The principle of erbium glass laser rangefinder

Laser ranging technology has the advantages of easy operation, simple system and can work both during day and night compared with general optical ranging technology. Compared with radar ranging, laser ranging has good anti-interference and high accuracy, and the laser has good ability to resist electromagnetic wave interference. Its superiority in laser ranging is more obvious when the detection distance is longer. Optical ranging technology is a distance measurement technique that uses laser pulses or continuous wave laser beams directed at the target to measure the distance to the target. The more commonly used laser ranging methods are triangulation, pulse method and phase method laser ranging.

1.Triangulation laser ranging

Laser displacement sensor measurement method is called laser triangle reflection method, the accuracy of laser rangefinder is certain, the same rangefinder measuring 10 meters and 100 meters accuracy is the same. And laser triangle reflection method measurement accuracy is related to the range, the longer the range, the lower the accuracy.

Precision sensors for non-contact position and displacement measurement using the laser triangulation principle and echo analysis principle. It is widely used in industrial measurement of position, displacement, thickness, radius, shape, vibration, distance and other geometric quantities. The semiconductor laser 1 is focused by the lens 2 on the object to be measured 6. The reflected light is collected by the lens 3 and projected onto the CCD array 4; the signal processor 5 calculates the position of the light point on the array 4 to get the distance from the object by the triangular function.

Figure 1. Laser triangulation schematic

The laser emitter shoots the visible red laser through the lens to the surface of the object, and the laser reflected by the object passes through the receiver lens and is accepted by the internal CCD linear camera, which can “see” the light point at different angles depending on the distance. According to the distance between the laser and the camera, the digital signal processor will be able to calculate the distance between the sensor and the measured object.At the same time, the position of the light beam at the receiving element is processed by analog and digital circuits and analyzed by a microprocessor to calculate the corresponding output value and output a standard data signal proportionally within the analog window set by the user. If a switching output is used, it conducts within the set window and cuts off outside the window. In addition, separate detection windows can be set for analog and switching outputs.Independent detection windows are available for analog and switching outputs. Commonly used for rail, product thickness, flatness, dimensions, etc.

2.Pulse method laser ranging

The pulse method of distance measurement is as follows: the laser emitted by the rangefinder is reflected by the measured object and then received by the rangefinder, which also records the round-trip time of the laser. The product of the speed of light and the round trip time is half of the distance between the rangefinder and the measured object. The accuracy of the pulse method of distance measurement is generally in the range of +/- 1 meter. In addition, the measurement blind area of such rangefinders is generally about 15 meters. With the development of optoelectronics technology, the accuracy is improving and the blind spot is shrinking. This method is mainly used for terrain measurement, tactical front range measurement, missile operation orbit tracking, LIDAR range measurement, and artificial satellite, earth and moon distance measurement, etc.

Fig. 2. The principle diagram of pulse method measurement

The principle is that a laser transmitting system emits a pulsed laser of very short duration, which is reflected by the target object after passing through the distance to be measured L. The transmitting pulsed laser signal is received by the photodetector in the laser receiving system, and the time interval circuit derives the distance between the target object and the emitted distance L by calculating the time t between the laser emission and the arrival of the return signal. its accuracy depends on: the rising edge of the laser pulse, the the bandwidth of the receiving channel, the detector signal-to-noise ratio and the time interval accuracy.

3.Phase laser rangefinder

The phase laser rangefinder uses the frequency of the radio band to amplitude modulate the laser beam and measure the phase delay caused by the modulated light going to and from the line once, and then converts the distance represented by this phase delay according to the wavelength of the modulated light. In other words, the time required for the light to travel to and from the line is measured by an indirect method, as shown in Figure 3.

Figure 3. phased laser ranging principle

The blue is the emitted light and the red is the reflected light. It is clear from the figure that when the emitted light is emitted after encountering the wall, the reflected waveform is actually a mirror image of the waveform after not encountering the wall, so the size of the phase difference is not related to the speed of light, but to when the emitted light encounters the obstacle.

