JPH0640597B2 - Laser light source device - Google Patents

Description

TECHNICAL FIELD OF THE INVENTION The present invention relates to a laser light source having a constant oscillation frequency and capable of arbitrarily varying a spectral width.

(Prior Art and Problems Thereof) As a coherent optical communication system, a long-distance communication system using a wavelength band of 1.55 μm is drawing attention as a next-generation optical communication system. In particular, it has been clarified that the transmission band, which is a problem in constructing an optical communication system, is generally determined by the chromatic dispersion of the optical fiber transmission line. Therefore, in recent years, since the oscillation and spectrum width of the laser light source are also important factors that determine the transmission band, the development of narrowing the oscillation spectrum width of the laser light source has been actively conducted.

FIG. 1 is a block diagram of a conventional laser light source device, in which the frequency of the output light of the laser light source is made as constant as possible and the spectrum width is made as narrow as possible. In FIG. 1, 1 is a laser oscillator of the Fabry-Perot resonance type or distributed feedback type, 6 is a laser drive unit that changes the excitation state of the laser oscillator 1 to change the oscillation frequency (oscillation wavelength),
Reference numeral 3 is a reflector formed of a reflecting mirror or a diffraction grating for reflecting and returning the monitor light L ₃ of the laser oscillator 1, 2 is a semitransparent reflecting mirror for branching the main output light L ₂ of the laser oscillator 1, 4 Is a frequency measuring unit for measuring the oscillation frequency, 5 is a frequency control unit for measuring a deviation from a desired oscillation frequency and controlling the laser driving unit 6, and 7 is a branched output light L ₄
Mirror for reflecting light, 8 is the branched main output light L ₂
A spectral width measuring unit for measuring the spectral width of
Reference numeral 9 is a spectrum width control unit for controlling the spectrum width.

In this laser light source device, in order to make the spectrum width as narrow as possible and to make the oscillation frequency constant, the excitation state of the laser drive unit 6 is adjusted in advance so that the desired oscillation frequency is obtained. , The distance between the laser oscillator 1 and the reflector 3, that is, the phase of the feedback light L ₁ is adjusted, or the inclination angle of the reflector 3 is changed to adjust the amplitude of the feedback light L ₁ to reduce the spectral width. Was doing.

Here, in order to keep the oscillation frequency constant regardless of the adjustment of the spectrum width described above, it is necessary to use the light (light that travels in the same direction as the feedback light L ₁ ) that circulates to oscillate in the laser oscillator 1. it is necessary to the phase theta ₀ and the phase theta ₁ of the feedback light L ₁ is in phase or anti-phase. However, after adjusting the oscillation frequency in the conventional method described above, by adjusting the tilt angle or the distance of the reflector 3 in order to narrow the spectral width, the laser oscillator will also vary the phase theta ₁ of the feedback light L ₁ 1 The oscillating frequency of shifts. Therefore, there is a problem that it is extremely difficult to adjust the spectrum width and the oscillating frequency alternately so as to converge to a desired value.

Further, the conventional method for stabilizing the oscillation frequency or the spectrum width in the laser light source device also has the following problems. In the following description, the stabilization of the oscillation frequency and the stabilization of the spectrum width are the same in principle, so the frequency stabilization will be described as an example.

FIG. 2 schematically shows the structure of a conventional frequency-stabilized light source. In FIG. 2, 1 is a laser oscillator,
Reference numeral 4 is a frequency measuring unit, which is composed of an absorption cell or a resonator 41 utilizing a gas absorption line and a light receiver 42, 5 is a frequency control unit, and 6 is a laser driving unit. M ₁ is a semitransparent mirror. L ₀ , L ₂ , L ₄ , and L ₅ marked by double lines are light rays, and arrows indicate their directions. S ₁ marked with a solid line,
S ₂ is the flow of the electric signal, and the arrow also shows the direction of the signal flow. Here, the absorption cell or the resonator 41 using the absorption line of gas is regarded as a band pass filter or a band element filter having a narrow band in the vicinity of the center frequency of absorption, the resonance frequency of the resonator, and the anti-resonance frequency. Therefore, this will be referred to as "narrowband filter 41" hereinafter for simplicity. Also, 2, 3,
7, 8 and 9 are the same as in FIG.

In addition, in order to actually create a frequency stabilized light source,
In addition to the components shown in the drawing, optical elements for geometrically operating light, such as lenses and reflecting mirrors, are necessary. However, since these are naturally expected to be used, they are not particularly described. It is actually present if it is not present, but the description is omitted.
This applies to all the figures below.

