Synthesis of supergratings by fourier methods

Details Synthesis of supergratings by fourier methods Synthesis of supergratings by fourier methods

2002-05-13

2003-12-02

Juba, John (Department: 2872)

Optical: systems and elements

Diffraction

Using fourier transform spatial filtering

C385S037000, C398S087000

Reexamination Certificate

active

06657786

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for computationally synthesizing supergratings in waveguides to give desired reflectance spectra, and more particularly the method relates to synthesizing supergratings using Fourier analysis.
2. Prior Art
Gratings are optical devices used to achieve wavelength-dependent characteristics by means of optical interference effects. These wavelength-dependent optical characteristics can, for instance, serve to reflect light of a specific wavelength while transmitting light at all other wavelengths. Such characteristics are useful in a wide range of situations, including the extraction of individual wavelength-channels in Wavelength Division Multiplexed (WDM) optical communication systems, or providing wavelength-specific feedback for tunable or multi-wavelength semiconductor lasers.
The term “multi-wavelength grating” generally refers to a grating that is capable of controlling optical characteristics at a number of wavelengths, such as a grating that reflects light at several select wavelengths (which may correspond to specific optical communication channels) while remaining transparent to light at other wavelengths. In some situations, however, there is a need to set the optical characteristics for a continuous range of wavelengths, rather than at specific wavelength values. This is the case when trying to compensate for the unevenness of optical gain profiles in laser cavities and optical amplifiers by means of a grating. This latter type of specification is usually difficult to meet with traditional grating technologies.
Gratings are usually implemented by modulating (varying) the effective index of refraction of a wave-guiding structure. The variation of refractive index along the length of the grating is often referred to as the “index profile” of the grating. These changes in index of refraction cause incident light to be reflected.
In the case of an abrupt interface between two index values, light incident directly on the interface is reflected according to the Fresnel reflection law:
E
r
E
i
=
n
i
-
n
i
+
1
n
i
+
n
i
+
1
(Eq. 1)
where E, and E
r
are the incident and reflected electric fields at the interface, respectively, and n
i
and n
j
+
j
are the refractive index values on either side of the interface, see FIG.
1
. Although this reflection phenomenon is most striking for refractive index steps, it also occurs in a more complicated form with a continuous refractive index. Grating devices utilizing both types of reflection phenomena exist. A grating derives its wavelength-dependent character from optical interference effects. This phenomenon is illustrated in FIG.
2
: incident light is reflected by each grating feature (step change in index of refraction) and interferes, either constructively or destructively, to generate a wavelength-dependent reflectance spectrum. At a certain wavelength, all the individually weak reflections add up constructively, leading to strong grating reflectance. At a different wavelength, however, the phase relation between the individual reflections is different and the beams may add up to produce little or no grating reflectance, transmitting most of the light.
Gratings may be “written” into the optical wave-guide in a variety of different ways, depending primarily on the material used. Fiber or glass guides, for example, often make use of photorefractiveness, a property of specially prepared glasses that allows their refractive index to be varied by exposing them to high intensity light (typically in the ultraviolet). Semiconductor gratings, on the other hand, are usually implemented as surface-relief gratings by etching a grating pattern into the surface of the semiconductor guide (which may then be buried following subsequent deposition). Etching the surface of the waveguide does not affect its real refractive index as photoinscription does, but rather varies the guide's effective index. Nevertheless, this difference does not affect the operation of the grating.
A simple and common grating device known as a Bragg Grating is illustrated in FIG.
3
. The Bragg Grating consists of a periodic variation in refractive index and acts as a reflector for a single wavelength of light related to the periodicity (known as pitch, A) of the index pattern. It is frequently used in both semiconductor systems and fiber-optic systems, where it is known as a Fiber Bragg Grating. The Bragg Grating can actually reflect at several wavelengths, corresponding to overtones of its fundamental pitch, which satisfy the relation:
&lgr;=2
&Lgr;n
eff
/N
  (Eq. 2)
where N is a positive integer (typically 1 for the design wavelength) and the average effective index n
eff
is generally wavelength-dependent. However, these higher-order wavelengths tend to be at quite different spectral regions than the fundamental, thus not making the Bragg Grating useful as a multi-wavelength reflector. Moreover, these higher-order wavelengths cannot be tuned independently of one another.
Wavelength Division Multiplexing (WDM) is a technology where many communication channels are encoded into a single optical cable by utilizing different wavelengths of light. Gratings are often used to separate or process these channels. Older grating technologies can process one wavelength at a time, forcing devices intended to process multiple wavelengths to employ a cascade of single-wavelength gratings. This is not an attractive solution because, on top of the additional losses that each grating creates, even a single grating occupies a considerable amount of space by today's standards of integration. It is thus desired to have a single device capable of processing several wavelengths in a space-efficient manner.
A similar situation occurs in the realm of semiconductor lasers. It is widely accepted that WDM technology would greatly benefit from lasers capable of generating light at several wavelengths. The output wavelength of semiconductor lasers is largely determined by the presence of “feedback elements” around or inside the laser gain section, which act to reflect light at the desired wavelength back into the laser. For multi-wavelength operation, multi-wavelength feedback is needed. Again, single-wavelength grating technology can only address this demand with a cascade of Bragg Gratings, leading to the same (if not more notable) loss and space problems mentioned above. The conclusion is the same: there exist a need for a single device capable of generating multi-wavelength reflection and transmission spectra in a space- and resource-efficient manner.
There are several multi-wavelength grating technologies: analog superimposed gratings, Sampled Gratings (SG), Super-Structure Gratings (SSG) and Binary Supergratings (BSG). The binary supergrating is also known as a binary superimposed grating, for historical reasons. BSG development was originally motivated by a desire to emulate the superposition of multiple conventional Bragg gratings, hence the term “binary superimposed grating”. Since then, synthesis techniques have evolved to allow the emulation of arbitrary diffraction characteristics, a flexibility better captured by the term “binary supergrating”.
Analog superimposed gratings are a generalization of the Bragg Grating and are rooted in a principle of superposition: a grating profile consisting of the sum of the index profiles of single-wavelength gratings reflects at all of its constituent wavelengths. Such a grating relies on an analog index variation, that is, a refractive index that changes continuously along the grating length (FIG.
4
). It is this analog nature of the profile that limits the functionality of these gratings: it is difficult to inscribe strong analog gratings using the photorefractive effect, as the change of index under illumination varies non-linearly with stronger exposures, making the writing process difficult. In semiconductors, where surface relief gratings are used, the situation is even worse as it is currently impossible to reprod

Affiliated with

Also associated with

Rating

Synthesis of supergratings by fourier methods does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Synthesis of supergratings by fourier methods, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Synthesis of supergratings by fourier methods will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-3154446