Integrated intermediate infrared TDLAS detection device and method

Abstract

The invention discloses an integrated mid-infrared TDLAS (tunable diode laser absorption spectroscopy) detection device and method, and relates to the technical field of laser spectrum. Comprising a local device and a far-end device, the local device comprises a controller, a DFB1 laser, a DFB2 laser and a detector, the DFB1 laser, the DFB2 laser and the detector are connected with the controller, the far-end device comprises a substrate, and the substrate is provided with a first nonlinear waveguide, a waveguide extension line, a filter and a second nonlinear waveguide which are sequentially connected. The DFB1 laser and the DFB2 laser are respectively connected with the first optical fiber, the first optical fiber is connected with the first nonlinear waveguide, the far-end device is connected with the second optical fiber through the second nonlinear waveguide, and the second optical fiber is connected with the detector. According to the invention, the integration degree is high, the detection sensitivity of the TDLAS technology is improved, and the cost is reduced.

Description

Technical Field

[0001] The present invention relates to the field of laser spectroscopy technology, and in particular to an integrated mid-infrared TDLAS detection device and method. Background Art

[0002] TDLAS is the abbreviation of Tunable Diode Laser Absorption Spectroscopy. This technology mainly utilizes the narrow line width and wavelength of tunable semiconductor lasers that change with the injected current. By modulating the wavelength of the laser, the wavelength of the laser is scanned across the absorption peak of the gas molecules to be measured. Based on the Beer-Lambert law, the gas molecules absorb the modulated laser, and the concentration of the gas molecules is measured according to the absorption amount.

[0003] Tuned semiconductor absorption spectroscopy (TDLAS) technology is a type of laser absorption spectroscopy (LAS) technology. According to the different driving forms of the laser, laser absorption spectroscopy (LAS) technology can be divided into: direct absorption method and modulation absorption method. The modulation absorption method has a higher detection efficiency than the direct absorption method because it uses the second harmonic 2f signal to filter the noise. The specific method is: a sawtooth wave driving current signal needs to be given to the laser, and a 2f harmonic signal needs to be loaded onto the driving current. It has high sensitivity and can avoid low-frequency interference. It is further divided into wavelength modulation and frequency modulation. The wavelength modulation type requires a larger tuning range, and the frequency modulation type requires a very high scanning frequency and modulation frequency. The technology is complex and the sensitivity is higher.

[0004] Semiconductor lasers are in the near-infrared band, which limits the application scope of this technology. Because many gases such as methane and hydrogen sulfide have strong absorption peaks in the mid-infrared band of 3μm-5μm, which is at least two orders of magnitude higher than the near-infrared band, the use of mid-infrared lasers for TDLAS detection can effectively improve the sensitivity. Methods for generating mid-infrared lasers include QCL (quantum cascade laser), optical parametric oscillation, etc., and the technical solutions are generally complex, costly, and difficult to integrate. Summary of the invention

[0005] In view of the above problems, the present invention provides an integrated mid-infrared TDLAS detection device with simple structure, convenient processing and improved reliability.

[0006] The technical solution of the present invention is: an integrated mid-infrared TDLAS detection device, including a local device and a remote device,

[0007] The local device includes a controller and a DFB1 laser, a DFB2 laser and a detector connected to the controller.

[0008] The remote device comprises a substrate, on which are arranged a nonlinear waveguide 1, a waveguide extension line, a filter and a nonlinear waveguide 2 which are connected in sequence.

[0009] The DFB1 laser and the DFB2 laser are respectively connected to optical fiber one, the optical fiber one is connected to nonlinear waveguide one, the remote device is connected to optical fiber two through nonlinear waveguide two, and the optical fiber two is connected to the detector.

[0010] The detector is a near infrared detector.

[0011] The substrate is made of silicon dioxide.

[0012] The nonlinear waveguide 1, the waveguide extension line and the nonlinear waveguide 2 are thin-film lithium niobate waveguides respectively.

[0013] The waveguide extension line is in a continuously bent shape.

[0014] An integrated mid-infrared TDLAS detection method, comprising:

[0015] The controller is used to provide current and voltage signals to the DFB1 laser and the DFB2 laser and to obtain electrical signals from the detector.

