JPS6233542B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an optical fiber dispersion measuring device that measures the wavelength dispersion of an optical fiber using a laser beam that oscillates continuously or almost continuously, and in particular,
This is a newly developed device that can measure wavelength dispersion with high accuracy on the order of sub-picoseconds (10 ^-13 seconds)/Å.

One of the factors that determines the transmission information capacity in optical communications using optical fibers is the dispersion of optical transmission by optical fibers, and among these, chromatic dispersion, the property that the propagation speed of light differs depending on the optical wavelength or optical frequency, is the essential property. It is something like that. Therefore, measurement of this chromatic dispersion is extremely important for inspecting optical fibers, determining transmission capacity, or improving optical fibers.

Conventionally, such chromatic dispersion has been measured using a measurement method in which chromatic dispersion is determined from the transmission spread of the pulse width of an ultrashort optical pulse or the difference in transmission time between two optical pulses with different wavelengths. However, the former method has the drawback that not only the measurement results vary depending on the properties of the optical pulse, but also the measurement accuracy itself is not very good.The latter method requires a large-scale optical pulse generator. In addition, measurement methods using these optical pulses require the use of optical pulses with optical energy or number of photons above a certain value, taking into account the SN ratio of the measurement system. Since it is necessary to measure the transmission delay and spread of optical pulses with picosecond precision using ultrashort optical pulses of about picoseconds (10 ^-12 seconds), the peak power of the light becomes extremely high. As a result, the nonlinearity of the optical fiber itself cannot be ignored, and it also has the disadvantage that it is difficult to accurately measure wavelength dispersion.

The purpose of the present invention is to eliminate the above-mentioned conventional drawbacks, use continuous light or nearly continuous light as input light, and only need to observe small pulsations of about gigahertz (10 ⁹ Hz) in the output light, and As a light source, light with a large peak power is not required and a normal laser oscillator can be used, so it does not require a large-scale device, and high measurement accuracy can be obtained, which is difficult to achieve with the current state of technology. Using a photodetector with a facile subnanosecond time response,
Optical fiber fraction enables detection with sub-picosecond/Å accuracy, which is currently the world's highest measurement accuracy, and also enables dispersion measurement for each wavelength using laser light of different wavelengths or laser light of variable wavelength. The purpose of this invention is to provide a measuring device.

That is, the optical fiber dispersion measuring device of the present invention is
Laser light with a phase modulation output of an optical phase modulator or a frequency modulation output with a 1/4 period shift of the sine wave phase of the phase modulation output is injected into the optical fiber of the object and transmitted, and the wavelength dispersion of the optical fiber is Based on
A photodetector and an optical waveform analyzer or frequency The present invention is characterized in that the measurement is performed using an analyzer and the measured value is converted into wavelength dispersion of the optical fiber.

In the following, the invention will be explained in detail by way of example embodiments with reference to the drawings.

First, in the configuration example of the optical fiber dispersion measuring device of _the present invention shown in FIG.
The optical phase modulator 3 driven by the optical phase modulator 3 performs phase modulation or frequency modulation in which the phase of the phase modulation is shifted by 1/4 period. The index Γ of this phase modulation or frequency modulation is determined by measuring a part of the modulated laser beam split by the beam splitter 4 with an optical spectrum analyzer 5 using a swept Fabry-Perot interferometer or the like. is led to the dispersion conversion circuit 6.

