Optical Wavelength Conversion Techniques And Investigation On Reflectivity Spectrum Of Phase Shifted Bragg Grating In Polarization Maintaining Fiber

Parallel with the increasing demand of the society for communication has come a fantastic spurt in optical communication technology. The communication network characterized with optical transmission and electronic exchange is developing to all-optical network of optical transmission and exchange. All-optical network , in which information is carried by fiber, increases transmitting capacity through wavelength-division multiplexed (WDM)technology. In the WDM network, the number of optical wavelength decides the channel capacity. While the window of fiber communication is wide, the wavelength number used is limited by a number of causes, as a result it is not capable of supporting a good quantity of nodes. When the signals from two channels with the. same wavelength share oneoutput port, block will happen. One of the solutions to these problems is by means of optical exchange-wavelength conversion. All-optical Wavelength converter is a key component of optical-fiber network. What is characteristic is that the optical wavelength carrying information becomes another one. Therefore, wavelength is used and allocated again and is avoided being competitive, i.e. the capacity of communication network is enlarged.The thesis mainly investigates wavelength conversion taking the use of FWM based on semiconductor laser and amplifier. And at the same time the thesis also makes theoretical and experimental studies on fiber Brag grating used in wavelength converter. Here are the specific studies:1. Optical Wavelength Conversion Based on Semiconductor Laser The up-conversion and down-conversion of wavelength is obtained by taking use of four-wave mixing (FWM) in semiconductor laser. Because pump laser itself can be used as mixing medium, the wavelength conversion based on laser is simple in structure. Figure 1 shows the scheme of experiment and output spectrum.In the distributed feedback (DFB) laser, though four-wave mixing occurs under larger detuning, the converted signal is very weak. Wavelength conversion with large efficiency is limited in small detuning and what's more the efficiency dropped rapidly with detuning increasing. The reason why the lower efficiency of FWM in semiconductor laser occurs is that gain saturates when the laser emits. So the efficiency of FWM drops due to small nonlinear coefficient resulted from gain saturation. When the polarazition state of input signal changes, the power of the signal converted varies greatly. The above reasons make it restricted that optical wavelength conversion is implemented by FWM technique in semiconductor laser.2. Wavelength Converter Based on Four-Wave Mixing inSemiconductor Optical AmplifiersThe mechanism of wavelength conversion in semiconductor devices includes the following : effect on shelf-phase modulation of semiconductor optical amplifier (SOA), on cross-gain modulation and four-wave mixing insemiconductor lasers or optical amplifiers. In the above mechanism, FWM is noted with great interest by scientists all over the world, because wavelength conversion based on FWM is transparent in bit rate and modulation format. Because the pump wavelength is fixed in semiconductor laser, continual tuning on wavelength conversion is restricted. In SOA, pump is coming from external or itself with tuning ability, and its wavelength varies conveniently, so scanning wavelength shifts continuously. That is why FWM in SOA is paid great attention by people.To obtain wavelength conversion with widely tuning range and flattening conversion efficiency, we used the FWM scheme with orthogonal pump.In Fig. 3, the input signal and two pumps were coupled into the SOA via optical couples. Polarization controllers were used to align pumpl and the input signal to the TE mode of the SOA, and Pump 2 coming from tunable tothe TM mode. In the experiment, efficiency η on wavelength conversion wasmeasured as indicated with the full dots in Fig. 4. In comparing with the theoretical result shown in Equation (1) below, we found that the relativeconversion efficiency function R(△λ) is lower according to theoretical calculation. η= P1 +P2 +GTE+GTE+GTM+R(△λ)Fig. 4. Conversion efficiency and gain versus scanning wavelength. Full and empty dots: experimental and theoretical for conversion efficiency, respectively. Triangle, plus and square: gain of pump2, pump1 and signal, respectively. Solid line: total sum of gains.We measure the relative conversion efficiency function R (△λ) because it is not the same as that of theoretical calculation, shown in Fig. 5. Accordingto the result measured from the experiment, R(△λ) is -33.0 dBm, oncondition that △λ is 0.4nm.The conversion efficiency calculated from Equation (1) is shown by the empty dots in Fig. 4, here R(△λ) =-33.0 dBm. As can be seen in Fig. 4, the theoretical result is in good agreement with experimental result.Fig. 5. Measured conversion efficiency η (empty square) and relative conversion efficiency R(△λ) (full square) versus wavelength shift.The relative conversion efficiency measured in this study should be useful for the efficiency of wavelength converter estimated. This is very meaningful to the design of gain semiconductor optical amplifiers. Due to high relative conversion efficiency function and orthogonal-pump FWM scheme, the conversion efficiency reaches up to -8.7 dB, and its variation is < 0.9 dB over C-band and < 4.5 dB over L-band. The experimental result shows that the wavelength conversion may be realized by the use of orthogonal-pump FWM in SOA. In addition, the result that FWM is obtainedwithin the rang from 1500 nm to 1640 nm demonstrates that the technique for optical wavelength conversion is applied not only in the present channel bandwidth but also in the future for widening channel bandwidth.3. Polarization-Independent Wavelength Conversion with DoublePumpsTo obtain large wavelength conversion range, the scheme of FWM with orthogonal pumps in SOA is investigated instead of codirectional polarization pumps. Although broad wavelength conversion is got by two orthogonal-polarized beams, the uncontrollable nature of the polarization state of the input signal, in the real communication network, induces the uncertainty of the conversion efficiency. The investigations on solving such problem