Femtosecond laser source for far-position measurement terahertz time-domain spectrometer

Abstract

The femtosecond laser source comprises a seed laser, an optical fiber isolator, an optical fiber filter, a pulse primary amplifier and a beam splitter which are connected in sequence, and the femtosecond laser source is divided into a detection output branch and an emission output branch after passing through the beam splitter. The frequency spectrum of the output laser pulse is narrowed through the filter, so that the influence of high-order dispersion on a wide spectrum is reduced, and the pulse is ensured to be transmitted to a far-end application. And then, performing pre-chirp processing on the pulse by using a constant dispersion optical fiber to ensure that the pulse chirp is positive, and obtaining a pulse soliton by using pulse self-compression to obtain a pulse soliton with high-peak power output so as to provide enough phase modulation for subsequent linear chirp spectrum spreading. And finally, performing linear chirp spectrum spreading on the soliton pulse by using a normal dispersion optical fiber, and then performing time domain compression by using an abnormal dispersion optical fiber to obtain sub-hundred femtosecond pulse output. According to the invention, the robustness of the system is improved, so that the system can adapt to severe working environments such as explosion, salt mist and the like.

Description

Technical Field

[0001] The invention belongs to the technical field of lasers, and in particular relates to a femtosecond laser source for a remote-measurement terahertz time-domain spectrometer. Background Art

[0002] Terahertz time-domain spectrometer based on ultrafast femtosecond laser is an important tool for terahertz analysis and measurement. It has wide applications in basic scientific research and industrial fields such as material research, non-destructive testing, spectral measurement, and semiconductor defect detection. At present, the laser sources based on ultrafast femtosecond lasers are generally solid-state titanium sapphire femtosecond lasers and erbium-doped fiber femtosecond lasers around 1550nm. Erbium-doped fiber femtosecond lasers have received increasing attention in recent years due to their high stability, good robustness, compact structure, and flexible use.

[0003] The average power, pulse width, pulse shape and stability of femtosecond lasers directly determine the performance indicators of terahertz time-domain spectrometers, such as the center frequency and bandwidth. Compared with solid-state titanium sapphire femtosecond lasers, fiber-type femtosecond lasers transmit laser pulses through optical fibers, and it is necessary to consider the effects of optical fiber loss, dispersion and nonlinearity on the output pulses. In single-mode optical fibers, the transmission loss of optical fibers is less than 1dB / km, and for short-distance transmission, the loss can be ignored. For pulses transmitted in optical fibers, one of the main factors affecting their pulse width is dispersion, which can cause pulse broadening and deformation. In order to eliminate the influence of dispersion on the transmitted pulses, the femtosecond laser pulses must be dispersion compensated. Generally, optical components such as dispersion compensating fibers (DCF), prism pairs, gratings and chirped mirrors are used for dispersion compensation. During the dispersion compensation process, since the second-order dispersion and third-order dispersion of the dispersion compensation device cannot perfectly compensate for the various orders of dispersion of the transmission optical fiber (such as SMF-28 or PM1550, etc.), as the transmission distance increases, the residual dispersion causes the pulse deformation to increase, thereby affecting the performance indicators of the terahertz time-domain spectrometer, resulting in the current femtosecond laser-based terahertz time-domain spectrometer cannot be used in remote measurement scenarios. Summary of the invention

[0004] In order to overcome the shortcomings of the prior art, the present invention provides a femtosecond laser source for a terahertz time-domain spectrometer for remote measurement, including a seed laser, an optical fiber isolator, an optical fiber filter, a pulse first-stage amplifier, and a beam splitter connected in sequence, and divided into a detection output branch and an emission output branch after passing through the beam splitter. The output laser pulse first passes through a filter to narrow the spectrum to reduce the influence of high-order dispersion on the wide spectrum to ensure that the pulse is transmitted to the remote application. Then, the pulse is pre-chirped using a constant dispersion optical fiber to ensure that the pulse chirp is positive, and pulse self-compression is used to obtain pulse solitons, and pulse solitons with high peak power output are obtained to provide sufficient phase modulation for subsequent linear chirp spectrum expansion. Finally, the soliton pulse is linearly chirped using a constant dispersion optical fiber, and then time domain compression is performed using an anomalous dispersion optical fiber to obtain a sub-hundredth femtosecond pulse output. The present invention improves the robustness of the system, so that the system can adapt to harsh working environments such as explosions and salt spray.

[0005] The technical solution adopted by the present invention to solve the technical problem is as follows:

[0006] A femtosecond laser source for a remote-measurement terahertz time-domain spectrometer comprises a seed laser, an optical fiber isolator, an optical fiber filter, a pulse primary amplifier, and a beam splitter, which are connected in sequence and are divided into a detection output branch and an emission output branch after passing through the beam splitter;

[0007] The detection output branch includes a pulse secondary amplifier, a pulse time delay line, a first dispersion compensating fiber, a soliton self-compression fiber, a second dispersion compensating fiber and a transmission fiber connected in sequence; the emission output branch includes a pulse secondary amplifier, a first dispersion compensating fiber, a soliton self-compression fiber, a second dispersion compensating fiber and a transmission fiber connected in sequence;

[0008] The output of the seed laser is amplified in one stage, and the pulsed laser after the first stage amplification is split by a beam splitter and used as a detection output branch and an emission output branch respectively; the detection output branch is firstly amplified by a pulsed secondary amplifier pumped forward and backward, and the amplified power is adjusted according to the insertion loss of the pulse time delay line;

[0009] The relative time delay difference between the detection output branch pulses and the transmission output branch pulses is compensated by the pulse time delay line of the detection output branch.

