JPS61105529A - Polarization controller - Google Patents

Abstract

PURPOSE:To reduce influences of noise upon a control circuit to extend the dynamic range and improve the precision and the speed of control by synthesizing a local oscillated light with monitor lights separated from a signal light and subjecting the synthetic light to heterodyne or homodyne detection and controlling the polarization state of the signal light on a basis of the polarization state obtained by this heterodyne or homodyne detection. CONSTITUTION:A local oscillated light F is synthesized with monitor lights D and E, and the beat output is detected to detect the polarization state of a signal light A. that is, the polarization state is detected by heterodyne detection. Since an electric signal including information related to light intensities detected by photodetectors 6, 7, 11, and 12 is processed in an intermediate frequency band, a drift generated in case of DC amplification is not generated in an amplifying circuit of this electric signal. Since the detected optical signal is an AC, there are no influences of dark currents upon photodetectors 6, 7, 11, and 12, and influences of thermal noise amplifiers 61, 71, 111, and 112 are reduced considerably, and the dynamic range is extended.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a device for controlling the plane of polarization of signal light. In particular, the present invention relates to a device that controls the plane of polarization of signal light at low reception power and at high speed.

[Conventional technology]

In order to control the polarization state of the signal light, it is necessary to detect the polarization state of the signal light. In order to detect the polarization state of the controlled light, a conventional polarization control device branches a part of the controlled light and directly detects each polarized wave output of the controlled light. Furthermore, the output from this detection is fed back to a polarization state control element provided in the optical path of the controlled light to perform polarization control of the controlled light.

FIG. 6 shows a block diagram of a conventional polarization control device.

Incoming signal light A is separated into output light B and monitor light C by beam splitter 3 after passing through A wavelength plate 1 and wave plate 2 . The monitor light C is further separated into two monitor lights E by a beam splitter 4.

The monitor light is separated into orthogonal polarization components by the Wollaston prism 5, and the difference between the two orthogonal polarization components is detected by the photodetector 6.7 and the differential amplifier 8. The monitor light E passes through the two-wavelength plate 9, and the orthogonal polarization components are separated by the Wollaston prism IO. 12 and a differential amplifier 13, the difference between the two orthogonal polarization components is detected. Here, the principal axes of the two Wollaston prisms 5 and 10 are arranged at an angle of 45'' with respect to each other, and the principal axes of the Wollaston prism 5 and the A wavelength plate 9 are arranged to coincide with each other.

The output of the differential amplifier 8 is input to the wave plate rotation control circuit 14. The wave plate rotation control circuit 14 rotates the axial direction of the wave plate 2 so that the output of the differential amplifier 8 becomes "0", that is, the difference between the orthogonal polarization components of the monitor light becomes "0". Control rotation. As a result, the principal axis direction of the signal light A is
It can be made to coincide with the main axis direction of the Wollaston prism 10.

The output of the differential amplifier 13 is input to the wave plate rotation control circuit 15. The wave plate rotation control circuit 15 controls the rotation of the x wave plate l in the axial direction so that the output of the differential amplifier 13 becomes "0". Thereby, the signal light A is controlled to be linearly polarized light.

Such a conventional polarization control device can control the polarization state of the signal light A to be linearly polarized light that coincides with the main axis direction of the Wollaston prism IO. Furthermore, by determining the rotational direction of the wave plate 2 depending on the positive/negative output of the differential amplifier 8, the maximum output direction of the controlled light A can be made to coincide with any one of the two main axes of the Wollaston prism 10. be able to.

[Problem that the invention seeks to solve]

In such a conventional polarization control device, since the differential amplifiers 8 and 13 are DC amplifiers, there is a drawback that a drift occurs in the output thereof. That is, there is a drawback that an error occurs in the detected polarization state and the polarization control accuracy deteriorates. Additionally, optical signals in the 1 to 24 bands are mainly used in optical transmission, and the germanium photodetectors used to detect optical signals in these wavelength bands have a large dark current, which limits their dynamic range and causes polarization. It is not possible to improve the detection accuracy of wave conditions,
Therefore, in order to improve the signal-to-noise ratio due to the dark current of the photodetector, which had the disadvantage of limiting polarization control accuracy,
Although it is possible to attach an integrator with a sufficiently large time constant to the outputs of the differential amplifiers 8 and 13, the provision of such an integrator has the disadvantage that the response time of polarization control becomes longer.

