A low-noise asynchronous optical sampling system with a wide adjustable repetition rate difference

Abstract

This invention discloses an asynchronous optical sampling system with low noise and a wide adjustable repetition rate difference, belonging to the field of laser and terahertz technology. The system includes a laser source module, a terahertz emission module, a terahertz receiving module, and a mirror assembly. The laser source module forms a ring laser cavity through a pump unit, a wavelength division multiplexing unit, and a gain unit. Utilizing an isolator-free design within the cavity, a natural repetition rate difference exists between clockwise and counterclockwise pulsed lasers. A wide range of adjustment of the repetition rate difference and noise control are achieved through a frequency adjustment unit and an electronic control unit. This invention uses a single laser source module instead of the traditional dual laser, simplifying the system structure, reducing costs, and achieving continuous adjustment of the repetition rate difference within the range of 0.1kHz to 10kHz, with a repetition rate difference stability of ±0.1% and laser phase noise ≤100fs. It combines the advantages of fast acquisition speed, small size, and low noise, making it suitable for terahertz spectral detection and material analysis.

Description

Technical Field

[0001] This invention relates to the field of laser and terahertz technology, and more specifically, to an asynchronous optical sampling system with low noise and a wide range of adjustable repetition rate difference. Background Technology

[0002] Terahertz waves, with a frequency range of 0.1–10 THz and a wavelength of 30 μm–3 mm, lie between infrared and millimeter waves. Because the rotational frequencies of many light molecules, the vibrational frequencies of functional groups in macromolecules, and the resonant frequencies of biological macromolecules are located in this frequency band, terahertz spectroscopy can be used for biochemical molecular detection and material analysis and identification. Terahertz time-domain spectrometers determine the composition by focusing a terahertz beam onto the analyte and measuring its absorption spectrum.

[0003] Classical terahertz time-domain spectrometers use mechanical delay devices to scan terahertz pulses point by point, resulting in slow sampling speed and poor stability. Although asynchronous optical sampling systems use electronic scanning to improve speed, they require two femtosecond lasers with similar repetition frequencies, leading to increased system size and cost, as well as limited adjustment range for repetition frequency difference and high noise. Utility Model Content

[0004] The problem this invention addresses is how to provide an asynchronous optical sampling system with a wide range of adjustable repetition rate difference, low noise, faster acquisition speed compared to the classic THZ-TDS system, and lower size and cost compared to the traditional ASOPS-THz-TDS system.

[0005] To address the above problems, this utility model provides a fiber laser, comprising:

[0006] To address the aforementioned issues, this invention provides an asynchronous optical sampling system with low noise and a wide adjustable repetition rate difference, comprising: a laser source module, a terahertz emission module, a terahertz receiving module, and a mirror assembly; the laser source module outputs two pulsed lasers, one of which serves as a probe light directly to the terahertz receiving module, and the other pulsed laser serves as a pump light to the terahertz emission module, generating a terahertz wave, which is then projected onto the terahertz receiving module after passing through the mirror assembly.

[0007] The laser source module includes: a pump unit, a wavelength division multiplexing unit, a gain unit, a coupling output unit, a saturable absorption unit, and a transmission optical fiber; the pump unit, the wavelength division multiplexing unit, the gain unit, the coupling output unit, and the saturable absorption unit are connected through the transmission optical fiber to form a ring laser cavity;

[0008] The output of the pump unit is connected to the pump input of the wavelength division multiplexing unit, the output of the wavelength division multiplexing unit is connected to one end of the gain unit, the other end of the gain unit is connected to the input of the coupling output unit, the first output of the coupling output unit is connected to the saturable absorption unit, and the second output of the coupling output unit is used to output pulsed laser light in the clockwise and counterclockwise directions within the ring laser cavity.

[0009] Preferably, the laser source module further includes a frequency adjustment unit, and at least one section of the transmission optical fiber in the annular laser cavity has both ends fixed to the frequency adjustment unit.

[0010] Preferably, the second output terminal of the coupling output unit is used to connect to the input terminal of the electronic control unit, and the input terminal of the frequency adjustment unit is used to connect to the output terminal of the electronic control unit.

[0011] Preferably, the saturable absorption unit includes a semiconductor saturable absorption mirror.

[0012] Preferably, the gain unit comprises erbium-doped gain fiber.

[0013] Preferably, the reflector assembly includes a first reflector unit, a second reflector unit, a third reflector unit, and a fourth reflector unit; the terahertz wave from the terahertz transmitting module passes sequentially through the first reflector unit, the second reflector unit, the third reflector unit, and the fourth reflector unit before being projected onto the terahertz receiving module, wherein the object to be tested is placed between the second reflector unit and the third reflector unit.

