CN101706361B - Acousto-optic tunable filter diffraction efficiency test system and test method - Google Patents

Abstract

The invention discloses a system for testing the diffraction efficiency of an acousto-optic tunable filter. The system consists of a wavelength tunable laser, a neutral density filter, an aperture, a beam splitter, a two-dimensional electric turntable, and an energy meter. The wavelength tunable laser can generate a laser beam with continuously adjustable wavelength. After passing through the neutral density filter and the aperture, it is divided into two laser beams with a fixed beam splitting ratio by the beam splitter. The reflected beam energy is used as the reference energy; the transmitted beam Enter the AOTF, receive the diffracted light energy when the RF drive is applied to it, and receive the directly transmitted energy when the drive is not applied, so as to calculate the diffraction efficiency of the AOTF. At the same time, the measurement of the aperture angle can be realized by changing the incident light angle through a two-dimensional electric turntable. The device of the invention is characterized by simple principle and strong operability, and can meet the requirement of AOTF testing continuous wavelengths, and at the same time, using a beam splitter to generate a reference beam can greatly improve test accuracy.

Description

Acousto-optic tunable filter diffraction efficiency test system and test method

Technical field:

The invention relates to optical measurement technology, in particular to an acousto-optic tunable filter diffraction efficiency test system and method.

Background technique:

Acousto-optic tunable filter (AOTF) is a narrow-band tunable filter, which is a spectroscopic device made according to the principle of acousto-optic interaction. Wavelength scanning is realized by changing the frequency-selective splitting wavelength of the RF driver applied to the crystal. At present, this technology has been widely used in non-imaging and imaging spectroscopy instruments.

AOTF's spectroscopic principle: As shown in Figure 1 and Figure 2, when a beam of polychromatic light passes through a high-frequency vibrating optically elastic crystal, monochromatic light of a certain wavelength will be diffracted inside the crystal to It is transmitted from the crystal at a certain angle, and the polychromatic light that has not been diffracted is directly transmitted through the crystal along the direction of the original light propagation, thereby achieving the purpose of light splitting. When the crystal vibration frequency changes, the wavelength of the monochromatic light that can be transmitted also changes accordingly. Both isotropic and anisotropic Bragg diffraction can be used for optical filter devices, but the use of isotropic crystals as optical filters is extremely poor in practicability, and it has strict requirements on the parallelism of incident light (the opening angle is within one thousandth of an arc Within), small deviations will also lead to a significant broadening of the spectral passband, and the diffraction angles corresponding to different wavelengths are also different. Therefore, AOTF uses birefringent crystal anisotropic Bragg diffraction. In the anisotropic AOTF, when the incident angle of the light incident on the AOTF has a small change _δθi , since the change of the birefringence with the angle just compensates the momentum mismatch caused by the angle change, the momentum matching can still be maintained The conditions are approximately true, and the acousto-optic diffraction is not very sensitive to the change of the incident light within a certain angle, so that an AOTF with a large angular aperture can be made.

AOTF Diffraction Efficiency Testing Technology: According to AOTF’s spectroscopic principle, there are high requirements for the collimation and beam diameter of the incident beam. In order to meet a certain measurement accuracy, the diameter and emission angle of the beam should be as small as possible. Moreover, the test of the diffraction efficiency of the AOTF needs to achieve full-band coverage.

At present, domestic methods for testing the diffraction efficiency of AOTF mainly focus on using a continuous broad-spectrum light source (such as a tungsten halogen lamp) to test the spectral characteristics of the outgoing light through a spectral receiving system. The schematic diagram of the testing device is shown in Figure 3. First, the light source is collimated, the polarizer is used to generate vertically polarized light incident, and the spectrometer is used to receive the 0-order light spectrum. Assuming that the spectral intensity of the 0th order light is I ₀ when no radio frequency signal is added, the minimum value of the 0th order light after being driven by radio frequency is I, and the diffraction efficiency T=(I ₀ -I)/I ₀ . When the driving power is fixed, the spectrum scanning can be performed by changing the AOTF RF driving frequency. The defect of this method is that the parallelism of the light source after collimation is not as good as that of the laser, and the test results of the diffraction efficiency of the AOTF will be greatly changed by the incident laser light at different angles, so the results tested by this method are different from the actual diffraction efficiency of the AOTF. have difference.

If a laser is used as the light source, it is only necessary to use the energy receiving system to conveniently measure and calculate the light energy at all levels, so as to obtain the AOTF diffraction efficiency. However, with this test method, a single-wavelength laser cannot meet the requirements of AOTF continuous spectrum testing, and this test method has relatively high requirements for the stability of laser energy. Therefore, at present, lasers are used as light sources for AOTF diffraction efficiency tests only in individual fixed bands, which cannot meet the needs of AOTF full-band tests.

