Terahertz photoconductive antenna transmitter and detector based on frequency doubling

Abstract

The invention discloses a pair of terahertz photoconductive antenna transmitters and detectors based on frequency multiplication, relates to the technical field of terahertz, and is realized through the following technical scheme: the main body structure of the terahertz photoconductive antenna transmitter comprises an optical fiber collimation module, a focusing lens, an optical frequency multiplication module, a transmitter photoconductive antenna, a detector photoconductive antenna, a shell and a bottom plate. According to the terahertz photoconductive antenna transmitter and detector based on frequency multiplication, an optical frequency multiplication module is introduced before pump light and detection light enter a semiconductor switch, so that detection femtosecond pulse laser with the wavelength of 1560 nm (+ / -40 nm) can be subjected to frequency multiplication to form femtosecond pulse laser with the wavelength of 780 nm (+ / -20 nm), and the detection femtosecond pulse laser with the wavelength of 780 nm (+ / -20 nm) can be converted into femtosecond pulse laser with the wavelength of 780 nm (+ / -20 nm). Therefore, GaAs (or LT-GaAs) which is low in cost and mature in process can be selected to replace InGaAs which is high in cost to serve as a semiconductor switch material, and the method is a feasible low-cost optical fiber terahertz photoconductive antenna detector integration scheme.

Description

Technical Field

[0001] The invention relates to the field of terahertz technology, and in particular to a pair of terahertz photoconductive antenna transmitters and detectors based on frequency doubling. Background Art

[0002] Terahertz waves refer to electromagnetic waves with a frequency of 0.1 to 10 THz (1 THz = 1012 Hz), which are located between microwave and infrared bands. It has the advantages of strong penetration, low photon energy, and fingerprint spectrum, which makes it have extremely important application prospects in the fields of material identification, security inspection, non-destructive testing of materials and structures, in vivo examination of biological tissues, wireless communication, etc. For broadband terahertz time-domain pulses, photoconductive sampling is one of the most effective detection methods. In photoconductive sampling, specially processed semiconductor materials are required. Photoconductive switches based on such materials can be excited by femtosecond pulse lasers to generate photogenerated carriers. These carriers form currents under the action of the terahertz pulse electric field. The terahertz pulses can be measured by detecting the changes in current. In order to achieve good photoconductive switch detection efficiency, the photon energy of the femtosecond laser must be higher than the energy band gap of the semiconductor material to excite the photogenerated carriers. Therefore, the selection of matching laser wavelengths and semiconductor materials becomes the key to terahertz photoconductive detectors.

[0003] At present, there are two main types of relatively mature terahertz photoconductive antennas: one is a fiber-coupled terahertz photoconductive antenna excited by a 1560nm (±40nm) wavelength femtosecond laser. The 1560nm band femtosecond laser has stable performance and compact structure, and this band can be transmitted by all-fiber, and the optical path reliability and stability are very high. However, its disadvantage is that the semiconductor material corresponding to the 1560nm wavelength is usually InGaAs, which is difficult to grow and has a complicated preparation process. The other is a terahertz photoconductive antenna excited by a 780nm (±20nm) wavelength femtosecond laser. The semiconductor material corresponding to the 780nm wavelength is usually GaAs or low-temperature grown LT-GaAs. These two materials are relatively simple to prepare and have relatively low costs, but there is a defect that the optical fiber technology in this band is not mature enough, and optical fiber transmission cannot be used. Generally, it is a free-space system. Therefore, the stability and reliability of the optical path are poor, it is greatly affected by the environment, and the requirements for the use scenario are relatively strict. Therefore, the present invention seeks to avoid the technical barriers of GaInAS materials and designs a pair of terahertz photoconductive antenna transmitter and detector based on frequency doubling. Summary of the invention

[0004] The present invention provides a pair of terahertz photoconductive antenna transmitter and detector based on frequency doubling to solve the problems raised in the above background technology.

