JPH0626264B2 - Variable wavelength light source - Google Patents

Description

TECHNICAL FIELD The present invention relates to improvement of characteristics and functions of a tunable laser source.

<< Conventional Technology >> FIG. 10 is a configuration block diagram showing an example of a conventional variable wavelength light source. The output light of the semiconductor laser LD1 is incident on the diffraction grating DG via the condenser lens LS2, and the first-order diffracted light P1 is emitted.
Returns to the semiconductor laser LD1. P2 is the 0th order diffracted light. When the diffraction grating DG is rotated, the wavelength of the first-order diffracted light returning to the semiconductor laser LD1 changes, so that the oscillation wavelength can be controlled. As the output to the outside, the output light from the other end of the semiconductor laser LD1 is extracted via the lens LS1.

<< Problems to be Solved by the Invention >> However, in the variable wavelength light source having such a configuration, when the rotation angle of the diffraction grating and the wavelength of the output light are made to correspond to each other, the wavelength of the He-Ne laser or the like becomes a clear standard. There is a problem that only one or several points can be calibrated using the light source.

The relationship between the diffraction angle θ and the wavelength λ is λ = k sin θ (1) (k is a proportional constant). That is, since the rotation angle and the wavelength have no comparison relationship, the error is small near the calibration point,
There was a problem that it would grow larger in the distance.

The present invention has been made to solve such a problem, and an object thereof is to realize a variable wavelength light source capable of accurately knowing an oscillation wavelength over a wide band.

<< Means for Solving Problems >> A tunable wavelength light source according to the present invention includes a tunable wavelength laser light source in which the wavelength of output light changes in response to an input signal, and a marker light source that outputs multimode oscillation laser light. It is characterized in that the wavelength of the output light of the variable wavelength laser light source is calibrated with the output light generated at a predetermined wavelength interval of the marker light source.

<< Examples >> Hereinafter, the present invention will be described in detail with reference to the drawings.

FIG. 1 is a configuration block diagram showing an embodiment of a variable wavelength light source according to the present invention. In the variable wavelength light source 1, 11 is a variable wavelength laser light source that outputs light having a wavelength corresponding to an input signal, 12 is an optical frequency marker light source (hereinafter, referred to as marker light source) that outputs marker light at fixed wavelength intervals, and 13 is a wavelength. Is a reference wavelength laser light source (hereinafter referred to as a reference light source) that outputs a constant light, 14 is a half mirror that makes the output light of the variable wavelength laser light source 11 incident and separates into two directions, and 15 is a reflection of this half mirror 14. A half mirror that receives light and the output light of the marker light source 12, 16 is a half mirror that receives the transmitted light of the half mirror 15 and the output light of the reference light source 13, and 17 is a PIN photodiode or an avalanche photodiode. A photodetector for injecting reflected light and transmitted light of the half mirror 15,
Reference numeral 8 is a bandpass amplifier for inputting the electric output of the photodetector 17, 19 is a photodetector similar to 17 for injecting reflected light and transmitted light of the half mirror 16, and 20 is an electric output of the photodetector 19. Is a bandpass amplifier for inputting. In the signal processing unit 2, reference numerals 21 and 22 are electric spectrum analyzers to which the outputs of the bandpass amplifiers 18 and 20 are input, respectively.

The operation of the variable wavelength light source having the above configuration will be described below. Part of the output light of the variable wavelength laser light source 11 passes through the half mirror 14 to become output light, and the other part is reflected and enters the half mirror 15. Part of this light is reflected by the half mirror 15, interferes with the output light of the marker light source 12 that has passed through the half mirror 15, and enters the photodetector 17.
The photodetection unit 17 converts the electrical signal having a difference in frequency between the two lights by heterodyne detection. Part of the electric output of the photodetector 17 passes through the bandpass characteristic of the filter 18, and the electric spectrum analyzer 21 causes the tunable laser light a
And the marker light b is displayed. The other part of the light (light from the variable wavelength light source) that has passed through the half mirror 15 is reflected by the half mirror 16, interferes with the output light of the reference light source 13 that passes through the half mirror 16, and enters the photodetector 19. To do.
The photodetection unit 19 converts the electric signal having a difference in frequency between the two lights by heterodyne detection. A part of the electric output of the photodetector 19 passes through the bandpass characteristic of the filter 20, and the electric spectrum analyzer 22 causes the variable wavelength laser light a to pass.
And the reference light c is displayed. The frequency interval of the marker light and the frequency interval of the marker light and the variable wavelength light can be accurately known from the display screen of the electric spectrum analyzer 21, and the variable wavelength light can be known from the relationship with the reference light on the display screen of the electric spectrum analyzer 22. You can know the frequency of. For example, electric spectrum analyzer 2
When the beat signal appears at OHz on the display screen of No. 2, the wavelength of the variable wavelength laser is equal to the wavelength of the reference wavelength laser.

