A self-injection terahertz optical frequency comb system

Abstract

The present application relates to a kind of self-injection terahertz light frequency comb system, wherein, terahertz quantum cascade laser is as light frequency comb source, beat frequency signal generated by self beat frequency is transmitted into first T-type biasing device;First T-type biasing device is used to transmit beat frequency signal to wave divider;Wave divider is used to divide beat frequency signal into two ways, one way is transmitted to circulator and participates in self injection, another way is connected to spectrum analyzer and carries out real-time detection;Circulator is used to keep the one-way transmission of beat frequency signal participating in self injection in loop;Phase shifter is used to phase match beat frequency signal participating in self injection;Second T-type biasing device is used to inject phase-matched beat frequency signal into terahertz quantum cascade laser;Spectrum analyzer is used to detect the self-injection beat frequency signal obtained in real time.The present application can improve the stability of system, meet the requirement as high-frequency optoelectronic oscillator.

Description

A self-injected terahertz optical frequency comb system Technical Field

[0001] This invention relates to the field of semiconductor optoelectronic device application technology, and in particular to a self-injected terahertz optical frequency comb system. Background Technology

[0002] With the rapid development of global fifth-generation (5G) wireless communication, radar surveillance systems, and other technologies, microwave frequencies are constantly increasing, leading to a growing demand for high-frequency, high-spectral-purity, and frequency-transmittable radio frequency (RF) signals. This, in turn, creates a need for lightweight, small-sized, and low-cost RF signal generators. However, traditional microwave technology cannot adapt to the transmission and processing of high-frequency, wide-bandwidth signals, giving rise to microwave photonics technology. Photonic devices can generate high-frequency electromagnetic signals in the microwave, millimeter-wave, and terahertz bands, and possess advantages such as high frequency, good phase noise performance, compact structure, and easy integration. Among these, the opto-oscillator (OEO) is considered a key technology in microwave photonics due to its advantage of generating stable high-frequency signals with ultra-low phase noise.

[0003] Terahertz quantum cascade lasers (THzQCLs), as semiconductor electrically pumped radiation sources, possess advantages such as high power and ease of integration, making them an ideal platform for realizing terahertz optical frequency combs. An optical frequency comb refers to a series of highly stable and equally spaced laser sources distributed in the frequency domain, exhibiting a strictly periodic envelope function curve in the time domain. The interaction between adjacent modes of the optical frequency comb generates a beat frequency signal located in the microwave band. This signal is used to characterize the stability and inter-mode coherence of the QCL optical frequency comb, and can also serve as a stabilizing microwave signal. THzQCLs can be used as important devices for realizing optoelectronic oscillators.

[0004] Optical frequency combs are often affected by external factors such as temperature, humidity, current, and mechanical vibration, preventing them from fully utilizing their advantages of wide bandwidth, low phase noise, and high frequency stability. In the radio frequency band, the instability of optical frequency combs manifests as jitter in the beat frequency signal, limiting the generation of ultra-stable, low phase noise microwave signals. Radio frequency injection is often used to stabilize the beat frequency signal, but because radio frequency injection requires an additional microwave signal source device, it not only increases costs, but the spectral purity, stability, and phase noise of the microwave signal source directly affect the generation of high-quality microwave signals, making it difficult to completely stabilize the beat frequency signal. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide a self-injected terahertz optical frequency comb system that can improve the stability of the system and meet the requirements as a high-frequency optoelectronic oscillator.

