JPH01276784A - Variable frequency light source - Google Patents

Abstract

PURPOSE:To accurately measure an output light frequency by performing the calibration of a Fabry-Perot interferometer, the adjustment of an output laser light, and the measurement of spectral characteristics of the interferometer by use of three lasers respectively. CONSTITUTION:First, a contact C of a switch 32 is connected to a contact B, and a Fabry-Perot interferometer is calibrated on the basis of an output from a laser 20. Then, the contact c of the switch 32 is connected to a contact A. An optical detector 26 detects a beat of the output lights from the laser 20 and a laser 35, and detects the number of the beats by a counter 33. In this state, as the temperature of a constant temperature chamber 37 is varied by a temperature control means 38, the frequency of the output beam from a laser 36 can be fixed to that of a peak of a light passing through the interferometer 4. Additionally, the frequencies of the output lights from the lasers 20, 35 are detected by the optical detector 26 on whether or not they are coincident with each other, and at the same time the number of times of the fluctuations of an output from the detector 26 is measured by the counter 33 until the frequencies of the lasers 35, 36 are coincident. The frequency of the output light can accurately be measured on the basis of the just mentioned number of time, the characteristics of the interferometer 4 and the frequency of the laser 20.

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to a variable frequency light source that can vary the frequency of its output light, and more particularly to a variable frequency light source that can determine the frequency of its output light with high precision.

<Prior Art> Semiconductor lasers are widely used as light sources because they are small, easy to install with brackets, and highly reliable. However, semiconductor lasers have drawbacks in that it is difficult to vary the frequency, that is, the wavelength, of their output light, and that their optical spectrum has a wide half-width. In order to improve this drawback, the applicant proposed a variable wavelength light source using a semiconductor laser in Japanese Patent Application No. 62-256502. The configuration of this variable wavelength light source will be explained below based on FIG. In FIG. 3, output light from a semiconductor laser 1 whose output light has a stable wavelength and is polarized in the direction of the arrow passes through a polarizing beam splitter 2 and a beam splitter 3 and enters a Fabry-Perot interferometer 4. The transmitted light from the Fabry-Perot interferometer 4 is incident on a polarizing beam splitter 5, and the intensity of the transmitted light is detected by a photodetector 6. Photodetector 6
The output is synchronously rectified by the lock-in amplifier 7 using the output of the oscillator 8 (frequency f□2) as a reference signal, added to the output of the oscillator 8, and applied to the piezoelectric element 9.

The piezoelectric element 9 is a semi-transparent mirror 411 of the Fabry-Perot interferometer 4.
412 interval is made variable. On the other hand, semiconductor laser 10
The output light is polarized perpendicular to the plane of the paper. This output light is focused by a lens 11, split into two by a beam splitter 12, and the reflected light is taken out as the output light of this variable wavelength light source.

Further, the transmitted light is reflected by the polarizing beam splitter 2 and enters the Fabry-Perot interferometer 4. Since the polarization directions of semiconductor lasers 1 and 10 differ from each other by 90°,
These lights can be combined and separated by a polarizing beam splitter 2.5. The intensity of the output light of the semiconductor laser 10 separated by the polarizing beam splitter 5 is detected by the photodetector 13 and synchronously rectified by the lock-in amplifier 14 using the output A of the oscillator 8 as a reference signal. Control. Further, the output of the photodetector 13 is input to a counter 15 and counted.

The combined light of the output lights of the semiconductor lasers 1 and 10 split by the beam splitter 3 is incident on the photodetector 16, and its intensity is detected. The output is input to the detection circuit 18 via the filter 17, and the output is input to the counter 15.

In such a configuration, only the output light of the semiconductor laser 1 is separated and incident on the photodetector 6 by the polarizing beam splitter 5. Since the Fabry-Perot interferometer 4 transmits only light of a specific wavelength related to the interval, the half-transparent mirror of the Fabry-Perot interferometer 4 is 411.
By controlling the interval 412, the interval between the Fabry-Perot interferometers can be calibrated with the wavelength of the output light of the semiconductor laser 1. Further, the output light of the semiconductor laser 10 passes through the Fabry-Perot interferometer 4, and the semiconductor laser 10 is controlled by the photodetector 13 and the lock-in amplifier 14 so that the intensity of the output light is constant. is fixed at one maximum value or shoulder of the peak of the transmitted light of the Fabry-Perot interferometer 4, and its wavelength can be kept constant. Furthermore, the ambient temperature of the semiconductor laser 10 is changed, and the photodetector 16 or the like detects that the wavelengths of the output lights of the semiconductor lasers 1 and 10 match, resets the counter ]-5, and the light enters the photodetector 13. By counting the number of times the intensity of the output light changes, the wavelength of the output light can be calculated using the FSR (Free 5pectrun R) of a Fabry-Perot interferometer.
It can be changed accurately in steps of (ange). That is, the wavelength of the output light of the semiconductor laser 1 is λ. ,
When the count value of the counter 15 is N, the wavelength λ of this variable wavelength light source is determined by C/λ=C/λ, +N*FSR C: speed of light.

<Problems to be Solved by the Invention> However, although the accuracy of the wavelength spacing of the output light of such a variable wavelength light source is determined by the accuracy of the F'SR of the Fabry-Perot interferometer 4, it is difficult to accurately measure the FSR. In order to do this, it is necessary to perform calibration by accurately measuring the frequency each time the power is turned on, which poses the problem of difficulty in determining the exact wavelength.

Furthermore, since it is not possible to determine which of the peaks of the transmitted light of the Fabry-Perot interferometer 4 should be fixed, there is also the problem that the absolute value of the wavelength cannot be determined.

<Object of the Invention> An object of the invention is to provide a variable frequency light source that can accurately determine the frequency of output light.

Means for Solving the Problems> In order to solve the above problems, the present invention provides a method of combining the output light of the second laser and the first laser whose output light frequency is stable, and the third laser. The output light of the laser is input to a Fabry-Perot interferometer, and the output light of the combined light is detected by the first and second photodetectors, and the output of the first photodetector is used to detect Fabry-Perot interference. The distance between the semi-transparent mirrors of the meter is controlled, the second laser is controlled by the output of the second photodetector, and the number of changes in the output is detected by the first counter. In addition, the intensity of the output light of the third laser emitted from the Fabry-Perot interferometer is detected by a third photodetector, and this output is used to detect the intensity of the output light of the third laser.
control the laser. Further, the intensity of the combined light of the output light of the third laser and the output light of the second laser is detected by a fourth photodetector, and the output frequency of the fourth photodetector is detected by a second counter. Measure with. In such a configuration, the FSR of the Fabry-Perot interferometer is determined from the frequency difference between the output lights of the second laser and the third laser, and this FSR is
When the output light of the SR and the second laser is swept,
The frequency of the output light is determined from the count value of the counter No. 1.

<Embodiment> FIG. 1 shows the configuration of an embodiment of a variable frequency light source according to the present invention. The same elements as in Figure 3 are given the same symbols,
The explanation will be omitted. In FIG. 1, 20 is a first laser, which outputs light with a constant frequency. This first laser connects a semiconductor laser 201 to a beam splitter 20
2, the transmitted light is outputted, and the reflected light is input to an absorption cell 203 containing a standard material that absorbs only light of a specific frequency, and the intensity of the transmitted light is detected by a photodetector 20.
4, and the frequency of the output light of the semiconductor laser 201 is controlled by the control means 205 so that the transmitted light intensity is minimized. In this way, the output light of the first laser 20 is stabilized at the absorption frequency of the standard substance. Note that the output light of the semiconductor laser 201 is modulated by adding the output of the oscillator 21 that oscillates the output of the frequency fnl and the output of the control means 205 in an adder 206, and the control means 205 performs synchronous rectification using the same signal. This is aimed at achieving high stability. It is assumed that this output light is polarized in the direction of arrow 22.

The output light of the first laser 20 is transmitted through a beam splitter 23,
The light passes through the polarizing beam splitter 2 and enters the Fabry-Perot interferometer 4. The transmitted light from the Fabry-Perot interferometer 4 passes through a polarizing beam splitter 5 and is split into two by a beam splitter 24, each of which is sent to a first photodetector 25.
, is incident on the second photodetector 26. First photodetector 2
The output of 5 is input to the control means 27. This control means 2
7 has a frequency f which is the output of the oscillator 21 as a reference signal.
The signal of l11 is input, and synchronous rectification is performed using this signal.

Therefore, only the intensity of the output light from the first laser 20 can be detected. The signal V1, which is the output of the control means 27 and the output of the oscillator 29, is input to an adder 28 and added together. The output of the adder 28 and the output frequency f42 of the oscillator 30 are input to the adder 31 and added, and the output is sent to the piezoelectric element 9.
is input to the semi-transparent mirror 411 of the Fabry-Perot interferometer 4.
412 intervals. Therefore, all the light passing through the Fabry-Perot interferometer 4 is modulated at the frequency 'f\fm2.