In the apparatus shown in FIG. 1, the variable wavelength laser light source 11 changes the wavelength by changing the injection current or temperature of the semiconductor laser, and one mirror of the external resonator is a diffraction grating as in the conventional example of FIG. It is possible to use one that changes the rotation angle to change the wavelength and various other materials.

FIG. 2 is a configuration block diagram showing an embodiment of the variable wavelength laser light source 11. In the figure, LD2 is a semiconductor laser,
AR1 and AR2 are antireflection coating portions provided at both ends of the semiconductor laser LD2, and LS3 is the antireflection coating portion A.
A lens for collimating the light emitted from R1 into parallel light, a beam splitter BS1 for reflecting the light passing through the lens LS3 and outputting resonance light to the outside, and a lens LS4 for converting the light emitted from the non-reflection coating part AR2. A lens for making parallel light, UM1 is a first ultrasonic modulator on which light passing through this lens LS4 is incident, and UM2 is this ultrasonic modulator UM1.
The second ultrasonic modulator on which the output light from the device is incident, M1 is a mirror that reflects the light emitted from the ultrasonic modulator UM2, and DR1 is an oscillator that excites the ultrasonic modulators UM1 and UM2 at a frequency F. is there. The light emitted from the non-reflective coating portion AR1 of the semiconductor laser LD2 is collimated by the lens LS3, reflected by the beam splitter BS1, and the reflected light returns to the original optical path and enters the semiconductor laser LD2 again. The light of the frequency f ₀₁ emitted from the non-reflection coating part AR2 is collimated by the lens LS4 and is incident on the first ultrasonic modulator UM1. The wavelength of light that satisfies a specific incident angle and emission angle with respect to a diffraction grating formed by ultrasonic waves changes if the wavelength of ultrasonic waves changes. The incident light undergoes a Doppler shift due to the ultrasonic waves during diffraction, and the frequency of the + 1st order diffracted light (the direction of the ultrasonic waves is the same as the direction of diffraction) is f ₀₁ + F. The light emitted from the ultrasonic modulator UM1 is diffracted again by the ultrasonic modulator UM2. Ultrasonic modulator UM
In FIG. 2, the relationship between the traveling wave of the ultrasonic wave and the diffracted light is
In contrast to the case of M1, the -1st-order diffracted light is generated, so the Doppler shift amount is -F, and the frequency of the output light of the ultrasonic modulator UM2 is f ₀₁ + F-F = f ₀₁ . The output light of the ultrasonic modulator UM2 is reflected by the mirror M1 and then subjected to Doppler shift by the ultrasonic modulator UM2 so that its frequency is f.
After becoming _01- F, the ultrasonic modulator UM1 produces f ₀₁ -F.
Since + F = f ₀₁ and the original frequency f ₀₁ is returned to the semiconductor laser LD2, the resonance state continues. By changing the wavelength (frequency F) of the ultrasonic wave with such a configuration, the wavelength of the resonating light can be swept. The resonated light is output to the outside via the beam splitter BS1.