[0016] The DFB1 laser generates a laser with a wavelength of λ1, and the DFB2 laser generates a laser with a wavelength of λ2.

[0017] Optical fiber one transmits two wavelengths of laser light to a remote device; optical fiber two is connected to a detector, and the detector receives the optical signal returned by optical fiber two.

[0018] The nonlinear waveguide 1 in the remote device performs a difference frequency on the two wavelength lasers of λ1 and λ2 through the difference frequency effect to generate a mid-infrared detection light with a wavelength of λ3;

[0019] The waveguide extension line is exposed in the gas as an optical window, and is used to extend the wavelength λ3 laser optical path;

[0020] The waveguide extension line has three wavelength lasers of λ1, λ2 and λ3. The laser of λ2 wavelength is filtered out by the filter, leaving the lasers of λ1 and λ3 to enter the nonlinear waveguide 2.

[0021] In nonlinear waveguide 2, the lasers of λ1 and λ3 undergo secondary frequency difference to generate lasers with a wavelength of λ2, which are exported through optical fiber 2.

[0022] The nonlinear waveguide 1 and the nonlinear waveguide 2 have the same structure.

[0023] The two frequency difference processes both satisfy the following equation:

[0024]

[0025] Wherein, K is the reciprocal lattice vector, Λ is the period, λ1, λ2, λ3 are the three input laser wavelengths, and n1, n2, n3 are the refractive indices of the corresponding three wavelengths.

[0026] During operation, the present invention includes a local device and a remote device, wherein a controller in the local device provides current and voltage signals to the laser and acquires electrical signals from the detector, optical fiber 1 is connected to the DFB1 and DFB2 lasers to transmit lasers of two wavelengths to the remote device; optical fiber 2 is connected to the detector to transmit the laser of the remote device to the local device.

[0027] An integrated nonlinear waveguide is used in the remote device to realize mid-infrared laser through difference frequency, and the detection optical path is increased through an integrated waveguide extension line.

[0028] The invention has a high degree of integration, improves the detection sensitivity of the TDLAS technology, and reduces the cost. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] In order to more clearly illustrate the specific implementation of the present invention or the technical solution in the prior art, the following briefly introduces the drawings required for the specific implementation or the prior art description. In the drawings, each part is not necessarily drawn according to the actual scale.

[0030] Figure 1 It is a structural schematic diagram of the present invention,

[0031] Figure 2 is a schematic diagram of the structure of the remote device.

[0032] Figure 3 It is the wavelength conversion logic diagram of the remote device.

[0033] Figure 4 is a schematic diagram of the three-dimensional structure of the remote device.

[0034] In the figure, 1 is the substrate, 2 is the nonlinear waveguide 1, 3 is the waveguide extension line, 4 is the filter, and 5 is the nonlinear waveguide 2. DETAILED DESCRIPTION

[0035] Embodiments of the present invention are described in detail below, examples of which are shown in the accompanying drawings, wherein the same or similar reference numerals throughout represent the same or similar elements or elements having the same or similar functions. The embodiments described below with reference to the accompanying drawings are exemplary and are only used to explain the present invention, and cannot be understood as limiting the present invention.

[0036] In the description of the present invention, it should be understood that the terms "upper", "lower", "left", "right", "vertical", "horizontal", etc., indicating the orientation or positional relationship, are based on the orientation or positional relationship shown in the drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as limiting the present invention. In the description of the present invention, unless otherwise specified, "plurality" means two or more.

[0037] In the description of the present invention, it should be noted that, unless otherwise clearly specified and limited, the terms "installed", "connected", and "connected" should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection, or it can be indirectly connected through an intermediate medium, or it can be the internal communication of two components. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0038] The present invention Figure 1 As shown, an integrated mid-infrared TDLAS detection device includes a local device, an optical fiber and a remote device. The local device is composed of a DFB1 laser, a DFB2 laser, a detector and a controller. The DFB1 laser generates a laser with a wavelength of λ1, the DFB2 laser generates a laser with a wavelength of λ2, the detector receives the optical signal returned by the optical fiber, and the controller provides the current and voltage signals of the DFB laser (i.e., DFB1 and DFB2) and obtains the electrical signal of the detector. The optical fiber is composed of optical fiber 1 and optical fiber 2. Optical fiber 1 is connected to the DFB1 laser and the DFB2 laser to transmit the lasers of the two wavelengths to the remote device; optical fiber 2 is connected to the detector to transmit the laser of the remote device to the local device.