[0006] The technical solution adopted by this invention to solve its technical problem is as follows: A self-injected terahertz optical frequency comb system is provided, comprising a terahertz quantum cascade laser, a first T-type bias, a wavelength divider, a circulator, a phase shifter, and a second T-type bias. The terahertz quantum cascade laser serves as the optical frequency comb source, generating a beat frequency signal through self-beating and transmitting it to the first T-type bias. The first T-type bias is used to transmit the beat frequency signal to the wavelength divider. The wavelength divider is used to split the beat frequency signal into two paths: one path is transmitted to the circulator for self-injection, and the other path is connected to a spectrum analyzer for real-time detection. The circulator is used to maintain unidirectional transmission of the beat frequency signal participating in self-injection within the loop. The phase shifter is used to perform phase matching on the beat frequency signal participating in self-injection. The second T-type bias is used to inject the phase-matched beat frequency signal into the terahertz quantum cascade laser. The spectrum analyzer is used to detect the obtained self-injected beat frequency signal in real time.

[0007] The lasing port and receiving port of the terahertz quantum cascade laser are respectively connected to the microstrip line. The electrodes of the terahertz quantum cascade laser are connected to the ceramic sheet. The ceramic sheet and the microstrip line are encapsulated on a heat sink and fixed to an H-shaped sample holder by the heat sink.

[0008] The grooves at both ends of the H-shaped sample holder are used to fix the microstrip line, thereby connecting the microstrip line to the high-frequency coaxial cable.

[0009] The mixing port of the first T-type bias is connected to the microstrip line of the terahertz quantum cascade laser, and the AC port is connected to the wave demultiplexer.

[0010] A bandpass filter and a low-noise amplifier are provided between the first T-type bias and the splitter. The bandpass filter is used to filter out signals other than the beat frequency signal, and the low-noise amplifier is used to amplify the beat frequency signal.

[0011] A low-noise amplifier is provided between the phase shifter and the second T-type bias, and the low-noise amplifier is used to amplify the beat frequency signal after phase matching.

[0012] The mixing port of the second T-type bias is connected to the low-noise amplifier, the DC port is connected to the current source, and the AC port is connected to the microstrip line of the injection port of the terahertz quantum cascade laser.

[0013] Beneficial effects

[0014] By employing the above-mentioned technical solution, this invention has the following advantages and positive effects compared with the prior art: This invention, by filtering and amplifying the beat frequency signal, directly injects the beat frequency signal from the optical frequency comb back into the laser, ultimately obtaining a stable beat frequency signal. This is manifested in a significant reduction in the linewidth and phase noise of the beat frequency signal, greatly improving the stability of the output RF signal. This invention eliminates the need for external RF injection of the laser's beat frequency signal, simplifying the system and improving system stability through self-injection. It enables the application of the THzQCL self-injection system as an optoelectronic oscillator, obtaining a high-frequency signal with ultra-low phase noise. Attached Figure Description

[0015] Figure 1 is a schematic diagram of an embodiment of the present invention;

[0016] Figure 2 is a structural schematic diagram of an embodiment of the present invention;

[0017] Figure 3 is a structural diagram of the sample holder according to an embodiment of the present invention. Detailed Implementation

[0018] The present invention will be further illustrated below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. Furthermore, it should be understood that after reading the teachings of this invention, those skilled in the art can make various alterations or modifications to the invention, and these equivalent forms also fall within the scope defined by the appended claims.

[0019] The embodiments of the present invention relate to a self-injected terahertz optical frequency comb system. The system uses a THz-band QCL as a radiation source. The emitted electrical beat frequency signal is self-injected into the optical frequency comb through a feedback loop. The phase of the beat frequency signal is adjusted by a phase shifter to obtain a high-stability radio frequency signal with low phase noise, thereby realizing the application of the THz QCL self-injection system as an optoelectronic oscillator.

[0020] As shown in Figure 1, the principle of this implementation method is as follows:

[0021] The optical frequency comb is offset by the carrier envelope frequency f. ceo and repetition frequency f rep Two parameters are determined, where the Nth comb tooth can be represented as:

[0022] f = f ceo +Nf rep

[0023] Where the repetition frequency f rep In the microwave frequency band, it manifests as a beat frequency signal f. B Numerically f B equal to f rep This is determined by the coherence between the optical frequency comb modes.

