General multi-function optical filter

Details General multi-function optical filter General multi-function optical filter

1999-01-08

2001-10-16

Lee, John D. (Department: 2874)

Optical waveguides

With optical coupler

Plural

C385S014000, C385S015000

Reexamination Certificate

active

06304689

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a multi-function optical filter intended for multi-wavelength optical communication system and high density wavelength-division multiplexed (WDM) network systems. Besides the optical spectrum region, the invention described herein can also be applied to other electromagnetic spectra such as microwave, milliwave, etc.
2. Description of Prior Art
Optical filters are key devices for multi-wavelength optical communication systems and high density wavelength-division multiplexed (WDM) network systems. In order to maximize the present 30 nm communication window supported by an erbium fiber amplifier for WDM network systems, there is an increasing demand for new tunable optical filters that have the following requirements; (1) wide free spectral range (FSR>30 nm), (2) narrow linewidth (&Dgr;&lgr;
FWHM
<0.3 nm), (3) High SNR or unity contrast or visibility, (4) tunable and (5) fiber compatible.
Furthermore, for next generation, highly complex combination of optical TDM-WDM network systems, an additional requirement, namely (6) programmability or multi-function capability, would be necessary to make a compact, versatile, economical and most importantly intelligent network system. Toward this goal, most if not all key devices in the network system such as an optical filter, an add/drop multiplexer (ADM), etc. need to be programmable, at best, or need to possess multi-function capabilities.
At present, most optical filters used in optical communication fields are dedicated, single-function devices. These single-function optical filters can be divided into two classes depending on the physical mechanism used in the filtering process namely; (1) grating-based filter and (2) resonant-based filter. Examples of grating-based filters are fiber bragg-grating (FBG) filter [F. Bakhati and P. Sansonetti, “Design and realization of multiple quarter-wave phase shifts UV-written bandpass filters in optical fibers”,
J. Lightwave Technol. vol.
15, 1433-1437, 1997], acousto-opto tunable filter (AOTF) [J. Jackel, J. Baran, A. d'Alessandro, and D. Smith, “A passband-flattened acousto-optic filter”,
IEEE Photonics Tech. Letter, Vol.
7, 318-320, 1995] and array-waveguide grating filter (AWGF) [H. Takahashi, K. Oda, H. Toba, Y. Inoue, “Transmission Characteristics of Arrayed Waveguide N×N Wavelength Multiplexer”,
J. Lightwave Technol vol.
13, 447-455, 1995]. Examples for resonant-based filters are multilayer dielectric film [P. H. Lissberger and A. K. Roy, “Narrowband position-tuned multilayer interference filter for use in single-mode-fiber systems”,
Electr. Lett. Vol.
21. No. 18, 798-799, 1985], Mach-Zehnder interferometer (MZI) [M. Kuznetsov, “Cascaded Coupler Mach-Zehnder Channel Dropping Filter for Wavelength-Division Multiplexed Optical Systems”,
J. Lightwave Technol. vol.
12, 227-230, 1994 ], ring resonator [B. E. Little, S. T. Chu, H.A. Haus, J. Foresi, and J.-P. Laine, “Microring resonator Channel Dropping Filters”,
J. Lightwave Technol., vol.
LT-15, pp. 998-1005, 1997], Fox-Smith resonator [P. Urquhart, “Compound optical-fiber-based resonator,”
J. Opt. Soc. Amer. A, vol.
5, pp. 803-812, 1988], and Fabry-Perot resonator [J. Stone and L. Stulz, “Pigtailed high-finesse tunable fiber Fabry-Perot interferometers with large, medium and small free spectral ranges”,
Elect. Lett. Vol.
23, 781-782, 1987].
From among these different types of optical filters, fiber ring resonator (RR) and fiber Fabry-Perot resonator (FPR) have the highest potential to realize the above first 5 requirements in a compact and stable configuration. Unfortunately, these filters lack programmability or multi-function capability. Optical filters with some programmability in principle like a cascaded optical fiber lattice are impractical to implement because of the numerous phases and coupling parameters that need to be controlled.
SUMMARY OF THE INVENTION
This invention presents a new multi-function optical filter that can function as a channel passing (CP) filter, a channel dropping (CD) filter and a bandpass (BP) filter depending on the interferometer arm length difference &Dgr;L or ratio &ggr;=&Dgr;L/d and reflectance R. CP filter accesses one channel of WDM signal and leave other channels undisturbed while CD filter performs the reverse functions. BP filter, on the other hand, allows only certain specified band of the spectrum to be transmitted while blocking other spectrum bands. Unlike other optical filters, the transmitted intensity outputs for all these three different functions are accessible in a single port. The first 5 requirements mentioned before can also be realized with this invention together with the multi-function capability.
Other unique features of this invention are (1) narrow linewidths for both MGTI-based CP and CD filters compared with a typical FPR having similar parameters, (2) unity contrast for all three functions and (3) excellent, near-perfect box-like response function for a BP filter.
This invention provides a multi-function optical filter, in which one of the reflecting mirrors of a Michelson interferometer or Tynman-Green interferometer is replaced by a Gires-Tournois resonator (GTR) having a resonator spacing d, that depends on the interferometer arm length difference &Dgr;L or &ggr; (ratio of &Dgr;L/d) and reflectance R. Another implementation of the multi-function optical filter is a Mach-Zehnder interferometer in which one of the arms of Mach-Zehnder the interferometer contains a ring resonator with effective resonator perimeter length p=2d, and the coupling constant of the said ring resonator.
The multi-function optical filter is tunable by making the resonator spacing d variable by using a PZT as a base stage for one of the mirror of the resonator, or using either liquid crystal, thermo-optical, electro-optical, and light-sensitive material as an element inside the resonator, or using a micromachined fabrication of the resonator.
The multi-function optical filter is made or implemented using fiber, waveguide or bulk optics configurations.
The multi-function optical filter function as an optical Channel Passing (CP) filter when ratio &ggr; is equal to n&lgr;
0
/2d, wherein n stands for an integer including 0.
The multi-function optical filter also function as an optical Channel Dropping (CD) filter for some wavelength range &Dgr;&lgr; when ratio &ggr; is equal to (2n+1)&lgr;
0
/4d wherein n stands for an integer and &lgr;
0
for an arbitrary fixed center wavelength of the light source.
The multi-function optical filter also function as a Bandpass filter when ratio &ggr; is equal to or approximately to equal to 0.5.
Further features of the invention, its nature and various advantages will be more apparent from the accompanying drawings and following detailed description of the invention.