[0005] In order to solve the above technical problems, the technical solution adopted by the present invention is:

[0006] A pair of terahertz photoconductive antenna transmitter and detector based on frequency doubling is realized by the following technical scheme: its main structure includes a fiber collimator module, a focusing lens, an optical frequency doubling module, a transmitter photoconductive antenna, a detector photoconductive antenna, a shell and a bottom plate; the femtosecond laser is divided into two beams after passing through a half-wave plate and a polarization beam splitter prism, the reflected light is the pump light, and the transmitted light is the detection light; the fiber collimator is located at the front end of the focusing lens, which is used to parallelly guide the pulse light transmitted in the optical fiber, and the two are as close as possible, and the distance range is generally set to 0 to 10 mm; the pump light of the transmitter and the detection light of the detector pass through the fiber collimator, the focusing lens and the optical frequency doubling module in turn, and enter the photoconductive antenna of the transmitter and the detector at the rear end of the optical frequency doubling module;

[0007] The specific working process of the transmitter and detector is as follows:

[0008] S1. The femtosecond laser pulse with a wavelength of 1520nm-1600nm as the detection light transmitted through the optical fiber is collimated by the optical fiber collimator and then emits parallel light. Then, the laser is focused on the optical frequency doubling module through the focusing lens;

[0009] S2, by controlling the polarization period and operating temperature of the frequency doubling crystal in the frequency doubling module, the frequency doubling module can double the 1560nm input light to 780nm output light with high frequency doubling efficiency;

[0010] S3. The frequency-doubled 780nm wavelength pulse laser is incident on the GaAs photoconductive antenna to generate photogenerated carriers. The emitter generates strong terahertz pulse radiation with adjustable power under the action of bias voltage. The photogenerated carriers in the detector generate transient induced current driven by the incident terahertz electric field. The intensity of the induced current is proportional to the terahertz electric field intensity. At the same time, the current output circuit external to the photoconductive antenna outputs the generated transient current to the signal acquisition device, and then the complete terahertz pulse time domain waveform can be obtained through sampling and measurement methods.

[0011] Preferably, the frequency doubling module can be selected from but not limited to periodically poled lithium niobate crystal PPLN, magnesium oxide-doped periodically poled lithium niobate crystal MgO:PPLN or PPMgO:LN, periodically poled potassium titanyl phosphate PPKTP; at the same time, a suitable crystal length is selected to adapt to the femtosecond pulse laser used in the present invention to reduce the negative effect of the crystal on the femtosecond laser.

[0012] Preferably, considering the broadening effect of the frequency doubling module on high peak power femtosecond pulses, a frequency doubling module with small pulse broadening and high damage threshold is selected, ultimately ensuring a narrow pulse width and a wide spectrum range of the terahertz radiation.

[0013] Preferably, the frequency doubling module is integrated with a temperature control unit to ensure its stable conversion efficiency at the central wavelength.

[0014] Preferably, in order to reduce the Fresnel loss caused by the high refractive index of the frequency doubling crystal, anti-reflection films should be added to both the input and output end faces of the crystal.

[0015] Preferably, the semiconductor material used for the photoconductive antenna includes mature GaAs and LT-GaAs, and the central wavelength of the exciting femtosecond pulse light is 780nm±20nm.

[0016] Preferably, the size of the light spot irradiated onto the frequency doubling module is controlled by adjusting the focal length of the focusing lens and configuring the relative positions of the components inside the transmitter to achieve the best conversion efficiency.

[0017] Preferably, the femtosecond laser pulse will introduce negative chirp before being injected into the device to neutralize the pulse broadening effect of the optical pulse during transmission. At the same time, the injected femtosecond pulse light should be e-polarized.

[0018] Due to the adoption of the above technical solution, the present invention has the following technical advances compared with the prior art:

[0019] 1. The present invention provides a pair of terahertz photoconductive antenna transmitter and detector based on frequency doubling, and proposes that before the pump light and the detection light enter the semiconductor switch, an optical frequency doubling module is introduced, so that the detection femtosecond pulse laser with a wavelength of 1560nm (±40nm) can be frequency-doubled to a femtosecond pulse laser with a wavelength of 780nm (±20nm), so that GaAs (or LT-GaAs) with low cost and mature technology can be selected to replace the expensive InGaAs as the semiconductor switch material, which is a feasible low-cost fiber terahertz photoconductive antenna detector integration solution.

[0020] 2. The present invention provides a pair of terahertz photoconductive antenna transmitter and detector based on frequency doubling. The detection light adopts a femtosecond pulse laser with a wavelength of 1560nm (±40nm). All-fiber transmission can be realized in the instrument, overcoming the shortcomings of poor free space stability and poor environmental adaptability.

[0021] 3. The present invention provides a pair of terahertz photoconductive antenna transmitters and detectors based on frequency doubling. Based on the temperature control and optimized selection (length and polarization period selection) of the frequency doubling crystal, it can ensure stable and efficient frequency doubling conversion efficiency and ensure the minimum pulse broadening effect, thereby obtaining terahertz pulse radiation with stable power and pulse width output.

[0022] 4. The present invention provides a pair of terahertz photoconductive antenna transmitter and detector based on frequency doubling. The detection light adopts a femtosecond pulse laser with a wavelength of 1560nm (±40nm). The remote optical frequency doubling is achieved outside the cavity. Compared with the frequency doubling inside the cavity, it has better output power stability and lower transmission loss, which is conducive to the formation of high-power and reliable photoconductive antenna transmitter products. All-fiber transmission can be realized in the instrument, overcoming the shortcomings of poor stability in free space and poor environmental adaptability.

[0023] 5. The present invention provides a pair of terahertz photoconductive antenna transmitter and detector based on frequency doubling. Before the detection light enters the semiconductor switch, an optical frequency doubling module is introduced, which can double the detection femtosecond pulse laser with a wavelength of 1560nm (±40nm) to a femtosecond pulse laser with a wavelength of 780nm (±20nm). It can avoid choosing expensive InGaAs as the semiconductor material of the photoconductive switch, and choose GaAs (or LT-GaAs) with simple preparation process and low price. It is of great significance to reduce the cost of terahertz instruments and realize the localization of 1560nm band terahertz photoconductive antenna detectors.

[0024] 6. The present invention provides a pair of terahertz photoconductive antenna transmitter and detector based on frequency doubling, which have simple structure, ingenious design, low preparation cost, and are suitable for industrial production. At the same time, they are environmentally friendly and have extremely broad market prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 It is a structural schematic diagram of the present invention;

[0026] Figure 2 Schematic diagram of the transmitter photoconductive antenna and the detector photoconductive antenna of the present invention;

[0027] Figure 3 It is a normalized response function line diagram of the present invention.

[0028] In the figure: 1. Fiber collimator; 2. Focusing lens; 3. Optical frequency doubling module; 4. Transmitter photoconductive antenna; 5. Detector photoconductive antenna; 6. Housing; 7. Bottom plate. DETAILED DESCRIPTION

[0029] In order to make the technical means, creative features, objectives and effects achieved by the present invention easy to understand, the present invention is further explained below in conjunction with specific implementation methods.

[0030] like Figure 1-Figure 3As shown, a pair of terahertz photoconductive antenna transmitter and detector based on frequency doubling is realized by the following technical scheme: its main structure includes a fiber collimator module, a focusing lens 2, an optical frequency doubling module 3, a transmitter photoconductive antenna 4, a detector photoconductive antenna 5, a housing 6 and a bottom plate 7; the femtosecond laser is divided into two beams after passing through a half-wave plate and a polarization beam splitter prism, the reflected light is the pump light, and the transmitted light is the detection light; the fiber collimator 1 is located at the front end of the focusing lens 2, and is used to parallelly guide the pulse light transmitted in the optical fiber, and the two are as close as possible, and the distance range is generally set to 0-10mm; the pump light of the transmitter and the detection light of the detector pass through the fiber collimator 1, the focusing lens 2 and the optical frequency doubling module 3 in turn, and enter the photoconductive antenna of the transmitter and the detector at the rear end of the optical frequency doubling module 3;

[0031] The main structure of the fiber coupling module includes a fiber collimator 1 and a focusing lens 2; the final photoconductive antenna transmitter and detector adopt an integral packaging shell to facilitate product integration;

[0032] In the transmitter scenario, a transmitter photoconductive antenna 4 is also provided on the rear side of the optical frequency doubling module 3 , and the detection light is finally transmitted to the transmitter photoconductive antenna 4 via the optical frequency doubling module 3 .

[0033] In the detector scene, a detector photoconductive antenna 5 is also provided at the rear side of the optical frequency doubling module 3 , and the detection light is finally transmitted to the detector photoconductive antenna 5 via the optical frequency doubling module 3 .

[0034] The specific working process of the transmitter and detector is as follows:

[0035] S1, the femtosecond laser pulse with a wavelength of 1520nm-1600nm as the detection light transmitted through the optical fiber is collimated by the optical fiber collimator 1 and then emits parallel light, and then the laser is focused on the optical frequency doubling module 3 through the focusing lens 2;

[0036] S2, by controlling the polarization period and operating temperature of the frequency doubling crystal in the frequency doubling module, the frequency doubling module can double the 1560nm input light to 780nm output light with high frequency doubling efficiency;

[0037] S3. The frequency-doubled 780nm wavelength pulse laser is incident on the GaAs photoconductive antenna to generate photogenerated carriers. The emitter generates strong terahertz pulse radiation with adjustable power under the action of bias voltage. The photogenerated carriers in the detector generate transient induced current driven by the incident terahertz electric field. The intensity of the induced current is proportional to the terahertz electric field intensity. At the same time, the current output circuit external to the photoconductive antenna outputs the generated transient current to the signal acquisition device, and then the complete terahertz pulse time domain waveform can be obtained through sampling and measurement methods.

