A novel off-axis external cavity is designed for laser diode array to improve the beam quality. In this external cavity, a circle aperture with variable size is used as a spatial filter. The diameter of aperture is optimized to 1.2mm and the off-axis angle of external cavity is optimized at 2.6 deg. In the optimal case, the beam parameter product (BPP) of laser diode array is reduced to 121 mm.mrad from 1050 mm.mrad with external cavity optical efficiency of 81%.
© 2007 Optical Society of America
Due to their compactness, low cost, relative ease of operation and excellent efficiency, high-power laser diode arrays (LDA) are attractive as optical sources, with many applications in industry, medicine, and science [1–5]. Unfortunately, the poor beam quality is a still limiting factor for many applications such as efficient coupling to optical fibers or nonlinear generation of new frequencies. So it is important to improve the beam quality. This poor beam quality is a consequence of the highly elongated emitting region, ~1µm×10mm, consisting of a linear array of individual emitters. The net result is a combined output beam that is nearly diffraction limited in the plane orthogonal to the plane of the array but that has a divergence ≥1000× diffraction limited in the array plane. This large mismatch in the beam-quality factors for orthogonal planes renders the output extremely difficult to focus into the small circular spot required by many applications (e.g., end pumping). Various investigations have been developed to improve spatial beam quality. A common technique in improving beam quality of laser diode bars in the slow axis is beam shaping such as the shaping of two mirrors , beam-shaping prism groups , and so on. In the fast axis, which is perpendicular to the array direction of a laser diode bar, the beam displays good quality and can be collimated to several milliradians by use of a microcylindrical lens. The external-cavity technique is also an effective method for laser diode bar to improve the beam quality. Xin Gao et al. reported an external cavity with a stripe mirror. The strip mirror is coated with a 500-µm pitch period, half of which (250 µm) is high reflection for feedback and the other half of which is antireflection for the output window for each laser diode element . However, the stripe mirror need be specially made. In this paper, we demonstrate an external cavity that consists of normal optical elements. In order to enhance further the mode selectivity of the setup, we place a circle aperture with variable size as a spatial filter (SF) in this external cavity.
The experimental setup is shown in Fig. 1.
In our experiment, we used a commercial 17-W, 1cm-long, 49 -emitter LDA at 808nm. The dimension of each emitter is 1 µm by 80 µm with a separation between emitters of 200 µm. Light emitted perpendicular to the array (fast axis) has a divergence of 40 deg, and light emitted parallel to array (slow axis) has a divergence of 10 deg with multi-spatial mode. The asymmetric divergence of the individual emitters requires use of extra collimation optics. An aspheric lens with short focal length is used as fast axis collimator (FAC) which collimated the rapidly diverging light along y direction. The array parallel to x-axis (slow axis).
The setup of the external cavity is designed off axis. A high-reflective mirror is used as feedback element. The front facet of diode laser array is antireflection coated, so there is a laser resonator formed for each array element between the high-reflector coating on the back facet and high-reflective mirror. Thus, 49 off-axis external-cavity laser diodes are formed by feedback with a high-reflective mirror. Each external cavity laser diode is separated into a feedback branch and an output branch. The feedback branch is made up of lens1, lens2 and high-reflective mirror. And the output branch is made up of lens1 and lens3. The f/number of lens1 is f/1.875 with a focal length of 150mm. The f/number of both lens2 and lens3 is f/2 with a focal length of 40mm. The LDA is placed in the front focal plane of lens1. The lens1 acts as a transform lens that transform the near field of LDA into far field and the lens2 can be used as a collimating lens. It is well known around tens of modes oscillate in single-element laser diode on the freely running condition. Each mode has a double-lobe intensity profile in the far field [8–12]. For a single-element laser diode, the beam quality improvement by using off-axis external cavity technique can be understood in the following way:
Due to the different radiation angles of various spatial modes, a single lobe of one of the spatial modes may be chosen by adjustment of high-reflective mirror tilt. The selected lobe of this mode is feedback into the laser diode, the other lobe of this mode is amplified and coupled out of the external cavity as output beam of laser system and all the other modes are suppressed effectively[10–12]. By this, each diode will oscillate in a narrow mode range. Due to the common feedback element shared by all emitters, the output beam of all emitters will propagate in the same direction. As a result, the beam quality is improved drastically.
In order to enhance further the mode selectivity of the setup, we place a circle aperture as a spatial filter(SF) in back focal plane of lens1. The size of aperture is variable and the aperture can be moved along the back focal plane of lens1. By varying the size of aperture, moving the aperture and adjusting high-reflection mirror tilt, we can obtain the optimal laser output.