[0024] From the perspective of phase matching, consider the nth and (n+1)th modes of the optical frequency comb. The wave vector of the nth mode is k. n The wave vector of the (n+1)th mode is k. n+1 The wave vector of the beat frequency signal is k B Phase matching satisfies the following relationship:

[0025] k n+1 =k n +k B

[0026] In an optical frequency comb, satisfying the phase matching condition is sufficient to satisfy the phase matching condition for four-wave mixing, which means satisfying the following relationship:

[0027] k n+1 +k n-1 =2k n

[0028] Where k n+1 k n and k n-1 These represent the wave vectors of the three consecutive optical frequency comb modes. Therefore, phase adjustment of the beat frequency signal can enhance the four-wave mixing lock between the optical frequency comb modes, thereby obtaining a stable beat frequency signal in the microwave frequency domain.

[0029] As shown in Figure 2, the self-injected terahertz optical frequency comb system in this embodiment includes a terahertz quantum cascade laser, a T-type bias 1, a wave demultiplexer, a circulator, a phase shifter, and a T-type bias 2.

[0030] The lasing port and receiving port of the terahertz quantum cascade laser are connected to microstrip lines, and the upper electrode of the terahertz quantum cascade laser is connected to a ceramic sheet, all encapsulated on a heat sink and fixed to an H-shaped sample holder via the heat sink. As shown in Figure 3, the H-shaped sample holder ensures heat dissipation for the laser encapsulated on the heat sink and is fixed in a cryogenic cooling system to ensure normal operation of the laser. The grooves at both ends of the H-shaped sample holder are used to fix the connection between the microstrip lines at both ends of the laser and the high-frequency coaxial cable for transmitting and receiving beat frequency signals. A current source is connected to the laser via the ceramic sheet, providing power to the laser-emitted optical frequency comb spectrum and controlling the power supply current.

[0031] This embodiment uses a free-running terahertz quantum cascade laser to normally emit an optical frequency comb spectrum, obtaining a beat frequency signal in the radio frequency band. The beat frequency signal is then phase-adjusted by a phase shifter and injected back into the laser's feedback loop port. It interacts with the original beat frequency signal within the laser's resonant cavity, ultimately yielding a stable beat frequency signal. In this embodiment, no additional external radio frequency injection or other operations are required for the optical frequency comb to obtain a highly stable beat frequency signal. The stable beat frequency signal generated in this embodiment is obtained by the self-probing mechanism of the optical frequency comb.

[0032] In the transmission path, the beat frequency signal from the laser optical frequency comb in the radio frequency band is transmitted to T-type bias 1 via a microstrip line. After passing through the AC port of T-type bias 1, it is transmitted to a bandpass filter. The function of T-type bias 1 is to avoid interference from DC signals, and the bandpass filter is used to filter out noise around the beat frequency signal in the frequency domain. The beat frequency signal is then transmitted to low-noise amplifier 1 via the bandpass filter to amplify the beat frequency signal obtained by the self-detection of the optical frequency comb. Low-noise amplifier 1 is connected to a demultiplexer, which splits the beat frequency signal into two paths. One path connects to a spectrum analyzer for real-time detection of the beat frequency signal, and the other path connects to a feedback loop for self-injection.

[0033] In the feedback path, the beat frequency signal used for self-injection, after being split by the demultiplexer, is first transmitted to the looper to ensure unidirectional signal transmission and avoid interference caused by reverse signal transmission. The looper is connected to the phase shifter, which adjusts the phase of the beat frequency signal in the feedback path. The phase-adjusted beat frequency signal is input to the second low-noise amplifier to amplify the beat frequency signal after phase compensation by the phase shifter and attenuation by the demultiplexer, in order to meet the power requirements of self-injection. The T-type bias 2 receives the phase-adjusted and amplified beat frequency signal, and its DC terminal is connected to a current source to increase the power of the optical frequency comb beat frequency signal. The beat frequency signal is then connected to the receiving port of the laser via the AC port through a microstrip line.