[0038] Before being injected into the device, the femtosecond laser pulse will introduce a certain negative chirp to neutralize the pulse broadening effect of the optical pulse during optical fiber transmission.

[0039] The frequency doubling module can be selected from but not limited to periodically poled lithium niobate crystal PPLN, magnesium oxide-doped periodically poled lithium niobate crystal MgO:PPLN or PPMgO:LN, and periodically poled potassium titanyl phosphate PPKTP; at the same time, a suitable crystal length is selected to adapt to the femtosecond pulse laser used in the present invention to reduce the negative effect of the crystal on the femtosecond laser.

[0040] Considering the broadening effect of the optical frequency doubling module 3 on high peak power femtosecond pulses, a double frequency doubling module with small pulse broadening and high damage threshold is selected, ultimately ensuring a narrow pulse width and a wide spectrum range of the terahertz radiation.

[0041] The optical frequency doubling module 3 is integrated with a temperature control unit to ensure its stable conversion efficiency at the central wavelength.

[0042] In order to reduce the Fresnel loss caused by the high refractive index of the frequency doubling crystal, anti-reflection films should be added to both the input and output end faces of the crystal.

[0043] The semiconductor materials used in the detector photoconductive antenna 5 include mature GaAs and LT-GaAs, and the central wavelength of the exciting femtosecond pulse light is 780nm±20nm.

[0044] By adjusting the focal length of the focusing lens 2 and configuring the relative positions of the components inside the transmitter, the size of the light spot irradiated on the frequency doubling module is controlled to achieve the best conversion efficiency.

[0045] Before the femtosecond laser pulse is injected into the device, negative chirp will be introduced to neutralize the pulse broadening effect of the optical pulse during transmission. At the same time, the injected femtosecond pulse light should be e-polarized.

[0046] The maximum pump power of the transmitting and detecting antennas used is 80mW and 50mW respectively, while the output power of the femtosecond laser is 350mW to 500mW, so we must carefully control the power of the pump light and the detection light to avoid damaging the antenna. At the same time, before the femtosecond enters the system, it must pass through an attenuation plate to ensure that the attenuated power is less than 120mW.

[0047] The material of the transmitter photoconductive antenna is GaAs, the electrode shape is "II", and the antenna electrode width is L e =20μm, the antenna spacing is L=50μm, and the spot size focused near the positive pole of the antenna is 12.5μm.

[0048] The photoconductive antenna material at the detector end is LT-GaAs, and the electrode is evaporated into an "H" type. In the figure, L = 2L m +Ls And L S =5μm,L m =22.5μm,L e =20μm,L s , L m and L e The meaning is that the femtosecond pulse is irradiated between the two electrodes, the two poles of the antenna are connected to an ammeter and then to an amplifier to detect the current generated between the two poles.

[0049] The focused spot size of the terahertz pulse is a function of the frequency. The smaller the frequency, the larger the spot size. m and L e , so that they are 45μm and 20μm respectively, and the L s , it can be seen that reducing L s The center frequency of the geometric response function can be moved to the high frequency part, and the amplitude of all frequencies increases; the main reason for the increase in amplitude is L s Reducing the antenna resistance will increase the current in the antenna; so in order to obtain a larger signal and a larger bandwidth, try to choose a smaller antenna spacing.

[0050] The emitted light generates strong terahertz radiation under bias through the optical frequency doubling module, and the detection light is transmitted to the photoconductive antenna through the optical frequency doubling module, generating photogenerated carriers to form an induced current, the intensity of which is proportional to the terahertz electric field intensity. At the same time, the current output circuit connected to the photoconductive antenna outputs the generated transient current to the signal acquisition device, and then the complete terahertz pulse time domain waveform can be obtained through sampling and measurement methods. Seeking to avoid the technical barriers of GaInAS materials, the design provides a pair of terahertz photoconductive antenna transmitter and detector designs based on frequency doubling; its main structure is simple, modular design, and its application prospects are extremely broad.

[0051] The above shows and describes the basic principles and main features of the present invention and the advantages of the present invention. It should be understood by those skilled in the art that the present invention is not limited to the above embodiments. The above embodiments and descriptions are only for explaining the principles of the present invention. Without departing from the spirit and scope of the present invention, the present invention may have various changes and improvements, which fall within the scope of the present invention to be protected. The scope of protection of the present invention is defined by the attached claims and their equivalents.

