Polarization state adjustment based pulse laser delay device and method

Abstract

The application belongs to the technical field of laser, and proposes a pulse laser delay device and method based on polarization state adjustment, which comprises a light splitting device composed of a coaxially arranged Faraday isolator and a half-wave plate, and a delay device composed of two mirrors, an electro-optic crystal and a 45° polarizer, wherein one of the mirrors is connected with a one-dimensional displacement table. After the laser is changed into s light by the light splitting device, the s light is reflected by the polarizer into the electro-optic crystal, and when a quarter-wave voltage is applied by the electro-optic drive, the s light is converted into p light to enter the resonant cavity. After the voltage is removed, the laser is reflected between the two mirrors for multiple times to realize delay. After the voltage is applied again, the laser is changed back into s light, which is reflected by the polarizer and escapes from the hole of the Faraday isolator to be output. Through the fine adjustment of the optical path by the displacement table and the control of the electric signal, the fine continuous adjustment of the delay of ns to μs level can be realized, and the long delay adjustment of high-power pulse laser is suitable.

Description

Technical Field

[0001] This invention relates to the field of laser technology, and in particular to a pulsed laser delay device and method based on polarization state adjustment. Background Technology

[0002] The statements in this section are merely background information related to the present invention and do not necessarily constitute prior art.

[0003] In optical production and research, pulsed laser delay adjustment is a common and important requirement. Fields such as sum-frequency amplification, optical parametric amplification (OPA), and laser processing all require adjustment of laser pulse delay. Currently, common methods for adjusting optical pulse delay include extending the spatial optical path, constructing a ring cavity using a polarizing beam splitter (PBS), and using delay chips that integrate optical delay devices.

[0004] Extending the optical path is typically suitable for picosecond-level pulse delay adjustment, with a maximum adjustment time of a few nanoseconds. When used for nanosecond-level delay adjustment, this structure requires optical paths of several meters, hindering laser miniaturization, resulting in transmission losses, and the long optical path leads to poor equipment stability, which is detrimental to production practice. The ring cavity constructed using a PBS (Polypolarized Surface Mount Technology) confines the laser pulse within the ring cavity structure through polarization state adjustment. After a specific delay, the polarization state is adjusted by an electro-optic switch to release the pulse. Ideally, this structure can achieve pulse adjustment for any length of time. However, in practice, due to its complex structure, laser loss is significant. As the laser repeatedly passes through the PBS, waveplate, and mirrors, the laser power rapidly attenuates within the ring cavity. Assuming a 5% power loss per pass through the ring structure, after 18 passes, the laser power is less than 40% of its initial value. If the ring cavity optical path is 3 meters, the corresponding delay for 18 passes is 180 ns, making longer delay adjustments impossible. Furthermore, the mirrors are all placed at 45°, making it inconvenient to adjust the ring cavity optical path and hindering continuous adjustable delay. Delay chips with integrated optical delay devices are usually achieved by miniaturizing, refining, and integrating several structures that extend optical paths and adjust delay. They are currently quite mature and can adjust delays for a relatively long period of time. However, they are usually used to process optical signals and are widely used in the field of optical communication. The disadvantage is that they have a low damage threshold and cannot be used to transmit or adjust high-power laser pulses. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides a pulsed laser delay device and method based on polarization state adjustment. This device can achieve long delay adjustment of pulsed lasers at the ns to μs level within a relatively small space, has a higher damage threshold, can adjust high-power lasers, and can achieve continuous and adjustable delay within a certain range through a one-dimensional displacement stage.

[0006] To achieve the above objectives, the present invention adopts the following technical solution: In a first aspect, the present invention provides a pulsed laser delay device based on polarization state adjustment.

[0007] A pulsed laser delay device based on polarization state adjustment includes a delayed laser beam splitter, a delay device, and a laser delay fine-tuning device. The delayed laser beam splitter includes a Faraday isolator and a half-wave plate, with the Faraday isolator and the half-wave plate arranged coaxially. The delay device includes a first reflector, a second reflector, an electro-optic crystal, and a 45° polarizer. The electro-optic crystal is located between the first and second reflectors. The front and rear end faces of the electro-optic crystal are parallel and coaxial with the reflecting surfaces of the first and second reflectors. The 45° polarizer is located between the electro-optic crystal and the half-wave plate. The optical axis of the 45° polarizer forms a 45° angle with the beam propagation direction. The optical axis of the delay device is perpendicular to the optical axis of the delay laser beam splitter. The electro-optic crystal is connected to an electro-optic drive. The laser delay fine-tuning device includes a one-dimensional displacement stage, a second reflector fixedly mounted on the one-dimensional displacement stage, and the movement direction of the one-dimensional displacement stage is parallel to the optical axis of the delay device.

[0008] In one implementation of the first aspect of the present invention, the Faraday isolator includes a first polarization beamsplitter, a second polarization beamsplitter, and a magneto-optical crystal located between the two. The first polarization beamsplitter, the magneto-optical crystal, and the second polarization beamsplitter are arranged sequentially along the optical path. The Faraday isolator is provided with a lateral escape aperture for outputting delayed laser light.

[0009] In one implementation of the first aspect of the present invention, the half-wave plate is an air-gap zero-order half-wave plate, which can be rotated around the optical axis to adjust the polarization state of the incident laser, so that the polarization state of the laser entering the 45° polarizer is vertical.

[0010] In one implementation of the first aspect of the present invention, the 45° polarizer is a thin-film polarizer. When the laser is incident at a 45° angle, it reflects the laser with vertical polarization and transmits the laser with horizontal polarization.

