Terahertz multimode laser mode resolution measurement system

Abstract

The invention relates to a terahertz multimode laser mode resolution measurement system, and the system comprises a terahertz quantum cascade laser which is used for generating multimode laser; the guiding module is used for guiding the multi-mode laser into the reflecting module; the reflection module adopts a reflection type blazed grating and is used for reflecting a preset laser mode in the multimode laser back into the terahertz quantum cascade laser according to an original light path; and the control acquisition module is used for acquiring the terminal voltage of the terahertz quantum cascade laser and controlling the current source of the terahertz quantum cascade laser based on the terminal voltage. According to the invention, rapid detection of the intensity of the single laser mode under the multi-mode condition is realized.

Description

Technical Field

[0001] The invention relates to the technical field of laser feedback interference measurement, and in particular to a terahertz multi-mode laser mode resolution measurement system. Background Art

[0002] In 1963, King and Steward first used self-mixing technology for laser metrology, and suppressed unwanted resonant laser modes through feedback from a third external cavity mirror ("self-mixing"). The radiation returned from a reflector was collected by a photosensitive detector. When operating in continuous wave mode, this effect was observable when only 0.1% of the radiation was reflected from an external mirror as far as 10m from the laser. This device has great potential in measuring physical parameters that change the optical path length, including measuring physical length and speed, and measuring changes in the propagation medium, such as changes in pressure or changes in the composition of the medium in the external cavity. Since the successful first demonstration, this technology has been studied in many fields such as biological tissue imaging, gas composition and concentration detection, and material detection. The self-mixing effect refers to the mixing of electromagnetic waves in the cavity after the electromagnetic waves in the cavity interact with the electromagnetic waves outside the cavity and are re-injected into the laser cavity. It is a very common phenomenon that can occur in any type of laser. Among them, research reports have been published in different types of lasers, including gas lasers, in-plane semiconductor diode lasers, vertical cavity surface emitting lasers (VCSELs), mid-infrared and terahertz quantum cascade lasers (THz QCLs), interband cascade lasers, optical fibers and fiber ring lasers, solid-state lasers, and quantum dot lasers. Monitoring the change in the voltage at the laser end is a common means of obtaining optical feedback signals, which brings great convenience and benefits, especially when the detector required for laser radiation is bulky, expensive, or complicated to use. Lang and Kobayashi gave the core model of semiconductor lasers undergoing optical feedback in their work in 1980. This model well reflects the essence of laser dynamics under optical feedback and is still the basis of the system model. To this day, theoretical research and practical demonstration of laser feedback interferometry schemes are still continuing.

[0003] There are many types of lasers suitable for self-mixing technology, among which terahertz quantum cascade lasers (THz QCL) are attractive radiation sources in this field. Because they have compact size, high emission power (>2.4W in pulse mode and >100mW in continuous mode), narrow instantaneous free-running linewidth (<30kHz), and spectral coverage of 0.7-5.4THz. THz QCL is a new type of terahertz laser radiation source. It is a monopole quantum device based on solid-state semiconductors, which relies on the transition of electrons between conduction band energy levels in quantum structures to radiate photons. In the self-mixing technology, terahertz radiation is intentionally reflected back into the THz QCL, causing interference of the laser inside, which leads to changes in the voltage at the THz QCL terminal. This change is very sensitive to the amplitude and phase of the reflected radiation field, which supports the wide application of this technology, including imaging and material analysis, and related research has been widely carried out. As a semiconductor laser, the lasing characteristics of THz QCL are sensitive to changes in the carrier density in the laser cavity. When the carrier density changes, the intracavity refractive index, gain spectrum, lasing frequency, and light field amplitude will all change until the system forms a new stable state. Changes in the driving current have a direct impact on the carrier density, which is an important factor that causes changes in lasing characteristics. When working with a multi-mode THz QCL, it can output higher power, expand the lasing spectrum range, and enhance detection capabilities. However, the superposition of multiple laser mode intensities also increases the difficulty of resolving the intensity of a single laser mode. The multi-mode laser will be reflected back to the laser cavity along the system optical path, and will also cause problems such as mode competition. Additional auxiliary scanning is usually required to achieve resolution of different frequencies, but it is difficult to meet the real-time monitoring of a single mode. Summary of the invention

[0004] The technical problem to be solved by the present invention is to provide a terahertz multi-mode laser mode resolution measurement system, which can realize the rapid detection of the intensity of a single laser mode under multi-mode conditions.