are significant.Fig. 6. Conversion scheme with polarization-diversityThe scheme of polarization-independent wavelength conversion with external pump sources is shown in Fig. 6. The signal and pump1 are split into two orthogonal-polarized beams by the polarization beam-splitter 1 (PBS1). Similarly, the tunable pump 2 is also split into two orthogonal-polarizedbeams by the PBS2. After being transmitted through OC2 and OC3, the signal, pump 1 from PBS1 and pump2 from PBS2 are injected into the opposite facets of SOA. The polarization state of signal and the Pump 1 is aligned with the TM (TE) mode in the SOA by adjusting PC4 (PC5). Fiber PC7 and PC6 make the polarization states of pump 2 parallel to the TE mode and TM mode of SOA, respectively. In this way, along each of the propagation directions, polarization states of the two pumps are orthogonal. The two SOA outputs go back through the same path of the input and then are injected back in the PBS1 before they are coupled into one beam. The output from the Port 3 of the optical circulator is detected by OSA.Fig. 7 Output spectra for input signal polarization producing (a) maximum or (b) minimum power of the converted signal Figure 7(a) and (b) show the maximum and minimum converted signal power at the output of the circulator when the incident signal has an arbitrary polarization state. The maximal change of the converted signal power is less than 1.3 dBm.Figure 8 indicates the relationship between the conversion efficiency andthe wavelength of pump 2. When the wavelength of pump 2 changes between 1510 nm and 1610 nm, the conversion efficiency is between -25.0 dBm and -35.0 dBm.Fig. 8. Output conversion efficiency versus scanning wavelength4. Polarization-Independent Wavelength Conversion Using a Single-RingFiber Laser with a Semiconductor Optical AmplifierIn the above experiment of polarization-independent wavelength conversion, ultrabroad-band wavelength converter with flattening conversion efficiency is able to be implemented, however, two pumps are needed. Therefore, we design a new experimental scheme that ultrabroad-band wavelength converter with flattening conversion efficiency may be realized by making use of FWM in ring fiber laser, and eliminate one of the external pumps. In the present experiment with self pump, we first resolve polarization-independent wavelength conversion for the arbitrary polarization state of the signal. Due to its simplicity and practicality, the wavelength converter is promising in the future.Figure 9. Experimental setup.Fig. 9 shows the scheme of polarization-independent wavelength conversion. The signal and the pump 1 waves are split into two orthogonal-polarized parts by the polarization beam-splitter 1 (PBS1). The two parts are then each transmitted into one facet of the SOA, where they are counter-propagating with each other. Another tunable pump2 comes from the SOA ring laser in which a FBG is introduced for both reflecting mirror and choosing wavelength operating in ring laser. In order to obtain counter-propagating waves that is stimulated in the fiber ring, the polarization of pump waves from P2 split by PBS2 is aligned with the same TM mode in the SOA by adjusting PC6 and PC7. The polarization of waves from S and P1 split by PBSl is aligned similarly with TE mode by adjusting PC4 and PC5. In this way, P1 and P2 will have orthogonal polarization in each propagating direction. The converted signals coming from the opposite facets of the SOA, together with two pump beams and the input signal, go back through the samepath of the pump2 and are combined at the polarizing beam splitter (PBS2). The output of the converted signal is obtained from a 3-dB optical coupler (OC2) and analyzed by an optical spectrum analyzer (OSA).Fig. 10 shows the experimental results obtained from the output port of the OC2. Fig. 10(a) and (b) show the maximum and minimum output power of converted signal (C) during the total polarization range of the signal, respectively. The maximum variation of the conjugate power is less than 1.3 dBm.Fig. 10. Output spectra at OC2 for signal polarization producing (a) maximum or (b) minimum of the conjugate wave power.Fig. 11 shows the measured conversion efficiency η versus wavelengthshifts. The conversion efficiency, defined as the output signal power at OC2 divided by the input signal power at the SOA, is measured within the range from -18.5 dBm to -22.0 dBm. The changes of conversion efficiency is < 3.5 dB over the wavelength range from 1529 nm to 1569 nm (C-band). The wavelength range of the tunable FBG used in this work limits the maximum wavelength shift in the measurement.Fig. 11. Conversion efficiency versus scanning wavelength5. The Application of Fiber Brag GratingIn the experiment, besides the converted signal in the output waves, there are other components that are filtered out by optical filter. Because the converted signal changes with input signal, such a filter should be tuned. The optical filter with a tunable range of 10 nm is implemented using FBG whose wavelength reflected is controlled by refractive index and grating period that are changed by strain.In wavelength conversion, fiber grating also plays the role of the component of pump sources with tuning and filtering. In addition to itsapplication in the above figure 9, FBG also becomes filtering component of external pump resource with tuning. A single frequency narrow-width fiber laser is obtained with 10 mW over a range of 35 nm (c-band).6. Investigation on Reflected Spectrum of Fiber Grating with Phase Shifted.In the above wavelength converters, one of external pump resources is eliminated if ordinary FBG is used in the ring fiber laser. If Brag grating is written on polarization-maintaining fiber, its transmission spectrum is characterized as very narrow linewidth and orthogonal polarization. If PM FBG replaced ordinary FBG in ring fiber laser, wavelength converter with orthogonal-polarization pumps would be realized instead of any external pump resource. We theoretically discuss reflected spectrum of PM FBG with phase shifted, due to the present restriction of technology.Figure 12 shows the reflected spectrum of PM FBG with single phase shifted.Continuous and dashed lines express transmission peaks of fast-axis mode and slow-axismode, respectively, (a) small difference of refractive index; (b) large difference ofrefractive index...