IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 10, NO. 1, JANUARY 1998 123 Low-Coherence Interferometry Using a Self-Mixing Super-Luminescent Diode L. Rovati, Member, IEEE and F. Docchio, Member, IEEE Abstract— A novel low-coherence interferometer (LCI) using a self-mixing superluminescent diode is described. The optical scheme proposed uses the photodiode on the back-face of a commercial super luminescent diode to detect the interference signal from a Fizeau interferometer. Interesting characteristics are low cost and small size, high stability and resolution. The proposed interferometer is expected to have several industrial as well as medical applications. Index Terms—Low-coherence interferometry, optical feedback. L OW-COHERENCE interferometry (LCI) is a well-known interferometric technique proposed for a large number of industrial and medical applications [1], [3]. LCI, also known as white light interferometry (WLI), is a noncontact method for absolute distance measurements. The main component of a low-coherence interferometer is the low-coherence optical source. For LCI, most popular, compact and reliable solid-state sources commercially available are super luminescent diodes (SLD). Like laser diodes (LD), SLD’s have a monitor photodiode placed in the same case of the optical source. This pho- todetector is generally used to monitor the optical power at the emitting junction back-face, in order to stabilise the emitted optical power. Recently, a different use of the monitor photodiode has been proposed: any light that is reﬂected back to the emitting junction causes a certain variation of the emitted optical power, and this variation can be detected using the monitor photodiode. This technique, so far used only with LD’s, is known as self-mixing laser diode (SM- LD). SM-LD has been proposed for different applications such as relative interferometer, Doppler velocimeter, displacement sensors, etc. [4], [6]. We observed similar phenomena using SLD (SM-SLD) and, in the present letter, we propose to exploit it for LCI. The main difference between SM-LD and SM-SLD is that in the former technique an external reﬂector is placed at a distance shorter than the output coherence length, whereas in the latter, two external reﬂectors are placed at a distance much longer then the output coherence length of the optical source. The basic setup of the proposed interferometer is shown in Fig. 1. In this interferometric scheme the light from the SLD is collimated by lens L1 and then focused by lens L2 onto the target TG. Part of the excitation light is back-reﬂected by a Manuscript received July 7, 1997; revised October 3, 1997. The authors are with the Dipartimento di Elettronica per l’Automazione, Facolt` a di Ingegneria, Universit` a degli Studi di Brescia, Via Branze 38, 25123 Brescia, Italy. Publisher Item Identiﬁer S 1041-1135(98)00478-9. Fig. 1. Principle layout and block diagram of the control electronics of the low-coherence interferometer using self-mixing super-luminescent diode. SLD: superluminescent diode; L1: collimating lens; SS: semireﬂecting slab; L2: focusing lens; TG: target; TC: Peltier temperature control; CG: sta- bilised current generator; PC: Peltier cell; PD: monitor photodiode; TP: transimpedance preampliﬁer; EDC: envelope detection circuit; ON: offset nulling circuit. semireﬂecting slab SS. This light, such as the light reﬂected by TG, retraces the main beam back to the emitting junction causing a level variation of the emitted power. The beams reﬂected by TG and by SS do not interfere since their optical path difference (OPD) is much higher than the coherence length of the optical source. However, the emitting junction interface (semiconductor–air) provides a second reﬂection of the incoming beams which retrace again the main beam back to SS and TG. Therefore, supposing the coherence length of the SLD to be equal to zero, and SS, L1, L2 ideally thin, interference phenomena in the emitting junction can be observed if the following geometrical condition is satisﬁed: (1) where, according to Fig. 1, is the distance between the SLD and focusing lens L2, the distance between lens L2, and semireﬂecting slab SS and is the focal length of L2. The main interference signal is achieved setting 2 and 1, hence, (1) becomes: (2) The intensity of the higher interference orders decreases exponentially as , where is the SLD output mirror re- ﬂectance. Since is of the order of 10 their contribution results negligible. For this interferometer we set the optical path difference (OPD), deﬁned as (3) equal to 0. According to (2), interference phenomena in the emitting junction can exclusively be observed when a reﬂecting plane 1041–1135/98$10.00  1998 IEEE