The present invention relates to optical modules, more particularly to configurations in which the gain of a signal increases with the increase in input signal power.
The recent development of the microstructured optical fiber, in which a high index core region is surrounded by cladding having a mix of silica and air, offers new fiber properties by virtue of the large refractive-index contrast that exists between glass and air.
Reference is made to Section 4.3.6 of Takanori Okoshi, “Optical Fibers”, Academic Press (1982) for discussion of the modes of an optical fiber. As described therein, HE11 mode is the fundamental mode and the others are the higher-order modes.
A properly designed microstructured optical fiber can support numerous transverse spatial modes that are essentially decoupled from one another, as described in U.S. Pat. No. 6,400,866. In one disclosed embodiment, an optical pump propagates in one transverse mode, and an optical signal in another. A microstructured optical fiber, which can support several decoupled transverse modes, can be used to phase match nonlinear optical processes such as second harmonic generation and four-wave mixing. An optical pump and an optical signal are coupled into respective transverse modes of a microstructured optical fiber having decoupled transverse modes. As a result, a third signal is generated, which may be, for example, a sum or difference frequency signal, a second or third harmonic signal, signals at frequencies above and below the pump frequency. While this patent suggests the usage of higher-order modes of a microstructured optical fiber for nonlinear optical processes, there is no suggestion of how, or recognition that, optical signal regeneration can be obtained free from frequency conversion.
A paper entitled All-Optical Data Regeneration Based On Self-Phase Modulation Effect, P. V. Mamyshev, ECOC'98 p.475 (1998) describes feeding a pulsed optical signal of return-to-zero (RZ) format into a nonlinear medium, where self-phase modulation (SPM) broadens the spectra of the signal. The output from the nonlinear medium passes through an optical filter having a central frequency offset from that of the original signal. By this configuration, the noise in “zeros” and the amplitude fluctuation in “ones” of the signal is suppressed, so that the signal is regenerated. However, the regenerated signal must have a frequency different from that of the original signal, so that the complexity of optical system increases by the incorporation of the regenerating module. Also, it is difficult to incorporate this regenerating module into existing optical systems.
The need thus exists in the prior art for an optical module of simple configuration that provides an increase in signal gain with increase of signal input power and that can phase match pump radiation and signal.
The present invention fulfills the above described, at least in part, by provision of a module that propagates a signal over an optical fiber with a pump lightwave that has a frequency three times of that of the signal and a phase index equal to that of the signal. Phase matching between the pump and signal is realized by employing a microstructured optical fiber. The following terms are defined to aid description of microstructured fiber characteristics.
A main medium is a medium that can constitute an optical fiber by itself. On the other hand, a sub medium is not necessarily able to constitute an optical fiber by itself. For example, glasses and polymers can be used as a main medium or a sub medium, while liquids, gases and vacuum can be used as a sub medium but not a main medium.
The average refractive index of a region composed of several media i (i=1 . . . M) is defined by the following formulas:
wherein n[i] and f[i] are respectively the refractive index and the volume of medium i.
The phase index of a mode is the ratio of the light speed in vacuum to the phase velocity of the mode. The nonlinear refractive index n(2) is related to the component χ(3)xxxx of the third-order susceptibility tensor by n(2)=(3/8n) χ(3)xxxx in the case of linearly polarized lightwave, where n is the refractive index. A wavelength of light is that in vacuum unless specified otherwise.
Advantages of the present invention will become readily apparent from the following detailed description, simply by way of illustration of the best mode contemplated of carrying out the invention. The invention is capable of other and different embodiments, and its several details are capable of modifications in various obvious respects, all without departing from the invention. Accordingly, the drawing and description are illustrative in nature, not restrictive.
The present invention is illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawing and in which like reference numerals refer to similar elements and in which:
Among the higher order modes, a HE1m (m>1) mode is desirable for use in the present invention, because the pump-signal overlap integral with the HE11 mode signal is higher for a pump of a HE1m (m>1) mode than the pumps of the other higher order modes, where the pump-signal overlap integral f is defined by the following expression:
where E1 and E2 are respective electric field vectors of the signal and the pump, dA is an area fraction, * shows complex conjugate, and the integral region is the whole cross section. The inner product (●) of vector Ea and Eb is defined by
(Ea·Eb)=Ea,xEb,x+Ea,yEb,y,
where Ea,x and Ea,y are the components of Ea along x and y coordinates, which are within the cross section of the optical fiber. In the pump-signal overlap integral here, z component of electric field, which is along the fiber length, is neglected because it is typically small. It is known that the efficiency of a nonlinear optical effect is in proportion to such an overlap integral, as described in Section 10.2 of Govind P. Agrawal, “Nonlinear Fiber Optics,” Academic Press.
In embodiment 1, the 2nd, 3rd, and 4th modes at the pump wavelength λ2 shown in
In the present embodiment, the core radius is 0.43 μm, so that the phase indices of the fundamental mode at λ1 and the second mode (lowest of the higher-order modes) at λ2 are matched. The signal at λ1 is launched into the first port in the fundamental mode, and coupled to the fundamental mode of the optical fiber via the multiplexing means. The pump at λ2 is launched into the second port in the second mode, and coupled to a higher-order mode of the optical fiber via the multiplexing means. It is preferable to employ a surface emitting laser diode as an optical source generating an optical pump in higher-order modes.
The propagation of the signal and pump in the optical fiber can be described by the following expressions (1), and (2).
where E1 and E2 are respectively the slowly-varying field amplitudes of the signal and pump, z is the coordinate along the fiber, i is the imaginary unit, n(2) is the nonlinear refractive index, k1 and k2 are respectively the wavenumbers of the signal and pump, and E* denotes a complex conjugate of E. The expressions (1), and (2) are based on the assumption that self-phase modulation, cross-phase modulation, and attenuation by the fiber are not significant.
By taking the assumption that |E1|<<|E2|, which is usually valid, and that the variation in E2 is hence not significant, it follows from expression (1) that
P1 and P2 are respectively the optical powers of the signal and pump, and A is given from the effective core areas of the signal A1 and pump A2 by
A=√{square root over (A1A2)}
As shown in
While the fiber length is 1 km in the present embodiment, a similar effect to improve SNR can be obtained with a shorter fiber when the optical fiber has a higher nonlinear refractive index, for example higher than 10−19 m2/W. Such a high nonlinear refractive index can be realized using, for example, a compound glass or silica glass doped with bismuth.
While this invention has been described in connection with what is presently considered to be the most practical and preferred embodiments, the invention is not limited to the disclosed embodiments, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.
Number | Date | Country | Kind |
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2002-348551 | Nov 2002 | JP | national |
Number | Name | Date | Kind |
---|---|---|---|
3875422 | Stolen | Apr 1975 | A |
5802236 | DiGiovanni et al. | Sep 1998 | A |
6400866 | Ranka et al. | Jun 2002 | B2 |
6453094 | Yue | Sep 2002 | B1 |
6693737 | Islam | Feb 2004 | B2 |
6813066 | Islam | Nov 2004 | B2 |
6847758 | Watanabe | Jan 2005 | B1 |
Number | Date | Country |
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WO 0239161 | May 2002 | WO |
Number | Date | Country | |
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20040105639 A1 | Jun 2004 | US |