An ultra-narrow linewidth laser generating device

Abstract

The application discloses a kind of ultra-narrow linewidth laser generating device, the device includes optical self-injection locking loop and ultra-stable cavity phase-locked loop, optical self-injection locking loop improves the quality factor of laser resonator to narrow linewidth, ultra-stable cavity phase-locked loop locks the phase of light signal in optical self-injection, realizes laser frequency and optical resonator synchronization, further reduces the frequency noise of laser and improves the linewidth narrowing rate of laser by feedback control laser drive current;At the same time, due to the stable control of ultra-stable cavity phase-locked loop to laser frequency, the phenomenon of mode jump caused by laser frequency drift is also avoided, and the stability of the system is improved.Therefore, the device can provide stable narrow linewidth laser source for fiber communication, laser radar, industrial device processing, medical treatment and other fields.

Description

TECHNICAL FIELD

[0001] The application belongs to the field of optoelectronic technology, and particularly relates to an ultra-narrow linewidth laser generating device. BACKGROUND

[0002] Laser is short for light amplification by stimulated emission of radiation. It refers to that a particle at a high energy level is excited by an external photon, and jumps to a low energy level, and releases a photon beam highly consistent with the optical characteristics of the excitation photon. Therefore, laser has the advantages of monochromaticity and directionality compared with ordinary light sources, and is widely used in optical fiber communication, laser radar, industrial device processing, medical treatment and other fields. For an ideal laser, only stimulated emission exists, the energy loss of the high energy level atom is supplemented in the stimulated process, and the generated light wave is superimposed with the original light wave, so that the amplitude of the light wave in the resonant cavity always remains constant, and therefore the linewidth should tend to zero. However, due to the influence of spontaneous emission, the gain of stimulated emission in the laser is slightly smaller than the total loss in the cavity, and then the incoherent light wave is superimposed with the coherent light generated by the stimulated emission, resulting in the broadening of the linewidth of the laser. And in practical application, due to the process and environmental factors, the laser inevitably appears frequency drift and phase jitter, which is reflected in the power spectrum, that is, the so-called broadening. Taking coherent optical communication as an example, the broadening of the laser linewidth will cause the deterioration of the signal-to-noise ratio at the receiving end, and then affect the quality of communication. Therefore, narrow linewidth laser is an indispensable part of laser application. At present, the linewidth of the mature and commercial distributed feedback semiconductor laser is about megahertz, which is sufficient for general application, but it is slightly insufficient for some precise optical measurement and high-quality microwave photon signal generation and other applications. And the commercial narrow linewidth laser is very expensive, which greatly increases the cost of the system. Therefore, researchers hope to narrow the linewidth of the laser through other external structures.

[0003] Optical self-injection locking is a method to narrow the linewidth of a laser. It only needs to couple a part of the laser output back into the laser through a circulator to narrow the linewidth of the laser. Therefore, the optical self-injection locking system has a very simple structure and can achieve a high linewidth narrowing rate. However, the self-injection method is equivalent to increasing the ring length of the resonant cavity to obtain a higher resonant cavity quality factor (Q value). Therefore, the longer ring length will introduce more phase jitter, and at the same time, cause the free spectral range to decrease, more longitudinal modes will meet the starting condition of the oscillator, which will result in a lower side mode suppression ratio, which will greatly limit the frequency stability of the laser. Therefore, researchers propose that a narrow-band optical filter can be added to the feedback loop to filter out unwanted modes. Common narrow-band filters include Fabry-Perot resonant cavities, grating resonant cavities, whispering gallery mode resonant cavities, etc. However, the smaller free spectral range also puts higher requirements on the quality factor of the narrow-band filter, and with the increase of the feedback injection light, the linewidth narrowing rate of the optical self-injection locking will tend to be saturated, generally around several hundred to one thousand times. SUMMARY

[0004] In view of the above, the present application provides an ultra-narrow linewidth laser generating device. The device has a higher linewidth narrowing rate than optical self-injection locking and lower frequency noise, and avoids the use of very high Q value optical resonant cavities, reducing the cost of the system.

