JPH0513399B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <<Industrial Application>> The present invention relates to an optical frequency synthesizer/sweeper that generates coherent light with controlled frequency, phase, amplitude, and polarization.

<<Prior Art>> Conventional laser light sources with a wavelength sweeping function include the following (FIGS. 12 and 13).

B. It uses the temperature characteristics of the wavelength of a semiconductor laser, and sweeps the wavelength by changing the temperature of the laser diode. FIG. 12 is an explanatory diagram showing the principle, in which the output wavelength of the laser diode LD is swept by controlling the constant temperature bath TB with the temperature control means TC. The sweep width is several tens of nanometers.

(b) Taking advantage of the wide gain range of the dye laser,
A device that sweeps the oscillation wavelength by rotating the prism inside the resonator. In FIG. 13, M is a mirror;
CC is a pigment cell, L is a lens, P is a prism,
HM is a half mirror. The sweep width is approx.
It is 100nm.

≪Problems to be solved by the invention≫ However, with the tunable wavelength laser light source configured as above, the wavelength accuracy is at most 1 nm (300G
Hz). In the field of coherent optical communication and applied optical measurement in the future, frequency measurement with an accuracy of MHz or less will be required, so the above light sources cannot be used in coherent optical measuring instruments.

The present invention was made to solve these problems, and an object of the present invention is to realize an optical frequency synthesizer/sweeper that can obtain a coherent optical output with high optical frequency accuracy, high stability, and high spectral purity. .

<<Means for Solving the Problems>> The optical frequency synthesizer sweeper according to the first aspect of the present invention includes a reference wavelength light source section and controls the wavelength of the optical output to a wavelength corresponding to the oscillation wavelength of the reference wavelength light source section. It is characterized by comprising an optical frequency PLL section.

The optical frequency synthesizer sweeper according to the second aspect of the present invention includes a reference wavelength light source section, an optical frequency PLL section that controls the wavelength of the optical output to a wavelength corresponding to the oscillation wavelength of the reference wavelength light source section, and the optical frequency
It is characterized by comprising an optical modulation section that modulates the output light of the PLL section, and an optical amplification section that amplifies the output light of the optical modulation section.

<<Operation>> According to the optical frequency synthesizer/sweeper configured as above, based on the principle of optical frequency PLL,
The wavelength of the optical output of the optical frequency PLL section can be made variable.

≪Example≫ The present invention will be explained in detail below using the drawings.

FIG. 1 shows an optical frequency synthesizer according to the present invention.
1 is a configuration block diagram showing an embodiment of a sweeper. FIG. 1 is a reference wavelength light source unit whose wavelength is stabilized; 2
3 is an optical frequency PLL section that inputs the output light of this reference wavelength light source section 1, 3 is an optical modulation section that modulates the output light of this optical frequency PLL section 2, and 4 is a light that amplifies the output light of this optical modulation section 3. This is the amplification section. optical frequency PLL
In the section 2, 21 is an optical heterodyne detection section whose one input is the output light of the reference wavelength light source section 1;
2 is a variable wavelength light source section whose oscillation wavelength of output light is controlled by the output of the optical heterodyne detection section 21;
3 is an optical frequency shifter section that shifts the frequency of the output light of this variable wavelength light source section 22; 24 is an optical frequency shifter section that multiplies the frequency of the output light of this optical frequency shifter section 23 and transmits the output light to the optical heterodyne detection section 21;
This is an optical frequency multiplier that takes the other input as the other input. Here, the optical frequency shifter section 23 and the optical frequency multiplier section 2
Reference numeral 4 constitutes an optical frequency conversion section that inputs light related to the output light of the variable wavelength light source section 22 and converts the optical frequency to another optical frequency.

The operation of the device having such a configuration will be explained next.
The output light of the reference wavelength light source section 1 is transmitted to the optical frequency PLL section 2.
, the optical frequency PLL section 2 fixes (locks) the wavelength of its optical output to the wavelength corresponding to the oscillation wavelength of the reference wavelength light source section 1. That is, the optical heterodyne detection section 21 compares the output light from the reference wavelength light source section 1 and the output light from the optical frequency multiplication section 24, and controls the variable wavelength light source section 22 so that the difference between them becomes small. The optical frequency shifter section 23 in the feedback circuit adds an offset frequency to the output light of the variable wavelength light source section 22, and the optical frequency multiplier section 24 adds an offset frequency to the output light frequency of the variable wavelength light source section 22 and the reference wavelength light source section 1.
Determine the ratio of the output optical frequencies. The optical modulation section 3 modulates the output light of the optical frequency PLL section 2, and the optical amplification section 4 amplifies the output light of this optical modulation section to generate the output of the optical frequency synthesizer sweeper.

