JP5278966B2 - Optical signal processing method and optical signal processing apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To acquire a phase discrimination signal, whose quality is equal to or higher than that acquired by using two ISBT optical gates, by using a single ISBT optical gate. <P>SOLUTION: The optical signal processing method and the optical signal processing device are characterized in that an optical signal 1 is divided into a first divided optical signal 1 and a second divided optical signal 1 so as to be made incident on the ISBT optical gate from mutually opposed directions, and also, the optical signal 2 is absorptively made incident on the ISBT optical gate, and an output difference from the ISBT optical gate between the first divided optical signal 1 and the second divided optical signal 1 is acquired as the phase discrimination signal. <P>COPYRIGHT: (C)2011,JPO&amp;INPIT

Description

  The present invention relates to an optical signal processing method and an optical signal processing device for obtaining a phase discrimination signal.

  Although the transmission rate required for optical communication is increasing year by year due to the rapid development of the Internet, the operation speed of electronic devices is reaching its limit. Therefore, research on optical devices using the nonlinear optical effect as an element capable of operating at a higher speed than electronic devices is underway. In particular, all-optical gate switches that control the opening and closing of light (signal light) with light (control light) are attracting attention from the viewpoint of low power consumption, and apply intersubband transition (ISBT) in semiconductor quantum wells. The optical gate switch can perform a terabit-class high-speed operation at three orders of magnitude faster than that using a normal semiconductor bandgap transition.

In a communication apparatus, phase synchronization (PLL) that matches the frequency of the clock signal on the reception side with the clock frequency on the transmission side is an essential technique. The PLL is a technique that detects the phase difference between two signals and applies feedback control to the oscillator so that the phase difference or the difference frequency is constant. For an electrical signal, a phase difference can be detected using a phase comparator represented by a double balanced mixer (DBM) (see FIG. 3).
In order to apply a PLL to two optical signals, an element for detecting a phase difference between the optical signals is required, but an ISBT optical gate can be used to realize such an optical phase comparator.
So far, a method has been proposed in which a phase difference between an electric signal (frequency = f ₁ ) and an optical signal (repetition frequency = f ₂ ) is detected (see FIG. 4) and phase synchronization is performed on them. In this case, a relationship satisfying the following expression can be realized.
f ₂ = m × f ₁ + Δf (m = integer, Δf: offset frequency)
When an ISBT optical gate is used, a phase discrimination signal can be detected from two optical signals exceeding 160 GHz. (See Figure 5)

The ISBT element transmits linearly polarized light (TE light) perpendicular to the quantum well surface, but absorbs parallel linearly polarized light (TM light). Initially, the ISBT optical gate used saturable absorption, but it has been found that higher efficiency can be obtained by using cross-phase modulation (XPM) by subsequent studies (Non-patent Document 1).
XPM is a phenomenon in which the phase of TE light passing therethrough changes because the refractive index changes when ISBT absorbs TM light.
In particular, when TM light is a short pulse, the spectrum of TE light spreads through the ISBT due to the high-speed phase modulation effect.
In order to use an ISBT element as an optical gate, it is necessary to convert a phase change generated in TE light into an intensity change. So far, a spectrum is extracted by a method using an Mach-Zehnder (MZ) interferometer and an optical filter. A method has been devised.

A configuration example of an ISBT optical gate using XPM is shown in FIG. 6 (Non-Patent Document 2). Here, the ISBT element is arranged and used on one optical path of the MZ interferometer.
The signal light is input from port A in a state having TE polarization. The control light is input from the port B in a state having TM polarization, and is absorbed when passing through the ISBT. The signal light is divided by an incident side beam splitter (BS), and after traveling through two optical paths, is combined at the output side BS. Here, when there is no control light, the M-Z interferometer is adjusted so that two light waves completely cancel each other at port D and signal light is output only from port C. However, when the control light is incident, a phase change occurs in the signal light transmitted through the ISBT, so that the interference condition at the time of multiplexing is broken, and a part of the signal light is output from the port D. Therefore, the signal light is output to the port D only when the control light is ON.

The optical gate outputs an optical signal only when signal light and control light are incident simultaneously. Therefore, when two optical signals having the same repetition frequency are incident on the optical gate as signal light and control light, respectively, the output light intensity changes according to the phase difference. If this is photoelectrically converted, phase discrimination will occur. A signal can be obtained.
However, since the phase discrimination signals always have the same polarity, that is, the DC components overlap (see FIG. 7), they cannot be used as they are for feedback control such as PLL. In order to obtain a phase discrimination signal whose polarity changes according to the phase, it is necessary to remove this DC component. The simplest method is to monitor the signal light intensity and subtract a proportional voltage from the output voltage of the light gate (see FIG. 8). However, the waveform of the phase discrimination signal obtained by this method is not necessarily positive / negative symmetric.
Even if the phase difference is constant, the output voltage changes depending on the intensity of the control light, the operating point of the MZ interferometer, and the like, so that a practical phase comparator is difficult to realize.

On the other hand, when two optical gates are used, the problem relating to the asymmetry of the output signal can be solved (see FIG. 9). The signal light is divided into two and made incident on the two optical gates with a phase difference of 180 °, and the output is photoelectrically converted. Since these two electric signals have the same waveform and are out of phase by 180 °, if the difference is taken, a positive and negative symmetrical electric signal centered on 0 volt can be obtained.
However, this method uses two optical gates, which complicates the system and consumes twice as much optical power. Further, fluctuations in the output voltage due to fluctuations in the operating point of each MZ interferometer are not canceled but rather increase.

H. Tsuchida et al., Opt. Lett., 32, 751 (2007). R. Akimoto et al., ECOC2008, P.2.03 (2008).

  The present invention has been made in view of the above-mentioned problems of the prior art, and the object of the present invention is to use a single ISBT optical gate, which is equivalent to or higher than the case where two ISBT optical gates are used. It is to obtain a quality phase discrimination signal.

The above problem is solved by the following means.
(1) The optical signal 1 is divided into two parts, a first divided optical signal 1 and a second divided optical signal 1, which are incident on the ISBT optical gate from opposite directions, and the optical signal 2 is incident on the ISBT optical gate and absorbed. An optical signal processing method characterized in that a difference between outputs from the ISBT optical gate of the first divided optical signal 1 and the second divided optical signal 1 is obtained as a phase discrimination signal.
(2) The means for dividing the TE-polarized optical signal 1 into two parts, the first divided optical signal 1 and the second divided optical signal 1, and the first divided optical signal 1 and the second divided optical signal incident from opposite directions. An optical interferometer having an ISBT element that receives 1 and a TM-polarized optical signal 2 and that converts a phase change generated in the first divided optical signal 1 and the second divided optical signal 1 passing through the ISBT element into an intensity change. , First photoelectric conversion means for receiving the output of the first divided optical signal 1 from the optical interferometer, and second photoelectric conversion means for receiving the output of the second divided optical signal 1 from the optical interferometer; And a means for obtaining a phase discrimination signal from a difference between output signals from the first and second photoelectric conversion means.
(3) Means for dividing the TE-polarized optical signal 1 into the first divided optical signal 1 and the second divided optical signal 1, and the first divided optical signal 1 and the second divided optical signal incident from opposite directions to each other 1 and an TMBT-polarized optical signal 2, a first filter that extracts a spectral component added by XPM from the first divided optical signal 1 transmitted through the ISBT element, and a second divided optical signal transmitted through the ISBT element A second filter that cuts out the spectral component added by XPM from 1, a first photoelectric conversion means that receives the output of the first split optical signal 1 cut out by the first filter, and a second filter Optical signal processing including second photoelectric conversion means for receiving the output of the second divided optical signal 1 and means for obtaining a phase discrimination signal from the difference between the output signals from the first and second photoelectric conversion means Location.

  According to the optical signal processing method and the optical signal processing apparatus of the present invention, since a single ISBT optical gate is used, the configuration is simpler than that of a system using two ISBT optical gates, and power consumption is also reduced. Can also be reduced. Further, it is possible to obtain a signal output with excellent long-term stability as compared with a method using two ISBT optical gates.

First configuration example for generating a phase discrimination signal using a single ISBT Second configuration example for generating a phase discrimination signal using a single ISBT The figure explaining the phase comparator of the conventional electric signal The figure explaining the phase comparator of an electric signal and an optical signal The figure explaining the phase comparator of an optical signal The figure explaining the structural example of the ISBT optical gate switch using cross phase modulation (XPM) Diagram for explaining basic operation of ISBT optical gate The figure explaining the simplest optical mixer using one ISBT optical gate The figure explaining the optical mixer using two ISBT optical gates

  The optical signal processing method and optical signal processing apparatus of the present invention will be described in detail below with reference to the drawings. However, the present invention is not construed as being limited thereto, and does not depart from the scope of the present invention. Insofar, various changes, modifications and improvements can be made based on the knowledge of those skilled in the art.

  FIG. 1 shows a first configuration example for generating a phase discrimination signal using a single ISBT. The two optical signals whose phases are compared are called optical signal 1 and optical signal 2, respectively. Optical signal 1 and optical signal 2 are used as signal light and control light, respectively, for the ISBT optical gate. The ISBT optical gate is composed of an MZ interferometer and one ISBT element placed on the optical path.

The optical signal 1 is TE-polarized, and is split into two by the first duplexer 5, and they are incident on the MZ interferometer from opposite directions. The MZ interferometer is composed of two demultiplexers 8 and 9 and two optical paths (optical path 1 and optical path 2), and is connected to two optical input / output units 6 and 7. The first optical input / output device 6 has a role of making the first split optical signal 1 incident on the MZ interferometer and outputting the second split optical signal 1 from the interferometer, and the second optical input / output device 7. Has a role of making the second split optical signal 1 enter the MZ interferometer and outputting the first split optical signal 1 from the interferometer.
The M-Z interferometer has two optical path lengths adjusted so that no signal light is output when no control light is present.

The optical signal 2 is TM-polarized and is superimposed on the first split optical signal 1 using the multiplexer 10. The optical signal 2 is absorbed by the ISBT. At this time, the phase of the optical signal 1 passing through the optical path 2 is shifted. Due to this phase shift, when the pulse of the optical signal 2 passes through the ISBT, the first and second divided optical signals 1 are output from the MZ interferometer. The incident timing of the first divided optical signal 1 and the second divided optical signal 1 is appropriately shifted so that these pulses do not enter the ISBT at the same time.
The first and second split optical signals 1 output from the MZ interferometer are converted into electrical signals using the first and second photoelectric converters 1 and 3, respectively. Since these two electric signals have the same waveform and are shifted in phase, when the difference is taken by the differential amplifier 4, a positive / negative symmetrical phase discrimination signal centered on 0 volts is obtained.

Here, a multiplexer 10 is used as a means for superimposing the optical signal 2 on the first divided optical signal 1, and two input / output devices 6 are used as means for coupling the first and second divided optical signals 1 to the MZ interferometer. 7 is used, but these parts can be omitted if the duplexers 8 and 9 constituting the optical interferometer also serve as the role.
The MZ interferometer is used to convert the phase change that occurs in the optical signal 1 into an intensity change, but it can be replaced with another type of optical interferometer (such as a Michelson interferometer or Sagnac interferometer). Also good.

  FIG. 2 shows a second configuration example for generating a phase discrimination signal using a single ISBT. The two optical signals whose phases are compared are called optical signal 1 and optical signal 2, respectively. The optical signal 1 and the optical signal 2 are used as signal light and control light, respectively.

The optical signal 1 is TE-polarized, and is split into two (first and second split optical signals 1) by the duplexer 17, and they are incident on the ISBT element 11 from opposite directions. Here, the timings of the first and second divided optical signals 1 are appropriately shifted so that those pulses do not enter the ISBT at the same time.
Two optical input / output devices are connected to the ISBT element. The first optical input / output unit 18 has a role of causing the first split optical signal 1 to enter the ISBT element and outputting the second split optical signal 1 from the ISBT element, and the second optical input / output unit 19 has the second split light signal. The signal 1 is incident on the ISBT element, and the first split optical signal 1 is output from the ISBT element.

