JP2005172688A - Standard apparatus for optical wavelength - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To simplify a controlling system of apparatus and to miniature the apparatus itself as well as to improve stability of wavelength in apparatus in a standard apparatus for optical wavelength. <P>SOLUTION: In the standard apparatus for optical wavelength equipped with a plane of incidence 2a and a plane of emission 2b formed with the Brewster window, an optical wavelength standard cell 2 encapsulated with absorbing material so as to absorb incident light in the specified optical wavelength zone injected from the plane of incidence, and a pair of total reflection mirrors 4 arranged in opposed position intervening both the plane of incidence and the plane of emission of the optical wavelength standard cell 2, incident light is forced to inject into interior of the optical wavelength standard cell 2 and then multiply reflected light resulting from its several reciprocations inside the optical wavelength standard cell 2 is emitted out of the optical wavelength standard cell 2, after transmitted light from the optical wavelength standard cell 2 is reflected multiply by the pair of total reflection mirrors 4. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an optical wavelength reference device, and more particularly to an optical wavelength reference device that gives a wavelength reference to a wavelength-stabilized light source using a wavelength measuring device or a laser resonator of an optical wave in an optical communication wavelength band. is there.

  Conventionally, as a technique for stabilizing the absolute wavelength (or frequency) of light, the center of the absorption spectrum of an atom or molecule is used as a wavelength reference, and the laser wavelength obtained from a single longitudinal mode laser light source is matched with this optical wavelength reference. Control technology has been studied, and an optical wavelength reference cell in which atoms or molecules are sealed has been developed as a reference device for such an optical wavelength.

  FIG. 13 is a schematic diagram illustrating an example of a configuration of an optical wavelength reference device in a conventional optical communication wavelength band. In this figure, this optical wavelength reference device 100 includes an optical wavelength reference cell 101 in which an acetylene molecule or hydrogen cyanide molecule having an absorption spectrum in the 1.5 μm band of the optical communication wavelength is sealed, and a pair of optical wavelength reference cells 101. It is constituted by a Fabry-Perot optical resonator 102 in which total reflection mirrors are arranged to face each other.

  The Fabry-Perot optical resonator 102 is configured to use a saturated absorption line with a narrow Doppler-free line width as an optical wavelength reference in the optical wavelength reference cell 101, and the intensity of light in the optical wavelength reference cell 101. And the light output wavelength is observed based on the saturation characteristics of the light absorption effect of the molecules in the light wavelength reference cell 101.

  FIG. 14 is a configuration diagram showing an example of the configuration of a wavelength-stabilized laser beam using a conventional optical wavelength reference device. In the figure, the output light from the wavelength tunable laser 105 which is a stabilizing light source is input to the optical wavelength reference device 103, and the difference between the oscillation wavelength of the wavelength tunable laser 105 and the reference wavelength of the optical wavelength reference device 103 is expressed as Fabry Detection is performed by a phase sensitive detection system including a piezoelectric element 104a for changing the resonator length of the Perot optical resonator 102, a photodetector 112, a signal generator 113, and a lock-in amplifier 114, and the detected signal is a negative feedback circuit. The resonant wavelength of the Fabry-Perot optical resonator 102 is stabilized at the reference wavelength of the optical wavelength reference device 103 by feeding back to the applied voltage of the piezoelectric element 105 b using 115. Further, a Fabry-Perot optical resonator is provided by another negative feedback control system including a frequency modulator 106, an optical splitter 107, a signal generator 108, a photodetector 109, an electric mixer 110, and a negative feedback circuit 111. The resonator length of the wavelength tunable laser 105 is negatively feedback controlled in synchronization with the resonator length of 102. At this time, the resonator length of the wavelength tunable laser 105 is controlled by the return light from the optical wavelength reference device 103 branched by the optical splitter 107. Thus, in the conventional wavelength stabilization laser, the oscillation wavelength of the wavelength tunable laser 105 is stabilized at the reference wavelength of the optical wavelength reference device 103 using these two negative feedback control systems.

Next, processing performed by the above-described phase sensitive detection system will be described. Here, FIG. 15 is an explanatory diagram for explaining the concept of phase-sensitive detection. By applying a modulation signal from the signal generator 113 to the piezoelectric element 104a, the resonator length of the Fabry-Perot optical resonator 102 is modulated, and at the same time, the resonance of the wavelength tunable laser 105 is synchronized with the resonator length. The captain is also changed. Therefore, a modulated laser beam that is frequency-modulated with the modulation frequency of the signal generator 113 is output from the wavelength tunable laser 105. The horizontal axis in FIG. 15 is the center wavelength λ _S of this modulated laser beam, and the vertical axis is the optical output P ₀ from the optical wavelength reference device 103. K1 is a characteristic curve showing an example of the absorption characteristic of the optical wavelength reference cell 101. This characteristic curve K1 means that the wavelength light corresponding to the point A1 has a larger absorption characteristic than the wavelength light corresponding to the points A2 and A3. On the other hand, the waveform shown at the bottom of the horizontal axis indicates the modulated laser light applied to the optical wavelength reference cell 101. The phase-sensitive detector output, since modulated laser beam itself is modulated (slight variation) at the modulation frequency f _m around the application point, as shown in the figure, the wavelength variation around the application point occurs waveform It becomes.

Now, in the case where the wavelength light corresponding to a point A1 on the curve K1 shown in FIG. 15 is incident, the output of the beat frequency of the frequency 2f _m as shown in the left side of the vertical axis is detected. On the other hand, the wavelength light corresponding to a point A2 or A3 on the curve K1 is the case of incident, the output of the beat frequency of the modulation frequency f _m is detected.

FIG. 16 is a diagram showing an outline of an output waveform output from the phase sensitive detection system. Now, the horizontal axis represents the center wavelength lambda _S of the modulated laser light, taking the components synchronizing the modulation frequency f _m of the output of the phase-sensitive detection system on the vertical axis, at point A1 of FIG. 15, the frequency f _m Synchronization to the components that are not observed, because the synchronous component is observed to the point A2, A3 at the frequency f _m in the figure, so that the curve K2 shown in FIG. 16. Thus, modulated to a frequency f _m to the synchronous component is zero, more specifically, controls and the optical wavelength reference device 103 tunable laser 105 based on a differential signal of the absorption characteristics obtained by phase-sensitive detection By doing so, it can function as a stable optical wavelength reference device.

  The above-described optical wavelength reference device is a type of reference device that inputs modulated laser light to an optical wavelength reference cell. In contrast, the optical wavelength reference cell for shielding the optical wavelength reference cell from an external magnetic field is covered with a magnetic shield tank, and magnetic field modulation is applied instead of the modulated laser beam as an incident signal for obtaining the optical wavelength reference. There is also an apparatus that uses light (for example, Patent Document 1).

JP-A-10-290041 (page 4-5, FIG. 1, etc.)

  However, in a wavelength-stabilized laser light source configured using a conventional optical wavelength reference cell device, it is necessary to synchronize the length of the Fabry-Perot optical resonator constituting the optical wavelength reference cell device with the resonator length of the laser light source. was there. Therefore, in this wavelength-stabilized laser light source, the control system that synchronizes the resonator length of the laser light source and the length of the Fabry-Bello optical resonator, and the resonance wavelength of the Fabry-Perot optical resonator are set to the wavelength of the optical wavelength reference cell. It is necessary to simultaneously operate the control system synchronized with the reference, and the configuration is complicated.

  Further, in order to detect the center of the molecular absorption spectrum serving as the wavelength reference, it has been necessary to use frequency-modulated light as described above. Therefore, since the frequency of the laser beam itself to be stabilized fluctuates, in terms of application, for example, when performing wavelength measurement, its resolution and measurement accuracy are reduced, so that its usefulness is limited. There was a problem.

  On the other hand, the above-mentioned Patent Document 1 is characterized in that unmodulated light is used as the LD oscillation light in order to obtain a more stable wavelength stabilization frequency. Is used. For this reason, a magnetic shield tank, a Zeeman modulation coil, etc. are required to suppress the influence of magnetic field fluctuations, which has the disadvantage that the structure becomes precise and complicated, and the configuration of the control system becomes complicated. . In addition, there is a drawback that the cost increases with the complexity of these structures and control systems.

  The present invention has been made in view of the above-described conventional drawbacks, and an object thereof is to provide an optical wavelength reference device having excellent wavelength stability without modulating the laser beam itself to be stabilized. It is another object of the present invention to provide an optical wavelength reference device having a simple structure and configured with a simple control system. It is another object of the present invention to provide an optical wavelength reference device that is smaller than the conventional one.

  Therefore, in order to solve the above-described problems, the present invention uses a pair of total reflection mirrors arranged so that both end faces of the optical wavelength reference cell have Brewster windows and face the outside of the optical wavelength reference cell. Since the structure in which the light reciprocates multiple times in the reference cell multiple times with low loss is designed to increase the interaction distance between molecules and light waves in the optical wavelength reference cell, without using a Fabry-Perot optical resonator, The present invention provides an optical wavelength reference device that has excellent wavelength stability and is configured with a simple structure and a simple control system.

  In the present invention, the end face of the optical wavelength reference cell in which the Brewster window is formed causes about 4 Fresnel reflections that occur each time light passes between the material (glass) of the optical wavelength reference cell and air. % Can be suppressed, and even when light reciprocates a plurality of times in the optical wavelength reference cell, the large optical loss generated by the conventional method is greatly reduced. As a result, it is possible to increase the interaction distance between molecules and light waves in the wavelength reference cell with almost no reduction in light intensity, and to provide an optical wavelength reference device that is smaller than the conventional one.

  That is, the optical wavelength reference device according to claim 1 of the present invention includes an incident surface and an output surface formed by a Brewster window, and absorbs incident light in a predetermined light wavelength band incident from the incident surface. And a pair of total reflection mirrors arranged opposite to each other with the incident surface and the output surface of the optical wavelength reference cell interposed therebetween, and the incident light is converted into the optical wavelength. Incident light enters the reference cell, the reflected light from the optical wavelength reference cell is multiple-reflected by the pair of total reflection mirrors, and the multiple reflected light reciprocated a plurality of times in the optical wavelength reference cell is transmitted to the optical wavelength reference cell. It is characterized by output to the outside.

  An optical wavelength reference device according to a second aspect of the present invention is the optical wavelength reference device according to the above invention, wherein an input end for allowing incident light to enter the optical wavelength reference cell and the multiple reflected light are input to the optical wavelength reference cell. It is further characterized by further comprising polarization-maintaining optical fibers respectively connected to output terminals for outputting to the outside.

  According to a third aspect of the present invention, there is provided an optical wavelength reference apparatus including an incident surface and an output surface formed by a Brewster window for absorbing incident light in a predetermined light wavelength band incident from the incident surface. A light wavelength reference cell in which a light absorbing material is enclosed, a pair of total reflection mirrors disposed opposite to each other with an incident surface and an output surface of the light wavelength reference cell interposed therebetween, and incident on the inside of the light wavelength reference cell A first polarization maintaining optical fiber connected to an input / output terminal for inputting light and outputting the multiple reflected light to the outside of the optical wavelength reference cell; and the first polarization maintaining light. A polarization maintaining optical circulator connected to a fiber, for guiding light from a light source to the optical wavelength reference cell, and separating and outputting return light from the optical wavelength reference cell to an optical path different from the light source; and the polarization Connected to a holding optical circulator, An optical amplifier for adjusting an input level of incident light; a second polarization maintaining optical fiber for temporarily outputting the multiple reflected light outside the optical wavelength reference cell; and the second polarization A polarization-maintaining optical fiber Bragg grating is provided, which is connected to the holding optical fiber and temporarily re-enters the multi-reflected light output to the optical wavelength reference cell.

  The optical wavelength reference apparatus according to claim 4 of the present invention is the optical wavelength reference device according to the above invention, wherein the optical wavelength reference device is set so that the absorption characteristics of molecules in the optical wavelength reference cell do not reach a saturation region. The light intensity incident on the reference cell is controlled by the amplification factor of the optical amplifier and / or the reflectance of the polarization maintaining optical fiber Bragg grating.

  The optical wavelength reference apparatus according to claim 5 of the present invention is the optical wavelength reference device according to the above-described invention, wherein hole burning is caused in the molecules in the optical wavelength reference cell, and the return from the polarization maintaining optical fiber Bragg grating is provided. The incident light intensity to the optical wavelength reference cell is controlled by the amplification factor of the optical amplifier and / or the reflectance of the polarization maintaining optical fiber Bragg grating so that the light becomes linear weak light. .

  The optical wavelength reference apparatus according to claim 6 of the present invention is characterized in that, in the above invention, the optical amplifier is an erbium-doped fiber optical amplifier.

  The optical wavelength reference device according to claim 7 of the present invention is characterized in that, in the above invention, the predetermined optical wavelength band is an optical communication wavelength band.

  The optical wavelength reference apparatus according to claim 8 of the present invention is characterized in that, in the above invention, the absorbing material is an acetylene molecule or a hydrogen cyanide molecule.

  According to the optical wavelength reference device of the present invention, the optical wavelength reference cell is formed by using a pair of total reflection mirrors arranged so that both end faces of the optical wavelength reference cell are Brewster windows and face the outside of the optical wavelength reference cell. Since the structure is such that light is reciprocated multiple times with low loss inside, and the interaction distance between molecules and light waves in the optical wavelength reference cell is increased, without using a Fabry-Perot optical resonator, There is an effect that it is possible to provide an optical wavelength reference device that is excellent in wavelength stability and smaller than the conventional one.

  Hereinafter, an optical wavelength reference apparatus according to an embodiment of the present invention will be described with reference to the drawings. Note that the present invention is not limited to the embodiments.

(Embodiment 1)
FIG. 1-1 is a schematic diagram (top view) showing the configuration of the optical wavelength reference device according to the first embodiment, and FIG. 1-2 is a side view thereof. The optical wavelength reference devices shown in these figures are the end faces (Brewster windows) 2a, 2b of the optical wavelength reference cell having a Brewster angle inclined to prevent reflection of incident light and outgoing light from itself. Is used to couple light to the optical wavelength reference cell 2 in which acetylene molecules or hydrogen cyanide molecules having an absorption spectrum in the optical communication wavelength band (1.5 μm band) are enclosed, and the polarization maintaining optical fiber 3. A coupler 4a, 4b, a pair of total reflection mirrors 5 disposed opposite to both ends of the optical wavelength reference cell 2 so that the light reciprocates a plurality of times inside the optical wavelength reference cell 2, and an optical wavelength reference An outer box 6 containing the cell 2 and the total reflection mirror 5 is provided.

  As shown in FIG. 1-1, the incident position of incident light (attachment position of the optical coupler 4a) and the extraction position of outgoing light (attachment position of the optical coupler 4b) are the outer box 6 as viewed from above. It is displaced in the short direction. The reason is to take out light that has reciprocated a plurality of times within the optical wavelength reference cell 2. Therefore, the polarization maintaining optical fiber 3 to be coupled to the optical couplers 4a and 4b is inclined by a predetermined angle (α). The incident inclination angle α (or the outgoing inclination angle) is an angle for efficiently reciprocating the light wavelength reference cell 2 between the light beams reflected by the total reflection mirror 5 without being mutually blocked. It is determined by the distance between the mirrors 5 and the beam diameter of the light. Details will be described later.

FIG. 2 is a schematic diagram showing an example of a configuration for obtaining a wavelength-stabilized laser beam using the optical wavelength reference apparatus of the present invention. In the drawing, laser light (unmodulated laser light) is input from a laser device 20 to a phase modulator 21, and modulated laser light that is phase-modulated by the phase modulator 21 is input to the optical wavelength reference device 1. The optical output (P ₀ ) of the optical wavelength reference device 1 is detected by the phase sensitive detection means 22. The phase sensitive detection means 22 controls the laser device 20 so that the light output (P ₀ ) from the optical wavelength reference device 1 becomes small. In this way, the unmodulated laser light output from the laser device 20 becomes an output in a desired wavelength band, that is, an optical wavelength reference output.

  Here, it compares with the case where a wavelength stabilization laser beam is obtained using the conventional optical wavelength reference | standard apparatus shown in FIG. In the related art, the laser beam output from the laser device 103 must be a modulated laser beam. This is related to the use of a Fabry-Perot optical resonator. In other words, when a control system is configured using a Fabry-Perot optical resonator, it is necessary to synchronize the resonator length of the laser light source and the Fabry-Perot optical resonator length. The light inevitably became modulated laser light. On the other hand, in the present invention, unmodulated laser light can be used as the laser light output from the laser device 20. This is because the present invention can be configured without using a Fabry-Perot optical resonator, and as shown in FIG. 2, the non-modulated light output from the laser device 20 is phase-modulated by the phase modulator 21, and this phase-modulated output is output. This is because it is possible to adopt a configuration that outputs to the optical wavelength reference device 1.

  Next, the operation mechanism of the present invention will be described. In FIGS. 1-1 and 1-2, the single polarized light from the polarization maintaining optical fiber 3 is collimated into a parallel beam by the optical coupler 4a and enters the optical wavelength reference cell 2 through the Brewster window 2a. A part of the incident light is absorbed by molecules inside the optical wavelength reference cell 2, and a part of the incident light is emitted from the Brewster window 2b. The emitted light is reflected by one total reflection mirror 5 arranged outside the optical wavelength reference cell 2, and returns to the optical wavelength reference cell 2 again through the Brewster window 2b. After being emitted from the Brewster window 2a after interaction with the molecules inside 2, the light is reflected by the other total reflection mirror 5 arranged outside the optical wavelength reference cell 2, and returns to the optical wavelength reference cell 2 again. . After this operation is repeated a plurality of times, the light is coupled to the polarization maintaining optical fiber 3 via the optical coupler 4b and output to the outside of the optical wavelength reference cell 2.

Next, let us consider the optical loss that light receives each time it passes through the optical wavelength reference cell 2 once. FIGS. 3A and 3B schematically show how light enters the end face (incident surface) of the optical wavelength reference cell 2 in which the Brewster window is formed. More specifically, FIG. 3A is a perspective view illustrating a state in which TM wave light is incident at a Brewster angle θ, and FIG. 3B is a side view thereof. Here, an axis obtained by projecting the incident optical axis onto the Brewster window is defined as an x axis, an axis orthogonal to the x axis in the plane of the Brewster window is defined as a y axis, and a z axis is defined as an axis orthogonal to the xy plane. Define. If defined in this way, the electric field component E _TM of light and the magnetic field component H _{TM of} light are expressed by E _TM = (E _x , 0, E _z ) and H _TM = (0, H _y , 0), respectively. be able to.

4A is a perspective view illustrating a state in which light is incident at an incident angle α in a plane including the incident optical axis and the y axis with respect to FIG. 3A, and FIG. It is the side view. Here, the incident angle α is equal to the polarization maintaining optical fiber 3 without being blocked by the total reflection mirror 5 arranged on the output end side of the optical wavelength reference cell 2 in FIGS. 1-1 and 1-2. This is the angle required to optically couple to Now, if the distance between a pair of opposing total reflection mirrors is L and the beam diameter of light is w, the required minimum angle α _min is given by the following equation.

  At this time, since the electric field of incident light does not include the y-axis component, it is constant regardless of the incident angle α, whereas the magnetic field component has an x component, a y component, and a z component depending on the incident angle α.

  Here, the light loss when passing through the optical wavelength reference cell 2 once is considered to be the light loss when light enters the Brewster window from the air, considering that there are four boundary surfaces between the air and the Brewster window. Can be calculated by multiplying the reflection loss received by 4 times. Next, an attempt is made to calculate the light reflectance between the air and the Brewster window.

  FIG. 5 is a diagram in which a rotational coordinate system of the xy plane and the x′y ′ plane is defined on the perspective view of FIG. 4-1. As shown in FIG. 5, the x′y ′ coordinate obtained by rotating the xy coordinate by an angle φ formed by the x ′ axis projected from the optical axis of the light incident at the incident angle α on the plane of the Brewster window and the original x axis. Since the incident light can be separated into TE wave and TM wave and expressed, the light reflectance can be easily calculated. Hereinafter, an electromagnetic field of incident light is defined in the xyz coordinate system, and then coordinate conversion is performed to x′y′z coordinates to derive the light reflectance at the boundary surface of the air-Brewster window.

  In FIG. 5, the electromagnetic field of incident light can be expressed by the following equation in the xyz coordinate system.

Here, E ₀ is the amplitude of the electric field, and W is the wave impedance. Further, the wave number vector k of incident light is given by the following equation, where the magnitude is k ₀ . However, the inside of () has shown the component of the x direction, the y direction, and the z direction, respectively.

  Also, the rotation angle φ of the coordinates is given by the following equation using the incident angle α of the incident optical axis.

  Next, when the electromagnetic field of incident light is described in the x′y′z coordinates, the above equations (1) to (3) can be expressed as the following equations.

  Here, the angle θ ′ is an angle formed by the incident optical axis and the z axis in the x′z plane, and is given by the relationship between the Brewster angle θ and the following equation.

By the way, the problem of reflection / transmission of an optical electric field with respect to a TE wave and a TM wave at two dielectric interfaces having different refractive indexes is generally well known. Assuming n ₁ and n ₂ , the reflectance of the optical electric field at the interface is given by the following equation.

  Therefore, by considering the component ratio of the TM wave and the TE wave in the equation (6), the light reflectance R can be obtained by the following equation using the equation (10).

  As described above, considering that there are four interfaces between the air and the Brewster window every time it passes through the optical wavelength reference cell 2, the optical loss of the optical wavelength reference cell 2 is expressed by the equation (11). Using the light reflectance R, it is given by

For example, when the Brewster window of the optical wavelength reference cell 2 is made of quartz glass, n _l = 1.0 (air) and n ₂ = 1.5 (glass), so the Brewster angle θ is , Θ = tan ⁻¹ (n ₁ / n ₂ ) = 56.3 degrees. FIG. 6 shows the result of calculating the optical loss (equivalent to one pass) using the equations (9), (11), and (12) under the conditions, using the incident angle α of light as a parameter. . In the figure, for comparison, a calculation result in a case where each end face of the optical wavelength reference cell 2 is not a Brewster window (that is, θ = 0 degree) is also shown.

For example, when the distance L between the total reflection mirrors arranged opposite to each other is 150 mm and the beam diameter w of the incident light is 2 mm, the incident angle α is tan ⁻¹ (w / 2L) = 0 from Equation (1). It is necessary to set it to 38 degrees or more. Therefore, the incident angle α is set to 1 degree, and the optical loss at this incident angle is obtained from FIG. First, when each end face of the optical wavelength reference cell 2 is not a Brewster window, there is a loss of about 0.7 dB (about 15%), but in the case of a Brewster window, about 3.5 × 10 ^−4. There is only a loss of dB (about 0.008%).

  FIG. 7 is a graph showing a result of calculating the relationship of the optical loss with respect to the number of times the light passes inside the optical wavelength reference cell when the incident angle (α) is 1 degree. For example, consider a case where light is reciprocated 10 times (20 times) inside the optical wavelength reference cell 2 by the total reflection mirror 5. When each end face of the optical wavelength reference cell 2 is not a Brewster window, there is a loss of about 14.2 dB (about 96.2%), whereas in the case of a Brewster window, at most about 0.007 dB. There is only a loss of (about 0.16%). As described above, it can be seen that the loss can be suppressed to a level where there is no practical problem by providing the Brewster window.

  Next, the maximum number of passes through which light can pass through the optical wavelength reference cell 2 will be considered. This maximum number of passes is given by 2 × (D / w−1) using the beam diameter w of light, where D is the diameter of the cross section of the optical wavelength reference cell 2 (long axis length in the case of an ellipse). . For example, when the diameter D of the cross section of the optical wavelength reference cell 2 is 20 mm and the light beam diameter w is 2 mm, the maximum number of passes is 18. At this time, when the length of the wavelength reference cell is 100 mm, the interaction distance between the light and the internal molecule of the optical wavelength reference cell 2 is 1.8 m, and a very long interaction length can be realized. For this reason, the internal pressure of the optical wavelength reference cell 2 can be lowered, and an absorption line with a narrow line width can be observed.

  Finally, optical fiber coupling at the input / output end of the optical wavelength reference device 1 will be considered. Since the end face (incident / exit surface) of the optical wavelength reference cell 2 of the present invention is formed by a Brewster window, it is important to fix the wave-breaking state of incident light to the TM wave. For the optical coupling, the polarization maintaining optical fiber 3 is suitable. By using the polarization-maintaining optical fiber 3, a polarization-maintaining cell is obtained, and the stability is greatly improved.

  In the above-described embodiment, as shown in FIG. 1-1, the incident angle of light to the optical wavelength reference device 1 is inclined by a predetermined angle, but FIG. 8-1 and FIG. As in the optical wavelength reference device 14 shown in FIG. 4, the incident angle can be made incident without an inclination. In this case, in order to achieve the same maximum number of passages with the same size as the optical wavelength reference device 1 shown in FIG. 1-1, the inclination angle of the total reflection mirror 5 is set to α as shown in FIG. What is necessary is just to set to / 2.

  As described above, according to the optical wavelength reference device of this embodiment, both end faces of the optical wavelength reference cell are Brewster windows, and the input / output ends of the optical wavelength reference device are coupled by polarization maintaining optical fibers. By doing so, it is possible to reduce the optical loss in the optical wavelength reference cell and the connection loss between the various optical fiber elements and this apparatus.

  Further, according to the optical wavelength reference device of this embodiment, the optical wavelength reference device is configured such that light is reciprocated a plurality of times in the optical wavelength reference cell using the opposing total reflection mirrors arranged outside the optical wavelength reference cell. By increasing the interaction distance between molecules and light waves in the cell, the pressure width can be reduced as a low-pressure cell, so that a narrow absorption line can be observed without using a Fabry-Perot optical resonator. As a result, two control systems conventionally required when configuring a wavelength-stabilized light source can be made one.

  Furthermore, according to the optical wavelength reference device of this embodiment, light is transmitted once between the material (glass) of the optical wavelength reference cell and the air by both end faces of the optical wavelength reference cell formed by the Brewster window. The Fresnel reflection that occurs each time it passes can be suppressed by about 4%. For this reason, when light is reciprocated a plurality of times inside the optical wavelength reference cell, a large optical loss occurs in the conventional method. However, in the present invention, the optical wavelength reference cell does not substantially attenuate the light intensity. The interaction distance between molecules and light waves can be increased. Thus, by using the multiple reflection mechanism in the configuration of the optical wavelength reference cell, the optical wavelength reference cell can be reduced in size and increased in sensitivity.

  In addition, according to the optical wavelength reference apparatus of this embodiment, it is necessary to modulate the resonator length of the laser light source when configuring a phase sensitive detection system for detecting the center of the molecular absorption spectrum serving as the wavelength reference. In addition, since it is possible to externally modulate the output light from the laser device, unmodulated laser output light can be obtained, and the wavelength resolution and measurement accuracy when applied to various optical measurements are increased. be able to.

(Embodiment 2)
FIG. 9A is a schematic diagram (top view) illustrating the configuration of the optical wavelength reference device according to the second embodiment, and FIG. 9B is a side view thereof. The optical wavelength reference device 11 of this embodiment shown in the figure is added to the polarization maintaining optical fiber 3 on the optical coupler 4b side (reflection end side) of the optical wavelength reference device 11 in addition to the configuration of the first embodiment. A polarization maintaining optical fiber Bragg grating 10 is provided. On the other hand, on the optical coupler 4a side (input end side) of the optical wavelength reference device 11, the light from the light source is incident on the optical wavelength reference cell 2, and the return light from the optical wavelength reference cell 2 is output as outgoing light. A polarization-maintaining optical circulator 7 and an erbium-doped fiber optical amplifier 8 connected to the polarization-maintaining optical circulator 7. Other configurations are the same as or equivalent to the configurations of the first embodiment shown in FIGS. 1-1 and 1-2, and these portions are denoted by the same reference numerals.

  In the optical wavelength reference device 11 of this embodiment, light from a light source is amplified by an erbium-doped fiber optical amplifier 8 and is incident on the optical wavelength reference cell 2 via the polarization maintaining optical circulator 7. The light emitted from the reference cell can be branched out from the incident light and extracted to the outside. Further, by adjusting the amplification factor of the erbium-doped fiber optical amplifier 8 installed on the input end side of the optical wavelength reference cell 2 and the reflectance of the polarization maintaining optical fiber Bragg grating 10 installed on the reflection end side thereof, The measurement sensitivity of the absorption line can be increased.

  The bandwidth of the polarization maintaining optical fiber Bragg grating 10 is as wide as several GHz, while the band of the absorption characteristics of the optical wavelength reference cell 2 shown in FIG. 15 is on the order of several hundred MHz. Therefore, a complicated control mechanism such as temperature control for the polarization maintaining optical fiber Bragg grating 10 is unnecessary.

  As described above, according to the optical wavelength reference apparatus of this embodiment, the light reflected by the polarization-maintaining optical fiber Bragg grating is incident in the opposite direction from the reflection end side of the optical wavelength reference cell, and both the lights are incident. Propagating in the direction can double the interaction distance between the internal molecules of the optical wavelength reference cell and the optical wave, and the optical wavelength is more accurate than the optical wavelength reference device that propagates in a single direction. A reference device can be realized. In addition, since this interaction distance can be doubled, the apparatus can be further downsized when realizing the same performance as in the first embodiment.

  In the optical wavelength reference device 11 of this embodiment, the polarization maintaining optical fiber Bragg grating 10 is used as a mechanism for returning the light from the reflection end side of the optical wavelength reference cell 2 in the reverse direction. Alternatively, for example, the configuration may be such that the reflected light level-adjusted using a total reflection mirror or an optical attenuator is returned to the input side at the position of the optical coupler 4b inside the optical wavelength reference cell 2.

  In this embodiment, the polarization maintaining optical fiber 3 to be coupled to the optical coupler 4a is made incident at a predetermined angle (α) as in the optical wavelength reference device 1 shown in FIG. Although shown, it goes without saying that it may be incident without having an incident angle as in the optical wavelength reference device 14 shown in FIG.

(Embodiment 3)
FIG. 10-1 is a schematic diagram (top view) illustrating the configuration of the optical wavelength reference device according to the third embodiment, and FIG. 10-2 is a side view thereof. The optical wavelength reference device 12 of this embodiment shown in the figure is the same as or equivalent to the second embodiment in terms of the configuration, but the embodiment differs in that the incident light intensity to the optical wavelength reference cell 2 is different. This is a difference from 2. Hereinafter, the operation mechanism of the third embodiment will be described.

  When the incident intensity of light to the optical wavelength reference cell 2 is increased to a certain extent, a phenomenon called hole burning occurs. In this state, when weak linear weak light is incident in the opposite direction to the incident light, a sharp dip called a saturated absorption line appears in the absorption characteristics of the molecules in the cell. In this embodiment, the saturated absorption line is used to obtain a highly accurate optical wavelength reference output.

FIG. 11 is a diagram conceptually showing a waveform of the absorption characteristic when hole burning occurs in the optical wavelength reference cell. In the figure, the horizontal axis represents the center wavelength λ _S of the modulated laser beam, and the vertical axis represents the optical output P ₀ from the optical wavelength reference device 12. In the characteristic curve K3, the above-described saturated absorption line is generated near the center. As shown in the figure, the waveform of the absorption characteristic when hole burning occurs (that is, the saturated absorption line) compared to the waveform near the minimum value of the absorption characteristic when hole burning does not occur (broken line in the figure). ) Has a steeper characteristic, so that a narrower absorption line (saturated absorption line width) can be observed.

  When hole burning occurs, if the intensity of re-incident light opposite to the incident light is increased too much, the detection sensitivity is lowered. FIG. 12 is a diagram for explaining the relationship between the intensity of re-incident light and detection accuracy. When the intensity of the re-incident light is weak, a saturated absorption line can be generated as shown by K3 (solid line), so that a narrower absorption line can be observed.

  On the other hand, when the intensity of the re-incident light is relatively strong, both sides of the central portion of the saturated absorption line are raised as indicated by K4 (broken line), and the waveform of the saturated absorption line becomes dull. In this state, high-sensitivity observation cannot be performed and narrow absorption lines cannot be observed. On the contrary, since the characteristic near the extreme value of the waveform K3 deteriorates, it becomes impossible to observe the narrow absorption line (linear absorption line width) that can be observed in the second embodiment. Therefore, the level adjustment of the reincident light is important. .

  The erbium-doped fiber optical amplifier 8 and the polarization maintaining optical fiber Bragg grating 10 play a particularly important role in optimally setting the intensity of the re-incident light. In the second embodiment, in order to observe the linear absorption line width, the amplification factor of the erbium-doped fiber optical amplifier 8 and the polarization maintaining optical fiber Bragg grating 10 are set so as not to saturate the internal molecules of the optical wavelength reference cell 2. Narrow absorption lines could be observed by optimally setting the reflectivity of.

  On the other hand, in this embodiment, in order to observe the saturated absorption line width, the internal molecules of the optical wavelength reference cell 2 are shifted to the saturated region to cause a state of hole burning, and by re-incident light. Narrower absorption lines can be observed by optimally setting the amplification factor of the erbium-doped fiber optical amplifier 8 and the reflectivity of the polarization maintaining optical fiber Bragg grating 10 so as not to deteriorate the SN ratio of the generated saturated absorption line. It becomes possible.

  Thus, in the optical wavelength reference device 11 of this embodiment, the light from the light source is amplified by the erbium-doped fiber optical amplifier 8 and incident on the optical wavelength reference cell 2 via the polarization maintaining optical circulator 7. Thus, the outgoing light from the optical wavelength reference cell can be branched out from the incident light and extracted to the outside. Further, by adjusting the amplification degree of the erbium-doped fiber optical amplifier 8 installed at the input end of the optical wavelength reference cell 2 and the reflectance of the polarization maintaining optical fiber Bragg grating 10 installed at the output end thereof, an optimum is achieved. Saturated absorption lines can be measured.

  As described above, according to the optical wavelength reference apparatus of this embodiment, the internal molecules of the optical wavelength reference cell are shifted to the saturation region to cause a state of hole burning, and the saturated absorption line generated by re-incident light. Since the amplification factor of the erbium-doped fiber optical amplifier 8 and the reflectivity of the polarization-maintaining optical fiber Bragg grating 10 are optimally set so as not to deteriorate the SN ratio of the optical fiber, the optical wavelength reference device with higher accuracy can be obtained. Can be realized.

  In the optical wavelength reference device 11 of this embodiment, the polarization maintaining optical fiber Bragg grating 10 is used as a mechanism for returning from the output end side of the optical wavelength reference cell 2 in the reverse direction. For example, the configuration may be such that the reflected light whose level is adjusted using a total reflection mirror, an optical attenuator, or the like is returned to the input side at the position of the optical coupler 4b inside the optical wavelength reference cell 2.

  In this embodiment, the polarization maintaining optical fiber 3 to be coupled to the optical coupler 4a is made incident at a predetermined angle (α) as in the optical wavelength reference device 1 shown in FIG. Although shown, it goes without saying that it may be incident without having an incident angle as in the optical wavelength reference device 14 shown in FIG.

  As described above, the optical wavelength reference device according to the present invention is useful for a wavelength stabilized laser light source, an optical wavelength measuring device, and the like, in particular, downsizing of these wavelength stabilized laser light source and optical wavelength measuring device, Contributes to high stability, high sensitivity, and high accuracy.

1 is a schematic diagram (top view) illustrating a configuration of an optical wavelength reference device according to a first embodiment; 1 is a schematic diagram (side view) showing a configuration of an optical wavelength reference device according to a first embodiment; It is a schematic diagram which shows an example of the structure for obtaining a wavelength stabilization laser beam using the optical wavelength reference | standard apparatus of this invention. It is a perspective view which shows a mode that the light of TM wave injects with Brewster angle (theta). It is a side view which shows a mode that the light of TM wave injects with Brewster angle (theta). It is a perspective view which shows a mode that the light entered with the angle of incident angle (alpha) within the plane containing an incident optical axis and a y-axis. It is a side view which shows a mode that the light entered with the angle of incident angle (alpha) in the plane containing an incident optical axis and a y-axis. It is the figure which defined the rotational coordinate system of xy plane and x'y 'plane on the perspective view of FIG. It is a graph which shows an example of the result of having calculated the optical loss (equivalent to one pass) using Formula (9), Formula (11), and Formula (12) by using the incident angle α of light as a parameter. It is the graph which showed an example of the result of having calculated the relationship of the optical loss with respect to the frequency | count of light passage inside an optical wavelength reference cell, when an incident angle ((alpha)) is 1 degree. FIG. 6 is a schematic diagram (top view) showing a configuration of a modified example of the first embodiment. FIG. 6 is a schematic diagram (side view) showing a configuration of a modified example of the first embodiment. FIG. 6 is a schematic diagram (top view) illustrating a configuration of an optical wavelength reference device according to a second embodiment; It is a schematic diagram (side view) which shows the structure of the optical wavelength reference | standard apparatus concerning Embodiment 2. FIG. FIG. 6 is a schematic diagram (top view) illustrating a configuration of an optical wavelength reference device according to a third embodiment. It is a schematic diagram (side view) which shows the structure of the optical wavelength reference | standard apparatus concerning Embodiment 3. FIG. It is a figure which shows notionally the waveform of the absorption characteristic when hole burning arises in the optical wavelength reference cell. It is a figure for demonstrating the relationship between the intensity | strength of re-incident light, and a detection accuracy. It is a schematic diagram which shows an example of a structure of the optical wavelength reference apparatus in the conventional optical communication wavelength band. It is a block diagram which shows an example of a structure of the wavelength stabilization laser beam using the conventional optical wavelength reference | standard apparatus. It is explanatory drawing for demonstrating the concept of a phase sensitive detection. It is the figure which showed the outline | summary of the output waveform output from a phase sensitive detection system.

Explanation of symbols

1, 11, 12, 14, 103 Optical wavelength reference device 2, 101 Optical wavelength reference cell 2a, 2b Brewster window 3 Polarization maintaining optical fiber 5 Total reflection mirror 6 Outer box 7 Polarization maintaining optical circulator 8 Addition of erbium Fiber optical amplifier 10 polarization maintaining optical fiber Bragg grating 20 laser device 21 phase modulator 22 phase sensitive detection means 102 Fabry-Perot optical resonator 104a, b piezoelectric element 105 wavelength tunable laser 106 frequency modulator 107 optical splitter 108, 113 Signal Generator 109, 112 Photodetector 110 Electric Mixer 111, 115 Negative Feedback Circuit 114 Lock-in Amplifier

Claims (8)

An optical wavelength reference cell comprising an entrance surface and an exit surface formed by a Brewster window, in which an absorbing material for absorbing incident light in a predetermined optical wavelength band incident from the entrance surface is enclosed;
A pair of total reflection mirrors arranged opposite to each other across the incident surface and the output surface of the light wavelength reference cell;
With
Multiple reflected light in which the incident light is incident into the optical wavelength reference cell, the transmitted light from the optical wavelength reference cell is multiple-reflected by the pair of total reflection mirrors, and reciprocated in the optical wavelength reference cell a plurality of times Is output to the outside of the optical wavelength reference cell.   A polarization maintaining optical fiber connected to an input end for allowing incident light to enter the optical wavelength reference cell and an output end for outputting the multiple reflected light to the outside of the optical wavelength reference cell; The optical wavelength reference device according to claim 1, further comprising: An optical wavelength reference cell comprising an entrance surface and an exit surface formed by a Brewster window, in which an absorbing material for absorbing incident light in a predetermined optical wavelength band incident from the entrance surface is enclosed;
A pair of total reflection mirrors arranged opposite to each other across the incident surface and the output surface of the light wavelength reference cell;
A first polarization maintaining optical fiber connected to an input / output terminal for causing incident light to be incident inside the optical wavelength reference cell and outputting the multiple reflected light to the outside of the optical wavelength reference cell;
Polarized wave connected to the first polarization maintaining optical fiber, for guiding light from a light source to the optical wavelength reference cell, and separating and outputting return light from the optical wavelength reference cell to an optical path different from the light source A holding optical circulator;
An optical amplifier connected to the polarization maintaining optical circulator for adjusting the input level of incident light;
A second polarization maintaining optical fiber for temporarily outputting the multiple reflected light outside the optical wavelength reference cell;
A polarization-maintaining optical fiber Bragg grating connected to the second polarization-maintaining optical fiber for re-entering the temporarily reflected multiple reflected light into the optical wavelength reference cell;
An optical wavelength reference device comprising:   The incident light intensity to the optical wavelength reference cell is set so that the absorption characteristics of the molecules in the optical wavelength reference cell do not reach the saturation region. 4. The optical wavelength reference device according to claim 3, wherein the optical wavelength reference device is controlled by a reflectance of the optical fiber Bragg grating.   The incident light intensity to the optical wavelength reference cell is caused to cause hole burning in the molecules in the optical wavelength reference cell, and the return light from the polarization maintaining optical fiber Bragg grating becomes linear weak light. 4. The optical wavelength reference device according to claim 3, wherein the optical wavelength reference device is controlled by an amplification factor of an optical amplifier and / or a reflectance of the polarization maintaining optical fiber Bragg grating.   The optical wavelength reference apparatus according to claim 3, wherein the optical amplifier is an erbium-doped fiber optical amplifier.   The optical wavelength reference apparatus according to claim 1, wherein the predetermined optical wavelength band is an optical communication wavelength band.   The optical wavelength reference apparatus according to claim 1, wherein the absorbing material is an acetylene molecule or a hydrogen cyanide molecule.

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