GB2192070A - Optical attenuator - Google Patents

Abstract

An optical attenuator comprises an isosceles right triangular prism 62 having a part BDXY of one face ABCD adjacent the right angle apex CD metallised with silver or gold of layer thickness 46+/-5nm. Light 60 incident on the metal layer from within the prism 62 excites a surface plasmon for a narrow range of angles of incidence for a particular wavelength of light. The prism 62 is rotated about an axis through its centroid parallel to its apex CD to tune through one side of the plasmon resonance. This results in the phenomenon of attenuated total reflection in which energy is coupled from the light beam to the surface plasmon despite total internal reflection occurring. The light beam is accordingly attenuated to a degree varying with angle of incidence. The prism 62 may be accompanied by a retroreflector 64 arranged to return light from the prism 62 back to it for a second transit in the reverse direction. This nullifies the shift in beam direction associated with prism rotation. One or more additional reflectors (66, 78, 80) may be employed to render the output or attenuated beam parallel to or if necessary collinear with the input beam. <IMAGE>

Description

SPECIFICATION Optical attenuator This invention relates to an optical attenuator, ie a device for varying the intensity of a visible or infra red light beam.

Optical attenuators are required for many applications, such as for example adjustment of laser beam intensities to convenient values. The properties of an ideal attenuator would include a large dynamic transmission range. It should not significantly either distort or steer (re-direct) the light beam, nor exhibit light polarisation sensitivity. Furthermore, the power handling capacity and spectral range should be as large as possible, and the mechanical design should be simple and rugged. All presently known attenuators fall short of this ideal to a greater or lesser extent.

One known form of attenuator comprises a wedge profile metal layer such as inconel or aluminium deposited on a glass block or other transparent dielectric. The metal layer is semitransparent, and transmittivity varies with wedge thickness. Attenuation is varied by altering the wedge thickness presented to the light beam. In practice, such an attenuator is fragile, difficult to manufacture accurately, and produces light beam distortion. This is because the maximum wedge thickness is much less than a wavelength of visible light. A metal film with an accurately graded wedge profile thickness is very difficult to manufacture. Conventional metal deposition techniques produce films with varying thickness and pinholes at the very low thickness values required. At best the film produced will have a thickness profile consisting of some random scatter about the wedge characteristic required.Since a laser beam in particular has a finite diameter even when focussed, the wedge thickness presented will vary across the beam and the beam intensity profile (normally Gaussian) will be distorted. Furthermore, the metal film is fragile and easily damaged irretrievably by contact or exposure to solvents or hostile vapours. The power handling capacity is undesirably low, being typically less than 1 watt for aluminium and in the order of milliwatts for inconel. Against this, the spectral range is fair, being 0.4 to 1 .0,um aluminium and 0.38 to2,um for inconel, and calibration at a specific wavelength is straightforward.

It is also known to employ attenuators operating on the basis of polarization selective properties. One such consists of a dichroic sheet which absorbs light polarized in one direction but not that in the orthogonal direction. This attenuator is restricted to the wavelength interval 0.4-0.65Am and attenuation varies with wavelength in this interval. The attenuator suface is fragile and easily damaged. Moreover, power handling capacity is very low, a few milliwatts, since energy absorption is relied on to obtain attenuation and the device has low thermal capacity. A further polarization selective attenuator comprises a dielectric coated cube divided along a cube diagonai at which the optical axis direction changes abruptly. The axis change results in transmission of one polarization and reflection of that orthogonal.Polarizations intermediate these are partly transmitted and partly reflected, ie their components resolved in one direction are transmitted. This attenuator has a large spectral range (0.18-2.7,um) and power handling capacity (~10 watts), but the design is complex and expensive to engineer.

Further known attenuators are based on interference filter techniques. One such comprises a muitilayer ZnS filter configured as a Fabry Perot etalon having a wedge-shaped inter-mirror region. Movement of the etalon across a light beam varies the transmission. This device is specific to a very narrow wavelength interval, and in practice would be designed for use with a specific laser wavelength. Moreover, a special spatial filter is required to obtain a good light beam profile. The etalon is not easy to construct, since an accurately wedge-shaped inter-mirror region is difficult to otain. It does however have a fair power handling capacity in the order of 1 watt.