[0011] In one implementation of the first aspect of the present invention, the two ends of the electro-optic crystal are coated with anti-reflection films, and the electro-optic drive is used to apply a quarter-wave voltage to the electro-optic crystal, so that the laser passing through the electro-optic crystal generates a quarter-wavelength optical path difference between the o-light and e-light.

[0012] In one implementation of the first aspect of the present invention, the electro-optic drive is capable of outputting a pulse voltage with a specific period and duty cycle, wherein the rise time and fall time of the pulse voltage are both less than four nanoseconds.

[0013] In one implementation of the first aspect of the present invention, the surfaces of the first and second reflectors are both coated with a high-reflectivity film, and the laser reflectivity corresponding to the wavelength at a zero-degree incident angle is greater than 99.9%.

[0014] In one implementation of the first aspect of the present invention, a one-dimensional displacement stage includes a movable part and a fixed part. The movable part is driven to move along a straight line by a motor, gear or spring-pull structure, thereby changing the distance between the first reflector and the second reflector.

[0015] Secondly, the present invention provides a pulsed laser delay method based on polarization state adjustment.

[0016] A pulsed laser delay method based on polarization state adjustment, utilizing the pulsed laser delay device based on polarization state adjustment of the first aspect of the present invention, includes the following steps: The laser beam enters from the Faraday isolator and passes through the half-wave plate, forming vertically polarized light. Vertically polarized light is reflected by a 45° polarizer and enters the electro-optic crystal; Electro-optic drive applies a quarter-wave voltage to the electro-optic crystal, causing vertically polarized light to be converted into circularly polarized light, which is then reflected by the first mirror and passes through the electro-optic crystal again to be converted into horizontally polarized light. Horizontally polarized light passes through a 45° polarizer and is reflected multiple times between the first and second reflecting mirrors; After the preset delay time is reached, the electro-optic drive applies a quarter-wave voltage to the electro-optic crystal again, causing the horizontally polarized light to be reflected by the second mirror and converted into circularly polarized light after passing through the electro-optic crystal. After being reflected by the first mirror and passing through the electro-optic crystal, it is converted into vertically polarized light. Vertically polarized light is reflected by a 45° polarizer and passes through a half-wave plate before exiting through the exit aperture of a Faraday isolator.

[0017] In one implementation of the second aspect of the present invention, the distance between the first and second reflectors is changed by controlling the movement of a one-dimensional displacement stage, while the time interval of the voltage applied by the electro-optic drive is adjusted to achieve continuous fine-tuning of the laser delay time.

[0018] Compared with the prior art, the beneficial effects of the present invention are: By employing a mechanism combining polarization state adjustment with an electro-optic crystal switch, this invention achieves mechanically motion-free switching of pulsed laser delay. Traditional delay devices often rely on moving mirrors to change the optical path, a method limited by mechanical inertia during high-speed switching, making it difficult to meet high-frequency modulation requirements. This invention utilizes the characteristic of electro-optic crystals to rapidly change the laser polarization state under voltage control, combined with a 45° polarizer as an optical path switch, allowing the laser to freely circulate in the storage loop or be released at a specific time. This design eliminates delays and vibrations caused by mechanical component movements, significantly improving the system's response speed and stability. Simultaneously, since the optical path switching is entirely controlled by electrical signals, the device can easily synchronize with external control systems, achieving precise timing matching. This is particularly suitable for terahertz radiation generation scenarios requiring rapid, repetitive pulse width modulation or time gating, ensuring the time accuracy and waveform quality of the output laser pulse.

[0019] Advantages of additional aspects of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. Attached Figure Description

[0020] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an improper limitation of the invention.

[0021] Figure 1 A schematic diagram of a pulsed laser delay device based on polarization state adjustment provided as an exemplary embodiment of the present invention; Among them, 1. Faraday isolator; 2. Half-wave plate; 3. Electro-optic crystal; 4. 45° polarizer; 5. First reflecting mirror; 6. Second reflecting mirror; 7. One-dimensional displacement stage; 8. Electro-optic drive. Detailed Implementation

[0022] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0023] It should be noted that the following detailed descriptions are exemplary and intended to provide further illustration of the invention. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0024] In a preferred embodiment of the present invention, a pulsed laser delay device based on polarization state adjustment is proposed, mainly applied to the delay adjustment of laser pulses with a repetition frequency of 1 kHz and a wavelength of 1064 nm. Its core objective is to achieve long delays from nanoseconds to microseconds by precisely controlling the laser polarization state through an electro-optic crystal, combined with multiple reflections within the optical resonant cavity, while maintaining the device's miniaturization, high damage threshold, and low loss characteristics. The overall architecture of this pulsed laser delay device based on polarization state adjustment presents a compact and logically rigorous layout in space, mainly composed of three functional modules organically combined: a delay laser beam splitter, a delay device, and a laser delay fine-tuning device. These three functional modules do not exist in isolation, but rather form a unified optical system through strict optical axis alignment and mechanical connection.

[0025] like Figure 1 As shown, the time-delay laser beam splitter, serving as a crucial hub for the optical path's entry and exit, comprises a Faraday isolator 1 and a half-wave plate 2 as its core components. In this embodiment, the Faraday isolator 1 and the half-wave plate 2 are configured coaxially, meaning their optical central axes coincide, and this axis is defined as the principal optical axis direction of the entire system. The Faraday isolator 1 utilizes the Faraday effect to alter the laser's polarization state and achieves spatial separation between the input and output lasers through its unique escape aperture structure. The half-wave plate 2 is positioned adjacent to the Faraday isolator 1 and is used to pre-adjust or post-adjust the laser's polarization direction during the initial entry into the system or the later exit of the system, ensuring that the laser enters the subsequent time-delay processing stage with the correct polarization state or is efficiently extracted from the system.

