A quantum cascade laser modulation bandwidth test system and method

Abstract

The present invention relates to a quantum cascade laser modulation bandwidth test system and method, wherein the test system comprises a detection quantum cascade laser, a first T-type bias device, a second T-type bias device, a radio frequency source and a spectrum analyzer; the sub-cascade laser to be measured is optically coupled with the detection quantum cascade laser, so that the eigenfrequency of the sub-cascade laser to be measured is mixed with the optical frequency comb frequency line of the detection quantum cascade laser to generate a beat frequency signal; the radio frequency source is used to generate a modulation signal, and the modulation frequency is adjusted with a preset frequency as the interval, so that the sub-cascade laser to be measured generates a modulation sideband; the spectrum analyzer is used to record the beat frequency signal of the modulation sideband and the optical frequency comb frequency line. The present invention can accurately characterize the modulation bandwidth of the quantum cascade laser.

Description

Technical Field

[0001] The invention relates to the technical field of semiconductor optoelectronic device applications, and in particular to a quantum cascade laser modulation bandwidth testing system and method. Background Art

[0002] Laser modulation bandwidth is one of the keys to determine the rate of direct modulation communication systems. The relaxation oscillation frequency of the laser is usually taken as its 3dB modulation bandwidth. When the modulation frequency is greater than the relaxation oscillation frequency, the modulation response of the laser will drop sharply. The fundamental reason for the relaxation oscillation phenomenon is stimulated radiation and gain saturation: the interaction between the inversion particle number and the photon number density jointly determines the transient response of the laser to the pumping conditions. In theory, the transfer function of the laser can be obtained by solving the rate equation containing small signal perturbations, and then the relaxation oscillation frequency equation can be obtained, which mainly includes three characteristic time parameters: upper energy level electron lifetime, photon lifetime and stimulated emission lifetime. Traditional interband semiconductor lasers generate photons through electron-hole recombination, and the upper energy level electron lifetime is long. The relaxation oscillation equation has a real part solution, so it has relaxation oscillation phenomenon. The quantum cascade laser is a semiconductor electrically pumped laser based on the generation of photons by interband electron transitions in the conduction band. The carriers of the unipolar quantum cascade laser are only electrons, which tend to a steady state through phonon scattering, and the upper energy level electron lifetime is very short, in the order of picoseconds. The relaxation oscillation frequency equation obtained by solving the rate equation has no real solution. The modulation of the quantum cascade laser to the external small signal is an ultrafast overdamped process without relaxation oscillation. Therefore, if the parasitic effect is not considered, the direct modulation bandwidth of the quantum cascade laser is as high as 100 GHz. This high-speed characteristic makes quantum cascade lasers have great application potential in the field of high-speed and high-capacity communications.

[0003] The modulation bandwidth test of quantum cascade lasers is an important prerequisite for realizing applications. The microwave rectification method is widely used at present, which is a method based on the inherent nonlinear current-voltage (IV) characteristics of quantum cascade lasers. The quantum cascade laser is modulated with currents of different frequencies, and the voltage change on the bias circuit is extracted to reflect the modulation response. Obviously, this is an indirect electrical measurement method, and its results are often affected by factors such as loss and impedance matching, and cannot truly reflect the modulation response bandwidth of the quantum cascade laser. Optical measurement is the most direct method, such as using a Fourier transform infrared spectrometer (FTIR) to measure the modulation sidebands of the device, but this method is limited by the sensitivity and resolution of FTIR, especially under high-frequency modulation. The sideband amplitude of the laser is small and cannot break through the background noise of FTIR, thus causing measurement errors. High-speed and high-sensitivity detectors are the main bottlenecks for optical testing of the modulation bandwidth of quantum cascade lasers. Summary of the invention

[0004] The technical problem to be solved by the present invention is to provide a quantum cascade laser modulation bandwidth testing system and method, which can accurately characterize the quantum cascade laser modulation bandwidth.