It is an object of the present invention to provide an alternative form of optical attenuator.

The present invention provides an optical attenuator including an isosceles prism having a right angle apex, a metallisation layer upon a prism face adjacent that apex, the layer and prism material being appropriate for surface plasmon generation, and means for rotating the prism about an axis through its centroid parallel to the prism apex and through an angular excursion encompassing at least part of a surface plasmon resonance, the dimensions of the prism being appropriate for light at angles of incidence on the metallised face within one degree of resonance to be reflected at a second prism face adjacent the apex. The attenuator produces variable attenuation by virtue of the phenomenon of attenuated total reflection.Despite being totally internally reflected within the prism, the light beam is attenuated by diversion of energy to a surface plasmon to a degree which varies over a narrow range of angles of incidence. The attenuator has the advantages of simplicity of design, cheapness, ease of construction and broad band performance. It also has good power handling capacity, and does not distort a light beam intensity profile. Moreover, the attenuated or output light beam emerges parallel to its input direction, albeit with a lateral shift varying with prism rotation.

In a preferred embodiment, the invention includes means for compensating for variation in output beam lateral shift with prism rotation. Such means may comprise a retroreflector arranged to return the output beam to the prism for a second transit, the return beam being displaced from the output beam in a direction parallel to the prism apex. This arrangement provides a second transit light beam which is invariant in position when the prism rotates. One or more reflectors may be arranged to render the second transit beam parallel to, and if necessary, collinear with the input beam.

The attenuator prism preferably has a refractive index at least equal to 1.7, and may be made of Schott glass of refractive index 1.805. The metallisation layer may be gold, silver, copper or aluminium.

The attenuator may incorporate prism rotating means coarsely adjustable for locating a plasmon resonance and finely adjustable for tuning attenuation through at least part of the resonance.

In order that the invention might be more fully understood, embodiments thereof will now be described, with reference to the accompanying drawings, in which: Figure 1 is a schematic sectional view of an attenuator of the invention; Figure 2 shows the Fig. 1 attenuator in an experimental arrangement for test purposes; Figure 3 illustrates attenuation/incidence angle graphs for the Fig. 1 attenuator at two wavelengths; Figure 4 illustrates attenuation characteristics of two Fig. 1 attenuators with differing metalisation layers; Figure 5 shows attenuation characteristics for two Fig. 1 attenutators with differing thicknesses of metallisation; Figure 6 is a part perspective, part sectional view of an attenuator of the invention arranged for substantially constant output beam direction; and Figure 7 shows laser beam intensity profiles after attenuation to varying degrees by the Fig. 1 attenuator.

Referring to Fig. 1, there is shown a sectional view of an optical attenuator 10 of the invention. The attenuator 10 comprises a right triangular isosceles prism 12 having a silver film 14 on one shorter-sided surface. The prism 12 is of Schott glass of refractive index 1.805. The film 14 is 45+5no in thickness, and comprises silver of 99.99% purity.

The attenuator 10 was constructed as follows. The prism 12 was cleaned in acetone and then degreased in isopropyl alcohol. Silver was evaporated on to the prism 12 under a vacuum of 106 torr. Film thickness was controlled by means of a quartz crystal film thickness monitor of commercially available kind.

The attenuator 10 operates as follows. A plane-polarized input beam 16 is incident on the film-prism interface at a point 18 and reflected along a path 20 to a second prism surface 22.

The beam is reflected once more at 24 to an output path 26 parallel to the input beam 16. The plane of incidence at the point 18 is the plane of the drawing, ie the plane of cross-section of the prism 12. The input beam 16 is polarized in this plane, as indicated by the electric vector arrow 28. This is referred to as p-polarization.

