Figure 6.21: Time output signal of the FBG based receiver with the POF link
A peak-to-peak voltage Vpp ≈ 8 mV is measured. The shape is irregular because the TLD was not tuned to obtain the same shape as in the back-to-back system. The measured Vppfor the back-to-back configuration is measured to be 10 mW. The difference is a reduction of the time signal of about 20 %. This could be because the TLD was not tuned to obtain the same shape as in a back-to-back system or we may conclude that this effects is because of the reduction on contrast ratio due to the operation in a multimode link.
6.3 RF linewidth
An important issue is the size of the linewidth of the generated microwave carriers. In this section the linewidth is measured for the FP based receiver with the POF link and for the FBG based receiver with the POF link.
6.3.1 FP based receiver with the POF link
The measuring setup used is shown in Figure 6.4. The resolution bandwidth of the RFSA is chosen as small as possible. Due to the very small movement in carrier frequency, it was not possible to measure in a smaller frequency span than 1 kHz. The carrier frequency would change while measuring. The linewidth of the 17.2 GHz signal can be seen in Figure 6.22.
Figure 6.22: Linewidth measurement of the 17.2 GHz carrier of FP based receiver with the POF link
The 3dB linewidth of the generated 17.2 GHz carrier is lower than 100 Hz. The resolution bandwidth of the RFSA was set to 100 Hz, so we are basically measuring the bandwidth of the RFSA. Anyway, we can safely state that the 3 dB linewidth of the generated 17.2 GHz carrier for a FP based receiver with the POF link is less than 100 Hz.
6.3.2 FBG based receiver with the POF link
We want to see what the differences on the linewidth of the generated carriers by the FP and FBG based receivers respectively. The measuring setup is shown in Figure 6.10. The measured linewidth of the 17.2 GHz carrier is shown in Figure 6.23.
6.3. RF LINEWIDTH 67
Figure 6.23: Linewidth measurement of the 17.2 GHz carrier of FBG based receiver with the POF link
The 3 dB linewidth of the generated 17.2 GHz carrier is lower than 100 Hz. The RFSA parameters are equal to the RFSA parameters set of the FP based receiver linewidth measurement. The linewidth of the 17.2 GHz carrier generated by the FP based receiver is equal to the linewidth of the 17.2 GHz carrier generated by the FBG based receiver.
Furthermore we can conclude that the (dispersive) POF link has no effect on the electrical linewidth.
Chapter 7
Conclusions and Recommendations
Two full multimode Fabry-Perot (FP) and Fabry-Perot Fiber Bragg Grating (FP-FBG) based radio over POF receivers are designed, constructed and are operational. The perfor-mance of both the FP and the FP-FBG based receivers are measured for a back-to-back system and for a system with 300m of POF as a link.
Fabry-Perot based receiver
• The 3mm thick wafer Fabry-Perot etalon shows no contrast because the flatness of the two (sawed and manually polished) mirrors are not sufficient.
• The power of the 17.2 GHz generated carrier in a back-to-back system with the 400 µm thick InP FP fluctuates with a maximum of 39 dB in 30 minutes. A slow modal noise time constant is observed due to the on- and off-switching of the air conditioning and because of other yet unknown system factors as well.
• The power of the 17.2 GHz generated carrier in a system with the 400 µm thick InP FP and 300m of POF as a transmission link fluctuates with a maximum of 31 dB in 30 minutes. The RF power carrier fluctuates because of modal noise due to spatial filtering of the light in the POF taper itself and due to spatial filtering at the interface of the output of the imaging and coupling system and receiving fiber, because the output spotsize is larger than the 50 µm core receiving fiber. The air conditioning stimulates this effect as well.
• The linewidth of the 17.2 GHz carrier in a system with 300m of POF as a transmis-sion link is less than 100 Hz.
• The pinhole finesse is increased by tapering the POF fiber.
• The calculated pinhole finesse is better in our solid FP wafer because the high refractive index of the wafer will reduce the beam divergence in the FP itself due to refraction.
69
• The contrast ratios of the transmission peaks are equal over a broad wavelength range.
• The wavelength separations of the transmission peaks are quite equal over a broad wavelength range.
