Optical: systems and elements – Optical frequency converter – Dielectric optical waveguide type
Reexamination Certificate
2000-09-07
2002-10-01
Font, Frank G. (Department: 2877)
Optical: systems and elements
Optical frequency converter
Dielectric optical waveguide type
C359S330000, C359S326000
Reexamination Certificate
active
06459525
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a fiberoptic broadband wavelength converter used as a key device for constructing an optical communication network, particularly in WDM (wavelength division multiplexing), and a wavelength converting optical fiber and a pump source used in such an apparatus.
2. Related Background Art
It is possible to generate, from three lights having frequencies f
1
, f
2
, f
3
, light having frequency f
4
different from these frequencies, by using FWM (four wave mixing) based on third-order nonlinear polarization in an optical fiber. In this case, the frequency f
4
is determined by three frequencies f
1
, f
2
, f
3
and a relationship (f
4
+f
1
=f
2
+f
3
) is established. Here, particularly, in case of f
1
=f
2
, it is referred to as “DFWM (degenerated four wave mixing)”, where generated light having frequency f
4
is called as idler light.
The FWM and DFWM have been applied to wavelength conversion and dispersion compensation by phase conjugate light. For example, when pump having a wavelength &lgr;
p
(=c/f
p
) is combined by an optical fiber through which optical signal having a wavelength &lgr;
s
(=c/f
s
) is propagated, by a coupler, at output end of the optical fiber, the idler light is generated by DFWM, as well as the optical signal and the pump. “c” in &lgr;
p
(=c/f
p
) is a speed of light in vacuum. Since the idler light is the same as the optical signal except that it has a wavelength different from that of the optical signal and has property of phase conjugation, when the pump and the optical signal are removed from the output light from the optical fiber by using a filter to pick up only the idler light, it is possible to realize a wavelength converter of the optical signal.
Nowadays, in WDM communication, it has been attempted that a bandwidth used to the optical communication has been become broader than that of the conventional EDFA (Erbium-Doped Fiber amplifier). The bandwidth of the conventional EDFA is typically inside the region of 1530 nm-1560 nm; so-called C-band. As one example, the bandwidth has been expanded to 1570 nm-1610 nm (referred as L-band) by using specially designed EDFA, Raman amplifier and so on. Concerning the broadband WDM optical communication networks based on such amplifiers, interconnection of the two independent WDM systems that are composed of the signals inside the different wavelength region, will be required. In this situation, all-optical signal processing is required and a broadband wavelength converter is expected to enhance the flexibility of the networks. Until now, a fiberoptic broadband (36 nm half width of the half maximum) wavelength converter using fiber DFWM was reported.
It is known that the bandwidth of the fiberoptic wavelength converter using the DFWM has infinite conversion bandwidth in principle under the condition that the pump wavelength coincides with the zero-dispersion wavelength of the fiber. However, in truth the conversion bandwidth is actually limited into the finite wavelength region because of the following five obstacles.
The first obstacle is chromatic dispersion variance of the optical fiber along the longitudinal direction. It is known that inhomogeneous distribution of the zero-dispersion wavelength seriously deteriorates the conversion efficiency. In other words, efficient idler generation is not expected under the large variance of the zero-dispersion wavelength.
The second obstacle is PMD (polarization-mode dispersion) of the fiber. Because of the fiber PMD difference of the SOP (state of polarization) of the pump and signal becomes larger as the lightwaves propagates. It is well-known that it is preferable to coincide the SOP of the signal with the pump in order to generate the idler light efficiently by the FWM in the optical fiber. Further, it is also known that generation efficiency of the idler light becomes zero when the SOP of the optical signal and pump are orthogonal. However, because of the following reasons, it is difficult to coincide the SOP of the signal and pump perfectly along the entire fiber length. Even if the SOP of the optical signal and the pump are coincided carefully at the input end of the optical fiber, unless PMF (polarization maintaining fiber) is used as the optical fiber and linearly polarization along an optical axis of polarization is launched into such a fiber, phase of the incident lights are changed during the propagation. In general, since there is PMD, i.e., birefringence in the optical fiber, the polarization state is not preserved. Further, since the birefringence is small and is distributed inhomogeneously along the longitudinal direction, there is no optical axis in the practical sense. Even when generalized inherent SOP such as principal state of polarization is chosen, since the magnitude of the birefringence itself is small and is thermally unstable, a stable wavelength conversion is impossible. In general, phase change of the lightwave during propagation induces the change of the SOP. When we put difference of the wavelength between the optical signal and the pump as &Dgr;&lgr;, phase difference &Dgr;&phgr; (a quantity representing difference of SOP between pump and signal) is represented as:
Δ
φ
=
-
2
π
λ
p
2
·
Δ
n
·
Δ
λ
·
L
∝
Δλ
·
L
(
3
)
where, &Dgr;n is birefringence induced refractive index difference, L is the fiber length, and &lgr;
p
is a wavelength of the pump.
As can be seen from the above equation (3), the phase difference &Dgr;&phgr; is proportional to both the wavelength difference &Dgr;&lgr; and the length L of the optical fiber. Accordingly, the larger the wavelength differences &Dgr;&lgr; increases, the larger the influence of change of polarization increases and it becomes more difficult to avoid the deterioration of the conversion efficiency due to PMD during the propagation. In order to solve this problem, it has been attempted that the phase difference between the pump and the signal be decreased by reducing contribution of L in the above equation (3) by using a polarization maintaining high nonlinearity optical fiber or extremely shortening the length L of the high nonlinearity optical fiber without polarization maintaining characteristics.
The third obstacle is the fact that the pump wavelength and the zero dispersion wavelength cannot be equalized exactly. Although the conversion bandwidth becomes infinite only when the wavelength of the pump is completely coincided with the zero dispersion wavelength of the optical fiber, even if they are slightly deviated from each other, the infinite converting bandwidth cannot be realized. However, for the practical sense, it is almost impossible to completely equalize the wavelength of the pump to the zero dispersion wavelength of the optical fiber.
The fourth obstacle is effect (high order effect of dispersion) of the fourth-order group velocity dispersion of the fiber. In general, in order to generate DFWM efficiently, the following phase matching conditions must be satisfied regarding both frequency of light and the propagation constant &bgr;:
2&ohgr;
p
=&ohgr;
s
+&ohgr;
c
(4)
2&bgr;(&ohgr;
p
)=&bgr;(&ohgr;
s
)+&bgr;(&ohgr;
c
) (5)
Where, &ohgr; is angular frequency and has a relationship between the angular frequency and the frequency f is &ohgr;=2&pgr;f.
In general, when the DFWM in the optical fiber is considered, phase matching of the frequency and phase matching of the propagation constant must be satisfied simultaneously. In this case, since the phase matching of the frequency can easily be realized, we should concentrate on realizing the phase matching of the propagation constant. When the broadband wavelength conversion based on the DFWM is considered, phase mismatch &Dgr;&bgr; of the propagation constant &bgr; is represented as follo
Aso Osamu
Namiki Shu
Font Frank G.
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Punnoose Roy M.
The Furukawa Electric Co. Ltd.
LandOfFree
Optical fiber type wide bandwidth wavelength converter and... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Optical fiber type wide bandwidth wavelength converter and..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Optical fiber type wide bandwidth wavelength converter and... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2929220