SUMMARY OF THE INVENTION An object of the present invention is to solve the above-mentioned drawbacks and provide a polarization control device that can control the polarization state of low-power ratio control light with a high-speed response time.

[Means for solving problems]

The polarization control circuit of the present invention includes means for branching monitor light from signal light, means for separating this monitor light into two orthogonal polarization components, and a means for splitting the polarization components separated by this means into optical and electrical components. means for converting, means for detecting the intensity difference between the orthogonal polarization components based on the intensity difference of the electrical signal converted by the means, and controlling the polarization state of the signal light based on the output of the detecting means. A polarization control device comprising: means for generating locally oscillated light; and means for combining the monitor light branched by the branching means and the locally oscillated light, and detecting the intensity difference. The means for detecting a beat signal of the separated polarized wave component and the locally oscillated light is characterized in that the means for detecting a beat signal of the separated polarized wave component and the locally oscillated light.

[Effect]

The polarization state detection device of the present invention combines local oscillation light with monitor light separated from signal light, performs heterogain detection or homodyne detection on this, and detects the signal light based on the polarization state obtained thereby. Control polarization state.

〔Example〕

FIG. 1 is a block diagram of a polarization control device according to a first embodiment of the present invention.

After passing through the l-wave plate 1 and l-wave plate 2, the signal light A is separated into output light B and monitor light C by the beam splinter 3. The monitor light C is multiplexed by the beam splitter 4 with the local oscillation light F output from the local oscillation optical oscillator 16, and is separated into two monitor lights E.

The monitor light is separated into orthogonal polarization components by the Wollaston prism 5, detected by a photodetector 6.7, and converted into an electrical signal. The monitor light E passes through the l wavelength plate 9,
Orthogonal polarization components are separated by the Wollaston prism 10,
Photodetector 1). 12 into an electrical signal. Here, the principal axes of the two Wollaston prisms 5.10 are arranged at an angle of 45'' to each other, and the Wollaston prisms 5.
The main axis direction of the 1-wave plate 9 and the main axis direction of the l-wave plate 9 are arranged to coincide with each other.

The photodetector 6 is connected to one input terminal of the differential amplifier 8 via an amplifier 61, a rectifier circuit 62, and a low-pass filter 63. The photodetector 7 includes an amplifier 71 and a rectifier circuit 72.
and is connected to the other input terminal of the differential amplifier 8 via the low-pass filter 73. The output of the differential amplifier 8 is input to the wave plate rotation control circuit 14. Wave plate rotation control circuit 1
4 controls the rotation of the axial direction of the l-wave plate 2.

The photodetector 1) is connected to one input terminal of a differential amplifier 13 via an amplifier 1) 1, a rectifier circuit 1) 2 and a low-pass filter 1) 3. The photodetector 12 is an amplifier 121
, via the rectifier circuit 122 and the low-pass filter 123,
It is connected to the other input terminal of the differential amplifier 13. The output of the differential amplifier 13 is input to the wave plate rotation control circuit 15. The wave plate rotation control circuit 15 controls the rotation of the l-wave plate 1 in the axial direction.

Control of the polarization state of signal light A is the same as in the conventional example. That is, the wave plate rotation control circuit 14 mechanically controls the rotation of the l-wave plate 2 in the axial direction so that the output of the differential amplifier 8 becomes "0". As a result, the principal axis direction of the signal light A is
The main axis direction of the Wollaston prism 10 is made to coincide with the main axis direction. The wave plate rotation control circuit 15 controls the output of the differential amplifier 13 to be "0".
” The axial direction of the l-wave plate 1 is mechanically controlled to rotate so that Thereby, the signal light A is controlled to be linearly polarized light.

The feature of the present invention is that the local oscillation light F is combined with the monitor light E, and the polarization state of the signal light A is detected by detecting its beat output. That is, the polarization state is detected by heterodyne detection.

In this example, the electrical signals containing information related to the light intensity detected by the photodetectors 6.7.1) and 12 are processed in an intermediate frequency band. Therefore, this electric signal amplification circuit does not generate a drift that occurs in the case of DC amplification. In addition, since the detected optical signal is AC, there is no influence of dark current of photodetector 6.7.1) and 12 (and amplifier 61.71.1), and the influence of thermal noise of 1 and 121 is extremely small. This allows the dynamic range to be expanded.

This example will be explained quantitatively.

