JPH0295224A - Laser frequency meter - Google Patents

Abstract

PURPOSE:To measure a laser frequency with high accuracy and fast response by providing a phase modulator, an optical means, a Fabri-Perot etalon, a photodetector, 1st and 2nd amplifying means, 1st and 2nd memory circuits, a sweeping means, and a frequency arithmetic circuit. CONSTITUTION:The phase modulator 2 imposes phase modulation upon laser light to be measured with a frequency f1 and the optical means 4 multiplexes the output light of the modulator 2 and reference laser light which is phase- modulated with a frequency f2 and has a known oscillation frequency. The photodetector 8 detects light which is transmitted through the Fabri-Perot etalon 7. The 1st amplifying means 9 detects the phase of the output of the detector 8 with the frequency f1 and its output is stored in the 1st memory circuit 11. The 2nd amplifying means 10 detects the phase of the output of the detector 8 with the frequency f2 and its output is stored in the 2nd memory circuit 12. A frequency arithmetic circuit 14 computes the frequency of the laser light to be measured from the outputs of the circuits 11 and 12 corresponding to variation in the mirror interval of the Fabri-Perot etalon by a switching means 73.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to an improvement in a laser frequency meter that determines the optical frequency of a laser.

(Prior Art) Conventionally, when measuring the optical frequency of a laser beam, the wavelength of the laser is generally measured and the frequency is determined by calculation.

FIG. 4 is a first conventional example of a wavelength meter for measuring the wavelength of a laser, and is a diagram illustrating the basic configuration of one that utilizes a diffraction grating.

When the light to be measured is incident on the diffraction grating, the incident angle θ of the light is changed by rotating the diffraction grating 31 around the rotation center 32. The wavelength of the light incident on the photodetector 33 depends on the angle of incidence θ, so by measuring θ, the wavelength value λ of the light to be measured can be determined.
8 can be kept constant.

FIG. 5 is a second conventional example of a wavelength meter, which is a diagram illustrating the basic configuration of one that utilizes a Michelson interferometer.

Half mirror 41 detects a measured light beam of unknown wavelength λ8 and a known wavelength λ. f reference light beam (e.g. He-Ne
The zero-combined light that enters the Meigelson interferometer is separated into two directions by a half mirror 42, one of which is reflected by a movable mirror 43 and the other by a fixed mirror 44. The light is incident on the photodetector 45 (separation means for separating the light to be measured and the reference light is omitted in the figure). Movable mirror 4
3 moves by Δ, interference fringes appear on the photodetector 45 as changes in brightness and darkness. At this time, the number of interference fringe changes of the light to be measured is M,
The number of interference fringe changes of the reference light is M. , then the following formula holds.

Δft-M-λ/2 in M-λref/2x
x ref... (1) Therefore, λ = (M, ef/M)・λref'...
(2) XX The wavelength λ8 of the light to be measured can be measured.

(Problems to be Solved by the Invention) However, each of the above-mentioned conventional methods has mechanically movable parts, so high-speed response is poor. In addition, in the case of the method shown in FIG. 4, mechanical precision affects measurement precision, so it is difficult to achieve high precision and is susceptible to changes over time.

Furthermore, since the method shown in FIG. 5 also has a long movable distance, it is difficult to achieve high accuracy. In order to improve accuracy, the distance between the diffraction grating and the photodetector must be increased in the case of Figure 4, and the movable distance must be increased by 2 in the case of Figure 5, but in both cases the optical system becomes larger. cause problems. Also large fabri
There is also a method of precisely measuring wavelengths by pressure sweep using a bellows interferometer, but this is not practical because it is large and requires a vacuum pump, which takes a long time to measure.

The present invention has been made to solve these problems, and an object of the present invention is to realize a laser frequency meter that can measure laser frequency with high precision and high speed response with a simple configuration.

