JP2847019B2 - Multi-dither adaptive optics - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an adaptive optics apparatus for correcting distortion (wavefront distortion) of an equal phase plane of a light wave of a laser or the like.
In particular, the present invention relates to a multi-dither adaptive optics device whose characteristics do not change over time.

[0002]

2. Description of the Related Art When observing an astronomical object with a telescope, weak light reaching the earth from a star passes through the atmosphere and is captured by the telescope. However, since the density of the atmosphere is uneven, the wavefront of light fluctuates. For this reason, the image is blurred, and high-precision observation cannot be performed. Thus, for example, US Pat.
No. 1103, No. 3975629, No. 397585
No. 3,988,608, No. 4016415, etc., will be used.

This adaptive optics apparatus uses a deformable mirror whose reflection surface can be arbitrarily shaped, and distorts the shape of the reflection surface in accordance with the fluctuation of the wavefront, and reflects the incident light whose wavefront fluctuates on the reflection surface. By doing so, the wavefront is aligned. The control to deform the reflecting surface of the deformable mirror is to take out a part of the reflected light from the reflecting surface, collect it, detect the light intensity with a photodetector, and calculate the fluctuation of the wavefront from this light intensity. Do with.

In order to control the wavefront by distorting the shape of the reflecting surface, it is necessary to know which part of the reflecting surface reflects the light of the wavefront that fluctuates. Therefore, the reflecting surface is divided into a plurality of regions. Then, the reflection surface is oscillated at a different frequency for each region to modulate the phase of the reflected light, and the reflection position on the reflection surface is obtained from the frequency component in the data detected by the photodetector.

[0005]

The above prior art describes the configuration of a multi-dither type adaptive optics device, but the control device is mostly composed of an analog circuit. In particular, the circuit portion that modulates the phase of the reflection surface is an analog circuit that matches the characteristics of the frequency to be phase-modulated. However, there is a problem that the analog circuit has a large circuit scale and the entire device becomes large. In addition, the frequency changes over time, and the frequency for phase modulation deviates from the set frequency. It is necessary to replace the analog circuit whose characteristics have changed in this way, and there is also a problem that maintenance and inspection are troublesome. In addition, since each frequency set in the analog circuit is fixed, and it is necessary to use a different frequency for each area of the reflection surface, an attempt was made to change the frequency for phase modulation to another frequency and to test. If
There is also a problem that the entire analog circuit must be replaced, and the degree of freedom of the device is small.

SUMMARY OF THE INVENTION It is an object of the present invention to solve the problems of performance degradation due to aging of circuit elements in an analog circuit, enlargement of a circuit, and freedom of a device, and to reduce quantization errors.
And to provide a multi-dither adaptive optical device Tsu.

[0007]

SUMMARY OF THE INVENTION An object of the present invention is to provide a deformable mirror having a reflecting surface for reflecting and emitting incident light and a plurality of actuators for changing the shape of the reflecting surface, and a light wave reflected by the reflecting surface. The waveform distortion is detected from the focused focus intensity, and the amount of displacement of each actuator for correcting the waveform distortion is obtained, and each actuator is oscillated at a different frequency at each displacement position of each actuator to perform phase modulation on the reflected light. in the multi-dither method adaptive optical device and a wavefront control device for providing extracts the frequency component of only the phase modulation signal included in the signal of the focal intensity
Automatically keeps the phase modulation signal always above a certain level
An amplifier for adjusting the gain, an analog-to-digital converter for converting a signal output from the amplifier into a digital value, and the same number of actuators as the actuators, each of which receives the digital value and detects a phase modulation signal. Respond to self
A plurality of digital signal processing units for determining the displacement command value of the actuator to be operated and each digital signal processing unit
This is achieved by providing a central control device to be managed in the wavefront control device.

