JPH0810162B2 - Optical sampling waveform measuring device - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an optical sampling waveform measuring apparatus for measuring an optical pulse waveform used in intensity-modulated optical communication or the like using sum frequency light generation, and more particularly, to an optical sampling waveform measuring apparatus. The present invention relates to an apparatus for automatically compensating for changes in the polarization state of a measurement light pulse and measuring the light pulse waveform.

[Prior Art] Conventionally, in the observation of an optical pulse using sum frequency light generation by a second-order nonlinear optical system, an optical pulse to be observed and a sampling optical pulse having a narrower width are guided to a nonlinear optical crystal, and both The method of extracting the cross-correlation signal as sum frequency light is used.

FIG. 5 shows the change in the relative position of the measured pulse and the sampling pulse with time in this method, and the low-speed cross-correlation waveform obtained thereby. The repetition frequency of the sampling pulse is the repetition frequency f of the measured pulse.
Such operation becomes possible by slightly lowering it from [Hz], that is, by Δf [Hz].

In general, this frequency difference is much lower than the sampling repetition frequency f−Δf [Hz], so the actual sampling operation is much closer to continuous sampling than that shown in the figure. Since the band of the cross-correlation waveform is about this frequency difference, there is an advantage that a high-speed photodetector is not required.

Now, the nonlinear optical crystal as the sum frequency light generating element operates optimally only for the incident light of a specific linearly polarized light. For example, a crystal in which extraordinary rays (or ordinary rays) generate the maximum sum frequency light is called a "type I" crystal.
In this case, the intensity of the cross-correlation waveform (sum frequency light) shown in FIG. 5 is the intensity of the sampling pulse and the pulse under measurement (Is, I
m) of the component (Is) corresponding to extraordinary light (or ordinary light)
proportional to the product of e, Ime) (or Iso, Imo). Therefore, in order to obtain the maximum sum frequency light, the sampling pulse and the pulse under measurement must be linearly polarized light corresponding to the extraordinary light (or ordinary light). In particular, since many of the waveguide type crystals that are said to have high sum frequency light generation efficiency perform type I operation, the sampling pulse and measured pulse are linearly polarized light corresponding to extraordinary light (or ordinary light). The need is particularly great.

On the other hand, the polarization state of an optical pulse is defined by the birefringence of the optical fiber in which it propagates.

The single mode optical fiber used for measurement also has birefringence, and the bending state of the optical fiber and the external pressure affect the polarization state of light at the exit end. Therefore, even if it is a pulse from a light source with a clear polarization characteristic, the polarization state at the input part of the measuring instrument is not uniquely determined, but rather randomly, as long as it is transmitted through a normal optical fiber that does not have a polarization maintaining characteristic. fluctuate.

However, this fluctuation is about the same as the fluctuation speed of the disturbance applied to the fiber.
It can be said that it is rarely faster than z.

When the type I crystal is used, it is equivalent to that the efficiency changes effectively when the polarization state of the optical pulse to be observed changes, and the amplitude of the observed correlation waveform fluctuates, and correct waveform data cannot be obtained. It will be.

[Problems to be Solved by the Invention] When the optical fiber is stationary, the birefringence of the optical fiber does not change, so that the polarization state at the exit end does not change. In this case, the polarization state at the incident end face of the type I crystal can be optimized by using a polarization compensating circuit in which a Babinet-Soleil compensator and a fiber loop are combined.

For this reason, until now, a polarization control circuit combining fiber loops was inserted into the optical fiber transmission line to optimize the incident polarization condition to the type I crystal, and then the measurement was performed without touching the optical fiber. Was going on.

FIG. 6 shows a configuration example of such a conventional optical sampling optical waveform measuring device.

In FIG. 6, reference numeral 1 is an oscillator having a sampling frequency f−Δf, and 2 is a sampling pulse generator that generates sampling pulse light in accordance with an oscillation output from the oscillator 1. The sampling pulse light is made incident on the polarization controller 3 to adjust the polarization state of the sampling pulse light to an optimum state.

On the other hand, the oscillation output from the sampling frequency oscillator 1
It is led to the RF mixer 4, and the electric pulse of the frequency f from the pulse train generation unit 5 to be measured as an external device is mixed with the oscillation output described above. The mixed output is a low pass filter 6
To a trigger signal by extracting the component of the frequency Δf and supplying the trigger signal to the display unit 7.