[0026] Following the time-delay laser beam splitter is the core component of this device: the time-delay unit. This time-delay unit is spatially perpendicular to the time-delay laser beam splitter; specifically, the optical axis of the time-delay unit intersects the optical axis of the time-delay laser beam splitter perpendicularly. This orthogonal arrangement not only saves space but, more importantly, utilizes the characteristics of polarization elements to achieve optical path switching. The internal structure of the time-delay unit is extremely precise, mainly consisting of a first reflecting mirror 5, a second reflecting mirror 6, an electro-optic crystal 3 located between the two reflecting mirrors, and a 45° polarizer 4 placed in the optical path. Positionally, the electro-optic crystal 3 is precisely positioned between the first reflecting mirror 5 and the second reflecting mirror 6, with its front and rear faces parallel to the reflecting surfaces of the first reflecting mirror 5 and the second reflecting mirror 6, respectively. Furthermore, all four planes (the two mirror surfaces and the two end faces of the crystal) are perpendicular to the optical axis of the time-delay unit, thus ensuring the coaxiality of the laser propagation within the cavity and avoiding beam deflection or loss due to angular deviations.

[0027] In this implementation, a 45° polarizer 4 is positioned in the optical path between the electro-optic crystal 3 and the time-delay laser beam splitter, with its polarization transmission axis forming a 45° angle with the optical axis of the time-delay device. This specific angle setting is crucial for enabling the laser to switch between a "stored" state and a "released" state. The electro-optic crystal 3 is connected to an electro-optic driver 8 via a high-voltage cable. The electro-optic driver 8 is responsible for applying precisely controlled voltage signals, particularly quarter-wave voltages, to the electro-optic crystal 3 to change the polarization state of the laser passing through the electro-optic crystal 3.

[0028] To achieve fine-tuning of the delay time, this invention also includes a laser delay fine-tuning device. The core component of this device is a one-dimensional displacement stage 7. Mechanically, one of the mirrors in the delay device, specifically the second mirror 6 in this embodiment, is rigidly fixed to the sliding block of the one-dimensional displacement stage 7. The installation direction of the one-dimensional displacement stage 7 is carefully designed, with its movement direction parallel to the optical axis of the delay device. By driving the one-dimensional displacement stage 7, the position of the second mirror 6 relative to the first mirror 5 can be precisely changed, thereby fine-tuning the distance between the two mirrors, i.e., changing the optical path length of the optical resonant cavity. This mechanical fine-tuning, combined with the signal timing adjustment of the electro-optic drive 8, achieves high-precision control of the laser delay.

[0029] In this embodiment, the Faraday isolator 1 is a Faraday isolator specifically designed for the 1064nm band. The aperture of the Faraday isolator 1 is set to 5mm to ensure it can accommodate the laser beam to be processed without truncation. Its isolation index is over 30dB, meaning it can effectively prevent back-propagating laser light from returning to the source, protecting the laser from damage. Simultaneously, the insertion loss of the Faraday isolator 1 is controlled below 0.5dB, ensuring extremely high energy transmission efficiency. Internally, the Faraday isolator 1 consists of two upper and lower polarizing beam splitters (PBS) and a centrally located magneto-optical crystal. Its working principle is based on the Faraday rotation effect of the magneto-optical crystal. This effect states that when linearly polarized light passes through a magneto-optical crystal under the influence of a magnetic field, its polarization plane rotates, and the rotation angle depends only on the direction of the magnetic field and the crystal properties, and is independent of the direction of light propagation. Therefore, when the laser first passes through the magneto-optical crystal in the forward direction, the polarization plane rotates by 45°; when the laser is reflected and passes through the magneto-optical crystal again in the reverse direction, the polarization plane continues to rotate by 45° in the same direction, for a cumulative rotation of 90°. The p-beam, which could originally pass through the upper polarizing beam splitter, is transformed into s-beam by rotating 90° after passing through twice. Since the s-beam cannot pass through the polarizing beam splitter, it is guided to exit from the side exit aperture, thus perfectly achieving spatial separation of the input and output optical paths.

[0030] The half-wave plate 2, coaxially arranged with the Faraday isolator 1, is, in this embodiment, an air-gap zero-order waveplate matched to the 1064nm wavelength. This type of waveplate has extremely low temperature sensitivity and a high damage threshold, making it ideal for high-power laser applications. The aperture of the half-wave plate 2 is designed to be 20mm, much larger than the laser beam diameter, to avoid edge diffraction effects. Its wavefront distortion is strictly controlled within λ / 10, ensuring the integrity of the beam quality. The main function of the half-wave plate 2 is to adjust the polarization direction of the incident laser, converting it to a specific linear polarization state, such as s-beam, before it enters the subsequent optical path, so that it can be reflected by the subsequent 45° polarizer 4 into the delay cavity.

[0031] The core component of the delay device, the electro-optic crystal 3, is a DKDP (potassium dihydrogen phosphate) electro-optic crystal in this embodiment. DKDP crystals possess excellent electro-optic coefficients and high damage thresholds, making them ideal materials for processing high-power pulsed lasers. For a 1064nm wavelength laser, the quarter-wave voltage of this electro-optic crystal 3 is approximately 3200V. This means that when the electro-optic driver 8 applies a voltage of 3200V to the electro-optic crystal 3, the electro-optic crystal 3 can convert linearly polarized light into circularly polarized light, or vice versa, depending on the polarization direction of the incident light and the orientation of the crystal's fast and slow axes. The electro-optic crystal 3 is placed in the optical path between the first reflecting mirror 5 and the second reflecting mirror 6, and its light-transmitting surface is coated with an anti-reflection film to reduce surface reflection loss.