[0034] The beat frequency signal in the feedback path is self-injected into the laser after phase adjustment by a phase shifter. Within the laser resonant cavity, it interacts with the original beat frequency signal, achieving phase matching between the laser's optical frequency comb modes. This enhances the coherence between modes, resulting in a more stable beat frequency signal detected by the spectrum analyzer, characterized by less frequency jitter, lower phase noise, and a narrower linewidth. The entire self-injection system acts as an optoelectronic oscillator, generating a highly stable radio frequency signal with low phase noise.

[0035] Therefore, this invention eliminates the need for external radio frequency injection into the laser's beat frequency signal. A stable beat frequency signal can be obtained by directly injecting the beat frequency signal obtained from the THzQCL self-detection back into the laser. This self-injection method simplifies the system while improving its stability, and is of great significance for realizing the application of the THzQCL self-injection system as an optoelectronic oscillator.

[0036] The method for measuring the stable beat frequency signal obtained from the self-injected terahertz optical frequency comb system using the above-mentioned device includes the following steps:

[0037] Step S1: Provide a terahertz quantum cascade laser and an H-shaped sample holder. Microstrip lines for impedance matching are provided 2mm from the front and rear ends of the laser resonant cavity. The microstrip lines are connected to the front and rear ends of the laser via gold wires and to a high-frequency coaxial cable via grooves in the novel H-shaped sample holder. A ceramic plate is connected to the laser via gold wires and powered by a current source connected to a probe. The laser is placed in a working temperature environment (liquid helium cooling environment). The laser's lasing frequency shifts linearly with the driving current. The center frequency of the optical frequency comb is 4.2 THz, and the repetition frequency of the optical frequency comb is 6.15 GHz, i.e., the beat frequency signal frequency is 6.15 GHz.

[0038] Step S2: Provide two T-type biasers. The operating frequency band of the T-type biasers is 100kHz to 40GHz. The mixing port of T-type biaser 1 is connected to the microstrip line of the lasing port through a high-frequency coaxial cable, and the AC port is connected to the bandpass filter through a high-frequency coaxial cable. The mixing port and AC port of T-type biaser 2 are connected to the low-noise amplifier 2 and the injection port microstrip line through high-frequency coaxial cables, respectively.

[0039] Step S3: Provide a bandpass filter and two microwave low-noise amplifiers. The passband frequency of the bandpass filter is 5.6GHz to 7GHz, and the gain of the two microwave low-noise amplifiers is 30dB. Both operate in the range of 1 to 18GHz. The low-noise amplifiers are connected to other devices via high-frequency coaxial cables.

[0040] Step S4: Provide a demultiplexer, a looper, and a phase shifter. The demultiplexer operates in the frequency band of 1GHz to 40GHz. Input port 1 of the demultiplexer is connected to a microwave low-noise amplifier, output port 3 (with 10dB power attenuation) is connected to a spectrum analyzer, and output port 4 is connected to the looper. The looper's input port is connected to the demultiplexer, and its output port is connected to the phase shifter. The phase shifter operates in the frequency band of 4GHz to 8GHz. All connections are made using high-frequency coaxial cables.

[0041] Step S5: Provide a temperature controller, a spectrum analyzer, two current sources, and a computer. The computer is equipped with a LabVIEW program to control the DC sources; the current sources are connected to the DC bias port of T-type biaser 2 via BNC cables to increase the power of the beat frequency signal; the computer is connected to the laser via a USB cable. Current source 1 powers the laser, and current source 2 powers the microwave amplifier.

[0042] Step S6: The current source powers the laser. During the test, the laser's drive current is controlled by the LabVIEW program on the computer, the temperature controller keeps the laser's operating temperature constant, and the optical frequency comb remains stable.

