Optical waveguides – With optical coupler – Input/output coupler
Reexamination Certificate
2002-06-28
2004-11-09
Bruce, David V. (Department: 2882)
Optical waveguides
With optical coupler
Input/output coupler
Reexamination Certificate
active
06816649
ABSTRACT:
This invention relates to an apparatus and a method for fabrication of optical waveguide Bragg gratings and the use of diffraction gratings in such fabrication.
Waveguide Bragg gratings are popular components used as wavelength selective filters in fiber-optic communication systems [1]. They are also popular for fiber-optic sensors because of their small size combined with the sensitivity of their reflection or transmission properties (typically the peak reflection wavelength) to strain, temperature and other mechanisms that change the fiber refractive index [2].
Fiber Bragg gratings (FBG) consist of a periodic modulation of the refractive index in the core (or more precisely within the modefield cross section) of an optical fiber [3]. The period of this modulation equals &Lgr;
B
=&lgr;
B
/2n where n is the fiber refractive index and &lgr;
B
is the optical Bragg wavelength at which the local reflectivity has its maximum. The index modulation is usually produced by illuminating the fiber core with ultraviolet (UV) light from a laser with wavelength &lgr;
UV
in the range from 190 to 300 nm and with a spatial intensity modulation period &Lgr;
IF
=&Lgr;
B
. Typically, a frequency doubled Argon ion laser with wavelength &lgr;
UV
≈244 nm may be used.
The intensity modulation can be created by splitting the laser beam and recombining the beams on the fiber via a mirror [4, 5] or lens [6] arrangement at an angle &agr;
Fiber
=2arc sin(&lgr;
UV
/&Lgr;
IF
). The UV beamsplitter can either be formed by a semitransparent mirror, a diffractive phase mask (spatially modulating the UV phase), or an amplitude mask (spatially modulating the UV amplitude). The angular separation between the +1 and −1 order diffracted beams from such a mask equals &agr;
Mask
=2arc sin(&lgr;
UV
/&Lgr;
Mask
), where &Lgr;
Mask
is the mask period at the position where the mask is illuminated.
It is also possible to produce gratings by placing the fiber in the near field behind a diffractive mask where diffractive orders still overlap. However, there are disadvantages with the near field approach. First, the zero and higher order diffracted beams from available non-ideal masks will cause an unwanted background exposure which limits the available index modulation amplitude in a fiber with a limited photoinduced index change. Second, nonlinearities in the photosensitivity (index change versus UV exposure energy) at high exposure levels may induce background index variations due to the mixing of three or more diffractive orders. These background index changes will depend on the relative phases of the mixed beams within the fiber, and it will therefore depend on variations in the distance between the fiber and the phase mask in the order of &Lgr;
IF
. In practice it is very hard to avoid that the distance varies by many &Lgr;
IF
, hence the resulting background index variations will cause errors in the effective grating phase seen by light propagating along the fiber waveguide. A third problem arises if one wants to use scanning techniques where a small diameter UV beam is scanned across the fiber and mask to enable spatially dependent modulation of the grating phase and amplitude by moving and dithering the mask during the scan, as suggested in [7]. In this configuration (and also in the scanning fiber technique [8] discussed in more detail below) the fiber must be placed very close to the mask (typically at a distance of 100-300 &mgr;m) to ensure overlap between the +1 and −1 order diffracted beams. The need for such a small spacing induces a high risk of damaging the delicate mask surface, as well as potential problems with static electric forces between the fiber and the mask, as discussed in [6].
Various stepwise grating exposure techniques have been disclosed. Two similar systems are disclosed in [8, 9], where the fiber is moved through a stationary UV interference pattern in the direction parallel to the fiber axis while short pulses are fired from the UV source at regular intervals when the phase of the UV intensity pattern relative to the fiber matches the wanted index modulation phase. A disadvantage of this pulsed technique is that the duty-cycle of the pulsing UV source must be low, preferably <20-30% in [8] and even lower in [9], to enable high visibility of the exposure and thus low background index change. With limited peak UV intensity available, for instance due to limited source power available or to damage limitations of the fiber or the UV optics, this will cause significantly longer production times compared to approaches using continuous exposure. Short writing time per FBG is generally advantageous, since it minimizes grating errors caused by slow drift in the relative positions of components in the FBG production system.
An alternative version of the step writing technique where the UV source is operated in a continuous wave mode is disclosed in [4]. In this approach the interference fringes are moved at the same speed as the fiber by moving the diffractive mask, but the mask position is reset each time the fiber has moved some multiple of Bragg periods. This technique can allow for shorter writing times than the techniques described in [8, 9].
A potential problem with the step writing techniques is that the periodic pulsing of the UV source or the resetting of the mask will tend to cause a super periodic modulation of the grating strength and/or phase with a super period length that corresponds to the pulse or resetting period. If the resetting period equals ML
B
where M is an integer, this will cause grating sideband reflections at wavelengths that correspond to Bragg wave numbers which are separated from the nominal Bragg wave number 2p/L
B
by integers of 2p/(M
LB
). There are two reasons why M preferably should be a small number. First, the sideband separation can usually be made so large by increasing M that the sidebands do not cause any problem for the FBG applications of interest. Second, as discussed in [9] the strength of the sideband reflections will be reduced when M is reduced due to averaging. This is because the number of step exposures at each position on the fiber is increased.
In the mask stepping approach, maximizing the speed and accuracy by which the mask position is reset will also contribute to minimize the super periodic modulation and the grating sideband reflectivity. In order to minimize M and to maximize the resetting speed it is thus desirable that the fringe position can be modulated with a large bandwidth. This may limit the size of the mask, as the mechanical resonance frequencies are typically inversely proportional to the size of the modulated device (the mask).
It is possible with the step writing techniques to impose discrete phase-shifts in the index modulation by suddenly changing the phase of the interference fringes as the fiber is scanned through the UV spot. The fiber section that is illuminated by the UV spot when the fringe phase changes will in this case be partially exposed with both fringe phases, and the grating phase-shift will thus occur gradually across this section. Apodization of the grating strength can be implemented without perturbing the grating phase or the mean exposure per grating period by modulating the phase of the fringes relative to the fiber while the fiber is scanned through the UV spot, without perturbing the mean phase value.
It is also possible to stretch or compress the Bragg period &Lgr;
B
slightly relative to the UV interference period &Lgr;
IF
=&lgr;
UV
sin(&agr;
Fiber
/2) by gradually changing the fringe phase relative to the fiber as the UV spot is scanned along the fiber. The Bragg wavelength shift achievable by this method is limited to about &Dgr;&lgr;
B,max
=&Lgr;
IF
2
/(4 L
Spot
) where L
Spot
is the 3 dB UV interference spot size [5]. When the shift increases above this limit the visibility o
Artman Thomas R
Bruce David V.
Moser, Patterson & Sheridan L.L.P.
Optoplan AS
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