The signal light of one of the two polarization components separated by the Wollaston prism 5 or 10 will be explained. The electric field, power, and angular frequency of this signal light are represented by Es, P, and ω, respectively, and the electric field, power, and angular frequency of the locally oscillated light are represented by Et, Pt, and ω, respectively. Furthermore, the phase difference between the electric fields of the signal light and the local oscillation light is δ, and the time is t. At this time, the electric field E of the signal light and the electric field EL of the local oscillation light are Es oc7pvcosω, t ---
-----=(1) ELoe7p, cos(ω,,t
+δ) −−−−−−(2). The current flowing through the photodetector 6 due to square law detection is Ioc (Ps+ PL+ 2-JPsPtcosCΔ
ωt+δ)−−−−−−・(3) It is expressed as follows. Here, Δω represents 1ω, −ω, and 1.

Therefore, the power PI% of the beat output of the signal light and the local oscillation light is P s ocP s P t
−・・−14) By measuring the power P of the signal light,
, an output proportional to .

Therefore, the output from the photodetector 6.7 is transferred to the amplifier 61.
．． 71, rectifier circuit 62.72 and low pass filter 63.7
Amplification, rectification, and low-pass filtering are performed by the photodetectors 1), .3, and the outputs are compared by the differential amplifier 8. 12, the output from amplifier 1) 1.121, rectifier circuit 1) 2.12
2 and low-pass filter 1) By amplifying, rectifying, and low-pass filtering using 3.123 and comparing the outputs with differential amplifier 13, the polarization state of signal light A is detected as in the conventional example. Can be done.

The signal-to-noise ratio for polarization state detection in this embodiment is mainly determined by the ratio between the beat output power PI expressed by equation (4) and shot noise due to locally oscillated light. PIN optimally designed according to the frequency band of signal light A
Signal-to-noise ratio (
S/N) o and the signal-to-noise ratio (S/N) c during hetero gain detection according to the embodiment of the present invention are (S/N)o = ((ηe / h ν)"Ps"
)÷((2e(L +Ps ηe/hν)+iN”
) Δν], −−−−−−・(5) (S/N)c = ((2Ps PL (ηe/h
ν)”)÷((2e(in Ps W e/h V +
Ps ηe/h v )+ 1, lR)Δν)
-----------(6) Here, η: quantum efficiency of the photodetector, e: electron charge, h blank constant, C: frequency of input light, P: power of signal light, PL: power of local oscillation light, i4: power of photodetector dark current, iH: equivalent input noise of the amplifier, Δshi: frequency band. Equivalent input noise 1N' is expressed as iH"J-τ・Δν using a constant C determined by the amplifier. When using the current PIN type photodiode and field effect transistor amplifier, 1a=1n
A.

C=lO-'A''/Hz, and when a semiconductor laser is used as the local oscillation optical oscillator 16, a local oscillation optical output power PLO of about OdBm can be obtained.With a transmission speed of about 100 Mb/s, In coherent transmission, the power of controlled light A is -60 dB
m. Therefore, it is necessary to reduce the optical power of the monitor light for polarization control to about -80 dBm or less.

FIG. 2 shows the signal-to-noise ratio versus frequency band for direct detection and heterodyne detection.

If a signal-to-noise ratio of 20 dB is required to accurately detect the polarization state, as shown in Fig. 2, the band is 2 kHz in conventional direct detection, but in this example, the band is 350 kHz.
A kHz band is obtained. Therefore, by performing heterogain detection, polarization can be controlled about 100 times faster.

Figure 3 shows the experimental results of the tracking characteristics of this example, where the signal power was -70.3 dBm and the local oscillation optical power was -17.3 dBm.
The polarization principal axis deviation θ1 and ellipticity χ before control and the polarization principal axis deviation θ after control at 6 dB-.

and ellipticity χ. and

This experiment was performed using a psi; type laser diode.
Separate the output light of 5- into two parts, use one as a local oscillation light, and use the other as a 150M acousto-optic frequency filter.
A pseudo-hetero gain transmission device was used in which the signal light was shifted in Hz. The signal light is input to the polarization control device through a +w single mode optical fiber and a variable attenuator.

FIG. 4 shows the main parts of the polarization control device according to the second embodiment of the present invention.
7 is a configuration diagram. Although FIG. 3 shows a configuration for performing homodyne detection and a configuration for detecting the polarization state of monitor light, the configuration for detecting the polarization state of monitor light E is also equivalent.

The local oscillation light F output from the local oscillation optical oscillator 16 passes through the phase modulator 17, and is multiplexed into two monitor lights E by the beam splinter 4.

The monitor light is separated into orthogonal polarization components by the Wollaston prism 5, and photoelectrically converted by the photodetector 6.7.

