Laser Ranging Technology - Laser Pulse Ranging Knowledge Sharing

I. Principles of Laser Rangefinding Technology

Laser Beam Emission: A laser diode or other laser devices, stimulated by electric current, produce a high-density, high-monochromaticity, and highly directional laser beam.

1. Laser rangefinding technology relies on the relationship between the speed of light and time, utilizing high-speed laser beams reflected from a target and returned to the origin to calculate distance. Its principles mainly encompass the following aspects:

2. Laser Beam Transmission and Reception: The laser beam is emitted by an optical system, illuminating the target and reflecting back to a detector.

3. Speed of Light: The speed of light in a vacuum is approximately 299,792,458 meters per second, forming the foundation of laser rangefinding.

4. Time Calculation: The rangefinder records the time interval from emission to reception.

5. Distance Calculation: Utilizing the formula "speed equals distance divided by time," the distance between the target and the rangefinder can be calculated using the recorded time interval and the speed of light.

II. Methods of Using Laser Rangefinding Technology

While the application of laser rangefinding technology is relatively straightforward, attention to detail is crucial to ensure accurate and reliable measurements.

1. Proper Use of Laser Rangefinder: Firstly, operators should thoroughly read the rangefinder's manual and familiarize themselves with its functions and operations. Before measuring, ensure that the laser beam is aimed at the target and adjust the focus for precision.

2. Avoiding Measurement Errors: To guarantee accuracy, it's essential to minimize interference during measurements. For instance, ensure there are no obstacles between the rangefinder and the target, and reduce disturbances from atmospheric humidity, smoke, or fog.

3. Selecting the Appropriate Measurement Mode: Laser rangefinders often offer various measurement modes, such as single-point or continuous measurement. Choose the suitable mode based on specific needs.

4. Assessing Measurement Results: After measuring, carefully evaluate the accuracy of the results. Taking multiple measurements and averaging them can help reduce errors. Additionally, comparing the results with other measurement methods can verify the rangefinder's accuracy.

III. Applications of Laser Rangefinding Technology

Laser rangefinding technology finds widespread applications in various fields. Here are some common scenarios where it's used:

1. Architectural Surveying: In the field of architecture, laser rangefinding technology aids in measuring and drawing floor plans and three-dimensional maps of buildings. It also facilitates precise dimension measurements and layout planning.

2. Geological Exploration: In geological surveys, laser rangefinders are utilized to measure terrain elevations and ground height differences, providing valuable data support for geological exploration and disaster warning systems.

3. Military Applications: Laser rangefinding technology has extensive applications in the military, especially in identifying and measuring distant targets, such as long-range artillery fire and drone navigation.

4. Robot Navigation: This technology is employed in robot navigation and sensing. By measuring the distance between the robot and obstacles, it helps robots avoid collisions and plan optimal paths.

5. Smart Homes: Laser rangefinding technology contributes to human sensing and distance detection in smart homes, enabling smart devices to better respond to human behavior and needs.

Based on different measurement methods, laser rangefinding can be categorized into pulse rangefinding and phase rangefinding. The former offers lower measurement accuracy, suitable for military and engineering surveys where precision isn't a top priority. The latter provides higher accuracy and is widely used in geodetic and engineering surveys.

Introduction to Laser Pulse Rangefinding

1. Principle of Laser Pulse Rangefinding

Since the speed of light is a constant and light travels in a straight line, by measuring the round-trip travel time of the light beam over the distance to be measured, the straight-line distance between two points can be calculated. The principle of laser pulse rangefinding is to control the timer to start by emitting a laser pulse and stop by receiving the returned laser pulse. This measures the round-trip travel time of the laser beam over the distance to be measured, completing the ranging process.

2. Structure of Laser Pulse Rangefinder

The simplified structural diagram of the laser pulse rangefinder is shown in Figure 6-18. Its working process is roughly as follows: When the rangefinder is aimed at the target, the laser emits a strong and narrow light pulse. This light pulse passes through the transmitting telescope to compress the divergence angle. Taking the ruby laser as an example, its beam divergence angle is generally a few milliradians, which is compressed to a few tenths of a milliradian through the transmitting telescope. Such a light pulse, when shot to a distance of 10km, will only form a light spot with a diameter of a few meters. When the light pulse is emitted, a tiny portion of it is immediately reflected by two mirrors into the receiving telescope. After passing through a filter, it reaches the photoelectric converter, turning the light pulse into an electrical signal. This electrical pulse, after amplification and shaping, is sent to the time measurement system to start timing. The light pulse directed towards the target will always have a portion reflected back due to the target's diffuse reflection. This reflected light enters the receiving telescope, passes through the filter, photoelectric converter, amplification and shaping circuit, and enters the time measurement system to stop the timing. The time recorded by the time measurement system is calculated and directly displayed on the monitor, showing the distance from the rangefinder to the target.