[0005] An ultra-narrow linewidth laser generating device, comprising an optical self-injection locking loop and an ultra-stable cavity phase-locked loop; the optical self-injection locking loop is used to improve the resonant quality factor of the laser resonant cavity and narrow the linewidth of the laser; the ultra-stable cavity phase-locked loop is used to stabilize the phase of the optical signal of the optical self-injection locking loop, and further reduce the frequency noise of the laser through feedback control of the laser current; the combination of the optical self-injection locking loop and the ultra-stable cavity phase-locked loop realizes the output of ultra-narrow linewidth laser with high stability of oscillation frequency.

[0006] The optical self-injection locking loop comprises a laser, a fiber circulator, two 1x2 fiber couplers, a phase modulator, an ultra-stable optical resonant cavity, a polarization controller, an erbium-doped fiber amplifier, an optical bandpass filter, and a piezoelectric ceramic controller; the ultra-stable cavity phase-locked loop comprises a laser, a fiber circulator, two 1x2 fiber couplers, a phase modulator, an ultra-stable optical resonant cavity, an avalanche photodiode, an electrical bandpass filter, an analog signal source, an electrical power divider, a mixer, a loop filter, an amplifier, a servo circuit, an addition circuit, and a constant current source; the optical self-injection locking loop and the ultra-stable cavity phase-locked loop have common components, including a laser, a fiber circulator, two 1x2 fiber couplers, a phase modulator, and an optical resonant cavity.

[0007] Further, the optical self-injection locking loop comprises:

[0008] a laser, which generates an optical signal L1 to a second port of a fiber loop;

[0009] a fiber loop, which realizes one-way transmission of the optical signal L1 from the second port of the fiber loop to a third port, and the output optical signal of the third port is denoted as L2, and the optical signal L2 is transmitted to a first 1x2 fiber coupler;

[0010] the first 1x2 fiber coupler is used for dividing the optical signal L2 into two optical signals L31 and L32, the optical signal L31 is transmitted to a phase modulator, and the optical signal L32 is a narrow linewidth laser signal output by the device;

[0011] the phase modulator modulates an electrical signal E22 onto the optical signal L31 to generate an optical signal L4, and the optical signal L4 is transmitted to an optical resonant cavity;

[0012] the optical resonant cavity, after the optical signal L4 passes through the optical resonant cavity, two sidebands of the optical signal L4 will produce different phase changes, at this time the optical signal is denoted as L5, and the optical signal L5 is transmitted to a second 1x2 fiber coupler;

[0013] the second 1x2 fiber coupler is used for dividing the optical signal L5 into two optical signals L61 and L62, the optical signal L61 is transmitted to an erbium-doped fiber amplifier, and the optical signal L62 is transmitted to an avalanche photodiode;

[0014] the erbium-doped fiber amplifier is used for amplifying the optical signal L61 into an optical signal L7, and the optical signal L7 is transmitted to an optical bandpass filter;

[0015] the optical bandpass filter is used for reducing the spontaneous emission noise introduced by the erbium-doped fiber amplifier in the optical signal L7, and outputs an optical signal L8 to a polarization controller;

[0016] the polarization controller is used for adjusting the polarization state of the optical self-injection locking loop, and the optical signal L8 is output as an optical signal L9 to a piezoelectric ceramic controller after passing through the polarization controller;

[0017] the piezoelectric ceramic controller is controlled by an electrical signal E7, can finely adjust the length of the optical self-injection locking loop, and outputs an optical signal L10 to the first port of the fiber loop;

[0018] the optical signal L10 enters the first port of the fiber loop and is transmitted one-way to the second port, thereby injecting back into the laser to form a complete optical self-injection locking loop;

[0019] the super-stable cavity phase-locked loop comprises:

[0020] laser, generating optical signal L1 to the second port of the fiber-optic circulator;

[0021] fiber-optic circulator, realizing one-way transmission of optical signal L1 from the second port of the fiber-optic circulator to the third port, the output optical signal of the third port being denoted as L2, and optical signal L2 being transmitted to the first 1×2 fiber-optic coupler;

[0022] the first 1×2 fiber-optic coupler, for splitting optical signal L2 into two optical signals L31 and L32, optical signal L31 being transmitted to the phase modulator, and optical signal L32 being the narrow linewidth laser signal output by the device;

[0023] the phase modulator, for modulating electrical signal E22 onto optical signal L31 to generate optical signal L4, and transmitting optical signal L4 to the optical resonant cavity;

[0024] the optical resonant cavity, for causing different phase changes of two sidebands of optical signal L4 after passing through the optical resonant cavity, at which time the optical signal is denoted as L5, and optical signal L5 being transmitted to the second 1×2 fiber-optic coupler;

[0025] the second 1×2 fiber-optic coupler, for splitting optical signal L5 into two optical signals L61 and L62, optical signal L61 being transmitted to the erbium-doped fiber amplifier, and optical signal L62 being transmitted to the avalanche photodiode;

[0026] the avalanche photodiode, for converting optical signal L62 into electrical signal E1, and transmitting electrical signal E1 to the electrical band-pass filter;

[0027] the electrical band-pass filter, for filtering electrical signal E1, and outputting electrical signal E2 to the RF port of the mixer;

[0028] the analog signal source, for generating reference electrical signal E3, and transmitting electrical signal E3 to the electrical power divider;

[0029] the electrical power divider, for splitting electrical signal E3 into electrical signals E41 and E42 which are equal in power and have a phase difference of 90°, electrical signal E42 being transmitted to the phase modulator, and electrical signal E41 being transmitted to the local oscillator port of the mixer;

[0030] the mixer, for comparing the phase difference between electrical signal E41 and electrical signal E2 to obtain electrical signal E5, and transmitting electrical signal E5 to the loop filter;

[0031] the loop filter, for performing integral processing on electrical signal E5 to generate electrical signals E61 and E62, electrical signal E61 being transmitted to the amplifier, and electrical signal E62 being transmitted to the servo circuit;

[0032] the amplifier, for amplifying control signal E61 to generate electrical signal E7 to control the piezoelectric ceramic controller;

[0033] Servo circuit, which converts the electrical signal E62 into the current signal E8, and transmits to the addition circuit;

[0034] Addition circuit, which adds the current signal E8 and the current signal E9 output by the constant current source to obtain the current E10 for feedback control of the laser;

[0035] The electrical signal E7 is used to control the piezoelectric ceramic controller to control the phase of the optical self-injection locking loop optical signal, and the electrical signal E10 is used to feedback control the current of the laser to reduce the phase noise of the laser, both of which realize the closed-loop control of the super-stable cavity phase-locked loop.

[0036] The device of the present application comprises an optical self-injection locking loop and a super-stable cavity phase-locked loop, the optical self-injection locking loop improves the quality factor of the laser resonant cavity to narrow the linewidth, and the super-stable cavity phase-locked loop adjusts the loop length of the optical self-injection locking and the driving current of the laser, realizes the synchronization of the laser frequency and the optical resonant cavity, thereby further reduces the frequency noise of the laser and improves the linewidth narrowing rate of the laser; at the same time, due to the stable control of the super-stable cavity phase-locked loop on the laser frequency, the phenomenon of mode jumping caused by the frequency drift of the laser is also avoided, and the stability of the system is improved. Therefore, the device of the present application can provide a stable narrow-linewidth laser source for the fields of optical fiber communication, laser radar, industrial device processing, medical treatment, etc. BRIEF DESCRIPTION OF DRAWINGS

[0037] Figure 1 It is a specific structure diagram of the device of the present application.

[0038] In the figure: 1—laser, 2—optical fiber circulator, 3—first 1×2 optical coupler, 4—phase modulator, 5—optical resonant cavity, 6—second 1×2 optical coupler, 7—erbium-doped fiber amplifier, 8—optical band-pass filter, 9—polarization controller, 10—piezoelectric ceramic controller, 11—avalanche photodiode, 12—electrical band-pass filter, 13—mixer, 14—analog signal source, 15—electrical power divider, 16—loop filter, 17—electrical amplifier, 18—servo circuit, 19—addition circuit, 20—constant current source.

[0039] Figure 2 It is an s-domain model block diagram of the device of the present application. DETAILED DESCRIPTION

[0040] In order to more specifically describe the present application, the technical solutions of the present application are described in detail below in combination with the drawings and specific embodiments.

[0041] As Figure 1As shown, the optical self-injection locking loop is composed of a laser 1, a fiber loop 2, a first 1x2 fiber coupler 3, a phase modulator 4, an optical resonator 5, a second 1x2 fiber coupler 6, an erbium-doped fiber amplifier 7, an optical band-pass filter 8, a polarization controller 9, and a piezoelectric ceramic controller 10, which are connected by optical fibers; the light output from the laser is returned to the inside of the laser after passing through the optical self-injection locking loop, which increases the ring length of the laser resonator and further improves the quality factor of the laser, thereby reducing the linewidth of the laser; however, due to the effects of the increased ring length and gain competition, the adjacent modes can also meet the oscillation condition, resulting in a jump of the oscillation mode, and therefore the optical resonator in the loop filters the unnecessary modes, which can improve the stability of the system; in the composition of the super-stable cavity phase-locked loop, the laser 1, the fiber loop 2, the first 1x2 fiber coupler 3, the phase modulator 4, the optical resonator 5, the second 1x2 fiber coupler 6, and the avalanche photodiode 11 are connected by optical fibers, the electrical band-pass filter 12, the analog signal source 14, the electrical power divider 15, and the mixer 13 are connected by coaxial cables, and the loop filter 16, the amplifier 17, the servo circuit 18, the addition circuit 19, and the constant current source 20 are connected by copper wires.

[0042] The specific working mode is as follows: first, the frequency of the laser is adjusted to the vicinity of the resonant frequency of the super-stable optical cavity, the super-stable cavity phase-locked loop converts the information of the frequency offset of the laser from the optical resonator into a voltage signal, and at the same time, when the laser is optically self-injection locked, the frequency of the laser will meet the oscillation condition after the ring length is increased, i.e., determined by the ring length, so that the laser frequency can be synchronized with the optical resonator by fine-tuning the ring length, the frequency noise of the laser caused by the ring jitter is reduced, and the high-frequency component of the control signal is used to control the driving current of the laser, thereby reducing the frequency noise caused by the current noise; therefore, the frequency noise of the laser can be further suppressed, i.e., the linewidth of the laser is further narrowed.

[0043] In this embodiment, the s-domain model is as shown in the figure. Figure 2 As can be seen from the figure, when there is no control of the super-stable cavity phase-locked loop, the input noise after passing through the optical self-injection locking structure is the output noise of the system, and therefore the gain of the noise suppression is K SIL , and K SIL is much smaller than 1, indicating that the optical self-injection locking suppresses the frequency noise. After the super-stable cavity phase-locked loop is added for feedback control, the frequency noise suppression gain of the closed-loop system can be derived as follows:

[0044]

[0045] wherein represents the output frequency noise of the laser, represents the input frequency noise of the laser, and KPDH representing the frequency discrimination gain of the super-stable phase-locked loop, LF representing the transfer function of the loop filter, VTF representing the frequency modulation response of the laser current, PZT representing the frequency response of the piezoelectric ceramic controller, LTF representing the response of the laser frequency to the loop length in the optical self-injection locking; thus, it can be obtained that, compared with a single optical self-injection locking system, the optical phase noise suppression ratio of the structure proposed by the present application can be increased by 1+K PDH F LF (K SIL F VTF +K LTF F PZT times.

[0046] The above description of the embodiments is for the purpose of enabling a person of ordinary skill in the art to understand and apply the present application. It is obvious for those skilled in the art to make various modifications to the above embodiments and apply the general principles described herein to other embodiments without creative labor. Therefore, the present application is not limited to the above embodiments, and the improvements and modifications made to the present application by those skilled in the art according to the disclosure of the present application should be within the protection scope of the present application.

