JP3206993B2 - Bidirectional optical space transmission equipment - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a two-way optical space transmission apparatus for performing two-way information transmission by optical radio to a remote place.

[0002]

2. Description of the Related Art In general, when performing bidirectional information transmission by optical radio to a remote place, respective optical transmitters are correctly oriented toward each other's optical receiver, and the arrival direction of an incident light beam. Must be detected accurately. In addition, during the information transmission, unless the tracking operation for maintaining the pointing and the detection correctly is continued, the optical axis may be affected by various external effects such as wind, solar radiation, and artificial influence on the device. Gap,
Communication may be interrupted.

For this reason, a conventional two-way optical space transmission apparatus is configured as shown in FIG. 8, for example.
The light transmitting unit 1 combines a transmission signal (main signal) S1 and an auxiliary signal (hereinafter, referred to as a pilot signal) Ps for detecting an angle error, and then converts the combined signal into a light beam. The light is deflected by the mirror angle drive mechanism 4 via the system 2 and the beam splitter 3, and transmitted via the transmission / reception lens 5.

At the time of reception, the receiving light is transmitted and received by a transmission / reception lens 5, a mirror angle driving mechanism 4, a beam splitter 3,
The light is guided to a beam splitter 6 and split into two, and the reflected light is detected by a light receiving unit 8 via a light receiving unit optical system 7.
Further, an error signal of the transmitted light is detected by an angle error detection unit 10 via an angle error detection unit optical system 9, and the mirror drive control unit 11 drives the mirror angle drive mechanism unit 4 based on the error signal, and the transmission optical system Detects an error signal associated with the degree of deviation between the axis of light and the direction of arrival of the received light, drives the control servo mechanism with this signal, matches the axis of the transmission optical system with the direction of arrival, and emits a light beam in that direction. I do. The tracking function is realized by performing this operation on each of the devices facing each other.

Here, as a two-dimensional angle error detector, photodetectors having the same characteristics are arranged in each of the four quadrants as shown in FIG. 9, and the position of the spot of the received light is determined by each photodetector. In general, it is determined from the sum and difference of outputs.
Control is performed so that the outputs of the four photodetectors are all equal.

[0006] For example, when the light emitting element is a laser diode, receive beams two-dimensional Gaussian distribution G next portrait corresponding to the radiation pattern of the laser diode as shown in FIG. 10, the strength is at the center of the 1 / e ² A line connecting a certain position inscribes the beam boundary. When the intensity distribution of the reception beam is uniform, the position of the center of the reception beam is (X, Y) with respect to the center of the reception optical system as shown in FIG. Are equal, the axis of the receiving optical system coincides with the arrival direction of the receiving beam.

[0007]

However, if the intensity distribution of the received beam is not uniform, the axis of the optical system will remain in the original direction of arrival even if the signals from the four photodetectors are all equal. When the amount exceeds the allowable value at the time of system design, the target tracking accuracy cannot be achieved. is there.

[0008] The effect of the above-mentioned non-uniformity of the received beam intensity distribution can be reduced by greatly increasing the beam diameter of the light beam at the receiving point, but the spatial power density of the light beam reaching the receiving side is reduced. However, there is a problem that good communication cannot be performed. Therefore, the light beam diameter is generally determined in consideration of transmission / reception power, transmission distance, and communication quality.

An object of the present invention is to solve the above-mentioned problems,
It is an object of the present invention to provide a two-way optical space transmission device which realizes a highly accurate tracking function in a light emitting element having a non-uniform radiation intensity.

[0010]

According to the present invention, there is provided a two-way optical space transmission apparatus comprising: a main signal having a frequency bandwidth B ₁ (Hz); a main signal having a frequency bandwidth B ₂ (H);
z) is a device that performs bidirectional information transmission using a light flux modulated based on a signal superimposed with a pilot signal for detecting an angle error, and is a device that is disposed opposite to the device. A transmission optical system for transmitting a transmission light beam toward the transmission / reception lens, a transmission / reception lens having an aperture δ for taking a part of a light beam transmitted from a device arranged opposite to the reception light beam into the device, and a reception signal received by the transmission / reception lens. A light splitter that splits a light beam into a first light beam and a second light beam, a first light detector that receives the first light beam and detects the main signal, and receives the second light beam A second photodetector that detects an angle error signal indicating a deviation between the arrival direction of the received light beam and the optical axis of the transmission optical system based on the pilot signal;
Control means for correcting a deviation between the direction of arrival of the received light beam and the optical axis of the transmission optical system based on the angle error signal detected by the second photodetector, wherein the transmission / reception lens is arranged in front of the transmission / reception lens. In a two-way optical space transmission device that functions as a diaphragm that takes in a light beam having a light beam diameter D into the device as a received light beam having a light beam diameter δ, the light splitter includes:
A total reflection mirror portion having an aperture d that partially reflects a received light beam and guides the received light beam to the second photodetector; and a first light beam that transmits the received light beam excluding a portion reflected by the total reflection mirror portion. A partial reflection mirror having a transmission light use area to be incident on the photodetector, wherein the diameter d of the total reflection mirror portion is an upper limit of 1/15 times the light beam diameter D at the front surface of the transmission / reception lens, and δ. × (B ₂ / B ₁ ) It is characterized in that it is set to be in a range between the lower limit indicated by ^1/2 . .

[0011]

The two-way optical space transmission apparatus having the above-described configuration is:
The diameter d of the total reflection mirror portion is set to an upper limit value of 1/15 times the luminous flux diameter D in front of the transmission / reception lens, and δ × (B ₂ /
B ₁ ) The angular error of the optical axis of the transmission beam is reduced by being set to be in a range between the lower limit indicated by _１ ^／ .

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described in detail with reference to the embodiments shown in FIGS. FIG. 1 is a configuration diagram of the first embodiment. A transmission signal (main signal) S1 and a pilot signal Ps are input to a light transmitting unit 21. On the optical path of the light transmitting section 21, a light transmitting section optical system 22, a beam splitter 23, and a mirror angle driving mechanism section 24 are sequentially arranged, and a transmission / reception lens 25 is provided in the reflection direction of the mirror angle driving mechanism section 24. I have. In the reflection direction of the beam splitter 23, a partial reflection mirror 26, a main signal light receiving unit optical system 27, and a main signal light receiving unit (first photodetector) 28 are sequentially arranged. Has an angle error detector optical system 29,
An angle error detector (second photodetector) 30 is arranged. Further, the output of the angle error detection section 30 is connected to the mirror angle drive mechanism section 24 via the mirror drive control section 31.

As shown in FIGS. 2 and 3, the partial reflection mirror 26 includes a transmitted light use area 26a and a total reflection mirror section 26.
b, and a total reflection film is deposited on the total reflection mirror portion 26b. The transmitted light use area 26a and the total reflection mirror section 26b are both elliptical so as to be circular when the partial reflection mirror 26 is viewed from the main signal receiving section optical system 27.

The pilot signal Ps is for detecting an angle error, and a narrow band signal such as a sine wave outside the transmission signal band is used as the pilot signal Ps. As a result, it is possible to reduce an optical signal output required to obtain a required S / N ratio of the angle error signal.

The transmission signal (main signal) S1 is converted into an optical signal after being superimposed on the pilot signal Ps in the light transmitting section 21, and transmitted through the light transmitting section optical system 22 and the beam splitter 23 to be transmitted to the mirror angle driving mechanism section. The light is deflected by 24, condensed by a transmission / reception lens 25, and transmitted.

At the time of reception, the optical signal from the partner device is reflected by the beam splitter 23 via the transmission / reception lens 25 and the mirror angle drive mechanism 24, and is reflected by the partial reflection mirror 26.
Is split into two light beams. The first light beam (main signal detection light beam) transmitted through the partial reflection mirror 26 is passed through the main signal light receiving portion optical system 27 to the main signal light receiving portion (first detector) 2.
8 and becomes a received signal S2. The second light flux reflected by the partial reflection mirror 26 is passed through an angle error detection unit optical system 29 to an angle error detection unit (second light detector) 3.
An angle error of 0 is detected. This angle error is input to the mirror drive control section 31 and drives the mirror angle drive mechanism section 24 to make the optical axis coincide with the incident direction of the reception signal S2.

FIG. 4 is a detailed explanatory diagram when the reception beam is divided into two using the partial reflection mirror 26, and shows the reception beam B reaching the front surface of the transmission / reception lens 25 and the total reflection mirror portion 26b. FIG. 5 shows the light beam diameter D of the received beam and the aperture of the total reflection mirror portion 26b (that is, the light beam diameter of the second light beam reflected by the total reflection mirror portion 26b) d.
, And the center (X, Y) of the reception beam B
And the relative received power difference with respect to X, that is, the received power of the photodetectors arranged in the four quadrants is defined as P1 to
{(P1 + P4)-(P2 + P3)} / (P1 + P2 + P3 +
It is a graph which shows the relationship of the value of P4). In this case, the relative received power difference becomes largest when Y = 0 for an arbitrary value of X.