L ₂ is the output light of the laser oscillator 1, and the output light L ₂ is a semitransparent mirror M ₁
Is divided into rays L ₀ and L _4, and the ray L ₄ is input to the narrow band filter 41. Rays emerging from narrowband filter 41
The light receiver 42 receives L ₅ and converts it into an electric signal. In this frequency stabilized light source, the oscillation frequency of the laser oscillator 1
If f ₁ matches the center frequency f _c of the narrow band filter 41 (the absorption center frequency in the case of an absorption cell, the resonance frequency in the case of a resonator), the intensity of the light beam L ₅ is the band pass of the narrow band filter 41. The maximum or minimum depending on the type or the band stop type. If the oscillation frequency f ₁ of the laser oscillator 1 is different from the center frequency f _c, according to whether the narrow-band filter 41 is a band elimination or a band-pass, light L ₅
Intensity decreases or increases. Therefore, if the amplitude of the electric signal S ₁ corresponding to the intensity of the light beam L ₅ is monitored, it is known whether the oscillation frequency f ₁ of the laser resonator 1 matches the center frequency f _c of the narrow band filter 41. be able to. At this time, if the oscillation frequency f ₁ of the laser oscillator 1 can be changed according to the electric signal S ₂ from the frequency control unit 5, the oscillation frequency is slightly changed in a certain direction, for example, in the higher direction, and the electric signal S in that case is changed. _By observing the change in the amplitude of ₁ , it is possible to know in which direction the oscillation frequency f ₁ is displaced with respect to the center frequency f _c of the narrow band filter 41, and according to this information, the electric signal S ₂ is adjusted. , The oscillation frequency f ₁ can be corrected to match the center frequency. However, the speed at which the frequency is changed must be sufficiently lower than the speed at which the amplitude, frequency, etc. are modulated (frequency) when the frequency-stabilized light source is used.

Another way to stabilize the oscillation frequency f _1, the oscillation frequency f ₁ of the laser 1 there is a method to keep the frequency modulation. In this method, the oscillation frequency f ₁ is modulated so that the maximum frequency shift is constant and the frequency periodically changes in the vicinity of the center frequency. At this time, the optimum maximum frequency shift is determined from the frequency characteristic of the narrow band filter 41. Also in this case, similarly to the above-mentioned method, the frequency (1 / cycle) for modulating the oscillation frequency f ₁ is sufficiently higher than the frequency for modulating the amplitude, frequency, etc. when using this frequency stabilized light source. Must be low.

Further, if the frequency is stabilized by using the absorption line of gas and only the spectral width is to be changed in that state, the amplitude is changed by the conventional method of changing the amplitude of the feedback light mechanically or electro-optically. And the phase changed slightly, and it was virtually impossible to change only the amplitude. Therefore, it was not suitable as a means for changing the spectral width in a light source with highly stabilized frequency.

As can be seen from the above description, in any conventional frequency-stabilized light source, the average frequency is stabilized at a constant frequency determined by the characteristics of the narrow band filter 41, but the frequency f ₁ at each time is
Has a limited application because it fluctuates periodically around this constant frequency or in a manner determined by a control method. Also,
The latter method has a problem that the response speed of the control system 5 for stabilizing the frequency is limited by the frequency for modulating the oscillation frequency f ₁ of the laser oscillator ₁ for frequency control, so that the utilization is further narrowed. It was

(Objects and Features of the Invention) The present invention has been made in order to solve the above-mentioned drawbacks of the prior art, and it is possible to adjust only the spectrum width independently to an arbitrary value while keeping the oscillation frequency of the laser light source constant. An object is to provide a laser light source device.

A second object of the present invention is to provide a laser light source device capable of stabilizing the oscillation frequency or spectrum width of the laser light source without being limited by the modulation speed when the output light of the laser light source is amplitude- or frequency-modulated and used. The purpose is to do.

The first feature of the present invention is to adjust the amplitude by changing the polarization plane of the monitor light of the laser light source, thereby independently adjusting the amplitude and phase of the feedback light to control only the spectral width to an arbitrary value. I have made it possible.

The second feature of the present invention is that the measurement is performed using an ultrasonic optical modulator for frequency shifting the oscillation frequency in the frequency measuring unit (spectral width measuring unit) of the laser light source.

(Structure of the Invention) The present invention will be described in detail below with reference to the drawings.

First, a means capable of independently adjusting the oscillation frequency and the spectrum width, which is the first object of the present invention, will be described. In order to clarify the difference from the conventional configuration, only the configuration of the reflector 3 having a configuration different from that of the conventional configuration shown in FIG. 1 will be described.

