Wavelength stabilization technology for high power semiconductor lasers

1. Introduction

As a mature laser light source, high- power semiconductor laser systems are widely used in material processing and solid-state laser pumping . Although high-power semiconductors have many advantages such as high conversion efficiency, high power, high 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 bars is about 3 to 6nm, and the peak wavelength will drift due to the influence of operating current and operating temperature.

Typically, Nd-doped solid crystals are pumped for their relatively wide 808nm absorption band, and standard semiconductor laser systems can easily meet the spectral requirements of 808nm pumping. However, in the past few years, with the continuous increase in the operating current and power of semiconductor laser bars, a larger wavelength drift has been generated in the process of rising from the threshold current to the operating current. In order to ensure stable and effective pumping over the entire operating range, it is necessary to control the spectrum of the pump semiconductor laser so that its spectral bandwidth always matches the absorption bandwidth of the activated laser medium.

On the other hand, the rapid development of fiber lasers has also increased the demand for pump sources at other wavelengths. For example, a standard ytterbium-doped fiber laser with a pump wavelength of around 1080nm requires a 915nm, 940nm, and 980nm fiber-coupled semiconductor laser system, especially the 980nm pump region, because ytterbium-doped materials have a higher absorption coefficient and a narrower absorption bandwidth in this pump region.

Another new pump wavelength is to pump Nd:YVO4 at 888nm. Compared with 808nm pumping, the advantage of 888nm pumping is that this wavelength is in the isotropic absorption region, that is, it has the same absorption coefficient in all polarization directions and has a small quantum loss. [1]

One of the applications with the highest demands on spectral linewidth is the optical pumping of alkali metal vapor lasers (such as rubidium or cesium), which require linewidths of approximately 10 GHz. For these applications, efficient pumping requires controlling the spectrum of the semiconductor laser pump source. [2]

Another disadvantage of high-power semiconductor laser systems consisting of multiple semiconductor laser bars is the relatively poor beam quality and brightness B, where B is defined by equation (1). The brightness of a semiconductor laser beam is determined by the laser power P and the beam parameter product (BPP) a in the slow and fast axis directions. [3]

The output beam of a common large-area semiconductor laser bar is characterized by highly asymmetric parameters 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 in the slow axis direction of a standard 10mm large-area semiconductor laser bar is between 400 and 500mm•mrad, far exceeding the diffraction limit.

In recent years, the brightness of semiconductor laser bars has been significantly improved by increasing the output power per emitter and reducing the slow-axis divergence angle. These advances have led to new semiconductor laser designs with fewer emitters and larger emitter spacing. These mini-bars offer advantages over traditional 10 mm large-area semiconductor laser bars. [4]

Further enhancement of the brightness of semiconductor laser systems is achieved through polarization coupling and wavelength multiplexing. Polarization coupling can only increase the brightness by a factor of two, while wavelength multiplexing technology is limited by the number of available wavelengths n. In fact, wavelength multiplexing performs power expansion at the expense of spectral brightness.

Wavelength multiplexing of standard semiconductor laser sources, as well as wavelength couplers based on non-dielectric films, requires a spectral width of about 30 nm. By using semiconductor laser sources with stable narrowband emission spectra and volume holographic gratings as combined units, the spectral distance can be significantly reduced to 3 nm. [5] As a result, for a given spectral range, the number of semiconductor laser bars that can be multiplexed increases, which in turn increases the brightness.

The biggest advantage of spectrally stabilized semiconductor laser modules is that they are less sensitive to operating temperature and operating current, making the cooling system simpler. In addition, the specification requirements for chip materials are also reduced, improving the wafer utilization rate in production; and it also eliminates the wavelength change ("red shift") caused by the increase in the operating 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.

Different wavelength stabilization techniques are described below.

2. Basic Concepts of Wavelength Stabilization

2.1 Wavelength stabilization method

In the past, researchers have explored a number of different approaches to improve the spectral brightness of semiconductor laser bars. These approaches can be divided into laser internal and laser external solutions. Internal solutions integrate wavelength stabilization structures into the semiconductor laser bar, while external solutions separate volume holographic gratings from Bragg gratings to stabilize the wavelength.

Distributed feedback semiconductor lasers (DFB) are a typical example of an internal wavelength stabilization solution, where the grating for selective spectral feedback is integrated into the active region structure of the laser bar. In this way, the wavelength drift with temperature is reduced to about 0.08nm/K and the spectral bandwidth is reduced to less than 1nm. [6,7,8] Obviously, the manufacturing process of such DFB-semiconductor lasers is more complicated, resulting in increased costs. Another disadvantage of this laser is reduced efficiency.

In addition to internal wavelength stabilization solutions, researchers have also explored solutions that use external components to achieve wavelength stabilization. An example of an external wavelength stabilization component is a thick grating based on photothermorefractive (PTR) inorganic glass. This grating uses a periodic change in the refractive index under ultraviolet light to record an efficient Bragg grating in this photosensitive glass. Such volume diffraction gratings are sold by different vendors under slightly different names, such as volume Bragg grating (VBG) [9], volume holographic grating (VHG) [10], or volume Bragg grating laser (VOBLA) [11].