(Means for Solving the Problems) A laser frequency meter according to the present invention includes a phase modulator that phase modulates a laser beam to be measured at a first frequency, and a phase modulator that modulates the phase of the output light of the phase modulator and a second frequency. an optical means for combining the reference laser beam with a known oscillation frequency, a Fabry-Perot etalon into which the output light of the optical means is incident, and a photodetector for detecting the light transmitted through the Fabry-Perot etalon. a first amplification means for phase-detecting the output of the photodetector at the first frequency; a first memory circuit for storing the output of the first amplification means; a second amplification means for phase detection at the second frequency; a second memory circuit for storing the output of the second amplification means; and a sweep means for minutely changing the mirror spacing of the Fabry-Perot etalon. The present invention is characterized by comprising a frequency calculation circuit that calculates the frequency of the laser beam to be measured based on the first memory circuit output and the second memory circuit output corresponding to the change in mirror spacing caused by the sweeping means.

(Function) The transmitted light of the Fabry-Perot etalon is detected by a photodetector, and the first differential signal of the interference signal of the measured laser beam and the reference laser beam is detected by each amplification means, and is transmitted through a memory circuit. The frequency of the laser beam to be measured is calculated.

(Example) The present invention will be explained in detail below using the drawings.

FIG. 1 is a block diagram showing an embodiment of a laser frequency meter according to the present invention. 1 refers to a semiconductor laser whose oscillation frequency is a stable and known frequency, for example, a semiconductor laser whose output frequency is controlled to the Rb absorption line (λ = 780.244nl).
). 2.3 is made of an electro-optic crystal such as LiNbO3, which modulates the phase of the laser beam to be measured and the reference laser beam, respectively. In the second phase modulator, 15 is a first oscillator that excites the first phase modulator 2 at a first frequency f, and 16 is a first oscillator that excites the second phase modulator 3 at a second frequency f2.
A second oscillator 4, which is excited by The half mirrors 15 and 6 constituting the optical means for combining are pinholes into which the output light of the half mirror 4 is incident, and 7 is a Fabry-Perot etalon into which the light passing through the pinhole 5.6 is incident. The Fabry-Perot etalon 7 consists of two semi-transparent concave mirrors 71.7 arranged so that the focal point of each is on the mirror surface of the other.
2 and a piezoelectric actuator 73 made of PZT or the like that constitutes an insertion means for minutely sweeping the mirror interval, and is configured in a vacuum chamber 76. 13 is a triangular wave oscillator of 0.511z, for example, which drives the piezoelectric actuator 73; 8 is a photodetector into which the light transmitted through the Fabry-Perot etalon 7 is incident; 9 is a photodetector 8 that uses the output of the oscillator 15 as a reference signal; A lock-in amplifier 10 constitutes a first amplification means for phase-detecting the output signal, and a lock-in amplifier 10 constitutes a second amplification means for phase-detecting the output signal of the photodetector 8 using the output of the oscillator 16 as a reference signal. , 11
12 is a second memory circuit that stores the output signal of the lock-in amplifier 10; 14 stores the output of the memory circuits 9 and 10 and the output of the oscillator 13; This is a frequency calculation circuit that receives input and calculates the frequency value of the light to be measured.

The operation of the laser frequency meter configured as described above will be explained next. The laser beam to be measured undergoes phase modulation by the phase modulator 2 at a frequency f, and the reference laser beam undergoes phase modulation by the phase modulator 3 at a frequency f2.The phase modulated laser beam to be measured and the reference laser beam are half mirrored. -4 is combined. The optical axes of the measured laser beam and the reference laser beam are adjusted so that they each pass through both pinholes 5.6. As a result, when the measured laser beam is transmitted through the half mirror 4 and the reference laser beam is reflected and combined, it passes through the pinhole 5.6 and enters the Fabry-Perot etalon 7 through the same optical path.