FIG. 3 is a configuration block diagram showing a second embodiment of the variable wavelength laser light source 11. The same parts as those in FIG. 2 are designated by the same reference numerals and the description thereof will be omitted. BS2 is a beam splitter EO1 that splits the light emitted from the lens LS4 into two directions.
Is an electro-optical element that makes the light transmitted through the beam splitter BS2 incident, V1 is a signal source that controls the electro-optical element EO1, M1 is a mirror that reflects the light emitted from the electro-optical element EO1, and EO2 is the beam splitter BS2. An electro-optical element that receives the light reflected by M.sub.2, a mirror M2 that reflects the light emitted from the electro-optical element EO2, and a signal source V2 that controls the electro-optical element EO2. The lengths of the electro-optical elements EO1 and EO2 in the optical path direction are l ₁ and l, respectively.
₂ , the refractive indices are n ₁ and n ₂ , respectively, and the beam splitter B
Assuming that the distance excluding l ₁ along the optical path between S1 and M1 is L ₁ and the distance excluding l ₂ along the optical path between the beam splitters BS1 and M2 is L ₂ and q is an integer, the oscillation frequency f in this case is f. _{02 f 02 = q · c / 2} | (L 1 + n 1 (V 1) l 1) - (L 2 + n 2 (V 2) l 2) | ... becomes (2). That is, by changing the electric field strength of the electro-optical element EO1 or EO2 by the signal source V1 or V2, the refractive index n ₁
Alternatively, by changing n ₂ , the oscillation frequency f ₀₂
Can be swept over a wide range.

FIG. 4 is a configuration block diagram showing an embodiment of the marker light source 12 of the apparatus shown in FIG. In the marker light source 12a, L
D3 is a semiconductor laser whose both ends are AR-coated (non-reflection coating), LS5 and LS6 are meter lenses for collimating the output light of this semiconductor laser LD3, and HM1 and HM2 are these lenses LS5 and LS6.
AT is an attenuator through which the light output from the semi-transmissive mirror LS6 passes. The output light transmitted through the semi-transmissive mirror HM1 is condensed by the lens LS7, and is output to the APD (Avalanch).
It is detected by the photodetector PD1 composed of e Photodiode). The output electric signal of the photodetector PD1 is an amplifier A.
After being amplified by 1, the waveform is monitored by the spectrum analyzer SA.

FIG. 4 The operation of the apparatus will be described below. Semiconductor laser LD
The spontaneous emission light gain curve of the output light of No. 3 is the dotted line d in FIG.
become that way. The light output from both end surfaces of the semiconductor laser LD3 is collimated by the lenses LS5 and LS6, respectively, and resonates between the semi-transmissive mirrors HM1 and HM2. Assuming that the resonator length (distance between the semi-transmissive mirrors HM1 and HM2) is L, the speed of light is c, and the refractive index is n, the free spectral range of the external resonator is determined by c / 2nL, as shown by the dotted line in FIG. Q increases every c / 2nL. As a result, the marker output light output from the attenuator AT becomes as shown by the solid line f in FIG. 5 (multimode oscillation). The wavelength interval λ _χ of the marker output light can be accurately read by the electric spectrum analyzer SA as the frequency interval ν _χ . If the resonator length L is changed, the wavelength interval λ _χ of the marker output can be changed. For example, when L = 10 mm, the frequency interval ν _χ is ν
_χ = c / 2L = 15 GHz. If necessary, an external resonator may be placed in a constant temperature bath or the like to provide stable frequency intervals.

FIG. 6 is a configuration block diagram showing an embodiment of the reference wavelength laser light source 13. In the figure, LD4 is a semiconductor laser, BS3 is a beam splitter on which the output light of this semiconductor laser LD4 is incident, and CL1 is this beam splitter B.
An absorption cell in which a standard substance which makes the reflected light of S3 incident is encapsulated, PD2 is a light receiving element to which the output light of this absorbing cell CL1 is made incident, and LA1 inputs the electric output of this light receiving element PD2 and outputs the corresponding output. A lock-in amplifier that controls the current of the semiconductor laser LD4, and a DR2 is an oscillator that frequency-modulates the current of the semiconductor laser LD4 and supplies the phase detection frequency of the lock-in amplifier LA1. The transmitted light of the beam splitter BS3 becomes the output light of this reference wavelength laser light source. As the standard substance, C _s ,
Any substance such as R _b , NH ₃ and H ₂ O can be used.

