JPH0279034A - Optical reception system for optical heterodyne detection - Google Patents

Abstract

PURPOSE:To enable good-sensitivity optical reception with simple constitution by switching the plane of polarization of local oscillation light to orthogonal directions at a predetermined period and performing heterodyne detection, and then adding the heterodyne-detected orthogonal components and receiving signal light. CONSTITUTION:An orthogonal polarized wave changeover switch 18 switches the plane of polarization of the local oscillation light L to orthogonal planes Fx and Fy of polarization for the heterodyne detection. Then the base band signal Sd obtained when the plane of polarization of the local oscillation light Lf is in an X-axial direction and the base band signal Sd obtained when the plane of polarization of the local oscillation light Lf is in a Y-axial direction are added at least once by the storage addition part 15a of a signal processor 15. Therefore, back scattered light Le can be received invariably in a state wherein efficiency is 1. Consequently, the circuit constitution is simple and the high-accuracy optical reception is enabled.

Description

Detailed Description of the Invention (1) Purpose of the Invention [Field of Industrial Application] Akira Motoiso relates to an optical reception method of optical heterodyne detection in which signal light and local light oscillated from a local oscillator are detected using a beat signal. It is something.

[Prior art] Optical heterodyne detection uses a beat signal of signal light and local light oscillated from a local oscillator to detect the signal, so it is an excellent method for receiving weak signals and is used in various N1 measuring instruments. ing.

FIG. 5 shows the configuration of an optical fiber backscattered light measuring device using a conventional heterodyne detection method.

In the figure, 1 is a laser light source drive circuit, 2 is a laser light source, 3 is an optical splitter, 4 is an acousto-optic optical switch, 5 is an optical splitter, 6 is an optical fiber to be measured, 7 is an optical multiplexer, 8 1 is an acousto-optic optical switch driving circuit that drives the acousto-optic optical switch 4; 9 is a reference signal generator; 10 is a light receiver;
is a beat signal amplifier, 12 is a band pass filter, 13
is a detector, 14 is a low-pass filter, and 15 is an accumulation adder 15a and a signal processor! ! a signal processor having a first part 15b;
16 is a display, A is a light receiver 10, and a beat signal amplifier 11
．． Bandpass filter 12. This is an optical receiving section composed of a detector 13 and a low-pass filter 14.

Conventionally, in this type of four-meter measuring device, a laser light source 2 is controlled by a laser light source driving circuit 1 to output continuous oscillation so as to obtain an output light La having good coherency, that is, a narrow spectral linewidth and a stable oscillation frequency rlfo. , Laser light it!
The output light 1a of 2 is split into two by an optical splitter 3, one of the lights is guided to an acousto-optic optical switch 4, and an ultrasonic drive signal sb intensity-modulated with a pulse waveform is sent to an acousto-optic optical switch 4. The signal light 1-b guided to the acousto-optic optical switch 4 in addition to the acousto-optic element is shifted in frequency by the ultrasonic frequency fa due to light diffraction and optical frequency shift due to the Shosuga optical effect. It is converted into an optical pulse signal 1c.

This frequency f. +fa optical pulse signal and C to optical splitter 5
is guided to the optical fiber 6 to be measured as a light pulse ld.

When the optical pulse ld propagates in the optical fiber 6 to be measured, backscattered light 1e that returns to the input end side of the Ray scattering light generated at each portion in the longitudinal direction of the optical fiber 6 to be measured.
is guided to the light receiver 10 side via the optical splitter 5. This optical frequency f. 10f, the backscattered light IIe, and the optical frequency f divided into the other by the optical splitter 3. The local light L" is multiplexed by the optical multiplexer 7 and heterodyne detected by the optical receiver 10. As a result, the output of the optical receiver 10 has optical frequencies f.+f,
A beat signal @SC with a difference frequency f between the backscattered light Le and the local light L with an optical frequency @fo is obtained, and this electrical signal is the intensity of the backscattered light 1e returning from the optical fiber 6 under test. Intensity modulation is performed using a distributed waveform.Therefore, this beat signal SC is filtered by a beat signal amplifier 11, passed through a bandpass filter 12 with a center frequency of fa, and square-law detection etc. are performed by a detector 13. The baseband signal S is the intensity distribution waveform of the backscattered light 1e along the longitudinal direction of the optical fiber 6 to be measured via the low-pass filter 14.
(f @ can be obtained.

