Semiconductor laser frequency stabilizing and line width narrowing method

Abstract

The invention discloses a frequency stabilization and line width narrowing method for a semiconductor laser, and relates to the application of the semiconductor laser in a PDH frequency locking technology, and the method comprises the steps: S1, carrying out the first beam splitting of transmission and reflection of a light beam of a DFB laser, reflecting the reflected light beam back into the laser, and suppressing random phase noise generated by spontaneous radiation; s2, the transmission beam subjected to phase noise suppression is subjected to second-time beam splitting of transmission and reflection, the transmission beam subjected to second-time beam splitting is isolated and then passes through an electro-optic phase modulator, a carrier generated by the electro-optic phase modulator is coupled to an ultra-stable cavity, an interference signal generated by the carrier and a sideband reflection signal serves as a frequency discrimination signal, and the frequency discrimination signal is transmitted to the ultra-stable cavity; locking the carrier frequency to the longitudinal mode frequency of the ultra-stable cavity; and S3, the reflected light beam in the second beam splitting is coupled to an IQ modulator, carrier suppression single-side-band laser is generated, and the frequency-tunable narrow-linewidth frequency ultra-stable light source is obtained by changing the frequency of the radio frequency signal.

Description

Technical Field

[0001] The present invention relates to a method for frequency stabilization and linewidth narrowing of a semiconductor laser. Background Art

[0002] Semiconductor lasers have a large application market. With their flexible center frequency, tunable characteristics, and small volume advantages, they will play more important roles in the future.

[0003] However, due to the short cavity length of the semiconductor laser resonator and short photon lifetime, its linewidth enhancement factor is relatively large, resulting in worse phase and frequency noise of the laser compared to solid or gas lasers with longer cavity lengths. At the same time, semiconductor lasers are easily affected by environmental temperature, current noise, etc. Therefore, the performance of semiconductor lasers, such as directivity, monochromaticity, and coherence, is relatively poor, and their linewidth is usually in the MHz order of magnitude.

[0004] The PDH frequency locking technology (i.e., phase modulation optical heterodyne frequency stabilization) requires that the free-running linewidth of the laser is smaller than the linewidth of the external resonator. However, the free-running linewidth of semiconductor lasers is usually greater than MHz, and the frequency reference of an external resonator with a linewidth in the MHz order of magnitude is of little significance.

[0005] In view of the large free-running linewidth of existing semiconductor lasers, the inapplicability of existing mainstream PDH to semiconductor lasers, and the frequency scanning requirements in practical applications of semiconductor lasers, it is necessary to provide a method for frequency stabilization and linewidth narrowing of semiconductor lasers. Summary of the Invention

[0006] The object of the present invention is to provide a method for frequency stabilization and linewidth narrowing of a semiconductor laser to solve the problems of the difficulty in applying existing semiconductor lasers in PDH frequency locking technology and practical frequency scanning.

[0007] To this end, the present invention provides a method for frequency stabilization and linewidth narrowing of a semiconductor laser, including the steps of: S1. Perform the first beam splitting of the beam of the DFB laser for transmission and reflection. For the reflected beam in the first beam splitting, reflect it back to the resonator of the laser, and suppress the random phase noise generated by spontaneous emission through weak optical feedback; S2. Perform the second beam splitting of the transmitted beam after phase noise suppression for transmission and reflection. For the transmitted beam in the second beam splitting, first isolate it and then pass it through an electro-optic phase modulator, and then couple the generated carrier to an ultra-stable cavity. Among them, the interference signal generated by the carrier and the sideband reflection signal is used as the frequency discrimination signal, and the carrier frequency is locked to the longitudinal mode frequency of the ultra-stable cavity; S3. Couple the reflected beam in the second beam splitting to an IQ modulator to generate carrier-suppressed single-sideband laser. Among them, the frequency difference between the single-sideband and the carrier is determined by the frequency of the radio frequency signal. By changing the frequency of the radio frequency signal, a narrow-linewidth frequency ultra-stable light source with tunable frequency is obtained.