The output light of the semiconductor laser LD4 is the beam splitter BS3.
Is reflected by and is incident on the absorption cell CL1 and CL in the absorption cell CL
It is absorbed by the standard substance of 1. The absorption amount is determined by the light receiving element P
It is detected by D2 and is fed back to the current of the semiconductor laser LD4 via the lock-in amplifier LA1. Since the output wavelength of the semiconductor laser LD4 is locked to the absorption spectrum line of the standard substance, a highly stable and highly accurate reference wavelength light source can be realized.

According to the method used in the apparatus shown in FIG. 6, the absorption spectrum becomes relatively thick due to the Doppler shift according to the linear absorption method, but the saturated absorption method (Hori, Kadota, Kitano, Yabuzaki, Ogawa: Saturated absorption spectroscopy was used. Frequency stabilization of the semiconductor laser, by the Technical Report OQE82-116), the absorption line of the ultrafine structure hidden by the Doppler shift is detected, and the oscillation wavelength of the semiconductor laser LD4 is locked to this to make it even more stable. be able to.

In addition, as the reference light source in FIG. 1, a laser having a known wavelength, for example, a He-Ne laser can be used.

According to the tunable wavelength light source having such a configuration, by adding the marker light source and the reference light source to the tunable wavelength laser light source, the oscillation wavelength of the tunable wavelength laser can be accurately known in a wide range.

If an electric spectrum analyzer is used, the oscillation wavelength can be used as a frequency and can be detected with high precision and high resolution.

Although the electric spectrum analyzer is used in the above embodiment, the variable wavelength can be approximately obtained by using the optical spectrum analyzer as follows. FIG. 7 is an explanatory view showing a display screen when each output light of the variable wavelength laser light source, the marker light source and the reference wavelength light source of the apparatus of FIG. 1 is observed by the optical spectrum analyzer. In the figure, the wavelength λ ₀ of the reference light g and the wavelength interval λ of the marker light
₃ is known exactly in advance. The wavelength λ _x of the variable wavelength light is calculated by using the lengths y ₂ and y _{3 of the} respective portions corresponding to the position of the reference light and the lengths y ₁ and y ₄ of the respective portions corresponding to the position of the variable wavelength light on the display screen. It can be calculated approximately as follows.

λ _x = λ ₁ + 2λ ₃ + λ ₂ + λ ₀ = λ ₃ y ₃ / y ₂ + 2λ ₃ + λ ₃ y ₄ / y ₁ + λ ₀ = λ ₃ (y ₃ / y ₂ + 2 + y ₄ / y ₁ + λ ₀ ... (3) Fig. 8 is a configuration block diagram showing another configuration example of the marker light source 12 of the Fig. 1. In the marker light source 12b,
LS7 and LS8 are rod lenses that make the output light of the semiconductor laser LD4 parallel light, and M3 is this rod lens LS.
7, EO3 is a mirror provided at one end of the electro-optical crystal, EO3 is an electro-optical crystal that receives the output light of the rod lens LS8, V3 is a control signal source connected to an electrode of the electro-optical crystal EO3, and HM
Reference numeral 3 is a semi-transmissive mirror that reflects the output light of the electro-optic crystal EO3.

In the apparatus having the above structure, the mirror M3 and the semi-transmissive mirror HM3 form a resonator on both sides of the semiconductor laser LD4, and the resonator length is equivalently changed when the refractive index of the electro-optic crystal EO3 is changed by the applied voltage. The wavelength interval of the marker output light can be easily changed by the output of the control signal source V3.

A single semiconductor laser that oscillates in multiple modes may be used as the marker light source 12 of the apparatus shown in FIG. In this case,
The marker wavelength interval can be changed by changing the temperature or current of the semiconductor laser.

Further, in the apparatus shown in FIG. 1, the respective lights are combined in real time using the half mirror, but the present invention is not limited to this, and it is also possible to switch and display them in a time division manner.

FIG. 9 is a configuration block diagram showing a coherent optical spectrum analyzer which is one application example of the variable wavelength light source according to the present invention. The same parts as those of the device shown in FIG. Reference numeral 1a is a variable wavelength light source, 31 is a half mirror that allows the output light of the variable wavelength light source 1a to enter from one side and the measured light to enter from the other side, and 32 denotes a PIN photodiode, an avalanche photodiode or the like. A photodetector for inputting output light to perform heterodyne detection, a filter circuit 33 for inputting an electric output of the photodetector 32 having a bandpass characteristic, and an amplifier 34 for inputting an output of the filter circuit 33, Is a signal processing for inputting the output of the amplifier 34
The display unit 36 is a sweep signal source for controlling the sweep of the signal processing / display unit 35 and the variable wavelength laser light source 11 of the variable wavelength light source 1a.

