All-optical wavelength converter circuit

A method and apparatus for transferring information of an optical information-bearing signal from a first wavelength to a second wavelength. The method is implemented in an all-optical wavelength converter circuit which includes a laser diode in communication with a polarization controller. An information-bearing signal having a first wavelength is input to the circuit. A polarization controller adjusts the polarization of the information-bearing signal. The laser diode receives the polarization-adjusted information-bearing signal and generates a converted information-bearing signal by transferring the information of the polarization-adjusted information-bearing signal from the first wavelength to the second wavelength. The polarization controller receives the converted information-bearing signal from the laser diode, and polarizes the converted information-bearing signal.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to an optical circuit that transfers information of an incoming information-bearing signal from a first wavelength to a second wavelength.

2. Background Information

Optical communication systems provide significant communications capacity using a point-to-point architecture, whereby one transmitter is connected directly to a remote receiver through a single optical fiber. One widely accepted approach used to further expand such communications capacity is implemented using a Wavelength Division Multiplex (“WDM”) system. In a WDM system, sources with different wavelengths (colors) used to carry their own information are combined and transmitted through a single optical fiber. Therefore, a WDM system employing 16 different wavelengths channels can increase the capacity of the optical fiber by 16 fold.

As optical systems evolve and become optical networks with more complex topologies (e.g., ring, star, etc.), there is a need to find ways to manage different channels having different wavelengths. Wavelength converters are used in dynamic and standard WDM networks to optically transfer information from one wavelength carrier to another. Such wavelength converters add flexibility to the networks by routing and reconfiguring channels therein.

Some current wavelength converters, also known as optical transponders, process an incoming optical signal at a first wavelength by converting the optical signal to an electrical signal. The electrical signal is then regenerated back to the optical domain at a different wavelength (i.e., Optical-Electrical-Optical, or OEO). This process is expensive, complex and has limited applications since it is limited to operating at a specific data rate.

Other known wavelength converters implement an all-optical methodology which uses techniques such as Four-Wave Mixing (“FWM”), Cross-Phase Modulation, Cross-Gain Modulation (“XGM”), or the like. These techniques require complex circuitry and are inefficient, thus leading to high conversion loss. For example, known wavelength converters which use XGM generally require a semiconductor optical amplifier (“SOA”) acting as a mixer in addition to a laser used as an oscillator. Such systems tend to be complicated and costly.

SUMMARY OF THE INVENTION

Briefly stated, according to a first aspect of the present invention, a method uses an optical circuit to transfer information of an information-bearing signal from a first wavelength to a second wavelength. The optical circuit has an input port, an output port, and a polarization controller in communication with a laser diode. The method includes inputting an information-bearing signal having a first wavelength into the input port. The polarization controller receives the information-bearing signal and adjusts the polarization of the information-bearing signal. The laser diode receives the polarization-adjusted information-bearing signal from the polarization controller and generates a converted information-bearing signal by transferring the information of the polarization-adjusted information-bearing signal from the first wavelength to the second wavelength. The converted information-bearing signal is output from the output port.

According to a second aspect of the present invention, an all-optical wavelength converter circuit for transferring information of an information-bearing signal from a first wavelength to a second wavelength includes a polarization controller for receiving an information-bearing signal having the first wavelength and adjusting the polarization of the information-bearing signal. A laser diode in communication with the polarization controller generates a converted information-bearing signal by transferring the information of the polarization-adjusted information-bearing signal from the first wavelength to the second wavelength.

According to a third aspect of the present invention, an all-optical wavelength converter circuit for transferring information of an information-bearing signal from a first wavelength to a second wavelength includes a laser diode in communication with the information-bearing signal. The laser diode transfers the information of the information-bearing signal from the first wavelength to the second wavelength using cross-gain modulation.

According to a fourth aspect of the present invention, an optical wavelength switch comprises an input for receiving at least a first information-bearing signal having a first wavelength and a second information-bearing signal having a second wavelength. The switch includes at least two all-optical wavelength converter circuits. Each wavelength converter circuit includes a laser diode. The laser diode of one of the wavelength converter circuits is in communication with one of the first and second information-bearing signals. The laser diode of the other of the wavelength converter circuits is in communication with the other of the first and second information-bearing signals. The laser diode in each of the wavelength converter circuits transfers information of the respective information-bearing signal to another wavelength using cross-gain modulation. The switch includes an output for transmitting the first and second information-bearing signals, such that the first information-bearing signal has the second wavelength and the second information-bearing signal has the first wavelength.

DETAILED DESCRIPTION OF THE INVENTION

Referring toFIG. 1, an all-optical wavelength converter circuit, generally designated100, in accordance with a first preferred embodiment of the present invention is shown. The converter circuit100transfers information of an incoming information-bearing signal from a first wavelength λAto a second wavelength λB. In the description that follows, the phrase “wavelength conversion” and references thereto are used interchangeably with and have the same meaning as “transferring information of an information-bearing signal from a first wavelength to a second wavelength.”