  The optical signal 2 is TM-polarized and is superimposed on the first split optical signal 1 by a multiplexer. The optical signal 2 is absorbed by the ISBT. At this time, the first and second split optical signals 1 passing through the ISBT are subjected to cross-phase modulation (XPM) to broaden their spectrum.

  The first split optical signal 1 transmitted through the ISBT is cut out of the spectral component added by XPM by the first band pass filter 12, and the signal is converted into an electric signal by the first photoelectric converter 13. The second split optical signal 1 that has passed through the ISBT is cut out of the spectral component added by XPM by the second bandpass filter 14, and the signal is converted into an electric signal by the second photoelectric converter 15. Since these two electric signals have the same waveform and are shifted by the phase, when the difference is taken by the differential amplifier 16, a positive / negative symmetrical phase discrimination signal centered on 0 volts is obtained.

  The present invention uses a single ISBT optical gate to obtain a positive and negative symmetric electrical signal centered at 0 volts. For this purpose, the optical signal is divided into two and made incident on the ISBT optical gate from opposite directions, and the difference between their outputs is taken. In this method, the configuration is simple and the power consumption is reduced as compared with the case where two ISBT optical gates are used. Further, since the two optical signals pass through the same ISBT optical gate, even if the operating environment of the optical gate changes, the effect is canceled out, so that a stable phase discrimination signal can be obtained.

  INDUSTRIAL APPLICABILITY The present invention can be used for a PLL that synchronizes the repetition frequency of a pulse light source on the receiving side with a data transmission rate in optical communication or the like.

DESCRIPTION OF SYMBOLS 1 1st photoelectric converter 2 ISBT element 3 2nd photoelectric converter 4 Differential amplifier 5 1st splitter 6 1st input / output device 7 2nd input / output device 8 2nd splitter 9 Third demultiplexer 10 Multiplexer 11 ISBT element 12 First band pass filter 13 First photoelectric converter 14 Second band pass filter 15 Second photoelectric converter 16 Differential amplifier 17 Demultiplexer 18 1st input / output device 19 2nd input / output device 20 multiplexer

Claims (3)

  The optical signal 1 is divided into two parts, a first divided optical signal 1 and a second divided optical signal 1, and incident on the ISBT optical gate from opposite directions, and the optical signal 2 is incident on the ISBT optical gate and absorbed. An optical signal processing method characterized in that a difference between outputs of the split optical signal 1 and the second split optical signal 1 from the ISBT optical gate is obtained as a phase discrimination signal.   A means for splitting the TE-polarized optical signal 1 into two parts, a first split optical signal 1 and a second split optical signal 1, and a first split optical signal 1 and a second split optical signal 1 that are incident in opposite directions, and TM. An optical interferometer having an ISBT element for receiving the polarized optical signal 2 and converting a phase change generated in the first divided optical signal 1 and the second divided optical signal 1 passing through the ISBT element into an intensity change; A first photoelectric conversion means for receiving the output of the first split optical signal 1 from the interferometer; a second photoelectric conversion means for receiving the output of the second split optical signal 1 from the optical interferometer; And a means for obtaining a phase discrimination signal from a difference between output signals from the second photoelectric conversion means.   A means for splitting the TE-polarized optical signal 1 into two parts, a first split optical signal 1 and a second split optical signal 1, and a first split optical signal 1 and a second split optical signal 1 that are incident in opposite directions, and TM. An ISBT element that receives the polarized optical signal 2, a first filter that cuts out a spectral component added by XPM from the first split optical signal 1 that has passed through the ISBT element, and an XPM from the second split optical signal 1 that has passed through the ISBT element. A second filter that cuts out the spectral component added by the first filter, a first photoelectric conversion means that receives the output of the first split optical signal 1 cut out by the first filter, and a second filter that is cut out by the second filter. An optical signal processing apparatus comprising: a second photoelectric conversion unit that receives an output of the divided optical signal 1; and a unit that obtains a phase discrimination signal from a difference between output signals from the first and second photoelectric conversion units.

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