[0032] The 45° polarizer 4 located between the electro-optic crystal 3 and the Faraday isolator 1 is a thin-film polarizer (TFP) in the 1064nm band, selected in this embodiment. This 45° polarizer 4 exhibits extremely high performance, with a transmittance of over 99.9% for p-light and a reflectance of over 99% for s-light. This high extinction ratio ensures low loss during laser circulation within the cavity and accurate routing when entering and exiting the cavity. The installation angle of the 45° polarizer 4 is precisely calibrated so that its transmission axis forms a 45° angle with the optical axis of the delay device (i.e., the direction of laser propagation within the cavity). This angle setting allows the laser to selectively transmit or be reflected after polarization state conversion via the electro-optic crystal 3, depending on whether a voltage is applied.

[0033] In this embodiment, the two mirrors constituting the optical resonant cavity, namely the first mirror 5 and the second mirror 6, are both total internal reflection mirrors made of fused silica substrate. Fused silica has an extremely low coefficient of thermal expansion and a high damage threshold, enabling it to withstand prolonged irradiation by high-power lasers. Both mirrors are coated with a high-reflectivity film for a wavelength of 1064 nm, achieving a reflectivity of up to 99.99%. This high reflectivity is crucial for achieving multiple reflections of the laser within the cavity without significant attenuation. The first mirror 5 and the second mirror 6 are positioned opposite each other, with their reflecting surfaces strictly parallel, forming a Fabry-Perot type linear resonant cavity structure. The laser reflects back and forth within this cavity, with each round trip corresponding to a certain time delay.

[0034] In this embodiment, a motorized precision one-dimensional displacement stage 7 is used to drive the movement of the second reflector 6. The stage 7 has a travel of 50 mm, sufficient to cover the required delay adjustment range. Its positioning accuracy is as high as ±1 μm, meaning it can adjust the position of the second reflector 6 with extremely fine precision, thereby achieving micron-level control over the optical path length. The movement direction of the one-dimensional displacement stage 7 is strictly parallel to the optical axis of the delay device, ensuring the stability of the optical axis during adjustment and preventing the introduction of additional angular deviations. The one-dimensional displacement stage 7 is typically fixed on an optical platform or bracket, and the first reflector 5 is securely mounted on the moving platform of the one-dimensional displacement stage 7 using a dedicated mirror mount.

[0035] The electro-optic driver 8, responsible for providing high-voltage pulses to the electro-optic crystal 3, utilizes a high-performance high-voltage pulse power supply in this embodiment. The output voltage range of the electro-optic driver 8 covers 0 to 5 kV, fully meeting the 3200 V operating voltage required by the electro-optic crystal 3. More importantly, the rise and fall times of the electro-optic driver 8 are both less than 3 ns, enabling it to maintain precise synchronization with nanosecond-level laser pulses. The electro-optic driver 8 can maintain a specific delay output with the laser pulse source via an external trigger signal, ensuring that the voltage is in the correct state (applied or removed) the instant the laser arrives at the electro-optic crystal 3. The electro-optic driver 8 is connected to the electrodes of the electro-optic crystal 3 via shielded high-voltage cables to reduce electromagnetic interference and ensure operational safety.

[0036] During the assembly of the device, the spatial positions and connections of each component are strictly defined. First, the Faraday isolator 1, half-wave plate 2, 45° polarizer 4, electro-optic crystal 3, and first reflector 5 are sequentially fixed onto the optical platform. During the fixing process, a high-precision laser collimator is used to ensure that the central axis of each component is completely coaxial with the preset laser optical path. Subsequently, the second reflector 6 is fixed on the sliding block of the one-dimensional displacement stage 7, and the one-dimensional displacement stage 7 is installed on the optical platform so that it is directly opposite the first reflector 5. By adjusting the one-dimensional displacement stage 7, the initial distance between the first reflector 5 and the second reflector 6 is set to a specific value, for example, 1.5 meters in this embodiment. At this time, the optical path L between the two mirrors is 1.5 meters (one way) or 3 meters (round trip). Finally, the electro-optic drive 8 is reliably connected to the electrodes of the electro-optic crystal 3 through a high-voltage cable, completing the physical assembly of the entire device.

[0037] During the parameter setting phase, detailed calculations and settings were performed to meet the specific requirements of a repetition frequency f=1kHz and a required additional delay t=300ns. Since the laser repetition frequency is 1kHz, its period T=1 / f=1ms=10 6 To achieve a 300ns delay, precise control of the voltage application timing of the electro-optic driver 8 is required. The calculated duty cycle for voltage application is 99.97%, meaning that within a complete 1ms cycle, the electro-optic driver 8 has 300ns of no voltage application (low level) and 999700ns of a quarter-wave voltage of 3200V (high level). The specific timing logic is as follows: Assume the moment before the laser first enters the electro-optic crystal 3 to the left is defined as t=0, at which point the electro-optic driver 8 is already in a voltage-application state (high level); the laser undergoes a series of optical processes, including being reflected by the 45° polarizer 4, passing through the electro-optic crystal 3, being reflected by the first reflecting mirror 5, and passing through the electro-optic crystal 3 again, until it completely passes through the electro-optic crystal 3, becomes p-light, and passes through the 45° polarizer 4 into the region between the two mirrors, denoted as t1; after time t1, the high voltage of the electro-optic driver 8 must be immediately removed, leaving the electro-optic crystal 3 in a voltage-free state for 300ns. Within this 300ns period, the laser beam, in p-light state, freely reflects back and forth between the two mirrors. When the 300ns delay ends, at time t1+300ns, the electro-optic drive 8 must apply a high-level voltage again. Considering the continuity of the cycle, the rising edge time of the electro-optic drive 8 should be set to t=999700ns-t1 (relative to the start of the next cycle). More intuitively, this means maintaining a high voltage for most of each cycle (999700ns), removing the high voltage only during the 300ns window when the laser needs to be "stored" within the cavity.

[0038] In this implementation, the operation begins with the laser incident phase. A laser pulse with a repetition frequency of 1 kHz and a wavelength of 1064 nm is incident from the input of the Faraday isolator 1 and enters the device. At this point, the polarization state of the laser may be arbitrary or undefined. Next is the initial polarization state adjustment phase, where the laser beam passes through a coaxially arranged half-wave plate 2. The function of the half-wave plate 2 is to precisely convert the polarization state of the incident laser into s-beam (relative to the definition of the 45° polarizer 4). This s-beam continues to propagate forward and is incident on the 45° polarizer 4. Since the polarization direction of the s-beam is perpendicular to the transmission axis of the 45° polarizer 4 (or at a specific angle causing reflection), the s-beam is totally internally reflected by the 45° polarizer 4, changing its propagation direction by 90°, thus entering the optical axis direction of the delay device, i.e., directed towards the electro-optic crystal 3.

[0039] The first stage, voltage application, then begins. At this point, the electro-optic drive 8 is in the voltage application phase, applying a quarter-wave voltage of 3.2 kV (3200 V) to the electro-optic crystal 3. After passing through the electro-optic crystal 3, the s-beam's polarization state changes from linearly polarized to circularly polarized. This circularly polarized light continues to propagate along the optical axis until it reaches the first reflecting mirror 5. The high-reflectivity coating of the first reflecting mirror 5 reflects the circularly polarized light back along its original path, causing it to re-enter the electro-optic crystal 3. When the circularly polarized light passes through the electro-optic crystal 3 with the applied quarter-wave voltage for the second time, its polarization state changes again, changing from circularly polarized to linearly polarized p-beam. At this time, the polarization direction of the p-beam is parallel to the transmission axis of the 45° polarizer 4. Therefore, when this p-beam reaches the 45° polarizer 4, it is no longer reflected but passes directly through the 45° polarizer 4, entering the open region formed by the first reflecting mirror 5 and the second reflecting mirror 6.

[0040] Next comes the crucial delay maintenance phase. Once the laser passes through the 45° polarizer 4 and enters the space between the two mirrors, the electro-optic drive 8 immediately enters a voltage-free period, the duration of which is precisely set to 300 ns. During this period, there is no voltage on the electro-optic crystal 3, which acts as an isotropic medium and does not change the polarization state of the laser passing through it. Therefore, the laser always remains in a p-light state. Since p-light can pass through the 45° polarizer 4 without loss (theoretically), and both mirrors between the first reflector 5 and the second reflector 6 are highly reflective, the laser is trapped within this optical resonant cavity, constantly reflecting back and forth between the first reflector 5 and the second reflector 6. As set, the distance between the two mirrors is 1.5 meters, the speed of light is approximately 3 × 10^8 meters / second, and the time required for the laser to travel 1.5 meters one way is approximately 5 ns. Therefore, 5 ns elapse for each complete one-way propagation of the laser between the two mirrors (i.e., reflection from one mirror to another). To achieve a total delay of 300 ns, the laser needs to be reflected approximately 60 times within this cavity (300 ns / 5 ns = 60 one-way trips, or 30 round trips). Within this 300 ns time window, no voltage is applied to the electro-optic crystal 3, and the laser is essentially "imprisoned" within the mirror cavity, accumulating optical path over time, thus achieving the pulse time delay.

[0041] After the delay reaches the preset 300ns, the delay ends and output phase begins. At this time, the electro-optic drive 8 re-enters the voltage application period, applying a quarter-wave voltage of 3200V to the electro-optic crystal 3 again. The p-beam propagating within the cavity is now traveling from right to left, i.e., from the second reflector 6 towards the first reflector 5, and passes through the electro-optic crystal 3 again. After passing through the electro-optic crystal 3 with the applied quarter-wave voltage, the p-beam is converted into circularly polarized light. This circularly polarized light continues to propagate to the first reflector 5, is reflected by the high-reflectivity film, and then passes back through the electro-optic crystal 3. After passing through the voltaged electro-optic crystal 3 for the second time, the circularly polarized light is converted back into linearly polarized s-beam. This s-beam then propagates to the 45° polarizer 4. Since the s-beam cannot pass through the 45° polarizer 4, it is reflected by the 45° polarizer 4, changing its propagation direction by 90° and propagating upwards. This reflected s-beam then enters the Faraday isolator 1 in the delay laser beam splitter. As before, the magneto-optical crystal inside Faraday isolator 1 causes the polarization plane of this back-propagating light to rotate by another 45° (a total of 90°), changing it from p-beam to s-beam (relative to the PBS inside Faraday isolator 1). This prevents it from returning from the original entrance port and instead outputs it through the exit hole on the side of Faraday isolator 1. This completes one cycle of delay adjustment, successfully delaying the laser pulse by 300 ns before output. At this point, the electro-optical driver 8 will continue to maintain the applied voltage for a total of 999700 ns until the next laser pulse enters the system, starting a new cycle.