Fabry-Perot Fiber Bragg Grating based receiver
• The power of the 17.2 GHz generated carrier in a back-to-back system fluctuates with a maximum of 14 dB in 30 minutes. A very fast time constant is observed.
• The power of the 17.2 GHz generated carrier in a system with 300m of POF as a transmission link fluctuates with a maximum of 40 dB in 30 minutes because of modal noise due to spatial filtering of the light at the transition from the large core POF to the 62.5µm FBG and due to spatial filtering at interface of the output of the FBG and the 50 µm receiving fiber (which is connected to the photodetector).
The air conditioning stimulates this effect as well because the FP-FBG is highly temperature sensitive.
• The linewidth of the 17.2 GHz carrier in a system with 300m of POF as a transmis-sion link is also less than 100 Hz.
• The contrast ratios of the transmission peaks are not equal. But if two peaks are used to sweep through, this effect can be neglected.
• The wavelength separations of the transmission peaks are not equal.
Comparison between the Fabry-Perot and the Fabry-Perot Fiber Gragg Grating based receivers
The FP-FBG in comparison with the FP has the advantages of the ease of handling, the small size, easy coupling and low loss. The FP has the advantage that the filter can be operated over a broad wavelength range, that the peak separation and spacing is constant, and that the imaging and coupling system is very flexible for using other FP filters. A table was used to qualify the performance on multiple aspects qualitatively.
FP based receiver FP-FBG based receiver
Table 7.1: Comparison on performance of the FP and the FP-FBG based receeivers
Recommendations
Due to the fact that both filters have their advantages and disadvantages, the actual decision to use which type of filter depends on the overall system requirements. Anyhow, recommendations can be done on how to improve both filters.
FP etalons can be found on the market, while FP-FBGs have to be made by cooperation with other research institutes such as the university of Valencia. On the other hand, the FP-FBG has major advantages on coupling loss and its ease of handling.
Recommendations on the Fabry-Perot etalons
• Increase the very low reflectivity finesse by increasing the reflectivity of the FP etalon mirrors to at least 60% to make the pinhole finesse the only limiting factor on total finesse if a contrast ratio of about 3 dB is needed.
• The pinhole finesse and ’walk-off’ can be improved by using a collimating lens with a larger focal length (but that requires a larger FP aperture). Or the pinhole finesse can be improved by making the input fiber core diameter smaller (e.g. by tapering).
Or the refractive index of the medium between the FP mirrors (such in our solid wafer FP) can be increased to increase the refraction and thus the divergence of the light beam.
• To obtain a FP based receiver with a finesse of 4.5 (about 10 dB contrast ratio), if the pinhole finesse is the limiting factor (and thus equal to 4.5), together with the 120 µm POF, the focal length of the collimating lens should be 13.6 mm if the emitting fiber is in the focal point of the collimating lens (worst-case situation). To let the pinhole finesse be the limiting factor, the reflectivity finesse should be larger than 20, which means that a reflectivity of 98 % is needed. These figures are for an air-spaced FP.
• Investigate the requirement for the optical periodic filter’s optimum contrast ratio needed in the system with the system parameters as used in the laboratory.
• Fabricate or buy a large (thick) Fabry-Perot etalon to make use of lenses with a high focal length to overcome the limits in pinhole finesse when higher contrast ratios are desirable.
• If lenses with larger focal lengths are desirable (for larger FP etalons), use aspheric lenses because of their excellent compensation for spherical aberrations (and thus their excellent coupling efficiency and reduction of the divergence of the nearly col-limated light beam).
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Recommendations on the fabry-Perot fiber Bragg grating filter
• Ask the university of Valencia for the possibilities for the fabrication of a 120 µm or 50 µm core FP-FBG to reduce the points where spatial filtering is done from two to one.
• Optimize the non constant contrast ratios and peak separations of the FBG as well as the operation range.
