Chirped optical fibre grating

Details Chirped optical fibre grating Chirped optical fibre grating

1998-06-18

2001-07-24

Healy, Brian (Department: 2874)

Optical waveguides

With optical coupler

Input/output coupler

C385S123000

Reexamination Certificate

active

06266463

ABSTRACT:

BACKGROUND OF THE INVENTION
This invention relates to chirped optical fibre gratings.
Chirped optical fibre Bragg gratings are useful in, for example, dispersion compensation in optical fibre transmission links.
In a chirped optical fibre grating the pitch of the refractive index variations (which form the grating) varies with distance along the grating, which leads to a dispersive effect. The sign of the dispersion is dependent on which end of the grating light is launched into: pulses incident from the long wavelength end suffer negative dispersion in reflection, whilst pulses launched into the short wavelength end experience a positive dispersion in reflection.
This type of grating can suffer problems due to cladding mode losses. In order to explain this problem, cladding modes in optical fibres will first be described. Then, cladding modes in a uniform pitch (unchirped) fibre grating will be described, before the problem which occurs due to cladding mode losses in chirped optical fibre gratings is discussed.
The core of a single-mode optical fibre is designed to support a single polarisation-degenerate guided radiation mode for optical wavelengths longer than that defined by the so-called cut-off condition. Other than this fundamental (LP
01
) mode, which is guided by the core-cladding interface, there exists a set of discrete higher-order radiation modes (LP
0n
) which are supported by the cladding of the fibre and are guided by the cladding-air, or cladding-coating interface.
The spatial extent of the cladding mode field increases with the mode order and there is a corresponding reduction in the overlap between the mode and the core area. As the cladding has a lower refractive index than the core, radiation propagating as a cladding mode will experience a lower group index (and hence a smaller propagation constant, &bgr;) than radiation propagating in the fundamental LP
01
mode. Modes supported predominantly in the cladding of a fibre are highly susceptible to out-coupling from the fibre if there are any small defects in the cladding-air (or cladding-coating) interface. Radiation in a cladding mode thus may be guided for just a few centimeters before it is out-coupled, making propagation in cladding modes extremely lossy.
The condition for reflection by a uniform fibre grating is the so-called Bragg phase-matching condition, which is met when the propagation constant of the grating, K, is equal to the sum of the forward- and backward-propagating mode constants, &bgr;
+01
&bgr;
−01
:
K=&bgr;
+
+&bgr;
−01
This occurs at the so-called Bragg wavelength. However, the condition for phase matching between a fundamental forward-propagating guided mode and a backward-propagating cladding mode is also met for radiation of wavelengths away from the fundamental Bragg wavelength.
The propagation constants of cladding modes are smaller than those of the fundamental LP
01
mode and so coupling into a backward propagating cladding mode may occur from a forward propagating LP
01
mode of a larger propagation constant (shorter wavelength), where:
K=&bgr;
+01
+&bgr;
−0n
where &bgr;
−0n
is the propagation constant of the nth backward propagating cladding mode.
This coupling of radiation from fundamental guided modes to cladding modes is visible as a series of discrete losses on the short wavelength side of the main Bragg reflection peak, shown in
FIG. 1
of the accompanying drawings. The position and strength of cladding mode losses relative to the main Bragg reflection is determined by the refractive index geometry of the fibre host to the grating. The first cladding mode is observed at wavelengths typically a few nanometers short of the Bragg wavelength.
The way in which cladding mode losses affect chirped gratings will now be discussed.
It is often desirable to use chirped fibre gratings with bandwidths greater than the separation of the first cladding mode from the main Bragg reflection to induce a negative chirp (i.e. pulses are launched into, and reflected from, the long wavelength end of the grating). In many systems it is also quite critical that the relative intensities of pulses reflected by the grating should not vary across the useable bandwidth.
This presents a problem where the shortest wavelength of the grating's useable bandwidth is below that of the first cladding mode associated with the longest spatial period of the grating. In this case, a fraction of the short wavelength light is coupled from its fundamental forward-propagating mode into a lossy cladding mode. This effect is generally seen only when using fibre Bragg gratings in a negative dispersion sense as, in this configuration, short wavelength light has to propagate through the “longer wavelength” portions of the grating, where the propagation constant is appropriate for coupling shorter wavelengths to a cladding mode) before arriving at the appropriate part of the grating for Bragg reflection of the short wavelength light.
The distributed nature of spatial frequencies in a chirped fibre grating means that the losses caused by the coupling of short wavelengths into cladding modes are integrated along the length of the grating.
In a chirped grating which should have a uniform reflectivity across the useable band, the coupling of shorter wavelengths into cladding modes actually causes a sloped response extending from the short wavelength end of the grating to the wavelength of the first cladding mode associated with the longest wavelength of the grating.
FIGS. 2
a
and
2
b
of the accompanying drawings illustrate this effect on the spectral response of a high-quality 7.5 nm (nanometer) bandwidth chirped fibre grating (total chirp 8.54 nm) when used in the negative dispersion sense, compared to the flat-top spectral response observed from the same grating used in the positive dispersion sense.
In particular,
FIG. 2
a
illustrates the reflection response when light is introduced from the short wavelength end of the grating, while
FIG. 2
b
illustrates the corresponding reflection response when light is introduced from the long wavelength end of the same grating.
For comparison with later results from prototype embodiments of the invention,
FIG. 2
c
illustrates the transmission response of the grating and
FIG. 2
d
illustrates the dispersion of the grating.
The size of this lossy effect is determined by the strength of coupling to the cladding modes, which is, for a given fibre, determined by the coupling constant of the grating. For strong chirped gratings the loss may be as much as several dB (decibels) in reflection, which is more than enough to cause significant problems in applications sensitive to in-band intensity non-uniformities.
SUMMARY OF THE INVENTION
This invention provides a method of fabricating a chirped optical fibre grating so that the grating has a predetermined desired wavelength-dependent response across an operational bandwidth, the method comprising apodising the grating so that a degree of apodisation at a longitudinal position along the grating is dependent upon the desired response at the optical wavelength reflected at that longitudinal position along the grating.
Although it would in theory be possible to alleviate the problems described above by both shifting the first cladding mode away from the Bragg wavelength and reducing coupling to cladding modes from LP
01
modes by way of a suitable fibre design (such as high NA, or depressed-cladding fibres), it is not possible in practice to achieve a great enough spectral shift and strong enough attenuation of the losses induced by coupling to cladding modes to allow strong chirped fibre gratings to have a flat reflection spectrum for bandwidths of greater than several nanometers without some careful and deliberate compensation of cladding mode effects.
In contrast, in the invention, because the phase-matching wavelengths of a chirped fibre grating are spatially distributed along its length, a controlled variation of the local grating coupling-constant can be used to

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