2.1 –

General synoptic of the system

The system is composed of the following elements (Fig.1):

Fig.1:

Synoptic of the system

The laser source is a DFB diode emitting at 1.55 μm. This wavelength allows us to use telecom components (diodes, fibers, couplers…). The source is regulated in current and temperature and frequency stabilized on a molecular absorption line. Flying models are under production at CSO Mesure for the IASI project [Pujol 97].

The electronic unit includes the global power supply and the fringe counting system.The current and temperature regulations of the source are driven by a portable PC.

Optical source and electronic treatment are routed by optical fibre and electrical cable.

2.2 –

Measuring head principle

For each measuring head, the general principle used squares with a Michelson interferometer operation.

The beam is injected into the optical circuit of the head and then divided into two beams: the reference one and the measuring one.

The measuring beam propagates in the air or the vacuum and is reflected on the corner cube, associated with the object of which we want to measure the displacement along the beam propagation axis.

The measuring beam coming back is injected in the optical circuit and overlaps with the reference beam with the tilt half-angle γ (Fig.2).

Fig.2:

interference area

At the beam overlapping area level, the optical intensity is done by:

with the gaussian envelope expression,

ϕ: phase shift due to the corner cube position,

: phase shift due to beam tilt with ii the interference fringe spacing,

λ: wavelength;

2.n.Δ: optical phase difference, n refractive index of the propagation medium (air or vacuum) and Δ: the corner cube position with respect to the sensor,

n_eff: refractive index of the overlapping medium,

γ: tilt half-angle.

The result is an interference pattern moving when the corner cube moves along the measuring axis (Fig.3).

Fig.3:

Fringes moving with the object displacement

Four signals taken from the interference pattern allow the object displacement measurement.

Two other ones are taken respectively from the measuring and the reference arms (photometry outputs). They enable us to control the optical power injected into the measuring head by the stabilized laser source and the optical power received in the measuring arm.

2.3 –

Optical circuit

The interferometric circuit made in integrated optics on glass is the following (Fig.4):

Fig.4:

Scheme of the optical circuit

The optical circuit includes:

In order to obtain a precise phase shift between detection signals, those waveguides are placed:

2.4

Measuring head

In order to favour sensor miniaturization, we have placed in the measuring head only the optical circuit, interference fringes detection (photodiodes), photometric outputs and their treatment (Fig.5):

Fig.5:

scheme of a measuring head

At the output of the four photodiodes placed directly at the back of the guides, currents are changed into voltage signals and then amplified in order to have the same average amplitude and the same modulation depth. Those amplification coefficients are theoretically independent of the contrast and can be hold constant over the whole measuring range.

Two summing amplifiers provide the following signals:

Thus, the result is two quadrature phase shifted signals, with the same amplitude A and no offset. Those signals are called Sin and Cos.

2.5

Treatment in the electronic unit

The general principle used for the displacement measurement consists in generating a TTL signal for every sinusoidal signal positive part and in counting every front. The frequency of these TTL signals gives the resolution.

The use of quadrature phase shift signals allows value and direction displacement measurement with a λ/8 resolution, which corresponds to 200 nm.

So, in order to improve the resolution, at the electric cable output we create two other signals from Sin (or Cos) by adding and substracting a DC component proportional to cos(π/4).

After a logic treatment, the device can generate TTL signals which are injected into a 32 bits counter (because of the measurement number to carry out), enabling to achieve the required resolution of the displacement measurement (λ/16, i.e. 100 nm).

3.1 –

Optical circuit transmission results

Optical circuits have been realized by GeeO.

Transmission measurements are made after 5° faces polishing.

The obtained results are shown in the following table, indications in brackets being injection point and measuring point, as represented in Fig.4. Values are given in dB.

Circuit n°1 is the first complete tested circuit and we notice that measuring and reference arms are not well balanced. Circuit n°2 (with a reajusted process) has a better ratio and less losses. In theory, V1 and V4 (V2 and V3) should be equal. It seems that waveguides are not placed symmetrically according to gaussian beam profiles and that reference and measuring beams don’t overlap exactly.

3.2 –

Interferometric signal characterization

3.2.1 –

Fringe spacing

The fringe spacing value ii done by equation (2.3) is determined by circuit design. In order to avoid coupling between waveguides, d must be larger than 21 μm and so ii larger than 28 μm. The value measured by GeeO is: