Beam Circularizing And Astigmatism Correction Methods For Semiconductor Laser Diodes - Online Article
The most commonly used method to circularize the elliptical beam is to use a pair of anamorphic correcting prisms, as shown in Fig.1. The prisms can enlarge or reduce the beam size in one direction while keeping the beam size in another direction unchanged. The enlargement or reduction rate can be adjusted by adjusting the angles of the two prisms. By properly adjusting the angles of the prisms and using a circular aperture, it is not difficult to circularize an elliptical beam.
Fig.1 Circularizing beam with anamorphic correcting prisms
The most commonly used method to correct astigmatism is to use a very weak cylindrical lens after the aspheric collimating lens, as shown in Fig.2. When the orientation and focal length of the cylindrical lens are right, this method can collimate the beam in the direction θy, while not altering θx. The aspheric lens cannot do this alone due to the astigmatism.
The Advantages & Disadvantages of Correcting the Beam
There are mainly two methods of correcting the beam.
- By using Anamorphic Prisms
- By using Weak Cylindrical Lens
The main advantages of employing both methods are:
- Easy to use: No complex adjustment is required.
- Low power loss: The only loss is prism and lens surface reflection. The total loss is about 30% ~ 50%, if the surfaces are antireflection coated.
- Low cost: The ease of use results in labor cost savings. Since there is low power loss, a lower power, and less expensive, laser diode can be used.
There are mainly two disadvantages of employing these methods:
- High beam wave front distortion and strong light scattering: Since the beam must pass through eight glass surfaces of two prisms, one cylindrical lens, and one aspheric lens, the surface defects greatly reduce the beam's quality.
- Residual astigmatism: The magnitude of astigmatism varies even between two diodes of the same type, and usually takes a value from a few microns for index guided, lower power laser diodes to tens of microns for gain-guided, wide-stripe, high power laser diodes. In reality, however, the number of cylindrical lens with different focal lengths are limited so, astigmatism is often either under-corrected or over-corrected.
Fig.2 using a cylindrical lens to correct astigmatism
An Alternative Method to Correct the Beam
An alternative method to correct the beam is that will both circularize the elliptical beam and correct the astigmatism, is to use a piece of single mode fiber as shown in Fig.3. The laser beam is coupled by two collimating lens into the single mode fiber and the output beam from the fiber is collimated by the third collimating lens. The fiber has a low transverse mode or, in other words, only contains one spot. Optical fiber theory indicates that the fiber used must be much longer than the laser wavelength. This sufficiently "mixes up" the beam as it passes through the fiber so that the beam can get rid of all the spatial characteristics it had upon entering the fiber. In this way, the quality and spatial characteristics of the beam output from the fiber can be totally determined by the surface quality and shape of the output end of the fiber. So, the elliptical cross section and astigmatism of the beam before entering the fiber, although varying from diode to diode, does not affect the spatial characteristics of the beam output from the fiber.
The fiber core has a circular cross section and a constant diameter in any radial direction. The equation, θ= 4λ / πd reveals that the beam output from the fiber also has a constant divergent angle in any radial direction. This means the beam from a single mode fiber has a circular cross section. Because the divergent angle of the beam is a constant in any radial direction, tracing any beam backwards will reach the same imaginary point source. In other words, the beam output from the fiber has no astigmatism. Fig.3 Laser beam optically corrected using an optical fiber.
Fig.3 Laser beam optically corrected using an optical fiber
The advantages of using this method are:
- Small wave front distortion and low light scattering. The wave front quality and the scattering level of the collimated beam are affected only by the quality of three glass surfaces; the two surfaces of the third aspheric lens plus the surface of the output end of the fiber, which is only a few square microns in size and usually of high quality. The effects of these three surfaces are much smaller than those of the eight surfaces described above.
- No residual astigmatism. Since the divergent angle of the beam output from the fiber is determined only by the cross section shape of the fiber core, and the core is circular, the beam output from the fiber has no residual astigmatism no matter how large astigmatism the beam had upon entering the fiber.
The disadvantages are:
- High power loss. Since the core of a single mode fiber is only a few square microns in size, the optical coupling between the beam and the fiber is inefficient. The resulting optical power loss for the system is usually between 50% to 70%.
- High cost. The difficulty of coupling the beam into such a small fiber core results in increased labor and mechanical parts cost. Also, to offset the loss of optical power, a more expensive higher power laser diode must be used.
- Large size. To prevent light leakage from the fiber and obtain sufficient mechanical strength, the fiber must be wound on a solid cylinder. This, combined with the three collimating lens and the precision mechanical parts occupy a volume larger than the volume occupied by the other solution of lens and prisms.
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