CN114725767A - Electro-optical Q-switching based on relaxor ferroelectric single crystal - Google Patents

Abstract

An electro-optic Q-switch based on a relaxor ferroelectric single crystal belongs to the technical field of lasers. The invention aims at the problems of high cost and difficulty in miniaturization of the existing electro-optic Q-switch. The method comprises the following steps: YVO (YVO) after laser pulse generated by the fiber coupled semiconductor laser is transmitted by the back reflector₄The crystal realizes wavelength conversion and gain amplification, and linearly polarized light is output through the polarization beam splitter; when the PIN-PMN-PT crystal has no external voltage, linearly polarized light passes through the quarter wave plate, the eighth wave plate and the PIN-PMN-PT crystal after phase delay, and cannot pass through the polarization beam splitter after being reflected by the output mirror and then reversely transmitted; the resonant cavity is in a low Q state; when voltage is applied to the PIN-PMN-PT crystal, linearly polarized light passes through the quarter wave plate, the eighth wave plate and the PIN-PMN-PT crystal, transmitted light is reflected by the output mirror and then reversely propagates to the rear reflecting mirror; the cavity is in a high Q state. The invention realizes low driving voltage and miniaturized Q switch.

Description

Electro-optical Q-switching based on relaxor ferroelectric single crystal

technical field

The invention relates to an electro-optical Q-switching switch based on a relaxor ferroelectric single crystal, and belongs to the technical field of lasers.

Background technique

Pulsed lasers have the advantage of high peak power and are widely used in civil and military fields such as laser processing, ranging, communications, and infrared countermeasures.

The electro-optical Q-switch is the core device of the pulsed laser, which has the advantages of short switching time and high efficiency, and can be used to obtain laser output with narrow pulse width and high energy. The working principle of the electro-optical Q-switch is to use the electro-optical effect of the crystal. Initially, a 1/4λ wave voltage is applied to the crystal, the switch is turned off, and the laser resonator is in a low-Q state, which is the energy storage stage. After the energy storage is completed, the voltage is removed, so that the energy in the resonator is released quickly, and the pulsed laser with high peak power is output in a high-Q state.

LiNbO3 (LN) and KD ₂ PO ₄ (DKDP) crystals are currently the two most used crystals in electro-optical Q-switches. Among them, the optical damage threshold of LN crystal is low, about 100MWcm ^-2 , which is not suitable for high-power lasers; DKDP crystal has moisture absorption characteristics, which requires complex moisture-proof technology, which increases the insertion loss of the device. At the same time, the effective electro-optic coefficients rc of LN and DKDP crystals are about 21 and _24pmV ^-1 respectively, which require high driving voltage or large-sized crystals to meet the application requirements. Therefore, the existing electro-optical Q-switch needs to be equipped with a high-voltage power supply, which has the disadvantages of high cost and difficulty in miniaturization, which has become a key obstacle to improving the performance of the device.

SUMMARY OF THE INVENTION

Aiming at the problems of high cost and difficulty in miniaturization that the existing electro-optical Q-switch needs to be configured with a high-voltage power supply and large-sized crystals, the present invention provides an electro-optical Q-switch based on a relaxor ferroelectric single crystal.

An electro-optical Q-switching switch based on relaxor ferroelectric single crystal of the present invention includes a fiber-coupled semiconductor laser, a back reflector, an Nd:YVO ₄ crystal, a polarization beam splitter, a quarter-wave plate, an eighth Waveplate, PIN-PMN-PT crystal and output mirror,

The fiber-coupled semiconductor laser is used to generate laser pulses with the expected center wavelength. The laser pulses are transmitted through the rear mirror to realize wavelength conversion and gain amplification through the Nd:YVO ₄ crystal, and then output linear polarization through the polarization beam splitter. Light;

When the PIN-PMN-PT crystal has no applied voltage:

