JPH08166610A - Multiplex optical frequency com generator - Google Patents

Abstract

PURPOSE: To provide a multiplex optical frequency com generator widened in the generation width of side band waves. CONSTITUTION: The phases of laser beams varying in wavelength from each other made incident via an optical coupler 102 from plural laser beam sources 101A to 101D of an optical phase modulator 112 formed with an incident end reflection film 117 and an exit end reflection film 118 at a light incident end and a light exit end are modulated according to the modulation signals of a frequency (fm) from an oscillator 111, by which the side bands of the laser beams are generated at every frequency (fm) interval of the modulation signals on the low frequency side and the high frequency side around the frequency of the laser beams.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical frequency comb generator for generating a laser beam having a sideband with a constant frequency interval, and more particularly to a multiple optical frequency comb generator using an optical phase modulator. .

[0002]

2. Description of the Related Art When heterodyne detection of two laser beams is performed to measure or control the difference frequency, the band is limited by the band of the light receiving element and is about several tens GHz, so an optical frequency comb generator is used. A wideband heterodyne detection system is constructed by using this. The optical frequency comb generator generates several hundred sidebands of the incident laser light at equal frequency intervals, and the frequency stability of the generated sidebands is almost the same as that of the original laser light. Therefore, if this sideband wave and another laser beam are subjected to heterodyne detection, a broadband heterodyne detection system over several THz can be constructed.

A conventional optical frequency comb generator is shown in FIG.
As shown in FIG. 2, lithium niobate (LiNbO ₃ ) is provided in an optical resonator 33 in which two reflecting mirrors 31 and 32 are installed to face each other and a microwave waveguide 34 into which a modulation signal of microwaves is injected. And a bulk type optical phase modulator 36 having an electro-optic crystal substrate 35 installed therein, and the optical phase modulator 36 is installed on the optical path of laser light that reciprocates in the optical resonator 33. Was becoming.

[0004]

However, the conventional optical frequency comb generator having the above structure has the following problems. First, since the optical phase modulator 36 is installed on the optical path of the laser light that reciprocates in the optical resonator 33,
Since there are two incident losses and two outgoing losses per round trip, the finesse of the optical resonator 33 is reduced and the sideband wave generation efficiency is extremely low. Secondly, the two that make up the optical resonator 33
A holder is required to hold the reflecting mirrors 31 and 32 in parallel with each other accurately, which increases the size of the entire optical frequency comb generator.

Therefore, the applicant of the present invention filed Japanese Patent Application No. 5-203.
As No. 441, an optical frequency comb generator has been previously proposed, which has high efficiency, low cost, and good accuracy without adjustment, for example, by having a configuration as shown in FIG.

FIG. 1 is a block diagram showing a heterodyne detection system constructed using an optical frequency comb generator 10 according to Japanese Patent Application No. 5-203441. This heterodyne detection system includes a first laser light source 1 that emits a laser beam for heterodyne detection, a second laser light source 1 that emits a laser beam for measurement, and a heterodyne from the first laser light source 1. The optical frequency comb generator 10 on which the laser light for detection is incident, the optical frequency comb generated by the optical frequency comb generator 10 and the laser light for measurement from the second laser light source 2 are mixed. Optical system 3 and a photodetector 4 that receives the laser light mixed by the optical system 3, and detects the difference frequency component between the optical frequency comb and the laser light to be measured as the heterodyne detection output. Output from the container 4.

The optical frequency comb generator 10 includes an oscillator 1
A waveguide type optical phase modulator 12 that modulates the phase of the incident laser light according to the modulation signal of frequency fm from 1 is provided.

The waveguide type optical phase modulator 12 comprises an electro-optic crystal substrate 13 such as lithium niobate (LiNbO ₃ ) as shown in FIG. On the electro-optic crystal substrate 13, a waveguide 14 is formed along the optical axis, and electrodes 15 and 16 are formed on the waveguide 14 and on both sides of the waveguide 14 along the optical axis. ing. Also,
On the electro-optic crystal substrate 13, chromium, gold, aluminum or a dielectric multilayer film is vapor-deposited on a light incident end and a light emitting end which are perpendicular to the optical axis. The reflective film 18 is formed.

A waveguide type optical phase modulator 1 having such a structure
2, the incident end reflection film 17 and the emission end reflection film 18
Is the waveguide 14 from the incident end through the incident end reflection film 17.
A Fabry-Perot etalon that resonates the laser light incident on the inside of the waveguide 14 is configured.

In the waveguide type optical phase modulator 12, the modulation signal of the frequency fm from the oscillator 11 is applied to the electrodes 15 and 16, and the electric field corresponding to the modulation signal is applied to the waveguide 14. By doing so, the phase of the laser light incident on the waveguide 14 from the incident end is modulated according to the modulation signal. Moreover, in the above-mentioned waveguide type optical phase modulator 12, the electric field due to the modulation signal can be concentrated in the waveguide 14 to efficiently modulate the phase of the laser light with an extremely small required power for modulation.

In the optical frequency comb generator having such a configuration, the laser light having the frequency fs as shown in FIG. 3 is made incident on the waveguide type optical phase modulator 12, so that as shown in FIG. Sideband waves fs ± nfm are generated at intervals of the frequency fm of the modulation signal on the low frequency side and the high frequency side centering on the frequency fs of the laser light.

Therefore, an object of the present invention is to further widen the width of generation of sidebands in the optical frequency comb generator having such a configuration.

Another object of the present invention is to combine a plurality of optical frequency combs so that all sidebands can have frequency stability with only one frequency reference. To provide a comb generator.

[0014]

In order to solve the above-mentioned problems, a multiple optical frequency comb generator according to the present invention comprises a plurality of laser light sources for emitting laser lights having different wavelengths,
An oscillator for generating a modulation signal of a frequency fm, an optical phase modulator for modulating the phases of laser lights having different wavelengths, which are incident from the plurality of laser light sources, according to a modulation signal from the oscillator, and the optical phase modulation Of the wavelengths emitted from the plurality of laser light sources, which are formed at the light incident end and the light emitting end of the container and resonate the incident laser light inside the optical phase modulator. An optical coupler that combines different laser lights and makes them incident on the optical phase modulator through the incident end reflection film, and has a low frequency side centered on the frequency of each laser light incident on the optical phase modulator. And a sideband wave of light is generated on the high frequency side at intervals of the frequency fm of the modulated signal.

The multiple optical frequency comb generator according to the present invention is characterized in that the optical phase modulator comprises a waveguide type optical phase modulator.

Further, the multiple optical frequency comb generator according to the present invention is characterized in that the optical coupler comprises an optical fiber coupler.

Further, in the multiple optical frequency comb generator according to the present invention, with respect to the optical frequency comb emitted from the optical phase modulator through the emission end reflection film, the laser light emitted from two laser light sources is used. A hederodyne signal detector for detecting a hederodyne component of the sideband is provided, and the hederodyne signal detected by the hederodyne signal detector is negatively fed back to one of the two laser light sources.

[0018]

In the multiple optical frequency comb generator according to the present invention, the optical phase modulator itself having the incident end reflection film and the emission end reflection film formed at the light incident end and the light emission end functions as an optical resonator. The optical phase modulator phase-modulates laser light having different wavelengths, which are incident from a plurality of laser light sources through an optical coupler, so that the frequency of each laser light is centered on a low frequency side and a high frequency side. Sidebands of light are generated at intervals of frequency fm of the modulated signal. Further, the waveguide type optical phase modulator can concentrate the electric field due to the modulation signal in the waveguide.

With respect to the optical frequency comb emitted from the optical phase modulator via the emission end reflection film, the hederodyne component of the sideband for the laser light emitted from the two laser light sources is detected, and the two lasers are detected. By negatively feeding back one of the light sources, the sidebands of the laser light are coupled to each other.

[0020]

Embodiments of the multiplex optical frequency comb generator according to the present invention will now be described in detail with reference to the drawings.

The multiple optical frequency comb generator according to the present invention comprises:
For example, it is configured as shown in FIG.

The multiple optical frequency comb generator 100 shown in FIG. 5 is a waveguide type optical frequency comb generator (WG-OFCG: Wavegauid) for constructing the heterodyne detection system shown in FIG.
e Type Optical Frequency Comb Generator), in which the frequency fm from the oscillator 111 is
The waveguide type optical phase modulator 112 that modulates the phase of the incident laser light according to the modulated signal of 4 is provided, and the four laser light sources 101A, 101B, 101C and 101D have different wavelengths through the optical coupler 102. λ _A , λ _B , λ
The laser light of _C and λ _D is generated by the waveguide type optical phase modulator 112.
It is designed to be incident on.

Multiple optical frequency comb generator 10 of this embodiment
0, the four laser light sources 101A, 101
B, 101C and 101D are laser diodes each having a different oscillation frequency, and have different wavelengths λ _A =
1.50 μm, λ _B = 1.52 μm, λ _C = 1.54 μm
, Λ _D = 1.56 μm laser light is emitted.

The optical coupler 102 is composed of, for example, an optical fiber coupler, and the four laser light sources 10 are provided.
The laser lights of different wavelengths λ _A , λ _B , λ _C , and λ _D emitted from 1A, 101B, 101C, and 101D are combined and incident on the waveguide type optical phase modulator 112. There is.

Further, the oscillator 111 supplies a modulation signal having a frequency of fm = 13.012 GHz and a power of 2 W to the waveguide type optical phase modulator 112 via the amplifier 113.

Further, the above-mentioned waveguide type optical phase modulator 112 is used.
Is an electro-optic crystal substrate such as lithium niobate (LiNbO ₃ ). A waveguide is formed on the electro-optic crystal substrate along the optical axis, and electrodes are formed on the waveguide and on both sides of the waveguide along the optical axis. Further, on the electro-optic crystal substrate, chromium, gold, aluminum or a dielectric multilayer film is vapor-deposited on a light incident end and a light emitting end perpendicular to the optical axis,
An incident end reflection film 117 and an emission end reflection film 118 are formed.

A waveguide type optical phase modulator 1 having such a structure
In FIG. 12, the incident end reflection film 117 and the emission end reflection film 118 are Fabry-Perot etalons that resonate each laser light incident on the waveguide from the incident end through the incidence end reflection film 117 inside the waveguide. I am configuring.

This waveguide type optical phase modulator 112
Then, the modulation signal of the frequency fm from the oscillator 111 is applied to the electrode, and the electric field corresponding to the modulation signal is applied to the waveguide, so that each laser incident on the waveguide from the incident end. The phase of light is modulated according to the modulation signal. Moreover, the waveguide type optical phase modulator 11
In No. 2, the electric field generated by the modulation signal can be concentrated in the waveguide, and the phase of each laser beam can be efficiently modulated with extremely small required modulation power.

In the multiplex optical frequency comb generator having such a structure, the frequency C of the laser light is generated by making the laser light having the wavelength λ enter the waveguide type optical phase modulator 112.
A sideband C / λ ± nfm is generated at intervals of frequency fm of the modulated signal on the low frequency side and the high frequency side centering on / λ (C is the speed of light).

In the multiplex optical frequency comb generator 100 having such a configuration, when the FSR is 1.86 GHz and the finesse is 10, the four laser light sources 10 described above are used.
Laser light having different wavelengths λ _A , λ _B , λ _C , and λ _D from 1A, 101B, 101C, and 101D enters the waveguide type optical phase modulator 112, and the waveguide type optical phase modulator 1
When the emitted light from 12 is observed by the optical spectrum analyzer 120, as shown in A of FIG. 6, in the non-modulation state in which the modulation signal is not input, that is, the sideband is not generated,
While the laser light has four frequencies C / λ _A , C / λ _B , C / λ _C and C / λ _D corresponding to the wavelengths λ _A , λ _B , λ _C and λ _D , the above modulation In a modulation state in which a signal is input, that is, a sideband generation state, as shown in FIG. 6B, four frequencies C / λ _A , C / λ _B corresponding to the wavelengths λ _A , λ _B , λ _C , and λ _D are set. , C / λ _C , C / λ _D were generated as sidebands, and an optical frequency comb in which sidebands were generated over a range of about 80 nm could be obtained.

Here, each curve shown in A and B of FIG. 6 shows the transmission loss of the optical fiber, and the sideband is generated in most of the range where the transmission loss is the minimum.

A fiber-coupled WG-OFCG in which a polarization-maintaining fiber is connected to the Ti-LiNbO ₃ waveguide surface.
Frequency of 13.014 GHz, phase modulation index 3
The WG-OFCG, which is driven at π, is made to enter the laser light having the oscillation wavelengths of 1.538 μm and 1.562 μm from the two laser light sources through the optical fiber coupler, and the generated sideband envelope is measured by the optical spectrum analyzer. As a result of observation, an optical frequency comb over a wide range of 40 nm (= 5 THz) could be obtained as shown in FIG.

The multiplex optical frequency comb generator 200 according to the present invention is constructed, for example, as shown in FIG.

The multiple optical frequency comb generator 200 shown in FIG. 8 is a waveguide type optical frequency comb generator (WG-OFCG: Wavegauid) for constructing the heterodyne detection system shown in FIG.
e Type Optical Frequency Comb Generator), in which the frequency fm from the oscillator 211 is applied.
Depending on the modulation signal, comprising a waveguide type optical phase modulator 212 to modulate the phase of the incident laser beam, the two laser light sources 201A, mutually different wavelengths through an optical coupler 202 from 201B lambda _A, lambda _B laser light is optical coupler 2
The light emitted from the waveguide type optical phase modulator 212 is detected by the photodetector 221 while being incident on the waveguide type optical phase modulator 212 via 02, and the detection output is to the laser light source 201B. Negative feedback is provided. The waveguide type optical phase modulator 212 includes an incident end reflection film 21 which constitutes a Fabry-Perot etalon for resonating each laser light incident on the waveguide from the incident end inside the waveguide.
7 and the exit end reflection film 218 are provided.

The multiple optical frequency comb generator 20 of this embodiment
At 0, the photodetector 221 guides the laser light emitted from the two laser light sources 201A and 201B and having different wavelengths λ _A and λ _B according to the modulation signal of the frequency fm from the oscillator 211. Waveguide type optical phase modulator 2
With respect to the outgoing light that has been phase-modulated by 12, the above wavelengths λ _A and λ
_A heterodyne signal between two optical frequency combs that generate sidebands having center frequencies of two frequencies C / λ _A and C / λ _B corresponding to _B is detected, and the heterodyne signal is transmitted to the laser light source 201B. Negative feedback is provided.

Here, the above-mentioned waveguide type optical phase modulator 212
_A heterodyne signal when the slopes of two optical frequency combs that generate sidebands with the center frequencies of C / λ _A and C / λ _B obtained as the output light of the two are overlapped as shown in FIG. Is the superposition of the heterodyne signals of many sideband pairs. Then, only the signals in the regions A, B, and C shown in FIG. 9 in which the powers of both sideband pairs are equal to or higher than the white noise level are valid,
In the region B, the phases of the heterodyne signals are deviated from each other by π, so that they all cancel each other. Therefore, only the heterodyne signals of the areas A and C are detected.

The above-mentioned two laser light sources 201A and 201B
From the frequency difference C / λ _A −C / λ _B of each laser light incident on the waveguide type optical phase modulator 212 to 0.44 THz.
When the output light of the waveguide type optical phase modulator 212 when (= 3.6 nm) is observed by the optical spectrum analyzer 231, it is found that in the unmodulated state in which the modulated signal is not input, that is, the sideband wave is not generated. As shown in A of FIG. 10, two frequencies C / λ corresponding to the wavelengths λ _A and λ _B are set.
_In contrast to the _A and C / λ _B laser beams, in the modulation state in which the modulation signal is input, that is, in the sideband generation state, as shown in FIG. 10B, the wavelengths correspond to the wavelengths λ _A and λ _B. Two
It was possible to generate sidebands having two frequencies C / λ _A and C / λ _B as center frequencies.

The line width of the heterodyne signal of one sideband wave pair for each laser beam from each laser light source 1A, 1B having an oscillation line width of 1 MHz is 2 MHz. A heterodyne signal having a line width of 2 MHz is detected from the light emitted from the waveguide type optical phase modulator 212. The above-mentioned waveguide type optical phase modulator 21
For the output light of 2, the difference frequency of the sideband pair is 40 MHz.
When the heterodyne signal as z was observed by the RF spectrum analyzer 232, as shown in FIG. 11, a heterodyne signal having a line width of 2 MHz was observed without deterioration due to superposition.

In the multiple optical frequency comb generator of this embodiment, the heterodyne signal detected by the photodetector 221 is negatively fed back to the laser light source 201B.
The two frequency combs are combined. Therefore, by stabilizing the oscillation frequency of the laser light source 201A, it is possible to impart frequency stability to all sidebands by only having one frequency reference.

[0040]

In the multiple optical frequency comb generator according to the present invention, the optical phase modulator itself in which the incident end reflection film and the emission end reflection film are formed at the light incident end and the light emission end functions as an optical resonator. . The optical phase modulator phase-modulates laser light having different wavelengths, which are incident from a plurality of laser light sources through an optical coupler, so that the frequency of each laser light is centered on a low frequency side and a high frequency side. Since the sidebands of light are generated at intervals of the frequency fm of the modulated signal, it is possible to obtain an optical frequency comb over a wide range. In addition, the waveguide type optical phase modulator can concentrate the electric field by the modulation signal in the waveguide and efficiently modulate the phase of each laser light with a very small required modulation power.

Further, the multiple optical frequency comb generator according to the present invention, with respect to the optical frequency comb emitted from the optical phase modulator through the emission end reflection film, with respect to the laser light emitted from two laser light sources. The two frequency combs can be coupled by detecting the hederodyne component of the sideband and performing negative feedback to one of the two laser light sources. Therefore, by stabilizing the oscillation frequency of one of the laser light sources, it is possible to impart frequency stability to all sidebands by only having one frequency reference.

[Brief description of drawings]

FIG. 1 is a block diagram showing a heterodyne detection system to which a multiple optical frequency comb generator according to the present invention is applied.

FIG. 2 is a perspective view schematically showing the configuration of the previously proposed optical frequency comb generator.

FIG. 3 is a spectrum diagram of laser light incident on the optical frequency comb generator.

FIG. 4 is a spectrum diagram of an optical frequency comb generated by the optical frequency comb generator.

FIG. 5 is a block diagram schematically showing a configuration of a multiplex optical frequency comb generator according to the present invention.

6 is a characteristic diagram showing a result of observing a sideband non-generation state and a sideband generation state with respect to the laser light emitted from the multiplex optical frequency comb generator shown in FIG. 5, using an optical spectrum analyzer.

FIG. 7 is a characteristic diagram showing an envelope of a sideband wave obtained by observing laser light emitted from another multiplex optical frequency comb generator according to the present invention with an optical spectrum analyzer.

FIG. 8 is a block diagram schematically showing another configuration of the multiplex optical frequency comb generator according to the present invention.

9 is a diagram schematically showing a sideband wave generation state for explaining detection of a heterodyne signal of an optical frequency comb of laser light emitted from the multiplex optical frequency comb generator shown in FIG.

10 is a characteristic diagram showing a result of observing a sideband non-generation state and a sideband generation state of the laser light emitted from the multiplex optical frequency comb generator shown in FIG. 8 with an optical spectrum analyzer.

11 is a characteristic diagram showing a result of observing a heterodyne signal of laser light emitted from the multiplex optical frequency comb generator shown in FIG. 8 with an RF spectrum analyzer.

FIG. 12 is a perspective view schematically showing a configuration of a conventional optical frequency comb generator.

[Explanation of symbols]

 100,200 Multiple optical frequency comb generator 101A-101D, 201A, 201B Laser light source 102,202 Optical fiber coupler 111,211 Oscillator 117,217 Incident reflection film 118,228 Emission reflection film 221 Photodetector

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification number Office reference number FI Technical indication location G02F 1/035 H01S 3/131 3/133 H04J 14/00 14/02 (72) Inventor Takashi Saito Description 381-1 Hayashi, Atsugi City, Kanagawa Prefecture Corpo Emerald 201 (72) Inventor Eric Duran 794-302 Suenaga, Takatsu-ku, Kawasaki City, Kanagawa Prefecture

Claims (4)

[Claims] 1. A plurality of laser light sources that emit laser lights having different wavelengths, an oscillator that generates a modulation signal of a frequency fm, and phases of laser lights having different wavelengths that are incident from the plurality of laser light sources. An optical phase modulator that modulates according to a modulation signal from an oscillator, and an incident end that is formed at a light incident end and a light emitting end of the optical phase modulator and resonates the incident laser light inside the optical phase modulator. A reflection film and an emission end reflection film, and an optical coupler that combines laser beams of different wavelengths emitted from the plurality of laser light sources and makes them incident on the optical phase modulator through the incidence end reflection film, Multiple optical frequencies, characterized in that sidebands of light are generated at low frequency side and high frequency side centered on the frequency of each laser beam incident on the optical phase modulator at intervals of frequency fm of the modulation signal. Beam generator. 2. The multiplex optical frequency comb generator according to claim 1, wherein the optical phase modulator comprises a waveguide type optical phase modulator. 3. The multiple optical frequency comb generator according to claim 1, wherein the optical coupler comprises an optical fiber coupler. 4. A hederodyne signal detector for detecting a hederodyne component of a sideband of laser light emitted from two laser light sources for an optical frequency comb emitted from the optical phase modulator via the emission end reflection film. The multi-optical frequency comb generator according to claim 1, wherein the hederodyne signal detected by the hederodyne signal detector is negatively fed back to one of the two laser light sources.