The light incident on the Fabry-Perot etalon 7 is the concave mirror 7
It goes back and forth between 1 and 72 three times, interferes with the incident light, and forms a concave mirror 72.
pass through. That is, since the focal points of the concave mirrors 71 and 72 are on each other's mirror surfaces, the incident light is reflected by the concave mirror 72, reaches the focal point 74, is reflected there, is reflected by the concave mirror 72, becomes parallel to the incident light, and is further reflected by the concave mirror 71. focus 75
It is reflected by the concave surface 1A71, returns to the same path as the incident light, and interferes with the incident light. Fabry Perrault Etalon 7
The transmitted light is detected by a photodetector 8. The output of the photodetector 8 is synchronously detected in lock-in amplifiers 9 and 10 at frequencies f, , f2, respectively, and the lock-in amplifier 9
From here is the Fabry-Perot etalon 7 of the laser beam to be measured.
A first-order differential waveform of a transmitted signal of the reference laser beam is outputted, and a first-order differential waveform of a transmitted signal of the reference laser beam of the Fabry-Perot etalon 7 is obtained from the lock-in amplifier 10. Fabbri
Since the mirror spacing of the Perot etalon 7 is swept with a triangular wave by the oscillator 13, the relationship between the mirror sweep length and the output signal of the lock-in amplifier 9.10 is as shown in FIG. However, the optical frequency of the reference laser beam is 0.78 μm1,
An example is shown in which the optical frequency of the laser beam to be measured is 1.55 μm and the sweep frequency of the triangular wave is 0.511z. These first-order differential waveforms are stored in memory circuits 11 and 12, respectively, and the frequency calculation circuit 14 calculates the laser frequency to be measured from the outputs of the memory circuits 11 and 12 and the output of the oscillator 13.

Next, from the first-order differential waveform shown in FIG.
A method of calculating the laser beam wavelength λ8 to be measured will be explained below. However, the interference peak position of the two light beams that have passed through the Fabry-Perot etalon 7 coincides with the cello-crossing point of the first-order differential waveform in FIG. Since the Fabry-Perot etalon 7 has the configuration described above, the interval between the interference peaks is λ8/6. λ,. , /6, the sweep distance corresponding to the number of interference peaks M of the measured light is △2, the number of interference peaks of the reference light corresponding to the sweep distance △e is M , the fraction of the mirror sweep distance of the reference ef light is α, When β is assumed, the following formula holds true.

ΔQ=(λ/6)−(M−1>=(λref/
x 6) −(M,., −t > 10 α 1 β... (3) Therefore, λ = λ − (Mr8.-1)/ (Mx-1)
+x ref 6 (α + β) / (Mx 1) (nm)... (
4) The optical frequency fx to be measured is obtained by the following formula.

f = c/λ8...(5) Next, α,
We will show how to find β. Piezoelectric actuator 7 when the etalon transmitted light of the laser beam to be measured reaches interference peaks a and b
The voltages applied to the piezoelectric actuator 73 when the etalon transmitted light of the reference laser beam reaches the interference peaks c and d are respectively V and Vd.

Since the interference peak interval of the reference laser beam is constant at λref/6, the mirror sweep length ΔX is determined by the piezoelectric actuator 73.
It is a function of the applied voltage ■, and Δx=G (V)
...(6) can be expressed. Therefore, α and β can be calculated using the following equations.

α=G (V) −G (V) ...(
7)b d β-G(V)-G(V)...(8)a As a numerical example in the above embodiment, for example, λref
=780,244nn and λ =1.55μ11 can be measured.

According to a laser frequency meter with such a configuration, Fabry
Since the Perot etalon is used, the finesse of the interference fringes is better than when using a Michelson interferometer.
se: sharpness of interference peak) is high.

Therefore, the applied voltages Va to Vd at the interference peaks can be detected with high precision, and the fractions α and β of the mirror sweep distance can also be measured with high precision. Therefore, even if the mirror sweep is minute, highly accurate measurements can be made.

In addition, since the etalon transmission signals of the reference laser beam and the measured laser beam are separated by different modulation frequencies, the photodetector is
There is no need for optical means to separate the two light beams. Further, even if the reference laser beam and the measured laser beam have the same wavelength, the signals can be separated, so measurement is possible.

Further, regardless of the polarization plane of the laser beam to be measured, the transmitted signals of the two light beams can be separated. For example, the oscillation frequency can be measured even with a circular wave laser beam.