The linearly polarized light enters the PIN-PMN-PT crystal after phase delay by the quarter wave plate and the eighth wave plate, and the transmitted light after the phase delay of the incident light by the PIN-PMN-PT crystal is reflected by the output mirror. , after the phase delay is reversed by the PIN-PMN-PT crystal, the phase delay caused by the birefringence effect of the PIN-PMN-PT crystal is compensated by the eighth wave plate, and then the phase delay caused by the birefringence effect of the PIN-PMN-PT crystal is compensated by the eighth wave plate, The polarization direction of the laser beam arriving at the polarization beam splitter is perpendicular to the polarization direction of the incident laser beam from the polarization beam splitter, and can no longer pass through the polarization beam splitter; at this time, the working substance in the Nd:YVO ₄ crystal continues to accumulate, and the laser resonator is at a low level at the current stage. Q state;

When a voltage is applied to the PIN-PMN-PT crystal, the PIN-PMN-PT crystal is equivalent to a quarter wave plate:

The linearly polarized light is phase-delayed by a quarter-wave plate and an eighth-wave plate, and then enters the PIN-PMN-PT crystal for phase delay. After the transmitted light of the PIN-PMN-PT crystal is reflected by the output mirror, Reverse through the PIN-PMN-PT crystal, the eighth wave plate and the quarter wave plate, the polarization direction of the laser reaching the polarization beam splitter through the quarter wave plate and the polarization direction of the incident laser beam from the polarization beam splitter In the same way, the laser that reaches the polarizing beam splitter in the reverse direction passes through the polarizing beam splitter and then reaches the rear mirror through the Nd:YVO4 crystal; the laser resonator is in a high-Q state at the current stage, and after a strong laser oscillation is established in the laser resonator, the output The mirror outputs laser pulses.

According to the electro-optical Q-switch based on the relaxor ferroelectric single crystal of the present invention, the center wavelength of the laser pulse generated by the fiber-coupled semiconductor laser is 808 nm, the repetition frequency is 10 Hz to 2 kHz, and the pulse duration is 100 μs.

According to the electro-optical Q-switching switch based on the relaxor ferroelectric single crystal of the present invention, the fiber diameter of the fiber-coupled semiconductor laser is 200 μm, and the numerical aperture is 0.22.

According to the electro-optical Q-switch based on relaxor ferroelectric single crystal of the present invention, the Nd:YVO4 crystal includes a 0.5at.% doped Nd:YVO4 crystal with a size of 3mm×3mm×5mm cut in the α direction _; The output light surface when the light is transmitted in the forward direction is polished and oriented at the Brewster angle.

According to the electro-optical Q-switch based on relaxor ferroelectric single crystal of the present invention, the eighth wave plate is used to compensate the optical phase delay caused by birefringence when the PIN-PMN-PT crystal has no applied voltage.

According to the electro-optical Q-switching switch based on the relaxor ferroelectric single crystal of the present invention, the PIN-PMN-PT crystal transmits light along the [100] crystal direction, and two corresponding crystal faces are coated with anti-reflection films.

According to the electro-optical Q-switching switch based on relaxor ferroelectric single crystal of the present invention, the anti-reflection film includes an inner layer film and an outer layer film, the inner layer film is a HfO ₂ film, and the outer layer film is a SiO ₂ film, both of which are made of ionic Coating by electron beam evaporation method of beam assisted deposition.

According to the electro-optical Q-switching switch based on the relaxor ferroelectric single crystal of the present invention, the process of coating the PIN-PMN-PT crystal with an anti-reflection film includes:

Use 99.95% _HfO2 target and 99.99% _SiO2 target;

During coating, the vacuum state pressure was kept below 3×10 ^-3 Pa, the working pressure was maintained at 1×10 ^-2 Pa, and oxygen was transported into the deposition chamber at a rate of 12 sccm through the Kauffman ion source.

According to the electro-optical Q-switching switch based on the relaxor ferroelectric single crystal of the present invention, the PIN-PMN-PT crystal is kept at 30° C. during the process of coating the anti-reflection film.

Beneficial effects of the present invention: The present invention provides an electro-optical Q-switch based on relaxor ferroelectric single crystal PIN-PMN-PT. It has been verified that compared with commercial DKDP and LN crystal Q switches, the switch volume can be reduced by one At the same time, the working voltage is reduced from 1300 to 3200V to 200V, thereby alleviating the electromagnetic interference problem of high-voltage pulses. At the same time, the PIN-PMN-PT crystal has a high optical damage threshold, about 500MWcm ^-2 , and does not require a moisture-proof process, so it can effectively achieve low driving voltage and miniaturization of Q-switching.

