JPH021526A - Light dispersion measuring instrument - Google Patents

Abstract

PURPOSE:To measure the dispersion of 1mum band by using a nonlinear crystal for a wavelength conversion and a picosecond shutter. CONSTITUTION:A variable optical pulse of a variable frequency omega2 (wavelength lambda2 = light velocity C/frequency omega2) which is obtained from a reference optical pulse of a frequency omega1 goes into an optical fiber 3 for measuring dispersion, and thereafter, goes into a nonlinear crystal 11. As for this nonlinear crystal 11, when optical pulses of frequencies omega1, omega2 which are inputted are superposed in the nonlinear crystal 11, an intense optical pulse of a frequency (omega1+omega2) is generated as an output. Accordingly, since the frequencies omega1, omega2 are already known, a point where two pulses are superposed by taking notice of only the frequency of (omega1+omega2) and moving a light delay path 8. Subsequently, the frequency omega2 is varied by DELTAomega, and an optical pulse having a frequency of (omega2+DELTAomega) is made incident on the optical fiber 3. Next, by moving the light delay path 8 and taking notice of the frequency of (omega1+omega2+DELTAomega), a point where both the pulses are superposed is derived. From a position of the light delay path in such a case, a delay difference L/C is derived.

Description

DETAILED DESCRIPTION OF THE INVENTION (1) Technical Field of the Invention The present invention relates to an optical dispersion measuring device having a resolution on the order of picoseconds.

(2) Prior art and its problems The wavelength of light used in optical fiber communication and the relay distance during relay transmission are determined by the transmission loss and band characteristics of the optical fiber. In particular, the dispersion characteristics of optical fibers cause waveform distortion, which limits the transmission speed during digital transmission. Therefore, even if an extremely low-loss optical fiber is used, the relay distance may be limited by the dispersion characteristics of the optical fiber, and measurement of the dispersion characteristics of the optical fiber is as important as measurement of transmission loss. These measurements are usually made using short lengths of fiber, and the results are applied to the long fibers based on their length ratios, so small measurement errors in the short lengths of fiber are also accounted for in the long lengths. This results in a large error in the dispersion characteristics of the fiber. For the above reasons, a measuring device with sufficiently high accuracy is required in measuring the dispersion characteristics of short fibers.

A conventional measuring device of this type is constructed as shown in FIG. An optical pulse generated from a picosecond optical pulse generator 1 is split into two by a beam splitter 2, one of which passes through an optical delay path 8 and enters a Kerr shutter 9, and the other enters an optical fiber 3 to be measured. The Kerr shutter 9 consists of polarizers 4.5 that are perpendicular to each other and a Kerr cell 6 filled with a substance that causes birefringence due to the optical force effect.The optical pulse that has passed through the optical delay path 8 is incident on the Kerr cell 6. Only occasionally will the substance in the Kerr cell 6 exhibit birefringence, and the Kerr shutter 9 will open and the light will be received by the light receiver 7.

The optical delay path 8 made of, for example, a prism is moved so that the light pulse after passing through the optical fiber 3 coincides with the opening of the Kerr shutter 9 and the maximum power is received, and this optical delay path 8 is used as a reference. The amount of delay of the pulse from the reference point is measured for each wavelength depending on how much it has moved from the point, and the dispersion characteristics are determined from the results. This conventional measuring device cannot efficiently pass the output light from the optical fiber unless the power of the optical pulse for opening the carshaft shutter is several hundred MW/cd or more, and in the long wavelength band of 1 μm or more, There is no optical receiver 7 for measuring the power of picosecond pulses with high precision. Therefore, the measurement wavelength band is 1 pTI.
It has the disadvantage that it cannot be used for measurements at wavelengths in the 1, 3 μm, and 1.55 pm bands, which are considered promising for optical fiber communications.

(3) Purpose of the Invention In order to solve the drawbacks of these conventional techniques, the present invention uses a nonlinear crystal instead of a Kerr shutter, and receives the light that has passed through the optical transmission medium to be measured using sum frequency optical mixing. The present invention provides an optical dispersion measurement device that converts light into a wavelength band that can be received by a device.

(4) Structure and operation of the invention The present invention will be explained in detail below with reference to the drawings.

FIG. 2 is a diagram for explaining the present invention in detail, in which a picosecond optical pulse generator 1a serving as a light source has an optical frequency ωI (
An optical pulse generator 1-1 that generates a reference optical pulse with a constant wavelength λ y = speed of light C/frequency ω1), and a parametric oscillator 1-2 that generates a variable wavelength optical pulse that is synchronized with the reference optical pulse. It's getting better. A light pulse with an optical frequency (hereinafter simply referred to as "frequency") ω is used as a reference, and after passing through an optical delay path 8, it is guided to a nonlinear crystal 11 such as KDP or Li1O3 via a beam splitter 10. It will be destroyed. On the other hand, the variable frequency ω2 (wavelength λ2 = speed of light C/
A variable light pulse of frequency ω2) enters the optical fiber 3 whose dispersion is to be measured and then enters the nonlinear crystal 11.

This nonlinear crystal 11 only generates optical pulses with frequencies ω1゜ω2, 2ω1, 2ω2 as outputs when the input optical pulses of frequency ω and frequency ω2 do not overlap in the nonlinear crystal 11. However, when both input optical pulses overlap in the nonlinear crystal 11, it is an optical nonlinear effect element that generates a strong optical pulse at a frequency (ω1+ω2) in addition to the optical pulse at the above frequency as an output. Therefore, since the frequencies ω and ω2 are known, it is possible to move the optical delay path 8 focusing only on the frequency (ω1+ω2) and find the point where the two pulses overlap. Next, when the frequency ω2 is changed by Δω and an optical pulse with a frequency of (ω2 + Δω) is made to enter the optical fiber 3, the amount of delay between the optical pulses with the frequency ω and the frequency (ω2 + Δω) due to the dispersion characteristics of the optical fiber 3 are different, and the two pulses no longer overlap in the nonlinear crystal 11. Therefore, ω1+ω2+ moves the optical delay path 8.
Focusing on the frequency of Δω), find the point where both pulses overlap. If the position of this optical delay path 8 deviates from the previous position by, for example, L, then the optical pulse of frequency ω2 and the frequency (ω2
The delay difference with the variable optical pulse of +Δω) is obtained from L/C (C: speed of light). Since L can be measured with an accuracy of several tens of microns, the amount of delay can be measured with an accuracy of picoseconds or less. By changing the frequency ω2 in this way,
Delayed arrival at each frequency can be measured, and optical dispersion characteristics can be determined from the values.

When following the system shown in Figure 2, it is possible to measure the optical dispersion characteristics of the optical transmission medium to be measured with high precision, but a reference optical transmission path is required outside of the optical transmission medium to be measured. Moreover, due to changes in external environmental conditions, variations occur in the reference light pulse and measurement light pulse propagating between both optical transmission lines, resulting in errors.

The present invention can overcome this drawback of the measuring device according to the system shown in FIG. 2, and FIG. 3 shows an embodiment thereof. In this embodiment, the variable optical pulse of frequency ω2 from the light source 1a and the output of the reference optical pulse from the optical delay path 8 into which the reference optical pulse of constant wavelength (frequency ω, ) is input are transmitted to the beam splitter 10. synthesized by
The combined output is input to the optical fiber 3 which becomes the optical transmission line to be measured.

In FIG. 2, the reference light pulse is guided directly to the nonlinear crystal 11 without passing through the optical fiber 3, but if the wavelengths of the variable light pulse and the reference light pulse are different, the reference light pulse is guided as shown in FIG. It is also possible to measure the pulse by passing it through the optical fiber 3 in the same way as the measurement light pulse, and thereby it is possible to eliminate variations between the reference light pulse and the measurement light pulse.

The other measurement principles are similar to those explained with reference to FIG.

It is known that in a single mode fiber, which has only one propagable mode among optical fibers, the propagation speed differs depending on the direction of the polarization plane, but theoretically this speed difference is 10 to 20 picoseconds/ Although it was previously considered impossible to measure with short fibers, these can now be measured using this device. Furthermore, the elongation rate of an optical fiber due to tension when using a short fiber can also be accurately measured by using this device.

(5) As explained in detail, according to the present invention, a nonlinear crystal is used for wavelength conversion and picosecond shutter, so dispersion measurement using training picosecond pulses was considered impossible. IP
In addition to being able to measure band dispersion with high precision, it is possible to prevent errors due to changes in the external environment because the reference light pulse and measurement light pulse propagate through the same optical transmission path (optical fiber 3). Furthermore, since the peak power of the pulses used is only about several KW/c+fl, there is an advantage that it is easy to handle.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing an example of a conventional optical dispersion measuring device, FIG. 2 is a system diagram for explaining the present invention in detail, and FIG. 3 is a block diagram showing an embodiment of the present invention. 1.1a... Optical pulse generator (optical rX), 1-1...
・Optical pulse generator, ■-2... Parametric oscillator (optical wavelength converter), ■-3... Beam splitter, ■-4... Optical wavelength converter, 2... Beam splitter, 3. ...Optical fiber (light transmission medium to be measured), 4,5...Polarizer, 6...Kersel, 7.
... Light receiver, 8... Optical delay path, 9... Kerr shutter, 11... Nonlinear crystal, 10... Beam splinter, 12... Light receiver.

Claims (1)

[Claims] (1) A light source for generating a reference light pulse with a constant wavelength and a short time width; and a variable wavelength light pulse with a short time width and a different wavelength that is synchronized with the reference light pulse; a variable optical delay path whose delay amount is variable and which is incident on one end thereof; an optical transmission medium to be measured in which the output of the other end of the variable optical delay path and the variable pulse are incident on one end; and the optical transmission medium to be measured. The light to be measured is transmitted so that when each optical output pulse on the other end side of the medium is received and the optical pulses overlap, the component of the sum of each frequency corresponding to the wavelength of each optical pulse becomes the maximum output. an optical nonlinear effect element disposed on the other end side of the medium and a light receiver for detecting the components of the sum, each frequency corresponding to the wavelength when the wavelength of the variable optical pulse is sequentially changed; An optical dispersion measuring device configured to measure optical dispersion of the transmission medium to be measured based on a delay amount of the variable optical delay path that is adjusted so that the sum component has a maximum output.