[0039] like Figure 2 As shown, the core of the remote device has a substrate, a nonlinear waveguide 1, a waveguide extension line, a filter, and a nonlinear waveguide 2. In addition, it should also have peripheral mechanical and optical packaging structures, such as a substrate fixing structure, an optical fiber coupling structure, an optical window, etc. Since these peripheral structures are conventional means, they are not the focus of this patent.

[0040] Among them, the function of the nonlinear waveguide 1 is to perform frequency difference on the two wavelength lasers of λ1 and λ2 through the difference frequency effect to generate mid-infrared detection light with a wavelength of λ3.

[0041] The function of the waveguide extension line is to extend the wavelength λ3 laser optical path through a repeated bending structure. The laser transmitted in the strip waveguide has an evanescent field, which is distributed outside the waveguide. If there is a gas outside the waveguide that absorbs the λ3 wavelength laser, the intensity of the laser transmitted in the waveguide will be affected.

[0042] An integrated mid-infrared TDLAS detection method, comprising:

[0043] The controller is used to provide current and voltage signals to the DFB1 laser and the DFB2 laser and to obtain electrical signals from the detector.

[0044] The DFB1 laser generates a laser with a wavelength of λ1, and the DFB2 laser generates a laser with a wavelength of λ2.

[0045] Optical fiber one transmits two wavelengths of laser light to a remote device; optical fiber two is connected to a detector, and the detector receives the optical signal returned by optical fiber two.

[0046] The waveguide extension line is realized by an integrated waveguide. The waveguide extension line should be exposed to the gas as an optical window in the packaging structure. The waveguide extension line has three wavelength lasers of λ1, λ2 and λ3. The laser of λ2 wavelength is filtered out by a filter, leaving only the lasers of λ1 and λ3 to enter the nonlinear waveguide 2. In the nonlinear waveguide 2, the lasers of λ1 and λ3 are frequency-differenced again to generate lasers of wavelength λ2, which are exported through optical fiber 2.

[0047] The all-fiber input and output of the present invention avoids the danger of gas explosion and combustion caused by short-circuit heating of electronic devices.

[0048] like Figure 3 As shown, the λ3 after the difference frequency is a mid-infrared laser, which is used for gas detection. After passing through the gas, the λ3 laser is absorbed by the gas and carries the gas information, and then the remaining λ2 laser is filtered out after filtering, and then the difference frequency of the λ1 and λ3 lasers is converted into the λ2 laser, and the gas information of the λ3 laser is converted into the gas information of the λ2 laser. The purpose of the above process is to perform mid-infrared gas detection at the remote device, while only transmitting near-infrared laser in the optical fiber, avoiding the problem of expensive transmission of mid-infrared.

[0049] The laser wavelengths in the second optical fiber are λ1 and λ2, which are transmitted back to the local device and received by the detector. The detector is a near-infrared detector, such as a germanium detector. In the application, a filter or a dichroic mirror is placed in front of the detector to filter the λ1 wavelength laser before entering the detector to avoid interference from the λ1 wavelength laser.

[0050] The λ1 and λ2 wavelength lasers in the present invention are both generated by DFB lasers. The wavelength of the λ1 wavelength laser remains unchanged, and the wavelength of the λ2 wavelength laser is frequency modulated to meet the TDLAS method. Specifically, the DFB2 laser should be current modulated to change the λ2 wavelength. The current modulation of the superposition of triangular wave and sine wave should be performed according to the TDLAS method. The above TDLAS methods are all mature technologies.

[0051] After the wavelength modulation of λ2 is performed, the wavelength modulation of λ2 is transferred to the mid-infrared laser of λ3 through the difference frequency process. Therefore, the mid-infrared laser of λ3 has the modulation effect of the superposition of the above-mentioned triangular wave and sine wave, and the near-infrared TDLAS is transferred to the mid-infrared band.

[0052] The λ3 mid-infrared laser with TDLAS signal passes through the waveguide extension line, is absorbed by the gas, carries the TDLAS signal of the gas, and enters the filter.