[0005] The technical solution adopted by the present invention to solve the technical problem is as follows: a quantum cascade laser modulation bandwidth test system is provided, comprising a detection quantum cascade laser, a first T-type bias device, a second T-type bias device, a radio frequency source and a spectrum analyzer; the first T-type bias device has a hybrid port connected to a sub-cascade laser to be measured, an AC port connected to the radio frequency source, and a DC port connected to a current source; the second T-type bias device has a hybrid port connected to the detection quantum cascade laser, an AC port connected to the spectrum analyzer, and a DC port connected to the current source; the sub-cascade laser to be measured is optically coupled with the detection quantum cascade laser, so that the eigenfrequency of the sub-cascade laser to be measured is mixed with the optical frequency comb frequency line of the detection quantum cascade laser to generate a beat frequency signal; the radio frequency source is used to generate a modulation signal, and the modulation frequency is adjusted with a preset frequency as an interval, so that the sub-cascade laser to be measured generates a modulation sideband; the spectrum analyzer is used to record the beat frequency signal of the modulation sideband and the optical frequency comb frequency line.

[0006] The detection quantum cascade laser and the to-be-measured quantum cascade laser have the same active region structure.

[0007] The gain ranges of the detection quantum cascade laser and the to-be-measured sub-cascade laser overlap.

[0008] The sub-cascade laser to be measured and the quantum cascade laser for detection are optically coupled through an optical circuit, and the optical circuit includes a first off-axis parabolic mirror and a second off-axis parabolic mirror. The first off-axis parabolic mirror is used to collect the terahertz beam of the sub-cascade laser to be measured, and convert the divergent terahertz beam into a parallel terahertz beam and transmit it to the second off-axis parabolic mirror; the second off-axis parabolic mirror is used to collect the parallel terahertz beam and converge it into the resonant cavity of the quantum cascade laser for detection.

[0009] A low noise amplifier is also arranged between the AC port of the second T-type bias device and the spectrum analyzer.

[0010] The detection quantum cascade laser is connected to the hybrid port of the second T-type bias device through wire bonding, an impedance matcher, and a high-frequency coaxial line.

[0011] The technical solution adopted by the present invention to solve the technical problem is: to provide a quantum cascade laser modulation bandwidth testing method, comprising the following steps:

[0012] Optically coupling the detection quantum cascade laser with the sub-cascade laser to be measured;

[0013] Connecting the hybrid port of the first T-type bias switch to the sub-cascade laser to be measured, connecting the AC port to the radio frequency source, and connecting the DC port to the power supply, so that the sub-cascade laser to be measured operates in a single-mode mode;

[0014] Connecting the hybrid port of the second T-type bias switch to the detection quantum cascade laser, connecting the AC port to a spectrum analyzer, and connecting the DC port to a power supply, so that the detection quantum cascade laser operates in an optical frequency comb mode;

[0015] Finding and recording the beat frequency signal f1 between the sub-cascade laser to be measured and the detection quantum cascade laser on the spectrum analyzer;

[0016] Setting the output power and output frequency of the radio frequency source;

[0017] The spectrum analyzer searches for and records the beat frequency signal of the modulation sideband and the optical frequency comb frequency line near the beat frequency signal f1 plus the output frequency;

[0018] Keeping the output power of the RF source unchanged, increasing the output frequency, wherein the increase value is a preset frequency, searching for and recording the beat frequency signal between the modulation sideband and the optical frequency comb frequency line on the spectrum analyzer, and repeating this step until the beat frequency signal between the modulation sideband and the optical frequency comb frequency line cannot be detected by the spectrum analyzer;

[0019] According to the beat frequency signals of the modulation sideband and the optical frequency comb frequency line recorded by the spectrum analyzer, half of the frequency with the maximum amplitude is taken as the modulation bandwidth of the sub-cascade laser to be measured.

