JPH08184502A - Light waveform measuring equipment - Google Patents

Abstract

PURPOSE: To realize measurement of light waveform with a high S/N ratio by performing heterodyne detection for detecting the beat of the output light from a nonlinear optical element and a local oscillation light of oscillation frequency in order to detect the mixture of four light waves from the output light of the nonlinear optical element. CONSTITUTION: A light waveform measuring circuit comprises a local oscillation light source 14, a polarization controller 15, an optical coupler 16, a band-pass filter(BPF) 17, and a squaring unit 18, as a specific constitution. An output light from a nonlinear optical element 10 controlled by a polarization controller 13 is coupled through an optical coupler 16 with an output light from the local oscillation light source 14 controlled by the polarization controller 15, and five types of light are transduced through an opto-electric transducer 11. The mixture 10a of four light waves is detected by means of a BPF 17 for the frequency band δ [Hz] of the mixed light 10a and a local oscillation light 14a. Since deterioration of S/N due to the optical filter is avoided through heterodyne detection, the light waveform can be measured well at a high speed while reducing the averaging operation.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention mainly relates to an optical waveform measuring method for evaluating high speed transmission characteristics of functional circuits, elements, materials and the like of a system for realizing ultra high speed optical communication.

[0002]

2. Description of the Related Art Next, FIG. 2 shows the configuration of an ultrahigh-speed optical waveform measuring device according to the prior art. 2, 1 and 2 are signal generators and 3
Is a mixer, 4 is a signal light source, 5 / 8.13 is a polarization controller,
Reference numeral 6 is an object to be measured (hereinafter referred to as DUT), 7 is a pulse light source, 9 is an optical coupler, 10 is a nonlinear optical element, 11 is a photoelectric converter, 12 is a display unit, and 20 is an optical filter. D
The UT 6 is, for example, an optical waveguide, an optical switch, an optical filter,
An optical circuit or a circuit including a light source.

In FIG. 2, the signal generator 1 generates an electric signal having a high repetition frequency f ₀ [Hz]. The signal light source 4 receives the output of the signal generator 1 and outputs an optical signal 4a having a repetition frequency f ₀ [Hz]. The oscillation frequency of the signal light source 4 is ν _S
[Hz].

The signal generator 2 generates an electric signal having a very low repetition frequency Δf [Hz]. The mixer 3 has a difference f ₀ −Δ between the two repetition frequencies of the signal generator 1 and the signal generator 2.
An electric signal having a frequency of f [Hz] is generated. The pulse light source 7 receives the output of the signal generator 2 as input, and the repetition frequency f ₀ −
The pulsed light 7a of Δf [Hz] is output. The oscillation frequency of the pulse light source 7a is assumed to be ν _P [Hz].

The optical signal 7a of the pulse light source 7 is a polarization controller 8
Input to the optical coupler 9 via the optical signal 4a of the signal light source 4
Is input to the DUT 6 via the polarization controller 5, and the output of the DUT 6 is input to the optical coupler 9 and multiplexed. Polarization controller 5
Numeral 8 is for controlling the polarization states of both incident lights so that the four-wave mixing light generation efficiency of the nonlinear optical element 10 is maximized.

The nonlinear optical element 10 receives the output of the optical coupler 9 as an input, and only when the signal light 4a and the pulsed light 7a are present at the same time, the four-wave mixed light 10a having an oscillation frequency of 2ν _P -ν _S is generated.
And four-wave mixed light 10b having an oscillation frequency of 2ν _S −ν _P is generated.

The repetition frequency of the four-wave mixed lights 10a and 10b is f ₀ -Δf [Hz] as in the pulsed light 7a.
Further, since the light intensity of the four-wave mixed light 10a is proportional to the product of the square of the light intensity of the pulsed light 7a and the light intensity of the signal light 4a, if the light intensity of the pulsed light 7a is kept constant, the The light intensity of the mixed light 10a is proportional to the light intensity of the signal light 4a.

The output light of the non-linear optical element 10 is input to the optical filter 20 via the polarization controller 13, and the signal light 4a other than the four-wave mixing light 10a, the pulsed light 7a and the four-wave mixing light 10b are removed. . The polarization controller 13 is the optical filter 2
This is for controlling the polarization state of the output light of 0 through non-linear optics so as to match the polarization property of 0. That is, FIG.
Therefore, the high-speed optical transmission characteristics of the DUT6 are as follows:
It is evaluated by the change of the signal waveform depending on the presence or absence of 6.