As will be described later in more detail, at some angle of incidence Sup the incident beam 16 excites a transverse magnetic (TM) wave known as a surface plasmon. The surface plasmon propagates along the boundary between a metal and a dielectric. Excitation of the plasmon results in energy being drawn from the incident beam, and the intensity reflected to the output path 26 falls to a low value. As the angle of incidence or input 0 changes from Sp, energy coupling to the surface plasmon falls and the reflected intensity rises. Accordingiy, for a fixed input beam direction, rotation of the prism 12 about an axis through its centroid 30 perpendicular to the plane of Fig. 1 results in varying attenuation of the input beam 16.

Referring now to Fig. 2, in which parts previously described are like referenced, there is shown an experimental arrangment for determining attenuation characteristics. The attenuator 10 receives an input beam 16 via polarizers 32 from a laser 34. The polarisers 32 are arranged to polarize the laser beam 16 in the plane of incidence, ie that of the drawing. Light passes from the prism 12 via a mirror 36 and a neutral density filter 38 to a vidicon detector 40. The vidicon 40 comprises a two-dimensional silicon photodiode array and detects the attenuator output beam. This permits the output beam to be detected despite its lateral shift in response to prism rotation.

Fig. 3 shows two graphs 50 and 52 of reflected intensity as a percentage of input beam intensity plotted against angle of incidence 0 in degrees. The graphs 50 and 52 are theoretical calculations corresponding to the attentuation properties of the attenuator 10 at two wavelengths, 623.8nm and 452.0nm. Graph 50 falls from 96.5% to 1.5% transmission in the interval 0=34.3 to 34.9 , and thereafter rises slightly more slowly to reach 90% reflectance at about 0=36.5 . Graph 52 has a minimum of 3% reflectance at 37.3 and rises to a maximum of 93% reflectance as 0 passes below 34.8 . In the region 0=36.5 to 37 , Graph 52 is close to linearity with a slope of about - 100% per degree. The effect of moving to shorter wavelengths is to broaden the plasmon resonance and to move its centre to a larger angle of incidence. Fig.

3 demonstrates that variable attenuation is obtainable from the attenuator 10 simply by rotating it through a small angle about its centroid. Dynamic ranges (ratio of maximum to minimum reflectance) of factors of 64 and 31 are obtained at 623.8nm and 452.0nm respectively.

Fig. 4 shows graph 50 repeated together with an equivalent graph 54 calculated theoretically for an attenuator having a gold film 14. The latter attenuator was assumed to be in other respects (including film thickness) equivalent to attenuator 12 of Fig. 1. Both graphs were calculated for a wavelength of 623.8nm. It can be seen that the effect of changing from a silver to a gold film is to broaden the plasmon resonance and to move its centre to a larger angle of incidence. This is similar to the effect of reducing the operating wavelength.

Fig. 5 shows graph 50 repeated but plotted on an expanded incidence angle scale. A further graph 56 is also shown for an attenuator similar to attenuator 10, but which had a silver film thickness of 56nm. It can be seen that the effect of increasing film thickness is to reduce the magnitude of the plasmon resonance and render it more sharply tuned to angle of incidence.

The surface plasmon phenomenon employed to furnish an attenuator in accordance with the invention arises as follows. As has been said, a surface plasmon is a TM wave which propagates along the boundary between a metal and a dielectric. The electric field associated with the plasmon oscillates sinusoidally in directions parallel to the boundary, but falls off exponentially in directions perpendicular to the boundary.

A surface plasmon cannot be excited directly by light from free space. This is because the frequency-wave vector or co-k relationship for surface plasmons exhibits dispersion, ie frequency dependent velocity. Since free space is non-dispersive, a surface plasmon may match the frequency or wave vector of incident light, but not both simultaneously. This is equivalent to simultaneous conservation of energy and momentum being unobtainable. The method of excitation of surface plasmons known in the prior art is referred to as Attenuated Total Reflection (ATR). This involves directing a light beam through a dielectric material on to a boundary of that material coated with a thin metal layer. The light beam must be polarized in the plane of incidence. This is known as p-polarization.In the absence of the metal layer, the light beam would be totally internally reflected at the boundary but an evanescent wave would extend beyond the boundary. The presence of the metal layer permits the light beam to couple to the surface plasmon via the evanescent wave, provided that the wave vector k8 of the evanescent wave at the dielectric/metal interface is matched to the surface plasmon wave vector ksp. The expressions for these wave vectors are as follows: ke=(co/c)(n sin 0) (1)

where angular frequency of light and surface plasmon; 8=angle of incidence or input of light beam at dielectric-metal boundary; n=refractive index of dielectric material; ed=dielectric constant of dielectric material; and em=real part of the dielectric constant of the metal.

