Fundamentals of fiber lasers

Fiber lasers are composed of three basic elements: pump source, gain medium, and resonator. The pump source generally uses semiconductor laser diode (LD), the gain medium is rare earth-doped fiber (usually ytterbium-doped fiber, YDF), and the resonator is generally composed of a grating and a gain fiber. The pumped light generated by multiple laser diodes enters the resonator through the forward and backward beam combiners, and the gain fiber forms a particle number reversal and generates radiation after absorbing the pumped light, and the resulting radiation light undergoes excitation amplification and forms a stable laser output through the backward beam combiner. 

The resonator   basically structured

The fiber laser resonator is mainly composed of three parts: high reflection grating, ytterbium-doped fiber, and low reflection fiber grating. 

The basic structure of ytterbium-doped optical fiber is shown in the figure, from the inside to the outside is the core, the inner cladding layer, the outer layer and the coating layer. The fiber core is doped with the rare earth element “ytterbium”. The pumped light is injected into the inner envelope and repeatedly passes through the core, where it is absorbed by ytterbium ions, thereby converting into a 1070 nm fiber laser. 

The light grid not only plays a positive feedback role in the resonant cavity structure, but also has the role of mode selection. Photons bounce back and forth between the two gratings, repeatedly passing through the gain medium, each time firing a new photon, and eventually the beam becomes more and more energetic. Only light of a specific wavelength and direction can be output from a low-reflection grating.

What is 976nm

After understanding the above basic structure and principle, we first start from the characteristics of the gain fiber, the following figure shows the absorption and emission spectrum of the ytterbium-doped fiber, it can be clearly seen that there are two strong absorption peaks in the 915nm and 976nm bands, so the pumped optical band usually used for ytterbium-doped fiber lasers is 915nm or 975nm. Among them, the absorption peak of 975 nm is higher, which is about 3 times that of 915 nm.

The 976nm has a higher electro-optical conversion rate

Since the absorption of 976nm is 3 times that of 915nm, the 1070nm laser that produces the same power consumes less 976nm pump light. The pump light is converted from electrical energy, which means that the use of 976nm pump source consumes less electrical energy, higher photoelectric conversion rate, and more efficient energy saving. Comprehensive analysis shows that the electro-optical conversion rate of 915nm is about 30%, while the electro-optical conversion rate of 976nm can reach more than 42%.

976nm has a higher optical conversion rate

According to Planck’s quantum hypothesis: photons have energy, the amount of energy depends on the frequency 

v (that is, the reciprocal of the wavelength), the smaller the wavelength, the greater the frequency, the greater the energy. When the ytterbium ion absorbs 915nm or 976nm pump light to emit a 1070nm laser, it will produce a loss of energy, which we call a quantum loss, and the energy lost will directly produce heat, affecting the stability of the laser. 

When using a 976 nm pump light emission 1070 nm laser, the quantum loss rate of the electron transition is about 8.8% (1-976/1070), while the pump light emission of 915 nm is used  At 1070 nm laser, the quantum loss rate of electron transition is about 14.5% (1-915/1070). It can be seen that the 976nm pump has a greater increase in the optical conversion rate than the 915nm pump source.

Why other friends widely use 915nm pump source

Since ytterbium-doped fibers absorb light better at 976 nm wavelengths and have higher light conversion, why did most lasers, including now, use 915 nm pump sources?

Looking closely at the above figure, we can find that the absorption peak of 976 nm is relatively high but very narrow; while the absorption peak at 915 nm is relatively low and wide, and the height of the absorption peak is only one-third of 976 nm, but the absorption spectrum width is about 5 of 976 nm Times or so. For 976nm, a slight change in wavelength can seriously affect the absorption rate.

The output wavelength of the laser diode will drift with temperature. Typically, the temperature drift coefficient is about 0.31 nm/°C. This is reflected in the fact that temperature changes have a greater impact on the performance of the laser.

From the perspective of energy conversion, lasers contain the following three transformations:

(1) The 220V/380V electricity is converted into various levels of voltage through the power supply to drive the pump source

(2) The pump source converts electrical energy into pump light

(3) The pump light outputs the laser after the gain of the resonator is amplified.

