CN207217989U - A two-stage weakly modulated F‑P cavity - Google Patents

Abstract

The utility model discloses a kind of weak modulation F P chambers of two-part, including two sections of different optical fiber, completely reflecting mirror and dichroic mirror；Two sections of different optical fiber are connected by way of physics docking between each other；Two sections of respective other ends of optical fiber are placed with dichroic mirror and completely reflecting mirror respectively；The burnishing surface of dichroic mirror and completely reflecting mirror fiber alignments different from two sections respectively constitutes two weak modulation F P chambers for having interaction；Wherein, dichroic mirror is more than 90% to the transmission of pumping light power, is less than 10% to the transmission of flashlight；The length of two sections of different optical fiber, which is respectively less than, is equal to 10cm.The utility model breaches limitation of the conventional method to F P fineness of cavity, by suppression of the fan-shaped envelope to longitudinal mode caused by two F P chambers interactions, suppresses fundamental frequency, directly generates two stable frequencys multiplication.The utility model is simple and compact for structure, and modulation depth have dropped two orders of magnitude, and the realization, particle in astronomicalc optics frequency comb accelerate integrated chip of laser system etc., are with a wide range of applications.

Description

A Two-stage Weakly Modulated F-P Cavity

technical field

The utility model relates to the technical field of high-precision F-P cavity, in particular to a two-stage weakly modulated Fabry-Perot cavity (hereinafter referred to as F-P cavity) capable of doubling the pulse repetition frequency.

Background technique

In order to meet the needs of spectral calibration in astronomical exploration and better realize the precise detection of Earth-like planets, how to double the pulse repetition frequency of laser output has always been a key topic of concern. In recent years, people's attention to outer space has been increasing, and the research on repetition rate doubling technology has gradually become a research hotspot.

Repetition frequency doubling technology has two ideas: one is to introduce two parallel dichroic mirrors outside the cavity, and increase the repetition rate by constructing an extra-cavity F-P cavity. This method was first proposed by Sizer et al. in 1989, and later in It has been confirmed in a series of experiments; the second is to construct an F-P cavity or an equivalent F-P cavity in the cavity to directly realize the ultra-high repetition frequency pulse output. Although the current repetition frequency doubling technology has made some progress in the direction of high repetition frequency broadband spectrum, the existing high-precision F-P cavity filters are prone to detuning and complex feedback control systems due to their frequency selectivity. , Irregular pulse trains and other issues have many limitations in practical applications.

For an FP cavity with mirrors with reflectivity R ₁ and R ₂ placed at both ends, its fineness can be expressed as: F=π(R ₁ R ₂ ) ^1/4 /(1-R ₁ R ₂ ^{1 /2} ). At present, the FP cavity filter used to increase the pulse repetition frequency generally has a fineness in the range of 10 ² to 10 ⁴ in order to effectively suppress unwanted pulses. Although this suppression is doubled when pulses travel back and forth within the FP cavity, it has been reported that an FP cavity fineness of at least 400 is required to achieve 50 dB of anharmonic suppression. However, high finesse creates difficulties in keeping the peaks of the filter transmission consistent with the desired mode, and its cumulative effect causes phase shifts.

In order to solve these problems and overcome the defects of the high-precision F-P cavity in doubling the pulse repetition frequency, the utility model proposes a new type of two-stage weakly modulated F-P cavity, which breaks through the limit of the previous fineness requirements, allowing fine The accuracy is reduced by two orders of magnitude, and it is possible to directly generate frequency doubling with a high signal-to-noise ratio comparable to the quality of a high-definition F-P cavity in a mode-locked laser.

Utility model content

One of the purposes of this utility model is to propose a weakly modulated F-P cavity located in the laser cavity that can double the pulse repetition frequency, and its weakly modulated depth is two orders of magnitude lower than that of the traditional F-P cavity reported.

The purpose of this utility model is achieved by at least one of the following solutions.

A two-stage weak modulation F-P cavity, including two different optical fibers, a total reflection mirror and a dichroic mirror; the two ends of the two different optical fibers are mirror-polished, and the two different optical fibers are physically connected to each other. Connection; the other ends of two different optical fibers are respectively placed with a dichroic mirror and a total reflection mirror; the dichromatic mirror and the total reflection mirror are respectively connected to the polished surfaces of two different optical fibers to form two interacting weakly modulated F-P A cavity; wherein, the dichromatic mirror transmits more than 90% of the pump light power and less than 10% of the signal light; the lengths of the two different optical fibers are both less than or equal to 10cm.

Furthermore, after the pump light is input, the two F-P cavities interact to generate a fan-shaped periodic envelope; this envelope can suppress the fundamental frequency and directly generate double frequency.

