How to Solve the Problem of High Reflection of Fiber Laser?

What is high reflection?

Generally speaking, high reflection refers to the low resistivity of processed materials, smooth surface and low absorption rate of the near-infrared laser.

As shown in the figure, it is the absorption coefficient of common metal elements, which leads to the emission of a large number of lasers.

In addition, most laser use scenarios are perpendicular to the material or a small inclination angle.

As a result, the return laser re-enters the output head, and even part of the return light is coupled into the energy transfer fiber, which is transmitted back to the inside of the laser along the fiber, resulting in the continuous high temperature of the core components in the laser.

What is high reflection

Specific implementation scheme of anti-high reflection

In the process of high reflection material processing, once too high reflection laser enters the laser source, there will be a certain risk of damage to the cutting head or welding head and the laser source itself.

Especially for high-power fiber laser products, the return laser power will be significantly higher than that of low-power laser products, and the risk of damage will be higher.

When cutting high reflection material, when the material is not penetrated, high-power return light will return to the inside of the laser and damage the laser.

Aiming at the above problems, Raycus laser R & D personnel have designed a four-level anti-high reflection light protection design, and will add various return light monitoring functions to ensure the real-time protection of the laser in case of abnormal processing.

01 Class IV anti-high reflection light design

Anti-high reflection design of optical cable output head

QBH, QD and QP optical fiber output heads designed by Raycus laser can efficiently convert uncontrollable return light into absorbable light and heat, and improve the heat absorption and heat dissipation capacity of the output head, so as to avoid the impact of return light on internal components to the greatest extent.

For the laser processing system, the return laser will first affect the output optical cable head, causing the risk of heating and even damage to the output optical cable.

In order to ensure the safety of the output optical cable, we have integrated a primary return light stripping device inside the output optical cable on the basis of the original design, as shown in the figure.

Anti high reflection design of optical cable output head

The anti-high reflection design added to the output end of the optical cable can peel off most of the returned laser at the first time and reduce the damage to the internal optical structure of the laser.

At the same time, combined with the water-cooling design, it can effectively absorb the stripped returned laser and avoid its thermal impact on the output head of the optical cable.

Two-stage anti-reflective design in beam combining module

Multi-module high-power laser products are mainly based on multi-unit module beam combinations.

When cutting high reflection materials, after the return light passes through the primary anti-high reflection design at the optical cable, some light may still be transmitted reversely to the inside of the beam combination through the output optical cable.

Therefore, considering the safety and reliability of the optical devices and optical path design in the beam combination, a two-stage anti-high reflection protection design is added in the beam combination, as shown in the figure.

Two-stage anti reflective design in beam combining module

The return laser is stripped step by step to ensure the safety of the anti-high reflection protection optical devices and reduce the impact of the return laser on the optical path structure of the laser.

Anti-reflective design in the unit module

Because there is an optical resonator inside the unit module, once the low-power return laser enters the unit module and is continuously amplified by the resonator, it will seriously affect the optical stability of the laser and increase the risk of laser damage.

In order to further improve the stability and reliability of the laser during high reverse cutting, combined with the optical path design inside the unit module, the anti-high reflection device of the unit module is added to improve the reliability of the module, as shown in the figure.

Anti reflective design in unit module

02 Multiple high anti-reflection software monitoring design (imported successively)

High reflection software protection is mainly high reflection monitoring protection, which refers to the monitoring or protection mechanism of laser operation state when the strong retroreflection generated by the laser enters the laser optical system when processing high reflection materials, resulting in instability of laser operation or damage to optical devices.

Although the anti-high reflection device can ensure that the laser can still work normally without damage within a certain return laser power threshold, the laser will still have the risk of damage after exceeding this threshold.

Therefore, in order to ensure that the laser can be protected in time when the return laser is too high, multiple high anti-software protection design is added in the multi-module high-power laser product:

Design of high anti-interference protection for output optical cable

When the return optical power is high, part of the returned laser will return to the output optical cable along the original optical path, and the other part will directly incident on the front end of the output optical cable.

Through the research on the return laser transmission and the monitoring function integration technology, we have integrated the optical cable temperature monitoring multiple detection functions such as laser monitoring and return light monitoring are used for real-time monitoring of return laser.

Once the return laser power exceeds the range that the laser can bear, the laser will turn off the light and give an alarm in time to ensure that the laser will not be damaged.

At the same time, it can also remind the customer that there is an abnormality in processing.

Beam combining and optical module high anti-protection design

Considering that there are two kinds of return laser: core laser and cladding laser, in addition to the high anti-protection design of the output optical cable, we also added a high anti-detection module inside the beam combination to detect the reverse transmission laser power and monitor the power of the return laser in real-time.

This high reflection detection module can detect not only the cladding laser, but also the core laser, which can effectively avoid the risk of laser damage caused by the core return laser when only the cladding return laser is detected.

Once there is a high return laser in the laser, it can also turn off the laser and give an alarm in time to ensure the safety and reliability of the laser.

03 Cutting test

Due to the low laser absorptivity and high thermal conductivity of red copper, the surface of red copper is always in a mirror state, and a continuous stream of laser returns to the optical fiber output head, so as to verify the anti high reflection ability of the new optical fiber output head.

Cutting aluminum

Cutting aluminum

Cutting red copper and abnormal cutting

Cutting red copper and abnormal cutting

As long as the operation is proper, Raycus laser can control high reflection materials such as aluminum plate, brass and red copper, with efficient cutting and good section effect.

For the abnormal processing test of high reflection, it is to test the anti-high reflection ability of the laser.

By marking back and forth on the red copper plate, it does not penetrate the red copper plate.

At this time, the return light is very high, and the laser can still be used normally, indicating that the Raycus laser has a strong anti-high reflection ability.

Signal display

Signal display

Through the above high reflection test, it can be seen that the high inversion monitoring signal can be transmitted to the display screen in real time during the cutting process.

Under the condition of abnormal processing, the high inverse signal can be monitored, and its signal is within the laser threshold.

When the high reflection exceeds a certain threshold, the machine alarm will be triggered to remind the staff to check whether the operation is wrong.

For cutting high reflection materials, the suggestions below are put forward by the process engineer:

(1) When cutting brass, red copper and other materials, the speed should be conservative and leave some space, not too extreme.

(2) Red copper must be cut with oxygen, not nitrogen or air.

When the following problems occur, please stop the machine immediately for inspection.

(1) The lower protective mirror is polluted.

(2) Cut the high reflection material and the perforation is not penetrated.

(3) Cutting high reflection material, the perforation penetrated, but the cutting did not cut through.

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