Invention content:

The purpose of the present invention is to provide a device for measuring AOTF diffraction efficiency, which solves the technical problems of measurement errors caused by unstable laser energy and inconsistent responses of energy meter probes.

As shown in accompanying drawing 4, the present invention uses EKSPLA NT342/1/UV wavelength tunable laser 1 as the test light source, and the light beam is incident on the beam splitter 4 after passing through the neutral density filter 2 and the aperture hole 3, and the beam splitter The mirror 4 splits the incident light into transmitted light and reflected reference light with a fixed beam splitting ratio. The optical path is adjusted so that the transmitted light enters the AOTF71 to be tested vertically, and the AOTF driver 72 applies a driving frequency to the AOTF71. The energy meter probe 61 receives the beam energy passing through the AOTF 71 , and the energy meter probe 62 receives the energy of the reference beam synchronously.

Specific method: adjust the wavelength of the laser, add a certain radio frequency drive to the AOTF71, the energy of the diffracted light received by the energy meter probe 61 is E ₁ , and the energy of the reference beam received by the energy meter probe 62 synchronously is E ₂ , when E ₁ /E ₂ is the largest is the diffraction center wavelength corresponding to the RF driving frequency.

Without RF drive, the AOTF directly emitted light energy E ₁ ' received by the energy meter probe 61, and the reflected beam energy synchronously received by the energy meter probe 62 is E ₂ ', and the calculation formula of the diffraction efficiency is as follows:

$\frac{{\mathrm{E.}}_1}{\mathrm{\η}{\mathrm{E.}}_2}=\frac{{{\mathrm{E.}}_1}^{\′}}{{{\mathrm{E.}}_2}^{\′}},$ but $\mathrm{\η}=\frac{{\mathrm{E.}}_1{{\mathrm{E.}}_2}^{\′}}{{\mathrm{E.}}_2{{\mathrm{E.}}_1}^{\′}}---(1-1)$

The incident light angle of AOTF71 can be changed by the rotation of the two-dimensional electric turntable 5, and when E ₁ /E ₂ drops to half of the maximum value, the corresponding angle change is the crystal aperture angle.

Wavelength tunable laser 1: Provide continuously adjustable laser wavelength from 210 to 2300nm, the laser divergence angle is less than 0.5mrad, the spectral scanning interval in the 210 to 709nm band is 0.1nm, and the spectral scanning interval in the 710 to 2300nm band is 1nm.

The neutral density filter 2 and the aperture 3 can control the incident light energy and limit the size of the laser spot. The function of the beam splitter 4 is to generate a reference beam, so as to greatly improve the test accuracy. The energy of the wavelength tunable laser is unstable, and the energy value of a single measurement is not comparable. The probe responsivity of the two energy meters is inconsistent, and the number of wavelengths to be tested in the experiment is large, so it is difficult to calibrate the probe response coefficient at each wavelength. The beam splitter added in the test light path can generate the reflected reference beam and monitor its energy. The beam splitting ratio of the beam splitter at the specified wavelength is constant, so the ratio of the energy entering AOTF71 to the energy of the reference beam is constant. The beam splitter plays a role in improving the test accuracy. Since the light transmission efficiency of AOTF71 is constant at a specified wavelength, the ratio of the diffracted light energy to the reference beam energy can be used instead of directly testing the diffracted light energy.

The two-dimensional motorized turntable 5 can adjust the angle of incident light, one is used for optical path adjustment, and the other is used for aperture angle measurement.

The advantage of this patent is:

1) The present invention uses a wavelength tunable laser as the incident light source, which can meet the requirements of AOTF for high-density, small-interval wavelength testing.

2) In the present invention, the beam splitter is used to split the incident light energy, and the energy of the reference beam is used as monitoring to offset the test error caused by the instability of the laser energy and the inconsistent response rate of the energy meter probe, thereby greatly improving the test accuracy of the AOTF diffraction efficiency.

Description of drawings:

Fig. 1 Vector diagram of anisotropic AOTF.

Figure 2 AOTF spectroscopic diagram.

Fig. 3 is a schematic diagram of the wide light source AOTF diffraction efficiency test system.

Fig. 4 is a schematic diagram of the AOTF diffraction efficiency test system.

Detailed ways:

The following is a preferred embodiment of the present invention provided according to FIG. 4, which is used to illustrate the structural features and implementation methods of the present invention, rather than to limit the scope of the present invention.