The present invention contributes to the development of applications in many fields, such as ultra-small, low-power lidar, sensing functions of small and medium-sized robots for autonomous driving, and precision medical and scientific equipment requiring high stability.

Description of drawings

FIG. 1 is a schematic structural diagram of an electro-optical Q-switched switch based on a relaxor ferroelectric single crystal according to the present invention.

Detailed ways

The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative work fall within the protection scope of the present invention.

It should be noted that the embodiments of the present invention and the features of the embodiments may be combined with each other under the condition of no conflict.

The present invention will be further described below with reference to the accompanying drawings and specific embodiments, but it is not intended to limit the present invention.

1, the present invention provides an electro-optical Q-switched switch based on a relaxor ferroelectric single crystal, which is characterized by comprising a fiber-coupled semiconductor laser 1, a back reflector 2, and a Nd:YVO ₄ crystal 3. Polarizing beam splitter 4, quarter wave plate 5, eighth wave plate 6, PIN-PMN-PT crystal 7 and output mirror 8,

The fiber-coupled semiconductor laser 1 is used to generate a laser pulse of the expected center wavelength, and the laser output after the laser pulse is transmitted by the rear mirror 2 realizes wavelength conversion and gain amplification through the Nd:YVO ₄ crystal 3, and then passes through the polarization beam splitter. 4 output linearly polarized light;

When PIN-PMN-PT crystal 7 has no applied voltage:

The linearly polarized light enters the PIN-PMN-PT crystal 7 after being phase-delayed by the quarter-wave plate 5 and the one-eighth-wave plate 6, and the transmitted light after the phase delay of the incident light by the PIN-PMN-PT crystal 7 is After the output mirror 8 is reflected, the phase is delayed by the PIN-PMN-PT crystal 7 in the reverse direction, and then the phase delay caused by the birefringence effect of the PIN-PMN-PT crystal 7 is compensated by the one-eighth wave plate 6. , and then the polarization direction of the laser that reaches the polarization beam splitter 4 through the quarter wave plate 5 is perpendicular to the polarization direction of the incident laser light of the polarization beam splitter 4, and can no longer pass through the polarization beam splitter 4; at this time, the Nd:YVO ₄ crystal 3 The medium working substance continues to accumulate, and the laser resonator is in a low-Q state at the current stage;

When a voltage is applied to the PIN-PMN-PT crystal 7, the PIN-PMN-PT crystal 7 is equivalent to a quarter wave plate:

After the linearly polarized light is phase delayed by the quarter wave plate 5 and the eighth wave plate 6, it enters the PIN-PMN-PT crystal 7 for phase delay, and the transmitted light of the PIN-PMN-PT crystal 7 is output by the output mirror. 8 After the reflection, it passes through the PIN-PMN-PT crystal 7, the eighth-wave plate 6 and the quarter-wave plate 5 in the reverse direction, and reaches the polarization direction of the laser beam of the polarization beam splitter 4 through the quarter-wave plate 5 The polarization direction of the incident laser light is the same as that of the polarizing beam splitter 4. The laser light that reaches the polarizing beam splitter 4 in the opposite direction passes through the polarizing beam splitter 4 and then reaches the rear reflector 2 through the Nd:YVO4 crystal 3; the laser resonator is at a high Q at the current stage. After the strong laser oscillation is established in the laser resonator, the laser pulse is output through the output mirror 8 . A quarter wave plate 5 is used to provide a phase retardation of π/4.

In this embodiment, a laser resonator is formed between the rear mirror 2 and the output mirror 8 .

As an example, the center wavelength of the laser pulse generated by the fiber-coupled semiconductor laser 1 is 808 nm, the repetition frequency is 10 Hz to 2 kHz, the pulse duration is 100 μs, and the pump pulse energy is as high as 3.7 mJ.

The 808nm central wavelength pump light can be absorbed and converted into 1064nm output light by Nd:YVO4 crystal 3. During the process, Nd:YVO4 crystal 3 is used as the gain medium in the laser resonator. To avoid thermal deposition effects, the repetition frequency was set from 10 Hz to 2 kHz and the pulse duration was set at 100 μs.

