Provided is a wavelength converter having a simple and low-cost structure. The wavelength converter generates converted light having a wavelength different from that of input light. The wavelength converter includes (1) a pump light source that outputs pump light, (2) an optical multiplexer that combines and outputs input light and the pump light, (3) a first optical fiber that receives and guides the input light and the pump light that have been combined and output by the optical multiplexer and generates converted light using a nonlinear optical phenomenon that occurs while the first optical fiber guides the input light and the pump light, the first optical fiber being coiled, and (4) zero-dispersion wavelength adjusting means that adjusts a zero-dispersion wavelength of the first optical fiber.

The present invention is a method of processing an optical signal, including the steps of (a) inputting signal light into a first nonlinear optical medium to broaden the spectrum of the signal light through self phase modulation occurring in the first nonlinear optical medium, thereby obtaining first spectrally broadened light (supercontinuum light), (b) compensating for chromatic dispersion effected on the first spectrally broadened light obtained in the step (a), and (c) inputting the first spectrally broadened light processed by the step (b) into a second nonlinear optical medium to broaden the spectrum of the first spectrally broadened light through self phase modulation occurring in the second nonlinear optical medium, thereby obtaining second spectrally broadened light.

The invention is related to a parametric amplifier comprising an amplifying fiber (3) connected to a pump laser (8) with a feedback control device (6) for adapting the pump laser power and wavelength. This amplifier is used in a transmission system. In addition the invention comprises a method for controlling the pump laser source in an iterative process to optimize the gain of the amplifier.

An optical fiber comprising a core (1) and a cladding region (5) which covers an outer periphery of the core (1), having a zero-dispersion wavelength in a wavelength range of 1.4 µm to 1.65 µm, and being in a single mode in that zero-dispersion wavelength, wherein GeO₂ is doped in the core (1) in a quantity such that a relative refractive index difference of the core (1) becomes not less than 1.8%, the cladding region (5) includes first, second, and third cladding regions (2, 3, 4), and a refractive index of the second cladding region (3) is smaller than those of the first cladding region (2) and the third cladding region (4).

A wavelength converter which employs an optical fiber and has high converter efficiency. The polarization planes of a signal light and an exciting light outputted from a laser diode (LD) (103) are respectively controlled by polarization controllers (PC's) (101 and 104) and the phases of the lights are respectively modulated by phase modulators (PM's) (102 and 105) in accordance with modulation signals outputted from an oscillator (110). Then, the output lights from the PM's (102 and 105) are multiplexed by a coupler (106). After the multiplexed signal light and exciting light are amplified by an optical amplifier (EDFA) (107), they are inputted to a dispersion shift fiber (DSP) (108). After wavelength transformation (four light waves mixing (FWM)) is practiced in the DSP, an FWM light is outputted through a band-pass filter (BPF) (109).

A multi-mode interference coupler is disclosed for coupling light between ports. The MMI coupler is a planar waveguide having a first input port for launching light into a core of the planar waveguide which has first refractive index n₁. The waveguide is dimensioned to have a response that confines light launched therein to a single mode in one dimension, and multi-mode in another dimension. Two other ports are provided for receiving light launched into and propagating through the core from the input port. A polymer cladding of a second material having a refractive index n₂ covers at least a portion of the core. A heater is provided and is thermally coupled to the cladding for heating the cladding when a control signal is applied.

The present invention relates to a variable optical filter that can be used to filter an incoming signal, attenuate an incoming signal or in one configuration switch an incoming signal from one path to another. The present invention has found that an accurate and economical variable optical filter can be created by using an elastomeric material having a high coefficient of expansion in cooperation with a means for locally varying the temperature of the elastomeric material as an actuator for moving a reflective surface within the optical filter. The actuator can be operated in a controlled manner for example, to effect a tilt of the reflective surface for switching or attenuating an optical signal, or to vary the resonant wavelengths of a resonant cavity between partially reflective surfaces. In accordance with the invention there is provided, a variable optical filter comprising an input port and an output port; a first at least partially reflective surface disposed to receive a beam of light launched from the input port; an elastomeric material for supporting and varying the position of the at least partially reflective surface with respect to the input port; a heater for applying variable amounts of heat to the elastomeric material to move or pivot the at least partially reflective surface relative to the input port; and, control means for controlling the heater and for providing a signal to apply variable amounts of heat.

A variable light filtering arrangement (10) includes at least one optical fiber section (11) including a waveguiding core (13), and at least one permanent Bragg grating region (12) in the optical fiber section. The grating region includes a plurality of grating elements constituted by periodic refractive index variations of a predetermined initial periodicity and cumulatively redirecting, of the light launched into the core for guided propagation therein, that having an axial wavelength within a narrow band around a central wavelength that is determined by the periodicity and refractive index variations of the grating elements. At least one of the periodicity and refractive index variations of the grating region is controlledly modified in such a manner as to temporarily change the central wavelength within a predetermined wavelength range.

Insbesondere für optische Überlagerungsempfänger ist es notwendig, die Polarisation des lokal erzeugten Lichtes der des Empfangslichtes nachzuregeln. Die Polarisationsregelung erfolgt dabei durch Verstellung der Verzögerung doppelbrechender Elemente, die nach einer Anzahl gleichgerichteter Verstellschritte an eine Verstellgrenze gelangen können und zurückgestellt werden müssen. Dabei ergibt sich das Problem, daß schnelle Änderungen der Polarisation des Eingangslichtes auch während Rückstellphase berücksichtigt werden müssen, außerdem sollten auch nichtideale doppelbrechende Elemente verwendet werden. Erfindungsgemäß werden vier doppelbrechende Elemente verwendet, von denen die im Lichtweg ersten beiden Elemente der normalen Regelung und die beiden folgenden Elemente der Regelung während der Rückstellprozedur dienen.