[0005] The technical solution adopted by the present invention to solve the technical problem is: to provide a terahertz multi-mode laser mode resolution measurement system, comprising:

[0006] Terahertz quantum cascade lasers for multimode laser generation;

[0007] A guiding module, used for introducing the multi-mode laser into a reflecting module;

[0008] A reflection module, using a reflective blazed grating, is used to reflect the preset laser mode in the multi-mode laser back to the terahertz quantum cascade laser according to the original optical path;

[0009] The control acquisition module is used to acquire the terminal voltage of the terahertz quantum cascade laser and control the current source of the terahertz quantum cascade laser based on the terminal voltage.

[0010] The reflective blazed grating adjusts the incident angle of the multi-mode laser through angle adjustment, so that the preset laser mode has the maximum diffraction efficiency, and the preset laser mode in the multi-mode laser is reflected back into the terahertz quantum cascade laser according to the original optical path.

[0011] The guiding module is an off-axis parabolic mirror.

[0012] A reflection delay module is also provided between the guiding module and the reflection module, and the reflection delay module is used to increase the optical path.

[0013] The reflection delay module includes a displacement platform, on which are disposed two 45° reflection mirrors, through which the multi-mode laser can be directed toward the reflection module; the control acquisition module is also used to control the displacement platform based on the terminal voltage.

[0014] The control acquisition module includes an amplifier and a data acquisition card connected in sequence, the amplifier is used to amplify the terminal voltage of the terahertz quantum cascade laser; the data acquisition card is used to collect the amplified terminal voltage; the data acquisition card, displacement platform and current source perform real-time communication and integrated control through the Labview program.

[0015] Beneficial Effects

[0016] Due to the adoption of the above technical solution, the present invention has the following advantages and positive effects compared with the prior art: the present invention introduces a reflective grating in the reflection module to achieve rapid mode resolution, that is, by adjusting the grating angle, a required laser mode can be reflected back to the laser, thereby achieving rapid detection of the intensity of a single laser mode under multi-mode conditions, and solving the problems of difficult mode resolution and poor real-time performance in terahertz laser systems. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 This is a schematic diagram of the self-mixing interference effect;

[0018] Figure 2 is a schematic diagram of a terahertz multi-mode laser mode resolution measurement system according to an embodiment of the present invention;

[0019] Figure 3 is a multimode self-mixing time-domain interferogram measured by an embodiment of the present invention;

[0020] Figure 4 is a multi-mode laser spectrum diagram at different currents measured by an embodiment of the present invention;

[0021] Figure 5 is a graph of laser mode changes at different grating angles measured by an embodiment of the present invention;

[0022] Figure 6 It is a schematic diagram of mode selection of two main frequencies under 0.95A current condition measured by an embodiment of the present invention. DETAILED DESCRIPTION

[0023] 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.

[0024] The embodiments of the present invention relate to a terahertz multimode laser mode resolution measurement system, which uses the self-mixing interference effect to directly resolve the laser frequency information from the interference signal. The basic principle is that light is emitted from the laser, transmitted to an external target, and after being partially reflected from the external target, it is retransmitted back to the laser. A portion of it re-enters the laser cavity, and the re-injected light interacts (mixes) with the resonant mode of the laser, such as Figure 1 As shown. The reinjected light is mixed in the laser cavity, disturbing the electric field in the cavity, transmitting this information from outside the laser cavity, and then becomes measurable by disturbing the laser operating parameters. For example, changes in gain lead to changes in optical power, lasing frequency, and laser terminal voltage. Since the reinjected light carries information such as the reflection intensity and phase of the target object, this information can be obtained by modulating the output performance of the laser by the reinjected light, thereby realizing the detection of the target object. Compared with other interference measurement methods, the measurement method based on the self-mixing interference effect has a simple optical path, does not require complex and expensive detectors, has high sensitivity, and has very broad application prospects. The dynamic process of the interaction between the laser light field and the gain medium can be described by the rate equation model proposed by Lang and Kobayashi for complex fields and carrier density:

[0025]

[0026] Where E(t) is the complex envelope of the electric field, according to e jωt fast oscillation, G is the gain term, which is related to the carrier density N, Γ is the optical confinement factor, ω is the angular frequency of the laser mode, ω m is the cavity resonant angular frequency, τ p is the photon lifetime in the laser cavity, τ ext is the external cavity round trip time, is the feedback coupling rate. The carrier density N is expressed as:

[0027]

[0028] Where I(t) is the laser driving current, q is the electron charge, V is the cavity volume, and τ n is the carrier lifetime, G is the gain in the laser cavity, and S(t) is the photon density in the laser cavity. These two equations together describe the laser dynamics under optical feedback. At the same time, the full rate equation can more clearly understand the changes in carriers in semiconductor lasers:

[0029]

[0030] Assuming that the stimulated emission rate is approximately equal to the inverse of the photon lifetime above the threshold (spontaneous emission is negligible), a specific formula can be obtained:

[0031]

[0032] Among them, P total (t) is the total optical power emitted by the laser surface, h is Planck's constant, v g is the laser cavity group velocity, α m is the effective mirror loss, V p is the effective cavity volume occupied by the photon, L in is the laser cavity length, R1 is the front reflector in the cavity, and R2 is the rear reflector in the cavity. By linking the optical output power change with the rate equation model, it can be observed that the photon density is proportional to the output optical power. The terminal voltage V of the semiconductor laser SM It can be expressed as:

[0033]

[0034] Among them, k B is the Boltzmann constant, T is the temperature, N i is the intrinsic carrier density of the active region.

[0035] Multimode laser means that the laser emits in multiple laser modes simultaneously. Since the resonant cavity adopts Fabry-Pérot type, the frequency interval between two adjacent longitudinal modes is c / (2n eff L), where c is the speed of light, L is the length of the resonant cavity, and n effis the effective refractive index of the gain medium. Since the laser spectra of all modes are superimposed, it is difficult to analyze the spectrum without any reference. In order to overcome this problem, the laser mode must be separated. One of the effective ways is to use a monochromator for mode resolution, but this solution is very inconvenient and complicated for THzQCLs that require expensive and bulky refrigeration detectors. Therefore, this embodiment proposes a terahertz multi-mode laser mode resolution measurement system based on self-mixing technology. It is more convenient and compact, and more importantly, it does not require an external detector and can achieve self-detection. The coherence characteristics of the self-mixing interference effect provide a theoretical basis for multi-mode laser self-mixing. Light of different modes is coherent with the light fed back and affects the bias voltage of the laser according to the phase relationship. Compared with the single-mode case, the multi-mode self-mixing situation is more complicated, involving interactions between modes, spatial distribution of gain, and three-dimensional transport processes of carriers, etc. In particular, the influence of the intensity of the self-mixing signal on the stability of the laser mode is crucial to the resolution and noise level of the laser spectrum.

[0036] like Figure 2 As shown, the terahertz multi-mode laser mode resolution measurement system of this embodiment includes:

[0037] Terahertz quantum cascade laser 1, used to generate multi-mode laser;

[0038] A guiding module 2, used for introducing the multi-mode laser into a reflecting module 4;

[0039] A reflection module 4, which uses a reflective blazed grating, is used to reflect the preset laser mode in the multi-mode laser back to the terahertz quantum cascade laser 1 according to the original optical path;

[0040] The control acquisition module 3 is used to acquire the terminal voltage of the terahertz quantum cascade laser 1 and control the current source of the terahertz quantum cascade laser 1 based on the terminal voltage.

[0041] In this embodiment, a reflection delay module 5 is further provided between the guiding module 2 and the reflection module 4, and the reflection delay module 5 is used to increase the optical path. The reflection delay module 5 includes a displacement platform, on which two 45° reflection mirrors are provided, and the multi-mode laser can be directed to the reflection module through the two 45° reflection mirrors; the control acquisition module is also used to control the displacement platform based on the terminal voltage.

[0042] In this embodiment, a wide-spectrum multimode THzQCL is used to expand the spectral range. The change of carriers in the laser cavity will cause the change of the lasing characteristics of the THzQCL until a new stable state is formed, and the driving current is an important factor that directly affects the change of carriers. However, THzQCL has a very narrow linewidth enhancement factor α and a gain recovery time of the order of picoseconds (ps), and has better dynamic stability than traditional semiconductor lasers. Therefore, the measurement method based on THzQCL as the radiation source has been widely studied in many fields. By adding the tuning ranges of multiple modes, the spectral coverage can be expanded exponentially, and the frequency coverage range is roughly between 4.1THz-4.3THz, thereby greatly enhancing the recognition and resolution capabilities of multiple modes.

[0043] The multi-mode laser is emitted from the end face of the THz QCL, and is first collimated by the guide module (i.e., a 1-inch off-axis parabolic mirror) and enters the reflection delay module 5. In the reflection delay module 5, two 45° reflectors are symmetrically placed on a high-precision displacement platform to increase the optical path and thus improve the resolution of the spectrum. Figure 2 When the reflective blazed grating is replaced by a reflective plane mirror and the driving current is changed, the self-mixing signal (see Figure 3 ). Terahertz spectroscopy can be measured by adjusting the path length of the feedback light, and the self-mixing signal will change periodically, producing "interference fringes", such as Figure 4 As shown. By measuring the v LFI -ΔL ext By Fourier transforming the curve, we can get the emission spectrum of THz QCL, such as Figure 4 This method allows the laser to be used as both a light source and a detector, making the measurement of the spectrum more accurate, sensitive, and efficient. It also avoids the use of additional liquid helium-cooled THz detectors, greatly simplifying the spectral system and making it compact and portable.

[0044] This embodiment uses a reflective blazed grating to quickly detect different laser modes generated under different current conditions, thereby achieving rapid frequency resolution. Figure 2As shown in the figure, in the reflection module, a customized reflective blazed grating is used. Its function is to adjust the incident angle of the multi-mode laser emitted by the THz QCL by rotating the reflective blazed grating. The reflective blazed grating has the maximum diffraction efficiency for light of different frequencies at different angles. Therefore, a certain laser mode required can be reflected back to the THz QCL according to the original optical path, thereby realizing the rapid detection of the intensity of a single laser mode under the condition of multi-mode laser emission, and this is the same as the effect of the single-mode self-mixing effect. By adjusting the frequency and grating angle of the THz QCL in a linked manner, different modes of the multi-mode laser can be quickly analyzed and monitored.

[0045] The control acquisition module in this embodiment includes a low-noise amplifier and a data acquisition card connected in sequence. The signal of periodic weak changes in the terminal voltage of the THz QCL caused by the self-mixing interference effect is amplified by the low-noise amplifier and then input into the data acquisition card for acquisition. The data acquisition card, high-precision displacement platform and current source are communicated and integrated in real time through the Labview program. Under a given current condition, when the displacement platform starts to move, the data acquisition card starts to collect data. When the displacement platform stops moving, the data acquisition card stops collecting data and proceeds to the next current value, and so on and so forth until all current conditions are measured. Under the conditions of current of 0.95A and temperature of 20K, the data acquisition card, high-precision displacement platform and current source are controlled in real time. Figure 2 The system shown in the figure first measures the emission spectrum of the THz QCL, under which the laser is in multi-mode lasing. The reflector at the reflection module is replaced with a reflective blazed grating. When the reflective blazed grating is placed at different angles, it will select and distinguish light of different frequency components. Using the Littrow incidence method, specific frequency components at specific angles will be returned along the original incident path. Figure 5 As shown, in the process of rotating the reflective blazed grating, the two main lasing modes (4.2320702THz and 4.25288912THz) under the current condition of 0.95A change from mode feedback of 4.2320702THz to the appearance of 4.25288912THz mode as the reflective blazed grating rotates. When the rotation angle is 0°52', only one mode of 4.25288912THz is fed back to the laser. Figure 6 It shows that when the incident angle is 25.064° and 25.86°, compared with the full emission laser mode under the current condition, the two main modes in the laser are fed back respectively, and it is consistent with the theoretical simulation results. Under other currents, different modes are also distinguished. It can be seen that based on the self-mixing effect, the laser mode can be well distinguished, multi-mode laser emission can be achieved, and the frequency mode is selectively returned.

[0046] It is not difficult to find that the terahertz multimode laser mode resolution measurement system of this embodiment can not only measure the terahertz spectrum more accurately and more portable, but also quickly resolve and monitor the single mode of the multimode laser through the designed reflective blazed grating, which provides a new method for multi-mode resolution in terahertz spectroscopy technology.

Claims

1. A terahertz multimode laser mode resolution measurement system, characterized in that: include: Terahertz quantum cascade lasers for multimode laser generation; A guiding module, used for introducing the multi-mode laser into a reflecting module; A reflection module, using a reflective blazed grating, is used to reflect the preset laser mode in the multi-mode laser back to the terahertz quantum cascade laser according to the original optical path; The control acquisition module is used to acquire the terminal voltage of the terahertz quantum cascade laser and control the current source of the terahertz quantum cascade laser based on the terminal voltage.

2. The terahertz multimode laser mode resolution measurement system according to claim 1, characterized in that: The reflective blazed grating adjusts the incident angle of the multi-mode laser through angle adjustment, so that the preset laser mode has the maximum diffraction efficiency, and the preset laser mode in the multi-mode laser is reflected back into the terahertz quantum cascade laser according to the original optical path.

3. The terahertz multimode laser mode resolution measurement system according to claim 1, characterized in that: The guiding module is an off-axis parabolic mirror.

4. The terahertz multimode laser mode resolution measurement system according to claim 1, characterized in that: A reflection delay module is also provided between the guiding module and the reflection module, and the reflection delay module is used to increase the optical path.

5. The terahertz multimode laser mode resolution measurement system according to claim 4, characterized in that: The reflection delay module includes a displacement platform, on which are disposed two 45° reflection mirrors, through which the multi-mode laser can be directed toward the reflection module; the control acquisition module is also used to control the displacement platform based on the terminal voltage.

6. The terahertz multimode laser mode resolution measurement system according to claim 5, characterized in that: The control acquisition module includes an amplifier and a data acquisition card connected in sequence, the amplifier is used to amplify the terminal voltage of the terahertz quantum cascade laser; the data acquisition card is used to collect the amplified terminal voltage; the data acquisition card, displacement platform and current source perform real-time communication and integrated control through the Labview program.

Images