[0042] Regarding the implementation of continuous adjustment, this invention provides a flexible fine-tuning mechanism. If the user needs to shorten the delay time, the optical path L between the second reflector 6 and the first reflector 5 can be reduced by controlling the one-dimensional displacement stage 7. Reducing the distance L means a shorter time required for a single round trip of the laser, thus shortening the total delay for the same number of reflections, or requiring more reflections for the same total delay (but this is limited by losses). Simultaneously, the period without voltage application must be shortened synchronously via the electro-optic drive 8 to match the new optical path and the required total delay. Conversely, if the delay needs to be extended, the one-dimensional displacement stage 7 can be driven to increase the optical path L, and the period without voltage application of the electro-optic drive 8 can be extended accordingly. Since the one-dimensional displacement stage 7 has a positioning accuracy of ±1μm, and the electro-optic drive 8 has nanosecond-level timing control capability, the two work together to achieve high-precision, continuously adjustable laser delay. This dual adjustment mechanism not only covers coarse adjustment (by changing the cavity length) but also achieves fine adjustment (by electrical signal timing), greatly expanding the applicability of the device.

[0043] To further improve the delay accuracy of the device and reduce the impact of intracavity laser power attenuation on delay stability, and addressing the issues of high loss in traditional ring cavities and precision deviation caused by the coupling of delay adjustment with optical path / power loss, laser intracavity loss compensation algorithms and delay time precision matching algorithms are introduced into some other implementation methods. Through quantitative calculation, the linkage control of optical path, loss, and delay time is achieved.

[0044] The formula for calculating the percentage of residual power after multiple reflections of laser light between the first and second reflecting mirrors is as follows: (1); Where η is the ratio of the remaining power of the laser after n single-pass reflections to the initial power; δ is the comprehensive loss coefficient of the laser passing through the delay device in a single pass; n represents the number of single-pass reflections of the laser between the two mirrors (times, a positive integer); α represents the fixed transmission efficiency of Faraday isolator 1 and half-wave plate 2. In other implementations, the effective reflection count threshold is obtained based on the remaining power percentage, including: (2); in, The maximum number of single-pass reflections required to maintain effective laser power is the core control threshold for loss compensation; This represents the minimum percentage of remaining power that allows the laser to achieve an effective delay.

[0045] In practical applications, the theoretical number of reflections is calculated based on the required delay time. ,like The distance between the two mirrors is increased by using a one-dimensional displacement stage. This reduces the number of reflections required to achieve the same delay, thus reducing the actual number of reflections. This ensures the laser remains within its effective power range; if the laser wavelength or optical components are replaced, only recalibration is required. and This allows for rapid calculation of new... This improves the versatility of the device.

[0046] The above method quantifies the power attenuation law of the laser within the cavity, accurately determines the maximum number of reflections required for the laser to maintain effective power, and avoids the laser power becoming too low to be usable due to blindly increasing the number of reflections. Compared to the traditional ring cavity without loss compensation, this method uses a comprehensive loss coefficient... The precise calibration reduces single-shot loss and significantly increases the maximum effective number of reflections; it provides a power constraint basis for setting the delay time and realizes the quantitative correlation between power loss, number of reflections, and delay time.

[0047] In some other implementations, the theoretical delay time of the laser between the two mirrors is: (3); For laser The theoretical delay time (s) of a single-pass reflection; The actual distance (m) between the first reflecting mirror 5 and the second reflecting mirror 6 is precisely controlled by a one-dimensional displacement stage. y is the speed of light in a vacuum (m / s). β is the equivalent refractive index of the optical medium.

[0048] In practical applications, if the delay time needs to be adjusted, first determine the maximum effective single-pass reflection count. Then, based on the preset delay... The required single-path optical path L is calculated by reversing the theoretical delay formula (3), and the distance between the two mirrors is precisely adjusted to this distance using a one-dimensional displacement stage; at the same time, the voltage-free period of electro-optic drive is set as the preset delay. This enables precise matching of delay time. When ambient temperature and mechanical stress cause slight deformation of the spacing L, the change in L is detected in real time by a one-dimensional displacement stage. The change in delay t is then quickly calculated by substituting the value into the formula. Finally, the electro-optic drive synchronously fine-tunes the voltage-free period to achieve real-time compensation of the delay time.

[0049] In this way, precise linkage control of optical path length (one-dimensional displacement stage), number of reflections (loss constraint), and delay time (electro-optic drive) is achieved; the effective number of reflections is limited based on loss constraint to avoid excessive power attenuation and improve delay stability; it is compatible with the micron-level control of the one-dimensional displacement stage and the ns-level timing control of the electro-optic drive, achieving dual protection of continuous fine-tuning and precise setting of delay time.

[0050] The pulsed laser delay device based on polarization state adjustment proposed in this invention offers significant advantages over traditional methods that extend the free-space optical path (such as folded optical paths or fiber delay lines) to achieve delay. Firstly, in terms of size, this invention significantly reduces the physical dimensions required for long delays by utilizing a high-reflectivity mirror to achieve multiple round trips of the laser within a limited space. Traditional methods often require optical paths of tens or even hundreds of meters to achieve μs-level delays, occupying enormous space; however, this invention achieves a long delay adjustment of 1.2 μs with a delay structure length of only 2 meters, a figure far exceeding the adjustment limit of traditional ring cavities. Secondly, in terms of loss, this invention significantly reduces the loss of a single laser pass by selecting coated elements with high damage thresholds and high reflectivity. Data shows that for common wavelengths, this device can reduce the loss of a single laser pass to below 0.5%. This low-loss characteristic allows the laser to maintain a remaining power of over 40% even after up to 180 reflections within the cavity, which is crucial for high-power laser applications.