[0020] Beneficial Effects

[0021] Due to the adoption of the above-mentioned technical solution, the present invention has the following advantages and positive effects compared with the prior art: the method adopted by the present invention is an optical testing method, which can directly test the modulation bandwidth and avoid the potential accuracy problems of indirect electrical testing such as microwave rectification. The various components used in the system of the present invention are microwave products that have been successfully commercialized and can be flexibly selected according to actual needs. The selectable operating frequency of the microwave products will not limit the lowest detection frequency of the entire system, so the modulation bandwidth of the sub-cascade laser to be measured can be fully tested. The present invention uses a quantum cascade laser with the same active region structure as the sub-cascade laser to be measured as a detector to avoid the limitation of the detector on the system frequency. The present invention can also ensure that the detection bandwidth of the quantum cascade laser as a detector is greater than the modulation bandwidth of the sub-cascade laser to be measured by adjusting the device size. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 is a test principle diagram of an embodiment of the present invention;

[0023] Figure 2 It is a schematic structural diagram of an embodiment of the present invention. DETAILED DESCRIPTION

[0024] The present invention will be further described below in conjunction with specific embodiments. It should be understood that these embodiments are only used to illustrate the present invention and are not intended to limit the scope of the present invention. In addition, it should be understood that after reading the content taught by the present invention, those skilled in the art can make various changes or modifications to the present invention, and these equivalent forms fall within the scope limited by the appended claims of the application equally.

[0025] The embodiment of the present invention relates to a quantum cascade laser modulation bandwidth test system, and the test system is based on the self-detection mechanism of the quantum cascade laser.

[0026] In order to ensure that the modulation sideband of the sub-cascade laser to be measured produces a beat frequency in the frequency domain with the quantum cascade laser used as a detector, a quantum cascade laser optical frequency comb is used as a detector, which can provide sufficient spectrum bandwidth and high frequency stability to improve the resolution of the modulation bandwidth test.

[0027] When the sub-cascade laser to be measured is in free running (unmodulated state), the eigenfrequency of the sub-cascade laser to be measured is mixed with the frequency line of the optical frequency comb, which will generate a beat signal, which is obtained by the self-detection of the optical frequency comb. According to different beat processes, down-conversion signals of different frequencies will be generated. For example, the beat between the single-mode eigenfrequency and the nearest optical frequency comb frequency line will generate a down-conversion signal f1, and the beat between the single-mode eigenfrequency and the second closest optical frequency comb frequency line will generate a down-conversion signal f2 (f1+f2=the comb tooth spacing of the optical frequency comb).

[0028] When the sub-stage cascade laser to be measured is directly modulated, the modulation frequency is f m , will be at a distance f on both sides of the eigenfrequency m Two modulation sidebands are generated at f1+f, and the modulation sidebands will also beat with the frequency line of the optical frequency comb. m and f1-f m At f2+f m and f2-f m The down-conversion frequency line will also be generated at this point. This embodiment is to continuously change the modulation frequency f m , use a spectrum analyzer to record the down-converted signal of the sideband, such as f1+f m , reflecting the modulation bandwidth of the sub-cascade laser to be measured.

[0029] like Figure 1As shown in the figure, on the left is the measurement of the sub-cascade laser (single mode, only one frequency line in the optical band) running freely and the quantum cascade laser optical frequency comb (multi-mode, a series of equally spaced frequency lines in the optical band, the frequency lines are f rep The beat frequency of the single-mode frequency line and the nth comb tooth of the optical frequency comb is generated by beating each other, and different beat frequencies are generated in the microwave band according to different beating processes. For example, the beat frequency of the single-mode frequency line and the nth comb tooth of the optical frequency comb is generated by beating f1, and the beat frequency of the single-mode frequency line and the n+1th comb tooth of the optical frequency comb is generated by beating f2, and f1+f2=f rep On the right is the sub-cascade laser to be measured, which is excited by a frequency of f m RF signal modulation (generating two sidebands in the optical band, the distance between the sidebands and the single-mode frequency line is f m ) and then with a quantum cascade laser frequency comb (multi-mode, with a series of equally spaced frequency lines in the optical band, with the frequency lines being f rep The modulation sideband will also beat with the optical frequency comb, generating different beat frequencies in the microwave band. For example, the left modulation sideband beats with the n+1th comb tooth of the optical frequency comb to generate f2+f m The right modulation sideband beats with the n+1th comb tooth of the optical frequency comb to generate f2-f m The test method of this embodiment is to detect the frequency of the modulation sideband and the beat frequency of the optical frequency comb teeth, reflecting the modulation bandwidth of the quantum cascade laser. Figure 1 In the process, the down-conversion process from the optical band to the microwave band is realized by the quantum cascade laser self-detection mechanism.

