Optical: systems and elements – Holographic system or element – Hardware for producing a hologram
Reexamination Certificate
2000-04-13
2002-08-20
Spyrou, Cassandra (Department: 2872)
Optical: systems and elements
Holographic system or element
Hardware for producing a hologram
C359S034000, C359S001000, C385S037000
Reexamination Certificate
active
06437886
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a device and a method for recording an interference pattern, such as holographic gratings or the like, in a photosensitive medium.
BACKGROUND
Optical elements of all kinds are often manufactured by impinging a light beam on a photosensitive medium to modify its optical, chemical or mechanical properties. A widely popular application of such a technique is to make diffraction gratings for use in spectroscopic, metrological and optical communication instruments, as well as for laser systems and a multitude of other systems having a use for such diffraction gratings.
Manufacture of gratings by interferometric methods has been well known for many years. The most common of such methods is by using two expanded collimated laser beams incident on a photosensitive plate. After exposure, the photosensitive plate is processed to obtain volume or relief structure grating, depending on the photosensitive material used. Assuming perfect collimation, the resulting gratings have uniform pitch. The recorded pitch has a value of
Λ
=
λ
sin
θ
1
-
sin
θ
2
where &Lgr; is the pitch, &lgr; is the wavelength of the laser and &thgr;
1
and &thgr;
2
are the angles of incidence of both beams with respect to the normal of the recording plane.
This method has many drawbacks. To collimate a beam, the initial small diameter laser beam is firstly cleaned and made divergent by the use of a spatial filter composed of a microscope objective and a pinhole. A lens or a parabolic mirror of proper diameter and focal length is then used to collimate the diverging beam. If this lens or mirror does not have a very good surface quality, the resulting beam will be noisy and some speckle pattern will be recorded in the grating, thereby generating noise in the application. Also, since the laser beam generally has a gaussian intensity profile, the beam must be sufficiently expanded in order to have a quasi-uniform illumination over the entire recording area. These problems are of more importance if large (over 50 mm) gratings are to be manufactured. The bigger the grating, the larger the optics.
In order to eliminate the noise problem, one can use non-collimated light. Using diverging beams, directly from the spatial filters, assures noise-free gratings. The problem with this configuration is that the pitch of the produced grating is not uniform. It has a hyperbolic-like pitch variation. European patent application no. 0 682 272 (MASANORI et al) discloses a grating manufacturing method using diverging beams. Although this technique is generally known in the field, it gives recording conditions to obtain less than 1% of variation in the recorded pattern interval.
Yet another problem for the two expanded (collimated or not) beams configuration is the stability of the interference pattern on the recording plane. Mechanical vibrations and air movements will cause some distortion or fringe displacement in the sinusoidal pattern. If this displacement is over a tenth of the pitch of the grating, the exposure will tend to erase itself by averaging of intensity. Again, the larger the recording area, and the longer the optical path of each beam, the more difficult it is to have a stable system. Although some stabilisation techniques are currently available (see for example FRELJLICH et al., Appl. Opt. 27(10), 1967 (1988) or TYUCHEV et al., J.Opt. Techno. 61(11), 839 (1994)), they will generally stabilise the pattern in a small area close to the recording area and will not assure stability over the full recording area.
To eliminate the need for large collimating optics, MOLLERNAUER, TOMLINSON, Appl. Opt. 16(3), 555 (1977) presents a technique using small scanning laser beams. Using a beamsplitter and a very flat mirror, the beam is split into two beams, which are then redirected to interfere on the recording plane. By precisely moving the mirror laterally, it is possible to move the interference area over the whole recording surface without disturbing the interference pattern. The drawbacks of this technique are that it requires a very stable linear translation stage to move the scanning mirror, because any angular deviation (as low as 10
−4
rad) will displace the interference pattern, which will erase itself by averaging.
Another configuration is based on a grating interferometer that uses a single point source and three hyperbolic gratings, such as disclosed in U.S. Pat. No. 5,394,266 (HIBINO). These three gratings are made by using standard two-point source interferometers as discussed earlier. By proper combination of a diverging point source and the three hyperbolic gratings, the curvature of the incident spherical wavefront, diffracted by the hyperbolic gratings results in an almost distortion-free interference pattern on a photosensitive layer. The resulting recorded grating has a very low grating line distortion compared to the standard two-point source interferometer. Also a grating interferometer with three gratings does not require a highly coherent source. A quasi-monochromatic light source is sufficient, since each of the two rays interfering in the recording plane is generated from a single point on the first grating. Using a low coherence source also reduces the noise recorded in the grating, since the optical path length of the diffused or scattered light (by imperfections on the gratings surfaces) is different from the optical path length of the recording light beams, thus lowering the speckle effect. The main disadvantage of this technique is that it requires a very tight alignment since a low coherence source is used. Of course, a highly coherent laser light can be used but then a higher noise may be generated.
Another kind of grating interferometer uses only a single grating, commonly called a phase mask. As presented in U.S. Pat. No. 5,367,588 (HILL et al), the phase mask is held in close proximity to the recording plane (an optical fibre in this specific case). The laser illumination of this mask will generate two main diffraction orders (other diffraction orders are weak). These two beams overlap in a small region close to the mask, generating the interference pattern, having a pitch half of the mask pitch. This configuration is very stable and does not require a laser with a very high coherence, since the interference zone is close to the mask.
In this technique, the whole length of the recording area is illuminated simultaneously. Based on the same phase mask principle, MARTIN, OUELLETTE, Elec. Lett., 30, 811 (1994) uses a translating focused illuminating beam to record the length of the grating, so that one does not need to expand the laser beam for writing longer gratings. Since the recording layer (again an optical fibre) is held in contact with the phase mask, the phase of the fringe pattern has a very low sensitivity to beam angular displacement so that a standard translation stage is sufficient to move the beam along the phase mask-fibre assembly. This technique also allows local control of the exposure level by varying laser light intensity or scanning speed.
The phase mask technique is very efficient for writing gratings in optical fibres. But to manufacture gratings for spectroscopic applications, the noise generated by higher order of diffracted orders during recording, will decrease the overall performance of the gratings.
U.S. Pat. No. 5,066,133 (BRIENZA) discloses a technique for producing extended length gratings using an endless strip grating in the form of a loop. A laser light passing through this grating is split into two beams, which are then redirected to interfere on an optical fibre to record a grating. By moving the strip grating at exactly the same speed as the optical fibre, the interference pattern stays stationary relative to the fibre so that an almost infinite length grating can be written. Although this principle seems interesting, there may be problems to maintain the exact same speed for both moving parts and the generation of a long flexible strip grati
Langlois Pierre
Trepanier Francois
Boutsikaris Leo
Institut National d'Optique
Merchant & Gould P.C.
Spyrou Cassandra
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