(Embodiment 1) FIG. 3 is a main part of a first embodiment of the present invention, and is a configuration diagram of a feedback optical unit for optically returning the monitor light L ₃ of the laser oscillator 1. In FIG. 3, 31 is a polarizer for passing the linearly polarized wave from the laser oscillator 1, 32 is a Faraday rotator for obtaining an arbitrary plane of polarization, 33 is a Faraday rotator 32 for applying a magnetic field. The magnetic field applying unit, 34 is a reflector for reflecting light, such as a reflecting mirror or a diffraction grating, L ₃ , L ₃₁ , L ₃₂ , L ₁₂ , L ₁₁ and L ₁ represent light rays, and arrows indicate light rays. The triple line M indicates a control line for applying a magnetic field from the magnetic field applying unit 33 to the Faraday rotator 32.

Next, the operation will be described. The output light L ₃ of the laser oscillator 1 passes through the polarizer 31, the output light L ₃₁ passes through the Faraday rotator 32, the output light L ₃₂ is reflected by the reflector 34, and the reflected light L ₁₂ is again the Faraday. After passing through the rotator 32, the output light L ₁₁ thereof passes through the polarizer 31 and is returned to the laser oscillator 1 as the feedback light L ₁ . At this time, among the light rays L ₃ , only the polarization component having the same polarization plane as the polarization plane of the light passed by the polarizer 31 passes through the same polarizer ₃₁ as the light ray L ₃₁ . Therefore, if the direction of the plane of polarization of the light ray L ₁ is different from the direction of the plane of polarization of the light passed by the polarizer 31, the angle θ _i is different,
₃₁ is attenuated by cos ² (θ _i ) with respect to the ray L ₃ . Light L ₃₁ is the light L ₃₂ whose polarization plane is subjected to rotation theta _f corresponding to the magnitude of the magnetization of the Faraday rotator 32 when passing through the Faraday rotator 32. The light ray L ₃₂ is reflected by the reflector 34 to become a light ray L ₁₂ and is again a Faraday rotator.
Upon passing through 32, it receives rotation θ _f of the plane of polarization and enters the polarizer 31 as a light beam L ₁₁ . Since light L ₁₁ is the direction of the plane of polarization to light rays L ₃₁ is rotated by 2 [Theta] _f, light L ₁₁ receives only attenuation cos ² (2θ _f) to light rays L _31. Therefore,
The maximum value of the amplitude of the feedback light L ₁ can be changed by changing the angle θ _i . Further, if the strength of the magnetic field applied to the Faraday rotator 32 is changed by the magnetic field application unit 33, the angle θ _f changes, and when the angle θ _f changes from 0 degree to 45 degrees, the amplitude of the feedback light L ₁ is the maximum value. To the minimum value. Furthermore, the angle θ
_{When f} changes from 45 degrees to 90 degrees, the amplitude of the feedback light L ₁ changes from the minimum value to the maximum value. However, in a case where there is between the case and the angle theta _f is 45 degrees the angle theta _f is between 0 to 45 degrees up to 90 degrees, the backward light L ₁ phase is different by 180 degrees. Here, since the extinction ratios of the polarizer 31 and the Faraday rotator 32 are usually not 0, the amplitude of the feedback light L ₁ does not become 0. Further, in order to change the Faraday rotation angle θ _f , the magnetic field generated by the magnetic field applying unit 33 must be changed by an external command or signal given to the unit 33.

As described above, in the present invention, the amplitude of the feedback light L ₁ can be adjusted only by controlling the plane of polarization, so that the phase can be kept constant regardless of the adjustment of the amplitude.

Next, a method of adjusting the phase will be described. The phase is adjusted by moving the position of the reflecting mirror 34 closer to or farther from the laser oscillator 1 as in the conventional case. In this case, backward light L ₁ phase theta ₁ and light orbiting laser oscillator within 1 phase theta ₀
And are extremely important because of the stability of the transmission frequency.

FIG. 4 shows the phase θ _t of the light (combined light of the feedback light and the light circulating in the laser oscillator) traveling in the same direction as the feedback light from the outside at an appropriate place inside the laser oscillator. Is. In the figure, the solid lines C ₁ , C ₂ , C ₃ , C ₄ , and C ₅ represent the phase θ ₁ of the feedback light L ₁ from the outside and the phase θ of the combined light when the phase θ ₀ without feedback light is in phase. _{At t} , the intensity of the return light becomes weaker in this order. The solid line C ₆ is the synthesized light in the absence of feedback light L ₁ from the external phase θ _t, C _t, C ₈ in the phase theta _t when the phase theta ₀ and the phase theta ₁ is reverse-phase, the intensity of the returned light Is stronger in the order of the solid lines C ₇ and C ₈ .

Here, when there is no return light from the outside, it is assumed that the laser oscillator oscillates at a frequency f _c where the solid line C ₅ crosses the horizontal axis. In order for the laser oscillator 1 to stably oscillate around one frequency, it is necessary that the phase θ _t does not become 0 at two or more frequencies close to each other. Therefore, the intensity and phase of the feedback light L ₀ must be controlled so that the state represented by the solid lines C ₁ and C ₈ does not occur.