Acousto-optic tunable filter diffraction efficiency test system includes the following parts:

1) Wavelength tunable laser 1: In this embodiment, EKSPLANT342/1/UV wavelength tunable laser 1 is selected as the light source. The laser can generate 210-2300nm continuously tunable laser beam.

2) Neutral density filter 2: Spiricon neutral density filter is selected in this embodiment.

3) Diaphragm hole 3: Daheng Optoelectronics GCM-57 variable diaphragm is selected for this implementation.

4) Beam splitter 4: In this embodiment, a beam splitter with a fixed beam splitting ratio of 1:1 (600-1200 nm) is selected. The beam splitting ratio will vary in other bands, but for a fixed wavelength, the beam splitting ratio is constant.

5) Two-dimensional electric turntable 5: Lianyi 148×142 two-dimensional electric turntable is used in this embodiment, the adjustment range is 360°, the transmission ratio of the motor is 1:360, the minimum reading of the scale is 0.1°, and the full-step resolution of the motor is 0.005°.

6) Energy meter probes 61 and 62: In this embodiment, the EPM1000 energy meter head of Coherent Company of the United States is selected, and the probes are J4-09 and J45LP-MB.

In this embodiment, a visible/near-infrared AOTF 71 and a matching radio frequency driver 72 are selected as the AOTF components from Brimrose in the United States. The wavelength of the laser is selected, and the two-dimensional electric turntable 5 is adjusted so that the beam emitted by the wavelength-tunable laser 1 passes through the neutral density filter 2, the aperture 3, and the beam splitter 4, and then enters the AOTF 71 vertically, and is received by the energy meter probe 62. The reference beam reflected by the beam mirror. When the AOTF 71 is not driven, the energy meter probe 61 receives the 0-level light energy behind the AOTF71; when the AOTF71 is driven, the energy meter probe 61 receives the diffracted light energy behind the AOTF71. During the test, the two energy meter probes 61, 62 receive synchronously. The diffraction efficiency of AOTF71 is calculated from the above formula (1-1) for calculating the diffraction efficiency.

As mentioned above, the test method of this test system is simple and operable, and it is an ideal AOTF diffraction efficiency test device.

Claims (3)

1. An acousto-optic tunable filter AOTF diffraction efficiency test system, it comprises light source, neutral density filter (2), aperture hole (3), beam splitter mirror (4), two-dimensional electric turntable ( 5), the first energy meter probe (61), the second energy meter probe (62), it is characterized in that: the described light source in the described testing system adopts wavelength tunable laser (1); Described wavelength tunable laser (1) The emitted laser beam passes through the neutral density filter (2) and aperture (3) in sequence, and then the incident light is divided into transmitted light and reflected reference light with a fixed beam splitting ratio by the beam splitter (4). The light is vertically incident on the AOTF (71) to be measured, the AOTF driver (72) applies radio frequency drive to the AOTF (71), the first energy meter probe (61) receives the beam energy passing through the AOTF (71), and the second energy meter probe (62) Synchronously receive the energy of the reflected reference beam, and the second energy meter probe (62) measures the laser energy of the reflected reference beam for compensating laser energy instability and the first energy meter probe (61) and the second energy meter probe ( 62) Measurement errors caused by inconsistent responses. 2. An acousto-optic tunable filter AOTF diffraction efficiency test system according to claim 1, characterized in that: the continuously adjustable laser wavelength range of the wavelength tunable laser (1) is 210-2300nm, and the laser The divergence angle is less than 0.5mrad, the spectral scanning interval of the 210-709nm band is 0.1nm, and the spectral scanning interval of the 710-2300nm band is 1nm. 3. an acousto-optic tunable filter AOTF diffraction efficiency testing method based on the system described in claim 1, is characterized in that comprising the following steps: A. Adjust the wavelength of the laser, add a certain radio frequency drive to the AOTF (71), measure the diffracted light energy E ₁ on the measurement optical path through the first energy meter probe (61), and measure the reference energy through the second energy meter probe (62) Reflected beam energy E ₂ on the optical path; B. Without RF drive, measure the direct outgoing light energy E ₁ ′ on the measurement optical path through the first energy meter probe (61), and measure the reflected beam energy E on the reference optical path through the second energy meter probe (62) ₂ '; C. Calculate the AOTF diffraction efficiency, the calculation formula is as follows: $\mathrm{<span\; class="notranslate">\; \&eta;</span>}<span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; E.</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}{{\mathrm{<span\; class="notranslate">\; E.</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}}^{<span\; class="notranslate">\; \&prime;</span>}}{{\mathrm{<span\; class="notranslate">\; E.</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}{{\mathrm{<span\; class="notranslate">\; E.</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}}^{<span\; class="notranslate">\; \&prime;</span>}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; .</span>$

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