Rear mirror 2 has high reflectivity (>99.5%) at 1064 nm and high transmittance (>99.6%) at 808 nm, and output mirror 8 has a reflectivity of 10% at 1064 nm wavelength.

In this embodiment, by applying a voltage control signal to the PIN-PMN-PT crystal 7, that is, a pulse _Vπ/2 voltage with a repetition frequency of 10Hz to 2kHz, the Q switch is in an operating state. When there is no applied voltage on the crystal, the laser passes through the quarter-wave plate twice, and the polarization direction reaching the polarizer is perpendicular to the incident polarization. The phase delay caused by the birefringence of the crystal is compensated by the eighth-wave plate, so it cannot pass through the polarization. device. At this time, the working substance in the Nd:YVO4 crystal continues to accumulate, and the laser resonator is in a low-Q state at the current stage. When the working voltage acts on the PIN-PMN-PT crystal, the crystal is equivalent to a quarter-wave plate. After the laser is reflected by the mirror and passes through the crystal and the quarter-wave plate twice, the polarization state of the laser remains unchanged. A strong laser oscillation is established in the resonator, the resonator is in a high-Q state, and a pulsed laser with a wavelength of 1064 nm is emitted from the output end of the resonator.

As an example, the fiber diameter of the fiber-coupled semiconductor laser 1 is 200 μm, and the numerical aperture is 0.22. It can be adapted to the resonant cavity formed between the rear reflector 2 and the output mirror 8 .

As an example, the Nd:YVO4 crystal ₃ includes a 0.5at.% doped Nd:YVO4 crystal with a size of 3mm×3mm×5mm cut in the α direction; the output light surface of the Nd:YVO4 crystal 3 when the light is transmitted in the forward direction is a polished surface and oriented at Brewster's angle. The Nd:YVO4 crystal 3 is used as the gain medium of the oscillator to prevent parasitic oscillation.

Further, the eighth wave plate 6 is used to compensate the optical phase delay caused by birefringence when the PIN-PMN-PT crystal 7 has no applied voltage.

The PIN-PMN-PT crystal 7 is a biaxial crystal in the orthorhombic phase. Compared with the uniaxial crystals LN and DKDP, the biaxial crystal will produce phase delay due to the birefringence effect. Therefore, an eighth-wave plate 6 is added to the resonator, and its position is adjusted to fully compensate the phase delay caused by birefringence of the PIN-PMN-PT crystal 7, ensuring that the oscillation is turned off in the non-operating mode and the laser cavity is in high loss. state, so that the laser output power is zero. Using the eighth wave plate 6 to compensate the phase has the advantages of simple structure and continuous adjustment.

Further, the PIN-PMN-PT crystal 7 passes light along the [100] crystal direction, and two corresponding crystal faces are coated with anti-reflection films.

In order to reduce the loss of light intensity caused by reflection on the crystal surface, anti-reflection films are coated on the two clear surfaces of the PIN-PMN-PT crystal 7, so that the transmittance of the crystal reaches 99.6% at a wavelength of 1064 nm.

As an example, the anti-reflection film includes an inner layer film and an outer layer film, the inner layer film is a HfO ₂ film, and the outer layer film is a SiO ₂ film, both of which are coated by the electron beam evaporation method of ion beam assisted deposition.

Further, the process of anti-reflection coating on PIN-PMN-PT crystal 7 includes:

Use 99.95% _HfO2 target and 99.99% _SiO2 target;

When coating, during the evaporation of the HfO ₂ target and the SiO ₂ target, the vacuum state pressure was kept below 3×10 ^-3 Pa, the working pressure was kept at 1×10 ^-2 Pa, and the oxygen was passed through the Kaufman ion source at 12sccm. transport into the deposition chamber.

During the entire deposition process, the temperature of the PIN-PMN-PT crystal 7 was 30 °C, which was much lower than the phase transition temperature of the PIN-PMN-PT crystal to ensure that the state of the electrical domains inside the crystal was not changed.

In this embodiment, the PIN-PMN-PT crystal 7 is kept at 30° C. during the anti-reflection coating process.

