Patent Application
20090290827
The present application relates to optical communications, in particular to a nonlinear optical loop mirror.
A nonlinear optical loop mirror has found many applications in optical signal processing such as optical switching, sampling, demultiplexing, wavelength conversion, pulse shaping, and signal regeneration.
Conventionally, operation of the nonlinear optical loop mirror is based on nonlinear phase modulation due to the nonlinear optical medium NL, which will be described in brief. An optical signal (such as a continuous-wave (CW) light, an optical pulse, etc.) is input into the first optical path 12. If the coupling ratio of the optical coupler 16 is set to be 1:1, the optical signal will be divided into two components having the same power by the optical coupler 16. The two components propagate in the loop optical path 18 clockwise and counterclockwise, respectively. In the nonlinear optical loop mirror 10 shown in
In one aspect, there is disclosed a nonlinear optical loop mirror. The nonlinear optical loop mirror may comprise: an optical coupler which includes a first optical path and a second optical path coupled to each other; and a loop optical path configured to connect the first and second optical paths. The loop optical path may be provided with a nonlinear element configured to vary a wavelength of an optical signal and a linear element configured to produce a wavelength dependent time delay for an optical signal.
In the other aspect, there is disclosed a delay interferometer for demodulating a differential phase-shift-keying (DPSK) signal. The delay interferometer may comprise: an optical coupler which includes a first optical path and a second optical path coupled to each other; and a loop optical path configured to connect the first and second optical paths. The loop optical path may be provided with a nonlinear element configured to vary a wavelength of an optical signal and a linear element configured to produce a wavelength dependent time delay for an optical signal.
The nonlinear optical loop mirror according to the present application does not rely on the difference of power levels of the two components in order to control the output. It allows for the same nonlinear element and linear element for the two components and is thus relatively stable since the two components share the same physical path and experience the same environmental disturbances. The nonlinear optical loop mirror as disclosed is applicable for many applications. In particular, it may function as a delay interferometer in detection of DPSK signals at variable bit-rates.
a) and (b) are eye diagrams of demodulation of RZ DPSK signals with 10-Gb/s and 20-Gb/s, respectively, by using the nonlinear optical loop mirror showing in
a) and (b) are graphs showing optical spectra of FWM for the 10-Gb/s and 20-Gb/s RZ DPSK signals, respectively;
Hereinafter, a detailed description of implementations will be given with reference to the appended drawings and embodiments.
Referring to
When an optical signal enters into the nonlinear optical loop mirror 100 through the first optical path 102, as shown in
Accordingly, the component traveling clockwise (a “clockwise component”) and the component traveling counterclockwise (a “counterclockwise component”) may have the same wavelength and power. The clockwise component first propagates through the nonlinear element 110 and the wavelength thereof may be varied through the processing of the nonlinear element 110. An output of the nonlinear element 110 is then directed to the linear element 112 that produces a wavelength dependent time delay for the clockwise component. Therefore, this component being processed by the nonlinear element 110 and the linear element 112 may have a variable optical delay dependent on the variation of the wavelength produced by the nonlinear element 110. On the other hand, the counterclockwise component will travel in turn through the linear element 112 and the nonlinear element 110. It can be understood that, since the wavelength of the counterclockwise component before propagating through the linear element 112 is fixed, this component being processed by the linear element 112 and the nonlinear element 110 may have a fixed optical delay. The two components may then meet again at the optical coupler 106. The counterclockwise component which has a fixed optical delay and the clockwise component which has a variable optical delay interfere at the optical coupler 106.
In an embodiment of the present application, the nonlinear optical loop mirror 100 as described with respect to
In the embodiment in which the nonlinear optical loop mirror 100 is used as a delay interferometer in a DPSK demodulator, the signal to be processed by the nonlinear optical loop mirror 100 may comprise an input signal and a control signal. In an illustrative example, the control signal may be a tunable CW light. As stated above, when the signal to be processed including the input signal and the control signal enters into the nonlinear optical loop mirror 100 through the first optical path 102, the signal to be processed is divided into a clockwise component and a counterclockwise component with the same wavelength and power.
The clockwise component first propagates through the nonlinear element 110 where the FWM occurs between the control signal and the input signal. In the process of the FWM, a dynamic index grating may be created through beating between the input signal and the control signal. The grating then scatters or modulates the control signal. During the scattering or modulation process, the wavelength of the clockwise component may be varied and the phase and amplitude information of the input signal may be transferred. Therefore, the FWM may preserve both phase and amplitude information of the input signal, and change the wavelength of the clockwise component. An output of the nonlinear element 110 has an optical angular frequency ωf=2ωc−ωs, where ωc and ωs are the optical angular frequencies of the control signal and the input signal, respectively. Accordingly, the wavelength of the output of the nonlinear element 110 in the clockwise component is variable according to the wavelength of the control signal.
