JPH01307287A - Variable-frequency light source - Google Patents

Abstract

PURPOSE:To stabilize the frequency of output light from a semiconductor laser to a high degree for a prolonged term, and to vary the frequency continuously by phase-modulating the output light by an output from an oscillator, projecting the phase-modulated output light to a Fabry-Perot interferometer and controlling the FM side-band of an output from the interferometer so as to coincide with one of the peaks of the transmitted light of the interferometer. CONSTITUTION:Output light from a semiconductor laser 20 is phase-modulated or frequency-modulated on the basis of an output from a highly stable oscillator 23 by a phase modulation means 22 or a frequency modulation means, the modulated light is projected to a Fabry-Perot interferometer 24, and the intensity of the transmitted light is detected by a photodetection means 27. The frequency of output light from the semiconductor laser 20 is controlled so as to coincide with one of the peaks of the intensity of the transmitted light of the Fabry-Perot interferometer 24 by a first control means 28. The FSR(Free Spectral Range) of the Fabry-Perot interferometer 24 is controlled in relation to an output from the oscillator 23 by a second control means 30. Accordingly, the frequency of output light can be changed continuously with excellent accuracy.

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 particularly to a variable frequency light source that can continuously and precisely vary the frequency of its output light. It is related to.

<Prior Art> Semiconductor lasers are widely used as light sources because they are small, easy to handle, and highly reliable.

However, semiconductor lasers have a drawback in that it is difficult to vary the frequency, or wavelength, of their output light with high precision. In order to improve this drawback, the applicant filed a patent application in 1983.
In the specification of 25-6502, a variable wavelength light source using a semiconductor laser was proposed. This variable wavelength light source will be explained below based on FIG. In Fig. 5, the output light of the semiconductor laser 1 with a stable pile length is transmitted to the polarizing beam splitter 2.
The transmitted light passes through the beam splitter 3 and enters the Fabry-Bello interferometer 4, and the transmitted light passes through the polarizing beam splitter 5 and enters the photodetector 6, where its light intensity is detected. The output of the photodetector 6 is the output of the oscillator 8 (in frequency, 2)
is synchronously rectified by lock-in amplifier 7 as a reference signal,
It is added to the output of the oscillator 8 and applied to the piezoelectric element 9, and the interval between the semi-transparent mirrors 411 and 412 of the Fabry-Bero interferometer 4 is controlled. On the other hand, the output light of the semiconductor laser 10 is focused by a lens 11 and 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 Bellow interferometer 4, and is reflected by the polarizing beam splitter 5 and enters the photodetector 13. Since the output light of the semiconductor laser 1 is polarized in the direction of the page, and the output light of the semiconductor laser 10 is polarized perpendicular to the page,
Multiplexing and separation can be performed using a polarizing beam splitter 2.5. The output of the photodetector 13 is synchronously rectified by a lock-in amplifier 14 using the output A of the oscillator 8 as a reference signal to control the semiconductor laser 10. It is also input to the counter 15 and counted. The photodetector 16 includes a semiconductor laser 1
The combined light of the output lights of 1 and 10 is input, 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 reset terminal R of the counter 15.

In such a configuration, since the Fabry-Perot interferometer 4 transmits only light of a specific wavelength related to its spacing, the semitransparent mirrors 411 and 412 of the Fabry-Perot interferometer 4 are adjusted so that the output of the photodetector 6 is maximized. By controlling the spacing with the piezoelectric element 9, the spacing of the Fabry-Bello interferometer 4 can be calibrated using the wavelength of the output light of the semiconductor laser 1. Furthermore, the output light of the semiconductor laser 10 is transmitted to a Fabry-Perot interferometer 4.
Since the semiconductor laser 10 is controlled so that the intensity of the output light is constant, the output light is fixed at the maximum value or the shoulder of one of the peaks of the transmitted light of the Fabry-Bérot interferometer 4. It is possible to keep the wavelength 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, and the counter 15 is reset. By counting the number of times the intensity of
, 'the interval between the peaks of transmitted light)'. 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 C/λ=C/λo+N
*FSR C: Determined at the speed of light.

<Problem to be solved by the invention> However, such a tunable wavelength light source requires two semiconductor lasers, and one of them must have a stable output light frequency, so the configuration is difficult. The problem was that it was complicated and expensive.

Another problem is that although the wavelength of the output light can be changed at intervals of the FSR of the Fabry-Bello interferometer, it cannot be changed continuously.

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

Means for Solving the Problems> In order to solve the above problems, the present invention phase modulates or frequency modulates the output light of a semiconductor laser using a phase modulation means or a frequency modulation means based on the output of a highly stable oscillator, The phase-modulated or frequency-modulated light is incident on a seven-abbreviation interferometer, and the intensity of the transmitted light is detected by a photodetector. Further, the first control means controls the frequency of the output light of the semiconductor laser to match one of the peaks of the transmitted light intensity of the Fabry-Perot interferometer, and the second control means controls the frequency of the output light of the semiconductor laser to match one of the peaks of the transmitted light intensity of the Fabry-Perot interferometer. The FSR of the oscillator is controlled in relation to the output of the oscillator.