Spectral width as a change in phase theta _t in the vicinity of the phase theta _t is an integer multiple of 0 or 360 degrees is steep narrows. Therefore, in the example shown in FIG. ₄ , the spectrum width becomes wider as the intensity of the feedback light is changed as shown by the solid lines C ₂ , C ₃ , C ₄ , C ₅ , C ₆ , and C ₇ . In particular, in the case represented by the solid line C ₇ , the spectral width may be extremely wide. Further, as a whole, it is necessary to lengthen the optical path length of the feedback optical system 3 in order to narrow the spectrum width. In this case, it is necessary to adjust the light quantity of the return light at the same time so that the state represented by the solid lines C ₁ and C ₈ in the figure does not occur.

As in this example, if the rotation of the plane of polarization when passing through the Faraday rotator 32 is adjusted to change the phase and intensity of the return light, the solid line C ₂ in FIG. 4 is obtained. Solid line from state
It is possible to obtain arbitrary states up to the state represented by C ₇ .

For example, to get the state of the solid line C ₆ , Faraday rotator 32
The rotation angle θ _{fr of the} plane of polarization when passing through two round trips is 90 degrees. Further, the condition of the rotation angle of the Faraday rotator 32 in the range from the solid line C ₂ to the solid line C ₇ can be approximately represented by the following equation.

Let r be the reflectance of the end face of the laser 1 on the side where the reflector 3 is coupled,
The transmittance is t, the effective reflectance from the reflector 3 when the rotation angle of the Faraday rotator 32 is 0 is R, the effective resonator length of the laser is 1, and the effective optical path length of the reflector 3 is Is L, and when L >> 1 and R << 1, the deviation Δθ _fr of the rotation angle θ _fr from 90 degrees satisfies the condition −Γ <sin (Δθ _fr ) <4.60 Γ (1) There is a need. here, Is.

FIG. 5 shows a configuration example of another feedback optical system in the present invention. This feedback optical system is obtained by omitting the polarizer 31 in the feedback optical system shown in FIG. Therefore,
In the case of this feedback optical system, it is a condition that the laser oscillator itself has polarization selectivity so that it can be used. 32, 33, 34,
Doublet _{L 1, L 3, L 12} , L 32 means similar to those in the embodiment shown so far. The operation is the same as that of the embodiment shown in FIG. 3 except that the laser oscillator itself has a function of selecting the polarized light and changing the effective intensity of the feedback light.

6 to 8 show an example of the configuration of the feedback optical system in the present invention. In Figures 6 to 8, 3
4, 35 and 36 are reflectors or diffraction gratings. 37 is a semitransparent mirror or a polarizing beam splitter. Reference numeral 38 is a polarization-maintaining optical fiber or an optical fiber polarizer made of a polarization-maintaining optical fiber, and 39 is a lens.

The operation of the feedback optical system shown in FIGS. 6 and 7 is such that the output light L ₃ of the laser oscillator is split into two light beams by the semitransparent mirror or the deflecting beam splitter 37, and each light beam rotates its polarization plane. Receiving the reflected light, the distance between the reflecting mirror or the diffraction grating 34 and 35, the distance between the reflecting mirror or the diffraction grating 36 and the semitransparent mirror or the polarization beam splitter 37. The feedback optical system is basically the same as the feedback optical system shown in FIG. 3 or 5 except that the phase of the feedback light can be changed by simultaneously changing the distance.

In the embodiment shown in FIG. 8 also, the polarization-maintaining optical fiber 38 is used as the polarizer 31, or the polarization-maintaining optical fiber 38 is used to shorten the distance over which light propagates in space. Basically, the same operation as that of the feedback optical system shown in FIG. 3 or 5 is performed except for.

FIG. 9 and FIG. 10 show that the Faraday rotator 32 generates an appropriate magnetic field for an arbitrary Faraday rotation angle of the Faraday rotator 32 by combining the light with and without the rotation of the plane of polarization.
It is an example of a feedback optical system in the present invention in which the intensity of the feedback light can be made substantially zero in a state of being applied to. The meanings of components, lines, etc. in the figure are the same as those in the embodiments shown so far.

In any of the embodiments, the output light L ^{3 of the} laser oscillator 1 is
Is divided into two by using a semitransparent mirror or a polarization beam splitter, and one is passed through the Faraday rotator 32 to rotate its plane of polarization. The other is directly reflected by a reflecting mirror or a diffraction grating 35, synthesized by a semitransparent mirror or a polarizing beam splitter 37, and returned to the laser oscillator 1. At this time, the semitransparent mirror or the diffraction grating 37 appropriately divides the light ray L ₃ or the reflectance of the reflection mirror or the diffraction grating 35, and the light reflected by the reflection mirror or the diffraction grating 34 and If the phase difference of the light rays reflected by is properly selected (180 degrees), the hysteresis of the rotation angle of the Faraday rotator 32 does not matter so much. You can

As described above, the present invention uses the feedback optical system as a means for changing the amplitude of the monitor light L ₃ of the laser oscillator 1 and adjusts it within the range given by the equation (1) to keep the oscillation frequency constant. And the spectral width can be changed to any desired value.