Further, the modulation depths are all less than 10%, and the fineness is less than 100.

Further, the two different sections of optical fiber are respectively an active optical fiber and a passive optical fiber. After the two ends of the two different optical fibers are polished into a mirror surface, one of the ends is connected to each other by mechanical docking; the active optical fiber is not connected to the passive optical fiber. The other end of the source fiber connection is placed with a total reflection mirror; the other end of the passive fiber that is not connected to the active fiber is placed with a dichromatic mirror; the active fiber and the passive fiber form two closely connected and interacting weak Modulation F-P cavity.

Further, the described two-stage weakly modulated F-P cavity is a dichromatic mirror from left to right, the first ceramic tube, common single-mode optical fiber, the second ceramic tube, the third ceramic tube, the gain fiber, the fourth Ceramic tubes and total reflection mirrors; the inner diameter of each ceramic tube is the same as the outer diameter of the optical fiber, and ordinary single-mode optical fiber and gain optical fiber are passed through the ceramic tube; the end faces of each ceramic tube are polished and polished; the dichroic mirror pairs the pump The transmittance of the pump light is 95%, and the transmittance of the signal light is 5%; the flat end surface of the dichromatic mirror and the second ceramic tube constitutes the first weakly modulated F-P cavity, and the total reflection mirror and the third ceramic tube The flat end face constitutes the second weakly modulated F-P cavity; the second ceramic tube and the third ceramic tube are mechanically connected to connect two sections of optical fiber, that is, a common single-mode optical fiber and a gain optical fiber.

Further, the described two-stage weakly modulated F-P cavity is a dichromatic mirror, a first glass tube, a common single-mode fiber, a second glass tube, a gain fiber, and a total reflection mirror from left to right; wherein, each The inner diameter of the glass tube is the same as the outer diameter of the optical fiber. The optical fiber passes through the glass tube. The end surface of the glass tube is ground and then mirror polished; the transmittance of the dichromatic mirror to the pump light is 90%, and the transmittance of the signal light to The ratio is 5%; the dichromatic mirror and the flat end surface of the other end of the first glass tube form the first weak modulation F-P cavity, and the total reflection mirror and the flat end surface of the second glass tube form the second weak modulation F-P cavity; A glass tube and a second glass tube are mechanically connected to connect the two sections of optical fibers together.

Compared with the prior art, the utility model has the following advantages:

The utility model proposes a new method for doubling the pulse repetition frequency by constructing a two-stage weakly modulated F-P cavity. It breaks through the limitation of the traditional method on the fineness of F-P cavity, suppresses the generation of the fundamental frequency through the fan-shaped envelope generated by the interaction of two F-P cavities on the longitudinal mode, and directly realizes the doubling of the repetition frequency. Compared with the ordinary F-P cavity, the two-stage F-P cavity has a simple and compact structure, and the modulation depth is reduced by two orders of magnitude. It has broad application prospects in the realization of astronomical optical frequency combs and the chip integration of particle-accelerated laser systems.

Description of drawings

Fig. 1 is a diagram of the two-stage F-P chamber structure device in Embodiment 1 of the present utility model.

Fig. 2 is a diagram of the two-stage F-P cavity structure device in Embodiment 2 of the utility model.

Fig. 3 is a diagram of the two-stage F-P cavity structure device in the test example of the utility model.

Fig. 4 is a spectrum diagram of the output pulse in the test example.

Fig. 5 is a time-domain diagram of the output pulse in the test example.

Fig. 6 is an autocorrelation diagram of output pulses in the test example.

Fig. 7 is a reflection curve diagram of the F-P cavity 1 in the test example.

Fig. 8 is a reflection curve diagram of F-P cavity 2 in the test example.

Fig. 9 is a result diagram of the interaction of two F-P cavities in the test case.

Detailed ways

Below in conjunction with embodiment and accompanying drawing, the utility model is described in further detail. However, the embodiments of the present invention are not limited thereto.

Example 1

Fig. 1 is the structural diagram of the two-stage F-P cavity of the present embodiment, and from left to right are the dichromatic mirror 1, the first ceramic tube 2, the common single-mode optical fiber (SMF 28e) 3, the second ceramic tube 4, the third Ceramic tube 5, gain optical fiber 6, fourth ceramic tube 7, total reflection mirror 8; wherein the inner diameter of each ceramic tube is the same as the outer diameter of the optical fiber, and the common single-mode optical fiber and gain optical fiber are passed in the ceramic tube; the end faces of each ceramic tube are ground After flattening, carry out mirror polishing treatment; the transmittance of the dichromatic mirror to the pump light is 95%, and the transmittance to the signal light is 5%; the dichromatic mirror and the flat end surface of the second ceramic tube 4 constitute the first weak The modulation F-P cavity 1, the flat end surface of the total reflection mirror and the third ceramic tube 5 constitutes the second weak modulation F-P cavity 2; the second ceramic tube 4 and the third ceramic tube 5 are mechanically connected, and the two sections of optical fibers are ordinary Single-mode fiber and gain fiber are connected together.