Phased laser rangefinders are generally used in precision ranging. Due to its high accuracy, generally in the millimeter range, in order to reflect the signal effectively and to limit the measured target to a specific point commensurate with the accuracy of the instrument, for such rangefinders are equipped with reflectors called cooperative targets.

4.Interferometric rangingmethod

Interferometry is a method of ranging using various interferometers based on the interference principle of light waves. The emergence of laser has made the application of interferometric ranging method more extensive. The basic principle of laser interferometry is shown in Figure 4：

Figure 4. Interferometric measurement principle diagram

The light from the laser is divided into two beams by the beam splitter, one beam is shot to the fixed reference arm of the interferometer, and the reference beam is formed after the return of the reference mirror M1; the other beam is shot to the measurement arm of the interferometer, and the reflector M2 in the measurement arm will move with the displacement of the measured object, and this beam forms the measurement beam after the return of the measurement mirror; the interference signal is formed by the superposition of the measurement beam and the reference beam on each other. The number of light and dark changes of the interference signal directly corresponds to the displacement of the measuring mirror, which can be expressed as: L=Nλ/2. Therefore, the value of the measured displacement L can be derived from the counting of N by the starting counting point of the pair N issued by the photoelectric microscope.

5.Erbium Glass Laser Rangefinder：

Erbium glass laser rangefinder is generally used in industrial and military rangefinding. Its small size and high accuracy, also known as the human eye safety rangefinder, is now widely used. The following is a small 4km small erbium glass laser rangefinder principle rangefinding capability calculation developed and produced by Erdi Laser Ltd.

Targets and condition requirements

Visibility≥8km

Humidity≤80%

For vehicles with 2.3m×2.3m dimension

Reflectivity=0.3

Ranging ability≥4km

Analysis and verification

The main parameters that affect ranging ability are peak power of lasers, divergence angle, transmitting and receiving transmittance, wavelength of laser, etc.

For this laser rangefinder, it takes ≥40kw peak power of lasers, 0.5mrad divergence angle, 1535nm wavelength, transmitting transmittance≥90%, receiving transmittance≥80% and 25mm receiving aperture.

It is a laser rangefinder for small targets, ranging ability can be calculated by the following formula. Ranging formula for small targets：

Detectable optical power

Transmitting power of laser rangefinder(40kw)

Transmitting transmittance(0.9)

Receiving transmittance(0.8)

Optical receiving area(25mm receiving aperture)

Effective reflection area of targets(5.29㎡)

Target reflectivity(0.3)

Atmosphere attenuation coefficient(0.2)

Visibility(according to testing condition)

Distance to targets

As long as detectable optical power that reflected by targets is larger than minimum detectable power, a laser rangefinder is able to range distance to a target. For a laser rangefinder with 1535nm wavelength, generally, the minimum detectable power(M.D.S) of APD is 5×10-9W.

Under 8km visibility with 6km distance to targets, the minimum detectable power is almost close to M.D.S of APD(5×10-9W), therefore, under a condition with 8km visibility, a laser rangefinder can range distance for (2.3m×2.3m) targets up to 6km(might be close or less than 6km).

Technical parameter

Item	Technical parameter	Instruction
Working wavelength	1535±5nm
Ranging range	50m~4km	2.3m×2.3m vehicle target, 0.3 diffuse reflectance, visibility ≥5km;
	50m~8km	Energy intensity ≥10km, 0.3 large reflectivity target;
Ranging accuracy	±2m	3δ
Ranging frequency	1 ~ 10 Hz is adjustable
Accuracy rate	≥98%
Divergence angle	≤0.5mrad
Receiving caliber	25mm
Communication Interface	422、485、232、TTL（3.3V）
Voltage	DC9~36V
Working power consumption	≤1.2W(@1hz)	Normal temperature test
Standby power consumption	≤0.2W	Normal temperature test
Size	≤65mm×44mm×35mm
Weight	≤75g