Claims

1. An ultra-narrow linewidth laser generating device, comprising an optical self-injection locking loop and an ultra-stable cavity phase-locked loop; the optical self-injection locking loop is used to improve the resonance quality factor of the laser resonant cavity and narrow the linewidth of the laser; the ultra-stable cavity phase-locked loop is used to stabilize the phase of the optical signal of the optical self-injection locking loop, and further reduce the frequency noise of the laser by feedback control of the laser current; the combination of the optical self-injection locking loop and the ultra-stable cavity phase-locked loop realizes the output of the ultra-narrow linewidth laser with high frequency stability; The optical self-injection locking loop comprises: a laser, generating an optical signal L1 to a second port of a fiber circulator; the fiber circulator, realizing one-way transmission of the optical signal L1 from the second port to the third port of the fiber circulator, the output optical signal of the third port being recorded as L2, and the optical signal L2 being transmitted to a first 1×2 optical fiber coupler; the first 1×2 optical fiber coupler, used to divide the optical signal L2 into two optical signals L31 and L32, the optical signal L31 being transmitted to a phase modulator, and the optical signal L32 being a narrow linewidth laser signal output by the device; the phase modulator, modulating an electrical signal E22 onto the optical signal L31 to generate an optical signal L4, and transmitting the optical signal L4 to an optical resonant cavity; the optical resonant cavity, after the optical signal L4 passes through the optical resonant cavity, two sidebands of the optical signal L4 will produce different phase changes, at this time the optical signal is recorded as L5, and the optical signal L5 is transmitted to a second 1×2 optical fiber coupler; the second 1×2 optical fiber coupler, used to divide the optical signal L5 into two optical signals L61 and L62, the optical signal L61 being transmitted to an erbium-doped fiber amplifier, and the optical signal L62 being transmitted to an avalanche photodiode; the erbium-doped fiber amplifier, used to amplify the optical signal L61 into an optical signal L7, and the optical signal L7 being transmitted to an optical bandpass filter; the optical bandpass filter, used to reduce the spontaneous emission noise introduced by the erbium-doped fiber amplifier in the optical signal L7, and output an optical signal L8 to a polarization controller; the polarization controller, used to adjust the polarization state of the optical self-injection locking loop, and output an optical signal L9 to a piezoelectric ceramic controller after the optical signal L8 passes through the polarization controller; the piezoelectric ceramic controller, controlled by an electrical signal E7, can fine-tune the length of the optical self-injection locking loop, and output an optical signal L10 to the first port of the fiber circulator; the optical signal L10 enters the first port of the fiber circulator and is transmitted one-way to the second port, thereby injecting back into the laser to form a complete optical self-injection locking loop.

2. The generating device of claim 1, wherein: The ultra-stable cavity phase-locked loop comprises: a laser, generating an optical signal L1 to a second port of a fiber circulator; the fiber circulator, realizing one-way transmission of the optical signal L1 from the second port to the third port of the fiber circulator, the output optical signal of the third port being recorded as L2, and the optical signal L2 being transmitted to a first 1×2 optical fiber coupler; the first 1×2 optical fiber coupler, used to divide the optical signal L2 into two optical signals L31 and L32, the optical signal L31 being transmitted to a phase modulator, and the optical signal L32 being a narrow linewidth laser signal output by the device; the phase modulator, modulating an electrical signal E22 onto the optical signal L31 to generate an optical signal L4, and transmitting the optical signal L4 to an optical resonant cavity; An optical resonant cavity, after the optical signal L4 passes through the optical resonant cavity, two sidebands of the optical signal L4 will produce different phase changes, at this time the optical signal is recorded as L5, and the optical signal L5 is transmitted to the second 1×2 optical fiber coupler; The second 1×2 optical fiber coupler is used to divide the optical signal L5 into two optical signals L61 and L62, the optical signal L61 is transmitted to the erbium-doped fiber amplifier, and the optical signal L62 is transmitted to the avalanche photodiode; The avalanche photodiode is used to convert the optical signal L62 into an electrical signal E1 and transmit it to the electrical band-pass filter; The electrical band-pass filter filters the electrical signal E1 and outputs an electrical signal E2 to the mixer radio frequency port; An analog signal source is used to generate a reference electrical signal E3, which is transmitted to the electrical power divider; The electrical power divider divides the electrical signal E3 into electrical signals E41 and E42 with equal power and a phase difference of 90°, the electrical signal E42 is transmitted to the phase modulator, and the electrical signal E41 is transmitted to the local oscillator port of the mixer; The mixer is used to compare the phase difference between the electrical signal E41 and the electrical signal E2, and obtains an electrical signal E5, which is transmitted to the loop filter; The loop filter integrates the electrical signal E5 to generate electrical signals E61 and E62, the electrical signal E61 is transmitted to the amplifier, and the electrical signal E62 is transmitted to the servo circuit; The amplifier amplifies the electrical signal E61 to generate an electrical signal E7 to control the piezoelectric ceramic controller; The servo circuit converts the electrical signal E62 into a current signal E8 and transmits it to the summing circuit; The summing circuit adds the current signal E8 and the current signal E9 output by the constant current source to obtain an electrical signal E10 for feedback control of the laser; The electrical signal E7 is used to control the piezoelectric ceramic controller to control the phase of the optical self-injection locked loop optical signal, and the electrical signal E10 is used to feedback control the current of the laser to reduce the phase noise of the laser, both of which realize the closed-loop control of the ultra-stable cavity phase-locked loop.

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