Further, a relative received power difference with respect to Y when X is a parameter {(P1 + P2)-(P3 + P4)} / (P1 + P2
+ P3 + P4) also has the same tendency, but since the received beam has a greater intensity distribution gradient in the parallel transverse mode than in the vertical transverse mode, the relative received power difference is larger than the value for X using the aforementioned Y as a parameter. Become smaller.

When the intensity of the reception beam B has a two-dimensional Gaussian distribution, the angle error detector (second photodetector) 30
When the center of the spot of the receiving beam is aligned with the center of the four photodetectors in the above, a relative reception power difference is generated, and the control servo mechanism is driven by an error signal generated by the relative reception power difference, and the value of the error signal Is controlled to be 0.

At this time, the center of the spot of the reception beam B is shifted from the center of the four photodetectors by Δx, and this Δx is the focal length of the angle error detection section optical system 29 and the reception on the photodetectors. If the spot diameter of the beam B is known, it corresponds to the angle error amount between the original arrival direction of the reception beam and the reception optical system, that is, the light transmission direction. This angle error amount becomes the tracking deviation of the tracking system as it is, other factors that cause the tracking deviation, such as the optical axis alignment error between the four photodetectors and the light emitting elements, and the imbalance in the sensitivity of the four photodetectors. together,
The tracking deviation generated in the tracking control system is greatly related to the tracking accuracy, and the allowable value of the tracking deviation is calculated at the time of designing the system. Therefore, it is necessary to determine the diameter d of the total reflection mirror portion 26b with respect to the light beam diameter D of the reception beam so that the angle error amount is equal to or less than the allowable value.

FIG. 6 is a graph showing the above-mentioned angle error amount with respect to the X axis.
, The ratio D / d is used as a parameter, the focal length of the angle error detection unit optical system 29 is set to 200 mm, and the spot diameter of the reception beam incident on the photodetector is set to 150 μm which is considered to be a general value in this system. Considering the realizable values of the optical axis alignment error and the sensitivity imbalance, in a two-way optical space transmission system that requires an accuracy tracking function, the amount of angular error due to the non-uniform intensity distribution of the received beam is considered. The tolerance is less than 20 μrad,
That is, the ratio D / d needs to be 15 or more. For example,
Assuming that the beam diameter D of the received beam in consideration of transmission / reception power, transmission distance, and communication quality is 1 m, the upper limit of the diameter d of the total reflection mirror portion 26b when the inside of the device is viewed from the antenna in the optical axis direction is 66. 7 mm.

Further, the lower limit value of the aperture d of the total reflection mirror section 26b is set to a value corresponding to the bandwidth of the main signal and the pilot signal when the photodetector, load resistance, and noise figure of the amplifier are equal to each other. Determined by the ratio. Here, the transmitting / receiving lens 25 has a light flux diameter D on the front surface of the transmitting / receiving lens 25.
This serves as a diaphragm when the light beam arriving at step (1) is taken into the apparatus as a received light beam. That is, the transmission / reception lens 25
Let δ be the aperture of, the light beam diameter of the received light beam also becomes δ. The light beam received by the transmission / reception lens 25 is transmitted to the total reflection mirror unit 26.
It is assumed that the light passes through the transmitted light use area 26a excluding b.
If the bandwidth of the main signal is B ₁ (Hz) and the bandwidth of the pilot signal is B ₂ (Hz), the optical signal output required to make the S / N value of the pilot signal equal to the main signal is the main signal. B ₂ / B ₁ times the signal.

Accordingly, the ratio of the diameter d of the total reflection mirror portion 26b to the diameter δ of the transmission / reception lens 25 is (B ₂ /
B ₁ ) It is necessary to be ^以上 or more. For example, the _{B 1} 100 MHz, the _{B 2} 10 KHz, if the diameter δ of the receiving lens 25 and 10 cm, diameter d of the total reflection mirror portion 26b when the in the apparatus as viewed in the optical axis direction from the transmitting and receiving lenses 25
Is 1 mm.