The output of the nonlinear element 110 is then directed to the linear element 112 that introduces a wavelength dependent time delay by the GVD. It can be understood that optical signals with different wavelength propagate at different speeds in a GVD medium due to the difference in refractive index for different wavelengths. Therefore, the propagating time of a signal is dependent on its wavelength launched to the GVD medium, which results in a wavelength dependent optical delay. The combination of the FWM at the nonlinear element 110 and the GVD at the linear element 112 thus forms into a tunable optical delay line inside the nonlinear optical loop mirror 100. By changing the wavelength of the control signal, the amount of optical delay can be varied.
For the counterclockwise component, the GVD is first introduced at the linear element 112 before the FWM takes effect at the nonlinear element 110. It can be understood that the amount of optical delay for the counterclockwise component is fixed since the FWM occurs after the GVD.
At the optical coupler 106, the counterclockwise component which has a fixed optical delay and the clockwise component which has a variable optical delay meet again and interfere. An output signal of the nonlinear optical loop mirror 100 is determined by the relative phase difference between the clockwise and counterclockwise components. It can be understood that, if the two components have the same phase, the interference taking place at the optical coupler 106 may be deemed as a constructive interference (i.e. no output at the second optical path 104); and if the two components are 180° out of phase, the interference taking place at the optical 106 may be deemed as a destructive interference (i.e. no output at the first optical path 102). In other words, a constructive interference occurs at the second optical path 104 and a destructive interference occurs at the first optical path 102. If the two components have a phase difference between 0 and 180°, a part of output will be at first optical path 102 and the other part will be at the second optical path 104.
The relative optical delay Δt between the clockwise and counterclockwise components is given by Eq (1) as following:
Δt=|λf−λs|×DGVD×LGVD=2|λc−λs|×DGVD×LGVD (1)
Where λf, λs, and λc are the wavelength of the output of the nonlinear element 110, the input signal and the control signal, DGVD and LGVD are the dispersion and the length of the linear element 112, respectively.
Hereafter, it will be discussed how the nonlinear optical loop mirror 100 works as a delay interferometer in demodulating DPSK signals with variable bit-rates.
With the development of optical communications, advanced modulation formats are being explored to carry data signals in optical fibers. In particular, a DPSK modulation format is attracting much interest. For a detection of DPSK signals, a demodulation approach is required to convert phase modulation into intensity modulation. Examples of the demodulation approaches include the use of a phase-shifted fiber Bragg grating, an optical discriminator filter, an all-fiber delay-line interferometer, a birefringent fiber loop, and so on. In other words, a common way for the detection of a DPSK signal is to use a component called a “delay interferometer” that converts the phase information into intensity information before the signal is directed to an optical receiver. In addition, the demodulation of a DPSK signal requires the interference between adjacent bits. However, a limitation of the existing “delay interferometer” is that it is not tunable, that is, it is built to operate at a fixed data bit-rate of communication.
It is known that the demodulation of a DPSK signal requires the interference between adjacent bits. In particular, the interference between the adjacent bits may convert the phase information in a DPSK signal into intensity information. In the nonlinear optical loop mirror 100 of an embodiment of the present application, the signal to be processed is split into two counter propagating components. The clockwise component has a variable delay, while the counter-clockwise component has a fixed delay. The delay may be adjusted such that the two counter-propagating components have a relative delay (delay difference) equal to one-bit period for a specific DPSK signal. When the two counter-propagating components meet again at the optical coupler 106, interference occurs. Since the relative delay between the two counter-propagating components is one-bit, interference between the adjacent bits is achieved.
In a specific example, a 64-m dispersion flattened photonic crystal fiber (PCF) may be used as the nonlinear element 110 to introduce the FWM between the input signal and the control signal. It is experimentally demonstrated that a wavelength variation of 20 nm is possible between the signal to be processed and the output of the nonlinear element 110 owing to low-dispersion and dispersion flattened characteristics of the PCF to make the conversion efficiency to be within a 3-dB variation. Hence, a large tuning range may be supported since the wavelength variation can essentially be tuned from 0 to 20 nm. In addition, the nonlinear optical loop mirror 100 may use, for example, a 600-m standard single mode fiber as the linear element 112 to provide an approximately 10 ps/nm GVD at the wavelength range of interest.