Since the backscattered light Le of the Ley scattered light generated in the optical fiber 6 to be measured is extremely weak light, the intensity distribution waveform of the backscattered light 1-e obtained at the output of the low-pass filter 14 has a lot of noise. Contains. Therefore, in order to improve the ratio of signal power to noise power, that is, the S/N, the signal processor 15 performs A/D conversion in the accumulation and addition section 15a, and then performs accumulation and addition to improve the S/N. The backscattered light, which has been subjected to signal processing in the processing section 15b, is output as an e waveform to the display 16. At this time, the timing of the incidence of the optical pulse ld into the optical fiber 6 to be measured and the timing of signal processing of the received backscattered light 1e must be synchronized, and in this case, the synchronization signal @Sa is the reference It is provided by a signal generator 9.

Compared to the direct detection method, such an isolated heterodyne detection method can achieve detection sensitivity at the shot noise limit, greatly improving light receiving sensitivity, and is expected to lead to the realization of a backscattered light measuring device with a wide dynamic range. be done.

[Problems to be Solved by the Invention] However, in the conventional configuration shown in FIG. 5, the polarization plane of the returning backscattered light 1-e matches the polarization plane of the local light r, which enters the light receiver 10. However, the plane of polarization of the backscattered light Le of the Ley scattered light generated within the optical fiber 6 to be measured is not constant along the longitudinal direction of the optical fiber 6 to be measured, but is polarized at any angle. It has a wave front. Therefore, when the backscattered light 1-e having a polarization plane orthogonal to the polarization plane of the local light Lf is incident, the signal component of the backscattered light 1e obtained by heterodyne detection by the light receiver 10 becomes zero.

As described above, with conventional measuring devices, the advantages of heterodyne detection are not fully utilized, and the measurement result of the distribution waveform of the backscattered light 1e of the Leigh scattered light along the longitudinal direction of the optical fiber 6 to be measured is Since it largely depends on the plane of polarization of the backscattered light Le, there is a drawback that the loss distribution along the longitudinal direction and the break point cannot be detected with high precision.

On the other hand, polarization diversity of optical fiber communication - Optical reception system % formula % Figure 6 shows the conventional polarization diper FIG. 2 is a block diagram of a city-optical reception system. Here, 1o is the signal light propagated through the optical fiber transmission line FO, 21 is the local oscillator laser that emits the local light and oscillates 1, and 22 is the signal light, which is an optical multiplexer for combining 0 and the local light L1. 23 is a polarization separation element for separating the combined signal light LO and local light 1 into orthogonal polarization components;
24a and 24b convert the beat components of the light obtained by combining orthogonal signal light 1o and local light L1 into electrical signals 81. S
2, a light receiving section 25 for converting and detecting the received electrical signal 8 which is the output of the i section 24a, 24b;
A synthesis/demodulation circuit that performs diversity synthesis and demodulation for 1.82 and outputs a data output signal S3, F1~
F4 is an optical fiber.

Therefore, in the polarization diversity optical reception method shown in Fig. 6, since the polarization planes of the local light and the received light are matched and detected, stable light reception is possible even when the polarization state of the received light changes. It has the advantage of being able to obtain high sensitivity and detecting waves in real time. However, the optical receiver 24a
and 24b are the light receivers 10. and 24b shown in FIG. Beat signal amplifier 11. It is necessary to have the same configuration as the optical receiver A consisting of a bandpass filter 12° detector 13 and a low-pass filter 14, which has the disadvantage that \A@ becomes extremely complicated. Therefore, even if the polarization diversity optical reception method shown in FIG. 6 is simply applied to the measuring device shown in FIG. 5, there is a drawback that the device becomes large and the visual measuring device becomes expensive.

The present invention has been made in order to solve the above-mentioned drawbacks of the prior art, and it is an object of the present invention to provide an optical reception system of optical heterodyne detection that has a simple device configuration and is capable of highly accurate optical reception.

