JP2583410B2 - Optical frequency spectrum analyzer - Google Patents

Description

The present invention relates to a high-precision and high-resolution optical high-frequency spectrum analyzer.

<< Conventional Technology >> Conventional optical frequency spectrum analyzers include the following.

I. Using a diffraction grating or prism as a spectroscope.

B. Using a Fabry-Perot resonator as a spectroscope: As shown in FIG. 16, two half mirrors HM are arranged to face each other to form a resonator. Assuming that the speed of light is c and the distance between the two half mirrors is L, this resonator has a resonance frequency at a frequency interval of c / 2L. When the light to be measured enters the left half mirror HM, the light having the same frequency as the resonance frequency is transmitted and enters the photodetector PD. When the resonance frequency is swept by vibrating the half mirror HM with PZT or the like, the spectrum of the light to be measured can be observed from the output of the photodetector PD.

<< Problems to be Solved by the Invention >> However, the optical frequency spectrum analyzer of the above-mentioned method A has a bad wavelength resolution of about 0.1 nm (equivalent to 30 GHz) and an absolute accuracy of about 2 nm (equivalent to 600 GHz). Further, the optical frequency spectrum analyzer of the method of b) has a frequency resolution of several tens of MHz. Absolute wavelength can be measured by inputting light of reference wavelength and measuring,
Very difficult to handle, poor accuracy (adjustment of mirror parallelism and vertical incidence, frequency error due to fluctuation of mirror spacing, etc.). There is also a disadvantage that laser beams oscillating in a plurality of modes cannot be measured simultaneously.

In the future coherent optical communication field and optical applied measurement field
Since the frequency measurement with high accuracy and high resolution of 1 MHz or less is required, each of the above methods is insufficient.

The present invention has been made to solve such a problem, and an object of the present invention is to realize an optical frequency spectrum analyzer having high accuracy and high resolution with absolute accuracy.

<< Means for Solving the Problems >> An optical frequency spectrum analyzer according to the present invention includes a polarization control unit (1) that receives a measured light having a frequency (ωi) and controls a polarization plane of the incident light. A reference wavelength light source (310) for outputting a reference optical signal having an absolute frequency, a sweep signal generator (330) for outputting an electric signal that changes at a constant cycle, and an optical signal from the reference wavelength light source An optical heterodyne detection unit (321) that outputs an electric signal related to an optical frequency difference from the other input, and an electric signal output from the optical heterodyne detection unit and an electric signal from a sweep signal generator (MX1) which mixes the output signal and the variable wavelength light source unit (32) which converts the output of the electric signal from the mixer into an optical signal and outputs it as output light of a frequency (ωo) and a feedback optical signal.
2) and a frequency shifter (323) which receives a feedback optical signal from the variable wavelength light source unit and converts the frequency of the optical signal to an optical signal having a frequency shifted, and a frequency of the optical signal from the optical frequency shifter. A local oscillator (3) having an optical frequency multiplying circuit (324) for multiplying the other input of the optical heterodyne detection unit and configured independently of the light source of the measured light; A light combining unit (HM1) for inputting the output light of the polarization control unit and the output light of the polarization control unit and combining the two output lights; and inputting the optical signal output from the light combination unit to output light of the local oscillator and polarization control. An optical heterodyne detection unit (4) that converts the signal into an electric signal related to a frequency difference (ωo−ωi) from the output light of the unit, and an optical signal input from the optical heterodyne detection unit, Generates frequency-related signals A band-pass filter (5), a detection unit (6) for inputting an electric signal output from the band-pass filter, and an output from a sweep signal generator in the local oscillator which receives an output of the detection unit as an input. A signal processing / display unit (7) input to the frequency axis as a sweep signal, wherein a power of a frequency spectrum and an absolute value of a frequency in the measured light are measured.

<< Operation >> The local oscillator having the above configuration outputs broadband output light, and combines such output light and the measured light to perform optical heterodyne detection, so that it has absolute accuracy, high accuracy, and high resolution. It is possible to realize an optical frequency spectrum analyzer.

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

FIG. 1 is a block diagram showing the configuration of an embodiment of an optical frequency spectrum analyzer according to the present invention. 1 is a polarization control unit using a magneto-optical effect crystal (YIG, lead glass, etc.) or the like, into which light to be measured is incident, 2 is an optical amplification unit to which the output light of this polarization control unit 1 is input, 3 is a local oscillation unit HM1 is a half mirror which is an example of a synthesizing means for inputting and synthesizing the output light of the local oscillation unit 3 and the optical amplification unit 2, and 4 is a PIN photodiode or an avalanche photodiode, etc. An optical heterodyne detection unit for inputting light, 5 is a filter unit that receives and amplifies the electric output of the heterodyne detection unit 4 and has bandpass characteristics, 6 is a detection unit that receives the electric output of the filter unit 5, and 7 Is a signal processing / display unit for inputting the electric output of the detection unit 6. In the local oscillating unit 3, 330 is a sweep signal generator, 310 is a reference wavelength light source unit, and 320 is the optical output of the reference wavelength light source unit 310.
Is an optical frequency PLL unit whose frequency sweep is controlled by the controller and whose output is output to the half mirror HM1. The optical amplifier 2
GaAlAs laser (780nm band) and InGaAsP laser (1500nm band)
The following three types can be used.

(A) A resonator type semiconductor laser amplifier in which a bias current near the oscillation threshold is passed, signal light is incident on a laser diode, and linear light amplification is performed by stimulated emission.

(B) An optical injection locking amplifier that controls the optical frequency and phase of oscillating light by injecting signal light into an oscillating laser diode.

(C) A traveling waveform laser amplifier in which both end faces of a laser diode chip are nonreflectively coated and optically amplified only by passing signal light.

The operation of the optical frequency spectrum analyzer having the above configuration will be described in detail below.

As the local oscillation unit 3, here, the optical frequency PLL unit 320 (output optical frequency ω _O ) controls the wavelength of the optical output to a wavelength corresponding to the oscillation wavelength of the reference wavelength light source unit 310 (output optical frequency ω _S ), and performs sweeping. Using an optical frequency synthesizer sweeper (detailed later) that sweeps the wavelength of the optical output of the optical frequency PLL section at the output of the signal generator 330, outputs local oscillation light with high accuracy, high stability, and high spectral purity. .

When the light to be measured having the frequency ωi enters the polarization controller 1,
By controlling the applied magnetic field utilizing the optical rotation of the magneto-optical effect crystal, the polarization plane of the incident light is controlled to be the same as the local oscillation light. The optical output of the polarization controller 1 is amplified by the optical amplifier 2, and then the light is output from the local oscillator 3 by the half mirror HM 1.
And the difference between the two frequencies ω _O −ω _i ′ (here, ω _i ′ = ω _i ) in the optical heterodyne detection unit 4.
Is converted to an electric signal having a frequency of A part of the electrical output of the optical heterodyne detection unit 4 passes according to the band pass characteristics of the filter 5 and is extracted by the detection unit 6 as power. The signal processing / display unit 7 inputs the electric output of the detection unit 6 as a power signal, inputs a signal related to the sweep from the sweep signal generator 330 as a frequency axis signal, and displays the spectrum of the measured light. An operation example of the optical frequency is shown below.

ω _S wavelength: 780 nm (locks the laser diode wavelength to the Rb absorption line) ω _O wavelength: 1560 nm ± 50 nm ω _i wavelength: 1560 nm ± 50 nm In this operation example, the measured light is the optimum wavelength for optical fiber communication. This is particularly effective for measuring the light emission characteristics (absolute wavelength, spectrum distribution, spectrum width) of the laser diode for optical communication.

FIG. 1 shows a case where a pulse synchronizing signal is applied to a sweep signal generator 330 in order to show a case where the spectrum is measured using pulsed light as incident light. FIG. 2 is a time chart for explaining the operation in this case. A trigger signal (FIG. 2 (B)) synchronized with the pulse light is input to the sweep signal generator 330 of the local oscillator 3, and the output frequency ω _O of the optical frequency PLL is synchronized with the trigger signal (FIG. 2 (A)). Sweep in a step-like manner. It sends a signal (similar to FIG. 2 (A)) corresponding to the sweep frequency omega _O to the signal processing and display unit 7 at the same time. As a result, the power spectrum of ω _O at one point is measured for each pulse light, and after the sweep, the entire spectrum of the pulse light as shown in the explanatory diagram of FIG. 3 can be output.

According to the configuration as described in example of FIG. 1, the frequency resolution of the optical frequency spectrum analyzer is determined by the spectral width and bandwidth of the filter unit 5 of the output frequency omega _O of local oscillator 3. The spectrum width of ω _O is determined by the variable wavelength light source of the optical frequency synthesizer, and is described later (FIG. 10 to FIG. 10).
By using an external cavity laser diode as shown in FIG. 14), excellent frequency resolution (10 ^-12 ) can be obtained.

High accuracy (100KHz) and high stability ( ^10-12 ) in absolute accuracy
The optical frequency spectrum analyzer can be obtained.

 There is also an advantage that the measurement of the light pulse is easy.

Note W-N _i (tungsten, nickel) in the optical heterodyne detection section 4 can also be used point contact diodes and Josephson element.

Further, in the above embodiment, a band-pass filter is used as the filter unit 5, but the present invention is not limited to this, and a low-pass filter may be used. In that case, along with the sweep of ω _O ,
The optical power of ω _i ′ where ω _i ′ = ω _O is detected.

FIG. 4 is a block diagram showing another example of the configuration of the optical amplifier 2. OC1 local oscillator light output frequency omega _L using the second wavelength stabilizing light source, OA optical amplifier optical output of said polarization control unit 1 is input, OX1 light output and the local oscillator of the optical amplifier OA OC1 Is an optical frequency mixer using a non-linear optical crystal for inputting the optical output of the optical frequency converter. According to such a configuration, the optical output frequency ω _i ′ of the optical frequency mixer OX1 becomes ω _i ′ = ω _i + ω _L due to the nonlinear optical effect. The local oscillator OC1, the optical frequency synthesizer sweeper described later to output a highly accurate frequency omega _L (Figure 7) is optimal. Measurement frequency range By using such a light amplifier section can be enlarged in addition to sweep range of omega _O. If a local oscillator OC1 that can output a plurality of frequencies ω _L1 , ω _L2 ,... Is used, a wider sweep range can be obtained.

FIG. 5 is a configuration block diagram showing another configuration example of the local oscillator 3. Variable wavelength light source 322a is controlled by a sweep signal generator 330, the high-precision absolute wavelength MK1～MK3, highly stable fixing of the optical frequency omega _O1 marker light source ～Omega _O3 and outputs respectively, the HM30~HM33 the variable It is a half mirror for combining the output of the wavelength light source 322a and the outputs of the marker light sources MK1 to MK3. When the local oscillator 3 having such a configuration is used, a marker signal appears at a position corresponding to the output frequencies ω _{O1 to} ω _O3 of the marker light source on the display of the signal processing / display unit 7, and the frequency axis can be scaled. it can. As the marker light sources MK1 to MK3, reference wavelength light sources used in an optical frequency synthesizer sweeper described later (FIG. 7) include Rb, C
High precision and high stability can be achieved by using a laser diode locked to each of the ultrafine structures of the D ₁ and D ₂ lines, which are absorption lines such as s, to provide a plurality of light sources. As the variable wavelength light source 322a, an optical frequency synthesizer sweeper described later (FIG. 7 and below), a variable wavelength laser diode described later (FIGS. 10 to 14), and the like can be used.

FIG. 6 is a block diagram showing a configuration example of an optical frequency synthesizer / sweeper used in the local oscillation section 3 of FIG. Reference numeral 310 denotes a reference wavelength light source unit whose wavelength is stabilized, and 320 denotes an optical frequency PLL unit that receives output light of the reference wavelength light source unit 310 and generates a synthesizer output. In the optical frequency PLL unit 320, reference numeral 321 denotes an optical heterodyne detection unit having the output light of the reference wavelength light source unit 310 as one input, and MX1 denotes an electric signal output from the optical heterodyne detection unit 321 and a signal from the sweep signal generator 330. A mixer 322 for mixing the electric signal with the electric signal is a variable wavelength light source unit that converts the output of the electric signal from the mixer MX1 into an optical signal and outputs it as output light of a frequency (ω _O ) and a feedback optical signal.

An optical frequency shifter 323 receives the feedback optical signal from the variable wavelength light source section 322 and converts the frequency of the optical signal into an optical signal having a frequency shifted. An optical frequency shifter 324 multiplies the frequency of the optical signal from the optical frequency shifter 323. Optical heterodyne detector 321
Is an optical frequency multiplication circuit having the other input of the optical frequency multiplication circuit.

The operation of the device having such a configuration will be described below. When the output light of the reference wavelength light source unit 310 is input to the optical frequency PLL unit 320, the optical frequency PLL unit 320 fixes (locks) the wavelength of the optical output to a wavelength corresponding to the oscillation wavelength of the reference wavelength light source unit 310. That is, the optical heterodyne detection unit 321 compares the frequency of the output light from the reference wavelength light source unit 310 with the frequency of the output light from the optical frequency multiplication unit 324, and controls the variable wavelength light source unit 322 so that the difference becomes small. The optical frequency shifter unit 323 in the feedback circuit adds an offset frequency to the output light of the variable wavelength light source unit 322, and the optical frequency multiplication unit 324 generates the variable wavelength light source unit 3
The ratio between the output light frequency of the reference wavelength light source unit 310 and the output light frequency of the reference wavelength light source unit 310 is determined.

FIG. 7 is a configuration block diagram showing a more specific example of the configuration of FIG. In the reference wavelength light source section 310, L
D1 is a laser diode, CL is an absorption cell in which Rb gas or Cs gas is sealed and the output light of the laser diode LD1 is incident, HM1 is a half mirror into which the output light of the absorption cell CL is incident, and PD1 is the half mirror HM1. A photodiode for inputting reflected light, A1 is a control circuit for inputting an electric output of the photodiode PD1 and controlling a current of the laser diode LD1 with an output corresponding thereto, IS1 is a light transmitted through the half mirror HM1. An isolator OA1 for preventing return light is an optical amplifying element to which light passing through the isolator IS1 is input. In the optical frequency PLL unit 320, HM2 is a half mirror that receives the output light of the reference wavelength light source unit 310, and PD2 constitutes an optical heterodyne detection unit 321 and is a PIN photodiode or an avalanche diode that receives the transmitted light of the half mirror HM2. Photodiode consisting of
MX1 is a mixer (mixing) circuit for mixing the electric signal output from the sweep signal generator 330 with the electric output of the optical heterodyne detection unit PD2. This mixer (mixing) circuit MX1
In the variable wavelength light source unit 322 to which the output of the mixer circuit MX1 is connected, FC is an optical frequency modulation circuit to which the output of the mixer circuit MX1 is connected.
L1 to VL3 are tunable laser diodes for inputting the output of the optical frequency modulation circuit FC, IS2 is composed of YIG (Yttrium Iron Garnet), an isolator through which the output light of the tunable laser diodes VL1 to VL3 passes, OS
An optical switch 1 receives light passing through a plurality of (three in FIG. 7) isolators IS2. HM3 is a half mirror on which the output light of the optical switch OS1 is incident, OA2 is an optical amplifier element for inputting the reflected light of the half mirror HM3, and UM1 constitutes an optical frequency shifter unit 323 and outputs the output light of the optical amplifier element OA2. The input ultrasonic modulator, NL constitutes the optical frequency multiplication unit 324, an optical waveguide using a nonlinear material to input the output light of the ultrasonic modulator UM1, and OA3 amplifies the output light of the optical waveguide NL. This is an optical amplifying element that outputs to the half mirror HM2.

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

As described below, the reference wavelength light source unit 310 controls the oscillation wavelength of the laser diode to the absorption line of the Rb (or Cs) atom to achieve high accuracy and high stability (10 ^-12 or more) in absolute wavelength. . The output light of the laser diode LD1 is absorbed when the wavelength of the output light of the LD1 coincides with the absorption line of the Rb gas (or Cs gas) when passing through the absorption cell CL, and as shown in the characteristic curve of FIG. The absorption characteristics as shown appear. Figure 9 is a diagram showing the energy levels of Rb gas, the absorption line of Rb is D ₂ line is 780 nm, D ₁ line is 795 nm.

For this reason, in FIG. 6, a variable wavelength light source which is also an output of the local oscillator 3 when doubled by the optical frequency multiplier 324 is shown.
The outputs of 322 are 1560 nm and 1590 nm, respectively, which is convenient because it matches 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 of the absorption cell CL reflected by the half mirror HM1 is detected by the photodetector PD1, and the control circuit A1 controls the current of the laser diode LD1 in accordance with the output of the photodetector PD1, thereby Lock the output wavelength of LD1 to the absorption center. For example, when it is desired to lock to the point a in FIG. 8A, the control circuit A1 uses a lock-in or the like to obtain a differential waveform of FIG. 8A at a point b on the characteristic curve of FIG. 8B. (Point at which the differential waveform value becomes 0). This method is called a linear absorption method, and the absorption spectrum becomes thick as shown in FIG. 8 (A), but the saturated absorption method (Hori, Kadota, Kitano, Yabuzaki,
Ogawa: Frequency stabilization of a semiconductor laser using saturation absorption spectroscopy, IEICE technical report OQE82-116) detects the absorption line of the ultrafine structure hidden by the Doppler shift, and uses this to detect the oscillation wavelength of the laser diode LD1. Locking makes it more stable. The temperature of the laser diode LD1 is 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 external reflection from entering the laser diode LD1 and becoming noise. The output light of the isolator IS1 is amplified by the optical amplifier OA1 as needed.

The optical frequency PLL section 320 has a function of locking the oscillation wavelength of the variable wavelength light source section 322 with a predetermined ratio and a predetermined offset with respect to the oscillation wavelength of the reference wavelength light source section 310, as described below. The output light of the reference wavelength light source unit 310 passes through the half mirror HM2 and enters the photodiode PD2 of the optical heterodyne detection unit 321. The feedback light from the optical frequency multiplier 324 is also transmitted to the half mirror H via the optical amplifier OA3.
After being reflected by M2, it enters the photodiode PD2. Assuming that the output of the reference wavelength light source unit 310 and the optical frequency of the feedback light are ω _S and ω ₁ , respectively, the frequency ω ₂ of the output electric signal of the optical heterodyne detection unit 321 is ω ₂ = | ω _S −ω ₁
| When the output frequency of the oscillator EC and omega _3, the output omega ₄ of the mixer circuit (phase detection circuit) MX1 is optical heterodyne detection section 321 outputs the frequency omega ₂ in the offset frequency is added to ω _{4 =} ω ₂ -ω ₃ of Becomes Output electrical signal omega ₄ of the mixer circuit MX1 is inputted to the optical frequency modulation circuit FC of the variable wavelength light source 322, the optical frequency of the tunable laser diode VL1~VL3 as optical frequency modulation circuit FC becomes omega _{4 =} 0 Control.

Here, as the variable wavelength laser diodes VL1 to VL3,
A resonator is constructed using the reflection from the diffraction grating built in the laser diode chip, and the oscillation frequency is determined by the pitch of the diffraction grating.
uted Feedback) laser or DBR (Distributed Bragg R)
eflector) A type of laser, ADFB (Acoustic DFB) laser (Yamanishi M, et.al .: GaAs Acoustic Distribute)
d Feedback Lasers, Jpn. J. Appl. Phys., Suppl. 18-1,
p.355, 1979). The ADFB laser is orthogonal to the diffraction grating in the DBR laser and has a surface acoustic wave (SAW)
Is generated, and a ring resonator of light is formed by Bragg diffraction using the diffraction grating and the SAW built in the chip. SAW
Is swept, the resonance wavelength of the link resonator changes, and the oscillation wavelength can be swept. In this configuration example, the oscillation wavelength is in the 1560 nm band. DFB, DBR with long resonator length
Also, ADFB lasers have the advantage that the oscillation spectrum is narrow and the spectral purity is good.

If the variable wavelength range of one ADFB laser is not sufficient, a plurality of ADFB lasers (VL1 to VL3) can be used as shown in FIG. 7 and switched by an optical switch or an optical multiplexer. That is, the output lights of the tunable laser diodes VL1 to VL3 are respectively passed through the optical switch OS1 via the return light preventing isolator IS2.
And a predetermined variable wavelength range is selected. Part of the output light of the optical switch OS1 is reflected by the half mirror HM3 and enters the optical amplifier OA2.

The output light of the optical amplifier OA2 is input to the optical frequency shifter unit 323, enters the ultrasonic modulator UM1, and outputs Bragg's s-order diffracted light. When the ultrasonic wave frequency supplied from a reference frequency source such as a crystal oscillator and omega _5, the optical frequency of the diffracted light is shifted by sω _5.

The output light of the optical frequency shifter 323 enters the optical frequency multiplier 324 and outputs the second harmonic of the input light from the optical waveguide NL using a nonlinear material. That is, the output of the variable wavelength laser diode of 1560 nm is input via the optical amplifier and the second harmonic
It outputs 780nm. As a waveguide, a four-layer slab optical waveguide of air-TiO ₂ -ZnS-glass using a nonlinear thin film of ZnS and a linear thin film of TiO ₂ is used to efficiently generate a nonlinear effect. Although the second harmonic is used in this embodiment, any n-th high frequency can be used.

The output light of the optical frequency multiplier 324 is amplified by the optical amplifier OA3 and then combined with the output light from the reference wavelength light source 310 by the half mirror HM2 as feedback light as described above.

Thus operation, the optical frequency ωo of the optical output of the optical frequency PLL unit 320 _{ω O = (ω S ± ω} 3) / n becomes ± sω ₅ (although code is not the same order). However, in this embodiment, the optical frequency multiplication number n = 2. That is, ω _O is locked to a highly accurate and stable optical frequency ωωs at an absolute wavelength via a predetermined ratio n, and furthermore, an arbitrary frequency ω ₃ / n or ω ₅
An optical frequency having an offset only. ω ₃ or ω
_{If 5} is swept, high-precision optical frequency sweep can be realized.
Here, since ω ₃ and ω ₅ are electric signals, high accuracy and high stability can be easily obtained.

In the above embodiment, the optical amplifiers OA1 to OA3 are the same as the optical amplifiers used in the optical amplifier 2 in FIG. 1 (GaAlAs laser (780 nm band) or InGaAsP laser (1500 nm)).
Band) etc.) is used.

In the above embodiment, the positions of the optical frequency shifter unit 323 and the optical frequency multiplying unit 324 are interchanged, and the optical frequency PLL unit is replaced.
The frequency omega _O of the light output of _{320 ω O = (ω S ±} ω 3 ± sω 5) / may be n.

In the optical frequency PLL section 320, the mixer circuit MX1 and the optical frequency shifter section 323 are both for adding an offset frequency, and one of them can be omitted.

In the optical frequency PLL section 320, if the multiplication number n is 1, the optical frequency multiplication section 324 can be omitted.

Further, ω ₃ '= ω ₃ instead of omega ₃ in Figure 7 device
If a frequency signal of + Ω (Ω is an FM modulation frequency when a lock-in amplifier is used) is input to the mixer MX1, unnecessary FM modulation components can be removed from the optical output of the optical frequency PLL unit 320.

Further, in the above embodiment, the reference wavelength
b or Cs absorption lines are used, but not limited to these, and any absorption line with high accuracy and high stability at absolute wavelengths, such as
NH ₃ or H ₂ O absorption lines (1500 nm band) can also be used.
In this case, the optical frequency multiplier 324 becomes unnecessary. The wavelength can be stabilized using a known Fabry-Perot resonator as a wavelength detector, but the characteristics are better when a quantum standard absorption line as described above is used.

Further, the variable wavelength laser diodes VL1 to VL3 are not limited to ADFB and the like as in the above embodiment, and an external resonator using a diffraction grating is added outside the laser diode chip,
The wavelength may be varied by rotating the diffraction grating and utilizing its wavelength selectivity. External cavity laser diodes have the advantage of a narrow spectrum.

In addition, as tunable laser diodes VL1 to VL3, the tenth
As shown in the figure, a resonator in which a wavelength-selective element is inserted may be used. In the figure, LD2 is a semiconductor laser, 51, 52
Is a non-reflection coating provided at both ends of the semiconductor laser LD2, LS1 is a lens that makes light emitted from the non-reflection coating 51 parallel, and M1 is a mirror that reflects light passing through the lens LS1. , LS2 is a lens that collimates the light emitted from the non-reflection coating section 52, UM2 is a first ultrasonic modulator into which light passing through the lens LS2 is incident, and UM3 is an emission from the ultrasonic modulator UM2. M2 is a mirror that reflects the light emitted from the ultrasonic modulator UM3, and DR1 is an oscillator that excites the ultrasonic modulators UM2 and UM3 at a frequency F. FIG. 11 is an operation explanatory diagram showing a state of wavelength selection and frequency sweep operation by the ultrasonic modulators UM2 and UM3 in the apparatus of FIG. The light emitted from the non-reflection coating part 51 of the semiconductor laser LD2 is the lens L
The light is collimated by S1 and 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 of the frequency f _O1 emitted from the non-reflection coating section 52 is converted into parallel light by the lens LS2, and the first ultrasonic modulator UM2
Incident on. At this time, from the diffraction conditions, the following expression is obtained between the incident angle θ _i1 to the diffraction grating 63 generated by the ultrasonic wave 61, the output angle θ _O1 after diffraction, the light wavelength λ _O and the ultrasonic wave wavelength Λ _O. Have a relationship.

sin θ _i1 + sin θ _O1 = λ _O / Λ _O (1) That is, the wavelength λ _O of light passing through the optical path that satisfies the specific incident angle θi ₁ and emission angle θ _O1 changes when the wavelength Λ _{O of the} ultrasonic wave changes. I do. The emitted light is subjected to a Doppler shift due to the ultrasonic wave. In this case, the frequency is f _O1 +
It becomes F. The light emitted from the ultrasonic modulator UM2 is diffracted again by the ultrasonic modulator UM3. As described above, there is the following relationship between the incident angle θ _i2 to the diffraction grating 64 generated by the ultrasonic wave 62, the output angle θ _O2 after diffraction, the wavelength λ _{O of} light, and the wavelength Λ _O of ultrasonic wave. .

Since _{sinθ i2 + sinθ O2 = λ O} / Λ O ... (2) However (2) lambda _O changes caused by Doppler shift of the ultrasonic modulator UM2 in equation smaller are ignored. Here, the relationship between the traveling wave 62 of the ultrasonic wave and the diffracted light is represented by an ultrasonic modulator.
If the reverse of UM2, since the -1 order diffracted light, the Doppler shift amount -F, and the frequency of the light emitted ultrasonic modulator UM3 becomes _{f O1 + F-F = f} O1. Ultrasonic modulator UM
The outgoing light of No. 3 is reflected by the mirror M2, reverses the original optical path, and enters the semiconductor laser LD2 again. When going backwards,
Doppler shift in frequency of the light emitted UM3 is f _O1 -F next, the frequency of the light emitted UM2 returns to the semiconductor laser LD2 becomes f _O1 -F + F = f _O1 and the original frequency f _O1, resonant state is maintained I do. In order to increase the diffraction efficiency, the Bragg incidence condition is satisfied, and the incident angle θ _i1 ,
The following relationship is established between the outgoing angle θ _O1 , the incident angle θ _i2, and the outgoing angle θ _O2 .

θ _i1 = θ _O1 = θ _i2 = θ _O2 If the wavelength Λo of the ultrasonic wave is changed in such a configuration, θ _i1 ,
The wavelength λo of light that satisfies θ _O1 , θ _i2 , and θ _O2 can be swept as in the following equation.

As _{sinθ i1 + sinθ O1 = (λ} O + Δλ) / (Λ O + ΔΛ) The tunable laser diode VL1-VL3, 12
As shown in the figure, a resonator in which an element capable of controlling the refractive index is inserted may be used. The same parts as those in FIG. 10 are denoted by the same parts, and the description is omitted. EO1 is an electro-optical element made of LiNbO ₃ (lithium niobate) or the like and having a double-sided anti-reflection coating to receive the output light of the lens LS2.
Power supply for controlling 1. The light emitted from the semiconductor laser LD2 becomes parallel light by the lens LS2, passes through the electro-optical element E01, is reflected by the mirror M2, reverses the original optical path, and enters the semiconductor laser LD2 again. This results in mirror M1 and mirror M
A resonator can be formed between the two. Assuming that the distance excluding the length l along the optical path 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, the oscillation frequency fo ₂ Is expressed as f _O2 = p · c / 2 (L + n (V) l) (3) That is, the refractive index n can be changed by changing the electric field strength of the electro-optical element EO1 by the power supply 71, and as a result, the oscillation frequency f _O2 can be swept.

FIG. 13 is a block diagram showing a configuration in which the tunable laser diode shown in FIG. 12 is a double resonator type. The same parts as those in FIG. 12 are denoted by the same reference numerals and description thereof will be omitted. BS1
Is a beam splitter that separates outgoing light from the lens LS2 in two directions, EO2 is an electro-optical element that receives light transmitted through the beam splitter BS1, M2 is a mirror that reflects the outgoing light of this electro-optical element EO2, and EO3 is The beam splitter B
An electro-optical element that receives the light reflected by S1 and a mirror M3 that reflects the light emitted from the electro-optical element EO3. The lengths of the electro-optical elements EO2 and EO3 in the optical path direction are l ₁ and l ₂ , the refractive indexes are n ₁ and n ₂ , respectively, and the distance excluding l ₁ along the optical path between the mirrors M ₁ and M 2 is L ₁ , the mirror Assuming that the distance excluding l ₂ along the optical path of M1 and M3 turns is L ₂ and q is an integer, the oscillation frequency f in this case is
_O3 is _{f O3 = q · c / 2} | - | a _{... (4) (L 1 +} n 1 (V 1) l 1) (L 2 + n 2 (V 2) l 2). Since the denominator of the equation (4) can be made smaller than that of the equation (3), the variable range of the oscillation frequency can be made larger than in the case of the apparatus shown in FIG.

FIG. 14 is a block diagram showing a configuration in which the tunable laser diode of FIG. 12 is integrated on one chip. 91 is GaAl
A laser diode composed of As, InGaAsP, etc., 92 is an optical amplifier provided at the junction of this laser diode 91, 93 is also a waveguide type external resonator, and 94 and 95 are provided at both ends of the laser diode 91. The mirror 96 is an electrode provided on the surface of the laser diode 91 corresponding to the optical amplifier 92, and 97 is an electrode provided on the surface of the laser diode 91 corresponding to the waveguide type external resonator 93. is there. Electrode 96
A current I _LD is injected into the junction through the optical amplifier 92 to generate laser light, and an electrode 97 is connected to the waveguide type external resonator 93.
Changing the refractive index of the waveguiding Michigata external cavity 93 passing a current I _F through by sweeping the oscillation frequency. The lengths along the junction of the optical amplifier 92 and the waveguide type external resonator 93 are respectively
When l ₃ , l ₄ and the refractive index are respectively n ₃ , n ₄ , and r are integers,
The oscillation frequency f _O4 is f _O4 = r · c / 2 (n ₃ l ₃ + n ₄ (I _F ) l ₄ ).

In Figure 7 device, as in the device of FIG. 1, the optical heterodyne detection section 321 W-N _i (tungsten, nickel) may be used point contact diodes and Josephson element. Since these elements have both a multiplication function and a mixer function, ωs, ω ₁ , and ω ₃ can be input simultaneously, and the mixer circuit MX1 in FIG. 7 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 multiple). In addition, ω ₄ = ω _S −2ω ₁ ± mω ₃ can be satisfied. In this case, the optical frequency multiplication number 324 is unnecessary.

FIG. 15 is a configuration block diagram showing another configuration example of the optical heterodyne detection unit 321. OC2 local oscillator light output frequency omega _L using the second wavelength stabilizing light source, OX2 input light output and light output of the optical frequency multiplier 324 of the local oscillator OC2 via the optical amplifier OA3 An optical frequency mixer using a nonlinear optical crystal, OD1 is a PIN photodiode or an avalanche photo diode which inputs the optical output of the optical frequency mixer OX2 and the output light from the reference wavelength light source section 310 and outputs the light to the variable wavelength light source section 322. It is a photodetector composed of a diode or the like. According to such a configuration, the optical output frequency omega ₆ of the optical frequency mixer OX2 by nonlinear optical effect,
ω ₆ = ω ₁ + ω _L In the configuration of FIG. 7, the optical frequency multiplying unit 324 (excluding the offset frequency) causes ω _S
= Ω ₁ = nω Only a limited ω ₁ determined by _O can be obtained, but the configuration of FIG. 15 can output light of various wavelengths. For example, using the absorption line of Rb, the wavelength of ω _S is changed to λ _S = 780 n
If the wavelength of ω _L is selected as λ _L = 852 nm using the absorption lines of m and Cs, the relationship ω _S = ω when the feedback loop is balanced
_6, there is a relationship of 1 / λ _S = 1 / λ ₁ + 1 / λ _L between the respective wavelengths λ _S , λ ₁ , and λ _L of ω _S , ω ₁ , and ω _L , so that λ ₁ =
It becomes 9230 nm.

The optical frequency synthesizer sweeper described above with reference to FIGS. 6 to 15 has the following advantages.

(B) The optical output is Rb, C
can be locked to absorption lines such as s, and a quantum standard with a stability of 10 ^-12 or more (conventional frequency standards are Cs (9 GHz), Rb (6
GHz) utilizing microwave resonance).

(B) Long cavity length as a tunable laser diode
Since the ADFB and the external resonator type laser diode are used, the Q of the resonator is high, and the oscillation spectrum width can be narrowed.

(C) Since the principle of the high-frequency PLL is used, a high-precision high-frequency sweep can be performed.

(D) By using Rb absorption lines (780 nm, 795 nm) and the doubling method, it is possible to output 1500 nm band light with the smallest optical transmission loss in optical communication fibers with high accuracy and high stability. Excellent practicality.

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

<< Effects of the Invention >> As described below, according to the present invention, it is possible to realize an optical frequency spectrum analyzer having high accuracy and high resolution with absolute accuracy.

[Brief description of the drawings]

FIG. 1 is a block diagram showing the configuration of an embodiment of an optical frequency spectrum analyzer according to the present invention, FIG. 2 is a time chart showing one mode of operation of the apparatus in FIG. 1, FIG. 4 and 5 are configuration block diagrams showing a modification of a part of the device shown in FIG. 1, FIG. 6 is a configuration block diagram showing one configuration example of a local oscillator 3 of the device shown in FIG. 1, and FIG. Is a configuration block diagram showing a configuration example that embodies the configuration of FIG. 6,
FIG. 8 is a characteristic curve diagram for explaining the operation of the FIG. 7 apparatus, FIG. 9 is an explanatory diagram for explaining the operation of the FIG. 7 apparatus, and FIGS. 10 and 12 to 14 are FIG. FIG. 7 is an explanatory diagram showing another example of the configuration of the tunable laser diodes VL1 to VL3 in FIG. 7, FIG. 11 is an operation explanatory diagram for explaining the operation of the FIG. 10 device, and FIG. FIG. 16 and FIG. 17 are explanatory diagrams of the principle for showing a conventional optical frequency spectrum analyzer. 3 local oscillator 4 optical heterodyne detector 5
... Filter section, 6 ... Detection section, 7 ... Signal processing / display section.

 ──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-58-182524 (JP, A) JP-A-57-103380 (JP, A) JP-B-5-13399 (JP, B2) JP-B 5-5- 23613 (JP, B2) Applied Physics, 50 [1] (1981) 29− 34

Claims (1)

(57) [Claims] 1. A polarization control section (1) for receiving a light to be measured having a frequency (ωi) and controlling a polarization plane of the incident light, and a reference for outputting a reference optical signal having a known absolute frequency. A wavelength light source (310), a sweep signal generator (330) for outputting an electric signal that changes at a constant period, and an optical signal from a reference wavelength light source as one input and relating to an optical frequency difference between the other input. An optical heterodyne detector (321) that outputs an electric signal, a mixer (MX1) that mixes the electric signal output from the optical heterodyne detector with an electric signal from the sweep signal generator, and an electric signal from the mixer A variable wavelength light source section (322) for converting the output of the optical signal into an optical signal and outputting the output light as a return light signal and an output light having a frequency (ωo).
And an optical frequency shifter (323) which receives a feedback optical signal from the variable wavelength light source unit and converts the frequency of the optical signal to an optical signal having a frequency shifted, and multiplies the frequency of the optical signal from the optical frequency shifter. A local oscillator (3) having an optical frequency multiplying circuit (324) as the other input of the optical heterodyne detection unit and configured independently of the light source of the measured light; A light combining means (HM1) for inputting the output light and the output light of the polarization control unit and combining the two output lights; inputting the optical signal output from the light combining means to output light of the local oscillator and a polarization control unit; An optical heterodyne detector (4) that converts the signal into an electric signal related to the frequency difference (ωo−ωi) with the output light of the optical signal; and the frequency of the required light to be measured among the electric signals input from the optical heterodyne detector Outputs signals related to A band-pass filter (5), a detection unit (6) for inputting an electric signal output from the band-pass filter, and an output from a sweep signal generator in the local oscillator which receives an output of the detection unit as an input. A signal processing / display unit (7) input to the frequency axis as a sweep signal, wherein the power and absolute value of the frequency of the frequency spectrum of the measured light are measured. analyzer.