JP7329241B2 - Fabry-Perot Etalon, Wavelength Variation Detector Using Same, and Wavelength Meter - Google Patents

Description

The present invention relates to a wavelength locker module, a wavelength variation detector and a wavelength meter using a Fabry-Perot etalon (etalon).

2. Description of the Related Art In recent years, in the field of spectroscopic measurement, a variable wavelength light source capable of varying the wavelength of oscillating light has attracted attention. In addition, wavelength tunable light sources are required to have a wide wavelength tunable range for the purpose of expanding the spectral range. There is a demand for higher accuracy of the quantity.

By the way, a semiconductor laser (ECDL) using an external resonator is used as a wavelength tunable light source capable of tunable over a wide band. be. Therefore, a wavelength locker module that detects the output light of the ECDL and stabilizes its oscillation wavelength is required.

Also, the oscillation wavelength of the ECDL is defined by the angle of the wavelength dispersive element that constitutes the external cavity, but it always contains an indefiniteness of several times (several to 10 GHz) of the external cavity mode, and the wavelength changes. In this case, a wavelength calibration module is required as a scale for the amount of change.

By the way, when correcting the wavelength fluctuation at a certain wavelength to stabilize the oscillation wavelength of the laser, it is common to use the transmission signal of an optical element (bandpass filter, etalon, etc.) whose transmittance changes with wavelength change. is.

For example, Patent Document 1 below discloses a method that enables wavelength stabilization with only one specific wavelength using a band-pass filter, and Patent Document 2 below discloses a method using an etalon at a plurality of wide wavelengths. Methods are described that allow for wavelength stabilization. In either method, the output of light A that has passed through an element whose transmittance changes and the output of light B that does not enter the element is received by a photoelectric conversion element (PD), and the difference between the two signals is used to obtain a specific It is characterized by generating a signal crossing 0 in the wavelength and performing wavelength stabilization by PID control using it as an error signal.

JP-A-10-209546 Japanese Patent Application Laid-Open No. 2003-204111

However, in the wavelength locker module of the prior art described above, although it is possible to stably lock the wavelength at a specific wavelength, there is a problem in coping with a wide wavelength band. For example, the method using a band-pass filter described in Patent Document 1 is effective only for a specific wavelength. Moreover, even with the method using the etalon described in Patent Document 2, it is difficult to cope with a wide wavelength band. this is,
In the case of the method using the difference between the signal A that passes through the etalon and the signal B that does not enter the etalon, when the wavelength changes significantly beyond several tens of nanometers, the chromatic aberration and transmission characteristics of the etalon and its introduction optical system change. , B intensity ratio and the amplitude of the etalon transmission signal change, it is necessary to trim the intensity ratio of the A and B signals at each wavelength to be wavelength-locked. This is not suitable for use with a light source that varies the wavelength over a wide band.

SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to provide a wavelength locker module and an etalon used therein, which solves the above-described problems and enables accurate wavelength locking in a wide wavelength band.

A Fabry-Perot etalon according to one aspect of the present invention for solving the above-mentioned problems is provided with two partially reflective elements having partially reflective surfaces that reflect part of incident light and transmit part of the incident light, and are installed facing each other in parallel. It is a Fabry-Perot etalon, characterized by having a phase plate between partially reflective elements.

Further, a wavelength change detector according to another aspect of the present invention includes two partially reflective elements having partially reflective surfaces that reflect part of incident light and transmit part of the incident light, and are installed facing each other in parallel, A Fabry-Perot etalon having a phase plate between two partially reflecting elements, a polarizing optical path splitting element that splits the light emitted from the Fabry-Perot etalon according to the polarization state, and a polarizing optical path splitting element. It has two photoelectric conversion elements arranged on the exit surface side.

Further, a wavelength change detector according to another aspect of the present invention includes two partially reflective elements having partially reflective surfaces that reflect part of incident light and transmit part of the incident light, and are installed facing each other in parallel, A Fabry-Perot etalon having a λ/16 waveplate between partially reflective elements, and a laser beam linearly polarized at an angle of 45 degrees with respect to the delay axis of the λ/16 waveplate is transmitted through the Fabry-Perot etalon or A polarization type optical path branching element that separates the light into polarized light components parallel and orthogonal to the delay axis of the wave plate after reflection, and a signal corresponding to the amount of light of each polarized component separated by the polarization type optical path branching element is output. and a controller for processing the signal of the photoelectric conversion element, and can quantitatively detect the change in the oscillation wavelength of the laser light using the output.

As described above, according to the present invention, it is possible to provide a wavelength locker module and an etalon used therein, which can accurately lock wavelengths in a wide wavelength band. This makes it possible to stabilize the oscillation wavelength of the wavelength tunable light source for a long period of time, and to accurately obtain the amount of wavelength change when the oscillation wavelength of the wavelength tunable laser is changed. Further, according to the present invention, it is possible to provide a wavelength change amount detector capable of detecting a wavelength change amount with high precision, and a wavelength meter using the same.

1 is a schematic diagram of an optical arrangement of a Fabry-Perot etalon according to Embodiment 1. FIG. FIG. 4 is a diagram showing the wavelength dependence of transmitted light obtained by the Fabry-Perot etalon according to Embodiment 1; FIG. 4 is a diagram showing an example of the quotient of the sum and the difference in the intensity of transmitted light obtained by the Fabry-Perot etalon according to the first embodiment; FIG. 10 is a diagram schematically showing the optical arrangement of a Fabry-Perot etalon according to Embodiment 2; FIG. 11 is a diagram showing an outline of a wavelength variation detector according to Embodiment 3; FIG. 10 is a diagram showing an interference waveform of a Fabry-Perot etalon according to Embodiment 3; FIG. 3 is a photograph of a Fabry-Perot etalon fabricated in Examples. 10 is a graph showing values near 1060 nm of x-polarized output PDa and y-polarized output PDb obtained by the Fabry-Perot etalon fabricated in the example. FIG. 4 is a diagram showing error signal waveforms obtained by the Fabry-Perot etalon fabricated in the example. FIG. 4 is a diagram showing respective outputs of PDa and b from 990 nm to 1080 nm obtained by the Fabry-Perot etalon fabricated in the example. FIG. 4 is a diagram showing respective outputs of PDa and b from 990 nm to 1080 nm obtained by the Fabry-Perot etalon fabricated in the example. FIG. 10 is a diagram showing a wavelength change when the wavelength is locked to 1064 nm in an example.

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. However, the present invention can be implemented in many different forms, and is not limited to the description of the embodiments and application examples shown below.

(Embodiment 1)
FIG. 1 is a diagram schematically showing an optical arrangement of a wavelength-locked module (hereinafter referred to as "this module") 1 using a Fabry-Perot etalon (hereinafter referred to as "this etalon") 2 according to this embodiment. As shown in the figure, the etalon 2 in the module 1 has two partially reflective elements 3a and 3b, which have partially reflective surfaces that reflect a portion of incident light and transmit a portion of the incident light, and are installed facing each other in parallel. and has a phase plate 4 between the two partially reflecting elements 3a, 3b.

The etalon 2 further includes a polarizing optical path splitting element 5 outside the pair of partial reflection elements 3 and on the side opposite to the side on which light is incident. Photoelectric conversion elements 6a and 6b are provided for converting respective amounts of light separated by the element 5 into electric signals.

Further, the etalon 2 is provided with a control device 7 electrically connected to the photoelectric conversion elements 6a and 6b, which can perform predetermined processing as described later and output the result to the outside.

In the etalon 2, the partially reflecting elements 3a and 3b have partially reflecting surfaces 31a and 31b that partially reflect and partially transmit incident light, as described above. In the etalon 2, the partially reflecting elements 3a and 3b are installed so that the partially reflecting surfaces 31a and 31b face each other and face each other, forming a hollow Fabry-Perot etalon. Examples of the partial reflection elements 3a and 3b are not limited as long as they have the above functions, and examples include half mirrors, beam splitters, and beam samplers.

Further, in the etalon 2, it is desirable that the rear surfaces 32a and 32b of the partially reflective surfaces 31a and 31b have at least one of a non-reflective coated surface and a wedge. By doing so, it is possible to prevent light that has once exited the pair of partial reflection elements 31a and 31b from entering the pair of partial reflection elements 31a and 31b again.

Further, in the phase plates 3a, 3b of the etalon 2, the distance between the partially reflecting surfaces 31a, 31b is not limited as long as the effect of the etalon 2 can be obtained, and can be adjusted as appropriate.

The material of the partial reflection members 3a and 3b in the etalon 2 is not particularly limited as long as it has the above functions, but it is desirable that it be a thermally stable material. etc. are preferred.

In addition, the phase plate 4 in the etalon 1 is a member that can change the phase of incident light, and by utilizing this phase change, the effects of the etalon described above and later can be achieved. can be done.

Specifically, in the etalon 2, the phase plate 4 has a finite difference in the free spectral range (FSR) for light with a polarization component parallel to the delay axis of the phase plate 4 and a polarization component orthogonal to it. is preferred. Here, "having a finite difference" means that the incident light has different spectral responses, that is, different wavelengths at which constructive and destructive interference occurs.

In the etalon 2, the phase plate 4 is preferably a λ/8 wavelength plate that delays the polarized light parallel to the delay axis by ⅛ wavelength. The effect of this will be described later.

Moreover, in the etalon 2 , the delay axis of the wave plate 4 is preferably set orthogonally or parallel to the plane of incidence of the polarizing optical path splitting element 5 . In this figure, for convenience, the delay axis of the wave plate is drawn in the y direction.

In the module 1, the polarizing optical path splitting element 5 is a member capable of splitting light by changing the traveling direction of light according to the polarization state of incident light. The configuration of the polarizing optical path splitting element 5 is not limited as long as it has the above function, and examples thereof include a polarizing beam splitter, a beam displacer, and a Glan-Thompson.

Moreover, in the module 1 , it is preferable that the plane of incidence of the polarizing optical path splitting element 5 is arranged parallel or perpendicular to the delay axis of the phase plate 3 . By doing so, it is possible to more reliably separate the light that has undergone a phase change into different polarized light.

The module 1 further includes photoelectric conversion elements 6a and 6b for converting the respective amounts of light separated by the polarizing optical path separation element 5 into electrical signals. Although the photoelectric conversion elements 6a and 6b are not limited as long as they have this function, they are preferably photodiodes, for example.

Further, the light (incident light) 8 incident on the etalon 2 of the module 1 is laser light and linearly polarized light forming an angle of 45 degrees with the plane of incidence of the polarizing optical path separation element 5. preferable. By doing so, the intensity of the light reaching the photoelectric conversion elements 6a and 6b after being separated by the polarization separation element 5 becomes 1:1, thereby minimizing the influence of noise and simplifying signal processing. become. Light transmitted through the partial reflection surfaces 31a and 31b of the partial reflection elements 3a and 3b without being reflected once and light reflected once each are separated into x and y polarized components by the polarization type optical path separation element 5, and then photoelectrically generated. They interfere at the conversion elements 6a and 6b to produce an etalon transmission waveform. At this time, the optical path length of the etalon for the polarized component along the delay axis of the wave plate is longer than the optical path length for the polarized component perpendicular to the delay axis by λ/4. The wavelength dependence of the transmitted light is relatively shifted by half FSR as shown in FIG. Since the quotient of the difference and the sum of these signals becomes a signal that crosses 0 as shown in FIG. 3, PID control by negative feedback is possible.

The error signal is equivalent to obtaining the difference between the transmission signals of two etalons whose FSR differs by λ/4. Since this is realized by inserting a wave plate into one etalon, it goes without saying that it is economical and space-saving. In addition, since they share exactly the same optical axis until just before they are split by the polarizing optical path splitting element 5, the change with respect to external disturbances such as heat and vibration is exactly the same, and the wavelength dependence of each optic is also the same. This enables stable wavelength locking and wavelength calibration.

The signal processing described above can be realized by the control device 7 described above. An example of the control device 7 is not limited as long as it can perform the above processing, but for example, an information processing device such as a computer equipped with a CPU, a hard disk, a memory, etc., or an IC capable of performing the above processing. A stacked dedicated device or the like is suitable.

(Embodiment 2)
This embodiment is substantially the same as the first embodiment, but differs in the direction of light emitted from the Fabry-Perot etalon and the optical arrangement therefor. Main differences will be described below. The parts whose description is omitted are the same as those in the first embodiment.

FIG. 4 shows the optical system of the wavelength-locked module 1 according to this embodiment. As shown in the figure, this module includes two partial reflection elements 3a and 3b having partial reflection surfaces 31a and 31b and a phase plate 4 disposed between them, as in the first embodiment. It includes a hollow Fabry-Perot etalon, a non-polarizing optical path splitting element 9 provided outside the Fabry-Perot etalon, a polarizing optical path splitting element 5, and photoelectric conversion elements 6a and 6b for converting the amount of light into electrical signals. Configured. As in the first embodiment, the partial reflection elements 3a and 3b are installed so that the partial reflection surfaces 31a and 31b face each other.

In addition, in this lock module, a non-polarizing optical path separation element 9 is provided on the light incident side, and the light emitted from this etalon is reflected by the non-polarization type optical path separation element to , in that a polarizing optical path splitting element 5 is provided.

Further, in the present module 1, the control device 7 is connected to the photoelectric conversion elements 6a and 6b in the same manner as described above, performs predetermined processing, and outputs signals.

Further, in this module 1, it is preferable that the polarization of the incident light 8 with respect to the incident plane of the polarizing optical path splitting element 5 and the delay axis of the wavelength plate 2 have the same relationship as in the first embodiment.

Further, the phase plate 4 is preferably a λ/8 wavelength plate that delays the polarized light parallel to the delay axis by ⅛ wavelength, as in the first embodiment.

In the module 1 , the incident light 8 is caused to enter the non-polarizing optical path splitter 9 once before entering the etalon 2 . Then, the light reflected by the partial reflection surface of one partial reflection element 3a and the light reflected by the partial reflection surface of the other partial reflection element 2b are separated from the incident optical axis by the non-polarizing optical path separation element 9. , and after being split into x and y polarized light components by the polarizing optical path splitting element 5, they interfere on the photoelectric conversion elements 5a and 5b to generate an etalon transmission waveform.

That is, in this embodiment, the wavelength dependence of the transmitted light obtained by the photoelectric conversion elements 5a and 5b is relatively shifted by half FSR as shown in FIG. Since the quotient of the difference and the sum of these signals becomes a signal crossing 0 as shown in FIG. 3, PID control by negative feedback becomes possible.

Since the etalon reflected light is used in this arrangement, the dark part of the interference waveform can be made zero regardless of the reflectance of the partially reflecting surface. Therefore, by using an uncoated surface or the like having a reflectance of about 4% as the partially reflective surface, it is possible to obtain an error signal that approximates a sine curve. As a result, wavelength locking can be performed at equal intervals regardless of which point of the rising/falling slope of the error signal is locked, so that the same effect as using an etalon having twice the length can be obtained.

(Embodiment 3)
This embodiment relates to a wavelength change amount detector that detects the amount of change in wavelength. FIG. 5 shows a layout diagram of a wavelength variation detector (hereinafter referred to as "the present detector") 1 according to the present embodiment. However, the structure of this detector is generally the same as that of the above-described embodiment, and mainly different points will be described below. The parts whose description is omitted are the same as those in the first embodiment.

In this embodiment, the pair of partial reflection elements 3a and 3b are installed so that the partial reflection surfaces 31a and 31b face each other. The phase plate 4 disposed therebetween is a λ/16 wavelength plate that delays the polarized light parallel to the delay axis by 1/16 wavelength.

Further, in this embodiment, an optical path separation element 9 is provided outside the Fabry-Perot etalon 2 to separate the light emitted from the Fabry-Perot etalon 2 . The optical path splitting element 9 may be of a non-polarizing type, or may be of a polarizing type. One of the lights separated by the optical path separation element 9 enters the photoelectric conversion element 6c, the other enters the other optical path separation element 5, enters the photoelectric conversion elements 6a and 6b, and is converted into electrical signals. be done.

In this embodiment, the wavelength dependence of the transmitted light obtained by the photoelectric conversion elements 6a and 6b is an etalon interference waveform shifted by a quarter of the FSR as shown in FIG. Assuming that the wavelength giving one maximum point of the 6a output is _λ0 and the frequency change therefrom is Δν, the 6a, 6b and 7 outputs are expressed by the following equations.

Output of 4a: X = 2A cos(θ/2)^2 = A(1 + cos θ)
Output of 4b: Y = 2Acos(θ/2-λ/8)^2
=A(1+cos(θ−λ/4))
=A(1+sin θ)
Output of 6c: A

At this time, θ is θ=Δν/FSR×2π, where FSR is the fringe interval of the etalon. Then, by calculating these outputs as follows, the frequency difference from λ ₀ can be obtained in the range of ±FSR/4.

x = XA = A cos θ
y=YA=A sin θ
arctan(y/x)=θ=Δν/FSR×2π

If the frequency change of the incident laser is sufficiently continuous with respect to the FSR (as one measure, if the change is finer than one-tenth of the FSR), even if the frequency changes in the range exceeding ±FSR/4 , ±FSR/4 to expand the frequency range and catch the frequency change.

In the arrangement of the second embodiment, the frequency change can be quantitatively obtained by the fringe spacing (FSR) of the etalon, and in this arrangement, it is possible to quantitatively evaluate with high accuracy up to a resolution of about 1/100 of the FSR. It is possible.

Further, the wavelength change amount of this embodiment can be used as a wavelength meter by combining with a reference light source. Specifically, this wavelength variation detector is combined with a reference light source that outputs a known wavelength, and the wavelength of the incident laser light is calculated based on the wavelength of the reference light source and the output of the wavelength variation detector. By doing so, it can be made to function as a wavelength meter. Alternatively, the wavelength change detector according to this embodiment is combined with a wavelength detector capable of detecting the absolute wavelength of one or more specific wavelengths, and the wavelength of the incident laser light is detected by the absolute wavelength and the wavelength change amount detector. It is possible to function as a wavelength meter by calculating based on the output of .

Here, the Fabry-Perot etalon according to the above embodiment was actually manufactured and its effect was confirmed. In this example, the optical arrangement described in the second embodiment was produced.

FIG. 7 shows a photograph of the example. λ-Master 1040 manufactured by Spectra Quest Lab Co., Ltd. was used as a wavelength-locking target laser, and a linearly polarized output was introduced into the wavelength locker module system through a polarization-preserving fiber.

A beam sampler BSF05-B manufactured by Thorlabs was used as a partially reflective element, and an etalon was constructed with an uncoated surface (reflectance of 4%) as a partially reflective surface. The distance between the etalons was defined by the lens barrel using a super-invar wire rod, which is a low-thermal-expansion material. The distance between the partially reflecting surfaces of the etalon is about 15 cm, and the FSR at this time is 1 GHz (3.3 pm).

A 1/8 wavelength plate was used as the phase plate 2, and a 1040 nm zero-order wavelength plate was custom-ordered by CRYSLASER in China.

A polarization-independent half beam splitter cube BS014 manufactured by Thorlabs was used as a polarization-independent optical path branching optical element.

As a polarizing optical path branching optical element, a wide-band polarizing beam splitter cube CCM5-PBS203 manufactured by Thorlabs was used.

Then, a photodetector PDA100A manufactured by Thorlabs was used to detect the amount of light, an error signal was calculated using a dedicated circuit or the like, and feedback control was performed on the oscillation wavelength of λ-Master1040, which is the wavelength lock target. The λ-Master 1040 has a function of externally modulating the LD current value, and can easily perform feedback control by adding an appropriate gain to the error signal.

FIG. 8 shows the values of the x-polarized output PDa and the y-polarized output PDb near 1060 nm. It can be seen that the wavelength dependence of PDa,b is shifted by half a mountain at the same period, and a finite difference in FSR occurs due to the λ/8 wavelength plate.

FIG. 9 shows an error signal that is actually output by calculating (PDa-PDb)/(PDa+PDb). An error signal swinging positively and negatively with respect to zero is obtained, and it is easy to perform wavelength locking by feedback using this. Also, the use of a 4% reflective surface as a partially reflective surface results in a signal that resembles a sine curve, with the zero crossing wavelengths evenly spaced regardless of the up/down slope. As a result, it is possible to lock wavelengths at equal intervals with a double wavelength pitch.

10 and 11 show respective outputs of PDa and b from 990 nm to 1080 nm. Both show substantially the same wavelength dependence, and wavelength locking is possible over the entire oscillation wavelength range of λ-Master 1040 without the need for gain adjustment of PDa and b. The intensity fluctuation seen at a period of about 1 nm, which is larger than the FSR of 3.3 pm of the etalon, is due to the interference of the beam splitter, etc., but since PDa and b show exactly the same dependence, it is canceled by the calculation of the error signal, and the wavelength is locked. does not pose any hindrance to

Also, FIG. 12 shows the wavelength change when the wavelength is locked to 1064 nm using this wavelength locker module. The wavelength change was measured using a wavelength meter A771 (wavelength accuracy: 60 MHz) manufactured by Bristol. The black solid line indicates the data when the wavelength is locked, and the gray solid line indicates the data when the wavelength is not locked. Without locking, the output wavelength drifts due to changes in ambient temperature, etc., but with wavelength locking, a stable oscillation wavelength is obtained within the accuracy of the wavelength meter.

As described above, the effect of the present invention could be confirmed by this example.

INDUSTRIAL APPLICABILITY The present invention has industrial applicability as a Fabry-Perot etalon and a wavelength locker module using the same. In addition, there is industrial applicability as a wavelength variation detector and a wavelength meter using this Fabry-Perot etalon.

Claims (6)

A Fabry-Perot etalon having two partially reflective elements having partially reflective surfaces that reflect a portion of incident light and transmit a portion of the incident light are placed in parallel and facing each other, and a phase plate is provided between the two partially reflective elements. There is
A Fabry-Perot etalon, wherein the phase plate is a λ/8 waveplate or a λ/16 waveplate. A Fabry-Perot etalon having a phase plate between two partially reflective elements having a partially reflective surface that reflects a part of incident light and transmits a part of the incident light, and that is installed in parallel facing each other. ,
a polarizing optical path splitting element that splits light emitted from the Fabry-Perot etalon according to the polarization state;
A wavelength change amount detector comprising two photoelectric conversion elements arranged on respective exit surface sides of the polarizing optical path splitting element. 3. A wavelength variation detector according to claim 2 , further comprising a control device connected to each of said two photoelectric conversion elements for processing output signals of said two photoelectric conversion elements. Two partially reflective elements having partially reflective surfaces that reflect part of the incident light and transmit part of it are installed in parallel facing each other, and a λ/8 wave plate or λ/16 wave plate is placed between the partially reflective elements. a Fabry-Perot etalon having
A laser beam linearly polarized at an angle of 45 degrees with respect to the delay axis of the λ/8 wave plate or the λ/16 wave plate passes through or is reflected by the Fabry-Perot etalon, and then passes through the delay axis of the wave plate. a polarizing optical path branching element that separates into parallel and orthogonal polarization components, respectively;
a photoelectric conversion element that outputs a signal corresponding to the amount of light of each polarized component separated by the polarization type optical path branching element;
and a control device that processes the signal of the photoelectric conversion element,
A wavelength change amount detector capable of quantitatively detecting a change in the oscillation wavelength of the laser light using the output. a wavelength variation detector according to claim 4 ;
a reference light source outputting a known wavelength;
A wavelength meter for calculating the wavelength of the incident laser light based on the wavelength of the reference light source and the output of the wavelength variation detector. a wavelength variation detector according to claim 4 ;
a wavelength detector capable of detecting absolute wavelengths for one or more specific wavelengths;
A wavelength meter for calculating the wavelength of the incident laser light based on the absolute wavelength and the output of the wavelength variation detector.