[0008]

The resistance and the capacitor as circuit elements are apt to change over time, and if a synchronous detector required for the control device is manufactured by an analog circuit, these elements will be used in large numbers. On the other hand, if the synchronous detector is constituted by the digital processing unit, performance degradation due to aging hardly occurs due to digital processing. However, simply using a digital processing unit slows down processing and causes a quantization error. Therefore, high-speed processing can be performed by configuring a synchronous detector with a plurality of digital processing units so that processing is executed independently and in parallel. Further, by amplifying the optical signal with an amplifier having an automatic gain adjuster, it is possible to always suppress a quantization error generated when converting the signal into a digital signal to a certain level.

[0009]

An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is a configuration diagram of a wavefront control device according to an embodiment of the present invention, and FIG. 2 is a configuration diagram of a multi-dither type adaptive optics device using the wavefront control device shown in FIG.
First, the multi-dither type adaptive optics device shown in FIG. 2 will be described.

[0010] The compensation optical device, distorted Les - The wavefront 1 0 a
Is incident on the deformable mirror 15 and reflected by the deformable mirror 15, thereby correcting the laser wavefront.
It outputs a laser beam having a uniform laser wavefront. In the multi-dither type adaptive optics apparatus, the reflecting surface of the deformable mirror 15 is divided into a plurality of regions, and the reflected beam is phase-modulated at different frequencies for each small aperture of the laser beam corresponding to each divided region. A part of the reflected light is taken out through the beam splitter 11 while the reflected light from the deformable mirror 15 is being emitted, and the reflected light is condensed by a lens 12. The light intensity at the focal point of the lens 12 is detected by a photodetector. Detect at 14.

The optical signal 1 detected by the photodetector 14 is input to the wavefront controller 8. The wavefront controller 8 controls the optical signal 1
The number of phase modulation signals equal to the number of the small apertures of the beam (the number of the divided areas) included in the above is separated and detected, and the displacement amount of each aperture of the shape variable mirror 15 of each aperture is determined from each phase modulation signal. I do. Further, the wavefront control device 8 generates a signal (a signal of a different frequency for each aperture) for performing phase modulation by the deformable mirror 15, and adds the signal and the displacement amount of each aperture to a command voltage. 5 is output to the driving power source 6 to realize laser wavefront correction and laser phase modulation by the deformable mirror 15.

As the deformable mirror 15, two types of structures as shown in FIG. 3 have been proposed. One is that a mirror 16 is provided at the tip of each actuator 17 such as a piezoelectric element.
Is a divided type deformable mirror 15a, and the other is an integral type deformable mirror 15b in which one thin mirror 16 is bonded to each end of a plurality of actuators 17. The present invention to be described later can be applied to any type of deformable mirror. In multi-dither type adaptive optics,
Since the same number of phase-modulated signals as the number of the actuators 17 are generated, the wavefront controller 8 requires the same number of synchronous detectors as the actuators 17 in the prior art. The greater the number of divisions of the reflecting surface, that is, the number of actuators, the better the compensation efficiency. However, when the wavefront control device 8 is configured by an analog circuit, the same number of synchronous detectors as the actuator 17 must be provided, and the configuration becomes large-scale. It becomes a problem.

Therefore, in an embodiment of the present invention, a synchronous detection circuit is constituted by a digital circuit instead of an analog circuit.
FIG. 1 shows the configuration of the wavefront control device 8 according to this embodiment.
In this embodiment, when an optical signal 1 is input, it is converted into a digital signal 3 by an A / D converter 2. The wavefront controller 8 is provided with the same number of digital signal processing units 4a to 4n as the number of actuators, and the digital signal 3 is input to each of the digital signal processing units 4a to 4n. Each digital signal processing section 4 has a command voltage 5a of an actuator 17 corresponding to itself, as described in detail later.
To 5n are determined, and the command voltages 5a to 5n are
Output to

Since the frequency band of the phase modulation frequency ranges from several kHz to several tens of kHz, all calculations must be performed in a few microseconds or less to detect the phase modulation signal by digital processing. The prior art wavefront controller simply processes with an A / D converter and a digital computer, and does not take into account high-speed calculations. Therefore, it has been difficult to process with a single digital computer. However, in the embodiment of the present invention,
Since the same number of digital signal processing units 4a to 4n are provided, high-speed processing is possible.