[0014] Preferably, the first reflecting unit, the second reflecting unit, the third reflecting unit, and the fourth reflecting unit are all off-axis parabolic reflectors.

[0015] Preferably, the asynchronous optical sampling system further includes an electronic control unit, which is connected to the laser source module.

[0016] Preferably, both the terahertz transmitting module and the terahertz receiving module are photoconductive antennas.

[0017] Compared with the prior art, the present invention has at least the following beneficial effects:

[0018] This invention couples a pump unit into a ring laser cavity via a wavelength division multiplexing unit, which acts on the gain unit to excite laser light of a specific wavelength. Under the mode-locking mechanism of the saturable absorber unit, femtosecond pulsed laser light is formed within the laser cavity. Since there is no optical isolator within the cavity, two femtosecond pulsed laser lights, one clockwise and one counterclockwise, exist simultaneously within the ring cavity. These two femtosecond pulsed laser lights naturally have similar but different pulse repetition frequencies, and are output through a coupled output unit. Simultaneously, the cavity length can be adjusted over a wide range via a frequency adjustment unit, thereby achieving a wide range of adjustable repetition frequency difference between the two output laser lights. Furthermore, real-time monitoring and feedback control by the electronic control unit reduces system noise. This invention uses a single laser source module to meet the application requirements of an asynchronous optical sampling system. It avoids the disadvantages of using mechanical delay devices in classic terahertz time-domain spectrometers, such as slow acquisition speed, slightly poor stability and accuracy, and inconvenience in integration, thus achieving a faster acquisition speed. At the same time, it simplifies the complexity of the asynchronous optical sampling system, reduces the system size and weight, lowers the system cost, and also achieves a wide range of adjustable repetition rate difference and low-noise operation. Attached Figure Description

[0019] Figure 1 This is a schematic diagram of a partial structure of the laser source module of this utility model;

[0020] Figure 2 This is a schematic diagram of one embodiment of the asynchronous optical sampling system of this utility model;

[0021] Figure 3 The time-domain waveform and spectrum of the terahertz wave obtained using the asynchronous optical sampling system of this invention are shown.

[0022] Reference numerals: 01. Terahertz transmitting module; 02. Electronic control unit; 03. Laser source module; 04. Terahertz receiving module; 05. First reflection unit; 06. Second reflection unit; 07. Item under test; 08. Third reflection unit; 09. Fourth reflection unit; 31. Pumping unit; 32. Wavelength division multiplexing unit; 33. Gain unit; 34. Transmission fiber; 35. Coupled output unit; 36. Saturable absorption unit; 37. Frequency adjustment unit. Detailed Implementation

[0023] To make the technical means, creative features, objectives and effects of this utility model easier to understand, the present utility model will be further described below in conjunction with specific embodiments.

[0024] Specific structure and working principle of the laser source module

[0025] Referring to Figure 1, the core structure of the laser source module 03 in this embodiment includes a pump unit 31, a wavelength division multiplexing unit 32, a gain unit 33, a coupling output unit 35, a saturable absorption unit 36, a transmission fiber 34, and a frequency adjustment unit 37. Each unit is connected via the transmission fiber 34 to form a ring laser cavity, wherein:

[0026] Pump unit 31 uses a fiber-coupled semiconductor laser with an average power of 400mW and a center wavelength of 976nm. Its output end is fused to the pump input end of wavelength division multiplexing unit 32 to provide excitation energy for the system.

[0027] The wavelength division multiplexing unit 32 uses a 980 / 1550nm wavelength beam splitter to couple the 976nm pump light to the gain unit 33, while isolating the 1550nm wavelength light in the laser cavity.

[0028] Gain unit 33 is a 0.5m long erbium-doped gain fiber (EDF) that amplifies optical signals in the 1550nm band under the action of pump light.

[0029] The coupling output unit 35 is a 2×2 fiber optic coupler in the 1550nm band. Its first output end is connected to the saturable absorption unit 36, and its second output end serves as the laser output port.

[0030] The saturable absorption unit 36 ​​employs a projected semiconductor saturable absorber mirror (SESAM) to achieve laser mode-locking and generate femtosecond pulses.

[0031] Single-mode polarization-maintaining fiber is selected for transmission fiber 34 to construct a ring cavity optical path and ensure the stability of the laser mode.

[0032] The frequency adjustment unit 37 is a linear piezoelectric ceramic actuator (PZT). A 10cm long transmission optical fiber 34 is fixed at both ends to the surface of the PZT and is driven by the electronic control unit 02 to achieve fine adjustment of the cavity length.

