1.

Introduction

There continues to be interest in low-cost optical modulators for applications in information technology (e.g., optical communications and optical data storage). The most widely used modulators (e.g., ${\mathrm{LiNbO}}_3$ electro-optic type) are expensive. Optical modulators that use reflective membranes that move under electrostatic or magnetostatic forces have potentially lower cost. The electrostatic type has received considerable attention, and experiments with electrostatically driven devices have shown that membrane type modulators can be driven at megahertz frequencies.^{1, 2} The electrostatic approach requires tens of volts and is not directly compatible with silicon very large scale integration (VLSI), which can supply about 1 V or less. This letter describes optical phase modulators with reflective membranes that are magnetically controlled and can be driven by lower voltages. The modulator can be constructed entirely on a semiconductor chip, scaled to micron dimensions to form arrays, and fabricated with conventional processes.

2.

Theory

A schematic for a magnetically actuated optical phase modulator is shown in Fig. 1. The force per unit volume on the reflective magnetic film is

Fig. 1

Cross section of the flexible mirror, including Mylar® substrate and ${\mathrm{Fe}}_{0.55}{\mathrm{Ni}}_{0.45}$ reflective magnetic film.

The deflection of a film is proportional to the applied pressure provided the deflection is small (i.e., less than or on the order of the thickness of the membrane). The maximum deflection, at the center of a circular magnetic membrane of radius $R$ clamped at its edges, is

3.

Mirror Fabrication and Experimental Setup

For this study, electron beam evaporation with an $\mathrm{Fe}(55\%)∕\mathrm{Ni}(45\%)$ source was used to deposit a 70-nm-thick, reflective, magnetic film onto a $2.5\text{-}\mu \mathrm{m}$ thick Mylar® substrate. Mylar® (polyethylene terephthalate) was chosen for its flexibility and ability to withstand heating $(\text{melting point}=254°\mathrm{C})$ during the evaporation process. The Mylar- $\mathrm{Fe}∕\mathrm{Ni}$ composite film was glued to the surface of an aluminum plate over a hole of radius 4.1 mm.

The setup for measuring mirror deflection is pictured in Fig. 2. A 629-turn, 2-cm-long electromagnet was fabricated by wrapping copper wire around a wood post of diameter 12 mm. The resistance of the coil was $7.9\Omega$ . Magnetic field strength was measured by replacing the sample with a thin gaussmeter probe. A helium-neon laser with a wavelength of $0.633\mu \mathrm{m}$ and a Twyman-Green interferometer were used to measure the deflection of the reflective membrane when current was applied to the electromagnet. Figure 2 also shows how a phase modulator can be converted to an amplitude modulator using interference. The laser beam was tightly focused onto the center of the membrane so that the interferometer measured the maximum deflection.

Fig. 2

Experimental setup for measuring the deflection of a flexible mirror in the field of an electromagnet.

4.

Results and Analysis of Experiment

For small separations between the membrane and the electromagnet, a current of 0.5 A and a corresponding drive voltage of just 4 V deflected the membrane by an amount sufficient to change the optical phase of the helium neon beam by $\pi$ . In order to better understand the operation of the modulator, membrane deflection was measured as the distance from the membrane to the front of the electromagnet (the plane of the first magnetic loop) was varied from 1.0 to 6.0 mm, as shown in Fig. 3. For these measurements, a coil current of 2 A was used to produce magnetic fields and the corresponding drive voltage was about 16 V. The optical phase shift for the reflected helium-neon beam is also shown in Fig. 3.

Fig. 3

Measured deflection of the flexible mirror and corresponding phase shift for the reflected optical beam.

The magnitude of the magnetic field was measured for distances from the front of the electromagnet that ranged from 1.732 to 7.732 mm. It was not possible to bring the center of the magnetic probe any closer to the electromagnet because of the finite thickness of the probe housing. The experimental values for the magnetic inductance were fit with a function that was the sum of a constant and an exponential. The function was analytically differentiated to produce an expression for the field gradient. Figure 4 shows the measured deflection as a function of the magnitude of the corresponding field gradient. The relation is nearly linear, as predicted by Eq. 1. Using the slope for the data in Fig. 4 and a value of $1.6\mathrm{A}∕\mathrm{m}$ for the saturated magnetization,⁵ Eq. 1 gives a value for the stiffness $D$ of the film of $3.1\times {10}^{-6}\mathrm{N}∕\mathrm{m}$ . The relatively large value for $D$ suggests that the drive voltage for the modulator can be further reduced by decreasing the stiffness of the membrane, as discussed in the next section.

Fig. 4

Membrane deflection versus the magnitude of the gradient of the applied magnetic inductance.

5.

A Scaled, Optimized Optical Phase Modulator

The results presented in the previous sections include the information needed to predict the performance of an optimized phase modulator, scaled to smaller size for fabrication into arrays. We consider a highly flexible membrane made of Mylar®, with a small reflecting disk of $\mathrm{Fe}(55\%)∕\mathrm{Ni}(45\%)$ alloy deposited just on the center portion of the polymer. This geometry allows the membrane to be as flexible as possible, although a moderately sized magnetic disk will be required in order to produce sufficient force to deflect the membrane. The proposed device shares similarities with the mechanical aspects of an actuator described by de Bhailís ⁶ We consider a $1.5\text{-}\mu \mathrm{m}$ -thick polymer film with a radius of $100\mu \mathrm{m}$ and with a $2\text{-}\mu \mathrm{m}$ -thick magnetic disk of $3\mu \mathrm{m}$ radius. A 15-turn electromagnet of radius $5\mu \mathrm{m}$ is positioned $2.5\mu \mathrm{m}$ from the center of the membrane. We estimate the current required to produce a $\pi$ phase shift for a light at a wavelength of 1540 nm to be about 140 mA. The current could be supplied by a drive voltage of 1 V provided the resistance of the electromagnet is no greater than $7\Omega$ .