On the other hand, the output of the second photodetector 26 is input to the common contact C of the switch 32. A first counter 33 is connected to the contact A of this switch 32, and a control means 34 is connected to the contact B, and the output of the second photodetector 26 is selectively input to the first counter 33 and the control means 34. do. The output of the control means 34 is input to the second laser 35 to control the frequency of its output light. The output light of the second laser 35 is input to the beam sinter 23 . The first laser 20 is the arrow 2
2, the second laser 35 is polarized in the direction of arrow 45. These lights are multiplexed by the beam splitter 23 and input to the polarizing beam splitter 2. 36 is the third
The racer is stored in a constant temperature bath 37. The temperature of this constant temperature bath 37 is precisely controlled by temperature control means 38. The output light of the third laser is split into two by a beam splitter 39, and the reflected light is taken out to the outside as output light. The transmitted light is further split into two by the beam splitter 40, and the transmitted light is input to the polarizing beam splitter 2. The reflected light is incident on the beam splitter 41 and transmitted through the beam splitter 23 to the second laser 35.
The intensity of the output light is measured by the fourth photodetector 42. The output of the fourth photodetector 42 is input to the second counter 43. The output of the photodetector 13 is transmitted to the control means 4
4 is input. The control means 44 receives the signal V of the oscillator 29 and the signal of frequency fm2 which is the output of the oscillator 30. The output of the control means 44 is the third laser 3
6, and the frequency of its output light is controlled. It is assumed that the second and third lasers 35 and 36 are semiconductor lasers, and the output light of the third laser 36 is polarized in a direction perpendicular to the plane of the paper. Therefore, the combined light of the first laser 20 and the second laser 35 and the output light of the third laser 36 can be combined by the polarizing beam splitter 2 and travel the same path, and can be combined by the polarizing beam splitter 5. It can be separated.

Next, the operation of this embodiment will be explained. The operation of this embodiment includes an FSR measurement mode and a variable frequency mode. First, the FSR measurement mode will be explained based on FIG. 2. Figure 2 shows the characteristics of the Fabry-Perot interferometer 4, where the horizontal axis represents the interval between the semi-transparent mirrors 411 and 412, and the vertical axis represents the frequency of transmitted light.The Fabry-Perot interferometer 4 resonates at the black dots. Then, the intensity of the transmitted light becomes maximum. In the FSR measurement mode, the common contact C of the switch 32 is connected to the contact B. First, the signal v1 is set to low level. Since the signal v1 is applied to the piezoelectric cable 9, the interval between the semi-transparent mirrors becomes I73, and the control means 27 controls the frequency f1 of the output light of the J-th laser 20 to be the mode of point a of the Fabry-Perot interferometer 4. Finely adjust the interval between the semi-transparent mirrors so that The control means 44 also fixes the frequency of the output light of the third laser 36 to the point in which the first laser 20 is in the fixed adjacent mode. The optical frequency at this time is assumed to be f3. The frequency of the output light of the second laser 35 is fixed.

Next, when the signal V[ goes high, the spacing of the semi-transparent mirrors becomes I5
It becomes ゜. The control means 27 again finely adjusts the interval so that the usable wave number f1 of the output light of the laser 20 of the tube 1 becomes the mode of point C of the 7-application Perot interferometer 4. Furthermore, the control means 44 fixes the frequency of the output light of the third laser to f3, and the control means 34 fixes the frequency of the output light of the second laser 35 to point d, which is the adjacent mode of point C. The frequency of the output light from the second laser 35 at this time is assumed to be f2.

The output lights of the second and third lasers 35 and 36 are combined by a beam splitter 41, and the beats are sent to the fourth photodetector 4.
By repeating these operations, (f - r3) can be measured by the second counter 43. F when the interval between semi-transparent mirrors is L,,l,
Let SR be f) SIt2 and fFSR3, respectively, the number of modes entering between and l-53 be N, the number of modes at point a be m, and the speed of light be C. From the characteristics of the Fabry-Perot interferometer, f1SR2=C /(2L2) fFS113= C/ (2L, 3) L = m C/ (2f 1) L = <rn N) C/ (2f 1) holds true. From now on, C/(2f)-C/(2f)FS
R3T-SR2 = NC/(2f1), f = f + (f2-f3), FSR
2FSR3 f ) (3 at f2-) to 5R3 t' = JIf 12-f3)/N)...(1)
FSR31 is required. As described above, f - N is known and (f f3) can be found from the second counter, so FSR can be found accurately.