(2) Structure of the Invention [Means for Solving the Problems] The optical reception method of optical heterodyne detection of the present invention is an optical reception method of optical heterodyne detection in which the optical heterodyne detection is detected by a beat signal of signal light and local light oscillated from a local oscillator. In the reception method, a laser light source is configured in which a signal light oscillator that oscillates the signal light and the local oscillator are configured in one oscillator, and an incident light emitted from the laser light source is transmitted as diffracted light whose traveling direction changes. It has an acousto-optical element that optically switches to non-diffracted light whose direction does not change, and a control means that controls switching of the acousto-optical element at a predetermined cycle, and the control means controls the switching of the acousto-optical element at a predetermined period. The diffracted light and the non-diffracted light when off are configured to be switchable and controllable, respectively, to be used as signal light and local light. On the other hand, a polarization plane changeover switch disposed in the optical path of the local light beam switches the polarization plane of the local light beam to an orthogonal direction at a predetermined period to perform heterodyne detection, and then the optical heterodyne detection is performed. The signal light is received by accumulating and adding the respective orthogonal components at least once.

In short, the first feature of the present invention is that after heterodyne detection is performed by switching the polarization plane of the local light to cross H in the orthogonal direction at a predetermined period, each of the heterodyne detected orthogonal components is The purpose is to receive the signal light by accumulating and adding the signal light.

The second feature of the present invention is that the optical splitter 3 used in the conventional device is omitted by using the re-output light when the Meikyo optical switch 4 turns on and off, and the insertion loss of the optical splitter 3 is reduced. This prevents this problem and simplifies the device configuration.

[Implementation Example 1] A first embodiment of the present invention will be described in detail using the drawings.

FIG. 1 is a configuration diagram of an optical fiber backscattered light measuring device using an optical reception method of optical heterodyne detection according to the present invention, in which 17 is an optical fiber delay line, 1B is an orthogonal polarization switch, and 19 is an orthogonal polarization switch. This is a base switch drive circuit, and the other components are the same as the conventional components shown in FIG.

However, in the conventional configuration, the acousto-optic optical switch 4 uses only the optical signal when it is on, but in the present invention, the optical splitter 3 is omitted by using the re-output light when it is on and off. It becomes.

[Creation 1] First, a first embodiment of the present invention will be described.

To perform heterodyne detection as described above, the maximum detection sensitivity is obtained when the polarization plane of the backscattered light Le, which is the optical signal to be detected, and the polarization plane of the local light emitted f match, and these polarization planes If they are orthogonal to each other, heterodyne detection cannot be performed. The electric field strength ES of the backscattered light 1e to be detected now is constant, the polarization plane (hereinafter referred to as the polarization plane Fs J) is at an arbitrary point on the circumference of the broken line, and the electric field of the linearly polarized local light Lf The intensity E is constant and the plane of polarization is as shown in Figure 2 (a).
And as shown in (b), the intensity of the baseband signal sd obtained at the output of the detector 13 of the optical receiver A at a certain angle on the y-axis or y-axis is shown in FIGS. 3(a) and (b). It comes to show. First, in the case of Fig. 2 (a), the magnitude of the beat signal @sc obtained by heterodyne detection with the optical receiver 1o is:
Proportional to ES -EX -CO3θ. However, ES is the electric field strength of the backscattered light Le, EX is the electric field strength of the local light 1-f having a polarization plane in the X-axis direction (hereinafter referred to as "polarization plane Fx J"), and θ is the polarization plane of the backscattered light Le. It is the angle between FS and the polarization plane Fx of the local light. In this case, the baseband signal MSd obtained at the output of the detector 13 that performs square-law detection etc.
The strength of is proportional to cos 2θ, and the relationship between θ and the standardized baseband signal Sd strength is as shown in FIG. 3(a). Similarly, in the case of FIG. 2(b), the magnitude of the beat signal SC obtained by heterodyne detection is ES −EV−
It is proportional to sin θ. However, Ey is the plane of polarization in the Y-axis direction (
Hereinafter, the local light 1-f having the "polarization plane Fy J"
is the electric field strength, and θ is the angle between the polarization plane Fs of the backscattered light Le and the y-axis. Therefore, the strength of the baseband signal Sd is proportional to sin 2θ, and θ and baseband signal S
The relationship between the intensity of d is as shown in FIG. 3(b). In this way, in conventional optical receiving systems that do not have polarization compensation, the angle θ
The efficiency of heterodyne detection ranges between 0 and 1 depending on .

