Single Mode and Multi-Mode Optical Fibers Optiwave [PDF]

Zitiervorschau

Experiment Name: Single Mode and Multimode Optical Fibers

Date: 5/3/2019

Group: Ali Ra'ad Mustafa Kareem Muhammed Fadhil Ameer Toran

Procedure: 1. The following transmitter and receiver circuits were connected, with a direct transmission channel and the properties below. The properties of the transmitted and received signals were observed. Bit rate 2.5Gbps Sequence Length 128bits Samples per bit 64 Laser frequency 1552.52nm (third window)

2. The transmission channel was changed to a 50km single mode optical fiber. Then the length was changed to 300km. The properties of the transmitted and received signals were observed in both cases. 3. The transmission channel was changed to a multimode optical fiber, and the properties of the transmitted and received signals were observed in the following cases: Case 1: Bit rate 2.5Gbps Laser frequency 1552.52nm (third window) Length of fiber 1km Case 2:

Bit rate Laser frequency Length of fiber Case 3: Bit rate Laser frequency Length of fiber Case 4: Bit rate Laser frequency Length of fiber Case 5: Bit rate Laser frequency Length of fiber Case 6: Bit rate Laser frequency Length of fiber

2.5Gbps 850nm (first window) 1km 100Mbps 850nm (first window) 1km 100Mbps 850nm (first window) 100m 10Gbps 850nm (first window) 100m 100Gbps 850nm (first window) 100m

Results: 1. Direct transmission:

Figure 1: Pulse Generator Output

Figure 2: Optical Modulator Output (time domain)

Figure 3: Optical Modulator Output (frequency domain)

Figure 4: Photodetector Output (noise and signal)

Figure 5: Photodetector Output (noise)

Figure 6: Low Pass Filter Output (noise and signal)

Figure 7: Low Pass Filter Output (noise)

2. Single Mode Fiber (50km):

3. Single Mode Fiber (300km)

4. Multimode Fiber case 1:

5. Multimode Fiber case 2:

6. Multimode Fiber case 3:

7. Multimode Fiber case 4:

8. Multimode Fiber case 5:

9. Multimode Fiber case 6:

Discussion: 1. In the direct transmission channel, almost all of the sent power is received with very low losses, with the attenuation occurring due to noise and free space losses. 2. In the single mode fiber case, when the length was 50km, the signal arrived with almost only 10% of its original power, but it still maintained its original form. When the length of the fiber was changed to 300km, the signal arrived with 1% of its original power and lost its shape due to mixing with the noise floor. These power losses occur due to the chromatic dispersion phenomenon occurring within the fiber, with the Dispersion constant being 16.75ps/nm/km, and a spectral width of almost 1nm (from figure 3) we can expect a delta ∆T: ∆T = Dc*L*∆λ = 16.75*50*1000*10^(-9) = 837.5 µs for L=50km However, when L=300: ∆T = Dc*L*∆λ = 16.75*300*1000*10^(-9) = 5025 µs = 5.025 ms which is relatively high. 3. In the multimode fiber case 1, the signal lost almost all of its power and mixed with the noise floor. This happened due to the high operational frequency of the laser (1552.52nm) which led to a very high number of modes in the fiber, which itself led to high intermodal dispersion: V = 2*a*π*sqrt(n1^2-n2^2)/λ M = V^2/2 (SI) M = v^2/4 (GI)

This attenuating effect was solved in case 2, when the frequency of the laser was changed to 850nm, lowering the number of modes and the intermodal dispersion. In case 3, we notice that lowering the bit rate to 100MHz offered a relatively better performance, because by lowering the bit rate, we increase the time of the single bit, lowering the effect of ∆T. In case 4, the length was decreased, which led to an almost 50% increase in the received power. This happened because by decreasing the length, dispersion also decreased: ∆T = L*n1^2*∆/c*n2 Cases 5 and 6 do the opposite of case 3, the bit rate was raised to 10GHz and 100GHz, well beyond the fiber's BL constant.