Next, the variable frequency mode will be explained. In this case, the common contact C of the switch 32 is connected to the contact A, and the signal V+- is fixed at a low level. The second photodetector 26 detects the beat of the output light of the first laser 20 and the second laser 35, and the number of beats is detected by the first counter 33. In this state, when the temperature of the thermostatic chamber 37 is changed by the temperature control means 38, the output light of the third laser 36 will appear at one of the peaks of the transmitted light of the Fabry-Perot interferometer 4, as explained in FIG. The frequency of can be fixed. At this time, when the frequency of the output light of the second laser 35 is swept by changing its driving current, etc., the output of the second photodetector 26 increases and decreases with the period of the FSR of the Fabry-Perot interferometer 4. The second photodetector 26 detects whether the frequencies of the output lights of the first laser 20 and the second laser 35 match, and then the second laser 35 and the third laser are detected by the output of the fourth photodetector 42. The first counter 33 measures the number of increases and decreases in the output of the second photodetector 26 until the frequencies of the output light from the laser 36 match.

This measured value is calculated by the first laser 20 and the third laser 36.
represents the mode number of the frequency difference of the output light, and its value is M. The frequency f3 of the output light of the third laser 36, that is, the output light of the variable frequency light source of this embodiment, is determined by f3 "fl + M" FSR3. I can do it. Since fFSR3 can be accurately determined from the above equation (1), the frequency, that is, the wavelength of the output light can be accurately determined.

Note that in the variable frequency mode, the third laser 36
It is also possible to continuously change the frequency of the output light and output the output of the photodetector 26 to the outside at the same time. In this case, the output of the photodetector 26 is the Fabry-Perot interferometer 4
Since its output changes at intervals of FSR, it can be used as a marker.

Further, in this embodiment, the interval between the semi-transparent mirrors 411 and 412 is varied by the piezoelectric element 9, but the Fabry-Perot
An electro-optical element may be inserted into the interferometer 4 to change the effective length. Furthermore, a solid etalon may be used to change the temperature.

<Effects of the Invention> As explained above in detail based on the embodiments, in this invention, three lasers are used, and the first laser outputs reference light to calibrate the Fabry-Perot interferometer. The frequency of the output light of the third laser was fixed at the peak of the transmitted light of the Fabry-Perot interferometer, and the FSR of this Fabry-Perot interferometer was further measured using the second laser. Therefore, since the FSR can be measured with high precision, there is an effect that the frequency of the output light can be determined accurately.

In addition, in the variable frequency mode, the frequency of the output light of the second laser is always swept to measure the number of modes between the frequencies of the first laser, which is the reference frequency light source, and the output light. Another advantage is that the frequency of the output light can be determined no matter which mode of the Fabry-Perot interferometer the laser is fixed to.

[Brief explanation of the drawing]

FIG. 1 is a configuration diagram showing an embodiment of the variable frequency light source according to the present invention, FIG. 2 is a characteristic curve diagram showing the characteristics of a Fabry-Perot interferometer, and FIG. 3 is a configuration showing the configuration of a conventional variable frequency light source. It is a diagram. 13... Third photodetector, 20... First laser, 25... First photodetector, 26... Second photodetector, 27.34.44... Control means, 33...first
counter, 35... second laser, 36... third laser, 42... fourth photodetector, 43...
Second counter. Figure 2 Interval between half-transparent mirrors

Claims (1)

[Claims] A first laser whose output light frequency is stable, a second and third laser whose output light frequency can be varied, a light obtained by combining the output lights of the first and second lasers, and a light beam which is a combination of the output light of the first and second lasers, and the third laser whose output light frequency is variable. a Fabry-Perot interferometer into which the output light of the laser No. 3 is incident; first and second photodetectors that detect the intensity of the combined light transmitted through the Fabry-Perot interferometer; a first counter into which the output of the detector is selectively input; a third photodetector that detects the intensity of the output light of the third laser transmitted through the Fabry-Perot interferometer; It has a fourth photodetector that detects the intensity of the combined light of the output light of the laser and the output light of the second laser, and a second counter into which the output of the fourth photodetector is input. , controlling the Fabry-Perot interferometer with the output of the first photodetector, and determining the characteristics of the Fabry-Perot interferometer from the frequency difference between the output lights of the second laser and the third laser; A variable frequency light source characterized in that the frequency of the output light is determined by sweeping the frequency of the output light of the laser.