[0008] First, the present invention uses a plane mirror optical feedback to narrow the linewidth of a semiconductor laser to below 100 kHz, and then cooperates with an external resonant cavity with a 100 kHz linewidth to achieve a laser frequency output with a linewidth below 50 kHz. On this basis, combined with the carrier suppression single sideband scanning technology, the scanning of the laser frequency is realized, and a laser output with a narrow linewidth, high stability, and tunability is obtained.

[0009] In addition to the purposes, features, and advantages described above, the present invention has other purposes, features, and advantages. The following will refer to the drawings to further elaborate on the present invention in detail. Description of the Drawings

[0010] The specification drawings forming a part of this application are used to provide a further understanding of the present invention. The schematic embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation of the present invention. In the drawings:

[0011] Figure 1 is a schematic structural diagram of a DFB laser frequency stabilization and linewidth narrowing system;

[0012] Figure 2 is a schematic structural diagram of a plane mirror optical feedback laser phase noise suppression subsystem;

[0013] Figure 3 is a schematic structural diagram of an ultra-stable cavity phase modulation optical heterodyne frequency stabilization subsystem;

[0014] Figure 4 is a schematic structural diagram of a carrier suppression single sideband frequency shift laser frequency tuning subsystem;

[0015] Figure 5 is a schematic structural diagram of the DFB laser frequency stabilization and linewidth narrowing system according to an embodiment of the present invention;

[0016] Figure 6 shows the drift amount of the laser output frequency before and after frequency locking;

[0017] Figure 7 shows the frequency noise spectrum of the laser output before and after linewidth narrowing measured based on the β-line segmentation method;

[0018] Figure 8 shows the laser wavelength scanning result. Detailed Embodiments

[0019] The following will refer to the drawings and combine with embodiments to elaborate on the present invention in detail.

[0020] With reference to Figures 1 to 8 , the method for stabilizing the frequency and narrowing the linewidth of a semiconductor laser according to the present invention includes the following steps S1 - S3.

[0021] S1. Perform the first beam splitting on the beam of the DFB laser for transmission and reflection. For the reflected beam in the first beam splitting, reflect it back into the resonator of the laser to increase the mode competition advantage of photons with the same phase in the internal resonator of the laser and suppress the random phase noise generated by spontaneous emission.

[0022] S2. Perform the second beam splitting on the transmitted beam after phase noise suppression for transmission and reflection. For the transmitted beam in the second beam splitting, first isolate it and then pass it through an electro-optic phase modulator. Then, couple the generated carrier to the ultra-stable cavity. Among them, the interference signal generated by the carrier and the sideband reflection signal is used as the frequency discrimination signal, and the carrier frequency is locked to the longitudinal mode frequency of the ultra-stable cavity.

[0023] S3. Couple the reflected beam in the second beam splitting to the IQ modulator to generate carrier-suppressed single-sideband laser. Among them, the frequency difference between the single-sideband and the carrier is determined by the frequency of the radio frequency signal. By changing the frequency of the radio frequency signal, a frequency-tunable narrow linewidth frequency ultra-stable light source is obtained.

[0024] The semiconductor laser frequency stabilization and linewidth narrowing system of this embodiment includes a plane mirror optical feedback laser phase noise suppression subsystem, an ultra-stable cavity phase modulation optical heterodyne frequency stabilization subsystem, and a carrier-suppressed single-sideband frequency shift laser frequency tuning subsystem.

[0025] The semiconductor laser is split into three laser beams by a beam splitter, and is respectively coupled with the plane mirror optical feedback laser phase noise suppression subsystem through free space transmission, coupled with the ultra-stable cavity phase modulation optical heterodyne frequency stabilization subsystem through free space transmission, and coupled with the carrier-suppressed single-sideband frequency shift laser frequency tuning subsystem through a single-mode optical fiber. A frequency-tunable narrow linewidth ultra-stable light source is obtained at the output end of the carrier-suppressed single-sideband frequency shift laser frequency tuning subsystem.