Next, FIG. 3 shows the relationship between the power of the output light from each part of FIG. 2 and the oscillation frequency. Figure 3A is the output light of the optical coupler 9, the oscillation frequency [nu _P of the oscillation frequency [nu _S and pulsed light 7a of the signal light 4a is set to ν _S> ν _P as an example, [nu _S <
It may be ν _P. FIG. 3B shows the output light of the nonlinear optical element 10. By inputting the output light of the optical coupler 9, the four-wave mixed light 10a.1 is provided on both sides of the signal light 4a and the pulsed light 7a.
0b occurs. FIG. 3C shows the output light of the optical filter 20, which is the final optical output of the optical filter 20 which passes only 10a having a high output light level of the two four-wave mixed lights. The output light of the optical filter 20 is the photoelectric converter 11
Is input to the display unit 12, is photoelectrically converted, and is displayed on the display unit 12.

Next, FIG. 5 shows the relative positions of the signal light 4a and the pulsed light 7a with respect to time. The relative positions of the two lights are deviated by Δt = Δf / f ₀ ² for each repeating cycle, and after the period of Δf elapses, the relative positions of the two lights match again. Therefore, if the output signal of Δf of the signal generator 2 is used as a trigger to reproduce the light intensity of the four-wave mixing light 10a which is the interaction light of both lights, that is, the output signal of the photoelectric converter 11, the waveform is reproduced. The envelope of the optical waveform as shown in Fig. 4 is obtained, and the time axis of the signal optical waveform is expanded to f ₀ / Δf times to observe the ultrahigh-speed signal optical waveform without being limited to the band of the photoelectric converter 11. it can.

[0011]

In FIG. 3B, the signal light 4a is used.
The frequencies of the pulsed light 7a and the four-wave mixed light 10a and 10b are close to each other, and a spectroscope is usually used as the optical filter 20 for separating them. However, the insertion of the spectroscope causes a loss in the output four-wave mixed light 10a, and its level becomes about 0.1 times the input level. Originally, since the nonlinear optical phenomenon has a low generation efficiency, the obtained light intensity becomes weaker and the SN ratio deteriorates. Therefore, the SN ratio is practically improved by the averaging process.
An object of the present invention is to provide an apparatus for performing an optical waveform measurement with a good SN ratio by performing heterodyne detection on the four-wave mixed light 10a.

[0012]

In order to achieve this object, the present invention provides a signal generator 1 for generating an electric signal having a repetition frequency f ₀ [Hz] and an electric signal having a repetition frequency Δf [Hz]. The signal generator 2 and the outputs of the signal generator 1 and the signal generator 2 as input, and the repetition frequency f ₀ −Δf
A mixer 3 for generating an electric signal of [Hz], a signal light source 4 of an oscillating frequency ν _S [Hz] for receiving an output of the signal generator 1 and emitting an optical signal of a repetition frequency f ₀ [Hz], and a signal. Light source 4
The optical pulse signal having the repetition frequency f ₀ -Δf [Hz] as the input, the polarization controller 5 for controlling the polarization state and inputting it to the DUT 6 and the output of the mixer 3 as the input. Pulsed light source 7 having an oscillation frequency of ν _P [Hz], a polarization controller 8 that controls the polarization state by using the output light pulse signal of the pulsed light source 7, and an output optical signal from the DUT 6. And an optical coupler 9 for combining the optical pulse signals emitted from the polarization controller 8
And an input of the combined optical signal of the optical coupler 9, and a part of the input combined optical signal is oscillated at a frequency of 2ν _P −ν _S [Hz], 2ν _S −ν _P.
Non-linear optical element 10 that converts into four-wave mixed light of [Hz] and outputs most of the input combined optical signal and four-wave mixed light, and the output light of non-linear optical element 10 as input, and controls the polarization state A polarization controller 13, an opto-electric converter 11 for converting only the four-wave mixed light having an oscillation frequency of 2ν _P −ν _S [Hz] from the output light of the nonlinear optical element 10, and converting the light intensity into an electric signal.
The photoelectric converter 1 using the output signal of the signal generator 2 as a trigger
In the optical waveform measuring device provided with the display unit 12 for measuring and displaying the output electric signal of No. 1, the oscillation frequency ν _L = 2ν _P −ν _S
A local light source 14 of + δ [Hz], a polarization controller 15 for inputting the outgoing light of the local light source 14 and controlling the polarization state, and an output light of the polarization controller 13 and the polarization controller 14. The optical coupler 16 which combines and outputs to the opto-electric converter 11, the band pass filter 17 which detects the δ [Hz] band electric signal from the output electric signal of the opto-electric converter 11, and the output of the band pass filter 17 A squarer 18 that squares an electric signal and outputs the squared electric signal to the display unit 12 is provided.

