Super-luminescent diodes brighten up inspection

We have developed a technique based on self-mixing super-luminescent diodes (SM-SLD) that is robust enough to be used in an industrial glass-manufacturing environment for defect and high-precision thickness measurements in sheet glass.^1-3 Compared to conventional optical coherence methods,⁴ the SM-SLD technique offers enhanced signal with reduced electronic noise and interference, suitable for a manufacturing environment.

In the self-mixing configuration, the SLD works as both emitter and detector, with the monitor photodiode detecting the interferometric signal. The interferometric signal is thus optically amplified by the high-gain active medium, so preamplification is less sensitive to environmental noise and electromagnetic interference.

In an SM-SLD system (inset), light from the SLD is collimated by the lens and passes through a semireflecting slab (SS) to reflect off of the target. Target reflections interfere with light reflected from the SS to create fringes at the monitor photo diode.

During operation, the collimated beam from an SLD is back-reflected in the emitting junction by a semireflecting slab (SS) and by the target (see figure). The two beams reflected by the target and by the SS do not interfere since their optical path difference is much longer than the coherence length of the optical source. However, different beams travel in the double cavity formed by the emitting junction interface (semiconductor-air), SS, and the target. In particular, assuming the coherence length of the SLD to be equal to zero, SS and the lens ideally thin, and l_r=l_s, the beam reflected by the target interferes with the beam that transits the cavity formed by the emitting junction interface and the SS.

If the reflectivity of the SS is 0.5, the power amplitude P_R of the main interferometric order is

where P_o is the SLD emitted optical power, G_s is the single-pass gain of the SLD emitting junction, R_out is the reflectivity of the SLD emitting junction-air interface, and R_tg is the target reflectivity. Compared to the standard Michelson interferometer, the major advantage of the SM-SLD scheme is that the interferometric signal is optically amplified by a factor . Typical values for G_S and R_out for a commercial SLD are 5000 and 1 x 10^-4 to 4 x 10^-4, respectively, so the theoretical signal enhancement factor of this system should be on the order of 50 to 100.

Using this working principle, we developed an innovative optical sensor for the quality inspection of glass slabs. The main interferometric signal is used to measure the distance between the sensor and the two glass-slab interfaces. Beams reflected by the air-glass and glass-air interfaces retrace the excitation beam direction through the SS and back to the SLD cavity. Interferometric signals are obtained when the reflecting interfaces are placed at a distance l_r=l_s from the SS. By moving the SS, different planes of the target satisfy the above equality and contribute to interference phenomena in the emitting junction. We analyze target reflectance profile as a function of the scanning coordinate.

The developed system consists of two main blocks: the optical head and the control unit. The optical head contains all the interferometer optical components, a stepper motor, an SLD driver, and a preamplification network. The control unit controls the stepper motor and processes the interferometric signal, generating a digital signal having a duration proportional to the thickness of the slab under test. oe

References

1. L. Rovati, and F. Docchio, IEEE Phot. Tech. Lett. 10, 123 (1998).

2. L.Rovati, L. Pollonini, et al, Rev. Sci. Instrum. 73(9), 3386 (2002).

3. L. Rovati , Patent Appl. No. PCT/EP99/02264 (1999).

4. U. Schnell, R. Dändlinkerand, et al., Opt. Lett. 21, 528 (1996).