The output of the photodetector 6 is input to a peak detector 64 via an amplifier 61, a rectifier circuit 62, and a low-pass filter 63. The output of the photodetector 7 is transmitted through an amplifier 71, a rectifier circuit 72,
and is input to a peak detector 74 via a low-pass filter 73. The output of the peak detector 64.74 is sent to the differential amplifier 8.
is input. Differential amplifier 8 and wave plate rotation control circuit 14
A sample holding circuit 81 is inserted between the two.

The phase modulator 17 is composed of LiNb0:+ or the like, and modulates the locally oscillated light F at a speed sufficiently lower than the transmission speed. Peak detectors 64 and 74 detect the peak value of the beat output within the modulation period. These two outputs are compared by a differential amplifier 8 to detect the polarization state.

In this embodiment, polarization control is performed at high speed by homodyne detection.

FIG. 5 is a block diagram of main parts of a polarization control device according to a third embodiment of the present invention.

In coherent transmission, the received signal light A' and local oscillation light F are combined by a beam splitter 3 on the optical signal receiving side. At this time, a part of the signal light A' is transmitted to the beam splinter 3.
A part of the local oscillation light F is reflected by the beam splitter 3.
Transmit. In this embodiment, the light generated by this is used as the monitor light C. This monitor light C is separated into two monitor lights E by a beam splinter 4, each of which enters a Wollaston prism 5.10, and the polarization state is detected in the same manner as in the above embodiment. In this embodiment, since unnecessary components generated by receiving optical signals of coherent transmission can be used as monitor light, the polarization state can be detected and controlled without substantially causing loss in the transmission path.

〔Effect of the invention〕

As explained above, according to the polarization control device of the present invention,
The influence of noise on the control circuit can be reduced, its dynamic range can be increased, and control accuracy and speed can be improved. As a result, the polarization state of the optical signal,
In particular, the polarization state within a single mode optical fiber can be controlled more configurably and at higher speed than with a direct detection method. Further, when used in a coherent transmission receiving device, loss due to monitor light can be substantially eliminated.

By installing the device of the present invention on the receiving side of a single-mode optical fiber for coherent transmission and using it for polarization matching with local oscillation light, stable transmission can be achieved without using a polarization-maintaining optical fiber as a transmission path. This has the effect of realizing coherent transmission.

[Brief explanation of the drawing]

FIG. 1 is a book configuration diagram of a polarization control device according to a first embodiment of the present invention. FIG. 2 is a diagram showing the signal-to-noise ratio for frequency bands. FIG. 3 is a diagram showing tracking characteristics. FIG. 4 is a block diagram of main parts of a polarization control device according to a second embodiment of the present invention. FIG. 5 is a block diagram of main parts of a polarization control device according to a third embodiment of the present invention. FIG. 6 is a block diagram of a conventional polarization control device. l...x wavelength plate, 2...2 wavelength plate, 3.4...beam splitter, 5.1o...Wollaston prism, 6.7...photodetector, 8...difference dynamic amplifier, 9...
・A wavelength plate, 1). 12... Photodetector, 13... Differential amplifier, 14.15... Wave plate rotation control circuit, 16
... Local oscillation optical oscillator, 17 ... Phase modulator, 61
．． 71.1) 1.121...Amplifier, 62.72.1
)2.122... Rectifier circuit, 63.73.1)3.1
23...Low pass filter, 64.74...Peak detector, 81...Sample holding circuit.

Claims (3)

[Claims] (1) means for branching monitor light from signal light; means for separating this monitor light into two orthogonal polarization components; means for photoelectrically converting the polarization components separated by this means; means for detecting the intensity difference between the orthogonal polarization components based on the intensity difference of the electrical signal converted by the means; and means for controlling the polarization state of the signal light based on the output of the detecting means. The polarization control device includes means for generating locally oscillated light, and means for multiplexing the monitor light branched by the branching means and the locally oscillated light, and the means for detecting the intensity difference includes the above-mentioned separating means. A polarization control device comprising: means for detecting a beat signal of the polarized wave component and the locally oscillated light. (2) Polarization control according to claim (1), wherein the means for branching the monitor light and the means for multiplexing the monitor light and the local oscillation light are constituted by one common beam splitter. Device. (3) The polarization according to claim (1), wherein the means for generating locally oscillated light includes means for modulating the phase of the locally oscillated light, and the means for detecting includes homodyne detection means for detecting a peak value. Wave control device.