The wavelength converter circuit100includes a laser diode115which is used as a gain medium that internally generates a probe continuous wave signal λB, rather than receiving the probe signal from an external source as in conventional wavelength converter circuits. The incoming information-bearing signal having the first wavelength is preferably routed through a routing device105and coupled to gain medium of the laser diode115. The routing device105may be an optical circulator, an optical directional coupler, an optical diplexer, an optical beamsplitter or any other device generally known in the art for routing an optical signal. A multiplex device may also be used to separate the first and second wavelengths. The incoming information-bearing signal may also be directly coupled to the laser diode115through back-facet coupling and/or through side coupling. The information-bearing signal may be in the form of either a digital information signal or an analog information signal.

According to the present invention, the laser diode115is preferably a non-isolated distributed feedback (“DFB”) laser diode with high slope efficiency and high threshold current. Using such a DFB laser makes it easier to couple the incoming signal into the gain medium of the laser diode115by providing high laser-fiber coupling and high laser output coupling, respectively. The laser diode115preferably has side modes having a WDM frequency spacing that matches an International Telecommunications Union (“ITU”) grid channel spacing (e.g., 50 and 100 GHz) to facilitate easier coupling of an incoming signal. The laser diode115may be tuned by adjusting an input bias current Iband by adjusting the temperature of the laser. Alternatively, the laser diode115may be a Fabry-Perot laser diode, an external cavity laser, a pulsed laser diode, a solid-state laser (e.g., microchip laser) or a fiber laser without departing from the spirit and scope of the present invention.

Unlike conventional wavelength converter circuits (e.g., those circuits which depend on SOAs), the wavelength conversion of the information-bearing signal according to the present invention preferably takes place within the laser diode115itself. In the present invention, the laser diode115preferably functions as both a mixer and an oscillator. The laser diode115preferably uses the gain depletion process of XGM taking place in the gain medium of the laser diode115to complete the conversion. Thus, conversion efficiency is maximized by using minimal input power to transfer information from the first wavelength λA(e.g., 1550.92 nm, channel33of the ITU grid) to the second wavelength λB(e.g., 1560.61 nm, channel21of the ITU grid). The wavelength conversion may be extended between different optical communication windows as well, allowing for conversion between the 1310 and 1550 nm windows, and vice-versa. Preferably, the wavelength conversion is implemented completely within the optical domain, independent of the data rate of the information-bearing signal. Thus, the wavelength converter circuit100provides additional flexibility and cost savings compared to conventional wavelength circuits.

Referring toFIG. 2, an all-optical wavelength converter circuit, generally designated200, in accordance with a second preferred embodiment of the present invention is shown. Elements in the converter circuit200which are similar to those discussed with respect to the converter circuit100are labeled with the same reference numerals.

The wavelength converter circuit200includes a polarization control circuit210which receives the information-bearing signal having the first wavelength λA. The polarization controller210is in communication with the laser diode115, such that the information-bearing signal input to the converter circuit200undergoes a polarization process prior to being input to the laser diode115. Since the laser diode may be polarization sensitive, adjusting the polarization of the information-bearing signal so that it matches the polarization of the laser diode115reduces loss of the signal during conversion. The wavelength of the information-bearing signal at the first wavelength may also be adjusted to match one of the side-modes of the converting laser, thereby increasing the coupling coefficient and the XGM phenomenon, and consequently reducing the conversion loss. The first wavelength may also be slightly off the side-mode wavelength to take into consideration any laser detuning effect due to the XGM.

The laser diode115generates a converted information-bearing signal by changing the first wavelength λAof the polarization-adjusted information-bearing signal to the second wavelength λB, similar to the converter circuit100. After conversion, the laser diode115transmits the converted information-bearing signal to the polarization controller210which adjusts the polarization of the converted information-bearing signal. The wavelength converter circuit200thus allows the use of low cost, commercially available off-the-shelf laser diodes which provide optimum power transfer efficiency, thereby minimizing conversion loss. Alternatively, instead of using a polarization controller210, a laser diode115having a symmetrical or quasi-symmetrical gain medium cross-section is also suitable as the conversion medium, and would relax or avoid the need of a polarization controller, while still minimizing conversion loss. Those skilled in the art will recognize that the polarization of the converted information-bearing signal need not be adjusted after being output from the laser diode115if there are no additional elements in the converter circuit200which are polarization sensitive. That is, the converted information-bearing signal may be output from the laser diode115, bypassing the polarization controller210, directly to the router105for transmission to the output port (see, for example,FIG. 6).