[0030] The quantum cascade laser modulation bandwidth test system of this embodiment at least includes a detection quantum cascade laser (hereinafter referred to as QCLcomb), a first T-type bias device, a second T-type bias device, a radio frequency source and a spectrum analyzer.

[0031] The modulation method of the sub-cascade laser to be measured (hereinafter referred to as QCLUT) is direct modulation, and the modulation signal is a radio frequency signal of a certain power. The radio frequency modulated QCLUT will produce two modulation sidebands, and the distance between the modulation sideband and the intrinsic mode is the frequency of the modulation signal.

[0032] The RF source is used to generate a modulation signal, and the modulation signal is injected into the QCLUT through the first T-type bias device. The hybrid port of the first T-type bias device is connected to the QCLUT, the AC port is connected to the RF source, and the DC port is connected to the power supply to ensure the working bias of the QCLUT.

[0033] The detection quantum cascade laser operates in an optical frequency comb mode, that is, its spectrum is composed of equally spaced patterns. Here, the present embodiment does not limit the method for forming an optical frequency comb with a quantum cascade laser. The gain ranges of QCLUT and QCLcomb overlap, that is, the emission spectrum of QCLUT is covered by the emission spectrum of QCLcomb. The modulated pulse generated by QCLUT is injected into the QCLcomb resonant cavity through an optical circuit. QCLcomb is connected to the hybrid port of the second T-type bias device, the AC port of the second T-type bias device is connected to the spectrum analyzer, and the DC port is connected to the power supply to ensure the working bias of QCLcomb. Optionally, in the present embodiment, QCLcomb integrates an impedance matching element to achieve low-loss high-frequency signal extraction.

[0034] In order to improve the coupling efficiency between the QCLUT and the QCLcomb, an optical circuit can be used when coupling the QCLUT and the QCLcomb, such as a collimation focusing circuit formed by off-axis parabolic mirrors. The QCLUT and the QCLcomb can also be placed face to face to form a direct coupling circuit.

[0035] In order to increase the power of the mixed signal detected by the QCLcomb, microwave components such as low-noise amplifiers and filters can be used to amplify and filter the mixed signal.

[0036] The present invention is further illustrated by a specific embodiment below.

[0037] In this embodiment, the QCLUT and QCLcomb are based on the same active region structure, with a gain center frequency of 4.2THz, wherein the waveguide size of the QCLUT is 150μm×1.5mm, and it is single-mode lasing under free operation. The waveguide size of the QCLcomb is 150μm×6mm, and thanks to the strong nonlinearity of the active region of the quantum cascade laser, the QCLcomb device realizes the optical frequency comb mode operation by the four-wave mixing effect under free operation.

[0038] like Figure 2 As shown, the QCLUT is used as a device under test and is connected to the mixed port (AC+DC) of the first T-type bias device through wire bonding and high-frequency coaxial lines. The DC port of the first T-type bias device is connected to a constant voltage source to apply a working bias to the QCLUT. The AC port of the first T-type bias device is connected to an RF source via a high-frequency coaxial line, and the fixed-power RF signal generated by the RF source modulates the QCLUT through the first T-type bias device.

[0039] QCLUT works in single-mode during free operation, and its spectrum characteristic is that there is only one frequency line of 4.2THz in the frequency domain.

[0040] The modulation signal generated by the RF source directly modulates the QCLUT, the modulation signal remains unchanged at 3dBm, and the modulation frequency is swept from 0 to 20GHz at intervals of 50MHz. The spectrum of the directly modulated QCLUT changes, and its characteristics are: with 4.2THz as the center, a modulation sideband is generated on both sides, and the distance between the modulation sideband and 4.2THz is the modulation signal frequency of the RF source.

