Educación, capital humano y productividad

7.1.1.1. Mirror

In order to build a compact system and to have the cavity as far from hemispherical condition as possible, the coupling mirror had to be relatively small, but cutting and coating a curved mirror on such a scale is difficult, so it becomes hard to find a manufacturer that will agree to make such a mirror. Jakob Reichel’s team was able to find a company to coat mirrors much smaller (several 100th ofµm of curvature)

and on a much smaller system (the fiber’s surface) ([19], [26]). However, it is difficult to procure them in small quantities. Ultimately, I engaged the company Altechna ([20]) to create these mirrors (Table 7.1). The mirrors are 3mm thick and have

a 5mm radius of curvature with a diameter of 3mm. The back face also has a

7.1 The OPO Cavity, Description of the Experiment Mirror a) b) M9 lens c) Ring Piezo Mirror Secure Screw Lens Holder Screw Secure Screw Z X

Figure 7.3.: a) Schematic of the mirror holder. A 1-inch ring with an aperture and

a screw thread that allows us to mount a lens is glued to a ring piezo actuator and a smaller cylinder holding the mirror. b) An image of the mirror holder on a standard mirror mount. c) The other face of the mirror mount with a standard mounted lens fixed on it.

curvature to reduce somewhat the divergence of the beam. Without that, the beam becomes very large after the cavity, with issues of clipping of the beam on the mirror mount.

Material: BK7

Diameter: 8 mm (+0/-0.1 mm)

Mirror diameter: 3 mm (+/-0.1 mm)

Radius of curvature: Concave ROC1: 5 mm (spherical) Convex ROC2: 7 mm (spherical)

Surface quality: 40/20 S/D

Surface figure: L/4@633nm Centering: <3 arcmin Protective chamfers: 0,2 mm x 45 deg

Coatings: S1 (ROC1):

PR(R=95%+/-1.5%)@532nm + HR(R>99,5%)@1064 nm, AOI=0 deg S2 (ROC2): AR(R<0.25%)@1064nm + 532nm, AOI=0 deg

Price: 240 EUR/pc (for 5) / 180 EUR/pc

(for 10)

Table 7.1.: Mirrors characteristics.

7.1.1.2. Mirror Mount

The mount for the mirror needed to be compact as well as compatible with a 1- inch mirror mount. It significantly reduces the stability of the system, but on the other hand, it allows us to change the configuration of the cavity to avoid eventual

Chapter 7 Experimental Method problems of conception, and it gives two rotational degrees of freedom that could have been useful. In practice, they were not very useful, and they could have been easily replaced by a monolithic block which would allow the system to be lower and would enhance mechanical stability.

The mirror mount is placed on a positioning stage (Nanomax [67]) with a little alu- minum plate to match the screws. This system is quite handy because it allows very easy modification of the whole system in case of a problem. The 3 axis positioning (0.5mm core screws + micro-metric precision screws) makes the alignment of the cavity very fast and easy. The average time to align the system is a few hours. Moreover, the long piezo in the cavity axis is very useful to have access to several Free Spectral Ranges (FSRs). But once the cavity is aligned, it is quite unlikely to be adjusted again. A good solution would be to hold the system with the Nanomax, to align it, and to use glue to fix everything in place indefinitely. Gluing cavities is a technique used by Jakob Reichel’s team ([43]) . With a very slow-hardening glue, we can flood the system and realign it a little bit with the Nanomax during the hardening to compensate for the expansion of the glue. The glue expands mostly at the beginning, and less and less during the process. I have not tried this technique with the system, but it is very likely that we will consider it for the next generation of miniOPO.

The mount allows the control of the three degrees of translation and the degree of rotation around the Z axis (vertical) and around the X axis (Figure 7.5 (c)).

Mostly, it is only the translations that are useful. I wanted to have as many degrees of freedom as possible for the mirror. This allows us to really know which degrees of freedom are useful, and which ones can be removed later, and it makes it easier to adapt to conception mistakes.

The mirror mount is made from two cylindrical plastic parts connected by a ring piezo transducer from Noliac (NAC2125-A01 ([28])). All the parts are glued to each other using epoxy.

The part which is holding the mirror is a bit smaller than one inch so it can go through a standard mirror mount. It has a little notch so as not to have the head of the screw in the way of the mount. The notch also needs to be big enough to allow the passage of the head of a screw driver. The screw that I used was an M2 screw. The other part is a 1 inch cylinder that fits in the mirror mount. I added to it an M9 screw thread to fit a mounted Thorlabs lens. The beam is diverging rapidly, so I wanted to be able to get a lens in the middle of the piezo ring to colimate the beam with a minimal beam waist to avoid clipping.

All the system is made of plastic and the part which is fixed to the mirror mount is a bit light, it does not make a very good counterweight for the piezo. Making this part heavier could certainly increase the piezo response of the mirror.

We have two piezos for the same directions: the ring piezo from the mirror mount is a fast piezo but with a small range (I can see just a little bit more than an FSR at

7.1 The OPO Cavity, Description of the Experiment

1064nm); and the Nanomax piezo in the Z axis, which has a very long range (20µm),

but is very slow. To control the resonance of the system, we used the combination of both piezos. -0.08 -0.07 -0.06 -0.05 -0.04 -0.03 -0.02 -0.01 0 101 102 103 amplitude frequency -30 -25 -20 -15 -10 -5 0 101 102 103 phase frequency

Figure 7.4.: In red, the Thorlabs Nanomax piezo frequency response; in blue, the

Noliac piezo frequency response. The Thorlabs piezo has a resonant frequency at 150hz which is not visible here.