JPH0260972B2 - - Google Patents

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 ultra-low-loss optical fiber is used, the dispersion characteristics of the optical fiber may limit the relay distance, so measuring the dispersion characteristics of the optical fiber is as important as measuring the transmission loss. It is. These measurements are usually made using short fiber lengths and applied to the long fibers based on the length ratio of the results obtained, in which case small measurement errors in the short fibers are also accounted for in the long fibers. This appears as a large error in the fiber's dispersion characteristics. For the above reasons, when measuring the dispersion characteristics of short fibers,
A measuring device with sufficiently high precision is required.

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 car shutter 9, and the other enters an optical fiber 3 to be measured. The car shutter 9 has polarizers 4 orthogonal to each other,
5 and a Kerr cell 6 filled with a substance that causes birefringence due to the optical Kerr effect, and the substance in the Kerr cell 6 exhibits birefringence only when the optical pulse that has passed through the optical delay path 8 is incident on the Kerr cell 6. As a result, the car shutter 9 opens and the light is received by the light receiver 7.
The optical delay path 8 made of, for example, a prism is moved so that the optical pulse after passing through the optical fiber 3 coincides with the opening of the car shutter 9 and the maximum power is received, and the optical delay path 8 is set as a reference point. The amount of delay of the pulse from the reference point is measured for each wavelength based on how much it has moved from the reference 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 optical pulse power for the car shutter aperture is several hundred MW/cm ² or more, and the long wavelength band of 1 μm or more is required. There is no optical receiver 7 that can accurately measure the power of picosecond pulses. For this reason, the measurement wavelength band is limited to 1 μm or less, and it has the disadvantage that it cannot be used for measurements at wavelengths in the 1.3 μm and 1.55 μm 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 car shutter and converts the light that has passed through the optical transmission medium to be measured into a light receiver using sum frequency optical mixing. The present invention provides an optical dispersion measuring device that converts light into a wavelength band that can receive light and then performs measurement.

(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 principle of the present invention, in which a picosecond optical pulse generator 1a serving as a light source is
Optical pulse generator _1-1 that generates a reference optical pulse with a constant optical frequency ω 1 (wavelength λ ₁ = speed of light C/frequency ω ₁ )
and a parametric oscillator 1-2 that generates a variable wavelength optical pulse synchronized with the reference optical pulse.
It's getting more and more familiar. A light pulse with an optical frequency (hereinafter simply referred to as "frequency") ω ₁ is used as a reference, and after passing through an optical delay path 8, it is sent to a nonlinear crystal 11 such as KDP or LiIO ₃ via a beam splitter 10. guided by. On the other hand _, the variable frequency ω ₂ (wavelength λ ₂ =
A variable light pulse of light speed C/frequency ω ₂ ) enters the optical fiber 3 whose dispersion is to be measured and then enters the nonlinear crystal 11 . This nonlinear crystal 11 outputs optical pulses with frequencies _{ω 1 ,} ω ₂ , 2ω ₁ , and 2ω ₂ when the input optical pulses with frequencies ω 1 and ω ₂ do not overlap in the nonlinear crystal 11 . However, when both input optical pulses overlap in the nonlinear crystal 11, an optical nonlinear effect occurs in which a strong optical pulse with a frequency of (ω ₁ +ω ₂ ) is generated in addition to the optical pulse with the above frequency as an output. It is element. Therefore, since the frequencies ω ₁ and ω ₂ are known, (ω ₁ +ω ₂ )
The optical delay path 8 is moved focusing only on the frequency of 2.
The point where two pulses overlap can be found. Next, when the frequency ω ₂ is changed by Δω and an optical pulse with a frequency of (ω ₂ +Δω) is made to enter the optical fiber 3, the dispersion characteristics of the optical fiber 3 cause the difference between the frequency ω ₂ and the frequency (ω ₂ +Δω). The amount of delay of the optical pulses is different, and the two pulses no longer overlap in the nonlinear crystal 11. Therefore, the optical delay path 8 is moved (ω ₁ +ω ₂ +
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, the delay difference between the optical pulse of frequency ω ₂ and the variable optical pulse of frequency (ω ₁ +Δω) is L/C (C: speed of light) More demanded. 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 ω ₂ in this manner, the delay difference at each frequency can be measured, and the optical dispersion characteristic can be determined from the value.

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, variations in the reference light pulse and measurement light pulse propagating between the two optical transmission lines occur due to changes in external environmental conditions, 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 ω ₂ 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 ₁ ) is input are as follows.
The beam splitter 10 combines the signals, and the combined output is input to the optical fiber 3, which serves as the optical transmission path 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 pico. It is small (sec/Km), and until now it was considered impossible to measure it with short fibers, but with this device it becomes possible to measure it. 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) Effects of the Invention As explained above, according to the present invention, since a nonlinear crystal is used for wavelength conversion and picosecond shutter, dispersion measurement using picosecond pulses has been considered impossible until now. In addition to being able to measure dispersion in the 1 μm band with high accuracy, it is also possible to prevent errors caused by changes in the external environment because the reference light pulse and measurement light pulse propagate through the same optical transmission path (optical fiber 3). Moreover, it has the advantage that it is easy to handle because the peak power of the pulse used only needs to be about several KW/cm ² .

[Brief explanation of drawings]

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 principle of the present invention, and FIG. 3 is a block diagram showing an embodiment of the present invention. 1, 1a... Optical pulse generator (light source), 1-1
...Optical pulse generator, 1-2... Parametric oscillator (optical wavelength converter), 1-3... Beam splitter, 1-4... 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 ... Car shutter, 10 ... Beam splitter, 11 ... Nonlinear crystal, 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 in synchronization with the reference light pulse; a variable optical delay path whose delay amount is variable; an optical transmission medium to be measured into which the output of the other end of the variable optical delay path and the variable pulse are input; The optical transmission medium to be measured is configured such that when each optical output pulse at the other end 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 and a light receiver for detecting a component of the sum; An optical dispersion measurement device configured to measure optical dispersion of the transmission medium under test from a delay amount of the variable optical delay path that is adjusted so that the sum component has a maximum output.