In order to avoid the phase transition due to the increase of the crystal temperature during the coating process and the change of the internal domain structure state, the temperature of the PIN-PMN-PT crystal 7 should always be kept near room temperature during coating.

Specific examples:

为：12mm×3.4mm；相比于DKDP晶体Q开关尺寸

为15mm×18mm和LN的Q开关尺寸

为22mm×22mm，减小了超过一个数量级。A Q-switch was fabricated using a PIN-PMN-PT crystal with dimensions of 5mm × 5mm × 1.5mm and compared with commercial DKDP and LN crystal Q-switches: Obtaining PIN-PMN-PT crystal Q-switch dimensions

is: 12mm × 3.4mm; compared to the size of the DKDP crystal Q switch

Q-switch dimensions of 15mm x 18mm and LN

It is 22mm×22mm, which is reduced by more than one order of magnitude.

The reason for the small size of the Q-switch of the PIN-PMN-PT crystal is its large effective electro-optical coefficient rc, which is _670pmV ^-1 ; theoretically, reducing the length of the Q-switch light-passing direction will lead to an increase in the operating voltage. According to the electro-optical Q-switching The formula for switching operating voltage:

where V _π/2 is the quarter-wave voltage, that is, the working voltage, λ is the wavelength, _d is the distance between the two electrodes of the PIN-PMN-PT crystal, n is the refractive index, rc is the effective electro-optic coefficient, l is the length of the PIN-PMN-PT crystal in the light-passing direction. Due to the large effective electro-optic coefficient of PIN-PMN-PT crystal, the working voltage is only 0.2kV, which is 16 times and 6.5 times lower than that of DKDP and LN, respectively.

According to the structure shown in Figure 1, the Q-switch made of PIN-PMN-PT crystal is tested. At a repetition rate of 1 kHz and a pump energy of 3.7 mJ, the output pulse width of the PIN-PMN-PT crystal is 1.8 ns, which is a significant improvement over commercial Q-switching performance. The PIN-PMN-PT crystal can output a smaller pulse width, which is due to the positive correlation between the pulse width and the propagation time of the laser in the cavity, and the small size of the PIN-PMN-PT crystal reduces the Q-switch insertion. The extra optical path is the product of the length of the electro-optic crystal in the laser transmission direction and the refractive index of the crystal. Therefore, the Q-switching time of the PIN-PMN-PT crystal in the cavity is shorter, which is beneficial to the reduction of the output laser pulse width.

Peak power is another important performance parameter of the Q-switch. The PIN-PMN-PT crystal Q-switch can output a maximum peak power of 154kW at a repetition rate of 1kHz and a pump energy of 3.7mJ, which is comparable to the commercial DKDP and LN crystal Q-switches. Peak power is almost the same. This result further indicates that the PIN-PMN-PT crystal meets the criteria for commercial Q-switching.

Although the invention has been described herein with reference to specific embodiments, it is to be understood that these embodiments are merely illustrative of the principles and applications of the invention. It should therefore be understood that many modifications may be made to the exemplary embodiments and other arrangements may be devised without departing from the spirit and scope of the invention as defined by the appended claims. It should be understood that the features described in the various dependent claims and herein may be combined in different ways than are described in the original claims. It will also be appreciated that features described in connection with a single embodiment may be used in other described embodiments.

Claims (9)