In contrast to internal solutions, external wavelength stabilization does not require any modifications to the chip structure, i.e. standard large-area semiconductor laser bars can be wavelength stabilized by external volume holographic gratings. This is an important advantage of external solutions. In addition, external wavelength stabilization achieves smaller temperature drift and spectral bandwidth than internal solutions: the temperature drift can be reduced to about 0.01 nm/K and the spectral width to less than 0.3 nm. However, an important disadvantage of external wavelength stabilization is the need for sensitive and highly aligned VHGs.

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

Figure 1: Typical alignment tolerances for the components shown in the figure. Typical
tolerances for rotation: x-axis: x-axis; y-axis: y-axis; z-axis: z-axis

2.2 Effect of semiconductor laser parameters on external wavelength stabilization performance

In order to obtain an effective and stable wavelength stabilization solution, the relevant parameters of the semiconductor laser bar must be carefully controlled. These parameters include the reflectivity of the output surface anti-reflection film, the emitter structure, the cavity length, the smile effect, the angular emission characteristics and the installation technology. These parameters will affect the wavelength drift with the operating current and the operating temperature.

The performance of the VHG can be optimized by refractive index modulation, varying the spatial frequency and thickness. These three independent parameters determine the Bragg angle, diffraction efficiency, spectral and angular selectivity of the grating. In principle, these VHG parameters must be optimized separately for each configuration. However, as a rule of thumb, for most commonly used semiconductor laser bars, the VHG reflectivity is around 20%. Of course, for a given current, the output power of a wavelength-stabilized bar will be reduced due to the insertion of a VHG compared to a semiconductor bar without wavelength stabilization. A VHG with higher reflectivity will increase the locking range at the expense of higher power loss. This means that the optimization of wavelength stabilization always requires a trade-off between locking range and power loss. Furthermore, it is important to note that the selection of the optimal reflectivity also depends on the application requirements. For some applications, the VHG needs to be optimized to obtain a large locking range, while for applications with fixed operating conditions, lower losses may be required.

As mentioned earlier, the most common external wavelength stabilization solution is to place a separate block VHG directly after 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 ultimately causes the reflected light to shift 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 the Smile effect on the optical feedback of a semiconductor laser bar using volume holographic grating wavelength stabilization technology.
Off optical axis: Off
optical axis On optical axis: On optical axis
Diode bar with smile: Diode laser bar with smile effect
Reflected intensity: Reflected intensity
Optical feedback by VHG: Optical feedback of VHG
Optical axis: Optical axis

One way to overcome the sensitivity to the smile effect is to integrate the grating structure into the FAC. [12] Such a component is insensitive to the smile effect and misalignment. Due to the larger divergence angle of the uncollimated beam and the small angular selectivity of the grating, only a small part of the beam is reflected back into the semiconductor laser cavity. In the case of misalignment or the presence of the smile effect, another part of the beam will be reflected to provide feedback. In contrast, an ideal case of integrating the grating into the FAC is to have precise collimation and no smile effect, in which case almost all the light reflected from the VHG is coupled into the semiconductor laser cavity. On the other hand, this means that in order to achieve effective wavelength locking, the reflectivity of the VHG-FAC needs to be significantly improved to 70%.

The greater advantage of the FAC with integrated VHG is that only one independent component needs to be operated and adjusted. One disadvantage of the VHG-FAC is the relatively low refractive index (n=1.45) of the quartz-based PTR material. FACs are usually made of high-refractive-index materials such as S-TiH53 or N-LAF21. If a material with a lower refractive index is used, a smaller radius of curvature can be used for the same focal length, which will affect the lens aberrations under high numerical aperture working conditions.

References:
1. L. McD onagh et. al.; “High-efficiency 60 W TEM00 Nd:YVO4 oscillator pumped at 888 nm”; Optics Letters Vol. 31, pp. 3297 (2006)
2. A. Gourevitch et. al.; “Continuous wave, 30 W laser-diode bar with 10 GHz linewidth for Rb laser pumping”; Optics Letters Vol. 33, pp. 702 (2008)
3. Friedrich Bachmann, Peter Loosen, Reinhart Poprawe „High Power Diode Lasers ", pp.121-123, pp.162-174, Springer Series in Optical Sciences (2007)
4. M. Haag et. al.; "Novel high-brightness fiber coup led diode laser device"; Proc. SPIE Vol. 6456, 6456-28 (2007)
5. C. Wessling et. al.; “Dense wavelength multiplexing for a high power diode laser”; Proc. SPIE Vol. 6104, 6104-21 (2006)
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9. BL Volodin et. al.; "Wavelength stabilization and spectrum narrowing of high-power multimode laser diodes and arrays by use of volume Bragg gratings"; Optics Letters Vol. 29, pp. 1891 (2004)
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-brightness narrow-line laser diode source with volume Bragg- grating feedback"; Proc. SPIE Vol. 5711, pp. 166 (2005)
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