JPH0769220B2 - Optical sampling waveform measuring device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of application] The present invention is an optical device for measuring an optical pulse waveform used in optical communication or the like with high time resolution by using sum frequency light generation by a second-order nonlinear optical material. The present invention relates to a sampling waveform measuring device.

[Prior Art] Conventionally, sum frequency light generation (SF
In the optical sampling waveform measurement using G), a method of guiding the optical pulse to be observed and the sampling optical pulse with a narrower width to the nonlinear optical crystal and extracting the cross-correlation signal between them as sum frequency light (SF light) I'm using it. Here, SFG is a phenomenon in which when two light waves having angular frequencies ω ₁ and ω ₂ are incident on a nonlinear optical material, light having a sum angular frequency ω ₁ + ω ₂ is generated.

FIG. 5 shows the principle of the optical waveform measuring method by optical sampling using this SFG. The relative time relationship between the measured optical pulse (A) and the sampling optical pulse (B) and 3 shows a low-speed cross-correlation waveform (C) obtained by By making the repetition frequency of the sampling light pulse (B) slightly lower (or higher) than the repetition frequency f _C [Hz] of the measured light pulse (A), that is, Δf (Hz), FIG. ), The cross-correlation signal waveform of Δf (Hz) is obtained. Therefore,
Since the time axis of the waveform of the optical pulse to be measured (A) can be enlarged and measured, extremely high-speed optical pulse waveform measurement can be performed without band limitation of the photodetector.

In the example of FIG. 5 described above, the case where the frequency of the measured light pulse and the frequency of the sampling light pulse are almost equal is shown, but the sampling frequency itself may be in the vicinity of 1 / n (f _O / nHz) of an integer (n). .

FIG. 6 shows a specific configuration example of a conventional optical sampling waveform measuring apparatus. Here, in the sampling light source 1, a sampling light pulse train having a repetition frequency f _S = f _O −Δf (where f _O is the repetition frequency of the measured light) and a pulse width narrower than that of the measured light signal (see FIG. 5 (B )) Is generated, the polarization direction controller 2 controls the polarization direction of the sampling light pulse to be linearly polarized light, and then is guided to the multiplexer 4. Further, the measured optical pulse train of repetition frequency f _O (FIG. 5 (A)) also directs the polarization direction by the polarization direction controller 3 to the multiplexer 4 After controlling the sampling light and the same direction of linearly polarized light.

Two optical pulse trains of the same polarization combined by the multiplexer 4
It is incident on a second-order nonlinear optical material 5 having type I phase matching, and a sum-frequency light pulse of two lights, that is, a cross-correlation signal (FIG. 5 (C)) is generated there. In the material having type I phase matching, as shown in FIG.
SF light is generated. The cross-correlation signal is detected by the photodetector 7 and converted into an electric signal, which is then averaged by the electric signal processing system 8 and displayed on the display screen 9.

In FIG. 6, the arrow in the circle represents the polarization direction of light.
Here, referring to the non-linear optical material used for the type I non-linear optical crystal 5, conventionally, LiNbO ₃ and LiID ₃ having the type I phase matching are used because of their ease of configuration and adjustment.
Etc. were used. However, in the case of materials having type I phase matching, the SFG of the measured light and the sampling light is
In addition, the sampling light itself and the measured light themselves are substantially phase-matched, so that each second harmonic (SH) light is also generated and becomes background light.

Now consider the signal-to-noise ratio (SN ratio) of a conventional optical sampling device using a material having type I phase matching. The signal power at the electric stage of each sampling value is the square of the photocurrent (i _SF ² ) due to the generated SF light (signal light). On the other hand, the noise power is the sum of the shot noise N _sh (∝i _SF + i _b ) due to the total photocurrent including the background photocurrent i _b and the thermal noise N _th of the light receiving circuit system. Here, since N _sh >> N _th can be obtained by using a photomultiplier having a large multiplication factor of the light receiving element, APD, or the like, the SN ratio of optical sampling can be approximated by the following equation.

Here, η is the SFG conversion efficiency, P _sig and P _sam are the powers of the measured light and the sampling light, respectively, and A is a proportional constant. This expression under normal conditions of use P _sig << P _sam the ^{_{S / N = ηP sig 2 /}} A (2) next, SN ratio is no longer dependent on the P _sam, means that determined by the value of P _sig and eta.

[Problems to be Solved by the Invention] The characteristic (a) shown in FIG. 7 illustrates the relationship between the sampling light power and the SN ratio in the case of the type I material. In optical sampling using a normal type I material, if P _sig = 1 mW and η = 10 ^-4 , only an S / N ratio of 10 ^-4 can be obtained. Therefore, conventionally, the sampling measurement values are averaged many times to improve the SN ratio. For this reason, in the past, only the optical signal could be measured periodically with the same waveform. By the way, in the case of an intensity-modulated optical signal like the waveform (A) in FIG. 8, as a cross-correlation signal between the measured optical pulse (A) and the sampling optical pulse (B) shown in FIG. A sum frequency light pulse (C) as shown in FIG. 8 (C) is taken out. In this case, since the light modulated to "1" and "0" is averaged, there is a drawback that the eye pattern measurement that represents the waveform of the mark space important for evaluating the transmission waveform cannot be measured. .