[0051] Furthermore, this invention boasts a high damage threshold. Thanks to the use of a fused silica substrate, large-aperture optical elements, and high-temperature resistant, damage-resistant components such as an air-gap zero-order waveplate, the device can withstand high-power laser irradiation, solving the problem of low damage thresholds caused by the use of optical fibers or other easily damaged materials in existing adjustment methods. By further improving the coating process, enhancing the performance of the mirrors and polarizers, or simply increasing the length of the delay structure (i.e., increasing the distance between the two mirrors), the delay duration achievable by this invention can be further increased in the future, demonstrating broad application prospects.

[0052] In terms of specific implementation details, attention must be paid to the tolerance fit between various components and environmental stability. For example, the coaxiality error of the Faraday isolator 1, half-wave plate 2, 45° polarizer 4, electro-optic crystal 3, and the first and second reflectors 5 and 6 should be controlled at the milliradian level to prevent the beam from deviating from the optical axis after multiple reflections. The straightness of the guide rail of the one-dimensional displacement stage 7 also directly affects the stability of the optical cavity, requiring the selection of high-precision products. The high-voltage output stability of the electro-optic drive 8 is equally critical; voltage fluctuations can cause deviations in the quarter-wave condition, leading to a decrease in polarization state conversion efficiency and increased losses. Therefore, in practical engineering, a feedback control loop is usually introduced to monitor the output power and dynamically adjust the voltage value of the electro-optic drive 8 and the position of the one-dimensional displacement stage 7 to compensate for the effects of environmental temperature changes, mechanical vibrations, and other factors.

[0053] Regarding the placement orientation of the electro-optic crystal 3, its sensing principal axes (fast and slow axes) must form a 45° angle with the polarization direction of the incident s-light. This is necessary to effectively convert linearly polarized light into circularly polarized light when a quarter-wave voltage is applied. This angle needs to be finely adjusted during assembly by rotating the support of the electro-optic crystal 3, and the optimal angle is confirmed by observing the extinction ratio using an analyzer. Similarly, the angle of the 45° polarizer 4 must also be precisely calibrated to ensure that its transmission axis is strictly aligned with the optical axis of the delay device at 45° and matches the sensing principal axis of the electro-optic crystal 3 to achieve optimal switching contrast.

[0054] In the operation of the laser delay fine-tuning device, the movement of the one-dimensional displacement stage 7 is not merely a simple change in distance; it also involves matching the resonant cavity modes. Although this invention primarily utilizes the reflection principle of geometric optics, diffraction effects must also be considered under extremely high precision requirements. Therefore, the stepper motor driver of the one-dimensional displacement stage 7 should possess a smooth motion curve to avoid mechanical impacts during movement, which could lead to cavity detuning or beam jitter.

[0055] Considering the complexities that may be encountered in practical applications, this embodiment also implicitly describes the system's fault tolerance and scalability. For example, if the laser wavelength changes, simply replacing the corresponding half-wave plate 2, 45° polarizer 4, electro-optic crystal 3, and reflector coating, and recalibrating the quarter-wave voltage value of the electro-optic drive 8, allows for rapid adaptation to the new operating wavelength, demonstrating the device's excellent versatility. For lasers with higher repetition rates, this device remains applicable as long as the response speed (rise / fall) and switching frequency of the electro-optic drive 8 can keep up. Furthermore, if a greater delay is required, a multi-stage series approach can be used, connecting multiple such delay devices end-to-end, with the output of the previous device serving as the input of the next, thereby doubling the delay time without infinitely increasing the length of a single cavity. This provides a clear technical path for future system upgrades.

[0056] In terms of safety, due to the involvement of 3200V high voltage, the connecting cables between the electro-optical driver 8 and the electro-optical crystal 3 must have good insulation performance and shielding measures to prevent high voltage breakdown or electromagnetic radiation interference to surrounding electronic equipment. Meanwhile, the presence of the Faraday isolator 1 not only serves to split the beam but also effectively prevents reflected light from flowing back and damaging the laser source, thus improving the safety of the entire system. The one-dimensional displacement stage 7 should be equipped with mechanical or software limits at its extreme positions to prevent overshoot damage to precision optical components.

[0057] In summary, this invention utilizes the birefringence properties of the electro-optic crystal 3 under voltage control, combined with the polarization selectivity of the 45° polarizer 4, to construct a controllable "optical switch" and "optical storage" unit. When there is no voltage, the optical cavity is transparent to p-light, and the laser is trapped; when there is voltage, the polarization state rotates, and the laser is released or injected. This polarization-state-adjustment-based mechanism avoids the drawbacks of slow mechanical shutter speeds and short lifespans, and overcomes the problems of large delay dispersion and strong nonlinear effects in pure optical fibers, providing a novel and efficient solution for delay adjustment of high-power pulsed lasers.

[0058] To address the potential delay drift issue during long-term operation, this invention introduces a one-dimensional displacement stage for fine adjustment of the first reflector's position. In optical resonant cavities or delay loops, fluctuations in ambient temperature or minute deformations of mechanical structures can cause significant changes in the optical path length, thus affecting delay accuracy. By mounting the core reflector on a high-precision one-dimensional displacement stage, operators or automatic control systems can fine-tune the reflector's position along the optical axis based on real-time monitoring results, thereby compensating for optical path errors. This combination of mechanical fine-tuning and electro-optic fast switching retains the high-speed advantage of electro-optic modulation while compensating for its shortcomings in absolute optical path locking. It allows the device to set a base delay within a wide range and perform high-resolution fine-tuning based on this, ensuring that the laser pulse delay remains at the set value under different working environments and long-term operation conditions, greatly enhancing the device's reliability and adaptability.