As the Faraday rotator 32, any Faraday rotator 32 having a large Faraday rotation angle can be used. For example, rare earth iron garnet such as yttrium iron garnet or gadolinium iron garnet, or Faraday glass may be used. When using a material with hysteresis such as a ferromagnetic material or a ferrimagnetic material, it is necessary to use it in a place where the strength of the applied magnetic field is high to some extent. For that purpose, the intensity of the feedback light when the magnetic field is not applied may be made sufficiently large. To avoid this problem, it is one method to use a material with good linearity such as Faraday glass.

As the polarizer 31, prisms using a birefringent crystal, a polarization beam splitter using a dielectric multilayer film, or a polarization-preserving optical fiber polarizer, a polarizing plate using a dichroic material Etc. can be used.

As described above, according to the present invention, the Faraday rotator 32 is the polarizer.
By controlling the amplitude of light using an optical element having polarization characteristics such as 31 (hereinafter, referred to as "feedback optical system"),
The amplitude and phase of light can be adjusted independently.

The present invention is based on the above-mentioned polarizer 31 and Faraday rotator.
The configuration is not limited to the combination with 32, and the amplitude and phase of light can be independently adjusted in the configurations shown in FIGS. 4 to 10 below.

Second Embodiment Next, a means for stabilizing the oscillation frequency or the spectrum width of the laser light source, which is the second object of the present invention, will be described in detail.

FIG. 11 is a block diagram of a frequency-stabilized light source according to the present invention. The difference from FIG. 2 (conventional example) is that an ultrasonic optical modulator 43-1 for performing frequency shift is provided in front of a narrow band filter 41. AC electrical signal for exciting ultrasonic wave in ultrasonic optical modulator 43-1
That is, the frequency modulator ₄₃ including the electric signal source 43-2 for supplying S ₄₃ is provided.

The present embodiment will be described below with reference to the drawings. The output light L ₂ of the laser oscillator 1 is converted into light rays L ₀ and L ₄ by using a semitransparent mirror M _1.
Split into and. The light ray L ₀ is used as the output light of this stabilizing light source, or is further divided and one is used as the output light and the other is used for stabilizing the amplitude of the output light by monitoring the amplitude thereof. When the light beam L ₄ is input to the frequency modulator 43, at least a part of the light beam L ₄ ′ having a frequency shift is input.
Is output as. The magnitude of the frequency shift can be controlled by the electric signal S ₃ output by the oscillation frequency controller 5. Ray of light
L ₄ ′ passes through the narrow band filter 41 and is output as a light beam L ₅ . The light beam L ₅ is received by the light receiver 42, and its intensity is measured by the electric signal S ₁
Convert to. Oscillation frequency of laser oscillator 1, that is, light beam
If the frequency of L ₂ (the frequencies of the light rays L ₀ and L ₄ are the same) is f ₁ and the frequency shift received by the frequency modulator 43 is f _a , the frequency f ₄ of the light ray L ₄ ′ is f ₄ ′ = f ₁ + f _a or f ₄ ′
= F ₁ + fa. The intensity of ray L ₅ is the frequency f ₄ of ray L ₄ ′.
Is equal to the center frequency f _c of the narrow band filter 41 (the absorption center frequency in the case of an absorption cell, the resonance frequency in the case of a resonator), the intensity of the light beam L ₅ is It is a maximum or a minimum depending on whether it is a band stop type. If one the frequency f ₄ is different from the frequency f _c, the intensity of the light beam L ₅ represents, depending narrowband filter 41 is a band elimination or a band-pass frequency f ₄ is the frequency f _c It will decrease or increase compared to when it is done. Thus, if the monitoring the amplitude of the electric signals S ₁ corresponding to the intensity of the light beam L _5, to know whether the frequency f ₄ of the light beam L ₄ 'is equal to the center frequency f _c of the narrow-band filter 41 You can At this time, according to the electric signal S ₃ from the oscillation frequency control unit 5, the frequency modulator 43 causes the frequency f _{4 of the} light beam L ₄ ′ to f ₄
If the frequency shift f _a given to
By slightly changing the frequency shift in a certain direction, for example, in the increasing direction, and observing the change in the amplitude of the electric signal S _{1 in} that case, the frequency f ₄ of the light beam L ₄ ′ is changed to the center frequency f _{c of the} narrow band filter 41. It is possible to know in which direction the displacement is. Since the magnitude of the frequency shift that the frequency modulator 43 gives to the light beam L ₄ ′ is known, the electrical signal S ₂ can be adjusted according to this information to stabilize the oscillation frequency f ₁ . The range of frequencies in which the oscillation frequency f ₁ is stabilized is determined by the center frequency f _c of the narrow band filter 41 and the frequency shift that the frequency modulator 43 can give to the light beam L ₄ ′. When the absolute value of the maximum frequency shift that can be given to the frequency f ₄ of the light beam L ₄ ′ by the frequency modulator 43 is f _a max and the absolute value of the minimum frequency shift is f _a min, the oscillation of the laser oscillator 1 is assumed. The frequency f ₁ is calculated by using the center frequency f _c of the narrow band filter 41 and these frequencies f _a max and f _a min.
_c− f _a max <f ₁ <f _c −f _a min (when f ₄ ′ = f ₁ + f _a )
Alternatively, f _c + f _a min <f ₁ <f _c + f _a max (f ₄ ′ = f ₁ −f
_In the case of a), it can be stabilized at any frequency in the range.