Therefore, when the linearly polarized local light whose plane of polarization is fixed in a certain direction is received by r, the backscattered light 1e is scattered and returned from each part of the optical fiber 6 to be measured along the direction of the force.
Since the polarization plane Fs has various angles depending on each part, it is not always possible to receive it with high efficiency.

FIGS. 2(C) and 3(C) show the polarization plane F of the local light beam f when using the optical receiving system according to the present invention shown in FIG.
It is a diagram showing the relationship between x, F y and the angle formed by the polarization plane Fs of backscattered light e, and the relationship between θ and baseband signal @Sd intensity. In the optical reception system of the present invention, as shown in FIG. 1, local light is emitted using an orthogonal polarization changeover switch 18, and the plane of polarization is sequentially switched between Fx and Fy so that the planes of polarization are orthogonal.

The switching timing may be changed every time the optical pulses L and d are input into the optical fiber 6 to be measured and a waveform of the backscattered light 1-e is obtained. The intensity distribution waveform of the backscattered light 1-e that is incident on the backscattered light 1-e that is incident on the backscattered light 1-e is maintained at the polarization plane 1"x in the X-axis direction for a certain number of times, and then the intensity distribution of the backscattered light Le that is made the same number of times is This operation may be repeated while maintaining the polarization plane Fy in the Y-axis direction while obtaining the waveform.

The switching signal of this switch is generated by the orthogonal polarization changeover switch drive circuit 19 (q) based on the output synchronization signal Sa of the reference signal generator 9. The baseband signal Sd after detection of the incoming backscattered light 1e is the local light Lf, although it differs depending on whether the plane of polarization is along the X-axis or the Y-axis.
Baseband signal 3 obtained when the plane of polarization is in the X-axis direction
d and the baseband signal Sd obtained when the polarization plane of the local light Lf is in the Y-axis direction.
By adding at least once in Figure 3 (C
), the backscattered light 1-e can always be received with an efficiency of 1 regardless of the value of θ. Further, the orthogonal polarization changeover switch 18 can be easily realized by using an optical element such as niobium oxide (LiNb03) having the Kerr effect. As described above, according to the present invention, the optical fiber to be measured 6
The extremely weak backscattered light 1e that is scattered along the longitudinal direction and returns can be always efficiently heterodyne detected regardless of the direction of its polarization plane Fs, making it possible to perform high-performance backscattered light measurement H.
It is possible to realize the

It should be noted that the local light emitted during the heterodyne detection used here
The polarization compensation method using orthogonal polarization switching is not limited to optical fiber transgressively scattered light acquisition, but is also used in applications such as spectrometers in the field of remote sensing, laser radar devices used for observing atmospheric molecules, etc. The present invention can be widely applied to devices that perform accumulation and addition processing on band signals Sd.

[Example 2] Next, an acousto-optic optical switch 4 which is a second example of the present invention
We will explain the case using .

FIG. 4 is an explanatory diagram of the operation of the acousto-optic optical switch 4 used in the method of the present invention. In the figure, 4a- is an acoustic optical element, la is the output light from the Rade light source 2, LC is the diffracted signal light, Lf is the non-diffracted local light, and sb is the ultrasonic driving signal.

[Function 2] In the method of the present invention, when an ultrasonic driving signal 4b with a carrier frequency f8 is applied to an 8-tone optical element 4a having a high acousto-optic effect such as tellurium dioxide (TeO2), photoelasticity is generated within the optical element. The effect causes density and density of the refractive index, and the optical frequency f
. The incident output light a is output as a diffracted signal light LC.

At this time, the optical frequency undergoes a Doppler shift and the ultrasonic frequency f is added to it, resulting in fo+f8.