[0033] When the 976nm light output from pump unit 31 is injected into gain unit 33 via wavelength division multiplexing unit 32, it excites erbium-doped fiber to generate spontaneous emission light in the 1550nm band. This light enters saturable absorber unit 36 ​​via coupling output unit 35, forming a mode-locked pulse under the nonlinear absorption of SESAM. Since no optical isolator is installed in the ring cavity, clockwise and counterclockwise pulsed lasers coexist, and a natural repetition rate difference is generated due to optical path asymmetry. The second output terminal of coupling output unit 35 outputs two pulsed lasers simultaneously, with a clockwise pulse repetition rate of 46.241425MHz and a counterclockwise pulse repetition rate of 46.241265MHz, and an initial repetition rate difference of approximately 160Hz.

[0034] Frequency repetition rate adjustment and noise control mechanism

[0035] Electronic control unit 02 achieves wide-range adjustment of repetition frequency difference and low-noise control through the following process:

[0036] Frequency sampling: The two lasers output from the second output terminal of the coupling output unit 35 are converted into electrical signals by a photodetector and input to the frequency counter of the electronic control unit 02.

[0037] Feedback control: The electronic control unit 02 calculates the frequency difference between the two signals, compares it with the set value (such as an adjustable range of 0.1kHz to 10kHz), and generates a PID control signal.

[0038] Cavity length adjustment: The control signal drives the PZT extension and retraction of the frequency adjustment unit 37, changing the length of the transmission optical fiber 34 fixed thereon, thereby adjusting the length of the annular cavity. For every 1μm change in cavity length, the repetition frequency changes by approximately 1.6kHz, achieving a wide range of adjustment of the repetition frequency difference.

[0039] Noise suppression: By monitoring frequency fluctuations in real time, the electronic control unit 02 dynamically adjusts the PZT at a sampling rate of 10kHz to control the frequency repetition rate difference stability within ±0.1%, thereby reducing the impact of laser phase noise on terahertz sampling.

[0040] Overall architecture of asynchronous optical sampling system

[0041] As shown in Figure 2, a complete asynchronous optical sampling system consists of the following parts:

[0042] Laser source module 03: Outputs two femtosecond pulse lasers with adjustable repetition rate difference. One (clockwise, 15.7mW) is amplified to 128mW and pulse width compressed to 150fs before being used as a probe light and directly connected to terahertz receiver module 04. The other (counterclockwise, 3.4mW) is amplified to 97mW and pulse width compressed to 120fs before being used as a pump light and connected to terahertz transmitter module 01.

[0043] Terahertz Transmitter Module 01: It adopts a TERA15-TX-FC photoconductive antenna, which is excited by pump light to generate terahertz waves with a pulse energy of about 1nJ and a center frequency of 0.3THz.

[0044] The mirror assembly consists of four off-axis parabolic mirrors (MPD229H-M01).

[0045] First reflecting unit 05 (focal length 100mm) collimates terahertz waves;

[0046] The second reflection unit 06 (focal length 150mm) focuses terahertz waves onto the object under test 07 (such as a 0.5mm thick polyethylene sheet).

[0047] The third reflection unit 08 (focal length 150mm) collects terahertz waves passing through the object under test;

[0048] The fourth reflection unit 09 (focal length 100mm) focuses terahertz waves to the terahertz receiving module 04.

[0049] Terahertz receiver module 04: TERA15-RX-FC photoconductive antenna, where the probe light and terahertz wave generate a difference frequency signal, which is then acquired by a lock-in amplifier to generate a terahertz time-domain waveform.

[0050] Sampling process and experimental results

[0051] During system operation, the repetition frequency difference between the two femtosecond lasers output by the laser source module 03 is set to 1 kHz, and the electronic control unit 02 maintains this difference stable via PZT. The pump light excites the terahertz emission module 01 to generate a terahertz wave, which, after being transmitted through the reflector group, carries the absorption information of the object under test 07 to the receiving module 04. The time delay between the probe light and the terahertz wave is determined by the repetition frequency difference between the two lasers, and 1000 samples can be completed per second, significantly higher than traditional mechanical delay systems (which require several minutes).

[0052] As shown in Figure 3, the measured terahertz wave time-domain waveform signal-to-noise ratio (SNR) is better than 60 dB, and the spectral resolution reaches 0.1 cm⁻¹. By adjusting the setting value of the electronic control unit O2, the repetition rate difference can be continuously adjusted within the range of 0.1 kHz to 10 kHz, corresponding to a time scan range of 0.1 ns to 10 ns, meeting the detection requirements of different samples. For example, when detecting a 0.1 mm thick biological tissue section, a 5 kHz repetition rate difference can complete the full spectrum acquisition within 200 ms, and due to the low system noise, it can resolve 0.5% of the terahertz absorption change.

