JP3311031B2 - Optical pulse signal reproducing apparatus and method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an apparatus and a method for efficiently reproducing (demultiplexing) information of an optical pulse signal ultra-high-speed modulated by time division multiplexing.

[0002]

2. Description of the Related Art Conventionally, an optical time-division multiplexing modulation (Optical Time-D) utilizing an ultra-high band characteristic of light has been known.
Omain Multiplexing, hereafter OTD
The M) method is known. This method has the advantage that transmission and switching at very high bit rates (gigabit / second or more) in optical networks can be performed using conventional electronics. As an element that determines the upper limit of the modulation frequency, there is a speed at which the optical pulse signal information that has been time-division multiplexed modulated is reproduced. Hitherto, it has been proposed to reproduce OTDM signal information at a higher speed by a pure optical method using a nonlinear optical effect as compared with the conventional electro-optical method.

[0003] As an example, FIG. 3 shows Lin and Smi.
th (for details, see Applied
Physics Letters 59,2802-2
FIG. 804 is a schematic diagram for describing a method of reproducing an OTDM signal using a second-order nonlinear optical effect (described in 804 (1991)). In the figure, consider a light pulse signal train A and the optical pulse signal train B are time division multiplexed frequency omega ₁ of the signal light beam as OTDM signal. The signal light beam is superimposed on the sample light beam having the frequency ω ₂ via the beam splitter 10. Here, the timing of the sample light beam with the signal light beam of the frequency ω ₁ is adjusted in order to first detect the information of the signal sequence A.

Next, these two light beams enter a nonlinear optical crystal 11 capable of generating a sum or difference frequency by a second-order nonlinear optical effect. In this case, the polarization of the signal light beam and the sample light beam and the crystal orientation of the nonlinear optical crystal 11 can generate a sum frequency of ω ₃ = ω ₂ + ω ₁ or a difference frequency of ω ₃ = ω ₂ −ω _1. It is set as follows. Therefore, after passing through the nonlinear optical crystal 11, the signal light flux and the sample light flux overlap each other in time, that is, at the time corresponding to the signal train A, the frequency ω ₃ of the same as that of the signal train A is obtained. A time-series signal light beam will be included. The light beam of the frequency omega ₃ corresponding to the signal sequence A is detected through the frequency cut filter 12 for removing the light beam splitter 10 and the frequency omega ₁ and omega _2.

Next, the signal light beam transmitted through the beam splitter 10 is time-delayed by exactly one pulse of the signal light beam from the sample light beam by a dispersive element 13 having a different group velocity depending on the wavelength, such as a diffraction grating pair. Nonlinear optical crystal 14
Incident on. Here, similarly to the nonlinear optical crystal 11, the crystal orientation of the nonlinear optical crystal 14 is such that a sum frequency of ω ₃ = ω ₂ + ω ₁ or a difference frequency of ω ₃ = ω ₂ −ω ₁ can be generated. Is set. Therefore, the after passing through the nonlinear optical crystal 14, it may include the time-series light beam frequency omega ₃ signal light beam and the sample beam is equivalent to the light flux of the overlapping portion, i.e. the signal sequence B in time become. The light beam corresponding to the signal train B is detected via the beam splitter 10 and the frequency cut filter 15 for removing the light beams of the frequencies ω ₁ and ω ₂ . As described above, by using the harmonic generation method, which is a nonlinear optical effect having an ultrafast response speed of picoseconds or less, the OTD
It is possible to reproduce each signal sequence information from the M signal purely optically at a very high speed.

[0006]

However, in the above method, it is necessary to precisely control the polarization state of two light beams incident on the nonlinear crystal in order to efficiently generate a sum frequency or a difference frequency. For this purpose, either using a polarization maintaining fiber to the optical fiber transmission media of the signal light beam, it is necessary to add a polarization control function for the polarization-maintaining signal beam in the signal reproducing system of FIG. 3,
There is a disadvantage that the entire system becomes complicated and expensive.

Accordingly, it is an object of the present invention to reproduce an optical pulse signal from a time-division multiplexed optical pulse train on the order of picoseconds, thereby reproducing information of a desired optical pulse signal sequence and eliminating a change in polarization of a signal light beam. It is to provide an apparatus and a method.

[0008]

According to the present invention, there is provided an optical pulse signal reproducing method for reproducing each signal data pulse train from a time-division multiplexed optical data pulse train in real time. The optical data pulse train is spatially spread and its wave front is inclined, and the optical data pulse train is incident on a medium having a two-photon absorption effect so as to spatially intersect with the spatially expanded probe pulse. The method is characterized in that output light having received a nonlinear transmittance depending on the degree of temporal overlap between the data pulse and the probe pulse is detected as spatial pattern information.

In the optical pulse signal reproducing apparatus according to the present invention for achieving the above object, the optical pulse signal reproducing apparatus for reproducing each signal data pulse train from a time-division multiplexed optical data pulse train in real time, Means for expanding the pulse train spatially and tilting the wave front, means for expanding the probe pulse spatially, and a medium having a two-photon absorption effect in which the optical data pulse train and the probe pulse are made to intersect spatially, The method is characterized in that output light from the medium, which has received a nonlinear transmittance depending on the time overlap of the time-division multiplexed individual data pulse and probe pulse, is detected as spatial pattern information.