On the other hand, when the ultrasonic wave is not applied, that is, when it is off, the incident output light a passes through the optical element as it is as the non-diffracted local light 1r. The frequency of the light at this time is the incident light f. is the same as Therefore, an acousto-optic optical switch 4 as shown in FIG.
By utilizing the characteristics of the optical splitter 3, which was a problem in the past,
It is possible to obtain the signal light LC and the local light 1-f that have a constant frequency necessary for hetero gain detection without the branching loss of optical power caused by the l! of the configuration. ! At the same time as lignification, the detection effect of the backscattered light Le is enhanced.

In FIG. 1, since the local light Lf does not exist while the acousto-optic optical switch 4 is on, the switch 4 turns off and the local light Lf is generated until it reaches the optical multiplexer 7.
Backscattered light generated in the optical fiber 6 to be measured 1. An optical fiber delay line 17 is placed between the acousto-optic optical switch 4 and the optical splitter 5 to provide an optical pulse width, that is, a propagation time corresponding to the time during which the switch 4 is on, so that the signal e does not return.

(3) Effects of the invention Thus, the present invention performs heterodyne detection by switching the polarization plane of local light to orthogonal directions at a predetermined period, and then accumulates and adds the heterodyne-detected orthogonal components. By receiving the signal light, it is possible to realize an optical reception system with a simple configuration using only one system of the optical receiver A and with good reception sensitivity.

In addition, by using the re-output light when the blue optical switch 4 turns on and off, the optical splitter 3 is omitted, preventing insertion loss of the optical splitter 3 and simplifying the device configuration. It becomes possible to do so.

Therefore, the present invention can be applied to a measuring device capable of accumulating and adding baseband signals and an optical receiving system such as optical communication, and has great effects.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of the optical fiber backscattered light measurement method according to the present invention, and FIGS. The relationship between the angle formed by light emission and the baseband signal intensity is shown in Figure 4. Figure 4 is an operational diagram of the acousto-optic optical switch used in the present invention, and Figure 5 is a diagram of the optical fiber backscattered light measurement method using conventional heterodyne detection. 6 is a block diagram of a conventional polarization diversity optical reception system. 1... Laser light source drive circuit 2... Laser light source 3... Optical splitter 4...
Acousto-optic optical switch 4a... Acousto-optic element 5... Optical splitter 6...
Optical fiber to be measured 7... Optical multiplexer 8... Acousto-optic optical switch drive circuit 9... Reference signal generator 10
... Photoreceiver 11 ... Beat signal amplifier 12 ... Band pass filter 13 ... Detector 14 ... Low pass filter 15 ... Signal processor 15a ... Accumulation adder 1
5b...Signal processing unit 16...Display unit 17...
Optical fiber delay line 18... Orthogonal polarization changeover switch 19... Orthogonal polarization changeover switch drive circuit 21... Local oscillator laser 22... Optical multiplexer 23... Polarization separation elements 24a, 24b.・・Optical receiving section 25 ・・Synthesizing/demodulating circuit la ・・Output light Lb ・・Signal light C ・・
・Light pulse signal 1d...Light pulse le...Backscattered light 1-f...Local light Sa...Synchronization signal
sb...Ultrasonic drive signal Sc-...Beat signal Sd...Baseband signal Fig. 2 Fig. 3

Claims (1)

[Claims] 1. In an optical reception method of optical heterodyne detection in which signal light and local light emitted from a local oscillator are detected by a beat signal, the polarization plane switching switch arranged in the optical path of the local light , after performing heterodyne detection by alternately switching the polarization plane of the local light in orthogonal directions at a predetermined period, cumulatively adding the orthogonal components detected by the optical heterodyne at least once, and receiving the signal light. Optical heterodyne detection optical reception method 2 is characterized in that the optical heterodyne detection optical reception method detects from a beat signal of a signal light and local light oscillated from a local oscillator, wherein the signal light oscillates the signal light. a laser light source configured with a single oscillator and the local oscillator, and an acoustic type that optically switches the output light from the laser light source into diffracted light whose traveling direction changes and non-diffracted light whose traveling direction does not change. an optical element; and a control means for switching and controlling the acoustic optical element at a predetermined period, and converts the diffracted light 1 when the acoustic optical element is on and the non-diffracted light when the acoustic optical element is off into signal lights, respectively. and an optical reception system for optical heterodyne detection, characterized in that it is configured to perform switching control so as to be used for local light.