Polarization control of UV writing

Details Polarization control of UV writing Polarization control of UV writing

2001-05-21

2004-06-08

Robinson, Mark A. (Department: 2872)

Optical: systems and elements

Diffraction

From grating

C385S037000

Reexamination Certificate

active

06747798

ABSTRACT:

FIELD OF THE INVENTION
The present invention is directed generally to a method and apparatus for forming refractive index gratings in a medium, and more particularly to a method and apparatus that uses polarization control of the exposing beams for forming refractive index gratings in the medium.
BACKGROUND
Certain optical media, including at least some silica-based waveguides, can be modified by exposure to electromagnetic radiation in an appropriate spectral range. The exposure of the optical media may induce refractive index changes affecting the optical properties in the illuminated portions of the optical medium.
Refractive index changes can be induced in photosensitive optical media. The photosensitivity means that the incident electromagnetic radiation interacts, at least to some degree, with the matter constituting the medium, implying an absorption of the electromagnetic radiation in the medium. Hence, the photosensitivity of the optical medium, and the strength of the changes in the refractive index, are dependent of the chemical composition of the medium. Germanosilicates are widely used for photosensitive waveguides, but other materials and/or other dopants than germanium may also give the desired photosensitivity.
Ideally, the photo-induced change in the refractive index, &Dgr;n, is linearly dependent upon the fluence of radiation on the photosensitive medium. The fluence, &phgr;(r), is the amount of energy per unit area and is defined as &phgr;(r)=∫I(t) dt, where I(t) is the intensity of the applied radiation at time t, for a position r. Hence both the fluence and the intensity are used for characterizing the radiation. The dependency of &Dgr;n on &phgr;(r) diverges from the ideal linear dependency for some material compositions and/or for high intensities.
If the incident radiation field forms a pattern on the medium, the induced changes in the refractive index may form a corresponding pattern. For example, an interference pattern in the incident radiation field may form a periodic pattern in the photosensitive medium, such as a periodic pattern forming one or more Bragg gratings.
FIG. 1A
illustrates a typical method for writing a periodic index pattern in a waveguide such as an optical fiber or a planar waveguide. A laser beam
102
of actinic radiation is directed through a phase mask
104
, through a cladding layer
108
of the medium
106
and into the core
110
. The phase mask
104
generates an interference pattern with a period half that of its surface relief pattern
105
. Index of refraction changes in the core
110
occur predominantly at the bright fringes of the interference pattern, thus creating a periodic variation
112
in the refractive index grating, also referred to as a grating, in the core
110
. The laser beam
102
may be translated along the medium
106
in order to write a longer grating
112
, for a given width of beam
102
. The actinic radiation used is typically UV or near UV radiation, but other wavelength ranges may be used, depending on the wavelength sensitivity of the photosensitive species in the core
110
.
The refractive index grating
112
may operate as a spectrally selective reflector or transmitter for electromagnetic radiation propagating along the core
110
. In general, the spectral response of the refractive index grating
110
is determined by a number of different parameters, including the shape of the grating, the period of the refractive index modulation, the variation in the period (also referred to as chirp), phase relations, amplitude modulations and the like.
In a simple approach, the shape and size of the grating
112
, may be described as the effective refractive index, n(r), as a function of position. In a simple form, the effective refractive index may be given as n(r)=n
0
+&Dgr;n(r), where n
0
is independent of position. This expression may not, however, be adequate in all contexts since the refractive index of a medium may also depend on the frequency and the polarization of the light propagating in the medium, as well as number of other parameters.
Various methods for controlling the writing of the refractive index grating have been proposed and utilized in the prior art. These methods have been based on such parameters as control of the laser intensity, scan speed, pulse rate, or using a controlled vibration of the phase mask or sample, or some combination of these.
A frequently used technique to improve the spectral response when writing refractive index gratings in waveguide structures is apodisation. A frequently encountered problem during apodisation is chirp. The period &Lgr; of a periodic grating is the optical distance between amplitude peaks in the periodic structure. However, the optical distance between two points is also dependent on the mean refractive index in the region between the two points. Hence, when the refractive index modulation is written, the mean index and thereby the optical distance and the period may change throughout the grating structure. Since the amplitude of the grating modulation typically varies over the periodic structure, for example to obtain apodization, the mean index and hence the period seen by radiation propagating in the medium varies, and the grating is subject to “chirp”. This is illustrated in
FIG. 1B
wherein the oscillating curve
152
shows the periodic structure and the solid curve
154
represents the mean index change along the grating.
U.S. Pat. No. 5,830,622 discloses a method for modulating the mean index by providing a method for forming an optical grating using two steps. In the first step, the periodic grating structure is written in a glass. Subsequent or prior thereto, a region concomitant to the grating structure is illuminated with radiation having a predetermined spatial distribution, intensity, wavelength etc. in order to raise and/or modulate the mean refractive index of the region. A disadvantage of the method for controlling the writing of refractive index gratings disclosed in U.S. Pat. No. 5,830,622 is that two separate exposures are required.
WO 97/21120 discloses a method for writing a refractive index grating by creating an interference pattern in the medium between two beams. The two beams are formed from one beam by deflecting parts of the beam to generate two beams, which are controlled simultaneously by overlapping the beam paths of the two beams.
It is a disadvantage of the existing methods for controlling the writing of refractive index gratings that the material is non-reciprocal meaning that the photosensitivity is changed during the first exposure and the change in photosensitivity is a non-linear function of the locally applied fluence. Therefore it is nearly impossible to raise the mean refractive index to a constant level and/or maintain the desired refractive index amplitude simultaneously.
It is a disadvantage of the other existing vibration-based methods for controlling the writing of refractive index gratings that interferometric (submicron) stability is needed for the entire writing set-up.
SUMMARY OF THE INVENTION
The present invention is directed to a system and method for controlling the writing of refractive index structures such as gratings in an optical waveguide, and in particular to a system and method that may use only a single writing step. The present invention is further directed to a system and method for controlling the writing of refractive index structures that do not require interferometric stability of the control elements.
One embodiment of the invention is directed to a method for changing a refractive index of a first part of a medium. The method includes simultaneously illuminating the first part of the medium with at least part of a first beam of electromagnetic radiation and at least part of a second beam of electromagnetic radiation, wherein the first beam has a first polarization state and a first wavevector and the second beam has a second polarization state different from the first polarization state, and a second wavevector different from the first wavevect

Affiliated with

Also associated with

Rating

Polarization control of UV writing does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Polarization control of UV writing, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Polarization control of UV writing will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-3351883