A DPSK signal to be demodulated as the input signal is first combined with a CW light from a tunable laser. The combined signal is launched to the nonlinear optical loop mirror 100 in which the FWM and GVD take place. It is known that since the FWM occurs more effectively for the control signal and input signal with high power, the combined signal may be boosted by a fiber amplifier (not shown) before launching to the nonlinear optical loop mirror 100. It can be understood that different bit rates are associated with different bit periods. To demodulate the DPSK signal, the relative delay between the two interference branches of the nonlinear optical loop mirror 100 should thus match with the bit period for a one-bit demodulation. Therefore, by adjusting the wavelength of the control signal, the relative delay between the two branches of the nonlinear optical loop mirror 100 is variable so that demodulation at different bit rates for DPSK signals is achieved. The nonlinear optical loop mirror according to an embodiment of the present application may thus provide continuous optical delay between two interfering branches. In an illustrative example that the nonlinear optical loop mirror may produce up to 20 nm variation between the signal to be processed and the FWM output as well as the approximately 10 ps/nm GVD, a 200 ps relative delay can be achieved. Thus, the nonlinear optical loop mirror can serve as a delay interferometer in a DPSK demodulator for any bit-rate above 5 Gb/s.
In an illustrative example, a 10-Gb/s RZ DPSK signal at 1553.8 nm is used as the input signal. The control signal is tuned to a wavelength of 1548.8 nm. The wavelength difference of the DPSK input signal and the CW control is 5 nm and thus a relative delay of Δt=100 ps is obtained according to Eq. (1). The delay is of the amount required for demodulating a 10-Gb/s DPSK signal.
As another illustrative example, to investigate the performance of the nonlinear optical loop mirror in demodulating a DPSK signal at variable bit-rates, a 20-Gb/s RZ DPSK signal is used as the input signal. In this example, the wavelength of the control signal is tuned to be 1551.28 nm. The wavelength difference between the input signal and the control signal is 2.5 nm, resulting in Δt=50 ps that can be used for the demodulation of a 20-Gb/s DPSK signal.
a) and (b) show, respectively, eye diagrams of the resultant signals of the demodulation of RZ DPSK signals with 10-Gb/s and 20-Gb/s by using the nonlinear optical loop mirror 100 of an embodiment of the present application. Since each of the widely opened eye diagrams in
It can be observed in
Besides the demodulation of RZ DPSK signals, it is experimentally demonstrated that a NRZ DPSK signal can be demodulated with different bit-delays and without observing error floor.
It can be understood that some additional components may be provided in the nonlinear optical loop mirror 100 to improve the performance of demodulating DPSK signals. For example, in order to separate the FWM output from the both the input signal and the control signal, a tunable optical bandpass filter 114 may be provided at the second optical path 104 for filtering out the demodulated DPSK signal, as shown in
Although it has been discussed that the nonlinear optical loop mirror according to the present application is applied in the demodulation of the DPSK signals, it can be understood that it also has some other functions. For example, with control of the tunable delay, the nonlinear optical loop mirror can also function as a bit-rate multiplier for RZ signals. It is known that a bit-rate multiplier requires a temporal interleaving of two lower bit rate inputs. The conventional approach is to split the signal into two branches and then recombine the branches after introducing a half-bit relative delay between them. The delay is normally determined by the length of fiber and is only slightly tunable through stretching of the fiber. With the nonlinear optical loop mirror according to the present application, it is possible to split the signal into two branches, while the delay is governed by the nonlinear element 110 and the linear element 112 as described before. By adjusting the wavelength to obtain a half-bit delay, a bit-rate multiplier for RZ signals is obtained. Besides, all-optical reshaping (that is, to reshape a temporal profile of a signal.) or conversion among different data formats (that is, a conversion between different data formats, e.g. RZ, NRZ, CSRZ, etc.) can be potentially achieved with the nonlinear optical loop mirror since the temporal and spectral properties of the output depend on the amount of overlap between the two branches.
Although the above descriptions include many specific arrangements and parameters, it should be noted that these specific arrangements and parameters only serve to illustrate one embodiment of the present application. This should not be considered as the limitations on the scope of the invention. It can be understood by those skilled in the art that various modifications, additions and substitutions may be made thereto without departing from the scope and spirit of the present invention. For example, as described above, a CW light is selected as the control signal of which the wavelength is varied to control the variation of the wavelength of the clockwise component. However, other type of signal can be used as the control signal. For example, a pulsed optical signal can be used if a return-to-zero data output after the DPSK demodulation is targeted to be obtained. Therefore, the scope of the present invention should be construed on the basis of the appended claims.
| Number | Date | Country | |
|---|---|---|---|
| 61055327 | May 2008 | US |
