Analysis of wavelength stabilization technology of

2022-10-21
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Analysis of wavelength stabilization technology of high-power semiconductor lasers

as a mature laser source, high-power semiconductor laser system is widely used in the field of material processing and solid-state laser pumping. Although high-power semiconductors have many advantages such as high conversion efficiency, high power, strong reliability, long life, small size and low cost, the relatively poor spectral brightness is a disadvantage that cannot be ignored. The typical spectral bandwidth of semiconductor laser bar is about 3 ~ 6nm, and the peak wavelength will drift due to the influence of working current and temperature

generally, Nd doped solid crystals pump their relatively wide 808nm absorption band. Standard semiconductor laser systems can easily meet the spectral requirements of 808nm pumping. However, in the past few years, with the continuous improvement of the working current and power of the semiconductor laser bar, greater wavelength drift has occurred in the process of rising from the threshold current to the working current. In order to ensure stable and effective pumping in the whole working range, it is necessary to control the spectrum of the pumped semiconductor laser so that its spectral bandwidth always matches the absorption bandwidth of the active laser medium

on the other hand, the rapid development of fiber lasers has also increased the demand for pump sources of other wavelengths. For example, a standard ytterbium doped fiber laser with a pump wavelength of about 1080nm requires fiber coupled semiconductor laser systems of 915nm, 940nm and 980nm, especially in the 980nm Pump region, because the ytterbium doped material has a high absorption coefficient and a narrow absorption bandwidth in the pump region

generally, the typical spectral width of high-power semiconductor laser module is about 3 ~ 6nm, and its central wavelength will drift with the change of temperature and driving current, which is a great obstacle for pump applications with small absorption bandwidth. The wavelength stability of high-power semiconductor laser module is very important for effectively pumping solid-state lasers with narrow absorption bandwidth

volume holographic grating wavelength stability technology can help high-power semiconductor laser module achieve stable wavelength. Of course, in order to achieve reliable wavelength stability, we must carefully select the relevant parameters of volume holographic grating and semiconductor laser module

another new pump wavelength is nd:yvo4 pumped at 888nm. Compared with 808nm pump, the advantage of 888nm pump is that the wavelength is in the isotropic absorption region, that is, it has the same absorption coefficient in all polarization directions, and the quantum loss is small. [1]

one of the applications with the highest requirements for spectral linewidth is the optical pumping of alkali metal vapor lasers (such as rubidium or cesium), which requires a linewidth of about 10GHz. For these applications, in order to achieve effective pumping, it is necessary to control the spectrum of the semiconductor laser pump source. [2]

another disadvantage of the high-power semiconductor laser system composed of multiple semiconductor laser bar bars is the relatively poor beam quality and brightness B. the following formula is the definition of B. The brightness of the semiconductor laser beam is determined by the laser power P and the beam parameter product (BPP) in the slow axis and fast axis directions. [3]

the output beam of bar bar bar of ordinary large area semiconductor laser is characterized by parameters that are highly asymmetric for beam size and divergence angle. The beam quality in the fast axis direction is about 1mm · mrad, which is close to the diffraction limit; However, the beam quality of standard 10mm large area semiconductor laser in the slow axis direction of bar is between 400 ~ 500mm · mrad, which far exceeds the diffraction limit

in recent years, by increasing the output power of each emitter and reducing the slow axis divergence angle, the brightness of the semiconductor laser bar has been significantly improved. These developments have led to the design of new semiconductor lasers with reduced number of emitters and increased spacing between emitters. These mini bar bars have more advantages than the traditional 10mm large area semiconductor laser bar bars. [4]

the brightness of semiconductor laser system is further enhanced through polarization coupling and wavelength multiplexing. Polarization coupling can only increase the brightness by twice a unit coefficient, while wavelength multiplexing technology is limited by the number of available wavelengths n. In fact, the power through wavelength multiplexing is mostly due to the air expansion in the oil pump at the expense of spectral brightness

wavelength multiplexing of standard semiconductor laser sources and wavelength couplers based on non dielectric films require a spectral width of about 30nm. By using a semiconductor laser source with a stable narrow-band emission spectrum and a volume holographic grating as a combination unit, the spectral distance can be significantly reduced to 3nm. [5] As a result, for a given spectral range, the number of semiconductor laser bar bars that can be multiplexed increases, thereby enhancing the brightness

The greater advantage of the semiconductor laser module with stable spectrum is that its sensitivity to working temperature and current is reduced, which makes the cooling system simpler. In addition, the specification requirements for chip materials are also reduced, which improves the utilization of wafers in production; It also eliminates the wavelength change ("red shift") caused by the increase of the working time of the semiconductor laser. However, it should be pointed out that all these advantages depend on the locking range of the volume holographic grating

basic concept of wavelength stabilization

method of wavelength stabilization

in the past, it was 5 The effective experimental width (mm): 400 improves the spectral brightness of the bar bar bar of the semiconductor laser. Researchers have discussed some different methods. These methods can be divided into laser internal and external solutions. The internal solution integrates the wavelength stabilization structure into the semiconductor laser bar, while the external solution separates the volume holographic grating from the Bragg grating to stabilize the wavelength