Further, the order of the peak of the transmitted light of the Fabry-Perot interferometer is determined by measuring the number of changes in the output of the light detecting means with a counter, and thereby the absolute value of the frequency of the output light of the semiconductor laser is determined. This is what I did.

<Embodiment> FIG. 1 shows an embodiment of a variable frequency light source according to the present invention. In FIG. 1, 20 is a semiconductor laser, the output light of which is split into two by a half mirror 21 and reflected The light is output to the outside as output light from this variable frequency light source. 22 is a phase modulation means, into which the output light of the semiconductor laser 20 that has passed through the half mirror 21 is incident. 23 is an oscillator that generates a highly stable frequency signal and can change its frequency, and its output (1 in frequency)
is input to the phase modulation means 22 The phase modulation means 22 is made of, for example, a crystal of lithium niobate (LiNO3), and phase-modulates the incident light based on the output of the oscillator 23. Reference numeral 24 denotes a Fapry Bellows interferometer, into which the output light of the phase modulation means 22 is incident. This Fabry Bellows interferometer 24 is composed of two semi-transparent M241.242,
Moreover, it is arranged within the vacuum chamber 25. Therefore, the refractive index around the Fabry-Perot interferometer 24 is 1. The vacuum chamber 25 is used to prevent errors due to fluctuations in the refractive index by placing the Fabry-Perot interferometer 24 in a vacuum. 26 is a piezoelectric element, and semi-transparent mirror 241.24
2. Change the interval slightly. 27 is a light detection means;
The transmitted light of the Fabry-Perot interferometer 24 is incident, and its light intensity is converted into an electrical signal. 28 is a first control means, which is composed of a double balanced mixer 281 and a control section 282. The output of the photodetector 27 and the output A (frequency f2) of the oscillator 29 are input as a reference signal to the double balanced mixer 281, and the output of the photodetector 27 is synchronously detected with this reference signal. The output of this double-balanced mixer 281 is input to a control section 282, and a control signal is calculated by proportional-integral calculation or the like. This control section 282
The output of is input to the semiconductor laser 20, and its injection current is controlled. 30 is a second control means, which is composed of a double balanced mixer 301 and a control section 302. The double balanced mixer 301 includes a light detection means 27.
The output of the oscillator 23 is inputted as a reference signal, and the output of the photodetector 27 is synchronously detected using this reference signal. The control unit 302 calculates a control signal from the output of the double-balanced mixer 301 by proportional-integral calculation or the like. Reference numeral 31 denotes an adder, into which the outputs of the second control means 30 and the oscillator 29 are input, and heats these outputs. This added output is input to the piezoelectric element 26.

32 is a counter to which the output of the light detection means 27 is input, and detects the number of changes in this output.

Next, the operation of this embodiment will be explained. First, the phase modulation means 22 is made not to operate.

Since the output of the oscillator 29 is input to the adder 31, the interval between the semi-transparent mirrors 241 and 242 changes with the frequency f2,
The transmitted light intensity of the Fabry-Perot interferometer 24 operates the zero-order phase modulation means 22 which is modulated at the frequency f2. Second
As shown in Figure (A), the first control means 28 locks the center frequency f of the output light of the semiconductor laser 20 to one of the peaks of the transmitted light intensity of the Fabry-Bello interferometer 24, but the FM side Bands f, ±f1 do not coincide with this peak and are shifted by ±δ. If the sidebands are limited to one on each side of the carrier and the photodetector 27 performs square-law detection, the photodetector 27 does not respond to the optical frequency f, so its signal output l is =a [-Jo 2(m) / 2+1G2(-
δ)+G (δ) ) J (m)/2+(G (
-δ)-G(δ) > Jo (m) Jl(m)
cos (2x f 12 (m)G(-δ)G(δ
) cos(4t ) J 1 πf1t)] α: Constant m: Modulation index Jo: 0th order Bessel function of the first kind J1: Represented by a first order Bessel function of the first kind.

When this signal is synchronously detected at frequency f, the output V0
is vo=to1Jo (m)Jl (m)(G (-δ)-
G (δ)) = K (g (-δ) + g2 (-δ) gl (δ) - g2
(δ)). However, K1 is a constant, K=1Jo (m), 11 (m), and gl (δ) is the amount of light transmitted through the Fabry-Bello interferometer 4 of the in-phase component of the incident light, as shown in Figure 3 (A). There is an even function with the characteristic, K2 (δ) is ±π/2 with the incident light
The amount of transmitted light of the shifted light through the Fabry-Bello interferometer 4 is an odd function having the characteristics shown in FIG. 3(B). Therefore, Vo = 2Kg2 (δ), and if δ is very small, K2 (δ) is proportional to δ. Therefore, the output of the optical detection means 27 is synchronously detected with the output of the oscillator 23 by the double balanced mixer 301. ■
. is proportional to δ. Therefore, the control unit 302
By controlling the piezoelectric element 26 and adjusting the interval between the semitransparent mirrors of the Fabry-Perot interferometer 24 so that the PSR of the Fabry-Perot interferometer 24 becomes zero, as shown in FIG.
Since the output frequency of the 0 oscillator 23 is highly stable, FSR can be kept accurately constant. Since the semiconductor laser 20 is fixed at the same peak of the transmitted light of the Fabry-Bello interferometer 24, if its order is m, the frequency f1 of the output light of the semiconductor laser 20 is f = mf1... -(1) [The frequency can be varied by varying 1 with the frequency t& of the oscillator 23. Since the output frequency of the oscillator 23 can be made relatively low, it is relatively easy to vary the frequency with high stability. Furthermore, since the half mirror 21 extracts the light before being phase modulated by the phase modulator 22 as output light, highly pure light can be obtained. In addition, when the power is turned on, the semi-transparent mirror 241.242
The interval is set to zero and is gradually changed to a predetermined interval, and the number of changes in the transmitted light intensity of the Fabry-Bello interferometer 24, that is, the output of the light detection means 27 at that time, is measured by the counter 32. Then, the intensity of the transmitted light changes every time the interval between the semi-transparent fi 241 and 242 changes by λ/2 (λ: wavelength of light), so the order m of the peak of the transmitted light can be obtained from the output of the 6 oscillator 23. frequency f1
By determining by another means, the absolute value of the frequency ft of the output light can be determined from the above equation (1).

FIG. 3 shows another embodiment of the present invention. In this embodiment, the output of the oscillator 23 is input to a double-balanced mixer 281, and the output of the oscillator 29 is input to a double-balanced mixer 301.

Even in this case, the same configuration can be achieved.

Note that in these embodiments, the FM sideband of the frequency of the output light of the semiconductor laser 20 was fixed to the peak of adjacent transmitted light, but it was fixed to the peak of one or more peaks. When the peak is fixed at n peaks, the FSR of the Fabry-Bello interferometer 24 becomes FSR=f1/n.

Further, as the oscillator 23, an oscillator whose reference is the output of an external oscillator such as a cesium beam frequency standard may be used.

Furthermore, frequency transformation means may be used instead of the phase modulation means 22.

<Effects of the Invention> As explained above in detail based on the embodiments, in this invention, the output light of a semiconductor laser is phase-modulated by the output of a highly stable oscillator, and enters the Fabry-Perot interferometer, and the output light is The Fabry-Bero interferometer is controlled so that the FM sideband of the FM sideband coincides with one of the peaks of the transmitted light, and the frequency of the output light of the semiconductor laser is varied by varying the output frequency of the oscillator. This is what I did. Therefore, the frequency of the output light can be made highly stable over a long period of time and can be varied continuously.

In addition, since it can be configured with one semiconductor laser,
It can be made small.

Furthermore, by simply changing the characteristics of the Fabry-Perot interferometer, such as by changing its coating, it can be used as a variable frequency light source for any frequency regardless of the characteristics of the semiconductor laser.

[Brief explanation of the drawing]

FIG. 1 is a configuration diagram showing one embodiment of a variable frequency light source according to the present invention, FIGS. 2 and 3 are characteristic curve diagrams showing the characteristics of a Fabry-Perot interferometer, and FIG. 4 is a diagram showing another embodiment of the present invention. FIG. 5 is a block diagram showing the structure of a conventional variable wavelength light source. 20... Semiconductor laser, 22... Phase modulation means, 2
3.29... Oscillator, 24... Fabry-Bello interferometer, 26... Piezoelectric element, 27... Photodetection means, 28
... first control means, 30 ... second control means, 3
2...Counter. Figure 3 (A) (B)

Claims (2)

[Claims] (1) A semiconductor laser, an oscillator, a phase modulation means or frequency modulation means that receives the output light of the semiconductor laser and modulates the phase or frequency of the light with the output of the oscillator, and the phase modulation means or frequency modulation means A Fabry-Perot interferometer receives the output light of the Fabry-Perot interferometer, a photodetection means receives the transmitted light of the Fabry-Perot interferometer and detects the light intensity, and an output of the photodetection means enters the output light of the semiconductor laser. a first control means for controlling the frequency to one of the peaks of transmitted light intensity of the Fabry-Perot interferometer; and a second control means for controlling the characteristics of the Fabry-Perot interferometer to which the output of the light detection means is input. A variable frequency light source characterized by having. (2) The variable frequency light source according to claim 1, further comprising a counter that receives the output of the light detection means and counts the number of changes in the output.