[0053] Summary of key technological advantages

[0054] Single laser dual-output: Through the ring cavity isolator-free design, two femtosecond lasers with different repetition rates are naturally generated, replacing the dual lasers of the traditional ASOPS system, reducing costs by more than 50%.

[0055] Wide range of frequency repetition rate adjustment: By using PZT to adjust the cavity length, the frequency repetition rate can be continuously adjusted from 0.1kHz to 10kHz, covering the main application scenarios of terahertz time-domain spectroscopy.

[0056] Low noise control: The electronic control unit provides real-time feedback adjustment, achieving a repetition rate difference stability of ±0.1% and laser phase noise ≤100fs, ensuring the accuracy of terahertz signal detection.

[0057] All-fiber design: The entire path from laser generation to terahertz detection is transmitted through optical fiber, which has strong resistance to environmental interference and reduces the system size by 60% compared to traditional ASOPS.

[0058] The foregoing has shown and described the basic principles, main features, and advantages of this utility model. Those skilled in the art should understand that this utility model is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of this utility model. Various changes and modifications can be made to this utility model without departing from its spirit and scope, and all such changes and modifications fall within the scope of the claims. The scope of protection of this utility model is defined by the appended claims and their equivalents.

Claims

1. A low-noise asynchronous optical sampling system with large range of repetition rate difference and adjustable, characterized in that, include: The system comprises a laser source module, a terahertz emission module, a terahertz receiving module, and a reflector assembly. The laser source module outputs two pulsed laser beams. One pulsed laser beam is used as a probe beam and directly passes to the terahertz receiving module. The other pulsed laser beam is used as a pump beam and passes to the terahertz emission module, generating a terahertz wave. After passing through the reflector assembly, the wave is projected onto the terahertz receiving module.

2. The low-noise asynchronous optical sampling system with large range of repetition rate difference adjustment according to claim 1, wherein, The laser source module includes a pump unit, a wavelength division multiplexing unit, a gain unit, a coupling output unit, a saturable absorber unit, and a transmission optical fiber. The pump unit, the wavelength division multiplexing unit, the gain unit, the coupling output unit, and the saturable absorber unit are connected through the transmission optical fiber to form a ring laser cavity. The output end of the pump unit is connected to the pump input end of the wavelength division multiplexing unit, the output end of the wavelength division multiplexing unit is connected to one end of the gain unit, the other end of the gain unit is connected to the input end of the coupling output unit, the first output end of the coupling output unit is connected to the saturable absorber unit, and the second output end of the coupling output unit is used to output clockwise and counterclockwise pulsed laser light within the ring laser cavity.

3. The low-noise asynchronous optical sampling system with large range of repetition rate difference adjustment according to claim 2, characterized in that, The laser source module further includes a frequency adjustment unit, and at least one section of the transmission optical fiber in the annular laser cavity has both ends fixed to the frequency adjustment unit.

4. The low-noise asynchronous optical sampling system with a wide adjustable repetition rate difference according to claim 3, characterized in that, The second output terminal of the coupling output unit is used to connect to the input terminal of the electronic control unit, and the input terminal of the frequency adjustment unit is used to connect to the output terminal of the electronic control unit.

5. The low-noise asynchronous optical sampling system with a wide adjustable repetition rate difference according to claim 2, characterized in that, The saturable absorption unit includes a semiconductor saturable absorption mirror.

6. The low-noise asynchronous optical sampling system with a wide adjustable repetition rate difference according to claim 2, characterized in that, The gain unit includes erbium-doped gain fiber.

7. The low-noise asynchronous optical sampling system with a wide adjustable repetition rate difference according to claim 1, characterized in that, The reflector assembly includes a first reflector unit, a second reflector unit, a third reflector unit, and a fourth reflector unit. Terahertz waves from the terahertz transmitting module pass sequentially through the first reflector unit, the second reflector unit, the third reflector unit, and the fourth reflector unit before being projected onto the terahertz receiving module. The object to be tested is placed between the second reflector unit and the third reflector unit.

8. The low-noise asynchronous optical sampling system with a wide adjustable repetition rate difference according to claim 7, characterized in that, The first reflecting unit, the second reflecting unit, the third reflecting unit, and the fourth reflecting unit are all off-axis parabolic reflectors.

9. The low-noise asynchronous optical sampling system with a wide adjustable repetition rate difference according to claim 4, characterized in that, It also includes an electronic control unit, which is connected to the laser source module.

10. The low-noise asynchronous optical sampling system with a wide adjustable repetition rate difference according to claim 1, characterized in that, Both the terahertz transmitting module and the terahertz receiving module are photoconductive antennas.

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