distributed feedback semiconductor laser (DFB) is a typical example of using an internal wavelength stabilization solution. The grating for selective spectral feedback is integrated into the active region structure of the laser bar. In this way, the drift index of wavelength with temperature will be reduced to about 0.08nm/k, and the spectral bandwidth will be reduced to less than 1nm. [6,7,8] it is obvious that the manufacturing process of this DFB semiconductor laser is more complex, resulting in increased costs. Another disadvantage of this laser is its low efficiency

in addition to the internal wavelength stabilization scheme, researchers also discussed the solution of wavelength stabilization through external components. An example of an external wavelength stabilizing element is a thick volume grating based on photothermal refractive (PTR) inorganic glass. This kind of grating can record high-efficiency Bragg grating in this photosensitive glass through the periodic change of refractive index under UV irradiation. There are different manufacturers selling this volume diffraction grating on the market, but the names are slightly different, such as volume Bragg grating (VBG) [9], volume holographic grating (VHG) [10], or volume Bragg grating laser (vobla) [11]

contrary to the internal solution, the external wavelength stabilization does not require any modification to the chip structure, that is, the wavelength stabilization of the standard large-area semiconductor laser bar can be achieved through the external volume holographic grating. This is an important advantage of external solutions. In addition, compared with the internal solution, the external wavelength stabilization solution can obtain smaller temperature drift and spectral bandwidth: the temperature drift can be reduced to about 0.01nm/k, and the spectral width can be reduced to less than 0.3nm. However, an important disadvantage of the external wavelength stabilization scheme is the need for sensitive and highly aligned VHG

Figure 1 shows the typical composition of a semiconductor laser bar with an external wavelength stabilization scheme. The angular sensitivity of VHG is conducive to reducing the divergence of the bar bar bar of the semiconductor laser, especially using the fast axis collimating lens (FAC) to collimate the beam in the fast axis direction. VHG will significantly improve optical feedback. VHG is placed directly after fac. The table in Figure 1 shows the typical alignment tolerances required for effective wavelength stabilization

Figure 1 Typical composition of semiconductor laser bar with wavelength stabilization scheme, VHG is placed directly behind fast axis collimating lens (FAC). The table shows the typical alignment tolerance of the composition shown in the figure

the influence of semiconductor laser parameters on the external wavelength stability performance

in order to obtain an effective and stable wavelength stability scheme, the relevant parameters of the semiconductor laser bar must be carefully controlled, including the reflectivity of the antireflection film on the output surface, emitter structure, cavity length, smile effect, angular emission characteristics and installation technology, These parameters will affect the drift of wavelength with working current and temperature

VHG performance can be optimized by refractive index modulation and changing spatial frequency and thickness. These three independent parameters determine the Bragg angle, diffraction efficiency, spectrum and angle selectivity of the grating. In principle, these VHG parameters must be optimized separately for each configuration. However, according to experience, for most commonly used semiconductor laser bar, VHG reflectivity is about 20%. Of course, compared with the semiconductor bar bar without wavelength stabilization scheme, for a given current, the bar with wavelength stabilization scheme will reduce the output power due to the insertion of a VHG. VHG with higher reflectivity will increase the locking range at the cost of higher power loss. This means that the optimization of wavelength stability always needs to make a trade-off between locking range and power loss. In addition, it is important to note that the selection of the best reflectivity also depends on the application needs. For some applications, VHG needs to be optimized to obtain a large locking range, while for fixed working conditions, it may require low loss

as mentioned earlier, the most common external wavelength stabilization scheme is to place a separate block VHG directly behind the fast axis collimating lens. An important disadvantage of this layout is its sensitivity to the smile effect. Due to the smile effect, some emitters are not exactly on the optical axis, resulting in a deflection angle after collimation, which eventually leads to the offset of the reflected light relative to the initial position of the emitter (see Figure 2). Emitters that are not on the optical axis will receive less optical feedback, as shown in the right figure in Figure 2

Figure 2 Influence of smile effect on the optical feedback of semiconductor laser bar using volume holographic grating wavelength stabilization technology

one way to overcome the sensitivity of smile effect is to integrate the grating structure into fac. [12] Such elements are not sensitive to the smile effect and non collimation. Due to the larger divergence angle of the uncollimated beam and the small angle selectivity of the grating, only a small part of the beam is reflected back into the semiconductor laser cavity. In the case of non collimation or smile effect, another part of the beam will be reflected to provide feedback. On the contrary, integrating the grating into fac, an ideal situation of this scheme is to have accurate collimation and no smile effect. At this time, almost all the light reflected from VHG is coupled to the semiconductor laser cavity. On the other hand, this means that in order to obtain effective wavelength locking, the reflectivity of vhg-fac needs to be significantly increased to 70%

the greater advantage of fac with VHG integration is that only one independent component needs to be operated and adjusted. One disadvantage of vhg-fac is the relatively low refractive index (n=1.45) of PTR materials based on quartz. Fac is usually made of high refractive index materials such as s-tih53 or n-laf21. If materials with low refractive index are used, for the same

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