[0026] Under the action of weak feedback of external reflected light, the semiconductor laser increases the mode competition advantage of photons with the same phase in the internal resonator of the laser, and will suppress the random phase noise generated by spontaneous emission.

[0027] For the laser after optical feedback phase noise suppression, couple the carrier generated by passing through the electro-optic phase modulator to the ultra-stable cavity, and use the interference signal generated by the carrier and the sideband reflection signal as the frequency discrimination signal to lock the carrier frequency to the longitudinal mode frequency of the ultra-stable cavity.

[0028] The laser after phase noise suppression and frequency stabilization is coupled to the IQ modulator. The five electro-optic phase modulators inside the IQ modulator generate carrier-suppressed single-sideband laser under the action of a bias controller. The frequency difference between the single-sideband and the carrier is determined by the frequency of the radio frequency signal. By changing the frequency of the radio frequency signal, a frequency-tunable narrow linewidth frequency ultra-stable light source can be obtained.

[0029] The light output from a 1550 nm DFB semiconductor laser passes through a half-wave plate (λ / 2). By rotating the λ / 2 wave plate, 90% of the laser light passes through the polarization beam splitter, and 10% of the light passes through the polarization beam splitter 1, then passes through a quarter-wave plate (λ / 4) and is reflected to the mirror.

[0030] The mirror is installed on Figure 2 the displacement adjustment stage as shown. The displacement adjustment stage includes a linear sliding guide, a guide rail slider, a three-dimensional adjustment frame provided on the guide rail slider, and a piezoelectric ceramic mounted on the three-dimensional adjustment frame. The mirror is provided on the piezoelectric ceramic. The piezoelectric ceramic can be controlled to adjust the telescopic amount to achieve the effect of fine-tuning the distance between the mirror and the laser.

[0031] The light reflected by the mirror passes through the λ / 4 wave plate twice and will be completely reflected back into the laser inside the polarization beam splitter 1, realizing the suppression of laser phase noise.

[0032] The transmitted light of the polarization beam splitter 1 passes through the λ / 2 wave plate. By adjusting the polarization direction of the λ / 2 wave plate, 50% of the light passing through the polarization beam splitter 2 is reflected and 50% of the light is transmitted. The reflected light enters the carrier suppression single-sideband frequency shift laser frequency tuning system, and the transmitted light enters the electro-optic phase modulation laser frequency stabilization system.

[0033] The transmitted light passes through an isolator. The isolator isolates all reflected light and prevents the light from feeding back into the laser.

[0034] As Figure 3 shown, the light passing through the isolator passes through an electro-optic modulator. The electro-optic modulator is driven by a radio frequency source with a driving frequency of 30 MHz.

[0035] The light passing through the electro-optic phase modulator then passes through a λ / 2 wave plate and a polarization beam splitter 3. By adjusting the polarization angle of the λ / 2 wave plate, the transmitted light of the polarization beam splitting prism is maximized.

[0036] The transmitted light of the polarization beam splitter 3 then passes through a λ / 4 wave plate and a matching lens and is coupled to a certain longitudinal mode of the ultra-stable cavity.

[0037] The reflected light from the front mirror of the ultra-stable cavity includes direct reflected light and leakage light inside the resonant cavity. This part of the light passes through the λ / 4 wave plate again, forming light with a polarization direction perpendicular to the incident light and being reflected at the polarization beam splitter 3.

[0038] The reflected light is captured by the photodetector 1 and demodulated under the action of the mixer with the radio frequency source signal driving the electro-optic modulator to obtain an error signal.

[0039] The radio frequency signal source passes through a phase modulator. The phase modulator is adjusted so that the frequency discrimination error of the error signal is maximized.