Referring toFIGS. 3–5, variations of the wavelength converter circuit200are shown. InFIG. 3the incoming information-bearing signal having the first wavelength λAis preferably amplified by an amplifier320prior to being input to the routing device105. The wavelength converter circuit200may also include an optical bandpass filter325which selects the second wavelength λBif higher suppression of the first wavelength λAis required. The filter325may, for example, be a tunable optical bandpass filter (“TOBPF”). Thus, the routing device105directs the information-bearing signal having the first wavelength to the polarization controller210, and directs the converted information-bearing signal to the filter325. Those skilled in the art will recognize that the wavelength converter circuit200ofFIG. 3may be implemented with an additional amplifier430, such that the filtered converted information-bearing signal is also amplified at the output port (seeFIG. 4). Alternatively, the wavelength converter circuit200may not amplify the incoming information-bearing signal prior to conversion, while still including the filter325and the amplifier430to filter and amplify the converted information-bearing signal (seeFIG. 5). As noted above, any of the variations of the converter circuit200shown inFIGS. 3–5may couple the converted information-bearing signal directly to the router105, bypassing the polarization controller210.

The wavelength converter circuits100,200discussed with reference toFIGS. 1–5output the converted information-bearing signal with a data signal which is inverted with respect to the information-bearing signal having the first wavelength. In some instances, the device receiving the converted information-bearing signal may be sensitive to such an inverted data signal. Thus, two wavelength converters100,200may be cascaded back to back (not shown), thereby re-inverting the data signal of the converted information-bearing signal and producing a second converted information-bearing signal which has a wavelength which is different than the first wavelength, but which retains the original, un-inverted data signal.

Those skilled in the art will also recognize that the wavelength converter circuits100,200discussed above may be implemented with devices or elements designed to increase the signal to noise ratio of the converted information-bearing signal upon output from the laser diode115.

A preferred method of converting the wavelength of an information-bearing signal to another wavelength in accordance with the present invention is shown in the flow chart ofFIG. 6. The method uses an optical circuit, such as any of the all-optical wavelength converter circuits as previously discussed with respect toFIGS. 1–5which convert the wavelength λAof an information-bearing signal to another wavelength λB. The steps inFIG. 6are mostly self-explanatory, and thus no detailed discussion of each step is provided. However, all or portions of steps610,615,620,635,640and645are optional or may be altered depending upon the particular configuration of the wavelength converter circuit100,200being used. That is, as previously described, the wavelength converter circuits100,200may include different combinations of a polarization controller210, a filter325and amplifiers320,430. Thus, the method ofFIG. 6could be implemented simply using steps605,625and630(reflecting the converter circuit100ofFIG. 1) without departing from the spirit and scope of the present invention.

Referring toFIGS. 7 and 8, optical cross connect (“OXC”) devices formed from wavelength converters in accordance with the present invention are shown.FIG. 7shows a wavelength switch, generally designated700, which includes two multiplexed incoming information-bearing signals having wavelengths λAand λB. The information-bearing signals are optionally amplified by an optical amplifier710and separated by an optical WDM filter720. The information-bearing signal having initial wavelength λAis applied to a wavelength converter730, which is similar to the converters100,200discussed above, such that the information is transferred to a signal having a final wavelength λB. An optical bandpass filter740may also be included to select the final wavelength λBif higher suppression of the initial wavelength λAis required. Similarly, the information-bearing signal having initial wavelength λBis applied to a second wavelength converter730, which transfers the information to a signal having a final wavelength λA. A second optical bandpass filter740may also be included to help select the final wavelength λAif higher suppression of the initial wavelength λBis required. The two final wavelengths are optionally recombined using a WDM filter750and amplified by a second amplifier710. The converted signals are output via an output port of the wavelength switch700.

Similar to the wavelength switch700,FIG. 8shows an array of all-optical wavelength converters forming an wavelength switch800, in accordance with another embodiment of the present invention. A set of N multiplexed incoming information-bearing signals having wavelengths λAto λNare amplified by an optional optical amplifier810and separated by an OXC device820according to instructions from a controller860. The OXC device820determines to which port, P1to PN, each incoming information-bearing signal is connected, and thus to which of the N available wavelengths each incoming signal is converted. The information-bearing signal having an initial wavelength λAis thus applied to a wavelength converter830, which is similar to the converters100,200discussed above, such that the information is transferred to a signal having a final wavelength within the remaining N multiplexed wavelengths λBto λN. The wavelength switch800may also include a optical bandpass filter840to select the desired final wavelength if higher suppression of the initial wavelength λAis required. The wavelength conversion is realized for the other initial wavelengths in the same fashion, as specified by the controller860. The final wavelengths are optionally recombined using a wavelength insensitive optical WDM filter850and amplified by a second amplifier810. The converted signals are output via an output port of the wavelength switch800. Many different configurations of the switches700,800are possible without departing from the spirit and scope of the present invention, so long as the switches700,800employ wavelength converters730,830which are similar to the converters100,200described above.