JPH0523613B2 - - Google Patents

Description

[Detailed Description of the Invention] <<Industrial Application Field>> The present invention is an optical fiber, an optical waveguide, a wavelength demultiplexer, an optical switch, an optical component for measuring optical transmission characteristics, optical reflection characteristics, etc. of optical components such as OEIC. Regarding frequency network analyzers.

<<Prior Art>> FIG. 12 is a block diagram showing a conventional optical fiber loss wavelength characteristic measuring device. variable wavelength light source
The output light from VL enters the fiber to be measured MF, and after the output light is detected by a photodetector PD, it is output to the amplification/display means DP. The wavelength characteristics of optical fiber loss are measured from the change in optical power when the output wavelength of the variable wavelength light source VL is swept.

FIG. 13 is a block diagram showing a conventional optical fiber wavelength dispersion characteristic measuring device. variable wavelength light source
VL and the reference wavelength light source SL are amplitude modulated by a frequency modulation signal source E. Measured fiber MF and reference wavelength light source to which the output light of variable wavelength light source VL is added
The output optical power of the reference fiber SF to which the output light of the SL is added is detected by the optical detection unit PD, and the phase difference between the frequency components of both is detected by the phase measurement unit PS, thereby determining the propagation delay with respect to the wavelength of the fiber under test MF. Measure time.

<<Problems to be Solved by the Invention>> However, the measuring instrument configured as described above has a drawback in that it cannot measure the phase propagation characteristics of light with high precision. Also, it can measure long optical paths such as fibers, but it cannot measure short waveguides. Performance tests of optical fibers, optical waveguides, wavelength demultiplexers, optical switches, OEICs, etc., which are important components of future coherent optical application technology, require measurement of propagation characteristics (loss, gain, phase, delay) and reflection characteristics. Although important, the above-mentioned measuring instruments are insufficient.

The present invention was made to solve these problems, and an object of the present invention is to realize an optical frequency network analyzer that can measure amplitude, phase characteristics, etc. with high precision.

<Means for Solving the Problems> The present invention provides a first optical output that sweeps the frequency and a second optical output that shifts the first optical output by a certain frequency.
an optical frequency sweeper that generates a light output and emits a first light output to a measurement target;
The amplified signal and the first signal reflected from the measurement target
The optical frequency of the first and second optical outputs obtained by inputting the optical output and the second optical output, which are signals obtained by deflecting and amplifying the optical output of A first electrical signal obtained by converting a signal having a frequency corresponding to the difference into an electrical signal by heterodyne detection, and the optical output reflected from the measurement target and the second optical output are both obtained by inputting the optical frequency. a first optical heterodyne detection unit that outputs a second electrical signal obtained by converting a signal having a frequency corresponding to the difference in frequency into an electrical signal by heterodyne detection, and inputs the first optical output and the second optical output; and a second optical heterodyne detection unit that converts a signal having a frequency corresponding to the difference between the optical frequencies of the two optical signals obtained by inputting these input signals into an electrical signal by heterodyne detection and outputs the electrical signal, a first filter section that receives first and second electrical signals output from the first optical heterodyne detection section and amplifies each signal; a second filter section for inputting and amplifying a signal; a comparing means for comparing the electrical signal from the first filter section and the electrical signal from the second filter section and outputting the result as an electrical signal; An optical frequency network analyzer is characterized in that it is equipped with a signal processing means that inputs the electrical signal output of the comparison means and processes the signal, and is capable of simultaneously measuring the propagation characteristics and reflection characteristics of the measurement target with ease and high precision. be.

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

FIG. 1 shows an optical frequency network according to the present invention.
1 is a configuration block diagram showing one embodiment of an analyzer. FIG. 1 is an optical frequency sweeper described later (FIGS. 2 to 11) that generates frequency-sweeping output light; 23;
2 is an optical heterodyne detection section which inputs the first and second output lights of this optical frequency sweeper 1; 24 is a filter section consisting of a bandpass filter which inputs the electrical output of this optical heterodyne detection section 23; An optical directional coupler that inputs the first output light of the sweeper 1; 3 an output end that outputs the output light from the optical directional coupler 2; and 10, a measurement unit that inputs the output light from the output end 3. The object, 4, is an incident end through which the emitted light from the measurement object 10 is incident, 41
42 is a polarization control unit using a magneto-optic effect crystal (YIG, lead glass, etc.) that inputs the incident light from the incident end 4; 42 is an optical amplification unit that inputs the output light of this polarization control unit 41; The optical amplifying section 42 and the second optical frequency sweeper 1 are composed of a PIN photodiode, an avalanche photodiode, etc.
an optical heterodyne detection unit which inputs bits of 44;
Reference numeral 45 denotes a filter section consisting of a bandpass filter that inputs and amplifies the electrical output of the optical heterodyne detection section 43, 45 an amplitude comparator section that inputs the electrical outputs from the filter sections 44 and 24, and 46 the filter section 44 and 24. A phase comparator 31 receives the electrical output from the optical directional coupler 2;
32 is a polarization control section similar to this polarization control section 31.
an optical amplifying section similar to 42 which inputs the output light of 33;
3 is an optical heterodyne detection unit similar to 43 which inputs the optical amplification unit 32 and the second output light of the optical frequency sweeper 1, and 3 is a bandpass similar to 44 which inputs the electrical output of this optical heterodyne detection unit 33. A filter unit consisting of a filter; 35 is an amplitude comparison unit similar to 45 that inputs the electrical outputs from the filter units 34 and 24; 36 is a phase comparison unit similar to 46 that inputs the electrical outputs from the filter units 34 and 24; section, 50 is the amplitude comparison section 35,
45 and a signal processing/display section into which the electrical outputs of the phase comparators 36 and 46 are input. 33, 43 are the first optical heterodyne detection sections; 34, 44 are the first optical heterodyne detection sections;
23 is the second optical heterodyne detection section, 24 is the second filter section, 35, 3 is the second filter section.
Reference numerals 6, 45, and 46 constitute comparison means, and 50 constitutes signal processing means, respectively. Optical amplification section 32, 42
is composed of a GaAlAs laser (780 nm band) or an InGaAsP laser (1500 nm band), and the following three types can be used.

(a) This is called a resonator-type semiconductor laser amplifier, and it performs linear optical amplification through stimulated emission by flowing 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 a signal light into an 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.

The operation of the optical frequency network analyzer configured as described above will now be described in detail.

The optical frequency sweeper 1 frequency-sweeps the optical output and outputs it with high accuracy, high stability, and high spectral purity (details will be described later). Frequency of optical frequency sweeper 1 ω ₀
The first optical output of is incident on the measurement object 10 via the optical directional coupler 2 and the output end 3, and the measurement object 10
The emitted light passes through the input end 4 to the polarization control unit 41
Enter. The polarization control unit 41 controls the polarization plane of the input light to be the same polarization plane as the locally oscillated light (the second optical output) by controlling the applied magnetic field using the optical rotation of the magneto-optic effect crystal. do. The optical output of the polarization control section 41 is amplified by the optical amplification section 42 and then combined with the local oscillation light from the optical frequency sweeper 1 by a half mirror or the like (not shown), and the optical heterodyne detection section 43 detects the difference between the two frequencies ( ω ₀ +Δω)−ω ₀
is converted into an electrical signal with a frequency of =Δω. The electrical output of the optical heterodyne detection section 43 is transmitted to the filter 4.
A part of the signal passes through the bandpass characteristic of No. 4. Also, the first output light (frequency ω ₀ ) from the optical frequency sweeper 1
is a direct local oscillation light (frequency ω ₀
+Δω), and the optical heterodyne detection section 23
It is converted into an electrical signal with a frequency equal to the difference Δω between the two frequencies. Part of the electrical output of the optical heterodyne detection unit 23 passes through the bandpass characteristic of the filter 24 and becomes a reference signal. The electric signal output of the filter 44 that has been affected by the characteristics of the object to be measured and the reference signal output of the filter 24 that has not been affected by the characteristics of the object to be measured are compared in amplitude by an amplitude comparison section 45, and are then compared in amplitude by a phase comparison section 46. The phases of the two are compared. Amplitude comparison section 45 and phase comparison section 4
The electrical output of 6 is subjected to signal processing in a signal processing/display unit 50, and as a result, the propagation characteristics of the measurement target are displayed. The reflected light output from the optical coupler 2 from the measurement target 10 via the output end 3 is also controlled by the polarization control unit 31,
The optical amplification section 32, optical heterodyne detection section 33, filter 34, amplitude comparison section 35, phase comparison section 36, and signal processing/display section 50 perform similar processing, and as a result, the reflection characteristics of the measurement target are displayed. When an optical waveguide is the object of measurement, the propagation loss of the waveguide, the wavelength characteristics of the phase difference, etc. can be measured. When an optical fiber is to be measured, a fiber with short wavelength characteristics such as propagation loss and delay can be used for measurement. When measuring a laser diode optical amplifier, wavelength characteristics of amplification gain, phase delay, etc. can be measured. Also, the reflection loss at the optical connection point can be measured from the characteristics of the reflected light.

According to the optical frequency network analyzer having such a configuration, amplitude, phase, wavelength characteristics, etc. can be measured with high precision.

Also, the propagation characteristics of the measurement target (loss, phase, delay, etc.)
Gain, etc.) and reflection characteristics can be measured simultaneously and easily.

Note that a W-Ni (tungsten, nickel) point contact diode or Josephson element can also be used for the optical heterodyne detection sections 23, 33, and 43.

Further, in the above embodiment, the filter portions 24, 3
A bandpass filter was used as 4 and 44, but
The present invention is not limited to this, and a low-pass filter may be used. In that case, Δω=0.

FIG. 2 is a configuration block diagram showing an optical frequency synthesizer sweeper which is an example of the configuration of the optical frequency sweeper 1 shown in FIG. 11 is a reference wavelength light source section whose wavelength is stabilized, and 12 is this reference wavelength light source section 1.
1 is an optical frequency PLL section that inputs the output light of the optical frequency PLL section 12, 13 is an optical modulation section that modulates the output light of this optical frequency PLL section 12, 14 is an optical amplification section that amplifies the output light of this optical modulation section 13, and 15 is the aforementioned optical frequency PLL section. This is an optical frequency shifter section that shifts the output frequency of the optical amplification section 14. In the optical frequency PLL section 12, 121 is an optical heterodyne detection section which receives the output light of the reference wavelength light source section 11 as one input, and 122 is a variable wavelength whose oscillation wavelength of the output light is controlled by the output of this optical heterodyne detection section 121. A light source section, 123 is an optical frequency shifter section that shifts the frequency of the output light of the variable wavelength light source section 122, and 124 is an optical frequency shifter section that multiplies the frequency of the output light of the optical frequency shifter section 123 and transmits the output light to the optical heterodyne detection section. 121 is an optical frequency multiplier that receives the other input.

The operation of the device having such a configuration will be explained next.
When the output light from the reference wavelength light source section 11 is input to the optical frequency PLL section 12, the optical frequency PLL section 12 fixes (locks) the wavelength of the optical output to the wavelength corresponding to the oscillation wavelength of the reference wavelength light source section 11. In other words, the optical heterodyne detection section 121 uses the reference wavelength light source section 11.
The output light from the optical frequency multiplier 124 is compared with the output light from the optical frequency multiplier 124, and the variable wavelength light source 122 is controlled so as to reduce the difference. The optical frequency shifter section 123 in the feedback circuit is the variable wavelength light source section 1.
By adding an offset frequency to the output light of 22, the optical frequency multiplier 124 determines the ratio of the output optical frequency of the variable wavelength light source 122 and the output optical frequency of the reference wavelength light source 11. The optical modulation section 13 is the optical frequency PLL section 12
The optical amplifying section 14 modulates the output light of the optical frequency PLL section 12, and the optical modulating section 13 modulates the output light of the optical frequency PLL section 12.
The optical amplification section 14 amplifies the output light of the optical modulation section 13 to generate an output of the optical frequency synthesizer sweeper (as a first optical output), and the optical frequency shifter section 15 amplifies the output light of the optical amplification section 14. The output light whose frequency is shifted by Δω is generated (as a local oscillation light output).

FIG. 3 is a block diagram showing a further embodiment of the configuration shown in FIG. In the reference wavelength light source section 11, 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 HM1
An isolator for preventing return light, OA1, through which the transmitted light passes through, is an optical amplification element into which the light that has passed through the isolator IS1 is input. In the optical frequency PLL section 12, HM2 is the reference wavelength light source section 11.
PD2 constitutes the optical heterodyne detection section 121 and inputs the transmitted light of the half mirror HM2. EC is a photodiode consisting of a PIN photodiode, avalanche diode, etc., and EC receives the reference frequency from a crystal or the like. The oscillator MX1, which receives input and generates an electrical signal of a predetermined frequency, is a mixer (mixing) circuit to which the electrical output of the oscillator EC and the electrical output of the optical heterodyne detection section PD2 are connected. This mixer (mixing)
Variable wavelength light source section 12 to which the output of circuit MX1 is connected
2, EC is an optical frequency modulation circuit to which the output of the mixer circuit MX1 is connected, VL1 to VL3 are variable wavelength laser diodes to which the output of this optical frequency modulation circuit FC is input, and IS2 is YIG (yttrium iron garnet). OS1 is an isolator through which the output light from the variable wavelength laser diodes VL1 to VL3 passes, and is an optical switch through which light that has passed through a plurality of (three in FIG. 3) isolators IS2 enters. HM3 is this light switch
A half mirror into which the output light of OS1 enters, OA2 is an optical amplification element into which the incident light of this half mirror HM3 is input, and UM1 constitutes an optical frequency shifter section 123, and ultrasonic modulation which inputs the output light of the optical amplification element OA2. NL is an optical waveguide using a nonlinear material that constitutes an optical frequency multiplier section and inputs the output light of this optical frequency shifter section, and OA3 is an optical amplification element that amplifies the output light of this optical waveguide NL. The optical frequency
In the optical modulation section 13 into which the output light of the PLL section 12 is input, AM1 and PM1 are phase modulators such as amplitude modulators each using an electro-optic crystal such as LiNbO ₃ , and LM1 is a phase modulator such as an amplitude modulator using a magneto-optic crystal such as YIG. It is a polarization modulator. OA4 is an optical amplification element that constitutes the optical amplification section 14 and amplifies the output light of the optical modulation section 13. The optical frequency shifter section 15 is composed of an ultrasonic modulator similar to 123.

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

The reference wavelength light source section 11 is as described below.
The oscillation wavelength of the laser diode is controlled by the absorption line of the Rb (or Cs) atom to achieve high precision and high stability (10 ^-12 or more) at the absolute wavelength. The output light of laser diode LD1 is absorbed when the wavelength of the output light of LD1 matches the absorption line of Rb gas (or Cs gas) when passing through absorption cell CL, as shown in the characteristic curve diagram in Figure 4A. Absorption characteristics appear. Fifth
The figure is an explanatory diagram showing the energy level of Rb gas.
The absorption lines of Rb are 780 nm for the D ₂ line and 795 nm for the D ₁ line, and when multiplied by 2, they become 1560 nm and 1590 nm, respectively, which is convenient because they coincide 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 optical applied 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 detects a laser diode in accordance with the output of the photodetector PD1.
By controlling the current of LD1, the absorption center
Lock the output wavelength of LD1. For example, if you want to lock to point a in FIG. 4A, control circuit A1 uses lock-in up, etc. to lock the differential waveform in FIG. 4A to point b in FIG. 4B (point where the differential waveform value becomes 0). ). This method is called the linear absorption method, and the absorption spectrum becomes thicker as shown in Figure 4A, but the saturated absorption method (Hori, Kadota, Kitano, Yabusaki, Ogawa:
Frequency stabilization of semiconductor lasers using saturated absorption spectroscopy, IEICE Technical Report OQE82-116) detects absorption lines in the ultrafine structure hidden by Doppler shift, and locks the oscillation wavelength of laser diode LD1 to this. It becomes even more stable. Note that the temperature of the laser diode LD1 is stabilized within a constant temperature range.
The light transmitted through half mirror HM1 is an isolator
Enter 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 isolator IS1 is transmitted to optical amplification element OA1 as needed.
is amplified.

As described below, the optical frequency PLL section 12
It has a function of locking the oscillation wavelength of the variable wavelength light source section 122 to the oscillation wavelength of the reference wavelength light source section 11 at a predetermined ratio and a predetermined offset. The output light of the reference wavelength light source section 11 is a half mirror HM2.
and enters the photodiode PD2 of the optical heterodyne detection section 121. Optical frequency multiplier 1
The feedback light from 24 is also passed through optical amplification element OA3.
After being reflected by half mirror HM2, it enters photodiode PD2. Reference wavelength light source section 1
1 and the optical frequencies of the feedback light are respectively ω _s and ω ₁ , the frequency ω ₂ of the output electrical signal of the optical heterodyne detection section 121 is ω ₂ = |ω _s −
ω ₁ | If the output frequency of the oscillator EC is ω ₃ , then the output frequency of the mixer circuit (phase detection circuit) MX1 is ω ₄
The offset frequency is added to the output frequency ω ₂ of the optical heterodyne detection section 121, and ω ₄ =ω ₂ −ω ₃
becomes. The output electrical signal ω ₄ of the mixer circuit MX1 is input to the optical frequency modulation circuit FC of the variable wavelength light source section 122, and the optical frequency modulation circuit FC adjusts the optical frequency of the variable wavelength laser diodes VL1 to VL3 so that ω ₄ =0. Control. 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. DFB
(Distributed Feedback) Laser and DBR
(Distributed Bragg Reflector) A type of laser.
ADFB (Acoustic DFB) Laser (Yamanishi
M, et.al.: GaAs Acoustic Distributed
Feedback Lasers, Jpn.J.Appl.Phys., Suppl.18
-1, p.355, 1979). The ADFB laser generates a surface acoustic wave (SAW) perpendicular to the diffraction grating in the DBR laser, and the SAW and the diffraction grating built into the chip form a ring resonator of light through black diffraction.
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 1560 nm.
DFB, DBR, and ADFB lasers with long cavity lengths have the advantage of narrow oscillation spectra and good spectral purity. If the tunable wavelength range of one ADFB laser is insufficient, use multiple ADFB lasers as shown in Figure 3.
ADFB lasers (VL1 to VL3) are used and can be switched using an optical switch or optical multiplexer. That is, the output lights of the variable wavelength laser diodes VL1 to VL3 are inputted to the optical switch OS1 via the isolator IS2 for preventing return light, respectively, and are selected within a predetermined variable wavelength range. A part of the output light from the optical switch OS1 is reflected by the half mirror HM3 and input to the optical amplification element OA2.

The output light of the optical amplification element OA2 is input to the optical frequency shifter section 123, 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 123 enters the optical frequency multiplier 124 and is connected to an optical waveguide using a nonlinear material.
NL 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 −TiO ₂ −ZnS − using a nonlinear thin film of ZnS and a linear thin film of TiO ₂ as waveguides
A four-layer glass slab optical waveguide is used to efficiently generate nonlinear effects. Although this embodiment uses second-order harmonics, any n-order harmonics may be used.

The output light of the optical frequency multiplier 124 is transmitted through an optical amplification element.
After being amplified by the OA3, as described above, it is combined with the output light from the reference wavelength light source section 11 by the half mirror HM2 as feedback light.

Through the above operation, the optical frequency ω ₀ of the optical output of the optical frequency PLL unit 12 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 12 is transmitted to the optical modulation section 13.
input, amplitude modulated by amplitude modulator AM1, phase modulated by phase modulator PM1, and polarization modulator
LM1 changes the polarization direction. The optical output of the optical modulation section 13 is amplified by the optical amplification element OA4 of the optical amplification section 14, and then becomes a synthesizer output (first optical output). Further, the output frequency of the optical output of the optical amplification element OA4 is shifted by Δω by the ultrasonic modulator of the optical frequency shifter section 15, and is output as locally oscillated light (second optical output) with a frequency of ω ₀ +Δω.

In the above configuration example, optical amplification elements OA1 to
The OA 4 uses the same amplification sections 32 and 42 as described above.

In the above configuration example, the positions of the optical frequency shifter section 123 and the optical frequency multiplier section 124 are swapped, and the frequency ω ₀ of the optical output of the optical frequency PLL section 12 is changed to ω ₀ (ω _s ±ω ₃ ±sω ₅ )/ It may be n.

Further, in the optical frequency PLL section 12, the mixer circuit MX1 and the optical frequency shifter section 123 are both for adding an offset frequency.
Either one can also be omitted.

In addition, in the optical frequency PLL section 12, the multiplication number n
If is set to 1, the optical frequency multiplier 124 can be omitted.

Further, in the above configuration example, the reference wavelength light source section 11 uses an absorption line of Rb or Cs, but this is not limited to these, and any absorption line with high precision and high stability at an absolute wavelength can be used, such as NH ₃ or H ₂ Absorption line of O (1500nm
Obi) can also be used. In this case, the optical frequency multiplier 124 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.

Also, in the device shown in Fig. 3, instead of ω ₃ ω ₃ ′=ω ₃ +
If a frequency signal of Ω (Ω is the FM modulation frequency when a lock-in amplifier is used in the reference wavelength light source section 11) is input to the mixer circuit MX1, the optical frequency
Unnecessary FM modulation components can be removed from the optical output of the PLL section 12.

Further, the variable wavelength laser diodes VL1 to VL3 are not limited to the ADFB as in the above configuration example, but can be made by adding an external resonator using a diffraction grating outside 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. 6 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. 7 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. 6. 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 a frequency of ₀₁ emitted from the anti-reflection coating portion 52 is converted into parallel light by a lens LS2, and enters the first ultrasonic modulator UM2. At this time, from the diffraction conditions, the angle of incidence θ _i1 of the ultrasonic wave 61 to the diffraction grating 63, 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 the light that passes through the optical path that satisfies the specific incident end angle θ _i1 and exit angle θ ₀₁ is the wavelength Λ ₀ of the ultrasonic wave. If it changes, it will change. The emitted light undergoes a Doppler shift due to the ultrasound, and in this case it becomes +1st order diffracted light (the direction of the ultrasound and the direction of diffraction are the same)
Therefore, its frequency is ₀₁ +F. The light emitted from the ultrasonic modulator UM2 is the ultrasonic modulator UM.
It diffracts again at 3. 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
₀₁ +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 from UM3 becomes ₀₁ -F due to the Doppler shift, and the frequency of the emitted light from UM2 becomes ₀₁ -F + F = ₀₁ , which returns to the original frequency of ₀₁ and returns to the semiconductor laser LD2, so the resonance state continues. do.
In addition, in order to increase the diffraction efficiency, the black incidence condition is satisfied, and when the wavelength of the ultrasonic wave is Λ ₀ , 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 . 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 of light that resonates while satisfying θ _i1 , θ ₀₁ , θ _i2 , θ _02
0
can be swept as shown below.

sinθ _i1 + sinθ ₀₁ = (λ ₀ + Δλ) / (Λ ₀ + ΔΛ) Alternatively, as the variable wavelength laser diodes VL1 to VL3, as shown in Fig. 8, an element that can control the refractive index may be inserted into the resonator. good. The same parts as in FIG. 6 are given the same symbols and their explanations 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 XS2, 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. Let L be the distance between mirror M1 and mirror M2 excluding the length l of electro-optic element EO1 along the optical path, n be the refractive index of electro-optic element EO1, c be the speed of light, and p be an integer, then the oscillation frequency ₀₂ is ₀₂ =pC/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, and as a result, the oscillation frequency ₀₂
can be swept.

FIG. 9 is a structural block diagram showing a double resonator type tunable wavelength laser diode of FIG. 8. The same parts as in FIG. 8 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 l ₂ is L ₂ and q is an integer, then the oscillation frequency ₀₃ in this case is 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. 10 is a configuration diagram showing the tunable wavelength laser diode of FIG. 8 integrated on one chip. 91 is a laser diode made of GaAlAs, InGaAsP, etc.; 92 is an optical amplification section provided at the junction of this laser diode 91; 93 is a waveguide external resonator; 94 and 95 are located at both ends of the laser diode 91. The mirror that was created, 9
Reference numeral 6 designates an electrode provided on the surface of the laser diode 91 corresponding to the optical amplifying section 92, and reference numeral 97 represents an electrode provided on the surface of the laser diode 91 corresponding to the waveguide type external resonator 93. The junction _ILD is injected through the electrode 96 to generate laser light in the optical amplification section 92, and the waveguide-shaped external resonator 93
A current I _F is passed through the electrode 97 to change the refractive index of the waveguide external resonator 93 and sweep the oscillation frequency. Optical amplification section 92 and waveguide external resonator 9
If the length along the junction of 3 is l ₃ and l ₄ respectively, the refractive index is n ₃ and n ₄ , and r is an integer, then the oscillation frequency ₀₄ is 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 121. These elements have both multiplier and mixer functions, so ω _s , ω ₁ , and ω ₃ can be input simultaneously, and the third
The mixer circuit MX1 in the figure becomes unnecessary. In this case, the output of these elements, ie, the input signal of the optical frequency modulation circuit FC, is ω ₄ =ω _s −ω ₁ ±mω ₃ (m is a multiplication number). Furthermore, it is also possible to set ω ₄ =ω _s −2ω ₁ ±mω ₃ , and in this case, the optical frequency multiplier 124 becomes unnecessary.

FIG. 11 is a block diagram showing another example of the configuration of the optical heterodyne detection section 121. OC is second
A local oscillator with an optical output frequency ω _L using a wavelength stabilized light source, OX is a nonlinear optical crystal into which the optical output of the local oscillator OC and the optical output of the optical frequency multiplier 124 are inputted via the optical amplification element OA3. The optical frequency mixer using the OD is a PIN photodiode or avalanche photodiode that inputs the optical output of the optical frequency mixer OX and the output light from the reference wavelength light source section 11 and outputs it to the variable wavelength light source section 122. This is a photodetector. According to this configuration, the optical output frequency ω ₆ of the optical frequency mixer OX is ω ₆ = ω ₁ due to the nonlinear optical effect.
= ω _L. In the configuration shown in Fig. 3, the optical frequency multiplier allows ω _s = ω ₁ (apart from the offset frequency).
Although only a limited amount of ω ₁ determined by =nω ₀ can be obtained, the configuration shown in FIG. 11 can output light of various wavelengths. For example, using the absorption line of Rb, the wavelength λ _s of ω _s =
Using the absorption line of Cs at 780nm, the wavelength of ω _L is determined as λ _L =
If 852 nm is selected, the relationship between the feedback loop balance ω _s = ω ₆ and the respective wavelengths λ _s , λ ₁ , λ _{L of ω s , ω 1 , ω L is 1 /} λ _s = 1. /λ ₁ +1/λ _L
Since there is a relationship, λ ₁ =9230 nm.

The optical frequency synthesizer/sweeper as explained above has the following features: (a) Its optical output is highly accurate and highly stable in terms of absolute wavelength.
Can be locked to absorption lines such as Rb and Cs,
It is possible to obtain a quantum standard with stability of 10 ^-12 or higher (conventional frequency standards utilize microwave resonance of Cs (9 GHz) and R _b (6 GHz)).

(b) Also, variable wavelength laser diodes VL1 to VL3
Since an ADFB with a long resonator length or an external cavity laser diode is used, the Q of the resonator is high and the oscillation spectrum width can be narrowed.

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

(d) Also, by using Rb absorption lines (780nm, 795nm) and using 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 high stability. , excellent in practicality.

(E) Various optical frequencies can be output by the configuration shown in FIG. 11.

An example of optical frequency operation in an optical frequency network analyzer configured as described in the embodiment of FIG. 1 will be described below.

Wavelength of ω _s : 780nm (the wavelength of the laser diode is R _b
) Wavelength of ω ₀ : 1560nm±50nm Frequency of Δω: 100MHz This operation example is when the measurement light is the optimum wavelength for optical fiber communication, and is particularly effective for measuring optical communication equipment. .

Note that in the embodiment of the optical frequency network analyzer described above, an optical frequency synthesizer sweeper is used as the optical frequency sweeper, but the present invention is not limited to this, and a high-precision sweeper that is not synthesized may be used.

Further, in the above embodiment, the reference signal of the comparing means is obtained using the second optical heterodyne detection section 23 and the second filter section 24, but the invention is not limited to this, for example, the optical frequency synthesizer shown in FIG. A modulation electrical signal corresponding to the shift frequency Δω applied to the optical frequency shifter 15 of the sweeper may be used. In this case, the configuration can be simplified by omitting the second optical heterodyne detection section and the second filter section.

In addition, the light emitted from the optical frequency network analyzer to the measurement target is not limited to continuous light; it is also possible to measure the wavelength characteristics of pulsed light by using pulsed light and sweeping the optical frequency in synchronization with this pulsed light. can.

<<Effects of the Invention>> As described above, according to the present invention, it is possible to realize an optical frequency network analyzer that can measure amplitude, phase characteristics, etc. with high precision.

[Brief explanation of the drawing]

FIG. 1 is a configuration block diagram showing one embodiment of the present invention, FIG. 2 is a configuration block diagram showing a configuration example of an optical frequency sweeper 1 used in the present invention, and FIG.
4 is a characteristic curve diagram for explaining the operation of the device shown in FIG. 3, and FIG. 5 is an explanation for explaining the operation of the device shown in FIG. 3. 6 and 8 to 10 are configuration explanatory diagrams showing other configuration examples of the tunable wavelength laser diode in FIG. 3, and FIG. 7 is an operational explanation for explaining the operation of the device shown in FIG. 6. 11 is a block diagram showing a modification of a part of the device shown in FIG. 3, FIG. 12 is a block diagram showing a conventional optical fiber loss wavelength characteristic measuring instrument, and FIG. 1 is a block diagram showing a fiber wavelength dispersion characteristic measuring device; FIG. 1... Optical frequency sweeper, 10... Measurement target,
23... second optical heterodyne detection section, 24...
Second filter section, 33, 43... First optical heterodyne detection section, 34, 44... First filter section, 35, 36, 45, 46... Comparison means, 50
...Signal processing means.

Claims (1)

[Claims] 1. A first frequency-sweeping optical output;
an optical frequency sweeper that generates a second optical output obtained by shifting the optical output of the target by a certain frequency and emits the first optical output to a measurement target;
The amplified signal and the first signal reflected from the measurement target
The optical frequency of the first and second optical outputs obtained by inputting the optical output and the second optical output, which are signals obtained by deflecting and amplifying the optical output of A first electrical signal obtained by converting a signal having a frequency corresponding to the difference into an electrical signal by heterodyne detection, and the optical output reflected from the measurement target and the second optical output are both obtained by inputting the optical frequency. a first optical heterodyne detection unit that outputs a second electrical signal obtained by converting a signal having a frequency corresponding to the difference in frequency into an electrical signal by heterodyne detection, and inputs the first optical output and the second optical output; and a second optical heterodyne detection unit that converts a signal having a frequency corresponding to the difference between the optical frequencies of the two optical signals obtained by inputting these input signals into an electrical signal by heterodyne detection and outputs the electrical signal, a first filter section that receives first and second electrical signals output from the first optical heterodyne detection section and amplifies each signal; a second filter section for inputting and amplifying a signal; a comparing means for comparing the electrical signal from the first filter section and the electrical signal from the second filter section and outputting the result as an electrical signal; An optical frequency network analyzer characterized in that it is equipped with a signal processing means that inputs the electrical signal output of the comparison means and processes the signal, and is capable of simultaneously measuring the propagation characteristics and reflection characteristics of a measurement object easily and with high precision.