[0040] The error signal generates a DC signal after passing through a low-pass filter. After passing through a PID controller, it is superimposed on the current signal driving the laser to form a negative feedback loop, achieving the locking of the laser frequency to the longitudinal mode of the resonant cavity. The PID controller (i.e., the adder) adds the error signal to the laser driving current to eliminate the frequency jitter of the laser. In the actual experimental system, behind the filter is the PID control and the laser current controller.

[0041] The reflected light of the polarization beam splitter 2 is coupled to the fiber receiving lens, changing from free space transmission to single-mode fiber transmission.

[0042] As Figure 4 shown, the single-mode fiber is connected to the IQ modulator.

[0043] The RF source in the carrier-suppressed single-sideband frequency-shifted laser frequency tuning system generates two in-phase RF signals of 0 - 20 GHz.

[0044] One of them is directly connected to the IQ modulator through the RF driver 1, and the other is connected to the IQ modulator through the RF driver 2 after passing through a 90-degree phase shifter.

[0045] The output light of the IQ modulator passes through a proportional beam splitter. 1% of the light enters the bias controller to control the three bias voltages of the IQ modulation, forming a carrier-suppressed single-sideband laser, and 99% of the light is output as a narrow-linewidth ultra-stable light source.

[0046] Changing the frequency of the RF source in the carrier-suppressed single-sideband frequency-shifted laser frequency tuning system will change the frequency of the output laser. The frequency difference between the output laser and the output of the DFB laser is determined by the frequency of the RF source.

[0047] The system of the present invention obtains a frequency-tunable narrow-linewidth ultra-stable light source at the output end of the carrier-suppressed single-sideband frequency-shifted laser frequency tuning subsystem.

[0048] Figure 6 Shows the drift amount of the laser output frequency before and after frequency locking. Measuring equipment: Bristol 520 - 1680 nm wavelength meter. From Figure 6 it can be seen that the drift of the laser output frequency is suppressed after frequency locking; Figure 7 Shows the frequency noise spectrum of the laser output before and after linewidth narrowing measured based on the β-line splitting method. From Figure 7 it can be seen that the free-running output laser linewidth is 33 MHz, and the output laser linewidth after linewidth narrowing is 26 kHz. Among them, the ordinate FNPSD represents the frequency noise power spectral density, with the unit Hz 2 / Hz. Figure 8The wavelength scanning results are shown. Measuring device: Bristol wavelength meter with a wavelength range of 520 - 1680 nm, indicating that within the modulation frequency range of 0 - 20 GHz, the single-sideband scanning range can respond linearly, showing that the laser after frequency stabilization and linewidth narrowing has a continuous frequency scanning function.

[0049] The above are only embodiments of the present invention and are not used to limit the present invention. For those skilled in the art, the present invention can have various changes and modifications. Any modification, replacement, improvement, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A method for frequency stabilization and linewidth narrowing of a semiconductor laser, characterized in that, Including the steps: S1. Perform the first beam splitting of the beam of the DFB laser for transmission and reflection. For the reflected beam in the first beam splitting, reflect it back to the resonator of the laser, and suppress the random phase noise generated by spontaneous emission through weak optical feedback; S2. Perform the second beam splitting of the transmitted beam after phase noise suppression for transmission and reflection. For the transmitted beam in the second beam splitting, first isolate it and then pass it through an electro-optic phase modulator, and then couple the generated carrier to the ultra-stable cavity. Among them, the interference signal generated by the carrier and the sideband reflection signal is used as the frequency discrimination signal, and the carrier frequency is locked to the longitudinal mode frequency of the ultra-stable cavity; S3. Couple the reflected beam in the second beam splitting to the IQ modulator to generate carrier-suppressed single-sideband laser. Among them, the frequency difference between the single sideband and the carrier is determined by the frequency of the radio frequency signal. By changing the frequency of the radio frequency signal, a frequency-tunable narrow linewidth frequency ultra-stable light source is obtained.

2. The method for stabilizing the frequency and narrowing the linewidth of a semiconductor laser according to claim 1, wherein Step S1 is implemented by a planar mirror optical feedback laser phase noise suppression subsystem.