[0010] Preferably, the seed laser adopts a full polarization-maintaining fiber structure.

[0011] Preferably, the output pulse power of the seed laser is in the mW range.

[0012] Preferably, the pulse first-stage amplifier adopts a front-to-back pumping structure.

[0013] Preferably, the first dispersion compensating optical fiber, the soliton self-compression optical fiber, the second dispersion compensating optical fiber and the transmission optical fiber are all polarization-maintaining optical fibers.

[0014] Preferably, the pulse time delay line is in the order of several centimeters to tens of centimeters.

[0015] Preferably, the output power of the detection output branch and the transmission output branch is less than the maximum tolerable power of the terahertz antenna.

[0016] The beneficial effects of the present invention are as follows:

[0017] The present invention improves the robustness of the system, so that the system can adapt to harsh working environments such as explosions, salt spray, etc. Secondly, the light source of the present invention uses linear chirp spectrum expansion technology and time domain compression technology to transmit and compress pulses. Different optical fiber combinations can be selected according to different output powers and transmission lengths to meet the requirements of different transmission lengths, and can meet the measurement application requirements of different distances. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 is a schematic diagram of a femtosecond laser source of the present invention;

[0019] Figure 2 This is a flow chart of a femtosecond laser generating sub-hundred-femtosecond pulse lasers;

[0020] Figure 3 is the output pulse of the transmitting output branch and the corresponding spectrum;

[0021] Figure 4 It is the output pulse and corresponding spectrum of the detection output branch. DETAILED DESCRIPTION

[0022] The present invention is further described below in conjunction with the accompanying drawings and embodiments.

[0023] When the terahertz time-domain spectrometer performs spectral detection and analysis, for scenes that require remote measurement, such as gunpowder explosion fields and toxic gas fields, the laser source of the terahertz time-domain spectrometer needs to be transmitted to the remote end. In order to ensure the performance indicators of the terahertz time-domain spectrometer, the light source pulse needs to be sub-hundred femtosecond pulses during transmission, and the relative time delay between the detection end and the emission end pulses must be compensated by the delay line on the detection output branch. This solves the problem of light source pulse deformation and broadening caused by the accumulation of high-order dispersion of the transmission fiber during remote measurements (greater than 10m).

[0024] The present invention proposes a femtosecond laser source, in which the output laser pulse is firstly narrowed by a filter to reduce the influence of high-order dispersion on the wide spectrum, so as to ensure that the pulse is transmitted to the remote application (greater than 10m). Then, the pulse is pre-chirped using a constant dispersion fiber to ensure that the pulse chirp is positive, and pulse self-compression is used to obtain pulse solitons, and pulse solitons with high peak power output are obtained to provide sufficient phase modulation for subsequent linear chirp spectrum expansion. Finally, the soliton pulse is linearly chirped using a constant dispersion fiber, and then time-domain compression is performed using an anomalous dispersion fiber to obtain a sub-hundred-femtosecond pulse output.

[0025] like Figure 1 As shown, the femtosecond laser source for a terahertz time-domain spectrometer suitable for remote measurement includes a seed laser, a fiber isolator, a fiber filter, a pulse primary amplifier, a beam splitter, a detection output branch, and an emission output branch. The detection output branch includes a pulse secondary amplifier, a pulse time delay line, a first dispersion compensating fiber, a soliton self-compression fiber (PM1550), a second dispersion compensating fiber, and a transmission fiber; the emission output branch includes a pulse secondary amplifier, a first dispersion compensating fiber, a soliton self-compression fiber (PM1550), a second dispersion compensating fiber, and a transmission fiber.

[0026] The seed laser adopts a fully polarization-maintaining fiber structure. Generally speaking, the seed laser output pulse power is in the order of several mW. Before the detection light and emission light are split, the seed laser output is first amplified, and a front-to-back pumping structure is generally used. After the first-stage amplification, the pulsed laser is split by the beam splitter and used as the detection output branch and the emission output branch respectively. Among them, the detection output branch is firstly power amplified by the front-to-back pumped amplifier, and the amplified power is adjusted according to the insertion loss of the time delay line. In order to improve the reliability of the system, the first dispersion compensating fiber, soliton self-compression fiber, second dispersion compensating fiber and transmission fiber used in the system are all polarization-maintaining fibers.

[0027] The relative time delay difference between the detection output branch and the emission output branch pulse can be compensated by the pulse time delay line of the detection output branch. Generally speaking, the selected pulse time delay line is in the order of a few centimeters to tens of centimeters, so the optical fiber length difference between the two detection output branches is also in this order of magnitude.

[0028] The output power of the detection output branch and the transmission output branch needs to be less than the maximum withstand power of the terahertz antenna. Therefore, the output power of the pulse second amplifier of the detection output branch and the transmission output branch needs to be adjusted according to the above maximum withstand power. At the same time, the output power of the pulse second amplifier also directly affects the length distribution of the dispersion compensation fiber and the soliton self-compression fiber.

[0029] Example:

[0030] The schematic diagram of the system structure of a femtosecond laser source for a remote measurement terahertz time-domain spectrometer designed by the present invention is as follows: Figure 1 As shown, it includes: an erbium-doped fiber femtosecond laser, a fiber isolator, a fiber filter, a pulse primary amplifier, a beam splitter, a detection output branch, and a transmission output branch, etc. The detection output branch includes a pulse secondary amplifier, a pulse time delay line, a first dispersion compensating fiber, a soliton self-compression fiber (PM1550), a second dispersion compensating fiber, and a transmission fiber, etc.; the transmission output branch includes a pulse secondary amplifier, a first dispersion compensating fiber, a soliton self-compression fiber (PM1550), a second dispersion compensating fiber, and a transmission fiber, etc. In order to ensure that the pulse time delay difference between the detection output branch and the transmission output branch can be compensated by the pulse time delay line in the detection output branch, and the output power can meet the power requirements of the detection antenna and the transmission antenna, the parameters of the dispersion compensating fiber and the soliton self-compression fiber used in the two branches are the same.

[0031] The workflow of the femtosecond laser source is as follows Figure 2 As shown, the specific process is as follows: The erbium-doped fiber femtosecond laser adopts a fully polarization-maintaining fiber structure based on nonlinear amplifying ring mirror mode locking. The initial spectrum output by the erbium-doped fiber femtosecond laser first enters the fiber isolator to prevent the return light from damaging the laser. The output laser enters the fiber filter for spectrum narrowing processing to reduce the influence of high-order dispersion on wide-spectrum pulses. The narrowed pulse is divided into two output branches by a beam splitter, one detection output branch and one emission output branch. The emission output branch also first passes through a pulse secondary amplifier for power amplification, and then enters the first dispersion-compensating fiber and soliton self-compression fiber for linear chirp spectrum expansion, and then enters the second dispersion-compensating fiber and transmission fiber for time-domain compression, and finally obtains sub-hundredth femtosecond pulse laser output. The output pulse and spectrum distribution are shown as follows. Figure 3 The detection output branch first passes through the pulse secondary amplifier to amplify the power, and then passes through the pulse time delay line to adjust the time delay difference between the detection output branch and the emission output branch, and then enters the first dispersion compensation fiber and soliton self-compression fiber for linear chirp spectrum expansion, and then enters the second dispersion compensation fiber and transmission fiber for time domain compression, and finally obtains sub-hundred-femtosecond pulse laser output.

[0032] The final output pulse and spectrum distribution are as follows: Figure 4 shown.

Claims

1. A femtosecond laser source for a remote terahertz time-domain spectrometer, characterized in that: It includes a seed laser, a fiber isolator, a fiber filter, a pulse primary amplifier, and a beam splitter which are connected in sequence, and are divided into a detection output branch and a transmission output branch after passing through the beam splitter; The detection output branch includes a pulse secondary amplifier, a pulse time delay line, a first dispersion compensating fiber, a soliton self-compression fiber, a second dispersion compensating fiber and a transmission fiber connected in sequence; the emission output branch includes a pulse secondary amplifier, a first dispersion compensating fiber, a soliton self-compression fiber, a second dispersion compensating fiber and a transmission fiber connected in sequence; The output of the seed laser is amplified in one stage, and the pulsed laser after the first stage amplification is split by a beam splitter and used as a detection output branch and an emission output branch respectively; the detection output branch is firstly amplified by a pulsed secondary amplifier pumped forward and backward, and the amplified power is adjusted according to the insertion loss of the pulse time delay line; The relative time delay difference between the detection output branch pulses and the transmission output branch pulses is compensated by the pulse time delay line of the detection output branch.

2. The femtosecond laser source for a remote terahertz time-domain spectrometer according to claim 1, characterized in that: The seed laser adopts a full polarization-maintaining optical fiber structure.

3. The femtosecond laser source for a remote terahertz time-domain spectrometer according to claim 1, characterized in that: The output pulse power of the seed laser is in the mW range.

4. The femtosecond laser source for a remote terahertz time-domain spectrometer according to claim 1, characterized in that: The pulse first-stage amplifier adopts a front-to-back pumping structure.

5. The femtosecond laser source for a remote terahertz time-domain spectrometer according to claim 1, characterized in that: The first dispersion compensating optical fiber, the soliton self-compression optical fiber, the second dispersion compensating optical fiber and the transmission optical fiber are all polarization-maintaining optical fibers.

6. The femtosecond laser source for a remote terahertz time-domain spectrometer according to claim 1, characterized in that: The pulse time delay line is in the order of several centimeters to tens of centimeters.

7. The femtosecond laser source for a remote terahertz time-domain spectrometer according to claim 1, characterized in that: The output power of the detection output branch and the transmission output branch is less than the maximum tolerable power of the terahertz antenna.

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