The main signal receiving section (first photodetector) 2
If the photodetector and the circuit configuration of the angle error detector 8 and the angle error detection section (second photodetector) 30 change, the lower limit changes accordingly. However, if these configurations are determined, the lower limit can be easily calculated. . For example, an avalanche photodiode with a multiplication factor of 100 and an angle error detector 3
0 of the photodetector is a PIN photodiode, the noise figure of the amplifier in each reception circuit of the main signal and the pilot signal Ps is set to be the same, B ₁ is 100 MHz, the diameter of the receiving lens 25 B ₂ is at 10 KHz [delta] Is 10 cm, the lower limit of the diameter d of the total reflection mirror 26 b is 10 mm.
Becomes Thus, the diameter d of the total reflection mirror portion 26b when the inside of the apparatus is viewed in the optical axis direction from the transmission / reception lens 25.
Can be determined.

The received light captured by the transmission / reception lens 25 is split into two by the partial reflection mirror 26, and the aperture d of the total reflection mirror 26b guided to the angle error detection unit optical system 29.
Although the method of determining the upper limit value and the lower limit value has been described, the angle error detection unit optical system 29 may be provided separately from the transmission / reception lens 25 as shown in FIG. That is, the lens aperture of the angle error detection unit optical system 29 is set to the aforementioned total reflection mirror unit 26b.
The same effect can be obtained by making the diameter d the same. In this case, it is difficult to align the optical axes of the transmission / reception lens 25 and the angle error detection unit optical system 29.
Since all of the received light captured by the transmission / reception lens 25 is guided to the main signal detection unit, the output of the received light signal can be used more effectively than in the embodiment in FIG.

[0026]

As described above, in the two-way optical space transmission apparatus according to the present invention, the upper limit of the aperture of the optical system for guiding the received light to the angle error detector is set at 1/15 of the beam diameter at the receiving point. By setting the lower limit value to be the square root of the pilot signal bandwidth with respect to the main signal bandwidth of the transmission / reception lens, even when using a light emitting element whose intensity distribution of the radiation pattern is not uniform,
A highly accurate tracking function can be realized.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of an embodiment.

FIG. 2 is a sectional view of a partial reflection mirror.

FIG. 3 is a plan view of a partial reflection mirror.

FIG. 4 is an explanatory diagram of a center of a reception beam and a total reflection mirror unit.

FIG. 5 is a graph showing a relative received power difference with respect to X.

FIG. 6 is a graph showing an angle error amount with respect to X;

FIG. 7 is a configuration diagram of another embodiment.

FIG. 8 is a configuration diagram of a conventional example.

FIG. 9 is an explanatory diagram of a conventional two-dimensional angle error detector.

FIG. 10 is an explanatory diagram of a radiation pattern of a laser diode.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 21 Light transmission part 22 Light transmission part optical system 23 Beam splitter 24 Mirror angle drive mechanism part 25 Transmission / reception lens 26 Partial reflection mirror 27 Light reception part optical system 28 Light reception part 29 Angle error detection part optical system 30 Angle error detection part 31 Mirror drive Control unit

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int. Cl. ⁷ , DB name) H04B 10/00-10/28 H04J 14/00-14/08 G02B 7/00

Claims (1)

(57) [Claims] 1. A main signal having a frequency bandwidth B ₁ (Hz)
And has a frequency bandwidth B ₂ (Hz), and detects an angular error.
Signal based on the signal superimposed with the pilot signal for
A device that performs bidirectional information transmission using modulated light beams.
To transmit the transmitted light beam to the device
Outgoing transmission optics and transmitted from opposing devices.
Aperture that takes in a part of the emitted light flux into the device as a received light flux
a transmitting / receiving lens having δ, and
Splits the received light beam into a first light beam and a second light beam
A light splitter for receiving the first light flux and the main signal
And a first photodetector for detecting the second light flux.
The direction of arrival and the direction of arrival of the received light beam based on the pilot signal
Detects angle error signal indicating deviation from optical axis of transmission optical system
A second photodetector, and a second photodetector detected by the second photodetector.
Arrival direction of received light beam and transmission optics based on angle error signal
Control means for correcting deviation from the optical axis of the system.
The receiving lens reaches the front of the transmitting and receiving lens with the luminous flux diameter D.
Into the device as a received light beam having a light beam diameter δ
A two-way optical space transmission device that functions as an aperture
The light splitter partially reflects the received light beam and
A total reflection mirror portion having an aperture d leading to the second photodetector;
And the received light flux excluding the portion reflected by the total reflection mirror section
Use of transmitted light that transmits light and enters the first photodetector
And a partial reflection mirror having an area.
The aperture d of the lens unit is the light in front of the transmitting / receiving lens.
The upper limit of 1/15 times the bundle diameter D, and δ × (B ₂ / B ₁ )
Set to be in the range between the lower limit indicated by ^1/2
Bidirectional optical atmospheric link system, characterized in that the.