[0059] The combined design of the Faraday isolator 1 and the half-wave plate 2 in this invention effectively solves the key problems of laser feedback interference and polarization state purity. In high-power or high-sensitivity laser systems, if the reflected light from the back end returns to the light source, it can easily cause instability or even damage to the laser. This invention utilizes the non-reciprocal property of the Faraday isolator 1 to ensure that the laser can only be transmitted in one direction, cutting off the harmful optical feedback path at the source. At the same time, the rotatable air-gap zero-order half-wave plate provides a flexible polarization state pre-adjustment function. Since the subsequent electro-optic switch and polarization beam splitter have strict requirements on the polarization direction of the incident light, the introduction of the half-wave plate allows users to precisely optimize the polarization angle of the laser entering the delay loop according to the actual optical path loss and extinction ratio requirements. This pre-optimization not only improves the contrast of optical path switching and reduces stray light noise, but also maximizes energy utilization, enabling the entire delay system to maintain efficient and pure beam output characteristics even under complex operating conditions.

[0060] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A pulsed laser delay device based on polarization state adjustment, characterized in that, It includes a time-delay laser beam splitter, a time-delay device, and a laser time-delay fine-tuning device. The time-delay laser beam splitter includes a Faraday isolator and a half-wave plate, with the Faraday isolator and the half-wave plate arranged coaxially. The delay device includes a first reflector, a second reflector, an electro-optic crystal, and a 45° polarizer. The electro-optic crystal is located between the first and second reflectors. The front and rear end faces of the electro-optic crystal are parallel and coaxial with the reflecting surfaces of the first and second reflectors. The 45° polarizer is located between the electro-optic crystal and the half-wave plate. The optical axis of the 45° polarizer forms a 45° angle with the beam propagation direction. The optical axis of the delay device is perpendicular to the optical axis of the delay laser beam splitter. The electro-optic crystal is connected to an electro-optic drive. The laser delay fine-tuning device includes a one-dimensional displacement stage, a second reflector fixedly mounted on the one-dimensional displacement stage, and the movement direction of the one-dimensional displacement stage is parallel to the optical axis of the delay device.

2. The pulsed laser delay device based on polarization state adjustment as described in claim 1, characterized in that, The Faraday isolator consists of a first polarization beam splitter, a second polarization beam splitter, and a magneto-optical crystal located between them. The first polarization beam splitter, the magneto-optical crystal, and the second polarization beam splitter are arranged sequentially along the optical path. The Faraday isolator is provided with a lateral escape aperture for outputting delayed laser light.

3. The pulsed laser delay device based on polarization state adjustment as described in claim 1, characterized in that, The half-wave plate is an air gap zero-order half-wave plate. The half-wave plate can rotate around the optical axis to adjust the polarization state of the incident laser, so that the laser entering the 45° polarizer is polarized in the vertical direction.

4. The pulsed laser delay device based on polarization state adjustment as described in claim 1, characterized in that, A 45° polarizer is a thin-film polarizer. When a laser is incident at a 45° angle, it reflects the vertically polarized laser and transmits the horizontally polarized laser.

5. The pulsed laser delay device based on polarization state adjustment as described in claim 1, characterized in that, The electro-optic crystal has anti-reflection coatings on both ends. The electro-optic drive is used to apply a quarter-wave voltage to the electro-optic crystal, so that the laser light passing through the electro-optic crystal produces a quarter-wavelength optical path difference between the o-light and e-light.

6. The pulsed laser delay device based on polarization state adjustment as described in claim 1, characterized in that, Electro-optic drive can output pulse voltage with a specific period and duty cycle, and the rise time and fall time of the pulse voltage are both less than four nanoseconds.

7. The pulsed laser delay device based on polarization state adjustment as described in claim 1, characterized in that, Both the first and second reflectors are coated with a high-reflectivity film, and the laser reflectivity at the corresponding wavelength is greater than 99.9% at a zero-degree incident angle.

8. The pulsed laser delay device based on polarization state adjustment as described in claim 1, characterized in that, A one-dimensional displacement stage consists of a movable part and a fixed part. The movable part is driven to move in a straight line by a motor, gear, or spring-pull structure, thereby changing the distance between the first reflector and the second reflector.

9. A pulsed laser delay method based on polarization state adjustment, characterized in that, The pulsed laser delay device based on polarization state adjustment according to any one of claims 1 to 8 includes the following steps: The laser beam enters from the Faraday isolator and passes through the half-wave plate, forming vertically polarized light. Vertically polarized light is reflected by a 45° polarizer and enters the electro-optic crystal; Electro-optic drive applies a quarter-wave voltage to the electro-optic crystal, causing vertically polarized light to be converted into circularly polarized light, which is then reflected by the first mirror and passes through the electro-optic crystal again to be converted into horizontally polarized light. Horizontally polarized light passes through a 45° polarizer and is reflected multiple times between the first and second reflecting mirrors; After the preset delay time is reached, the electro-optic drive applies a quarter-wave voltage to the electro-optic crystal again, causing the horizontally polarized light to be converted into circularly polarized light and then reflected by the first mirror before passing through the electro-optic crystal again and being converted into vertically polarized light. Vertically polarized light is reflected by a 45° polarizer and passes through a half-wave plate before exiting through the exit aperture of a Faraday isolator.

10. The pulsed laser delay method based on polarization state adjustment as described in claim 9, characterized in that, By controlling the movement of a one-dimensional displacement stage to change the distance between the second and first reflectors, and simultaneously adjusting the time interval of the voltage applied by the electro-optic drive, continuous fine-tuning of the laser delay time can be achieved.

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