signal at its sequentially different portions and to regenerate a high-frequency signal from the extracted many samples in a low-frequency region.
A time resolution in the sampling is uniquely determined by the pulse width of a trigger. At present, the band of a maximum-performance electrical sampling measurement device is limited by an electrical band, and it is about 50 GHz. Accordingly, the time resolution is about 20 ps at most.
Usually, in the case of measuring the eye pattern of an optical signal, the optical signal is once converted into an electrical signal by an opto/electrical converter, and the eye pattern of this electrical signal is next measured. Accordingly, even though an opto/electrical converter having a wide band greater than 50 GHz is used, the eye pattern of an optical signal having a width smaller than 20 ps cannot be measured.
As shown in
FIG. 1
, the terms of “sampling” means obtaining “AND” of an input signal and a trigger. In the case of
FIG. 1
, a pulse pattern generator (PPG)
4
generates a modulating signal according to a clock from an oscillator
2
, and an LN (lithium niobate) optical modulator
6
modulates CW light (continuous-wave light) from a laser diode (LD)
8
according to the above modulating signal. An optical signal obtained by this modulation is transmitted by an optical fiber
10
, and the transmitted optical signal is converted into electrical data by an O/E converter (opto/electrical converter)
12
. Then, an electrical sampling circuit
14
measures the waveform of the input electrical data from the O/E converter
12
by using the clock from the oscillator
2
as a trigger.
As the O/E converter
12
, an optical receiver having a band of about 0.60 GHz has already been put to practical use, and as the electrical sampling circuit
14
, a device having a band of about 50 GHz has already been put to practical use. Accordingly, a time resolution of about 20 ps can be realized.
By obtaining the electrical “AND” as mentioned above, the eye pattern of a certain level high-speed signal can be stably measured. However, it is difficult to apply the electrical “AND” to the measurement of the eye pattern of a higher-speed signal of 100 Gb/s or more requiring a higher time resolution.
As a method for remarkably improving the time resolution, an optical sampling method is known, in which short pulse light of the order of picoseconds is used as the above-mentioned trigger, and an input optical signal and this optical trigger are input into a nonlinear medium to optically examine the cross correlation therebetween. To realize a high time resolution in the optical sampling method, ultrashort pulses of light with less phase noise are required as the optical trigger, and a nonlinear medium having ultrahigh-speed characteristics and ultrawide-band characteristics is indispensable as an AND circuit.
In the optical sampling method, the time resolution is determined by the pulse width and jitter of the optical trigger and by the response speed and group velocity dispersion of the nonlinear medium. It has been reported to use a nonlinear medium having sufficiently high-speed response characteristics and small group velocity dispersion, thereby effecting optical sampling with a time resolution of the order of picoseconds.
For example, optical sampling can be performed by using KTP having high-speed response characteristics of the order of subpicoseconds and a group delay difference of the order of subpicoseconds (interaction length: several millimeters) as the nonlinear medium and by adopting a method (SFG) of generating light corresponding to a sum frequency of an optical signal whose waveform is to be measured (which signal will be hereinafter referred to as “subject optical signal”) and an optical trigger. The time resolution is limited by the pulse width of the optical trigger, and it is about 8 ps with an S/N ratio of 22 dB. Accordingly, the waveform of an optical signal of 25 Gb/s can be measured. In general, an inorganic nonlinear medium such as KTP has a problem that the conversion efficiency is low.
As an improvement in the conversion efficiency, a method of using a nonlinear organic crystal as the nonlinear medium has been reported in IEE Electronics Letters, vol. 32, issue 24, Nov. 21, 1996, pp. 2256-2258. In this method, an organic crystal (AANP) having a high conversion efficiency about 10 times or more that of an inorganic nonlinear medium is used to improve the conversion efficiency, and an optical trigger of 0.4 ps is used to achieve a time resolution of 0.9 ps.
As another method for improving the conversion efficiency, a method of utilizing four-wave mixing (FWM) generated in a semiconductor optical amplifier (SOA) is known (IEEE Photonics Technology Letters, vol. 11, no. 11, 1999, pp. 1402-1404). In this method, a time resolution of 1.7 ps has been achieved. However, there is a future problem of whether or not accurate measurement can be made on an optical signal having an arbitrary pattern, because a pattern effect due to a carrier density modulation effect is exhibited in a SOA. Other methods of utilizing FWM generated in an optical fiber have been reported (IEE Electronics Letters, vol. 27, issue 16, Aug. 1, 1991, pp. 1440-1441 and J. Li, et al., “300 Gbit/s eye-diagram measurement by optical sampling using fiber based parametric amplification”, Optical Fiber Communication Conference, Mar. 17-22, 2001, Anaheim, Calif.). In the latter report, the measurement of an eye pattern corresponding to 300 Gb/s is made.
The reason why the time resolution is limited by a group velocity will now be described with reference to FIG.
2
. As shown in
FIG. 2
, group delays &tgr;
sig
and &tgr;
tri
per unit length of an optical fiber are produced in the optical signal and the optical trigger, respectively, by the group velocity. In general, &tgr;
sig
≠&tgr;
tri
because the group velocity differs according to wavelength, so that a relative temporal difference (walk-off) is induced between the optical signal and the optical trigger in the optical fiber.
To efficiently generate FWM, the wavelength &lgr;
tri
of the optical trigger is generally set equal to the zero-dispersion wavelength &lgr;
0
of the optical fiber. Accordingly, when the wavelength &lgr;
sig
of the optical signal is substantially equal to the zero-dispersion wavelength &lgr;
0
, the optical signal and the optical trigger cannot be separated from each other, and the waveform of the optical signal cannot therefore be observed near the zero-dispersion wavelength &lgr;
0
.
As means for basically eliminating the limitation to the time resolution by the group velocity, there has been proposed a method of using cross-phase modulation (XPM) generated in a nonlinear optical loop mirror (NOLM) (IEE Electronics Letters, vol. 27, issue 3, Jan. 31, 1991, pp. 204-205). The principle of sampling an optical signal by using a NOLM will now be described with reference to FIG.
3
.
The NOLM has a first optical coupler in
Futami Fumio
Watanabe Shigeki
Fujitsu Limited
Lee John D.
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