The angle Ap at which the wave vector of the light beam matches that of the surface plasmon, ie k8=k5p, is given by combining (1) and (2) above:

In order to produce a well-defined surface plasmon resonance, the real part of the metal dielectric constant must be negative and much larger than the corresponding imaginary part. If this condition is not satisfied, the surface plasmon would be heavily damped and the resonance broadened. The result is that the metal layer would experience a temperature rise. This would be unacceptable for the purpose of modulating laser beams of appreciable power, since damage to the metal layer would occur. Furthermore modulation dynamic range would be reduced.Metals having acceptably high values of dielectric constant in the visible and/or infra-red include Ag, Au, Cu and Al; Ct)m must be negative, and its modulus should preferably be greater than or equal to 10. For practical purposes, Hop should not be close to 90" to avoid the difficulties of setting up an optical system to operate close to glancing incidence at the prism-metal interface. By inspection of Equation (3) this requires n to be large. In practice, n should be at least equal to 1.7.

The metal layer should be sufficiently thick to ensure that the suface plasmon is not significantly perturbed by the adjacent dielectric material, and Equations (1) and (2) are based on this assumption. The layer should however be sufficiently thin to produce a significant effect in terms of energy transfer to a surface plasmon. In practice, the thickness limits are 40-60nm, and preferably 46 + 5no.

The embodiment of the invention previously described suffers from the disadvantage that the incident light beam is shifted or steered as the prism 12 is rotated. The beam remains parallel to its original direction, but undergoes a lateral shift. The size of the shift depends on the angle through which the prism is rotated, and on the distance of the point of incidence from the prism right angle apex. The latter assumes rotation about the prism centroid. Since the rotation required to produce maximum to minimum attenuation is small, less than or equal to 10 in Fig. 5 for example, the beam shift may be tolerable for many purposes. Frequently however it is desirable to provide an attenuator in which the output beam propagates the same direction as the input beam and is unaffected by prism rotation.

Referring now to Fig. 6, there is shown a further attenuator 60 of the invention arranged for substantially constant output beam direction. The attenuator 60 includes a 45" isosceles right triangular prism 62 shown in perspective and having vertices A to F. The prism 62 is arranged with its triangular and rectangular faces horizontal and vertical respectively. The rectangular face ABCD is partially metallised between edge BD and line XY for surface plasmon generation as previously described.

The attenuator 60 includes two further 45" isosceles right triangular prisms 64 and 66 shown in side elevation, prism 66 being arranged above and to the right of prism 64. These prisms have illustrated vertices G to I and K to M respectively. An input light beam from a light source (not shown) such as a laser passes along an input path 68 through face ABEF of prism 62 to metallised face ABCD. Here the light beam is reflected to face CDEF and emerges through face ABEF. It then passes along path 70 for reflection at faces GH and HI of prism 64, and returns along path 72 to prism 62 having undergone a vertical displacement. After second reflections at prism faces ABEF and ABCD, the light beam passes along path 74 to prism 66.Here it undergoes a further vertical displacement by virtue of reflection at faces KL and LM, and emerges along path 76 which passes above prism 62. The light is then reflected at successive mirrors 78 and 80 to an output path 82.

The light paths 68 to 75 and 82 are accurately parallel to one another and are perpendicular to the apex CD of prism 62. In particular, the input and output paths 68 and 82 lie in the same straight line.