1. An electro-optical Q-switched switch based on a relaxor ferroelectric single crystal, characterized in that it comprises a fiber-coupled semiconductor laser (1), a rear reflector (2), a Nd:YVO crystal ( ₃ ), a polarization beam splitter ( 4), quarter wave plate (5), eighth wave plate (6), PIN-PMN-PT crystal (7) and output mirror (8), The fiber-coupled semiconductor laser (1) is used to generate a laser pulse of a desired center wavelength, and the laser output after being transmitted by the rear reflector (2) realizes wavelength conversion and gain amplification through an Nd:YVO ₄ crystal (3), Then output the linearly polarized light through the polarization beam splitter (4); When the PIN-PMN-PT crystal (7) has no applied voltage: The linearly polarized light enters the PIN-PMN-PT crystal (7) after phase delay by the quarter wave plate (5) and the eighth wave plate (6). The PIN-PMN-PT crystal (7) responds to the incident light. After the transmitted light after the phase delay is reflected by the output mirror (8), the phase delay is reversed by the PIN-PMN-PT crystal (7), and then the PIN-PMN-PT is reflected by an eighth wave plate (6). The crystal (7) performs phase compensation due to the phase delay caused by the birefringence effect, and then reaches the polarization direction of the laser beam of the polarization beam splitter (4) through the quarter-wave plate (5) and the incident laser beam from the polarization beam splitter (4). The polarization direction of the laser is vertical, and can no longer pass through the polarization beam splitter (4); at this time, the working material continues to accumulate in the Nd:YVO ₄ crystal (3), and the laser resonator is in a low-Q state at the current stage; When a voltage is applied to the PIN-PMN-PT crystal (7), the PIN-PMN-PT crystal (7) is equivalent to a quarter wave plate: After the linearly polarized light is phase-delayed by a quarter-wave plate (5) and an eighth-wave plate (6), it enters the PIN-PMN-PT crystal (7) for phase delay, and the PIN-PMN-PT crystal ( After the transmitted light of 7) is reflected by the output mirror (8), it passes through the PIN-PMN-PT crystal (7), the eighth wave plate (6) and the quarter wave plate (5) in the reverse direction. The polarization direction of the laser light reaching the polarization beam splitter (4) by the half-wave plate (5) is the same as the polarization direction of the incident laser light in the polarization beam splitter (4), and the laser light reaching the polarization beam splitter (4) in the opposite direction passes through the polarization splitter. The beamer (4) reaches the rear mirror (2) through the Nd:YVO4 crystal (3); the laser resonator is in a high-Q state at the current stage, and after a strong laser oscillation is established in the laser resonator, the output mirror (8) outputs laser pulse. 2. The electro-optical Q-switched switch based on relaxor ferroelectric single crystal according to claim 1, wherein, The center wavelength of the laser pulse generated by the fiber-coupled semiconductor laser (1) is 808 nm, the repetition frequency is 10 Hz to 2 kHz, and the pulse duration is 100 μs. 3. The electro-optical Q-switched switch based on relaxor ferroelectric single crystal according to claim 2, wherein, The fiber diameter of the fiber-coupled semiconductor laser (1) is 200 μm, and the numerical aperture is 0.22. 4. The electro-optical Q-switched switch based on relaxor ferroelectric single crystal according to claim 3, wherein, The Nd:YVO4 crystal (3) includes a 0.5at.% doped Nd:YVO4 crystal with a size of 3mm×3mm×5mm cut in the α direction _; the output light surface of the Nd:YVO4 crystal (3) when the light is transmitted in the forward direction is polished face and oriented at Brewster's angle. 5 . The electro-optical Q-switched switch based on relaxor ferroelectric single crystal according to claim 4 , wherein, The eighth wave plate (6) is used for compensating the optical phase delay caused by birefringence when the PIN-PMN-PT crystal (7) has no applied voltage. 6 . The electro-optical Q-switched switch based on relaxor ferroelectric single crystal according to claim 5 , wherein, The PIN-PMN-PT crystal (7) passes light along the [100] crystal direction, and the two corresponding crystal faces are coated with anti-reflection films. 7 . The electro-optical Q-switched switch based on relaxor ferroelectric single crystal according to claim 6 , wherein, The anti-reflection film includes an inner layer film and an outer layer film, the inner layer film is a HfO ₂ film, and the outer layer film is a SiO ₂ film, both of which are coated by the electron beam evaporation method of ion beam assisted deposition. 8 . The electro-optical Q-switched switch based on relaxor ferroelectric single crystal according to claim 7 , wherein, The process of coating the PIN-PMN-PT crystal (7) with anti-reflection film includes: Use 99.95% _HfO2 target and 99.99% _SiO2 target; During coating, the vacuum state pressure was kept below 3×10 ^-3 Pa, the working pressure was maintained at 1×10 ^-2 Pa, and oxygen was transported into the deposition chamber at a rate of 12 sccm through the Kauffman ion source. 9 . The electro-optical Q-switched switch based on relaxor ferroelectric single crystal according to claim 8 , wherein, During the anti-reflection coating process, the PIN-PMN-PT crystal (7) was kept at 30°C.

Images