JP3070326B2 - Second harmonic generator and light source comprising the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a technique for stabilizing the oscillation frequency of a semiconductor laser used for optical communication in the 1.5 .mu.m band, and more particularly to a second harmonic generator used as a frequency reference and a light source comprising the same.

[0002]

2. Description of the Related Art As a method for expanding the capacity of optical communication, an optical frequency multiplexing method (FDM; Frequency Domain Multiplexing) for multiplexing using a plurality of lights having slightly different wavelengths has been studied. In this method, the frequency of these lights, that is, the oscillation frequency of the light source, must be precisely controlled. A semiconductor laser is used as the light source, but the oscillation frequency of the semiconductor laser is easily affected by the driving current and the operating temperature. For this reason, it is necessary to actually detect the laser beam, compare the frequency with the frequency reference, and perform control to minimize the difference between the two.

[0003] For this purpose, there is a frequency control method using the absorption line of an atom or a molecule as a reference. That is, a part of the laser light is transmitted through gaseous atoms or molecules having an absorption line at a frequency substantially equal to the oscillation frequency,
The drive current is feedback-controlled so that the output of the transmitted light at this time is minimized.

By the way, from the viewpoint of transmission loss in a silica-based optical fiber, the transmission wavelength in current optical communication is 1.5 times.
Although the μm band is the mainstream, there is no suitable atom as a gas having a characteristic absorption line in this wavelength band, and it cannot use molecular beam absorption such as water (H ₂ O) or ammonia (NH ₃ ) Not get. However, the absorption coefficient of molecular beam is small because it is based on the rotational or vibrational motion of molecules. As a result, it is necessary to detect weak absorption, so the absorption container filled with these gases becomes large, and the control circuit is complicated.

On the other hand, a device for controlling the frequency, that is, stabilizing the frequency using the atomic absorption at a wavelength of 0.78 μm of rubidium (Rb) and the second harmonic of a laser beam of 1.5 μm is disclosed in Japanese Unexamined Patent Publication No. -137494. That is, a second harmonic is generated by a fundamental wave having a wavelength of 1.56 μm, and the second harmonic is transmitted through an absorption container filled with gaseous rubidium so that the intensity of the transmitted light at this time is minimized. Controls the laser drive current. As a result, the semiconductor laser is controlled so that the wavelength of the output light becomes 1.56 μm. The laser light whose wavelength, that is, the frequency is thus controlled, can be used as the reference frequency light of the optical frequency multiplexing method. In addition, the second harmonic can be used as short-wavelength coherent light.

By utilizing the atomic absorption line of rubidium as described above, it is possible to reduce the size of the absorption container and simplify the control circuit.

[0007]

In the frequency stabilizing device disclosed in the above publication, it is necessary to generate the second harmonic of the laser beam having the target reference frequency. Such a second
It is well known that a nonlinear optical material such as a crystal such as lithium niobate or lithium titanate (LiNbO ₃ ) is used as the harmonic generation means. Generally, the conversion efficiency of the second harmonic in the nonlinear optical crystal is proportional to the power of the incident laser light. The output of a semiconductor laser is not large enough. Therefore, the nonlinear optical crystal, that is, the second
It is desirable to provide an optical waveguide in the harmonic generation element to increase the optical power density as much as possible so that high conversion efficiency can be obtained.

On the other hand, when the fundamental wave emitted from the second harmonic generation element is used as the reference frequency signal light, it is necessary to efficiently separate the second harmonic from the fundamental wave. For this purpose, basically, the following two separation methods can be considered.

(1) As shown in FIG. 4, a part of the fundamental wave before entering the second harmonic generating element 1 is separated by a filter 2A, coupled to an optical fiber 3, and the rest is converted to a second harmonic. The light is incident on the element 1.

(2) As shown in FIG. 5, the second harmonic and the fundamental wave emitted from the second harmonic generator 1 are filtered by a filter 2B.
And the fundamental wave is coupled to the optical fiber 3,
Harmonics are used for frequency control.

In the method according to the configuration of FIG. 4, since the power of the fundamental wave incident on the second harmonic generation element 1 is low,
High conversion efficiency cannot be obtained. On the other hand, in the method according to the configuration of FIG. 5, the fundamental wave of the guided mode, which is easily coupled to the optical fiber 3, is once emitted into space, condensed by the lens 5D, and then coupled to the optical fiber 3. Misalignment and lens 5
Loss due to reflection and absorption by C and 5D occurs.

An object of the present invention is to solve the conventional problems in the frequency stabilizing method using the second harmonic as described above.

[0013]

An object of the present invention is to provide a first end face made of a non-linear optical material and on which a fundamental wave is incident.
A second harmonic generation element having a second end face opposing the end face of the first and second light sources and having at least an optical waveguide for guiding the incident fundamental wave and generating a second harmonic of the fundamental wave; A first filter having an optical characteristic of transmitting the fundamental wave and reflecting a second harmonic of the fundamental wave and being provided in close contact with the second end face, and being incident on the first end face; And a second harmonic generator according to the present invention, comprising: an optical unit for coupling the fundamental wave to the optical waveguide. The second harmonic, which has an optical characteristic of reflecting a harmonic and is disposed in front of the first end face and is emitted from the second harmonic generation element after being reflected by the first filter, is used as the second harmonic. 1 to an optical path different from the fundamental wave incident on the end face. Only second harmonic generation device, according to the present invention characterized by comprising a second filter for reflecting or, and either the second harmonic generator described above,
A semiconductor laser for generating the fundamental wave incident on the first end face; and means for controlling an oscillation frequency of the semiconductor laser based on the second harmonic emitted from the second harmonic generation element. This is achieved by any of the light sources according to the present invention.

[0014]

A filter having an optical characteristic of transmitting a fundamental wave and reflecting the second harmonic is formed in close contact with an end face of the second harmonic generation element opposite to the light incident surface. Thereby, since the entire power of the fundamental wave is input to the second harmonic generation element, the fundamental wave emitted from the second harmonic generation element can maintain a high conversion efficiency to the second harmonic. An optical fiber can be directly coupled to the second harmonic generation element without using an optical system for separating the second harmonic, and the fundamental wave can be transmitted without loss due to optical axis shift or lens absorption or reflection. Highly efficient transmission.

[0015]

FIG. 1 is an explanatory view of an embodiment of the present invention. The second harmonic generation element 1 is made of a non-linear optical material such as a LiTaO ₃ crystal. The distance between the opposing end faces 1a and 1b parallel to the C-axis of the crystal, that is, the element length is about 20 mm. Assuming that the end face 1a is a face on which a fundamental wave having a wavelength λ ₀ = 1.56 μm is incident, at least the end of the optical waveguide on the end face 1b transmits the fundamental wave and the second harmonic (wavelength λ
A filter 11 that totally reflects ( ₁ = 0.78 μm) is formed in close contact. Filter 11 having such desired optical characteristics
May be formed using a dielectric multilayer film as is well known. As described above, in order to achieve high second harmonic conversion efficiency, it is necessary to increase the power density of the fundamental wave in the second harmonic generation element. For this purpose, as shown in FIG. 2, the end face 1a and the end face
An optical waveguide 14 is provided between the optical waveguide 1b and the optical waveguide 1b. And the end face
A fundamental wave is made incident on the end of the optical waveguide 14 in 1a.

The second harmonic generated inside the second harmonic generation element 1
The harmonic (SH) is emitted from the end of the optical waveguide on the end face 1a to the outside of the second harmonic generation element 1. The second harmonic is separated and extracted from the fundamental wave incident on the second harmonic generation element 1. In the present embodiment, a filter 12 that transmits the fundamental wave and totally reflects the second harmonic is installed in front of the end face 1a. As the filter 12, the filter in FIG.
The same thing as 2B may be used. Second harmonic generation element 1
The second harmonic emitted from the filter 12
It is reflected in a direction different from the fundamental wave incident on 1a. The second harmonic separated in this way enters the oscillation frequency control means 20, which will be described later.

The fundamental wave passes through the filter 11 and is emitted outside the second harmonic generation device 1. Here, the fundamental wave is directly incident on the optical fiber 3 by installing the optical fiber 3 so as to be in close contact with the end of the optical waveguide 14 on the end face 1b via the filter 11. The fundamental wave thus combined with the optical fiber 3 is used as a reference frequency signal light in the optical communication based on the optical frequency multiplex system as described above.

A second ferroelectric crystal such as LiTaO ₃
The conversion efficiency of the second harmonic can be improved by forming a domain-inverted region 15 that partially inverts the polarization of the harmonic generation element 1 at a predetermined period along the optical waveguide 14. That is, because the fundamental wave and the second harmonic traveling in the optical waveguide 14 have different speeds, a phase shift occurs, and the fundamental waves interfere with each other periodically. By forming the domain-inverted region 15 for inverting the polarization direction of the second harmonic generation element 1 for each interference distance, cancellation by interference is eliminated and conversion efficiency is improved.

The method of forming the optical waveguide 14 and the domain-inverted region 15 as described above by the proton exchange method is described in another application by the present inventors (Japanese Patent Application Nos. 3-243722 and 4-058352). In more detail.

The element length is 20 mm, and the array period is 15 μm.
Of LiTaO ₃ crystal with domain-inverted regions 15 formed
As shown in FIG. 1, a fundamental wave light having a wavelength of 1.56 μm emitted from a semiconductor laser 4 was incident on the harmonic generation device 1. The fundamental light emitted from the semiconductor laser 4 is collimated into a parallel beam by a lens 5A, passes through an isolator 6 and a filter 12, and then passes through a lens 5B.
As a result, the light is focused on the end face 1a of the second harmonic generation element 1.
The isolator 6 is provided in order to prevent a problem that the operation of the semiconductor laser 4 becomes unstable due to the re-incident of the light reflected and returned by the optical member arranged thereafter.

A part of the fundamental wave light incident on the second harmonic generation element 1 is converted to a second harmonic light having a wavelength of 0.78 μm, and propagates along the waveguide with the fundamental wave. At the end face 1 b of the second harmonic generation element 1, only the fundamental light transmitted through the filter 11 is coupled to the optical fiber 3. The second harmonic light is reflected by the filter 11, goes back through the waveguide, and is emitted from the end face 1a. The second harmonic light emitted from the second harmonic generation element 1 is reflected by the filter 12 in a direction different from the optical path of the fundamental light.

The second harmonic generation device 10 of the present invention comprising the second harmonic generation element 1 and the filters 11 and 12 generates 0.1 mW second harmonic light from 50 mW fundamental wave light. The power of the fundamental light is an input value at the end face 1a of the second harmonic generation element 1, and the power of the second harmonic light is a value after reflection by the filter 12.

According to the present invention, the second harmonic generator
By combining 10 with a semiconductor laser 4 and a control device 20 for controlling the oscillation frequency thereof, as shown in FIG. 1, a light source whose frequency is stabilized is provided.

The oscillating frequency control device 20 is composed of rubidium (Rb)
The cell has a rubidium cell 21 filled with a gas. The rubidium cell 21 has a wavelength of 1.56 μm output from the semiconductor laser 4.
It exhibits strong atomic absorption at a wavelength of 0.78 μm corresponding to the second harmonic of the fundamental light. Therefore, the second harmonic light generated by the second harmonic generator 10 is separated and extracted by the filter 12 as described above, and is incident on the rubidium cell 21. The second harmonic light transmitted through the rubidium cell 21 is detected by the photodetector 22, and the current control circuit 23 controls the drive current for the semiconductor laser 4 so that the transmitted light intensity is minimized. Thus, the output light wavelength of the semiconductor laser 4 is controlled so that the wavelength of the second harmonic light coincides with the absorption line of the rubidium cell 21. This control effectively operates not only for the wavelength fluctuation of the output light of the semiconductor laser 4 due to the fluctuation of the driving current but also for the wavelength fluctuation due to the temperature fluctuation. The oscillation circuit 25 and the synchronous detection circuit 26 control the output light wavelength,
For example, when the wavelength of the second harmonic light deviates from the absorption line of the rubidium cell 21, a general method for determining whether the current control circuit 23 controls the drive current to increase or decrease. Means.

In the above embodiment, the end face 1a and the end face 1b of the second harmonic generation element 1 are parallel to each other, and the optical path of the fundamental wave light in the second harmonic generation element 1 is at least perpendicular to the end face 1b. It is assumed that. Therefore, as shown in FIG. 3, for example, the end face 1b of the second harmonic generation element 1 is
It is formed to be inclined with respect to the fundamental wave incident here. A filter 11 that transmits the fundamental wave light and reflects the second harmonic light is formed on the end face 1b as in the above-described embodiment.

According to the structure shown in FIG. 3, the second harmonic light reflected by the filter 11 travels in a direction different from the optical path of the fundamental light, and travels in a direction different from the optical path of the fundamental light incident on the end face 1a. The light is emitted outside the second harmonic generation element 1. Therefore, the filter 12 for separating the second harmonic light from the fundamental light in the embodiment of FIG. 1 can be omitted. Also in the structure of FIG. 3, optical waveguides 14a and 14b for propagating the fundamental light and the second harmonic light are provided between the end faces 1a and 1b in order to increase the conversion efficiency.

Various modifications can be made to the structure of FIG.
For example, on the optical axis of the second harmonic light reflected by the filter 11, an end face perpendicular to this optical axis is formed in the second harmonic generation element, that is, the exit face of the second harmonic light is formed on the end face 1a. And so on.

[0028]

According to the present invention, since the fundamental wave is made incident on the second harmonic generating element without being divided, a high second harmonic conversion efficiency can be obtained. Also, since the fundamental wave can be directly coupled to the optical fiber without passing through a lens or filter,
Not only is optical axis alignment easy, but also optical axis misalignment or loss due to surface reflection or absorption of the lens is avoided. Thus, it is possible to provide a small-sized second harmonic generator having high conversion efficiency and coupling efficiency, and a small and stabilized frequency reference light source configured using the same.

[Brief description of the drawings]

FIG. 1 is a block diagram for explaining an embodiment of the present invention.

FIG. 2 is a schematic diagram for explaining another embodiment of the present invention.

FIG. 3 is a schematic view for explaining still another embodiment of the present invention.

FIG. 4 is an explanatory diagram of a conventional problem (part 1).

FIG. 5 is an explanatory view of a conventional problem (part 2).

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 2nd harmonic generation element 1a, 1b End surface 2, 11, 12 Filter 3 Optical fiber 4 Semiconductor laser 5 Lens 6 Isolator 10 2nd harmonic generation device 14, 14a, 14b Optical waveguide 15 Polarization inversion area 20 Oscillation frequency control device 21 Rubidium cell 22 Photodetector 23 Current control circuit 25 Oscillator 26 Synchronous detector

──────────────────────────────────────────────────続 き Continuation of the front page (58) Field surveyed (Int. Cl. ⁷ , DB name) H01S 5/00-5/50 G02F 1/37 JICST file (JOIS)

Claims (7)

(57) [Claims] 1. An optical waveguide comprising a non-linear optical material and having a first end face on which a fundamental wave is incident and a second end face opposite to the first end face, for guiding at least the incident fundamental wave. A second harmonic generating element provided with a wave path and generating a second harmonic of the fundamental wave, having an optical characteristic of transmitting the fundamental wave and reflecting the second harmonic of the fundamental wave; A first filter provided in close contact with the second end face; and an optical means for coupling the fundamental wave incident on the first end face to the optical waveguide. 2 harmonic generator. 2. The second filter, having an optical property of transmitting the fundamental wave and reflecting the second harmonic, being disposed in front of the first end face and being reflected by the first filter, after being reflected by the first filter. The second emission from the harmonic generation element
The second harmonic generator according to claim 1, further comprising a second filter that reflects the harmonic toward an optical path different from the fundamental wave incident on the first end face. 3. The second optical waveguide, wherein the second end face is formed to be inclined with respect to the fundamental wave incident thereon and guides the second harmonic reflected by the first filter. The second harmonic generator according to claim 1, further comprising: 4. An optical fiber for transmitting the fundamental wave emitted from the second harmonic generation element is tightly coupled to the second end face via the first filter. The second harmonic generator according to claim 1, wherein 5. The second harmonic generation element is made of a ferroelectric crystal, and polarization inversion means for partially inverting the polarization of the ferroelectric crystal is provided periodically along the optical waveguide. The second harmonic generator according to claim 1, wherein: 6. The second harmonic generation device according to claim 1, a semiconductor laser that generates the fundamental wave incident on the first end face, and the second harmonic generation device. Means for controlling the oscillation frequency of the semiconductor laser based on the second harmonic emitted from the light source. 7. The oscillation frequency control means includes: an absorption container filled with a gas absorbing the second harmonic emitted from the second harmonic generation element; and the second harmonic transmitted through the absorption container. Photoelectric conversion means for receiving a signal and outputting a signal corresponding to the intensity thereof; and controlling a parameter on which the oscillation frequency of the oscillation laser depends so as to minimize the intensity of the second harmonic transmitted through the absorption container. 7. The light source according to claim 6, further comprising: a parameter control unit that performs the parameter control.