The length L ₁ of the ordinary single-mode fiber used in this example is 5 cm, and the length L ₂ of the gain fiber is 6 cm.

Example 2

Fig. 2 is the structural diagram of the two-stage F-P cavity of the present embodiment, and from left to right are dichromatic mirror 1, first glass tube 202, ordinary single-mode fiber (SMF 28e) 3, second glass tube 204, gain fiber 5. Total reflection mirror 6; wherein, the inner diameter of each glass tube is the same as the outer diameter of the optical fiber, the optical fiber is passed through the glass tube, and the end face of the glass tube is polished and polished; the transmittance of the dichromatic mirror to the pump light is 90%, and the transmittance to signal light is 5%; the flat end face of the dichromatic mirror and the other end of the first glass tube 202 constitutes the first weakly modulated F-P cavity, and the total reflection mirror and the flat end face of the second glass tube 204 other end The flat end face constitutes the second weak modulation F-P cavity; the first glass tube 202 and the second glass tube 204 connect the two sections of optical fibers together by means of mechanical butt joint.

The length L ₁ of the ordinary single-mode fiber used in this example is 4.5 cm, and the length L ₂ of the gain fiber is 3.6 cm.

Test case:

Figure 3 is the structure diagram of the two-stage FP cavity in this test example. From left to right, there are dichromatic mirror 1, the first ceramic tube 2, ordinary single-mode fiber 3, the second ceramic tube 4, and erbium-ytterbium co-doped phosphate Gain fiber 305, total reflection mirror 306. Among them, the length L ₁ of the ordinary single-mode silica fiber is 3.6 cm, the length L ₂ of the gain fiber is 3.3 cm, the inner diameter of the ceramic tube is the same as the outer diameter of the optical fiber, and the optical fiber passes through the ceramic tube. The end face of the ceramic tube is ground and then mirror polished.

In order to reduce the collimation loss as much as possible, the dichromatic mirror is directly coated on the fiber end surface of the ceramic tube 2 by means of plasma sputtering. The transmittance of the dichromatic mirror to signal light at 1564 nm is 0.7%, and the transmittance to pump light at 976 nm is 95%. The other end of the first ceramic tube 2 is physically connected to the second ceramic tube 4 to connect the ordinary single-mode optical fiber 3 , that is, the ordinary single-mode silica optical fiber and the erbium-ytterbium co-doped phosphate gain fiber 305 together.

The total reflection mirror 306 placed at the far right is a saturable absorption mirror (SESAM), which has a strong reflection effect on light near 1550nm, and a reflectivity of 90.8% for signal light at 1564nm.

Inject pump light with a wavelength of 976nm and a power of 850mW from the direction of the arrow in Figure 3, and use equipment such as an oscilloscope, spectrum analyzer, and autocorrelator to detect the outgoing light.

The test results are as follows:

Figure 4, Figure 5, and Figure 6 are the spectrum diagram, time-domain diagram and autocorrelation diagram of the output pulse in the test example, respectively. In the test example, the equivalent length of the mode-locked laser cavity is 6.9 cm. According to the calculation of the relationship B=c/(nL) between the speed of light c, the cavity length L, the fundamental frequency repetition frequency B and the optical fiber refractive index n, it can be known that the laser cavity The fundamental repetition frequency is about 1.44GH. From the time domain and spectrogram, it can be seen that the repetition frequency of the output pulse is about 2.8 GHz, which is twice the fundamental frequency. In addition, it can be seen from the spectrogram and autocorrelation diagram that the signal-to-noise ratio of the doubled pulse is as high as 75dB, and the pulse width is 3.9ps, indicating that it has a very high pulse quality.

In the test case, the theoretical mechanism of the repetition frequency doubling in the two-stage weakly modulated F-P cavity is as follows:

Since the refractive index of the phosphate gain fiber used is different from that of the commercial silica fiber, after being ground and mirror polished, and connected together by mechanical butt joint, there is a reflectance of about 1564nm between the end faces. 4% weak reflection. therefore. The dichromatic mirror 1 and the flat end surface of the other end of the first ceramic tube 2 constitute a first weakly modulated F-P cavity, while the S ESAM and the flat end surface of the other end of the first ceramic tube 4 constitute a second weakly modulated F-P cavity. According to the above data, the fineness of these two weakly modulated F-P cavities can be calculated to be 1.6% and 1.8%, respectively. For these two weakly modulated F-P cavities, the reflection function of pulse transmission in them can be described mathematically by the following function:

Among them, u is the amplitude of the electric field, z is the longitudinal spacing of the fiber, t is the relaxation time, and ω is the angular frequency. β ₂ , γ, g and Ω are the second-order dispersion constant, nonlinear coefficient, saturable gain coefficient and gain bandwidth constant, respectively. R _a is the reflection coefficient of the SESAM in saturation.