3. Results and discussion
In this paper, the beam parameter product (BPP, i.e., beam width ×beam full divergence) is used to evaluate beam quality of LDA. Figure 2 shows a typical BPP(in the slow axis) versus current for free-running LDA and external cavity feedback laser diode array(ECLDA). At the drive current of 17A, the BPP measured 1050 mm mrad and 152 mm mrad without and with external cavity respectively. When a SF with the optimal diameter (1.2mm) is inserted at the optimal position in the feedback branch, the BPP is reduced to 121mm mrad. In the following, we will discuss how to choose the optimal diameter of SF and determine the optimal position of SF. With the drive current increasing, the BPP of ECLDA and of free-running LDA is increased. The BPP of ECLDA with a SF is almost unchanged. For the ECLDA, this shows that the extra gain obtained from the higher drive current can overcome the feedback effect introduced by the external cavity . As a result, more extra modes will oscillate and the beam quality will be degraded. A SF placed in the feedback branch will eliminate some extra modes. Thus, the beam quality of laser output with a SF is almost unchanged.
The output power of ECLDA measured 2.63W at the drive current of 17A or 85% of the radiated power of the free-running state; while the output power of diode array with the external cavity and a SF is 2.52W that is 81% of the radiated power of the free-running state.
Here, we define the external cavity optical efficiency as the ratio of output power of the ECLDA to the output power of the free-running LDA. The output power of 2.63W seems rather low for a 17-W LDA. This can be explained as following: The LDA used in experiment has a standard antireflection coating on the output facet with a reflectivity about 5%. For the ECLDA, due to reflection of about 5 percent in the output facet of LDA, at high current, the external feedback effect is overcome by extra gain obtained from higher driver current. As a result, the beam quality will be degraded. In order to obtain higher power output with good beam quality, output facet of LDA need be specially antireflection coated.
Figure 3 shows the BPP variation of ECLDA versus various aperture positions (the diameter of aperture is fixed to 1.2 mm).
The aperture position is specified by the distance between optical axis and center of aperture. As shown in Fig. 3, the distance between optical axis and center of aperture has been changed from 3.5mm to 9nm. We can see clearly that when the aperture is moved to about 6.8mm from optical axis and the laser output with best beam quality is obtained. The off-axis angle of external cavity is defined as following:
here D is the distance between optical axis and center of aperture and f 1 is the focal length of lens1. According to the formula (1), the optimal off-axis angle of external cavity is calculated to θ=2.6 deg.
Figure 4 shows BPP variation of ECLDA versus various aperture sizes (the distance between optical axis and center of aperture is fixed to 6.8mm). We can see clearly from figure 4 that the optimal diameter of aperture is about 1.2 mm. When the diameter of aperture varied from 1mm to 1.5 mm, the beam displays good quality.
When the diameter of aperture is less than 1mm, with the size of aperture reducing, the beam quality drastically degraded. A major factor leading to the problem is geometric aberrations of optical elements in external cavity. Because of geometric aberrations of lens1, beams from different emitters traveling in the same direction can not converge at a common point at aperture position but a blur. Therefore, with the size of aperture reducing, some beams can not pass through the aperture. Also, because of geometric aberrations of lens2, some feedback beams can not pass through the aperture. These will lead to a great loss of feedback. As a result, the external cavity effect is reduced and the beam quality is degraded. When the diameter of aperture is more than 1.5mm and less than 4 mm, with the size of aperture increasing, the beam quality gradually degraded. This is because the function of mode-selection of SF gradually weakened, with the size of SF increasing. When the diameter of aperture is more than 4 mm, the function of mode-selection of SF is almost out of action. Therefore, with the size of aperture increasing, the beam quality is almost unchanged.
In conclusion, we have described a novel off-axis external cavity for laser diode array to improve the beam quality. A normal high-reflective mirror is used as feedback element. In order to enhance further the mode selectivity of the setup, a circle aperture with variable size is used as a SF in the external cavity. The diameter of SF is optimized to 1.2mm and the off-axis angle of external cavity is optimized at 2.6 deg. In the optimal case, the BPP of the laser diode array is reduced from 1050mm.mrad to 121mm.mrad with external cavity optical efficiency of 81%. It is expected that the output power may be increased significantly with good beam quality if the output facet of LDA can be specially antireflection coated.
References and links
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