As is clear from the above description, the output light L ₀ of the frequency-stabilized light source according to the present invention is not subjected to any modulation. The control system for stabilizing the oscillation frequency f ₁ of the laser light source 1, also when used by modulating the output light of the frequency-stabilized light source for some purposes, quite a modulator for modulating the output light Since they operate independently, the operating speed of the control system for stabilizing the frequency is not limited by external factors. Furthermore, the frequency of the output light L ₀ is
It can be seen that it can be changed by the frequency shift given to the ray L ₄ ′ at 43.

As the ultrasonic optical modulator 43-1, as long as it is given a frequency shift f _a which requires the ray L _4, it may be of any type. However, if a frequency-stabilized light source is to be constructed as simply as possible, the modulation efficiency depending on the magnitude of the given frequency shift f _a (the output light that has undergone the frequency shift relative to the intensity of the input light) over the required frequency range. It is desirable that there is little change in the intensity ratio). If it is not necessary to change the frequency of the output light of the frequency stabilizing light source, the frequency band of the ultrasonic light modulator does not need to be so wide. However, when it is desired to change the frequency of the output light L ₂ within a certain frequency range, it is necessary to widen the band according to the desired frequency range. As a method of realizing an ultrasonic optical modulator with a wide frequency band, an optically anisotropic crystal (such as tellurium dioxide TeO ₂ ) is used to efficiently diffract light and There is a method of satisfying the angle condition that the traveling direction of the ultrasonic wave should be within a certain frequency range even if the frequency of the ultrasonic wave changes. As another method, a plurality of ultrasonic light modulators 43-1 may be connected in series and used. Generally, a high-efficiency ultrasonic optical modulator 43-1 has a diffraction efficiency of 90% or more,
For example, even if 10 pieces are used, 30% or more of the folded lattice can be obtained. Therefore, the frequency shift provided by one ultrasonic light modulator is
Even with a frequency of about 200MHz, 10 units will allow a frequency shift of about 2GHz.

As the electric signal source 43-2, an alternating current electric signal of a required electric power can be supplied to the ultrasonic optical modulator over a required frequency range, and the frequency of the ultrasonic wave, that is, a frequency shift f according to an electric signal inputted from the outside. Any value can be used as long as it can change _a . As a method of changing the frequency of the ultrasonic wave, the electric signal source 43-2 may be a simple power amplifier, and the electric signal S ₃ having the same frequency as the frequency shift to be given may be sent from the transmission frequency control device, or The electric signal S ₃ may be a DC electric signal, and the electric signal source 43-2 may be composed of an oscillator whose oscillation frequency changes according to the level of the DC electric signal and a power amplifier for amplifying the output signal of the oscillator. . Further, the electric signal S ₃ may be an appropriate digital signal, and the electric signal source 43-2 may be composed of an oscillator capable of controlling the oscillation frequency by the digital signal and a power amplifier for amplifying the output signal of the oscillator. Good.

In the frequency modulator 43 used in the present invention, the frequency conversion f _a given to the light beam L ₄ needs to be accurately known. Therefore, as the electric signal S ₄₃ , the output of the oscillator whose frequency is stabilized or the output of which the frequency is modulated,
Alternatively, those obtained by amplifying them are desirable.

Further, FIG. 12 is a block diagram of the frequency control unit 5 used in the frequency stabilized light source of the present invention, 51 is a frequency modulator 43-
1 is a controller for controlling 1; 52 is a synchronizing signal generator for generating a synchronizing signal of a constant cycle and supplying it to the controller 51 and a synchronizing detector 53 described later; 53 is an electric signal output from the photodetector 42
It is a synchronous detector for amplifying or attenuating the amplitude of S ₁ to an appropriate amplitude.

The synchronization signal generator 52 generates a synchronization signal having a constant cycle, and supplies the synchronization signal S ₅₃ and the synchronization signal S ₅₂ to the controller 51 and the synchronization detector 53, respectively. The controller 51 generates an electric signal S ₃ according to the synchronization signal 53, and the frequency modulator 43 has the maximum value f _a max with the same period as the synchronization signal generated by the synchronization signal generator 52.
And a frequency shift f such that the minimum value f _a min is constant.
_The frequency modulator 43 is controlled so that a is given to the light beam L ₄ . Ray L ₄ ′ that has undergone frequency modulation (that is, frequency shift)
Is near the center frequency f _c of the narrow band filter 41, the component in-phase or in-phase with the synchronizing signal of the electric signal S ₁ output from the photodetector 42 is extracted by the synchronous detector 53 and is extracted. The amplitudes of the in-phase component and the anti-phase component correspond to the deviation of the average frequency of the light beam L ₄ ′ from the center frequency f _c of the narrow band filter 41. The synchronous detector 53 outputs the amplitude of this in-phase component or anti-phase component as an electric signal S ₅₁ . The electric signal S ₅₁ may be a voltage or current having a value corresponding to the amplitude of the in-phase or anti-phase component, or may be a digital signal encoded corresponding to the same amplitude. Controller 51, an electric signal based on the information about the deviation from the center frequency f _c of the narrow band filter 41 of the average frequency of the applied by an electrical signal S ₅₁ light L ₄ 'S ₂
And the amplitude frequency f ₁ of the laser oscillator 1 is changed by this signal S ₂ to control the light beam L ₄ ′ so that the average frequency coincides with the center frequency f _c of the narrow band filter 41. At this time, the oscillation frequency f ₁ of the laser oscillator 1, the center frequency f _c of the narrowband filter 41 is divided by the difference of the average value of the frequency shift f _a frequency modulator 43 provides the ray L _4. Therefore, by changing the average value of the frequency shift f _a frequency modulator 43 provides, it is possible to vary the oscillation frequency of the laser oscillator 1.

In the above description, the configuration for stabilizing the oscillation frequency of the laser light source 1 has been described, but a configuration for stabilizing the spectrum width of the laser light source 1 to an arbitrary value is also possible.
For example, the spectrum width measuring unit 8 may monotonically change the frequency shift of the ultrasonic modulator 43-1 built in the frequency measuring unit 4 between a predetermined minimum value and maximum value. The simplest method is to change the frequency shift between the minimum value and the maximum value so that the rate of change per unit time is constant.

13 and 14 are block diagrams of the spectrum width control unit 9 used in the spectrum width stabilizing light source in the present invention.

In FIG. 13, reference numeral 90 is a spectrum width controller for controlling the frequency shift of the ultrasonic light source modulator 43-1 so as to be performed with a desired temporal change, and 91 is a signal S from the spectrum width measuring unit 8. _An integrator for integrating the signal generated based on ₈ for a predetermined time, 92 is the signal S ₈ and the reference signal S _r
And the adder.

The time when the integration in the integrator 91 starts and the time when the integration ends are
The signal S ₈₀ provides the spectral width controller 90 to the integrator 91. This time is a time at which the frequency shift reaches a predetermined maximum value and minimum value. Therefore, at which time the integration starts and at which time the integration ends is determined by the method of changing the frequency shift. For example, when changing the frequency transition from the minimum value to the maximum value repeatedly, the time when the frequency transition starts increasing from the minimum value is the time when the integration starts, and the time when the frequency transition reaches the maximum value is the integration. The time to end.

If the signal S ₈ is a baseband signal having a level corresponding to the intensity of the ray L ₅ , the integrated signal is the signal S ₈
It may be itself or may be one that has been subjected to appropriate processing such as subtracting an appropriate baseband signal from the signal S ₈ .

The spectrum width controller 90 compares the output signal S ₉₁ of the integrator ₉₁ or the integration result indicated by the signal S ₉₁ with the measurement value, and makes the spectrum width larger or smaller than the current value depending on the magnitude. The feedback optical system 3 directly controls at least one of the amplitude and / or the phase of the feedback light fed back to the laser oscillator 1 based on the signal S ₃ , or the information for controlling the feedback optical system 3 is supplied. Give to.
If necessary, the spectrum width controller 90 may directly or indirectly control the laser oscillator driving unit 6 by the signal S ₂ to change the oscillation condition or the temperature of the laser oscillator 1.