[0013]

[Function] 4 required from the output light of the nonlinear optical element 10
In order to detect the light wave mixed light 10a, the nonlinear optical element 10
Output light and the beat light of the local light 14a having the oscillation frequency ν _L are detected by heterodyne detection.

[0014]

1 is a block diagram of an optical waveform measuring circuit according to the present invention. In FIG. 1, reference numeral 14 is a local light source, 15 is a polarization controller, 16 is an optical coupler, 17 is a bandpass filter (hereinafter referred to as BPF), 18 is a squarer, and others are shown in FIG.
Is the same as That is, the configuration of FIG.
And the polarization controller 1 for inputting the output light 14a of the local light source 14
5 and a polarization controller 1 inserted in place of the optical filter 20.
An optical coupler 16 that multiplexes the output of the polarization controller 15 and the output of the polarization controller 15 is provided, and the four-wave mixed light 10a of the electric signal converted by the photoelectric converter 11 is detected by the BPF 17.

Next, the waveform of each part in the optical waveform measuring device having the configuration of FIG. 1 will be described with reference to FIGS. 3 and 4. The input / output light is the same as in the conventional example of FIG. 2 up to FIGS.
The output light of the nonlinear optical element 10 controlled by the polarization controller 13 is combined with the output light of the local light source 14 controlled by the polarization controller 15 by the optical coupler 16, and five kinds of light are optoelectrically converted. The waveform is converted by the converter 11 and has a waveform as shown in FIG. Here, the four-wave mixed light 10a is detected by the BPF 17 in the beat frequency δ [Hz] band of the four-wave mixed light 10a and the local light 14a.

As the non-linear optical element 10, for example, 1.
A 55 μm zero-dispersion optical fiber can be used. In this case, when the oscillation frequency ν _P of the pulsed light 7a is matched with the zero-dispersion frequency of the 1.55 μm zero-dispersion optical fiber, the maximum four-wave mixing light generation efficiency is increased. can get. A traveling wave LD amplifier can also be used as the non-linear optical element 10 as an active element.

Next, FIG. 7 shows a block diagram of an embodiment of the light sources. FIG. 7A shows an example of the signal light source 4. As the light source, a stable DFB-LD with a narrow line width is used. The necessary signal waveform is generated by a Mach-Zehnder waveguide type optical intensity modulator or the like. Further, an erbium-doped fiber amplifier (hereinafter referred to as EDFA) is used to increase the signal light intensity.

FIG. 7B shows an embodiment of the pulse light source 7. A mode-lock type erbium-doped fiber laser or the like is effective as a short pulse light source. EDFA is used to increase the pulsed light intensity. Note that the oscillation frequency of short pulsed light is often quite wide. For example, if the width is 100 G [Hz] and B = 10 ⁷ [Hz], the sensitivity of heterodyne detection is 1.
Since it deteriorates by 0 ^-4, it is necessary to be careful to use a short pulse light source whose optical frequency width is as narrow as possible.

FIG. 7C shows an embodiment of the local light source 14. Four
A variable wavelength LD is used to shift the oscillation frequency 2ν _P −ν _S and δ [Hz] of the mixed light wave 10a to control the oscillation frequency ν _L.

Next, the SN ratios of the direct detection system having the configuration of FIG. 2 and the heterodyne detection system having the configuration of FIG. 1 will be compared. It is assumed that the photosensitivity of photoelectric conversion is almost 1 in the wavelength band of 1.5 μm, and the light intensity of the output four-wave mixed light 10a of the nonlinear optical element 10 is P _{S. In} the case of the direct detection system configuration shown in FIG. , SN ratio of the output of the photoelectric converter 11 the loss of the optical filter 20 is also taken into consideration becomes ^{_{SNR IM = P S 2 / [}} 100 (i n 2) B] ··· (1). Here, i _n is the input conversion noise current density, and B is the bandwidth.

On the other hand, in the case of the heterodyne detection system shown in FIG.
The shot noise due to 4a becomes the main noise, and the SN ratio of the output of the BPF 17 is SNR _H = P _S / (eB) (2). However, e has an elementary charge of 1.6 × 10 ⁻¹⁹ C.