Further, the measured optical pulse having the frequency f from the measured pulse train generating section 5 is guided to the polarization control section 8 and the polarization state of the optical pulse is adjusted to the optimum state here. In the polarization control units 3 and 8, the sampling light pulse and the measured light pulse whose polarization states have been adjusted are guided to the type I nonlinear crystal 10 via the coupler 9, where the low-speed cross-correlation output light of the frequency Δf is output. To get The output light is received by the low-speed light receiver 11 and converted into an electric signal, which is then supplied to the signal processing unit 12 for appropriate signal processing and then supplied to the display unit 7. The display unit 7 displays the output from the signal processing unit 12 as a waveform as shown in FIG. 4 at the timing of the trigger signal.

The thick solid line in FIG. 6 shows the flow of optical signals, and the thin solid line shows the flow of electrical signals.

The coupler 9 superimposes another sampling pulse having a pulse width narrower than the optical pulse on the optical pulse to be measured having the repetition frequency f, and the superposed output light is optimally operated only for specific linearly polarized light. When it is incident on the crystal 10 and the delay of this sampling pulse is further swept, the cross-correlation waveform drawn by the sum frequency optical power generated in proportion to the overlapping portion of both optical pulses is displayed on the display device 7. , Measure the incident light pulse waveform.

However, it is almost impossible to touch the optical fiber in the actual measurement, and it is necessary to constantly monitor the polarization state in order to maintain the optimum polarization state.

In the above measuring apparatus, in the case of a pulse train to be measured whose oscillation frequency changes (chirves) within a pulse like an optical pulse from a semiconductor laser, when a birefringent medium such as an optical fiber is passed, The polarization state within the pulse will also change. In this case, there is always a portion that cannot be fully compensated by the above polarization compensation circuit, and the observed waveform deviates significantly from the true waveform.

Therefore, in order to perform accurate waveform observation with high sensitivity on an incident optical pulse of any polarization state, it is necessary to develop an optical sampling waveform measuring device whose output signal does not depend on the incident polarization state.

The simplest is to prepare two nonlinear crystals and allocate these two nonlinear crystals to their respective polarization components. This is the same as providing two sampling measurement systems. In addition, in this method, it is necessary to match the sensitivities of the two measurement systems, which is problematic in terms of stability, reliability, and economy.

Therefore, an object of the present invention is to solve the above-mentioned problems in an optical sampling waveform measuring apparatus using a sum frequency light generation technique, and automatically compensate the amplitude fluctuation of the measured waveform due to the random fluctuation of the polarization state. An optical sampling waveform measuring device is provided.

[Means for Solving the Problem] In order to achieve the above object, the present invention superimposes another sampling light pulse having a width narrower than that of the incident light pulse to be measured, which has repetition, When the superimposed output light is incident on a nonlinear optical crystal that operates optimally only for a specific linearly polarized light and the delay of the sampling light pulse is swept, the sum frequency that is generated in proportion to the overlapping portion of these two light pulses In an optical sampling waveform measuring device that measures the waveform of an incident light pulse from the cross-correlation waveform drawn by optical power, the incident light pulses are orthogonal to each other 2
A polarization splitting means for splitting into one linearly polarized light component, a delaying means for relatively delaying one of the split light pulses by a time larger than the pulse width, and an optical element of the same kind as the polarization splitting means, Coupling means for coupling on the same optical axis from the delay means, means for forming temporally separated pulses corresponding to two linearly polarized components orthogonal to each other from the combined output, and the separated pulses. Means for automatically compensating the amplitude fluctuation of the measurement waveform due to the fluctuation of the polarization state of the incident light pulse by adding.

[Operation] In the present invention, the incident optical pulse is separated into two linearly polarized pulses which are orthogonal to each other, the optical sampling is performed separately, and then the respective measurement waveforms are added. That is, in the present invention, one crystal is operated in time division with respect to the light pulses of two linearly polarized light components orthogonal to each other of the light pulse to be measured,
The measured waveforms of the quadrature components serially obtained in time are electrically added.