For the first F-P cavity, the reflection curve calculated by the above formula is shown in Fig. 7 .

For the second F-P cavity, the reflection curve calculated by the above formula is shown in FIG. 8 .

The result of the interaction between the two F-P cavities can be obtained by performing the cross product operation of the above two functions (Figure 9), which is also the theoretical value of the output result in this test example. It can be seen that the fan shape generated by the interaction of the two F-P cavities suppresses the longitudinal mode very well, suppresses the fundamental frequency, and directly generates double the repetition frequency. In the fan-shaped envelope, the interval between small peaks is about 2.86GHz, and the overall width of the fan-shaped envelope is 36.19GHz, which is in full agreement with the pulse performance test results mentioned above.

The above-mentioned embodiment is one of the embodiments of the present utility model, but the embodiment of the present utility model is not limited by the embodiments and test examples, and any other changes made without departing from the spirit and principle of the present utility model , modification, replacement, combination, and simplification, all should be equivalent replacement methods, and are all included in the protection scope of the present utility model.

Claims (5)

1. A two-section weakly modulated F-P cavity is characterized in that it comprises two sections of different optical fibers, total reflection mirrors and dichromatic mirrors; the two ends of the two sections of different optical fibers are mirror-polished, and the two sections of different optical fibers are connected to each other It is connected by physical docking; the other ends of the two different optical fibers are respectively placed with a dichromatic mirror and a total reflection mirror; the dichromatic mirror and the total reflection mirror are respectively connected with the polished surfaces of two different optical fibers to form two mutual Functional weakly modulated F-P cavity; wherein, the dichromatic mirror transmits more than 90% of the pump light power and less than 10% of the signal light; the length of the two different optical fibers is less than or equal to 10cm. 2. A two-stage weakly modulated F-P cavity according to claim 1, characterized in that after the pump light is input, the two F-P cavities interact to produce a fan-shaped periodic envelope; the envelope can suppress Fundamental frequency, directly produces double frequency. 3. a kind of two-section weakly modulated F-P cavity according to claim 1, is characterized in that, two sections of different optical fibers are respectively one section of active optical fiber and one section of passive optical fiber, and the two ends of two sections of different optical fibers are polished into After the mirror surface, one of the ends is connected to each other by mechanical docking; the other end of the active fiber that is not connected to the passive fiber is placed with a total reflection mirror; the other end of the passive fiber that is not connected to the active fiber is placed with a dichromatic mirror; The active optical fiber and the passive optical fiber form two closely connected and interacting weakly modulated F-P cavities respectively. 4. A two-stage weakly modulated F-P cavity according to claim 1, characterized in that, from left to right, there are dichromatic mirrors, the first ceramic tube, common single-mode optical fiber, the second ceramic tube, the third Ceramic tube, gain fiber, fourth ceramic tube, and total reflection mirror; the inner diameter of each ceramic tube is the same as the outer diameter of the optical fiber, and ordinary single-mode optical fiber and gain optical fiber are passed through the ceramic tube; the end faces of each ceramic tube are polished and then mirrored Polishing treatment; the transmittance of the dichromatic mirror to the pump light is 95%, and the transmittance to the signal light is 5%; the dichromatic mirror and the flat end surface of the second ceramic tube constitute the first weakly modulated F-P cavity, and the entire The reflector and the flat end face of the third ceramic tube constitute the second weakly modulated F-P cavity; the second ceramic tube and the third ceramic tube are mechanically connected to connect two sections of optical fiber, that is, a common single-mode optical fiber and a gain optical fiber. 5. A kind of two-stage weakly modulated F-P cavity according to claim 1, characterized in that, from left to right, there are dichromatic mirror, first glass tube, common single-mode fiber, second glass tube, gain fiber , total reflection mirror; wherein, the inner diameter of each glass tube is the same as the outer diameter of the optical fiber, the optical fiber is passed through the glass tube, and the end face of the glass tube is polished and polished; the transmittance of the dichromatic mirror to the pump light is 90 %, the transmittance to signal light is 5%; the dichromatic mirror and the flat end surface at the other end of the first glass tube form the first weakly modulated F-P cavity, and the total reflection mirror and the flat end surface at the other end of the second glass tube form a The second weakly modulated F-P cavity; the first glass tube and the second glass tube are mechanically connected to connect the two sections of optical fiber.