Here, for comparison, the SN ratio becomes 1 4
The light intensity of the light wave mixed light 10a respectively P _IM, when the _{P H, P H / P IM} = e [B / (i n 2)] 1/2 / 10 ··· (3) , and the typical value As B = 10 ⁷ [Hz], i _n ² = 10
^-22 Substituting [A ² / Hz], the light intensity of _{P H / P IM = 5 ×} 10 -6 next, SNR = 1 become four-wave mixing light 10a is towards the heterodyne detection directly by conventional configuration It can be seen that it may be 5 × 10 ⁻⁶ times smaller than the detection.

[0023]

According to the present invention, the S by the optical filter is
Since N deterioration is avoided by heterodyne detection, good high-speed optical waveform measurement can be performed with less averaging processing.

[Brief description of drawings]

FIG. 1 is a configuration diagram showing an embodiment of the present invention.

FIG. 2 is a configuration diagram showing a conventional embodiment.

FIG. 3 is a diagram showing the relationship between the output light power of each part of FIG. 2 and the oscillation frequency.

FIG. 4 is a diagram showing the relationship between the power of combined light and the oscillation frequency of the optical coupler 16 according to the present invention.

FIG. 5 is a relationship diagram between signal light and pulsed light whose repetition frequency is deviated by Δf (Hz).

FIG. 6 is an optical waveform of sampled signal light.

FIG. 7 is a configuration diagram showing an embodiment of a light source.

[Explanation of symbols]

 1.2 Signal generator 3 Mixer 4 Signal light source 5/8 13/15 Polarization controller 6 DUT 7 Pulse light source 9/16 Optical coupler 10 Non-linear optical element 11 Photoelectric converter 12 Display section 14 Local light source 17 Band Pass filter 18 Squarer 20 Optical filter

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI technical indication H04B 10/152 10/142 10/04 10/06

Claims (1)

[Claims] 1. A first signal generator 1 for generating an electric signal of a repetition frequency f ₀ [Hz], a second signal generator 2 for generating an electric signal of a repetition frequency Δf [Hz], and a first signal generator 1. Of the signal generator 1 and the second signal generator 2 as inputs, and a mixer 3 for generating an electric signal having a repetition frequency f ₀ -Δf [Hz] and an output of the first signal generator 1 as inputs. , The signal light source 4 having an oscillation frequency ν _S [Hz] that emits an optical signal having a repetition frequency f ₀ [Hz], and the optical signal emitted from the signal light source 4 are input, and the polarization state is controlled to the DUT 6. The first polarization controller 5 to be input and the output of the mixer 3 are input, and the repetition frequency f ₀ −Δf
A pulsed light source 7 having an oscillation frequency ν _P [Hz] that emits an optical pulse signal of [Hz], and a second polarization controller 8 that receives the emitted optical pulse signal of the pulsed light source 7 and controls the polarization state. , A first optical coupler 9 for multiplexing the output optical signal from the device under test 6 and the output optical pulse signal from the second polarization controller 8, and the combined optical signal of the first optical coupler 9 as input , Oscillation frequency 2ν _P −ν _S [Hz], 2ν _S −ν
Non-linear optical element 10 that converts the four-wave mixed light of _P [Hz] and outputs the most of the input combined optical signal and the four-wave mixed light, and controls the polarization state by using the output light of the non-linear optical element 10 as input From the output light of the third polarization controller 13 and the nonlinear optical element 10, the oscillation frequency 2ν _P −ν
_The opto-electric converter 11 that inputs only the four-wave mixed light of _S [Hz] and converts the light intensity into an electric signal, and the output signal of the opto-electric converter 11 using the output signal of the second signal generator 2 as a trigger In an optical waveform measuring device including a display unit 12 for measuring and displaying a signal, a local light source 14 having an oscillation frequency ν _L = 2ν _P −ν _S + δ [Hz]
And the output light of the third polarization controller 13 and the fourth polarization controller 14 for inputting the light emitted from the local light source 14 and controlling the polarization state. A second optical coupler 16 that combines and outputs to the photoelectric converter 11, a bandpass filter 17 that detects an electrical signal in the δ [Hz] band from an electrical signal output from the photoelectric converter 11, and a bandpass filter An optical waveform measuring device comprising a squarer 18 that squares the output electric signal of 17 and outputs the squared output to the display unit 12.