It can be realized only in specific wavelength bands and materials,
By optimizing the polarization state of the sampling pulse using a so-called "type II" nonlinear optical crystal, both ordinary and extraordinary light propagating in the crystal can contribute to the generation of sum frequency light. Thus, it is possible to realize the generation of sum frequency light that is not affected by the change in the polarization state of incident light.

As described above, in the present invention, the optical pulse waveform can be stably measured by automatically compensating the amplitude variation of the measurement waveform due to the random variation of the polarization state.

EXAMPLES The present invention will be described in detail below with reference to the drawings.

1 and 2 show the configuration of the first embodiment of the present invention.

The first embodiment is applicable when the pulse width is sufficiently smaller than the pulse repetition period t.

As shown in FIG. 1, an incident light pulse having a repetition period t as an optical signal to be measured is guided to the Rochon prism 21,
Here, after splitting into orthogonal linearly polarized light components and propagating paths of different lengths, one optical pulse is delayed by D / 2, and then both optical pulses are guided to the second Rochon prism 22 again. Combine with. If the optical path difference is made larger than the pulse width, two linearly polarized light pulses orthogonal to each other can be obtained by temporally separating the optical path difference D. If the reflection loss and absorption loss due to the optical components used here are ignored, the sum of the powers of these two light pulses is equal to the power of the incident light pulse.

These light pulses are observed by a conventional optical sampling method using a general "Type I" nonlinear optical crystal 10. However, the linear polarization direction of the incident light pulse needs to be 45 degrees with respect to the polarization direction in which the crystal 10 shows the maximum efficiency. Therefore, both of the light pulses described above are passed through the analyzer 23 in the deflection direction of the crystal 10. This ensures that each polarization component is equal
It will make a contribution of 1/2. If the crystal 10 also functions as the analyzer 23, the analyzer 23 can be omitted.

In this way, the optical output obtained from the analyzer 23 is passed through the coupler 9 such as a semi-transparent mirror together with the sampling pulse light to be superimposed on the sampling pulse, and the superimposed optical output is made incident on the nonlinear optical crystal 10. Here, when the sweep period of the sampling pulse is T, the pulse waveform in the observed cross-correlation output also has a period T, and two pulses that are TD / t apart in time corresponding to each polarization component are periods. Observed within T. If T is on the order of several tens of msec, the polarization is considered to be constant within this period.

Therefore, in the signal processing unit 12 that processes the cross-correlation output from the photodetector 11, for example, as shown in FIG. 2, the observed electrical waveform signal regarding this cross-correlation is supplied to the branching device 24 and further divided into two. One of the signals is passed through the delay device 25 to delay the two pulses corresponding to the respective polarization components so that they overlap each other, and then the adder 26 adds the two signals and discards the non-overlapping parts. According to this, optical sampling that does not depend on the polarization state of the incident light pulse can be realized.

Also, a lotion prism as a polarization separation prism
Instead of 21, the single-mode fiber with large polarization dispersion is prepared, and the transmission polarization plane of the analyzer 23 in the latter stage is
The same effect can be obtained by arranging so as to form 45 degrees.

If the pulse width is not sufficiently smaller than the repetition period t of the optical pulse, the second pulse shown in FIG.
The examples are valid.

As shown in FIG. 3, after the incident light pulse is passed through the polarization polarizing prism 31 and divided into two light pulses having two linearly polarized light beams orthogonal to each other, these light pulses are guided to the optical switch 32,
Here, the switching signal synchronized with the sampling cycle is switched so that only one of the optical pulses is extracted, and the two polarization components are temporally divided and output as shown by A and B in the figure. The optical pulse switched in this way is sampled and measured with the same configuration as in the first embodiment.

In the signal processing unit 12, the waveforms A and B before and after the switching thus obtained are added as shown in FIG. As a result, the waveforms of the respective polarization components are added, so that an optical sampling measurement waveform that does not depend on the polarization state of the incident light pulse can be realized.

In yet another embodiment of the present invention, as the nonlinear optical crystal that is the sum frequency light generating element, a crystal of a type that can achieve the so-called "type II" matching condition can be used. This type of non-linear optical crystal is one in which the average value of the refractive index of ordinary light and extraordinary light at the wavelength of the fundamental wave is equal to the refractive index at the wavelength of the second harmonic light, and is not generally available. However, at a specific crystal and wavelength where this condition is realized, an optical sampling optical waveform measuring device that does not depend on the polarization state of the incident light pulse can be realized as described below.