Now, most of the modulated laser light is injected into the optical fiber 7, which is the object to be examined, and transmitted. However,
In an optical fiber exhibiting wavelength dispersion, the group transmission speed of optical energy differs depending on the frequency or wavelength of the light, so the transmission speed of frequency-modulated light is equivalently modulated corresponding to its instantaneous frequency. I will receive it. For example, in the normal dispersion region of an optical fiber, the higher the optical frequency, that is, the shorter the wavelength, the higher the refractive index and the slower the transmission speed of light; conversely, the lower the optical frequency, that is, the longer the wavelength, the lower the refractive index. becomes lower and the transmission speed of light becomes faster.
Therefore, even if the intensity of light is constant over time, if phase modulation or frequency modulation is applied, during transmission through an optical fiber, the higher optical frequencies will be delayed and the lower optical frequencies will be delayed. As a result, in the part of the optical fiber during transmission where the frequency changes from high to low over time, that is, in the negative frequency chirping region, optical power is gathered from the previous and following parts. As the optical power increases, the optical power density also increases, and conversely, in the region where the frequency temporally changes from low to high, that is, in the positive frequency chirping region, the optical power is dispersed to the front and rear parts, and the optical power increases. Therefore, of course, the optical power density also decreases. In other words, similar to the conversion of velocity modulation of electrons into intensity modulation in a klystron, due to the above-mentioned bunching effect, the frequency modulation of light, which results in velocity modulation, is converted into density modulation, which in turn converts into intensity modulation. Become. Furthermore, simply expressed, the dispersion of the optical fiber converts some of the FM of the light into AM.

Note that in the anomalous dispersion region of an optical fiber, the higher the optical frequency, the lower the refractive index and the faster the transmission speed of light, so contrary to the normal dispersion region, the optical power and therefore the optical power density increase. is the positive frequency chirping region. However, in any case, the optical transmission output of optical fiber is
Intensity modulation components appear as described above.
Waveform analysis of such an optical transmission output waveform is performed by a high-speed photodetector 8 and a frequency analyzer 9 or other waveform analysis device for its detected output electrical signal. Note that if the frequency analyzer 9 is used, it is possible to determine the magnitudes of the fluctuation component P ₁ and the DC component P ₀ of the fundamental frequency f _n included as the modulation frequency in the intensity modulation signal waveform of the photodetection output, and the ratio of the magnitudes of these components.
P ₁ /P ₀ is calculated. Furthermore, the modulation degree M of the intensity modulation signal waveform can be determined using a high-speed photodetector and a waveform analysis device such as an oscilloscope without using a frequency analyzer. Furthermore, in practice, in most cases, when the modulation degree of the intensity modulation occurring in the optical transmission output is shallow, that is, when M<<1, the intensity modulation degree M becomes approximately equal to the component ratio P ₁ /P ₀ . Furthermore, under such conditions, the modulation degree M or the component ratio P ₁ /P ₀ is the ratio of the transmission time τ required for light to propagate through the optical fiber depending on the transmission optical frequency ν, that is, the transmission time τ is proportional to the frequency dispersion or wavelength dispersion ∂τ/∂ν. The mathematical reasoning is simplified below.

The instantaneous phase θ _{(ts) of the modulated laser beam that enters the optical fiber 7 via the optical phase modulator 3 at time t=t s is} expressed by the following equation (1).

θ _(ts) = θ ₀ + Γ sin (2πf _n t _s ) (1) where Γ is the modulation index of phase modulation or frequency modulation, and f _n is the modulation frequency, that is, the fundamental frequency. Therefore, the instantaneous optical frequency ν(t _s ) at time t=t _s is given by the following equation (2), where ν ₀ is the laser light frequency before modulation.

ν(t _s )=ν ₀ +(1/2π)∂θ/∂t=ν ₀ +Γf _n cos(2πf _n t _s ) (2) The time t _a when this modulated laser beam passes through the optical fiber 7 is , is expressed by the following equations (3) and (4).

t _a =t _s +π (3) τ=l/v _g =l∂β/∂ω (4) Here, τ is the time required for the light to propagate through the fiber 7, and the frequency ν of the light is Therefore, it depends on the time t _s at which the light is incident. Further, l is the fiber length, v _g is the group transmission velocity of light, β is the propagation constant of light in the optical fiber, and ω is the angular frequency of light, which is 2π times the optical frequency ν. Note that, if the transmission time required for the laser beam of frequency ν ₀ to propagate through the optical fiber 7 is τ ₀ , then the instantaneous frequency ν(t
The time t _a when the light _s ) is emitted from the optical fiber 7 is
Approximately, the following equation (5) is obtained.

