JPH02257026A - Laser frequency stability measuring instrument - Google Patents

Abstract

PURPOSE:To easily measure the frequency stability of an optional laser beam by performing a step sweeping till the output wavelength of a variable wavelength light source becomes the nearest to the wavelength of the light to be measured in an arithmetic operation controller. CONSTITUTION:The laser beam to be measured with a frequency fx is made incident on one input end of an optical coupler 22 and the output light beam with the frequency fs from the variable wavelength light source 20 is made incident on the other input end of the optical coupler 22 through an optical fiber. And the light which is multiplexed by the coupler 22 is inputted in a photodetector 23 and the frequency difference between two laser light beams fx-fs is detected. The beat signal from the photodetector 23 is amplified 24 and the frequency fb is measured in a frequency counter 25. And the stability of the variance of erlang, etc., is arithmetically operated based on the fluctuation in the frequency fb in the arithmetic operation controller 26. When the output frequency fs of the light source 20 closes to the frequency fx of the laser beam to be measured by the output of the step sweeping in the controller 26, the frequency fb is detected 23 and the sweeping is stopped in the controller 26.

Description

DETAILED DESCRIPTION OF THE INVENTION <Field of Industrial Application> The present invention relates to the realization of a device for easily measuring the frequency stability of a laser.

<Prior Art> Conventionally, when measuring the frequency stability of laser light, the following method is generally used.

(1) Obtain the beat frequency by combining with a stable reference laser and evaluate based on its stability.

(2) Make two identical lasers and find the relative stability of the output of the two lasers to Be 1 from the frequency.

<Problem to be solved by the invention> However, in case (1), it is not possible to measure the stability of a laser at a frequency far from that of the reference laser, and in case (2), it is difficult to make two units, and the relative The problem is that we only know the relative stability.

The present invention has been made to solve the above problems, and another object of the present invention is to realize an apparatus for easily measuring the frequency stability of any laser beam.

A device for determining the laser frequency stability according to the present invention controls the transmission wavelength of the Fabry-Perot etalon to the wavelength of the output light of the reference wavelength light source, and A tunable wavelength light source configured such that the output wavelength of a semiconductor laser is controlled as one of the transmission wavelengths, and the output wavelength is varied in steps by using the current or temperature of the semiconductor laser as a sweep input, and the output of the tunable wavelength light source. A combining means for combining the light and the light to be measured, a photodetector for detecting a signal from the output light of the combining means, and a Be1 frequency of the output signal of the photodetector. and a calculation control means for calculating stability from the output of the frequency counter, and configured to step sweep the output wavelength of the variable wavelength light source until the calculation control means or the output wavelength of the variable wavelength light source becomes closest to the wavelength of the light to be measured. It is characterized by what it did.

Function> The arithmetic control means sweeps the output wavelength of the variable wavelength light source until it becomes closest to the wavelength of the light to be measured, and the frequency stability of the light to be measured can be measured using the output light of the variable wavelength light source as a reference.

<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 stable nail measuring device according to the present invention. In 1 ml,
21 is an optical system that introduces the laser beam to be measured into the optical fiber;
20 is a variable frequency light source which will be described later; 22 is an optical coupler that inputs and combines the output light of the optical system 21 and the output light of the variable frequency light source 20 via an optical fiber; and 23 is the multiplexed output light of the optical coupler 22; An incident photodetector, 24 an amplifier for amplifying the electrical signal output of the photodetector 23, 25 a frequency counter for measuring the beat frequency of the output of the amplifier 2/1, 26 a microprocessor, etc. This is an arithmetic and control device that sweeps the output frequency of the light source 20 and calculates stability from the output of the frequency counter 25. In the variable frequency light source 20, 1 is a reference wavelength light source whose wavelength is made very stable by controlling the wavelength of a semiconductor laser to an atomic absorption line such as Cs or Rb, and 2 is this reference wavelength light source 1.
4 is a Fabry-Perot etalon that receives the output light of this polarizing beam splitter 2, and 5 is a second polarizing beam splitter that receives the output light of this Fabry-Perot etalon 4. A polarizing beam splitter, 6 a light receiving element such as a photodiode which receives the transmitted light from the polarizing beam splitter 5, 7 a control circuit comprising a lock-in amplifier etc. which inputs the electric output of the light receiving element 6, and 9 a control circuit. A piezoelectric element such as PZT which inputs the output of 7 and drives the Fabry-Perot etalon 4, 10
11 is a semiconductor laser; 11 is a lens that parallelizes the output light of this semiconductor laser 10; and 13 is a beam splitter 12 that converts the output light of this lens 11 into a polarizing beam splitter 2. Fabry-Perot etalon 4 and polarizing beam splitter 5
The light-receiving element 14, such as a photodiode, which is incident through the light-receiving element 13, is a control circuit composed of a lock-in amplifier, etc., which inputs the electrical output of the light-receiving element 13 and controls the current injected into the semiconductor laser 10 using the output. As the reference wavelength light source 1,
For example, the apparatus described in Japanese Patent Application No. IL-11894 filed by the same applicant may be used.

The operation of the apparatus configured as described above will now be explained using the frequency spectrum diagrams 1 to 1 in FIG. A laser beam to be measured with a frequency of 1.chi. enters one input port of an optical coupler 22 via an optical system 21 and an optical fiber. Further, the output light of the frequency fS from the variable wavelength light source 20 enters the other input coupler 1- of the optical coupler 22 via the optical fiber. Optical coupler 2
The light multiplexed in step 2 enters the photodetector 23, and the frequency difference (beat frequency) fχ-fs between the two laser beams is detected. The beat signal from the photodetector 23 is amplified by an amplifier 24, and its frequency fb is measured by a frequency counter 25. The control calculation device 26 calculates the stability of Allan dispersion and the like from the fluctuation of this frequency fb.

In the variable frequency light source 20, the output light from the reference wavelength light source 1 is converted into a Fabry-Perot beam via a polarization beam splitter 2.
It enters etalon 4. Fabry Perrault Etalon 4
is control port II! As the resonator length is periodically changed by a modulation signal superimposed on the output of 87, the transmitted light intensity is modulated. The transmitted light is detected by the light receiving element 6 through the polarizing beam splitter 5, and synchronously detected by the control circuit '7.
Feedback is applied to the voltage applied to the piezoelectric element 9, and the peak of any one of the transmission wavelengths of the Fabry-Perot etalon 4 is controlled to the oscillation wavelength of the reference wavelength light source 1 (Fig. 2 (A) (I-
3)). The output light of the semiconductor laser 10 becomes parallel light by the lens 11, passes through the beam splitter 12, is reflected by the polarizing beam splitter 2, enters the same optical path as the reference wavelength light, enters the Fabry-Perot etalon 4, and becomes the reference wavelength light. It undergoes modulation in the same way as light, and the transmitted light is reflected by the polarizing beam splitter 5, separated from the reference wavelength light, and enters the light receiving element 13. By making the polarization directions of the output light of the reference wavelength light source 1 and the output light of the semiconductor laser 10 perpendicular to each other, and installing the polarizing beam splitters 2 and 5 so that the former is transmitted and the latter is reflected, the polarization of both lights is Synthesis and separation are possible. The output of the light receiving element 13 is synchronously detected by the control circuit 14 and then fed back to the injection current of the semiconductor laser 10, and the oscillation wavelength of the semiconductor laser 10 is controlled to one of the transmission wavelengths of the Fabry-Perot etalon 4 (second (a) In Figure (C), a part of the output light from the semiconductor laser 10 is reflected by the six-beam splitter 12, focused by the lens 15, and output to the optical fiber.

In this state, when the control circuit 14 changes the injection current of the semiconductor laser 10 using the sweep signal from the arithmetic control means 26,
While the control loop that controls the wavelength of the semiconductor laser 10 is working, the wavelength change caused by the injected current is negatively fed back to the current to perform a correction operation, so the wavelength remains constant, but the control loop becomes saturated and breaks. The wavelength jumps to the next transmission wavelength peak of the Fabry-Perot etalon 4. By repeating this operation, the wavelength can be changed in steps.

The variable wavelength light source 20 is controlled by the sweep output of the control calculation device 26.
When the output frequency fs approaches the frequency fχ of the laser beam to be measured, the beat frequency fb is detected by the photodetector 23, and at this time the arithmetic control means 26 stops the sweep. As a result, the output frequency of the semiconductor laser 10 is locked to the nearest resonant peak of the Fabry-Perot etalon (Figure 2(C) and (D)). In this state, Bee 1-
The frequency fluctuation of the laser beam to be measured is estimated from the fluctuation of the frequency fb=fχ−fχ. Here, the stability is measured based on the stability of the variable wavelength light source 20, but the wavelength of each step of the output light of the variable wavelength light source 20 is Fabry-Perot.
The peak interval corresponds to the transmission wavelength peak of the etalon 4, and the peak interval is the FSR (Free Spec) of the Fabry-Perot etalon 4.
trumRange) and is based on the absorption line of a standard substance such as Rb, so it has very high accuracy.

In the above configuration, if the F S 1% of the Fabry-Perot etalon 4 is, for example, 2 GHz, the bit frequency fb is at most 1. GHz, so the amplifier 24,
The counter 25 and the like may have any frequency band within that range.

The laser frequency stability measuring device with such a configuration has good stability because it is a variable frequency light source or a light source that can be stabilized in steps. In experiments, frequency stability up to 10-g rods can be measured.

In addition, since the Fabry-Perot-Era 1 controller is used as an intermediary, the variable range of the semiconductor laser 10 is controlled by the reference laser 1.
It is possible to measure a laser beam of any wavelength different from that of the laser beam.

In the above embodiment, the laser beam to be measured and the variable frequency light source light are combined using an optical fiber or a fiber coupler, but they may be combined using a beam splitter or a polarizing plate without using an optical fiber.

Furthermore, the mirrors of the Fabry-Perot etalon 4 in Fig. 1 may be ordinary parallel mirrors, but if two concave mirrors are used so that their focal points are on the mirror surface of the other,
You can make it smaller. Alternatively, a plane mirror may be placed opposite the concave mirror.

Moreover, the input deviation of the beat signal of the photodetector 23 to the spectrum analyzer can also be used to measure the spectrum width.

Further, the oscillation wavelength can also be swept in steps by manually controlling the laser temperature of the semiconductor laser 10. Alternatively, a control loop may be configured by feeding back the laser temperature, and the oscillation wavelength of the semiconductor laser 10 may be changed in steps by inputting the laser injection current.

Furthermore, instead of using a piezoelectric element to change the resonator length of the Fabry-Perot etalon 4, an electro-optical element may be placed between the mirrors of the Fabry-Perot etalon 4 and the voltage may be fed back.

<Effects of the Invention> As described above, according to the present invention, a laser frequency stability measuring device that easily measures the frequency stability of any laser beam can be realized with a simple configuration.

[Brief explanation of drawings]

FIG. 1 shows one of the laser frequency stability measuring devices according to the present invention.
FIG. 2 is a block diagram showing the configuration of the embodiment, and FIG. 2 is a characteristic curve diagram for explaining the operation of the device shown in FIG. DESCRIPTION OF SYMBOLS 1... Reference wavelength light source, 4... Fabry-Perot etalon, 10... Semiconductor laser, 20... Variable wavelength light...combining means, 23... Photodetector,... Frequency ( A) First laser output (D) Laser light to be measured Fig. 2

Claims (1)

[Claims] controlling the transmission wavelength of the Fabry-Perot etalon to be the wavelength of the output light of a reference wavelength light source; controlling the output wavelength of the semiconductor laser to one of the transmission wavelengths of the Fabry-Perot etalon; and controlling the current or temperature of the semiconductor laser. a variable wavelength light source configured such that the output wavelength is variable in steps with a sweep input; a combining means for combining the output light of the variable wavelength light source with the light to be measured; A photodetector for detecting a beat signal from the photodetector, a frequency counter for measuring the beat frequency of the output signal of the photodetector, and an arithmetic control means for calculating stability from the output of the frequency counter, the arithmetic control means for calculating the stability. A laser frequency stability measuring device characterized in that the output wavelength of a variable wavelength light source is step-swept until it becomes closest to the wavelength of the light to be measured.