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patent: 5706079 (1998-01-01), Kersey
patent: 5982518 (1999-11-01), Mizrahi
patent: 6222958 (2001-04-01), Paiam
patent: 52018378 (1977-10-01), None
Inoue et al. Wavelength conversion using a light injected DFB-LD and a Mach-Zehnder filter with a ring resonator, IEEE Photonics Technology Letters, vol.: 7 No. 9, Sep. 1995.*
F. Bakhti, et al., Journal of Lightwave Technology, vol. 15, No. 8, pp. 1433-1437, “Design and Realization of Multiple Quarter-Wave Phase-Shifts UV-Written Bandpass Filters in Optical Fibers”, Aug. 1997.
Janet L. Jackel, et al., IEEE Photonics Technology Letters, vol. 7, No. 3, pp. 318-320, “A Passband-Flattened Acousto-Optic Filter”, Mar. 1995.
Hiroshi Takahashi, et al., Journal of Lightwave Technology, vol. 13, No. 3, pp. 447-455, “Transmission Characteristics of Arrayed Waveguide N X N Wavelength Multiplexer”, Mar. 1995.
P. H. Lissberger, et al., Electronics Letters, vol. 21, No. 18, pp. 798-799, “Narrowband Position-Tuned Multilayer Interference Filter for Use in Single-Mode-Fibre Sy

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