3. Requirements for Light Pulses in Laser Pulse Rangefinders

To expand the measurement range and improve measurement accuracy, the rangefinder has the following requirements for light pulses:

(1) The light pulse should have sufficient intensity.

Regardless of how the beam's directionality is improved, it will inevitably have a certain divergence. Coupled with the air's absorption and scattering of light, the farther the target, the weaker the reflected light, or it may not even be received. To measure longer distances, the light source should emit light with high power density.

(2) The directionality of the light pulse should be good.

This serves two purposes. On the one hand, it can concentrate the light energy within a smaller solid angle, ensuring a longer shooting distance while improving confidentiality. On the other hand, it can accurately determine the target's location.

(3) The monochromaticity of the light pulse should be good.

Whether it's day or night, there will always be various stray lights in the air, which are often much stronger than the reflected light signal. If these stray lights and the light signal enter the receiving system together, measurement becomes impossible. The filter in Figure 6-18 allows only monochromatic light from the light signal to pass through, blocking other frequencies of stray light. Obviously, the better the monochromaticity of the light pulse, the better the filtering effect, effectively improving the signal-to-noise ratio of the receiving system and ensuring measurement accuracy.

(4) The width of the light pulse should be narrow.

The width of a light pulse refers to the time interval between the "occurrence" and "extinction" of the flash. A narrower light pulse width can avoid overlap between the reflected light and the emitted light. Due to the high speed of light, if the measurement distance from the rangefinder to the target is 15km, the round-trip time for the light pulse is only one ten-thousandth of a second. Therefore, the light pulse width must be much less than one ten-thousandth of a second for normal measurement. For closer distances, the light pulse needs to be even narrower.

Currently, lasers used in rangefinders include ruby lasers, neodymium glass lasers, carbon dioxide lasers, semiconductor lasers, etc. Solid-state lasers are commonly used as pulsed light sources in long-range rangefinders, while semiconductor lasers are more commonly used in short-range rangefinders.

4. Generation of Laser Giant Pulses

The power of the light pulses used in ranging is very high, typically with a peak power of over one megawatt and a pulse width of less than tens of nanoseconds. Such light pulses are often called "giant pulses." The general laser pulses are not giant pulses. They have a wider pulse width (about 1ms) and insufficient pulse power, so they cannot meet ranging requirements. By adopting the Q-switching technique introduced in Section 4.6, the laser can be made to meet the ranging requirements.

5. Distance Display

The round-trip time of pulses in pulse ranging is very short, so high-frequency oscillating crystals are usually used to record the number of vibrations for timing. [Note: There seems to be a formatting error or missing information after "A.10" in the original text. Assuming it's a reference to a figure or diagram, it can be ignored or replaced with an appropriate description in the translation.] The principle block diagram of such equipment is shown in the figure.

When the emitted reference optical pulse enters the receiver and is converted into an electrical pulse, it is input into the "main gate" (main gate circuit) in Figure 6-19, which simultaneously opens the main gate. At this time, the electrical pulses generated by the quartz crystal oscillator pass through the main gate and enter the counter, which begins counting. Simultaneously, the digital display continuously indicates the number of electrical pulses recorded by the counter. When the reflected optical pulse signal enters the receiver and is converted into an electrical pulse that is input into the main gate, the main gate immediately closes, and the electrical pulse signal generated by the quartz crystal oscillator can no longer enter the counter, causing the counter to stop counting. The number displayed on the monitor represents the number of electrical pulses generated by the oscillator during the time from when the optical pulse was emitted until it returned.

The ranging accuracy of laser pulse rangefinders is mostly in the order of "meters", which is suitable for certain projects in military and engineering surveys where high precision is not required. Pulse methods are also utilized for long-distance spatial measurements because, for remote spaces, a measurement error in the order of "meters" is already considered to be quite high in precision.

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