t _a =t _s +τ ₀ + [∂τ/∂ν]〓 ₌ 〓 ₀ (ν(t _s )−ν ₀ )=t _s +τ ₀ +Γf _n [∂τ/∂ν]〓 ₌ 〓 ₀ cos (2πf _n t) (5) Therefore, the light that has entered the optical fiber 7 from the optical phase modulator 3 from time t = t _s to t = t _s + δt _s is from time t = t _a to t = t _a Since it is emitted from the optical fiber 7 during the period up to +δt _a ,
The output light intensity I(t _a ) is the input light intensity I(t
_s ) is multiplied by δt _s /δt _a , and therefore, the following equation (6) is obtained from the above equation (5).

I(t _a ) = I(t _s ) δt _s / δt _a = I(t _s )/{1-2πf _n ² Γ [∂τ/∂ν] ₌ = ₀ sin(2πf _n t)} = I (t _s ) {1+2πf _n ² Γ [∂τ/∂ν] _{= 0} sin (2πf _n t)} = I (t _s ) {1+Msin (2πf _n t)} = P ₀ + P ₁ sin (2πf _n t) (6) In short, the modulation degree M or the component ratio P ₁ /P ₀
is the frequency dependence of transmission time τ (∂τ/∂ν) 〓
₌ 〓 Proportional to ₀ , M(P ₁ /P ₀ )2πf _n ² Γ(∂τ/∂ν)〓 ₌ 〓 ₀ (7) Therefore, (∂τ/∂ν)〓 ₌ 〓 ₀ M/ (2πf _n ² Γ) (P ₁ /P ₀ )/(2πf _n ² Γ) (8).

Therefore, the modulation frequency f _n is known, and the modulation index Γ can be measured in advance. In addition, a part of the incident light is split by a beam splitter 4 and constructed using a swept Fabry-Perot interferometer or the like. It is also possible to monitor and measure the modulation index Γ in real time by guiding it to the optical spectrum analyzer 5. Therefore, in the measuring device of the present invention, the modulation frequency f _n and the modulation index Γ are combined with the modulation degree M determined by the measurement of the transmitted output light or the value of the component ratio P ₁ /P ₀ . In addition, the required chromatic dispersion value (∂ τ／∂ν)〓 ₌ 〓 ₀ will be calculated and displayed as output. Note that this wavelength fraction value represents how much the optical fiber transmission time differs when the optical frequency differs by one unit frequency.
Further, by considering the length of the optical fiber to be measured in this calculation result and dividing the obtained wavelength fraction value by the fiber length l, the chromatic dispersion value per unit length of the optical fiber can be obtained.

Furthermore, if a microwave as a modulation signal is applied in a pulsed manner to the optical phase modulator 3, and the time length of the microwave application is set to be shorter than the optical fiber transmission time, for example, 5 msec/Km, the microwave modulation is When the laser beam is detected and measured after propagating through the optical fiber, the application of the microwave has already stopped, so the transmission of the microwave and the microwave component carried by the laser beam, that is, the modulated component It is possible to temporally separate the detection of The proportion of time that the modulating microwave is applied to the optical frequency modulator or the optical frequency modulator, that is, the duty ratio, becomes smaller. Therefore, these effects make it possible to apply large microwave power even to optical modulators whose allowable electrical input is limited due to thermal breakdown. In other words, when microwaves are applied in pulses, the allowable microwave input of the optical modulator naturally increases, and if a large microwave modulation power is used,
The modulation index Γ can be increased. On the other hand, if direct mixing of microwaves from parts other than the carrier light is prevented during optical detection, the modulation degree M or the component ratio
The measurement accuracy of P ₁ /P ₀ is improved. In addition, as can be seen from equation (7), if the modulation index is large, the modulation index M or component ratio to be measured for the same wavelength dispersion value is
Since the value of P ₁ /P ₀ increases, measurement sensitivity and measurement accuracy improve. In this case, it is preferable to use a chopper circuit 10 as an electrical or optical switch circuit that operates synchronously, as shown in FIG.

