3.1. Requirements of optics
Although the optics for an SM sensor is very important, many former researchers had not investigated scrupulously them. For most references, they employed a single lens as the collimating optics except few researchers who used complex lenses of 2 or more lenses (e.g. [11]).
There are two defaults for the optics of a singlet—one is the weak optical feedback strength due to small aperture of the lens and the other is the poor adjustment of the measurement distance. The optical system of several lenses is indispensable for improving the quality of SMSs or adjusting the measurement distance in wide range. Hence we put forth a design method for the SM displacement sensor which ensures a high SN ratio and a variable effective focal length.
The important requirements for such optics are as follows:
a) The optics should focus the laser beam into as small spot as possible as explained in Sec. 2.
b) The focal length of the optics should vary in a wide range (from 0.5 to 5 m in the paper).
We examine a design method for the optics meeting with such requirements below.
3.2. Design of optics
The optical system for the above requirements consists of two sub-systems, that is, optics for formation of parallel beam and focusing optics. The former transforms the divergent beam from the LD into a parallel beam and the latter converges the parallel beam into the target surface. The former and latter optics are composed of two lenses, respectively, and therefore the resultant optics consists of four lenses.
We design the optics using the professional optical design software, ZEMAX.
First we design the parallel beam formation optics for an LD HL6312G (Hitach) at a lasing wavelength of 635 nm, which is used as the SM sensor in our experiment. Note that the typical value of the beam divergence perpendicular to the junction, θ⊥=31°, which is taken into account in the design. The entrance pupil diameter in the design is set to be 15 mm.
Fig. 5 shows the layout of the parallel beam formation optics designed for the above conditions and Table 1 gives the calculated values of the design parameters corresponding to Fig. 5.
Then we design the focusing optics. This optical system should focus, for different measurement distances (the distance between the output facet of the LD and the target), the laser beam exiting the parallel beam formation optics shown in Fig. 5 on the target surface into the smallest spot possible as explained in Sec. 2. Note that a good-quality SMS is achieved when the focused spot size is less than 1 mm according to our experience. In order to satisfy the requirement for variable measurement distances, we use a Multi-Configuration Editor supported by ZEMAX, where the distance between the rear face of the last lens in the optical system and the target surface (the thickness of the surface 8 just before the image surface in Table 2) is set as 0.5, 1, 1.5, 3 and 5 m, respectively. Then we determined the values of the design parameters for the focusing optics by the optimization tool supported by ZEMAX.
Fig. 6 shows the resultant optics of the parallel beam formation optics and focusing optics and Table 2 and 3 gives the values of the design parameters corresponding to Fig. 6
Table 1. Values of design parameters for layout in Fig. 5.
Surface
|
Radius
|
Thickness
|
Material
|
Diameter
|
0
|
Infinity
|
0.998
|
-
|
0.000
|
1
|
-1.670E+004
|
3.844
|
N-BK7
|
0.600
|
2
|
-2.965
|
17.716
|
-
|
3.276
|
3
|
214.300
|
8.990
|
N-BK7
|
6.724
|
4
|
-20.897
|
2.114
|
-
|
7.637
|
We emphasize that variable focal length for any measurement distances in a given range can be guaranteed, as seen in Table 3, by adjusting only the space between the third lens and the fourth lens in the final optics, leading to great convenient for practical application. Note that the thickness of surface 4, the distance between the second and the third lens in the final optics, is almost constant in Fig. 6 and Table 3. Fig. 7 shows the spot diagrams of the final optics for five measurement positions given. As shown in Fig. 7, the spot sizes of all configurations are much less than 1 mm, thereby meeting the first requirements of the design. We emphasize again that the feedback optical intensity is enough for such spot sizes.
Table 2. Values of design parameters for final optics.
Surface
|
Radius
|
Thickness
|
Material
|
Diameter
|
0
|
Infinity
|
0.998
|
-
|
0.000
|
1
|
-1.670E+004
|
3.844
|
N-BK7
|
0.600
|
2
|
-2.965
|
17.716
|
-
|
3.276
|
3
|
214.300
|
8.990
|
N-BK7
|
6.724
|
4
|
-20.897
|
2.114
|
-
|
7.637
|
5
|
-11.350
|
2.004
|
N-BK7
|
7.657
|
6
|
-13.614
|
31.507
|
|
8.121
|
7
|
143.22
|
2.544
|
N-BK7
|
10.079
|
8
|
-99.510
|
500.00
|
|
10.114
|
Table 3. Calculated values in Multi-Configuration Editor (which are for different measurement distances, 50, 100, 150, 300, and 500 cm, respectively).
Thickness
|
Number of configuration
|
1
|
2
|
3
|
4
|
5
|
Thickness of surface 4
|
2.114
|
2.120
|
2.118
|
2.113
|
2.130
|
Thickness of surface 6
|
31.507
|
12.595
|
7.356
|
2.481
|
0.620
|
Thickness of surface 7
|
500
|
1000
|
1500
|
3000
|
5000
|