More specifically, the output light having undergone the non-linear transmittance is photoelectrically detected after passing through a spatial mask having a transmittance change.

According to the present invention, since information of an OTDM signal is converted into spatial pattern information by pure optical means and each time-division optical signal information can be reproduced in parallel, the information reproduction of an ultra-high-speed OTDM signal can be performed. Becomes possible. Furthermore, in the present invention, since the reproduction efficiency is insensitive to the change in the polarization of the optical signal, reproduction with a good S / N ratio can be achieved with a simple configuration.

[0012]

FIG. 1 is a schematic diagram showing a main part of an embodiment of an optical pulse signal reproducing method according to the present invention. An OTDM signal light beam 2 composed of a time-series optical pulse signal A (A1, A2, ...) and a time-series optical pulse signal B (B1, B2, ...)
1 is spread in a sheet beam shape in one direction (z direction in FIG. 1) by the beam expander 22. At this time, the wave front of the pulse of the time-series signal (A1, B1, A2, B2,...) Spreads parallel to the z direction as shown in the figure. After that, the signal light beam 21 passes through the beam splitter 23 and is 90 ° with respect to the optical axis of the signal light beam 21.
The light is reflected in the direction opposite to the incident direction by the blazed diffraction grating 24 tilted by a certain angle from the degree. At this time,
The signal beam 21 due to reflection from the blazed diffraction grating 24
Of the light pulse in the z direction is tilted in the x direction. The reason for this is that the blazed diffraction grating 24 is tilted from the optical axis of the signal light beam 21 by a certain angle from 90 degrees. Thereafter, the reflected signal light beam 21 is reflected by the beam splitter 23, and becomes a signal light beam in which the wavefront of the x-direction optical pulse propagating in the z direction is inclined.

On the other hand, the light beam 25 of the probe pulse which is independently prepared and propagates in the z direction is a signal light beam 21 in this example.
It is set so as to be incident at a low height in the y direction so as not to overlap with the beam splitter 23, is spread in the x direction by the beam expander 26, and transmits through the beam splitter 23 with a wave front perpendicular to the propagation direction (z direction). I do. The wave front of the light beam 25 of the probe pulse is such that the signal light beam 2 crosses the signal light beam 21 having the inclined wave front temporally as shown in FIG.
The timing with time 1 is adjusted.

Both the signal light beam 21 and the probe pulse light beam 25 are condensed on the nonlinear optical medium 28 in the y direction by the cylindrical lens 27. Here, in the non-linear optical medium 28, with respect to the x direction, it becomes a converging line in the same direction (x direction) as the sheet beam. This nonlinear optical medium 28
As a material having a two-flux absorption effect with respect to the wavelength used, for example, a semiconductor such as GaAs or CdTe, or an insulator such as barium titanate, SBN or KNSBN can be used. When transmitting through the nonlinear optical medium 28, the signal light beam 21 and the light beam 25 of the probe pulse have a light absorption coefficient which increases in proportion to their light intensity due to a two-photon absorption effect, and the transmittance decreases. Therefore, in a portion where the signal light beam 21 and the probe pulse light beam 25 temporally overlap each other, the signal light beam 21 and the probe pulse light beam 25
Will be further reduced.

It is important to note that the nonlinear optical medium 28
Can be selected so that the two-photon absorption effect depends only on the light intensity, and there is no coherence between the two pulses (in the case of light from two independent light sources as in the embodiment of the present invention). ) Means that an equivalent two-photon absorption effect can be obtained without actually depending on the relative polarization state of the two pulses. Therefore, in the method of the present invention, there is almost no influence of the change in the polarization of the signal light beam 21 as compared with the conventional example shown in FIG.

FIG. 2 shows that the transmittance from the nonlinear optical medium (semiconductor GaAs in this example) is reduced by the two-photon absorption effect due to the temporal overlap (relative time difference) of the two light pulses as described above. This is an experimental example showing that this is the case. Here, a pulse having a full width at half maximum of 28.4 picoseconds is 1.
As a function of the relative time difference between two light pulses (pump light and probe light) from a 064 micron YAG laser, the change in the transmittance of the probe light from the absence of the pump light is expressed as a normalized transmittance. Is shown. From this figure, it can be seen that the transmittance of the probe light decreases most when the pump light and the probe light coincide with each other in time.

Returning to FIG. 1, in a portion where the signal light beam 21 and the probe pulse light beam 25 temporally overlap, the transmittance of the signal light beam 21 and the probe pulse light beam 25 is further reduced due to the two-photon absorption effect. . Signal beam 21
Is detected by using the spatial pattern of the light beam 25 of the probe pulse after transmission through the imaging lens system 29 using the detector 30, and the A1, B1, A2, and B of the signal light beam 21 are detected.
It is possible to detect the presence or absence of a pulse in one data set including the pulse trains of 2,. In this case, in the spatial pattern of the light beam 25 of the probe pulse, the portion where the amount of transmitted light is reduced corresponds to the presence of the bit pulse in the signal light beam 21.