In order to solve the above-mentioned problems, an object of the present invention is to perform individual sampling by removing background light and multiplying the power of incident light to a nonlinear optical material in an optical sampling optical waveform measuring apparatus using SFG. An object is to provide an optical sampling waveform device configured to improve the SN ratio of values and measure an eye pattern.

[Means for Solving the Problem] In order to achieve such an object, the present invention provides a sampling light generator for generating a train of sampling light pulses having a pulse width narrower than that of the light signal under measurement, and a light signal under measurement. And a sampling optical pulse, and a nonlinear optical material that receives the multiplexed output from the multiplexer and generates a sum frequency light of the optical signal under measurement and the sampling optical pulse, and a nonlinear optical material In the optical sampling waveform measuring instrument including a photodetector that detects the sum-frequency light generated in the above and converts it into an electrical signal, the polarization directions of the non-measurement optical signal and the sampling optical pulse are orthogonal to each other in the non-linear optical material. It is a non-linear optical material that sometimes phase-matches to generate sum frequency light.
An optical amplifier for optically amplifying at least one of the power of the measured optical signal and the power of the sampling optical pulse is disposed on the light incident side to the nonlinear optical material.

[Operation] The present invention removes background light using a nonlinear optical material that causes phase matching of SFG when the polarization directions of two lights are orthogonal to each other, and uses an optical amplifier to measure the measured optical power or The optical waveform can be sampled and measured with a high S / N ratio by multiplying the sampling optical power or both optical powers. It differs from the prior art in the use of nonlinear optical materials and optical amplifiers that provide Type II phase matching.

Embodiments Embodiments of the present invention will be described in detail below with reference to the drawings.

(Embodiments 1 and 2) FIGS. 1 and 2 are views showing Embodiments 1 and 2 of the present invention, respectively. The illustrated construction is shown in a portion surrounded by a broken line in FIG. 6 showing a conventional technique. Deploy. Other parts in this embodiment are the same as the conventional ones. FIG. 1 shows Example 1 for amplifying sampling light, and FIG. 2 shows Example 2 for amplifying light to be measured.

1 and 2, 2 and 3 are polarization direction controllers for controlling the polarization directions of the sampling light and the measured light, respectively, 4 is a multiplexer for combining the measured light and the sampling light, and 10 is a multiplexer. 11 is a rare earth-doped fiber type optical amplifier, which is a type II nonlinear optical material that causes SFG by phase matching when the polarization directions of two lights are orthogonal to each other.

In the first embodiment of FIG. 1, the sampling light is amplified by the optical amplifier 11 and then guided to the multiplexer 4 via the polarization direction controller 2. The measured light is guided to the multiplexer 4 via the polarization direction controller 3. The combined output light from the combiner 4 is guided to the nonlinear optical material 10.

In Example 2 of FIG. 2, the sampling light is guided to the demultiplexer 4 via the polarization direction controller 2. The measured light is amplified by the optical amplifier 11 and then guided to the multiplexer 4 via the polarization direction controller 3. The combined output light from the multiplexer 4 is guided to the type II nonlinear optical material 10.

Specifically, as the nonlinear optical material 10 causing the type II phase-matched SFG, KTP, LiIO ₃ , LiNbO ₃ , KNbO ₃ or the like may be used as a bulk crystal, or, instead of the bulk crystal, a waveguide structure is used. Any form of non-linear optical element may be used.

Further, although the rare earth-doped fiber type optical amplifier is used as the optical amplifier 11 in this embodiment, a semiconductor laser amplifier or a fiber Raman amplifier may be used instead.
However, in that case, since the optical amplifier 11 has polarization direction dependency, it is necessary to dispose the polarization direction controller 2 or 3 before or after the optical amplifier 11.

In these examples, the polarization controllers 2 and 3 adjust the polarization directions of the sampling light and the measured light so that they are orthogonal to each other, so that the SF light, which is a correlation signal between the sampling light and the measured light, is phase-matched. However, the background light (SH light of the measured light itself and SH light of the sampling light itself) does not occur because it belongs to the type I phase matching in the same polarization direction.
Therefore, since i _b = 0 in the equation (1), the equation (1) representing the SN ratio in this case is expressed as the following equation.

The result of calculating the relationship between the sampling light power and the SN ratio using this is shown as the characteristic (b) in FIG. However,
Here, the peak power of the measured light is assumed to be 1 mW.

As is clear from FIG. 7, in the case of type II, the SN ratio can be made higher than that of the conventional type I, and the SN ratio can be further improved by increasing the sampling light power.