That is, the laser beam is chopped with an optical chopper so that the laser beam is supplied to the modulator only when the microwave is applied to the modulator, in synchronization with an electric switch that turns on and off the application of the microwave.
Furthermore, in synchronization with a delay approximately equal to the optical fiber transmission time of the laser beam, an electric chopper or the like is activated before the frequency analyzer 9 as shown in the figure, or an optical chopper is activated before the photodetector 8. By activating the , the signal transmission path is opened to allow the optical detection output signal to pass only during the period when optical transmission output is obtained, and the signal transmission path is closed during other periods to prevent unnecessary signals and noise from entering. It is preferable to prevent

(Numerical example of measurement sensitivity) Even with the current optical modulation technology, it is considered that it is fully possible to realize a modulation frequency of 2 GHz and a modulation index Γ = 5π for light with a wavelength of 1 μm. Also, the modulation degree 1
% intensity modulation signal waveform is easy to observe. Therefore, in the above equation (8), f _n =2×10 ⁹ Hz, Γ
=5π and M=0.01, the required dispersion value (∂τ/∂ν) = _{= 0} becomes 2.5×10 ⁻²³ sec/Hz. Therefore, for light with a wavelength of 1 μm, 1 Å
corresponds to approximately 30 GHz, so this fractional value corresponds to 0.75 ps/Å. Even in a single mode optical fiber, which is said to have the lowest dispersion, the dispersion value is on the order of several ps/Å per 1 km. Therefore, if a specimen with a fiber length of 1 km is used, it is possible to sufficiently observe the mode of dispersion. is possible.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing an example of the configuration of an optical fiber dispersion measuring device according to the present invention. 1... Laser oscillator, 2... Microwave power source for modulation, 3... Optical phase modulator, 4... Beam splitter, 5... Optical spectrum analyzer, 6... Dispersion conversion circuit, 7... Test object Optical fiber, 8...Photodetector, 9...Frequency analyzer, 10...Chopper circuit, _fn ...Phase modulation frequency, Γ...Phase modulation index, M...Intensity modulation degree of optical transmission output waveform, _P1 / _P0
...Ratio between the fluctuation component of the fundamental frequency f _n and the DC component in the optical transmission output waveform, (∂τ/∂ν) _{= = 0}
...Dispersion of an optical fiber at optical frequency ν ₀ .

Claims (1)

[Scope of Claims] 1. A laser beam having a phase modulated output of an optical phase modulator or a frequency modulated output obtained by shifting the sine wave phase of the phase modulated output by 1/4 period is injected into an optical fiber of a subject and transmitted, Based on the wavelength dispersion of the optical fiber, the modulation degree of optical intensity modulation that occurs in the transmitted output light according to phase modulation or frequency modulation, or the ratio of the magnitude of the fluctuation component of the fundamental frequency in the optical intensity modulation waveform to the DC component. An optical fiber dispersion measurement device, characterized in that the measurement is performed using a detector and an optical waveform analyzer or a frequency analyzer, and the measured value is converted into wavelength dispersion of the optical fiber. 2. In the measuring device according to claim 1, the index of the phase modulation or the frequency modulation is Γ, the transmission time of the optical fiber is τ,
The frequency of the laser beam is ν ₀ , the frequency of the transmitted output light is ν, the modulation degree in the intensity modulation is M, the fundamental frequency is f _n , and the magnitude of the DC component is
P ₀ and the magnitude of the fluctuation component of the fundamental frequency f _n as P ₁ , (∂τ/∂ν) ₌ 0 ₀ M/(2πf _n ² Γ) (P ₁ /P ₀ )/(2πf _n ² Γ) An optical fiber dispersion measuring device characterized in that the conversion is performed using the formula: