CN110649462B - A method for compressing the spectral linewidth of quantum cascade lasers - Google Patents

Abstract

The invention relates to a method for compressing the spectral line width of a quantum cascade laser. An optical feedback device is arranged on the output optical path of the quantum cascade laser. The invention is characterized in that the output light of the quantum cascade laser passes through the optical feedback device. The laser cavity of the cascade laser is coupled with the output light of the quantum cascade laser, and the optical feedback ratio reaches the strong optical feedback. At the same time, the optical feedback phase is not controlled, so the strong optical feedback compression is used under the condition of no feedback phase control. Spectral linewidths of quantum cascade lasers. Compared with the traditional optical feedback solution, the present invention does not need to use a piezoelectric sensor to control the phase of the optical feedback, so it has the advantages of simple device and low cost.

Description

A method for compressing the spectral linewidth of quantum cascade lasers

technical field

The invention relates to a method for stabilizing the frequency of a laser in the technical field of semiconductor optoelectronics, in particular to a method for compressing the spectral line width of a quantum cascade laser.

Background technique

A quantum cascade laser is a semiconductor laser light source that realizes stimulated emission based on the transition of electrons between subbands in the conduction band. Its radiation spectrum covers the mid-infrared and terahertz bands, and has important application value in the fields of molecular spectroscopy, gas detection, free-space optical communication, and optoelectronic countermeasures. The above applications usually require the use of single longitudinal mode quantum cascade lasers, including single mode excited Fabry-Perot cavity lasers, distributed feedback lasers, distributed Bragg reflection lasers, vertical cavity surface emitting lasers, and external cavity lasers. Due to the noise generated by spontaneous emission in the laser, the noise of the laser current source and the noise of the laser temperature controller, the laser output from the above single longitudinal mode quantum cascade laser has a limited spectral linewidth, usually in the order of megahertz. This linewidth can meet the general gas detection requirements, but it cannot meet the detection needs of high-resolution molecular spectroscopy, small gas molecules with narrow absorption peaks, etc. Therefore, it is necessary to compress the spectral linewidth of quantum cascade lasers.

Common methods of compressing the spectral linewidth of quantum cascade lasers include: I. Phase-lock the laser to the absorption peak of a certain gas, and control the current of the laser to stabilize the frequency by detecting the error signal generated by the frequency change of the laser [Williams et al. , Opt. Lett. 24, 1844 (1999)]. But this method requires the laser frequency to approximate the absorption peak frequency of the gas, so the laser loses tunability. II. Phase-lock the laser to a certain longitudinal mode of the high-quality optical cavity, and generate an error signal when the frequency of the laser is inconsistent with the frequency of the longitudinal mode of the optical cavity, thereby controlling the current of the laser to stabilize the frequency [Taubman et al., Spectr. Acta 60, 3457 (2004)]. But this method requires phase modulation of the laser. III. Phase-lock the laser to an optical frequency comb in the near-infrared band by difference frequency or sum frequency technology [Argence et al., Nature Photon. 9,456 (2015)]. However, the experimental setup required for this method is extremely complex. Relatively simple methods include: frequency stabilization by detecting changes in quantum cascade laser voltage [Patent No. WO2014/198707A1; Tombez et al., Opt. Lett. 38, 5079 (2013); Sergachev, Opt. Lett. 39, 6411 (2014)], and phase locking a laser to an optical delay line [Shehzad et al., Opt. Lett. 44, 3470 (2019)]. All the above methods require active frequency stabilization of the laser, and the present invention provides a simple method for passive frequency stabilization of the quantum cascade laser by using strong light feedback.

As shown in Figure 1, optical feedback refers to the phenomenon in which part of the light returns to the laser cavity and couples with the output light of the laser due to the existence of a certain reflective device such as a plane mirror on the output optical path of the laser. The spectral behavior of a laser is related to the optical feedback ratio and optical feedback phase. The optical feedback ratio is defined as the ratio of the optical power of the reflected light to the optical power of the free-running laser. The optical feedback phase is determined by the distance between the laser and the reflector. Under the condition of strong light feedback (the feedback ratio is greater than about 1%), the general semiconductor laser will be unstable, and the spectral linewidth will increase sharply. Under weak light feedback conditions (feedback ratios less than about 1%), the laser spectral linewidth is compressed when the feedback phases are in-phase or nearly in-phase [Schunk and Petermann, IEEE J. Quantum. Electron. 24, 1242 (1988)]. When the initial phase of the feedback is reversed or approximately reversed, the laser spectral linewidth broadens. Therefore, the linewidth of the laser can be compressed under the condition that weak optical feedback is used and the phase of the optical feedback is precisely controlled. As shown in Fig. 1, the optical feedback phase is usually controlled by a piezoelectric sensor [Dahmani et al., Opt. Lett. 12, 876 (1987)]. However, the stability and reliability of the above systems are poor, so the effect on the linewidth compression of semiconductor lasers is general.

SUMMARY OF THE INVENTION

The spectral linewidth of free-running quantum cascade lasers cannot meet the requirements of high-resolution molecular spectroscopy and detection of narrow absorption peaks of small gas molecules. Therefore, the purpose of the present invention is to propose a method for compressing the spectral linewidth of a quantum cascade laser based on strong light feedback.

In order to achieve the above object, the technical solution of the present invention is to provide a method for compressing the spectral linewidth of a quantum cascade laser, and an optical feedback device is arranged on the output optical path of the quantum cascade laser, which is characterized in that the output light of the quantum cascade laser is After passing through the optical feedback device, part of the light returns to the laser cavity of the quantum cascade laser and is coupled with the output light of the quantum cascade laser. The optical feedback ratio achieves strong optical feedback. At the same time, the phase of the optical feedback is not controlled, so that there is no feedback Compression of the spectral linewidth of a quantum cascade laser using intense light feedback under phase control.

Preferably, a mains-driven current source, a mains-driven voltage source, a battery-driven low-noise current source or a battery-driven low-noise voltage source are used to power the quantum cascade laser.

Preferably, the optical feedback device adopts a partially reflective optical lens, or adopts a polarizer that has a certain angle with the polarization direction of the output light of the quantum cascade laser.

Preferably, the optical feedback device includes a beam splitter, a polarizer, a polarizer and a reflector, wherein:

The beam splitter is used to divide the output light of the quantum cascade laser into two paths, one for optical feedback and the other for line width measurement;

The polarization direction of the first polarizer is kept consistent with the polarization direction of the output light of the quantum cascade laser;

The magnitude of the light feedback ratio is controlled by rotating the second polarizer to change its angle with the first polarizer.

Preferably, a line width measuring device for measuring the line width is also included, and a line of light for the line width measurement split by the beam splitter is input to the line width measuring device.

Preferably, a beam collimating mirror is also included, and the output light of the quantum cascade laser is collimated by the beam collimating mirror and then input to the beam splitting mirror.

Preferably, the output wavelength of the quantum cascade laser is in the mid-infrared band or the terahertz band, and the output mode is a single longitudinal mode.

Compared with the traditional optical feedback solution, the present invention does not need to use a piezoelectric sensor to control the phase of the optical feedback, so it has the advantages of simple device and low cost. In addition, the present invention is not sensitive to the inevitable phase jitter of optical feedback, and thus has the advantage of high stability. On the other hand, the present invention uses strong light feedback (the feedback ratio is greater than about 1%), while the traditional optical feedback scheme can only use weak light feedback (the feedback ratio is greater than about 1%). The line width compression effect is good. Compared with the three commonly used solutions of compressed quantum cascade lasers, the solution proposed by the present invention greatly simplifies the complexity and cost of the device.

Description of drawings

Figure 1 is a schematic diagram of the frequency stabilization of a semiconductor laser by traditional weak light feedback. This device requires a piezoelectric sensor to precisely control the position of the flat mirror, and the intensity of the optical feedback is weak (the feedback ratio is less than about 1%).

FIG. 2 is a graph showing the relationship between the spectral line width compression ratio of the quantum cascade laser of the present invention, the optical feedback ratio and the optical feedback phase. The linewidth compression ratio is defined as the ratio of the linewidth of the free-running laser to the linewidth of the laser under optical feedback.

FIG. 3 is a schematic diagram of frequency stabilization of a quantum cascade laser by strong light feedback according to the present invention. The device does not require piezoelectric sensors to control the position of the flat mirror, and the optical feedback intensity is strong (the feedback ratio is greater than about 1%).

FIG. 4 is an effect diagram of an embodiment of the present invention using strong light feedback to compress the spectral linewidth of a quantum cascade laser.

FIG. 5 is a diagram of an embodiment of the present invention using strong light feedback to compress the spectral linewidth of a quantum cascade laser and its measurement.

FIG. 6 is an effect diagram of an embodiment of the present invention using strong light feedback to suppress frequency noise of a quantum cascade laser.

Detailed ways

The present invention will be further described below in conjunction with specific embodiments. It should be understood that these examples are only used to illustrate the present invention and not 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 also fall within the scope defined by the appended claims of the present application.

As shown in Fig. 2, under weak optical feedback conditions (feedback ratio less than about 1%), the spectral linewidth of the quantum cascade laser may be compressed or broadened depending on the optical feedback phase. Under strong optical feedback conditions (feedback ratio greater than about 1%), the linewidth of the quantum cascade laser is compressed for any optical feedback phase. Therefore, under the condition of strong light feedback, the linewidth of the quantum cascade laser can be effectively compressed without any phase control devices including piezoelectric sensors. Based on the above analysis, the principle of the technical solution proposed by the present invention is shown in FIG. 3 . The solution mainly includes a quantum cascade laser and a plane mirror, but does not include and does not require the piezoelectric sensor in FIG. 1 . Using this scheme, Fig. 4 shows the compression effect of the strong light feedback on the spectral linewidth of the quantum cascade laser under different feedback ratios. Under free-running conditions, the spectral linewidth of the quantum cascade laser is 7.8 MHz. With a feedback ratio of 42%, the spectral linewidth of the quantum cascade laser is compressed to 107kHz. This scheme compresses the linewidth of the quantum cascade laser by about 73 times, which is far superior to the 2.8 times of the voltage stabilization scheme of the quantum cascade laser in the patent WO2014/198707A1.

A specific implementation of the present invention is shown in FIG. 5 . It should be noted that the present invention is not limited to the embodiments described below, and other similar devices can also be used to achieve the same functions based on the principles and principles proposed by the present invention.

The device in FIG. 5 includes three main components: I. Light source part; II. Optical feedback device; III. Line width measurement device.

I. The light source part includes a laser driving power supply 1 , a quantum cascade laser 2 and a beam collimating mirror 3 . The laser driving power source 1 used in this embodiment is a current source driven by commercial power. Quantum Cascade Laser 2 is a Fabry-Perot cavity laser. The output wavelength of the quantum cascade laser 2 is in the mid-infrared band, and the output mode is a single longitudinal mode. The beam collimating mirror 3 is an aspherical lens, which collimates the laser beam output by the quantum cascade laser.

In addition to the above technical solutions, the laser driving power source 1 can also use a commercial power-driven voltage source, or a battery-driven low-noise current source, or a battery-driven low-noise voltage source to power the quantum cascade laser 2 . The quantum cascade laser 2 may also be a laser that can output a single longitudinal mode, such as a distributed feedback laser, a distributed Bragg reflection laser, a vertical cavity surface emitting laser, and an external cavity laser. The laser output wavelength can be in the terahertz band. The beam collimating mirror 3 is not a necessary element in the embodiment, and those skilled in the art may also not use the beam collimating mirror 3 .

II. The optical feedback device includes a beam splitter 4 , a polarizer 5 , a polarizer 6 and a reflector 7 . The beam splitter 4 divides the laser beam collimated by the beam collimator 3 into two paths: one path is used for light feedback, and the other path is used for line width measurement. In this embodiment, the polarization direction of the polarizer 5 is kept consistent with the polarization direction of the laser output light, and the polarization direction of the laser output light is hereinafter referred to as the TM direction for short. The polarizer 6 is used to control the intensity of the feedback light. The optical feedback ratio can be controlled by rotating the polarizer 6 to change the angle between the polarizer 6 and the polarizer 5 . The mirror 7 is a flat mirror.

In addition to the above technical solutions, there are many other implementations of the optical feedback device, such as using a partially reflective optical lens, or using a polarizer with a certain angle to the TM direction to achieve optical feedback.

III. The line width measurement device includes an optical isolator 8 , a frequency discriminator 9 , a photodetector 10 and an electrical spectrum analyzer 11 . In this embodiment, the optical isolator 8 is used to avoid optical feedback generated by the device in the measurement optical path. The frequency discriminator 9 uses a CO gas cell in order to convert the frequency noise of the quantum cascade laser 2 into intensity noise. The photodetector 10 converts the converted intensity noise into an electrical signal. The spectrum analyzer 11 displays the frequency noise spectrum of the quantum cascade laser 2 in the frequency domain. The spectral linewidth of the quantum cascade laser 2 is obtained by analyzing its frequency noise spectrum.

In addition to the above technical solutions, the linewidth measurement device can be obtained by mixing with other narrow linewidth lasers such as optical frequency combs.

Fig. 6 shows the effect diagram of using the present invention to suppress the frequency noise of the quantum cascade laser. Strong light feedback reduces frequency noise below 100MHz relative to a free-running laser. Especially the frequency noise below 100kHz is suppressed by about 40dB. The spectral linewidth of the laser is determined by the frequency noise higher than the β line in the figure. By integrating this part of the frequency noise, the change of the spectral linewidth of the quantum cascade laser with the feedback ratio in Figure 4 is obtained (for the specific method, refer to [Domenico et al. al. Appl. Opt. 49, 4801 (2010).]).

Claims (7)

1. A method for compressing the spectral line width of a quantum cascade laser is characterized in that an optical feedback device is arranged on an output optical path of the quantum cascade laser, the output light of the quantum cascade laser is subjected to light splitting after passing through the optical feedback device and then is returned to a laser cavity of the quantum cascade laser to be coupled with the output light of the quantum cascade laser, the optical feedback ratio achieves strong light feedback, meanwhile, the optical feedback phase is not controlled, and therefore the strong light feedback is used for compressing the spectral line width of the quantum cascade laser under the condition of no feedback phase control.

2. The method of claim 1, wherein the quantum cascade laser is powered by a mains-driven current source, a mains-driven voltage source, a battery-driven low-noise current source, or a battery-driven low-noise voltage source.

3. The method as claimed in claim 1, wherein the optical feedback device employs a partially reflective optical lens or a polarizer having an angle with respect to the polarization direction of the output light of the qc laser.

4. A method of compressing the spectral linewidth of a quantum cascade laser as claimed in claim 1 wherein said optical feedback means comprises a beam splitter, a first polarizer, a second polarizer, and a mirror, wherein:

the beam splitter is used for splitting the output light of the quantum cascade laser into two paths, one path is used for optical feedback, and the other path is used for line width measurement;

the polarization direction of the first polarizing film is kept consistent with the polarization direction of output light of the quantum cascade laser;

and the size of the optical feedback ratio is controlled by rotating the polarizing plate II to change the included angle between the polarizing plate II and the polarizing plate I.

5. The method of claim 4, further comprising a line width measuring device for measuring line width, wherein the line width measuring device is inputted with a path of light for measuring line width split by the beam splitter.

6. The method of claim 4, further comprising a beam collimator, wherein the output light of the quantum cascade laser is collimated by the beam collimator and then input to the beam splitter.

7. The method of claim 1, wherein the output wavelength of the quantum cascade laser is in the mid-infrared band or the terahertz band, and the output mode is a single longitudinal mode.

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