The operation of the optical spectrum analyzer having the above configuration will be described below. In the tunable wavelength light source 1 a, the output lights of the tunable wavelength laser light source 11, the marker light source 12, and the reference light source 13 whose wavelength sweeping is controlled by the sweep signal source 36 are combined by the half mirrors 14, 15 and 16 to be output from the half mirror 14. It is incident on the half mirror 31. The light to be measured is combined with the output light of the variable wavelength light source 1a by the half mirror 31, and is converted into an electric signal having a frequency of the difference between the two frequencies by the photodetector 32. A part of the electric output of the photodetection unit 32 passes through the bandpass characteristic of the filter circuit 33 and the amplification unit 34
Is taken out as power. The signal processing / display unit 35 inputs the control signal from the sweep signal source 36 as a frequency axis signal and the electric output of the amplification unit 34 as a power signal to spectrum the measured light j, the reference light k and the marker light l. indicate.

Although the band pass filter is used as the filter circuit 33 in the above application example, the present invention is not limited to this, and a low pass filter may be used.

According to the above application example, since the reference light and the marker light are displayed (or recorded) together with the measurement data, the number of intervals of the marker light is counted from the wavelength of the reference light, and the wavelength can be easily known by performing interpolation. be able to.

<< Effects of the Invention >> As described above, according to the present invention, it is possible to realize a variable wavelength light source capable of accurately knowing an oscillation wavelength over a wide band with a simple configuration. Further, since the relationship between the input signal of the tunable wavelength laser light source and the oscillation wavelength is not required to be accurate, the structure of the tunable wavelength laser light source is simple.

[Brief description of drawings]

FIG. 1 is a configuration block diagram showing an embodiment of a variable wavelength light source according to the present invention, and FIGS. 2 to 4 and 6 are partial configuration block diagrams for explaining a part of the apparatus shown in FIG. FIG. 5 is a characteristic curve diagram for explaining the operation of the apparatus shown in FIG. 4, FIG.
FIG. 1 is a characteristic curve diagram for explaining the operation of the apparatus shown in FIG. 1, FIG. 8 is a block diagram showing a configuration example of a part of the apparatus shown in FIG. 1, and FIG. 9 is a variable wavelength light source according to the present invention. FIG. 10 is a configuration block diagram showing one application example of FIG. 1, and FIG. 10 is a configuration block diagram showing a conventional variable wavelength light source. 1, 1a ... Variable wavelength light source, 11, 11a, 11b ...
Tunable laser light source, 12,12a, 12b ...... marker light source, λ ₃ ...... wavelength interval, 13,13a ...... reference wavelength laser light source.

Claims (6)

[Claims] 1. A variable wavelength laser light source that changes the wavelength of output light in response to an input signal, and a marker light source that outputs multimode oscillation laser light. The wavelength of the output light of the variable wavelength laser light source is a marker light source. A variable wavelength light source characterized in that the output light generated at a predetermined wavelength interval is calibrated. 2. The variable wavelength light source according to claim 1, wherein the marker light source is constituted by combining a semiconductor laser and an external resonator. 3. The variable wavelength light source according to claim 2, wherein the wavelength interval of the marker light source output is changed by changing the external resonator length. 4. The variable wavelength light source according to claim 2, wherein an electro-optical element is inserted in the external resonator to electrically change the equivalent resonator length. 5. The variable wavelength light source according to claim 1, further comprising a reference wavelength laser light source for generating an optical output of a constant wavelength within an output band of the variable wavelength laser light source. 6. A variable optical electric signal, a marker electric signal and a reference electric signal are obtained by causing the variable wavelength laser light source output, the marker light source output and the reference wavelength laser light source output to interfere, heterodyne detected and then passed through a band pass filter. The variable wavelength light source according to claim 5, configured as described above.