In the above-described embodiment, the same number of digital signal processing units 4a to 4n as the number of actuators constitute the wavefront control device 8
However, it is of course possible to reduce the number of digital signal processing units within the range permitted by the allowable value of the operation time. In this case, the command voltage 5 of each of the plurality of actuators 17 is determined by one digital signal processing unit. The digital signal processing unit is realized by a microcomputer or a digital signal processor.

Next, a processing algorithm in the digital signal processing section will be described with reference to FIG. The flowchart shown in FIG. 4 is for a wavefront control device having the same number of digital signal processors as the number of actuators.

First, as initial settings, a bias voltage VBm, a phase modulation frequency fm, a control gain km, etc. of the actuator 17 of the deformable mirror 15 are set. Next, the digital signal 3 is read. Then, a reference phase modulation signal shown in the following equation 1 is generated.

[0018]

Where: ψ: phase modulation amplitude ωm: phase modulation angular frequency (= 2πfm) t: time

Next, in order to detect a phase modulation signal having a frequency fm contained in the digital signal 3, a calculation represented by the following equation 2 is performed.

[0020]

Ym = (signal 3) · sin (ωmt + θ) Synchronous detection is performed.

Here, θ represents the phase difference, and is set to the phase difference between the reference phase modulation signal shown in Equation 1 generated by the digital signal processing unit and the phase modulation signal of the frequency fm included in the digital signal 3. This phase difference is set by measuring in advance. The DC component of the calculation result ym of the equation (2) is the amplitude value of the phase modulation signal, and excess harmonic components of the ym are removed by filtering. The above is the part of the synchronous detection processing for detecting the phase modulation signal in the processing algorithm.

The detected phase modulation signal is proportional to the phase difference between the laser beam of the phase-modulated part and the other part of the laser beam. Therefore, it can be understood that the laser wavefront can be corrected by controlling all the phase modulation signals to be zero. Therefore, the control voltage vm is calculated so that the feedback control is performed so that the phase modulation signal detected by each digital signal processing unit becomes minimum. This control calculation may be realized by a control method such as integral control, proportional control, first-order lag control, or the like, using the phase modulation signal as a deviation signal of the control system.

Next, the command voltage Vm of the drive voltage 7 of the deformable mirror 15 is determined by the following equation (3).

[0024]

## EQU3 ## where VBm is the initially set bias voltage of the actuator 17, and the third term of the equation (3) is the drive voltage for phase modulation. With the processing algorithm described above, the wavefront control of the multi-dither type adaptive optics device becomes possible.

When digital signal processing is performed by the above-described wavefront controller 8 to output the command voltage 5, it is necessary to convert a digital signal into an analog signal. FIG. 5 shows one embodiment. In this embodiment, the phase modulation signal 21 and the control voltage 22 calculated and output by the digital signal processing circuit 4 are individually output. Then, the respective signals 21 and 22 are converted into analog signals by the D / A converters 19a and 19b, and thereafter, only the phase modulation signal 21 is reduced to, for example, 1/20 by the amplifier 20. Make 5.

Assuming that the laser wavelength is λ, the phase modulation signal 2
1 is about λ / 20, whereas the control voltage 22
Is designed to be about several λ. Therefore, the command voltage V expressed by the equation 3 calculated by the digital signal processing circuit 4
When m is converted to an analog signal by one D / A converter, a high-resolution D / A converter is required in consideration of the quantization error of the phase modulation signal 21. On the other hand, according to the configuration of FIG. 5, since the amplitude of the phase modulation signal 21 is adjusted by the amplifier 20, the D / A converter (19a, 19
b).