List of Figures
1.1 Schematic diagram of the Radio over POF system. . . . 3
1.2 Generated microwave signals for sinusoidal sweeping of the laser wavelength. 4 1.3 Generated microwave signals for triangular sweeping of the laser wavelength. 4 1.4 Relative powers of the harmonic components as function of the mirror reflec-tivity R of a Fabry-Perot optical filter for a triangular sweep signal.. . . 5
1.5 Schematic diagram of the radio over POF bi-directional system.. . . 5
1.6 Schematic diagram of the radio over POF receiver and its issues. . . . 6
2.1 Determination of the core area which can be lighted . . . 12
2.2 Loss as function of the wavelength for PMMA and PF based POF. . . . 13
2.3 Photograph of POF endface, and IR photograph of 1310nm light in core.. . . 15
2.4 Near field intensity profile at the output of 300m of POF with 1300nm sin-glemode launch . . . 15
3.3 A large beam divergence occurs when placing the emitting object in the focal plane of the lens. . . . 20
3.4 The beam divergence is reduced by placing the emitting object further away from the lens its focal plane. . . . 20
3.5 Spherical aberrations in a lens. . . . 21
3.6 Chromatic aberration in a lens. . . . 21
3.7 Achromatic lens: Compensated for spherical and chromatic aberrations . . . 22
3.8 Aspheric lens: Compensates for spherical aberrations . . . 22
3.9 Operating principle of an anti-reflection coating . . . 23
3.10 Model of the imaging system for generating a parallel beam and for coupling power from the POF to fiber patch cables by using different lens-types. . . . 25
3.11 Drawing of the imaging and coupling system. . . . 27
3.12 General setup for coupling light from the POF to different fiber patch cables by using different lens-types. . . . 28
4.1 Fabry-Perot operating principle and parameters. . . . 33
4.2 Fabry-Perot transmission curve model. . . . 34 73
4.3 Measuring setup for the Fabry-Perot wafers. . . . 37 4.4 Transmission curve and loss of the 3mm thick Fabry-Perot wafer. . . . 38 4.5 Loss, contrast ratio and free spectral range of the 400 µm thick Fabry-Perot
wafer filter. . . . 39 4.6 Shifting the entire transmission curve in wavelength by rotating the filter 0.5◦. 40 4.7 Fabry-Perot Fiber Bragg Grating made in 62.5 µm core multimode fiber.. . . 41 4.8 Measuring setup for the Fabry-Perot Fiber Bragg Grating . . . 41 4.9 Measurement of the FP-FBG filter . . . 42 5.1 Microwave waveguide filter. . . . 48 5.2 Output spectrum of the 17.2 GHz waveguide filter when measured with an
electrical network analyzer.. . . 49 5.3 Bandwidth and loss of the 17.2 GHz waveguide filter. . . . 49 6.1 Measurement setup for measuring the RF carrier power stability and RF
carrier generation for the FP based receiver in a back-to-back system. . . . . 52 6.2 Carrier generation with the FP based receiver in a back-to-back system . . . 53 6.3 RF carrier stability of the 17.2 GHz carrier of the FP based receiver.. . . 54 6.4 Measurement setup for measuring the full RF spectrum and 17.2 GHz carrier
power stability for a FP based receiver with the POF link. . . . 54 6.5 Full receiver output RF spectrum of the FP based receiver with the POF link 55 6.6 RF carrier stability at 17.2 GHz of the FP based receiver with the POF link. 56 6.7 Measurement setup for measuring the RF carrier power stability and RF
trace for the FBG based receiver. . . . 56 6.8 Full RF spectrum of the FBG based receiver . . . 57 6.9 RF carrier stability at 17.2 GHz of the FBG based receiver . . . 58 6.10 Measurement setup for measuring the RF carrier power stability and RF
trace for a FBG receiver with POF link. . . . 58 6.11 Full receiver output RF spectrum of the FBG based receiver with the POF link 59 6.12 RF carrier stability at 17.2 GHz of the FBG based receiver with the POF link 59 6.13 Shape of the electrical time signal for an arbitrary fsw and Psw choice. . . . . 60 6.14 Shape of the electrical time signal for a chosen fsw and Psw such that an
electrical time signal is obtained without discontinuities. . . . 61 6.15 Measurement setup for measuring the time plot of a FP receiver in a
back-to-back configuration. . . . 61 6.16 Time output signal of the FP based receiver in a back-to-back configuration
with Psw = -6 dBm . . . 62 6.17 Time output signal of the FP based receiver in a back-to-back configuration
with Psw = -13.8 dBm . . . 63 6.18 Measurement setup for measuring the time plot of a FBG receiver in a
back-to-back configuration. . . . 63 6.19 Time output signal of the FBG based receiver in a back-to-back configuration 64 6.20 Measurement setup for measuring the time plot of a FBG receiver with the