Claims

1. A pair of terahertz photoconductive antenna transmitter and detector based on frequency doubling, characterized in that: The invention is realized by the following technical scheme: its main structure comprises an optical fiber collimation module, a focusing lens (2), an optical frequency doubling module (3), a transmitter photoconductive antenna (4), a detector photoconductive antenna (5), a housing (6) and a bottom plate (7); after passing through a half-wave plate and a polarization beam splitter, the femtosecond laser is divided into two beams, the reflected light is the pump light, and the transmitted light is the detection light; the optical fiber collimator (1) is located at the front end of the focusing lens (2) to parallelly guide the pulse light transmitted in the optical fiber, and the two are as close as possible, and the distance range is generally set to 0 to 10 mm; the pump light of the transmitter and the detection light of the detector pass through the optical fiber collimator (1), the focusing lens (2) and the optical frequency doubling module (3) in sequence, and enter the photoconductive antenna of the transmitter and the detector at the rear end of the optical frequency doubling module (3); The specific working process of the transmitter and detector is as follows: S1, a femtosecond laser pulse with a wavelength of 1520nm-1600nm as a detection light transmitted through an optical fiber is collimated by an optical fiber collimator (1) and then emits parallel light, and then the laser is focused on an optical frequency doubling module (3) through a focusing lens (2); S2, by controlling the polarization period and operating temperature of the frequency doubling crystal in the frequency doubling module, the frequency doubling module can double the 1560nm input light to 780nm output light with high frequency doubling efficiency; S3, the frequency-doubled 780nm wavelength pulse laser is incident on the GaAs photoconductive antenna to generate photogenerated carriers; The transmitter generates strong terahertz pulse radiation with adjustable power under the action of bias voltage; the photogenerated carriers in the detector generate transient induced current driven by the incident terahertz electric field, and the intensity of the induced current is proportional to the intensity of the terahertz electric field. At the same time, the current output circuit external to the photoconductive antenna outputs the generated transient current to the signal acquisition device, and then the complete terahertz pulse time domain waveform can be obtained through sampling and measurement methods.

2. The pair of terahertz photoconductive antenna transmitter and detector based on frequency doubling according to claim 1, characterized in that: The frequency doubling module can be selected from but not limited to periodically poled lithium niobate crystal PPLN, magnesium oxide-doped periodically poled lithium niobate crystal MgO:PPLN or PPMgO:LN, and periodically poled potassium titanyl phosphate PPKTP; at the same time, a suitable crystal length is selected to adapt to the femtosecond pulse laser used in the present invention, thereby reducing the negative effect of the crystal on the femtosecond laser.

3. The pair of terahertz photoconductive antenna transmitter and detector based on frequency doubling according to claim 2, characterized in that: Considering the broadening effect of the frequency doubling module on high peak power femtosecond pulses, a double frequency doubling module with small pulse broadening and high damage threshold is selected, ultimately ensuring a narrow pulse width and a wide spectrum range of terahertz radiation.

4. The pair of terahertz photoconductive antenna transmitter and detector based on frequency doubling according to claim 3, characterized in that: The frequency doubling module is integrated with a temperature control unit to ensure its stable conversion efficiency at the central wavelength.

5. The pair of terahertz photoconductive antenna transmitter and detector based on frequency doubling according to claim 4, characterized in that: In order to reduce the Fresnel loss caused by the high refractive index of the frequency doubling crystal, anti-reflection films should be added to both the input and output end faces of the crystal.

6. The pair of terahertz photoconductive antenna transmitter and detector based on frequency doubling according to claim 5, characterized in that: The semiconductor material used for the photoconductive antenna includes mature GaAs and LT-GaAs, and the central wavelength of the exciting femtosecond pulse light is 780nm±20nm.

7. The pair of terahertz photoconductive antenna transmitter and detector based on frequency doubling according to claim 6, characterized in that: By adjusting the focal length of the focusing lens (2) and configuring the relative positions of the components inside the transmitter, the size of the light spot irradiated onto the frequency doubling module is controlled to achieve the best conversion efficiency.

8. The pair of terahertz photoconductive antenna transmitter and detector based on frequency doubling according to claim 7, characterized in that: The femtosecond laser pulse will introduce negative chirp before being injected into the device to neutralize the pulse broadening effect of the optical pulse during transmission. At the same time, the injected femtosecond pulse light should be e-polarized.

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