In the above-described embodiment, the case where the actuator 17 of the deformable mirror 15 is one in the driving direction of the mirror 16 has been described. Next, as shown in FIG.
Referring to FIG. 6, an embodiment using a deformable mirror for driving the phase modulation actuator 17a and the control phase modulation actuator 17b in an overlapping manner will be described.

In this embodiment, the phase modulation signal 21 and the control voltage 22 output from the digital signal processing circuit 4 are converted into analog signals by the respective D / A converters 19a and 19b and output to the drive power supply 6. It is. Instead of lowering the amplitude of the phase modulation signal 21 using an amplifier as in the embodiment of FIG. 5, the length of the phase modulation actuator 17a is made shorter than that of the control actuator 17b. The effect has been obtained. The present invention can be implemented by such a configuration.

FIG. 8 is a block diagram of a wavefront control device according to another embodiment of the present invention. In the present embodiment, in addition to the digital signal processing units 4a to 4n for calculating each command voltage 5, a CP for managing and controlling these digital signal processing units 4a to 4n
U25 is provided. The CPU 25 stores information such as a modulation frequency and a bias voltage necessary for signal processing in each of the digital signal processing units 4a to 4n in the dual access RAMs 23a to 23n.
The data is transferred to each of the digital signal processing units 4a to 4n via 23n, and the state of each of the digital signal processing units 4a to 4n is managed. By providing such a CPU 25, the processing contents of the digital signal processing units 4a to 4n can be easily changed, and a highly reliable device can be obtained by managing the state.

Although the information is transferred to the digital signal processing unit via the dual access RAM 23 in the embodiment of FIG. 8, it is also possible to transfer the information from each digital signal processing unit directly to the CPU 25 via the bus 24. .

In the above-described embodiment, the number of digital signal processing units 4 must be at most the same as the number of actuators 17 of the deformable mirror 15. Here, for example, as shown in FIG. 10, a module including digital signal processors 4a to 4n is defined as one digital signal processor 27, and as shown in FIG.
When a through 27n are connected to the management CPU 25 via the bus 24, the device can be easily expanded, and the length of the signal line for transferring the digital signal 3 to each digital signal processing unit 4 can be reduced. .

Next, still another embodiment of the present invention will be described with reference to FIGS. In FIG. 11, an optical signal 1 is input to an AGC-equipped amplifier 28, amplified to an optimum size, and sent to an A / D converter 2. The subsequent operation is the same as in the above-described embodiment. When performing digital signal processing, quantization errors that occur when converting an analog signal to a digital signal poses a problem. In order to always minimize this quantization error, in this embodiment, A
The optical signal 1 is amplified by the amplifier with GC 28.

FIG. 12 is a diagram showing an example of the configuration of the amplifier 28 with AGC. The optical signal 1 is amplified by the preamplifier 29, and a BPF (Band Pass Filter) for passing only the frequency component of the phase modulation signal included in the optical signal 1 is provided.
Amplify based on the instruction of 4. In the gain control circuit 34,
The amplitude value of the phase modulation signal detected by the square detector 32 is LP
The optimum gain is determined from the signal from which the high-frequency component has been removed by F (low-pass filter) and the current gain of the variable amplifier 31.

With the above operation, the signal strength can be always maintained at a certain level or more. The intensity of the optical signal 1 changes depending on the change in the density of the medium through which the laser propagates.
Since the phase modulation signal included in the
It is important to amplify the optical signal 1 using the amplifier with C 28.

[0035]

According to the present invention, the influence of performance deterioration due to aging of circuit elements is reduced, the digital signal processing speed is improved, the size of the device is reduced, and quantization errors due to digital signal processing are reduced. Can be.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of a wavefront control device showing one embodiment of the present invention.

FIG. 2 is a configuration diagram of a multi-dither type adaptive optics device showing one embodiment of the present invention.

FIG. 3 is a structural diagram of a deformable mirror constituting the adaptive optics device.

FIG. 4 is a flowchart of a processing algorithm of digital signal processing performed by a wavefront controller according to an embodiment of the present invention.
It is.