[0043] Step S7: Due to the coherence between the optical frequency comb modes of the quantum cascade laser, a beat frequency signal can be observed near 6.2 GHz on the spectrum analyzer. The drive current of the laser can be changed using the LabVIEW program on the computer. During self-injection, a stronger beat frequency signal should be selected as much as possible. The phase of the phase shifter is adjusted, and the change of the beat frequency signal is observed in real time on the spectrum analyzer. After dynamic adjustment, a beat frequency signal with a significantly narrower linewidth is observed on the spectrum analyzer, indicating that the phase shifter has completed phase adjustment and the laser has achieved self-injection.

[0044] It is easy to see that this invention utilizes mature microwave technologies such as detection, filtering, and amplification to divide the beat frequency signal of the laser optical frequency comb. After adjusting the phase of the beat frequency signal using a phase shifter, it is self-injected into the laser. The interaction within the resonant cavity enhances the coherence between the optical frequency comb modes, ultimately resulting in a stable beat frequency signal. This is manifested in a significant reduction in the beat frequency signal linewidth and phase noise within the radio frequency band, greatly improving the system stability. This invention eliminates the need for external radio frequency injection into the optical frequency comb, achieving improved system stability through self-injection. It holds promise as a device for generating ultra-low noise, stable radio frequency signals as an optoelectronic oscillator, and has significant application prospects in the field of microwave photonics.

Claims

1. A self-injected terahertz optical frequency comb system, characterized in that, The system includes a terahertz quantum cascade laser, a first T-type bias, a wavelength division multiplexer (WDM), a circulator, a phase shifter, and a second T-type bias. The terahertz quantum cascade laser acts as an optical frequency comb source, generating a beat frequency signal through self-beating and transmitting it to the first T-type bias. The beat frequency signal is obtained using the optical frequency comb source output by the terahertz quantum cascade laser in the radio frequency band. The first T-type bias is used to transmit the beat frequency signal to the WDM. The WDM splits the beat frequency signal into two paths: one path is transmitted to the circulator for self-injection, and the other path is connected to a spectrum analyzer for real-time detection. The circulator maintains unidirectional transmission of the beat frequency signal participating in self-injection within the loop. The phase shifter performs phase matching on the beat frequency signal participating in self-injection. The second T-type bias injects the phase-matched beat frequency signal into the terahertz quantum cascade laser. The spectrum analyzer is used to detect the obtained self-injected beat frequency signal in real time.

2. The self-injected terahertz optical frequency comb system according to claim 1, characterized in that, The lasing port and receiving port of the terahertz quantum cascade laser are respectively connected to the microstrip line. The electrodes of the terahertz quantum cascade laser are connected to the ceramic sheet. The ceramic sheet and the microstrip line are encapsulated on a heat sink and fixed to an H-shaped sample holder by the heat sink.

3. The self-injected terahertz optical frequency comb system according to claim 2, characterized in that, The grooves at both ends of the H-shaped sample holder are used to fix the microstrip line, thereby connecting the microstrip line to the high-frequency coaxial cable.

4. The self-injected terahertz optical frequency comb system according to claim 1, characterized in that, The mixing port of the first T-type bias is connected to the microstrip line of the terahertz quantum cascade laser, and the AC port is connected to the wave demultiplexer.

5. The self-injected terahertz optical frequency comb system according to claim 1, characterized in that, A bandpass filter and a low-noise amplifier are provided between the first T-type bias and the splitter. The bandpass filter is used to filter out signals other than the beat frequency signal, and the low-noise amplifier is used to amplify the beat frequency signal.

6. The self-injected terahertz optical frequency comb system according to claim 1, characterized in that, A low-noise amplifier is provided between the phase shifter and the second T-type bias, and the low-noise amplifier is used to amplify the beat frequency signal after phase matching.

7. The self-injected terahertz optical frequency comb system according to claim 6, characterized in that, The mixing port of the second T-type bias is connected to the low-noise amplifier, the DC port is connected to the current source, and the AC port is connected to the microstrip line of the injection port of the terahertz quantum cascade laser.

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