Similarly to FIG. 13, FIG. 14 shows an outline of a configuration example of the spectrum width control unit 9. Of the components and signals, those other than the adder 92, the output signal S ₉₂ of the adder ₉₂ , and the reference signal Sr are the same as those in FIG. The difference from the embodiment shown in FIG. 13 is that the adder 92 can add the output signal S ₈ of the light receiver 41 and the reference signal Sr and output as a signal S ₉₂ . If the reference signal Sr having a polarity opposite to that of the signal S ₈ is prepared, the output signal S ₉₁ of the integrator 91 can represent the difference between the directly set spectrum width and the actual spectrum width. it can. In this case, the spectrum width can be controlled by adjusting the magnitude of the reference signal Sr.

(Effect of the Invention) As described above, according to the present invention, the amplitude of the feedback light is adjusted by the polarization element independently of the phase, so that the oscillation frequency of the laser light source 1 is kept constant and the spectral width is selected to an arbitrary value. Therefore, it can be used as a test light source for an optical communication system or a mode hopping noise source.

Further, according to the present invention, the main signal light itself output from the laser light source 1 is frequency-modulated by using the ultrasonic optical modulator 43-1 that shifts the frequency in the frequency measuring unit 4 (spectral width measuring unit 8). It becomes possible to stabilize the frequency (spectral width) without being affected by the control speed of frequency stabilization, and the effect is extremely large.

[Brief description of drawings]

FIG. 1 is a block diagram showing an example of a conventional laser light source device, FIG. 2 is a block diagram showing an example of a conventional frequency stabilizing light source, and FIGS. 3, 5 to 10 show the present invention. FIG. 4 is a block system diagram showing a configuration example of a feedback optical unit used in a laser light source device, FIG. 4 is a characteristic diagram of phases and oscillation frequencies of light circulating in the laser oscillator and feedback light from the outside in the present invention, 11 is a block diagram showing another embodiment of the frequency stabilizing light source according to the present invention, FIG. 12 is a block diagram showing a configuration example of a frequency control unit used in the frequency stabilizing light source of the present invention,
13 and 14 are block diagrams showing a configuration example of the spectrum width control unit used in the spectrum width stabilizing light source of the present invention. 1 ... Laser oscillator, 2 ... Semi-transparent reflecting mirror, 3 ... Reflector (feedback optical system), 4 ... Frequency measuring unit, 5 ... Frequency control unit, 6 ... Laser driving unit, 7 ... Mirror , 8 ... Spectral width measurement unit, 9 ... Spectral width control unit, 31 ... Polarizer, 32 ... Faraday rotator, 33 ... Magnetic field application unit, 3
4,35,36 …… Reflector or diffraction grating, 37 …… Semi-transparent mirror or polarizing beam splitter, 38 …… Optical fiber polarizer, 39
...... Lens, 41 …… Narrow band filter, 42 …… Receiver, 43
...... Frequency modulator, 43-1 …… Ultrasonic wave, 43-2 …… Electrical signal source, 51 …… Controller, 52 …… Synchronous signal generator, 53 ……
Synchronous detector, 90 ... Spectral width controller, 91 ... Integrator, 92 ... Adder.

Claims (2)

[Claims] 1. A laser light source device for adjusting the oscillation frequency and spectrum width of the laser oscillator by changing the amplitude and phase of the monitor light of the monitor light and the main output light output from the laser oscillator. Polarization plane changing means for arbitrarily rotating the polarization plane of the monitor light of a linearly polarized wave determined in advance by the strength of the magnetic field, and the monitor light whose polarization plane is changed by the polarization plane changing means is reflected and the polarization plane is changed again. A feedback optical system having at least a reflecting means for entering the wavefront varying means,
The phase of the monitor light reflected by the reflecting means and incident on the laser oscillator by changing the distance between the laser oscillator and the reflecting means and the phase of the light circulating in the laser oscillator are opposite in phase. Alternatively, the laser light source device is configured so as to obtain the desired spectrum width by changing the intensity of the monitor light by the polarization plane changing means after the phases are made in-phase and the desired oscillation frequency is obtained. . 2. The constant oscillation frequency or spectrum width is controlled by controlling the excitation state of the laser oscillator or the intensity and phase of monitor light while measuring the oscillation frequency or spectrum width of the laser oscillator by the frequency measuring section or the spectrum width measuring section. In the stable laser light source device, the polarization plane changing means for controlling the intensity of the monitor light by changing the polarization plane by the intensity of the magnetic field applied to the monitor light, and the main output light of the laser oscillator are branched. At least one ultrasonic optical modulator that gives a frequency shift to the signal light obtained by the above, frequency comparison means for comparing the deviation between the frequency of the frequency-shifted signal light and the desired oscillation frequency, and the frequency comparison An optical-electrical conversion means for converting the optical signal obtained by the means into an electric signal, and a known frequency shift of the ultrasonic optical modulator. Is controlled to stabilize the oscillation frequency of the laser oscillator by controlling the excitation state of the laser oscillator or the intensity of the monitor light so that the electric signal becomes maximum or minimum from the value of Laser light source device.