FIG. 5 is a configuration diagram of a wavefront control device showing one embodiment of the present invention.

FIG. 6 is a configuration diagram of a wavefront control device showing one embodiment of the present invention.

FIG. 7 is a structural diagram of a deformable mirror constituting the adaptive optics device.

FIG. 8 is a configuration diagram of a wavefront control device showing one embodiment of the present invention.

FIG. 9 is a configuration diagram of a wavefront control device showing one embodiment of the present invention.

FIG. 10 is a configuration diagram of a digital signal processing device according to an embodiment of the present invention.

FIG. 11 is a configuration diagram of a wavefront control device showing one embodiment of the present invention.

FIG. 12 is a configuration diagram of an amplifier with AGC showing one embodiment of the present invention.

[Explanation of symbols]

1: optical signal, 2: A / D converter, 3: digital signal,
4 digital signal processing unit, 5 command voltage, 6 drive power supply, 7 drive voltage, 8 wavefront controller, 9 incident laser beam, 10 laser wavefront, 11 beam splitter -, 12: lens, 13: outgoing laser beam, 14: photodetector, 15: deformable mirror, 16: mirror, 17: actuator, 18: digital signal processing circuit, 19: D
/ A converter, 20: amplifier, 21: phase modulation signal, 22
... Control signal, 23 ... Dual access RAM, 24 ... Bus, 25 ... CPU, 26 ... Storage device, 27 ... Digital signal processing device, 28 ... Amplifier with AGC, 29 ... Preamplifier, 30 ... BPF, 31 ... Variable amplifier, 32 ... Square detector, 33 ... LPF, 34 ... Gain control circuit.

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Hideaki Saito 4-18-10 Kikari, Inzai-machi, Inba-gun, Chiba (72) Inventor Masakatsu Sugii 3-20-21, Chuo-cho, Higashi-Kurume-shi, Tokyo (72) Invention Yasuharu Mine 2-3-15-201 Soka, Soka City, Saitama Prefecture (56) References JP-A-4-84477 (JP, A) JP-A-3-226708 (JP, A)

Claims (4)

(57) [Claims] 1. A shape-variable mirror having a reflecting surface that reflects and emits incident light and a plurality of actuators that change the shape of the reflecting surface, and a focus intensity that captures and converges light waves reflected by the reflecting surface. A wavefront control device for detecting a waveform distortion and correcting the waveform distortion, obtaining a displacement amount of each of the actuators, and vibrating each actuator at a different frequency at each displacement position of each actuator to apply phase modulation to reflected light. In the dither type adaptive optics device, the phase modulation signal
And the phase modulation signal is always
Amplifier that automatically adjusts the gain so that
And analog-to-digital conversion means for converting a signal output from the width unit into a digital value, wherein the actuator and the same number
A plurality of de Ijitaru signal processing unit provided, each of which determines the displacement amount command value of the actuator corresponding to the self-detects the phase modulated signal takes in the digital values, each di
A multi-dither type adaptive optics device, wherein a central control device for integrally managing a digital signal processing unit is provided in the wavefront control device. 2. The system according to claim 1, wherein
Each of the digital signal processing units is a dual access RA.
A multi-dither type adaptive optics device, characterized in that information is exchanged via M. 3. An apparatus according to claim 1 or claim 2, the phase
The modulation signal and wavefront correction signal are converted to digital signal processing units.
Digitally generated by the digital
Phase modulation after converting to analog signal with log converter
Reduce the amplitude of the analog signal for
A multi-dither type adaptive optics device, comprising means for controlling each actuator with the calculated signal . 4. The method according to claim 1, wherein
Actuator for wavefront correction and actuator for wavefront correction
Actuator with smaller displacement than actuator
And a phase modulation signal.
The wavefront correction signal is independently decoded by each digital signal processor.
Digital signal and then digital-to-analog converter
Each actuator is controlled by converting it to an analog signal.
Multi dither method adaptive optical apparatus characterized in that a means that.