POF link. . . . 64 6.21 Time output signal of the FBG based receiver with the POF link . . . 65
LIST OF FIGURES 75
6.22 Linewidth measurement of the 17.2 GHz carrier of FP based receiver with the POF link . . . 66 6.23 Linewidth measurement of the 17.2 GHz carrier of FBG based receiver with
the POF link . . . 67 A.1 Setup for measuring the laser stability. . . 77 A.2 Laser Stability . . . 77 B.1 Modal noise when POF connected to power meter sensor by using a SC fiber
adapter. . . . 79 B.2 Modal noise when POF placed closely in front of the AQ2717 power sensor. . 80 C.1 First mechanical setup . . . 81 D.1 Mounting plate for mounting the opto-mechanical components (figures are in
millimeters) . . . 83 E.1 Mounting rings for mounting the lenses into a 0.5 inch lens tube. . . . 85 F.1 Market search for a high-bandwidth multimode photodetector. . . . 87
Appendix A
Laser Stability
Firstly the stability of the laser is measured. The setup can be seen in Figure A.1. The output of the laser is by means of a single mode path cable connected to the power meter (an isolator was included but did not give other measurement results). Every 10 seconds a power value is stored for over 30 minutes. The laser output is single mode pigtailed, so
Figure A.1: Setup for measuring the laser stability
only one mode exists and no modal noise can be present. However, we want to see if the laser is stable, and does not influence the measurements which are preformed later on. We can see in Figure A.2 that the laser is very stable.
Figure A.2: Laser Stability: Average Power = 10.002 dBm; Minimum Power = 9.996 dBm;
Maximum Power = 10.009 dBm; Power fluctuation = 0.013 dBm
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Appendix B
Power Sensor Modal Noise
A question was raised if the measured modal noise inside the POF was introduced not by the POF, but by spatial filtering in the power sensor itself. The light from the POF could fall partly outside the active area of the photodetector inside the power sensor head.
This would result in spatial beam filtering and so introduce the modal noise. The output power of the POF was measured with the AQ2717 power sensor head. The AQ2717 has a photodetector with an active area of about 5mm and is designed to accept light from large aperture fibers [21]. An SC fiber adapter was used to connect the POF to the power sensor head. To measure if all power from the POF is collected by the AQ2717 power sensor, 2 measurements where done. One measurement was done with the POF connected to the fiber adapter and the other measurement was done in which the POF was placed directly in front of the photodetector inside the power sensor head by means of an XYZ translator stage. With the last setup it is definite that all power is collected from the POF by the photodetector due to the small separation of the POF endface and the photodetector. In Figures B.1 and B.2 the measurement results are shown. The output
Figure B.1: Modal noise in POF with POF connected to AQ2717 power sensor by using an SC fiber adapter: Average Power = -0.3561 dBm; Minimum Power = -0.5020 dBm;
Maximum Power = -0.1820 dBm; Power fluctuation = 0.3200 dB
79
Figure B.2: Modal noise in POF with POF placed closely in front of AQ2717 power sensor:
Average Power = -0.4798 dBm; Minimum Power = -0.7320 dBm; Maximum Power = -0.3100 dBm; Power fluctuation = 0.4420 dB
of the two measurement give the same power fluctuations The measurements were done a few times. The results are for both measurements very dependent on the environment.
Measurements where done with the air conditioning turned on and off, in the weekend, in the morning, afternoon and evening. Each time, there was no difference in measured power fluctuations between measuring with the SC fiber adapter connected to the power sensor, and measuring with the POF placed very close to the power sensor’s active area.
We can conclude that all the power from the POF is collected when using the SC fiber adapter to connect the POF to the power sensor head.
Appendix C
First version mechanical setup
Figure C.1: First mechanical setup
81
Appendix D
Mounting plate
Figure D.1: Mounting plate for mounting the opto-mechanical components (figures are in millimeters)
83
Appendix E
Lens mounting rings
Figure E.1: Mounting rings for mounting the lenses into a 0.5 inch lens tube.
85
Appendix F
Photodetector market search
Figure F.1: Market search for a high-bandwidth multimode photodetector.
87
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