In Example 1 of FIG. 1, assuming that a semiconductor laser pulse having a peak power of 100 mW is used as the sampling light and this sampling light is amplified by 30 dB (peak power 100 W) by the rare earth light-doped fiber type optical amplifier 11, FIG. From the characteristic (b), the S / N ratio can be improved to about 10 and individual sampling values can be determined. Moreover, even if the measured light is amplified by the optical amplifier 11 as shown in FIG.
The same effect can be expected from equation (3).

(Embodiments 3 and 4) FIGS. 3 and 4 are views showing Embodiments 3 and 4 of the present invention, respectively, in which case both the sampling light and the measured light are optically amplified.

In the embodiment shown in FIG. 3, the measured light and the sampling light are separately amplified by the optical amplifiers 12 and 13, and the amplified output lights are guided to the multiplexer 4 via the polarization direction controllers 2 and 3, respectively. Type the combined output light from the combiner 4
II Lead to nonlinear optical material 10.

In the embodiment of FIG. 4, the measured light and the sampling light are set to 1
Amplify with two optical amplifiers. That is, the sampling light is guided to the demultiplexer 4 via the polarization direction controller 2. The measured light is guided to the multiplexer 4 via the polarization direction controller 3. The combined output light from the combiner 4 is amplified by the optical amplifier 14 and then guided to the nonlinear optical crystal 10.

In these embodiments, the optical amplifiers 12, 13 and 14 are the first
The optical amplifier has the same structure as the rare earth-doped fiber type optical amplifier 11 shown in FIG.

In these embodiments, the product of the amplification factor of the optical amplifier 12 and 13 or 14 with respect to the sampling light power and the measured light power contributes to the improvement of the SN ratio. Sampling light power 10
0mW, measured optical power 1mW, amplification rate of each optical power is 30dB
Then, the SN ratio becomes as high as 10 ⁴ . At this time, assuming that the SN ratio is about 10, the measurable optical power of the measured light is about 1 μW. That is, a semiconductor laser is used as the sampling light, and the measured light with a micro power of about μW is sufficient to determine each sampling value.
It becomes possible to measure with the SN ratio.

[Effects of the Invention] As described above, the present invention uses the type II nonlinear optical material and the optical amplifier that cause SFG by phase matching when the polarization directions of two lights are orthogonal to each other. Since the power of SF light can be increased without the generation of background light, a significant improvement in the SN ratio can be expected. Therefore, according to the present invention, since it is possible to achieve high sensitivity of optical sampling waveform measurement, it is possible to reduce the number of times of averaging processing, and further it is possible to perform eye pattern measurement. Furthermore, in the present invention, since the semiconductor laser can be used as the sampling light source, the sampling frequencies can be easily synchronized, and further, the device can be downsized and the cost can be easily reduced.

[Brief description of drawings]

FIG. 1 shows the first embodiment of the optical sampling waveform measuring apparatus of the present invention.
2 is a block diagram showing the configuration of the optical sampling waveform measuring apparatus according to the second embodiment of the present invention.
3 is a block diagram showing the configuration of the optical sampling waveform measuring apparatus according to the third embodiment of the present invention.
FIG. 4 is a block diagram showing the configuration of FIG. 4, and FIG. 4 is a fourth embodiment of the optical sampling waveform measuring apparatus of the present invention.
5 is a block diagram showing the configuration of a conventional optical sampling waveform measuring method by optical sampling, FIG. 6 is a block diagram showing the configuration of an example of a conventional optical sampling waveform measuring apparatus, and FIG. FIG. 8 is a signal waveform diagram showing an example of measured waveforms when the intensity modulated optical pulse train is optically sampled by calculating the relationship between the sampling light power and the SN ratio. 1 ... Sampling light source, 2, 3 ... Polarization direction controller, 4 ... Multiplexer, 5 ... Non-linear optical material for generating sum frequency light of type I phase matching, 6 ... Optical filter, 7 ... Optical receiver, 8 ... Electrical signal processing system, 9 ... Display screen, 10 ... Nonlinear optical material for generating sum frequency light of type II phase matching, 11,12,13,14 ... Rare earth doped fiber optical amplifier .

Claims (1)

[Claims] 1. A sampling light generator for generating a train of sampling light pulses having a pulse width narrower than that of an optical signal to be measured,
A combiner for combining the measured optical signal and the sampling optical pulse, and a combined output from the combiner to generate sum frequency light of the measured optical signal and the sampling optical pulse. In the optical sampling waveform measuring instrument comprising: a non-linear optical material, and a photodetector that detects the sum frequency light generated in the non-linear optical material and converts it into an electric signal, the non-linear optical material is the non-measurement optical signal and A nonlinear optical material that generates a sum frequency light by phase matching when the deflection directions of the sampling light pulse are orthogonal to each other, and the power of at least one of the power of the measured optical signal and the power of the sampling light pulse. An optical sampling waveform measuring device, wherein an optical amplifier for optically amplifying the above is disposed on the light incident side to the nonlinear optical material.