[0053] At this time, the laser with wavelength λ2 has no effect and is filtered out by the filter. Then, through the difference frequency process, the mid-infrared laser with wavelength λ3 and the near-infrared laser with wavelength λ1 are frequency-differencing to generate near-infrared laser with wavelength λ2 again. The TDLAS signal of the gas is transferred to the near-infrared laser with wavelength λ2 and detected after being transmitted through optical fiber 2.

[0054] like Figure 4 As shown, the three-dimensional structure of the remote device adopts the material system of thin-film lithium niobate waveguide LNOI, in which the substrate adopts low-refractive-index silicon dioxide, and the thin-film lithium niobate is processed into a strip waveguide through conventional photolithography, etching, FIB and other processes, and the thickness and width of the waveguide are in the order of hundreds of nanometers to microns.

[0055] The coupling between the optical fiber and the waveguide in the present invention can adopt the structures such as the grating coupler and the tapered coupler that have been reported. The filter adopts the waveguide coupler, and the wavelength λ2 is coupled out and then discarded by designing the waveguide spacing. The above methods are all mature methods.

[0056] The waveguide extension line in the present invention may be formed by multiple foldings or by deformations of other shapes, such as a U-shaped waveguide.

[0057] The structures of the nonlinear waveguide 1 and the nonlinear waveguide 2 in the present invention are exactly the same. In order to achieve the momentum conservation of the difference frequency, according to the quasi-phase matching technology, the reciprocal lattice vector can be designed according to the relationship between the three wavelengths:

[0058]

[0059] Wherein, K is the reciprocal lattice vector, Λ is the period, λ1, λ2, λ3 are the three input laser wavelengths, and n1, n2, n3 are the refractive indices of the corresponding three wavelengths.

[0060] Specifically, the reciprocal lattice vector is realized by room temperature electric field polarization technology of lithium niobate. Since both difference frequency processes satisfy equation (1), the structural parameters of the two nonlinear waveguides are the same.

[0061] The λ2 laser is transmitted back to the local device via the optical fiber, and the sine wave is detected by the near-infrared detector, and then the frequency-doubled electrical signal is obtained by the signal processing technology known to TDLAS, and the gas concentration is obtained by the frequency-doubled signal intensity. Since the traditional TDLAS technical solutions are very mature, the present invention will not repeat them.

[0062] The present invention is further described below with examples.

[0063] The present invention adopts a material system of thin-film lithium niobate (LNOI), and the substrate material is silicon dioxide with a thickness of 0.5 mm. A 1 μm thick lithium niobate film is prepared by LNOI processes such as injection, bonding, and stripping. The lithium niobate waveguide on the substrate is prepared by a micromachining process, and the waveguide width is 1 μm. Specifically, the length of nonlinear waveguide 1 and nonlinear waveguide 2 is 10 mm, the area of ​​the waveguide extension line is 10 mm×10 mm, and the waveguide spacing is 1 μm. In this way, a straight waveguide with a length of 10 mm can be arranged every 2 μm, extending the original 10 mm waveguide by 5000 times to 50 m.

[0064] In this example, the detected gas is methane, whose infrared absorption peak is near 3342nm. DFB1 with a wavelength of λ1=1064nm and an output power of 10mW, DFB2 with a wavelength of λ2=1560.9nm and an output power of 10mW are used, and the difference frequency output target is a mid-infrared laser with a wavelength of λ3=3342nm. The above DFB1 and DFB2 are output through optical fibers, coupled into one optical fiber by a fiber combiner, and enter the remote device after optical fiber transmission.

[0065] The DFB2 is modulated by current, for example, a triangle wave of 80 mA ± 20 mA at a frequency of 100 Hz, superimposed with a sine wave of 5 mA at 1 kHz, to generate corresponding wavelength tuning. The wavelength tuning generates a difference frequency through nonlinear waveguide 1, and the wavelength tuning is transferred to λ3.

[0066] According to the above wavelength design and the effective refractive index of the waveguide, the polarization period Λ=17.6μm can be calculated according to formula (1). The lithium niobate of nonlinear waveguide 1 and nonlinear waveguide 2 is domain-inverted by room temperature electric field polarization technology, and the inversion period is 17.6μm.