The prism 62 is arranged to be rotatable about an axis 84 through its centroid parallel to apex CD. The angular excursion of this rotation is arranged such that the angle of incidence of the input beam at prism face ABCD is adjustable through a range of values. This range is chosen to be that over which surface plasmon generation at the metallised face ABCD varies attenuation from a low to a high value. Means for rotating the prism 62 are not illustrated, since such devices are very well known in the art of optical instruments. A suitable arrangment would comprise a prism support table rotatable by a backlash-free differential micrometer screw drive.

The latter would be arranged for fine adjustment to enable attenuation to be varied, and coarse adjustment to select the range of angles of incidence covering a plasmon resonance at any required wavelength. From Fig. 3, the latter is wavelength dependent.

The attenuator 60 operates as follows. He input beam travels along path 68 as has been said, and experiences a lateral shift before emerging from prism 62 on path 70. Upon rotation of prism 62 through a small angle to change the degree of attenuation, path 70 will be displaced laterally towards or away from path 68 in accordance with the direction of rotation. Return path 72 from prism 64 to prism 62 will undergo a like displacement. Accordingly, the second reflection at each of the prism faces ABEF and ABCD is vertically above the respective first reflection. Light path 74 from prism 62 to prism 66 is accordingly invariant in position. The lateral shift of light path 70 produced by rotation of prism 62 about its centroid is exactly compensated by the corresponding shift of light path 72.Furthermore, since light path 74 is invariant under rotation of prism 62, so also are light paths 76 and 82 since prism 66 and mirrors 78 and 80 are fixed in position. The attenuator 60 accordingly provides in-line input and output beams independently of degree of attenuation.

Referring now to Fig. 7, there are shown three graphs 90, 92 and 94 of laser beam intensity I (arbitrary units) plotted as a function of distance relative to beam centre. The graphs were obtained after attenuating a constant intensity He-Ne laser beam (TEMoo mode) by differing degrees using an attenuator as shown in Fig. 2. It can be seen that the three graphs 90, 92 and 94 are Gaussian to within measurement accuracy. This demonstrates that an attenuator of the invention produces an attenuated beam which is substantially undistorted, ie the original intensity profile is preserved.

The optical power handling capacity of the attenuator of Fig. 1 was investigated using a 514nm, 0.8 watt Argon-ion laser beam. The beam was incident on the prism/metal boundary of the attenuator at the plasmon resonance or maximum attenuation angle for in excess of 5 hours.

The attenuation was over 90%, corresponding to over 0.72 watts diverted to surface plasmon energy. No measurable shift in the resonant incidence angle was detected, nor any damage to the metal film. The invention accordingly provides a stable and reproducible attenuator with good power handling capacity. If additional dynamic range of attenuation is required, tvçó or more attenuators of the invention may be employed in tandem, eg attenuators such 'as 60.

Claims (9)

1. An optical attenuator including an isosceles prism having a right angle apex, a metallisation layer upon a prism face adjacent that apex, the layer and prism material being appropriate for surface plasmon generation, and means for rotating the prism about an axis through its centroid parallel to the prism apex and through an angular excursion encompassing at least part of a surface plasmon resonance, the dimensions of the prism being appropriate for light at angles of incidence on the metallised face within one degree of resonance to be reflected at a second prism face adjacent the apex.

2. An attenuator according to Claim 1 including means for compensating for light beam displacement upon prism rotation.

3. An attenuator according to Claim 2 wherein the compensating means is arranged to receive light from the second prism face and return it thereto for incidence parallel to the preceding reflection direction and with a displacement parallel to the prism apex.

4. An attenuator according to Claim 3 including reflecting means disposed to receive light after a second passage through the prism and render it parallel to an input light direction.

5. An attenuator according to any preceding claim wherein the prism has a refractive index equal to at least 1.7.

6. An attenuator according to Claim 5 wherein the prism is of Schott glass with a refractive index of 1.805.

7. An attenuator according to any preceding claim wherein the metallisation layer is gold, silver, copper or aluminium.

8. An attenuator according to any preceding claim including prism rotating means coarsely adjustable for locating a plasmon resonance and finely adjustable for tuning attenuation through at least part of the resonance.

9. An optical attenuator substantially as herein described with reference to Fig. 1 or 6.