[0041] In this embodiment, QCLcomb is used as a detector and is connected to the mixed port (AC+DC) of the second T-type bias device through wire bonding, impedance matching, and high-frequency coaxial lines. The DC port of the second T-type bias device is connected to a constant voltage source to apply a working bias to the QCLcomb. The AC port of the second T-type bias device is connected to a spectrum analyzer, and the mixed signal detected by the QCLcomb is amplified by a low-noise amplifier before being detected by the spectrum analyzer.

[0042] QCLcomb works in free-running mode, that is, in optical frequency comb mode. Its spectrum characteristics are: between 4.1THz and 4.4THz, with nearly 50 frequency lines evenly distributed at intervals of 6GHz.

[0043] In this embodiment, QCLcomb and QCLUT are optically coupled through an optical circuit, which is composed of two off-axis parabolic mirrors: the first off-axis parabolic mirror collects the terahertz pulses of QCLUT, converts the terahertz light with a certain divergence angle into parallel light, and transmits it to the second off-axis parabolic mirror; the second off-axis parabolic mirror converges the collected parallel terahertz light into QCLcomb. At the same time, in order to reduce water vapor absorption, the entire optical circuit system is placed in an atmosphere filled with nitrogen, thereby reducing the propagation loss of terahertz light.

[0044] When the quantum cascade laser modulation bandwidth test system of this embodiment is used for testing, the following steps are included:

[0045] S1, build a collimating and converging optical circuit consisting of two off-axis parabolic mirrors, and place the light-emitting end faces of QCLUT and QCLcomb at the focal points of the two off-axis parabolic mirrors respectively;

[0046] S2, the QCLUT is connected to the hybrid port of the first T-type bias device, the AC port is connected to the RF source, the DC port is connected to the power supply, and a suitable bias voltage is applied to make the QCLUT work;

[0047] S3, QCLcomb is connected to the hybrid port of the second T-type bias device, the AC port is connected to the spectrum analyzer, the DC port is connected to the power supply, and a suitable bias voltage is applied to make QCLcomb work in the optical frequency comb mode;

[0048] S4, find the QCLUT and QCLcomb beat frequency signal f1 on the spectrum analyzer and record it:

[0049] S5, set the RF source output power to 3dBm, and the output frequency to 50MHz;

[0050] S6, the spectrum analyzer searches for and records the beat frequency signal between the modulation sideband and the optical frequency comb frequency line near f1+50MHz;

[0051] S7, keep the output power of the RF source unchanged and increase the output frequency by 50MHz;

[0052] S8, searching for and recording the beat frequency signals of the modulation sideband and the optical frequency comb frequency line on the spectrum analyzer;

[0053] S9, repeating steps S7 and S8 until the beat frequency signal between the modulation sideband and the optical frequency comb frequency line can no longer be detected by the spectrum analyzer;

[0054] S10, data processing, based on the beat frequency signals of the modulation sideband and the optical frequency comb frequency line recorded by the spectrum analyzer, take half of the frequency with the maximum amplitude as the 3dB modulation bandwidth of the QCLUT.

[0055] It is not difficult to find that the method adopted by the present invention is an optical testing method, which can directly test the modulation bandwidth and avoid potential accuracy problems of indirect electrical testing such as microwave rectification. The various components used in the system of the present invention are microwave products that have been successfully commercialized and can be flexibly selected according to actual needs. The selectable operating frequency of the microwave product will not limit the minimum detection frequency of the entire system, so the modulation bandwidth of the sub-cascade laser to be measured can be fully tested. The present invention uses a quantum cascade laser with the same active region structure as the sub-cascade laser to be measured as a detector to avoid the limitation of the detector on the system frequency. The present invention can also ensure that the detection bandwidth of the quantum cascade laser as a detector is greater than the modulation bandwidth of the sub-cascade laser to be measured by adjusting the device size.

Claims

1. A quantum cascade laser modulation bandwidth test system, characterized in that: The invention comprises a quantum cascade laser for detection, a first T-type bias device, a second T-type bias device, a radio frequency source and a spectrum analyzer; the hybrid port of the first T-type bias device is connected to the sub-cascade laser to be measured, the AC port is connected to the radio frequency source, and the DC port is connected to a current source; the hybrid port of the second T-type bias device is connected to the quantum cascade laser for detection, the AC port is connected to the spectrum analyzer, and the DC port is connected to the current source; the sub-cascade laser to be measured is optically coupled with the quantum cascade laser for detection, so that the eigenfrequency of the sub-cascade laser to be measured is mixed with the optical frequency comb frequency line of the quantum cascade laser for detection to generate a beat frequency signal; the radio frequency source is used to generate a modulation signal, and the modulation frequency is adjusted with a preset frequency as the interval, so that the sub-cascade laser to be measured generates a modulation sideband; the spectrum analyzer is used to record the beat frequency signal of the modulation sideband and the optical frequency comb frequency line.

2. The quantum cascade laser modulation bandwidth test system according to claim 1, characterized in that: The detection quantum cascade laser and the to-be-measured quantum cascade laser have the same active region structure.

3. The quantum cascade laser modulation bandwidth test system according to claim 1, characterized in that: The gain ranges of the detection quantum cascade laser and the to-be-measured sub-cascade laser overlap.

4. The quantum cascade laser modulation bandwidth test system according to claim 1, characterized in that: The sub-cascade laser to be measured and the quantum cascade laser for detection are optically coupled through an optical circuit, and the optical circuit includes a first off-axis parabolic mirror and a second off-axis parabolic mirror. The first off-axis parabolic mirror is used to collect the terahertz beam of the sub-cascade laser to be measured, and convert the divergent terahertz beam into a parallel terahertz beam and transmit it to the second off-axis parabolic mirror; the second off-axis parabolic mirror is used to collect the parallel terahertz beam and converge it into the resonant cavity of the quantum cascade laser for detection.

5. The quantum cascade laser modulation bandwidth test system according to claim 1, characterized in that: A low noise amplifier is also arranged between the AC port of the second T-type bias device and the spectrum analyzer.

6. The quantum cascade laser modulation bandwidth test system according to claim 1, characterized in that: The detection quantum cascade laser is connected to the hybrid port of the second T-type bias device through wire bonding, an impedance matcher, and a high-frequency coaxial line.

7. A quantum cascade laser modulation bandwidth test method, characterized in that: The following steps are involved: Optically coupling the detection quantum cascade laser with the sub-cascade laser to be measured; Connecting the hybrid port of the first T-type bias switch to the sub-cascade laser to be measured, connecting the AC port to the radio frequency source, and connecting the DC port to the power supply, so that the sub-cascade laser to be measured operates in a single-mode mode; Connect the hybrid port of the second T-type bias switch to the detection quantum cascade laser, connect the AC port to the spectrum analyzer, and connect the DC port to the power supply, so that the detection quantum cascade laser works in the optical frequency comb mode; search for the beat frequency signal f1 between the sub-cascade laser to be measured and the detection quantum cascade laser on the spectrum analyzer and record it; Setting the output power and output frequency of the radio frequency source; The spectrum analyzer searches for and records the beat frequency signal of the modulation sideband and the optical frequency comb frequency line near the beat frequency signal f1 plus the output frequency; Keeping the output power of the RF source unchanged, increasing the output frequency, wherein the increase value is a preset frequency, searching for and recording the beat frequency signal between the modulation sideband and the optical frequency comb frequency line on the spectrum analyzer, and repeating this step until the beat frequency signal between the modulation sideband and the optical frequency comb frequency line cannot be detected by the spectrum analyzer; According to the beat frequency signals of the modulation sideband and the optical frequency comb frequency line recorded by the spectrum analyzer, half of the frequency with the maximum amplitude is taken as the modulation bandwidth of the sub-cascade laser to be measured.

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