[0067] Through optical packaging, the core of the remote device is packaged with the optical fiber, optical window, and mechanical structure. The waveguide extension line is exposed in the optical window and contacts the methane gas to be measured. The evanescent wave of the waveguide is absorbed, and the power of the mid-infrared laser transmitted in the waveguide changes. Since the wavelength of λ3 is in tuning, the mid-infrared laser power also changes sinusoidally. In addition, since the peak of the methane absorption peak has a nonlinear bend, high-order harmonics are generated, and the strongest is the second harmonic. This harmonic is transferred to the near-infrared wavelength of λ2 through the difference frequency of the nonlinear waveguide 2, and is transmitted back to the detector of the local device through the optical fiber. The above-mentioned detectors are indium gallium arsenide and germanium detectors.

[0068] Regarding the contents disclosed in this case, there are a few points that need to be explained:

[0069] (1) The drawings of the embodiments disclosed in this case only involve the structures involved in the embodiments disclosed in this case, and other structures can refer to the general design;

[0070] (2) In the absence of conflict, the embodiments disclosed in this case and the features therein may be combined with each other to obtain new embodiments.

[0071] The above are only specific implementation methods disclosed in this case, but the protection scope of the present disclosure is not limited thereto. The protection scope disclosed in this case should be based on the protection scope of the claims.

Claims

1. An integrated mid-infrared TDLAS detection device, characterized in that: Including local devices and remote devices, The local device includes a controller and a DFB1 laser, a DFB2 laser and a detector connected to the controller. The remote device comprises a substrate, on which are arranged a nonlinear waveguide 1, a waveguide extension line, a filter and a nonlinear waveguide 2 which are connected in sequence. The DFB1 laser and the DFB2 laser are respectively connected to optical fiber one, the optical fiber one is connected to nonlinear waveguide one, the remote device is connected to optical fiber two through nonlinear waveguide two, and the optical fiber two is connected to the detector.

2. The integrated mid-infrared TDLAS detection device according to claim 1, characterized in that: The detector is a near infrared detector.

3. The integrated mid-infrared TDLAS detection device according to claim 1, characterized in that: The substrate is made of silicon dioxide.

4. The integrated mid-infrared TDLAS detection device according to claim 1, characterized in that: The nonlinear waveguide 1, the waveguide extension line and the nonlinear waveguide 2 are thin-film lithium niobate waveguides respectively.

5. The integrated mid-infrared TDLAS detection device according to claim 1, characterized in that: The waveguide extension line is in a continuously bent shape.

6. An integrated mid-infrared TDLAS detection method, characterized in that: An integrated mid-infrared TDLAS detection device as described in any one of claims 1 to 5, comprising: The controller is used to provide current and voltage signals to the DFB1 laser and the DFB2 laser and to obtain electrical signals from the detector. The DFB1 laser generates a laser with a wavelength of λ1, and the DFB2 laser generates a laser with a wavelength of λ2. Optical fiber one transmits two wavelengths of laser light to a remote device; optical fiber two is connected to a detector, and the detector receives the optical signal returned by optical fiber two.

7. An integrated mid-infrared TDLAS detection method according to claim 6, characterized in that: The nonlinear waveguide 1 in the remote device performs a difference frequency on the two wavelength lasers of λ1 and λ2 through the difference frequency effect to generate a mid-infrared detection light with a wavelength of λ3; The waveguide extension line is exposed in the gas as an optical window, and is used to extend the wavelength λ3 laser optical path; The waveguide extension line has three wavelength lasers of λ1, λ2 and λ3. The laser of λ2 wavelength is filtered out by the filter, leaving the lasers of λ1 and λ3 to enter the nonlinear waveguide 2. In nonlinear waveguide 2, the lasers of λ1 and λ3 undergo secondary frequency difference to generate lasers with a wavelength of λ2, which are exported through optical fiber 2.

8. An integrated mid-infrared TDLAS detection method according to claim 6, characterized in that: The nonlinear waveguide 1 and the nonlinear waveguide 2 have the same structure.

9. The integrated mid-infrared TDLAS detection method according to claim 6, characterized in that: The two frequency difference processes both satisfy the following equation: Wherein, K is the reciprocal lattice vector, Λ is the period, λ1, λ2, λ3 are the three input laser wavelengths, and n1, n2, n3 are the refractive indices of the corresponding three wavelengths.

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