All three of these conversion processes have certain energy losses, and most of the energy lost is converted into heat. The change in temperature will not only affect the absorption of pumped light by the ytterbium-doped fiber, reducing the efficiency of the laser, but also the unabsorbed pumped light will also cause damage to other components in the optical path.

The absorption peak of the ytterbium-doped fiber in the 915nm band is wider, and the wavelength drift of the laser diode has little effect on the absorption efficiency, and the laser is more stable and easier to control for the design of fiber lasers. As a result, the 915nm pump solution has been widely used in the fiber laser market in the past.

GW laser for the breakthrough and optimization of 976nm technology

GW Laser Tech has been committed to the research of 976nm technology since its inception, and has made technological breakthroughs and optimizations in the following aspects.

As mentioned above, with the gradual increase of fiber laser power, heat control becomes particularly important, and taking effective measures to dynamically control the heat inside the laser and suppress the generation of thermal effects has become a key element for the stable operation of 976nm pump lasers.

The heat generation of fiber lasers mainly comes from the two parts of the pump source and the ytterbium-doped fiber. Guanghui Laser creatively uses the front and back sides of the water-cooled plate to separate the pump source and the ytterbium-doped optical fiber on both sides, and carries out water-cooled heat dissipation for all hot spots that need heat dissipation of the laser, overcoming the limitations of the traditional water-cooled structure compact and the overall heat dissipation conflict; at the same time, several fans are installed on the water-cooled surface, and the combination of water-cooled and air-cooled can accelerate the internal temperature and humidity balance, accelerate system heat dissipation, and reduce local over-hot spots.

Fiber lasers are spliced to achieve the connection of fiber optic devices. In order for lasers to achieve higher power specifications, high-quality fiber splicing is very important. Losses at weld locations lead to reduced efficiency, deterioration in beam quality, and damage to pumps, fibers, and fiber optic devices. Guanghui Laser adopts unique weld point thermal management technology to overcome the technical problems of welding point and apply for related patents.

Fiber lasers are usually industrial processed in a high temperature, high humidity and dusty environment, and gain fibers and key solder joints are prone to aging in this environment for a long time, affecting beam quality and output efficiency. Guanghui Laser independently developed 976nm two-way pump optical fiber winding reel structure, the use of sealing rings and covers to seal the internal fiber and welding points in the winding reel, on the one hand to ensure that the heavy heat load at both ends of the fiber can be effectively dissipated, on the other hand for the external environment temperature and humidity has a certain isolation, can reduce the occurrence of condensation, and prevent dust from entering to improve the stability of the laser. At the same time, different loop radius can effectively filter out different specific high-order modes, and the output spot quality will be closer to single mode.

In addition, the optical energy density of the 976 nm wavelength is high, so the return light and residual light are highly likely to damage the internal key components, so their processing is also crucial. Guanghui Laser’s ABR anti-high reflection technology with independent intellectual property rights technically solves the problem of device damage by designing residual and return light stripping devices at key positions inside the laser and ensures the stable operation of high-power lasers.

Economic value of 976nm energy efficiency

Taking the 12kW high-power laser of Guanghui Laser 976nm technology as an example, compared with the 915nm pump laser of the same power of other friends, in the same use environment, the annual power saving is more than 60,000 yuan (estimated in the following table, the specific situation prevails). In today’s high-power laser price continues to decline, the competitive advantage is extremely obvious.

976nm is the general trend of the 10,000-watt laser era

As fiber lasers become more and more powerful, the drawbacks of using 915nm pump sources are becoming more and more prominent. Due to the low absorption efficiency of ytterbium fibers to the 915nm wavelength, in order to make the laser output higher power, more 915nm pumps are used, which means:

(1) The increase in the volume and weight of the laser machine;

(2) Greater electricity consumption;

(3) Generate more heat, require a more powerful chiller;

(4) The overall design difficulty of the laser is increased.

When the output power is high to a certain extent, the use of 915nm pumping schemes becomes extremely complex. Therefore, high-power fiber lasers need to adopt a more efficient pumping scheme, and 976nm pumping is the trend of the times.

Article from GW LaserTech