Furthermore, three times as many interference fringes can be observed for the same sweep length compared to a Fabry-Perot etalon made of a plane mirror. Therefore, as is clear from equation (4), the wavelength measurement accuracy is 3
Improve twice.

Furthermore, since the mirror spacing is swept using PZT, high-speed sweeping is possible, and measurement time can be shortened.

Furthermore, compared to a wavelength needle using a diffraction grating or a Michelson interferometer, the configuration can be smaller and simpler.

Furthermore, since the wavelength of the reference laser beam has absolute accuracy, the wavelength value of the laser beam to be measured also has absolute accuracy. Therefore, unlike a diffraction grating wavelength meter, there is no need for calibration.

In the above embodiment, two concave mirrors whose focal points are on each other's mirror surfaces are used to improve the accuracy of the sweep length, but it is also possible to use a Fabry-Perot etalon using a plane mirror.

Furthermore, in the above embodiment, if a modulated output laser is used as the reference laser 1, the phase modulator 3 can be omitted.

FIG. 3 is a modification of the laser frequency meter shown in FIG. 1. In FIG. 1 is the same mirror as in FIG. 1, and 77 is a semi-transparent plane mirror arranged parallel to this concave mirror 71. This plane mirror 77 is pseudo-concave mirror 7 shown by a dotted line at a position symmetrical to the concave mirror 71 with respect to the mirror surface 79 of the plane mirror 77.
It produces the same effect as having 8. That is, the distance between the concave mirror 71 and the plane g77, and the distance between the plane mirror 77 and the pseudo concave surface 'I
J: This pseudo concave mirror 78 is the same distance from A78.
The plane mirror 77 is positioned so that the focal point 74 of the plane is on the mirror surface of the concave surface MA71.

Since the characteristics of the concave mirror 71 and the pseudo concave surface IA78 are the same, by doing this, the focal point of the concave surface a71 will be on the mirror surface of the pseudo concave mirror 78. Since the operation is the same as that in FIG. 1, the explanation will be omitted. According to the Fabry-Perot etalon having such a configuration, the distance between the mirrors can be further halved compared to the case shown in FIG.

(Effects of the Invention) As described above, according to the present invention, a laser frequency meter capable of measuring laser frequency with high precision and high speed response can be realized with a simple configuration.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of the configuration of a laser frequency meter according to an embodiment of the present invention, FIG. 2 is an explanatory diagram showing the operation of the device shown in FIG. 1, and FIG. 3 is a modification of the device shown in FIG. 1. The main part configuration block diagram shown in FIGS. 4 and 5 are principle diagrams showing a conventional laser frequency meter. 2... Phase modulator, 4... Optical means, 7... Fabry-Perot etalon, 8... Photodetector, 9...
First amplification means, 10...Second amplification means, 11...
・First memory circuit, 12...Second memory circuit, 1
4... Frequency calculation circuit, 73... Sweeping means, f, .
...first frequency, f2...second frequency. Diagram? 9 Yaku Nori Shiro 4 anti-utatu ffiri C5

Claims (1)

[Claims] a phase modulator that phase-modulates a laser beam to be measured at a first frequency; and an optical means that multiplexes the output light of the phase modulator with a reference laser beam that is phase-modulated at a second frequency and whose oscillation frequency is known. , a Fabry-Perot etalon into which the output light of the optical means is incident, a photodetector that detects the light transmitted through the Fabry-Perot etalon, and a phase detection of the output of the photodetector at the first frequency. a first amplification means for detecting the output of the first amplification means; a first memory circuit for storing the output of the first amplification means; a second amplification means for phase-detecting the output of the photodetector at the second frequency; a second memory circuit for storing the output of the second amplification means; a sweeping means for minutely changing the mirror spacing of the Fabry-Perot etalon;
A laser frequency calculator characterized by comprising a frequency calculation circuit that calculates the frequency of the laser beam to be measured based on the first memory circuit output corresponding to the change in mirror spacing caused by the sweeping means and the second memory circuit output. Total.