FIG. 2 is a block diagram showing a second embodiment of the present invention, which is a further embodiment of the configuration shown in FIG. In the reference wavelength light source section 1, LD1 is a laser diode, CL is an absorption cell filled with Rb gas or Cs gas and receives the output light of the laser diode LD1, HM1 is a half mirror into which the output light of the absorption cell CL enters; PD1 is a photodiode that inputs the reflected light of this half mirror HM1, A1 is a control circuit that inputs the electric output of this photodiode PD1 and controls the current of the laser diode LD1 with the corresponding output, IS1 is the half mirror An isolator for preventing return light, OA1, through which the light transmitted through HM1 passes, is an optical amplification element into which the light that has passed through this isolator IS1 is input. In the optical frequency PLL section 2, HM2 is a half mirror into which the output light of the reference wavelength light source section 1 is input, and PD2 is a PIN which constitutes the optical heterodyne detection section 21 and inputs the transmitted light of the half mirror HM2.
A photodiode consisting of a photodiode or an avalanche diode, EC is an oscillator that generates an electrical signal of a predetermined frequency by inputting a reference frequency from a crystal, etc., and MX1 is a combination of the electrical output of this oscillator EC and the optical heterodyne detector PD2. It is a mixer (mixing) circuit to which the electrical output is connected. In the variable wavelength light source section 22 to which the output of the mixer (mixing) circuit MX1 is connected, FC is connected to the mixer circuit MX1.
Optical frequency modulation circuit, VL, to which the output of MX1 is connected
1 to VL3 are variable wavelength laser diodes that input the output of this optical frequency modulation circuit FC, and IS2 is a YIG
(yttrium iron garnet) and the variable wavelength laser diodes VL1 to VL3
The isolator OS1 through which the output light of is passed is an optical switch through which the light that has passed through a plurality of (three in FIG. 2) isolators IS2 is incident. HM3 is a half mirror into which the output light of this optical switch OS1 enters, OA2 is an optical amplification element into which the reflected light of this half mirror HM3 is input, and UM1 constitutes an optical frequency shifter section 23, which converts the output light of the optical amplification element OA2. The input ultrasonic modulator, NL is an optical waveguide using a nonlinear material that constitutes an optical frequency multiplier and inputs the output light of this optical frequency shifter, and OA3 is an optical amplification element that amplifies the output light of this optical waveguide NL. It is. In the optical modulation section 3 into which the output light of the optical frequency PLL section 2 is input, AM1 and PM1 are amplitude modulators and phase modulators using electro-optic crystals such as LiNbO ₃ , respectively, and LM1 is a magneto-optic crystal such as YIG. This is the polarization modulator used. OA 4 is an optical amplification element that constitutes the optical amplification section 4 and amplifies the output light of the optical modulation section 3 .

The operation of the apparatus having such a configuration will be described in detail below.

The reference wavelength light source section 1 has Rb as described below.
(or Cs) atomic absorption line to control the oscillation wavelength of the laser diode to achieve high precision and high stability (10 ^-12 or more) at the absolute wavelength. laser diode
When the output light of LD1 passes through the absorption cell CL,
The wavelength of the output light of LD1 is Rb gas (or Cs gas)
When the absorption line coincides with the absorption line of , it is absorbed, and an absorption characteristic as shown in the characteristic curve diagram of FIG. 3A appears. Figure 4 is
This is an explanatory diagram showing the energy level of Rb gas. The absorption lines of Rb are the _D2 line at 780nm and the _D1 line at 795nm.
When multiplied by 2, they become 1560nm and 1590nm, respectively.
This is convenient because it coincides with the 1500 nm band, which is the optical fiber communication wavelength. This is also a wavelength range that is easy to use in the field of photoreaction measurement. The part of the output light from the absorption cell CL that is reflected by the half mirror HM1 is detected by the photodetector PD1, and the control circuit A1 switches the laser diode LD1 in response to the output of the photodetector PD1.
By controlling the current of LD1 at the absorption center
locks the output wavelength of For example, a in Figure 3A
If it is desired to lock to a point, the control circuit A1 uses a lock-in amplifier or the like to fix it at point b (the point where the differential waveform value becomes 0) in FIG. 3B, which is the differential waveform in FIG. 3A. This method is called the linear absorption method, and the absorption spectrum becomes thicker as shown in Figure 3A. , IEICE Technical Report OQE82-116), detecting the absorption line of the ultrafine structure hidden by the Doppler shift and locking the oscillation wavelength of the laser diode LD1 to this will further increase stability. Furthermore, the laser diode LD
1 is temperature stabilized in a constant temperature bath. The light transmitted through the half mirror HM1 enters the isolator IS1. The isolator IS1 prevents return light due to reflection from the outside from entering the laser diode LD1 and causing noise. The output light of the isolator IS1 is amplified by the optical amplification element OA1 as necessary.