3. The method for stabilizing the frequency and narrowing the linewidth of a semiconductor laser according to claim 2, wherein The planar mirror optical feedback laser phase noise suppression subsystem includes a λ / 2 wave plate, a polarization beam splitter 1 arranged in sequence along the optical path of the laser, a λ / 4 wave plate arranged in sequence on the reflected optical path of the polarization beam splitter, and a displaceable mirror, wherein the reflected light of the mirror is reflected back to the internal resonator of the laser to achieve laser phase noise suppression.

4. The method for stabilizing the frequency and narrowing the linewidth of a semiconductor laser according to claim 3, wherein By rotating the λ / 2 wave plate and the λ / 4 wave plate, adjust the ratio and size between the transmitted beam and the reflected beam split by the polarization beam splitter 1.

5. The method for stabilizing the frequency and narrowing the linewidth of a semiconductor laser according to claim 2, characterized in that, The planar mirror optical feedback laser phase noise suppression subsystem further includes a displacement adjustment stage. The displacement adjustment stage includes a linear sliding guide, a guide rail slider, a three-dimensional adjustment frame arranged on the guide rail slider, and a piezoelectric ceramic installed on the three-dimensional adjustment frame, wherein the mirror is arranged on the piezoelectric ceramic.

6. The method for stabilizing the frequency and narrowing the linewidth of a semiconductor laser according to claim 1, characterized in that, Step S2 is implemented by an ultra-stable cavity phase modulation optical heterodyne frequency stabilization subsystem.

7. The method for stabilizing the frequency and narrowing the linewidth of a semiconductor laser according to claim 6, characterized in that The ultra-stable cavity phase modulation optical heterodyne frequency stabilization subsystem includes an isolator, an electro-optic modulator, a λ / 2 wave plate, a polarization beam splitter 3, a λ / 4 wave plate on the transmitted optical path of the polarization beam splitter 3, a lens, an ultra-stable cavity, a photodetector on the reflected optical path of the polarization beam splitter 3, a mixer, a low-pass filter, a PID controller. Among them, the reflected light of the front mirror of the ultra-stable cavity is reflected at the polarization beam splitter 3, and the reflected light is captured by the photodetector 1 and demodulated with the radio frequency source signal driving the electro-optic modulator under the action of the mixer to obtain an error signal. The error signal generates a DC signal after passing through the low-pass filter, and after passing through the PID controller, it is superimposed on the current signal driving the laser to form a negative feedback loop to achieve locking the laser frequency to the longitudinal mode of the resonator.

8. The method for stabilizing the frequency and narrowing the linewidth of a semiconductor laser according to claim 7, characterized in that, It further includes a phase modulator. Among them, the radio frequency source signal driving the electro-optic modulator passes through the phase modulator, and the phase modulator is adjusted to make the frequency discrimination error of the error signal the largest.

9. The method for frequency stabilization and linewidth narrowing of a semiconductor laser according to claim 1, characterized in that, Step S3 is implemented by a carrier-suppressed single-sideband frequency-shifted laser frequency tuning subsystem.

10. The method for stabilizing the frequency and narrowing the linewidth of a semiconductor laser according to claim 9, wherein The carrier-suppressed single-sideband frequency-shifted laser frequency tuning subsystem includes an optical fiber, an IQ modulator, a radio frequency signal source, a radio frequency driver 1, a radio frequency driver 2, a 90° phase shifter, an optical fiber coupler, and a proportional beam splitter. The reflected beam of the second beam splitting is coupled to a single-mode optical fiber, and the single-mode optical fiber is connected to the IQ modulator. The radio frequency signal source generates two signals. One signal passes through the radio frequency driver 1 and is connected to the IQ modulator, and the other signal passes through the 90-degree phase shifter and is then connected to the IQ modulator through the radio frequency driver 2. The output light of the IQ modulator passes through a proportional beam splitter, and 1% of the light enters the bias controller to control the three bias voltages of the IQ modulation, forming a carrier-suppressed single-sideband laser.

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