First, in a type II crystal, extraordinary light and ordinary light, which are orthogonal to each other, contribute to the generation of sum frequency light. That is, the ordinary light component (Iso) of the sampling light and the extraordinary light component (Im) of the measured light
e) The process of generation of sum-wave light proportional to the product and the process of generation of sum-frequency light between the extraordinary light component (Ise) of the sampling light and the ordinary light component (Imo) of the measured light simultaneously proceed. . Therefore, if the sampling light is also incident with the same amplitude as the extraordinary light and the ordinary light, if it is incident as linearly polarized light or circularly polarized light with a polarization plane inclined by 45 degrees with respect to the polarization plane of the ordinary light, the incident light pulse Whatever the polarization state is, it is possible to realize the stable sampling by automatically compensating the amplitude variation of the measurement waveform due to the variation of the polarization state.

[Effects of the Invention] As described above, according to the present invention, it is possible to automatically compensate the amplitude fluctuation of the measurement waveform due to the random fluctuation of the polarization state.

If the random change of the polarization state is very slow, there is practically no problem even if linear polarization is compensated with a Babinet-Soleil compensator, but if the polarization state changes within one pulse, However, such a method is inconvenient, and for the first time, according to the present invention, the amplitude variation of the measurement shape due to the variation of the polarization state can be automatically compensated.

[Brief description of drawings]

FIG. 1 is a block diagram showing a processing system for an optical signal in the first embodiment of the present invention, FIG. 2 is an explanatory view showing an electric signal processing procedure in the first embodiment of the present invention, and FIG. FIG. 4 is a configuration diagram showing a processing system for an optical signal in the second embodiment, FIG. 4 is an explanatory diagram showing an electric signal processing procedure in the second embodiment of the present invention, and FIG. 5 is a time chart of a measured pulse and a sampling pulse. FIG. 6 is a block diagram showing a configuration example of a conventional optical sampling optical waveform measuring device, which shows a change in relative position and a low-speed cross-correlation waveform obtained thereby. 1 ... Sampling frequency oscillator, 2 ... Sampling pulse generator, 3 ... Polarization controller, 4 ... RF mixer, 5 ... Measured pulse train generator, 6 ... Low-pass filter, 7 ... Display, 8 ...... Polarization control unit, 9 ...... Coupler, 10 …… I nonlinear optical crystal, 11 …… Slow-speed light receiver, 12 …… Signal processing unit, 21,22 …… Rochon prism, 23 …… Analyzer, 24 ・ ・ ・… Splitter, 25 …… Delayer, 26 …… Adder, 31 …… Polarization separation prism 32 …… Optical switch.

Claims (2)

[Claims] 1. A non-linear optical system which superimposes another sampling optical pulse having a width narrower than that of the incident optical pulse to be measured having repetitions, and optimally operates the superposed output light only for a specific linearly polarized light. From the cross-correlation waveform drawn by the sum frequency optical power generated in proportion to the overlapping portion of these two optical pulses when the optical pulse is made incident on the optical crystal and the delay of the sampling optical pulse is swept, In an optical sampling waveform measuring device for measuring a waveform, a polarization separation means for separating the incident light pulse into two linearly polarized light components orthogonal to each other, and one of the separated light pulses are relative to each other for a time larger than the pulse width. Delaying means for delaying the optical pulses, a combining means for combining the temporally separated optical pulses from the delaying means on the same optical axis, and a combined output thereof Means for forming temporally separated pulses corresponding to two linearly polarized light components orthogonal to each other, and by adding the separated pulses, the amplitude of the measured waveform due to the variation of the polarization state of the incident light pulse An optical sampling waveform measuring device comprising means for automatically compensating for fluctuations. 2. A means for superimposing an incident light pulse to be measured having repetitions and a sampling pulse having a linear polarization or a circular polarization direction inclined by 45 degrees with respect to the polarization direction of the ordinary light in the crystal, Means for generating sum frequency light proportional to the product of the intensity of ordinary light and extraordinary light, and measuring the waveform of the incident light pulse from the cross-correlation waveform drawn by the power of the sum frequency light. Sampling waveform measuring device.