In this embodiment, a signal sequence A and a signal sequence B
This can be distinguished from the above-mentioned case by separating and reproducing by electrical processing after photoelectric conversion by the detector 30, or by placing a spatial mask on the detector 30 whose transmittance is encoded corresponding to the timing of a desired signal sequence. Can be separated and regenerated. In the case of the latter method, in order to reproduce the signal train A and the signal train B independently, the reproducing system shown in FIG. 1 may be configured independently, or the signal light beam transmitted through the beam splitter 23 may be used. A pair of the signal pulse 21 and the probe pulse 25 may be divided into two, and a spatial pattern corresponding to the signal sequence A and the signal sequence B may be prepared and detected independently.

As described above, by converting the time-series signal information of the light pulse into the spatial pattern information, the data set interval can be made smaller than the response speed of the detector and the relaxation time of the two-photon absorption effect (usually on the order of nanoseconds). As long as the response speed of the detector is sufficiently slower than the light pulse width, the light pulse train can be detected in parallel. For this purpose, one data set composed of a time-division multiplexed optical pulse signal sequence may be, for example, nanoseconds or less. As an example, consider the case of regenerating an optical pulse train consisting of 500 bits per data set every other nanosecond, and the signal detection speed according to the method of the present invention is a very high detection data rate of 250 Gbit / s. As described above, according to the present invention, the probe light beam 25 and the signal light beam 2 are used in order to utilize the two-photon absorption effect in the nonlinear optical medium 28.
There is no requirement for the condition of the relative polarization direction of 1 and there is an advantage that the disadvantage of the conventional system shown in FIG. 3 can be avoided.

[0020]

As described above, according to the present invention,
A signal beam consisting of an optical pulse train of a time-division multiplexed superluminous beam is superimposed on a probe pulse beam in a medium having a two-photon absorption effect, and a response is detected to detect a change in the spatial pattern of the transmitted probe pulse beam. Even when a photodetector having a low speed is used, information of a desired optical pulse signal sequence can be reproduced from an optical pulse sequence that has been subjected to time division multiplex modulation on the order of picoseconds. Further, in this case, since the two-photon absorption effect is used, a change in the polarization of the signal light beam does not cause a problem, so that there is an advantage that there is no need to control the polarization of the signal light beam or transmit the signal light beam through the polarization maintaining optical fiber. .

[Brief description of the drawings]

FIG. 1 is a schematic view of a main part showing the principle of an optical pulse signal reproducing method according to the present invention.

FIG. 2 is a diagram showing an experimental example of a change in transmittance due to a two-photon absorption effect of a probe light pulse when two light pulses of a pump and a probe are used.

FIG. 3 is a schematic diagram of a main part for describing a conventional method of detecting a time division multiplexed modulated optical signal.

[Explanation of symbols]

 Reference Signs List 21 OTDM signal light beam 22, 26 Beam expander 23 Beam splitter 24 Blazed diffraction grating 25 Probe pulse 27 Cylindrical lens 28 Nonlinear optical medium 29 Imaging lens system 30 Detector

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-5-72047 (JP, A) Kazuhiro Ema, 27p-R-13 Measurement of ultrafast waveform using third-order nonlinearity, The Physical Society of Japan Proceedings of the 46th Annual Meeting, Japan, The Physical Society of Japan, Second Volume, p. 358 (58) Fields investigated (Int.Cl. ⁷ , DB name) H04B 10/00-10/28 H04J 14/00-14/08 G02F 1/00

Claims (4)

(57) [Claims] 1. An optical pulse signal regeneration method for regenerating each signal data pulse train in real time from a time-division multiplexed optical data pulse train, wherein said optical data pulse train is spatially spread and its wave front is inclined, and Injected into a medium having a two-photon absorption effect so as to spatially intersect with the expanded probe pulse, and received a non-linear transmittance depending on the time overlap of each time-multiplexed individual data pulse and probe pulse. An optical pulse signal reproducing method characterized by detecting output light as information of a spatial pattern. Wherein the optical pulse signal reproduction method according to claim 1, wherein the detecting the nonlinear transmittance photoelectric output light received. 3. An optical pulse signal regenerating apparatus for regenerating each signal data pulse train in real time from a time-division multiplexed optical data pulse train, means for expanding the optical data pulse train spatially and inclining the wavefront, Means for expanding a probe pulse, a medium having a two-photon absorption effect in which the optical data pulse train and the probe pulse are made to intersect spatially, and the time-division multiplexing of individual data pulses and probe pulses An optical pulse signal reproducing device, wherein output light from the medium having received a nonlinear transmittance depending on the degree of overlap is detected as information on a spatial pattern. 4. An output light receiving said nonlinear transmittance is photoelectrically detected.
4. The optical pulse signal according to claim 3,
Raw equipment .