As described below, the optical frequency PLL section 2 has a function of locking the oscillation wavelength of the variable wavelength light source section 22 to the oscillation wavelength of the reference wavelength light source section 1 at a predetermined ratio and a predetermined offset. The output light of the reference wavelength light source section 1 is transmitted through the half mirror HM2 and is then sent to the photodiode of the optical heterodyne detection section 21.
It enters PD2. The feedback light from the optical frequency multiplier 24 is also reflected by the half mirror HM2 via the optical amplification element OA3, and then transmitted to the photodiode.
It enters PD2. The output of the reference wavelength light source section 1 and the optical frequency of the feedback light are respectively ω _s ,
When ω ₁ is assumed, the frequency ω ₂ of the output electrical signal of the optical heterodyne detection section 21 becomes ω ₂ =|ω _s −ω ₁ |.
When the output frequency of the oscillator EC is ω ₃ , the output ω ₄ of the mixer circuit (phase detection circuit) MX1 is obtained by adding an offset frequency to the output frequency ω ₂ of the optical heterodyne detection section 21 to obtain ω ₄ =ω ₂ −ω _{3 .} becomes. The output electrical signal ω ₄ of the mixer circuit MX1 is the variable wavelength light source section 2.
The optical frequency modulation circuit FC controls the optical frequencies of the variable wavelength laser diodes VL1 to VL3 so that ω ₄ =0.
Here, for the variable wavelength laser diodes VL1 to VL3, a resonator is constructed using reflection from a diffraction grating built into the laser diode chip, and the oscillation frequency is determined by the pitch of the diffraction grating, so the wavelength is relatively stable. ADFB (Acoustic DFB) laser (Yamanishi M, et.al.: GaAs Acoustic) is a type of DFB (Distributed Feedback) laser and DBR (Distributed Bragg Reflector) laser.
Distributed Feedback Lasers, Jpn.J.Appl.
Phys., Suppl. 18-1, p. 355, 1979). The ADFB laser generates surface acoustic waves (SAW) perpendicular to the diffraction grating inside the DBR laser, and the diffraction grating built into the chip
A ring resonator of light is formed by Bragg diffraction with SAW. Sweeping the wavelength of the SAW changes the resonance wavelength of the ring resonator, making it possible to sweep the oscillation wavelength. In this example, the oscillation wavelength is 1560nm.
It is worn as an obi. DFB, DBR, etc. with long resonator length
ADFB lasers also have the advantage of having a narrow oscillation spectrum and good spectral purity. one
If the variable wavelength range of the ADFB laser is insufficient, use multiple ADFB lasers VL1 to VL as shown in Figure 2.
3 and can be switched using an optical switch or optical multiplexer. i.e. tunable laser diode
The output lights of VL1 to VL3 are each input to an optical switch OS1 via an isolator IS2 for preventing return light, and are selected from a predetermined variable wavelength range. A part of the output light of optical switch OS1 is a half mirror.
It is reflected by HM3 and input to optical amplification element OA2.

The output light of the optical amplification element OA2 is input to the optical frequency shifter section 23, and then input to the ultrasonic modulator UM1.
Outputs Bragg's s-order diffracted light. If the frequency of the ultrasonic wave supplied from a reference frequency source such as a crystal oscillator is _ω5 , then the optical frequency of the diffracted light is shifted by _sω5 .

The output light from the optical frequency shifter section 23 enters the optical frequency multiplier section 24 and is connected to an optical waveguide NL using a nonlinear material.
outputs the second harmonic of the input light. i.e.
The tunable wavelength laser diode output of 1560 nm is input through an optical amplifier, and the second harmonic of 780 nm is output. Air− _TiO2 −ZnS using a nonlinear thin film of ZnS and a linear thin film of _TiO2 as waveguides
-Using a four-layer glass slab optical waveguide, nonlinear effects are efficiently generated. Although this embodiment uses second-order harmonics, any n-order harmonics may be used.

The output light of the optical frequency multiplier 24 is transmitted to the optical amplification element OA.
After being amplified in step 3, as described above, the reference wavelength light source unit 1 is sent to the half mirror HM2 as feedback light.
It merges with the output light from.

Through the above operation, the optical frequency ω ₀ of the optical output of the optical frequency PLL unit 2 becomes ω ₀ =(ω _s ±ω ₃ )/n±sω ₅ (note that the signs are not in the same order). However, in this embodiment, the optical frequency multiplication number n=2. In other words, ω ₀ is the absolute wavelength and a highly accurate and highly stable optical frequency ω _s
It becomes an optical frequency that is locked by a predetermined ratio n and further offset by an arbitrary frequency ω ₃ /n or ω ₅ . By sweeping ω ₃ or ω ₅ , highly accurate optical frequency sweeping can be achieved. Here ω ₃ , ω ₅
Since is an electrical signal, high precision and high stability can be easily obtained.

The optical output of the optical frequency PLL section 2 is input to the optical modulation section 3, where it is amplitude modulated by the amplitude modulator AM1, phase modulated by the phase modulator PM1, and then modulated by the polarization modulator LM1.
The direction of polarization is changed by After the optical output of the optical modulator 3 is amplified by the optical amplifying element OA4 of the optical amplifying unit 4,
This is the synthesizer output.

In the above embodiment, the optical amplification elements OA1 to
OA4 uses GaAlAs laser (780nm band) or InGaAsP
Consists of lasers (1500nm band), etc., and the following three
A system can be used.

(a) Called a resonator-type semiconductor laser amplifier, it conducts linear optical amplification through stimulated emission by passing a bias current near the oscillation threshold and inputting signal light into the laser diode.

(b) This is called an optical injection locked amplifier, which controls the optical frequency and phase of the oscillated light by inputting signal light into the oscillating laser diode.

(c) It is called a traveling wave laser amplifier, and both ends of the laser diode chip are coated with anti-reflection coating, and light is amplified only by passing the signal light.

In the above embodiment, the positions of the optical frequency shifter section 23 and the optical frequency multiplier section 24 are swapped, and the frequency ω ₀ of the optical output of the optical frequency PLL section 2 is changed to ω ₀ =(ω _s ±ω ₃ ±sω ₅ )/n.

In addition, in the optical frequency PLL section 2, a mixer circuit
Both the MX1 and the optical frequency shifter section 23 are for adding an offset frequency, and either one can be omitted.

Furthermore, in the optical frequency PLL section 2, if the multiplication number n is set to 1, the optical frequency multiplication section 24 can be omitted.

In addition, in the above embodiment, in the reference wavelength light source section,
Although the absorption line of Rb or Cs is used, it is not limited to these, but any absorption line with high precision and high stability at the absolute wavelength, such as the absorption line of NH ₃ or H ₂ O (1500 nm band)
You can also use In this case, the optical frequency multiplier 24 becomes unnecessary. Although it is possible to stabilize the wavelength by using a known Fabry-Perot resonator as a wavelength detector, the characteristics are better if the quantum standard absorption line as described above is used.

Further, the variable wavelength laser diodes VL1 to VL3 are not limited to ADFBs as in the above embodiments, but may include an external resonator using a diffraction grating added to the outside of the laser diode chip, and rotating the diffraction grating.
It may be possible to use the wavelength selectivity to make the wavelength variable. External cavity laser diodes have the excellent feature of narrow spectrum.

Moreover, as the variable wavelength laser diodes VL1 to VL3, those in which a wavelength selective element is inserted into a resonator as shown in FIG. 5 may be used. In the figure
LD2 is a semiconductor laser, 51 and 52 are anti-reflection coating parts provided at both ends of this semiconductor laser LD2,
LS1 is a lens that converts the light emitted from this non-reflection coating portion 51 into parallel light, and M1 is this lens LS.
1 is a mirror on which the light passing through LS2 is reflected; LS2 is a lens that converts the light emitted from the anti-reflection coating section 52 into parallel light; UM2 is a first ultrasonic modulator into which the light passing through lens LS2 is incident; UM3 is a second ultrasonic modulator into which the light emitted from this ultrasonic modulator UM2 enters, and M2 is this ultrasonic modulator UM.
DR1 is an oscillator that excites the ultrasonic modulators UM2 and UM3 at a frequency F. FIG. 6 is an explanatory diagram showing the wavelength selection and frequency sweeping operations performed by the ultrasonic modulators UM2 and UM3 in the apparatus shown in FIG. 5. Anti-reflection coating part 5 of semiconductor laser LD2
The light emitted from 1 is made into parallel light by lens LS1,
It is reflected by mirror M1. The reflected light from the mirror M1 returns along the optical path and enters the semiconductor laser LD2 again. The light having the frequency f ₀₁ emitted from the anti-reflection coating portion 52 is converted into parallel light by the lens LS2, and is incident on the first ultrasonic modulator UM2. At this time, from the diffraction conditions, the angle of incidence θ _i1 to the diffraction grating 63 generated by the ultrasonic wave 61, the output angle θ ₀₁ after diffraction, the wavelength λ ₀ of the light, and the wavelength Λ ₀ of the ultrasonic wave are determined by the following equation. There is a relationship.

sinθ _i1 + sinθ ₀₁ = λ ₀ / Λ ₀ ...(1) In other words, the wavelength λ ₀ of light passing through an optical path that satisfies the specific incident angle θ _i1 and exit angle θ ₀₁ will change if the ultrasonic wavelength Λ ₀ changes. Change. The emitted light undergoes a Doppler shift due to the ultrasonic wave, and in this case is +1st order diffracted light (the direction of the ultrasonic wave is the same as the diffracted direction), so its frequency becomes f ₀₁ +F. The light emitted from the ultrasonic modulator UM2 is diffracted again by the ultrasonic modulator UM3. Similarly to the above, there is a relationship as shown in the following equation between the incident angle θ _i2 of the ultrasonic wave 62 on the diffraction grating 64, the output angle θ ₀₂ after diffraction, the wavelength λ ₀ of the light, and the wavelength Λ ₀ of the ultrasonic wave. .

sinθ _i2 +sinθ ₀₂ =λ ₀ /Λ ₀ (2) However, in equation (2), the change in λ ₀ due to the Doppler shift of the ultrasonic modulator UM2 is ignored because it is small. Here, the relationship between the ultrasonic traveling wave 62 and the diffracted light is opposite to that in the ultrasonic modulator UM2, and -
Since it is the first-order diffracted light, the Doppler shift amount is -F
Therefore, the frequency of the emitted light from the ultrasonic modulator UM3 is
f ₀₁ +F−F=f ₀₁ . The emitted light from the ultrasonic modulator UM3 is reflected by the mirror M2, travels back along the original optical path, and enters the semiconductor laser LD2 again. When going backwards, the frequency of the emitted light of UM3 becomes f ₀₁ −F due to Doppler shift, and the frequency of the emitted light of UM2 becomes
Since the original frequency f ₀₁ becomes f ₀₁ −F+F=f ₀₁ and returns to the semiconductor laser LD2, the resonance state continues.
In order to increase the diffraction efficiency, the Bragg incidence condition is satisfied, and the following relationship holds between the incident angle θ _i1 , the output angle θ ₀₁ , the incident angle θ _i2 , and the output angle θ ₀₂ when the wavelength of _the ultrasonic wave is Λ 0. That's what I do.

θ _i1 = θ ₀₁ = θ _i2 = θ ₀₂ If you change the wavelength Λ ₀ of the ultrasonic wave with this configuration,
Wavelength λ ₀ of light that resonates while satisfying θ _i1 , θ ₀₁ , θ ₂ , θ ₀₂
can be swept as shown in the following equation.

sinθ _i1 + sinθ ₀₁ = (λ ₀ + Δλ) / (Λ ₀ + ΔΛ) Alternatively, as the variable wavelength laser diodes VL1 to VL3, a device in which an element that can control the refractive index is inserted into the resonator as shown in Fig. 7 may be used. good. The same parts as in FIG. 5 are given the same symbols and their explanation will be omitted. EO1 is an electro-optical element made of LiNbO ₃ (lithium niobate) etc. and has anti-reflection coating on both sides and receives the output light from lens LS2, and 71 is this electro-optical element.
This is the power supply that controls EO1. Semiconductor laser LD2
The emitted light becomes parallel light by lens LS2, passes through electro-optical element EO1, is reflected by mirror M2, and then goes back along the original optical path and returns to the semiconductor laser.
Injects into LD2. As a result, a resonator can be constructed between mirror M1 and mirror M2. If the distance excluding the length l of the electro-optical element EO1 between the mirror M1 and the mirror M2 is L, the refractive index of the electro-optical element EO1 is n, the speed of light is c, and p is an integer, then the oscillation frequency f ₀₂ becomes f ₀₂ =p・c/2(L+n(v)l)...(3). In other words, the electro-optical element is powered by the power source 71.
By changing the electric field strength of EO1, the refractive index n can be changed, resulting in the oscillation frequency f ₀₂
can be swept.

FIG. 8 is a structural block diagram showing a double resonator type tunable wavelength laser diode of FIG. 7. The same parts as in FIG. 7 are given the same symbols and the explanation is omitted. BS1 is a beam splitter that separates the light emitted from lens LS2 into two directions, EO2 is an electro-optical element that receives the light that has passed through this beam splitter BS1, and M2 is this electro-optical element EO.
2, EO3 is an electro-optical element that receives the light reflected by the beam splitter BS1, and M3 is a mirror that reflects the output light of this electro-optical element EO3. Electro-optical element EO2,
The length of EO3 in the optical path direction is l ₁ and l ₂ respectively, the refractive index is n ₁ and n ₂ respectively, the distance excluding l ₁ along the optical path between mirrors M1 and M2 is L ₁ , and the distance between mirrors M1 and M3 is If the distance along the optical path excluding ₂ is L ₂ and q is an integer, then the oscillation frequency f ₀₃ in this case is f ₀₃ = q・c/2 | (L ₁ + n ₁ (V ₁ ) l ₁ ) − (L ₂ + n ₂ (V ₂ ) l ₂ ) | ...(4). Since the denominator of equation (4) can be made smaller than that of equation (3), the variable range of the oscillation frequency can be made larger than in the case of the device shown in FIG.

FIG. 9 is a configuration diagram showing the tunable wavelength laser diode of FIG. 7 integrated on one chip. 91 is a laser diode made of GaAlAs, InGaAsP, etc.; 92 is an optical amplification unit provided at the junction of this laser diode 91; 93 is a waveguide-type external resonator; 94 and 95 are located at both ends of the laser diode 91. The mirror that was born, 96
The laser diode 9 corresponds to the light width portion 92.
An electrode 97 provided on the surface of the laser diode 91 corresponds to the waveguide external resonator 93.
This is an electrode provided on the surface of the A current I _LD is injected into the junction via the electrode 96 to generate laser light in the optical amplification section 92 , and the waveguide external resonator 9
3 through an electrode 97 to change the _refractive index of the waveguide external resonator 93 and sweep the oscillation frequency. Assuming that the lengths along the junction of the optical amplifying section 92 and the waveguide external resonator 93 are l ₃ and l ₄ respectively, and the refractive indexes are n ₃ , n ₄ and r are integers, then the oscillation frequency f ₀₄ is f ₀₄ = r·c/2 (n ₃ l ₃ + n ₄ (I _F ) l ₄ ).

Further, a W-Ni (tungsten, nickel) point contact diode or Josephson element can also be used for the optical heterodyne detection section 21. Since these elements have both multiplier and mixer functions, ω _s ,
ω ₁ and ω ₃ can be input simultaneously, and the mixer circuit MX1 in FIG. 2 becomes unnecessary. In this case, the outputs of these elements, that is, the input signals of the optical frequency modulation circuit FC, are ω ₄ =ω _s −ω ₁ ±mω ₃ (m is a multiplier). It is also possible to set ω ₄ =ω _s −2ω ₁ ±mω ₃ , and in this case, the optical frequency multiplier 24 becomes unnecessary.

FIG. 10 is a block diagram showing another example of the structure of the optical heterodyne detection section 21. OC is a local oscillator with an optical output frequency ω _L using a second wavelength stabilized light source, and OX is the optical output of this local oscillator OC and the optical output of the optical frequency multiplier 24 inputted via the optical amplification element OA3. An optical frequency mixer using a nonlinear optical crystal, OD is an optical frequency mixer using a nonlinear optical crystal.
A PIN that inputs the optical output of OX and the output light from the reference wavelength light source section 1 and outputs it to the variable wavelength light source section 22.
A photodetector consisting of a photodiode or an avalanche photodiode. According to such a configuration, the optical output frequency ω ₆ of the optical frequency mixer OX becomes ω ₆ =ω ₁ +ω _L due to the nonlinear optical effect. In the configuration shown in Figure 2, the optical frequency multiplier allows
Although only a limited ω ₁ determined by ω _s =ω ₁ =nω ₀ can be obtained (aside from the offset frequency), the configuration shown in FIG. 10 can output light of various wavelengths. For example, using the absorption line of Rb, the wavelength of ω _s is λ _s = 780 nm,
If the wavelength of ω _L is chosen as λ _L = 852 nm using the absorption line of Cs, then from the relationship ω _s = ω ₆ when the feedback loop is balanced, the respective wavelengths λ _s of ω _s , ω ₁ , and ω _L ,
Since there is a relationship between λ ₁ and λ _L of 1/λ _s =1/λ ₁ +1/λ _L , λ ₁ =9230 nm.

FIG. 11 is a configuration block diagram showing an optical frequency synthesizer sweeper capable of simultaneously outputting two optical frequencies, which is a third embodiment of the present invention embodying the configuration of FIG. 1. As the reference wavelength light source section 1, a two-wavelength stabilized laser diode using saturation absorption spectroscopy (see the above document) is used. i.e.
LD11 and LD12 are laser diodes that generate laser outputs of different wavelengths, HM4 is a half mirror that combines the outputs of the laser diodes LD11 and LD12, and HM5 is this half mirror HM4.
A half mirror that separates the output light into two directions, CL
is an absorption cell similar to that shown in Fig. 2 where the transmitted light of this half mirror HM5 enters, and HM6 is this absorption cell.
Half mirror where the light emitted from CL enters, IS1
M4 is an isolator for preventing return light that passes the output light of the half mirror HM6, M4 is a mirror that receives the reflected light of the half mirror HM5, and HM7
is a half mirror that receives the reflected light from this mirror M4, LS3 is an aperture that receives the transmitted light from this half mirror HM7, M5 is a mirror that receives the output light of this aperture LS3, and PD11 is a half mirror that receives the output light of this mirror M5. a photodetector incident through the mirror HM6, the absorption cell CL and the half mirror HM5;
PD12 is a photodetector into which the light reflected by the half mirror HM7 enters through the absorption cell CL, A2 is a differential amplifier that calculates the difference between the electrical outputs of the photodetectors PD11 and PD12, and LA1 and LA2 are the differential amplifiers. Input the output of amplifier A2 and connect the laser diode LD.
A lock-in amplifier with a laser diode driving circuit outputs outputs to LD11 and LD12, respectively, and IS1 is an isolator for preventing return light through which the output light of the half mirror HM6 passes.

Only the differences from the device shown in FIG. 2 in the optical frequency PLL section will be described below. MX11 and MX12
are mixers that input the electrical output of the optical heterodyne detection unit 21 and the FM modulation frequencies Ω _A and Ω _B , respectively. In the variable wavelength light source section 22, FC1, FC2
is an optical frequency modulation circuit which inputs the outputs of the mixers MX11 and MX12 and has LPF characteristics,
VL4 and VL5 are the optical frequency modulation circuits respectively
tunable wavelength laser diodes whose oscillation frequencies are controlled by the outputs of FC1 and FC2; IS21 and IS22 are isolators for preventing return light that pass the optical outputs of the tunable wavelength diodes VL4 and VL5, respectively;
OS2 is an optical multiplexer that receives and combines the optical outputs of the isolators IS21 and IS22. The other parts are the same as the configuration shown in FIG.

The operation of the device having such a configuration will be explained next.
The oscillation frequencies of the optical outputs of the two laser diodes LD11 and LD12 are assumed to be ω _A +Ω _A and ω _B +Ω _B. These two beams are combined by a half mirror NH4 and then separated into two directions by a half mirror HM5. The light transmitted through the half mirror HM5 is absorbed into the absorption cell as saturated light.
After passing through CL, it passes through half mirror HM6 and is output to the optical frequency PLL section via isolator IS1. The reflected light from half mirror HM5 is reflected by mirror M4 and separated into two directions by half mirror HM7. The light transmitted through the half mirror HM7 is focused by the aperture LS3 and then reflected by the half mirror HM6, becoming a probe light that is much narrower than the saturated light and entering the absorption cell CL, where the Doppler spread is accompanied by a sharp depression due to the saturation effect. After being absorbed, it is reflected by half mirror HM5 and enters photodetector PD11. The light reflected by the half mirror HM7 enters the absorption cell CL from the vertical direction as a reference light, undergoes absorption with Doppler spread, and then enters the photodetector PD12. The differential amplifier A2 calculates the difference between the electrical outputs of the photodetectors PD11 and PD12, and sends the difference signal output to the two lock-in amplifiers LA1 and
Input to LA2. Lock-in amplifier LA1 is Ω _A
By using synchronous rectification as a reference frequency and controlling the laser diode LD11 by detecting only the Ω _A component, for example, in the absorption line of F=1 in Fig. 3, it is possible to detect the ultrafine structure of the absorption line hidden by the Doppler shift. Lock at the center of any one of r to t in Fig. 4. Similarly, the lock-in amplifier LA2 performs synchronous rectification using Ω _B as a reference frequency, detects only the Ω _B component, and controls the laser diode LD12. It locks at the center of any one of o to q in FIG. 4, which is the absorption line of the ultrafine structure. In this way, the oscillation frequency ω _A +Ω _A , ω _B
A two-wavelength stabilized light source of +Ω _B can be obtained. The two-wavelength reference optical output output from the reference wavelength light source section 1 is input to the optical frequency PLL section, and the optical heterodyne detection section 2
1, optical heterodyne detection is performed together with the optical output from the optical frequency multiplier 24, and the frequency is |ω _A −ω _1A +Ω _A
|, |ω _B −ω _1B +Ω _B |, |ω _A −ω _B +Ω _A +Ω _B |, |
Detection outputs of ω _A −ω _1B +Ω _A |, |ω _B −ω _1A +Ω _B | (ω _1A and ω _1B are two frequencies of the optical output of the optical frequency multiplier 24) are obtained. When the optical frequency PLL unit 2 is operating, ω _A ω _1A , ω _B ω _1B , and Ω _A and Ω _B are several k.
Hz, ω Since the difference between _A and ω _B is 6.8 _GHz as shown in Figure 4, by giving the photodetector PD2 a low-pass characteristic, |ω _A −ω _1A +Ω _A |, |ω _B −ω _1B +
Ω _B | Only frequency components can be extracted. 2
The two mixer circuits MX11 and MX12 mix the output electrical signal of the optical frequency heterodyne detection unit 21 with input signals of frequencies Ω _A and Ω _B , respectively, and output signals ω _4A =|ω _A −ω _1A |, ω _4B =|, respectively. ω _B −
ω _1B | is generated. In the variable wavelength light source section 22, the two optical frequency modulation circuits FC1 and FC2 are configured using variable wavelength diodes so that the output signals ω _4A and ω _4B of the mixer circuits MX11 and MX12 are 0, respectively.
Controls the oscillation frequency of VL4 and VL5. The optical outputs of the variable wavelength diodes VL4 and VL5 are sent to the optical multiplexer OS2 via isolators IS21 and IS22, respectively.
and synthesized into two optical frequencies ω _A /n±
A light output consisting of sω ₅ , ω _B /n±sω ₅ is generated.
This optical output is not FM modulated at frequencies Ω _A and Ω _B.

In the above embodiment, the case of a synthesizer sweeper with two frequencies is shown, but the present invention is not limited to two frequencies, but can be similarly applied to any plurality of frequencies.

In addition, in the above embodiment, the reference wavelength light source section 1 uses the saturated absorption method, but the absorption center of F=1 and F=2 in FIG.
The wavelength may also be locked. In this case, the reference wavelength light source section 1 shown in Fig. 2 inputs the light incident on the absorption cell CL to
The reference wavelength light source section 1 shown in FIG. 11 uses a light beam and two lock-in amplifiers.

Furthermore, in Fig. 11, only the ultrasonic modulator UM1 is used for offset and sweeping of the optical frequency, but shift frequencies ω _3A and ω _3B are used instead of the input frequencies Ω _A and Ω _B of the mixer circuits MX11 and MX12. The added ω _3A +Ω _A and ω _3B +Ω _B may be used. in this case,
The two optical frequencies of the optical output are respectively (ω _A ±
ω _3A )/n±sω ₅ and (ω _B ±ω _3B )/n±sω ₅ , so two frequencies can be swept simultaneously at ω ₅ , and two frequencies can be swept simultaneously by sweeping ω _3A and ω _3B independently. can also be swept independently.

<<Effects of the Invention>> As shown in the above embodiments, the optical frequency synthesizer/sweeper of the present invention is capable of locking its optical output to the absorption lines of Rb, Cs, etc. at the absolute wavelength with high precision and high stability. quantum standards with stability above 10 ^-12 (traditional frequency standards are Cs (9GHz), Rb
(using 6GHz microwave resonance) can be obtained.

Furthermore, since an ADFB or external cavity type laser diode with a long cavity length is used as the variable wavelength laser diode, the Q of the cavity is high and the oscillation spectrum width can be narrowed.

Also, since it uses the principle of optical frequency PLL,
Highly accurate optical frequency sweep is possible.

In addition, by using Rb absorption lines (780nm, 795nm) and the doubling method, it is possible to output light in the 1500nm band, which has the lowest optical transmission loss among optical communication fibers, with high precision and stability, making it dual-purpose. Excellent.

With the configuration shown in FIG. 10, various optical frequencies can be output.

Furthermore, the configuration shown in FIG. 11 allows a plurality of optical frequencies to be output simultaneously and swept independently.

Further, as in the configuration shown in FIG. 11, unnecessary FM modulation components can be removed from the optical output. Even in the case of Figure 2, ω ₃ ′ = ω ₃ + Ω (Ω is the FM modulation frequency when using a lock-in amplifier) is set in the mixer circuit.
It can be removed in the same way by inputting it into MX1.

As described above, according to the present invention, it is possible to realize an optical frequency synthesizer/sweeper that can obtain a coherent optical output with high optical frequency accuracy, high stability, and high spectral purity with a simple configuration.

[Brief explanation of drawings]

FIG. 1 is a configuration block diagram showing the basic configuration of one embodiment of the present invention, FIG. 2 is a configuration block diagram showing a second embodiment of the present invention embodying the configuration of FIG. 1,
Fig. 3 is a characteristic curve diagram for explaining the operation of the apparatus shown in Fig. 2, Fig. 4 is an explanatory diagram for explaining the operation of the apparatus shown in Fig. 2, and Figs. 2
A configuration explanatory diagram showing another embodiment of the tunable wavelength laser diode in the figure, FIG. 6 is an operation explanatory diagram for explaining the operation of the device in FIG. 5, and FIG. 10 is a partial modification of the device in FIG. 2. FIG. 11 is a configuration block diagram showing a third embodiment of the present invention, and FIGS. 12 and 13 are principle explanatory diagrams showing a conventional tunable wavelength laser light source. be. 1... Reference wavelength light source section, 2... Optical frequency PLL
section, 3... optical modulation section, 4... optical amplification section.

Claims (1)

[Claims] 1. A reference wavelength light source section whose output light wavelength is controlled to be constant, and an optical frequency PLL which controls the wavelength of the optical output to a wavelength corresponding to the oscillation wavelength of the reference wavelength light source section.
The optical frequency PLL section includes a variable wavelength light source section, and a light source that inputs light related to the output light of the variable wavelength light source section and converts the optical frequency into another optical frequency corresponding thereto. A frequency converter inputs light related to the output light of the optical frequency converter and the output light of the reference wavelength light source, and changes the oscillation wavelength of the output light of the variable wavelength light source by a signal related to the electrical output. What is claimed is: 1. An optical frequency synthesizer/sweeper, comprising: an optical heterodyne detection section for controlling the optical frequency PLL section; 2. The optical frequency synthesizer according to claim 1, wherein the reference wavelength light source section has a plurality of oscillation wavelengths, and the optical frequency PLL section controls the wavelengths of the optical output to wavelengths corresponding to the plurality of oscillation wavelengths. Sweeper. 3. The optical frequency synthesizer/sweeper according to claim 1, wherein the reference wavelength light source section uses a laser diode whose oscillation wavelength is controlled based on the absorption spectrum of Rb atoms or Cs atoms. 4 D2 of Rb atom (780nm) as reference wavelength light source part
A patent claim in which an optical frequency PLL section outputs light in a wavelength band twice that of each oscillation wavelength, using a laser diode whose oscillation wavelength is controlled to absorb at least one of the absorption spectra of the line and the D1 line (795 nm). The optical frequency synthesizer/sweeper according to item 1. 5. The optical frequency synthesizer sweeper according to claim 1, wherein the optical frequency conversion section includes an optical frequency shifter section that shifts the frequency of light related to the output light of the variable wavelength light source section. 6. The optical frequency synthesizer/sweeper according to claim 1, wherein the optical frequency conversion section includes an optical frequency multiplication section that multiplies the frequency of light related to the output light of the variable wavelength light source section. 7. A patent in which the optical frequency PLL section includes a mixer circuit that inputs the electrical output of the optical heterodyne detection section, and the oscillation wavelength of the output light of the variable wavelength light source section is controlled by a signal related to the electrical output of the mixer circuit. An optical frequency synthesizer sweeper according to claim 1. 8. Claims 5, 6, and 8, wherein the tunable wavelength light source section includes a tunable wavelength laser diode in a resonator that has wavelength selectivity by utilizing light diffraction by an ultrasonic modulator. The optical frequency synthesizer/sweeper according to item 7. 9. An optical frequency synthesizer/sweeper according to claims 5, 6, and 7, comprising a tunable laser diode in which the tunable wavelength light source section can control the refractive index of the optical path within the resonator. . 10 A reference wavelength light source section in which the wavelength of output light is controlled to be constant, and an optical frequency that controls the wavelength of the optical output to a wavelength corresponding to the oscillation wavelength of this reference wavelength light source section.
and a PLL section, and the optical frequency PLL
The section includes a variable wavelength light source section, an optical frequency conversion section that inputs light related to the output light of the variable wavelength light source section and converts the optical frequency to another corresponding optical frequency, and the optical frequency conversion section. an optical heterodyne detection section that receives light related to the output light of the variable wavelength light source section and the output light of the reference wavelength light source section, and controls the oscillation wavelength of the output light of the variable wavelength light source section by a signal related to the electrical output thereof. , the output wavelength of the optical frequency PLL section is made variable based on the output wavelength of the reference wavelength light source section,
Further, it includes an optical modulation section that modulates the optical output of the variable wavelength light source section, and an optical amplification section that amplifies the optical output of the optical modulation section, and the wavelength of the optical output can be varied by a signal input to the optical modulation section. An optical frequency synthesizer/sweeper characterized by being configured as follows.